Introduction to Basic Manufacturing Processes and ... - always yours

Welding 329
Where
H = heat generated in joules
I = current in Amp.
R = resistance in ohms
t = time of current flow in seconds.
17.7.1 Types of Resistance welding
The major types of resistance welding are given as under:
(1) Spot Welding
(2) Seam Welding
(3) Projection Welding
(4) Resistance Butt Welding
(5) Flash Butt Welding
(6) Percussion Welding
(7) High Frequency Resistance Welding
(8) High Frequency Induction Welding
Some of the above important welding processes are discussed as under,
17.7.1.1 Spot Welding
In this process overlapping sheets are joined by local fusion at one or more spots, by the
concentration of current flowing between two electrodes. This is the most widely used resistance
welding process. A typical resistance spot welding machine is shown in Fig. 17.23. It essentially
consists of two electrodes, out of which one is fixed. The other electrode is fixed to a rocker
arm (to provide mechanical advantage) for transmitting the mechanical force from a pneumatic
cylinder. This is the simplest type of arrangement. The other possibility is that of a pneumatic
or hydraulic cylinder being directly connected to the electrode without any rocker arm. For
welding large assemblies such as car bodies, portable spot welding machines are used. Here
the electrode holders and the pneumatic pressurizing system are present in the form of a
portable assembly which is taken to the place, where the spot is to be made. The electric
current, compressed air and the cooling water needed for the electrodes is supplied through
cables and hoses from the main welding machine to the portable unit. In spot welding, a
satisfactory weld is obtained when a proper current density is maintained. The current
density depends on the contact area between the electrode and the work-piece. With the
continuous use, if the tip becomes upset and- the contact area increases, the current density
will be lowered and consequently the weld is obtained over a large area. This would not be able
to melt the metal and hence there would be no proper fusion. A resistance welding schedule
is the sequence of events that normally take place in each of the welds. The events are:
1. The squeeze time is the time required for the electrodes to align and clamp the two
work-pieces together under them and provide the necessary electrical contact.
2. The weld time is the time of the current flow through the work-pieces till they are
heated to the melting temperature.
3. The hold time is the time when the pressure is to be maintained on the molten
metal without the electric current. During this time, the pieces are expected to be
forged welded.