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Tackling the repair of mixed-material vehicles

Friday, September 1, 2017 - 06:00
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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.

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