Application of ultrasound for fatigue testing of lightweight alloys

The use of aluminium and magnesium alloys offers a great potential for weig ht reduction in automotive applications. Load-bearing car components are su bjected 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 th e fatigue properties of a high-pressure, die-cast magnesium alloy AZ91 hp a nd a post-forged, cast-aluminium alloy AlSi7Mg0.3 in ambient air and saltwa ter (5 wt% sodium chloride) spray. In ambient air, fatigue cracks in AZ91 h p emanate from voids, and it is possible to correlate void areas with the n umbers 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 mu m(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 becom es more pronounced.