Characteristics of open die forging in an AZ31 Mg alloy and a relation between microstructure and mechanical properties of the forged alloy have been investigated for an as-received specimen and an extruded specimen, where the as-received specimen consisted of both elongated coarse grains and equiaxed small grains, and the extruded specimen consisted of only equiaxed small grains. Each specimen was forged at 323 to 673 K. In addition, tensile tests of the forged specimens were carried out at room temperature and the microstructure was observed using an optical microscope. In the forging tests, the specimens could be forged from 18 mm to 2.5 mm in thickness at forging temperatures of more than 423 K. In particular, the specimen could be forged without surface cracks when the grains were refined by extrusion prior to forging. However, for the as-received specimen, some cracks were observed at the edge of the specimens forged at less than 573 K. Therefore, it is suggested that the microstructure of the pre-forged specimens needs to be homogeneous, in order to attain a good surface quality, when forging is carried out at temperatures less than 573 K. The grain size of the forged specimens decreased with decreasing forging temperature. It should be noted that a very small grain size of about 3 μm was attained by forging at 473 K. The tensile strength and the 0.2% proof stress of the forged specimens increased with decreasing forging temperature due to grain refinement by forging.
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