HVOF-Sprayed Alloy In718 – The Influence of Process Parameters on the Microstructure and Mechanical Properties

New near-net-shape structures of alloy Inconel 718 processed by HVOF spraying require optimum mechanical properties. Dominant factors defining the material quality are the particle properties velocity and temperature adjusted by the HVOF process parameters. Based on theoretical analysis of the HVOF process, experiments were performed with a defined variation of primary process parameters, producing coating samples of alloy 718 and measuring the particle velocities. Microstructural and X-ray analysis shows that in coatings with a high fraction of molten phase and high velocity, mainly divalent and spinell-type oxides are formed during particle impact on the substrate. Due to severe oxidation of the y'/y''- forming elements Ti, Al and Nb, precipitation-hardening effects of In 718 coatings are low. This leads to merely mediocre mechanical properties. The reduction of the molten phase to nearly zero leads to a drastic decrease of the oxide formation. The hardening γ'/γ'' phases are precipitated homogeneously in the Ni-base matrix. Strength values comparable to cast and wrought alloy In718 are attained by spraying with a low molten-phase fraction and high particle velocity. However, extensive intergranular 8-phase precipitation due to too high an Nb content of the powder causes only mediocre fracture elongation. Coatings up to 10 mm thick have been sprayed. The construction effort and hence the costs and weight of combustion chambers for hypersonic propulsion systems are to be reduced through direct thermal spraying of the loadbearing metallic pressure jacket onto the tubular cooling system. As a semifinished product, the selected Inconel 718 alloy exhibits good mechanical properties in the cryogenic temperature range as well as under higher thermal loads, and is commercially available in powder form. Aging serves to increase the strength up to the range of 1,200 N/mm2. For the sprayed In718 version, coating thicknesses in the centimeter range, a porosity < 1% and mechanical properties comparable with those of the cast version are required. The objective of the research work is to optimize spray-process control so that the resultant structural thick layers meet the design as well as the material requirements with respect to combustion-chamber technology. This necessitates elaborating the dominant microstructural parameters influencing the mechanical properties and the effect on them of the spraying process, and correlating them with the particle-condition parameters and the process parameters [1].