Method for manufacturing steel objects by powder bed fusion

Abstract

This provides an improved process for manufacturing steel objects by powder bed fusion. [Solution] A process for manufacturing a steel object by powder bed fusion, comprising: a) a step of providing a steel powder layer 316L containing 200 ppm or less of nitrogen, wherein the powder used has not undergone any special atomization or chemical pretreatment; and b) 0 ≤ p Ar The partial pressure of argon (Ar) p for which < 1 Ar and 0 <p N2 At partial pressures of dinitrogen (N2) such that p ≤ 1 Ar +p N2 The process is characterized by comprising the steps of: c) solidifying a powder layer by powder bed fusion in a controlled atmosphere where = 1; and c) manufacturing a steel object by repeating steps a) and b) as necessary.

Description

[Technical Field]

[0001] This invention relates to the field of manufacturing steel objects by powder bed fusion.

[0002] The present invention can be used in a wide range of industrial fields, including the power generation, aerospace, and automotive industries. [Background technology]

[0003] Methods involved in manufacturing steel objects using powder bed fusion have been the subject of extensive research over several years.

[0004] The parameters accessible and modifiable within commercially available equipment for implementing such methods (e.g., laser power, laser scanning speed in the case of laser powder bed fusion), and the optimization of the shape of the equipment used to implement these methods, have a significant impact on the performance (mechanical properties, etc.) of the manufactured object. Mastering these various parameters is an essential part of the technical expertise of equipment manufacturers.

[0005] Furthermore, studies have shown that the physicochemical properties of the steel powder used also significantly affect the properties of the objects produced in this manner. For example, it has been shown that changing the content of trace elements, particularly nitrogen and oxygen, in the 316L of steel powder can lead to significant changes in the microstructural properties of the resulting object (especially grain size, texture, and precipitation within the metal matrix), and can ultimately affect the performance of the object (such as mechanical properties).

[0006] Given the current state of knowledge, controlling the performance of the final object related to these microstructural properties involves specific pretreatment processes for the powders used and loaded into the equipment used to carry out powder bed fusion.

[0007] To control the properties of the final product, existing options based on the physicochemistry of the powder include the following: - Changing the powder atomization method (which is usually done by the powder manufacturer and supplier) (this is complex and potentially costly), and / or - Insert one or more pre-treatment steps for the powder to eliminate trace amounts of undesirable chemical species (this increases manufacturing time and costs).

[0008] It should be noted that in the latest technology, the nitrogen content is generally set, for example, before solidification occurs during powder atomization or powder pretreatment. Therefore, those skilled in the art use steel powder with a certain nitrogen content and then solidify it. As a result, steel is obtained with microstructural properties that depend on the nitrogen content of the supplied steel powder, and solidification does not affect this nitrogen content.

[0009] Furthermore, in some cases, it may be necessary to manufacture parts with different microstructural properties. The usual approach in this case is to change the powder during manufacturing, but this results in a loss of productivity and increased complexity (e.g., protocols for interrupting and restarting manufacturing). The various powders used may be atomized or pre-treated in different ways. [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] One objective of the present invention is to provide an improved process for manufacturing steel objects by powder bed fusion.

[0011] In particular, one objective of the present invention is to propose a method for manufacturing steel objects by powder bed fusion that allows for the modification of the microstructural properties of the manufactured object during manufacturing without interrupting the manufacturing process. [Means for solving the problem]

[0012] To solve the above-mentioned objectives, the present invention provides a method for manufacturing a steel object by powder bed fusion, a) Providing a 316L steel powder layer containing nitrogen at 200 ppm or less, where the powder used has not undergone special atomization treatment or chemical pretreatment; b) In a controlled atmosphere where 0 ≦ p Ar <1 for the argon (Ar) partial pressure p Ar and 0 < p N2 ≦ 1 for the dinitrogen (N2) partial pressure, where p Ar + p N2 = 1, solidifying the powder layer by powder bed fusion; c) Optionally repeating steps a) and b) to produce a steel object. A method is proposed, characterized by including these steps.

[0013] The method according to the present invention can include at least one of the following additional features, either alone or in combination. - The steel powder supplied in step a) contains nitrogen at a maximum of 170 ppm, preferably at a maximum of 150 ppm; - In step b), varying the partial pressure of dinitrogen (p N2 ); - Step b) is carried out using powder bed fusion selected from laser powder bed fusion and concentrated energy deposition; - Step b) is carried out using laser powder bed fusion with the following parameters: - Laser power: 50 - 200 W; - Throughput rate: 0.3 - 3 m / s; - Thickness of the powder layer: 10 - 100 microns; and - Strand spacing: 10 - 150 microns - In step c), varying the partial pressure of dinitrogen.

[0014] Further objects and features of the present invention will become clearer in the following description made with reference to the accompanying drawings.

Brief Description of the Drawings

[0015] [Figure 1] Schematic diagram of the main stages of the method according to the present invention. [Figure 2] It is a diagram showing an apparatus for executing the method according to the present invention using laser powder bed fusion technology. [Figure 3a] It is a crystal orientation map generated by electron backscatter diffraction of steel 316L obtained using a method according to the prior art. [Figure 3b] It shows a crystal orientation map generated by electron backscatter diffraction of steel 316L obtained using the method according to the present invention. [Figure 4] It shows different crystal orientation maps generated by electron backscatter diffraction of steel 316L obtained using a method according to the prior art, and different nitrogen contents in the powder according to images (a) to (e).

Embodiments for Carrying Out the Invention

[0016] Detailed Description of the Invention The present invention relates to a method for manufacturing a steel object by powder bed fusion. The method includes the following steps: a) Providing a layer of steel powder 316L containing nitrogen of 200 ppm or less, where the powder used has not undergone special atomization treatment or chemical pretreatment; b) Solidifying the powder layer by powder bed fusion in a controlled atmosphere where the argon (Ar) partial pressure p satisfies 0 ≦ p Ar < 1 and the dinitrogen (N2) partial pressure p satisfies 0 < p Ar ≦ 1, and p N2 + p Ar = 1; N2 c) Repeating steps a) and b) as necessary to manufacture a steel object. c) Optionally repeating steps a) and b) to manufacture a steel object.

[0017] Typically, powder bed fusion is performed in a controlled atmosphere of argon (p Ar = 1).

[0018] However, the present invention demonstrates that changing the atmosphere in which steel powder with a low nitrogen content (<200 ppm) is solidified affects the granular structure of the formed steel, and therefore, in particular, the mechanical properties of the steel products thus manufactured. This is done without requiring any special atomization or chemical pretreatment of the powder used. In the context of the present invention, the nitrogen content in the steel powder can be changed during solidification by powder bed fusion.

[0019] Therefore, the present invention is even less obvious than the existing prior art, particularly in that it makes it uncertain to those skilled in the art that the solidification rate, especially in high powder bed fusion (particularly laser powder bed fusion), can be controlled or predicted, as well as the interaction between dinitrogen in the enclosure and the molten material and its diffusion in the solidified material at the starting point of microstructural modification. The rate difference between these two phenomena (solidification on the one hand, and diffusion on the other) is quite significant.

[0020] Advantageously, the steel powder supplied in step a) contains up to 170 ppm of nitrogen, and more advantageously, up to 150 ppm of nitrogen.

[0021] Furthermore, within the scope of the present invention, in step b), the partial pressure of dinitrogen (p N2 ) can be changed.

[0022] This makes it possible to gradually control and modify the granular structure (grain size, texture) of the solidified steel along the powder layer during solidification. This allows for the creation of steel objects with gradients in the granular structure, and therefore gradients in the mechanical properties depending on the solidification direction. In particular, when solidification is performed using powder scanning technology, a non-limiting example in laser powder bed fusion, it is possible to create two gradients defined by two orthogonal X and Y directions on the plane of the powder bed.

[0023] Similarly, during step c), the partial pressure of dinitrogen (p N2 It is possible to change ). This means when step b) is repeated, i.e., when another powder layer is solidified on top of the previous powder layer.

[0024] This makes it possible to gradually control and modify the granular structure (grain size, texture) of the solidified steel layer by the powder layer during solidification. As a result, a steel object can be obtained that has a gradient in the granular structure, and therefore a gradient in the mechanical properties corresponding to the stacking direction of these layers. This gradient is defined along the Z direction perpendicular to the X and Y directions.

[0025] Powder bed fusion is selected from laser powder bed fusion (L-PBF) and directed energy deposition (DED).

[0026] Furthermore, when the method according to the present invention is carried out by laser powder bed fusion, the following parameters can be provided: - Laser output: 50~200W; - Slew rate: 0.3~3 m / s; -Powder layer thickness: 10-100 microns; and - Strand distance: 10-150 microns.

[0027] Figure 2 shows an apparatus that can carry out the method according to the present invention, particularly in the case of laser powder bed fusion. This same apparatus can also carry out the prior art method.

[0028] The APR apparatus features an enclosure ECT configured to spread a controlled atmosphere. Inside the enclosure is a container containing powder PDR, which is moved by a piston PST1. The movement of this container allows for the extraction of a predetermined amount of powder. This powder PDR can be transported by a scraper RCT to a separate container used to solidify the powder using a laser beam FL. This separate container is mounted on a piston PST2 to descend as the powder layer is transported for the production of a steel object OBJ. The laser beam FL is generated by a laser LSR, and its output beam is tuned by an optical assembly OPT before being directed towards the powder layer to be solidified. The atmosphere inside enclosure ECT can be controlled with appropriate amounts of dinitrogen and argon, managed by dedicated flowmeters DM1 and DM2, respectively.

[0029] The aforementioned advantages of the method according to the present invention will be better understood in light of the tests presented below (Test 2 vs. Test 1 and additional tests).

[0030] Test 1 (Prior Art) The steel powder 316L is composed of particles (spherical particles) with a particle size in the range of 15 to 45 μm and a nitrogen content of 200 ppm or less (low nitrogen content). This powder is supplied to the powder reservoir RP in the manufacturing equipment, in this case the TruPrint® 1000 ytterbium-doped fiber laser machine (wavelength 1064 nm, laser spot 55 μm).

[0031] Next, the low-nitrogen steel powder 316L is solidified in a manufacturing enclosure under a controlled argon (Ar) atmosphere. The argon is industrial grade and has a purity of over 99.99% by volume. The parameters used for solidification are as follows: - Laser output: 165W; - Slew rate: 450 mm / s; -Layer thickness: 30 μm; and - Strand-to-strand distance: 60 μm.

[0032] Test 2 (Test according to the present invention) In Test 2, solidification is carried out under a controlled atmosphere of dinitrogen (N2). The dinitrogen (N2) is industrial grade and has a purity of over 99.99% by volume. Everything else is the same as in Test 1, in particular the type of powder used, the equipment used, and the solidification parameters applied.

[0033] Figures 3(a) and 3(b) show the microstructure characteristics of the steel product 316L manufactured in this manner compared to the prior art (Test 1) and the present invention (Test 2), respectively.

[0034] These figures show crystal orientation maps generated by electron backscatter diffraction (EBSD) of steel 316L.

[0035] Compared to a material solidified under an argon atmosphere (Figure 3(a), prior art), the solidification of steel 316L produced by laser powder bed fusion from powder with a nitrogen content of less than 200 ppm in an enclosure filled with dinitrogen (Figure 3(b), present invention) results in a significant change in the granular structure of the final product.

[0036] In this case, comparing the structure in Figure 3(b) with the structure in Figure 3(a), we can see the following: - The particle morphology in Figure 3(b) shows a columnar structure oriented in the construction direction (BD), but this structure is quasi-equal-axis in Figure 3(a). - The grain size is a coefficient multiple of 2.5 to 4 between Figure 3(b) and Figure 3(a). - The collective organization is shown in Figure 3(b) <110> It is more pronounced in the direction.

[0037] Therefore, the comparison of the two tests (Test 1 / Test 2) demonstrates that changes in the atmosphere in which the steel powder is solidified affect the granular structure of the formed steel, and thus, in particular, the mechanical properties of the steel products manufactured in this manner. Furthermore, in the context of the present invention, the powder used has not undergone any specific atomization or chemical pretreatment.

[0038] Additional tests Other tests were carried out using prior art methods, more precisely under the conditions described above for Test 1, except that they were performed using 316L of steel powder with different nitrogen content (a) 93±7 ppm, (b) 169±22 ppm, (c) 292±23 ppm, (d) 452±53 ppm, and (e) 558±22 ppm.

[0039] The crystal orientation maps obtained by electron backscatter diffraction (EBSD) of steel 316L for cases (a) to (e) are shown in Figure 4.

[0040] When nitrogen is gradually added to 316 L of steel powder, it can be seen that a transition is possible from a quasi-equal-axle structure (case (a) with a low nitrogen content (<200 ppm): a structure similar to that in Figure 3(a)) to a columnar structure (case (c) with a nitrogen content >200 ppm, passing through an intermediate state in case (b)). Cases (d) and (e) (nitrogen content >200 ppm in the powder) are also columnar, but are characterized by a coarser granular structure than that observed in case (c).

[0041] Incidentally, it should be noted that the columnar structure of steel 316L obtained in Figure 3(b) (the present invention) is similar to that of case (c) in Figure 4, and more generally, it can be considered similar to that of cases (d) and (e) in Figure 4.

[0042] Therefore, from all these tests, it is understood that, for example, whether conventional steel powder with a low nitrogen content is pretreated to add nitrogen according to prior art methods and then solidified in an argon atmosphere, or whether conventional powder containing a low amount of nitrogen (<200 ppm) is started according to the present invention and then solidified in a nitrogen atmosphere, the granular structure and equivalent mechanical properties of the manufactured steel product are defined in either case, so starting with steel powder having a predetermined amount of nitrogen is hardly important.

[0043] Therefore, to the extent that the results shown in Figure 4 for different nitrogen content of the powder can be replaced with the present invention, we start with conventional powders with low nitrogen content (<200 ppm), and then p Ar +p N2 =1 and argon (p Ar Dinitrogen (p N2 By adjusting the partial pressure of ), Figure 3(a) (prior art / outside the scope of the present invention) and Figure 3(b) (partial pressure of dinitrogen p N2 It is possible to obtain all types of intermediate structures between those with =1) and those with =1).

[0044] In particular, it is understood that by gradually adding dinitrogen (N2) into the enclosure of the apparatus for performing laser powder bed fusion, starting from an argon (Ar) atmosphere, it is possible to transition from one granular structure to another during production, especially to subsequent depositional layers. This involves changing the partial pressure of dinitrogen during the production of the object. As a result, it is possible to produce a steel object in which the granular structure develops according to the gradient of the depositional direction of the continuously deposited powder layers for solidification.

Claims

1. A method for manufacturing steel objects by powder bed fusion, a) A step of providing a steel powder layer 316L containing up to 200 ppm of nitrogen, wherein the powder used has not undergone any special atomization or chemical pretreatment. b) 0 ≤ p Ａｒ The partial pressure of argon (Ar) p is less than 1. Ａｒ and 0 < p Ｎ２ Dinitrogen (N) ≤ 1 ２ ) At partial pressure, p Ａｒ +p Ｎ２ A step of solidifying the powder layer by powder bed fusion in a controlled atmosphere where = 1, c) A method characterized by comprising the step of manufacturing the steel object by repeating steps a) and b) as necessary.

2. The method according to claim 1, wherein the steel powder supplied in step a) contains a maximum of 170 ppm of nitrogen, preferably a maximum of 150 ppm of nitrogen.

3. In step b), the partial pressure of dinitrogen (p Ｎ２ The method according to claim 1 or 2, which changes ).

4. The method according to any one of claims 1 to 3, wherein step b) is carried out using powder bed fusion selected from laser powder bed fusion and concentrated energy deposition.

5. Step b) uses laser powder bed fusion with the following parameters: - Laser output: 50-200W; - Slew rate: 0.3-3 m / s; - Thickness of the powder layer: 10 to 100 microns; and - Distance between strands: 10-150 microns, The method according to any one of claims 1 to 4, as implemented in [location].

6. In step c), the partial pressure of dinitrogen (p Ｎ２ The method according to any one of claims 1 to 5, which changes ).

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