Hybrid Wire Arc-Directed Energy Deposition of Commercially Pure Titanium in Monolithic and Bimetallic Configurations

This study presents a novel approach to address the challenges associated with additive manufacturing of commercially pure titanium (CPTi) and its integration with stainless steel (SS316L) in bimetallic configurations. Despite CPTi's excellent corrosion resistance, biocompatibility, and mechanical properties, its high reactivity at elevated temperatures and tendency to form brittle intermetallics with steel pose significant processing challenges. We introduce an innovative localized shielding method for printing CPTi in monolithic and bimetallic configurations using a cold metal transfer (CMT)-based hybrid wire arc-directed energy deposition (WA-DED) process. A Cu-based alloy is used as an intermediate bond layer (IBL) between SS316L and CPTi.
Single-bead experiments were conducted to optimize process parameters. Defect-free monolithic and bimetallic structures are printed using these optimal process parameters. Monolithic and vertical bimetallic structures' phase composition, microstructure, and room temperature mechanical properties were investigated in as-processed (AP) and heat-treated (HT) conditions. Heat treatment proved effective in reducing residual stress in the CPTi region by over 90% for monolithic and nearly 80% for bimetallic configurations. X-ray diffraction (XRD) analysis revealed a preferred orientation along the (002) planes in both AP and HT specimens. Microstructural examination showed α grains in the CPTi region for all conditions. Notably, no cracking was observed at the bimetallic interfaces. The bimetallic interface between Cu-alloy and CPTi was nearly 100µm thick, while the same between Cu-alloy and SS316L was 5-20µm. The EDS revealed the diffusion of Cu at the SS316 and CPTi interface. The average microhardness at the interface of Cu-alloy and CPTi was 434±6 HV_0.2 and the same at the Cu-alloy and SS316L was 389±14 HV_0.2. The Mechanical testing demonstrated that the average yield strength of bimetallic structures was approximately 55% of the monolithic CPTi yield strength. To showcase the potential applications of this technique, we fabricated prototype radial bimetallic structures featuring an SS316L tube core with CPTi reinforcement coating. Our findings highlight the feasibility of printing reactive materials like CPTi in complex configurations using the proposed localized shielding method. This approach enables the production of defect-free monolithic and bimetallic structures, offering unprecedented design flexibility for additive manufacturing applications. The successful integration of CPTi with SS316L opens new avenues for developing tailored materials with enhanced properties for various industries, including aerospace, biomedical, and energy sectors.