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Since cold spraying (CS) technology was discovered, it has shown prominent potential in fabricating compact metallic coatings. However, the coating performance cannot meet the needs of the industry in some cases. For this, in this article, with taking copper as an example, the technology and performance characteristics of cold sprayed copper coatings at home and abroad in recent years were reviewed. The effects of four post - treatment processes on the microstructure and properties of cold sprayed copper coatings were discussed, and the applications of cold sprayed copper coatings in main fields were given. Finally, the problems and solutions still existing in the preparation of copper by cold spraying were analyzed and prospected.

Irregular (i.e. non-spherical) morphology powder is more cost-efficient to produce than spherical shaped powder. However, its reduced levels of flowability limit the wide application ranges of laser beam powder bed fusion (LPBF). To address this issue, a novel powder sheet additive manufacturing concept (MAPS) is proposed. Herein, a pre-manufactured metal particle-polymer binder composite (i.e. powder sheet) feedstock is employed as a raw material. The printability of irregularly shaped powder particles in sheet (MAPS) format was physically investigated using a range of different process parameters. The manufacturing process was observed by high-speed imaging. Microstructural and chemical element characterisations of the irregularly shaped powder particle print were then compared against those prints which were conducted using sheet-based (MAPS) spherical powder morphologies. The results indicated that the geometric accuracy and density of irregular powder morphology sheet printing improved when using a negative defocus strategy of the laser beam. A negative defocusing strategy allowed for the melting mode of the material to be transformed from keyhole to a more favourable conduction state. High speed imaging revealed that more spatter and vapour plume were observed with the increase in the magnitude of the negative defocus. The multi-morphology 304 L stainless steel (SS304) samples were printed in a single printer using an efficient method for the first time, i.e. printing spherical SS304 material on top of irregular SS304. EDX results indicated an insignificant change of chemical elements between the spherical and irregular prints. EBSD results revealed that columnar grains could grow through the irregular-spherical transition zone and similar grain size can form between spherical and irregular prints. The results of this study provide insights into the optimum printing configurations for powder sheet additive manufacturing using a cost-effective solution of irregular morphology material. [ABSTRACT FROM AUTHOR]

FeCoNiCr+x(TiC) (x: mass fraction; x = 5, 10, 15, respectively) composite high-entropy alloy (HEA) coatings were fabricated by the plasma cladding method, aiming to investigate the effects of different mass fractions of TiC ceramic particles on the phase composition, microstructure evolution, hardness, and wear resistance performance of HEA coatings. The results showed that the FeCoNiCr coating consisted of a single FCC phase, and FeCoNiCr-TiC coatings consisted of FCC, TiC, and carbide phases. The Vickers hardness of the FeCoNiCr-TiC coatings increased with additional TiC ceramic particles, and was raised from 150 to 365 HV when the added TiC powder mass fraction was 15 wt.%. The coating with the highest hardness had the lowest mass loss of 1.8 × 10-6 (mm3/N × m), which is only 11% of the FeCoNiCr coating. However, cracks appear on the surface after wear. The number of the wear particles was relatively reduced when the TiC content in the coating was 10% wt.%, while the surface did not show any microcracks. The wear mechanisms of the FeCoNiCr-TiC coating were adhesive wear and abrasive wear. [ABSTRACT FROM AUTHOR]

(CoCrFeNi)94Ti1.5Al4.5 high entropy alloy (HEA) coatings were prepared on Q235 steel substrate by plasma cladding and plasma spraying method to investigate the effect of minor addition of Ti, Al elements and different microstructure evolution on the corrosion resistance. After 18 h of annealing at 800 °C, the microstructure of the cladded HEA coatings presented typical dendrite structure, and the precipitated phases appeared in the interdendrite region. The corrosion current densities for the cladded CoCrFeNi coating, (CoCrFeNi)94Ti1.5Al4.5 coatings without and with annealing and plasma sprayed (CoCrFeNi)94Ti1.5Al4.5 HEA coatings were 0.040, 0.166, 0.016, and 1.276 μA/cm2, respectively, while the corrosion potentials were approximately the same. The corrosion resistance of the cladded coatings was better than that of the sprayed coating. With the addition of Ti and Al elements, the passive region was broadened. The corrosion resistance was improved after annealing, whereas galvanic corrosion was easily formed between precipitates and dendrite region, which was prone to pitting corrosion. [ABSTRACT FROM AUTHOR]

The microstructure and composition evolution of FeCoNiCrMn high entropy alloy during extreme deformation with a strain rate of 108–109 s−1 induced by cold spray technology was investigated. In the region experiencing extremely large strain and high strain rate deformation, ultrafine nanograins with an average size of smaller than 100 nm were formed due to the occurrence of dynamic recrystallization. Additionally, the rapid redistribution of segregated Mn and Ni elements in extremely deformed FeCoNiCrMn was found for the first time. The reason for this phenomenon is the increased grain boundary area and dislocation density caused by extreme plastic deformation. The redistribution of Mn and Ni elements was found in extremely deformed FeCoNiCrMn. This finding enriches our knowledge on the microstructure evolution mechanism of high entropy alloys under extreme deformation. [ABSTRACT FROM AUTHOR]

Ultrasonic surface mechanical attrition treatment (SMAT) was employed to modify the surface microstructural layer of SLM Ti6Al4V ELI biomedical material to improve the fatigue performance. The SMAT method can introduce a nanostructured layer in the SLM sample surface by imposing high-strain-rate plastic deformation. The nanostructured layer improves the mechanical strength of the SMAT-affected zone and induces compressive residual stress parallel with the surface which suppress the initiation of cracks. As a result, the specimen after SMAT exhibits significantly higher fatigue strength than the non-treated sample in both low- and high-cycling regime. This research demonstrates that surface mechanical attrition treatment (SMAT) is a promising post-processing technology for selective laser melting. SMAT improves the fatigue strength of selective laser melted Ti6Al4V by 100%. [ABSTRACT FROM AUTHOR]

In this paper, the microstructure and mechanical anisotropy of additively manufactured cold spray copper are investigated. Two nozzle scanning strategies (bidirectional and cross-hatching strategy) were employed to explore the optimum strategy for minimizing the anisotropy of cold sprayed deposits. The experimental results indicate that cold sprayed deposits showed anisotropy of microstructure and mechanical properties and that the anisotropic level can be affected by heat treatment and nozzle scanning strategy. For the microstructure, deposited copper particles exhibited an equiaxed shape through the XY plane but a lens-like shape in the XZ and YZ planes. After annealing, the microstructure anisotropy was minimised through recrystallization across the inter-particle boundaries. Both the bidirectional and cross-hatching scanning strategies appeared similar in regards to microstructure anisotropy which suggests that scanning strategies may not affect the microstructure anisotropy of the cold sprayed deposits. However, as for the mechanical properties, the scanning strategies significantly affected the mechanical anisotropy of the deposit. Mechanical anisotropy is much more prominent in the deposit produced with the bidirectional strategy when compared the deposit produced with the cross-hatching strategy. The main reason for the mechanical anisotropy is the existence of inter-track interfaces in a single-layer deposit which reduces the deposit cohesion strength (responsible for the difference between the X and Y directions) and different fracture modes during the tensile test (responsible for the difference between the XY plane and Z direction). In addition, the applied annealing treatment had no substantial effect on mechanical anisotropy. [ABSTRACT FROM AUTHOR]

A hybrid additive manufacturing technology for fabricating functionally graded materials (FGMs) is proposed in this paper. The new process represents a combination of two existing additive manufacturing processes, selective laser melting (SLM) and cold spraying (CS). The targeted experiment of Al and Al + Al 2 O 3 deposited onto SLM Ti6Al4V via CS reveals that the hybrid additive manufacturing process can produce thick, dense and machinable FGMs composed of non-weldable metals without intermetallic phase formation at the multi-materials interface. The SLM Ti6Al4V part exhibited fully acicular martensitic microstructure in contrast with α + β microstructure in the Ti6Al4V feedstock, while the grain structure of the CS Al part had no significant change as compared with the Al feedstock. Due to the phase transformation of the SLM part and work hardening of the CS part, the overall hardness of the FMGs was higher than that of the feedstock. [ABSTRACT FROM AUTHOR]

Experimental investigation was carried out to explore the formation mechanism of nickel-aluminum intertwining interface in cold spray, and to characterize the microstructure of deposited nickel particles at the intertwining interface. Shear stress was found to induce the intertwining interface through elongating and breaking of the nickel particles at the coating-substrate interface. The in-situ temperature measurement indicated that the temperature at the intertwining interface did not exceed the recrystallization temperature of nickel during the entire deposition process, suggesting that the nickel particles at the intertwining interface were in solid state rather than thermally softened viscous state. Electron channeling contrast (ECC) and electron backscatter diffraction (EBSD) imaging revealed a development of elongated subgrain (200 nm < D < 1 μm) and localized equiaxed ultrafine grain (D < 200 nm) microstructure within the highly deformed and fractured nickel particles at the intertwining interface. Such microstructures were induced by the dislocation accumulation due to the high strain/strain-rate plastic deformation and grain refinement caused by adiabatic temperature rise, respectively. Moreover, equiaxed ultrafine grains were also found to localize within a shear band near the center of the nickel particles, which experimentally confirms the existence of shear stress at the intertwining interface. [ABSTRACT FROM AUTHOR]

WC-Co metal matrix composite is frequently applied in the form of coating to prevent the underlying base materials from serious wear. Cold spray has been successfully used to produce WC-Co coatings in recent years, showing great potential. However, due to the lack of sufficient Co matrix phase for plastic deformation, the fabrication of cold sprayed WC-Co coating naturally requires expensive propulsive gas or very high working parameters, significantly increasing the manufacturing difficulty and cost. This paper aims to use conventional high pressure cold spray to fabricate WC-Co-Ni wear-resistance coatings under moderate working parameters, and to clarify the coating formation mechanism. To achieve this objective, mechanically mixed porous WC-17Co and dense Ni powders were selected as the feedstock with different WC mass fractions, F1 (41.5 wt%), F2 (64.5 wt%) and F3 (74.7 wt%). Working parameters were set at a moderate level (nitrogen, 3.0MPa and 350°C). Experimental results show that the WC reinforcements had no phase transformation and were completely retained in the WC-Co-Ni coatings as compared with in the feedstock. Fracture of the porous WC-Co particles during the deposition process was found to be the reason for such high WC retainability, dominating the coating formation mechanism. Tribological performance test shows that the coating hardness, toughness and wear-resistance performance improved as the WC content increased from the F1 to F3 coating. Wear mechanism analysis demonstrates that the F3 coating exhibited a completely different wear mechanism from the F1 and F2 coatings, thus the F3 coating had the best wear-resistance performance. [ABSTRACT FROM AUTHOR]

Diamond-reinforced metal matrix composites (DMMC) have great potential for wear-resistance applications due to the superior hardness of the diamond component. Cold spray as an emerging coating technique is able to fabricate coatings or bulk materials without exceeding the material melting point, thereby significantly lowering the risk of oxidation, phase transformation, and excessive thermal residual stress. In this paper, thick DMMC coatings were deposited onto aluminum alloy substrate via cold spray of three feedstock powders: copper-clad diamond and pure copper, and their mixtures. It was found that, due to its low processing temperature, cold spray is able to prevent graphitization of the diamond in the DMMC coatings. Further to that, the original diamond phase was almost completely retained in the DMMC coatings. In case of the coatings fabricated from copper-clad diamond powders only, its mass fraction reached 43 wt%, i.e. value higher than in any previous studies using conventional pre-mixed powders. Furthermore, it was found that the added copper content powders acted as a buffer, effectively preventing the fracture of the diamond particles in the coating. Finally, the wear test on the coatings showed that the cold sprayed DMMC coatings had excellent wear-resistance properties due to the diamond reinforcement. [ABSTRACT FROM AUTHOR]

Cold Gas Dynamic Spray or Supersonic Cold Spray, or simply 'Cold Spray', is an emerging technology for rapidly building thin films, thick coatings and large-scale additive manufacturing at relatively low temperatures. In a cold spray process, particles are accelerated to supersonic speeds by a propellant gas and impact a substrate, thus producing a strong bonding with the substrate and subsequently forming a deposit via layer-by-layer buildup. The scalability and low cost of this method make it promising for many applications in industry, such as metal component surface repair/enhancement/restoration and functional coatings for electrical, thermal, biomedical, energy storage, and nuclear plant applications. However, cold sprayed deposits usually require post process treatments to further modify their microstructures and mechanical properties in order to obtain the desired performances. A number of studies have been carried out on this topic. Here, recent progress in different post process treatments on cold sprayed deposits is reviewed, including heat treatment, friction-stir processing, shot peening, and laser re-melting. The effects of these post treatments on the microstructure, residual stress and mechanical properties of cold sprayed deposits are discussed. [ABSTRACT FROM AUTHOR]

The cold spray technique was employed in this study to produce pure nickel (Ni) and nickel-Inconel 718 powder deposits on duplex stainless steel substrates. High quality coatings were manufactured using nitrogen as the propellant gas. The coatings exhibited satisfactory Inconel 718 (In718) retention within the Ni matrix due to the highly ductile binder phase necessary for fabrication. The characterization of the coating microstructures was implemented by means of X-ray diffraction, electron microscopy, energy-dispersive X-ray spectroscopy, and microhardness testing. In addition, the erosion resistance of both coatings was evaluated by performing cavitation erosion tests, with the analysis of the eroded surfaces revealing different erosion mechanisms for each coating. The results demonstrated the efficiency of the cold spray technique for use in the production of metal-metal matrix composite coatings and the potential of In718 use in applications which demand enhanced cavitation erosion resistance. • Inconel 718 addition increased the porosity and hardness of the coating. • The cavitation erosion performance was significantly improved. • Inconel 718 offers less sites which may trigger material detachment. • Cavitation erosion revealed a fatigue nature which weakens the metallic bonding. [ABSTRACT FROM AUTHOR]

Fusion based metal additive manufacturing (AM) techniques such as selective laser melting (SLM) offer many advantages when compared to traditional manufacturing techniques, however, are often limited by the low build rates achievable, particularly for the manufacture of large components at scale. This may be alleviated by combining SLM with other more rapid AM processes such as cold spray (CS). Therefore, in this work, a hybrid AM process combining SLM and CS was utilised for the production of CS-SLM hybrid components. The proposed hybrid process allows the fabrication of complex geometries with SLM and the rapid manufacture of simple geometries using CS. The hybrid parts were manufactured by depositing a thick 316L stainless steel structure onto an SLM 316L stainless steel part via CS, followed by heat treatment to modify the microstructure and improve the CS-SLM adhesion. The microstructure, phase composition and mechanical properties of the as-fabricated and heat-treated part were studied using various materials characterisation methods. Based on the experimental results and analysis, it was found that the CS part had a grain structure similar to the feedstock in the as-fabricated state, while the SLM part was characterised by cellular subgrains confined in coarse grain structures. Due to the 'fusion' nature of the process, the SLM part delivered improved mechanical properties when compared to the CS part; however, this difference was reduced after heat treatment through the improvement in the tensile strength of the CS part by over 200% for both helium and nitrogen. Also, heat treatment improved the CS-SLM adhesive strength due to enhanced interface diffusion. Overall, this study demonstrates that the proposed hybrid AM is a promising technique for the manufacture of free-standing CS-SLM components. [ABSTRACT FROM AUTHOR]

Nickel-based Inconel 718 is a very good candidate for selective laser melting (SLM). During the SLM process, Inconel 718 develops a complex and heterogeneous microstructure. A deep understanding of the microstructural features of the as-built SLM material is essential for the design of a proper post-process heat treatment. In this study, the microstructure of as-built SLM Inconel 718 was investigated at different length scales using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Electron backscatter diffraction (EBSD) was also used to analyze the grain morphology and crystallographic texture. Grains elongated in the build direction and crossing several deposited layers were observed. The grains are not constrained by the laser tracks or by the melt pools, which indicates epitaxial growth controls the solidification. Each grain is composed of fine columnar dendrites that develop along one of their <100> axes oriented in the direction of the local thermal gradient. Consequently, prominent <100> crystallographic texture was observed and the dendrites tend to grow to the build direction or with occasional change of 90° at the edge of the melt pools. At the dendrite length scale, the microsegregation of the alloying elements, interdendritic precipitates, and dislocations was also detected. [ABSTRACT FROM AUTHOR]

Abstract This study presents an evaluation of the effects of pore diameter and porosity on the mechanical properties and biocompatibility of Ti6Al4V ELI periodic lattice structures, fabricated using SLM technology. Lattice structures of titanium alloys are in high demand for biomedical applications and are particularly useful as bone substitutes. A series of lattice structures with pore diameters of 500, 600, and 700 μm and porosities of 60% and 70% were designed by repeating an octahedral unit cell. Based on SEM and micro-CT observations, good morphological agreement was detected between the original designs and the SLM-produced structures. Microstructural analysis using TEM showed that the typically acicular α′ martensitic microstructure was obtained in the strut, which contributes to the brittle behavior of the lattice structure. Uniaxial compression tests were conducted, and the deformation behavior was recorded using a digital camera. Finite element analysis (FEA) of compression process was also conducted to enhance the understanding of the deformation mechanism. The surface chemistry of the lattice structure was analyzed using XPS methodology. The cytocompatibility of the lattice was also investigated with an in vitro test. The results show that the lattice structures with biocompatible surfaces have a comparable compressive strength (71–190 MPa) and elastic modulus (2.1–4.7 GPa) to trabecular bone. Highlights • Ti6Al4V ELI lattice structure was fabricated by selective laser melting. • As-fabricated lattice structure was examined by using X-ray computed tomography. • SLM Lattice structure shows mechanical properties comparable to trabecular bone. • Compression test of lattice structure was studied by finite element analysis. • An in vitro test shows good biocompatibility of Ti6Al4V lattice structure. [ABSTRACT FROM AUTHOR]

This study focuses on the successful fabrication of three distinct types of CoCrFeNi high entropy alloy (HEA) coatings through cold spray (CS) technology, with an emphasis on analyzing the impact of varying crucial CS parameters (spraying temperature and the range of powder particle size), on the coating's microstructure and tribological properties. Contrasted with conventional thermal spraying techniques, lower operational temperature in CS safeguards the materials from undergoing oxidation or phase transitions that are typically induced by high-temperature conditions. Additionally, the high-velocity impact of particles onto the substrate within CS process triggers plastic deformation, resulting in the creation of coatings that are characterized by heightened hardness, and greater density. Such coatings exhibit significantly enhanced performance and durability. The cocktail effect observed in CoCrFeNi HEA is reflected in a suite of exceptional properties that markedly surpass those exhibited by traditional alloys. Chiefly, this phenomenon is manifested through the alloy's exceptionally high hardness and dense structure, positioning CoCrFeNi HEA as a promising candidate for applications in high-wear scenarios. Experimental outcomes indicate that when smaller powder particles and higher spraying temperatures are employed, the porosity of CSed CoCrFeNi HEA coatings was observed to decrease by nearly an order of magnitude, concomitant with a 22.46% enhancement in microhardness. This improvement in microhardness translates into a significant reduction of over 72% in the wear rate, underscoring the positive correlation between enhanced microstructural integrity and wear resistance properties. By meticulously tuning spraying temperature and powder particle size, the resulting microstructure can be rendered increasingly dense and refined. [ABSTRACT FROM AUTHOR]

This work elaborated the microstructure and wear behavior of laser cladding (LC) FeCoCrNiAl 0.5 Ti 0.5 high-entropy alloys (HEAs) coatings on AISI 1045 steel substrates. The microstructure of the HEAs coatings is mainly comprised of a body-centered-cubic (BCC) + face-centered-cubic (FCC) dual-phase structure. Besides, the coating exhibites high hardness. During the friction process, the FCC phase was more prone to deformation and peeling than BCC structure. As the alloying elements (such as Al, Ti, and Cr) tend to form oxide film at high temperatures during friction, the friction process of the LC FeCoCrNiAl 0.5 Ti 0.5 coating was mainly controlled by oxidative wear and adhesive wear mechanisms. Friction test results showed that the coating owned excellent wear resistance and the wear rate of the HEAs coating was only 6.53 % of the wear rate of the steel substrate. • The tribological behavior of FCC and BCC phases was investigated. • Competitive relationship between the FCC + BCC dual phases in the wear mechanism of HEAs coating was investigated. • The multiscale relationship of elements oxidation, phase deformation and dislocation movement was studied. [ABSTRACT FROM AUTHOR]

This work elaborated the microstructure and corrosion behaviors of laser cladding (LC) FeCoCrNiAl 0.5 Ti 0.5 high-entropy alloys (HEAs) coating on AISI 1045 steel substrate. It revealed that the LC FeCoCrNiAl 0.5 Ti 0.5 HEAs coatings were composed of dendritic region (DR) and interdendritic region (IR). Furthermore, DR was mainly composed of BCC phase while IR was composed of FCC phase. There was also the transformation of BCC to L2 1 structure detected due to the addition of Ti element. Due to the uniform microstructure and addition of corrosion-resistant elements (such as Cr), the dual-phase HEAs coatings exhibited pretty good corrosion resistance. A dense passive film would form during the corrosion process to protect the inner structure. It was demonstrated that the corrosion rate of the coating was only 32.8% of that of the substrate. However, the pores formed during the LC process on the surface might be the initial site for pitting corrosion. • Formation of FCC+BCC dual-phase structure observed in LC FeCoCrNiAl 0.5 Ti 0.5 HEAs coating was investigated. • Passivation process of LC FeCoCrNiAl 0.5 Ti 0.5 HEAs coating during electrochemical tests was illustrated. • Pitting corrosion mechanism of LC FeCoCrNiAl 0.5 Ti 0.5 HEAs coating was studied. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of cold sprayed aluminum coatings on copper substrates were studied in this paper. The impact and deposition mechanisms of single and multiple particles during cold spraying were investigated using finite element modeling. The results of single particle model show that the maximum equivalent plastic strain is located at the contact surface between particle and substrate, decreasing from the jet site to the center of particles, and the equivalent plastic strain at the top of particles is almost zero. The results of multi-particle model show that the regions with large equivalent plastic strain include the contact surface between particle and substrate and the region between particles. The experimental results show that particle deposition has a significant impact on compaction during the spraying process. When the spraying distance is set to a fixed value of 30 mm, and the gas pressure is 2.2 MPa, 2.6 MPa, 3.0 MPa and 3.4 MPa, the coating porosity is always stable at about 1 %. The bonding strength of coating increases with the increase of gas pressure. When the gas pressure increases from 2.2 MPa to 3.4 MPa, the coating bonding strength has increased by 142.0 %, and the maximum value of bonding strength between the coating and the substrate is 16.9 MPa. When the gas pressure is 3.4 MPa with the gas preheating temperature of 300 °C, and the spraying distance is 20 mm, 30 mm and 40 mm, the coating porosity is 1.74 %, 0.95 % and 1.21 %, respectively. The maximum value of bonding strength between the coating and the substrate is 21.3 MPa under the spraying distance of 30 mm. • The maximum plastic strain of single particle model is located at the interface between particle and substrate. • The plastic strain of the multi-particle model is concentrated in the interface between particle and substrate. • The effect of particle deposition on compaction is very significant in the process of spraying. • The bonding strength of coating increases with the increase of gas pressure. [ABSTRACT FROM AUTHOR]

In this work, equimolar FeCoNiCrMn high-entropy alloy (HEA) was fabricated by solid-state cold spray additive manufacturing (CSAM) technique and post-annealed. The microstructure evolution and mechanical properties of the deposits before and after annealing were systematically evaluated. The tensile test results revealed that the deposit annealed at 1000 °C for 2 h exhibited an excellent combination of tensile strength and ductility, with a doubled ultimate tensile strength of 455 MPa compared with the as-sprayed and significantly improved elongation (28%). The improved strength can be attributed to the healing of micropores and interparticle interfaces in the deposit due to the interface diffusion induced by high temperature. The recovery of ductility is the result of a significantly decreased dislocation density and enhanced metallurgical bonding at interfacial regions. For the first time, the current work demonstrated that the combination of CSAM and post-annealing strategy could achieve the strength-ductility synergy of HEA deposits. The results of in-situ analysis show that the mechanism of tensile deformation process includes particle separation, and it can be inferred that the preparation of dense sediments combined with post-treatment is the direction of further optimization. • Combination of CSAM and post-annealing strategy could achieve the strength-ductility synergy of HEA deposits. • The microstructure evolution and mechanical properties of the deposits were systematically evaluated. • The deposit annealed at 1000 °C for 2 hours has the best mechanical property of 455 MPa and 28 %. • The improved strength can be attributed to the healing of micropores and interparticle interfaces. [ABSTRACT FROM AUTHOR]

Dual or multi-phase high-entropy alloys (HEAs) with nano-precipitates are of great importance to overcome the strength-ductility tradeoff of structural materials. In this work, Al 0.5 CrFeNi 2.5 Si 0.25 HEAs were designed and fabricated using vacuum induction melting. The microstructure, phase evolution and compressive behavior of Al 0.5 CrFeNi 2.5 Si 0.25 HEAs before and after annealing at various temperatures (750 °C, 850 °C, 1050 °C, and 1200 °C) were investigated. The as-casted and annealed Al 0.5 CrFeNi 2.5 Si 0.25 HEAs constitute a dual-phase (FCC and BCC) composition with L1 2 and σ phases. With increasing annealing temperature (750 °C–1200 °C), the proportion of the BCC matrix containing Cr-rich nanoparticles and strip-shaped σ phases gradually increased. The Cr-rich nanoparticles were gradually refined. Partial σ phase dissolved after heat treatment at 1050 °C and 1200 °C. The variation of the precipitated phases at different temperatures led to the occurrence of strengthening, which makes the alloys overcome the tradeoff between compressive strength and ductility. Under the 750 °C–1200 °C annealing, the compressive strength increased from 2288 MPa to 2584 MPa, accompanied by an increase in fracture strain from 14.9% to 23.4%. [Display omitted] • As the annealing temperature increases, Si separation promotes an increase in the proportion of the DR region. • The variations in microstructure and phase proportion leading to precipitation strengthening and fine grain strengthening. • The alloys improved the compressive strength and plasticity simultaneously by heat treatment. [ABSTRACT FROM AUTHOR]

In this work, equiatomic CoCrFeNi high-entropy alloy (HEA) was fabricated by solid-state cold spray additive manufacturing technology and then post-spray annealed at the temperature range of 500–1000 °C for 2 h. By adjusting the annealing temperature, four types of deposits (i.e., as-sprayed, recovered (500 °C), partially recrystallized (700 °C), and fully recrystallized (1000 °C) deposits) were obtained, and their microstructure, compressive and tensile properties were systematically explored. The as-sprayed deposit exhibited high compressive yield strength due to the dislocation strengthening and grain boundary strengthening effects but fractured within the elastic deformation regime in the tensile test. Such significant tension-compression asymmetry can be attributed to the difference in the sensitivity of the deposit to interior defects (i.e., pores and particle boundaries) under tensile and compressive loads. Only recover annealing hardly influenced the microstructure and mechanical properties of the deposits. While recrystallization annealing could trigger enhanced interface diffusion and the resultant metallurgical bonding, as evidenced by the improved deposit density and less visible interparticle interfaces. The partially recrystallized and fully recrystallized deposits exhibited an excellent combination of compressive strength and ductility. While the fully recrystallized deposit exhibited almost equal tensile and compressive yield strength and the best recovery of tensile ductility, indicating the weakened tension-compression asymmetry. [Display omitted] • Bulk CoCrFeNi high-entropy alloy was fabricated by cold spraying and post-spray annealing. • The tensile and compressive properties of cold-sprayed high-entropy alloy deposits were first reported. • The inherent defects led to significantly different mechanical performances under tensile and compressive loads. • Recrystallization annealing can achieve an excellent combination of compressive strength and ductility. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effect of specific laser parameters under the same energy density is studied. • The L-PBF NiTi sample in this work has excellent superelasticity. • The influence of parameters on thermal history pool was analyzed by FEA. • The crystallographic texture and nanoprecipitates are the determining factors. • The results of this work can guide the performance control of NiTi alloy. As the key parameter in the laser powder bed fusion (L-PBF), the laser energy density can directly determine the comprehensive quality of NiTi shape memory alloys (SMAs). However, the specific role of laser parameters under the energy density value is rarely discussed. In this work, the effect of specific laser power and scanning speed variation on the properties of NiTi alloy under the same energy density (62.5 J/mm3) was studied. Results show that the phase transformation temperatures decrease as the Ni content was increasing with the simultaneous increases in laser power and scanning speed. Under the influence of Ni content, the SMA samples with low phase transformation temperatures were fully austenite at room temperature. In the cyclic compression test at A f + 15 °C, the sample built under the lowest laser power and scanning speed showed the best superelasticity effect. Higher scanning speed resulted in a stronger <1 0 0>//BD texture, and the increase of laser power produced a deeper molten pool, which remelted the solidified part at the bottom and further enhanced the <1 0 0>//BD texture. The TEM characterization shows that more Ni 4 Ti 3 precipitates with larger sizes are obtained in the sample with the lowest laser power and scanning speed. The finite element analysis shows the lowest peak temperature in the molten pool under the lowest laser power and scanning speed, which can promote the precipitation of Ni 4 Ti 3 particles. The promoted formation of Ni 4 Ti 3 precipitates enhanced the austenite stability and further enhanced the superelasticity of NiTi alloy fabricated at the lowest laser power. The present work shows that the specific laser parameter under the constant energy density value can further improve the comprehensive performance of L-PBF NiTi SMAs. [ABSTRACT FROM AUTHOR]

The complex thermal history during the laser direct energy deposition (L-DED) can result in the inhomogeneous microstructure of the Ti-47Al-2Cr-2Nb (TiAl) alloy. This work investigates the grain structure and metastable microstructure evolution of L-DED TiAl samples with different deposit shapes of single-track, thin-wall, and bulk. Numerical simulations were performed for all samples to reveal the influence of thermal history on microstructural evolution. Among the three samples, the single-track sample has unique dendritic and cellular structures due to the highest cooling rate. Meanwhile, incompletely precipitated lamellar γ was found at the boundary and inside the α 2 grains. And residual B2 phase is also present in the single-track sample. As for the thin-wall and bulk samples, they exhibit fully lamellar structure, and contain only γ and α 2 phases at room temperature. During the L-DED process, the temperature gradient and cooling rate inside both samples decreased with the elevation of the deposition height. However, due to the different thermal histories, the temperature gradient and cooling rate do not change in the same way. This leads to a large difference in grain structure and metastable microstructure between the thin-wall and bulk samples. Also, these changes affect the mechanical properties of TiAl alloys. In summary, this study can provide potential guidance for a tailored microstructure in L-DED TiAl alloy. • Microstructure evolution of L-DED TiAl alloy was studied in three deposit shapes. • Different metastable microstructures were found at different building heights. • Grains in heat affected zone are influenced by cooling rate and thermal cycles. • The thermal history caused by the deposit shapes can determine the phase content. • The changes in microstructure of L-DED TiAl alloy are also reflected in hardness. [ABSTRACT FROM AUTHOR]

Selective laser melting (SLM) is one of the current rapid fabrication technology methods which has wide potential application in the aerospace, medical, consumer products and automotive industries. Currently, ceramic materials are not used as widely as metal and polymer materials due to the high melting point, high-temperature strength and low thermal conductivity, which influence the microstructure and density of ceramic samples during SLM fabrication. The most effective method of reducing cracks is the preheating at high temperature of the ceramic powder during SLM process. This paper presents the selective melting of yttria-stabilized zirconia (ZrO 2 –Y 2 O 3 93–7) ceramic using a 1 μm wavelength fibre laser with high-temperature preheating at 1500–2500 °C, and an additional CHEVAL Nd-YAG laser for the preheating of the powder bed before scanning. In this paper, the influence of different laser powers and different scanning velocities on the microstructure, relative density and deformation of the ceramic sample is investigated; in particular, the effect of preheating on the morphology of the micro-cracks is discussed. Experimental results show that high-temperature preheating in 10 mm diameter range is possible with the Nd-YAG laser, and that orderly cracks are transformed into disordered little cracks by the high-temperature preheating. With preheating to 1500 °C, 2000 °C and 2500 °C, the relative density of the sample made by mixing fine powder (9–22.5 μm, 20 wt%) and coarse powder (22.5–45 μm, 80 wt%) is increased by 84% (without preheating) to 90–91%. The transformation of the monoclinic and cubic structures to a tetragonal structure is observed during the process of melting and cooling, and increasing the preheating temperature to 1500 °C, 2000 °C and 2500 °C is more suited to the formation of tetragonal crystals. [ABSTRACT FROM AUTHOR]

• The effect of preheating on the tensile property of deposits was firstly reported. • The tensile strength of Fe deposits prepared with preheated powder nearly doubled. • In-process powder preheating can effectively strengthen cold sprayed deposits. Cold sprayed Fe deposits were manufactured with non-preheated and preheated powders, respectively. The microstructure of the deposits was characterized by electron backscattered diffraction. The tensile property of the deposits was then evaluated. The results show that the preheated Fe powder deforms more extensively than the non-preheated powder during cold spraying due to their thermal softening. The ultimate tensile strength of the deposits made with preheated powder approximately doubles to 109.42 ± 50.56 MPa due to the enhanced intersplat bonding. However, the deposits in both cases exhibit poor ductility. The increased dislocation density and the nano-sized oxides on the preheated powder surface are considered to be the cause of low ductility of the deposits fabricated with preheated powder. [ABSTRACT FROM AUTHOR]

It is known that the reactive nitrogen atmosphere can effectively modulate the microstructure of laser powder bed fusion (L-PBF) of Ti6Al4V alloys. In the present work, high nitrogen contents of 15 and 25 vol% were applied in argon atmospheres to investigate the influence on microstructure and mechanical performances. The enhanced nitrogen solid solution was achieved with nitrogen content ranging 374 ppm and 456 ppm (parts per million) in the Ti6Al4V alloys, respectively. Besides, the EBSD (electron back-scattered diffraction) characterization shows the significant refinement of acicular martensite α′ and the EBSD phase reconstruction shows the interrupting epitaxial growth of the β phase with the increasing nitrogen content in the atmosphere. The KAM (kernel average misorientation) diagrams with an increasingly higher value indicate an increased micro-strain and dislocation density at higher nitrogen content in the atmosphere. Meanwhile, the high magnification TEM further confirms the martensite α' refinement and also the formation of β lamella at 25 vol% nitrogen content in L-PBF atmosphere. Such microstructure evolution can be mainly attributed to the more severe thermal cycling with a higher thermal gradient and cooling rate under the atmosphere with higher nitrogen content. By applying the 25 vol% nitrogen in the atmosphere, the L-PBF Ti6Al4V alloys exhibit a significant increase in maximum compressive strength of ∼1885 MPa and maximum compressive strain of ∼10%, respectively. The underlying strengthening mechanism can be attributed to the refinement of martensite structure, solid solution, and higher dislocation density. The present work shows that the atmosphere can effectively modulate the microstructure and improve the mechanical property of L-PBF alloys. [ABSTRACT FROM AUTHOR]

High entropy alloys (HEAs) are novel materials that have been extensively studied in recent years. In this work, Al 2 O 3 particles reinforced FeCoNiCrMn HEA composite coatings were fabricated by cold spray. The microstructure, mechanical and tribological properties of the composites coating were studied and compared with those of the pure FeCoNiCrMn coating. The results indicate that cold spray is a promising process to fabricate HEA composite coatings. The composite coatings made in this work had higher hardness than pure FeCoNiCrMn coating due to the reinforcing effect of well distributed Al 2 O 3 particles. The composite coatings also had improved wear-resistance properties with nearly 50% reduction in wear rate as compared to the pure FeCoNiCrMn coating. The improvement was due to the formation of tribo-layer which can effectively withstand material loss. The results also reveal that the main wear mechanisms for the composite coatings were dominated by adhesive wear in comparison to abrasive wear for the pure FeCoNiCrMn coating. This study proves the feasibility of cold spray for the fabrication of high-performance HEA composite coatings. • Cold spray is capable of depositing Al 2 O 3 reinforced FeCoNiCrMn HEA composite coatings. • Composite coatings had higher hardness than pure FeCoNiCrMn coating. • Composite coatings had higher wear-resistance properties than pure FeCoNiCrMn coating. [ABSTRACT FROM AUTHOR]

NiCrBSi alloys have a huge potential for applications in the field of various industries due to their high hardness and excellent wear resistance performance. In this experiment, the wear behaviors of the plasma-sprayed Ni60 and 5 wt % SiO 2 –Ni60 composite coatings under dry and water-lubrication sliding conditions were comparatively investigated utilizing a ball-on-disk tribometer at room temperature. Furthermore, the phase compositions and worn tracks of the two as-sprayed coatings before and after the friction test were inspected through X-ray diffraction and scanning electron microscopy to demonstrate the main wear mechanisms of both coatings. And the results manifested that the incorporating of the SiO 2 nanoparticles into Ni60 alloy played a critical effect on improving the wear-resistance performance of the 5 wt % SiO 2 –Ni60 composite coating, especially under the water-lubrication sliding condition. The reason was found to be the filling and polishing effect of nanoparticles, together with the cooling and water lubrication effect of deionized water, in the course of friction test. Moreover, the model diagram of the wear behaviors of the 5 wt % SiO 2 –Ni60 composite coating was also established to comprehensively analyze the stress of nanoparticles during friction. • The nanoparticles greatly improve wear behaviors of the 5 wt % SiO 2 –Ni60 coating. • The addition of nanoparticles into alloys offers the filling and polishing effect. • The model diagram of wear behaviors of 5 wt % SiO 2 –Ni60 coating was established. • The stresses of SiO 2 nanoparticles in the process of friction were also analyzed. [ABSTRACT FROM AUTHOR]

In this paper, Ti6Al4V octahedron lattice structures with top cap and bottom base were fabricated through selective laser melting (SLM). The structural integrity, microstructural evolution, compressive properties and failure mechanism of as-fabricated (AF) and vacuum annealing treated (VAT) samples were studied through both experiments and numerical modeling. The X-ray computed tomography analysis revealed that vacuum annealing had an insignificant effect on internal pore elimination and porosity reduction. For both the AF and VAT samples, larger pores exhibited more irregular shape than smaller pores. The microstructural analysis suggested that vacuum annealing was able to transform acicular α/α' martensites into uniformly distributed lamellar α + β phases and also to cause the formation of nano-particle precipitation and dislocation. The compressive test indicted that the lattice structure with confined top and bottom had much better compressive properties than those without. Also, vacuum annealing significantly improved the compressive strength by 26% due to the synergistic effect of residual stress relief, nano-particle precipitation and dislocation strengthening. The ductility of the VAT sample was also improved as compared to the AF sample, which was mainly attributed to the formation of α + β phases. Unlabelled Image • Ti6Al4V octahedron lattices were fabricated by SLM for aviation and aerospace applications. • Microstructural evolution during both SLM and VAT processes were analyzed in detail. • Confined compression deformation behaviors were investigated through experimental and numerical approaches. • The lattice structure with confined top and bottom had much better compressive properties than those without. • Vacuum annealing significantly enhanced the compressive strength by 26% and improved the ductility. [ABSTRACT FROM AUTHOR]

In this study, a new precipitation hardening stainless steel (PHSS), C-X stainless steel, was manufactured using selective laser melting (SLM) technology. Following SLM fabrication, a series of heat treatments were applied to improve the mechanical properties of the as-built samples. The microstructure precipitates distribution and evolution, and mechanical properties of SLM C-X stainless steels in the as-built and heat-treated conditions were systematically studied using scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscope (TEM). The XRD spectrum revealed that solution treatment resulted in the formation of a complete martensite phase, and a reverted austenite (γ') phase formed after aging treatment. The TEM analysis indicated that numerous dislocations and nanoprecipitates were dispersed within the martensite matrix for both the as-built and aged samples. The rod-like NiAl precipitates with a size range of 3–25 nm for the as-built samples and 7–30 nm for the solution-aged samples were determined through high-resolution TEM (HRTEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDS). Furthermore, the microhardness of the SLM C-X stainless steel parts was found to significantly improve from 350 HV 0.2 in the as-built state to 510 HV 0.2 in the solution-aged state. The ultimate tensile strength (UTS) of the SLM C-X stainless steel parts also increased from 1043 MPa in the as-built state to 1601 MPa after solution-aging heat treatment. Image 1 [ABSTRACT FROM AUTHOR]