Microstructural and mechanical characteristics of laser coatings

Abstract

Abstract The microstructure and mechanical properties of Ni-base alloy, Co-base alloy and (Ni+WC) coatings were investigated experimentally by using X-ray diffraction, optical microscopy, quantitative analysis by X-ray spectroscopy and Vickers microhardness (HV0.3) measurements. Coatings were manufactured by laser cladding on a medium carbon steel Z38CDV5 (3431) substrate using a 5.2 kW continuous wave CO2 laser. Coatings were metallurgically bonded to the substrate, and the processed thickness was up to 2 mm in each case. Solidification planar front, cellular and dendrite structures were observed in the case of a Co-base alloy coating. In contrast, a homogeneous structure was observed in the case of a Ni-base alloy coating. A microstructure containing a δ phase ferrite is observed only in the substrate coated with Ni-base alloys. The high hardness of the Co-base alloys coating is attributed to the presence of M7C3-type carbides (essentially chromium-rich carbides) dispersed in the γ-(Co) phase matrix. The lower hardness of Ni-base alloys coatings is related to the presence of a γ-(Ni, Fe) solid solution and to low concentrations of Cr and C. The metal matrix composite (Ni+WC) coatings are dense, with good mechanical properties (Young modulus, hardness), but contain many cracks. A relative deformation higher than 50% is achieved in the case of Ni-base alloys but with few cracks. Hardness values higher than 450 HV are measured after a large plastic deformation.