Research On Homogenization Treatment And Low Temperature Exstrusion Sheet Of AZ91 Magnesium Alloy

Magnesium alloy extruded sheet has showed great superiority in the structure parts of aerospace sector. The homogenization treatment and thermal stability of as cast magnesium alloy were studied in this paper. AZ91 magnesium alloy sheet was formed by low temperature extrusion. The influence of extrusion and heat treatment on microstructure evolution and mechanical properties of magnesium alloy was revealed. The relationship between the precipitates and strengthening mechanism was analyzed which provided certain theoretical and practical basis for the second time plate rolling plastic deformation in the next step.AZ91 magnesium alloy as cast rod was taken as the raw material with a following process: homogenization treatment of cast rod, thermal stability research and sheet extrusion. In the experiment, the microstructure and mechanical properties were analyzed. Firstly, the dendrite segregation and coarse second phase were eliminated by the uniform treatment. And then the reasonable plastic heating temperature range of the rod was obtained by the thermal stability test. The AZ91 magnesium alloy sheet was formed by extrusion process and typical process route of the magnesium alloy sheet was obtained at last.The results showed that the secondary phase dendrite segregation at the grain boundary of the original as cast AZ91 magnesium alloy rod was eliminated by the uniform annealing treatment at 410℃×12h. The second phase existed in the matrix in a homogeneous layer. The mechanical properties of magnesium alloy were improved that the tensile strength was increased from 120.33 MPa to 188.59 MPa, and the elongation was increased from 3.92% to 5.91%. The fracture surface of the original cast specimen showed a strong tendency to brittle fracture, brittle and ductile fracture after homogenization treatment instead. The strengthening mechanism of as cast magnesium alloy after homogenization annealing was the second phase strengthening.Extrusion plate was analyzed by EBSD: grain size variable and proportion of high angle grain boundaries increased with the increase of the extrusion temperature; silk fabric of extrusion sheet had a obvious preferred orientation which mainly alongs the direction {0001}∥TD.According to the XRD diffraction pattern of as cast and extruded AZ91 magnesium alloy, the microstructure of magnesium alloy was composed of matrix α-Mg and β-Mg17Al12 precipitates. But according to the analysis of EDS data, it could be known that the Mn-Al phase, Al-Si phase and Mg-Si phase were mixed in the extruded sheet matrix of 100℃. It was easy to become the source of the crack in the deformation process which leaded to the poor elongation at room temperature.Two typical extrusion processes were got by sheets extruded at low temperature: tensile strength was 326.84 MPa and elongation was 18.12% when the extrusion temperature was room temperature; tensile strength was 326.20 MPa and elongation was 19.68% when the extrusion temperature was 200℃. Low temperature extrusion plate mainly depended on the twin to induce plasticity. The fracture mode of extruded plate was ductile fracture. The strengthening mechanism was mainly diffusion of fine grain strengthening and inside the dispersed second phase strengthening caused by point.