Patent Publication Number: US-2018043621-A1

Title: Apparatus for producing a three-dimensional workpiece with temperature-controlled shielding gas

Description:
The present invention relates to an apparatus for producing a three-dimensional workpiece by irradiating layers of a raw material powder with electromagnetic or particle radiation. Furthermore, the invention relates to a method for operating an apparatus of this kind and to a shielding gas control system for such an apparatus. 
     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 laser radiation in a site selective manner in dependence on the desired geometry of the workpiece that is to be produced. The laser 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 laser treatment, until the workpiece has the desired shape and size. Selective laser melting or laser sintering can be used in particular for the production of prototypes, tools, replacement parts or medical prostheses, such as, for example, dental or orthopaedic prostheses, on the basis of CAD data. 
     It is further known to provide shielding gas to the process chamber of an apparatus of this kind to protect the irradiated area from undesired reactions with the surrounding atmosphere and, typically, from reactions with oxygen. Suitable shielding gases may be inert or semi-inert gases. 
     A further relevant parameter is the temperature within the process chamber of a respective apparatus. EP 2 859 973 A1 describes a powder processing arrangement for use in an apparatus for producing three-dimensional workpieces by selectively irradiating a raw material powder with electromagnetic or particle radiation, wherein a carrier element comprises a build section adapted to carry a raw material powder layer while being selectively irradiated with electromagnetic or particle radiation and at least one transfer section adapted to carry a raw material powder layer prior to being selectively irradiated with electromagnetic or particle radiation. A heating device is adapted to pre-heat the raw material powder carried by the transfer section of the carrier element prior to being applied to the build section of the carrier element so as to form the raw material powder layer to be selectively irradiated with electromagnetic or particle radiation. 
     Further, EP 2 878 409 A1 discloses a method and a device for controlling an irradiation system for use in an apparatus for producing a three-dimensional workpiece and comprising a first and the second irradiation unit. A first and a second irradiation area are defined on a surface of the carrier adapted to receive a layer of raw material powder. A layer of raw material powder applied onto the carrier in the first irradiation area is irradiated by the first irradiation unit, wherein the operation of the first irradiation unit is controlled in such a manner that the raw material powder is pre-heated. Thereafter, the layer of raw material powder applied onto the carrier in the first irradiation area is irradiated by means of the second irradiation unit, wherein the operation of the second irradiation unit is controlled in such a manner that the raw material powder is heated to a temperature which allows sintering and/or melting of the raw material powder in order to generate a layer of the three-dimensional workpiece. While the first irradiation unit irradiates a layer of raw material powder applied onto the carrier in the first irradiation area, a layer of raw material powder applied onto the carrier in the second irradiation area is irradiated by the second irradiation unit. Furthermore, while the first irradiation unit irradiates a layer of raw material powder applied onto the carrier in the second irradiation area, a layer of raw material powder applied onto the carrier in the first irradiation area is irradiated by the second irradiation unit. 
     The invention is directed at the object of providing an apparatus for producing a three-dimensional workpiece by irradiating layers of a raw material powder with electromagnetic or particle radiation which allows the production of a particularly high-quality three-dimensional workpiece. Furthermore, the invention is directed at the object of providing a method for operating an apparatus of this kind. 
     This object is addressed by an apparatus as defined in claim  1  and a method as defined in claim  15 . 
     An apparatus for producing a three-dimensional workpiece comprises a carrier being arranged in a process chamber of the apparatus and adapted to receive a layer of raw material powder. The carrier may be a rigidly fixed carrier. Preferably, however, the carrier is designed to be displaceable in vertical direction so that, with increasing construction height of a workpiece, as it is built up in layers from the raw material powder, the carrier can be moved downwards in the vertical direction. 
     The process chamber may be sealable against the ambient atmosphere, i.e. against the environment surrounding the process chamber, in order to be able to maintain a controlled atmosphere, in particular an inert atmosphere within the process chamber. The raw material powder to be received on the carrier preferably is a metallic powder, in particular a metal alloy powder, but may also be a ceramic powder or a powder containing different materials. The powder may have any suitable particle size or particle size distribution. It is, however, preferable to process powders of particle sizes &lt;100 μm. 
     The apparatus further comprises an irradiation device for selectively irradiating electromagnetic or particle radiation onto the raw material powder applied onto the carrier in order to produce the workpiece from said raw material powder by an additive layer construction method (i.e, irradiating predetermined irradiating sites to produce single workpiece layers). The irradiation device may be configured to heat the raw material powder upon irradiation to a specific temperature which allows a site-selective sintering and/or melting of the raw material powder in order to generate a layer of the three-dimensional workpiece. The apparatus may be configured to, in a generally known manner, carry out a cyclic process by adding a further layer of raw material onto the carrier, and specifically onto the just produced workpiece layer, after the irradiation device has completed producing a layer. Following that, the irradiation device can again perform site-selective irradiation to produce a further workpiece layer on top of the previous one. This can be repeated until the workpiece is completed. 
     The irradiation device may comprise a sintering/melting radiation source, such as a laser source, and at least one optical unit for guiding and/or processing a sintering/melting radiation beam emitted by the sintering/melting radiation source. The optical unit may comprise optical elements such as an object lens, in particular an f-theta lens, and a scanner unit, the scanner unit preferably comprising a diffractive optical element and a deflection mirror. The irradiation device may comprise only one irradiation unit or a plurality of irradiation units each being adapted to emit electromagnetic or particle radiation which allows a site-selective sintering and/or melting of the raw material powder from a sintering/melting radiation emission plane. 
     Still further, the apparatus comprises a shielding gas supply system adapted to supply a shielding gas to the process chamber. The shielding gas supply system may comprise a gas line or gas tube connected to an entry site at the process chamber. Furthermore, the shielding gas supply system may comprise a gas flow generating device, such as a compressor, for supplying the shielding gas to the process chamber. 
     In one example, the shielding gas supply system comprises a shielding gas circuit for supplying shielding gas to the process chamber and also collecting shielding gas leaving said process chamber. The shielding gas circuit may comprise a line circuit connecting an entry site of the process chamber with an exit site thereof. In this context, the shielding gas circuit may further comprise a respective first section for supplying shielding gas to the process chamber and a second section for collecting shielding gas leaving the process chamber, said first and second section being connected by a connecting section which may comprise components such as a shielding gas reservoir or a gas flow generating device as previously discussed. 
     Finally, the apparatus further comprises a shielding gas control system that is adapted to control the temperature of the shielding gas. The shielding gas control system may comprise a control unit, and preferably an electronic control unit. Said control unit may be provided separately from or integrated into a central control unit of the apparatus. The control unit can be configured to provide control signals to existing actuators or control elements of the apparatus that are configured to act on the temperature of the shielding gas. Alternatively or in addition thereto, the shielding gas control system may comprise own actuators or control elements for influencing the temperature of the shielding gas. These may be controlled by a control unit of the apparatus, for example an existing central control unit, and/or by a separate control unit of the shielding gas control system as previously discussed. 
     The shielding gas control system can generally be configured to control the shielding gas temperature in a feed-forward manner. Also, the shielding gas control system may be configured to control the shielding gas temperature according to a feedback loop, i.e., actually regulating said temperature. This may involve providing at least one suitable temperature sensing means in the apparatus, e.g. close to or at an entry or exit site of the process chamber. In one example, the shielding gas control system is configured to selectively switch between feed-forward and feedback control depending, for example, on current operating conditions of the apparatus. 
     Furthermore, the gas control system may be configured to, e.g. by itself or in cooperation with the gas supply system, direct at least part of the stream of temperature-controlled shielding gas to an irradiation site of the irradiation device, i.e., to a currently produced workpiece layer. In this context, the gas control system may further be configured to adjust or regulate the stream direction of the temperature-controlled shielding gas in accordance with the ongoing production process, so as to maintain a stream of shielding gas along said irradiation site. 
     According to a preferred embodiment, the shielding gas control system is configured as a unit and/or module separate from the remainder of the apparatus. In this case, the unit or module may further be configured to be electronically connectable to an existing control unit and/or power supply of the apparatus. Overall, this can allow for easy retrofitting of existing devices. 
     In summary, the inventors have thus discovered that temperature management in existing apparatuses of the present kind is not sufficient for guaranteeing a high product quality. More precisely, the inventors have discovered that improper heating and/or cooling of the process chamber and, in particular, of the raw material powder accommodated therein or of the at least partially formed workpiece, may lead to problems such as creating a high temperature gradients within the workpiece. This may result in considerable internal stresses as well as varying material characteristics along the workpiece&#39;s height. 
     Specifically, the inventors have discovered that when using known carrier heating units, which are typically provided to counteract temperature gradients being formed in the workpiece, the temperature may significantly decrease from regions near said carrier to regions further away therefrom. In other words, with increasing build height of the workpiece relative to the carrier, the temperature gradient further increases especially when viewed in a vertical direction relative to the carrier. Consequently, workpiece regions near the carrier may be maintained at a comparatively high temperature, whereas workpiece regions near new layers being formed are comparatively cold. 
     The previously discussed known additional heating devices for the raw material powder, however, increase the costs and complexity of the system and can still be insufficient for preventing the formation of significant temperature gradients. Similar considerations apply with regard to the known additional irradiation devices for pre-heating the raw material powder from above. 
     On the other hand, situations may arise in which a temperature gradient in the workpiece results from its geometry and the undesired accumulation of heat in certain areas. In this case, cooling may be required to avoid significant temperature gradients in the workpiece. Again, the inventors have discovered that known solutions are insufficient in this regard, in particular if simultaneous heating by means of a carrier heating device is to be maintained. 
     Therefore, in an attempt to improve product quality, the inventors have come up with an alternative solution for heating and/or cooling the process chamber and workpiece, said solution being based on the above described shielding gas control system. Specifically, the inventors have discovered that by adjusting the temperature of the shielding gas according to given needs, cheap and reliable heating and/or cooling can be achieved without having to substantially reconfigure existing apparatus designs. Also, this heating and/or cooling may be provided directly near or at the workpiece, since the shielding gas is typically supplied so as to stream along upper or new layers of said workpiece being formed. 
     Note that the temperature control of the shielding gas may also be used to heat/cool further components of a shielding gas circuit or of structures adjacent thereto. For example, the shielding gas circuit may comprise a filter device which serves to filter the particulate impurities from the shielding gas flowing though the shielding gas circuit prior to the shielding gas being recirculated to the process chamber. Accordingly, the temperature of said filter device may equally be controlled by means of the shielding gas and, preferably, said filter device can be cooled for protective purposes with help of the temperature-controlled shielding gas. 
     According to a preferred embodiment, the shielding gas control system is further adapted to raise the temperature of the shielding gas. Accordingly, the shielding gas control system may be adapted to selectively heat the shielding gas. This way, the shielding gas may transfer additional heat to the surrounding when entering the process chamber and streaming along the raw material and workpiece layers accommodated therein. Specifically, the shielding gas may be heated to a temperature which is above the temperature of the workpiece or, at least, above the temperature of a region of the workpiece along which the shielding gas streams. Such a region may comprise an uppermost or currently produced layer of the workpiece. Note that the inventors have discovered that by pre-heating the shielding gas, product quality is further improved since remaining humidity in the process chamber can be removed. Such humidity may otherwise negatively impact the melting/sintering process. 
     Alternatively or in addition thereto, the shielding gas control system may be adapted to lower the temperature of the shielding gas. Accordingly, the shielding gas control system may be adapted to selectively cool the shielding gas. This way, the shielding gas may remove excess heat from its surrounding when entering the process chamber and streaming along the raw material and workpiece layers accommodated therein. Specifically, the shielding gas may be cooled to a temperature which is below the temperature of the workpiece or, at least, below the temperature of a region of the workpiece along which the shielding gas streams. Such a region may comprise an uppermost or currently produced layer of the workpiece. 
     Note that the gas control system can generally be configured to selectively switch between heating or cooling of the shielding gas depending on current needs, such as current operating states or process parameters of the apparatus and/or given workpiece parameters of the workpiece to be produced. Examples of such parameters will be discussed below. 
     According to a preferred embodiment, the shielding gas control system is adapted to control the temperature of the shielding gas prior to entering the process chamber. For example, the shielding gas control system may perform and/or initiate heating of the shielding gas at or close to an entry site of the shielding gas to said process chamber. This increases the chances of the shielding gas reaching a current production/irradiating site with a desired temperature due to the shortened travel distance. 
     Likewise, the shielding gas control system may be adapted to control the temperature of the shielding gas in the process chamber. This may be achieved, for example, by heating and/or cooling elements being provided in the process chamber and dose to or at the entry and/or exit site thereto. 
     Still further, the shielding gas control system can be adapted to control the temperature of the shielding gas after leaving the process chamber. For example, the shielding gas control system may perform and/or initiate cooling of the shielding gas at or close to an exit site of the shielding gas. This way, a large portion of the excess heat of the shielding gas can be collected directly after leaving the process chamber. Said heat may further be used, for example, by a line circuit of the of the shielding gas control system as detailed below. Also, this limits the risk of undesired heating of adjacent components within the apparatus when guiding the shielding gas (e.g. in a respective line circuit) through the apparatus back to the entry site at the process chamber. 
     Note that the above described cooling and/or heating close to the exit and/or entry site may comprise limiting a travel distance of the shielding gas between the respective entry/exit site and a site of heating/cooling to not more than 2 meters, not more than 1 meter, not more than 60 centimeters or not more than 20 centimeters. 
     According to a preferred embodiment, the shielding gas control system comprises at least one temperature control element that is configured to act on the shielding gas to adjust the temperature thereof. Said temperature control element may comprise or be configured as a (thermal) actuator, such as a heater or cooler. Furthermore, it may be provided alone or in addition to existing temperature control elements of the apparatus to perform the desired temperature control. As explained above, the temperature control element may either be controlled by a central (electronic) control unit of the apparatus or by a separate control unit of the shielding gas control system. Also, it may be generally controlled to selectively adjust the amount of exchanged heat between the temperature control element and the shielding gas according to current needs. 
     In one example, the temperature control element is configured as a heat exchanger. Said heat exchanger may in a generally known manner carry a heat exchange medium, such as a cooling and/or heating liquid or a coolant/refrigerant. Furthermore, it may interact with the shielding gas so as to achieve a heat exchange between the shielding gas and said medium, thus providing heating and/or cooling of said gas. Preferably, the heat exchanger is configured as a heat exchanger coil which is wound around a gas line carrying the shielding gas. 
     The shielding gas control system may further comprise a line circuit connecting a first and second temperature control element, the second temperature control element being configured to remove heat from the shielding gas and the line circuit being configured to transfer at least part of said heat to the first temperature control element for heating the shielding gas. The line circuit may comprise tubes and/or connecting lines between said temperature control elements. Moreover, the line circuit may comprise a suitable heat exchange medium, such as a heating liquid, to transfer the heat between the temperature control elements as well as to and from the shielding gas. In a generally known manner, the line circuit may further comprise components such as a pump or compressor for creating a desired flow of the heat exchange medium through the line circuit and, preferably, a circulation of said medium in the circuit. 
     The second temperature control element may be positioned close to an exit site of the shielding gas from the process chamber. The first temperature control element, on the other hand, may be positioned close an entry site of the shielding gas to said process chamber. This way, heat can be at least partially recycled within the line circuit to selectively heat up the shielding gas prior to entering the process chamber. Similarly, if a cooling of the shielding gas prior to entering the process chamber is desired, the second temperature control element may be positioned close the entry site and the first temperature control element may be positioned close to the exit site. Note that for cooling, it may generally be sufficient to only provide one temperature control element which removes heat from the shielding gas near the entry site. Also, the proximity to the entry and exit sites is merely optional and the positions of the temperature control elements may be chosen differently, e.g. depending on the heat-sensitivity of adjacent devices and structures. 
     According to a preferred embodiment, the shielding gas control system is further adapted to control the flow rate of the shielding gas to and/or from the process chamber. For doing so, the shielding gas control system may provide control signals for prompting existing actuators of the apparatus, and in particular of the gas supply system, to adjust the flow rate in a desired manner. For example, the shielding gas control system may prompt a flow generating means of the gas supply system, such as a compressor, to vary the flow rate in a desired manner. Likewise, valves of said gas supply system may be controlled to open and/or close or vary their opening degrees due to signals from the shielding gas control system. Alternatively or in addition thereto, the gas control system may comprise own actuators for adjusting the flow rate, such as an additional valve which may be arranged close to or at an entry site of the shielding gas to the gas chamber. 
     By varying said flow rate in addition to controlling the temperature of the shielding gas, and additional parameter is available for controlling the temperature in the process chamber and especially of the workpiece being formed therein. For example, increasing the flow rate at low temperatures of the shielding gas may help to pro-mote cooling, whereas decreased flow rates may be beneficial for heating the workpiece. 
     According to one example, the shielding gas control system is adapted to controlling the temperature and/or flow rate of the shielding gas in response to at least one process parameter. For doing so, the apparatus may further comprise a control unit that is adapted to setting and/or monitoring at least one process parameter. As previously explained, said control unit may be formed or be comprised by a central control unit of apparatus and, preferably, at least partially control the shielding gas control system. In case the shielding gas control system is configured as a separate module, the control unit may also be directly integrated in said module. Moreover, the control unit may be connected in a well-known manner to sensors of the apparatus to determine relevant process parameters and/or to actuators of the apparatus to set such process parameters. Also, the control unit may be configured to determine such process parameters from an operating program of the apparatus, such as a CAD/CAM- or NC-program for producing the current workpiece. These information may be used by the shielding gas control system to set an appropriate amount of heating/cooling of the shielding gas as well as adjusting the flow rate thereof to meet current needs. 
     The process parameter may generally relate to any process parameters defined by or relating to the apparatus and/or to any dynamically changing parameters of the process and/or workpiece, such as the current build height of the workpiece being produced by the apparatus. Said build height may relate to a vertical or orthogonal height of the workpiece relative to the carrier, i.e., a distance of a most recently formed workpiece layer to said carrier, and may increase with every new workpiece layer being formed during the production process. A respective process parameter for indicating the build height may be derived, for example, from determining a position of a vertical moving axis of the carrier. Also, a number of workpiece layers having been irradiated by the irradiation device or an elapsed build time of the workpiece may be considered. Both of these options can equally allow to infer on a current build height of the workpiece. 
     Moreover, if one of said process parameters indicates an increasing height of the workpiece, the shielding gas control system may detect a risk of an unduly high temperature gradient being formed in the workpiece. This relates in particular to a case in which an additional heating unit is provided at the carrier, the heat of which being increasingly incapable of reaching upper regions of the workpiece remote from said carrier. Accordingly, the shielding gas control system may initiate suitable countermeasures, such as increasing the shielding gas temperature in or prior to entering the process chamber or reducing a flow rate thereof. 
     Furthermore, the process parameter may indicate a measured or estimated heat distribution within the workpiece. Such estimated heat distributions may be pre-stored or calculated on the basis of simulations. If the shielding gas control system detects, for example based on a current build height of the workpiece, that a region or layer of the workpiece is reached which is marked by an (estimated) critical temperature or temperature gradient, it can adjust the temperature and/or flow rate of the shielding gas accordingly. 
     According to a further aspect, the shielding gas control system is adapted to control the temperature and/or flow rate of the shielding gas in response to at least one workpiece parameter relating to the workpiece to be produced by the apparatus. Such workpiece parameters may generally relate to any parameter of the finished workpiece or parameters which remain substantially stable until production is completed, such as a cross-sectional area of a currently produced workpiece layer. Furthermore, for determining said workpiece parameters, the apparatus may comprise a control unit that is adapted to storing and/or determining such workpiece parameters. Again, the control unit may be configured according to any of the aspects discussed above. Furthermore, the workpiece parameters may be stored in or determined from any suitable data format, such as NC- or CAD-data, wherein said determination may involve calculations based on these data. Likewise, the workpiece parameters may be pre-stored as a discrete number in any suitable format. 
     In this context, the workpiece parameter may indicate a workpiece shape, and in particular a cross-sectional shape, and/or at least one workpiece dimension, such as the workpiece height or width. The shape of the workpiece may relate to its overall three-dimensional shape or merely to a shape of its cross-section or the cross-sectional variations along its height. The height and/or width, on the other hand, may be defined relative to the carrier, wherein the height can be defined vertically or orthogonally relative to said carrier (cf. build height discussed above). The width can be defined as extending substantially in parallel to the carrier and/or orthogonally to the height. Thus, the width may generally define a material thickness transverse to the workpiece&#39;s height or, in other words, a width of a cross-section of the workpiece. In one example, the width relates to the workpiece dimension when viewed from an entry to an exit site of the shielding gas to and from the process chamber, or, in other words, the distance across the workpiece&#39;s cross-section along which the shielding gas streams. 
     Note that the control unit may update the workpiece parameter depending on a current production state, such as a current build height or a current workpiece layer being formed. This can be used, for example, to identify a suitable workpiece parameter relating to the layer being currently produced, such as the current workpiece cross-section along which the shielding gas will stream. Depending on the width, size, shape and/or area of said current workpiece cross-section, the shielding gas control system can adapt its settings accordingly. 
     When performing a control based on a workpiece parameter, the gas supply system may generally be configured to (in advance or during an ongoing production process) identify areas of the workpiece which are sensitive to accumulating excessive amounts of heat or are sensitive to cooling out very quickly. In other words, regions (i.e., layer sections) of the workpiece which are sensitive to creating undesired temperature gradients can be identified and the settings of the shielding gas control system can be adapted accordingly as soon as these regions or layers are reached during production. Specifically, if reaching a workpiece region which is prone to excessive heat accumulation, the shielding gas control system may decrease a temperature of the shielding gas supplied to the process chamber and/or increase the flow rate thereof. 
     According to a further embodiment, the apparatus further comprises a heating unit and/or cooling unit and the shielding gas control system can be adapted to control the temperature and/or flow rate of the shielding gas in response to an operating state of the heating and/or cooling unit. In other words, the shielding gas control system may be adapted to adjust its settings based on the settings of additional heating and/or cooling units being provided in the apparatus. For example, if additional heating is already provided by a respective unit, further heating of the shielding gas by means of the shielding gas control unit may be controlled in dependence on said already provided heat. 
     In one example, the heating unit is provided in form of a carrier heating unit as discussed above. Such heating units may be attached to or integrated in the carrier to transfer heat to the workpiece. 
     Of course, it can also be considered to (additionally or alternatively) control such additional heating and/or cooling units of the apparatus based on current settings of the shielding gas control system. 
     The invention further relates to a shielding gas control system for an apparatus, said apparatus being adapted to produce a three-dimensional workpiece and said apparatus comprising:
         a carrier being arranged in a process chamber of the apparatus and adapted to receive a layer of raw material powder;   an irradiation device for selectively irradiating electromagnetic or particle radiation onto the raw material powder applied onto the carrier in order to produce the workpiece from said raw material powder by an additive layer construction method; and   a shielding gas supply system adapted to supply a shielding gas to the process chamber,
 
wherein the shielding gas control system is adapted to control the temperature and/or flow rate of the shielding gas in accordance with one of the above discussed aspects. According to this solution, the shielding gas control system may be provided as a separately configured and/or separately mountable module which is generally beneficial for retrofitting existing apparatuses.
       

     Furthermore, the invention relates to a method of operating an apparatus, said apparatus being adapted to produce a three-dimensional workpiece and said apparatus comprising:
         a carrier being arranged in a process chamber of the apparatus and adapted to receive a layer of raw material powder;   an irradiation device for selectively irradiating electromagnetic or particle radiation onto the raw material powder applied onto the carrier in order to produce the workpiece from said raw material powder by an additive layer construction method;   a shielding gas supply system adapted to supply a shielding gas to the process chamber,
 
wherein the method comprises the step of controlling the temperature and/or flow rate of the shielding gas in accordance any of the above aspects. Specifically, the method according to the present invention may involve any additional step to provide any of the functions and effects as well as realise any of the operating states and control activities of the above discussed apparatus and, in particular, of the shielding gas control system.
       

     For example, the method may include a step of increasing a shielding gas temperature in accordance with an increasing build height of the workpiece. This may generally take place in a stepwise, continuous and/or proportional manner or involve only a single adjustment upon surpassing a predefined threshold value of said build height. Likewise, the method may include a step of decreasing a shielding gas temperature in accordance with increasing workpiece dimensions, in particular cross-sectional dimensions, or if layer sections with an estimated high heat accumulation are reached. 
    
    
     
       Preferred embodiments of the invention are explained in greater detail below with reference to the accompanying schematic drawings, in which: 
         FIG. 1  shows a schematic representation of an apparatus for producing three-dimensional workpieces according to an embodiment of the invention, 
         FIG. 2A-C  show schematic views of a workpiece being produced by the apparatus of  FIG. 1  under different operating conditions. 
     
    
    
       FIG. 1  shows an apparatus  10  for producing three-dimensional workpieces  16  by powder bed fusion. Generally, only selective components of the apparatus  10  are presently depicted and the apparatus  10  comprises further standard components of this as such known category of device to produce three-dimensional workpieces  16  by selective laser melting. 
     As can be seen in  FIG. 1 , the apparatus  10  comprises a process chamber  12 . The process chamber  12  is sealable against the ambient atmosphere, i.e. against the environment surrounding the process chamber  12 . A powder application device (not shown), which is disposed in the process chamber  12 , serves to apply a raw material powder  13  onto a carrier  14 . The carrier  14  is designed to be displaceable in a vertical direction V so that, with increasing build height H of a workpiece  16 , as it is built up in layers from the raw material powder  13  on the carrier  14 , the carrier  14  can be moved downwards. For forming said layers, the apparatus  10  further comprises a non-depicted irradiation device for selectively irradiating a laser radiation L onto the layer of raw material powder  13  applied onto the carrier  14 . Also, a carrier heating unit  15  is provided which is attached to an underside of the carrier  14  to provide additional heating to the workpiece  16  from below. 
     In the state depicted in  FIG. 1 , the production process has been ongoing for a certain time so that a block-shaped workpiece  16  has already been partially formed on the carrier  14 . The workpiece  16  is thus marked by a current build height H extending vertically from an upper surface of the carrier  14 . Furthermore, the workpiece  16  is marked by a width W extending along said upper surface of the carrier  14  and orthogonally to the build height H. 
     The apparatus  10  further comprises a shielding gas supply system  20 . Again, only selective components of said shielding gas supply system  20  are presently depicted, said system  20  being otherwise configured according to known solutions. As can be gathered from  FIG. 1 , the shielding gas supply system  20  comprises a line circuit  22  for guiding shielding gas to an entry site  24  to the process chamber  12 . Said entry site  24  is configured as a simple aperture in an outer sidewall of the process chamber  12 . Furthermore, the line circuit  22  extends from an exit site  26  from the process chamber  12 , which again is configured as a simple aperture. This way, shielding gas can be supplied to the entry site  24  of the process chamber  12 , stream through the process chamber  12  as indicated by arrow S, and then leave the process chamber  12  through the exit site  26  by being collected in the line circuit  22 . Following that, the shielding gas is transported through the line circuit  22  by means of a non-illustrated compressor to again reach the entry site  24 . Thus, an overall circulating movement of the shielding gas through the apparatus  10  is generated. 
     The apparatus  10  further comprises a shielding gas control system  30 , again only selective components of which are presently displayed. The shielding gas control system  30  comprises a line circuit  32  being configured separately from the line circuit  22  of the gas supply system  20  and accommodating a heat exchange medium. Said heat exchange medium is circulated in the line circuit  32  by means of a pump  34  (alternatively, a compressor may be used). Furthermore, the line circuit  32  comprises two temperature control elements in form of a first heat exchanger coil  36  and a second heat exchanger coil  38  which are connected by means of the line circuit  32 . The first heat exchanger coil  36  is arranged near the entry site  24  to the process chamber  12  and accommodates a section of the line circuit  22  of the gas supply system  20  leading to said entry site  24  (i.e., is wound around a respective section of the gas supply line circuit  22 ). The second heat exchanger coil  38 , on the other hand, is arranged near the exit site  26  from the process chamber  12  and similarly accommodates a section of the line circuit  22  of the gas supply system  20  extending from said exit site  26 . Note, however, that the heat exchanger coils  36 , 38  can also be positioned further remote from the entry and/or exit sites  24 , 26  and may e.g. be positioned depending on the heat-sensitivity of adjacent devices and structures. 
     As indicated by arrow P, the pump  34  causes a circulation of the heat exchange medium in an opposite direction compared to the shielding gas stream S from the second heat exchanger coil  38  to the first heat exchanger coil  36 . Specifically, the heat exchange medium is made to circulate from the second to the first heat exchanger coil  38 ,  36  over a shorter distance compared to the opposite flow from the first to the second heat exchanger coil  36 ,  38  along arrow R. 
     In the depicted embodiment, the first heat exchanger coil  36  acts as a heating unit for the shielding gas in line circuit  22  and the second heat exchanger coil  38  acts as a cooling unit for said shielding gas. More precisely, during operation of the apparatus  10 , the first heat exchanger coil  36  heats the shielding gas travelling through said coil  36  in the line circuit  22  prior to entering the gas chamber  12  via entry site  24 . Following that, the shielding gas streams towards the exit site  26  along arrow S. As indicated by arrow Q, the shielding gas transfers part of its heat to the workpiece  16  during this process which, at least on its upper surface facing away from the carrier  14  (i.e., at its most recently produced layer), has a temperature which is lower than that of the shielding gas. The shielding gas thus cools down when streaming through the process chamber  12  while the workpiece  16  heats up. Consequently, the shielding gas reaches the exit site  26  and the second heat exchanger coil  38  at a lower temperature compared to its previous temperature at the entry site  24 . 
     Following that, the shielding gas travels in the line circuit  22  through the second heat exchanger coil  38 . Said coil  38  removes remaining excess heat from the shielding gas by means of the heat exchange medium in the line circuit  32  of the shielding gas control system  30 . Specifically, in the second heat exchanger coil  38 , the heat exchange medium is heated up by absorbing heat from the shielding gas to then transfer said heat to the first heat exchanger coil  36  by being circulated through the line circuit  32  via the pump  34 . During this process, the pump  34  may supply additional heat to the heat exchange medium by means of an integrated heating unit. This may in particular be necessary during start-up of the apparatus  10 . 
     After heating the shielding gas when flowing through the first heat exchanger coil  36 , the heat exchange medium has cooled down again and circulates back to the second heat exchanger coil  38  along arrow R. Due to having cooled down, it can then provide the desired cooling effect in the second heat exchanger coil  38 . Note that the line circuit  32  comprises a non-depicted cooling unit along arrow R that can selectively be activated to reduce the temperature of the heat exchange medium prior to reaching the second heat exchanger coil  38 . This helps to increase the desired cooling effect. 
     The shielding gas control system  30  further comprises a non-depicted control unit for regulating the temperature of the shielding gas by means of the first and second heat exchanger coils  36 ,  38 . For doing so, the flow rate of the heat exchange medium through the line circuit  32  can be adjusted, e.g. via the pump  34 . Likewise, the settings of the additional heating and cooling units provided in said line circuit  32  can be adjusted. Furthermore, the control unit of the shielding gas control system  30  can generate control signals for the non-depicted compressor of the shielding gas supply system  20  for adjusting a flow rate of the shielding gas through the process chamber  12  along arrow S. Overall, the shielding gas control system  30  can thus selectively increase or decrease the temperature of the shielding gas as well as its flow rate according to given needs. 
     Examples of a temperature control by means of the shielding gas control system  30  will now be explained with reference to  FIGS. 2A-C .  FIG. 2A  shows partial views of the carrier  14 , the workpiece  16  being formed thereon as well as the carrier heating unit  15  as explained with reference to  FIG. 1 . Furthermore, the current building height H and width W of the workpiece  16  are again indicated. During production of the workpiece  16 , the carrier heating unit  15  provides heat from below, so that the workpiece  16  has a comparatively high temperature T 1  near the carrier  14 . This temperature decreases if moving further away from the carrier  14  along the build height H as indicated by temperature T 2  in  FIG. 2A . Accordingly, a temperature gradient (T 1 &gt;T 2 ) is formed in the workpiece  16 , said temperature gradient increasing with an increasing build height H. Note that even though the carrier heating unit  15  contributes to a temperature gradient being formed in the workpiece  16 , it still limits said temperature gradient compared to a situation without any additional heating from below and the only heat being transferred to the workpiece  16  by means of the laser irradiation L. 
     In  FIG. 2A , the shielding gas control system  30  operates in an idle mode and does not perform any dedicated temperature control of the shielding gas. Therefore, the shielding gas streaming along arrow  5 , which upon entering the gas chamber  12  through the entry site  24  of  FIG. 1  has approximately room temperature, will actually pick up additional heat from the workpiece  16  as indicated by arrow Q. This means, first of all, that the shielding gas heats up when streaming through the process chamber  12 . In addition, the temperature T 2  of the workpiece  16  near its upper end remote from the carrier  14  will even further decrease, thus increasing the temperature gradient within the workpiece  16 . This may lead to significant internal stresses in the workpiece  16 , thus diminishing product quality. 
       FIG. 2B , to the contrary, depicts a state in which the shielding gas streaming along arrow S is temperature-controlled by means of the shielding gas control system  30 . Specifically, as described with reference to  FIG. 1 , the shielding gas is now heated up prior to entering the process chamber  12 . Again, the carrier heating unit  15  is provided which increases a temperature T 1  of the workpiece near said carrier  14 . Yet, the shielding gas control system  30  now controls the temperature of the shielding gas to be above the temperature T 2  near the upper end of the workpiece  16  (i.e., near its currently produced layer). 
     Suitable temperature values for the shielding gas for being above said workpiece temperature T 2  can generally be previously determined, for example, with help of experiments, simulations or calculations. Also, they can be dynamically calculated, e.g. based on the current build height H. 
     As a result, heat is transferred from said shielding gas to the workpiece  16  as indicated by arrow Q. First of all, this means that the shielding gas cools down when streaming through the process chamber  12  (cf. discussion of  FIG. 1  above). Second of all, however, this means that the upper end of the workpiece  16  is now deliberately heated by means of said shielding gas, thus increasing the temperature T 2  of the workpiece  16  in this region as compared to the situation in  FIG. 2A . This way, the temperature gradient (T 1 &gt;T 2 ) within the workpiece  16  is significantly decreased, thus limiting internal stresses. 
     Consequently, a temperature T 3  in the middle part along the build height H of the workpiece  16  is now actually lower than the temperatures T 1  and T 2  near the lower and upper ends. The differences between said temperatures T 1 , T 2  and T 3  are, however, much smaller compared to the non-temperature-controlled situation of  FIG. 2A , thus leading to overall smaller temperature gradients being formed within the workpiece  16 . 
     Note that in case of  FIG. 2B , the shielding gas control system  30  further increases the temperature of the shielding gas based on the continuously increasing build height H. This helps to make up for the decreasing amount of heat from the carrier heating unit  15  that reaches the upper region of the workpiece  16  at larger build heights H (cf. discussion of  FIG. 2A ). 
       FIG. 2C  depicts a case in which a different workpiece  16  is produced, said workpiece  16  requiring cooling by means of the shielding gas. Again, the workpiece  16  is formed on a carrier  14  which is heated from below by means of a carrier heating unit  15 . Similar to above, this leads to a locally increased temperature T 1  of the workpiece  16  near the carrier  14 . Yet, in the depicted state, the workpiece  10  further comprises support structures  40 . Such support structures  40  are well known in selective laser melting and help to stabilise a workpiece during production. Also, they may generally be provided for transferring heat away from upper workpiece layers and towards the carrier  14 . After the production is completed, the supports are typically removed from the workpiece  16 . 
     As can be gathered from  FIG. 2C , the support structures  40  are integrally formed with and support an overhanging projection  42  of the workpiece  16 . Said projection  42  locally increases the width W and thus the cross-section of the workpiece  42 . This leads to a locally increased heat transfer from the laser irradiation L to the workpiece  16  when producing the layers of the projection  42  since the amount of absorbed irradiation L increases at larger cross-sections. Accordingly, the temperature T 1 /T 2  difference along the build height H of the workpiece  16  may actually be acceptable. 
     Yet, this is only the case as long as the support structures  40  help to equalise the temperature gradient between these regions of the workpiece  16 . Specifically, the support structures  40  have to be designed sufficiently large to ensure that the accumulated heat in the overhanging projection  42  is conducted to cooler regions of the workpiece  16 , such as its middle part. This means that if the support structures were not present or dimensioned too small, the temperature T 2  near the upper end of the workpiece  16  (i.e., near the currently produced layer) could be undesirably high. This may even lead to an opposite temperature gradient compared to  FIG. 2A  (T 2 &gt;T 1 ) or at least to an undesirably high temperature gradient compared to a temperature T 3  in the middle part of the workpiece  16  (T 2 &gt;T 3 ). 
     In such cases, the shielding gas control system  30  will, in an opposite manner compared to  FIG. 1 , control the first heat exchanger coil  36  to operate as a cooling unit. Consequently, the shielding gas enters the process chamber  12  at a reduced temperature compared to a temperature T 2  at an upper end of the workpiece  16 . Thus, when streaming along arrow  5 , the shielding gas picks up heat from said upper end as indicated by arrow Q in  FIG. 2C . Consequently, the shielding gas temperature is raised when reaching the exit site  26  of the process chamber  12  compared to its temperature at the entry site  24 . Additionally, the temperature T 2  near the currently produced layer of the workpiece  16  is decreased, thus decreasing the temperature gradient compared to temperatures T 3  and/or T 1 . 
     Note that it is very conceivable that such support structures  40  as depicted in  FIG. 2C  may be omitted altogether when producing the workpiece  16 . This may be the case when repairing an already existing workpiece  16  (i.e., the support structures having already been removed during the initial production) or if omitting said support structures  40  for cost reasons. Yet, as shown above, the shielding gas control system  30  according to the invention is configured to prevent the formation of undesired temperature gradients even in case of such missing support structures  40  by cooling the shielding gas appropriately. 
     Furthermore, depending on the size, shape, width W and/or area of the cross-section of the projection  42  for a current building height H, the shielding gas control system  30  can generally adjust its settings appropriately. Specifically, cooling of the shielding gas prior to entering the process chamber  12  is increased with increasing cross-sectional dimensions of the workpiece  16 , for example, in a stepwise manner when reaching the first layer for producing the projection  42 .