Ti-Fe intermetallics analysis and control in joining titanium alloy and stainless steel by Laser Metal Deposition

There are urgent needs to join titanium alloy and stainless steel so that their outstanding mechanical properties can be utilized integratedly in the aeronautics and astronautics industries. However, direct fusion joining two alloys can cause brittle Ti-Fe intermetallics which compromise the mechanical properties of diffusion bonds between titanium alloys and stainless steel. Therefore, filler metals are required as transition layers. In this research, stainless steel metallic powder was directly deposited on the titanium alloy substrate by laser beam, the Ti-Fe intermetallic phases formed in this process were investigated through analyzing fracture morphology, phase identification, and Vickers Hardness Number (VHN). After that, Laser Metal Deposition (LMD) was applied to explore a new fabricating process to join Ti6Al4V and SS316. A transition composition route was introduced (Ti6Al4V → V → Cr → Fe → SS316) to avoid the intermetallic phase between Ti6Al4V and SS316. A thin wall sample was fabricated via LMD following the transition composition route. X-ray Diffraction (XRD) tests were conducted. The results demonstrate that the generation of intermetallic phases is effectively avoided following the composition route. Microstructure characterization and composition distribution analysis were performed via Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometry (EDS). The SEM results indicated that rapid solidification results in the elongated microstructure. The EDS result can reflect the transition composition route design. Besides, the diffusions of metals are detected in the EDS results. Vickers hardness test was executed to observe the VHN distribution from Ti6Al4V to SS316. Vickers hardness result showed that there was no significant formation of intermetallic phases. Comparing with directly depositing SS316 on Ti6Al4V, the usage of the new transition route can eliminate the Ti-Fe intermetallics effectively.