Aluminum frame repair: IT'S ELEMENTAL

Aluminum has been part of auto manufacturing for years. Repairing vehicles made with this element requires an understanding of how aluminum parts react to heat, pulling and more.

Aluminum is not new to automobile manufacturing; it has been used for years, primarily to lighten vehicles and thus make them more fuel-efficient. Today vehicles that use aluminum are grouped into two categories.

The first group includes vehicles built with varying aluminum parts. Examples of aluminum uses in this group are the outer body panels found on vehicles including the Land Rover, Range Rover and Mercedes-Benz SL500; aluminum hoods on Ford F150s; and GM vehicles such as the Escalade, Yukon and Tahoe, which use aluminum rear closure panels.

The second group includes aluminum-intensive vehicles that use larger amounts of aluminum in parts such as strut towers, pillars, drive train mounts, engine cradles, rails and rocker panels. Vehicles including the Audi A8, Acura NSX, Honda Insight, 03 Jaguar XJ and the Mercedes-Benz CL Class use aluminum as a structural component. They use aluminum exterior panels attached with mechanical fasteners, bonding, weld bonding, bonding with mechanical fasteners, clinching and squeeze-type resistance welding. Aluminum-intensive vehicles use sheet (the most widely used), extruded and cast aluminum.

The use of aluminum parts on highly produced vehicles such as the F150 and some General Motors vehicles has been restricted to outer parts that can be removed and replaced. When damaged, these parts have largely been replaced instead of repaired, with little or no difficulty. However, repair attempts using traditional steel methods and tools have met with unforeseen failure.

As the use of aluminum in the manufacturing of vehicles has increased and we now find aluminum used as structural parts, technicians must learn new repair processes for them as well. In the past we learned that the different types of steel were sensitive to heat. Different types of aluminum also are sensitive to heat. The way that aluminum responds to the pulling forces is also somewhat different than steel, and different tests and precautions must be used when repairing it.

Aluminum in its pure form is not strong enough for use in structural parts. Other elements such as copper, magnesium and zinc are mixed with aluminum to alloy it, thus making it stronger. Alloyed aluminum is grouped into two categories: heat-treatable and non-heat-treatable. The heat-treatable group includes: 2000 series (alloyed with copper) used often for body panels; 6000 series (alloyed with magnesium), also used for body panels; and 7000 series (alloyed with both zinc and magnesium, making it very strong), used for applications such as bumper reinforcements.

Non-heat-treatable aluminum includes: 1000 series (nearly 99 percent pure), which is very soft and used for electrical wiring; 3000 series (alloyed with magnesium), used for interior structures; 4000 series (alloyed with silicon), often used for electrode welding wire; and 5000 series (alloyed with magnesium), often used for inner structural parts.

Aluminums soften when heated, thus allowing technicians to repair collision-deformed aluminum more easily. Both heat-treatable and non-heat-treatable aluminum alloys may be heated to repair, but as with steel, these alloys may also be damaged with heat if the recommended temperature is exceeded. Unlike steel, aluminum does not change color as it heats, thus making it nearly impossible to judge temperature by eye as the material is heated. Aluminum also has a critical temperature at which it "anneals" or permanently softens. This temperature is below its melting temperature, but above its repair threshold.

Technicians must be very diligent when using heat to repair aluminum to stay within the repair temperature limits. Heating time though, unlike some steels, is not a factor when repairing aluminum. As long as the temperatures are maintained within the repair threshold and the aluminum is allowed to cool naturally, no damage will result from heating. Water or compressed air quenching of hot aluminum will crystallize the alloy, thus changing its mechanical properties.

Heating aluminum can be monitored using heat detection crayons, heat detection paint, heat monitoring strips, non-contact thermometers or thermocouples found as probes on a digital volt meter. Because of the high temperatures needed to reach the relatively low threshold (400-570 degrees F), careful monitoring should be observed.

Aluminum involved in a collision may crack from the stress of impact. When repaired, it may also crack from the stress of the repair. Cracks in aluminum structural parts should not be repaired. The part should be replaced. Cracks may be very small and difficult to detect, so repair technicians will need to use a dye penetrate to detect small cracks.

Dye penetrate use is a three-step process. First, the technician cleans the area. Some dye penetrates come with a specific cleaning agent; if none is available, wax and grease remover should be used. The area must be thoroughly cleaned to remove any contaminants that may block the dye from entering the crack. After the area is clean, the dye is applied. This dye (often red) flows over the area that is being inspected and thus seeps into any small cracks that may exist. The dye is then wiped off. Next, the developer is applied; any cracks will be visible through the developer. Developer is a weld contaminator and should be completely cleaned off after use.

Though cracked structural parts should not be repaired, some damaged areas may be repairable. Areas that have received indirect damage, bent areas in non-collapse zones, damage without visual deformation and bends that are removed with heat (within the heat repair temperature window) may be repaired.

Welding of aluminum during a repair process is significantly different than welding steel. Technicians who weld automotive structural aluminum should always be trained, tested and certified as aluminum welders. All manufacturer repair recommendations should be found and followed to assure that the repairs are properly completed.

As one can see, structural aluminum and structural steel share some of the same repair techniques, but the processes are vastly different. Technicians should be trained specifically for aluminum repair and certified as aluminum welders. Technicians must be able to test and inspect the repairs that they complete to assure their repair quality, and also in order to be aware of and be able to eliminate galvanic corrosion.

The use of aluminum in the manufacturing of vehicles will in all likelihood increase in the future, and technicians must continually train to stay current with the required repair techniques and processes.