STUDY OF MICROSTRUCTURE AND MECHANICAL PROPERTIES OF ALUMINIUM ALLOYS CASTINGS MADE BY RHEOCASTING PROCESS

Abstract

The influence of vigorous agitation of A1..1+.5 Cu alloy (wt/percent) prior to pouring in the die has boon studied on micro-structure and mechanical properties of the casting. The melt was agitated at two different speeds of 500 and 550 RPM. This was followed up by pouring with different fraction solid so that slurries with varying viscosities were produced. The following significant conclusions were dram: (1) Excessive refinement of grains in the eauiaxed zone has been observed with increase in shear rate. There is an increase in solid precipitation with increasing fraction solid. - (2) The microstructure study showed that at lower fraction solid grains have an open dendrLtic structure but at higher fraction solid, grains becomes more compact and rounded. The CuAl2 compound is found to precipitate mostly at grain boundaries. The CuAl2 constituent as well as eutectic struc tare have tendancy to refine with increasing speed. (3) The ultimate tensile strength (.ZT.TS ), and Brinell hardness number (BHN) were found to first rise and then fall with increase of fraction solid. There was a signifi-cant increase in tensile strength of rheocasting as compared with the strength of conventional castings. (71) The percentage elongation however showed completely apposite behaviour as compared • with that of tensile strength. Potential Engineering advantages of this par-tially solid liquid slurries are fine grain size, lower casting temperature, less heat of fusion to be removed, less porosity and improved homogenity. Also the ability to vary viscosity over a wide range might be of use in reducing turbulence and spray fo ima tion during mold filling.