Chapter 8 Gas Metal and Flux Cored Arc Welding 233 Copyright Goodheart-Willcox Co., Inc. 8.15 Gas Metal Arc Spot Welding A gas metal arc welding power source and arc weld- ing gun can be used to produce a weld in one small spot, called a spot weld. Metals commonly welded with the gas metal arc spot welding process are low-carbon steel, stainless steel, aluminum, copper-based metals, and magnesium. A GMAW spot weld is a weld made at a single point on overlapping pieces. Although gas metal arc spot welding is generally done on metals under 1/16″ (1.6 mm) thick, metals up to 3/16″ (5 mm) thick can be spot welded using the GMAW process. A manually controlled spot weld is made by posi- tioning the welding wire above the location where the spot or tack is to be made. The trigger on the welding gun is pulled. No movement of the gun takes place. After the weld pool is formed, the trigger is released. See Figure 8-72. The welder sets the voltage and wire feed speed as is done when running a weld bead. The weld time is controlled by the welder. This technique is used to tack parts together. It can also be used to weld through a thin metal sheet to join it to a piece of metal beneath the thin top sheet. The weld time can be controlled by the welding power source if it is equipped with timer controls. The welder sets the desired weld time on the power source. When the gun trigger is pressed, the arc takes place for a set and controlled time so current fl ows for the same duration for every spot weld. A completed spot weld is shown in Figure 8-73. Spot welds on thin metals can be made in any position using the GMAW process. As the metal thick- ness increases, spot welding with the GMAW process is limited to the fl at welding position. Weld quality and uniformity are not as good as that possible with resistance spot welding. The big advantage of spot welding with GMAW over resistance spot welding is that access to only one side of the parts is required. 8.16 GMAW/FCAW Troubleshooting Figure 8-74 lists many typical troubles that may occur during GMAW and actions that can be taken to cor- rect them. To use this chart, fi nd the problem in the left column. The center column lists possible causes of the problem. Methods to correct the problem are listed in the third column. When troubleshooting, fi rst identify items that are known to cause a problem and are easy to correct. These include removing spatter from the nozzle, replacing a worn contact tip, and making sure the correct shield- ing gas and fl ow rate are being used. These common causes of problems should be checked fi rst, especially if the equipment was making good welds prior to issues arising. The second set of causes to check are equipment setup and welding technique. These include items like wire feed speed, voltage settings, too long an arc length, and improper gun angles or travel speed. One common problem is having the incorrect weld- ing polarity. GMAW uses DCEP. FCAW can use either DCEP or DCEN, depending on the electrode. Com- mon FCAW–S electrodes use DCEN. Pro Tip A welder can forget to switch the polarity when switching electrodes. If the equipment does not weld well, ensure that the polarity is correct for the electrode wire being used. If a cause is still not found after checking these items, check the more diffi cult items. These include worn cables, grounding issues, problems with the equipment, and problems with the design of the weld. Spot welds on two overlapping pieces Goodheart-Willcox Publisher Figure 8-72. Several spot welds. Notice the depth of penetration. Goodheart-Willcox Publisher Figure 8-73. A completed spot weld made with GMAW.