The aim of this study was to investigate the effects of the cryogenic treatment (CT) using liquid nitrogen on tensile properties and microstructures of the 2024-T351 aluminum alloy. Tensile tests were carried out, and tensile fractures were observed using a scanning electron microscope (SEM). The microstructure evolution of 2024-T351 subjected to CT was also studied using both an optic microscope (OM) and a SEM. The components of the second phase were tested with an energy dispersive spectrometer (EDS). The results showed that both the ultimate strength and the yield strength of the 2024-T351 aluminum alloy could be improved through CT without the sacrifice of elongation. In addition, tensile fractures showed that the plasticity of 2024-T351 aluminum might also be improved, as the dimples in the fracture of the CTed specimens were markedly more uniform compared with the untreated specimen. The phenomenon of grains refinement (GR) was found through microstructure observation. It was also found that the second phases were distributed more uniformly after CT. A conceivable mechanism concerning the shrinking effect and crystal grain movement was raised to explain the experimental phenomena. The effects of CT on residual stress in the 2024-T351 aluminum alloy are discussed herein. Measurements showed that tensile residual stress in 2024-T351 was removed, and slight compressive residual stress was generated after CT. This may also contribute to the improvement of the tensile properties of the alloy.

Laser cladding was used to prepare three composite coatings, i.e., Inconel 718 nickel-based superalloy (IN718), IN718-50 wt.% WC created by adding tungsten carbide (WC) particles and IN718-50 wt.% WC assisted by ultrasonic vibration. The phase composition, microstructure evolution, microhardness, residual stress and tribological properties of the three coatings were studied. The addition of WC enhances hardness and improves tribological properties, but also causes aggregation and considerable formation of carbides associated with rough structure. Ultrasonic vibration greatly refines the solidification microstructure, as it can break the growing dendrites, reduce the aggregation of reinforced particles and refine solidified structure. The average microhardness of the latter two composite coatings was increased by 36.37% and 57.15%, respectively, compared with the first IN718 coating, and the last composite coating (ultrasonic assistance) had the lowest COF (0.494). The WC particles and refined carbides converted the wear mechanism from adhesive wear to abrasive wear. In addition, the resultant stress on the surface of the composite coating roughly doubled after adding 50 wt.% WC, and only increased by 49.53% with ultrasonic treatment. The simulation results indicate that acoustic cavitation mainly occurs in the middle and bottom of the molten pool and proper frequency ultrasonic is conducive to the generation of the cavitation effect.

Laser cladding coating with wolfram carbide (WC) as enhanced particles can improve the performance of nickel-based materials. However, there still exists several problems, such as serious element segregation and unequal distribution of the reinforcement phase. In order to improve the mechanical properties further, IN718/WC coatings were prepared by ultrasonic vibration-assisted laser cladding. The effects of ultrasonic vibration on the ceramic distribution, microstructure, and wear performance were systematically studied. The results show that ultrasonic vibration can promote the uniform distribution of WC particles without changing the phase composition of the coating. The cavitation and acoustic flow induced by ultrasonic vibration interrupt the growth of columnar dendrites and refine the grains. In addition, the microhardness of the ultrasonic vibration-assisted coating is enhanced by 15.6% to 475 HV_0.2. The average coefficient of friction (COF) of the ultrasonic vibration-assisted coating is 0.452, which is 17.5% lower than that of the unassisted coating. The wear volume of the coating is decreased significantly, and the wear characteristics change from severe adhesive wear to the slight coexistence of abrasive wear and adhesive wear.