Sliding friction and wear of magnesium alloy AZ91D produced by two different methods

Alloy AZ91D is a leading magnesium alloy used for structural applications. It contains aluminum and zinc as principal alloying elements. This alloy is normally die-cast, but recent developments in semi-solid injection molding (Thixomolding®), which offers certain processing advantages, produces a slightly different microstructure than die-casting, and it was of interest to determine whether the two processing routes would measurably affect the friction and wear of AZ91D. The present work involved ambient air, room temperature testing of die-cast (DC) and Thixomolded® (ThM) AZ91D, in both unidirectional and reciprocating sliding motion, using stainless steel type 440C as the counterface. After running-in, the average sliding friction coefficients in both types of test fell into the range of 0.29–0.35, irrespective of processing method. The formation of a built-up edge raised the friction slightly in unidirectional tests compared with reciprocating tests. The average wear rate of the ThM alloys in reciprocating sliding was approximately 25% lower than that for DC alloys. However, the wear rates of the magnesium specimens in unidirectional sliding were comparable for DC and ThM materials. Owing to the transfer of magnesium, there was no measurable wear on the stainless steel 440C balls. The wear mechanism during sliding involves the formation of thin, narrow shards along the edges of wear grooves which break off to produce loose particles.