Fatigue behavior of thixomolded® magnesium AZ91D using ultrasonic techniques

The fatigue behavior of the Thixomolded® magnesium alloy AZ91D has been examined using ultrasonic fatigue testing techniques at frequencies of approximately 20 kHz and for lifetimes as long as 10⁹ cycles. An apparent endurance limit of approximately 65-70 MPa is observed. Comparison with the fatigue behavior of AZ91 produced by conventional die casting indicates that the Thixomolded® aterial has an endurance limit substantially higher than that of die cast material. This is attributed primarily to reduced porosity associated with the thixotropic processing technique. Fractographic analyses indicate that fatigue cracks leading to failure initiate at internal porosity in approximately 75% of tests and at the as-molded surface in the remainder of tests. Fractographic studies indicate that, in general, longer fatigue lifetimes are associated with smaller fracture initiation sites. It was also observed that a bimodal distribution of fatigue lives occurred, especially at lower stresses. This was analyzed using cumulative distribution functions, which confirmed the existence of dual failure populations.