Containerless melting and characterization of cast magnesium AZ31-B alloy at low superheat in the magnetic suspension melting process

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This study deals with containerless induction melting and characterization of cast magnesium AZ31-B alloy at low superheat via the Magnetic Suspension Melting (MSM) process. The operating conditions for melting and confinement of a 63.5mm molten column of the alloy with respect to current and frequency were found to range from 770-830 Ampere and 3552-3600 Hz, respectively. The macro/micro-structure, oxide entrapment, segregation of alloying elements, and the formation of intermetallic precipitates in MSM cast AZ31-B alloy in a unidirectional bottom chilled ceramic mold were investigated. For a baseline comparison, the alloy was also cast using conventional methods at a superheat of 60°C. Analysis of cooling curves showed that the cooling rate during solidification is essentially constant at about 1°C/s. The growth velocity, V, for MSM casting produced at low superheat is almost constant during solidification, around 1 mm/s, while the thermal gradient, G, decreases with increasing solid fraction from 2.07°C/mm at f=0, to 1.10°C/mm at f=0.5. In contrast, V for castings produced at high superheat is four times smaller than that for low superheat castings--the conditions that favor equiaxed dendritic solidification morphology. Metallographic examination of the MSM alloy cast at low superheat shows no evidence of oxide formation. It was also found that casting at this low superheat produced a fine globular grain structure compared to the equiaxed dendritic structure in conventionally cast alloys at high superheat. The average grain sizes for 5 and 8°C MSM produced castings were 83.96 and 94.81μm, respectively. The 60°C superheat castings were found to have a much larger average grain size of 334.42μm. Elemental segregation analysis was performed and showed the presence of primary-α Mg and secondary-α Al rich Mg phases, along with γMg_17 Al_12 and Al_8 Mn_5 intermetallic phases. For the globular structures, the intermetallic phases were found to form in the secondary-α phase along the grain boundaries. Comparatively, the dendritic entrapment of the secondary-α phase was found to lead to intermetallic phase formation in the matrix of the grains.

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Materials science