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Abstract: The major second phase (β-Mg12Al17) that is present in the most commonly used cast magnesium alloy, AZ91, has a strong effect on various properties of the alloy. Thus how casting conditions affect the amount of this phase formed needs to be studied. In the present study, casting of AZ91 was conducted allowing various cooling conditions by employing a chill. Total fraction of β-phase (f β ), the fraction of the eutectic β phase (f β-eut ) formed during eutectic solidification, and the fraction of discontinuous β precipitates (f β-d ) formed during subsequent solid state cooling were quantitatively measured, respectively. It was found that f β was not significantly affected by the variation of cooling history in different locations of the casting. However, f β-eut and f β-d are strongly location-dependent, decreasing and increasing, respectively, towards the chill end of the casting. Explanation on this is given taking into accounts the effects of casting/thermal conditions on solidification and subsequent discontinuous precipitation. [Copyright &y& Elsevier]

In this study, constitutional liquation in AZ91 cast alloy at 440°C and 465°C was experimentally simulated. Metallurgical examination was conducted to reveal the morphology of the constitutional liquid and dihedral angle was measured to evaluate the wettability of the liquid on grain boundary. Dihedral angle was found to be initially ~30 degree and thus grain boundaries were incomplete wet. During annealing, the metastable constitutional liquid resolidified, followed by grain growth. The extent of grain boundary wetting by the remaining liquid increased slightly with treating time. The rate of resolidification was also evaluated and the factors affecting this rate were discussed. [ABSTRACT FROM AUTHOR]

Abstract: During gas tungsten arc (GTA) welding of AZ91 Mg cast alloy, constitutional liquid forms locally in the original interdendritic regions in the partially melted zone (PMZ). The PMZ re-solidification behaviour has not been well understood. In this study, the gradual change of the re-solidification microstructure within PMZ from base metal side to weld metal side was characterised. High cooling rate experiments using Gleeble thermal simulator were also conducted to understand the morphological change of the α-Mg/β-Mg17Al12 phase interface formed during re-solidification after partial melting. It was found that the original partially divorced eutectic structure has become a more regular eutectic phase in most of the PMZ, although close to the fusion boundary the re-solidified eutectic is again a divorced one. Proceeding the eutectic re-solidification, if the degree of partial melting is sufficiently high, α-Mg re-solidified with a cellular growth, resulting in a serrated interface between α-Mg and α-Mg/β-Mg17Al12 in the weld sample and between α-Mg and β-Mg17Al12 (fully divorced eutectic) in Gleeble samples. The morphological changes affected by the peak temperature and cooling rate are also explained. [Copyright &y& Elsevier]