Abstract:
Melting energy exemplified by an arc ( 24 ) is delivered to a metal alloy material ( 22, 23 ), forming a melt pool ( 26 ). A metal oxide material ( 34 ) is delivered ( 33 ) to the melt pool and dispersed therein. The melting energy and oxide deliveries are controlled ( 44 ) to melt the alloy material, but not to melt at least most of the metal oxide material. The deliveries may be controlled so that the melting energy does not intercept the metal oxide delivery. The melting energy may be controlled to create a temperature of the melt pool that does not reach the melting point of the metal oxide. Deliveries of the melting energy and the oxide may alternate so they do not overlap in time. A cold metal transfer apparatus ( 22 ) and process ( 18, 19, 20 ) may be used for example in combination with an oxide particle pulse delivery device ( 42, 46 ).

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of metal-component fabrication and repair, and more particularly to the formation of oxide dispersion strengthened (ODS) alloys. 
       BACKGROUND OF THE INVENTION 
       [0002]    Oxide dispersion strengthened (ODS) alloys possess superior properties for high temperature applications; especially ODS alloys formed of superalloy materials. ODS alloys are distinguished from conventional alloys by the presence of dispersoids of fine particles and by an elongated grain shape which generally develops during a recrystallization heat treatment and/or hot and cold working. This particular grain structure enhances the high temperature deformation characteristics of ODS alloys by inhibiting the accumulation of inter-granular damage. As result of this and other properties, components fabricated from ODS alloys exhibit improved high-temperature creep strength and improved oxidation resistance as compared to conventional alloys. 
         [0003]    The term “superalloy” is used herein as is understood in the art to describe a highly corrosion and oxidation resistant alloy that exhibits excellent mechanical strength and resistance to creep at high temperatures, as well as good surface stability. Superalloys typically include a base alloying element of nickel, cobalt or nickel-iron. Examples of superalloys include alloys sold under the trademarks and brand names Hastelloy, Inconel alloys (e.g., IN 700, IN 738, IN 792, IN 939), Rene alloys (e.g., Rene N5, Rene 80, Rene 142), Haynes alloys, Mar M, CM 247, CM 247 LC, C 263, 718, X-750, ECY 768, 282, X45, PWA 1483 and CMSX (e.g., CMSX-4, CMSX-8, CMSX-10) single crystal alloys. 
         [0004]    ODS alloys, especially superalloys, are very difficult to weld and repair by conventional techniques (e.g., gas tungsten arc welding, laser welding, electron beam welding, etc.). Such fusion welding causes significant loss of strength. The alloys are furthermore difficult and uneconomical to process by less traditional processes such as friction welding. 
         [0005]    ODS alloys are manufactured by mechanically alloying mixtures of powders. For example, metal powder such as alloys of iron aluminide, iron chromium, iron-chromium-aluminum, nickel chromium, or nickel aluminide, and oxides such as yttria (Y 2 O 3 ) or alumina (Al 2 O 3 ) are impacted in a ball mill. Shearing and smearing of the powders produces a fine mixture. A sealed container of the powder is then hot isostatically pressed and hot formed into a desired shape. High temperature heat treatment then provides stress relief and enlarges the grain size. Extraordinary strength is achievable with ODS materials. However, ODS processing is slow, expensive and provides limited control of part geometry. In addition, joining or repair of ODS parts is very difficult. Conventional arc and energy beam processes cause the fine oxide particles to segregate or coalesce, which degrades the result. Nickel based ODS superalloys are especially difficult to cold work and recrystallize. 
         [0006]    Additional challenges associated with ODS alloys involve general shaping and joining of these materials. Shaping and joining techniques which preserve the microstructure and intrinsic strength of ODS alloys are severely limited, which often curtails their ability to be incorporated into high-temperature, load-bearing structures. For example, excessive heating of ODS alloys can cause the oxide to coalesce, leading to agglomeration such that the oxide dispersoids may no longer be effective in resisting slip at the grain boundaries. Melting of ODS alloys also results in “slagging off” of the oxide dispersoids reducing their strengthening ability. Since most ODS alloys derive their strength from an elongated grain structure, such disruption of the grain structure reduces strength. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention is explained in the following description in view of the drawings that show: 
           [0008]      FIG. 1A-1D  illustrate apparatus and steps in a known cold metal transfer process. 
           [0009]      FIG. 2  schematically illustrates aspects of a method and apparatus for injecting a metal oxide into an alloy melt pool. 
           [0010]      FIGS. 3A-B  illustrate an embodiment of an additive cold metal transfer process in which oxide injection alternates with the melting energy to avoid intercepting the oxide particles with the arc. 
           [0011]      FIGS. 4A-B  illustrate an arc welding embodiment in which oxide injection alternates with the melting energy. 
           [0012]      FIG. 5  shows overlapping areas of melting energy and oxide injection at the surface of the melt pool in an embodiment. 
           [0013]      FIG. 6  shows an oxide pulse driver comprising a rotating oxide carrier cylinder with a gap that aligns with a gap in a housing to eject an oxide particle pulse. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]      FIG. 1A-1D  illustrate basic apparatus and steps in a known cold metal transfer process  20 . In  FIG. 1A , a consumable electrode  22  approaches an electrically conductive substrate  23 , establishing an arc  24  that melts a melt pool  26  on a surface  28  of the substrate and creates a melt drop  30  of alloy filler material on the electrode tip. The melt pool solidifies into a deposit  32  on the substrate. In  FIG. 1B  the consumable electrode  22  is advanced  18  toward the melt pool. In  FIG. 1C  the melt drop  30  touches the melt pool, extinguishing the arc. Electrical current is prevented from spiking during the short circuit by a controller (not shown). In  FIG. 1D  the melt drop  30  adheres to the melt pool, and the electrode  22  is retracted  19 . This pulls the drop  30  off of the electrode into the melt pool by surface tension, thus adding the drop as alloy filler material to the melt pool. This technology minimizes spatter and excess heating compared to other arc welding techniques, while providing fast deposition rates. 
         [0015]      FIG. 2  schematically illustrates an embodiment  20 A of a method and apparatus for injecting  33  a metal oxide into a melt pool  26  formed by cold metal transfer. The melt pool has a presently heated portion  26 A and a presently unheated portion  26 B. The unheated portion  26 B may be a trailing portion of the melt pool after the electrode  22  has passed, i.e. resulting from movement of the electrode  22  to the right in the figure relative to the substrate  23 . The metal oxide may be formed into a powder and transported by an inert carrier gas. At least most of the oxide  34  may be directed by a nozzle  36  onto or into the unheated portion  26 B of the melt pool  26  so that at least most of the metal oxide is not directly intercepted by the arc  24 . This prevents the oxide from melting or permits only superficial melting, and avoids or minimizes coalescing, thus allowing a substantially uniform distribution of small oxide dispersoids  38 B in the deposit, which maximizes their effectiveness in resisting slippage at the alloy grain boundaries. The consumable electrode  22  may comprise filler metal to constitute the metal alloy matrix surrounding the deposit dispersoids. 
         [0016]    The substrate  23  may be made of an oxide dispersion strengthened (ODS) alloy with dispersoids  38 A, and the ODS deposit  32  may be formed of material matching the substrate for buildup or repair, or the deposit  32  may be formed of material different than substrate for substrate surface enhancement. ODS cladding, layering, or welding may be applied to a substrate that is or is not ODS. In another embodiment, the substrate may be replaced with a fugitive or removable support surface for additive manufacturing that builds a component by successive ODS layering starting with a first layer on the support surface. 
         [0017]    Particles of the metal oxide  34  may contain at least one metal oxide having a higher melting point than the filler alloy of the electrode  22 . Exemplary metal oxides include oxides of aluminum, calcium, cerium, chromium, cobalt, hafnium, lanthanum, magnesium, nickel, silicon, titanium, tantalum, thorium, yttrium and zirconium. Mixtures of oxide particles containing different metal oxides and/or having different particle sizes may be used, or the oxide particles may be changed over time as the deposition progresses in space. The oxide particles may be directed  33  into the melt pool  26  from above as shown with nozzle  36 . Alternately, not shown, they may be injected directly into the melt pool  26  from below its surface via a refractory nozzle to avoid contact with the atmosphere. 
         [0018]    Excessive heating of the oxide can lead to coalescence and slagging of dispersoids  30 B, adversely affecting the oxide distribution in the ODS deposit. Optimal size, shape and distribution of the dispersoids  30 B may be attained in part by adjusting the alloy melting energy by adjusting the intensity of the arc and the on-off dwell proportions of the arc. The power and dwell times of the arc may be controlled so that the melt pool meets or exceeds the melting point of the filler alloy of the electrode  22  and the substrate, but does not reach the melting point of the oxide particles. This prevents oxide coalescence and slagging. 
         [0019]      FIGS. 3A-B  illustrate an additive cold metal transfer embodiment  20 B in which oxide injection  33  alternates with the arc  24 . In  FIG. 3A  the arc  24  is present and the oxide injection is stopped by a controller  44 . The oxide may flow continuously in a recirculation channel  40  in a direction perpendicular to the page to maintain particle suspension in a carrier gas. In  FIG. 3B  the arc is extinguished when the melt drop  30  touches the melt pool  26 , as in a cold metal transfer process. At this time the oxide is injected  33  by an oxide particle pulse driver  42 , for example an acoustic driver, electrostatic particle deflector, or solenoid. This separation in timing of the injection  33  and the arc  24  prevents interception of the oxide by the arc, and avoids melting the oxide with the arc. Timing of the injection may be coordinated by alternately switching the pulse driver and the arc power. Such switching may optionally be triggered by a short circuit detector in the controller  44 . The electrode  22  may be automatically advanced  18  toward and retracted from the melt pool multiple times per second—for example at least 10 times per second in some embodiments and up to 130 times per second in some embodiments. This action creates turbulence and forced convection in the melt pool that thoroughly mixes the oxide particles therein before solidification. At least some of the particles  33  may be injected to intersect the melt drop  30  so that they are carried into the melt pool  26  with the material of the melt drop  30 . 
         [0020]      FIGS. 4A-B  illustrate another embodiment  20 C in which oxide injection  33  alternates with an arc  24 . In this example, a non-consumable electrode  45  creates an oxide dispersion strengthened autogenous weld or layer  47  on a non-ODS substrate  49 . Alternately, an energy beam such as laser or electron beam, or a consumable electrode as in  FIGS. 3A-B  may be used. In addition to a non-consumable electrode or energy source, a supplemental filler metal may be fed. The process of  FIGS. 4A-B  includes no filler material, but it serves to form a layer  47  containing dispersoids  38 B. In  FIG. 4B  the arc  24  is switched off and the oxide material  34  is injected  33  by an oxide particle pulse driver  42 . Here the oxide injection overlaps the arc in space, but they are mutually exclusive in time so the oxide is not directly intercepted by the arc. Overlapping in space allows injecting the particles into the most active part of the melt pool to facilitate mixing before solidification. 
         [0021]      FIG. 5  shows an overlap between the melting energy  24  and the oxide injection  33  at the surface of the melt pool  26 . For example the area of the melting energy at the surface of the melt pool may be at least 40% or 60% overlapped by the area of the oxide injection at the surface of the melt pool or the area of the oxide injection at the surface of the melt pool may be at least 40% or 60% overlapped by the area of the melting energy at the surface of the melt pool. 
         [0022]    The distribution of the dispersoids  38 B in the ODS alloy deposit  32  may be controlled by altering the velocity and concentration of particles of the oxide material  34  injected into the melt pool  26 . Increasing the velocity or concentration of the oxide particles fired into the melt pool increases the proportion of dispersoids in the resulting deposit  32 . Increasing velocity can also provide a more uniform distribution when the melt pool is especially viscous. 
         [0023]      FIG. 6  shows an oxide pulse driver  46  having a rotating oxide carrier cylinder  48  or wheel with a gap  50  that aligns with a gap  52  in a housing  54  to produce a pulse  56  of oxide  34 . The rotation rate of the carrier cylinder may be synchronized with the cyclic translation of the electrode tip in a cold metal transfer process or with the alloy melting energy in any process such that the oxide pulse does not overlap the melting energy in time. The carrier cylinder  48  may be part of a recirculation circuit as previously described. 
         [0024]    Herein, cold metal transfer and pulsed arc welding are illustrated as exemplary. Alternate technologies that can provide the alloy melting energy include pulsed gas metal arc welding, pulsed gas tungsten arc welding, pulsed tip tungsten inert gas welding (pulsed tip TIG), and pulsed energy beams, including for example a laser beam, a particle beam, a charged-particle beam, a molecular beam, etc. The cold metal transfer process is advantageous because of its mechanical mixing of the melt pool by rapid repetitive dipping of the electrode tip, high deposit control and relatively low heat. In addition to welding and cladding, it can form an extensive variety of additive deposition forms and wall growth directions. Tip tungsten inert gas welding may also be advantageous because of superimposed mechanical oscillation of feed wire helping to agitate the molten weld pool and promote oxide distribution therein. The on/off switching of the alloy melting energy described herein includes in some embodiments switching between a first energy level (on) and a second energy level (off) that is less than 50% of the first energy level. 
         [0025]    Embodiments of the present disclosure enable the formation and repair of ODS superalloy components. However the invention is not confined to such materials and may also be applied to other ODS materials. 
         [0026]    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.