Abstract:
A method and apparatus for manufacturing a three-dimensional object by additive layer manufacturing. The method includes providing layers of material in powder form on a support inside a chamber, and irradiating each layer with a beam before providing the subsequent layer. A gas atmosphere is maintained inside the chamber during the irradiation steps. The pressure and/or the composition of the gas atmosphere is controlled where at least two different gas atmospheres having different predetermined pressures and/or compositions are inside the chamber during irradiation of the layers, the beam spot size on the layers is controlled such that at least two different beam spot sizes are utilized during irradiation, and/or the temperature of the gas atmosphere inside the chamber and/or of the layer being irradiated is controlled such that at least two different temperatures of the gas atmosphere and/or of the layer being irradiated are present during irradiation of the layers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of European Application No. EP 12192307.2 and to U.S. Provisional Application No. 61/725,154, both of which were filed on Nov. 12, 2012, the entire disclosures of which are both incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a method of manufacturing a three-dimensional object by additive layer manufacturing (ALM), in particular selective laser melting (SLM) or direct material deposition (DMD), and to a corresponding apparatus for manufacturing a three-dimensional object by additive layer manufacturing. 
       BACKGROUND 
       [0003]    Additive layer manufacturing is increasingly used for rapidly manufacturing prototype or even final components and is then also referred to as rapid prototyping and rapid manufacturing, respectively. In contrast to conventional manufacturing methods involving removal of material from a block of material by, e.g., cutting, drilling or other machining processes, additive layer manufacturing directly constructs a desired three-dimensional object layer by layer from a digital representation of the object. It is also known as 3D printing. 
         [0004]    A typical additive layer manufacturing method comprises providing a thin layer of material from which the product is to be manufactured in powder form on a support plate, melting, curing or sintering the powder in those portions of the layer corresponding to the product being manufactured by means of laser irradiation, subsequently providing a further thin layer of the material on top of the initial layer and again melting, curing or sintering the powder of the layer in those portions of the layer corresponding to the product being manufactured by means of laser irradiation, and repeating the process until the complete object is obtained. In each layer the powder not corresponding to the product is not irradiated and remains in powder form, so that it can be removed from the object at a later stage. The support plate may be provided by a movable table that—after each irradiation of a layer—is lowered a distance equal to the thickness of that layer to provide for a defined starting condition for the provision of each layer. 
         [0005]    In this regard, it is to be noted that it is in principle possible that the individual layers are not entire or continuous layers of material, but comprise material only in the areas corresponding to the object being manufactured or in selected regions comprising those areas. 
         [0006]    Known additive layer manufacturing methods as described above are carried out in chambers in which a tightly controlled constant inert gas atmosphere, e.g. argon, is maintained in order to avoid as far as possible reactions between the layers and surrounding gases upon laser irradiation. 
         [0007]    Particular additive layer manufacturing methods of this type are also referred to as selective laser melting (SLM) or direct material deposition (DMD). In this regard, it is noted that instead of using a laser beam it is also possible to use an electron beam for the same purposes. A particular additive layer manufacturing method utilizing an electron beam is also referred to as electron beam melting (EBM). 
         [0008]    As noted above, the object is built up layer by layer in a three-dimensional manner. This makes it possible to efficiently and rapidly manufacture different highly complex objects from various materials, in particular metal materials, but also plastic materials and ceramic materials, using one and the same apparatus. For example, highly complex grid or honeycomb structures which are difficult to produce using other techniques can be easily manufactured. As compared to traditional methods, the complexity of the object has only little influence on the manufacturing costs. 
         [0009]    In spite of these advantages known additive layer manufacturing methods also have a major drawback in that it is difficult and complicated to provide for different material characteristics in different parts of the manufactured object. For example, the material can be changed between different layers, but such change is time consuming, difficult to implement and adds to the manufacturing costs. Additive layer manufacturing methods have this drawback in common with other common manufacturing techniques, such as, e.g., molding, casting, forging or cutting. It is in general difficult or impossible to manufacture graduated objects. 
       SUMMARY 
       [0010]    It is therefore an object of the present invention to provide a method and a system for manufacturing a three-dimensional object by additive layer manufacturing enabling to selectively change the material properties in different parts of the object being manufactured in a simple, rapid and cost efficient manner. 
         [0011]    This object is achieved by a method having the features of claims  1  and by an apparatus having the features of claim  13 . Advantageous embodiments of the method and the apparatus are the subject-matter of the respective dependent claims. 
         [0012]    According to the present invention a method of manufacturing a three-dimensional object by additive layer manufacturing, comprising—in accordance with the prior art methods described above—successively providing a plurality of layers of material in powder form, one on top of the other, on a support means inside a chamber, which is also referred to as build chamber, and irradiating each layer with a laser or particle beam prior to providing the subsequent layer. Although using a laser beam is preferred, in some applications the use of a particle beam may also be advantageous. For example, provided that low pressures can be present inside the chamber, the irradiation may be effected by means of an electron beam. 
         [0013]    Typically the layers have thicknesses in the range of 20 to 100 μm, wherein the thicknesses are selected based on the desired surface finish quality and processing speed. Each layer is irradiated selectively only in those portions of the layer corresponding to the three-dimensional object being manufactured, and the irradiation is carried out in such a manner that the material of the respective layer is melted or sintered locally in the irradiated portions. This local melting or sintering serves to fuse the powder particles in the irradiated zones together and to the preceding layer. 
         [0014]    During the entire manufacturing process or at least during each of the laser or particle beam irradiation steps a gas atmosphere having a controlled pressure and composition is maintained inside the chamber. However, different from known additive manufacturing methods the pressure and composition of the gas atmosphere inside the chamber, the beam spot size during irradiation of each layer and/or the temperature of the gas atmosphere and/or of the layers are selectively changed in the course of the manufacturing process or at least during each of the laser or particle beam irradiation steps in order to selectively achieve different characteristics for different layers in the three-dimensional object manufactured. 
         [0015]    According to a first option, at least one of the pressure and the composition of the gas atmosphere inside the chamber is controlled such that at least two different gas atmospheres having different predetermined pressures and/or compositions are present inside the chamber during the irradiation of different ones of the layers. In other words, the pressure, the composition or the pressure and composition of the gas atmosphere inside the chamber is changed in the course of the process such that the gas atmosphere in which one or more of the layers are irradiated is different from the gas atmosphere in which others of the layers are irradiated. Thus, two or more different gas atmospheres may be used for irradiating different subsets of the plurality of layers. This necessitates that the gas atmosphere is changed between the irradiation of one or more pairs of adjacent layers. 
         [0016]    According to a second option, which can be realized as an alternative to or in combination with the first option, the beam spot size of the laser beam or the particle beam used for irradiating each of the layers is controlled such that at least two different beam spot sizes are utilized during the irradiation of different ones of the layers. Thus, two or more different beam spot sizes may be used for irradiating different subsets of the plurality of layers. This necessitates that the beam spot size is changed between the irradiation of one or more pairs of adjacent layers. As conventionally used, in the present application the term beam spot size designates the size of the beam spot on the surface of the layer currently irradiated, which surface is the surface onto which the beam is incident. For example, the beam spot size may, in particular, be the area of the beam spot. The beam spot size generally determines the size or area of the irradiation-induced melting zone. 
         [0017]    According to a third option, which can be realized as an alternative to both the first and second option or in combination with the only the first option, only the second option or both the first and second options, the temperature of the gas atmosphere inside the chamber and/or of the layer being irradiated is controlled such that at least two different temperatures of the gas atmosphere and/or of the layer being irradiated are present during the irradiation of different ones of the layers. In other words, the gas atmosphere temperature, the layer temperature or both the gas atmosphere temperature and the layer temperature is changed in the course of the process such that the respective temperature or temperatures during irradiation of some of the layers is or are different from the respective temperatures during irradiation of others of the layers. Thus, two or more different gas atmosphere temperatures and/or layer temperatures may be used for irradiating different subsets of the plurality of layers. This necessitates that the respective temperature is changed between the irradiation of one or more pairs of adjacent layers. 
         [0018]    In a preferred embodiment only the first of the above three options or at least the first of the above three options is realized. In further embodiments only the second option, only the third option or both the second and third options are realized. 
         [0019]    It has been found that by selectively using different gas atmospheres for different layers it is easily possible to selectively influence and adjust the material properties of the corresponding portions of the object manufactured. In particular, controlled amounts of particular reactive gases can be mixed with an inert gas, such as argon, which reactive gases have a controlled impact on the material properties of the respective layers, such as, e.g., strength and/or ductility. The necessary changes in the gas atmosphere can be effected rapidly and in a simple manner while adding only little costs to the method and apparatus. Aside from the adaptation and control of the gas atmosphere the selective adjustment of the material properties does not require additional work steps and is implemented as part of the normal manufacturing process. 
         [0020]    Similar considerations also apply to the above parameters beam spot size and temperature. By selectively using different beam spot sizes, gas atmosphere temperatures and/or layer temperatures for different layers it is likewise easily possible to selectively influence and adjust the material properties of the corresponding portions of the object manufactured. In particular, the amount of intake of gases from the gas atmosphere by the layers, in particular of specific reactive gases mixed with an inert gas, such as argon, which reactive gases have a controlled impact on the material properties of the respective layers, such as, e.g., strength and/or ductility, can be efficiently controlled by suitably selecting the above parameters. The necessary changes in the beam spot size and/or the temperatures can be effected rapidly and in a simple manner while adding only little costs to the method and apparatus. Aside from the adaptation and control of one or more of these parameters the selective adjustment of the material properties does not require additional work steps and is implemented as part of the normal manufacturing process. 
         [0021]    In a preferred embodiment the plurality of layers used for building up the object is constituted by two or more different groups, i.e. sets or subsets, of layers, each group including only one layer or multiple adjacent layers. Thus, each layer of the plurality of layers is included in one and only one of the groups. Making use of the first of the above three options, the pressure, the composition or the pressure and composition of the gas atmosphere inside the chamber is then changed between adjacent groups, but for all layers belonging to the same group the same gas atmosphere having the same pressure and composition is maintained inside the chamber during their irradiation. Consequently, for at least one of the groups a different gas atmosphere having a different predetermined pressure and/or composition is present inside the chamber during the irradiation of the corresponding layers as compared to the other group or groups. In other words, at least one of the pressure and the composition of the gas atmosphere inside the chamber is controlled such that for at least two different groups of layers different gas atmospheres having different predetermined pressures and/or compositions are present inside the chamber during the irradiation of the layers of the different groups. If there are more than two groups it is possible that different gas atmospheres are used for all groups, or some of the groups, which are not adjacent to each other, may use the same gas atmosphere. For example, the latter case may be used for objects comprising one the one hand functionally loaded portions, which require a high strength and are formed using a first gas atmosphere, and intermediate connecting portions, which are preferably ductile in order to avoid problems with small gaps and are formed using a different second gas atmosphere. 
         [0022]    In a further preferred embodiment, which may be combined with the preceding embodiment, the plurality of layers used for building up the object is constituted by two or more different groups, i.e. sets or subsets, of layers, each group including only one layer or multiple adjacent layers. Thus, each layer of the plurality of layers is included in one and only one of the groups. If this embodiment is used in combination with the preceding embodiment, the groups may be the same or different with respect to the two embodiments. Making use of the second of the above three options, the beam spot size is then changed between adjacent groups, but for all layers belonging to the same group the same beam spot size is maintained during their irradiation. Consequently, for at least one of the groups a different beam spot size is used during the irradiation of the corresponding layers as compared to the other group or groups. In other words, the beam spot size is controlled such that for at least two different groups of layers different beam spot sizes are utilized during the irradiation of the layers of the different groups. If there are more than two groups it is possible that different beam spot sizes are used for all groups, or some of the groups, which are not adjacent to each other, may use the same beam spot size. For example, the latter case may be used for objects comprising one the one hand functionally loaded portions, which require a high strength and are formed using a first beam spot size, and intermediate connecting portions, which are preferably ductile in order to avoid problems with small gaps and are formed using a different beam spot size. 
         [0023]    Similarly, in a further preferred embodiment, which may be combined with each or both of the two preceding embodiments, the plurality of layers used for building up the object is constituted by two or more different groups, i.e. sets or subsets, of layers, each group including only one layer or multiple adjacent layers. Thus, each layer of the plurality of layers is included in one and only one of the groups. If this embodiment is used in combination with one or both of the two preceding embodiments, the groups may be the same or different with respect to all or some of the three embodiments. Making use of the third of the above three options, the gas atmosphere temperature and/or layer temperature is then changed between adjacent groups, but for all layers belonging to the same group the same gas atmosphere temperature and/or layer temperature is maintained during their irradiation. Consequently, for at least one of the groups a different gas atmosphere temperature and/or layer temperature is used during the irradiation of the corresponding layers as compared to the other group or groups. In other words, the gas atmosphere temperature and/or layer temperature is controlled such that for at least two different groups of layers different gas atmosphere temperatures and/or layer temperatures are utilized during the irradiation of the layers of the different groups. If there are more than two groups it is possible that different gas atmosphere temperatures and/or layer temperatures are used for all groups, or some of the groups, which are not adjacent to each other, may use the same gas atmosphere temperature and/or layer temperature. For example, the latter case may be used for objects comprising one the one hand functionally loaded portions, which require a high strength and are formed using a first gas atmosphere temperature and/or layer temperature, and intermediate connecting portions, which are preferably ductile in order to avoid problems with small gaps and are formed using a different gas atmosphere temperature and/or layer temperature. 
         [0024]    In principle it is possible that in the above manner each layer of e.g. typically thousands of layers building up the object is selectively provided with individual material properties. Thus, depending on the embodiment, the gas atmosphere, the beam spot size, the gas atmosphere temperature and/or the layer temperature would then be changed between each two layers. In that case each group would include only one layer, and in the extreme case different gas atmospheres, beam spot sizes, gas atmosphere temperatures and/or layer temperatures would be used for all of the layers. 
         [0025]    However, in practice it is preferred that the number of groups is limited and that most or all of the groups include more than one layer. For example, the total number of groups may be two or three in order to limit the impact on the simplicity and speed of the manufacturing process. This approach results in an object having a corresponding number of continuous portions having different material properties. 
         [0026]    In these embodiments, it is advantageously possible to select for the group of layers including the first layer provided and/or for the group of layers including the last layer provided, i.e. for one or both of the groups including a layer defining an outer surface of the manufactured object, the corresponding gas atmosphere, the beam spot size, the gas atmosphere temperature and/or the layer temperature in such a manner that desired physical and/or chemical characteristics of the respective surface are obtained. Thus, it is easily possible to provide the object with specific functional surfaces. If only the properties of one or both surfaces shall be influenced, it is preferable to use only two and three groups, respectively, wherein the group including the layer or layers defining the surface or surfaces at issue may be chosen to include as few layers as possible. In any case, functional surfaces can be generated without post-processing during the normal manufacturing process in the same manner in which bulk properties of the object are controlled. 
         [0027]    In a further preferred embodiment, which may be combined with any of the above embodiments, the pressure, the composition or the pressure and composition of the gas atmosphere inside the chamber is gradually changed between first and second predetermined values for a plurality of adjacent ones of the layers when moving from the first to the last layer of that plurality of adjacent layers. In other words, using the above terminology, each layer of that plurality of layers forms an own group or set (including only the corresponding layer). A first predetermined gas atmosphere is selected for the layer of the plurality of adjacent layers which is provided first. For each subsequent layer of the plurality of adjacent layers the gas atmosphere is gradually changed as compared to the gas atmosphere used for the previous layer such that for the last layer of the plurality of adjacent layers a second predetermined gas atmosphere different from the first pre-determined gas atmosphere is achieved. In this manner it is advantageously possible to construct portions of the object exhibiting a gradient in one or more material properties, and the gradual changes from layer to layer and the first and second predetermined gas atmospheres are preferably chosen to obtain a desired gradient in one or more specific material properties. 
         [0028]    In a further preferred embodiment, which may be combined with any of the above embodiments, the same applies to the beam spot size, i.e. the beam spot size is gradually changed between first and second predetermined values when moving from the first to the last layer of the plurality of adjacent layers. 
         [0029]    Similarly, in a further preferred embodiment, which may be combined with any of the above embodiments, the same applies to the gas atmosphere temperature and/or the layer temperature, i.e. the gas atmosphere temperature and/or the layer temperature is gradually changed between first and second predetermined values when moving from the first to the last layer of the plurality of adjacent layers. 
         [0030]    In a preferred embodiment making use of the first of the above three options the composition of the gas atmosphere inside the chamber is controlled such that all or at least some of the different gas atmospheres comprise different oxygen and/or different nitrogen levels. Oxygen and nitrogen are particular examples of reactive gases which are suitable for influencing the material properties of specific materials. 
         [0031]    For example, in a preferred embodiment in which the material in powder form is or comprises titanium or a titanium alloy, preferably Ti 6 Al 4 V, the oxygen level is suitable to control the strength and the ductility provided by the corresponding layers. In particular, it has been found that an increase in oxygen level results in increased strength but decreased ductility, and that a decrease in oxygen level results in decreased strength but increased ductility. 
         [0032]    As a further example, in a preferred embodiment in which the material in powder form is or comprises steel, the nitrogen level is suitable to control various material properties, such as the corrosion resistance. 
         [0033]    In a preferred embodiment making use of the first of the above three options the pressure, the composition or the pressure and composition of the different gas atmospheres are selected such that the layers irradiated under different atmospheres have different physical characteristics. 
         [0034]    In a preferred embodiment making use of the second of the above three options the beam spot size is selected such that the layers irradiated with different beam spot sizes have different physical characteristics. 
         [0035]    In a preferred embodiment making use of the third of the above three options the gas atmosphere temperature, the layer temperature or both the gas atmosphere and layer temperatures are selected such that the layers irradiated using different gas atmosphere and/or layer temperatures have different physical characteristics. 
         [0036]    In a preferred embodiment the material in powder form is selected from the group consisting of metal material, plastic material, ceramic material and glass material. In this regard, the material in powder form may include only a single specific one of these materials or a mixture of one or more different materials. 
         [0037]    The above-described method can be advantageously carried out using an apparatus which comprises a housing defining a chamber, a gas supply system adapted for introducing gas into the chamber, a gas venting system adapted for venting gas from the chamber, a support means disposed inside the chamber, a powder delivery means for providing the plurality of layers of material in powder form one on top of the other on the support means, a temperature control means adapted for selectively controlling the temperature of the gas atmosphere present inside the chamber and/or of the layers during irradiation thereof, an irradiation device, such as an electron beam device or preferably a laser device, adapted for irradiating each of the layers provided by the powder delivery means on the support means with a laser or particle beam, a beam spot size control means adapted for selectively controlling the spot size of a beam emitted by the irradiation device on the layers during irradiation thereof, a beam movement means—i.e. a laser or particle beam movement means—adapted for selectively irradiating only portions of each of the layers provided by the powder delivery means on the support means, a storage means for storing a digital representation of a three-dimensional object in the form of a plurality of layers, and a control unit operatively coupled to the gas supply system, the gas venting system, the powder delivery means, the temperature control means, the irradiation device, the beam spot size control means, the beam movement means and the storage means and adapted for operating the powder delivery means, the irradiation device and the beam movement means to manufacture a three-dimensional object in accordance with a digital representation of the object stored in the storage means. To this extent the apparatus corresponds to known apparatuses for additive layer manufacturing. 
         [0038]    The storage means is further adapted for storing, for each digital representation of a three-dimensional object stored in the storage means and as a function of the layers of the digital representation, pressure data, in particular digital pressure data, composition data, in particular digital composition data, or both pressure and composition data representative of different gas atmospheres having different predetermined pressures and/or compositions, beam spot size data, in particular digital beam spot data, representative of different beam spot sizes, and/or temperature data, in particular digital temperature data, representative of different gas atmosphere temperatures and/or layer temperatures. Thus, the digital file stored in the storage means for defining layer by layer the structure of an object to be manufactured is extended with data defining the gas atmospheres to be used for the individual layers, the beam spot sizes to be used for the individual layers, and/or the gas atmosphere temperatures and/or layer temperatures to be used for the individual layers. The latter data can be provided, for example, as separate data sets for each layer, as data defining the above-described groups and the gas atmospheres, the beam spot sizes, the gas atmosphere temperatures and/or the layer temperatures for the groups, or as data describing if and how the gas atmosphere, the beam spot size, the gas atmosphere temperature and/or the layer temperature is to be changed when moving from one layer to the next. For each particular application, the choice of the type of data stored in the storage means depends, of course, on which of the three above-mentioned options are to be realized in the respective method to be carried out with the apparatus. 
         [0039]    Finally, the control unit is further adapted for automatically controlling the pressure, the composition or the pressure and composition of the gas atmosphere inside the chamber in accordance with pressure and/or composition data stored in the storage means for the three-dimensional object being manufactured, automatically controlling the beam spot size in accordance with beam spot size data stored in the storage means for the three-dimensional object being manufactured, and/or automatically controlling the gas atmosphere temperature and/or the layer temperature in accordance with temperature data stored in the storage means for the three-dimensional object being manufactured. This requires, of course, that the control unit is adapted for reading out the pressure and/or composition data, the beam spot data and/or the temperature data from the storage means. For achieving the above control the control unit is operatively coupled with the gas supply system and the gas venting system (at least if the first option is to be realized), with the beam spot size control means (at least if the second option is to be realized) and/or with the temperature control means (at least if the third option is to be realized) and is adapted for automatically controlling them in a suitable manner. 
         [0040]    In this regard, the gas supply system preferably includes gas storage means for one gas or several different gases and suitable valve means allowing for selective introduction of the respective gases into the chamber and for selectively opening and sealingly closing the chamber towards the gas supply system. Further, the gas venting system preferably comprises one or more valves for selectively opening and closing the chamber towards the environment or a gas receptacle, and preferably also a pump adapted for pumping selective amounts of gas out of the chamber. 
         [0041]    Further, the beam spot size control means preferably includes an adjustable focusing means, such as an adjustable lens system, or alternatively or in addition a means adapted for adjusting the distance between the irradiation device and the irradiated surface of the layer being irradiated, i.e. the upper surface of the topmost layer or the surface of the layer onto which the beam is incident. In this respect the irradiation device includes a focusing means for focusing the beam onto the surface, and the adjustable distance is the distance—along the beam path—between the focusing means and the surface. The distance may be adjusted by either moving the focussing means or, more preferably, by suitably moving the support means, which must already be movable for receiving the various layers. 
         [0042]    Further, the temperature control means preferably includes a heating and/or cooling device thermally coupled to the gas atmosphere and/or the layers. With respect to the coupling to the layers, the heating and/or cooling device may e.g. be thermally coupled to the support means. 
         [0043]    Further, it is preferred that the apparatus comprises a gas atmosphere detector system adapted for detecting the pressure, the composition or both the pressure and composition of the gas atmosphere present inside the chamber. The control unit is then operatively coupled with the gas atmosphere detector system and in use receives detection signals characteristic of the pressure and/or composition from the gas atmosphere detection system, and the control unit is adapted for controlling the gas atmosphere on the basis of the received detection signals. 
         [0044]    Further, it is preferred that the apparatus comprises a temperature detector system adapted for detecting the gas atmosphere temperature, the layer temperature or both the gas atmosphere temperature and the layer temperature. The control unit is then operatively coupled with the temperature detector system and in use receives detection signals characteristic of the gas atmosphere temperature and/or layer temperature from the temperature detection system, and the control unit is adapted for controlling the respective temperature on the basis of the received detection signals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    In the following an embodiment of the invention is explained in more detail with reference to the drawings. 
           [0046]      FIG. 1  is a schematic representation of an apparatus according to the invention for manufacturing a three-dimensional object by selective layer melting. 
           [0047]      FIG. 2  is a schematic representation of a three-dimensional object to be manufactured. 
           [0048]      FIG. 3  is a flow chart of an embodiment of a method according to the invention for manufacturing a three-dimensional object by additive layer manufacturing. 
       
    
    
     DETAILED DESCRIPTION 
       [0049]    The apparatus  1  for selective laser melting (SLM) shown in  FIG. 1  comprises a housing  2  defining an interior chamber  3 . In the bottom wall  4  of the housing  2  two integrated powder containers  5  are provided, each having a bottom provided by a movable powder feed piston  6 . Further, a portion of the bottom wall  4  of the housing  2  is defined by a movable build platform  7 . More particularly, the build platform  7  is movable upwardly and downwardly inside a channel-shaped extension  8  of the housing  2  and sealingly engages the channel walls thereof. 
         [0050]    In operation powder stored in the powder containers  5  is fed into the chamber  3  by moving upwardly one or both of the powder feed pistons  6  and is distributed as a thin layer on the top surface of the build platform  7  or of a partial object  11  disposed thereon by operating a powder spreading roller  9  which is movable in the horizontal direction. In this regard, prior to operating the powder spreading roller  9  the build platform  7  is moved downwardly inside the channel  8  such that the vertical distance between the upper end  10  or the bottom wall  4  of the housing  2  and the top surface of the build platform  7  or a partial object  11  disposed thereon is identical to the thickness of the powder layer to be distributed. 
         [0051]    After each powder layer has been distributed a laser  12  is operated to irradiate the layer with a laser beam  13 . The laser beam  13  is moved over the layer by means of a movable mirror  14 , and the laser  12  and the mirror  14  are operated in such a manner that only selective portions of the layer are irradiated. In those portions the powder melts and forms a part of a three-dimensional object corresponding to the respective layer. 
         [0052]    Following the irradiation the above steps are repeated, i.e. the build platform  7  is moved downwardly by a distance corresponding to the thickness of the subsequent layer, and the subsequent layer is provided on top of the previous layer by means of the powder feed pistons and the powder spreading roller  9  and is irradiated by means of the laser  12  and the mirror  14 . 
         [0053]    The above process is carried out automatically under the control of a control unit  18 . For this purpose, the control unit  18  is operatively coupled to the powder feed pistons  6 , the build platform  7 , the powder spreading roller  9 , the laser  12  and the mirror  14  (for reasons of clarity of the Figure these couplings are not shown in the Figure) such that it can move and operate these elements as described above. The control is effected on the basis of digital data stored in a memory  19  of the control unit  18 . For manufacturing a particular three-dimensional object, digital data are stored in the memory  19  describing layer for layer the structure of the object. 
         [0054]    During irradiation of each of the layers a defined gas atmosphere is maintained inside the chamber  3 . For this purpose, the apparatus  1  comprises a gas supply system  15  and a gas venting system  16 . The gas supply system  15  comprises suitable tanks or containers for one or more gases and one or more valves and pumps for selectively introducing gas from one or more of the tanks or containers into the chamber  3 . The gas venting system  16  comprises one or more valves and pumps for removing gas from the chamber  3 . Further, a detector  17  is disposed inside the chamber  3 , which detector  17  is operable to detect particular characteristics of the gas atmosphere present inside the chamber  3  and to provide corresponding detection signals. 
         [0055]    As shown in  FIG. 1 , the gas supply system  15 , the gas venting system  16  and the detector  17  are operatively coupled to the control unit  18  such that in operation the control unit  18  can send control signals to the gas supply system  15  and the gas venting system  16  and can receive status signals from the gas supply system  15  and the gas venting system  16  and the detection signals provided by the detector  17 . 
         [0056]    This allows for an automatic control of the gas atmosphere by the control unit  18 . This control is likewise effected on the basis of digital data stored in the memory  19  of the control unit  18 . For manufacturing a particular three-dimensional object, in addition to the digital data mentioned above further digital data are stored in the memory  19  describing the gas atmosphere to be created and maintained in the chamber  3  for the individual layers. For example, digital data may be stored which divide the plurality of layers represented by the other digital data stored for the object in the memory  19  into two or more separate groups or sets, each including a plurality of adjacent layers.  FIG. 2  schematically illustrates a layered representation of an object to be manufactured, wherein three groups  20   a ,  20   b ,  20   c  of layers are defined. Further, digital data are stored for each group defining the gas atmosphere to be maintained during the irradiation of the layers belonging to the respective group. On the basis of these digital data, which retrieved by the control unit  18  from the memory  19 , a corresponding control of the gas atmosphere is effected as described above. 
         [0057]    As can be taken from the example shown in  FIG. 2 , it is therefore possible to selectively provide the two surface regions corresponding to the groups  20   a  and  20   c  with characteristics different from the characteristics of the bulk group  20   b.    
         [0058]    According to the illustrated embodiment, the apparatus  1  may also include a heating and cooling device  21  disposed inside the chamber  3  and adapted for measuring the temperature of the gas atmosphere inside the chamber  3 . Such a heating and cooling device  21  can then be used for maintaining during irradiation of each of the layers a defined gas atmosphere temperature inside the chamber  3 . Further, the detector  17  or a separate detector is then also configured to detect the gas atmosphere temperature inside the chamber  3  and to provide corresponding detection signals to the control unit  18 . As shown in  FIG. 1 , the heating and cooling device  21  is likewise coupled to the control unit  18 , such that in operation the control unit  18  can send control signals to the heating and cooling device  21  and can receive detection signals provided by the detector  17 . 
         [0059]    This allows for an automatic control of the gas atmosphere temperature by the control unit  18 . The control is effected in a manner corresponding to the automatic control of the gas atmosphere, i.e. for manufacturing a particular three-dimensional object, in addition to the digital data mentioned above further digital data are stored in the memory  19  describing the gas atmosphere temperature to be established and maintained in the chamber  3  for the individual layers. The effects, advantages and possibilities are the same as the one described above for the automatic control of the gas atmosphere. 
         [0060]    According to the illustrated embodiment the control unit  18  may also be adapted for positioning the build platform  7 , after each layer has been provided on top of the build platform  7  or the preceding layers, such that the upper surface of the topmost layer is located at a height resulting in a particular beam spot size. This allows for an automatic control of the beam spot size by the control unit  18 . The control is effected in a manner corresponding to the automatic control of the gas atmosphere and the automatic control of the gas atmosphere temperature, i.e. for manufacturing a particular three-dimensional object, in addition to the digital data mentioned above further digital data are stored in the memory  19  describing the beam spot size to be established and maintained for the individual layers. The effects, advantages and possibilities are the same as the one described above for the automatic control of the gas atmosphere. 
         [0061]      FIG. 3  shows an embodiment of a method  30  of manufacturing a defined three-dimensional object using the apparatus  1 . 
         [0062]    In step  31  digital data are stored in the memory  19  describing layer for layer the structure of the object. These data are adapted for providing information to the control unit  18  allowing it to control the powder feed pistons  6 , the build platform  7 , the powder spreading roller  9 , the laser  12  and the mirror  14  are operated such that the final object has the desired structure. 
         [0063]    In step  32  digital data are stored in the memory  19  defining two or more groups  20   a ,  20   b ,  20   c  of layers, wherein all layers building up the object are divided into these groups  20   a ,  20   b ,  20   c  (see also  FIG. 2 ). 
         [0064]    Moreover, in step  33  digital data are stored in the memory  19  defining the pressure and composition of the gas atmosphere to be used for these groups  20   a ,  20   b ,  20   c . Thus, the digital data define different gas atmospheres and which of the gas atmospheres is to be used for each of the groups  20   a ,  20   b ,  20   c . The number of gas atmospheres may be equal to or smaller than the number of groups  20   a ,  20   b ,  20   c . In the latter case, the same gas atmosphere is used for two or more non-adjacent ones of the groups  20   a ,  20   b ,  20   c.    
         [0065]    The storing of the various digital data in the memory  19  may e.g. be carried out by inserting a removable data carrier storing the data into a corresponding reading device provided in the control unit  18 , wherein the control unit  18  is operable for transferring the data from the removable data carrier to the memory  19 . In addition or alternatively, the control unit  18  may be connected or connectable to a wired or wireless data transmission network over which the digital data to be stored in the memory  19  can be received by the control unit  18 . 
         [0066]    Next, based on the digital data the gas atmosphere in the chamber  3  is controlled to have the pressure and composition of the gas atmosphere associated with the group  20   a ,  20   b ,  20   c  to which the current layer belongs (step  34 ). At the beginning of the process this is the group  20   a  to which the very first layer to be provided on the top surface of the build platform  7  belongs. Once this has been done, the build platform  7  is positioned in the above-described manner to receive the current layer of powder material (step  35 ), and the powder feed pistons  6  and the powder spreading roller  9  are operated to provide the layer of powder material on the build platform  7  (step  36 ). The laser  12  and the mirror  14  are then operated to irradiate the layer in accordance with the corresponding structural digital data associated with the respective layer (step  37 ). 
         [0067]    Following the irradiation of each of the layers it is determined whether the current layer is the last layer (step  38 ) and the process is ended if that is the case. Otherwise, it is determined if the subsequent layer belongs to a different group  20   b  associated with a different gas atmosphere. In the affirmative, the method reverts to step  34  for creating the different gas atmosphere, and otherwise the method reverts to step  35  for positioning the build platform  7  for receipt of the subsequent layer (step  39 ). 
         [0068]    The above steps are repeated until the last layer has been irradiated and the object is completed (step  40 ). 
         [0069]    A control of the gas atmosphere temperature and/or the beam size can be achieved similarly.