Spot welding is one
of the oldest welding processes. It is used in a wide range of industries but
notably for the assembly of sheet steel vehicle bodies. This is a type of
resistance welding where the spot welds are made at regular intervals on
overlapping sheets of metal. Spot welding is primarily used for joining parts
that are normally up to 3 mm in thickness. Thickness of the parts to be welded
should be equal or the ratio of thickness should be less than 3:1. The strength
of the joint depends on the number and size of the welds. Spot-weld diameters
range from 3 mm to 12.5 mm.
How spot welding works
welding is one form of resistance welding, which is a method of welding two or
more metal sheets together without using any filler material by applying
pressure and heat to the area to be welded. The process is used for joining
sheet materials and uses shaped copper alloy electrodes to apply pressure and
convey the electrical current through the workpieces. In all forms of resistance
welding, the parts are locally heated. The material between the electrodes
yields and is squeezed together. It then melts, destroying the interface between
the parts. The current is switched off and the "nugget" of molten
materials solidifies forming the joint.
create heat, copper electrodes pass an electric current through the workpieces.
The heat generated depends on the electrical resistance and thermal conductivity
of the metal, and the time that the current is applied. The heat generated is
expressed by the equation:
where E is the heat energy,
I is the current, R is the electrical resistance and t is the time that the
current is applied.
is used for electrodes because it has a low resistance and high thermal
conductivity compared to most metals. This ensures that the heat is generated in
the workpieces instead of the electrodes.
for spot welding
Steel has a higher
electrical resistivity and lower thermal conductivity than the copper electrodes,
making welding relatively easy. Low carbon steel is most suitable for spot
welding. Higher carbon content or alloy steel tend to form hard welds that are
brittle and could crack. Aluminium has an electrical resistivity and thermal
conductivity that is closer to that of copper. However, aluminium's melting
point is much lower than that of copper, making welding possible. Higher levels
of current must be used for welding aluminium because of its low resistivity.
Galvanized steel (i.e. steel coated with zinc to prevent
corrosion) requires a different welding
approach than uncoated steel. The zinc coating must first be melted off before
the steel is joined. Zinc has a low melting point, so a pulse of current before
welding will accomplish this. During the weld, the zinc can combine with the
steel and lower its resistivity. Therefore, higher levels of current are required
to weld galvanized steel.
How to determine welding
parameters for spot welding?