Abstract:
A method for consolidating a preform made of composite material. The preform and a compliant metal alloy sheet are placed between less compliant matched confronting forming/molding surfaces with the preform being sandwiched between the metal alloy sheet and a matched confronting surface. The matched confronting surfaces and compliant metal alloy sheet are heated until the preform reaches at least its consolidation temperature. During heating, force is applied so that the matched confronting surfaces exert sufficient compressive force on the preform and metal alloy sheet to cause the composite material to consolidate at the consolidation temperature. The metal alloy sheet has a tensile yield point in a range of 25-300 psi at the consolidation temperature at a strain rate of about 1% to 10% strain per minute.

Description:
BACKGROUND 
       [0001]    This disclosure generally relates to fabrication of parts made of composite material. More specifically, this disclosure relates to apparatus and methods for consolidating and forming a pre-form made of fiber-reinforced plastic material (also referred to herein as a composite preform) to reduce voids and/or porosity. 
         [0002]    Fiber-reinforced organic resin matrix composites have a high strength-to-weight ratio or a high stiffness-to-weight ratio and desirable fatigue characteristics that make them increasingly popular as a replacement for metal in aerospace applications. Organic resin composites may comprise thermoplastics or thermosetting plastics. 
         [0003]    Prepregs combine continuous, woven, or chopped reinforcing fibers with an uncured, matrix resin, and usually comprise fiber sheets with a thin film of the matrix. Sheets of prepreg generally are placed (laid-up) by hand or with fiber placement machines directly upon a tool or die having a forming surface contoured to the desired shape of the completed part or are laid-up in a flat sheet which is then draped and formed over the tool or die to the contour of the tool. Then the resin in the prepreg layup is consolidated (i.e., pressed to remove any air, gas, or vapor) and cured (i.e., chemically converted to its final form usually through chain-extension) in a vacuum bag process in an autoclave (i.e., a pressure oven) to complete the part. 
         [0004]    In hot press forming, the prepreg is laid-up, bagged (if necessary) and placed between matched metal tools that include forming surfaces that define the internal, external, or both mold lines of the completed part. The tools and composite preform are placed within a press and then the tools and preform are heated under pressure to produce a consolidated, net-shaped part. 
         [0005]    It is known to consolidate and form composite preforms using inductively heated consolidation tools. Induction heating is a process in which an electrically conducting object (usually a metal) is heated by electromagnetic induction. During such heating, eddy currents are generated within the metal and the electrical resistance of the metal leads to Joule heating thereof. An induction heater typically comprises an electromagnet through which a high-frequency alternating current is passed. Most organic matrix composites require a susceptor in or adjacent to the composite material preform to achieve the necessary heating for consolidation or forming. The susceptor is heated inductively and transfers its heat principally through conduction to the preform sandwiched between opposing susceptor facesheets. During heating under pressure, the number of voids and/or the porosity of a composite preform can be reduced. 
         [0006]    Recycled graphite fibers can be used in the fabrication of composite aircraft parts, such as lightweight seat back components. First, a matte product is fabricated using recycled graphite fibers and virgin thermoplastic fibers; then the matte product is consolidated and formed in matched tooling comprising opposing susceptor facesheets. These matte products produced with recycled graphite fibers can exhibit a rather matted and tangled fiber architecture. These products can also have undesirable unevenness and thickness/density variations. Furthermore, since the graphite fibers are tangled, they do not facilitate the flow of the thermoplastic material. These characteristics lead to the formation of voids and/or porosity in the final composite product due to the fact that thickness and density distribution are not uniform across the matte product. The formation of voids and/or porosity is further aided by the fact that the fibers are entangled and flow is limited and unable to “heal” porosity very effectively. Even prepreg has some variation in thickness and density distribution, which could lead to the formation of voids and/or porosity during matched tool molding of prepreg composites. 
         [0007]    Accordingly, there is a need for a method and an apparatus that can reduce the number of voids and/or the porosity during the consolidation and formation of a composite preform having uneven thickness and/or density distribution. 
       SUMMARY 
       [0008]    The subject matter disclosed herein is directed to a method for reducing the number of voids and/or porosity during the consolidation and formation of a composite preform having uneven thickness and/or density distribution, such as a matte product comprising recycled graphite fibers and virgin thermoplastic fibers. This method is carried out using an apparatus that comprises matched molding tools. The apparatus further comprises a compliant layer that is situated between the composite preform and one of the matched molding tools for the purpose of providing a more even pressure over the entire area of the preform during the consolidation process. The compliant layer should have an offset tensile yield point (0.2% of the strain) in a range of 25-300 psi at the temperature of consolidation of the preform at strain rates of about 1% to 10% strain per minute. 
         [0009]    In accordance with one embodiment, a sheet of magnesium base alloy is used to act as the compliant layer or shim to compensate for uneven thickness or density over the area of a composite preform, for example, a matte product comprising recycled graphite fibers and virgin thermoplastic fibers. Magnesium base alloy makes an excellent candidate for a compliant layer for high-performance thermoplastic resins due to the fact that some magnesium alloys become very soft at temperatures useful for assisting the consolidation and molding of thermoplastic composites (i.e., 600-750° F.) and do not melt until above 1000° F. As the temperature and pressure increase inside the apparatus during consolidation of the composite preform, the magnesium alloy sheet softens and forms into the areas of relatively lower pressure. The magnesium alloy sheet can be reused due to the soft nature of the material. Other alloys can be used instead of a magnesium base alloy provided that the alloy has an offset tensile yield point (0.2% of the strain) in a range of 25-300 psi at the temperature of consolidation of the preform at strain rates of about 1% to 10% strain per minute 
         [0010]    In accordance with one aspect, a method is disclosed for consolidating a preform made of composite material with reduced number of voids and/or porosity. The preform and a compliant metal alloy sheet are placed between less compliant matched confronting forming/molding surfaces with the preform being sandwiched between the metal alloy sheet and one of the matched confronting surfaces. The matched confronting surfaces are inductively heated (which heats the compliant metal alloy sheet by conduction) until the preform reaches at least a consolidation temperature of the composite material. During heating, force is applied so that the matched confronting surfaces exert sufficient compressive force on the preform and metal alloy sheet to cause the composite material to consolidate at the consolidation temperature. The compliant metal alloy sheet has a tensile yield point in a range of 25-300 psi at the consolidation temperature at a strain rate of about 1% to 10% strain per minute. 
         [0011]    Another aspect of the disclosed subject matter is an apparatus for consolidating a preform made of composite material at a consolidation temperature, comprising: first and second tool assemblies having matched confronting surfaces; a metal alloy sheet disposed between said matched confronting surfaces, wherein said metal alloy sheet has a tensile yield point in a range of 25-300 psi at the consolidation temperature at a strain rate of about 10% strain per minute; means for heating at least the matched confronting surfaces of the first and second tool assemblies; and means for applying force to one or both of the first and second tool assemblies so that the matched confronting surfaces are capable of exerting compressive force on the preform and metal alloy sheet. 
         [0012]    A further aspect is a method for consolidating a composite preform made of recycled graphite fibers and organic resin fibers, comprising: placing the composite preform and a metal alloy sheet between matched confronting surfaces of first and second tool assemblies, the matched confronting surfaces being less compliant than the metal alloy sheet; heating the matched confronting surfaces of the first and second tool assemblies and the metal alloy sheet during a heating cycle; and applying force to one or both of the first and second tool assemblies so that the matched confronting surfaces exert sufficient compressive force on the composite preform and metal alloy sheet to cause thermal coupling of one matched surface to one side of the composite preform, the other side of the composite preform to the metal alloy sheet, and the metal alloy sheet to the other matched confronting surface. The force is applied during at least a portion of the heating cycle. The metal alloy sheet has a tensile yield point in a range of 25-300 psi at a consolidation temperature at a strain rate of about 1% to 10% strain per minute. 
         [0013]    Other aspects of the invention are disclosed and claimed below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Various embodiments will be hereinafter described with reference to drawings for the purpose of illustrating the foregoing and other aspects of the invention. 
           [0015]      FIG. 1  is a diagram showing a sectional view of portions of a known apparatus, the apparatus comprising upper and lower tool assemblies with matched surfaces designed to consolidate and form a composite preform. The tool assemblies are shown in their retracted positions and the preform is shown in an uncompressed state. 
           [0016]      FIG. 2  is a diagram showing a sectional view of the apparatus depicted in  FIG. 1 , except that the tool assemblies are in their extended positions with the preform compressed therebetween. 
           [0017]      FIG. 3  is a diagram showing an end view of a portion of a lower tooling die in accordance with one embodiment. 
           [0018]      FIG. 4  is a diagram showing a sectional view of a portion of the lower tooling partially depicted in  FIG. 3 , the section being taken along line  4 - 4  seen in  FIG. 3 . 
           [0019]      FIG. 5  is a diagram showing the placement of an unconsolidated matte product between a pair of opposing susceptors in their retracted positions, the matte product comprising recycled graphite fibers and virgin thermoplastic fibers. 
           [0020]      FIG. 6  is a diagram showing a consolidated matte product between a pair of susceptors in their extended positions. 
           [0021]      FIG. 7  is a diagram showing a compliant layer and a consolidated matte product between a pair of susceptors in their extended positions. 
           [0022]      FIG. 8  is a graph showing the effect of temperature on the tensile properties of a magnesium base alloy having a chemical composition of 2.5-3.5% aluminum; 0.7-1.3% zinc; 0.20-1.0% manganese; balance magnesium. The 0.2% proof stress curve has been extrapolated to the right of the vertical axis labeled “ ELONGATION ” to show the anticipated effect of temperatures in excess of 300° C. on the 0.2% proof stress. 
           [0023]      FIG. 9  is a block diagram showing components of a system comprising upper and lower tool assemblies with matched surfaces and a compliant metal alloy sheet disposed therebetween for use in consolidating composite preforms. 
       
    
    
       [0024]    Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals. 
       DETAILED DESCRIPTION 
       [0025]    The following detailed disclosure describes a method and an apparatus for consolidating and molding/forming a composite preform having an uneven thickness and/or density distribution. In accordance with the disclosed method, a compliant layer is placed between the composite preform and one heated consolidation tool. The compliant layer is designed to distribute the molding pressure more evenly over the entire area of the uneven preform during the consolidation process. 
         [0026]    One known apparatus for matched tool consolidation of composite preforms is partly depicted in  FIGS. 1 and 2 .  FIG. 1  shows the apparatus in a pre-consolidation stage, while  FIG. 2  shows the apparatus while consolidation is under way. The apparatus comprises a lower die frame  2 , a lower tooling die  4  supported by the lower die frame  2  and having a first contoured die surface  6 , an upper die frame  8 , and an upper tooling die  10  supported by the upper die frame  8  and having a second contoured die surface  12  which is complementary to the first contoured die surface  6 . The contoured die surfaces  6  and  12  may define a complex shape different than what is depicted in  FIGS. 1 and 2 . However, the novel means disclosed herein also have application when the die surfaces are planar. The die frames  2  and  8  may be coupled to hydraulic actuators (not shown in  FIGS. 1 and 2 ), which move the dies toward and away from each other. In addition, one or more induction coils (not shown in  FIGS. 1 and 2 ) may extend through each of the tooling dies  4  and  10  to form an induction heater for raising the temperature of the resin in a composite preform to at least its consolidation temperature. A thermal control system (not shown) may be connected to the induction coils. 
         [0027]    Still referring to  FIGS. 1 and 2 , the apparatus further comprises a lower susceptor  18  and an upper susceptor  20  made of electrically and thermally conductive material. The susceptors and the induction coils are positioned so that the susceptors can be heated by electromagnetic induction. The lower susceptor  18  may generally conform to the first contoured die surface  6  and the upper susceptor  20  may generally conform to the second contoured die surface  12 . In some cases, it is preferred that the temperature at which a composite preform is consolidated should not exceed a certain temperature. To this end, susceptors  18  and  20  are preferably so-called “smart susceptors”. A smart susceptor is constructed of a material, or materials, that generate heat efficiently until reaching a threshold (i.e., Curie) temperature. As portions of the smart susceptor reach the Curie temperature, the magnetic permeability of those portions falls to unity (i.e., the susceptor becomes paramagnetic) at the Curie temperature. This drop in magnetic permeability has two effects: it limits the generation of heat by those portions at the Curie temperature, and it shifts the magnetic flux to the lower temperature portions, causing those portions below the Curie temperature to more quickly heat up to the Curie temperature. Accordingly, thermal uniformity of the heated preform during the forming process can be achieved irrespective of the input power fed to the induction coils by judiciously selecting the material for the susceptor. In accordance with one embodiment, each susceptor is a layer or sheet of magnetically permeable material. Preferred magnetically permeable materials for constructing the susceptors include ferromagnetic materials that have an approximately 10-fold decrease in magnetic permeability when heated to a temperature higher than the Curie temperature. Such a large drop in permeability at the critical temperature promotes temperature control of the susceptor and, as a result, temperature control of the part being manufactured. Ferromagnetic materials include iron, cobalt, nickel, gadolinium and dysprosium, and alloys thereof. 
         [0028]    The consolidation/molding apparatus shown in  FIGS. 1 and 2  further comprises a cooling system  14  comprising respective sets of cooling conduits  16  distributed in the tooling dies  4  and  10 . Each set of coolant conduits  16  is coupled via respective manifolds to a source of cooling medium, which may liquid, gas or a gas/liquid mixture such as mist or aerosol. 
         [0029]    In a typical implementation of a composite consolidation and molding process, the composite preform  22  is initially positioned between the upper and lower tooling dies of the stacked tooling apparatus, as shown in  FIG. 1 . Then the tooling dies  4  and  10  are moved toward each other, as shown in  FIG. 2 , as the induction coils heat the susceptors  18  and  20 . Therefore, as the tooling dies close toward each other, the susceptors rapidly heat the composite preform  22 . During this process, the composite preform will be molded by the opposing contoured (or planar) surfaces of the susceptors  18  and  20 . 
         [0030]    After a predetermined interval of time, the cooling system  14  will be operated to apply a cooling medium to the tooling dies  4  and  10 , thereby also cooling the susceptors  18  and  20  and the composite preform  22  therebetween. The composite preform  22  remains sandwiched between the susceptors for a predetermined period of time until complete cooling of the composite preform has occurred. This allows the molded and consolidated composite preform  22  to retain the structural shape which is defined by the contoured surfaces of the susceptors  18  and  20 . The tooling dies are then opened and the composite preform can be removed. The formed and cooled composite preform is removed from the stacked tooling apparatus without loss of dimensional accuracy when it is cooled at an appropriate property-enhancing rate. 
         [0031]      FIG. 3  is an end view of a portion of a lower tooling die  4  in accordance with one embodiment. The upper tooling die may have a similar construction. Each tooling die comprises a multiplicity of cavities  32 , which may be mutually parallel.  FIG. 3  shows only two such cavities  32 , the upper portion of each cavity  32  having a portion of a respective turn of an induction coil  34  which passes through the uppermost portion of the cavity. 
         [0032]    The sectional view shown in  FIG. 4  is taken along line  4 - 4  seen in  FIG. 3  and passes through a cavity  32 , but not through the portion of inductive coil  34  therein. One or more coils can be used. As the parts requiring fabrication get bigger, it may be necessary to break the coil into multiple coils connected in parallel in order to limit the voltage required by each coil. Without the smart susceptors, control of the current (and resulting temperature) to each parallel coil could become problematic. For the sake of simplicity,  FIGS. 3 and 4  show a portion of a lower tooling die for which the corresponding portion of the attached susceptor is horizontal rather than angled. 
         [0033]    Still referring to  FIGS. 3 and 4 , the lower tooling die may comprise a lamination of alternating metal (e.g., stainless steel) plates  28  and dielectric spacers  30  which are trimmed to appropriate dimensions to form a plurality of parallel longitudinal cavities  32  in which the turns of one or more induction coils  34  reside. Each metal plate  28  may have a thickness in the range of about 0.0625 to about 0.5 inch. Air gaps  36  (see  FIG. 4 ) may be provided between the upper portions of metal plates  28  to facilitate cooling of the susceptors. The stacked metal plates  28  may be attached to each other using clamps (not shown), fasteners (not shown) and/or other suitable means known to persons skilled in the art. The stacked metal plates  28  may be selected based on their electrical and thermal properties and may be transparent to the electromagnetic field produced by the induction coils. 
         [0034]    As best seen in  FIG. 4 , the smart susceptor  18  is attached directly to the metal plates  28  of the lower tooling die. (The smart susceptor  20  seen in  FIG. 1  is likewise attached directly to the metal plates of the upper tooling die.) In accordance with one embodiment, the metal plates  28  are made of austenitic (non-magnetic) stainless steel and each plate is 0.185 inch thick, which is suitable when the induction heater is operated at a frequency of 1 to 3 kHz and with smart susceptors having thicknesses of 0.125 inch. These lamination plates can be thicker than 0.185 inch for lower induction heater frequencies and would need to be thinner when using higher frequencies. These laminations can (and should) have a space  36  between them to allow the quenching fluid (gas or liquid) to have direct impingement against the surface of the heated susceptor  18 . This spacing is dictated by the thickness and strength of the smart susceptor surface shell and the consolidation pressures used. In addition, the susceptors do not require an electrical connection to one another. The metal plates  28  are interleaved with dielectric spacers  30  except near the susceptor and in places that are needed to allow the quenching medium to flow to the susceptor. The same considerations apply to the upper tooling die and the susceptor attached thereto. 
         [0035]    Preferably each induction coil  34  is fabricated from copper tubing which is lightly drawn. A lightly drawn condition of the tubing enables precision bending by numerically controlled bending machines. Numerically controlled bending of the tubes allows accurate placement of the tubing relative to the changing contours of the susceptors, thereby improving the degree to which the each susceptor is uniformly inductively coupled to the induction heater across the length and width of the susceptor. However, it should be understood that the compliant layer disclosed hereinafter can be employed also in cases wherein the susceptors are planar rather than concave/convex. Optionally the coils  34  also remove thermal energy by serving as a conduit for a coolant fluid, such as water. After being bent and installed, the coils include straight tubing sections connected by flexible tubing sections. The flexible tubing sections connect the straight tubing sections and also allow the dies to be separated. The accurate placement of the tubing of the induction coils  34  promotes uniformity in the amount of heat generated by the magnetic flux field and the amount of heat removed by flow of the coolant fluid. 
         [0036]    As disclosed in U.S. Pat. No. 6,528,771, the induction coils  34  can be connected to a temperature control system that includes a power supply, a controlling element, a sensor and a fluid coolant supply preferably containing water (not shown). The power supply supplies an alternating current to the induction coils  34  which causes the coils to generate the electromagnetic flux field. The fluid coolant supply supplies water to the induction coils  34  for circulation through the coils and the removal of thermal energy from the dies. The sensor is capable of measuring the power supplied by the power supply. Alternatively, or in addition to measuring the power supply, the sensor may include a voltmeter that can measure the voltage drop across the induction coils  34 . The controlling element receives the sensor output and uses the measurements in a feedback loop to adjust the power being supplied by the power supply. The controlling element can include hardware, software, firmware, or a combination thereof that is capable of using feedback to adjust the voltage output by the power supply. 
         [0037]    The system described with reference to  FIGS. 1-4  can be enhanced to facilitate the processing of composite preforms having uneven thickness and/or density across the extent of the preform by adding a compliant layer between the composite preform and one susceptor. For example, it is known to fabricate aircraft parts from a matte product that comprises recycled graphite fibers and virgin thermoplastic fibers.  FIG. 5  shows the placement of such a matte product  40  (in an unconsolidated state) between a pair of opposing susceptors  18  and  20  in their retracted positions. The matte product  40  can have a matted and tangled fiber architecture and undesirable unevenness and thickness/density variations. Furthermore, since the graphite fibers are tangled, they do not facilitate the flow of the thermoplastic material. As seen in  FIG. 6 , which shows the matte product  40  at the end of the consolidation process, these characteristics can lead to the formation of voids  42  and/or porosity in the final composite product due to the fact that thickness and density distribution were not uniform across the matte product. The formation of voids and/or porosity is further aided by the fact that the fibers are entangled and flow is limited and unable to “heal” porosity very effectively. 
         [0038]    In accordance with various embodiments, the consolidation apparatus further comprises a compliant layer that is situated between the composite preform and one of the matched molding tools for the purpose of providing a more even pressure over the entire area of the preform during the consolidation process. The compliant layer should have an offset tensile yield point (0.2% of the strain) in a range of 25-300 psi at the temperature of consolidation of the preform at strain rates of about 1% to 10% strain per minute. 
         [0039]    In accordance with one embodiment shown in  FIG. 7 , a sheet  44  of magnesium base alloy approximately 0.125 inch thick is used to act as the compliant layer or shim to compensate for uneven thickness or density over the area of a matte product  40  comprising recycled graphite fibers and virgin thermoplastic fibers. The matte product  40  and the magnesium alloy sheet  44  are shown sandwiched between a pair of susceptors  18  and  20  in their extended position, i.e., the molding apparatus is closed. A magnesium base alloy is selected which becomes very soft at temperatures useful for assisting the consolidation and molding of thermoplastic composites (i.e., 600-750° F.) and does not melt until above 1000° F. As the temperature and pressure increase inside the molding apparatus during consolidation of the resulting composite product, the magnesium alloy sheet  44  softens and forms into the areas of relatively lower pressure. The magnesium alloy sheet  44  can be reused due to the soft nature of the material. 
         [0040]    More specifically, a suitable magnesium alloy sheet material is Elektron AZ31B Sheet, which is commercially available from Magnesium Elektron UK in Manchester, England. AZ31B is a wrought magnesium base alloy, is non-magnetic, has high electrical and thermal conductivity, and has a melting range of 1050-1170° F. Superplastic forming of AZ31B can occur during the preform consolidation process. The chemical composition of AZ31B magnesium base alloy is: 2.5-3.5% aluminum; 0.7-1.3% zinc; 0.20-1.0% manganese; balance magnesium.  FIG. 8  is a graph showing the effect of temperature on the tensile properties of this particular magnesium base alloy. The 0.2% proof stress curve has been extrapolated to the right of the vertical axis labeled “ ELONGATION ” to show the anticipated effect of temperatures in excess of 300° C. on the 0.2% proof stress. 
         [0041]    Other magnesium base alloys can be used instead of AZ31B provided that the alloy has an offset tensile yield point (0.2% of the strain) in a range of 25-300 psi at the temperature of consolidation of the preform at strain rates of about 1% to 10% strain per minute. Alternatively, metal alloys having a base element different than magnesium, such as aluminum, can be used provided that they have the aforementioned tensile yield property. 
         [0042]    A system incorporating a compliant layer  44  the type described above is shown in  FIG. 9 . In this embodiment, the compliant layer  44  is attached to the upper tool die  10  by means of clamps, fasteners or other known means (not shown in  FIG. 9 ), with an upper susceptor  20  disposed between the compliant layer  44  and the upper tool die  10 . Alternatively, the compliant layer could be attached to the lower tool die  4  with the lower susceptor  18  disposed therebetween. During the consolidation process, the upper and lower tool dies are moved toward each other by hydraulic actuators  46 , which tool closing motion is indicated by arrows in  FIG. 9 . Electrical power is supplied to the induction coils (not shown) by a power supply  48  in the manner previously described. After consolidation and cooling, the hydraulic actuators  46  move the tool dies apart to allow removal of the consolidated product from the mold. The compliant layer can be reused. 
         [0043]    A compliant layer of the type described above also has application in the consolidation and forming/molding of composite preforms other than the matte product described herein. For example, the compliant layer can be used in the consolidation of composite preforms that comprise reinforcing fibers embedded in a matrix made of either thermoplastic or thermosetting plastic material. 
         [0044]    While the invention has been described with reference to various embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. For example, a compliant layer can be used with heated consolidation tools that lack susceptors. In cases wherein the tooling dies have heated matched surfaces, the compliant layer can be placed between one of those heated matched surfaces and the composite preform. In addition, many modifications may be made to adapt a particular situation to the teachings herein without departing from the essential scope thereof. Therefore it is intended that the claims not be limited to the particular embodiments disclosed. 
         [0045]    The method claims set forth hereinafter should not be construed to require that the steps recited therein be performed in alphabetical order or in the order in which they are recited, and should not be construed to exclude two or more steps being performed concurrently.