SPRAY FORMING OF AI-Si-Pb ALLOYS AND THEIR CHARACTERIZATION

Abstract

Processing of Al—Pb-Si alloys by the conventional casting techniques is difficult due to liquid immiscibility and segregation of lead during melt solidification. To overcome these problems, spray forming can be used which also possesses several advantages like microstructural control together with producing a near net shape preform in a less number of processing steps. In present study a convergent-divergent nozzle to atomize the melt was designed and fabricated. Before spray forming, a detailed gas field study of this nozzle was carried out using a pitot tube. Then, Al-Si-Pb alloys were spray formed in the form of a disc shape by using this nozzle and then their characteristics such as thickness uniformity of the disc, microstructure, porosity, hardness, strength, wear etc. were studied. The spray droplets were deposited over a rotating copper substrate to achieve a disc shape preform. The thickness uniformity of the preform was found to depend on the preform distance from the nozzle, preform offset distance from the axis of the atomizer and inclination of the preform from the horizontal plane. Mathematical equations were developed to represent the thickness variation of the preform with aforementioned parameters. The porosity, hardness, strength, wear and microstructural studies of the Al-Si-Pb spray deposited alloys were conducted for different lead content, top to bottom distance of the deposit and the distance from centre to periphery of the deposit. Overspray and worn debris particles were also analyzed. Particles/droplets of two different types were observed to deposit on the substrate. First type particles were aluminum rich and second type were lead rich. Optical micrographs were taken at four different locations of the deposit viz. (a) top (b) middle (c) bottom and (d) peripheral regions. The size of the aluminum grains was almost same at the bottom and top regions of the deposit whereas it was lower at the. peripheral region. In middle region the grain size was a little bit coarser than at the top or bottom region. In scanning electron micrograph the grain boundaries were not clear with the increase in lead content and maximum amount of lead was distributed along the grain boundaries. The lead was uniformly dispersed in the deposit as revealed by color dot map.