Additive manufacturing process for a metal part

Abstract

Additive manufacturing process for a metal part. The invention relates to an additive manufacturing process for a metal part, by selectively melting at least one layer (10) of a metal powder using a laser beam (12) controlled by a control system. The additive manufacturing process comprises at least: A determination step, during which a plurality of parameter sets are determined; A fabrication step, during which at least one sample is produced, for each parameter set, by additive manufacturing with such parameter set; An analysis step, during which the sample is analyzed to obtain a distribution of at least one dimension (D1, D2) of the pores (20) of the sample; A production step, during which, for each parameter set, a production rate (Vf) of the part with the parameter set is obtained; A selection step.during which one of the parameter sets (Js) is selected from the characteristic quantities of the dimension distribution (D1, D2) of the pores (20) and the manufacturing speed (Vf) associated with each parameter set, and an additive manufacturing step of the part with the selected parameter set. Figure to be published with the abbreviation: Figure 5,

Description

Title of the invention: Additive manufacturing process for a metal part Technical field of the invention

[0001] The invention relates to the field of additive manufacturing, in particular additive manufacturing by selective laser powder bed fusion, also known by the acronym "SLS" for "Selective Laser Sintering" in English.

[0002] More specifically, the invention relates to a method for manufacturing a mechanical part by additive manufacturing. Prior art

[0003] A method is known consisting of manufacturing at least one part, in particular one or more metallic part(s), by selective melting of successive layers of metallic powder by means of a laser beam, controlled by a control and information processing system, in which the three-dimensional coordinates of the points of the successive layers to be produced are recorded to form the part.

[0004] A selective melting step of successive layers of powder involves a number of parameters influencing the final structure of the material and, thus, the mechanical properties of the manufactured part.

[0005] Such parameters include variables relating to the laser beam, such as a laser beam radius and average power, to the path of the powder layer, such as a path speed and a gap between two adjacent scanning lines, or to the granular material, such as a grain size and a thickness of successive powder layers.

[0006] The selection of a set of parameters for a selective melting process is generally based on the manufacture of samples with several sets of parameters and a measurement of the density of the material by optical analysis of a polished surface of each sample.

[0007] Poroses detected by image analysis are used to define the quality and resistance of the material for the sample produced.

[0008] However, such a technique, although effective, does not allow for sufficient discrimination between different sets of parameters that yield satisfactory results. Indeed, simply measuring the density of the samples proves insufficient to effectively predict the final behavior of the part.

[0009] A visual comparison of the images by an operator is also not acceptable, as it is rarely repeatable between different operators. Presentation of the invention

[0010] The invention aims to remedy such drawbacks by enabling the development of efficient parameter sets that optimize both the manufacturing speed of a part and the quality of the material.

[0011] To this end, the invention relates to a method for the additive manufacturing of a metal part, by selectively melting at least one layer of a metal powder using a laser beam controlled by a control system. More specifically, the additive manufacturing method comprises at least: - A determination step, during which a plurality of parameter sets are determined, in particular each parameter set comprising at least a laser beam power, a scanning speed of each powder layer by the laser beam, a gap between two laser beam scanning lines on each layer and / or a thickness of each layer, - A manufacturing step, during which at least one sample is produced, for each set of parameters, by additive manufacturing with such a set of parameters, - An analysis step, during which the sample is analyzed to obtain a distribution of at least one dimension of the pore size of the sample, - A determination step, during which, for each set of parameters, a manufacturing speed for the part with that set of parameters is obtained, - A selection step, during which one of the parameter sets is selected from the characteristic quantities of the pore size distribution and the manufacturing speed associated with each parameter set, and - An additive manufacturing step of the part with the selected set of parameters.

[0012] Such a process allows for rapid and complete access to information relating to the porosity of the samples and thus to select a set of parameters for the manufacture of the part offering the best compromise between material quality and manufacturing speed.

[0013] Advantageously, the parameter set comprises the following four parameters: - the power of the laser beam, - a scanning speed of each layer of powder by the laser beam, - a gap between two laser beam scan lines on each powder layer and / or - a thickness of each layer of powder.

[0014] It is recalled that a pore is an empty space contained within a solid material. Consequently, Porosity is a ratio between the volume occupied by the pores and the total volume of the pores and the solid material.

[0015] The analysis step may include a calculation step during which at least one characteristic quantity of the distribution associated with the set of parameters used to make the sample is calculated.

[0016] The analysis step may include acquiring a two-dimensional image of a sample surface and analyzing the image to identify at least one pore of the sample.

[0017] Such a feature makes it possible to identify each pore of a representative surface of the sample, in order to determine quickly and reliably the distribution of at least one dimension of the pores of the sample.

[0018] The dimension is, for example, a larger diameter of the pore, a smaller diameter of the pore, a surface area of ​​the pore, and / or an indication of a spherical or linear character of the pore.

[0019] Image analysis is, for example, a greyscale analysis involving a discrimination threshold between pores and solid material.

[0020] The pore dimension obtained in the analysis step can be a larger pore diameter or a smaller pore diameter.

[0021] Such a characteristic makes it possible to obtain indications of the linear or spherical nature of each of the observed pores, and thus to characterize the dominant type of porosity in the material. Different types of porosity are more or less critical for different intended roles of the part.

[0022] The terms "largest diameter" and "smallest diameter" of the pore are designated by the name Feyret diameter, and correspond substantially to the respective diameters of a circle circumscribed about the pore and a circle inscribed about the pore.

[0023] The analysis step may also include a sorting step, during which the set of parameters associated with the sample comprising at least one pore whose largest diameter is greater than a limit value is eliminated.

[0024] Such a characteristic makes it possible to directly eliminate parameter sets that result in porosity detrimental to the mechanical strength of the part. The limit value is, for example, equal to 100 micrometers.

[0025] The presence of linear type pores with such dimensions is highly limiting for the fatigue resistance of parts subjected to mechanical stresses.

[0026] During the selection step, the characteristic quantities of each distribution of at least one dimension of the pores may include a minimum, a first quartile, a median, a third quartile and / or a maximum of the distribution.

[0027] Such a characteristic makes it possible to characterize the distribution of pore dimensions by a small number of quantities concentrating the information, in order to facilitate the treatment.

[0028] The additive manufacturing process may include a step of graphically representing the minima, first quartiles, medians, third quartiles and / or maxima associated with each set of parameters in the form of box-and-whisker diagrams, ordered according to the manufacturing speed with the associated set of parameters.

[0029] Such a feature makes it possible to simplify the analysis by presenting the results obtained for the different samples in a clear manner, thus facilitating the selection of a set of parameters.

[0030] The selection step may include determining at least one condition relating to the pore size distributions and selecting the set of parameters associated with the highest manufacturing speed and respecting each of the conditions.

[0031] Such a feature makes it possible to simplify the selection step and to select a set of parameters allowing a high manufacturing speed.

[0032] The conditions can be chosen according to the intended role of the part and / or the mechanical and / or thermal constraints to which the part will be exposed.

[0033] Such a feature makes it possible to adapt the selection criteria of the parameter set to the intended role of the part, in order to obtain the one allowing the best manufacturing speed.

[0034] For example, a part designed to withstand repeated mechanical stresses will be more sensitive to high porosity, particularly high linear porosity, i.e., porosity with large diameter values. In comparison, a part subjected to only low mechanical and / or thermal stresses presents less risk of fatigue and can exhibit higher and more linear porosity. Brief description of the figures

[0035] The invention will be better understood and other features and advantages will become apparent upon reading the following detailed description, which includes embodiments given by way of illustration with reference to the accompanying figures, presented as non-limiting examples, which may serve to complete the understanding of the invention and the explanation of its implementation and, where appropriate, contribute to its definition, on which: - [Fig.1] is a schematic perspective view of an additive manufacturing process by selective laser beam melting on a bed of metal powder according to the invention; - [Fig.2] is a detailed cross-sectional view of a pore of a part obtained by fa additive construction; - [Fig.3] is a graphical representation of a one-dimensional distribution pores in a sample obtained by additive manufacturing; - [Fig.4] is a graphical representation of characteristic quantities of the distribution of [Fig. 3]; and - [Fig.5] is a graphical representation of the characteristic quantities obtained for several samples during a selection step. Detailed description of the invention

[0036] An additive manufacturing process for a metal part from a metal powder will be described. This type of additive manufacturing process is useful for parts with complex three-dimensional geometries, for example heat exchangers, particularly for aeronautical applications.

[0037] The metal powder used in such an additive manufacturing process is, for example, composed of an aluminum alloy, in particular that designated by the acronym AlSi7Mg0.6, comprising 7% by mass of silicon and 0.7% by mass of magnesium.

[0038] The additive manufacturing process according to the invention includes preliminary steps aimed at determining a set of parameters for the additive manufacturing of the part, allowing a good compromise to be obtained between, on the one hand, the quality and resistance of the material and, on the other hand, the speed of manufacture.

[0039] Figure 1 is a schematic perspective view of an additive manufacturing process by selective laser beam melting on a bed of metal powder according to the invention. More specifically, Figure 1 illustrates an additive manufacturing step of the part, implemented using an additive manufacturing device, as well as the various parameters influencing the additive manufacturing step.

[0040] The additive manufacturing device comprises, in a known manner, a hopper containing a metal powder deposited in at least one layer 10, in particular in successive layers 10, by means of a scraper. The layer 10 is substantially flat and extends in a substantially horizontal XY plane.

[0041] The successive layers 10 can be stacked along an elevation direction Z.

[0042] The layer 10 has a thickness H controlled by a control system of the additive manufacturing device, in particular by adjusting the movements of the vat from one layer 10 to the next.

[0043] The additive manufacturing device also includes a laser source and optics (not shown), adapted to generate and drive a laser beam 12 so as to form a movable point of incidence 14 on the layer 10.

[0044] The point of incidence 14 sweeps a predetermined part of the layer 10 in order to locally melt the layer 10 to add material to the manufactured part.

[0045] The power of the laser beam 12 is also controlled by a control system, so as to vary the depth of powder melting. power is, for example, an average power calculated over a cross-section of the laser beam 12.

[0046] For each layer 10, the beam 12 scans the predetermined part of the layer 10, in particular along lines parallel to a scanning direction, in particular spaced along the spacing direction.

[0047] The scanning direction is, for example, parallel to a first transverse direction X and the spacing direction is, for example, parallel to a second transverse direction Y perpendicular to the first transverse direction X.

[0048] In the example shown, the scanning and spacing directions are alternately parallel to the first and second transverse directions X, Y from one layer 10 to the next.

[0049] The point of incidence 14 follows the path lines at a speed V, which determines the time taken by the beam to traverse the entire predetermined region of each layer 10.

[0050] The scanning lines are spaced according to the spacing direction of a predetermined pitch E, also called vector gap, in particular chosen substantially equal to a width of the point of incidence 14 according to the spacing direction.

[0051] An additive manufacturing parameter set is defined by selecting values ​​for the parameters mentioned above. Since most of such parameters are interdependent, selecting values ​​for the following four parameters makes it possible to constitute a sufficient parameter set for manufacturing the part, namely: the power P of the laser beam, the scanning speed V, the gap vector E and the thickness H of the layer 10.

[0052] Other combinations of parameters are conceivable to characterize the additive manufacturing step and form a set of parameters within the meaning of the invention.

[0053] The additive manufacturing process includes a step of determining a plurality of sets of parameters, suitable for testing in order to select from among them the one that will be most suitable for manufacturing the mechanical part by additive manufacturing.

[0054] The parameter sets can be determined on the basis of known values ​​of the parameters used for previous manufacture of similar parts, and selected in order to sweep ranges of values ​​to find an optimal combination.

[0055] The additive manufacturing process then includes a step of producing at least one sample, for each set of parameters, by additive manufacturing with such a set of parameters.

[0056] In particular, the sample has a simple geometry and allows for ob- easy preservation, for example a roughly parallelepiped geometry.

[0057] The dimensions of the sample are determined to present surfaces of sufficient size to be representative of the material during a statistical porosity analysis.

[0058] One of the sample surfaces is then polished to allow the acquisition of at least one clear image of the surface enabling image analysis.

[0059] The additive manufacturing process then includes a sample analysis step.

[0060] The sample analysis step can be performed using an optical method. In such a case, the optical method may include, for example, acquiring a two-dimensional image of the sample surface, preferably the polished surface of the sample. Consequently, the sample analysis step may include analyzing the image thus obtained by thresholding to distinguish between a solid part of the material and porosity.

[0061] Other methods are conceivable, for example three-dimensional observation by X-ray tomography, allowing the entire volume of the sample to be recreated.

[0062] The sample analysis step makes it possible to identify each of the pores on the surface of the sample and to analyze a geometry of the pores.

[0063] The sample analysis step may also include obtaining a distribution of at least one dimension of the pores of the sample.

[0064] An example is shown in [Fig. 2], which is a detailed cross-sectional view of a pore 20 of the part obtained by additive manufacturing. The pore 20 opens in the surface, in particular the polished surface, of the sample.

[0065] Figure 2 shows two examples of characteristic dimensions of pore 20: - a larger diameter Dl, which is also a diameter of a circle circumscribed CC around pore 20, considered for example in a plane of the image, and - a smaller diameter D2, which is also a diameter of an inscribed circle CI at pore 20, considered for example in the plane of the image.

[0066] By convention, pore 20 is considered as - of a spherical nature if the ratio D1 / D2 is less than 2, and - of a linear nature if the ratio D1 / D2 is greater than or equal to 2.

[0067] The spherical or linear nature of the pore 20 can also play the role of a pore dimension according to the invention.

[0068] The largest diameter Dl and the smallest diameter D2 are determined for each of the pores 20 of the two-dimensional image. A distribution of each of the largest diameters Dl and the smallest diameters D2 is obtained for each sample analyzed.

[0069] Fig. 3 is a graphical representation of the distribution of the largest pore diameter DI, in micrometers, obtained for such a sample obtained by additive manufacturing, represented both in the form of a histogram and of LogNormal and Gamma distribution type regression curves.

[0070] Such representations make it possible to retain a large part of the information by making it observable, but do not facilitate the comparison of a large number of samples.

[0071] To remedy this, the analysis step may then include a calculation step of at least one characteristic quantity of the distribution associated with each set of parameters used to make the analyzed samples.

[0072] As shown in [Fig.4] as a graphical representation of characteristic quantities of the distribution of [Fig.3], the characteristic quantities of the distribution include, for example, a minimum Min, a first quartile Q1, a median Med, a third quartile Q3 and a maximum Max of said distribution.

[0073] The characteristic quantities are represented in the form of a box-and-whisker plot, or Tukey diagram. Such a diagram allows for a rapid evaluation of the different quantities and facilitates the comparison of several samples.

[0074] The diagram is advantageously supplemented with additional information obtained through image analysis, such as a number N of distinct pores detected.

[0075] Advantageously, the additive manufacturing process may include a sorting step, during which the parameter sets associated with each sample containing at least one pore whose largest diameter (ID) exceeds a limit value are eliminated. The limit value is determined, in particular, based on the intended role of the mechanical part and / or the mechanical and / or thermal stresses to which it will be exposed. For example, the limit value may be 100 micrometers.

[0076] The additive manufacturing process also includes a obtaining step, during which, for each set of parameters, a manufacturing speed Vf of the part with such a set of parameters is obtained.

[0077] The manufacturing speed Vf of the part is determined by calculating a volume of material aggregated by the laser beam 12, as a function of the scanning speed V and the layer thickness H for such a set of parameters.

[0078] The additive manufacturing process then includes a step of selecting one of the parameter sets Js from the characteristic quantities and manufacturing speeds obtained previously for each parameter set.

[0079] The selection may include, for example, a step of graphically representing the minima, the first quartiles, the medians, the third quartile and / or the

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[0088] Maximum values ​​associated with each set of parameters are shown as box-and-whisker plots. The plots can be ordered according to the manufacturing speed with the associated parameter set. A graphical representation of the characteristic values ​​obtained for several samples during a selection step is shown in [Fig. 5]. Specifically, [Fig. 5] shows the characteristic values ​​of the distributions of the largest pore diameters (ID) 20, obtained for fifteen distinct samples. The diagrams are plotted on the x-axis according to the manufacturing rate (Vf), expressed in cubic centimeters per hour. The total number (N) of pores detected is also shown on this graph. Such a graphical representation allows for a good visualization of the trade-off between material quality and associated manufacturing speed. In the example shown in [Fig.5], for a part requiring good mechanical strength, the selected parameter set Js is associated with the pore distribution 20 exhibiting the fewest largest diameters Dl, and a very non-linear character, which indicates good resistance to mechanical and thermal stresses. The JavaScript parameter set includes - a laser beam power P equal to 414 Watts, - a vector deviation E equal to 0.08 millimeters, - a layer thickness H equal to 0.06 millimeters and - a scanning speed V equal to 1.65 meters per second. The associated production rate Vf is equal to 28.5 cubic centimeters per hour. If lower mechanical resistance is acceptable but production speed is important, another compromise can be chosen, for example with a second set of Js' parameters. A third set of parameters could be determined according to this method, offering a material quality even better than that obtained with the second set of parameters Js', but less than that obtained with the first set of parameters Js, for a manufacturing speed Vf equal to 54.4 cubic centimeters per hour. The third set of parameters includes - a laser beam power P equal to 440 Watts, - a vector deviation E equal to 0.12 millimeters, - a layer thickness H equal to 0.09 millimeters and - a scanning speed V equal to 1.4 meters per second. Alternatively, the selection step may include determining at least one condition relating to the distributions, the pore size, and selecting the parameter set associated with the highest manufacturing speed and satisfying each of the conditions.

[0089] Such conditions can, for example, be determined by mechanical and / or thermal tests.

[0090] The conditions are also chosen according to an intended role of the part and / or the type of constraints associated.

[0091] For example, if the part is under high mechanical and / or thermal stress, such as a heat exchanger, greater importance will be given to the quality and robustness of the material, whereas if the part is under less stress, such as an equipment housing, less fatigue resistance will be considered.

[0092] The additive manufacturing process finally includes an additive manufacturing step of the part with the selected set of parameters.

Claims

Demands

1. A method for additively manufacturing a metal part by selectively melting at least one layer (10) of a metal powder using a laser beam (12) controlled by a control system, the additive manufacturing process comprising at least: - A determination step, during which a plurality of parameter sets are determined, in particular each parameter set comprising at least: • a power (P) of the laser beam (12), • a scanning speed (V) of the layer (10) by the laser beam (12), • a gap (E) between two scan lines of the laser beam (12) on each layer (10) and / or • a thickness (H) of each layer (10), - A manufacturing step, during which at least one sample is produced, for each set of parameters, by additive manufacturing with such a set of parameters, - An analysis step, during which the sample is analyzed to obtain a distribution of at least one dimension (D1, D2) of the pores (20) of the sample, - A obtaining step, during which, for each set of parameters, a manufacturing speed (Vf) of the part with the parameter set is obtained, - A selection step, during which one of the parameter sets (Js) is selected from the characteristic quantities of the pore size distribution (Dl, D2) (20) and the manufacturing speed (Vf) associated with each parameter set, and - An additive manufacturing step of the part with the selected set of parameters.

2. Additive manufacturing process according to the preceding claim, wherein the analysis step includes a calculation step during which at least one characteristic quantity of the distribution associated with the set of parameters used to make the sample is calculated.

3. An additive manufacturing method according to any one of the preceding claims, wherein the analysis step includes acquiring a two-dimensional image of a sample surface and analyzing the image to identify at least one pore (20) of the sample.

4. An additive manufacturing process according to any one of the preceding claims, wherein the pore dimension (20) obtained at the analysis step is a larger diameter (D1) of the pore (20) or a smaller diameter (D2) of the pore (20).

5. An additive manufacturing method according to the preceding claim, wherein the analysis step includes a sorting step, during which the set of parameters associated with the sample comprising at least one pore (20) whose largest diameter (Dl) is greater than a limit value is eliminated.

6. An additive manufacturing method according to any one of the preceding claims, wherein during the selection step, the characteristic quantities of each distribution of the pore size (20) include a minimum (Min), a first quartile (Q1), a median (Med), a third quartile (Q3) and / or a maximum (Max) of the distribution.

7. Additive manufacturing method according to the preceding claim, comprising a step of graphically representing the minima (Min), first quartiles (Q1), medians (Med), third quartiles (Q3) and / or maxima (Max) associated with each set of parameters in the form of box-and-whisker diagrams, ordered according to the manufacturing speed (Vf) with the associated set of parameters.

8. An additive manufacturing method according to any one of the preceding claims, wherein the selection step includes determining at least one condition relating to the size distributions (Dl, D2) of the pores (20) and selecting the parameter set associated with the highest manufacturing speed (Vf) and satisfying each of the conditions.

9. An additive manufacturing method according to the preceding claim, wherein the conditions are chosen according to an intended role of the part and / or the mechanical and / or thermal stresses to which the part will be exposed.

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