Warm Spray Research Articles

Al-based amorphous coatings by warm spraying: Numerical simulation and experimental validation

Al-based fully amorphous coatings with low porosity were successfully produced via warm spraying, combining numerical simulation and experimental validation. A warm spray gun model, employing computational fluid dynamics, was developed to simulate the warm spraying process. The study delved into the effects of reactant flow rate, oxygen/fuel (O/F) ratio, coolant flow rate, spray distance, particle size, particle shape, particle injection velocity, and particle injection angle on the gas flow field parameters (pressure, temperature, and velocity) and particle in-flight behaviors (temperature, velocity, and in-flight trajectory). The optimum spraying parameters (OSP) predicted by the simulation results were determined: 0.008740 kg/s for reactant flow rate, 2.7 for the O/F ratio, 0.007356 kg/s for coolant flow rate, 142 mm for spraying distance, 10–35 μm for particle size range, spherical for particle shape, 10 m/s for particle injection velocity, and 0° for particle injection angle. Subsequently, warm spraying experiments based on the OSP yielded Al-based fully amorphous coatings with a porosity of 0.08 %. This work provides valuable insights for the production of Al-based fully amorphous coatings with minimal porosity through warm spraying.

Microstructure characteristics of warm spray additively manufactured Inconel 718 superalloys and correlation with mechanical performance

This study focuses on Inconel 718, one of the most popular Ni-based superalloys for extreme environments, with high demand for in-situ repair and additive manufacturing. Highly dense Inconel 718 was solid-state deposited using warm spray (WS) without expensive and non-renewable helium. The role of nitrogen flow rate as a key factor in controlling the particle temperature was investigated to illustrate that lowering the nitrogen flow rate facilitates particle deformation, thus reducing surface roughness and densifying the Inconel 718 deposits. Another benefit is cost savings. Like cold spray, the larger the particle deformation, the more pronounced the process hardening (manifested as higher hardness); however, heat treatment can eliminate it effectively. The presented technique has great potential for high-quality and low-cost solid-state deposition of hard-to-deform metallic materials.

Comparison of Microstructure, Microhardness, Fracture Toughness, and Abrasive Wear of WC-17Co Coatings Formed in Various Spraying Ways

WC-Co cermet materials serving as protective coatings are widely used in many fields. Conventional WC-17Co coatings were formed in high-velocity oxygen-fuel (HVOF), warm spraying (WS), and cold spraying (CS), respectively. Deposition behavior of a single WC-17Co particle, as well as the microstructure, microhardness, fracture toughness, and abrasive wear of WC-17Co coatings formed in various spraying ways were investigated. The results show that the deposition behavior of a single WC-17Co particle was different after it was deposited onto a Q235 steel substrate in various spraying ways. The WC-17Co splat deposited by HVOF showed a center hump and some molten areas, as well as some radial splashes presented at the edge of the splat. The WC-17Co splat deposited by WS presented a flattened morphology with no molten areas. However, the WC-17Co splat deposited by CS remained nearly spherical in shape and embedded into the substrate to a certain depth. All the WC-17Co coatings had the same phase compositions with that of feedstock. The microstructure of all the WC-17Co coatings was dense with no cracks or abscission phenomena between the coatings and substrate. Moreover, fine WC particles were formed in the coatings due to the fracture of coarse WC particles, and the content of fine WC particles in the cold-sprayed coating was significantly more than the other coatings. A stripe structure was formed by the slippage of fine WC particles with a plastic flow of Co binder in the warm-sprayed and cold-sprayed coatings. More fine WC particles, as well as the stripe structure, in the coatings were conducive to improve the microhardness and fracture toughness of the coating. The microhardness and fracture toughness of the cold-sprayed WC-17Co coating were the highest among the coatings. The main wear mechanism of all coatings was the groove and some peel-offs. The cold-sprayed WC-17Co coating with the lowest wear loss presented the highest wear resistance among the coatings.

Numerical study on the behavior of titanium particles in the process of warm spraying

Numerical study on the behavior of titanium particles in the process of warm spraying

A Comparative Study of Aluminium and Titanium Warm Sprayed Coatings on AZ91E Magnesium Alloy.

Aluminium (Al) and titanium (Ti) coatings were applied on AZ91E magnesium alloy using a low-pressure warm spray (WS) method. The deposition was completed using three different nitrogen flow rates (NFR) for both coatings. NFR effects on coating microstructure and other physical properties were systematically studied. Microstructural characterization was performed using scanning electron microscopy (SEM), and the porosity was estimated using two methods—image analysis and X-ray microtomography. The coating adhesion strength, wear resistance, and hardness were examined. The protective properties of the coatings were verified via a salt spray test. Decreasing NFR during coating deposition produced more dense and compact coatings. However, these conditions increased the oxidation of the powder. Al coatings showed lower hardness and wear resistance than Ti coatings, although they are more suitable for corrosion protection due to their low porosity and high compactness.

Sensitivity Analysis of Process Parameters of Warm Spraying Process Based on Response Surface Method

Sensitivity Analysis of Process Parameters of Warm Spraying Process Based on Response Surface Method

Detecting the defects of warm-sprayed Ti-6Al-4V coating using Eddy current testing method

Thermal spay coatings have been widely used to provide structural components with protection from harsh environments such as corrosion, heat, and wear resistance. However, delamination, pores or peeling defects may exist in an as-fabricated coating which influence the mechanical properties of the coating. We developed an eddy current testing (ECT) system using a special ECT probe. The ECT probe was composed of one detection coil and two excitation coils. The lower excitation coil was used to produce the excitation magnetic field in the specimen and the upper excitation coil was used to compensate the background signal. In this study, the feasibility of our ECT system to inspect the Ti-6Al-4V warm spray (WS) coatings were investigated. The artificial defects embedded between the Ti-6Al-4V coating and the substrate were successfully detected using the ECT system.

Warm Spray Deposition for Additive Manufacturing

Warm Spray Deposition for Additive Manufacturing

Mechanical properties of warm sprayed HATi bio-ceramic composite coatings

To explore a new approach for fabricating the load bearing implants with the combination of bioactivity, biocompatibility, and mechanical properties, mechanically mixed hydroxyapatite (HA) and titanium (Ti) powders containing 30, 50, and 70 wt% Ti were sprayed onto a 316L stainless steel substrate using a warm spray (WS) process. The microstructures, phase compositions, chemical structures, and mechanical properties of WS HATi composite coatings were comprehensively investigated and compared to those of WS HA coating. Experimental results indicate that the cross-sectional microstructures of WS HATi composite coatings present typical lamellar structures composed of curved stripes formed by well-deformed and oxidized Ti splats and limited deformed HA splats, and are significantly influenced by the Ti content in the original powders. Phase constitutions of the composite coatings mainly consist of HA, Ti, TiO2, and TiO. Chemical structures of HA in the composite coatings deposited using powders with Ti content less than 30% are similar to the structures in the original powder. The microhardness, elastic modulus, and bond strength of the coatings increased from 0.32 ± 0.15 GPa to 1.41 ± 0.31 GPa, from 1.37 ± 0.28 GPa to 23.28 ± 3.45 GPa, and from 17.3 ± 2.2 MPa to 34.8 ± 3.2 MPa, respectively. The abrasive wear weight loss of the coatings on Al2O3 abrasive paper decreased from 2.9 mg to 1 mg, as the addition of Ti particles in original powders increased from 0 to 70%.

Residual Stress in High-Velocity Impact Coatings: Parametric Finite Element Analysis Approach

High-velocity impact coatings are produced by techniques such as HVOF, HVAF, cold spraying, warm spraying, and supersonic plasma spraying. All these processes have in common the impact of particles at high velocities that produce peening of the surface and induce compressive residual stresses in the radial and axial orientations of the impact. If the process involves a significant heat input to the particles, quenching of splats and thermal mismatch between coating and substrate adds residual stress to the peening, and subsequently defines the final stress state. Through a parametric finite element model of coating formation, physical variables—including particle temperature and velocity, particle mass, particle morphology and deposition temperature—are studied to observe their effect on residual stresses, and define their possible manipulation to design coatings of desired average residual stress. To allow key parameter selection, a contour map of SS316 feedstock deposited on SS316 substrate is produced based on the parametric modeling of particle impact (via an explicit FE model) and the subsequent layer-by-layer coating formation (via an implicit FE model) employing ABAQUS® code. The Johnson–Cook model for high strain, strain rate and temperature is used as the constitutive equation for the study of impact and rapid cooling.

Influence of powder properties on residual stresses formed in high-pressure liquid fuel HVOF sprayed WC-CoCr coatings

This paper presents a systematic study of the effect of various WC-CoCr powders on the residual stresses of the high pressure HVOF sprayed coating. As the residual stresses are recognized to play a significant role in the mechanical and fatigue resistance of the coating, it is understandable that their management is important for damage tolerant coating design. Several studies have recently shown that processes, which produce high particle kinetic energy and lower particle temperature, such as Warm spray, HVAF and high-pressure HVOF processes, generate higher peening stresses and therefore final residual stresses is more compressive compared to lower kinetic energy HVOF systems. In addition to the spraying process, powder properties are known to be one of the most important variables in thermal spraying. Nevertheless, only few studies can be found on the effect of powder properties on residual stresses. The aim of this study was to understand the effect of different powder properties on the formation of residual stress. In situ monitoring was utilized to record curvature and temperature during spraying and to calculate coating residual stresses. This approach is a useful tool for understanding of residual stresses during the thermal spraying process enabling their manipulation. It was found that the powders, with only minor differences in density and particle size, produced a significant difference of about 350 MPa in the stress states of the coatings. The combined effect of spray powder properties and spray parameters on residual stress was almost 560 MPa.

The Effect of Ti-6Al-4V Coating Structure Deposited by Warm Spray on Defect Sensitivity of Laser Ultrasonic Testing

The Effect of Ti-6Al-4V Coating Structure Deposited by Warm Spray on Defect Sensitivity of Laser Ultrasonic Testing

Correction to: Microstructure and Oxidation Performance of TiAl-(Cr, Nb, Ta) Coatings Fabricated by Warm Spray and High-Velocity Oxy-Fuel Spraying

Correction to: Microstructure and Oxidation Performance of TiAl-(Cr, Nb, Ta) Coatings Fabricated by Warm Spray and High-Velocity Oxy-Fuel Spraying

Microstructure and corrosion resistance of warm sprayed titanium coatings with polymer sealing for corrosion protection of AZ91E magnesium alloy

Thermally sprayed coatings are one of the potential ways to increase corrosion performance of Mg alloys dedicated for aircraft applications. In our research, a novel thermal spray process called Warm Spraying (WS) was used to deposit Ti coatings onto AZ91E magnesium alloy. WS gives us the possibility to control the temperature of the sprayed particles by changing the nitrogen flow rate (NFR) what as a result determine the porosity of WS coatings. The microstructure, porosity and oxygen content of coatings deposited under three different NFR was determined. A relation between porosity, oxygen content and NFR was revealed. The corrosion testing in 3.5% NaCl solution of as-sprayed coatings revealed their poor corrosion resistance that led to fast degradation of the substrate. The following post-treatment with epoxy-based polymer resulted in a significant improvement of the corrosion performance of Ti warm sprayed coatings.

Microstructure and Oxidation Performance of TiAl-(Cr, Nb, Ta) Coatings Fabricated by Warm Spray and High-Velocity Oxy-Fuel Spraying

To improve the oxidation resistance of near α-titanium alloy IMI834, TiAl-(Cr, Nb, Ta) coatings were deposited by applying high-velocity oxy-fuel (HVOF) and warm spray (WS). Comparison was made in terms of microstructure, surface morphology as well as isothermal and cyclic oxidation behaviors in the air at 750 °C up to 100 h and 100 cycles, respectively. The results show that smoother and less oxidized coatings were deposited by warm spraying. The microstructure of all coatings underwent an appreciable change during the oxidation tests, as in as-sprayed state it occurred in the nonequilibrium state. It was revealed that a small difference in the initial oxidation between the two spraying processes as well as microstructure, the level of porosity and surface roughness significantly influences the oxidation kinetics of the sprayed coatings at high temperature, which should affect the service lifetime as an oxidation-resistant layer for potential applications. After exposure at 750 °C in air, rutile TiO2 was found in addition to α-Al2O3 in the oxide scale formed on the HVOF and warm sprayed coatings. However, isothermal and cyclic oxidation tests of all WS TiAl-(Cr, Nb, Ta) coatings showed improved oxidation resistance of IMI 834 as well as good adherence to the substrate alloy.

Effects of Combustion Model and Chemical Kinetics in Numerical Modeling of Hydrogen-Fueled Dual-Stage HVOF System

The present work examines the effect of utilizing different combustion models and chemical kinetics in predicting the properties of gas and particle phases in a hydrogen-fueled, dual-stage high-velocity oxy-fuel (HVOF) thermal spray system. For this purpose, effects of two combustion models, eddy dissipation concept (EDC) and eddy dissipation model (EDM), on the temperature and velocity fields in the system are studied. The computations using EDC model are performed for detailed and reduced chemical kinetics and for a range of mixture from lean to rich. It is found that EDC with multi-step reaction mechanism predicts higher temperatures for the flow and particle in the warm spray system. In contrast to EDC, the EDM with one-step global reaction shows extra heat release outside the HVOF barrel for rich mixtures which leads to unphysical higher prediction of particle temperature. The simulations using EDC model with detailed and reduced chemical kinetics show some exothermic reactions in converging-divergent nozzle of the system. The heat release from these reactions has profound impacts on the flow and particle temperatures and affects the gas dynamic behavior of flow considerably. Finally, it is discussed that moving toward rich mixtures is more reliable way to control the particles temperature.

Corrosion Resistance of Aluminum Coatings Deposited by Warm Spraying on AZ91E Magnesium Alloy

The corrosion resistance of aluminum coatings on Mg alloy (AZ91E) substrate was investigated in 3.5% NaCl solution. The Al coatings were deposited using a warm spraying (WS) method under three diff...

Fabrication and Oxidation Resistance of TiAl Matrix Coatings Reinforced with Silicide Precipitates Produced by Heat Treatment of Warm Sprayed Coatings

Ti-Al-based intermetallics are promising candidates as coating materials for thermal protection systems in aerospace vehicles; they can operate just below the temperatures where ceramics are commonly used, and their main advantage is the fact that they are lighter than most other alloys, such as MCrAlY. Therefore, Ti-Al-Si alloy coatings with five compositions were manufactured by spraying pure Ti and Al-12 wt.% Si powders using warm spray process. Two-stage hot pressing at 600 and 1000 °C was applied to the deposits in order to obtain titanium aluminide intermetallic phases. The microstructure, chemical composition, and phase composition of the as-deposited and hot-pressed coatings were investigated using SEM, EDS, and XRD. Applying of hot pressing enabled the formation of dense coatings with porosity around 0.5% and hard Ti5(Si,Al)3 silicide precipitates. It was found that the Ti5(Si,Al)3 silicides existed in two types of morphologies, i.e., as large particles connected together and as small isolated particles dispersed in the matrix. Furthermore, the produced coatings exhibited good isothermal and cyclic oxidation resistance at a temperature of 750 °C for 100 h.

Microstructures and Properties of Warm-Sprayed Carbonated Hydroxyapatite Coatings

Carbonated hydroxyapatite (CHA) coatings were deposited onto 316L stainless steel substrates using an in-house developed warm spraying system. Microstructures of the coatings were comprehensively investigated. Microhardness, tensile strength and wear resistance of the CHA coatings were examined. In addition, bioactivities of the coatings were studied after immersing in simulated body fluid (SBF). Results show that the as-sprayed coatings exhibited typical lamellar architectures consisting of partially melted and flattened splats, i.e., with molten shells and un-molten cores of original powders. The CHA coatings had nearly identical Ca/P ratios, crystalline structures and phase constitutions to those of the feedstock powders, indicating that undesired decompositions caused by overheating can be avoided by employing the warm spraying process. Microhardness and tensile strength of as-sprayed coatings were around 690 and 11.4-20.6 MPa, respectively. Moreover, the warm-sprayed CHA coating exhibited a high resistance against abrasion wear when sliding took place with polymers. After being immersed in Hank’s SBF for 28 and 60 days, new apatite was formed on the coating surface corroborating the good biocompatibility of the coating.

Manufacturing and Properties of Al-Al2O3 Composite Coating Layer Using Warm Spray Process

Properties of coatings produced by warm spray were investigated in order to utilize this technique as a repair method for Al tire molds. Al-(0-10 %)Al2O3 composite powder was sprayed on Al substrate by warm spraying, and the microstructure and mechanical properties of the composite coating layer were investigated. For comparative study, the properties of the coating produced by plasma spray, which is a relatively high-temperature spraying process, were also investigated. The composite coating layers produced by the two spray techniques exhibited significantly different morphology, perhaps due to their different process temperatures and velocities of particles. Whereas the Al2O3 particles in the warm sprayed coating layer maintained their initial shape before the spray, flattened and irregular shape Al2O3 particles were distributed in the plasma sprayed coating layer. The coating layer produced by warm spray showed significantly higher adhesive strength compared to that produced by plasma spray. Hardness was also higher in the warm sprayed coating layer compared to the plasma sprayed one. Moreover, with increasing the fraction of Al2O3, hardness gradually increased in both spray coating processes. In conclusion, an Al-Al2O3 composite coating layer with good mechanical properties was successfully produced by warm spray.