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 produce three-dimensional workpieces, further the invention relates to an irradiation system of this kind, and finally the invention relates to an apparatus for producing three-dimensional workpieces. [Background technology]

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

[0003] An exemplary apparatus for manufacturing three-dimensional workpieces by powder bed fusion, as described, for example, in WO 2019 / 141381, comprises a carrier configured to receive a plurality of 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. Upstream of the spatial light modulator, polarization means are provided for performing linear polarization of the laser beam in order to provide the spatial light modulator with a linearly polarized laser beam.

[0004] When building a three-dimensional workpiece 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 evaporation of the raw material creates vapor capillaries through which the laser beam penetrates deeper into the raw material.

[0005] The object of the present invention is to provide a method for operating an irradiation system for irradiating a layer of raw material powder with a laser in order to manufacture a three-dimensional workpiece, and an irradiation system of this kind 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 of 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 on 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] DOP, as used herein, 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 linearly polarized and non-linearly polarized (e.g., unpolarized) components, 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, the first threshold being any of 99%, 98%, 97%, 96%, or 95%. Alternatively or in addition, the linearly polarized laser light may have a DOP equal to or more than a second threshold, the second threshold being 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% produces the best results. Thus, 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 powder layer is applied onto the surface of the carrier by a powder application device, which moves over the carrier to distribute the raw powder. The carrier may be a rigidly fixed carrier. However, preferably, the carrier is configured to be vertically movable, whereby the carrier can move vertically downwards, building up layer by layer of raw powder as the build height of the workpiece increases. Furthermore, the carrier may comprise a cooling device and / or a heating device configured to cool and / or heat the carrier.

[0011] The carrier and the powder application device can be accommodated in a process chamber that can be sealed against the surrounding 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 can be passed through the process chamber and directed over the raw powder layer applied on the carrier before being discharged from the process chamber through a gas outlet. The raw powder applied on the carrier in the process chamber is preferably a metal powder, in particular a metal alloy powder, but can also be a ceramic powder or a powder containing different materials. The powder can have any suitable particle size or particle size distribution. However, it is preferred that the particle size of the processed end product is less than 100 μm.

[0012] The illumination system may have a laser beam source that emits at least one linearly polarized laser light (e.g., DOP > 95%). In particular, the laser beam source of the illumination system may emit linearly polarized laser light (e.g., DOP > 95%) 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 nm to 1090 nm or 1530 nm to 1610 nm, i.e. "infrared" laser light. 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 light, which is converted into linearly polarized laser light (e.g., DOP > 30%) by a suitable polarizing device, such as, for example, a polarizer or a polarizing beam splitter cube. When splitting the laser light into two or more partial beams with different polarizations using one or more beam splitter cubes, only one partial beam can be used as the illumination beam and 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 can be modified, in particular their polarization or DOP can be modified. Additionally or alternatively, multiple beams can 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 powder layer with a single laser beam. However, it is also conceivable that the irradiation system irradiates the raw powder layer with two or more laser beams. When the irradiation system irradiates the raw powder layer with two or more laser beams, at least one laser beam is 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., 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 powder layer may be irradiated by suitable 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, in particular one or more optical elements. The deflection mirror can lower 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 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), for example, by 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) depending on 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., as applied by the laser source or applied to 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 a plurality of scan vectors as further described below); and (v) the incidence angle (e.g., an incidence angle as further described below).

[0018] In a method of operating an irradiation system, 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. The term "plane of polarization" as used herein is defined by the propagation vector of the incident laser beam and the electric field vector of the electromagnetic laser light wave, and thus the polarization plane coincides with the vibration plane of the electric field of the electromagnetic laser light wave. The term "plane of incidence" as used herein refers to the plane defined by the propagation vector of the incident laser beam and the surface normal that extends perpendicular to the material surface on which the incident laser beam strikes. 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 the electric field vector aligned on 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 incidence plane 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 incidence plane 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 incidence plane 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 taking into account and actively controlling the absorption of the laser energy by the raw material, the process stability during the production of three-dimensional workpieces can be improved. Furthermore, by increasing the absorption of the laser energy in a controlled manner, the process productivity can be increased. 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, for example, Cu and Cu alloys, can be processed by irradiating the respective raw material powder 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 incidence plane 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 incidence plane 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 according to the orientation of the plane of incidence of the linearly polarized laser light on the material, such that the polarization plane is oriented substantially parallel to the plane of incidence, in other words, 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 to be p-polarized.

[0022] In the p-polarized state, the absorption of the energy of the linearly polarized laser light by the raw 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 the linearly polarized laser light in the p-polarized state is generally higher than that of the randomly polarized laser light. Thus, by controlling the orientation of the polarization plane relative to the orientation of the plane of incidence to result in the p-polarized state, the absorption of the 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 energy of the laser light. 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. Thus, the DOP of the linearly polarized laser light in addition to the orientation of the polarization plane of the linearly polarized laser light can be controlled to control the amount of laser energy absorbed by the irradiated powder material.

[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 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 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 the evaporation of the raw material heated by the absorption of the energy of 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, the focal diameter and the focal shape of the incident laser beam, as well as the scanning speed and scanning direction of the laser beam and / or at least one parameter of the 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 irradiating the raw material powder layer. Using a predetermined (e.g., pre-measured) list of capillary sizes and shapes associated with various combinations of the aforementioned parameters, the orientation of the inner wall surface of the capillary 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 of the linearly polarized laser light.

[0024] Instead of estimating the size and shape of the capillary using such a predefined list, the orientation of the capillary inner wall surface may be determined based on the direction of the trajectory of the (e.g., molten powder) material ejected (e.g., from the capillary) from the raw powder layer. 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 comprise a so-called on-axis camera. The on-axis camera may be arranged to acquire images via an optical unit. Alternatively, the imaging system may comprise a camera arranged and configured to acquire images of the entire powder layer. Further details of the determination of the trajectory and / or orientation of the capillary 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 inner wall surface of the capillary. 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 the respective droplets or microdroplets.

[0025] The imaging system can be used to determine the orientation of the inner wall without detecting the 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 contour of the capillary detected. 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 is longer the larger the angle between the inner wall and the line of sight of the imaging system. Thus, 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 contour of the capillary) 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 principal angle may then be determined based on the multiple angles (e.g., as an average, median, or weighted average), and the principal angle may then be assumed to be the angle of the inner wall and / or may be assumed to represent the orientation of the inner 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. beforehand) using, for example, 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 be performed, for example, by an on-axis (e.g. OCT) measurement sensor.

[0027] In general, the median or average (e.g., weighted average) of multiple orientations of the inner wall surface may 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 components of the irradiated light are 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 on the raw material powder layer penetrates into the capillary and impinges on the inner wall surface of the capillary, which has a different orientation from the upper surface of the raw material powder layer. For example, the inner wall surface of the capillary on which the laser beam impinges may extend at an angle of about 45° to 90°, preferably about 60° to 80°, with respect to the upper surface of the raw material powder layer, which is usually 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 inner wall surface of the capillary, which is formed by the interaction of the laser beam with the raw material, allows a particularly reliable and accurate control of the absorption of laser energy by the raw material.

[0030] When the scanning direction of the laser beam across the raw material powder layer changes, the direction of the plane of incidence of the linearly polarized laser light on the raw material usually changes. Therefore, when controlling the direction of the polarization plane of the linearly polarized laser light according to the direction of the plane of incidence of the linearly polarized laser light on the raw material, it is preferable that at least one of (i) the DOP of the linearly polarized laser light and (ii) the direction of the polarization plane of the linearly polarized laser light is 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, for example a wave plate, in particular a half-wave plate, may be rotated. Alternatively, a collimator arranged 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, in particular a pair of deflecting mirrors, may be used to deflect the beam of linearly polarized laser light as required. Also, two pairs of polarizing mirrors may be used to deflect the beam of linearly polarized laser light, 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 arranged 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 scanning pattern in which a beam of linearly polarized laser light is directed across the raw material powder layer. By analyzing the scanning pattern, a change in the scanning 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 necessary and synchronized with the operation of the scanner unit to scan the laser beam across the raw material powder layer. The analysis of the scanning pattern may be performed in situ before starting the manufacturing of the three-dimensional workpiece and / or during the manufacturing 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 state, the 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 that extends 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 incidence angle 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 incidence angle depends on the orientation of the raw material surface on which the incident laser beam impinges.

[0035] Besides the incidence angle of the linearly polarized laser beam, several further process parameters can affect the absorption of laser energy by the feedstock. These parameters can include the power of the beam of linearly polarized laser light, the focal diameter and the focal shape. Furthermore, the scanning speed, scanning mode (forward or backward), scanning direction and the scanning 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 directed across the surface of the feedstock powder layer through the process chamber, particularly the volumetric flow rate and flow velocity of the gas flow, as well as the type of gas, can affect the 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 incidence 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 the laser energy by the material, on the one hand a particularly reliable control of the absorption can be achieved and overheating of the material can be avoided. On the other hand, the efficiency of the procedure can be increased. For example, a decrease in absorption caused by a reduction in the laser power and / or an increase in the scanning speed can be compensated by an increase in absorption at a suitable angle of incidence. Furthermore, for certain materials, the energy imparted to the material can be increased in a controlled manner by a suitable control of the angle of incidence, for example to enable the processing of these materials.

[0038] The linearly polarized laser beam may 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 scanned consecutively, 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 direction of the polarization plane of the linearly polarized laser light and / or the DOP of the linearly polarized laser light. 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 direction of the polarization plane of the linearly polarized laser light and / or the DOP of the linearly polarized laser light. Thus, a change in the orientation of the polarization plane of the linearly polarized laser light and / or the DOP of the linearly polarized laser light may only be required if the vector direction, i.e., the direction in which the vector extends, is changed. Such a scanning strategy can reduce the "number of updates" of the orientation of the polarization plane of the linearly polarized laser light and / or the DOP of the linearly polarized laser light. As a result, the update process can be simplified.

[0039] In an embodiment of the method of operating the irradiation system, a first section of the raw powder layer may 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 powder layer may be selectively irradiated with randomized (e.g., 0%, 10% or 15% DOP), radially polarized laser light, and / or azimuthally polarized laser light. The second section may be a section of the raw powder layer that is intended to be irradiated according to a scanning strategy that requires frequent and / or fast updating of either or both (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 may be a section of the raw powder layer that is intended to be irradiated according to a scanning pattern that includes a high density of short scan vectors and / or scan vectors showing multiple directions, and / or may be a section of the raw powder layer that is intended to be irradiated at a high scanning speed.

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

[0041] In a further embodiment of the method of 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 the 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. The term "overlapping section" as used herein 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 scanning vectors are scanned according to a trailing scan mode, or all scanning vectors are scanned according to a leading scan mode. Thus, in the overlapping sections, the absorption of laser light by the raw material is not affected by the scanning mode and can therefore be more reliably controlled.

[0042] In order to manufacture a three-dimensional workpiece, an irradiation system for irradiating a layer of raw material powder with a laser beam is configured to selectively irradiate at least a portion of the raw material powder layer applied on a carrier with linearly polarized laser beam 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 beam according to the direction of the incident plane of the linearly polarized laser beam 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 raw material according to the direction of the incident plane so that the polarization plane of the linearly polarized laser light on the raw 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 light in response to an orientation of a plane of incidence of the linearly polarized laser light on the feedstock. The DOP may be controlled to adjust the amount of energy of the linearly polarized component of the laser light in a p-polarized state (e.g., the component having 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, 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 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.

[0047] Specifically, the controller 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 the manufacturing of the three-dimensional workpiece and / or in situ during the manufacturing of the three-dimensional workpiece.

[0048] The control device may further be configured to control 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 in which the linearly polarized laser beam is directed across the raw material powder layer, and / or at least one parameter of a gas flow directed across the raw material powder layer, in response to the incident angle of the linearly polarized laser beam on the raw material.

[0049] As described above, the linearly polarized laser light may have a DOP less than or equal to a first threshold, the first threshold being any of 99%, 98%, 97%, 96%, or 95%. The linearly polarized laser light may have a DOP greater than or equal to a second threshold, the second threshold being any of 30%, 60%, 75%, 85%, 90%, or 95%. The first threshold may be 99% and the second threshold may be 85%, where 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 the 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 deflection mirror configured to reduce the DOP of the incident light upon reflection.

[0051] The control device is configured to control the scanner unit such 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 powder layer may be a hatch section of the workpiece layer produced by selectively irradiating the raw powder layer, and the second section of the raw powder layer may be a contour section of the workpiece layer produced by selectively irradiating the raw 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-mentioned irradiation system.

[0056] Preferred embodiments of the invention will now be explained in further detail with reference to the accompanying schematic drawings. [Brief description 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. [Diagram 2] 2 shows an illumination system used in the apparatus of FIG. 1; [Diagram 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 a 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 PREFERRED EMBODIMENTS

[0058] FIG. 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 applicator 104 for applying raw material powder onto the carrier 102. The carrier 102 and the powder applicator 104 are housed in a process chamber 106 that can be sealed against the surrounding atmosphere. The carrier 102 is vertically movable within a built-in cylinder 108, and the carrier 102 can be moved downward as the stacking height of the workpieces 110, which are stacked layer by layer from the raw material powder on the carrier 102, increases. The carrier 102 may be equipped with a heater and / or a cooler.

[0059] The apparatus 100 further comprises an irradiation system 10 for selectively irradiating the raw powder layer 11 applied on the carrier 102 with laser light. In the embodiment of the apparatus 100 shown in FIG. 1, the irradiation system 10 comprises two laser beam sources 12a, 12b, each configured to irradiate a laser beam 14a, 14b. 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 comprises only one laser beam source and only one optical unit, so that only a single laser beam is emitted. A control device 18 is provided for controlling the operation of the irradiation system 10 and also for controlling the components of the apparatus 100, such as, for example, the powder application device 104.

[0060] A controlled gas atmosphere, preferably an inert gas atmosphere, is formed in 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 can then be cooled or heated.

[0061] During operation of the device 100 for manufacturing three-dimensional workpieces, a layer 11 of raw powder is applied onto the carrier 102 by the powder application device 104. To apply the layer 11 of raw 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 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 powder onto the carrier 102 and selectively irradiating the layer 11 of raw 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 by the irradiation system 10 onto the raw powder layer 11 is a 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, for example 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 nm to 1090 nm or 1530 nm to 1610 nm, i.e. "infrared" laser light. The polarizing device 20 is used for rotating 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 powder layer 11 by the 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. Thus, the linearly polarized laser light irradiated to the raw material powder layer 11 has a DOP of 99% or less. The DOP of the linearly polarized laser light forming the laser beam 14a and irradiated to 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 powder by the laser beam 14a irradiated on the raw powder layer 11 melts and / or sinters the raw powder. Specifically, a molten pool of molten raw material is generated in the area where the laser beam 14a irradiates the raw powder. Furthermore, the vapor capillary 24, which is 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 on the raw material. The laser beam 14a contains unpolarized components since the DOP is 99% or less. These are reflected multiple times in the capillary 24, which results in the expansion of the capillary 24. Thus, the mechanical and temporal stability of the capillary 24 is improved, and the risk of melting or raw powder material flying out of the capillary 24 can be reduced compared to when the DOP of the laser beam 14a is more 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 top surface 28 of the raw powder layer 11. In the exemplary embodiment shown in FIG. 3, the inner wall surface 26 of the capillary on which the laser beam strikes extends at an angle γ of about 75° to 80° with respect to the top surface 28 of the raw powder layer 11, which is oriented substantially parallel to the surface of the carrier 102. As described above in detail, 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 the 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 on 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 incidence angle 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 controller 18 controls the orientation of the polarization plane of the linearly polarized laser beam 14a in response to the orientation of the incident plane of the linearly polarized laser beam 14a relative to the material. Specifically, the controller 18 controls the orientation of the polarization plane of the linearly polarized laser beam 14a in response to the orientation of the incident plane of the linearly polarized laser beam 14a relative to the inner wall surface 26 of the capillary tube 24.

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

[0068] When the scanning direction S of the laser beam 14a crossing the raw material powder layer 11 changes, the orientation of the incident plane of the linearly polarized laser beam 14a with respect to the raw material changes. Therefore, when controlling the orientation of the polarization plane of the linearly polarized laser beam 14a according to the orientation of the incident plane of the linearly polarized laser beam 14a with respect 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 polarization plane of the linearly polarized laser beam 14a according to the scanning direction S of the linearly polarized laser beam 14a crossing the raw material powder layer 11.

[0069] 2, updating the orientation of the polarization plane of the linearly polarized laser beam 14a is accomplished by appropriately rotating the polarizing device 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 the production of the three-dimensional workpiece 110 and / or in situ during the 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 a number of scanning vectors indicating a first direction are scanned consecutively, 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 polarizing device 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 reduce the number of adjustments by the optical unit 16a that need to be performed to update the DOP of the linearly polarized laser beam 14a.

[0071] FIG. 4 also shows that the absorption of the energy of the linearly polarized laser light by the raw material depends strongly on the incidence angle α of the laser beam 14a on the raw material. The incidence angle α depends on the deflection angle of the incident laser beam 14a with respect to the surface normal extending perpendicular to the upper surface 28 of the raw material powder layer 11, i.e., on the operating state of the scanner unit 22. Furthermore, the incidence angle α depends on the orientation of the raw material surface 26 on which the incident laser beam 14a impinges. In the p-polarized state, the absorption increases with increasing incidence angle α, at least for incidence angles α between about 10° and 80°. The relationship between absorption, incidence angle and polarization may also be material and temperature dependent.

[0072] In order to ensure that the absorption of the laser energy by the raw material can be particularly controlled, the control device 18 takes into account, in addition to the incidence angle α of the linearly polarized laser beam 14a, several further process parameters that may affect the absorption of the laser energy by the raw material when controlling the operation of the irradiation system 10. In particular, the control device 18 controls at least one of the power, the focal diameter and the focal shape of the linearly polarized laser beam 14a, and / or at least one of the scanning speed, the scanning mode, the scanning direction S and the 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 be the same 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 such 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 the scanning vectors are scanned according to the same scanning mode. In particular, in the overlapping sections of the raw material powder layer 11, all the scanning vectors are scanned according to either the backward scanning mode or the 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 the randomized laser beam 14b, the radially polarized laser beam 14b, and / or the azimuthally polarized laser beam 14b. In such a case, the first section of the raw powder layer 11 may be selectively irradiated with the linearly polarized laser beam (e.g., with a DOP of 99% or less and 30% or more), and the second section of the raw powder layer 11 may be selectively irradiated with the randomized laser beam (e.g., with a DOP of 0%, 10% or 15%), the radially polarized laser beam, and / or the azimuthally polarized laser beam. Specifically, the first section of the raw powder layer 11 may be a hatch section of the work layer generated by selectively irradiating the raw powder layer 11, and the second section of the raw powder layer 11 may be a contour section of the work layer generated by selectively irradiating the raw powder layer 11.

Claims

1. A method for operating an irradiation system (10) that irradiates a raw material powder layer with laser light to manufacture a three-dimensional workpiece (110), the method comprising: selectively irradiating at least a portion of a raw material powder layer (11) applied on a carrier (102) with linearly polarized laser light having a polarization degree DOP of 99% or less and 30% or more; and controlling 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.

2. 2. The method of claim 1, wherein 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 material such that the polarization plane is oriented substantially parallel to the plane of incidence.

3. The method according to claim 1 or 2, wherein the DOP of the linearly polarized laser light is controlled depending on an orientation of the plane of incidence of the linearly polarized laser light on the raw material.

4. The method according to any one of claims 1 to 3, 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 an inner wall surface (26) of a capillary (24) extending from a surface (28) of the raw material powder layer (11) into the volume of the raw material powder layer (11) and formed by the interaction of the linearly polarized laser light with the raw material.

5. The method according to any one of claims 1 to 4, wherein 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 on 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 in accordance with a scanning direction (S) of the linearly polarized laser light crossing the raw material powder layer (11).

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 scanning pattern in which a beam (14a, 14b) of the linearly polarized laser light is directed across the raw material powder layer (11), and the analysis of the scanning pattern is performed before starting production of the three-dimensional work (110) and / or in situ during production of the three-dimensional work (110).

7. 7. The method according to claim 1, wherein at least one of the parameters of the DOP, power, focal diameter and focal shape of the beams (14a, 14b) of linearly polarized laser light and / or at least one of the parameters of the scanning speed, scanning direction (S), scanning mode and scanning pattern with which the beams (14a, 14b) of linearly polarized laser light are directed across the raw material powder layer and / or at least one parameter of a gas flow directed across the raw material powder layer are controlled in response to an incidence angle (α) of the beams (14a, 14b) of linearly polarized laser light with respect to the raw material.

8. 8. 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 any of 99%, 98%, 97%, 96% or 95%.

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

10. The method according to claims 8 and 9, wherein the first threshold is 99% and the second threshold is 85%.

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

12. 12. The method of claim 1, wherein the DOP of the linearly polarized laser light is set or controlled in response 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 (14a, 14b) moves across the raw material powder layer (11).

13. The method according to any one of claims 1 to 12, wherein the DOP of the linearly polarized laser light is determined or adjusted by one or more optical elements arranged in a path of laser light emitted from a laser beam source of the illumination system (10).

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. The method according to any one of claims 1 to 14, wherein a beam (14a, 14b) of linearly polarized laser light is scanned across the raw powder layer (11) according to a scanning strategy in which a number of scanning vectors indicative of a first vector direction are successively scanned, followed by at least one scanning vector indicative of a second vector direction different from the first vector direction.

16. The method according to any one of claims 1 to 15, wherein a first section of the raw powder layer (11) 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 powder layer (11) is selectively irradiated with random laser light, radially polarized laser light, and / or azimuthally polarized laser light.

17. 17. The method according to claim 16, wherein the first section of the raw powder layer (11) is a hatch section of a work layer generated by selectively irradiating the raw powder layer (11) and / or the second section of the raw powder layer (11) is a contour section of a work layer generated by selectively irradiating the raw powder layer (11).

18. The method according to any one of claims 1 to 17, wherein a plurality of beams of linearly polarized laser light (14a, 14b) are scanned across overlapping sections of the raw material powder layer (11) according to a scanning strategy, all scanning vectors being scanned according to the same scanning mode.

19. An irradiation system (10) that irradiates a raw material powder layer with laser light to manufacture a three-dimensional workpiece (110), the irradiation system (10) being configured to selectively irradiate at least a portion of a raw material powder layer (11) applied on a carrier (102) with linearly polarized laser light having a polarization degree DOP of 99% or less and 30% or more, the irradiation system (10) having a control device (18) 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. The control device (18) - controlling the orientation of the plane of polarization of the linearly polarized laser light in response to the orientation of the plane of incidence of the linearly polarized laser light on the material such that the plane of polarization is oriented substantially parallel to the plane of incidence, and / or - 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 material, and / or - The irradiation system (10) of claim 19, 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 incidence plane of the linearly polarized laser light on an inner wall surface (26) of a capillary (24) extending from a surface (28) of the raw material powder layer (11) into the volume of the raw material powder layer (11) and formed by the interaction of the linearly polarized laser light with the raw material.

21. The control device (18) is 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 a scanning direction of the linearly polarized laser light across the raw material powder layer (11) when controlling the orientation of the polarization plane of the linearly polarized laser light in accordance with an orientation of the incident plane of the linearly polarized laser light on the raw material.

22. The irradiation system (10) of claim 21, wherein the control device (18) is 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 scanning pattern in which the beam (14a, 14b) of the linearly polarized laser light is directed across the raw material powder layer (11), and the analysis of the scanning pattern is performed before starting production of the three-dimensional work (110) and / or in situ during production of the three-dimensional work (110).

23. 23. The irradiation system (10) of any one of claims 19 to 22, wherein the control device (18) is configured to control at least one of the DOP, power, focal diameter and focal shape of the beams (14a, 14b) of linearly polarized laser light, and / or at least one of the scanning speed, scanning direction (S), scanning mode and scanning pattern with which the beams (14a, 14b) of linearly polarized laser light are directed across the raw material powder layer (11), and / or at least one parameter of a gas flow directed across the raw material powder layer as a function of the incidence angle (α) of the beams (14a, 14b) of linearly polarized laser light with respect to the raw material.

24. 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%; The illumination system (10) according to any one of claims 19 to 23, wherein the linearly polarized laser light has a DOP equal to or greater than a second threshold value, the second threshold value being either 30%, 60%, 75%, 85%, 90% or 95%.

25. 25. The illumination system (10) of claim 24, wherein the first threshold is 99% and the second threshold is 85%, and optionally the raw material powder is a metal powder.

26. a laser beam source, one or more optical elements arranged in a path of laser light emitted from said laser beam source, The illumination system (10) according to any one of claims 19 to 25, wherein the one or more optical elements are configured to determine or adjust the DOP of the linearly polarized laser light.

27. 27. The illumination system (10) of claim 26, 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.

28. The control device (18) - configured to control a scanning unit (22) such that a beam (14a, 14b) of linearly polarized laser light is scanned across said raw material powder layer (11) according to a scanning strategy in which a number of scanning vectors indicative of a first vector direction are successively scanned followed by at least one scanning vector indicative of a second vector direction different from said first vector direction, - The irradiation system (10) according to any one of claims 19 to 27, configured to control the irradiation system (10) so that a first section of the raw powder layer (11) 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 powder layer (11) is selectively irradiated with random laser light, radially polarized laser light and / or azimuthally polarized laser light, the first section of the raw powder layer (11) being in particular a hatch section of a work layer generated by selectively irradiating the raw powder layer (11) and / or the second section of the raw powder layer (11) being in particular a contour section of a work layer generated by selectively irradiating the raw powder layer (11).

29. The illumination system (10) of any one of claims 19 to 28, wherein the control device (18) is configured to control the scanning unit (22) such that a plurality of beams (14a, 14b) of linearly polarized laser light are scanned across overlapping sections of the raw material powder layer (11) according to a scanning strategy, and all scanning vectors are scanned according to the same scanning mode.

30. An apparatus (100) for manufacturing a three-dimensional workpiece (110), comprising an irradiation system (10) according to any one of claims 19 to 29.

Images