FRACTURE AND FATIGUE STUDIES OF FRICTION STIR WELDED JOINTS OF ALUMINIUM ALLOYS

Abstract

High-strength aluminium alloys generally present low weldability because of the poor solidification microstructure, porosity in the fusion zone and loss in mechanical properties when welded by fusion welding processes which otherwise can be welded successfully by comparatively newly developed process called friction stir welding (FSW). Friction-stir welding (FSW) is an autogenous solid-state welding process in which the material being welded is not melted and recast as in case of conventional fusion welding processes but plastically deformed, extruded and forged to form weld joint at temperatures below melting point. In FSW a rotating tool with a shoulder and terminating into a specially designed pin, moves along the faying surfaces of two rigidly clamped plates placed on a backing plate. The localized heating produced by friction at the shoulder and to a lesser extent at the pin surface softens the material around the pin. This plasticized soft material is transported from the front of the tool to the trailing edge due to tool rotation and translation where it is forged into a monolithic joint. The purpose of this dissertation is develop the FSW joints of different aluminium alloys and their mechanical and metallurgical characterization. Friction stir welding is first time developed in this department. The friction stir butt welds between plates of size 150mm x 50mm x 6mm of different grade of aluminium alloys like commercial structural aluminium, AI-Zn-Mg (A 7039) alloy and Al-Mg-Mn (A 5086) alloy were obtained. The weld sample then post weld heat treated for solution heat treatment at 550° C for 4 hours, water quenched and then artificial aged at 190° C for 6 hours. Microstructural and mechanical characterization of these weld sample in as welded and post weld heat treated condition were carried out. Optical microscope and scanning electron microscope used to determine the microstructure and other unique features associated with different weld zones. EDAX, XRD and EPMA analysis was carried out to understand the elemental composition of base and welded material. Mechanical properties of these welds examined by Vickers microhardness test (Hv), tensile test, fatigue test and charpy impact test.