Korrosionsermüdung von Aluminium- und Magnesium-gußlegierungen

The fatigue properties of the aluminium cast alloys AlSiTMg and AlSi10Mg, magnesium high pressure die-cast AZ91 hp and AM60 hp, as well as low-pressure permanent mould casting AZ91 hp were determined at numbers of cycles to failure between 10 ⁵ and 10 ⁹ in ambient air and in saltwater spray. The investigations were performed at constant amplitudes and in addition at varying load amplitudes for aluminium cast alloy AlSilOMg and low-pressure permanent mould casting AZ91 hp with a high-frequency testing facility (20 kHz ultrasound). The SN-curves of the aluminium cast alloys and of the magnesium low-pressure permanent mould casting are steeper in corrosive environment than in laboratory air. No endurance limit could be detected up to 10 ⁹ cycles in both environments. The magnesium high pressure die-cast alloys do not have an endurance limit in saltwater spray, whereas an endurance limit could be detected in laboratory air. The life time curves for inservice loading conditions are about parallel to the SN-curves for ambient air, but steeper for corrosive environment. The reason for the reduced fatigue properties in corrosive environment are accelerated crack initiation and higher crack propagation rates. Material defects may lead to a large scatter of the fatigue data for testing in air as well as in saltwater spray. Corrosion pits are formed on the surface of both light-weight alloys at higher number of cycles. They become crack initiation sites.