Abstract:
A system is provided for attaching a layer of a metal to other layers in a material stack-up using a metal mechanical fastener that does not allow for direct contact between the fastener and the metal layer, thereby neutralizing any galvanic reaction between the metal sheet and the fastener and avoiding corrosion. The metal layer may be any of several metals, including magnesium. The fastener is a rivet, a screw or a bolt. The disclosed inventive concept uses an insulating layer to insulate the magnesium layer. In one embodiment, a layer of a thin metal is formed over the insulating layer. Where the fastener is a rivet and the metal is magnesium, the insulating layer and thin metal provide a barrier between the magnesium layer and the rivet prongs following rivet insertion, thus isolating the rivet. The system enables a greater application of fastener joining, particularly with magnesium and mixed material joining.

Description:
TECHNICAL FIELD 
       [0001]    The disclosed inventive concept relates generally to mechanical fastening systems for joining a mixed material stack-up. More particularly, the disclosed inventive concept relates to a system that mechanically joins a mixed material stack-up having a magnesium upper layer where corrosion between the fastener and the magnesium upper layer is avoided by insulating the fastener from the magnesium upper layer. 
       BACKGROUND OF THE INVENTION 
       [0002]    The automobile manufacturing industry is constantly faced with new challenges in a wide array of areas including vehicle safety, reliability, durability and cost. Perhaps the greatest challenge faced by the automobile industry today is the need to improve fuel mileage to both decrease carbon emissions and increase fuel economy for both environmental and cost reasons, all without compromising safety, power or durability. In 2011, new fuel economy requirements were imposed that establish a US vehicle fleet average of 54.5 miles per gallon by 2025. As the industry moves to that target year fuel annual economy requirements will be ramped up for different-sized vehicles. 
         [0003]    Efforts have been made to increase fuel economy for vehicles. These efforts can be divided into two approaches: the “supply” side and the “demand” side. 
         [0004]    On the supply side attention is drawn to improving energy conversion efficiency through use of, for example, electric or hybrid-electric drive trains. In addition, new vehicle drive trains, including smaller engines and more efficient transmission having multiple gears and transfer cases, are being developed and employed. Other technologies, including start-stop and engine cylinder deactivation strategies, are also proving effective at decreasing fuel consumption. Improved transmissions with multiple gears are also important elements to increased fuel consumption efficiencies. 
         [0005]    On the demand side weight reduction is key, though other aspects, such as improved aerodynamics and drag reduction, are also important. Conventional vehicles, particularly trucks, rely on steel components. For over 100 years the material of choice for most vehicles is steel. Today steel makes up about 60% of the average car by weight. 
         [0006]    Despite the improvement in steel composition the weight of steel regardless of type remains significant. It is also possible to reduce vehicle weight when steel is used by reducing component thickness. However, at a certain point it is no longer practical to reduce steel thickness regardless of the steel grade used. The use of high strength steel or advanced, high strength steel does not improve the realization that there are limits to how much vehicle weight can be reduced by steel thickness reduction without compromising vehicle performance. 
         [0007]    Thus as the automotive industry continues to focus on light weighting vehicles to meet customer expectations on fuel economy and CAFE requirements, interest in alternative materials including aluminum intensive vehicle applications has increased. This is because vehicle weight reduction is most directly accomplished through substituting lighter materials for currently used steel parts. However, a limited variety of materials is available as a substitute for automotive steel. One such material is carbon fiber which is both lightweight and strong. 
         [0008]    Accordingly, much attention is drawn to the use of light-weight metals, such as magnesium, which demonstrate significant weight advantages over traditional steel. The use of magnesium not only provides weight reduction but also results in good crash performance. Research has shown that in collisions magnesium can perform well. 
         [0009]    In body-in-white structures, joining methods have traditionally relied on resistance-spot welding (e.g., in steel structures). In the case of magnesium as a vehicle material used in conjunction with other metals, joining applications often rely on self-piercing rivet (SPR) technology that use an engineered rivet. According to SPR technology, self-piercing rivets do not require a drilled or punched hole, unlike solid rivets and blind rivets. 
         [0010]    One advantage of SPR technology is that it is a high production volume assembly process. Further, it is compatible with an adhesive, where both methods can be used in conjunction. 
         [0011]    As mixed material joining continues to grow magnesium applications continue to develop. One concern related to the riveting of magnesium when it is present in the top sheet of the material stack up is the corrosion concern between the fastener and the magnesium top sheet. Eliminating this concern would increase the usage of magnesium sheeting in vehicle assembly. 
         [0012]    As in so many areas of vehicle technology there is always room for improvement related to the mechanical fastening of magnesium materials. 
       SUMMARY OF THE INVENTION 
       [0013]    The disclosed inventive concept overcomes the problems associated with known systems and methods for joining materials together of which the upper sheet is a metal, such as magnesium. The disclosed inventive concept provides a solution that allows the use of metal mechanical fasteners to mechanically join a mixed material stack-up having a metal, such as magnesium as a top layer where corrosion is avoided. The metal mechanical fastener is selected from the group consisting of rivets, screws and bolts. Rivets include self-piercing rivets, blind rivets, and solid rivets. Screws include, for example, flow drill screws. 
         [0014]    The disclosed inventive concept provides for the use of an isolation barrier between the fastener and the top layer of material in the joint. The fastener is inserted through the isolation barrier. One example of a barrier is a layer of aluminum. Another example of the barrier is a layer of tape. After the fastener is inserted, the barrier provides a barrier between the top sheet and the fastener, thus isolating the fastener. 
         [0015]    An advantage of the disclosed inventive concept is that it enables a greater application of joining mechanical fasteners, particularly with metals such as magnesium and mixed material joining. Another advantage of the disclosed inventive concept is is that it can be used on any material to isolate the metal fastener from the sheet of metal beneath the barrier, specifically when there is a large difference in galvanic potential between the substrate material (that may include carbon fiber material) and the metal fastener. It is a lower cost and lighter weight solution than using an additional piece of metal on top of the magnesium to sandwich the joint. The disclosed inventive concept also eliminates concern from vehicle operations in regards to trying to locate an additional material for use in production. 
         [0016]    The above advantages and other advantages and features will be readily apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    For a more complete understanding of this invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention wherein: 
           [0018]      FIG. 1A  is a schematic illustration of the first step of a self-piercing rivet process according to the prior art in which the blankholder and the punch are in position above the rivet prior to pressure being applied to the punch; 
           [0019]      FIG. 1B  is a schematic illustration of the second step of the self-piercing rivet process according to the prior art in which initial pressure has been applied to the punch; 
           [0020]      FIG. 1C  is a schematic illustration of the third step of the self-piercing rivet process according to the prior art in which the rivet has pierced the upper layer and is interlocked into the lower layer; 
           [0021]      FIG. 1D  is a schematic illustration of the fourth step of the self-piercing rivet process according to the prior art in which the rivet process has been completed and the punch and blankholder have been removed; 
           [0022]      FIG. 2  is a schematic view of a joint according to the prior art in which a metal top sheet is riveted to a lower sheet; 
           [0023]      FIG. 3  is a schematic view of a joint according to a first embodiment of the disclosed inventive concept in which an insulating layer and an upper metal layer have been formed over the metal top sheet; 
           [0024]      FIG. 4  is a schematic view of a joint according to a second embodiment of the disclosed inventive concept in which an insulating layer has been formed over a magnesium layer; and 
           [0025]      FIG. 5  is a schematic view of a joint according to a third embodiment of the disclosed inventive concept in which an insulating layer is formed between a metal top layer and a magnesium layer. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    In the following figures, the same reference numerals will be used to refer to the same components. In the following description, various operating parameters and components are described for different constructed embodiments. These specific parameters and components are included as examples and are not meant to be limiting. 
         [0027]    The disclosed inventive concept may find use in any number of applications where plural layers of the same or dissimilar materials are being attached. Accordingly, the disclosed inventive concept may be used in the production of automotive vehicles and trucks. 
         [0028]    The use of metal mechanical fasteners, such as self-piercing rivets, in the assembly of plural components is a known technique as illustrated in  FIGS. 1A through 1D . These figures schematically show steps involved in the self-piercing rivet process. As the rivet is inserted into the stack, the material deforms into the die and the resultant form is called a “button.” 
         [0029]    As illustrated in  FIG. 1A , the first step of a self-piercing rivet process according to the prior art is illustrated. A first layer  10  is shown in position over a second layer  12 . A rivet  14  is illustrated in position over the first layer  10 . A punch  16  and a blankholder  18  are illustrated in position with the rivet  14  prior to pressure being applied to the punch  16 . A die  20  is in position beneath the second layer  12 . 
         [0030]    In  FIG. 1B , the second step of the self-piercing rivet process according to the prior art is illustrated. In this step, initial pressure has been applied to the punch  16  and the rivet  14  is shown beginning to deform the first layer  10  and the second layer  12 . 
         [0031]    In  FIG. 1C , the third step of the self-piercing rivet process according to the prior art is illustrated. In this step, the punch  16  has been fully inserted through the blankholder  18  such that the rivet  14  pierced the first layer  10  and forms the second layer  12 . 
         [0032]    In  FIG. 1D , the fourth step of the self-piercing rivet process according to the prior art is illustrated. In this step, the rivet  14  is shown fully inserted through the first layer  10  and a button is formed in the second layer  12 . The punch  16  and the blankholder  18  have been moved out of contact with the first layer  10 . 
         [0033]    The self-piercing rivet process shown in  FIGS. 1A through 1D  and described in conjunction therewith may be used in the joining of any number of materials, including magnesium. This is consistent with the general increase industry-wide of the joining of mixed materials with one such material being magnesium. 
         [0034]    Referring to  FIG. 2 , a schematic view of a prior art joint, generally illustrated as  30 , is shown. A metal top sheet  32  is fitted over a bottom sheet  34 . A self-piercing rivet  36  having a rivet head  37 , a rivet shank  38  and a rivet foot  39  is inserted in a known manner to thereby attach together the metal top sheet  32  and the bottom sheet  34 . 
         [0035]    While the magnesium top sheet  32  may be successfully attached to the bottom sheet  34  using the self-piercing rivet  36 , one concern related to the riveting of the magnesium top sheet  32  in the material stack-up is the corrosion concern between the rivet head  37  and the magnesium top sheet  32  because of direct metal-to-metal contact. Such corrosion may lead to failure of the mechanical attachment and separation of the joined materials. 
         [0036]    The disclosed inventive concept provides a practical and cost-effective solution to the problem of corrosion that results when a metal rivet is used to join a metal top sheet, such as a magnesium top sheet, with one or more additional layers. The disclosed inventive concept may be used with a material to isolate the metal fastener from the top metal sheet, and specifically is useful when there is a large difference in galvanic potential between the substate material and the metal mechanical fastener. The solution of the disclosed inventive concept is illustrated in  FIG. 3  in which a schematic view of a joint, generally illustrated as  40 , is shown. A metal top sheet  42  composed of a metal, such as, but not limited to magnesium, is placed over a bottom sheet  44 . The bottom sheet  44  may be of any of a variety of materials including metals (such as steel or, more particularly, carbon steel grade [DP800]) or carbon-fiber composites. 
         [0037]    An insulating layer  46  is applied over the metal top sheet  42 . The insulating layer  46  may be applied over the entire metal top sheet  42  or only over areas into which rivets will be pierced. The insulating layer  46  may be composed of any of several materials, including, but not limited to, aluminum or synthetic cellulose acetate. Preferably the insulating layer  46  has an adhesive layer for easy attachment to the metal top sheet  42 . The adhesive layer may be composed of, for example, synthetic rubber resin and may include a release tape (not shown) that is removed prior to attachment to the metal top sheet  42 . 
         [0038]    A relatively thin outer metal layer  48  composed of any of several metals is applied over the insulating layer  46 . A metal mechanical fastener  50  metal top is inserted in a known manner to thereby attach together the outer metal layer  48 , the tape layer  46 , metal top sheet  42  and the bottom sheet  44 . The metal mechanical fastener  50  may be a rivet, a screw or a bolt. Possible choices for a rivet include a self-piercing rivet, a blind rivet and a solid rivet. The screw may be of several types, and may be, for example, a flow drill screw. The illustrated metal mechanical fastener  50  is provided for suggestive purposes only and is not intended as being limiting. The illustrated metal mechanical fastener  50  includes a rivet head  51 , a rivet shank  52 , and a rivet foot  54 . The outer metal layer  48  is provided to protect the insulating layer  46  and to prevent damage to the insulating layer  46  that might otherwise result upon insertion of the metal mechanical fastener  50  into the layers of material. 
         [0039]    According to the disclosed inventive concept, the insulating layer  46  and the outer metal layer  48  provide a protective barrier between the metal mechanical fastener  50  and the rivet head  51  and the metal sheet  42 , thus preventing direct contact between the metal mechanical fastener  50  and the rivet head  51  and the metal sheet  42 . Without direct contact of the metal mechanical fastener  50  and the rivet head  51  with the metal sheet  42 , the galvanic potential is neutralized and corrosion is prevented. Thus the disclosed inventive concept provides a practical and cost-effective solution to the problem of corrosion when certain metals, such as magnesium sheets, are used in a material stack-up that is fastened together by a metal mechanical fastener. 
         [0040]      FIG. 4  illustrates an alternative embodiment of the disclosed inventive concept in which a schematic view of a joint, generally illustrated as  60 , is shown. A top sheet  62 , such as aluminum or tape, is placed over a magnesium layer  64 . The top sheet  62  may be composed of any of several materials, including, but not limited to aluminum or synthetic cellulose acetate. The top sheet  62  may have an adhesive later for easy attachment to the magnesium layer  64 . The adhesive layer may be composed of, for example, synthetic rubber resin and may include a release tape (not shown) that is removed prior to attachment to the magnesium layer  64 . The magnesium layer  64  is placed over a base layer  66 . The base layer  66  may be of any of a variety of metals such as, but not limited to, steel, particularly carbon steel grade (DP800). 
         [0041]    A metal mechanical fastener  68  metal top is inserted in a known manner to thereby attach together the top sheet  62 , the magnesium layer  64 , and the base layer  66 . The metal mechanical fastener  68  may be a rivet, a screw or a bolt. Possible choices for a rivet, include a self-piercing rivet, a blind rivet and a solid rivet. 
         [0042]    The screw may be of any of several types, and may be, for example, a flow drill screw. The illustrated mechanical fastener  68  is provided for suggestive purposes only and is not intended as being limiting. The illustrated metal mechanical fastener  68  includes a rivet head  70 , a rivet shank  72 , and a rivet foot  74 . 
         [0043]    According to the disclosed inventive concept of  FIG. 4 , the top sheet layer  62  provide a protective barrier between the metal mechanical fastener  68  and the rivet head  70  and the magnesium layer  64 . Without direct contact of the metal mechanical fastener  68  and the rivet head  70  with the magnesium layer  64 , the galvanic potential is neutralized and corrosion is prevented. 
         [0044]      FIG. 5  illustrates an additional alternate embodiment of the disclosed inventive concept in which a schematic view of a joint, generally illustrated as  80 , is shown. A metal top layer  82  is provided as the outer layer. The metal top layer  82  may be of any type of metal, including steel. A magnesium layer  84  is positioned over a metal layer  86 . The metal layer  86  may be any of several metals including, but not limited to, steel, such as carbon steel grade (DP800). 
         [0045]    An insulating layer  88  is positioned between the metal top layer  82  and the magnesium layer  84 . The insulating layer  88  may be composed of any of several materials, including, but not limited to, aluminum or synthetic cellulose acetate. Preferably, the insulating layer  88  has an adhesive layer on one or both sides. The adhesive layer(s) may be composed of any of several materials, including, but not limited to, synthetic cellulose acetate. The adhesive layer may be composed of, for example, synthetic rubber resin and may include a release tape (not shown) that is removed prior to installation. 
         [0046]    A metal mechanical fastener  90  is inserted in a known manner to thereby attach together the metal top layer  82 , the insulating layer  88 , the magnesium layer  84 , and the metal layer  86 . The metal mechanical fastener  90  may be a rivet, a screw or a bolt. Possible choices for a rivet include a self-piercing rivet, a blind rivet and a solid rivet. The screw may be of several types, and may be, for example, a flow drill screw. The illustrated metal mechanical fastener  90  includes a rivet head  92 , a pivot shank  94 , and a pivot foot  96 . The metal top layer  82  is provided to protect the insulating layer  88  and to prevent damage to the insulating layer  88  that might otherwise result upon insertion of the metal mechanical fastener  90  into the layers of material. 
         [0047]    According to this embodiment of the disclosed inventive concept, the insulating layer  88  and the metal top layer  82  provide a protective barrier between the metal mechanical fastener  90  and the rivet head  92  and the magnesium layer  84 , thus preventing direct contact between the metal mechanical fastener  90  and the rivet head  92  and the magnesium layer  84 . Without direct contact of the metal mechanical fastener  90  and the rivet head  92  with the magnesium layer  84 , the galvanic potential is neutralized and corrosion is prevented. Thus this embodiment of the disclosed inventive concept also provides a practical and cost-effective solution to the problem of corrosion when certain materials, such as magnesium sheets, are used in a material stack-up that is fastened together by a metal mechanical fastener. 
         [0048]    For at least the above reasons the disclosed invention as set forth above overcomes the challenges faced by known methods and systems for the mechanical fastening together a stack-up of material having a metal top layer, such as a magnesium top layer. However, 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 true spirit and fair scope of the invention as defined by the following claims.