EFFECTS OF THERMOMECHANICAL PROCESSING ON THE MICROSTRUCTURE, TENSILE STRENGTH AND PLASTICITY INDICES OF 6063 ALUMINIUM ALLOY

Abstract

An investigation has been carried out on the effect of the variation of thermomechanical processing parameters on the microstructure, tensile strengths and plasticity indices of 6063 Aluminium alloy. It is intended to determine the possibility of developing a high tensile strength plasticity indices alloy through refining of microstructure by thermomechanical ageing route, thereby expanding the horizon of its applications in machine building, automobile, architecture and civil engineering industries. The as received alloy samples were solution heat treated (SHT) at 530°C for 2hours and quenched in water to room temperature. The study involved initial characterization of the thermoplastically worked AI-Mg-Si alloy, after solution heat treatment, as a singular operation employing varying degrees of plastic deformation (10%-50%) at different working temperatures (28°C, 160°C, 180°C, 200°C, 220°C and 240°C). Similarly, artificial ageing of the solution heat treated alloy samples was carried out at different ageing times (1-20hrs) and temperatures (160oC, 180°C, 200°C, and 220°C), followed by the characterization of the aged alloy samples. Both thermoplastic working and artificial ageing were carried out as preliminary studies. Finally, the solution heat treated alloy was subjected to various thermomechanical ageing treatments in order to give room for a exhaustive assessment of the effects of process variables (degree of preageing, % deformation, deformation temperature, final ageing time and temperature) on the microstructure, tensile strengths and plasticity indices of the 6063 Aluminium alloy. Tensile and impact tests were carried out on the processed alloy samples as means of property assessment, as well as microstructural examinations using the optical and transmission electron microscopes. Attempt was also made to develop mathematical models capable of relating tensile strength and plasticity as a function of processing parameters. The results show that thermomechanical processing has significant effect on the microstructure, tensile strengths and plasticity of the alloy. Precisely the properties were enhanced within certain ranges of the values of the processing parameters, while same properties were impoverished within some ranges of the values of the processing variables. However, thermomechanical ageing processing route TMAIIIC(9hrs preageing time, 30% warm deformation, 10hrs final ageing time and water quenched to room temperature), produced the best results in terms of tensile strength-plasticity indices of the alloy. It is therefore concluded that thermomechanical ageing techniques could be a highly useful tool for developing higher strength-plasticity property in age-hardenable alloys, as demonstrated in the studied 6063 Aluminium alloy. Likewise, improved understanding of the fundamental mechanisms of alloy strengthening by thermomechanical ageing can by appropriate process control, help in the creative task of alloy development. The success of an alloy development effort, of course, always depends on the competition with existing materials, and on the search for new demanding applications on the part of industrial alloy users. Scientifically and technologically, thermomechanical ageing is ready for such a challenge, in particular for age hardenable alloys.