Pistons for high power output demand its material to possess the properties of low thermal expansion, anti-seizure, wear resistance, high thermal conductivity, high creep and fatigue strength and high strength to weight ratio. Aluminium silicon alloys has excellent characteristics as a piston material. Due to design constraints in engines for heat dissipation and engine temperatures upto 300 ° C, the need for the study on the effect of thermal behaviour of the piston alloy during engine operation becomes important.
However a piston operating at 150° C with aluminum silicon alloy gradually loses its attained hardness in T6 condition and is not a constant during engine operation. This decrease in hardness of the alloy due to exposure to temperature and time is due to the phenomena called “age softening”. This phenomena occurs if the equilibrium phase diagram reveals partial solid solubility of the alloying element, at a higher temperature than at lower temperature.
Large decrease in hardness of the piston leads to piston wear and seizure. In order to retain hardness during softening, copper was added to the existing aluminium silicon alloy. Though maximum hardening effect can be obtained by addition of 3 to 6 weight %, the amount of copper addition is optimized to 3.4 weight percent, due to reasons discussed in the paper. The age hardening and softening behaviour data is determined and in the heat-treated condition the new alloy reveal peak hardness significantly higher than the existing alloy substantiated by presence of phases in scanning electron micrographs. As the time to attain peak hardness and the peak hardness of the new alloy is found to be higher than the existing alloy, the onset of age softening behaviour is delayed and also the hardness at the offset of the age softening is increased. Effect of softening of alloys at different temperatures was studied and master curves are plotted for hardness versus time at constant temperature. It was found that the hardness of the new alloy was higher at all the temperatures, the hardness decreasing with increasing temperatures.
Pistons made of new and existing alloy were heat treated for engine endurance. Hardness of the piston was measured at number of locations across the cross section of the piston before and after the engine test. There was a drop in hardness of the piston alloy at all locations and the hardness is minimum in the top center of the piston. This hardness decrease data is very important to estimate the piston temperature during operation. The temperatures during operation of the piston can be determined from the master curves of hardness versus time plotted. The hardness profile of the engine tested piston with new alloy is about 25 percent higher than the existing alloy, thus the age softening is retarded.