MIG Welding Defects
November 20, 2018 @ 3:54:pm
In a previous post, we discussed how the “weld metal” tensile strength would yield same result in either MIG or TIG welding. This assumes there are no weld defects in either method. The operational control and the skill of the operator make the difference in the welds. While TIG offers the greatest control of either method, it not always the right method for every shop or every build. Be sure to understand where and how typical weld defects occur before choosing the method for your shop.
Typical weld defects found in MIG welding are lack of fusion, burn through, crater/ crater cracks, undercut and porosity in the weld metal. These defects can be caused by a number of factors, such as wire feed, speed, travel speed, improper voltage, improper shielding gas, improper shielding gas flow rate, along with defects that occur from improper welding techniques, improper weld preparation and defective equipment.
The starting and stopping points in MIG are common areas to see weld defects. Lack of fusion, sometimes referred to as cold lapping, occurs when there is little or no fusion between the weld metal and the base metal. This is normally caused by lack of heat input at the start of the weld. One way to correct this is by striking the arc slightly ahead (about 1.5 times the weld size) of the desired start point and quickly backing up to the desired start point. Lack of fusion may also show up anywhere in a weld due to poor welding technique also. Another common defect that occurs when stopping is a crater / crater crack, found at the end of a weld. This can be counteracted by welding to the desired stopping point and then slightly back (about 1.5 times the weld size). This will fill the groove or crater in the weld while also slowing the cooling rate by extending the coverage of the shielding gas.
Undercutting, is a groove or crater that occurs near the toe of the weld. When this weld flaw occurs, the weld metal fails to fill in that grooved area, resulting in a weak weld that is prone to cracking along the toe of the weld. Excessive heat, as well as poor welding techniques can both lead to undercutting.
Downhill progression welding can also cause weld defects especially on thicker materials, such as lack of fusion. Lack of fusion may occur in downhill progression when the weld puddle gets in front of the arc.
Burn through is a defect that occurs when the weld metal burns through the base material. This is normally caused by excess heat input. Excess heat input can also cause other issues such as an increased HAZ or, heat affected zone, even where burn through is not present. The increased HAZ can diminish the strength of the material.
Porosity in the weld metal, another common weld defect, occurs when gases become trapped in the weld metal or along the surface. This can cause weak welds. Porosity can be caused by a number of variables such as contamination in the atmosphere or the base material, defective equipment, too much or too little gas flow, or an improper welding technique. Porosity can be common in MIG welding because of the fast travel speeds and relatively small nozzles which can result in compromised shielding gas coverage. Weld filler metal selection can also be a factor in helping eliminating porosity depending on the percentage of deoxiding elements and base metals being welded.