Effect of Ageing on the Microstructure and Mechanical Properties of Al-Si Alloys with Copper Additions

Abstract

Aluminum has a density approximately one-third that of cast-iron or steel and have proved effective alternatives for many engineering applications. The good strength to weight ratio offered by Al-Si and Al-Si-Cu alloys has made these alloys popular as they present opportunities for weight reduction in automotive applications. The mechanical properties of aluminum can be enhanced by adding alloying elements such as silicon, copper, magnesium, zinc etc. Silicon as an alloying element has the ability of to reduce density and coefficient of thermal expansion, improve hardness, mechanical properties such as modulus and strength, thermal stability, wear resistance and corrosion resistance of aluminum. Further it also improves castability of aluminum also. This has created considerable interest among the materials and manufacturing engineers to explore the Al–Si family of alloys for possible applications in automotive, electrical and aerospace industries. Based on Si content Al-Si alloys are generally classified as hypoeutectic (up to 11%), eutectic (11 to 13 %) and hypereutectic (13 to 19 %) alloys. Grain refinement and modification of the alloy using suitable refiner and modifier improves the mechanical properties further. Addition of copper to Al-Si alloys induces precipitation of CuAl2 particles and depending on cooling rate and modifier level, this phase appears as blocky CuAl2 or fine eutectic colonies at the grain boundaries. Hence, an addition of Cu to Al-Si alloys improves the tensile strength. Solution heat treatment and ageing in Al-Si-Cu alloys, forms variants of CuAl2 leading to still better hardness and mechanical properties. The present work is carried out to investigate the influence of addition of copper (4.5 wt.%), combined modification and grain refinement, effect of T6 heat treatment process on hypoeutectic (7% Si), eutectic (12% Si) and hypereutectic Al-Si (15% Si) alloys. Al-1Ti-3B is used as grain refiner while Sr and P are used as modifiers. The effect of these processes on microstructure of Al-Si alloys, mechanical properties and tribological properties are investigated.IV Al-1Ti-3B grain refiner showed a markedly positive effect on the refinement of α-Al phase. Addition of Al-10Sr modifier was responsible for the modification of Si. The micro structural changes due to modification and grain refinement led to an improvement in UTS and % elongation of all the three types of alloys considered in this work. For Al-7Si the UTS increased from 154 MPa to 171 MPa (11%) while % elongation showed marked improvement increasing from 7.6 to 11.9 (56.6%). The corresponding improvement in UTS values for Al-12Si and Al-15Si were 18% and 22% and % improvement in elongation were 57 and 28, respectively. Addition of 4.5 wt.% Cu to Al-Si alloys was responsible for improved tensile strength and also resulted in improved sliding wear properties. This is attributed to the presence of CuAl2 as particle or eutectic in the Al interdendritic region. A combined addition of Al-1Ti-3B and Al-10Sr to hypoeutectic and eutectic alloys resulted in conversion of large α-Al grains in to equiaxed grains, plate like eutectic Si in to fine particles and changed the coarse CuAl2 morphology to fine eutectic colonies of Cu + CuAl2. A combined addition of Al-1Ti-3B grain refiner and AlP and Al-10Sr modifier to Al- 15Si-4.5Cu alloy provided a favorable microstructural change that led to superior mechanical and sliding wear properties. The heat treatment schedule applied in this study consisted of solution heat treatment at a temperature of 500°C for 6h; quenching in water at room temperature; ageing at a temperature of 200°C for varying time periods. Experimental results showed that heat treatment influenced the mechanical properties to a good extent in both the as-cast as well as combined modified and refined alloy. The Al-7Si-4.5Cu alloy showed a peak hardness of 126 BHN in as-cast condition where as the modified alloy showed 138 BHN. The UTS of melt treated Al-7Si-4.5Cu alloy increased from 208 MPa to 262 MPa (26% improvement), Similar trends were also observed for the other two alloys. The refinement during solution treatment and subsequent precipitation of fine CuAl2 particles may be the main reason for this improvement. The increase in strength due to Cu addition and age hardening of the alloy is balanced out by corresponding decrease in ductility. For 7Si alloy the ductility reduced by 22%V on adding Cu and further reduced by 22% on ageing for 20 h. However, the ductility is improved to a certain extent by grain refinement and modification. During dry sliding wear test, melt treatment reduced the wear volume loss of as-cast Al-7Si alloy by 25% and with Cu addition and melt treatment, the loss reduced by 40%. The solution treatment and ageing for 20 h improved the wear resistance substantially, with wear volume loss reducing by 79% as compared to as-cast Al-7Si alloy. Similar results were also observed for 12Si and 15Si alloys.