Investigation on cold spray based additive manufacturing of IN 718 superalloy

Abstract

Cold spray is a cutting-edge additive manufacturing and coating technique that employs high-speed gas to propel powdered particles toward a substrate at low temperatures. Unlike conventional thermal spray methods, this solid-state process prevents particle melting, instead enabling adhesion through plastic deformation upon impact. By preserving the feedstock material’s original properties, cold spray minimizes oxidation and thermal stresses, resulting in dense, strong coatings. Its suitability for temperature-sensitive materials like titanium, aluminum, copper, and nickel alloys makes it valuable in applications that requires high-quality coatings with minimal porosity. Cold spray based additive manufacturing (CSAM), is rapidly evolving driven by several pivotal factors. Ongoing research focuses on refining parameters to enhance deposition efficiency and material properties, while advancements in powder metallurgy expand the range of compatible materials. Investments in advanced cold spray systems with automation and precision are boosting throughput and repeatability, making cold spray an integral part of industrial production processes. Integrating cold spray technology into aerospace, automotive, and defense sectors underscores its capability to meet stringent quality standards. Researchers are exploring simultaneous deposition of multiple materials for creating complex, multifunctional components. Leveraging digitalization tools and simulation software optimizes processes and predicts material behavior, complemented by standardization efforts ensuring reliability and uniformity. With these advancements, cold spray technology emerges as a versatile and widely embraced manufacturing solution across various industries. In the context of additive manufacturing, the application of cold spraying technique is very recent. The process appears to be promising due to its unparalleled advantages. However, the applicability of cold spray process for components at industrial length-scale such as energy, aviation, oil and gas, locomotives and healthcare etc. is unknown and needs to be explored to scale-up the process successfully. Moreover, understanding the influence of process parameters on the microstructure evolution, mechanical behavior as well as degradation behavior under simulated operating conditions is critical for advancing this technology in a real sense. This research aims to develop a comprehensive approach and database for fabricating components using Inconel 718 (IN 718), a precipitation-hardened nickel-chromium-iron-based superalloy renowned for its exceptional mechanical properties across a wide temperature range. IN 718 exhibits outstanding tensile, fatigue, creep, and rupture strength, along with excellent resistance to corrosion and oxidation. However, conventional manufacturing methods face significant challenges with IN 718 due to issues such as excessive tool wear, work hardening, and low material removal rates during machining. These challenges are compounded by difficulties in high-temperature forming processes caused by the segregation of refractory elements like niobium (Nb) and molybdenum (Mo), leading to Laves phase formation, which compromises material strength and ductility. Advanced manufacturing techniques are necessary for fabricating intricate components from IN 718. Several studies have explored the microstructure of additively manufactured (AMed) IN 718 using various metal additive manufacturing (MAM) processes. However, AMed IN 718 exhibits inferior fatigue performance due to additive manufacturing induced defects like porosity and subpar surface quality, along with notable variations in microstructure and texture leading to mechanical response anisotropy. Traditional thermal spray techniques result in coatings with high porosity levels. Cold Spray emerges as a promising substitute for additive manufacturing IN 718 components, effectively addressing limitations seen in conventional methods and offering potential enhancements in efficiency and performance. In this research, our first objective was to establish a proof-of-concept by fabricating thick plates of materials that typically posed challenges in welding, forming, casting, or traditional fabrication methods. To achieve this, we focused on commercially pure Ti and Ni-20Cr alloy. Employing pre- and post-heat treatment procedures, we investigated their effects on structure-property relations. Our initial study involved evaluating thick layers, ranging from 6 to 16 mm, of pure Ti and Ni-20Cr alloy fabricated through high-pressure cold spray process. These layers were then meticulously compared with their conventional as-cast counterparts. The successful deposition of such thick layers underscored the immense potential of cold spray in additive manufacturing of components. Additionally, each heat treatment option enhanced specific properties in the deposits. Depending on the application of the product, a suitable treatment could be selected to achieve desirable properties. For instance, if better tensile strength was required, one could opt for substrate heating (SH) pre-treatment to complement the cold spray process. Conversely, if enhanced ductility with reduced porosity was the goal, hot isostatic pressing (HIP) post-treatment could serve as a suitable supplement to the cold-sprayed deposits. Our comprehensive analyses, encompassing microstructural examinations, mechanical property evaluations, and high temperature cyclic oxidation studies, consistently indicated that cold spraying offered a viable and promising route for additive manufacturing, particularly for producing thick layers at significantly higher production rates. In the subsequent study involving the second objective, IN 718, a nickel-based superalloy, was chosen for pipe fabrication. A two-level full factorial design was employed to analyze the velocity ratio (the ratio of particle impact velocity at standoff distance (SoD) to critical velocity) for effective cold spray deposition of IN 718. Cold spraying high-strength materials like IN 718 posed challenges due to limited deformability, impacting coating quality. To achieve properties similar to bulk materials, maintaining a particle impact velocity at SoD above the critical velocity was crucial, with a velocity ratio within the 1.1-2 range for deposition efficiency of 80-100 %. Powder feed rate (PFR) and SoD were kept constant at 30 g/min and 25 mm, respectively. Statistical analysis revealed PFR as a non-significant parameter, while SoD had a minimal positive effect on the velocity ratio. A lower SoD helped maintain a consistent cold spray footprint diameter, ensuring better deposition accuracy and minimizing material oxidation. Thus, meticulous adjustment of process parameters was necessary to achieve elevated particle impact velocities and temperatures, facilitating the formation of maximum well-bonded interfaces between particles and particle-substrate. In our ongoing study, we rigorously explored various cold spray process parameters at both low and high levels using a comprehensive full factorial two-level approach to pinpoint optimized parameters for effective cold spray deposition of IN 718. In the third objective, we conducted an examination of how different injection angles (90 deg, 30 deg and 0 deg) affected the performance of the cold spray process using IN 718 powder particles and nitrogen gas. The computational modeling including gas flow field simulations and particle trajectory analyses is utilized to understand how these angles influenced various aspects of the process. The chosen turbulence model, k-ω SST (shear stress transport), was commended for its accuracy in describing turbulent flows, making it suitable for capturing the complex dynamics of gas-particle interaction within the cold spray nozzle. Among the angles tested, the 30 deg injection angle emerged as the most favorable due to several factors. This angle facilitated improved heat transfer, resulting in a higher gas temperature and a more uniform distribution within the gas flow, which was crucial for maintaining consistent deposition quality. Additionally, the 30 deg angle produced higher exit velocities for the nitrogen gas, improving its ability to accelerate and entrain powder particles. Furthermore, the gas dynamics at this angle increased drag forces on the powder particles, aiding in their acceleration and entrainment. As a result, the IN 718 powder particles experienced elevated velocities and temperatures, enhancing their kinetic energy and adhesion upon impacting the substrate. The combined effect of these factors resulted in enhanced deposition efficiency at the 30 deg injection angle, highlighting the importance of angle optimization in achieving desired outcomes, especially in applications requiring precise track generation within complex geometries. In the fourth objective, a discrete phase computational fluid dynamics analysis was conducted to simulate the gas flow and particle behavior during the cold spray deposition of IN 718 onto an SS 304 pipe. During the analysis, researchers observed a bow shock phenomenon at the impingement zone where the supersonic jet interacted with the SS 304 substrate pipe. This bow shock caused an increase in gas density and a significant reduction in gas velocity, while the velocity of IN 718 powder particles experienced a relatively smaller decrease. Following this observation, researchers determined the average impact velocity and temperature of the IN 718 powder particles at SoD, serving as crucial input parameters for establishing boundary conditions in single- and multi-particle explicit impact simulation studies of IN 718 onto the SS 304 substrate. Further observations revealed jetting phenomena during the cold spraying process, particularly evident across the D10, D50, and D90 diametric distributions of the IN 718 powder feedstock. Larger particles with diameter above critical diameter underwent notable shape changes due to jetting, contrasting with sub-critical smaller particles that predominantly embedded within the SS 304 substrate. Experimental validation of the coating microstructure, both in single- and multi particle impact scenarios, underscored the effectiveness of computational fluid dynamics and impact deformation studies in elucidating the underlying processes. In the last study, researchers utilized optimized cold spray process parameters and calibrated robot configurations, along with tilting turntable settings, to fabricate a 3D standalone pipe. The resulting IN 718 thick pipe deposits measured 6 inches in length, with a nominal bore of 60.3 mm and a thickness of 5 mm respectively. Microstructural analysis revealed deposits characterized by dense structures and minimal porosity. The density, microhardness values and mechanical properties of cold spray additively manufactured (CSAMed) IN 718 thick pipe deposits closely resembled those of bulk IN 718. Notably, no oxide formation was observed in energy dispersive spectroscopy (EDS) analysis, and this was further confirmed through X-ray diffraction (XRD). Additionally, all the phases identified in the powder feedstock were retained in the CSAMed IN 718 thick pipe deposits, indicating the absence of phase transformations during the cold spray process. δ phase precipitates were detected in scanning electron microscopy (SEM) microstructure, which was also confirmed through EDS and XRD analysis. Initial microstructural and mechanical characterization suggested that cold spray could effectively replace conventional processes for fabricating IN 718 pipes, offering numerous advantages.