Abstract:
A method for forming an article by introducing one or more powders into a bag. Vacuum is applied to the bag. The bag is sealed. The one or more powders in the sealed bag are forged.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Benefit is claimed of U.S. Patent Application Ser. No. 61/789,353, filed Mar. 15, 2013, and entitled “Powder Metallurgy Alloy Forging”, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length. 
     
    
     BACKGROUND 
       [0002]    This disclosure relates to aluminum aerospace alloys. More particularly, the disclosure relates to forging of aluminum alloys. 
         [0003]    Forging of powder-origin metal alloys including aluminum alloys is known. 
         [0004]    Nonlimiting examples are aluminum alloys have included aluminum-based structural amorphous metals (SAM) and icosahedral phase (I-phase) strengthened aluminum. 
         [0005]    SAM and I-phase aluminum have been proposed for aerospace uses including forged airfoil elements such as fan blades. These alloys are produced via rapid solidification using a variety of techniques; examples being comninution of melt-spun ribbon or gas-atomization of powder. Using gas-atomization as an example, the powder is poured into a containment can which then contains both powder and the gas from atomization. An example of a can is one which is fabricated of commercially available aluminum (such as 6061-T8) and is ten inches in diameter by sixty inches tall with a 0.125 inch wall thickness (25 cm in diameter by 1.5 m tall with a 3.2 mm wall thickness). 
         [0006]    Once the can is filled with powder, it is processed through a degassing cycle, where the can is heated (e.g., to 700° F. (371° C.)) while maintaining a vacuum to remove moisture and contaminants. Once an acceptable vacuum level is achieved (e.g., 5×10 −6  Torr), the can is sealed. The degassing cycle can take an exemplary ten hours to two weeks to achieve the desired outgassing, depending on cleanliness and contaminants in the powder, can, and vacuum system. 
         [0007]    Once degassed, the SAM/I-phase alloy-filled can is moved to a hydraulic compaction press, while still hot (e.g., 700° F. (371° C.)) and a load is applied to compact the alloy can. With the exemplary commercial can above, exemplary compaction is to approximately ten inches in diameter by twenty-four inches tall by 0.250 inch wall thickness (25 cm diameter by 61 cm tall with a 6.4 mm wall thickness). This compaction cycle does not fully compact the alloy powder into a solid billet, but is estimated to be approximately 60% compacted. 
         [0008]    After compaction, the billet cools to room temperature. 
         [0009]    Solid metal pieces (e.g., of 6061-T8 aluminum, ten inches (25.4 cm) in diameter) are secured (e.g., welded) to the ends of the compacted can. The pieces act as a leader/front (e.g., six inches (15 cm) thick) and follower/rear (e.g., four inches (10 cm) thick) to aid in the compaction/extrusion process. Without the leader and follower, breaching the can during extrusion would result in powder loss and contamination due to oxidation. 
         [0010]    The SAM/I-phase alloys are then ready for solidification via extrusion. The can and the welded leader and follower are then heated in an oven to achieve an elevated internal temperature (e.g., 675° F. (357° C.)), then placed into an extrusion press and pressed against a flat/blind die to fully compact the alloy powder into a solid state. The blind die is removed from the press, and an extrusion die is loaded into the press. Typically an extrusion ratio of 10:1 is used for processing the SAM/I-phase alloy into an extruded log. An issue with the extrusion process is the adiabatic heating caused by the friction of the alloy being pushed through a small diameter hole in comparison with the large (e.g., ten inch (25.4 cm)) starting diameter. Depending on the speed of the press, the adiabatic heating can reach as high as 850+° F. (454+° C.). Any temperature exposure above a certain amount (e.g., 700° F. (371° C.)) has a detrimental effect on material properties of the SAM/I-phase alloys. After extrusion, the extrudate is cooled to room temperature. 
         [0011]    Once the SAM/I-phase alloys are extruded, the leader and follower material is cut away and discarded. The now fully dense alloy log is then cut into individual pieces, referred to as “mults” or “forging mults”, and machined (removing the can) to be true in diameter and length (e.g., nine inches (22.9 cm) long by 3.5 inches (8.9 cm) in diameter in the example). The fully dense mults are now ready for forging. 
         [0012]    There are an exemplary three forging steps to producing airfoils/fan blades from the SAM/I-phase alloy mults. For the first forging step, mults are placed in a mult furnace at 675° F. (357° C.) for a short isothermal soak at temperature. A single mult is then placed in a “blocker” die (which was previously heated in a die furnace) that has also been heated to 675° F. (357° C.), and forged in a hydraulic press producing a rectangular shape approximately eleven inches long by three inches wide by 1.75 inches thick. 
         [0013]    For the second forging step, the rectangular shape is removed from the blocker die and put back into the mult furnace at 675° F. (357° C.), soaked, then transferred to a hot (e.g., 675° F. (357° C.)) preform die (which was previously heated in the same die furnace as the blocker die) and forged into the preform shape. 
         [0014]    For the third forging step, the preform shape is removed from the preform die, placed in the mult furnace, reheated and soaked at 675° F. (357° C.), then placed in the hot final form die (which was previously heated in the same die furnace as the blocker and preform dies), and forged into the final shape, producing the desired near net-shape fan blade. 
         [0015]    Final machining, surface treatments (e.g., peening), and coating processes may follow. 
         [0016]    Attempts have been made to simplify this process. One example is US Patent Application Publication 2005/0147520A1. 
       SUMMARY 
       [0017]    One aspect of the disclosure involves a method for forming an article. One or more powders are introduced to a bag. Vacuum is applied to the bag and the bag is then sealed. The one or more powders in the sealed bag are then forged directly. 
         [0018]    In one or more embodiments of any of the foregoing embodiments, the forging is a first forging in a first die and the method further comprises a second forging in a second die. 
         [0019]    In one or more embodiments of any of the foregoing embodiments, a third forging in a third die. 
         [0020]    In one or more embodiments of any of the foregoing embodiments, the first forging, the second forging, and the third forging are the only forgings. 
         [0021]    In one or more embodiments of any of the foregoing embodiments, the method of further comprises: a heat soak between the first forging and the second forging; and a heat soak between the second forging and the third forging. 
         [0022]    In one or more embodiments of any of the foregoing embodiments, there is no die extrusion step. 
         [0023]    In one or more embodiments of any of the foregoing embodiments, a machining is after the second forging. 
         [0024]    In one or more embodiments of any of the foregoing embodiments, the article is a blade comprising an airfoil and an attachment root. 
         [0025]    In one or more embodiments of any of the foregoing embodiments, the article comprises an aluminum alloy. 
         [0026]    In one or more embodiments of any of the foregoing embodiments, the aluminum alloy is a SAM alloy comprising Al with Y, Ni, Co, Zr. 
         [0027]    In one or more embodiments of any of the foregoing embodiments, the aluminum alloy is an I-phase alloy comprising Al with Cr, Mn, Co, and Zr. 
         [0028]    In one or more embodiments of any of the foregoing embodiments, a peak temperature is less than 500° C. 
         [0029]    In one or more embodiments of any of the foregoing embodiments, the bag comprises an aluminum alloy having a wall thickness less than 6.5 mm. 
         [0030]    In one or more embodiments of any of the foregoing embodiments, the bag comprises an aluminum alloy having a wall thickness less than 1.5 mm. 
         [0031]    In one or more embodiments of any of the foregoing embodiments, the bag comprises an inner surface which is separated from the outer surface by a thin-walled metal that, unlike a can, is collapsible or partially collapsible due to atmospheric pressure on the outer surface when the inner surface is subjected to a vacuum. 
         [0032]    In one or more embodiments of any of the foregoing embodiments, the bag comprises a polymer having a wall thickness less than 2 mm. 
         [0033]    In one or more embodiments of any of the foregoing embodiments, the sealing comprises either liquid or solid state welding. 
         [0034]    Another aspect is an article produced by the method. 
         [0035]    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 
         [0036]      FIG. 1  is a partially schematic longitudinal sectional view of a gas turbine engine. 
           [0037]      FIG. 2  is a view of a fan blade of the engine of  FIG. 1 . 
           [0038]      FIG. 3  is a plan view of an example pattern for forming a bag for forging a blade. 
           [0039]      FIG. 4  is a simplified view of an example bag. 
           [0040]      FIG. 5  is a longitudinal sectional view of a bag. 
           [0041]      FIG. 6  is a schematic view of an apparatus for filling a bag. 
           [0042]      FIG. 7  is a schematic view of an apparatus for final depressurization of the bag. 
           [0043]      FIG. 8  is a schematic view of a first die in an open condition. 
           [0044]      FIG. 9  is a schematic view of the first die in a closed condition. 
           [0045]      FIG. 10  is a schematic view of a second die in an open condition. 
           [0046]      FIG. 11  is a schematic view of a second die in a closed condition. 
           [0047]      FIG. 12  is a schematic view of a third die in an open condition. 
           [0048]      FIG. 13  is a schematic view of a third die in a closed condition. 
       
    
    
       [0049]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0050]      FIG. 1  is a schematic view of selected portions of an example gas turbine engine  10  suspended from an engine pylon  12  of an aircraft, as is typical of an aircraft designed for subsonic operation. The gas turbine engine  10  is circumferentially disposed about an engine centerline, or axial centerline axis A. The gas turbine engine  10  includes a fan  14 , a compressor  16  having a low pressure compressor section  16   a  and a high pressure compressor section  16   b,  a combustion section  18 , and a turbine  20  having a high pressure turbine section  20   b  and a low pressure turbine section  20   a.  As is known, air compressed in the compressors  16   a,    16   b  is mixed with fuel that is burned in the combustion section  18  and expanded in the turbines  20   a  and  20   b.  The turbines  20   a  and  20   b  are coupled for rotation with, respectively, rotors  22   a  and  22   b  (e.g., spools) to rotationally drive the compressors  16   a ,  16   b  and the fan  14  in response to the expansion. In this example, the rotor  22   a  drives the fan  14  through a gear train  24 . 
         [0051]    In the example shown, the gas turbine engine  10  is a high bypass geared turbofan arrangement. In one example, the bypass ratio is greater than 10:1, and the fan  14  diameter is substantially larger than the diameter of the low pressure compressor  16   a  and the low pressure turbine  20   a  has a pressure ratio that is greater than 5:1. The gear train  24  can be any known suitable gear system, such as a planetary gear system with orbiting planet gears, planetary system with non-orbiting planet gears, or other type of gear system. In the disclosed example, the gear train  24  has a constant gear ratio. Given this description, one of ordinary skill in the art will recognize that the above parameters are only exemplary and that the disclosed examples are applicable to other engine arrangements or other types of gas turbine engines. 
         [0052]    An outer housing, nacelle  28 , (also commonly referred to as a fan nacelle) extends circumferentially about the fan  14 . A generally annular fan bypass passage  30  extends between the nacelle  28  and an inner housing, inner cowl  34 , which generally surrounds the compressors  16   a,    16   b  and turbines  20   a ,  20   b.  The gas turbine engine  10  also includes guide vanes  29  (shown schematically). 
         [0053]    In operation, the fan  14  draws air into the gas turbine engine  10  as a core flow, C, and into the bypass passage  30  as a bypass air flow, D. In one example, approximately 80 percent of the airflow entering the nacelle  28  becomes bypass airflow D. A rear exhaust  36  discharges the bypass air flow D from the gas turbine engine  10 . The core flow C is discharged from a passage between the inner cowl  34  and a tail cone  38 . A significant amount of thrust may be provided by the bypass airflow D due to the high bypass ratio. 
         [0054]    As can be appreciated, the gas turbine engine  10  may include airfoil components in one or more of the sections of the engine. An example is an aluminum alloy fan blade  60  ( FIG. 2 ) having an airfoil portion  62  and a root portion  64  for mounting the airfoil component in the gas turbine engine  10  (e.g., to a fan hub). The airfoil portion extends from an inboard end  66  at the root (or an intervening platform) to an outboard end  68  (e.g., a tip shown as a free (unshrouded) tip). The airfoil extends streamwise from a leading edge  70  to a trailing edge  72  and has a pressure side  74  and a suction side  76 . 
         [0055]    In the following example, instead of using a baseline can, the processing of SAM/I-phase alloy powder uses a flexible, individual blade-sized containment bag  310  ( FIG. 4 ) to compact and forge into an aircraft fan blade. The bag may be made of aluminum foil (e.g., an aluminum alloy having a characteristic thickness 1.5 mm or less, more particularly 1.0 mm or less or an exemplary 0.4-1.0 mm). The exemplary bag for making a particular blade is, eleven inches long by three inches wide by three inches tall by 0.030 inch wall thickness (28 cm by 7.6 cm by 7.6 cm by 0.8 mm wall thickness). The exemplary bag is constructed by picking a shape, cutting a flat pattern  300  ( FIG. 3 ) for the three-dimensional shape out via laser, or something as simple as scissors, depending on the gage. The cut shape is then folded and tack welded to form the 3-dimensional shape which can then be welded using either solid or liquid state processes. A tube  320  ( FIG. 4 ) is then welded onto the bag and the other end has a quick disconnect valve  322  ( FIG. 6 ) on it which can be opened to allow for evacuation of either gas or air from the bag, or allow for filling of the bag with an inert gas. The seams will be where the bag is joined together after being cut from the sheet. These seams might be as simple as those to make a right circular cylinder, or they may occur in such a way as to define a blocker shape that can be used to start blocker forge operations or preferably, a final forge operation. The only fitting is envisioned to be on an end for evacuation/pressurization purposes. The end would be chosen such that minimal deformation of the fitting would occur, thus precluding the possibility that the powder becomes exposed to air prior to the establishment of a high relative density (which then provides for closing off any remaining porosity from the outside environment). The powder enters the bag through the tube. 
         [0056]    The exemplary bag has an internal volume of 300 in 3  (4.9 liters). 
         [0057]    Alternatively, the bag may be made of a high temperature polymer (e.g., KAPTON polyimide foil). 
         [0058]    Exemplary SAM alloys are found in U.S. Pat. No. 6,248,453 B1, U.S. Pat. No. 6,974,510 B2 and U.S. Pat. No. 7,413,621 B2 and generally comprise Al with Y, Ni, Co, and Zr. Exemplary I-phase powders generally comprise Al with Cr, Mn, Co, and Zr. The exemplary bag is filled with about 30 kg of such alloys. A broader weight range for blades of a generally similar type for different engine sizes is 10-80 kg, but for a broader part range 1-500 kg. 
         [0059]    The exemplary bag replaces a much larger baseline containment can and also allows for omitting steps in the baseline process. Of particular relevance, not only from a cost savings aspect, but from a materials property standpoint, is the elimination of the extrusion process and the adiabatic heating of the material therein. Also, time at temperature during thermal processing, in general, reduces the strength characteristics of the SAM/I-phase alloy, and is additive. 
         [0060]    Thus, this process may eliminate significant time at temperature over the baseline to better maintain strength. 
         [0061]    In an exemplary process, a powder source  330  ( FIG. 6 ) containing metal powder  331  may be in a form such as a static or dynamic degassing unit (or any closed container with a valve opening) containing the powder.  FIG. 6  further shows a vacuum source  332  and an inert gas source  400 . The vacuum source  332 , the powder source  330 , and the gas source  400  are coupled to a port for connection to the valve  322 . One or more valves  334 ,  335 , and  336  selectively control flow amongst the powder source  330 , the vacuum source  332 , the gas source  400 , and the bag  310 . In the exemplary embodiment, there is a cross joint  340  coupling these four locations. Initially, all communication is blocked (e.g., the valves  334 ,  335 , and  336  are closed). The valve  335  may then be opened and the vacuum source  332  (e.g., a pump) evacuates the cross joint  340 . The pump (mechanical or other) evacuates the cross to a pressure on the order 10 −3  to 10 −4  Torr. Thereafter, the valve  322  is opened. This evacuates air from the bag. If the bag is sufficiently thin, it will be crushed by atmospheric pressure. In this case, valve  335  can be closed and valve  336  can be opened to fill the bag  310  with inert gas from gas source  400 , thereby returning it to its original shape. Repeating this procedure two to three times will assure that the bag is free of contaminants. Optionally, the bag may be heated. The heating helps further drive off any moisture on the inner wall surface of the bag. If heated, the bag may then be cooled to room temperature, and in this case, is still under pressure from the inert gas. The valve  322  is then closed to close off communication with the bag. The valve  336  is then closed to close off communication with the gas source. 
         [0062]    Thereafter, the valve  334  may be opened. This opening causes powder to flow from the source  330  into the cross  340 . Thereafter, the valve  322  may be opened causing the powder to fall into the bag  310 , filling the bag, while simultaneously displacing the inert gas in the bag. With the bag full, the valves  322  and  334  may be closed and the bag disengaged. If the bag is not crushed by atmospheric pressure, it can be seen that appropriate opening and closing of valves  322 ,  335 , and  336  could create a vacuum in the bag  310 . Keeping valves  335  and  336  closed and valve  322  open, the vacuum in bag  310 , once valve  334  is opened, would further encourage, rather than depending on gravity alone as was the situation in the prior case, the filling of bag  310  due to the pressure differential between bag  310  and powder source  330 . 
         [0063]    The bags may then be evacuated (for the case of a collapsible bag)/further evacuated (for the case of a rigid bag) to reduce/further reduce their pressure (e.g., with a stronger vacuum pump (e.g., diffusion, cryo-pumps)  350  ( FIG. 7 )). The vacuum pump may be connected to a manifold  352  for simultaneously evacuating multiple bags. The trunk and branch of the manifold may each have a valve for flow control. 
         [0064]    For degassing, the filled bag is then connected to an associated branch of the vacuum manifold  352  (e.g., to which many such bags (typically ten to sixteen bags) are connected via respective branches  354 )). Ideally, all bags would be attached to this system simultaneously. The system valves would then be opened to remove the air between the system valves and the bag valves. The bag valves would then be opened and the residual gas in the bags removed. For the case where a bag or bags were added to the system, the system and bag valves would be closed to the existing bags, and then opened for the new bags. Once the pressure for the new bags reached the pressure of the existing bags, then all valves could be opened to all bags. While this would not be as efficient as the method for placing all bags on the manifold simultaneously, it would allow for late or additional bags to be evacuated. 
         [0065]    Again, the pump  350  may optionally pre-evacuate the manifold. Then the valves are opened to evacuate the bags. For yet further evacuation of water or other undesirable contaminants, the bags may further be heated during this process. 
         [0066]    When acceptable vacuum level is achieved (e.g., 1×10 −6  Torr) the vacuum tube  320  on the bag is crimped and welded. The bag is then disconnected from the vacuum source  350 . Due to the reduced powder volume in a bag (e.g., about 300 in 3  (4.9 liters) in one example) versus a can (e.g., 4500 in 3  (74 liters), the outgassing cycle time and time at temperature may be reduced to less than two hours, saving fourteen or more hours to days of heat exposure. 
         [0067]    After degassing and sealing, the filled bag is ready for forging. This eliminates several steps in the prior art, including an exemplary three thermal cycles during forging. Also, this eliminates the adiabatic heating induced in the extrusion process, which has the most dramatic effect on material properties of SAM/I-phase alloys. Thus, temperatures may be kept to no more than an exemplary 752° F. (400° C.) contrasted with the baseline adiabatic heating to 850° F. (454° C.). 
         [0068]    Filled bags are placed in a mult furnace at an exemplary 675° F. (357° C.) for a short isothermal soak at temperature in order to get a uniform thermal distribution in the bag. Then, while still at temperature, the filled bags  310  are each (e.g., consecutively or using separate die sets) placed in a “blocker” die  600  ( FIGS. 8 &amp; 9 ) that has been heated in a die furnace to 675° F. (357° C.), and forged in a hydraulic press producing a rectangular shape  604  (a first precursor of the blade) approximately eleven inches long by three inches wide by 1.75 inches thick (28 cm long by 7.6 cm wide by 4.4 cm thick). This step combines the compaction, compaction/extrusion, and blocking process in one step, eliminating thermal cycles, containment can materials, leaders, followers, machining, and thermal processing. 
         [0069]    The rectangular shape  604  is then placed into the mult furnace at an exemplary 675° F. (357° C.), soaked, then transferred to a hot (e.g., 675° F. (357° C.)) preform die  610  ( FIGS. 10 and 11 ) (heated in a die furnace) and forged into the preform shape  606  (as a second precursor of the blade). This step forms the initial distinct precursors of the airfoil and root. 
         [0070]    Finally the preform shape  606  is placed in the mult furnace, heated to 675° F. (357° C.), then placed in the hot final form die  620  ( FIGS. 12 and 13 ) (which was heated in the die furnace), and forged into the final forged shape  608 , producing the desired near net shape fan blade as a third precursor of the blade with further defined airfoil and root portions. 
         [0071]    Thereafter, machining may create the final blade shape. In some embodiments the bag may become a part of the forged structure. For example, on the one hand, one might want to have an aluminum based bag containing zinc or chromium that has superior corrosion resistance than the baseline powder alloy; but, on the other hand, one might want to machine the bag away. The bag could be aluminum based and the powder could be conventional aluminum alloys including 6000, 2000, 7000, or more recently, Al—Li alloys. On the other hand, the bag could be composed of a SAM/I-phase alloy that has the same composition as the powder. Or maybe the bag is an I-phase alloy with better corrosion resistance, but reduced mechanical properties relative to the powder alloy. The surface, whether composed of the bag or not, would be machined, peened, and coated as appropriate. 
         [0072]    Due to the nature of forging powder, it may be the case that the temperature can be reduced. This would further reduce thermal coarsening of the microstructure that leads to reduced properties. It would also lower the time of exposure, thereby further reducing microstructural coarsening, since less time is needed to heat up to lower forging temperatures. The 675° F. (357° C.) temperature range has been chosen due to the fact that this was the lowest possible temperature that SAM alloys could be extruded successfully. Forging of powder may be performed as low as 482° F. (250° C.) 
         [0073]    The foregoing exemplary process, including bag sizing is specific to one particular airfoil/blade part number. This process can be used on different airfoil shapes and sizes, but will result in different bag sizes to address size and volume differences. Other parameters which may be varied for different purposes include bag thickness, temperatures, time at temperature, press speeds, etc. 
         [0074]    The use of “first”, “second”, and the like in the following claims is for differentiation only and does not necessarily indicate relative or absolute importance or temporal order. 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. 
         [0075]    One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, details of the part being manufactured or details of baseline equipment being re-used will influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.