| Wrought 7075 aluminum alloy with super-high strength is considered as one of important substitute materials to replace steel in the lightweighting automobile industry. So it is important to investigate the preparation process of the high-strength Al alloy for auto parts. Due to its inhomogeneous structure and serious segregation for the alloy, it’s difficult to get qualified components by normal casting method, and defects such as shrinkage and porosity occur more easily in die-casting process as the parameters are chosen improperly. These problems have seriously limited the application of 7075 aluminum alloy in the production of automobile part. So this paper mainly focuses on developing the rheo-diecasting process of 7075 aluminum alloy by spiral electromagnetic stirring.The purification treatment of 7075 aluminum alloy melt by rotation degassing process was studied. Effects of degassing parameters such as rotation speed, degassing time, holding time and degassing temperature on melt quality were studied. The results show that the rotation degassing method can effectively remove the hydrogen in the alloy melt, and degassing effect can be improved as the rotation speed increasing. Prolonging degassing time can reduce the hydrogen content of the melt, which don’t change significantly as the degassing time increased more. The hydrogen content of the degassed melt increases with the holding time, but it increases less and maintains at a low level. The hydrogen content increases sharply with an increase of melt temperature. The optimization parameters are as follows: rotation speed of 300r/min, degassing time of 14 min, holding time of 7min and degassing temperature of 750℃.The effects of spiral electromagnetic stirring on solidification behavior of 7075 aluminum alloy casting were studied and compared with normal electromagnetic stirring and normal diecasting. The results show that, the microstructure with spiral A-EMS is mainly made of fine equiaxed grain, the average grain size is about 76μm, and also composition segregation is alleviated greatly; the microstructure with normal EMS and without EMS, however, the microstructures consist of coarse dendritic grains, the average grain sizes are 94μm and 118μm, respectively, and also composition segregation is serious, especially in case without EMS.The filling and solidification process of the engine connecting rods made of 7075 alloy was numerical simulated by Magmasoft software to provide guidance for parameter optimization. The results show that the computational model in this study can predict the defects in the casting well, and the casting with integrity can be obtained at the pouring temperature of 640℃, extruding speed of 3m/s and mould temperature of 150℃.Rheo-diecasting process by combining spiral electromagnetic stirring and normal diecasting was developed, and the microstructure and mechanical properties of automobile connecting rods manufactured by rheo-diecasting and normal diecasting with the optimized parameters were compared. The results show that, the average grain sizes of rheo-diecast rods and normal-diecast rods are 34μm and 59μm, respectively. In case of rheo-diecasting, microstructure is finer and uniform, and elements segregation(Zn, Mg, Cu) can be remarkably alleviated, and much more hardness of 19 HB can be achieved, and the tensile strength, the yield strength and elongation of automobile connecting rod are 507 MPa, 464 MPa, 4.5%, respectively. However, in case of normal diecasting, tensile strength, yield strength, and elongation of auto rod are 444 MPa, 420 MPa, and 2.5%, respectively. |
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