The importance of test welds

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There is an often-used word in our industry: "Change." I use it a lot, too, but what if we stepped away from something new for a moment and focused on something that is not new? I hope I have your interest, even if it's not good news. There are times when we can and must pause from all of the new things being thrown at us and revisit a not-so-new process that far too many shops are not performing. As I've traveled around the country the past few years, the subject of performing test welds has come up too often in the wrong context. I've been a bit surprised at the lack of understanding on the subject. We'll cover those misconceptions and more importantly the reasons behind why it needs to be done.  

Anyone who knows me knows I'm always thorough in explaining why, and, no, it's not just because the manufacturer says so; that is only part of the reason for performing them. More importantly, test welds should be part of your SOP or process for any type of welding you perform on a vehicle regardless if the vehicle manufacturer specifies it or not. More on that later. 

I'll provide some examples of what may be specified by a vehicle manufacturer. It's worth noting that these guidelines will typically be found, depending on your information provider, in body shop basics, weld conditions, or welding guidelines in a body repair manual. They may not be included in that panel/part procedure you've acquired for that particular operation. FCA discusses test welds beginning in its Types of Welding section in a body repair manual for a 2017 Jeep Renegade:  

Learning how to create weld coupons, refer to Test Weld Coupon and then performing a destructive test using these coupons, is the key to successfully using STRSW. FCA US LLC requires a physical test using test coupons and the methods outlined in this document to test welds prior to making repairs.  

The "Test Weld Coupon” text is a hyperlink in the manual and will take you to this description (partially edited to be brief): 

Select the proper spot welding tong/arm which provides the best access to the areas of the vehicles where the spot welds are to be made. 

1. Obtain metal of the same thickness and coating (IE., bare, galvanneal, or galvanized) to be welded. This metal will be used for spot welder set up. Damaged sheet metal taken from the vehicle being repaired may be used if it is from the area from which the work is to take place. The sheet metal must be flat, free of cracks, wrinkles and scored metal. 

2. Using the procedure outlined in Test Weld Coupon, prepare the test coupons. 

3. Clean and prepare both mating coupons. If using adhesives, verify the recommendations of the adhesive manufacturer. All contaminates such as rust scale, dirt, paint, and existing sealers and adhesives must be removed. Remove any E-coat within 25 mm. (1 in.) of where the welds are to be placed. 

4. If the panel joint originally contained structural adhesive, it should be applied to the coupon at this time. 

5. Install the equipment manufacturers recommended electrode tips. 

6. Adjust the welding electrode tip force, and clamp time per manufacturer recommendations.  NOTE: Galvanneal and galvanized coated steel will require more force 

7. As shown in Test Weld Coupon, place first weld at a position at least 12.5 mm. (0.5 in.) away from end weld coupons. Then make the second weld. The weld spacing should be the same distance as the original welds or the closest existing weld, whichever is the least on the vehicle being repaired. 

8. Destructively test the second weld to determine the size of the resistance spot weld produced (see examples in Test Weld Coupon. If the weld is insufficient, adjust the welder per the welder manufacturer recommendations and repeat steps 7, 8 and 9 until the proper weld size is achieved. 

Some key takeaways from this excerpt include using the same type of metal that the actual welding will take place on, using adhesive on the test weld if it's going to be used during the process, and performing a destructive test of the weld. In simpler terms: use the same metal you will be welding and mimic what you are going to do on the vehicle. If you are doing different scenarios, they would each call for a separate weld test. We'll dive deeper into the destructive portion of the test in a bit. Below is an excerpt from the body repair manual for a 2011 BMW X5: 

Preparations for MAG and spot-welding  

Remove the paint coating in an area of approx. 30 mm around the weld seam or spot. 

• The zinc layer underneath must be removed during the MAG welding method. 

• It is not necessary to remove the zinc layer underneath during spot-welding. 

Remove the paint coating on the reverse side of the weld seam or spot. 

• Contaminants will otherwise enter the weld pool via the root of the weld seam. 

• An insufficient electron flow prevents an optimum spot-weld joint. 

Coat all metal overlaps and weld spot flanges with welding primer. 

To determine the optimum electrode contact force for spot-welding, carry out spot-weld shear tests on sample sheet metal. 

In order to keep the electron flow short during MAG welding on the body, you must, if possible, attach the ground terminal directly to the component to be welded.  

I used this example to show just how little may be published. The test weld is mentioned in only one line, but it's no less important!  

The basics 

So, I've given you some edited examples of what you may see a manufacturer call out. Some examples may describe the destructive portion of the test. If they don't, we will cover some of the basics now. If it is a spot weld made using STRSW (squeeze-type resistance spot welding) or a plug weld made by GMAW (gas metal arc welding, which includes MAG/MIG welding) you will want to verify the top layer of metal tearing a hole in the base layer (what you welded the part/panel too). It's worth noting that when performing plug welds with the GMAW it's unlikely the base/host metal will not tear a hole unless the plug weld you made completely covers the hole you are welding. Voids, porosity, and other defects will likely cause the weld to fail as well. If it is an open butt joint weld you would want to fold the two pieces of metal you joined onto their faces and verify the weld does not break. The same is true for a MIG brazed open butt joint. For a butt with a backer on aluminum or steel, you will want the two top layers to break off of the weld and the weld needs to remain attached to the base metal.  

Test welds need to be done every time before welding any panels or parts on a vehicle. The only way a technician is going to know if the weld they just performed on a vehicle is a good, quality, and safe weld is to perform it on some scrap cut off of the vehicle and do a visual and destructive test to verify its performance. It also allows the technician to get the settings correct on the machine and not have to make adjustments as they are welding on the vehicle. That sounds a little harsh, but it's been the reality far too long. Too many times I've seen the welding process start on the car, SUV, or truck and the technician adjusts the welder after a few welds have been completed. The other aspect of ensuring the validity of the weld is to go beyond the appearance of the weld and verify that not only does it meet potential visual criteria, but it also will perform as it should in a subsequent collision. This is true for any form of joining metal by welding, whether it be using an STRSW, MIG, or MAG welding for the various forms of joining we may use. Plug welds (MIG or MAG), spot welding, stitch, fillet, or any form with any type of machine should be recreated and proven successful before actually welding on the vehicle.  

Regardless of what the manufacturer may or may not require it is an important process to be performed every time. It not only should be done to ensure the vehicle is repaired properly but it will also increase a technician’s efficiency. If the equipment is set up properly, including the settings, the actual time spent welding on a vehicle will decrease typically. It is also important to destruct the test weld to verify the machine is operating properly regardless if it “passes” the weld. I'm referring specifically to the smart STRSWs now available to us today. Some may sense the type of metal, thickness, or if adhesive is being used. Those machines are wonderful — there is no doubt about that — but rest assured the only true way to know if the weld is successful is via a destructive test.   

Advanced materials and processes 

As we have more commonly used welding processes the need for weld testing increases. I'll toss out some different processes for you to ponder: MIG Brazing (Silicon Bronze), STRSW, aluminum (MIG), and steel (MAG). Within steel and aluminum welding there are more types: plug, open butt, fillet, butt with a backer, etc. STRSW has multiple options as well: two pieces of metal or more and with or without weld bonding. I will suggest that if the manufacturers do not publish guidelines, I would set the test welds up as they are to be performed on the vehicle. For example, if you are using STRSW and performing weld bonding I would have the adhesive between the test metal as it would be on the vehicle. 

Another consideration is that of checking the welds as you go along when welding on a vehicle. There are two common aspects to this. The first is visually inspecting the welds. I-CAR has this to say on the subject in their Back To Basics series within their Repairability Technical Support (RTS) portal:  

Sometimes going back to basics can make the difference between a quality repair and a failed repair. When it comes to inspecting repairs for quality, there are many things to look for in order to avoid failures throughout the repair process. 

One of the things that should be inspected are welds. It is important to check welds for quality, as this may have an impact on the way the repair will stand the test of time or another collision. Some things to look for include porosity, penetration, cleanliness, and incomplete welds. Any of these defects will weaken the weld and may cause a repair to fail. Always make practice welds on scrap material from the damaged part before welding the new part onto the vehicle. Take time throughout the repair to check welds before moving on to the next process. 

There are many more items that should be inspected for quality during the repair process. Always follow OEM procedures to ensure a complete, safe, and quality repair. 

Wedge test 

Another aspect of checking the quality of your welds would be that of a “wedge test.” It may be designated by an OEM while welding on the vehicle. The wedge test is typically performed when making welds with an STRSW, but could be used similarly if the plug welds were that of GMAW (MIG/MAG). The test is performed by placing a wedge-shaped object — a panel separator is usually available and works quite well — in between the two panels being tested (if there are multiple layers, they would need to be tested separately) and making sure the metal deforms around the weld. If the weld is not sufficient, it will usually pop loose quickly. Even if not called out by the manufacturer the wedge test is a good way to make sure the machine is continually performing as it did initially. Making this a regular practice is very efficient overall as any glitches in the process are discovered quickly. If there were no published guidelines, I would do this test every five to 10 spot welds I made. It's fairly quick to do as an in-process procedure. If the welds being tested are sufficient, it's a common practice to gently hammer and dolly the area back flat after the area is deformed during the test. 

As promised, I've given some examples of what the manufacturers may call out for weld testing and I've hopefully ingrained the need to perform them regardless of an OEM requirement. It truly is a process that has multiple benefits. Increased efficiency is certainly one, but the most important is safety.  

It may not have been good news that recent observations have highlighted the need for our industry to take this process to the top of the to do list every single time we perform welds on a vehicle. It should be done regardless if it is in writing from the manufacturer.  

Everything we do should have a sharp focus on how that car, truck, or SUV would react in a collision after repairs are made. You might have passed the test to get that cool sticker, magnet, or certificate, but are the welds you make on that vehicle today and tomorrow going to pass the test when someone's life depends on it?