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Improvements of coating properties and residual stress states in arc sprayed aluminium bronze coatings by using an alternative gas mixture

: Hauer, M.; Henkel, K.-M.

Deutscher Verband für Schweißen und Verwandte Verfahren e.V. -DVS-:
International Thermal Spray Conference & Exposition, ITSC 2017 : Düsseldorf/Germany, June 7-9, 2017
Düsseldorf: DVS Media, 2017 (DVS-Berichte 336)
ISBN: 978-3-96144-000-9
ISBN: 3-96144-000-X
International Thermal Spray Conference & Exposition (ITSC) <2017, Düsseldorf>
Fraunhofer IGP ()

Arc spraying is a simple and cost-effective thermal spray technology, which is determined by only a few influencing factors. Arc voltage, current and gas flow, pressure and type mainly affect coating quality. Besides, residual stresses in the coatings are strongly influenced by these parameters and the spray kinematics.
Since arc spraying is already established for repairs of large structures, the process is also considered for restoration of eroded ship rudders. Cavitation erosion, the formation and collapse of vapor bubbles caused by pressure oscillations, has a great impact on these damages. Typically, compressed air is used as atomizing gas, which on the one hand results in partly large oxides in the coatings. On the other hand, there is a significant melting loss of alloying elements, having a high oxygen affinity, during the spray process. This negatively affects cavitation erosion resistance of aluminum bronzes. Furthermore, tensile residual stresses near the coating surfaces, mainly induced by quenching stresses, contribute to this material loss.
Recent process developments revealed that use of different atomizing gases improves coating quality. Gases like nitrogen and its mixtures with hydrogen create a less reactive atmosphere and diminish the temperature of the particles in the spray jet. Hence, less oxidation occurs and residual stresses are reduced due to less quenching stresses.
In the work presented, the highly cavitation erosion resistant propeller alloys CuAl9Ni5Fe4Mn and CuMn13Al8Fe3Ni2 were arc sprayed using a mixture of nitrogen and 2 % hydrogen as atomizing gas. Residual stresses were measured by modified hole-drilling method using ESPI. Microstructural, chemical and mechanical analyses were realized to examine adhesive and cohesive properties. Additionally, analyses of the cavitation erosion behavior were carried out. In comparison to coatings sprayed with pressurized air, the results of the study show superior coating qualities with regard to microstructure, cavitation erosion resistance and residual stresses.