Flash butt welding is one of the resistance welding processes employed to join metals. In flash butt welding process, the ends of the piece to be welded are connected to the secondary circuit of a transformer, while one piece is held firmly by a clamping device attached to a stationary platen; the other piece is clamped to a movable platen. The platen travel is continuous starting at the time of flashing and progressing until upset. At upset period the platens are rapidly squeezed together for upsetting, the current may be immediately terminated. The material being joined is clamped rigidly in the dies, and the specimens are separated by a suitable air gap. Then the movable platen is advanced slowly until contact is made.
The surfaces to be welded are allowed to touch when heavy currents pass through the peaks or asperities of the edges providing resistive heat (many short-circuits randomly located over the opposing interfaces) to the edges. This portion of the process is known as the flashing period, its objective being the establishment of a suitable temperature distribution in the work to assure proper forging action during the subsequent upset period of the cycle. These asperities start melting and, at greater velocities, the molten bridges are broken and thrown off as flash particles from joint.
This cycle of the formation and collapse of bridges goes on as the movable platen advances. When the conductive heat was sufficiently heated the metal behind the faying surfaces on either side to ensure adequate plasticity, the flashing current is stopped and surfaces are butted against each other at greater force. This portion of the operation is known as the upset period. This action ensures that the molten metal oxides and other impurities are extruded out of the surfaces to be joined and satisfactory welding takes place.
Basic components of the machine:
1. Clamping Mechanism
2. Forging Mechanism
3. A transformer
(This will reduce the mains supply voltage from 400/500 Volts to a suitable welding voltage between 4 and 12 Volts and make available sufficient current to heat the components being welded. The welding current required varies between 30,000 amps to 80,000 amps depending on cross sectional area of the rail being welded)
History of Flash Welding:
Flash butt welding technique spread too many countries during the 1930s but much of this development work came to a standstill during the war years particularly in the U.K. and on the Continent. However, by 1950 the flash butt welding of rails was common place in all major railroads throughout the world.
Features of Flash Welding:
Basic Metallurgy – Forging Operation
Heat Affected Zone – 40 – 60 mm
Nominal welding transformers power – 600 kVA
Upsetting and stripping force – 800 kN
Typical welding cycle within – 150 s
Advantages of flash welding:
1. Joint obtained is clean, as filler metal is not use in this process.
2. Produces defects free joint. Oxides, scales and other impurities are thrown out of the weld joint due to high pressure applied at elevated temperature.
3. Reduces maintenance costs
4. Faster installation
5. Lowest life cycle cost
6. Saves track time
7. Eliminates corrugation
8. No weld filler material
9. Smaller heat affected zone
10. Smaller annealed zone
11. Consistent hardness
12. Highest fatigue resistance
13. Average life equal to the rail
14. 25% savings over Thermite welding
15. Large cross sectioned shape materials can be welded in a short time
Disadvantages of flash butt welding:
1. The process is suitable for parts with similar cross sectional area
2. Joint preparation is must for proper heating of work pieces to take place
Applications of Flash Butt Welding:
1. Used for producing joints in long tubes and pipes
2. Flash butt welding is widely employed in the automotive, air craft, and several other engineering industries. Some examples of its use on wheel rims for automobiles, long welded rails, etc.