Study On Laser Cladding Nickel Based Alloy And Nano-composite Coatings

Nano & Micro CeO₂ or Sm2O3/Ni-based alloy coatings, deposited on low carbon steel substrate by laser cladding are introduced. Cross-sections of single track and multi-track, single and multi-layer of the coatings have been examined to reveal microstructure and phase composition using optical microscope, scanning electron microscope (SEM, including EDS microanalysis) and X-ray diffraction instrument (XRD). Microhardness profile, electrochemical corrosion and wear resistance of the coatings were also tested. The 3D finite element models were set up for the coatings with the theories of elastic-plastic deformation and nonlinear transient heat transfer. The temperature and stress field of Ni-based alloy coating were simulated by the MSC. Marc 2005 non-linearity finite element software.The results showed that smooth coatings were prepared with proper processing parameters (P=2.0 kW, V=180 mm/min) by adding 1.5% nano-CeO₂.Homogeneous distribution of alloy elements and microstructure was obtained by mechanically grinding the mixture of nano-CeO₂/ Sm2O3 and Ni-based alloy powder. There were primary phaseγ-Ni, Cr₂₃C₆ and Ni₃B in Ni-based alloy coatings. CeNi₅ appeared with nano-CeO₂ addition, and Fe₇Sm, Ni₃Si existed in Sm₂O₃/Ni-based alloy coatings. Directional solidification of dendrite was found at the interface in 1.5% micro-CeO₂ / Ni-based alloy coatings, whereas fine and multi-orientation growth of dendrite appeared in 1.5% nano-CeO₂ / Ni-based alloy coatings. Finer and actinomorphous dendrite was found at the interface by adding 3% nano-CeO₂. At the bonding area of 1.5% Sm₂O₃/Ni-based alloy coatings, dendrite growth was observed by adding micro powders where equiaxed dendrite was shown with nano powder addition. With rare earth oxide powder addition, the width of the metallurgical bonding was smaller than that of Ni-based alloy coatings.The hardness and wear resistance of the coatings were improved greatly with nano-CeO₂ addition. With increase amount of powder, the mechanism of wear was found from adhesive wear to abrasive wear. In acid corrosion solution, extractive corrosion was found in Ni-based alloy coatings and micro-CeO₂/ Ni-based alloy coatings, but uniform corrosion in nano-CeO₂/ Ni-based alloy coatings. The corrosion resistance was greatly improved with 1.5 % nano-CeO₂ addition. Compared with the coatings with nano-CeO₂ addition, the microhardness and wear resistance decreased but the corrosion velocity increased in the coatings with nano-Sm₂O₃ addition.The simulation results showed that with same laser power density, the mean stress in the coatings decreased by reducing laser scanning speed. Stress concentration area and average value of the thermal stress in the coatings could be effectively reduced by preheating the substrate, controlling time interval between tracks and adjusting scanning path. Cracks produced at the beginning of the second track by stress concentration which direction was 45°angle to that of laser scanning speed.