In the production of electrical contacts, the move from solid precious metal rivets to bimetal rivets has been one of the most significant cost-saving innovations of the last century. Bimetal rivets, which feature a silver or silver-alloy head bonded to a copper shank, provide the necessary electrical performance at the contact point while utilizing cheaper, highly conductive copper for the bulk of the component. The heart of this manufacturing process is “Cold-Heading”—a high-speed, high-precision technique that ensures both structural integrity and material efficiency.
What is Cold-Heading?
Cold-heading, also known as cold forming, is a forging process where metal wire is shaped at room temperature using a series of dies and punches. For bimetal rivets, the process involves two different wires: a precious metal wire (like AgNi or AgSnO2) for the contact face and a copper wire for the shank and base.
The process begins with precisely cutting “slugs” from both wires. These slugs are then placed into a die. A high-pressure punch strikes the silver slug, forcing it to bond with the copper slug while simultaneously shaping the rivet head.
The Secret to a Perfect Bond
The most critical aspect of a bimetal rivet is the bond between the silver and the copper. If the bond is weak, the rivet can delaminate under the thermal stress of electrical switching, leading to catastrophic failure of the switch or relay.
Cold-heading achieves a “cold weld” or molecular bond through extreme pressure. As the punch deforms the metals, the surface oxide layers are broken, allowing the atoms of the silver alloy and the copper to intermingle at the interface. This creates a bond that is often stronger than the parent metals themselves. Advanced cold-heading machines use multi-station dies to gradually shape the rivet, ensuring that the precious metal layer is evenly distributed across the contact face.
Material Utilization and Cost Efficiency
The primary driver for using bimetal rivets is cost. By using a silver-copper bimetal structure, manufacturers can reduce the precious metal content of a rivet by 50% to 80% without any loss in electrical performance.
Cold-heading is an “additive” process in terms of shape—meaning it moves material rather than removing it. Unlike machining (turning), which produces significant scrap, cold-heading has a material utilization rate of nearly 100%. This is particularly vital when dealing with silver, where every milligram of scrap represents a direct hit to the bottom line.
Structural Integrity and Precision
Beyond cost, cold-heading improves the mechanical properties of the rivet. The process of cold-working the metal increases its hardness and tensile strength through work-hardening. This results in a rivet that is more resistant to the mechanical shocks of high-speed relay operations.
Modern cold-heading equipment can produce bimetal rivets at speeds of up to 400 pieces per minute with tolerances as tight as ±0.02mm. This level of precision is essential for automated assembly lines where rivets are fed into stamping machines to be crimped onto contact bridges.
Conclusion
The cold-heading process for bimetal rivets is a masterclass in manufacturing efficiency. It combines the science of metallurgy with high-speed mechanical engineering to produce components that are both high-performing and economical. For industries ranging from automotive to home automation, cold-headed bimetal rivets remain the standard for reliable electrical switching.
