Abstract:
A method for treating a deposited titanium-base material from an initial condition to a treated condition includes a rapid heating and a rapid cooling. The heating is from a first temperature to a second temperature, the first temperature being below a β transus and the second temperature being below above the β transus. The cooling is from the second temperature to a third temperature below an equilibrium β transus.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to deposition of Ti-based materials. More particularly, the invention relates to addressing deposition defects.  
         [0002]     A growing art exists regarding the deposition of Ti-based materials. For example, electron beam physical vapor deposition (EBPVD) may be used to build a coating or structural condensate of a Ti alloy atop a substrate of like or dissimilar nominal composition. Such techniques may be used in the aerospace industry for the repair or remanufacture of damaged or worn parts such as gas turbine engine components (e.g., blades, vanes, seals, and the like).  
         [0003]     Deposition defects, however, potentially compromise the condensate integrity. One group of such defects arises when a droplet of material is spattered onto the substrate or the accumulating condensate. The melt pool may contain additives not intended to vaporize and accumulate in the condensate. For example, U.S. Pat. No. 5,474,809 discloses use of refractory elements in the melt pool. Once the droplet lands on the surface (of the substrate or the accumulating condensate) further deposition builds atop the droplet and the adjacent surface. Along the sides of the droplet, there may be microstructural discontinuities in the accumulating material due to the relative orientation of the sides of the droplet. As further material accumulates, these discontinuities may continue to build all the way to the final condensate surface.  
       SUMMARY OF THE INVENTION  
       [0004]     One aspect of the invention involves a method for treating a deposited titanium-base material from an initial condition to a treated condition. The material is heated from a first temperature to a second temperature. The first temperature is below an equilibrium β transus. The second temperature is above the equilibrium β transus. The heating includes a portion at a rate in excess of 5° C./s. The material is cooled from the second temperature to a third temperature below the equilibrium β transus.  
         [0005]     In various implementations, the heating may be to a peak at least 10° C. above a non-equilibrium β transus. The material may be above the equilibrium β transus for a brief period (e.g., no more than 2.0 seconds). The heating and cooling may have sufficient rates to maintain a characteristic grain size of at least a matrix of the material below 100 μm. In the initial condition, the material may include a number of defects having trunks with microstructures distinct from a microstructure of a matrix of the material. In the treated condition, the trunks&#39; microstructures may be essentially integrated with the matrix microstructure.  
         [0006]     In one group of implementations, the heating may be to a peak 10-50° C. above a non-equilibrium β transus. The material may be above the equilibrium β transus for a very brief period (e.g., no more than 1.0 seconds). In another group of potentially overlapping implementations, the heating may be to 1-30° C. above the equilibrium β transus for a somewhat longer period (e.g., 1.0-5.0 seconds). The cooling may be sufficiently rapid to limit β growth to a characteristic size smaller than 100 μm. One group of materials consist in largest weight parts of titanium, aluminum, and vanadium. An exemplary material thickness may be at least 2.0 mm (e.g., for relatively thick structural and repair material, contrasted with thinner coatings).  
         [0007]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is an optical micrograph of a Ti-6Al-4V condensate atop a like substrate and showing defects.  
         [0009]      FIG. 2  is a view of a healed condensate.  
         [0010]      FIG. 3  is a temperature-time diagram showing healing processes. 
     
    
       [0011]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0012]      FIG. 1  shows a condensate  20  accumulated atop a surface  22  of a substrate  24 . Exemplary condensate thickness may be from less than 0.2 μmm (e.g., for thin coatings) to in excess of 2 mm (at least locally—e.g., for structural condensates such as certain repairs). The condensate has a first defect  26  triggered by a spattered molybdenum droplet  28  that landed atop the surface  22 . Exemplary droplet sizes are 30-500 μm (measured as a characteristic (mean/median/mode) transverse dimension). The defect comprises a trunk  30  extending from the droplet  28  toward the condensate surface (not shown). A second defect  32  is shown and may have been triggered by a droplet below the cut surface of the view.  
         [0013]     The exemplary deposition is of nominal Ti-6Al-4V condensate atop a like substrate. Alternate depositions may include Ti-6Al-2Sn-4Zr-2Mo and Ti-8Al-1Mo-1V. The deposition may be from a melted ingot at least partially through a pool containing one or more refractory or other elements which may be essentially non-consumed during deposition (e.g., a pool formed from a 30%Mo-70%Zr mixture). Accordingly, the droplets may tend to have composition similar to the surface layers of the pool. In the absence of the non-consumed pool additive, the droplet  28  might have a similar composition to the ingot yet still produce similar defects. Many droplets in systems using an Mo-containing pool would have Mo concentrations of at least 10% by weight; others at least 20%. This may be somewhat less than the Mo percentage of the non-expending pool material to reflect possible dilution by deposition material elements in the pool.  
         [0014]     In the exemplary implementation, the substrate  28  has an α-β microstructure of medium to coarse grains (e.g., 10-40 μm characteristic grain size (e.g., mean) or about ASTM 10.5-6.5). An exemplary 10-20% by weight of the substrate is β phase with the remainder essentially a phase. The condensate matrix (away from the defects) also has an α-β microstructure but of very fine grains (e.g., acicular α grains of 5-10 m in length and 2-5 μm in thickness, lengthwise oriented along the condensate growth/deposition direction). The trunk size will depend, in substantial part, upon the droplet size. Exemplary trunk diameters are from about 20 μm to about 50 μm. However, much larger trunks are possible. The trunks have a columnar α-β microstructure. This microstructure may have a characteristic grain size several times greater than that in the matrix and the grains may be elongated in the direction of accumulation (i.e., away from the substrate). Particularly in the case of very large diameter trunks (e.g., in excess of 100 μm in diameter), there may be porosity around the trunk. The grain discontinuity at the trunk-matrix interface and the particular alignment of trunk grains may cause structural weaknesses affecting, inter alia, ductility, fracture toughness, fatigue resistance, fretting fatigue resistance, corrosion resistance, wear resistance, crack nucleation resistance, and the like.  
         [0015]     We have, accordingly, developed heat treatment regimes for healing workpieces having such defects.  FIG. 2  shows a condensate  50  after healing. In this view, the substrate is not shown. The condensate surface  52  is, however, shown. A defect within the condensate had been caused by a droplet  54 . The resultant trunk had propagated all the way to the surface forming a bulge  56 . The healed condensate, however, shows essentially no remaining artifacts of the defect other than the original droplet  54  and bulge  56 . The trunk has been microstructurally integrated with the condensate matrix as a fine β equiaxed microstructure (e.g., grain size of 10-100 μm (˜ASTM 10.5-3.5)). Additionally, laminar variations in chemistry or microstructure may be diminished or eliminated, thereby increasing the isotropy of the condensate mechanical properties. The bulge  56  may be removed (e.g., during a subsequent surface machining). The healing permits the workpiece (e.g., a blade, vane, seal, or the like) to be operated at ambient temperature (e.g., 0-40° C.) and/or at elevated temperatures (e.g., above 250° C., such as in the range of 300-500° C.) essentially without increased chances of failure.  
       EXAMPLE 1  
       [0016]     In a first exemplary healing method, the workpiece is initially at room temperature (condition/location  100 ) on the temperature against time plot of  FIG. 3 .  FIG. 3  further shows a line  102  representing the equilibrium β transus (T e   β ) as well as curves  104  and  106  of non-equilibrium β transus temperatures for the condensate and substrate, respectively (specific to the microstructural and thermal history of this Example 1). The equilibrium β transus temperature is a function of chemistry only. Because the exemplary condensate and substrate have the same chemistry, they share the same equilibrium β transus temperature (e.g., 960-1010° C. for Ti-6Al-4V). The non-equilibrium β transus temperature is a function of composition, grain size/morphology, and heating rate. The smaller the grains, the lower the transus temperature. The more rapid the heating rate, the higher the transus temperature. At very slow heating rates, the equilibrium transus temperature equals the non-equilibrium transus temperature. Given very fine condensate grains, medium to coarse substrate grains, and a heating rate of approximately 100° C./sec, it is estimated that the difference between the non-equilibrium β transus of the two structures is about 100° C.  
         [0017]     In a first stage, the workpiece is heated  109  moderately above the condensate non-equilibrium β transus (condition/location  110 ). This heating may be under vacuum or in an inert atmosphere. This heating is advantageously rapid (e.g., occurring at a rate of 5-100° C./s or greater) so as to prevent excessive grain growth. Excessive grain growth (e.g., above 150 μm (˜ASTM 2.5) or even 100 μm (˜ASTM 3.5), depending upon the application) is disadvantageous because it excessively reduces structural properties including one or more of ductility, fracture toughness, fatigue resistance, fretting fatigue resistance, corrosion resistance, wear resistance, crack nucleation resistance, and the like. The heating reaches a peak of ΔT 1  above the condensate non-equilibrium β transus of approximately 10-50° C. The heating substantially converts the condensate microstructure to β. The upper limit on ΔT 1  will reflect the microstructural/thermal history and is advantageously sufficiently low to avoid excessive β grain growth (in view of time considerations discussed below). The lower limit is advantageously high enough to provide essentially complete transition to the β phase (α+β to β). The substrate may be essentially unaffected.  
         [0018]     In a second stage, the workpiece is rapidly cooled  111  back to room temperature (condition/location  112 ). This heating may be under the same vacuum or atmosphere as the first stage. During this cooling, metastable martensite may accumulate in the condensate and the substrate. The cooling is advantageously sufficiently rapid to further limit β (grain growth. The rapid heating and cooling maintain the condensate above its equilibrium β transus for a time interval sufficiently brief to avoid the excessive β grain growth noted above while providing the β phase transition. An exemplary time interval above the equilibrium β transus is 1.0 seconds or less. Once below the β transus, the β will transform to α+β in a β-transformed (also known as transformed β) microstructure. This microstructure is characterized by preservation of the boundaries of the prior β grains with a (e.g., in needle- or platelet-like form) in a β matrix within such boundaries. The rapid heating and cooling may provide a sufficiently short time at elevated temperature (in view of the magnitude of such temperature) to greatly limit oxidation even if the procedure is performed in an ambient atmosphere rather than under vacuum or an inert atmosphere. Thus, especially for workpieces to be exposed to relatively low thermal and mechanical stresses (e.g., some non-rotating turbine engine parts), there may be greater environmental flexibility during the heating and/or cooling.  
         [0019]     An optional third stage (not shown in  FIG. 3 ) involves annealing/aging by heating the workpiece to an annealing temperature (e.g., in the vicinity of 500-600° C. for the exemplary Ti alloy). The exemplary annealing/aging is for a period of 1-24 hours and is effective to eliminate the martensite but without producing β grain growth. This may leave the condensate as essentially fine grain β (e.g., broadly smaller than 150 μm, more narrowly smaller than 100 μm, and preferably smaller than 50 μm (˜ASTM 2.5, 3.5, and 5.5, respectively)) plus the droplets and leaves the substrate as essentially medium to coarse grain β-β. However, the cooling of the second stage may be sufficiently slow to avoid martensite formation, in which case it is particularly appropriate to omit the aging/annealing.  
       EXAMPLE 2  
       [0020]     In a second exemplary healing method, the initial heating stage  119  is similarly rapid but to a temperature above the equilibrium β transus but below the condensate non-equilibrium β transus. The resulting condition/location  120  may be at a temperature of ΔT 2  above the equilibrium β transus (e.g., by 10-30° C.). The upper limit on this range is advantageously effective to avoid excessive β grain growth. The lower limit on this range is advantageously high enough to provide essentially complete transition to the β phase (α+β to β). The substrate may be essentially unaffected.  
         [0021]     Rather than being immediately followed by rapid cooling, the workpiece is maintained  121  at such a temperature for a moderate time interval (e.g., of about two seconds, more broadly 1.5-4.0 seconds or 1.0-5.0 seconds) to achieve a condition/location  122 . It is during this time interval that the condensate microstructure changes to fine β as in condition/location  110 . Thereafter, a rapid cooling  123  may transition the workpiece to a condition/location  124  similar to  112  and may, in turn, be followed by a similar annealing/aging if appropriate or desired.  
         [0022]     The heating and cooling may be performed by a variety of techniques. One family of rapid heating techniques provides highly local heating (exemplary such techniques include induction heating, laser heating, electron beam heating, and the like). Such heating facilitates heating of the condensate with less substantial heating of the substrate, thereby minimizing any structural effects on the substrate. The desired heating depth may be controlled in view of the coating thickness by means including frequency control of induction heating, beam intensity of laser or electron beam heating, and the like. With some implementations of such heating, the heating may progress across the condensate (e.g., by moving or reorienting the workpiece relative to the heating source). Direct electrical resistance heating may be used to more generally heat the workpiece. The exemplary rapid cooling may be performed by forced cooling with an inert gas (especially when the heating is performed under vacuum), forced air cooling, liquid quench (e.g., in oil or water), and the like.  
         [0023]     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, details of the chemical composition of the particular substrate and condensate and of the physical configuration of the substrate and thickness of the condensate may influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.