Research On Microstructure And Properties Of AZ31 Magnesium Alloy Sheets Under Different Constrained Groove Pressing Conditions

Magnesium alloy is the lightest metal structural materials in industrial applications and has been widely used in aerospace manufacturing, military manufacturing, medical equipment and other industries because of its low density, high specific strength and easy to recycle. However, magnesium alloy is limited to be used due to its HCP crystal structure making it has less slip system and poor ductility at room temperature. Studies have shown that Grain refinement can not only improve material’s strength while improving its plasticity. Therefore, study the grain refinement of magnesium alloy has important significance to generalize the use of magnesium alloy. Constrained groove pressing(CGP) is an effective way to refine the sheet metal grains. In this paper, AZ31 magnesium alloy sheets were fabricated by CGP to increasing their strength and improving their ductility. Finite element simulation was carried out for analyzing the accumulation of equivalent strain of AZ31 magnesium alloy processed by different CGP ways. The metallographic tests and unidirectional tensile tests were used to study the influence of microstructure and properties of AZ31 magnesium alloy sheets fabricated by different CGP conditions including CGP temperature, pass, deformation way, deformation rate and annealing process. The main conclusions are as follows:1)The different annealing treatment of grains refinement of AZ31 magnesium alloys before CGP was determined by analyzing the microstructure and mechanical properties. It was found that the sample was annealed at 275 ℃ for 60 minutes, the grains were small and the microstructure showed homogeneous, the average grain size of the sample was decreased from 24.83μmμm to 19.59μm, the elongation was increased from 14.57% to 18.59%. The treatment of 275℃×60min was used as the annealing treatment of grains refinement of AZ31 magnesium alloys before CGP.2) Deform-3D finite element simulation was carried out for analyzing the accumulation of equivalent strain of AZ31 magnesium alloy processed by different CGP ways, such as ―parallel CGP, 180° cross CGP and 90° cross CGP‖.The results show that: The equivalent strain values after the three deformation ways are 2.40、2.40、2.39 and the difference between the maximum and minimum are 0.47、0.44、0.43, so the three deformation ways almost have the same deformation capacity from the point view of mechanics.3)AZ31 magnesium alloy sheets were deformed by different CGP ways under the initial strain rate of 10-2s-1, at 250℃、300℃. The results show that: with the increase of temperature and CGP passes, the grain size become smaller, and the microstructure become more homogeneous. After two passes by three CGP ways at 250℃,the grain size has less difference and the microstructure is more homogeneous by the latter two cross CGP ways. The smallest grain size was 4.47μm after 90° cross CGP.4) AZ31 magnesium alloy sheets were deformed by two passes CGP. for parallel CGP, the mechanical properties are decreased after one CGP pass, because of the inhomogeneous microstructure. After two passes of parallel CGP, the yield and tensile strength are improved due to the fine and homogeneous microstructure, but elongation decreased slightly. The mechanical properties after 180° cross CGP and 90° cross CGP are similar, but the tensile strength and yield strength are decreased and elongation is improved comparing with the parallel CGP. Probably because of the orientation of crystal grains are changed after different deformation ways.5) AZ31 magnesium alloy sheets were deformed by one pass parallel CGP under the different initial strain rate at 250℃. The results show that: it is benefit to obtain the fine equiaxed grains and better mechanical properties with the decreasing strain rate. The strength and elongation of AZ31 magnesium alloy sheets are improved after recrystallization annealing. The magnesium alloy sheets CGPed under the strain rate of 10-3s-1 have better comprehensive mechanical properties after annealing. Tensile strength and yield strength are improved from 245MPa、145MPa to 340MPa、181MPa,and the elongation is increased from 18.59% to 21.34% compared with the as-received sheets.