Lighter Doors Make a Massive Difference for 2025 (To Say Nothing of the Rest of the Closures)

Gary S. Vasilash

Here’s a look at a study conducted to get a sense of just how much mass can be taken out of closures through the use of mixed materials rather than just one. You may be surprised at what they determined.

Although the acronym for the Coalition for Automotive Lightweight Materials, which is under the aegis of the Center for Automotive Research (cargroup.org), is CALM, while it might be an exaggeration to say that those who are looking at the future of automotive materials are agitated, being in a calm state is probably something that they’re not feeling at the moment. But a co-development subgroup* of CALM decided that they would work to make an assessment of the challenge being faced by the auto industry in terms of reducing vehicle weight in part to help meet fuel economy standards. The group knows that there will be modifications to powertrain, aerodynamics and other elements of vehicles, but they also reckon that a 10 percent vehicle mass reduction can result in fuel economy improvements on the order of 6 to 8 percent.
 

Mass matters. A lot.
So they did the research and developed a report titled “Mixed Materials Solutions: Alternative Materials for Door Assemblies” that shows how selecting different materials and performing the requisite process changes can achieve a greater than 10 percent mass reduction for doors. (This should be requisite reading for those who are involved—OEMs and suppliers alike—in trying to figure out how to reach the still-likely 2025 CAFE regulations.)

While the door is but one of the closure panels on a vehicle, it is a representative part. CALM points out that closures—doors, liftgates, decklids, hoods—“represent a significant opportunity for weight reduction,” and because these are panels that are added to a body structure, there is an opportunity to make them with materials other than that of the base structure. This is certainly not something new, as for years there have been examples of predominantly steel cars, trucks and crossovers with aluminum hoods or polymer or magnesium liftgates.

And this works to good purpose due to the mass reduction of the vehicle. It isn’t simply a matter of reducing the weight of that particular part, as there is also the opportunity to downsize or down-gauge related components, such as the brackets, struts, hinges, motors and other parts associated with that closure panel. Going further, were there to be an across-the-board mass reduction due to changes in the materials for closure panels, this could result in everything from a smaller engine (less mass to move) to smaller brakes (less mass to stop).

The CALM group started with a study that the U.S. Environmental Protection Agency (EPA) had conducted for it by FEV that was published in mid-2014, “Silverado 1500 Mass-Reduction and Cost Analysis Project Review.” That study looked at materials and methods that could contribute to mass savings. The door assembly assessment FEV conducted determined that the door outer, door inner, crash intrusion beam, beltline reinforcement and hinges accounted for 80 percent of the mass of the completed door.

Using “readily available” aluminum alloys and gauges, the determination was made by the FEV researchers that the weight could be reduced from 29 kg to 18.8 kg.

So the CALM group used that as the starting point for their assessment. However, they knew that they weren’t going to be doing the finite element analysis and physical prototyping that would be required to engineer an actual door, but they did create an analysis tool, the “Weight Approximation Model.” The algorithm that’s used takes into account factors like the part area, volume, material grade and material thickness. The group didn’t have the means by which the hinge system could be analyzed, so they concentrated on the other four parts leaving the hinges as they are.

What occurred is something of a step-by-step approach to achieving a lighter structure.

For example, they made a change of materials for both the crash intrusion beam and beltline reinforcement of the Silverado door. For the former they changed a ferritic bainite beam to a boron beam and the latter from a DP500 to a dual-phase 980. That reduced the mass by just over 6 kg. They changed the outer to a DP 440 bake-hardenable, which allowed them to reduce the overall mass by 2.8 kg.

But the group then went at the door from a mixed-material point of view.

For example, rather than a steel impact beam, they looked at aluminum, which can reduce the mass by 1 kg. They assessed glass fiber-reinforced inners, which helped bring the mass down to 17.7 kg. Deploy an aluminum outer: 17.1 kg. But use glass fiber-reinforced materials for the outer and the beltline reinforcement and it goes to 16.5 kg. 

On they went, to the point where the door consists of a glass fiber door outer panel, a continuous fiber carbon fiber-reinforced plastic (CFRP) door inner, an aluminum intrusion beam and a chopped fiber epoxy resin beltline reinforcement panel. The steel hinges are still the original steel hinges.

This gets the mass of the assembly down to 11.7 kg. Which is a significant change from the 18.8 kg that they targeted, to say nothing of the 29 kg that the actual model year 2011 Silverado has.

They even came up with an assembly that combines carbon fiber, adhesives and structural foams to get the mass to 9.7 kg.

It should be pointed out that this wasn’t simply an exercise that had them mixing and matching materials. A large part of the endeavor included looking at the various processes required to actually produce the various components, as well as to take into account creating the assembly. This led to the conclusion that there would be several different approaches taken both on the assembly line (i.e., greater use of adhesives and fasteners) as well as in the paint shop (i.e., using thin-film surface treatments to facilitate mixed-materials).

What’s more, various companies within the working group have created their own distinctive approaches to creating new modules and components for doors that can provide notable mass reductions—even if they’re not down to the 11.7 kg vicinity.

One of the three authors of “Mixed Materials Solutions: Alternative Materials for Door Assemblies” is Shashank Modi, a research engineer at CAR. (His two colleagues on the report are Mark Stevens and Matthew Chess.)

When I inquired as to whether he sees mixed materials as the norm going forward, he pointed out that the real question goes to the point of whether the subject is body-in-white (BIW) and closure panels because otherwise, every vehicle is a mixed-material vehicle (steel, glass, rubber, plastic, fabric, etc.).

However, if it is BIW and closures, then he sees that there will be three types of vehicles in the future fleet:
1. Steel-intensive vehicles
2. Aluminum-intensive vehicles
3. Mixed-material vehicles

The last will be the most in terms of the approach.

Modi explained, for example, “It really depends on the market segment the vehicle falls in and the countries where it will be made and sold. For an entry level, low-cost vehicle which is produced globally, it will make more sense to stick with steel.” He added, “Mixed materials in the body-in-white and closures is seen as an effective strategy to lightweight for most of the fleet. It is really about the right material at the right place philosophy.”  

*Members of the group participating in the study: American Chemistry Council, Aluminum Association, Aisin, AK Steel, Altair Engineering, AP&T, ARaymond Automotive, Axalta Coating Systems, BASF, Böllhoff, Covestro, Dana, Dow Automotive Systems, DuPont Performance Polymers, Eisenmann, Faurecia Automotive Exteriors, Henkel, Hexion, ITW Automotive, Pittsburgh Glass Works, PPG, Rifast Systems, SABIC, Shiloh Industries, Sika.