What is cold pressure welding?

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 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. Reverend 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 Reverend 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.  

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 formed 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

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 no 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

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 that 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.

The role of dies

The dies play an important role in the cold butt weld process. Firstly, they must grip the material firmly and, therefore, the inside of the cavity is either etched with an electric pencil or, when the die is to be used for welding large pieces of aluminium, grip marks are put in the cavity before the die is heat treated.

The gap between the two faces, or noses, of the die is also extremely important. If it is too large, the material will just collapse or bend away. This dimension is taken care of during manufacture and cannot be changed.

Finally, there is the offset of the die noses, which has the effect of making the weld look out of line around the circumference of the material. The purpose of the offset is to break the flash into two halves, so that removal is easy: otherwise the flash is likely to remain as a loose ring around the material and have to be cut off. The noses of the die also have to be sharp enough to virtually pinch off the flash around the weld, again to ensure that the complete flash can be easily removed.

The hardness and the temper of the die are most important as well. In the early days of cold welding, die breakage was very common and long after a machine was designed to weld 8mm copper rod, there were problems in containing the necessary forces within a die of this size.

PWM has been producing dies to extremely high standards and tolerances for over 40 years. As wire technology has improved, so has the demand for precision. PWM’s on-going research and development programme has enabled it to produce dies that are capable of joining extremely fine wire. PWM was the first company outside the USA to develop a die that could be used in conventional cold welders to join wire as fine as 0.08mm (0.003145″) in diameter. Individually handmade in matched sets to the highest possible tolerances by skilled craftsmen, PWM’s industry standard dies can now be produced for wire sizes between 0.08 (0.003145″) and 6.50mm (0.256″). Dies can also be manufactured to suit round or profile wires and rods according to customers’ specifications.

PWM dies can also be manufactured to suit various profiles, as long as the profile allows the die to be made in two halves – which is necessary for the removal of the welded wire – and the cross-sectional area is within the capacity of the machine.

It is also possible to weld two different wire sizes together. Generally, the larger diameter should not be more than 30% greater than the smaller. If the copper is considerably smaller in diameter than the aluminium, the copper will merely embed itself into the aluminium and no weld will be achieved.