Effect of heat treatment on residual stress and wear behaviors of the TiNi/Ti2Ni based laser cladding composite coatings

Abstract

The TiNi/Ti2Ni based composite coatings reinforced by TiC and TiB2 were prepared on Ti6Al4V at different circumstance temperatures (25°C, 400°C, 600°C, and 800°C) by laser cladding, then were preserved for 3h. Macromorphologies and microstructures of the coatings were examined through an optical microscope (OM), an X-ray diffractometer (XRD), a scanning electron microscope (SEM), and an energy dispersive spectrometer (EDS). Residual stresses along the depth direction of the coatings were measured by the nanoindentation method, and wear behaviors of the coatings were also investigated using an ultra-functional wear testing machine. Results showed that the coatings were mainly composed of TiNi/Ti2Ni as the matrix and TiC/TiB2 as the reinforcement. A small amount of Cr2Ti was formed in the coatings prepared at 400°C and 600°C. Besides that, Ti3Al was also observed in the coating prepared at 800°C. The tensile stress existed in the coatings prepared at 25°C, 400°C and 600°C when the coating prepared at 800°C was regarded as the stress-free reference. The average residual stress in the surface of coating prepared at 25°C reached the largest value of about 2.79GPa and presented a decreasing tendency with increasing the circumstance temperature (1.03GPa at 400°C, 0.52GPa at 600°C, and 0GPa at 800°C). It revealed that the rise in circumstance temperature contributed to the reduction in cracking susceptibility in the laser cladding coating. However, the wear volumes of the coatings were increased with increasing the circumstance temperature (0.1912 mm3 at 25°C, 0.2828 mm3 at 400°C, 0.3732 mm3 at 600°C, and 0.6073 mm3 at 800°C) due to the weakening in strain-hardening effect and the reduction in reinforcement density. The wear mechanism of the coatings was transformed from the single brittle-debonding into the combination of micro-cutting and brittle-debonding when the circumstance temperature was changed from room temperature to high temperature. The suitable circumstance temperature should be 600°C, at which a comparatively high wear resistance was maintained on the premise that the residual stress was effectively relieved.