Plasma generators are frequently used in a wide field of applications. These applications include thermal coating, plasma-welding, preprocessing of surfaces, sterilization and propulsion systems. The industrial use of plasma generators demands a high level of accuracy and reproducibility which has to be monitored throughout the process. Rather than analyzing results of e.g. a coating process (by electron microscopic) one prefers to have an online assessment of the plasma generator. The plasma generated is defined by various parameters (e.g., temperature, conductivity) which can be measured with suitable measuring systems. Determining all plasma parameters in one setup is very costly and requires a big setup. In industrial facilities it is often suitable to measure one or two physical quantities and derive the plasma parameters from these measurements. This paper focuses on the evaluation of the electric current consumed by the plasma generator and the sound emitted by the whole system. A quick analyzation of these signals - that may result in an online assessment -was accomplished by using the wavelet transform. Multiple experiments were performed to gain reliable and significant data (behavior of the arc/plasma).

The main goal of the project was to investigate the influence of different gas properties, in combination with different spraying parameters, on the wire arc spraying process. For this purpose, investigations with several gas compositions (pure nitrogen as well as nitrogen combined with hydrogen or ethene – compared to compressed air) have been carried out for different spraying materials which are currently being used in today’s industries, such as copper, carbon (St0.8) and stainless steels (316L). The preheating of the process gas as well as the variation of the gas pressure up to 1,400 kPa (14 bar) were also subject to research. The resulting coating properties have been analyzed in terms of oxide content, porosity and hardness as well as deposition efficiency and adhesive tensile strength. Additionally, in order to enhance the process stability, a system to detect the cause of cold shuts has been developed.

Coatings produced by thermal spray processes are determined by particle parameters, which can be investigated using various diagnostic systems. In the first part of the paper the different PSI (Particle Shape Imaging) setup are described. Thereby a summary of former publications, presentation of measurements on different thermal spray processes and suggestions of data error evaluations with special difficulties at the particle investigation are shown. In the second part of the paper the new developed mobile experimental setup of PSI is focused. Four important particle parameters (velocity, size, shape and position) can be measured and evaluated. In this part the results and test of new different illumination sources, optical configurations and trigger units are discussed. Furthermore interesting ideas of picture processing and data evaluation are depicted and explained in more detail. First comparison of the results measured with DPV2000, SprayWatch and additional Voltage-Current-High-Speed- Camera investigations are shown. The complete paper concludes with an outlook for future combination of further established diagnostic systems (i.e. find the optimal distance between substrate and thermal spray system).

This study investigates the effect of different gas compositions on the size, velocity, and temperature of particles in a wire arc gas jet. In the experiments, two wire materials (316L and G3Si1) were sprayed via compressed air, nitrogen, and a nitrogen-hydrogen mixture and high-speed shadow imaging was used to record in-flight particle characteristics. Deposition efficiency was also measured along with the hardness and oxide content of the coatings. The spraying process, equipment, and test methods employed in the study are described and the results are presented and discussed.

This study investigates arc behavior associated with twin-wire arc spraying and the effect of pulsed current power on arc formation and coating properties. It is shown through false color images and by calculations that arc length is directly related to voltage and that a “natural” frequency can be obtained from voltage fluctuations. By applying current pulses at this frequency, arc movement along the wire tips is effectively controlled because the arc reignites with each current pulse. This results in coatings with lower oxide content and a microstructure nearly free of the lamellae typically found in dc arc spray deposits. Further improvements are likely to be achieved by optimizing the configuration of the wire guides relative to the gas nozzle.

This study investigates the cause of process instabilities in large and small scale arc spraying production equipment and in an experimental spray gun. Arc voltage and current, voltage drop at the wire contact sleeves, and wire feed velocity are recorded and spectrum analysis is used to predict the separation of unmelted wire pieces, or cold shots, during spraying with a view to quality assurance. A high-speed camera with pulsed LED illumination is used to observe droplet detachment and spray jet generation. For each case, applied diagnostics are presented and the results are interpreted with respect to the physical causes of instability and how to avoid them.

One approach for controlling the twin wire arc spray (TWAS) process is to use optical properties of the particle beam like length or brightness of the 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 can be detected through observation of optical properties 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 so the coating properties change significantly. Thus, the goal is to detect these optical deviations and compensate occurring errors by adjusting appropriate process parameters for the wire arc spray unit. One cost effective method for monitoring optical properties of the particle beam is to apply the process diagnostic system PFI (Particle Flux Imaging): PFI fits an ellipse to an image of a particle beam thereby defining easy to analyze characteristic parameters by relating optical beam properties to ellipse parameters. Using artificial neural networks (ANN) mathematical relations between ellipse and process parameters can be defined. Thus 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. In this paper, it will be shown that different process parameters can lead to particle beams with the same PFI parameters.

A real-time measuring system has been developed to record and analyze the fluctuations of burner voltage and current. The objective is to identify the occurrence of stripping unmolten wire pieces (cold shuts) and to detect corresponding spraying conditions. To identify voltage and current fluctuations causing cold shuts, high speed camera shots are synchronized with electrical measurements to analyze spraying parameters which were assumed leading to cold shuts. As a result, different cold shut conditions could be identified. The electrical cold shut identification then triggers a camera system, to capture online the size and velocity of cold shuts. Based on electrical measurements it is intended to detect wear of spraying equipment which leads to enhanced cold shuts occurrence during industrial spraying process.

In order to improve the wire arc spray process an inverter power source (PS) from gas metal arc welding (GMAW) process was used to evaluate the influence of current modulation on the formation of coating particles. Using the inverter PS allowed the application of high current pulses with varying amplitude and frequency. It was shown that particle formation can be limited to the high current phases, and that a strong interaction with the gas flow can be observed. The investigations suggest that using this technology new parameters may be introduced to control the wire arc spray process.

This paper presents an innovative method for thermal spray particle diagnostics. In the demonstration, twelve CCD cameras are arranged around the spray cone created by a wire arc gun. The cameras are aligned radially with 15° offsets and are synchronized with a hardwired trigger. Images simultaneously captured from different directions contain information from which the location and trajectory of the particles can be determined. In addition to describing the setup of the camera system, the paper also develops the mathematics required for image processing and the calculation of 3D particle trajectories and velocity distributions.

Much of the research being done in arc wire spraying deals with new nozzle geometries and secondary atomizing gas flows. This study, instead, focuses on pulsed current power. Spray trials are conducted using a conventional electric arc gun and an inverter that produces large current pulses with controlled amplitude and frequency. Droplet formation and detachment are monitored with a high-speed camera that records shadow images synchronized with current and voltage measurements. The images are processed into analog signals that are converted to the frequency domain and the Fourier coefficients are plotted, providing a relative measure of the influence of pulse frequency on droplet ablation. The results suggest that arc spraying can be improved using pulsed current power and that gas pressure and pulse time have greater influence than pulse frequency.

Traditionally, large moulds for manufacturing of CFRP (carbon fiber reinforced plastics) parts are machined from a solid metal block making this way of manufacturing very energy and time consuming. Using wire arc spraying thin-walled moulds can be produced by spraying onto an original mould and separating the coating. In order to create a reliable and high quality product the manufacturing process needs to be highly reproducible. Thus the spraying process requires monitoring and control, which can be done using artificial neural networks (ANN). In our approach, for monitoring the process the diagnostic system PFI (Particle Flux Imaging) is used to characterize the spray particle stream, which is essentially achieved by fitting an ellipse to an image of the particle stream. Comparing deviations from a reference ellipse recorded for an “optimal” coating process provides data that can subsequently be used for process control. Investigations performed by the method of design of experiments (DOE) show a very strong correlation of the parameters pressure, current, and voltage with certain parts of the PFI data: for example the semi-minor axis of the ellipse depends linearly on voltage and current but quadratic on pressure. These results can further on be used to control the coating process by ANN. This paper discusses the application of this method and its feasibility for industrial use.

The main goal of this work is to improve the coating properties of three-cathode atmospheric plasma sprayed coatings with respect to porosity and residual stresses. This was done by use of numerical simulation coupled with advanced diagnostic methods. A numerical model for the triple injection of alumina feedstock, as well as acceleration and heating of the powder particles in the characteristic threefold symmetrical plasma jet cross section produced by a three-cathode-plasma torch was developed. The modeling results for the standard injector’s position “0” were calculated and experimentally verified by Laser Doppler Anemometry (LDA). Based on the criteria defined for concentrated feedstock transport and homogeneous thermal treatment of powder particles in the plasma jet, the optimal injection position was found. In the next step a previously developed, coupled CFD-FEM-simulation model was used for simulations of the coating build-up, describing flattening, solidification and deformation due to shrinkage for alumina particles on a rough substrate surface.

New developments in the field of thermal spraying systems (increased particle velocities, enhanced process stability) are leading to improved coating properties. At the same time innovations in the field of feedstock materials are supporting this trend. The combination of modern thermal spraying systems and new material concepts has led to a renaissance of Fe-based feedstocks. Using modern APS or HVOF systems, it is now possible to compete with classical materials for wear and corrosion applications like Ni basis (e.g. NiCrBSi) or metal matrix composites (MMC, e.g. WC/Co or Cr 3 C 2 /NiCr). The work described in this paper focuses on that combination and intends to give an analysis of the in-flight particle and spray jet properties achievable with two different modern thermal spraying systems (kerosene driven HVOF system K2, 3- cathodes APS system TriplexPro-200/-210) using Fe-based powders. The velocity fields are measured with the Laser Doppler Anemometry (LDA). Additionally, resulting coatings are analyzed metallographically with regard to their properties and a correlation with the particle in-flight properties is given. The experimental work is accompanied by computational fluid dynamics (CFD) simulations of spray jet and particle velocities, leading to a comprehensive analysis and characterization of the achievable particle properties with state-of-the-art HVOF and APS systems.

Properties and performance of coatings deposited by the Cold Gas Spraying process are affected by particle properties, which can be changed by using various spraying parameters. Different diagnostic systems can be applied to investigate these parameters. By means of the commercial SprayWatch system, particle properties (velocity, temperature, size, number density) can be measured and evaluated. In comparison the Laser Doppler Anemometry (LDA) system determines velocity and number density. Using both systems the velocity has been measured for a range of parameters, including different materials, different gas properties and nozzles thus offering the possibility to compare the two different diagnostic systems. In the present paper only the results of different gas properties will be shown.

The goal of this research group is to homogenize properties of three-cathode plasma sprayed coatings on basis of numerical simulations and advanced diagnostics. Results of the first project phase as well as an outlook to future work are presented. A numerical model for investigation of plasma flow in the free jet, produced by three-cathode torch was developed. Modelling results are verified by plasma diagnostics (Computer Tomography). In order to include particle shrinking effects, coating formation simulation is accomplished by a newly developed model, based on Computational Fluid Dynamics coupled with the Finite Element method, whereat diagnostics carried out in the fields of particle diagnostics. During the next phase of the project, the investigation of the plasma free jet and particle injection by advanced diagnostics and simulation respectively is scheduled. In a subsequent stage the transition from conventional particles to suspensions will be considered. Coating formation simulations are scaled up to dimensions of macroscopic tensile tests. By combining these overarching investigations, appropriate process parameters for homogenized coatings will be obtained.

In cold spray and thermal spray applications, one of the primary factors affecting coating deposition is the location where particles are injected into the gas jet. Therefore, a detailed knowledge of the gas flow distribution is required. Non-resonant laser scattering allows to spatially resolve the distribution of drift velocity and mass density within the flow, particularly at locations close to the injector. Based on laser scattering, this paper presents a new diagnostic that locally measures drift velocity, as well as a relative mass density distribution of a gas stream. Its application is mainly focused on cold gas flows, where velocity measurements in a supersonic nozzle, obtained by means of laser scattering, correlate well with theoretical calculations and particle image velocimetry (PIV) experimental results.

Powder injection parameters like gas flow, injection angle and injector position strongly influence the particle beam and thus coating properties. The interaction of the injection conditions on particle properties based on DPV-2000 measurements using the single-cathode F4 torch is presented. Furthermore, the investigation of the plasma plume by emission computer tomography is described when operating the three-cathode TriplexPro torch. By this imaging technology, the three-dimensional shape of the radiating plasma jet is reproduced based on images achieved from three CCD cameras rotating around the plume axis It is shown how the formation of the plasma jet changes with plasma parameters and how this knowledge can be used to optimize particle injection.

In the area of atmospheric plasma spraying, newly-developed triple-cathode technologies offer the potential to homogenize the coating properties with respect to porosity and residual stresses. Focused on numerical simulation, combined with advanced diagnostics, the goal of this research group is to adjust these properties systematically. A numerical model that couples fluid dynamic, electro-magnetic and thermal phenomena for a three-cathode torch was developed to investigate the plasma and the electric arc behaviour inside the torch. With help of self-developed computer tomography equipment, which is based on emission spectroscopy, combined with the solution of the Saha equation in thermodynamical equilibrium, it is now possible to reconstruct the 3- dimensional temperature distribution close to the torch outlet. This measurement allows us to confirm the torch numerical modelling. Coating formation is simulated by coupled computational fluid dynamics (CFD) and FEM simulation, so that fluid structure interaction is taken into account. This innovative approach has the advantage to predict residual stresses which occur during cooling and moreover the shrinking effects can be considered. By simulation of the individual regions, in combination with experimental results, which also include the particle velocity, diameter and surface temperature, the corresponding process parameters can be obtained for the desired coating properties.

In order to homogenize the properties of APS sprayed coatings, the spray process was investigated using numerical simulations combined with innovative diagnostic techniques. The process was subdivided into three areas: the plasma torch, the free jet, and coating formation. By simulating these areas separately and combining the results, appropriate process parameters for homogenized coatings were obtained. For a comprehensive computation of coating formation which, besides the impact, flattening, and solidification of particles, includes the mechanical properties of the coating, a volume of fluid algorithm is coupled with a finite element model. In order to verify the modeling of the plasma jet and to provide input data for the coating formation, diagnostic efforts were concentrated on measuring the gas temperature of the plasma as well as particle shape, velocity, and temperature. The results of spatially resolved 3D analysis employing an innovative tomography system are presented and compared with the numerical results.