Abstract:
A hopper ( 30 ) holds a metal and flux powder ( 24 ). A filler tube ( 28 ) conveys the powder from the hopper. Compressed gas ( 36 ) is injected into the powder to fluidize and convey the powder through the filler tube. The hopper may be vibrated ( 34 ) to prevent clumping. A gas permeable envelope ( 29 ) surrounds the filler tube and is filled with powder as it moves off the end of the filler tube. The gas escapes from the permeable envelope. Feed mechanisms ( 54, 56, 66, 74 ) may feed gas permeable sheets ( 55, 57 ) over opposite sides of the filler tube. A seaming device ( 58 A-B,  78 A-B) may seam the sheets along their edges to form the gas permeable envelope surrounding the filler tube. Closing ( 40 ) and cutting ( 42 ) machines close and cut the envelope, forming a packet ( 22 ) containing the powder.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of additive manufacturing, and more specifically to the production of packets of powdered metal or powdered metal and flux for preplacement of filler material for laser deposition in solid freeform fabrication and repair, and particularly for laser cladding on superalloy turbine components. 
       BACKGROUND OF THE INVENTION 
       [0002]    Solid Freeform Fabrication (SFF) technologies produce functional metal parts by layer-wise accumulation and consolidation of feed material (e.g. powder or wire), allowing parts to be produced with a high geometric freedom directly from a CAD model. The feed material is called “filler” because it provides additive material that forms a bead or layer for repair or fabrication. A group of SFF technologies known as direct metal laser fabrication (DMLF) utilizes lasers to consolidate powder. Other groups use tungsten inert gas (TIG), metal inert gas (MIG), or electron beam technologies. 
         [0003]    In additive manufacturing, a component is fabricated by building it in layers. Each layer is melted, sintered, or otherwise integrated onto a previous layer. Each layer may be modeled as a slice of a numeric solid model of the component. Superalloy materials are among the most difficult materials to fabricate and repair due to their susceptibility to melt solidification cracking and strain age cracking. The present inventors have previously disclosed methods for successfully depositing the difficult to weld superalloys by selective laser melting (SLM) of superalloy material in the presence of flux material, such as described in United States patent application publication number US 2013/0140278 A1, incorporated by reference herein. The filler material may be delivered to the point of processing as a filler wire or strip or powder. Powder may be delivered continuously, most commonly assisted by a delivery gas such as air, nitrogen or an inert gas, or it may be preplaced on the processing surface. Advantages of preplacement include:
       a precise amount of filler material can be located at the point of processing;   there is minimal wastage; and   complications associated with continuous feeding of material are avoided, such as the need for a delivery gas and restrictions on the size of the particles that can be delivered. The disadvantages of preplacement include:   only relatively horizontal surface can be processed; and   preplacement is somewhat labor intensive and generally slower than continuous processing.       
 
         [0009]    The present inventors have previously disclosed the idea of preplacing powder in the form of powder packets. Powder packets are convenient and efficient in terms of tooling and labor compared to preplacing loose powder, so high throughput processes are possible. Packets are also more conveniently retained on a non-horizontal or curved surface. Thus, the number of commercial applications of additive manufacturing utilizing powder packets is expected to increase rapidly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention is explained in the following description in view of the drawings that show: 
           [0011]      FIG. 1  is a side sectional view of a filler packet containing a powdered filler material for laser deposition. 
           [0012]      FIG. 2  shows a filler packet production machine and process. 
           [0013]      FIG. 3  is a sectional view of multiple filler packets interconnected to form a filler preform for laser deposition. 
           [0014]      FIG. 4  illustrates a machine for continuous production of filler packets for laser deposition. 
           [0015]      FIG. 5  is a perspective view of a sewing machine presser foot with longitudinal and transverse stitching needles and slots for upper feed dogs. 
           [0016]      FIG. 6  shows a needle plate corresponding to the presser foot of  FIG. 5  with lower feed dogs. 
           [0017]      FIG. 7  shows a quilted filler packet as produced by the machine of  FIGS. 4-6 . 
           [0018]      FIG. 8  illustrates a machine that forms a continuous braided tube around a feed tube serving as a mandrel, and fills the braided tube to form filler packets. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The present inventors have found that the use of powder packets during an additive manufacturing process is very desirable, but that the production of those packets can be very time consuming and labor intensive, particularly to form filler packets for gas turbine engine applications where manufacturing and repair tolerances may be very tight. Accordingly, the inventors have developed techniques and devices for producing filler powder packets that advance the art and that facilitate the commercial implementation of this technology. 
         [0020]    The present inventors utilize packets of powdered filler material for laser deposition in solid freeform fabrication and repair of difficult to weld superalloy components. The packets contain metal and flux for preplacement on an article being repaired or fabricated. A laser beam melts the metal into a layer and fuses it to the article. The flux facilitates metal fusion and provides a slag blanket that shields the melt pool from air. It also traps heat, which speeds and facilitates melting, reduces power input, and slows cooling, making solidification more consistent. The flux scavenges contaminants such as oxides. The packets can hold the powder in position on an inclined and/or curved surface, unlike open powder beds, and prevents powder scattering and shifting during processing. The term “metal” is used herein in a general sense and is meant to include pure metals as well as metal alloys. 
         [0021]      FIG. 1  is a side sectional view of a filler packet  20  for selective laser melting, including a closed envelope  22  containing a powdered filler material  24 . The envelope may be cylindrical or flat as later shown. Each end of the envelope is closed with a respective closure  26 A,  26 B, such as a tie, stitch, adhesive, or melt. Commercially available adhesives include zirconia silica adhesives and alumina silica adhesives. The powder particles may constitute respective metal and flux particles mixed in a pre-determined volume ratio or the particles may constitute metal particles coated with or containing flux, or the powder may be metal particles alone with fluxing action provided by the material of the envelope  22 , or the powder may be flux particles alone. The envelope may be formed of gas-permeable sheet material with perforations smaller than the particles, including woven and non-woven sheets. The envelope material may contribute to the flux, and should not create detrimental smoke and ash. For example a woven or non-woven fabric of alumina or silica fiber may be used. The powder  24  may be unbound, meaning loose, as opposed to consolidated, compacted, or sintered into a block. A benefit of unbound particles is that laser energy penetrates to a greater depth by reflection between the particles than with a solid filler preform. 
         [0022]      FIG. 2  shows a packet filling machine and process in which a filler tube  28  from a powder hopper  30  is inserted into an open end  32  of a gas permeable tube or envelope  29 . The powder may be fluidized by a vibration mechanism  34 . Compressed gas  36 , such as air, may be injected into the hopper to convey the powder through the filler tube. If the gas is injected into the lower 40% or 20% of the hopper, and under the top surface of the powder  24  as shown, it also fluidizes the powder to prevent clumping and clogging. Such energized powder flow allows the filler tube to be non-vertical and curved as desired without blockage. The gas permeable envelope  29  filters the particles, keeping them inside while allowing the gas to escape  38 . The bottom end of the envelope is closed  26 A before filling, and the top end  32  is closed  26 B after filling and removal of the filler tube. The filler tube may be withdrawn from the gas permeable tube as filling progresses, either by moving the filler tube upward or moving the gas permeable tube downward or both. An automated closing mechanism  40  may be provided, such as a stitching machine, an adhesive machine, a heat sealer, a twist tie machine, or a stapler. Stitching thread may match the sheet material, such as alumina or silica fiber. Alternately, materials such as polyester and/or cotton thread can be used. Twist ties and staples may be made for example of a ductile subset of constituents of the filler metal, or of other material such as plastic or metal wire, including steel or aluminum. An automated cutter  42  may be provided, such as a knife or scissor device. An automated product conveyor or feed mechanism  44  may be provided, such as opposed wheels or belts that draw the gas permeable envelope  29  distally over the filler tube  28 . A length of the gas permeable tube may be preloaded onto the filler tube as indicated by the gathered portion  48 . The process may be controlled by an electronic controller  50  such as a microprocessor. 
         [0023]      FIG. 3  is a sectional view of several small cylindrical filler packets  20  attached or positioned relative to each other side-by-side, and optionally stacked vertically, to form a preform  52  that covers a desired surface area. Subsets of the packets may contain respectively different filler constituents, for example structural superalloy constituents  24 A, metallic bond coat constituents  24 B, ceramic thermal barrier constituents  24 C, and laser energy blocking material such as graphite  24 D. This allows concurrent laser deposition of multiple types of additive materials over an area, and optionally, multiple layers. The blocking material  24 D provides a precise edge to the resulting layers. The packets  20  may be attached to each other by adhesive cement or by stitching, or they may be grouped together by a temporary support. 
         [0024]      FIG. 4  shows a machine for continuous production of filler packets. A filler tube  28  descends from a hopper  30 . The filler tube may bend to a horizontal orientation aligned with a fabrication table (not shown), although this is not a requirement. Compressed air  36 A,  36 B may be injected into the hopper, particularly into a lower 40% or 20% of the hopper, to fluidize the filler powder and prevent clumping and clogging. It may be injected at multiple locations in the hopper, particularly at lower corners thereof  36 B if the hopper has such corners. Two spools  54 ,  56  holding gas-permeable sheet material for the packets may be provided on first and second opposite sides of the filler tube  28 . Respective gas permeable sheets  55 ,  57  may unspool to cover the first and second sides of the filler tube. The adjacent side edges of the sheets are seamed together  58 A,  58 B to form the gas permeable tube or envelope  29  around the filler tube  28 . A seaming mechanism for joining the sheet edges is represented symbolically by opposed rollers. This may be for example an adhesive applicator, a heat fuser, or a sewing machine on each edge. Progression of the sheets  55 ,  57  may be performed by a sheet feed mechanism such as feed dogs on opposite sides the emerging packet as later shown, or by the previously mentioned sealing rollers  58 A,  58 B. The filler tube  28  may have a laterally elongated transverse section as shown. Such a flat tubular shape of the filler tube  28  provides a resulting flat shape of the envelope, which may be retained by quilting across the packet as later shown. 
         [0025]      FIG. 5  shows a sewing machine presser foot  62  and two needles  64  for stitching along a feed-wise direction  63 , and slots  66  for upper feed dogs. Transverse stitching may be done by one or more needles  68  in respective transverse slots  70 . Alternately a separate transversely moving presser foot and needle plate apparatus may be provided for transverse stitching. 
         [0026]      FIG. 6  shows a needle plate  72  corresponding to the presser foot of  FIG. 5 . It may have lower feed dogs  74  and a transverse stitching slot  76 . The gas permeable envelope  29  formed by the sheets  55 ,  57  of  FIG. 4  may pass between the presser foot and needle plate of  FIGS. 5 and 6  for conveyance, flat forming, and stitching as the gas permeable tube is filled with the metal and flux powder. The transverse stitching apparatus  68 ,  70 ,  76  may be controlled to close the ends of the filler packet and/or to quilt the filler packet as it is drawn away from the filler tube by the feed dogs. Powder delivery may be interrupted to form a relatively empty section of the envelope in the region of the stitching. 
         [0027]      FIG. 7  shows a flat filler packet  20 B with stitched side edges  78 A,  78 B, stitched end closures  79 A,  79 B and quilting stitches  80 ,  81  as produced by the machine of  FIGS. 4-6 . 
         [0028]      FIG. 8  shows a tube braiding machine  82  that forms a gas permeable braided tube  84  by winding a first circular array of strings  86 A,  87 A and a second circular array of strings  88 ,  89  in opposite directions around a mandrel  90 , which in this embodiment is or contains a feed tube of the present invention. Mechanisms of tube braiding machines are known in the prior art, and are therefore not detailed here. For example see U.S. Pat. Nos. 4,130,046 and 4,372,191. In such machines, an outer circular array of bobbins  91 A,  93 A rotates around the mandrel  90  in a first direction on a first rotating element  95 . An inner circular array of bobbins  92 ,  94  rotates around the mandrel in the opposite direction on a second rotating element  96 . The outer/inner bobbins effectively trade places radially each time as they pass each other. This is illustrated by outer bobbins  91 A and  93 A shifting radially  96  to alternate inner positions  91  B and  93 B respectively. However, in one embodiment, the outer strings  86 A,  87 A may be guided to the radially inner position  86 B,  87 B on guide arms without moving the outer bobbins  91 A,  93 A radially. For example, see U.S. Pat. No. 4,130,046. The present invention adds filling of the braided tube  84  with filler material  24  through the hollow mandrel  90  to produce braided packets  20 C that can be used for solid freeform fabrication and repair as previously described. Closures  26 A, feed wheels  44 , a cutter  42 , and a closer  40  may be provided as previously described. The filler material  24  may be conveyed through the filler tube by gravity and/or compressed gas  36  as previously described. 
         [0029]    Alternately (not shown), a helical tube forming machine may be used to create a gas permeable tube for the filler packets from a tape of gas permeable material wrapped helically around a mandrel that also serves as a filler feed tube in accordance with the present invention. The helical tape may be seamed or sealed along overlapping edges in the helical winding. Such machines are known in the prior art, and are not shown here. For one example, see U.S. Pat. No. 3,793,929. 
         [0030]    Filler packets as produced herein to include metal and flux powders have the following benefits in additive manufacturing processes such as selective laser cladding of superalloy materials: 
         [0031]    a) Can build on existing 3-D surfaces. Not limited to horizontal flat surfaces. 
         [0032]    b) High build rate, such as over 3 or 4 mm per layer. 
         [0033]    c) Usable for superalloy metals that are difficult to weld. 
         [0034]    d) Robust process that is adaptable to new damage modes. 
         [0035]    e) No pre-heating or fast cooling of article being repaired or built is needed. 
         [0036]    f) No shielding of the melt pool by inert gas is needed. 
         [0037]    g) Less or no waste of powdered filler due to scattering. 
         [0038]    h) Wide range of powder sizes. 
         [0039]    i) Reduced sensitivity to the powder production method. 
         [0040]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.