CHARACTERIZATION OF SPRAY FORMED ALUMINUM ALLOYS

Abstract

Processing of Aluminum alloys (Al- Graphite and Al-Si-Pb) by the conventional casting techniques is difficult due to liquid immiscibility and segregation of second phase particles during melt solidification. To overcome these problems spray forming can be used which also possesses several advantages like micro structural control together it producing a near net shape preform in less number of processing steps. In present study characterization of spray formed Al-8Graphite and Al-6Si-l0Pb were studied. Optical micrographs were taken at three different locations of the deposit viz. (a) top (b) middle and (c) bottom regions. The size of the aluminum grains was almost same at the bottom and middle regions of the deposit where as it was lower at the top region. The lead and graphite was uniformly distributed in the deposit as revealed by color dot maps. The hardness of both Al-Graphite and Al-Si-Pb are calculated and it was found that the hardness of Al-Si-Pb is more compared with Al-Graphite alloy. Preform hardness is almost constant with the increase in distance from bottom to top of the deposit. Hardness was not affected by the grain size although it decreased with the increase in porosity of the deposit. In order to reduce the porosity, the strips were rolled to various thickness deformations in the range 20-80%. During rolling, both densification and deformation in the Al—graphite deposit take place simultaneously. In the initial stages of rolling (i.e. about 20% thickness deformation), the metal flow in the Al—graphite deposit is mainly in the thickness direction resulting in the removal of porosity by rearrangement and restacking of spray deposited particles. Beyond this, plastic deformation becomes the predominant mechanism of densification during rolling. As a result the pores are removed by a process involving pore elongation in the direction of rolling followed by fragmentation into several smaller size pores.