Veröffentlichungen und Vorträge

DC plasma jet CVD is applied to synthesize coatings from liquid state single source precursors on metallic substrates. For SiBCN coating synthesis hexamethylcyclotrisilazanoboranes with Si/B ratio 3:1, 3:2 and 3:3 are used. SiCNTi coatings are produced with tris(dimethylamino)- and tris(diethylamino)hexamethylcyclotrisilazanotitanium. Attempts to deposit SiCNZr coatings by use of tris(diethylamino)hexamethylcyclotrisilazanozironium are not successful due to too high precursor viscosity. Vacuum plasma sprayed nickel, molybdenum and copper interlayers prove to permit effective suppression of nitride and boride reaction zone formation, which is observed during direct SiBCN coating deposition on mild steel substrates. By use of adapted process parameters coatings with columnar or dense morphology and amorphous or nanocrystalline structure can be synthesized. Deposition rates exceeding 2000 my m/h are achieved. By relative movement between DC plasma torch and substrate surface the coated area is enlarged. Simultaneous improvement of coating density and homogeneity is observed.

Laser cladding offers several advantages as low dilution, low thermal influence and distortion of the components and a fine microstructure due to the high cooling rate. Nevertheless the set of laser cladding in the industrial production is nowadays limited to few applications, as the efficiency of laser sources is still too low and the running costs are too high. The absorption behaviour of metal surfaces, as for example steel, towards diode laser (up to approx. 42%) is very high compared to Nd:YAG (approx. 35%) and CO2-Laser sources (approx. 11%), so that some benefits can be expected. One of the disadvantages of diode laser was the low intensity and the difficulty to couple it in a fibre. The development of the last years reached to overcome these boundaries. High power diode lasers are nowadays available till 6 kW and can be coupled in fibres, allowing an easier processing. Investigations regarding laser alloying with high power diode laser confirm the appreciable reliability of this laser class. In order to improve efficiency of laser surface processing the hybrid process plasma-augmented-laser-cladding (PALC) with an Nd:YAG laser source was presented and confirmed that due to the pre-heating of the powder through the plasma transferred arc and the activation of the metallic surface a higher processing velocity (factor three) and efficiency could be reached by summing only one third of the energy with a PTA-equipment. Further more when processing with higher velocity the energy loss due to conductivity decreases so that a further positive increase of efficiency can be depicted. The suitability of fibre coupled high power diode laser for PALC hasn’t yet been investigated, even if the set of this laser type would be very advantageous. - In this paper investigations regarding the process design and the necessary set up of the hybrid PALC-process with a 3 kW fibre coupled high power diode laser and wire coating metal are reported.

In the last years laser beam cladding was recognised as a high quality coating process. Low thermal influence, high cooling rates, metallic bonding, minimal surface roughness are only some of the positive aspects. On the other hand the process efficiency is very low and the running costs in comparison with PTA or Thermal Spraying are high. Attempts to improve the productivity, aim to enhance the efficiency of the beam source itself or to optimize the energy management. This is for example possible through a higher coating speed or through hybrid setups, which allow the use of an additional preheat source (e. g. plasma-assisted laser cladding). Regarding flexibility, defined as the capability of a process to answer in a short time to application requests, the effectiveness of laser cladding can be widely increased through free forming (shaping) of coatings without additional clamping devices. The geometric shape of the coating seam is mainly defined by gravity and surface tension of the melt. An additional force, as for example Lorentz force, can optimize the geometry and improve the process conditions. Wide seams allow for example a low number of overlapping layers when coating large areas and rise the specific area deposition rate. Slim coatings on the contrary are advantageous when generating 3&Ohaak;dimensional structures. The induced force depends from the applied magnetic field and the flow of an electric current. The presented investigations clarify the physical background, the interaction of the magnetic force and the geometric shape of the coating and the possibility to apply them for technical coatings. The application of an external force made it possible to increase the efficiency of the laser cladding process and to gain more interest of the industry.

Due to the boiling temperature of zinc (907 deg C), which is lower than the steel melting point, the welding of zinc coated steel sheets presents many difficulties. As a result of the violent evaporation of zinc, pores in the weld seam are present after solidification and the zinc coating near the weld is damaged. ZnAl-alloys are characterized through low melting temperature, which are comparable to the melting point of zinc, so that the damaging of the zinc coating can be reduced. Investigations carried out with ZnAl-materials for joining zinc coated steel sheets as DC04ZE75/75 and DX56Z are reported. The process reliability of laser soldering with ZnAl-alloys and a Nd:YAG as well as a diode laser is reported. The low surface tension leads to a wide bearing section. The edge welds were evaluated through tensile tests. Soldering with ZnAl-wires reduces the damaging of zinc coatings. Due to the low melting point of these alloys, which is lower than the boiling point of pure zinc, it is possible to process zinc coated steels. The best results were achieved for Nd:YAG-laser with a power of about 1500 W to 1800 W and a ratio of wire-/process-velocity of 2:1 to 1:1. For the use of diode lasers the results are similar, but a higher laser power and a smaller ratio of wire-/process velocity are needed. The optimization of the process parameters, especially a higher ratio of wire-/process velocity, allowed a better filling of the soldering gap. If the ratio is too high the soldering material is only partly molten and results in an incomplete bonding.The damage of the zinc layer was reduced to a very small region of about 100 micrometer or was avoided completely. The high capillary pressure and the low viscosity of the zinc melt leads to an extreme wide bonding area, which can reach values up to 4 mm. The transition from the soldering material to the steel sheet is very uniform, thus the notch effect is reduced.