Microstructure and Mechanical Properties of Nickel-Aluminum Bronze Coating on 17-4PH Stainless Steel by Laser Cladding

Bimetallic copper-steel composite could be an effective structural material to improve the performance of traditional nickel-aluminum bronze (NAB) ship propeller due to its high structural strength and corrosion resistance. In this work, the defect-free NAB coatings has been successfully fabricated by laser direct depositing technique on the 17-4PH stainless steel substrate. The phase constitution, microstructure characteristics and hardness properties were investigated in details. The XRD results showed that the coatings mainly consisted of α-Cu, Fe and intermetallic κ phases despite the diffraction peaks shifted more than 0.5°, which may due to the influence of the Ni, Fe and Al atoms dissolved into Cu-matrix. The microstructures of the coatings were affected significantly by laser energy density according to SEM and EDS results. The top region of the coating was more undercooled during solidification, therefore the grains at this region was much finer than that at the bottom region. The higher energy input would lead to coarser grains. Fe-rich dendrites and spherical particles were found in the Cu matrix, which could be a result of liquid separation. The hardness of the coating is in the range of 204 HV 0.2 –266 HV 0.2 which is higher than traditional as- cast NAB. The uneven distribution of Fe-rich phases as well as the hard κ phases could be the main reasons for the fluctuations of the hardness value. Tensile fracture occurred at bronze side, not at transition zone, which shows there is a good interfacial bonding between the two metals produced by laser cladding.