| In recent years, the application of lightweight material in transportation vehicles is a main development direction. Aluminum alloys exhibit low density, high strength-to-weight ratio, good corrosion resistance and good recyclability and are regarded as an important lightweight material. However, the formability of aluminum alloys panel is limited at room temperature, and hot stamping technology of aluminum alloys can enhance formability of material and produce complex-shaped components.Hot stamping technology is employed to manufacture aluminum alloy parts effectively and efficiently. During hot stamping process, the blank was firstly heated to solution heat treatment temperature and soated at this temperature for a period of time. Then, the hot blank was moved to cold dies for deformation. The part was quenched in the tools, followed by ageing process to improve the strength. There are many science and engineer topics in this relatively novel technology, such as viscoplastic characteristics and deformation behaviour at elevated temperature, effect of hot stamping on mechanical property of material, and influence of process parameters on formability of hot stamped parts. Thus, the research on the law of formability and property for hot stamping and process optimisation were conducted in this paper.Thermal deformation tests of AA6082 aluminum alloy were conducted by thermal-mechanical Gleeble simulator to investigate the thermal deformation behavior. The mechanism for deep-drop stress was studied. Through observing the surface of specimen near crack and fracture morphology, it is revealed that the damage and fracture mechanism is micro-void nucleation, growth and coalescence, which leads to the final crack. Based on this observation and corresponding damage and crack mechanism, a damage evolution equation was developed, considering strain, strain rate, deformation temperature and so on. The equation was then implemented into the dislocation-based unified viscoplastic constitutive equations, to establish a damage constitutive model. Material constants in the model were determined by genetic algorithm tool. The model can accurately present the thermal behavior of AA6082 and deep-drop part of flow stress.The influence of hot deformation and strain rate on the part quality was investigated using thermal-mechanical simulator machine. The property evolution during hot forming process was analyzed. And the property variation of the hot-formed parts in ageing process was measured. Then, a strengthening model was developed based on strengthening theory. The model can predict the part hardness for various strains, strain rates and ageing periods.The effect of solution heat treatment, quenching and ageing were studied, based on hot stamping process to further enhance the understanding on the law of property of hot stamping aluminum alloy. It mainly focused on the influence of solution temperature and time, quenching method, ageing technique on fracture morphology and final quality of specimen. As the controversy between ultimate tensile strength and ductility exists, the multi-objective genetic algorithm NSGA-II was employed for optimization, to obtain optimal process parameters.The tool of hot cup drawing was machined and a corresponding finite element model was established. The experiments and simulations of hot cup drawing were carried out. The forming defects and damage crack mechanism were analyzed. The influence of process parameters on the minimum thickness and thickness deviation of hot-stamped part was quantified, using Taguchi method and mathematical statistics. Blank holder force (BHF) and friction coefficient have significant effect on minimum thickness, and the influence contribution is about 35.5% and 34.9%, respectively. The punch velocity affects thickness deviation markedly with of influence contribution of 40.4%. Then, further researches were carried out on the effect of BHF, friction coefficient and punch velocity. In the following part, the influence of BHF, friction coefficient and punch velocity on thickness distribution, minimum thickness, thickness deviation, strain path and failure mode was detailed analyzed. By using user-defined subroutine, the model can be implemented into the main program of the FE (finite element) software via VUMAT (Variable User MATerial), to predict the property evolution for hot stamping of aluminum alloy.The formability of hot stamping an automotive panel part was analyzed in this paper. A corresponding FE model was established. Stress distitution, strain distribution, thickness distribution, forming precision and thinning for the hot-stamped automotive panel were analyzed using numerical simulations. In order to solve defects occurred in aluminum alloy hot stamping, such as thinning and forming precision error, response surface method along with central composite design was employed to analyze the effect of two forming factors, namely initial blank temperature and blank holder force (BHF) on the response values, namely, maximum thinning rate and maximum forming precision error. Results show that maximum thinning rate increases with increasing initial blank temperature and BHF; maximum forming precision error decreases with increasing initial blank temperature and BHF. Conflict exists between thinning and forming precision error. Multi-objective optimization was conducted to get Pareto optimal solutions. Both maximum thinning rate and maximum forming precision error value were kept at the low level, and optimal ranges of process parameters were determined. The UG software was used to design the hot stamping tool for an automotive panel part. Then, the machine program was genetated, modified and input into a D165 high-speed milling machine center, which was used to machine the surface of the tool. After assembling hot stamping tool, the hot stamping experiments of the automotive panel part can be conducted. Real experimental results show that the reasonable process parameters can avoid crack and decrease forming precision error. Good agreements between simulated and experimental results are performed. The part was formed with good forming precision and quality. The research and analysis on hot stamping of automotive panel part of aluminum alloy can provide theoretical reference for hot stamping other panel components. |
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