Abstract
Mechanical properties of aluminium - silicon - zinc alloys for castings are closely conected with their composition or their micro - structure. These relations are detailed on several figures, as to pure alloys cast in sand or permanent moulds and alloys containing a modifying agent of 0.5 per cent chromium cast in permanent moulds. Larger contents of zinc along the eutectic line in the diagram increase the tensile strength, or α of aluminium - rich solid solution and eutectic silicon is the main variable on the strength. The hardness becomes higher with increasing contents of zinc & silicon almost regardless the eutectic line. Chromium addition to the alloys enlarges equi-strength ranges (25 kg / mm2 and 30 kg / mm2) and equi-hardness ranges in the figures. The alloys cast in sand moulds give less tensile strength by 20 per cent and less hardness than the alloys cast in permanent moulds. Silicon contents in sand castings which give the maximum strength are less than in permanent mould castings. Such differences may be caused by structural changes of silicon with different cooling rate.
