The high deposition rate additive manufacture of nickel superalloys and metal matrix composites

Abstract

The deposition rate of Additive Manufacture (AM) processes are a significant factor for the economic production of metallic materials by AM. Higher deposition rates must be achieved if the technology as a whole and nickel alloys in particular are to be more widely adopted within industry. This thesis investigates the potential of two techniques, high power (>1kW) laser beams within a powder bed laser melting (LM) configuration and Plasma Transferred Arc Welding (PTAW) using a wire fed approach for the deposition of Inconel 625, a widely used nickel superalloy. The processing parameters required for stable deposition of material in both single welds and multiple layers was determined, and the deposited material characterised. High deposition rate powder bed LM using 500μm layer thicknesses was conducted and a process stability map for single welds was characterised. Multi-layer multi-weld samples achieved an acceptable relative material density of 99.8%, using a reduction in laser power with increasing height as a thermal control strategy to achieve a deposition rate of 0.023cc/s, an order of magnitude increase in productivity over existing low deposition rate powder bed LM (0.0036cc/s). Deposition at 500μm layers was found to impart a secondary alignment to the microstructure due to a lower ratio of beam diameter vs. layer thickness, thus conductive cooling into previously deposited weld tracks within the same layer becomes significant. PTAW deposition of Inconel 625 was investigated and a process map characterising single bead on plate experiments has been compiled and presented. Deposition strategies for multi-layer, multi-weld features have also been investigated and the importance of thermal control due to thermal isolation from the substrate shown. PTAW deposited material has been characterised by tensile testing at elevated temperatures using both conventional tensile tests and by electrical resistance heating of specimens using a Gleeble thermo-mechanical simulator, validating the novel use of infra-red thermography to measure the thermal gauge length. In addition to a need for increased deposition rate, the limits of material performance in AM with respect to nickel alloys are currently constrained by superalloy related weldability and cracking problems. The work presented in this thesis examines the potential for production of an Inconel 625 based Metal Matrix Composite, which may offer benefits to material properties. Candidate ceramic reinforcement materials were identified and a feasibility study was conducted, identifying TiC as the most promising candidate. Feedstock powders were mixed and assessed, mixing TiC directly with Inconel 625 and mixing pure Ti and carbon in the form of graphite with Inconel 625 to investigate an in-situ reactive processing route. The process parameter windows were characterised for both MMC feedstocks at both 100μm and 500μm layer thickness and with the use of pre-heating to establish the relationships present. Process stability maps were created and significantly the presence of TiC affected the ability of the laser to penetrate the powder bed, not due to its high melting point, but due to its high absorptivity which results in greater melting within the powder bed which hinders penetration and wetting with the substrate. The in-situ forming of TiC was partially successful, but unwanted Mo2C carbides were formed and the matrix structure affected due to the homogenous presence of carbon during processing. The power density of the laser beam was identified as the critical factor in determining the dissolution and re-precipitation behaviour of TiC within the matrix, as opposed to the commonly used energy density metric