Development of nickel-titanium graded composition components

Purpose - Three layer-additive manufacturing methods were evaluated to producing nickel-titanium graded composition material. One potential application is fabrication of attachment clips that join thermal protection systems to launch vehicle structure. Thermal gradients during flight generate excessive bending and shear loads that limit the service lifetime of the Inconel clips currently used. It is envisioned that a graded composition component could be tailored to reduce the stress concentrations. Design/methodology/approach - Deposits with nearly continuous composition grade were built from Ti-6-4 and Inconel 718 powder using laser direct metal deposition. Layered deposits were produced by flat wire welding from Ti-6-4 and Inconel 718 wire. Ultrasonic consolidation was used to produce layered deposits from pure nickel and commercially pure titanium foils. Microstructure, bond line morphology, chemical composition, and reaction phases were characterized. Findings - All three manufacturing methods require further development before graded composition material can be reliably produced. Laser direct metal deposition samples exhibited coarse dendritic microstructures and significant elemental segregation. Chemistries varied from calculated targets by up to 20 percent and macroscopic cracking occurred for chemistries greater than 60 percent Inconel 718. Flat wire welded deposits exhibited good mixing between the wire layers, however brittle cracking occurred adjacent to a 5 mum thick reaction layer. Ultrasonically-consolidated samples demonstrated metallurgical bonding between pure Ni and commercially pure (CP) Ti foils, with material reaction limited to a 1 mum layer. Originality/value - Producing nickel-titanium graded composition materials had not been attempted by the selected manufacturing methods. A refined experimental program is needed to resolve the remaining technical issues.