Effect of extrusion conditions on grain refinement and fatigue behaviour in magnesium alloys

Abstract

This paper describes the grain refinement due to controlled extrusion and the fatigue behaviour of the extruded materials in three magnesium alloys, AZ31B, AZ61A and AZ80. First, in order to investigate the effect of working temperature, billets of AZ31B and AZ61A were extruded at an extrusion ratio of 67 under controlled conditions with three different working temperatures. It was found that grain refinement was attained in both alloys whose grain size decreased with decreasing working temperature. Rotary bending fatigue tests were performed using smooth specimens. In AZ31B, fatigue strength increased with decreasing grain size, but in AZ61A, it did not depend on grain size. Fatigue crack initiation and subsequent small crack growth were examined in AZ31B. Consequently, grain refinement improved both crack initiation resistance and small crack growth resistance, resulting in the increase in fatigue strength. Furthermore, it was indicated that fatigue strength was expressed properly by the Hall–Petch relationship in AZ31B, but not in AZ61A. Then, in order to investigate the effect of extrusion ratio, billets of AZ61A and AZ80 were extruded at three different extrusion ratios under controlled conditions. It was found that grain size decreased with increasing extrusion ratio in both alloys, but mechanical properties were not affected significantly by extrusion ratio, i.e. grain size, and the grain size dependence of fatigue strength was different between alloys. In AZ61A, fatigue strength was higher in the material extruded at lower extrusion ratio (coarser grain), but in AZ80, in the material extruded at higher extrusion ratio (finer grain). Fatigue crack initiation behaviour was consistent with the tendency of fatigue strength, where crack initiation delayed in the material extruded at lower extrusion ratio in AZ61A and in the material at higher extrusion ratio in AZ80. Furthermore, the observed grain size dependence of fatigue strength in both alloys was discussed on the basis of a texture formed by extrusion and the presence of inclusions from which cracks generated.