Laser cladding has obvious advantages in forming, preparing coatings and repairing metal parts. However, what follows is that the internal defects in its microstructure and poor mechanical properties need to be further studied. A research institute uses ultrasonic-assisted processing for laser cladding to improve the performance of cladding parts.
Based on the role of ultrasonic sonication and cavitation effects on the metal solidification process, the vibration parameters are used to study the degree of subcooling and inoculation rate of the molten pool metal. The experimental results confirmed that after the application of ultrasonic, the grain size is significantly smaller than that obtained when only laser cladding is used.
When the ultrasonic vibration amplitude is 25 μm, the grain size can be refined 0.522 times that of no vibration. The structure and chemical composition of the precipitated phase have been significantly changed. In addition, the effect of high-frequency vibration on the mechanical properties of the cladding layer was also analyzed through comparative experiments. The results show that the application of high-frequency vibration can effectively reduce the pores, and at the same time improve the microhardness and friction properties of the parts. Quantitatively speaking, the friction coefficient is 0.628 when ultrasonic vibration is used and the amplitude is 25 μm, and it is 0.709 when there is no ultrasonic vibration.
Laser cladding technology, often used as a direct energy deposition technology, has recently become a relatively active research field in the world because of its preparation, repair and strengthening capabilities. Among different laser cladding, laser cladding of nickel-based superalloys, such as impeller blades in aero engines, is a particularly attractive research area. Nickel-based superalloys are widely used in the preparation of turbine blades, turbine discs and combustion chambers of aircraft engines. This is because of the high-temperature strength, fatigue properties, oxidation resistance and heat corrosion resistance of the alloy.
However, the laser cladding of nickel-based superalloys also faces huge challenges because of the uneven microstructure and high porosity in the alloy. In order to overcome this challenge, in recent years, ultrasonic vibration technology has been introduced as an auxiliary technology for laser cladding of nickel-based alloys, which can improve its internal microstructure and mechanical properties (micro-hardness and friction properties). ) And reduce the defects formed during the deposition process.
In the process of ultrasonic-assisted laser cladding, the sound pressure changes periodically with the application of ultrasonic waves. Based on the changes in sound pressure, the degree of subcooling and the incubation rate increase with the increase in the amplitude and frequency of ultrasonic vibration. Compared with laser cladding without ultrasonic vibration, the columnar crystals are mainly concentrated at the bottom of the cladding layer. The middle and top of the cladding layer become equiaxed crystals with the application of ultrasonic waves. As the vibration amplitude increases, the grain size is significantly refined, and the number and composition of Laves phases (Nb and Mo) decrease.
In addition, due to the refinement of the crystal grains, the microhardness is improved, and the number of Laves phases and pores decrease with the application of ultrasonic waves. When the ultrasonic vibration amplitude is 25μm, the microhardness reaches the maximum value of 215HV. In addition, the microhardness on the top of the coating is greater than that on the bottom. The main reason is that the columnar crystals are mainly concentrated at the bottom, while the middle and top are mainly equiaxed crystals. As for the heat-affected zone, it can be found that the microhardness gradually increases with the increase of the vibration amplitude. However, the influence of laser power and scanning speed on microstructure, microhardness and heat-affected zone is very insignificant.
As for the friction performance, when the ultrasonic vibration amplitude is 25 μm, the friction coefficient is the smallest, which is 0.628 times that of no ultrasonic vibration. It is 0.709 times of traditional craftsmanship. This means that the coefficient of friction can be reduced by introducing ultrasonic vibration. In addition, the depth of friction is also the smallest when the vibration amplitude is 25 μm, which also reflects that the surface friction damage of the sample is the smallest.
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