Tig Mig Plasma

8/9/2019 Tig Mig Plasma

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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.

Tig Mig Plasma

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