Ultrasonic-driven grain uniformity of commercially pure nickel in continuous heat input directed energy deposition

Abstract

Wire arc-based metal additive manufacturing (AM) commonly results columnar grain structures aligned with the build direction, causing anisotropic mechanical properties and performance degradation. To mitigate the formation of columnar grain structures, ultrasonic vibration (UV) at 20 kHz was applied during the continuous heat input directed energy deposition (DED) process, targeting microstrucrual refinement in high-purity Ni alloy. The application of UV disrupted thermal gradients and facilitated dynamic recrystallization, transitioning columnar grains into equiaxed grains near the substrate. Electron backscatter diffraction (EBSD) analysis revealed a marked reduction in crystallographic texture, showing randomized grain orientations in UV-treated samples, contrasting with the strong anisotropy in untreated samples. High-speed imaging revealed localized vibrations at the interface between the metal bead and molten metal, forming band-like structures that progressively refined dendrites during solidification. These findings demonstrate the potential of UV to refine grain structures, enhance isotropy, and improve the mechanical properties of AM-produced materials.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-29468-3.

Introduction

Additive manufacturing (AM) using directed energy deposition (DED) techniques has gained increasing attention in industrial sectors such as shipbuiding^1,2, aerospace³, and nuclear energy⁴ due to its ability to fabricate large-scale metallic components with high deposition rates and design flexibility^5,6.

However, the thermal cycles inherent in additive manufacturing, characterized by repeated melting and solidification during layer by layer deposition, typically result in columnar grain structures aligned with the build direction due to directional heat flow and steep thermal gradients^7–9. Due to the distinct thermal history inherent to the WAAM process, differences in microstructural anisotropy and mechanical properties—such as yield strength, elongation, and fatigue life—are observed compared to conventionally cast component^10,11. To overcome this limitation, ongoing research has focused on enhancing mechanical properties by refining the microstructure through the application of ultrasonic vibrations (UVs) during the process, without altering the alloy composition.

Previous studies have demonstrated the potential of UV in promoting grain refinement and improving mechanical performance in metal additive manufacturing. For instance, Todaro et al. applied high-intensity UV during the AM of Ti-6Al-4 V, achieving a complete transition from columnar to fine (~ 100 μm) equiaxed grains, which led to a 12% increase in yield stress and tensile strength¹². A similar approach was extended to the nickel-based superalloy Inconel 625, where UV effectively suppressed the formation of columnar grains typically observed in AM builds.

Chen et al. further expanded the understanding of UV-induced grain refinement to welding processes, specifically investigating its effects in the weld zones of pure aluminum during gas tungsten arc welding (GTAW)¹³. Their findings attributed the refinement mechanisms to cavitation and high-pressure shock waves, and identified a nonlinear relationship between ultrasonic amplitude and grain fragmentation. Notably, their results also highlighted the influence of process parameters, such as welding current, on the extent of grain refinement.

Building on these insights, Ji et al. employed three-dimensional computational fluid dynamics (CFD) simulations to analyze the effects of UV on molten metal behavior and solidification dynamics in a wire-based DED system¹⁴. Their study emphasized the roles of acoustic resonance and shock wave propagation in transforming columnar grains into fine equiaxed structures. Moreover, UV was shown to improve thermal uniformity, broaden the overheated zone, and suppress excessive grain growth—further reinforcing its potential as an effective tool for crystalline structure control in AM.

In this study, the effect of ultrasonic-induced vibrations on grain refinement in a commercially pure nickel alloy with a face-centered cubic (FCC) structure was investigated under a continuous heat input DED process¹⁵.

Unlike conventional methods where metal beads are deposited in a horizontal direction, the continuous heat input DED process builds the metallic structure in a vertical rod-like form. Consequently, since the deposition direction is aligned with the axis of the sonotrode generating ultrasonic vibrations, it is possible to evaluate the variation in vibration effects and grain refinement behavior along the build height.

This study differs from previous UV-assisted AM research by leveraging a continuous heat-input DED process that maintains a molten state during vertical deposition, enabling in-situ investigation of ultrasonic vibration propagation and attenuation along the build height. This unique configuration allows direct correlation of height-dependent ultrasonic energy with grain refinement, texture evolution, and mechanical anisotropy, offering insights not captured in conventional horizontal layer-by-layer builds.

The ultrasonic vibration, initiated at the sonotrode, propagated upward through the vertically grown structure and into the molten metal. High-speed imaging revealed localized vibration phenomena at the interface between the solid structure and the molten metal. Vibration analysis confirmed that these interfacial physical effects influenced vibration intensity and acted as driving factors for microstructural transformation.

Materials and methods

Figure 1 illustrates the wire-based DED system and the experimental method for AM incorporating UV process. The wire-based DED system consists of a six-axis robot(ABB IRB 1520ID, ABB, Switzerland) and the arc plasma heat source (TPS 500i CMT, Fronius International, Austria) has a continuous heat input DED mode (M3DPen), and experimental conditions are as follows: current, 120 A; voltage 15.2 V; wire feed rate 3.2 mpm; travel speed 5 mm/s; Ar gas flow rate 15 L/min; arc length 15 mm shown in Fig. 1(a). Figure 1(b) shows an ultrasonic equipment operating at frequency 20 kHz with a 20 μm amplitude. The metal wire used Ni alloy(KW-M61, KISWEL, Republic of Korea) as specified in Table 1. The substrate used in the experiment was stainless steel 316 L with dimensions of 140 mm x 40 mm x 3 mm.

Fig. 1.

Schematic and high-speed camera images of wire-based DED using UV. (a) Scheme of wire-based DED system, (b) Scheme of a rod-shaped metal structure and UV equipment and (c-f) High-speed camera images of metal structure manufacturing using continuous heat input DED process without cooling time a one cycle.

Table 1.

The chemical composition of Ni alloy metal wire in weight%.

C	Si	Mn	P	S	Cr	Ni	Mo	Ti
0.01	0.27	0.4	-	-	-	95.8	-	3.19

The behavior images of the molten metal were obtained using a high-speed camera with 50–4,000 fps (Photron, FASTCAM MINI AX100, Japan). Tensile test (High-precision UTM, Mrig, Republic of Korea) was performed at room temperature with a strain rate of 10^− 3 s^− 1. Figure 1(c–f) presents high-speed camera images of specimens fabricated using the continuous heat input DED process. Unlike conventional horizontal layer-by-layer AM, the continuous heat-input DED process used in this study maintains a molten metal throughout deposition, producing a continuous vertical build. This configuration allows height-resolved analysis of ultrasonic propagation and its influence on microstructural evolution.

The deposited metallic rod was approximately 50 mm in total height and exhibited bead diameter of 10 mm the applied process conditions.

Owing to the continuous heat-input mode, the molten metal was maintained throughout deposition, producing a continuous vertical build rather than discrete stacked layers.

Therefore, the layer number could not be distinctly defined.

A rod-shaped metal specimen was deposited on the substrate in situ a frequency of 20 kHz and an amplitude of 20 μm. (See Supplementary Movie S1)

The solid and liquid regions were automatically distinguished using the segment anything model (SAM) based on the clear boundaries observed in the captured high-speed images.

The molten metal appears brighter due to strong thermal radiation, while the previously solidified region appears darker due to lower thermal intensity.

This visual contrast allows SAM’s automatic mask generator to precisely detect the solid–liquid interface. An intersection over union (IoU) threshold of 0.62, stability score of 0.95, and minimum area of 40,000 pixels were applied to minimize false detections.

Results and discussion

Figure 2 provides a schematic representation of the molten metal behavior observed via high-speed image and the mechanism of grain refinement driven by UV. Figure 2(a–b) depicts the behavior of molten metal under UV influence when molten metal transfer is inactive, as captured by a high-speed camera. The specimen is divided into two regions: a lower solid section, where solidification progresses through conduction, and an upper liquid section, which remains in a molten metal state due to continuous arc plasma heat input. When the arc is off, surface waves with localized vibration zones with high amplitude are observed at the interface between the metal bead (solid) and the molten metal (liquid). (See Supplementary Movie S2).

Fig. 2.

Schematic illustration and high-speed camera images showing localized vibration and grain refinement mechanisms induced by UV at the solid–liquid interface. (a-b) High-speed camera images of localized vibration phenomena in interface of molten metal induced by UV and (c) Mechanism of grain refinement by UV.

This phenomenon of surface wave is attributed to the UV transmitted from the metal bead, leading to cavitation at the interface, interference of reflected waves, and the combined effects of molten metal density, surface tension, and viscosity^16–20. The solid-liquid interface, a critical region for dendrite formation during the solidification process, exhibits significant amplitude fluctuations.(See Supplementary Movie S3).

Figure 2(c) presents a schematic diagram illustrating the mechanism of grain refinement in molten metal under UV. During continuous heat input DED process, UV induces a localized increase in amplitude at the solid-liquid interface, where dendrite growth occurs. The localized amplification of ultrasonic vibrations is primarily attributed to (1) wave interference caused by acoustic impedance mismatch at solid–liquid interfaces^21,22, (2) cavitation collapse producing intense local thermal and pressure spikes^23,24, and (3) acoustic streaming that enhances melt pool convection and promotes grain fragmentation^25,26. The localized vibration zones responsible for grain refinement occur at the interface between the deposited bead and the molten metal, and are progressively shifted upward as solidification proceeds. While direct measurements of cavitation and acoustic streaming were not performed, the observed localized vibration patterns and the resulting equiaxed grain structure suggest the synergistic action of wave interference, cavitation-induced pressure spikes, and melt pool convection. Future studies incorporating real-time acoustic mapping and numerical modeling are necessary to validate these hypotheses quantitatively.

To evaluate the effect of UV on mechanical properties, mini-tensile tests were performed on layered materials in the direction perpendicular to the build direction (i.e., the normal direction [ND] or transverse direction [TD]). Digital image correlation (DIC, ARAMIS 5 M, GOM GmbH, Germany) was employed for accurate strain measurement during deformation, replacing the use of an extensometer. The geometry of the mini-tensile specimens, along with strain mapping results and corresponding displacement profiles, is shown in the inset. The stress-strain curves in Fig. 3(a) indicate that the Ni-UV sample exhibits approximately a 10% increase in yield strength (𝜎_𝑦=155 ± 2 MPa) compared to the Ni-NV sample (𝜎_𝑦=141 ± 1 Mpa). Although the elongation of Ni-UV decreased slightly, both the yield strength and tensile strength improved, which is a significant finding. This is particularly significant as most loading scenarios for three-dimensional structures involve forces acting perpendicular to the build direction. The true stress–true strain curves and strain hardening rate (SHR) curves in Fig. 3(b) provide further insights into the mechanical behavior of the layered materials. UV facilitated the formation of finer equiaxed grains at the solidification front, increasing the grain boundary area. This increase introduced additional barriers to dislocation motion, resulting in higher SHR values during plastic deformation beyond the yield point compared to the Ni-NV sample. However, strain localization caused a sharp decrease in SHR, leading the Ni-UV sample to reach the Considère criterion threshold at a lower strain value, indicating an earlier onset of necking.

Fig. 3.

(a) Stress-strain curves obtained from mini-tensile tests of Ni-UV and Ni-NV samples and (b) true stress-true strain curves (dotted line) and strength hardening rate (solid line). The Ni-UV sample exhibits higher yield strength compared to the Ni-NV sample. Digital Image Correlation (DIC) analysis was performed to accurately measure the strain during deformation, as shown in the inset. The insets also display the geometry of the mini-tensile specimens, along with the strain mapping results and corresponding displacement profiles.

Figure 4 presents the EBSD analysis results, highlighting the influence of UV on the microstructure of Ni-based alloys, with a focus on grain orientation and texture evolution. The Ni-UV sample exhibits a uniform microstructure with equiaxed grains and random orientations. UV effectively refined the microstructure by promoting dynamic recrystallization during the build process, reducing the anisotropy typically associated with AM and promoting a more isotropic grain structure. As illustrated in Fig. 4(a), the Ni-UV sample exhibits a random grain orientation distribution, indicative of a reduced texture and more isotropic microstructure. In contrast, the Ni-NV sample retains pronounced grain elongation along the building direction (BD), reflecting the columnar grain structures typically associated with conventional AM processes. The absence of ultrasonic vibration allowed the persistence of strong anisotropic grain orientation, resulting in a texture dominated by grains aligned with the BD. These elongated grains reflect a solidification-dominated microstructure with limited recrystallization or grain refinement.

Fig. 4.

EBSD analysis result: (a) Inverse pole figure (IPF) maps of Ni-based alloys showing grain orientation differences. The left panel (Ni-UV) exhibits a uniform grain distribution with diverse grain orientations, while the right panel (Ni-NV) reveals a significant grain elongation along the building direction (BD). The key directions of BD, transverse direction (TD), and sample direction (SD) are marked; (b) Orientation distribution function (ODF) plots presented as IPFs in X1, Y1, and Z1 directions for top, middle, and bottom regions of the samples. The color intensity represents the degree of texture, with differences in grain orientation distribution highlighted for Ni-UV and Ni-NV samples.

Further insights from Fig. 4(b) reveal that the inverse pole figures (IPFs) for Ni-UV demonstrate randomized and uniform grain orientations across the X1, Y1, and Z1 directions, regardless of position within the build. This uniformity is further corroborated by the orientation distribution function (ODF) plots, where the reduced intensity of the (100) orientation confirms that UV effectively suppressed the texture commonly observed in Ni-NV samples. UV disrupted thermal gradients during solidification, promoting grain nucleation in multiple directions instead of the unidirectional growth observed in Ni-NV. In contrast, the ODF plots for Ni-NV highlight strong crystallographic texture, particularly along the BD, as indicated by the high color intensity in the (001) and (101) orientations across all regions. The absence of UV allowed columnar growth driven by the thermal gradients inherent to AM, resulting in an anisotropic microstructure with strong directional dependence of mechanical properties. These findings highlight the significant metallurgical impact of UV during AM. In the Ni-UV sample, UV not only disrupted thermal gradients but also induced localized plastic deformation, facilitating dynamic recrystallization and grain refinement. This process eliminated the strong anisotropic grain orientation and texture typically associated with additively manufactured alloys. Conversely, the Ni-NV sample retained unrefined columnar grains aligned along the BD, reflecting the absence of such refinements. The refined and isotropic grain structure in the Ni-UV sample aligns with the mechanical test results shown in Fig. 3, which demonstrate improved yield strength and tensile properties compared to Ni-NV. Furthermore, the reduction in anisotropy in Ni-UV supports the enhanced consistency of mechanical properties, underscoring the potential of UV to address anisotropy-related challenges in additively manufactured materials.

Figure 5 illustrates the relationship between UV intensity and grain orientation, comparing the fraction of grains near X1 (100) within a 15° range for Ni-UV and Ni-NV samples. The graph shows that UV intensity decreases with height, resulting in a reduced fraction of grains with the specific orientation in higher regions of the sample. Ultra sonic intensity I as a function of the build height h is given by¹²:

Fig. 5.

Comparison of the fraction of grains with a specific orientation near X1 (100) within a 15˚ range for Ni-UV and Ni-NV samples, along with the ultrasonic vibration intensity profile. The fractions were extracted from the Orientation Distribution Function (ODF) data within a 15˚ deviation from the ideal orientation across the build height (bottom, middle, and top).

where ρ = 8,900 kg m^− 3 is the solid nickel density²⁷, c = 6,040 m/s is the sound velocity in solid nickel²⁸, f is the frequency, A₀ is the amplitude and h is build height.

We confirmed that grain refinement is greatly affected by intensity, heat input, and build height in continuous heat input DED processes used for fabricating bulk structures in the previous study¹⁵. Specifically, as the build height increases along the build direction, the formation of equiaxed grains is influenced by thermal gradients. To further understand these effects, we diverged from previous studies by assuming a solid-state condition to calculate the intensity. This approach allowed us to isolate the role of UV amplitude, which is strongest near the substrate, where it promotes recrystallization and disrupts the thermal gradient-driven columnar growth typically observed in additively manufactured materials. At the bottom of the sample, stronger ultrasonic vibration leads to finer, more sequiaxed grains and a reduced intensity of crystallographic texture, as seen in the higher fraction of randomized grains in the ODF data. As height increases, the UV attenuates, leading to directional grain growth driven by solidification and cooling effects from the free surface. Additionally, as continuous heat input is applied during the build process, the temperature gradient gradually decreases, particularly in the upper regions of the specimen. This reduction in temperature gradient promotes the growth of equiaxed grains near the top layers. The combined effect of reduced UV and decreased temperature gradient at higher build heights results in larger, more directionally aligned grains. The microstructural images further illustrate these trends, showing that grains in the top region are larger and more directionally aligned compared to the bottom, where finer and more isotropic grains are observed. ODF plots confirm these observations, revealing that the fraction of grains oriented near X1 (100) decreases with height, aligning with the reduction in ultrasonic vibration intensity. Overall, the results demonstrate that ultrasonic vibration effectively refines grains and reduces texture near the substrate, while the combination of weaker vibration and reduced temperature gradient at higher build heights fosters anisotropic grain growth and a stronger crystallographic texture.

Regarding the variation of acoustic velocity across liquid and solid phases, as well as its potential dependence on microstructural evolution, the ultrasonic intensity in this study was calculated using the acoustic properties of solid-state nickel. During the DED process, ultrasonic waves generated at the substrate predominantly propagate through the already-solidified region of the specimen before reaching the molten metal at the top. Accordingly, the propagation behavior is governed by the acoustic characteristics of the solid phase.

Although temperature gradients and evolving crystallographic textures may influence the local sound velocity, the purpose of this calculation is to illustrate how the position-dependent variation in ultrasonic intensity could affect microstructural formation. The intensity profile derived under the solid-state assumption is therefore used to provide qualitative insight into its potential correlation with the observed grain evolution.

Conclusion

This study demonstrated a novel approach for in situ microstructural control during continuous heat input directed energy deposition (DED) of a commercially pure nickel alloy by introducing 20 kHz ultrasonic vibration (UV). The application of UV effectively disrupted the thermal gradient at the solid–liquid interface and promoted dynamic recrystallization, resulting in a transition from columnar to equiaxed grains near the substrate. EBSD and ODF analyses confirmed randomized crystallographic texture and enhanced isotropy in the UV-treated samples. Mechanical testing revealed a ~ 10% improvement in yield strength, which is attributed to the refined grain structure. High-speed imaging captured localized vibration phenomena at the interface, supporting a grain refinement mechanism involving cavitation, acoustic streaming, and acoustic impedance-induced resonance. These findings demonstrate the potential of UV as a powerful in situ modifier to enhance the structural and mechanical quality of AM-fabricated metals without altering alloy composition.

However, the attenuation of ultrasonic intensity with increasing build height limited the grain refinement effect in the upper regions of the specimen, where directional growth reemerged. To overcome this limitation, future work should explore strategies to maintain uniform UV energy delivery throughout the build, such as multi-point excitation systems, adaptive amplitude control, or integration of feedback-based thermal regulation. Furthermore, computational models—such as CFD and thermomechanical-acoustic simulations—are needed to quantitatively analyze melt pool dynamics and energy propagation under UV conditions. Finally, extending this methodology to other crystal structures (e.g., BCC and HCP alloys) would help assess the broader applicability and generalizability of the proposed approach.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Chan Kyu Kim wrote the main manuscript draft, conducted the investigation, performed formal analysis, and contributed to conceptualization. Gitae Park reviewed and edited the manuscript and participated in the investigation. SeungHyeok Chung conducted the investigation and curated the data. Young Tae Cho and Seok Kim contributed to methodology development and formal analysis. Sang Woo Song supervised the project, provided resources, managed the project administration, contributed to methodology and conceptualization, acquired funding, and reviewed and edited the manuscript. All authors reviewed the manuscript.

Funding

This work was supported by the Fundamental Research Programs of the Korea Institute of Materials Science (KIMS, Grant No. PNKA 370). This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Climate, Energy & Environment (MCEE) of the Republic of Korea (No.20251K00000020).

Data availability

The data of this study are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.