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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.
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.
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.
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.
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.
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.
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.
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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.
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
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.
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.
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.
Other aspects of the invention are disclosed and claimed below.
BRIEF DESCRIPTION OF THE DRAWINGS
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Various embodiments will be hereinafter described with reference to drawings for the purpose of illustrating the foregoing and other aspects of the invention.
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.
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.
FIG. 3 is a diagram showing an end view of a portion of a lower tooling die in accordance with one embodiment.
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.
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.
FIG. 6 is a diagram showing a consolidated matte product between a pair of susceptors in their extended positions.
FIG. 7 is a diagram showing a compliant layer and a consolidated matte product between a pair of susceptors in their extended positions.
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.
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.
Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals.
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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.
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.
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.
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.