Microstructure and Properties of Ultrasonic/Laser Hybrid In-situ Alloying Ti-B4C Coatings on Aluminum Surfaces

Abstract:

Using Ti-B₄C as the alloying material, an in-situ various ceramic particles reinforced aluminum matrix composite coating was prepared on the surface of 7075 aluminum alloy using ultrasonic vibration assisted laser alloying process. The effects of different ultrasonic amplitudes (0, 50, 70 and 90 μm) on the overall morphology, phase composition, microstructure, microhardness and wear resistance of the coatings were investigated. The mechanism of ultrasonic vibration on the size, morphology and spatial distribution of in-situ reinforcement phases in the laser alloying process was analyzed. The results show that assisted ultrasonic vibration during laser alloying does not change the phase composition of the coatings, but promotes the in-situ chemical reactions in the molten pool. The diffraction peaks of in-situ synthesized ceramics reinforcement phases such as TiB₂ and TiC are increased with the rise of ultrasonic amplitude. The strong convection in the molten pool produced by the ultrasonic vibrations results in an improvement of the coating surface macro-morphology, a slight increase in roughness, and an increase in coating thickness and dilution. With the increase in ultrasonic amplitude, the acoustic cavitation effect and the acoustic flow effect are enhanced, and the microstructure of the coating is significantly refined. The in-situ synthesized reinforcement particles are not only increased in number but also more uniformly and spatially distributed. Both in-situ synthesized TiB₂ and TiC are well bonded to the α-Al, with interfacial mismatch rates of about 5.83% and 6.81%, respectively, suggesting that they can act as effective nucleation sites to promote grain refinement. The microhardness of the coatings is significantly increased and more uniformly distributed after applying assisted ultrasonic vibration. The average microhardness of the coatings at an ultrasonic amplitude of 70 μm is about 703.87 HV_0.2, which is 5.77 and 1.66 times higher than the hardness of the coatings in the substrate and without ultrasonic vibration, respectively. At this time, the coating has the optimum wear resistance, and the wear mechanism is only slight abrasive wear. The wear resistance is approximately 11.8 times that of the substrate, and 4.1 times that of the coating without ultrasonic vibration.