Twin Wire Arc Spraying Process Research Articles

Influence of Zn and Mg alloying elements on the mechanical properties of Al coating deposited via twin wire arc spray process

Protective coating has been one of the most effective corrosion mitigation methods and utilized to protect steel substrates for decades. Among various coating techniques, the twin wire arc spray (TWAS) process has been widely adopted in surface engineering to protect components due to its cost efficiency and flexibility. Apart from the coating parameters, coating composition plays a crucial role in obtaining superior coating properties. Previous studies have demonstrated that Al-Zn coating produced high roughness and low adhesion to substrates. In this study, two different wires inputs; Al-Mg and Zn were used to produce a pseudo-alloy of Al-Zn-Mg alloy coating on mild steel substrates where the addition of Zn and Mg elements have been the focus. Stand-off distance (SOD) parameters were first varied at 100, 200, 300, and 500 mm and it was found that coatings produced at 100 mm SOD produced the best quality thus were further investigated in this study. The effect on the properties of developed coating layer on microstructure, thickness, hardness, roughness, and adhesion was investigated. The microstructure and morphological analyses were conducted using SEM and the phase analysis using XRD. Results have shown that the Al-Zn-Mg coating prepared has demonstrated the best coating quality compared to pure Al coating. The average thickness of the coating at 100 mm SOD was recorded to be the lowest at 250 μm with the highest hardness and adhesion values, 135 Hv and 9.3 MPa, respectively and the lowest surface roughness of 5.8 µm.

Evaluation of the Mechanical and Corrosion Behavior of Twin Wire Arc Sprayed Ni-Al Coatings with Different Al and Mo Content

In this study, the surface properties of marine structures were improved by applying a twin wire arc spray process to high-strength low alloys. The effect of Al and Mo contents in Ni-Al coatings on their mechanical and corrosion behaviors was analyzed using hardness tests, electrochemical experiments, and immersion tests. The increase in Al content resulted in the formation of oxides and intermetallic compounds, leading to a significant improvement in the mechanical properties by approximately 222 HV. Despite a fine galvanic phenomenon causing a decrease in corrosion resistance by up to 8.91%, a higher Al content demonstrated the highest corrosion resistance after high-temperature exposure, with an enhancement of approximately 20.9%, attributed to the formation of an oxide film generated by intermetallic phase transformation. However, the mechanical properties experienced a reduction of 134.3 HV. This study demonstrated a correlation between the microstructure of the coating layers that form passivation films and their respective mechanical and corrosion properties. It also revealed that the content of Al and Mo significantly affects the mechanical and corrosion behavior of Ni-Al coatings.

Influence of twin wire arc spraying and machine hammer peening on the corrosion fatigue of ZnAl4 coatings on S355 J2C + C substrate

ZnAl-based coatings are used as corrosion protection for offshore application, e.g., offshore wind turbines. These applications are subjected to superimposed mechanical and corrosive stresses, e.g., wind, tide, seawater and mist. Thermally sprayed coatings processed by twin wire arc spray process and the mechanical post-treatment process machine hammer peening (MHP) were investigated. The coating consisting of ZnAl4 was deposited on a S355 J2C + C substrate. In addition to two conditions produced with different process parameters during MHP post-treatment, uncoated and as-sprayed conditions were evaluated as reference. The aim was to determine the effect of the coating and its properties, such as hardness, porosity and roughness, on the corrosion and corrosion fatigue behavior. In this study, potentiodynamic polarization tests and instrumented corrosion fatigue tests using a self-designed corrosion cell were performed in 3.5% NaCl solution. Mechanical and electrochemical measurement techniques in combination with fractographic and microstructural analyses were used to characterize corrosion fatigue damage mechanisms.MHP resulted in a uniform coating thickness with lower roughness and porosity in comparison to the as-sprayed coatings, which were further reduced by adjusting the process parameters of MHP. The electrochemical potential of the ZnAl coating provides passive corrosion protection for the substrate. The coating system improved corrosion fatigue resistance at high cycle fatigue regime. While as-sprayed and MHP post-treated specimens run out at 280 MPa, uncoated specimens run out at 240 MPa. The open circuit potential (OCP) curves plotted during the corrosion fatigue tests can be correlated with the damage mechanisms. Crack initiation and propagation in the coating and crack propagation in the substrate can be identified from the OCP curves.

The Effect of Argon as Atomization Gas on the Microstructure, Machine Hammer Peening Post-Treatment, and Corrosion Behavior of Twin Wire Arc Sprayed (TWAS) ZnAl4 Coatings

In the twin wire arc spraying (TWAS) process, it is common to use compressed air as atomizing gas. Nitrogen or argon also are used to reduce oxidation and improve coating performance. The heat required to melt the feedstock material depends on the electrical conductivity of the wires used and the ionization energy of both the feedstock material and atomization gas. In the case of ZnAl4, no phase changes were recorded in the obtained coatings by using either compressed air or argon as atomization gas. This fact has led to the assumption that the melting behavior of ZnAl4 with its low melting and evaporating temperature is different from materials with a higher melting point, such as Fe and Ni, which also explains the unexpected compressive residual stresses in the as-sprayed conditions. The heavier atomization gas, argon, led to slightly higher compressive stresses and oxide content. Compressed air as atomization gas led to lower porosity, decreased surface roughness, and better corrosion resistance. In the case of argon, Al precipitated in the form of small particles. The post-treatment machine hammer peening (MHP) has induced horizontal cracks in compressed air sprayed coatings. These cracks were mainly initiated in the oxidized Al phase.

Advanced corrosion resistant cylinder-bore coatings

This paper presents a novel approach to enhance the corrosion resistance of cylinder-bore coatings for internal combustion engines by changing surface characteristics and microstructures of the coating and utilizing alternative materials. The approach of this study refers to the knowledge of the complex gas-flow conditions during the internal-diameter coating of cylinder-bores using a twin-wire arc-spray process. Selective influencing of the gas flow enables the preparation of sample coatings with inhomogeneous and more homogeneous microstructures and surfaces. The various coatings were exposed to corrosive environments using the Kesternich testing method and an alternating immersion test. Significant improvements in corrosion resistance have been demonstrated by up to 50 % through altering the microstructure of the coating and almost 100 % by additional changing the spraying material. The study reveals an increased protection of cylinder-bore coatings against corrosive attacks through a homogeneous microstructure prepared in oxygen-poor coating conditions.

Metallization of Porous Polyethylene Using a Wire-Arc Spray Process for Heat Transfer Applications.

Metallization of polyethylene (PE) using thermal spray techniques has proved difficult due to its low melting point and softness. In this study, metallic coatings were applied on porous polyethylene substrates using a twin wire-arc spray process. Commercially available polyethylene sheets, 3 mm in thickness, were used as substrates. Copper (Cu), aluminum (Al), and zinc (Zn) were successfully deposited on the porous polymer, without prior surface preparation, to form coatings with thickness of about 400 µm. Coating surface morphology and cross-sections were examined using a scanning electron microscope. Individual metal splats on the porous and non-porous substrates were observed to study the differences in the bonding mechanisms. The adhesion strength and electrical resistivity of the coatings on porous PE were evaluated. It was found that the bond strength of all three metallic coatings was found to be higher than the ultimate fracture strength of the porous. These results suggest that porosity in the polymer helps to overcome the challenges of metallizing polyethylene and provides a significant reduction in the weight of the polymer. Therefore, all these properties aided in fabricating an extremely lightweight, composite material with potential use in thermal management applications.

In Situ Acoustic Monitoring of Thermal Spray Process Using High-Frequency Impulse Measurements

In order to guarantee their protective function, thermal spray coatings must be free from cracks, which expose the substrate surface to, e.g., corrosive media. Cracks in thermal spray coatings are usually formed because of tensile residual stresses. Most commonly, the crack occurrence is determined after the thermal spraying process by examination of metallographic cross sections of the coating. Recent efforts focus on in situ monitoring of crack formation by means of acoustic emission analysis. However, the acoustic signals related to crack propagation can be absorbed by the noise of the thermal spraying process. In this work, a high-frequency impulse measurement technique was applied to separate different acoustic sources by visualizing the characteristic signal of crack formation via quasi-real-time Fourier analysis. The investigations were carried out on a twin wire arc spraying process, utilizing FeCrBSi as a coating material. The impact of the process parameters on the acoustic emission spectrum was studied. Acoustic emission analysis enables to obtain global and integral information on the formed cracks. The coating morphology and coating defects were inspected using light microscopy on metallographic cross sections. Additionally, the resulting crack patterns were imaged in 3D by means of x-ray microtomography.

Process Parameter Settings and Their Effect on Residual Stresses in WC/W2C Reinforced Iron-Based Arc Sprayed Coatings

Residual stresses have been a major source of concern, as they are an inevitable consequence of manufacturing and fabrication processes. The magnitude of these stresses is often as high as, or at least, comparable to the yield strength of the material. In terms of arc sprayed coatings, the utilization of bore hole drilling methods presents some practical disadvantages as mechanical parameters (Poisson’s ratio, Young’s modulus) need to be identified in order to determine the residual stress distribution. Curvature techniques using Almen strips are cost- and time-effective methods that can be used for analytical quality assurance. Within the scope of this work, a quantitative study of the amount of residual stresses induced in a twin wire arc spraying (TWAS) process for a given combination of process parameters was conducted using the incremental bore hole drilling method, as well as the curvature method including Almen strips. Therefore, the effect of the primary gas pressure, substrate preheating temperature, and handling parameters, such as the spray angle and gun velocity, which influence the coating deposition as well as the heat input into the substrate, are examined. The experiments were carried out by using an iron-based cored wire with cast tungsten carbides as filling. The results of both methods are in an acceptable accordance with each other. Different stress fields were observed depending on the parameter settings.

A Study on the Tribological Behavior of Vanadium-Doped Arc Sprayed Coatings

The formation of thin reactive films in sliding contacts under elevated temperature provides enhanced tribological properties since the formation of Magneli phases leads to the ability of self-lubricating behavior. This phenomenon was studied for vanadium-doped coating systems which were produced using CVD and PVD technology. Vanadium-containing arc sprayed coatings were not widely examined so far. The aim of this study was to characterize Fe-V coatings deposited by the Twin Wire Arc Spraying process with respect to their oxidation behavior at elevated temperatures and to correlate the formation of oxides to the tribological properties. Dry sliding experiments were performed in the temperature range between 25 and 750 °C. The Fe-V coating possesses a reduced coefficient of friction and wear coefficient (k) at 650 and 750 °C, which were significant lower when compared to conventional Fe-based coatings. The evolution of oxide phases was identified in situ by x-ray diffraction for the investigated temperature range. Further oxidation of (pre-oxidized) arc sprayed Fe-V coatings, as verified by differential thermal analysis and thermo-gravimetric analysis, starts at about 500 °C.

Established and Adapted Diagnostic Tools for Investigation of a Special Twin-Wire Arc Spraying Process

In the LDS® (Lichtbogendrahtspritzen) process, a twin-wire arc spraying (TWAS) process developed by Daimler AG, the gas injection and feed to the arc play a crucial role in separating the molten particles from the wire ends. This paper describes an investigation of the gas and particle behavior according to individual LDS® process parameters. Coating problems are not considered. The measurements are separated into two different parts: “cold” (without arc and particles) and “hot” (with arc and particles). The results provide the first detailed understanding of the effect of different LDS® process parameters. A correlation between the gas parameter settings and the particle beam properties was found. Using established and adapted diagnostic tools, as also applied for conventional TWAS processes, this special LDS® process was investigated and the results (gas and particle behavior) validated, thereby allowing explanation and comparison of the diagnostic methods, which is the main focus of this paper. Based on error analysis, individual instabilities, limits, and deviations during the gas determinations and particle measurements are explained in more detail. The paper concludes with presentation of the first particle-shadow diagnostic results and main statements regarding these investigations.

Wear behavior of thermally sprayed Zn/15Al, Al and Inconel 625 coatings on carbon steel

Abstract In this study, Zn/15Al, Al and Inconel 625 coatings were applied on low carbon steel substrates by twin wire arc spray process as a corrosion and wear protection barrier against the marine atmosphere. The wear behavior of the coating layer was characterized by a pin-on-disk apparatus at a combination condition of three normal loads and two sliding speeds. The wear tests were conducted in a 3.5 wt.% NaCl solution to evaluate the wear resistance of the coatings in a corrosive environment. The results suggested that the Inconel 625 coating had a better wear resistance in a 3.5 wt.% NaCl solution as compared to the ZnAl and Al specimens. Zn/15Al coatings revealed less wear resistant in a corrosive environment due to the rupture of the oxide film on the coating surface during wear. For the sacrificial protection coatings of Zn/15Al and Al, the characteristics of surface oxide film play a vital role in the wear resistance of the coating materials.

Twin wire arc spray process modification for production of porous metallic coatings

Abstract One approach to protecting surfaces exposed to high temperature gas flows is to form a boundary layer of cooler gas along the surface, typically by allowing cool gas to flow from internal passages through a pattern of holes drilled in the surface of the component. The production of this pattern of fine holes is time consuming and expensive. In this study we have explored the feasibility of creating high fractions of open porosity extending through the outer face of a metallic foam core sandwich structure heat shield by controlling the porosity content of the thermally sprayed faces. The faces were fabricated by twin wire arc spraying of alloy 625 on the filled surface of the metallic foam core. Modification of the spray process was required to obtain the desired level of open porosity in the coatings, since manipulation of the twin wire arc gun operating parameters alone was not sufficient. An external nozzle was mounted in line with the gun axis to spray polyester powder particles, as a pore generating agent, onto the substrate surface during coating deposition. The effects of twin wire arc gun operating parameters and the polyester powder spray conditions were studied using statistical design of experiments to increase the porosity of the deposits. It was found that the feeding rate of the polyester powder is the most significant factor affecting the coating porosity content, while the gun operating parameters have a relatively small influence. Embedding polyester particles in the coating by spraying through the external nozzle resulted in a pore content of up to approximately 20%.

Controlling the Twin Wire Arc Spray Process Using Artificial Neural Networks (ANN)

One approach for controlling the twin wire arc spray process is to use optical properties of the particle beam as input parameters for a process control. The idea is that changes in the process like eroded contact nozzles or variations of current, voltage, and/or atomizing gas pressure may be detected through observation of the particle beam. It can be assumed that if these properties deviate significantly from those obtained from a beam recorded for an optimal coating process, the spray particle and thus the coating properties change significantly. The goal is to detect these deviations and compensate the occurring errors by adjusting appropriate process parameters for the wire arc spray unit. One method for monitoring optical properties is to apply the diagnostic system particle flux imaging (PFI): PFI fits an ellipse to an image of a particle beam thereby defining easy to analyze characteristical parameters by relating optical beam properties to ellipse parameters. Using artificial neural networks (ANN), mathematical relations between ellipse and process parameters can be defined. It will be shown that in the case of a process disturbance through the use of an ANN-based control new process parameters can be computed to compensate particle beam deviations.

Tribological properties of twin wire arc spray coated aluminum cylinder liner

Abstract The effects of honing process on the friction and wear behaviors of Twin Wire Arc Spray (TWAS) coated aluminum cylinder liners were investigated using a pin-on-reciprocating type of a tribotester. Two types of coated cylinder liners were prepared for the tests: Type I — smooth honing (SH) with ~ 40° honing angle, and Type II — helical structure honing (HSH) with ~ 140° honing angle. The aluminum cylinder liners were coated with an Fe0.8C wire by the TWAS process. In un-lubricated condition, Type II specimen showed lower coefficient of friction (COF) compared to Type I specimen. This result was due to the fact that the groove of Type II was sufficiently large to trap the wear particles that may otherwise contribute to three body abrasive wear. In lubricated condition, Type I showed lower COF due to its lower roughness in comparison to Type II. The experimental results indicate that TWAS process can be effectively utilized for engine applications in conjunction with optimum honing process for the cylinder liner.

Studying the Effect of the Air-Cap Configuration in Twin-Wire Arc-Spraying Process on the Obtained Flow Characteristics Using Design of Experiment Oriented Fluid Simulation

The computational fluid dynamics approach is adopted in this work using the design of experiments to reveal the effect of the air-cap configurations on the obtained gas velocity, the shear stresses, the high velocity zone, and the convergence of the obtained spraying plume in the twin-wire arc-spraying process. The parameters, which are revealed to optimize the air-cap configuration, are the throat diameter, the convergence angle of the throat inlet, the throat length, and the distance between the throat outlet and the intersection point of the approaching wires. The throat length is dependent upon the other configuration parameters. Outlet gas velocity, the turbulence in the flow, and the exerted shear stresses at wire tips are directly affected by the dominating flow regimes near the intersection point of the approaching wires. The presence of wires and the contact tips in the gas flow has enormous impact on the obtained flow characteristics. Air-cap throat diameter and the distance between throat outlet and intersection point determine the shape and length of the obtained high velocity zone in the spraying plum.

Effect of Porosity Content of Arc-Sprayed Alloy 625 Skins on the Flexural Behavior of Nickel Foam Core Sandwich Structures

Metallic foam core sandwich structures have been of particular interest for engineering applications in recent decades because of their unique mechanical and physical properties. Thermal spraying techniques have been recently introduced as a novel low-cost method for production of these structures with complex shapes. One of the potential applications of the metallic foam core sandwich structures prepared by thermal spray techniques is as heat shield devices. Open porosity in the microstructure of the coating may allow the cooling efficiency of the heat shield to be improved through the film cooling phenomenon. A modified twin wire-arc spraying process was employed to deposit high temperature resistant alloy 625 coatings with a high percentage of the open porosity. The effect of skin porosity on the mechanical properties (flexural rigidity) of the sandwich structures was studied using a four-point bending test. It was concluded from the four-point bending test results that increase in the porosity content of the coatings leads to decrease in the flexural rigidity of the sandwich panels. The ductility of the porous and conventional arc-sprayed alloy 625 coatings was improved after heat treatment at 1100 °C for 3 h.

3D µCT and SEM Analysis of Resolidified Tips of Cored Wires Used in Twin-Wire Arc Spraying

In twin-wire arc spraying (TWAS), the in-flight particles are atomized from a melting bath which generates an inhomogeneous spraying plume. This inhomogeneity is due to the fact that these particles are generated by the impingement of fast continuous flowing air upon the melting tips of electrically conductive wires. This work aims to contribute to the understanding of the initiation of such particles in the TWAS process. For this purpose, cored wires filled with W-rich particles were sprayed. After interrupting the TWAS process, the tips of these cored wires were imaged by 3D µCT and scanning electron microscopy in order to analyze how the filling powder interacts with the melted part of the sheath. The analysis of the 3D tomograms shows that the resolidified melting bath of the cored wires is interspersed with both spherical and irregular-shaped W-rich particles. This irregular shape suggests a partial melting of the W-rich particles.

Quality Designed Twin Wire Arc Spraying of Aluminum Bores

After 125 years of development in combustion engines, the attractiveness of these powerplants still gains a great deal of attention. The efficiency of engines has been increased continuously through numerous innovations during the last years. Especially in the field of motor engineering, consequent friction optimization leads to cost-effective fuel consumption advantages and a CO2 reduction. This is the motivation and adjusting lever of NANOSLIDE® from Mercedes-Benz. The twin wire arc-spraying process of the aluminum bore creates a thin, iron-carbon-alloyed coating which is surface-finished through honing. Due to the continuous development in engines, the coating strategies must be adapted in parallel to achieve a quality-conformed coating result. The most important factors to this end are the controlled indemnification of a minimal coating thickness and a homogeneous coating deposition of the complete bore. A specific system enables the measuring and adjusting of the part and the central plunging of the coating torch into the bore to achieve a homogeneous coating thickness. Before and after measurement of the bore diameter enables conclusions about the coating thickness. A software tool specifically developed for coating deposition can transfer this information to a model that predicts the coating deposition as a function of the coating strategy.

Metal Matrix Composites Deposition in Twin Wire Arc Spraying Utilizing an External Powder Injection Composition

The powder injection parameters, the location of the injection port, as well as the metal matrix composites are important features, which determine the deposition efficiency and embedding behavior of hard materials in the surrounding matrix of the twin wire arc-spraying process. This study investigates the applicability of external powder injection and aims to determine whether the powder injection parameters, the location, and the material combination (composition of the matrix as well as hard material) need to be specifically tailored. Therefore, the position of the injection port in relation to the arc zone was altered along the spraying axis and perpendicular to the arc. The axial position of the injection port determines the thermal activation of the injected powder. An injection behind the arc, close to the nozzle outlet, seems to enhance the thermal activation. The optimal injection positions of different hard materials in combination with zinc-, nickel- and iron-based matrices were found to be closer to the arc zone utilizing a high-speed camera system. The powder size, the mass of the particle, the carrier gas flow, and the electric insulation of the hard material affect the perpendicular position of the radial injection port. These findings show that the local powder injection, the wetting behavior of particles in the realm of the molten pool as well as the atomization behavior of the molten pool all affect the embedding behavior of the hard material in the surrounded metallic matrix. Hardness measurement by means of nanoindentation and EDX analysis along transition zones were utilized to estimate the bonding strength. The observation of a diffusion zone indicates a strong metallurgical bonding for boron carbides embedded in steel matrix.

Adapting of the Background-Oriented Schlieren (BOS) Technique in the Characterization of the Flow Regimes in Thermal Spraying Processes

In thermal spraying technique, the changes in the in-flight particle velocities are considered to be only a function of the drag forces caused by the dominating flow regimes in the spray jet. Therefore, the correct understanding of the aerodynamic phenomena occurred at nozzle out let and at the substrate interface is an important task in the targeted improvement in the nozzle and air-cap design as well as in the spraying process in total. The presented work deals with the adapting of an innovative technique for the flow characterization called background-oriented Schlieren. The flow regimes in twin wire arc spraying (TWAS) and high velocity oxygen fuel (HVOF) were analyzed with this technique. The interfering of the atomization gas flow with the intersected wires causes in case of TWAS process a deformation of the jet shape. It leads also to areas with different aero dynamic forces. The configurations of the outlet air-caps in TWAS effect predominantly the outlet flow characteristics. The ratio between fuel and oxygen determine the dominating flow regimes in the HVOF spraying jet. Enhanced understanding of the aerodynamics at outlet and at the substrate interface could lead to a targeted improvement in thermal spraying processes.