Not Your Father's Sheet Metal

It begins with increased legislative pressure to make vehicles safer for passengers in a collision, resulting in the addition of reinforcements and inner structures to body panels, as well as safety-related components like airbags. Then there is the increasing number of electronic and computer-related components appearing on today’s vehicle, as well as the consumer’s seemingly insatiable lust for larger, bulkier vehicles.

All of these developments come with a curbside weight cost, and to address the issue without sacrificing safety, manufacturers are incorporating a diverse group of metals into the composition of body panels and structural parts. Many of these metals are recent developments. The Society of Automotive Engineers reports that 80 percent of today’s vehicular steels were developed in the 1990s. Among these are aluminum and a family of high strength steels that are lighter and thinner, yet stronger—three to five times as strong—as mild steel. It’s a triumph of innovation, but staying on top of this ongoing metallurgical evolution is tough. This is not your father’s sheet metal.

In past months, ABRN’s “Metal Shop” section has touched on some of the issues surrounding the new wave of high-tech metals. This month we’ll expand upon and consolidate this information in an effort to provide perspective on how to proceed when you encounter a repair—or think you have encountered a repair—that involves these substances. We’ll start with aluminum and move on to advanced high strength steels.

Addressing Aluminum

From a repairability standpoint, aluminum has become a relatively known quantity. It lacks the metallurgical variety of the new high strength steels and its use in vehicle parts is limited due to its expense—about $1,700 per ton, compared to $400 per ton for steel.

Nevertheless, aluminum retains a niche popularity in the market with a number of high profile applications, most notably the all-aluminum Audi A8 and aluminum-intensive Jaguar XJ series. It is not unusual for flat body panels to incorporate the material, as in the case of the hood of the Ford F-150 pickup, and the rear deck lid of the Chevrolet Malibu Maxx. The metal also remains a popular choice with manufacturers of hybrids.

When repaired, aluminum displays its own set of unique characteristics. It is less forgiving than mild steel and can easily crack or break if overworked, a mistake that’s easy for a repairer to make. “Aluminum has ‘less memory’ than mild steel,” says Steve Marks, research coordinator for the I-CAR Tech Centre. “While a mild steel panel has a strong tendency to return to its previous shape when pulled or straightened, aluminum doesn’t. If you don’t carefully plan your hammer hits, you may find yourself going over the same panel area too many times trying to get it right, which leads to unwanted work hardening and, ultimately, tearing.”

Metal being heated using an induction heater. Heating some of the advanced high strength steels may not be an option during straightening.Photo courtesy of I-CAR

Heating is recommended to make aluminum more malleable and less brittle during a repair. In lieu of specific recommendations from the automaker, I-CAR suggests heating the metal to a temperature ranging from 400°F to 570°F. This ensures the metal will only temporarily soften and return to original panel strength after cooling.

Caution must be exercised when applying heat, however, as aluminum begins to anneal at around 650°F, meaning its essential metallurgy will be changed, permanently softening and weakening it. “The heat sensitivity of aluminum—and all exotic metals—makes the monitoring of temperature during the repair important,” Marks says. “An infrared thermometer, heat crayons or heat stickers are the best tools to use.”

Depending on the application, an induction heater, heat gun or the open flame from a propane or oxy fuel torch can be used to heat shrink aluminum. A heat gun may work for heating thin body panels, but may not provide enough thermal output to heat a larger area on a thicker structural panel.

The Aluminum Association, who partnered with I-CAR to create standard procedures for aluminum repair, recommends the following guidelines for working with aluminum:

Handle with care. Aluminum scratches easily. Parts usually come carefully packaged and should be unpacked with equal care before they are brought to the workplace. Never work the metal on a steel table or the floor unless a piece of plywood is first laid down. Storage is important, too. Aluminum has a thin but dense coat of aluminum oxide that can trap dirt over time. For welding preparation, cleaning with a stainless steel brush is recommended. Wait no more than one day to commence working with “cleaned” aluminum, and make sure it is stored indoors in an environment immune from sudden changes in temperature.

Tools. When possible, use softer tools made of plastic and wood. Hammers, dollies and spoons should have smooth faces. Wood, leather and plastic hammers are preferred.

Avoid contamination. Because residual steel particles can easily embed into aluminum and contaminate it, separate sets of tools for use with aluminum are a must. Sanding discs, cutting tools, files and even shop towels cannot be used interchangeably between aluminum and steel jobs. Make sure to clean power tools when moving between aluminum and steel repairs.

Cutting. The Aluminum Association advises that cutting aluminum is more analogous to wood working than working with steel. It recommends the use of reciprocating and band saws, used in concert with blades that are specifically designed for aluminum. High blade speeds work best. Oxy fuel cutting is not recommended because the oxide film that coats the piece, and the aluminum itself, melt at different temperatures. This kind of cutting is also problematic in that the heat it generates is conducted rapidly by aluminum, resulting in a ragged cut that will have to be further trimmed. Plasma arc cutting, on the other hand, is acceptable because of the lower temperatures it generates.

Grinding, sanding and pulling. Light sanding with orbital or dual-action sanders with open-coat sanding works fine on aluminum surfaces, providing disks no coarser than 80-grit are used. Files and stones can be used to trim edges. Filing should be done carefully to avoid gouging the surface. A long sanding board, typically used for sanding filler, effectively highlights high spots without removing metal.

Rivets. Unlike traditional sheet metal, some aluminum panels forgo rivet holes to achieve an appearance that is more pleasing to the eye. This characteristic may require the use of a self-piercing riveting tool capable of delivering a maximum of 11,000 pounds of pressure. This is enough to rivet several aluminum pieces together.

This image shows e-coat that has been burned away from the backside of a panel as a result of welding and heating.Photo courtesy of I-CAR

In some aluminum riveting applications, an adhesive may be applied in the area of the rivet to provide additional strength and impact resistance to the area being joined, which may be either body or structural. The adhesive is applied before assembly, using a suitable surface pre-treatment.

“The BMW 5 series and the Audi 8 are two good examples of this combination of techniques,” says Mike West, owner of Southtowne Auto Rebuild in Seattle. “The result is a bond that’s up to five times as strong as a spot weld that doesn’t affect standard riveting procedure.”

Advanced High Strength Steels

As defined by the UltraLight Steel Auto Body Advanced Vehicle Concepts (ULSAB-AVC) program, an initiative funded by an international consortium of steel producers, high strength steels (HSS) are defined as steels with yield strengths from 210-550 MPa. [MPa stands for “megapascal.” One megapascal equals approximately 145 pounds per square inch (psi).] It’s a category that includes the high strength, low alloy (HSLA) steels that have been around since the unibody revolution of the late 1970s, and have come to be defined as conventional.

On the cutting edge of metallurgical technology are advanced high strength steels (AHSS). The AHSS family consists of alloys with yield strengths from 550 MPa to 1,400 MPa, and though a variety of steel types falls under the AHSS umbrella, they—as a class—tend to be thinner, harder and better at absorbing impact than mild steels and more conventional high strength steels.

The AHSS presence on contemporary vehicles is growing. It is used for both body and structural parts. Models called out by ULSAB-VAC for innovative use of AHSS steel include the latest models of the Chevrolet Malibu, Chrysler Pacifica, Ford F-150, Honda Civic, Volvo XC90 and Porsche Cayenne. AHSS is incorporated into hybrids, such as the Toyota Prius, as well. Since you are likely confronting these steels on a daily basis, how should you proceed?

Start by understanding that when it comes to repairing AHSS there is no “one size fits all” approach. Different alloys retain different properties, and the landscape changes constantly as new steels are engineered. “You can almost say alloys are customized to address specific vehicle applications,” says Marks. “Repairers need to proceed with caution and be conservative in their approach. When planning a repair, time is an ally, not an enemy.” 

That isn’t to say general guidelines for the new wave of metals don’t exist, they do, beginning with rules that address their hardness and thinness. Though they tend to have more of a “metal memory” than aluminum, the potential for overworking AHSS remains an issue because the panels take more effort to move and stretch. Complex bending and shaping is difficult.

Unlike aluminum, heat shrinking is not a reliable option because of the material’s sensitivity to heat. According to West, “The dime-sized cherry-red area technicians have grown accustomed to seeing when they heat shrink mild steels can be a waning beacon with advanced high strength steels. It could well signal that you’re over an acceptable temperature threshold.”

As an alternative to heat shrinking, and to avoid overworking and possible weakening of the panel, use a hammer off dolly technique to avoid excessive upward or downward panel movement. As a general rule, softer tools, like those used in the repair of aluminum, are recommended.

Other things to keep in mind when you are working with advanced high strength steel include:

Favor resistance welding. Given the heat-sensitivity of AHHS, it is usually preferred over GMA/MIG welding. Resistance welders dispense a powerful charge at the point of contact, which translates to less heat in the immediate point of contact. Temperatures will not travel farther than you want them to. The composition of the panel will not be compromised.

The end of pick-and-filing? The thinness of AHSS discourages another traditional method for panel straightening, the pick-and-file technique. Using a pick hammer to elevate low spots in a panel leaves raised pickmarks on the panel’s rear that were traditionally filed off. The thinness of some new steels (24 to 26 gauge, as compared to 20) makes this procedure problematic, in that you run the risk of filing through. If pick-and-file work is going to be achieved, it is better to use a flat sanding board with 80-grit sandpaper to smooth the part down. This allows you to see the high and low spots on a surface without removing unnecessary metal.

Use caution when anchoring pulls. Some AHHS alloys are so strong (see sidebar on boron steel) that they can cause unexpected damage during an anchored pull. “Steels can be so rigid that they can remain in position while adjacent panels start to move in the direction of the pull,” explains Marks. “When this happens, you run the risk of the adjacent panels tearing away.”

Understand the effect of weld-on studs during pulls. A popular method of straightening involves the use of weld-on studs, affixed to the rear of the panel as attachment points for pulling. In the case of AHSS, this procedure may not be ideal, as the heat generated may soften the relatively thin metal and affect its strength. In addition, this process may result in the burn-off of corrosion protection.

Corrosion protection is extremely important. Although always an important part of restoring a damaged vehicle to equal or better condition, corrosion protection is even more important for maintaining the integrity of thin, high strength steel panels. Apply weld-through primer on the bare steel, carefully removing the primer from the direct weld area. Do not remove primer from the area beyond the immediate weld area. Apply epoxy primer to the parts affected after welding.

Corrosion protection materials being applied to the inside of a structural part. This is especially important with the thin gauge steels in use todayPhoto courtesy of I-CAR

Applying weld-through primer to weld flanges before spot welding.Photo courtesy of I-CAR

Using STRSW rather than MIG welding to minimize heat effect on HSS parts.Photo courtesy of I-CAR

In addition to I-CAR and the growing number of classes at vo-tech schools that keep up-to-date on AHSS procedures, manufacturers are a key resource for finding information about new steels. The great majority provide “for pay” Web sites that allow subscribers access to up-to-the-minute information about repairability and other vehicle-related matters.

West, for example, keeps his technicians and himself abreast of the latest developments in high strength steel technology by accessing the National Automotive Service Task Force (NASTF) Web site (www.nastf.org), which provides a gateway to all existing OEM service Web sites.

“In this day and age, the more you think you know about steel, the less you find out you do,” says West. “But you can’t ignore the fact that more of these parts are coming down the pike because they’ve done a good job of solving weight issues, while improving the crashworthiness of the car. It takes a lot of energy to keep up, but the more you learn the more you find you can apply, or—at the very least—anticipate the next time you run into sheet metal you’ve never seen.”

— Thanks to I-CAR and the Aluminum Association for their invaluable help in researching this article.