Study On Microstructures And Microhardness Of The AZ91Alloys Prepared By Ball Milling/Powder Metallurgy

Crystal structure of magnesium is hexagonal close packed(hcp), and its strength is very sensitive to grains sizes. Fine grain magnesium alloys which prepared by powder metallurgy have many superiorities over the traditional processing methods, for example, higher strength and hardness. In this paper, AZ91is selected as the research material, is ball-milled to prepare AZ91powders. The particle diameters of the prepared powder are tiny and uniform. At last, At last, the AZ91powder is cold pressed and sintered in order to prepare bulk material.The experiment parameters were designed by Orthogonal Method to study systematacially the effect of the ball-milling technology on the grains sizes and refining mechanism of AZ91powders. And the morphology, partical size and composition of the powder prepared by ball-milling were characterized by. The effect of ball milling conditions such as milling time, ball/powder weight ratio, the speed of revolution and process controlling agent on milling results was studied. The particle size distribution and morphology of the powders were described by means of laser particle analyzer and scanning electronic microscope(SEM). Experimental results showed that with the increase of the time, revolving speed and ball/powder weight ratio, the granularity of alloy powder decreases markedly, and the appearance features are also significantly changed. The ball milling parameters with rotational speed rate of350r/min, milling time of8h, ball/powder ratio of50:1was considered as the optimum ball milling technology, and d50of the powder prepared by the above technology is14.79μm.Then, the powder prepared by the above technology is cold pressed and sintered. The effect of cold pressing and sintering process on density and microhardness was studied. The dependence of density and microhardness on sintering time and sintering temperature were discussed detailedly in this paper. It is thought that sintered at500℃, densification mechanism of the alloy is diffusion of atoms; When sintering temperature is above500℃, the densification mechanism is liquid phase sintering, and it is very useful to improved the density of the alloys. The relative density of the alloy is highest, i.e.0.99, when it is sintered at580℃for4h. The grains sizes of the alloy are very fine, about5μm, much smaller than common magnesium alloys’; and the microhardness is improved obviously, from80HV to102HV. Fine grain AZ91magnesium alloys was prepared by powder metallurgy in this paper. Hoping this article will be able to work the certain reference function to future work.