TECHNICAL KNOWLEDGE
WHAT IS RESISTANCE WELDING PROCESS ?
Resistance welding, sometimes called electric resistance
welding is a process which metals can be joined together
by applying pressure and conducting a strong electric
current through the metal combination to heat up the
welding joint and melt the metals, forging them together.
HOW DOES RESISTANCE WELDING WORK?
Resistance welding is generally used to join two plain metal work pieces together. An electric current is delivered
to the metal sheets (or any work pieces being joined) through weld electrodes which apply force to the sheets
This force is then converted to heat. The heat is generated so that it melts the metal at the point where they join –the point of ‘resistance’ between the surfaces.
The electrode then extracts heat from the molten weld area which forms a weld nugget at the point where it
solidifies. A force is applied before, during and after a current is applied, which confines the contact area.
TYPES OF RESISTANCE WELDING
There are many resistance welding processes with different uses, such as
SPOT WELDING
PROJECTION WELDING
SEAM WELDING
BUTT WELDING
FLASH BUTT WELDING
Each one has a different specific welding application that makes it optimal for a particular situation.
SPOT WELDING
Spot welding is a resistance welding process. This welding
process is used primarily for welding two or more metal
sheets together by applying pressure and heat from an
electric current to the weld area.
It works by contacting copper alloy electrodes to the sheet
surfaces, whereby pressure and electric current are applied
and heat is generated by the passage of current through
resistive materials such as low carbon steels.
HOW DOES SPOT WELDING WORK ?
A form of resistance welding, spot welding is one of the oldest welding processes whereby two or more sheets of
metal are welded together without the use of any filler material.
The process involves applying pressure and heat to the weld area using shaped alloy copper electrodes which
convey an electrical current through the weld pieces. The material melts, fusing the parts together at which point
the current is turned off, pressure from the electrodes is maintained and the molten “nugget” solidifies to form the
joint.
The welding heat is generated by the electric current, which is transferred to the workpiece through copper
alloy electrodes. Copper is used for the electrodes as it has a high thermal conductivity and low electrical
resistance compared to most other metals, ensuring that the heat is generated preferentially in the work
pieces rather than the electrodes.
WHERE IS SPOT WELDING USED?
Spot welding has applications in a number of industries,
including automotive, aerospace, rail, white goods, metal
furniture, electronics, medical building and construction.
Given the ease with which spot welding can be automated
when combined with robots and manipulation systems, it
is the most common joining process in high volume
manufacturing lines and has in particular been the main
joining process in the construction of steel cars for over 100
years.
PROJECTION WELDING
Resistance projection welding is a form of resistance
welding where the electricity, force, and weld time are
concentrated on raised ‘projections’ across the surface
Projection welding is generally used for welds using thicker
materials than the thinner metal pieces that spot welding is
usually required for, and often not metals
HOW DOES PROJECTION WELDING WORK ?
Projection welding current and weld force are localized on the weld points unlike spot welding where the weld is
determined by the size or shape of electrode. Projections for projection welding can be produced by embossing on
sheet metal, machining or cold forging on a solid piece and press punch on the edge of a piece.
WHERE IS PROJECTION WELDING USED?
Projection welding has wider applications than spot welding;
from welding nuts and bolts to sheet components to as big
as welding bearing bars and cross bars to form a grating
panel.
SEAM WELDING
Resistance seam welding is a variant of the basic resistance
spot welding process. In spot welding a weld nugget is
produced by passing an electric current through the sheet
materials to be welded while they are held together under a
mechanical force between shaped copper electrodes in a lap
configuration.
Fusion is produced where the sheet surfaces are in contact
because this is the point of highest electrical resistance and
hence the point where the temperature is highest.
HOW DOES SEAM WELDING WORK ?
In seam welding a series of overlapping nuggets is produced, usually by replacing the conventional spot welding
electrodes by wheels that turn as the parts to be welded are fed between them. The process thus produces a
continuous leaktight seam.
WHERE IS SEAM WELDING USED?
Resistance seam welding can be used to make gas- or
fluid-tight joints in a variety of sheet metal fabrications.
Steel fuel tanks for motor vehicles are a prime example. It
is also used in making tin cans, steel drums and domestic
radiators.
BUTT WELDING / FLASH BUTT WELDING
Butt Welding / Flash Butt Welding is a resistance welding that does not require overlap of weld material. It is
commonly used to join same or different cross section in tube, bar, rod, sheet or plates. Butt weld is also called
Upset weld.
Heat for welding is the resistance to electric current created through contacts between the moveable part
and the fixed part. As the weld part touches, the material starts to flash and sparks (metal loses) from the weld
area.
The flashing process heat up the cross section of the weld part, then comes the butt weld (Upset) process
which is is to accelerate the movable part towards the fixed part with higher current. Rapid speed and
continuous pressure will weld two pieces together.
