Tackling the repair of mixed-material vehicles

Sept. 1, 2017
Both BMW and General Motors (GM) have mixed-material vehicles that provide some insight into what we might expect to see more of in the future. This article will take a closer look at those vehicles and the repair strategies used by each vehicle makers.

Earlier this year, I-CAR wrote about some of the reasons behind why we might expect to see more mixed-material vehicles in the future (http://bit.ly/2tgyhBv). That article covered some of the challenges vehicle makers battle between meeting Corporate Average Fuel Economy (CAFÉ) requirements and consumers’ demand for safer, more comfortable vehicles.

Both BMW and General Motors (GM) have mixed-material vehicles that provide some insight into what we might expect to see more of in the future. This article will take a closer look at those vehicles and the repair strategies used by each vehicle makers.

Figure 1 - The Cadillac CT6 employs a variety of steels and aluminum in its unitized structure.

Cadillac CT6

Cadillac’s CT6 sedan is comprised of both aluminum and steel. (Figure 1) On this vehicle, you will find aluminum parts attached to a steel part, and then another aluminum part. An example of this is the roof assembly. On the CT6, the outer roof skin is constructed of 6000 series aluminum, with the front roof brace being identified as dual phase (DP) ultra-high strength steel (UHSS). The two middle roof braces are high strength low alloy steel (HSLA), followed by a fourth brace which, is 5000 series aluminum. With this setup repairers will need to be able to identify which parts are constructed of aluminum and which are steel. Cadillac has addressed possible confusion with a structure identification document, in their repair information. This document identifies what materials were used on each part of the body.

Cadillac has used many different attachment methods to incorporate the mixed materials. Some of the attachment methods will not be able to be duplicated during collision repairs. From the factory, the CT6 uses a combination of aluminum and steel spot welds, aluminum laser welds, self-piercing rivets (SPR), flow drill screws, and adhesives. For part replacement, blind rivets, flow form rivets, and flow drill screws are used along with adhesives.

So, what happens when the CT6 is damaged in a collision? Can any parts of the vehicle be repaired? Cadillac has address this question in the body repair manual. Cadillac breaks down the different materials that the vehicle is constructed of and provides repairability guidelines. Some examples include aluminum extrusions and castings which require replacement if damaged. (Figure 2) GM guidelines for dual phase (DP), mild, and high strength low alloy (HSLA) steels allow cold repairs, if the damage does not include kinks.

If the damaged part is not a candidate for repair, Cadillac has provided many replacement procedures for the CT6. These procedures are available on both the free (http://www.genuinegmparts.com) and subscription (https://www.acdelcotds.com) repair information websites.

Figure 2 - If there is damage to any of the castings on the CT6, part replacement is required.

Cadillac does not have a sectioning procedure for the front lower rail. Instead they have a partial replacement procedure, in front of the firewall and at the middle of the rail. For this procedure, you will be required to grind off the existing welds, and then duplicate the factory welds in the original locations, with the same lengths.

Cadillac has sectioning procedures for both the outer quarter panel and the outer rocker panel on the uniside. The outer rocker panel procedure requires removing SPRs, flow drill screws, and factory spotwelds. Reattachment is accomplished by replacing the spotwelds and SPRs with flow form rivets and structural adhesive. The new part is trimmed to create an overlap for all the section joints. This joint is adhesive only with, flow form rivets and flow drill screws in the pinchwelds.

For the rear rail, Cadillac has both a full rail replacement and a partial replacement procedure. Replacing the entire rear rail requires removing the rail all the way to the passenger compartment. For a less intrusive repair, if the extent of damage allows, the partial rail replacement procedure can be used. This procedure separates the rear section of the rear rail at a factory seam, near the spare tire tub. The new rail end is then slid into the front section and attached with structural adhesive, flow drill screws, and structural rivets.

BMW 7 Series

The most recent version of BMW’s 7 Series is another example of a vehicle constructed of mixed materials (Figure 3). The 7 Series uses aluminum for exterior and closure panels such as the hood, doors, trunk, and outer roof skin. The outer uniside and reinforcements are steel. Extruded aluminum is used for the front lower rails and cast aluminum for the strut towers. There are also carbon fiber reinforcements inside the roof rail and the roof bow.

Figure 3 - The BMW 7 Series is constructed of different types of steel, aluminum and carbon fiber.

BMW uses flow drill screws, rivets, adhesives, and welds. The flow drill screws are one time use fasteners, and might be replaced using a different type of fastener, per the repair procedure. For example, the flow drill screws used on the front lower rail, are replaced by blind rivets when replacing the front lower rail.

When developing a repair plan for the 7 Series, keep in mind that BMW has guidelines for repairability of the different materials used. For aluminum outer body panels, heat may be used for straightening. Using a heat gun is the recommended tool for straightening. However, for extruded or cast aluminum parts, straightening, heating, or reshaping is not an option. Damaged castings and extrusions must be replaced, following the BMW repair procedures. According to BMW, if the adhesives are heated above 120° C or about 248°F, the adhesive will be destroyed. Additionally, when heated or reshaped, the aluminum can lose up to 40% of its original strength.

Steel parts of the unitized structure can be straightened as long as they are repaired cold, and the steel is not high-strength steel (HSS). BMW warns that heating of steel body components is not permitted. As with the aluminum structure, BMW says that heating can cause the material to lose up to 40% of its strength.

The carbon fiber reinforcements do not offer any repairability. If damaged, complete part replacement is the only option.

BMW has provided many different replacement procedures for the 7 Series when parts cannot be repaired. The front lower rail replacement procedure involves cutting off the damaged rail, in front of the mounting location. Cutting off the damaged part will allow for easier access to remove the remaining portion of the front lower rail. Next, the flow drill screws are removed along with the welded connections. Blind rivets are used to replace the flow drill screws, along with weld bonding, in the welded areas.

For removing carbon fiber parts, BMW has special precautions that must be followed, so that additional damage does not occur. The carbon fiber parts are attached with blind rivets and adhesives. After drilling out the blind rivets, BMW recommend using a heat gun to release the adhesive.

They also recommend to start removing the carbon fiber part in the center of the part. Starting from the sides, can cause delamination of the carbon fiber part. Another warning given by BMW is to not use infrared lights, in the area of carbon fiber. Infrared light can heat the metal behind the carbon fiber too quickly and damage the backside of the carbon fiber.

If you are interested in learning more about the new 7 Series, I-CAR has created a 360° video that highlights the 7 Series. This video covers material usage, replacement procedures, and calibration. You can find this video by visiting the I-CAR Repairability Technical Support portal


Steel-intensive vehicles will most likely continue to dominate the market for several more years, and we’ll surely see more aluminum-intensive vehicles in the immediate future, but the long-term outlook could be quite different. As vehicle engineers continue to look to trim pounds, and oftentimes ounces, from vehicle structures, it’s quite likely that we’ll be working on more mixed-material vehicles. The technology found on the CT6 and 7 series are undoubtedly going to make their way into other vehicles in the vehicle maker line up. Vehicle engineers are going to choose the material that best suits their needs to achieve their desired goal. Collision repair professionals are going to have to adapt to these changes, identify which materials are being used, understand each material’s repairability limits, and ensure that we’re following the vehicle maker procedures for complete, safe, and quality repairs. These changes will also require investments in new equipment and training to ensure the proper repair techniques are being used.

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