Dressing GMA (MIG) Welds
There's a lot of information in the field on making and testing GMA (MIG) welds, but not too much on dressing the welds after they're made on a vehicle. Any GMA (MIG) usually requires dressing the top surface of the weld.
Does Dressing Reduce Strength?
There is a belief by some, that dressing a weld reduces the strength of that weld. If you look at a cross-section of an undressed plug weld, it looks similar to the shape of a blind rivet,
with the crown of the weld the head of the rivet (see Figure 1). Once the weld is dressed, it would be like grinding off the head of the rivet. Is the joint as strong? With a rivet, the answer is no, but then again a rivet is not fused to the metal around it. A good plug weld is fused to the metal around it (see Figure 2).
We made several plug welds on identical size coupon strips so we could pull them on our tensile testing equipment, some dressed flush and undressed (see Video). In general, there was an insignificant difference in tensile strength between those that were undressed and those that were dressed flush. Some of the dressed welds started failing on the top piece, rather than starting to pull a nugget out of the base metal. These welds had porosity or some other inclusion that was revealed after dressing and we don't believe the dressing to be a factor.
This video shows an undressed plug weld (left) and a dressed plug weld (right) being pulled with tensile test equipment.
We found little difference between the two.
So how far do you dress a weld? The short answer is so that it is flush with the surface. It's important to not reduce the thickness of the base metal, because that will weaken the structure. The only way to prevent that is to use light pressure and check your progress often.
As far as tool selection, a common initial tool to use is an angle grinder, with a 36 or 50 grit disc depending on the size of the bead or nugget (see Figure 3). The disc is lightly applied to the weld and moved slowly back and forth. If there are multiple welds, like a row of plug welds, they are ground one at a time. On a butt joint, again use a light touch and slow movement. It doesn't matter if you move the grinder across the weld or lengthwise down the weld, whatever works best for you. The important part is always being conscious and careful to not grind into the base metal.
Another tool that can be used is a cutoff wheel, but this tool is not as precise a tool as an anglegrinder (see Figure 4). There's a temptation to use this tool exclusively because it's aggressive, as in fast, but it's difficult to control, especially when you get close to the base metal. If used at all, it should only be the first step, then followed with an angle grinder.
When using a cutoff wheel, just like the angle grinder use light pressure. Hold the tool with two hands for better control. Start at a high RPM and walk the edge slowly across the surface of the weld, like you're planing off the surface of the bead. Stop when there is still a slight profile left on the weld. Do not touch the base metal surrounding the weld at all. This should keep you out of trouble. Again, follow this up with an angle grinder to get flush with the base metal.
Still another tool that can be used is a carbide rotary file. There are a couple different profiles available (see Figure 5). This is used when the plug weld is in a recess or access won't allow accessfor a disc. This tool is difficult to control, so use extreme caution oor you could damage an adjacent area. See Video You can finish the dressing with a hand file, maybe progressing through a couple different threads until the surface is flush as it needs to be.
Nearly all GMA (MIG) welds require dressing. The goal of dressing GMA (MIG) welds is to make the surface flush. Gouging below the surface of the base metal not only looks bad, it weakens the steel. Keep this in mind whenever doing this process.
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