Abstract:
A method for manufacturing a powder comprises vaporizing a solvent; passing a metallic powder and a polymer powder through the solvent vapor to mix the metallic powder with the polymer powder; and removing the solvent.

Description:
BACKGROUND 
       [0001]    This disclosure relates to agglomerate powders for thermal spray. More particularly, it relates to such powders for forming gaspath leakage seals for gas turbine engines. 
         [0002]    Gas turbine engines, such as those used to power modern commercial and military aircraft, generally include one or more compressor sections to pressurize an airflow, a combustor section for burning hydrocarbon fuel in the presence of the pressurized air, and one or more turbine sections to extract energy from the resultant combustion gases. The airflow flows along a gaspath through the gas turbine engine. 
         [0003]    The gas turbine engine includes a plurality of rotors arranged along an axis of rotation of the gas turbine engine. The rotors are positioned in a case, with the rotors and case having designed clearances between the case and tips of rotor blades of the rotors. It is desired to maintain the clearances within a selected range during operation of the gas turbine engine as deviation from the selected range can have a negative effect on gas turbine engine performance. For each blade stage, the case typically includes an outer airseal located in the case immediately outboard (radially) of the blade tips to aid in maintaining the clearances within the selected range. 
         [0004]    Within the compressor section(s), temperature typically progressively increases from upstream to downstream along the gaspath. Particularly, in relatively downstream stages, heating of the airseals becomes a problem. U.S. patent application Ser. No. 14/947,494, of Leslie et al., entitled “Outer Airseal for Gas Turbine Engine”, and filed Nov. 20, 2015 (&#39;494 application), the disclosure of which is incorporated by reference in its entirety herein as if set forth at length, discusses several problems associated with heat transfer to outer airseals and several solutions. 
         [0005]    The airseal typically has an abradable coating along its inner diameter (ID) surface. In relatively downstream stages of the compressor where the blades have nickel-based superalloy substrates, the abradable coating material may be applied to a bondcoat along the metallic substrate of the outer airseal. For relatively upstream sections where the compressor blades comprise titanium-based substrates (a potential source of fire) systems have been proposed with a fire-resistant thermal barrier layer intervening between the bondcoat and the abradable material. An example of such a coating is found in U.S. Pat. No. 8,777,562 of Strock et al., issued Jul. 15, 2014 and entitled “Blade Air Seal with Integral Barrier”. 
         [0006]    Among coating application techniques are thermal spray processes such as air plasma spray. Typically, the plasma spray process involves a single feedstock outlet discharging a mixture of coating constituents and fugitive porosity former in to a plasma jet. Proposals have been made to segregate the porosity former and introduce that through a relatively downstream outlet while the matrix and solid lubricant are introduced from a conventionally located upstream outlet. Examples of these are found in U.S. Pat. No. 4,696,855, of Petit, Jr. et al., issued Sep. 29, 1987, and entitled “Multiple Port Plasma Spray Apparatus and Method for Providing Sprayed Abradable Coatings”, and U.S. Pat. No. 4,299,865, of Clingman et al., issued Nov. 10, 1981 and entitled “Abradable Ceramic Seal and Method of Making Same”. U.S. Pat. No. 4,386,112, of Eaton et al., issued May 31, 1983, and entitled “Co-Spray Abrasive Coating” shows separate introduction of matrix and abrasive in an abrasive coating. 
       SUMMARY 
       [0007]    One aspect of the disclosure involves a method for manufacturing a powder, the method comprising: vaporizing a solvent; passing a metallic powder and a polymer powder through the solvent vapor to mix the metallic powder with the polymer powder; and removing said solvent. 
         [0008]    A further embodiment may additionally and/or alternatively include the metallic powder being a second metallic powder. The method further comprises driving a flow of a first metallic powder along a flowpath. The method further comprises introducing the second metallic powder and polymer powder to the flow of the first metallic powder. 
         [0009]    A further embodiment may additionally and/or alternatively include vibratory mixing of the mixed second metallic powder and polymer powder and the first metallic powder to produce a blend. 
         [0010]    A further embodiment may additionally and/or alternatively include size classifying the blend. 
         [0011]    A further embodiment may additionally and/or alternatively include the size classifying comprising: feeding back undersize particles to a source of the first metallic powder or the second metallic powder; and crushing oversize particles and feeding the crushed particles back into a classifier performing the size classifying. 
         [0012]    A further embodiment may additionally and/or alternatively include controlling a temperature of the first metallic powder to a first temperature; controlling a temperature of the second metallic powder to a second temperature; and controlling a temperature of the polymer powder to a third temperature. 
         [0013]    A further embodiment may additionally and/or alternatively include the first temperature being greater than the second temperature and the second temperature being greater than the third temperature. 
         [0014]    A further embodiment may additionally and/or alternatively include the first temperature being equal to or greater than a dew point of the solvent vapor; the second temperature being equal to or less than the dew point; and the third temperature being less than the dew point. 
         [0015]    A further embodiment may additionally and/or alternatively include the controlling the temperature of the first metallic powder to the first temperature comprising heating; the controlling the temperature of the second metallic powder to the second temperature comprising heating; and the controlling the temperature of the polymer powder to the third temperature comprising cooling. 
         [0016]    A further embodiment may additionally and/or alternatively include an overlapping powder delivery process of the second metallic powder and the polymer powder providing: the mixing of the second metallic powder and the polymer powder; and the introduction of the mixed second metallic powder and polymer powder to the flow of the first metallic powder. 
         [0017]    A further embodiment may additionally and/or alternatively include the overlapping powder delivery process comprising: delivering the second metallic powder onto the flow of the first metallic powder over a first footprint; and delivering the polymer powder over a second footprint within the first footprint. 
         [0018]    A further embodiment may additionally and/or alternatively include the overlapping powder delivery process comprises overlapping spraying. 
         [0019]    A further embodiment may additionally and/or alternatively include the first metallic powder and the second metallic powder being of alloys of the same composition. 
         [0020]    A further embodiment may additionally and/or alternatively include maintaining the solvent vapor at a partial pressure of at least 50% of a chamber atmosphere. 
         [0021]    A further embodiment may additionally and/or alternatively include comprising acetone. 
         [0022]    A further embodiment may additionally and/or alternatively include passing a non-metallic filler through the solvent vapor. 
         [0023]    Another aspect of the disclosure involves an apparatus for manufacturing a powder. The apparatus comprises: a chamber; a temperature control system for the chamber interior; a vibratory conveyor within the chamber; a first powder source within or coupled to the chamber; a first powder flowpath from the first powder source through the chamber and passing along the vibratory conveyor; a second powder source within or coupled to the chamber; a second powder flowpath from the second powder source merging with the first powder flowpath along the vibratory conveyor; a third powder source within or coupled to the chamber; a third powder flowpath from the third powder source merging with the first powder flowpath along the vibratory conveyor; and a vaporizer within the chamber or coupled thereto to deliver vaporized liquid to the chamber. 
         [0024]    A further embodiment may additionally and/or alternatively include: a classifier; and a return flowpath from the classifier to the first flowpath. 
         [0025]    A further embodiment may additionally and/or alternatively include the return flowpath passing through a crusher. 
         [0026]    A further embodiment may additionally and/or alternatively include the temperature control system comprising a heater. 
         [0027]    The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a partially schematic view of a powder-producing apparatus. 
           [0029]      FIG. 1A  is an enlarged view of a classifier of the apparatus of  FIG. 1 . 
           [0030]      FIG. 2  is a schematic sectional view of a first agglomerate. 
           [0031]      FIG. 3  is a schematic sectional view of a second agglomerate. 
       
    
    
       [0032]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0033]      FIG. 1  shows an apparatus  20  for producing a feedstock for thermal spray processes. The exemplary feedstock is a powder. The particles of the powder are, themselves made up of multiple particles of constituent powders. In one group of embodiments, the constituent powders comprise one or more metallic powders for ultimately forming a matrix of the sprayed material. Additional powders include a fugitive porosity former (“fugitive”) to form porosity in the ultimate coating (e.g., after a bake-out or chemical removal) and may include other non-metallic powders such as solid lubricants or other fillers to affect friability or other properties of the sprayed material. 
         [0034]    For example,  FIGS. 2 and 3  respectively schematically show a two-component powder agglomerate particle  200  and a three-component powder agglomerate particle  201 . Each has a core  202  formed by a single particle of the fugitive. A shell  204 ,  205  is formed of multiple particles, namely alloy particles  206  in  FIG. 2  and a combination of alloy particles  206  and non-metallic filler particles  208  in  FIG. 3 . 
         [0035]    The agglomerate component particles may be modeled as spheres. If the core  202  of agglomerate  200  has a diameter D 1  and the alloy particles each have a diameter D 2 , the overall particle may be modeled as having a diameter D=D 2 +2D 1 . Maximum theoretical coverage of the core surface may not be achieved in practice. Thus, some fraction of this may be used to calculate parameters that represent overall by volume alloy content to, in turn, predict overall by volume alloy content in the ultimate coating. Similar calculations may be performed for the three-component agglomerate particle  201 . A sample calculation calculates the surface of a sphere of radius R=0.5(D 1 +D 2 ) as 4πR 2 . An amount of the alloy particles may be calculated to cover that area up to a packing limit. An exemplary modelling models the alloy particles&#39; cross-sectional area as covering 60% of that sphere surface which is a little less than the hexagonal close packed or square array packing. The table below uses that 60% figure as a fractional coverage of 0.6. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Component Dimensions and Relative Volumes 
               
             
          
           
               
                   
                   
                   
                 Assumed 
                 Fugitive 
                 Alloy 
               
               
                   
                   
                   
                 Fractional 
                 Volume 
                 Volume 
               
               
                 D1 (μm) 
                 D2 (μm) 
                 D (μm) 
                 coverage 
                 Fraction 
                 Fraction 
               
               
                   
               
             
          
           
               
                 50 
                 22 
                 94 
                 0.60 
                 0.077 
                 0.923 
               
               
                 100 
                 22 
                 144 
                 0.60 
                 0.188 
                 0.812 
               
               
                 150 
                 22 
                 194 
                 0.60 
                 0.282 
                 0.718 
               
               
                 50 
                 16 
                 82 
                 0.60 
                 0.157 
                 0.843 
               
               
                 100 
                 16 
                 132 
                 0.60 
                 0.326 
                 0.674 
               
               
                 150 
                 16 
                 182 
                 0.60 
                 0.444 
                 0.556 
               
               
                 50 
                 11 
                 72 
                 0.60 
                 0.316 
                 0.684 
               
               
                 100 
                 11 
                 122 
                 0.60 
                 0.528 
                 0.472 
               
               
                 150 
                 11 
                 172 
                 0.60 
                 0.642 
                 0.358 
               
               
                 50 
                 9 
                 68 
                 0.60 
                 0.425 
                 0.575 
               
               
                 100 
                 9 
                 118 
                 0.60 
                 0.634 
                 0.366 
               
               
                 150 
                 9 
                 168 
                 0.60 
                 0.733 
                 0.267 
               
               
                 50 
                 6 
                 62 
                 0.60 
                 0.649 
                 0.351 
               
               
                 100 
                 6 
                 112 
                 0.60 
                 0.805 
                 0.195 
               
               
                 150 
                 6 
                 162 
                 0.60 
                 0.865 
                 0.135 
               
               
                   
               
             
          
         
       
     
         [0036]    For a three-component system, a similar approximation may be used. 
         [0037]    The apparatus  20  comprises a chamber  22  having a wall structure  24  (e.g., top, bottom sides, and ends). The chamber has an interior  26  generally bounded by an interior surface  28  of the wall structure. A temperature control system  30  may control the temperature of the interior surface for purposes such as causing or preventing condensation of solvent vapor. As is discussed below, the solvent renders the fugitive porosity former sticky to adhere the other powders thereto. Thus, depending on implementation, the system  30  may serve as or be a heater, cooler, or both. Exemplary systems  30  are thermoelectric systems, vapor compression systems (having a heat exchanger integrated with the wall structure), or resistance or other heaters. 
         [0038]    The apparatus  20  includes several sources of the constituent powders. In an example of a metallic matrix-forming powder and a polymeric fugitive porosity forming powder (fugitive powder), a source  40  may provide metallic powder  41  (e.g., a Cu—Ni alloy (e.g., Cu26Ni8.5Al4Cr) or an MCrAlY (although the Y may be eliminated in lower temperature engine locations) and a source  42  may provide the fugitive powder  43  (e.g., polymethyl methacrylate (PMMA)). The sources may comprise a reservoir  44 ,  46  of the respective powders and a feed mechanism  48 ,  50 . Exemplary feed mechanisms are spray feed mechanisms using a carrier gas from a carrier gas source  52 ,  54  (although shown separately, the carrier gas sources may at least partially overlap such as using one or more gas cylinders in common) and discharging respective sprays  53 ,  55  through respective nozzles  56 ,  58 . As is discussed below, the spray discharge produces mixing of the powders. Exemplary carrier gas is nitrogen. Nitrogen (or an inert carrier gas) serves to limit oxygen when using flammable solvent. If using water solvent (e.g., with PVA fugitive particles), air may be used. 
         [0039]    To form agglomerates, the atmosphere within the chamber contains vapors of a solvent for the fugitive powder. The vapors may be provided by a vaporizer  60  within or communicating with the chamber interior. The vaporizer may comprise a reservoir or other body of solvent  62  and a heater  64  (e.g., resistive) for heating and vaporizing the solvent. Another vaporizer example uses the bottom of the chamber as the vaporizer (e.g., a heated pool of solvent along the bottom). As the spray  55  passes through the solvent vapor, its particles pick up solvent and become sticky to clump with the particles of the spray  53 . A conveyor  70  passes the mixed particles downstream along a main flowpath for further processing such as solvent removal and classification (discussed below). 
         [0040]    The atmosphere in the chamber may be up to 100% solvent vapor (in which case the temperatures will all be related to the boiling point of the solvent (at atmospheric pressure)). In practice, the atmosphere will have some other gases. These gases would include the carrier gas(es) plus any other gases which may come in via the powder introduction, plus leakage, residual air, outgassing, and the like. Such gases give the atmosphere some reduced vapor pressure (fraction) of the solvent. 
         [0041]    To the extent that the atmosphere is &lt;100% solvent, then the temperature of condensation (dew point) will be reduced. Exemplary solvent vapor partial pressure is at least 50% of the chamber atmosphere or at least 75% or at least 90%. 
         [0042]    The exemplary conveyor  70  is a vibratory conveyor having a bed  72  and a vibration mechanism  74  (e.g., motor-driven, piezoelectric, pneumatic, or the like. The exemplary conveyor uses a metallic powder to protect the bed  72 . A source  80  provides this metallic powder  81  and may include a reservoir  82  and a feed mechanism  84  an exemplary feed mechanism is a feedscrew or other non-spray system, although spray systems are alternatives. The powders  41  and  81  may be the same, or they may be the same alloy but differing in morphology due to feedback issues (discussed below) or preprocessing of the powder  41  (discussed below) or may be more fundamentally different such as differing alloys or differing size. In one differing size example, the particles of the powder  81  may be very large such that they do not pass through a classifier and get recycled back to the source  80  (perhaps with some of the fugitive powder  43  and alloy powder  41 ). 
         [0043]    In this example, a flowpath  500  extends downstream from the source  80 . The flowpath extends along the bed  72 . The alloy powder  81  forms a base layer  88  atop the bed  72  and the alloy powder  41  is sprayed atop the base layer along a footprint  90 . The footprint  90  at least partially overlaps with a footprint  92  of the spray  55  of the fugitive powder  43 . In the illustrated example, the footprint  90  leads the footprint  92  (i.e., the upstream extreme of the footprint  90  is upstream of the upstream extreme of the footprint  92 ). This means the powder  43  generally lands atop the powder  41 . In the exemplary embodiment, the downstream (along the flowpath) extreme of the footprint  90  is downstream of the downstream extreme of the footprint  92 . This may help fully embed the fugitive powder  43  in the alloy powder  41 . Nevertheless, particular morphologies of final feedstock may be obtained by varying the footprints as well as the particle sizes and flow rates of the constituent powders. 
         [0044]    The vibratory conveyor  70  may cause some portion of the powder  81  to mix with the other powders in the final spray feedstock. For this reason, the chemistry and particle size of the powder  81  may be chosen for its role in the ultimate spray feedstock. At its simplest, this consideration suggests using the same particle size and chemistry as for the powder  41 . 
         [0045]    In another example, there is no separate source  80 . Instead, the source  40  may have a relatively larger footprint  90  extending further upstream of the footprint  92 . 
         [0046]    As noted above, the mixed powders pass along the flowpath for further processing.  FIG. 1A  shows further details of a processing system  120 . The flowpath leads to one or more collectors  122 .  FIG. 1A  shows an exemplary pair of collectors which may selectively receive powder passing along the flowpath (e.g., via a diverter such as a manually-controlled or actuator-driven flapper  124 ). The exemplary flowpath thus has respective branches through the respective collectors. Along the branches, the collectors each have an upstream isolation valve  126  and a downstream isolation valve  128  (e.g., pinch valves along tubing). 
         [0047]    The exemplary collectors  122  also serve to remove the solvent. While one collector is open to the flowpath and receiving powder, the other is closed to the flowpath and doing solvent removal. The exemplary collectors have heating jackets  130  (e.g., resistive) or other heating elements for heating the powder to vaporize the solvent. A vacuum and condenser unit  140  (e.g., having a vacuum pump and a condensing heat exchanger (e.g., refrigerated or merely exchanging to ambient conditions)) draws the solvent vapor off. After solvent removal from a given collector, its downstream isolation valve  128  may be opened to discharge the de-solvented powder for further processing. After discharge, the downstream isolation valve  128  may then be closed and the upstream isolation valve opened to receive a subsequent charge of powder. 
         [0048]    In the exemplary system, the flowpath branches reunite downstream of the downstream isolation valves  128  and the flowpath proceeds to a classifier  150 . The exemplary classifier is a two-stage classifier with feedback. Each stage may comprise a screen or other foraminate medium. The first stage medium  152  splits a flow  154  of over-size particles from a remainder flow  156  which passes to the second stage. In the exemplary embodiment, the flow  154  passes to a crusher (e.g., rolling mill)  158  whose output flow  160  is fed back to the first stage. The second stage medium  162  separates a flow  164  of acceptable size particles and passes a flow  166  of under-size particles. The flow  164  may pass to a hopper  168  and then to packaging and distribution for use as a spray feedstock. 
         [0049]    The flow  166  may pass back along a return flowpath  520  to one or both of the sources  40  and  80 . The particular one may depend on a number of factors (e.g., degree of agglomeration, size and composition of the powders  41  and  81 ). Thus, this feedback or return flow  166  may account for some of the difference between the sources  40  and  80  even if the majority of powder in each is coming as identical fresh powder. If the bed powder (alloy powder  81 ) is sized to differentiate it from the agglomerates and source powders  41  and  43  (e.g., is larger than the agglomerates), it may require additional segregation for reuse (e.g., an additional coarse stage before those shown). 
         [0050]    Temperature management of the respective powder sources may play a role in achieving desired final powder properties. Due to its introduction upstream along the conveyor  70 , the alloy powder  81  may be referred to as a first powder. As noted above, the first powder (if included) may serve to protect the conveyor bed  72  surface. The first powder may be heated (e.g., by a heater  170  such as a resistive heating jacket ( FIG. 1 )) to a point where the vapor will not condense on it. This heating may be just to or above the boiling point or dew point of the solvent (e.g., by 5-15° F. (2.8-8.3° C.) above dew point), but slightly higher may help limit clumping of the agglomerates together by boiling off some or all of the solvent that arrives with the powders landing on it from the sprays  53  and  55 . 
         [0051]    Another exemplary heater  170  is a vapor jacket with controlled pressure (temperature for condensation of the vapor is proportional to the pressure (partial pressure of the vapor)). If the solvent vapor is used in the vapor jacket, then small adjustment from atmospheric pressure will result in temperature that is just above or below the dew point of the pure vapor at atmospheric pressure (thus, one may have precise control). 
         [0052]    The alloy powder  41  forms a second powder at a temperature that may be selected to control condensation (e.g., less than dew point, such as a room temperature powder introduced to a 100° F. (38° C.) chamber). Control to this temperature may be a heating or a cooling/chilling jacket  172  (e.g., via a heat pump system  182 ). Cooling would allow the introduction of more solvent which may be desirable in some situations. 
         [0053]    The fugitive powder is cooled (e.g., via a heat exchanger  174  associated with a pump system  184 ). This may be to a temperature of 0° C. (32° F.). This induces condensation of solvent on the fugitive powder spray. This temperature may be less than that of the metallic powder due to lower heat capacity of the fugitive and/or a desire to get a higher condensation directly on the fugitive than directly on the alloy powder. 
         [0054]    If a nonmetallic filler (e.g., solid lubricant such as hBN or an alternative non-lubricant for coating friability/abradability, e.g., one or more oxides such as a metal oxide and/or rare earth oxide) is to be introduced there are a number of options. Some options involve preblending the nonmetallic filler with the alloy powder of the source  40  (and/or source  80  if present). Other options involve a source (not shown) similar to the sources  40  and  42  containing the filler. For example, this source may have a spray footprint larger than the footprint  92  (e.g., coextensive with the footprint  90 ). 
         [0055]    The use of “first”, “second”, and the like in the following claims is for differentiation within the claim only and does not necessarily indicate relative or absolute importance or temporal order. Similarly, the identification in a claim of one element as “first” (or the like) does not preclude such “first” element from identifying an element that is referred to as “second” (or the like) in another claim or in the description. 
         [0056]    Where a measure is given in English units followed by a parenthetical containing SI or other units, the parenthetical&#39;s units are a conversion and should not imply a degree of precision not found in the English units. 
         [0057]    One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, when applied to an existing baseline process, details of such baseline may influence details of particular implementations. Accordingly, other embodiments are within the scope of the following claims.