Effect of melt conditioning on heat treatment and mechanical properties of AZ31 alloy strips produced by twin roll casting

Abstract

In the present investigation, magnesium strips were produced by twin roll casting (TRC) and melt conditioned twin roll casting (MC-TRC) processes. Detailed optical microscopy studies were carried out on as-cast and homogenized TRC and MC-TRC strips. The results showed uniform, fine and equiaxed grain structure was observed for MC-TRC samples in as-cast condition. Whereas, coarse columnar grains with centreline segregation were observed in the case of as-cast TRC samples. The solidification mechanisms for TRC and MC-TRC have been found completely divergent. The homogenized TRC and MC-TRC samples were subjected to tensile test at elevated temperature (250 to 400C). At 250C, MC-TRC sample showed significant improvement in strength and ductility. However, at higher temperatures the tensile properties were almost comparable, despite of TRC samples having larger grains compared to MC-TRC samples. The mechanism of deformation has been explained by detailed fractures surface and sub-surface analysis carried out by scanning electron and optical microscopy. Homogenized MC-TRC samples were formed (hot stamping) into engineering component without any trace of crack on its surface. Whereas, TRC samples cracked in several places during hot stamping process.

Influence of intensive melt shearing on subsequent hot rolling and the mechanical properties of twin roll cast AZ31 strips

“…The optical microstructure of the samples prepared by TRC and MC-TRC processes are shown elsewhere [3]. The microstructures showed cast dendritic structure in both the cases.…”

“…%) alloy strips were produced by MC-TRC process [3]. The samples were homogenized at 400 C for one hour before hot rolling at the same temperature.…”

“…However, only a small quantity of material can be produced by these processes and also they are expensive and consume more energy. Twin roll casting (TRC) has been demonstrated to be a process capable of producing Mg sheets at significantly reduced cost [3,4]. However, the quality of the Mg sheets produced by the TRC process is limited by the formation of coarse columnar dendritic grains and centreline segregation, which reduces the strength and ductility of Mg-alloys.…”

“…However, the quality of the Mg sheets produced by the TRC process is limited by the formation of coarse columnar dendritic grains and centreline segregation, which reduces the strength and ductility of Mg-alloys. To encounter such problems, melt condition technique of intensive melt shearing by external field was employed before TRC process [3]. The strip obtained from MC-TRC process had fine equi-axed grain structure without any centreline segregation.…”

“…TRC samples have several casting defects [5], which has been minimized or eliminated by MC-TRC process. However, MC-TRC process failed to eliminate porosity inside the strips [3]. Such cast defects can be catastrophic during forming process, as it can initiate cracks.…”