Despite their light weight, 2.3 times lighter than Al, polymers are limited to application with low thermal, wear, and abrasion demands. The enhancement of the functional surfaces of the polymers using thermal spraying techniques is a challenging task due to the thermal degradation of polymers, the low wettability, and the disparate atomic properties. The twin-wire arc spraying (TWAS) process comprises two contradictory features. Almost all spraying particles are in a molten state on the one hand, and on the other hand, the spray plume has the lowest heat output among the different thermal spraying techniques. Therefore, it is a promising spraying technique for the required surface improvement. The surface of the 3D-printed parts was metalized using two successive layers. The first layer is a TWAS coating made of low-melting ZnAl 4 to avoid thermal degradation and provide a bond coat. The topcoat is also applied using a TWAS process and was made out of Ni-WC-Co as cored wires. The top hard coating has improved the wear resistance of the polymers by 14.6 times. The erosion of the coated and uncoated specimens was determined using a low-pressure cold gas spray gun. Ni-WC-Co coating led to more than five times higher erosion resistance.

Metal structures in offshore facilities are usually protected from corrosion using Zn-Al coatings even though they are subjected to collective stress conditions. This paper evaluates a post-treatment called machine hammer peening and its effect on surface finish, induced residual stresses, and near-surface microstructure of thermally sprayed ZnAl4 coatings. As expected, coating roughness was reduced from about Rz = 53.5 μm in the as-sprayed condition to 10.4 μm after treatment and coating densification was revealed in the near-surface zone. Residual stresses, which were surprisingly compressive in the as-sprayed condition, were likewise affected by the peening process, reaching a maximum of 200 MPa. The influence of peening direction and other such parameters were also investigated as part of the study.

In the present work, three different APS alumina coatings were fabricated using three fused and crushed alumina powders of different particle size fine, medium and coarse. The influence of the particle size on thermal properties and micro-structural features of the produced coating were investigated by thermal insulation test and detailed image analysis technique, respectively. The analyzed micro-structural features include the total porosity, pore size (fine, medium, and large) and cracks. All types of cracks were considered in calculations as voids and were evaluated according to their sizes as pores. All spray parameters except the particle size were fixed throughout the spraying process. The results revealed that the fine starting powder has produced the densest coating with the lowest total porosity and that the total porosity increases with an increasing particle size. This was expected as powders of smaller particle size will reach a higher in-flight temperature and velocity than powders of bigger particle sizes as long as the same spray parameters are applied. However, a detailed image analysis investigation on the three produced coatings showed that the fraction of fine pores and cracks versus the total porosity is substantially higher in coatings produced by using fine starting powders than those produced using medium and coarse powders. In this work, a connection between the thermal insulation and the porosity fraction, which includes fine pores and cracks, was revealed.

The super-elasticity behavior of a NiTi-shape memory alloy (SMA) is very promising regarding cavitation resistance. The need of high vacuum conditions by thermal spraying processes, to avoid oxidation, has always been and still is the main obstacle for the widespread of NiTi as a coating material. This work deals with studying the effect of the different shroud concepts on the obtained oxide content and the phases of the obtained twin wire arc sprayed (TWAS) coatings. The concepts include the use of argon as a shield in gas shroud (GS) as well as the use of an extended air cap attachment as a massive shroud (MS). The use of MS-concept led to a significant decrease in oxide content and therefore was selected to spray pre-alloyed NiTi-SMA wires. The standoff distance between the MS-outlet and the substrate surface shows also an effect on the obtained phases and thus on the behavior of the obtained coatings. At lower standoff distance a pseudo-elastic behavior was obtained and therefore a higher cavitation and wear resistance. The use of argon as atomization and shield gas with a massive shroud could be a cost-effective alternative for vacuum process in case of spraying NiTi-SMA pre-alloyed feedstock materials.

In this study, different air-cap configurations and shroud designs are developed and tested in twin wire arc spraying (TWAS) trails with the intent of increasing the collision probability of in-flight particles and thereby controlling coating composition and microstructure. In order to obtain greater insight on alloying effects and a better understanding of the coating build-up process, solid nickel and solid iron wires are used as feedstock materials and simultaneously sprayed on medium carbon steel substrates. The effect of polarity reversal and the use of a secondary atomization gas are also assessed. Detailed test results are presented in the paper along with in-depth analysis.

The computational fluid dynamic approach is adopted in this work, using L16-Taguchi matrix, to study the effect of different secondary atomization gas outlet configurations on the gas velocity, jet divergence, and pressure distribution at cap outlet. The spraying process variables that are integrated in this study are primary and secondary atomization gas pressure, PG and SG respectively. In addition, the geometrical variables of the SG air-cap like the position, the number and the angle of the outlet holes for SG are a part of the L16-Taguchi matrix. The effect of the process variables and geometrical design variations are analyzed on the obtained gas flow characteristics. Increasing the number of the SG outlet holes leads to a higher gas velocity at the cap outlet. The amount and the angle of the SG outlet holes have a direct effect on the plume divergence. The SG outlet angle determines the distance between the flow intersection point (PG-flow and SG-flow) and the air-cap outlet. Increasing the SG outlet angle leads to a reduction of the gas velocity. The use of Design of Experiment (DoE) in the optimization of the air-cap design by implementing CFD-simulation was proved to be a very useful and efficient tool to design high performance air-caps of twin-wire arc-spraying.

In this work, CFD simulations are used to evaluate air cap configurations for twin wire arc spraying (TWAS). Investigators employed a design of experiments (DoE) approach to identify air cap parameters with the greatest impact on gas velocity, jet convergence, and pressure distribution. The ones selected for study are the convergence angle, the length and diameter of the throat, and the distance between the air cap outlet and the point where the wires intersect. In all configurations studied, the spray wires deflected the flow of the primary gas and narrowed the cross-section of the plume along one axis. The effects of each air cap parameter are discussed in the paper along with possible design improvements.

The aim of this study is to determine how various factors, including process parameters and nozzle configurations, affect the shape and size of the spray jet in twin wire arc spraying. In the experiments, steel specimens were sprayed using an iron-based cored wire with a fused tungsten carbide filling. In-flight particle temperature and velocity and fluctuations in voltage and current were measured during spraying. The shape of the thermal spray spot and the 3D footprint of the plume were determined by means of image analysis and tactile surface profiling methods. The results obtained show that spray plume characteristics, and thus particle distribution, are heavily influenced by secondary gas flow, particularly the number, location, and angle of atomization outlet holes in the secondary gas nozzle.

The homogeneity of thermal spray plumes is mostly dependent on the type of feedstock used. Powdery feedstocks, for example, promote homogeneity. If in-flight particles are atomized from a melting bath, however, as in twin wire arc spraying (TWAS), the spray jet is less homogeneous due to the fact that particles are generated by the impingement of an airflow on the melting tips of electrically conducting wires. This work aims to contribute to the understanding of the initiation of such particles in the TWAS process. To that end, cored wires filled with W-rich particles were sprayed, then the process was halted and the wire tips were examined to analyze how the filling powder interacts with the melted part of the velum. 3D tomograms show that the resolidified melt bath is interspersed with spherical and irregular-shaped W-rich particles. The irregular shape implies a partial melting of the W-rich particles.

This paper presents a thickness measurement method that can be used during thermal spraying. The new method is based on photogrammetry and image reconstruction and is able to measure complex 3D shapes with continuous contours. Initial results demonstrate the nondestructive nature of the method as well as its accuracy, versatility, and speed.

This study demonstrates an experimental setup in which acoustic emission sensing is used to monitor a twin wire arc spraying (TWAS) process. Emitted acoustic signals were recorded by broadband sensors attached to the spray nozzle and mounted under the substrate. Sensor outputs were converted from the time domain to the frequency domain by fast Fourier analysis. Acoustic emission amplitude plots were produced and are correlated with gas pressure, arc voltage, in-flight particle velocity and temperature, coating thickness, and crack formation due to cooling.

Various approaches are presented in this paper to adapt conventional twin wire arc spraying (TWAS) for the production of smooth and finely structured coatings. Higher particle velocities were achieved by modifying spraying nozzle geometry. New geometries that incorporate a Laval shape produced the highest particle velocities while also eliminating overspray and extending the high-velocity region. This led to a more focused spraying plume and a change in optimal spraying distance, which was found to be more than 200 mm based on coating roughness. Some of the new nozzles exhibited evidence of particle deposition on the inner walls, which can restrict plume flow if not addressed. The problem is related to the position of the Laval throat in the spray plume as well as changes in gas pressure.

This study investigates the potential of external powder injection for producing functionally graded coatings by twin wire arc spraying. In spray trials, the position of the injection port was altered along the spray axis and perpendicular to the arc and different powders and carrier gases were used. Real-time images were captured by a high-speed camera during spraying to detect correlations between gas flow rates, hard particle wetting, and atomization of the molten pool. The optimal location for injection was found to be dependent on the size and density of the powder and the flow rate of the carrier gas. In the case of embedding B 4 C in a Fe-based matrix, a strong metallurgical bond was formed, confirming that powder injection is a viable approach for controlling the composition of twin wire arc sprayed coatings.

One of the greatest obstacles for a wide distribution of thermal spraying techniques is the lack of online control over the spraying process. The thermally sprayed coatings are optimized by an empirical modification of the spraying parameters and the subsequent correlation of these parameters to the obtained coatings. Some intrinsic parameters, such as the fluctuations in twin wire arc spraying and wear in the atomization nozzle, are not adjustable. Even though they have an enormous impact on the obtained coating quality, they are often scientifically neglected for reasons of simplification. In this work, acoustic emission analysis is utilized to study the effect of uncontrollable parameters on acoustic signals. In order to enable an easy determination of the changes in the acoustic signals, the acoustic sensors were mounted on the spraying nozzle as well as on the substrate. At increased current, a lower acoustic emission is recorded. A correlation between uncontrollable parameters, the acoustic signals, and the obtained coating quality was observed. This research contributes to the online control of the spraying process.

Owing to the arc ignition in twin wire arc spraying (TWAS) the wire tips are heated in three different zones. The outer part of the wire tips (contact zone) is heated directly by the arc ignition (zone I). The wires material in this zone becomes fully liquefied. The heat propagation phenomenon raises the temperature of the area immediately adjacent (towards the fed wires) generating a doughy material (zone II). Next to the doughy zone (towards the fed wires) the transferred heat softens the wire material causing a permanent deformation (zone III). The deformation is due to the exerted aerodynamic forces of the atomization gas pressure. A high speed imaging system was used to observe the melting behavior, metal break up, and particle formation under different operating conditions. The liquidus metal in zone I is directly atomized in the form of smaller droplets. Their size is a function of the specific properties of the molten metal and the exerting aerodynamic forces. The doughy area (zone II) is the origin of the extruded metal sheets at the anode and cathode side. The extruded metal sheets in case of cored wires are shorter than the ones observed by solid wires. The extruded metal sheets support the re-ignition of the arc and therefore enhance the process stability in twin wire arc spraying. In this study the effects of adjustable parameters and powder filling on melting behavior, particle formation and process instability were revealed and a comparison between solid and cored wires was made. The findings can improve the accuracy of TWAS process modeling and enhance the atomization of metal droplets through the adoption of specific nozzle geometry modifications.

The composition of the cored wires is inhomogeneous and contains solid velum as well as a powder filling which strongly influences the particle formation, in-flight particle behavior, the coating microstructure, and consequently the behavior of the desired coating. To study the effect of particle size distribution of the filling powder in cored wires the parameters of the twin wire arc spraying process such as current, voltage, and atomization air pressure are changed for different intervals of particle size distributions (-45µm+25µm, and -95µm+63µm). Fluctuations in arc voltage and current are measured and found to be higher at smaller particle sizes. The characteristics of inflight particles showed a higher particle velocity in case of smaller particle sizes. The particle temperature is higher in case of bigger particle sizes. The splats tend to form a pancake shape in case of smaller particle sizes. Therefore, the lamellas are more homogenous and the porosity is low. This investigation is important for deep understanding of twin wire arc spraying with cored wires.

Thermal spraying is a material processing technique, which is based on the combination of thermal and kinetic energy. The used feedstock is melted in a hot flame and the melt is atomized and accelerated by means of atomization or process gases. The formed particles are rapidly solidified and consolidate to form splats as they hit a pre-treated substrate. The splats pile one-on-top-of-other forming lamellas and creating the final coating. In the work presented here a combination of cored wire (WC as filling powder) and massive wire (copper) were simultaneously sprayed using the twin wire arc spraying (TWAS) process. 3D micro tomography was used in order to gain knowledge about splat formation and layer build-up. Due to the high attenuation coefficient of tungsten in comparison with copper and carbon tungsten-rich particles and splats can easily be spotted in the tomogram of the coating layer. It turns out that besides irregular formed flat splats also ball-shaped particles exist in the coating layer which suggests that the spherical particles impacted on the substrate in an un-molten state. By 3D data processing tungsten-rich particles were visualized to analyze their spatial distributions as well as their geometric parameters were quantified. This work aims at contributing to the understanding of spraying processes.

Functionally graded coatings (FGC), in which the composition and properties vary gradually from the bond layer to the top layer, are introduced in this study. Using a mixing nozzle, a graded coating was generated employing twin wire spraying (TWAS) process. The deposition started with a base layer of massive metallic wires for better adhesion to the substrate. At following top layers hard material, in form of powders, was injected to the massive wires to enhance the wear protection. The results show that microstructure, porosity, and compositions are gradually varied in the coatings. This is a clear indication for the better performance in as-sprayed FGC than coatings sprayed by means of cored wires. The approach was to grade the coating composition from pure metallic to composite with higher content of hard material particles. The goal of the study is to articulate the needed coating performances by customizing the layers deposited regarding to their position.

Computer simulations of thermal spray coating processes can accelerate the development of new products by minimizing the need for prototypes. An important aim of these simulations is the calculation of the coating distribution on the surface of a given workpiece with respect to a given movement path of the spray gun. In this paper, a novel approach for computing coating distribution on arbitrarily complex freeform surfaces is presented. In contrast to approaches that implement symmetric deposition models, the presented concept is based on a rotationally asymmetric model, making it particularly well suited for wire arc spraying as turbulences caused by electrodes in the gas flow often result in asymmetric coating distributions. In order to obtain the required knowledge base to derive the deposition model and verify the simulation, basic experiments were made. The simulation concept and experimental setup are presented in the paper along with the results.

The intent of this study is to gain a better understanding of the twin wire arc spray process, particularly the correlation between spray path or gun movement and coating quality. Coatings were deposited on steel substrates using solid nickel and cored WC-Cr wire while moving the gun in various directions relative to the orientation of the electrodes. The coatings obtained are evaluated based on porosity, hardness, thickness, adhesion strength, and microstructure. It is observed that gun movements cause inhomogeneities in the spray cone that affect different coating properties in different ways depending on the direction of the movement and gas pressure.