Water turbine parts damaged by cavitation erosion (CE) and/or slurry erosion (SE) may cause excessive operational costs for plants worldwide. The damages can be reduced by choosing more resistant materials and right technology in the first-production or at repair and overhaul. Thermal spray technologies have a great potential in the field of repairing works. Thick multilayered coatings deposited by wire electric arc spraying (WAS) has been developed and applied as CE and SE protection at the repair of stationary Francis turbine blades. Repair technology by WAS was performed on large eroded areas (1-3 m 3 ) of preguide blades of Francis turbine: 1) local damaged depths 30-35 mm maximum were repaired by sprayed materials, 2) subsequently wire arc spraying of functional coating was applied. Three types of functional coatings with total thickness 10 mm a) duplex high - Cr stainless steel with NiAl bond coat, b) graded NiAl - Cr stainless steel coatings, and c) multilayered graded NiAl - Cr stainless steel coatings were compared by means of stress measurements and structural analysis. The coating structure influences very strongly the residual stress level and adhesive-cohesive strength. Multilayered graded NiAl - Cr stainless steel coatings showed the best results and were sprayed on water turbine blades in 4 Czech water power station during regular cut-off repair periods. After 30 - 36 months' continuous operation, Francis turbine blades repaired by WAS technology show better behaviour in comparison with original material from the point of wear resistance, reliability, cost-effect and life-time.

Iron-based wire feedstocks represent a technical as well as economical alternative to carbide reinforced feedstocks for wear protection applications. To assess the potential of such feedstocks, iron-based cored wires were developed with up to 6 wt% boron. The feedstocks were deposited by electric arc spraying, forming hard, partially amorphous coatings with embedded nanocrystalline boride precipitations. To further improve wear resistance, chromium carbide was blended into the powder filling in some wires. Coatings produced from all feedstocks were evaluated by means of optical microscopy, X-ray diffraction, and microhardness measurements.

Among the different wires used in arc spraying, copper is a material of choice in some applications. Its malleability is used to allow an easy machining procedure after spraying. This article focuses on the limitations of the oxidation of copper during arc spraying and its influence on coating process and properties. The aim of this series of experiments was to improve coating properties of copper sprayed with the electrical wire arc spraying process by substituting compressed air with nitrogen. These experiments show that coating properties, as well as electric wire arc spraying process, are strongly influenced by the gas employed as the atomising element.

The manufacture of tooling using the electric arc spray process to spray steel directly onto a master pattern offers substantial reductions in the lead times required to make complex tooling for polymer injection moulding and other applications. The process of spray forming is fast, efficient, and low cost, and has been shown to be dimensionally accurate with proper control over the residual stresses that develop during spraying. Poor dimensional control because of high internal stresses in thick arc sprayed steel coatings is well known, but these problems can be avoided by the use of correct spraying conditions. This paper describes the steps of the spray forming process used to make tooling for polymer injection moulding. The spray forming route competed directly with a traditional method for toolmaking and considerably reduced the lead time from order to completion. The tooling produced by spray forming has been operating commercially in production in the U.S. and has to date produced in the region of half a million parts without appreciable wear. The incorporation of contoured cooling channels during spraying has enabled plastic injection moulding cycle times to be decreased by 15%.

The use of cored wires for thermal spraying is a relatively new development that is being rapidly utilized for arc spray in a wide variety of applications. This paper will discuss the existing applications and industries in which cored wire coatings are used. Additionally, this presentation will cover the effect cored wires have had on the use of other types of thermal spray equipment. The paper will close with a trend analysis that discusses the applications of the future.

Electric arc sprayed coatings with a disperse lubricating phase have potential for use in a variety of industrial applications as bearing materials including low friction coatings for drill string joints in the oil industry to reduce casing wear and lower drilling torque. This paper describes the optimisation of electric arc spray parameters for Fe-0.06wt.%C that will subsequently be used as a self-lubricating coating matrix. The effect of electric arc spray parameters on the microstructure of a Fe-0.06wt.%C matrix has been characterised in terms of deposition rate, temperature during manufacture, porosity and microhardness. It has been shown that the local coating temperature during directly affects the final coating porosity, grain size, grain morphology and microhardness. The most effective parameter in controlling coating temperature was the coating deposition rate. The Fe0.06wt.% C coating microstructure was primary equiaxed ferrite with a dispersion of spherodised Fe3C particles formed from the in-situ tempering of the as-sprayed martensite or bainite during spraying. A fuller analytical treatment of these phenomena is given elsewhere (13). Fe-0.06wt.%C powder particles microstructure was primarily bainitic or martensitic. A small number of Fe-0.06wt.%C powder particles showed a dendritic phase which has been proposed as retained austenite because of austenite stabilisation by fine grains and the N2 atmosphere, or an unidentified nitride layer.

An experimental study of twin-wire electric arc spraying of zinc and aluminum coatings demonstrates the suitability of the process for anticorrosion applications. Experiments were conducted using Box-type full-factorial designs. Operating parameters were varied around the following process parameters: nozzle diameter, nozzle geometry, and system pressure. A systematic design of experiments displayed the range of processing conditions and their effect on the resultant coatings. The coatings were characterized with hardness and deposition efficiency tests, and optical metallography. Coating properties are quantified with respect to roughness, hardness, porosity, thickness, bond strength, and microstructure. The features of the coatings are correlated with the process changes. Selected analytical calculations and process diagnostics of the meltpool dynamics are presented.

For the last seven years, Oxford University and the Ford Motor Co. have been researching jointly the development of the large-scale spray forming of steel tooling capable for use in mass production, particularly for the pressing of sheet metal in automotive applications. These investigations have involved: comprehensive microstructure and property studies, modelling of shape evolution and heat flow, real-time feedback control of tool temperature to eliminate tool distortion, high speed imaging and particle image velocimetry of droplet deposition on 3D shapes, the testing of full-scale tools for different applications in the production environment, and detailed studies of the cost and time savings realised for different tooling applications. This paper will provide an overview of the scientific and technical progress to date, present latest results, and describe the current state-of-the-art. Many of the insights described have relevance and applicability across the family of thermal spray processes and applications.

This paper evaluates various methods for adjusting the degree of oxidation in arc-sprayed layers. Oxidation control is primarily achieved through the use of a nonoxidizing gas, such as argon or nitrogen, especially in combination with a fine nozzle. In the case of Ni18Cr6Al2Mn deposits, the measured oxide content varied from 0.85 to 3.41 wt% based on the atomizing gas, nozzle, and stand-off distance used. Paper includes a German-language abstract.

Increase of the pressure of air flow from 0.6 to 1.2 MPa provides the growth of its speed from 300 to 600 m/s. The increase of air flow speed reduces the flight time of drop from the arc to the substrate and them against the prepared surface at higher temperatures. Increase of the pressure of air provides reduction the size of drops and oxides between lamellas as well as improves the mechanical characteristics.

Microstructures and properties of electrical arc sprayed cored-wired coatings were investigated and compared in this work. Three types of cored-wired studied were WC-Ni, WC-Fe and nano WC-Fe The as-received cored-wire were characterized prior to be sprayed. In-flight particles were collected during spraying in order to reveal particle morphology and size distribution forming splats. Size distribution analysis of the filler particles showed that nanostructured WC-Fe had a comparable mean filler particle size with that of the WC-Fe and were larger than that of the WC-Ni. The difference in filler particle size of starting cored-wire was well correlated with the in-flight particle size produced. Nano WC-Fe cored wire coating had the lowest porosity and roughness compared to others with a comparable hardness to WC-Ni cored wire coating with rather different coating microstructures.

In the paper, a newly composite TiB 2 /Al 2 O 3 coating prepared by High Velocity Electric Wire Arc Spraying and cored wire with ceramic powders composed of refractory Al 2 O 3 and TiB 2 filling inside was introduced. The microstructure and composition of the TiB 2 /Al 2 O 3 coating were investigated by metallgraphic microscope, scanning electron microscope and X-rayed diffraction, Bonding strength and hardness and mechanics properties were tested. The experimental results show the TiB 2 /Al 2 O 3 composite coating possessed high hardness and higher bonding strength with substrate and excellent comprehensive mechanical properties. It can be used for improving shoot-proof properties in the surface of arms.

This paper examines the in-flight oxidation of molten aluminum sprayed in air using the twin-wire electric arc (TWEA) thermal spray process. The oxidation reaction of aluminum in air is highly exothermic and is represented by a heat generation term in the energy balance. Aerodynamic shear at the droplet surface: (1) enhances the amount of in-flight oxidation by promoting entrainment and mixing of the surface oxides within the droplet, and (2) causes a continuous heat generation effect due to the exothermic oxidation reaction that sustains droplet temperature as compared to a droplet without internal circulation. This continual source of heat input keeps the droplets in a liquid state during flight. A linear rate law based on the Mott-Cabrera theory was used to estimate the growth of the surface oxide layer formed during droplet flight. An explanation is provided for the elevated, nearly constant surface temperature (~ 2000 °C) of the droplets during flight to the substrate and it is shown that the majority of oxide content in the coating is produced during flight, rather than after deposition. The calculated oxide volume fraction of an average droplet at impact agrees well with the experimentally determined oxide content for a typical TWEA-sprayed aluminum coating, which ranges from 3.3 to 12.7%.

The use of thermally sprayed coatings to improve wear and corrosion resistance of industrial components has been found to be an effective and viable choice. Coatings based on several materials can be used for such applications, including situations when they are combined, using distinct deposition methods. The analysis for choice of the ideal coating and application process should generally take into account both coating performance and cost. In this work a comparative study of three commercial coating materials is performed. The used coating materials are High Chromium (HCr), High Chromium + 420 Stainless Steel and 420 Stainless Steel + 1080 Carbon Steel wires applied by Electric Wire Arc Spaying. The obtained coatings are tested for wear (ASTM-G-65-91 rubber wheel test) and corrosion (ANSI/ASTM-B117 and Electrochemical Measurements). Coatings microstructure, microhardness and adhesion (ASTM-C633-85) are also evaluated.

The high-velocity oxy-fuel (HVOF) gun represented a major development towards forming dense coatings. But, compared to the electric arc-spray process, the HVOF process is more difficult to apply and in general more costly. Therefore, some process development efforts have aimed at exploiting the attributes of both the electric arc-spray technique combined with those of the HVOF technique. Specifically, this hybrid process utilizes the electric arc spray benefits of using wire stock and high deposition rates combined with the higher coating densities obtained using HVOF. This paper presents an in-depth, in-flight particle characterization of a hybrid spray gun. A DPV-2000 particle diagnostics sensor was used to measure particle velocity, temperature, size and distribution. The influence of feed material, electric arc-spray parameters, and HVOF parameters on the particle characteristics is presented. It is observed that the velocity of the particles in the hybrid mode are slightly lower than the velocity of particles by HVOF alone but significantly higher than typical electric arc-spray velocities. In addition, the particle temperature in the hybrid mode isn’t significantly different than those by HVOF only. The particles produced by the hybrid gun tend to be more uniform and smaller compared to traditional electric arc-spray coatings.

Within the Aircraft MRO (Maintenance, Repair and Overhaul) business KLM Royal Dutch Airlines Engineering & Maintenance Division has been involved with Thermal Spraying since the late 60’s. The latest procurement on thermal spraying is the state of the art light weight electric arc spray gun (the EM-14) for the closed-loop electric arc spray system of Praxair TAFA called the CoArc. With this new piece of equipment KLM is up-to-date in the high-tech approach of applying thermal spray coatings on aircraft engines with a electrical twin-wire arc spray system. Close teamwork between an end-user in aircraft industry (KLM) and a thermal spray supplier (Praxair) resulted in a new state of the art arc spray gun. This paper shows the first spray results with the EM-14 gun.

This paper presents process development efforts aimed at exploiting the benefits of both the electric arc-spray technique and the High Velocity Oxygen Fuel (HVOF) technique. This so-called hybrid process offers the benefits of using wire stock and high deposition rates of an electric arc-spray gun combined with the higher coating densities obtained using a HVOF gun. Various configurations starting from a single wire arc to 4-wire arc in combination with HVOF have been investigated. Experiments with single material wires to multi-material wire combinations have also been investigated. The feed rates of individual electric arc wire pairs and HVOF powder/wire are controlled independently allowing the creation of functional gradient materials (FGM). This gun has been used to create alloys as well as patterned deposits of different materials. The ability to create in-situ alloys and patterned deposits with excellent quality has been demonstrated.

Thermal spray of Zn, Zn/Al, or Al is extensively used to make anticorrosion coatings on steel structures. Twin arc spray and wire flame spray are the two most practised processes to achieve such coatings. This paper presents measurements of particle emissions generated by these two processes. Sampling and analysis of aerosols generated by both processes have been carried out inside the exhaust ductwork using various instruments: an ELPI impactor, a CNC (Condensation Nucleus Counter), a TEOM microbalance and sampling filters allowing sampling for SEM observations. Electric arc spraying produced much more fumes of ultra fine particles than flame spraying. Aluminum spraying also produces large fume quantities compared to the Zn spraying under the same conditions. The aerosol comprised submicron particles and 95% of the numerical particle size distribution was less than 100 nm. The nanometric nature of the fume particles was confirmed by observations on the SEM. The strong dilution caused by compressed air has the effect of strongly limiting particle coagulation and, in turn, the size of the agglomerated particles. Electric arc spray has taken market share versus wire flame spray for Zn, ZnAl, or Al spraying, but this study shows that it generates much more particle emissions.

HVAF Arc process deposited coatings of dual wire stock, fused by an electric arc and atomized by a high-velocity jet of air and gaseous fuel combustion products. The HVAF jet was generated in toroidal chamber, where a hot catalytic insert activated the combustion process. Such atomization produced fine, 5-20 micron, spray particles accelerated to over 200 m/s. Excess of fuel in the arc zone prevented rapid oxidation of fused material. The process is specifically beneficial for deposition of high-quality coatings of aluminum, corrosion resistant nickel-base alloys and hardface Fe-Cr-B-C cored wires.

A method for the production of particle reinforced coatings by wire arc spraying will be presented in this paper. This technology is based on twin-wire electric arc spraying (TWEA) process. Here, additional particles were injected into the atomizing gas stream and sprayed in a non molten state along with wire feedstock material onto the substrate. According to the reinforcing particles, the process can be applied to produce coatings with a high wear resistance as well as a high surface roughness. In a wide range of applications, these coating characteristics are required. Due to economic constraints, coatings of large surfaces have to be done in short times at low costs. Based on wire arc spraying, the thermal spray process with the highest deposition performance, the mentioned industry requirements can be fulfilled.