The Cold Welding Process by PWM
Cold welding close up
Cold welding is a form of solid phase welding, which is unique because it is carried out at ambient temperatures. Other forms of solid phase welding are carried out at elevated temperatures. The Egyptians prepared iron by hammering a metal sponge in order to weld the red-hot particles together and blacksmiths have also hammer welded wrought iron for centuries – but these types of welding are always carried out at high temperatures.
In 1724, a British clergyman, Reverend Desaguliers, managed to create a permanent bond between two lead balls without using any kind of heat, simply by twisting them together. His was the first successful British attempt at what is now known as cold pressure welding.
Today, cold welding accounts for a large proportion of all the welding processes within the wire and cable industry. Many wire and cable manufacturers prefer a welding process that is clean, quiet and ‘green’, and that enables them to join non-ferrous materials without heat, flux or fillers, saving costs and materials.
The cold weld process is straightforward, reducing the need for operator training. Small wire sizes can be joined in seconds using a manually operated cold welding machine; bonding large rod sections takes a little longer and requires the use of a powered machine - but these are far more energy efficient than using an electrical flash butt welder. Cold welding produces permanent welds stronger than the parent material, without loss of electrical integrity, and can be used to join strip and profile as well as round wire. Most non-ferrous materials can be cold welded, as well as various alloys, and the process can also be used to join dissimilar metals, such as aluminium to copper.
Multi upset principle
Today’s cold pressure welding technique employs a ‘multi upset’ principle, perfected by a British company, GEC. When the material is inserted in the die, each time the machine is activated, the material is gripped by the die and fed forward. As they are pushed against each other, the two opposing faces of the material are stretched and enlarged over their entire surface area. The oxide and other surface impurities are forced outward from the core of the material and a bond is effected. A minium of four ‘upsets’ is recommended to ensure all impurities are squeezed out of the interfaces.
Simple set up
The ends of the wire or rod do not need any preparation prior to welding and the alignment of the two butt ends happens automatically as the material is placed in the die. There is no heat setting to be arrived at, no gap setting to be made (this is built into the die) and no spring pressure to be set. Any one of these things set incorrectly on a resistance butt welder would result in a weld failure.
How a cold weld happens
The currently accepted hypothesis that accounts for a cold weld taking place is as follows:
The atoms of metals are held together by a metallic ‘bond’, so called because it is peculiar to metallic substances. The bond can be described as a ‘cloud’ of free negatively charged electrons, enveloping ionised positively charged atoms into a unit as a result of attractive forces.
So, if two metallic surfaces are brought together with only a few angstroms separation (there are 300 million angstroms to one centimetre), interaction between the free electrons and ionised atoms can occur. This will eliminate the potential barrier, allowing the electron cloud to become common. This, in turn, results in a bond and therefore a weld.
A simpler way of explaining this rather complex process is that if two surfaces (both surfaces being atomically clean and atomically flat when considered on an atomic scale) are put together under pressure, a bond is effected equal in strength to that of the parent material.
To help wire and cable manufacturers understand the cold weld process, British company PWM (Pressure Welding Machines) has developed a series of online video demonstrations, which can be viewed at www.pwmltd.co.uk. The videos show large cold welders for joining rod 5mm to 30mm (.197" to 1.181”) in action, as well as smaller machines, the HP200 and M101, for bonding wire 2mm to 6.50mm (.079” to .256”) and 1mm to 5mm (.040” to .197”) respectively.
PWM, which specialises in the design and manufacture of cold pressure welding machines and dies, has been supplying high-performance cold welding equipment to wire and cable manufacturers worldwide for over 30 years. The PWM product range includes hand-held, bench and trolley-mounted machines, as well as large free-standing models for cold welding large rod sections. Both manual and powered cold welding machines are available, with capacities ranging from 0.10mm (.0039”) up to 30mm (1.181”).
To ensure total accuracy, all PWM cold welding machines and dies are designed and made in-house by PWM’s own team of skilled engineers to stringent quality standards. The dies, which play a major role in the cold weld process, are individually hand-made in matched sets. They can be made in industry standard sizes or custom made to suit individual applications.
Joe Snee Associates Inc. is the exclusive USA and Canadian distributor for PWM: email@example.com.