Which Weld is Right?

STRSW vs. MIG: More than just alphabet soup. Make sure your welding jobs are up to Chrysler Standards.

Since the introduction of special high-strength steels and coated steels in the early 1980s, body shop technicians have been required to use Gas Metal Arc Welding (GMAW) to install and repair body panels on unibody vehicles (this type of welding is also known as Metal Inert Gas, or MIG, welding). While this type of welding will create a strong joint with a limited heat-affected zone, there is a concern that corrosion can occur when MIG welding is done, among other concerns. Squeeze Type Resistance Spot Welding (STRSW) is now an option for field repairs, particularly when the repair work is done in a cosmetic location.

We’re going to examine these two types of welding, looking at the pros and cons for each method. Both types of welding methods have specific applications, so we’ll consider where each is appropriate. But first, let’s review the requirements for a welding repair.

Requirements of a Welding Repair

The major construction of a unibody vehicle consists of welded panels that create the supporting structure for all components and assemblies of the vehicle. When a vehicle is damaged in a collision, the number one requirement of any welding or weld bonding repair is to restore the unibody to its original OEM condition. The materials and technology used should duplicate the original OEM conditions as much as possible. With regard to welds, the technician must ensure that the welds are replaced in the same size, quantity and location.

A significant amount of structural adhesive is used when the unibody is assembled at the factory. The primary goal is to improve joint strength. It might be difficult to determine if the material between the panels is an adhesive or a sealer. If there is any doubt as to which material it is, use a two-component, corrosion inhibiting, structural adhesive. This adhesive must meet Chrysler standard MS CD 507.

It should be noted that MIG welding is not recommended within 1 inch of an adhesive as the welding heat will destroy the adhesive. STRSW, on the other hand, can weld through this adhesive without destroying its properties.

Chrysler recommends that the same quantity of welds as the original panel be used during repair, but the placement of the new welds should be within 1/4" of the original weld locations. Placing new welds over an original weld location can lead to metal fatigue or poor weld quality.

Listed below are a few key points to remember:

1. Poor part fit-up will adversely affect weld quality, regardless of the method.
2. Clamps should be used to bring parts together and hold the parts in position.
3. When using STRSW, clamps should be insulated.
4. Number, size and location of welds should closely duplicate the original assembly.
5. Surface of steel parts should be clean and free of scale, rust, paint, or cured adhesives/sealers and any other contaminant that can affect weld quality.
6. Weld-thru primers are not recommended when STRSW and MIG welding is used.

STRSW and MIG Welding

STRSW (Squeeze Type Resistance Spot Welding) relies on the resistance of the material being welded to create heat as the current passes through it.

Welding with this method is pretty straightforward. The materials being welded are squeezed together. Current is passed through these materials and the resistance causes of build-up of heat. Combined with the force of the tips, this heat allows the materials to fuse together. To complete the weld, the current is shut off, while the force of the tips holds the pieces together for a short period of time known as the cool-down cycle. When this cycle is complete, the pressure is released and the weld is finished. Remember, proper tip dressing is necessary to control weld quality and consistency.

In order to successfully weld using this method, the technician must learn how to create weld coupons shown in Figure 1. Then destructive tests must be performed on the coupons. This is key to successfully using STRSW. Chrysler requires a physical test of the weld coupons in order to test welds prior to making repairs.

Gas Metal Arc Welding (GMAW) is an arc welding process in which an electrode is fed through a weld gun and is surrounded by a shielding gas. The term MIG comes from early uses of this method on aluminum in which argon was used as the shielding gas. Since argon is an inert gas, the process became known as Metal Inert Gas, or MIG, welding. This method of welding is the most commonly used for unibody repair.

When STRSW is used in conjunction with a structural adhesive, the method is known as weld bonding. This procedure provides the customer with a superior repair compared to the traditional plug/puddle MIG welding process. Remember, though, structural adhesives should not be used in a joint where the original weld did not use an adhesive.

Applications of STRSW and MIG Welding

Due to advances in equipment technology, STRSW is no longer restricted to light gauge sheet metal. Heavier gauges of High Strength Low Alloy (HSLA) and coated steel currently used in underbody structures can now be welded in the field. Don’t forget that testing must be done on each combination to ensure weld quality and strength. The test joint must be an exact duplicate of the original joint, including layering and sealer or adhesive application.

The equipment used for STRSW must be able to produce two-sided welds. Also, the equipment must have the ability to create welds that comply with Chrysler minimum weld nugget diameters listed in the Minimum Weld Nugget Requirements chart. This chart can be found in the Chrysler service publication, Welding and Weld Bonding, number 81-316-0740 (www.techauthority.com).

Finally, the technician must have the appropriate sheet metal measuring equipment. This will ensure that the welds made meet the minimum weld nugget size for the panels being welded.

When it is not practical to use STRSW for a sheet metal repair, MIG welding can be used. Also, all truck frame repairs should be done with MIG welding. The most common use of MIG welding on uncoated or galvanneal coated sheet will utilize a 75 percent argon/25 percent CO2 shielding gas and an AWS specification ER 07ER70S-6 wire.

When welding galvanized material, flux core arc welding (FCAW) using AWS specification E71T-GS wire should be used to avoid weld porosity from the zinc.

While some 110V MIG welding equipment might be adequate for welding thin gauge sheet metal, it is recommended that a 220V MIG welder with a minimum output capacity of 150 amps be used (to avoid equipment limitations, 250 amp output capacity is suggested).

As with STRSW, weld coupons identical to the repair situation need to be made. These coupons should then be destructively tested to ensure that quality welds are being made (see Figure 1).

There are some limitations with MIG welding. First of all, you cannot MIG weld through paints, sealers, or adhesives. Also, the zinc used in coated steels can reduce weld strength due to porosity (see the note above on FCAW). Finally, due to the heat-affected zone, structural adhesives cannot be applied within 1 inch of MIG welds.