Regulating Ti/Cu Interfacial Reactions via an Ultra-Fine Laser Beam for High-Strength Ti-6Al-4V/Incoenel 718 Dissimilar Joints

Ti/Ni dissimilar structures leverage the complementary properties of titanium and nickel alloys, making them attractive for aerospace and related applications. A common approach to join these materials involves laser welding with a single Cu interlayer or multilayer interlayers inserted at the Ti–Ni interface. Nevertheless, such joints are often compromised by the formation of brittle intermetallic compounds (IMCs), resulting in limited tensile strengths typically in the range of 300–400 MPa. In this study, an innovative and patented laser welding process was introduced to reconfigure the interfacial microstructure by precisely controlling the power density distribution and thermal field. The method employs a single-mode ultra-fine laser beam (spot diameter ≈ 0.04 mm) as the heat source, combined with a single Cu interlayer. For comparison, laser welding with a conventional 0.4 mm spot was also performed. To further suppress undesirable interfacial reactions, the beam position was varied between the Cu/IN718 interface and the center of the Cu interlayer. Finite element thermal simulations were utilized to quantitatively assess fusion widths, peak temperatures, and cooling rates. These were coupled with BSE/EDS, TEM, XRD, and nanoindentation analyses to characterize the phase composition, elemental distribution, and localized mechanical properties within the Ti-side transition layer.The results demonstrate that the proposed process considerably increases cooling rates, minimizes base metal melting, and effectively restricts the diffusion of Ni, Fe, and Cr toward the Ti/Cu interface. As a result, the transition layer on the Ti side evolves from a non-uniform bilayer containing brittle IMCs into a Ti–Cu gradient structure predominantly composed of Ti₂Cu, TiCu, and TiCu₂, exhibiting continuous composition and hardness profiles. When the beam was aligned with the Cu/IN718 interface, a thin and straight transition zone formed. Conversely, focusing the beam at the center of the Cu interlayer promoted enhanced melting of TC4 and Ti diffusion into Cu-rich regions, generating a tortuous gradient transition layer reinforced by Ti-based IMCs. This microstructural configuration altered crack propagation paths and mitigated stress concentration at the interface. Consequently, the joint tensile strength increased from approximately 260 MPa (large-spot welding) to about 430 MPa (ultra-fine spot at Cu/IN718 interface) and 538.8 MPa (ultra-fine spot at Cu interlayer center), substantially surpassing the reported performance of conventional Ti/Ni joints. This work presents a novel and promising laser welding strategy for high-quality laser joining of dissimilar material structures.