Thermal spray is a widespread technology on the way to large scale production. Driven by industrial importance, companies of all sizes employ this technology, with research mainly driven to improve quality and reduce cost. In order to produce high-quality coatings, efforts have been made to provide on-line process control. To find out which parameters affect coating formation and thus which of these parameters should be monitored high level cost intensive diagnostic systems have to be used. This work introduces selected diagnostic systems for the on-line process control under the condition of serial production: shadow imaging for the particles (PSI), a system to measure particle velocities (PVI) and an integral working system using a qualitatively image algorithm (PFI).

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.

In most plasma spraying SMEs a One-Cathode-One-Anode-Plasma-Generator (OCOAPG) is used due to its cost-effectiveness. To achieve high deposition rates, the highest possible fraction of the injected powder has to be melted and accelerated towards the substrate. Adequate to the amount and size of the particles, a sufficiently long and reproducible residence time in plasma is therefore needed. This can be achieved by a long plasma jet with little or no temporal variation in length and temperature. In OCOAPG an arc is operated between a cathode and a central tubular anode, which causes different instabilities in the effluent plasma jet. Due to the instable interaction between the plasma jet and the carrier gas jet continuously incorporating the powder, fluctuations occur resulting in reduced coating quality. Coating systems with a higher amount of electrodes (and hence using several arcs) show higher stability and therefore can provide higher coating quality. However, due to their complexity and cost intensity, the investment hurdle for SMEs usually turns out to be too high. Recently, research steps to improve the plasma spraying process with OCOAPG have been undertaken by using a controllable current source to create a uniform particle gas jet interaction. As the movement of the anodic arc attachment point can be actively controlled by current pulses, the plasma jet can be lengthened and shortened at periodic intervals. Furthermore, by pulsing the particle delivery synchronously to the power modulation an improved particle penetration and consistent residence times can be achieved. First diagnostic results, including coatings, are presented and discussed within the paper.

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 paper demonstrates the use of two laser cladding automation tools, one that automates laser power and one that automates laser head positioning. Both tools are based on intelligent cameras that evaluate recordings from the cladding zone through the optical path of the laser. The laser power tool monitors and controls local emission from the laser cladding process. The position control tool detects distances between edges on component surfaces and generates an error correction signal based on edge-geometry and material data and various process settings. Examples in which the closed-loop tools are used to clad thin-walled tubes with different alloys and fill irregular grooves on shafts are presented.

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.

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.

One important aspect concerning the coating of surfaces using thermal spray is the improvement of the injection of material particles into the gas jet and thus the deposition efficiency. Therefore a better knowledge of the temperature distribution within the jet is relevant in order to optimize spraying conditions. Particularly interesting is the existence of a well-defined threefold finger structure in the plasma jet produced by triple electrode torches, which allows an efficient injection of coating material due to the existence of zones with higher and lower viscosity. The jet structure, however, lacks rotational symmetry and can therefore not be analyzed by systems relying on the validity of the Abel inversion, thus new systems have to be developed. In this work an innovative tomography device is described that has been designed for this purpose. By circling half around the plasma jet and taking simultaneously intensity images under different view orientations, a three-dimensional intensity distribution of the jet is generated, which can be used to determine the temperature distribution.

Increasing erosion of the anode wall of a single-cathode F4 torch changes the fluctuation pattern of the plasma jet. Trajectories and temperature history of particles injected into the plasma become accordingly modified. In this work such fluctuations are characterised through spectral analysis of the torch voltage as well as of the light intensity of the jet outside the torch. Instead of considering the evolution of the main peaks in the frequency spectrum for both voltage and optical signals, the variation with increasing torch wear of the spectrum’s higher frequency fraction (above 20kHz) is focused on. The evolution of the fractal dimension in this high frequency range exhibits a reliable correlation with the erosion of the anode wall. This is confirmed further by the development of a simple stochastic model for the motion of the arc root along the anode wall. This model yields, depending on the probability for a pronounced arc root jump, a fractal dimension and an evolution just within the range observed in the measurements. Additionally, tracking the fluctuations at different locations outside the torch enables the isolation of the fluctuation dissipation due to jet viscosity and velocity.

Modern thermal spray equipments and diagnostic tools give a lot of relevant process data. Currently only single process parameters (e.g. particle temperature) are recorded and analysed in order to keep the parameters constant. However, even this simple kind of process control is not state of the art in every spray shop. The issue of this publication is the development of a method for prediction of material- and component-properties for thermal spraying. This method will allow an offline process control of the complex coating process. For the development of this kind of process control numerous experiments will be been carried out with both HVOF- and APS-processes using the Particle-Flux-Imaging diagnostic tool (PFI). The PFI-system is the basis for the process control to be developed. In comparison to other diagnostic tools it is easy to handle and cheaper. The set up of experiments and the data evaluation were carried out by means of statistical DOE [1]. The first series of experiments were designed to determine the significant interrelations between these influencing parameters. Based on the results from the first series of experiments, the significant parameters were further manipulated during spraying the second series of experiments The correlation between the coating properties (hardness, porosity) and the main influencing parameters are the basis for the creation of an offline process control.

With the goal to improve the plasma spraying technique a considerable number of plasma gun types using different physical principles have been developed in the past. In this paper for conventional DC discharge plasma torches widely used in industry, some aspects of operation as arc fluctuations and non-symmetric plasma jets are discussed. Different construction principles with single or multiple electrodes as well as with one-piece or cascaded nozzles are compared. Finally a new torch concept is presented.

For the effective development and optimization of plasma torches and plasma processes a knowledge of the spatial distribution of plasma properties within the generated plasma jets is useful. Information about some of these properties also in case of arbitrary distribution can be achieved by computer tomography (CT) using radiation emitted by the plasma jet. The stationarity of the plasma jet is a necessary condition for the CT technique described in this paper. By CT in principle the radiation from a small cross-sectional disk of the plasma jet is recorded successively under different directions perpendicular to the torch axis. From the recorded data the spatial distribution of radiation emissivity within the jet is calculated using an algebraic recursive algorithm. The CT was applied for the investigation of a TRIPLEX II plasma jet giving the following results: The jet is characterized by a very high degree of stationarity and exhibits a definite triple symmetry, which can be described by three partial plasma jets. Measuring their positions the influence of the arc current on the TRIPLEX plasma jet rotation was determined.

This paper provides an overview of two particle imaging techniques that have proven useful in the development and implementation of thermal spraying processes. It covers the basics of each method, including the hardware setup, the interpretation of output data, and example applications. Paper includes a German-language abstract.

The innovative diagnostic system, PFI, records the radiation of the plasma jet as well as of the luminous particle flux. A desktop computer immediately converts these brightness distributions to a set of ellipses. The simple set-up of the system and the fast algorithm enables the utilization of the system in serial production. A statistical design of experiments (DOE) was applied for various process parameters to correlate the determined characteristic ellipses to the process parameters. Measurements of the process by DPV2000 and properties of the resultant coating were used to validate the established correlations. The results demonstrate the suitability of the diagnostic method PFI for quality control and quality assurance in serial production.

This paper presents a low-cost in situ diagnostic method that monitors and controls thermal spray processes using a CCD camera and a PC. The method, called particle flux imaging (PFI), records light emitted by thermal spray particles and the hot propellent carrier-medium in which they are conveyed. Brightness distributions corresponding to temperature and density profiles are represented by sets of ellipses that are compared in real time to a reference image. An image analysis algorithm adjusts relevant spray parameters based on the comparisons, maintaining a constant and unchanged spraying process.

Plasma spraying is a well established process for producing ceramic and metallic coatings for many technical applications. The quality of the coatings and the efficiency of the process depend on the powder and on the operating parameters as well as on the plasma torch properties. Whereas many efforts have been made on creating novel powders and on optimizing the operational parameters, the principle, however, of DC plasma torches has remained unchanged for many years. This was the reason for developing a novel DC plasma torch system. This paper presents the construction and operating principle of this innovative torch system. Its high performance is demonstrated in selected ceramic coatings. Paper includes a German-language abstract.

The quality and the deposition efficiency of coatings produced in thermal spraying processes are influenced by many parameters. Therefore normally the process optimization is laborious and time consuming. To reduce these efforts the low cost In-situ diagnostic method PFI was developed. A stationary CCD camera records the radiation of the hot propellant carrier-medium and of the luminous particle flux. A connected PC immediately converts the brightness distributions to a false colour image corresponding to the temperature and density profiles of the carrier-medium and of the particle flux. Utilizing a reference image the parameters can be adjusted for a prompt and successful optimization of the spray process. The performance of the PFI- method is demonstrated in some examples. Paper text in German.

Dc plasma torches typically use a mixture of inert and molecular gases when spraying high melting powders. The addition of molecular gases increases the enthalpy of plasma jets, but it also produces arc root fluctuations that can cause variations in injected powders. This paper describes an innovative plasma torch system characterized by a long nozzle and three parallel cathodes. The nozzle consists of several electrically insulated rings and a ring-shaped anode. By adding more rings, the arc gap and voltage can be increased along with the enthalpy of the plasma jet. The results of various tests, comparing the spray rates and deposition efficiencies of new and conventional torches, are presented in the paper.