Miniaturization and performance improvements of electronic devices in recent decades have significantly increased heat dissipation rates. To overcome this, researchers have developed heat sinks with miniature fluid channels to maintain small device footprints with increased heat transfer performance. These channels are often fabricated using either subtractive fabrication methods, such as etching or micro-milling, or additive methods such as direct metal laser sintering (DMLS). These methods are limited by their long processing times, low geometric accuracy, or high cost. To overcome these limitations, a novel additive manufacturing method is developed using twin wire-arc spray. Wire-arc spray was used to build complex aluminum structures with length scales varying from 0.5 mm to 74 mm. Surface structures were built on a metal plate by spraying aluminum through a 3D printed polymer mask. Internal flow passages were made by filling surface channels with a water-soluble polyvinyl alcohol (PVA) paste that was allowed to harden, spraying metal over it, and then dissolving the PVA. The influence of wire-arc spray process parameters, such as standoff distance and scanning speed, on coating solid PVA with aluminum, were also investigated.

The development of efficient ice mitigation systems for surfaces exposed to atmospheric ice has been in progress for decades. The need for passive anti-icing systems is essential as current ice mitigation systems require a substantial amount of energy and their implementation involves complex manufacturing considerations. Fluorinated polymer coatings are among the candidates for passive anti-icing systems. While many processes have been investigated to produce them, these methods can be costly, time consuming and can cause thermal damage to the substrate. The current work aims to explore a green and cheap alternative approach by using cold spray. Furthermore, the cold spray process offers advantages such as being a portable easy to perform solid-state coating process for eventual repairs. This work uses computational and experimental approaches to design and test a new dedicated nozzle for the efficient deposition of adhesive perfluoroalkoxy alkane. Computational results reveal that for the same operating conditions, the use of the new nozzle design increases particle impact temperature, improving the deposition of the feedstock material, as confirmed experimentally. The wetting behaviour, ice nucleation time and ice adhesion strength were compared for 6 different surface types, including bare aluminum, various polymer materials and the cold spray perfluoroalkoxy alkane coating on aluminium substrate. Results indicate that the as-sprayed coating performs as both a superhydrophobic and icephobic surface.

Cold spray (CS), a solid-state depositing technique, has recently demonstrated promising application in additive manufacture (AM). Compared with fusion based AM technique, cold spray can eliminate solidification defects and is appropriate to fabricate some materials that are difficult for high energy beam methods, such as Aluminium. In the cold spray process, extreme plastic deformation will occur, which triggers the severe dynamic recrystallization and result in the formation of ultrafine grain structure. For a dense CSed component, the grain structure highly influences its performance. Especially for the grain structure in the region around the impact interface, which decides the bonding of particles. However, due to the extreme processing condition of CS and complicated deformation history around the interface, it is challenging to carryout systematic study on the factors that influence the finial grain structure and make a prediction on the final grain size in this region. Here, a Monte Carlo model was built to simulate the dynamic recrystallization in the Aluminium cold spray process. The influence of impact velocity and single/multiparticle impact on the final grain structure in the interface was investigated comprehensively and independently. And the average grain size on the impact interface predicted by the modelling agreed well with reported experimental results.

In cold spray (CS) additive manufacturing process, micrometer scale particles accelerated through a supersonic nozzle are targeted on a surface with velocities in the rage of 300-1500 m/s in solid state. The impact energy of the particles leads them to deform plastically with high shear energy near the impact interface and adhere to the surface metallurgically, mechanically, and chemically. Using CS, deposition of metals, metal matrix composites, and polymers are achieved with high adhesive/cohesive strength and low porosity. Sensitivity of the CS additive manufacturing process to the variabilities in the process parameters are still being understood. Among the process parameters, particle morphology can have significant implications on drag forces, and therefore, on the particle impact velocity. This in turn affects the deposition efficiency (DE) and the quality of products. In this work, a new approach is introduced for computing DE by incorporating particle sphericity and its variation into one-dimensional numerical models. Size, sphericity, and the variability of size and sphericity of aluminum, copper, titanium, and tantalum particles are measured from static optical microscope images. The data is used for predicting impact velocity, temperature, and DE. The model results are then compared with particle velocity measurements.

In a novel approach for guiding elements of sawing machines wear resistant coatings were applied on open-porous AlSi 7 Mg substrates by means of high velocity suspension flame spraying (HVSFS). The challenge is to establish a wear resistant coating but simultaneously maintain the open-porous structure that is necessary to serve as a permeable structure for liquid cooling or lubricant media under operation. In a first approach, a water-based suspension containing a mixed Al 2 O 3 -TiO 2 powder for HVSFS was used to deposit dense and well adherent mixed-oxide coatings. As the substrates exhibit an open-porous structure and a well-defined pore size distribution, transpiration cooling through the pores is possible and even necessary in order to ensure a low thermal impact on the fragile pore structure, preserving the open-porosity of the substrate. The coatings are characterized and compared by the means of light microscopy and hardness indentation.

Metals were deposited on components made by 3-D printing with polyvinyl alcohol (PVA), a water-soluble polymer. The polymer was then dissolved, leaving a metal layer whose surface topography was the negative of that of the polymer. This is a rapid and low-cost alternative to 3D printing directly using metal, but to succeed it is essential for the sprayed metal to adhere to the polymer substrate. Tests were done in which aluminum and copper were sprayed using a twin-wire arc spray system onto 3D printed coupons, 50 mm x 50 mm in size, made from polylactic acid (PLA), PLA mixed with metal (aluminum, copper) or carbon fiber, and PVA. Adhesion depended on substrate roughness (minimum 1-2 μm) and substrate temperature (above the glass transition temperature but below the melting temperature of the polymer). It was shown that surface features could be made with high resolution on metal components using this technique.

The scanning acoustic microscopy (SAM) technique is used for studying the character and interface quality of cold sprayed Fe coatings deposited onto notched Al-based substrates. Three notch geometries were used: a rectangular notch, a trapezoidal notch with a flat bed, and a trapezoidal notch with a cylindrical bed. Scanning electron micrographs demonstrated an increased porosity and cracks at the areas where the spraying direction was not perpendicular to the surface of the substrate. The SAM measurements were then performed on thin plates cut vertically across the notches such that the scanned area included the locations of the increased porosity and their surroundings. The resulting distributions of longitudinal wave velocities and their attenuation revealed that the affected area is more complex and the mechanical response of the coatings could be limited not only at areas of the visible porosity, but also in their vicinity.

Low pressure cold spraying is an attractive technique for onsite metal coating fabrication due to its compactness and portability. However, the bonding strength of the coating prepared by low pressure cold spraying is generally low, which restricts the further applications in engineering and industrial fields. To improve the bonding strength, pre-treatment on substrate surface can be an effective procedure. In this study, a low-temperature plasma treatment was applied to a pretreatment technique, and the effect of the treatment on particle bonding was compared with that of a laser treatment. Copper coatings on aluminum and copper substrates were selected and studied as basic metal materials. The SEM observation results show that the particle adhesion rate significantly increases by the laser and plasma treatments, due to the removal of the native oxide films on the substrates. The particle bonding on the plasma-treated substrate reveals better interfacial adhesion with less gap compared with the laser-treated one. The pre-treatment by low-temperature plasma can be an attractive technique to assist the cold spraying process due to the oxide removal ability and no thermal effect which can apply a wide range of materials.

Biofouling has been persisting as a worldwide problem due to the difficulties in finding efficient environment-friendly antifouling coatings for long-term applications. Developing novel coatings with desired antifouling properties has been one of the research goals for surface coating community. Recently hydrogel coating was proposed to serve as antifouling layer, for it offers the advantages of the ease of incorporating green biocides, and resisting attachment of microorganisms by its soft surface. Yet poor adhesion of the hydrogel on steel surfaces is a big concern. In this study, porous matrix aluminum coatings were fabricated by cored wire arc spray, and the sizes of the pores in the aluminum (Al) coatings were controlled by altering the size of the cored powder of sodium chloride. Silicone hydrogel was further deposited on the porous coating. The hydrogel penetrated into the open pores of the porous Al coatings, and the porous Al structure significantly enhanced the adhesion of the hydrogel. In addition, hydrogel coating exhibited very encouraging antifouling properties.

This paper discusses the challenges of constructing mathematical models of physicochemical and heat-mass transfer processes associated with reactive heterogeneous materials used in laser additive manufacturing. The results of calculations of thermocapillary convection induced by laser heating in an aluminum melt with an admixture of nickel particles are presented. Models of interphase and chemical interactions with the formation of intermediate phases and intermetallic compounds on nickel particles added to the melt during laser alloying or cladding are proposed, which make it possible to calculate the composition of intermetallic phases in the trace of the beam after crystallization and cooling.

Cold spraying of automotive engine blocks requires a gun adaptation for inner diameter spraying with a very short nozzle. In this work, 316L coatings are sprayed with such a gun and the behavior of particles impacting aluminum and stainless steel surfaces is studied in order to understand the factors that affect coating adhesion and cohesion. Correlations between spraying parameters and coating properties were investigated via design of experiments and the effect of process parameters on deposition efficiency and coating thickness was optimized for mass production. Post-process honing was also employed as part of the study and smooth coatings with small pores were obtained.

This study investigates how Al and Ta diffusion affects the growth of surface oxides in NiCoCrAlYTa coatings and the interdiffusion that occurs between coatings and single-crystal substrates in high-temperature oxidation processes. HVOF-sprayed layers were tested at temperatures between 900 °C and 1100 °C and the corresponding oxidation behavior of Al and Ta was assessed. It was found that higher amounts of Al in MCrAlY coatings promote the selective oxidation of Al 2 O 3 and that the addition of Ta increases the stability of the γ’-Ni 3 (Al,Ta) phase.

This paper is the second part of a study on how Al and Ta diffusion affects the oxidation of NiCoCrAlYTa coatings. Thermodynamic and diffusion simulations of the coatings with different additions of Al and Ta predicted the development of a typical γ, γ’, ß-phase microstructure and suggested that the ß-NiAl phase was depleted as Al diffused into the substrate. The simulations also indicated that Ta could diffuse back to γ’-Ni 3 (Al,Ta) phase in the substrate with a γ’ depletion due to inward diffusion of Co and Cr from the HVOF-sprayed deposit. How this process impacts microstructure development is discussed in detail.

In this study, aluminum coatings were cold sprayed, with and without laser assistance, on laser-textured aluminum 6060 and Fe52 steel substrates. The results indicate that laser texturing makes for a cleaner coating interface than grit blasting and that the benefits are greatest when spraying on harder substrate materials. For the steel substrate, the optimized topography achieved through laser texturing assisted in particle deformation, leading to the formation of a much tighter coating structure. Laser-assisted cold spraying, in turn, improved deposition efficiency as well as coating density and adhesion. Separately or together, the two processes have proven to be beneficial for cold spraying.

This study focuses on the relationship between porosity and leak tightness in cold-sprayed aluminum. Aluminum coatings with 0.2-9% porosity were produced by cold spraying and evaluated via helium leak testing. Multiscale porosity was determined through SEM and TEM analyses and shows good correlation with leak test results. The mechanisms involved in the creation of porosity were investigated as well through finite element analysis of single and multi-particle impacts.

Cold spray is a technology with great potential for additive manufacturing applications. Due to the high levels of plastic deformation experienced by the powder during the coating process, any deposit will require heat treatment post-spraying to improve ductility and fatigue strength. In extreme cases, the residual stresses from coating can cause delamination or compromise the bond strength when subsequent cold spray layers are deposited. This work details the use of a commercial CO 2 laser cutter to perform a surface heat treatment on single lines of cold sprayed aluminium, to relieve residual stresses. The effect of laser power and traverse speed on material hardness is quantified, and compared with as sprayed deposits. The results shown in this work demonstrate the potential for in-process heat treatment to reduce post-processing time and improve coating quality by reducing residual stresses.

Cold-spray process (CS) showed suitable properties to weld together compatible and incompatible materials. Since it has been intensively studied as additive technique using metallic powders on metallic surfaces, and only recently with polymeric powders, we compare, in a very preliminary attempt, simulated data of the impact of an aluminum particle onto aluminum substrate, with the case of an ultra-high molecular weight polyethylene (UHMWPE) particle on the same substrate. It is noteworthy that such polymer cannot be processed by classical means (extrusion, injection molding, etc.), and CS appears to be very promising as alternative technique to powder sintering (applicability on large surface area, surface curvature).

A three-dimensional transient heat conduction model was developed to evaluate analytically the surface temperature profile of substrates that were exposed to the impingement of a moving time-dependent heat source such as a cold spray nozzle or thermal spray torch. The estimated surface temperature profile was compared to the experimental results of the surface temperature of an aluminium substrate under impingement of a moving air jet generated by a low-pressure cold spray system. Close agreement between the analytical model and the experimental data was found. It was also found that the velocity of the moving heat source significantly affected both the profile and maximum temperature of the substrate. A non-dimensional characteristic velocity was defined and considered to take into account the effect of the velocity of the moving heat source, to broaden the application of the model to a wide range of materials and conditions. It was found that as the non-dimensional characteristic velocity increased, the thermal energy that was conducted into the substrate decreased. It was concluded that, with knowledge of the characteristic velocity, the analytical model was capable of predicting the spatial and temporal surface temperature of the substrate exposed to a moving heat source produced by spray nozzles and torches.

This research demonstrates the use of cold spray (CS) as an additive manufacturing process to manufacture reflective aluminium coatings. Nitrogen was used as a carrier gas at various gas heating temperatures. Following deposition, the coatings were finished using a number of machining and/or polishing processes to surface roughness values of 20-150 nm. The samples were characterised with respect to total reflectivity within the wavelength range of 400-1800 nm, porosity, surface roughness, and density. The reflectivity of the coatings approached that of bulk material, and 99% dense coatings were obtained. Increasing the gas heating temperature did not decrease the porosity with the lowest gas heating temperature found to deliver the best reflectivity. This work demonstrates that CS can be used to coat thin layers of aluminium onto various materials, which can be subsequently polished to create composite reflectors. This provides a novel reflector with the reflectivity of aluminium, and the structural and thermal properties of the substrate material, allowing for greater flexibility in the manufacture of reflectors.

This paper reports on the performance evaluation of stainless steel (SS) thermal spray coatings aimed at shielding lightweight aluminum (Al) brake rotor disks from excessive heat and providing an adequate tribological surface in contact with brake pads. Coating wear, corrosion and heat resistance performances were evaluated using pin-on-disk, cyclic corrosion tests and thermal cycling using a custom laser rig, respectively. Arc spray optimized coatings displayed lower or equivalent wear rates when compared with the baseline gray cast iron disks, with similar frictional behavior. However, arc spray coating exhibited low adhesion which limits the maximum coating thicknesses achievable and leads to early coating spalling after about 1000 thermal cycles. Arc sprayed coatings also corroded and delaminated under corrosion tests. Optimized cold spray coatings present high corrosion resistance and could resist above 10,000 thermal cycles without spalling. However, cold spray coatings exhibit wear rates at least 4 times those of the cast iron. Taking advantage of both types of coatings, it was found that the production of a duplex coating made of a cold spray bond coat and an arc spray top coat could meet the requirements for protecting Al disks, with near 50% weight reduction.