Patent Publication Number: US-2016229552-A1

Title: Intermetallic and composite metallic gap filler

Description:
FIELD 
     This invention relates to a gap filler positioned between structural components of a structure, and more particularly, the gap filler which provides electrical conductivity between the structural components of an aircraft. 
     BACKGROUND 
     Structures and particularly aircraft are designed to withstand lightning strikes and maintain their structural integrity. Traditional construction of aircraft, for example, included metallic structural elements being secured together with metallic fasteners. The fasteners were electrically grounded to the metallic structural elements with the metallic fasteners being in contact with the metallic structural elements. This arrangement provided electrical conductivity between the fastener and the structural element thereby not electrically isolating the fastener from the structural elements. Isolating the fastener would otherwise provide an undesired electrostatic force between the fastener and the structural element upon the occurrence of a lightning strike to the aircraft. 
     Aircraft are more recently being constructed of structural components made of a composite material. The composite material comprises a matrix material, often a resin, and of a fiber material such as carbon fiber. The resin is not generally not as electrically conductive in contrast to the fiber material. This composite material is often carbon fiber reinforced plastic (“CFRP”). The CFRP structural elements are secured together with fasteners, such as, metallic bolts. A bolt used to fasten a structural element constructed of CFRP may not necessarily be electrically grounded to the CFRP structural element. Rough surfaces of the bolt and rough surfaces of the structural element can create gaps between the surface of the metallic bolt and that of the electrically conductive fiber. This condition can lead to an electrostatic force build up between the fastener and the structural element constructed of CFRP. 
     Currently, in an attempt to prevent an electrostatic force build-up between the fastener and the structural element, the fasteners or bolts are coated with an electrically insulating sealant material before they are secured to the structural element constructed of CFRP material. Thus, at the time of a lightning strike to the aircraft, the insulating of the fastener from the structural element prevents an undesired electrostatic build-up between the fastener and the structural element. However, coating the fasteners with the sealant and then securing them to the structural element takes a great deal of care, time and expense compared to the traditional securement of metallic structural elements with metallic fasteners. The sealant must completely cover the surface of the fastener, otherwise, at the time of a lightning strike to the aircraft, current could travel from the fibers of the structural element to the fastener through an opening or breach of the sealant. Current that passes through such opening will charge the fastener and create an imbalance of the charge between the fastener and the fiber of the CFRP. This condition can create undesired electrostatic forces between the bolt and the fibers of the CFRP. Additional, care needs to be taken in securing the fastener such that sealant is not removed in the process. Furthermore, once the fastener is installed, careful inspection needs to be made that no opening through the sealant was created during the securement process of the fastener. Thus, careful steps must be taken in coating the fastener, securing the fastener and inspecting the fastener in the fastening process. These steps contribute to cost of the assembly the aircraft. 
     Other attempts to prevent undesired electrostatic force build-up between the CFRP material of the structural element and the metallic fastener or bolt have been attempted. These attempts were not feasible. For example, the use of a metallic solder could not be employed. Even though solder would provide electrical conductivity and structural integrity between the bolt and the CFRP material, the integrity of the CFRP material is structurally compromised at a temperature of over two hundred and fifty four degrees Fahrenheit (254° F.), which is below the melting temperature of many solders. Alternatively, a conductive adhesive could be employed between the bolt and the CFRP material. However, the adhesive has a more resistive conductivity than a metallic bond. This resistivity could result in a charge build up between the two surfaces of the bolt and the CFRP material and result in an undesired electrostatic discharge. 
     A soft metallic insert could also be employed and positioned between the two surfaces of the bolt and the CFRP material. However, this may improve electrical conductivity between the two surfaces but creates a structural weak point between the two surfaces and is limited in its ability to conform to the two surfaces. Alternatively, welding the bolt to the CFRP structural component is not feasible, where one surface, that of the CFRP structural component, is non-metallic. 
     There is a need for an inexpensive and reliable gap filler that will conform to the rough surfaces of the fastener or bolt and the structural element constructed of CFRP. The gap filler will need to provide electrical conductivity between the fastener or bolt and fibers of the CFRP material to ground the fastener to the structural element constructed of CFRP material. 
     SUMMARY 
     An example of a structural assembly of an aircraft includes a structural element constructed of a composite material which includes a matrix material and a plurality of fibers positioned to extend through the matrix material in which at least a portion of the plurality of fibers are accessible from a surface of the structural element. A fastener secures the structural element to a structural component. A metal structure comprising gallium is positioned in contact with a surface of the fastener. The metal structure extends from the surface of the fastener and contacts at least a portion of the at least a portion of the plurality of fibers. 
     An example of a method for assembling a structure which includes the step of providing a structural element constructed of a composite material which includes a matrix material and plurality of fibers positioned to extend through the matrix material wherein at least a portion of the plurality of fibers are accessible from a surface of the structural element. Another step includes mixing a liquid metal alloy comprising gallium with at least one of a solid metal or solid metal alloy forming a slurry. The method further includes the step of applying the slurry onto a surface of a fastener and onto at least a portion of the at least of the portion of the plurality of fibers. Another step of the method includes securing the structural element with the fastener to a structural component wherein the slurry is positioned in contact with the surface of the fastener and interconnects the surface of the fastener with the at least a portion of the at least a portion of the plurality of fibers. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF SUMMARY OF THE DRAWINGS 
         FIG. 1  is a side elevation view of an aircraft; 
         FIG. 2  is an exemplary schematic fragmentary view of a selected portion of the aircraft in  FIG. 1 , wherein a cross section is shown of a fastener connecting structural elements constructed of a composite material to a structural component with a metallic gap filler positioned in contact with and interconnecting a surface of the fastener with fibers of the composite material of the structural elements; 
         FIG. 3  is Table 1 showing sample compositions of gap fillers comprising gallium based alloys of liquid metal combined with a pure solid metal or with a solid metal alloy; 
         FIG. 4  is a graph showing free corrosion rates for selective samples of metallic gap fillers from Table 1 of  FIG. 3 ; 
         FIG. 5  is a graph showing galvanic corrosion rates for selective samples of metallic gap fillers from Table 1 of  FIG. 3 ; and 
         FIG. 6  is a Table 2 showing free and galvanic corrosion rates, along with relative galvanic to free corrosion rates for selective samples of metallic gap fillers from Table 1 along with copper foil and aluminum 6061 alloy. 
     
    
    
     While various embodiments have been described above, this disclosure is not intended to be limited thereto. Variations can be made to the disclosed embodiments that are still within the scope of the appended claims. 
     DETAILED DESCRIPTION 
     As earlier discussed, it is important to assemble structures that resist damage when struck by lightning. An example of such assembled structures include aircraft. It is desired that electrostatic force build up is not created between metal fasteners and structural elements of the assembled aircraft at the time of a lightning strike. With aircraft being now selectively assembled with composite materials the grounding of a metal fastener with conductive fibers of the composite material of a structural element is needed. 
     Referring to  FIG. 1 , an example of an assembled structure, aircraft  10  is shown. Aircraft  10  includes various sections to its assembly. These sections, in this example, include a fuselage  12  and wings  14  extending from opposing sides of fuselage  12 . Fuselage  12  also includes a nose section  16  and an opposing tail section  18 . Each of these sections of aircraft  10  can be selectively constructed with structural elements constructed of composite materials. 
     In referring to  FIG. 2 , an exemplary securement of structural element  20  is shown for an assembly of a selective section of aircraft  10 . Structural element  20  is constructed of a composite material, which includes a matrix material  22  and a plurality of fibers  24  positioned to extend through the matrix material  22 . 
     In this example shown in  FIG. 2 , structural element  20  overlies another structural element  26 , which is also constructed of the composite material including a matrix material  28  and a plurality of fibers  30  which are also positioned to extend through matrix material  28 . Both structural element  20  and other structural element  26  are secured, to structural component  32 . Structural component  32  is another structural item of an assembly of a section of aircraft  10 . Structural component  32  can be constructed of a composite material, metal or the like. In this example, structural component  32  is a portion of a metallic frame positioned within a select section of aircraft  10 . 
     Plurality of fibers  24  and  30  in this example are constructed of electrically conductive material such as carbon. Matrix material  22  and  28  in this example is constructed of one of a thermoplastic resin such as polypropylene, polyethylene and nylon or thermosetting resin such as an epoxy. 
     Structural element  20  and another structural element  26  are secured to structural component  32  with fastener  34 . Fastener  34  is a securement item, which can secure two or more items together, such as a bolt, screw, pin or the like. In this example, fastener  34  is a bolt, which includes head  36 , threaded shaft  38  (threads not shown) and threaded nut  40  (threads not shown). Fastener or bolt  34  is constructed of metal, such as carbon steel, titanium alloy or the like. 
     In this example, through-hole  42  is positioned through structural component  32 . Through-hole, in this example, was pre-formed in structural component  32 . Through-hole  42  may also be positioned through structural component  32  by drilling through structural component  32 . Through-hole  44  and recess section  54  of structural element  20  and through-hole  46  of other structural element  26  have been formed with drilling through structural element  20  and other structural element  26 , in this example. In drilling through composite material of structural element  20  and another structural element  26 , rough surface  48  of structural element  20  and rough surface  50  of other structural element  26  are formed. Additionally, rough surface  52  is formed in recess section  54  of structural element  20 . 
     Plurality of fibers  24 , which extend through matrix material  22  of structural element  20  are schematically represented, in  FIG. 2 . In this example, plurality of fibers  24  extend through matrix material  22  and are positioned throughout structural element  20 , extending in the length direction, width direction and layered in the thickness direction of structural element  20 . Plurality of fibers  24  in  FIG. 2  are schematically shown extending in a length direction and layered in the thickness direction of structural element  20  without showing plurality of fibers  24  extending in a width direction within matrix  22 . 
     With the formation of a discrete through-hole  44  and recess section  54  through structural element  20 , through-hole  44  and recess section  54  engages at least a portion  25  of the total of the plurality of fibers  24 , which are positioned throughout structural element  20 . In  FIG. 2  the at least a portion  25  of the plurality of fibers  24  of structural element  20  are schematically shown positioned at and along rough surface  48  of through-hole  44  and at and along rough surface  52  of recess section  54 . Portion  25  of plurality of fibers  24 , are also positioned about through-hole  44  and recess section  54 , but are not shown. The at least a portion  25  of plurality of fibers  24 , in  FIG. 2 , are shown schematically extending to rough surface  48  at through-hole  44  and to rough surface  52  at recess section  54 . However, with through-hole  44  and recess section  54  formed, in this example, by drilling through structural element  20 , ends of the at least a portion  25  of the plurality of fibers  24  positioned at through-hole  44  and recess section  54  may extend from, be flush with or be recessed from rough surface  48  and rough surface  52 . Regardless of the position of an end of a fiber of the at least a portion  25  of plurality of fibers  24  relative to rough surfaces  48  and  52 , the ends will be accessible from rough surface  48  in through-hole  44  and from rough surface  52  of recess section  54 . 
     Other structural element  26  with plurality of fibers  30  are similarly configured and schematically presented as described above for structural element  20  with plurality of fibers  24 . Plurality of fibers  30  are schematically shown to extend through matrix material  28  and are positioned throughout another structural element  26 , extending in the length direction, width direction and layered in the thickness direction of another structural element  26  as the plurality of fibers  24  were configured within structural element  20  and schematically shown. Plurality of fibers  30 , in this example, are shown extending in a length direction and layered in the thickness direction of another structural element  26  without showing plurality of fibers  24  extending in a width direction within matrix  28 . 
     With the formation of a discrete through-hole  46  through structural element  26 , through-hole  46  engages at least a portion  47  of the total of the plurality of fibers  30 , which are positioned throughout another structural element  26 . The at least a portion  47  of the plurality of fibers  30  of structural element  26  are schematically shown positioned at and along rough surface  50  of through-hole  46 . The at least a portion  47  of plurality of fibers  30 , are also positioned about through-hole  46  but are not shown. The at least a portion  47  of plurality of fibers  30  are shown schematically extending to rough surface  50  at through-hole  46 . However, with through-hole  46  formed, in this example, by drilling through structural element  26 , ends of the at least a portion  47  of the plurality of fibers  30  positioned at through-hole  46  may extend from, be flush with or be recessed from rough surface  50 . Regardless of the position of an end of a fiber of the at least a portion  47  of plurality of fibers  30  relative to rough surface  50 , the ends will be accessible from rough surface  50  in through-hole  46 . 
     In forming recessed portion  54  and through-hole  44  of structural element  20  and forming through-hole  46  of other structural element  26 , these openings are dimensioned to be slightly larger than fastener  34  to enable fastener  34  to extend through structural element  20  and another structural element  26 . For the same reason, through-hole  42  of structural component  32  is similarly slightly larger than the dimension of fastener  34 . A gap  55  is formed between head  36  and threaded shaft  38 , on the one hand, and rough surfaces  48 ,  50  and  52  of structural element  20 , recess portion  54  of structural element  20  and another structural element  26 , on the other hand. Gap  55  is also formed between structural component  32  and fastener or bolt  34 , as seen in  FIG. 2 . 
     A metal structure  56  is positioned within and conforms to the shape of gap  55 . Metal is a solid material that is typically malleable, fusible, and ductile, with good electrical and thermal conductivity. Metal structure  56  is in contact with surface  57  of fastener  34 , which includes surface  58  of head  36  and surface  60  of threaded shaft  38 . Metal structure  56  extends across gap  55  from surfaces  58  and  60  and contacts at least a portion of the at least a portion  25  of plurality of fibers  24  associated with structural element  20  and recess section  54  of structural element  20 . This configuration of metal structure  56  establishes an electrical connection with the fibers and fastener  34 , thereby grounding fastener  34 . Similarly, metal structure  56  contacts surface  57  of fastener  34 , which includes surface  60  and extends across gap  55  and contacts at least a portion of the at least a portion  47  of plurality of fibers  30  associated with another structural element  26 . Likewise, this configuration of metal structure  56  establishes an electrical connection with the fibers and fastener  34 , thereby grounding fastener  34 . Also, in this example, metal structure  56  contacts surface  57  of fastener  34 , which includes surface  60  and extends across gap  55  and contacts surface  62  of structural component  32  at through-hole  42 . This configuration of metal structure  56  also establishes and electrical connection between fastener  34  and structural component  32 . 
     Metal structure  56 , as described above, extends from the above-described surfaces of fastener  34  across gap  55  to the above described at least a portion of the at least a portion  25  and  47  of plurality of fibers  24  and  30 , respectively, and to structural component  32 . Metal structure  56  extends in a range between twenty-five micrometers (25 μm) and one millimeter (1 mm) in accomplishing these electrical connections. In extending across gap  55 , metal structure  56  comes into contact and conforms to rough surfaces  48 ,  50  and  52 . As will be described below, metal structure  56  is applied in a slurry or paste-like consistency to the above described surfaces of fastener  34  and to rough surfaces  48 ,  50  and  52  and surface  62 . In application of metal structure to rough surfaces  48 ,  50  and  52  at least a portion of the at least a portion  25  and  47  of plurality of fibers  24  and  30 , respectively, come into contact with metal structure  56 . In securing structural element  20  and other structural element  26  to structural component  32  with fastener  34  the paste-like consistency of metal structure  56  is positioned within and occupies gap  55  and provides a continuous electrical connection between fastener  34  and the above described fibers of the composite material. Metal structure also provides an electrical connection between fastener  34  and structural component  32 . 
     A method for assembling a structure, such as in this example, aircraft  10 , includes providing structural element  20 . In this example, other structural element  26  is additionally provided. Structural element  20  as is other structural element  26  are constructed of a composite material which includes, as described earlier, a matrix material  22  for structural element  20  and matrix material  28  for other structural element  26 . A plurality of fibers  24  extend through matrix material  22  of structural element  20  and plurality of fibers  30  extend through matrix material  28 . At least a portion  25  of the plurality of fibers  24 , as described above, are accessible from rough surfaces  48  and  52  associated with structural element  20  and at least a portion  47  of plurality of fibers  30 , as described above, are accessible from rough surface  50  associated with structural element  26 . With structural element  20 , other structural element  26 , structural component  32  and fastener  34  available to assemble, mixing a liquid metal alloy containing gallium with at least one of a solid metal and solid metal alloy can be conducted. In this example, a Wig-L-Bug dental amalgamator is employed. This mixture forms a slurry. 
     The slurry or paste-like mixture is applied to surface  57 , including surfaces  58  and  60  of fastener  34  and onto at least a portion of the at least a portion  25  and  47  of the plurality of fibers  24  and  30 , respectively, by applying the paste-like material to rough surfaces  48 ,  50  and  52 . The paste-like material is also applied to surface  62  of structural component  32 . With the paste-like material of metal structure  56  properly applied, securing structural element  20 , along with in this embodiment, other structural element  26 , to structural component  32  with fastener  34  can proceed, as described above. With securing fastener or bolt  34 , gap  55  becomes occupied and filled with the paste-like consistency of metal structure  56  and conforms to the shape of gap  55 . As will be described below, the paste-like material over a period of time sets and solidifies. Metal structure  56  provides the needed electrical connection between fastener  34  and the fibers and the structural component  32  and grounds fastener  34 . 
     Metal structure  56 , which contains a gallium alloy, is initially in a liquid state at approximately room temperature and is then mixed with a metal powder or film to form a slurry, wherein a peritectic bond formed in which gallium within the liquid metal alloy diffuses into a solid metal such as a pure metal or metal alloy. The initial mixture forms a slurry or paste-like mixture that transforms into an alloy with a higher melting temperature. Over time, the slurry or paste-like consistency cures into a solid. 
     Referring to  FIG. 3 , Table 1 is shown which includes samples of compositions for metal structure  56 . As can be seen in Table 1, gallium alloys are formed with combining gallium with one or both of tin and indium. These metal alloys are initially in a liquid state at about room temperature below thirty degrees Centigrade (30° C.). The liquid metal alloy containing gallium is then mixed with a solid metal or solid metal alloy. The solid metal or solid metal alloy to be mixed with the liquid metal alloy is either in a powder or film state. The dimension of the particle size or the film thicknesses are between fifty nanometers (50 nm) and one hundred micro meters (100 μm). As can be seen in Table 1 of  FIG. 3 , the solid metal mixed with the gallium alloy are either pure nickel, pure copper or pure silver. A solid metal alloy, of bronze, can selectively be used to mix with the liquid gallium metal alloy. Table 1 shows the material elemental weight ratio of the chemical components to be mixed to create each sample of metal structure  56 . 
     With the mixing of the gallium alloy with the solid metal or solid metal alloy, the resulting slurry or paste-like consistency provides the user the ability to properly apply the material to conform to the roughened surfaces of the fastener and the composite material. Once securement is completed using fastener  34 , the material or metal structure  56  cures into a solid state. The assembled structure, or in this example, aircraft  10 , can then be used for flight. 
     An additional mechanical reinforcing phase can be added to the slurry of the mixture of the liquid gallium metal alloy with a solid metal or solid metal alloy. This mechanical phase will provide enhanced shear resistance to the cured solidified alloy. This mechanical phase material can selectively include one of a pure cobalt, pure tungsten, pure molybdenum or pure titanium or of an alloy of titanium, such as AMS 4911, or stainless steel, such as  302  or  316 . 
     Referring now to  FIG. 4 , the graph shows free corrosion test results of five select samples of metal structure  56  which appear in Table 1 of  FIG. 3 . Corrosion resistance is important for metal structure  56 , which will perform in the outside environment exposed to varying environmental conditions. The testing criteria assessment uses a commercial 3-electrode corrosion test cell. In addition to the tested samples, which comprised the working electrode, the cell contained a platinum mesh counter electrode and a silver wire reference electrode. The electrolyte (the corrosive environment) was three per cent by weight (3.0 wt %) Sodium Chloride (NaCl) solution exposed to laboratory air, i.e. containing dissolved oxygen. Using the fixturing in the cell, a surface area of one square centimeter (1 cm×1 cm) of each sample was exposed to the electrolyte. Standard linear polarization measurements were performed around the (open circuit) corrosion potential using −10 mV to +10 mV potential sweeps. The data was fit to straight lines to obtain the polarization resistance (R p , Ohms). Relative corrosion rates were expressed using the inverse of the polarization resistance. As can be seen in the graph, the testing in at least one instance extended to eighty hours. 
     Free corrosion rates are given by the inverse of the polarization resistance are shown in this graph in  FIG. 4 . The lower the inverse polarization resistance values on this graph indicates a greater resistance to corrosion. All of the copper containing metal alloys had equivalent corrosion rates regardless of the composition of the metal when the same liquid metal alloy (Ga/In or Ga/In/Sn) was used. The gallium liquid metal alloy had some effect. The gallium/tin liquid metal alloy was more corrosion resistant than gallium/indium liquid metals. The difference is attributable to tin being more oxidatively stable than indium. As can be noted, the silver containing solidified metals were more stable than any indium containing material. 
     Referring to  FIG. 5 , this graph shows galvanic corrosion testing results for three selected samples of metal structure  56  from Table 1 of  FIG. 3 . Galvanic testing is conducted on samples of metal structure  56  since metal structure  56  is in constant electrical contact with conductive carbon fiber of the composite material. 
     The testing criteria includes placing each sample in contact with carbon fiber reinforced plastic (“CFRP”) and using linear polarization measurements performed on combined test sample/CFRP electrodes. This approach treats the galvanic couple as a single electrode and enables a relative comparison of the free corrosion and galvanic corrosion rates. The CFRP was prepared by cutting a one square centimeter piece (1 cm 2 ) from a CFRP panel. One face was ground to expose the carbon fiber. The edges and back were sealed with 5 minute epoxy cured overnight at fifty degrees Centigrade (50° C.). The entire CRFP piece was immersed in the electrolyte adjacent to the test sample. Electrical connection between the test sample and the CFRP was made using a jumper from the test sample to an epoxy sealed threaded rod connected to the CFRP. The area for the test samples was 0.785 square centimeters (0.785 cm 2 ) and the area of the CFRP was two centimeters by two centimeters (2 cm×2 cm) which equals four square centimeters (4 cm 2 ). Thus, the area ratio of CFRP/metal alloy was 5:1. 
     In referring to the graph in  FIG. 5 , the initial eighteen to twenty two hours portrayed on the graph are free corrosion test results for these three samples. The above galvanic test was then initiated, which indicated a steep rise in the corrosion rates for each sample in this graph. As time progressed, the corrosion rate of each of the three samples substantially leveled and reached a steady state. Again, the higher position on this graph indicates a greater corrosion rate. As can be seen, the gallium/tin with copper alloy and the gallium/indium with copper alloy have similar corrosion rates. The gallium/indium and silver alloy had the lowest corrosion rate. 
     In referring to  FIG. 6 , Table 2 of corrosion rates of five samples are provided. The first three samples are sample compositions from Table 1 of  FIG. 3  and are found in the corrosion tests in the graphs set forth in  FIGS. 4 and 5 . The next sample in Table 2 is copper foil and the final sample is aluminum alloy 6061. This table shows in the first column the free corrosion rate. In the second column is shown the galvanic corrosion rate. In the third column, is shown a ratio of the galvanic corrosion rate in ratio to free corrosion rate. The gallium liquid alloy mixed with copper solid metal samples have slightly greater galvanic corrosion rates than copper foil and gallium liquid alloy mixed with silver solid metal, but significantly less corrosion rate than aluminum alloy 6061. 
     While various embodiments have been described above, this disclosure is not intended to be limited thereto. Variations can be made to the disclosed embodiments that are still within the scope of the appended claims.