HVAF sprayed Inco 718 coatings for repair applications

Purpose and goal

The potential utility of high velocity air fuel (HVAF) spraying for repair and re-manufacture has been assessed and compared with high velocity oxy-fuel (HVOF) spraying, the currently used repair method. Tensile testing of grooved Alloy 718 specimens re-built by the HVAF and HVOF routes as well as bulk Alloy 718 specimens was carried out. Tensile tests results according to SS-EN ISO 6892-1:2009, using strain rates of 0.0023/s up to 0.2% strain and 0.008/s up to failure, suggest that HVAF repair of Alloy 718 parts is possible and can yield properties at least as good as the HVOF process.

Expected results and effects

Although the Young´s modulus of samples was reduced by the presence of coatings as compared to bulk material in both cases, the HVAF built specimens were observed to yield values higher than corresponding HVOF built specimens. During tensile testing, the HVAF coatings were also noted to withstand higher strain than HVOF coatings before visible coating delamination could be seen. This higher adhesion of the HVAF coatings, leading to a higher stiffness of samples, was noted despite tensile testing not being the optimal loading case and is promising for further testing under compression.

Planned approach and implementation

Efforts to build free-standing specimens by HVAF led to deformation of samples during spraying. To simulate repair, 3 mm thick Alloy 718 sheets were water-jet cut, and grooves milled into them for subsequent build-up by HVAF and HVOF. Using a suitable fixture with masking tools, the grooves were filled by spraying a 1,8 mm thick coating by HVAF and HVOF. All coatings were ground flush with the sheet surface and water-jet cut into dog bone shape for tensile testing. Blank Alloy 718 samples were used as reference. 10 samples per condition were tested to have a reasonable level of statistics.