CHARACTERIZATION OF SPRAY DEPOSITED COLD ROLLED Al-Si-Pb ALLOYS

Abstract

Al-Si-Pb systems are potential materials for bearing alloys because they possess superior performance like high hardness, good thermal conductivity, low wear rate, high corrosion resistance and low cost of production than other aluminum bearing alloys. Processing of these alloys by the conventional casting techniques is difficult due to liquid immiscibility and segregation of lead during melt solidification. To overcome these problems, spray deposition technique can be used which possesses several advantages like microstructural control together with producing a preform in a less number of processing steps. However, the major drawback of this technique is lower densification with poor mechanical properties. Therefore, the preform prepared by spray deposition technique must be further densified and deformed plastically in order to prepare fully dense metal sheets with improved mechanical properties. Rolling is most effective method used to densify the material because of having both hydrostatic and deviatoric components of the applied stress field. In present study Al-Si-Pb alloys were spray deposited in the form of a disc shape and then cold rolled to various percentages of thickness reductions. Further, their characteristics such as microstructure, porosity, hardness, wear and corrosion rate were studied. The porosity, hardness, corrosion, wear and microstructural studies of the Al-Si-Pb spray deposited alloys were conducted from centre to periphery of the preform, for different lead content and for different percentage of thickness reduction. Microstructural analysis of worn surfaces and worn debris were also performed. Samples from the central and peripheral regions of the preform were cut down and then cold rolled to different thickness reductions for its microstructural study. The size of the aluminum grains was almost same at the top and centre regions of the deposit whereas it was lower at the peripheral region. The lead was uniformly dispersed in the deposit and higher fraction of lead was distributed along the grain boundaries. The size of lead particles was found to increase linearly with the increase in lead content. The size of these particles was also higher at the centre as compared to that of periphery of the preform. Grains were observed to be elongated in the rolling direction after 60 and 80 % thickness reductions and aspect ratio of Al-grains was increased with the increase in thickness reduction. Pore size was reduced with the increase in thickness reduction and it was almost eliminated after 80 % thickness reduction. However, it was increased with the increase in Pb content. The porosity was measured for different percentage of thickness reductions at different locations of the preform. Two types of porosity were found in the samples, one was called as casting porosity and the other as crack porosity. The total porosity was decreased with the increase in thickness reduction however it was increased where the rate of increase in crack porosity was higher than the rate of decrease in casting porosity with the increase in thickness reduction. The porosity of the spray deposited alloy was found to decrease with the decrease in lead content and from centre to periphery of the preform. Also, it was unaffected along the thickness of the preform. The hardness of the spray deposit was observed to increase with the increase in thickness reduction and it was decreased with the increase in lead content. Preform hardness was increased from centre to periphery of the deposit. Hardness was affected by the grain size and porosity of the preform. It was found to decrease with the increase in porosity and grain size of the deposit. Friction and wear testing of spray deposited cold rolled samples were investigated using a pin on disk type wear testing machine. The spray deposited cold rolled samples with different lead content were machined into cylindrical pins having 5 mm diameter and 3 or 2.5 or 2 mm height. Wear rate behavior with applied load was observed to depict three stages, in the second stage rate of increase in wear rate was the lowest and in the third stage it was the highest. Also, the wear rate was decreased with the increase in thickness reduction irrespective of Pb content however it was increased where the porosity was increased. A linear relationship was found between wear rate and porosity of the samples. The wear rate was decreased from centre to periphery of the preform. It was decreased linearly with the increase in lead content up to 20 % and after that it was increased. The coefficient of friction was decreased rapidly up to the load of 30 N and beyond this load the friction coefficient was almost constant. The coefficient of friction was lowest for 20 % Pb content and it was decreased with the increase in thickness reduction. A linear relationship was observed between hardness and coefficient of friction of the samples. Corrosion tests were performed in aqueous 3.5 % NaCl (analytical grade, 99.5% purity) solution. A conventional electrochemical flat bottom cell was used for electrochemical investigations and all potentiodynamic polarisation tests were performed in a potentiostat at ambient temperature at a scan rate of 0.166 mV/s. A Teflon O-ring sealed the sample to the cell exposing an area of 1 cm2 to the solution. Corrosion rate was found to decrease with the increase in thickness reduction however it was increased where the porosity was increased. Also, it was decreased with the increase in Pb content and also from centre to periphery of the preform.