| 7075aluminum alloy with high strength is widely used in the aerospace, aviation and military industry. Due to the high content of alloy elements, the solidification structure is non-uniform and coarse and alloy elements segregation is serious, resulting in being craze when casting and hot rolling, which seriously limits its use and development. By applying physical fields on alloy melt, mandatory uniform solidification technology can promote the melt flowing and shearing and make the temperature field, component field and tiny heterogeneous particles uniformly distribute in the melt, which will make the alloy melt overall nucleate, effectively refine the grain and reduce segregation. So this paper was concerned on the technology and application research on mandatory uniform solidification of7075aluminium alloy.We studied the effect of twin-screw stirring process on the melt physical fields and solidification structure of7075aluminum alloy. The twin-screw stirring process can make the melt flowing rate and shearing rate very high, the temperature and composition fields very uniform, and the effect of mandatory uniform solidification very well. The optimal process parameters of twin-screw stirring were the intensity of100rpm, the stirring time of60s and the pouring temperature of650℃. Under this process condition, the7075alloy grain was very uniformly fine and the average grain size was55μm. The alloy chemical composition segregation greatly abated. The segregation rates of Zn, Mg and Cu elements were0.71%,0.87%and2.1%, respectively. The deficiencies of twin-screw stirring process were that the stirring device was easy to be corroded and expensive to be maintained, and it cannot process high temperature alloy (melting point higher than1000℃). So it is not suitable for mass production of parts, and it is difficult to realize industrialization production.The electromagnetic stirring has the advantages of high density and cleanliness of energy, superior responsiveness and controllability, easy automation, high energy utilization rate, and etc. So it obtains more widely commercial application. We studied the effect of the annular electromagnetic stirring process on the physical fields and solidification structure of7075aluminum alloy. Compared with the conventional electromagnetic stirring process, annular electromagnetic stirring process can avoid the lower part of magnetic induction intensity, alternate the disadvantage of electromagnetic field skin effect into advantage and make full use of the advantage of high magnetic induction intensity on the skin effect layer. Also it increased the cooling area of the melt center and made the melt temperature and composition fields more uniform. But because of the intrinsic characteristics of electromagnetic stirring, electromagnetic force in the melt showed the circumferential distribution. The melt flow was also the circumferential movement. The circumferential flow was strong, but the axial flow and radial flow were weak. It was inevitable that the temperature gradient and composition gradient distributed along the radial and axial part.On the basis of the above research, this paper presented the spiral electromagnetic stirring technology which could overcome the shortcomings of mechanical stirring for easy pollution and difficult application. Under the condition of spiral electromagnetic stirring, the flow velocity and shearing rate of melt increased significantly, the temperature gradient significantly decreased, and the turbulence of melt flow became more intensive. The optimal structures of the screw were:the screw diameter of40mm, the screw thread height of12mm, the screw thread angle of65°and the screw thread pitch of55mm. The optimal process parameters of spiral electromagnetic stirring were:the stirring current of30A, the stirring frequency of50Hz and the pouring temperature of650℃. And under this process condition, the7075alloy solidification microstructure became more uniformly fine, and alloy chemical composition segregation greatly abated. The segregation rates of Zn, Mg and Cu elements were1.5%,3.3%and3.4%, respectively.On the basis of the above experiment research, it was further understood that the effect of mandatory uniform solidification technology on the solidification behavior of alloy melt. It was established that the description of controlled nucleation and growth model under the action of mandatory uniform solidification. Mandatory uniform solidification technology can effectively improve the homogeneous nucleation rate and heterogeneous nucleation rate of the alloy melt, improve the survival rate of the nuclus, make the nucleation outbreak in the whole melt, and reduce or even eliminate the undercooling region in the forefront of alloy solid-liquid interface. Finally, it limited the dendrite growth and made the solidification microstructure become fine equiaxed grains.Finally on the basis of the above research, the application of the spiral electromagnetic stirring technology on rheo-diecasting was carried on. The results showed that the spiral electromagnetic stirring technology can well applied on the alloy rheo-diecasting forming. Compared with common diecasting forming, under the condition of spiral electromagnetic stirring, the microstructure of rheo-diecasting part distributed uniformly fine and showed little difference in different position. Brinell hardness of rheo-diecasting parts increased significantly, and also showed little difference in different positions. The solidification microstructure and mechanical property were similar with that in the condition of twin-screw mechanical stirring, but the spiral electromagnetic stirring technology obviously increased the production efficiency. In order to overcome the shortcoming of spiral electromagnetic stirring technology for the low automation degree, it was designed that automatic spiral electromagnetic stirring technology prototype, which had the advantages of automatic collection, automatic processing and automatic pouring and laid the foundation for the ultimate realization of industrial production. |
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