Friction stir rivet and method of joining therewith

Abstract

A friction stir rivet having a body, a mandrel, and a mechanical interface therebetween, is disclosed. The body has an elongated cylindrical shank, a cap at a first end, and an axial hole therethrough. The mandrel has an elongated shaft defining an axis and a head at one end thereof, the head having an effective outside diameter greater than the effective outside diameter of the shaft, the shaft being disposed within the axial hole of the body, the mandrel head being disposed at an opposite end of the body to that of the cap, and the end of the mandrel head having a flat surface oriented substantially perpendicular to the axis of the shaft. The mechanical interface between the body and the mandrel is such that the body rotates in response to rotation of the mandrel.

1 . A friction stir rivet, comprising: 
 a body having an elongated cylindrical shank, a cap at a first end, and an axial hole therethrough;    a mandrel having an elongated shaft defining an axis and a head at one end thereof, the head having an effective outside diameter greater than the effective outside diameter of the shaft, the shaft being disposed within the axial hole of the body, the mandrel head being disposed at an opposite end of the body to that of the cap, and the end of the mandrel head having a flat surface oriented substantially perpendicular to the axis of the shaft; and    a mechanical interface between the body and the mandrel such that the body rotates in response to rotation of the mandrel.    
   
   
       2 . The rivet of  claim 1 , wherein: 
 the flat surface oriented substantially perpendicular to the axis of the shaft is perpendicular to within plus-or-minus two degrees thereof.    
   
   
       3 . The rivet of  claim 1 , wherein: 
 the flat surface has an effective outside diameter that is equal to or greater than about 80% of the effective outside diameter of the mandrel head.    
   
   
       4 . The rivet of  claim 3 , wherein: 
 the flat surface extends over about 100% of the effective diameter of the end of the mandrel head.    
   
   
       5 . The rivet of  claim 1 , wherein: 
 the shaft has a first region having a first tensile strength and a second region having a second tensile strength, the second tensile strength being less than the first tensile strength, and the second region being disposed proximate the cap.    
   
   
       6 . The rivet of  claim 1 , wherein: 
 the mandrel comprises a material having a microstructure that substantially maintains its strength during a short-term temperature excursion, such that subsequent to a short-term temperature excursion, and in response to the mandrel head being pulled into the body, the mandrel has sufficient strength to upset the body prior to the mandrel severing.    
   
   
       7 . The rivet of  claim 1 , wherein: 
 the mandrel comprises a material having a martensite microstructure.    
   
   
       8 . The rivet of  claim 1 , wherein: 
 the mandrel comprises a material having a bainite microstructure.    
   
   
       9 . The rivet of  claim 1 , wherein: 
 the mandrel comprises a material having a cold drawn microstructure.    
   
   
       10 . The rivet of  claim 1 , wherein: 
 the body comprises a deformed region thereby defining the mechanical interface.    
   
   
       11 . The rivet of  claim 1 , wherein: 
 the mandrel comprises a deformed region thereby defining the mechanical interface.    
   
   
       12 . The rivet of  claim 1 , wherein: 
 the mandrel and the body have non-circular cross sections perpendicular to the axis of rotation defining a non-circular annulus such that the outer surface of the mandrel engages the inner surface of the body in response to rotation of the mandrel thereby defining the mechanical interface.    
   
   
       13 . The rivet of  claim 1 , wherein: 
 the shaft has an effective outside diameter equal to or greater than the effective inside diameter of the shank at a process temperature greater than room temperature thereby defining the mechanical interface.    
   
   
       14 . The rivet of  claim 1 , wherein: 
 the mandrel and body comprise copper, titanium, iron, or any alloy comprising at least one of the foregoing.    
   
   
       15 . The rivet of  claim 1 , wherein: 
 the mandrel comprises medium carbon steel or high carbon steel.    
   
   
       16 . The rivet of  claim 1 , wherein: 
 the mandrel comprises a medium carbon steel having equal to or greater than about 0.29 weight % carbon and equal to or less than about 0.53 weight % carbon that is quenched to form a martensite microstructure and is then tempered at a temperature of equal to or greater than about 450 deg-C. for equal to or greater than about 30 minutes.    
   
   
       17 . The rivet of  claim 16 , wherein: 
 the mandrel is tempered at a temperature of equal to or greater than about 500 deg-C. for equal to or greater than about 30 minutes.    
   
   
       18 . The rivet of  claim 1 , wherein: 
 the mandrel comprises a material that undergoes a change in tensile strength in response to the friction stir process that is equal to or less than about 5% change.    
   
   
       19 . The rivet of  claim 18 , wherein: 
 the mandrel comprises a material that undergoes a change in tensile strength in response to the friction stir process that is equal to or less than about 1% change.    
   
   
       20 . A method of joining upper and lower workpieces fluidly bondable at a point of engagement, comprising: 
 positioning a friction stir rivet at the point of engagement of the workpieces, the rivet comprising: a body having an elongated cylindrical shank, a cap at a first end, and an axial hole therethrough; a mandrel having an elongated shaft defining an axis and a head at one end thereof, the head having an effective outside diameter greater than the effective outside diameter of the shaft, the shaft being disposed within the axial hole of the body, the mandrel head being disposed at an opposite end of the body to that of the cap, and the end of the mandrel head having a flat surface oriented substantially perpendicular to the axis of the shaft; and, a mechanical interface between the body and the mandrel such that the body rotates in response to rotation of the mandrel;    rotating the mandrel about its rotational axis, driving the rivet toward and into the workpieces such that resultant frictional heating between the rivet and the workpieces causes the materials of the workpieces to soften at a process temperature thereby providing a friction stirred displaceable path for the rivet to traverse, and driving the rivet along the displaceable path until the cap is seated against the workpieces;    stopping further rotation of the mandrel and allowing the workpieces and mandrel to cool below the process temperature, thereby permitting the softened workpieces to harden; and    axially loading the mandrel with sufficient force to drive the mandrel head into the end of the body thereby upsetting and expanding the body end to create an interference between the body and the underside of the lowermost workpiece and to cause the mandrel shaft to sever at a point internal to the body and proximate the cap, thereby resulting in the workpieces being held together by the fluidly bonded materials of the workpieces, the differential thermal contraction of the workpieces and the rivet, and the mechanical loading between the mandrel and the body, at the point of engagement.    
   
   
       21 . The method of  claim 20 , wherein the rotating comprises: 
 rotating the mandrel at equal to or less than about 12,000 revolutions per minute.    
   
   
       22 . The method of  claim 20 , wherein the driving comprises: 
 driving the rivet at a rate equal to or greater than about 6 millimeters per minute and equal to or less than about 150 millimeters per minute.    
   
   
       23 . The method of  claim 20 , further comprising: 
 holding the mandrel rotation at about 12,000 revolutions per minute for about 2 seconds subsequent to the cap being seated against the workpieces.    
   
   
       24 . The method of  claim 20 , wherein the resultant frictional heating is initiated by the friction stir interaction between the flat surface of the mandrel and the workpieces.  
   
   
       25 . The method of  claim 20 , wherein the flat surface extends over equal to or greater than about 80% of the effective diameter of the end of the mandrel head, thereby displacing the material along the displaceable path in such a manner as to reduce the tendency for the displaced material to penetrate the region between the workpieces as the rivet is driven into the workpieces.  
   
   
       26 . The method of  claim 25 , wherein the flat surface extends over about 100% of the effective diameter of the end of the mandrel head.  
   
   
       27 . The method of  claim 20 , wherein the driving the rivet into the workpieces comprises: 
 driving the rivet absent a preexisting hole in the workpieces.    
   
   
       28 . The method of  claim 20 , wherein the causing the materials of the workpieces to soften at a process temperature comprises: 
 causing the materials to soften at a process temperature that is substantially lower than the melting temperature of the rivet.    
   
   
       29 . The method of  claim 20 , further comprising: 
 driving the rivet along the displaceable path until the underside of the cap is in loaded contact with one of the workpieces; and    friction stirring and partially penetrating the cap into the one of the workpieces.