Investigation On Reheating Process And Mechanical Model Of Aluminum Alloys During Semi-solid Forming

Semi-solid billets of6061and A356aluminum alloys were prepared by liquidus semi-continuous casting and vertical electromagnetic continuous casting. A novel two-stage reheating process for thixoforming has been developed in this work to investigate evolution mechanism of the semi-solid microstructure during reheating process. It turns out that the semi-solid microstructure with a refined equivalent diameter, better spherical degree, lower coarsening rate constant of solid grains and reduced possibility of defect caused by entrapped liquid can be obtained by two-stage reheating process in comparison with traditional reheating process. Uniform, fine and spheroid microstructure with favourable thixotropic behavior for6061wrought aluminum alloy and A356casting aluminum alloy were obtain using this two-stage reheating process. Meanwhile, it provides experimental foundation for establishment of representative volume element (RVE) in macro-micro constitutive modelling for semi-solid alumimum alloys.Based on the empirical relationship, a two-phase law incorporated the influence of liquid phase has been presented by inducing an adjustment factor related liquid. In order to further consider the macroscopic deformation mechanism and the microstructure evolution of solid and liquid phases, a macro-micro semi-solid modeling has been proposed to model the thixotropic behavior more comprehensively during the semi-solid forming process. This model can then be used to investigate the global thixotropic behavior and the local effect of solid and liquid phase in the semi-solid aluminum alloy. Thixotropic compression tests on semi-solid billets of6061aluminum alloy were carried out using Gleeble3800dynamic material testing machine. Three-stage deformation behavior on stress-strain curves including strain hardening, strain softening and steady-state were addressed. Dynamic mechanical analysis were performed to find out the elastic modulus at different temperatures. Based on above experimental results, macro-micro semi-solid modeling, along with empirical relationship and two-phase law, was verified by means of secondary development of finite element software ABAQUS platform. It turns out that the theoretical results of macro-micro semi-solid modeling show a better agreement with the experimental results than the results of the other two. The deviations among these three constitutive laws become more obvious as the liquid fraction increases in the semi-solid aluminum alloy. Macro-micro semi-solid modeling approach proposed in this study is more accurate and has a wider scope of application compared to empirical relationship and two-phase law.According to the experiment results of the stress-strain curves at different temperatures and strain rates, empirical constitutive equation of6061semi-solid aluminum alloy was established by multiple regression. The relationship between apparent viscosity and shear rate was investigated in this work, and it was proved that power law can be applied to predict the flow behavior of6061semi-solid slurry. The parameters acquired in numerical simulation, such as apparent viscosity and the shear rate at different temperatures, were identified by viscosity model. It provids theoretical guidance for precisely determining the process paramerers during the semi-solid die-casting. The commercial simulation software AnyCasting was applied to optimize die design and process parameters during the semi-solid die-casting process of semi-solid automobile engine bracket. The effects of the thickness of ingate, filling speed and filling temperature of semi-solid aluminum alloy during semi-solid die-casting process were analyzed. The filling characterization of semi-solid slurry and casting defect were studied as well.