Method for additive manufacturing of metal parts

Abstract

The present invention relates to a method for additive manufacturing of metal parts by selective sintering of at least one layer (10) of metal powder by a laser beam (12) controlled by a control system, the method for additive manufacturing comprising at least a determination step in which a plurality of parameter sets are determined, a fabrication step in which, for each parameter set, at least one sample is fabricated by additive manufacturing using such parameter set, an analysis step in which the sample is analyzed to obtain a distribution of at least one dimension (D1, D2) of the pores (20) of the sample, an acquisition step in which, for each parameter set, a production rate (Vf) for manufacturing the part using that parameter set is obtained, a selection step in which one of the parameter sets (Js) is selected based on characteristic values ​​for the distribution of dimensions (D1, D2) of the pores (20) and for the production rate (Vf) associated with each parameter set, and a step of additive manufacturing of the part using the selected parameter set.

Description

[Technical Field]

[0001] The present invention relates to the field of additive manufacturing, in particular additive manufacturing by selective laser sintering over a powder bed, also known by the acronym "SLS" for "selective laser sintering."

[0002] More particularly, the present invention relates to a method for producing mechanical parts by additive manufacturing. [Background technology]

[0003] A method is known for producing at least one component, in particular one or more metal components, by selective sintering of successive layers of metal powder using a laser beam controlled by a control and information processing system, the control and information processing system storing the three-dimensional coordinates of each point of the successive layers produced to form the component.

[0004] The step of selective sintering of successive layers of powder involves several parameters that influence the final structure of the material and, therefore, the mechanical properties of the manufactured part.

[0005] Such parameters include variables related to the laser beam, such as beam radius and average power, variables related to the path within the powder layer, such as the distance between two adjacent scan lines and path speed, or variables related to the granular material, such as the particle size and thickness of successive powder layers.

[0006] Selecting a parameter set for a selective sintering process is typically based on preparing samples using several parameter sets and measuring the density of the material by optical analysis of the polished surface of each sample.

[0007] The porosity detected by image analysis is used to define the material quality and strength of the fabricated sample.

[0008] However, while effective, such techniques do not provide sufficient discrimination between different parameter sets to yield satisfactory results. In fact, simply measuring the density of a sample is insufficient to effectively predict the final behavior of a part.

[0009] Visual comparison of images by operators is also unacceptable as there is little reproducibility between different operators. Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention aims to overcome such drawbacks by enabling the development of an effective set of parameters that optimizes both the production rate and material quality of the part. [Means for solving the problem]

[0011] To this end, the present invention relates to a method for additive manufacturing of metal parts by selective sintering of at least one layer of metal powder by means of a laser beam controlled by a control system. More specifically, the method for additive manufacturing comprises at least - a determining step in which a plurality of parameter sets are determined, in particular each parameter set including at least the laser beam power, the scanning speed at which the laser beam scans each powder layer, the distance between two scanning lines of the laser beam on each layer, and / or the thickness of each layer; - a fabrication step, in which for each parameter set, at least one sample is fabricated by additive manufacturing using such parameter set; - an analysis step in which the sample is analyzed to obtain a distribution of at least one dimension of the pores of the sample; - an obtaining step in which, for each parameter set, a production rate for producing a part using that parameter set is obtained; a selection step in which one of the parameter sets is selected based on characteristic values ​​for the pore size distribution and for the production rate associated with each parameter set; -Steps for additive manufacturing of the part using the selected parameter set Includes:

[0012] Such a method provides fast and comprehensive access to information related to the porosity of a sample, thus making it possible to select a set of parameters for manufacturing a part that achieves 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, the scanning speed at which the laser beam scans each powder layer; the spacing between two scan lines of the laser beam on each powder layer, and / or -Thickness of each powder layer Includes:

[0014] Note that pores are voids within a solid material, and therefore porosity is the ratio of the volume occupied by the pores to the total volume of the pores and the solid material.

[0015] The analyzing step may include a calculating step in which at least one characteristic value of the distribution associated with the parameter set used to create the sample is calculated.

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

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

[0018] The dimensions may be, for example, maximum pore diameter, minimum pore diameter, pore surface area, and / or an indication of whether the pores are spherical or linear in character.

[0019] The image analysis is, for example, a grayscale analysis with a threshold to distinguish between pores and solid material.

[0020] The pore size obtained in the analysis step may be the maximum pore diameter or the minimum pore diameter.

[0021] Such features provide information about the linear or spherical nature of each of the observed pores and therefore characterize the predominant porosity type in the material. Different porosity types are more or less important for different intended roles of the part.

[0022] The terms "maximum diameter" and "minimum diameter" of a pore are called Feret diameters and correspond substantially to the diameters of the circles circumscribing and inscribing the pore, respectively.

[0023] The analyzing step may also include a filtering step, during which parameter sets associated with samples containing at least one pore with a maximum diameter greater than a threshold value are eliminated.

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

[0025] The presence of linear type pores with such dimensions significantly limits the fatigue resistance of parts subjected to mechanical stress.

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

[0027] Such a feature allows the pore size distribution to be characterized by a small number of values ​​that concentrates the information for ease of processing.

[0028] The method for additive manufacturing may include graphically representing the minimum, first quartile, median, third quartile, and / or maximum associated with each parameter set in the form of a box plot arranged according to the production rate using the associated parameter set.

[0029] Such a feature simplifies the analysis by clearly presenting the results obtained for different samples, thus facilitating the selection of parameter sets.

[0030] The selecting step may include determining at least one condition related to pore size distribution and selecting a parameter set that is associated with the highest production rate and satisfies each of the conditions.

[0031] Such a feature simplifies the selection step and makes it possible to select a parameter set that allows for high production rates.

[0032] The conditions may be chosen according to the intended role of the part and / or the mechanical and / or thermal stresses to which the part will be subjected.

[0033] Such a feature allows a criterion for selecting a parameter set adapted to the intended role of the part in order to obtain a set that allows the best production speed.

[0034] For example, parts intended to be subjected to repeated mechanical stresses are more susceptible to significant porosity, particularly significant linear porosity, i.e., porosity with a large value for the largest diameter. In comparison, parts that are subjected to little mechanical and / or thermal stresses are at lower risk of fatigue and may exhibit higher, more linear porosity.

[0035] The invention will be better understood and other features and advantages will become more apparent on reading the following detailed description, which includes embodiments given by way of illustration with reference to the accompanying drawings, presented as non-limiting examples, which may help to complete the understanding of the invention and the description of its implementations and, where appropriate, contribute to its definition. [Brief explanation of the drawings]

[0036] [Figure 1] FIG. 1 is a schematic perspective view of a method for additive manufacturing by selective laser sintering on a metal powder bed in accordance with the present invention. [Figure 2] FIG. 1 shows a detailed cross-sectional view of a pore in a part obtained by additive manufacturing. [Figure 3] 1 is a graphical representation of the distribution of pore sizes within samples obtained by additive manufacturing. [Figure 4] 4 is a graphical representation of characteristic values ​​of the distribution of FIG. 3. [Figure 5] 1 is a graphical representation of the property values ​​obtained for several samples during the selection step. DETAILED DESCRIPTION OF THE INVENTION

[0037] A method for additive manufacturing of metal parts by using metal powders is described. This type of additive manufacturing method is useful for parts with complex three-dimensional shapes, such as heat exchangers, particularly for aerospace applications.

[0038] The metal powder used in such a method for additive manufacturing is, for example, an aluminum alloy, in particular an aluminum alloy called AlSi7Mg0.6, which contains 7% by weight of silicon and 0.7% by weight of magnesium.

[0039] The method for additive manufacturing according to the invention comprises a preliminary step aimed at determining a set of parameters for the additive manufacturing of the part, which makes it possible to reach a good compromise between material quality and strength on the one hand and manufacturing speed on the other hand.

[0040] Figure 1 is a schematic perspective view of a method for additive manufacturing using selective laser sintering on a metal powder bed in accordance with the present invention. More specifically, Figure 1 illustrates the steps in the additive manufacturing of a part performed by an additive manufacturing apparatus, as well as various parameters that affect the additive manufacturing steps.

[0041] The additive manufacturing device comprises, in a known manner, a tank containing metal powder which is deposited by a scraper into at least one layer 10, in particular a continuous layer 10. The layer 10 is substantially flat and extends in a substantially horizontal plane XY.

[0042] The continuous layers 10 can be stacked in the height direction Z.

[0043] The layer 10 has a thickness H, which is controlled by the control system of the additive manufacturing device, in particular by regulating the movement of the tank from one layer 10 to the next layer 10.

[0044] The additive manufacturing apparatus also includes a laser source and optics (not shown) adapted to generate and control a laser beam 12 to form a movable point of incidence 14 on the layer 10 .

[0045] The incident point 14 sweeps over a predetermined portion of the layer 10 to locally melt the layer 10 to add material to the manufactured part.

[0046] The power of the laser beam 12 is also controlled by the control system to vary the melting depth of the powder, the power being, for example, the average power calculated over the cross section of the laser beam 12.

[0047] For each layer 10, the beam 12 scans a predetermined portion of the layer 10, particularly along lines spaced apart in the spacing direction and particularly parallel to the scanning direction.

[0048] 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 that is perpendicular to the first transverse direction X.

[0049] In the illustrated example, the scan and spacing directions alternate from one layer 10 to the next layer 10 parallel to a first transverse direction X and a second transverse direction Y.

[0050] The point of incidence 14 moves along the path line with a velocity V, which determines the time it takes the beam to travel across a given area of ​​each layer 10 .

[0051] The scan lines are spaced apart in the spacing direction by a predetermined pitch E, also called vector spacing, chosen in particular to be substantially equal to the width of the incident points 14 in the spacing direction.

[0052] By selecting values ​​for the above parameters, a set of additive manufacturing parameters is defined. Since most of these parameters are interdependent, the following four parameters are defined: - power P of the laser beam, - scanning speed V, -vector spacing E, and - thickness H of layer 10 Selecting values ​​for allows for creating a parameter set sufficient to manufacture the part.

[0053] Other combinations of parameters are also conceivable for characterizing an additive manufacturing step and forming a parameter set within the meaning of the present invention.

[0054] A method for additive manufacturing includes determining a plurality of parameter sets that can be tested to select the parameters therefrom that are most suitable for manufacturing a mechanical part by additive manufacturing.

[0055] Parameter sets may be determined based on known parameter values ​​used in previous production of similar parts and selected to sweep across a range of values ​​to find the optimum combination.

[0056] The method for additive manufacturing then includes creating at least one sample for each parameter set by additive manufacturing using such parameter sets.

[0057] In particular, the sample has a simple shape, for example a substantially rectangular parallelepiped shape, that allows for easy observation.

[0058] The sample dimensions are determined to present a surface of sufficient size to represent the material in the statistical porosity analysis.

[0059] One of the surfaces of the sample is then polished to allow for capturing at least one clear image of the surface and to allow for image analysis.

[0060] The method for additive manufacturing then includes a sample analysis step.

[0061] The sample analysis step may be performed using optical methods. In such cases, the optical methods may, for example, include capturing a two-dimensional image of the sample surface, preferably a polished surface of the sample. The sample analysis step may then include using thresholding to analyze the acquired image in order to distinguish between solid portions and porosity of the material.

[0062] Other methods are possible, for example three-dimensional observation by X-ray tomography, which allows the entire volume of the sample to be reconstructed.

[0063] The sample analysis step allows for the identification of each of the pores on the sample surface and for the analysis of the shape of the pores.

[0064] The sample analyzing step may also include obtaining a distribution of at least one dimension of pores of the sample.

[0065] An example is shown in Figure 2, which is a detailed cross-sectional view of a pore 20 in a part obtained by additive manufacturing. The opening of the pore 20 is in the surface of the sample, in particular the polished surface.

[0066] FIG. 2 shows two examples of characteristic dimensions of pores 20: a maximum diameter D1, which is also the diameter of a circle CC circumscribing around the pore 20, for example considered in the plane of the image; a minimum diameter D2, which is also the diameter of the circle CI inscribed in the pore 20, considered for example in the plane of the image; Shows.

[0067] By convention, the pores 20 are - considered spherical if the ratio D1 / D2 is less than 2; - If the ratio D1 / D2 is 2 or more, it is considered linear.

[0068] The spherical or linear nature of the pores 20 may also function as the pore size according to the present invention.

[0069] A maximum diameter D1 and a minimum diameter D2 are determined for each of the pores 20 in the two-dimensional image. For each analyzed sample, a distribution of each of the maximum diameter D1 and minimum diameter D2 is obtained.

[0070] Figure 3 is a graphical representation of the distribution of maximum pore diameters D1 in micrometers obtained for such samples obtained by additive manufacturing, represented as both a histogram and a log-normal and gamma distribution type regression curve.

[0071] Such a display allows most of the information to be preserved by visualizing it, but does not facilitate comparison of multiple samples.

[0072] To remedy this, the analyzing step may then include calculating at least one characteristic value of the distribution associated with each parameter set used to create the analyzed sample.

[0073] As shown in Figure 4, which is a graphical representation of the characteristic values ​​of the distribution of Figure 3, the characteristic values ​​of the distribution include, for example, the minimum value Min, the first quartile Q1, the median Med, the third quartile Q3, and the maximum value Max of the distribution. The characteristic values ​​are represented in the form of a box plot or Tukey box plot. Such plots allow for quick evaluation of different values ​​and facilitate comparison of multiple samples.

[0074] The plot is advantageously complemented with additional information obtained by image analysis, such as the number N of distinct pores detected.

[0075] Advantageously, the method for additive manufacturing may comprise a sorting step in which parameter sets associated with each sample containing at least one pore whose maximum diameter D1 is greater than a threshold value are eliminated. The threshold value is determined in particular according to the intended role of the mechanical part and / or the mechanical and / or thermal stresses to which the mechanical part is subjected. For example, the threshold value may be equal to 100 micrometers.

[0076] The method for additive manufacturing also includes an obtaining step in which, for each parameter set, a manufacturing rate Vf of the part using such parameter set is obtained.

[0077] The part production rate Vf is determined by calculating the volume of material deposited 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 method for additive manufacturing then includes selecting one of the parameter sets Js based on previously obtained characteristic values ​​and manufacturing rates for each parameter set.

[0079] The selection may include, for example, graphically representing the minimum, first quartile, median, third quartile, and / or maximum values ​​associated with each parameter set in the form of a box plot, and the plots may be ordered according to the production rate using the associated parameter set.

[0080] Such a graphical representation of the characteristic values ​​obtained for several samples during the selection step is presented in FIG. 5. More specifically, FIG. 5 shows the characteristic values ​​of the distribution of the diameter D1 of the largest pores 20 obtained for 15 separate samples. The plot is arranged on the abscissa according to the production rate Vf expressed in cubic centimeters per hour. The total number N of detected pores is also shown on this graph.

[0081] Such a graphical representation provides a good visualization of the trade-off between material quality and associated production speed.

[0082] In the example shown in Figure 5, for a part requiring good mechanical strength, the selected parameter set Js is associated with a distribution of pores 20 with the smallest maximum diameter D1 and highly nonlinear properties, which indicates good resistance to mechanical and thermal stresses.

[0083] The parameter set Js is -414 watts laser beam power P, -0.08 mm vector spacing E, - a layer thickness H of 0.06 mm, and -1.65 meters per second scanning speed V Includes:

[0084] The associated production rate Vf is 28.5 cubic centimeters per hour.

[0085] If lower mechanical strength is acceptable but speed of fabrication is important, another compromise can be chosen, for example using a second parameter set Js'.

[0086] Using this method, a third parameter set can be determined which, for a production rate Vf equal to 54.4 cubic centimeters per hour, provides a material of quality even better than that obtained with the second parameter set Js' but lower than that obtained with the first parameter set Js.

[0087] The third set of parameters is -laser beam power P, equal to 440 watts Vector spacing E, equal to -0.12 mm - the layer thickness H equal to 0.09 mm, and - Scanning speed V equal to 1.4 meters per second Includes:

[0088] Alternatively, the selecting step may include determining at least one condition related to the distribution of pore sizes and selecting a parameter set that is associated with the highest production rate and satisfies each of the conditions.

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

[0090] The conditions are also selected based on the intended role of the part and / or the type of stress involved.

[0091] For example, if the part is subjected to high mechanical and / or thermal stresses, such as a heat exchanger, material quality and robustness will be more important, whereas if the part is subjected to low stresses, such as an equipment housing, lower fatigue resistance will be a consideration.

[0092] Finally, the method for additive manufacturing includes additively manufacturing the part using the selected parameter set.

Claims

1. 1. A method for additive manufacturing of a metal part by selective sintering of at least one layer (10) of metal powder by a laser beam (12) controlled by a control system, the method for additive manufacturing comprising at least a determination step in which a plurality of parameter sets are determined, in particular each parameter set comprising at least power (P) of the laser beam (12), the scanning speed (V) at which the laser beam (12) scans the layer (10), the distance (E) between the two scanning lines of the laser beam (12) on each layer (10), and / or Thickness (H) of each layer (10) a determining step including: - a fabrication step, in which for each parameter set, at least one sample is fabricated by additive manufacturing using such parameter set; an analysis step in which the sample is analyzed in order to obtain a distribution of at least one dimension (D1, D2) of the pores (20) of the sample; - an obtaining step in which, for each parameter set, a production velocity (Vf) for the production of a part using that parameter set is obtained; a selection step in which one of the parameter sets (Js) is selected based on characteristic values ​​for the distribution of the dimensions (D1, D2) of the pores (20) and for the production rate (Vf) associated with each parameter set; - additive manufacturing of the part using the selected parameter set; 1. A method for additive manufacturing comprising:

2. 10. The method for additive manufacturing of claim 1, wherein the analyzing step includes a calculating step in which at least one characteristic value of a distribution associated with a parameter set used to create the sample is calculated.

3. 3. The method for additive manufacturing of claim 1 or 2, wherein the analyzing step comprises collecting a two-dimensional image of a surface of the sample and analyzing the image to identify at least one pore (20) in the sample.

4. 4. The method for additive manufacturing according to any one of claims 1 to 3, wherein the dimension of the pore (20) obtained in the analysis step is the maximum diameter (D1) of the pore (20) or the minimum diameter (D2) of the pore (20).

5. 5. The method for additive manufacturing according to claim 4, wherein the analyzing step comprises a screening step, during which parameter sets associated with samples comprising at least one pore (20) having a maximum diameter (D1) greater than a threshold value are eliminated.

6. 6. The method for additive manufacturing according to any one of claims 1 to 5, wherein during the selection step the characteristic value of each distribution of pore (20) dimensions comprises the minimum (Min), first quartile (Q1), median (Med), third quartile (Q3), and / or maximum value (Max) of the distribution.

7. 7. The method for additive manufacturing of claim 6, comprising graphically representing the minimum (Min), first quartile (Q1), median (Med), third quartile (Q3), and / or maximum value (Max) associated with each parameter set in the form of a box plot ordered by the production rate (Vf) using the associated parameter set.

8. 8. The method for additive manufacturing according to any one of claims 1 to 7, wherein the selecting step comprises determining at least one condition related to the distribution of the dimensions (D1, D2) of the pores (20) and selecting a parameter set related to a maximum production rate (Vf) and satisfying each of the conditions.

9. 9. The method for additive manufacturing according to claim 8, wherein the conditions are chosen according to the intended role of the part and / or the mechanical and / or thermal stresses to which the part will be subjected.

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