Abstract:
A method of fabricating a metal/composite hybrid laminate is provided. One or more layered arrangements are stacked on a solid base to form a layered structure. Each layered arrangement is defined by a fibrous material and a perforated metal sheet. A resin in its liquid state is introduced along a portion of the layered structure while a differential pressure is applied across the laminate structure until the resin permeates the fibrous material of each layered arrangement and fills perforations in each perforated metal sheet. The resin is cured thereby yielding a metal/composite hybrid laminate.

Description:
ORIGIN OF THE INVENTION 
     This invention was made by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods of metal/composite laminate fabrication. More specifically, the invention is a fabrication method involving resin infusion of a layered metal/composite hybrid and the resulting metal/composite hybrid laminate. 
     2. Description of the Related Art 
     Metal/composite hybrid laminates provide a combination of structural and functional properties for a variety of applications to include aerospace structures. When comparing a structure made from a metal/composite hybrid laminate with one made from just the parent metal, the hybrid laminate-based structure is lighter in weight, has improved load bearing ability, is stiffer, and has improved fatigue properties. When comparing the hybrid laminate-based structure with one made from just the parent composite, the hybrid laminate-based structure has improved impact resistance, damage tolerance, and permeation resistance. 
     Currently, metal/composite hybrid laminates are prepared by compressing (e.g., using a press, autoclave, etc.) layers of metal sheets interleaved with layers of fibrous sheets previously impregnated with a resin. The fibrous sheets can be comprised of unidirectionally-arranged fibers or a mesh of woven fibers. The layered structure is typically placed in a mold prior to compression processing thereof. However, both autoclave and press molding techniques require complex tooling and are limited in size/shape owing to the size limitations of autoclave or press molding equipment. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method of fabricating a metal/composite hybrid laminate. 
     Another object of the present invention is to provide a method capable of being used to make relatively large, shaped metal/composite hybrid laminate-based structures. 
     Still another object of the present invention is to provide a metal/composite hybrid laminate. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, a method of fabricating a metal/composite hybrid laminate is provided. At least one layered arrangement is stacked on a solid base to form a layered structure. Each layered arrangement is defined by a fibrous material and a perforated metal sheet with the layered arrangement&#39;s fibrous material being closer to the solid base than the layered arrangement&#39;s perforated metal sheet. A resin in its liquid state is introduced along a portion of the layered structure. A differential pressure is induced across the laminate structure until the resin permeates the fibrous material of each layered arrangement and fills perforations in each perforated metal sheet. The resin is cured thereby yielding a metal/composite hybrid laminate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a vacuum assisted resin transfer molding set-up for fabricating a metal/composite hybrid laminate in accordance with the present invention; 
         FIG. 2  is a cross-sectional view taken along line  2 - 2  in  FIG. 1  during operation of the vacuum assisted resin transfer molding set-up; 
         FIG. 3  is a planar view of one of the perforated metal sheets in the hybrid laminate; 
         FIG. 4  is a cross-sectional view of an embodiment of a metal/composite hybrid laminate fabricated in accordance with the present invention; 
         FIG. 5  is a cross-sectional view of another embodiment of a metal/composite hybrid laminate fabricated in accordance with the present invention; and 
         FIG. 6  is a side view of a shaped support used to fabricate a shaped metal/composite hybrid laminate in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, simultaneous reference will initially be made to  FIGS. 1 and 2 . A vacuum assisted resin transfer molding set-up (referenced generally by numeral  100 ) is illustrated with a preform  10  of a metal/composite hybrid laminate that is to be fabricated in accordance with the present invention. Pursuant to the ensuing description, one of ordinary skill in the art will readily recognize that set-up  100  is simply one embodiment of an equipment arrangement that can be used to fabricate the present invention&#39;s metal/composite hybrid laminate. Accordingly, it is to be understood that the fabrication method and resulting metal/composite hybrid are not limited by the particular configuration of the processing equipment. 
     Preform  10  is a multi-layer structure that includes a base layer  12  and at least one layered arrangement  14  (e.g., two are illustrated in  FIG. 2 ) of a fibrous material  14 A and a perforated metal sheet  14 B. Base layer  12  is a solid material that is typically a solid sheet or foil or metal. For each layered arrangement  14 , fibrous material  14 A is closer to base layer  12  than its corresponding perforated metal sheet  14 B. Each layer of fibrous material  14 A is an arrangement of fibers that, through processing in accordance with the present invention, will become the composite portion of the metal/composite hybrid laminate. In general, fibrous material  14 A is any porous fibrous arrangement to include unidirectionally-arranged fibers or an open woven mesh that is permeable with respect to a liquid resin as will be explained further below. Suitable choices for fibrous material  14 A include, but are not limited to, unidirectionally extending glass fibers, graphite fibers, KEVLAR® fibers, SPECTRA® fibers, M5® fibers, ZYLON® fibers, or other suitable fibers, or open mesh fabrics made from such fibers. Base layer  12  and each perforated metal sheet  14 B is any suitable metal (e.g., steel, aluminum, titanium, etc.) in sheet or foil form that will become the metal portion of the metal/composite hybrid laminate. The perforated metal sheets could also be surface treated to alter or tailor the adhesion between layers depending on the required level of adhesion required for the particular application. 
     Referring additionally to  FIG. 3 , each perforated metal sheet  14 B has an arrangement of holes  16  formed therethrough that will become pathways for transverse-plane resin transfer during processing and that will provide an improved means of bonding in the ultimate metal/composite hybrid laminate. The particular size, shape, and arrangement of holes  16  will be governed by the desired processing and ultimate application of the metal/composite hybrid laminate and are, therefore, not limitations of the present invention. In general, the size of holes  16  must be large enough to permit resin transfer therethrough yet small enough so as not to negatively impact the structural integrity of the ultimate metal/composite hybrid laminate. The shape of holes  16  can be circular, elliptical, square, rectangular, slotted, etc., without departing from the scope of the present invention. Similarly, the arrangement of holes  16  can be varied without departing from the scope of the present invention. Further, the arrangement of holes  16  can be the same between layered arrangements  14  (in which case holes  16  will be aligned throughout preform  10  and the ultimate metal/composite hybrid laminate), or the arrangement of holes  16  can be different between layered arrangements  14  (in which case holes  16  will be misaligned throughout preform  10  as illustrated in  FIG. 2  and the ultimate metal/composite hybrid laminate). 
     Set-up  100  includes the following:
         a tool or support  102  that may be coated or covered with a non-stick material (not shown) on which preform  10  rests,   a resin reservoir  104  containing suitable resin (e.g., epoxy, cyanate ester, bismaleimide, polyimide, etc.),   a resin distribution arrangement coupled to resin reservoir  104  that includes a delivery conduit  106  and a planar and porous resin distribution media  108  that is positioned over preform  10  and that receives resin via conduit  106  and readily facilitates resin distribution over the area of media  108  (although not shown to preserve clarity in the illustration, a release material is typically placed between preform  10  and distribution media  108  to facilitate removal of media  108  after cure as would be well known in the art), and   a vacuum application arrangement that includes a vacuum  110  and a vacuum bag  112  sealed to tool  102  over the top of preform  10  and distribution media  108 .       

     In operation, preform  10  is positioned on tool  102  with distribution media  108  being arranged over the top of preform  10 , i.e., the top or exposed one of perforated metal sheets  14 B. Suitable choices for the distribution media  108  include, but are not limited to, PLASTINET® bi-planar nylon-6 mesh available from Applied Extrusion Technology for low temperature infusions and metal, such as aluminum, screen for high temperature applications. Vacuum bag  112  is sealed in place about the periphery thereof and vacuum  110  is turned on. As a result, resin (represented by flow arrows  114 ) is drawn from reservoir  104  to one end of distribution media  108  and then across to the other end of distribution media  108 . Other means of positively providing or introducing resin  114  to distribution media  108  could be used without departing form the scope of the present invention. The vacuum force generated by vacuum  110  is applied to the lower portion of preform  10  near base layer  12 . In this way, the vacuum force is drawn transversely through preform  10  via fibrous material  14 A and holes  16  in perforated metal sheets  14 B. As a result, resin  114  flows transversely through preform  10  via fibrous material  14 A and holes  16 . The vacuum force is applied until each fibrous material  14 A is permeated with resin  114  and holes  16  are filled with resin  114 . 
     The resulting preform  10  with resin  114  impregnated therein is cured in accordance with the curing specifications of the particular resin and then removed from set-up  100 . For example, curing typically takes place on tool  102 , although sometimes a free-standing post cure is performed after an initial cure depending on the particular resin system. The resulting metal/composite hybrid laminate  20  is illustrated in  FIG. 4  with cured resin  114  (i.e., represented by the “stippling” marks) filling holes  16  and permeating fibrous material  14 A. Processing in accordance with the present invention provides that cured resin  114  is contiguous throughout laminate  20  thereby improving the intra-adhesion properties of the laminate. As shown in  FIG. 4 , holes  16  can be misaligned throughout laminate  20 . However, the present invention can also be used to fabricate a metal/composite hybrid laminate  30  ( FIG. 5 ) where holes  16  are aligned with one another throughout the laminate. Furthermore, laminate  30  also illustrates that resin  114  can have reinforcing inclusions  116  mixed therein. Inclusions  116  are any conventional reinforcing material (e.g., chopped fibers, carbon nanotubes, etc.) that are small enough to be mixed in (liquid) resin  114  and pass through holes  16  and fibrous material  14 A during the resin infusion processing portion of the present invention. 
     For ease of illustration and description, tool  102  was illustrated as a flat support. However, the present invention is not so limited as the tool or support can be shaped as illustrated in  FIG. 6  where tool  202  defines a shape to which preform  10  conforms when placed therein/thereon. Processing in this configuration is the same as previously described. 
     By way of example, a flat hybrid laminate was fabricated using three layers of stainless steel foil with two layers of 5-harness satin biaxial woven fabric composed of HEXCEL® 6k IM7 carbon fiber tows sandwiched between each stainless steel foil. The stainless steel foils were 5 inches×5 inches and 0.003 inches thick. Each graphite fabric layer was 6 inches×6 inches and 0.0134 inches thick prior to infusion. Flow pathways were introduced by machining with a number 80 wire drill bit to an approximate diameter of 0.0134 inches in a staggered pattern approximately one inch apart. The hybrid laminate was subjected to non-destructive testing including thermography and x-ray analysis. The test results indicated a high quality laminate having very low void content. 
     By way of further example, a curved hybrid laminate, with a radius of curvature ranging from 6 inches on one side to 5.5 inches on the other side, was fabricated. The laminate was 8 inches wide and 10 inches long on the smaller radius side and 12 inches long on the larger radius side. The same metal foil as the earlier example and two stacks of multi-axial warp knit (MAWK) carbon fabric were utilized. The same flow pathway size and pattern as the earlier example was utilized and the compacted thickness of each stack of MAWK fabric was 0.055 inches. Non-destructive testing including thermography and x-ray analysis indicated a quality hybrid laminate having very low void content. 
     The advantages of the present invention are numerous. The processing method provides for the fabrication of a variety of size/shape metal/composite hybrid laminate structures without the drawbacks associated with conventional autoclave or press molding techniques. The resulting metal/composite hybrid laminate has improved adhesion between the constituent layers thereof and is mechanically improved as the cured resin simultaneously bonds to the metal surfaces and holds the assembly together via its contiguous presence in the metal sheets&#39; perforations. The contiguous presence of the resin in the metal sheet&#39;s perforations provides a through-the-thickness reinforcement that can improve impact resistance and damage tolerance. As mentioned above, alternative embodiments could be configured to provide a reduced adhesion strength at the surface between layers. Such reduced adhesion could be utilized as a means of energy absorption by delamination of the layers while maintaining structural integrity with the through-the-thickness, inclusion-reinforced, resin-filled perforations. 
     Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.