Dressing GMA Welds

Figure 1 - This cross-section of an installed blind rivet shows that a major part of the strength of the joint is the rivet head.

There's a lot of information in the field on making and testing GMA welds, but not too much on dressing the welds after they're made on a vehicle. Any GMA weld 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).

Figure 2 - This cross-section of an undressed plug weld shows that the weld has fused the top and bottom pieces, and grinding off the nugget crown would have little difference in the joint strength.Figure 2 - This cross-section of an undressed plug weld shows that the weld has fused the top and bottom pieces, and grinding off the nugget crown would have little difference in the joint strength.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.

The Process
948Figure 3 - An angle grinder is an effective tool for dressing welds.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 angle 947Figure 4 - A cutoff wheel, which is used on the edge of the wheel, is not as precise as an angle grinder.grinder (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 carbide949Figure 5 - The different profiles of carbide rotary files allows accessing welds that were made in recesses. 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 access for a disc. This tool is difficult to control, so use extreme caution or 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.



Conclusion
Nearly all GMA welds require dressing. The goal of dressing GMA 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.

Additional I-CAR Collision Repair News you may find helpful:


Related I-CAR Courses

  • I-CAR Repairers Realm: Steering Column Inspection After A Collision - Now Available Friday, 1 November 2024

    I-CAR had a discussion on consideration for steering column inspection after a collision.

  • OEM Emergency Response Guide: Polestar Thursday, 31 October 2024

    Are you looking for OEM emergency response guides (ERGs)? The Repairability Technical Support (RTS) team has located these guides and either houses them on the RTS website, provides links to...

  • Back-To-Basics: Stationary Glass Installation Considerations Wednesday, 30 October 2024

    Sometimes going back-to-basics can make the difference between a quality repair and a failed repair. When replacing stationary glass, there are extra considerations to perform a complete and quality...

  • One-Time-Use Parts And Fasteners: Genesis Friday, 25 October 2024

    What is a one-time-use part, and how do you determine if a fastener, clip, or part can be reused? These are questions that we are confronted with in the collision industry quite a lot. The other big...

  • OEM Linking Pin: Toyota/Lexus Sectioning Clarification Thursday, 24 October 2024

    As part of the I-CAR Repairability Technical Support (RTS) OEM linking pin activity, we are helping to connect the collision repair industry to the vehicle makers. A common question when it comes to...

  • Refinishing Park Assist Sensors: Jaguar Land Rover Tuesday, 22 October 2024

    Park assist sensors are part of the park assist system, just one of many advanced driver assistance systems (ADAS) found on vehicles today. Damaged sensors are typically replaced and may require...

  • I-CAR At SEMA 2024 Friday, 18 October 2024

    Attending the SEMA Show in Las Vegas this year? Well, I-CAR will be providing many different opportunities to connect with you at this year’s show.

  • Non-SRS Wiring Repair: BMW/Mini Thursday, 17 October 2024

    What is a non-SRS wiring repair? It is the repairing of an electrical component that is not part of the supplemental restraints system circuit (usually identified by yellow wire looms). As...

  • Paint Maker ADAS Radar Optimized Color Resources: BASF Wednesday, 16 October 2024

    Changes in advanced driver assistance systems (ADAS) radar technology are introducing new refinishing considerations. We know that paint film thickness can affect the performance of ADAS radars, but...

  • Repairer Driven News: BMW Adds Structural Steel Parts To Restricted List Thursday, 10 October 2024

    Repairer Driven News (RDN) has released an article highlighting BMW adding structural steel parts to their restricted list.