Veröffentlichungen und Vorträge

Laser cladding is a very promising technology from a metallurgical point of view. The benefits are low heat input into the substrate, high cooling rates resulting in a fine microstructure of the coating and an excellent metallurgical bonding to the substrate. The main disadvantage of laser cladding, however, is poor process efficiency, which causes high processing costs. In order to gain interest on the industrial level an optimisation of the energy management, for example reducing the energy loss through conduction, is necessary. Improved process efficiency and reduced processing time can be realized by a hybrid technology of coupling laser with a plasma arc. The feed can be coaxial to the plasma gun in form of powder. The main advantage of this process is that the processing area as well as the powder is preheated and activated by a transferred arc. The laser power is mainly used for heating up the feedstock material till melting temperature. The experimental results prove the theoretical considerations. Compared to laser cladding an increase in cladding speed and a lower energy input are reached. Further the efficiency towards material can be nearly doubled, reducing stock costs. At last the heat-affected zone is reduced, which offers the possibility to process crack sensitivity materials.

Low running costs, high spray rates and efficiency make electric arc spraying a good tool for coating large areas with high production rates. The main applications are in the field of corrosion and wear protection of large structures, e. g. parts for bridges or offshore industry. New applications are expected for high quality coatings produced by cored wires. Disadvantages of the arc spraying process are that only electrically conductive wires can be processed and the lower particle velocity in comparison to other thermal spray processes like HVOF or APS. Depending on the process parameters the oxidation of particles has a negative effect on the mechanical and the electrochemical properties of the coating, too. In this paper some investigations with new and flexible power supply systems for arc spraying are presented. The particle size and the morphology of the coating can be optimised, due to the possibility of changing the current generator characteristic and modulating the power by pulsing up to 500 Hz. The oxidation of particles can be reduced by a lower heat input based on lower spray voltage. For a higher quality of the coating microstructure investigations with dynamic generators were performed for Zn, Al, ZnAl, 110MnCrTi8 and Al. An enhancement of the process stability was achieved, too.

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.