Distortion in a sintered 7000 series aluminium alloy

The 7000 series aluminium alloys processed using elemental powder mixtures are prone to distortion, which is manifest as hourglassing or waisting in c ylindrical specimens. By characterising the density distribution using hard ness measurements, it is shown that the green density is not evenly distrib uted through a part, even though aluminium is relatively soft and readily c ompacted. Because the density equilibrates during sintering, the non-unifor m green density leads to distortion. The cause of this distortion is a resu lt of differential shrinkage, which occurs during sintering as well as on s olidification during cooling from sintering. Distortion can be controlled b y increasing the compaction pressure, which homogenises the green density a nd does not affect the tensile properties.