Patent Publication Number: US-2023146425-A1

Title: Material deposition unit for powder build-up welding

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP2021/052570 (WO 2021/156317 A1), filed on Feb. 3, 2021, and claims benefit to German Patent Application No. DE 10 2020 103 175.0, filed on Feb. 7, 2020. The aforementioned applications are hereby incorporated by reference herein. 
    
    
     FIELD 
     Embodiments of the invention relate to a material deposition unit. 
     BACKGROUND 
     Laser build-up welding (also Laser Metal Deposition (LMD), Direct Metal Deposition (DMD) or Direct Energy Deposition (DED)) is a generative manufacturing method for metallic structures. 
     In principle, laser build-up welding is carried out as follows: On a component surface, by means of a laser a melt pool is created or a base material forming the component surface is heated. When reference is made to “melt pool” below, what is also meant by this is a general process zone which comprises a heated or molten base material. The melt pool can for example melt a few micrometers of the base material, but greater melting depths are also conventional. Metal powder is introduced in an automated manner by means of a powder discharge device, usually in the form of a nozzle. The result is beads or material layers which are welded to one another and produce structures on existing or new basic bodies or components. 
     (Laser) build-up welding makes it possible for example to apply 3D structures to existing or new, possibly also uneven, surfaces. Geometry modifications can be easily implemented in this way. By modifying the powder and/or the powder composition, it is possible to switch between various materials in one work process. It is also possible to blend the powder used from different materials and thereby to create alloys. To provide wear-resistant layers, it is for example possible to feed a matrix material in powder form which melts in the melt pool and in addition to feed a hard material, which typically does not melt at the temperatures prevailing in the melt pool, likewise in powder form. 
     What is referred to as extreme high-speed laser build-up welding (EHLA) is already known from DE 10 2011 100 456 B4. According to this method, a significant increase in the achievable machining speed compared to conventional laser build-up welding is obtained in that at least one completely molten filler material is fed to a melt pool present on a surface to be machined. For this purpose, the filler material, which initially is present in particular as a powder, is melted by means of a laser beam at a distance from the melt pool of greater than zero and then fed to the melt pool in completely liquid form. In this respect, the same laser beam can melt the filler material, in particular the powder, at the stated distance from the melt pool and heat the melt pool. The laser beam incident on the melt pool thus also causes the filler material to melt at the stated distance from the melt pool. This is performed by moving the melt pool and a focus of the laser beam parallel to one another relative to the surface at a speed of at least 20 m/min. Furthermore, in the case of a powdered filler material, the powder density can be set in particular such that a laser power output of the laser beam in the melt pool is less than 60% of the laser power output before the laser beam makes contact with the powder. 
     During laser build-up welding, it is conventional to use a material deposition unit having a laser unit which is configured to direct a laser beam onto a workpiece, and having a powder discharge device which is configured to discharge powder in a directed form onto the workpiece. 
     In this context, the powder discharge device is conventionally designed in such a way that it discharges the material powder in the direction of the workpiece via an annular die or multiple powder discharge units, as disclosed e.g. in US 5961862 A, which may be in the form of powder-outlet openings, for example. This results in one powder jet or a plurality of powder jets. These powder jets are focused in a material focal zone. In this context, the existing systems are sensitive to the distance of the powder discharge device and the powder focal position from the workpiece and to the combination of angle of contact (angle at which the laser beam is directed on the workpiece), distance and diameter of the material focus. 
     Moreover, the distribution of the powder over the individual powder discharge units constitutes a problem. The powder mass stream that is fed should be divided as uniformly as possible over the individual powder discharge units, it being the intention to keep the installation space needed for the powder discharge device as small as possible. 
     SUMMARY 
     Embodiments of the present invention provide a material deposition unit that includes a radiation unit designed to emit electromagnetic radiation in a directed manner onto a workpiece along a beam axis extending in a beam direction. The material deposition unit further includes a powder discharge device that has multiple powder discharge units configured to discharge powder in a directed form onto the workpiece through powder-outlet openings. The material deposition unit further includes a powder division unit having multiple powder channels. A number of powder channels corresponds to a number of powder discharge units. The powder division unit is designed to distribute a central powder stream guided to a feed channel uniformly over the powder channels. Each respective powder channel is connected to a respective powder discharge unit by an exchangeable connecting element. At least one powder discharge unit has an exchangeable powder discharge element. The powder discharge element is elongate, has a first end and a second end, and is arranged at least partially within the corresponding powder discharge unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Subject matter of the present disclosure will be described in even greater detail below based on the exemplary figures. All features described and/or illustrated herein can be used alone or combined in different combinations. The features and advantages of various embodiments will become apparent by reading the following detailed description with reference to the attached drawings, which illustrate the following: 
         FIG.  1    shows a side view of a powder discharge device of a material deposition unit according to embodiments of the invention; 
         FIG.  2    shows a cross section, from the side, of the powder discharge device according to  FIG.  1   ; 
         FIG.  3    shows a perspective view of the powder discharge device according to  FIG.  1   ; 
         FIG.  4    shows a cross section, from the side, of a first functional zone of the powder division unit according to embodiment of the present invention; 
         FIG.  5    shows a cross section, from the side, of a second functional zone of the powder division unit according to embodiment of the present invention; 
         FIG.  6    shows a plan view of a separating part of the powder division unit according to embodiment of the present invention; 
         FIG.  7    shows a cross section, from the side, of a separating part according to  FIG.  6   ; 
         FIG.  8    shows a perspective view of a separating part according to  FIG.  6    according to embodiment of the present invention; 
         FIG.  9    shows a cross section, from the side, of a separating part according to  FIG.  6    in a state connected to a second functional zone according to  FIG.  5    according to embodiment of the present invention; 
         FIG.  10    shows a cross section, from the side, of a powder discharge device according to  FIG.  1    in a state connected to a separating part according to  FIG.  6   ; 
       _ FIG.  11    shows a cross section, from the side, according to  FIG.  10    with a powder discharge element according to embodiment of the present invention; and 
         FIG.  12    shows a cross section, from the side, according to  FIG.  10    with a further exemplary embodiment of a powder discharge element according to embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The material deposition unit according to embodiments of the invention comprises: A radiation unit designed to emit electromagnetic radiation in a directed manner, in particular a laser unit, a powder discharge device and a powder division unit. 
     The radiation unit, in particular laser unit, is configured to direct electromagnetic radiation, in particular a laser beam, onto a workpiece along a beam axis extending in a beam direction, and in particular to focus it there (it may also be provided that defocusing is effected at the workpiece). On the workpiece, the radiation, in particular the laser beam impinging there, in particular focused there, creates a weld pool or a melt pool. 
     The powder discharge device is configured to discharge a material powder, which typically is or comprises a metallic or ceramic powder, onto the workpiece. Typically, this is realized by a powder/gas jet. In this respect, the powder discharge device comprises multiple, in particular at least seven, in particular exactly seven, powder discharge units, which are configured to discharge the powder through powder-outlet openings in directed form (for example in the form of a jet or multiple jets) onto the workpiece. 
     Typically, the material deposition unit is designed in such a way that the powder discharge units (in particular in the form of channels to be directed toward one another) are uniformly distributed circumferentially around the beam axis. This results in particular in a preferred uniform build-up behavior of the material deposition unit. 
     The powder division unit comprises multiple powder channels. In this context, the number of powder channels corresponds to the number of powder discharge units. In this way, for example, when there are seven powder discharge units, seven powder channels are provided. The powder division unit distributes a central powder stream uniformly over the individual powder channels. The powder division unit is typically rotationally symmetrical or has a rotationally symmetrical cross section. 
     The powder channels are each connected to the individual powder discharge units by means of an exchangeable connecting element. Thus, for example, when there are seven powder channels or powder discharge units, seven connecting elements are likewise provided. The connecting elements are preferably in the form of flexible hoses. 
     At least one powder discharge unit has an exchangeable powder discharge element. It is preferably the case that all powder discharge units have a respective powder discharge element. Such a powder discharge element is preferably a small tube. The powder discharge element is elongate and has a first end and a second end. The powder discharge element is arranged at least partially within the corresponding powder discharge unit. 
     The material of the powder discharge element is preferably a material that reflects near infrared radiation, in particular with a high coefficient of heat conduction. The material is preferably a hard metal alloy, e.g. copper alloy. It is consequently possible to reduce wear and lengthen the service life (time in use). Moreover, this makes it possible to not have to additionally package the powder discharge element. 
     The connecting elements allow the powder division unit to be decoupled from the powder discharge unit, therefore making pivoting or tilting possible, for example. Moreover, dividing the central powder stream into multiple, in particular seven, discrete individual powder jets achieves a sufficiently high flow velocity along the powder conveying section, in particular within the powder channels, the connecting elements, the powder discharge units and powder discharge elements, with the result that the individual powder/gas jets are not influenced or are scarcely influenced by gravitational force or other acting forces. This increases the flexibility and the areas of use of the material deposition unit. 
     At least one powder discharge unit may be configured in such a way that a powder discharge element arranged at least partially in this powder discharge unit can be exchanged for another powder discharge element. In this respect, the two exchanged powder discharge elements can have the same outside diameter. This is due to the fact that the two powder discharge elements have to fit at least partially into the same powder discharge unit. However, the two powder discharge elements can have a different inside diameter and/or different lengths. 
     The second end of at least one powder discharge element may be arranged in the region of the powder-outlet opening of the corresponding powder discharge unit. In particular, it may be arranged flush with the powder-outlet opening, that is to say terminate flush with the powder-outlet opening. In other words, the second end of a powder discharge element may lead into the powder-outlet opening. 
     However, the second end of at least one powder discharge element may also be arranged outside the corresponding powder discharge unit. This makes the handling involved in exchanging the individual powder discharge elements easier. It is possible to grip the second end of the powder discharge element, for example using tongs, and pull it out of the powder discharge unit. 
     Advantageously, at least one connecting element may be detachably fixed by means of plug-in connections to a powder discharge unit and/or to a powder channel. The plug-in connections make it possible to easily exchange a connecting element without using a tool. This results in a reduction of possible downtimes (time in which a machine is not working), for example caused by maintenance and/or cleaning. Other types of connections are likewise conceivable, such as e.g. screw connections, bayonet connection, snap connection, etc. 
     Advantageously, at least one connecting element is at least partially rigid, flexible, straight and/or has at least one curvature. A connecting element may thus be configured as a metallic, rigid tube connection. This may be straight or have at least one bend. A connecting element may also be a flexible plastic hose or a metallic flex hose. Other materials and forms may also be used. This makes it possible to arrange the connecting elements flexibly, with the result that the degree of movement between the powder discharge device and the powder division unit can be increased. This results in greater flexibility of the material deposition unit. 
     Advantageously, the powder division unit has a longitudinal axis and comprises:
     a first functional zone, which is elongate and has a first end, a second end and a round inside diameter, and   a second functional zone, which is elongate and has a first end, a second end and a round inside diameter.   

     In this respect, the second end of the first functional zone leads in line into the first end of the second functional zone. 
     The longitudinal axis of the powder division unit may be different from the beam axis or coincide with the beam axis. 
     Advantageously, the inside diameter of the first functional zone is constant at least in certain portions, in particular at least in a portion through to the second end. The inside diameter of the first functional zone is preferably constant over the entire length of the first functional zone. The inside diameter of the first functional zone is selected to be relatively small compared to the length of the first functional zone. Thus, it may for example be 4 mm, the length being 100 mm. An inside diameter that is small compared to the length forces the powder particles into a trajectory running coaxially with the longitudinal axis of the powder division unit. This avoids undesired turbulences of the powder stream. 
     Advantageously, the second functional zone comprises a first portion and a second portion. In this respect, the second portion has an inside diameter larger than the first portion. In this way, an expansion zone is established in which the coaxially aligned particles are prepared for separation into the individual powder channels, in that they are slowed down and distributed over a larger cross section. In this context, the transition from the first portion to the second portion (and therefore the enlargement of the inside diameter) may have a configuration becoming larger in one stage or multiple stages, conically or in a curve shape. 
     The first and the second functional zone may be in the form of separate elements. They may both consist of solid, non-flexible material. Thus, for example, the first functional zone may be configured as a metal tube and the second functional zone may be made from acrylic glass. The two functional zones may be connected detachably to one another, e.g. by means of a plug-in or screw connection. In the assembled-together state, the two functional zones are aligned coaxially with one another, so that they have a common longitudinal axis. 
     Advantageously, the inside diameter of the first functional zone in the region of the second end of the first functional zone and the inside diameter of the second functional zone in the region of the first end of the second functional zone are the same. In other words, the inside diameter of the first functional zone at the second end of the first functional zone is the same as the inside diameter of the first portion of the second functional zone. This means that the powder particles in the first portion of the second functional zone also remain on their trajectory coaxial with the longitudinal axis of the powder division unit, into which trajectory they have been forced in the first functional zone. 
     Advantageously, the powder division unit has a separating part. The separating part has a top side and a bottom side. The powder channels are arranged in this separating part. They are preferably arranged rotationally symmetrically in relation to the longitudinal axis of the powder division unit. 
     The separating part is preferably in the form of a separate element and arranged on the second end of the second functional zone. The separating part may also be fixed by way of a detachable connection (see above). 
     The powder channels preferably each extend from the top side of the separating part to the bottom side of the separating part. They may be straight and run radially outward, with the result that they each span an angle Alpha with the longitudinal axis of the powder division unit. The angle Alpha is preferably smaller than 45°, in particular 40°. The angle Alpha is preferably larger than 5°, in particular 10°, in particular 15°. 
     Advantageously, an elevated region is arranged on the top side of the separating part, in particular centrally on the top side. In addition, inlet openings of the individual powder channels are arranged on the top side. In this respect, the inlet openings of the powder channels are arranged on the elevated region. The inlet openings may be uniformly arranged circumferentially around the longitudinal axis of the powder division unit. The inlet openings may be uniformly arranged circumferentially around a frustoconical projection on the elevated region. 
     As already mentioned, a frustoconical projection may be arranged on the elevated region. The lateral surface of the frustoconical projection may be designed to slope toward the nearest inlet opening of the respective powder channel. A pyramidal shape of the projection is likewise conceivable. In this case, the pyramid has as many sides as there are inlet openings. Each side of the pyramid is directed toward an inlet opening and the side face slopes toward the respective inlet opening. 
     The powder stream is thus uniformly divided at a point on the top side of the separating part over all powder channels present. 
     Advantageously, at least one powder channel may have at least two different inside diameters in certain portions. The changing inside diameter within a powder channel makes it possible to influence the trajectory and the flow velocity of the powder particles. 
     Advantageously, at least one powder discharge element may be arranged coaxially with the corresponding powder discharge unit. 
     The powder discharge device preferably has a bottom edge. In this respect, the distance between the bottom edge and the workpiece may be 12.5 mm or more. In particular, this distance is exactly 12.5 mm. With this small distance from the workpiece, it is possible to ensure a better shielding-gas covering required for laser build-up welding. 
     Further features, possible applications and advantages of the invention will become apparent from the following description of the exemplary embodiment of the invention, which will be discussed on the basis of the drawing, it being possible for the features to be essential to the invention both individually and in different combinations, without this being explicitly pointed out again. 
     In the following figures, corresponding components and elements bear the same reference signs. For the sake of better clarity, all reference signs are not reproduced in all of the figures. 
     The material deposition unit according to embodiments of the invention comprises a radiation unit, not illustrated, which is arranged in a powder discharge device  12 . The radiation unit is designed to emit electromagnetic radiation, in particular laser radiation, in a directed manner onto a workpiece along a beam axis  10  extending in a beam direction. 
       FIG.  1    shows a side view of the powder discharge device  12  of the material deposition unit according to embodiment of the invention. The powder discharge device  12  has multiple powder discharge units  14 . 
       FIG.  2    shows a section through the powder discharge device  12  according to  FIG.  1    along the line denoted A-A in  FIG.  1   . The powder discharge units  14  are in the form of channels  40  in the powder discharge device  12 . The channels  40  each have a widened portion  44  on an inlet side  42 . Each widened portion  44  bears an internal thread  46 . 
     The powder streams directed by the powder discharge units  14  onto the workpiece to be machined pass through the channels  40  and leave them through powder-outlet openings  64 . 
     The electromagnetic radiation from the radiation unit runs along the beam axis  10  through the powder discharge device  12  and leaves it at an opening  15 . The workpiece to be machined is arranged below the opening  15 . The radiation unit is configured in such a way that the electromagnetic radiation is focused on the surface of the workpiece to be machined. It may also be focused above the workpiece. 
     The powder discharge units  14  serve to discharge a powder in the form of respective powder jets in a directed form onto the workpiece. The powder discharge units  14  extend, as illustrated on the right in  FIG.  2   , in such a way that they are at an angle with respect to the beam axis  10 . The powder discharge device  12  is configured in such a way that the powder jets from the powder discharge units  14  intersect at a common focus. This focus may correspond to the focus of the electromagnetic radiation from the radiation unit. However, the focal zones of the electromagnetic radiation and the powder jets may also be spaced apart from one another, in particular along the beam axis  10 . 
     The powder discharge units  14  have a stepped form at their end remote from the common powder focus. In this way, it is possible to be able to arrange a respective connecting element at this end (screwing into the internal thread  46 ; a plug-in connection is also conceivable). The inside diameter of the connecting element and the inside diameter of the respective powder discharge unit  14  are in particular the same here. A constant inside diameter in the region of the connection between the respective connecting element and the respective powder discharge unit  14  can therefore be realized. 
       FIG.  3    shows a perspective view of the powder discharge device  12  according to  FIG.  1   . The powder discharge device  12  shown comprises seven powder discharge units  14 . The powder discharge units  14  are uniformly distributed circumferentially around the powder discharge device  12 . 
     The material deposition unit further comprises a powder division unit. In the present case, the powder division unit has two functional zones. 
       FIG.  4    shows a cross section, from the side, of a first functional zone  18  of the powder division unit. In the present case, the first functional zone  18  is arranged in a first functional element  48 . The first functional element  48  is in the form of an elongate metal tube with a constant inside diameter. The first functional zone  18  has a first end  22  and a second end  24 . During operation of the material deposition unit, the fed powder stream is conducted from the first end  22  through the first functional zone  18  in the direction of the second end  24 . The first functional zone  18  has an inside diameter which is small compared to its length. Consequently, the powder particles within the first functional zone  18  are forced onto a trajectory running axially in relation to the longitudinal extent of the first functional zone  18 . Undesired turbulences of the powder stream or of the powder particles can thus be avoided or at least reduced. 
       FIG.  5    shows a cross section, from the side, of a second functional zone  20  of the powder division unit. In the present case, the second functional zone  20  is arranged in a second functional element  50 . In the present case, the second functional element  50  is configured in the form of an acrylic glass tube. The second functional zone  20  has a first end  26  and a second end  28 . In the present case, the first end  26  of the second functional zone  20  is configured such that the second end  24  of the first functional zone  18  can be arranged in line on the first end  26  of the second functional zone  20 . For this, the second end  24  of the first functional zone  18  has a stepped form. The first end  26  of the second functional zone  20  has an accordingly corresponding stepped form. The facing ends (second end  24  and first end  26 ) of the first functional element  48  and the second functional element  50  complement one another. The facing ends of the first functional element  48  and the second functional element  50  can be plugged into one another, with the result that the second end  24  of the first functional element  48  is received in the first end  26  of the second functional element  50 . 
     The first functional zone  18  and the second functional zone  20  are part of a feed channel  21  via which powder particles can be transported by means of a gas stream. 
     The second functional zone  20  has a first portion  27  and a second portion  29 . The inside diameter of the first portion  27  is smaller than the inside diameter of the second portion  29 . In the present case, the inside diameter of the first portion  27  is the same as the inside diameter of the first functional zone  18 . The first portion  27  of the second functional zone  20  thus constitutes an extension of the first functional zone  18 , as it were. 
     The second portion  29  of the second functional zone  20  establishes an expansion zone. The enlargement of the inside diameter distributes the powder particles over a larger cross-sectional area. This reduces the flow velocity of the powder particles. 
     The powder division unit also comprises a separating part  30 . 
       FIG.  6    shows a plan view of the separating part  30  of the powder division unit. The separating part  30  is in the form of a separate element, in the same way as the first functional zone  18  and the second functional zone  20 . The separating part  30  has a circular top side  32  and a likewise circular bottom side  34 , shown in  FIG.  7   . Inlet openings  38  of the individual powder channels  16  are arranged on the top side  32 . They are uniformly distributed in the circumferential direction. 
       FIG.  7    shows a section through the separating part  30  according to  FIG.  6   . Here, the section illustrated in  FIG.  7    runs along the line denoted B-B in  FIG.  6   . 
     The top side  32  of the separating part  30  has a stepped form. The second end  28  of the second functional zone  20  has a corresponding stepped form. The separating part  30  can be arranged in an accurately fitting manner on the second functional zone  20 . During operation, the longitudinal axis of the powder division unit thus runs through the center of the top side  32  of the separating part  30 . 
     The separating part  30  has an elevated region  52 , in the center of which a frustoconical projection  54  is arranged. The inlet openings  38  of the powder channels  16  lead into the elevated region  52  and are arranged around the frustoconical projection  54 . The lateral surface of the frustoconical projection  54  slopes toward the inlet openings  38  of the powder channels  16 . 
     The second portion  29  of the second functional zone  20  leads into a receiving region  36  of the second functional zone  20 . The receiving region  36  complements the elevated region  52 , with the result that it can be inserted into the receiving region (see  FIG.  9   ). 
     The powder channels  16  extend, as illustrated on the left in  FIG.  7   , from the top side  32  of the separating part  30  to the bottom side  34  of the separating part  30 . In the present case, the powder channels  16  are straight. The powder channels  16  each have a stepped form at the end opposite the inlet opening  38 . In this way, it is possible to be able to arrange a respective connecting element at this end (plug-in connection), the inside diameter of the connecting element and the inside diameter of the respective powder channels  16  being the same. A constant inside diameter in the region of the connection between the respective connecting element and the respective powder channel  16  can therefore be realized. 
     The inside diameter within a powder channel  16  and within a powder discharge unit  14  interacting therewith and within a connecting element connecting the two to one another may be constant. 
     As can also be seen in  FIG.  7   , the powder channels  16  are arranged radially spreading out in the direction of the bottom side  34  of the separating part  30 . In other words, each powder channel  16  spans an angle Alpha with the longitudinal axis of the powder division unit. In this exemplary embodiment, this angle Alpha is less than 45°. 
     During operation of the material deposition unit, the powder mass stream is conducted through the first functional zone  18  and through the second functional zone  20 , until it impinges on the top side  32  of the separating part  30 . Here, the powder stream is distributed over the individual powder channels  16 . The individual powder particles pass through the inlet openings  38  into the individual powder channels  16 . 
       FIG.  8    shows a perspective view of the separating part  30  according to  FIG.  6   . It can be seen here that the inlet openings  38  are arranged approx. half in the region of the frustoconical projection  54  and approx. half outside the frustoconical projection  54  on the surface  32  of the separating part  30  or of the elevated region  52 . 
       FIG.  9    shows the separating part  30  in a state connected to the second functional element  50 . 
       FIG.  10    shows the powder discharge device  12  in a state connected to the separating part  30 . The powder discharge device  12  and the separating part  30  are connected by way of one connecting element  56  per powder discharge unit  14  or per powder channel  16 . In the present case, only the connecting element  56  located in the sectional plane is shown. The connecting element  56  is plugged in the separating part  30  by means of a first coupling unit and screwed in the powder discharge device  12  by means of a second coupling unit  59 . The connecting element  56  may be in the form of a flexurally rigid metallic tube or else a flexible hose connection. 
       FIG.  11    shows a cross section, from the side, according to  FIG.  10    with a powder discharge element  60 . In the present case, the powder discharge element  60  is in the form of a small tube  62 . The powder discharge element  60  is plugged and pressed into the corresponding powder discharge unit  14 . The powder discharge element  60  has a first end  66  and a second end  68 . The powder discharge element  60  is aligned in such a way that it is aligned with its second end  68  in the direction of the workpiece. 
     In the present case, the powder discharge device  12  illustrated has a stepped offset  69  around the opening  15 . This makes it possible to prevent the powder discharge element  60 , or the small tube  62 , slipping through in the direction of the workpiece. 
     In the present case, the length of the powder discharge element  60  is selected in such a way that a region  72  projects out of the powder-outlet opening  64 . This exposed region  72  can make it easier to exchange the powder discharge element  60 . For this, it is advantageous to not provide the above described offset  69 , or to make the offset  69  as small as possible, with the result that it is not an obstacle or is an obstacle to the smallest possible extent when the powder discharge element  60  is being exchanged. To exchange the illustrated powder discharge element  60 , it is possible, for example, to use tongs to grip the powder discharge element  60  in the exposed region  72  and pull it out of the powder discharge unit  14  through the powder-outlet opening  64 . Such pulling out of the powder discharge element  60  is often easier, since dirt possibly clings to the powder discharge element  60  and makes it more difficult to push it inward through the powder discharge unit  14 , in the direction of the inlet side  42 . 
     The powder discharge device  12  shown in  FIG.  11    has a bottom edge  70 . In the exemplary embodiment illustrated, the edge  70  constitutes the deepest point of the powder discharge device  12 . In the case of EHLA, the distance between the workpiece and this edge  70  is preferably 12.5 mm or more. At this small distance, a better shielding-gas covering can be ensured. 
       FIG.  12    shows a cross section, from the side, according to  FIG.  10    with a further exemplary embodiment of a powder discharge element  61 . The exemplary embodiment of the powder discharge element  61  differs from the powder discharge element  60  shown in  FIG.  11    in that it has a larger inside diameter. It is possible to select or set a desired inside diameter by mutually exchanging the powder discharge elements  60  and  61 . 
     The inside diameter of the powder discharge element  60  or  61  is decisive for the diameter of the powder stream flowing through the powder discharge element  60 ,  61 . In this way, by varying the inside diameters of the powder discharge elements  60 ,  61 , it is possible to set the diameter of the individual, in the present case seven, powder streams. 
     Varying the individual powder stream diameters makes it possible to vary the common powder focus diameter. If, for example, larger inside diameters of the powder discharge elements  60 ,  61  are selected, this results in larger powder diameters of the individual, in the present case seven, powder streams, which in turn results in a larger common powder focus diameter. Correspondingly, smaller inside diameters of the powder discharge elements  60 ,  61  result in a smaller common powder focus diameter. 
     Since the two mutually exchanged powder discharge elements  60 ,  61  must at least partially fit into the same powder discharge unit  14 , the powder discharge elements  60 ,  61  preferably have the same outside diameter. 
     While subject matter of the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Any statement made herein characterizing the invention is also to be considered illustrative or exemplary and not restrictive as the invention is defined by the claims. It will be understood that changes and modifications may be made, by those of ordinary skill in the art, within the scope of the following claims, which may include any combination of features from different embodiments described above. 
     The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.