FLASH
WELDING
Flash welding is a resistance welding
process in which joint is produced over the entire area of the abutting
surfaces simultaneously.
Flash
welding is used for joining many ferrous and non-ferrous alloys except for alloys
of cast iron, lead, zinc, antimony and bismuth. Flash is produced during the
process hence it is named as flash welding.
PRINCIPLE
In this method two metals are butted
together and current is passed through the joint which heats the ends to red
hot temperature. The metals are then separated till an arc is developed between
them and the metals begin to melt. The metals are butted under high pressure
with the current still flowing.
EQUIPMENT
The flash welding system consists of
(a) The main frame to which the welding
machine is attached. On this a fixed platen and movable platen is attached.
(b) Movable platen mounted on the
machine
(c) Two clamps hold the job
(d) Welding transformer
(e) Controls for controlling the motion
of the movable platen and temperature for flashing.
In flash welding process, the ends of
the piece to be welded are connected to the secondary circuit of a step-down transformer
to get a high current and low voltage.
PROCESS
1. One piece is held firmly by a
clamping device attached to a stationary platen (test bench) and the other
piece is clamped to a movable platen.
2. The surfaces to be welded are allowed
to touch when heavy currents pass through the peaks of the edges which provide
resistive heat to the edges.
3. This duration of this process period
is known as the flashing period.
4. The objective of is to make a suitable
temperature distribution in the work piece and to assure proper forging action
during the subsequent upset period of the cycle.
5. The platen travel is continuous
starting at the time of flashing and progressing until upset.
6. The edges start melting and, at
greater velocities, the molten bridges are broken and thrown off as flash
particles from joint.
7. This cycle of the formation and
collapse of bridges goes on as the movable platen advances.
8. When the conductive heat was
sufficiently heated the metal behind the faying surfaces [the meaning of a
faying surface is one of the surfaces that are in contact at a joint] on either
side to ensure adequate plasticity, the flashing current is stopped and
surfaces are butted against each other at greater force.
9. This portion of the operation is
known as the upset period.
10. 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.
ADVANTAGES
1. Filler metals are not required.
2. The impurities are removed by
flashing and metal ejcection.
3. This process eliminates corrugation
(corrugation means the act of shaping into parallel ridges and grooves)
4.
Smaller heat affected and annealed zone
5. Large cross sectioned shape materials
can be welded in a short time.
DISADVANTAGES
1. Similar cross section work pieces can
be welded.
2. Metal is lost during flashing and
upsetting.
3. Zinc and its alloys, cast iron, lead
and its alloys are not flash welded.
4. For removal of flash and molten
particles a special equipment is required which involves heavy expenses
5. Joint preparation is must for proper
heating of work pieces to take place.
APPLICATIONS
1. Used for producing joints in long
tubes and pipes
2. In petroleum industry oil drilling
pipe is attached with fittings by flash welding.
3. It is applied in automotive and
aircraft industry.
4. It is applied in household
appliances, refrigerators and framing implements.
5. Dissimilar metals may be welded
provided their characteristics are similar.
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