In this paper, we evaluate the potential of ultrasonic atomization as a new feedstock manufacturing technique for cold spray by comparing the cold spray performance of an experimental stainless steel 316L powder obtained from ultrasonic atomization with a commercial stainless steel 316L powder produced through gas atomization.

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 widespread use of additive manufacturing and modern powder-based technologies (thermal spraying, hardfacing, sintering) encourages the search for alternative routes enhancing the development of metal and metal alloy powders. The state-of-the-art powder production processes, like gas, water or plasma atomization, are dedicated to mass production, which limits the availability of new powder compositions with desired characteristics. In this study, stainless steel powders were investigated. The powders were atomized by an in-house developed ultrasonic (UT) atomization set-up, called ULTRAMIZER. In this system, the atomization of melt is possible by using a high-power ultrasonic field. The atomized powders were characterized in terms of morphology and particle size distribution (PSD). The powder features were then correlated with operating parameters of: (i) UT atomization system, mainly frequency and root mean square power (RMS), and (ii) the orientation of the atomization plate against the melting system, by means of distance and tilting angle. The study shows that the ultrasonic atomization allows producing nearly spherical, defect-free powder particles, with a very narrow and controllable size distribution. These are important advantages over other metal powder production methods, especially when it comes to the development of new types of powder.

The use of metal powders is dynamically increasing in many different fields of research and industry. The development of additive manufacturing technology or innovative thermal spray processes still enhance the range of possible applications. This means that there is a big demand for high quality metal powder materials. Currently, the atomization methods, like water or gas atomization, seem to be most established technologies for metal powder production. This work concerns the development of an innovative technology of metal powder manufacturing, namely ultrasonic atomization. First, the general idea of ultrasonic atomization is discussed. The designing of sonotrode, in order to generate appropriate ultrasonic field responsible for metal stream atomization, is discussed. Then, the ultrasonic set-up was verified by using non-contact fiber optic displacement sensor. Finally, the developed system was preliminary tested under water loading and confirmed positively in terms of ultrasonic atomization capabilities.

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.

This paper addresses a need for information on nanoparticle emissions and related issues such as worker exposure, filtration efficiency, and dustiness. A survey has been conducted on the working conditions and safety measures used in thermal spray companies and the results compared to scientific literature and previous surveys. Responses to questions on matters of health and safety reveal a lack of information and awareness of the risks posed by the emissions of ultrafine particles generated by thermal spraying processes.

Wire atomization processes used to make refractory and high temperature alloy powders are relatively expensive due to the cost of feedstock, energy, and gas. A new process based on Transferred Arc Wire Atomization technology, however, has the potential to overcome these problems. This paper introduces the innovative process which, in combination with hydrogen generation, presents new opportunities for several alloys that can be more easily processed by plasma wire atomization. The new approach shows promise to reduce both fixed and variable costs for certain refractory and high temperature materials.

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 study, yttria films with high thermal shock resistance were synthesized from a metal-EDTA complex by means of combustion flame spraying. A rotating stage and various cooling agents were used to control substrate temperature during deposition. Although thermally extreme environments were employed during synthesis, the obtained films showed only a few cracks and some minor peeling in their microstructures. In the case of a Y 2 O 3 film synthesized using substrate rotation and water atomization, the porosity was found to be 22.8% and the temperature of the film immediately after deposition was 453 °C, owing to a high heat of evaporation in the cooling water.

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.

Internal coating of cylinders has always been a challenge for cars engineers. Driven for more than two decades now by the ecological and economical constrains applied to the automotive industry, it constitutes a dynamic way of research and development for industrial applications. One of the most economical processes for this kind of coatings is the rotating twin-wire arc spray (TWAS) system. Meanwhile the actual quality and the performances of the corresponding coatings still leave place for some improvements. Therefore, in the work presented here, attention was paid to the second atomization phenomena in a TWAS system considering the influence of the gas flow parameters on the particles’ morphology and deposition behavior. Numerical modeling of the plume and comparisons between several designs of the second atomizing units were also considered.

Comprehensive study of the wire arc thermal spray technology will allow for better design and optimization of guns. In wire arc spray, a feed of two electrically-charged wires are melted using an arc. This bath of molten metal goes through an atomization process with a high pressure air being blown upon it. Flow of air will then carry the generated molten drops and deposits them on the substrate. The focus of this study is on the numerical simulation of wire arc sprays using ANSYS FLUENT software. Effects of geometrical parameters on resulting flow conditions and flow circulations inside the gun are studied. Simulation results help in better parameter selection for effective wire arc coating.

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.

Liquid injection plasma spraying is of growing interest for thermal spray applications like thermal barrier coatings and solid oxide fuel cells, since finely structured coatings offer improved properties over conventionally spray ones, for example lower thermal diffusivity and higher catalytic activity. One challenge is the optimization and understanding of the injection process. With a new high speed shadowgraphy setup, the injection and atomization of individual drops was observed and described in detail in this work which is, to our best knowledge, not reported before. A drop atomization cone model is derived from observations. A new modelling approach is developed which allows the prediction of the drop atomization cones by analytical calculations. The simulations are compared to measurements and deviations are explained by neglected effects which will be included in further developments of this model.

Thermally sprayed cermet powder coatings as well as bulk cermet materials sintered of carbide/metal powder blends are widely used in applications with severe abrasive wear conditions. A cost-saving alternative can be provided by using iron-based melt-atomised hard alloy powder feedstocks. Among them, commercial alloys containing high amounts of vanadium and carbon obtain outstanding wear resistance due to their high volume fraction of finely dispersed, hard vanadium carbides. However, their performance is still exceeded by cemented carbides. A further improvement of the wear properties of hard alloys basically can be attained by increasing their carbide content, concurrently considering the limitations of the melting and atomisation process regarding the melting temperature. A possible solution can be provided by alloying the basic system Fe-V-C with an additional strong carbide former like niobium. Subject of this work is the comparing investigation of the technologically important melting equilibria in the systems Fe-V-C and Fe-V-C-Nb.

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.

Thermal spraying is a material processing technique, which is based on the combination of thermal and kinetic energy. The used feedstock is melted in a hot flame and the melt is atomized and accelerated by means of atomization or process gases. The formed particles are rapidly solidified and consolidate to form splats as they hit a pre-treated substrate. The splats pile one-on-top-of-other forming lamellas and creating the final coating. In the work presented here a combination of cored wire (WC as filling powder) and massive wire (copper) were simultaneously sprayed using the twin wire arc spraying (TWAS) process. 3D micro tomography was used in order to gain knowledge about splat formation and layer build-up. Due to the high attenuation coefficient of tungsten in comparison with copper and carbon tungsten-rich particles and splats can easily be spotted in the tomogram of the coating layer. It turns out that besides irregular formed flat splats also ball-shaped particles exist in the coating layer which suggests that the spherical particles impacted on the substrate in an un-molten state. By 3D data processing tungsten-rich particles were visualized to analyze their spatial distributions as well as their geometric parameters were quantified. This work aims at contributing to the understanding of spraying processes.