Laser-based additive manufacturing device and method for crystallization control of metallic glasses

Abstract

The present invention relates to a novel method for laser power bed fusion (LPBF) manufacturing of three-dimensional (3D) objects from metallic glasses, wherein the crystalline fraction of the resulting 3D printed object can be controlled by optimizing one or more LPBF process parameters prior to printing. In a further aspect, the invention also provides an LPBF printing device adapted to carry out the method.

Description

[0001] DESCRIPTION

[0002] LASER-BASED ADDITIVE MANUFACTURING DEVICE AND METHOD FOR CRYSTALLIZATION CONTROL OF METALLIC GLASSES

[0003] FIELD OF THE INVENTION

[0004] The present invention belongs to the technical field of additive manufacturing. More specifically, the invention relates to a laser-based power bed fusion (LPBF) method for processing metallic glasses to control or avoid the creation of crystalline regions, as well as to a corresponding LPBF device configured to carry out such a method.

[0005] BACKGROUND OF THE INVENTION

[0006] Metallic glasses (MGs) are materials with high potential of application to a wide number of fields, such as aerospace, biomedicine, precision manufacturing, communication electronics, and chemical catalysis. Their homogeneous amorphous microstructure, free of crystalline defects, provides them with high strength and elasticity, hardness, and low hysteresis losses. The absence of grain boundaries also leads to excellent corrosion and wear resistance. However, a number of significant obstacles stand in the way of the extensive structural uses of MGs, including the difficulty of producing parts with large sizes and complex geometries due to their poor glass forming ability (i.e., their tendency to crystallize upon solidification), as well as their low ductility.

[0007] MGs are often produced by cooling metallic liquids sufficiently fast to avoid crystallization. This need limits the manufacturing of MGs to relatively thin geometries (wires, thin ribbons, and powders) using techniques such as vapor deposition, melt spinning, and atomization. These geometries are not suitable for structural applications. Hence, the key to the development of the MG field depends on figuring out how to overcome the size and shape constraints posed by the required high cooling rates.

[0008] Powder-based additive manufacturing (AM) methods, such as laser powder bed fusion (LPBF) (also known as selective laser melting, laser-based powder bed fusion or direct melting laser sintering), have attracted great attention from researchers to achieve freeform high-performance metallic glasses. Particularly in LPBF, a layer of metal powder is melted and solidified using a laser to create a solid layer of metal, with consecutive 2D layers being joined to preceding layers to build up a fully solid 3D part according to a 3D computer-aided design model. This layer-by-layer deposition, combined with extremely high cooling rates (approximately of 105– 108K / s), makes LPBF a viable solution to overcome the size and geometry limitations associated with metallic glasses processing.

[0009] Nonetheless, it is widely recognized that AM, and hence, LPBF, have limitations for the processing of metallic glasses. These include:

[0010] the generation of residual stresses and internal defects, affecting the quality of the final part;

[0011] the extension of the Heat Affected Zone (HAZ) originated in the vicinity of the processed layer to the cold layers beneath it, resulting often in undesired devitrification; and

[0012] process parameters dependent on the shape and size of the MG part to be printed, which, despite using the same material, lead to experimentally time-consuming parameters optimizations for achieving certain mechanical properties, electromagnetic behavior, and (micro)structure (including the ratio between crystalline / amorphous fractions) in the printed part.

[0013] Crystallization control of MGs during AM has then become an important issue in this field, particularly for MGs containing ferromagnetic transition metals (i.e., Fe, Co, or Ni), valued for their soft magnetic and mechanical properties, and for Zr-based MGs prized for their strength, hardness, and corrosion resistance, both of which generally exhibit limited glassforming ability.

[0014] BRIEF DESCRIPTION OF THE INVENTION

[0015] The present invention provides an adaptive scanning strategy for laser-based powder bed fusion (LPBF) processing of metallic glasses, enabling control over the resulting crystalline fraction of the printed part by optimizing one or more LPBF process parameters prior to printing.

[0016] Accordingly, in a first aspect, the present invention relates to a method for controlling crystallization in LPBF printing of a three-dimensional (3D) object from a metallic glass, wherein said method comprises performing the following steps in any technically possible order:

[0017] a) providing a metallic glass; b) creating a 3D CAD model for the 3D object to be printed;

[0018] c) defining at least one LPBF process parameter selected from: laser power, laser scan speed, hatch distance, powder layer thickness, laser spot size, laser exposure time, laser point distance, laser duty cycle, and time between adjacent laser scan tracks; d) defining a laser scan path;

[0019] e) defining at least one crystallization metric at which crystallization of the metallic glass provided in step a) is expected to occur, selected from: cooling rate, solidification rate, and incubation time, wherein incubation time is the amount of time during which a point of the metallic glass remains within a temperature range between the crystallization temperature and the liquidus temperature of said metallic glass;

[0020] f) simulating the LPBF printing of the 3D object with a shape and size according to the 3D CAD model defined in step b), under the LPBF process parameters defined in step c) and following the laser scan path defined in step d), using a thermo- metallurgical predictive model adapted to:

[0021] f.1) calculate a temperature evolution within a build region of the object (1) during LPBF printing through a heat transfer equation; and

[0022] f.2) calculate, based on the temperature evolution calculated in step f.1), at least one crystallization metric selected from: cooling rate, solidification rate, and incubation time;

[0023] g) comparing the simulated crystallization-metric value / s obtained in step f) with the crystallization-metric value / s defined in step e);

[0024] h) adjusting the LPBF process parameter / s defined in step c) according to the comparison performed in step g) so as to obtain a 3D printed object with a controlled crystalline fraction;

[0025] i) printing the 3D object with a shape and size according to the 3D CAD model defined in step b), under the LPBF process parameter / s adjusted in step i) and following the laser scan path defined in step d).

[0026] In a preferred embodiment of the method of the invention, step a) comprises providing a metallic glass in powder-form.

[0027] In another preferred embodiment of the method of the invention, the metallic glass comprises a ferromagnetic transition metal-based alloy or a Zr-based alloy. More preferably, a Fe- or Zr-based alloy. In another preferred embodiment of the method of the invention, step c) comprises defining at least one LPBF process parameter selected from:

[0028] a laser power between 20 and 500 W;

[0029] a laser scan speed between 100 and 4000 mm / s;

[0030] a hatch distance between 10 and 150 µm;

[0031] a powder layer thickness between 10 and 70 µm;

[0032] a laser spot size between 10 and 500 µm;

[0033] a laser exposure time between 20-1000 µs;

[0034] a laser point distance between 10 and 100 µm;

[0035] a laser duty cycle between 70-100%; and

[0036] a time between adjacent laser scan tracks of up to 1000 ms.

[0037] More preferably, step c) comprises defining at least one LPBF process parameter selected from:

[0038] a laser power between 35 and 80 W;

[0039] a laser scan speed between 300 and 700 mm / s;

[0040] a hatch distance between 60 and 150 µm;

[0041] a powder layer thickness between 25 and 50 µm;

[0042] a laser spot size between 60 and 90 µm;

[0043] a laser exposure time between 100-250 µs;

[0044] a laser point distance between 50 and 90 µm;

[0045] a laser duty cycle between 90-100%; and

[0046] a time between adjacent laser scan tracks between 1-30 ms.

[0047] In particular embodiments of the invention, step c) comprises defining at least the following LPBF process parameters: laser power, hatch distance, powder layer thickness, laser spot size, and time between adjacent laser scan tracks. In another particular embodiment of the invention, step c) further comprises defining one or more of the following LPBF process parameters: laser exposure time, laser point distance, and laser scan speed.

[0048] In particular embodiments of the method of the invention, step d) comprises defining a laser scan path according to any of the following scan strategies: stripes, chessboard, bidirectional, meander, double-scan, or point-random. Preferably, step d) comprises defining a laser scan path following a meander scan strategy, wherein neighboring vectors of the laser scan path are scanned with a constant hatching distance in the opposite direction over the entire metallic glass powder layer deposited on a build platform of an LPBF device. More preferably, the laser scan path is of meander type with a rotation of the laser scan direction between 60-90° after each deposited metallic glass powder layer.

[0049] In another preferred embodiment of the method of the invention, step i) comprises performing an optimization of the LPBF process parameters using a training model according to a machine learning or an artificial intelligence algorithm, wherein the training model comprises a dataset including a combination of LPBF processing parameters, 3D object’s shapes and sizes, and resulting crystalline fraction.

[0050] In another preferred embodiment of the method of the invention, step e) is performed based on at least one selected from:

[0051] the phase change diagram of the metallic glass;

[0052] the crystallization temperature and the liquidus temperature of the metallic glass; and

[0053] experimental calibration based on measurements of crystalline fractions and / or melt-pool and heat-affected-zone (HAZ) dimensions obtained from 3D-printed objects manufactured using different combinations of LPBF process parameters.

[0054] In a second aspect, the invention relates to an LPBF device for printing a 3D object from a metallic glass comprising:

[0055] a metallic glass powder reservoir, adapted for containing a plurality of metallic glass powder particles;

[0056] dispensing means, connected to the metallic glass powder reservoir and adapted for depositing a plurality of consecutive layers of the metallic glass powder particles contained in the reservoir on a build platform;

[0057] laser emission means, adapted for emitting a laser beam over the deposited consecutive layers of the metallic glass powder particles on the build platform; and a chamber, adapted for containing the metallic glass powder reservoir, the dispensing means, the build platform and the laser emission means, and for maintaining a controlled atmosphere during the LPBF printing of the 3D object.

[0058] Advantageously, said device further comprises a control unit, communicatively connected to the metallic glass powder reservoir, the dispensing means and the laser emission means, wherein said control unit is adapted to implement the method of the invention. Said computer unit comprises:

[0059] design modelling means for creating a 3D computer-aided design (CAD) model of the 3D object to be printed;

[0060] LPBF process parameters selecting means for selecting one or more LPBF process parameters;

[0061] laser scan path selecting means for selecting a scan path for the laser beam emitted by the laser emission means; and,

[0062] software means adapted for simulating the LPBF printing of the 3D object with a shape and size according to the 3D CAD model defined by the design modelling means, under the LPBF process parameters defined by the LPBF process parameters selecting means and following the laser scan path defined by the laser scan path selecting means, so as to adjust the LPBF process parameters to print a 3D object with a controlled crystalline fraction.

[0063] In a third aspect, the invention relates to a computer program comprising instructions which, when executed in the control unit of the LPBF device herein described, enables performing either steps b) to h) or steps b) to i) of the method of the invention.

[0064] Finally, the invention relates to the following additional aspects:

[0065] 1. A laser-based powder bed fusion (LPBF) device for printing a three-dimensional (3D) object (1) from a metallic glass comprising:

[0066] - a metallic glass powder reservoir (2), adapted for containing a plurality of metallic glass powder particles (3);

[0067] - dispensing means (4), connected to the metallic glass powder reservoir (2) and adapted for depositing a plurality of consecutive layers of the metallic glass powder particles (3) contained in the reservoir (2) on a build platform (5);

[0068] - laser emission means (6), adapted for emitting a laser beam (6’) over the deposited consecutive layers of the metallic glass powder particles (3) on the build platform (5);

[0069] - a chamber (7), adapted for containing the metallic glass powder reservoir (2), the dispensing means (4), the build platform (5) and the laser emission means (6), and for maintaining a controlled atmosphere during the LPBF printing of the 3D object (1);

[0070] and characterized in that said device further comprises a control unit (8), communicatively connected to the metallic glass powder reservoir (2), the dispensing means (4) and the laser emission means (6), wherein said control unit (8) further comprises: - design modelling means (9) for creating a 3D computer-aided design (CAD) model of the 3D object (1) to be printed;

[0071] - LPBF process parameters selecting means (10) for selecting one or more LPBF process parameters;

[0072] - laser scan path selecting means (11) for selecting a scan path for the laser beam (6’) emitted by the laser emission means (6); and,

[0073] - software means (12) adapted for simulating the LPBF printing of the 3D object with a shape and size according to the 3D CAD model created by the design modelling means (9) under the LPBF process parameters defined by the LPBF process parameters selecting means (10) and following the laser scan path defined by the laser scan path selecting means (11), so as to adjust the LPBF process parameters to print a 3D object (1) with a determined crystalline fraction.

[0074] A method for LPBF printing a 3D object (1) from a metallic glass by means of the device according to claim 1, characterized in that said method comprises performing the following steps in any technically possible order:

[0075] a) providing a metallic glass;

[0076] b) creating a 3D CAD model of the 3D object (1) to be printed by the design modelling means (9);

[0077] c) defining a plurality of LPBF process parameters by the LPBF process parameters selecting means (10);

[0078] d) defining a laser scan path by the laser scan path selecting means (11);

[0079] e) setting one or more of the following metrics at which crystallization of the metallic glass provided in step a) is expected to occur: cooling rate, solidification rate, and / or incubation time, wherein incubation time is the amount of time that the metallic glass is within a temperature range limited by the crystallization temperature and the liquidous temperature of said metallic glass;

[0080] f) simulating by the software means (12) the LPBF printing of the 3D object (1) with a shape and size according to the 3D CAD model defined in step b) under the LPBF parameters defined in step c) and following the laser scan path defined in step d); g) calculating the simulated values for cooling rate, solidification rate and / or incubation time based on the local thermal profiles simulated in step f);

[0081] h) comparing the simulated values obtained in step g) with the values set in step e); i) adjusting the LPBF process parameters defined in step c) according to the comparison performed in step h) so as to obtain a 3D printed object (1) with a determined crystalline fraction; j) printing the 3D object (1) with a shape and size according to the 3D CAD model defined in step b) under the LPBF parameters adjusted in step i) and following the laser scan path defined in step d).

[0082] 3. The method according to claim 2, wherein step a) comprises providing a metallic glass in powder-form.

[0083] 4. The method according to any of claims 2 or 3, wherein the metallic glass comprises a ferromagnetic transition metal-based alloy.

[0084] 5. The method according to claim 4, wherein the metallic glass comprises a Fe-based alloy.

[0085] 6. The method according to any of claims 2 to 5, wherein step c) comprises defining one or more of the following LPBF process parameters: laser power, laser scan speed, hatch distance, powder layer thickness, laser spot size, laser exposure time, laser point distance, laser duty cycle, and / or time between adjacent laser scan tracks.

[0086] 7. The method according to claim 6, wherein step c) comprises defining the following LPBF process parameters: laser power, hatch distance, powder layer thickness, laser spot size, laser exposure time, laser point distance, and time between adjacent laser scan tracks.

[0087] 8. The method according to claim 6, wherein step c) comprises defining:

[0088] - a laser power between 20 and 500 W; and / or

[0089] - a laser scan speed between 100 and 4000 mm / s; and / or

[0090] - a hatch distance between 10 and 100 µm; and / or

[0091] - a powder layer thickness between 10 and 70 µm; and / or

[0092] - a laser spot size between 10 and 500 µm; and / or

[0093] - a laser exposure time between 20-1000 µs; and / or

[0094] - a laser point distance between 10 and 100 µm; and / or

[0095] - a laser duty cycle between 70-100%; and / or

[0096] - a time between adjacent laser scan tracks between 0-1000 ms.

[0097] 9. The method according to claim 7, wherein step c) comprises defining:

[0098] - a laser power between 35 and 65 W; and / or - a laser scan speed between 300 and 700 mm / s; and / or

[0099] - a hatch distance between 60 and 90 µm; and / or

[0100] - a powder layer thickness between 25 and 50 µm; and / or

[0101] - a laser spot size between 60 and 90 µm; and / or

[0102] - a laser exposure time between 100-250 µs; and / or

[0103] - a laser point distance between 50 and 90 µm; and / or

[0104] - a laser duty cycle between 90-100%; and / or

[0105] - a time between adjacent laser scan tracks between 1-30 ms.

[0106] The method according to any of claims 2 to 9, wherein step d) comprises defining a laser scan path according to any of the following scan strategies: stripes, chessboard, bi-directional, meander, double-scan, or point-random.

[0107] The method according to claim 10, wherein step d) comprises defining a laser scan path following a meander scan strategy, wherein neighboring vectors of the laser scan path are scanned with a constant hatching distance in the opposite direction over the entire metallic glass powder layer deposited on the build platform (5).

[0108] The method according to claim 11, wherein in step d) a laser scan path of meander type with a rotation of the laser scan direction between 60-90° after each deposited metallic glass powder layer on the build platform (5) is defined.

[0109] The method according to any of claims 2-12, wherein steps f) to h) are performed using a thermo-metallurgical model.

[0110] The method according to any of claims 2-13, wherein step i) comprises performing an optimization of the LPBF process parameters using a training model according to a machine learning or an artificial intelligence algorithm, wherein the training model comprises a dataset including a combination of LPBF processing parameters, 3D object’s (1) shapes and sizes, and resulting crystalline fraction.

[0111] A computer program comprising instructions which, when executed in the control unit (8) of a LPBF device according to claim 1, enables performing:

[0112] steps b) to h) of a method according to any of claims 2 to 13; or

[0113] steps b) to i) of a method according to claim 14. All the terms and embodiments described anywhere in this document are equally applicable to all aspects of the invention. It should be noted that, as used in the specification and in the appended claims, the singular forms “a”, “an”, and “the” include their plural referents unless the context clearly indicates otherwise. Similarly, the term “comprises” or “comprising” as used herein also describes “consists of” or “consisting of” in accordance with generally accepted patent practice.

[0114] DESCRIPTION OF THE DRAWINGS

[0115] The foregoing and other features and advantages will be more fully understood from the detailed description of the invention, as well as from examples referring to the attached figures, which are described in the following paragraphs, wherein:

[0116] Figure 1 shows the device of the invention according to one of its preferred embodiments.

[0117] Figure 2a shows maximum incubation time and experimentally measured crystalline fraction of a metallic glass part for different laser exposure time values calculated using the method of the invention. Figure 2b shows the relationship between the simulation performed by the method of the invention and experimental results.

[0118] Figure 3 shows: (a) LPBF-printed cubic samples of a Fe-based metallic glass with different sizes and different jump delays assigned, (b, c) Close view of 4 mm samples printed with a jump delay of 1 ms which show severe warping (d, e) Close view of 2 mm samples printed with a jump delay of 1 ms which failed due to excessive warping during the process.

[0119] Figure 4 shows a comparison between simulated incubation time and experimentally measured crystalline fraction for samples of different sizes and increasing jump delays. The error bars correspond to the + / - 3% measurement error in crystalline fraction from the instrument used to quantify the crystalline fraction of LPBF-printed samples.

[0120] Figure 5 illustrates Example 3, showing local crystallization reduction in a Zr-based metallic glass by the method of the invention: (a) Cylindrical tensile test specimen LPBF-printed from a Zr-based metallic glass; (b) Local crystallization observed in the printed specimen by optical microscopy; and (c) simulation of the printed geometry, showing the predicted crystallization patterns. Figure 6 illustrates the results of Example 3, showing the correlation between experimental measurements and simulations for local crystallization reduction in a Zr-based metallic glass. The crystallization enthalpy and the incubation time are presented for the unoptimized sample (Sample 1) and the optimized sample (Sample 2), demonstrating the effect of LPBF process parameter optimization according to the method of the invention.

[0121] NUMERICAL REFERENCES USED IN THE DRAWINGS

[0122] In order to provide a better understanding of the technical features of the invention, Figure 1 is accompanied by a series of numerical references which, with an illustrative and nonlimiting character, are hereby represented:

[0123] 1 3D printed object

[0124] 2 Metallic glass powder reservoir

[0125] 3 Metallic glass powder particles

[0126] 4 Dispensing means

[0127] 5 Build platform

[0128] 6 Laser emission means

[0129] 6’ Laser beam

[0130] 7 Chamber

[0131] 8 Control unit

[0132] 9 Design modelling means

[0133] 10 LPBF process parameters selecting means

[0134] 11 Laser scan path selecting means

[0135] 12 Software means

[0136]

[0137] DETAILED DESCRIPTION OF THE INVENTION

[0138] The present invention provides a novel method for laser powder bed fusion (LPBF) manufacturing of three-dimensional (3D) objects (1) from metallic glasses, wherein the crystalline fraction of the resulting 3D printed object (1) can be controlled by using the optimal combination of LPBF process parameters found by simulation prior printing. In a further aspect, the invention also provides an LPBF printing device adapted to carry out the method.

[0139] In the context of the instant invention, the terms “3D object” or “3D printed object” shall be understood as a LPBF-printed solid shape from a metallic glass that have three dimensions including length, depth and width. The terms “object” and “part” will be used interchangeably throughout this text.

[0140] Likewise, the term “metallic glass” shall be understood as a material with a completely or partially disordered atomic-scale structure, wherein said material comprises either a pure metal, an alloy (i.e., a combination of two or more metallic elements), or a combination of one or more metallic elements and one or more metalloids. The term “metallic glass” comprises, without limitation, Fe-, Co-, Ni-, Zr-, AI-, Cu-, Pd-, La-, Mg-, Y-, Pt-, Ce-, Nd-, Ca-, Hf-, Sm-, Au-, Pr-, Sr-, Zn-, AI-, and Ti-based alloys. In particular embodiments, the metallic glass is selected from Fe-, Co-, Ni-, Zr-, AI-, Cu-, Pd-, La-, Mg-, Y-, Pt-, Ce-, Nd-, Ca-, Hf-, Sm-, Au-, Pr-, Sr-, Zn-, AI-, and Ti-based alloys. Preferably, the metallic glass comprises a ferromagnetic transition metal as the main constituent (i.e., Fe-, Co-, or Ni) or a Zr-based alloy.

[0141] Specific examples of metallic glasses suitable for the present invention include, but are not limited to: Fe-P-B, (Fe, Co, Ni)-Si-B, (Fe, Co, Ni)-Mo-C, (Fe, Co, Ni)-Zr-B, (Fe, Co, Ni)-Hf-B, (Fe, Co, Ni)-Nb-B, Fe-Co-P-B, Fe-Co-Si-B, Fe-Si-B, Fe-Zr-B, Fe-Hf-B, Fe-Ti-B-Cu, Fe-Zr-B-Cu, Fe-Hf-B-Cu, Fe-Nb-B-Cu, Fe-Ta-B-Cu, (Fe, Co)-Nb-B-Cu, (Fe, Co)-Hf-B-Cu, (Fe, Co)-Zr-B-Cu, Fe-(Nb, Cr, Mo)-(AI, Ga)-(P, B, C), Fe-Co-(Zr, Hf, Nb)-(Mo, W)-B, Fe-Co-Nd-Dy-B, Fe-Co-Ga-(P, C, B), Fe-(Co, Cr, Mo, Ga)-(P, C, B), Fe-Ni-P-B, Fe-B-Si-(Zr, Nb), Fe-(Co, Cr, Mo, Ga, Sb)-C-B-P, Fe-Ni-P-B, or any combination thereof. More preferably, the metallic glass comprises a Fe- or a Zr-based alloy. In a particular embodiment of the invention, the metallic glass is Fe73.7B11Si11Cr2.3C2[atomic, at., %]. In another particular embodiment of the invention, the metallic glass is Zr59.3Cu28.8Al10.4Nb1.5[at%].

[0142] The method of the invention initially requires a selection of at least one LPBF process parameter, as well as the definition of a laser scan path to be followed during printing.

[0143] Within the scope of interpretation of the present invention, the term “LPBF process parameter” refers to any measurable input element in a laser powder bed fusion process that affects the process output. The term “LPBF process parameter” includes any of the following parameters:

[0144] laser power (P), i.e., the energy delivered by the laser beam per unit of time and per unit of area. In particular embodiments, the P is initially set between 20-500 W, between 30-250 W, or between 35-160 W. Preferably, the P is initially set between 35-80 W. In particular embodiments, the P is initially set at 20 W, 30 W, 35 W, 40 W, 50 W, 60 W, 65 W, 80 W, 100 W, 160 W, 250 W, or 500 W.

[0145] laser scan speed (v), i.e., how fast the laser beam moves across the metallic glass powder layer. In particular embodiments, the v is initially set between 100-4000 mm / s, between 250-2000 mm / s, or between 300-1000 mm / s. Preferably, the v is initially set between 300-700 mm / s. In particular embodiments, the v is initially set at 100 mm / s, 250 mm / s, 300 mm / s, 350 mm / s, 500 mm / s, 650 mm / s, 900 mm / s, 1000 mm / s, 2000 mm / s, or 4000 mm / s.

[0146] hatch distance (h), i.e., the length between the centre of sequential laser scan tracks during the LPBF process. In particular embodiments, the h is initially set between 10-150 pm, between 20-150 pm, or between 40-150 pm. Preferably, the h is initially set between 60-150 pm. In particular embodiments, the h is initially set at 10 pm, 20 pm, 30 pm, 40 pm, 50 pm, 60 pm, 80 pm, 90 pm, 100 pm, 130 pm or 150 pm. powder layer thickness (t), i.e., how thick is the metallic glass powder material dispensed over the build platform. In particular embodiments, the t is initially set between 10-70 pm, between 20-60 pm, or between 25-50 pm. Preferably, the t is initially set between 25-50 pm. In particular embodiments, the t is initially set at 10 pm, 20 pm, 25 pm, 30 pm, 40 pm, 50 pm, 60 pm or 70 pm.

[0147] laser spot size, i.e., the diameter of the laser beam measured at the exit face of the laser emission means (6). In particular embodiments, the laser spot size is initially set between 10-500 pm, between 50-150 pm, or between 60-90 pm. Preferably, the laser spot size is initially set between 60-90 pm. In particular embodiments, the laser spot size is initially set at 10 pm, 20 pm, 30 pm, 40 pm, 50 pm, 60 pm, 62 pm, 65 pm, 80 pm, 85 pm, 90 pm, 150 pm, or 500 pm.

[0148] laser exposure time (ET), i.e., the period that the laser beam is heating a metallic glass layer dispensed by the dispensing means over the build platform. In particular embodiments, the ET is initially set between 20-1000 ps, between 50-500 ps, or between, 80-300 ps. Preferably, the ET is initially set between 100-250 ps. In particular embodiments, the ET is initially set at 20 ps, 50 ps, 80 ps, 100 ps, 120 ps, 150 ps, 220 ps, 250 ps, 500 ps, or 1000 ps. laser point distance (PD), i.e., the distance between two consecutive laser exposures over a metallic glass layer. In particular embodiments, the PD is initially set between 10-100 pm, or between 50-90 pm. Preferably, the PD is initially set between 50-90 pm. In particular embodiments, the PD is initially set at 10 pm, 20 pm, 30 pm, 40 pm, 50 pm, 60 pm, 70 pm, 80 pm, 90 pm, or 100 pm.

[0149] laser duty cycle (DC), i.e., the amount of time that the laser emission means are actively emitting laser light. In particular embodiments, the DC is initially set between 70-100%, or between 90-100%. Preferably, the DC is initially set between 90-100%. In particular embodiments, the DC is initially set at 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%.

[0150] - jump delay, i.e., time between adjacent laser scan tracks. In particular embodiments, the time between adjacent laser scan tracks is initially set up to 1000 ms, between 1-500 ms, between 1-300 ms, or between 1-50 ms. Preferably, the jump delay is initially set between 1-30 ms. In particular embodiments, the time between adjacent laser scan tracks is initially set at 1 ms, 5 ms, 5.5 ms, 6.5 ms, 7.5 ms, 9 ms, 10 ms, 12.5 ms, 13 ms, 14.5 ms, 15 ms, 18 ms, 21.5 ms, 25 ms, 50 ms, 300 ms, 500 ms, or 1000 ms.

[0151] In a preferred embodiment of the method of the invention, step c) comprises defining at least the following LPBF process parameters: laser power, hatch distance, powder layer thickness, laser spot size, and time between adjacent laser scan tracks. In particular embodiments of the invention, step c) further comprises defining one or more of the following LPBF process parameters: laser exposure time, laser point distance, and laser scan speed. In a particular embodiment, step c) further comprises defining laser exposure time and laser point distance. In a particular embodiment, step c) further comprises defining laser scan speed.

[0152] Within the scope of interpretation of the present invention, the term “laser scan path” will be understood as the spatial moving pattern of the laser beam over the scanning surface; i.e., the metallic glass powder layer. In particular embodiments, the laser scan path is selected from the following scan strategies: stripes, chessboard, bi-directional, meander, doublescan, or point-random [see Zrodowski, L., et al. New approach to amorphization of alloys with low glass forming ability via selective laser melting. J. Alloys Compd., 2019, vol. 771, p. 769-776], Preferably, the laser scan path is meander. Under this scan strategy, neighboring vectors of the laser scan path are scanned with a constant hatching distance in the opposite direction over the entire metallic glass powder layer. In particular embodiments, the laser scan path is meander with a rotation of the laser scan direction between 0°-200°, between 60°-180°, or between 60°-90° after the deposition of a new metallic glass powder layer on a build platform of a LPBF device. In particular embodiments, the laser scan direction is rotated 0°, 60°, 90°, or 180°. More preferably, the laser scan path is meander with a rotation of the laser scan direction between 60°-90°. In particular embodiments of the invention, the laser scan path is meander with a 67° rotation of the laser scan direction after the deposition of a new metallic glass powder layer.

[0153] In addition to defining the LPBF process parameters and the laser scan path, the method of the invention initially comprises designing and modelling the 3D object (1) to be printed, and subsequently slicing the 3D CAD model in the required number of layers according to the selected powder layer thickness (f)..

[0154] The next step of the method of the invention comprises simulating the LPBF printing process of the 3D object (1) with a shape and size according to the designed 3D CAD model, using the initially selected LPBF process parameters and following the previously defined laser scan path. A thermo-metallurgical model is employed to perform this simulation, predicting both the local temperature evolution and the expected crystallization behavior of the metallic glass.

[0155] The thermal part of the model computes the evolution of temperature in the 3D printed part (1) while a moving surface or volumetric heat source scans the cross section of the current printed layer during LPBF printing. The heat source is defined based on laser power, scan speed, exposure time, point distance, spot size, scan path, and the material properties of the metallic glass, such as density, absorptivity, specific heat capacity, latent heat, and thermal conductivity. The main objectives of the thermal simulation are to estimate the melt pool dimensions (i.e., the region of molten metallic glass near the laser / material interface) and to simulate the thermal profile in the Heat Affected Zone (HAZ; i.e., the area of the 3D printed object (1) that has not been melted but has undergone changes in properties due to exposure to elevated temperatures). The classical heat transfer equation is used to compute the temperature evolution in the metallic glass, as shown in Equation [1]:

[0156] ρcp∂T / ∂t = ∇ · (k∇T) + Ql+ Qø

[0157]

[0158] [1] where p is the density, cpis the specific heat capacity, k is the conductivity, T is the temperature, t is time, QLis the absorbed heat due to the laser incidence, and Q0is the absorbed / released heat due to melting / solidification of the metallic glass. The total heat source term therefore accounts for both laser input and phase- change contributions.

[0159] The laser source contribution, Qi is computed as follows [2]:

[0160] Ql= αpGlIz / Hl[2] where αpis the laser absorptivity, Gl= (2P / πφ²) exp(-2((x-xl)²+(y-yl)²) / φ²) is the Gaussian

[0161]

[0162] 2

[0163] distribution at the irradiated surface, Iz= (1 / 0.75)[-2.25((zl-z) / Hl)² + 1.5((zl-z) / Hl) + 0.75] is a parabolic decay along the laser penetration direction, Hlis the laser penetration depth, P is the laser power, φ is the diameter of the laser spot, and (xl, yl) are the coordinates of the center of the laser spot that irradiates the surface located at zl, referred to the Cartesian coordinate system (x, y, z).

[0164] The phase-change contribution, Q0, is computed as:

[0165] Qφ= ρLf∂fs / ∂t [3] where Lfis the latent heat of fusion and fsis the solid volume fraction that is computed by the metallurgical part of the model.

[0166] A Newton’s law boundary condition (Eq. [4]) is applied to couple the 3D part (1) with the build platform, surrounding powder particles and chamber’s atmosphere of an LPBF printing device:

[0167] qc= -hc(T - T∞) [4] where qcis the normal heat flux, hcis the interfacial heat transfer coefficient, and T∞is the temperature of the environment. The metallurgical part of the model assesses the formation of crystalline regions in the simulated 3D printed object (1) based on the local temperature profiles computed by the thermal part. It accounts for phase transformations of the metallic glass during the LPBF printing, including solid-to-liquid and liquid-to-solid transitions. During melting, the solid phase may be treated with the physical properties of either the metallic glass powder (to simulate the melting of powder particles) or a bulk metallic glass (to simulate remelting of previously consolidated tracks or layers). In this context, the term “bulk metallic glass” refers to non-particulate solid metallic glass material, preferably comprising a single consolidated layer or a stack of consolidated layers.

[0168] Crystallization is assessed based on the calculation of at least one crystallization metric selected from:

[0169] cooling rate: the rate at which the metallic glass decreases in temperature after laser exposure;

[0170] solidification rate: the rate at which the melt pool solidifies; and / or

[0171] incubation time: the amount of time during which a point of the metallic glass remains within a temperature range between the crystallization temperature (Tx) and the liquidus temperature (Ti) of said metallic glass.

[0172] Within the scope of the present invention the term “crystallization temperature” shall be interpreted as the temperature at which the amorphous solid structure of the metallic glass begins to transition into a crystalline phase upon heating. Likewise, the term “liquidus temperature” shall be understood as the temperature above which the material is fully molten and exists entirely in the liquid phase.

[0173] Critical values for these crystallization metrics (i.e., at which crystallization of the metallic glass is expected to occur) are set based on at least one selected from:

[0174] the phase change diagram of the metallic glass;

[0175] the crystallization temperature and the liquidus temperature of the metallic glass; and

[0176] experimental calibration based on measurements of crystalline fractions and / or melt-pool and heat-affected-zone (HAZ) dimensions obtained from 3D-printed objects manufactured using different combinations of LPBF process parameters.

[0177] The thermo-metallurgical model is solved over the entire 3D part (1) using analytical or numerical methods, such as finite elements, finite difference, finite volumes, and spectralbased methods. This approach allows prediction of crystalline region formation during LPBF printing by comparing the simulated crystallization-metric value / s with the corresponding critical value / s. Based on these predictions, the initially selected LPBF process parameters are then adjusted to ensure a controlled crystalline fraction in the 3D printed object (1). This can be done either manually or automatically. In the first case, the user of the method of the invention will evaluate the results of the simulations performed for the same 3D CAD model but with different combinations of LPBF process parameters, comparing them and choosing the combination that minimizes the above-mentioned crystallization metrics. In the latter case, a regression is performed by software means resulting in an optimal combination of the LPBF process parameters, whose values can even be exported in a file or printed.

[0178] Finally, after optimizing the initially selected LPBF process parameters according to the simulated results, the LPBF printing of the metallic glass 3D part (1) is performed.

[0179] In a second aspect, the invention relates to an LPBF device configured to carry out the method of the invention. A preferred embodiment of the device of the invention is schematically represented in Figure 1. Said device shares the following elements with a known LPBF device:

[0180] a metallic glass powder reservoir (2), adapted for storing a plurality of metallic glass powder particles (3).

[0181] dispensing means (4), connected to the metallic glass powder reservoir (2) and adapted for depositing a plurality of consecutive layers of the metallic glass powder particles (3) contained in the reservoir (2) on a build platform (5).

[0182] laser emission means (6), adapted for emitting a laser beam (6’) over the deposited consecutive layers of the metallic glass powder particles (3) on the build platform (5).

[0183] a chamber (7), adapted for containing the metallic glass powder reservoir (2), the dispensing means (4), the build platform (5) and the laser emission means (6), and for maintaining a controlled atmosphere during the printing process of the 3D object (1).

[0184] In the context of the instant invention, the term “reservoir” shall be understood as a hollow object used for storing a metallic glass powder. In different embodiments of the device of the invention, the metallic glass powder reservoir (2) can be, for instance, a hopper or a silo. Preferably, the metallic glass powder reservoir includes a piston or an embolus to push said particles towards the dispensing means (4). Likewise, the expression “controlled atmosphere” shall be understood as a gas or mixture of gases whose composition is such that minimizes the risk of metallic glass surface oxidation during the LPBF printing process. Preferably, said controlled atmosphere comprises vacuum, nitrogen, argon, hydrogen or any combination thereof. However, other gas or inert gases able to minimize the risk of material oxidation during the LPBF printing process can also be employed in the context of the invention.

[0185] In different embodiments of the device of the invention, the dispensing means (4) comprises a rubber, metal or ceramic wiper, a blade, a roller or a re-coater.

[0186] In different embodiments of the invention, the laser emission means (6) comprises one or more ytterbium fiber lasers (A = 1070-1080 nm), or one or more gallium nitride-based diode lasers (A = 360-521 nm). Under the scope of the present invention, the term “ytterbium fiber laser” shall be understood as a type of laser where the gain medium is a host material doped with ytterbium (Yb3+) ions. Likewise, the term “gallium nitride-based diode laser” refers to a type of semiconductor laser that utilizes gallium nitride (GaN) as the active medium to produce coherent light in the visible light spectrum. In a particular embodiment of the invention, the laser emission means (6) is a pulsed wave ytterbium fiber laser with a wavelength of 1075 nm or a continuous wave ytterbium fiber laser with a wavelength of 1080 nm.

[0187] Advantageously over the prior art LPBF devices, the device of the invention further comprises a control unit (8), communicatively connected to the metallic glass powder reservoir (2), the dispensing means (4) and the laser emission means (6), wherein said control unit (8) is adapted to implement the method herein described. The control unit (8) comprises:

[0188] design modelling means (9) for creating a 3D computer-aided design (CAD) model of the 3D object (1) to be printed;

[0189] LPBF process parameters selecting means (10) for selecting one or more LPBF process parameters;

[0190] laser scan path selecting means (11) for selecting a scan path for the laser beam (6’) emitted by the laser emission means (6); and,

[0191] software means (12) adapted for simulating the LPBF printing of the 3D object (1) with a shape and size according to the 3D CAD model created by the design modelling means (9), under the LPBF process parameters defined by the LPBF process parameters selecting means (10) and following the laser scan path defined by the laser scan path selecting means (11).

[0192] Thanks to this control unit (8), the device of the invention allows to perform LPBF process parameters optimization iterations in a simulated environment to control the crystalline fraction in the resulting 3D printed object (1), saving time, energy, material and operating costs. The simulations can give rise to different combinations of optimal LPBF process parameters depending on the dimensions and shape of the analyzed region of the 3D printed part (1).

[0193] The design modelling means (9) comprises commercially available CAD-type software. Said design modelling means (9) are selected from: AutoCAD, Autodesk Fusion, SOLIDWORKS, CATIA, Tinkercad, FreeCAD, Creo, BricsCAD, SketchUP, or Siemens NX. Nonetheless, other CAD-type software oriented to drawing and modeling in 3D can also be employed in the context of the invention.

[0194] In preferred embodiments of the invention, LPBF process parameters optimization is performed using a training model (local or cloud-based) according to a machine-learning (ML) or an artificial intelligence (Al) algorithm, which can also be fed and improved by the actual data obtained in a LPBF printing process using the device of the invention. In this way, the system can be further optimized in time, so as to obtain more precise LPBF process parameters. Said training model comprises a dataset that includes different combination of LPBF processing parameters according to the shape and size of the 3D part to be printed, and the resulting crystalline fraction. Among the machine learning algorithms that could be used are the following: tree-based models like Random Forests (RF) or Gradient Boosting Machines (GBM), Bayesian Optimization, genetic algorithms, multifidelity optimization, convolutional neural networks, fully connected neural networks or reinforcement learning. However, other ML or Al algorithm known in the state of the art can also be employed in the context of the invention.

[0195] Once the LPBF process parameters have been optimized, the 3D printing of the metallic glass 3D part (1) is carried out. A layer of metallic glass powder particles (3) from the reservoir (2) is deposited onto the build platform (5) by the dispensed means (4), and a laser beam (6’) is emitted by the laser emission means (6) over the deposited layer, scanning the layer following the laser scan path defined by the laser scan path selecting means (11). The layer is thereby melted and subsequently solidified at extremely high cooling rates (approximately of 105– 108K / s). Consecutive 2D layers are deposited and melted over the preceding layers to build a fully consolidated 3D object (1) in accordance with the 3D CAD design model created by the design modelling means (9).

[0196] In a third aspect, the invention relates to a computer program comprising instructions which, when executed in the control unit (8) of the LPBF device herein described, enables performing either steps b) to h) or steps b) to i) of the method of the invention.

[0197] EXAMPLES

[0198] The following invention is hereby described by way of the following examples, which are to be construed as merely illustrative and not limitative of the scope of the invention.

[0199] Example 1: LPBF printing of a 3D object from a Fe-based metallic glass by the method of the invention.

[0200] A Fe-based metallic glass powder with nominal composition Fe73.7B11Si11Cr2.3C2[at. %] was used for printing a 3D object with a conventional LPBF system equipped with a pulsed wave ytterbium fiber laser (A=1075 nm). The 3D CAD model used as input geometry was an 8 x 8 x 9.5 mm cuboid. The LPBF printing process was performed following a laser scan path of meander type with a 67° rotation among subsequent layers, and selecting the LPBF process parameters described below:

[0201] laser power (P) = 40 W;

[0202] hatch distance ( / ?) = 80 pm;

[0203] powder layer thickness (t) = 30 pm;

[0204] laser spot size = 62 pm;

[0205] laser exposure time (ET) = 220 ps;

[0206] laser point distance (PD) = 80 pm; and

[0207] - jump delay = 1 ms.

[0208] This combination of LPBF process parameters yielded a 3D object with a crystalline fraction of 89%.

[0209] By using the method of the invention, the simulation performed by the software means of the device of the invention indicated that, for the aforementioned combination of LPBF process parameters, the incubation time is 6·10-4s. The model suggests that, by reducing ET to 150 ps and 120 ps, while keeping all other LPBF process parameters constant, the incubation time is reduced to 5.15·10-4s and 4.45·10-4s, respectively.

[0210] The experimental validation consisted of printing a 3D part with the same geometry and LPBF process parameters except for the exposure time, where the new values of 150 ps and 120 ps were applied to the LPBF system instructions. The crystalline fractions measured on the resulting parts were 70% (for ET = 150 ps) and 54% (for ET = 120 ps). Figure 2a shows the numerically calculated incubation time and the experimentally measured crystalline fraction for increasing exposure time values. Figure 2b depicts the positive relationship between the simulation metric and the experimental results.

[0211] Hence, it can be concluded that the method of the invention can be successfully used to reduce the crystalline fraction and adapt LPBF processing parameters accordingly wherein the shape and size of the 3D object to be printed is fixed.

[0212] Example 2: Effect of 3D object’s size on LPBF process parameters

[0213] Several prism-shaped 3D objects with different square cross-sections (i.e., 8x8 mm, 6x6 mm, 4x4 mm, 2x2 mm) were printed from a Fe-based metallic glass powder with nominal composition Fe73.7B11Si11Cr2.3C2[at. %] using a conventional LPBF system equipped with a pulsed wave ytterbium fiber laser (A=1075 nm). The LPBF printing process was performed following a laser scan path of meander type with a 67° rotation among subsequent layers, and under the same LPBF process parameters, which are described below:

[0214] laser power (P) = 60 W;

[0215] hatch distance ( / ?) = 80 pm;

[0216] powder layer thickness (t) = 30 pm;

[0217] laser spot size = 85 pm.

[0218] laser exposure time (ET) = 150 ps; and

[0219] laser point distance (PD) = 80 pm.

[0220] Fig. 3(a) shows a build plate with the aforementioned Fe-based metallic glass samples after LPBF processing; in particular: one prism-shaped sample of 8 mm-side, five prism-shaped samples of 6 mm-side, ten prism-shaped samples of 4 mm-side, and twelve prism-shaped samples of 2 mm-side. The 8 mm-side sample is placed in the upper left corner of the build plate, and it is visible that the prism shape is retained after processing, suggesting that the combination of LPBF process parameters applied is within the viable range of LPBF process parameters (known as “processability window”). On this sample, the jump delay was 1 ms. When the sample size is reduced to 4 mm and the jump delay is kept constant along with the rest of the LPBF process parameters, the samples warp and the corners break off, as can be seen on Fig. 3 (b, c). Furthermore, the effect is more severe when the sample size is reduced to 2 mm, as depicted in Fig. 3 (d, e), where the samples failed during the printing process due to excessive warping. Further LPBF printings were performed varying the jump delay, while the above-mentioned LPBF process parameters were kept constant. Increasingly large jump delay values were given for samples with decreasing sizes in order to compensate for the heat accumulation. Specifically:

[0221] 6 mm-side samples were assigned jump delays of 5 ms, 5.5 ms, 6.5 ms, 7.5 ms; 4 mm-side samples were assigned jump delays of 9, 10, 12.5, 15 ms; and 2mm-side samples were assigned jump delays of 13, 14.5, 18, 21.5 and 25 ms.

[0222] 2mm- and 4 mm- side samples with higher jump delay values successfully retained their shape during the printing process. It becomes clear that smaller samples need an adjustment of LPBF process parameters, particularly the jump delay.

[0223] The numerical model of the method of the invention was used to calculate an incubation time which can be closely related to the crystalline fraction of printed samples. Fig. 4 presents the simulated incubation time (squares) for each sample size with respect to the jump delay values that were applied in the LPBF experiment. On each sample size plot, the measured crystalline fractions (circles) of printed samples are plotted with respect to the applied jump delays. The error bars represent the measurement error associated with the instrument used to quantify the crystalline fractions of printed specimens. The simulation results (labeled as ‘sims’) show a very close agreement with the experiments, being within the experimental error in all cases for samples in the 6- and 4-mm groups (Fig. 4 (a, b)), and in most cases for samples in the 2 mm group (Fig. 4(c)).

[0224] Therefore, the method of the invention has proven to be effective to adjust a LPBF process parameter that closely matches the degree of crystallization in LPBF-processed metallic glass parts of different geometries.

[0225] Example 3: Local crystallization reduction on aZr-based metallic glass by the method of the invention

[0226] A commercial Zr-based metallic glass with nominal composition Zr59.3Cu28.8Al10.4Nb1.5[at%] was processed by LPBF using a conventional LPBF system equipped with a continuous wave ytterbium fiber laser (A=1080 nm). The most relevant processing parameters used initially (sample 1) were:

[0227] laser power (P) = 70 W;

[0228] laser scan speed (v) = 400 mm / s

[0229] hatch distance (h) = 130 pm;

[0230] powder layer thickness (t) = 30 pm;

[0231] laser spot size = 85 pm;

[0232] - jump delay = 1 ms.

[0233] The scan path selected was meander with a 67° rotation among subsequent layers and the geometry of the sample was a cylindrical tensile test specimen with several changes in diameter along its height, as shown in Fig. 5a. This combination of parameters caused local crystallization, which was observed by optical microscopy and is illustrated in Fig. 5b. DSC measurements performed in two of the apparently affected border regions revealed an average enthalpy of crystallization of 3300 J / atom g.

[0234] The printed geometry was then simulated and a higher incubation time (T) was found in the regions where the diameter of the specimen increased, as depicted on Fig. 5c. The average calculated T in the critical regions identified on this specimen was 1.5 s.

[0235] The optimization was carried out according to the steps of the invention, varying several processing parameters. Finally, the following parameters were selected for sample 2: P = 50 W, v = 500 mm / s, h = 130 pm, t = 30 pm, laser spot size = 85 pm and jump delay = 1 ms. With this combination of parameters, the thermo-metallurgical model calculated T = 0.32 s in the same regions were T was calculated for Sample 1. Sample 2 was then fabricated using the optimized parameters and DSC measurements were performed in the same locations as on Sample 1. The resulting average enthalpy of crystallization was found to be 3475 J / atom g. This increase in enthalpy confirms that there has been an increase in amorphous volume, which crystallized during the DSC test. This suggest that, despite Sample 1 having a relatively high amorphous content, it was successfully improved using the current method. The correlation between experiments can be seen on Figure 6, where the crystallization enthalpy and the incubation time are presented for the unoptimized sample (Sample 1) and the optimized sample (Sample 2).

[0236] Note about the use of crystallization enthalpy: The most reliable method to calculate the amorphous or crystalline fraction is by means of differential scanning calorimetry (DSC) measurements. This technique consists of applying a constant heating rate to a small material sample and record the enthalpy peaks caused by phase transformations (such as transition from amorphous to crystalline). The enthalpy of crystallization (AH) can therefore be calculated for any partially or fully amorphous material. If one has a reference of a fully amorphous sample, such a melt-spun ribbon, the amorphous fraction of any semi-crystalline sample can be calculated, by taking the ratio of the crystallization enthalpies of the sample and the reference. In the case of using a ribbon as reference: AM% =ΔH / ΔH* 100. However, when the amorphous content of a sample

[0237] is either too high or too low, the quantification of AH is not very accurate, and the AM% cannot be easily calculated. This was the case in this example, where one can simply compare the crystallization enthalpies of different samples and compare those with the simulation result.

Claims

CLAIMS1. A method for controlling crystallization in laser-based powder bed fusion (LPBF) printing of a three-dimensional (3D) object (1) from a metallic glass, characterized in that said method comprises performing the following steps in any technically possible order: a) providing a metallic glass;b) creating a 3D CAD model of the 3D object (1) to be printed;c) defining at least one LPBF process parameter selected from: laser power, laser scan speed, hatch distance, powder layer thickness, laser spot size, laser exposure time, laser point distance, laser duty cycle, and time between adjacent laser scan tracks;d) defining a laser scan path;e) defining at least one crystallization metric at which crystallization of the metallic glass provided in step a) is expected to occur, selected from: cooling rate, solidification rate, and incubation time, wherein incubation time is the amount of time during which a point of the metallic glass remains within a temperature range between the crystallization temperature and the liquidus temperature of said metallic glass;f) simulating the LPBF printing of the 3D object (1) with a shape and size according to the 3D CAD model defined in step b), under the LPBF parameters defined in step c) and following the laser scan path defined in step d), using a thermo- metallurgical predictive model adapted to:f.1) calculate a temperature evolution within a build region of the object (1) during LPBF printing through a heat transfer equation; andg) f.2) calculate, based on the temperature evolution calculated in step f.1), at least one crystallization metric selected from: cooling rate, solidification rate, and incubation time;comparing the simulated crystallization-metric value / s obtained in step f) with the crystallization-metric value / s defined in step e);h) adjusting the LPBF process parameter / s defined in step c) according to the comparison performed in step g), so as to obtain a 3D printed object (1) with a controlled crystalline fraction;i) printing the 3D object (1) with a shape and size according to the 3D CAD model defined in step b), under the LPBF parameter / s adjusted in step h) and following the laser scan path defined in step d).

2. The method according to claim 1, wherein step a) comprises providing a metallic glassin powder-form.

3. The method according to any of claims 1 or 2, wherein the metallic glass comprises a ferromagnetic transition metal-based alloy or a Zr-based alloy.

4. The method according to claim 3, wherein the metallic glass comprises a Fe- or a Zr- based alloy.

5. The method according to any of claims 1 to 4, wherein step c) comprises defining at least one LPBF process parameter selected from:- a laser power between 20 and 500 W;- a laser scan speed between 100 and 4000 mm / s;- a hatch distance between 10 and 150 µm;- a powder layer thickness between 10 and 70 µm;- a laser spot size between 10 and 500 µm;- a laser exposure time between 20-1000 µs;- a laser point distance between 10 and 100 µm;- a laser duty cycle between 70-100%; and- a time between adjacent laser scan tracks of up to1000 ms.

6. The method according to claim 5, wherein step c) comprises defining at least one LPBF process parameter selected from:- a laser power between 35 and 80 W;- a laser scan speed between 300 and 700 mm / s;- a hatch distance between 60 and 150 µm;- a powder layer thickness between 25 and 50 µm;- a laser spot size between 60 and 90 µm;- a laser exposure time between 100-250 µs;- a laser point distance between 50 and 90 µm;- a laser duty cycle between 90-100%; and- a time between adjacent laser scan tracks between 1-30 ms.

7. The method according to any of claims 1 to 6, wherein step c) comprises defining at least the following LPBF process parameters: laser power, hatch distance, powder layer thickness, laser spot size, and time between adjacent laser scan tracks.

8. The method according to claim 7, wherein step c) further comprises defining one or more of the following LPBF process parameters: laser exposure time, laser point distance, and laser scan speed.

9. The method according to any of claims 1 to 8, wherein step d) comprises defining a laser scan path according to any of the following scan strategies: stripes, chessboard, bi-directional, meander, double-scan, or point-random.

10. The method according to claim 9, wherein step d) comprises defining a laser scan path following a meander scan strategy, wherein neighboring vectors of the laser scan path are scanned with a constant hatching distance in the opposite direction over the entire metallic glass powder layer deposited on a build platform of a LPBF device.

11. The method according to any of claims 1 to 10, wherein step i) comprises performing an optimization of the LPBF process parameters using a training model according to a machine learning or an artificial intelligence algorithm, wherein the training model comprises a dataset including a combination of LPBF processing parameters, 3D object’s (1) shapes and sizes, and resulting crystalline fraction.

12. The method according to any of claims 1 to 11, wherein step e) is performed based on at least one selected from:the phase change diagram of the metallic glass;the crystallization temperature and the liquidus temperature of the metallic glass; andexperimental calibration based on measurements of crystalline fractions and / or melt-pool and heat-affected-zone (HAZ) dimensions obtained from 3D-printed objects manufactured using different combinations of LPBF process parameters.

13. A LPBF device for printing a 3D object (1) from a metallic glass comprising:- a metallic glass powder reservoir (2), adapted for containing a plurality of metallic glass powder particles (3);- dispensing means (4), connected to the metallic glass powder reservoir (2) and adapted for depositing a plurality of consecutive layers of the metallic glass powder particles (3) contained in the reservoir (2) on a build platform (5);- laser emission means (6), adapted for emitting a laser beam (6’) over the deposited consecutive layers of the metallic glass powder particles (3) on thebuild platform (5);- a chamber (7), adapted for containing the metallic glass powder reservoir (2), the dispensing means (4), the build platform (5) and the laser emission means (6), and for maintaining a controlled atmosphere during the LPBF printing of the 3D object (1);and characterized in that said device further comprises a control unit (8), communicatively connected to the metallic glass powder reservoir (2), the dispensing means (4) and the laser emission means (6), wherein said control unit (8) is adapted to implement the method according to any of claims 1 to 12.

14. The LPBF device according to claim 13, wherein the control unit (8) comprises:- design modelling means (9) for creating a 3D computer-aided design (CAD) model of the 3D object (1) to be printed;- LPBF process parameters selecting means (10) for selecting one or more LPBF process parameters;- laser scan path selecting means (11) for selecting a scan path for the laser beam (6’) emitted by the laser emission means (6); and,- software means (12) adapted for simulating the LPBF printing of the 3D object with a shape and size according to the 3D CAD model created by the design modelling means (9), under the LPBF process parameters defined by the LPBF process parameters selecting means (10), and following the laser scan path defined by the laser scan path selecting means (11), so as to adjust the LPBF process parameters to print a 3D object (1) with a controlled crystalline fraction.

15. A computer program comprising instructions which, when executed in the control unit (8) of a LPBF device according to any of claims 13 or 14, enables performing:steps b) to h) of a method according to any of claims 1 to 10 or 12; orsteps b) to i) of a method according to claim 11.

Images