The microstructures of 10 hemispheres east from AlSi7Mg alloy with a low and counter pressure at different molten metal and mould temperatures and various pressure differences are investigated. The secondary dendrites arm spacings (DAS), local crystallisation times (LCT), Si-particles sizes and porosity are determined. A numerical simulation of the porosity formation is carried out. A simple criterion, which is taking into account the temperatures of both the molten metal and the mould in the first moment of the mould filling, is created. It offers a possibility for an evaluation of the influence of the thermal conditions on the structure formation of the hemispheres.
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