The Effect On Microstructure And Cracking Sensitivity Of Laser Cladding Coating By Substrate Preheating

The study aimed to effectively reduce the cracking susceptibility of the laser clad coating by enhancing the environmental temperature during laser cladding,and reveal the residual stress distribution in different depths of the coating.The Ti Ni/Ti2 Ni based coatings were prepared on Ti6Al4 V by laser cladding at different environmental temperatures of 25,400,600 and 800 °C.The changes in residual stress along the depth of the coatings were investigated in detail by the nanoindentation method.Results showed that the residual tensile stress of about 2.90 GPa average value in the coating prepared at 25 oC was largest.With the increase in environmental temperature,the average value was reduced to 1.34 GPa(400 °C),0.70 GPa(600 °C)and 0 GPa(800 °C).For all the coatings,the residual stress was increased with increasing the distance from the coating surface.Enhancing the environmental temperature contributed to reducing the cracking susceptibility of the coatings.Ti Ni/Ti2 Ni based composite coatings reinforced by Ti C and Ti B2 were prepared on the Ti6Al4 V substrate at different circumstance temperatures(25 °C,400 °C,600 °C and 800 °C)by laser cladding,then were preserved for 3 h.The residual stresses along the depth of the coatings were measured by the nanoindentation method and wear behaviors of the coatings were investigated using an ultra-functional wear testing machine in detail.Results showed that the coatings prepared at different circumstance temperatures were mainly composed of Ti Ni/Ti2 Ni as the matrix and Ti C/Ti B2 as reinforcement.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 2.79 GPa and presented the decreasing tendency with increasing the circumstance temperatures(1.03GPa at 400 °C,0.52 GPa at 600 °C and 0 GPa at 800 °C).It revealed that the rise in circumstance temperature will contribute to the reduction in cracking susceptibility in the coatings.However,the wear volumes of the coatings were increased with increasing the circumstance temperatures(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.The wear mechanism of the coatings was transformed from the single brittle-debonding into the combination of the micro-cutting and brittle-debonding when the processing temperature was changed from room temperature to high temperature.The processing temperature should be lower than 600 °C,at which the comparatively high wear resistance was maintained on the premise that the residual stress was effectively relieved.