Quantitative Description of the Microstructure of Aluminium Foundry Alloys

Abstract

In this paper, the quantitative description of the microstructure of hypoeutectic aluminium-silicon foundry alloys is treated. First, the microstructure is described qualitatively and the important microconstituents are defined. Then, programs for the measurement of the corresponding microstructural parameters are developed on the automatic image analyser Quantimet-900. These developments have originally been performed on hypoeutectic aluminium-silicon alloys. The microconstituents considered to be relevant are the aluminium and aluminium-silicon phase, the silicon and intermetallic particles as well as the microporosity. The corresponding microstructural parameters chosen are the area fraction of eutectic phase, the mean chord of the aluminium and aluminium-silicon eutectic phase, the area fraction, length, maximum length, breadth, shape factor and distribution of the silicon + intermetallic particles as well as the volume fraction, length, maximum length and shape factor of the micropores. There exists a clear relationship between the mean chord and the secondary dendrite arm spacing of the aluminium phase. Since, contrary to the secondary dendrite arm spacing, the mean chord can be measured automatically, this latter parameter is chosen to represent the aluminium phase. These programs developed on the Quantimet-900 for the quantitative description of the microstructure are then applied to commercial hypoeutectic aluminium-silicon foundry alloys. A set of test specimens with a rather large variation in the solidification conditions, i.e. local solidification time and local thermal gradient, have been cast and then analysed metallographically in a quantitative way, using the Quantimet-900 programs mentioned above. The alloys considered are AlSi7Mg0.3 (A356) unmodified, modified with strontium or sodium or refined with antimony, with an iron content varying between 0.03 and 0.17%, as well as the strontium modified alloys AlSi9Mg0.3 and AlSi11Mg0.2. All the alloys have been studied in the T6 temper. Graphic representations of the variation of the microstructural parameters of these alloys with the solidification conditions are shown and the corresponding regression analyses performed. The quantitative microstructural analysis of these alloys is part of a detailed study of the so-called triangle of relationships relating the processing parameters, the mechanical properties and the microstructure.