Study On Microstructure And Properties Of Al₂O₃ Based Coatings By Laser Cladding Assisted High Frequency Light Vibration

In this paper, Al₂O₃/TiO₂ coating have been prepared on Ti-6Al-4V alloy substrate from nanostructured Al₂O₃/TiO₂ powder under laser cladding and high frequency light vibration composite laser cladding process, respectively. The process of laser cladding Al₂O₃/TiO₂ coating on titanium alloy surface were explored, the structure, phase composition and microhardness of coatings were also investigated and tested, respectively. While Al₂O₃/NiTi powder were fabricated by the mixed powder milling process, Al₂O₃/NiTi coating were prepared on the surface of titanium alloy by laser cladding process. The effect of Al₂O₃ to microstructure and phase composition of the coating were analyzed, the microhardness and friction and wear performance of the coating were tested at the same time.The microstructure of laser cladding Al₂O₃/TiO₂ coating presented streamlined features. Larger unmelt particles in the coating tend to distribute to near the surface of the the coating zone, and unmelt particles close to the coating and substrate interface zone is small. The coating and substrate formed good combination. The internal microstructure of coating was density and uniform without obvious cracks and pores. The coating consists of Ti, AlTi3, α-Al₂O₃ and γ-Al₂O₃ phase. The actinomorphic structure in coating is unmelt or partially melted particles, and surrounding materials formed melting structure after fully melting. The line scanning of the coating show that the interface region composition of coating and substrate changes in small range. But composition in the coating change bigger which caused by convection in molten pool. The microhardness of the substrate is 360 390 HV0.3, while the coatings reach 1000 1300 HV0.3, which is about two times higher than that of the substrate.Under the effect of high frequency light vibration, laser cladding Al₂O₃/TiO₂ coating on the surface of continuous, smooth, less surface defects. Compared with the direct laser cladding process, the coating is easier to shape, surface macroscopic quality is better, the structure of the coating is compact, the combined coating with the titanium alloy substrate is better. Under different vibration frequency, the microstructure of the laser cladding Al₂O₃/TiO₂ coating presented obvious regional features. Dendrites become broken dendrites in internal coating, and microstructure was dendrites and some broken dendrites at bonding part of coating. When vibration frequency is 500 Hz, mostly microstructure of coating was broken grains, there is also part of isometric crystal and a small number of branches. The coating consists of α-Al₂O₃, Al2TiO5, Ti, AlTi3, Brookite-TiO₂ and Rutile-TiO₂ phase. The microhardness of the coating reach 1200 HV0.3, and the microhardness of substrate is 360³90 HV0.3. The microhardness of the coating is about two times higher than that of the substrate.The combination of laser cladding Al₂O₃/NiTi coating and substrate region has obvious bright line, which showing a good metallurgical bonding of coating and substrate. The microstructure of Al₂O₃/NiTi coating shows a three-dimensional net structure, which made of white and black structure. EDAX and XRD results show that the main phase of NiTi coating is NiTi₂ and NiTi phases, while the main phase of Al₂O₃/NiTi coating was NiTi₂, Ti and a small amount of Al₂O₃ phase. Bright white structure was NiTi₂, black structure and black flower-shaped dendrite structure was Ti-rich structure. The microhardness of NiTi coating reach 720 HV0.1, and the microhardness of Al₂O₃/NiTi coating reach 760⁸00 HV0.1. Friction and wear curves have four stages including running-in period moderate growth, low, rapid growth and stable period. The wear loss of the coating was less than that of the substrate. However the coefficient of friction was higher than substrate. Al₂O₃/NiTi coating can improve the wear resistance of substrate.