Collision Repair Training | United States
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Current Events/News: Advantage Online: 2002 Archives
DAIMLERCHRYSLER EXPANDS THE USE OF REPAIR SPOT WELDING August 12, 2002 -A new publication from DaimlerChrysler details the use of squeeze-type resistance spot welding (STRSW) during collision repair, both with and without the help of structural adhesives in the joint. Also, STRSW is no longer restricted to exterior panels on DaimlerChrysler vehicles. Underbody structural parts, including rails, can now be spot welded during repairs, provided successful test welds are performed for each application to ensure weld quality and strength.In 1999, DaimlerChrysler became the first domestic vehicle maker (among DaimlerChrysler, Ford, and General Motors) to recommend the use of STRSW for collision repair on vehicles built in North America. The use was very specific. STRSW could only be used on outer "cosmetic" panels as an alternative to GMA (MIG) welding and, most importantly, could only be used in combination with adhesives in a process known as weld bonding (see "Weld Bonding Repair" in the September-October 1999 (6.6M) issue, and "Performing A Weld Bonding Procedure" in the July-August 2000 (9.3M) issue of the I-CAR Advantage). In this second of which is expected to be an ongoing series of documents on the subject of the use of STRSW, it’s stated that STRSW equipment makers have made significant advances. These advances "now enable repair facilities to duplicate an OEM joint utilizing the same process for repair that the OEM uses during production–both with and without structural adhesives." Adhesives are still required in the repair joint in combination with spot welds if adhesives were used in combination with spot welds in the original joint. This is done often. It’s pointed out in the publication that a significant amount of structural adhesive is used by DaimlerChrysler to improve joint strength. It’s recognized that it may be difficult to determine if material between panels is a sealer or adhesive, so if in doubt, use an adhesive. A two-part, corrosion-inhibiting, structural adhesive is recommended. The structural adhesive must meet or exceed DaimlerChrysler MS CD507. If the original joint obviously did not use adhesive, do NOT use adhesive in the repair joint. In other words, do not try to strengthen the joint. This could have a negative effect on the vehicle as a whole, such as altering the collapse sequence of frame rails. Also, it’s stated in the publication that DaimlerChrysler "DOES NOT endorse or condone the use of structural adhesives alone in the replacement of body panels." STRSW Process According to DaimlerChrysler, the number one requirement of any welding repair is to restore the vehicle to its OEM condition. One of the requirements in regards to STRSW is duplicating the same number of welds within 6 mm (1/4") of the original locations. Locating repair spot welds in the same weld locations as the original weld locations may lead to metal fatigue or poor weld quality.
Before welding, surfaces must be clean and free of corrosion or coatings that could adversely affect weld quality. This includes removal of any E-coat within 25 mm (1") of any welds. A plastic-coated abrasive wheel can be used to remove the E-coat without damaging the galvanized coating. On joints where structural adhesive will be used, all E-coat must be removed where the adhesive will be applied. The use of weld-through primer is NOT recommended. Proper corrosion protection must be restored when repairs are complete. Proper tip dressing must be done to control weld quality and consistency. STRSW Equipment Requirements Each brand and model of STRSW equipment has a limit for the capacity of material that can be welded. The power supply available to the facility will also have a significant impact on equipment performance. DaimlerChrysler recommends that STRSW equipment be 220 volt, preferably three-phase. Before any equipment is purchased, it’s recommended that the repair facility check the power supply that is available. This includes contacting the utility provider and an electrician to see if three-phase power can be provided. The publication states that "if there is no three-phase power supply, the traditional 220V single phase must be utilized, and the reductions in equipment capability, efficiency, and speed accepted."
Test Welds Learning how to create weld coupons, making test welds, and then performing a destructive peel test using these coupons before welding on the vehicle, is the key to successfully using STRSW. The test must produce the minimum nugget diameter for the metal thickness being used, or STRSW cannot be used for that joint. DaimlerChrysler requires following a specific procedure for making and destructively testing the samples. The test joint must be an exact duplicate of the original joint, regarding number of layers, material thicknesses, and application of sealers or adhesives. All of these factors can have an impact on nugget size. Damaged sheet metal taken from the vehicle being repaired in the same area where the weld repair is to take place can be used. The sheet metal must be flat, and free of cracks, wrinkles, or scored metal.
Cut the coupons to about 25 x 100 mm (1 X 4"). Then prepare the coupons the same as the repair area on the vehicle will be prepared, by cleaning and removing coatings. Apply adhesive if adhesive will be used on the vehicle joint. Equip the welder with the arm set that will be used on the vehicle, set the welder following the equipment maker’s recommendations, then make one weld at one end, at least 12.5 mm (1/2") from the edge. Try to center the weld on the 25 mm (1") width. Then make a second weld on the sample toward the other end. The distance between the two welds (pitch) should try to equal the spacing of the original welds on that area of the vehicle, slightly less if anything, but not a greater distance. Then peel test the second weld (see Figure 7).
The reason two welds are made, and only the second weld destructively tested, is to allow for the current shunting effect that occurs between adjacent spot welds. With the first weld, the current can only flow through the panels, and the adhesive if applicable. With the second weld, some of the current flows through the weld that was just made, so the current level likely needs to be adjusted upward. Since there’s usually an adjacent weld for most of the welds that will be made on the vehicle, it is this setting that the welder should be set to when welding on the vehicle.
Training The DaimlerChrysler publication makes it clear that training is required before STRSW is used, preferably before equipment is purchased. The technician must be well versed in how STRSW equipment operates, how adjustments are made, and the effects those adjustments have on the weld. Equipment maintenance must also be understood. Training must also include weld testing, using test coupons and how to perform the destructive peel test. It must be understood that good physical appearance alone does not necessarily mean a quality weld. The training should include STRSW welding in general, and when the equipment is purchased, equipment-specific training. I-CAR offers a general course on STRSW equipment. "Welding And Cutting Steel-Program 4" is a four-hour program specific to the STRSW process. The program includes a module on the process of weld bonding. Watch for it in your area. |
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