Solid-state joining of titanium alloy to stainless steel

Abstract

Titanium has a wide variety of applications in a multitude of industries but can be costly in large quantities. To reduce the amount of titanium alloys needed and therefore the cost of materials, some suggest designing parts made with both titanium alloys and stainless steels. Current joining methods produce intermetallic compounds (intermetallics) at the joint interface which are detrimental to joint strength. In this study, dissimilar joints between 436L stainless steel and a titanium alloy, Ti 1.2 ASN, were made by the solid-state welding methods of vaporizing foil actuator welding (VFAW) and mash seam resistance welding (MSW). A Nb interlayer was used in the MSW process as a diffusion barrier due to the relative higher heat input and longer welding time in comparison to the VFAW process. The welds were evaluated to identify correlations between microstructural and mechanical properties. Microstructural characterization of the base materials and solid-state joints was performed by optical microscopy, scanning electron microscopy (SEM) with energy dispersive x-ray spectrometry (EDS) and hardness testing using both Vickers microhardness and nanoindentation. Mechanical properties were tested by means of tensile and tensile-shear testing with in-situ digital imaging correlation (DIC) to map localized strain. In the VFAW joints, jet-trapped zones and discrete regions of Ti-Fe intermetallic compounds were observed along the weld interface. The maximum shear strength was observed to be about 227 MPa and failure occurred through the Ti 1.2 ASN base material. In the MSW joints, the 436L stainless steel reacted with the Nb interlayer and formed a hard reaction layer ranging from 42 μm to 480 μm in thickness that was rich in Cr, Fe, and Nb. No reaction between Ti 1.2 ASN alloy and the Nb interlayer was observed. The maximum tensile strength for this samples was about 428 MPa. The MSW joints did not fail at the joint interface despite the presence of a hard reaction layer, this could be partly attributed to the larger thickness in the lap-joint geometry than that of the base materials.