Slaying the Aluminum Dragon

Drawn from the innards of the earth, raw aluminum ore was discovered in significant quantity in 1824. It then became reliably used industrially by the turn of this century. Gradually, aluminum became part of automobile production. Why? Simply put: Aluminum's lighter weight, when compared with steel, furthered its use.

But unfortunately, a large problem occurred when constructing and repairing aluminum car bodies. Because of oxygen contamination, quality welding was difficult. As a result, nuts, bolts and other fasteners held the metal together. Though heli-arc welding emerged in 1942, this new welding method didn't immediately come into widespread use.

By 1950, the aircraft industry spawned a new era of electric welding. The inert gas, or TIG, system emerged. This welding technology furthered aluminum use in automobile production, thus opening the door for the development of MIG, plasma and other new types of metal welding. Now, living in a time that realizes the need for lightweight, fuel-efficient vehicles, we are beginning to see aluminum body panels taking up more space in body shop repair bays.

I-CAR instructors agree that it's important to understand the properties of aluminum. Compared to steel, aluminum is much lighter and is a better electrical conductor. It also conducts and transfers heat very well. Aluminum cannot be magnetized because it contains no iron. But it is softer and more easily scratched and almost too easily sanded. Finally, this metal's hard, thin oxide layer gives it natural corrosion resistance.

Like a Winged Bird

Having both silicon and magnesium as its main alloying elements, 6000-series has improved in strength and formability. This series often forms outer panels on hoods, outer and inner deck lids where character lines are pronounced. With some all-aluminum vehicles, it may be used for other exterior panels. Finally, the 7000 aluminum series contains zinc. This alloy has high strength, which is put to use in bumpers, bumper reinforcements and door intrusion beams.

Now, how does an aluminum dragon handle heat? There are non-heat treatable and heatable aluminums. Non-heat treatable aluminum alloys include the 1000, 3000, 4000 and 5000 series. While these may harden by working, they cannot be heat-strengthened. If these become hardened when working on them, you can anneal them with heat. But take note that each temperature cycling weakens the metal. If heated, cool the metal gradually. These parts are sometimes used for structural body parts and retain excellent formability in weld areas.

A Fire Breather?

The real fire breathers are the heat-treatable alloys. These include the 2000, 6000 and 7000 series. They are strengthened by work hardening, however, and are annealed with heat. Some may even be strengthened by using heat. The heat-treatable alloys have varying temperature limits. But if these limits are exceeded, they often lose strength in weld-affected areas.

The heat treatables should not be preheated before welding. It is also important to note that they may increase in strength and brittleness during a paint bake cycle. The heat-treatable family may be as hard and strong as a dragon's scales-vying with some steels. They are often used in exterior body panels, bumpers, bumper reinforcements and door intrusion beams.

For basic metalworking, you will use most of the same tools you use to repair steel, but it is preferable to have a separate set, as you won't want to contaminate your aluminum tools with steel particles that may contaminate aluminum surfaces. This means cutting tools, sanding discs and pads, sandpaper and even shop towels used with steel body parts must be properly prepared and cleaned before working on aluminum panels. You must also thoroughly clean metal-forming hammers, dollies and other tools that are used for both metals.

Bouncing Off the Surface

Avoid using use serrated, shrinking hammers. Remember: Aluminum is normally softer. With all tools, watch for gouging possibilities. The faces and edges of metal hammers should be smooth and polished. Often, you may find it preferable to use wood, plastic or leather hammers. Round off and smooth file edges to prevent gouging the aluminum surface.

To remove paint, do not use coarse grinding discs of less than 80 grit. Select open-coat discs and beware of clogged discs that can overheat and smear a soft aluminum surface. When installing the disc, use foam backing pads. And definitely opt for an orbital or dual-action sander, leaving your rotary grinder in the box.

If necessary, cut aluminum using a mechanical tool. Do not use an oxyacetylene or plasma torch. Follow the equipment and vehicle makers' recommendations.

As with a steel part, make a good metal-working start by pushing and bumping with the appropriately contoured tool. Keep hammering to a minimum because hammering stretches the softer metal faster than you are used to with steel. Bump and lift to a roughly correct contour. Work until a springy resistance prevents metal movement.

Next, find any high spot and shrink the metal. Cold shrinking is nearly impossible on tempered aluminum. Heat shrink carefully, keeping in mind that aluminum won't glow. Use heat crayons judiciously to control the heat you apply. With aluminum, forget heating and water splashing. Don't use a shrink hammer. Shrinking aluminum is subtle. To shrink aluminum, choose a hammer of the right size, shape and weight, or use a spoon or slapper and dolly. Next, use mechanical advantage. Some off-dolly straightening as the simple leverage of working the low spot, used against a high one, shrinks the metal.

You can use a slapper to cover two high spots simultaneously. Cross-hatch the high spot with the low temperature indicator crayon and set up an oxyacetylene torch with a soft, reducing flame. Work quickly and limit the temperature, working the heated metal. Bump the high area down with the hammer or slapper while supporting the neighboring low area with the dolly. Work the metal while it cools. The metal shrinks until it is cool to the touch. Use this heat and bump method until you restore the proper shape. Practice and carefully observe the progress.

When the area's high spots are down and roughness remains, the next step is planishing. Using a hammer, roll the metal to shape. Use off-dolly hammering to smooth the work. You can take planished aluminum to a high finish, even to a polish. Be patient.

Combination spot weld and bonded joints have high strength, and you can replace these with plug welds and adhesive bonding during the repair. Just make sure the adhesive used between the plug welds won't contaminate the weld. Always replace adhesive bonded parts following the vehicle manufacturer's instructions. Surface prep and clean them, and then follow the adhesive manufacturer's guidance. Use fasteners or clamps to secure the joint while the adhesive cures. You may need to apply some heat, depending on the type of adhesive you use.

Double-Edge Sword

If you need to secure an aluminum part using pop rivets or blind rivets, use rivets that require hammer-style forming of the shank to lock in place. Some shops use high-speed automatic punch riveters. These can rivet at a rate similar to spot welding. To prevent corrosion, use stainless steel or aluminum alloy rivets. If steel rivets become necessary, make sure the rivets are coated to prevent galvanic corrosion.

To replace rivets, use a hand-held rivet gun. Sometimes you may use rivets with a countersunk head to fit flush with panel surfaces. But know this, this requires use rivets of the same material, strength and quality as the manufacturer.

When welding aluminum, use heat control. Aluminum spreads heat much faster than steel and does not change color when heated. In addition, aluminum has a relatively low melting point of 660??C. It has a thermal expansion about twice that of steel.

Use Your Shield

Never heat aluminum past the vehicle manufacturer's recommendation. Whenever the carmaker's specs are not available, refer to The Aluminum Association's guidelines. Always keep repair temperatures below annealing temperatures by making high temperature times short. Use an oxy-acetylene torch carefully, with a neutral flame. Use temperature indicators and, if necessary, heat sink compounds to protect the surrounding metal.

Make sure the part is not magnesium. Magnesium parts should not be welded. They will burn and cause a dangerous fire hazard. Identify aluminum using a stainless steel wire brush: Aluminum returns a shiny silver finish while magnesium turns a dull gray.

When welding, use the correct helmet eye filter plate to thwart ultraviolet (UV) rays that will damage your eyes. The Aluminum Association recommends Grade 10 for the minimum filter lens. (The higher the number, the darker the shade.) Note that the required lens shade may depend on the type of shielding gas and voltage and current (amperage) settings. Generally, use darker filter plates for welding aluminum than steel because you use higher amperage when welding aluminum. Also remember not to get flashed. Put your face shield down before striking an arc. Or, use a speed filter plate. And warn others in the vicinity before striking an arc.

Proper tension on your MIG drive rolls is as important when welding aluminum as it is with steel. Too much tension distorts soft aluminum wire, and too little tension may cause feeding problems. Also, use drive rolls designed for aluminum wires. Drive rolls with a U-groove are preferred.

To shield the aluminum, select 100 percent argon gas, and obtain a regulator matched to the shielding gas. Adjust the flow meter to 25-40cfh, with fine-tuning depending on the type of transfer method, thickness of the aluminum and the nozzle size you use. When MIG welding aluminum, The Aluminum Association recommends the spray arc or pulsed spray arc transfer methods.

Use a spool gun that will allow the use of soft, small-diameter electrode wires. As for the type of electrode wire, consult the vehicle's body repair manual because some have specific recommendations. The best welding results are obtained by using new electrode wire. After first use, store the wire in a dry area, in its original container or in a sealed plastic bag.

Many collision repair facilities usually convert an existing MIG unit and dedicate it to welding aluminum. The welding machine may have a spool gun and a push-pull feed. It may sport a light, compact torch rather than a spool gun with high amperage to allow spray transfer. An output of 200 amps at 30 percent duty cycle is recommended as a pulse feature to control heat spread when welding on thin aluminum. To reduce cold start and crater defects, MIG welders for aluminum have a relatively low amperage and voltage and allow only short-circuit transfer. You also want to equip your machine with long plug-in cables that allow you to place the equipment a long distance from the power supply.

To tune the welder, complete a practice weld or two. Watch for a steady arc, and listen for a steady crackle with short-circuit transfer or a humming sound with spray arc transfer. Practice with the same thickness and alloy as the metal that will be welded, and always use room temperature test pieces. Maintain correct electrode stick-out, and keep a contact distance of 3 mm to 5 mm (1/8 in. to 3/16 in.). Then visually inspect your test welds, but be careful because aluminum remains hot for a long time after welding. After checking visually, conduct destructive tests to determine the strength of your test welds.

Ready for Battle?

Unlike steel, welding gun direction for aluminum requires using only the push, or forehand, technique. This helps direct the shielding gas toward the front of the weld puddle and provides an arc cleaning action. Normally, you want to use a faster travel speed than you would for steel, and you need to speed up as welding progresses.

To begin pre-weld cleaning, use a clean, oil-free plastic woven pad. Remove paint, undercoating and corrosion protection. Clean with wax and grease remover. Use a grinder only when absolutely necessary. If a grinder is used for cleaning, do not adversely reduce the thickness of the aluminum. Preferably, use a disc sander with 80-grit to 120-grit sandpaper. Lastly, stroke the surface using a stainless steel wire brush.

Now you're ready to fight the dragon. Aluminum welds require longer beads than steel. Run beads as far as possible without creating heat. Control heat buildup using a temporary or permanent backing plate and heat sink compounds. Also use continuous MIG welds, including the fillet or lap weld, the butt joint weld with a backing or the groove weld (this is the same as the butt joint with backing weld, but the mating panel edges are ground to a taper).

Striking a Blow

When you start a continuous weld in aluminum, cut off the ball on the end of the welding wire before striking the arc. Because the weld starts too cold, begin welding off the joint and move the torch into the seam. Preheat the metal, and hesitate slightly at the start point.

Finish with crater crack prevention techniques by increasing your speed just before breaking the arc. Then move the welding gun backward at the end of the pass to fill the crater, thus breaking and restriking the area to build up the crater area before hardening sets in. You may also use run-off tabs to end your bead. Afterward use a dye penetrant to check for cracks and flaws.

When applying body filler to aluminum, follow the product maker's requirements for surface preparation and oxide removal. Some vehicle makers recommend applying body fillers over an epoxy primer. When this is the case, follow the paint manufacturer's surface prep recommendations. Use an epoxy primer compatible with the paint system you use.

Body Fillers

You will need to clean the surface by washing it with soap and water and then cleaning it with wax and grease remover. Remove undercoating, corrosion, dirt, oil and grease. Next, hand sand with a woven plastic pad. If needed, sand with an orbital or dual-action sander, making sure all pads and discs are kept separate from those used on steel.

Oxidation results from exposing metal to oxygen and moisture or contaminants. Aluminum starts oxidizing immediately after contact with air. Oxide forms, providing a protective coating. But, if desired, further oxidation may be prevented or reduced by treating the metal and applying primer.

Putting on Armor

Now, we know that corrosion may be caused by a galvanic reaction. When refinishing aluminum, refer to the vehicle maker's recommendations for corrosion protection. Preserve the original coating as much as possible, and don't remove any e-coat unless directed by the vehicle maker. Use only products suggested for aluminum, and follow the manufacturer's requirements for surface preparation and application.

Make sure the surface is dry and at room temperature. Sand as needed, and use a metal conditioner or conversion coating. Often you may use a self-etching or wash primer or a two-part epoxy primer. Cover the surface with a two-part epoxy primer before applying plastic fillers. (Two-part epoxy primer may be required as a base for body fillers.)

If used, apply a metal conditioner formulated for aluminum. Many contain strong chemicals, so use appropriate personal safety protection. Etch the metal using the product as directed, then neutralize with water as appropriate. The conditioner is usually followed with a conversion coating. But this method isn't for closed panels and interior surfaces because of washing difficulty, and don't use the coating methods over or under body filler. Also, be sure not to touch the aluminum surface with your bare hands after chemical treating.

This procedure is not required if you use self-etching primer because it eliminates separate metal conditioner and the need for conversion coating. Self-etching primer can also be used on closed section areas and interior surfaces. After preparing the surface and applying the recommended primer, the remaining refinishing steps are the same as those used on steel parts.

Now take a bow. You've triumphed like St. George over the dragon.

Just don't put your foot up on this monster aluminum job to mark your victory.