Method for manufacturing an object by additive manufacturing of powder layers

Abstract

Disclosed is a method for additive manufacturing of an object from a powder layer, the method comprising the steps of: projecting an energy beam in a spot onto the surface of the powder layer to melt the powder (200), scanning the surface with the energy beam such that the spot moves on the surface with a movement comprising a translation along a longitudinal scanning direction and an oscillation having at least one component in an oscillation direction (202), adjusting the focusing of the energy beam during scanning according to the translation along the longitudinal scanning direction without considering the component of the oscillation in the oscillation direction (204).

Description

Technical Field

[0001] The present invention relates to a method for additive manufacturing of an object from powder layers and an apparatus suitable for implementing the method. Background Art

[0002] Additive manufacturing refers to manufacturing an object by melting powder layers stacked on top of each other. These powder layers correspond to the respective cross-sections of the object to be manufactured.

[0003] To melt the powder layer, an energy source projects an energy beam in the form of a light spot onto the surface of the powder layer, and melting occurs at the light spot. Then, the energy beam is controlled to scan the surface so that this melting spreads over the entire surface of the powder layer.

[0004] Typically, the energy beam scans the respective regions of the surface in the longitudinal direction and in an alternating outward and return direction.

[0005] Furthermore, it has been proposed to control the energy source such that the light spot does not travel on the surface with a completely straight translational movement in the longitudinal direction, but with a movement including translational and oscillatory movements in the longitudinal direction. The oscillatory movement oscillates particularly in the transverse direction at a relatively high frequency and a relatively low amplitude to enlarge the melt pool. The oscillatory movement is typically obtained by directing the energy beam in a pendulum-like manner to a certain angular range.

[0006] Furthermore, it is desirable to finely control the size of the light spot to avoid excessive fluctuations in the amount of energy applied to the powder layer per unit area. The size of the light spot depends on the distance covered by the energy beam between the energy source and the surface and itself varies according to the inclination angle of the energy beam with respect to the surface. Therefore, the oscillatory movement contributes to the size of the light spot changing at a relatively high frequency. To illustrate this, Figure 1 the path of the energy beam projected onto the surface of a plane perpendicular to the longitudinal direction is depicted. In Figure 1 the transverse direction is horizontal. The path of the energy beam can move around an axis parallel to the longitudinal direction and pass through a fixed point P. The markings used are as follows:

[0007] · S is the center of the light spot where the energy beam is projected onto the surface.

[0008] · r is the minimum distance between point P and point S.

[0009] · α is the half angle of oscillation of the energy beam.

[0010] · L is the distance between point P and the surface S covered by the energy beam inclined at an angle α.

[0011] · A is half of the amplitude of the oscillation of point S in the transverse direction in the plane.

[0012] During the oscillation, the distance covered by the energy beam between point P and point S changes by a distance difference d, resulting in:

[0013]

[0014] The value of the distance difference d is very small. For example, for r = 700 mm and A = 0.3 mm, a value of d = 0.06 μm is obtained.

[0015] In order to maintain a completely constant spot size during scanning, the focusing device must take into account the oscillation and thus must take into account this very small distance difference d. Summary of the Invention

[0016] An object of the present invention is to be able to finely control the amount of energy per unit area applied by an energy beam to a powder layer during the additive manufacturing of an object, while obtaining an enlarged melt pool without premature wear of the energy source emitting the energy beam.

[0017] To this end, one aspect of the present invention proposes a method for additive manufacturing an object from a powder layer, the method comprising the following steps:

[0018] - Projecting an energy beam in the form of a spot onto the surface of the powder layer to melt the powder,

[0019] - Scanning the surface with the energy beam such that the spot moves across the surface in a movement comprising a translation along a longitudinal scanning direction and an oscillation having at least one component along an oscillation direction,

[0020] - Adjusting the focusing of the energy beam during scanning according to the translation along the longitudinal scanning direction without considering the component of the oscillation movement along the oscillation direction.

[0021] A focusing device that adjusts the focusing of the energy beam according to the oscillation enables a theoretically time-invariant spot size to be obtained. However, the inventors have found that a focusing device configured in this way wears out very quickly due to the relatively high frequency and relatively low amplitude of the oscillation.

[0022] Therefore, by adjusting the focusing of the energy beam during scanning without considering the component of the oscillation movement along the oscillation direction, as in the method according to the first aspect, this premature wear can be avoided. However, adjusting the focusing of the energy beam according to the translation along the longitudinal direction can indeed limit the significant fluctuations in the spot size caused by the translational movement. Therefore, the amount of energy applied per unit area using the method according to the first aspect varies in an acceptable proportion.

[0023] The method according to the first aspect may further comprise the following optional features, considering the optional features individually or in combination with each other where the combination of optional features is technically feasible.

[0024] Preferably, the oscillatory movement includes a lateral component along a lateral scan direction perpendicular to the longitudinal scan direction, and the focusing of the energy beam is adjusted without considering the lateral component of the oscillatory movement.

[0025] Preferably, the lateral component of the oscillatory movement oscillates at a frequency of at least 1 kHz.

[0026] Preferably, the lateral component of the oscillatory movement oscillates with an amplitude between 100 microns and 2 millimeters.

[0027] Preferably, the oscillatory movement includes a longitudinal component along the longitudinal scan direction, and the focusing of the energy beam is adjusted without considering the longitudinal component of the oscillatory movement.

[0028] Preferably, the lateral component of the oscillatory movement oscillates at a frequency of at least 1 kHz.

[0029] Preferably, the longitudinal component of the oscillatory movement oscillates with an amplitude between 100 microns and 2 millimeters.

[0030] Preferably, the path includes successive loops offset from each other along the longitudinal scan direction.

[0031] Preferably, the focusing of the energy beam is adjusted using pre-computed focusing parameter values computed prior to projecting the energy beam, each pre-computed focusing parameter value being associated with the position of a spot on the surface.

[0032] A second aspect of the present invention also provides an apparatus for additive manufacturing an object from a powder layer, the apparatus comprising an energy source configured to:

[0033] - project an energy beam in the form of a spot onto the surface of the powder layer to melt the powder,

[0034] - control the scanning of the surface with the energy beam such that the spot travels over the surface with a movement comprising a translation along the longitudinal scan direction and an oscillatory movement having at least one component along an oscillatory direction,

[0035] - adjust the focusing of the energy beam during the scanning according to the translation along the longitudinal scan direction without considering the component of the oscillatory movement along the oscillatory direction. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Other features, objects, and advantages of the present invention will become apparent from the following description, which is illustrative and non-limiting and should be read in conjunction with the accompanying drawings, in which:

[0037] Figure 1 (already discussed) graphically depicts the travel of an oscillatory energy beam projected onto a surface in the lateral direction.

[0038] Figure 2 is a schematic view of an additive manufacturing apparatus according to an embodiment.

[0039] Figure 3 is a flowchart of steps of an additive manufacturing method according to an embodiment.

[0040] Figure 4 depicts the path followed by a light spot obtained by projecting an energy beam onto a surface during the implementation of the method with respect to Figure 3 In all the figures, similar elements have the same reference numerals.

[0041] In all the figures, similar elements have the same reference numerals. DETAILED DESCRIPTION

[0042] Additive Manufacturing Apparatus

[0043] Referring to Figure 2 , the additive manufacturing apparatus includes an energy source 1 and a support 140.

[0044] The support 140 has a free surface (which is generally flat) extending in two directions: a longitudinal direction and a transverse direction perpendicular to the longitudinal direction. Hereinafter, the longitudinal direction will conventionally be denoted as X and the transverse direction as Y.

[0045] The free surface of the support 140 is intended to serve as a support surface 140 for a powder layer 150 or a plurality of powder layers 150 stacked on top of each other.

[0046] Generally, the energy source 1 is designed to project an energy beam towards the support 140. When a powder layer 150 is deposited on the support 140, this energy beam is projected in the form of a light spot onto the upper surface of the powder layer 150.

[0047] The energy source 1 particularly includes a generating device 110 configured to generate an energy beam. For example, the generating device 110 is a laser source; the thus-generated energy beam is a laser beam containing photons, or in other words, a light beam. Alternatively, the generating device 110 is of the Electron Beam Melting (EBM) type designed to generate an electron beam. Hereinafter, the non-limiting case of a laser beam will be considered.

[0048] The energy source 1 further includes a focusing device designed to adjust the focusing of the light beam. Thus, this focusing device is capable of changing the size of the light spot at which the energy beam is projected onto the upper surface of the powder layer 150 deposited on the support 140.

[0049] For example, the focusing device includes a focusing element 1102 and a focusing lens 1101, and the focusing lens 1101 is capable of translational movement relative to the focusing element parallel to the optical axis of the lens. The focusing lens 1101 is located downstream of the energy beam generating device 110. Hereinafter, the terms "upstream" and "downstream" implicitly refer to the propagation direction of the energy beam along the optical path extending from the generating device 110 to the support 140.

[0050] The focusing device includes an actuator for moving the focusing lens 1101 relative to the focusing element 1102.

[0051] The energy source 1 further includes a scanning device 130, and the scanning device 130 is designed to direct the energy beam such that the spot projected by the energy beam can move relative to the support 140 on the surface of the powder layer 150 in the longitudinal direction and the transverse direction.

[0052] The scanning device 130 is located downstream of the focusing device.

[0053] For example, the scanning device 130 includes a first scanning mirror 131 and a second scanning mirror 132. The first scanning mirror 131 is capable of rotationally moving relative to the support 140 about a first rotation axis 133, and the second scanning mirror 132 is capable of rotationally moving relative to the support 140 about a second rotation axis 134, and the second rotation axis 134 is different from the first rotation axis 133. One of the two scanning mirrors 131, 132 is located downstream of the other scanning mirror such that the energy beam from the generating device 110 is sequentially reflected by the two scanning mirrors before being redirected towards the support 140.

[0054] As a variant embodiment, the scanning device 130 includes a single scanning mirror that is capable of rotationally moving relative to the support 140 about a first rotation axis 133 and a second rotation axis 134. In this case, the single scanning mirror is positioned such that the energy beam from the generating device 110 is reflected by the scanning mirror before being redirected towards the support 140.

[0055] The scanning device 130 further includes at least one actuator (one actuator is used for each scanning mirror). The purpose of each actuator is to rotationally move the scanning mirror about at least one rotation axis and within the range of the scanning angle.

[0056] For example, the range of the scanning angle is adapted such that the spot covers the entire surface of the powder layer 150, or at least most of it.

[0057] For a given configuration of the scanning device, the central axis of the energy beam emitted from the generating device 110 intercepts the surface of the support 140 at a specific point. Therefore, there is a mathematical relationship between the coordinates (x, y) of this point and the angular positions of the scanning mirrors 131, 132.

[0058] In particular, the scanning device 130 is configured to cause a composite movement of the light spot projected onto the surface of the powder layer 150. This composite movement includes translations along a longitudinal scanning direction, alternately in an outward direction and a return direction opposite to the outward direction, the choice of the longitudinal scanning direction being independent of the longitudinal and transverse directions of the support 140.

[0059] The composite movement further includes an oscillatory movement of the light spot along at least one oscillatory direction on the surface of the powder layer 150 deposited on the support 140.

[0060] For example, the laser source 110 and the scanning device 130 are arranged such that the rate of surface melting (that is, the area of the powder layer 150 covered by the laser spot per unit time) is greater than 1000 cm 2 / min, for example greater than 2000 cm 2 / min, for example greater than 4000 cm 2 / min, for example less than 15000 cm 2 / min, for example less than 10000 cm 2 / min, for example on the order of 6000 cm 2 / min.

[0061] For example, the scanning device 130 is configured such that the travel rate of the light spot is between 0.5 and 10 m / s, for example, between 1 and 5 m / s, for example, equal to 1 or 2 m / s.

[0062] The energy source 1 further includes a control unit (not shown) configured to control the focusing device and the scanning device 130. This control unit is in particular configured to control the respective actuators of these different devices.

[0063] The control unit may include or be connected to a memory that stores a table of pre-calculated focusing parameter values calculated for various coordinate pairs (x, y) in the plane of the free surface of the support 140. Thus, when the light spot is centered on a point with the coordinates (x, y) of the surface of the support, the control unit is configured to control the focusing device using the focusing parameter value associated with this coordinate pair in the table of pre-calculated values.

[0064] Additive manufacturing method

[0065] Reference Figure 3 , the additive manufacturing method using the above-described device includes the following steps.

[0066] Deposit at least one powder layer 150 on the support 140, as Figure 1 shown. The powder layer 150 has a free surface extending along the longitudinal direction and the transverse direction of the support.

[0067] For example, the particle size of the powder particles ranges from 10 to 100 μm, for example, from 20 to 60 μm, for example, equal to 40 μm.

[0068] For example, the material of the powder layer 150 or each powder layer 150 has a flux between 0.5 and 10 J / mm 2 and, for example, between 1 and 5 J / mm 2 and, for example, equal to 2 J / mm 2 .

[0069] The material of the powder layer 150 or each powder layer 150 may comprise titanium and / or aluminum and / or Inconel and / or stainless steel and / or maraging steel. The material of the powder layer 150 or each powder layer 150 may consist of titanium and / or aluminum and / or Inconel and / or stainless steel and / or maraging steel.

[0070] The generating device 110 is activated to emit an energy beam. The energy beam passes through the focusing device and the scanning device 130 (step 200) before being projected onto the free surface of the powder layer 150 in the form of a light spot. Thus, the powder layer 150 is heated in the area of the light spot to melt its particles.

[0071] The scanning device 130 directs the energy beam so that the light spot translates along the longitudinal scanning direction on the surface (step 202).

[0072] During step 202, the scanning device 130 oscillates the energy beam so that this translation is modulated by an oscillatory movement; thus, the light spot moves according to the aforementioned composite movement.

[0073] The oscillatory movement can be implemented in various ways.

[0074] In a first embodiment, the oscillatory movement is performed only along the transverse scanning direction, which is perpendicular to the longitudinal scanning direction. Thus, the path followed by the light spot is a zigzag path.

[0075] In a second embodiment, the oscillatory movement includes a transverse oscillatory component along the transverse scanning direction and a longitudinal oscillatory component along the longitudinal scanning direction, the transverse scanning direction being perpendicular to the longitudinal scanning direction. In other words, such an oscillatory movement causes the light spot to oscillate on the surface of the powder layer 150 not only along the transverse scanning direction but also along the longitudinal scanning direction.

[0076] The combination of these two components makes it possible to define a two-dimensional oscillatory movement, thus defining the light spot path, which includes consecutive patterns offset from each other along the longitudinal scanning direction, the shape of these patterns depending on the specific parameters of the two components, in particular their frequency, amplitude and phase shift.

[0077] For example, when the two components of the oscillation oscillate at the same frequency, the oscillatory movement can be circular or elliptical. By combining such circular or elliptical movement with the above-described translation performed by the scanning device 130, it is possible to cause the light spot to follow a path on the surface of the powder layer 150 that includes consecutive loops offset from each other in the longitudinal direction, as Figure 4 shown. In Figure 4 , the arrow of the dashed line represents the above-described translational movement of the scanning device in the longitudinal direction.

[0078] In a variant embodiment, the oscillatory movement can have other shapes, for example, the shape of the number eight or the infinity symbol (i.e., the number eight written horizontally). Then, the light spot follows a path that includes a continuous pattern more complex than a simple loop.

[0079] When the oscillatory movement has a transverse component, the transverse component preferably oscillates at a frequency of at least 1 kHz. When the energy beam is a laser beam, this frequency is typically between 1 kHz and 10 kHz, or, when the energy beam is an electron beam, this frequency is typically between 1 kHz and 100 kHz. In addition, the transverse component of the oscillatory movement can oscillate with an amplitude between 100 microns and 2 mm.

[0080] Similarly, when the oscillatory movement has a longitudinal component, the longitudinal component preferably oscillates at a frequency of at least 1 kHz. When the energy beam is a laser beam, this frequency is typically between 1 kHz and 10 kHz, or, when the energy beam is an electron beam, this frequency is typically between 1 kHz and 100 kHz. In addition, the transverse component of the oscillatory movement can oscillate with an amplitude between 100 microns and 2 mm.

[0081] During the scanning process, it is required that the focusing device adjusts the focus of the energy beam. To make such an adjustment, the focusing lens 1101 is translated relative to the focusing element 1102, which has the effect of moving the image focal plane of the optical system formed by the energy source 1 relative to the surface of the powder layer 150.

[0082] The focus adjustment performed by the focusing device takes into account the scanning performed by the scanning device 130 (which translates the light spot in the longitudinal direction). Therefore, the fluctuations in the light spot size caused by the scanning are limited by this adjustment.

[0083] More specifically, the focal length of the energy beam adjusted by the focus adjustment varies according to the angular position of the scanning mirror of the scanning device.

[0084] As previously mentioned, for a given configuration of the scanning device 130, the central axis of the energy beam projected by the energy source 1 intercepts the surface of the support 140 at a specific point of coordinates (x, y). Therefore, there is a mathematical relationship between the coordinates (x, y) of this point and the angular position of the scanning mirror. By considering the average thickness of the powder layer 150 deposited on the substrate, the parameter values for the focusing device to change the focus of the energy beam during scanning can be pre-calculated and stored in the memory used by the control unit. Therefore, the control unit does not need to perform calculations to instruct the focusing device.

[0085] Conversely, the focusing adjustment performed by the focusing device does not take into account the oscillations generated by the scanning device 130. More specifically, the focal length of the energy beam is independent of the angular position of the energy beam caused by the oscillations.

[0086] In other words, the focusing device is configured to act as if there are no oscillations of the energy beam.

[0087] The above steps are repeated on multiple adjacent regions of the surface along the lateral scanning direction. These regions are scanned along the longitudinal scanning direction, but alternately in the outward direction and the return direction to accelerate the process of two-dimensional scanning of the surface of the powder layer 150.

[0088] The above method can be the subject of other variant embodiments.

[0089] First, the energy beam can be oscillated by other types of oscillation devices other than the oscillation device 120 described herein, and can be focused by other types of focusing devices other than the aforementioned focusing device. The above method can be applied to any type of energy source that can change the focus of the energy beam projected onto the surface in the form of a light spot and can move the light spot on the surface with a composite movement including translational and oscillatory movements, regardless of the internal structure of the energy source (especially the structure capable of generating different components of the composite movement).

[0090] Second, in the embodiment of the method described herein (which is a preferred embodiment), the focus of the energy beam is adjusted without considering the components of the oscillatory movement. This has the advantage of avoiding premature wear of the energy source (especially the focusing device). As a variant embodiment, when the oscillatory movement has two components (one lateral and one longitudinal), it can be envisaged that the focus of the energy beam depends only on one of the two components of the oscillatory movement. Of course, this results in greater wear of the energy source, but the control of the focus of the energy beam is more precise.

[0091] Third, although pre-calculated focusing parameter values are highly advantageous to avoid a heavy computational burden, these values can still be calculated dynamically during scanning.

Claims

1. A method for additive manufacturing of an object from a powder layer, the method comprising the steps of: · Projecting an energy beam in the form of a light spot onto the surface of the powder layer to melt the powder (200), · Scanning the surface with the energy beam such that the light spot travels in a movement including a translation along a longitudinal scanning direction and an oscillatory movement having at least one component along an oscillatory direction (202), - Adjusting the focusing of the energy beam during scanning according to the translation along the longitudinal scanning direction without considering the component of the oscillatory movement along the oscillatory direction (204).

2. The method according to claim 1, wherein, The oscillatory movement includes a transverse component along a transverse scanning direction perpendicular to the longitudinal scanning direction, and wherein the focusing of the energy beam is adjusted without considering the transverse component of the oscillatory movement.

3. The method according to claim 2, wherein The transverse component of the oscillatory movement oscillates at a frequency of at least 1 kHz.

4. The method according to any one of claims 2 and 3, wherein The transverse component of the oscillatory movement oscillates with an amplitude between 100 microns and 2 millimeters.

5. The method according to claim 1, wherein, The oscillatory movement includes a longitudinal component along the longitudinal scanning direction, and wherein the focusing of the energy beam is adjusted without considering the longitudinal component of the oscillatory movement.

6. The method according to claim 5, wherein The transverse component of the oscillatory movement oscillates at a frequency of at least 1 kHz.

7. The method according to any one of claims 5 and 6, wherein The longitudinal component of the oscillatory movement oscillates with an amplitude between 100 microns and 2 millimeters.

8. The method according to any one of claims 1 to 3, wherein, The path includes consecutive loops offset from each other along the longitudinal scanning direction.

9. The method according to claim 1, wherein Adjusting the focusing of the energy beam using pre-calculated focusing parameter values calculated before projecting the energy beam, each pre-calculated focusing parameter value being associated with the position of the light spot on the surface.

10. An apparatus for additive manufacturing of an object from a powder layer, the apparatus comprising an energy source configured to project an energy beam in the form of a light spot onto the surface of the powder layer to melt the powder, the energy source including a control unit configured to: · Control the scanning of the surface with the energy beam such that the light spot travels in a movement including a translation along a longitudinal scanning direction and an oscillatory movement having at least one component along an oscillatory direction, · Adjust the focusing of the energy beam during scanning according to the translation along the longitudinal scanning direction without considering the component of the oscillatory movement along the oscillatory direction.

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