Aluminum Authority

Story and Photos contributed by I-CAR

Where to look Examples of vehicles with structural parts that are covered in this article include the following 1997–2003 Audi A8 2004 Audi A8 2004 Jaguar XJ 2003 Honda Insight 1990–2003 Acura NSX

Repair vs. Replace? As stated earlier, the kink vs. bend rule does not apply to structural aluminum parts. In most cases, damaged structural aluminum will be straightened to repair indirect damage, and the parts with visual deformation will be removed and replaced. Damage without visual deformation and bends that can be removed with heat within the repair temperature window are types of damage that may be repairable. Sheet aluminum provides the most repair options. Sheet aluminum can be straightened, sectioned or replaced as a complete assembly. Damage that would require part replacement includes rips or tears. Extrusions offer limited repairability and generally will require sectioning or replacement if visually damaged. Cast parts are generally not repaired, nor sectioned. Visual damage that would require part replacement includes cracks in the casting itself. Ultimately, the vehicle maker should be referenced for specific recommendations. For example, BMW recommends that extruded and cast aluminum parts that have visible or measurable signs of deformation must be replaced. Dye penetrant is one tool that can be used to locate cracks that are not visible to the unaided eye. After dye penetrant is used, it must be removed from the weld zone to prevent contamination.

The front rail extension (right) for the 2003 Honda Insight is a multi-celled extruded part. Above, the attachment point for the front rail extension on the 2004 Jaguar XJ is a casting.

Replacement rivets supplied by the vehicle maker include a Jaguar self-piercing rivet, an Audi solid rivet, a BMW blind rivet and an Audi blind rivet.

Self-piercing rivets with different appearances may be located next to each other on the same part.

As aluminum vehicles become more common so does the need for technical expertise. Working with aluminum requires a new kind of repair knowledge that technicians need to know in order to succeed.

Working with structural aluminum not only requires getting accustomed with the different properties of aluminum compared to steel, but also different repair procedures and equipment. Structural aluminum parts are not inspected, repaired and replaced the same way as steel parts. For starters, the kink vs. bend rule no longer applies. In fact, there is no general rule for repair vs. replace. The process that was used to form the parts plays an important role in the repair. Some structural aluminum parts are replaced using a different attachment method than what was originally used. Some structural parts are attached using bolts or rivets. Some welded joints are staggered instead of continuously welded. Some structural parts may be sectioned using an adhesively bonded joint or a rivet-bonded joint. And the list goes on.
This article is a follow-up to the January ABRN article, “The Aluminum Age.” It focuses on forms of aluminum structural parts, actual applications of welding processes and other joining methods, sectioning procedures for structural aluminum, and other considerations to keep in mind when doing collision repairs on aluminum structures.

Structural Aluminum Alloys
Structural aluminum parts are not 100 percent aluminum. Different elements are added to the aluminum to provide characteristics such as strength and corrosion resistance. Once the elements have been added, it becomes an aluminum alloy that is defined by a standardized numbering system. Wrought alloys use a four-digit designation and cast alloys use a three-digit designation. This information becomes helpful when selecting the best electrode wire for GMA (MIG) welding a specific type of aluminum. A body repair manual may list the series of alloys used or the type of electrode that should be used for GMA (MIG) welding repairs.
The physical and mechanical properties of an aluminum alloy are known as its temper. The temper is identified by the suffix that follows the alloy number, such as “H-11” or “T-6.” The higher the number, the harder the alloy.

Aluminum Forms
Structural aluminum parts may be sheet, extrusions or castings. Sheet aluminum is formed by rolling and stamping the aluminum into shape. Characteristics of sheet aluminum include raised, ribbed, dimpled or stretched areas made by the forming process. Sheet aluminum parts may have welded, riveted or rivet-bonded seams joined at a pinchweld. Sheet aluminum is typically thinner than the extrusions or castings on a vehicle. An example of a sheet metal part is the outer uniside on the 2004 Audi A8.
Extrusions are formed by pressing heated aluminum through a die. Extruded parts are seamless and do not have pinched flanges. Extruded parts are a continuous form, such as a tube, that may have one or multiple cells. Some extruded parts are a cutoff of a longer extrusion. An example of an extruded part is the rocker panel on the 2003 Honda Insight.

Cast parts are formed by pouring melted aluminum into a mold. Cast parts may have a variable wall thickness throughout the part. The surface is typically textured but may be smooth. Cast parts may have machined areas such as threaded holes and are typically thicker than sheet metal or extrusions. An example of a cast part is the strut tower on the 2004 Jaguar XJ.

Straightening Aluminum
The memory of aluminum is different than structural steel. Aluminum has less memory than steel and will tend to stay in the existing shape and state, whereas steel has a memory and will return more readily to the pre-damaged state.

Structural aluminum may also be thicker than structural steel. Therefore, greater pressures may be required because it bends less easily and cracks more easily, making it harder to straighten. It may be necessary to slightly over-pull the aluminum to compensate for the amount of spring back. Controlled heat is typically required to straighten structural aluminum parts.

Using Heat to Straighten
Aluminum alloys are classified as heat-treatable or non-heat-treatable. This refers to the treatment the metal received during the manufacturing process. Both heat-treatable and non-heat-treatable alloys can be heated during the repair process. Too much heat, however, can weaken the strength characteristics of the aluminum alloy. The repair temperature window for most aluminum alloys is 200–300°C (400–570°F). The approximate melting point of most aluminum alloys is 640°C (1,184°F). This number is significantly lower compared with the melting point of steel, which is 1,500°C (2,732°F).

Another factor that makes aluminum different from steel is the fact that aluminum does not change color when it is heated to the melting point. This is why it is important to use heat indicators such as heat detection crayons, heat-detection paint, heat- monitoring strips, a thermocouple with a DVOM, or a noncontact thermometer. When using a noncontact thermometer, leave the coating on the metal to ensure that the accuracy of the thermometer is not compromised.

The cumulative amount of time that heat is applied is not as critical on aluminum as it is with steel. The total amount of time that the aluminum is heated in order to change the temper of a heat-treatable alloy is far greater than the amount required to repair a part, as long as the heat is kept in the repair temperature window.
A heated panel should be allowed to cool naturally. Quenching with water and cooling with compressed air can cause the aluminum to crystallize and may change the mechanical properties of the metal.

Heat also spreads faster on aluminum than steel. If the part will be replaced, the repair temperature upper limit may be exceeded, but the heat spread should be considered so that parts that will not be replaced are not damaged from the heat.

All of these subjects on straightening and heating aluminum are covered in two I–CAR Enhanced Delivery programs: Cosmetic Straightening Aluminum (STA01) and Structural Straightening Aluminum (SSA01). Watch for them in your area or request a class at the I-CAR Web site at www.i-car.com.

Aluminum Corrosion
Aluminum has a natural barrier against corrosion. When bare aluminum comes in contact with the air, it creates an oxide film that seals the aluminum. This oxide film thickens over time and protects the underlying bare aluminum surface.

Galvanic corrosion is a result of two dissimilar metals coming in contact with each other in the presence of an electrolyte such as water. This type of corrosion may also be called contact corrosion or sacrificial corrosion. The vehicle maker typically recommends certain procedures using dedicated tools in a dedicated area to reduce the risk of this type of corrosion between aluminum and steel. We discuss these dedicated tools and repair areas in more detail below.

Corrosion Protection
Like steel, aluminum requires corrosion-protection coatings to prevent corrosion and restore appearance. Corrosion protection should be applied to welded, riveted, heated and bare metal areas following vehicle makers’ recommendations. Corrosion protection should be considered when there is a potential for bare steel to come in contact with bare aluminum. If slight damage occurs to the aluminum or the steel, causing bare metal to be exposed, galvanic corrosion may occur. Corrosion-resistant primers and topcoats are typically recommended for bare exterior areas. The recommended corrosion-resistant primer or anti-corrosion compounds should be applied to enclosed areas and seam sealers should be applied to duplicate the original appearance and to seal out moisture.

Dedicated Repair Area and Tools
Some vehicle makers, such as Audi and Jaguar, require a dedicated aluminum repair area and aluminum-specific tools. The dedicated repair area should be sectioned off from the rest of the shop and only aluminum repairs should be made in that area. It is generally recommended that aluminum hand tools and sanding, grinding, cutting and drilling tools be kept separate from steel tools. Some of the same power tools may be used for steel and aluminum but must be cleaned before use.

Additional restrictions include that some vehicle makers release the fixtures as well as structural parts required for the repair of an aluminum vehicle only to approved repair facilities. Structural repairs on these vehicles may require the vehicles be sent to an approved facility.

Specialty tools may be recommended to measure and straighten aluminum vehicles. Magnetic target attachments cannot be attached to an aluminum frame. Some aluminum vehicles have a horizontal style pinchweld that will require special anchoring clamps and some aluminum vehicle makers use rivets in the place of resistance spot welds on the pinchweld. Special clamps may be required that have spaces for rivets between the clamping surfaces.

To prevent possible contamination and galvanic corrosion between the steel jaws of a pinchweld clamp and the aluminum of the vehicle body, an aluminum plate can be placed between the steel clamp and the aluminum pinchweld. Another way to prevent contamination is to have a dedicated set of clamp jaws for aluminum vehicles only. After structural repairs, it is necessary to refinish the clamping area to prevent corrosion.

Grinding, Sanding, Cutting, and Drilling
Besides keeping the tools separate from steel, there are additional considerations when grinding, sanding, cutting or drilling aluminum.

Aluminum should be cut and drilled at high speeds and ground and sanded at slow speeds using light pressure. When aluminum is sanded at high speed using heavy pressure, the metal is excessively heated. The metal may smear, the abrasive may clog more quickly and the metal is thinned more easily. Lubricants may be recommended to prevent this from happening. Lubricants are available in the form of a block, liquid or paste. Vehicle makers may recommend specific lubricants when working with aluminum. After use, the lubricant must be properly removed from the repair area. If not removed, it may contaminate the weld, adhesive bond or topcoats.
It is also recommended that no coarser than P80 grit be used when grinding or sanding the aluminum part itself, while coarser grits may be used to remove welds.
Aluminum dust is also a concern. Aluminum dust particles are nontoxic, but they are lung and eye irritants and should be collected with an air extraction system. Compressed air should not be used because the dust is lightweight and will float in the air for an extended period of time. Aluminum dust can also be dangerous. Although unlikely, in the correct concentrations with other elements, such as steel, it can be explosive.

Replacing Bolted Parts
Some aluminum structural parts, such as the front rail extensions on the 2004 Jaguar XJ, are bolted to the front of the rail. Fasteners, inserts and small parts may be made of aluminum or steel. Bolts are typically made of coated steel, which can create a galvanic corrosion issue. If there is a damaged coating on a fastener, it cannot be repaired and the bolt should be replaced. Audi is one vehicle maker that recommends always using new fasteners if the old fasteners are removed.

Audi also recommends that pneumatic and electric tools only be used to loosen bolts. They warn against using these types of tools to tighten bolts, especially when installed into light alloy parts. Following installation, bolts should be torqued to specification. It is important to use a torque wrench to prevent damaging softer aluminum alloys.

Replacing Welded Parts
Welds that are typically used during original construction include spot welds, plug welds, fillet welds and slot welds. The recommended welding process for repair is GMA (MIG) welding. Spot welds and plug welds are replaced with plug welds because squeeze-type resistance spot welding requires an excessive amount of current. Always refer to vehicle maker’s recommendations. BMW is one vehicle maker that warns against using aluminum plug welds for repair.

Aluminum vehicles are constructed using more fillet welds than steel vehicles. The original fillet welds on structural aluminum parts are typically staggered, not continuous. Staggered welds are typically replaced with staggered welds of the same specifications. Fillet welds are also used for the original and replacement of slot welds.

An example of where welds are not replaced with welds is on the outer A-pillar for the 1997–2003 Audi A8 where it is recommended that a fillet-welded seam be replaced with blind rivet bonding. When a welded joint is replaced with rivet bonding on the BMW 5 Series, it is recommended that electro-magnetic compatibility screws be installed to ensure that the chassis ground is complete. These are special fasteners supplied by BMW that are covered with seam sealer following installation.

Before making welds on the vehicle, test welds are made on the same type and thickness of aluminum and in the same position. Test welds should be visually inspected and destructively tested to verify the integrity of the weld.

When making welds on aluminum, remember that the aluminum oxide should be removed to achieve proper penetration, and weld-through primer is not used.
After welds have been made, check for cracks using dye penetrant. If a crack is found, the weld will have to be removed and the parts welded back together.

Unlike steel, not all aluminum welds require dressing. The weld appearance should duplicate the original. Many of the fillet-welded factory seams on aluminum vehicles are not dressed; therefore, they are not dressed during repairs. Sectioning joints and plug welds may require dressing to hide the repair. Honda states that hidden weld areas do not require dressing.

Replacing Riveted Parts
Rivet-bonded parts are attached with rivets and adhesives. The vehicle maker recommends the type of rivet and adhesive that should be used. The recommended rivet may be blind, solid, self-piercing, flush-mounted, protruding head, aluminum or coated steel.

A vehicle maker’s adhesive application recommendation may be different from the product maker. The vehicle maker’s recommendations should be followed. If no vehicle maker’s recommendations exist, follow the product maker’s recommendations. Vehicle maker’s recommendations may include additional part preparation steps. For example, Audi recommends using a grinding stone and applying primer, and Jaguar recommends a flame treatment process and applying primer before the adhesive.
Both Audi and BMW recommend that adhesives be used whenever rivets are installed for repairs, even if adhesives were not originally used. Rivet-bonded parts are typically replaced using a rivet-bonding procedure. The difference may be that a different type of rivet or a plug weld is used for replacement. For example, it may be recommended that self-piercing rivets be replaced with self-piercing rivets, protruding head-coated steel blind rivets, all-aluminum flush-mounted blind rivets, or all-aluminum flush-mounted solid rivets.

Be aware that there are some areas where rivets are originally installed without adhesive and adhesives are not recommended during replacement. An example is the radiator core support on the 2004 Jaguar XJ.

Self-Piercing Rivets
The type of rivet has to be identified before it can be properly removed. A body repair manual may be used to identify the type of rivet. Self-piercing rivets are used during original assembly and may also be referred to as punch, Henrob® or tubular rivets. Self-piercing rivets are made of coated steel. The backside of a self-piercing rivet typically looks dimpled but may look similar to a solid rivet. The backside of a self-piercing rivet may look different if it is located in a thick flange.

The methods of removing self-piercing rivets include pressing them out using a special tool, drilling them out, grinding them down and punching them out, or pulling them out using a special stud welder. Audi is one vehicle maker that warns against drilling or grinding down self-piercing rivets.

Self-piercing rivet installation does not require a hole, but it does require a special tool. During installation, the rivet pierces the top sheet and spreads outward into the bottom sheet. The self-piercing rivet should not penetrate through the bottom sheet. Replacement self-piercing rivets are installed a specific distance from the original self-piercing rivet removal hole. Different lengths may be recommended depending on the flange thickness. For example, Jaguar recommends adding 80 percent of the diameter of the head of the self-piercing rivet to the total panel thickness in the joint location to determine the proper length self-piercing rivet that should be used. Audi, on the other hand, recommends replacing self-piercing rivets with all-aluminum, flush-mounted blind or solid rivets, as specified.

Blind Rivets
Blind rivets may also be called Pop® rivets. During blind rivet installation, a hole is required, the rivet is placed in the hole, and the rivet gun grabs the mandrel of the rivet and uses it to compress the body of the rivet. When finished, the mandrel breaks off flush with the blind rivet head. A special rivet gun may be required to properly install some blind rivets.

The blind rivets that are recommended by Jaguar are the coated, steel- protruding head type and Audi recommends an all-aluminum flush-mounted type. Different lengths may be recommended depending on the flange thickness. For example, Jaguar recommends adding 6 mm to the total panel thickness in the joint location to determine the proper length blind rivet while Audi only has one blind rivet length.

Flush-mounted rivet installation requires countersinking the installation hole. Audi recommends a 100-degree angle for the rivet to nest properly in the hole. Countersinking may be done using a drill bit or a dimpling tool.

Solid Rivets
Solid rivets may also be called straight, conventional or solid shank rivets. An all-aluminum, flush-mounted rivet is typically recommended by the vehicle maker for structural repairs. Solid rivets are installed using a rivet gun with special dies to create the proper dome-shaped bucktail on the backside of the panel.

When selecting the correct solid rivet, total panel thickness is used to determine the length of the rivet that should be used. Audi recommends two different lengths of solid rivets. The shorter rivet is used for a two-panel joint and the longer rivet is used for a three-panel joint.

Solid rivets are used to assemble aluminum aircraft. Therefore, some of the tools that are used to assemble aircraft are also being used to repair aluminum vehicles. One of these tools is a cleco. Clecos are a type of clamp that is designed to be temporarily installed in a bolt, rivet or drilled hole to hold the panels together. Depending on the design, clecos are installed using cleco pliers, wing nuts or speed nuts, and may be color-coded according to size.

Sectioning Rivet-Bonded Parts
When rivet-bonded parts are sectioned, a traditional welded joint or a rivet-bonded joint may be recommended. If a welded joint is recommended, adhesive application must be kept a minimum distance from the weld location. If a rivet-bonded joint is recommended, flush-mounted blind rivets are used in the place of plug weld holes. It may also be recommended that the edges of the joint be tapered. This tapered area will be covered with an aluminum body filler to finish the joint.

Adhesively Bonded Sectioning Joint
An adhesively bonded sectioning joint is used when sectioning the front rail on the 2004 BMW 5 Series. This specialty joint uses BMW-specific expandable inserts. To make this joint, the insert is covered in adhesive and inserted halfway into each end of the rail. A bolt through the center of the insert is used to expand it. After the adhesive has cured, the bolt is removed, and seam sealer is applied to the joint area.

Repairing Structural Aluminum Parts
Repairing structural aluminum introduces new and different concepts for the collision repair technician. All of the aluminum vehicle makers have different recommendations. The vehicle-specific body repair manual is an essential resource for the proper repair of these vehicles. Another information source for technicians is I-CAR. There are two programs for repairing structural aluminum parts. The first is called Structural Aluminum Design And Repair Processes (SPA01). The second is Structural Aluminum Repair Processes (SPA02). Watch for them in your area, or request a class at the I-CAR Web site at www.i-car.com. m


This article was submitted by I-CAR, the Inter Industry Conference on Auto Collision Repair. I-CAR is a collision repair training organization dedicated to improving the quality and safety of collision repairs for the ultimate benefit of consumers.