Innovative Structural Materials

A Japanese national project to develop technologies for multi-materialization was launched in October 2013, to reduce significantly the weight of transportation equipment. The Innovative Structural Materials Association (ISMA) was established as an organization to promote the project of “Research and Development of Innovative Structural Materials” for a period of 10 years, and was disbanded in March 2023 after demonstrating a 50% weight reduction of automobile bodies through innovative technologies. In this project, world-leading innovative results were achieved in a total of seven fields, ranging from the development of innovative materials, analytical evaluation technology to ensure material reliability, computational science to guide material performance, joining technologies necessary for manufacturing, design technology essential for product realization, and the recycling and Life Cycle Assessment (LCA). “Innovative Structural Materials for Multi-Materialization” is designed to pass on the science and technology to the next generation of engineers and researchers for further development. The main results of this project are introduced with a brief description of the basic knowledge of each field so that readers unfamiliar with this field can understand materials research. We also expect that it will be used as a reference in various fields as a precedent for multi-materialization in the future.

It is well known that the trade-off relationship exists between strength and ductility in metallic materials. We developed technologies to obtain materials with both high strength and high ductility: innovative steel sheets with tensile strength of 1.5 GPa and elongation of 20% and high-strength innovative 5000 series and 6000 series aluminum alloys with Sc precipitates. For innovative magnesium alloys, we have completed the evaluation of a prototype hermetic fatigue test structure for a full-size (5 m long) high-speed rail car using a flame-retardant magnesium alloy. For innovative titanium alloys, we developed innovative refining and manufacturing processes to reduce production costs. Thermoset carbon fiber reinforced plastic requires heating and curing in a high-temperature autoclave for several hours, resulting in an increase of the production costs and limited use in automotive applications. The LFT-D (Long Fiber Thermoplastics-Direct) process, in which thermoplastic resin and relatively long carbon fiber are mixed and pressed at high speed, was adopted to develop a high productive manufacturing process, and trial prototypes of chassis and floor panels were manufactured on trial. We succeeded in introducing innovative carbon fibers, by developing new precursor compounds to get flame resistant polymer threads for pre-carbonization, and also by developing new microwave carbonization process technology.

Materials integration (MI) is an approach that links the four elements of materials, Processing, Structure, Properties and Performance, on a computer by integrating experiments, calculations, theory and data science. MI is a data-driven type of materials development, in which all calculations from the initial inputs of materials, processing and use conditions, to the final output, i.e., the performance (life prediction, failure probability, etc.) of the member of interest, are performed instantly and in an integrated manner by connecting multiple computational modules so that the output of one module becomes the input of the next module, and automating the exchange of that data, to realize substantial reductions in the cost and time required for materials development. Although the term “materials informatics” is sometimes used as a synonym for “materials integration,” much of the research classified as materials informatics gives the impression of a search for materials focused on their structures and properties. However, in materials integration, treatment that links the four above-mentioned elements of materials is essential. The technical overview of materials integration and efforts related to MI in the Project are described in detail in Chap. 3.

As mentioned in Chap. 2, the Project has been conducting materials development of ultra-high strength steel sheets of medium- and high-carbon steel, aluminum alloys, magnesium alloys, titanium alloys, CFRTP and other materials that will contribute to reduction of the car body weight. At the same time, it has been also developing welding and joining methods for ultra-high strength steel sheets and 3 combinations of dissimilar materials: steel to aluminum alloy, aluminum alloy to CFRTP, and steel to CFRTP. In this chapter, 6 types of joining processes, which are welding, brazing, friction joining, interface-melt joining, adhesive bonding and mechanical fastening, are described for multi-material structure car body consisting of the newly-developed materials and existing materials. The joint strengths of the feasible joining processes developed are mainly focused together with the target values for joining of ultra-high strength steels and dissimilar materials respectively.

The knowledge on steel corrosion, galvanic corrosion of dissimilar materials, hydrogen embrittlement of steel, and nondestructive testing techniques in automobiles is summarized, and an overview of the issues and research results related to them in this project is given. Chemical and physical stresses during vehicle use cause various degradations. In particular, corrosion and hydrogen embrittlement are difficult to evaluate. The outlines of the developments on this project including electrochemical measurement techniques on the surfaces of microstructures of 1.5 GPa high strength steel containing high carbon, galvanic corrosion of multi-material automobile bodies, standard test methods for evaluating hydrogen embrittlement, and analysis techniques for crack propagation morphology were presented. Nondestructive testing is positioned to eliminate defective products in the process and to estimate the remaining life of the product. In the production process, it is desirable to be able to eliminate defective products online or in-line without affecting the product, and it is necessary to introduce equipment that can identify defective parts nondestructively without relying on human labor. In addition, to estimate the remaining life expectancy, studies to correlate various measurements with life expectancy are needed, and it is expected that many producers will cooperate in this research in many cases.

Multi-materials structures, in which various materials are arranged by selecting the optimum material for the optimum part, are indispensable for weight reduction of automobiles and other transportation equipment. However, integrated technology development, beginning with the establishment of design techniques for optimization of the multi-materials structure, including modeling of joints of various materials, is urgently required. In the Project, a design tool utilizing topology optimization, which allows changes in topology (shape and hollow parts) was developed and applied to actual multi-materials design. Details concerning these design technologies are described in this chapter, “Structural Design Technologies.”

To confirm the potential for practical application of the innovative materials and innovative joining technologies developed in this project, we examined the performance of the requirements for practical application, such as formability, joining, coating, and rust prevention, through trial production of parts. Specifically, innovative steel sheet was used for the A-pillar, tailored blank fabricated using FSW for the B-pillar outer and inner panels, innovative aluminum for the side member and sill reinforcement member, innovative magnesium for the hood, CFRP/CFRTP panels for the roof, and LFT-D material for the floor, prototypes and evaluations were conducted by applying aluminum and CFRTP dissimilar material joining to the doors. Crash analysis at the actual vehicle level was conducted, and it was confirmed that the performance was equivalent to that of the base vehicle. In summary, the results of the evaluation of the component prototypes indicated that the innovative materials and innovative joining technology have high potential for practical application.

Recycling and Life Cycle Assessment (LCA) are important issues for future materials development. Here, in the narrow sense, “recycling” means materials recycling, and is a recycling method in which waste is reused as the raw material for new products. Development of recycling technologies for aluminum alloys and CFRP products was carried out as part of the Project. In substitution of automotive materials for auto body weight reduction, it is necessary to consider the effects on society as a whole. In the Project, we created an LCA model with a system boundary extended spatially and temporally, which can evaluate the environmental, social and economic impacts in Japanese society as a whole up to the year 2050, together with the necessary database, and developed an evaluation tool that demonstrates the model and its database. Technology development related to recycling and LCA is described in detail in this Chap. 8, “Recycling and Life Cycle Assessment (LCA).”

In this chapter, we will discuss the management of projects that will lead to future prospects, including methods for evaluating the progress of project tasks and methods for accumulating, managing, and utilizing project data and other information that will lead to future development. The evaluation method is the use of the Technology Readiness Level (TRL), and the use of data is the establishment of center functions that connect technical research institutes in each field.