Grain refinement in aluminium containing magnesium alloys

Abstract

The novel grain refiners developed in this research could be broadly classified into borides and carbides. The motive behind choosing MgB2, AlB2 and their master alloys Mg-MgB2 and Mg-AlB2 as the grain refiners was driven by the crystallographic matching of the hexagonal borides with the magnesium crystal structure. Apart from this lightweight borides, denser borides such as CrB and WB have also shown excellent grain refinement in AZ91, AM50 and AZ31 alloys. It is suggested that the grain refinement effect in the magnesium alloys could be improved through a combined addition of bismuth along with the boride. The carbon based grain refiners were also explored as they are well-established for the grain refinement of aluminium containing magnesium alloys. The new carbon based grain refiners identified through this research are B4C, Mg-B4C, Mg-3Ti-1C. Magnesium matrix was chosen for the development of each of these master alloys to eliminate any impurity contamination during the grain refiner addition to the magnesium melt. The pressureless melt infiltration techniques was involved in the development of Mg-MgB2, Mg-AlB2 and Mg-B4C while, the ‘halide salt route’ was adopted for producing Mg-3Ti-1C master alloys. The application of ultrasonic cavitation for the development of a new Al-1.5B-2C master alloy was shown to be effective for a homogenous distribution of the intermetallic phases in the form of Al3BC. The potential heterogeneous nucleating sites proposed in the commercial aluminium containing magnesium alloys are MgB2 for Mg-MgB2 master alloy; Mg1-xAlxB2 (0.10 < x < 0.18) for AlB2 grain refiner; MgB2C2 for B4C grain refiner; CrB and WB for their individual powder additions respectively; Ti2AlC for Mg-3Ti-1C master alloy; Al3BC and Al4C3 for the Al-1.5B-2C master alloy.