Impact of varying tool position on the intermetallic compound formation, metallographic/mechanical characteristics of dissimilar DH36 steel, and aluminum alloy friction stir welds

In this study, an attempt was made to join one type of high-strength shipbuilding grade steel, i.e., DH36 steel with 6061aluminum alloy (AA6061), using the friction stir welding technique. The effect of tool offset on micro/macrostructure and mechanical properties of the welded joints was investigated. The intermetallic compound (IMC) formation in the various regions, such as aluminum/steel interface, stir zone (SZ), and mixed zone (MZ), was examined through energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis. It was observed that the ultimate tensile strength (UTS) increased with an increase in tool offset from 0.4 to 1.5 mm and then reduced with a further increase in tool offset from 1.5 to 2.5 mm. Sample S4 (i.e., tool offset of 1.5 mm) exhibited the maximum UTS of 207.6 MPa, which was about 76.16% of the AA6061 base material. The lower UTS at other tool offsets could be attributed to the cross-sectional defects, i.e., voids and cracks at the interface, inadequate IMC layer thickness, distribution of steel fragments, and IMCs inside the SZ. The AlFe and AlFe3 IMCs were observed in MZ at a tool offset of 0.4–1.5 mm. However, Al2Fe and AlFe were identified in MZ at a tool offset of 1.8 mm and 2.2 mm. The aluminum/steel interface exhibited the Al3Fe and Al5Fe2 IMCs. These IMCs significantly affected the mechanical properties of the welded joints. The irregular distribution of hardness was attributed to the scattering of steel fragments and Al3Fe IMCs inside the AA6061 matrix.