# In situ porosity imaging with synchrotron X-ray Tomography during laser rescanning of Zr-based metallic glass by Laser Powder Bed Fusion

Camille Pauzon, Rémi Daudin, Pierre Lhuissier, Xavier Bataillon, Pierre Lapouge, Pierre Hébrard, Patrice Peyre, Frédéric Coste, Lucas Varoto, Elodie Boller, et al.

► To cite this version:

Camille Pauzon, Rémi Daudin, Pierre Lhuissier, Xavier Bataillon, Pierre Lapouge, et al. In situ porosity imaging with synchrotron X-ray Tomography during laser rescanning of Zr-based metal-lic glass by Laser Powder Bed Fusion. Additive Manufacturing Letters, 2025, 15, pp.100318. 10.1016/j.addlet.2025.100318. hal-05216282

HAL Id: hal-05216282

https://hal.science/hal-05216282v1

Submitted on 20 Aug 2025

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Short Communication

# In situ porosity imaging with synchrotron X-ray tomography during laser rescanning of Zr-based metallic glass by laser powder bed fusion

Camille Pauzon<sup>a, *, †</sup>, Rémi Daudin<sup>a</sup>, Pierre Lhuissier<sup>a</sup>, Xavier Bataillon<sup>a</sup>, Pierre Lapouge<sup>b</sup>, Pierre Hébrard<sup>b</sup>, Patrice Peyre<sup>b</sup>, Frédéric Coste<sup>b</sup>, Lucas Varoto<sup>a</sup>, Elodie Boller<sup>c</sup>, Jean-Jacques Blandin<sup>a</sup>

ARTICLE INFO

Keywords:

Bulk metallic glasses

Laser powder bed fusion

Porosity control

Synchrotron X-ray microtomography

In situ

A B S T R A C T

When laser powder bed fusion (LPBF) is applied to bulk metallic glasses rather than traditional crystalline alloys, one has to avoid conditions that could cause crystals to form. To achieve a balance between the porosity content and devitrification in the heat affected zone, it is common practice to process such material with a thin layer thickness, and thereby reduce the laser power necessary for melting. In this manufacturing regime, lack-of-fusion defects typically subsist. This work investigates how laser rescanning can densify metallic glasses while still ensuring their amorphous nature. Synchrotron X-ray Computed Tomography during LPBF allows imaging in situ the pores upon the glass construction. This non-destructive cutting-edge technique helps understanding the consolidation mechanism associated with rescanning and in particular its effect on layer surface roughness and the homogeneity of the powder recoating. Applied to the well-established Zr-Cu-Al-Nb grade, this work paves the way towards the adoption of less thermally stable glasses for LPBF, and the control of defect distribution. In particular, it is revealed that the hatch spacing effect is of primary importance in the production of viscous materials such as glasses, and that laser rescanning allows the surface of the deposited layer to be smoothed, improving consolidation without associated crystallisation.

1. Introduction

Free of crystalline defects such as grain boundaries and dislocations, bulk metallic glasses (BMGs) exhibit exceptional chemical, physical and mechanical properties, with for example corrosion resistance, yield and fracture strength, largely exceeding that of their crystalline counterparts. Synthesized under rapid cooling conditions by melt spinning or injection casting, their properties make them relevant for various applications ranging from the micromechanical industry to healthcare. Recently, additive manufacturing (AM) and in particular laser powder bed fusion (LPBF) has widened up the complexity and part size of producible BMG components. However, depending on the metallic glass composition, challenges concerning their LPBF processability still need to be addressed to ensure their interesting properties are withheld.

Unlike traditional crystalline alloys, LPBF of BMGs faces an additional constraint related to laser-induced crystallisation. To avoid the

nucleation and growth of crystals upon processing, laser parameters can be tuned to find a compromise between the formation of lack-of-fusion (l-o-f) defects and the crystallisation onset. On the one hand, l-o-f defects have been identified as the primary cause for metallic glass components failure upon uniaxial tensile testing [1], and impact testing [2]. As recalled by Tang *et al.* [3], l-o-f defects are porosities generated at low laser energy densities due to incomplete melting, originating from insufficient overlap between successive melt pools. On the other hand, crystallisation is observed as devitrification products within the heat affected zone (HAZ) of the deposited material [4], affecting the material's structural properties and homogeneity. This narrows the acceptable processing window for metallic glasses considerably. Research on LPBF of BMGs has focused primarily on the Zr-Cu-Al-Nb composition, which does not face issues such as cracking encountered for Fe-based alloys as reviewed for example by Zhang *et al.* [5] and reported by Luo *et al.* for a Fe-Co-B-Si-Nb grade [6]. For this grade, the common

<footnote><sup>a</sup> Université Grenoble Alpes, CNRS, Grenoble INP, SIMaP laboratory, Grenoble, France</footnote>

<footnote><sup>b</sup> Ecole Nationale Supérieure des Arts et Métiers, PIMM, UMR 8006 CNRS, Paris, France</footnote>

<footnote><sup>c</sup> ESRF- The European Synchrotron, CS40220 Grenoble, France</footnote>

<footnote>* Corresponding author.
E-mail address: pauzon@chalmers.se (C. Pauzon).</footnote>
processing approach is to work with thin layer thicknesses, i.e. 20 µm. It permits to consolidate the powder layers with low laser power, i.e. below 100 W. Several authors have shown that increasing the laser power yields a higher crystalline fraction for Zr-based BMGs, e.g. for the present alloy [7], for the Zr-Cu-Ni-Al-Ti system [8]. A complementary strategy is laser rescanning or scanning twice each deposited layer, as employed by Marattukalam et al. [9] and Larsson et al. [10] for the development of biocompatible components. Regrettably, the motivations for this approach are either unstated or lacking supporting evidence. The adoption or not of the rescanning strategy (e.g. as not employed by Sohrabi et al. [2], Wegner et al. [11] or even Kosiba et al. [12]), raises the question of its purpose and necessity, for example when considering productivity matters and more interestingly for metallic glasses compositions that are more challenging to process by LPBF. Since the above-mentioned works were conducted on different machines and with different process parameters, a systematic study is needed to understand the role of laser rescanning for LPBF of metallic glasses.

Rescanning strategies have also been investigated for LPBF of traditional crystalline alloys to promote their chemical homogeneity (e. g. for a mixture of Ti-6Al-4V and Cu [13], or a mixture of Ni and steel powders [14]), to control microstructures and create architecture materials for example for Ni-based [15] or stainless steel [16]. In addition, De Formanoir et al. [17] proposed to laser rescan with a defocused laser in order to heal keyhole porosities. Hence, the status of the literature enables to raise potential areas over which rescanning may act positively: chemical homogeneity and defects.

To begin with, the present work aims to elucidate the impact of laser multiple scanning on defects, namely l-o-f pores, and to verify whether and how rescanning permits the control of the consolidation of the metallic glass. To achieve this purpose, the well-established $Zr_{59.3}Cu_{28.8}Al_{10.4}Nb_{1.5}$ grade is selected for the investigation. It should be noted that this composition must not necessarily be processed at low laser energy since it is rather thermally stable when LPBF processed. The present goal is to use this grade as a model alloy to pave the way for potentially more complex grades, more sensitive to devitrification, and that could benefit from a low-energy multiple scanning strategy. To this aim, a combination of a low laser power (50 W) and a relatively large hatch distance (100 µm) was chosen in order to remain in the l-o-f regime and study the effect of laser rescanning on the potential consolidation mechanisms during the build of $Zr_{59.3}Cu_{28.8}Al_{10.4}Nb_{1.5}$ parts.

While post-mortem conventional metallography and standard X-ray computed tomography are substantial tools for assessing the porosity of printed parts in 2D and 3D respectively, these techniques are limited in providing information on the origin of the pores during the LPBF construction. These can provide a precise and statistical characterization of the pore population, but do not allow it to be related to specific events and phenomena that occur during printing. In addition, despite the fact that pore position is directly related to construction height, large and complex l-o-f defects may be difficult to attribute to a specific time of the built when looking only at the post-mortem state of the samples. This limitation becomes even more critical for more advanced laser processing strategies such as laser rescanning, and it is essential to decompose the building sequence into individual steps to clarify the dynamic consolidation mechanisms. In addition, this decomposition must be carried out in a timely manner so as not to interfere too much with the expected construction schedule. Hence, the high brilliance of X-ray synchrotron source and the miniature LPBF experimental rig developed by Lhuissier et al. [18] are employed to perform in situ X-ray Computed Tomography (XCT) during the LPBF construction and capture 3D information at intermediate stages of the build. Installed at the BM18 beamline of the European Synchrotron Radiation Facility (ESRF), the system allows the collection of 3D information about the build with a time and a microns scale resolution compatible with the process. The results reveal the consolidation mechanisms associated with laser rescanning using laser strategies in the l-o-f regime. Post-mortem ex situ

evaluation of the produced samples completes the understanding of the effect of multiple scanning on the processing of metallic glasses.

## 2. Materials and methods

### 2.1. In situ synchrotron X-ray computed tomography

In recent years, synchrotron X-ray imaging techniques have permitted to gain fundamental insight into LPBF and in particular pore formation [19]. Herein, in situ synchrotron X-ray computed tomography (XCT) was conducted during LPBF using a miniature machine developed and presented by Lhuissier et al. in [20]. Fig. 1a introduces the different steps of a typical in situ SCT measurement cycle during LPBF. In the present work, the SCT recording is started once a few layers are built and a manufacturing steady state is established. SCT images are collected after the powder recoating, after the laser scanning and after the multiple laser scans. Hence, steps 3 and 4 in Fig. 1c are repeated as long as laser rescanning is applied. The method is in situ because it takes place during the construction but it is not operando as the recording is not simultaneous to the laser scanning.

Fig. 1b presents the system as installed at the ESRF BM18 beamline. This LPBF machine is designed to SCT image the process area at different key times of the build. Fig. 1c introduces its structure. The walls of the machine build chamber are made from a polycarbonate tube transparent to X-rays and the machine is mounted on a motorized stage present at the beamline to perform a 180° rotation. The process chamber is inertized with a continuous Ar flow of 99.9995% purity over the build area, thereby ensuring a residual oxygen level of about 100 ppm $O_2$ [18]. The motors of the miniature LPBF machine control the recoater and the build plate movements. The latter is made from Ti Grade 5 and is herein also referred to as the build piston. The system operates with a continuous redPOWER® 1kW laser (SPI Lasers) with a wavelength of 1070 nm, whose power was calibrated in the range of interest prior to the experiments. The laser position on the build piston during manufacturing is controlled by the motorized galvanometric mirrors of a scan head (model HSC I 14 from Trumpf Laser Marking Systems). Its focus is realised by an f-theta lens with focal distance 420 mm. The voxel size is 2.02 µm, the X-ray energy 100 keV, and 3000 projections were acquired over 180°. The acquisition time was approximately 2.5 min. The data collected during the beamtime has for digital identifier of object the following "doi.esrf.fr/10.15151/ESRF-ES-1106935016". Re-constructions were conducted by the filtered back projection method using Nabu. The reconstructed images may have a slightly blurry appearance owing to the fine and dense powder material employed.

### 2.2. Material and laser processing

The powder employed has for nominal composition $Zr_{59.3}Cu_{28.8}Al_{10.4}Nb_{1.5}$ in at.-% (Amloy-Zr01, formerly known under the trade name AMZ4, supplied by Hereaus). Its particle size distribution is in the range of 15 µm to 45 µm, with the virgin feedstock sieved through a 63 µm woven wire cloth prior to processing. As presented in [21], the virgin powder has a rather spherical and satellite-free morphology, with seldom-observed filaments and more elongated particles resulting from the atomisation of the viscous material.

For manufacturing, the laser parameters were defined on the basis of a literature review of the work carried out on the alloy under study, which indicated that typically thin layer thicknesses are used to limit the energy required to melt the material, as well as a high scanning speed to reduce the energy input. The hatch distance is adjusted accordingly and is typically in the order of 100 µm. Therefore, the layer thickness is set to 20 µm, the scanning speed to 2000 mm/s. The laser is put in its low power mode and the power is maintained at 50 W for the duration of the study with an unchanged hatch distance of 100 µm. Utilising a low power and a relatively large hatch distance is expected to generate l-o-f defects but also to allow avoiding the higher laser energy region, where
<figure><img src="image_2.png" /><figcaption>Fig. 1. a) Schematics of the typical experimental cycle during in situ synchrotron SCT and LPBF. b) Picture of the LPBF miniature system integration at the beamline BM18 of the ESRF. c) 3D CAD-view of the miniature system.</figcaption></figure>

devitrification occurs, and to limit elemental vaporization. Vaporization
can result in changes to the composition, as well as the entrainment of
powder particles near the melt pool. This can contribute to local denu-
dation and spatter generation [22]. As depicted in [21], the main type of
Zr-Cu-Al-Nb spatter belongs to the entrained particles category, which
undergo complex heating when travelling close to the laser-melt pool
interaction zone. Such particles are partially crystalline, and the for-
mation of both Cu₂Zr₄O nanocrystals and coarser α-Zr(O) dendrites was
reported. Hence, there is a two-fold hazard when working at higher laser
power and generating spatters: with first, the potential transfer of
crystals from the spatter particles to the as-built material upon their
redeposition; and second, the possible formation of stochastic 1-o-f de-
fects with locally larger powder thickness as observed in [21] for the
studied material, and for other more common crystalline alloys such as
Alloy 718 [23] or Hastelloy X [24].

On a single build piston of 8 mm in diameter, 4 cubic samples of
dimension 1,8 × 1,8 mm² were produced, without baseplate preheating.
One sample was built following standard LPBF approach with a single
laser scan per layer, and a rotated scan strategy by 90° between layers; a
second sample with an additional scanning on each layer; and two more
samples with respectively a total of 4 and 8 laser scans per layer. Hence,
for the layers of interest, SCT images were collected after powder
recoating, and after the 1st, 2nd, 4th and 8th laser scans. Each laser scan of
a layer is parallel to and overlaid on the first one, without any lateral
shift.

2.3. *XCT image analysis*

The segmentation of the SCT images and in particular of the
deposited material and surrounding powder bed was performed with the
machine-learning pixel classification workflow of the *ilastik* open-source
software [25]. Pixel labels are assigned based on user annotations and
pixel features, such as grey level, edges between different regions and
image texture, in order to distinguish the deposited material, the
recoated powder and the porosity. A machine-learning random forest
classifier is trained interactively from the manual segmentation. The rest
of the analysis of the segmented porosity was conducted with the
“Analysis 3D” plugin in ImageJ [26], considering an analysed volume of
approximately 0,4 mm³. A minimum void size of at least 8 voxels is
considered to avoid false segmentation due to possible artefacts. Hence,
pores with minimum equivalent diameter of around 5 µm were detected.

To further study the uniformity of the part top surface and of the
deposited powder layer, the “Z Project” tool in ImageJ was used on the
stack of the segmented part images with filled porosity and on the stack
of segmented powder layer images. This allows appreciating the local
thickness of the part and of the powder layer. Finally, the Avizo software
was used for 3D volume renderings.

2.4. X-ray diffraction

X-ray diffraction (XRD) of the samples produced at the beamline was
performed using a RIGAKU SMARTLAB (1.5041 Å) diffractometer,
scanning between 15° and 95°, with a 0.05° step size. The produced
cubic samples were first separated from the baseplate. Their bottom
surfaces were then ground and polished for measurement.

## 3. Results and discussion

The post-mortem evaluation of samples built with the single laser scan and the rescan strategy reveals a reduction of the porosity by half already with a single laser rescan, from 2,3% to 1,2%. In addition, the laboratory XRD measurements confirm the samples remain amorphous upon laser rescanning.

### 3.1. Origin of l-o-f pores in single scan LPBF

The origin and the development of pores in the single laser scan sample is presented with in situ XCT images in Fig. 2. A region of interest (ROI) in a slice parallel to the building direction (BD) is displayed at different times of the construction. The layer N in Fig. 2a corresponds to the 12th deposited layer, and hence a built height of about 240 µm. At this stage, two types of surface asperities are pointed out: a large one in red, and regularly-spaced smaller ones in yellow. With the next layers, see Fig. 2b,c, these asperities become more pronounced. The ones pointed out by the yellow arrows are spaced laterally by about 100 µm and are located in between laser beads. These form parallel ridges which are perpendicular to the observation plane. The convex shape of the laser beads in between these ridges are connected to the high surface tension of the material. In the literature, this is referred to as the hatch spacing effect. Now, looking at layer N+5, these hatch spacing pits observed since layer N+1 have led to the formation of evenly spaced pores. In contrast, the larger one pointed by the red arrow since layer N is still open to the surface and filled with powder particles upon recoating, see Fig. 2d. At this location, the volume of powder particles to be consolidated largely overpasses that corresponding to the nominal layer thickness of 20 µm. In turn, the laser energy can only partially consolidate these regions, which are comparable to large pockets of powder particles. Finally, after the deposition of layer N+8, one is able to observe the resulting l-o-f defect initiated from this large surface asperity, spanning across several deposited layers in the Z direction and percolating at least between two adjacent laser beads. From these in situ observations, one can gather that the origin of porosity is related to the surface asperities of the previously consolidated layer and that the larger the asperity the larger the resulting l-o-f defect. The formation of the surface asperities is itself connected to the properties of the processed glass, namely its viscosity, and the low energy laser parameters employed.

### 3.2. Consolidation mechanisms with LPBF rescan strategy

The XCT images of the cubes built with a rescan strategy are analysed to understand the consolidation mechanisms associated with laser rescanning. Fig. 3a-c displays a slice of interest after the 1st and 2nd laser scan of the layer M, and after the 2nd laser scan of the layer M+6. After the 1st laser scan of layer M, regularly-spaced small surface asperities, as seen in the single scan sample, due to the hatch-spacing effect are observed and pointed out with yellow arrows. The 2nd laser scan on that layer appears to have smoothen the top surface, see Fig. 3b. As a result of the erasing of these small surface asperities, no evenly spaced pores are formed at these locations when the construction proceeds, see Fig. 3c. The effect of this 2nd laser scan is comparable to that sought when laser polishing is employed to achieve superior surface finish of rough components [27]. By running the Python code developed by Steinhilber et al [28], on the final segmented top surfaces, we confirm that the areal average roughness (Sa) is decreased from approximately 30 µm in case of single laser scan to 20 µm in case of laser rescan sample. Confronting the current process conditions to established understanding of laser polishing, the mechanism for surface roughness reduction is similar to shallow surface melting [29]. Shallow surface melting occurs when only the top surface is melted and the melted layer thickness is less than the “peak-valley” distance of the surface asperities. In that regime, mass transfer from peaks to valleys occurs under the capillary pressure. The valleys are filled with molten metal from peaks, resulting in a roughness reduction. In addition, fewer large surface pits and associated l-o-f pores are developing with the rescan strategy. The formation of one of these remaining pits is pointed out in Fig. 3 with a red arrow. Its size is significantly smaller than that of the one pointed out in red in Fig. 2f, with the single scan strategy. Hence, the rescan strategy limits the depth of the surface asperities formed and, in turn, fewer large l-o-f pores are formed. The fact that some pores subsist with the rescan strategy suggests that a critical pit/asperity size leading to pore formation with these laser parameters exists.

As the top surface is smoothened with the 2nd laser scan, see Fig. 3b, it is expected that less large pockets of powder particles overpassing the nominal layer thickness of 20 µm should form, as seen in Fig. 2d. To verify this point, volumes of interest in the parts built with the single laser scan and the rescan strategies are studied. The consolidated material and powder particles in these volumes are segmented by image analysis. Fig. 4a,b presents the local part height and Fig. 4c,d the local layer thickness. This examination points out the correspondence of the local part height minima with the local layer thickness maxima for the

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Slices parallel to the building direction (BD) of an ROI of the sample printed with a single laser scan strategy. It shows the origin of l-o-f pore formation upon several construction steps. The arrows in the sample's bulk highlight specific defect features transferred from layer to layer: in red a large l-o-f defect and in yellow smaller l-o-f defects resulting from the hatching-effect pits formation.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 3. Slices parallel to the building direction (BD) of the sample printed with a laser rescan strategy showing several steps in its construction: a) after the 1st laser scan of layer M, b) after the 2nd laser scan of layer M, and c) after the 2nd laser scan of layer M+6.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. Two volumes of interest consisting of the built part and freshly deposited powder particles are studied for the single (a-c-e) and the rescan strategy (b-d-f). The local part height is shown in a and b), the local layer thickness in c and d) and the local volume renderings (view from below the surface) in e and f). The segmented powder layer is displayed in red and the built part in grey transparency. The red arrow points out the local part height minima and local layer thickness maxima resulting from the hatch-spacing effect in the single laser scan sample. The yellow dashed lines are pointed out as local profiles are presented in the following figure.</figcaption></figure>

single scan strategy, see red arrows in Fig. 4a-c. In other words, it depicts the formation of powder pockets at the large surface asperities. The arrangement of these powder pockets in a grid pattern is a direct result of the 90° scan vector rotation applied between layers and the ridge formation between laser passes, i.e. the hatch spacing effect. The 90° scan vector rotation is particularly suited for the purpose of the experiment and the in situ porosity formation tracking. Still, the authors would recommend for future work to implement a 67° rotation, which

would ensure a more homogeneous thermal history throughout the samples. Its effect on the porosity content and distribution should be explored.

Similar observations can be drawn for the laser rescan strategy, but the local part height minima are more localised and smaller, meaning the ridges are shallower. In addition, the local layer thickness maximum (i.e. powder pocket height) is reduced by about half. This can also be appreciated from the volume renderings presented in Fig. 4e,f, where the pockets filled with powder are visible in red. Overall, the recoated layer thickness appears more homogeneous on the rescan sample surface with a reduced amount of large powder pockets at the build part surface asperities. Animated volume renderings are presented in Supplementary Videos.

Fig. 5 provides with line profiles from the locations indicated by the yellow dashed lines in Fig. 4. This allows a more quantitative assessment of the differences between the two samples in terms of local layer thickness and part height, the latter reflecting the top surface roughness. Firstly, it can be seen that under both processing conditions, the effective local layer thickness exceeds the required height of 20 µm, see Fig. 5a. This is only related to the local part height as the recoater and sample stage movements of the LPBF replicator system are properly controlled to achieve the 20 µm layers. In addition, Fig. 5b permits to appreciate more clearly the presence of open porosity in the single scan sample, as the local part height is zero at a distance of approximately 200 µm along the horizontal axis. In contrast, the double scan sample shows a much smoother local part height profile.

Fig. 6 presents in more detail the post-mortem evaluation of the porosity of the two studied samples. The distributions of the pore volume are presented alongside representative pores rendered in 3D. The single laser scan sample features a larger pore population (i.e. 595 pores versus 529 pores) than the rescanned one. Few gas pores with sizes about 1·10⁴ µm³ are noted, see grey pore in Fig. 6. These account for less than 1% of the total porosity of both samples. In addition, an OCT scan of the powder in a glass capillary permits to quantify the gas porosity in the feedstock to be less than 0.8%. This suggests that gas pore formation is not determinant for the metallic glass consolidation during LPBF. More interestingly, it appears that 40% of the total porosity of the single laser scan sample correspond to pores with a volume larger than 10⁶ µm³, while no such large pores are observed in the rescanned sample. In other words, the single scan sample has a larger population of big l-o-f pores which contributes greatly to its final porosity content. The latter have morphologies similar to the red pore presented in Fig. 6 and correspond to l-o-f pores developing across several laser beads and deposited layers, as pointed out in red in Fig. 2. Intermediate size l-o-f pores similar to the one rendered in yellow in Fig. 6 are observed in both samples and emerge from less pronounced surface asperities. The beneficial effect of the laser rescan on the large l-o-f pores raises one's interest to investigate an even larger number of rescanning. Thus, the present work continues with the analysis of samples built with 4 and 8 laser scans.
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Examples of profiles of a) the local layer thickness and b) the local part height for the samples scanned 1 and 2 times. The profiles correspond to the location highlighted by the yellow dashed line in the previous **Fig. 4**.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** a) Distributions of the pores volume within the samples built with a single laser scan strategy and a rescan strategy; b) Insert depicting the distribution of the larger pores in both specimens.</figcaption></figure>

### 3.3. Perspectives

As established previously, the efficiency of the consolidation is directly correlated to the surface asperities smoothening. Hence, the volume close to the surface of each sample is reconstructed after each laser scan to appreciate the evolution of the smoothening, see Fig. 7. The laser polishing effect of the 2^nd^ laser scan is evident on all the constructed samples and is confirmed by the displayed slices perpendicular to the laser scanning axis for the single rescan sample. Despite a few visible changes with additional rescans, the overall aspect of the top surfaces remains relatively constant after the 2^nd^ laser scan. This is consistent with the post-mortem porosity measurement which highlights a stable consolidation state of the samples (in the range of 1,2% - 0,9% porosity), while the material remains XRD amorphous, see Fig. 8. The broad peaks observed in the XRD patterns are typical of an amorphous sample and are connected to the short-range order within the material. Complementary future work should be conducted to construct larger specimens allowing to assess the thermal stability of the samples by differential scanning calorimetry i.e. relaxation state, and investigate a possible finer effect of rescanning on the thermal history of the samples. The present sample mass of few mg do not permit to measure

representative thermal properties by such means.

Locally where the asperities are very pronounced, the 4^th^ and 8^th^ laser scans have shown to still permit to smoothen the layer out a little bit more. Still, under these processing conditions and for this alloy, it is not beneficial to apply more than 2 laser scans per layer to increase the overall density of the samples, even if some further local consolidation mechanisms could be observed.

In summary, the top surface changes only slightly from its original state after the 2^nd^ laser scan. From this state, the absorption of the laser energy is expected to decay with the better consolidation of the layer, as multiple reflections are reduced on the smoother surface [30], that explains the stable surface aspect and porosity levels with subsequent rescans. This laser absorption decay shall be quantified in future work.

The rescanning strategy appears to have an overall smoothening effect on the entire top surface, reducing the depth of surface asperities, which can lead to powder particle pockets and in turn large l-o-f defects formation. Future work shall address the possible side effects of multiple scanning on chemical homogeneity and residual stresses. In addition, it would be of interest to study the influence of the time imposed between the laser scans on heat accumulation in order to identify best practices for industrial production. Herein, sufficient time is provided between
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Volume renderings of an ROI located around the top surface of the processed volumes depicting the consolidation evolution for the 4 printed samples, from top to bottom: the single scan sample, the rescanned sample, the samples scanned 4 and 8 times. For the rescanned sample, one slice perpendicular to the laser scanning direction is displayed before and after the laser rescan.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 8.** X-ray diffraction patterns collected on the bottom polished surfaces of the Zr-Cu-Al-Nb samples scanned one to eight times.</figcaption></figure>

each laser scan for all parts to cool down. Still, the present results confirm that guidelines for the production of glass samples on a conventionally sized LPBF system should include a laser rescanning strategy on each layer in order to achieve high density. Future work includes repeatability testing of the hereby presented laser rescanning strategy on larger industrial machine, as well as a more in depth study of its influence on the structural and mechanical properties of the produced glass.

The rescanning approach is promising for metallic glasses other than the Zr-Cu-Al-Nb grade that are more sensitive to devitrification under the laser heat input, and with higher porosity level after one laser scan. It is likely that further density improvement at low power can also be achieved, for example by modifying the hatch spacing effect or by introducing an offset of the laser pattern position, even between

consecutive laser pass within the same layer. Additionally, the absorption decay may be compensated by increasing the energy density of the rescanning or adjusting the laser focus position. Furthermore, the recent simulation work of Yang *et al.* [31] provides additional inspiration for the optimisation of scanning strategies. The authors demonstrate that the heat accumulation and the crystallisation can be controlled by implementing a line jumping approach, which entails rearranging the sequence of laser passes of a single layer. This method allows for the restriction of adjacent reheating. Finally, the present work shows that advanced laser scanning strategies can be developed to control the size, shape and spacing of porosities. Future work to model the consolidation mechanisms active during laser rescanning of metallic glasses will support the rapid development of manufacturing strategies for these challenging materials to process, complementing their rapid evaluation thanks to in situ synchrotron XCT and LPBF replicator systems as demonstrated herein.

## 4. Conclusions

Focused on the well-established Zr₅₉.₃Cu₂₈.₈Al₁₀.₄Nb₁.₅ (at.%) grade, the present study is a proof-of-concept to have a better understanding of the strategy of laser rescanning towards the consolidation challenge of BMGs processed by LPBF. This process is constrained by a narrow parameter window and the part's density is limited by l-o-f porosities. This hinders the further development of the metallic glass portfolio for AM with less stable grades. In such a context, laser rescanning can be a fruitful way to reduce porosity while minimizing crystallization. Thanks to synchrotron XCT, the origin of the l-o-f porosity was clarified and in situ consolidation mechanisms were identified. The main outcomes are as follows:

* In the present processing regime, the porosity originates primarily from surface asperities of the last deposited layer caused by the hatch spacing effect and exacerbated by the high viscosity of the glass and the low energy laser parameters employed. Locally powder particles accumulate in pits forming pockets larger than the nominal layer thickness of 20 µm.
* The 1st laser rescan smoothes the rough top surface of the last consolidated layer. It allows for a more even powder recoating of the next layer, limiting the formation of large l-o-f defects.

* The subsequent laser rescans have a limited effect on the surface asperities and thus overall density, as the surface reflectivity increases.

The consolidation mechanism observed is interesting to be tailored for future less stable glasses, starting with a lower initial density (because of a higher constraint on energy input or a higher viscosity) and for tailored porosity distributions such as for ductility control. Some optimisation strategies envisioned include increasing laser energy of the rescan, adjusting the laser focus, the scanning pattern, sequence and rotation.

### CRediT authorship contribution statement

**Camille Pauzon:** Writing – review & editing, Writing – original draft, Visualization, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Rémi Daudin:** Writing – review & editing, Validation, Supervision, Methodology, Investigation, Formal analysis, Conceptualization. **Pierre Lhuissier:** Visualization, Software, Resources, Methodology, Data curation, Conceptualization. **Xavier Bataillon:** Resources, Methodology, Investigation. **Pierre Lapouge:** Resources, Methodology, Investigation. **Pierre Hébrard:** Resources, Methodology, Investigation. **Patrice Peyre:** Resources. **Frédéric Coste:** Resources. **Lucas Varoto:** Investigation. **Elodie Boller:** Resources, Methodology, Investigation, Formal analysis. **Jean-Jacques Blandin:** Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization.

### Declaration of competing interest

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

### Acknowledgments

Funded by the European Union under the grant agreement "ROSAMA2" n°101059435 (MSCA-PF). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or REA. Neither the European Union nor the granting authority can be held responsible for them. This work also benefited from the investments made by the Center of Excellence of Multifunctional Architectured Materials "CEMAM" n°AN-10-LABX-44-01, and from the facilities available on the characterization platform CMTC of Grenoble INP-UGA. We give a special thanks to Stéphane Coindeau for his expertise and support with the post-mortem XRD of the samples. The beamtime at the ESRF on the BM18 beamline was granted in the frame of the LTP MA4928: Correlative Imaging for Additive Manufacturing of Metallic Materials (CIAM3). The data collected during the beamtime has for digital identifier of object the following "doi.esrf.fr/10.15151/ESRF-ES-1106935016".

### Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.addlet.2025.100318.

### Data availability

Data will be made available on request.

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