Method for additive manufacturing of a metal part

Abstract

The invention relates to a method for additive manufacturing of a metal part by selectively melting at least one layer (10) of a metal powder by means of a laser beam (12) which is controlled by a control system, the additive manufacturing method comprising at least: - a determination step, during which a plurality of sets of parameters are determined; - a production step, during which at least one sample is produced, for each set of parameters, by additive manufacturing using such a set of parameters; - an analysis step, during which the sample is analysed to obtain a distribution of at least one dimension (D1, D2) of pores (20) of the sample; - an obtaining step, during which, for each set of parameters, a speed (Vf) for manufacturing the part using the set of parameters is obtained; - a selection step, during which one of the sets of parameters (Js) is selected from the characteristic quantities of the distribution of the dimension (D1, D2) of the pores (20) and of the manufacturing speed (Vf) associated with each set of parameters; and - a step of additive manufacturing of the part using the selected set of parameters.

Description

[0001] DESCRIPTION

[0002] TITLE: Additive manufacturing process for a metal part

[0003] Technical field of the invention

[0004] The invention relates to the field of additive manufacturing, in particular additive manufacturing by selective laser melting on a powder bed, also known by the acronym "SLS" for "Selective Laser Sintering" in English.

[0005] More specifically, the invention relates to a method of manufacturing a mechanical part by additive manufacturing.

[0006] State of the prior art

[0007] A method is known which consists of manufacturing at least one part, in particular one or more metal part(s), by selective melting of successive layers of metal 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.

[0008] A step of selective fusion of successive layers of powder involves a certain number of parameters influencing the final structure of the material and, thus, the mechanical properties of the manufactured part.

[0009] Such parameters include variables related to the laser beam, such as a laser beam radius and an average power, to the path of the powder layer, such as a path speed and a gap between two neighboring scan lines, or to the granular material, such as a grain size and a thickness of successive powder layers.

[0010] The selection of a parameter set for a selective melting process is generally based on manufacturing samples with several parameter sets and measuring the material density by optical analysis of a polished surface of each sample.

[0011] Porosities detected by image analysis are used to define material quality and strength for the produced sample.

[0012] However, such a technique, although effective, does not allow sufficient discrimination between different sets of parameters to give satisfactory results. Indeed, the simple measurement of the density of the samples proves insufficient to effectively predict the final behavior of the part. A visual comparison of the images by an operator is also not acceptable, as it is rarely repeatable between different operators.

[0013] Presentation of the invention

[0014] The invention aims to overcome such drawbacks by enabling the development of effective parameter sets to optimize both the manufacturing speed of a part and the quality of the material.

[0015] To this end, the invention relates to a method for additive manufacturing of a metal part, by selective melting of at least one layer of a metal powder by means of a laser beam controlled by a control system. More specifically, the additive manufacturing method comprises at least:

[0016] A determining step, during which a plurality of parameter sets are determined, in particular each parameter set comprising at least a power of the laser beam, a scanning speed of each layer of powder by the laser beam, a gap between two scanning lines of the laser beam on each layer and / or a thickness of each layer,

[0017] A production step, during which at least one sample is produced, for each set of parameters, by additive manufacturing with such a set of parameters,

[0018] An analysis step, in which the sample is analyzed to obtain a distribution of at least one pore size of the sample,

[0019] An obtaining step, during which, for each set of parameters, a manufacturing speed of the part with the set of parameters is obtained,

[0020] 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

[0021] An additive manufacturing step of the part with the selected parameter set.

[0022] Such a process allows for rapid and complete access to information relating to the porosity of 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.

[0023] Advantageously, the parameter set includes the following four parameters:

[0024] - a laser beam power,

[0025] - a scanning speed of each layer of powder by the laser beam,

[0026] - a gap between two scanning lines of the laser beam on each layer of powder and / or

[0027] - a thickness of each layer of powder. Remember that a pore is an empty space included in a solid material. Consequently, a porosity is a ratio between a volume occupied by the pores on a total volume of the pores and the solid material.

[0028] 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 produce the sample is calculated.

[0029] The analyzing step may include acquiring a two-dimensional image of a surface of the sample and analyzing the image to identify at least one pore of the sample.

[0030] Such a feature makes it possible to identify each pore of a representative surface of the sample, in order to quickly and reliably determine the distribution of at least one dimension of the pores of the sample.

[0031] The dimension is, for example, a largest pore diameter, a smallest pore diameter, a pore surface area, and / or an indication of a spherical or linear character of the pore.

[0032] Image analysis is, for example, a grayscale analysis involving a discrimination threshold between pores and solid material.

[0033] The pore size obtained in the analysis step can be a larger pore diameter or a smaller pore diameter.

[0034] Such a characteristic makes it possible to obtain indications on the linear character, or the spherical character of each of the pores observed, and thus to characterize the dominant type of porosity in the material. Different types of porosity are more or less critical for different roles intended for the part.

[0035] The terms "largest diameter" and "smallest diameter" of the pore are designated by the name of Feyret diameter, and correspond approximately to the respective diameters of a circle circumscribed to the pore and a circle inscribed in the pore.

[0036] The analysis step may also include a sorting step, during which the parameter set associated with the sample comprising at least one pore whose largest diameter is greater than a limit value is eliminated.

[0037] Such a characteristic makes it possible to directly exclude parameter sets that produce porosities that are harmful to the mechanical strength of the part. The limit value is, for example, 100 micrometers.

[0038] The presence of linear pores of such dimensions is highly limiting for the fatigue resistance of parts subjected to mechanical stress.

[0039] During the selection step, the characteristic quantities of each distribution of at least one pore dimension may include a minimum, a first quartile, a median, a third quartile and / or a maximum of the distribution. Such a characteristic makes it possible to characterize the distribution of the pore dimension by a small number of quantities concentrating the information, in order to facilitate its processing.

[0040] The additive manufacturing process may include a step of graphically representing the minimums, first quartiles, medians, third quartiles and / or maximums 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.

[0041] 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 parameter set. The selection step may include determining at least one condition relating to the pore size distributions and selecting the parameter set associated with the highest manufacturing speed and meeting each of the conditions.

[0042] Such a feature makes it possible to simplify the selection step and to select a set of parameters allowing a significant manufacturing speed.

[0043] Conditions can be chosen based on an intended role of the part and / or the mechanical and / or thermal stresses to which the part will be exposed.

[0044] Such a feature allows 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.

[0045] For example, a part designed to undergo repeated mechanical stresses will be more sensitive to significant porosity, particularly significant linear porosity, i.e., having large values ​​of larger diameter. In comparison, a part subjected to only a few mechanical and / or thermal stresses presents less risk of fatigue and may have higher and more linear porosity.

[0046] Brief description of the figures

[0047] The invention will be better understood and other characteristics and advantages will become apparent upon reading the detailed description which follows, comprising embodiments given for illustrative purposes with reference to the appended figures, presented as non-limiting examples, which may serve to complete the understanding of the invention and the description of its implementation and, where appropriate, contribute to its definition, in which:

[0048] - [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;

[0049] - [Fig. 2] is a detailed sectional view of a pore of a part obtained by additive manufacturing;

[0050] - [Fig. 3] is a graphical representation of a distribution of a pore size in a sample obtained by additive manufacturing; - [Fig. 4] is a graphical representation of characteristic quantities of the distribution of Figure 3; and

[0051] - [Fig. 5] is a graphical representation of the characteristic quantities obtained for several samples during a selection step.

[0052] Detailed description of the invention

[0053] A process for additive manufacturing of 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.

[0054] 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 AISi7Mg0.6, comprising 7% by mass of silicon and 0.7% by mass of magnesium.

[0055] The additive manufacturing method according to the invention comprises preliminary steps aimed at determining a set of parameters for the additive manufacturing of the part, making it possible to obtain a good compromise between, on the one hand, quality and resistance of the material and, on the other hand, manufacturing speed.

[0056] Figure 1 is a schematic perspective view of an additive manufacturing process by selective laser beam melting on a metal powder bed according to the invention. More specifically, Figure 1 illustrates a step of additive manufacturing of the part, implemented by means of an additive manufacturing device, as well as the different parameters influencing the additive manufacturing step.

[0057] The additive manufacturing device comprises, in a known manner, a tank 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 plane XY.

[0058] Successive layers 10 can be stacked in a Z elevation direction.

[0059] Layer 10 has a thickness H controlled by a control system of the additive manufacturing device, in particular by adjusting the movements of the tank from one layer 10 to the next.

[0060] The additive manufacturing device also comprises a laser source and optics (not shown), adapted to generate and control a laser beam 12 so as to form a movable point of incidence 14 on the layer 10.

[0061] The point of incidence 14 scans a predetermined portion of the layer 10 in order to locally melt the layer 10 to add material to the manufactured part. A power of the laser beam 12 is also controlled by a control system, so as to vary the melting depth of the powder. The power is, for example, an average power calculated over a cross-section of the laser beam 12.

[0062] 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 according to the spacing direction.

[0063] 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.

[0064] 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.

[0065] The point of incidence 14 follows the path lines at a speed V, which determines the time taken by the beam to travel the entire predetermined region of each layer 10.

[0066] The scanning lines are spaced along the spacing direction by a predetermined pitch E, also called vector spacing, in particular chosen to be substantially equal to a width of the point of incidence 14 along the spacing direction.

[0067] An additive manufacturing parameter set is defined by selecting values ​​for the parameters mentioned above. Since most of these parameters are interdependent, selecting values ​​for the following four parameters allows for a sufficient parameter set for manufacturing the part, namely:

[0068] - the power P of the laser beam,

[0069] - the scanning speed V,

[0070] - the vector deviation E and

[0071] - the thickness H of layer 10.

[0072] Other combinations of parameters can be envisaged to characterize the additive manufacturing step and form a set of parameters within the meaning of the invention.

[0073] The additive manufacturing process comprises a step of determining a plurality of sets of parameters, capable of being tested in order to select from among them the one which will be most suitable for manufacturing the mechanical part by additive manufacturing.

[0074] Parameter sets can be determined based on known parameter values ​​used for previous builds of similar parts, and selected to scan ranges of values ​​to find an optimal combination.

[0075] The additive manufacturing method then comprises a step of producing at least one sample, for each set of parameters, by additive manufacturing with such a set of parameters. In particular, the sample has a simple geometry allowing easy observation, for example a substantially parallelepiped geometry.

[0076] The sample dimensions are determined to present surfaces of sufficient size to be representative of the material during a statistical porosity analysis.

[0077] One of the sample surfaces is then polished to allow the acquisition of at least one clear image of the surface allowing image analysis.

[0078] The additive manufacturing process then includes a sample analysis step.

[0079] The sample analysis step may be performed by an optical method. In such a case, the optical method may comprise, for example, an acquisition of a two-dimensional image of the surface of the sample, preferably the polished surface of the sample. Subsequently, the sample analysis step may comprise an analysis of the image thus obtained by thresholding to discriminate a solid part of the material and a porosity.

[0080] Other methods are possible, for example three-dimensional observation by X-ray tomography, allowing the entire volume of the sample to be recreated.

[0081] The sample analysis step allows each of the pores on the sample surface to be identified and a pore geometry to be analyzed.

[0082] The step of analyzing the sample may also include obtaining a distribution of at least one pore size of the sample.

[0083] An example is shown in Figure 2 which is a detailed sectional view of a pore 20 of the part obtained by additive manufacturing. The pore 20 opens into the surface, in particular the polished surface, of the sample.

[0084] Figure 2 shows two examples of characteristic dimensions of pore 20:

[0085] - a larger diameter D1, which is also a diameter of a circumscribed circle CC at the pore 20, considered for example in a plane of the image, and

[0086] - a smaller diameter D2, which is also a diameter of an inscribed circle Cl at pore 20, considered for example in the plane of the image.

[0087] By convention, pore 20 is considered as

[0088] - spherical in nature if the D1 / D2 ratio is less than 2, and

[0089] - linear in nature if the ratio D1 / D2 is greater than or equal to 2.

[0090] The spherical or linear nature of the pore 20 may also act as a pore dimension according to the invention.

[0091] The largest diameter D1 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 D1 and the smallest diameters 2 is obtained for each sample analyzed. Figure 3 is a graphical representation of the distribution of the largest pore diameter D1, in micrometers, obtained for such a sample obtained by additive manufacturing, represented both in the form of a histogram and regression curves of LogNormal and Gamma distribution type.

[0092] Such representations allow a large part of the information to be retained by making it observable, but do not facilitate the comparison of a large number of samples. To remedy this, the analysis step can then include a step of calculating at least one characteristic quantity of the distribution associated with each set of parameters used to produce the analyzed samples.

[0093] As shown in Figure 4 as a graphical representation of characteristic quantities of the distribution of Figure 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.

[0094] Characteristic quantities are represented in the form of a box-and-whisker plot, or Tukey diagram. Such a diagram allows a rapid evaluation of the different quantities and facilitates the comparison of several samples.

[0095] The diagram is advantageously supplemented with additional information obtained through image analysis, such as a number N of distinct pores detected.

[0096] Advantageously, the additive manufacturing process may comprise a sorting step, during which the parameter sets associated with each sample comprising at least one pore whose largest diameter D1 is greater than a limit value are eliminated. The limit value is, in particular, determined according to the intended role of the mechanical part and / or the mechanical and / or thermal constraints to which it will be exposed. For example, the limit value may be equal to 100 micrometers.

[0097] The additive manufacturing process also includes an obtaining step, during which, for each set of parameters, a manufacturing speed Vf of the part with such a set of parameters is obtained.

[0098] 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.

[0099] 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.

[0100] The selection may include, for example, a step of graphically representing the minimums, first quartiles, medians, third quartiles and / or maximums associated with each parameter set in the form of box and whisker diagrams. The diagrams may be ordered according to the manufacturing speed with the associated parameter set.

[0101] Such a graphical representation of the characteristic quantities obtained for several samples during a selection step is presented in Figure 5. More specifically, Figure 5 shows the characteristic quantities of the distributions of the largest diameters D1 of pore 20, obtained for fifteen distinct samples. The diagrams are classified on the abscissa according to the manufacturing speed Vf, expressed in cubic centimeters per hour. The total number N of pores detected is also represented on this graph.

[0102] Such a graphical representation allows a good visualization of the compromise between the quality of the material and the associated manufacturing speed.

[0103] In the example shown in Figure 5, for a part requiring good mechanical strength, the selected parameter set Js is associated with the pore distribution 20 having the fewest largest diameters D1, and a very non-linear character, which indicates good resistance to mechanical and thermal stresses.

[0104] The Js parameter set includes

[0105] - a power P of the laser beam 12 equal to 414 Watts,

[0106] - a vector deviation E equal to 0.08 millimeters,

[0107] - a layer thickness H equal to 0.06 millimeters and

[0108] - a scanning speed V equal to 1.65 meters per second.

[0109] The associated production speed Vf is equal to 28.5 cubic centimeters per hour.

[0110] If lower mechanical strength is acceptable but production speed is important, another compromise can be chosen, for example with a second set of Js' parameters.

[0111] A third set of parameters could be determined using this method, providing an even better material quality 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.

[0112] The third set of parameters includes

[0113] - a power P of the laser beam equal to 440 Watts,

[0114] - a vector deviation E equal to 0.12 millimeters,

[0115] - a layer thickness H equal to 0.09 millimeters and

[0116] - a scanning speed V equal to 1.4 meters per second.

[0117] Alternatively, the selection step may comprise determining at least one condition relating to the pore size distributions and selecting the parameter set associated with the highest manufacturing speed and meeting each of the conditions. Such conditions may, for example, be determined by mechanical and / or thermal testing.

[0118] The conditions are also chosen based on the intended role of the part and / or the associated type of stresses. For example, if the part is highly mechanically and / or thermally stressed, such as a heat exchanger, greater importance will be given to the quality and robustness of the material, while if the part is less stressed, such as an equipment casing, lower fatigue resistance will be considered.

[0119] The additive manufacturing process finally includes a step of additive manufacturing of the part with the selected set of parameters.

Claims

CLAIMS 1. Method for additive manufacturing of a metal part, by selective melting of at least one layer (10) of a metal powder by means of a laser beam (12) controlled by a control system, the additive manufacturing method comprising at least: - A determination step, during which a plurality of sets of parameters are determined, in particular each set of parameters comprising at least: o a power (P) of the laser beam (12), o a scanning speed (V) of the layer (10) by the laser beam (12), o a gap (E) between two scanning lines of the laser beam (12) on each layer (10) and / or o a thickness (H) of each layer (10), A production 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, An obtaining step, during which, for each set of parameters, a manufacturing speed (Vf) of the part with the set of parameters is obtained, A selection step, during which one of the parameter sets (Js) is selected from the characteristic quantities of the distribution of the dimension (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.

2. Additive manufacturing method according to the preceding claim, in which the analysis step comprises a calculation step during which at least one characteristic quantity of the distribution associated with the set of parameters used to produce the sample is calculated.

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

4. Additive manufacturing method according to any one of the preceding claims, wherein the dimension of the pores (20) obtained in the analysis step is a larger diameter (D1) of the pore (20) or a smaller diameter (D2) of the pore (20).

5. Additive manufacturing method according to the preceding claim, in which the analysis step comprises a sorting step, during which the set of parameters associated with the sample comprising at least one pore (20) whose largest diameter (D1) is greater than a limit value is eliminated.

6. Additive manufacturing method according to one of the preceding claims, wherein during the selection step, the characteristic quantities of each distribution of the pore size (20) comprise 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 graphical representation of the minimums (Min), the first quartiles (Q1), the medians (Med), the third quartiles (Q3) and / or the maximums (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. Additive manufacturing method according to one of the preceding claims, in which the selection step comprises the determination of at least one condition relating to the distributions of the dimension (D1, D2) of the pores (20) and the selection of the set of parameters associated with the highest manufacturing speed (Vf) and respecting each of the conditions.

9. Additive manufacturing method according to the preceding claim, in which the conditions are chosen according to an intended role of the part and / or the mechanical and / or thermal constraints to which the part will be exposed.

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