Severe plastic deformation (SPD) is the main feature of the Cold Spray (CS) process, which might result in producing metal grain refinement under extensive hydrostatic pressure and high strain rate loading conditions. In this study, an anisotropic strain gradient plasticity model (SGP) is presented to predict materials behavior in CS process. The enhanced dislocation densities produced throughout particle deformation affect coating material properties and modify their thermodynamic characteristics and kinetics of resistance to plastic deformations. This study also demonstrates that the SGP model can describe the experimentally observed trends and account for homogenization of the accumulated strains under dynamic recrystallization conditions. The evolution of statistically stored dislocation density through the characteristic material length scale parameter is in good agreement with experimental results and data reported by other research groups. The proposed SGP modeling is suggested as an express method to evaluate the advanced coating and additively manufactured materials, and powder feedstock used in thermal spray and 3D manufacturing applications.

Atmospheric plasma spray is considered as one of the most efficient methods for forming FeAl intermetallic coatings. But the performance of plasma-sprayed FeAl coatings was remarkably limited because of oxidation and phase transformation during the preparation. In the present work, FeAl intermetallic coatings were prepared by atmospheric plasma spray combined with dry-ice blasting. The microstructure, oxidation and porosity of FeAl intermetallic coatings were investigated. In addition, XRD measurements were also employed to illustrate the lattice-scale performance, e.g., dislocation density. The temperatures during plasma spray were also measured using an infrared pyrometer system. The results show that a denser B2-FeAl coating with a lower content of oxide and lower phase transformation can be achieved because of the cryogenic effect and the mechanical effect of dry-ice blasting. Moreover, the microhardness of FeAl coating was nearly increased by 72%, due to the lower porosity and higher dislocation density.

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.

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 processes of microstructural evolution in titanium during deposition by cold spray were investigated. Titanium particles underwent plastic deformation at very high strain rates, resulting in (a) extensive refinement and (b) large inhomogeneities in microstructure. There were large nanostructured areas (several micrometres in extent) with high dislocation densities and subgrain sizes less than 100 nm. Dense dislocation tangles and elongated subgrains were observed in other places. It is hoped that by further understanding the microstructural changes that occur during cold spray, improvements in mechanical properties may be realised.

cBN/NiCrAl nanocomposite coating was deposited by cold spraying using mechanically alloyed composite powders. To examine the thermal stability of coating microstructure, the nanocomposite coating was annealed at different temperatures from 750 to 1000°C. The microstructure of composite coatings was characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results showed that the nanostructure was retained in the coating when the annealing temperature was lower than 825°C which is 0.7 times of the melting point of NiCrAl matrix. The dislocation density significantly reduced when annealing temperature was higher than 750°C. The cBN particle growth became significant when the annealing temperature was higher than 825°C. The effects of crystal grain refinement and work-hardening strengthening mechanisms were quantitatively estimated as the function of annealing temperature. The effect of annealing temperature on the contribution of different strengthening mechanisms to coating hardness was discussed.

In this study, two copper powders with different size distribution are applied to aluminum substrates using cold gas dynamic spraying. The powders are sprayed with helium gas at ambient temperature and at 200 °C. Investigators measure oxide content in the powders and correlate it with in-flight particle velocity, deposition efficiency, particle deformation, and coating properties including microhardness, structure, and dislocation density.

High-entropy alloys are of great interest due to their unique phase structure. They are constructed with five or more principal alloying elements in equimolar or near-equimolar ratios and thus derive their performance from multiple elements rather than one. In this work, solid-state cold spraying is used for the first time to produce a FeCoNiCrMn high-entropy alloy coating. As a low-temperature process, cold spraying completely retained the high-entropy phase structure in the coating without any phase transformation. Examination shows that the grains underwent significant refinement due to dynamic recrystallization and that the coatings are much harder than the feedstock powder because of increased dislocation density and grain boundaries.

The Cold spray method of material deposition is widely used for surface enhancement, to improve properties such as corrosion and wear. A detailed microstructural analysis of cold sprayed Ni based coatings (IN625 (Inconel 625 is a Trademark of Huntington alloys corporation) using the transmission electron microscope, revealed the occurrence of three distinct types of microstructures in the as sprayed condition, adjacent to each other. These include the occurrence of large (> 1 μm) grains having a high dislocation density, along with regions comprising shear bands (20-30 nm wide) and twins with a large aspect ratio (> 1000), along with locations having a very fine grained structure (20-30 nm). The crystallite size, using a Hall-Williamson plot measures an average 50 nm. The substrate deformation indicates a bilayer dynamic recrystallization, as a means to accommodate the strain. The microscopy studies will serve to correlate the bonding mechanism of cold sprayed IN625 on 4130 steel.

Two copper powders were deposited with CGT3000 cold gas dynamic spray system on aluminium substrates. The X-ray diffraction patterns allow the characterisation of the microstructure such as grain size, strain in the coating and dislocation densities. Both powders and coatings have been fully characterised. Three methods have been used to interpret the X-ray patterns: the Warren-Averbach method, the Hall Williamson method and the modified Hall-Williamson method. A comparison between the state of the powders before and after deposition will give an insight on the metallurgical processes that take place during the formation of the coating. The influence of the grain size distribution will also be discussed. This article is a follow up of the publication done at ITSC2006: “Comparison Between Coatings from two Different Copper Powders: Mechanical Properties, Hardness and Bond Strength”.

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.

High entropy alloys, as a novel alloy system, demonstrated excellent mechanical performance. However, despite its excellent mechanical performance, the strength-ductility trade-off effect still limit its performance. In recent decades, it has been found that heterogenous or gradient microstructure can efficiently solve the conflict. Cold spray is a promising method to create heterogenous microstructure with high efficiency and low cost. In this work, equiatomic FeCoNiCrMn HEA was deposited by cold spray and the microstructure was systematically investigated by transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD). In cold spray, a gradient microstructure was formed and segregated Ni and Mn in starting particle were also redistributed. Moreover, twinning in ultra-fine nanograins were detected in the region close to the impact interface. Compared with severe deformation of other low SFE metals, for FeCoNiCrMn HEA, twinning in nanograins also highly related to the grain size.

Tantalum and silver are recognized for their outstanding biocompatibility and antibacterial ability, respectively. However, owing to their distinct chemical and physical properties, synthesizing alloys and composites by using Ta and Ag presents a considerable challenge. In this study, Ta-Ag composites, exhibiting good antibacterial ability, were successfully produced by using a solid-state cold spray technique. Notably, intriguing correlations were observed between Ag microstructure and antibacterial ability. To unravel this correlation, a comprehensive experimental and simulation analyze were conducted. It is found that the volume ratio of Ta to Ag in the feedstock powder result in different deformation histories for Ag during the cold spray process. This, in turn, leads to the formation of distinctive Ag microstructures within Ta-Ag composites. The varied Ag microstructures results in different Ag dissolution ability and the formation of an insoluble AgCl layer exhibiting varying morphologies, when Ag exposed in a high chorine ion environment, like in human body fluids. This consequently influences the concentration of Ag ion and ultimately determines antibacterial ability. The study demonstrates that Ag release rate and the related antibacterial properties could be alternatively controlled by changing Ag contains or by creating different deposition process by adjusting CS parameter.

In this work, the fundamental mechanisms for ceramic particle deformation in aerosol deposition were investigated. We hypothesized that pre-existing defects affect ceramic particle deformation under compression. Preliminary results showed that near defect-free, sub-micron, single crystal alumina (Al 2 O 3 ) particles exhibited dislocation nucleation and motion along with significant plastic deformation, shape change, and cracking in compression at room temperature. In contrast, highly defected, micron-sized alumina particles exhibit no observable change in shape before fracture and fragmentation. Particle deformation mechanisms, identified through this work, provide insight into feedstock design for solid state alumina deposition using the aerosol deposition process.

Cold spray, a new member of the thermal spray process family, can be used to prepare dense, thick metal coatings. It has tremendous potential as a spray forming process. However, it is well known that significant cold work occurs during the cold spray deposition process. This cold work results in hard coatings but relatively brittle bulk deposits. We have investigated the mechanical properties of cold sprayed aluminum and the effect of annealing on those properties. Cold spray coatings approximately one centimeter thick were prepared using three different feedstock powders: Valimet H-10, Valimet H-20, and Broadman Flomaster. ASTM E8 tensile specimens were machined from these coatings and tested using standard tensile testing procedures. Each material was tested in two conditions: as-sprayed and after a 300°C, 22 h air anneal. The as-sprayed material showed high ultimate strength and low ductility, < 1% elongation. The annealed samples showed a reduction in ultimate strength but a dramatic increase in ductility, up to 10% elongation. The annealed samples exhibited mechanical properties similar to wrought 1100 H14 aluminum. Microstructural examination and fractography clearly showed a change in fracture mechanism between the as-sprayed and annealed material. These results indicate good potential for cold spray as a bulk forming process.

Deposition of copper by cold gas dynamic spraying has attracted much interest in recent years because of the capability to deposit low porosity oxide free coatings. However, it is generally found that as-deposited copper has a significantly greater hardness, and potentially lower ductility, than bulk material. This paper will describe work undertaken to investigate the effect of annealing heat treatments on the structure and mechanical properties of freestanding cold sprayed copper. After de-bonding from substrates these tracks were annealed for one hour at a range of temperatures up to 600 °C. Optical microscopy, scanning electron microscopy and X-ray diffraction were all employed to examine the microstructure. The peak widths in XRD were analysed according to the Hall – Williamson method so that changes in grain size and microstrain (i.e. dislocation content) could be quantified. Mechanical behaviour of the deposits was studied by microhardness measurements and tensile testing. The influences of annealing on mechanical properties are rationalised in terms of microstructure evolution and its effect on strengthening and recrystallization mechanisms in metals. The softening behaviour of cold sprayed Cu is explained considering the low stacking fault energy of Cu and the possibility of dynamic recystallization occurring during spraying.

An earlier study reported an investigation of the mechanical properties of cold sprayed aluminum and the effect of annealing on those properties. In that study, cold spray coatings approximately one centimeter thick were prepared using three different feedstock powders: Valimet H-10, Valimet H-20, and Brodmann Flomaster. ASTM E8 tensile specimens were machined from these coatings. Each material was tested in two conditions: as-sprayed and after a 300°C, 22 hour air anneal. The as-sprayed material showed a high ultimate strength and low ductility, < 1% elongation. The annealed samples showed a reduction in the ultimate strength but a dramatic increase in ductility, up to 10% elongation. Microstructural examinations and fractography clearly showed a change in the fracture mechanism between the as-sprayed and annealed material, but insufficient data was available to conclusively explain the ductility increase at that time. Since then, Kikuchi mapping of the Valimet H-10 material in the as-sprayed and annealed conditions has been conducted. Kikuchi mapping allows indexing of grains, identification of grain boundaries, and phase identification using backscattered diffraction patterns in an SEM. The data shows that significant recrystallization within the splats upon annealing has occurred. No significant crystal growth across splat boundaries is observed. The data demonstrate that the mechanism of ductility increase in annealed cold spray deposits is recrystallization of the base aluminum material.

In this investigation, carbon nanotube (CNT) aluminum composite coatings are produced by kinetic spraying. Electrical and mechanical properties are evaluated and compared with those of cold-sprayed pure aluminum coatings. The CNT aluminum composite coatings exhibited higher conductivity and hardness than the pure aluminum, which is attributed to the high electrical conductivity and dispersion hardening effects of embedded CNTs in the aluminum matrix.

Cold sprayed copper coatings have been widely studied but the effect of substrate properties and spraying conditions on the bond strength still needs further investigations. Using nanoindentation, it is possible to compare the effect of spray parameters on the change in hardness of the coating due to the difference in size distribution of the powder. Deposition of 2 copper powders on aluminium and Ta6V substrates has been done at different stagnation temperatures and pressures using nitrogen as process gas. Hardness profiles show how the speed of the particles induces shot peening effects on the surface. The interaction between the hot gas jet and the surface may modify the hardness profiles. The effect on the substrate hardness has also been investigated.

Thermally-sprayed LZ/YSZ double-layer coatings are promising candidate for the next generation thermal barrier coatings (TBCs) due to exceedingly low thermal conductivity and superior high-temperature phase stability. However, a delamination failure at LZ and YSZ interface were widely observed during TBCs service. Till today, the interfacial microstructure between LZ and YSZ remains unclear. In the present study, LZ splats were deposited on YSZ substrate to serve as a LZ/YSZ interface. The interfacial microstructure was explored by focused ion beam (FIB) and high-resolution transmission electron microscope (HR-TEM). The interfacial defects at splat interface were clearly observed and thoroughly discussed. These results would shed light on deeply understanding the interfacial failure of double-layer LZ/YSZ coatings.