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→Composite Contact Rivets
Clad rivets, for which only a part of the head (composite or bimetal rivets) or also the shank end (tri-metal rivets) are composed of contact material – with the balance of the body mostly being copper – have replaced the solid rivet for many applications because of economical considerations. The cost savings depend on the contact material and its required volume for a specific application. These composite rivets are also produced scrap-less from wire material on special machinery with two process variations utilized.
During ''cold bonding'' the process of heading the bond between the contact material and the copper is achieved without external heat energy by high plastic deformation at the face surfaces of the two wire segments (<xr id="fig:Cold_bonding_of_bimetall_rivets"/><!--(Fig. 3.1)-->). <figure id="fig:Cold_bonding_of_bimetall_rivets">[[File:Cold_bonding_of_bimetall_rivets.jpg|right|thumb|Figure 4: Cold bonding of bimetall rivets (schematic)]]</figure> The bonding pressure must be high enough to move the lattice components of the two metals within a few atom radii, so that the adhesion forces between atoms become effective.
Therefore the head to shank diameter ratio of 2:1 must be closely met for a strong bond between the two metals.
During ''hot bonding'' the required heat energy is applied by a short term electrical current pulse (<xr id="fig:Hot_bonding_of_bimetal_rivets"/> <!--(Fig. 3.2)-->). <figure id="fig:Hot_bonding_of_bimetal_rivets">
[[File:Hot_bonding_of_bimetal_rivets.jpg|right|thumb|Hot bonding of bimetall rivets (schematic)]]
</figure> In the case of Ag and Cu a molten eutectic alloy of silver and copper is formed in the constriction area between the two wire ends. When using metal oxide containing contact materials, the non-soluble oxide particles tend to coagulate and the bonding strength between the component materials is greatly reduced. Therefore the cold bonding technology is preferred for these contact materials. During cold bonding, the required high surface deformation ratio can be reduced for the hot bonding process which allows the head to shank diameter ratio to be reduced below 2:1.
For composite rivets with AgPd alloys, as well as alloys on the basis of Au, Pd and Pt, the methods named above cannot be used because of the very different work hardening of these materials, compared to the base material copper. The starting material for such composite rivets is clad strip material, from which the contact rivets are formed in multiple steps of press-forming and stamping. Similar processes are used for larger contact rivets with head diameters > 8 mm and Ag-based contact materials.
*Typical contact shapes for composite rivets (<xr id="fig:Typical_contact_shapes_for_composite_rivets"/>)
<figure id="fig:Typical_contact_shapes_for_composite_rivets">
[[File:Typical_contact_shapes_for_composite_rivets.jpg|right|thumb|Typical contact shapes for composite rivets]]
*Base materials <br /> Cu <br />
*Dimensional ranges (<xr id="fig:Dimensional_ranges"/> ) <br />These parameters cannot be chosen independently of each other. They depend mainly on the mechanical properties of the contact material. Before specifying the final dimensions we recommend to consult with the contact manufacturer. <br />
<figure id="fig:Dimensional_ranges">
[[File:Dimensional_ranges.jpg|right|thumb|Dimensional ranges]]
