High-strength aluminum alloys are widely used in the aerospace and national defense fields owing to their light weight, high specific strength, and corrosion resistance. However, owing to their material characteristics, high-strength aluminum alloys are often accompanied by coarse columnar crystals and a serious hot cracking tendency in the additive manufacturing process, causing the additive manufacturing of high-strength aluminum alloys to lag behind that of other alloy materials. Compared to conventional Gaussian laser additive manufacturing, swing laser additive manufacturing has a more stable laser keyhole and a smaller molten pool temperature gradient. It can also stabilize the deposition process of aluminum alloys and reduce deposition defects. Recently, oscillating lasers have been widely used in welding and additive manufacturing. In addition to the oscillating laser, high-intensity ultrasonic vibration is widely used to improve the microstructure of metals owing to its cavitation and acoustic flow effects in the molten pool. The deposition formation and microstructure properties of oscillating laser wire additive manufactured sample under ultrasonic-assisted conditions are studied to address the difficult problem of microstructure property control in the laser wire additive manufacturing of high-strength aluminum alloys. The effect of ultrasonic parameters on the formation and microstructure of 2319 aluminum alloy is systematically studied using ultrasonic-assisted swing laser additive manufacturing technology. The effect of ultrasound on the microstructures of multilayer deposited samples and the behavior and mechanism of the molten pool are analyzed.
The test materials are an aluminum alloy welding wire (diameter of 1.2 mm) and a 6061 aluminum alloy substrate (size of 200 mm×100 mm×10 mm). The welding head, wire feeder, and ultrasonic horn remain stationary, the substrate moves horizontally at a constant speed, and the distance between the molten pool and ultrasonic input position is fixed to achieve a constant ultrasonic intensity input (Fig.1). Table 2 lists single pass deposition process parameters. Because ultrasonic waves can produce cavitation, acoustic flow, and mechanical vibrations in the molten pool, they produce a series of strengthening effects on the molten pool. After the test, we compare the macroscopic cross-sections, microstructures, grain orientations, grain sizes, elemental distributions, and phase compositions of the samples using optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscope (EDS), and X-ray diffraction (XRD). We analyze the mechanism of ultrasonic molten pool strengthening based on the grain orientation of the sample, element distribution, and phase composition.
The influence of ultrasonic power on the deposition morphology is generally small but significantly influences the grain size. When the ultrasonic power proportion is 40%, the deposition morphology and grain size are the best in the same group (Figs.3 and 4). The increase in the ultrasonic amplitude leads to an enlargement of the maximum sound pressure region. As the amplitude increases, the area of the maximum sound pressure zone gradually approaches that of the full molten pool, and the area of the maximum sound pressure zone does not change with the increase in amplitude. The ultrasonic amplitude distribution on the substrate is a typical standing-field wave distribution. When the molten pool is close to the ultrasonic input position, the cavitation range of the molten pool increases. When the ultrasonic action interval increases, the cavitation area in the molten pool gradually decreases until the maximum sound pressure value in the molten pool is lower than the cavitation threshold, and cavitation cannot occur. Therefore, the average grain size decreases when the ultrasonic action distance is relatively small, and the effect is better when the ultrasonic beam is located behind the molten pool (Fig.6). The pressure of the amplitude transformer affects the flow in the molten pool by affecting the droplet transition frequency. With an increase in the pressure of the horn, the mechanical vibration energy of the ultrasound is enhanced, which accelerates the transition of the droplet on the welding wire to the molten pool and promotes liquid flow in the molten pool. When the horn pressure increases, the role of promoting droplet transition remains unchanged, and the mechanical vibration effect of the ultrasonic wave reaches a certain saturation state.
The microstructures and composition distributions of multilayer deposited specimens with ultrasonic assistance differ significantly from those without ultrasonic assistance. The ultrasonically assisted multilayer deposition samples show a decrease in the macroscopic defects (Figs.9 and 10), a significant decrease in the grain size of the deposition structure (Fig.11), mainly fine equiaxed crystals, and a decrease in the average grain size of 64% (99.6 μm). The introduction of ultrasound results in more Al2Cu in the multilayer sample. Al2Cu, as the strengthening phase in the 2319 aluminum alloy structure, prevents slip and dislocation (Fig.13). The strengthening effect of ultrasonic waves on the molten pool is essentially realized through cavitation and sound flow effects, which influence each other. The cavitation effect essentially involves multiple cycles of bubble expansion and rupture before the solidification of the molten pool, resulting in fatigue fracture of the grain. The sound flow effect is caused by the attenuation of ultrasound in the molten pool and the sound pressure difference, leading to a low solute flow.
An ultrasonic-assisted swing laser is used for the additive deposition of a 2319 high-strength aluminum alloy. Compared with single-pass swing laser additive deposition, introducing ultrasound can effectively reduce deposition defects, promote melt flow, inhibit columnar grain growth, refine grains, and improve microstructural properties. Simultaneously, the ultrasonic assistance increases the content of the strengthening phase to a certain extent and inhibits the expansion of strain cracks. The ultrasonic parameters also clearly influence the macroscopic morphology and microstructure of the deposited samples.
