Temperature control technology for 3D workpieces produced by additive manufacturing

Abstract

A method for controlling the temperature of a three-dimensional workpiece produced by additive manufacturing is provided. The method includes, during a workpiece manufacturing process, maintaining a temperature of a bottom portion of the workpiece within a first predetermined range including a predetermined set temperature, irradiating a layer of material on top of the workpiece with at least one energy beam to solidify the irradiated material to form a top layer of the workpiece, and maintaining a temperature of the top layer of the workpiece within a second predetermined range including the predetermined set temperature. A corresponding apparatus for controlling the temperature of a three-dimensional workpiece produced by additive manufacturing is also provided.

Description

[Technical Field]

[0001] The present invention generally relates to temperature control techniques for three-dimensional workpieces produced by additive manufacturing processes, which may be, but are not limited to, powder bed fusion processes such as selective laser sintering, selective laser melting, or electron beam melting. [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 radiation (e.g., laser or particle light) in a selective manner depending on the desired shape of the workpiece to be manufactured. The radiation that penetrates the powder layer heats the raw material powder particles, resulting in their melting or sintering. Additional layers of raw material powder are then applied successively to the previously irradiated carrier layer 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 components, or medical prostheses, such as dental or orthopedic prostheses, based on CAD data. Examples of powder bed fusion techniques include selective laser melting, selective laser sintering, and electron beam melting.

[0003] Apparatuses for manufacturing one or more workpieces according to the above techniques are known. For example, EP2961549A1 and EP2878402A1 describe apparatuses for manufacturing three-dimensional workpieces according to the selective laser melting technique. The general principles described in these documents are also applicable to the technique of the present disclosure. Summary of the Invention [Problem to be solved by the invention]

[0004] In particular, during the construction of a workpiece having a complex shape and / or height (i.e., a workpiece having a large extension in the z-direction perpendicular to the substrate plate or carrier) using one of the aforementioned additive manufacturing techniques, large temperature gradients may exist, particularly large temperature differences between the substrate plate and the top layer of the workpiece (also referred to herein as the "top layer").

[0005] In the following, the term "temperature gradient" is used to refer to the spatial temperature gradient along the vertical axis (z-axis) of the workpiece.

[0006] For example, large temperature gradients can cause degradation of material properties and high tensions within the workpiece.

[0007] Accordingly, the present invention aims to provide a technique that solves at least one of the above-mentioned problems and / or other related problems. In particular, but not exclusively, a technique is desired that reduces thermal gradients within a workpiece produced by additive manufacturing, thereby improving the material properties of the finished workpiece.

[0008] This object is addressed by the subject matter of the independent claims. Advantageous embodiments are set forth in the dependent claims. [Means for solving the problem]

[0009] According to a first aspect, there is provided a method for controlling the temperature of a three-dimensional workpiece produced by additive manufacturing, the method including: maintaining a temperature of a bottom portion of the workpiece within a first predetermined range including a predetermined set temperature during a manufacturing process of the workpiece; irradiating a material layer on top of the workpiece with at least one energy beam to solidify the irradiated material to form a top layer of the workpiece; and maintaining a temperature of the top layer of the workpiece within a second predetermined range including the predetermined set temperature.

[0010] One or more of the following features of the method aspects may also be applicable to the apparatus of the apparatus aspects described below: In this disclosure, whenever the term "workpiece" is used, it refers to a "three-dimensional workpiece."

[0011] The additive manufacturing process that produces the workpiece may be additive manufacturing from a powder bed, such as selective laser sintering or selective laser melting, or other additive manufacturing processes in which the workpiece is constructed from raw materials (e.g., in powder, granular, and / or liquid form).

[0012] The predetermined set temperature may be stored in a memory, in particular in a memory of the additive manufacturing device that performs the method, and may be entered by a user or may be predetermined (in particular calculated), for example by a control unit or other computing device of the additive manufacturing device.

[0013] The maintaining step may include taking measures to affect the temperature of the bottom of the workpiece, such as heating or cooling, but the temperature of the bottom of the workpiece may also be maintained within the first predetermined range by providing one or more suitable support structures and / or by providing the workpiece with an appropriate shape.

[0014] The first predetermined range can be up to 5 K, up to 10 K, up to 20 K, up to 30 K, up to 40 K, up to 50 K, up to 70 K, or up to 100 K. It may be desirable to bring the temperature of the bottom of the workpiece as close as possible to a predetermined set temperature. The predetermined set temperature can be in the middle of the first predetermined range. However, the predetermined set temperature can also be one of the limits of the first predetermined range. For example, the predetermined set temperature can be Tset, and the first predetermined temperature range can range from Tset - Ttolerance_1 to Tset + Ttolerance_2. The values ​​of Ttolerance_1 and Ttolerance_2 can be the same or different.

[0015] To maintain the temperature at the bottom of the workpiece, the temperature at the bottom of the workpiece may be measured or estimated (eg, non-contact) and closed-loop temperature control may be implemented.

[0016] The bottom of the workpiece may include the bottom layer of the workpiece. When referring to a layer of the workpiece in this disclosure, it refers to the solidified region of the workpiece resulting from the respective feedstock layer. The bottom of the workpiece may be in direct contact with the carrier of the additive manufacturing device, or may be embedded in the feedstock powder, i.e., solidified on top of the unsolidified feedstock layer. Furthermore, the bottom of the workpiece may be supported by, include, or consist of one or more support structures.

[0017] The step of applying at least one energy beam can be performed as is common in the field of selective laser melting or selective laser sintering. Accordingly, the energy beam can be a laser beam. However, the energy beam can also be a particle beam, an electron beam, or any other suitable energy beam configured to melt or sinter the raw material powder at a desired location. For this purpose, a scanning optical system can be provided to laterally position the energy beam on the irradiated layer of the workpiece. In this disclosure, the lateral direction refers to a direction in the x-y plane parallel to the carrier of the additive manufacturing apparatus. The z-axis is perpendicular to the x-y plane and corresponds to the height axis (i.e., the axis perpendicular to the raw material layers deposited on the carrier). Furthermore, multiple energy beams can be applied simultaneously or sequentially. For example, multiple energy beams that can be independently steered in the x- and y-directions can be provided. For this purpose, a separate scanning optical system can be provided for each energy beam. Each energy beam can be emitted from a corresponding beam source (e.g., a laser source). Furthermore, the multiple energy beams can have different wavelengths, different laser powers, and / or different beam profiles. For example, the shell portion of the workpiece can be irradiated with a different energy beam than the core portion of the workpiece.

[0018] The upper workpiece material layer is a material layer deposited on top of the previously irradiated workpiece layer.

[0019] In this disclosure, when referring to the temperature of the top layer of a workpiece, this may refer to the temperature of the top surface of the workpiece. The temperature of the top layer of a workpiece may be averaged over time and / or space (i.e., different locations on the top layer). The temperature of the top layer of a workpiece may be measured and / or determined a predetermined time after irradiating the top layer, for example, immediately before the next material layer is deposited. Furthermore, the temperature of the top layer of a workpiece may refer to the spatially averaged temperature of the top layer, excluding one or more potential melt pools (which will always be hotter than the rest of the layer). To exclude the temperature peaks of one or more melt pools, the temperature of the top layer may be the median of the temperatures measured and / or considered throughout the top layer.

[0020] Maintaining the temperature of the top layer may mean taking steps to ensure that this temperature is at or close to a set temperature.

[0021] The second predetermined range can be the same as the first predetermined range. The second predetermined range can be up to 5 K, up to 10 K, up to 20 K, up to 30 K, up to 40 K, up to 50 K, up to 70 K, or up to 100 K. It may be desirable to keep the temperature of the top of the workpiece as close as possible to a predetermined set temperature. The predetermined set temperature can be in the middle of the second predetermined range. However, the predetermined set temperature can also be one of the limits of the second predetermined range. For example, the predetermined set temperature can be Tset, and the second predetermined temperature range can range from Tset - Ttolerance_3 to Tset + Ttolerance_4. The values ​​of Ttolerance_3 and Ttolerance_4 can be the same or different.

[0022] Maintaining the temperature of the lowermost part of the workpiece may include heating a carrier on which the workpiece is placed, particularly to a temperature within a first predetermined range.

[0023] Heating of the carrier can be achieved via a heating element integrated into the carrier or provided below the carrier. The heating element can be, for example, an electric heating element in the form of a heating coil. The heating element can be configured to uniformly heat the carrier to a desired temperature corresponding to a predetermined set temperature. In other words, the heating element can be configured to heat the carrier to a temperature within a first predetermined range.

[0024] The carrier may be fixed in its vertical position during the building process. Alternatively, the carrier may be a vertically movable carrier of the additive manufacturing device, which can be lowered during the building process to deposit a new material layer on top of the previous material layer. The term carrier can be interchangeable with base plate or substrate plate. In either case, the carrier may have a flat surface on which the workpiece is built. This surface may be heated in the step of heating the carrier.

[0025] The temperature of the bottom of the workpiece can be maintained using, for example, open-loop or closed-loop temperature control of the carrier temperature. For example, a heating element that heats the carrier can be activated at predetermined time intervals to achieve and maintain a set temperature. In other words, in open-loop temperature control of the available carrier temperature, heating times, non-heating times, and / or heating power values ​​are stored in a look-up table for desired temperature values. Furthermore, a temperature sensor configured to measure the temperature of the carrier can be provided. This temperature sensor can be integrated into the carrier and / or in contact with the carrier. In this case, the output of the temperature sensor can be used for closed-loop temperature control of the carrier temperature. For example, if the temperature of the carrier measured by the temperature sensor falls below a predetermined threshold (depending on the set temperature), the heating element is activated to heat the carrier to the desired set temperature. The predetermined threshold can be equal to the lower limit of a first predetermined range. In this way, the temperature is maintained within the first predetermined range, particularly throughout the entire workpiece building process. The build process can be defined as the time from the start of deposition of the first material layer on the carrier to the end of irradiation of the last material layer. Optionally, the build process can include an additional predetermined cooling period after irradiation of the last material layer. During this cooling period, the temperatures of the bottom and top layers of the workpiece can be maintained within their respective ranges, for example, to achieve a desired microstructure of the workpiece.

[0026] To maintain the temperature of the bottom of the workpiece within the first predetermined range, it may be necessary to set the set temperature of the carrier slightly higher than the desired set temperature of the bottom of the workpiece due to additional heat sinking (especially if the bottom of the workpiece is not in direct contact with the carrier or has a small contact area with the carrier). Therefore, maintaining the temperature of the bottom of the workpiece within the first predetermined range may include maintaining the carrier on which the workpiece is built within a third predetermined range that is different from the first predetermined range. The upper and lower limits of the third predetermined range may be greater than the upper and lower limits of the first predetermined range, particularly by a fixed offset (e.g., 5K, 10K, or 15K).

[0027] The method may further include determining the predetermined set temperature based on building data that defines the shape of the workpiece to be built.

[0028] The set temperature may be affected by several parameters, including the shape of the workpiece to be built, and may further include at least one of the following: the material or material composition used, the height of the workpiece, and one or more irradiation parameters such as wavelength, beam power, and / or beam profile.

[0029] The set temperature can be determined by analyzing the build data (CAD data, SLM data, etc.). A mathematical model may be used to determine the predetermined set temperature. Furthermore, the set temperature can also be determined using past process values ​​(e.g., measured temperatures) as input parameters. For example, an artificial intelligence engine can be trained and implemented to determine the set temperature.

[0030] The step of determining the predetermined set temperature may include storing the predetermined set temperature in a memory, for example, a memory of the additive manufacturing device. Furthermore, a first predetermined range may be determined based on the determined set temperature. For example, the first predetermined range may be determined such that the set temperature is in the middle of the first predetermined range. Furthermore, a second predetermined range may be determined based on the determined set temperature. For example, the second predetermined range may be determined such that the set temperature is in the middle of the second predetermined range.

[0031] Maintaining the temperature of the top layer of the workpiece may include at least one of heating the top layer of the workpiece and / or a layer of raw material above the workpiece and introducing a period of reduced irradiation power.

[0032] In other words, there are at least two options for maintaining the temperature of the top layer: heating and cooling (i.e., introducing periods of reduced irradiation power). Heating can be achieved actively using a heating device. Cooling can be achieved passively by removing heat from the workpiece (e.g., thermal radiation or convection).

[0033] Maintaining the temperature of the top layer can be performed by open-loop or closed-loop control. In open-loop control, predetermined actions are taken that are known to reach the desired setpoint temperature (i.e., the temperature of the top layer within a second predetermined range). For example, the heating time, non-heating time, and / or heating power can be stored in a look-up table for multiple desired temperature values. Similarly, the length of time that the irradiance power is reduced and / or values ​​of other parameters can be stored in a look-up table for multiple desired temperature values.

[0034] In the closed loop, the temperature of the top layer is measured, for example, by a contact temperature sensor, a non-contact temperature sensor, a camera, a pyrometer, etc. Based on the measured temperature, a cooling or heating action for the top layer is initiated. For example, if the measured temperature is below the lower limit of a second predetermined range, the top layer is heated by a heating device (top layer heating device). If the measured temperature is above the upper limit of the second predetermined range, at least one additional period of reduced irradiation power (e.g., a waiting period without irradiation) is added.

[0035] Heating the top layer of the workpiece can include increasing the irradiation power of at least one energy beam. The irradiation power (e.g., laser power) can be increased to a value that ensures that the amount of heat introduced into the top layer is equal to the amount of heat released from the top layer from the start of coating the current material layer to the start of coating the next material layer. In other words, the irradiation power can be increased to maintain the temperature of the top layer within a second predetermined range.

[0036] Furthermore, the above-described measures for maintaining the temperature of the top layer in closed-loop operation may be implemented for the current top layer of the workpiece where the temperature measurement is performed, but the temperature maintenance measures may also be implemented for the layer next to the layer where the temperature measurement was performed, i.e., subsequent layers.

[0037] The method may further include calculating the amount of heat introduced into the top layer by the at least one energy beam, calculating the amount of heat released from the top layer during irradiation of the top layer, and if the amount of heat introduced into the top layer is greater than the amount of heat released from the top layer, introducing a time period during which the irradiation power is reduced during which additional heat can be released from the top layer, and / or if the amount of heat released from the top layer is greater than the amount of heat introduced into the top layer, heating the top layer of the workpiece and / or a raw material layer above the workpiece so that additional heat is introduced into the top layer.

[0038] Calculating the amount of heat introduced into and released from the top layer can be part of the open-loop control of the temperature of the top layer. Thus, this process can be performed without measuring the temperature of the top layer. However, measuring the temperature of the top layer can provide an additional step to verify that the control performed is correct and that the temperature of the top layer is maintained at the desired value.

[0039] Thus, a combination of open-loop and closed-loop control can be performed, for example, open-loop control can be performed for one or more initial iteration steps, and then switched to closed-loop control.

[0040] The calculation of the amount of heat introduced into the top layer and / or the calculation of the amount of heat released from the top layer can be performed using a mathematical model and / or an artificial intelligence engine. Input parameters to the mathematical model and / or the artificial intelligence engine can be one or more of the shape of the top layer to be irradiated, the area of ​​the top layer to be irradiated, the power (e.g., laser power) of the energy beam, the beam profile of the energy beam, the number of energy beams irradiating the top layer, the shape of the underlying solidified workpiece layer, the presence or absence of gas flow, the gas flow rate, etc.

[0041] The amount of heat introduced into the top layer may be the amount of heat introduced by at least one energy beam during irradiation of the entire top layer. The amount of heat released from the top layer may be the amount of heat released from the top layer during the time from the start of irradiation of the top layer to the start of irradiation of the next layer. This time corresponds to the sum of the irradiation time and the fire-to-fire time during which the next layer is coated. More precisely, the amount of heat released from the top layer may be the amount of heat released from the top layer without taking into account the time during which the irradiation power decreases. Similarly, the amount of heat introduced into the top layer may be the amount of heat introduced only by irradiation with at least one energy beam, without taking into account additional heating (e.g., heating by a top layer heating device). Therefore, the calculated amount of heat introduced into and / or released from the top layer may be calculated without taking into account means for introducing / removing additional heat.

[0042] The predetermined waiting time may have a predetermined time (e.g., expressed in seconds or milliseconds). The predetermined time may be calculated so that the additional heat is released from the top layer within this time. A mathematical model may be used for this purpose.

[0043] If the amount of heat introduced into the top layer is greater than the amount of heat released from the top layer, the additional amount of heat released from the top layer may be equal to the difference between the amount of heat introduced into the top layer and the amount of heat released from the top layer. Additionally or alternatively, if the amount of heat released from the top layer is greater than the amount of heat introduced into the top layer, the additional amount of heat introduced into the top layer may be equal to the difference between the amount of heat released from the top layer and the amount of heat introduced into the top layer.

[0044] In this way, it can be ensured that the sum of the heat introduced into the top layer is equal to the sum of the heat released from the top layer. In other words, for example, for each top layer, the sum of the heat introduced into the top layer is net zero. This can also be referred to as a net-zero concept.

[0045] Heating of the top layer of the workpiece can be achieved by thermal radiation.

[0046] For this purpose, a separate heating device (top layer heating device) may be provided. This heating device may be fixed or mobile. It is also possible to provide one or more additional laser beams, for example, to heat the top layer using a laser beam with a laser power that is not sufficient to generate a melt pool. In addition, infrared radiation may be used to heat the top layer.

[0047] The thermal radiation can be provided by a device attached to a powder coating apparatus configured to coat the top layer of the workpiece.

[0048] Thus, the heating device may be a top layer heating device configured to move with the powder coating device. For example, additional heat may be introduced to the top layer as the top layer material is applied by the coating device, although the coating and heating may occur at different times.

[0049] The time during which the irradiation power is reduced may be a predetermined waiting time during which the raw material is not irradiated.

[0050] In other words, during a predetermined waiting time, all energy beams may be switched off (e.g., by a shutter) or directed into a beam trap. Furthermore, so-called ghost parts may be added to the build data, which correspond to unirradiated (and therefore virtual) workpieces.

[0051] The time during which the irradiation power is reduced may be a period during which at least one irradiation parameter is changed to reduce the irradiation power, the irradiation parameter being at least one of the number of effective energy beams, laser power, scanning speed, and beam profile.

[0052] During the time when the irradiation power is reduced, the irradiation power introduced to the top layer may be greater than zero, but may be less than if the time when the irradiation power is not reduced is not introduced. In other words, during the time when the irradiation power is reduced, it is not necessary to completely turn off the irradiation to the top layer, as in the case of a predetermined waiting time. The irradiation power may also be reduced during this time, specifically, by the following method. Irradiating the top layer with reduced irradiation power may require irradiation of the top layer for a predetermined time longer than if the time when the irradiation power is not reduced is not introduced. The period during which the irradiation power is reduced may be calculated so that the amount of heat that can be released from the top layer during the predetermined time is equal to the (calculated) additional heat amount.

[0053] The period during which the irradiation power is reduced can be introduced between the time when irradiation of the top layer is completed and the time when powder coating of the next layer on the top layer is started, between the time when coating of the next layer on the top layer is completed and the time when irradiation of the next layer is started, or during irradiation of the top layer.

[0054] In other words, there are multiple options for introducing one or more periods of reduced irradiation power. The period of irradiation power can also be divided into two or more periods, for example, one period after coating and before irradiation, and another period after irradiation and before coating the next layer. Furthermore, one or more periods of reduced irradiation power can also be introduced between successive irradiation vectors (e.g., a hatch pattern). There are multiple options for introducing one or more periods of reduced irradiation power, such as turning off one or more energy beams at one or more predetermined times before, during, or after irradiation of the top layer.

[0055] The temperature of the top layer of the workpiece can be maintained by closed loop control.

[0056] The temperature of the top layer can be measured. Based on the measured temperature, it can be determined whether to introduce a time during which the irradiation power is reduced or whether to heat the top layer. The set temperature of the closed-loop control can be equal to the set temperature of the bottom of the workpiece (e.g., the set temperature of the carrier on which the workpiece is built). For example, if the measured temperature is below the lower limit of a first predetermined range, additional heating is performed, for example, by a top layer heating device. Furthermore, if the measured temperature is above the upper limit of the first predetermined range, a time during which the irradiation power is reduced is introduced.

[0057] The method can be carried out by an additive manufacturing apparatus, in particular an apparatus for selective laser melting or selective laser sintering.

[0058] The additive manufacturing apparatus may comprise all of the usual components of such an apparatus, such as a build chamber, a build cylinder, a movable carrier, an irradiation unit for irradiating one or more energy beams, a gas circulation line with a corresponding circulation device (e.g., a fan or blower), one or more filters, etc.

[0059] The method may further include determining a predetermined set temperature for defining the microstructure during the build process.

[0060] In this way, the cooling rate can be determined during the cooling process of the workpiece, and the microstructure of the workpiece can be controlled. In particular, during this cooling process, the predetermined set temperature can be dynamically controllable, i.e., can change during the cooling process. More precisely, the predetermined set temperature can comprise a predetermined, time-dependent set temperature, which is set, for example, by a control unit of the additive manufacturing device.

[0061] According to a second aspect, there is provided a temperature control device for a three-dimensional workpiece produced by additive manufacturing. The device includes a control unit and an irradiation device that irradiates at least one energy beam. The control unit is configured to maintain the temperature of a bottom portion of the workpiece within a first predetermined range including a predetermined set temperature during the workpiece manufacturing process, and to instruct the irradiation device to irradiate a topmost layer of material of the workpiece with at least one energy beam to solidify the irradiated material and form a topmost layer of the workpiece, and to maintain the temperature of the topmost layer of the workpiece within a second predetermined range including the predetermined set temperature.

[0062] All of the above aspects and details described with respect to the method aspect (first aspect) are also applicable to the apparatus aspect. In particular, the apparatus of the second aspect may be configured to perform the method of the first aspect, in which case one or more of the details described above with respect to the method aspect are implemented.

[0063] The apparatus may further comprise a carrier heating device, and maintaining the temperature of the bottom of the workpiece includes instructing the carrier heating device to heat the carrier on which the workpiece is constructed, particularly to a temperature within a first predetermined range.

[0064] The control unit may further be configured to determine the predetermined set temperature based on building data that defines the shape of the workpiece to be built.

[0065] The apparatus may further include a top layer heating device, and maintaining the temperature of the top layer of the workpiece includes at least one of instructing the top layer heating device to heat the top layer of the workpiece and / or a raw material layer on top of the workpiece, and introducing a period in which the irradiation power is reduced.

[0066] The control unit may further be configured to calculate the amount of heat introduced into the top layer by the at least one energy beam, calculate the amount of heat emitted from the top layer during irradiation of the top layer, and, if the amount of heat introduced into the top layer is greater than the amount of heat emitted from the top layer, introduce a time period for reducing the irradiation power during which additional heat can be emitted from the top layer, and / or, if the amount of heat emitted from the top layer is greater than the amount of heat introduced into the top layer, instruct the top layer heating device to heat the top layer of the workpiece and / or a raw material layer above the workpiece so that additional heat is introduced into the top layer.

[0067] If the amount of heat introduced into the top layer is greater than the amount of heat released from the top layer, the additional amount of heat released from the top layer can be equal to the difference between the amount of heat introduced into the top layer and the amount of heat released from the top layer. Additionally or alternatively, if the amount of heat released from the top layer is greater than the amount of heat introduced into the top layer, the additional amount of heat introduced into the top layer can be equal to the difference between the amount of heat released from the top layer and the amount of heat introduced into the top layer.

[0068] The heating device may be configured to heat the top layer of the workpiece by thermal radiation.

[0069] The top layer heating device is attached to a powder coating device configured to coat the top layer of the workpiece.

[0070] The time during which the irradiation power is reduced may be a predetermined waiting period during which no irradiation of the source material occurs.

[0071] The time during which the irradiation power is reduced can be the time during which at least one irradiation parameter is changed to reduce the irradiation power, the irradiation parameter being at least one of the number of active energy beams, laser power, scanning speed and beam profile.

[0072] The irradiation power reduction time can be introduced between the time irradiation of the top layer is completed and the time powder coating of the next layer on the top layer is started, between the time coating of the next layer on the top layer is completed and the time irradiation of the next layer is started, or during irradiation of the top layer.

[0073] The control unit may be configured to maintain the temperature of the top layer of the workpiece by closed-loop control.

[0074] The apparatus may be an additive manufacturing apparatus, in particular an apparatus for selective laser melting or selective laser sintering.

[0075] The control unit can be configured to determine a predetermined set point temperature for defining the microstructures during the building process.

[0076] 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]

[0077] [Figure 1] FIG. 1 is a schematic side view of an additive manufacturing apparatus with one laser beam, according to an embodiment of the present disclosure. [Figure 2] FIG. 1 is a schematic side view of an additive manufacturing apparatus with two laser beams, according to an embodiment of the present disclosure. [Figure 3] FIG. 10 shows details of a build cylinder with a carrier and a workpiece with a support, applicable to embodiments of the present disclosure. [Figure 4] FIG. 1 illustrates a flowchart of a method according to an embodiment of the present disclosure. [Figure 5]FIG. 1 illustrates a control unit with modules according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0078] 1 is a schematic diagram of an additive manufacturing apparatus 10 for manufacturing a three-dimensional workpiece 12. The apparatus 10 is generally well known to those skilled in the art, except for the temperature control method and some optional details programmed into a control unit 40 of the apparatus 10. The apparatus 10 is, for example, a typical additive manufacturing apparatus, and the temperature control method according to the present disclosure is programmed into the control unit 40 of the apparatus 10. Optionally, some additional structural elements may be provided that are not necessarily common in typical additive manufacturing apparatuses, such as a top layer heating device.

[0079] The principles of the apparatus 10 are well known to those skilled in the art of additive manufacturing and will only be briefly described. For example, such an apparatus 10 may be an apparatus for selective laser melting or an apparatus for selective laser sintering, in which one or more laser beams 14 can be used to selectively irradiate and solidify subsequent layers of raw material powder.

[0080] An apparatus 10 for performing a selective laser melting process as described below is provided as an example. A typical feature of powder bed fusion is that raw material powder is applied in layers, and each layer is selectively irradiated and solidified to produce one layer of the workpiece 12 to be manufactured. After removing excess powder and after any post-processing steps (e.g., removal of one or more support structures), the final workpiece 12 is obtained.

[0081] It should be noted that the technique of the present disclosure is not limited to powder bed fusion technology, and liquid or granular raw materials can also be used. Furthermore, the technique of the present disclosure is not limited to irradiation with a laser beam. Instead, any other energy beam (e.g., particle beam, electron beam, etc.) that can solidify the raw material powder used can be used. However, in the specific embodiment described below, the raw material is a raw material powder (e.g., metal powder, ceramic powder, or plastic powder), and at least one energy beam that solidifies the raw material powder is a laser beam.

[0082] FIG. 1 shows an apparatus 10 for manufacturing a three-dimensional workpiece 12 by selective laser melting. The apparatus 10 includes a process chamber 16. The process chamber 16 can be sealed against the ambient atmosphere, i.e., against the environment surrounding the process chamber 16. A powder coating device 18 disposed within the process chamber 16 applies raw material powder onto a carrier 20. For this purpose, the powder coating device 18 can include at least one of a roller, a blade, and a raw material hopper. The powder coating device 18 is configured to apply a uniform layer of raw material onto a previously applied and irradiated layer of raw material. "Uniform" here particularly means a uniform layer thickness. The layer thickness can be selected, for example, by vertical positioning of the powder coating device 18 and / or the carrier 20.

[0083] A vertical movement unit 22 is provided to displace the carrier 20 in the vertical direction, so that as the build height of the workpieces 12 stacked in layers from the raw material powder on the carrier 20 increases, the carrier 20 can be moved vertically downward.

[0084] The ability of the carrier 20 to be moved by the vertical movement unit 22 is well known in the field of selective laser melting and will not be described in detail here. As an alternative to a movable carrier 20, the carrier 20 may be provided as a static (or fixed) carrier (particularly with respect to the vertical z-direction), in which case the irradiation device 24 (see below) and the processing chamber 16 are configured to be moved upward during the build process (i.e., as the build height of the workpiece 12 increases). Furthermore, both the carrier 20 and the irradiation device 24 may be independently movable along the z-direction.

[0085] The carrier surface of the carrier 20 defines a horizontal plane (xy plane), and the direction perpendicular to this plane is defined as the vertical or build direction (z direction). Thus, each top layer of raw material powder and each layer of the workpiece 12 extends in a plane parallel to the horizontal plane (xy plane) defined above.

[0086] The apparatus 10 further includes a gas inlet 26 for supplying an inert gas (e.g., argon) into the processing chamber 16. A gas outlet 27 is also provided to form a gas circuit, thereby generating a continuous (horizontal) gas flow through the processing chamber 16. In a preferred embodiment, a unidirectional laminar gas flow is generated over the top layer of raw material powder. The gas flow generated by the gas inlet 26 and the gas outlet 27 helps remove molten spatter and / or other unwanted contaminant particles from the top layer of the workpiece 12, improving the quality of the workpiece 12. The gas flow also removes heat from the workpiece 12 by convection.

[0087] Additionally, a temperature measurement device 28 is disposed within the process chamber 16 for determining the temperature of the top layer 13 of the workpiece 12. This device 28 is optional and may only be used for closed-loop control (see detailed description below). The temperature measurement device 28 may comprise a pyrometer configured to detect thermal radiation emitted from the top layer 13 of the workpiece 12. In particular, the temperature measurement device 28 is configured to generate a heat map of the top layer 13, thereby determining a temperature value for each location on the top layer 13. The temperature measurement device 28 provides the measured temperature values ​​to the control unit 40, which may then further process these temperature values, for example, by generating a time series of temperature values ​​and / or determining an average temperature value.

[0088] As an alternative to the non-contact temperature measurement device 28, a contact thermometer may be provided that is configured to contact the top layer 13 at a desired time and / or at a desired location to obtain one or more temperature values ​​indicative of the temperature of the top layer 13.

[0089] The apparatus 10 further includes an irradiation device 24 for selectively irradiating the laser beam 14 onto the top layer of the raw material powder applied on the carrier 20. The irradiation device 24 allows the raw material powder applied on the carrier 20 to be laser-irradiated in a selective manner depending on the desired shape of the workpiece 12 to be manufactured. In the embodiment of Fig. 1, the irradiation device 24 includes exactly one irradiation unit 24a, which is configured to irradiate exactly one laser beam 14 at a time.

[0090] The irradiation unit 24a includes a scanning unit 30 configured to selectively irradiate the raw material powder provided on the carrier 20 with the laser beam 14. The scanning unit 30 is controlled by a control unit 40 of the apparatus 10. The scanning unit 30 may include a single mirror that is tiltable about two perpendicular axes. Alternatively, the scanning unit 30 may include two tiltable mirrors, each configured to tilt about a corresponding axis. The tiltable mirrors may be, for example, galvanometer mirrors.

[0091] The irradiation unit 24a is supplied with laser light from a laser light source 32. The laser light source 32 may be provided inside the irradiation unit 24a or may be provided outside the irradiation unit 24a as shown in FIG. 1. In the former case, the laser light source 32 can be considered to be part of the irradiation unit 24a. In the latter case, the laser beam is generated by the laser light source 32 and guided into the irradiation unit 24a via an optical fiber 34. Alternatively, the laser beam may be guided into the irradiation unit 24a through air or a vacuum, for example, by using one or more mirrors.

[0092] A laser beam is directed from the laser source 32 towards the scanning unit 30. The laser source 32 may include, for example, a diode-pumped ytterbium fiber laser that emits laser light at a wavelength of approximately 1070-1080 nm (i.e., in the infrared wavelength range).

[0093] The illumination unit 24a further includes two lenses 36 and 38, which are configured to focus the laser beam 14 at a desired focal position along the z-axis. In the embodiment shown in FIG. 1, both lenses 36 and 38 have positive refractive power. The lens 38, which is upstream in the beam path, is configured to collimate the laser light emitted by the fiber 34, thereby generating a collimated or substantially collimated laser beam. The lens 36, which is downstream in the beam path, is configured to focus the collimated (or substantially collimated) laser beam at a desired z-position.

[0094] The processing chamber 16 is provided with a top layer heating device 42. The top layer heating device 42 is configured to heat the top layer (i.e., the top layer 13) of the workpiece 12 being built and / or to heat a new powder layer to be deposited on the top layer 13. The top layer heating device 42 is configured to heat the top layer 13 by thermal radiation. The amount of thermal radiation emitted from the top layer heating device 42 is controllable by the control unit 42. The top layer heating device 42 is configured, for example, to irradiate the top layer 13 with thermal radiation (infrared rays) in order to heat the top layer 13. Heating as discussed herein is synonymous with introducing heat, particularly a predetermined amount of heat.

[0095] The top layer heating device 42 is attached to the powder coating device 18 and moves horizontally together with the powder coating device 18. This allows powder coating and heating to be performed simultaneously. It is also possible to perform powder coating and heating separately, in which case the horizontal movement device provided on the powder coating device 18 is shared with the top layer heating device 42.

[0096] Alternatively, the top layer heating device 42 may be provided with separate (horizontal and / or vertical) movement devices to allow it to move independently relative to the powder coating device 18 .

[0097] Furthermore, other types of top layer heating devices can be implemented. For example, a contact-type top layer heating device that heats the top layer 13 while contacting it can be provided. Furthermore, for example, one or more laser light sources can be provided as the top layer heating device, and heat can be introduced by irradiating a predetermined portion of the top layer with laser beams generated by the one or more laser light sources (without melting the top layer). Furthermore, at least one irradiation beam 14 that performs the building job can also perform (additional) heating of the top layer. In this case, the control unit 40 increases the irradiation power of the irradiation beam 14 by a predetermined amount.

[0098] A carrier heater 44 is incorporated into the carrier 20. The carrier heater may be provided on the bottom, one side, or both sides of the carrier. Note that the carrier 20 itself may include multiple plates and / or other structures. The multiple plates may be referred to as a plate package. The topmost plate of these plates may be referred to as a substrate plate. The carrier heater 44 may be provided on one of the plates of the plate package, or between two plates of the plate package.

[0099] The carrier heating device 44 is configured to heat the carrier 20 to a predetermined temperature. For this purpose, a temperature sensor (not shown) can be incorporated into or contacted with the carrier 20 to measure the temperature of the carrier 20. In this way, a closed-loop temperature control of the carrier 20 can be performed, for example, by a control unit. When the temperature falls below a lower limit (of the second predetermined range), the carrier heating device 44 introduces heat into the carrier 20, and when the temperature rises above an upper limit (of the second predetermined range), heating is stopped. However, other methods for maintaining the temperature of the carrier 20 are also possible. For example, predetermined heating and / or non-heating times and / or predetermined heating power values ​​for multiple temperature values ​​can be stored in a look-up table. If a specific temperature value needs to be set (as a set temperature), the heating and / or non-heating times and / or heating power can be read from the look-up table (open-loop temperature control of the carrier).

[0100] The top layer heating device 42 and the carrier heating device 44 are optional. That is, the present technology will function even if one or more of these devices are omitted. For example, if an appropriate set temperature is selected and only a period of reduced irradiation power is introduced, additional heating by the heating device 42 is not necessary. Furthermore, if the temperature of the bottom 15 of the workpiece can be maintained without introducing additional heat by the carrier heating device 44, the carrier heating device 44 can be omitted. For example, this is the case when an appropriate support structure is selected for the workpiece 12, as shown in FIG. 3.

[0101] The control unit 40 includes a processor and a memory in which instructions for controlling the individual components of the apparatus 10 are stored. For example, the control unit 40 can be configured to control one or more of the temperature measurement device 28, the vertical movement unit 22, the powder coating device 18, the gas flow rates provided by the gas inlet 26 and the gas outlet 27, the irradiation device 24, the top layer heating device 42, and the carrier heating device 44. A user input / output interface can be provided and is connected or connectable to the control unit 40. The control unit 40 also has an interface for receiving workpiece data representing the three-dimensional shape of the workpiece 12 to be manufactured.

[0102] 1 and 2 are merely schematic and not limiting. The control unit 40 may be located at any suitable location within the device 10, or may be located at a location remote from the device 10 (e.g., integrated into a network structure such as a LAN). Furthermore, the control unit 40, or at least a part thereof, may be provided in the form of a cloud computing device.

[0103] FIG. 2 shows a different embodiment of an apparatus 10 similar to the embodiment of the apparatus 10 of FIG. 1. The only difference between the two apparatuses 10 is that the irradiation apparatus 24 of the apparatus 10 of FIG. 2 includes two irradiation units 24a and 24b instead of the single irradiation unit 24a of the apparatus 10 of FIG. 1. However, the remainder of the apparatus 10 of FIG. 2 has the same components and functions as those described above with respect to FIG. 1, and therefore a repetition of this description will be omitted. Furthermore, the components of the irradiation units 24a and 24b are given the same reference numerals as in FIG. 2. However, the suffixes "a" and "b" are used to distinguish between the components of the irradiation unit 24a (suffix a) and the components of the further irradiation unit 24b (suffix b). The functions of the individual components within the irradiation units 24a and 24b are the same as those described above with respect to the irradiation apparatus 24 of FIG. 1.

[0104] Hereinafter, the use of reference numerals without the suffix (a or b) also refers to the respective elements with the suffixes a and b, unless otherwise specified. For example, when referring to "scanning unit 30," it also refers to scanning units 30a and 30b.

[0105] In the apparatus 10 of FIG. 2 , the irradiation unit 24a is configured to scan a first predetermined area (i.e., a first scanning field) of the top powder layer. Similarly, the additional irradiation unit 24b is configured to scan a second predetermined area (i.e., a second scanning field) of the top powder layer. The first scanning field and the second scanning field overlap each other in an overlapping region. In other words, there is a region of the top powder layer (i.e., an overlapping region) that can be reached and selectively irradiated by both laser beams 14a and 14b. The first scanning field and the second scanning field may each be rectangular or circular, and the size and / or shape of each scanning field may be predetermined by the movement range of the scanning unit 30a of the irradiation unit 24a and the scanning unit 30b of the irradiation unit 24b. Furthermore, the overlapping region can cover the entire top powder layer, thereby allowing each of the laser beams 14a and 14b to reach every point on the top powder layer.

[0106] To produce the three-dimensional workpiece 12, both laser beams 14a and 14b can simultaneously irradiate different portions of the same powder layer, with each of laser beams 14a and 14b irradiating a portion of the workpiece 12 in a corresponding scan field. In this manner, the workpiece 12 can be constructed more quickly than if only one laser beam 14 were used (see, e.g., FIG. 1 ). Furthermore, different laser beams 14a and 14b may be used, for example, to solidify the shell and core portions of the workpiece 12. The different laser beams have different beam parameters, such as laser power, wavelength, and / or beam profile.

[0107] For example, one of the laser beams 14a, 14b can be turned off while the illumination power is reduced.

[0108] FIG. 3 shows details of a build cylinder (i.e., the volume through which carrier 20 moves) that can be used in both the apparatuses shown in FIGS. 1 and / or 2. As an alternative to, or in addition to, carrier heating device 44, the temperature of lower portion 15 of workpiece 12 can be maintained by calculating and constructing a suitable carrier structure 46. In other words, constructing a corresponding carrier structure 46 can provide a mathematical model for determining and appropriately setting the temperature of lower portion 15 of workpiece 12. Because carrier structure 46 removes a controlled amount of heat from workpiece 12, lower portion 15 maintains a temperature within a second predetermined range, even when additional heat is introduced into top layer 13 of workpiece 12 by laser beam 14.

[0109] A temperature control method according to the present invention will now be described in more detail. The method is carried out by any of the devices 10 of Figures 1 and 2 (and optionally also Figure 3). In particular, the main parts of the method are carried out by the control unit 40 of the respective device.

[0110] To build a workpiece 12 using selective laser melting technology, optical power (i.e., laser power) must be introduced into raw material powder (e.g., metal powder). The raw material powder melts and solidifies to form the workpiece 12. Part of the optical power introduced into the process chamber 16 is reflected by the powder and the melt pool generated by the laser beam 14, while the remaining optical power reaches the workpiece 12. From there, multiple heat sinks are generated: radiation from the workpiece surface (i.e., top layer 13), primary heat transfer to the walls of the process chamber 16, convection into the gas flow (generated by the gas outlet 26 and gas inlet 27), removal of metal spatter condensing in the gas flow, and heat transfer to the carrier 20 via the workpiece 12 (e.g., heat loss to the build cylinder via the powder). The heat distribution among the heat sinks depends on the process. Relevant factors include the raw material powder material, material parameters, and the workpiece geometry (especially the irradiated surface). In particular, for materials with low thermal conductivity (i.e., low heat transfer), the following facts apply depending on the workpiece height (i.e., extension along the z-axis):

[0111] When the workpiece 12 is low (has little extension in the z direction), the thermal resistance from the top layer 13 of the workpiece 12 to the carrier 20 is small, so a relatively large amount of heat reaches the carrier 20 by heat transfer through the workpiece 12 (i.e., in the z direction). On the other hand, when the workpiece 12 is tall (has a large extension in the z direction), the thermal resistance of the workpiece 12 is large, so a smaller amount of heat reaches the carrier 20 by heat transfer through the workpiece 12. In other words, a larger temperature difference is required to transfer the same amount of heat energy, which may cause problems during the process.

[0112] The thermal resistance of the workpiece 12 physically always generates a temperature gradient within the workpiece 12 relative to the temperature of the carrier 20 as long as optical power is introduced into the workpiece 12. A temperature gradient is undesirable because it induces tension in the material and can impair material properties (depending on the magnitude of the gradient). The two factors mentioned above directly determine the magnitude of the temperature gradient. First, the greater the thermal resistance, i.e., the poorer the thermal conductivity of the material, the greater the height (z-direction) of the workpiece 12, and the more unfavorable the workpiece's shape (e.g., the presence of narrow sections), the greater the temperature gradient. Second, the greater the temperature gradient, the greater the amount of heat introduced into the workpiece 12.

[0113] The second factor in particular can be controlled by the techniques of the present disclosure.

[0114] Net Zero Concept The net-zero concept described herein is used to maintain zero "net" heat introduced into the workpiece 12 during the process of forming one layer. When this goal is achieved, there are no temperature gradients, and at least the aforementioned and other related problems are resolved. Under the assumption that heat loss through the (unconsolidated) powder is negligible (justified by the relatively low thermal conductivity of powder), the above formulation is equivalent to considering the net energy balance per layer of the top layer 13. Therefore, to realize the net-zero concept, the following applies:

[0115] JPEG2026502901000002.jpg1295

[0116] In the above equation, E is the energy of the top layer 13, and dE / dt is its change over time. This is over the formation period of the layer, i.e., from the start time t n From the start time t of layer n+1 n+1 It is integrated up to Q in is the amount of heat introduced into the top layer 13 of the workpiece during the formation of the layer, and Q out is the amount of heat released from the top layer 13. The variables marked with "dot" are the heat flows, i.e., their respective time derivatives.

[0117] In either case, to generate (i.e., print) the workpiece 12, it is necessary to melt the powder, so optical power must be introduced into the workpiece 12 and the top powder layer. This input heat amount Q in depends on the build parameters, the irradiation area, and other factors. Based on these factors, Q can be calculated with relatively high accuracy (relative error less than 5%), for example, at the beginning of the build process. in Those skilled in the art can calculate the input heat Q based on one or more input parameters such as laser power, beam profile, irradiation time, and irradiation area. in Therefore, this disclosure will not go into further detail on how to calculate the heat input.

[0118] To achieve net zero, the amount of heat Q released from the top layer 13 out is the input heat Q in As mentioned above, the heat sinks available for this purpose are radiation from the workpiece 12, metal spatter, and convection into the gas flow. In particular, the radiation heat and convection heat depend on the top layer temperature, i.e., the temperature of the top layer 13 of the workpiece 12.

[0119] Optical Load Fluctuations During the fabrication process (build job), the optical load fluctuates due to changes in process parameters such as the irradiation area and build parameters, which directly affect the optical load. In this disclosure, this is referred to as the input heat amount Q in Therefore, to satisfy the net zero concept, the output heat quantity Q out It is also necessary to change the input heat quantity Q in and / or output heat Q out To affect this, you have the following options: a) The top layer 13 of the workpiece 12 and / or the raw material layer on top of the workpiece 12 is heated, for example, by a top layer heating device 42 (hereinafter also referred to as "RadHeat"). in directly impacts. b) Adding periods during which the irradiating power is reduced, especially waiting periods. This reduces Q out Affects. c) Add work. This is Q in and Q out Affects. d) Adjusting a parameter, in particular changing the laser power of the laser beam 14, while leaving other parameters (in particular all other parameters) unchanged, or changing a qualified parameter to a different qualified parameter and / or changing a parameter set to a different parameter set. This is called Q in Affects. e) Changing and / or adding support structures, which affect the resistance of the workpiece and therefore the temperature of the top layer of the workpiece 12, and therefore indirectly affecting Q out Affects.

[0120] Constant workpiece temperature The net-zero concept ensures that the energy of the top layer 13 does not change during the process of each layer, and therefore energy is introduced into the entire workpiece 12. This is equivalent to maintaining the temperature of the top layer 13 of the workpiece 12. The temperature to be maintained constant is referred to herein as the net-zero temperature or set temperature. The set temperature can be set by the user based on the user's experience, based on calculations by the user, or optionally using a mathematical mode. The set temperature can also be determined and set automatically by the control unit 40 of the device 10 based on a mathematical model. Examples of determining and setting the set temperature are given below.

[0121] The details of the above parameters required to achieve a net-zero temperature can be determined mathematically (open-loop control) before the build process. However, it may be preferable to achieve and / or maintain the set temperature with closed-loop control. A model estimating the above influences (e.g., heating of the top layer 13, time of irradiation power reduction, parameter changes, etc.) can provide an initial model-based input (initial value) for temperature control. During the process, the temperature of the top layer is ideally measured, and the control is adapted accordingly (to maintain the set temperature).

[0122] In the present disclosure, it is important that the set point temperature of the bottom 15 of the workpiece 12 matches the set point temperature of the top layer 13. In other words, a first temperature range in which the temperature of the bottom 15 of the workpiece 12 is maintained includes the same set point temperature as a second temperature range in which the temperature of the top layer 13 of the workpiece 12 is maintained. In further terms, the concepts of the present disclosure aim to provide the same set point temperature at the bottom 5 and top layer 13 of the workpiece 12. In this way, temperature gradients across the workpiece (along the z-axis) can be reduced, or at least avoided. In one embodiment, the set point temperature of the carrier heating device 44 matches the set point temperature of the top layer 13.

[0123] Detailed examples of temperature control will be described below, but the present technology is not limited to these examples.

[0124] Example 1 (prior art) The build process is performed using IN718 material with high power parameters ranging from 5 to 40% of the irradiation area (i.e., 5 to 40% of the total irradiable area, i.e., the total irradiation area) and a height (along the z-direction) of 600 mm. For IN718, the preheat temperature can be 200°C. For a 40% irradiation area, an 8 kW optical load (power) is introduced into the process chamber 16, with approximately 4 kW reaching the workpiece 12. Without the technology disclosed herein, the thermal resistance of the workpiece 12 would be significantly affected by the height of the workpiece 12 being built. Furthermore, the low thermal conductivity of IN718 further exacerbates this effect. As a result, the temperature of the workpiece 12 near the carrier 20 is approximately equal to the preheat temperature of 200°C, with higher temperatures in the upper layers. For a specific AM device 10, a rough estimate without considering the heat sink, process chamber 16, and gas flow suggests that the temperature of the top layer 13 would be 1000°C. This means that the temperature gradient from the bottom to the top of the workpiece 12 exceeds 800 K. This results in a large difference in material properties and elongation (dimensional accuracy).

[0125] Net-zero solution with heating of the top layer 13 and / or additional waiting time IN718 can withstand relatively high temperatures. 600°C is selected as the net-zero temperature (set temperature). At this temperature, radiation from the workpiece 12 plays a key role; radiation is very powerful. Furthermore, convection into the gas flow also becomes important. According to a calculation example based on a mathematical model, the radiation and convection terms, together with other heat losses from the top layer 13, are calculated to be 3.5 kW. For example, if a job is built with a 40% exposure area, 4 kW of energy is introduced into the workpiece 12 in 60 seconds, resulting in 240 kW. Due to the 600°C temperature, 3.5 kW of heat is lost during the layer process (the sum of the exposure time and inter-exposure time). The inter-exposure time is defined as the time between the end of exposure of one layer and the start of exposure of the next layer. For example, during this inter-exposure time, the next layer is coated on top of the previous layer.

[0126] If the irradiation time is about 8.5 seconds, (60 seconds + 8.5 seconds) × 3.5 kW = 239.75 kWs is lost, and the amount of heat is approximately the same (i.e., the amount of heat introduced Q in = 240 kW and heat loss Q out = 239.75kW).

[0127] However, a device with a shorter inter-irradiation time (eg, 7 seconds) would require an additional wait time of 1.5 seconds for 40% of the irradiated area.

[0128] Therefore, this additional waiting time can be introduced at any point in the treatment process of the current layer (top layer 13). For example, an additional waiting time of 1.5 seconds is introduced at the end of the irradiation and before the inter-irradiation time (i.e., before the next layer is coated).

[0129] Furthermore, the open-loop control described above can be supported by a closed-loop control, in which a temperature measuring device 28 measures the temperature of the top layer 13 of the workpiece 12. If the measured temperature is still above 600°C after an additional waiting time of 1.5 seconds, an additional waiting time can be introduced.

[0130] For layers with an irradiation area of ​​less than 40%, the optical load is significantly smaller. For example, if the irradiation area of ​​the top layer 13 is 5%, the optical load on the process chamber 16 is approximately 2 kW, which means that the optical load on the workpiece 12 is approximately 1 kW. Here, the irradiation time is only 20 seconds. Therefore, the heat introduced to the workpiece 12 is only 20 kWs. Because the net temperature is 600°C, 3.5 kW is lost (as in the previous example). Because the inter-irradiation time is 7 seconds, the heat loss is (20 seconds + 7 seconds) × 3.5 kW = 94.5 kWs. This difference in heat can be compensated for by heating the top layer 13 with the top layer heating device 42. In this case, the top layer heating device 42 needs to irradiate a heat amount of 94.5 kWs - 20 kWs = 74.5 kWs. In this case, it is assumed that the top layer heater 42 travels with the powder coating apparatus 18 and heating occurs within the inter-irradiation time, so the top layer heater 42 does not require any additional time.

[0131] In summary, the carrier 20 is heated to 600°C, and this temperature is maintained by the carrier heating device 44. Therefore, the workpiece 12 has a temperature of 600°C near the carrier 20, i.e., at its bottom 15. By introducing an additional waiting time or by heating the top 13, the temperature of the top 13 is controlled to achieve the desired top temperature (set temperature) of 600°C. Therefore, it is ensured that the temperatures of the top 13 of the workpiece 12 and the bottom 15 of the workpiece 12 are maintained at the set temperature of 600°C. The reason this temperature is maintained is because the introduced heat quantity Q in and heat loss Q out Because they are equal.

[0132] Solutions with net-zero concepts and only additional waiting times Similar to the above solution for heating the top layer, overheating can be avoided by selecting a lower set temperature. In this case, the set temperature is selected to be 300°C. Therefore, the carrier 20 is heated and maintained at 300°C. A mathematical model determines that in order to achieve a temperature of 300°C for the workpiece 12 after the coating process, the temperature of the workpiece 12 must be increased to 310°C before a new powder layer is coated. After irradiating a layer, the temperature of the top layer is measured (monitored) during an additional waiting period. As soon as 310°C is reached, coating of the next layer begins. In this way, the temperatures of the top layer 13 and bottom layer 15 of the workpiece 12 are again identical, and no gradient occurs. The additional waiting period is used for controlled cooling of the workpiece by radiation and convection. The waiting period is set long enough so that the heat introduced (e.g., 4 kW for 60 seconds at 40% of the irradiated area) is equal to the heat loss to the gas flow and the walls of the processing chamber 16. This solution may be easier to implement compared to the above solution using the top layer heater 42 (RadHeat) because it does not require the top layer heater 42, but it may require longer wait times. Setting a higher net zero temperature reduces wait times, but achieving a high net zero temperature may be more difficult.

[0133] It should be noted that the above example refers to maintaining a set temperature. However, it is not always possible to maintain a precisely constant set temperature. Therefore, for the present technology to function, it is sufficient that the temperature of the bottom 15 of the workpiece 12 is maintained within a first predetermined temperature range and the temperature of the top 13 of the workpiece 12 is maintained within a second predetermined temperature range. Both the first and second predetermined temperature ranges include the set temperature. For example, the set temperature may be located in the center of each of the first and second temperature ranges, and the first and second temperature ranges may be identical. The range of each of the first and second temperature ranges may be, for example, 1 K, 2 K, 5 K, 10 K, 20 K, 30 K, 40 K, or 50 K.

[0134] Any combination of the above options is possible and covered by the present technology. For example, if other parameters are used depending on the build height, the heat load will change, and therefore the potential wait time and heat differential to be applied by the top layer heater 42 will also change.

[0135] A further improvement is provided as follows. The above discussion is based on the assumption that heat loss on the powder is negligible. Only in that case would the temperatures at the top and bottom of the workpiece 12 be the same, and no gradient would exist. However, in reality, this heat loss exists, and the central region of the workpiece 12 may be slightly cooler. This effect can be avoided by heating the build cylinder. Therefore, according to an embodiment of the present technology, a build cylinder heating device is provided, and the control unit 40 is configured to heat the build cylinder to a set temperature by the build cylinder heating device. In this case, no heat flow to the build cylinder occurs. This concept is similar to the method for maintaining the temperature of the carrier 20, but in addition, the system edges reach and maintain a set temperature to avoid temperature gradients.

[0136] Selecting the temperature setting In principle, almost any temperature can be selected as the setpoint temperature (net-zero temperature). However, an inappropriate setpoint temperature may involve drawbacks, for example, with regard to the required waiting time, the required additional work (powder consumption), the required heat input (energy consumption), the heat resistance of the components of the device 10, etc. A suitable setpoint temperature can be determined empirically, through experiments, research, etc., or based on models. More precisely, the setpoint temperature is calculated by the control unit 40 based on input parameters, which may include one or more parameters, for example, the material used, the geometry of the workpiece, etc.

[0137] Furthermore, it is also possible to solve a multidimensional optimization problem that shows, for example, the relationship between standby time and energy consumption at different temperatures. An appropriate set temperature can be automatically selected by the control unit 40.

[0138] FIG. 4 shows a flowchart of a method for controlling the temperature of a three-dimensional workpiece 12 produced by additive manufacturing according to an embodiment of the present disclosure.

[0139] The method is performed by an additive manufacturing apparatus (eg, apparatus 10 shown and described in FIGS. 1-3) during the build process of a three-dimensional workpiece 12.

[0140] The method begins with step 50, which involves maintaining the temperature of the bottom 15 of the workpiece 12 within a first predetermined range, including a predetermined set temperature, during the build process of the workpiece 12. Based on the above discussion, there are two main options for how to maintain the temperature. First, a suitable support structure 46 is provided, which ensures that the temperature of the bottom 15 is maintained within the first predetermined range during the build process. Second, the bottom 15 of the workpiece 12 is mounted adjacent to or directly on the carrier 20 of the apparatus 10, and the carrier 20 is heated via the carrier heating device 44. In this second case, heating and maintenance can be performed in an open-loop or closed-loop manner. In an open-loop manner, the heating time and / or heating power are predetermined and read from a look-up table (for the desired set temperature). The look-up table can be stored in the memory of the control unit 40. In a closed-loop manner, the temperature of the carrier 20 is measured, and the carrier heating device 44 is activated if the temperature is too low, or deactivated if the temperature is too high. In this way, the temperature of the carrier 20 can be maintained constant, at least within a first predetermined range.

[0141] In step 52, at least one energy beam is applied to a layer of raw material on top of the workpiece 12, solidifying the irradiated raw material to form the top layer 13 of the workpiece 12. This step necessarily introduces heat into the workpiece 12 to form at least one melt pool for solidifying the raw material at a desired location. The irradiation can be performed using only one or multiple energy beams, particularly laser beams.

[0142] In step 54, the temperature of the top layer 13 of the workpieces 12 is maintained within a second predetermined range that includes the predetermined set temperature. This maintenance step can typically be performed by increasing the amount of heat input and the amount of heat loss. A first option can be performed by heating the top layer 13 of the workpieces 12 and / or the material layer above the workpieces 12 with the top layer heating device 42. A second option can be performed by introducing one or more additional periods during which the irradiation power is reduced, in particular additional waiting periods without irradiation.

[0143] FIG. 5 is a schematic diagram of a control unit 40 in one of the devices 10 of FIG. 1 or FIG. 2. The control unit 40 includes a number of modules 60-64, each of which may be represented as hardware and / or software. In one example, the control unit 40 includes a processor and memory. The memory stores instructions that cause the processor to perform the method shown in FIG. 4. To this end, the software stored in the memory may be considered to include modules 62-64 shown in FIG. 5.

[0144] In detail, these modules are: A first maintenance module 60 maintains the temperature of the bottom of the workpiece within a first predetermined range including a predetermined set temperature during the workpiece building process. an instruction module 62 that instructs the irradiation device to irradiate at least one energy beam onto a layer of material on top of the workpiece to solidify the irradiated material and form a top layer of the workpiece; A second maintenance module 64 that maintains the temperature of the top layer of the workpiece within a second predetermined range that includes the predetermined set point temperature.

[0145] Details described herein with respect to individual method steps are also applicable to the corresponding modules 60-64 of the control unit 40. In other words, the control unit 40 is configured to perform the method of Figure 4 above, and details of this method described herein apply, as appropriate, to the software of the control unit and / or to the corresponding and / or additional modules shown in Figure 5.

[0146] According to some embodiments, the advantages of the techniques disclosed herein are as follows:

[0147] Uniform expansion of the workpiece 12, which can result in, for example, predictable warpage. Typically, in prior art, there is no nearly constant temperature to solve the inverse problem of determining the optimal shape of the workpiece 12, so it can be very difficult to initially calculate the temperature the workpiece 12 should have to have the desired shape after cooling. This is possible if the temperature is constant. If expansion compensation is not performed, the workpiece will warp because the upper layers are hotter and therefore expand more before cooling. This problem can be eliminated or mitigated by the present technique.

[0148] The desired set temperature is the same (in terms of time and space) from the beginning to the end of the build job, improving material properties: since there is no temperature difference, the parameters that are good for the first layer are also good for the last layer.

[0149] Design freedom The combination of the above features allows for complete design freedom. Conventional techniques have not been able to produce workpieces that are inadequately supported or that have a small lower irradiated area and a large upper irradiated area (inverted pyramid shape). In both cases, the problem is the same: the thermal resistance of the workpiece, especially the upper layer, is so high relative to the main heat sink (carrier) that the heat cannot be dissipated. As a result, the workpiece 12 can become locally very hot, expand, protrude from the powder layer, and even break. With this technology, this problem can be completely avoided if the temperature along the workpiece 12 is constant. With the Net Zero Concept, this problem does not occur because the carrier 20 does not have a heat sink.

Claims

1. 1. A method for temperature control of a three-dimensional workpiece produced by additive manufacturing, comprising: maintaining a temperature of a lowermost portion of the workpiece within a first predetermined range including a predetermined set temperature during a manufacturing process of the workpiece; irradiating a layer of raw material on top of the workpiece with at least one energy beam to solidify the irradiated raw material to form a top layer of the workpiece; maintaining the temperature of the top layer of the workpiece within a second predetermined range that includes the predetermined set temperature; A method comprising:

2. 2. The method of claim 1, wherein maintaining the temperature of the lowermost part of the workpiece comprises heating a carrier on which the workpiece is built, in particular to a temperature within the first predetermined range.

3. The method of claim 1 or 2, further comprising determining the predetermined set temperature based on build data that defines the shape of the workpiece to be built.

4. The method according to any one of claims 1 to 3, wherein maintaining the temperature of the top layer of the workpiece includes at least one of heating the top layer of the workpiece and / or a raw material layer on the top of the workpiece, and introducing a period in which irradiation power is reduced.

5. calculating the amount of heat introduced into the top layer by the at least one energy beam; calculating the amount of heat released from the top layer during irradiation of the top layer; If the amount of heat introduced into the top layer is greater than the amount of heat released from the top layer, introducing a period of reduced irradiation power during which additional heat can be released from the top layer; and / or When the amount of heat released from the top layer is greater than the amount of heat introduced into the top layer, heating the top layer of the workpiece and / or the upper raw material layer of the workpiece so that additional heat is introduced into the top layer; The method of claim 4 further comprising:

6. if the amount of heat introduced into the top layer is greater than the amount of heat released from the top layer, the additional amount of heat released from the top layer is equal to the difference between the amount of heat introduced into the top layer and the amount of heat released from the top layer; and / or 6. The method of claim 5, wherein if the amount of heat emitted from the top layer is greater than the amount of heat introduced into the top layer, the additional amount of heat introduced into the top layer is equal to the difference between the amount of heat emitted from the top layer and the amount of heat introduced into the top layer.

7. The method according to any one of claims 4 to 6, wherein heating the top layer of the workpiece is performed by thermal radiation.

8. The method of claim 7 , wherein the thermal radiation is provided by a device attached to a powder coating device configured to coat the top layer of the workpiece.

9. The method according to any one of claims 4 to 8, wherein the time during which the irradiation power is reduced is a predetermined waiting time during which no irradiation of the raw material is performed.

10. The method according to any one of claims 4 to 8, wherein the time during which the irradiation power is reduced is a time during which at least one irradiation parameter is changed to reduce the irradiation power, and the irradiation parameter is at least one of the number of active energy beams, laser power, scanning speed, and beam profile.

11. The method according to any one of claims 4 to 10, wherein the time period during which the irradiation power is reduced is introduced between the time when irradiation of the top layer is completed and the time when powder coating of a next layer on the top layer is started, between the time when coating of a next layer on the top layer is completed and the time when irradiation of the next layer is started, or during irradiation of the top layer.

12. The method according to any one of claims 1 to 10, wherein the temperature of the top layer of the workpiece is maintained by closed-loop control.

13. The method according to any one of claims 1 to 12, wherein the method is performed by an additive manufacturing apparatus, in particular an apparatus for selective laser melting or selective laser sintering.

14. The method of any one of claims 1 to 13, further comprising determining said predetermined set temperature for defining a microstructure during a building process.

15. An apparatus for temperature control of a three-dimensional workpiece produced by additive manufacturing, comprising: a control unit; an irradiation device for irradiating at least one energy beam; The control unit maintaining a temperature of a lowermost portion of the workpiece within a first predetermined range including a predetermined set temperature during a manufacturing process of the workpiece; instructing the irradiation device to irradiate at least one energy beam onto a layer of raw material on top of the workpiece to solidify the irradiated raw material and form a top layer of the workpiece; configured to maintain the temperature of the top layer of the workpiece within a second predetermined range that includes the predetermined set temperature; Device.

16. Further comprising a carrier heating device; 16. The apparatus of claim 15, wherein maintaining the temperature of the bottom of the workpiece includes instructing the carrier heating device to heat a carrier on which the workpiece is built, particularly to a temperature within the first predetermined range.

17. 17. The apparatus according to claim 15 or 16, wherein the control unit is configured to determine the predetermined set temperature based on build data defining the shape of the workpiece to be built.

18. Further comprising a top layer heating device; The apparatus of any one of claims 15 to 17, wherein maintaining the temperature of the top layer of the workpiece includes at least one of instructing the top layer heating device to heat the top layer of the workpiece and / or a raw material layer on top of the workpiece, and introducing a period of time in which irradiation power is reduced.

19. The control unit further comprises: calculating the amount of heat introduced into the top layer by the at least one energy beam; calculating the amount of heat released from the top layer during irradiation of the top layer; If the amount of heat introduced into the top layer is greater than the amount of heat released from the top layer, introducing a period of reduced irradiation power during which additional heat can be released from the top layer; and / or 19. The apparatus of claim 18, wherein the apparatus is configured to heat the top layer of the workpiece and / or the upper raw material layer of the workpiece such that additional heat is introduced into the top layer if the amount of heat emitted from the top layer is greater than the amount of heat introduced into the top layer.

20. if the amount of heat introduced into the top layer is greater than the amount of heat released from the top layer, the additional amount of heat released from the top layer is equal to the difference between the amount of heat introduced into the top layer and the amount of heat released from the top layer; and / or 20. The apparatus of claim 19, wherein if the amount of heat emitted from the top layer is greater than the amount of heat introduced into the top layer, the additional amount of heat introduced into the top layer is equal to the difference between the amount of heat emitted from the top layer and the amount of heat introduced into the top layer.

21. The apparatus according to any one of claims 18 to 20, wherein the top layer heating device is configured to heat the top layer of the workpiece by thermal radiation.

22. 22. The apparatus of claim 21, wherein the top layer heating device is attached to a powder coating device configured to coat the top layer of the workpiece.

23. 23. The apparatus according to any one of claims 18 to 22, wherein the time during which the irradiation power is reduced is a predetermined waiting time during which no irradiation of the material takes place.

24. The apparatus according to any one of claims 18 to 22, wherein the time during which the irradiation power is reduced is a time during which at least one irradiation parameter is changed to reduce the irradiation power, and the irradiation parameter is at least one of the number of active energy beams, laser power, scanning speed and beam profile.

25. The apparatus according to any one of claims 18 to 24, wherein the time period during which the irradiation power is reduced is introduced between the time when irradiation of the top layer is completed and the time when powder coating of a next layer on the top layer is started, between the time when coating of a next layer on the top layer is completed and the time when irradiation of the next layer is started, or during irradiation of the top layer.

26. The apparatus according to any one of claims 15 to 25, wherein the control unit is configured to maintain the temperature of the top layer of the workpiece by closed-loop control.

27. The apparatus according to any one of claims 15 to 25, wherein the apparatus is an additive manufacturing apparatus, in particular an apparatus for selective laser melting or selective laser sintering.

28. The apparatus of any one of claims 15 to 27, wherein the control unit is configured to determine the predetermined set temperature for defining a microstructure during a building process.

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