Installing Mopar Sheet Metal: Part Two
In Mopar® Magazine’s September/October Body Shop article we covered two topics. First, we explained the advantages of using Mopar collision parts to repair a collision-damaged vehicle. Secondly, we reviewed the basics of hanging Mopar sheet metal using mechanical means (nuts and bolts). In this follow-up article, we’ll finish the discussion on hanging sheet metal by reviewing the other methods—welding and weld bonding.
WELDING BASICS AND GUIDELINES
Welding sheet metal on a car body has been performed for a long time, but major changes occurred about 25-years-ago with the introduction of special high-strength and coated steels. As a result, Gas Metal Arc Welding (GMAW), also known as Metal Inert Gas (MIG) welding became the required welding technique to install body panels on unibody vehicles. Furthermore, structural adhesives and Squeeze-Type Resistance Spot Welding (STRSW) has become an option in cosmetic repair locations. This is due to significant advances by STRSW equipment manufacturers.
The number one requirement for welding or weld bonding repair is to restore the vehicle to its original condition. The materials and technology used should duplicate the OEM conditions as much as possible. In order to achieve this goal, the technician must ensure the following:
- Panel shingling is the same as the original
- Equivalent sealers or adhesives are used
- Welds are replaced in the same size, quantity and location
A significant amount of structural adhesive is used when a car is built. This is done to improve joint strength. When performing a repair, it might be difficult to determine if the material between panels is an adhesive or a sealer. For this reason, follow this general rule: If in doubt, use a two-component, corrosion inhibiting, structural adhesive. This adhesive must meet or exceed Chrysler MS CD 507.
Remember, MIG welding is not recommended within 1 inch of an adhesive as it creates heat that will destroy that adhesive. STRSW, on the other hand, can weld through the adhesive and not destroy its properties. Also, Chrysler recommends the same quantity of welds as the original panel, but placement of the new weld should be within 1/4" of the original locations. The weld should not be on the original site to prevent metal fatigue or poor weld quality.
When welding to hang sheet metal, remember the following key points:
- Poor part fit-up will adversely affect weld quality; early weld failure can occur due to excessive metal stretching around the nugget
- Clamps should be used to bring parts together and to hold parts in place
- Clamps should be insulated when using STRSW to control weld shunting; place an insulating material, such as cardboard, between the clamp jaw and panel
- Number, size and location of welds should closely duplicate the original assembly
- The surface of steel parts should be clean and free of scale, rust, paint, cured adhesives/sealers, and any other contaminants that can adversely affect the quality of the weld joint; this includes the removal of any E-coat applied to the replacement sheet metal within 1 inch of the weld (proper corrosion protection must be restored when the repair is done)
- Weld-thru primers are not recommended when STRSW and MIG welding is used
WELDING PROCESSES
The three main welding methods are STRSW (spot welding), MIG welding and weld bonding. Shielded Metal Arc Welding (SMAW), also known as stick welding, should never be used for sheet metal repairs.
STRSW, due to advances in equipment technology, is no longer restricted to light gauge sheet metal repair. Heavier gauges of high-strength and coated steel can now be welded using this process, but this technique is beyond the scope of this article. Remember that this equipment must be able to produce two-sided welds and have the ability to create welds that comply with the nugget diameters suggested by Chrysler. If you use this method, you must have the appropriate equipment to measure the nugget size to ensure it meets minimum weld nug
