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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 $${R}_{c}^{2}=0.7$$ R c 2 = 0.7 (or $${R}_{c}=0.83$$ 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.

Selective Laser Melting is an additive manufacturing practice that permits the production of metal alloy-based parts. While facilitating the design of complex geometry, SLM leads to the fabrication of a unique material structure that showcases distinct behavioural characteristics relative to their traditional methods of material manufacture. Defects that are innate to SLM inspire the presence of a compositional outlook that is inhomogeneous in nature and only serves to hinder part efficiency. Thus, the Theory of Critical Distances offers a refreshed proposal to evaluating notched Ti-6Al-4V material produced by additive manufacturing processes. Key principles of the theory’s working mechanisms are outlined. Subsequently, symmetrical notches of contrasting size are assessed. Findings reveal that the Theory of Critical Distances is adequately compatible with accurate fatigue prediction of SLM Ti-6Al-4V in its as-built state. Additionally, fracture surface analysis reveals that crack initiation is predominantly a surface-based phenomenon. Hereby, increased focus must be given to the quality of processed material that is located at the externalities of additively manufactured components, in order to enhance their service life capabilities. This will induce an increasingly uniform material structure that will allow for more predictable behavioural characteristics.

About 3 mm thick five-element equimolar high-entropy alloy (HEA) FeCoCrNiMn was successfully deposited by solid-state cold spraying (CS). The high-temperature oxidation behavior of the CSed HEA was investigated at 700-900 °C. Heat treatment was performed on the CSed HEA before oxidation to heal the incomplete interfaces between the deposited particles. Results show that the microstructure of the CSed HEA is characterized by grain refinement and abundant interparticle incomplete interfaces. Post-spray heat treatment promotes recrystallization and grain growth in the CSed HEA. After oxidation testing, the oxide scales are composed of multi-layers: a Mn2O3 (or Mn3O4) outer layer, a Mn-Cr spinel intermediate layer and a Cr2O3 inner layer. The CSed HEA exhibits higher parabolic rate constants and more favorable internal oxidation than the bulk HEAs that have similar compositions in the literature. Such a discrepancy becomes pronounced at higher temperatures. The grain refinement and numerous particle boundaries are responsible for such a distinctive performance of the CSed HEA.

INCONEL 718 superalloy (IN 718) is frequently used in highly aggressive environments, such as aerospace and gas turbine engines, where excellent mechanical properties, creep-, fatigue- and oxidation-resistance performance at high and cryogenic temperatures are required. Recent studies have successfully cold sprayed IN 718, showing great potential mainly in maintenance and repairing fields. However, due to the low plastic deformation, the manufacture of IN 718 cold sprayed coatings often requires the use of expensive propulsive gases or high working parameters to enhance deposition efficiency, with a significant increase in production costs. This paper investigates for the first time the addition of Ni to IN 718 powders in order to increase plastic deformation and interparticle bonding strength. Four composite coatings were deposited via a high-pressure cold spray process using nitrogen as propulsive gas, considering different IN 718 mass fractions in the feedstock: C1 (0 wt%), C2 (25 wt%), C3 (50 wt%), C4 (75 wt%). The coatings are examined in terms of microstructural characteristics and tribological performance. The addition of IN 718 particles significantly improves the mechanical properties of the coatings, despite an increase in porosity, which however does not exceed 1%. The tribological performance of the four coatings is investigated using a pin-on-disk test, demonstrating that the coating wear resistance behaviour improved as the IN 718 content increased. Analysis of the wear mechanism shows that C4 coating has a different wear behaviour than the other coatings, thus achieving the best wear-resistance performance.

Additive manufacturing techniques such as selective laser melting enable the production of customised components with high geometrical freedom. However, SLM results in a material condition with different properties to their conventionally manufactured counterparts. The presence of process-inherent defects can significantly impact the degradation of part performance. Hereby, a novel approach to assessing notched SLM Ti-6Al-4V material via a critical distance theory is presented. Geometrical notches of varying size are evaluated. Results show that the Theory of Critical Distances is appropriately applicable to fatigue prediction of SLM Ti-6Al-4V in its as-built state.

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 CuAl2 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.

Cold Spray(CS) is an emerging process, attracting much interest both as a coating and additive technique due to its solid-state nature and high deposition rates. The key element of any CS apparatus is the supersonic nozzle. The work presented here, for the first time, will explore in detail the relationship between particle flow, and its acceleration in the nozzle using Computational Fluid Dynamics and experimental measurements obtained with a Particle Tracking Velocimetry apparatus. We have developed modelling parameters capable to accurately predict the particle flow exit speed and space distribution, leading to considerably improved nozzle performances.

Technological progress in hybrid bearings developed high wear and abrasion resistant materials for rolling elements. The manufacturing process of bearing balls presents new challenges, as nowadays, it requires time-consuming and costly processes. In this frame, the bearing manufacturing industry is demanding improvements in materials, geometry, and processes. This work aims to investigate new abrasive coatings for grinding wheels for Si3N4 ball manufacturing. Tribological pin on disk tests are performed on samples of grinding materials (disk) versus a Si3N4 ball (pin). Two samples of specimens coated with an electrodeposited diamond and diamond-reinforced metal matrix composite are examined to measure the abrasion rate and the wear resistance of Silicon Nitride Si3N4 balls, considering the influence of sliding speed and the effect of coating deposition on diamond particle density and granulometry. The measurements estimated the specific wear coefficient k, the height wear surface h, and the wear rate u of the Si3N4 balls. The results pointed out that by increasing the sliding speed, the abraded volume increases for both the coatings. The parameters affecting the abrasion effectiveness of both the coatings are the surface roughness, the abrasive particle dimension, and the sliding speed.

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.

This work details new insights into the in-process densification mechanism of cold spray Al coatings. The results show a trend counter to common observations: coating plastic deformation levels and coating density decreases with an increase in particle impact velocity. A lower particle impact velocity and the consequent lower deposition efficiency (DE) results in greater tamping energy per unit volume of deposit, which is the primary reason for the observed trend. This is the first time that DE has been shown to have a non-linear impact on the density of a cold spray coating, with particle in-process tamping being the primary mechanism for coating densification.

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.

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.

Manufacturing parameters are critical for the fabrication of cold sprayed deposits with desirable microstructure and excellent performance. In CSAM, key manufacturing parameters that determine the deposition process include gas pressure, gas temperature, gas type, powder feed rate, nozzle traverse speed, scanning step, standoff distance, spray angle and trajectory. These parameters must work in unison and under careful control to produce high-quality deposits. In this chapter, the aforementioned key manufacturing parameters and their roles in cold spray deposition are introduced and the relationship among these parameters are discussed.

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., The research was supported by Enterprise Ireland (EI) and SchuF Valve Technology GmbH (IP2018 0730)

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 HV0.2 in the as-built state to 510 HV0.2 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.

The complexity of technical applications involving particle-laden gas flows often impedes a holistic understanding of the interplay of underlying physical aspects. The usual approach is to isolate phenomena and study them individually, forcing researchers to ignore realistic circumstances. An example is the coating formation process Cold Spray (CS), in which high Stokes number particles are accelerated in a supersonic nozzle and free jet. Along the flow, the particle laden gas undergoes extreme changes, e.g. in velocity, temperature and volume fraction, through a series of interconnected flow events for a poly-disperse distribution of particle sizes. A consequence of this complexity is an under-representation of research on the fluid-mechanics in this field. This manuscript aims to build a bridge between empirical testing in CS and more fundamental aspects of the relevant fluid mechanics: Parameters associated with the multi-phase character of the process are not yet well understood. In this sense, the particle mass loading is often ignored, so are particle-particle interactions and the strong variations of local conditions. Previous work has indicated that there is no conclusive understanding of the phase interaction mechanisms in CS beyond high-Mach number drag laws. In order to start filling this gap, the first experiment for the direct observation of the particle behaviour within a CS nozzle is designed. Rectangular de-Laval nozzles are manufactured from quartz glass and particle tracking velocimetry (PTV) is used to obtain particle velocities and positions at the injection zone, throughout the nozzle and in the supersonic jet under varying operating conditions. This study firstly reports the direct measurement of particle injection, acceleration, and dispersion in CS. It suggests physical explanations based on a statistical evaluation of the observed data and forms the basis for more fundamental understanding of CS fluid mechanics.

Selective Laser Melting (SLM) is an additive manufacturing (AM) technique which has been heavily investigated for the processing of Ti-6Al-4V (Ti64) which is used in the biomedical, aerospace and other industries. To date the SLM processing of this material has been inhibited by the requirement of post processes due to three primary challenges of martensitic microstructures, undesired porosity and residual stresses which are present in the as-built state. This work identifies the state of the art in process optimisation which is being used to confront these challenges in the as-built state with a view to removing the reliance on post processing. Regarding process optimisation, maximising part density is the primary goal due to the negative influence of pores on fracture and fatigue properties. To accomplish this, a high energy input is required which results in high cooling rates during processing. It is these cooling rates which are instrumental in the microstructural evolution and residual stress production. Accordingly novel methods have been proposed which aim to maintain the necessary high level of energy input but control the cooling rates to tailor the microstructure and reduce residual stresses. Research gaps have been identified pertaining to all three of these challenges when considering mechanical properties of as-built components. Thus in its current state post processes remain critical, however promising techniques in early stage development provide encouragement going forward.

High-performance metal additive manufacturing (AM) has been extensively investigated in recent years because of its unique advantages over traditional manufacturing processes. AM has been applied to form complex components of Ti, Fe or Ni alloys. However, for other nonferrous alloys such as Al alloys, Mg alloys and Cu alloys, AM may not be appropriate because of its melting nature during processing by laser, electron beam, and/or arc. Cold spraying (CS) has been widely accepted as a promising solid-state coating technique in last decade for its mass production of high-quality metals and alloys, and/or metal matrix composites coatings. It is now recognized as a useful and powerful tool for AM, but the related research work has just started. This review summarized the literature on the state-of-the-art and problems for CS as an AM and repairing technique.

This paper reveals new insights on how to achieve direct deposition of functional materials onto silicon wafers for cooling purposes. Manufacturing heat spreaders directly onto a microprocessor will negate the need for Thermal Interface Materials (TIMs) which often account for the highest resistances in a typical CPU thermal circuit. In this work we demonstrate that Cold Spray can be tailored to directly print onto a silicon wafer with a layered structure of aluminum, copper and diamond. The thermal performance of the copper/diamond coating was also separately measured using an in-house testing apparatus showing heat-transfer enhancements in agreement with theoretical predictions.

Confined jet array impingement boiling heat transfer is investigated for water at atmospheric pressure and inlet subcooling of 8 °C for flow rates ranging between 0.33 and 0.67 L/min. Three jet configurations consisting of 2 × 2, 3 × 3 and 5 × 5 arrays of 1.0 mm diameter jets were tested for a jet-to-target spacing of 2 mm. A 15 mm × 15 mm plane copper surface was used as the heat transfer surface which formed a confined channel with the upper jet orifice plate. For a Reynolds number (Re) range of 900 ≤ Re ≤ 11,800, tests were performed by fixing the flow rate and progressively increasing the heat flux until the Critical Heat Flux (CHF) was reached. The results show that the single phase heat transfer coefficient increases with increasing Re and is reasonably predicted by a recent jet array heat transfer correlation. However, once fully developed boiling conditions are achieved, the heat transfer is generally insensitive to the flow rate and is predicted by the well-established Cooper pool boiling correlation. Consistent with earlier confined boiling studies, this suggests that the dominant heat transfer mechanisms are associated with the local bubble activity opposed to convective influences. Contrastingly, both the Onset of Nucleate Boiling (ONB) and CHF were found to be strongly dependent on the jet velocity. For the latter, it is related to the ability of the flow to clear the confined channel of vapour. CHF predictions are compared with the measurements showing good agreement.

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. Keywords: Kinetic spray, Tungsten carbide cobalt, Nickel, Tribology, Sliding wear rate, Tribofilm

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.

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.

Jet array impingement heat transfer is investigated for two phase forced convection of water at atmospheric pressure and subcooling of 7 °C with flow rates up to 660 mL/min. A jet array consisting of nine 1 mm jets with 5 mm inter-jet spacing and a 2 mm jet to target spacing was employed to cool a 15 mm by 15 mm heated surface. A linear micro-groove and a radial micro-groove surface were investigated and compared to a flat surface. The results show that the heat transfer performance of the impinging jet is insensitive to Reynolds number for fully developed boiling. A maximum heat transfer coefficient of h = 230 kW/m 2 K was achieved with the radial micro-groove surface, transporting a substantial heat flux of 380 W/cm 2 . This was a 2.3-fold improvement compared with the flat surface. The linear micro-groove surface also performed well, achieving a 2-fold enhancement. Finally, the performance of the jet array impingement onto micro-grooved surfaces is compared with other recent compact water cooled two phase heat exchanger concepts from the recent literature showing comparable thermal performance.

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.

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.

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.

This paper describes the application of Cold Spray to the deposition of a diamond grade pre-coated with Cu and Ni. Diamond powders are therefore used as the sole feedstock without the addition of separate binders (ductile phases) in cold spraying. The experimental results showed that it was possible to manufacture thick metal-diamond composite coatings with high diamond fraction in the coating but without phase change or graphitization. Results presented in this paper demonstrate the feasibility of a new methodology for the deposition of metal-diamond/ceramic composite coating with the cold spray technique.

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.

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.

Cold Spray (CS) of copper particles onto polymers has been validated as an effective tool for maintaining surface integrity in bioenvironments. CS requires limited heat input, can be applied locally or in large areas. The key parameters are particle penetration depth and copper surface coverage. However, the process parameters that can optimise the coating performance with deposition have not been comprehensively explored. In this paper, copper particles were deposited onto two polymers used in marine applications. A detailed analysis was carried out to correlate the key surface properties to the process parameters so as to determine the optimum conditions.

It is known that ZnO is an n-type semiconductor with photocatalytic performances under ultraviolet light irradiation. Constructing a superior structure for a modified electron band has been one of the major research goals for photocatalytic ZnO. Here we report a new technical route for making nano-ZnO coatings with a porous topographic morphology. The coatings were fabricated by plasma spraying the mixture of suspension and solution liquid precursors. Pre-loading of ZnO and Zn powders in the precursor was carried out for the purpose of tailoring the structure of the coatings. The coatings in micron thicknesses showed a porous skeleton and a fluffy top layer consisting of ultrafine ZnO grains. Photocatalytic testing by measuring the degradation of methylene blue revealed significantly enhanced activities of the coatings deposited using the ZnO/Zn loaded precursor. The hybrid-structured ZnO coatings exhibited a narrowed band gap and modified oxygen defects as compared to those deposited from the single liquid feedstock. The results shed light on a one-step easy thermal spray fabrication of polytropic nanostructured functional coatings by employing solid powder-loaded liquid as the starting feedstock.

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.

Cold Spray is a novel technology for the application of coatings onto a variety of substrate materials. In this method, melting temperatures are not crossed and the bonding is realized by the acceleration of powder particles through a carrier gas in a converging-diverging nozzle and their high energy impact over a substrate material. The critical aspect of this technology is the acceleration process and the multiphase nature of it. Three different nozzle designs were experimented under constant conditions and their performance simulated using Computational Fluid Dynamics tools. The Deposition Efficiency was measured using titanium as feedstock material and it was shown that it decreases with the cross-sectional throat area of the nozzle. Computational results based on a one-way coupled multiphase approach did not agree with this observation, while more sophisticated modelling techniques with two-way couplings can partially provide high-quality outcomes, in agreement with experimental data.

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.

Ni60-WC particles are used to improve the wear resistance of hard-facing steel due to their high hardness. An emerging technology that combines laser with cold spraying to deposit the hard-facing coatings is known as supersonic laser deposition. In this study, Ni60-WC is deposited on low-carbon steel using SLD. The microstructure and performance of the coatings are investigated through SEM, optical microscopy, EDS, XRD, microhardness and pin-on-disc wear tests. The experimental results of the coating processed with the optimal parameters are compared to those of the coating deposited using laser cladding.

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.

Coating technologies play a critical role in the worldwide manufacturing industry. The ability to form layers of specific materials onto engineering components to enhance mechanical and physical properties has numerous applications, ranging from corrosion protection, repair, hardfacing, down to purely aesthetic purposes. Cold spray is an innovative technology, which allows for the manufacturing of coatings in a solid state manner; hence, feedstock properties can be fully preserved. Its working principles relies upon the acceleration of powders up to supersonic velocities, and the subsequent generation of high energy impacts on a substrate which triggers the coating formation. This paper presents deposition efficiency (DE) results from four different supersonic nozzles when using titanium as feedstock material. DE is the most critical parameter to assess the performance of cold spray nozzles. A theoretical analysis through computational fluid dynamics (CFD) is carried out so to compare numerical re...

Cold spray (CS) is attracting interest of research and industry due to its rapid, solid-state particle deposition process and respective advantages over conventional deposition technologies. The acceleration of the particles is critical to the efficiency of CS, and previous investigations rarely consider the particle feed rate. However, because higher particle loadings are typically used in the process, the effect of this cannot be assumed negligible. This study therefore investigates the particle velocities in the supersonic jet of an advanced CS system at low- and high pressure levels and varying particle feed rates using particle image velocimetry. The particle dispersion and velocity evolution along the jet axis were investigated for several feedstock materials. It was found that the average particle velocity noticeably decreases with increasing particulate loading in all cases. The velocity distribution and particle dispersion were also observed to be influenced by the feed rate. Effects are driven by both mass loading and volume fraction, depending on the feedstock’s particle velocity parameter. Increased particle feed rates hence affect the magnitude and distribution of impact velocity and consequently the efficiency of CS. In particular, numerical models neglecting this interconnection are required to be further improved, based on these experimental studies.

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.

A concept based upon Equal Channel Angular Extrusion (ECAE) is developed and introduced in the form of a Universal Re-usable Energy Absorption Device ‘UREAD’. In impact situations the device utilises the energy required to extrude deformable materials through the shear planes of a set of intersecting channels and hence provides the means to protect engineering structures. The impact force is absorbed through the resistance of a deformable material and the energy is dissipated through an operational stroke. This paper examines the use of this new concept under dynamic loading. The device performance and usability during dynamic impacts are tested in a landing frame type experiment where the effectiveness of the technique in reducing impact loads and energy are also examined.

Near fully dense tungsten coatings onto molybdenum substrates have been demonstrated using the Supersonic Laser Deposition (SLD) process. This is a characteristic that is not readily achievable with refractory materials. The tensile strength of the tungsten deposited coatings, 724 MPa, is comparable to that of wrought tungsten with no evidence of melting or substrate grain growth. The results have shown that SLD is able to deposit tungsten with unique interface bonding and desirable properties as opposed to other deposition processes for refractory materials.