In this article, the microstructure of AZ91C magnesium alloy castings produced by investment casting was investigated. Castings were studied in as-cast state and after T4 and T6 heat treatment. The identification of all phases in microstructure was carried out using a scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS) analysis. The influence of cooling rate after casting, temperature of solution annealing, temperature of artificial aging and cooling rate after heat treatment on the microstructural characteristics such as secondary dendrite arm spacing (SDAS) or grain size was studied. The SDAS analyses were carried out using a light microscopy with the subsequent image analysis. Except of Mg-Al solid solution and Mg17Al12 precipitates, the microstructure contains other phases based on Mg-Si and (Al,Si)-Mn in globular or needle morphology. The SDAS in as-cast state was measured and was of tens of micrometers, but after T4 heat treatment, the gran size exceeded 100 µm and the Mg17Al12 phase was still undissolved along the grain boundaries in samples with thicker wall dimension. After T6 heat treatment, continuous and discontinuous precipitates of Mg17Al12 phase were formed, most of them were nearby the grain boundaries proving the inhomogeneous distribution of Al atoms in solid solution after T4 heat treatment.Keywords: Magnesium alloy, AZ91, investment casting, microstructure, heat treatment
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