Technique for temperature control of a three-dimensional workpiece generated via additive manufacturing

Abstract

A method for temperature control of a three-dimensional workpiece generated via additive manufacturing is provided. The method comprises maintaining a temperature of a lowermost part of the workpiece during a build process of the workpiece within a first predefined range including a predefined set temperature, irradiating an energy beam into a raw material layer at a top portion of the workpiece to solidify the irradiated raw material and to form an uppermost layer of the workpiece, and maintaining a temperature of the uppermost layer of the workpiece such that the temperature of the uppermost layer of the workpiece is within a second predefined range including the predefined set temperature. Further, a corresponding device for temperature control of a three-dimensional workpiece generated via additive manufacturing is provided.

Description

[0001] Technique for temperature control of a three-dimensional workpiece generated via additive manufacturing

[0002] The present invention generally relates to a technique for temperature control of a three-dimensional workpiece generated via additive manufacturing. The process of additive manufacturing may be, without limitation, powder bed fusion, such as selective laser sintering, selective laser melting, or electron beam melting.

[0003] Powder bed fusion is an additive layering process by which pulverulent, in particular metallic and / or ceramic raw materials can be processed to three-dimensional workpieces of complex shapes. To that end, a raw material powder layer is applied onto a carrier and subjected to radiation (e.g., laser or particle radiation) in a site-selective manner in dependence on the desired geometry of the workpiece that is to be produced. The radiation penetrating into the powder layer causes heating and consequently melting or sintering of the raw material powder particles. Further raw material powder layers are then applied successively to the layer on the carrier that has already been subjected to radiation treatment, until the workpiece has the desired shape and size. Powder bed fusion may be employed for the production of prototypes, tools, replacement parts, high value components, or medical prostheses, such as, for example, dental or orthopedic prostheses, on the basis of CAD data. Examples for powder bed fusion techniques include selective laser melting, selective laser sintering, and electron beam melting.

[0004] Apparatuses are known for producing one or more workpieces according to the above technique. For example, EP 2 961 549 Al and EP 2 878 402 Al, respectively, describe an apparatus for producing a three-dimensional workpiece according to the technique of selective laser melting. The general principles described above and in these documents may also apply to the technique of the present disclosure.

[0005] In particular during the build of a complex shaped and / or high workpiece (i.e., a workpiece having a large extension in a z-direction perpendicular to a substrate plate or carrier) via one of the aforementioned additive manufacturing techniques, a large temperature gradient may exist, in particular a large temperature difference may exist between a substrate plate and an uppermost layer (also referred to herein as "top layer") of the workpiece. In the following, the term "temperature gradient" is used for a spatial temperature gradient along the vertical axis (z-axis) of a workpiece.

[0006] Large temperature gradients, e.g., may lead to deteriorated material properties and strong tension within such workpieces.

[0007] The invention is therefore directed at the object of providing a technique that solves at least one of the aforementioned problems and / or other related problems. In particular, and without limitation, a technique is desired, which reduces temperature gradients within a workpiece built via additive manufacturing and which thereby may help to improve material properties of the finished workpiece.

[0008] This object is addressed 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 generated via additive manufacturing is provided. The method comprises maintaining a temperature of a lowermost part of the workpiece during a build process of the workpiece within a first predefined range including a predefined set temperature, irradiating at least one energy beam into a raw material layer at a top portion of the workpiece to solidify the irradiated raw material and to form an uppermost layer of the workpiece, and maintaining a temperature of the uppermost layer of the workpiece such that the temperature of the uppermost layer of the workpiece is within a second predefined range including the predefined set temperature.

[0010] One or more of the following features of the method aspect may also apply to the device of the device aspect described below. When, in the present disclosure, the term "workpiece" is used, it always refers to the "three-dimensional workpiece"

[0011] The process of additive manufacturing, via which the workpiece is generated, may be additive manufacturing from a powder bed, such as selective laser sintering or selective laser melting, or any other additive manufacturing process, where a workpiece is build up from raw material (e.g., in the form of powder, granulate, and / or liquid).

[0012] The predefined set temperature may be stored in a memory, in particular, in a memory of an additive manufacturing device carrying out the method. The predefined set temperature may have been input by a user or may have been determined (in particular, calculated) in advance, e.g., by a control unit of the additive manufacturing device or by another computing device.

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

[0014] The first predefined range may be at most 5 K, at most 10 K, at most 20 K, at most 30 K, at most 40 K, at most 50 K, at most 70 K, or at most 100 K. It may be desired to maintain the temperature of the lowermost part of the workpiece as close to the predefined set temperature as possible. The predefined set temperature may be in the middle of the first predefined range. However, the predefined set temperature may also be one of the limits of the first predefined range. For example, the predefined set temperature may be Tset and the first predefined temperature range may extend from Tset—Ttolerance_l tO Tset + Ttolerance_2- The Values Of Ttolerance_l and Ttoler- ance_2 may be identical or may be different from each other.

[0015] For maintaining the temperature of the lowermost part of the workpiece, the temperature of the lowermost part of the workpiece may be measured (e.g., contactless) or estimated and a closed-loop temperature control may be implemented.

[0016] The lowermost part of the workpiece may include a lowest layer of the workpiece. When in the present disclosure layers of the workpiece are mentioned, it is referred to solidified regions of the workpiece resulting from respective raw material layers. The lowermost part of the workpiece may directly contact a carrier of the additive manufacturing device. It may also be embedded into the raw material powder, i.e., it may be solidified on top of an unsolidified raw material layer. Further, it may be supported by one or more support structures or may even include or consist of support structures.

[0017] The step of irradiating the at least one energy beam may be carried out as it is common in the field of selective laser melting or selective laser sintering. Hence, the energy beam may be a laser beam. The energy beam may, however, also be a particle beam, an electron beam, or any other suitable energy beam configured to melt or sinter the raw material powder at desired locations. To this end, a scanning optic may be provided for laterally positioning the energy beam on an irradiated layer of the workpiece. In the present disclosure, lateral directions mean directions in an x-y- plane, which is parallel to a carrier of the additive manufacturing device. A z-axis is provided perpendicular to the x-y-plane and corresponds to a height axis (i.e., an axis perpendicular to the raw material layers deposited onto the carrier). Further, more than one energy beams may be irradiated simultaneously or subsequently. For example, a plurality of energy beams may be provided, which are independently steerable in x- and y-direction. To this end, an individual scanning optic may be provided for each energy beam. Each energy beam may be emitted by a corresponding beam source (e.g., laser source). Further, the plurality of energy beams may have different wavelengths, different laser powers, and / or different beam profiles. For example, irradiation of a shell part of the workpiece may be carried out with a different energy beam than a core part of the workpiece.

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

[0019] When the present disclosure refers to a temperature of an uppermost layer of the workpiece, this may mean a temperature of a top surface of the workpiece. The temperature of the uppermost layer of the workpiece may be averaged over time and / or over space (i.e., different locations on the uppermost layer). The temperature of the uppermost layer of the workpiece may be measured and / or determined after a predefined time has passed after an irradiation of the uppermost layer, e.g., directly before a next raw material layer is deposited. Further, the temperature of the uppermost layer of the workpiece may refer to a spatial average temperature of the uppermost layer, excluding one or more potential melt pools (which always will be hotter than the rest of the layer). In order to exclude temperature peaks of one or more melt pools, the temperature of the uppermost layer may be a median value of temperatures measured and / or considered over the entire uppermost layer.

[0020] Maintaining the temperature of the uppermost layer may mean that measures are taken that ensure that this temperature is identical or close to the set temperature.

[0021] The second predefined range may correspond to the first predefined range. The second predefined range may be at most 5 K, at most 10 K, at most 20 K, at most 30 K, at most 40 K, at most 50 K, at most 70 K, or at most 100 K. It may be desired to maintain the temperature of the uppermost layer of the workpiece as close to the predefined set temperature as possible. The predefined set temperature may be in the middle of the second predefined range. However, the predefined set temperature may also be one of the limits of the second predefined range. For example, the predefined set temperature may be Tset and the second predefined temperature range may extend from Tset—Ttolerance_3 tO Tset + Ttolerance_4- The Values Of Ttolerance_3 and Ttol- erance_4 may be identical or may be different from each other.

[0022] Maintaining the temperature of the lowermost part of the workpiece may comprise heating a carrier, on which the workpiece is built, in particular to a temperature within the first predefined range.

[0023] The heating of the carrier may be carried out via a heating element integrated in the carrier or provided below the carrier. The heating element may be an electric heating element provided, e.g., in the form of a heating coil. The heating element may be configured to homogeneously heat the carrier to a desired temperature corresponding to the predefined set temperature. In other words, the heating element may be configured to heat the carrier to a temperature within the first predefined range.

[0024] The carrier may be stationary during the build process in regard of its vertical position or may be a vertically movable carrier of an additive manufacturing device, that is lowered during the build process, such that a new raw material layer can be provided on top of the previous raw material layer. The term carrier may be replaced with base plate or substrate plate. In any case, the carrier may comprise 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 lowermost part of the workpiece may be maintained, e.g., by using an open-loop or closed-loop temperature control of a temperature of the carrier. For example, a heating element heating the carrier may be activated in predefined time periods in order to obtain and maintain the set temperature. In other words, an open-loop temperature control of the temperature of the carrier may be provided, wherein heating times, non-heating times, and / or heating power values may be stored in a look-up table for a desired temperature value. Further, a temperature sensor may be provided that is configured to measure a temperature of the carrier. This temperature sensor may be integrated into the carrier and / or may contact the carrier. In this case, an output of the temperature sensor may be used for closed-loop temperature control of the temperature of the carrier. For example, in case the temperature of the carrier measured with the temperature sensor drops below a predefined threshold value (which depends on the set temperature), the heating element is activated in order to heat the carrier to the desired set temperature. The predefined threshold value may correspond to a lower limit of the first predefined range. In this way, the temperature is maintained within the first predefined range, in particular during an entire build process of the workpiece. The build process may be defined as a time when a deposition of a first raw material layer onto the carrier starts to a time when an irradiation of a last raw material layer stops. Optionally, the build process may comprise an additional predefined cooling time after the irradiation of the last raw material layer. During said cooling time, the temperature of the lowermost part of the workpiece and the temperature of the uppermost layer may still be maintained in the respective range, e.g., in order to achieve a desired microstructure of the workpiece.

[0026] In order to maintain the temperature of the lowermost part of the workpiece within the first predefined range, a set temperature of the carrier may have to be set slightly higher than the desired set temperature of the lowermost part of the work piece, due to additional heat sinks (in particular, in case the lowermost part of the workpiece does not directly contact the carrier or has a rather small contact area to the carrier). Hence, maintaining the temperature of the lowermost part of the workpiece within the first predefined range may comprise maintaining a carrier, on which the workpiece is built, in a third predefined range different from the first predefined range. The upper and lower limits of the third predefined range may be larger than the upper and lower limits of the first predefined range, in particular, by a fixed offset (e.g., 5 K, 10 K, or 15 K).

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

[0028] A plurality of parameters may have an influence on the set temperature. The geometry of the workpiece to be built is one of these parameters. Those parameters may further include at least one of a used material or material composition, a workpiece height, one or more irradiation parameters such as wavelength, beam power, and / or beam profile, etc.

[0029] The set temperature may be determined by analyzing the build data (e.g., CAD data or SLM data). A mathematical model may be used for determining the predefined set temperature. Further, historical process values (e.g., measured temperatures) may be input parameters for the determination of the set temperature. For example, an artificial intelligence engine may be trained and implemented for determining the set temperature. The step of determining the predefined set temperature may comprise storing the predefined set temperature into a memory, e.g., a memory of the additive manufacturing device. Further, the first predefined range may be determined, based on the determined set temperature. For example, the first predefined range may be determined such that the set temperature is in the center of the first predefined range. Further, the second predefined range may be determined, based on the determined set temperature. For example, the second predefined range may be determined such that the set temperature is in the center of the second predefined range.

[0030] Maintaining the temperature of the uppermost layer of the workpiece may comprise at least one of heating the uppermost layer of the workpiece and / or the raw material layer at the top portion of the workpiece and introducing a period of reduced irradiation power.

[0031] In other words, there are at least two options of maintaining the temperature of the uppermost layer, namely heating and cooling (i.e., introducing the period of reduced irradiation power). The heating may be carried out actively via a heating device. The cooling may be carried out passively by allowing heat to be removed from the workpiece (e.g., via heat radiation or convection).

[0032] Maintaining the temperature of the uppermost layer may be carried out via openloop control or closed-loop control. In open-loop, predefined measures are taken that are known to lead to the desired set temperature (i.e., a temperature of the uppermost layer within the second predefined range. For example, heating times, nonheating times, and / or heating powers may be stored in a look-up table for a plurality of desired temperature values. Similarly, values for a length and / or other parameters of a period of reduced irradiation power may be stored in a look-up table for a plurality of desired temperature values.

[0033] In closed-loop, the temperature of the uppermost layer is measured, e.g., via a contact temperature sensor, a non-contact temperature sensor, a camera, a pyrometric measurement device, etc. Based on the measured temperature, measures are initiated that lead to cooling or heating of the uppermost layer. For example, in case the measured temperature is below a lower limit of the second predefined range, the uppermost layer is heated via a heating device (top layer heating device). In case the measured temperature is above an upper limit of the second predefined range, at least one additional period of reduced irradiation power is introduced (e.g., a waiting period in which no irradiation is carried out). Heating the uppermost layer of the workpiece may comprise increasing an irradiation power of the at least one energy beam. The irradiation power (e.g., laser power) may be increased to a value that ensures that an amount of heat introduced into the uppermost layer is equal to an amount of heat leaving the uppermost layer during a time when coating of a current raw material layer starts to a time when coating of a subsequent raw material layer starts. In other words, the irradiation power may be increased such that the temperature of the uppermost layer is maintained within the second predefined range.

[0034] Further, the above-described measures for maintaining the temperature of the uppermost layer in closed-loop operation may be carried out in a current uppermost layer of the work piece, on which temperature measurements are carried out. However, the measures for maintaining the temperature may also be carried out in a subsequent layer, i.e., in a layer following the layer in which the temperature measurements have been carried out.

[0035] The method may further comprise calculating an amount of heat introduced into the uppermost layer via the at least one energy beam, calculating an amount of heat leaving the uppermost layer during irradiation of the uppermost layer, if the amount of heat introduced into the uppermost layer is larger than the amount of heat leaving the uppermost layer, introducing the period of reduced irradiation power during which an additional amount of heat can leave the uppermost layer, and / or if the amount of heat leaving the uppermost layer is larger than the amount of heat introduced into the uppermost layer, heating the uppermost layer of the workpiece and / or the raw material layer at the top portion of the workpiece such that an additional amount of heat is introduced into the uppermost layer.

[0036] Calculating the amount of heat introduced into the uppermost layer and the amount of heat leaving the uppermost layer may be part of an open-loop control of the temperature of the uppermost layer. The process may therefore be carried out without measuring the temperature of the uppermost layer. However, measuring the temperature of the uppermost layer may provide an additional step of checking whether the performed control was correct and led to the desired maintaining of the temperature of the uppermost layer.

[0037] Hence, a combination of open-loop and closed-loop control may be implemented. For example, an open-loop control may be carried out for one or more initial iteration steps and, after that, a closed-loop control may take over. The calculating of the amount of heat introduced into the uppermost layer and / or the calculating of the amount of heat leaving the uppermost layer may be carried out by using a mathematical model and / or an artificial intelligence engine. Input parameters for the mathematical model and / or the artificial intelligence engine may be one or more of a geometry of the uppermost layer to be irradiated, an area of the uppermost layer to be irradiated, a power of the energy beam (e.g., a laser power), a beam profile of the energy beam, a number of energy beams irradiating the uppermost layer, a geometry of underlying solidified workpiece layers, a presence of a gas flow, a speed of a gas flow, etc.

[0038] The amount of heat introduced into the uppermost layer may be the amount of heat introduced by the at least one energy beam during irradiation of the entire uppermost layer. The amount of heat leaving the uppermost layer may be the amount of heat leaving the uppermost layer from a beginning of the irradiation of the uppermost layer to the beginning of irradiation of a subsequent layer. This period may correspond to an irradiation time plus a fire-to-fire time, during which the subsequent layer is coated. More precisely, the amount of heat leaving the uppermost layer may be the amount of heat leaving the uppermost layer without any period(s) of reduced irradiation power being considered. Similarly, the amount of heat introduced into the uppermost layer may be the amount of heat introduced solely by irradiation via the at least one energy beam without any additional heating (e.g., by a top layer heating device) being considered. Thus, the calculated amount of heat introduced and / or leaving the uppermost layer may be calculated without considering any measures to introduce / remove additional heat.

[0039] The predefined waiting period may have a predefined time duration (indicated, e.g., in seconds or milliseconds). The predefined time duration may be calculated such that, during this time duration, the additional amount of heat can leave the uppermost layer. For this purpose, a mathematical model may be used.

[0040] If the amount of heat introduced into the uppermost layer is larger than the amount of heat leaving the uppermost layer, the additional amount of heat leaving the uppermost layer may correspond to the difference of 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 larger 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 of the amount of heat leaving the uppermost layer and the amount of heat introduced into the uppermost layer.

[0041] In this way, it can be ensured that a sum of heat introduced into the uppermost layer is equal to a sum of heat leaving the uppermost layer. In other words, a net sum of heat introduced into the uppermost layer, e.g., for each considered uppermost layer, is zero. This may also be referred to as net-zero concept.

[0042] The heating of the uppermost layer of the workpiece may be carried out via heat radiation.

[0043] To this end, a respective heating device (top layer heating device) may be provided. This heating device may be stationary or movable. It is, e.g., possible to provide one or more additional laser beams that carry out the heating of the uppermost layer with laser radiation having a laser power that is not sufficient to generate a melt pool. Further, infrared radiation may be used to heat the uppermost layer.

[0044] The heat radiation may be radiated by a device attached to a powder coating device configured to apply the uppermost layer of the workpiece.

[0045] The heating device may therefore be a top layer heating device configured to be moved together with the powder coating device. For example, the additional heat may be introduced into the uppermost layer at the time the raw material of the uppermost layer is coated via the coating device. However, coating and heating may also be carried out at different times.

[0046] The period of reduced irradiation power may be a predefined waiting period during which no irradiation of raw material is carried out.

[0047] In other words, during the predefined waiting period, all energy beams may be switched off (e.g., via a shutter) or may be guided into a beam trap. Further, a so- called ghost part may be added to the build data, which corresponds to a non-irradi- ated (and, therefore, virtual) workpiece.

[0048] The period of reduced irradiation power 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 number of active energy beams, laser power, scanning speed, and beam profile. During the period of reduced irradiation power, the irradiation power introduced into the uppermost layer may be larger than zero but smaller than without introducing the period of reduced irradiation power. In other words, during the period of reduced irradiation power, not the complete irradiation of the uppermost layer has to be switched off as it is the case for the predefined waiting period. It is also possible that the irradiation power is reduced during this period, in particular, in the following way: Irradiating the uppermost layer with reduced irradiation power may mean that irradiation of the uppermost layer takes a predefined time longer than without the period of reduced irradiation power. The period of reduced irradiation power may be calculated such that the amount of heat that can leave the uppermost layer during the predefined time corresponds to the (calculated) additional amount of heat.

[0049] The period of reduced irradiation power may be introduced between a time when an irradiation of the uppermost layer is finished and a time when a powder coating of a next layer on top of the uppermost layer begins, between a time when a coating of a next layer on top of the uppermost layer is finished and an irradiation of the next layer begins, or during irradiation of the uppermost layer.

[0050] In other words, there are a plurality of options for introducing one or more periods of reduced irradiation power. The period of irradiation power may also be split into two or more periods. For example, one period is introduced after coating and before irradiation and one period is introduced after irradiation and before coating of a next layer. Further, one or more periods of reduced irradiation power may be introduced between consecutive irradiation vectors (e.g., of a hatch pattern). For introducing one or more periods of reduced irradiation power, multiple options exist, such as switching off one or more energy beams at one or more predefined times before, during, or after irradiation of the uppermost layer.

[0051] The temperature of the uppermost layer of the workpiece may be maintained via a closed-loop control.

[0052] The temperature of the uppermost layer may be measured. Based on the measured temperature, it may be decided whether a period of reduced irradiation power is introduced or whether the uppermost layer is heated. A set temperature of the closed- loop control may correspond to the set temperature of the lowermost part of the workpiece (e.g., a set temperature of a carrier on which the workpiece is built). For example, in case the measured temperature is below a lower limit of the first predefined range, additional heating is carried out, e.g., via a top layer heating device. Further, in case the measured temperature is above an upper limit of the first predefined range, a period of reduced irradiation power is introduced.

[0053] The method may be carried out by an additive manufacturing device and, in particular, by a device for selective laser melting or selective laser sintering.

[0054] The additive manufacturing device may comprise all the usual components of such a device, 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 a corresponding circulation device (e.g., a fan or blower), one or more filters, etc.

[0055] The method may further comprise determining the predefined set temperature to define a microstructure during a build process.

[0056] In this way, a cooling rate may be defined and the microstructure of the workpiece can be controlled during a cooling down process of the workpiece. In particular, during this cooling down process, the predefined set temperature may be controlled dynamically, i.e., it may change during the cooling down process. More precisely, the predefined set temperature may comprise a predefined time-dependent set temperature that is set, e.g., by a control unit of the additive manufacturing device.

[0057] According to a second aspect, a device for temperature control of a three-dimensional workpiece generated via additive manufacturing is provided. The device comprises a control unit and an irradiation device for irradiating at least one energy beam. The control unit is configured to maintain a temperature of a lowermost part of the workpiece during a build process of the workpiece within a first predefined range including a predefined set temperature, instruct the irradiation device to irradiate at least one energy beam into a raw material layer at a top portion of the workpiece to solidify the irradiated raw material and to form an uppermost layer of the workpiece, and maintain a temperature of the uppermost layer of the workpiece such that the temperature of the uppermost layer of the workpiece is within a second predefined range including the predefined set temperature.

[0058] All of the above aspects and details discussed with regard to the method aspect (first aspect) may apply to the device aspect. In particular, the device of the second aspect may be configured to carry out the method of the first aspect, wherein one or more of the details discussed above with regard to the method aspect are implemented. The device may further comprise a carrier heating device, wherein maintaining the temperature of the lowermost part of the workpiece comprises instructing the carrier heating device to heat a carrier, on which the workpiece is built, in particular to a temperature within the first predefined range.

[0059] The control unit may be further configured to determine the predefined set temperature based on build data defining a geometry of the workpiece to be built.

[0060] The device may further comprise a top layer heating device, wherein maintaining the temperature of the uppermost layer of the workpiece comprises at least one of instructing the top layer heating device to heat the uppermost layer of the workpiece and / or the raw material layer at the top portion of the workpiece and introducing a period of reduced irradiation power.

[0061] The control unit may be further configured to calculate an amount of heat introduced into the uppermost layer via the at least one energy beam, calculate an amount of heat leaving the uppermost layer during irradiation of the uppermost layer, if the amount of heat introduced into the uppermost layer is larger than the amount of heat leaving the uppermost layer, introduce the period of reduced irradiation power during which an additional amount of heat can leave the uppermost layer, and / or if the amount of heat leaving the uppermost layer is larger than the amount of heat introduced into the uppermost layer, instruct the top layer heating device to heat the uppermost layer of the workpiece and / or the raw material layer at the top portion of the workpiece such that an additional amount of heat is introduced into the uppermost layer.

[0062] If the amount of heat introduced into the uppermost layer is larger than the amount of heat leaving the uppermost layer, the additional amount of heat leaving the uppermost layer may correspond to the difference of 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 larger 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 of the amount of heat leaving the uppermost layer and the amount of heat introduced into the uppermost layer.

[0063] The heating device may be configured to carry out the heating of the uppermost layer of the workpiece via heat radiation. The top layer heating device is attached to a powder coating device configured to apply the uppermost layer of the workpiece.

[0064] The period of reduced irradiation power may be a predefined waiting period during which no irradiation of raw material is carried out.

[0065] The period of reduced irradiation power 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 number of active energy beams, laser power, scanning speed, and beam profile.

[0066] The period of reduced irradiation power may be introduced between a time when an irradiation of the uppermost layer is finished and a time when a powder coating of a next layer on top of the uppermost layer begins, between a time when a coating of a next layer on top of the uppermost layer is finished and an irradiation of the next layer begins, or during irradiation of the uppermost layer.

[0067] The control unit may be configured to maintain the temperature of the uppermost layer of the workpiece via a closed loop control.

[0068] The device may be an additive manufacturing device and, in particular, a device for selective laser melting or selective laser sintering.

[0069] The control unit may be configured to determine the predefined set temperature to define a microstructure during a build process.

[0070] Preferred embodiments of the invention are described in greater detail with reference to the appended schematic drawings, wherein

[0071] Fig. 1 shows a schematic side view of an additive manufacturing device with one laser beam, according to an embodiment of the present disclosure;

[0072] Fig. 2 shows a schematic side view of an additive manufacturing device with two laser beams, according to an embodiment of the present disclosure;

[0073] Fig. 3 shows a detail of a build cylinder with a carrier and a workpiece with supports that can be applied to embodiments of the present disclosure; Fig. 4 shows a flowchart of a method according to an embodiment of the present disclosure; and

[0074] Fig. 5 shows a control unit with modules according to an embodiment of the present disclosure.

[0075] Fig. 1 shows a schematic representation of an additive manufacturing device 10 for producing a three-dimensional workpiece 12. The device 10 is, in general, apart from the specific method for temperature control, programmed into a control unit 40 of the device 10, and some optional details, well-known to the person skilled in the art. The device 10 may be, e.g., a typical additive manufacturing apparatus, wherein the method for temperature control, according to the present disclosure, is programmed into a control unit 40 of the device 10. Optionally, some additional structural elements may be provided, such as a top layer heating device, which is not necessarily common for typical additive manufacturing devices.

[0076] The principles of the device 10 are well-known to the person skilled in the art in the field of additive manufacturing and will only be described briefly. For example, such a device 10 may be a device for selective laser melting or a device for selective laser sintering, wherein one or more laser beams 14 may be used for selectively irradiating and solidifying subsequent layers of raw material powder.

[0077] The device 10 for carrying out a process of selective laser melting as described below may serve as an example. Typical features of powder bed fusion are that a raw material powder is applied in layers and each layer is selectively irradiated and solidified in order to generate one layer of a workpiece 12 to be produced. After removing excess powder, and after optional steps of post processing (e.g., removing one or more support structures), the final workpiece 12 is obtained.

[0078] It should be noted that the technique of the present disclosure is not limited to powder bed fusion techniques and that also liquid or granulate raw material may be used. Further, 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.) may be used, which is capable to solidify the used raw material powder. However, according to the specific embodiments described below, the raw material is a raw material powder (e.g., metallic powder, ceramic powder, or plastic powder) and the at least one energy beam that solidifies the raw material powder is a laser beam. Fig. 1 shows a device 10 for producing a three-dimensional workpiece 12 by selective laser melting. The device 10 comprises a process chamber 16. The process chamber 16 is sealable against the ambient atmosphere, i.e. against the environment surrounding the process chamber 16. A powder coating device 18 (also referred to as powder application device), which is arranged in the process chamber 16, serves to apply a raw material powder onto a carrier 20. To this end, the powder coating device 18 may 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 layer of raw material. In this context, uniform particularly means that the layer has a uniform thickness. The layer thickness can be chosen, e.g., by vertically positioning the powder coating device 18 and / or the carrier 20.

[0079] A vertical movement unit 22 is provided, such that the carrier 20 can be displaced in a vertical direction so that, with increasing construction height of the workpiece 12, as it is built up in layers from the raw material powder on the carrier 20, the carrier 20 can be moved downwards in the vertical direction.

[0080] 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 herein. As an alternative to the movable carrier 20, the carrier 20 may be provided as stationary (or fixed) carrier (in particular, with regard to the vertical z-direction), wherein the irradiation device 24 (see below) and the process chamber 16 are configured to be moved upwards during the build process (i.e., with increasing construction height of the workpiece 12). Further, both the carrier 20 and the irradiation device 24 may be individually movable along the z-direction.

[0081] A carrier surface of the carrier 20 defines a horizontal plane (an x-y-plane), wherein a direction perpendicular to said plane is defined as a vertical direction or build direction (z-direction). Hence, each uppermost layer of raw material powder and each layer of the workpiece 12 extend in a plane parallel to the horizontal plane (x-y- plane) defined above.

[0082] The apparatus 10 further comprises a gas inlet 26 for supplying an inert gas (e.g., argon) into the process chamber 16. Further, a gas outlet is provided, such that a continuous (horizontal) stream of gas may be generated through the process chamber 16 by implementing a gas circuit. In a preferred embodiment, a unidirectional laminar gas flow is generated over the uppermost raw material powder layer. The gas flow generated by the gas inlet 26 and gas outlet 27 may serve to take away melt spatter and / or other undesired dirt particles from the uppermost layer of the workpiece 12 in order to improve a quality of the workpiece 12. The gas flow also removes heat from the workpiece 12 via convection.

[0083] Further, a temperature measurement device 28 is arranged in the process chamber 16, for determining a temperature of an uppermost layer 13 of the workpiece 12. The device 28 is optional and may only be used for closed-loop control (see the below detailed description). The temperature measurement device 28 may comprise a pyrometer configured to detect thermal radiation emitted from the uppermost layer 13 of the workpiece 12. In particular, the temperature measurement device 28 may be configured to generate a heat map of the uppermost layer 13, such that for each position of the uppermost layer 13, a temperature value is determined. The temperature measurement device 28 provides the determined temperature values to the control unit 40, such that the control unit can further process them, e.g., by generating a time series of temperature values and / or by determining an average temperature value.

[0084] 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 desired times and / or desired locations in order to obtain one or more temperature values indicating a temperature of the uppermost layer 13.

[0085] The device 10 further comprises an irradiation device 24 for selectively irradiating the laser beam 14 onto the uppermost layer of raw material powder applied onto the carrier 20. By means of the irradiation device 24, the raw material powder applied onto the carrier 20 may be subjected to laser radiation in a site-selective manner in dependence on the desired geometry of the workpiece 12 that is to be produced. 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.

[0086] The irradiation unit 24a comprises a scanning unit 30 configured to selectively irradiate the laser beam 14 onto the raw material powder applied onto the carrier 20. The scanning unit 30 is controlled by a control unit 40 of the device 10. The scanning unit 30 may comprise one mirror tiltable with regard to two perpendicular axes. Alternatively, the scanning unit 30 may comprise two tiltable mirrors, each configured to be tilted with regard to a corresponding axis. The tiltable mirrors may be, e.g., galvanometer mirrors. The irradiation unit 24a is supplied with laser radiation from a laser beam source 32. The laser beam source 32 may be provided within the irradiation unit 24a or outside the irradiation unit 24a, as shown in Fig. 1. In the first case, the laser beam source 32 may be regarded as being part of the irradiation unit 24a. In the latter case, the laser beam is generated by the laser beam 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 the air or through a vacuum, e.g., by using one or more mirrors.

[0087] From the laser beam source 32, the laser beam is directed to the scanning unit 30. The laser beam source 32 may, for example, comprise a diode pumped Ytterbium fiber laser emitting laser light at a wavelength of approximately 1070 to 1080 nm (i.e., in the infrared wavelength range).

[0088] The irradiation unit 24a further comprises two lenses 36 and 38, which are configured to focus the laser beam 14 onto a desired focus position along the z-axis. In the embodiment shown in Fig. 1, both lenses 36 and 38 have positive refractive power. The lens 38 further upstream of the beam path is configured to collimate the laser light emitted by the fiber 34, such that a collimated or substantially collimated laser beam is generated. The lens 36 further downstream of the beam path is configured to focus the collimated (or substantially collimated) laser beam onto a desired z-posi- tion.

[0089] A top layer heating device 42 is provided in the process chamber 16. The top layer heating device 42 is configured to heat a top layer (i.e., an uppermost layer 13) of the currently built workpiece 12 and / or to 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 via heat radiation. An amount of heat radiation irradiated by the top layer heating device 42 can be controlled by the control unit 42. The top layer heating device 42 is, e.g., configured to emit thermal radiation (infrared radiation) to the uppermost layer 13 in order to heat the uppermost layer. Heating, as discussed herein is equal to introducing heat, in particular, a predefined amount of heat.

[0090] The top layer heating device 42 is attached to the powder coating device 18. The top layer heating device 42 horizontally moves together with the powder coating device 18. In this way, powder coating and heating may be carried out at the same time. However, powder coating and heating may be carried out at different times but a horizontal movement device provided for the powder coating device 18 may be shared with the top player heating device 42.

[0091] Alternatively, the top layer heating device 42 may be provided with an individual (horizontal and / or vertical) movement device, such that it can be independently moved with regard to the powder coating device 18.

[0092] Further, other types of top layer heating devices may be implemented. For example, a contact top layer heating device may be provided, which heats the uppermost layer 13 while contacting it. Further, e.g., one or more laser beam sources may be provided as top layer heating device, wherein the laser beam(s) generated by the one or more laser sources can be directed to predefined sections of the uppermost layer (without melting the uppermost layer) in order to introduce heat. Still further, the at least one irradiation beam 14 carrying out the build job may carry out the (extra) heating of the uppermost layer. In this case, the control unit 40 increases an irradiation power of the irradiation beam 14 by a predefined amount.

[0093] A carrier heating device 44 is integrated into the carrier 20. The carrier heating device may be also provided below or at one or more sides of the carrier. It should be noted that the carrier 20 itself may comprise a plurality of plates and / or other structures. The plurality of plates may be referred to as plate package. An uppermost plate of these plates may be referred to as substrate plate. The carrier heating device 44 may be provided in one of the plates of the plate package or between two plates of the plate package.

[0094] The carrier heating device 44 is configured to heat the carrier 44 to a predefined temperature. For this purpose, a temperature sensor (not shown) may be integrated into the carrier 20 or may contact the carrier 20 for measuring a temperature of the carrier 20. In this way, a closed-loop temperature control of the carrier 20 may be carried out, e.g., by the control unit. When the temperature drops below a lower limit (of a second predefined range), the carrier heating device 44 introduces heat into the carrier 20 and when the temperature raises above an upper limit (of a second predefined range), the heating is stopped. However, other methods of maintaining a temperature of the carrier 20 are possible. For example, predefined heating and / or non-heating times and / or predefined heating power values may be stored in a look-up table for a plurality of temperature values. In case a particular temperature value shall be set (as set temperature), the heating times and / or non-heating times and / or the heating power may be read from the look-up table (open-loop temperature control of the carrier).

[0095] The top layer heating device 42 as well as the carrier heating device 44 are optional. That means, one or more of these devices may be omitted and the technique can still function. For example, in case a suitable set temperature is chosen and only periods of reduced irradiation power are introduced, no additional heating via heating device 42 is necessary. Further, the carrier heating device 44 may be omitted in case a temperature of a lowermost part 15 of the workpiece is maintained without introducing additional heat via a carrier heating device 44. For example, as shown in Fig. 3, this may be the case when a suitable support structure for the workpiece 12 is chosen.

[0096] The control unit 40 comprises a processor and a memory, wherein, on the memory, instructions are stored for controlling the individual components of the device 10. For example, the control unit 40 may be configured to control one or more of the temperature measurement device 28, the vertical movement unit 22, the powder coating device 18, a 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. A user input and output interface may be provided and connected or connectable to the control device 40. Further, the control unit 40 has an interface to receive workpiece data representative of a three-dimensional shape of the workpiece 12 to be produced.

[0097] It should be noted that the position of the control unit 40 in Figures 1 and 2 is purely schematic and it is not limiting. The control unit 40 may be located at any suitable position of the device 10 or even remote from it (e.g., integrated into a network structure such as a LAN). Further, the control unit 40 or at least part of the control unit 40 may be provided in the form of a cloud computing device.

[0098] Fig. 2 shows a different embodiment of a device 10, similar to the device 10 of the embodiment of Fig. 1. As only difference between the two devices 10, the irradiation device 24 of the device 10 of Fig. 2 comprises two irradiation units 24a and 24b instead of the one irradiation unit 24a of the apparatus 10 of Fig. 1. However, the rest of the device 10 of Fig. 2 has the same components and functions as those discussed above with regard to Fig. 1, such that a repetition of this description is omitted. Further, the components of the irradiation units 24a and 24b have the same reference signs as those of Fig. 2. However, suffixes "a" and "b" are used in order to distinguish between components of the irradiation unit 24a (suffix a) and the components of the further irradiation unit 24b (suffix b). The function of the individual components within the irradiation units 24a and 24b is the same as discussed above with regard to the irradiation device 24 of Fig. 1.

[0099] In the following, the use of a reference sign without suffix (a or b) also refers to the respective elements with the suffixes a and b, if not explicitly indicated otherwise. For example, when it is referred to the "scanning unit 30", it is thereby also referred to the scanning units 30a and 30b.

[0100] The device 10 of Fig. 2 is configured such that the irradiation unit 24a is configured to scan a first predefined region (i.e., a first scan field) of an uppermost powder layer. Similarly, the further irradiation unit 24b is configured to scan a second predefined region (i.e., a second scan field) of the uppermost powder layer. The first scan field and the second scan field overlap each other in an overlap area. In other words, there is a region of the uppermost powder layer that can be reached and selectively irradiated by both laser beams 14a and 14b (i.e., the overlap area). The first scan field and the second scan field may be each rectangular or circular and a size and / or shape of the respective scan field may be predefined by a range of movement of the respective scanning unit 30a and 30b of the irradiation units 24a and 24b, respectively. Further, it is possible that the overlap area covers the entire uppermost powder layer, such that each of the laser beams 14a and 14b may reach every point on the uppermost powder layer.

[0101] For producing the three-dimensional workpiece 12, both laser beams 14a and 14b can simultaneously irradiate different sections of the same powder layer, wherein each laser beam 14a and 14b irradiates a section of the workpiece 12 in its corresponding scan field. In this way, the workpiece 12 can be built up faster than in a case where only one laser beam 14 is used (see, e.g., Fig. 1). Further, different laser beams 14a and 14b may be used, e.g., for solidifying a shell portion and a core portion of the workpiece 12. The different laser beams have different beam parameters, such as laser power, wavelength, and / or beam profile.

[0102] For example, during a time of reduced irradiation power, one of the laser beams 14a, 14b may be switched off. Fig. 3 shows a detail of a build cylinder (i.e., the volume in which the carrier 20 is moved), which can be used for both devices shown in Fig. 1 and / or Fig. 2. As an alternative or as an addition to the carrier heating device 44, the temperature of the lower part 15 of the workpiece 12 can be maintained by calculating and building a suitable carrier structure 46. In other words, a mathematical model may be provided according to which the temperature of the lower part 15 of the workpiece 12 can be determined and suitable set by building 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 part 15 maintains a temperature within a second predefined range, although additional heat is introduced by the laser beam(s) 14 to the uppermost layer 13 of the workpiece 12.

[0103] In the following, the method for temperature control according to the present disclosure is explained in further detail. The method is carried out by one of the devices 10 of Fig. 1 and Fig. 2 (and, optionally, according to Fig. 3). In particular, main parts of the method are carried out by the control unit 40 of the respective device.

[0104] In order to build a workpiece 12 via selective laser melting technology, optical power (i.e., laser power) has to be introduced into raw material powder (e.g., metal powder). The raw material powder melts and forms a workpiece 12 after solidifying. The optical power introduced into the process chamber 16 is partly reflected from the powder and the melt pool generated by the laser beam 14; the rest of the optical power reaches the workpiece 12. From there, a plurality of heat sinks exist: radiation from the work piece surface (i.e., from its uppermost layer 13), primary into walls of the process chamber 16, convection into the gas flow (generated by gas outlet 26 and gas inlet 27), metal spatter carried away, which condense in the gas flow, heat transfer through the workpiece 12 into the carrier 20 such as heat loss through the powder into the build cylinder. The distribution of the heat between the heat sinks is process-dependent. Relevant factors are, e.g., the used material of the raw material powder, the material parameter, and the workpiece geometry (inter alia, irradiation surface). For materials having low thermal conductivity (i.e., low heat transfer), in particular, the following facts apply, dependent from a workpiece height (i.e., extension along the z-axis):

[0105] For low (small extension in z-direction) workpieces 12, the thermal resistance, from the uppermost layer 13 to the carrier 20, of the workpiece 12 is small, such that a relatively large amount of heat reaches the carrier 20 via heat transfer through the workpiece 12 (i.e., in z-direction). For high (large extension in z-direction) workpieces 12, the thermal resistance of the workpiece 12 is high, such that a low amount of heat is transferred through the workpiece 12 and reaches the carrier 20. In other words, in order to transfer the same thermal energy, a significantly larger temperature difference is necessary, which may lead to problems during the process.

[0106] The thermal resistance of the workpiece 12 leads, physically, always to a temperature gradient within the workpiece 12, relative to a temperature of the carrier 20, as long as optical power is introduced into the workpiece 12. A temperature gradient is undesired, since it may lead to material tension and may impair material properties (in dependence from how large the gradient is). The two mentioned factors directly proportionally determine how large the temperature gradient is: Firstly, it is larger, the larger the thermal resistance is, i.e., the worse the thermal conductivity of the material is, the higher (z-direction) the workpiece 12 is, and the more unfavorable the workpiece geometry is (e.g., due to narrow portions). Secondly, the temperature gradient is larger, the more heat is introduced into the workpiece 12.

[0107] In particular the second factor may be controlled by the technique of the present disclosure.

[0108] Net-zero concept: The net-zero concept described herein is used in order to maintain the "net" heat introduced into the workpiece 12 during the course of forming one layer at zero. When this goal is achieved, no temperature gradient exists, such that at least the aforementioned problems and other related problems may be solved. Under the assumption that heat loss via the (unsolidified) powder is negligible (which is justified due to relatively low heat conductivity of powder), the above formulation is equivalent to consider the net energy balance of the uppermost layer 13 over one layer. Thus, in order to fulfil the net-zero concept, the following shall apply:

[0109] In the above equation, E is the energy in the uppermost layer 13 and dE / dt is its variation over time. This is integrated over the duration of a layer, i.e., from a beginning time tn of a layer n to a beginning time tn+i of a layer n+1. Qin is the heat introduced during the course of a layer into the uppermost workpiece layer 13, Qout is the heat leaving the uppermost layer 13. Variables indicated with "dot" are heat flows, i.e., the respective derivation with respect to time. In order to generate (i.e., print) the workpiece 12, in any case, optical power has to be introduced into the workpiece 12 and the uppermost powder layer, respectively, since powder has to be melted. This input heat Qin depends on the build parameter, the irradiated area and further factors. It can be calculated, on the basis of these factors, e.g., at the beginning of a build process with relatively high precision (relative error less than 5 %). The person skilled in the art knows how to calculate the input heat Qin on the basis of one or more input parameters such as laser power, beam profile, irradiation time, irradiated area, etc. The process of calculating the input heat will therefore not be explained in further detail in the present disclosure.

[0110] In order to achieve the net-zero, it has to be ensured that the heat Qout leaving the uppermost layer 13 is equal to the input heat Qin. As mentioned above, the heat sinks available for this purpose are radiation from the workpiece 12, metal spatter, as well as convection into the gas flow. In particular the heat radiation and the convection depend on the top layer temperature, i.e., the temperature of the uppermost layer 13 of the workpiece 12.

[0111] Fluctuations in the optical load: In build processes (build jobs), the optical load fluctuates, since process parameters such as the irradiated area or the build parameter change, which has a direct influence on the optical load. With regard to the present disclosure, this means that the input heat Qin changes, such that also the output heat Qout has to change in order to meet the net-zero concept. Amongst others, the following options exist in order to influence the input and / or output heat Qin / Qout: a) Heating of the uppermost layer 13 of the workpiece 12 and / or the raw material layer at the top portion of the workpiece 12, e.g., via the top layer heating device 42 (also referred to herein as "RadHeat"). This has direct influence on Qin. b) Additional periods of reduced irradiation power, in particular, additional waiting periods. This has influence on Qout. c) Additional workpieces. This has influence on Qin and Qout. d) Adaption of parameters, in particular, changing a laser power of the laser beam 14, while other parameters (in particular, all other parameters) remain unchanged. Or: change between different qualified parameters and / or parameter sets. This has influence on Qin. e) Changed and / or additional support structure(s). This has influence on a workpiece resistance and, thus, on the temperature of the uppermost layer of the workpiece 12 and, thus, indirectly on Qout. Constant workpiece temperature: The net-zero concept ensures that, during the course of a layer, no change of the energy in the respective uppermost layer 13 takes place and, thus, is 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 shall be kept constant is referred to herein as net-zero temperature or set temperature. The set temperature can be set by a user, either based on the user's experience or based on a calculation made by the user, optionally with the help of a mathematical mode. It is also possible that the set temperature is automatically determined and set by the control unit 40 of the device 10, on the basis of a mathematical model. Examples of determining and setting the set temperature will follow.

[0112] The above parameter specifications, that are needed to achieve the net-zero temperature may be determined before the build process on the basis of a mathematical mode (open-loop control). However, it may be favorable that the set temperature is achieved and / or maintained via closed-loop control. A model that estimates the above influences (e.g., heating of the uppermost layer 13, periods of reduced irradiation power, changed parameter, etc.) may provide a first model-based input for the temperature control (initial value). During the process, the temperature of the uppermost layer is ideally measured and the control is adapted accordingly (such that the set temperature is maintained).

[0113] It is crucial for the present disclosure that the set temperature of the lowermost part 15 of the workpiece 12 corresponds to the set temperature of the uppermost layer 13. In other words, a first temperature range, in which the temperature of the lowermost part 15 of the workpiece 12 is maintained includes the same set temperature as a second temperature range in which the temperature of the uppermost layer 13 of the workpiece 12 is maintained. In still other words, the concept of the present disclosure aims at providing the same set temperature at the lowermost part 5 and at the uppermost layer 13 of the workpiece 12. In this way, a temperature gradient through the workpiece (along the z-axis) can be reduced or at least avoided. In one embodiment, a set temperature of a carrier heating device 44 corresponds to a set temperature of the uppermost layer 13.

[0114] In the following, detailed examples of a temperature control are provided, wherein the present technique is not limit to these examples. Example 1 (prior art): A build process is carried out with the material IN718 and with a high-power parameter ranging from 5 to 40 % irradiated area (i.e., 5 to 40 % of a possible total area that can be irradiated, i.e., a total irradiation area) and is 600 mm high (along the z-axis). For IN718, a pre-heating temperature of 200°C may be used. For 40 % irradiated area, an optical load (power) of 8 kW is introduced into the process chamber 16, wherein approximately 4 kW reach the workpiece 12. Without the technique disclosed herein, the large height of the workpiece 12 to be build would lead to a strong influence of the thermal resistance of the workpiece 12. Further, IN718 has a bad heat conductivity, which enhances the aforementioned effect. As a consequence, the temperature of the workpiece 12, near the carrier 20, almost corresponds to the pre-heating temperature of 200°C and would be much higher in upper layers. A rough estimation without consideration of the heat sinks process chamber 16 and gas flow, which has been done for a specific additive manufacturing device 10, leads to temperatures of 1000°C for the uppermost layer 13. This means that the temperature gradient would be larger than 800 K from a lowermost part to an uppermost part of the workpiece 12. This would lead to massive differences in the material properties and in the extension (size accuracy).

[0115] Solutions with net-zero concept, via heating of an uppermost layer 13 and / or with additional waiting times: IN718 can withstand relatively high temperatures. As net- zero temperature (set temperature), 600°C is chosen. At this temperature, (thermal) radiation from the workpiece 12 plays an important role, i.e., it is significantly strong. Further, also convection into the gas flow grows in importance. This leads, according to an example calculation based on a mathematical model, to the fact that the radiation and convection term, in sum with other heat losses from the uppermost layer 13, is calculated as 3,5 kW. When the job is built with 40 % irradiated area, e.g., 4 KW is introduced for 60 s into the workpiece 12, thus, an energy of 240 kWs. Due to the temperature of 600°C, 3,5 kW of heat is lost during the duration of the layer, which means the irradiation time plus a fire-to-fire time. The fire-to-fire time is defined as the time it takes from the end of irradiation of one layer to the beginning of irradiation of a next layer. During the fire-to-fire time, e.g., the next layer is coated onto the previous layer.

[0116] With a fire-to-fire time of approximately 8,5 s, (60 s + 8,5 s) * 3,5 kW = 239,75 kWs are lost, such that the amounts of heat are approximately the same (i.e., the amount of introduced heat Qin = 240 kWs and the amount of lost heat Qout = 239,75 kWs is approximately the same. However, in a machine with a shorter fire-to-fire time of, for example, 7 s, an additional waiting period of 1,5 s would be necessary for 40 % irradiated area.

[0117] Hence, this additional waiting time can be introduced at any time of the process of processing the current layer (uppermost layer 13). For example, the additional waiting time of 1,5 s is introduced at the end of the irradiation and before the fire-to-fire time (i.e., before the next layer is coated).

[0118] Further, a closed-loop control may support the above open-loop control. In closed- loop, the temperature measurement device 28 measures the temperature of the uppermost layer 13 of the workpiece 12. In case the measured temperature is still larger than 600°C after 1,5 s of additional waiting time, a further additional waiting period may be introduced.

[0119] For a layer with less than 40 % irradiated area, the optical load is significantly smaller. For example, an uppermost layer 13 of 5 % irradiated area is considered with an optical load of approximately 2 kW into the process chamber 16, which means approximately 1 kW into the workpiece 12. Here, the irradiation duration is only 20 s. Thus, only 20 kWs of heat is introduced into the workpiece 12. Due to the net-zero temperature of 600°C, 3,5 kW are lost (as in the previous example). With the fire-to-fire time of 7 s, this leads to (20 s + 7 s) * 3,5 kW = 94,5 kWs of lost heat. This difference in heat amounts can be compensated via heating the uppermost layer 13 with the top layer heating device 42. In the present case, it has to irradiate an amount of heat of 94,5 kWs - 20 kWs = 74,5 kWs. In this case, it is assumed that the top layer heating device 42 does not need any extra time since it moves together with the powder coating device 18 and the heating is thus carried out during the fire-to-fire time.

[0120] It can be summarized that the carrier 20 is heated to 600°C and this temperature is maintained via the carrier heating device 44. The workpiece 12 therefore has a temperature of 600°C close to the carrier 20, i.e., in its lowermost part 15. The desired top layer temperature (set temperature) of 600°C is achieved via control of the temperature of the uppermost layer 13 by introducing additional waiting times or by heating the uppermost layer 13. Thus, it can be ensured that the temperature in the top layer 13 of the workpiece 12 and in the lowermost part 15 of the workpiece 12 is maintained at the set temperature of 600°C. This temperature is maintained since the amount of introduced heat Qin and the amount of lost heat Qout is the same. Solution with net-zero concept and only additional waiting times: Similar to the above solution with heating of the uppermost layer, the heating may be avoided when a lower set temperature is chosen. In the present case, the set temperature is chosen to be 300°C. Thus, the carrier 20 is heated and maintained at 300°C. From a mathematical model, it is known that the workpiece 12 must have a temperature of 310°C before a new powder layer is coated, in order to have a temperature of 300°C after the coating process. After the irradiation of a layer, the temperature of the uppermost layer is measured (surveilled) during an additional waiting time. As soon as it is 310°C, the coating of the next layer is started. Due to this method, the temperature of the workpiece 12 in its uppermost layer 13 and in its lowermost section 15 is again identical, such that no gradient results. The additional waiting time is used for controlled cooling of the workpiece via radiation and convection. The waiting time is respectively so long that the introduced heat (e.g., 4 kW * 60 s at 40 % irradiated area) is equal to the heat losses into the gas flow and the walls of the process chamber 16. This solution might be easier to implement as compared to the above solution involving a top layer heating device 42, since no top layer heating device 42 (RadHeat) is needed. However, it might require longer waiting times. The waiting times are shorter the higher the net-zero temperature is set, but a higher net-zero temperature may be more difficult to achieve.

[0121] It should be noted that in the above examples, it is mentioned that the set temperature is maintained. However, keeping the set temperature exactly constant may not be possible. Hence, it is sufficient for the functioning of the present technique, that the temperature of the lowermost part 15 of the workpiece 12 is maintained within a first predefined temperature range and the temperature of the uppermost layer 13 of the workpiece 12 is maintained within a second predefined temperature range. Both the first and second predefined temperature range include the set temperature. For example, the set temperature may be in the middle of each of the first and second temperature range and the first and second temperature ranges may be identical. An extension of each of the first and second temperature range may be, e.g., 1 K, 2K, 5 K, 10 K, 20 K, 30 K, 40 K, or 50 K.

[0122] All combinations of the above options are possible and covered by the present technique. For example, when other parameters are used in dependence on the build height, the thermal load changes and, thereby, potential waiting times and the heat difference that has to be irradiated via the top layer heating device 42, etc. A further improvement is defined as follows: The above considerations are based on the assumption that the heat loss over the powder can be neglected. Only then, the temperature at the uppermost part and the lowermost part of the workpiece 12 is the same and no gradient exists. However, in reality, this heat loss is existent and may cause the workpiece 12 to be slightly colder in a center region of the workpiece 12. This effect can be avoided by providing a build cylinder heating. Hence, according to embodiments of the present technique, a build cylinder heating device is provided and the control unit 40 is configured to heat the build cylinder, via the build cylinder heating device, to the set temperature. In this case, no heat stream exists into the build cylinder. This concept is similar to maintaining a temperature of the carrier 20, but additionally, further system edges are brought to and maintained at the set temperature in order to avoid thermal gradients.

[0123] Choice of the set temperature: In principle, almost any temperature may be chosen as set temperature (net-zero temperature). However, an inappropriately set temperature may have disadvantages, e.g., regarding the required waiting time, the required additional workpieces (powder consumption), the required heat to be introduced (energy consumption), temperature resistance of elements of the device 10, etc. Suitable set temperatures may be determined empirically or model-based via experiments or studies, etc. More precisely, the set temperature may be calculated by the control unit 40 on the basis of input parameters, e.g., involving one or more of the used material, the geometry of the workpiece, etc.

[0124] Further, it is possible to solve a multi-dimensional optimization problem, which indicates, e.g., waiting times vs. energy consumption for different temperatures. A choice of a suitable set temperature can be made automatically by the control unit 40.

[0125] Fig. 4 shows a flowchart of a method for temperature control of a three-dimensional workpiece 12 generated via additive manufacturing, according to an embodiment of the present disclosure.

[0126] The method is carried out by an additive manufacturing device (such as the device 10 shown and discussed with regard to Figures 1 to 3) during a build process of a three-dimensional workpiece 12. The method starts with a step of maintaining 50 a temperature of a lowermost part 15 of the workpiece 12 during a build process of the workpiece 12 within a first predefined range including a predefined set temperature. According to the above description, there are two main options, how the temperature may be maintained. Firstly, a suitable support structure 46 is provided, which ensures that the temperature of the lowermost part 15 stays within the first predefined range during the build process. Secondly, the lowermost part 15 of the workpiece 12 is provided close to or directly on the carrier 20 of the device 10 and the carrier 20 is heated via a carrier heating device 44. In this second case, the heating and maintaining may be carried out in open-loop or closed-loop. In open-loop, heating times and / or a heating power are predefined and read from a look-up table (for the desired set temperature). The look-up table may be stored in a memory of the control unit 40. In closed-loop, the temperature of the carrier 20 is measured and the carrier heating device 44 is activated when it is too low and it is deactivated when it is too high. In this way, a constant temperature of the carrier 20 may be maintained, at least within the limits of the first predefined range.

[0127] In a step 52, at least one energy beam is irradiated into a raw material layer at a top portion of the workpiece 12 to solidify the irradiated raw material and to form an uppermost layer 13 of the workpiece 12. In this step, heat is necessarily introduced into the workpiece 12 in order to form at least one melt pool for solidifying the raw material at desired locations. Irradiation may take place with only one or with more than one energy beams, in particular laser beams.

[0128] In a step 54, a 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 predefined range including the predefined set temperature. The step of maintaining may be generally carried out by raising an amount of introduced heat and by raising an amount of lost heat. The first option can be carried out by heating the uppermost layer of the workpiece 12 and / or the raw material layer at the top portion of the workpiece 12 via a top layer heating device 42. The second option can be carried out by introducing one or more additional periods of reduced irradiation power, in particular, additional waiting times during which no irradiation is carried out.

[0129] Fig. 5 shows a schematic representation of the control unit 40 of one of the devices 10 of Fig. 1 or Fig 2. The control unit 40 comprises several modules 60 to 64, each of which may be represented in the form of hardware and / or software. In one example, the control unit 40 comprises a processor and a memory. On the memory, instructions are stored, which cause the processor to carry out the method shown in Fig. 4. For this purpose, the software stored on the memory may be considered to comprise the modules 62 to 64 shown in Fig. 5.

[0130] In detail, these modules are:

[0131] First maintaining module 60 for maintaining a temperature of a lowermost part of the workpiece during a build process of the workpiece within a first predefined range including a predefined set temperature.

[0132] Instructing module 62 for instructing the irradiation device to irradiate at least one energy beam into a raw material layer at a top portion of the workpiece to solidify the irradiated raw material and to form an uppermost layer of the workpiece.

[0133] Second maintaining module 64 for maintaining a temperature of the uppermost layer of the workpiece such that the temperature of the uppermost layer of the workpiece is within a second predefined range including the predefined set temperature.

[0134] The details discussed herein with regard to the individual method steps may also apply to the corresponding modules 60 to 64 of the control unit 40. In other words, the control unit 40 is configured to carry out the method of Fig. 4 above, wherein the details of this method discussed herein apply accordingly to the software of the control unit and / or to the corresponding modules shown in Fig. 5 and / or to additional modules.

[0135] According to some embodiments, advantages of the technique disclosed herein may comprise the following.

[0136] Equal expansion of the workpiece 12. From this, e.g., a predictable warpage follows. Usually, in the prior art, there is no almost constant temperature with which an inverse problem of optimal form finding of the workpiece 12 can be solved, such that initial calculations, which form a workpiece 12 shall have in order to have a desired shape in cooled down state, may be very difficult. With constant temperatures, this is possible. When no compensation of the expansion is carried out, workpieces are warped, since they are warmer in higher layers and, therefore, more expanded before they cool down. This problem may be eliminated or reduced with the present technique. Better material properties, since the desired set-temperature is (with regard to time and space) identical from a beginning to an end of the build job. Parameters, which are suitable for the first layer, are also suitable for the last layer of the job since not thermal difference exists.

[0137] Design freedom: From the combination of the above items, total design freedom results. In the prior art, it is not possible to build badly supported workpieces or workpieces that have a small irradiated area at a lower part and a large irradiated area at an upper part (upside-down pyramid). The problem is the same for both cases: The thermal resistance of the workpiece, in particular of the upper layers, to the main heat sink (the carrier) is so large that the heat cannot dissipate. Accordingly, the workpiece 12 gets very hot locally, expands, protrudes from the powder layer, which may lead to a crash. According to the present technique, this problem may be totally avoided when the temperature along the workpiece 12 is constant. The problem does not exist since, with the net-zero concept, the carrier 20 is no heat sink.

Claims

Claims1. A method for temperature control of a three-dimensional workpiece generated via additive manufacturing, the method comprising: maintaining a temperature of a lowermost part of the workpiece during a build process of the workpiece within a first predefined range including a predefined set temperature; irradiating at least one energy beam into a raw material layer at a top portion of the workpiece to solidify the irradiated raw material and to form an uppermost layer of the workpiece; and maintaining a temperature of the uppermost layer of the workpiece such that the temperature of the uppermost layer of the workpiece is within a second predefined range including the predefined set temperature.

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 predefined range.

3. The method of claim 1 or 2, further comprising: determining the predefined set temperature based on build data defining a geometry of the workpiece to be built.

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

5. The method of claim 4, further comprising: calculating an amount of heat introduced into the uppermost layer via the at least one energy beam; calculating an amount of heat leaving the uppermost layer during irradiation of the uppermost layer; if the amount of heat introduced into the uppermost layer is larger than the amount of heat leaving the uppermost layer, introducing the period of reduced irradiation power during which an additional amount of heat can leave the uppermost layer; and / orif the amount of heat leaving the uppermost layer is larger than the amount of heat introduced into the uppermost layer, heating the uppermost layer of the workpiece and / or the raw material layer at the top portion of the workpiece such that an additional amount of heat is introduced into the uppermost layer.

6. The method of claim 5, wherein, if the amount of heat introduced into the uppermost layer is larger than the amount of heat leaving the uppermost layer, the additional amount of heat leaving the uppermost layer corresponds to the difference of the amount of heat introduced into the uppermost layer and the amount of heat leaving the uppermost layer, and / or if the amount of heat leaving the uppermost layer is larger than the amount of heat introduced into the uppermost layer, the additional amount of heat introduced into the uppermost layer corresponds to the difference of the amount of heat leaving the uppermost layer and the amount of heat introduced into the uppermost layer.

7. The method of any of claims 4 to 6, wherein the heating of the uppermost layer of the workpiece is carried out via heat radiation.

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

9. The method of any of claims 4 to 8, wherein the period of reduced irradiation power is a predefined waiting period during which no irradiation of raw material is carried out.

10. The method of any of claims 4 to 8, wherein the period of reduced irradiation power 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 number of active energy beams, laser power, scanning speed, and beam profile.

11. The method of any of claims 4 to 10, wherein the period of reduced irradiation power is introduced between a time when an irradiation of the uppermost layer is finished and a time when a powder coating of a next layer on top of the uppermost layer begins, between a time when a coating of a next layer on top of the uppermost layer is finished and an irradiation of the next layer begins, or during irradiation of the uppermost layer.

12. The method of any of claims 1 to 11, wherein the temperature of the uppermost layer of the workpiece is maintained via a closed-loop control.

13. The method of any of claims 1 to 12, wherein the method is carried out by an additive manufacturing device and, in particular, by a device for selective laser melting or selective laser sintering.

14. The method of any of claims 1 to 13, further comprising determining the predefined set temperature to define a microstructure during a build process.

15. A device for temperature control of a three-dimensional workpiece generated via additive manufacturing, the device comprising: a control unit; and an irradiation device for irradiating at least one energy beam; wherein the control unit is configured to: maintain a temperature of a lowermost part of the workpiece during a build process of the workpiece within a first predefined range including a predefined set temperature; instruct the irradiation device to irradiate at least one energy beam into a raw material layer at a top portion of the workpiece to solidify the irradiated raw material and to form an uppermost layer of the workpiece; and maintain a temperature of the uppermost layer of the workpiece such that the temperature of the uppermost layer of the workpiece is within a second predefined range including the predefined set temperature.

16. The device of claim 15, further comprising a carrier heating device, wherein maintaining the temperature of the lowermost part of the workpiece comprises instructing the carrier heating device to heat a carrier, on which the workpiece is built, in particular to a temperature within the first predefined range.

17. The device of claim 15 or 16, wherein the control unit is further configured to: determine the predefined set temperature based on build data defining a geometry of the workpiece to be built.

18. The device of any of claims 15 to 17, further comprising a top layer heating device,wherein maintaining the temperature of the uppermost layer of the workpiece comprises at least one of instructing the top layer heating device to heat the uppermost layer of the workpiece and / or the raw material layer at the top portion of the workpiece and introducing a period of reduced irradiation power.

19. The device of claim 18, wherein the control unit is further configured to: calculate an amount of heat introduced into the uppermost layer via the at least one energy beam; calculate an amount of heat leaving the uppermost layer during irradiation of the uppermost layer; if the amount of heat introduced into the uppermost layer is larger than the amount of heat leaving the uppermost layer, introduce the period of reduced irradiation power during which an additional amount of heat can leave the uppermost layer; and / or if the amount of heat leaving the uppermost layer is larger than the amount of heat introduced into the uppermost layer, instruct the top layer heating device to heat the uppermost layer of the workpiece and / or the raw material layer at the top portion of the workpiece such that an additional amount of heat is introduced into the uppermost layer.

20. The device of claim 19, wherein, if the amount of heat introduced into the uppermost layer is larger than the amount of heat leaving the uppermost layer, the additional amount of heat leaving the uppermost layer corresponds to the difference of the amount of heat introduced into the uppermost layer and the amount of heat leaving the uppermost layer, and / or if the amount of heat leaving the uppermost layer is larger than the amount of heat introduced into the uppermost layer, the additional amount of heat introduced into the uppermost layer corresponds to the difference of the amount of heat leaving the uppermost layer and the amount of heat introduced into the uppermost layer.

21. The device of any of claims 18 to 20, wherein the heating device is configured to carry out the heating of the uppermost layer of the workpiece via heat radiation.

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

23. The device of any of claims 18 to 22, wherein the period of reduced irradiation power is a predefined waiting period during which no irradiation of raw material is carried out.

24. The device of any of claims 18 to 22, wherein the period of reduced irradiation power 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 number of active energy beams, laser power, scanning speed, and beam profile.

25. The device of any of claims 18 to 24, wherein the period of reduced irradiation power is introduced between a time when an irradiation of the uppermost layer is finished and a time when a powder coating of a next layer on top of the uppermost layer begins, between a time when a coating of a next layer on top of the uppermost layer is finished and an irradiation of the next layer begins, or during irradiation of the uppermost layer.

26. The device of any of claims 15 to 25, wherein the control unit is configured to maintain the temperature of the uppermost layer of the workpiece via a closed loop control.

27. The device of any 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. The device of any of claims 15 to 27, wherein the control unit is configured to determinine the predefined set temperature to define a microstructure during a build process.

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