Twin Wire Arc Spraying Research Articles

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.

Properties of protective NiCr coatings, deposited by Cold Spray technology

The NiCr coatings, deposited by modern Cold Spray technology, and by well-established Twin Wire Arc Spraying and High Velocity Oxygen Fuel technologies, was studied in terms of microstructure, mechanical, tribological and corrosion properties. Compared to other thermal spraying technologies, CS coatings achieve higher cohesive strength and associated tribological properties. The dense structure is responsible for the coatings’ high corrosion protection ability. The influence of CS technological parameters was found negligible, except the usage of nozzle for internal diameter spraying, which results to significant increase of coatings porosity.

Comparison of high temperature corrosion resistance of NiCr-based coatings applied by High Velocity Oxygen Fuel and Twin Arc Spray processes

High temperature corrosion in a molten sodium sulfate salt environment is a serious problem of coal-fired boiler components. The promising possibility for surface protection is to apply thermally sprayed coatings. However, the key to success is to choose the optimal material and the proper spraying technology for a given application. Therefore, this study is aimed at a comparison of the high temperature behaviour of two protecting NiCr-based coatings sprayed by different technologies on 1.4903 (W.nr.) chrome-molybdenum steel. NiCrMoWFe was deposited by High Velocity Oxygen Fuel technology (HVOF). Twin wire arc spray technology (TWAS) was used to deposit multi-layered “multicoat” consisting of NiCrMoNbTa and NiCrTi with ceramic overlay Armaguard coated by brush. Chrome-molybdenum steel 1.7715 (W.nr.) commonly used in coal-fired boiler was also exposed to the high temperature corrosion test to evaluate the potential benefit of the coating. Coated as well as the uncoated specimens were exposed to the molten salt environment 18% Na2SO4 and 82% Fe2(SO4)3 at 690 °C under cyclic condition. The thermogravimetric approach was used to determine kinetics of corrosion. The X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyze the corrosion products and determine the corrosion mechanism. The results of high temperature corrosion resistance show that NiCrMoWFe coated by HVOF and multicoat sprayed by TWAS was comparable.

Comparative investigation on thermal corrosion of alloy coatings in simulated waste incinerator environment

The thermal corrosion test and ash adhesion test were performed in this work to quantify the corrosion resistance of coatings prepared by plasma transferred arc welding (PTA), twin wire arc spray (TWAS) and high-velocity oxygen-fuel spray (HVOF) technologies. Results showed that TWAS coatings would fall off above 550 °C. PTA and HVOF coatings were excellent even at 650 °C. For HVOF-NiCr coatings, as Cr content increased from 7.22 % to 21.55 %, the corrosion resistance was enhanced linearly. This study revealed the anti-corrosion mechanism and provided a guidance for coating application in boilers.

Prediction of Thermal Spray Coatings Performance in Marine Environments by Combination of Laboratory and Field Tests

Cost-effective corrosion mitigation of offshore steel structures can be achieved by thermal spray coatings. These coatings, when comprised of Al, Zn and their alloys, provide a physical barrier against the environment when intact, and cathodic protection to underlying steel when damaged. Due to the complexity of marine environments, laboratory tests should be combined with field work in order to understand the corrosion protection offered by these coatings. The work presented here was carried out with thermal spray coatings of aluminum alloys (AA1050, AA1100, Al-5Mg) and Zn-15Al prepared by Twin Wire Arc Spray onto low carbon steel substrates. The resulting coatings were ~300 μm in thickness, and 5% of surface area defects were artificially machined in order to expose the steel substrate, simulating mechanical damage or erosion of the coating. Electrochemical data collected over a 90 days period showed a good correlation between laboratory and real marine environment results. Aluminum alloys showed better corrosion protection in fully immersed conditions, while zinc alloys performed better in atmospheric and splash zones. Overall, these results aim to improve design of thermal spray coatings to protect carbon steel in marine environments.

The FeCr-Based Coating by On-Site Twin-Wire Arc Spraying for Proactive Maintenance of Power Plant Components

There is high demand for refurbishment of damaged mechanical components in power plants. Twin-wire arc spraying (TWAS) is promising for on-site proactive maintenance. In this paper, FeCr-based coating was prepared on AISI 1020 carbon steel substrate by TWAS. The coating microstructure was characterized in terms of porosity, oxides and constituent phases. The wear behavior of the coating was evaluated by ball-on-disk wear test at both room temperature (RT) and tube service temperature of 300 °C. The results show that the coating is characterized with composite-like structure of oxide boundaries and FeCr solid solution, with hard phase of borides embedded within FeCr phase, resulting an average microhardness of 10.80 GPa. Tested at RT and 300 °C, the weight loss of the coating indicates much better wear resistance than substrate steel. The morphology of the wear scars demonstrates that the wear mechanism of the coating transformed from furrow and fracture spalling at RT to adhesive and abrasive wear at high temperature. The high hardness and low oxidation rate of the coating contribute to excellent wear resistant. The FeCr-based coating by on-site TWAS has been successfully applied on boiler tubes, large-sized blades of slurry pumps and various fans.

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.

Application of flash-pulse thermography methods for quantitative thickness inspection of coatings made by different thermal spraying technologies

Flash-pulse thermography is a nondestructive testing method, which is based on thermal excitation and response analysis of a tested sample. It is the traditional method for a non-destructive inspection of discontinuities and inhomogeneities at a sample surface, which is also applicable for a qualitative inspection of coating thickness variations. Possibilities of a quantitative inspection of the coating thickness are investigated numerically and experimentally in this contribution. Pulse-phase and power-function methods are compared and an ambiguity of the pulse-phase evaluation of coating thickness is presented on numerical computation results. Measurement and evaluation procedures are introduced and inspection results are presented for Fe-Cr, Sn-Sb-Cu and WC-CoCr coatings made by TWAS (Twin Wire Arc Spray), flame spraying and HVOF (High Velocity Oxygen Fuel) technologies, respectively. Sensitivity of a response of the power-function method to a change of thickness of the inspected coatings in a connection with their structure is presented. It is demonstrated, that a sensitivity of about 10 μm can be achieved for the analyzed coatings of a thickness in the range 74–1240 μm.

Effect of spraying parameters on surface roughness, deposition efficiency, and microstructure of electric arc sprayed brass coating

Thermal, chemical, and metallurgical surface treatment techniques are being applied to give materials the desirable properties for numerous applications in service. Nowadays, electric arc spraying of copper alloys has gained importance in surface engineering owing to their excellent mechanical properties. However, electric arc sprayed metallic coating usually has high coating roughness and low deposition efficiency upon adherence to the substrate. In this article, the brass coating was deposited onto a mild steel substrate using a twin wire arc spraying technique. The spraying parameters current, voltage, and spray off distance were varied during the deposition process. To gain an understanding, the effect of these spraying parameters on the characteristics of the coating was investigated in terms of its surface roughness, deposition efficiency, and microstructure. The primary purpose of this research is to optimize the process in terms of coating roughness and deposition efficiency. Based on the results obtained, it was observed that an increase in voltage and spray off distance resulted in an increase in coating roughness, whilst an increase in current resulted in a decrease in coating roughness. Deposition efficiency was also greatly affected by the process parameters and decreased significantly with the longer spray off distance. Furthermore, the deposition efficiency increased with the decrease in surface roughness and vice versa. The best results in terms of low coating roughness, high deposition efficiency, and optimized microstructure were found at 24V, 240A, and 100 mm. The use of small diameter feedstock, supersonic nozzle incorporated in equipment, better surface preparation, and optimized process parameters gave better results as compared to the previous study in the field of electric arc spray coating.

Thermal spray deposition of aluminum and zinc coatings on thermoplastics

Coatings of aluminum and zinc were applied on two thermoplastic materials, ultra-high molecular weight polyethylene (PE) and polytetrafluoroethylene (PTFE), using twin wire-arc spray spraying. Samples were either smooth (Ra = 0.2 μm) or roughened by grit blasting (Ra = 1.6 μm). Before the start of spraying samples were kept at either room temperature or preheated to 55 °C for PE and 95 °C for PTFE, so that the peak temperature during spraying reached the glass transition temperature of the polymer. Surface temperature was monitored during spraying using thermocouples. Coating adhesion strength was measured using pull tests. Single splats of metal on samples were examined using SEM images. Zinc coatings, about 260 ± 20 μm thick, formed on both polymer samples irrespective of their surface roughness. Adhesion strength was significantly higher on rough surfaces than smooth surfaces. Increasing the initial sample temperature also enhanced adhesion strength. Aluminum adhered only to the PTFE but not the PE. Aluminum droplets have a higher melting point than zinc and melted the sample upon impact, inhibiting mechanical bonding. These results suggest that the metal adhesion to thermoplastics can be significantly improved by pre-heating polymer samples prior to thermal spraying.

CFD Enhanced Thermal Spray Process for Coating of Cylinder Bores of Car Engines

As the automotive industry shifts toward hybrid and alternative drive concepts like electromobility, it remains vital to continuously improve internal combustion technology. Internal combustion applications represent the largest portion of transportation technologies. Continuous development and improvement of passenger car engines focus on reducing emission by weight reduction and enhanced efficiency. To gain a competitive advantage in present and future engines, Mercedes-Benz AG developed the innovative NANOSLIDE® technology that uses thermal spray technology, i.e., twin wire arc spray, to coat the internal diameter of cylinder bores. Computational fluid dynamics (CFD) simulation is utilized to investigate the complex gas flow in the cylinder bore of the crankcase during the coating process. Extensively experiments are performed and analyzed to compare the results with the quantitative analysis in order to enhance the properties of the cylinder coating. The combination of simulation and coating experiments led to the development of uniform distributed layer adhesion strength throughout the length of the cylinder bore.

Thermal wave interferometry measurements and microstructural analysis of twin wire arc spray cylinder coatings for passenger car engines

With intent to increase the efficiency of combustion engines in passenger cars, thermally sprayed cylinder bore coatings replaced cast iron liners as cylinder running surfaces over the past years. Since the functionality of an engine depends on the life-long integrity of these coatings only a few 100μm thick, monitoring their physical properties in series production is most important. This contribution focuses on the non-destructive testing of thermally sprayed coatings by thermal wave interferometry (TWI), investigating the relationship between thermal and microstructural properties. Microstructural characteristics such as laterally expanded splat interfaces or spherical pores correlate with macroscopic quality criteria such as adhesion, cohesion, or corrosion resistance. Further on, especially splat interfaces act as thermal barriers within the coatings. Hence, TWI thermal diffusivity measurements were compared to optical microscopy images of polished cross sections and three-dimensional X-ray computed tomography data at multiple positions in a crankcase. Both, microscopy images as well as computed tomography data were analyzed quantitatively by image processing algorithms. Due to this, local variations in thermal diffusivity were found, which revealed a significant correlation with microstructural characteristics. Further on, a significant influence of the amount, size, and shape of microstructural volume defects on the local thermal diffusivity of thermally sprayed coatings is carried out in this contribution.

Tribological and Microstructural Properties of Carbon Steel Coatings Fabricated by Wire Arc Spray

Thermal spray is a widely used process employed for the reduction of engine weight by replacing heavy cast iron liners inserted in cylinder bores. In particular, twin wire arc spray (TWAS) is one of the most popular thermal spray processes due to its reasonable cost and high deposition rate. In this study, ferrous coatings were fabricated by using TWAS with two variables: gas type and initial carbon content in wire. The investigation was focused on the relationship between variables and their effects on the oxidation, microstructure, and tribological properties. Especially, for evaluation of the tribological properties, a reciprocating sliding wear test was designed by considering realistic engine conditions. Individual splats forming the coating layer undergo oxidation which results in oxide formation, loss of alloying elements, and heat generation during spraying. From the results of experiments, it was figured out that the oxidation state is controlled by gas type, and its effects on microstructure and coating properties vary depending on the initial carbon content of the wire. Finally, it was confirmed that the wear resistance of the ferrous coating containing FeO and more carbon was better than the other for its improved frictional property and high hardness.

Hot corrosion behavior of wire-arc sprayed NiCrB coatings

A series of NiCrB coating was deposited using the twin wire arc spraying technique with a newly designed NiCrB (B: 0–4 wt%) cored wire, aiming to protect the boiler tubes used in the waste-to-energy plant. The hot corrosion behavior of the coatings was investigated with the molten salt (Na2SO4 + 10% NaCl, wt%) at 800 °C for 200 h using the thermogravimetric technique. The SEM/EDS and XRD results showed that the addition of element B alleviated the oxidation tendency in the as-deposited coatings. Sufficient free stage Cr atoms diffused to coating surface timely to form protective scale, which inhibited further rapid corrosion, owing to low oxides in the coating. In addition, the corrosion resistance was influenced by the B content of the coating. In the present study, the coating with 1–2% wt% B had the best hot corrosion performance.

Antibacterial Efficacy of Wire Arc Sprayed Copper Coatings Against Various Pathogens

The antimicrobial effect of copper (Cu) as well as its potentiality to reduce healthcare-associated infections is well recognized. In this study, a twin wire arc spray gun has been used to produce antibacterial copper coatings on stainless-steel (316L) surfaces. The thickness of coating was 120 ± 30 μm in average. In parallel, series of coating formation simulations were made using Comsol Multiphysics. The coating morphology was examined by scanning electron microscope (SEM) and its structure determined by x-ray diffraction (XRD). Surface roughness measurements were carried out on as-sprayed and polished surfaces by using a 3-D Profilometer. The coating antibacterial efficacy has been investigated considering standard and clinically isolated cultures such as standard ATCC 25922 Escherichia coli (E. coli), standard ATCC 29213 Staphylococcus Aureus (Staph. Aureus), clinically isolated Pseudomonas aeruginosa, Vancomycin-resistant Enterococcus (VRE) and Methicillin-resistant Staphylococcus aureus (MRSA). The predictions of simulations matched with the monitored data with an error below 10%. The coatings exhibited excellent antibacterial properties for all the pathogen types used.

Atomization Characteristics of Droplet and Morphologies of Arc Sprayed Ni-Al Particles and Composition Coatings

Droplet atomization characteristics and its morphology in twin-wire arc sprayed (TWAS) Ni-Al were explored in depth. Both numerical model and theoretical method were built for calculating the process of the droplet deformation and breakup in air flow, which based on the standard k-e model and the volume of fluid (VOF) dual-phase flow model. The atomization characteristics of double droplets and three droplets were also studied in the atomization process by simulation. The micro structure and the morphology of the composite coatings were analyzed by SEM and TEM. The experimental results indicate that TWAS droplets are the process of explosive breakup or two steps breakup. Morphology of the TWAS prepared Ni-20wt.%Al and Ni-5wt.%Al particles was accordingly characterized by SEM. The phase compositions of the Ni-Al particles were obtained by EDS. By testing the velocity of sprayed particles, the change rule of particles with spray pressure was obtained. Ni twins are discovered in the Ni-Al coating due to rapid cooling in TWAS process.

Microstructure and Flight Behaviors of Droplet and its Solidification in Twin-Wire Arc Sprayed Ni-Al Composite Coatings

Droplet flight and solidification behaviors during twin-wire arc sprayed (TWAS) composite coatings were systematically investigated. Both theoretical model and numerical method were established for calculating the droplet deformation, breakup and solidification process in air flow based on the volume of fluid (VOF) dual-phase flow model jointed with the standard k-e model. The experimental simulation results indicate that TWAS droplet is broken through explosion or two steps breaking process. The calculation of TWAS gas flight dynamics demonstrates that the TWAS particles are accelerated at first and then slowed down. Microstructure of the TWAS prepared Ni-5wt.%Al and Ni-20wt.%Al composite coating was accordingly characterized by XRD, SEM and TEM, so the phase compositions of the Ni-Al composite coatings were obtained. TEM analysis also showed that an amorphous phase was formed according to the characteristic of diffraction ring in Ni matrix solid solution at an original state.

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.

Heat treated twin wire arc spray AISI 420 coatings under dry and wet abrasive wear

The influence of applying two different heat treatments such as: deep cryogenic and tempering on dry/wet abrasive wear resistance of twin wire arc spray martensitic AISI 420 coatings was evaluated by using a modified rubber wheel type test apparatus. A load dependency was observed on the abrasive wear rate behavior of both; dry and wet tests. Three body (rolling) and two body (sliding) wear mechanisms were identified in dry conditions, prevailing rolling at lower and higher loads. However, at higher loads, more presence of grooving and pits formation was observed. Coatings tempered at 205 °C/1 h displayed better wear resistance than cryogenic treated ones. A change in wear mechanism between dry and wet conditions was observed; two body wear mechanism predominated respect to three body. In both; dry and wet conditions the microstructure (several inter-splat oxides) as well as strain and residual stress promotes brittle material removal which was more evident in cryogenic and as-sprayed samples during dry test and at higher loads in wet conditions.

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.