Abstract:
A permanent faster is disclosed that utilizes an amorphous metal alloy for a retaining collar that is thermoplastically formed on install. The fastener includes a headed pin with locking grooves disposed thereon and a collar having a cylindrical inner wall. Head geometry may be any suitable shape; countersunk, counter bore, flat, etc., and may also be non-rotationally symmetric. The pin is disposed into aligned holes through the work pieces to be secured, and the collar is disposed about the pin over the locking grooves. The collar is heated into a thermoplastic region, something only allowed because of the amorphous metal material properties. The cylindrical wall of the collar is then radically compressed into the locking grooves to affix the collar on the pin. The amorphous metal properties allow use of a smaller fastener, and the thermoplastic region is reached at a temperature which does not damage composite work pieces.

Description:
BACKGROUND 
     Field 
     This invention relates generally to a deformable permanent fastener and, more particularly, to a permanent fastener for joining two work pieces, where the fastener includes a headed pin passing through holes in the work pieces with locking grooves on a shank portion, and a retaining collar composed of an amorphous metal alloy, where the collar is heated into a thermoplastic region and radially compressed such that the collar deforms to engage with the locking grooves on the shank portion of the pin. 
     Discussion 
     Various industries, including aviation, general construction, electronics, and general manufacturing, use fasteners for a number of different purposes. For example, in the aviation industry, aircraft structures are often comprised of two or more panels—which may be made of the same material or different materials, and which must be permanently fastened together. 
     Many different types of fasteners for joining two work pieces have been developed over the years—including everything from old-fashioned pounded rivets, to fasteners made of modern materials. Traditional one-piece or two-piece fasteners requiring plastic deformation upon installation have been used in various applications. However, in the aircraft industry, these fasteners require multiple production steps to ensure proper grain boundary control in the metal, and multiple installation and post-processing steps such as machining and inspection. These procedural steps add time and cost to the usage of plastically deformed metal fasteners. In addition, these fasteners use materials with grain boundaries which are inherently susceptible to onset and propagation of corrosion, stress concentrations and fatigue. 
     Threaded fasteners do not involve plastic deformation upon installation, but typically employ materials with the same limitations and susceptibilities as described above for plastically deformed metal fasteners. In addition, threaded fasteners are prone to inconsistent clamp-up force, and are also vulnerable to creep or loosening due to vibrations and shock experienced during service life. 
     In a highly fatigue-sensitive environment such as an aircraft structure, a fastener is needed which offers reduced installation time and fewer installation steps than traditional plastically-formed fasteners, better fastener material properties, more fastener geometric options, improved fastener gripping strength, more consistent clamping force, and resistance to vibration and shock. These features can be achieved with the amorphous metal fastener designs disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of an amorphous metal permanent fastener including a pin and a retaining collar, according to one embodiment of the invention; 
         FIG. 2  is an illustration of two work pieces which have been brought together in preparation for installation of the fastener of  FIG. 1 ; 
         FIG. 3  is an illustration of the fastener of  FIG. 1  in a first installation step, where the pin is inserted through the work pieces; 
         FIG. 4  is an illustration of the fastener of  FIG. 1  in a second installation step, where the retaining collar—which has been heated into a thermoplastic material region—is positioned over a shank of the pin; 
         FIG. 5  is an illustration of the fastener of  FIG. 1  in a third installation step, where the retaining collar has been thermoplastically formed via compression onto the shank of the pin; 
         FIG. 6  is an illustration of the fastener of  FIG. 1  in a final installation step, where the collar has cooled and a pulling tip has been removed from the pin; 
         FIG. 7  is an illustration of a second embodiment of an amorphous metal permanent fastener, where the shank of the pin has a knurled shape; 
         FIG. 8  is an illustration of a third embodiment of an amorphous metal permanent fastener, where the shank of the pin has a fluted shape; 
         FIG. 9  is an illustration of the amorphous metal permanent fastener of  FIG. 7 , after installation, where the retaining collar has been thermoplastically formed onto the shank of the pin and into a countersink cone on the lower work piece; and 
         FIG. 10  is a flowchart diagram of a method for joining work pieces using an amorphous metal fastener. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to an amorphous metal permanent fastener utilizing a thermoplastically swaged retainer is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, some embodiments discussed below are described in the context of joining panels for assembly of an aircraft. However, the disclosed fastener is also suitable for use in any other application where work pieces need to be permanently joined—such as automotive, military vehicles, machinery, building construction, etc. 
     Many different types of fasteners for joining work pieces have been developed over the years. These include threaded fasteners which are removable, and permanent one-piece or two-piece metal fasteners requiring plastic deformation upon installation. However, all of these fasteners exhibit one or more undesirable characteristics—such as susceptibility to onset and propagation of corrosion, stress concentrations and fatigue, inconsistent clamp-up force, vulnerability to creep or loosening due to vibrations and shock experienced during service life, multiple production steps to ensure proper grain boundary control in the fastener metal, and multiple installation and post-processing steps such as machining and inspection. 
     However, the advent of amorphous metals has enabled the development of a fastener which overcomes the undesirable characteristics described above. 
     An amorphous metal is a solid metallic material, usually an alloy, with a disordered atomic-scale structure. Most ordinary metals and alloys are crystalline in their solid state, which means they have a highly ordered arrangement of atoms. Amorphous metals, however, are non-crystalline, and have a glass-like structure. But unlike common glasses, such as window glass, which are typically insulators, amorphous metals have good electrical conductivity, and other physical properties which make them useful in structural applications. 
     There are several ways in which amorphous metals can be produced, including extremely rapid cooling, physical vapor deposition, solid-state reaction, ion irradiation, and mechanical alloying. Amorphous metals are also known metallic glass or glassy metals. One example of an amorphous metal alloy includes: an alloy of 77.5% palladium, 6% copper, and 16.5% silicon; and an alloy of 41.2% zirconium, 13.8% titanium, 12.5% copper, 10% nickel, and 22.5% beryllium. Other examples of amorphous metal alloys include compositions containing varying percentages of the elements iron, molybdenum, nickel, chromium, phosphorus, carbon, boron and silicon. Still other amorphous metal alloy compositions include silver, aluminum, and other elements. The specific amorphous metal alloy used for the fastener disclosed below will depend on the properties needed for a particular application. 
     Amorphous metal alloys contain atoms of significantly different sizes, leading to low free volume (and therefore up to orders of magnitude higher viscosity than other metals and alloys) in molten state. The viscosity prevents the atoms moving enough to form an ordered lattice. The material structure also results in low shrinkage during cooling, and resistance to plastic deformation in the solid state. The absence of grain boundaries, the weak spots of crystalline materials, leads to better resistance to wear and corrosion. Amorphous metals, while technically glasses, are also much tougher and less brittle than oxide glasses and ceramics. 
     From a practical point of view, the amorphous structure of amorphous metals gives them two important properties. First, like other kinds of glasses, they experience a glass transition into a super-cooled semi-liquid state upon heating. In this thermoplastic state, the material has softened enough to allow dramatic re-shaping of a component before cooling. Second, the amorphous atomic structure means that amorphous metals do not have the crystalline defects called dislocations that govern many of the mechanical properties of more common alloys. One consequence of this is that amorphous metals can be much stronger (3-4 times or more) than their crystalline counterparts. Another consequence is that amorphous metals are less susceptible to onset and propagation of corrosion, stress concentrations and fatigue. These properties of amorphous metals can be exploited in a fastener design with a combination of features and characteristics previously unavailable, as detailed below. 
       FIG. 1  is an illustration of a permanent fastener  100  made of an amorphous metal, according to one embodiment of the present invention. The fastener  100  includes a fastener pin  110  and a retaining collar  140 . The pin includes a head portion  112 , which may have a traditional conical counter-sink shape, a counter-bore shape, or any other shape suitable for the intended application. Other shapes may include non-countersunk (raised) heads, and heads with an end-view shape which is not rotationally symmetrical (for example, oval or rectangular) in order to prevent fastener rotation. The pin  110  also includes a shank portion  114  with mechanical grip-inducing features that will be explained in detail below. In the fastener  100 , the grip-inducing feature on the shank portion  114  is provided by circumferential locking grooves  116  which in this embodiment do not continue all the way around the circumference of the pin  110 . The pin  110  further includes an optional pulling tip  118 , also discussed below. 
     The retaining collar  140  has a tapered cylindrical shape, with a cylindrical hole  142  through its center. The hole  142  has a diameter just large enough to fit over the shank portion  114  of the pin  110 . A wall  144  of the retaining collar  140  must have enough thickness to allow for substantial deformation during plastic forming without excessive thinning of material in the wall  144 . Furthermore, the wall  144  must be thick enough to provide structural rigidity and gripping strength in the fastener  100  after installation. The taper on the exterior surface of the wall  144  allows for simultaneous application of radial compression and upward force on the collar  140  by a tool used during installation. 
       FIG. 2  is an illustration of two work pieces  150  and  160  which have been brought together in preparation for installation of the fastener  100 . The work pieces  150  and  160  include aligned holes  152  therethrough, where the holes  152  are just large enough in diameter to allow the pin  110  to be inserted. The work pieces  150  and  160  also include a countersink cone or other feature as needed to accommodate the head  112  of the pin  110 . The work pieces  150  and  160  can be any sheet material which needs to be fastened together, including metal and composite materials. The fastener  100  is particularly well suited for use with work pieces  150 / 160  made of composite materials, which are widely used in the aircraft/aerospace industries and increasingly in other industries. In most applications, many of the fasteners  100  will be used to secure the work pieces  150 / 160  together. The fastener  100  is also suitable for securing work piece assemblies containing multiple (more than two) layers. 
     In the fastener  100 , the retaining collar  140  must be comprised of an amorphous metal, as the collar  140  will be heated into the thermoplastic region of the material and then compression formed onto the shank  114 . Amorphous metal provides the distinct advantage that the temperature required to reach the thermoplastic region of the material is far lower than softening temperatures for traditional metals. For example, one amorphous metal suitable for the retainer  140  reaches the thermoplastic region at around 200° C. The retaining collar  140  at 200° C. can be pressed against a work piece made of a composite material without melting or damaging the work piece. On the other hand, a retainer made from a traditional steel would have to be heated to a temperature of at least 800°-1000° C. in order to soften the retainer material, and at that temperature the composite material of the work piece would be destroyed on contact. Furthermore, if a traditional steel retainer were to be used without heating to the point of material softening, the forces required to plastically deform the retainer onto the pin of fastener would be so high that the work pieces would be mechanically damaged. These reasons provide the motivation for making the retaining collar  140  from an amorphous metal. 
     The pin  110  of the fastener  100  may also be comprised of an amorphous metal, or may be comprised of a traditional metal such as stainless steel. Amorphous metal provides several desirable features in the pin  110 . One desirable feature of the pin  110  when made from amorphous metal is the ability to make the pin  110  smaller in diameter for a given amount of desired holding strength. This is due to the lack of grain structure in amorphous metals, which make the pin  110  less susceptible to stress concentrations and corrosion. Another desirable feature of the pin  110  when made from amorphous metal is that the pin  110  can be made in a casting process, or a process which is actually more similar to injection molding, rather than machining and/or heading as done for traditional metal fasteners. It is much simpler and less costly to create molds for a variety of different pin shapes than to set up machining and heading tools for the same variety of shapes. Furthermore, injection molding of the pin  110  results in a part without the high residual stresses produced by machining and heading operations on traditional metal fasteners. 
       FIGS. 3-6  show the fastener  100  in four different stages of installation in the work pieces  150 / 160 .  FIG. 3  is an illustration of the amorphous metal fastener  100  in a first installation step, where the pin  110  is inserted through the holes in the work pieces  150 / 160 .  FIG. 4  is an illustration of the fastener  100  in a second installation step, where the retaining collar  140  is positioned over the shank portion  114  of the pin  110 . At this step, the collar  140  must be heated into the thermoplastic material region where it can be plastically deformed under fairly low stress. The collar  140  can be directly heated by the tool which is used to compress the collar  140  onto the pin  110 . For example, the tool could include an inductive heating element for heating the collar  140 . Alternately, the collar  140  can be preheated in a separate oven or other heating apparatus, and transferred to the tool for forming onto the pin  110 . Care must be taken to heat the collar  140  to a working temperature in the thermoplastic region, but not to a temperature which would cause it to melt. 
     At the step shown in  FIG. 4 , the heated collar  140  is pressed upward and compressed radially inward, as shown by arrows  170 . Simultaneously, the pulling tip  118  is used to pull downward (arrow  180 ) on the pin  110 , which both resists the upward push of the collar  140  and provides a clamping pre-load in the work pieces  150 / 160 . Due to the compressive force it is experiencing and the thermoplastic material properties it possesses, the cylindrical wall  144  of the collar  140  is radically compressed into the locking grooves  116  to affix the collar  140  on the pin  110 . At that point, the collar  140  must be rapidly cooled back to its solid amorphous metal state. Care must be taken not to allow the collar  140  to cool too slowly, which would allow crystalline structure to form. A cooling gas or liquid could be applied to the collar  140  if necessary to achieve the desired rate of cooling. 
       FIG. 5  is an illustration of the fastener  100  in a third installation step, where the retaining collar  140  has been thermoplastically formed via compression onto the shank  114  of the pin  110 . It can be seen in  FIG. 5  that the inside diameter of the collar  140  has deformed into the grooves  116  on the pin  110 . This dramatic deformation of the collar  140  provides tremendous mechanical grip in the fastener  100  when installed. It can also be seen that the outside diameter of the collar  140  has deformed to take the shape of the tool that was used to compress the collar  140  in the previous step. The shape of the tool and the outside of the collar  140  is not significant, other than that the tool shape used to most effectively compress the collar  140  may be dependent upon the design of the mechanical grip features on the shank  114  of the pin  110 . 
       FIG. 6  is an illustration of the fastener  100  in a final installation step, where the collar has fully cooled and the pulling tip  118  has been removed from the pin  110 . The pulling tip  118  may be designed with a stress riser undercut at its base, which would allow the pulling tip  118  to be snapped or broken off of the pin  110  by applying a lateral force and/or bending moment. As shown in  FIG. 6 , the fastener  100  installation is complete, including a residual tension in the pin  110  and a clamping pre-load in the work pieces  150 / 160 . 
       FIG. 7  is an illustration of a pin  210  for a fastener  200 , which is a second embodiment of an amorphous metal permanent fastener. On the pin  210 , a shank  212  includes a grip feature which has a knurled shape. The knurled shape of the shank  212  would be extremely effective in preventing rotation of the retaining collar  140  relative to the pin  210 . 
       FIG. 8  is an illustration of a pin  310  for a fastener  300 , which is a third embodiment of an amorphous metal permanent fastener. On the pin  310 , a shank  312  includes a grip feature which has a fluted or scalloped shape formed by a plurality of axial grooves on the pin  310 . The axial grooves would not extend all the way to the end of the shank  312 ; a full-diameter shoulder would need to be provided in order to prevent the collar  140  from pulling off of the end of the pin  310 . The shape of the shank  312  would be very effective in preventing rotation of the retaining collar  140  relative to the pin  310 , and also provide very positive axial grip of the collar  140  on the pin  310 . 
     The number and type of mechanical grip features and shapes which can be used on the pin shank of the disclosed amorphous metal fastener are virtually limitless. The pin shank designs can include rotationally symmetrical shapes, and non-rotationally symmetrical shapes which prevent rotation of the retaining collar  140  relative to the pin. The pin shank shapes are enabled by the large-scale deformations of the retaining collar  140  which are made possible through the use of amorphous metal alloys. 
       FIG. 9  is an illustration of the amorphous metal permanent fastener  200  of  FIG. 7 , after installation, where a retaining collar  240  is used which has a different shape than the retaining collar  140 . The retaining collar  240  has a conical shape designed to fit into a countersink cone on the bottom surface of the work piece  160 . The countersink cone on the bottom surface of the work piece  160  may be provided in order to improve fastener performance through increased contact surface area, or the countersink cone on the bottom surface of the work piece  160  may simply be a shape which was used with a legacy fastener of some sort. In either case, the retaining collar  240  can be shaped to generally fit the countersink cone, and further formed into the countersink cone via plastic deformation during installation. 
       FIG. 10  is a flowchart diagram  400  of a method for joining work pieces using an amorphous metal fastener. At box  402 , the work pieces  150 / 160  are arranged in a position as they are to be joined, with holes aligned. At box  404 , the fastener pin  110  is inserted through the work pieces  150 / 160 , so that the head  112  of the pin  110  contacts the outside surface of the work piece  150  and the shank portion  114  of the pin  110  extends clear of the work piece  160 . 
     At box  406 , the amorphous metal retaining collar  140  is heated to a working temperature which causes the retaining collar  140  to have thermoplastic material properties. The heating can be performed within the installation tool, or separately in an oven or other heating unit. At box  408 , the retaining collar  140  is placed over the shank portion  114  of the fastener pin  110 , such that the retaining collar  140  surrounds the locking geometric features (grooves  116 ) on the shank portion  114  of the pin  110 . At box  410 , the fastener pin  110  is pulled while simultaneously applying a force to the conical outer wall of the retaining collar  140  which counteracts the pulling of the fastener pin  110  and also compresses the retaining collar  140  onto the locking grooves  116  on the shank portion  114  of the pin  110 . As discussed above, the collar  140  is then cooled rapidly enough to preserve its amorphous metal properties. The pulling tip  118  can also be removed from the fastener pin  110 . 
     The amorphous metal fastener described above provides numerous advantages to any manufacturer needing to secure together two or more composite panels. These advantages include the ability to make the fastener smaller due to the desirable material properties of the amorphous metal, particularly the absence of a crystalline grain structure resulting in improved resistance to corrosion and stress concentrations. In addition, the amorphous metal retaining collar, which can be heated to a thermoplastic region of the material without risk of damaging the composite work piece, can be dramatically deformed onto the shank of the pin upon installation. The dramatic plastic deformation of the collar provides excellent mechanical grip of the fastener, further improving performance for a given size. Finally, the amorphous metal fastener can be injection molded in a number of different sizes and shapes with a minimum of tooling investment compared to traditional machined and headed metal fasteners. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.