In cold spray applications, optimum process conditions to accelerate particles may vary with different densities of the feedstock. These conditions could depend on the geometry of the spray nozzle, suggesting possible benefits of material-specific nozzle designs. The present study developed a nozzle geometry optimization concept based on three-dimensional computational fluid dynamics (3D-CFD) simulations to provide a specific nozzle design. Applying a design of experiments (DoE) approach, the proposed model seeks an optimal nozzle geometry, using aluminum Al6061 and pure copper with mean particle diameters of 40 µm as examples. Different geometry parameters were varied to reach the highest particle velocities before impact on the substrate, such as the nozzle’s divergent section length, throat cross section, and expansion ratio. The process gas was nitrogen with set stagnation pressure and temperature of 5 MPa and 500 °C, respectively. For high particle impact velocities, the simulation identified the divergent section length as the most influential parameter, followed by the throat cross section. In addition, the results show that the expansion ratio must be carefully tuned to avoid over-expansion of the gas already inside the nozzle, which is detrimental to the particle acceleration.

In cold spray, successful bonding occurs when particle impact velocities exceed the critical velocity. The description of the critical velocity includes temperature upon impact and material properties, relying on tabulated data of bulk material. However, rapid solidification of powder particles during gas atomization results in higher strengths than reached by respective bulk materials, causing an underestimation of the critical velocity. Thus, a readjustment of the semiempirical calibration constants can 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

Cold spraying (CS) of high-strength materials, e.g., Inconel ® 625 is still challenging due to the limited material deformability and thus high critical velocities for achieving bonding. Further fine-tuning and optimization of cold spray process parameters are required, to reach higher particle impact velocities and temperatures, while avoiding nozzle clogging. Only then, sufficiently high amounts of well-bonded particle–substrate and particle–particle interfaces can be achieved, assuring high cohesive strength and minimum amounts of porosities. In this study, Inconel ® 625 powder was cold sprayed on carbon steel substrates, using N 2 as propellant gas under different spray parameter sets and different powder sizes for a systematic evaluation. Coating microstructure, porosity, electrical conductivity, hardness, cohesive strength, and residual stress were characterized in as-sprayed condition. Increasing the process gas temperature or pressure leads to low coating porosity of less than 1% and higher electrical conductivity. The as-sprayed coatings show microstructures with highly deformed particles. X-ray diffraction reveals that powder and deposits are present as γ -solid-solution phase without any precipitations. The deposits show high microhardness and compressive residual stresses, which is attributed to work hardening and peening effects. The optimized deposits reach almost bulk material properties and are thus well suited for industrial applications.

Abstract Robot-guided cold spraying is currently developing as a technique with great potential for the repair of metallic components, particularly for depositing heat- and oxidation-sensitive materials. In this regard, the use of automation and robotics enables flexible control of the repair process. To ensure an optimal repair process, it is essential to consider the various requirements of robot-guided cold spraying already during the simulative planning phase. However, conventional robotic repair trajectories often do not fully consider the geometric constraints of material deposition, efficient material use, and the underlying limitations of robot kinematics. This work proposes the application of trajectory optimization by mathematical optimization for repair by robot-guided cold spraying. In this context, the optimal repair strategy must handle the constant material flow by the spray jet, which inevitably couples local material deposition with the robot motion. For this purpose, decision variables, objective function, constraints and a material deposition model are formulated to control the amount of deposited material accordingly. The goal is to generate an optimized trajectory that incorporates the requirements of cold spraying and robot kinematics to guarantee high-quality repair and efficient material use. This includes minimizing excess material and minimizing the jerk of the robot motion. The results demonstrate successful application of the trajectory optimization for component repair by cold spraying.

Abstract In cold spray, optimum process conditions to accelerate particles vary with different densities and melting temperatures of the materials. Therefore, material-specific nozzle designs are required. In the present study, a nozzle geometry optimization concept based on 3D-CFD simulations was developed to provide a specific nozzle design for a given material. Al6061 and pure copper with mean particle diameters of 40 μm were taken as examples. Together with a design of experiments (DoE) approach, the model seeks for the optimal nozzle geometry. In order to reach the highest particle velocity prior to impact upon the substrate, different geometry parameters were varied, such as the nozzle throat cross section, the aspect ratio, and the nozzle divergent section length. The process gas was nitrogen with set stagnation pressure and temperature of 50 bars and 500 °C. For both materials, the simulation identified nozzle divergent section length as the most influential parameter, followed by the throat cross-section. The aspect ratio must be tuned to avoid over expansion of the gas in the nozzle.

Abstract Thick deposits were produced from pure Al powder of three different sieve sizes using cold spraying at the same process parameters. The in-plane mechanical and fracture properties of the deposits were investigated using bending of small specimens in four specimen orientations. It was shown that increasing the Al particle size by approximately 50% and 100% leads to small, but statistically significant differences of yield strength. Further, the increase in the powder particle size led to higher fracture toughness KIC but lower fatigue crack growth threshold ΔKthr. This can be attributed to two different fracture mechanisms in the cold sprayed deposits. A trans-particular fracture in the near-threshold fatigue regime is controlled by the microstructure and work hardening of the particles. At higher cyclic loads and in quasi-static regime, the particle decohesion and the resulting crack path determine the fracture behavior instead. However, the observed effect of particle size was rather small, much smaller than the effect of spray process parameters observed in the previous research.

Cold gas spraying nowadays receives much interest for additive manufacturing due to its high deposition rate. Associated structural applications define high requirements regarding mechanical properties and failure tolerances. Up to present, micro-flat tensile (MFT) or tubular coating tensile (TCT) tests are well-established for determining deposit strengths. Due to particular stress states during testing, both provide slightly different information. While MFT tests can provide information on strain and ductility, stress concentration in TCT tests requires to apply a notch factor for calculating the ultimate tensile strength. Here, we suggest that a suitable combination of both tests can provide additional information about tolerances against local stress concentrations in crack initiation and growth. Taking titanium and copper as model systems, results from MFT and TCT tests are evaluated over a wide range of spray parameter sets into regimes that allow for high deposit qualities, even reaching certain ductility. The correlation between the results reveals that the derived stress concentration sensitivity depends on the deposit quality and could eventually decrease to unity. In turn, the correlation to respective strain to failure data can supply information on underlying deformation mechanisms. These preliminary results thus provide strategies for tuning deposit toughness and give prerequisites for quality forecasts.

Cold spraying (CS) of high strength materials, e.g., Inconel 625 is still challenging due to the limited material deformability and thus high critical velocities. Further fine tuning and optimization of cold spray process parameters is required, to reach higher particle impact velocities as well as temperatures, while avoiding nozzle clogging. Only then, sufficiently high amounts of well-bonded particle-substrate and particle-particle interfaces can be achieved, assuring high cohesive strength and minimum amounts of porosities. In this study, Inconel 625 powder was cold sprayed on carbon steel substrates using N2 as propellant gas under different refined spray parameter sets and powder sizes for a systematic evaluation. Coating microstructure, porosity, electrical conductivity, hardness, cohesive strength and residual stress were characterized in as-sprayed condition. Increasing the process gas temperature or pressure leads to low coating porosity of less than 1 % and higher electrical conductivity. The as-sprayed coatings show microstructures with highly deformed particles and well bonded internal boundaries. X-ray diffraction reveals that powder and deposits are present as γ- solid-solution phase without any precipitations. By work hardening and peening effects, the deposits show high microhardness and compressive residual stresses. With close to bulk material properties, the optimized deposits should fulfill criteria for industrial applications.

Bismuth vanadate (BiVO 4 ) offers high photon efficiencies in solar photo-anodes, due to its suitable semiconductor band gap energies and associated visible light absorption. In well-tuned conditions, such anodes enable green hydrogen generation in photoelectrochemical water splitting cells. Bismuth vanadate films have to ensure high efficiencies in electron/hole pair generation and sufficiently high rates of charge transfer to the conducting substrate and the electrolyte, respectively. Thus, the tuning of coating properties has to aim for high phase purity, good layer integrity as well as optimum diffusion path lengths. In order to explore the potential of aerosol deposition to produce BiVO 4 films with high photoelectrochemical activity and to elucidate influences on microstructure and application properties, powder sizes and spraying parameters had to be tailored. By ball milling over durations of up to 20 min, particles sizes in the range from 8.3 down to 0.6 µm were obtained. With respect to spray conditions, the process gas pressure was varied from 1.0 to 2.1 bar corresponding to gas flow rates of 10-40 l/min. The wide range of powder sizes and parameters in aerosol deposition allowed for developing a window of deposition in order to derive the most promising combinations for layer build-up. Optimum parameter sets in application on stainless steel substrates were transferred to FTO-coated glass substrates for backlit cell layouts. The thickness and conductivity of the layers were adjusted to a layer thickness range of 200-500 nm in order to achieve maximum photocurrents. The production of homogeneous, large-scale prototypes demonstrates that aerosol deposition is suitable for processing layers for solar energy harvesting with high photo current densities of up to 3.55 mA/cm 2 .

Applications in thermal and kinetic spraying increasingly aim for coating of parts with complex geometries. So far, respective robot programming for the required path during deposition is usually adjusted individually in time-consuming procedures. Thus, it is essential to develop methods that allow a fast adaptation to part geometries and production conditions as well as possible quality control. To tackle these problems, this work addresses novel strategies for robot programming and post-spray analyses. The design of the method and workflow follows routes of smart manufacturing and should enable fast and accurate implementation into spray procedures. Here, the developed application can handle complex parts of arbitrary geometry in the form of CAD files. Supported features include (i) cutting the objects according to the object boundary, (ii) creating self-intersecting curves, (iii) generating a set of index-sequence-based spatial discrete points and (iv) reordering the discrete points to generate adaptive paths. Robot offline programming allows for process simulation, analysis and optimization of the robot kinematics. By optical scanning profilometry, the layer-by-layer deposit build-up could be monitored for quality control, as well as for the determination of the final overall coating thickness. The entire procedure was tested by cold spraying onto a complex workpiece, validating the capability of the proposed strategy. Based on the universal layout of the applied methods, the strategies can also be applied for thermal spraying in general, considering individual boundary conditions. With respect to cold spraying, the implementation framework of this study provides a good basis for part repair and additive manufacturing.

Due to its suitable semiconductor band gap energies and associated visible light absorption, bismuth vanadate offers high photon efficiencies in solar photo-anodes, enabling green hydrogen generation in photoelectrochemical water splitting cells. Respective bismuth vanadate films have to ensure high efficiencies in electron / hole pair generation, and sufficiently high rates of charge transfer, for both, electrons to the conducting substrate, as well as holes to the electrolyte. Thus, tuning of coating properties has to aim for high phase purity and good layer integrity. So far, respective films are mainly produced by thin film techniques, but at rather high costs and low deposition rates. Less costly processing routes are opened by thermal spraying or sol-gel techniques, however, these cannot guarantee the required phase purity or absence of remnants from the binder. As solid state and binderless alternative, Aerosol Deposition (AD) offers several advantages: comparative low costs, high deposition rates, no undesired phase transformations, and no impurities or residues that could reduce the photoelectrochemical activity. Under the scope of this research on photo-electrochemically active bismuth vanadate films, powder sizes were tailored by milling, and spray parameter sets like the process gas pressure were varied, in order to elucidate their influence on microstructure and application properties. Covering a wide parameter range in aerosol deposition allowed for the development of a window of deposition. Most promising combinations for layer build-up were derived. The results on stainless steel substrates were transferred to FTO-coated glass substrates, as needed in backlit cell layouts. For fine tuning of maximum photocurrents, layer thickness and conductivity were then systematically adjusted. Homogeneous large-scale prototypes demonstrate that aerosol deposition is suitable for processing layers for solar energy harvesting.

A melt pocket in the bonding zone of medium carbon steel bonded on low-carbon steel by explosive welding was investigated by laser assisted atom probe tomography. It was found that the structure is nanocrystalline and (sub-) grain boundaries are enriched with carbon. High hardness values in the melt pocket are attributed to a combination of (sub-) grain size and carbon distribution. (c) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Cold Spraying of high strength materials, i.e., Inconel 718 is still challenging due to the limited deformability of the material restricting the quality of deposits. Thus, process parameters must be tuned for reaching higher particle impact velocities and temperatures to allow for maximum amounts of well-bonded particle-substrate and particle-particle interfaces. In the present study, Inconel 718 powder was cold sprayed under varied process gas temperatures for a systematic study of the influence on the quality of thick deposits. In addition, one set of samples of each batch was exposed to post heat treatment procedures by hot isostatic pressing, thermal soft annealing, and aging for attaining hard bulk material properties. Deposits microstructure, porosity, electrical conductivity, hardness, and residual stress were analyzed in as-sprayed and as-heat treated conditions. Results are discussed in terms of the “coating quality parameter”, defined as the ratio between particle impact velocity and critical velocity. As-sprayed deposits exhibit microstructures with highly deformed particles and well bonded internal interfaces. X-ray diffraction reveals that powder and deposits present a γ-solid-solution phase, allowing to assume conventional softening behavior for estimating critical conditions for bonding. Increasing the process gas temperature leads to lower coating porosity and higher electrical conductivity. Deposits showed similarly high microhardness and compressive residual stresses, both caused by work hardening during cold spraying. Subsequent heat treatments improved the quality of internal interfaces, mostly for deposits with high values of “coating quality parameters”. By distinguishing influences on several coating properties, these results contribute to gain basic knowledge for successful manufacturing of Inconel 718 thick deposits by cold spraying, particularly concerning needed coating quality parameters for adjusting desired properties.

In this study, an experimental approach to investigate basic dependencies on impact and bonding of agglomerated ceramic particles in cold spraying is presented. Single impact morphologies of ceramic particles obtained from wipe tests are correlated with data obtained from powder compression experiments with a modified nanoindenter. Different feedstock powders of agglomerated TiO2-nanoparticles were used and also partially heat treated. The powder shapes and sizes prior and after the compression tests were analyzed by confocal microscopy. The single particle impacts were characterized by SEM. Besides the expected influence of substrate material, substrate temperature, and spray conditions, the deformation and bonding of ceramic particles to metal substrates critically depend on the powder properties. To which degree particles fracture or contribute to layer formation upon the high-energy impact is highly correlated to their individual deformation behaviour in quasi-static compression tests.

In current process techniques to manufacture sliding bearings, bronze components are soldered to the respective parts, which is cost and energy intensive. Apart from that, so far most bearing materials still contain lead, which in new applications is omitted by EU law to avoid associated health risks. The present study aims to offer solutions for both by using cold gas spraying as additive manufacturing technique for processing bearings directly onto steel parts and by applying that to new lead-free bronze alloys. A lead-free bronze alloy was processed as powders by gas atomization and classified to optimum sizes for cold spraying. During cold spraying, the process gas pressures and temperatures as well as the substrate temperature were varied with the aim to study influences by impact conditions and effective surface temperature on particle deformation and bonding. Respective coatings show low porosity, high hardness and high electrical conductivities. With properties similar to that of bulk cast material respectively manufactured parts should meet the requirements for new bearing applications.

AbstractThis chapter reviews the current understanding of high-pressure cold spraying for different materials, covering widely accepted general mechanisms for particle deposition and the processes and parameters involved. It begins by reviewing the mechanisms of bonding. An overview of the optimization of the critical process parameters for improving coating qualities is then provided. This is followed by a separate section dealing with bonding between different materials and addressing influences on adhesion to the substrate as well as the cohesion between dissimilar coating constituents. The knowledge of the basic science and mechanisms finally allows for discussion on the requirements for suitable cold spray equipment and of the parameter sets needed for successful coating deposition.

Abstract The modeling and simulation activities in the field of high-pressure cold spray can be divided into two main parts: solid mechanics and fluid dynamics. This chapter focuses on these parts of modeling work in cold spray research. The discussion covers the objective, principal concepts, methods, and outcome of modeling and simulation of particle impact and of in-flight history of particles in cold spraying. The concept of integration of particle impact and fluid flow modeling to optimize cold spray deposition for a given material is also explained.

A new range of applications in cold spraying is expected for bulk metallic glass (BMG) coatings. For retaining amorphous structures in cast multi-component BMG parts, typically high purity raw material must be used. The present investigation explores an alternative approach, where cold spraying is used to deposit a technical-grade binary amorphous alloy. This approach is shown to be potentially cost-effective and suitable for rapid manufacturing. For this purpose, amorphous Cu50Zr50 was chosen as a model alloy system, and cold spraying was performed using nitrogen as process gas. By a systematic variation of the spray parameter sets, the critical velocities for coating formation were determined experimentally. Based on the current models of bonding of amorphous Cu50Zr50 powder in cold spraying, a new, more comprehensive concept of bonding and rebound is presented, which also considers the presence of liquefied interfaces and quenching rates for resolidification. Results concerning impact morphologies and coating formation demonstrate that under suitable choice of spray conditions, well-adhering coatings with amorphous structure of the Cu50Zr50 powders can be obtained by cold spraying.

Cold spraying was used to build up coatings of ordered Fe40Al intermetallics, which are a kind of alloys which are being proposed for high temperature applications. The spray experiments were performed with chemically ordered powder feedstock of different size distributions to study influences from impact velocities. Stainless steel and Ti grade 1 substrates were used to investigate influences of the mechanical and physical properties of the substrate material. While coatings on stainless steel are mostly dense with good cohesion, those on titanium show some cracks perpendicular to the coating substrate interface. That might be due to the larger difference in the thermal expansion coefficients between the titanium substrate and the iron aluminide. Regarding the influence of spraying parameters, it has been found for both substrate materials, that when spraying the first layers, a uniform almost linear decrease in the coating thickness is observed when increasing the spraying distance. By increasing the number of spray layers, the increase in thickness asymptotically approaches a saturation limit. Thicker coatings were obtained on stainless steel substrates than on Ti substrates.