PROCESSING AND PROPERTIES OF Y2O3 REINFORCED Al-7075 COMPOSITES BY POWDER FORGING

Abstract

Aluminum alloy composites are in recent times finding applications in critical areas of aerospace as well as automobile industries. Aluminum alloy 7075 based composites are suitable for these applications as they offer a high strength to weight ratio coupled with good mechanical as well as tribological properties. The present investigation deals with fabrication and characterization of Al-7075 alloy matrix composites with micrometric as well as nanometric yttria (Y2O3) as reinforcement through powder forging route. The powder mixtures (Al-7075 and Y2O3 of various volume fractions) were uniaxially cold compacted in a closed rectangular die of cross section 47 x 22 mm using a hydraulic press at 350 MPa. The green compacts were then sintered at 610oC for 40 minutes under nitrogen atmosphere to obtain the sintered performs. Further, the hot forging of these preforms was carried out leading to forged samples of close to full density. Based on the above mentioned processing route, Al-7075 forged compacts and Al-7075 forged composites with varying micrometric and nanometric Y2O3 were fabricated. Four different studies were carried out. In the first study, microstructural evolution of the sintered preforms of Al-7075 during hot forging at four different temperatures (0.6, 0.7, 0.8 and 0.9 Tm) and under various strains (0.51, 0.92 and 1.14) were studied. The effect of strain at various forging temperatures on the particle morphology, interparticle porosity, prior particle boundary (oxide layer) and hardness were investigated. This was carried out to optimize the forging conditions i.e. forging temperature and strain. In the second study Al-7075 composites reinforced with 1, 3, 5, 10 and 15 vol % Y2O3 of micrometric size (~4 μm) and 0.1, 0.5, 1 and 3 vol % Y2O3 of nanometric size (~10 nm) were fabricated based on processing parameters optimized in the earlier study. Density, hardness, tensile and compressive properties were determined and correlated with the reinforcement size/content and microstructures. In the third study tribological properties (coefficient of friction, wear rate and wear surface morphology) of these composites were studied at three different loads of 10 N, 20 N and 30 N and sliding speeds of 1 m/s and 2 m/s. The fourth study dealt with machinability studies of these composites using a radial drilling machine with the characterization of drilling forces, cutting torque, surface roughness and chip geometry. The microstructural studies carried out at different temperatures and strains revealed that a temperature of 0.8 Tm and a true strain of 0.92 are required for the disruption of oxide layer during hot forging of the powder preforms. The hardness values reveal that the strain of 0.92 is sufficient ii for optimum deformation and consolidation of the particles even at lower forging temperatures of 0.6 Tm. The densification and mechanical properties of the composites were profoundly affected by the particle size and volume fraction of the Y2O3 reinforcement. The hardness with nano Y2O3 reinforcement was higher than that of micron Y2O3 in the T6 condition. Both the tensile strength and compressive strength values were higher with the incorporation of nano Y2O3 as compared to micron Y2O3 in the T6 condition. The hardness values as well as tensile strength in both the cases peaked at a volume fraction of 0.5% for nano Y2O3 and at a volume fraction of 5% for micron Y2O3. The basic wear mechanism of pure Al-7075 aluminum alloy and composites consisted of adhesive wear with plastic deformation followed by abrasive wear. Major wear in the samples was due to ploughing, oxidation and delamination. Wear resistance improved with the addition of Y2O3 reinforcement in aluminum alloy. Wear rate for all the composites decreased initially with addition of Y2O3, optimized at a fixed composition and then again increased with further addition of Y2O3. The lowest wear rate was achieved for 5 vol% of micrometric size and for 0.5 vol % for nanometric size Y2O3 addition. At a constant volume fraction, the overall wear was found to be lesser for the composites having nanometric Y2O3 as compared to micrometric Y2O3 for all the test conditions. The machining forces (thrust forces as well as cutting torque) exhibited a similar trend as that of the hardness of the material in both the solutionized as well as in the T6 condition. Moreover the thrust forces were higher in the T6 condition as compared to the solutionized condition due to the higher hardness in the peak aged condition. As compared to the Al-7075, the surface roughness was found to be higher for the composites in the solutionized condition. On the other hand the surface roughness was lower for the composites as compared to the pure alloy in the T6 condition.