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Electric Arc Welding
Electricity is passed through an
electrode which jumps between the
electrode and the work piece. This
causes an arc which produces great
heat melting the electrode and the
work piece causing the edges to fuse
together. (consumable electrode.)
Manual Metal Arc Welding.
Operation of the electrode
• The weld pool is protected from oxidation
by the gasses produced by melting the
chemicals on the electrode coating.
• This wire electrode also acts as a filler
material to fill the gap between the two parts
being joined.
• Its third function is to form a slag which
protects the weld area form cracking as it
allows the joint cool slowly and protect it
from oxidation.
Uses
This has many operational uses such as
repair work to constructional steel. It
is ideal for outdoor uses as the gasses
needed to form the shield are not
blown away.
The Transformer.
• This is a device used to change the
voltage of A.C. current. It can either
step up (increase) or step down
(decrease) the voltage. It is necessary to
change the voltage when welding
thicker or different types of steel.
Transformer-rectifier circuit
Transformer-rectifier circuit - A.C. to D.C.
•
•
•
The transformer circuit element of the circuit take
AC current and pass it through a step down
transformer reducing the voltage, the current is
increased.
The low voltage AC is then passed through a series
of diodes which only allow current flow in one
direction causing it to be changed into Direct
Current, this is called a bridge rectifier.
Finally the DC current is passed through a
‘smoothing’ capacitor which causes the current flow
more evenly.
Electric Arc Welding Safety
• Pg 370
Metal Arc Gas Shielded Welding.
(MAGS/MIG)
MIG (Metal Inert Gas) welding
• Also called MAGS (Metal Arc Gas Shielded)
welding
• It uses an inert gas such as Argon or CO2 to
shield the weld
• The bare wire electrode is fed continuously to
the weld - semi-automatic process
• Can be used on light and heavy plate.
• Does not produce a slag and can be used for
multi run welds and by robots.
Metal Arc Gas Shielded Welding.
(MAGS/MIG)
Electricity is passed through an electrode which
jumps between the electrode and the work
piece. The electrode is touched briefly against
the work, this causes an arc which produces
great heat melting the electrode and the work
piece causing the edges to fuse together.
Metal Arc Gas Shielded Welding.
(MAGS/MIG)
Uses.
• This is one of the most common ways
of welding. It can be used to weld
sheet metal as well as heavy plates. As
the electrode is fed automatically and
no slag is formed it is ideal for robotic
welding for example car production.
Tungsten Arc Gas Shielded Welding.
(TAGS/TIG)
Tungsten Arc Gas Shielded Welding.
(TAGS/TIG)
Principle.
• Two separate current flow in the circuit in this welding process.
One is for the arc and is similar to MAGS, the other is a high
frequency current used to start the arc. This means an arc is not
stuck by touching the work piece as before.
Operation.
1. The arc melts the two edges to be joined as well as the filler rod
forming the weld pool.
2. Gas is pumped through the nozzle protecting the weld pool
from oxidation.
3. The electrode is only to maintain the arc supplying the heat. A
consumable filler rod fed by the operator gives the extra metal
necessary for the weld pool.
Tungsten Arc Gas Shielded Welding.
(TAGS/TIG)
Uses.
• This is suitable for welding most metals and generally uses DC
current for the power source. However when welding aluminium
AC is used. This is necessary to breakdown the oxide layer on
the outside of the aluminium which has a high melting point. It
is also suitable for stainless steel.
Fluxes
• Active fluxes – these fluxes are able to dissolve oxide films
and will prevent oxides reforming. Zinc chloride is an active flux.
Active fluxes are corrosive. Active fluxes remaining on the surface
of the steel will corrode the surface in a very short time.
• Passive fluxes – all the joint surfaces must be thoroughly
cleaned before using a passive flux. It does not have the ability to
remove oxides from the surface.
Submerged Arc Welding (SAW)
Submerged Arc Welding (SAW)
Principle.
• Electricity is passed through an electrode which jumps between
the electrode and the work piece. The arc produces great heat
melting the electrode and the work piece causing the edges to
fuse together. The electrode is a wire fed consumable.
Operation.
1. The arc melts the two edges to be joined as well as the electrode
forming the weld pool.
2. The arc is submerged by a granulated flux released from a
hopper ahead of it. This flux melts with the heat of the arc
protecting the weld pool and forming a slag that controls
cooling.
Uses.
• This is an ideal automated welding process. It is ideal for long
runs. It is used for producing steel girders or beams used in
Electro-Slag Welding.
Electro-Slag Welding.
Principle.
• Electricity is passed between two couples on either side of the
plates. This causes the materials in the gap to melt as well as the
edges of the plates. This form the weld pool.
Operation.
1. Two pieces of metal are placed between the electrodes.
2. The electrodes are then closed on the piece causing the circuit to
complete.
3. The resistance in the metal between the electrodes causing them
to melt and fuse together.
Uses.
• This is generally used to join very thick plates together giving a
series of overlapping welds until the two plates are fully joined. It
is suitable for automatic welding as it is used on large plates
usually.
Resistance Welding. (Spot
Welding)
Resistance Welding. (Spot
Welding)
Principle.
•
Electricity is passed between two electrodes with the pieces to
be joined in between. As the electricity passes through the
material the resistance of the metals cause the metals to melt
fusing them together.
Operation.
1. Two pieces of metal are placed between the electrodes.
2. The electrodes are then closed on the piece causing the circuit
to complete.
3. The resistance in the metal between the electrodes causing
them to melt and fuse together.
Uses.
•
This is generally used to join light gauge steel sheet metal.
Sheets that are too thick will prevent the current from flowing
and will not work
Resistance Seam Welding.
Principle
•
This is exactly the same as spot welding except that a
continuous is produced.
Operation.
1. The first method is similar to spot welding. Here however the
spots are overlapped to form a continuous seam. This is called
Stitch welding.
2. The second method is to replace the electrodes with rollers
that cause a continuous seam to be produces.
Uses.
•
This is also used to weld sheet metal except that a continuous
seam is required perhaps the production of a water proof tank.
Welding Terms
Multi-runs.
• This is a method of welding two pieces of thick
metal together. A number of welding runs are
necessary to build up the gap between the two
pieces.
Flash back arrestor.
• This is a safety device used in gas welding. It
prevents the flame from the burning gas traveling
through the hose and into the tanks.
Dissolved acetylene.
• This is the form that acetylene is stored in the tank.
The tank is similar to a sponge which absorbs the
acetylene. Liquid acetylene would explode in the
tank.
Shielded arc.
• This is the protection of the weld pool by an inert
gas preventing oxidation of the weld.
Edge preparation.
• This is the preparation of the edges to be welded.
They should be free of rust and oil to allow for a
quality weld and should be tapered slightly to allow
for a good joint.
Slag inclusion.
• This is a fault where the slag drops into the weld
pool and becomes part of it weakening the weld.
This is caused by moving the welding apparatus to
slowly. Welding at the correct pace will prevent this.
Porosity.
• Porosity is the presence of air bubbles in the
weld. This is caused by moving the welding
apparatus to quickly not giving the weld pool a
chance to form properly. This is controlled by
welding at the correct pace.
Lack of penetration.
• This is again a problem with the weld pool. Due
to a lack of temperature the edges to be joined
do not melt to form the weld pool and fuse
together. This can be prevented by setting the
welder to the correct power setting.
Gas welding
• Metals are fused together and the gap is filled
with a filler rod
• Oxygen and acetylene are mixed together to
generate a flame to melt the weld joint.
• Oxygen is pressurised into the cylinder.
• Acetylene is dissolved in a porous material called
acetone, which will absorb 25 times it’s own
volume of Acetylene. Acetylene would explode
if it were stored under pressure.
Gas Welding
Oxy-acetylene flames