Joint strength and failure studies of dissimilar AA6061-AA7075 friction stir welds: Effects of tool pin, process parameters and preheating

Abstract

Abstract The present study aims at bracketing the effect of tool pin geometry, process parameters e.g. tool rotational speed, feed rate, and use of additional heat source (preheating) during dissimilar friction stir welding (FSW) between 6061-T6 and 7075-T651 aluminum alloys. Experimental designs following Taguchi orthogonal array and subsequent statistical analysis are carried out to assimilate the influence on joint tensile strength and flexural load. Detailed micrographic observations of the weld cross-section, joint failure studies are performed in addition to the fractographic analysis. Tool pin profiles and tool rotational speed are observed most prevailing factors and when suitable tool pin profiles are used, the tool rotational speed and feed rate are observed most significant. For the dissimilar FSW joints where a sound weld is obtained using suitable tool pin and process parameter combinations, the tensile failure of the joints observed from the base metal of the advancing side (AA6061-T6). On the flipside, failure is observed from the side of the stir zone when material flow, mixing and joining is inadequate. During flexural loading, initiation of failure is mostly observed from the vicinity of the void/tunnel region of the weld nugget. Study of fracture surfaces using scanning electron microscope shows the presence of numerous dimples in the sound bonded joints ensuring ductile failure of the joints under tensile loading. However tearing, rupture and brittle failure is observed for poorly bonded joints. Energy dispersive spectroscopy of fracture surface also shows the presence of oxides in aluminum matrix for poorly bonded joints resulting in brittle failure of the joints.