Aluminum repair

Al Thomas ABRN auto body repair collision repair aluminum repair It seems at least one thing has never changed throughout the years for collision repair technicians: They must continue to train and learn new repair methods, or at least continually upgrade or modify the techniques they've already perfected. New products and materials are being added, vehicles are being constructed from new materials, and they're being equipped with more sophisticated operating and safety systems. Heck, technicians once washed vehicles with water, and now they paint them with it.

Aluminum, though not new – it was used in early auto production and in the aviation industry for years – is being adapted more to current production vehicles, and not just the high-end ones.

Aluminum is found in body parts, frames and unibody construction, and it's being adapted to all areas of construction. Currently, technicians must know its specific repair methods and understand its specific repair requirements to be able to return a vehicle to its pre-accident condition. Aluminum parts are found in structural and nonstructural areas.

To repair aluminum body panels, technicians should know the essentials: the characteristics of aluminum, the precautions that must be observed when repairing or replacing it, and the different repair methods used during its repair.

Characteristics

Although aluminum is softer and will scratch more easily than steel, it can be harder than steel with the addition of one or more elements to make an alloy. Other beneficial characteristics are it:

is a better thermal conductor than steel;
will conduct electricity better;
can't be magnetized;
has a natural corrosion resistance;
melts at a lower temperature than steel;
will work-harden faster (producing cracking faster); and
is one-third the weight of steel.

Alloy series are numbered from 1000 (99 percent pure aluminum) through 7000, which is the highest strength. They're divided further into nonheat-treatable or heat-treatable categories.

The nonheat-treatable choices (series 1000, 3000, 4000 and 5000), which are strengthened only by work-hardening, don't crack easily when welded and are usually used for structural body panels.

Heat-treatable items (series 2000, 6000 and 7000) are strengthened by work-hardening and can be strengthened by heat but have various heat limits. These often are used for exterior body panels.

An aluminum body panel work-hardens faster than steel, which makes the panel more likely to develop fine cracks during the straightening process. These cracks can be so small they're not seen easily through normal inspection; therefore, a dye penetrant and developer should be used to reveal possible cracks.

Aluminum has no telltale color change before melting. When heat is used during the repair process, the surface temperature must be monitored closely. The use of heat crayons or heat paint can be used along with a noncontact thermometer, which is adjustable for use with aluminum surfaces. Heat is often used during the repair process at low temperatures (400 degrees F to 570 degrees F) to reduce work-hardening and subsequent cracking.

Aluminum forms an oxide covering almost immediately after being sanded or ground. The oxide is highly corrosion resistant unless contaminated by a dissimilar metal such as steel. Even steel dust can cause galvanic corrosion. This condition can be caused easily by aluminum contamination unless the repair area of aluminum is kept separate from the steel area. Many vehicle manufacturers recommend (and some demand) that aluminum repairs be done in an area isolated from the steel area (Fig. 1).

Alloy panel repair precautions

One of the most significant cautions to consider when repairing aluminum is dissimilar metal corrosion or galvanic corrosion. For galvanic corrosion to occur, three conditions must be present: 1. You need electrochemically dissimilar metals – in this case, steel and aluminum. 2. These metals must be in contact. Nothing can separate them. 3. They must be exposed to an electrolyte, such as water.

When these conditions occur, the more active of the two metals, aluminum, which is the anode, will corrode faster than the less active one, steel. The steel's (cathode) corrosive process will be retarded to the point of nearly stopping. This condition can be caused by:

leaving steel drill shavings behind after drilling,
using a steel pry bar to raise low spots in aluminum,
using bumping tools with a rough or damaged face,
leaving behind small steel particles in the repair site, or
using a steel wire brush to remove oxidation before welding aluminum.

To guard against galvanic corrosion when repairing aluminum body panels, technicians should dedicate a set of aluminum tools such as hammers, dollies, spoons, etc., to that type of job. Tools for aluminum repair should be made of aluminum, titanic plastic or stainless steel. Steel tools, which have been prepared and maintained correctly, can be used as long as they are not used to repair both substrates.

Hammers that will be used for aluminum repair must be dressed first to remove any nicks or mars in the face, particularly around the edges (Fig. 2). This procedure is followed by polishing the face through sanding to ensure a completely nick-free surface (Fig. 5). A polished tool reduces the possibility of galvanic corrosion and lessens the likelihood of producing scratches and dents in the surface of the softer aluminum panels. Even new tools can benefit from being filed and polished with grit paper as fine as P1000 to avoid nicks.

Tools made from aluminum, stainless steel and titanic don't cause galvanic corrosion but may have disadvantages. For example, aluminum dollies may not have the weight needed to do the job, and titanic tools may be prohibitively expensive, so stainless steel is used most often.

Aluminum panel repair is best performed when the surface is warmed to a threshold temperature of 400 degrees F to 570 degrees F. With only a 170-degree window, the temperature must be monitored carefully. This can be done in several ways. One method is with temperature monitoring paint or crayons, which some technicians find cumbersome and time consuming to perform. An alternative is to use a noncontact thermometer. Not all thermometers are capable of monitoring temperature on aluminum because of its reflective properties. Care must be taken to choose one that can be calibrated between the ungrounded painted surface and the paint-free aluminum surface.

Another precaution is that aluminum has a stronger memory and exceeds its point of elasticity sooner than steel. When this occurs, the resulting work-hardening causes the aluminum to become brittle, and cracking often occurs. Heating aluminum to this workable window of 400 to 570 degrees F allows the work-hardened aluminum to be less brittle, and the cracking is reduced. Also, workability increases.

To ensure the finished work (including welds) does not have cracking, a dye penetrant and developer should be used. Dye penetrant (often red) is applied to the repaired surface and wiped off. Follow this by applying the developer, which causes the otherwise undetectable imperfections to become visible. If any imperfection is noted, it must be repaired before proceeding further.

Other precautions are that bolted-on parts must be fastened with nonsteel fasteners or those that have a barrier between them and the aluminum. This barrier may be undisturbed coating (often green) to isolate the steel bolt from the aluminum. Fasteners with damaged coatings should not be reused. Plastic washers or sealants also are used to separate steel and aluminum. Fasteners may loosen over time, and many vehicle manufacturers have a recommended torx for all fasteners, including fender bolts.

Aluminum panel repair

Though the same techniques used for steel repair also are used with aluminum repairs, such as hammer-on-dolly (Fig. 4) and hammer-off-dolly (Fig. 3), caution must be taken because aluminum work hardens more quickly than steel, after which the repair becomes more difficult to move. To avoid this, aluminum must be heated to its repair threshold. Aluminum spreads heat faster than steel (four to six times) and doesn't change color as it's heated; therefore, heat-monitoring devices must be used. Aluminum melts at a lower temperature than steel, 1,220 degrees F, which is less than half the temperature to melt steel.

Minor damage often can be repaired with the addition of heat between 400 degrees F and 570 degrees F, though the vehicle maker's recommendations should be consulted and followed. Additional repairs can be made with normal techniques through often planishing (smoothing) with wooden hammers or leather covered slapping spoons.

Pulling methods are similar, though glue-on pulling tools are used often with aluminum repairs, as are aluminum weld-on devices. Monitored low heat is used to help the pulling process because less mechanical force is needed for the same results.

Heat application can be made from a heat gun, torch or carbide tool. However, when heating (other than heat-shrinking), the lower-temperature methods, such as heat gun and propane torch, work better than a carbide tip. Heat-shrinking also can be used for aluminum repair, although more care must be followed to stay within the heat tolerances. When heat-shrinking aluminum, quenching (quick cooling) with water and hammering can be used. A shrinking hammer, one with the check design cut in the head, shouldn't be used (Fig. 6).

Body fillers can be applied directly to the aluminum or over the epoxy primer, depending on the vehicle maker's recommendations. Precautions are needed because the aluminum oxide forms almost immediately after paint removal has been completed. The general recommendation is that bare aluminum must be covered as soon as possible (20 to 30 minutes) after it's exposed to air, and it never should be sanded with coarser than 80-grit abrasive. The plastic is worked in the usual fashion, primed and then prepared for paint.

Although aluminum panels may be new to some technicians, many repair techniques are the same as steel, yet there are slight variations. New or veteran technicians can learn minor variations to become accomplished aluminum panel repair technicians quickly.