Cold spray (CS) is a progressive method for the deposition of metals and alloys whose principles involve considerable plastic deformation of the produced material at extreme strain rates. Positron annihilation spectroscopy (PAS) is an analytical technique capable of studying deformation on the atomic scale level, even in extremely deformed materials. In our study, the PAS method was used to characterize the deformation character at the lattice level and quantify the open-volume defects in four cold sprayed metals: Al, Cu, Ni, and Ti. As counterparts, bulk samples of these materials with ultrafine-grained structures were also produced by high-pressure torsion (HPT), a process exceeding cold spray in the total deformation, but having several orders of magnitude smaller strain rates, and by a traditional cold rolling process. The results show that the CS and HPT processes lead to the formation of similar lattice defects (dislocations and vacancy clusters), and both exhibit significantly higher dislocation densities than conventionally cold-rolled materials. Further, the vacancy clusters present in CS and HPT materials were not present in the rolled counterparts due to the lower vacancy production rate.

The deformation behavior of particles plays a significant role in achieving adhesion during cold spray. The deformation behavior of the particles is associated with the fracture of the oxide layer and recrystallization, which are the key elements of the quality of cold spray. Studies of particle compression have been made to understand the deformation behavior of a particle. However, the deformation behavior of particle under controlled load and precise and high strain rate is yet to be studied. Here, we show the oxide layer fracture pattern and recrystallization regime under controlled load with a precise and high strain rate. We found that the cracks in the oxide layer initially appeared on the equator of the particle and propagated towards the edge of the top surface. Meanwhile, on the top surface, the circumferential crack was developed. On the other hand, the nanoindentation result showed that the compressed particle under a high strain rate has an unusual load-displacement behavior. Our results demonstrate that the oxide layer fracture behavior corresponds to the adhesion mechanism suggested by previous studies. Our study also revealed that recrystallization takes place within the particle under a high strain rate. We anticipate this finding to give a general insight into the deformation behavior of particles during cold spray. For instance, since the recrystallization behavior at a given strain rate can be predicted through this study, the resultant grain size and shape, which is associated with mechanical properties, can also be predicted. Furthermore, the amount of strain and strain rate to form optimal adhesion can be evaluated.

The Cold spray (CS) is a promising solid-state additive manufacturing method. The interesting physics involved in the CS process including cold, high strain rate, adiabatic and severe plastic deformation results in a unique and complex structure of CS deposits at different length scales that directly determines the properties of the deposits. Therefore process- structure properties (performance) (PSP) linkages explorations are pivotal. Integrated computational materials engineering (ICME) methods in complement with experimental analyses are required to evaluate materials properties and behaviour in PSP links exploration. Finite element modelling is used to simulate the thermomechanical response of materials and evolution of field variables in CS, i.e stress, strain, strain rate, and temperature, at structural scales. Molecular dynamics modellings of nano-particle impact have provided useful insights into atomic-scale phenomena of individual particle impact while the modelling of microstructure evolution in micro and mesoscale has yet to be investigated. In this study, we developed and implemented a thermodynamic phase field simulation method to capture the structure evolution of CS composite Ni-Ti deposit upon post-spray heat treatment (PSHT) in microstructure scale. The external or internal stimuli such as heat and strain either generated in the system because of phase transformation or stored as internal energy upon CS process are accounted for. The interface mobility and microstructure development are calculated by minimization of Gibbs free energy of the system. The comparison of the simulated microstructure with experimental results confirms that the phase field modelling precisely predicts the microstructure evolution of the CS deposits upon PSHT.

Cold spray additive manufacturing technology (CSAM) is a progressive method of 3D print of metals and alloys. Its inherent work principles allow production of the components below the material melting points, thereby avoiding several undesired material degradation processes. Among other inherently associated phenomena, the work principles of CSAM involve extreme plastic deformation of the materials, triggering formation of several types of lattice defects. Positron annihilation spectroscopy (PAS) is an analytical technique capable of studying deformation on the atomic scale level, even in extremely deformed materials. In our study, the first historical analysis of CSAM materials by PAS was carried out. For the demonstration, four different base metals were selected (Al, Cu, Ni, Ti). For these, the character of dislocations and vacancies was observed and the respective densities were quantified. The results show that the extremely high strain rate in the cold spray process prevents recovery of vacancies by diffusion to sinks. The deformation-induced vacancies agglomerate into small vacancy clusters. Hence, metals deposited using CSAM contain not only dislocations but also vacancy clusters. Both kinds of defects were detected by positron annihilation spectroscopy.

Tailoring strength and ductility in additive manufacturing or repair is key to successful applications. Therefore, cold spraying must be tuned for maximum amounts of well-bonded internal interfaces as well as sufficient softening of the highly workhardened deposit. Zinc (Zn) with its low melting temperature is an ideal model system to study phenomena associated with high strain rate deformation and local temperature distributions, both, in single impacts and thicker deposits. Bonding and recrystallization can be facilitated by covering selected wide parameter regimes in cold spraying. Despite the low temperatures, Zn single splats already show recrystallization at internal interfaces, the respective amounts then scaling with increasing process gas temperatures. At higher process temperatures, deposits are almost fully recrystallized. The recrystallization seems to improve bonding at internal and at deposit-substrate interfaces. Under optimum conditions, an ultimate deposit cohesive strength of up to 135 MPa and an elongation to failure of 18.4% are reached, comparable to that of laser-manufactured or bulk Zn parts. This demonstrates a welltuned interplay between high amounts of bonded interfaces and softening by recrystallization that allows for deriving bulk-like performance of cold sprayed material without additional posttreatments. Correlations between microstructures, mechanical properties, and fracture mechanisms supply information about prerequisites needed for reaching high ductility as obtained in damage and failure modes of deposits and bulk materials in global and local approaches.

Thermal spray technology keeps attracting several industries in both the manufacturing and repair sectors, thanks to its practicability and its reasonable processing time. Moreover, different kinds of materials can be successfully deposited to form coatings with potential excellent thermo-electro-mechanical properties. The resultant coating microstructure is completely different from the wrought powder material before the deposition process. In the case of metallic materials, the thermomechanical characteristics are quite dependent on the deposition conditions monitored from the spraying setup. One can mention gas temperature, impact velocity and angle, material combination, surface state, particles size, etc. Hence, one major factor which influences the final coating microstructural state is kinetic energy. In fact, in such processes where high velocity deposition is observed, intense grain refinement and sharp increase of the dislocation density are an outcome that is tightly related to high temperature and severe plastic deformation. Prediction of the mechanical properties of the produced coating is usually carried out using phenomenological models that describe very well the relationship between stress and strain under different conditions of temperature and strain rates. Most of these models fail, however, to describe the effect of the deformation mechanisms observed at ultra-high strain rates such as the viscous drag regime of dislocations or further the weak shock load regime, scenarios commonly observed in such processes. In the present paper, we present an enhanced physics-based model to describe the stress strengthening of metals upon impact and associated microstructure changes. We show that the model can accurately represent the desired effect of the dislocation drag. Modelling of the impact of a single copper particle onto a copper substrate is carried out to show the capability of the model to predict grain refinement and dislocation network modification.

Anisotropy of stress-strain behavior, fracture toughness, and fatigue crack growth rate of Ti6Al4V deposited by cold spray using nitrogen was studied. For that, flat deposits were tested with stress acting in the in-plane directions and tubular deposits were tested in the out-of-plane stress directions. In all tests, unified small-size specimens were used. It was shown that for the in-plane stress, the deposits can be considered isotropic, whereas the out-of-plane stress led to significantly lower values of the measured properties. The obtained results were related to fractography and microstructural analysis. While a combination of trans-particle and inter-particle fracture determined the fatigue properties in the near-threshold regime, at higher loads, inter-particle fracture was dominant. It was also shown that the different particle-to-stress orientations influenced the resulting fatigue and static properties.

Because of their high specific strength, carbon fiber reinforced plastics (CFRPs) are widely used in the aerospace industry. Metallization of CFRP by cold spraying as a surface modification method can improve the low thermal resistance and electrical conductivity of CFRP without the need for high heat input. Herein, we cold spray a Sn coating on cured CFRP substrates and examine the Sn/epoxy interface. The results suggest that the Sn coatings are successfully obtained at a gas temperature of 473 K and indicate no severe damage to the CFRP substrates. The stress and plastic strain distributions at the cross-section of the Sn/CFRP interface when a Sn particle is impacted onto the CFRP substrate are obtained using the finite element method.

Diamond-reinforced composites prepared by cold spray are emerging materials simultaneously featuring outstanding thermal conductivity and wear resistance. Their mechanical and fatigue properties relevant to perspective engineering applications were investigated using miniature bending specimens. Cold sprayed specimens with two different mass concentrations of diamond 20% and 50% in two metallic matrices (Al – lighter than diamond, Cu – heavier than diamond) were compared with the respective pure metal deposits. These pure metal coatings showed rather limited ductility. The diamond addition slightly improved ductility and fracture toughness of the Cu-based composites, having a small effect also on the fatigue crack growth resistance. In case of the Al composites, the ductility as well as fatigue crack growth resistance and fracture toughness have improved significantly. The static and fatigue failure mechanisms were fractographically analyzed and related to the microstructure of the coatings, observing that particle decohesion is the primary failure mechanism for both static and fatigue fracture.

Cold spraying is a promising process for fabricating functional coatings. Because of the solid-state particle deposition, the electrical and chemical properties of the coatings are similar to those of the bulk materials. Mechanical properties, on the other hand, differ from those of bulk materials due to severe plastic deformation of the particles. Residual stress may thus be an important variable to track during cold spraying although the formation mechanism is not entirely clear. In this study, the residual stress of metallic (copper) and ceramic (titania) coatings is measured during the cold spray process. The results are presented and discussed.

An internationally recognized best practice for disposing used nuclear fuels is to store them in specially designed containers in deep geological repositories. One type of spent fuel container is a carbon steel canister with a cold-sprayed copper coating. The aim of this study is to assess the impact of various factors on the ductility of this protective copper layer. The current investigation finds that there can be significant variability in ductility when feedstock powder size and chemical composition are changed while keeping spraying and heat treatment conditions constant. Test results show that the ductility of nitrogen-sprayed copper decreases with increasing hardness, but can be improved by raising annealing temperature from 300 to 600 °C. The effects of substrate geometry and process variations are discussed as well.

This study assesses the mechanical performance of cold-sprayed aluminum 6061 coatings heat treated using focused IR radiation. The heat treatment was performed in-process with the aim of improving the ductility and strength of the coatings. The properties of the heat-treated samples are compared to those achieved using traditional annealing and as measured in as-sprayed samples. It was found that the rapid IR heat treatment increased the ultimate tensile strength of the coatings by 52% and elongation at failure by 43%.

Cold spray is a technology with great potential for additive manufacturing applications. Due to the high levels of plastic deformation experienced by the powder during the coating process, any deposit will require heat treatment post-spraying to improve ductility and fatigue strength. In extreme cases, the residual stresses from coating can cause delamination or compromise the bond strength when subsequent cold spray layers are deposited. This work details the use of a commercial CO 2 laser cutter to perform a surface heat treatment on single lines of cold sprayed aluminium, to relieve residual stresses. The effect of laser power and traverse speed on material hardness is quantified, and compared with as sprayed deposits. The results shown in this work demonstrate the potential for in-process heat treatment to reduce post-processing time and improve coating quality by reducing residual stresses.

Repairing of Ni-alloy components using cold spray is being increasingly considered as an option in the aerospace industry. To further the understanding of the microstructure of Ni-alloy coatings and the bonding mechanism, gas atomised alloy 718 was sprayed onto carbon steel substrates to form 0.5mm thick coatings and single particle impacts. Spray trials were performed with different process parameters to compare the splat and coating morphology/microstructure and to optimise the parameters. The powder consumable, single particle impacts and coatings were characterised using SEM, EBSD, TEM and nanoscale XRF and XRD. Four-point bend tests were performed to test strength, ductility, cracking and de-bonding. Fine grains were observed in the substrate-particle interfaces caused by particle fragmentation, deformation and dynamic recrystallisation. Low angle grain boundaries and sub-grains form in the substrate due to strain induced by high energy impacts. The deposition efficiency, thickness, porosity, hardness and surface roughness of the coatings were measured and compared across all parameters. The porosity decreases notably (1.2% to 0.25%) and the hardness increases (410HV to 465 HV) with the increase in gas temperature and pressure. The results indicate that temperature has a larger effect on the coating properties compared to the pressure and that deformation has an important role in bonding.

The cold spray deposition mechanism of Ultra-High Molecular Weight Polyethylene (UHMWPE) requires a detailed understanding of strain rate sensitivity of UHMWPE. The yield and flow in UHMWPE are complex and sophisticated compared to metals due to their dependency on time, temperature, strain, strain-rate and the history of the processing. In this research work, powder-sintered UHMWPE of 10.5 Mg/mol was subjected to various strain-rates ranging from 10-2 s-1 to 103 s-1 via low compression strain-rate testing and Split Hopkinson Pressure Bar testing (SHPB). The experimental true stress-strain curves in compression of a sintered UHMWPE at pre-yield, yield point, and post-yield were analyzed. The pre-yield and yield point region, 10.5 Mg/mol UHMWPE exhibited an increase in the tangent modulus and the yield stress with an increase in strain rate. Further, the post-yield phenomenon in UHMWPE shows no apparent post-yield softening and shows an increase in the strain hardening with an increase in the strain-rate. The curves at increasingly higher strain rates showed an increasingly pronounced bi-linearity to its flow behavior with the rate of hardening increasing above 10~15% strain. Under the domain tested, the strain rate dependence for UHMWPE can be depicted by a logarithmic fit.

The Nuclear Waste Management Organization (NWMO) has proposed the concept of a deep geological repository (DGR) for the storage of Canada’s used nuclear fuel. A major engineered component is the used fuel container (UFC) consisting of a steel core coated with copper for corrosion resistance. The copper coating is required to have sufficient ductility and adhesion strength to the steel substrate for loading requirements under DGR conditions. The NWMO has identified two coating technologies for the application process: electrodeposition and cold spray. Electrodeposition is utilized to coat the bulk of the UFC components (i.e., hemi-spherical head and lower assembly). A portion of the hemi-spherical head and the lower assembly openings remain uncoated in order to facilitate the final assembly closure weld process after fuel loading. This area is then cold sprayed with copper to complete the coating on the steel. Since the cold sprayed coating is highly strained in the as-sprayed state, it requires a heat treatment to impart ductility. The ductility is assessed indirectly by measuring the hardness of the material before and after the heat treatment. A recent advancement on this front includes the implementation of an optimized band heat treatment method to prototype UFC’s.

Cold-sprayed copper coatings tend to be brittle and their electrical conductivity is inferior to that of the bulk material. In order to solve these problems, conventionally, it has been attempted to recover the metallic structure by heat treatment. This study, however, focuses on the effects of phosphorus and tin with the aim of improving cold spray copper coatings by optimizing the impurity content of Cu powder. It is shown that, by adjusting the content of P and Sn, dense copper coatings can be obtained with high ductility and electrical conductivity equal to that of the bulk material.

The application of metallic foam core sandwich structures in engineering components has been of particular interest in recent years because of their unique mechanical and thermal properties. Thermal spraying of the skin on the foam structure has recently been employed as a novel cost-efficient method for fabrication of these structures from refractory materials with complex shapes that could not otherwise be easily fabricated. The mechanical behavior of these structures under flexural loading is important in most applications. Previous studies have suggested that heat treatment of the thermally sprayed sandwich structures could improve the ductility of the skins and so affect the failure mode. In the present study the mechanical behavior of sandwich beams prepared from arc sprayed alloy 625 skin on 40 ppi nickel foam was characterized under four point bending. The ductility of the arc sprayed alloy 625 coatings was improved after heat treatment at 1100°C and 900°C while the yield point was reduced. Heat treatment of the sandwich beams reduced the danger of catastrophic failure.

In this work the Brazilian disc test technique is applied to mechanical characterization of cold-sprayed Cu-In-Ga deposits. The main advantage of the test is that the material can be tested in its end product form while naturally attending to the distribution of micro-structural phases and flaws by choosing an appropriate specimen size. The stress state of the test specimen can be obtained in analytical closed form and testing can be readily extended to obtain the stress-strain curve. While limited to low ductility materials the Brazilian disc test appears ideal for testing as-sprayed cold spray deposits due to their typically brittle nature.

This study evaluates the corrosion and wear resistance of WC-Co coatings produced by cold gas and HVOF spraying. Three WC-Co cermet powders varying in cobalt content were deposited on aluminum alloy substrates by both methods. The powders were characterized based on microstructure, particle size distribution, and phase composition, and the coatings based on cross-sectional microstructure, phase composition, and Vickers hardness. The coatings are also compared based on the results of ball-on-disk, rubber-wheel, and electrochemical testing, which shows that CGS has several advantages over HVOF spraying for the deposition of WC-Co coatings.