EoW July 2010

Straightening, cutting & welding

Precision under pressure

Multiple upset principle A buttwelding system employing what is

First identified in 1724, cold pressure welding has many advantages over electrical welding. The cold weld process is faster, cleaner and more energy efficient, and creates a reliable, permanent weld stronger than the parent material without loss of electrical integrity. Cold welding is a form of solid phase welding, unique because it is carried out at ambient temperatures. Other types of solid phase welding, such as the hammer welding of wrought iron carried out by blacksmiths for centuries, are conducted at elevated temperatures. However, although this involves high temperatures, the material is not molten, merely more ductile. How a weld is formed There have been several

known as the ‘multiple upset principle’ proved more effective. When the material is inserted in the dies, each time the machine is activated the material is gripped by the dies and fed forward. As the two opposing faces are forced against each other, they are stretched and enlarged over their entire surface area.

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.

This forces the oxides and other surface impurities outward from the core of the material and a metallurgical bond stronger than the parent material is effected. To ensure all impurities are forced out of the interfaces, four to six ‘upsets’ are recommended. Most non-ferrous metals can be cold welded as well as various alloys. The cold weld process can also be used to join dissimilar materials, for example, aluminium to copper, and materials of different sizes. High performance equipment PWM Ltd designs and manufactures high performance cold pressure welding machines and dies to suit a broad range of applications, from hand-held, manually operated welders up to electro-pneumatic and electro-hydraulic powered rod welders. Precision engineered to provide consistent, reliable welds and minimise materials wastage, PWM machines are very simple to operate and easy to maintain. PressureWelding Machines – UK Fax : +44 1233 820591 Email : pwm@btinternet.com Website : www.pwmltd.co.uk Flash photo ▲ ▲ : Close up of an aluminium weld showing the resulting flash where the excess material, and any impurities, has been squeezed out from the interfaces

Close-up weld ▲ ▲ : Close up of a cold weld on copper rod after deflashing and rubbing down ready for the drawing machine

The metallic bond The currently accepted hypothesis relates to the fact that the atoms of metals are held together by a metallic ‘bond’, so called because it is peculiar to metallic substances. This bond can be described as a ‘cloud’ of free negatively-charged electrons, which envelop ionised positively charged atoms into a unit as a result of attractive forces. So, if two metallic surfaces are brought together with a space of 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. A simpler way of explaining this process is to say that if two surfaces (both atomically clean and atomically flat when considered on an atomic scale) are put together, a bond is effected equal to the parent metal. In practice, however, bonding is virtually impossible under most conditions due to

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 has to be made as large as possible. Early attempts In early applications of cold pressure butt welding, the upset and radial displacement of the interfaces was undertaken in a single step. This was not very satisfactory, since the ends to be joined had first to be squared off and both surfaces had to be free of contamination. In addition, the amount of material projecting from the gripping dies could result in bending and lack of coaxiality, so spoiling the correct flow of metal.

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EuroWire – July 2010