Researches On Microstructure And Mechanical Properties Of Fine-grained WE43 Magnesium Alloy Processed By Equal Channel Angular Pressing

Due to the excellent corrosion resistance, biocompatibility, thermal stability and integrated mechanical properties, WE43 magnesium alloy shows wide application prospect as biomedical and structural material. However, the poor forming ability resulted from its HCP crystal structure severely limits the application of WE43 alloy. Grain refinement has been regarded as the effective methods to improve the strength and ductility of magnesium alloys. Therefore, the fined-grained WE43 alloy was prepared by equal channel angular pressing(ECAP) in this paper. The evolution of microstructure, texture and mechanical properties were studied during ECAP processes, especially superplastic behavior. Moreover, the processes of pre-extrusion and subsequent heat-treatment were applied to WE43 alloy processed by ECAP for further optimizing the microstructure and mechanical properties.Firstly, the evolution of microstructure and texture of solution-treated WE43 alloy during multi-pass ECAP processes were investigated and their influences on mechanical properties were also discussed. With the increase of ECAP passes, the fine-grained area constantly enlarged, the microstructure gradually became uniform and the final grain size reached to 1.5 μm. Meanwhile the secondary-phase β-Mg5 RE precipitated and the morphology had obvious transformations from plate-like to granular and spherical finally. The dislocation density increased greatly after ECAP for 1 pass and then decreased gradually. Moreover, the initial random orientation was converted to the strong(0002) basal texture gradually. Finally, the strength improved constantly and the ultimate tensile strength reached to the maximum of 322 MPa at ECAP for 12 passes. However, the elongation first rose and then dropped, which reached to the maximum of 18 % at ECAP for 4 passes. The variation of strength and elongation resulted from the corresponding evolution of microstructure and texture.Secondly, the high temperature tensile test was conducted on WE43 alloy processed by ECAP for 8 passes. The WE43 alloy exhibited excellent superplasticity and the elongation reached to the maximum of 1120 % at 723 K and 3×10-3 s-1. Meanwhile the WE43 alloy also achieved outstanding high strain rate superplasticity of the maximum of 960 % at 2×10-2 s-1 and 860 % at 1×10-1 s-1. This resulted from the uniform and equiaxed fine-grain microstructure and the strong pinning effect of thermally stable phase β to grain boundary. Furthermore, the β was softened and deformed due to the high deformation temperature, which effectively relieved the stress concentration. The strain rate sensitivity exponent m was about 0.4 and activation energy Q varied from 126 to 177 kJ/mol, suggesting that the grain boundary sliding controlled by lattice diffusion was the main deformation mechanism. Besides, the main failure mechanism was the nucleation, growth and coalescence of cavities.Finally, the influences of initial microstructures, including as-cast, solution-treated and pre-extruded, on microstructure and mechanical properties of WE43 alloy processed by ECAP were discussed. After ECAP process, the most eutectic phases of as-cast WE43 alloy were broken into small particles and concentrated distributed along deformation shear band. Therefore, the microstructure was not uniform and the bimodal microstructures were finally formed, which resulted in the relatively low ultimate tensile strength of 285 MPa and elongation of 12 %. The solution-treated WE43 alloy processed by ECAP achieved uniform fine-grain microstructure with the average grain size of 1.5 μm and dispersedly spherical thermal-stable phase, achieving the considerable ultimate tensile strength of 310 MPa and elongation of 16 %. After ECAP process, the grain size of pre-extruded WE43 alloy was further refined to 0.5 μm and a large number of dislocations existed in the grain interior, leading to the high ultimate tensile strength of 354 MPa and low elongation of 11 %. For improving the ductility of WE43 alloy processed by pre-extrusion and ECAP(E-ECAP), the subsequent annealing treatment was conducted and WE43 alloy obtained integrated mechanical properties with the ultimate tensile strength of 341 MPa and elongation of 17 % at annealing temperature of 623 K for 1 h. Moreover, the aging treatment was also conducted on WE43 alloy processed by E-ECAP for further promoting the ultimate tensile strength and the maximum of 365 MPa was achieved at peak-aging condition of 453 K for 8 h.