Metal melting forming technology plays a vital role in the development of high-end manufacturing industry. In response to the problems of pores, cracks and uneven organization in the process of metal melting forming, researchers proposed ultrasonic external energy field assisted metal melting forming technology to improve the quality and performance of metal melting forming. The mechanism and addition method of ultrasonic vibration in the process of metal melting forming are introduced in detail, especially for metal melting forming methods such as welding, surface cladding and additive manufacturing. The influence of ultrasonic vibration on the microstructure and performance of formed parts is elaborated in detail, and the development trend of ultrasonic vibration assisted metal melting forming technology is prospected.

Metal melting forming is a forming method that melts metal by heating to achieve alloying or metallurgical bonding, such as welding, surface cladding and additive manufacturing. In the fields of aerospace, shipbuilding and high-end equipment, metal melting forming technology plays an important role in the forming, connection and repair of metal structural parts. However, the characteristics of rapid solidification will lead to a large temperature gradient, which will affect the microstructure and performance of metal parts.
According to the metal solidification theory, when the metal melt is in equilibrium solidification or under a small temperature gradient, the grain growth is relatively gentle, and its shape is usually round or cellular. On the contrary, when the temperature gradient is large, the grains will grow rapidly due to rapid cooling, forming a dendritic structure. The solute redistribution inside the dendrite is uneven, resulting in segregation, which in turn affects the uniformity of the structure after the alloy solidifies. The characteristic of metal melting forming technology is that the metal material is melted in a small area using a high energy density heat source, and there is a relative displacement between the heat source and the area to be processed. The moving heat source causes the metal melt to solidify rapidly under extremely high temperature gradients, and the internal grains are usually columnar crystals or dendrites. These two grain forms are prone to form cracks and pores under the action of thermal stress, which seriously affects the performance of the workpiece. Therefore, it is of great significance to regulate the internal structure of metal melting forming parts and reduce internal defects.
In order to solve the above problems, researchers proposed to apply external physical fields during the metal solidification process, and use the interaction between the physical field and the metal melt to regulate the metal solidification structure. The external physical fields include electric fields, magnetic fields and ultrasonic fields. In 1869, researchers introduced mechanical vibration into the casting mold to break up coarse grains and refine the grains of the steel ingot. Ultrasonic vibration is a high-frequency mechanical vibration that can produce unique effects in the metal melt. Eskin studied the effect of ultrasonic vibration on high-purity aluminum and found that the solidification structure after ultrasonic treatment was significantly refined and the mechanical properties were improved. Ultrasonic vibration has the effects of refining grains, inhibiting segregation and reducing temperature gradients, and plays a positive role in the growth and solidification of grains. At present, the introduction of ultrasonic fields as external energy fields into metal melting forming technology has become one of the research hotspots. This paper discusses and summarizes the latest progress and challenges of ultrasonic vibration-assisted metal melting forming technology at home and abroad in recent years, with special attention to the combined application of ultrasonic vibration with metal melting forming technologies such as welding, surface cladding and additive manufacturing. First, the mechanism of ultrasonic vibration in metal melt is briefly introduced; then, according to the transmission characteristics of ultrasonic waves, the influence of ultrasonic vibration in different addition methods on metal melting forming technology is discussed. Then, according to the process characteristics of metal melting forming, the regulation effect of ultrasonic vibration on the microstructure and performance of formed parts is discussed for three forming technologies: welding, surface cladding and additive manufacturing. Finally, the key scientific issues and technical challenges faced by ultrasonic vibration-assisted metal melting forming technology are summarized.

Ultrasonic vibration assisted metal melting and forming mechanism

Ultrasonic vibration will produce periodic mechanical waves with a frequency greater than 20,000 Hz. Ultrasonic vibration in the metal melt causes collision and friction between liquid metal particles, resulting in cavitation effect, acoustic streaming effect, mechanical effect and thermal effect. The cavitation effect causes the cavitation bubble in the metal melt to rupture, and an instantaneous high-speed shock wave and high temperature are generated around the cavitation bubble, which in turn affects the surrounding solidifying grains. Wang et al. observed the rupture process of cavitation bubbles through high-speed photography. As shown in Figure 1, cavitation bubbles are usually formed at the top of dendrites. After the dendrites are formed, there is a certain stress threshold at the root of the dendrite. The instantaneous high temperature and high-velocity shock wave generated by the rupture of the cavitation bubble act on the top of the dendrite, causing the dendrite to bend and then break. Wang et al. used X-ray synchronous radiation technology to realize the real-time observation of ultrasonic cavitation bubbles during the solidification process of Bi Zn alloy under ultrasonic vibration. They found that the acoustic pressure in the melt determines the severity of cavitation bubbles in the melt. Under a pressure amplitude of 14.5 MPa, the cavitation bubbles implode within an ultrasonic cycle after nucleation and growth. In the pressure amplitude range of 0~0.33 MPa, the cavitation bubbles reach a stable state during the process of contraction and expansion, and can remain intact within thousands of ultrasonic cycles. There is a huge temperature gradient in the process of metal melting and forming. After solidification, it is very easy to produce thick dendrites. Dendrites induce texture, and then the mechanical properties of parts deteriorate. Therefore, grain refinement during solidification requires more nucleation sites. The cavitation effect can break large dendrites, and the broken dendrite wall arms can serve as new nucleation sites to promote grain refinement. In terms of numerical simulation, the researchers established the dynamic partial differential equation of cavitation bubbles and used MATLAB to solve the dynamic influence of ultrasonic parameters on cavitation bubbles. The rupture of cavitation bubbles requires sufficient negative pressure and expansion time. After the ultrasonic frequency increases, the expansion and contraction cycle of cavitation bubbles becomes shorter and more difficult to rupture. The increase in ultrasonic amplitude allows the cavitation bubble to receive more acoustic energy, the radius changes more during the expansion process, and the high-temperature shock wave generated after rupture is stronger.

On the other hand, ultrasonic vibration will also change the flow pattern and temperature gradient of the metal melt. Ultrasonic vibration generates an acoustic field in the metal melt, and an acoustic pressure gradient will be formed in the melt, which affects the flow pattern of the melt. The generation of acoustic streaming depends on the direction of application of ultrasonic vibration. The driving force of acoustic streaming is the acoustic pressure gradient in the melt, and the direction of acoustic streaming points to the direction of reduced acoustic pressure. Wang Chuanyu studied the acoustic streaming effect in the process of laser cladding IN718 coating, as shown in Figure 2. The results show that the acoustic streaming effect changes the flow trend of the metal melt. Unlike the Marangoni circulation, in the acoustic streaming flow mode, the axial jet generated by the substrate vibration reaches the middle of the metal melt and then deflects, forming a vortex in the middle and lower parts, while the other areas in the metal melt are almost static.
The cavitation effect, particle collision and particle friction caused by ultrasonic vibration will convert mechanical energy into thermal energy. The metal melt has an acoustic absorption effect. The pores in the melt will increase the absorption effect of the melt on the sound wave, causing the local temperature to rise and reducing the temperature gradient of the metal melt. Wei et al. used Fluent to simulate the effect of in-situ ultrasonic vibration on the flow and solidification behavior of laser cladding metal melt. The results show that the acoustic streaming effect generated by ultrasonic vibration reduces the temperature of the center of the metal melt. With the increase of ultrasonic power, the temperature gradient first decreases and then increases, and the solidification rate first increases and then decreases. The acoustic flow generated by ultrasonic vibration is coupled with temperature change. As the ultrasonic action time increases, the temperature change becomes more obvious. At the location with high flow velocity in the melt, the collision and friction between particles become more intense, and the temperature rises more. At the same time, the acoustic flow stirs the melt, which can reduce the temperature gradient of the melt and inhibit the segregation of elements.

Research status of ultrasonic vibration assisted metal melting and forming

Ultrasonic vibration introduction method
The ultrasonic vibration system consists of a transducer, a horn and a tool head. Among them, the transducer is responsible for converting electrical energy into mechanical vibration at ultrasonic frequency. The horn receives the mechanical vibration from the transducer and amplifies or focuses it according to application requirements. The tool head can be regarded as a secondary horn, which plays a key role in the metal melting and forming process. In metal melting and forming, if the horn is in direct contact with the metal melt, the high temperature environment will affect the working mode of the horn. Therefore, the role of the tool head is crucial. It is the medium for transmitting ultrasonic vibration and ensures that the ultrasonic vibration is transmitted to the metal melt under high temperature conditions. This structure helps maintain the effectiveness of ultrasonic vibration and protects key components from damage by extreme temperatures, thereby ensuring the stable operation and long-term reliability of the system, and providing an efficient and reliable ultrasonic vibration solution for metal melting and forming. In the process of welding, surface cladding and additive manufacturing, the heat source usually moves at a certain speed, resulting in a smaller size of the metal melt, so the device for adding ultrasonic vibration is more complicated. The ultrasonic introduction method can be roughly divided into direct contact and non-contact, as shown in Figure 3.
If the contact is not firm during the transmission of ultrasonic vibration, the transmission effect of ultrasonic vibration will be greatly reduced, so it is necessary to apply a preload at the contact position. Common preload adding methods are direct preload and thread preload. At present, the widely used ultrasonic introduction method is to apply ultrasound under the substrate or work platform. The specific method is to place the ultrasonic vibration unit under the substrate, the tool head directly contacts the substrate, and the contact position is preloaded by adjusting the relative position, so that the tool head drives the substrate to vibrate at high frequency.
When Guo Minhai clad YSK ceramic powder on the TC4 substrate, he added ultrasound from the bottom of the substrate. The preload force was added by direct preload. The laser sensor measured the substrate amplitude at the maximum power to be 3 μm. Lin Xin used high-speed photography and MATLAB image processing methods to analyze the influence of ultrasonic vibration on the morphology of the metal melt and the keyhole during laser welding, as shown in Figure 4. The high-speed photography results show that the increase in the amplitude rod pressure will aggravate the splashing phenomenon of the metal melt, the area fluctuation of the metal vapor plasma will be significantly enhanced, and the average area of ​​the metal vapor plasma ejected in the keyhole will increase. The process of metal melting and forming occurs on the upper surface, and the ultrasonic vibration will attenuate when it is transmitted to the substrate. At the same time, the ultrasonic vibration intensity at different positions on the upper surface of the substrate may also be different.
The preload force applied by adjusting the relative position is affected by the pressure of the contact position, and as the laser is heated and cooled, the volume change of the contact position will affect the transmission effect of the ultrasonic vibration. Therefore, the ultrasonic amplitude provided by this preload force is often small, and the vibration frequency of the substrate is lower than the frequency set by the ultrasonic vibration source.
Zhuang et al. connected the substrate and the horn through threads, and then laser remelted the substrate surface. The results of electron backscatter diffraction (EBSD) showed that the grain refinement in the remelting zone was obvious, and the cavitation effect and acoustic streaming effect caused by ultrasonic vibration changed the temperature gradient of the melt, and the coarse columnar crystals were transformed into fine equiaxed crystals. After remelting, the microhardness increased, and the surface friction coefficient decreased from 1.3 to 0.4. Lu Xijiang added bottom ultrasound in the process of laser deposition manufacturing IN939 high-temperature alloy, and independently designed a horn with a resonant mode through ANSYS. Then, the substrate and the top of the horn were fixed with threads. The results showed that the transmission effect of ultrasonic vibration was good, the porosity in the cladding layer was reduced, the grains were refined, and the texture strength was reduced. Thread pre-tightening can ensure the tight connection between the horn and the workpiece, and can transmit ultrasonic vibrations with higher energy and higher frequency.
Another way to add ultrasound is to apply ultrasound from the side of the substrate or workpiece, which is divided into longitudinal and transverse directions according to the direction of heat source movement. The ultrasonic vibration perpendicular to the direction of heat source movement drives the substrate or workpiece to move laterally. The ultrasonic amplitude of this transverse movement is generally small, but the frequency is high, which can ensure that the melt is subjected to stable ultrasonic vibration while solidifying stably. It is usually used to improve the organization and performance of additive parts. Rabiey et al. added ultrasonic vibration perpendicular to the direction of heat source movement in the process of laser deposition manufacturing of 316L stainless steel. The maximum amplitude of the amplitude transformer measured by the laser vibrometer was 3.49 μm and the frequency was 25 kHz. The results show that lateral ultrasound can reduce the porosity in the organization, refine the grains, and the ultrasonic vibration intensity and direction in the sample have a complex relationship. Gorunov added ultrasonic vibration perpendicular to the direction of heat source movement in the process of additive manufacturing TC4, and the frequencies of ultrasonic vibration were 22, 40, 60, 80, and 100 kHz respectively. It was found that the grain distribution of the straight-arm additive parts was not uniform, as shown in Figure 5. Equiaxed grains only appeared in some specific positions, which changed with the change of ultrasonic power and frequency. The propagation form of ultrasonic vibration is mainly transverse wave. After adding transverse ultrasonic vibration, the propagation direction of the wave is consistent with the additive manufacturing direction, and standing waves are generated in the additive parts. The transformation (CET) of columnar crystals to equiaxed crystals only occurs at the position of the standing wave antinode.
Zhu et al. added ultrasonic vibration along the direction of heat source movement during the laser cladding process of IN718 high-temperature alloy. Through high-speed photography and finite element simulation, it was found that the longitudinal ultrasonic vibration changed the flow mode of the metal melt, as shown in Figure 6. The morphology of the metal melt changes under the drag of longitudinal ultrasound, the length of the melt increases, the depth decreases, the Marangoni circulation generated by laser melting changes to a unidirectional reciprocating flow along the direction of ultrasonic vibration, and the temperature gradient of the melt also changes. However, longitudinal ultrasonic vibration is not conducive to multi-layer cladding, and strong ultrasonic vibration seriously affects the stability and forming quality of the melt during solidification. Adding ultrasonic vibration from above is a new technology. The ultrasonic fixture usually moves synchronously with the laser to keep the relative position of the ultrasonic vibration and the heat source unchanged, so that the ultrasonic vibration acts on the metal melt in a more direct way. The key to adding ultrasonic vibration from above is to adjust the relative position between the ultrasonic action point and the metal melt. Wu et al. built a follow-up ultrasonic vibration composite laser additive manufacturing test platform. Through phase analysis, it was determined that when the angle between the ultrasonic tool head and the laser optical path was 30°, the propagation effect of ultrasonic vibration was the best. However, the ultrasonic vibration device is large in size and has limited position. It cannot apply ultrasonic vibration to the entire additive part. It only transmits ultrasonic vibration through the formed part, which is similar to the principle of applying ultrasonic vibration from below. Li Yang also added follow-up ultrasonic vibration in the process of laser cladding TiC/TC4 composite cladding layer. As shown in Figure 7, the positions of the nodes and antinodes of ultrasonic vibration were determined by pre-powdering. It was found that compared with the absence of ultrasonic action, when the ultrasonic power was 1800 W, the mass fraction of unmelted TiC in the composite cladding layer decreased by 40.74%, and the grains were refined. However, when the preload between the tool head and the substrate was increased, the stability of the device deteriorated, and the propagation effect of ultrasonic vibration was not ideal. This is because the increase in the mass of the vibrating body increases the energy attenuation of ultrasonic vibration.
The addition of non-contact ultrasonic vibration gets rid of the limitation of preload. It is necessary to design a special tool head structure to transmit ultrasonic vibration through low-density media such as air or water to produce ultrasonic focusing effect at a specific position. However, the high temperature caused by the heat source will intensify the molecular motion of the low-density medium, increase the molecular spacing, and deteriorate the ultrasonic focusing effect, so non-contact ultrasonic vibration is usually coupled with other external energy fields. Zhou et al. coupled non-contact ultrasonic vibration with substrate preheating. Under the synergistic effect of ultrasonic vibration and preheating, the temperature gradient of the molten metal decreased, the cooling rate decreased, and the internal structure evolved from lamellar to granular.
At present, the addition of ultrasound is mainly contact-type, and the difference between different methods is reflected in the transmission direction of ultrasonic vibration in the molten metal. The transmission mode directly affects the effect of ultrasonic vibration in the molten metal, so each has its advantages and disadvantages. For example, under long-term work, by reducing the preload between the tool head and the workpiece surface, the combination of the tool head and the substrate is loosened, and a gap problem occurs. This leads to a significant decrease in the transmission efficiency of ultrasonic vibration and frequency attenuation, which gradually turns ultrasonic vibration into low-frequency mechanical vibration. On the other hand, the non-contact method uses ultrasonic focusing to transmit vibration to the melt, but the high temperature environment may affect the density of the air, thereby affecting the ultrasonic focusing effect. Table 1 lists the advantages and disadvantages of different addition methods. This comparison helps researchers to consider various factors more comprehensively when choosing an ultrasonic addition method and improve the application effect of ultrasonic vibration in metal melting and forming.

Ultrasonic assisted laser welding
Laser welding has the characteristics of large penetration depth, small heat affected zone, and high welding efficiency. The extremely high energy density causes the metal to vaporize instantly to form a keyhole. The flow of metal vapor in the keyhole and the surrounding molten material plays an important role in the welding process. The shape of the keyhole has a great influence on the welding quality. The cooling rate of the melt is large, and the internal metal vapor is easily formed after solidification. Lots of pores. At the same time, the heat transfer in the molten pool during laser welding significantly affects the shape of the molten pool, the melt flow in the molten pool, and the cooling rate of the molten pool. During laser deep penetration welding, the collapse and local penetration of the liquid metal will affect the welding process. Seam formation. Liu et al. introduced ultrasonic vibration into the laser welding process of 301 stainless steel. It was found that the addition of ultrasonic vibration changed the morphology of the weld, which changed from a red wine cup shape to a funnel shape, and a large number of columnar crystals in the weld area evolved into equiaxed crystals, as shown in Figure 8. Further hardness and tensile strength tests were conducted on the weld, and it was found that the average hardness of the weld zone increased by 16%, while the hardness of the heat-affected zone had no significant change. The stress-strain curve showed that the addition of ultrasonic vibration could improve the ductility of the weld.
Liu et al. simulated the effect of ultrasonic vibration on stress changes during welding. The results show that ultrasonic vibration can significantly weaken the tensile stress in the upper part of the weld and increase the compressive stress in the lower part of the weld, thus improving the mechanical properties of the weld. The acoustic flow effect caused by ultrasonic vibration plays a stirring role, making the heat distribution in the weld more uniform, while the shock wave generated by the cavitation effect will break the thick columnar crystals at the root of the weld, and the grain morphology of the weld will change. Zhang Jiaqi et al. added ultrasonic vibration during the laser-MIG hybrid welding (MIG, melting extremely inert gas welding) of steel and aluminum and found that when the ultrasonic power was 130~140 W, the internal porosity of the weld dropped from 0.25% to 0.21 %, the content of the brittle intermetallic compound FeAl3 in the weld is reduced, and the tensile property of the weld is increased by 12%. Welding is a process that uses a heat source to melt the materials to be welded and then forms a stable metallurgical bond during the cooling process. The stability of the metal melt during laser welding is the key to the entire welding process. The frequency of droplet transfer and the shape of the melt directly affect the forming quality of the weld. Research shows that the frequency of ultrasonic vibration will affect the transition frequency of the metal melt, and the amplitude will affect the shape of the metal melt when it solidifies. Chen et al. studied the effect of ultrasonic vibration impact position on droplet transfer. The results show that when the impact position is close to the metal melt, the metal melt shakes violently.
Although vibration helps to promote melting, it will produce undercuts at the side walls of the groove; when the distance between the impact position and the metal melt is appropriate, ultrasonic vibration is conducive to increasing the penetration depth and is less likely to cause poor interlayer fusion. On the other hand, ultrasonic vibration can increase the volume of the metal melt and the time for vapor to escape.
between. However, the molten pool area of ​​laser welding is small and the cooling rate is fast. Excessive ultrasonic vibration will produce a strong cavitation effect. A large amount of metal vapor accumulates inside the metal melt, making it difficult to completely discharge the molten pool in a very short time, resulting in Internal pores are formed, affecting the forming quality and mechanics of the weld.
performance.
The grain refinement of the weld caused by ultrasonic vibration is very obvious. During the welding process, the columnar crystals with preferential orientation that grow from the base material as the starting point break under the synergistic effect of cavitation effect and acoustic flow effect, increasing the metal melting rate. The nucleation points in the body promote the refinement of grains. The refinement of grains produces more grain boundaries and dislocations. Grain boundaries are good barriers against stress, and the pinning effect of dislocations can absorb stress during the stress transfer process, produce work hardening, and improve the tensile strength of the weld. stretch performance. In terms of microhardness, grain refinement usually increases the microhardness of the weld. Tarasov et al. observed microscopic defects in welds after adding ultrasonic vibrations using a transmission electron microscope (TEM) and found that ultrasonic vibrations increased the number of stacking faults in the weld structure. These stacking faults were split by dislocations. Accumulation increases the microhardness of the weld.
The introduction of ultrasonic vibration will affect droplet transfer, weld morphology, etc., and can regulate grain growth during the welding process and improve the performance of welded components. At present, the ultrasonic vibration-assisted welding process is relatively mature, but for the welding process with different heat sources, different equipment for adding ultrasonic vibration needs to be designed. How to promote the large-scale application of ultrasonic vibration-assisted welding still requires further research.

Ultrasonic-assisted laser cladding
Metal workpieces usually soften, deform, and corrode in extreme service environments such as aircraft engines, nuclear power plants, and deep sea. It is necessary to clad a high-temperature and corrosion-resistant protective layer on the surface of the workpiece. The ideal state of the cladding layer is uniform element distribution and fine grains. In the most widely used laser cladding or plasma arc cladding, there will be different grain sizes inside the cladding layer, which will then produce internal stress and element segregation. Among them, internal stress will produce cracks, affecting the continuity and performance of the cladding layer. At present, improvement methods include post-heat treatment, process parameter optimization, element composition control, etc., but the above methods are slightly cumbersome and the improvement cycle is long. Ultrasonic vibration can be added in real time during the cladding process, which is easy to control and adjust, and is used to improve defects in the cladding process.
-Ultrasonic-assisted cladding layer morphology and microstructure
The cladding layer and the base material form a stable metallurgical bond by melting, but the movement of the heat source and the uneven distribution of the powder during the cladding process will affect the heating of the cladding layer, and the cladding layer is not beautiful in shape and has sticky powder. The thermal effect generated by ultrasonic vibration can promote the melting of unmelted powder, and the cavitation effect and acoustic streaming effect can promote the heat flow circulation inside the metal melt, reduce the temperature gradient of each part of the cladding layer, and refine the grains. Yang et al. studied the effect of ultrasonic vibration on the microstructure of multi-layer laser cladding layer. The cladding process adopted was one layer with ultrasound and one layer without ultrasound. The EBSD results showed that there were a large number of columnar crystals inside the cladding layer when ultrasound was not added. After adding ultrasound, the grains of the cladding layer were transformed into equiaxed crystals, and the average grain size was refined from 98.51 μm to 13.91 μm. Zhang et al. clad TribaloyX-40 medium entropy alloy on TC4 substrate and applied ultrasonic vibration to the substrate from above, but did not directly introduce the metal melt. The grain size of the cladding layer was refined from 7.38 μm to 6.19 μm, and the number of small-angle grain boundaries increased from 5.8% to 31.1%. Xiao et al. used ultrasonic vibration-assisted laser cladding technology to prepare an iron-based composite amorphous alloy cladding layer, as shown in Figure 9. The columnar crystals at the bottom and middle of the cladding layer were broken and converted into equiaxed crystals by ultrasonic vibration, but the ultrasonic vibration intensity was attenuated, and the vibration effect at the top of the cladding layer was weak. There was still a large area of ​​amorphous area at the top of the cladding layer, which may be caused by the attenuation of ultrasonic vibration in the substrate.
Dilution rate is an important parameter affecting the quality of laser cladding. When the intensity of ultrasonic vibration is too large, the violent cavitation effect and acoustic streaming effect enhance the melting ability of the laser, and a large number of elements in the substrate are mixed with the cladding layer through violent dilution, which violates the law of low dilution rate of laser cladding and destroys the integrity of the cladding substrate. Wen et al. used ultrasonic-assisted laser cladding technology to design and prepare AlCoCrFeMn0.5Mo0.1Nbx eutectic high entropy alloy cladding layer, and the ultrasonic vibration was loaded from the top to the substrate. The results are shown in Figure 10. Ultrasonic vibration can change the morphology and dilution rate of the cladding layer. The cladding width and depth increase, the cladding height decreases, and the maximum dilution rate reaches 42.9%. Through the regional energy dispersive spectrometer (EDS) analysis, it can be seen that the content of iron element increases significantly. X-ray diffraction (XRD) phase analysis shows that the single-phase body-centered cubic (BCC) high-entropy alloy is transformed into a face-centered cubic (FCC) + BCC two-phase alloy. The high-entropy alloy in the cladding layer no longer has the organization and properties of the original high-entropy alloy.
-Ultrasonic-assisted laser cladding layer mechanical properties
The hardness of the cladding layer is closely related to the friction resistance. Researchers usually use grain refinement or the addition of reinforcement phase to improve the hardness of the cladding layer. However, the cladding process is a rapid solidification process. The temperature gradient in the metal melt is quite different, the grain size distribution is uneven, and the reinforcement phase particles are segregated due to solute redistribution, which cannot meet the requirements of improving the hardness of the cladding layer. The cavitation effect produced by ultrasonic vibration can effectively refine the grains, and the shock waves produced can break up the segregated reinforcement phase. At the same time, the acoustic streaming effect promotes the uniform distribution of the reinforcement phase. Zhuang et al. studied the cladding of 316L stainless steel on Q235 substrate. The results showed that ultrasonic vibration refined the microstructure of the cladding layer, and the average grain size was about 42% smaller than that of the cladding layer without ultrasonic vibration assistance; the friction coefficient of the cladding layer was reduced by 28%. Zhang et al. used ultrasonic vibration-assisted laser cladding technology to prepare TiB2 and TiC ceramic particle-reinforced iron-based composite cladding layers, as shown in Figure 11. It was found that the addition of ultrasonic vibration broke up large ceramic particles, making the distribution of ceramic particles in the cladding layer more uniform, and improving the microhardness and wear resistance of the cladding layer. However, excessive ultrasonic vibration will affect the stability of the metal melt during solidification, and the cladding layer is prone to cracks.
Wen et al. introduced ultrasonic vibration into laser cladding high entropy alloy technology and found that with the increase of Nb element content, the microhardness of the cladding layer increased from 756 HV to 869.9 HV. Han et al. found that in the process of laser cladding 316L, the addition of ultrasonic vibration would affect the friction resistance of the cladding layer. With the increase of ultrasonic amplitude, the friction coefficient of the cladding layer decreased from 0.713 to 0.517, but excessive amplitude would cause the metal melt to shake and the surface quality of the cladding layer would decrease. Zhao et al. studied the effect of ultrasonic vibration on laser cladding of refractory high entropy alloys and found that the thermal effect of ultrasonic vibration greatly reduced the amount of unmelted powder on the surface of the cladding layer and eliminated the cracks inside the cladding layer. In terms of hardness, ultrasonic vibration broke up the coarse grains, and the microhardness of the cladding layer increased from 718 HV to 980 HV. The friction coefficient of the cladding layer at 25, 600, 800 and 1000 ℃ decreased by 10.7%, 25%, 25% and 33% respectively. At 25, 600, 800 and 1000 ℃, the wear rate decreased by 15%, 20.1%, 36.2% and 40.3% respectively.
-Ultrasonic-assisted laser cladding layer corrosion resistance and oxidation resistance
In liquid corrosion or high temperature environment, the passivation layer formed on the surface of the cladding layer can effectively protect the base material from environmental corrosion. The corrosion-resistant elements such as Cr, Ni and Co in the cladding layer can promote the formation of the passivation layer, but there is a certain chemical potential gap between these elements and other elements, resulting in segregation. Adding ceramic phase can also improve the corrosion resistance and oxidation resistance of the cladding layer, but segregation will still occur. The acoustic streaming effect and cavitation effect generated by ultrasonic vibration can stir the melt, inhibit element segregation, and enhance phase segregation. Biswas et al. studied the effect of ultrasonic vibration on the corrosion resistance of laser cladding 2024Al alloy cladding layer. Due to the uneven distribution of alumina, plate-like corrosion marks appeared on the surface of the original cladding layer after corrosion, and the lack of passivation film formed plate gaps; after adding ultrasound, the corrosion surface was uniform and there was no obvious plate gap, but the potentiodynamic polarization curve showed that the corrosion resistance of the cladding layer deteriorated after adding ultrasound. Zhuang et al. studied the corrosion resistance of CrMnFeCoNi composite TiC particle cladding layer and found that the addition of ultrasonic vibration can make TiC particles evenly distributed in the cladding layer. When the TiC content is low, the potential of the self-corrosion galvanic cell formed in the cladding layer increases, and the corrosion resistance decreases; with the increase of TiC particle content, the segregation area is obvious, the internal dislocation of the cladding layer is reduced, and the corrosion resistance is improved. Fan et al. studied the effect of ultrasonic vibration on the electrochemical corrosion mechanism of AlCoCrFeNi and AlCoCuFe2.1Ni high entropy alloy cladding layers. In these two high entropy alloys, Cr and Cu elements determine the corrosion resistance of high entropy alloys. The corrosion mechanism is shown in Figure 12. Ultrasonic vibration refines the grains, and the increase in the number of grain boundaries provides more active corrosion points, but the Cr element segregates at the grain boundaries under the action of ultrasonic vibration, forming more passivation films during the corrosion process, thereby improving the corrosion resistance of the cladding layer. For AlCoCuFe2.1Ni, the Cu element segregates in large quantities at the grain boundaries, resulting in a large potential difference between the Cu-rich phase and the Cu-poor phase, which in turn leads to corrosion. Ultrasonic vibration significantly refines the grains inside the cladding layer, the Cu element segregation phenomenon is significantly weakened, the corrosion resistance is improved, and the corrosion surface composition is close to the original composition.
Ultrasonic vibration can improve the microstructure and properties of the surface cladding layer. In terms of microstructure, the acoustic streaming effect and thermal effect of ultrasonic vibration reduce the temperature gradient inside the metal melt, interrupt the coarse columnar crystals at the bottom of the cladding layer, increase the content of equiaxed crystals in the cladding layer, and inhibit element segregation. In terms of performance, grain refinement improves the mechanical properties of the cladding layer, and the element composition and element distribution of the alloy jointly affect the corrosion resistance of the cladding layer. At present, most of the research on ultrasonic-assisted laser cladding remains in the laboratory research stage, and a lot of practical research is still needed to promote the engineering application of ultrasonic vibration-assisted laser cladding.

Ultrasonic-assisted laser additive manufacturing
Additive manufacturing is a layer-by-layer metal forming method with the characteristics of high degree of freedom, high automation, fast forming speed, and low material loss. It is widely used in the manufacture of complex structural parts and molds. In the process of laser additive manufacturing, the material melts and solidifies layer by layer, resulting in heat accumulation. The grains continue to grow in the continuous heat accumulation, forming large-sized columnar crystals with high texture strength, which seriously affects the mechanical properties of the workpiece. In addition, defects such as cracks, pores, and residual internal stress often appear inside additively manufactured parts, requiring a lot of optimization or numerical simulation of process parameters, but such methods are slightly cumbersome. Ultrasonic vibration has the functions of degassing and grain refinement during welding and laser cladding, so it is used by researchers to improve the macroscopic morphology, microstructure and performance of additive parts.
-Ultrasonic-assisted laser additive manufacturing system
Laser additive manufacturing technology based on metal melting requires the integration of many equipment and systems, such as high-precision laser generation system, metal material transportation system, protective atmosphere generation system and parts post-processing system. The metal melt volume of laser additive manufacturing is very small and the solidification speed is high. How to add effective ultrasonic vibration in the short time from melting to solidification is a key issue. In addition, unlike laser welding and laser cladding, the manufacturing cycle of laser additive manufacturing is relatively long. When facing large parts and complex structural parts, long-term manufacturing will lead to a large amount of heat accumulation. The whole part repeatedly undergoes heating and cooling processes. The drastic change of temperature is not conducive to the addition and propagation of ultrasonic vibration. At the same time, in the process of additive manufacturing, the increase in part mass will also affect the stability of vibration frequency and amplitude.
In response to the above problems, researchers have designed a variety of ultrasonic systems to transmit ultrasonic vibrations. Todaro et al. focused on the mechanism of action of bottom high-intensity ultrasonic vibration in the additive manufacturing process of 316L stainless steel, and designed a resonant ultrasonic matrix through modal analysis. Since the mass of the ultrasonic matrix is ​​larger than that of the additive manufacturing parts, it can continuously generate 20 kHz high-frequency ultrasonic vibration while isolating the heat effect. In order to verify the reliability of the system, they established a correlation model between ultrasonic vibration intensity and forming height. The ultrasonic vibration still has a short-term periodic attenuation of intensity, indicating that the temperature change and mass change in the additive manufacturing process seriously affect the propagation of ultrasonic vibration.
From the perspective of the organization and performance characterization of the formed parts, high-intensity ultrasonic vibration has a significant regulating effect on the grain structure of laser additive manufacturing parts. As shown in Figure 13, the addition of ultrasonic vibration changes the direction of grain growth in the metal melt. The cavitation effect caused by high-intensity ultrasonic vibration reduces the temperature gradient at the front of the solid-liquid interface, reduces the temperature gradient difference of the melt, provides more nucleation sites, inhibits the growth of columnar crystals, and changes the inclination angle of the root grains from the original 29° to 48°. The grains are significantly refined and the texture strength is significantly reduced. The tensile stress-strain curve shows that the tensile strength increases from (980±13) MPa to (1094±18) MPa. Therefore, adding ultrasonic vibration from the bottom is a reliable way. Another idea to improve the intensity of ultrasonic vibration is to transmit ultrasonic vibration directly to the inside of the metal melt through a special tool head. This method requires the tool head to have good high temperature resistance. At present, this technology has been applied in the field of arc additive manufacturing. However, due to the viscosity of liquid metal, there are traces of tool head extraction on the surface of the prepared additive parts, and the surface quality of the additive parts is poor. Wang et al. designed an ultrasonic vibration device, coupled ultrasonic vibration with the laser additive manufacturing system through an extremely fine ultrasonic tool head, and numerically simulated the acoustic field of the metal melt. The results show that with the increase of ultrasonic power, the content of columnar crystals in the formed part decreases, the microhardness of the top of the part is not much different, but the hardness of the middle and lower parts of the part is significantly different, indicating that the propagation effect of ultrasonic vibration in the liquid is poor, and the effect of ultrasonic vibration in the part is not uniform. Therefore, it is necessary to conduct a more in-depth study on the addition method of extending into the molten pool.
-Ultrasonic-assisted laser additive manufacturing part structure and performance
Additive manufacturing is a process of alternating rapid solidification and thermal cycling. The metal melt undergoes repeated cooling and heating processes, and pores are easily generated between layers. Ultrasonic vibration can promote melt flow and reduce the porosity of additive parts. Wang et al. studied the effect of ultrasonic vibration frequency on the additive manufacturing process of IN718 high-temperature alloy, as shown in Figure 14. The results show that the addition of ultrasound is conducive to grain refinement and reduction of porosity. At the same time, the elastic modulus of the part is improved compared with that without ultrasound. However, too high an ultrasonic frequency will increase the porosity. This is because under high frequency conditions, the shortest time required for cavitation cannot be guaranteed, and the cavitation effect is suppressed.
Texture is an important feature of the grain structure of laser additive parts. A large number of columnar crystals produced by rapid solidification grow rapidly under a negative temperature gradient. Under the remelting effect of the laser, the columnar crystals can continue to grow upward along the heat flow direction until the additive manufacturing is completed. Such a structure is anisotropic, and the original cracks will also extend upward along the grain boundaries, thereby damaging the overall quality of the part. Ultrasonic vibration can interrupt columnar crystals that pass through multiple layers, refine the grains of the part, and reduce the temperature gradient, promote melt flow, and inhibit crack growth. Zhang Anfeng et al. studied the effect of ultrasonic vibration on the laser additive manufacturing process of TC4 alloy. It was found that the addition of ultrasound reduced the roughness of the surface of the formed part, and the coarse columnar crystals that crossed multiple deposition layers were refined, and the average grain size was reduced from 345 μm to 250 μm. Yuan et al. designed a follow-up ultrasonic-assisted laser additive manufacturing device and prepared ER 321 stainless steel. The results showed that after adding ultrasound, the texture strength in the forming layer was significantly reduced, which promoted the transformation of columnar crystals to equiaxed crystals; the microhardness of the formed part increased from 196 HV to 217 HV, the yield strength increased from 328 MPa to 367 MPa, and the tensile strength increased from 646 MPa to 668 MPa. Masaylo et al. suppressed the growth of columnar crystals by adjusting the ultrasonic power when manufacturing H13 tool steel by laser additive manufacturing, greatly reducing the anisotropy of the parts.
The melt flow induced by ultrasonic vibration plays a stirring role, which promotes the uniform distribution of the reinforcement phase in the organization. Niu et al. prepared a ceramic-based TC4 alloy by applying ultrasound from above. The results show that under the action of ultrasonic cavitation and acoustic streaming, the unmelted TiC content is reduced, the dendrite primary TiC is broken during solidification, and the distribution uniformity of primary TiC is improved; the average microhardness of the composite cladding layer reaches the maximum value (656.70 HV) under the action of ultrasonic vibration, which is 83.21% higher than that of the Ti6 Al4V matrix and 26.44% higher than that of the composite cladding layer without ultrasonic vibration. Hu et al. introduced ultrasound into the laser additive manufacturing process of zirconia-alumina ceramics. The results show that ultrasonic vibration can effectively inhibit the initiation and propagation of cracks in the formed parts by reducing thermal stress; ZrO2 aggregates and connects at the grain boundaries of Al2O3, and the network microstructure formed can effectively eliminate cracks.
Summarizing the above studies, it can be seen that the introduction of ultrasonic vibration can improve the defects of additive parts such as coarse grains, high texture strength, and internal crack propagation. However, additive manufacturing is a layer-by-layer preparation method. The space span of the formed parts is large and the structure is complex. The propagation effect of ultrasonic vibration will be affected by the structure of the parts. Therefore, the application of ultrasonic vibration in additive manufacturing technology still needs further research. Table 2 lists the specific role of ultrasonic vibration in different metal melting forming technologies.
-Ultrasonic-assisted laser additive manufacturing engineering application
Laser additive manufacturing technology has great advantages in manufacturing and repairing service parts and has received great attention. However, this technology involves complex melting and solidification dynamics, it is difficult to control the grain structure, and structural defects are prone to occur during the manufacturing process. In addition, the volume and mass changes of the parts have a significant impact on the laser heat conduction process, resulting in uneven internal structure of the parts. In order to solve these problems, a new strategy is to use high-intensity ultrasound to control the solidification process. This method provides a new idea for ultrasonic vibration-assisted laser additive manufacturing of large parts. When researchers used ultrasonic vibration to control the pores and grains of parts in laser additive manufacturing of 3 m3 industrial waste containers. The grain structure at the corner of the container is shown in Figure 15. Ultrasonic vibration eliminates a large number of pores in the original structure, reduces the grain size, reduces the temperature gradient of the solid-liquid interface during the solidification of the melt, and suppresses the generation of thermal cracks. However, the results achieved in this experiment are only proportional reductions in the original size, and do not completely solve the problem of uneven ultrasonic vibration transmission when manufacturing large parts. Therefore, in order to achieve a wider range of applications, the ultrasonic vibration system needs to be further optimized.

Years of research by researchers around the world have confirmed that, whether as an energy source or an auxiliary energy field, ultrasonic vibration technology has shown broad application prospects in the fields of connection, preparation and repair of metal and non-metal materials. The mechanism of action of ultrasonic vibration in metal melts is reviewed. For four metal melting forming methods, namely casting, welding, surface cladding and additive manufacturing, the introduction method of ultrasonic vibration and its influence on the microstructure and physical properties of the formed parts are introduced. Ultrasonic vibration can refine the coarse grains produced during rapid solidification, suppress defects, improve the macroscopic morphology and mechanical properties of welds and cladding layers, thereby avoiding the problems existing in the manufacturing process of a single energy field. However, the following issues still require in-depth research by scientific researchers.

Establish a more complete finite element model to simulate the interaction mechanism between ultrasonic vibration and metal melts. The multiple effects generated by ultrasonic vibration, such as cavitation and acoustic streaming, play an important role in improving the defects of metal melting formed parts and improving the performance of workpieces. However, these effects usually need to be described through experiments and theoretical speculations. Taking the cavitation effect and acoustic streaming effect as examples, how to simultaneously show these two effects in a more intuitive way is a challenge. Although finite element simulation provides an intuitive means, how to more accurately simulate and characterize the multiple effects produced by ultrasonic vibration has always been a hot topic of research. Ultrasonic vibration has the characteristics of high frequency and high energy, and these effects are generated based on its high frequency characteristics. Researchers usually use water, glycerin and other substances as research objects, and have conducted a lot of research on cavitation phenomena and acoustic streaming effects in static media, including theoretical derivation, finite element simulation and experimental verification. However, metal melting and forming is a dynamic process accompanied by drastic temperature changes. In the metal melt, there are non-isothermal flow phenomena such as Marangoni circulation and electromagnetic stirring. The entire metal melt is subjected to the composite action of multiple effects during the cooling process. Therefore, how to simultaneously characterize the flow characteristics of the metal melt itself and the composite effects produced by ultrasonic vibration requires further in-depth research. This may involve modeling the dynamic flow characteristics of the metal melt, as well as more accurate finite element simulation of the process of ultrasonic vibration affecting metal flow and solidification under high temperature and high energy conditions. This research will help us understand the mechanism of interaction between ultrasonic vibration and metal melt more comprehensively and deeply.

Further quantify the influence of various parameters of ultrasonic vibration on the microstructure and performance of formed parts. The adjustable parameters of ultrasonic vibration are amplitude and frequency. The frequency adjustment of ultrasonic vibration is relatively simple, usually achieved by controlling the feedback of the power supply. However, the adjustment of amplitude cannot be achieved by controlling the feedback of the power supply, and usually requires the use of laser vibrometers or in-situ sensors. At the same amplitude, the amplitude is positively correlated with the output power of the power supply, so most researchers use the output power of the power supply to characterize the amplitude of ultrasonic vibration. However, the output power of the power supply is a relative concept. The same amplitude may require different powers to produce in different power supplies or ultrasonic equipment, and it is impossible to normalize the independent variables. At present, most reports only study the influence of ultrasonic vibration addition on the microstructure and performance of parts, and there are relatively few studies on combining ultrasonic parameters with parameters in metal melting and forming for multi-factor process experiments. Therefore, it is necessary to further optimize the multi-factor experimental design of composite ultrasonic vibration to more comprehensively understand the influence of ultrasonic vibration on the metal melting and forming process. For example, the study of more accurate amplitude adjustment methods and the in-depth study of the relationship between ultrasonic vibration parameters and metal forming parameters.

The attenuation and uniformity of ultrasonic vibration in the metal melting and forming process need to be solved urgently. This process is accompanied by drastic temperature changes, which may affect the working state of the ultrasonic vibration device and thus the propagation effect of the ultrasonic vibration. As the temperature rises, the frequency of the ultrasonic vibration will change, and excessive temperature may even damage the ultrasonic components. In addition, whether it is a contact introduction method or a threaded connection introduction method, it is difficult for the ultrasonic vibration to be evenly covered when it propagates to the metal melt. In long and complex manufacturing processes such as surface cladding and additive manufacturing, the ultrasonic vibration effects at different positions may be significantly different, resulting in differences in the performance of the formed parts. Therefore, in order to improve the stability and consistency of ultrasonic vibration in the metal melting and forming process, the following measures need to be taken: 1) Research and develop ultrasonic vibration devices that can work stably in high temperature environments to reduce the impact of temperature on ultrasonic vibration frequency and components. 2) Explore more effective ultrasonic vibration propagation methods to ensure that ultrasonic vibrations can be evenly and effectively distributed throughout the metal melt. This research will help to promote ultrasonic vibrations to play a greater role in actual manufacturing.