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• The first review of strengthening strategies for cold-sprayed deposits. • Strengthening can be achieved through pre-process, in-process and post-process strategies. • Most processes only improve the density and strength of deposits. • Themo-mechanical-based processes improve both strength and ductility. Cold spray, as a solid-state additive manufacturing process, has been attracting increasing attention from both scientific and industrial communities. However, cold-sprayed deposits generally have unfavorable mechanical properties in their as-fabricated state compared to conventionally manufactured and fusion-based additive-manufactured counterparts due to the inherent microstructural defects in the deposits (e.g., porosity and incomplete interparticle bonding). This downside reduces its competitiveness and limits its wide applications as an additive manufacturing process. In the past years, many strengthening technologies have been developed or introduced to adjust the microstructure and improve the mechanical properties of cold-sprayed deposits. The term "strengthening" in this work specifically refers to improving the mechanical strength, particularly the tensile strength of the cold-sprayed bulk deposits. According to the stage that the strengthening technologies are used in the cold spray process, they can be classified into three categories: pre-process (e.g., powder heat treatment), in-process (e.g., powder heating, in-situ micro-forging, laser-assisted cold spray), and post-process (e.g., post heat treatment, hot isostatic pressing, hot rolling, friction stir processing). Therefore, a comprehensive review of these strengthening technologies is conducted to illuminate the possible correlations between the strengthening mechanisms and the resultant deposit microstructures and mechanical properties. This review paper aims to help researchers and engineers well understand the different strengthening methods and provide guidance for the cold spray community to develop new strengthening strategies for future high-quality mass production. [Display omitted] [ABSTRACT FROM AUTHOR]

• A repairing approach for restoring neutron shielding composite plates was presented. • Defected plate was subjected to cold sraying followed by hot rolling treatments. • Hot rolling resulted in improved microstructure of the as-sprayed deposit. • 3-point bend test confirmed good strength of the repaired material. Cold spray (CS) is considered as a new type of repairing technique for restoration or remanufacturing of a wide range of unserviceable engineering components due to its unique 'cold' characteristic. In this work, a viable repairing approach i.e. "cold spraying followed by hot rolling post-treatment" was successfully applied to restore defective neutron shielding B 4 C/6061 Al composite plates. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) results revealed that the coating-substrate interface is in a good bonding state without micro-cracks. Hot rolling post-treatment resulted in improved microstructure of the as-sprayed deposit with more uniform distribution of B 4 C particles in the matrix. Moreover, well bonded splat boundaries and strain free Al grains were evolved in the matrix due to enhanced splat deformation and continuous dynamic recrystallization (CDRX). Three-point bending test results revealed that the strength of the repaired material is at par with the in-service plates. The findings of this work could be considered as a great stride to further extend possible applications of CS for repairing engineering structural components. [ABSTRACT FROM AUTHOR]

[Display omitted] In this work, the Invar 36 alloys were manufactured using cold spray (CS) additive manufacturing technique. The systematic investigations were made on the microstructural evolution, thermal expansion and mechanical properties under as-sprayed (AS) and heat-treated (HT) conditions. XRD (X-ray diffraction) and ICP-AES (inductively coupled plasma atomic emission spectroscopy) analyses show that no phase transformation, oxidation, nor element content change have occurred. The X-ray computed tomography (XCT) exhibited a near fully dense structure with a porosity of 0.025% in the helium-produced sample under as-sprayed condition, whereas the nitrogen-produced samples produced at 5 MPa and 800 °C show more irregular pore defects. He-AS sample shows a more prominent grain refinement than that of nitrogen samples due to the more extensive plastic deformation. The post heat-treatment exhibited a promoted grain growth, inter-particle diffusion, as well as the formation of annealing twins. Between 25 °C and 200 °C, the nitrogen samples possessed lower CTE (coefficient of thermal expansion) values (1.53 × 10-6/°C) compared with those produced by casting and laser additive manufacturing. The He-AS samples exhibited a noticeable negative CTE value between 25 °C and 200 °C, which may due to the significant compressive residual stress (-272 MPa) compensating its displacement with temperature increase during CTE test. The N2-HT and He-HT Invar 36 samples present a notable balance between strength and ductility. In conclusion, the CS technique can be considered as a potential method to produce the Invar 36 component with high thermal and mechanical performance. [ABSTRACT FROM AUTHOR]

About 3 mm thick five-element equimolar high-entropy alloy (HEA) FeCoCrNiMn was successfully deposited by solid-state cold spraying (CS). The high-temperature oxidation behavior of the CSed HEA was investigated at 700-900 °C. Heat treatment was performed on the CSed HEA before oxidation to heal the incomplete interfaces between the deposited particles. Results show that the microstructure of the CSed HEA is characterized by grain refinement and abundant interparticle incomplete interfaces. Post-spray heat treatment promotes recrystallization and grain growth in the CSed HEA. After oxidation testing, the oxide scales are composed of multi-layers: a Mn 2 O 3 (or Mn 3 O 4) outer layer, a Mn-Cr spinel intermediate layer and a Cr 2 O 3 inner layer. The CSed HEA exhibits higher parabolic rate constants and more favorable internal oxidation than the bulk HEAs that have similar compositions in the literature. Such a discrepancy becomes pronounced at higher temperatures. The grain refinement and numerous particle boundaries are responsible for such a distinctive performance of the CSed HEA. [ABSTRACT FROM AUTHOR]

The oxide scale present on the feedstock particles is critical for inter-particle bond formation in the cold spray (CS) coating process, therefore, oxide scale break-up is a prerequisite for clean metallic contact which greatly improves the quality of inter-particle bonding within the deposited coating. In general, a spray powder which contains a thicker oxide scale on its surface (i.e., powders having high oxygen content) requires a higher critical particle velocity for coating formation, which also lowers the deposition efficiency (DE) making the whole process a challenging task. In this work, it is reported for the first time that an artificially oxidized copper (Cu) powder containing a high oxygen content of 0.81 wt.% with a thick surface oxide scale of 0.71 μm., can help achieve an astonishing increment in DE. A transition of surficial oxide scale evolution starting with crack initiations followed by segmenting to peeling-off was observed during the high velocity particle impact of the particles, which helps in achieving an astounding increment in DE. Single-particle deposit observations revealed that the thick oxide scale peels off from most of the sprayed powder surfaces during the high-velocity impact, which leaves a clean metallic surface on the deposited particle. This makes the successive particles to bond easily and thus leads to a higher DE. Further, owning to the peeling-off of the oxide scale from the feedstock particles, very few discontinuous oxide scale segments are retained at inter-particle boundaries ensuring a high electrical conductivity within the resulting deposit. Dependency of the oxide scale threshold thickness for peeling-off during the high velocity particle impact was also investigated. [ABSTRACT FROM AUTHOR]

This work focused on the role of Al 2 O 3 particles in the corrosion behavior of cold sprayed AA5083 aluminum alloy matrix composite (Al-MMC) coatings. The electrochemical characterization of the coatings was investigated in a 3.5 wt.% NaCl solution as a function of time. The results show that fragmentation of Al 2 O 3 particles is not clearly observed in the case of AA5083/20 vol.% Al 2 O 3 coating, while the broken Al 2 O 3 particles can be seen clearly in AA5083/40 vol.% Al 2 O 3 and AA5083/60 vol.% Al 2 O 3 coatings. The addition of 20 vol.% Al 2 O 3 particles yield the coating with the lowest porosity, and different volume fractions of Al 2 O 3 in the feedstock have important effects on the electrochemical behavior of composite coatings. The Al-MMC coating reinforced with 20 vol.% Al 2 O 3 particles exhibits the highest E corr and the lowest i corr compared with the other conditions. The order of current density is as follows: AA5083/20 vol.% Al 2 O 3 < AA5083 < AA5083/40 vol.% Al 2 O 3 < AA5083/60 vol.% Al 2 O 3. [ABSTRACT FROM AUTHOR]

In this study, three kinds of A380/Al 2 O 3 composite coatings were prepared by cold spray (CS) using spherical, irregular and spherical + irregular shaped Al 2 O 3 particulates separately mixed in the original A380 alloy powders. The influence of Al 2 O 3 particulates' morphology on the microstructural characteristics (i.e. retention of Al 2 O 3 content in coatings, coating/matrix interfacial bonding, pore size distribution and morphology etc.) and wear performance of the coatings was investigated by scanning electron microscopy (SEM), X-ray computed tomography (XCT) and 3-D optical profilometry. Results indicated that the spherical Al 2 O 3 shows obvious tamping effect during deposition process. As a result, the interface showed a wavy shape while the matrix and particulates were mechanical interlocked with much improved adhesion. In addition, the porosity of the coating was minimized and the pores exhibited curved spherical structure with reduced dimensions. The irregular Al 2 O 3 particles predominantly displayed the embedding effect together with fragmentation of Al 2 O 3 particulates. Consequently, poor coating/ matrix interfacial bonding, high porosity and the formation of angular-shaped pores were resulted in the coating. Dry sliding wear tests results revealed that the wear resistance of the coating is directly related with the retained content of Al 2 O 3 in the coating. The coating containing irregular Al 2 O 3 particulates displayed superior wear performance with its wear rate one seventh of that of the pure A380 alloy coating. The coating containing both kinds of Al 2 O 3 particulates showed mixed characteristics of above two kinds of Al 2 O 3 composite coatings. [ABSTRACT FROM AUTHOR]

Inter-particle bonding is an important factor affecting the property of cold sprayed metallic deposit. Because the interface bonding between particles in deposit is directly determined by plastic strain of particles during spraying, Cu deposits were made at series of impact velocities of 578 m s−1 to 745 m s−1 and 807 m s−1 to correlate particle impact condition with microstructure and properties of the deposits. Results show that as the average particle impact velocity increases from 578 m s-1 to 745 m s-1 and 807 m s-1, the deposition efficiency of feedstock powder increases from 58% to 84% and even to 95%. Although all three deposits reveal dense microstructure due to the high ductility of Cu, the deformation degree of the deposited particles remarkably increases with increasing impact velocity. The enhanced plastic deformation of the deposited particles leads to more dispersed oxide scale and thereby stronger inter-particle bonding with the strength of the deposit along the deposition direction increasing from 25.8 MPa to 148.5 MPa. The electrical and thermal conductivities at through-thickness direction of the deposit at particle impact velocity of 807 m s-1 are 78 % IACS, 295 W m−1 K−1, respectively. [ABSTRACT FROM AUTHOR]

In this study, friction stir processing (FSP) was employed to modify cold-sprayed (CSed) AA2024/Al 2 O 3 metal matrix composites (MMCs). Three different rotation speeds with a constant traverse speed were used for FSP. Microstructural analysis of the FSPed specimens reveals significant Al 2 O 3 particle refinement and improved particle distribution over the as-sprayed deposits. After FSP, a microstructural and mechanical gradient MMC through the thickness direction was obtained. Therefore, a hybrid technique combining these two solid-state processes, i.e. CS and FSP, was proposed to produce functionally gradient deposits. The Guinier-Preston-Bagaryatskii zone was dissolved during FSP, while the amounts at different rotation speeds were approximately the same, which is possibly due to the excellent thermal conductivity of the used Cu substrate. Mechanical property tests confirm that FSP can effectively improve the tensile performance and Vickers hardness of CSed AA2024/Al 2 O 3 MMCs. The properties can be further enhanced with a larger rotation speed with a maximum increase of 25.9% in ultimate tensile strength and 27.4% in elongation at 1500 rpm. Friction tests show that FSP decreases the wear resistance of CSed MMCs deposits due to the breakup of Al 2 O 3 particles. The average values and fluctuations of friction coefficients at different rotation speeds vary significantly. [ABSTRACT FROM AUTHOR]

In the present study, 304 stainless steel coatings were deposited on interstitial-free steel substrates by cold gas dynamic spray technology. The effect of gas temperature on microstructure, micro-hardness, cohesive strength, and electrochemical property of the coatings were investigated and compared. The results showed that increasing gas temperature had a great contribution to enhancing the bonding strength between the deposited particles and making the microstructure more density. Therefore, the porosity of the coatings decreased from 6%±0.5% to 2%±0.3%, and the tensile strength of the coatings increased from 56±4 MPa up to 73±3 MPa. In addition, the corrosion resistance of the coatings was also deeply influenced by process gas temperature. The corrosion kinetics of the coatings were affected by both of the plastic deformation of deposited particles and the porosity in the coatings. [Copyright &y& Elsevier]