MECHANICAL BEHAVIOUR OF ULTRAFINE GRAINED Al ALLOYS

Abstract

Materials with high strength to weight ratio and with good ductility are very attractive for advanced structural applications. An Al alloy (6XXX) belongs to this category. To enhance the strength of Al alloys further, rolling at cryogenic temperature is one of the best options as it produces ultrafine grained structures. It is well known that when the material undergo sever deformation, yield strength and fracture strength will increase. However, this increase in strength is usually accompanied by a loss in ductility .Achieving a combination of high strength and ductility is still a challenge. Therefore, the present work has been envisaged to investigate the mechanical properties of ultrafine grained Al alloys (A1-6061) produced by cryorolling. The optimum annealing and ageing conditions to improve both strength and ductility of Al 6061 alloys has been identified. In the first part of the present work, the solutionised plates of Al 6061 alloy was subjected to rolling at cryogenic temperature using liquid nitrogen to true strain of 2.2.The effect of cryorolling strain on microstructure and mechanical properties were investigated by using tensile tests, hardness tests, XRD, Optical micrograph study, OIM study(EBSD) ,DSC and TEM.A substantial improvement in strength (296 MPa) has been observed in ultrafine grained Al 6061 alloys as compared to its bulk counter part (140 MPa). In the second part of this work cryorolled Al 6061 alloys were subjected to short annealing and further ageing at different conditions. The microstructure and mechanical properties were investigated by using above mentioned techniques. The pre-cryorolled solid solution treatment combined with post-cryorolled short annealing (200°C for 5 min.) and then ageing treatment (120 °C for 46h) has been found to be the optimum processing condition to obtain the ultrafine grained microstructure with substantial improvement of tensile strength (372 MPa) and good tensile ductility (12.5%) in the Al 6061 alloy. The significant improvement of the mechanical properties of the cryorolled peakaged 6061 Al alloy has been observed as compared to its bulk Al 6061 alloys in the peak-aged condition (T6).