Development and characterization of Al-Si based functionally graded material through Directional Solidification

Abstract

Many structural components encounter service conditions and hence required material performance vary with location within the component. It is well known that abrupt transitions in materials composition and properties within a component often result in sharp local concentrations of stress, whether the stress is internal or applied externally. It is also known that these stress concentrations are greatly reduced if the transition from one material to the other is made gradual. By definition, functionally graded materials (FGM’s) are used to produce components featuring engineered gradual transitions in microstructure and/or composition, the presence of which is motivated by functional performance requirements that vary with location within a part. With functionally graded materials, these requirements are met in a manner that optimizes the overall performance of the component. The research on FGM’s is encouraged by the need for properties that are unavailable in any single material and the need for graded properties to offset adverse effects of discontinuities for layered materials. Directional solidification is the common method for fabricating FGM’s which is mainly a composite material which has high differences of density and low solubility on different phases or different materials of the same alloy. The main thrust in the present work is to fabricate FGM’s for solid valve lifter applications. The first phase of study was to fabricate Al-Si based and Al-Si reinforced with 2wt% graphite FGM’s, to accomplish this a novel directional solidification technique combined with lateral vibrations was developed. In the second phase of study, optimization of process parameters i.e. chill material, chill volume and pouring temperature were carried out using taguchi technique through the experiments performed according to the L27 orthogonal array. The optimization was carried out by investigating the influence of process parameters(chill material, chill volume and pouring temperature) on hardness values across the top and bottom portion of the cast specimen. The castings obtained by both with vibrations and without vibrationswere tested. From the obtained results it was revealed that optimized samples obtained with lateral vibrations exhibited better variation in properties compared to the ones obtained without vibrations In the third phase of study, evaluation of mechanical and tribological characteristics was carried out on the optimized sample of Al-Si based FGM and Al-Si reinforced with 2wt% graphite FGMs to understand the significance of lateral vibrations. The FGM’s have been analyzed for microstructure by the scanning electron microscope (SEM) morphologies which revealed that concentration of Si was more at the top portion. The spatial transition of Si can be attributed to the presence of the chill at the bottom and to the influence of lateral vibrations which has led to variation of properties within the structure, these observations were also supported by the hardness values. Ultimate tensile strength (UTS) results also showed shift in the properties from bottom to top portion of the cast. Wear analysis carried out at the top and the bottom portion of the FGM showed that there is a decrease in wear loss, coefficient of friction and specific wear rate at the top portion compared to the bottom portion. Worn-out surface analysis revealed that graphite addition had imparted the self-lubricating property at the top portion which could serve as anti-friction and anti-wear applications in the automotive sector.