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Fusion Welding Process
ARC-WELDING PROCESSES
- In arc welding, the heat is obtained from electrical energy
- By using AC or a DC power supply.
- The process involved can be either consumable or non- consumable electrode.
- An arc is produced between the tip work piece which need to be welded.
of electrode and the
- The arc produces temperatures approximately 5000 degreescelsius.
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Arc welding
Equipments:
• A welding generator (D.C.) or Transformer (A.C.)
• Two cables- one for work and one for electrode
•
Electrode holder• Electrode
• Protective shield
• Gloves
• Wire brush• Chipping hammer
• Goggles
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Arc Welding Equipments
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A r c S h i e l d i n g
•
At high temperature in AW, metals areandchemically reactive to oxygen, nitrogen,
hydrogen in air
– Mechanical properties of joint can be degraded by
these reactions
–
To protect operation, arc must be shielded from
surrounding air in AW processes
•
Arc shielding is accomplished by: –
S argon, helium, CO2 h i e l d i n g g
– F l u x
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Flux
A substance that prevents formation of oxides and
other contaminants in welding,
and facilitates removal
or dissolves them
Provides protective atmosphere for welding
Stabilizes arc
Reduces spattering
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Power Source in Arc Welding
DC arc welding is more expensive than AC welding.
DC W is generally preferred because of the control of heat
input offered by it.
70 % of heat is liberated near the anode ,30% cathode.
If more heat is required at w/p ( thicker w/p, high thermal
conductivity metals such as Al, Copper ) w/p can be
connected to anode –
Straight polarity current Electrode negative)
or DCEN ( Direct
It produces welds that are narrow and deep.
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If less heat is required at w/p, (thinner w/p)
w/p can be connected to negative. This is
referred as reverse polarity, or DCEP(direct
current Electrode positive )
The weld zone is shallower and widerDCEN DCEP
Fig. The effect of polarity on weld beads:
(a) dc current straight polarity; (b) dc current reverse polarity; (c) ac current.
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Comparison of AC & DC welding machines
AC machine ( Transformer) DC machine (Generator) 1.Efficency is more (80 to 85 %) Efficiency is less (30 to 60 %)
2.Power consumption is less Power consumption is more
3.Cost of equipment is less Cost of equipment is more
4. Any terminal can be connected Polarity is significant
to work or electrode
5.Voltage is higher, not safe Voltage is low, safer operation
6.Not suitable for welding Very much suitable for both
nonferrous metals ferrous & nonferrous metals
7.Not preferred for welding thin Preferred for welding thin sections
sections
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Arc welding
Advantages – Most efficient way to join
metals
–
Lowest-cost joiningmethod
– Affords lighter weight
through better utilization
of materials
–
Joins all commercial metals
– Provides design flexibility
Limitations • Manually applied, therefore
high labor cost.
Need high energy causingdanger
Not convenient for
disassembly.
Defects are hard to detect at
joints.
•
•
•
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Heat transfer in arc welding
H l
e VIv
H = heat input V = voltage applied
v = welding speed
l = weld length I = current
e = efficiency
• Heat input is
•
Heat input to melt a certain volumematerial is
of
u = specific energy required for melting Vm = volume of material melted
A = cross section of the weld
H uV m uAl
• Welding speed is
eVI
uAv
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Consider the situation where a welding operation is being
performed with V = 20volts, I = 200A and the cross-sectional area of the weld bead is 30 mm2. Estimate the welding speed if the
work piece and electrode are made of (a) aluminium, (b) carbon
steel, and (c) titanium. Use an efficiency of 75%.
0.75 20200
VISolution v e 34.5 mm/s
2.930
uA a)For aluminium,
v 8.1mm/sb)For carbon steel,
v 7.0 mm/sc)For titanium,
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Consumable Electrode AW Processes Non consumable Electrode Processes
• Gas Tungsten Arc
Welding (TIG)Plasma Arc Welding
Carbon Arc Welding
Stud Welding
• Shielded Metal Arc
Welding
• Gas Metal Arc
Welding(MIG)
• Flux-Cored Arc Welding• Electrogas Welding
• Submerged Arc Welding
•
•
•
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MIG (Metal Inert Gas)
or
GMAW (Gas Metal Arc Welding )
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Weld materials:
Carbon steels, low alloy steels, stainless
steels, most
copper alloys
aluminum alloys, zinc based
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MIG or GMAW
Consumable electrode is in the form of a wire reel which is fedat constant rate.
Weld area is shielded by an external source of gas.
Deoxidizers are present in the electrode to prevent oxidation.
Process is rapid, versatile and economical.
Shielding gas :Argon, Nitrogen, Helium
The temperatures generated are relatively low.
Suitable only for thin sheets which is less than 6mm.
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Gas Metal-Arc Welding
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TIG (Tungsten Inert Gas)
or
Gas Tungsten Arc Welding ( GTAW)
As the tungsten electrode is not consumed, a constant
and stable arc gap is maintained at a constant current
level
GTAW process is used for applications with aluminium,
magnesium & titanium
Cost of the inert gas is more expensive but provides high
quality welds and surface finish
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Power supply either 200A DC or 500A AC; depending onthe metals to be welded.
Generally, AC is suitable for aluminum and magnesium.
Thorium or zirconium may be used in the tungstenelectrodes to improve the electron emissioncharacteristics.
Contamination of the tungsten electrode by molten metal can cause discontinuities in the weld.
Therefore, contact between the electrode with the molten metal pool should be avoided.
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Gas Tungsten Arc Welding
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Gas Tungsten-Arc Welding
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Gas Tungsten Arc Welding
Advantages:
• High quality welds for suitable applications
• No spatter because no filler metal through arc
• Little or no post-weld cleaning because no flux
Disadvantages:
• Generally slower and more costly than consumable electrode
AW processes
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PLASMA-ARC WELDING (PAW)
In this welding operation, a concentrated plasma arc is
produced and directed towards the weld area.
The arc is stable and the temperature can reaches up to 28000degrees celsius.
PAW has less thermal distortion, and higher energy
concentration –
permitting deeper and narrower welds.
Plasma: it is an ionized hot gas composed of nearly equalnumber of electrons and ions.
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This plasma initiated between the tungsten electrode and the small orifice by low current pilot arc.
a
Operating current: usually below 100A.
Filler metal is fed into the arc during welding process.
There are two methods of plasma-arc welding:
a) Transferred-arc method Work piece being welded is part of the electrical
circuit. The arc transfers from the electrode to
the work piece.
b) Non-transferred method
The arc occurs between the electrode and the
nozzle. The heat is carried to the workpiece by
the plasma gas.
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Welding speeds from 120 to 1000 mm/min.
Can be welded with part thickness less than 6mm.
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Plasma-Arc Welding Process
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Welded Joints
Fig. Examples of
welded joints and
their terminology.