Page 18 - MetalForming March 2010

Gas-Tungsten-Arc-Welding Productivity
time by as much as 36 percent for a 3⁄8-in. fillet weld and 125 percent for a 1⁄8-in. 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 grind- ing and finishing.
To prevent overwelding, avoid over- designing weld joints–do not use a larg- er joint than is necessary to gain the appropriate strength for the applica- tion. 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 1⁄8-in. thick plate to a 1⁄4-in. plate, a 1⁄8-in. weld bead suffices.
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 ver- tical-down position on thin materials.
Tip #5: 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 ceri- ated, 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 wan- dering. To achieve this shape, grind the tungsten on a borazon or diamond grinding wheel specially designated for the job. Next, grind the taper on the electrode tip to a distance no more than 2.5 times the electrode diameter. For example, using a 1⁄8-in. electrode, grind a surface 1⁄4 to 5⁄16 in. long. This tip design will ease arc starting and help create a more focused arc.
When welding with low amperage on
thin materials (0.005 to 0.040 in. thick), grind the tungsten to a point. This allows welding current to transfer in a focused arc and helps prevent distortion. In particular, a pointed, ceriated tungsten electrode works well when welding alu- minum, as it provides 30 to 40 percent 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 perform- ance by preventing the tungsten from balling. First grind the tungsten to a taper (as explained above), then grind a 0.010- to 0.030-in. flat land on the end of the tungsten.
Note: When grinding thoriated tung- sten, be sure to control and collect any grinding dust, provide operators with an adequate ventilation system at the grinding station, and follow any man- ufacture’s warnings, instructions and material-safety datasheets. MF
16 METALFORMING / MARCH 2010
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