Three-dimensional Analysis And Organization Uniformity Of Semi-solid Extrusion 7075 Aluminum Alloy

As a new type of metal processing, semi-solid processing technology has been widely used in industries such as vehicles, machinery, the military industry, etc. In the production, the organization prone to be inhomogeneous in the process of semi-solid because of the semi-solid slurry is solid-liquid mixture. This inhomogeneity will result in the nonuniform of components property, which has bad influence on the quality of the components. So the study on the organization uniformity of semi-solid forming processing is meaningful. But the existing research datas are based on metallographic pictures, which is deviate from the actual three-dimensional shapes on the distribution of liquid phase and solid phase particles. The situation goes against the precise analysis. Therefore, it is very valuable to explore organization uniformity of semi-solid metal in the method of three-dimensional analysis.This study is based on semi-solid extrusion of the 7075 aluminum alloy. Firstly, through a large number of trials, extrusion process parameters of the research object are determined as follows:the remelting temperature is 590 ℃ and 600 ℃, remelting holding time is 5 min,10 min,15 min, the preheating temperature of the mould is 250℃ and 290 ℃ and the extrusion velocity is 20 mm/s. Then, influence rules of different parameters and different parts of workpieces on the microstructure uniformity is discussed through the experiment, meanwhile, the method to reconstruct three dimensional of the microstructure is explored and quantitative measure the 3d organization of extruded parts.The study leads to the following conclusions.(1) As the prolong of heat preservation time in the semi-solid temperature range, microstructure of the extruded alloy gradually transformed into granular semi-solid microstructure after recrystallization, and the process speed up with the improvement of heating temperature. Complete semi-solid organization can be obtained when the heat preservation time is 10 min and the heating temperature above 590℃. Further analysis of the uniformity of the organization shows that comparatively excellent semi-solid billets can be prepared with the method of heating temperature of 600 ℃ and heat preservation of 5 min.(2) Different process parameters lead to a significant change of non- uniformity of organization, the right controlling of the process parameters can largely reduce the level of organization non-uniform. There are non-uniformity of microstructure in the semi-solid extrusion forming process of 7075 aluminum alloy for both forward extrusion and backward extrusion. Too high or too low of isothermal temperature, isothermal time, mould preheating temperature are detrimental to the organization’s uniformity. Too long heat preservation time, too high heat preservation temperature and mold temperature will make semi-solid billet liquid rate increased, solid phase particles raised abnormally, the number of grain decreased and grain diameter enlarged. Otherwise will make solid rate exorbitant, grains existed in large granular, friction between the larger particles increased during extrusion and large size grain tangled under the pressure, ultimately affected the smoothness of solid-liquid flow seriously. This study indicates that the preferable extrusion organization can be obtained when extruded after keeping warm for 10 min under the 600 ℃ when the mold temperature is 290 ℃.(3) The semi-solid microstructure of 7075 aluminum alloy are reconstructed using serial section 3D reconstruction method of the software Mimics. The result shows the microstructure is a typical near spherulite organization. Three-dimensional reconstruction and 3D quantitative analysis of a single chip’s microstructure shows that the shape factor of a single chip is 0.46, the average equivalent diameter is 24.02mm. The results show that the analysis of semi-solid organization with three-dimensional reconstruction method is feasible.