Study On The Technique And Simulation Of The Thixo-Forming Of Aluminum Alloy

Semisolid metal forming (SSM or SSF) has an extensive applications in many industries, especially in the automobile industry and higher theoretical researching value as the formed parts possess more compact microstructure, less entrapment of gas and better mechanical properties, meanwhile it belongs to net shape forming and advanced materials manufacturing technique, and it also strides many fields, such as heat transfer, fluid mechanics, materials mechanics and computer mathematics etc.A356 aluminum alloy is the most suitable materials for the semi-solid forming and for the reduction of vehicle weight because it has a series of advantages, such as less density, higher strength to mass ratio and easer reutilization etc. According to the requires in the thixoforming of aluminum alloys, the billets must be heated quickly to the semi-solid state with less oxidation, more homogenous and precise temperature field, finer and rounder primary -Al grain, so that, only the induction heater can satisfy these strict conditions among several reheating methods.In this dissertation, combining with the experiments of induction heat and measurement, the finite element simulation has first been used to study in detail the effect of process parameters, such as heating power, frequency, heating time, soaking time and the size of billets etc, on the temperature fields in the heated billets of aluminum alloys, then the quantitative metallography, differential scanning calorimeter and thermo-calculation method have been applied to analyze the content of solid phase in billets at semi-solid state, meanwhile the coarsening and spherecizing mode of the primary a-Al grain during the induction heating and soaking have been studied by self-developed image analysis software. Based on these analyses, the reasonable process and parameters of induction heating of aluminum alloys can be acquired. Secondly, the evolution of the apparent viscosity of the semi-solid slurries under different cooling rates has been investigated with the Couette viscometer. In the following, the characteristic of mould in semi-solid die casting, especially the size and shape of the gate has been studied with finite element simulation and semi-solid die castingexperiments. On basis of the above researched results, the effects of process parameters of the semi-solid die casting, such as, shot speed and pressure, time of intensifying pressure, billet temperature, mould temperature and lubricant etc on the microstructure and properties of the die castings of aluminum alloy have been studied systematically, and the fracture mechanism of a part of the die castings has also been investigated with the scanning electron microscope (SEM). Thirdly, the die castings of A356 aluminum alloy have been solution treated and aged under series of conditions, and then the quantitative metallography and transmission electron microscope (TEM) are applied to analyze the causes that the properties of semi-solid die castings of aluminum alloy are superior to that of the liquid die castings and the related heat treatment strengthening mechanism. Lastly, the runner systems of the complex mould in semi-solid die casting, such as the moulds of producing the automobile parts of rear supported seat and inner sleeve etc, have been analyzed with finite element simulation, then batch of the complex parts have been produced by semi-solid die casting, the analysis of microstructure and properties, and bench test of the semi-solid formed parts have been conducted. The following conclusions can be obtained from all the above experiments and analyses.1. It is shown from the results simulated with modified commercial ANSYS finite element software that the model presented in the dissertation can effectively simulate induction heated temperature field in the billets of aluminum alloys with different sizes and kinds, so it is convenience to optimize the induction heating process and parameters of aluminum alloys by means of the simulation.2. The precision and homogeneity of temperature fields he...