Techniques for temperature control of three-dimensional

Abstract

A method for temperature control of a three-dimensional workpiece produced by additive manufacturing is provided. The method includes: during a build process of a workpiece, maintaining a temperature of a lowermost portion of the workpiece within a first predetermined range including a preset temperature; irradiating an energy beam to the raw material layer at the top of the workpiece to cure the irradiated raw material and form an uppermost layer of the workpiece; and maintaining the temperature of the uppermost layer of the workpiece such that the temperature of the uppermost layer of the workpiece is within a second predetermined range including the preset temperature. Furthermore, a corresponding apparatus for temperature control of a three-dimensional workpiece produced by additive manufacturing is provided.

Description

Technical Field

[0001] The present invention generally relates to a temperature control technique for a three-dimensional workpiece produced by additive manufacturing. The additive manufacturing process may be, but is not limited to, powder bed fusion, such as selective laser sintering, selective laser melting, or electron beam melting. Background Art

[0002] Powder bed fusion is an additive layering process by which powdered, in particular metallic and / or ceramic raw materials can be processed into three-dimensional workpieces with complex shapes. To this end, layers of raw material powder are applied to a carrier and then subjected to radiation (e.g. laser or particle radiation) in a position-selective manner depending on the desired geometry of the workpiece to be produced. The radiation penetrating the powder layer causes heating and thus causes the raw material powder particles to melt or sinter. Further layers of raw material powder are then applied in sequence to the layer on the carrier that has already been subjected to the radiation treatment until the workpiece has the desired shape and size. Powder bed fusion can be used to produce 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 technologies include selective laser melting, selective laser sintering and electron beam melting.

[0003] Devices for producing one or more workpieces according to the above-mentioned techniques are known. For example, EP 2 961 549 A1 and EP 2 878 402 A1 each describe devices for producing three-dimensional workpieces according to the selective laser melting technique. The general principles described above and in these documents can also be applied to the technology of the present disclosure.

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

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

[0006] Large temperature gradients can, for example, lead to deterioration of material properties and high tensions within such workpieces. Summary of the Invention

[0007] It is therefore an object of the present invention to provide a technique that addresses at least one of the above-identified problems and / or other related problems. Specifically, but not limiting of, a technique is desired that reduces temperature gradients within a workpiece constructed by additive manufacturing and thereby can help improve the material properties of the finished workpiece.

[0008] This object is solved by the subject matter of the independent claims. Advantageous embodiments are indicated in the dependent claims.

[0009] According to a first aspect, a method for temperature control of a three-dimensional workpiece produced by additive manufacturing is provided. The method includes: maintaining the temperature of a lowermost portion of the workpiece within a first predetermined range including a preset temperature during a build process of the workpiece; irradiating at least one energy beam into a layer of raw material at the top of the workpiece to solidify the irradiated raw material and form an uppermost layer of the workpiece; and maintaining the temperature of the uppermost layer of the workpiece so that the temperature of the uppermost layer of the workpiece is within a second predetermined range including the preset temperature.

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

[0011] The additive manufacturing process for producing the workpiece may be additive manufacturing from a powder bed, such as selective laser sintering or selective laser melting, or any other additive manufacturing process in which the workpiece is built up from a feedstock (e.g., in the form of powder, granules, and / or liquid).

[0012] The preset temperature may be stored in a memory, in particular in a memory of the additive manufacturing device performing the method. The preset temperature may have been input by a user, or may have been predetermined (in particular calculated), for example, by a control unit of the additive manufacturing device or by another computing device.

[0013] The maintaining step may involve taking measures that have an effect on the temperature of the lowermost portion of the workpiece, such as heating or cooling. However, by providing one or more suitable support structures and / or by providing a workpiece of suitable geometry, the temperature of the lowermost portion of the workpiece may also be maintained within the first predetermined range.

[0014] The first predetermined range may be at most 5K, at most 10K, at most 20K, at most 30K, at most 40K, at most 50K, at most 70K or at most 100K. It is desirable to keep the temperature of the lowermost portion of the workpiece as close as possible to the predetermined temperature. The predetermined temperature may be in the middle of the first predetermined range. However, the predetermined temperature may also be one of the extremes of the first predetermined range. For example, the predetermined temperature may be T 设定 , the first predetermined temperature range may be T 设定 -T 容限 _1 expands to T 设定 +T 容限 _2. T 容限 _1 and T 容限 The values of _2 can be the same or different.

[0015] To maintain the temperature of the lowermost portion of the workpiece, the temperature of the lowermost portion of the workpiece may be measured or estimated (eg, contactlessly) and closed-loop temperature control may be implemented.

[0016] The lowermost portion of the workpiece may comprise the lowermost layer of the workpiece. Reference to a layer of the workpiece in this disclosure refers to the solidified region of the workpiece resulting from the corresponding raw material layer. The lowermost portion of the workpiece may directly contact a carrier of the additive manufacturing apparatus. It may also be embedded in the raw material powder, i.e., it may solidify on top of an unsolidified raw material layer. Furthermore, it may be supported by one or more support structures, or even include or consist of a support structure.

[0017] The step of irradiating at least one energy beam can be performed as is common in the field of selective laser melting or selective laser sintering. Therefore, 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 the desired position. To this end, a scanning optical device can be provided for laterally positioning the energy beam on the irradiated layer of the workpiece. In the present disclosure, the lateral direction refers to the direction in the xy plane parallel to the carrier of the additive manufacturing device. The z-axis is arranged perpendicular to the xy plane and corresponds to the height axis (i.e., the axis perpendicular to the raw material layer deposited on the carrier). In addition, more than one energy beam can be irradiated simultaneously or sequentially. For example, multiple energy beams can be provided, which are independently controllable in the x-direction and the y-direction. To this end, a separate scanning optical device can be provided for each energy beam. Each energy beam can be emitted by a corresponding beam source (e.g., a laser source). In addition, the multiple energy beams can have different wavelengths, different laser powers, and / or different beam profiles. For example, a different energy beam than that used for the core of the workpiece can be used to irradiate the shell of the workpiece.

[0018] The layer of feedstock at the top of the workpiece is the layer of feedstock deposited on top of the previously irradiated layer of the workpiece.

[0019] When the present disclosure refers to the temperature of the top layer of a workpiece, this may mean the temperature of the top surface of the workpiece. The temperature of the top layer of the workpiece may be averaged over time and / or over space (i.e., different locations of the top layer). The temperature of the top layer of the workpiece may be measured and / or determined after a predetermined time has passed after irradiation of the top layer, for example, just before deposition of the next layer of raw material. Furthermore, the temperature of the top layer of the 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 temperature peaks in one or more melt pools, the temperature of the top layer may be the median of the temperatures measured and / or considered across the entire top layer.

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

[0021] The second predetermined range may correspond to the first predetermined range. The second predetermined range may be at most 5K, at most 10K, at most 20K, at most 30K, at most 40K, at most 50K, at most 70K or at most 100K. It is desirable to keep the temperature of the uppermost layer of the workpiece as close as possible to the preset temperature. The preset temperature may be in the middle of the second predetermined range. However, the preset temperature may also be one of the extremes of the second predetermined range. For example, the preset temperature may be T 设定 , the second predetermined temperature range may be T 设定 -T 容限 _3 Expand to T 设定 +T 容限 _4. T 容限 _3 and T 容限 The values of _4 can be the same or different.

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

[0023] The carrier can be heated using a heating element integrated into or positioned beneath the carrier. The heating element can be an electric heating element, for example, provided 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 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 can be fixed relative to its vertical position during the build process, or it can be a vertically movable carrier of the additive manufacturing apparatus that is lowered during the build process so that a new layer of raw material can be provided on top of a previous layer of raw material. The term carrier can be substituted with base plate or substrate. In any case, the carrier can include a flat surface on which the workpiece is built. This surface can be heated during the step of heating the carrier.

[0025] For example, the temperature of the lowest portion of the workpiece can be maintained by using open-loop or closed-loop temperature control of the carrier temperature. For example, a heating element that heats the carrier can be activated for a predetermined period of time to achieve and maintain a set temperature. In other words, open-loop temperature control of the carrier temperature can be provided, wherein heating times, non-heating times, and / or heating power values can be stored in a lookup table of desired temperature values. Furthermore, a temperature sensor configured to measure the carrier temperature can be provided. This temperature sensor can be integrated into the carrier and / or can be 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 carrier temperature measured by the temperature sensor falls below a predetermined threshold (which depends on the set temperature), the heating element is activated to heat the carrier to the desired set temperature. The predetermined threshold can correspond to the lower limit of a first predetermined range. In this way, the temperature is maintained within the first predetermined range, particularly during the entire build process of the workpiece. The build process can be defined as the time from the start of deposition of the first material layer onto the carrier to the time when irradiation of the last material layer stops. Optionally, the build process can include an additional predetermined cooling time after irradiation of the last material layer. During the cooling time, the temperature of the lowermost portion and the temperature of the uppermost layer of the workpiece may still be maintained within corresponding ranges, for example, to achieve a desired microstructure of the workpiece.

[0026] In order to maintain the temperature of the lowermost portion of the workpiece within the first predetermined range, the set temperature of the carrier may need to be set slightly higher than the desired set temperature of the lowermost portion of the workpiece due to the additional heat sink (particularly when the lowermost portion of the workpiece does not directly contact the carrier or has a relatively small contact area with the carrier). Therefore, maintaining the temperature of the lowermost portion of the workpiece within the first predetermined range may include maintaining the carrier on which the workpiece is being 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, and in particular, greater than, the upper and lower limits of the first predetermined range by a fixed offset (e.g., 5K, 10K, or 15K).

[0027] The method may further comprise determining the predetermined temperature based on build data defining the geometry of the workpiece to be built.

[0028] Several parameters can influence the set temperature. The geometry of the workpiece being built is one of these parameters. These parameters can also include: the material or material composition used; the height of the workpiece; or one or more illumination parameters, such as wavelength, beam power, and / or spot size.

[0029] The set temperature can be determined by analyzing build data (e.g., CAD data or SLM data). A mathematical model can be used to determine the preset temperature. Furthermore, historical process values (e.g., measured temperatures) can be input parameters for determining the set temperature. For example, an artificial intelligence engine can be trained and implemented to determine the set temperature.

[0030] The step of determining the preset temperature may include storing the preset temperature in a memory (e.g., a memory of the additive manufacturing apparatus). 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 at the center 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 at the center of the second predetermined range.

[0031] Maintaining the temperature of the uppermost layer of the workpiece may include at least one of heating the uppermost layer of the workpiece and / or a layer of feedstock at the top of the workpiece and introducing a reduced irradiation power for a period of time.

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

[0033] Maintaining the temperature of the uppermost layer can be accomplished by open-loop control or closed-loop control. In open-loop control, predetermined measures are taken that are known to produce the desired set temperature (i.e., the temperature of the uppermost layer is within the second predetermined range). For example, the heating time, non-heating time, and / or heating power can be stored in a lookup table for a plurality of desired temperature values. Similarly, the length of the reduced irradiation power period and / or the value of other parameters can be stored in a lookup table for a plurality of desired temperature values.

[0034] In closed-loop control, the temperature of the topmost layer is measured, for example, by a contact temperature sensor, a non-contact temperature sensor, a camera, a pyrometer, or the like. Based on the measured temperature, measures are initiated that result in cooling or heating of the topmost layer. For example, if the measured temperature is below the lower limit of a second predetermined range, the topmost 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 during which no irradiation is performed) is introduced.

[0035] Heating the uppermost layer of the workpiece may include increasing the irradiation power of at least one energy beam. The irradiation power (e.g., laser power) may be increased to a value that ensures that the amount of heat introduced into the uppermost layer is equal to the amount of heat leaving the uppermost layer between the time when the current raw material layer begins to be coated and the time when the subsequent raw material layer begins to be coated. In other words, the irradiation power may be increased such that the temperature of the uppermost layer remains within a second predetermined range.

[0036] Furthermore, the aforementioned measures for maintaining the temperature of the top layer in closed-loop operation can be performed in the current top layer of the workpiece, at which the temperature measurement is performed. However, the measures for maintaining the temperature can also be performed in subsequent layers, i.e., layers subsequent to the layer in which the temperature measurement has already been performed.

[0037] The method may further comprise calculating the amount of heat introduced into the uppermost layer by at least one energy beam, calculating the amount of heat leaving the uppermost layer during irradiation of the uppermost layer, introducing a reduced irradiation power for a period of time during which additional heat may leave the uppermost layer if the amount of heat introduced into the uppermost layer is greater than the amount of heat leaving the uppermost layer, and / or heating the uppermost layer of the workpiece and / or a layer of raw material on top of the workpiece such that additional heat is introduced into the uppermost layer if the amount of heat leaving the uppermost layer is greater than the amount of heat introduced into the uppermost layer.

[0038] Calculating the heat entering and leaving the top shelf can be part of an open-loop control of the top shelf temperature. Thus, the method can be performed without measuring the top shelf temperature. However, measuring the top shelf temperature provides an additional step to check that the control being performed is correct and produces the desired top shelf temperature.

[0039] Thus, a combination of open-loop control and closed-loop control may be implemented. For example, open-loop control may be performed for one or more initial iteration steps, and thereafter, closed-loop control may take over.

[0040] The calculation of the amount of heat introduced into the top layer and / or the amount of heat leaving the top layer can be performed using a mathematical model and / or an artificial intelligence engine. Input parameters to the mathematical model and / or artificial intelligence engine can be one or more of the geometry of the top layer to be irradiated, the area of the top layer to be irradiated, the power of the energy beam (e.g., laser power), the spot size of the energy beam, the number of energy beams irradiating the top layer, the geometry of the underlying workpiece layer being solidified, the presence of airflow, the velocity of the airflow, etc.

[0041] The heat introduced into the top layer may be the heat introduced by the at least one energy beam during irradiation of the entire top layer. The heat leaving the top layer may be the heat leaving the top layer during the period from the start of irradiation of the top layer to the start of irradiation of the subsequent layer. This period may correspond to the irradiation time plus the fire-to-fire time, during which the subsequent layer is applied. More precisely, the heat leaving the top layer may be the heat leaving the top layer without taking into account any periods of reduced irradiation power. Similarly, the heat introduced into the top layer may be the heat introduced solely by irradiation by the at least one energy beam, without taking into account any additional heating (e.g., by a top layer heating device). Therefore, the calculated amount of heat introduced and / or leaving the top layer can be calculated without taking into account any measures to introduce / remove additional heat.

[0042] The predetermined waiting period may have a predetermined duration (e.g., expressed in seconds or milliseconds). The predetermined duration may be calculated such that during this duration, an additional amount of heat may leave the uppermost layer. For this purpose, a mathematical model may be used.

[0043] If the amount of heat introduced into the uppermost layer is greater than the amount of heat leaving the uppermost layer, the additional amount of heat leaving the uppermost layer may correspond to the difference between the amount of heat introduced into the uppermost layer and the amount of heat leaving the uppermost layer. Additionally or alternatively, if the amount of heat leaving the uppermost layer is greater than the amount of heat introduced into the uppermost layer, the additional amount of heat introduced into the uppermost layer may correspond to the difference between the amount of heat leaving the uppermost layer and the amount of heat introduced into the uppermost layer.

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

[0045] The heating of the uppermost layer of the workpiece can be performed by thermal radiation.

[0046] For this purpose, a corresponding heating device (top layer heating device) can be provided. This heating device can be fixed or movable. For example, one or more additional laser beams can be provided to heat the top layer by irradiating the top layer with a laser power insufficient to generate a molten pool. Infrared radiation can also be used to heat the top layer.

[0047] The thermal radiation may be radiated by an apparatus attached to a powder coating apparatus configured to apply an uppermost layer of a workpiece.

[0048] Thus, the heating device can be a top heating device configured to move with the powder coating device. For example, additional heat can be introduced into the top layer while the top layer of raw material is being coated by the coating device. However, coating and heating can also be performed at different times.

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

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

[0051] The reduced irradiation power period may be a period in 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 light spot.

[0052] During the reduced illumination power period, the illumination power delivered to the uppermost layer can be greater than zero, but less than the power that would be delivered without the reduced illumination power period. In other words, during the reduced illumination power period, it is not necessary to completely shut down illumination of the uppermost layer, as is the case with the predetermined waiting period. It is also possible to reduce the illumination power during this period, particularly in such a way that irradiating the uppermost layer with the reduced illumination power can require a predetermined longer time than would be required without the reduced illumination power period. The reduced illumination power period can be calculated so that the amount of heat that can leave the uppermost layer within the predetermined time corresponds to the (calculated) additional heat.

[0053] A period of reduced irradiation power may be introduced between the time irradiation of the uppermost layer is completed and the time powder coating of the next layer on top of the uppermost layer begins, between the time coating of the next layer on top of the uppermost layer is completed and the time irradiation of the next layer begins, or during irradiation of the uppermost layer.

[0054] In other words, there are various options for introducing one or more periods of reduced irradiation power. A certain period of irradiation power can also be divided into two or more periods. For example, one period can be introduced after coating and before irradiation, and another period can be introduced after irradiation and before coating the next layer. In addition, one or more periods of reduced irradiation power can be introduced between consecutive irradiation vectors (e.g., of a hatching pattern). There are various 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 uppermost 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, a decision can be made as to whether to introduce a period of reduced irradiation power or whether to heat the top layer. The set temperature for the closed-loop control can correspond to the set temperature of the bottom portion 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 can be applied, for example, by a top layer heating device. Furthermore, if the measured temperature is above the upper limit of the first predetermined range, a period of reduced irradiation power can be applied.

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

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

[0059] The method may further include determining a predetermined temperature to define the microstructure during the building process.

[0060] In this way, during the cooling process of the workpiece, the cooling rate can be defined and the workpiece's microstructure can be controlled. In particular, during this cooling process, the preset temperature can be dynamically controlled, i.e., it can be changed during the cooling process. More specifically, the preset temperature can include a predetermined, time-dependent set temperature, for example, set by a control unit of the additive manufacturing apparatus.

[0061] According to a second aspect, an apparatus for temperature control of a three-dimensional workpiece produced by additive manufacturing is provided. The apparatus includes a control unit and an irradiation device for irradiating at least one energy beam. The control unit is configured to maintain the temperature of a lowermost portion of the workpiece within a first predetermined range that includes a preset temperature during the workpiece build process, instruct the irradiation device to irradiate the at least one energy beam onto a material layer at the top of the workpiece to solidify the irradiated material and form an uppermost layer of the workpiece, and maintain the temperature of the uppermost layer of the workpiece within a second predetermined range that includes the preset temperature.

[0062] All the above aspects and details discussed with respect to the method aspect (first aspect) may apply to the device aspect. Specifically, the device of the second aspect may be configured to perform the method of the first aspect, wherein one or more details discussed with respect to the method aspect are implemented.

[0063] The apparatus may further comprise a carrier heating apparatus, wherein maintaining the temperature of the lowermost portion of the workpiece comprises instructing the carrier heating apparatus to heat the carrier on which the workpiece is built, in particular to a temperature within the first predetermined range.

[0064] The control unit may further be configured to determine the preset temperature based on building data defining the geometry of the workpiece to be built.

[0065] The apparatus may further include a top layer heating apparatus, wherein maintaining the temperature of the uppermost layer of the workpiece comprises at least one of instructing the top layer heating apparatus to heat the uppermost layer of the workpiece and / or a layer of raw material at the top of the workpiece, and introducing a reduced irradiation power for a period of time.

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

[0067] If the amount of heat introduced into the uppermost layer is greater than the amount of heat leaving the uppermost layer, the additional amount of heat leaving the uppermost layer may correspond to the difference between the amount of heat introduced into the uppermost layer and the amount of heat leaving the uppermost layer. Additionally or alternatively, if the amount of heat leaving the uppermost layer is greater than the amount of heat introduced into the uppermost layer, the additional amount of heat introduced into the uppermost layer may correspond to the difference between the amount of heat leaving the uppermost layer and the amount of heat introduced into the uppermost layer.

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

[0069] The top layer heating apparatus is attached to a powder coating apparatus configured to apply an uppermost layer of the workpiece.

[0070] The certain period of reduced irradiation power may be a predetermined waiting period during which no irradiation of the raw material is performed.

[0071] The reduced irradiation power for a certain period may be a period in 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 spot.

[0072] A period of reduced irradiation power may be introduced between the time irradiation of the top layer is completed and the time coating of the next layer of powder on top of the top layer begins, between the time coating of the next layer on top of the top layer is completed and the time irradiation of the next layer begins, or during irradiation of the top layer.

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

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

[0075] The control unit may be configured to determine a preset temperature to define the microstructure during the building process. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] The preferred embodiment of the present invention will be described in more detail with reference to the accompanying schematic drawings, wherein

[0077] Figure 1 shows a schematic side view of an additive manufacturing apparatus with one laser beam according to an embodiment of the present disclosure;

[0078] Figure 2 shows a schematic side view of an additive manufacturing apparatus with two laser beams according to an embodiment of the present disclosure;

[0079] Figure 3 Details of a build cartridge with a carrier and a workpiece with supports that may be used in embodiments of the present disclosure are shown;

[0080] Figure 4 A flowchart illustrating a method according to an embodiment of the present disclosure; and

[0081] Figure 5 A control unit with modules according to an embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0082] Figure 1 A schematic diagram of an additive manufacturing apparatus 10 for producing a three-dimensional workpiece 12 is shown. Aside from the specific method for temperature control that is programmed into the control unit 40 of the apparatus 10, the apparatus 10 generally and with some optional details are known to those skilled in the art. The apparatus 10 may be, for example, a typical additive manufacturing apparatus, wherein the method for temperature control according to the present disclosure is programmed into the control unit 40 of the apparatus 10. Optionally, some additional structural elements, such as a top heating device, may be provided, which are not necessarily common to typical additive manufacturing apparatuses.

[0083] 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, wherein one or more laser beams 14 may be used to selectively irradiate and solidify subsequent layers of a feedstock powder.

[0084] The following is an example of an apparatus 10 that can be used to perform a selective laser melting process. A typical feature of powder bed fusion is that raw material powder is applied layer by layer, and each layer is selectively irradiated and cured to produce a layer of the workpiece 12 to be manufactured. After removing excess powder and after optional post-processing steps (e.g., removing one or more support structures), the final workpiece 12 is obtained.

[0085] It should be noted that the technology of the present disclosure is not limited to powder bed fusion technology, and liquid or granular raw materials can also be used. In addition, the technology of the present disclosure is not limited to irradiation with a laser beam. Instead, any other energy beam (e.g., a particle beam, an electron beam, etc.) that can solidify the raw material powder used can be used. However, according to the specific embodiment described below, the raw material is a raw material powder (e.g., a metal powder, a ceramic powder, or a plastic powder), and at least one energy beam that solidifies the raw material powder is a laser beam.

[0086] Figure 1 A device 10 for producing a three-dimensional workpiece 12 by selective laser melting is shown. The device 10 comprises a process chamber 16. The process chamber 16 is sealable with respect to the surrounding atmosphere, i.e. with respect to the environment around the process chamber 16. A powder coating device 18 (also referred to as a powder application device) arranged in the process chamber 16 serves to apply a raw material powder to a carrier 20. To this end, the powder coating device 18 can comprise 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 on top of a previously applied and irradiated raw material layer. In this context, uniform particularly means that the layer has a uniform thickness. For example, the layer thickness can be selected by vertically positioning the powder coating device 18 and / or the carrier 20.

[0087] The vertical movement unit 22 is provided to enable the carrier 20 to be displaced in the vertical direction, so that when the workpiece 12 is layered from the raw material powder on the carrier 20 , the carrier 20 can be moved downward in the vertical direction as the build height of the workpiece 12 increases.

[0088] Since the movability of the carrier 20 by means of the vertical movement unit 22 is well known in the field of selective laser melting, it will not be explained in detail here. As an alternative to a movable carrier 20, the carrier 20 can be provided as a fixed (or stationary) carrier (especially with respect to the vertical z-direction), wherein the irradiation device 24 (see below) and the process chamber 16 are configured to move upward during the build process (as the build height of the workpiece 12 increases). In addition, both the carrier 20 and the irradiation device 24 can be moved independently along the z-direction.

[0089] The carrier surface of the carrier 20 defines a horizontal plane (xy plane), wherein the direction perpendicular to the plane is defined as the vertical direction or build direction (z direction). Therefore, each uppermost layer of the raw material powder and each layer of the workpiece 12 extends in a plane parallel to the above-mentioned horizontal plane (xy plane).

[0090] The apparatus 10 also includes a gas inlet 26 for supplying an inert gas (e.g., argon) into the process chamber 16. Furthermore, a gas outlet 27 is provided so that a continuous (horizontal) gas flow can be generated through the process chamber 16 by implementing a gas circuit. In a preferred embodiment, a unidirectional laminar flow is generated over the uppermost layer of raw material powder. The gas flow generated by the gas inlet 26 and the gas outlet 27 can be used to remove melt spatter and / or other undesirable dirt particles from the uppermost layer of the workpiece 12, thereby improving the quality of the workpiece 12. The gas flow also removes heat from the workpiece 12 by convection.

[0091] Furthermore, a temperature measuring device 28 is arranged in the process chamber 16 for determining the temperature of the uppermost layer 13 of the workpiece 12. The device 28 is optional and may be used only for closed-loop control (see detailed description below). The temperature measuring device 28 may include a pyrometer configured to detect thermal radiation emitted from the uppermost layer 13 of the workpiece 12. In particular, the temperature measuring device 28 may be configured to generate a thermal map of the uppermost layer 13, thereby determining a temperature value for each position of the uppermost layer 13. The temperature measuring device 28 provides the determined temperature values to the control unit 40, enabling the control unit to further process them, for example, by generating a time series of temperature values and / or by determining an average temperature value.

[0092] As an alternative to the non-contact temperature measurement device 28 , a contact thermometer may be provided that is configured to contact the uppermost layer 13 at a desired time and / or desired location to obtain one or more temperature values indicative of the temperature of the uppermost layer 13 .

[0093] The apparatus 10 further comprises an irradiation device 24 for selectively irradiating the laser beam 14 onto the uppermost layer of the raw material powder applied to the carrier 20. By means of the irradiation device 24, the raw material powder applied to the carrier 20 can be subjected to laser irradiation in a position-selective manner according to the desired geometry of the workpiece 12 to be produced. Figure 1 In the embodiment, the irradiation device 24 includes exactly one irradiation unit 24 a configured to irradiate exactly one laser beam 14 at a time.

[0094] The irradiation unit 24a includes a scanning unit 30 configured to selectively irradiate the laser beam 14 onto the raw material powder applied to the carrier 20. The scanning unit 30 is controlled by the control unit 40 of the apparatus 10. The scanning unit 30 may include a single mirror that can be tilted relative to two perpendicular axes. Alternatively, the scanning unit 30 may include two tiltable mirrors, each tiltable mirror configured to tilt relative to a corresponding axis. The tiltable mirror may be, for example, a galvanometer mirror.

[0095] Laser radiation is supplied from the laser beam source 32 to the irradiation unit 24a. Figure 1As shown, laser beam source 32 can be disposed either inside or outside irradiation unit 24a. In the first case, laser beam source 32 can be considered part of irradiation unit 24a. In the latter case, the laser beam is generated by laser beam source 32 and guided into irradiation unit 24a by optical fiber 34. Alternatively, the laser beam can be guided into irradiation unit 24a through air or vacuum, for example, using one or more mirrors.

[0096] The laser beam is directed to the scanning unit 30 from a laser beam source 32. The laser beam source 32 may, for example, include a diode-pumped ytterbium fiber laser emitting laser light having a wavelength of approximately 1070 nm to 1080 nm (ie, in the infrared wavelength range).

[0097] The irradiation unit 24a further includes two lenses 36 and 38, which are configured to focus the laser beam 14 to a desired focal position along the z-axis. Figure 1 In the embodiment shown, both lenses 36 and 38 have positive refractive power. Lens 38, further upstream in the beam path, is configured to collimate the laser light emitted by optical fiber 34 so as to produce a collimated or substantially collimated laser beam. Lens 36, further downstream in the beam path, is configured to focus the collimated (or substantially collimated) laser beam to a desired z position.

[0098] A top layer heating device 42 is provided in the processing chamber 16. The top layer heating device 42 is configured to heat the top layer (i.e., the uppermost layer 13) of the currently constructed workpiece 12 and / or heat a new powder layer deposited on top of the uppermost layer 13. The top layer heating device 42 is configured to heat the uppermost layer 13 by thermal radiation. The amount of thermal radiation irradiated by the top layer heating device 42 can be controlled by the control unit 42. The top layer heating device 42 is configured, for example, to emit thermal radiation (infrared radiation) toward the uppermost layer 13 to heat the uppermost layer. As discussed herein, heating is equivalent to introducing heat, particularly a predetermined amount of heat.

[0099] The top heating device 42 is attached to the powder coating device 18. The top heating device 42 moves horizontally together with the powder coating device 18. In this way, powder coating and heating can be performed simultaneously. However, powder coating and heating can be performed at different times, but the horizontal movement device provided for the powder coating device 18 can be shared with the top heating device 42.

[0100] Alternatively, the top layer heating device 42 may be provided with a separate (horizontal and / or vertical) moving device, such that it can be moved independently relative to the powder coating device 18 .

[0101] Furthermore, other types of top layer heating devices may be used. For example, a contact top layer heating device may be provided that heats the top layer 13 while in contact with it. Furthermore, for example, one or more laser beam sources may be provided as the top layer heating device, wherein the laser beam generated by the one or more laser beam sources can be directed to a predetermined portion of the top layer (without melting the top layer) to introduce heat. Furthermore, at least one irradiation beam 14 performing the building operation may (additionally) heat the top layer. In this case, the control unit 40 increases the irradiation power of the irradiation beam 14 by a predetermined amount.

[0102] The carrier heating device 44 is integrated into the carrier 20. The carrier heating device can also be located below or on one or more sides of the carrier. It should be noted 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 base plate. The carrier heating device 44 may be located within one of the plate packages or between two plates in the plate package.

[0103] The carrier heating device 44 is configured to heat the carrier 44 to a predetermined temperature. To this end, a temperature sensor (not shown) may be integrated into the carrier 20 or may contact the carrier 20 to measure the temperature of the carrier 20. In this way, closed-loop temperature control of the carrier 20 can be performed, for example, by a control unit. When the temperature drops below the lower limit (of the second predetermined range), the carrier heating device 44 introduces heat into the carrier 20, and when the temperature rises above the upper limit (of the second predetermined range), heating is stopped. However, other methods of maintaining the temperature of the carrier 20 are also possible. For example, predetermined heating and / or non-heating times and / or predetermined heating powers can be stored in a lookup table for multiple temperature values. In the case where a specific temperature value should be set (as a set temperature), the heating time and / or non-heating time and / or heating power can be read from the lookup table (open-loop temperature control of the carrier).

[0104] The top layer heating device 42 and the carrier heating device 44 are optional. This means that one or more of these devices can be omitted and the technology can still function. For example, if a suitable set temperature is selected and only a reduced irradiation power period is introduced, no additional heating by the heating device 42 is required. In addition, if the temperature of the lowermost portion 15 of the workpiece is maintained without introducing additional heat by the carrier heating device 44, the carrier heating device 44 can be omitted. For example, if Figure 3 As shown, this may be the case when selecting a suitable support structure for the workpiece 12 .

[0105] The control unit 40 includes a processor and a memory, wherein the memory stores instructions for controlling the various components of the apparatus 10. For example, the control unit 40 can be configured to control one or more of the temperature measuring device 28, the vertical movement unit 22, the powder coating device 18, the gas flow supplied 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. User input and output interfaces may be provided and connected or connectable to the control device 40. In addition, the control unit 40 has an interface for receiving workpiece data representing the three-dimensional shape of the workpiece 12 to be manufactured.

[0106] It should be noted that Figure 1 and Figure 2 The location of control unit 40 in FIG. 1 is purely illustrative and non-limiting. Control unit 40 may be located at any suitable location on device 10 or even remote from device 10 (e.g., integrated into a network structure such as a LAN). Furthermore, control unit 40 or at least a portion of control unit 40 may be provided in the form of a cloud computing device.

[0107] Figure 2 Shows something like Figure 1 As the only difference between the two devices 10, Figure 2 The irradiation device 24 of the device 10 includes two irradiation units 24a and 24b instead of Figure 1 An irradiation unit 24a of the device 10. However, Figure 2 The remainder of the device 10 has the same Figure 1 The components and functions are the same as those discussed in the previous section, so repeated descriptions are omitted. Figure 2 The same reference numerals are used. However, the suffixes "a" and "b" are used to distinguish between the components of the illumination unit 24a (suffix a) and the components of the additional illumination unit 24b (suffix b). The functions of the various components within the illumination units 24a and 24b are the same as those described above with respect to Figure 1 The irradiation device 24 is the same as discussed above.

[0108] In the following, unless otherwise expressly stated, reference numerals without a suffix (a or b) are used to refer also to corresponding elements with the suffixes a and b. For example, when reference is made to "scanning unit 30", reference is thereby also made to scanning units 30a and 30b.

[0109] Figure 2The device 10 is configured so that the irradiation unit 24a is configured to scan a first predetermined area of the uppermost powder layer (i.e., the first scanning area). Similarly, the additional irradiation unit 24b is configured to scan a second predetermined area of the uppermost powder layer (i.e., the second scanning area). The first scanning area and the second scanning area overlap each other in the overlapping area. In other words, there is an area of the uppermost powder layer that can be reached and selectively irradiated by the two laser beams 14a and 14b (i.e., the overlapping area). The first scanning area and the second scanning area may each be rectangular or circular, and the size and / or shape of the corresponding scanning area may be predetermined by the movement range of the corresponding scanning units 30a and 30b of the irradiation units 24a and 24b, respectively. In addition, it is possible that the overlapping area covers the entire uppermost powder layer, so that each of the laser beams 14a and 14b can reach every point on the uppermost powder layer.

[0110] In order to produce a three-dimensional workpiece 12, two laser beams 14a and 14b can simultaneously irradiate different portions of the same powder layer, wherein each laser beam 14a and 14b irradiates a portion of the workpiece 12 in its corresponding scanning field. In this way, compared to the case where only one laser beam 14 is used (see, for example, Figure 1 ), the workpiece 12 can be built faster than with conventional solidification methods. In addition, different laser beams 14a and 14b can be used, for example, to solidify the shell and core of the workpiece 12. Different laser beams have different beam parameters, such as laser power, wavelength and / or spot size.

[0111] For example, during times of reduced illumination power, one of the laser beams 14a, 14b may be turned off.

[0112] Figure 3 Details of the build cylinder (ie the volume in which the carrier 20 moves) are shown, which can be used Figure 1 and / or Figure 2 . Alternatively or in addition to the carrier heating device 44, the temperature of the lower portion 15 of the workpiece 12 can be maintained by calculating and constructing a suitable carrier structure 46. In other words, a mathematical model can be provided from which the temperature of the lower portion 15 of the workpiece 12 can be determined and appropriately set by constructing a corresponding carrier structure 46. A controlled amount of heat is removed from the workpiece 12 by the carrier structure 46, such that the lowermost portion 15 is maintained at a temperature within the second predetermined range despite the additional heat introduced into the uppermost layer 13 of the workpiece 12 by the laser beam 14.

[0113] Hereinafter, the method for temperature control according to the present disclosure is explained in further detail. Figure 1 and Figure 2 of the devices 10 (and optionally, according to Figure 3In particular, the main part of the method is executed by the control unit 40 of the corresponding device.

[0114] In order to build a workpiece 12 using the selective laser melting technique, optical power (i.e., laser power) must be introduced into a raw material powder (e.g., metal powder). The raw material powder melts and, after solidification, forms the workpiece 12. The optical power introduced into the process chamber 16 is partially reflected from the molten pool generated by the powder and the laser beam 14; the remaining optical power reaches the workpiece 12. From there, there are multiple heat sinks: radiation from the workpiece surface (i.e., from its uppermost layer 13), mainly entering the process chamber 16, convection into the gas flow (generated by the gas outlet 26 and the gas inlet 27), metal spatter condensed in the gas flow and entrained, heat transfer through the workpiece 12 into the carrier 20, and heat losses such as through the powder into the build cylinder. The heat distribution between the heat sinks depends on the process. Relevant factors include, for example, the material used for the raw material powder, its material parameters, and the workpiece geometry (in particular, the irradiated surface). For materials with low thermal conductivity (i.e., low heat transfer), the following applies, depending on the workpiece height (i.e., the extension along the z-axis):

[0115] For a low (small extension in the z-direction) workpiece 12, the thermal resistance of the workpiece 12 from the uppermost layer 13 to the carrier 20 is relatively small, so that a relatively large amount of heat is transferred through the workpiece 12 by heat transfer (i.e., in the z-direction) to the carrier 20. For a high (large extension in the z-direction) workpiece 12, the thermal resistance of the workpiece 12 is high, so that a small amount of heat is transferred through the workpiece 12 and reaches the carrier 20. In other words, to transfer the same amount of thermal energy, a significantly larger temperature difference is required, which may cause problems in the process.

[0116] Whenever optical power is introduced into workpiece 12, the thermal resistance of workpiece 12 physically always results in a temperature gradient within workpiece 12 relative to the temperature of carrier 20. This temperature gradient is undesirable because it can cause material strain and potentially impair material properties (depending on how large the gradient is). The magnitude of the temperature gradient is directly proportional to the two factors mentioned above: first, the greater the thermal resistance, i.e., the poorer the thermal conductivity of the material, the taller the workpiece 12 (in the z-direction), and the more unfavorable the workpiece geometry (e.g., due to narrow sections). Second, the greater the temperature gradient, the more heat is introduced into workpiece 12.

[0117] Specifically, the second factor can be controlled by the technology of the present disclosure.

[0118] Net Zero Concept: The Net Zero concept described herein is intended to maintain the "net" heat introduced into the workpiece 12 during the process of forming a layer to zero. When this goal is achieved, there are no temperature gradients, thereby resolving at least the aforementioned and other related issues. Assuming that the heat loss from the (uncured) powder is negligible (which is reasonable due to the relatively low thermal conductivity of the powder), the above formula is equivalent to considering the net energy balance of the uppermost layer 13 on a layer. Therefore, in order to achieve the Net Zero concept, the following should apply:

[0119]

[0120] In the above formula, E is the energy in the uppermost layer 13 and dE / dt is its change with time. n To the start time t of layer n+1 n+1 Integrate. Q 入 is the heat introduced during the process of the layer entering the uppermost layer 13 of the workpiece, Q 出 is the amount of heat leaving the uppermost layer 13. The variable represented by the "dot" is the heat flow, ie the corresponding derivative with respect to time.

[0121] In any case, in order to produce (ie print) the workpiece 12, optical power must be introduced into the workpiece 12 and into the uppermost powder layer, since the powder must melt. 入 It depends on the build parameters, the irradiation area and other factors. It can be calculated based on these factors, for example, at the beginning of the build process with relatively high accuracy (relative error less than 5%). A person skilled in the art knows how to calculate the input heat Q based on one or more input parameters, such as laser power, spot size, irradiation time, irradiation area, etc. 入 Therefore, the process of calculating the input heat will not be explained in further detail in this disclosure.

[0122] To achieve net zero, it is necessary to ensure that the heat Q leaving the uppermost layer 13 出 Equal to the input heat Q 入 As mentioned above, heat sinks that can be used for this purpose are radiation from the workpiece 12, metal splashes, and convection into the air flow. In particular, heat radiation and convection depend on the top layer temperature, ie the temperature of the uppermost layer 13 of the workpiece 12.

[0123] Fluctuations in the optical load: During the build process (build job), the optical load fluctuates because process parameters, such as the irradiation area or build parameters, vary, which has a direct impact on the optical load. In relation to the present disclosure, this means that the input heat Q 入 Change so that the output heat Q 出 Changes must also be made to meet the net zero concept.

[0124] Among them, the following options exist to influence the input and / or output heat Q 入 / Q 出 :

[0125] a) Heating the uppermost layer 13 of the workpiece 12 and / or the raw material layer at the top of the workpiece 12, for example, by a top heating device 42 (also referred to herein as "RadHeat"). 入 Have a direct impact.

[0126] b) Reduced irradiation power for additional periods, especially additional waiting periods. This has an impact on Q 出 influential.

[0127] c) Additional artifacts. This is for Q 入 and Q 出 influential.

[0128] d) Adjustment of parameters, in particular changing the laser power of the laser beam 14, while other parameters (in particular all other parameters) remain unchanged. Or: changing between different qualified parameters and / or parameter sets. This has an impact on Q 入 influential.

[0129] e) Modified and / or additional support structures. This has an impact on the workpiece thermal resistance and therefore on the temperature of the uppermost layer of the workpiece 12 and thus indirectly on Q 出 influential.

[0130] Constant Workpiece Temperature: The net-zero concept ensures that no energy changes occur in the corresponding uppermost layer 13 during the layer process, and thus is not introduced into the entire workpiece 12. This is equivalent to maintaining the temperature of the uppermost layer 13 of the workpiece 12. The temperature that should be maintained constant is referred to herein as the net-zero temperature or set point temperature. The set point temperature can be set by the user based on user experience or calculations performed by the user, optionally with the aid of a mathematical model. It is also possible for the control unit 40 of the device 10 to automatically determine and set the set point temperature based on a mathematical model. An example of determining and setting the set point temperature is as follows.

[0131] The aforementioned parameter metrics required to achieve a net-zero temperature can be determined prior to the build process based on mathematical models (open-loop control). However, achieving and / or maintaining a set temperature through closed-loop control may be advantageous. A model estimating the aforementioned effects (e.g., heating of the topmost layer 13, reduced irradiation power periods, altered parameters, etc.) can provide the first model-based input (initial value) for temperature control. Ideally, the temperature of the topmost layer is measured during the process, and control is adjusted accordingly (so that the set temperature is maintained).

[0132] Crucial to the present disclosure is that the set temperature of the lowermost portion 15 of the workpiece 12 corresponds to the set temperature of the uppermost layer 13. In other words, the first temperature range for maintaining the temperature of the lowermost portion 15 of the workpiece 12 includes the same set temperature as the second temperature range for maintaining the temperature of the uppermost layer 13 of the workpiece 12. In other words, the concept of the present disclosure is to provide the same set temperature at the lowermost portion 5 and the uppermost 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 temperature of the carrier heating device 44 corresponds to the set temperature of the uppermost layer 13.

[0133] Hereinafter, detailed examples of temperature control are provided, wherein the present technology is not limited to these examples.

[0134] Example 1 (Prior Art): A build process was performed using material IN718 with high power parameters ranging from 5% to 40% of the irradiated area (i.e., 5% to 40% of the total possible area that can be irradiated) and a height of 600 mm (along the z-axis). For IN718, a preheat temperature of 200°C was used. For a 40% irradiated area, an optical load (power) of 8 kW was introduced into process chamber 16, of which approximately 4 kW reached workpiece 12. Without the disclosed techniques, the large height of workpiece 12 to be built would have a significant impact on the thermal resistance of workpiece 12. Furthermore, IN718 has poor thermal conductivity, which exacerbates this effect. Consequently, the temperature of workpiece 12 near carrier 20 corresponds approximately to the 200°C preheat temperature and is significantly higher in the upper layers. A rough estimate for a particular additive manufacturing apparatus 10, without considering the heatsink, process chamber 16, and airflow, results in a temperature of 1000°C for the uppermost layer 13. This means that the temperature gradient from the lowermost to the uppermost part of the workpiece 12 will be greater than 800 K. This will lead to large differences in material properties and elongation (dimensional accuracy).

[0135] The IN718 can withstand relatively high temperatures by adopting a net-zero concept: heating the top layer 13 and / or employing an additional waiting time. 600°C was chosen as the net-zero temperature (setpoint). At this temperature, (heat) radiation from the workpiece 12 plays a significant role, i.e., it is very strong. Furthermore, convection into the airflow becomes increasingly significant. According to an example calculation based on a mathematical model, this results in a calculation of 3.5 kW for the sum of radiation and convection terms, along with other heat losses from the top layer 13. When constructing a job with a 40% irradiation area, for example, 4 kW is introduced into the workpiece 12 for 60 seconds, thus introducing 240 kW of energy. At a temperature of 600°C, 3.5 kW of heat is lost over the duration of the layer, representing the irradiation time plus the fire-to-fire time. The fire-to-fire time is defined as the time from the end of irradiation of one layer to the start of irradiation of the next. During the fire-to-fire time, for example, the next layer is applied onto the previous one.

[0136] In the case where the fire-to-fire time is about 8.5 s, the loss is (60 s + 8.5 s) x 3.5 kW = 239.75 kW, so the heat is roughly the same (i.e., the heat introduced Q 入 = 240kW and heat loss Q 出 =239.75kW (roughly the same).

[0137] However, in a machine with a shorter fire to fire time (eg 7 s), an additional waiting period of 1.5 s would be required for an illuminated area of 40%.

[0138] Thus, this additional waiting period can be introduced at any time during the processing of the current layer (uppermost layer 13). For example, an additional waiting period of 1.5s is introduced at the end of irradiation and before the fire-to-fire time (ie before applying the next layer).

[0139] Furthermore, closed loop control can support the above open loop control. In closed loop control, the temperature measuring device 28 measures the temperature of the uppermost layer 13 of the workpiece 12. If the measured temperature is still greater than 600°C after an additional waiting period of 1.5s, a further additional waiting period can be introduced.

[0140] For layers with an irradiation area of less than 40%, the optical load is significantly smaller. For example, the top layer 13 with an irradiation area of 5% is considered to have an optical load of approximately 2 kW entering the process chamber 16, which means approximately 1 kW entering the workpiece 12. Here, the irradiation duration is only 20 seconds. Therefore, only 20 kW of heat is introduced into the workpiece 12. Due to the net zero temperature of 600°C, 3.5 kW is lost (as in the previous example). With a fire-to-fire time of 7 seconds, this results in a heat loss of (20 seconds + 7 seconds) × 3.5 kW = 94.5 kW. This difference in heat can be compensated by heating the top layer 13 with the top heating device 42. In the present case, it must irradiate 94.5 kW - 20 kW = 74.5 kW of heat. In this case, it is assumed that the top heating device 42 does not require any additional time because it moves together with the powder coating device 18 and therefore heats during the fire-to-fire time.

[0141] In summary, the carrier 20 is heated to 600° C. and this temperature is maintained by the carrier heating device 44. As a result, the workpiece 12 has a temperature of 600° C. near the carrier 20, i.e., at its lowermost portion 15. The desired top layer temperature (set temperature) of 600° C. is achieved by controlling the temperature of the uppermost layer 13 by introducing an additional waiting time or by heating the uppermost layer 13. Thus, it is ensured that the temperature in the top layer 13 of the workpiece 12 and in the lowermost portion 15 of the workpiece 12 is maintained at the set temperature of 600° C. Due to the introduced heat Q 入 and the heat loss Q 出 The same, so the temperature is maintained.

[0142] Solution using a net-zero concept and only increasing the waiting time: Similar to the above-mentioned solution for heating the top layer, heating can be avoided when a lower set temperature is selected. In the present case, a set temperature of 300°C is selected. Therefore, the carrier 20 is heated and maintained at 300°C. According to the mathematical model, it is known that the workpiece 12 must have a temperature of 310°C before a new powder layer is applied, so that it has a temperature of 300°C after the coating process. After irradiating this layer, the temperature of the top layer is measured (monitored) during an additional waiting time. Once 310°C is reached, the next layer is applied. Due to this method, the temperature of the workpiece 12 in its top layer 13 and in its bottom part 15 is also the same, so no gradient is generated. The additional waiting time is used for controlled cooling of the workpiece by radiation and convection. The length of the waiting time is such that the heat introduced (for example, 4 kW×60 s at 40% irradiation area) is equal to the heat lost to the incoming air flow and the walls of the process chamber 16. Compared to the above-mentioned solution involving the top heating device 42, this solution may be easier to implement because the top heating device 42 (RadHeat) is not required. However, it may require a longer waiting time. The higher the net zero temperature setting, the shorter the waiting time, but a higher net zero temperature may be more difficult to achieve.

[0143] It should be noted that, in the above-mentioned embodiment, reference is made to maintaining a set temperature. However, it may be impossible to keep the set temperature precisely constant. Therefore, for the function of the present technology, it is sufficient to maintain the temperature of the lowermost portion 15 of the workpiece 12 within a first predetermined temperature range and to maintain the temperature of the uppermost layer 13 of the workpiece 12 within a second predetermined temperature range. Both the first predetermined temperature range and the second predetermined temperature range include a set temperature. For example, the set temperature may be in the middle of each of the first temperature range and the second temperature range, and the first temperature range and the second temperature range may be the same. An extension of each of the first temperature range and the second temperature range may be, for example, 1K, 2K, 5K, 10K, 20K, 30K, 40K or 50K.

[0144] All combinations of the above options are possible and encompassed by the present technology. For example, when other parameters are used depending on the build height, the heat load changes, and thus the potential waiting time and the heat differential that must be radiated by the top heating device 42, etc. also change.

[0145] A further improvement is defined as follows: The above considerations are based on the assumption that heat losses on the powder can be ignored. Only in this way are the temperatures of the uppermost and lowermost parts of the workpiece 12 the same and there is no gradient. However, in reality, this heat loss exists and may cause the workpiece 12 to be slightly cooler in the central area of the workpiece 12. This effect can be avoided by providing build barrel heating. Therefore, according to an embodiment of the present technology, a build barrel heating device is provided, and the control unit 40 is configured to heat the build barrel to a set temperature by the build barrel heating device. In this case, there is no heat flow into the build barrel. The concept is similar to maintaining the temperature of the carrier 20, but additionally, in order to avoid thermal gradients, the other system edges are brought to and maintained at a set temperature.

[0146] Selection of a set temperature: In principle, nearly any temperature can be selected as the set temperature (net zero temperature). However, an inappropriate set temperature may have disadvantages, for example, with respect to the required waiting time, the additional workpiece required (powder consumption), the required heat introduced (energy consumption), the temperature resistance of the components of the apparatus 10, etc. A suitable set temperature can be determined empirically or based on a model, such as through experiments or studies. More precisely, the set temperature can be calculated by the control unit 40 based on one or more input parameters, such as those related to the material used, the geometry of the workpiece, etc.

[0147] Furthermore, it is possible to solve a multidimensional optimization problem which indicates, for example, energy consumption with respect to waiting time at different temperatures.The control unit 40 can automatically select a suitable set temperature.

[0148] Figure 4 A flow chart illustrating a method for temperature control of a three-dimensional workpiece 12 generated by additive manufacturing according to an embodiment of the present disclosure is shown.

[0149] The method is performed during the construction process of the three-dimensional workpiece 12 by an additive manufacturing device (such as Figures 1 to 3 10) as shown and discussed above.

[0150] The method begins with maintaining 50 the temperature of the lowermost portion 15 of the workpiece 12 within a first predetermined range, which includes a pre-set temperature, during the build process of the workpiece 12. As described above, there are two main options for maintaining the temperature. First, a suitable support structure 46 is provided that ensures that the temperature of the lowermost portion 15 remains within the first predetermined range during the build process. Second, the lowermost portion 15 of the workpiece 12 is positioned adjacent to or directly on a carrier 20 of the apparatus 10, and the carrier 20 is heated by a carrier heating device 44. In this second embodiment, heating and maintaining the temperature can be performed in an open-loop or closed-loop manner. In an open-loop approach, the heating time and / or heating power are predetermined and read from a lookup table (desired set temperature). The lookup table can be stored in the memory of the control unit 40. In a closed-loop approach, the temperature of the carrier 20 is measured, and the carrier heating device 44 can be activated if the temperature is too low and deactivated if the temperature is too high. In this way, a constant temperature of the carrier 20 can be maintained, at least within the limits of the first predetermined range.

[0151] In step 52, at least one energy beam is irradiated into the raw material layer at the top of the workpiece 12 to solidify the irradiated raw material and form the uppermost layer 13 of the workpiece 12. In this step, heat must be introduced into the workpiece 12 to form at least one molten pool for solidifying the raw material at the desired location. Irradiation can be performed using only one or more energy beams (especially laser beams).

[0152] In step 54, the temperature of the uppermost layer 13 of the workpiece 12 is maintained such that the temperature of the uppermost layer 13 of the workpiece 12 is within a second predetermined range, which includes the predetermined temperature. The maintaining step can generally be performed by increasing the amount of heat introduced and by increasing the amount of heat lost. The first option can be achieved by heating the uppermost layer of the workpiece 12 and / or the raw material layer at the top of the workpiece 12 using the top heating device 42. The second option can be achieved by introducing one or more additional periods of reduced irradiation power, in particular, additional waiting times during which no irradiation is performed.

[0153] Figure 5 Shown Figure 1 or Figure 2 Schematic diagram of a control unit 40 of one of the devices 10. The control unit 40 includes several modules 60 to 64, each of which can be represented in the form of hardware and / or software. In one embodiment, the control unit 40 includes a processor and a memory. Instructions are stored on the memory, which cause the processor to execute Figure 4 To this end, the software stored on the memory may be considered to include Figure 5 Modules 62 to 64 are shown in FIG.

[0154] Specifically, these modules are:

[0155] The first maintaining module 60 is configured to maintain the temperature of the lowermost portion of the workpiece within a first predetermined range including a preset temperature during the building process of the workpiece.

[0156] The instruction module 62 is used to instruct the irradiation device to irradiate at least one energy beam into the raw material layer at the top of the workpiece, so that the irradiated raw material is solidified and forms the uppermost layer of the workpiece.

[0157] The second maintaining module 64 is configured to maintain the temperature of the uppermost layer 13 of the workpiece 12 so that the temperature of the uppermost layer of the workpiece is within a second predetermined range including a preset temperature.

[0158] The details discussed herein regarding the individual method steps are also applicable to the corresponding modules 60 to 64 of the control unit 40. In other words, the control unit 40 is configured to perform the above Figure 4 method, wherein the details of the method discussed herein apply accordingly to the software of the control unit and / or Figure 5 The corresponding modules and / or additional modules shown in .

[0159] According to some embodiments, advantages of the technology disclosed herein may include the following.

[0160] The expansion of the workpiece 12 is equal. This results in, for example, predictable warping. Conventionally, the prior art lacks a nearly constant temperature at which the inverse problem of finding the optimal shape for the workpiece 12 can be solved, making initial calculations for forming the workpiece 12 with the desired shape in the cooled state extremely difficult. This is possible at a constant temperature. Without expansion compensation, the workpiece would warp because it is hotter in the upper layers and therefore expands more before cooling. This problem can be eliminated or reduced using this technology.

[0161] Better material properties, because the desired set temperature (in terms of time and space) is the same from the beginning to the end of the build job. The parameters that apply to the first layer also apply to the last layer of the job, because there is no thermal difference.

[0162] Design freedom: From the combination of the above items, a total design freedom arises. In the prior art, it was not possible to build a poorly supported workpiece, or a workpiece with a small irradiation area in the lower part and a large irradiation area in the upper part (inverted pyramid). The problem is the same in both cases: the thermal resistance of the workpiece, especially the upper layer, to the main heat sink (carrier) is so great that the heat cannot be dissipated. As a result, the workpiece 12 becomes very hot locally, expands, protrudes from the powder layer, which can lead to collisions. According to the present technology, this problem can be completely avoided when the temperature along the workpiece 12 is constant. This problem does not exist because, in the net zero concept, the carrier 20 is not a heat sink.

Claims

1. A method for temperature control of a three-dimensional workpiece produced by additive manufacturing, the method comprising: During a build process of a workpiece, maintaining a temperature of a lowermost portion of the workpiece within a first predetermined range including a preset temperature; irradiating at least one energy beam onto the raw material layer at the top of the workpiece to solidify the irradiated raw material and form the uppermost layer of the workpiece; as well as The temperature of the uppermost layer of the workpiece is maintained so that the temperature of the uppermost layer of the workpiece is within a second predetermined range including the preset temperature.

2. The method according to claim 1, wherein Maintaining the temperature of the lowermost portion of the workpiece includes heating a carrier on which the workpiece is built to a temperature particularly within the first predetermined range.

3. The method according to claim 1 or 2, further comprising: The predetermined temperature is determined based on build data defining the geometry of the workpiece to be built.

4. The method according to any one of claims 1 to 3, wherein Maintaining the temperature of the uppermost layer of the workpiece includes at least one of heating the uppermost layer of the workpiece and / or a layer of raw material at the top of the workpiece and introducing a reduced irradiation power for a period of time.

5. The method according to claim 4, further comprising: calculating the amount of heat introduced into the uppermost layer by the at least one energy beam; calculating the amount of heat leaving the uppermost layer during irradiation of the uppermost layer; When the amount of heat introduced into the uppermost layer is greater than the amount of heat leaving the uppermost layer, introducing a reduced irradiation power for a period of time during which the excess heat is able to leave the uppermost layer; and / or When the amount of heat leaving the uppermost layer is greater than the amount of heat introduced into the uppermost layer, the uppermost layer of the workpiece and / or the layer of raw material at the top of the workpiece is heated such that additional heat is introduced into the uppermost layer.

6. The method according to claim 5, wherein: When the amount of heat introduced into the uppermost layer is greater than the amount of heat leaving the uppermost layer, the additional amount of heat leaving the uppermost layer corresponds to the difference between the amount of heat introduced into the uppermost layer and the amount of heat leaving the uppermost layer; and / or When the amount of heat leaving the uppermost layer is greater than the amount of heat introduced into the uppermost layer, the additional heat introduced into the uppermost layer corresponds to the difference between the amount of heat leaving the uppermost layer and the amount of heat introduced into the uppermost layer.

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

8. The method according to claim 7, wherein: The heat radiation is radiated by an apparatus attached to a powder coating apparatus configured to apply an uppermost layer of the workpiece.

9. The method according to any one of claims 4 to 8, wherein The certain period of reduced irradiation power is a predetermined waiting period during which irradiation of the raw material is not performed.

10. The method according to any one of claims 4 to 8, wherein The reduced irradiation power for the certain period is a period in 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 light spot.

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

12. The method according to any one of claims 1 to 11, in, The temperature of the uppermost 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 carried out by means of an additive manufacturing device, and in particular by means of a device for selective laser melting or selective laser sintering.

14. The method according to any one of claims 1 to 13, further comprising determining the predetermined temperature to define a microstructure during a building process.

15. An apparatus for temperature control of a three-dimensional workpiece produced by additive manufacturing, the apparatus comprising: control unit; and an irradiation device for irradiating at least one energy beam; Wherein, the control unit is configured as follows: During a build process of a workpiece, maintaining a temperature of a lowermost portion of the workpiece within a first predetermined range including a preset temperature; instructing the irradiation device to irradiate at least one energy beam onto the raw material layer at the top of the workpiece, so that the irradiated raw material is solidified and forms the uppermost layer of the workpiece; as well as The temperature of the uppermost layer of the workpiece is maintained so that the temperature of the uppermost layer of the workpiece is within a second predetermined range including the preset temperature.

16. The apparatus according to claim 15, further comprising a carrier heating device, in, Maintaining the temperature of the lowermost portion of the workpiece includes instructing the carrier heating device to heat the carrier on which the workpiece is built, in particular to a temperature within the first predetermined range.

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

18. The apparatus according to any one of claims 15 to 17, further comprising a top layer heating device, in, Maintaining the temperature of the uppermost layer of the workpiece includes at least one of instructing the top heating device to heat the uppermost layer of the workpiece and / or a layer of raw material at the top of the workpiece, and introducing a reduced irradiation power for a period of time.

19. The apparatus according to claim 18, wherein The control unit is further configured to: calculating the amount of heat introduced into the uppermost layer by the at least one energy beam; calculating the amount of heat leaving the uppermost layer during irradiation of the uppermost layer; When the amount of heat introduced into the uppermost layer is greater than the amount of heat leaving the uppermost layer, introducing a reduced irradiation power for a period of time during which the excess heat is able to leave the uppermost layer; and / or When the heat leaving the uppermost layer is greater than the heat introduced into the uppermost layer, the top heating device is instructed to heat the uppermost layer of the workpiece and / or the raw material layer at the top of the workpiece so that additional heat is introduced into the uppermost layer.

20. The apparatus according to claim 19, wherein When the amount of heat introduced into the uppermost layer is greater than the amount of heat leaving the uppermost layer, the additional amount of heat leaving the uppermost layer corresponds to the difference between the amount of heat introduced into the uppermost layer and the amount of heat leaving the uppermost layer; and / or When the amount of heat leaving the uppermost layer is greater than the amount of heat introduced into the uppermost layer, the additional heat introduced into the uppermost layer corresponds to the difference between the amount of heat leaving the uppermost layer and the amount of heat introduced into the uppermost layer.

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

22. The apparatus according to claim 21, wherein The top layer heating apparatus is attached to a powder coating apparatus configured to apply an uppermost layer of the workpiece.

23. Apparatus according to any one of claims 18 to 22, wherein The certain period of reduced irradiation power is a predetermined waiting period during which irradiation of the raw material is not performed.

24. Apparatus according to any one of claims 18 to 22, wherein The reduced irradiation power for the certain period is a period in 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 light spot.

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

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

27. Apparatus according to any one of claims 15 to 26, wherein The device is an additive manufacturing device and, in particular, a device for selective laser melting or selective laser sintering.

28. Apparatus according to any one of claims 15 to 27, wherein The control unit is configured to determine the predetermined temperature to define a microstructure during a building process.

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