Multi-material Body Solutions: Possibilities and Manufacturing Challenges

June 1, 2016

In the automotive industry, mass reduction and lightweight design is a continuing trend that does not show signs of declining. When examing where to reduce weight in a vehicle, the body is a preferential sub-system due to its large contribution to overall mass and the stability of body composition over a specific model range. The automotive industry is moving toward a greater differentiation in materials, as can be seen in the different multi-material vehicle bodies recently introduced. But while mixing materials may contribute to a good compromise between weight reduction and vehicle cost, it also proposes a number of challenges.

Introduction

At the moment, considerable industrial attention is focused on reducing energy consumption and this is especially true in the automotive industry. A vehicle’s energy consumption can be divided into three discrete phases: production phase, use phase and end-of-life phase.

While development is not mentioned anywhere in these three phases (and occurs before the first phase, production, is initiated), decisions made during the development of a vehicle will affect energy consumption in the production, use and end-of-life phases.

Mass reduction of the vehicle clearly will affect energy consumption. But which parts of the vehicle contribute necessary mass and which can be lightweighted?

A vehicle can be divided into five different subsystems based on functionality; body, chassis, powertrain, interior and electrical system. Figure 1 shows that the body is the largest mass contributor to the finalized vehicle, with 40% of the total mass attributed to this subsystem. This makes the body a most interesting target for mass reduction. Also, since the body is more or less standardized throughout a model range, mass reduction in the body will contribute to a mass reduction to all trim levels of that model, whereas a mass reduction in interior or powertrain components might apply only to a select number of vehicles.

There are other ways of dividing the vehicle into sub-systems; one is to look at the mass as primary, secondary and tertiary mass. Here, the body is the primary mass; engine and drivetrain, suspension, wheels and fuel is secondary mass; all other mass (as from glass, electrical systems and interior) is tertiary mass. Primary mass reduction will enable a secondary mass reduction without affecting functionality, performance or vehicle characteristics — and this further emphasizes the vehicle body as a focus area for mass reduction, since it can be seen as an enabler for further mass loss on the vehicle.

Mass generally can be attributed to two factors: geometry and material density. In the application of vehicle design, material density could be translated into material selection. Usually, these two factors are combined, since material qualities other than density have to be considered. A vehicle is composed of a large number of different materials, but again, the body contributes to a significant part of the mass of the finalized product. Therefore, material distribution trends even on finalized vehicles can indicate what is happening with automotive bodies, as shown in Figure 2.

Also shown in Figure 2 is that further material differentiation is a continuing trend. Notably, the plastic and plastic composites as well as aluminum content have grown with time (from around 6% to 9-10%), while conventional steel each year comprises a smaller portion of the materials.

Body-material composition also varies throughout vehicle types and makes, with the upper segments of the market showing more differentiation than lower-priced markets. The sports and supercar segments, with cars like McLaren MP4-12C and Lexus LF-A, can be used as an example of this larger material composition diversity in the upper market segments.

Since no single material is best suited for all body components, a multi-material approach seems to be the best way to find an optimal compromise between requirements. However, existing design tools and methods have issues handling material properties and multi-material solutions because many tools have been developed strictly for one type of material and/or manufacturing method.

In turn, this suggests that different concepts might need different evaluation methods, making it hard to compare qualities between concepts if they are too dissimilar. Also, component geometry will need to differ between different material concepts in order to have an honest comparison. Adding the possibility of path dependency — and the fact that some solutions might be favored by existing manufacturing capability — the engineering task becomes extremely complex.

Method

This paper aims to show the need for integrated product and production development when looking at lightweight design in the automotive industry. These two research topics interact with the industrial result and the context to create a basis for this work.

In this project, study developed according to the process described in Figure 3. First, a preliminary general study was made to research the purpose of lightweight design and possibilities for mass-reduction approaches. Subsequently, an industry overview was performed to investigate current multi-material solutions. In parallel, a general process model for vehicle car bodies was created from empirical findings. This process was split into a number of sub processes based on value-adding activities, each for which different challenges with multi-material solutions were investigated via literature research. The material families selected were the same as have been investigated in earlier research found by the authors, while general material properties were taken from CES EduPack 2015. These challenges were then analyzed and put into demands on product development tools and methods.

In this paper, different approaches to the multi-material design body-in-white (BIW) are presented. General studies or studies not implemented in large-scale production are presented under the heading “Academic examples and research projects.” Designs that have been or will in the near future be implemented in large-scale production are presented under the headline “Industry examples.”

Academic examples and research projects

Different research projects point to a possible significant mass reduction from increasing polymer composites as structural material in vehicles and an increased use of aluminum in structural components.

Peterson and Peterson have shown that a significant mass reduction can be achieved, with the same unit manufacturing cost as existing designs, if the whole vehicle is designed for these changes. Other research projects also have shown that a multi-material approach can reduce body mass by more than 40% at a 35% cost increase — using only current technologies.

Industrial research projects show that a light-duty pickup truck can be lighweighted by as much as 33% by switching to an aluminum body, while a passenger car can be lightweighted by 23% and the BIW by 21% employing existing manufacturing and a multi-material design approach. Another project points to a 31% mass reduction by transitioning from steel to aluminum and composite body panels. Projects focusing on magnesium-intensive structures have shown that this material family also is promising for mass reduction without significantly increasing unit costs.

Industry examples

Some stakeholders in the automotive industry believe in increased usage of hybrid or multi-material designs. Different polymer-based materials have long been used for hang-ons such as hatches, hoods and fenders, while more recently, fiber-reinforced polymer composites have been used in roofs. Currently, a number of vehicles are being released with different multi-material solutions even further integrated into the bodies. A few are presented as follows:

BMW 7-Series: The body of the G11/G12 BMW 7-series is designed for lightweighting via a combination of steel, aluminum and carbon-fiber reinforced plastics (CFRP) in what the company calls the “BMW Carbon Core” approach. A number of components — among them door sills, B-pillars and roof beams — are either reinforced or replaced with CFRP panels shown as the darker body portions in Figure 4.

Cadillac CT6: The 2016 Cadillac CT6 (Figure 5) is built around what General Motors calls the “Fusion Frame,” a concept in which a steel center section is clad with aluminum panels for everything visible. Aluminum also is used for components such as crash bars.

Volvo XC90: Volvo’s XC90 utilizes several different high-strength steel grades as well as aluminum parts for a lightweight but strong body. Within the body structure, joints between these different steels and aluminum are present throughout, as can be seen in Figure 6. Aluminum also is used in the strut towers and the front crash bar, along with the hood and front quarters.

Mercedes-Benz C-Class: For the 2016 Mercedes-Benz C-Class (Figure 7), the company developed a body with all hang-ons (doors, hood, fenders, trunk lid) in aluminum, an aluminum roof and a body with an increased use of aluminum and high-strength steel (hot-formed as well as conventional).

Analysis of industry examples

As can be seen from these industry examples, there is no single or clear-cut method for integrating newer materials in the vehicle body. The chosen materials are not the same for each manufacturer (though aluminum and high-strength steel are widely used by all) and the position and design of the non-steel panels is not identical. This could suggest that this technology step has not yet matured, but also that there are internal factors within automotive manufacturing organizations that may affect the gains from design choices.

Manufacturing challenges

Based on study visits at automotive manufacturers, the authors developed a simple overview of the production process: the production plant is split into three different factories: A, B and C. In this breakdown, the A factory is the body manufacturing and welding facility, the B factory is the paint shop and the C factory is the main body shop where the vehicle is assembled.

Within the A-factory, two major groups of processes can be separated: forming and joining. In the B-factory, three major process steps can be performed: pretreatments, painting and curing. In the C-factory, many different types of assembly are performed.

Cost

Mårtensson has shown that to find a suitable compromise between performance and cost, the question of integrating versus differentiating structures must be addressed when looking at composite lightweight structures. This could also be transferred into multi-material solutions, where differentiating also could indicate different materials in two or more components.

When comparing different materials and different manufacturing techniques, the task of comparing manufacturing cost becomes increasingly more complex. Because tooling costs depend on the component geometry, this needs to be addressed in cost estimations.

General material properties

Materials can have very different qualities and many different properties. Two important factors in the A-factory, where body panels are formed and joined, are tensile strength and yield strength. These parameters explain how much force is needed to permanently alter the shape of the material at room temperature and how much force can be added before the material breaks.

Although there are outliers, metals in general are stronger than polymers in both tensile strength and yield strength. This means that the metals can withstand higher loads before deforming plastically and before rupture. This will affect both forming and joining in the A-factory and assembly in the C-factory.

Two relevant properties when looking at effects on the B-factory are maximum service temperature and coefficient of thermal expansion, due to the painting process. Although all metals have a relatively low thermal-expansion coefficient, the maximum service temperature differs greatly: from under 200°C to over 1000°C.

Looking at polymers, it becomes even harder to draw any conclusions that involve all materials in the family. Some materials have both maximum service temperature and thermal expansion coefficients similar to some metals, while others have very low service temperatures or reasonable service temperatures but relatively high thermal expansion coefficients compared to metals.

Joining

There are four types of joining processes; mechanical, chemical, thermal and hybrid processes.

Traditionally, car bodies have been manufactured in steel and resistance-welded. But when transitioning to joining dissimilar materials, as with a multi-material body, the number of design parameters increases due to an increase in number of relevant material properties. This means the joining process type could need to be revised, for example from welding to flow drill screws or adhesive joining technologies.

Often, there are multiple joining-process types that are possible, although only a few processes are preferable or realistic. The selection of joining method becomes interlinked with material selection and component geometry.

Painting and curing

The whole-surface finishing process, including pretreatments and painting, means that the body is heated for curing several times. The curing occurs by transporting the body through an insulated tunnel, where hot air is used to heat it. Since the body is constantly moving through the tunnel and minimal tunnel time is desired, this process is not guaranteed to heat the entire body to uniform temperature.

Mixed-model assembly lines

It is common to assemble several models or variants of a vehicle on the same line. This is defined as a mixed-model assembly (MMA) line and is characterized by its ability to utilize multi-skilled workers and automated tool changes between different variants of products. Since the variants of vehicles increase with the trend of customization, MMA-lines are required in order to increase capacity utilization.

Conclusions

To cope with new manufacturing challenges related to mass reduction via multi-material design, product development tools and methods need to help design engineers find solutions to previously unknown issues — or issues that have earlier been solved by clear-cut standards or design rules. Product development tools and methods need to be evaluated and possibly improved to identify potential manufacturing issues and solve them early in the design phases.

This article was adapted from SAE technical paper 2016-01-1332 authored by Fredrik Henriksson and Kerstin Johansen of Linköping University.

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