Effects Of Heat Treatment On The Microstructure And Properties Of Rolled AZ31 Magnesium Alloy Sheets

Closed-packed hexagonal lattice structure and only one basal slipping plane at room temperature make magnesium alloys have poor plasticity, deform difficultly and be prone to appear defects during deformation at room temperature, so their extensive application in industries is limited. At present, how to improve the plasticity and formability of magnesium alloys at room temperature has attracted peoples great attention. It is reported that grain refinement is an effective measure to improve plasticity of magnesium alloys. However, most researches are focused on the effect of annealing, solid solution, ageing treatment on as-cast magnesium alloys, there are less reports on the study how to improve mechanical properties of wrought magnesium alloys and then to enhance their formability and corrosion resistance. In the paper, the effects of heat treatment on microstructure, mechanical, bulging and corrosive properties were investigated for the purpose of obtaining an economic and efficient heat treatment technology to improve the formability and corrosion resistance at room temperature, further to extend the application of magnesium alloys in industries.Rolled AZ31 magnesium alloy sheets were heat treated at different conditions. The temperature of heat treatment was between 150℃and 400℃, the holding time was between 5 min and 120 min, the cooling methods were water cooling, air cooling, sand cooling, and furnace cooling respectively. In the paper, microstructure analysis and microhardness tests, mechanical tests, erichsen tests, static weight loss tests and electrochemical corrosion tests were used to study the effects of heat treatment parameters on the microstructure and properties of rolled AZ31 magnesium alloy sheets.The results of microstructure analysis show that static recrystallization occurred during the heat treatment, the grains tended to be homogeneous and fined, and twins disappeared after recrystallization. When held 15min at 350℃and cooled in the water, twins disappear completely, grains turn tiny and uniform, the average grain size was reduced to 4.6μm.The results of mechanical and Erichsen tests indicate that after heat treatment the microhardness value drapes, yield strength and anisotropy decreases obviously, yield ratio declines, ultimate tensile falls slightly, while tensile elongation and Erichsen value increase. After held 15 min at 350℃and cooled in the air, the microhardness value of rolled AZ31 magnesium alloy sheets is decreased by 11.0%, compared with as-received sheets, the tensile elongation is increased respectively to 34.3% and 33.1% in rolling direction (RD) and transverse direction (TD), while tensile elongation of as-received sheet are 25.1% and 23.3% respectively in RD and TD, associated with the respective reduction of yield ration from 0.719 to 0.660 in RD and from 0.742 to 0.656 in TD, Erichsen value is increased to 3.88mm from 2.50 of as-received sample.The results of corrosion tests demonstrate that the corrosion resistance property improves after heat treatment. As the grain size turn fined, the corrosion resistance get better. After held 15min at 350℃and cooled in the air, the rate of corrosion is reduced to 9.3mg/cm2·d compared with the 17.3mg/cm2·d of as-received sample, connected with the decrease of corrosion current density from 0.023mA to 0.001mA, and the increase of corrosion voltage from-1438mV to -1166mV, because twins disappear, static recrystallization is accomplished, grains is the tiny and uniform.In the paper, the research results show that when held 15min at 350℃and cooled in the air, ductility and corrosion resistance of the AZ31 magnesium alloy sheet used in the tests get improved at room temperature, and the best over-all properties can be obtained.