Nickel-based Gradient Coating on Copper Prepared by the Combination of Laser Alloying and Laser Cladding Technologies

Combining laser alloying and wide-band laser cladding processes, a gradient coating consisting of a Cu-Ni alloyed layer in middle and a NiCrBSi cladding layer on top was prepared successfully. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), microhardness tester and tribometer were used to investigate the phase constitutions, microstructures, microhardness and wear properties. The results show that the gradient coating is free of pores and cracks. All the interfaces between different layers achieve metallurgical bonded. In the alloyed layer the Ni alloying element and Cu base element are thoroughly miscible, resulting that the thermal conductivity of the alloyed layer is only 1/8 of that of the Cu substrate, which could promote the formation of molten pool in laser cladding. In the laser cladding layer, the phases are mainly composed of gamma- Ni, alpha-(Cu, Ni), M23C6 (M=Cr, Ni, Fe), CrB and Ni3Si, and the microstructures consist of dendrites, interdendritic eutectics, black flower-like phases and short rod-shaped phases. The microhardness of the whole gradient coating shows a gradual increase from 80 HV in bottom to 400 HV on top, indicating a characteristic of stress relaxation structure. The wear loss of the gradient coating is only 1/10 of that of the Cu substrate after being abraded for 400 m.