| Aging and grain refinement are two major strengthening procedures in magnesium alloys. In the paper, the precipitations behavior and mechanical properties of AZ91 alloy were investigated by a combination of fine-grain and heat treatment strengthening. Effect of grain size on microstructures of AZ91 treated by solid solution and aging treatment was investigated, and the relationship between precipitates and mechanical properties was analyzed.Microstructure of AZ91 alloy was observed and micro-hardness was tested after solution treatment. The discontinuously distributedβ-Mg17Al12 phase, which precipitated along the grain boundary was dissolved intoα-Mg matrix, during the solution treatment. Grain refinement can improve efficiency of the solid solution treatment. When the solution temperature was 420℃, if the holding time longer, theβ-Mg17Al12 phase was dissolved completely, but the grain size was coarsening. The hardness of alloy, which was dropped gradually when the holding time extended. The solution time is same, at different temperature, the microstructure evolution and hardness changes in the same trend of the law of the above.After study of microstructure and mechanical properties of AZ91 alloy, found that through establishment of a reasonable heat treatment process, at the same time to play a fine-grain and aging strengthening, can effectively improve the mechanical properties of AZ91 alloy, reduce the time spent. Grain refinement can improve the rate of continuousβphase during aging treatment, AZ91 alloy reached peak hardness by the shorter time and getted higher peak hardness. Fine-grained AZ91 alloy reaches peak hardness after 16h's ageing treatment and in coarse-grained AZ91 alloy, the ageing time for peak hardness was 60h.High temperature was fit for continuous precipitation ofβphase, when the low temperature was favor of discontinuous precipitation ofβphase. The hardness of discontinuous precipitation ofβphase is high by itself, but the major role in strengthening the alloy is the continuous precipitation ofβphase. When the aging time was same, as longer the aging time,the amount of P phase precipitates gradual increase, The continuous precipitates was uniformly precipitated in the matrix of fine-grained AZ91 alloy; while in the coarse-grained AZ91 alloy, the continuous precipitates was precipitated from grain boundary to crystal; discontinuousβphase was precipitated in the grain boundary only. The more the proportion of continuous precipitates, the higher the hardness,yield strength, the compression ratio decreased.Dual-Aging Process:360℃aging for 10 or 20 minutes first and then aging treatment 12h by 200℃. The results show that the whether the grain size of the alloy is smaller or larger, the micro-hardness,compressive strength and yield strength all higher then aging 12h by 200℃, which is the same as aging more then 16h by 200℃,shorted aging time. Aging at high temperature can promoted the nucleation and growth ofβphase, but long aging time will make the size ofβphase is large. Aging at e 360℃high temperature, Coarse-grained AZ91 aging time can extended to 20 meanwhile in fine-grained AZ91 alloy, aging time is for the benefits to 10min. Dual-aging treatment improved the mechanical properties of the AZ91 alloy, shortened the processing time and improved the efficiency of industrial production. |
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