Flash Welding Process | Flash Butt Welding Of Rails | Flash And Upset Welding

Flash welding:

01-flash butt  welding process

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.

01-flash welding - electrical resistance welding01-Flash Butt Welding - high frequency resistance welding

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)

01- Mobile flash butt Welding - flash butt welding of rails

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

01-flash butt welding of rails

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.

Welding Mechanical Engineering | Resistance Projection Welding | Projection Welding Over Spot Welding

Projection Welding:

The projection welding process is similar to spot welding except that the welding pressure, and welding current. Hence the welding heat are localized by making projection or embossments on one or both of the work pieces to be joined. Such projections are made at all points where a weld spot is desired.

01-projection welding

The projections have a diameter on the face equal to about the thickness of the stock and extend about 60 percentage of the stock thickness above the stock.

Operation:

The welding current is passed through the joint, welding heat is generated at these projections. Under the welding pressure the projections flatten allowing the two surfaces to be joined to come together. The melted projection becomes the weld.

01-projection welding machine - resistance projection welding process

The number of projections made in a joint should permit proper contact between the work pieces at the projections. The ideal number is three as the two sheets will always be in contact at three points. The maximum number of projections that can be satisfactorily handled is about six.

Special attention must be paid to the selection of correct pressing force at the beginning of the welding process. Use of excessive force causes the projection to collapse before the weld pool is created, which increases the contact surface and reduces current density. Variation in tensile strength of the workpiece may make welding more difficult, because it may result in projections of different sizes, in addition to which they flatten in different ways during welding.

When welding several projections at the same time, problems may occur in the heat balance of the joint or in the flattening of the projections. Problems can often be avoided by increasing the distance between projections. The recommended distance is four times the diameter of the projection.

Welding soft materials may be difficult if the workpiece thickness is less than 0.50 mm, because projections may collapse before welding current is applied.

For a successful projection weld the projections made on the parts should have the following characteristics:

1. The projections should be stiff enough to take the squeeze force before the current is passed.

2. The projections should have sufficient mass to heat a spot in the plane surface to welding temperature

3. The projections should collapse during welding without splashing between the sheets being welded.

4. The projections should be properly formed without any partial shearing.

5. It should be possible to form the projections without disturbing the other portions of the component.

01-projection welding fixtures - projection welding gun

Advantages of Projection Welding:

1. More than one spot weld can be made in a single operation, so the operation is very fast.

2. Welding current and pressure required is less

3. It helps in obtaining a satisfactory heat balance in welding of difficult to weld combinations of metals and thickness.

4. Closer spacing of welds is possible

5. Electrodes can be shaped to act as assembly fixtures for mass welding of parts

6. Uniform welds with good finish are produced.

7. Suitable for automation

8. Filler metals are not used. Hence clean weld joints are obtained

Disadvantages of projection welding:

1. Projections cannot be made in thin work pieces.

2. Thin work pieces cannot withstand the electrode pressure

3. Additional operation is required after the welding process is over.

4. Equipment is costlier

Applications of projection welding:

01-projection welding nuts - projection welding electrodes

1. A very common use of projection welding is the use of special nuts that have projections on the portion of the part to be welded to the assembly. Also, used for welding parts of refrigerator, condensers, refrigerator racks & grills, bushings, studs, nuts, handles etc..