Study On Several Key Problems Of Lightweight Design For Steel-aluminum Hybrid Structure Car Body And Its Applications

The lightweight of car body plays an important role in the development of lightweight vehicles, and it is one of the most effective methods for improving vehicle fuel economy and reducing harmful emissions. Compared to the traditional car body with mono steel material, the new concept of steel-aluminum hybrid car body structures can better take into account all aspects of requirements for lightweight effect, technology, crash safety, cost, et al. It represents the latest trends of the future development of car body structures. The meaning of steel-aluminum hybrid structure car-body is that for the traditional steel car-body frame structure, some parts are replaced by new lightweight materials, such as high strength steel (HSS), aluminum alloy sheets, to realize the high-strength and lightweight requirements under the premise of controlling the cost rise in a reasonable level. This kind of car-body structure can make full use of price, high strength and stiffness advantages of HSS, as well as lightweight and energy absorption advantages of aluminum alloy sheets. Its core idea is: according to the principle that the appropriate materials are used for the appropriate parts, for each specially selected car-body part, to determine its optimal material and structural dimensions. By adopting the methods of material replacement and structural improvement, this new concept is a typical application to the combination of two basic ways of the car body lightweight, and it completely conforms to the technology roadmap of car body lightweight development.Based on the analysis of the development process and methodology of traditional BIW with mono steel material, this paper proposes the development process and development methodology of the steel-aluminum hybrid materials BIW. Aiming at the matching and optimization problem of steel-aluminum hybrid materials, the theoretical model of combinational optimization of material selection and structural size for steel-aluminum hybrid car body is established and the approximate model-based optimization solution method is further given. Based on the test case concerning the multi-objective optimization for the lightweight and crashworthiness of a single-hat thin-walled beam made of steel-aluminum hybrid materials, three kinds of metamodels including quadratic polynomial, Kriging and radial basis function (RBF) are considered to solve such kind of combinational optimization problem, and their feasibility and suitability are emphatically investigated. The result shows that RBF is more suitable to be used as metamodel to solve the problem concerning material types and sheet thickness.In order to solve the problems of the connection between dissimilar materials in the steel–aluminum hybrid car-body, the experimental research and finite element (FE) simulation analysis are carried out to study the feasibility of joining the sheets of HSS SPFC590 and aluminum alloy A5052-H34 which are both commonly used in car-body by using mechanical clinching. Based on the feasibility of experimental verification, aiming at obtaining high-quality joints with optimal properties in shear and tensile strength, the multi-objective optimization for the key geometric parameters of the clinching tool is performed by comprehensively using the methods of experimental design, statistical analysis, Meta-modeling of response surface method (RSM) and genetic algorithm (GA). The goal is to maximize both of the neck thickness and undercut value of the clinching joint. The result of solution set for Pareto front is obtained, which can provide engineers with a wide range of possible options. Taking into account the failure mode in the experiment, the relative optimum schemes are recommended.By taking the crashworthiness of a single-hat thin-walled beam as a numerical example, the energy-absorption characteristics of several popular brands of HSS and aluminum alloys are analyzed under the condition of same mass and crushing length. The results show that the aluminum alloys can absorb more energy than HSS with the same mass. Then the crashworthiness of the S-shaped rail extracted from the frontal frame in a car is studied. In order to reduce the peak impact force while increasing the total absorbed energy, the hybrid materials are employed in that rail, where aluminum alloy is used for its front part and advanced high strength steel (AHSS) for its back. By designing 16 experiments based on orthogonal experiment, the effects of five in?uence factors with four levels on the crash performance of the steel–aluminumhybrid S-shaped front rail are emphatically investigated. These in?uence factors include the different material types of aluminum alloy and advanced high strength steel (AHSS), the sheet thicknesses of the two parts, and length proportion for the aluminum part. The research result shows that the use of steel–aluminum hybrid materials can reduce the peak impact force and the total weight for the S-shaped front rail, while the total absorbed energy can be greatly increased, so the crashworthiness and lightweight of the S-shaped front rail are significantly improved.Based on the regulation of Euro NCAP 40% offset deformable barrier frontal crash test, the crash simulation model of a SUV is established. Through the real car crash test, the car-body deformation mode and the vehicle collision acceleration are compared with that of the simulation model, and the reliability of the FE model is verified. In order to reduce the computing time of single FE simulation, the simplified model of the vehicle front-end structures is established to replace the whole vehicle time-consuming FE model. The steel-aluminum hybrid front rail is used in that simplified model and its crash properties are analyzed. Based on the front rail made of steel-aluminum hybrid materials, the theoretical model of multi-objective optimization is established by choosing the steel-aluminum hybrid material types and the sheet thicknesses of the bumper, crash-box, cross-member of the subframe, and the back part of the front rail as the design variables. The mathematical model is based on the uniform experimental design and the RBF approximate model, and the considered performance include the total mass, total absorbed energy, torsional stiffness of BIW, peak impact force tested from the back part of the front rail. The simulation results are analyzed by applying the multi-objective optimization scheme in the 40% offset crash FE model, the results show that the optimized steel-aluminum hybrid materials can improve the vehicle crash safety, meanwhile the car-body lightweight level can also be significantly enhanced, and ultimately the total mass of the research objects are reduced by 29.1%.