Laser cladding시 발생하는 crack 생성 및 전파 거동에 관한 연구

Abstract

The laser cladding process has been used to improve the surface properties such as wear and corrosion resistance of the materials. However, unwanted cracking problems can occur during and after laser process. In this study, the cracking behavior of Co-based stellite#6 mixed with WC+NiCr composite coatings was investigated according to change in the processing variables such as laser power, scan speed, and feeding rate. The microstructure of clad layer was composed of Co-based dendrite (γ) structures with γ+Cr7C3 eutectic and WC particles. The laser clad was susceptible to the hot cracking due to the presence of continuous γ+Cr7C3 low melting temperature eutectics along the subgrain boundaries. The number of cracks significantly increased with the increase of scan speed due to the change of thermal stress caused by the difference of cooling rate. Further, the increase of laser power and feeding rate promoted the formation of clad cracks by increasing the volume fraction of pores in the clad layer. The specimen overlaid with 100wt% stellite-6 displayed 200MPa maximum tensile stress due to contraction of the coating layer generated by solidification. In contrary, the specimen overlaid with 55wt% stellite-6 and 45wt% WC+NiCr showed higher 300MPa tensile stress than that overlaid with 100% stellite-6 by the difference of thermal expansion coefficient between the stellite-6 and WC particles during solidification. And it is able to generate fracture of the WC particles owing to low tensile strength of WC particles. Thus it is assumed that the clad cracks are created in the WC particles and propagate along the matrix. Hence WC particles are major initiation site of the clad cracks in the coating layer. Consequently, the residual stress affected the generation of clad cracks on three types of initiation site. First, the cracks were created by fracture of WC particles due to low tensile strength of WC particles. Large size pores also caused initiation of the clad cracks under the influence of the residual stress. Finally, solidification cracks propagated in a shape of brittle fracture by residual stress of the coating layer after solidification.