Investigation of Microstructure and Mechanical Properties of Friction Stir Extruded Joints of Dissimilar Materials

Abstract

Abstract The welding of steel and aluminum is difficult to join because of their differences in solid solubility, melting points, and formation of intermetallic compounds (IMCs), which may weaken the welded joints. Friction stir welding (FSW) has more feasibility for joining aluminum and mild steel (MS), but IMCs may form, affecting the joint properties. Friction stir extrusion (FSE) is a new derivative technique of FSW that reduces the tool wear and eliminates the formation of IMCs by forming mechanical interlocking joints at the interface. This present study applies the FSE process to join two dissimilar metals: Al6082 alloy and AISI 1020 MS. The FSE process is optimized by adjusting axial force, tool rotation rate, and traverse rate parameters. The mechanism of groove filling was observed and compared (at optimum parameters) with two different profile tools: one is a plain tapered tool and another is a tapered threaded tool; both were made of high-speed steel. The aluminum-MS joints were subjected to shear strength by conducting tension-shear tests using a dynamic servo-controlled fatigue testing machine, as per ASTM E466, Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials. The hardness at the joint is evaluated by conducting Rockwell hardness tests, as per ASTM E18, Standard Test Methods for Rockwell Hardness of Metallic Materials. The microstructural characterization is done using optical electron microscope (OEM) and scanning electron microscope (SEM). The obtained results concluded that 30 mm/min is the optimum traverse rate for complete filling of the groove at 14.7-kN axial force and a tool rotation rate of 1,000 rpm. Furthermore, the discussions were made using hardness tests, OEM, SEM, and energy-dispersive spectroscopy analysis.