This research proposes an experimental methodology towards visually observing high strain rate polymer deformation characteristics at scales relevant to cold spray particle impacts. Macro-scale (~ 3 mm) polymer impact testing via a light gas gun has shown evidence of cold spray indicative features at certain (material, particle/substrate temperature, velocity, etc.) conditions.

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.

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.

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.

This study investigates the impact behavior and consequences for coating formation in cold spraying of FeAl intermetallic compound powder. A range of spraying conditions was used to process single impacts in so-called wipe tests and for processing spray layers. In order to avoid brittle failure, high process gas temperatures and varied traverse speeds were used to account for thermal softening of spray particles and already adhering layers. Morphologies of as-impacted particles and partially removed single splats were subsequently investigated by SEM. The study of spray lines indicates that secondary impacts are only successful within an extremely narrow range of impact conditions. Within this narrow parameter regime, thicker and dense coatings are obtained. Hardness testing shows that the properties of the powders were retained.

In cold gas spraying, the powder is not molten before impact on the substrate. The bonding of the coating only depends on powder characteristics and impact conditions. To optimize coating microstructure and properties, spray conditions have to be tuned for a particular powder. The optimization procedure usually requires a systematic variation of spray conditions and an analysis of the sprayed coatings which is time consuming and costly. Therefore, alternative test methods which are less expensive and operate with similar load mechanisms on powder particles have to be developed. High strain rate deformation can be easily studied by explosive powder compaction. In this method, the powder is loaded by a shock wave and deformed under high strain rates. The bonding conditions of powder particles should be similar to those obtained in cold spraying. By a special design, shock loading in explosive powder compaction can cover a wide energy range in one single experiment. Therefore, the method appears feasible to determine the energy input required for successful bonding of particles. To evaluate the capability of the method, microstructural features of particle/particle interfaces are investigated and compared to those of cold sprayed coatings. In addition, the results can supply more information concerning the bonding mechanisms in cold gas spraying.

This paper represents an effort to systematize an understanding of the cold spray process and the suitability of material for such a process. The evaluation is based on a brief analysis of the powder particle impact and a literature research concerning shock compression phenomena in matter and related physical effects, such as impact heating and dynamic yielding. The FEM simulations performed permit estimating the maximum impact pressures, the deformation rates and the deformation kinetics during impact. The calculations can be verified experimentally and are supported by the data published. From a brief analysis of the equations of state applied to shock compression, key material parameters are derived and investigated. A parameterization of physical properties and correlation with the crystal types endeavors to provide a ranking of material suitability.

In cold spraying, in contrast to thermal spraying the coating material is not melted prior to the impingement onto a substrate. The powder particles are accelerated to high velocities by a supersonic gas jet. Even though the particles are in a solid state, they form a dense and solid bonded coating upon impact. In order to form a dense coating with sufficient adhesion to the substrate, the particles have to reach a certain velocity before hitting the substrate. This velocity is characteristic of the coating material and also depends on the particle temperature. A variety of experiments have been carried out with copper as spay material in order to determine the critical velocity for solid bonding of particles onto the substrate. To investigate the effect of spray parameters and nozzle geometry on the velocity and temperature of the particles, computational fluid dynamics was performed. The calculations allow a direct correlation between experimentally obtained deposition efficiencies and process parameters. Finite element modeling of the particle impact could relate successful bonding to high strain rate phenomena at the particle interface. In view of the above criteria an optimization strategy for cold spray process can be developed.

Two different coatings were studied in this work : vacuum plasma-sprayed NiCoCrAlYTa and electrodeposited NiCoCrAlYTa. These coatings were deposited on AM3 single crystal alloy. The tensile and creep properties of coated single crystal test specimens were investigated. Ductile-brittle transition temperatures (DBBTs) were determined from tensile tests. Creep tests were performed on cylindrical specimens and on thin flat specimens. All the coatings were examined before and after testing. The two tested coatings induce a ductile/brittle transition. Strain rate has a great influence on the transition temperature. The comparison between the two processes of deposition illustrates the strong influence of coating microstructure. A marked decrease in creep properties was observed for thin single crystal specimens but contrary to cylindrical specimens, the coating has a quite positive influence, so that the creep life of coated thin specimens is increased.