Interlayer thickening for development of laser-welded Ti-SS joint strength

Abstract

This paper investigates the mechanical strength and microstructural characterization of the laser beam-welded Ti6Al4V-304 stainless steel (SS) joints using different interlayer thicknesses at single and double interlayer structures. This study attempts to increase the Ti-SS joint strength through changing the Cu interlayer thickness. In single interlayer experiments using 0.5 mm-, 1 mm-, and 1.5 mm-thick Cu, the greatest strength was reached at a laser power of 4 kW using 1 mm-thick Cu. Energy dispersive spectroscopy (EDS) and X-Ray diffraction (XRD) characteristics showed that some FeTi and Fe2Ti were formed along the fracture propagation path of a 0.5 mm-thick Cu sample; however, no Fe-Ti IMCs or Cr2Ti could be found in the 1 mm-thick Cu sample. Meanwhile, αTi + FeTi and CuTi + CuTi2 compounds fractured in 0.5 mm- and 1 mm-thick Cu joints, respectively, after the tensile testing. Electron backscatter diffraction (EBSD) observation showed that the fracture happened through grain boundaries of CuTi and Cu3Ti2 compounds at a 0.5 mm-thick Cu joint; however, that happened approximately along the grain boundaries of Cu4Ti-Cu3Ti2 for the 1 mm-thick Cu joint. For double interlayer experiments, the XRD result of the SS-Cu0.5-V-Ti sample showed the formation of the Fe2Ti and FeTi at the fracture surface, while the fracture surface of the SS-Cu1-V-Ti sample primarily contained some Cu-Ti IMCs. The microstructures of the fracture edges indicated that the eutectic FeTi + Fe2Ti and eutectoid αTi + Ti2Cu fractured after the tensile testing at the SS-Cu0.5-V-Ti and SS-Cu1-V-Ti samples, respectively. EBSD observation indicated that the fracture happened primarily along the low angle grain boundaries of Cu3Ti2 and αTi compounds at the SS-Cu1-V-Ti joint; however, the fracture occurred intergranularly along the high angle grain boundaries of the Fe2Ti in the SS-Cu0.5-V-Ti joint. Investigation showed that the joint strength and also the elongation of the SS-Cu1-V-Ti joint are approximately 179% and 10 times greater than those found in the SS-Cu0.5-V-Ti joint.