The atoms of metals are held together by the metallic "bond" peculiar to metals. This 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. Thus, 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, effects a bond and therefore a weld.

Early Applications

In practice, however, bonding is virtually impossible under most conditions because of surface irregularities, organic surface contamination, and chemical films such as an oxides. For maximum weld efficiency, any form of contamination must be reduced to a minimum, while the area of contact (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 made in a single step. This technique had several disadvantages. The ends to be joined had first to be squared off. Both surfaces had to be kept free of contamination. And the amount of material projecting from the gripping die promoted bending at the same time that it hampered coaxiality, spoiling the flow of metal.

The Multi-Upset Technique

The system of butt welding developed by G. E. C. employs the "multi-upset principle." Here, each time the machine is activated the material inserted into the die 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. Oxides and other surface impurities are forced outward from the core of the material, and a bond effected. A minimum of four upsets is recommended to ensure that 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 and no gap setting to be determined, made as these are built into the die. Nor must any spring pressure be set. Any one of these, incorrectly set on a resistance butt welder, would result in a weld failure. The Metals Treatable by Cold Pressure Welding Cold pressure welding is restricted to non-ferrous materials, or to soft iron that has no carbon content. Most non-ferrous metals can be cold welded. While copper and aluminium are the most common, various alloys such as Aldrey; Tripple 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 popular M101 model can be either bench or trolley mounted for convenience. Manually operated, the M101 welds wire or strip measuring 1.00 to 3.60mm Ø (copper) and 1.00 to 5.00mm Ø (EC aluminium)

Welding Dissimilar Metals

The usual methods of joining such dissimilar metals as copper and aluminium - resistance welding, friction welding, flame brazing - will all result in a rapid breakdown of the joint. This breakdown or reaction in a copper/aluminium joint begins to take place as soon as the two metals are placed together. The problem lies with the oxides and the air space left between the interfaces during these methods of welding, not the dissimilarity between the metals themselves. But with cold pressure welding, these oxides and air spaces are squeezed out in the weld process. Since no heat is applied, only the metallurgical changes that take place at ambient temperatures occur.

Cold pressure welding provides the most satisfactory way of joining copper to aluminium without the formation of brittle intermetallic compounds. The quality is excellent because it produces a worked structure, as opposed to the cast structure obtained in fusion welding. Furthermore, there is no heat-affected zone with unsuitable properties. To test weld strength, most operators rely on the tensile tester. An alternative is a reverse bend test. The most stringent test, though, is to pass the weld through a number of dies in a wire drawing machine.

The Importance of Dies

Dies play an important role in the cold butt weld process. Firstly, they must grip the material firmly. To assist the grip of the die, the inside of the cavity is either etched with an electric pencil or, when the die is to be used for welding large sizes of aluminium, grip marks are put into the cavity before the die is heat treated. The gap between the two faces, or noses, of the die is also extremely important. Too large, and the material will just collapse or bend away. This dimension is imposed during manufacture and cannot be changed.

Also to be considered is offset of the die noses. This 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, thus ensuring that complete flash removal is a simple matter. The hardness and temper of the die are very important, as well. In the early days of cold welding, die breakage was very common. Even long after a machine was designed to weld 8mm copper rod, problems would arise in containing the necessary forces within a die of this size.

The free-standing heavy duty P1500 rod welder is an electro/hydraulic model capable of welding copper rod up to 25mm Ø and aluminium rod up to 30mm Ø

Welding Different Profiles and Diameters

Dies can be manufactured to suit various profiles, so long as the profile allows the die to be made in two halves (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 diameters together. Generally, the larger diameter should not be more than 30 per cent greater than the smaller. In practice, if the copper is considerably smaller in diameter than the aluminium, the copper will merely embed itself into the aluminium and no weld be achieved.

About Pressure Welding Machines

The British company Pressure Welding Machines Limited (PWM), founded in 1984, is a world market leader in the design and manufacture of cold pressure welding machines and dies. PWM's specialist staff have over 30 years' experience in the cold weld technique, and the company's research and development programme ensures that it continues to offer a comprehensive range of cold pressure welders. Additionally, PWM's network of experienced agents ensures fast and efficient service to the wire and cable industry world-wide, offering full back-up and after-sales service on all PWM products.

The PWM Product Range PWM's range includes cold welders suitable for a wide variety of applications, from lightweight hand-held models for welding fine wire and strip up to heavy-duty rod welders with capacities of up to 30mm in diameter. The four hand-held welders in the range will accommodate welding requirements from 0.08mm to 2.00mm copper/aluminium in either wire or strip form. The M10, M10 Superfine, and M25 and M30 models are all manually operated. The pneumatic HP30, which provides an effortless weld on wire or strip, is portable within the limits of the air supply. All are extremely reliable and easy to operate. PWM's bench-mounted welders, the BM10 Superfine, BM10, and BM30, also manually operated, have welding capacities between 0.08mm and 1.80mm copper/aluminium.

The two lengths of the wire to be welded are placed into the die

Other popular models, the HP100 (air/hydraulic) and P101 (pneumatic), have capacities of 1.00mm up to 5.00mm aluminium/copper and are trolley-mounted. The M101 is a manual bench machine but can be supplied trolley-mounted.

At the top of the range, the EP500 and the P1500 rod welders are heavy-duty free-standing machines capable of cold-welding aluminium/copper up to 30mm in diameter.

Latest Developments

PWM's full range of cold welders can be seen in action at Wire 2002 in Düsseldorf (Stand 9 B 41) in April. A new mobile air/hydraulic welder will be launched at the show, and visitors will have an opportunity to watch demonstrations of the cold welding technique and be able to assess its benefits for themselves.