SOME STUDIES OF MECHANICAL BEHAVIOUR OF WELDMENTS OF ULTRA FINE GRAINED (UFG) AL ALLOYS

Abstract

Materials with high strength to weigh ratio and with high fatigue strength are very attractive for advanced aerospace applications. An Al alloy (7XXX) belongs to this category. To enhance the strength of Al alloys further, rolling at cryogenic temperature is one of the best options as its produces ultrafine grained structures. It is well known that material undergo severe deformation, yield strength and fracture strength will increase. However, the weldability of Al 7075 is poor and it generally considers an unweldable by arc welding process. Therefore, the present work has been envisaged to investigate the weld joint efficiency in high strength UFG Al 7075 processed by cryorolled materials through Friction Stir Welding. In first part of the present work, the solutionised plates of Al 7075 alloy subjected to rolling at cryogenic temperature using liquid nitrogen to true strain of 0.91.The effect of cryorolling strain on microstructure and mechanical properties were investigated using tensile tests, hardness test and optical micrograph study. A substantial improvement in strength (550MPa) has been observed in ultrafine grained Al 7075alloys as compared to its bulk counterpart (242MPa),In the second part of this work cryorolled Al 7075 alloys were welded by friction stir welding. The optimum processed parameter used to weld cryorolled samples were investigated by bead on plate studies in cryorolled samples. In addition, microstructural and mechanical properties were investigated across bulk and cryorolled weld. The fine micro structure in thermo mechanically affected zone and higher hardness in heat affected zone has been observed in cryorolled weld sample as compared with bulk Al weld.