BodyShop Business
Aluminum Dent Repair – with a Stud Welder
Mike West
7/1/2006

Aluminum is not a new metal to the collision industry. The Model T Ford had an aluminum hood. The 1925 Pierce-Arrow was all aluminum. And many classic cars of the 1930s had aluminum parts on them. Why? Because aluminum allowed low-production automobile manufacturers to hand-form parts without the use of the costly stamping process. The easy formability of the softer-temper aluminum alloys made the process of forming sheet metal parts a very viable one, even though it required a high level of skill.

While many of these skills no longer exist in our industry, we have technology to help make up for it.

I like to say that aluminum repair isn’t difficult – it’s different. And that’s the primary idea to keep in mind when you begin. Aluminum repair, in its entirety, is a vast field that requires much study to become proficient in.

I’ve always followed the theory that success comes with understanding. If you understand the physical, mechanical and chemical reasons for something being the “way it is,” then you have a much higher probability of changing the “way it is,” successfully.

It takes some knowledge, and a different kind of knowledge, to properly repair aluminum. No big deal though because all that knowledge is readily available to you. Try some I-CAR aluminum repair courses. I recommend STA01 (Straightening Aluminum), PRA01 (Panel Replacement Aluminum), SSA01 (Structural Straightening Aluminum), WCA01, WCA02 (Welding and Cutting Aluminum course 1 and 2), and SPA01 and SPA02 (Structural Parts Aluminum course 1 and 2) for starters.

Why should you even get involved in aluminum repair? Good question. I guess the short answer is because you’re in the automotive repair industry. It’s what we do.

Oversimplistic? Think of it this way: The bumper covers on automobiles aren’t made from steel.

Therefore, they require a completely different repair process, one that definitely requires a good understanding of the process to successfully accomplish the repair. When someone attempts this process without that understanding, failure will be a probable result.

For this reason, many repairers have opted out of plastic bumper cover repair. Instead, they’re installing “reconditioned” bumper covers that somebody else may or may not have properly repaired. Where did these covers come from? These are the covers “we” didn’t want to repair. They came from “our” scrap piles. We lost valuable and significant repair work that we could use now. I like repairing these covers, and I don’t get comebacks either.

Will aluminum fenders, hoods and decklids be the next “reconditioned” parts for our industry? You know if there’s a need, some enterprising entrepreneur will fill that need, and the insurance industry will give him as much help as necessary. Let’s fill the need ourselves.

Understanding Aluminum
Aluminum is unlike steel in that it doesn’t have a “memory” or “desire” to return to its original shape once it’s damaged. It will want to remain in its damaged state, and you’ll need to apply different techniques to assist it in returning to its original state.

Wow! I realize I’m writing about this as if it were a living thing, but to me it is. After working daily with metals of all kinds for more than 40 years, they become part of your family. In fact, some of them are smarter than some of your family. (Yep, I’m firmly planted on that tree limb ... What’s that sawing sound I hear?!)

Aluminum also gets harder or stiffer when it’s damaged and again when it’s straightened. This is called “work hardening” and is a definite part of the understanding process.

Cosmetic exterior panels made from aluminum are generally made from heat-treated aluminum.

Heat-treated aluminum can range in hardness from a T0 (which is very soft and malleable) to T6 (so hard that when you attempt to bend it, it’ll break). Many outer aluminum body panels are temper T4, which is very strong and dent resistant.

The Process
For our project, we’ll employ the use of an aluminum stud welder to enable us to remove a dent in a Cadillac Escalade rear hatch, where access to the damage from the back side wasn’t possible.

Remember, this is an electrical welding process, so we need to follow the vehicle maker’s recommendations regarding the disabling of electronics and safety restraint systems. Follow the process and photos. It’ll be fun:

Step 1: Assess the damage and develop a repair plan. Gather all tools and lay them out in repair sequence. Our dent is softball size with a deeper center that will require stud welding.

Step 2: Apply heat. Notice the size of the dent is reducing because the heat is relieving stress that’s locking the dent into position. Aluminum dent repair should always be heated. The heat range is between 400 and 570 degrees F. Aluminum loses its temper once you exceed 570 degrees F and becomes permanently soft. It will melt and fall through at 1,174 degrees F.

Step 3: Notice the size of the dent as you continue to bring the dented metal up to the heat repair range. Because aluminum is such a good conductor of heat, it’ll warm up much faster than steel. Back off. Be gentle. Try not to burn the paint.

Step 4: Monitor your heat carefully during the heating process to avoid exceeding the upper range. Several methods are available for monitoring heat: heat crayons (two will be needed, one at 400 degrees F and another at 550 degrees F), a thermocouple (a wand that transmits digital readouts to a handheld calculator) and a non-contact thermometer (which reads heated surfaces at the point of a laser dot emitted from a thermometer). I have all three methods and prefer the non-contact thermometer for speed and utility.

A word about non-contact thermometers: They’re sensitive to shiny surfaces because of how the surface is read by the thermometer. A painted surface will give an accurate reading, while a bare sanded surface will not. This is important! The higher the temperature, the bigger the discrepancy between the painted surface and the shiny bare surface. The difference can exceed 100 degrees F at the high range.

Solution: Don’t point the non-contact thermometer at bare aluminum. If necessary, apply high heat black (barbeque paint works) to the repair area prior to heat. In the stud welding process, a black soot forms around the base of the stud that provides a suitable surface to use. You may also choose to reduce the area you grind paint from to allow for taking heat readings from nearby painted areas.

Note: I still like the non-contact thermometer. It’s handy.

Step 5: Take your heat close to the upper range for better results. While aluminum heats up quickly, it dissipates heat rapidly so the closer to the higher limit you are, the more time the metal has to relax and return to its original contour.

Step 6: Allow to cool. If heat transfer is an issue, you can rapid cool with compressed air (air blower) or wet rag (messy). Be aware that any adhesive areas in the heat zone will have to be re-bonded at the conclusion of the repair, as well as applying corrosion protection from burned inner surfaces. Note the dented area in the photo has been reduced to a small deep dent by just heat stress relieving.

Step 7: Sand/grind paint from repair area using 80-grit disc in grind mode.

Step 8: Vacuum, don’t blow. Cross contamination and galvanic corrosion are serious considerations when working with aluminum. Dissimilar metals (steel and aluminum) in contact with one another in the presence of an electrolyte (moisture) cause galvanic corrosion. It’s very important to make sure you vacuum up your filings, grindings and dust and don’t blow them around the shop, where they can land on cars of a dissimilar metal. Use fresh sanding abrasives to prevent cross contamination. Vacuum all your tools and wipe them down with a damp rag to remove all traces of metals.

Some manufacturers recommend separate tool sets marked “Aluminum Only.”

Be diligent and be conscientious. Nobody wants to make a great repair and then have it come back with corrosion bubbling under the paint. It’s ugly.

Step 9: Clean the sanded area, and you’re ready for studs.

Step 10: Bring out the aluminum stud welding machine. Set it up correctly on an old part of comparable thickness. This is important to prevent “blow through.” Start with a lower setting and test stud for pulling strength. This takes a little practice. These machines have a wide range of settings, so I recommend that when you find the proper range, mark it on the machine for future reference. The machine is unlike conventional steel stud welders and has two external ground cables that need to be applied to an area connected to the repair area. I used the hatch window weatherstrip pinchweld. My machine came without a cart, so I had to build one (another Saturday gone).

Step 11: Once the machine is properly set up, apply one or more studs to the dent area. I used two.

Step 12: Thread on pulling loops to threaded studs to allow for pull. Note the soot buildup around the base of stud. This will provide a suitable measuring surface for a non-contact thermometer, or you may apply high-heat paint in a nearby location.

Step 13: Heat the area of the dent carefully to the repair range of 400 to 570 degrees F. Monitor the heat accurately to avoid annealing.

Step 14: When the desired heat has been reached, apply the leverage bar and remove the dents while the panel is hot. Move quickly because aluminum cools quickly. That’s why racers like it for oil tanks and pans.

Step 15: Remove the threaded studs with side cutters. Don’t try to twist off, or you may tear a hole in the panel. Remember the physician’s oath, “First do no harm.”

Step 16: Grind down the remainder of the stud to the surface. Use 80 grit.

Step 17: For those of you who don’t recognize this, it’s a metalman’s vixen file. If you have one, it must be dulled on a wire wheel so it won’t remove too much metal. Dress off the area to reveal highs and lows. Use the file at a 3-degree angle and not straight back and forth to avoid file marks. Painters hate those.

Step 18: Grind the area with an 80-grit disc and check for highs and lows again. You may wish to apply an appropriate filler. In this case, it wasn’t necessary.

Step 19: Featheredge the repair area and apply appropriate primers and primer surfacers. There you have it. This repair represents a significantly different repair process than a steel part, with different time considerations. It’s a valuable process for any estimator to witness in order to properly understand the procedure.

Never Stop Learning
Collision repair is an ever-changing occupation, and those who don’t change with it will be left behind. It’s always better to learn new skills as you go and invest in new equipment/technology as the need arises than to wait and be overwhelmed after a couple of years. Changes don’t always come one at a time either, so it’s good to be ahead of the curve rather than playing catch-up.

Writer Mike West, a contributing editor to BodyShop Business, has been a shop owner for more than 30 years and a technician for more than 40 years. His shop in Seattle, Wash., has attained the I-CAR Gold Class distinction and the ASE Blue Seal of Excellence.

Comments? We want to hear from you! Please fax your comments to (330) 670-0874 or e-mail them to BodyShop Business editor Georgina K. Carson at gcarson@babcox.com.



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