Flash butt welding
Flash butt welding is a type of resistance welding without using any filler metal. It is used for joining two metal parts together using heat and force. Each of the two parts to be joined are clamped against an electrode, usually a copper alloy. The electrodes themselves being connected to the secondary side of a transformer. The ends are brought slowly together until they just touch. At this point a high current flows through the touching points, rapidly heating and melting the metal at the points of contact. The molten metal is then expelled by its own rapid expansion. This part of the welding cycle is called the flashing and generally creates a spectacular shower of sparks. Voltage used is generally low (typically between 4 and 20 volts) but the current usually very high, often in the tens of thousands of amps. The heat generated raises the temperature of either side of the joint. Once the temperature is above the forging temperature (typically around 1,250°C for steel) the ends are rapidly pushed together with great force. The high speed expels any remaining molten metal and the high force generates enough pressure at the joint (around 90 megapascals for steel) to 'forge weld' the ends together.
THE DIFFERENT CYCLES USED
Pre-Heating
Sometimes an optional pre-heating operation is used to heat the joint area using Joules energy from passing a current through the joint area directly without any flashing. This heats the whole area between the electrodes. Pre-heating itself is sometime preceded by a 'Burnoff' cycle which prepares the ends for better contact in the pre-heating cycle.
Flashing
An important parameter is to attain the proper forging temperature around the joint. This is generally dependent on the amount of heat generated by the flashing cycle. Flashing is typically determined by either a pre-set amount of time (seconds) or a pre-set distance to be flashed away (mm). Whereas pre-heating will heat the whole joint area that is between the electrodes, flashing will instead generate heat only at the joint interface. Heat will then be conducted back into the joint parent metal.
Butting a.k.a. Upsetting
Once the correct temperature profile has been generated then the ends of the joint are rapidly forced together. Initially a high velocity 'squirts' out any impure molten metal at the joint interface before it solidifies. Further movement as the joint cools swells the joint area through an upsetting action until the pressure at the joint, along with the temperature is sufficient to 'forge-weld' the ends together.
Hold Time
At the end of upsetting there is commonly a 'hold time' during which the joint is held still to allow the joint to cool and the two pieces of metal to completely bond.
Applications
Railway Lines (Flash butt welding machines are often transported to the work site on a road-rail vehicle)
Chains
Steel wheels
Sheets or rods of steel in rolling mills
Starter Rings
Busbar
See also
References
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