Important Repair Concerns when Replacing Structural Components and Subassemblies

Introduction

This bulletin will identify repair concerns and recommend precautions for replacing structural subassemblies on unibody vehicles. The concerns stem from Tech-Cor's collision repair research and case studies on actual vehicle repair situations.

With the advent and almost universal adoption of unitized vehicle construction by automobile manufacturers, it has become practical to replace entire corners, or complete front sections as subassemblies, rather than individual pieces to repair collision damage. This is partially due to the compact size and shape of these body components and how the vehicle structure is designed to absorb energy as the impact force is transmitted through the structural components, but also due to the complexity of the multi-piece welded structure. If damage is extensive, it may be cost effective to replace a left or right front corner apron, rail, and strut tower as a single subassembly, since damage to one of these components frequently means others are also damaged. Automobile manufacturers have reacted favorably to this situation by providing service subassemblies, to avoid having the repair technician assemble many small pieces to form a larger assembly.

Problems arise when these subassemblies are often removed from an assembly line process and shipped as service parts, before all necessary manufacturing operations have been performed. Welds, brackets and holes may be missing, since these operations or additional pieces would have been performed or added later in the production process.

Prior to installation, care must be taken to closely inspect all service parts and subassemblies. Compare the number and location of welds, brackets, etc. on the service part to the vehicle. Any discrepancy must be noted, then corrected during the component installation process. This bulletin will provide important reminders for the inspection and repair process, to help technicians achieve quality repairs.

Single Component or Subassembly Replacement

Before attempting to replace any welded on component, it is essential to have a complete understanding of the body structure in the area being serviced. Carefully inspect the service part, comparing it to the damaged component on the vehicle, noting all factory weld locations. The use of a propane torch, scraper and wire brush may be necessary to remove undercoating, seam sealer and paint in order to identify all weld locations. Always use appropriate safety equipment due to risk of fire and toxic fumes. Next, mark all welds requiring removal to replace the damaged component.

Thoroughly inspect all areas of the service part, comparing it to the damaged part on the vehicle for: the number and location of welds, weld spacing, multiple layered seams, brackets, mounting holes and overall part configuration. Be certain that each weld passes through all metal layers at each weld site.

The previous inspection step is very important, since welds are often omitted on the service part. Service subassemblies may be shipped with component pieces only tack welded in place for shipping and component location purposes, and may not be properly secured as in the production vehicle.

Areas to look for missing welds on replacement subassemblies are where the part attaches to a multi-panel seam, overlapping mating panels, or at reinforcement attaching locations. Welds may be missing on the service part since they would be welded at a later stage in the vehicle production. Factory resistance welding generally penetrates through all layers of a multi-panel seam.

Upon completion of a thorough inspection, mark all welds requiring removal and determine the best way to access each weld during its removal and subsequent replacement procedures. When determining which side of the weld nugget to drill out, drill from the side having the thinner metal layer if accessible, when replacing welded panels.

Several methods and tools can be used to remove welds. Depending on the accessibility, the method which best suites the particular situation and technician's preference should be utilized to remove each particular weld.

When removing welds, extreme care must be taken to remove only the weld nugget or damaged layer of metal of the part being replaced. Do NOT grind into or remove any of the base metal or pre-existing galvanized coatings from the part remaining in the vehicle. Removal of base metal will significantly weaken the weld joint. Removal of galvanized coatings will reduce the long term effectiveness of the vehicle's corrosion protection and the ability to properly manage energy in a subsequent collision.

Exercise caution when removing a deformed panel. Do NOT cut through internal or external reinforcements (which will remain with the vehicle), wiring harnesses and other undamaged panels.

Mating flanges should be cleaned and free from any: distortion, weld nugget debris, undercoating, seam sealers, and corrosion. A propane torch may be used with caution to aid in softening undercoating and seam sealer during removal. To avoid risk of fire and over heating the metal flange, Do NOT use an oxyfuel (oxyacetylene) gas torch.

During weld flange preparation, weld any tears in the metal being careful not to remove any pre-existing galvanized coatings or grind into the base metal when dressing down the weld. Prior to final assembly, the application of a zinc rich weld-through coating to the mating side of the prepared flange will help protect any areas where of the pre-existing galvanized coatings may have been removed.

Solvent clean new panels before the initial test fit installation, to remove any labels, dirt and oil that may contaminate the metal and resulting weld or adversely affect primer and paint adhesion.

When attaching new panels, remove the factory applied primer only from areas to be welded on the matting flanges, as it will contaminate the weld. Keep the factory primer intact between the welds. On occasion, it maybe necessary to remove the primer from the entire mating flange. In such cases, a zinc rich weld-through coating must be applied to the entire mating flange surface, if the metal is not already galvanized.

Whenever possible, weld the thinner panels to heavier gauge panels. Welding in this fashion reduces potential burn through and aids in obtaining a quality weld.

Always test fit replacement parts prior to final joint and welding preparation. This insures proper vehicle measurements, flange alignment and aids in marking weld locations. Then complete the joint preparation steps for final part fit-up and welding. Use clamps or sheet metal screws to hold parts during the test fit-up, and to ensure joints are clamped tightly prior to welding. If screws are used during fit-up, the holes must be welded closed.

Where any of the pre-existing galvanized coating has been removed or damaged, always use a zinc rich weld-through coating on both sides of the mating flanges prior to final assembly of replacement body components.

Weld-through coatings have extremely poor adhesion qualities and should never be used outside of mating flanges (except on surfaces such as internal reinforcements which fit tight against the side member). Epoxy or self - etching primer should be sprayed both internally and externally as the first step to protect the repaired area from corrosion after welding is completed.

Some service parts may have missing welds that are not accessible after the component is or attached to the vehicle. To ensure proper attachment of the subassembly, additional welds that are hidden from view in the final assembly must be applied to the service part prior to fit-up. An alternative method is to use a "step" plug weld to insure proper weld penetration through all layers of metal. A "step" plug weld is formed by drilling progressively larger holes in each metal layer, away from the base metal when preparing the plug weld site. Either method can be used to obtain adequate weld penetration through all layers of metal, when attaching the replacement components.

It is very important to obtain complete weld penetration through all metal layers on a multi-panel seam, since each component contributes to the overall strength and rigidity of the vehicle. (Refer to the section titled "Welding and Related Operations" for more detailed information on weld joint preparation and welding.) Then perform the necessary welding operations to secure the replacement part or subassembly.

Dress cosmetic and pitted welds enough to remove any pits since they are a prime source of future corrosion. Do not grind into the base metal when dressing welds, since it will reduce the weld's strength.

After all welding is completed, wash off any zinc rich weld-through coating that may have seeped out from between the welded seam prior to any painting operations. Any exposed zinc rich weld-through coating at a body seam will cause paint adhesion problems.

To ensure proper paint adhesion, clean all weld sites on both sides when possible with an abrasive blaster, a power wire brush or a 3M "Clean 'n Strip Disk" (or equivalent) to remove the weld slag and residue. Then wipe down the welded areas with wax and grease removing solvent.

Before proceeding direct a jet of compressed air into and around all welded areas. After all seams are thoroughly cleaned, prepare the surface for primer as directed by the paint manufacturer's instructions.

Painting a service part prior to welding it in place can be an effective method of completely coating all surfaces, insuring maximum corrosion protection. Pre-painting service parts may be the most effective way to properly paint hard to reach areas. This method may also save pre-paint preparation time and result in a more uniform application of the topcoat, due to no obstructions. Weld sites should be protected from primer and paint application. Freshly painted panels must also be protected during the welding process. After installation is complete, any damaged paint along the edges, from welding or installation handling, should be properly cleaned and repainted. Many times, a brush can be used to effectively accomplish the paint and caulking operations when spray application is difficult.

Automobile manufacturers protect the body shell against corrosion by using several chemical processes before any paint is applied. In addition, several seam sealers and other materials are applied to help protect the vehicle against corrosion. The steps used during manufacture are not practical or adaptable to the repair environment. Therefore, other materials have been developed for use in the collision repair shop.

A vehicle's corrosion protection can be damaged by a collision and during the collision repair process. Restoring the same level of corrosion protection during collision repair is very important. It is the repair technician's responsibility to ensure the repaired area of the vehicle is properly protected against corrosion.

Preventing visible corrosion or surface rust is only a small part of the overall corrosion protection necessary during collision repairs. Welded unibody structural components hold the vehicle together, by providing a structural foundation for the vehicle. The welded structure manages impact energy during a collision and vehicle loads during normal driving. Any corrosion of structural components or their attaching welds could eventually lead to a loss in the vehicle's structural integrity. Special care must be taken to prevent structural corrosion. If not protected, severe corrosion in repaired seams can occur in a short time. This corrosion may seriously jeopardize the effectiveness of automatic passenger restraint devices in the event of a second collision. To prevent future corrosion problems, all corrosion protection barriers and sealers must be renewed when repairing collision damaged.

The repair technician must have a thorough understanding of anti-corrosion materials and their application methods to completely restore a vehicle's corrosion protection to its pre-accident condition. These steps are necessary to ensure the structural strength and longevity of the repair. (Refer to Tech-Cor Research Bulletin 1986-7 "Restoring Corrosion Protection During Vehicle Repair" for detailed information.)

Refer to The I-CAR Collision Repair Course on "Replacement of Structural Parts" and "Restoring Corrosion Protection" for additional information.

Welding and Related Operations

All welding in this text will refer to fusion welding, using Gas Metal Arc Welding (GMAW) equipment. A more recognized term which is used in collision repair facilities is the Metal Inert Gas (MIG) welder, also referred to as a "wire feed welder." The focus will be on the MIG welder and its usage to weld structural body components during collision repair and structural component replacement. This is appropriate, since the collision repair industry standard as stated by the automobile manufacturers, Tech-Cor and I-CAR all advise MIG welding to be the only recommended method for replacing factory welds on structural unibody components.

Never use any oxyfuel (oxyacetylene) Gas Brazing/Welding or Resistance Spot Welding equipment for making structural welds on unitized automobiles.

MIG welders can be used with different shielding gases and wire types, or flux core wire with no shielding gas. The recommended shielding gas is a mixture of 75% Argon and 25% Carbon Dioxide gas (the gas mixture may vary slightly, depending upon the supplier), since it provides the best shielding and weld penetration for collision repair welding conditions. There are many types and sizes of welding wire that can be used in various MIG welders. It is very important that welding wire meets the American Welding Society (A.W.S.) standards for welding wire type and quality. The recommended MIG welding wire for unibody repair is 0.023 inch (0.584 mm) or 0.025 inch (0.635 mm) diameter, that meets A.W.S. wire specification "AWS-ER70S-6." This welding wire can produce quality welds on high strength and mild steel body components.

There are many other factors that will affect weld quality, such as: the technician's experience & technique, metal thickness & joint configuration, joint fit, weld sight preparation, weld site location & orientation, quality & condition of the welder, available line voltage to welder, and the proper welder adjustment settings.

When using MIG plug or continuous welding techniques, the technician must alternate welding locations in order to limit heat build-up and metal distortion. Heat build-up in high strength alloy steels (HSLA & HSS) or special heat treated steels can significantly reduce their strength.

The typical plug weld hole diameter should be 5/16 inch (8 mm) minimum, with a minimum of 1 inch (25 mm) spacing between plug welds. Continuous butt or edge welding should be performed in 1/2 - 3/4 inch (13-19 mm) alternating increments. Aside from the general guidelines above, follow the same weld spacing and location as the factory installed welds in areas other than section joints.

A multi-layered flange on the service part can be "pre-welded" together at the same locations where the part will be welded to the vehicle, so the weld will effectively penetrate all metal layers. Complete weld penetration is accomplished since the additional weld will be made on top of the previous weld location, to form a single weld nugget which attaches all metal layers together.

Another option is using a "step" plug weld, by step drilling the multi-layered flange on the service part, prior to fit up. Step drilling is a procedure for using successively larger holes, for each metal layer away from the base metal when plug welding. The top piece will have the largest hole. The plug weld hole diameter next to the base metal should be 5/16 inch (8 mm) for the first piece being attached, then use progressively larger holes in each additional layer. For the second piece being attached, use a 5/16 - 3/8 inch (8-10 mm) diameter hole, dependent on the specific metal thicknesses being welded. On thin panels 5/16 inch (8 mm) will be adequate, when it is a non-structural part. For thicker panels, roughly the same thickness as the base metal, when it is a structural part use a 3/8 inch (10 mm) diameter hole. This procedure allows for adequate weld penetration into the base metal and first metal layer being attached, without just filling the hole at the start of the plug weld operation. As the weld continues, the weld pool is tied into the second (or top) metal layer being attached. The end result of either method is a weld nugget connecting all metal layers at each weld location, similar to factory applied resistance spot welds during vehicle production.

Prior to any welding operations, it is imperative that the technician properly adjust the welder by making practice welds on like material and perform peel tests. These practice welds should use the same metal type, metal thickness and joint configuration as in the repair application being welded. When peeling apart the sample weld pieces, the metal pieces should first deform, then tear apart leaving the complete weld nugget on one piece of base metal. Perform additional weld test samples to establish proper welding technique and welder adjustment settings to obtain adequate weld penetration and quality.

Refer to The I-CAR Collision Repair Course on "Welding in Collision Repair" for additional information.

The material contained in this Information Bulletin does not supersede previous repair information published by Tech-Cor Since the information presented in this text is not directly linked to any specific vehicle damage or structural design, it must be considered educational material only.

The information provided in this bulletin is for educational purposes only. Although every effort has been made to ensure the accuracy of the information contained in this bulletin, Tech-Cor assumes no responsibility or liability for any repairs performed using information from any publication issued by Tech-Cor.

Any person performing repairs must determine whether any suggested or recommended procedures or repairs are suitable or appropriate for the particular vehicle being repaired. The repairer remains solely responsible for such determination, as well as for the proper completion of the repairs.

Reproduction of this bulletin is not permitted without the written approval of Tech-Cor.