Cold pressure welding
by Dave Nicholas
Introduction
All metals are surrounded by surface layers (oxide) which must be disrupted if they are to be welded. Cold pressure welding, carried out at ambient temperature, relies upon the use of high compressive pressures[1] (1400-2800N/mm2 for aluminium and at least double that value for copper). This provides interfacial deformations of 60% to 80% that break the oxide layers to expose fresh,
uncontaminated metal that makes contact. In this state, interatomic forces take over to produce the weld.
Materials welded
This joining method has been used for thousands of years and is applicable to most of the ductile metals and their alloys, such as copper, aluminium, lead, zinc, nickel, silver, platinum, gold, palladium, and cadmium. Published literature indicates
that variations of the methods have been used in production for joining like and dissimilar metal combinations for at least 40 years.
Joint types
The process can be applied to both lap and butt configurations[2]. Butt joints are primarily used for joining wires and rods in diameters from ~0.5mm up to 12mm, depending upon the metal being
joined. If the single butt (pressure) approach is utilised then the surfaces to be joined must be thoroughly cleaned. However, greater tolerance to the weld surface condition is possible if a multiple butt technique (that is, multiple pressure
applications) is used.
Where the application demands the joining of lapped sheets to themselves or bars, a series of small welds can be used. When calculating strength, the designer should consider that at least half the thickness of one of the sheets will be lost due
to the applied pressure.
It is also possible to make use of drawing processes[2] in which multilayer tubes or solid multilayer (clad) bars can be cold pressure welded. Similarly, roll bonding of metals is also
possible[3]. Specialist equipment manufacturers supply various forms of cold pressure welding systems.
Advantages and limitations
As the process is performed at ambient temperature, there are no thermal effects on the parts being joined, and the process is fast. It is simple and inexpensive to operate once dies have been produced. However, it is highly specialised with respect
to joint design and materials to be welded. As the welds are made in the 'solid state' they are difficult to inspect; thus reliance must be placed on process control. With the exception of butt welds, or welds where the contact surfaces are
sheared together, the thickness of the parts is reduced significantly at the weld.
Applications
Perhaps the greatest use of cold pressure welding has been for joining of wire, foil to wire, wire to bi-metals, and sealing of heat sensitive containers such as those containing explosives (detonators for example). Rod coils are butt welded to
permit continuity in post-weld drawing to smaller diameters. In the electronics industry, cold welding processes are used to seal tin plated steel crystal cans and copper packages for heat sensitive semiconductor devices. Glass packages are also
sealed using an indium or tin alloy interlayer.
An interesting application of the process is underground wire servicing where joins need to be made in hostile environments, such as in the presence of explosive gases.
Further information
Use SEARCH to identify other relevant information and knowledge. TWI Industrial Members have unrestricted access to all TWI content.
You can use the Weldasearch literature database to supplement what you find in JoinIT.
References
1. 'Cold welding', Welding Handbook, 8th Edition, Volume 2, Welding Processes, American Welding Society, 1991, pp 900-908.
2. K Thomas, 'Cold Welding', ASM Handbook, Volume 6, Welding, Brazing and Soldering, 1993, pp 307-310.
3. M G Nicholas and D B Milner, 'Roll Bonding of Aluminium', British Welding Journal, 1962, Volume 9, No. 8, August, pp 469-475.
|
