Irradiation system operation method, irradiation system, and three-dimensional workpiece manufacturing device

Abstract

In a method for operating an irradiation system (10) for irradiating a layer of raw material powder with laser light to manufacture a three-dimensional workpiece (110), at least a portion of a raw material powder layer (11) applied on a carrier (102) is selectively irradiated with linearly polarized laser light having a degree of polarization DOP of 99% or less and 30% or more. The orientation of the polarization plane of the linearly polarized laser light is controlled according to the orientation of the plane of incidence of the linearly polarized laser light on the raw material.

Description

[Technical Field]

[0001] The present invention relates to a method for operating an irradiation system for applying a laser beam to a layer of raw material powder in order to manufacture three-dimensional workpieces. The present invention further relates to an irradiation system of this kind. Finally, the present invention relates to an apparatus for manufacturing three-dimensional workpieces. [Background technology]

[0002] Powder bed fusion is an additive layering process that can process powdered raw materials, especially metal and / or ceramic, into three-dimensional workpieces with complex shapes. For this purpose, a layer of raw material powder is applied to a carrier, and the powder layer is irradiated with a laser in a selective manner depending on the desired shape of the workpiece to be manufactured. The laser irradiation that penetrates the powder layer heats the raw material powder particles, resulting in their melting or sintering. Further raw material powder layers are then successively applied to the layer already subjected to laser processing on the carrier until the workpiece has the desired shape and size. Powder bed fusion can be used to manufacture or repair prototypes, tools, replacement parts, high-value parts, or medical prostheses, such as dental or orthopedic prostheses, based on CAD data.

[0003] For example, an exemplary apparatus for manufacturing three-dimensional workpieces by powder bed fusion, as described in WO 2019 / 141381, includes a carrier configured to receive multiple layers of raw material and an irradiation unit configured to selectively irradiate the raw material on the carrier with a laser to manufacture the workpiece. The irradiation unit is provided with a spatial light modulator configured to split the laser beam into at least two sub-beams. A polarization means for linearly polarizing the laser beam is provided upstream of the spatial light modulator to provide a linearly polarized laser beam to the spatial light modulator.

[0004] When a three-dimensional workpiece is built on a carrier in a powder bed fusion machine, the laser light that falls on the raw material powder is absorbed, melting and / or sintering the raw material powder, creating a melt pool of molten raw material. Furthermore, the raw material vaporizes, forming vapor capillaries through which the laser beam penetrates deeper into the raw material.

[0005] The present invention relates to a method for operating an irradiation system for irradiating a layer of raw material powder with a laser to manufacture three-dimensional workpieces, and to an irradiation system of this type capable of efficiently manufacturing high-quality workpieces.Furthermore, the present invention relates to a three-dimensional workpiece manufacturing apparatus equipped with an irradiation system capable of efficiently manufacturing high-quality workpieces.

[0006] In a method for operating an irradiation system that irradiates a layer of raw material powder with a laser to manufacture a three-dimensional workpiece, at least a portion of the raw material powder layer applied to a carrier is selectively irradiated with a linearly polarized laser having a polarization degree DOP of 99% or less and 30% or more (i.e., 30%≦DOP≦99%). Summary of the Invention [Problem to be solved by the invention]

[0007] As used herein, DOP is a quantity that describes the linearly polarized portion of an electromagnetic wave. A completely linearly polarized wave has a DOP of 100%, while a completely unpolarized wave has a DOP of 0%. A partially linearly polarized wave can be described by a superposition of a linearly polarized component and a non-linearly polarized (e.g., unpolarized) component, and the DOP can be between 0% and 100%. DOP can be calculated as the percentage of the total power carried by the linearly polarized component of the electromagnetic wave.

[0008] The linearly polarized laser light may have a DOP equal to or less than a first threshold, where the first threshold is any of 99%, 98%, 97%, 96%, or 95%. Alternatively or additionally, the linearly polarized laser light may have a DOP equal to or greater than a second threshold, where the second threshold is any of 30%, 60%, 75%, 85%, 90%, or 95%. In other words, at least one of the following conditions may apply: (i) DOP≦first threshold; and (ii) second threshold≦DOP. [Means for solving the problem]

[0009] Experiments have shown that when using metal powders, a DOP between 85% and 99% provides the best results. Therefore, in one preferred embodiment, the first threshold is 99% and the second threshold is 85%. In this preferred embodiment, the raw powder may be a metal powder.

[0010] The raw material powder layer is applied onto the surface of the carrier by a powder application device, which moves over the carrier to distribute the raw material powder. The carrier may be a rigidly fixed carrier. However, preferably, the carrier is configured to be vertically movable, so that the carrier can move vertically downward, building up layers of raw material powder as the build height of the workpiece increases. Furthermore, the carrier may include a cooling device and / or a heating device configured to cool and / or heat the carrier.

[0011] The carrier and powder application device can be housed in a process chamber that can be sealed against the ambient atmosphere. An inert gas atmosphere can be formed in the process chamber by introducing a gas flow into the process chamber through a gas inlet. The gas flow passes through the process chamber, is directed across the raw powder layer applied to the carrier, and can then be discharged from the process chamber through a gas outlet. The raw powder applied to the carrier in the process chamber is preferably a metal powder, particularly a metal alloy powder, but may also be a ceramic powder or a powder containing a different material. The powder can have any suitable particle size or particle size distribution. However, it is preferred that the particle size of the processed material is less than 100 μm.

[0012] The illumination system may have a laser beam source that emits at least one linearly polarized laser beam (e.g., a DOP of more than 95%). In particular, the laser beam source of the illumination system may emit linearly polarized laser beam (e.g., a DOP of more than 95%) with a wavelength of 450 nm, i.e., "blue" laser beam, or laser beam (e.g., a DOP of more than 95%) with a wavelength of 532 nm, i.e., "green" laser beam, or laser beam (e.g., a wavelength in the range of 1000 nm to 1090 nm or 1530 nm to 1610 nm), i.e., "infrared" laser beam. However, it is also conceivable that the laser beam source of the illumination system emits at least one randomly polarized (i.e., non-polarized) laser beam, which is converted into linearly polarized laser beam (e.g., a DOP of more than 30%) by an appropriate polarizing device, such as a polarizer or a polarizing beam splitter cube. When one or more beam splitter cubes are used to split the laser beam into two or more partial beams with different polarizations, only one partial beam can be used as the illumination beam, while the other partial beams can be blocked. Alternatively, one or more partial beams can be directed to different illumination systems of one or more additive manufacturing devices. Additionally or alternatively, one or more partial beams may be modified, in particular their polarization or DOP may be modified. Additionally or alternatively, multiple beams may be modified to have the same polarization and combined before being collectively directed to the illumination systems.

[0013] The irradiation system may irradiate the raw material powder layer with a single laser beam. However, it is also conceivable that the irradiation system irradiates the raw material powder layer with two or more laser beams. When the irradiation system irradiates the raw material powder layer with two or more laser beams, at least one laser beam may be a linearly polarized laser light beam with a DOP of 99% or less and 30% or more, and at least one other laser beam may be a randomly polarized laser beam (e.g., a DOP of 0%, 10%, or 15%), a radially polarized laser beam, and / or an azimuthally polarized laser beam. The multiple laser beams irradiated by the irradiation system to the raw material powder layer may be irradiated by appropriate subunits of a laser beam source.

[0014] The illumination system may also comprise at least one optical unit for splitting, guiding and / or processing at least one laser beam emitted from the laser beam source. The optical unit may comprise one or more optical elements, such as an objective lens and a scanner unit, which preferably comprises a diffractive optical element and / or a deflection mirror. The one or more optical elements may be arranged in the path of the laser emitted from the laser beam source of the illumination system.

[0015] The DOP of the linearly polarized laser light can be determined and / or adjusted by an optical unit, particularly one or more optical elements. A deflection mirror can reduce the DOP of the incident light upon reflection. For example, linearly polarized laser light with a DOP of 99.5% can be emitted from a laser light source and reflected by a deflection mirror. The reflected laser light can have a lower DOP, for example, a DOP of 98%. The linearly polarized laser light with a DOP of 98% is then irradiated onto the raw material powder layer.

[0016] It should be understood that the DOP of the linearly polarized laser light used to irradiate the raw material powder layer can be statically predetermined, for example, by the optical properties of one or more optical elements. Alternatively, the DOP of the linearly polarized laser light used to irradiate the raw material powder layer can be dynamically adjusted (e.g., controlled) by, for example, controlling the movement, alignment, or optical properties of one or more optical elements. As a non-limiting example, a deflecting mirror can have reflective properties that vary across its reflective surface. The DOP of the reflected light can depend on the position and angle of the incident light on the reflective mirror surface.

[0017] The DOP of the linearly polarized laser light can be set (e.g., statically) or controlled (e.g., dynamically) according to one or more of: (i) the type or physical characteristics of the raw material or raw material powder; (ii) the intensity of the laser light (e.g., irradiated by a laser source or irradiated onto the raw material powder layer); (iii) the scan speed at which the linearly polarized laser light (14a, 14b) moves across the raw material powder layer (11); (iv) the scan vector (e.g., one of multiple scan vectors described further below); and (v) the angle of incidence (e.g., the angle of incidence described further below).

[0018] In a method for operating an irradiation system, the orientation of the polarization plane of linearly polarized laser light is controlled according to the orientation of the plane of incidence of the linearly polarized laser light on a material. As used herein, the term "plane of polarization" is defined by the propagation vector of the incident laser beam and the electric field vector of the electromagnetic laser light wave, and therefore the polarization plane coincides with the vibration plane of the electric field of the electromagnetic laser light wave. As used herein, the term "plane of incidence" refers to the plane defined by the propagation vector of the incident laser beam and the surface normal extending perpendicular to the surface of the material on which the incident laser beam impinges. For a linearly polarized electromagnetic wave with a DOP of 80%, it can be said that the total power delivered by the linearly polarized components of the electromagnetic wave with their electric field vectors aligned with the polarization plane is 80% of the total power delivered by the electromagnetic wave. The polarization plane can be defined by the maximum electric field vector of the laser light.

[0019] The orientation of the polarization plane of the linearly polarized laser light relative to the plane of incidence of the linearly polarized laser light on the raw material has a strong effect on the absorption of laser energy by the raw material, similar to the DOP of the linearly polarized laser light. Therefore, by controlling the orientation of the polarization plane according to the plane of incidence of the linearly polarized laser light on the raw material and, if necessary, controlling the DOP of the linearly polarized laser light according to the plane of incidence of the linearly polarized laser light on the raw material, it is possible to control the absorption of the energy of the linearly polarized laser light by the raw material.

[0020] By considering and actively controlling the absorption of laser energy by the raw material, process stability can be improved during the production of three-dimensional workpieces. Furthermore, by increasing the absorption of laser energy in a controlled manner, process productivity can be improved. As a result, high-quality workpieces can be produced particularly efficiently. Furthermore, materials that are currently difficult to process by laser or sintering / melting, such as Cu and Cu alloys, may be processable by irradiating the respective raw material powders with linearly polarized laser light while controlling the orientation of the polarization plane of the linearly polarized laser light depending on the orientation of the plane of incidence of the linearly polarized laser light on the raw material, and further, if necessary, controlling the DOP of the linearly polarized laser light depending on the orientation of the plane of incidence of the linearly polarized laser light on the raw material.

[0021] In a preferred embodiment of the method for operating the irradiation system, the orientation of the polarization plane of the linearly polarized laser light is controlled in response to the orientation of the plane of incidence of the linearly polarized laser light on the raw material, such that the polarization plane is oriented substantially parallel to the plane of incidence, i.e., the orientation of the polarization plane of the linearly polarized laser light is controlled in response to the orientation of the plane of incidence of the linearly polarized laser light on the raw material to be p-polarized.

[0022] In the p-polarized state, the absorption of linearly polarized laser light energy by the material is generally higher than in the s-polarized state, where the polarization plane extends perpendicular to the plane of incidence. Furthermore, the absorption of linearly polarized laser light in the p-polarized state is generally higher than that in the randomly polarized state. Therefore, by controlling the orientation of the polarization plane relative to the plane of incidence to achieve the p-polarized state, the absorption of laser energy can be increased in a controlled manner. Generally speaking, the greater the amount of energy delivered by the linearly polarized component of the laser light corresponding to the p-polarized state, the higher the absorption of the laser light energy. The amount of energy delivered by the linearly polarized component depends on the DOP of the laser light and the overall irradiation intensity. Increasing the DOP increases the amount of energy delivered by the linearly polarized component. Therefore, the amount of laser energy absorbed by the irradiated powder material can be controlled by controlling the DOP of the linearly polarized laser light in addition to the orientation of the polarization plane of the linearly polarized laser light.

[0023] Preferably, at least one of (i) the DOP of the linearly polarized laser light and (ii) the orientation of the polarization plane of the linearly polarized laser light is controlled according to the orientation of the plane of incidence of the linearly polarized laser light on the inner wall surface of a capillary extending from the surface of the raw material powder layer into the raw material powder layer and formed by the interaction of the linearly polarized laser light with the raw material. The capillary may be a vapor capillary formed by evaporation of the raw material heated by absorption of energy from the laser beam impinging on the raw material layer. The size and shape of the capillary may depend on various parameters, such as the power, focal diameter and shape of the incident laser beam, the scanning speed and scanning direction of the laser beam, and / or at least one parameter of a gas flow. The gas flow is directed across the raw material powder layer to establish a controlled atmosphere in the process chamber and to remove particulate impurities, such as droplets, smoke, or soot particles, generated when the raw material powder layer is irradiated. Using a predetermined (e.g., pre-measured) list of capillary sizes and shapes associated with various combinations of the aforementioned various parameters, the orientation of the capillary inner wall surface relative to the surface of the raw powder material layer can be estimated based on the various parameters used (or to be used) during irradiation with linearly polarized laser light.

[0024] Instead of using such a predetermined list to estimate the size and shape of the capillary, the orientation of the capillary's inner wall surface may be determined based on the direction of the trajectory of material (e.g., molten powder) ejected from the raw powder layer (e.g., from the capillary). The ejected material may be referred to as a "splash" or metal microdroplet. The trajectory may be determined based on two or more (e.g., video) images of the powder layer acquired by an imaging system (e.g., included in the apparatus described herein). The imaging system may include a so-called on-axis camera. The on-axis camera can be positioned to acquire images via an optical unit. Alternatively, the imaging system may include a camera positioned and configured to acquire images of the entire powder layer. Further details on determining the trajectory and / or orientation of the capillary's inner wall surface are described, for example, in Ly, S., Rubenchik, AM, Khairallah, SA, et al., "Metal vapor micro-jet controls material redistribution in laser powder bed fusion additional manufacturing," Sci Rep 7, 4085 (2017). The main direction can be determined based on the directions of multiple trajectories of separate droplets or microdroplets. The main direction can then be used to determine the (e.g., average) orientation of the capillary wall surface. The main direction may correspond to or be determined based on the average or median of the directions of the multiple trajectories. The average may be a weighted average. To determine the weighted average, the directions of the multiple trajectories may be weighted based on the size of each droplet or microdroplet.

[0025] The imaging system can be used to determine the orientation of the inner wall without detecting droplets and without determining the trajectory. In this case, the orientation of the inner wall can be determined based on the size of the capillary detected in the image acquired by the imaging system. The image can be acquired in a direction perpendicular to the surface of the material powder layer. Generally speaking, the larger the angle between the inner wall and the line of sight of the imaging system (e.g., normal to the surface of the material powder layer), the larger the detected capillary contour. For a laser beam with a circular cross section, the capillary has an elliptical contour in the image acquired by the imaging system, and the major axis of the elliptical contour becomes longer as the angle between the inner wall and the line of sight of the imaging system increases. Therefore, the orientation of the inner wall (e.g., the angle of the inner wall with respect to the surface of the powder material) can be determined based on the size and / or shape of the capillary (e.g., the capillary contour) detected in the image acquired by the imaging system. Multiple images acquired at different times or from different line of sight directions can be used to determine multiple angles of the inner wall of the capillary at different times. A dominant angle can then be determined (e.g., as an average, median, or weighted average) based on the multiple angles, and the dominant angle can then be assumed to be the angle of the interior wall and / or to represent the orientation of the interior wall.

[0026] It should be noted that the trajectory of the microdroplets does not have to be determined using a camera, but may be determined in another way (e.g., using a distance sensor). The shape of the capillary and / or the orientation of the inner wall surface may also be simulated (e.g., in advance), for example, using finite element simulation tools and / or computational fluid dynamics (CFD) simulation tools. In yet another variant, the depth of the capillary and / or the orientation of the inner wall surface may be measured using any applicable measurement technique, such as, for example, optical coherence tomography (OCT). The measurement may, for example, be performed by an on-axis (e.g., OCT) measurement sensor.

[0027] Generally, the median or average (e.g., weighted average) of multiple orientations of the inner wall surface can be determined and assumed to be the (e.g., correct or appropriate) orientation of the inner wall surface. The same applies to the determined capillary shape.

[0028] By using linearly polarized laser light with a DOP of 99% or less, the unpolarized component of the irradiated light is reflected multiple times within the capillary tube, expanding the capillary tube, making the capillary tube more stable and reducing the risk of material scattering compared to using linearly polarized laser light with a DOP of more than 99% (e.g., 100%).

[0029] The laser beam irradiated onto the raw material powder layer penetrates the capillary and strikes the capillary's inner wall, which has an orientation different from the upper surface of the raw material powder layer. For example, the capillary's inner wall, on which the laser beam strikes, may extend at an angle of about 45° to 90°, preferably about 60° to 80°, relative to the upper surface of the raw material powder layer, which is typically oriented substantially parallel to the surface of the carrier on which the raw material powder layer is applied. When controlling the orientation of the polarization plane of the linearly polarized laser light and / or the DOP of the linearly polarized laser light, taking into account the orientation of the plane of incidence of the linearly polarized laser light on the capillary's inner wall, which is formed by the interaction of the laser beam with the raw material, particularly reliable and accurate control of the absorption of laser energy by the raw material can be achieved.

[0030] When the scanning direction of the laser beam across the raw material powder layer changes, the orientation of the plane of incidence of the linearly polarized laser light onto the raw material usually changes. Therefore, when controlling the orientation of the plane of polarization of the linearly polarized laser light according to the orientation of the plane of incidence of the linearly polarized laser light onto the raw material, it is preferable that at least one of (i) the DOP of the linearly polarized laser light and (ii) the orientation of the plane of polarization of the linearly polarized laser light be updated according to the scanning direction of the linearly polarized laser light across the raw material powder layer.

[0031] To update the orientation of the polarization plane of the linearly polarized laser light, a polarizing device, such as a wave plate, particularly a half-wave plate, may be rotated. Alternatively, a collimator located downstream of the polarizing device in the optical path of the laser light emitted from the illumination beam source may be rotated. Furthermore, at least one deflecting mirror, particularly a pair of deflecting mirrors, may be used to deflect the linearly polarized laser light beam as needed. Alternatively, two pairs of deflecting mirrors may be used to deflect the linearly polarized laser light beam, one pair rotating the polarization plane and the other pair deflecting the beam. In a preferred embodiment, mirrors with a metal coating (e.g., aluminum, silver, gold, etc.) are used. The DOP can be controlled as described above using one or more optical elements located in the optical path of the laser light emitted from the illumination beam source.

[0032] At least one of (i) the DOP of the linearly polarized laser light and (ii) the orientation of the polarization plane of the linearly polarized laser light may be updated based on an analysis of a scan pattern in which a beam of linearly polarized laser light is directed across the raw material powder layer. By analyzing the scan pattern, changes in the scan direction of the laser beam across the raw material powder layer can be determined. As a result, at least one of (i) the DOP of the linearly polarized laser light and (ii) the orientation of the polarization plane of the linearly polarized laser light may be updated as needed and synchronized with the operation of the scanner unit scanning the laser beam across the raw material powder layer. The analysis of the scan pattern may be performed in situ before starting fabrication of the three-dimensional workpiece and / or during fabrication of the three-dimensional workpiece.

[0033] The absorption of linearly polarized laser light energy by a material is highly dependent on the angle of incidence of the laser beam on the material. For p-polarized states, absorption increases with increasing angle of incidence, at least for angles of incidence between approximately 10° and 80°. As used herein, the term "angle of incidence" refers to the angle between the propagation vector of the incident laser light and the surface normal extending perpendicular to the material surface on which the incident laser light strikes.

[0034] To selectively irradiate the raw material powder layer, i.e., to scan the laser beam across the raw material powder layer, the laser beam is deflected with respect to a surface normal extending perpendicular to the upper surface of the raw material powder layer. As a result, the angle of incidence depends on the deflection angle of the incident laser beam with respect to the surface normal extending perpendicular to the upper surface of the raw material powder layer, i.e., on the operating state of the scanner unit. Furthermore, the angle of incidence depends on the orientation of the raw material surface on which the incident laser beam impinges.

[0035] In addition to the angle of incidence of the linearly polarized laser beam, several additional process parameters can affect the absorption of laser energy by the feedstock. These parameters can include the power, focal diameter, and focal shape of the beam of linearly polarized laser light. Furthermore, the scan speed, scan mode (forward or backward), scan direction, and scan pattern in which the laser beam is directed across the feedstock powder layer can affect the absorption of laser energy by the feedstock. Also, the gas flow, particularly the volumetric flow rate and flow velocity of the gas flow, and the type of gas, directed through the process chamber across the surface of the feedstock powder layer can affect absorption.

[0036] Therefore, in a preferred embodiment of a method for operating an irradiation system, at least one of the DOP, power, focal diameter and focal shape of the linearly polarized laser beam, and / or at least one of the scanning speed, scanning mode, scanning direction and scanning pattern at which the linearly polarized laser beam is directed across the raw material powder layer, and / or at least one parameter of the gas flow directed across the raw material powder layer are controlled in response to the incident angle of the linearly polarized laser beam with respect to the raw material.

[0037] By relating the angle of incidence of the linearly polarized laser beam to further process parameters that influence the absorption of laser energy by the material, on the one hand, particularly reliable absorption control can be achieved and overheating of the material can be avoided. On the other hand, the efficiency of the procedure can be improved. For example, a decrease in absorption caused by a reduction in laser power and / or an increase in scanning speed can be compensated for by an increase in absorption at an appropriate angle of incidence. Furthermore, for certain materials, the energy imparted to the material can be increased in a controlled manner by appropriately controlling the angle of incidence, for example, to enable the processing of these materials.

[0038] The linearly polarized laser beam can be scanned across the raw material powder layer according to a scanning strategy. In the scanning strategy, multiple scanning vectors indicating a first vector direction are successively scanned, followed by at least one scanning vector indicating a second vector direction different from the first vector direction. The scanning vector indicating the first vector direction v1 is scanned "forward" in a direction +v1 and "backward" in a direction -v1 without changing the polarization plane of the linearly polarized laser beam and / or the DOP of the linearly polarized laser beam. Similarly, the scanning vector indicating the second vector direction v2 is scanned "forward" in a direction +v2 and "backward" in a direction -v2 without changing the polarization plane of the linearly polarized laser beam and / or the DOP of the linearly polarized laser beam. Therefore, changing the polarization plane of the linearly polarized laser beam and / or the DOP of the linearly polarized laser beam may be necessary only when the vector direction, i.e., the direction in which the vector extends, is changed. This scanning strategy can reduce the "number of updates" of the polarization plane of the linearly polarized laser beam and / or the DOP of the linearly polarized laser beam. As a result, the update process can be simplified.

[0039] In an embodiment of a method for operating an irradiation system, a first section of a raw material powder layer can be selectively irradiated with linearly polarized laser light having a DOP of 99% or less and 30% or more, and a second section of the raw material powder layer can be selectively irradiated with randomized laser light (e.g., having a DOP of 0%, 10%, or 15%), radially polarized laser light, and / or azimuthally polarized laser light. The second section can be a section of the raw material powder layer intended to be irradiated according to a scanning strategy that requires frequent and / or rapid updating of either or both of (i) the DOP of the linearly polarized laser light or (ii) the orientation of the polarization plane of the linearly polarized laser light. For example, the second section can be a section of the raw material powder layer intended to be irradiated according to a scan pattern including a high density of short scan vectors and / or scan vectors indicating multiple directions, and / or a section of the raw material powder layer intended to be irradiated at a high scan speed.

[0040] In particular, the first section of the raw material powder layer may be a hatch section of a workpiece layer produced by selectively irradiating the raw material powder layer. The second section of the raw material powder layer may be a contour section of a workpiece layer produced by selectively irradiating the raw material powder layer. As a result, the advantages of irradiating the raw material powder layer with linearly polarized laser light as described above can be realized in the hatch section, which typically forms the majority of the area of ​​the workpiece layer. At the same time, the difficulty that may arise when updating the orientation of the polarization plane and / or DOP of the linearly polarized laser light in the contour section can be avoided.

[0041] In a further embodiment of the method for operating the irradiation system, multiple linearly polarized laser beams (e.g., each having a DOP of 99% or less and 30% or more) can be scanned across overlapping sections of the raw material powder layer according to a scanning strategy, where all scan vectors are scanned according to the same scanning mode. As used herein, the term "overlapping section" defines a section of the raw material powder layer that can be irradiated with multiple laser beams. For example, in the overlapping sections of the raw material powder layer, all scan vectors are scanned according to a trailing scan mode, or all scan vectors are scanned according to a leading scan mode. Therefore, in the overlapping sections, the absorption of laser light by the raw material is not affected by the scan mode and can therefore be more reliably controlled.

[0042] An irradiation system for irradiating a layer of raw material powder with laser light to manufacture a three-dimensional workpiece is configured to selectively irradiate at least a portion of the raw material powder layer coated on a carrier with linearly polarized laser light having a DOP of 99% or less and 30% or more. The irradiation system has a control device configured to control the direction of the polarization plane of the linearly polarized laser light according to the direction of the incident plane of the linearly polarized laser light on the raw material.

[0043] The control device of the irradiation system can be configured to control the direction of the polarization plane of the linearly polarized laser light on the material in accordance with the direction of the incident plane so that the polarization plane of the linearly polarized laser light on the material is substantially parallel to the incident plane of the linearly polarized laser light, thereby achieving a p-polarized state.

[0044] The controller may be further configured to control the DOP of the linearly polarized laser beam in response to an orientation of the plane of incidence of the linearly polarized laser beam at the feedstock, and the DOP may be controlled to adjust the amount of energy of the linearly polarized component of the laser beam in the p-polarized state (e.g., the component with an E-field in the plane of incidence).

[0045] The control device may further be configured to control at least one of (i) the DOP of the linearly polarized laser light and (ii) the orientation of the polarization plane of the linearly polarized laser light depending on the orientation of the incident plane of the linearly polarized laser light on the inner wall surface of the capillary extending from the surface of the raw material powder layer into the volume of the raw material powder layer and formed by the interaction between the raw material and the linearly polarized laser light.

[0046] Furthermore, when controlling the orientation of the polarization plane of the linearly polarized laser light in accordance with the orientation of the incident plane of the linearly polarized laser light onto the raw material, the control device may be configured to update at least one of (i) the DOP of the linearly polarized laser light and (ii) the orientation of the polarization plane of the linearly polarized laser light in accordance with the scanning direction of the linearly polarized laser light across the raw material powder layer.

[0047] Specifically, the control device may be configured to update at least one of (i) the DOP of the linearly polarized laser light and (ii) the orientation of the polarization plane of the linearly polarized laser light based on an analysis of a scan pattern in which the linearly polarized laser beam is directed across the raw material powder layer. The analysis of the scan pattern may be performed before starting fabrication of the three-dimensional workpiece and / or in situ during fabrication of the three-dimensional workpiece.

[0048] The control device may further be configured to control, depending on the angle of incidence of the linearly polarized laser beam on the raw material, at least one of the power, focal diameter, and focal shape of the linearly polarized laser beam, and / or at least one of the scanning speed, scanning direction, scanning mode, and scanning pattern by which the linearly polarized laser beam is directed across the raw material powder layer, and / or at least one parameter of the gas flow directed across the raw material powder layer.

[0049] As described above, the linearly polarized laser light may have a DOP less than or equal to a first threshold, where the first threshold is 99%, 98%, 97%, 96%, or 95%. The linearly polarized laser light may have a DOP greater than or equal to a second threshold, where the second threshold is 30%, 60%, 75%, 85%, 90%, or 95%. The first threshold may be 99% and the second threshold may be 85%, in which case, optionally, the raw material is a metal.

[0050] As described above, the illumination system may include a laser beam source and one or more optical elements disposed in a path of laser light emitted from the laser beam source. The one or more optical elements may be configured to define or adjust the DOP of the linearly polarized laser light. The one or more optical elements may include a deflecting mirror configured to reduce the DOP of the incident light upon reflection.

[0051] The control device is configured to control the scanner unit so that the linearly polarized laser beam is scanned across the raw material powder layer according to a scanning strategy in which multiple scanning vectors indicating a first vector direction are scanned consecutively, followed by at least one scanning vector indicating a second vector direction different from the first vector direction.

[0052] Further, the control device may be configured to control the illumination system such that a first section of the raw material powder layer is selectively irradiated with linearly polarized laser light and a second section of the raw material powder layer is selectively irradiated with random laser light, radially polarized laser light, and / or azimuthally polarized laser light.

[0053] The first section of the raw material powder layer may be a hatch section of the workpiece layer created by selectively irradiating the raw material powder layer, and the second section of the raw material powder layer may be a contour section of the workpiece layer created by selectively irradiating the raw material powder layer.

[0054] The control device can be configured to control the scanner unit such that multiple linearly polarized laser beams (e.g., each having a DOP of 99% or less and 30% or more) are scanned across overlapping sections of the raw material powder layer according to a scanning strategy, where all scanning vectors are scanned according to the same scanning mode.

[0055] An apparatus for manufacturing a three-dimensional workpiece includes the above-described irradiation system.

[0056] Preferred embodiments of the present invention will now be described in more detail with reference to the accompanying schematic drawings. [Brief explanation of the drawings]

[0057] [Figure 1] This shows a device that manufactures three-dimensional workpieces by irradiating a layer of raw material powder with laser light. [Figure 2] 2 shows an illumination system used in the apparatus of FIG. 1; [Figure 3] 1 illustrates the interaction of a linearly polarized laser beam with a material. [Figure 4] 1 shows the dependence of the absorption of laser light by the raw material on the polarization state of the laser light and the angle of incidence of the laser light on the raw material. DETAILED DESCRIPTION OF THE INVENTION

[0058] 1 shows an apparatus 100 for manufacturing three-dimensional workpieces by a layer-by-layer process. The apparatus 100 includes a carrier 102 and a powder application device 104 for applying raw material powder onto the carrier 102. The carrier 102 and the powder application device 104 are housed in a process chamber 106 that can be sealed against the ambient atmosphere. The carrier 102 is vertically movable within a built-in cylinder 108, and the carrier 102 As the stack height of the workpieces 110 stacked layer by layer from the raw material powder increases, the carrier 102 can be moved downward. The carrier 102 may be equipped with a heater and / or a cooler.

[0059] The apparatus 100 further includes an irradiation system 10 for selectively irradiating the raw material powder layer 11 coated on the carrier 102 with laser light. In the embodiment of the apparatus 100 shown in FIG. 1 , the irradiation system 10 includes two laser beam sources 12a, 12b configured to emit laser beams 14a, 14b, respectively. Optical units 16a, 16b for guiding and processing the laser beams 14a, 14b emitted by the laser beam sources 12a, 12b are associated with the laser beam sources 12a, 12b, respectively. However, it is also conceivable that the irradiation system 10 includes only one laser beam source and only one optical unit, resulting in the emission of only a single laser beam. A controller 18 is provided for controlling the operation of the irradiation system 10 and for controlling components of the apparatus 100, such as the powder coating apparatus 104.

[0060] A controlled gas atmosphere, preferably an inert gas atmosphere, is formed within the process chamber 106 by supplying a shielding gas to the process chamber 106 via the process gas inlet 112. The gas is directed through the process chamber 106 and across the raw material powder layer 11 applied on the carrier 102, and then exhausted from the process chamber 106 via the process gas outlet 114. The flow direction of the shielding gas from the process gas inlet 112 through the process chamber 106 to the process gas outlet 114 is indicated by arrow F. The process gas is recirculated from the process gas outlet 114 back to the process gas inlet 112, where it may be cooled or heated.

[0061] During operation of the apparatus 100 for manufacturing three-dimensional workpieces, a layer 11 of raw material powder is applied onto the carrier 102 by the powder application device 104. To apply the layer 11 of raw material powder, the powder application device 104 moves across the carrier 102 under the control of the control unit 18. Then, again under the control of the control unit 18, the layer 11 of raw material powder is selectively irradiated with laser light by the irradiation device 10 according to the shape of the corresponding layer of the workpiece 110 to be manufactured. The steps of applying the layer 11 of raw material powder onto the carrier 102 and selectively irradiating the layer 11 of raw material powder with laser light according to the shape of the corresponding layer of the workpiece 110 to be manufactured are repeated until the workpiece 110 has the desired shape and size.

[0062] At least one of the laser beams 14a, 14b irradiated onto the raw material powder layer 11 by the irradiation system 10 is linearly polarized laser light (e.g., DOP 99.5%). A more detailed view of the laser beam source 12a and the optical unit 16a is shown in FIG. 2. The laser beam source 12a irradiates linearly polarized laser light, such as laser light with a wavelength of 450 nm, i.e., "blue" laser light, or laser light with a wavelength of 532 nm, i.e., "green" laser light, or laser light with a wavelength in the range of 1000 to 1090 nm or 1530 to 1610 nm, i.e., "infrared" laser light. The polarizer 20 is used to rotate the polarization plane and may be configured, for example, in the form of a rotatably mounted wave plate, in particular a half-wave plate. The linearly polarized laser beam 14a is scanned across the raw material powder layer 11 by a scanner unit 22. The optical unit 16a can define and / or adjust the DOP of the laser light irradiated by the laser beam source 12a. Therefore, the linearly polarized laser light irradiated onto the raw material powder layer 11 has a DOP of 99% or less. The DOP of the linearly polarized laser light that forms the laser beam 14a and irradiates the raw material powder layer 11 can be statically set or dynamically adjusted by the optical unit 16a to a value, for example, from 30% to 99%, preferably from 85% to 99%. The DOP of the laser beam 14a may be set or adjusted by the optical unit 16a based on a control signal from the control unit 18.

[0063] The laser energy introduced into the raw material powder by the laser beam 14a irradiated onto the raw material powder layer 11 melts and / or sinters the raw material powder. Specifically, a molten pool of molten raw material is generated in the area where the laser beam 14a irradiates the raw material powder. Furthermore, a vapor capillary 24, shown in detail in FIG. 3, is formed by the evaporation of the raw material heated by the absorption of the energy of the laser beam 14a irradiated onto the raw material. Since the laser beam 14a has a DOP of 99% or less, it contains unpolarized light components. These components are reflected multiple times within the capillary 24, resulting in the expansion of the capillary 24. Therefore, the mechanical and temporal stability of the capillary 24 is improved, and the risk of melting or the raw powder material scattering from the capillary 24 can be reduced compared to when the DOP of the laser beam 14a is greater than 99% (e.g., 100%).

[0064] The laser beam 14a penetrates into the capillary 24 and strikes an inner wall surface 26 of the capillary 24, which is oriented differently from the upper surface 28 of the raw material powder layer 11. In the exemplary embodiment shown in Figure 3, the inner wall surface 26 of the capillary on which the laser beam strikes extends at an angle γ of approximately 75° to 80° with respect to the upper surface 28 of the raw material powder layer 11, which is oriented substantially parallel to the surface of the carrier 102. As described in detail above, the angle γ can be determined, for example, based on the trajectory of the microdroplets ejected from the capillary, based on the shape and size of the capillary outline detected from an image of the capillary, based on OCT measurements, or based on CFD simulations.

[0065] 3, the laser beam 14a is scanned in a forward scanning mode across the raw material powder layer 11 in a scanning direction indicated by arrow S. The plane of incidence of the laser beam 14a onto the raw material, i.e., the inner wall surface 26 of the capillary tube 24, is defined by a propagation vector P of the incident laser beam 14a and a surface normal N extending perpendicular to the raw material surface 26 on which the incident laser beam 14a impinges. An angle of incidence α is defined between the propagation vector P of the incident laser beam 14a and the surface normal N.

[0066] During operation of the irradiation system 10, the control device 18 controls the orientation of the polarization plane of the linearly polarized laser beam 14a depending on the orientation of the plane of incidence of the linearly polarized laser beam 14a relative to the raw material. Specifically, the control device 18 controls the orientation of the polarization plane of the linearly polarized laser beam 14a depending on the orientation of the plane of incidence of the linearly polarized laser beam 14a relative to the inner wall surface 26 of the capillary tube 24.

[0067] As is clear from Figure 4, the orientation of the polarization plane of the linearly polarized laser beam relative to the plane of incidence of the linearly polarized laser beam on the raw material has a strong effect on the absorption of laser energy by the raw material. In the p-polarized state, defined by the polarization plane of the linearly polarized laser beam 14a being parallel to the plane of incidence of the linearly polarized laser beam on the raw material, the absorption of laser energy by the raw material is greater than the absorption of laser energy by the raw material in the s-polarized state, defined by the polarization plane of the linearly polarized laser beam 14a being perpendicular to the plane of incidence of the linearly polarized laser beam on the raw material. Furthermore, in the p-polarized state, the absorption of laser energy by the raw material is greater than the absorption of energy from randomly polarized laser beam. Therefore, the control device 18 controls the polarization plane of the linearly polarized laser beam 14a and, optionally, the DOP of the linearly polarized laser beam 14a depending on the orientation of the plane of incidence of the linearly polarized laser beam 14a on the raw material so that the polarization plane is substantially parallel to the plane of incidence, i.e., the p-polarized state.

[0068] The orientation of the plane of incidence of the linearly polarized laser beam 14a relative to the raw material changes when the scanning direction S of the laser beam 14a across the raw material powder layer 11 changes. Therefore, when controlling the orientation of the plane of polarization of the linearly polarized laser beam 14a in accordance with the orientation of the plane of incidence of the linearly polarized laser beam 14a relative to the raw material, the control device 18 updates at least one of (i) the DOP of the linearly polarized laser beam 14a and (ii) the orientation of the plane of polarization of the linearly polarized laser beam 14a in accordance with the scanning direction S of the linearly polarized laser beam 14a across the raw material powder layer 11.

[0069] 2, updating the orientation of the polarization plane of the linearly polarized laser beam 14a is achieved by appropriately rotating the polarizer 20. The controller 18 updates at least one of (i) the DOP of the linearly polarized laser beam 14a and (ii) the orientation of the polarization plane of the linearly polarized laser beam 14a based on an analysis of a scan pattern in which the linearly polarized laser beam 14a is directed across the raw material powder layer 11. The analysis of the scan pattern can be performed before starting production of the three-dimensional workpiece 110 and / or in situ during production of the three-dimensional workpiece 110.

[0070] To simplify updating at least one of (i) the DOP of the linearly polarized laser beam 14a and (ii) the orientation of the polarization plane of the linearly polarized laser beam 14a, the linearly polarized laser 14a is scanned across the raw material powder layer 11 according to a scanning strategy in which multiple scanning vectors indicating a first direction are successively scanned, followed by at least one scanning vector indicating a second direction different from the first direction. Such a scanning strategy can reduce the number of changes in the scanning direction S, i.e., the number of rotations of the polarizer 20 that need to be performed to update the orientation of the polarization plane of the linearly polarized laser beam 14a, and alternatively or additionally, can reduce the number of adjustments of the optical unit 16a that need to be performed to update the DOP of the linearly polarized laser beam 14a.

[0071] Figure 4 also shows that the absorption of linearly polarized laser light energy by the raw material depends significantly on the incident angle α of the laser beam 14a relative to the raw material. The incident angle α depends on the deflection angle of the incident laser beam 14a relative to the surface normal extending perpendicular to the upper surface 28 of the raw material powder layer 11, i.e., the operating state of the scanner unit 22. Furthermore, the incident angle α depends on the orientation of the raw material surface 26 onto which the incident laser beam 14a impinges. In the p-polarized state, absorption increases as the incident angle α increases, at least when the incident angle α is between approximately 10° and 80°. The relationship between absorption, incident angle, and polarization may also depend on the material and temperature.

[0072] To ensure particularly reliable control of the absorption of laser energy by the raw material, the control device 18 also takes into account several further process parameters that may affect the absorption of laser energy by the raw material, in addition to the incidence angle α of the linearly polarized laser beam 14a, when controlling the operation of the irradiation system 10. Specifically, the control device 18 controls at least one of the power, focal diameter, and focal shape of the linearly polarized laser beam 14a, and / or at least one of the scanning speed, scanning mode, scanning direction S, and scanning pattern with which the linearly polarized laser beam 14a is directed across the raw material powder layer 11, and / or at least one parameter of the gas flow directed across the raw material powder layer 11, depending on the incidence angle α of the linearly polarized laser beam 14a on the raw material.

[0073] The laser beam source 12b and the optical unit 16b may have the same configuration as the laser beam source 12a and the optical unit 16a, in which case the laser beam 14b is also a linearly polarized laser beam 14b. In such a case, the control device 18 controls the operation of the irradiation system 10 so that the linearly polarized laser beams 14a, 14b are scanned across the overlapping sections of the raw material powder layer 11 according to a scanning strategy, in which all scanning vectors are scanned according to the same scanning mode. In particular, in the overlapping sections of the raw material powder layer 11, all scanning vectors are scanned according to either a backward scanning mode or a forward scanning mode to eliminate the effect of the scanning mode on the absorption of laser energy by the raw material.

[0074] However, the laser beam source 12b and the optical unit 16b may be configured to irradiate a randomized laser beam 14b, a radially polarized laser beam 14b, and / or an azimuthally polarized laser beam 14b. In such a case, a first section of the raw material powder layer 11 may be selectively irradiated with linearly polarized laser light (e.g., a DOP of 99% or less and 30% or more), and a second section of the raw material powder layer 11 may be selectively irradiated with randomized laser light (e.g., a DOP of 0%, 10%, or 15%), radially polarized laser light, and / or azimuthally polarized laser light. Specifically, the first section of the raw material powder layer 11 may be a hatch section of the workpiece layer generated by selectively irradiating the raw material powder layer 11, and the second section of the raw material powder layer 11 may be a contour section of the workpiece layer generated by selectively irradiating the raw material powder layer 11.

Claims

1. A method for operating an irradiation system that irradiates a raw material powder layer with laser light to manufacture a three-dimensional workpiece, wherein at least a portion of the raw material powder layer coated on a carrier is selectively irradiated with linearly polarized laser light having a polarization degree DOP of 99% or less and 30% or more, and the orientation of the polarization plane of the linearly polarized laser light is controlled according to the orientation of the plane of incidence of the linearly polarized laser light on the raw material.

2. 2. The method of claim 1, wherein the orientation of the polarization plane of the linearly polarized laser light is controlled according to the orientation of the plane of incidence of the linearly polarized laser light on the material so that the polarization plane is oriented substantially parallel to the plane of incidence.

3. The method of claim 1 , wherein the DOP of the linearly polarized laser light is controlled depending on the orientation of the plane of incidence of the linearly polarized laser light on the material.

4. The method of claim 1, wherein at least one of (i) the DOP of the linearly polarized laser light and (ii) the orientation of the polarization plane of the linearly polarized laser light is controlled according to the orientation of the incident plane of the linearly polarized laser light on the inner wall surface of a capillary extending from the surface of the raw material powder layer into the volume of the raw material powder layer and formed by the interaction of the linearly polarized laser light with the raw material.

5. 2. The method of claim 1, wherein when controlling the orientation of the polarization plane of the linearly polarized laser light depending on the orientation of the incident plane of the linearly polarized laser light to the raw material, at least one of (i) the DOP of the linearly polarized laser light and (ii) the orientation of the polarization plane of the linearly polarized laser light is updated depending on the scanning direction of the linearly polarized laser light across the raw material powder layer.

6. 6. The method of claim 5, wherein at least one of (i) the DOP of the linearly polarized laser light and (ii) the orientation of the polarization plane of the linearly polarized laser light is updated based on an analysis of a scan pattern in which a beam of the linearly polarized laser light is directed across the raw material powder layer, and wherein the analysis of the scan pattern is performed before starting production of the three-dimensional workpiece and / or in situ during production of the three-dimensional workpiece.

7. 2. The method of claim 1, wherein at least one of the DOP, power, focal diameter and focal shape of the beam of linearly polarized laser light, and / or at least one of the scanning speed, scanning direction, scanning mode and scanning pattern with which the beam of linearly polarized laser light is directed across the raw material powder layer, and / or at least one parameter of the gas flow directed across the raw material powder layer are controlled in response to the incident angle of the beam of linearly polarized laser light with respect to the raw material.

8. 2. The method of claim 1, wherein the linearly polarized laser light has a DOP less than or equal to a first threshold, the first threshold being one of 99%, 98%, 97%, 96%, or 95%.

9. 2. The method of claim 1, wherein the linearly polarized laser light has a DOP greater than or equal to a second threshold, the second threshold being one of 30%, 60%, 75%, 85%, 90%, or 95%.

10. 9. The method of claim 8, wherein the first threshold is 99% and the linearly polarized laser light has a DOP greater than or equal to a second threshold, the second threshold being 85%.

11. The method of claim 10, wherein the raw material powder is a metal powder.

12. 2. The method of claim 1, wherein the DOP of the linearly polarized laser light is set or controlled according to one or more of: (i) the type of raw material; (ii) the intensity of the laser light; and (iii) a scanning speed at which the beam of linearly polarized laser light moves across the raw material powder layer.

13. 10. The method of claim 1, wherein the DOP of the linearly polarized laser light is determined or adjusted by one or more optical elements disposed in a path of laser light emitted from a laser beam source of the illumination system.

14. The method of claim 13 , wherein the one or more optical elements include a turning mirror that reduces a DOP of the incident light upon reflection of the incident light.

15. 2. The method of claim 1, wherein the beam of linearly polarized laser light is scanned across the raw material powder layer according to a scanning strategy, wherein the scanning strategy includes sequentially scanning multiple scanning vectors indicating a first vector direction, followed by scanning at least one scanning vector indicating a second vector direction different from the first vector direction.

16. 2. The method of claim 1, wherein a first section of the raw material powder layer is selectively irradiated with the linearly polarized laser light having a DOP of 99% or less and 30% or more, and a second section of the raw material powder layer is selectively irradiated with random laser light, radially polarized laser light, and / or azimuthally polarized laser light.

17. 17. The method of claim 16, wherein the first section of the raw material powder layer is a hatch section of a work layer generated by selectively irradiating the raw material powder layer, and / or the second section of the raw material powder layer is a contour section of a work layer generated by selectively irradiating the raw material powder layer.

18. 2. The method of claim 1, wherein multiple beams of linearly polarized laser light are scanned across overlapping sections of the raw material powder layer according to a scanning strategy, and all scanning vectors are scanned according to the same scanning mode.

19. An irradiation system that irradiates a raw material powder layer with laser light to manufacture a three-dimensional workpiece, the irradiation system being configured to selectively irradiate at least a portion of the raw material powder layer applied on a carrier with linearly polarized laser light having a polarization degree DOP of 99% or less and 30% or more, the irradiation system having a control device configured to control the orientation of the polarization plane of the linearly polarized laser light in accordance with the orientation of the plane of incidence of the linearly polarized laser light on the raw material.

20. An apparatus for manufacturing a three-dimensional workpiece, the apparatus comprising an irradiation system for irradiating a raw material powder layer with laser light to manufacture the three-dimensional workpiece, the irradiation system being configured to selectively irradiate at least a portion of the raw material powder layer applied on a carrier with linearly polarized laser light having a polarization degree DOP of 99% or less and 30% or more, the irradiation system having a control device configured to control the orientation of the polarization plane of the linearly polarized laser light according to the orientation of the plane of incidence of the linearly polarized laser light on the raw material.

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