Lightweight is one of the core technologies to realize energy saving, to improve the safety and driving performance of vehicles, and to provide added passenger comfort as well. It is of special significance for increasing cruising range of electric or hybrid-powered vehicles. Beyond these, lightweight components use less mass and hence reduce the primary energy requirement imbedded in the component. Therefore, lightweight has always been a central topic and an important direction in the automotive field whether in the past or in the future.

Lightweight design is one of the current key drivers to reduce the energy consumption of vehicles. Design methodologies for lightweight components, strategies utilizing materials with favorable specific properties and hybrid materials are used to increase the performance of parts for automotive applications. In this paper, various forming processes to produce light parts are described. Material lightweight design is discussed, covering the manufacturing processes to produce hybrid components like fiber–metal, polymer–metal and metal–metal composites, which can be used in subsequent deep drawing or combined forming processes. Approaches to increasing the specific strength and stiffness with thermomechanical forming processes as well as the in situ control of the microstructure of such components are presented. Structure lightweight design discusses possibilities to plastically form high-strength or high-performance materials like magnesium or titanium in sheet, profile and tube forming operations. To join those materials and/or dissimilar materials, new joining by forming technologies are shown. To economically produce lightweight parts with gears or functional elements, incremental sheet-bulk metal forming is presented. As an important part property, the damage evolution during the forming operations will be discussed to enable even lighter parts through a more reliable design. New methods for predicting and tailoring the mechanical properties like strength and residual stresses will be shown. The possibilities of system lightweight design with forming technologies are presented. A combination of additive manufacturing and forming to produce highly complex parts with integrated functions will be shown. The integration of functions by a hot extrusion process for the manufacturing of shape memory alloys is presented. An in-depth understanding of the newly developed processes, methodologies and effects allows for a more accurate dimensioning of components. This facilitates a reduction in the total mass and an increasing performance of vehicle components.

Martensitic steels are widely used in the automotive lightweight application but less understood in aspects of post-forming material properties. The steels show good ductility in roll forming but occasionally experience delayed (hydrogen) fracture issues, which are believed to be due to the formation of localized residual stress and a reduced product of strength and elongation. To characterize the effect of roll forming process on the formation of residual stress and material properties variation of martensitic steel components, this paper investigates the forming-induced longitudinal residual stress and material property variation in a roll-formed high-strength MS1180 automotive rocker panel. The finite element analysis results for residual stress are validated by neutron diffraction measurements. The numerical model is used to analyze the full evolution of residual stress during the roll forming process and the effect on material properties with major focus on the product of strength and elongation. It is found that the flower design, in particular the overbending stages, play a significant role in the formation of residual stress and the change in material properties. The product of strength and elongation is significantly reduced across the profile, in particular in the corners. The achieved understanding will assist researchers comprehend the material properties of roll-formed component and therefore assist future studies aimed at preventing the occurrence of hydrogen fracture.

The 5754 aluminum alloy has been widely used in the automotive industry to reduce the weight of vehicles. The weld-bonding (WB) process comprising resistance spot welding and adhesive bonding processes effectively improves the mechanical properties of joints. However, it is still a great challenge in the WB process to obtain high-quality and defect-free nuggets of aluminum alloys. In this study, the parameters of the WB process are optimized and the mechanism of generation of defects during WB is analyzed. The results show that the welding parameters have a significant effect on the nugget sizes, among which the welding current plays the most important role. The residual adhesive can easily cause defects during welding, e.g., expulsion and porosity in the nugget. This can be effectively avoided by optimizing the welding parameters. In addition, the gas in the joints is effectively reduced by adding an appropriate preheating pulse prior to welding, thus lowering the damage degree of the adhesive layer. As a result, welded joints with better weld nugget quality and more stable mechanical properties are obtained.

In industrial production, the standoff distance of magnetic pulse welding (MPW) is a critical parameter as it directly affects welding quality. However, the effects of standoff distance on the physical properties of MPW joints have not been investigated. Therefore, in this study, aluminum alloy (AA5182) sheets and high-strength low-alloy steel (HC340LA) sheets were welded through MPW at a discharge energy of 20 kJ, under various standoff distances. Thereafter, mechanical tests were performed on the MPW joints, and the results indicate that there is a significant change in the shear strength of the AA5182/HC340LA-welded joints with respect to the standoff distance. When the standoff distance ranges from 0.8 to 1.4 mm, the strength of the joint is higher than that of the base AA5182 sheet. Microscopic observations were conducted to analyze the interfacial morphology, element diffusion behavior, and microdefects on the welding interface of the AA5182/HC340LA joints. The AA5182/HC340LA joint with a standoff distance of 1.4 mm possesses the longest welded region and the largest interfacial wave. This interfacial wave pattern is suitable for achieving MPW joints with high shear strengths.

Self-piercing riveting (SPR) is a mature method to join dissimilar materials in vehicle body assembling. Friction self-piercing riveting (F-SPR) is a newly developed technology for joining low-ductility materials by combining SPR and friction stir spot welding processes. In this paper, the SPR and F-SPR were employed to join AA6061-T6 aluminum alloy and AZ31B magnesium alloy. The two processes were studied in parallel to investigate the effects of stack orientation on riveting force, macro-geometrical features, hardness distributions, and mechanical performance of the joints. The results indicate that both processes exhibit a better overall joint quality by riveting from AZ31B to AA6061-T6. Major cracking in the Mg sheet is produced when riveting from AA6061-T6 to AZ31B in the case of SPR, and the cracking is inhibited with the thermal softening effect by friction heat in the case of F-SPR. The F-SPR process requires approximately one-third of the riveting forces of the SPR process but exhibits a maximum of 45.4% and 59.1% higher tensile–shear strength for the stack orientation with AZ31B on top of AA6061-T6 and the opposite direction, respectively, than those of the SPR joints. The stack orientation of riveting from AZ31B to AA6061-T6 renders better cross-section quality and higher tensile–shear strength and is recommended for both processes.

To analyze the rollover safety, finite element models were established for the electric bus body frame, rollover simulation platform, living space, and bus rollover. The strength and stiffness of the body frame were calculated under four typical working conditions considering the main low-order elastic modal characteristics. The results indicate that the initial body frame of the electric bus satisfies the required structural strength, stiffness, modes, and rollover safety, and it has great potential for lightweight design. Sensitivity and structural contribution analyses were performed to determine the design variables for lightweight optimization of the body frame, and a mathematical model was established for multi-objective collaborative optimization design of the electric bus. Then, the radial basis function neural network was used to approximate the optimization model. Besides, the accuracy of the approximate model was verified, and the non-dominated sorting genetic algorithm II was employed to determine solutions for the lightweight optimization. Compared with the initial model, the mass of the optimized model is reduced by 240 kg (9.0%) without any changes in the materials of the body frame.

Lithium-ion batteries (LIBs) are widely employed in electric vehicles owing to their high power density, long cycle life, and environmental friendliness. However, LIBs are hazardous in the event of a crash, leading to thermal runaway. In this study, the basic structure of a battery module is analyzed to improve the crashworthiness of LIBs. A simplified finite element model of the battery module structure, which is a battery unit composed of two pouch cells and a cooling fin, is set up and verified by conducting module-level simulations. The simulation results reveal that the cooling fin in the battery module has the potential to absorb energy. Six sandwich configurations are introduced to modify the cooling fin. With a unidirectionally stiffened double hull USDH structure serving as an example, a parametric analysis is conducted, demonstrating that the sandwich height does not influence the areal density; a small height of 3 mm can make the material work sufficiently while avoiding early buckling of the structure. Further, the crashworthiness of different sandwich configurations with the same areal density and height is compared, leading to three deformation modes. USDH and circular core structures are found to be able to effectively reduce the peak force and improve the energy absorption ability.

Lightweight is an effective design strategy to conserve energy in automotive vehicles. It is a big challenge to evaluate the level of lightweight for passenger cars. This paper summarizes various evaluation methods for lightweight automotive vehicles. A lightweight index [Lv for internal combustion engine vehicles (ICEVs) and Lev for battery electric vehicles (BEVs)] is proposed to assess the lightweight of passenger cars. The proposed lightweight index is composed of the nominal density, weight-to-power ratio, and fuel consumption of footprint area (in the case of ICEVs) or electricity consumption of footprint area (in the case of BEVs). The validity and universality of the proposed lightweight index are demonstrated through a statistical analysis of 7018 ICEV and 326 BEV models. The calculation procedures of the standard partial regression coefficients of statistical multiple regression and elastic coefficients are employed in the proposed method. The results show that either Lv or Lev is most sensitive to the curb mass of the vehicles. The proposed lightweight index can help guide automakers in setting reasonable weight reduction targets during new product development. In addition, the proposed lightweight index can be applied to new hybrid electric vehicles with further efforts, to facilitate the development of lightweight automotive design.

The design of hybrid structure offers an attractive solution to enhance strength and structural stiffness as well as to achieve lightweight effect and cost reduction. The applications of steel–FRP (fiber-reinforced polymer) composites in transportation and civil engineering have been comprehensively reviewed. In order to apply hybrid structures to car body parts such as B-pillar, flexural performance of steel–FRP composites is investigated by means of three-point bending test in this study. An analytical model is deduced to calculate the initial stiffness, the bending load and the energy absorption of steel–FRP composites. Steel–CFRP (carbon fiber-reinforced polymer) and steel–AFRP (aramid fiber-reinforced polymer) composites are experimentally studied and discussed. The results demonstrate that the steel–FRP composites exhibit significantly higher load-carrying capabilities and initial stiffnesses along with larger energy absorptions in the bending process compared to the single steel sheet.