Dissimilar joining of A7075 aluminum and SS400 steel utilizing center-driven double-sided linear friction welding using mild steel as a center material: Processing, mechanical and microstructure characterization

Abstract

In this study, conventional direct linear friction welding of SS400 steel and A7075 aluminum was proven challenging because of absence of interfacial plastic deformation towards SS400 during joining, revealing several un-jointed regions throughout the joint interface, which eventually led to a poor joint strength of 77.6 MPa with interfacial fracture. Therefore, center-driven double-sided LFW (CDDS-LFW) is employed to effectively weld SS400 and A7075, with mild steel (MS) used as the center material. Using CDDS-LFW method, a highly efficient weld revealing 100 % joint efficiency concerning MS was obtained successfully by applying different pressures each side. Additionally, the fabricated weld exhibited a base metal fracture in the MS region away from both joint interfaces. The applied pressures were determined based on the cross-point concept after analyzing the thermal dependence behaviors of materials' strengths. A pressure of 50 MPa was applied at MS/SS400 interface, where the materials were simultaneously deformed and joined at high temperature, while 300 MPa was applied at MS/A7075 interface, where both materials were simultaneously deformed and joined at low temperature. This approach enabled the control of welding temperature at both joint interfaces by changing applied pressures on each side. Subsequent mechanical and microstructure investigations were carried out both at center and edge of the fabricated joint. SEM observation confirmed the absence of un-jointed regions and weld defects throughout both the joint interfaces of dissimilar CDDS-LFW weld, ensuring a sound joining. Moreover, microstructure evolution through EBSD analysis revealed the extremely fine-grained microstructure in the joint interface region compared to coarse grain base metal regions.