C Sridhar, Director-Technical, Advance Institute of Welding Technology, Chennai, Tamilnadu
By its very nature, gas-tungsten-arc welding (GTAW) is a relatively slow process. But it also is a very versatile one. In fact, it can be used to weld more materials than any other process, even exotic and heavier alloyed metals. It’s also ideal for thin materials, as it results in a relatively low amount of heat input, to prevent burn through. Plus, no matter the application, when performed properly GTAW can provide extremely high weld quality.
Achieving such positive results, however, is not always easy—success is as much a matter of training and practice as it is simple patience. Fortunately, arming yourself with a few tips along the can help you greatly improve the effectiveness of the process. After all, you don’t want an already slow welding process to become even slower.
Tip 1: Invert the Process
Using an inverter power source is one of the first steps a metal former can take to improve GTAW efficiency. Inverters operate by switching high-voltage low-amperage alternating current (AC) into direct current (DC) back and forth at a very high rate of speed—up to 50,000 times/sec. The overall result is a smooth arc that provides consistent welding performance.
Inverters also have frequency controls that allow the welder to determine the length of time that it takes the unit to complete one full current cycle (the combined time spent on electrode positive electrode negative), and to adjust the frequency from 20 to 400 Hz.
(Note: transformer-based power sources only produce an output of
60 Hz, the same frequency that comes from a wall power receptacle).
The inverter’s frequency feature helps improve welding efficiency by narrowing the focus of the arc, creating a narrow weld bead and minimal heat-affected zone (HAZ). With this feature, welders will spend less time and consume less filler metal completing each weld. A smaller HAZ minimizes the likelihood of burn through and the need for rework—a definite cost saver in any welding application.
Inverters also feature a balance control, which allows the welder to adjust how long the current spends in each part of the AC cycle—particularly useful when welding aluminum. He can adjust the balance control more toward the electrode-positive portion of the cycle, which helps to remove the oxide layer on the aluminum workpiece (referred to as ‘cleaning action’), or more toward the electrode-negative portion of the cycle, which increases weld penetration and travel speed.
Tip 2: Stay Cool and Flexible
Selecting the right GTAW torch for the application also can help make the process more efficient. First, select a torch with good insulation. Silicon-rubber insulation, for example, protects against high-frequency leakage and cracking that can lead to premature torch failure and downtime for torch changeover.
Also consider whether an air- or water-cooled GTAW torch is best suited to the application. Air-cooled models prove useful for low-amperage applications, say below 200 A, for welding materials less than 8.0 mm. thick, or for shops where welders tend to move around a lot, since these torches do not require an external cooler. Conversely, consider a water-cooled GTAW torch for applications in excess of 200 A. These torches help prevent overheating and allow welders to achieve faster travel speeds.
When selecting a GTAW torch, also consider the angles at which the welders must weld, since maneuvering around difficult joints can be time-consuming, not to mention uncomfortable. Most GTAW-torch manufacturers offer models with flexible necks that make the job easier in awkward positions.
Some torch-body styles also feature a modular design, which allows the welder to add a flexible neck and different head angles to an existing torch. These kits provide good joint access and can lower downtime associated with changing over different torches for multiple applications. Plus, you can save money on extra inventory.
Tip 3: Cover Yourself
Adding a gas lens to the GTAW torch (left) provides for an even flow of shielding gas when compared to a torch without a gas lens (right).
When possible, use a gas lens to replace the collet body of a standard GTAW torch. A gas lens helps hold the tungsten in place and creates the electrical contact necessary for proper current transfer. It also provides two other functions that can help improve efficiency: It improves shielding-gas coverage (see photos) and improves weld-joint accessibility.
Gas lenses typically consist of a copper or brass body that contains a layered mesh of stainless-steel screens. These screens distribute the shielding gas evenly around the tungsten electrode and along the weld puddle and arc to help prevent oxygen contamination that could lead to weld defects. As in any welding application, minimizing defects and their associated rework ensures that the welder can spend more time in production and less time fixing defects.
Gas lenses also allow the welder to extend the tungsten electrode further out from the nozzle. This additional electrode extension gives the welder a clearer look at the joint and arc, allowing him to have greater torch control and achieve better weld quality, particularly on critical applications or in hard-to-reach areas such as T, K and Y joints.
Gas lenses—which can be used with any shielding gas and are available for air- and water-cooled torches—also prove particularly helpful when welding on alloys highly reactive to atmospheric contaminants or for materials used in high-temperature applications.
Tip 4: Less can be more
Taking steps to prevent overwelding will significantly improve GTAW efficiency, and save money. Overwelding occurs when the welder deposits more weld metal in a joint than is required to obtain the necessary weld strength. It often results from poor joint fitup or preparation, improper welding parameters or from simple overcompensation —the welder believing that he needs more weld metal to fill the joint than is necessary.
Overwelding wastes shielding gas and filler metal, and increases welding time. For example, overwelding a fillet weld by a mere 1.5 mm can increase arc-on time by as much as 30 to 40 percent for a 10.00 mm fillet weld and 80 to 90 percent for a 3.00 mm weld. In addition, overwelding increases the amount of heat input into the base material, raising the risk of burnthrough or distortion and leading to costly and time-consuming rework. It may even increase the need for grinding and finishing which again an additional operation and increases cost of weld metal by another 20 to 25 percentage and also adds time factor.
To prevent overwelding, avoid over-designing weld joints–do not use a larger joint than is necessary to gain the appropriate strength for the application. A good rule of thumb: Make the leg of a fillet weld no wider than the thickness of the thinnest workpiece, and weld accordingly. For example, when joining a 3.00 mm thick plate to a 6.00 mm plate, a 3.00 mm weld bead suffices unless, the weld is designed for a dissimilar leg size. Also, know the size of the joint being welded. When in doubt, don’t guess—use a fillet gauge.
Lastly, proper joint preparation and tight fitup provide good defenses against overwelding, as does welding in the vertical-down position on thin materials.
Tip 5: Use Pulsed TIG process with fast pulse
When welding thin metal, the main objective is to avoid warping, burn-through and excessive heat affected zones while still ensuring the weld has sufficient mechanical strength for the application. The welding processes that provide the most control over heat is pulsed MIG, pulsed TIG.
For fabricators and others with bottom line goals, welding sheet metal often means a constant battle between productivity and equipment investment vs. burn-through, warping, excessive heat affected zones (HAZ) and weld appearance. For the individual occasionally welding sheet metal, success can be as simple as learning the proper techniques.
Pulsed Gas Tungsten Arc Welding – Pulsing between a high peak current and a low background current at frequencies of 100 to 500 pulses per second (PPS)—allows operators to accomplish one or more of the following:
- Reduce the bead width by upto 52 per cent
- Increase penetration by upto 34 per cent
- Reduce heat in-put by upto 60 per cent
- Increase welding travel speed upto 35 per cent
- Promote better weld quality
Pulsed TIG Waveforms: Operators set four variables when programming a pulsed TIG output: peak amperage, background amperage, pulses per second (PPS) and peak time.
DC TIG Comparisons: Slow travel speeds increase the width of heat-affected zone and can cause carbide precipitation on the backside of the weldment. Faster travel speeds, produced by high speed pulsed TIG, can alleviate weld quality issues while improving productivity.
Straight TIG, Pulsed TIG : The pulsed TIG weld bead took 30 percent less time to weld and it requires almost no clean-up. It also clearly indicates the reduced heat-affected zone.
Tip 6: Get to the Point
The type of tungsten used—which depends on the kind of power source selected and the type of material being welded—as well as the shape of the electrode tip can significantly impact process efficiency.
For AC and DC welding using an inverter power source and either a ceriated, lanthanated or thoriated tungsten electrode, grind the electrode to create a pointed or truncated tip. This provides the stable arc needed to achieve good welding performance and quality, while preventing contamination or arc wandering. To achieve this shape, grind the tungsten on a borazon or diamond grinding wheel specially designated for the job. Alternatively use a fine grain wheel with 100 to 120 grit size. Never use your bench grinder which has a coarse grain (grit size 40 or 60) wheel as it causes serrations and spoils the shape of taper and also consume tungsten metal. It also results in low focus of arc power on the job and diversion of current / heat. Next, grind the taper on Grind the taper on the tungsten-electrode tip to the electrode tip to a distance no more than 2.5 times taper on the tungsten-electrode tip to a distance the electrode diameter. For example, for 3.0 mm electrode, no more than 2.5 times the electrode diameter (A), grind a surface 6.00 to 8.00 mm long. The tip design will, for example, using a 3.0 mm electrode, grind a ease arc starting and help create a more focused arc surface 6.0 to 8.0 mm long. This tip design will ease when welding with low amperage on thin materials (1.00 arc starting and help create a more focused arc to 3.00mm thick) grind the tungsten to a point. Don’t grind circumferentially (C); grind longitudinally (B).
This will allows welding current to transfer in a focused arc and helps to prevent bdistortion. In particular, a pointed ceriated helps to prevent distortion. In particular, a pointed ceriated tungsten electrode works well when welding Aluminum, as it provide 30 to 40 per cent more amperage capacity than does pure tungsten before it begins to melt.
Note. Do not use a balled tungsten electrode tip for such an application.
On higher-current applications, grinding the tungsten to a truncated tip can help improve welding performance by preventing the tungsten from balling. First grind the tungsten to a taper (as explained above), then grind a 0.30 to 0.80 mm flat land on the end of the tungsten.
Note: When grinding throated tungsten, be sure to control and collect any grinding dust, provide operators with an adequate ventilation system at the grinding station, and follow any manufacture’s warnings, instructions and material safety datasheets.