Plasma Transferred Arc (PTA) welded coatings are used to improve surface properties of mechanical parts. Advantages are the high reliability of the process and the low dilution of substrate and coating material. Processing of surfaces by PTA welding is restricted at the time to flat horizontal position. Furthermore industry is interested in the development of strategies for coating with PTA in constraint position as complex 3-D parts could be then easily processed as well. Under commercial aspects, the process design can be optimized in order to increase process efficiency and to reduce heat input during the welding process. Process optimization involves the determination of guidelines for PTA welding in constraint positions as well. Modelling the process gives an alternative to reduce the experimental effort to optimize the welding process. Results of simulation studies of the PTA welding process will be given in the present work. It will be shown, that coating conditions can be optimized by varying plasma gas flow, heat input and heat flow, process speed or powder injection with regard to welding in constraint positions. The defined controlling of the PTA welding allows to modify process management with less experimental effort and to develop coating strategies for processing in different positions. In experimental investigations the developed coating strategies will be confirmed by producing PTA coatings in constraint position as well as complex 3-D parts.

Thermal sprayed coatings are widely used to improve wear and corrosion behavior of metallic surfaces. The coating characteristics depend on the morphology, which can be designed and adjusted for special applications. Therefore the knowledge of the interaction of process parameters with the resulting structure plays a very important role in the optimization of coating processes. The implementation of mathematical models allows to foresee the coating characteristics and enhance quality and process efficiency as well. In this paper, a model of the vacuum plasma spray process is presented. Theoretical studies show the influence of process parameters on temperature and velocity within the plasma jet. Heating and acceleration of particles by the plasma and following the spreading, superposition, cooling and solidification of particles on the substrate are investigated. The resulting structure depends on plasma properties, injection conditions, particle parameters and substrate properties. Systematic studies show the effect of parameter variation on the particle properties, cooling and solidification behavior and subsequently on the coating structure.

HVOF-, arc- and plasma sprayed coatings are widely used for wear protection. Today these type of layers are dominant if thin coatings from 50 up to 500 µm and low heat input into the work piece are required. The main disadvantage of thermally sprayed coatings is the adhesion to the substrate and the early failure when cyclic loaded. In both cases a metallurgical bonding to the substrate can improve the life cycle time. Plasma transferred arc (PTA) welded coatings show a metallurgical bonding to the substrate. The main disadvantages of this coating technology are the dilution of about 5%, the heat input into the substrate and that nowadays all welding positions seem to be impossible to carry out. In this paper the theoretical background for welding thin coatings (less than 500 µm) with a decreased dilution and in all welding positions is given and experimentally proved.

Lightweight materials such as Al and Ti alloys tend to show poor wear resistance. However, laser alloying of thermally sprayed coatings can be used to form intermetallic phases within the surface area to overcome this disadvantage and to build a metallurgical bond between substrate and coating. Such phases formed in an exothermic reaction may show excellent corrosion behaviour and wear resistance. These reactions can be used to influence the surface properties by remelting metallic coatings on Al or Ti substrates. With respect to the wear behaviour, Ti and Al intermetallics are of great interest. Ti and Al alloys were coated by Al, Ti, and Ni respectively. The different structures on the surface of the alloys depend first on the laser processing parameters resulting in the overheated melt and as well as the latent heat of the formed intermetallic phases. The experimental results clearly show that for short dwell times the latent heat dominates the solidification process and that at high solidification rates the microstructure formation becomes nearly independent from the process parameters. This effect is of special interest for industrial applications as quality requirements ask for robust processes. The paper discusses the metallurgical fundamentals of intermetallic phases and the energy balance of the solidification while giving a deep insight into the influence of different process parameters. Lastly, the properties of alloyed surfaces are discussed.

Titanium aluminide shows good potential for lightweight applications at high temperatures, provided various problems are overcome. The primary problem, low-temperature brittleness, can be resolved through a combination of laser cladding and directional solidification. This paper explains how Ti48Al2Cr alloy is applied and how process parameters must be chosen so that directional solidification occurs. This involves the use of FEM simulations, carried out in parallel with experimental work, to determine temperature gradients and cooling rates at the solidification front and in the melt pool and their effect on solidification structures. Paper text in German.

A large expansion of the applicability of thermally sprayed coatings will be achieved, if there is success in tailoring the coatings structure. This is particularly necessary for functional coatings with special electrochemical or electromechanical properties. The coating structure is determined by the solidification of the sprayed particles on the substrate respectively the earlier deposited particles. In addition to the particle parameters - particle size, velocity, degree of melting and temperature of the melt, the substrate surface conditions - roughness, temperature, activity - influence the spreading and subsequently the solidification of the particles and the formation of the coating. Theoretical parameter studies show the particular influence of the above mentioned parameters on the coatings microstructure. The influence of the remelting of deposited particles caused by the following particles is regarded. The amount of remelted volume depends on the particle feed rate, particle size distribution, heat content and substrate temperature. By controlling these parameters even directionally solidified coatings can be produced. The calculations are in good agreement with the experiments.

The properties of thermally sprayed coatings significantly depend on the alloy composition and the adjusted process parameters. In addition to the powder certificate it may be useful to analyse the chemical composition of the sprayed powder during the spraying process itself. The principle of composition analysis is similar to the chemical analysis in an ICP plasma but the boundary conditions are more complex because the sprayed powder should not be completely evaporated in a thermal spray process. Nevertheless all thermal spraying processes lead to a certain evaporation of the species and to excitation of atomic states. The transition into the ground state occurs under emission of characteristic lines. The intensity of these lines is influenced by the plasma temperature, the particle temperature, the temperature dependent evaporation rate of the alloying elements and the powder feed rate. In consideration of the boundary conditions and the information from a detailed analysis of the emitted spectra the lines can be used to quantify the chemical composition of the sprayed alloys online. The theory of the principle for on-line analysing the chemical composition will be deduced and the first experimental validation will be presented.

The crystal structure of the coating depends on the splashing and solidification of the sprayed particles. The splashing of the particles is simulated by commercial software taking temperature dependent parameters like melt viscosity and surface tension into account. This article provides the theoretical background and the experimental verification. The steps of simulation solidification, splashing, and heat and momentum transfer help to develop novel coating systems with defined microstructure and shorten the time to market. Experimental investigations were carried out using Ni and Ni/Cr 80/20 spray powders with a mean particle diameter of about 15 μm. The theoretical investigations to describe the flattening process of a droplet show the influence of particle velocity and temperature. It was shown that different material parameters, as latent heat or surface energy, have a significant influence on the simulation results.

Some applications of thermally sprayed coatings need a metallurgical bonding of substrate and coating. This can be reached by laser remelting of a thermally sprayed coating, which causes, on the other hand, a certain dilution of the substrate elements into the coating. This article discusses the influence of reaction enthalpies on the microstructure formation in the alloying systems Ni-Al and Ti-Al. Experimental work and simulation were done to examine the time constants of solidification influenced by laser dwell time and reaction enthalpy. It was observed that, for short dwell times, the reaction heat dominates the solidification process and the microstructure formation.