High Temperature-Creep Resistant Magnesium Alloys:Advances in Thixomolding Automotive Components

Applications of magnesium-based alloys to transmission and underhood automotive components have been limited by creep and bolt-load relaxation. A further concern is the relative expense of Mg alloys, such as AE42, that contain rare earth elements. These alloys still fall short of the performance of 380 Al. A new family of low-cost Mg alloys (ZAC) containing Zn, Al, and Ca has been developed and reported to approach AE42 and to outperform 380 Al; however, it exhibits castability concerns in die casting.

This study was designed to evaluate the performance of semi-solid injection-molded (Thixomolded®)¹ ZAC8506 bolt-load retention samples at temperatures up to 175°C. The Thixomolding® process has been reported to markedly improve the creep resistance of AZ91D as the volume fraction of solids is increased. The processing conditions were selected to produce bolt-load retention samples having solid fractions of 6% to 37% primary alpha. The bolt-load retention properties were then studied as a function of time at a torque level of 28 kN and at 150 and 175×C for periods of 100 h.

The study also included both Thixomolded and die cast AZ91D bolt-load retention samples for direct comparison, and corroboration of the test methodology for bolt-load retention. Elevated temperature measurements of both tensile and compressive properties were also carried out on the ZAC8506 alloy. The moldability of ZAC was evaluated using a relatively complex Delphi automotive electronics housing.