In cold gas spraying, successful bonding occurs when particle impact velocities exceed the critical velocity. The critical velocity formula depends on material properties and temperature upon impact, relying mainly on tabulated data of bulk material. However, rapid solidification of powder particles during gas atomization can result in strengths up to twice that of bulk materials, causing an underestimation of the critical velocity. Thus, a re-adjustment of the semi-empirical calibration constants could supply a more accurate prediction of the requested spray conditions for bonding. Using copper and aluminum as examples, experimentally determined particle strengths for various particle sizes were 43% and 81% higher than those of the corresponding soft bulk materials. Cold gas spraying was performed over a wide range of parameter sets, achieving deposition efficiencies ranging from 2% to 98%. Deposition efficiencies were plotted as functions of particle impact velocities and temperatures, as calculated by a fluid dynamic approach. By using deposition efficiencies of 50%, the critical velocities of the different powders and the corresponding semi-empirical constants were determined. Based on particle strengths, the results reveal slight material-dependent differences in the mechanical pre-factor. This allows for a more precise description of individual influences by particle strengths on critical velocities and thus coating properties. Nevertheless, the general description of the critical velocity based on bulk data with generalized empirical constants still proves to be a good approximation for predicting required parameter sets or interpreting achieved coating properties.

The quality of feedstock powder is a critical factor in determining the properties of coatings deposited using cold spray (CS). However, most commercially available powders are not designed or optimized for CS applications, making it challenging to tailor powders to desired quality. In this research, we investigated and compared the cold-sprayability of four different Cu powders produced by electrolysis and gasatomization methods. We assessed the powders' microstructure, particle morphology and size distribution to understand the effect of manufacturing methods on Cu powder characteristics. We also studied the flowability of the powders using the shear cell method and evaluated their mechanical properties and deformability for CS using nano-indentation. Our results showed that gas-atomized powders with equiaxed grains exhibited promising flowability and deformability for CS applications, outperforming the other powders tested. Specifically, the spherical morphology of gas-atomized powders provided less surface area than the irregular-shaped electrolytic powder, reducing the interaction of surface forces and contributing to smooth powder flow. Additionally, the gasatomized powder with small dendrites in the microstructure exhibited the highest nano-hardness value (HIT= 1.6±0.1 GPa), while the porous electrolytic Cu powder had the lowest value (HIT= 0.7±0.2 GPa). In conclusion, we found that gas-atomized Cu powders with equiaxed grains may hold promise as the optimised feedstock for CS application, considering both effective metrics of flowability and deformability.

The addition of refractory metals represents a promising development approach for future high-entropy alloys (HEAs). Niobium and molybdenum are particularly suitable for increasing hardness as well as wear and corrosion resistance. In the context of surface protection applications, eutectic alloys with their homogeneous property profile are of particular interest. In the present work, two eutectic HEAs (EHEAs) were developed for the starting Al 0.3 CoCrFeNi using electric arc furnace. Following mechanical and microstructural characterization, the two alloys Al 0.3 CoCrFeNiMo 0.75 and Al 0.3 CoCrFeNiNb 0.5 were identified. For thermal spray processing, powders were prepared by inert gas atomization. The coatings produced by high velocity oxy-fuel (HVOF) spraying were characterized and evaluated comparatively to the castings, allowing process-structure-property relationships to be derived. Based on the results, statements on possible application potential can be made.

In this work, a 2D axisymmetric model of gas atomization at unsteady state that accounts for break-up and solidification is used to simulate laser melting of gas atomized powder. With an optimal nozzle width of 0.6-1 mm and a nozzle angle of 30-32°, the yield of fine 15-45 μm stainless steel powder, suitable for selective laser melting, is shown to increase from 20% to 35%. The effect of laser power on the melting channel width, microstructure, and mechanical properties of the sample is also investigated.

Cold gas dynamic spray has significant potential for load-bearing repairs of high-value metallic components, as it is capable of producing pore and oxide-free deposits of significant thickness and with good levels of adhesion and mechanical strength. However, recently published research has shown that the rapid solidification experienced by gas atomised powders during manufacture can lead to a non-equilibrium powder microstructure, including clusters of dislocations as well as significant localised segregation of alloying elements within each particle. This paper reports on an investigation into the solution heat treatment of a precipitation hardenable aluminium alloy powder. The objective was to create a consistent and homogeneous powder phase composition and microstructure before cold spraying, with the expectation that this would also result in a more favorable heat treatment response of the cold spray deposits. Aluminium alloy 7075 gas atomized powders were solution heat treated at 450 °C for 5 hours in a sealed glass vial under vacuum and quenched in water. The powder particle microstructures were investigated using scanning electron microscopy with electron back scatter diffraction (SEM/BSE) and optical microscopy. The dendritic microstructure and solute segregation in the gas atomized powders was altered, with the heat-treated powder particles exhibiting a homogeneous distribution of solute atoms. The influence on the mechanical properties of the powder particles was studied using micro-indentation. The heat-treated powders exhibited a hardness decrease of nearly 25% compared to the as-received powders. This paper relates the behavior and the deformation of both as-received and heat-treated powders during spraying (single particle impacts), comparing the measured hardness with the deformation effect and the material jetting occurring upon impact.

In this work, the aim is to develop a cost-effective coating to protect cast iron and carbon steel from corrosion and wear. An alloy with a composition of FeCr25Mn10BC was designed that could be readily converted to powder form by gas atomization. Different sized powders were produced, characterized, and subsequently sprayed using a three-cathode air plasma generator. It was found that fine powders with fractions of -25 +10 μm and -10 μm had a much higher affinity to oxidation than coarser ones. Nevertheless, using suitable parameters, dense coatings with low oxide content could be realized even with the finest powder. The results show that full utilization of the powder is achievable due to the wide parameter window of three-cathode plasma spraying and that the average deposition efficiency is more than 70%. In addition to savings in material and processing costs, the new alloy system provides greater wear resistance than stainless steel coatings and exhibits significantly higher corrosion resistance than unprotected cast iron and carbon steel.

In this investigation, spherical Cr 7 C 3 -(Ni,Cr) 3 (Al,Cr) powder with different compositions was prepared by vacuum melting and gas atomization. During the solidification process, Cr 7 C 3 is in-situ synthesized and uniformly distributed in the Cr-doped Ni 3 Al phase. Coatings produced from the powders by HVOF spraying were characterized based on composition, microstructure, hardness, and tribological properties. The results show that the coatings compare well with commercial Cr 3 C 2 -NiCr coatings used on piston rings in heavy duty diesel engines. Optimization routes for further improvement are also discussed.

The computational fluid dynamic approach is adopted in this work, using L16-Taguchi matrix, to study the effect of different secondary atomization gas outlet configurations on the gas velocity, jet divergence, and pressure distribution at cap outlet. The spraying process variables that are integrated in this study are primary and secondary atomization gas pressure, PG and SG respectively. In addition, the geometrical variables of the SG air-cap like the position, the number and the angle of the outlet holes for SG are a part of the L16-Taguchi matrix. The effect of the process variables and geometrical design variations are analyzed on the obtained gas flow characteristics. Increasing the number of the SG outlet holes leads to a higher gas velocity at the cap outlet. The amount and the angle of the SG outlet holes have a direct effect on the plume divergence. The SG outlet angle determines the distance between the flow intersection point (PG-flow and SG-flow) and the air-cap outlet. Increasing the SG outlet angle leads to a reduction of the gas velocity. The use of Design of Experiment (DoE) in the optimization of the air-cap design by implementing CFD-simulation was proved to be a very useful and efficient tool to design high performance air-caps of twin-wire arc-spraying.

A cost effective manufacturing process for molds which are used to produce components of carbon fiber reinforced plastics (CFRP) is proposed. A wire arc spray process has been employed to reinforce a thin electroformed nickel shell by a several millimeter thick layer of thermally sprayed deposit, forming a vacuum tight mold system in a time saving and cost effective way. To achieve a low thermal expansion equivalent to CFRP, Fe 64 Ni 36 is used as spray material. Here, the main challenge is the successful control of distortion which occurs due to residual stresses. In this paper, the influence of process parameters on shell temperature and distortion distribution is discussed. Key parameters influencing the heat flow into the substrate leading to distortion like continuous cooling, atomizing gas and spray distance are addressed. Temperature measurements were performed using infrared pyrometry as well as by use of thermocouples. Distortion measurements were carried out by use of optical measurement devices recording 3D surface coordinates before and after the thermal spray process. Further, mechanical and thermophysical properties of the as-sprayed composite are part of the investigations to evaluate how the Fe 64 Ni 36 bulk material properties can be achieved. Differences between air atomized and inert gas atomized coatings are presented in detail.

To use the manifold possibilities that the arc spraying offers to deposit carbide layers, the knowledge of the particle characteristics are necessary. This work is focused on understanding the particle formation during arc spraying with cored wires. The influence of primary and secondary atomization gas on the particle formation as well as on the particle shape, particle dimension and grain size are analyzed. Melting behaviors of different cored wires under various spraying parameters are investigated. Correlations between coating properties and particle characteristics are established. Experimental approaches to determine the in-flight properties of the sprayed particles in combination with metallographic analyses were employed to establish these correlations.

Liquid metal atomization using a De Laval nozzle has proven its efficiency in producing fine and narrow sized powder. The modeling work of gas dynamics related to nozzle geometry has led to a better understanding of the effects of the processing parameters. During the emptying of the crucible, the decrease in the static height of the melt acts on the metal mass flow rate. An experimental study on the particle size distribution in the cross-section of the spray and its evolution during the process has confirmed the unsteadiness of the process. By establishing a model to fit the gas pressure to the mass flow rate evolution, an almost steady state can be reached for the process. This has brought us to reduce the mean particle size and to improve the narrowness of the as-atomized particle size distribution.

Gas atomized feedstock particles of an Al-13Co-26Ce alloy system were sprayed using the Cold Spray deposition technique. The microstructures of the coatings produced are examined and the mechanical characteristics, in particular the bending fatigue and the bond strength, of the Al-Co-Ce coatings are reported. The results show that the Al-Co-Ce coating improved the fatigue behavior of AA 2024-T3 specimens when compared to uncoated and Al clad specimens. During the bond strength tests, the bonding agent failed and no delamination of the coating from the substrate occurred. The microstructural features of the feedstock powder were also found in the coatings. The coatings contained amorphous and crystalline phase contents similar to the ones found in the feedstock powder. It is suggested that the increase in the fatigue properties can be attributed to the residual compressive stresses induced in the coatings and to the high adhesion strength of the coatings to the substrates.

This paper describes recent effort to synthesize Fe-based amorphous alloys coatings using Cold Gas Dynamic Spraying. Characterization of the gas atomized Fe-based (Fe-Cr-Mo-WC-Mn-Si-Zr-B) powder shows that fully amorphous powder is found when particle diameter is below 20 µm. The coatings produced were composed of the same microstructure as the one observed in the feedstock powder. The overall deformation suggests the occurrence of a localized deformation process at the particle/particle boundary and possible adiabatic deformation softening inside the powder particles during splat formation. The influence of the substrate material on the coating deposition process was also investigated. The synthesis of fully amorphous, porous free coatings using Cold Spray was demonstrated in this work.

The kinetic spray process is a coating process that involves impingement of a substrate by metallic particles at high velocities. In this work, we investigated the kinetically sprayed Ti coatings, which were deposited onto different metallic substrates using gas-atomized powders. The powder particles were characterized in terms of size distribution, morphology, hardness, and explosibilty index. The propellant gas used for coating deposition was compressed nitrogen. The substrates were placed inside an enclosure filled with nitrogen gas during deposition. It was observed that Ti coatings can be deposited at relatively high deposition efficiencies using large particles (median size~ 100 mm). Ti coatings with a wide range of thickness and good macroscopic appearance were readily prepared. The particle temperature appears to have strong effects on the coating formation; deposition efficiencies of ~90% were achievable. Microscopic examination of the coatings revealed deposited Ti particles with significantly lower deformation when compared to ductile materials such as Al and Cu. As a result, the Ti coatings exhibited a high degree of porosity. Several methods were exploited in order to make the Ti coatings denser, including the incorporation of heavy, hard particles for in-situ peening during the coating deposition, and post deposition laser heating. Abstract only; no full-text paper available.

The atomization of molten materials using hot gases has advantages in comparison to the conventional gas atomization techniques. Some of these advantages favour the hot gas technology for the powder production for thermal spraying. Firstly, a considerably higher output of fine powder, in particular within the important particle size range between 5 and 30 µm, is a result of hot (inert) gas atomization. The reasons are the strongly increased gas exit velocity and the higher overall temperatures inside the interaction zone of the gas and the melt droplets. The enhanced shear forces acting on the molten liquid and the prolonged liquid state of the atomized particles lead to a more efficient atomization. Secondly, the extended time regime for liquid droplets facilitates the atomization of highly viscous melts, such as oxide melts, and results in more spherical particle shapes with good flowability. Thirdly, oxide or nitride powders can be generated directly from the molten metal by the usage of hot reactive gases or gas components for atomization. This paper describes the special features of a 25 bar hot gas atomization technique with pre-heated gases up to 1200°C and discusses its potentials for the generation of powders in view of their suitability in thermal spray applications.

In this paper describes the design of the arc spray gun with secondary atomization, which using the mixture of propane and air as the main gas and the compressed air as the gas for secondary atomization. The test shows that in the high velocity airflow produced by the combustion of propane – air mixture, the atomized particle size is less than 20 micron. In the second- combustion thermal airflow, the high temperature area of the arc beam is obviously lengthened at the same time the combustion gas environment restrains the oxidation of the spraying materials. Because the speeds of particles are improved greatly, the coatings obtained are denser. Through practical application it shows that this system can obtain stable deposition efficiency, high bond strength, and low roughness and greatly reduce the cost of machining. Abstract only; no full-text paper available.

The excellent wear-resistant performance of cast iron coatings considerably depends on the formation of graphite structure with an inherent self-lubricating property. In the present study, two types of cast iron powders produced by gas- (GA) and water-atomization (WA) were deposited on an aluminium alloy substrate by atmospheric DC plasma spraying. WA powders are generally characterized by high oxygen content, irregular appearance and inexpensiveness compared with those of GA powders. Although alloying elements of silicon and aluminium work as a strong graphitizer and anti-oxidizer, graphite structures are not recognized in coatings sprayed with as-atomized high silicon and aluminium powders. Therefore, either pre-annealing of powders or post-annealing of coatings is required to achieve cast iron coatings containing graphite structure. A marked decrease in graphite occurs to the coatings with pre-annealed GA powder, since there exists precipitated graphite mainly on a GA powder surface. A short period of post-annealing is also valuable for graphitization. The weak oxide layers are observed in coating cross-sections with GA and WA powder, however, their oxidized levels are much lower than those with bearing steel powder containing low silicon and aluminium. Hence, graphitized cast iron coatings sprayed with inexpensive WA powder exhibit a splendid anti-wear performance.

A bulk amorphous NiTiZrSiSn produced using an inert gas atomization was sprayed onto the Cu substrate. As the oxygen to hydrogen gas fraction was increased, oxide phase fraction was increased at the expenditure of amorphous phase fraction. The phase evolution was mainly due to the in-flight particle oxidation according to flame gas composition. Tribological behaviors were investigated in view of friction coefficient and weight loss by a pin-on-disc dry sliding test. Both friction coefficient and weight loss were largely dependent on the phase composition of the coating. As the amorphous phase fraction was increased, the friction coefficient was decreased with the increase of the transfer film formation. On the other hands, major weight loss mechanism was changed from transfer film formation to pull-out of coat material as the amorphous phase fraction was decreased.

Most of the work published to date on thermally sprayed titanium has been carried out in controlled atmospheres, yielding little information about the reaction of titanium with nitrogen and oxygen. The aim of this study is to investigate the influence of atomization gas on the formation of titanium nitrides and oxides during wire arc spraying. In the experiments, three types of gases (air, nitrogen, and argon) are used to deposit Ti on steel substrates and the microstructure and composition of the coatings, as well as the wire feedstock, are assessed by means of SEM and XRD analysis. The effect of spraying distance on crystal structure and nitrogen content is also investigated in the case of the argon-atomized coating. Paper includes a German-language abstract.

In this investigation, gas atomized powders derived from Ni-Ti wires are vacuum plasma sprayed, producing free-standing coatings with different geometries and wall thicknesses. XRD measurements show that the dominant phases associated with the shape-memory effect and superelasticity remain during powder and coating production, although a problem with brittleness in uncured samples makes tensile test results unusable. Paper includes a German-language abstract.