A LIFETIME BOND

Joining metals with weld bonding is a well-used method of returning cars to pre-accident condition

Today, more than ever before, technicians in the collision repair industry have the ability to return a car to its pre-accident condition. This advancement is largely due to the acceptance and growth of the use of weld bonding — a method of joining metals together that involves squeeze-type resistance spot welding (STRSW) used in conjunction/combination with a structural adhesive. It can improve strength and stiffness; provide additional fixturing capabilities; allow for fewer welds; improve fatigue and impact resistance as well as cycle time; and even increase corrosion protection and NVH (noise, vibration and harshness) control properties. With weld-bonding, vehicles can be repaired to an undetectable factory appearance, while maintaining the original structural integrity.

"The increased usage of weld bonding has armed body shops with a repair method that restores damaged vehicles to pre-accident condition better than ever before," says Ed Staquet, LORD Corporation's senior technical service representative, Automotive Aftermarket. "We know the quality of repairs is improving to higher levels as these repaired vehicles are more difficult to detect, which is a good thing."

The popularity of weld bonding as a viable and efficient method for joining secondary, exterior auto body panels is rising in response to the increase in complexity of automotive metals. OEMs are progressively utilizing different materials in the construction of automobiles and trucks to promote safety, eliminate excessive vehicle weight and allow for better styling. Some metals, such as aluminum, laminated steel and advanced high-strength steels, may be difficult, if not impossible, to weld using conventional techniques. Therefore, the use of adhesives has become a recognized industry standard for joining similar and dissimilar metals, whether used alone, with rivets or in conjunction with compression welds.

For more than 25 years, the North American automotive collision repair market has used MIG welding (gas metal arc welding) as an accepted industry procedure. As compared with MIG welding, metal adhesives alone offer collision repair technicians a multitude of advantages such as corrosion protection, NVH reduction, elimination of welding damage, moisture barrier and workshop efficiencies.

In general, some metal bonding adhesives contain phosphating agents that stop the rusting process. Further, when compared with welding that concentrates stress at specific points, certain metal bonding adhesives uniformly distribute stress — making the repair area both stronger and quieter. A substantial benefit of bonding is the reduction of MIG welding damage due to panel warpage, glass burn and interior burns. Further, metal bonding products can form a moisture-barrier, which helps eliminate those pesky water leaks.

The European market

In contrast, Europe has utilized squeeze type resistance spot welding (STRSW). However, STRSW has not been seen a great deal in the United States because of the high level of voltage it requires. In North America, there is typically a 200 volt/single phase service available to the body shop, whereas in Europe, the voltage received is 400 volts and higher. The greater voltage in Europe allows body shops the ability to use high-powered compression resistance welding equipment.

Necessity being the mother of invention, welder manufacturers have made tremendous progress by developing cutting-edge equipment that works well within the North American electrical system. These systems are gradually becoming more and more popular. For example, in 1999,Chrysler recognized welding and weld bonding for exterior non-structural applications and as recent as 2006, Chrysler updated its weld bonding procedure to include cured adhesive applications.

Ford also has added detailed weld bonding procedures to its factory workshop manuals in section 501-35. Since some late model Ford vehicles contain as much as 160 linear feet of adhesive products, it was paramount that structural adhesives, foams and sealers be replaced when damaged. In response, Ford conducted a weld bonding study to better understand repairability issues. After extensive work, it was determined that certain room-temperature-cure adhesives can help duplicate factory appearance and performance when used with weld bonding. Further, the study validated that STRSW weld bonding procedures can help body shops repair damaged vehicles undetectably.

According to Gerry Bonanni, paint and body technical engineer at Ford Motor Company, weld bonding offers the body shop the ability to duplicate proven factory processes in the auto body repair field, and the value of improved repair quality by utilizing weld bonding is enormous.

"With the advent of near-future vehicles utilizing ultra high-strength steels, aluminum and alternative structures, the progressive body shop will do well to explore the opportunities of weld bonding and squeeze type resistance spot welding equipment," Bonanni says.

Weld bonding

Any automotive part (structural and/or non-structural) originally welded or weld bonded by the manufacturer with adhesives and/or sealers can be repaired using weld bonding. STRSW welders are often used with advanced steels as there is minimal heat affect damage compared to MIG welding. Since higher strength steels are typically heat sensitive, resistance spot welding affords technicians the ability to match the OEM weld appearance without sacrificing integrity to the high-strength metals being welded.

However, there are different types of STRSW equipment that can be used for weld bonding. At this time, the most common compression welder options are transformer and inverter. Transformer welders change AC power to DC current to obtain higher amperage, whereas inverter welders change the wave pattern from a sine wave to a square wave. It then rectifies from AC to DC to improve the weld duty cycle. Typically inverters are a better choice as they allow technicians the ability to perform nearly undetectable repairs. They also allow for a high-powered weld in a short amount of time, which minimizes the heat-affect zone on the adhesive and metal.

Although MIG weld bonding can be utilized in collision repair, the procedure may seem cumbersome. In a MIG weld bonding scenario, the technician applies adhesive to the bond locations of the part but skips areas in which a MIG weld would be placed. MIG weld bonding affords some of the same benefits of STRSW weld bonding such as corrosion protection in the bond areas and NVH advantages. However, issues such as the increased heat affect and longer repair times make STRSW weld bonding a better choice.

When choosing between MIG and STRSW for weld bonding though, a body shop owner must consider the joint design of the part and the type of equipment on-hand. STRSW has been proven to work well with weld bonding, but not all metal joints allow for STRSW gun access. In addition, the number of shops equipped with the latest in STRSW equipment is still fairly low as the welders are typically pricey and shop electrical upgrades are routinely required.

Implementation

Before implementing weld bonding into everyday practice, a shop must be aware of a few factors. First, the shop itself must be electrically equipped to handle resistance welders. Inverter welders require more shop power. There must be a dedicated circuit with a minimum of 60 amps, three-phase and 208/220-volt service to handle most available resistance welding equipment. The welder should be UL/CSA approved and conform to the electrical code in your local area. After obtaining a welder, keep in mind that voltage drop can cause welder performance problems.

Weld bonding has taken the automotive aftermarket to the next level. As more and more vehicles are built in the factory with weld bonded joint designs, it is incumbent on the collision industry to embrace weld bonding to fix vehicles in the manner in which they were manufactured. Technicians have pride in what they do and they now have the ability to rebuild cars using inverter STRSW welders and OEM approved structural adhesives.