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• HIP treatment improved the compressive properties of SLM lattice structure. • The improvement was due to reduced porosity and microstructure change by HIP. • HIP treatment is an effective method for post-processing SLM lattice structure. In this study, the compressive properties of selective laser melted (SLM) Ti6Al4V lattice structure were studied. Based on results and discussion, it was found that hot isostatic pressing (HIP) treatment significantly influenced the microstructure and compressive properties of the SLM Ti6Al4V lattice structure. HIP resulted in a transformation of acicular α′ martensite in the AF sample into a mixture of α and β lathes and a simultaneous reduction of internal porosity from 5.9% in the AF state to 0.5% after HIP treatment. As a consequence, the HIP samples exhibited significantly improved compressive strength and fracture strain as compared to the AF sample. [ABSTRACT FROM AUTHOR]

The study investigated the performance of CoCrFeMnNi high entropy alloy and tungsten carbide–nickel coatings under alumina particle impingement. The coatings were applied to 2205 duplex stainless steel substrates by cold spray using nitrogen. For the composite coatings, the amount of WC nanoparticles entrapped in the Ni matrix was estimated as 42.5 ± 8 vol.%. A cold-sprayed interlayer of 316 stainless steel promoted CoCrFeMnNi HEA deposition. Post-process annealing was performed on the coatings. After 3 hours of holding time, x-ray diffraction examination revealed no tungsten decarburization in the WC/Ni-Ni coating. Annealing peak temperatures for the WC/Ni–Ni and high entropy alloy coatings were 600 and 550 °C, respectively. The effect of SS316 annealing on subsequent HEA deposition and coating's erosion behavior was also investigated. Microhardness tests showed the HEA coating outperforming the WC/Ni-Ni. The coatings were subjected to impact erosion using alumina particles at three different approach angles: 30°, 60°, and 90°. To compare the different materials, average erosion values were calculated expressed in volume loss per grams of erodent. The as-sprayed WC/Ni–Ni was the most effective against 60° impingement angle, while HEA coatings demonstrated promising under impacts at 30° and 90°. Overall, the 30° impact orientation was identified as the most critical condition. Eroded surfaces were examined using optical microscopy (OM) and scanning electron microscopy (SEM). [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]

Lack of fusion (LOF) defects impact adversely on the mechanical properties of additively manufactured components produced via laser-based powder bed fusion. Following a stress-relieving heat treatment, the tensile properties and hardness of Ti6Al4V components were found to be negatively impacted by the presence of LOF defects. This work considers a geometrical-based inequality for the prediction of LOF defects. We critically evaluate an LOF criterion using both the experimentally and analytically obtained melt pool geometries. Experimentally, we determined melt pool dimensions by analysing a single-layer, multi-track deposition with oversized hatch spacing in order to establish depth and width from non-overlapping melt pools. Analytically, Rosenthal-based predictions of melt pool size (width and depth) are applied. To investigate LOF defects, we used hatch spacing as the main parameter variation to investigate defects while keeping all other controllable parameters unchanged. An original LOF criterion from the literature was found to be an adequate predictor of LOF defects when experimentally obtained melt pool geometry was used. Critically, however, the analytical expressions for melt pool geometry were found to be in error and this caused the LOF criterion to fail in predicting LOF defects in all cases where defects were observed experimentally. However, an adaptation to the LOF prediction criterion is proposed whereby it is recommended that a correction factor R c 2 = 0.7 (or R c = 0.83 ) is used with the analytically derived melt pool geometry. Furthermore, this correction is extended into the laser power versus scanning speed operating space to give minimum (corrected) line energy for LOF avoidance in Ti6Al4V components. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microstructure and mechanical performance of steel samples printed using a novel additive manufacturing approach (MAPS) are investigated. • Dense and defect-free samples for tensile tests are successfully MAPS manufactured. • MAPS manufactured samples have a higher mechanical strength than the laser beam powder bed fusion counterparts due to in-process alloying. • The relationship between microstructure and mechanical property in MAPS is explored. A novel approach of Metal Additive Manufacturing using Powder Sheet (MAPS) is developed and demonstrated. A flexible metal particle-polymeric binder composite sheet is employed as innovative feedstock in this new method and this approach offers enhanced health and safety. MAPS successfully printed dense samples (99.99%). The chemical composition of the printed alloy resulted enriched in carbon compared to the feedstock powder due to the C pick-up during MAPS, leading to distinct microstructures and enhanced mechanical properties compared to those of laser beam powder bed fusion (PBF-LB/M) benchmark samples. In particular, the microstructural examinations of the MAPS samples show a coarser carbide network stable even after thermal treatment at high temperatures, while the tensile tests revealed that MAPS samples have a higher mechanical strength than the PBF-LB/M counterparts but possess lower ductility. The modification in the chemical composition indicates a strong potential for in-process alloying through MAPS. The demonstrated MAPS approach offers a novel avenue for manufacturing functional metal components with bespoke compositions and resulting properties. [ABSTRACT FROM AUTHOR]

In this study, it is revealed for the first time that ultra-high-speed impact can trigger in-situ dehydrogenation at the impact interface of titanium hydride (TiH). This phenomenon leads to a phase transformation from brittle TiH to ductile Ti, causing a shift in the ductile-to-brittle transition from the impact interface to the particle interior. Inspired by this unique behavior, and considering the mechanical interactions between particles during the spray process, TiH powders were used as feedstock to successfully produce a large-scale random cellular structure through cold spray. A systematic investigation of the deposition process revealed that unbroken TiH particles interact with dehydrogenated ones, resulting in only the ductile portion of the TiH particle being deposited, while the brittle interior is spalled off, consequently forming the "walls" of the cellular structures. This work highlights the potential of TiH powders to advance cold spray technology, particularly in the creation of complex cellular structures. [Display omitted] [ABSTRACT FROM AUTHOR]

In this work, cold spray was used to produce Ta-xAg metal matrix composites (MMCs). The addition of Ag prevents Ta matrix from corrosion and simultaneously induces excellent antibacterial ability. Additionally, varied soft Ag volumes in feedstock powder led to a synergistic deposition phenomenon, profoundly affecting Ag dissolution and correlated antibacterial performance via a microstructural mechanism. Taking advantage of it, a robust antibacterial ability can be facilitated by minimal Ag usage. • Corrosion resistance and antibacterial Ta-Ag composites were fabricated by cold spray. •.The synergistic deposition process affects Ta-Ag composite dissolution mechanisms. • Enhanced Ta corrosion resistance and antibacterial properties achieved by adding Ag. [ABSTRACT FROM AUTHOR]

Abstract: Stellite 6® powders were deposited on carbon steel using Supersonic Laser Deposition. The microstructure and performance of the coatings were examined using SEM, optical microscopy, EDS, XRD, microhardness testing and pin-on-disc wear testing. The results showed that the microstructure and wear behaviour of the most successful SLD deposition conditions with N2 at a pressure of 30bar, a temperature of 450°C and a deposition power of 1.5kW were compared with that of optimised laser cladding. [Copyright &y& Elsevier]

Selective laser melting (SLM) of multi-material structures (MMS) is of significance because it allows for bespoke structural innovation and high-accuracy process tailoring. However, most of the currently developed loose powder-based SLM techniques for MMS are limited by the long changeover time and potential cross-contamination between materials. To address these issues, a novel Metal Additive Manufacturing using Powder Sheets (MAPS) technique is proposed for printing MMS within a single process. It utilizes flexible powder sheets as the feedstock material, which are composed of metal powder-polymer binder composites. The printability of MMS by MAPS is assessed through the fabrication of three-phase SS304-IN718-SS304 composites with increased geometric dimensions on the SS316 baseplates. The effects of part size on the evolution of the melt-pool morphology and the formation of defects during MAPS are investigated by experimental characterizations and computational modeling. The results show that when fabricating larger MMS, the use of a longer scan-vector easily leads to defects such as lack-of-fusion porosity, balling and cracks. This is due to the longer duration of inter-hatch cooling time, the reduced amount of thermal accumulation and the higher degree of residual stresses. By adopting an island scanning strategy, a defect-free large-size MMS with variations of chemical composition, microstructure and microhardness is successfully printed by MAPS. The proposed MAPS method offers a new solution for producing high-quality MMS. [ABSTRACT FROM AUTHOR]

The increasing trend of high strength low alloy steel application in the shipbuilding industry dictates the necessity for an investigation of their welds with conventional high strength steels. In this study, conventional and high strength low alloy naval steel plates 12 mm thick, were joined together in a butt joint configuration, using the metal cored arc welding technique and low heat input (1.0 kJ/mm). The weld was examined in terms of microhardness, microstructure, and tensile performance. The effect of multiple thermal cycles on the formation of primary and secondary microstructures was also discussed. The results revealed a smooth distribution of the microhardness across the different weld regions. Inside the heat affected zones (HAZs) and the weld metal various constituents were revealed, with ferrite in various forms (lath-like, acicular, semi-equiaxed, polygonal) appearing as the predominant phase, alongside martensite and austenite grains, and pearlite (lamellar and degenerated). The tensile testing indicated the base metal of the conventional high strength steel to be the weakest link of the weld, which was mainly attributed to its ferritic-pearlitic microstructure and the coarse grain size. The overall tensile performance of the weld remained at high levels, with a simultaneous reduction in the elongation after fracture in comparison to the base metals. [ABSTRACT FROM AUTHOR]

• Flattening ratio of a particular splat can be obtained by image analysis software. • The formula for calculating the overall fattening ratio is proposed. • The higher the FR, the better the wear performance, only applies to similar composite coatings. Cold spray is one of the promising coating deposition techniques for fabricating anti-wear composite coatings. These composite coatings contain an even distribution of the two phases, they are Ti-6Al-4V and Al splats if referring to the current study. With the help of image analysis software (Fiji), the flattening ratio (FR) of Al splats was found to be larger than that of Ti-6Al-4V in different composite coatings. The overall FR is more comprehensively correlated to fracture toughness and wear performance for the entire composite structure, rather than comparing the particular FR of Ti-6Al-4V or Al splats only. The higher the FR of the composite coating, the better its wear performance, but this phenomenon only applies to the same type of composite coating, because different composite coatings respond differently to wear. [ABSTRACT FROM AUTHOR]

• Dynamic cell – cold sprayed coating interaction were visualized by focused ion beam-scanning electron microscopy. • Apparent cellular sensing, adhesion, migration, and mineral dissolution after 3 h incubation of the cells were observed. • In vitro cell fusion and mineralization with the formation of an intermediate layer were evidenced. Cold spraying has been proved it is a promising method for biomaterials surface modification and additive manufacturing. Fundamental understanding of the cell-biomaterial interaction is essential for design and fabrication of biomaterials for their biomedical applications. Here we report the evolutional characteristics of the interfaces between osteoblast cells and cold sprayed hydroxyapatite-titanium composite coating. The in situ observation of the interfaces was carried out by focused ion beam-scanning electron microscopy, and influence of the surface characteristics of the coatings on the behaviors of protein adsorption and cell proliferation was examined. Apparent cellular sensing, adhesion, migration, and mineral dissolution after 3 h incubation of the cells were visualized. The predominant existence of fragment particles in the coatings, one of the exceptional features of cold sprayed coating, plays vital role in regulating the cell association and cellular uptake. The presence of Ti and HA fragments at the cell-coating interface had no significant effect on cell viability and cell proliferation. After 5 days culturing, in vitro cell fusion and mineralization with the formation of an intermediate layer were evidenced on the coating, suggesting rapid regeneration and accelerated establishment of bone-implant integration. These critical phenomena occurred at different periods of culturing would facilitate understanding the influence of specific surface features of biomaterials on cell responses, and shed light on the development of manufacturing techniques for biomaterials. [ABSTRACT FROM AUTHOR]

The cohesion mechanism of cold sprayed metal matrix composite coating reinforced with core-shell-structured particles was studied. Al and Cu-Ni-coated diamond were used as the matrix and the reinforcement, respectively. The two components were found metallurgically bonded as evidenced by a formation of CuAl 2 layer at the interface. Owing to this enhanced matrix-reinforcement bonding, the core-shelled diamond particles delivered superior deposition (i.e., higher diamond deposition efficiency, higher diamond content, better fracture resistance) than conventional uncoated diamond particles. Despite the metallurgical bonding, the coating cohesion strength was not improved, with the limiting factor likely being insufficient cohesion of the core-shelled diamonds. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We report the fatigue striation spacing of commercially pure titanium with respect to stress amplitude. Rotating bending fatigue tests were performed at 150, 175, 200 and 227 MPa in which samples did not fail at 150 MPa. No fatigue striation up to a certain crack length is observed for the studied conditions. For each condition, a minimum striation spacing value is found that decreases from 0.45 to 0.36 to 0.24 μm with increasing stress from 175 to 200 to 227 MPa, respectively. The minimum striation spacing remains constant up to a certain crack length for each condition. Thereafter, it starts increasing with crack length. It is quite understandable that the constant striation spacing over a certain crack length may not represent the macroscopic fatigue crack growth rate. The fatigue crack growth mechanism during the constant striation spacing region has been discussed in detail. [ABSTRACT FROM AUTHOR]

Cold spray is a solid-state metal deposition and additive manufacturing (AM) technology. The low processing temperatures in cold spraying result in less favorable properties of the deposits as compared to their fusion-based AM counterparts, particularly for materials with high strength such as 316L stainless steel. Therefore, annealing is generally required for the cold sprayed deposits to improve their mechanical properties. In this paper, three different annealing strategies are systematically investigated in order to optimize the microstructure and mechanical properties of cold sprayed 316L stainless steel deposits: air annealing, vacuum annealing, and hot isostatic pressing (HIP), the latter representing a method widely used for densification of components in fusion-based metal AM. The results indicate that the three annealing treatments improve the mechanical properties of the cold sprayed 316L stainless steel deposits through grain recrystallization and diffusion at oxide-free inter-particle interfaces. Such improvement is less pronounced for the air annealing as a formation of oxide inclusions impedes full inter-particle metallurgical bonding. This effect is suppressed in the vacuum annealing, resulting in a significant improvement in the tensile strength and ductility. Despite the significant improvement in the deposits' density, the HIP results in mechanical properties equivalent to those after vacuum annealing. The experimental results suggest that the strengthening of cold sprayed deposits is mainly dominated by the improved inter-particle bonding and particle grain structure rather than through a reduction of porosity. • Hot isostatic pressing (HIP) significantly reduced the porosity of cold sprayed 316L coating. • Vacuum annealing resulted in better mechanical properties than air annealing. • Air annealing led to oxidation at the inter-particle interfaces. • HIP resulted in equivalent mechanical properties as vacuum annealing. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Cold spray can be used to fabricate highly reflective Al coatings. • Increasing the gas heating temperature results in higher deposition efficiencies. • Low coating densities were delivered under the highest gas heating temperatures. • Porosity is the dominant factor that affects the reflectivity of the coatings. Abstract This research presents the results of an investigation into the additive manufacture of reflective Al coatings using cold spray (CS). This work is the first time that CS technology has been used to fabricate highly reflective surfaces. The coatings were characterised with respect to total reflectivity within the wavelength range of 400–1800 nm, surface roughness, particle deformation levels, and density. The experimental results indicate that raising the gas heating temperature increased the porosity and that the lowest gas heating temperature delivered the highest spectrally average reflectivity of 93.4% which was within 1% of bulk Al. This investigation demonstrates that CS can be used to coat thin layers of Al onto various materials, which can be subsequently polished to create composite reflectors. Hence, a reflector with the reflectivity of Al and the structural and thermal properties of the substrate material can be fabricated, allowing for greater design flexibility and the rapid manufacture of multi-material reflectors. [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]

In the present work the synthesis of multicomponent nanocrystalline Al 0.1- 0.5 (Mn)CoCrFeNi HEA powder feedstocks by mechanical alloying (MA) through high energy ball milling to produce HEA coatings with cold spraying were studied. Pure (>99.5) elemental powders of Al (Mn), Co, Cr, Cu, Fe & Ni were mixed in predetermined ratios and mechanically alloyed under Ar gas in a Planetary Ball Mill. MA HEA powder feedstocks were characterized by optical microscopy, SEM+EDAX, XRD & XPS to determine powder particle size, shape & distribution, particle surface condition, chemical composition, crystal & phase structures & microstructures. MA-processed Al-HEA powder feedstocks exhibited tri-modal particle size distribution (PSD) while Mn-HEA had four distinct PSD modes. Fine size fraction of the MA- HEA powders contained A2/B2 (BCC) harder phases with lattice parameters of 2.8831 and.2.8690 Å, respectively and coarse size fraction contained (ductile) softer A1 phase/Cu-rich regions combined (FCC) having a lattice parameter of 3.6181 Å. The ratio of the phases varied depending on the Al and Mn contents in the powder. For higher aluminium contents, there was more disordered A2/B2 phases and less disordered A1 phase in Al 0.1–0.5 HEA powder feedstocks. A1 phase contained Fe, Cr, Co and Ni; A2 phase contained Fe, Cr; and B2 phase contained Al, Ni elements in Al 0.1–0.5 HEA powder feedstocks. There were passive surface oxide films on the rough surfaces of MA HEA powder feedstocks consisting of a main mixture of surface oxides such as Al 2 O 3 , MnO, Cr 2 O 3 , Mn 2 O 3 , and minor oxides such as CuO, Fe 2 O 3 , NiO, Fe 3 O 4 , and a few CoCr 2 O 4 spinel oxide. Dual FCC-dual BCC phase prediction of thermodynamic stable phase formation criteria reported in the literature on MA-HEA powder feedstocks were consistent with the experimental results of the present study. [Display omitted] • Phase formation predictions. • Phase differentiation via LM, SEM, XRD deconvolution & PSD analyses. • HEA coating characterizations. [ABSTRACT FROM AUTHOR]

In this work, Ti6Al4V ELI samples were fabricated with previously optimized parameters by selective laser melting (SLM) and then heat-treated under vacuum (HT) or hot isostatic pressed (HIP). Optical microscopy (OM) and scanning electron microscopy (SEM) were used to characterize the microstructure. As-built microstructure was found to be dominated by columnar grains and acicular α′ martensite. Thermo-mechanical treatments allowed to modify the microstructure in the size and shape of the grains, as well in the volume fraction and chemical composition of the phases. Apart from the phase transformation, HIP treatment also allowed to significantly reduce the interior defects, such as pores and internal cracks. Tensile and fatigue properties of the Ti6Al4V ELI samples corresponding to the different microstructures were subsequently investigated. As-built samples exhibited a high tensile strength but a poor ductility with an elongation generally smaller than 6% and low fatigue resistance. HT samples displayed a reduced tensile strength but improved elongation behavior and relatively enhanced fatigue resistance versus as-built samples. The HIP treatment also resulted in a reduction in strength but improved furthermore the ductility and the fatigue behavior due to the elimination of internal defects. [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]

Cold spray is a solid-state coating deposition technology developed in the 1980s. In comparison with conventional thermal spray processes, cold spray can retain the original properties of feedstock, prevent the adverse influence on the underlying substrate materials, and produce very thick coatings. Coatings with nanostructure offer the potential for significant improvements in physical and mechanical properties as compared with conventional non-nanostructured coatings. Cold spray has also demonstrated great capability to produce coatings with nanostructure. This paper is aimed at providing a comprehensive overview of cold-sprayed metal coatings with nanostructure. A brief introduction of the cold spray technology is provided first. The nanocrystallization phenomenon in the conventional cold-sprayed metal coatings is then addressed. Thereafter, focus is switched to the microstructure and properties of the cold-sprayed nanocrystalline metal coatings, and the cold-sprayed nanomaterial-reinforced metal matrix composite (MMC) coatings. At the end, summary and future perspectives of the cold spray technology in producing metal coatings with nanostructure are concluded. [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]

• A modified Theory of Critical Distances method for fatigue prediction of AM Ti64. • Aim is to apply the TCD to the scenario of multiple stress concentrating features. • Point Method (PM) and Line (LM) Method forms of the TCD are implemented. • Fracture surfaces help to reveal material behaviour during the failure process. This work aims to apply the Theory of Critical Distances (TCD) to the fatigue assessment of additively manufactured (AM) Ti-6Al-4V material produced via the selective laser melting (SLM) process. Modified alternatives to traditional TCD methods are considered. In this sense, it is sought to develop a fatigue prediction model that is better suited to assessing the impact of multiple stress-rising features which are located in close proximity to each other. Hereby, consideration has been given to modelling process-inherent surface roughness in combination with an internally positioned artificial defect, shaped as a feature that is reminiscent of a pore. Simultaneously, the research also seeks to circumnavigate a potential issue with respect to the current TCD methodology. This concerns the matter of applying TCD practices to components whereby the area of interest for conducting stress-distance analytics is on a size scale that is smaller than that of the critical distance length parameter itself. Several different strategies were attempted as a way to try and achieve meaningful modifications to the TCD process. Results show that it is possible to overcome such challenges that can often present themselves during the fatigue appraisal of AM metal parts. In this sense, the optimal novel strategy that was experimented with returned average error margins of 13.7% or better. It is anticipated that such models may assist in further optimising the accuracy of service life evaluation for metallic AM components that are intended for industry. [ABSTRACT FROM AUTHOR]

Additive manufacturing techniques such as cold spray are translating from research laboratories into more mainstream high-end production systems. Similar to many additive processes, finishing still depends on removal processes. This research presents the results from investigations into aspects of the machinability of aluminum 6061 tubes manufactured with cold spray. Through the analysis of cutting forces and observations on chip formation and surface morphology, the effect of cutting speed, feed rate, and heat treatment was quantified, for both cold-sprayed and bulk aluminum 6061. High-speed video of chip formation shows changes in chip form for varying material and heat treatment, which is supported by the force data and quantitative imaging of the machined surface. The results shown in this paper demonstrate that parameters involved in cold spray directly impact on machinability and therefore have implications for machining parameters and strategy. [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]

In this paper, cold spray in conjunction with powder ball milling was used to fabricate GNP-reinforced copper MMC coatings without crossing materials' melting points. As a result, non-agglomerated and uniformly-distributed GNPs were included in the cold sprayed MMC powders and coating. No phase change and oxidation occurred during the coating fabrication. The friction coefficient of GNP-reinforced MMC coating reduced by approximately 20% compared to bulk copper. The cold sprayed MMCs also resulted in lower friction coefficient than spark plasma sintered MMCs. [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]

A new theory was proposed to explain the interfacial bonding mechanism of hard Ni coating onto soft Al substrate. The experimental results indicate that the metal-to-metal contact and the consequent metallurgical bonding at the coating/substrate interface were absent in the single particle depositing but could be achieved in the full coating deposition. Based on this, it is proposed that the particle peening effect breaks the cracked oxides that remained at the coating/substrate interface into nano-pieces and promotes further deformation of materials. Thus, the pores caused by the bridge-like oxides at the interface are filled and the discontinuous metal-to-metal contact is achieved. [ABSTRACT FROM AUTHOR]

Investigation on a single particle deposition onto a polished substrate is an effective way to well understand the coating/substrate bonding mechanism in cold spray. But existing studies on this topic are always limited to the particle surface morphology and cross-section observation. This paper presents a methodology to directly observe the fractured contact surface between the cold sprayed particle and substrate. By this means, the particle/substrate fractured contact surfaces at different spray angles were obtained and analyzed. It was found that in the perpendicular spray dimple-like features as a sign of metallurgical bonding were formed at the surrounding of the central region on the fractured contact surface. However, for the angular spray, such dimple-like features only located at a small area of one side. Further analysis suggested that the high contact pressure is the dominant factor determining the particle/substrate metallurgical bonding quality and thus the formation of dimple-like features. [ABSTRACT FROM AUTHOR]

Cold spraying is increasingly attracting attentions from both scientific and industrial communities due to its unique 'low-temperature' coating build-up process and its potential applications in the additive manufacturing across a variety of industries. The existing studies mainly focused on the following subjects: particle acceleration and heating, coating build-up, coating formation mechanism, coating properties, and coating applications, among which particle acceleration and heating can be regarded as the premise of the other subjects because it directly determines whether particles have sufficient energy to deposit and form the coating. Investigations on particle acceleration and heating behavior in cold spraying have been widely conducted both numerically and experimentally over decades, where many valuable conclusions were drawn. However, existing literature on this topic is vast; a systematical summery and review work is still lack so far. Besides, some curtail issues involved in modeling and experiments are still not quite clear, which needs to be further clarified. Hence, a comprehensive summary and review of the literature are very necessary. In this paper, the gas flow, particle acceleration, and heat transfer behavior in the cold spray process are systematically reviewed. Firstly, a brief introduction is given to introduce the early analytical models for predicting the gas flow and particle velocity in cold spraying. Subsequently, special attention is directed towards the application of computational fluid dynamics technique for cold spray modeling. Finally, the experimental observations and measurements in cold spraying are summarized. [ABSTRACT FROM AUTHOR]

This paper describes the application of cold spray to the deposition of a diamond grade pre-coated with Cu and Ni. This is the first time that pre-coated diamond powders are used as the sole feedstock without the addition of binders (ductile phases) in cold spraying. The experimental results showed that it was possible to manufacture thick metal–diamond composite coatings onto an Al alloy substrate with high diamond fraction in the coating and without phase change. Results from this paper also have demonstrated a new methodology for the deposition of metal–diamond/ceramic composite coating with the cold spray technique. [ABSTRACT FROM AUTHOR]

Cold spray as a relatively new surface modification technique has great potential in the industry due to its 'low working temperature'. The bonding quality between the cold-sprayed coating and the target substrate is one of the most important evaluation index for the coating performance. In this study, the interfacial bonding features between cold-sprayed Ni coatings and Cu are investigated to clarify the role of substrate temperature in the coating-substrate bonding. The finite element analysis model which can simulate the heat conduction through the coating-substrate interface is developed to model the particle deposition process on the substrate. The surface morphology and cross section of the experimentally deposited particles are observed by scanning electron microscope. The substrate surface oxidization behavior and coating-substrate interfacial atomic mixture are evaluated through the energy-dispersive spectroscopy line scans. The results indicate that heat conduction from the high-temperature substrate plays an important role in heating the interfacial region. The increased interfacial temperature significantly enhances the metallurgical bonding by improving the coating-substrate atomic mixture at the interface. Besides, the high-temperature substrate is also found to result in prominent metal jet and strong mechanical interlock due to the enhanced thermal softening effect. As a consequence, coating mass and coating-substrate bonding strength are promoted by using the high-temperature substrate. [ABSTRACT FROM AUTHOR]

Demand for new Ti alloys for AM is immense. Ti–5Al–5Mo–5V–3Cr (wt%), a β type Ti alloy is chosen for this study as it possesses excellent properties. Samples are manufactured using SLM technique. Relative density, surface morphologies and hardness of samples produced by varying laser processing parameters, e.g., laser scan speed and laser power, are thoroughly investigated. Top surface of the as-built samples shows adhered powder particles and discontinuous ripple patterns. A change in scan speed from 1200 to 1400 mm.s-1 drastically deteriorates top surface quality in terms of ripples patterns and adhered powders. Surface roughness (S a /R a) of the side surface of the produced samples shows a value of below 20 µm. Side surfaces show adhered powder particles, open surface pores, microcrack and interlayer crack. Substantial increase in adhered powder particles is evident when both laser power and laser scan speed are increased distinctively. These results are discussed with the help of the thermodynamic and metallurgical phenomena involved in the SLM processing. Hardness of the as-built samples is measured, the value is in agreement with conventional processing. This fundamental study will be useful for producing Ti-5553 alloy AM parts with optimum surface. • Relative density, surface morphologies and hardness of SLM produced Ti–5Al–5Mo–5V–3Cr alloy are investigated. • Over 99% of relative density has been achieved for AM processed Ti-5553. • The adhered powders, ripples, microcrack, pores are seen on the surfaces. • Laser scan speed and laser power affect surface characteristics of as-built Ti-5553. • Hardness number shows a value that is in good agreement with conventionally processed Ti-5553. [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]

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]

Cold spray deposition (CSD) emerged as an additive manufacturing process in recent years. It features the highest efficiency among all metal additive manufacturing processes. In this work, the deposition mechanism of brittle reinforcing particles during the CSD of metal matrix composites (MMCs) was investigated. The rebound and fragmentation of brittle microparticles during deposition were investigated at nanosecond level and with microscale resolution through in-situ observation. For the first time, the velocity that determines the transition from particle rebound to particle fragmentation was identified. Herein, this velocity is defined as "fragmentation velocity". The X-ray computed tomography on CSD deposits reveal that most of the diamonds in the deposits were fragments of the original particles and their volume fraction significantly depended on their impact velocities relative to the fragmentation velocity. Based on these findings along with microstructure characterizations, the deposition mechanism of brittle reinforcing particles during the CSD of MMCs is proposed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Experimental investigation was conducted to explore the formation conditions and provide new insights into the formation mechanisms of the unexplained intermixing phenomenon observed at the substrate-coatings interface of cold sprayed materials. The results indicate that the formation of intermixing interface significantly depends on the extent of plastic deformation at the coating-substrate interface, with severe deformation creating favorable conditions for the intermixing interface. Two factors have been identified to be critical for inducing the severe interfacial plastic deformation: low deposition efficiency and material properties. During low deposition efficiency cold spraying, most of the accelerated particles rebound after impact while inducing accumulative plastic deformation and thus intermixing at the coating-substrate interface. Considering the material properties, the coating materials must have sufficiently high density to attain enough kinetic energy for creating substantial plastic deformation of the first coating layer and the substrate upon their impact. Also, the substrate materials cannot be too hard so that plastic deformation can be induced. Based on the experimental analyses, the hypothesis of the intermixing interface formation mechanism is proposed in this paper. Unlabelled Image This work identified the conditions necessary to trigger the formation of intermixing interfaces, a unique phenomenon in cold spray, explained the underlying physical principles, and proposed a corresponding mechanistic explanation. • The unique microstructure feature forms at severe plastic deformation conditions. • Two critical factors were identified to trigger the intermixing interfaces: low deposition efficiency and material properties. • Low deposition efficiency induces accumulated plastic deformation by impacts of rebound particles at intermixing interface. • Sufficient density of particles is needed to induce required levels of strain. [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]

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]