Investigation on crack behavior of Ni60A alloy coating produced by coaxial laser cladding

Abstract

Crack is one of the common problems in preparation of NiCrBSi alloy coatings by laser cladding, especially for high hardness coatings. In this study, the crack behavior and prevention of Ni60A coating by coaxial laser cladding are investigated. The results indicate that the nonuniform hard Cr-rich precipitates and large residual tensile stress are the main reason for cracking of Ni60A coating. According to the difference of thermophysical properties and temperature between the coating and the substrate, the thermal stress models are established, in which the stress perpendicular to the scanning direction $$\sigma_{{\text{y}}}$$ is smaller than the stress along the scanning direction $$\sigma_{{\text{x}}}$$ . Due to the change in the relative relationship among $$\sigma_{{\text{x}}}$$ , $$\sigma_{{\text{y}}}$$ and fracture strength $$\sigma_{{\text{f}}}$$ , the crack distribution changes from network to parallel, until to no crack with the increasing line energy and the decreasing powder feed rate. And the angle $$\theta_{{0}}$$ between the direction of resultant stress of $$\sigma_{{\text{x}}}$$ and $$\sigma_{{\text{y}}}$$ and Y-axis is 52.85°, which fits with the direction of long and short cracks in network crack distribution. In addition, the substrate preheating can effectively reduce cracking rate of Ni60A coating and completely prevent crack when preheated up to 500 °C. Also, the crack-free coating can be obtained by the combination of preheating to 300 °C and placing insulated plank under the substrate, which can reduce thermal damage of the substrate and obtain higher microhardness.