The cyclic extrusion compression （CEC） process was introduced into the AM60B magnesium alloy. The use of the CEC process was favorable for producing finer microstructures. The results show that the microstructure can be effectively refined with increasing the number of CEC passes. Once a critical minimum grain size was achieved, subsequent passes did not have any noticeable refining effect. As expected, the fine-grained alloy has excellent mechanical properties. The micro-hardness, yield strength, ultimate tensile strength and elongation to failure of two-pass CEC formed alloy are 72.2, 183.7 MPa, 286.3 MPa and 14.0%, but those of as-cast alloy are 62.3, 64 MPa, 201 MPa and 11%, respectively. However, there is not a clear improvement of mechanical properties with further increase in number of CEC passes in AM60B alloy. The micro-hardness, yield strength, ultimate tensile strength and elongation to failure of four-pass CEC formed alloy are 73.5, 196 MPa, 297 MPa and 16%, respectively.
As a brazing foil, 4004 A1 alloy has good welding performance. However, the high Si content decreases the plasticity of the alloy. In order to improve the plasticity of 4004 A1 alloy and subsequently improve the productivity of 4004 AI foil, 4004 A1 alloy was modified by RE-Ba-Sb. As a comparison, the 4004 A1 alloy was also modified by RE with different addition. The tensile properties of the alloy reach the best when the addition of RE was 0.2%, in which the tensile strength and elongation were 194 MPa and 5%, respectively. For RE-Ba-Sb modification, the addition of three elements was optimized by orthogonal analysis. The results showed that the greatest impact parameter of RE-Ba-Sb modification was RE addition, followed by addition of Ba and Sb. The optimum addition amounts of RE, Ba and Sb obtained by orthogonal analyses were 0.01%, 0.3%, and 0.05%, respectively. The tensile strength and the elongation of 4004 A1 alloy modified by the optimal modification process were 224 MPa and 6%, respectively. The amount of RE addition in RE-Ba-Sb modification is lower than that in RE modification.
For thixoforming to be possible, the microstructure of the starting material must be non-dendritic, which can be ob- tained by the strain induced melt activation （SIMA） route. Based on the SIMA route, as-cast AZ91D alloy with the addition of yttrium was deformed by cyclic closed-die forging （CCDF）. Microstructure evolution of CCDF formed AZ91D-RE alloy during partial remelting were investigated. Furthermore, the mechanical properties of thixoformed AZ91D-RE magnesium alloy components were also studied. The results showed that prolonged holding time resulted in grain coarsening and the improvement in degree of spheroidization. The coarsening behaviour of solid grains in the semi-solid state obeyed Ostwald ripening mechanism. The coarsening rate constant of CCDF formed AZ91D-RE during partial remelting was 324 um3/s at 550 ℃. The value of yield strength, ultimate tensile strength and elongation to fracture of four-pass CCDF formed AZ91D-RE magnesium alloy were 214.9, 290.5 MPa and 14%, respectively. Then the four-pass CCDF formed alloys were used for thixoforming. After holding at 550 ℃ for 5 min, the values of yield strength, ultimate tensile strength and elongation to fracture of thixoformed component were 189.6 MPa, 274.6 MPa and 12%, respectively. However, prolonged holding time led to remarkable decrease in mechanical properties ofthixo- formed components.
In order to improve the plasticity of 4004 AI alloy and subsequently the productivity of 4004 AI foil, the research studied in detail the influence of the rotary impeller degassing process on the refining effect of 4004 AI alloy, in which the impacts of four major parameters： gas flow, rotational speed, refining time, and stewing time, on degassing rate of 4004 AI alloy was systematically studied by using an orthogonal experiment methodology. Results show that the rotational speed has the greatest impact on the degassing of 4004 AI alloy, followed by gas flow and refining time; stewing time has the least impact. The optimum purification parameters obtained by current orthogonal analysis were： rotor speed of 500 r.min1, inert gas flow of 0.4 mL.h-1, refining time of 15 min, and stewing time of 6 min. Degassing rate using the optimum parameters reaches 68%. In addition~ the comparison experiments among C2CI6 refining, rotary impeller degassing, and combined treatment of C2CI6 refining and rotary impeller degassing for 4004 AI alloy were performed. The experimental data indicated that the combined treatment of C2CI6 refining and rotary impeller degassing has the best degassing effect. Degassing rate of C2CI6 refining, rotary impeller degassing and combined refining treatment is 39%, 69.1% and 76.9%, respectively. The mechanical properties of the specimen refined by rotary impeller degassing were higher than those by C2CI6 refining, but lower than those by combined refining treatment.
Wang LipingCao GuojianGuo ErjunWang GuojunLü Xinyu
