A process for manufacturing a steel object by powder bed fusion.

Abstract

TITLE: Method for manufacturing a steel object by powder bed fusion. The invention relates to a method for manufacturing a steel object by powder bed fusion, characterized in that it comprises the following steps: a) providing a layer of steel powder containing at most 200 ppm of nitrogen, b) consolidating the powder layer by powder bed fusion under a controlled atmosphere at a partial pressure of Argon (Ar) pAr such that 0 ≤ pAr < 1 and a partial pressure of nitrogen (N2) such that 0 < pN2 ≤ 1, with pAr + pN2 = 1, c) repeating steps a) and b) as many times as necessary to manufacture the steel object. Figure for the abstract: Figure 1

Description

Title of the invention: Method for manufacturing a steel object by powder bed fusion. Technical field of the invention

[0001] The invention relates to the field of manufacturing a steel object by powder bed fusion.

[0002] The invention can find application in numerous industrial fields such as energy production, aeronautics, space, or even the automotive industry. Technical background

[0003] The processes for manufacturing a steel object by powder bed fusion have been the subject of in-depth studies for several years.

[0004] Thus, the parameters accessible and modifiable within commercially available equipment for implementing such processes (for example, for laser powder bed fusion: laser power, laser scanning speed, etc.) and the optimization of the geometries of the equipment used to implement these processes have a major impact on the performance of the objects thus manufactured (mechanical properties, etc.). Mastering these various parameters constitutes an integral part of the technical know-how of equipment manufacturers.

[0005] Furthermore, studies have shown that the physicochemical characteristics of the steel powders used also have a significant influence on the characteristics of the objects thus manufactured. For example, it has been demonstrated that modifying the content of minor elements, particularly nitrogen and oxygen, in a 316L steel powder leads to significant modifications in the microstructural characteristics of the object obtained after manufacturing (grain size, texture and precipitation within the metallic matrix in particular) which can also impact the performance of the final object obtained (mechanical characteristics, etc.).

[0006] In the current state of knowledge, controlling the performance of the final object, linked to these microstructural characteristics, involves specific pretreatment processes of the powders used and loaded into the equipment for implementing the powder bed fusion process.

[0007] To control the characteristics of the final product, existing options based on the physicochemical properties of powders consist of: - modify the powder atomization process (most often carried out by powder manufacturers and suppliers), which is complex and potentially expensive and / or to; - insert one or more pre-treatment steps of the powders, eliminating traces of undesirable chemical species, which makes manufacturing longer and more expensive.

[0008] Furthermore, in some cases, it may be necessary to manufacture a part whose microstructural properties are not uniform throughout. In this case, the known approach consists of changing the powder during manufacturing, which leads to a loss of productivity and increased complexity (production interruption and restart protocol, etc.). The different powders then used may, in particular, have been previously atomized or pretreated differently.

[0009] One objective of the invention is to propose an improved powder bed fusion manufacturing process for a steel object.

[0010] In particular, an objective of the invention is to propose a method for manufacturing a steel object by powder bed fusion allowing the microstructural properties of the object to be manufactured to be modified during manufacturing, without interrupting the manufacturing process. Summary of the invention

[0011] To achieve the aforementioned objective, the invention proposes a method for manufacturing a steel object by powder bed fusion, characterized in that it comprises the following steps: a) provide a layer of steel powder containing at most 200 ppm of nitrogen, b) consolidate the powder layer by powder bed fusion under a controlled atmosphere at a partial pressure of Argon (Ar) pAr such that 0 < Pat < 1 and a partial pressure of dinitrogen (N2) such that 0 < pN2 < 1, with Pat + Pn2 = 1, c) repeat steps a) and b) as many times as necessary to manufacture the steel object.

[0012] The method according to the invention may include at least one of the following additional features, taken alone or in combination: - the steel powder supplied in step a) contains at most 170 ppm of nitrogen, advantageously at most 150 ppm of nitrogen; - during step b), the partial pressure (pN2) of dinitrogen is varied; - step b) is implemented with a powder bed fusion is chosen including: laser powder bed fusion or concentrated energy deposition; - Step b) is implemented with laser powder bed fusion, with the following parameters: - Laser power: 50 to 200W, - scanning speed: 0.3 to 3 m / s, - thickness of a powder layer: 10 to 100 microns, - Inter-cord distance: 10 to 150 microns. - during step c), the partial pressure of nitrogen is varied. Brief description of the figures

[0013] Other objects and features of the invention will become clearer in the following description, made with reference to the accompanying figures, in which:

[0014] Fig. 1 is a schematic representation of the main steps of the process according to the invention;

[0015] Fig. 2 represents an apparatus for implementing the process according to the invention using the powder bed laser fusion technique;

[0016] Figure 3(a) is a crystal orientation map produced by electron backscattering diffraction of a 316L steel obtained according to a prior art process;

[0017] Figure 3(b) represents a crystal orientation map produced by electron backscattering diffraction of a 316L steel obtained according to a process according to the invention;

[0018] Fig. 4 represents different crystal orientation maps obtained by electron backscattering diffraction of a 316L steel obtained according to a prior art process, and for different nitrogen contents in the powder according to images (a) to (e). Detailed description of the invention

[0019] The invention relates to a method for manufacturing a steel object by powder bed fusion. The method comprises the following steps: a) provide a layer of steel powder containing at most 200 ppm of nitrogen, b) consolidate the powder layer by powder bed fusion under a controlled atmosphere at a partial pressure of Argon (Ar) pAr such that 0 < Pat < 1 and a partial pressure of dinitrogen (N2) such that 0 < pN2 < 1, with pAr + Pn2 = 1, c) repeat steps a) and b) as many times as necessary to manufacture the steel object.

[0020] Conventionally, powder bed fusion is carried out under a controlled Argon atmosphere (pAr = 1).

[0021] The invention shows, however, that modifying the atmosphere in which a low-nitrogen (< 200 ppm) steel powder is consolidated has an impact on the granular structure of the steel formed and therefore, in particular, on the mechanical properties of the steel product thus manufactured. This is achieved without the powder used requiring any special atomization or any chemical pretreatment.

[0022] Advantageously, the steel powder supplied in step a) shall contain at most 170 ppm of nitrogen, even more advantageously at most 150 ppm.

[0023] Furthermore, within the framework of the invention, it is possible to vary the partial pressure (pN2) of dinitrogen during step b).

[0024] This allows for the control and progressive modification of the granular structure (grain size, texture) of the consolidated steel along the powder layer during consolidation. A steel object can then be obtained with a gradient in its granular structure, and therefore in its mechanical properties, depending on the consolidation direction. In particular, if the consolidation is carried out using a powder scanning technique, which is the case, for example but not limited to, with laser powder bed fusion, it is possible to create two gradients, defined by the two orthogonal X and Y directions of the powder bed plane.

[0025] Similarly, within the framework of the invention, it is conceivable to vary the partial pressure (pN2) of nitrogen during step c). By this, it must be understood that when step b is repeated, i.e. when another layer of powder located above the previous one is consolidated.

[0026] This allows for the control and progressive modification of the granular structure (grain size, texture) of the consolidated steel, layer by layer of powder during consolidation. A steel object can then be obtained with a gradient in the granular structure, and therefore in the mechanical properties, depending on the direction of layer stacking. This gradient is defined along a Z direction perpendicular to the X and Y directions.

[0027] Powder bed fusion is chosen from: laser powder bed fusion (or L-PBF for "Laser Powder Bed Fusion" according to Anglo-Saxon terminology) or concentrated energy deposition (or DED for "Directed Energy Deposition" according to Anglo-Saxon terminology).

[0028] Furthermore, when the process according to the invention is implemented with laser powder bed fusion, the following parameters can be predicted: - Laser power: 50 to 200W, - scanning speed: 0.3 to 3 m / s, - thickness of a powder layer: 10 to 100 microns, - inter-bead distance: 10 to 150 microns.

[0029] Figure 2 shows an apparatus capable of implementing the process according to the invention, in the particular case of laser powder bed fusion. This same apparatus is also capable of implementing a process according to the prior art.

[0030] This APR device comprises an ECT chamber designed to maintain a controlled atmosphere. Within the chamber is a container of PDR powder, which can be moved by a PST1 piston. The movement of this piston allows the extraction of a predetermined quantity of powder. This PDR powder can be conveyed by an RCT scraper to another container used for powder consolidation by a FL laser beam. This other container is mounted on a PST2 piston to descend and As layers of powder are fed into the steel OBJ for fabrication, the FL laser beam is generated by an LSR laser. The output beam is conditioned by an OPT optical system before being directed onto the powder layer to be consolidated. The atmosphere within the ECT chamber can be controlled with an appropriate amount of nitrogen and argon, managed respectively by dedicated flow meters DM1 and DM2.

[0031] The aforementioned advantages of the process according to the invention can be better understood in support of the tests (test 2 versus test 1 and additional tests) presented below.

[0032] Test 1 (prior art reference).

[0033] The 316L steel powder consists of particles with a particle size ranging from 15 to 45 µm (spherical grains), with a nitrogen content less than or equal to 200 ppm (low nitrogen content). This powder is introduced into the RP powder reservoir in the manufacturing equipment, in this case a TruPrint® 1000 machine with an ytterbium-doped fiber laser (wavelength 1064 nm, laser spot 55 µm).

[0034] Next, the 316L low-nitrogen steel powder is consolidated under a controlled atmosphere with Argon (Ar) in the manufacturing chamber. The Argon is of industrial grade, with a purity greater than 99.99% by volume. The parameters used for consolidation are as follows: - laser power: 165 W - scanning speed: 450 mm / s - layer thickness: 30 µm - inter-bead distance: 60 µm.

[0035] Test 2 (according to the invention).

[0036] In this test 2, consolidation is carried out under a controlled nitrogen (N2) atmosphere. The nitrogen (N2) is of industrial grade, with a purity greater than 99.99% by volume. Everything else is identical to test 1, therefore in particular the nature of the powder used, the equipment employed, and the consolidation parameters implemented.

[0037] Figure 3(a) and Figure 3(b) show respectively the microstructural properties of the 316L steel product thus manufactured for reference to the prior art (test 1) and the invention (test 2).

[0038] These figures are crystal orientation maps produced by electron backscattering diffraction (or EBSD for "Electron Back Scattered Diffraction" according to Anglo-Saxon terminology) of 316L steel.

[0039] Compared to the material consolidated under an argon atmosphere (Figure 3(a) / prior art), the consolidation of a 316L steel produced by laser powder bed fusion from a powder having a nitrogen content of less than 200 ppm in a filled chamber of dinitrogen (figure 3(b) / invention) causes a significant change in the granular structure of the final product.

[0040] In the case presented, if we compare more precisely the structure of figure 3(b) to that of figure 3(a), we observe that: - the morphology of the grains presents a columnar structure oriented in the direction of construction (BD) on figure 3(b) whereas this structure is almost equiaxed on figure 3(a); - the grain size is multiplied by a factor between 2.5 and 4 between figure 3(b) and figure 3(a), - the texture is more strongly marked depending on the direction <110> in Figure 3(b).

[0041] Comparing the two tests (test 1 / test 2) thus demonstrates that modifying the atmosphere in which the steel powder is consolidated has an impact on the granular structure of the steel formed and, therefore, in particular, on the mechanical properties of the steel product thus manufactured. Furthermore, in the context of the invention, the powder used was not subjected to any particular atomization or any chemical pretreatment.

[0042] Additional tests.

[0043] Other tests were carried out with the prior art process, more specifically under the conditions mentioned above for test 1, but with 316L steel powders with different nitrogen contents, namely (a) 93 + 7 ppm, (b) 169 + 22 ppm, (c) 292 + 23 ppm, (d) 452 + 53 ppm and (e) 558 + 22 ppm.

[0044] The crystal orientation maps obtained by electron backscattering diffraction (or EBSD for "Electron Back Scattered Diffraction" according to Anglo-Saxon terminology) of 316L steel for cases (a) to (e) are provided in [Fig.4].

[0045] It is noted that the progressive addition of nitrogen in the 316L steel powder allows a transition from a quasi-equiaxed structure (case (a) with low nitrogen content: < 200 ppm; structure similar to that of figure 3(a)) to a columnar structure (case (c) with nitrogen content > 200 ppm, passing through an intermediate state which is that of case (b)). Cases (d) and (e) (nitrogen content > 200 ppm in the powder) are characterized by a grain structure that is also columnar, but coarser than that observed for case (c).

[0046] Incidentally, it is noted that the structure of the 316L steel obtained in figure 3(b) (invention), of columnar type, is similar to that of [Fig.4], case (c) and can more generally also be considered as similar to those of Fig.4, case (d) and case (e).

[0047] From all these tests, it is therefore also understood that it matters little to start with a steel powder with a predetermined quantity of nitrogen, for example because in accordance to a prior art method, a conventional low-nitrogen steel powder was pretreated to add nitrogen and then consolidated under an Argon atmosphere, or, according to the invention, to start from a conventional powder containing a small amount of nitrogen (< 200ppm) and then consolidated under a nitrogen atmosphere to define a granular structure and equivalent mechanical properties, in both cases, of the steel product manufactured.

[0048] Thus, insofar as the results shown in [Fig.4] for different nitrogen contents of the powder are therefore transposable to the invention so that by starting from a classic powder with low nitrogen content (< 200 ppm), then by adjusting a partial pressure of dinitrogen (pN2) relative to a partial pressure of Argon (par) with Pat + pN2 = 1, it is possible to obtain all types of intermediate structures between that of figure 3(a) (prior art / outside invention) and that of figure 3(b) (partial pressure of dinitrogen pN2 =1).

[0049] In particular, it is understood that it is possible, starting with an atmosphere containing Argon (Ar) and progressively adding nitrogen (N2) within the apparatus to perform laser powder bed fusion, to effect a transition from one granular structure to another during fabrication, notably layer by layer. This involves varying the partial pressure of nitrogen during the fabrication of the object. Consequently, this makes it possible to manufacture a steel object whose granular structure evolves along a gradient directed in the direction of deposition of the successively deposited powder layers for consolidation.

Claims

Demands

1. A method for manufacturing a steel object by powder bed fusion characterized in that it comprises the following steps: a. providing a layer of steel powder comprising at most 200 ppm of nitrogen, b. carrying out consolidation of the powder layer by powder bed fusion under a controlled atmosphere at a partial pressure of Argon (Ar) pAr such that 0 < Pat < 1 and a partial pressure of dinitrogen (N2) such that 0 < pN2 < 1, with Pat + Pn2 = 1, c. repeating steps a) and b) as many times as necessary to manufacture the steel object.

2. A process according to claim 1, wherein the steel powder supplied in step a) comprises at most 170 ppm of nitrogen, advantageously at most 150 ppm of nitrogen.

3. A method according to any one of the preceding claims, wherein in step b), the partial pressure (pN2) of nitrogen is varied.

4. A method according to any one of the preceding claims, wherein step b) is carried out with powder bed fusion selected from: laser powder bed fusion or concentrated energy deposition.

5. A method according to any one of the preceding claims, wherein step b) is carried out with a powder bed laser fusion, with the following parameters: - laser power: 50 to 200W, - scanning speed: 0.3 to 3 m / s, - powder layer thickness: 10 to 100 microns, - inter-bead distance: 10 to 150 microns.

6. A method according to any one of the preceding claims, wherein in step c), the partial pressure (pN2) of nitrogen is varied.

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