Bubble explosion induced melt pool instabilities in electron beam melting of aluminum alloy

Domain: neuroscience
Published: 2026-04-07
Editorial outcome: major_revision

Abstract
Electron beam melting (EBM) is an additive manufacturing technology that can process
materials and manufacture components otherwise impossible or uneconomical. However, defects,
including porosity and surface irregularities, are widely reported in EBM-built components, and
their formation mechanisms are not fully understood. Here, using in-situ high-speed synchrotron
X-ray imaging, we reveal that bubble explosions in Al6061 during EBM induce melt pool
instabilities contributing to defect formation. The melt pool and keyhole evolve through three
stages: (1) initial formation of a melt pool, (2) subsurface bubble formation and explosion, and (3)
periodic keyhole oscillation. During scanning, periodic bubble explosions can eject molten liquid
as spatters and disturb the vapor depression and melt pool, contributing to surface humping, that
may trigger lack-of-fusion defects in subsequent layers. The physical insights we report could
provide guidance for EBM machine development, process innovation, alloy design and model
development.

Introduction
Electron beam melting (EBM) additive manufacturing process utilizes a focused electron beam
to selectively melt and fuse feedstock powders layer by layer to create high value parts within a
vacuum chamber. It has emerged as an important technology in advanced manufacturing, offering
advantages of high energy efficiency for processing highly reflective metals (e.g., pure copper for
heat exchangers and electrical components)1–4, low residual stress for processing brittle materials
(e.g., TiAl for jet engine blades)5–10, clean environment for processing impurity-sensitive materials
(e.g., tungsten and refractory high entropy alloys for fusion energy applications)11–15, and fast beam
steering for programmable thermal control16,17. However, the adoption of EBM for mission critical
components remains constrained by processing defects, including porosity, surface irregularities
and spattering, which compromise part quality and reliability18,19.

In EBM, accelerated electrons bombard the pre-sintered powder bed, converting their kinetic
energy into heat and causing localized melting20. This interaction generates a melt pool. As the
melt pool advances, the loose pre-sintered powder bed will be melted and then solidified to form
a layer of the part. During melting and solidification, various defects can form21–26. Porosity is a
primary concern because it directly compromises mechanical performance27,28. It can originate
from gas trapped within powders, pore capture by the advancing solidification front, or lack-of-
fusion between successive tracks and layers caused by insufficient energy input29–32. Surface
irregularity is another major issue; its progressive accumulation across layers can reduce the
printability and uniformity of parts9,33. Such irregularities can result from powder bed
nonuniformity, adherence of partially melted particles or droplet spatter18,34, and balling35,36.

The defect formation mechanisms reported for EBM have all been observed in laser powder bed
fusion (LPBF)37–42. However, EBM fundamentally differs from LPBF in its heat source: photons
in LPBF penetrate only nanometers, causing 2D surface heating20, whereas high-energy electrons
in EBM can penetrate tens of microns, volumetrically depositing energy through scattering43,44. This fundamental difference raises a critical question: are there defect formation mechanisms and
process instabilities unique to EBM?

To address this question, we developed an EBM system for in-situ synchrotron X-ray imaging
and demonstrated its feasibility32,45. In this study, taking advantage of this in-situ characterization
system, we discovered that bubble explosions during EBM in Al6061, unique to EBM, destabilize
the melt pool and contribute to multiple types of defects. The findings provide critical mechanistic
insights into the origin of melt pool instabilities and defect formation in EBM, laying the
foundation for developing strategies to achieve defect-lean EBM additive manufacturing.

Results
Subsurface bubble formation and explosion
The dynamics of the melt pool and vapor depression in electron beam melting of Al6061 were
captured using a custom-built electron beam melting system integrated with high-speed
synchrotron X-ray imaging at beamline 32-ID of the Advanced Photon Source (Fig. 1a and details
of the setup can be found in Methods). During melting, the melt pool remained confined within
the boundaries of the Al6061 sample, ensuring the formation of a fully developed melt pool
without penetrating the 1.2 mm sample thickness (a representative surface profile of the melt track
is shown in Fig. 1b). Subsurface bubble formation and explosion were consistently observed in the
electron beam irradiated area across all processing conditions, including stationary melting,
shallow and deep vapor depression scanning (Fig. 2 and Supplementary Movies 1-3), and were
found to have impact on the melt pool and vapor depression dynamics.

Dynamics of melt pool and vapor depression
The dynamics of melt pool and vapor depression under the influence of subsurface bubble
formation and explosion were investigated. Three distinct stages were identified during stationary
melting: (i) initial formation of a melt pool (Fig. 3a-c), (ii) subsurface bubble formation and
explosion (Fig. 3d-j), and (iii) periodic keyhole oscillation (Fig. 3j-o, Supplementary Note 1,
Supplementary Fig. 1, and Supplementary Movie 4). The melt pool and vapor depression
dimensions were tracked frame-by-frame over time (Fig. 3p). The moments corresponding to the
transient states of melt pool and vapor depression shown in Fig. 3a-o are indicated by arrows along
the curves for reference.

Stage 1 is characterized by the gradual formation and deepening of the melt pool without the
appearance of a vapor depression. Stage 2 begins with the onset of rapid explosions, exceeding
10 kHz in frequency between 720 µs to 800 µs. Due to the short lifespan of individual bubbles and
limitations in imaging frame rate, the exact frequency was not determined. At this stage, the rapid
expansion of the melt pool and keyhole is primarily driven by rising vapor pressure within the
depression, which displaces molten material, similar to the mechanism of keyhole drilling
observed in laser powder bed fusion46. Bubble explosions may further amplify local pressure
fluctuations, promoting additional keyhole deepening. Stage 3 marks the onset of periodic keyhole
oscillation: while the width remains quasi-stable with fluctuations of approximately 20% of the
mean width, the depth undergoes pronounced oscillations exceeding 60% of the mean depth,
driven by periodic keyhole collapse at a frequency of approximately 2.5 kHz.

After the initiation of the melt pool and vapor depression, their evolution during scanning was
investigated. To reach the specified power level at the start of the scan track, at the starting point
of the scan track, the system was programmed to perform stationary melting for approximately 0.5
ms before initiating scanning. Once a vapor depression formed, the electron beam began scanning.
With a high scan speed of 1.2 m/s and an electron beam power of 426 W, frequent bubble
explosions were observed at the front wall of the vapor depression, with multiple explosions
occurring simultaneously within a single frame (Fig. 4 and Supplementary Movie 5). These
explosions generated surface waves at the vapor–liquid interface (Fig. 4j). The resulting unsteady
melt pool dynamics manifest as a wavy morphology on the rear wall of the vapor depression,
indicating a periodic and discontinuous driving force acting on the melt flow. Liquid metal
accumulated at the rear of the melt pool, producing periodic humping and a rough melt track
surface after solidification (Fig. 4i). The frequency of bubble explosion was quantified to exceed
24 kHz. Based on the number of explosions captured per frame, the front wall of the vapor
depression was categorized into three distinct states: no explosion, single explosion (Fig. 4j), and
multiple explosions (Fig. 4k), as summarized in Fig. 4l.

To further investigate the influence of energy density on bubble explosion dynamics and its
consequences, electron beam melting was performed at a higher energy density by reducing the
scan speed from 1.2 m/s to 0.6 m/s while maintaining the same electron beam power of 426 W.
Similar explosion events were observed on the front wall of the vapor depression, along with
explosion-induced surface waves at the vapor–liquid interface (Fig. 5 and Supplementary Movie
6). A wavy morphology was also observed on the rear wall of the vapor depression (Fig. 5a-f).
Following the onset of scanning, the melt pool and vapor depression gradually elongated (Fig. 5c
and d). As the scan continued, molten metal accumulated at the rear of the melt pool. After
solidification, the melt track exhibited a nonuniform height profile, with large humping. The
classification of front wall states is shown in Fig. 5j. The total explosion frequency exceeds 31 kHz,
with simultaneous multiple explosions occurring within a single frame at a frequency of
approximately 13 kHz. This frequency is notably higher than that observed under lower energy
density (Fig. 4l). Multiple explosions were consistently preceded by single-explosion events,
suggesting a potential cascade or triggering mechanism in the explosion dynamics.

Dynamics of spattering
The dynamics of spattering under the influence of subsurface bubble formation and explosion
were also investigated. Since spatter primarily arises from interactions between metallic vapor and
the melt pool surface or powder bed, pre-sintered powder bed samples were used to study
interactions among metal vapor, melt pool and powder. Following subsurface bubble explosions
(Fig. 6a and b), the melt pool became unstable within the subsequent frames, ejecting a liquid
ligament from the explosion site followed by spatters (Fig. 6c-f and Supplementary Movie 7). To
facilitate clearer identification of bubble explosions and the associated spatters, the original images
were processed and are shown in Fig. 6g and h. An additional example of bubble-explosion–
induced spattering is provided in Supplementary Note 2, Supplementary Fig. 2 and Supplementary
Movie 8. It is noted that strong evaporation at the vapor depression without bubble explosion can
also drive spatter ejection, as previously reported in both laser and electron beam powder bed
fusion processes34,47. The bubble explosions observed here represent an additional mechanism that
can contribute to spatter formation.

Methods
Materials
Al6061 was selected as the material for this study because of its good X-ray transparency for
in-situ high-speed X-ray imaging experiments58. Two types of samples were prepared for the
experiments: bare substrates and pre-sintered powder-bed samples. The bare substrate samples
were sectioned from commercial Al6061 plates (purchased from McMaster). Since powder bed
sintering is not the focus of this study, to accelerate the beamline experiments, the powder layer
was pre-sintered before loading into the chamber. The pre-sintered powder-bed Al6061 samples
were prepared by sintering one layer of Al6061 powders (15–45 μm) on a commercial Al6061
plate using laser heating under an argon atmosphere to minimize oxidation. As observed in the X-
ray images, the powder particles appear to have partially melted and coalesced into larger
agglomerates, with apparent diameters of approximately 200 μm. This coalescence results from
localized laser melting during the sintering process rather than the use of larger initial powder
particles. Both types of Al6061 samples were cut into pieces with dimensions of 58 mm (length)
x 10 mm (width) x 1.2 mm (thickness in the X-ray penetration direction), with the powdered
surface facing the incident beam during experiments.

Electron beam melting system
High-speed, high-resolution X-ray imaging was conducted using a custom-built electron beam
melting system integrated with beamline 32-ID-B at the Advanced Photon Source, Argonne
National Laboratory (Lemont, IL). The system comprises a vacuum chamber with multiple
customized viewports, an EBM unit, and a sample stage. The EBM gun operates in both continuous
mode (max power 600 W) and pulsed mode (peak power up to 1.2 kW). It features a LaB₆
single‑crystal cathode, magnetostatic beam‑shaping/focusing/deflection coils, and dual
turbopumps. The system offers a maximum emission current of 10 mA and a tunable accelerating
voltage range of 0–60 keV, enabling both single-track and stationary melting experiments on pre-
sintered powder beds and bare substrates. In the experiments, the electron beam has a diameter of
approximately 200 µm at the working plane, defined as the full width at half maximum (FWHM)
of a Gaussian energy distribution. The beam size was measured using a slit-scan method, in which
the beam was stepped across a slit of known width while measuring the transmitted current to
determine the rise/fall regions. Then the spot size was calculated using FWHM metric. The vacuum
system achieves a base pressure of ∼10⁻⁸ torr using copper‑gasket seals and features glassy‑carbon
X‑ray viewports (6 mm thick) that maintain both high vacuum integrity and high X‑ray
transparency. All experiments were performed under high vacuum (~10⁻⁵ torr), without helium
shielding gas.

In-situ high-speed synchrotron X-ray imaging
The X-ray imaging system at beamline 32‑ID utilized a pseudo pink beam generated by an
18‑mm‑period undulator, with a single harmonic energy peak centered at approximately 24 keV
with a bandwidth of 5–7%. The beam was transmitted through the sample and converted to visible
light via a LuAG:Ce scintillator (100 µm thick), then captured by a high‑speed camera (Photron
FastCam SA‑Z2100K). The imaging setup achieved a nominal spatial resolution of 2 μm per pixel,
with recording frame rates of 50 kHz, 100kHz and 135 kHz.

Surface morphology characterization
Surface profiles of single scan tracks were characterized using a VHX-5000 Digital Microscope
(KEYENCE Corporation of America) equipped with a fringe-projection optical system. This
technique enables full-field, three-dimensional mapping of surface topography of individual tracks.

Image processing
ImageJ 1.53t was employed for enhancing the contrast of the melt pool surface and keyhole
boundary, and quantifying melt pool and vapor depression dimensions59. The depths of the melt
pool and vapor depression were measured from sample surface to the deepest point of the melt
pool and vapor depression, respectively. And the vapor depression width was measured at the
vapor depression opening. The apparent boundaries of the melt pool and keyhole in the X-ray
images possess a width of 2 pixels (equivalent to 4 μm), resulting in an inherent uncertainty of ±
2 μm for all measurements. To clearly identify surface waves and bubble trajectory, the image
intensity at each pixel of Frame (i + 1) was divided by the intensity of corresponding pixel in
Frame (i), converting motionless parts in the image to a blank background60. More details about
image processing can be found in the Supplementary Information (Supplementary Note 4 and
Supplementary Fig. 4).

To quantify bubble explosion events, a customized detection and classification protocol was
employed. Explosions were identified using two complementary criteria applied to sequential
frames: (1) the sudden appearance of a localized, high-intensity bright region on the front wall of
the vapor depression, indicative of vapor release and liquid-film rupture; and (2) associated frame-
to-frame morphological changes at the same location, such as rapid expansion or distortion. Based
on these criteria, the front-wall state in each frame was classified into three categories: no
explosion, defined by the absence of a newly formed bright region; single explosion, characterized
by one spatially isolated bright-region cluster; and multiple explosions, indicated by two or more
spatially distinct bright-region clusters within the same frame. The primary source of uncertainty
in event counting arises from the limited temporal resolution relative to the explosion dynamics.
Because the frame interval is limited to 20 µs (50 kHz), explosions with lifetimes shorter than the
inter-frame time may be missed or ambiguously attributed to adjacent frames. Consequently, the
measured explosion frequency should be regarded as a lower estimate of the actual rate. For each
condition, the analysis is based on more than 200 data points extracted from high-speed imaging
frames.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript used in-situ high-speed high-resolution synchrotron X-ray imaging technology to observe the formation and explosion of subsurface bubbles in the molten pool during the electron beam powder bed fusion (EB-PBF) process of aluminum alloy Al6061, and investigated the mechanism of the subsurface bubble formation and the effects on the molten pool instabilities, and the morphology of the melt track. Since it is the first time that subsurface bubbles have been directly observed during the melting and deposition process of EB-PBF, this manuscript is of great significance for the research of EB-PBF technology.\nAlthough the classical EB process theory suggested that the energy absorption peak of the EB is several to tens of micrometers below the surface of the material, there is no experimental observation directly indicating the formation of micro-bubbles and its explosions below the surface. Moreover, for the aluminum alloy used in this paper as the experimental material, the frequency of bubble formation and explosion is very high, almost existing under all energy density conditions. Therefore, the findings of this paper are original.\nThis manuscript is valuable and an important reference for understanding the EB-PBF process, but more evidence is needed to prove that its conclusions are universal and can guide the process development for other materials. The main reason is that the aluminum alloy used in this experiment is an uncommon material for EB-PBF process with low density, low melting point and boiling point, which makes the subsurface bubble easy to form. On the contrary, the common materials for EB-PBF, such as titanium alloys, titanium aluminum alloys and superalloys, have higher melting points and boiling points, and higher difficulty to form the bubbles. So, more evidence is required to prove the existence of bubble formation and explosion phenomena in these materials as well. Here, the factors need to be considered not only the higher melting and boiling points of the material, but also the higher material density, which makes the position of the EB energy absorption peak closer to the surface. Therefore, more experimental research may be needed to verify whether the bubble formation and explosions in EB-PBF are “not material-specific and is expected to manifest across all material systems” (Page 15, line 308). Before the author provides more evidence, it is the best for the title and conclusion of this manuscript to focus on the EB-PBF process for aluminum alloys.\nAnother point worth discussing is, the manuscript mentioned in Page 16 line 314, the strategy of an appropriate EB accelerating voltage to shift the peak energy deposition closer to the surface to suppress melt pool instabilities induced by bubble explosion. Actually, shifting the energy deposition closer to the surface usually needs to lower the acceleration voltage of the EB. But existing research (Ref. 1) has shown that although increasing the EB acceleration voltage will deepen the position of energy absorption, it can also increase the total depth of EB energy absorption effectively, and making energy absorption more uniform at different depths. Meanwhile, keeping the constant power, the higher acceleration voltage will make the intensity of the EB low, which can further reduce the energy density of the EB energy absorption peak, and should be beneficial for reducing the bubble formation as well. Additionally, the exiting research (Ref. 1) indicated already that higher acceleration voltage of the EB could obtain higher density and fewer defects, which may be partially due to the reduction of bubble formation.\nIn addition to the above issues, the experimental methods, data analysis, interpretation and conclusions of the manuscript are quite appropriate.\nFinally, one more suggestion that the authors may be interested. Based on the engineering practice, the formation and explosion of the subsurface bubbles may not only cause melt pool instabilities, lack-of-fusion defects, and melt track irregularity, but also may trigger the phenomenon of \"smoking\", which wasn’t mentioned in the manuscript yet. Existing research (Ref. 2) has revealed that when EB encounters powder particles above the powder bed, it will charge the particles and be dispersed onto the powder bed outside the focal spot, and the charged particles will also fall down onto the powder bed, thereby triggering smoking. The spatters, generated by the subsurface bubble explosions, may be a trigger to cause smoking as well if they can fly to a higher altitude and coincidentally encounter the EB. This may explain the sudden occurrence of smoking during selective melting with a focused EB in the EB-PBF process. If recognized by the authors, it can be added to the background introduction of the manuscript.\nReference:\n1. \"Effects of the higher accelerating voltage on electron beam powder-bed based additive manufacturing of Ti6Al4V alloy.\" Additive Manufacturing 50 (2022): 102579.\n2. \"Multiple stages of smoking phenomenon in electron beam powder bed fusion process.\" Additive Manufacturing 66 (2023): 103434.", "reviewer_2": "The authors report the electron beam additive manufacturing specific process instability caused by bubble explosion. Further, the observations are connected and explained by the electron beam matter interaction.\nThe reported results provide important insights that will facilitate improved process control and final quality of manufactured components.\nEspecially figure 7 and 8 provide sound evidence of the claims.\n1. Further details on the prepared samples can be provided.\nHow was the sintering performed? Could there be gas or humidity entrapped in the samples?\n2. In line 42-43 the authors claim that boiling is always present in ebm processing. But steering the processing parameters allows for selection of keyhole or conduction mode melt pools. It is not necessary to process with keyhole.\n3. The sentence 249-250 should be rephrased.\n4. Line 308-309 is it also expected in tungsten?", "reviewer_3": "The authors present a highly novel and compelling study about a newly discovered bubble formation and explosion phenomenon that occurs during melt pool formation within Al6061 samples using EBM (better referred to as PBF-EB nowadays), enabled by high-speed synchrotron imaging with a specialized in-situ sample environment. The discovered phenomenon is novel and has never been reported on, making it highly suitable for publication.\nUnfortunately, the presentation of the phenomenon, as well as its discussion in its current state, is highly flawed and requires substantial revision. First of all, the lack of supplementary video material (which is often included with such in-situ studies) as well as appropriately processed imagery focusing on the phenomenon itself, make it highly difficult for the reader to confirm the claims and descriptions in the written paragraphs. The imagery should be improved for better clarity, focusing on the bubble explosion phenomenon and ideally video material should also be provided if possible. Next, the provided explanations for observed phenomena are highly superficial and lack sufficient theoretical and mathematical considerations to be convincing. Here, the authors seem to provide explanations that best suit the narrative of their article instead of considering the established complexity and nuance of melt pool dynamics in powder bed fusion additive manufacturing.\nAdditionally, the authors attribute essentially all observed melt pool dynamics (keyhole shape, periodic collapse, melt pool backflow, spattering, etc.) to the bubble explosion phenomenon, despite there being an established body of scientific work (predominantly in-situ imaging of laser powder bed fusion, as well as CFD simulations) providing much more elaborate and convincing explanations for many of the described phenomena. The article, in its current state, is also entirely devoid of a discussion of observed phenomena that considers previously established descriptions of melt pool dynamics in powder bed fusion AM which seems highly inappropriate, as many of the observed phenomena very closely resemble the findings of other research groups and studies. Since the provided explanations for observed phenomena are mostly phrased as factual truths and not as working hypotheses, the authors are advised to provide a much more elaborate analysis of their experimental findings to support their claims, in order to demonstrate a consideration of established research in this field, especially considering the novelty of their discovered phenomenon. Alternatively, if this turns out to be outside the scope of this article, it should be made clear that the bubble explosion phenomenon could potentially be a contributing factor in most of the observed melt pool dynamics and not their sole cause.\nConsidering the novelty of the presented findings, the article is definitely worthy of publication once these general concerns, as well as those described below have been addressed.\nAbstract\nl.19: The statement that bubble explosions are intrinsic to EBM is overselling the findings, as a single study with a single material is not sufficient evidence for the introduction of bubble explosions as an intrinsic mechanism of this AM technology.\nIntroduction\nl.44: EBM can create vapor depressions under certain conditions but does not necessarily lead to their formation in general.\nl.60: Similar to the Abstract, “intrinsic and unique to EBM” is overstating the significance of the presented findings.\nResults\nL.78: You do not provide a spot size definition for their stated electron beam spot size. Is the spot size 200 µm FWHM (Full Width Half Max), FWHP (Full Width Half Power), D4σ (4 standard deviations) or some other spot size definition?\nL.101: You state that the rapid melt pool and keyhole expansion are the result of bubble explosions, yet there is no reason for this being the case. The increase in both can easily be attributed to increasing vapor pressures which result in a displacement of the molten material and lead to an increase in heat flux into the still solid substrate.\nL. 107-110: Again, there is no necessary causal link between the shown vapor depression formation and the effect of bubble formation and explosion. The widening of the depression at the beginning could simply be the result of material transport phenomena. The fact that keyholes get more narrow as they deepen (e.g. common in laser powder bed fusion) can be explained by the necessarily higher vapor pressures to maintain them. Since the electron beam most likely follows a gaussian distribution, evaporation is strongest in its center and thus the keyhole gets more narrow as it deepens as a consequence, as only the energy flux density in the beam’s center is sufficient to displace the melt. While the bubble explosions could be a contributing factor, there is no compelling evidence to suggest that it dominates the melt pool dynamics. How many datasets did show the behavior presented in Fig. 3f – h, exactly? Is the shown example statistically significant? Have any calculations been performed to estimate the force exerted by bubble explosions onto the surrounding melt, compared to the force exerted by evaporation directly? Is there an underlying physical model that strongly suggests that bubble explosion could dominate melt pool displacement at all? If so, why was it not included in the manuscript?\nL.110-113: Based on Fig. 3j, the onset of keyhole collapse seems to be attributable to the cessation of bubble explosions. Is this assumption correct? If so, be aware that keyhole collapse is a phenomenon not unique to EBM and does happen in PBF-LB as well where this bubble explosion behavior as never been observed. See the study of Qu et al. (https://doi.org/10.1016/j.addlet.2022.100068) for reference. The periodic collapse can be explained by Marangoni convection induced material transport in the keyhole surrounding melt pool, as well as fluctuations in electron beam irradiation due to changes in beam incidence angles and the effect of surface tension on the shape of the keyhole.\nFig.4 & 5: The bubble formation and explosion events are extremely difficult to discern from the provided radiography data, so much so that the reader is forced to believe the described dynamics without being able to confirm them through the provided imagery. I highly suggest you either provide close-up imagery of at least some of the presented datasets, or better even provide video material for each dataset presented in your manuscript. Video material would allow a much better examination of the described phenomena and a much better understanding of them by readers. If you are not able to provide either video material or close-up imagery, please elaborate on why that is the case, as most high quality X-ray imaging studies in powder bed fusion published in recent years tend to do so for the mentioned reasons.\nL.162: The causal link between bubble explosions and spatter ejection is not evident from the imagery in Fig. 6. The evaporation which induces the visible vapor depression is itself strong enough to cause the visible spatter ejection and provides an easier and alternative explanation. In that regard, plenty of X-ray studies exist that confirm the link between evaporation and spatter ejection (both in PBF-LB & EB). As already mentioned, please provide better imagery or video material such that the described behavior is easier to observe.\nFig. 6: The particle sizes visible in Fig. 6 do not match the powder particle size distribution of 15 to 45 µm stated in your methods section. In your radiographs, the particles seem to be roughly an order of magnitude larger than the used Al6061 powder. This indicates that the powder particles were actually molten during “sintering” which caused them to coalesce into larger particles. Please provide a detailed explanation on how the samples were prepared, including how the pre-sintered powder layers were created and applied to the thin-walled bulk samples used in your study.\nDiscussion\nL192-194: The explanation of bubble formation is highly flawed from a physical perspective. First, you essentially attribute the formation of a bubble to the fact that the irradiated material liquefies and evaporates below the surface first which results in an accumulation of vapor pressure that leads to the formation of a bubble. While it might be true that the energy deposition is highest a few µm below the irradiated surface, the outgoing heat flux (as a result of thermal diffusion) is also much higher there as the heated volume is surrounded by highly conductive material from all sides. In contrast, the surface of the irradiated material can only conduct heat downwards. This is why liquefaction of material irradiated by an electron beam should still be expected to start from its surface, unless specific energy transport calculations suggest otherwise. Second, your assumption that the material reaches its boiling point first where energy deposition rate is highest is incorrect. The boiling point of a material strongly depends on the surrounding pressure. This means that even if the material gets heated from the inside, it does not necessarily start to boil there as well (or rather evaporate) as it is still surrounded by solid or liquid material. Since the surface of the irradiated material is in contact with the vacuum environment of your process chamber, it requires significantly lower temperatures to evaporate (or rather has a much lower boiling point) and thus should be expected to do so first. This means that while your explanation for the observed bubble formation behavior is not necessarily incorrect, it is not convincing unless necessary calculations or simulations are performed that show that evaporation is highly likely to initiate inside the irradiated material which would likely be a totally unprecedented discovery.\nL.207-209 (+ L.214-217): You seem to essentially attribute all observed melt pool dynamics to the formation and explosion of bubbles with no consideration for other sources of hydrodynamic effects. Aside from the higher energy absorption efficiency and penetration depth provided by the electron beam, your experimental setup very closely resembles that of research groups from APS (Zhao et al. 2017, https://doi.org/10.1038/s41598-017-03761-2), SSRL (Calta et al. 2018, https://doi.org/10.1063/1.5017236) and DLS (Leung et al. 2018, https://doi.org/10.1016/j.addma.2018.08.025). These and some other groups have published extensive studies of melt pool dynamics in PBF-LB and should not be disregarded entirely on the basis of a difference in used heat source alone. Additionally, studies about melt pool dynamics in EBM from other research groups also exist and should be considered for discussion as well. The fact that your submitted article is entirely devoid of a comparison between your findings and the existing body of work in this field also seems questionable in general. I therefore highly recommend a discussion of your work that considers findings by other research groups, especially considering the similarities in observed melt pool dynamics (aside from the presented bubble formation and explosion mechanism). This should also result in a much more nuanced understanding of the inducing effects of bubble formation and explosion discovered by your experiments.\nL.237- 239: Is the vapor plume inside the depression actually strong enough to sufficiently scatter the electron beam to induce collapse? Be reminded that the high acceleration voltage is sufficient to allow the electron beam to penetrate fairly deep into solid material. How about performing a rough calculation of the penetration depth of the electron beam for reasonable vapor pressures, based on the same equations you used to calculate the energy deposition profile in Fig. 7a (by assuming that the vapor has the same composition as the solid, just at a much lower density)? This would contextualize your conclusion by some actual numbers (even if they are based on estimates). Additionally, your explanation that electron scattering causes keyhole collapse does not match the general observation that keyholes start to collapse from the surface through the formation of a constriction. How about considering material transport phenomena inside the melt as well?\nL.285-287: The backwards melt flow visible in Fig. 9 is a fairly common phenomenon that does not rely on bubble explosion to occur. The backwards material ejection simply follows from the molten material having to flow around the depression, during which it gains momentum in the opposite direction of the moving electron beam/depression. Like previously stated, don’t just attribute all observations to bubble formation and explosion without considering more commonly known and often likely alternatives.\nL.293: Humping is a well known phenomenon in PBF-LB, where based on your provided explanation, bubble formation is not possible to occur. For reference, see Bhatt et al. 2023 (https://doi.org/10.1016/j.addma.2023.103809) for example.\nL.308: Your statement that the bubble formation mechanism is not material-specific is incorrect, especially considering that there is no evidence for it. While electron beams can penetrate all metallic materials, the extent of their ability to do so differs drastically between alloys. Al6061 is arguably the material exhibiting the most amount of electron beam penetration (of all commonly processed EBM alloys) due to its low density (and low average atomic number). More commonly processed materials in EBM, like Cu, steels, Ni-base superalloys and refractory alloys offer much lower beam penetration depths and have (if your provided explanation for bubble formation is assumed to be correct) therefore a much lower chance of exhibiting bubble formation. Even a previously published article by the same group (Yuan et al. 2024, https://doi.org/10.1016/j.addlet.2024.100239) where Ti6Al4V and Al6061 was used to perform in-situ X-ray imaging experiments, shows no signs of bubble formation and explosion. This suggests that the observed phenomenon is, contrary to stated, highly material specific and likely unique to the combination of Al6061 and EBM and that it does not occur all the time, as Al6061 was melted in this study using the same sample geometry, beam power and scan velocity without exhibiting the bubble formation phenomenon (other melt pool dynamics seem to be similar to those reported here though).\nL.314-316: The high beam penetration depth of EBM is arguably its biggest advantage over other additive manufacturing techniques, as it leads to a much better distribution of heat and lower evaporation during melting. Suggesting to remove this benefit to avoid bubble formation is highly counter-intuitive and would defy the purpose of EBM almost entirely.\nMethods\nL.333: Was any X-ray transparent confinement used, between the Al6061 samples were sandwiched? If not, did the melt pool stay confined between solid Al6061 during melting (i.e. did you make sure to create a melt pool thinner than the 1.2 mm sample thickness). Could you include an image of an Al6061 sample post experimental procedure in Fig. 1, such that the reader gets an expression of how melting affects the sample?\nL.342-343: As previously stated, could you please provide a spot size definition for your electron beam diameter of 200 µm?\nReferences\nL.448-449: Reference 34 (Bidola, P. M. et al. In situ synchrotron…) is wrongly referenced. The first author of this publication should be “Semjatov, N.” not “Bidola, P. M.”.\nFinal remark\nAssuming the described methodology is complete, the in-situ synchrotron imaging study was performed at room temperature. In EBM, processes are always performed at an elevated build temperature to enable sintering of powder particles with minimal energy input, in order to minimize the risk of smoke and reduce accumulation of thermal stress. As a result, solidification times are usually a few orders of magnitude larger than in PBF-LB which is one of the reasons why melt pool dynamics can differ drastically between the two techniques. Since elevated temperatures were not used in your study, solidification behavior should actually be more similar to that encountered in PBF-LB than in EBM (PBF-EB). How do you think does this affect the transferability of your findings to industrial PBF-EB processes? Would you expect to see the same melt pool dynamics, bubble formation and explosion behavior, as well as humping formation or would you expect differences?\nAside from my remarks, I find the presented experimental findings highly interesting and am looking forward to their publication once the necessary adjustments have been made.\nSincerely,\nNick Semjatov"}, "author_response": "Response to Reviewer’s comments\nWe deeply appreciate the editor’s and reviewers’ time and effort in providing constructive\ncomments and valuable advice on our manuscript. Over the past month, we have made our best\nefforts to address all the insightful comments and improve the paper accordingly. Below, we\nprovide a detailed description of the corrective actions in the revised manuscript and our point-by-\npoint responses to each reviewer’s comments (the revised sections are highlighted in different\ncolors).\nReviewer #1 (Remarks to the Author):\nThis manuscript used in-situ high-speed high-resolution synchrotron X-ray imaging technology\nto observe the formation and explosion of subsurface bubbles in the molten pool during the\nelectron beam powder bed fusion (EB-PBF) process of aluminum alloy Al6061 and investigated\nthe mechanism of the subsurface bubble formation and the effects on the molten pool instabilities,\nand the morphology of the melt track. Since it is the first time that subsurface bubbles have been\ndirectly observed during the melting and deposition process of EB-PBF, this manuscript is of great\nsignificance for the research of EB-PBF technology. Although the classical EB process theory\nsuggested that the energy absorption peak of the EB is several to tens of micrometers below the\nsurface of the material, there is no experimental observation directly indicating the formation of\nmicro-bubbles and its explosions below the surface. Moreover, for the aluminum alloy used in this\npaper as the experimental material, the frequency of bubble formation and explosion is very high,\nalmost existing under all energy density conditions. Therefore, the findings of this paper are\noriginal.\nResponse 1-1: We sincerely thank the reviewer for the thorough evaluation of our manuscript and\nthe encouraging comments. All revisions made in response to the reviewer’s comments have been\nhighlighted in blue in the revised manuscript.\nThis manuscript is valuable and an important reference for understanding the EB-PBF process,\nbut more evidence is needed to prove that its conclusions are universal and can guide the process\ndevelopment for other materials. The main reason is that the aluminum alloy used in this\nexperiment is an uncommon material for EB-PBF process with low density, low melting point and\nboiling point, which makes the subsurface bubble easy to form. On the contrary, the common\nmaterials for EB-PBF, such as titanium alloys, titanium aluminum alloys and superalloys, have\nhigher melting points and boiling points, and higher difficulty to form the bubbles. So, more\nevidence is required to prove the existence of bubble formation and explosion phenomena in these\nmaterials as well. Here, the factors need to be considered not only the higher melting and boiling\npoints of the material, but also the higher material density, which makes the position of the EB\nenergy absorption peak closer to the surface. Therefore, more experimental research may be\nneeded to verify whether the bubble formation and explosions in EB-PBF are “not material-\nspecific and is expected to manifest across all material systems” (Page 15, line 308). Before the\nauthor provides more evidence, it is best for the title and conclusion of this manuscript to focus on\nthe EB-PBF process for aluminum alloys.\nResponse 1-2: We thank the reviewer for the encouraging and insightful comments. We agree that\nbubble formation and explosion behaviors may be material-specific, as variations in physical\nproperties such as melting point, boiling point, atomic number, and density can significantly\ninfluence energy absorption and vaporization behavior during EB-PBF. Further experimental\ninvestigations on other alloy systems (e.g., Ti- and Ni-based alloys) are indeed necessary to verify\nthe generality of this phenomenon.\nAccordingly, we have revised the Title, Abstract, Introduction, Discussion, and Conclusion to\nfocus specifically on aluminum alloys, while clearly acknowledging the need for further validation\nacross additional material systems. The revised content is as follows.\nTitle (Lines 1 and 2): Bubble explosion induced melt pool instabilities in electron beam melting\nof aluminum alloy\nAbstract (Lines 18-20): Here, using in-situ high-speed synchrotron X-ray imaging, we reveal\nthat bubble explosions in Al6061 during EBM induce melt pool instabilities contributing to defect\nformation.\nIntroduction (Lines 58-60): In this study, taking advantage of this in-situ characterization system,\nwe discovered that bubble explosions during EBM in Al6061, unique to EBM, destabilize the melt\npool and contribute to multiple types of defects.\nDiscussion: We deleted the previous statement: “The melt pool instabilities induced by bubble\nexplosion originate from the energy deposition characteristics inherent to the high-energy electron\nbeam. This mechanism is not material-specific and is expected to manifest across all material\nsystems. However, the energy absorption behavior, including the electron beam peak energy\ndeposition depth and penetration depth, is highly material dependent.”\nConclusion (Lines 343-347): This work is expected to inspire the electron beam additive\nmanufacturing and welding community to advance modeling efforts for a quantitative\nunderstanding of bubble formation, explosion, and their effects, and to experimentally investigate,\nthrough in-situ monitoring, the occurrence of subsurface bubble explosions in high-atomic-number\nmaterials, thereby deepening the overall understanding of electron beam melting.\nAnother point worth discussing is, the manuscript mentioned in Page 16 line 314, the strategy\nof an appropriate EB accelerating voltage to shift the peak energy deposition closer to the surface\nto suppress melt pool instabilities induced by bubble explosion. Actually, shifting the energy\ndeposition closer to the surface usually needs to lower the acceleration voltage of the EB. But\nexisting research (Ref. 1) has shown that although increasing the EB accelerating voltage will\ndeepen the position of energy absorption, it can also increase the total depth of EB energy\nabsorption effectively, and make energy absorption more uniform at different depths. Meanwhile,\nkeeping the constant power, the higher acceleration voltage will make the intensity of the EB low,\nwhich can further reduce the energy density of the EB energy absorption peak, and should be\nbeneficial for reducing the bubble formation as well. Additionally, the exiting research (Ref. 1)\nindicated already that higher acceleration voltage of the EB could obtain higher density and fewer\ndefects, which may be partially due to the reduction of bubble formation.\nResponse 1-3: We thank the reviewer for this insightful comment regarding the influence of\nelectron beam accelerating voltage. We agree that increasing the accelerating voltage could deepen\nand broaden the region of energy absorption, resulting in more uniform energy distribution.\nAdditionally, at constant beam power, a higher accelerating voltage reduces beam intensity and\npeak energy density, which could suppress bubble formation and improve part density, as reported\nin the literature.\nIn the present work, the primary focus is on revealing the existence of subsurface bubble\nformation and explosion during the electron beam melting of Al6061. A systematic investigation\nof bubble formation and explosion under different accelerating voltages, and their effects on melt\npool dynamics, will be conducted in future studies. In the revised version, we have removed the\ndiscussion related to accelerating voltage strategy (Page 16, line 314 in the original manuscript)\nand added the following updated discussion.\nLines 333-336: Furthermore, the effect of electron beam acceleration voltage on bubble\nexplosion behavior and melt pool dynamics warrants systematic investigation, as higher\nacceleration voltages deepen and homogenize energy deposition, potentially reducing the\npropensity for subsurface bubble explosion58.\n58. Li, H., Yu, Y., Li, Y. & Lin, F. Effects of the higher accelerating voltage on electron beam\npowder-bed based additive manufacturing of Ti6Al4V alloy. Addit. Manuf. 50, 102579 (2022).\nIn addition to the above issues, the experimental methods, data analysis, interpretation and\nconclusions of the manuscript are quite appropriate.\nResponse 1-4: We thank the reviewer for the positive feedback.\nFinally, one more suggestion that the authors may be interested in. Based on the engineering\npractice, the formation and explosion of the subsurface bubbles may not only cause melt pool\ninstabilities, lack-of-fusion defects, and melt track irregularity, but also may trigger the\nphenomenon of \"smoking\", which wasn’t mentioned in the manuscript yet. Existing research (Ref.\n2) has revealed that when EB encounters powder particles above the powder bed, it will charge\nthe particles and be dispersed onto the powder bed outside the focal spot, and the charged particles\nwill also fall down onto the powder bed, thereby triggering smoking. The spatters, generated by\nthe subsurface bubble explosions, may be a trigger to cause smoking as well if they can fly to a\nhigher altitude and coincidentally encounter the EB. This may explain the sudden occurrence of\nsmoking during selective melting with a focused EB in the EB-PBF process. If recognized by the\nauthors, it can be added to the background introduction of the manuscript.\nResponse 1-5: We thank the reviewer for this thoughtful and insightful suggestion. We agree that\nexplosion-induced spattering could potentially contribute to the smoking phenomenon in EB-PBF.\nAs described in the mentioned studies, smoking can arise when charged particles generated above\nthe powder bed are dispersed by the electron beam and subsequently fall back, disturbing the\npowder layer. Spatters produced by subsurface bubble explosions may similarly interact with the\nelectron beam, become charged, and thus act as triggers for smoking events.\nas a potential link between bubble explosion–induced spattering and the onset of smoking.\nLines 226-230: Previous research has shown that when the electron beam interacts with powder\nparticles above the powder bed, these particles can become charged and dispersed beyond the focal\nspot before eventually falling back onto the powder bed, thereby triggering smoking54. Similarly,\nspatters produced by subsurface bubble explosions may interact with the electron beam, become\ncharged, and consequently act as potential initiators of smoking events.\n54. Wang, D., Zhao, D., Liang, X., Li, X. & Lin, F. Multiple stages of smoking phenomenon in\nelectron beam powder bed fusion process. Addit. Manuf. 66, 103434 (2023).\nReference:\n1. \"Effects of the higher accelerating voltage on electron beam powder-bed based additive\nmanufacturing of Ti6Al4V alloy.\" Additive Manufacturing 50 (2022): 102579.\n2. \"Multiple stages of smoking phenomenon in electron beam powder bed fusion process.\"\nAdditive Manufacturing 66 (2023): 103434.\nResponse 1-6: We thank the reviewer for providing these valuable references. They are highly\nrelevant and have strengthened both the current discussion and the context for future work. We\nhave incorporated the suggested citations into the revised manuscript.\nReviewer #2 (Remarks to the Author):\nThe authors report the electron beam additive manufacturing specific process instability caused\nby bubble explosion. Further, the observations are connected and explained by the electron beam\nmatter interaction.\nThe reported results provide important insights that will facilitate improved process control and\nfinal quality of manufactured components.\nEspecially figure 7 and 8 provide sound evidence of the claims.\nAll revisions made in response to your comments have been highlighted in green in the revised\nmanuscript.\n1.Further details on the prepared samples can be provided.\nHow was the sintering performed? Could there be gas or humidity entrapped in the samples?\nResponse 2-2: We thank the reviewer for this helpful suggestion. Additional details regarding\nsample and sintered-layer preparation have been included in the revised manuscript (lines 351–\n364)and are summarized below.\nPre-sintered powder-bed Al6061 samples were prepared by sintering one layer of Al6061\npowders (15–45 μm) on a commercial Al6061 plate using laser heating under an argon atmosphere\nto minimize oxidation. After pre-sintering, the samples were cut, and the exposed side surfaces\nwere ground to 800 grit and then cleaned in an ultrasonic bath with ethanol to remove potential\nsurface contaminants. For the bare plate samples, after cutting, the side and top surfaces of samples\nwere similarly ground to 800 grit and then cleaned in an ultrasonic bath with ethanol to remove\npotential surface contaminants. During the electron beam melting experiments, the samples were\nplaced inside a vacuum chamber at a pressure of 10⁻⁵ Torr, allowing any residual surface gas to be\nreleased.\nDuring X-ray imaging of electron beam melting process, no surface instabilities were observed.\nAll bubbles observed and reported in this study were generated beneath the liquid metal surface,\nsuch as in Fig. 8, and were nucleated within the front keyhole wall rather than on the top surface\nin any of our data, demonstrating that the samples did not entrap gas or humidity.\nTo avoid confusion for readers, clarification on sample and sintered-layer preparation was added\nto the revised manuscript, as shown below:\nLines 351-364: Al6061 was selected as the material for this study because of its good X-ray\ntransparency for in-situ high-speed X-ray imaging experiments52. Two types of samples were\nprepared for the experiments: bare substrates and pre-sintered powder-bed samples. The bare\nsubstrate samples were sectioned from commercial Al6061 plates (purchased from McMaster).\nSince powder bed sintering is not the focus of this study, to accelerate the beamline experiments,\n6\nthe powder layer was pre-sintering before loading into the chamber. The pre-sintered powder-bed\nAl6061 samples were prepared by sintering one layer of Al6061 powders (15–45 μm) on a\ncommercial Al6061 plate using laser heating under an argon atmosphere to minimize oxidation.\nAs observed in the X-ray images, the powder particles appear to have partially melted and\ncoalesced into larger agglomerates, with apparent diameters of approximately 200 μm. This\ncoalescence results from localized laser melting during the sintering process rather than the use of\nlarger initial powder particles. Both types of Al6061 samples were cut into pieces with dimensions\nof 58 mm (length) x 10 mm (width) x 1.2 mm (thickness in the X-ray penetration direction), with\nthe powdered surface facing the incident beam during experiments.\n2. In line 42-43 the authors claim that boiling is always present in ebm processing. But steering\nthe processing parameters allows for selection of keyhole or conduction mode melt pools. It is not\nnecessary to process with keyhole.\nResponse 2-3: We agree with the reviewer that electron beam powder bed fusion is typically\nconducted in the conduction mode, often without the formation of a vapor depression zone. The\nword “boiling” has been removed.\nLines 42 and 43: In EBM, accelerated electrons bombard the pre-sintered powder bed,\nconverting their kinetic energy into heat and causing localized melting.\n3. The sentence 249-250 should be rephrased.\nResponse 2-4: We thank the reviewer for pointing out this issue. The sentence has been rephrased:\nLines 270 and 271: On the inclined front wall of the vapor depression, instabilities induced by\nbubble explosion lead to the formation of protrusions (Fig. 8b).\n4. Line 308-309 is it also expected in tungsten?\nResponse 2-5: We thank the reviewer for this valuable comment. Based on our current results, we\ncannot determine whether bubble formation and explosion can occur in tungsten. The bubble\nformation and explosion behavior is likely to be material-specific, and it remains unknown whether\nit occurs in high–melting point, high–density materials such as tungsten. Significant differences in\nphysical properties, including melting and boiling points, vapor pressure, atomic number, and\n7\ndensity, can substantially influence the energy absorption profile and vaporization dynamics in\nEB-PBF.\nTo clarify this point, we have revised the Title, Abstract, Introduction, Discussion and\nConclusion to explicitly focus on aluminum alloys and to indicate that further experimental studies\non other alloy systems (e.g., W-based materials) are required to evaluate the generality of the\nsubsurface bubble formation phenomena.\nTitle (Lines 1 and 2): Bubble explosion induced melt pool instabilities in electron beam melting\nof aluminum alloy\nAbstract (Lines 18-20): Here, using in-situ high-speed synchrotron X-ray imaging, we reveal\nthat bubble explosions in Al6061 during EBM induce melt pool instabilities contributing to defect\nformation.\nIntroduction (Lines 58-60): In this study, taking advantage of this in-situ characterization system,\nwe discovered that bubble explosions during EBM in Al6061, unique to EBM, destabilize the melt\npool and contribute to multiple types of defects.\nDiscussion: We deleted the previous statement: “The melt pool instabilities induced by bubble\nexplosion originate from the energy deposition characteristics inherent to the high-energy electron\nbeam. This mechanism is not material-specific and is expected to manifest across all material\nsystems. However, the energy absorption behavior, including the electron beam peak energy\ndeposition depth and penetration depth, is highly material dependent.”\nConclusion (Lines 343-347): This work is expected to inspire the electron beam additive\nmanufacturing and welding community to advance modeling efforts for a quantitative\nunderstanding of bubble formation, explosion, and their effects, and to experimentally investigate,\nthrough in-situ monitoring, the occurrence of subsurface bubble explosions in high-atomic-number\nmaterials, thereby deepening the overall understanding of electron beam melting.\nThe authors present a highly novel and compelling study about a newly discovered bubble\nformation and explosion phenomenon that occurs during melt pool formation within Al6061\nsamples using EBM (better referred to as PBF-EB nowadays), enabled by high-speed synchrotron\nimaging with a specialized in-situ sample environment. The discovered phenomenon is novel and\nhas never been reported on, making it highly suitable for publication.\nResponse 3-1: Thank you very much for the thorough evaluation of our manuscript. We sincerely\nappreciate your evaluation and encouraging comments. The changes in the manuscript relevant to\nyour comments have been highlighted in purple.\nUnfortunately, the presentation of the phenomenon, as well as its discussion in its current state,\nis highly flawed and requires substantial revision. First of all, the lack of supplementary video\nmaterial (which is often included with such in-situ studies) as well as appropriately processed\nimagery focusing on the phenomenon itself, make it highly difficult for the reader to confirm the\nclaims and descriptions in the written paragraphs. The imagery should be improved for better\nclarity, focusing on the bubble explosion phenomenon and ideally video material should also be\nprovided if possible.\nResponse 3-2: We thank the reviewer for this helpful suggestion. Ten X-ray videos of the electron\nbeam melting process have been included in the Supplementary Materials, with each melting\nsequence presented in the manuscript paired with its corresponding video. The quality and design\nof the images have been improved.\nNext, the provided explanations for observed phenomena are highly superficial and lack\nsufficient theoretical and mathematical considerations to be convincing. Here, the authors seem to\nprovide explanations that best suit the narrative of their article instead of considering the\nestablished complexity and nuance of melt pool dynamics in powder bed fusion additive\nmanufacturing. Additionally, the authors attribute essentially all observed melt pool dynamics\n(keyhole shape, periodic collapse, melt pool backflow, spattering, etc.) to the bubble explosion\nphenomenon, despite there being an established body of scientific work (predominantly in-situ\nimaging of laser powder bed fusion, as well as CFD simulations) providing much more elaborate\nand convincing explanations for many of the described phenomena. The article, in its current state,\n9\nis also entirely devoid of a discussion of observed phenomena that considers previously established\ndescriptions of melt pool dynamics in powder bed fusion AM which seems highly inappropriate,\nas many of the observed phenomena very closely resemble the findings of other research groups\nand studies. Since the provided explanations for observed phenomena are mostly phrased as factual\ntruths and not as working hypotheses, the authors are advised to provide a much more elaborate\nanalysis of their experimental findings to support their claims, in order to demonstrate a\nconsideration of established research in this field, especially considering the novelty of their\ndiscovered phenomenon. Alternatively, if this turns out to be outside the scope of this article,\nit should be made clear that the bubble explosion phenomenon could potentially be a contributing\nfactor in most of the observed melt pool dynamics and not their sole cause.\nResponse 3-3: We sincerely thank the reviewer for this valuable and constructive comment.\nRegarding the first point:\nIn this study, our primary goal is to reveal a previously unreported phenomenon that may occur\nduring electron beam melting and to demonstrate its influence on melt pool dynamics. We agree\nwith the reviewer that more quantitative modeling and calculation are required to further elucidate\nthe underlying mechanisms of subsurface bubble formation, explosion, and their influence. Our\nobjective for this paper is to bring this phenomenon to the attention of researchers and engineers\nin the field, so that future efforts can collectively advance understanding through improved\nequipment design, process optimization, alloy development, and numerical modeling. Accordingly,\nwe have clarified this point in the revised manuscript:\nConclusion (Lines 343-347): This work is expected to inspire the electron beam additive\nmanufacturing and welding community to advance modeling efforts for a quantitative\nunderstanding of bubble formation, explosion, and their effects, and to experimentally investigate,\nthrough in-situ monitoring, the occurrence of subsurface bubble explosions in high-atomic-number\nmaterials, thereby deepening the overall understanding of electron beam melting.\nRegarding the second point:\nWe agree that the observed melt pool dynamics cannot be solely attributed to bubble formation\nand explosion. Other hydrodynamic effects—including Marangoni convection, recoil pressure,\n10\nsurface tension gradients, and thermocapillary-driven flow—also play essential roles in melt pool\ndynamics.\nIn the updated Results and Discussion sections, we provide a more comprehensive analysis that\nsituates our findings within the context of previous studies on melt pool dynamics under both laser\nand electron beam conditions. Simultaneously, in addition to bubble explosion, we now consider\nothers factors that can influence the melt pool dynamics, including vapor pressure, evaporation,\nRayleigh–Taylor instability and related mechanisms. We have added the following discussion to\nthe revised manuscript:\nRegarding the evolution of the melt pool and keyhole:\nLines 112-116: At this stage, the rapid expansion of the melt pool and keyhole is primarily\ndriven by rising vapor pressure within the depression, which displaces molten material and\nenhances heat flux into the surrounding solid, similar to the mechanism of keyhole drilling\nobserved in laser powder bed fusion46. Bubble explosions may further amplify local pressure\nfluctuations, promoting additional keyhole deepening.\n46. Cunningham, R. et al. Keyhole threshold and morphology in laser melting revealed by\nultrahigh-speed x-ray imaging. Science 363, 849–852 (2019).\nLines 234-235: These dynamics contribute to reshaping the melt pool and vapor depression\nthrough two coupled mechanisms.\nRegarding spattering:\nLines 165-177: The dynamics of spattering under the influence of subsurface bubble formation\nand explosion were also investigated. Since spatter primarily arises from interactions between\nmetallic vapor and the melt pool surface or powder bed, pre-sintered powder bed samples were\nused to study interactions among metal vapor, melt pool and powder. Following subsurface bubble\nexplosions (Fig. 6a and b), the melt pool became unstable within the subsequent frames, ejecting\na liquid ligament from the explosion site followed by spatters (Fig. 6c-f and Supplementary Movie\n7). To facilitate clearer identification of bubble explosions and the associated spatters, the original\nimages were processed and are shown in Fig. 6g and h. An additional example of bubble-\nexplosion–induced spattering with preheating prior to melting is provided in Supplementary Note\n11\n2, Supplementary Fig. 2 and Supplementary Movie 8. It is noted that strong evaporation at the\nvapor depression without bubble explosion can also drive spatter ejection, as supported by\nnumerous X-ray studies in both laser and electron beam powder bed fusion processes34,47. The\nbubble explosions observed here represent an additional mechanism that can contribute to spatter\nformation.\n34. Semjatov, N., Wahlmann, B. & Carolin, K. Multiple interaction electron beam powder bed\nfusion for controlling melt pool dynamics and improving surface quality. Addit. Manuf. 90,\n104316 (2024).\n47. Guo, Q. et al. Transient dynamics of powder spattering in laser powder bed fusion additive\nmanufacturing process revealed by in-situ high-speed high-energy x-ray imaging. Acta Mater. 151,\n169–180 (2018).\nRegarding melt flow:\nLine 300-305: Second, the backward melt flow observed in Fig. 9c, a phenomenon commonly\nreported in laser powder bed fusion, typically arises as molten metal circulates around the vapor\ndepression and gains momentum opposite to the scanning direction57. Bubble explosions occurring\nat the front wall may introduce additional momentum to the backward flow by generating transient\nvapor jets and surface waves that propagate along the vapor–liquid interface.\n57. Guo, Q. et al. In-situ full-field mapping of melt flow dynamics in laser metal additive\nmanufacturing. Addit. Manuf. 31, 100939 (2020).\nLines 306-311: As the melt flow propagates backward, periodic undulations develop along the\nmelt pool surface due to the Rayleigh–Taylor instability, as previously reported in laser and\nelectron beam powder bed fusion34,41. As the solidification front advances, these undulations\nevolve into humping structures along the melt track. In electron beam melting, intermittent\nmomentum impulses generated by bubble explosions further amplify this instability by perturbing\nthe melt pool surface. The combined effects of Rayleigh–Taylor and explosion-induced\ninstabilities thus promote periodic undulations, ultimately leading to the formation of humping\nstructures along the melt track (Fig. 9d and Supplementary Movie 5).\n12\n34. Semjatov, N., Wahlmann, B. & Carolin, K. Multiple interaction electron beam powder bed\nfusion for controlling melt pool dynamics and improving surface quality. Addit. Manuf. 90, 104316\n(2024).\n41. Bhatt, A. et al. In situ characterisation of surface roughness and its amplification during\nmultilayer single-track laser powder bed fusion additive manufacturing. Addit. Manuf. 77, 103809\n(2023).\nConsidering the novelty of the presented findings, the article is definitely worthy of publication\nonce these general concerns, as well as those described below have been addressed.\nResponse 3-4: We thank the reviewer for offering positive feedback and listing the issues with the\nmanuscript.\nAbstract\nl.19: The statement that bubble explosions are intrinsic to EBM is overselling the findings, as a\nsingle study with a single material is not sufficient evidence for the introduction of bubble\nexplosions as an intrinsic mechanism of this AM technology.\nResponse 3-5: We thank the reviewer’s critical comment. We agree with the reviewer. The word\n“intrinsic” has been removed.\nLines 18-20: Here, using in-situ high-speed synchrotron X-ray imaging, we reveal that bubble\nexplosions in Al6061 during EBM induce melt pool instabilities contributing to defect formation.\nIntroduction\nl.44: EBM can create vapor depressions under certain conditions but does not necessarily lead\nto their formation in general.\nResponse 3-6: We agree with the reviewer. The words “vapor depression” have been removed.\nLine 42: This interaction generates a melt pool.\n13\nl.60: Similar to the Abstract, “intrinsic and unique to EBM” is overstating the significance of\nthe presented findings.\nResponse 3-7: The word “intrinsic” has been removed. The revised text reads:\nLines 58-61: In this study, taking advantage of this in-situ characterization system, we\ndiscovered that bubble explosions during EBM in Al6061, unique to EBM, destabilize the melt\npool and contribute to multiple types of defects.\nResults\nL.78: You do not provide a spot size definition for their stated electron beam spot size. Is the\nspot size 200 µm FWHM (Full Width Half Max), FWHP (Full Width Half Power), D4σ (4 standard\ndeviations) or some other spot size definition?\nResponse 3-8: We thank the reviewer for this insightful comment. In the experiments, the electron\nbeam has a diameter of approximately 200 µm at the working plane, defined as the full width at\nhalf maximum (FWHM) of a Gaussian energy distribution. The beam information was measured\nusing a slit-scan method, in which the beam was stepped across a slit of known width while\nmeasuring the transmitted current to determine the rise/fall regions. Then the spot size was\ncalculated using the FWHM metric. This clarification has been incorporated into the revised\nmanuscript to ensure accuracy and consistency in the beam specification.\nLines 80-82: The experiments were conducted in a vacuum chamber using a beam with a\ndiameter of approximately 200 µm (full width at half maximum (FWHM) of a Gaussian energy\ndistribution).\nLines 371-375: In the experiments, the electron beam has a diameter of approximately 200 µm\nat the working plane, defined as the full width at half maximum (FWHM) of a Gaussian energy\ndistribution. The beam information was measured using a slit-scan method, in which the beam was\nstepped across a slit of known width while measuring the transmitted current to determine the\nrise/fall regions. Then the spot size was calculated using FWHM metric.\nL.101: You state that the rapid melt pool and keyhole expansion are the result of bubble\nexplosions, yet there is no reason for this being the case. The increase in both can easily be\n14\nattributed to increasing vapor pressures which result in a displacement of the molten material and\nlead to an increase in heat flux into the still solid substrate.\nResponse 3-9: We appreciate the reviewer for this valuable comment and agree that the rapid\nexpansion of the melt pool and keyhole can indeed be attributed to increasing vapor pressure,\nwhich displaces molten material and enhances heat transfer into the surrounding solid. We have\nrevised the relevant text to clarify that bubble explosion is not the sole cause but rather a possible\ncontributing factor. The content has been revised manuscript:\nLines 106-107: At this stage, violent bubble explosions can contribute to the rapid deepening\nand widening of the keyhole.\nLines 112-116: At this stage, the rapid expansion of the melt pool and keyhole is primarily\ndriven by rising vapor pressure within the depression, which displaces molten material and\nenhances heat flux into the surrounding solid, similar to the mechanism of keyhole drilling\nobserved in laser powder bed fusion46. Bubble explosions may further amplify local pressure\nfluctuations, promoting additional keyhole deepening.\n46. Cunningham, R. et al. Keyhole threshold and morphology in laser melting revealed by\nultrahigh-speed x-ray imaging. Science 363, 849–852 (2019).\nL. 107-110: Again, there is no necessary causal link between the shown vapor depression\nformation and the effect of bubble formation and explosion. The widening of the depression at the\nbeginning could simply be the result of material transport phenomena. The fact that keyholes get\nmore narrow as they deepen (e.g. common in laser powder bed fusion) can be explained by the\nnecessarily higher vapor pressures to maintain them. Since the electron beam most likely follows\na gaussian distribution, evaporation is strongest in its center and thus the keyhole gets more narrow\nas it deepens as a consequence, as only the energy flux density in the beam’s center is sufficient to\ndisplace the melt. While the bubble explosions could be a contributing factor, there is no\ncompelling evidence to suggest that it dominates the melt pool dynamics. How many datasets did\nshow the behavior presented in Fig. 3f – h, exactly? Is the shown example statistically significant?\nHave any calculations been performed to estimate the force exerted by bubble explosions onto the\nsurrounding melt, compared to the force exerted by evaporation directly? Is there an underlying\n15\nphysical model that strongly suggests that bubble explosion could dominate melt pool\ndisplacement at all? If so, why was it not included in the manuscript?\nResponse 3-10: We thank the reviewer for this insightful comment and agree that vapor depression\nformation and evolution can primarily result from material transport phenomena the increasing\nvapor pressure within the depression. We have revised the relevant discussion to clarify that bubble\nexplosions are not the sole cause of the deepening and widening of the keyhole, but rather an\nadditional contributing factor.\nIn the revised manuscript, we now state:\nLines 112-116: At this stage, the rapid expansion of the melt pool and keyhole is primarily\ndriven by rising vapor pressure within the depression, which displaces molten material and\nenhances heat flux into the surrounding solid, similar to the mechanism of keyhole drilling\nobserved in laser powder bed fusion46. Bubble explosions may further amplify local pressure\nfluctuations, promoting additional keyhole deepening.\n46. Cunningham, R. et al. Keyhole threshold and morphology in laser melting revealed by\nultrahigh-speed x-ray imaging. Science 363, 849–852 (2019).\nRegarding the representativeness of the data, we thank the reviewer for the insightful comment\nand agree that reproducibility and statistical relevance are essential. The experiments were\nperformed using our custom-built electron beam melting system at the Advanced Photon Source,\nArgonne National Laboratory, across four independent beamtime sessions (February 1–7, 2022;\nJune 1–9, 2022; October 25–28, 2022; and March 8–13, 2023), totaling 624 experimental hours\nand producing 318 datasets. The phenomena of bubble formation and explosion were consistently\nobserved in both Al6061 and AlSi10Mg alloys. Among the 41 stationary melting datasets, 15 were\nperformed under beam energy conditions sufficient to generate a keyhole. The bubble explosion\nand keyhole evolution behaviors were reproducibly captured across all these 15 stationary melting\nexperiments conducted under comparable conditions.\nRegarding the force estimation and the physical model, we acknowledge that a quantitative\nanalysis of the pressure dynamics induced by bubble explosions and a detailed physical model of\nthe associated melt pool response are beyond the current scope of this work. We have removed the\nprevious interpretation based solely on bubble formation and explosion and have expanded the\n16\ndiscussion to incorporate mechanisms that align more closely with observations from both EBM\nand LPBF systems. This limitation and the need for future modeling efforts have been clearly\nstated in the revised manuscript.\nLines 112-116: At this stage, the rapid expansion of the melt pool and keyhole is primarily\ndriven by rising vapor pressure within the depression, which displaces molten material and\nenhances heat flux into the surrounding solid, similar to the mechanism of keyhole drilling\nobserved in laser powder bed fusion46. Bubble explosions may further amplify local pressure\nfluctuations, promoting additional keyhole deepening.\n46. Cunningham, R. et al. Keyhole threshold and morphology in laser melting revealed by\nultrahigh-speed x-ray imaging. Science 363, 849–852 (2019).\nConclusion (Lines 343-347): This work is expected to inspire the electron beam additive\nmanufacturing and welding community to advance modeling efforts for a quantitative\nunderstanding of bubble formation, explosion, and their effects, and to experimentally investigate,\nthrough in-situ monitoring, the occurrence of subsurface bubble explosions in high-atomic-number\nmaterials, thereby deepening the overall understanding of electron beam melting.\nL.110-113: Based on Fig. 3j, the onset of keyhole collapse seems to be attributable to the\ncessation of bubble explosions. Is this assumption correct? If so, be aware that keyhole collapse is\na phenomenon not unique to EBM and does happen in PBF-LB as well where this bubble explosion\nbehavior has never been observed. See the study of Qu et al.\n(https://doi.org/10.1016/j.addlet.2022.100068) for reference. The periodic collapse can be\nexplained by Marangoni convection induced material transport in the keyhole surrounding melt\npool, as well as fluctuations in electron beam irradiation due to changes in beam incidence angles\nand the effect of surface tension on the shape of the keyhole.\nResponse 3-11: We thank the reviewer for this insightful comment. We would like to clarify that\nthe onset of keyhole collapse is not attributed to the cessation of bubble explosions. We agree with\nthe reviewer that the periodic collapse of the keyhole results from the complex interplay of\nMarangoni convection, recoil pressure, and surface tension, which should be consistent with the\nmechanisms observed in both EBM and PBF-LB processes, as also discussed in the referenced\nstudy by Qu et al. (2022).\nannotation in Fig. 3j that might have implied a direct causal relationship between bubble\nexplosions and keyhole collapse, as shown below. In addition, the discussion about keyhole\nperiodic collapse has been added.\nLines 256-261: During melting, recoil pressure from the dense metallic vapor drives the\nsurrounding liquid outward, forming a deep and narrow cavity (Fig. 2i and j). As melting continues,\nthe interplay among thermocapillary force, Marangoni convection, recoil pressure, and surface\ntension, similar to the mechanisms observed in laser powder bed fusion55,56, induces keyhole\ninstability. When the keyhole collapses, the high-energy electron beam reinitiates intense\nevaporation in the newly formed shallow keyhole, driving a cyclic collapse–regrowth instability\n(Fig. 2k–o).\n55. Qu, M. et al. Mitigating keyhole pore formation by nanoparticles during laser powder bed\nfusion additive manufacturing. Addit. Manuf. Lett. 3, 100068 (2022).\n56. Zhao, C. et al. Critical instability at moving keyhole tip generates porosity in laser melting.\nScience 370, 1080–1086 (2020).\nelectron beam stationary melting with an electron beam power of 426 W. (a-c) Stage 1: Initial\nformation of a melt pool during electron beam stationary melting. (d-j) Stage 2: Subsurface bubble\nformation and explosion. At this stage, violent bubble explosions can contribute to the rapid\ndeepening and widening of the keyhole. (j-o) Stage 3: Periodic keyhole oscillation. In this stage,\nthe keyhole depth starts to fluctuate periodically. p Melt pool and keyhole dimensions over time.\nFig.4 & 5: The bubble formation and explosion events are extremely difficult to discern from\nthe provided radiography data, so much so that the reader is forced to believe the described\ndynamics without being able to confirm them through the provided imagery. I highly suggest you\n19\neither provide close-up imagery of at least some of the presented datasets, or better even provide\nvideo material for each dataset presented in your manuscript. Video material would allow a much\nbetter examination of the described phenomena and a much better understanding of them by\nreaders. If you are not able to provide either video material or close-up imagery, please elaborate\non why that is the case, as most high quality X-ray imaging studies in powder bed fusion published\nin recent years tend to do so for the mentioned reasons.\nResponse 3-12: We thank the reviewer’s critical comment. Ten X-ray videos of electron beam\nmelting have been included in the Supplementary Materials, with each melting sequence presented\nin the manuscript paired with its corresponding video.\nL.162: The causal link between bubble explosions and spatter ejection is not evident from the\nimagery in Fig. 6. The evaporation which induces the visible vapor depression is itself strong\nenough to cause the visible spatter ejection and provides an easier and alternative explanation. In\nthat regard, plenty of X-ray studies exist that confirm the link between evaporation and spatter\nejection (both in PBF-LB & EB). As already mentioned, please provide better imagery or video\nmaterial such that the described behavior is easier to observe.\nResponse 3-13: We thank the reviewer for this valuable and insightful comment. We agree that\nstrong evaporation at the vapor depression can indeed drive spatter ejection, as supported by\nnumerous X-ray studies in both laser and electron beam powder bed fusion processes. Our\nobservations suggest that subsurface bubble explosions can be an additional mechanism that can\ncontribute to spatter formation. The related description has been revised accordingly in the\nmanuscript.\nWe have added the corresponding video (Supplementary Movie 7) and an additional example\nof bubble-explosion–induced spattering in the supplementary material. The new figure and its\naccompanying explanation have been added to the Supplementary Materials (Supplementary Note\n2, Supplementary Fig. 2 and Supplementary Movie 8).\nLines 172-177: An additional example of bubble-explosion–induced spattering with preheating\nprior to melting is provided in Supplementary Note 2, Supplementary Fig. 2 and Supplementary\nMovie 8. It is noted that strong evaporation at the vapor depression without bubble explosion can\nalso drive spatter ejection, as supported by numerous X-ray studies in both laser and electron beam\n20\npowder bed fusion processes34,47. The bubble explosions observed here represent an additional\nmechanism that can contribute to spatter formation.\n34. Semjatov, N., Wahlmann, B. & Carolin, K. Multiple interaction electron beam powder bed\nfusion for controlling melt pool dynamics and improving surface quality. Addit. Manuf. 90,\n104316 (2024).\n47. Guo, Q. et al. Transient dynamics of powder spattering in laser powder bed fusion additive\nmanufacturing process revealed by in-situ high-speed high-energy x-ray imaging. Acta Mater. 151,\n169–180 (2018).\nSupplementary Note 2: Dynamics of spattering\nIn addition to the spattering behavior associated with bubble formation and explosion shown in\nFig. 6 of the main text, a similar sequence is presented in Supplementary Fig. 2 and Supplementary\nMovie 8. Immediately after a subsurface bubble explosion (Supplementary Fig. 2a and b), the melt\npool becomes unstable within the subsequent frames, and a liquid ligament is ejected from the\nexplosion site (Supplementary Fig. 2c and d), leading to spatter formation (Supplementary Fig. 2e\nand f).\nSupplementary Figure 2. Dynamics of spattering under the influence of subsurface explosion\nduring electron beam scanning with an electron beam power of 426 W and a scan speed of\n1.0 m/s. (a and b) X-ray images showing bubble explosions on front vapor depression wall during\nelectron beam scanning. (c-f) X-ray images showing the dynamics of liquid ligament and\nspattering induced by bubble explosions during electron beam scanning. Prior to melting, a\n21\npreheating step was performed using the electron beam operated in pulse mode with a 0.4 duty\ncycle, a power of 318 W, a scan speed of 16 m/s, and a duration of 10 s. The duty cycle of a pulsed\nelectron beam is the fraction of each pulse period during which the beam is actively on, defined\nas: , where and are the durations of the beam-on and beam-off\non off\nt𝑡𝑜𝑜𝑜𝑜\nperi𝐷𝐷od𝐷𝐷s𝐷𝐷 𝐷𝐷w 𝑐𝑐it𝐷𝐷h𝑐𝑐in𝑐𝑐𝑐𝑐 o=ne p𝑡𝑡𝑜𝑜u𝑜𝑜l+s e𝑡𝑡,𝑜𝑜 𝑜𝑜r𝑜𝑜espectivel𝐷𝐷y. 𝐷𝐷\nFig. 6: The particle sizes visible in Fig. 6 do not match the powder particle size distribution of\n15 to 45 µm stated in your methods section. In your radiographs, the particles seem to be roughly\nan order of magnitude larger than the used Al6061 powder. This indicates that the powder particles\nwere actually molten during “sintering” which caused them to coalesce into larger particles. Please\nprovide a detailed explanation on how the samples were prepared, including how the pre-sintered\npowder layers were created and applied to the thin-walled bulk samples used in your study.\nResponse 3-14: We thank the reviewer for the suggestion. Additional details about sample and\nsintered-layer preparation have been included in the revised manuscript, as also shown below in\nthe end of this response.\nThe pre-sintered powder-bed Al6061 samples were prepared by sintering one layer of Al6061\npowders (15–45 μm) on a commercial Al6061 plate using laser heating under an argon atmosphere\nto minimize oxidation. As observed in the X-ray images, the powder particles appear to have\npartially melted and coalesced into larger agglomerates, with apparent diameters of approximately\n200 μm. This coalescence results from localized laser melting during the sintering process rather\nthan the use of larger initial powder particles.\nTo avoid confusion for readers, clarification on sample and sintered-layer preparation and\npowder size was added to the revised manuscript, as shown below:\nLines 351-364: Al6061 was selected as the material for this study because of its good X-ray\ntransparency for in-situ high-speed X-ray imaging experiments52. Two types of samples were\nprepared for the experiments: bare substrates and pre-sintered powder-bed samples. The bare\nsubstrate samples were sectioned from commercial Al6061 plates (purchased from McMaster).\nSince powder bed sintering is not the focus of this study, to accelerate the beamline experiments,\n6\nthe powder layer was pre-sintering before loading into the chamber. The pre-sintered powder-bed\nAl6061 samples were prepared by sintering one layer of Al6061 powders (15–45 μm) on a\n22\ncommercial Al6061 plate using laser heating under an argon atmosphere to minimize oxidation.\nAs observed in the X-ray images, the powder particles appear to have partially melted and\ncoalesced into larger agglomerates, with apparent diameters of approximately 200 μm. This\ncoalescence results from localized laser melting during the sintering process rather than the use of\nlarger initial powder particles. Both types of Al6061 samples were cut into pieces with dimensions\nof 58 mm (length) x 10 mm (width) x 1.2 mm (thickness in the X-ray penetration direction), with\nthe powdered surface facing the incident beam during experiments.\nDiscussion\nL192-194: The explanation of bubble formation is highly flawed from a physical perspective.\nFirst, you essentially attribute the formation of a bubble to the fact that the irradiated material\nliquefies and evaporates below the surface first which results in an accumulation of vapor pressure\nthat leads to the formation of a bubble. While it might be true that the energy deposition is highest\na few µm below the irradiated surface, the outgoing heat flux (as a result of thermal diffusion) is\nalso much higher there as the heated volume is surrounded by highly conductive material from all\nsides. In contrast, the surface of the irradiated material can only conduct heat downwards. This is\nwhy liquefaction of material irradiated by an electron beam should still be expected to start from\nits surface, unless specific energy transport calculations suggest otherwise. Second, your\nassumption that the material reaches its boiling point first where energy deposition rate is highest\nis incorrect. The boiling point of a material strongly depends on the surrounding pressure. This\nmeans that even if the material gets heated from the inside, it does not necessarily start to boil there\nas well (or rather evaporate) as it is still surrounded by solid or liquid material. Since the surface\nof the irradiated material is in contact with the vacuum environment of your process chamber, it\nrequires significantly lower temperatures to evaporate (or rather has a much lower boiling point)\nand thus should be expected to do so first. This means that while your explanation for the observed\nbubble formation behavior is not necessarily incorrect, it is not convincing unless necessary\ncalculations or simulations are performed that show that evaporation is highly likely to initiate\ninside the irradiated material which would likely be a totally unprecedented discovery.\nResponse 3-14: We thank the reviewer for this insightful and critical comment.\n(Computational Mechanics, DOI 10.1007/s00466-015-1170-1), who modeled the temperature\nfield in electron beam melting based on electron–atom interactions6. In their study, electron\ntrajectories were tracked, and energy deposition was computed from electron–atom collisions for\na Ti alloy (TC4) under a 60 kV acceleration voltage. The results showed that the maximum energy\ndensity occurs approximately 4.4 µm beneath the surface. During the initial formation of the melt\npool, the subsurface region (~4.4 µm below the surface) attains the highest temperature first,\nreaching on the order of 3900–6000 K. (see Fig. 10 in Yan’s paper). Following the same physical\nmechanism, a similar subsurface temperature maximum is expected to form during the melting of\nAl alloys. However, it should be noted that the simulation in Yan’s study did not consider the\npresence of a gas phase; thus, bubble nucleation and explosion phenomena were not captured in\ntheir model. Future studies focusing on bubble formation, explosion, and their underlying causes\nare warranted.\nWe have revised our interpretation in the revised manuscript as follows:\nLines 204-207: Thus, as heating progresses, the temperature at the maximum energy deposition\ndepth may reach the boiling point first. Simulation results by Yan et al. support this hypothesis,\nshowing that under 60 kV electron-beam irradiation of Ti-6Al-4V, the subsurface region (~4.4 µm\nbelow the surface) reaches the highest temperature first, on the order of 3900–6000 K43.\n43. Yan, W., Smith, J., Ge, W., Lin, F. & Kam, W. Multiscale modeling of electron beam and\nsubstrate interaction: a new heat source model. Comput. Mech. 56, 265–276 (2015).\nRegarding the second point on pressure and boiling temperature differences between the top\nsurface and peak absorption layer, we performed a simple estimation. Let the pressure on the top\nmelt pool surface be , and the pressure at the peak absorption layer be . Then,\ntop peak\n𝑃𝑃 𝑃𝑃\n= ,\ntop peak\n𝑃𝑃 𝑃𝑃 + ρgh\nwhere is the density of liquid aluminum ( kg/m ), is gravitational\nacceleration, and m is the distance between the top surface 3 and the 3 peak absorption layer7.\n𝜌𝜌 𝜌𝜌 ≈ 2.4×10 𝑔𝑔\nThe resulting pressure difference, Pa.\nℎ ≈ 10 𝜇𝜇\n𝜌𝜌𝑔𝑔ℎ ≈ 0.24\nvapor depression is always present, indicating that evaporation does not occur in a true vacuum\nwithin the irradiated region. However, the vapor pressure during electron beam melting has not\nbeen reported in literature and is difficult to determine accurately. \nQuantitative modeling and in situ diagnostics are necessary to further elucidate the mechanisms\nof subsurface bubble nucleation and explosion. Accordingly, we have clarified this point in the\nrevised manuscript:\nLines 207-209: This localized overheating could initiate vaporization and the nucleation of\nsubsurface bubbles filled with metal vapor (Fig. 3d). Future studies will focus on investigating the\nmechanisms underlying bubble formation.\nConclusion (Lines 343-347): This work is expected to inspire the electron beam additive\nmanufacturing and welding community to advance modeling efforts for a quantitative\nunderstanding of bubble formation, explosion, and their effects, and to experimentally investigate,\nthrough in-situ monitoring, the occurrence of subsurface bubble explosions in high-atomic-number\nmaterials, thereby deepening the overall understanding of electron beam melting.\nL.207-209 (+ L.214-217): You seem to essentially attribute all observed melt pool dynamics to\nthe formation and explosion of bubbles with no consideration for other sources of hydrodynamic\neffects. Aside from the higher energy absorption efficiency and penetration depth provided by the\nelectron beam, your experimental setup very closely resembles that of research groups from APS\n(Zhao et al. 2017, https://doi.org/10.1038/s41598-017-03761-2), SSRL (Calta et al. 2018,\nhttps://doi.org/10.1063/1.5017236) and DLS (Leung et al. 2018,\nhttps://doi.org/10.1016/j.addma.2018.08.025). These and some other groups have published\nextensive studies of melt pool dynamics in PBF-LB and should not be disregarded entirely on the\nbasis of a difference in used heat source alone. Additionally, studies about melt pool dynamics in\nEBM from other research groups also exist and should be considered for discussion as well. The\nfact that your submitted article is entirely devoid of a comparison between your findings and the\nexisting body of work in this field also seems questionable in general. I therefore highly\nrecommend a discussion of your work that considers findings by other research groups, especially\nconsidering the similarities in observed melt pool dynamics (aside from the presented bubble\nformation and explosion mechanism). This should also result in a much more nuanced\n25\nunderstanding of the inducing effects of bubble formation and explosion discovered by your\nexperiments.\nResponse 3-15: We sincerely thank the reviewer for this valuable and constructive comment. We\nagree that the observed melt pool dynamics cannot be solely attributed to bubble formation and\nexplosion. Other hydrodynamic effects—including Marangoni convection, recoil pressure, surface\ntension gradients, and thermocapillary-driven flow—also play essential roles in shaping melt pool\nbehavior. The references suggested by the reviewer are highly relevant and have been carefully\nreviewed and cited in the revised manuscript to strengthen the discussion.\nIn the updated Results and Discussion sections, we provide a more comprehensive analysis that\nsituates our findings within the context of previous studies on melt pool dynamics under both laser\nand electron beam conditions. In addition to bubble explosion, we considered others factors that\ncan influence the melt pool dynamics, including vapor pressure, evaporation, Rayleigh–Taylor\ninstability and related mechanisms.\nWe have added the following discussion to the revised manuscript:\nRegarding the evolution of the melt pool and keyhole:\nLines 112-116: At this stage, the rapid expansion of the melt pool and keyhole is primarily\ndriven by rising vapor pressure within the depression, which displaces molten material and\nenhances heat flux into the surrounding solid, similar to the mechanism of keyhole drilling\nobserved in laser powder bed fusion46. Bubble explosions may further amplify local pressure\nfluctuations, promoting additional keyhole deepening.\n46. Cunningham, R. et al. Keyhole threshold and morphology in laser melting revealed by\nultrahigh-speed x-ray imaging. Science 363, 849–852 (2019).\nLines 232-233: These dynamics contribute to reshaping the melt pool and vapor depression\nthrough two coupled mechanisms.\nLines 256-261: During melting, recoil pressure from the dense metallic vapor drives the\nsurrounding liquid outward, forming a deep and narrow cavity (Fig. 2i and j). As melting continues,\nthe interplay among thermocapillary force, Marangoni convection, recoil pressure, and surface\ntension, similar to the mechanisms observed in laser powder bed fusion55,56, induces keyhole\n26\ninstability. When the keyhole collapses, the high-energy electron beam reinitiates intense\nevaporation in the newly formed shallow keyhole, driving a cyclic collapse–regrowth instability\n(Fig. 2k–o).\n55. Qu, M. et al. Mitigating keyhole pore formation by nanoparticles during laser powder bed\nfusion additive manufacturing. Addit. Manuf. Lett. 3, 100068 (2022).\n56. Zhao, C. et al. Critical instability at moving keyhole tip generates porosity in laser melting.\nScience 370, 1080–1086 (2020).\nRegarding spattering:\nLines 165-177: The dynamics of spattering under the influence of subsurface bubble formation\nand explosion were also investigated. Since spatter primarily arises from interactions between\nmetallic vapor and the melt pool surface or powder bed, pre-sintered powder bed samples were\nused to study interactions among metal vapor, melt pool and powder. Following subsurface bubble\nexplosions (Fig. 6a and b), the melt pool became unstable within the subsequent frames, ejecting\na liquid ligament from the explosion site followed by spatters (Fig. 6c-f and Supplementary Movie\n7). To facilitate clearer identification of bubble explosions and the associated spatters, the original\nimages were processed and are shown in Fig. 6g and h. An additional example of bubble-\nexplosion–induced spattering with preheating prior to melting is provided in Supplementary Note\n11\n2, Supplementary Fig. 2 and Supplementary Movie 8. It is noted that strong evaporation at the\nvapor depression without bubble explosion can also drive spatter ejection, as supported by\nnumerous X-ray studies in both laser and electron beam powder bed fusion processes34,47. The\nbubble explosions observed here represent an additional mechanism that can contribute to spatter\nformation.\n34. Semjatov, N., Wahlmann, B. & Carolin, K. Multiple interaction electron beam powder bed\nfusion for controlling melt pool dynamics and improving surface quality. Addit. Manuf. 90,\n104316 (2024).\n47. Guo, Q. et al. Transient dynamics of powder spattering in laser powder bed fusion additive\nmanufacturing process revealed by in-situ high-speed high-energy x-ray imaging. Acta Mater. 151,\n169–180 (2018).\nLine 300-305: Second, the backward melt flow observed in Fig. 9c, a phenomenon commonly\nreported in laser powder bed fusion, typically arises as molten metal circulates around the vapor\ndepression and gains momentum opposite to the scanning direction. Bubble explosions occurring\nat the front wall may introduce additional momentum to the backward flow by generating transient\nvapor jets and surface waves that propagate along the vapor–liquid interface.\nLines 306-313: As the melt flow propagates backward, periodic undulations develop along the\nmelt pool surface due to the Rayleigh–Taylor instability, as previously reported in laser and\nelectron beam powder bed fusion34,41. As the solidification front advances, these undulations\nevolve into humping structures along the melt track. In electron beam melting, intermittent\nmomentum impulses generated by bubble explosions further amplify this instability by perturbing\nthe melt pool surface. The combined effects of Rayleigh–Taylor and explosion-induced\ninstabilities thus promote periodic undulations, ultimately leading to the formation of humping\nstructures along the melt track (Fig. 9d).\n34. Semjatov, N., Wahlmann, B. & Carolin, K. Multiple interaction electron beam powder bed\nfusion for controlling melt pool dynamics and improving surface quality. Addit. Manuf. 90, 104316\n(2024).\n41. Bhatt, A. et al. In situ characterisation of surface roughness and its amplification during\nmultilayer single-track laser powder bed fusion additive manufacturing. Addit. Manuf. 77, 103809\n(2023).\nL.237- 239: Is the vapor plume inside the depression actually strong enough to sufficiently\nscatter the electron beam to induce collapse? Be reminded that the high acceleration voltage is\nsufficient to allow the electron beam to penetrate fairly deep into solid material. How about\nperforming a rough calculation of the penetration depth of the electron beam for reasonable vapor\npressures, based on the same equations you used to calculate the energy deposition profile in Fig.\n7a (by assuming that the vapor has the same composition as the solid, just at a much lower density)?\nThis would contextualize your conclusion by some actual numbers (even if they are based on\nestimates). Additionally, your explanation that electron scattering causes keyhole collapse does\nnot match the general observation that keyholes start to collapse from the surface through the\n28\nformation of a constriction. How about considering material transport phenomena inside the melt\nas well?\nResponse 3-16: We appreciate the reviewer’s insightful suggestions.\nTo assess the plausibility of electron beam scattering by the vapor plume, we estimated the beam\npenetration depth under vapor-phase conditions.\nFirst, we used the ideal gas law to estimate the density of gaseous aluminum. The molar\n𝑃𝑃𝑃𝑃\nmass of aluminum is 𝜌𝜌 = 𝑅𝑅𝑅𝑅 , the gas constant , and the\n𝑘𝑘𝑘𝑘\ntemperature is taken as a𝑀𝑀lum=in0u.0m2’6s 9b8oi𝑚𝑚li𝑚𝑚n𝑚𝑚g point, .𝑅𝑅 = 8.314 𝐽𝐽/(𝑚𝑚𝑚𝑚𝑐𝑐∙𝐾𝐾)\nAs we mentioned, the vapor pressure during el𝑇𝑇ec=tro3n0 b1e3a 𝐾𝐾m melting has not been reported in\nliterature and is difficult to determine accurately. Here, we use reported vapor pressures from laser\npowder bed fusion as a rough order of magnitude estimate, while acknowledging that future work\nis needed to quantify the differences between laser and electron beam melting conditions. In laser\npowder bed fusion, the recoil pressure P is on the order of 50–90 kPa (Cullom T. et al.\nrecoil\nFrequency domain measurements of melt pool recoil force using modal analysis. Sci. Rep. 11, 1–\n11 (2021)). Under EBM’s low-pressure (vacuum) environment, the vapor pressure is expected to\nbe lower than 50 kPa. Using this value yields an approximate vapor density:\n3\n𝜌𝜌𝑣𝑣𝑣𝑣𝑣𝑣𝑚𝑚𝑣𝑣(50 𝑘𝑘𝑃𝑃𝑘𝑘) < 0.054 𝑘𝑘𝑔𝑔/𝑚𝑚\nCompared with the solid aluminum density ( ), the density ratio exceeds\n3\n.\n𝜌𝜌\nm𝑣𝑣𝑣𝑣o𝑣𝑣r𝑚𝑚e𝑣𝑣 t\n≈\nhan\n2 7\nfo\n0\n0\n𝑘𝑘\nto\ng\n/\n𝑚𝑚\nve orders of magnitude denser\n4\nthan its vapor phase.\n5.0×10\nThe penetration depth of 60 keV electrons in solid aluminum is approximately 30 µm\nsolid\n(Klassen, A., Bauereiß, A. & Körner, C. Modelling of electron beam absorption. J. Phys. D Appl.\n𝐷𝐷\nPhys. 065307, 065307 (2014)). Assuming penetration depth scales inversely with material density,\nthe corresponding penetration depth in aluminum vapor can be estimated as:\nsolid\nvapor solid\nvapor\n𝜌𝜌 −6 4\n𝐷𝐷 ≈ 𝐷𝐷 × > 30×10 m×5.0×10 ≈ 1.5m.\n𝜌𝜌\nelectron scattering by vapor alone is insufficient to induce keyhole collapse. Therefore, the\nobserved collapse is more plausibly attributed to fluid-dynamic instabilities within the melt—such\nas recoil pressure fluctuations, Marangoni convection, and surface-tension-driven flow—rather\nthan direct beam–plume interactions. Future quantitative analysis using Monte Carlo electron\ntransport simulations could further refine these estimates.\nIn response, we have carefully revised the related discussion in the Results section (Lines 274–\n279)to present a more physically grounded interpretation of the keyhole collapse mechanism. The\nrevised text now reads:\nLines 256-261: During melting, recoil pressure from the dense metallic vapor drives the\nsurrounding liquid outward, forming a deep and narrow cavity (Fig. 2i and j). As melting continues,\nthe interplay among thermocapillary force, Marangoni convection, recoil pressure, and surface\ntension, similar to the mechanisms observed in laser powder bed fusion, induces keyhole instability\n(Fig. 2k). When the keyhole collapses, the high-energy electron beam reinitiates intense\nevaporation in the newly formed shallow keyhole, driving a cyclic collapse–regrowth instability\n(Fig. 2k–o).\nL.285-287: The backwards melt flow visible in Fig. 9 is a fairly common phenomenon that does\nnot rely on bubble explosion to occur. The backwards material ejection simply follows from the\nmolten material having to flow around the depression, during which it gains momentum in the\nopposite direction of the moving electron beam/depression. Like previously stated, don’t just\nattribute all observations to bubble formation and explosion without considering more commonly\nknown and often likely alternatives.\nResponse 3-17: We thank the reviewer for this insightful comment. We agree that backward melt\nflow is a commonly observed phenomenon in laser powder bed fusion processes, and it can indeed\narise from hydrodynamic flow around the vapor depression as the molten material gains\nmomentum opposite to the beam scanning direction. In our revised manuscript, we have clarified\nthat bubble explosion can introduce additional momentum to the backward flow.\nLines 300-305: Second, the backward melt flow observed in Fig. 9c, a phenomenon commonly\nreported in laser powder bed fusion, typically arises as molten metal circulates around the vapor\n30\ndepression and gains momentum opposite to the scanning direction57. Bubble explosions occurring\nat the front wall may introduce additional momentum to the backward flow by generating transient\nvapor jets and surface waves that propagate along the vapor–liquid interface.\n57. Guo, Q. et al. In-situ full-field mapping of melt flow dynamics in laser metal additive\nmanufacturing. Addit. Manuf. 31, 100939 (2020).\nL.293: Humping is a well known phenomenon in PBF-LB, where based on your provided\nexplanation, bubble formation is not possible to occur. For reference, see Bhatt et al. 2023\n(https://doi.org/10.1016/j.addma.2023.103809) for example.\nResponse 3-18: We thank the reviewer for this insightful comment and for pointing out the\nrelevant reference. We agree that humping in laser powder bed fusion, where bubble formation\ndoes not occur, primarily arises from Rayleigh–Taylor instability. Our revised text now\nacknowledges this established mechanism and clarifies that bubble explosions are not the sole\ncause of humping in electron beam melting (EBM). Instead, we propose that in EBM, bubble-\ninduced transient pressure pulses act as an additional source of perturbation, superimposing on the\ninherent Rayleigh–Taylor instability. The revised paragraph (Lines 324–331) now reads as follows:\nLines 306-313: As the melt flow propagates backward, periodic undulations develop along the\nmelt pool surface due to the Rayleigh–Taylor instability, as previously reported in laser and\nelectron beam powder bed fusion34,41. As the solidification front advances, these undulations\nevolve into humping structures along the melt track. In electron beam melting, intermittent\nmomentum impulses generated by bubble explosions further amplify this instability by perturbing\nthe melt pool surface. The combined effects of Rayleigh–Taylor and explosion-induced\ninstabilities thus promote periodic undulations, ultimately leading to the formation of humping\nstructures along the melt track (Fig. 9d and Supplementary Movie 5).\n34. Semjatov, N., Wahlmann, B. & Carolin, K. Multiple interaction electron beam powder bed\nfusion for controlling melt pool dynamics and improving surface quality. Addit. Manuf. 90, 104316\n(2024).\n41. Bhatt, A. et al. In situ characterisation of surface roughness and its amplification during\nmultilayer single-track laser powder bed fusion additive manufacturing. Addit. Manuf. 77, 103809\n(2023).\nespecially considering that there is no evidence for it. While electron beams can penetrate all\nmetallic materials, the extent of their ability to do so differs drastically between alloys. Al6061 is\narguably the material exhibiting the most amount of electron beam penetration (of all commonly\nprocessed EBM alloys) due to its low density (and low average atomic number). More commonly\nprocessed materials in EBM, like Cu, steels, Ni-base superalloys and refractory alloys offer much\nlower beam penetration depths and have (if your provided explanation for bubble formation is\nassumed to be correct) therefore a much lower chance of exhibiting bubble formation. Even a\npreviously published article by the same group (Yuan et al. 2024,\nhttps://doi.org/10.1016/j.addlet.2024.100239) where Ti6Al4V and Al6061 was used to perform\nin-situ X-ray imaging experiments, shows no signs of bubble formation and explosion. This\nsuggests that the observed phenomenon is, contrary to stated, highly material specific and likely\nunique to the combination of Al6061 and EBM and that it does not occur all the time, as Al6061\nwas melted in this study using the same sample geometry, beam power and scan velocity without\nexhibiting the bubble formation phenomenon (other melt pool dynamics seem to be similar to those\nreported here though).\nResponse 3-19: We thank the reviewer for this insightful and detailed comment. We agree that\nthe bubble formation and explosion behavior should be material-specific due to differences in\nphysical properties, such as melting point, boiling point, atomic number, and density, which can\nstrongly influence electron beam energy absorption, penetration depth, and vaporization behavior\nduring EB-PBF. The high beam penetration depth and low boiling temperature of Al6061 make it\nmore susceptible to subsurface vaporization and bubble formation compared with denser materials\nwith high boiling points, such as Cu, Ni-based superalloys, or steels.\nFurther experimental studies on other alloy systems (e.g., Cu, Ni-based superalloys) are\nnecessary to determine the generality of the observed bubble explosion phenomenon and its\ndependence on material properties. To reflect this important point, we have revised the manuscript\nTitle, Abstract, Introduction, Discussion, and Conclusion to focus specifically on aluminum alloys\nand to clarify that the present findings are material-dependent, while acknowledging the need for\nfurther validation across different material systems.\nof aluminum alloy\nAbstract (Lines 19-21): Here, using in-situ high-speed synchrotron X-ray imaging, we reveal\nthat bubble explosions, unique to EBM, induce melt pool instabilities contributing to defect\nformation in Al6061.\nIntroduction (Lines 58-60): In this study, taking advantage of this in-situ characterization system,\nwe discovered that bubble explosions during EBM in Al6061, unique to EBM, destabilize the melt\npool and contribute to multiple types of defects.\nDiscussion: We deleted the previous statement: “The melt pool instabilities induced by bubble\nexplosion originate from the energy deposition characteristics inherent to the high-energy electron\nbeam. This mechanism is not material-specific and is expected to manifest across all material\nsystems. However, the energy absorption behavior, including the electron beam peak energy\ndeposition depth and penetration depth, is highly material-dependent.”\nConclusion (Lines 343-347): This work is expected to inspire the electron beam additive\nmanufacturing and welding community to advance modeling efforts for a quantitative\nunderstanding of bubble formation, explosion, and their effects, and to experimentally investigate,\nthrough in-situ monitoring, the occurrence of subsurface bubble explosions in high-atomic-number\nmaterials, thereby deepening the overall understanding of electron beam melting.\nWe appreciate the reviewer’s careful observation. In that earlier work, our primary focus was\non the formation of defects that directly evolve into porosity during electron beam melting. At that\nstage, our intent was not to present or interpret all possible phenomena occurring in electron beam\nmelting. Therefore, we limited the scope of the discussion and did not include the full sequence of\nsubsurface bubble formation and explosion in that paper.\nIn the present study, we systematically investigate and report this previously unaddressed\nphenomenon of subsurface bubble formation and explosion in detail. The difference in focus,\nrather than in experimental outcome, explains the absence of explicit bubble evolution images in\nour earlier publication.\nexample, in Fig. 3b and c of Yuan et al. (2024), ripples can be observed along the rear wall of the\nkeyhole. These features are explosion-induced surface waves, showing a similarity to the ripples\nobserved in Figs. 4 and 5 of the present manuscript. These consistencies suggest that while the\nprevious study did not explicitly discuss bubble dynamics, the underlying phenomena were indeed\npresent.\nL.314-316: The high beam penetration depth of EBM is arguably its biggest advantage over\nother additive manufacturing techniques, as it leads to a much better distribution of heat and lower\nevaporation during melting. Suggesting to remove this benefit to avoid bubble formation is highly\ncounter-intuitive and would defy the purpose of EBM almost entirely.\nResponse 3-20: We thank the reviewer for this insightful comment. We agree that the high beam\npenetration depth is indeed a key advantage of EBM, as it enables a more uniform heat distribution\nand reduces excessive evaporation during melting. We have removed the discussion suggesting\naccelerating voltage adjustment (Page 16, Line 314 in the original manuscript) in the revised\nmanuscript.\nMethods\nL.333: Was any X-ray transparent confinement used, between the Al6061 samples were\nsandwiched? If not, did the melt pool stay confined between solid Al6061 during melting (i.e. did\nyou make sure to create a melt pool thinner than the 1.2 mm sample thickness). Could you include\nan image of an Al6061 sample post experimental procedure in Fig. 1, such that the reader gets an\nexpression of how melting affects the sample?\nResponse 3-21: No confinement was used during the melting experiments. The thickness of the\nAl6061 samples was 1.2 mm. During melting, the melt pool remained confined within the\nboundaries of the Al6061 sample, ensuring the formation of a fully developed melt pool without\npenetrating the 1.2 mm sample thickness. An optical image of the melt track has been added to\nFig. 1, and the setup for the optical tests has been included in the Materials and Methods section.\nAl6061 sample, ensuring the formation of a fully developed melt pool without penetrating the 1.2\nmm sample thickness (a representative surface profile of the melt track is shown in Fig. 1b).\nFig. 1. In-situ electron beam melting high-speed synchrotron X-ray imaging experiment. (a)\nIllustration of the experimental setup of in-situ high-speed synchrotron X-ray imaging of electron\nbeam melting. (b) Typical surface profile of the melt track after electron beam melting. High-\nspeed X-ray imaging with a frame rate of 50 kHz, 100 kHz and 135 kHz, and spatial resolution of\n2 µm was used to capture the dynamics of vapor depression, melt pool and spatter during electron\nbeam melting. The experiments were conducted in a vacuum chamber using a beam with a\ndiameter of approximately 200 µm (full width at half maximum (FWHM) of a Gaussian energy\ndistribution).\nL.342-343: As previously stated, could you please provide a spot size definition for your\nelectron beam diameter of 200 µm?\nResponse 3-22: We thank the reviewer for this insightful comment. In the experiments, the\nelectron beam has a diameter of approximately 200 µm at the working plane, defined as the full\nwidth at half maximum (FWHM) of a Gaussian energy distribution. The beam information was\nmeasured using a slit-scan method, in which the beam was stepped across a slit of known width\nwhile measuring the transmitted current to determine the rise/fall regions. Then the spot size was\n35\ncalculated using the FWHM metric. This clarification has been incorporated into the revised\nmanuscript to ensure accuracy and consistency in the beam specification.\nLines 80-82: The experiments were conducted in a vacuum chamber using a beam with a\ndiameter of approximately 200 µm (full width at half maximum (FWHM) of a Gaussian energy\ndistribution).\nLines 371-376: In the experiments, the electron beam has a diameter of approximately 200 µm\nat the working plane, defined as the full width at half maximum (FWHM) of a Gaussian energy\ndistribution. The beam information was measured using a slit-scan method, in which the beam was\nstepped across a slit of known width while measuring the transmitted current to determine the\nrise/fall regions. Then the spot size was calculated using FWHM metric.\nReferences\nL.448-449: Reference 34 (Bidola, P. M. et al. In situ synchrotron…) is wrongly referenced. The\nfirst author of this publication should be “Semjatov, N.” not “Bidola, P. M.”.\nResponse 3-23: The mistake has been corrected.\nLines 487-489 Semjatov, N. et al. In-situ synchrotron imaging of powder consolidation and melt\npool dynamics in electron beam powder bed fusion. Addit. Manuf. 110, 104943 (2025).\nFinal remark\nAssuming the described methodology is complete, the in-situ synchrotron imaging study was\nperformed at room temperature. In EBM, processes are always performed at an elevated build\ntemperature to enable sintering of powder particles with minimal energy input, in order to\nminimize the risk of smoke and reduce accumulation of thermal stress. As a result, solidification\ntimes are usually a few orders of magnitude larger than in PBF-LB which is one of the reasons\nwhy melt pool dynamics can differ drastically between the two techniques. Since elevated\ntemperatures were not used in your study, solidification behavior should actually be more similar\nto that encountered in PBF-LB than in EBM (PBF-EB). How do you think does this affect the\n36\ntransferability of your findings to industrial PBF-EB processes? Would you expect to see the same\nmelt pool dynamics, bubble formation and explosion behavior, as well as humping formation or\nwould you expect differences?\nResponse 3-24: We agree with the reviewer that electron beam melting is always performed with\npreheating. Electron beam melting with preheating was also conducted during our beamline\nexperiments. Preheating was carried out by electron beam scanning with a lower electron beam\npower and a fast speed before melting. Subsurface bubble formation and explosions were observed,\naccompanied by similar melt pool dynamics (Supplementary Note 3 and Supplementary Fig. 3).\nThe figures and corresponding video of melting Al6061 with preheating have been added to the\nsupplementary materials.\nOur findings are relevant to industrial PBF-EB processes. The fundamental mechanisms\nrevealed in this study—such as subsurface bubble formation, vapor depression dynamics, and melt\npool instability—are governed by the same physical principles that operate in large-scale industrial\nsystems. Although the beam parameters, scanning conditions and sample dimensions may differ,\nthe underlying electron–matter interactions are similar. Therefore, the insights gained here are\nrelevant for understanding and optimizing industrial PBF-EB processing conditions.\nThe following contents have been added to the revised manuscript and supplementary materials:\nLines 326-329: The influence of preheating on melting dynamics, including subsurface bubble\nformation, melt pool behavior, and vapor depression evolution was also investigated. Subsurface\nbubble formation and explosions were observed, accompanied by similar melt pool dynamics\n(Supplementary Note 3 and Supplementary Fig. 3).\nSupplementary note 3: Influence of preheating on melting dynamics\nThe influence of preheating on melting dynamics, including subsurface bubble formation, melt\npool behavior, and vapor depression evolution was also investigated. Preheating was carried out\nby electron beam scanning with a lower electron beam power and a fast speed before melting.\nSubsurface bubble formation and explosions were observed, accompanied by melt pool dynamics\nsimilar to those observed without preheating (Supplementary Fig. 3a). Driven by the Plateau-\nRayleigh and explosion-induced instabilities, this elongated liquid column (i.e., the melt pool)\n37\neventually forms a humping structure on the melt track after solidification (Supplementary Fig.\n3b-d).\nSupplementary Fig. 3. Melting dynamics during electron beam scanning with preheating. (a-\nc) X-ray images showing the bubble explosion and melt pool dynamics during electron beam\nscanning. (d) X-ray image showing periodic humping after solidification. Prior to melting, a\npreheating step was performed using the electron beam operated in pulse mode with a 0.4 duty\ncycle, a power of 318 W, a scan speed of 16 m/s, and a duration of 10 s. Melting was then\nperformed with an electron beam power of 426 W and a scan speed of 1.0 m/s.\nAside from my remarks, I find the presented experimental findings highly interesting and am\nlooking forward to their publication once the necessary adjustments have been made.\nSincerely,\nNick Semjatov\nWe deeply appreciate the editor’s and reviewers’ time and effort in providing constructive\ncomments and valuable advice on our manuscript. We have made our best efforts to address all\nthe insightful comments. Below, we provide a detailed description of the corrective actions and\nour point-by-point responses to each reviewer’s comments.\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nThe authors responded well and made revisions to the reviewer's comments. The newly revised\npaper focuses on Al6061 aluminum alloys, which is more rigorous and comprehensive, and also\nprovides reference and inspiration for EBM processes of other materials. Therefore, I believe that\nthe paper can be accepted.\nResponse 1-1: We sincerely thank the reviewer for the thorough evaluation of our manuscript and\nthe encouraging recommendation for acceptance.\nReviewer #2 (Remarks to the Author):\nThe authors have addressed the reviewers’ comments in an appropriate and satisfactory way. In\nparticular, the hedging of the interpretations of the observations to the actual experiment improve\nthe manuscript. Further the clarification of the phenomenological nature, as well as the\nidentification of the limitations and scope of the work benefit the manuscript.\nThe presented results and their interpretation are more clear and categorized. The limitations of\nthe work and the directions of future work are pointed out, which strengthens the contribution of\nthe work to PBF-EB research.\nResponse 2-1: We thank the reviewer for the positive and constructive feedback. We appreciate\nthe reviewer’s recognition that the revisions have improved the clarity, scope, and overall\ncontribution of the manuscript.\nabout the observations in the videos.\nWith including this adjustment, the manuscript gives a clear and coherent presentation of\nphenomenological observations and situates these in the research field. It is therefore suitable for\nconsideration for publication.\nResponse 2-2: We thank the reviewer for this helpful suggestion. We have added visual markers\nand guidance cues to the videos to highlight key observations, such as the melt pool boundary,\nvapor depression, and bubble dynamics.\nReviewer #3 (Remarks to the Author):\nThis revision leaves the manuscript in a much better state. All of my remarks have been sufficiently\naddressed. With the inclusion of video material, the authors provide much stronger and easier to\nfollow evidence for their reported observations (the bubble formation and explosion phenomenon\nis much easier to see this way). The description of experimental procedures is also significantly\nimproved and the description of scientific impact and relevance to the field of PBF-EB\nappropriately adjusted. There is no further revision needed from my point of view. I am looking\nforward to the publication of this study.\nSincerely,\nNick Semjatov\nResponse 3-1: We sincerely thank the reviewer for the very positive and encouraging assessment.\nWe greatly appreciate the reviewer’s recognition of the improved presentation and relevance of\nthe work to the field of PBF-EB research.\n2"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors responded well and made revisions to the reviewer's comments. The newly revised paper focuses on Al6061 aluminum alloys, which is more rigorous and comprehensive, and also provides reference and inspiration for EBM processes of other materials. Therefore, I believe that the paper can be accepted.", "reviewer_2": "The authors have addressed the reviewers’ comments in an appropriate and satisfactory way. In particular, the hedging of the interpretations of the observations to the actual experiment improve the manuscript. Further the clarification of the phenomenological nature, as well as the identification of the limitations and scope of the work benefit the manuscript.\nThe presented results and their interpretation are more clear and categorized. The limitations of the work and the directions of future work are pointed out, which strengthens the contribution of the work to PBF-EB research.\nThe authors should include some markers/guidance to the eye to ensure that there is no confusion about the observations in the videos.\nWith including this adjustment, the manuscript gives a clear and coherent presentation of phenomenological observations and situates these in the research field. It is therefore suitable for consideration for publication.", "reviewer_3": "This revision leaves the manuscript in a much better state. All of my remarks have been sufficiently addressed. With the inclusion of video material, the authors provide much stronger and easier to follow evidence for their reported observations (the bubble formation and explosion phenomenon is much easier to see this way). The description of experimental procedures is also significantly improved and the description of scientific impact and relevance to the field of PBF-EB appropriately adjusted. There is no further revision needed from my point of view. I am looking forward to the publication of this study.\nSincerely,\nNick Semjatov"}, "author_response": "Response to Reviewer’s comments\nWe deeply appreciate the editor’s and reviewers’ time and effort in providing constructive\ncomments and valuable advice on our manuscript. We have made our best efforts to address all\nthe insightful comments. Below, we provide a detailed description of the corrective actions and\nour point-by-point responses to each reviewer’s comments.\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nThe authors responded well and made revisions to the reviewer's comments. The newly revised\npaper focuses on Al6061 aluminum alloys, which is more rigorous and comprehensive, and also\nprovides reference and inspiration for EBM processes of other materials. Therefore, I believe that\nthe paper can be accepted.\nResponse 1-1: We sincerely thank the reviewer for the thorough evaluation of our manuscript and\nthe encouraging recommendation for acceptance.\nReviewer #2 (Remarks to the Author):\nThe authors have addressed the reviewers’ comments in an appropriate and satisfactory way. In\nparticular, the hedging of the interpretations of the observations to the actual experiment improve\nthe manuscript. Further the clarification of the phenomenological nature, as well as the\nidentification of the limitations and scope of the work benefit the manuscript.\nThe presented results and their interpretation are more clear and categorized. The limitations of\nthe work and the directions of future work are pointed out, which strengthens the contribution of\nthe work to PBF-EB research.\nResponse 2-1: We thank the reviewer for the positive and constructive feedback. We appreciate\nthe reviewer’s recognition that the revisions have improved the clarity, scope, and overall\ncontribution of the manuscript.\nabout the observations in the videos.\nWith including this adjustment, the manuscript gives a clear and coherent presentation of\nphenomenological observations and situates these in the research field. It is therefore suitable for\nconsideration for publication.\nResponse 2-2: We thank the reviewer for this helpful suggestion. We have added visual markers\nand guidance cues to the videos to highlight key observations, such as the melt pool boundary,\nvapor depression, and bubble dynamics.\nReviewer #3 (Remarks to the Author):\nThis revision leaves the manuscript in a much better state. All of my remarks have been sufficiently\naddressed. With the inclusion of video material, the authors provide much stronger and easier to\nfollow evidence for their reported observations (the bubble formation and explosion phenomenon\nis much easier to see this way). The description of experimental procedures is also significantly\nimproved and the description of scientific impact and relevance to the field of PBF-EB\nappropriately adjusted. There is no further revision needed from my point of view. I am looking\nforward to the publication of this study.\nSincerely,\nNick Semjatov\nResponse 3-1: We sincerely thank the reviewer for the very positive and encouraging assessment.\nWe greatly appreciate the reviewer’s recognition of the improved presentation and relevance of\nthe work to the field of PBF-EB research.\n2"}]

Decision letter