Preparation Of Ultrafine Grain Mg-3Al-Zn Alloy By Mechanical Milling And Research On Its Microstructure And Properties

Magnesium alloys have significant value and great potential for applications among metal structural materials due to their minimum densities, high specific strength, high specific stiffness, easy recycle and other remarkable advantages. However, their plastic deformation abilities and plastic processing performances at room temperature are not good enough. Therefore, it is the key point that the application area and development of magnesium alloys can be further expanded by improving their strength, the plastic deformation and processing abilities by means of grain refinement.Variation laws of grain morphology and microstructure for the mixture of Mg, Al and Zn powders were investigated in the course of mechanical milling in this paper. It indicated that nano-crystalline Mg-3Al-Zn powders with around 40~80nm in diameters can be obtained through milling with proper techniques. The average grain size and its distribution of nano-crystalline powder on distinct milling time scales were measured by a particle size analyzer. The variation law of Mg-3Al-Zn alloy grain size with milling time was found by observing the morphology evolution and the variation of grain size using SEM and TEM, respectively. And a preliminary analysis on the structural defect of magnesium alloy caused by milling was carried out as well in this paper.A densification technique of nano-crystalline Mg-3Al-Zn powder was systemat-ically studied in this paper, and a magnesium alloy bar with a density of 98% was acquired by the combination of cold pressing and canning extrusion. The relationship between the density of the alloy preform and milling time and the change tendency of pressing force with the density of the alloy preform in the process of cold pressing were both analyzed in our experiments. Meanwhile, the connection between the density and grain size of alloy bar and milling time was surveyed. A compression test and observation of fracture morphology were performed for alloy bar. In addition, TEM was employed to observe the change of grain size under different milling time. The results showed that the grain size of alloy was around 200nm after eighty-hour milling. And the hardness of a Mg-3Al-Zn bar prepared by extruding at 150℃was 133.57HV.Based on the facts above, both a three-millimeter-thick sheet and a magnesium alloy wire with a diameter of 3mm were successfully fabricated through rolling and double extrusion of magnesium alloy bar obtained by canning extrusion. And the results suggested that the strength of the rolling bar was higher than that of twice extrusion bar, the highest is 390MPa for the rolling one; on the contrary, a high elongation around 17% at 200℃was observed for the double extrusion one.