Application of ultrasound for fatigue testing of lightweight alloys

Abstract

The use of aluminium and magnesium alloys offers a great potential for weight reduction in automotive applications. Load‐bearing car components are subjected to 10 8 cycles and more during service, and the high‐cycle fatigue properties of construction materials are therefore of great interest. The time‐saving ultrasound fatigue testing method has been used to study the fatigue properties of a high‐pressure, die‐cast magnesium alloy AZ91 hp and a post‐forged, cast‐aluminium alloy AlSi7Mg0.3 in ambient air and saltwater (5wt% sodium chloride) spray. In ambient air, fatigue cracks in AZ91 hp emanate from voids, and it is possible to correlate void areas with the numbers of cycles‐to‐failure. Post‐forging of AlSi7Mg0.3 reduces the numbers and size of voids. The remaining small voids (void areas smaller than 9000 μm 2 ) do not significantly reduce lifetimes. Saltwater deteriorates the fatigue properties of both the lightweight alloys. With increasing numbers of cycles, the influence of the corrosive liquid on fatigue strength becomes more pronounced.