Resistance Welding:
Resistance
welding is a process in which two or more parts are welded by the
coordinated and regulated use of heat and pressure. The heat in the
resistance welding process is generated by the resistance offered by the
work pieces to the flow of low voltage high density electric current.
The
pressure for welding is applied through the contacting electrodes. It
may be obtained by mechanical means, springs, air pressure or
hydraulically with the help of a pressure cylinder and piston
arrangement. The pressure generally used range from 30 to 55 MPa.
The
high density current is produced by using transformer in the welding
machine circuit. The transformer reduces the voltage to around 4 to 12
volts and raises the amperage to produce a current density of 45 to 52
VA / mm2 of the area to be welded based on a time of around 10 seconds.
AC
current has been found to be most convenient for resistance welding
because it is possible to obtain any desired combination of current and
voltage by using suitable transformer with different settings.
History:
Sometime
in the year 1885, Professor Thompson invented a process called electric
resistance welding. He discovered that to weld metals together, one
could fire an electric current through the metals while they were
tightly clamped together. When the current passed through the metals, it
would create such a high heat that the metals would melt and run
together and a weld would be made.
The total heat generated in the work pieces and electrodes can be expressed as:
H = I2 R t K
H = I V t K
Where,
H – Heat generated in the workpiece, Joules
I – Current, Amperes (5000 to 20000 A), although voltage is low (below 10 V)
R – Resistance, Ohms
t – Time over which the current is supplied, seconds (0.1 to 0.4 sec)
V – Voltage, volts
K
– A correction factor to account for the loss of heat due to radiation,
conduction and convection from the electrodes and work pieces.
The
current flowing to develop the joint determines the rate of heat
generation at the joint. High welding currents are required in
resistance welding to develop the necessary heat as the ohmic resistance
of any resistance welded joint is low.
Process Variables:
A typical resistance welding cycle consists of:
1. Squeeze
2. Weld
3. Hold periods
Operation:
The
pressure is applied and built up to the desired value over a period of
time. After the proper pressure value has been attained, current of
required magnitude is passed for pre-set period of time. Further the
interface resistance between upper sheet and lower electrode and lower
sheet are kept low by proper contact pressure. Heat is rapidly generated
at this interface where it is trapped and slowly dissipated. In a
properly controlled weld the welding heat is first generated at pin
points on sheet interfaces and then subsequently a weld nugget is formed
gradually.
Weld Defects:
Advantages:
1. Resistance welding is a production welding process most suitable for light gauge sheets which can be overlapped
2. Operation is quite fast, practically all metals can be resistance welded.
3. Cleaner workspace with less contaminants
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