The cavitation performance of wear resistant cermet coatings can deteriorate in a corrosive environment. This investigation therefore considered the cavitation resistance in seawater of thermally sprayed High Velocity Oxy Fuel (HVOF) WC-10Co-4Cr coatings deposited on two different substrate materials of carbon steel and austenitic stainless steel. Coatings were deposited using industrially optimised parameters. Cavitation tests were conducted following the ASTM G32 test method in indirect mode, where there was a gap of 0.5 mm between the sonicator and the test surface. A submersed copper cooling coil controlled the temperature of the seawater. The cumulative cavitation erosion mass loss and cavitation erosion rate results are reported. The eroded substrate and coating surfaces were analysed using Scanning Electron Microscopy (SEM) in combination with energy dispersive x-ray analysis (EDX) to understand the failure modes. Coating phases were identified using x-ray diffraction. Results are discussed in terms of the cavitation failure modes and cavitation erosion rates for both the substrate and coated surfaces.

Successful cold spray of tool steels and other hard steels would unlock several opportunities, including the repair of molds as well as ships and heavy industry components. However, the high hardness of typical atomized steel powders strongly limits their cold sprayability. It has been recently demonstrated that heat treatment in a rotating furnace can significantly improve H13 cold sprayability via softening and agglomeration. In this work, this powder modification method is extended to a range of transformation hardenable steels: 4340, SS420, A588, 1040 and P20. The results show that powder heat treatment improves the powder deposition efficiency and the quality of the final cold sprayed coating, probably as the result of the decreased powder micro-hardness. The effects of the powder heat treatment atmosphere, a key parameter, will also be presented and discussed.

The widespread use of additive manufacturing and modern powder-based technologies (thermal spraying, hardfacing, sintering) encourages the search for alternative routes enhancing the development of metal and metal alloy powders. The state-of-the-art powder production processes, like gas, water or plasma atomization, are dedicated to mass production, which limits the availability of new powder compositions with desired characteristics. In this study, stainless steel powders were investigated. The powders were atomized by an in-house developed ultrasonic (UT) atomization set-up, called ULTRAMIZER. In this system, the atomization of melt is possible by using a high-power ultrasonic field. The atomized powders were characterized in terms of morphology and particle size distribution (PSD). The powder features were then correlated with operating parameters of: (i) UT atomization system, mainly frequency and root mean square power (RMS), and (ii) the orientation of the atomization plate against the melting system, by means of distance and tilting angle. The study shows that the ultrasonic atomization allows producing nearly spherical, defect-free powder particles, with a very narrow and controllable size distribution. These are important advantages over other metal powder production methods, especially when it comes to the development of new types of powder.

High-pressure cold spraying has shown significant potential in manufacturing metallic composite coatings for a wide range of industrial applications, including wear and corrosion protection. Quasi-crystalline materials, in turn, are promising candidates due to their unique microstructural features. Combining these concepts, metallic composite coatings were generated using high-pressure cold spraying to produce functional and protective coatings. Several spray trials were done to detect the effect of compositions and size of quasi-crystalline feedstock materials mixed with metal powders, Al6061, and stainless steel 316L, on coating microstructure, integrity, and surface properties. A scanning electron microscope was used to examine the microstructure of the feedstock materials and composite coatings. A 3D surface optical profilometer was also used to investigate surface texture. The wettability of the coating surfaces was measured by static water contact angles using a droplet shape analyzer. Cold-sprayed quasi-crystalline composite coatings showed denser and well-integrated deposits with a random distribution of phases across the composite surface, indicating promising structural reliability and hydrophobic behavior.

The health of railway material is of paramount importance for the safety of rail transportation. Railways are subject to heavy mechanical loading and to harsh environments, causing corrosion and damage that can bring to failure. The cold spray process has a great potential as an efficient and portable refurbishment technology, with the advantage of avoiding thermal effects on the substrate. In this study, a proof of concept for the cold spraying of railway steel onto a similar material is presented. This represents a first step towards the development of a cold spray solution for railway repair. First, the as-atomized steel powder revealed to be hardly sprayable. A heat treatment was then optimized and applied to the powder to induce microstructural evolution and to improve deposition efficiency and material quality. Therefore, the refurbishment of damaged railway samples by cold spray was proven to be viable. Finally, mechanical testing assessed the restoration of the structural properties needed for the application.

Three different coatings were deposited using the Detonation Gun Spraying (DGS) technology from steel powders alone, and steel powers mixed with Fe3C and SiC particles, respectively. The microstructural characteristics of these coatings were examined and the hardness of each type of coating was studied. The morphology and structure of the feedstock powders were affected by the exposure to high temperature during the spraying process and rapid solidification of steel powders that resulted in the formation of an amorphous structure. The unreinforced steel coating had the highest hardness among the three types of coatings, possibly due to a higher degree of amorphization in the coating compared to the other two samples. The microstructural observation confirmed the formation of dense coatings with a layered structure with good connectivity between layers with minimum defects and porosities in the interfacial regions.

Industries developing cold-spray processes aim at producing dense and resistant coatings. Controlling microstructure and inter-particular fracture characteristics of sprayed coatings is essential to improve their properties. To do so, post-spraying heat treatment is a promising approach. This work addresses the development of such heat treatments and focuses on the analysis of recovery and recrystallization. Different heat treatment parameters were explored, namely, holding temperature and time, heating rate, and heating method. This approach revealed a competition between recrystallization and other microstructural evolution mechanisms, such as precipitation and porosity coalescence. An optimized heat treatment, allowing microstructural softening and adequate mechanical properties, was sought after. First, differential scanning calorimetry measurements applied to as-sprayed coatings enabled to identify recovery and recrystallization temperature ranges. Then, a variety of heat treatments was applied, involving long-time isothermal holdings as well as shorter cycles. Microstructure analysis and hardness measurements allowed making a first selection of treatment conditions.

Due to their excellent corrosion resistance, austenitic stainless steels are suitable for surface protection applications. However, the application potential is often limited by the low wear resistance. An interstitial hardening of the surface layer area can solve this problem for massive wrought alloys. Further potential for improvement lies in the transition to surface technology. For this purpose, powder feedstock of the stainless-steel grade AISI 316L was gas nitrocarburized at low temperatures. The formation of a metastable expanded austenitic phase was achieved. Subsequently, the processing was carried out by cold gas spraying. Due to the simultaneously high process kinetics and low thermal load, dense coatings were produced while maintaining the metastable state of the feedstock. When compared to solid reference systems, the scratch resistance saw a marked improvement.

The product quality of selective laser melting (SLM) is closely related to the alloy powder characteristics, including the size distribution and the oxygen content. In this work, the 316L stainless steel powder was prepared by a vacuum atomization furnace and sieved into a normal-sized distribution range from 15 to 53 μm with a median diameter of 37.4 μm, and a fine-sized distribution range from 10 to 38 μm with a median diameter of 18.9 μm. Then they were mixed with each other in different proportions. The results show that, under the condition of the same SLM parameters, the SLM part, with adding a large amount of fine-sized powder, has a lower density and strength, as well as more holes and spheroidized particles, compared with the SLM part with adding a small amount of finer-sized powder. Furthermore, the 316L stainless steel powder with a high oxygen content was prepared by a non-vacuum atomization furnace. Although the 316L stainless steel powder with a high oxygen content can be evenly spread in the SLM process, the surface layer of the powder is easy to form an oxide film during the cooling and solidification of powder inside the molten pool. Under the action of thermal stress, the small crack forms and expands along the oxide film, eventually leading to large cracks inside the melt channel.

A novel powder modification method based on the simultaneous softening and agglomeration of steel powders via heat treatment in a rotary tube furnace has been investigated as a means to improve the cold sprayability of H13 tool steel powder. By adjusting starting powder size and shape as well as heat treatment conditions (maximum temperature, cooling rate, and atmosphere), cold spray of H13 powder went from virtually no deposition to the production of thick dense deposits with a deposition efficiency of 70%. Powder agglomeration, surface state, microstructure evolution, and softening are identified as key factors determining powder deposition efficiency and resulting deposit microstructure.

In this study, two sizes of iron and stainless steel powders were binarily mixed into four groups with different weight percent fractions and the various mixtures and single-component powders were cold sprayed onto aluminum substrates. The deposition efficiencies (DE) of the powder mixtures and single-component powders were measured and are compared. The results show that the four binary mixtures exhibit different DE characteristics as a function of stainless steel wt% and that the small size mixtures have higher DE relative to the single-component iron powder. The difference is explained by particle-particle interactions (tamping and retention) that occur upon impact and only in the small size mixtures. The study also finds that changing spray parameters, such as feed rate, stand-off distance, gun travel speed, and gas temperature and pressure, has no effect on particle-particle interactions.

In this work, a 2D axisymmetric model of gas atomization at unsteady state that accounts for break-up and solidification is used to simulate laser melting of gas atomized powder. With an optimal nozzle width of 0.6-1 mm and a nozzle angle of 30-32°, the yield of fine 15-45 μm stainless steel powder, suitable for selective laser melting, is shown to increase from 20% to 35%. The effect of laser power on the melting channel width, microstructure, and mechanical properties of the sample is also investigated.

In this study, 43 μm 316L stainless steel and 23 μm commercial purity Fe feedstocks were used. The following coatings were made by cold spray: single component 316L, Fe, and their binary composites with nominal compositions of 20 wt.% Fe (20Fe), 50 wt.% Fe (50Fe) and 80 wt.% Fe (80Fe). The coatings were characterized (microstructure, flattening ratio, composition) and the cold sprayability metrics (DE, porosity, coating cohesion strength) were analyzed. Results show that the single component 316L coating has a much better DE and coating cohesion strength, and a slightly lower porosity as compared with the Fe coating, whereas all the composite coatings have the similar cohesion strength. Moreover, the 20Fe coating features the highest porosity and the lowest DE; 50Fe coating features the lowest porosity; and the 80Fe coating features the highest DE. To characterize the feedstock mixture composition, in addition to the usual approach of weight or volume fraction, the ratio of the 316L and Fe particle numbers in a mixture (i.e. particle number fraction), was calculated. Using this metric, the effects of the feedstock mixing composition on the cold sprayability of bimodal size 316L/Fe powder mixtures can be better explained.

Plasma spheroidization utilizing a plasma spray gun has been demonstrated for water-atomized stainless steel SUS316L powder. The angular particles were successfully spheroidized, and the D50 size of particles were decreased from 35 μm to 24 μm with 9 kW of processing power and to 15 μm with 17 kW of processing power. It was found that the high processing power of 17 kW generates a significant number of fine particles with the size of under 1 μm. By contrast, the powder formed on low processing power of 9 kW has better flow-ability and low cohesiveness, suggesting that an appropriate processing power exists to form the spherical powder suited for additive manufacturing

This paper considers the deposition of a commercial steel powder with a chemical composition that allows the coating to obtain an amorphous structure using thermal spray techniques. The processes used are characterized by high cooling speeds of the particles after the impact upon the substrate. The powders were sprayed with two different processes: cold gas spray (CGS) and high velocity oxyfuel (HVOF). A comparison between the samples obtained reveals that only the CGS coatings are completely amorphous; the HVOF samples exhibit nanocrystalline phases, detected with XRD analysis and SEM micrographs. Furthermore, the CGS coatings are more compact and show lower hardness with a comparable Young’s modulus. A hypothesis is that the formation of the amorphous structure is related to plastic deformation at impact (due to the high energy of the particles), rather than to the temperature; the mechanism could resemble that of a severe plastic deformation process. Additional thermal treatments and mechanical tests are in progress to investigate the toughness and other mechanical properties of the coatings.

In this work, single component 316L and Fe coatings, as well as mixed 316L/Fe coatings with a dual powder feeder to obtain various feedstock compositions, were deposited to measure the deposition efficiency (DE). Individual particle impact tests were performed on single component and composite coatings to understand the particle impact behaviors during deposition. Bond ratio (BR) were determined for the impact tests to correlate with the DE. Results show that the 316L powder has a better DE than Fe, whereas the DE of the mixed 316L/Fe powders increases with increasing feedstock Fe content. The BR results correspond well with the DE of single component powders and mixed powders. The BR of 316L impacts onto composite coatings decreases with increasing Fe content, while the BR of Fe impacts plateaus at a high value regardless of composite coating composition, which leads to the increase of overall mixture DE.

In this study, stainless steel powder is mixed with commercially pure iron and cold sprayed on steel in order to produce a metal composite with controlled properties. For these composites, porosity is very low, and annealing at 600-1100°C for an hour reduces it further. Annealing also sinters interparticle interfaces, leading to vastly improved fracture properties. Fully annealed single-component stainless steel exhibits a much higher strength than annealed CP iron, but adding just 20% stainless steel to iron produces a composite with the same fully annealed strength as that of stainless steel.

In this investigation, particle image velocimetry (PIV) and direct imaging are used to measure particle velocities during cold spraying. Four feedstock powders were sprayed, including Ni, WC-Co, carbonyl Fe, and Cr steel. Multiple exposures at 500 ns intervals were used to measure in-flight particle velocities via direct imaging with a high shutter speed camera. Velocimetry measurements were made with a double-pulse laser and a high-resolution camera. With the minimum frame straddling time set to 100 ns, a maximum particle velocity of 1052 m/s was measured.

Cold spray processing of stainless steel coatings, which represent a cost-effective method for wear and corrosion resistance, has been demonstrated as technically feasible. However, these coatings have very low tensile strength in the as-sprayed condition and may also exhibit a marginally higher wear rate. In this study, the cold spraying of 316L stainless steel coatings was investigated to assess the effect of powder size distribution and post-spray heat treatment on strength and wear properties. Coatings on aluminum and steel substrates were produced with a feedstock powder obtained in three particle size distributions. All coatings were deposited under the same conditions using nitrogen as the propellant gas, and then annealed at the optimum temperature. The microstructure and mechanical properties of both as-sprayed and heat-treated coatings were evaluated and the results are presented in the paper.

Thermal spraying (APS and HVOF) of an agglomerated nanostructured powder, based on the composition of a commercial martensitic steel, is introduced. The nanostructure of the produced powder is examined by means of microscopy and X-ray diffraction. The influence of the two different processes on crucial properties such as porosity, microhardness, adhesion, and wear resistance is studied. High wear resistance is noted for both coatings. The HVOF coating, especially, showed better wear performance in comparison with the APS coating and the bulk martensitic steel. The superiority of the HVOF coating over the APS coating regarding the aforementioned properties is attributed to a higher retention of the nanostructure of the starting powder, higher peening and relatively low oxidation.