Grain refinement and improvement of fatigue strength due to controlled extrusion in magnesium alloys

Abstract

This paper describes the grain refinement due to controlled extrusion and the fatigue behaviour of the extruded materials in two magnesium alloys, AZ61A and AZ31B. Billets of both alloys were extruded at an extrusion ratio of 67 under controlled conditions with three different outlet temperatures, i.e. working temperatures. It was found that grain refinement was attained in both alloys whose grain size decreased with decreasing working temperature, in particular remarkably in AZ31B for which the grain size of approximately 2μm was obtained. Thus it was concluded that extrusion was a very useful method for refining grain size in magnesium alloys. Rotating bending fatigue tests were then performed using smooth specimens of the extruded materials. In AZ61A, the fatigue strength of the extruded materials did not depend on grain size, while in AZ31B, increased with decreasing grain size. Fatigue crack initiation and subsequent small crack growth were examined in AZ31B. Consequently, grain refinement improved significantly both crack initiation resistance and small crack growth resistance, resulting in 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.