Changes

Clad rivets for which only a part of the head (composite or bimetal rivets) or alsothe shank end (tri-metal rivets) are composed of contact material – with thebalance of the body mostly being copper – have replaced for many applicationssolid rivet versions because of economical considerations. The cost savingsdepend on the contact material and its required volume for a specificapplication. These composite rivets are also produced scrap-less from wirematerial on special machinery with two process variations utilized.

During ''cold bonding'' and heading the bond between the contact material andthe copper is achieved without external heat energy by high plastic deformationat the face surfaces of the two wire segments ''(Picture Cold bonding of bimetall rivets)''. The bonding pressuremust be high enough to move the lattice components of the two metals within afew 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 astrong bond between the two metals. (s. Picture "Cold bonding of bimetall rivets")

During ''hot bonding'' the required heat energy is applied by a short term electricalcurrent pulse ''(Picture Hot bonding of bimetall rivets)''. In the case of Ag and Cu a molten eutectic alloy ofsilver and copper is formed in the constriction area between the two wire ends.When using metal oxide containing contact materials the non-soluble oxideparticles tend to coagulate and the bonding strength between the componentmaterials is greatly reduced. Therefore the cold bonding technology is preferredfor these contact materials. The during cold bonding required high surfacedeformation ratio can be reduced for the hot bonding process which allows thehead 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 above methods cannot be used because of the very different workhardening of these materials compared to the base material copper. Thestarting material for such composite rivets is clad strip material from which thecontact rivets are formed in multiple steps of press-forming and stamping.Similar processes are used for larger contact rivets with head diameters > 8 mmand Ag-based contact materials. (s. Picture "Hot bonding of bimetall rivets")