Abstract
The microstructural evolution and corresponding phase transformations have been investigated during partial remelting of AZ91D magnesium alloy refined by Al-Ti-B master alloy. The effect of heating temperature on semisolid microstructure has also been discussed. The results indicate that the microstructural evolution process includes four stages, the initial rapid coarsening, structure separation, spheroidization accompanied by coarsening and the final coarsening. Two or more equiaxed dendrites in the as-cast microstructure evolve into one spheroidal primary particle in the semisolid microstructure through the former three stages. The initial rapid coarsening results from the reaction of β→α, the structure separation is due to the α + β→L and α→L, the spheroidization is attributed to the α→L and the final coarsening is ascribed to the two reverse reactions of α→L and L→α. Rising the heating temperature during partial remelting is beneficial for obtaining small and spheroidal primary particles.
Highlights
Magnesium alloys have relatively small density and high specific strength and stiffness, and are very attractive for structural applications in transportation industry[1]
According to the equilibrium phase diagram of binary Mg-Al shown by Figure 122, it can be expected that the residual liquid amount is quite little during eutectic reaction because the Al content is far away from the Mg-Al binary eutectic point for AZ91D alloy
That is to say that the whole microstructural evolution of the AZ91D alloy includes four stages, the initial rapid coarsening, structure separation, spheroidization accompanied by coarsening and final coarsening
Summary
Magnesium alloys have relatively small density and high specific strength and stiffness, and are very attractive for structural applications in transportation industry[1]. Mechanical properties of the same samples of most of the magnesium alloys are inferior to those of another family of light alloys, aluminum alloys, and do not meet the requirements in many situations. To improve their mechanical properties, as known, thixoforming is a promising way by decreasing grain size and shrinkage porosities[2]. The chemical grain refining process produces the desired microstructures by adding grain refiner during traditional casting and a following partial remelting It is a relatively simple method because it does not need special treatments, such as stirring, spraying or deformation[2,3]. The result from the authors’ previous investigation indicates that its grain size can be decreased from 422 μm to 79 μm after being refined by an Al-Ti-B master alloy that has been commercially used to aluminium alloys[7]
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