Microstructure and Mechanical Properties of AZ91 Magnesium Cup Processed by a Combined Backward Extrusion and Constrained Ironing Method

Abstract

A combined metal forming process consisted of backward extrusion (BE) and constrained ironing (CI) is used to produce thin walled ultrafine grained (UFG) magnesium cups. In this new method, the initial thick-walled cup is formed from the bulk material using the BE process and then the CI process is used to produce a UFG thin-walled cups. The advantage of the CI process is applying compressive stresses that are suitable to form hard to deform materials like magnesium alloys without fracture while achieving higher thickness reduction ratio (TRR). The results showed that after this new combined method, the tensile strength raised to 233 MPa, from the initial values of 123 MPa. Simultaneous improvement in strength and ductility attributes to very high hydrostatic compressive stresses and also breakage of Mg17Al12 precipitates in to smaller parts that facilitate the movement of dislocation. Also, the hardness increased to about 233 MPa from the initial values of 58 HV. Significant grain refinement was also taken place and the grain size in the BE+CI sample reduced to ~1 μm from the initial value of ~150 μm due to imposing high value of strain. This combined method is very promising for processing of UFG thin-walled cup-shaped samples from hard to deform materials. SEM images illustrated the brittle fracture at unprocessed and BE samples with existence of wide crack and shallow-elongated dimples but BE+CI sample revealed brittle fracture with fewer cracks due to hydrostatic pressure.