SAFETY
As in all occupations, safety is paramount. Because there are numerous safety codes and regulations in place, we recommend that you always read all labels and the Owner’s Manual carefully before installing, operating, or servicing the unit. Read the safety information at the beginning of the manual and in each section. Also read and follow all applicable safety standards, especially ANSI Z49.1, Safety in Welding, Cutting, and Allied Processes.
ANSI Z49.1:, Safety in Welding, Cutting, and Allied Processes is available as a free download from the American Welding Society at: http://www.aws.org
Here is a list of additional safety standards and where to get them.
Safe Practices for the Preparation of Containers and Piping for Welding and Cutting, American Welding Society Standard AWS F4.1, from Global Engineering Documents (Phone: 1-877-413-5184, website: www.global.ihs.com).
National Electrical Code, NFPA Standard 70, from National Fire Protection Association, Quincy, MA 02269 (Phone: 1-800-344-3555, website: www.nfpa.org and www. sparky.org).
Safe Handling of Compressed Gases in Cylinders, CGA Pamphlet P-1, from Compressed Gas Association, 14501 George Carter Way, Suite 103 Chantilly, VA 20151(Phone: 703-788-2700, website:www.cganet.com).
Safety in Welding, Cutting, and Allied Processes, CSA Standard W117.2, from Canadian Standards Association, Standards Sales, 5060 Spectrum Way, Suite 100, Mississauga, Ontario, Canada L4W 5NS (Phone: 800-463-6727, website: www.csa-group.org).
Safe Practice For Occupational And Educational Eye And Face Protection, ANSI Standard Z87.1, from American National Standards Institute, 25 West 43rd Street, New York, NY 10036 (Phone: 212-642-4900, website: www.ansi.org).
Standard for Fire Prevention During Welding, Cutting, and Other Hot Work, NFPA Standard 51B, from National Fire Protection Association, Quincy, MA 02269 (Phone: 1-800-344-3555, website: www.nfpa.org.)
OSHA, Occupational Safety and Health Standards for General Industry, Title 29, Code of Federal Regulations (CFR), Part 1910, Subpart Q, and Part 1926, Subpart J, from U.S. Government Printing Office, Superintendent of Documents, P.O. Box 371954, Pittsburgh, PA 15250-7954 (Phone: 1-866-512-1800) (There are 10 OSHA Regional Offices—phone for Region 5, Chicago, is 312-353-2220, website: www.osha.gov).
Booklet, TLVs, Threshold Limit Values, from American Conference of Governmental Industrial Hygienists (ACGIH), 1330 Kemper Meadow Drive, Cincinnati, OH 45240 (Phone: 513−742−2020, website: www.acgih.org).
Towing a Trailer − Being Equipped for Safety, Publication from U.S. Department of Transportation, National Highway Traffic Safety Administration, 1200 New Jersey Ave. SE, Washington, D.C. 20590 (Phone: 1-800-424-9071, website: www.fhwa.dot.gov)
U.S. Consumer Product Safety Commission (CPSC), 4330 East West Highway, Bethesda, MD 20814 (Phone: 301-504-7923, website: www.cpsc.gov).
Applications Manual for the Revised NIOSH Lifting Equation, The National Institute for Occupational Safety and Health (NIOSH), 1600 Clifton Rd, Atlanta, GA 30333 (Phone: 1-800-232-4636,website: www.cdc.gov/NIOSH).
Prepared by the Miller Electric Mfg. Co. Training Department.
©2016 Miller Electric Mfg. Co.
The contents of this publication may not be reproduced without permission of Miller Electric Mfg. Co., Appleton, Wisconsin, U.S.A.
WARNINGThis document contains general information about the topics discussed herein. This document is not an application manual and does not contain a complete statement of all factors pertaining to those topics.
The installation, operation, and maintenance of arc welding equipment and the employment of procedures described in this document should be conducted only by qualified persons in accordance with applicable codes, safe practices, and manufacturer’s instructions.
Always be certain that work areas are clean and safe and that proper ventilation is used. Misuse of equipment and failure to observe applicable codes and safe practices can result in serious personal injury and property damage.
Welding Process and Filler Metals Training Series:
Welcome to the Welding Process and Filler Metals Training Series. This training series was developed for the purpose of providing a basic set of educational materials that can be used individually or in a classroom setting.
The topics covered in the series are:
Welding Processes
•Basic Electricity For Welding
•Engine-Driven Power Sources
•Shielded Metal Arc Welding
•Gas Tungsten Arc Welding
•Troubleshooting Welding Processes
Filler Metals
Please note, this series was not developed to teach the skill of welding or cutting, but rather to provide a foundation of general knowledge about the various processes and related topics.
The GTAW (TIG) Process
The necessary heat for Gas Tungsten Arc Welding (TIG) is produced by an electric arc maintained between a nonconsumable tungsten electrode and the part to be welded. The heat-affected zone, the molten metal, and the tungsten electrode are all shielded from the atmosphere by a blanket of inert gas fed through the GTAW torch. Inert gas is inactive, or deficient in active chemical properties. The inert shielding gas blankets the weld and excludes the active properties in the surrounding air. It does not burn, and has no effect on the metal. Inert gases such as argon and helium do not chemically react or combine with other gases. They possess no odor and are transparent, permitting the welder maximum visibility of the arc. In some instances a small amount of reactive gas such as hydrogen can be added to enhance travel speeds.
The GTAW process can produce temperatures of up to 35,000°F (19,426°C). If filler metal is required to make the weld, it may be added manually in the same manner as it is added in the oxyacetylene welding process. There are also a number of filler metal feeding systems available to accomplish the task automatically. Figure 1 shows the essentials of the manual GTAW process using a water cooled torch.
Figure 1, Essential Components Of A Water Cooled GTAW System
Advantages of the GTAW Process The greatest advantage of the GTAW process is that it will weld more kinds of metals and metal alloys than any other arc welding process. TIG can be used to weld most steels including stainless steel, nickel alloys such as Monel® and Inconel®, titanium, aluminum, magnesium, copper, brass, bronze, and even gold. GTAW can also weld dissimilar metals to one another such as copper to brass and stainless to mild steel.
The concentrated nature of the GTAW arc permits pinpoint control of heat input to the workpiece resulting in a narrow heat-affected zone. A high concentration of heat is an advantage when welding metals with high thermal conductivity such as aluminum and copper.
Figure 2, GTAW Welding Arc
Flux is not required with this process so there is no slag to obscure the welder’s vision of the molten weld pool. The finished weld will not have slag to remove between passes. Entrapment of slag in multiple pass welds is seldom seen.
No Sparks, Spatter, or Noise
In the GTAW process there is no transfer of metal across the arc. There are no molten globules of spatter to contend with, and no sparks produced if the material being welded is free of contaminants. Also, under normal conditions the GTAW arc is quiet without the usual cracks, pops, and buzzing of Shielded Metal Arc Welding (SMAW or Stick) and Gas Metal Arc Welding (GMAW or MIG). Generally, the only time noise will be a factor is when a pulsed arc or the AC welding mode is being used.
The GTAW process itself does not produce smoke or injurious fumes. If the base metal contains coatings or elements such as lead, zinc, nickel, or copper that produce fumes, these must be contended with as in any fusion welding process on these materials. If the base metal contains oil, grease, paint or other contaminants, smoke and fumes will be produced as the heat of the arc burns them away. The base material should be cleaned to make the conditions most desirable.
The GTAW process produces very high quality welds, however it is a slow process as compared to GMAW or even SMAW welding. GTAW requires a high skill level of the operator. Excellent hand-eye coordination is necessary and GTAW is difficult to learn. A high degree of patience is also needed for an operator to become proficient as this process is much slower and is very precise. GTAW is very susceptible to contamination from external sources. Oil, grease, dust, dirt, paint, and debris, must be removed prior to welding to prevent weld failures or weld discontinuities.
The arc rays produced by the GTAW process tend to be brighter than those produced by SMAW and GMAW due to the absence of visible fumes and smoke. The increased amounts of ultraviolet rays from the arc also cause the formation of ozone and nitrous oxides. Care should be taken to protect skin with the proper clothing and protect eyes with the correct shade lens in the welding hood. When welding in confined areas, concentrations of shielding gas may build up and displace oxygen. Make sure that these areas are ventilated properly. Always read and follow all applicable safety standards, especially ANSI Z49.1, Safety in Welding, Cutting, and Allied Processes available for free from the American Welding Society at: http://www.aws.org.
GTAW is a very clean process. It is considered to be one of the most metallurgically pure arc welding processes. It is desirable from an operator point of view because the operator has the control to finesse the weld to look just right. The welder must maintain good welding conditions by properly cleaning material, using clean filler metal and clean welding gloves, and by keeping oil, dirt and other contaminants away from the weld area. Cleanliness cannot be overemphasized, particularly on titanium, aluminum, and magnesium. These metals are more susceptible to contaminants than are ferrous metals. Porosity in aluminum welds has been shown to be caused by hydrogen. Consequently, it is important to eliminate all sources of hydrogen contamination such as moisture and hydrocarbons in the form of oils and paint.
If you have ever had the experience of hooking up a car battery backwards, you were no doubt surprised at the amount of sparks and heat that can be generated by a 12 volt battery.
When welding was first discovered in the early 1880s it was performed with batteries. (Some batteries used in early welding experiments reached room size proportions.) The first welding machine was developed by N. Benardos and S. Olszewski of Great Britain and was issued a British patent in 1885 (Figure 3). It used a carbon electrode and was powered by batteries, which were in turn charged with a dynamo, a machine that produces electric current by mechanical means.
Figure 3, Original Carbon Electrode Welding Apparatus — 1885
Figure 4 shows what a welding circuit using a battery as a power source would look like. Figure 4, A Simple Welding Circuit Showing Voltage Source and Current Flow
The two most basic parameters we deal with in welding are the amount of current in the circuit, and the amount of voltage pushing it. Current and voltage are further defined as follows:
•Current — The number of electrons flowing past a given point in one second. Measured in amperes (amps).
•Voltage — The amount of pressure induced in the circuit to produce current flow. Measured in voltage (volts).
Resistance in the welding circuit is represented mostly by the welding arc and to a lesser extent by the natural resistance of the cables, connections, and internal components. Even though the resistance caused by the connections and cables is much less than the resistance of the arc, they have a significant impact on the quality of the welding arc.
All connections and cables should be inspected and replaced routinely to ensure high arc quality and efficient use of the power supply. When cables and connections build up resistance, heat is generated and resistance builds exponentially. When current loses the ability to flow to the arc, the arc gets “cold” and the proper amount of energy is not available to make a sound weld. When this occurs the welder turn up the machine amperage and now the machine is delivering more power than is being used at the arc; the uses more energy than is required to make the weld.
Books have been written on the theory of current flow in an electrical circuit. For the sake of simplicity just remember that in welding we follow the theory that current flow is from negative to positive. Early researchers were surprised at the results obtained when the battery leads were switched when welding with DC current.
After alternating current (AC) became available for welding with the use of transformer power sources, welds produced were more difficult to accomplish and of lesser quality than those produced with direct current (DC). Although these AC transformer power sources greatly expanded the use of commercial power for SMAW (Stick), they could not be used for GTAW because as the current approached the zero value, the arc would extinguish.
Motor generators followed quickly. These were machines that consisted of an AC motor, that turned a generator, that produced DC for welding. The output of these machines could be used for both SMAW and GTAW.
Figure 5, Early Hobart Motor Generator
It was with a motor generator power source that GTAW was first accomplished in 1942 by V.H. Pavlecka and Russ Meredith while working for the Northrup Aviation Company. Pavlecka and Meredith were searching for a means to join magnesium, aluminum and nickel, which were coming into use in the military aircraft of that era (Figure 6).
Figure 6, The Original Torch and Some of the Tips Used by Pavlecka and Meredith to Produce the First GTAW Welds in 1942. The Torch Still Holds one of the Original Tungsten Electrodes Used in Those Experiments.
Although the selenium rectifier had been around for some time, it was the early 1950’s when rectifiers capable of handling current levels found in the welding circuit came about. The selenium rectifier had a profound effect on the welding industry. It allowed AC transformer power sources to produce DC. And it meant that an AC power source could now be used for GTAW welding as well as SMAW.
Adding high frequency to the weld circuit made AC power usable for TIG welding. The addition of this high voltage, low amperage, high kilohertz secondary circuit to the welding circuit provided a method to keep the AC arc established as the weld power AC cycle passes through zero. High frequency helps to stabilize the AC GTAW arc, and it also aids in arc starting without the risk of contamination. The later addition of remote current control, remote contactor control, and gas solenoid control devices evolved into the modern GTAW power source. Further advances such as Squarewave AC and Advanced Squarewave AC power sources have further refined the capabilities of this already versatile process.
Alternating current (AC) is an electrical current that has both positive and negative half-cycles. These components do not occur simultaneously, but alternately, thus the term alternating current. Current flows in one direction during one half of the cycle and reverses direction for the other half cycle. The half cycles are called the positive half and the negative half of the complete AC cycle.
Figure 7, 60 Hz AC Cycle as Seen on an Oscilloscope
The rate at which alternating current makes a complete cycle of reversals is termed frequency. Electrical power in the United States is delivered at a frequency of 60 cycles per second, or to use common terminology, 60 hertz (Hz). This means there are 120 reversals of current flow directions per second. The power input to an AC welding machine and other electrical equipment in North America is 60 Hz power. Outside of North America 50 Hz power is more commonly used. As this frequency increases, the magnetic effects accelerate and become more efficient for use in transformers, motors and other electrical devices. This is the fundamental principal on how an inverter welding power source works. In an inverter, increasing the AC welding output frequency has a major effect on welding arc performance. As frequencies increases, the arc becomes more stable, narrows, and becomes stiffer and more directional. Figure 8 represents various frequencies. 
Figure 8, An Oscilloscope Representation of 60, 120, and 2000 Hz Frequencies
The following sections discuss alternating current wave forms which represent the current flow in a circuit. Figure – 9 is what would be seen on an oscilloscope connected to a wall receptacle and shows the AC wave form known as a sine wave. This is the shape of the AC power supplied from the power company (single phase).
The wave forms represent the current flow as it builds in amount and time in the positive direction and then decreases in value before reaching zero. Then current changes direction and polarity, reaching a maximum negative value before rising to the zero value. This “hill” (positive half) and “valley” (negative half) together represent one cycle of alternating current. This is true for all AC wave forms.
Inverter AC welding power supplies have the ability to select four different wave shapes: sine, soft squarewave, advanced squarewave, and the triangular wave shape. Each wave shape produces different arc characteristics that have an affect on the weld. The welding arc width, penetration profile, and “feel” can all be controlled by changing the wave shape.
Sine Wave
Figure 9 is what would be seen on an oscilloscope connected to the output of an older GTAW welding power supply pre-dating 1976 when the squarewave essentially replaced the sine wave. There are very few models manufactured today that still produce an original AC sine wave output and those that do are no longer made for GTAW. The sine wave produces a very soft and wide arc for welding Aluminum. Figure 10 shows the shape of the sine wave on an inverter. The inverter based sine wave is changed to make the transition from EN to EP faster, helping eliminate rectification and arc outages without using a high frequency circuit. Figure 9, Standard Alternating Current Sine Wave
Figure 10, The Shape of a Sine Wave in an Advanced AC Inverter Power Supply
Squarewave / Soft Squarewave AC
The first squarewave AC output profile was patented in 1976 by Miller Electric Manufacturing for use in welding. Most GTAW power sources manufactured since, due to refinement of electronics, have the ability to rapidly make the transition between the positive and negative half cycles of alternating current. Today, the squarewave AC shape is considered to be the “conventional” AC delivery method for AC GTAW and is now referred to as a soft squarewave since there are now two types of square waves. Traditional transformer rectifier GTAW welding power supplies offer the soft squarewave where as inverter-based power supplies produce all of the wave shapes.
Figure 11, A Sine Wave (Blue) Compared With a Soft Squarewave (Green)
When welding with AC, the faster you transition between the two polarities (EN and EP), and the more time you spend at their maximum values, the more effective the machine is. Electronic circuitry makes it possible to make this transition almost instantaneous. Plus, the effective use of the energy stored in magnetic fields results in wave forms that are relatively square. They are not truly square due to electrical inefficiencies. However, the advanced squarewave GTAW welding power source has improved efficiencies and can produce a nearly perfect square wave shape. The arc is more stable than a sine wave and provides excellent puddle control.
Advanced Squarewave
An advanced squarewave is a nearly perfect squarewave and only available in an inverter based AC GTAW welding power source. Figure 12 shows an advanced squarewave shape. Inverters for GTAW, also referred to as advanced squarewave power supplies, incorporate fast switching electronics capable of switching current up to 50,000 times per second. This allows the inverter-type power source to be much more responsive to the needs of the welding arc. These electronic switches allow the output to change from positive to negative quickly whenever it is asked for. The output frequency of conventional squarewave and sine wave power sources is limited to 60 cycles per second, the same as the input power from the power company. An inverter-type power supply allows the AC welding frequency to be changed to meet the demands of the application. The advanced squarewave shape provides a sharp, responsive, dynamic welding arc for fine control when welding aluminum.
Figure 12, Advanced Squarewave
Triangular Wave
Advanced squarewave welding power supplies provide an additional wave shape to control the welding arc to meet the work demands. The triangular wave provides an arc dynamic that produces a “punch” to penetrate while reducing the overall heat input. This wave shape provides better control and reduced distortion on thin metals.
Figure 13, Triangular Wave Shape
Direct current is an electrical current that flows in one direction and has either a negative or positive polarity. A battery, for either a flashlight (dry cells) or an automobile (wet cells ), is a source of direct current, and has a positive and a negative terminal (pole). The conventional theory of electrical current flow, credited to Benjamin Franklin, states that electrons flow from the positive (+) terminal to the negative (–) terminal.
A direct current output welding machine also has a positive and a negative terminal. Polarity of the electrical current (or the direction of current flow) is determined by connecting the electrode cable, holder, and an electrode to either the positive or negative terminal. The work cable (and its clamp) is connected to the opposite terminal. The current flows from the negative (–) terminal to the positive (+) terminal in a single direction. This is the electron theory that is credited to Thomas Edison and is used in arc welding theory.
Polarity
The polarity of the direct current welding arc, or the direction of electrical current flow, is very important. The polarity of the current will have dramatic affects on the welding arc. The majority of GTAW is performed with the electrode connected to the negative (-) output terminal (pole) of the welding power source. Power sources with polarity switches will have the output terminals marked electrode and work, and make the connections internally. When the arc is established, electron flow is from negative to positive.
In DC arc welding, approximately 70% of the heat will be concentrated at the positive pole of the arc. Electrode negative produces more heat into the base metal with a nonconsumable electrode such as the tungsten used in the GTAW process. Because the polarity of DC is not always changing like alternating current, the arc is more stable with less fluctuation.
Figure 14, Heat Distribution for DCEN
Figure 15, Heat Distribution for DCEP
Direct Current Electrode Negative (DCEN)
Direct current electrode negative (DCEN) is used for TIG welding of nearly all metals. The torch is connected to the negative output of the power source and the work lead is connected to the positive output.
In a DCEN arc, approximately 70% of the heat will be concentrated at the positive side of the arc and the greatest amount of heat is distributed into the workpiece. This allows smaller tungsten electrodes to be used with DCEN than with DCEP or AC. It is also thought that the negatively charged electrons are striking the work and the positively charged gas ions are attracted toward the negative electrode. Figure 16 shows the affects of polarity with GTAW.
Figure 16, The Effects of DC Polarity and AC on Weld Characteristics.
Direct Current Electrode Positive (DCEP)
When welding with direct current electrode positive (DCEP), the torch is connected to the positive terminal on the welding power source. When using this polarity, the electron flow is still from negative to positive, however the electrode is now the positive side of the arc. In DCEP approximately 70% of the heat is distributed into the electrode and it becomes very hot. The electrode must be very large (even when low amperages are used) to prevent overheating and possible melting. A disadvantage of this polarity is that due to magnetic forces the arc will sometimes wander from side to side when making a fillet weld on two pieces of metal are at a close angle to one another. This phenomenon is similar to what is known as arc blow and can occur in DCEN, but DCEP polarity is more susceptible.
Generally DCEP is undesirable for GTAW and is rarely used except when welding aluminum or magnesium. Some non-ferrous metals, such as aluminum and magnesium, quickly form an oxide coating when exposed to the atmosphere. This material is formed in the same way rust accumulates on iron. It is a result of the interaction of the material with oxygen. The oxide that forms on aluminum is one of the hardest materials known. Before aluminum can be welded, this oxide, because it has a much higher melting point than the base metal, must be removed. The oxide can be removed by mechanical means like wire brushing or with a chemical cleaner, but as soon as the cleaning is stopped the oxides begin forming again. It is advantageous to have the “cleaning” take place while the welding is being done.
The oxide is removed by the welding arc during the electrode positive half of an AC cycle. The positively charged gas ions flow from the tungsten to the negative workpiece with DCEP. They strike the workpiece with sufficient force to break up and chip away the brittle aluminum oxide, and provide what is referred to as a “cleaning” action. It is important to note that although the term cleaning is used, it does not clean oil, grease, dirt, dust, and debris from the weld metal.
To weld with DCEP at 100 amperes a 1/4 in. diameter tungsten electrode would be required. This large electrode would naturally produce a wide weld pool resulting in the heat being widely spread over the joint area. By comparison, if DCEN were being used at 100 amperes a tungsten electrode of 1/16 in. would be sufficient.
Welding with Alternating Current Welding with alternating current is a combination of DCEP and DCEN. On an old sine wave-type welding power supply there was no control over how much time was spent on each polarity; they were essentially equal. When the squarewave was patented in 1976, the ability to skew the time spent in each polarity was added. This became known as balance control and it allowed the operator to skew the squarewave shape to control when the polarity change takes place during the AC cycle. Then came inverters which allowed the operator to make far more adjustment in the balance control. Inverters also allow the operator to control the frequency of the AC weld current as well as independently control how much amperage occurs on each half of the AC cycle. All of this control was essentially created to make welding aluminum more efficient.
Arc Rectification
When GTAW welding with alternating current, we find that the equal half cycle theory is not always exactly true. An oscilloscope will show that the electrode positive half cycle is of less magnitude than the electrode negative half cycle (Figure 17).
Figure 17, Rectified AC Sine Wave. Note the Positive Half Cycle is “Clipped Off”. The Missing Portion was Lost Due to Rectification of the Arc.
There are two theories accounting for this. One theory suggests the oxide coating on nonferrous metals such as aluminum acts as a rectifier, making it much more difficult for the electrons to flow from the work to the electrode, than from the electrode to the work.
The other theory is that molten, hot, clean aluminum does not emit electrons as easily as hot tungsten. This results in more current flowing from the hot tungsten to the clean molten weld pool (DCEN), and less current flowing from the clean molten weld pool to the electrode (DCEP). When any half of an AC cycle is “skipped”, or does not fully reach the amperage or the “shape” it is intended to, it is said to have “arc rectification”. Adding a high frequency circuit during the AC weld output helps eliminate arc rectification on a conventional sine or squarewave power source. Inverter power supplies switch the AC so quickly high frequency is not needed and rectification is almost nonexistent.
Balanced and Unbalanced Waveforms
Squarewave AC power sources have a front panel control that allows the welder to alter the squarewave shape by changing the time spent on each half of the AC cycle. A balanced AC wave shape spends equal amounts of cycle time in both the positive and negative half of the AC cycle. The total cycle time is the same as any cycle time, however this control is simply skewing the waveform to spend more or less time on the electrode negative half of the cycle. Figure 18 shows the effects of changing this adjustment.
Figure 18, Note the Variation In The Etching / Oxide Removal Bands For Balance .
Machines of this type are very common for TIG welding in industry today. A conventional squarewave machine has the ability to adjust the balance between 45 and 68% electrode negative. An inverter AC GTAW power supply will adjust between 30 and 99% electrode negative. (The frequency will determine the true ability to reach 99% without rectification.)
There are three terms commonly used to identify balance control: “max cleaning”, “max penetration”, and % EN. On a conventional squarewave power source, “max cleaning” spends more time in electrode positive causing the oxides on the aluminum to be removed aggressively. “Max penetration” adjusts the waveform to spend more time in the electrode negative half of the AC cycle limiting the removal of oxides.
On newer welding power supplies % EN is the term used and is the actual adjustment being set. The terms “max cleaning” and “max penetration” are a representation of the percentage of time the arc is spending on the negative half of the cycle. These terms are a poor description of what balance control is doing to the weld and %EN is the more accurate term.
When the balance control is set to produce the maximum time at electrode negative and the minimum time at electrode positive on conventional squarewave power supplies, the maximum value reached is 68% EN. On an inverter AC GTAW power supply the maximum is 99% EN. A setting of 99% EN can be used at any AC frequency, however, when that value is set above 90% EN the AC welding frequency will determine the ability of the power supply to deliver the “perfect” wave shape without any rectification.
Figure 19, Conventional Welding Power Supply Lower Frequency (Left) Advanced AC Welding Power Supply With Higher Frequency (Right)
Using a waveform with more electrode negative than electrode positive can provide the following benefits:
•Can use higher currents with smaller electrodes
•Potential use of a smaller gas cup and reduced shielding gas flow rate due to a slightly smaller weld pool
•Reduced heat input (lower amperage and faster travel speeds)
•Smaller heat affected zone
A balanced wave shape is when the balance control is set to produce equal amounts of time electrode negative and electrode positive. Thus, on 60 Hz power, 1/120th of a second is spent electrode negative (less oxide removal) and 1/120th of a second is spent electrode positive (more oxide removal). This is represented as 50% EN. A balanced wave setting produces a welding arc with the following characteristics:
•More oxide removal than a High %EN
•A bead shape somewhere between the high %EN and the low %EN
When the balance control is set to produce the maximum time at electrode positive and a minimum time at electrode negative on a conventional squarewave power source, the “maximum cleaning” setting is 45% EN and on an inverter AC GTAW machine the maximum cleaning setting is 30% EN.
Figure 20, A 60 Hz Fillet Weld (Left) and a 180 Hz Fillet Weld (Right)
Setting the machine to produce more time on the positive half of the cycle produces a welding arc with the following characteristics:
•The most aggressive oxide removal
•More heat on the tungsten
•The benefits of the balance control should be well understood and applied in an appropriate manner. Figure 19 shows actual welds made at a set current and travel speed with only the balance control being changed. Generally, welding with this setting set to near 70% EN for aluminum is best. With new clean aluminum, the oxide layer is fairly thin and a minimal amount of electrode positive is needed to remove the oxide layer.
Adjustable Frequency (Hz) Adjustable frequency on advanced squarewave welding power supplies gives the welder additional control of the weld and penetration profile by adjusting the AC welding output frequency. Alternating current makes constant reversals in direction of current flow. In other words, the polarity changes from DCEP to DCEN 60 times in one second. One complete reversal is termed a cycle, and is referred to as its frequency. In the United States, the frequency delivered by the power company is 60 Hz. On a conventional AC GTAW machine, the AC output frequency is limited by the input power frequency. On an Inverter AC GTAW machine, the AC welding frequency is adjustable between 20 Hz and 250 Hz and, depending on the model, up to 400 Hz.
The faster the current changes direction (increased frequency), the more the arc becomes constricted. Higher AC Welding frequencies increase the arc pressure making the arc more stable and directional. Figure 21 shows how frequency affects the weld profile. Increased frequency provides deep, penetrating, narrow welds. Increasing the AC welding frequency can also be beneficial in automated welding by reducing the amount of deflection and arc wandering that occurs when producing a fillet weld. Increased frequency will also provide a more driving arc, penetrating the base metal deeper than a lower AC frequency.
Figure 21, Effects of Frequency Adjustment
The frequency on an advanced squarewave power source can be adjusted down to 20 Hz. When the AC welding frequency is lowered, a wide, shallow, penetrating weld is produced. This is useful on thin butt welds or outside corner welds that require minimal melt thru. The lower AC frequency arc is softer and provides shallower penetration.
Independent Current Control
Independent current control allows the operator to control separately and independently the amperage of the electrode negative (penetration) and electrode positive (oxide removal) half cycles. Not all advanced squarewave power sources have this control.
The ability to independently control amperage with the advanced squarewave power source provides even more control of the AC weld. As with all of the adjustments that an advanced power supply provides, balancing heat input and the amount of etching is the key to controlling the weld. By controlling the amount of amperage on each half of the cycle the welder can create the ideal welding condition for the part being joined (Figure 22).
Figure 22, Effects of Independent Current Control
The benefits of advanced squarewave welding power sources go beyond increased travel speeds. This type of welding machine allows a narrower and deeper penetrating weld bead compared to that of squarewave or sine wave machines. The advanced squarewave AC is capable of welding thicker material than squarewave or sine wave power sources at a given amperage. Figure 19 shows an example of welds made with squarewave and advanced squarewave welding power sources. Note that with an advanced GTAW power supply the etched zone can be narrowed or eliminated.
Figure 23, With (Left) and Without (Right) the Use of FASTIG™ Flux for Enhanced Penetration
Figure 24, The Advanced Squarewave Welding Power Source Allows the Operator to Shape the Arc and Control the Weld Bead. Separately, or in Any Combination, the User Can Adjust the Balance Control, Frequency (Hz) and Independent Current Control to Achieve the Desired Weld and Bead Characteristics for Each Application.
Note: All forms of AC create audible arc noise. Many advanced squarewave AC combinations, while greatly improving desired weld performance, create noise that may be objectionable to some persons. Hearing protection is always recommended.
Welding Fluxes for GTAW
The GTAW process does not require a flux to perform the welding process. However, developments have been made in producing chemical fluxes that affect the weld pool and allow improved penetration on certain metals. The flux is applied prior to welding and at a given amperage penetration will be increased. These fluxes, designed specifically for GTAW, can also act as a heat shield to prevent heat from spreading to areas where the heat is not desired or they can provide a means to avoid purge gas on the back side of a weld. Fluxes used for GTAW will likely require qualification.
Gas Tungsten Arc Welding uses a non-consumable electrode. Since this tungsten electrode is not compatible with the metals being welded (unless you happen to be welding tungsten), it requires some unique methods to initiate the welding arc.
Gas Ionization
Gas ionization is a fundamental requirement for starting and maintaining a stable arc. An ionized gas is a gas that has been electrically charged and becomes a good conductor of electricity. There are two ways of electrically charging this gas. One way is to heat the gas to a high enough temperature and electrons will be dislodged from the gas atoms and the gas atoms will become positively charged gas ions. The heat of a welding arc is a good source for this thermal ionization. When AC welding with a conventional sine wave, as the current approaches zero there is not sufficient heat in the arc to keep the gas ionized and the arc extinguishes. Another ionization method used for GTAW is to apply enough voltage to the gas atom to dislodge the electrons from the gas atom to produce a positive gas ion. This method is known as high frequency.
High Frequency
When referring to high frequency we are talking about a high voltage/low amperage arc generated at a very high cycle or frequency rate, (not to be confused with the AC welding output frequency adjustment). Frequency rates of approximately 1 million Hz are typical. This high voltage allows for good arc starting and arc stability while the high frequency is generated. This high frequency voltage ionizes the shielding gas, thus providing a good path for the current to follow and the arc will jump the gap between the electrode and the workpiece. Once the high frequency arc is established the welding current now has a very conductive path to follow to the work. On materials sensitive to impurities, touching the tungsten to the work will contaminate the work and the tungsten. High frequency is used to start the arc without making contact with the work, eliminating this possible chance of contamination.
When alternating current first became available for SMAW, researchers immediately began looking for a means to assist the re-ignition of the arc during the transition between the positive and negative halves of the AC cycle. Shielded Metal Arc Welding electrodes at this time did not have arc stabilizers in the coating for AC welding to assist in the transition. It was found that the introduction of high frequency into the secondary welding circuit of the welding power source assured arc re-ignition. The high frequency is used to eliminate the effects of the arc outage. While the primary 60 Hz current is going through its zero point, the HF may go through many cycles, thus preventing the arc from stopping. A common misconception is that the high frequency itself is responsible for the oxide removal (or “cleaning” action) of the arc. But the high frequency only serves to re-ignite the arc. Figure 25 shows the relationship of a high frequency arc to the 60 cycle frequency of the primary current.
Figure 25, AC high frequency (not to scale) on top of a sine wave.
With GTAW, high frequency is used to stabilize the arc. Aluminum and magnesium are poorer emitters of electrons when they are hot and molten than the hot tungsten. Plus the area of current flow on the molten weld pool is so much larger than the area on the end of the tungsten. This causes the arc to have a tendency to wander and become unstable. Because the high frequency provides an ionized path for the current to follow, arc re-ignition is much easier and the arc becomes more stable. DC and inverter-based welding power sources use high frequency for starting the arc only where as a conventional AC GTAW power supply offers continuous high frequency to take advantage of its stabilizing characteristics.
High frequency is produced a spark gap oscillator. 115 volts ac is applied to a step-up transformer that increases the voltage to approximately 3000 volts while amperage is reduced to approximately 80 milliamperes. The voltage charges a capacitor, and when the capacitor voltage reaches a level high enough the electricity jumps across the spark gaps (Figure 26). After it discharges, the cycle repeats itself. The frequency of this charge/discharge oscillation is approximately 1 megahertz or 1,000,000 cycles per second.
Figure 26, A Spark Gap Oscillator
High frequency has a tendency to get into places where it’s not wanted and falls under control of the Federal Communication Commission (FCC). It can be a major interference problem with all types of electrical and electronic devices. Figure 27 and Figure 28 show high frequency issues and solutions. Generally, all electrically conductive metals within a 100 foot diameter sphere from the source of high frequency should be earth grounded. High frequency is on a mission to find earth ground and providing a short path to ground helps to eliminate the interference caused by it.
Figure 27, Correct High Frequency Equipment Installation
Figure 28, Incorrect High Frequency Equipment Installation
Pulse Mode HF
These machines utilize special circuitry to impose a high intensity pulse on the output circuit when the voltage is at a specific value. Lets assume we have a machine that provides this pulse when voltage is 30 volts or more. When not welding, welding power source voltage (or pressure) is at maximum because no current is being allowed to flow and the pulsing circuitry is enabled. As the electrode is brought near the work, the pulses help jump- start the arc and welding begins. Once the arc is started, weld circuit voltage typically drops to a value somewhere in the low teens to low twenties. The pulsing circuit senses this change and drops out. The pulse mode circuitry can also help stabilize the AC arc because it is enabled during times the voltage sine wave is transitioning through zero. The high intensity pulses do affect other electronic circuitry in the immediate vicinity, but the effect is not as pronounced as that of a high-frequency power source. You may find it necessary to move the electrode slightly closer to the workpiece to initiate the arc with pulse assist than you would with traditional high frequency arc starting methods.
Lift-Arc™
Lift-Arc™ allows the tungsten to be placed in direct contact with the metal to be welded. As the tungsten is lifted off the part, the arc is established. This is sometimes referred to as touch start. Little, if any, chance of contamination is possible due to this circuitry. When the Lift-Arc™ switch is activated, low power is supplied to the tungsten electrode. This low power allows some preheating of the tungsten when it is in initial contact with the part. Hot tungsten is a good emitter of electrons and this power level is low enough not to overheat the tungsten or melt the work, thus eliminating the possibility of contamination. Once the arc is established, the power source circuitry switches from the Lift-Arc™ mode to standard weld mode, (Figure 29).
Figure 29, Lift-Arc™ Starting procedure
Scratch Start
Scratch start is generally not a desired arc starting method as it can easily lead to poor weld quality. It is usually performed when doing GTAW DC welding with a power source designed for SMAW only. These machines are not equipped with an arc starter so the only way to start the arc is with direct contact of the tungsten electrode with the metal. This is done at full weld power level and generally results in contamination of the electrode and or weld pool. This method, as the name implies, is accomplished by scratching or striking the arc as would be done for Shielded Metal Arc Welding.
Capacitive Discharge
These machines produce a high voltage discharge from a bank of capacitors to establish the arc. The momentary spark created by these machines is not unlike a static discharge. Although capacitive discharge machines have good arc starting capability, they do not have the arc stabilization properties of high-frequency machines. They are typically used only for DC welding and not used on AC welding.
Pulsed GTAW welding produces an arc that rapidly changes between a high welding amperage and a low welding amperage. As peak amperage is reached, penetration is quickly achieved. Before the workpiece can become heat saturated, the amperage is reduced to the point where the pool is allowed to cool but current is sufficient to keep the arc established. The pulsed arc greatly reduces the need to adjust heat input as the weld progresses. This gives the welder much greater control when welding out of position and in situations where joints are of differing thicknesses. Pulse welding also helps to weld materials with high nickel content that produce a sluggish feel when welding without pulsing. The pulsing action provides a number of benefits over a standard non-pulsed arc. Reference Figure 30.
Figure 30, Conventional Pulse GTAW Wave Form
Some of the advantages of Pulsed GTAW are as follows:
Good penetration with less heat input
•Good control of the pool when welding out of position
•Ease of welding thin materials
•Ease of welding materials of dissimilar thickness
The pulsed waveform can be confused with AC wave shapes. The AC sine wave represents direction and cycle time of current flow and the peak amperage in the welding circuit. The pulsed waveform represents the amount and duration of two different output levels of the power source. The signal does not switch between positive and negative values as it does in the AC sine wave. This is not to say that AC cannot be pulsed between two different output levels, as there are applications and power sources capable of doing that.
The electronics in conventional and inverter-type power sources have inherently fast response times. They can easily control the welding output to provide a pulsed arc. The conventional GTAW machines are somewhat limited in speed as compared to the inverters. Pulsing can be controlled by add-on controls or built directly into the welding power source.
The basic controls for setting pulse parameters are as follows:
Peak Amperage — This value is usually set somewhat higher than it would be set for a non-pulsed GTAW weld.
Background Amperage — This value is set lower than peak amperage.
Pulses Per Second — Indicates the number of times per second that the weld current achieves peak amperage.
% On Time — This value is the pulse peak duration as a percentage of total time. It controls how long the peak amperage level is maintained before it drops to the background value.
Figure 31, Pulsing Waveform Adjustments
Pulsed Welding at High Cycle Rates
Although the majority of Pulsed GTAW welding is done in a frequency range of .5 to 20 pulses per second, there are applications where much higher pulse frequencies are utilized. The advantage of pulsing at high frequencies (200 to 500 pulses per second) is that this pulse provides a much “stiffer” arc. Arc stiffness is a measure of arc pressure. As pressure increases, the arc is less subject to wandering caused by magnetic fields (arc blow). Welding with higher pulsing frequencies has also proven beneficial by producing better agitation of the weld pool. This helps to float impurities to the surface resulting in a weld with better metallurgical properties. High speed pulsing is used in mechanized and automated applications where an arc with exceptional directional properties and stability is required. It is also used where a stable arc is required at very low amperages.
Figure 32, High Speed Pulse GTAW
Equipment for GTAW has advanced significantly since 1976 when the first AC Squarewave patent was issued to Miller Electric Mfg.. The early 1990s introduced the Aerowave® power supply allowing many of the advanced AC welding functions we now use today. The Aerowave® significantly impacted the way aluminum is TIG welded.
Figure 33, The Aerowave® Power Supply Was The First Machine Made By An American Company To Provide Advanced Ac Welding Functions Such As Variable Frequency, Extended Balance Control, Independent Amperage Adjustment And Operate On Single Or Three Phase Input Power.
There are a number of different options with regard to selecting and using the power source and the auxiliary components needed to produce a weld. With the many types of welding machines available, certain considerations must be made in order to select the right machine for the job. The ranges of voltage and amperage needed for a particular process must be determined. A welding machine can then be selected to meet these output needs.
Arc welding power sources are classified in terms of their output characteristics with regard to voltage and amperage. They can produce a constant current (CC), a constant voltage (CV) or both. Generally, machines manufactured today that produce both CC and CV are DC only and do not produce an AC welding arc.
Gas Tungsten Arc Welding power supplies manufactured today produce a constant current curve that is nearly a perfect constant current. Older power supplies produced a curved or drooper constant current curve. Figure 34 shows these two curves.
Figure 34, Volt-Ampere Output Curves For Smaw And Gtaw Power Supplies
The drooper curve is best used for SMAW welding since it helps the welder control the heat slightly by changing arc length. On today’s power supplies this curve represents the weld output when the SMAW mode is selected.
A near perfect constant current curve is used for GTAW because it maintains the set current flow in the weld circuit no matter how much the voltage (arc length) varies. A constant current power supply is required for GTAW because the filler metal is added manually and varying the welding amperage significantly impacts the weld quality.
The tenants of Ohm’s law (volts = amps x resistance) are exhibited in all welding circuits. Amperage is preset on the power supply; it becomes a fixed setting. Resistance is established by the cables and their connections, and also by the arc length. This arc length gap can vary the resistance by being larger or smaller and the type of shielding gas can also affect the amount of resistance generated in the weld circuit. Resistance levels are very low and do not vary much.
Voltage and resistance are changing values in the GTAW circuit. Arc voltage is dependent upon the physical distance the tungsten is from the weld pool. When the GTAW process is performed manually, the welder cannot maintain the perfect arc length. By allowing voltage to vary and adapt to the small changes in resistance the welding arc is maintained and is steady enough to produce repeatable welds.
Figure 35, 153 lb SCR Transformer (Left) Comparable 5 Pound Inverter Transformer (Right)
Squarewave Silicon-Controlled Rectifier (SCR) Power Sources
These types of power sources were introduced to the welding industry in the mid 1970s. They have now virtually replaced all the AC sine wave power sources for the GTAW process. This type of power source uses a large, heavy, 50 or 60 Hz transformer. They are typically very similar in size and weight to the older style mechanically or magnetically-controlled welding power sources that produced a sine wave AC output. The SCR power source has simple wave-shaping technology producing squarewave AC and possesses closed loop feedback for consistent weld output. We call these the Syncrowave® series of power supplies and refer to them as conventional technology today.
The Inverter Power Source Inverter power sources were not used as welding power supplies until the 1970s. Instead of operating at a common input power frequency of 50 or 60 Hz, inverters boost the frequency as much as 1000 times that of the input frequency. The boost in frequency causes less heat to be generated allowing for a drastic reduction in the size of the transformer (Figure 36). Inverter power supplies are much smaller and lighter in weight than a conventional transformer/rectifier power source.
Figure 36, Inverter Power Conversion Block Diagram
A major advantage of this type of machine is its primary power requirements. Some inverters can be used on either single or three-phase input power, and either 50 or 60 Hz. This is because the incoming primary power is rectified and converted to the extent that input power is not a critical factor. Some inverters, due to their unique circuitry, are multi-process machines capable of GTAW, GMAW, SMAW, FCAW (Flux Cored) and Carbon Arc Gouging. Although these inverters are capable of accomplishing these multi-processes, some are specifically designed for the GTAW process.
Figure 37, Comparison Of Conventional 350 Amps GTAW Power Supply Vs. An Advanced Squarewave Ac Power Supply
The first thing the inverter does is rectify the high voltage low amperage AC into DC. It is then filtered and fed to the inverter’s high-speed switching devices. Just like a light switch, they turn the power on and off. They can switch at a very fast rate, up to 60,000 times per second. This high voltage, low amperage, fast DC switching looks like AC to the transformer, which is many times smaller than a 60 Hz transformer. The transformer steps the voltage down and increases the amperage for welding. This low voltage, high amperage current is filtered for improved DC arc welding performance or converted to AC through the electronic polarity control. The AC or DC power is then provided to the TIG torch. The AC is fully adjustable and the DC is extremely smooth and very capable of being pulsed or sequenced.
Figure 38, 350 Amp Conventional (Left) And Advanced (Right) Squarewave GTAW Power Supplies
Advanced Squarewave Power Sources The name advanced squarewave power source is another reference to an AC/DC Inverter based power supply. The inverter power supply allows four major independently controllable functions of the welding output:
•Balance (% of time electrode is negative)
•Frequency in hertz (cycles per second)
•Electrode negative current level in amps
•Electrode positive current level in amps
The transition through zero amps on advanced squarewave power sources is much quicker than conventional squarewave machines; therefore, no high frequency is required even at low amperages. High frequency is called HF impulse on these machines. High frequency is only used to start the arc and should the machine see any rectification it will provide an impulse of high frequency if it is needed. These power supplies also allow for AC and DC GTAW without the use of any high frequency in Lift-Arc Mode.
Advanced squarewave welding power supplies also provide the ability to adjust the AC wave form four ways. A modified version of the sine wave, a soft squarewave, an advanced squarewave, and a triangular wave. Each wave shape provides additional control when TIG welding aluminum. The ability to control all of these aspects of the AC GTAW arc provides these benefits:
More efficient control resulting in higher travel speeds
•A narrow, deep-penetrating arc
•Ability to narrow or eliminate the white etched zone adjacent to the weld
•Reduced use of high frequency arc starts
•Improved arc starting (ability to control the time, amps and polarity of the initial arc strike moment)
The Engine-Driven Power Source Engine-driven power sources convert mechanical energy that is obtained from a gasoline or diesel engine into electrical power suitable for arc welding and/or auxiliary electrical power. For welding, two basic types of rotating power sources are used, the generator and the alternator. Both designs have a rotating member (called a rotor) and a system of magnetic field excitation circuits to produce the power.
There are three essential parts to generate electricity:
•Magnetic lines of force (magnetic field)
•Electrical current carrying conductor
•Relative motion between the magnetic field and the electrical current carrying conductor.
To generate electricity, there must be relative motion between a magnetic field and a current carrying conductor. Whenever a wire moves through the lines of force created by a magnetic field, or whenever the lines of force of a magnetic field are moved through a wire, a voltage is induced in the wire. This induced voltage causes electric current to flow when the circuit is complete. Thus, the principle of any rotating power source is that electrical current is produced in electrically conducting materials (coil) when they are moved through a magnetic field. Physically, it makes no difference whether the magnetic field moves or the conductor moves, just so the coil experiences a changing magnetic field intensity.
Some of the first electric arc welding power sources invented were the motor generator type that produced welding current by means of a rotor moving inside a stator. This is the same principle of current generation by means of moving a conductor through a magnetic field. The movement in these machines was provided by an electric motor, (Figure 5). The concept is still being put to good use by modern power sources that replaced the electric motor with gasoline or diesel engines. The most important feature of these electromechanical devices is they free the welder from dependence on commercial power, and give them the mobility to work nearly anywhere in the world. Most of these machines are welder generators that provide welding output and AC and/or DC power for the operation of lights and power tools.
Figure 39, Cut-Away View of an Engine-Driven Welder/Generator With an Added Air Compressor System
Duty cycle is important in the selection of a welding machine. The duty cycle of a welding power source is the actual operating time it may be used at its rated load without exceeding the temperature limits of the insulation in the component parts. In the United States, the duty cycle is based on a ten minute time period . In some parts of the world, Europe for example, the duty cycle is based on a five minute time period. Simply stated, if a power source is rated at a 50% duty cycle and it is operated at its rated output for five minutes, it must be allowed to cool for five minutes before operating again. The duty cycle is not accumulative. For example, a power source with a 50% duty cycle cannot be operated for thirty minutes then allowed to cool for 30 minutes. This violates the ten minute rule. Also a machine rated at 50% should not be operated at maximum for five minutes and then shut off. The cooling fan must be allowed to operate and cool the internal components, otherwise the machine could be damaged.
A power source with a 100% duty cycle may be operated at or below its rated output continuously. However, if the machine is operated above its rated output for a period of time, it has exceeded the duty cycle limitations.
Figure 40, Duty Cycle Example
Single-Phase — Three-Phase Power companies provide single-phase and three-phase power. Welding power supplies operate using single-phase, three-phase or sometimes either phase. A conventional GTAW power supply that only produces a DC arc will deliver a smoother DC arc with three-phase power than with single-phase. Conventional AC/DC GTAW machines can only operate from single-phase power but an inverter-based AC/DC GTAW machine can operate using single or three-phase input power.
Single-phase power uses two conductor wires and one ground wire. Single-phase input power has an AC sine wave with one line (Figure 41). This sine wave is what is transformed to a high amperage/low voltage supply for welding. Because of the way a conventional AC/DC GTAW machine changes the incoming power, AC weld output requires the input to be single-phase to match the single-phase welding output. When single phase is rectified to produce a DC arc, half of the AC waveform is flipped up to produce a DC current. The initial rectification from AC to DC produces a rough unstable DC welding current (arc). The welding current is then filtered to produce a smooth DC current for welding but it is still a little rough compared to a welding power supply that uses three-phase input power.
DC made from three-phase power is produced the same way as single-phase, but because there are three conductors that overlap each other the DC produced is much smoother to begin with than single-phase DC. When the three-phase DC current is filtered it becomes a very smooth DC current for welding.
Conventional AC weld output from a three-phase welding machine is not very practical because an AC welding arc can only be singe-phase. Producing a single phase AC arc from three-phase would require electronic control that eliminates two of the three phases, therefore making it impractical on a conventional machine. For an inverter power supply, since it is changing the power significantly and is essentially creating the output as two DC polarities, the inverter can produce AC weld output from single or three-phase input power. For an inverter-power source that operates on both single and three-phase the output rating will be different for each phase.
Figure 41, Single and Three-Phase AC Power Converted to Direct Current
Primary voltage is the input voltage supplied by the power company or auxiliary electrical power generator unit to the machine. The primary voltage is constant at every receptacle. This could be 120 (110/115), 208 (200), 230 (220/240), 460 (440/480) or 575 (600) VAC (volts of alternating current) with a frequency of 50 or 60 Hz. Welding power source transformers are designed to work with these voltages. These voltages may be single or three-phase (Figure 41). Many welding power sources are equipped with an input terminal board (Figure 42). This board is used to connect the welding power source to the line voltage supplied. This must be properly connected or the welding equipment can be severely damaged. If the power source is moved from location to location with different input voltages, re-linking this board will be required.
Figure 42, Single-Phase Input Terminal Board or Link Board
Other power sources are equipped with devices that will detect the input voltage and automatically set the equipment for proper operation. Two common types are referred to as Auto-Link® and Auto-Line™. Auto-Link® uses a sensing circuit to read the incoming voltage then it mechanically re-links the machine to operate from the power supplied. Auto-Line™ constantly monitors and maintains the appropriate voltage to the transformer. Since input voltage can affect output voltage on a conventional power supply, Auto-Line® eliminates this problem by constantly adapting to the input voltage to maintain a consistent output.
Gas Tungsten Arc Welding is a precise process and requires a number of components to make the process efficient and functional. There are a number of components needed to perform the process and some that are not necessarily required, but “nice to have”.
High-frequency arc starters and stabilizers are used with AC and DC GTAW welding power sources. Welding power supplies designed specifically for GTAW usually have this built into the equipment. To TIG weld on a SMAW welding power supply an external high frequency unit may be needed. High frequency is very useful when TIG welding to prevent contamination of the weld from the tungsten electrode.
Some DC GTAW power sources use Lift-Arc™ technology which allows for a clean arc initiation with minimal possibility of contamination from the tungsten electrode (without the use of high frequency). Some units will feature gas valves, time delay relays, and control circuits to regulate the flow of gas along with the high-frequency current.
Adding an arc starter to a power source not designed for GTAW (especially the AC type sine wave machines) will require special precautions. The HF unit adds additional heat to the power source causing the unit to operate differently than designed. This additional heat requires the machine to be derated from its original design by approximately 30% to prevent overheating.
Torches used for GTAW may be either water or air-cooled. High production or high amperage torches are usually water-cooled while lighter duty torches for low amperage applications may be air-cooled. Air-cooled torches use the ambient air and shielding gas to dissipate excess heat. The power cable on an air-cooled torch contains more copper than the cable on a water-cooled torch to help prevent the cable insulation from melting or possibly burning. As a result, air-cooled torch cables are heavier and less flexible compared to water-cooled torch cables. Air-cooled systems do not require an independent radiator cooling system and extra hoses that are associated with water-cooled systems. The simpler designs of air-cooled systems make them easier to operate, assemble, maintain, and transport. Air-cooled GTAW torches are rated between 50 and 300 amps. Figure 43 illustrates the typical air-cooled torch, showing the basic components.
Figure 43, 150 Amp Air-Cooled GTAW Torch
Water-cooled systems require a radiator cooling system which circulates water. Water-cooled torch and gun cables are lighter and more flexible because the cable contains less copper compared to air-cooled systems. Water-cooled TIG torches are rated between 125 and 500 amps.
The water-cooled torch is designed so that water is circulated through the torch, cooling it and the power cable. The power cable is contained inside a hose, and the water returning from the torch flows around the power cable providing the necessary cooling. The power cable can be relatively small, making the entire cable assembly light and easy to maneuver by the welder. When using a water-cooled torch, any lack of coolant flow in the system will cause the polyethylene or braided rubber sheath to melt or possibly burn the power cable.
Figure 44, A Water-Cooled GTAW Torch
On some GTAW torches a safety device known as a “fuse assembly” or “fuse link” can be installed in the power cable. This assembly contains a fuse, which is also cooled by the water. If there is no cooling water circulating, the fuse link will melt and prevent damage to other more expensive components. When the fuse link is replaced and water flow is maintained, welding can continue. This device is not used as much today but can still be found, especially for high amperage requirements.
GTAW Torch Components
The components that complete the GTAW torch are offered in a number of configurations and sizes. Selection of the components for the application is really based on understanding the differences and functions of each component.
Collet
The welding electrode is held in the torch by the collet. The collet is usually made of copper or a copper alloy. The collet’s grip on the electrode is secured when the torch cap is tightened in place. Good electrical contact between the collet and tungsten electrode is essential for good current transfer.
Collet Body
The collet body screws into the torch body. It is replaceable and is changed to accommodate various size tungsten electrodes and their respective collets. The collet body allows gas and weld current to flow through it. It is important to tighten the collet body (or gas lens) before anything else. A common mistake is to tighten the collet body with the tungsten inserted and the back cap about half tight. This only tightens the collet to the collet body and, as soon as the back cap is loosened, the collet body becomes loose. By not tightening the collet body correctly you can create unnecessary resistance in the torch head and produce excessive heat in the torch. This will also have an affect on the consistency of an arc start and the arc itself.
Figure 45, Collet Body for a 250A Water-Cooled Torch (Top) and a Collet Body for a 150A Air-Cooled Torch (Bottom)
Figure 46, Collet for a 250A Water-Cooled Torch (Top) and a Collet for a 150A Air-Cooled Torch (Bottom)
Figure 47, Gas Lens for a 250A Water-Cooled Torch (Top) and a Gas Lens for a 150A Air-Cooled Torch (Bottom)
Gas Lenses
A gas lens is a device that replaces the normal collet body. It attaches to the torch body and is used to reduce turbulence and produce a longer undisturbed flow of shielding gas. A gas lens will allow the welder to move the nozzle further away from the joint allowing increased visibility of the arc. A much larger diameter nozzle can be used, which will produce a large blanket of shielding gas. This can be very useful in welding material like titanium. The gas lens also enables the welder to reach joints with limited access such as inside corners. Figure 48 shows the difference of gas flow between a collet body and a gas lens. Figure 48, Erratic Gas Flow on a Collet Body (Left) Smooth Directed Gas Flow of a Gas Lens (Right)
Nozzles
Gas nozzles, or cups as they are better known, are made of various types of heat resistant materials in different shapes, diameters and lengths. The nozzles are either screwed into the torch head or pushed in place. Nozzles can be made of ceramic, metal, metal-jacketed ceramic, glass, or other materials. Ceramic is the most popular material, but is easily broken and must be replaced often. Nozzles used for automatic applications and high amperage situations often use a water-cooled metal design.
Gas nozzles or cups must be large enough to provide adequate shielding gas coverage to the weld pool and surrounding area. A nozzle of a given size will allow only a given amount of gas to flow before the flow becomes turbulent. When this occurs the effectiveness of the shielding gas is reduced, and nozzle size must then be increased to restore an effective non-turbulent flow of gas.
A large shielding gas envelope that reaches beyond the weld pool into the weld bead will help protect the weld as it cools. The longer the weld bead is in the shielding gas the less oxidation can occur on the surface of the weld. (Oxidation causes the colors we see on stainless steel welds.) Those colors indicate the temperature of the metal surface at the time the atmosphere replaced the shielding gas. The darker the color, the more oxidation. A larger cup will protect the puddle longer, allowing it to cool more and produce much less discoloration.
Figure 49, Nozzle for a 250A Water-Cooled Collet Body (Top) and a Nozzle for a 150A Air-Cooled Collet Body (Bottom)
Figure 50, Nozzle for a 250A Water-Cooled Gas Lens (Top) and a Nozzle for a 150A Air-Cooled Gas Lens (Bottom)
The correct coolant type and method of delivery is essential to ensure high quality welds and minimize component failure. There are two types of coolant used:
•Low conductivity coolant
Figure 51, Various Cooling Radiators/Re-circulators
Water free-flowing directly out of the tap from well or city water sources is not recommended to continuously cool the torch head. The cold tap water can be colder than the dew point and cause condensation to occur. This creates moisture inside the torch body leading to significant weld contamination until the torch temperature exceeds the dew point. Continuous flow of tap water is not recommended as a coolant because of its inherent mineral content, which can build up over a period of time and clog the small cooling orifices in the torch head. Conservation also dictates the use of less wasteful methods such as a coolant radiator re-circulating system.
Figure 52, Maintenance Schedule for Cooling Systems
Since high frequency is being used, the coolant should be deionized. A deionized coolant prevents the high frequency current from bleeding off prior to it getting to the arc. The coolant must also be able to stay liquid below freezing, but DO NOT USE ANTIFREEZE. Antifreeze contains leak preventatives and other additives and is electrically conductive. Antifreeze will dissipate HF causing poor arc starts. Antifreeze will also see the head of the torch as a “leak” and try to fix it by depositing the anti-leak compounds in the torch head causing the coolant flow to drop.
Some method of reducing algae growth is advisable. Low Conductivity coolants already have algae preventatives where deionized water is only water with the minerals removed. To prevent algae growth, change the coolant regularly. For low conductivity coolant, change it every year. For DI water change it every six months. All coolants must be clean. Otherwise, blocked passages may cause overheating and damage the equipment. It is advisable to use a water strainer or filter on the coolant supply source. This prevents scale, rust, and dirt from entering the hose assembly.
Figure 53, Coolant Filter
The rate of coolant flow through the torch is important. Rates that are too low may decrease cooling efficiency. Rates that are too high damage the torch and service line. The direction the coolant flows through the torch is a debatable topic. Coolant flowing from the coolant source directly through the water hose to the torch head cools the hot torch head first, but it can bring additional yet somewhat inconsequential heat to the power cable. By cooling the power cable first, (the most frequent component to need replacement) then the head, the decreased heat from the cable to the head will not affect the ability to remove the high heat generated at the head. This may extend the power cable’s life. Both cooling methods are used and will likely continue to be argued for some time.
Remote Control
Sometimes a welding application requires the welder to place a weld in a location where access to controls on the power source is limited. The welder may need to control the amount of current being used throughout the weld. Extra amperage may be required at the start to establish a weld pool more quickly on cold metal, or when making long welds on aluminum where weld current must be gradually reduced as the arc heats the work.
Most welding machines designed primarily for GTAW provide remote control capability. The remote control functions usually include weld output and current control. Generally, output and current control are located as separate switches on the machine’s front panel and can be operated independently if desired. By using a remote control device, the welder can go to a location away from the power source, activate the power source and its systems (gas flow, arc starter, etc.) and vary the amperage levels as desired.
Figure 54, Remote Foot Control
Remote output gives the welder control of open-circuit voltage (OCV). OCV is the voltage present at the output studs of the welding power source with no load attached. Once a torch is connected to the output, the electrode would be continuously energized if it were not for the output control. The remote output’s primary job then is to interrupt the weld circuit until the welder is prepared to start the arc.
When in the remote position, the current control switch on the welding power source works in conjunction with the main current control. If the main current control is set at 50%, the maximum output current available through the remote device is 50% of the maximum current available from the welding power source. To obtain full machine output current through the remote device, the main current control must be set at 100%. Understanding this relationship allows the welder to fine tune the remote control device for the work being done.
There are a variety of remote controls used for TIG welding. The most popular of the remote output and current controls is the foot pedal type (Figure 54). The foot pedal remote control operates much the same as the gas pedal in an automobile: weld current increases and the pedal is depressed. Figure 55, Wireless Foot Control
Another type of remote control that provides greater mobility is the finger-tip control. The finger-tip control can be integrated into or mounted to the handle of the torch (Figure 56). Finger-tip remote controls can have a momentary push button to apply the contactor or to control a sequencer. They can have a combination of a push button for the contactor and a roller for the amperage control, or a momentary or steady on rocker switch. The amperage controls can be roller balls, sliders, or push buttons, and combinations of all of these. There are also free standing remote controls available. However, these are usually used for Shielded Metal Arc Welding.
Figure 56, WeldCraft® 150 Amp Air-Cooled Torch With Integrated Amperage Dial and Contactor Switch
Running Gear and Cylinder Racks In order for the GTAW process to work most effectively, it is necessary to keep the TIG torch cable shorter than about 50 feet. A welding power source mounted on a running gear is easy to move to the work and helps keep the workshop clean. Having the welding power supply mounted a few additional inches off the floor also reduces the internal components potential exposure to dust.
Figure 57, Complete TigRunner System with Running Gear
Running gears made for welding systems will have a means of securing the gas cylinder. Cylinders are considered high-pressure vessels and must be protected from damage. A serious accident may occur if the cylinder cap is not in place and the cylinder is not secured. Always secure high-pressure cylinders.
Automated GTAW meets the increasing needs for high productivity and quality. Automatic welding can be performed with a number of welding processes in Three basic ways: fixed automation, flexible automation and programmable automation.
Fixed automation utilizes a dedicated machine specifically designed for arc welding the same specific parts on a continuous production basis. An operator loads a part into a fixture, presses a start button, and the part is automatically welded. The operator unloads the welded part, inspects the part, loads a new part, and starts the process again. (Typically GMAW or GTAW processes are used.)
Figure 58, A Jetline Seam Welding System
The parts to be welded may be rotated under the welding torch; or the welding torch may move across or around the part, usually in just one axis of movement. Parts to be welded are generally of a simple design.
Flexible automation is simply a variation of fixed automation but allows for some part variation such as a diameter change on a circumferential weld or a length adaptation on a longitudinal weld.
Programmable or robotic TIG welding is performed, however it is usually only used in very specialized applications. Robots move quickly over a work envelope to weld small component parts or large part assemblies. The robot can make the proper sized weld accurately and consistently with optimal speed and repeatability.
Robots offer the capability of making small production runs that can quickly be re-tooled for changeover; or they can be dedicated to making large volume production runs operating 24 hours a day / 7 days a week.
Whichever automation method is used, additional control is required over the welding sequence. A weld sequence is what happens when a signal is given to start and finish the welding operation (Figure 59). Figure 59, GTAW Sequence of Events
Preflow opens the gas valve to allow the shielding gas to flow prior to the arc start. The shielding gas provides a better path for the current to flow, improving the consistency of the arc start. Having the gas on before striking the arc ensures that the weld puddle will be protected.
•Initial Current is the welding current that establishes and stabilizes the arc. This level may be higher than the weld amperage or lower than the weld amperage. The current level at the start depends on the requirements of the application. Generally, this initial amperage is a low amperage to initialize and stabilize the arc before welding takes place.
•Initial slope is the rate at which the amperage moves from the initial amperage to the welding amperage. In other words, how long it take to go from a start amperage of 5 amps to a welding amperage of 125 amps.
•Weld/peak current is the amperage value at which the actual welding takes place. When doing GTAW-pulsed, this indicates the peak current level. In Figure 59 the dashed lines indicate the pulse current levels as they change from peak to background current.
•Final Slope is simply the opposite of initial slope. It is the amount of time to transition from the higher welding amperage level to the final lower amperage level.
•Final Amperage is the low amperage level that allows the weld to be properly terminated to prevent a crater crack. This final amperage, in combination with the final slope, is also referred to as crater fill.
•Postflow is the length of time the gas will continue to flow to protect the weld crater and the tungsten from atmospheric contamination.
All of the sequence times are controlled by the equipment and will happen the same way every time. When welding manually with a foot pedal, for example, this sequence of operation is being performed by the operator in the same way except the times can vary due to human inconsistencies.
Automation for GTAW requires precision in part fit-up and weld joint location. When everything is consistent prior to welding, the welds will be repeatable and consistent.
Microprocessors
Microprocessors provide the ability to control weld sequencing and can be used to control a number of other things. Devices such as PLCs (programmable logic controllers) and even simple welding machines use microprocessors to make decisions to ensure repeatability and functionality. Nearly all welding power sources manufactured today utilize a microprocessor to control the welding machine’s features and internal functions.
Figure 60, 9900 Touch Screen Weld Controller from Jetline® Engineering.
Microprocessor controllers usually have the ability to store numerous weld programs in memory, assuring repeatability as well as reducing set-up time. In an automated application, functions controlled by microprocessors might include the following:
•Pre-flow of shielding gas
•Arc starting currents polarity and time
•Initial current, initial time and initial slope
•Weld current, and weld time
•Pulse peak and background current
•Percent of on time (pulse)
•Final slope, final current, and final time
•Post-flow of shielding gas
•Programmable speed control with travel start and stop delay times
•A contact for a pneumatic torch with start and stop delay times
•Closed-loop control for better accuracy and display of travel speeds and welding parameters.
•Automatic correction from any deviation from the preset.
Arc Length Control System
Since arc length is critical on some applications, devices such as the Cyclomatic AVC-501 arc voltage control may be used in an automated system. Arc voltage control systems maintain a precise arc length by measuring arc voltage and comparing the measurements to the operator setting. It maintains the set arc length by moving the torch up or down as required using a motorized slide. Monitoring the voltage and using this data to control the arc length will provide for consistent weld appearance, profile, and penetration.
Cold Wire Feed System
A cold wire feeder adds the same filler metal as you would in the manual GTAW process only the filler metal is supplied on a spool similar to GMAW.In the same way, GTAW is generally considered a low-deposition process. However, by automating it and adding the filler wire in an automatic fashion its deposition rates can be increased. Increased weld deposition means higher travel speeds and more parts out the door at the end of the day.
Figure 61, Jetline® Cold Wire Feeder
Hot Wire Feed System
The hot wire process is used in applications where high deposition rates of the filler metal is desired. The process is used primarily with the GTAW or the PAW (Plasma Arc Welding) process.
Hot wire welding provides deposition rates normally associated with the GMAW (MIG) welding process but with the added advantages offered by GTAW and Plasma welding.
Figure 62, Jetline® Hot Wire Feed System
These advantages include better control of the heat input because the welding current is independent of the wire feed speed. This leads to reduced dilution in overlay welds and better side wall fusion in deep groove welds.
The wire is electrically preheated before it enters the weld pool, allowing it to easily melt and become part of the deposited weld. Shielding gas is supplied to the wire separate from the welding torch. The hot wire system provides enhanced control of heat input by requiring less energy at the arc to melt the wire. The system can be used for the surfacing or the joining of ferrous and non-ferrous alloys. The hot wire feeding system can also operate as a cold wire feed system.
Video Arc Monitoring
Video Monitoring Systems use the latest technology in color cameras to provide a magnified, high quality image of the welding arc, puddle, and joint area. These systems are designed to see where a welding operator usually can’t. Video monitoring systems allow the operator to not only safely view the weld area remotely, but also to change torch position by way of optional motorized slides. It is especially useful with long or large diameter parts or in applications where two or more welds are taking place simultaneously. Video monitoring can be used with nearly all mechanized arc welding processes.
Figure 63, Jetline® 14-in. LCD Video Monitor System Components
Seam Tracking
Laser and mechanical seam tracking are the two general methods used to maintain a consistent position of the torch over the weld joint in some automated GTAW applications. The seam tracker can be used to initially find the weld joint, track the joint while welding, and then retract the torch up and away from the part at weld completion.
Mechanical seam tracking uses an electromechanical probe that is mounted ahead of the torch. The probe has a tip that rides in the weld joint and, when deflected, causes the motorized slides to respond vertically or horizontally. Large tack welds may present a problem for guidance since the probe must trace the joint.
A laser system uses a camera that “sees” a feature in the laser stripe that is projected on the weld joint. This causes the cross slides to maintain a consistent torch-to-weld joint position. Tack welds and tight joint fit-up present no problem for laser tracking systems.
Magnetic Arc Control
Magnetic arc control uses magnetic fields to deflect and control the arc without moving the torch. It is useful for high speed automatic welding to even out the weld pool, prevent undercut, and promote uniform penetration. The oscillation and positioning effects of these magnetic fields on the arc improve weld appearance and weld bead profiles. It can be used with both ferrous as well as non-ferrous materials such as aluminum.
Mechanical Weld Oscillator
The mechanical weld oscillator automatically weaves the welding arc, increasing production rates and adding better arc control. It is typically a bolt on system that includes a control and motorized slide. The mechanical oscillator physically moves the welding torch to provide the weave pattern required for the application. Mechanical control of the torch movement provides smooth, stable, and precise oscillating movement of the welding torch. Using a mechanical oscillator has several advantages:
•Increases sidewall fusion, eliminates undercut
•Improves most overlay or cladding operations
•Increases productivity for multi-pass welds
•Improves weld quality and appearance
Unlike the GMAW or SMAW welding processes where the electrode serves as both the reinforcing filler metal and the current carrying electrode, the electrodes used for GTAW are not added to the weld puddle and are referred to as non-consumable electrodes. The GTAW electrode needs to have the ability to withstand the high temperature generated by the welding arc without overheating and becoming part of the weld. To achieve this, tungsten and tungsten alloys are used. This explains the name for the process.
Tungsten Electrodes for GTAW Tungsten is a very hard, steel-gray metal. It is a highly refractory metal that does not melt or vaporize in the heat of the arc. It has a melting point of 6170°F (3410°C), and a boiling point of 10,220°F (5600° C). Tungsten retains its hardness even when red hot.
Electrodes made of tungsten and tungsten alloys are secured within a GTAW torch to carry current to the welding arc. Tungsten is preferred for this process because it has the highest melting point of all metals.
The tungsten electrode establishes and maintains the arc. It is said to be “non-consumable” because the electrode is not melted and added to the weld pool. Care must be taken so that the tungsten does not contact the weld pool in any way. If the tungsten melts or is somehow broken off and added to the weld, it is referred to as a “tungsten inclusion”. Tungsten inclusions are considered defects and can be the cause of a rejected weld.
Figure 64, Typical Current Ranges for Various Tungsten Electrode Types, Applications, and Diameters
Accidentally dipping the tungsten into the weld pool or touching the filler metal to the tungsten during welding can also cause the tungsten to be contaminated and will have a detrimental effect on the welding arc quality. When the tungsten becomes contaminated the welding arc can become erratic, and maintaining a quality weld becomes difficult. If this happens the electrode needs to be reconditioned (Figure 65). Figure 65, A Pure Tungsten Electrode Contaminated by Aluminum Filler Metal
Tungsten electrodes for GTAW come in a variety of sizes and lengths. They may be composed of pure tungsten or a combination of tungsten and other elements and oxides. Electrodes are manufactured to specifications and standards developed by the American Welding Society (AWS) and ASTM International. Electrodes come in standard 0.01 in. through 0.25 in. diameters. The diameter of the tungsten electrode needed is often determined by the thickness of base metal being welded and the required amperage to make the weld. Figure 64 shows a chart that shows the recommended current capacity of each electrode diameter. The length of the tungsten electrode needed is often determined by the type of torch or the needs of a particular application. A 7-in. length is the most commonly used and purchased. For special applications some suppliers provide them in lengths cut to your specifications and may already have a specified preparation contour on the end.
Types of tungsten and tungsten alloy electrodes for GTAW are classified according to the chemical makeup of the particular electrode types. Figure 70 shows the common types of electrodes classified by the American Welding Society. The AWS classification system (Figure 66) uses letters and numbers to identify Tungsten Electrodes. The letter “E” is the designation for electrode. The “W” is the designation for the chemical element tungsten.
Figure 66, AWS Classification System for Tungsten Electrodes
The next one or two letters designate the alloying element used in the particular electrode. The “P” designates a pure tungsten electrode with no intentionally added alloying elements. The “Ce”, “La”, “Th”, and “Zr” designate tungsten electrodes alloyed with cerium, lanthanum, thorium, or zirconium, respectively.
The number “1”, “1.5” or “2” behind this alloy element indicates the approximate percentage of the alloy addition.
The last electrode designation, “EWG”, indicates a “general” classification for those tungsten electrodes that do not fit within the other categories. Obviously, two electrodes bearing the same “G” classification could be quite different, so the AWS requires that a manufacturer identify on the label the type and content of any alloy additions.
Electrodes are color-coded for ease of identification. Care should be exercised when working with these electrodes so the color-coding remains intact.
Use of Tungsten Electrodes With the choice of several alloy types and a variety of sizes, many factors must be considered when selecting the electrode. One of the main considerations is welding current. The welding current will be determined by several factors including base metal type and thickness, joint design, fit-up, position, shielding gas, type of torch, and other job quality specifications.
In cases where an incorrect electrode type, size, current, polarity, or technique is used, tungsten particles can be unintentionally transferred across the arc. The welding power source used may also affect the amount of tungsten which could be unintentionally transferred across the arc.
A machine designed specifically for GTAW welding will usually have characteristics advantageous for the process. Using a power supply designed for SMAW can produce excessive current surges or “spikes” that will cause “spitting” of tungsten. Excessive arc rectification on AC welding output will cause a half-wave effect, and could overheat the electrode and cause particles of tungsten to be transferred across the arc.
An electrode of a given diameter will have its greatest current carrying capacity with direct current electrode negative (DCEN), less with alternating current and the least with direct current electrode positive (DCEP) (see Figure 70). Tungsten has a high resistance to current flow and therefore, heats up during welding. In some applications the end of the electrode forms a molten hemisphere referred to as a ball (Figure 68). The balled end is characteristic of pure and zirconiated tungsten and is most desirable for AC welding. The ball is the only part of the electrode which should be this hot. If the ball gets too hot it will drop into the weld pool. This is referred to as tungsten “spitting”. To avoid this, the remainder of the electrode should be kept cool by selecting the appropriate type and diameter for the type and amount of welding current being used. Excessive electrode stickout beyond the collet will cause heat build-up in the electrode. Heat is generated by electrical resistance and the highly resistive air gap that produces the welding arc. Tungsten is not as good of a heat conductor as the copper that holds it so extra heat is generated in that short section of tungsten. The copper collet that holds the tungsten helps dissipate the heat generated in the tungsten. If the tungsten is moved further away from the collet more heat is generated in the tungsten electrode. In a water-cooled torch, heat is more rapidly dissipated from the collet assembly than in an air-cooled torch.
Figure 67, Cutaway View of a Water Cooled Torch
EWP (100% Tungsten, Green)
“Pure” tungsten electrodes do not have additional alloys and are comprised of a minimum of 99.5% tungsten. Pure tungsten provides good arc stability when using AC current with either balanced or unbalanced AC waves and continuous high-frequency. Pure tungsten electrodes are preferred for conventional AC sine wave and squarewave welding of aluminum and magnesium because they provide good arc stability with both argon and helium shielding gasses. Pure tungsten has less current-carrying capacity than alloyed electrodes and will form a ball on the end when heated by the arc.
Figure 68, A Balled Pure Tungsten
EWCe-2 (2% Cerium, Grey)
Commonly referred to as ceriated tungsten, EWCe-2 is alloyed with about 2% cerium. Cerium is a non-radioactive material and is one of the most abundant rare earth elements. The addition of this small percentage of cerium increases the electron emission qualities of the electrode which gives it a better starting characteristic and a higher current-carrying capacity than pure tungsten.
Ceriated tungsten is the best all-purpose electrode that will operate successfully with AC or DC electrode negative. Ceriated tungsten electrodes provide excellent arc stability and they have excellent arc starting properties at low DC welding current levels. Ceriated electrodes work excellent with the advanced squarewave AC welding power sources. This tungsten is recommended so that a shop can consolidate tungsten inventory and have one tungsten that can do it all.
EWLa-1 (1% Lanthanum, Black), EWLa-1.5 (1.5% Lanthanum, Gold) and EWLa-2 (2% Lanthanum, Blue)
Lanthanated electrodes have excellent arc starting characteristics, low-burn-off rates, excellent arc stability, and excellent arc re-ignition characteristics compared to pure tungsten. The addition of 1 – 2% lanthanum increases the maximum current carrying capacity of tungsten by approximately 50% for a given size electrode using alternating current. The higher the percentage of lanthanum, the more expensive the electrode.
Since lanthanated electrodes can operate at slightly different arc voltages than thoriated or ceriated tungsten electrodes, these slight changes may require welding parameters and procedures to be adjusted. The 1.5% content appears to most closely match the conductivity properties of 2% thoriated tungsten. Compared to cerium and thorium the lanthanum electrodes have less tip wear at given current levels. Lanthanated electrodes generally have longer life and provide greater resistance to tungsten contamination of the weld.
The lanthanum is dispersed evenly throughout the entire length of the electrode. It maintains a sharpened point well. This is an advantage for welding steel and stainless steel on DC, or the AC from advanced squarewave power sources. Thus, the lanthanated electrodes work well on AC or DC electrode negative with a pointed end or they can be balled for use with AC sine wave power sources.
EWTh-2 (2% Thorium, Red) and EWTh-1 (1% Thorium, Yellow)
Commonly referred to as 1 or 2% thoriated tungsten, these commonly used electrodes were the first to show better arc performance over pure tungsten for DC welding. However, thorium is a low-level radioactive material, so vapors, grinding dust, and disposal of the thorium raises health, safety and environmental concerns. The relatively small amount present has not been found to represent a health hazard. But if welding will be performed in confined spaces for prolonged periods of time, or if electrode grinding dust might be ingested, special precautions should be taken concerning proper ventilation. The welder should consult informed safety personnel and take the appropriate steps to avoid exposure.
The thoriated electrode does not ball as well as the pure tungsten, cerium or lanthanum electrodes. Instead, it forms several small projections across the face of the electrode when used on alternating current. These projections (Figure 69) are sometimes referred to as “warts”.
Figure 69, 2% Thoriated Tungsten with “Warts” From Advanced AC Welding
Thoriated electrodes work well with the advanced squarewave power sources when ground to a modified point. When used on AC, the arc wanders between the multiple projections and is undesirable. Ceriated, lanthanated, or high percentage thorium electrodes should be used.
However, the tendency of the “warts” to grow still presents issues. These electrodes are usually preferred for DC applications. In many DC applications, the electrode is ground to a taper or point. The thoriated electrode will retain the desired shape in DC applications where the pure tungsten would melt back and form a ball on the end. The thorium content in the electrode is responsible for increasing the life of this electrode over the pure tungsten (EWP).
Figure 70, Common Tungsten Types, Sizes, Applications, and Color Bands
EWZr-1 (1% Zirconium, Brown)
A zirconium oxide (zirconia) alloyed tungsten electrode is preferred for AC welding for extremely high quality work where even the smallest amounts of weld pool contamination cannot be tolerated. This is possible because the zirconium alloyed tungsten produces an extremely stable arc which resists tungsten spitting in the arc. The current-carrying capability is equal to, or slightly greater than, an equal sized cerium, lanthanum or thorium-alloyed electrode. Zirconium electrodes are typically used only for AC welding with a balled end.
EWG (Unspecified Alloy, Any Unused Color)
This classification covers tungsten electrodes containing unspecified additions of rare earth oxides or combinations of oxides. The purpose of the oxide additions is to change the nature or characteristics of the arc.
Some “rare earth” electrodes in this category contain various percentages of the 17 rare earth metals. One mixture is 98% tungsten, 1.5% lanthanum oxide, and a 0.5% special mixture of other rare earth oxides. Some of these electrodes work on AC and DC, last longer than thoriated tungsten, can use a smaller size diameter tungsten for the same job, can use a higher current than similar sized thoriated tungsten, reduce tungsten spitting, and are not radioactive.
Selecting and using an unspecified electrode may present issues with regard to code welding. If the tungsten type is an essential variable that requires re-qualification of a welding procedure, the specific alloys in these electrodes become important and may require a re-qualification should the formula change. Standardizing on a specified type of tungsten would eliminate this possibility.
Figure 71, Proper Grinding Techniques are Critical to Tungsten Life. Improper Preparation Can Significantly Affect the Welding Arc.
After the proper size and type of electrode has been selected, electrode preparation is critical to its useful life as well as its performance (before it needs to be re-prepared). The sharpening angle, referred to as the included angle, has an effect on the arc width, which in turn has an effect on the finished weld.
A course grinding finish can lead to arc wandering and a heavy oxide can form quickly, causing poor arc starting. Grinding along the shaft of the tungsten can also cause arc wandering and starting issues as well as contribute to the oxide that hinders arc starts. A consistent included angle, and a smooth grind finish only on the end to have the angle, will improve tungsten performance and contribute to a high degree of weld quality. Consistent smooth grind finishes will also improve the number of arc starts that can be performed with each tungsten preparation.
Tungsten is harder than most grinding wheels, therefore it is chipped away rather than cut away. The grinding surface should be made of an extremely hard material like diamond or borazon. The grinding marks should run lengthwise with the point (Figure 71). If the grinding is done on a coarse stone and the grinding marks are concentric with the electrode, there are a series of ridges on the surface of the ground area. There is a possibility of the small ridges melting off and floating across the arc. If the stone used for grinding is not clean, contaminating particles can be lodged in the grinding crevices and dislodge during welding, ending up in the weld deposit. The grinding wheel used on tungsten electrodes should be used for no other material. After welding, the surface of the tungsten should be shiny and bright. If it appears dull and hazy gray, an excessive amount of current for the diameter selected may be the cause. If the tungsten appears blue to purple or blackened, there is insufficient postflow time of the shielding gas. This means the surrounding atmosphere oxidized the electrode while still hot, and it is now contaminated with oxides. Continuing to weld with this condition can only result in the oxide flaking off and ending up in the weld deposit. A general rule for postflow is one second for each ten amperes of welding current. This is normally adequate to protect the tungsten and weld pool until they both cool below their oxidizing temperature.
Contamination of the electrode can occur in several ways in addition to the lack of postflow shielding gas. The most common form of contamination is contact between electrode and weld pool or the electrode and filler metal. Loss of shielding gas or contamination of the shielding gas due to leaking connections or damaged hoses causes electrode contamination. Excessive gas flow rates and nozzles that are dirty, chipped or broken cause turbulence of the shielding gas. This aspirates atmospheric air into the arc area also leading to contamination of the tungsten and weld pool.
Figure 72, 10X Magnification of a Perpendicular Hand Grind (Left) Hand Finish on a Typical Grinding Wheel (Middle) Finish on a Triad™ Tungsten Grinder (Right)
Figure 73, 20X Magnification of a Factory Finish (Left) Hand Finish on a Typical Grinding Wheel (Middle) Finish on a Triad™ Tungsten Grinder (Right)
The electrode that has been contaminated by contact with the weld pool or filler metal will have a deposit of the metal on the electrode. If this deposit is small, maintaining an arc on a scrap piece of material for a period of time may vaporize the deposit off the electrode. If the contamination cannot be removed in this manner, the preferred method is to re-prepare the electrode to remove the contamination. Breaking the contaminated tungsten off is not recommended as it may cause the tungsten to split lengthwise or bend the electrode. This may result in excessive electrode heating and a poorly shaped arc. Proper tungsten shaping and removal of contamination is a key to maintaining consistent welds. A properly prepared tungsten will reduce or eliminate arc wandering, splitting, spitting, and weld quality inconsistencies. Figure 74 shows a specially designed grinder for tungsten preparation. Figure 74, Weldcraft® Triad™ Tungsten Grinder
When too large of a diameter tungsten is selected for low amperage DC welding, poor arc starts and arc wandering will just never seem to go away. This is because tungsten is a poor conductor of electricity. Attempting to carry a 15 amp arc on a 3/32 in. diameter electrode can cause the arc to wander and creates difficulty in arc starting. Even the correct preparation of an electrode too large for the current level required can be the cause welding arc issues. Figure 75 shows the preferred shapes to prepare tungsten for GTAW.
Figure 75, Tungsten Preparation Techniques
DC Tungsten Preparation
A sharpened tungsten is preferred for direct current GTAW since 70% of the weld energy is directed to the positive half of the GTAW arc, and only 30% of the arc heat is directed to the tungsten.
A common practice in pointing electrodes is to grind the taper for a distance of about 2 electrode diameters in length to a sharp needle point for use on DC welding. Using 2 electrode diameters as a guideline yields approximately a 30° included angle. Using this rule for an 1/8 in. electrode, the ground surface would use 1/4in. of the length of the tungsten electrode. There are many theories and even more opinions on the proper included angle of the point. The actual application is the most important consideration when determining the configuration of the point. The second most important factor is to be sure that once a tungsten configuration is determined, you can consistently reproduce the preparation angles and the smoothness of the finish.
Generally, the welding arc emits from the tungsten at a 90° angle from the surface. So, contrary to common belief, a small included angle (15° needle point) does not produce a narrower weld bead, the arc is wider and more flared out causing a wider more shallow penetrating weld bead. A steeper angle like 30° will produce a slightly narrower arc column and weld bead, but is not easily noticeable until a 60° or higher included angle is used. By using a 30° included angle with a small truncated end (blunted back tip) you can help to constrict the arc in DC welding to produce a narrow arc column with a narrow deep penetrating weld bead. Figure 76 shows examples of various arcs and weld profiles produced by changing the included angle on the electrode. Figure 76, The Effects on Weld Bead Width and Penetration Depth With Varying Included Angles.
Figure 77, A Properly Prepared Tungsten With a 30° Included Angle With A Smooth Finish
Needle-pointed electrodes are usually preferred on very thin metals in the range of 0.005 in. to 0.040 in.. The benefit to having a needle point on thin metals is to flare out the arc column to produce a wide shallow penetrating weld bead. This helps the welder to avoid burning through the base metal and gives control back to the welder to ensure a quality weld can be produced.
Figure 78, A Properly Prepared Tungsten With a 30° Included Angle, Smooth Finish, and a Blunted End.
In other applications, the slightly blunted end is preferred because the extreme point may be melted off and end up in the weld deposit or simply to have a constricted arc to ensure deep penetration and a small narrow weld bead. In many applications, pointing is done to too large of an electrode diameter where a smaller electrode diameter should be used.
Conventional AC Tungsten Preparation
Electrodes for conventional AC welding, which includes sine wave and squarewave AC wave forms, should have a hemispheric or balled end. The diameter of the end should not exceed the diameter of the electrode by more than 1.5 times. As an example, an 1/8 in. electrode should form a ball no larger than 3/16 in. in diameter. If it becomes larger, because of excessive current, there is the possibility of it dropping into the weld pool causing a potentially rejectable weld defect. If the ball is excessively large, and the current is decreased before the molten tip drops off, the arc will tend to wander around on the large surface of the electrode tip. The arc becomes very hard to control as it wanders from side to side. If welding conditions are correct, a visual observation of the electrode should reveal a ball end of uniform shape and proper size. Ideally the smallest ball possible for the current you are welding with will help to minimize the wandering of the arc.
For improved arc focus set the balance control to favor the electrode negative side of the balance control and try a ceriated, lanthanated or thoriated tungsten with a modified point. This will keep the ball as small as possible to improve the stability of the conventional AC arc.
Figure 79, An Ideally-Sized Ball on Pure Tungsten for Conventional AC GTAW.
Advanced AC Tungsten Preparation
With the extended capability of balance control, AC frequency adjustment, and independent amperage control in an advanced AC GTAW power supply, the electrode shape is very nearly the same as that for DC electrode negative welding. The only difference is to prepare a ceriated, lanthanated, or thoriated tungsten the same way as you would for DC, except to add a small blunted end that will melt back slightly to form a very small ball on the end of the tungsten. The trick here is to find the angle and blunt back shape that will allow a ball to form that is equal to or only slightly larger than the diameter of the tungsten at that point. If you sharpen the tungsten to a needle point, the material that makes the needle point will melt back causing a ball that may wander off to the side instead of being concentric with the tungsten.
Figure 80, An Ideal Ball for a Higher Amperage Advanced AC Weld Setting Prepared Using a 30O Tapered and Blunted Tungsten
Figure 81, An Ideal Ball for a Lower Amperage Advanced AC Weld Setting Prepared Using a 30° Tapered and Blunted Tungsten
Sharpening the tungsten for use on an advanced AC machine improves the ability to focus the arc with greater localization of the heat into the work. Also, since the advanced AC machine is usually spending a minimum amount of amperage and time on the positive half of the cycle, a significantly lower amount of heat is generated on the tungsten allowing a much smaller ball to be formed. The smaller ball keeps the arc more stable and provides directional stability.
All arc welding processes utilize some method of protecting the molten weld pool from the atmosphere. Without this protection, the molten metal reacts with gases in the atmosphere and produces porosity in the weld. The weld deposits may even have undesired mechanical properties that greatly reduce weld strength.
The importance of atmospheric shielding is reflected in the fact that most arc welding processes take their names from the method used to provide the shielding; gas tungsten arc, gas metal arc, submerged arc, shielded metal arc, flux cored arc, etc.
For GTAW, Argon and Helium are two inert gases primarily used for shielding the weld pool. Shielding gases must be of high purity for welding applications. The purity required is at a level of 99.995% or greater. Although the primary function of the gas is to protect the weld pool from the atmosphere, the type of gas used has an influence on the characteristics and behavior of the arc and the
