Cold Spray Deposition Research Articles

Mesoscale modeling of jet initiation behavior and microstructural evolution during cold spray single particle impact

Quasi-coarse-grained dynamics (QCGD) simulations are carried out to investigate the mesoscale deformation behavior during the impact of a 20 µm pure aluminum particle onto a substrate of pure aluminum at time and length scales relevant to cold spray deposition. A rigorous analysis of the evolution of pressure, temperature, strain, flow stress and microstructure is carried out to investigate the jetting mechanisms over a range of process parameters (impact velocity and particle temperature). The QCGD simulations identify a critical role of the pressure wave propagation in the initiation of a jet, i.e. outward flow of material at the particle/substrate interface periphery (edge). Jetting is observed to initiate when the shock wave interacts with the edge and results in localized softening of the metal in this region. This localized softening enables outward flow of the material and is accompanied by a release of the pressures in the particle and the substrate at the interface. Observations of final splat microstructures of systems that showed jetting revealed several new “small” grains in the range of 2–4 µm. These grains are mainly found at the interface, suggesting that recrystallization is favored in cold sprayed impacts of aluminum.

Mechanically induced grain refinement, recovery and recrystallization of cold-sprayed iron aluminide coatings

FeAl coatings can be processed by cold spraying conditions only under the use of well-tuned parameter sets that guarantee shock-loading conditions, which provide sufficient ductility in iron aluminide intermetallic compounds and are capable to ensure sufficient heat creation and thus thermally softened interfaces for bonding. Original microstructures, present in the powder feedstock particles, locally change under the respectively applied loads and temperatures. Given the challenge due to the anomaly softening of such ordered alloys, the present study describes and discusses the differences of coating microstructures in terms of the effect of spraying parameters such as particle size, gas pressure, spraying distance and substrate heating. Electron backscatter diffraction analysis provides elucidation on the grain refinement mechanisms and internal particle strain upon particle impact. The microstructural characterization is complemented by the misorientation analysis within single grains, which is correlated with the spraying parameters and the respective thermal energy input. The misorientation analysis allows for analyzing grain refinement, internal grain bending and for elucidating associated stress relief mechanisms. The results show that the cold spray deposition of FeAl intermetallics can be explained by mechanically induced adiabatic shear instabilities as well as different recovery and recrystallization mechanisms causing a local stress relief.

Understanding the Cold Spray Deposition Characteristics of Mixed Metal Powders

ABSTRACTMixing metal powders in cold spray is of significant interest not only because it is a straightforward method to produce novel composites, but also it has been observed to generate beneficial effects, e.g. improved deposition efficiency (DE). However, the mechanisms behind DE improvements are still not clear fundamentally. In this paper, two examples of mixing metal powders effects in cold spray are introduced: 1) the first example focuses on the effects of different particle/substrate interactions which occurred during cold spray of SS/Fe mixed powders; 2) the second example presents the DE-improving effect of depositing mixed metal powders onto polymers. Various mechanisms associated with the cold spray deposition characteristics of mixed metal powders are discussed in this paper.

Cold spray deposition of aluminum coating onto an erodible material

Results of a pioneering experimental study of the cold spray method used for spraying aluminum traces onto the surface of a brittle erodible material (like brick) are reported. The spraying is performed using a sonic nozzle. Particle velocities near the axis of the two-phase jet are measured, and an approximating formula is derived to describe the dependence of the particle velocity on the stagnation pressure and temperature of the accelerating air. Interaction of the two-phase jet with the substrate surface is studied, and it is shown that an increase in the gas stagnation temperature transforms the process of substrate erosion a coating formation process. Parameter values that can be recommended as working ones for practical spraying are identified.

Effect of process parameters on the properties of Stellite-6 coatings deposited by Cold Gas Dynamic Spray

This study analyzes the effects of process parameters on mechanical and tribological properties of Stellite-6 coatings obtained by Cold Gas Dynamic Spray (CGDS). The gas pressure and temperature as well as the speed of the deposition torch were considered as significant process parameters. The aim is to overcome some technical issues arising in the cold spray deposition of hard anti-wear metallic coatings, such as stellite, due to their high strength and melting point. A High-Pressure CGDS equipment was used and systematic studies were carried out for a deeper understanding of the effects of all investigated process parameters. A particular focus has been put on the substrate temperature, that can be regarded as an indirect process parameter. This latter, in fact, was monitored in-situ during deposition, by infrared thermography (IR). The microstructure was analyzed by scanning electron microscopic observations. Mechanical properties were analyzed by instrumented micro- and nano-indentation measurements. Hardness (H) and Young's modulus (E) were considered as affective parameters to estimate the inter-particle cohesion strength and the work hardening of the coating. Results revealed that the substrate temperature, that is affected by the process parameters, plays a fundamental role in the coating formation process. In particular, the lower the scanning speed the higher the quality of the coatings, in terms of compactness, hardness and elastic modulus. Finally, tribological properties of the coatings were analyzed by pin-on-disk tests. Wear coefficient is strongly related to the strain-to-failure ratio (H/E) that is higher when high values of scanning speed were chosen. In fact, it was observed that decreasing scanning speed elastic modulus increases more quickly if compared with trend of the hardness response. In conclusion, it was demonstrated that both mechanical and tribological properties of CGDS Stellite-6 coatings are mainly affected by the impact temperature of the particle-substrate system and effective deposition windows were identified.

Cold spray (P, N, Mo)-TiO2 photocatalytic coatings: synthesis, microstructural characterization and photocatalytic properties

The (P, N, Mo)-TiO2 photocatalyst particles were intuitively synthesized using spray the sol-gel method followed by the spray drying method. Imperatively, the (P, N, Mo)-TiO2 photocatalyst coatings were deposited on aluminum substrate by cold spraying. The (P, N, Mo)-TiO2 powders were initially prepared by sol-gel method, which exhibited the characteristics of a polygonal particle with a primary particle size of 20 nm and a mean size of 2 μm. The synthesized (P, N, Mo)-TiO2 powders were agglomerated into spherical particles with a mean size of 5 μm by spray drying method. The agglomerated powders depicted a good flowability, wettability and were suitable for cold spray deposition. The successful (P, N, Mo)-TiO2 coatings exhibited good interfacial bonding with the substrate, with an average coating thickness of 15 μm. The properties of the synthesized (P, N, Mo)-TiO2 powders and coatings were analyzed by advanced modern techniques, including XRD, XPS, UV–vis, SEM and TEM. The photocatalytic performance of (P, N, Mo)-TiO2 coatings were investigated under visible light. Cold sprayed (P, N, Mo)-TiO2 coatings exhibited excellent photocatalytic activity in MB degradation under visible-light irradiation. The (P, N, Mo)-TiO2 coating sprayed at 500 °C revealed the highest photodegradation rate of 38% in 4 h. Complementarily, the doping mechanism and photocatalytic mechanism of (P, N, Mo)-TiO2 coatings were also effectively proposed and discussed.

Supersonically Sprayed Zn2SnO4/SnO2/CNT Nanocomposites for High-Performance Supercapacitor Electrodes

In this study, we demonstrate rapid and facile supersonic cold spray deposition of Zn2SnO4/SnO2/CNT nanocomposite supercapacitor electrodes with promising combinations of power and energy density. ...

Stress Corrosion Cracking Resistance of Cold-Sprayed Al 6061 Deposits Using a Newly Developed Test Fixture

The stress corrosion cracking (SCC) response of Al 6061 bulk deposits produced by high-pressure cold spray (HPCS) was investigated and compared to commercial wrought Al 6061-T6 material. Representative tensile coupons were stressed to 25%, 65% and 85% of their respective yield strength and exposed to ASTM B117 salt fog for 90 days. After exposure, the samples were mechanically tested to failure, and subsequently investigated for stress corrosion cracking via optical and scanning electron microscopy with energy-dispersive X-ray spectroscopy (EDS). The results were compared to the wrought Al 6061-T6 properties and correlated with the observed microstructures. Wrought samples showed the initiation of stress corrosion cracking, while the cold-sprayed deposits appeared to be unaffected or affected by general corrosion only. Optical microscopy revealed evidence of stress corrosion cracking in the form of intergranular corrosion in the wrought samples, while no significant corrosion was observed in the cold-sprayed deposits. Fractography revealed wrought samples failed due to multiple mechanisms, with predominant cleavage and intergranular failure, but cold-sprayed samples only failed by ductile dimple rupture. The difference in SCC response between the differently processed materials is attributed to the documented benefits of the cold spray process, which includes maintaining fine grain structure of the feedstock powder and high density after consolidation, low oxidation, and work hardening effect.

Influence of Cold Spray Nozzle Displacement Strategy on Microstructure and Mechanical Properties of Cu/SiC Composites Coating

In this work an influence of cold spray nozzle displacement parameters on the properties of copper-silicon carbide cold spray deposits is considered. In particular the influence of nozzle traverse speed and distance between deposited tracks on the coating porosity and behavior during compressive tests was analyzed. It was shown that cold spraying at low nozzle traverse speed leads to formation of thick tracks with quasi-triangular cross-section. As a consequence, the particle impact angle on the sides of spraying track increases that. Thus, the particle deformation at impact on the track periphery becomes insufficient and local porosity value rises. Increase of nozzle traverse speed allows increasing coating density and mechanical properties due to amelioration of particle deformation conditions. Compressive tests revealed significant anisotropy of mechanical properties of copper-silicon carbide cold spray deposits. In particular, compressive strength measured in vertical direction (perpendicular to the substrate) was significantly higher than one measured in horizontal plane (parallel to substrate). This anisotropy could be explained by the orientation of particle deformation pattern during impact.

Multi-Layer Cold Spray Coating: Strain Distribution

In this paper, we report the effect of multi-layer cold spray deposition on the residual stress formation in the coating and substrate. A method is proposed to separately measure the thermal and mechanical residual stresses induced in cold spray coating. Fiber Bragg Grating (FBG) sensors were employed for in situ monitoring of the strain evolution during the cold spray of multi-layer coating Al7075-Zn on AZ31B Magnesium substrates. Utilizing the capability of the FBG sensors in recording both thermal and mechanical strain gradients, first the effect of temperature on the substrate was investigated when the sample was only treated under carrier gas temperature. Then, the sensors were employed to evaluate the mechanical strain behavior of substrate during the coating process and cooling. Therefore, the effect of thermal mismatch on inducing mechanical strains was observable during the process. Finally, the interaction between the peening process of cold spray and thermal mismatch after cooling was studied. It is shown that the thermal expansion coefficient (CTE) plays a critical role in residual stress development in the substrate and consequently affects the mechanical properties of the coated sample. Hence, careful selection of layers in multilayer deposition can provide desired residual stress in the coating and substrate.

Architectured Multi-Metallic Structures Prepared by Cold Dynamic Spray Deposition

The architectured metallic materials are a class of composite materials that combines two or more metals/alloys with a specific spatial ordering (architecture). The main goal behind the preparation of such materials is to obtain properties that are not achievable by a single material. The internal architecture thus creates an extra degree of freedom in materials design. Based on theoretical considerations three aluminum alloy structures containing square, triangle and sinusoidal iron beam patterns have been prepared by a cold spray deposition technique. Strength properties difference and good bonding of the reinforcing Fe structure to the Al matrix has been found to be important for effective improvement of final properties. Incorporating about 30 vol. % structured iron beams into the Al matrix resulted in a macroscopic performance of the architectured multimaterial similar to Ti alloys.

Influence of the Metallic Cold Spray Deposition on the Low-Velocity Impact Behaviour of Composite Laminates

Recently, the growing attraction to the development of new eco-sustainable composite materials is driving the research interest toward the replacement of synthetic reinforcing fibres with natural ones and exploiting the inherent recyclability of thermoplastic resins even for uses in which thermosetting matrices are well consolidated (e.g. naval and aeronautical fields). Among the natural fibres, a growing interest of the research is addressed to basalt fibres. Focusing the attention on thermoplastic composites, many experimental findings already available in literature highlight the outstanding mechanical properties of composite materials including basalt fibres and their potentiality concerning glass ones. On the other hand, some issues are related to the surface properties of the bio-laminates: in particular, the wear ones, the flame resistance and the aesthetic appearance have to be improved to extend the application fields of these materials. Aiming to these goals, this paper deals with the study of the deposition ofaluminium coating through cold spray process on polypropylene/basalt fabric composite laminates. The specimens were obtained by film stacking, and compression moulding technology and their performances were studied in terms of low-velocity impact behaviour, considering the influence of the surface modification with the aluminium coating. The results obtained from the reference laminates and the coated ones are compared in terms of impact parameters: the aluminium deposition seems to affect the damage mechanism propagation even if the impact response seems to be similar in both conditions.

Inter-particle bonding in cold spray deposition of a gas-atomised and a solution heat-treated Al 6061 powder

The heat treatment of a number of gas-atomised aluminium alloys prior to cold spraying recently showed that the resultant microstructural modification was accompanied by an improvement in deposition; however, the relationship between the microstructural homogenisation occurring after recrystallisation and the increase in deposition efficiency and particle–particle bonding had not been investigated. In this study, Al 6061 gas-atomised feedstock powder, before and after solution heat treatment, was cold sprayed and these materials were characterised using electron backscatter diffraction and transmission electron microscopy. The solution heat-treated Al 6061 powder showed large stress-free grains as opposed to the as-atomised feedstock powder which exhibited smaller grains with the presence of dislocations. The coating produced from as-received powder exhibited a homogeneous distribution of misorientation and lattice defects throughout the particles, whereas the coating produced from solution heat-treated powder showed a strain concentration in the interfacial zones. This was attributed to the partial dissolution and the clustering of solute atoms, allowing the aluminium matrix to deform around the newly formed precipitates.

Fabrication of micro-/nano-structured super-hydrophobic fluorinated polymer coatings by cold-spray

A fabrication of superhydrophobic Perfluoroalkoxy Alkane (PFA) coatings on stainless steel surfaces by cold-spray technique has been demonstrated in this work. The in-situ supersonic impact deposition and consolidation of finely divided 20–40 μm powder material yielded hydrophobic thin coatings of PFA which exhibited contact angles of 125° but high roll off angles of 60°. The deposition efficiencies were less than 0.5%, coating was mechanically very fragile and non-uniform. However, the cold-spray deposition efficiency was subsequently increased to almost 15% by performing simple modification of particle surface morphology by introducing organosilane coated nanoceramic layers on the PFA particles and by laser texturing the stainless steel substrate surfaces to form mountain-valley micro-structures. The nanoceramic interlayers formed bridge bonds at the particle-particle interface and laser texturing achieved efficient coating-substrate interlock. The improved coatings exhibited super-hydrophobic character with 160° contact angle and 6° roll-off angle. The superhydrophobicity was seen to be as a result of micro-/nano-structured surface consisting of papillae like structures capped by a layer of hydrophobic nanoceramic. The coating also exhibited excellent water repellency.

In-situ creep deformation of cold-sprayed aluminum splats at elevated temperatures

This study provides new insights into temperature-dependent splat sliding mechanisms in cold-sprayed deposit by mechanical investigations at length scales comparable to the splats. In-situ indentation of 6061 Al splat structure is performed inside a scanning electron microscope from room temperature up to 400 °C. Indentation at 176 °C, which is the standard heat-treatment temperature, is characterized by enhanced nanohardness (1.65 GPa) as compared to room temperature (1.37 GPa). This is associated with the precipitate formation and improved splat-bonding due to solid-state diffusion. Further enhancement of temperature results in softening, with nanohardness ~70 MPa at 400 °C. The elastic modulus of the splat structure is ~50% of the ‘bulk’ modulus at room temperature and is attributed to energy-dissipation due to splat-sliding activated during indentation. ‘Recovery Resistance’ is computed to quantify energy-dissipation, and a five-fold jump in dissipation is noticed from room temperature to 400 °C. The elasto-plastic zone under the indenter becomes larger with temperature and comprises of multiple splats. Hence, the splat-sliding is found to be enhanced at higher temperatures. Indentation creep investigations show the transition of deformation mechanism from purely plastic (material pile-up) at lower temperatures to non-plastic (crack propagation) at 400 °C. Creep stress-exponents are computed and correspond to solute drag (~3) and dislocation creep (~4–7) mechanisms. At 400 °C, stress-concentration along the splat interfaces is observed in the vicinity of the indenter, eventually leading to crack initiation, propagation and splat delamination due to prolonged loading. These observations highlight that splat interfaces play a vital role in determining the deformation behavior of cold-sprayed materials.

Fatigue Bending Behavior of Cold-Sprayed Nickel-Based Superalloy Coatings

Cold-sprayed Ni-based superalloy coatings offer new possibilities for manufacturing and repairing damaged components, such as gas turbine blades or other parts of aircraft engines. This development shines a new light on the conventional additive manufacturing technologies and significantly broadens application fields of cold spray. The idea is that cold spray can contribute to improving the fatigue properties of manufacturing and repaired components. This study deals with the analysis of the microstructural and mechanical properties of IN625 cold-sprayed coatings on V-notched carbon steel substrate. Process conditions of 1000 °C and 50 bar were employed to produce coatings in V-notched (60° and 90°) samples in order to evaluate the fatigue crack behavior of the sprayed material. Bending tests were carried out in order to evaluate the crack propagation in the coatings during cyclic loading. The K factor was quantified for the two different notch geometries. After fatigue tests, the cracking mechanisms were observed through SEM. Optical microscopy, nanoindentation as a function of coating/substrate distance and corrosion tests were performed. Porosity measurements through image analyses were done to characterize the coatings’ quality. The results achieved demonstrate that cold spray deposition and repair can contribute to resistance and to the increase in the global fatigue life of cracked structures.

Effect of Heat Treatment on the Al-Cu Feedstock Powders for Cold Spray Deposition

This paper examines the effects of heat treatment on the Al-Cu feedstock powders used for cold spray deposition. In particular, we relate improvement of the powder to changes in the spray characteristics and microstructure of cold-sprayed coatings. The inert-gas-atomized Al-Cu powders were solutionized at 535 °C for 2 hours using a rotary furnace, followed by a water quench. Heat treatment of the powders not only eliminated most of the cellular solidification structure generated by the gas atomization process but also resulted in significant grain growth in the powder particles. After solution treatment, the copper content in the α-aluminum matrix increased as expected. After heat treatment, the powders were immediately cold sprayed onto AA2024 substrates. Heat treatment of Al-Cu feedstock powders significantly increased the deposition efficiency for all compositions tested. For heat-treated powders, the deposition efficiency systematically decreased with the increasing copper content, while the coating hardness increased. The formation of GP zones or θ″/θ′ strengthening precipitates through natural aging was not generally observed in the powder particles, but θ″/θ′ precipitates were observed in some cases for the cold-sprayed material with higher copper content.

Influence of Alloy Additions on the Microstructure, Texture, and Hardness of Low-Pressure Cold-Sprayed Al-Cu Alloys

This paper examines a series of Al-Cu binary alloy coatings, ranging from 2 to 5 weight percent copper, produced using low-pressure cold spray (CS) deposition with helium as the carrier gas. Binary Al-Cu alloy feedstock powder was produced through inert gas atomization and was sprayed over a variety of temperatures and pressures. Using helium gas, this set of Al-Cu alloys was successfully deposited as high-density coatings. Raising the carrier gas pressure increased the particle velocity and deposition efficiency (DE) in the case of spraying the Al-5 wt.% Cu powders. A clear composite deformation structure was formed in all coatings with clear prior particle centers surrounded by severely deformed regions with ultrafine grains. Microstructural deformation generated by the CS process produced a weak but clear fiber texture for both Al-2 wt.% Cu and Al-5 wt.% Cu coatings. The copper content of the feedstock powder directly influenced the coating hardness and porosity, while having no systematic effect on the DE.

Cold Spray Aluminum–Alumina Cermet Coatings: Effect of Alumina Morphology

The feedstock powder morphology has an important effect on the deposition behavior of cold spray coatings, and this effect is even more significant while spraying cermet coating by the mixture powders. The effect of alumina powder morphology on the deposition efficiency and coatings mechanical properties is investigated in the cold spray deposition of aluminum–alumina cermets. The deposition of aluminum mixed with spherical and angular alumina is studied and compared for six different feedstock powder compositions for each particle morphology. The addition of angular alumina particles in the feedstock powder induces an increase in deposition efficiency followed by a decrease as the alumina content increases beyond a specific value. This effect is not observed when spherical alumina is used. The creation of asperities during deposition was explored for the two alumina powder morphologies, and it was determined that spherical alumina does not produce intricate asperities at the coating surface, explaining the difference in deposition efficiencies. Coatings produced with spherical alumina were harder and showed a greater increase in adhesion strength than coatings produced with angular alumina for similar coating alumina content. These differences are attributed to a larger amount of plastic deformation of the matrix material in coatings sprayed with spherical alumina.

Particle deformation and microstructure evolution during cold spray of individual Al-Cu alloy powder particles

This paper examines the single particle impact process for a series of Al-Cu alloy powder particles with 2–5 wt% copper, performed using a low-pressure cold spray system with helium as the carrier gas. The cold spray deposition process is fundamentally controlled by the deformation processes, which occur during single particle impacts. Single particle depositions on steel substrates were produced for each composition over a range of gas temperatures (225–325 °C). Cross sections from single, impacted particles were produced using focused ion beam methods. The deformation microstructure in individual cold sprayed particles was studied using precession electron diffraction (PED) and transmission Kikuchi diffraction (TKD) techniques. For Al-Cu alloy particles, the amount of deformation estimated using a compression ratio did not show a significant difference with varying copper alloy additions. The single particles experienced large deformation upon impact, and ultrafine grains were formed at the particle/substrate interface via dynamic recrystallization. The particles were bonded to the steel substrate via a thin amorphous layer at the interface.