Optimization of Inconel 718 thick deposits by cold spray processing and annealing
Introduction
Cold spraying (CS) is a solid-state coating technology with the potential to be used as additive manufacturing (AM) or repairing technique owing to its high deposition rates, low cost and its feasibility to produce thick and compact deposits or parts [[1], [2], [3], [4], [5]]. As a solid-state technique, CS does not involve melting of the feedstock material. In comparison to other AM techniques (e.g., direct energy deposition or powder bed fusion), possible oxidation and phase transformations by solidification during processing can be avoided [6].
During CS, powder particles (i.e., typical sizes range from 15 to 50 μm) are accelerated in a supersonic gas flow (i.e., particle velocity of up to > 1000 m/s) by passing through a convergent-divergent de-Laval nozzle under the operation of a compressed and preheated process gas, typically N2 or in some instances He [6]. By effects associated with their severe plastic deformation during impact, the sprayed particles bond to the substrate or to the previously deposited sprayed layer [7]. By successive particle impact, depending on traverse speed and number of passes, uniform deposits are build-up in thickness ranging from 100 μm up to thick deposits of some centimeters, the latter being interesting for additive manufacturing applications [8].
In CS, successful bonding is achieved when the thermal softening dominates over the strain and strain-rate hardening effects at the impact zone, leading to the formation of adiabatic shear instabilities (ASI) [7]. High strain at local temperatures close to the melting point may cause metallurgical bonding at particle-to-particle and particle-to-substrate interfaces [7]. These conditions are achieved when the particle impact velocity (vp) exceeds a critical velocity (vcr), which depends on powder material properties (i.e., strength, density, melting temperature, heat capacity, etc.), and the particle impact temperature [9]. The amount of bonded area increases with the excess over vcr, improves its deposition efficiency and coating properties, such as tensile strength, electrical conductivity, among others. Reachable particle impact conditions (particle velocity and temperature) depend on spraying process parameters (i.e., gas temperature, gas pressure, nozzle design, stand-off distance) and feedstock materials features (i.e., density, size distribution, and morphology).
To select the optimum spraying process parameters, Assadi et al. demonstrated that the quality of CS coatings can be correlated with the ratio of particle velocity to critical velocity, called η value [10]. Favorable coating properties should be reached as soon as η approaches to a value above 1.5 [10].
Nowadays, CS is well established for ductile materials that show rather regular thermal softening behavior [11]. However, for high strength materials such as nickel-based superalloys, successful coating build-up could be restricted by their high strength leading to high critical velocities [12].
Recently, CS of Inconel 718 (IN718) has raised the interest of the research community. IN718 is a nickel-based superalloy known for its excellent corrosion resistance and high mechanical strength at elevated temperatures. At present, IN718 is extensively used in the aerospace industry for components that operate at temperatures up to 700 °C [13]. In the “as-aged” state, IN718 has limited plastic deformability and does not show thermal softening up to typical impact temperatures attainable in CS (i.e., ∼650 °C), making successful deposition of IN718 to a challenge. The situation could be different if the feedstock powder would retain a soft-annealed state, which would show a lower strength and larger strain to failure than an aged state, and as a solid solution, a rather conventional thermal softening behavior.
Due to the technical limitations processing decent quality IN718 coatings, there is limited work reported. Karthikeyan et al. first demonstrated the feasibility of IN718 deposits by CS in 2004 [14]. Marroco et al. produced dense IN718 coatings with relatively high hardness, but low bond strength values [15]. Kim et al. reported that the main obstacle to process thick deposits by CS is given by the presence of compressive residual stresses accumulated through the coating thickness that restricts interfacial bonding to the substrate [16].
Other investigations revealed that ductility in the as-sprayed state were relatively low, which can be improved by post-heat treatment. For example, Levasseur et al. produced IN718 coatings with a porosity level lower than 3% but with lack of interparticle bonding. Therefore, coating properties were improved by a sintering treatment, which increased its flexural strength and ductility [17]. Similar results were reported by Wong et al. [18]. Furthermore, Ma et al. [19] fabricated high-performance Inconel 718 alloy cold sprayed with different propelling gases. The results show that higher adhesive strength was obtained using helium instead of nitrogen. The Inconel 718 samples processed with nitrogen demonstrate that adhesive strength scales with the pressure of the propelling gas. Bagherifard et al. compared the microstructure and mechanical properties of as-build and aged IN718 samples fabricated by CS, and as-build selective laser melting (SLM) samples [20]. The results showed higher tensile strength in the aged CS samples as compared to the as-build SLM samples, reaching comparable mechanical strength and ductility than a reference bulk IN718 material [21].
In 2017, Mauer et al. stated a good comparison between experimentally determined particle velocities for IN718 powder cold sprayed with different particle sizes with values estimated by model calculations, and proved correlations of the deposition efficiency and η values [22].
Despite the work reported for CS IN718 so far, there is still a lack of understanding of the interplay between powder properties and process parameters to achieve needed coating properties for producing high-quality thick deposits. The process gas temperature plays a crucial role in the particle impact velocity and temperature, the latter also influencing the critical velocity. Together define the particle ability to deform upon impact and the amount of well-bonded interfaces for attaining good mechanical properties [6,23]. Since process gas temperature is one of the most critical process parameters during CS, in this work, the influence of the process gas temperature on various properties of thick IN718 deposits has been addressed in detail to reveal more general information on bonding features, and a possible forecast of expected coating performance using the concept of “quality parameter” (η value).
Section snippets
Powder analysis
Commercial IN718 powder produced by gas atomization (SANDVIK, Wales, United Kingdom) was used for cold spraying. The powder size distribution and morphology were analyzed by laser diffraction (LA-910 HORIBA, Kyoto, Japan) and by scanning electron microscopy (SEM) using a Quanta 650 microscope from FEI (Brno, Czech Republic), respectively. The XRD patterns of the powder were obtained using a D8 DISCOVER X-ray diffractometer from Bruker (Massachusetts, USA). The chemical composition of the powder
Powder microstructures and properties
Results of powder shapes and sizes are shown in the SEM-micrographs of Fig. 1. The feedstock powder has a spherical morphology, typical for gas atomization, with some small satellite particles attached to larger ones (Fig. 1a). As determined by laser diffraction, the size of IN718 powder shows a maximum population at 30 μm and display a unimodal distribution with d10 = 17 μm, d50 = 27 μm and d90 = 45 μm (Fig. 1b). Micrographs of powder cross-sections in Fig. 2 reveal dendritic and grain refined
Discussion
Based on the known IN718 bulk material properties [29], the challenge to cold spray this nickel-based superalloy to produce high-quality thick deposits is evident. Heat treatments to precipitate intermetallic phases such as γ' and γ'´, avoids thermal softening below 670 °C. Under these conditions, very high impact velocities and impact temperatures well above the softening temperature should be needed to build up deposits by CS [32,33]. In such a case, it would be nearly impossible to reach
Summary and conclusions
A comprehensive guideline is developed and tested for cold sprayed IN718 to obtain high-quality thick deposits for potential AM applications. Through a theoretical-experimental approach, process gas temperatures were tuned seeking to maximize the deposit properties, including porosity, hardness, electrical conductivity, and residual stress for as-sprayed and HIP-annealing conditions.
The main findings of this contribution are presented below:
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Analysis of powder microstructures and properties
Acknowledgments
The authors thank CONACYT for financial support. This project was funded by CONACYT Projects 272095, 279738 and 279738, CONACYT-AEM Project 275781 and carried out partially at CENAPROT, LIDTRA, LANIMFE, LISMA national laboratories. Also, support by C. Bauer from Impact Innovations GmbH for performing the spray experiments is greatly acknowledged. Moreover, authors thank teams from HSU and CINVESTAV, namely (in alphabetic order) T. Breckwoldt, M. Schulze, A. Jimenez and E. Urbina for technical
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