Microstructure and Mechanical Properties of Dissimilar Friction Stir Welding of AA6061-T6 and AA7075-T6 under Water Cooling Condition

The application of aluminum alloys has been growing in the past decades in many industries. The main group of aluminum alloys, which are most often used, are the wrought alloys. With the growth of wrought alloys within the industries, the friction stir welding (FSW) of these materials has become attractive. Many alloys have been subjected to similar and dissimilar FSW in different ambient conditions. Recently, FSW conducted in water cooling (WC) condition has showed improvement in mechanical property of the welded joints. However, lack of research in joining of dissimilar alloys in WC condition is obvious. In this dissertation FSW of dissimilar materials of AA6061-T6 and AA7075-T6 is investigated. The materials are meant to be in T6 (artificial aged hardening) condition. T6 materials are considerably harder and stronger due to formation of β phase (precipitates) unsolved in the supersaturated aluminum matrix during the hardening process. Properties of joined wrought aluminum alloys depend on FSW parameters due to generation of various heat signatures. Therefore, heat input during the process may count as a risk to the strength of materials if the generated heat is large and steady enough to dissolve the precipitates. Aim of this dissertation is to investigate dissimilar FSW under WC condition with variable parameters of traverse and rotational speed and to find the optimized process parameters. Ultimate tensile strength (UTS) analysis and hardness (H) were obtained. In addition, microstructure analysis of the joints were evaluated by transmit electron microscope (TEM), optical microscope (OM) and scan electron microscope (SEM). The results revealed that higher traverse speed and lower rotational speeds generate lower heat input which led to finer average grain size (Dav) thus larger H and UTS and lower elongation (El%). The grain boundaries and dislocations were identified as responsible for the higher H and UTS of the joints welded at lower heat input conditions. Moreover, the Hall–Petch relationship showed a deviation from its linear classical equation, which was due to the formation of substructures such as dislocations inside the grains. In comparison with the optimum condition, higher heat inputs caused grain growth and reduction in dislocation density and hence led to lower H and UTS and larger El%. The results show that by adopting WC FSW instead of FSW, 6% and 4% improvement in UTS and El% is achieved respectively. Furthermore, mathematical models were developed to predict the UTS, El%, H and Dav which are evaluated to be precise. Keywords: Friction Stir Welding, Aged Hardened Aluminum Alloys, Water Cooling.