Abstract:
The invention relates to a method for producing an attachment element (R) comprising a shaft ( 100 ) having a first end provided with a head ( 200 ) and a second end that is intended to be deformed in order to form a button ( 300 ) providing a bearing surface opposite the surface formed by the head ( 200 ), said shaft ( 100 ) having a nominal diameter (D). The method is characterised in that the element is sized such that the minimum length (L) of the shaft ( 100 ) to be deformed to form the button ( 300 ) is no less than  1.25  times the nominal diameter (D) of the shaft ( 100 ). The invention is suitable for use in the production of attachment elements, such as rivets.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of fastening elements such as rivets and particularly to adaptations for enhancing the mechanical features thereof. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    There are a plurality of rivets and manufacturing methods in the prior art liable to have the mechanical features sought. Nonetheless, the fatigue and tearing strength criteria applicable to rivets are increasingly stringent. 
         [0003]    However, while one solution to meet these criteria lies in increasing the rivet dimensions, such an increase involves the drawback of the appearance of cracks. 
       DESCRIPTION OF THE INVENTION 
       [0004]    For this reason, the applicant conducted research aimed at providing a solution for the most stringent criteria in respect of the fatigue and tearing strength criteria, on one hand, and at solving the technical problem of the appearance of cracks, on the other. 
         [0005]    This research resulted in the design of a method for manufacturing, using a metal wire, a fastening element such as that having a shank wherein a first end is equipped with a head, the second end being intended to be deformed to act as a knob forming a bearing surface to be positioned facing that formed by the head, said shank having a nominal diameter. This fastening element is designed such that the minimum length of the shank intended to be deformed to act as the knob is not less than 1.25 times the nominal diameter of the shank. 
         [0006]    The invention is remarkable in that it consists of
       obtaining, for the metal wire, a material grain adopting a size of 6 or finer according to the criteria defined by the ASTM E 112 standard,   performing, on said wire, a work-hardening step wherein the rate is greater than 30%, and   designing said element such that the minimum diameter of the knob obtained after deforming the shank is greater than 1.75 times the nominal diameter of the shank.       
 
         [0010]    These features ensure that a knob and thus a fastening element having a satisfactory fatigue strength are obtained. 
         [0011]    Obtaining a finer grain requires additional working operations on the material, rendering the wire production method longer and more costly. In this way, for example, the proposed work-hardening should be performed in a plurality of steps. Furthermore, a suitable heat treatment should be used. 
         [0012]    A person skilled in the art would not turn to such a solution requiring more operations and control of these base wire manufacturing operations before producing the rivet. A person skilled in the art facing the same technical problem would prefer to purchase a more expensive material such as titanium to obtain a fastener having the same features. 
         [0013]    According to one further particularly advantageous feature of the invention, the knob obtained after deforming the shank has a maximum thickness equal to 0.45 times the nominal diameter of the shank. Such a thickness associated with the diameter defined above makes it possible to obtain a fastening element meeting fatigue and tearing strength criteria. 
         [0014]    The invention also relates to the rivet having all or some of the features described above. 
         [0015]    In addition to the dimensional and grain features presented by the wire selected for manufacturing the fastening element according to the invention, further features suitable for obtaining a fastening element meeting the criteria without involving the risk of cracking, have also been devised. 
         [0016]    Further features of the manufacturing method are particularly based on the selection of the features of the wire from which the fastening element is forged and the constituent materials of said wire, which is subject to a method comprising specific work-hardening steps, and the element obtained is subject to a quenching step, etc. 
         [0017]    This wire is an aluminum alloy wire which, in addition to the aluminum (Al) compound, comprises additional elements such as: 
         [0018]    Chromium symbolized by CR, Copper symbolized by CU, Iron symbolized by FE, Magnesium symbolized by MG, Manganese symbolized by MN, Silicon symbolized by SI, Titanium symbolized by TI, Zinc symbolized by ZN, Zirconium symbolized by ZR, etc. The proportions described below are minimum or maximum percentages of the elements alone or a combination of elements. These compositions are suitable for taking on the sought mechanical properties for a fastener while preventing the presence of cracks particularly due to the copper content. They are also suitable for conducting the desired work-hardening and thus obtaining the fine grain sought. 
         [0019]    In this way, according to one particularly advantageous feature, the manufacturing method according to the invention is remarkable in that it consists of selecting, for the metal wire from which the fastening element is formed, an aluminum alloy wherein the additional elements to aluminum adopt as a percentage the following proportions: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 min 
                 max 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 CR 
                 0 
                 0.04 
               
               
                   
                 CU 
                 2 
                 2.5 
               
               
                   
                 FE 
                 0.011 
                 0.14 
               
               
                   
                 MG 
                 2.04  
                 2.3 
               
               
                   
                 MN 
                 0 
                 0.05 
               
               
                   
                 SI 
                 0.02  
                 0.12 
               
               
                   
                 TI 
                 0.02  
                 0.06 
               
               
                   
                 ZN 
                 6.1 
                 6.7 
               
               
                   
                 ZR 
                 0.08 
                 0.15 
               
               
                   
                   
               
             
          
         
       
     
         [0020]    According to a further particularly advantageous feature, the method consists of selecting, for the metal wire from which the fastening element is formed, an aluminum alloy wherein the additional elements to aluminum adopt as a percentage the following proportions: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 min 
                 max 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 CR 
                 0.05 
                 0.1 
               
               
                   
                 CU 
                 2.6 
                 3 
               
               
                   
                 FE 
                 0.34 
                 0.5 
               
               
                   
                 MG 
                 0.33 
                 0.5 
               
               
                   
                 MN 
                 0.1  
                 0.2 
               
               
                   
                 SI 
                 0.32 
                 0.8 
               
               
                   
                 TI 
                 0 
                 0.01 
               
               
                   
                 ZN 
                 0.1  
                 0.17 
               
               
                   
                 FE—MN 
                 0.47 
                 0.68 
               
               
                   
                 TI—ZR 
                 0.05 
                 0.15 
               
               
                   
                   
               
             
          
         
       
     
         [0021]    According to a further particularly advantageous feature, the method consists of selecting, for the metal wire from which the fastening element is formed, an aluminum alloy wherein the additional elements to aluminum adopt as a percentage the following proportions: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 min 
                 max 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 CR 
                 0 
                 0.03 
               
               
                   
                 CU 
                 3.5 
                 4.3 
               
               
                   
                 FE 
                 0.016 
                 0.3 
               
               
                   
                 MG 
                 0.43 
                 0.65 
               
               
                   
                 MN 
                 0.41  
                 0.63 
               
               
                   
                 SI 
                 0.21 
                 0.4 
               
               
                   
                 TI 
                 0.005 
                 0.08 
               
               
                   
                 ZN 
                 0.002 
                 0.04. 
               
               
                   
                   
               
             
          
         
       
     
         [0022]    The condition of the wire is also the subject of further specific selections. For example, according to one particularly advantageous feature, the method consists of selecting a wire which does not have inclusions (isolated nicks, abrasions, pitting, isolated minor inclusions, cold shuts, non-continuous scratches, tool marks, burrs, etc.) greater than 0.15 millimeters in depth. Furthermore, the method consists of selecting a wire wherein the depth of longitudinal continuous defects does not exceed 40 microns. 
         [0023]    A further feature of this method relates to the quenching step. According to a technological choice, the method consists of subjecting said element to a cooling step wherein the time for transferring the fastening elements from one medium wherein the temperature is equivalent to that of solution to a medium wherein the temperature is 20 degrees Celsius is less than 5 seconds. 
         [0024]    Finally, according to a further feature, the method consists of subjecting said element to a quenching step wherein the temperature variation during the quenching of the same batch of elements does not exceed 5 degrees Celsius. 
         [0025]    According to a further particularly advantageous feature of the invention, the shank end radius of the fastening element, i.e. the connecting radius between the cylindrical surface of the shank and the plane surface of the shank end, is proportional to the diameter of the shank itself according to a proportion wherein said radius is equal to the nominal diameter of the shank divided by a value ranging between 3 and 3.5. 
         [0026]    A rivet having an association of these features in the manufacturing method thereof is suitable not only for meeting fatigue and tearing strength criteria but also for preventing the presence of cracks. 
         [0027]    A further feature helping obtain a crack-free fastener consists of selecting, from the manufactured fasteners, a fastener wherein the shank particularly at the part to be deformed, i.e. at the shank end radius, does not have longitudinal continuous defects greater than 40 microns. 
         [0028]    It would thus appear that the invention consists of a series of feature selections both upstream from the manufacture of the fastening element for example in the choice of the features of the base wire and downstream, i.e. not only in the final dimensions but also in the final surface condition of the fastening element obtained. 
         [0029]    The invention thus also relates to the fastening element obtained according to all or some of the features of the method described above. 
         [0030]    The fundamental concepts of the invention described above in the most basic form thereof, further details and features will emerge more clearly on reading the following description and with reference to the appended figures, giving as a non-limiting example, an embodiment of a fastening element according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0031]      FIG. 1  is a schematic drawing of a sectional view of an embodiment of a rivet according to the invention fitted in an orifice passing through two parts to be assembled; 
           [0032]      FIG. 2  illustrates the assembly produced using the embodiment in  FIG. 1 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]    As illustrated in the drawing in  FIG. 1 , the rivet R is fitted in a hole T passing through two plates P 1  and P 2  to be assembled. This rivet R has a shank  100  wherein a first end is equipped with a head  200 , the second end being intended to be deformed to act as a knob  300  (see  FIG. 2 ) forming a bearing surface to be positioned facing that formed by the head  200 , said shank having a nominal diameter D. As illustrated before deformation, a portion of the shank  100  of the rivet R projects outside the hole T. According to the invention, so as to meet the fatigue and tearing strength criteria, the rivet R is designed such that the minimum shank length L intended to be deformed to act as the knob  300 , i.e. the minimum shank length projecting outside the hole T, is equal to 1.25 times the nominal diameter D of the shank  100 . 
         [0034]    According to a further particularly advantageous feature of the invention, the knob  300  obtained after deforming the shank  100  has a maximum thickness e equal to 0.45 times the nominal diameter D of the shank. 
         [0035]    According to a further feature, the minimum diameter d of the knob  300 , obtained after deformation, is greater than 1.75 times the nominal diameter D. 
         [0036]    Finally, the shank end radius r is equal to the nominal diameter D of the shank divided by a value ranging between 3 and 3.5. 
         [0037]    It is understood that the fastening element and the method have been described above and represented for the purposes of disclosure rather than limitation. Obviously, various adaptations, modifications and enhancements may be made to the example above, without leaving the scope of the invention.