Mechanical properties of thixomolded AZ91D magnesium alloy

Continuous Twin Screw Rheo-Extrusion of an AZ91D Magnesium Alloy

“…The secondary solidification of the remnant liquid took place during water quenching as the material was extruded out of the die assembly, forming a network of fine secondary a-Mg grains together with b-Mg 17 Al 12 intermetallics and a small amount of eutectic structures between a-Mg and b-Mg 17 Al 12 phases, in the matrix of primary a-Mg granular grains, as shown in Figures 6 and 9. It has been claimed in the literature [17,21,36] that both strength and ductility decrease with increasing solid fraction, corresponding to decreasing extrusion temperature in the present investigation. Czerwinski and co-workers [17] blame the increased microchemical and microstructural heterogeneity, in the form of Al and Zn segregation, Mg 17 Al 12 precipitates, and eutectic islands, for the decreased strength and ductility when primary solid content increases.…”

“…Czerwinski and co-workers [17] blame the increased microchemical and microstructural heterogeneity, in the form of Al and Zn segregation, Mg 17 Al 12 precipitates, and eutectic islands, for the decreased strength and ductility when primary solid content increases. However, in a thixomolding investigation, [21] increasing solid fraction was found to have resulted in reduced tensile strength and ductility, whereas lowering the processing temperature apparently improved mechanical properties. These findings are contradictory and no explanations have been given.…”

“…However, the overall mechanical performance of the as-rheo-extruded rods was comparable with some of the thixomolding and rheo-die-casting products of the same material. [16,17,21] V. DISCUSSION…”

“…The AZ91D was chosen because it has certain wroughtability in addition to its excellent castability, is easier to process than purer alloys, and has been widely studied in semisolid processing. [16][17][18][19][20][21][22] Besides, there is a niche market for AZ91 magnesium alloy welding rods, which are difficult to produce using conventional processes.…”

“…The primary particle size reduced gradually as the extrusion temperature decreased, and there seemed to be a turning point at 833 K (560°C), below which the particle size reduced more quickly with decreasing extrusion temperature. At 818 K (545°C), an [17][18][19][20][21] At relatively lower extrusion temperatures, plastic deformation took place and is considered to contribute to the particle size decrease. Although most of the deformation features are expected to be removed by dynamic recovery at semisolid temperatures, twins and dislocation cells were observed for samples extruded at relatively low temperatures.…”