
==== Front
Ultrason Sonochem
Ultrason Sonochem
Ultrasonics Sonochemistry
1350-4177
1873-2828
Elsevier

S1350-4177(24)00318-3
10.1016/j.ultsonch.2024.107070
107070
Ultrasound and SDGs
Ultrasonic field-assisted metal additive manufacturing (U-FAAM): Mechanisms, research and future directions
Li Xuekai a
Wang Wei b
Wu Yihong a
Zhou Donghu a
Kang Huijun ac
Guo Enyu ac
Li Jiehua d
Chen Zongning znchen@dlut.edu.cn
ac⁎
Xu Yanjin b
Wang Tongmin tmwang@dlut.edu.cn
ac⁎
a Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
b AVIC Manufacturing Technology Institute, Beijing 100024, China
c Ningbo Institute of Dalian University of Technology, Ningbo 315000, China
d Institute of Casting Research, Montanuniversität Leoben, Leoben A-8700, Austria
⁎ Corresponding authors at: School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China. znchen@dlut.edu.cntmwang@dlut.edu.cn
14 9 2024
12 2024
14 9 2024
111 10707027 6 2024
28 8 2024
12 9 2024
© 2024 The Author(s)
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Metal additive manufacturing (AM) is a disruptive technology that provides unprecedented design freedom and manufacturing flexibility for the forming of complex components. Despite its unparalleled advantages over traditional manufacturing methods, the existence of fatal issues still seriously hinders its large-scale industrial application. Against this backdrop, U-FAAM is emerging as a focus, integrating ultrasonic energy into conventional metal AM processes to harness distinctive advantages. This work offers an up-to-date, specialized review of U-FAAM, articulating the integrated modes, mechanisms, pivotal research achievements, and future development trends in a systematic manner. By synthesizing existing research, it highlights future directions in further optimizing process parameters, expanding material applicability, etc., to advance the industrial application and development of U-FAAM technology.

Keywords

Metal additive manufacturing
Ultrasonic field-assisted manufacturing
3D printing
U-FAAM mechanisms
Melt pool behavior
Research directions
==== Body
pmcNomenclature

AM Additive manufacturing

AI Artificial intelligence

CS Constitutional supercooling

CET Columnar to equiaxed transition

CFD Computational fluid dynamics

G Temperature gradient

LDED Laser-directed energy deposition

LOF Lack of fusion

LAM Laser additive manufacturing

LAGB Low angle boundary

ML Machine learning

U-FAAM Ultrasonic field-assisted metal additive manufacturing

UV Ultrasonic vibration

USMP Ultrasonic melt processing

UA Ultrasonic amplitude

UOP Ultrasonic generator output power

UF Ultrasonic frequency

UTS Ultimate tensile strength

WAAM Wire arc additive manufacturing

YS Yield strength

ΔT Undercooling rate

δ Fracture elongation

1 Introduction

The United Nations' “2030 Agenda for Sustainable Development” sets forth 17 Sustainable Development Goals (SDGs) that address urgent global priorities in environmental sustainability, inclusive economic growth, and social well-being [1]. The agenda highlights the crucial role of technological innovation in achieving these goals, particularly within the manufacturing sector. Traditional manufacturing, which heavily relies on fossil fuels, is notoriously energy-intensive and highly polluting, directly conflicting with the objectives of the SDGs [2], [3]. This conflict has accelerated the advancement of clean and efficient technologies, such as AM, which enhance resource usage efficiency and mitigate pollution, thereby supporting the attainment of the SDGs [4]. Consequently, international policy has increasingly prioritized the transformation of traditional manufacturing practices through technological innovation.

Representing a paradigm shift in the manufacturing sectors, Additive Manufacturing (AM), or 3D printing, builds objects layer by layer from digital models, setting it apart from traditional subtractive and formative manufacturing processes [5], [6], [7]. This manufacturing technology offers unparalleled advantages in terms of efficiency and environmental impact, thus positioning it as a revolutionary force in the aerospace, medical, automotive, and custom manufacturing industries [8], [9]. Table 1 presents a summary of the advantages of AM over traditional manufacturing methods. By leveraging AM, companies can boost efficiency and facilitate product innovation, while markedly diminishing their environmental footprint. This strategic approach is in alignment with the visionary prospect of sustainable and innovative manufacturing for the future.Table 1 Presents a summary of the advantages of AM over traditional manufacturing methods.

The advantages of AM	Traditional manufacturing methods	Instructions	
High design freedom [10]	Limitation	Manufacturing complex geometries beyond the capabilities of traditional methods [11]	
Customization and personalization [8]	Difficult to achieve	Customized products, suitable for small batch production of individual needs	
Reduce material waste [12]	Significant material wastage	Utilizes only the necessary materials, reducing waste compared to traditional subtractive methods	
Shorten production cycle [13]	Long production period	Directly from digital models to physical objects, signification shortening the time from design to finished product	
Reduce manufacturing cost [14]	High cost (during small-scale production)	For small batches and complex designs, the absence of expensive molds or cutting tools can reduce cost	
Supply chain simplification [15]	Supply chain complexity	On-demand production reduces inventory and transportation, simplifying the supply chain	

Despite the considerable potential and advantages of AM technology over traditional manufacturing, there are still some significant issues with metal AM printed materials in practical applications that can have a detrimental effect on the quality and performance of the final products. These drawbacks include: (1) Porosity and inclusions [16], [17]; (2) Large and highly columnar dendrites resulting in uneven microstructure [18]; (3) Residual stress and deformation [19], [20], [21]; (4) Poor interlayer adhesion [22], [23].

In order to address the aforementioned critical issues, researchers and engineers have conducted in-depth studies and technological innovations across multiple areas, including raw material powder quality control [24], [25], process optimization [26], and auxiliary processing [27], [28]. Against this backdrop, ultrasonic field assisted metal additive manufacturing (U-FAAM) has emerged as an innovative manufacturing technique that integrates ultrasonic energy with traditional metal AM methods [29]. This technology employs high-frequency ultrasonic vibration (UV) to facilitate solid-state bonding between metal powders or sheets, thereby enabling layer-by-layer deposition and overall shaping to produce complex metal components.

Currently, U-FAAM technology, with its outstanding advantages, occupies an indispensable position in the field of metal AM. This technology has been extensively applied to the printing process of a variety of metal materials, including aluminum alloys [30], [31], magnesium alloys [28], [32], titanium alloys [33], [34], [35], nickel-based superalloys [36], [37] and steel [38]. The ultrasonic field profoundly affects the flow dynamics, temperature distribution and solidification mode of the microstructure in the melt pool [39], [40]. The introduction of ultrasonic fields in AM brings numerous significant benefits to the printed materials, including: (1) reduced defects [41], [42]; (2) optimized microstructure [43]; (3) lowered thermal stress [35]; (4) improved interfacial bonding [44]; (5) increased material utilization [45]; (6) diversified material processing [15]. Together, these advantages collectively propel the application of U-FAAM technology in the field of metal manufacturing and have established its key position in modern industry.

The motivation of this work is to provide a comprehensive and in-depth review of the latest research developments and application prospects of U-FAAM. The work elaborates on the methods of integrating ultrasonic fields with traditional AM technologies, the interaction mechanisms between ultrasonic fields and materials, and the beneficial effects of ultrasonic fields in improving material density, microstructure, and mechanical properties in metal AM. Furthermore, the simulation and modelling of the U-FAAM process are thoroughly reviewed. Finally, this paper outlines the current research gaps and future directions in the field of U-FAAM, with the aim of providing researchers with a thorough understanding of this technology, facilitating its transition from the laboratory to industrialization, and accelerating its application in global sustainable development. We believe that as this technology continues to evolve and be applied, its significance in promoting global sustainability will increasingly grow.

2 Types of integration types of auxiliary ultrasonic fields with traditional metal AM techniques

As metal AM technology continues to evolve, the application of ultrasonic field assistance has emerged as a significant innovation. This technique integrates ultrasonic fields with traditional metal AM technologies, not only optimizing the quality of the forming process but also significantly enhancing material properties. This section provides a comprehensive review and summary of the integration methods and implementation techniques of ultrasonic assistance with various traditional metal AM technologies. It offers valuable insights for related research and practical applications.

In the field of U-FAAM, substrate UV amplitude modulation and moving UV amplitude modulation are regarded as two fundamental technological approaches. Fig. 1 demonstrates the widespread application of substrate UV amplitude modulation in conventional metal AM processes, encompassing WAAM [46], LDED [47] and LPBF [48]. Slightly differently, the LDED process can further subdivision of the construction depending on whether the UV direction is parallel or perpendicular to the scan direction [49], [43]. The utilization of moving UV amplitude modulation has only been documented in the WAAM and LDED processes [15]. Given its propensity to disrupt the powder bed and induce powder scattering, moving UV amplitude modulation is not a viable option for the LPBF process.Fig. 1 A systematic review and summary of the integration types and implementation methods of auxiliary ultrasonic fields in different traditional metal AM technologies [31], [48], [50], [51], [52], [53].

2.1 Substrate UV amplitude modulation technology

In substrate UV amplitude modulation for metal AM, ultrasonic equipment is positioned beneath the build platform to apply ultrasonic energy directly to the substrate [50], [54]. The resulting acoustic cavitation and streaming disrupt dendrite formation, refine grain structure, and minimize porosity, thereby enhancing the density and mechanical properties of the metal components [55]. Additionally, ultrasonic agitation can facilitate uniform stress distribution, reduce residual stress, and improve material toughness and fatigue resistance [56]. However, challenges exist, particularly in designing ultrasonic devices capable of high-temperature operations, as these require materials and structures resilient to extreme heat. Furthermore, for tall components, ultrasonic intensity decreases significantly with build height. Todaro et al. found that ultrasonic intensity drops from its peak to zero at approximately 62.5 mm [34]. Thus, maintaining microstructural refinement requires keeping the sample height within a feasible range. Precise control over the frequency, amplitude, and duration of UV is essential to ensure process stability and consistency.

2.2 Moving UV amplitude modulation technology

In the case of moving UV amplitude modulation for metal AM, the construction principle is to operate by synchronizing an ultrasonic device mounted on a mobile platform with an arc or laser beam moving along a predetermined path within the metal melt pool. The precise positioning system on the platform enables the ultrasonic waves to interact with the liquid metal at various locations within the pool, thereby enhancing the flow and homogeneity of the metal, improving interlayer bonding, and reducing defects and porosity. This results in an increase in the density and mechanical properties of the metal parts. Furthermore, the technology facilitates rapid heat dissipation, narrowing the width of the heat-affected zone and minimizing thermal stress and deformation [57]. In comparison to traditional substrate UV systems, this technology offers greater flexibility and maintains a constant ultrasonic intensity when manufacturing complex or large-scale components [32], [33].

Nevertheless, the implementation of moving UV amplitude modulation is not without its challenges. Firstly, direct contact between the ultrasonic head and the deposited material can result in contamination issues, such as the introduction of oxygen and carbon atoms [15]. Secondly, there is a necessity for precise coordination between the ultrasonic equipment and the mobile platform, which increases complexity and cost of the system. It is crucial to accurately control and optimize vibration parameters for different processing conditions. Despite these challenges, mobile UV technology demonstrates considerable potential for improving the quality of large, complex-shaped metal components in AM. Fig. 2 presents a summary of the advantages and disadvantages of each of the two amplitude modulation technologies.Fig. 2 Summarizes the respective advantages and disadvantages of two amplitude modulation techniques [50], [54].

3 Mechanisms of the auxiliary ultrasonic field in traditional liquid metals and traditional metal AM

Prior to undertaking a detailed examination of the interaction mechanisms between ultrasonic fields and materials in traditional metal AM, it is imperative to first elucidate the role of ultrasonic fields in the solidification processes of liquid or semi-solid metals and alloys. An understanding of the effects of ultrasonic fields on molten metals, including the facilitation of nucleation and solute homogenization, provides a foundation for comprehending their applications in metal AM [58], [59]. This understanding not only helps to uncover the significant impacts of ultrasonic fields on the microstructure and macroscopic properties of metals but also offers scientific evidence and references for the subsequent application of ultrasonic-assisted techniques in AM.

3.1 Mechanisms of ultrasonic field action on the solidification of liquid or semi-solid metals and alloys

In recent years, the application of power ultrasonic treatment in the solidification process of liquid or semi-solid metals and their alloys has garnered widespread attention in both the research domain and the casting industry. The application of ultrasonic melt processing (USMP) technology enables the induction of a series of significant and beneficial microstructural changes. These changes include the refinement of matrix grains and primary particles, enhanced uniformity of microstructure and chemical composition [60], [61], [62], [63]. Therefore, USMP technology is a promising economic technology, which provides an environmentally friendly method to improve the integrity and quality of the ingot.

3.1.1 Acoustic cavitation contributing microstructure refinement

One of the key applications of USMP is microstructural refinement. A critical theory underlying this effect is the cavitation-enhanced nucleation theory, which encompasses two sub-mechanisms [64]. The first mechanism involves the facilitation of solid-phase nucleation through alterations in local equilibrium conditions caused by the implosion of cavitation bubbles. When these bubbles collapse, they generate extremely high pressures (up to the GPa range), raising the solidification temperature and causing significant undercooling of the liquid phase, thereby forming stable nuclei [65], [66]. The second mechanism relates to heterogeneous nucleation during solidification, known as “inclusion activation”. This mechanism posits that the formation, growth, and subsequent collapse of cavitation bubbles near impurity particles can generate high-energy shock waves or liquid jets [67], [68]. These phenomena enhance the wettability between the melt and impurity particles (such as oxides in molten aluminum), thereby activating the impurity particles and rendering them effective nucleation sites for the solid phase [69].

An additional microstructural refinement theory is the cavitation-induced crystal fragmentation theory [70]. This theory suggests that crystal fragmentation in metal melts and organic transparent alloys occurs primarily through the following mechanisms: (i) cyclic acoustic pressure exerted by pulsating bubbles induces fatigue fracture [71], [72]; (ii) the implosion of bubbles causes primary dendrite fracture and mechanical damage due to remelting [73]; (iii) shock waves generated by bubble implosion and flow-induced stress lead to the local bending and breaking of primary and secondary arms [74]; (iv) acoustic streaming promotes the separation of fragments [75].

In recent years, the use of in situ synchrotron X-ray analysis has significantly advanced our understanding of the mechanisms behind cavitation-induced microstructure refinement. The oscillation and implosion of bubbles in liquid Bi-8 % Zn alloy, captured with exceptional clarity through ultrafast synchrotron X-ray imaging at the Advanced Photon Source (Fig. 3a) [75]. For the first time, Wang et al. [76] demonstrated the nucleation of primary Al2Cu particles on alumina film fragments (Fig. 3b), providing direct evidence for the long-debated role of ultrasonication in promoting oxide nucleation—a major breakthrough in the field. As shown in Fig. 3c, primary Al3Ti crystals were fragmented after 1680 bubble pulsation cycles, indicating that crack formation and propagation are linked to the fatigue mechanisms induced by cyclic acoustic pressure during bubble expansion, contraction, implosion, and rebound [71]. Additionally, related research captured phenomena such as dendrite remelting under acoustic streaming, dendrite fragmentation due to pulsating cavitation bubble impacts, bending and breaking of dendrite arms by oscillating cavitation bubbles, and acoustic streaming promoting the fragmentation and detachment of intermetallic phases (Fig. 3(d-f)) [73], [74]. These observations provide direct experimental evidence for the cavitation-induced fragmentation and refinement mechanisms of intermetallic phases.Fig. 3 (a) The implosion of a bubble beneath the ultrasonic electrode tip in a Bi-8% Zn alloy [75]; (b) the nucleation of primary Al2Cu crystals (dark) on alumina fragments (bright) in an Al-35 wt% Cu alloy, observed via in situ synchrotron radiation [76]; (c) the fragmentation of primary Al3Ti crystals under the influence of pulsating bubbles, with arrows indicating the initiation and propagation of fatigue cracks [71]; (d) the breaking of Al2Cu dendrites by pulsating bubbles, partially remelting due to acoustic streaming [73]; (e) the fragmentation of dendrite arms caused by the oscillation of cavitation bubbles in succinonitrile (SCN)-2- wt% acetone organic transparent alloy [74]; and (f) in situ observation of the fragmentation and detachment of primary Al2Cu intermetallic phases induced by acoustic streaming [73].

3.1.2 Acoustic streaming promoting solute homogenization

Acoustic streaming, driven by the nonlinear propagation of high-intensity sound waves in liquid media, plays a critical role in solute homogenization [77]. As these sound waves propagate through the liquid, they generate pressure gradients that prompt the formation of microscale flows and vortices [78]. These microscopic flows markedly enhance solute diffusion and mixing, thereby achieving a more rapid and uniform solute distribution. Additionally, sound waves can induce large-scale turbulence, further enhancing internal liquid mixing efficiency [79]. In systems exhibiting phase interfaces, acoustic streaming serves to boost interfacial mixing and mass transfer, speeding up the transfer of solute between phases. Furthermore, acoustic streaming prevents solute particles from settling and aggregating by maintaining a continuous flow, ensuring that suspended particles are evenly distributed [80]. These mechanisms collectively promote efficient solute homogenization in liquid media. As shown in Fig. 4(a-b), Wang et al. demonstrated that the application of ultrasound markedly reduced the segregation of major elements in the high entropy FeCoNi2Al0.9 alloy. This effect is primarily due to the enhancement of mass diffusion by ultrasonic-induced acoustic streaming at different amplitudes, which inhibits solute accumulation at the front of the growing eutectic interface [81]. Similarly, Hu et al. discovered that as the ultrasound dimension increased from 1D to 3D, the intragranular solute distribution of Al became more uniform. This is attributed to the strong cavitation and acoustic streaming effects, which significantly reduce the micro-segregation of Al, as shown in Fig. 4(c) [82].Fig. 4 (a-b) The solute distribution curve and degree of segregation in the eutectic structure of high-entropy FeCoNi2Al0.9 alloy following static and ultrasonic curing [81]; (c) The distribution of Al solute in the α-Mg primary grains of AZ91 magnesium alloy after static and ultrasonic curing with varying amplitude dimensions

Source [82].

3.2 Mechanisms of the ultrasonic fields on materials in conventional metal AM process

In the previous article, we conducted a comprehensive investigation into the mechanisms by which auxiliary ultrasonic fields enhance traditional liquid metals. In particular we focused on the role of acoustic cavitation and acoustic streaming effects in refining grain structure and homogenizing solute distribution. These studies provide valuable insights and foundational theories that are equally applicable and significantly instructive for U-FAAM. Building on our understanding of ultrasonic effects in liquid metals, we can delve deeper into their application in metal AM. The following details the mechanisms of auxiliary ultrasonic fields in traditional metal AM, demonstrating how insights gained from liquid metal research can further advance AM technologies.

It is noteworthy that in the process of U-FAAM, cavitation and acoustic streaming remain the two primary mechanisms through which ultrasonic fields interact with materials [73], [83]. As discussed in Section 3.1.1, the cavitation effect also plays a crucial role in refining the microstructure of metal materials in AM. Cavitation bubbles nucleate within a region beneath the acoustic electrode, known as the “cavitation zone” [84]. To initiate cavitation, it is necessary to overcome a certain pressure in the liquid, referred to as the “cavitation threshold”. The cavitation threshold (Pt), which can be evaluated by the following equation [85]:(1) Pt=P0-Pv+2332σR03P0-Pv+2σR0

Where P0, σ and R0 are the standard atmospheric pressure, the surface tension coefficient of the melt and the initial radius of the melt microbubble, respectively. Gas and vapor are contained within the bubbles usually, Pv is the saturated vapor pressure and can be described as [38]:(2) Pv=133.32×10-19710/T-1.27*lgT+13.27

The actual ultrasound-induced sound pressure (Pa) can be calculated by the following equation [34], [86]:(3) Pa=2ρIc

(4) I=12ρc2πfA2

where I, ρ, c, f, A represent the ultrasonic intensity, the density of the molten metals, the velocity of sound in the liquid, the angular frequency of the ultrasound and the amplitude of oscillation, respectively. Both the ultrasonic angular frequency and the amplitude of oscillation have a positive correlation with the ultrasonic intensity. To achieve significant microstructural refinement, both ultrasonic intensity and sound pressure should be higher than the threshold required for cavitation in the molten melt [71]. Only then can the melt pool effectively generate extensive cavitation, where the intense oscillation and collapse generate high pressure that fracture the dendrites at the solidification front into equiaxed crystals (Fig. 5a) [32].Fig. 5 The internal action mechanism inside the melt pool of the typical auxiliary ultrasonic wire-arc DED process (a) [32]; Schematic illustration of the CS zone ahead of a growing grain during AM (b) without UV and (c) with UV [92].

In addition to cavitation effects, acoustic streaming is another key factor influencing solidification. Notably, the ultrasonic-induced inertial force (Su) can be introduced into the melt pool during the UV process. The Su can be calculated using the following equation [87]:(5) Su=-ρf2Asinft

where t represents the time the ultrasound acts on the melt pool. The inertial forces generated by the ultrasound can stir and mix the molten pool, thereby accelerating fluid flow, increasing diffusion rates, and smoothing the temperature gradient of the melt pool [51]. Consequently, the UV can promote a more uniform distribution of alloy elements and alleviate thermal stress of deposited material. The degree of acoustic streaming is positively correlated with the ultrasonic intensity and the direction of UV. For U-FAAM, the ultrasound is typically applied vertically to achieve sufficient cavitation and acoustic streaming effects. In terms of the molten pool flow characteristics, the inertial force generated by UV drives the molten pool to flow forward and backward along the scanning direction, resulting in the bulge and depression appearing alternately on the surface of the molten pool [87].

During rapid solidification process, CS at the front of the solid–liquid transition interface plays a vital role in grain refinement [88], [89]. The rapid transformation of columnar grains into equiaxed grains requires sufficient CS to trigger this change when the driving force of intrinsic supercooling exceeds the degree of supercooling required for effective nucleation (ΔTcs ≥ ΔTn) [90], [91]. As shown in Fig. 5b, in the absence of ultrasound, the initial event occurs at time t1, i.e. epitaxial growth on the previous layer of partial remelting. A steep temperature gradient (G) limits ΔTcs ahead of the growing grain (the yellow region representing the temperature difference between the equilibrium liquid phase temperature TE and the actual melt temperature TA), impedes the activation of the effective nucleation point in front of the solid–liquid interface and promotes columnar growth up to time t3. At this juncture, columnar growth generates sufficient ΔTcs to activate effective nucleation agents, driving nucleation and the CET.

Solidification under high-intensity ultrasound is different (Fig. 5c). With ultrasound, the initial event is still the epitaxial growth at time t1. However, the acoustic flow effect causes the higher temperature melt to flow rapidly towards the molten pool boundary. The convection in the melt pool is strengthened, which effectively reduces the temperature gradient G. The decrease in temperature gradient caused by UV increases ΔTCS (yellow region) so that the transition time from columnar grains to equiaxial grains is significantly advanced to the early stage of time t2. This leads to a pronounced decrease in the size and volume fraction of columnar grains in samples treated with UV [92]. Overall, cavitation-induced fragmentation and/or enhanced nucleation, alongside reduced temperature gradient, fundamentally expand the ΔTCS region. These combined effects facilitate grain structure refinement and earlier CET in samples prepared with U-FAAM.

4 Research progress on the multiple influence of U-FAAM

The application of auxiliary ultrasonic fields in metal AM is garnering increasing attention. This technology not only assists in refining the morphology of the melt pool, but also significantly impacts the flow dynamics of the powder within the melt pool, the splashing behavior of molten metals, as well as temperature gradients [93], [94]. In addition, ultrasonic fields play a pivotal role in enhancing the density, microstructural evolution, and final mechanical properties of the manufactured parts [47], [95]. This section presents a comprehensive review of the latest advancements in ultrasonic field research in these aspects. Moreover, the underlying mechanisms and practical applications are analyzed with the aim of providing scientific insights and guidance for further optimization of ultrasonic metal AM technology.

4.1 Effect of auxiliary ultrasonic field on molten pool behavior

4.1.1 Molten pool morphology

Despite the significant advancements that have been made in metal AM technology, there is still a challenge in controlling the morphology of the melt pool [96]. This is particularly difficult when maintaining consistency in the shape and size of the melt pool under condition of high printing speeds and complex geometries [97], [98]. Moreover, the dynamic behavior of the melt pool is difficult to control precisely, often leading to significant variations in its morphology [99]. Consequently, the real-time effective monitoring and adjustment of the melt pool shape, as well as the maintenance of stable melt pool characteristics under varying materials and process parameters, represent crucial technical barriers to enhancing the reliability of metal AM and improving the performance of the final products [100]. In-depth research and optimization of mechanisms and methods for controlling the melt pool morphology are essential for improving the quality of manufactured products.

The utilization of an auxiliary ultrasonic field to improve the morphology of the melt pool through UV has been shown to have significant effects. The inertial forces generated by the UV impact the Marangoni forces within the molten pool. These inertial forces increase the fluidity of the molten pool and modify the surface tension gradient, thereby regulating the distribution and intensity of the Marangoni forces [51]. This adjustment facilitates a more uniform flow and distribution of the metal solution within the molten pool. As a result, the auxiliary ultrasonic field can optimize the shape and size of the molten pool, reduce morphological fluctuations, and demonstrate considerable potential as a technique for improving molten pool morphology. Researchers have already conducted studies on the influence of the ultrasonic assist field on molten pool morphology. For instance, Yang et al. [94], [87] employed in situ high-speed imaging to clearly capture critical phenomena of molten pool morphology, highlighting the distinctions between LDED and UVLDED. In contrast to the relatively convex and stable surface of the LDED molten pool, the UVLDED molten pool exhibited a larger size, a reduced angle of inclination to the substrate, a concave shape and a fish-scale pattern, together with a narrow and elongated region at the rear (t = 0 to t = 7.20 ms) (Fig. 6 (a-d)). Similarly, Jiang et al. [51] investigated the molten pool states during LDED and UVLDED at laser powers ranging from 600 to 1400 W and a scan speed of 6 mm/s, using high-speed camera. The results showed that the DED molten pool was stable and ellipsoidal, whereas the UVLDED melt pool exhibited a concave and spoon-shaped morphology (Fig. 6 (e-f)). In the LDED process, the Marangoni force is the primary driving force within the molten pool. Additionally, the disparity in the molecular distance between the metal molecules on the surface of the molten pool surface and the gas molecules results in the metal molecules with stronger intermolecular attraction clustering together, thereby reducing the surface area of the LDED molten pool. In the case of UVLDED, the height difference between the cladding layer and the substrate during deposition, coupled with the resultant force of the inertial forces generated by UV and the support forces from the substrate, gives rise to centrifugal motion of the molten pool, leading to a concave surface.Fig. 6 High-Definition Images of the Molten Pool: (a) Without UV and (b) With UV [94]; A sequence of optical images captures the instantaneous morphology of the molten pool at 3.6 ms intervals: (c) Without UV and (d) With UV [87]; When the laser power ranges from 600-1400 W and the scanning speed is set at 6 mm/s, the morphology of the molten pool is depicted as: (e) Without UV and (f) With UV [51].

4.1.2 Molten pool powder flow and molten metal splash

In the case of metal AM processes, the non-uniform flow of powder in the melt pool and the splattering of molten metal presents several challenges [101], [102]. Due to the weak surface tension and flow characteristics of the melt pool, the powder tends to distribute unevenly, resulting in inadequate fusion and compromising the quality of the clad layer. Furthermore, the molten metal surface is susceptible to the formation of splattered droplets when subjected to external forces [103]. The irregular dispersion of these droplets around the melt pool results in a reduction in the surface quality and processing efficiency of the final product [104]. These issues have the effect of reducing the uniformity and mechanical properties of the clad layer, thereby limiting the application and performance optimization of traditional AM techniques.

The application of an auxiliary ultrasonic field has been demonstrated to significantly improve powder flow in the melt pool and molten metal splattering by inducing cavitation and acoustic streaming. The application of UV enhances the fluidity of the melt pool, resulting in a more uniform distribution of powder and better fusion with the substrate [52]. This, in turn, leads to an improvement in the quality of the clad layer. Additionally, the high-frequency stirring and impact forces generated by UV increase the kinetic energy of the molten metal, resulting in a more frequent and controlled formation of splattered droplets. This reduces irregular splattering and its subsequent negative impact on surface quality. This synergistic effect enhances processing efficiency and optimizes the uniformity and mechanical properties of the clad layer. As shown in Fig. 7(a-b), Zhu et al. [94] observed the formation of droplets sputtered by molten metal through in-situ high-speed imaging. They found that when the surface tension of the molten metal is less than the external force, droplets separate from the surface. The powder particles in the molten pool move along the edge with the outward Marangoni flow. During the UV cladding process, the nature of the Marangoni flow is influenced by both cavitation and acoustic streaming. The reciprocating effect of the vibration and the deposition of powder onto the molten pool surface cause partial solidification of the surface, while the underlying layer remains molten, leading to a lamination phenomenon. This occurs because heat conduction between the cold powder on the surface and the high-temperature molten pool beneath increases the cooling rate. Yang et al. [87] found that metal droplets in the UVLDED process splatter more frequently. Compared to LDED, the UVLDED process shows a wider distribution of droplet diameters and a larger average diameter. In contrast, the LDED process exhibits a wider distribution of droplet speeds and a higher average speed (Fig. 7c). This indicates that the primary mechanisms of splatter formation are different between UVLDED and LDED. In the UVLDED process, the primary mechanism of splatter formation is the detachment of micro-droplets from the molten pool. Conversely, in the LDED process, most splatter is associated with local overheating, as evidenced by the presence of metal vapor, resulting in smaller and faster splatter.Fig. 7 The formation of powder flow and molten metal splash in molten pool at different time: (a) Without UV and (b) With UV [94]; (c) Splash formation, movement and quantitative behavior in LDED and UVLDED [87].

4.1.3 Molten pool temperature gradient

In conventional metal AM, the importance of regulating the molten pool temperature gradient cannot be overstated [105]. High temperature gradients can result in uneven cooling rates, leading to residual stresses and deformation that affect the precision and quality of the final part [106]. These temperature gradients can also cause microstructural inhomogeneities, such as the formation of undesirable phases or uneven grain structures, which reduce the mechanical properties and overall quality of the material [107]. Additionally, steep temperature gradients can exacerbate issues like cracking and delamination, particularly in materials sensitive to thermal cycling [108]. Therefore, optimizing the temperature gradient is a critical step in enhancing the performance and reliability of additively manufactured metal components.

The use of auxiliary ultrasonic fields is crucial for regulating the temperature gradient in the molten pool during metal AM. UV, through cavitation and acoustic streaming effects, enhance the mixing of the molten pool and promote rapid heat transfer, thereby reducing the temperature gradient [89], [109]. This results in uniform cooling, minimizing residual stress and deformation, and improving the dimensional accuracy and mechanical properties of the parts [42]. Additionally, it helps prevent microstructural inhomogeneities, cracking, and delamination [37]. Therefore, the ultrasonic field is a key technique for optimizing the temperature gradient and enhancing the performance and reliability of additively manufactured metal components. Relevant studies have confirmed these findings. For example, Wang et al. [110] investigated the impact of UV on the temperature gradient from the top to the bottom of the molten pool. They observed that the temperature gradient with UV is lower than without UV, with a smoother gradient in the center of the molten pool after UV treatment (Fig. 8a). This indicates that the acoustic streaming induced by UV can homogenize the temperature distribution within the molten pool, effectively reducing the temperature gradient during solidification. Similarly, Jiang et al. [51] compared the peak temperatures of the molten pool in the LDED and UVLDED processes, finding that under the same laser parameters, the peak temperature in the UVLDED process is approximately 100 K lower than in the LDED process (Fig. 8b). This reduction is attributed to the enhanced thermal exchange within the molten pool due to UV, leading to a more uniform temperature distribution. Additionally, the increased size of the molten pool accelerates heat dissipation, transferring more heat to the substrate and surrounding environment.Fig. 8 (a) The extent to which the temperature gradient of UV changes from the top to the bottom of the molten pool during solidification [110]; (b) When the laser power is within the range of 800–1400 W and the scanning speed is 10 mm/s. A comparison analysis attained of the maximum temperature attained by the molten pool in LDED and UVLDED processes [51].

4.2 Effect of ultrasonic field on densification

4.2.1 Porosity and densification

Due to the non-equilibrium rapid solidification and accompanying segregation phenomena experienced by materials during the AM process, common defects such as porosity and cracking are prevalent in metal parts processed by AM [111]. Porosity in materials can originate from multiple mechanisms: (i) Inadequate energy input leading to incomplete fusion, typically resulting in pores with irregular shapes [112]; (ii) Excessive energy input causing keyhole porosity, characterized by predominantly spherical pores located at the bottom of the melt pool, arising from fluctuations in keyhole stability and vapor pressure instability [113]; (iii) Gas porosity generated from powder or shielding gas entrainment into the melt pool, with these gas pores generally being smaller than those formed by keyhole phenomena or insufficient fusion [114].

In terms of porosity reduction and densification, the proper auxiliary UV assistance can significantly mitigate the formation of micropores within deposited materials. This phenomenon is primarily attributed to the induction of acoustic streaming and agitation mechanisms, whereby the acoustic field increases the fluidity of the melt pool. Consequently, bubbles are more likely to rise and escape from the melt pool prior to solidification, thereby substantially lowering the porosity [28]. Research indicates that acoustic streaming plays a critical role in porosity reduction, particularly under condition of high ultrasonic amplitude [115]. For instance, Huang et al. [28] used three-dimensional reconstructed XCT images to demonstrate the significant benefits of U-FAAM in substantially improving the volume porosity and defect distribution of directed energy deposited AZ31 magnesium alloys. Compared to samples processed without ultrasonic intervention, those subjected to ultrasonic assistance exhibited a notable reduction in volume porosity, with samples exposed to ultrasonic powers of 60 W and 90 W showing decreases of approximately 81 % and 90 %, respectively. The application of ultrasound not only reduced the size of the porosities but also facilitated the transition of defect types from large-scale lack of fusion (LOF) defects to smaller, more regular pores, and completely eliminated interlayer chain-like porosities, thus significantly benefiting the intrinsic quality of the material (Fig. 9a). This defect transformation is primarily attributed to the cavitation effect induced by ultrasound, along with shock waves and microjets stimulated by acoustic streaming, which intensified the orderly flow of the molten metal and facilitated defect elimination.Fig. 9 Effect of ultrasonic assistance on the porosity and densification of metal AM components. (a) Compares the volumetric porosity rates of AZ31 magnesium alloy with and without UV [28]; (b) CT scanning results of LDED and UVLDED on pore defects of cladding layer under different laser parameters [51], and (c) compares the porosity rates of 316L steel with and without UV [92].

In a parallel research endeavor, Jiang et al. [51] utilized CT scanning to investigate the porosity of cladding layers under different laser parameters. Their findings revealed that the average pore diameter in UVLDED decreased from 45 µm to 18 µm, while the porosity was reduced from approximately 0.28 % to 0.11 % (Fig. 9b). The number and volume of pores in the UVLDED cladding layer were significantly suppressed. This reduction in porosity can be attributed to the increased melt pool flow rate induced by UV, which facilitates the escape of larger pores from the melt pool. The resulting lower porosity helps to reduce stress concentration, thereby preventing the initiation and premature failure of cracks in the cladding layer. Concurrently, Todaro et al. [92] examined the impact of UV on defect formation in LDED-manufactured 316L steel, revealing that the strategic application of UV assistance can facilitate the production of flawless 316L steel specimens (Fig. 9c).

4.2.2 Cracks

In the field of metal AM, the emergence of cracks can lead to numerous critical issues, adversely affecting the performance and applicability of the manufactured components [116]. Cracks can be broadly categorized into the following types: (i) Thermal cracks form due to excessive internal thermal stress triggered by localized rapid heating and cooling during the AM process, predominantly occurring at overlapping solidification tracks or at the edges of melt pools [117]. (ii) Solidification cracks occur as the material cools, with temperature gradients and solid phase deformation causing stress that exceed the yield strength of the material, typically manifesting when the material has solidified yet remains at elevated temperatures [118]. (iii) Defect-induced cracks originate from internal flaws such as porosity and inclusions, which act as stress concentrators, undermining the overall integrity of the material and precipitating crack formation [119], [120].

To address the challenge of cracking, the industry has implemented a myriad of solutions, including strategic material selection, the adoption of pre-alloyed powders to improve printability, and substrate preheating to increase the initial temperature, thereby slowing the cooling rate of the melt pool and reducing the risk of thermal stress and cracking [20], [121]. Apart from these approaches, ultrasonic assisted technology has shown significant potential in preventing crack initiation by modifying melt pool dynamics, refining grain structure and optimizing the stress field. Despite the limited research into the effect of UV on cracking in additively manufactured components, the work of Cong et al. [122] in fabricating AISI 630 steel by LDED demonstrates a significant reduction in microcrack formation when UV is applied (Fig. 10a and b). Similarly, Zhang et al. [53] reported that the assistance of UV effectively enhances microstructural uniformity and reduces defects such as lack of fusion and microporosity (Fig. 10c and d). This improvement is attributed to the cavitation and acoustic streaming induced by UV, which alter the gas–liquid interface, eliminate gaseous porosities, and promote homogenization of the molten powder with the substrate. This leads to a decrease in residual stress and hence a reduction in microcrack development.Fig. 10 Effect of ultrasonic field on crack of AM parts. Comparison of microcracks in AISI 630 steel (a) without UV and (b) with UV [122]; Comparison of microcracks in NiTi alloys (c) without UV and (d) with UV [53]; Interfacial microcracks: (e) without UV and (f) with UV [95].

Interestingly, Li et al. [95] investigated defects at the bottom of the deposition layer under various conditions and found that, in the absence of ultrasonic assistance, the crack length at the bottom of the layer reached 98.22 µm (Fig. 10e). While ultrasonic assistance can inhibit crack formation, an excessive ultrasonic energy density can cause cracks to reappear (Fig. 10f). The thermal-physical property differences between the deposition layer and the substrate, along with residual stress, lead to the formation of microcracks at the bottom interface during rapid cooling. The acoustic streaming effect induced by ultrasonic excitation can influence the Marangoni effect, resulting in a more uniform melt pool flow and reducing the overall G. This reduction in G mitigates excessive internal stress, thereby eliminating the formation of microcracks at the bottom from the source.

4.3 Effect of ultrasonic field on microstructural evolutions

4.3.1 Grain refinement and CET

In the AM processes, the application of ultrasonic fields significantly enhances material microstructure and performance through grain refinement and CET. This advancement is primarily achieved by mechanisms such as increased nucleation rates, fragmentation of existing grains, optimization of liquid metal flow, and modulation of solidification dynamics [123]. These combined effects not only optimize the internal structure of materials but also vastly improve their overall properties. This represents a novel strategy for microstructural control and performance enhancement in the field of metal AM, offering profound implications for materials science and engineering.

Consequently, grain refinement and CET phenomena have been widely reported in alloys fabricated using UV assisted AM techniques. Research by Ji et al. [38] on the preparation of AISI 1045 steel using ultrasonically assisted WAAM demonstrated that an increase in ultrasonic amplitude (UA) leads to the propagation of grain refinement from the center to the periphery. Notably, at an amplitude of 12 μm, a significant portion of the deposition zone underwent grain refinement, with columnar grains transforming to non-oriented equiaxed grains and a dramatic reduction in grain size (Fig. 11a). Furthermore, Li et al. [28] investigated the impact of ultrasonic generator output power (UOP) on the microstructure of AZ31 magnesium alloy samples produced via LDED technology, revealing that samples at 60 W and 90 W output power displayed a fully equiaxed grain structure (Fig. 11b). Similarly, Todaro et al. [34] observed CET and grain refinement phenomena in both β-prior grains and α-laths in Ti6Al4V alloys produced through LDED assisted by UV (Fig. 11c). Wang et al. [124] also explored the effect of ultrasonic frequency (UF) on the microstructure of Inconel 718 alloys constructed through LDED, finding that with increasing UF, the grain distribution became more uniform, grain size was further reduced, resulting in a completely equiaxed grain structure, while the grain boundaries were significantly shortened (Fig. 11d). These findings effectively validate the role of externally applied ultrasonic fields in enhancing CET and grain refinement in metal AM processes.Fig. 11 Effect of ultrasonic assistance on grain refinement and the CET during the metal AM process: (a) AISI 1045 steel [38]; (b) AZ31 magnesium alloys [28]; (c) Ti6Al4V alloys [34]; and (d) Inconel 718 alloys [124].

4.3.2 Texture alleviation

The heterogeneity of texture density in metal 3D printing poses a significant challenge to its widespread application, particularly in the aerospace and automotive industries where the demand for high-performance materials is stringent. This issue notably undermines the mechanical properties of components [125], [126], [127]. In response, the research community has vigorously examined various strategies to mitigate this barrier, among which ultrasonic assistance technology has been validated as an effective solution. For example, under ultrasonic assistance, ER321 stainless steel fabricated using LWAM technology exhibited a significantly weakened < 001 > texture density and more randomized grain orientation (Fig. 12a) [128]. Similarly, Yi et al. [32] effectively prevented the strong {0001} texture in Mg-Al alloys prepared using LDED technology with UV assistance, resulting in an almost random crystal structure (Fig. 12b). Moreover, Todaro et al. significantly attenuated the α-phase texture and the strong < 001 > texture of prior-β grains in Ti-6Al-4 V alloys produced via ultrasonically assisted LDED technology (Fig. 12c) [34]. This can be attributed to the high-frequency vibrations induced by ultrasonic wave during the metal AM process, which act directly on the powder material or molten metal, causing micro-level flow and agitation. As a result, this enhances the fluidity of the melt pool, accelerates melting and solidification processes, thereby facilitating the homogenization of the texture.Fig. 12 Effect of ultrasonic assistance on the texture during the metal AM process: (a) ER321 stainless steel [128]; (b) Mg-Al alloys [32]; (c) Ti-6Al-4 V alloys [34].

4.3.3 Uniform dispersion of alloy elements and reinforcements

A critical challenge in advanced metal AM is ensuring the uniform dispersion of alloying elements and reinforcements within components, as this has a direct impact on the performance and reliability of the manufactured parts [129], [130], [131]. To address this issue, researchers have begun to explore novel technologies and methods. Ultrasonic assistance technology, which is recognized for its successful application in improving material mixing and facilitating chemical reactions in other fields, is considered a promising solution[132], [133]. The application of ultrasonic energy generates micro-level agitation within the material, promoting the redistribution of atoms and particles at the microscale, thereby achieving uniform dispersion of alloying elements and reinforcements in the metal matrix.

Wang et al. [134] employed UV-assisted Laser AM (LAM) to fabricate Inconel 718/Ti6Al4V composites. Their research indicated that the application of ultrasonic wave mitigated the segregation of Ni elements within the metal phase, effectively inhibiting the formation and reducing the content of Ti2Ni phases (Fig. 13a). Similarly, Zhang et al. [53] discovered a more uniform distribution of Ni and Ti elements in NiTi alloys prepared with UV-assisted LDED, without significant clustering observed (Fig. 13b). This outcome is likely attributed to the smoothing effect of thermal gradients and the promotion of fluid flow by UV, leading to a more uniform material distribution within the melt pool. Additionally, the use of UV has shown considerable promise in the fabrication of particle-reinforced metal matrix composites, particularly in the dispersal of reinforcing materials. With the assistance of UV, Ning et al. [135] noted a significant improvement in the dispersion of reinforcing particles in Ti-TiB composites created via UV-assisted LDED, as a result of the shear forces induced by the UV breaking up the TiB agglomerates during the deposition process (Fig. 13c).Fig. 13 Effect of ultrasonic assistance on the distribution of elements and reinforcements during the metal AM process: (a) Inconel 718/Ti6Al4V composites [134]; (b) NiTi alloys [53]; (c) Ti-TiB composites [135].

4.4 Effect of ultrasonic field on mechanical properties

Traditional metal AM processes are plagued by defects such as porosity, lack of fusion, and stress concentration, which severely degrade the mechanical performance of components [136], [137]. To address these challenges and improve the mechanical properties of components, U-FAAM technology has emerged. By applying UV during the AM process, it optimizes melt pool dynamics and improves microstructure quality, thereby significantly enhancing mechanical performance. Table 2 summarizes these observations. Todaro et al. [34] reported that the use of UV assistance in the LDED of Ti6Al4V alloy resulted in a 12 % increase in yield and ultimate tensile strength without a significant reduction in ductility (Fig. 14a). Furthermore, due to the rapid melting and solidification characteristics of the metal AM processes result in significant microstructural heterogeneity, such as columnar grain morphology and strong texture, which causes anisotropy in the mechanical properties of the final product in different directions. The application of UV effectively mitigates the mechanical anisotropy in AM components by CET and texture weakening. As illustrated in Fig. 14b, Yi et al. [32] found that compared to a similar alloy without UV treatment, Mg-Al alloys fabricated with DED under UV assistance exhibit markedly reduced mechanical orientation and alleviate the reduction in hardness along the build direction. This improvement can be attributed to grain refinement and a decrease in textural strength. Similarly, Yu et al. [138] reported that the application of UV significantly increased the hardness of the inner region of the print. Concurrently, the applied of UV significantly enhances the mechanical properties of the material and improves the anisotropy of the mechanical properties, and finally obtains the strength-plastic equilibrium deposition (Fig. 14c).Table 2 Mechanical properties of AM fabricated parts with auxiliary ultrasonic field.

AM	Auxiliary UV	Material	Frequency (kHz)	Amplitude (μm)	Mechanical property	Property with UV	Property without UV	Units	Property difference	Ref	
LDED	Substrate UV	NiTi alloy	25	/	Hardness	520	430	HV	20.9 %	[53]	
LDED	Moving UV	ER321 steel	20	16	YS
UTS
δ
Hardness	367
663
60
217	328
646
59
196	MPa
MPa
%
HV	11.9 %
2.6 %
1.7 %
11.1 %	[128]	
LDED	Substrate UV	Fe-Cr steel	/	/	Hardness	441	456	HV	3.4 %	[139]	
LDED	Substrate UV	CoCrFeMnNi alloy	20	/	YS	274	235	MPa	16.6 %	[140]	
LDED	Substrate UV	Al-Cu alloy	20	5	UTS
δ	274
12	198
10	MPa
%	38.3 %
20.0 %	[42]	
LDED	Substrate UV	Al-12Si alloy	20	25	YS
UTS
δ	107
227
12	102
194
8	MPa
MPa
%	4.9 %
17.0 %
50.0 %	[141]	
LDED	Substrate UV	AZ31 Mg alloy	15	30	YS
UTS
δ	91
236
19	70
163
6.6	MPa
MPa
%	30.0 %
45.0 %
189.0 %	[28]	
LDED	Substrate UV	Mg-Al alloy	/	25	YS
UTS
δ
Hardness	93
228
19
61	89
182
7
54	MPa
MPa
%
HV	4.5 %
25.3 %
171.4 %
13.0 %	[32]	
LDED	Substrate UV	Inconel 718	41	5	Hardness	280	236	HV	19 %	[124]	
LDED	Substrate UV	Ti-6Al-4 V	20	30	YS
UTS	1094
1137	980
1015	MPa	11.6 %
12.0 %	[34]	
LDED	Substrate UV	ER70S-6 alloy	/	15	YS
UTS
Hardness	490
628
271	442
567
245	MPa
MPa
HV	10.9 %
10.7 %
10.6 %	[47]	
LDED	Moving UV	24CrNiMoY alloy	40	30	UTS
δ
Hardness	1125
12
382	854
6
346	MPa
%
HV	31.7 %
100 %
10.6 %	[52]	
WAAM	Moving UV	ER321 steel	20	50	YS
UTS
δ
Hardness	425
694
50 %
223	380
667
48 %
195	MPa
MPa
%
HV	10.5 %
3.7 %
4.2 %
12.5 %	[142]	
WAAM	Moving UV	AA7075 MMNC	19.9	14.6	UTS
δ	296
11	203
8	MPa
%	45.8
37.5	[143]	
WAAM	Substrate UV	Inconel 625 alloy	20	/	UTS	797	650	MPa	22.5 %	[144]	
LPBF	Substrate UV	GH5188 superalloy	40	20	YS
UTS
δ
Hardness	731
1032
46
301	699
1007
43
288	MPa
MPa
%
HV	4.6 %
2.6 %
5.6 %
4.5 %	[48]	

Fig. 14 Effect of UV assistance on the mechanical properties of materials: (a) tensile properties of Ti6Al4V alloy with and without UV [34]; (b) Tensile performance of Mg-Al alloy with and without UV [32]; (c) Microhardness and tensile performance of 17–4 PH stainless steel with and without UV [138], and (d) tensile performance of GH5188 alloy with and without UV [48].

Interestingly, Yan et al. [48] found that for the LPBF process, ultrasound treatment must be performed at low intensity in order to mitigate the negative effects of ultrasound on the powder bed. Although this treatment can refine the grains of GH5188 alloy, it is unable to completely transform the long columnar crystals into equiaxial crystals. Consequently, the enhancement of the mechanical properties of materials is not pronounced, and the auxiliary effect of ultrasonic field is constrained (Fig. 14d). It is evident that further research is required to ascertain the full applicability of UV in the LPBF process.

5 Ultrasonic field-assisted metal AM simulation

The significance of simulation research in U-FAAM lies in its capacity to furnish an exhaustive comprehension and anticipation of intricate physical phenomena, including molten pool morphology, temperature distribution, fluid behavior, and pressure distribution [106], [67]. The application of simulation studies enables the optimization of ultrasonic parameters and process conditions, thereby reducing experimental costs and time while avoiding material waste from trial-and-error methods [33]. Furthermore, the utilization of simulation affords the elucidation of the dynamic effects of ultrasonic wave within the molten pool, thereby providing a scientific foundation for the advancement of process improvement and innovation[145], [146]. It is therefore evident that simulation research is an indispensable tool for improving the quality and efficiency of metal AM.

In recent years, significant progress has been made in the simulation research of U-FAAM, spanning both fundamental theories and practical applications. Jiang et al. [51] precisely predicted the length of the molten pool using models, revealing that the UVLDED molten pool is significantly longer than the LDED molten pool. An increase in laser power or a decrease in scanning speed results in an increase in the length of the molten pool. The influence of laser power on the molten pool length is greater than that of scanning speed because lower scanning speeds result in more powder deposition per unit area, which obstructs and reflects the laser, reducing thermal efficiency (Fig. 15a). Wang et al. [110] used COMSOL Multiphysics software to simulate the temperature distribution of the molten pool. They found that the temperature contour of the UVLDED molten pool is sparser than that of the DED molten pool. Additionally, the maximum temperature within the UVLDED molten pool is nearly 100 K lower than that of the DED molten pool (Fig. 15b).Fig. 15 (a) Simulation and experimental verification of molten pool length and cladding topography [51]; (b) To simulate the temperature distribution of the molten pool at different times [110]; (c) Simulation of the flow field of the molten pool at different moments in a droplet transfer cycle [56]; (d) Simulation of the pressure distribution in the molten pool at different times [56].

Ji et al. [56] employed three-dimensional CFD modelling to examine the impact of UV and non-UV conditions on the flow and pressure fields within a melt pool. The simulation results demonstrated that ultrasonics did not affect the state of fluid flow within the melt pool. However, under the influence of UV, there was a notable increase in both the maximum flow rate and the extent of the high-flow region, indicating that ultrasonic enhanced the fluid dynamics in the melt pool (Fig. 15c). With regard to the pressure within the melt pool, it was found to be almost uniform in the absence of UV, with the highest pressure located at the bottom center, slightly above atmospheric pressure, and the surface pressure equal to atmospheric pressure. In the presence of UV, the majority of regions within the melt pool exhibited a positive pressure profile. The maximum positive pressure was observed in the region below the center, while a small negative pressure zone formed at the bottom (Fig. 15d). This phenomenon occurs due to the transmission of the negative pressure phase of the ultrasonic wave through the substrate to the bottom of the melt pool, thereby creating a negative pressure area. As the ultrasonic continue to propagate, the negative pressure area expands in an upward direction until it encompasses nearly the entire melt pool, with the maximum negative pressure located just below the center of the pool. Subsequently, as the ultrasonic wave enters the positive pressure phase, the area and maximum value of negative pressure gradually decrease, while the area and maximum value of positive pressure increase. These studies demonstrate that ultrasonic wave significantly affect the thermal, flow, and pressure characteristics within the molten pool, providing essential insights for process optimization.

6 Future research directions and their relationship to the sustainable development Goals

6.1 Future research directions

U-FAAM technology integrates conventional AM with ultrasonic energy, presenting numerous challenges and opportunities for future research and development. This section will primarily focus on discussing key directions for future research and development, including process optimization, material applicability, and technological innovations in equipment, to improve the maturity and processing efficiency of U-FAAM technology. Fig. 16. provides an overview of the prospective research and development trajectory of U-FAAM.Fig. 16 The perspectives in future R&D of U-FAAM.

6.1.1 Process optimization and parameter control

Although U-FAAM technology has made significant advancements in enhancing the quality and applicability of 3D printed parts, it is still in its nascent stage and faces numerous challenges. The frequency, power, and amplitude of ultrasonic wave have a pronounced impact on the quality of printed materials [43], [140]. It is imperative that future research systematically explores the effects of on part density, microstructure, and mechanical properties, and precisely controls them to identify the optimal parameter set. Particularly, in substrate UV, the ultrasonic energy diminishes as parts accumulate, leading to enhanced material anisotropy that fails to meet industrial requirements. Therefore, developing adjustable amplitude techniques suitable for large component printing is a priority. Additionally, the cavitation and acoustic streaming effects induced by ultrasonic in the melt pool also impact the LDED process. The evolution of cavitation bubbles disrupts the Marangoni flow in the melt pool, leading to lamination phenomena[94]. To reduce defects caused by lamination, new ultrasonic-assisted deposition strategies are necessary, such as activating ultrasonic at specific layers or between layers.

The disparate in solidification rate and thermal gradient between LPBF and LDED processes result in distinct mechanisms through which ultrasonic affect microstructural organization in the LPBF process. Studies by Richter and others have shown that ultrasonic can influence grain structure even in the absence of cavitation, indicating that acoustic streaming may be a key driver of grain structure change under LPBF condition [147]. Consequently, further investigation into the impact mechanisms of ultrasonic on the LPBF process will be a focal point of future research. Currently, the application of UV in LPBF is limited mainly due to compatibility challenges with the powder layer. Ultrasonic excitation in the LPBF process may disrupt the powder layer, leading to uneven powder distribution [15]. Thus, the suitability of UV technology in LPBF remains uncertain and requires extensive validation in future studies.

In the future, the control of U-FAAM parameters will be dependent upon the optimization algorithms of AI and ML. By analyzing vast amounts of experimental data, these advanced technologies can rapidly identify the optimal set of process parameters, significantly enhancing manufacturing efficiency and product quality [148], [149]. AI and ML will be employed to deeply analyze the effects of ultrasonic frequency, power, and amplitude on material density, microstructure, and mechanical properties, and to construct predictive models for precise optimization. Furthermore, numerical simulations and computational modelling play a crucial role in process optimization, enabling the simulation of ultrasonic propagation in the melt pool and its effects, revealing the mechanisms by which cavitation and acoustic streaming influence metal flow and solidification behavior. By modelling temperature fields, stress distribution and microstructure evolution under various ultrasonic parameters, researchers can systematically assess the impact of each parameter on product performance, providing scientific guidance for parameter selection in actual manufacturing processes. Despite the challenges posed by equipment complexity and high cost, these technologies hold significant potential to enhance product performance and manufacturing efficiency, making them key directions for future development.

6.1.2 Research of material suitability

With the increasing industrial demand, the need for materials with special properties is continuously rising. Consequently, it is imperative that U-FAAM technology expands its research into the adaptability of new materials. The primary research focuses include: firstly, a systematic study of the impact of ultrasonic on the microstructure and macroscopic properties of innovative metals and their alloys. This involves experimental and numerical simulation analyses to elucidate the propagation mechanisms and effects of ultrasonic within the melt pool, exploring its role in grain refinement, phase transformation behavior, and defect control. Secondly, ultrasonic fields hold great potential in composite materials and multi-material systems [150]. The utilization of ultrasound can markedly enhance the propagation characteristics and wettability of a multitude of interfaces, thereby optimizing the interface bonding strength [151], [152]. Particularly in the preparation of particle-reinforced metal matrix composites, the application of ultrasonic fields has been proven to be an effective method for mitigating the agglomeration of reinforcing particles [153], [154]. Additionally, the use of renewable resources and environmentally friendly alloys is explored to advance the development of green materials, studying their forming behaviors and properties under ultrasonic assistance, and assessing their environmental friendliness and sustainability.

On this basis, establishing an optimized matching model for materials and ultrasonic process parameters, combined with advanced characterization techniques (such as electron microscopy, X-ray diffraction, and tomography) and numerical simulation methods (such as finite element analysis and molecular dynamics simulation). This will enable a comprehensive evaluation of the applicability and stability of different material systems in U-FAAM. Through real-time monitoring and feedback control, the process parameters can be optimized in real-time to achieve the maximize the performance of the material. Ultimately, in response to the demand of manufacturing complex parts, the development of intelligent control systems is necessary to facilitate the efficient production of multi-material and composite materials. This will promote the synergistic development of material innovation and process optimization, improving manufacturing efficiency, and ensure product quality in accordance with high industrial application standards.

6.1.3 Equipment and technology innovation

The U-FAAM technology is confronted with a number of challenges including an uneven transmission of ultrasonic energy, difficulty in precise parameter control, and issues with compatibility and long-term stability with various additive processes [29], [40]. It is recommended that future equipment and technological innovation focus on the development of advanced ultrasonic devices that are capable of precisely controlling frequency, power, and amplitude in order to minimize energy loss and ensure process stability. In the printing of large and complex parts, the presence of multiple layers and continuously changing geometries can cause variations in pressure nodes/antinodes and ultrasonic energy as the build position and height change [147]. Developing new amplitude modulation techniques or using multiple ultrasonic transducers to adjust the waveform might maintain a constant acoustic pressure in the current layer, potentially solving this issue [82], [155]. Notably, for ultrasonic devices employed in the LPBF process, it is advisable to avoid the formation of cavitation and keyholing, opting instead to utilize the acoustic streaming effect to optimize the grain structure. The low power requirements for maintaining tip vibration increase the feasibility of LPBF. Consequently, the compatibility of ultrasonic equipment with various fabrication processes becomes particularly crucial. Moreover, the future of metal AM will likely be dominated by multi-field interaction-assisted processes. The integration of multiple physical fields, such as ultrasonic, magnetic, and thermal fields, can facilitate a synergistic optimization of the manufacturing processes, leading to enhanced quality, efficiency, and sustainability. This approach is of great consequence for meeting the growing industrial demand for complex, high-performance components, and drives innovation and sustainable development in manufacturing technologies.

6.1.4 Standardization and quality assurance

The future research and development of U-FAAM technology will concentrate on the advancement of a comprehensive standardization system, quality control technologies, quality assessment and certification systems, and optimizing quality assurance frameworks. This encompasses the establishment of international and industry standards pertaining to equipment, materials, processes, and quality inspections, alongside the creation of operational procedures and quality control guidelines aimed at standardizing production operations and improving product consistency. Furthermore, the implementation of high-precision real-time monitoring and the development of non-destructive testing technologies, such as precise sensors and advanced imaging techniques, will enable the monitoring of the manufacturing processes. This will allow for real-time adjustments of process parameters and ensure the quality of the manufactured parts. Furthermore, a comprehensive quality evaluation and certification system will be established with the objective of enhancing market trust. The incorporation of big data and artificial intelligence technologies will facilitate the utilization of market and quality feedback to ensure the continuous enhancement of product quality and the optimization of quality management efficiency [156].

6.2 The relationship between future research directions and sustainable development goals

U-FAAM technology is of great consequence in propelling the manufacturing industry towards sustainability, in close alignment with the United Nations 2030 Sustainable Development Goals. At the outset, U-FAAM technology serves to enhances energy and material efficiency through process optimization and precise parameter control, thereby reducing waste and directly supporting responsible consumption and production (SDG 12). Additionally, this technology expands the scope for the utilization of materials, including recycled materials, by improving their microstructures, thus promoting sustainable industrialization (SDG 9) and supporting environmental protection (SDGs 13, 14, and 15). Technological and equipment innovations not only enhance the efficiency of production processes but also mitigate the environmental impact of industrial activities, further enabling the use of clean energy (SDG 7) and addressing climate change (SDG 13).

Furthermore, the implementation of standardization and quality assurance measures within the field of U-FAAM technology ensures product consistency and reliability, thereby laying the foundation for the sustainable industrialization globally. These standards not only facilitate the adoption and acceptance of the technology but also reinforce global cooperation by promoting inclusive growth (SDG 10) and supporting the industrialization of developing countries (SDG 9). In conclusion, the advancement and implementation of U-FAAM technology illustrate how technological innovation can facilitate the dual objectives of economic growth and environmental protection, thereby becoming a pivotal force in the advancement of the Sustainable Development Goals.

CRediT authorship contribution statement

Xuekai Li: Writing – original draft, Validation, Methodology, Investigation, Conceptualization. Wei Wang: Writing – review & editing, Methodology, Formal analysis. Yihong Wu: Writing – review & editing. Donghu Zhou: Writing – review & editing. Huijun Kang: Validation, Funding acquisition. Enyu Guo: Methodology, Funding acquisition. Jiehua Li: Writing – review & editing. Zongning Chen: Writing – review & editing, Funding acquisition, Conceptualization. Yanjin Xu: Writing – review & editing. Tongmin Wang: Supervision, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

No data was used for the research described in the article.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (No. 2022YFB3403701), the National Natural Science Foundation of China (Nos. 51971051, 52022017, U22A20174, and 52174356), the Science and Technology Plan Project of Liaoning Province (Nos. 2022010005-JH6/1001 and 2022JH2/1013), the Innovation Foundation of Science and Technology of Dalian (No. 2023JJ12GX021), the Major Science and Technology Projects of Longmen Laboratory (NO.231100220400) and the fundamental research funds for the Central Universities.
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