Development of Wire and Arc Additive Manufacturing Process for Deposition of Ni- Based Super Alloy

Abstract

Wire Arc Additive Manufacturing (WAAM) is an innovative form of 3D printing that uses an electric arc as a heat source to melt metal wire, which is then deposited layer by layer to create complex metal parts. This technique achieves high deposition rates and allows the building of large-scale components with significant cost and material savings compared to traditional manufacturing methods. In the past, a variety of materials have been fabricated through WAAM. However, very little work has been done using WAAM on nickel-based super-alloys. Nickel-based super-alloys are a class of metallic alloys known for their exceptional high-temperature strength, resistance to creep deformation, and resistance to corrosion and oxidation. The excellent combination of these properties makes these materials ideal for use in jet engines, gas turbines, power plants, and other high-temperature applications. Manufacturing these complex design components of nickel-based superalloys using traditional methods is challenging and expensive. To address this issue, the current research focuses on the feasibility and development of the WAAM process as a promising alternative for producing nickel-based components. The initial focus is on establishing a robust GMAW-based WAAM process. This involves optimizing the gas metal arc welding (GMAW) process parameters, a key element of WAAM. Through numerous trials, the main aim is to identify the ideal settings for factors such as current, voltage, and welding speed. This study uses a bi-directional, low-cost, GMAW-based wire arc additive manufacturing (WAAM) setup to deposit aerospace-grade super-alloy IN718. Orthogonally designed experiments are carried out to optimize process parameters for single and multilayer bead geometries. Additionally, ANOVA analysis and S/N ratio plots are employed to optimize process parameters for bead characteristics such as width, reinforcement, penetration, dilution, WRFF, WPSF, surface waviness, and effective area. Furthermore, the effect of heat input is discussed. The study results indicate that increased heat input increases bead parameters like bead width, reinforcement, dilution, penetration, and weld form factor while negatively impacting the weld shape factor and wetting angle. After optimizing process parameters, characterizations of WAAM-deposited materials were done based on metallurgical and mechanical testing. The metallurgical analysis includes optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The mechanical analyses were done based on hardness and tensile testing. These tests provide insights into the microstructure, phases present, elemental composition, grain orientation, and material strength. The microstructural examination reveals that a high heat input produces a coarse grain structure with an average grain size of 72.29 µm. In contrast, a low heat input produces a finer grain structure with an average grain size of 32.12 µm and smaller Laves phases. The XRD analysis shows the presence of NbC and TiC phases in the low heat input sample, enhancing the deposited material's mechanical properties. The multilayer wall structures fabricated with high heat input demonstrate more uniformity in shape and less waviness than those fabricated with low heat input. Tensile testing results indicate that the low heat input sample exhibits higher strength (775.82 MPa) than the high heat input sample (741.07 MPa). This increased strength is attributed to the smaller grain structure and higher hardness (278.11 HV) observed in the low heat input sample compared to the high heat input sample (257.26 HV). The EBSD analysis further confirms that the low heat input sample has a highly textured surface, more grain boundary lengths, and larger grain boundary orientation, which contributes to its superior mechanical properties. This study also provides insights into cooling environments and thermal management techniques for WAAM of IN718 components. The material was deposited under four different heat-input conditions, using either air or water cooling. The layers were deposited in a normal atmospheric environment with air cooling, while water cooling involved depositing the material inside a water tank with varying water levels. To validate the air- and water-cooling thermal management techniques, IN718 single-pass, and multilayer linear walls were deposited under four different heat-input conditions. Temperature profiles were recorded during single-layer depositions, and geometric and microstructural features were examined. The SEM analysis revealed that the microstructure in the building direction was non-homogeneous compared to that in the deposition direction. Additionally, water cooling significantly influenced bead characteristics, such as wall width and height. The grain size and anisotropy of the mechanical properties decreased with water cooling. Therefore, water cooling is economical and efficient in mitigating excessive heat accumulation in WAAM-deposited IN718 components. Further, this study also examines the effects of shielding gases at various heat inputs on the bead geometry, microstructure evolution, and mechanical properties of Inconel 718 deposited by WAAM. Bead-on-plate experiments were conducted with the CMT technique of GMAW, utilizing Taguchi’s L9 orthogonal array. The factor effects, their contributions, and optimal levels were analyzed using ANOVA and S/N ratio approaches. To determine heat input, the dynamic characteristics of the welding power source were recorded and processed. The study found increased heat input led to more pronounced bead geometry characteristics and a wider heat-affected zone (HAZ). Increasing the CO2 concentration in the Ar + CO2 mixture altered the weld texture from silver metallic (with pure Ar) to grey with 2.5% CO2 and light yellow with 20% CO2. Higher bead width, reinforcement, penetration, and dilution were observed with an Ar 20% CO2 mix. Beads formed with 2.5% CO2 were continuous and smoother at all heat inputs. Welds prepared in a 100% Ar shielding environment exhibited higher hardness than those with 2.5% CO2 and 20% CO2 mixtures. Higher concentrations of Nb-rich precipitates, identified through SEM-EDS analysis, contributed to increased strength. Finally, the project culminated in a comparative analysis of two WAAM variants: CMT-WAAM and GMAW-WAAM. CMT-WAAM provides a more controllable arc with lower spatter, potentially reducing porosity and improving bead geometry. A comparison of surface waviness and mechanical properties shows that waviness is lower, whereas strength is higher in CMT WAAM components. Understanding these distinctions empowers us to decide the most suitable WAAM approach for specific applications.