INVESTIGATION ON FRICTION STIR WELDING OF DISSIMILAR ALUMINIUM ALLOYS AA 7075 AND AA 1200 BUTT JOINTS

Abstract

Dissimilar aluminium alloys of AA1200 and AA7075 have significant applications in automobile sector, aerospace, defence and shipping industries. These alloys faces problems like hydrogen solubility, formation of aluminium oxides, solidification shrinkage etc when welded with fusion welding process. Friction stir welding is the process which is able to successfully weld these dissimilar alloys of aluminium. Process parameters like rotational speed, welding speed, tool design, vertical force plays important role in determining the joint properties. Rotational speed and welding speed has its major role in producing the necessary frictional heat to plasticise the material which on solidification produces the weld joint. After developing workable process parameters (process window) from preliminary welds, the welds are taken on rotational speed of 900 rpm to 2100 rpm and welding speed for 30 mm/min to 90 mm/min using the response surface methodology approach. The tool geometries were varied as tapered tool (TT) and tapered threaded tools (TTT). The tensile test performed shows that the ultimate tensile strength for the TT ranges from 118-152.48 MPa translating to a joint efficiency range of 65.78-84.71%, microhardness ranges from 77-99.72 HV, while for TTT the ultimate tensile strength ranges from 130.52-161.84% translating to a joint efficiency of 72.52-90.32 %, microhardness ranges from 77.8-105.75 HV . Rotational speed of 1500 rpm, Traverse speed of 60 mm/min and Tilt Angle of 2o were obtained as optimal welding parameters for the tapered (TT) and tapered threaded tools (TTT) and Tapered threaded tool weldment gave better mechanical properties. The result of the corrosion test performed revealed that TTT weldment performed better at optimal level in terms of corrosion resistance with a corrosion rate of 0.497 mm/year as against 0.698 mm/year obtained using the tapered tool (TT). The microstructures obtained using both tool geometries revealed presence of onion ring formation which is an indication of mixing or material coalescence at the optimal level of parameters. It was recommended that compressive and bending strengths of the weldment of the two dissimilar aluminium alloys should be evaluated, friction stir welding in lap configuration arrangement should be conducted on the two dissimilar alloys for other application purposes, further research should be performed on the effect of residual stress on the weldment and that the effect of the axial load on the mechanical properties of the weldments be investigated.