February 19, 2002
A guide to the gas metal arc welding (GMAW) process. The benefits of the process include its high deposition rate, efficient use of filler matel, elimination of slag and flux removal, and the reduction of smoke and fumes.
Gas metal arc welding (GMAW) is a semiautomatic welding process that uses a wire electrode fed through a welding gun.
This continuous wire feeding during welding frees up the welder and allows him or her to focus fully on the gun position so that the proper arc length is maintained. A typical GMAW torch is shown in Figure 1.
The benefits of the process include its high deposition rate, efficient use of filler metal, elimination of slag and flux removal, and reduction of smoke and fumes. A disadvantage is that GMAW requires more equipment, making it less portable than shielded metal arc welding (SMAW). Also, the shielding gas required in GMAW may make outdoor applications more challenging.
The following tips can help you prepare for the next GMAW job.
1. Select the correct shielding gas to get the best result from your filler metal. Solid-wire electrodes used in GMAW require a shielding gas to protect the molten weld puddle from impurities in the atmosphere, specifically oxygen and nitrogen. The ideal end result produces no slag, thereby significantly reducing cleanup time.
The most typical shielding gases used for most mild steel solid-wire applications are 100 percent carbon dioxide and 75 percent argon/25 percent carbon dioxide. These are most widely used for the globular and short circuiting transfer modes.
Advantages of both are outlined in Figure 2. When choosing a filler metal wire for GMAW, always read the manufacturer's recommendations for shielding gas selection.
2. Before starting any welding project, make sure the workpiece is as clean as possible. Use a clean cloth, wire brush, or sandpaper to remove rust, dirt, paint, grease, oil, or any other contaminant. Avoid cleaning solvents because of the risk of explosion, fire, or illness from toxic vapors.
3. Set the power source according to the manufacturer's instructions for wire polarity. A power source that is not set for the proper polarity may produce a poor-quality weld.
4. Follow wire electrode specifications to set the proper wire feed speed (amperage) and voltage. Any power source might require some fine-tuning. Test a few welds on scrap metal to check that the wire feed speed and voltage are set to perform accurately.
Wire speed that is too fast will cause excess metal to be deposited, wasting filler metal or resulting in possible burn-through. Wire speed that is set too slow will result in a weld that does not penetrate or fill the joint properly, and either may cause the wire to "burn back," or melt at the tip.
Voltage that is too high will create excessive spatter and a flatter, wider bead that is porous. Additionally, high voltage can cause undercutting, a groove melted into the workpiece that is not properly filled with weld metal. Voltage set too low produces a narrow weld bead that lacks proper penetration and fusion.
5. Align the proper electrode stick-out with the wire diameter being used. Stick-out is the length of unmelted wire coming out of the contact tip of the welding gun. It affects the amount of amperage drawn by the wire and the outcome of the weld. Determining how much stick-out to use depends on the wire's diameter.
For instance, a good guideline to follow is: for 0.024- and 0.030-inch wire, use 1/4- to 3/8-inch stick-out; for 0.035- and 0.045-inch wire, use 3/8- to 1/2-inch stick-out.
Make slight adjustments to the stick-out to fine-tune the amperage for the desired result. Lengthening the stick-out reduces the amperage slightly, while shortening the stick-out causes a slight increase in amperage.
Listen to the arc while welding. A good arc sounds consistent, like bacon frying. If excessive popping and cracking are heard, the electrode probably is sticking too far out of the gun, or the wire feed speed is too fast.
Even an experienced welder needs continuing technical education. The following tips can help with your future welding jobs.
1. Learn proper electrode angles. Be sure to position the wire electrode properly over the weld joint for maximum coverage, paying special attention to the work angle and the travel angle.
The work angle is the angle at which the wire is pointing at the weld joint. Lap and T-welds require a work angle of 45 degrees, while butt welds require a 90-degree work angle.
The travel angle is the angle of the wire as it travels along the weld path. For most wire welding applications, this angle is 15 to 30 degrees. The most common travel angle is called a drag angle in which the electrode points in a direction opposite that of the arc travel.
2. Learn to manipulate the welding gun effectively. For lap and T-welds, manipulate the gun to create a series of small ovals to produce good welding coverage. Take care not to move too far back into the weld puddle or fusion problems may occur.
For butt joints, manipulate the gun so that the electrode moves in a Z pattern while traveling along the workpiece (see Figure 3). This pattern is most effective because it produces a flatter weld, spreading the molten weld puddle evenly across the joint.
A Z pattern is most effective for butt joints because it produces a flatter weld.
3. Control the travel speed as you weld. Watch the molten weld puddle and listen to the arc for evidence of too-fast or too-slow travel.
Moving at a high travel speed or too quickly causes insufficient penetration, and popping sounds will be heard as the wire comes into contact with the cold metal just ahead of the puddle. Welding at low travel speeds or moving too slowly will cause the weld metal to pile up, resulting in poor fusion.
When one of the following problems is experienced during GMAW, try these solutions (one at a time):
Incomplete fusion. Incomplete fusion is a gap that occurs when the weld metal is not completely fused to the base metal. This can happen between the weld metal and the base metal or between passes in a multiple-pass weld. Solutions are to:
Porosity. Porosity is a gas pocket in the weld metal that may be scattered in small clusters or along the entire length of the weld. These voids, which can be internal and/or on the surface of the weld bead, weaken the weld. Solutions may be to:
Undercutting. Undercutting is a condition that occurs when a groove is melted in the base metal next to the toe or root of a weld that is not filled by the weld metal. A particular problem with fillet welds, undercutting produces a weaker joint at the toe of the weld, which may result in cracking. To correct this problem:
When overlapping occurs, the weld metal protrudes over the edge or toe of the weld bead. You can:
Metal particles expelled during welding that do not form a part of the weld is weld spatter. Excessive spatter creates a poor weld appearance, wastes electrodes, makes slag removal difficult, and can lead to incomplete fusion in multiple welds. Solutions are to:
Melt-through takes place when the arc melts through the bottom of the weld. Remedies are to:
This is insufficient flow of shielding gas to the welding area or blockage of the shielding gas flow, which causes many GMAW defects. To correct this problem:
Wire feed stoppages. Wire feed stoppage is the malfunction of a wire feed system that extinguishes the arc and creates an irregular weld bead. Compared to other continuous-wire-feed welding processes, GMAW has the most problems with wire feed stoppage because of the small-diameter electrode wires that are used. Solutions may be to:
GMAW requires a certain degree of welder skill to produce high-quality welds. Semiautomatic GMAW, for example, requires that the welder control the welding gun and the speed of travel.
However, the process generally takes less skill when compared to manual welding processes, such as SMAW, because the machine controls the arc length and feeds the filler wire.
Examples of good and bad welds are shown in Figure 4. A quality GMAW weld is the result of a good welding technique and proper choice of welding parameters.