IMPROVING THE SURFACE PROPERTIES OF INCONEL 718 BY APPLYING A CO_2 LASER HEAT TREATMENT TO A HIGH-VELOCITY OXY-FUEL COATING OF WC-CrCo POWDER

摘要

A micron-sized WC-CrCo powder was coated onto an IN718 substrate using high-velocity oxyfuel (HVOF) thermal spraying. To further improve the surface properties, the HVOF coating was heat-treated by a CO_2 laser. The surface properties of both the coating and the laserheated coating were then compared. The HVOF optimal coating process (OCP) for a coating with the highest surface hardness was determined with the Taguchi program. The friction and wear behaviors of the coating, an electrolytic hard chrome (EHC)-plated coating and IN718, were comparatively investigated via a reciprocating sliding wear test at both 25 and 450℃. The friction coefficient (FC) for all three samples decreased when the sliding surface temperature increased from 25 to 450℃. The FC of the coating decreased with increasing surface temperature: 0.33 ± 0.02 at 25℃ to 0.26 ± 0.02 at 450℃; the coating had the lowest FC among the three samples. At both temperatures, the coating wear depth (WD) was smaller than those of the EHC sample and IN718. At room temperature, WC-CrCo and the EHC coatings had good wear resistance and had only a shallow WD. IN718, however, had poor wear resistance with 50 μm deep grooves created from fretting corrosion that arose during the 1500 reciprocating slides (2.5Hz, 10min sliding wear test). At 450℃, the coating WDs were much shallower than those for the EHC coating and IN718: 0.5-μm deep grooves compared to 60–70-μm deep grooves. These results proved that the coating provided a protective coating for IN718 and other metal components. With the OCP coating fabricated from the powders on the IN718 surface, the surface hardness increased 316% from 399Hv to 1260Hv. Furthermore, by laser heating the coating surface for 0.6 s, the hardness increased 44% from 1260 ± 30Hv to 1820 ± 100Hv, porosity decreased more than five times from 2.2 ± 0.3% to 0.4 ± 0.1%, and the coating thickness decreased 17% from 300 to 250 μm. These results showed that both the WC-CrCo powder coating and the laser-heating improved the surface properties of IN718.