| Al/Fe joints have shown extensive applications in the fields like energy, ships,aircrafts, rail rolling stocks, and automobiles industries due to their significant advantages such as weight savings, and joining technology for them is a key to the engineering applications. As a novel welding process, friction stir welding(FSW)has been successfully applied to weld materials such as aluminum and magnesium alloys. Recently, FSW of dissimilar metal has been becoming a research hotspot.However, Al/Fe joints are difficult to be friction stir welded owing to their significant differences in physical and chemical properties; there exist some problems such as low joint efficiency. In this study, 3-mm-thick 6082-t6 aluminum alloy and 2-mm-thick Q235 A steel were friction stir lap welded, and the formation mechanism of surface appearance was explained. The influence of welding parameters on the joint microstructure and mechanical properties was investigated,and the formation mechanism of intermetallic compounds(IMCs) during FSW was clarified.The periodical surface wave is a prominent feature, and its formation mechanism is a critical factor in friction stir welds. The direct observations had been made on how wavy structures formed on the surface of the FSW processed samples. The three-dimensional(3D) morphology of surface wave was displayed by accurate depth from defocus(DFD). The material in leading and advancing side was forced to fill wave trough to form wave crest due to the shoulder sliding and material stacking action. The similar phenomenon was also confirmed in friction stir welding of other alloys. The wave was mainly influenced by the ratio of welding speed to rotation speed and material properties which in turn affected the mass of plastic material, and its shape changed from sine shape to checkmark shape once the ratio exceeded 0.1 mm/rev. A set of mathematical formulas were established to provide a description of the horizontal wavy structure, and these predicted that the variation of surface wave morphology was depending on the mass of plastic material transferred by the tool, which was affected by welding parameters and material properties.The microstructural characteristics of different regions in the Al/Fe joints as well as the dependence of joint properties on the microstructure were investigated.It was shown that the joint was composed of WNZ, TMAZ, HAZ, BM and IZ(Interfacial zone); the IZ characterized by vortex-like and swirl features, was composed of Fe or Al fragments, Al-Fe Al3-Fe/Al-Fe laminated composites, anddispersed Al/Fe composites; the joint ultimate tensile strength was low; the hardness distributions were consistent with the variation of the structures within the joint.The effect of welding process on microstructure and properties was clarified. With increasing the welding speed or decreasing the rotation speed, the proportion of IMCs and the density of laminated composites decreased, the joint strength increased. The tensile shear strength increases 43.8% with the increasing of the welding speed from 25 mm/min to 400 mm/min, and it reach the maximum 7400 N with the welding speed 200 mm/min and rotation speed 800 r/min.The formation thermodynamics of the IMCs at the Al/Fe interface during FSW were analyzed, and the formation mechanism of IMCs was presented. During FSW,the initial Al and Fe grains were elongated to form layered structures, and subsequently dynamic recrystallization occurred in Al and Fe. After pin passed,Fe Al3 sub-layer was first formed at the Al/Fe interface; while Fe Al3 grew towards the internal of Fe grains, Fe Al2 and Fe Al sub-layer was formed at the Fe/Fe Al3 interface, that is the formation of Al-Fe Al3-Fe Al2/Fe Al-Fe laminated structure. |
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