Jim Read this no I'm not kidding!
Click here for an animation of the process
Cold pressure 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 conducted at elevated temperatures, but - although these temperatures are high - the material is not molten, merely more ductile.
As early as 3,000 BC, the Egyptians prepared iron by hammering a metal sponge in order to weld the red-hot particles together. Blacksmiths have also hammer welded wrought iron for centuries. This type of welding was always carried out at high temperatures.
The first known example in Britain of hammer welding at ambient temperatures (therefore true cold pressure welding) dates back to the late Bronze Age, around 700 BC. The material used was gold, and gold boxes made by this process have been found during excavations.
DISCOVERY OF COLD PRESSURE WELDING
The electro/pneumatic EP500 rod welder is a heavy-duty machine that will weld nonferrous wire and strip from 5.00mm (.197") up to 12.50mm (.492").
The first scientific observation of cold pressure welding was made in 1724 by the Reverend J I Desaguliers. He demonstrated the phenomenon to the Royal Society and later published the details in the scientific journals of the time. Rev Desaguliers discovered that if he took two lead balls about 25mm each in diameter, pressed them together and twisted them, then the two pieces would join together. The joint strength was measured on a steelyard and although the results were erratic, good bonds were produced, with some as strong as the parent material.
After Rev Desaguliers' discovery in the 18th century, it appears that very little happened until the Second World War. This accelerated developments, especially in Germany, where light alloy cooler elements for aircraft were pressure welded, although it is understood that this welding was carried out at elevated temperatures.
A MAGICAL PROCESS
what-weldmat
Lengths of welded copper/aluminium rod
Seen for the first time, cold pressure welding can appear an almost magical process. People unfamiliar with it are often reluctant to accept a method of welding that does not involve heat or electricity and some form of flux to make the joins. After a demonstration, they inevitably ask, "How are the two pieces of metal joined?"
There have been several explanations as to the actual mechanism by which a cold pressure weld is obtained. For example, it has been suggested that it happens via recrystallisation or by an energy hypothesis, but most explanations have been either experimentally disproved or refuted on theoretical grounds.
The currently accepted hypothesis that accounts for a cold pressure weld taking place involves the atoms of metals being held together by the metallic 'bond', so called because it is peculiar to metallic substances. The bond can be described as a 'cloud' of free, negatively charged atoms into a unit as a result of attractive forces.
CREATING A WELD
Therefore, if two metallic surfaces are brought together with only a few angstroms separation, there being 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 awesome process is that if two surfaces are put together, both being anatomically clean and anatomically flat when considered on an atomic scale, a bond is effected equal to that of the parent material.
EARLY APPLICATIONS
The M10 hand-held, manually operated welder will join fine wire from 0.10mm (.0039") to 0.50mm (.0196")
In practice, however, bonding is virtually impossible under most conditions, because of surface irregularities, organic surface contamination and chemical films such as oxide films. In order to obtain maximum weld efficiency, any form of contamination must be reduced to a minimum, while the area of contact, the weld area, has to be made as large as possible.
In earlier applications of cold pressure butt welding, the upset and radial displacement of the interfaces was undertaken in a single step. This technique had several disadvantages: it was necessary to square off the ends to be joined; both surfaces had to be kept free of contamination; and the amount of material which projected from the gripping die was such that bending and lack of coaxiality could occur, thereby spoiling the correct flow of metal.
THE MULTI UPSET PRINCIPLE
Then came the system of butt welding developed by GEC, employing what is known as the 'multi upset principle'. When the material is inserted in the die, each time the machine is activated, the material is gripped by the die and fed forward.
In this way, the two opposing faces are stretched and enlarged over their entire surface area as they are pushed against each other. The oxide and other surface impurities are forced outward from the core of the material and a bond is effected. A minimum of four upsets is recommended to ensure all impurities are squeezed out of the interfaces.
The advantages of this type of welding are easily seen in practice. The ends of the wire or rod need no preparation prior to welding and the alignment of the two butt ends is automatic as the material is placed in the die. There is not heat setting to be arrived at, no gap setting to be made, as this is built into the die, and no spring pressure to be set. Any one of these things incorrectly set on a resistance butt welder would result in a weld failure.
SUITABLE METALS
what-weld
A cross section of a welded area, showing 0.315" (8mm) diameter copper rod joined to 0.374" (9.5mm) diameter aluminium rod
Cold pressure welding is restricted to nonferrous materials or, at best, soft iron that has no carbon content. Most nonferrous metals can be cold welded, and while copper and aluminium are the most common, various alloys such as Aldrey, Triple E, Constantan, 70/30 brass, zinc, silver and silver alloys, nickel, gold and many others have good cold weldability. Plated wires, including tinned copper, silver plated and nickel-plated, can all be welded to themselves or to plain copper.
The usual methods of joining dissimilar metals such as copper and aluminium, namely resistance welding, friction welding or flame brazing, will all result in a rapid breakdown of the joint. This reaction in a copper/aluminium joint begins to take place as soon as the two metals are placed together.
The problem is created by the oxides and the air space, which are left between the interfaces during these methods of welding, rather than by the dissimilarity between the metals themselves. However, with cold pressure welding, these oxides and air spaces are squeezed out in the weld process and, since no heat is applied, only the metallurgical changes than operate at ambient temperatures occur.
Cold pressure welding provides the most satisfactory way of joining copper to aluminium without the formation of brittle inter-metallic compounds. The quality is excellent because it produces a worked structure as opposed to the cast structure obtained in fusion welding. Also, there is no heat-affected zone with unsuitable properties.
To test weld strength, most people rely on a tensile tester. Alternatively, you can make a reverse bend test. However, the most stringent test is to pass the weld though a number of dies in a wire drawing machine.
