Cryo Effect During Solidification on the Tribological Behavior of Aluminum-Alloy/Glass (SiO2) Metal Matrix Composites

The present investigation aims at producing cast aluminum alloy/SiO2 composites in molds containing cryo cooled copper chills by dispersing particles in molten aluminum alloy above the liquidus temperature, the size of the particles dispersed being between 50 to 100 mu m. The dispersoid being added ranges from 3 to 12 wt% in steps of 3 wt%. The resulting composites cast using cryo cooled copper chills were tested for their wear behavior. Subzero cooled copper chills of different thickness (10-25mm) was used to study the effect of chilling rate on the wear behavior of the composite developed. The wear tests were carried out using a computerized pin on disc type wear testing machine as per ASTM 325 A 98 standards. The superior mechanical properties of the castings, particularly their ultimate tensile strength (UTS), hardness, and wear resistance are discussed in relation to their microstructure. Dispersoid content up to SiO2-9wt% was found to increase the mechanical properties and therefore SiO2-9wt% is considered as the optimum limit. Results of the investigation indicate that, strength, hardness and wear resistance of the composite developed is highly dependent on the chilling rate and also the dispersoid content of the composite. Wear resistance of cryo chilled Al alloy/SiO2 composites were remarkably improved by adding SiO2 particles. The effect of amounts of dispersoid content up to 3 wt% can be ignored for all the MMCs until a low sliding speed of 0.44 m/s. From SEM structural studies, wear surfaces of 9 wt% composite chilled using copper chill of 25 mm thickness at low sliding speed showed slight groove formation than those of the other chilled MMCs. Chill thickness (25 mm), coolant used (liquid nitrogen) and chill material (copper) however does significantly affect volumetric heat capacity of the chill and hence strength, hardness and wear resistance of the composite.