SPOT WELDING
Spot welding is a type of resistance welding used to weld various
sheet metals. Typically the sheets are in the 0.5-3.0 mm thickness
range. The process uses two shaped copper alloy electrodes to
concentrate welding current and force between the materials
to be welded. The result is a small "spot" that is
quickly heated to the melting point, forming a nugget of welded
metal after the current is removed. The amount of heat released
in the spot is determined by the amplitude and duration of the
current. The current and duration are chosen to match the material,
the sheet thickness and type of electrodes. Applying the current
for too long can result in molten metal being expelled as weld
splash, or can even burn a hole right through the materials
being welded.
Spot welding is typically used when welding steel sheet metal.
Thicker stock is difficult to heat up from a single spot,
as the heat can flow into the surrounding metal too easily.
Spot welding can be easily identified on many sheet metal
goods, such as metal pails. Aluminum alloys can also be spot
welded. However, their much higher thermal conductivity and
electrical conductivity mean that up to three times higher
welding currents are needed. This requires larger, more powerful,
and more expensive welding transformers.
Due to changes in the resistance of the metal as it starts
to liquefy, the welding process can be monitored in real-time
to ensure a perfect weld every time, using the most recent
advances in monitoring/feedback control equipment. The resistance
is measured indirectly, by measuring the voltage at and current
through the electrodes.
The voltage needed for the welding depends on the resistance
of the material to be welded, the sheet thickness and desired
size of the nugget. When welding a common combination like
1.0 + 1.0 mm sheet steel, the voltage between the electrodes
is only about 1.5 V at the start of the weld but can fall
as low as 1 V at the end of the weld. This drop in voltage
stems from the resistance reduction caused by the steel melting.
The open circuit voltage from the transformer is much higher
than this, typically in the 5-10 V range, but there is a very
large voltage drop in the electrodes and secondary side of
the transformer when the circuit is closed.
MIG WELDING
MIG (Metal Inert Gas) or as it even is called GMAW (Gas Metal
Arc Welding) uses an aluminium alloy wire as a combined electrode
and filler material. The filler metal is added continuously
and welding without filler-material is therefore not possible.
Since all welding parameters are controlled by the welding
machine, the process is also called semi-automatic welding.
The MIG-process uses a direct current power source, with
the electrode positive (DC, EP). By using a positive electrode,
the oxide layer is efficiently removed from the aluminium
surface, which is essential for avoiding lack of fusion and
oxide inclusions. The metal is transferred from the filler
wire to the weld bead by magnetic forces as small droplets,
spray transfer. This gives a deep penetration capability of
the process and makes it possible to weld in all positions.
It is important for the quality of the weld that the spray
transfer is obtained.
There are two different MIG-welding processes, conventional
MIG and pulsed MIG:
Conventional MIG uses a constant voltage DC power source.
Since the spray transfer is limited to a certain range of
arc current, the conventional MIG process has a lower limit
of arc current (or heat input). This also limits the application
of conventional MIG to weld material thicknesses above 4 mm.
Below 6 mm it is recommended that backing is used to control
the weld bead.
Pulsed MIG uses a DC power source with superimposed periodic
pulses of high current. During the low current level the arc
is maintained without metal transfer. During the high current
pulses the metal is transferred in the spray mode. In this
way pulsed MIG is possible to operate with lower average current
and heat input compared to conventional MIG. This makes it
possible to weld thinner sections and weld much easily in
difficult welding positions.
Recommended material thicknesses for MIG-welding
- Conventional MIG-welding: with or without backing: >
3 mm
- Pulsed MIG-welding: > 1 mm. Smaller thicknesses are
possible, however with thinner diameter on welding wire.
- For welding of thick plates preheating of 50ñ100ƒC
may be required to avoid lack of fusion.
Recommended welding positions for MIG-welding
- All welding positions are possible. Pulsed MIG-welding
is, however better in vertical and under-up positions.
Applications of MIG-welding
MIG-welding is a general purpose welding process for welding
of aluminium and applicable in most cases in all welding positions
from about 1 mm sheet thickness to thick walled sections.
MIG-welding also offers high quality welds with a high productivity.
There are two main variants conventional MIG-welding and pulsed
MIG-welding.
MIG Welding Benefits
- All position capability
- Higher deposition rates than SMAW
- Less operator skill required
- Long welds can be made without starts and stops
- Minimal post weld cleaning is required
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