Lecture 2 - Welding

Arc Shielding

▪ At high temperatures in AW, metals are chemically reactive

to oxygen, nitrogen, and 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:
▪ Shielding gases e.g., Argon, Helium, CO2
▪ Use of Flux
Flux

It is defined as a substance that prevents formation of

oxides and other contaminants in welding, or dissolves
them and facilitates removal. Its functions are;

▪ It provides protective atmosphere for welding

▪ It stabilizes the electric arc

▪ It reduces spattering
 Various Flux Application Methods

▪ Pouring granular flux onto welding operation

▪ Stick electrode coated with flux material that melts
during welding to cover operation
▪ Tubular electrodes in which flux is contained in the
core and released as electrode is consumed
Advantages of Flux Coating
1. It protects the welding zone from oxidation.
2. It produces low melting temperature slag, which dissolves the impurities
present in the metal like oxides and nitrides, and floats on the surface of the
weld pool.
3. It refines the grain size of the welded metal.
4. It adds alloying elements to the welded metal.
5. It stabilizes the arc by providing certain chemicals which have this ability.
6. It reduces the spattering of weld metal.
7. It concentrates the arc stream and reduces thermal losses. This result in
increased arc temperature.
8. It slows down the cooling rate of weld and accelerates hardening process.
9. It increases the rate of metal deposition and the penetration obtained.
Common Constituents of Electrode Coating
Power Source in Arc Welding

Direct current (DC) vs. Alternating current (AC)

▪ AC machines are less expensive to purchase and

operate, but generally restricted to ferrous metals

▪ DC equipment can be used on all metals and is generally

noted for better arc control
Alternating Current (from Transformer)
1. More efficiency
2. Power consumption is less
3. Cost of equipment is less
4. Higher voltage – hence not safe
5. Any terminal can be connected to the work or electrode

Direct Current (from Generator) (voltage required is 60 to 80 volts for

striking the arc and 15-25 V for maintaining the arc)
1. Power consumption and cost of equipment are more
2. Low voltage – safer operation
3. suitable for both ferrous and non-ferrous metals
4. Preferred for welding thin sections
5. Both bare and coated electrodes can be used
 Comparison of A.C. and D.C. arc welding

• There is no fixed polarity at the terminals when using AC and

they interchange in every cycle (the arc pulsates rapidly)

• In DC welding the electrode acts as one terminal and the

workpiece the other terminal (+ve or –ve) . The potential
difference is so adjusted that the heat developed at the +ve
terminal is higher, nearly 2/3rd and on the negative terminal
lower nearly 1/3 rd of total heat evolved.

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DC Arc Welding

• About 70% of the heat is liberated near the anode in DC arc

welding.
• If more heat is required at the workpiece side, such as for
thicker sheets or for the work materials which have higher
thermal conductivity such as aluminium and copper, the
workpiece can be made as anode, liberating large heat near
it.
• This is termed as 'straight polarity' or DCEN (direct
current electrode negative).

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DCEP

• For thinner materials, the polarity could be reversed

by making the workpiece as negative.
• This is termed as 'reversed polarity' or DCEP (direct
current electrode positive).
• In reversed polarity, the penetration is small.

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 Weld penetration

Penetration: It is the depth upto which the weld metal combines with
the base metal as measured from the top surface of the joint.

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Duty Cycle

Duty cycle refers to the percentage of welding time of total

welding cycle i.e. welding time divided by welding time plus and rest
time
The maximum current which can be drawn from a power
source at a given duty cycle depends upon
1) size of winding wire,
2) type of insulation and
3) cooling system of the power source.
In general, large diameter cable wire, high temperature resistant
insulation and forced cooling system allow high welding current
drawn from the welding source at a given duty cycle

The insulation is classified as A, E, B, F& G in increasing order of their

maximum allowable temperature 60, 75, 80, 100 &125 Deg C
 Numerical
1. A heat source transfers 3500 J/s to a metal surface for welding. The heated area
is approximately circular, and the heat intensity decreases with increasing radius
as follows: 50% of the power is transferred within a circle of diameter = 5 mm,
and 75% is transferred within a concentric circle of diameter = 10 mm. What are
the power densities in the (a) 5-mm-diameter inner circle and (b) 10-mm-
diameter ring that lies around the inner circle?

Solution: (a) Area A = (5)2/4 = 19.63 mm2

Power P = 0.50(3500) = 1750 J/s
Power density PD = (1750 J/s)/19.63 mm2 = 89.1 W/mm2
(b) A = (102 − 52)/4 = 58.9 mm2
Power P = (0.75 x 3500) − 1750 = 875 J/s
Power density PD = (875)/58.9 mm2 = 14.8 W/mm2
FUSION WELDING PROCESSES
ARC WELDING PROCESSES
Consumable Electrode AW Processes

▪ Shielded Metal Arc Welding

▪ Gas Metal Arc Welding

▪ Electrogas Welding

▪ Submerged Arc Welding

 Shielded Metal Arc Welding (SMAW)
▪ Uses a consumable electrode consisting of a filler metal rod
coated with chemicals that provide flux and shielding
▪ Sometimes called stick welding
Welding Stick in SMAW

▪ Composition of filler metal usually close to base metal

▪ Coating (flux): powdered cellulose (cotton and wood powder)
mixed with carbonates, and held together by a silicate binder
▪ Welding stick is clamped in electrode holder and connected
to power source
▪ Sticks are 225-450 mm long and 2.5 – 9.5 mm in dia
▪ Current 30 – 300 A and Voltage : 15 – 45 V
▪ Disadvantages of stick welding:
▪ Sticks must be periodically changed
▪ High current levels may melt coating prematurely
SMAW Applications

▪ Used for steels, stainless steels, cast irons, and

certain nonferrous alloys
▪ Not used or rarely used for aluminum and its
alloys, copper alloys, and Titanium
▪ Ship building, pipelines for maintenance work.
 Gas Metal Arc Welding (GMAW)/MIG Welding

▪ Uses a consumable bare metal

wire as electrode with shielding
by flooding arc with a gas
▪ Wire is fed continuously and
automatically from a spool
through the welding gun
▪ Shielding gases include argon and
helium for aluminum welding,
and CO2 for steel welding
▪ Bare electrode wire plus shielding
gases eliminate slag on weld bead
▪ No need for grinding and
cleaning of slag
GMAW Advantages over SMAW

▪ Higher arc time because of continuous wire electrode

▪ Better use of electrode filler metal than SMAW, as end of
stick cannot be used in SMAW
▪ Higher deposition rates due to continuous process
▪ Eliminates problem of slag removal, so no post processing
like manual grinding and cleaning are required.
▪ This process can be readily automated.
Flux-Cored Arc Welding (FCAW)

▪ Adaptation of shielded metal arc welding, to overcome

limitations of stick electrodes - two versions
▪ Self-shielded FCAW - core includes compounds
that produce shielding gases
▪ Gas-shielded FCAW - uses externally applied
shielding gases

▪ Electrode is a continuous consumable tubing (in coils)

containing flux and other ingredients (e.g.,alloying
elements) in its core
Flux-Cored Arc Welding (FCAW)
Presence or absence of externally supplied shielding gas
distinguishes: (1) self-shielded - core provides ingredients for
shielding, (2) gas-shielded - uses external shielding gases
Electrogas Welding (EGW)

▪ Uses a continuous consumable electrode, flux-cored wire

or bare wire with externally supplied shielding gases, and
molding shoes to contain the molten metal
▪ When flux-cored electrode wire is used and no external
gases are supplied, then EGW is a special case of
self-shielded FCAW (Flux Cored Arc Welding)
▪ When a bare electrode wire used with shielding gases
from external source, then EGW is a special case of
GMAW
 Electrogas Welding

EGW using flux-cored electrode wire: (a) front view with molding
shoe removed for clarity, (b) side view showing molding shoes on
both sides
Applications - EGW

• It is principally used in the construction of large storage tanks

and in ship building.
• Workpiece thickness from 12 – 75 mm are within the capacity
of EGW.
• It is always used in vertical orientation.
• Sometimes molding shoes are specifically designed based
upon the joint shapes involved.
Submerged Arc Welding (SAW)
▪ Uses a continuous, consumable bare wire electrode, with arc shielding
by a cover of granular flux
▪ Electrode wire is fed automatically from a coil
▪ Flux introduced into joint slightly ahead of arc by gravity from a
hopper
▪ Completely submerges welding operation, preventing sparks, spatter,
and radiation
SAW Applications and Products

▪ Steel fabrication of structural shapes (e.g., I-beams)

▪ Seams for large diameter pipes, tanks, and pressure
vessels
▪ Welded components for heavy machinery
▪ Most steels (except high-C steel), but not good for
nonferrous metals
 Non-consumable Electrode Processes

▪ Gas Tungsten Arc Welding

▪ Plasma Arc Welding

▪ Carbon Arc Welding

 Gas Tungsten Arc Welding (GTAW)
▪ Uses a non-consumable tungsten electrode and an inert gas (Argon) for
arc shielding.
▪ Melting point of Tungsten = 3410C (6170F)
▪ Used with or without a filler metal, when filler metal used, it is added to
weld pool from separate rod or wire
▪ Applications: For welding Aluminum and Stainless steel mostly
 Advantages and Disadvantages of GTAW

▪ 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
 Plasma Arc Welding (PAW)
▪ Special form of GTAW in which a constricted plasma arc is directed at weld
area
▪ Tungsten electrode is contained in a nozzle that focuses a high velocity stream
of inert gas (Argon) into arc region to form a high velocity, intensely hot
plasma arc stream
▪ Temperatures in PAW reach 18,000C, due to constriction of arc, producing a
plasma jet of small diameter and very high power density
Advantages and Disadvantages of PAW

▪ Advantages:
▪ Good arc stability and excellent weld quality
▪ Better penetration control than other AW processes
▪ High travel speeds
▪ Can be used to weld almost any metal including
Tungsten

▪ Disadvantages:
▪ High equipment cost
▪ Larger torch size than other AW processes, which
tends to restrict access in some joints
Resistance Welding (RW)

▪ A group of fusion welding

processes that use a
combination of heat and
pressure to accomplish
coalescence
▪ Heat generated by
electrical resistance to
current flow at junction to
be welded
▪ Principal RW process is
resistance spot welding
(RSW)
 Components in Resistance Spot Welding

▪ Parts to be welded (usually sheet metal)

▪ Two opposing electrodes
▪ Means of applying pressure to squeeze parts between
electrodes
▪ Power supply from which a controlled current can be
applied for a specified time duration
▪ Very high current involved (5000-20000 A), at low
voltage (approx. 8-10 V)
▪ Time at each step is around 0.1-0.4 sec, and the
resistance is around 0.1 mΩ
 Advantages and Drawbacks of Resistance Welding

▪ Advantages:
▪ No filler metal required
▪ High production rates possible
▪ Lends itself to mechanization and automation
▪ Lower operator skill level than for arc welding
▪ Good repeatability and reliability

▪ Disadvantages:
▪ High initial equipment cost
▪ Limited to lap joints for most RW processes
 Spot Welding Cycle

Cycle:
(1) parts inserted
between electrodes,
(2) electrodes close,
(3) current on,
(4) current off,
(5) electrodes opened

Sheets upto 3mm can

be welded
 Resistance Seam Welding (RSEW)

▪ Uses rotating wheel electrodes to produce a series of

overlapping spot welds along lap joint
▪ Can produce air-tight joints

Applications:
▪ Gasoline tanks
▪ Automobile mufflers
▪ Various sheet metal
containers
 Resistance Projection Welding (RPW)

▪ Resistance welding process in which coalescence occurs

at one or more small contact points on the parts
▪ Contact points determined by design of parts to be joined
▪ May consist of projections, embossments, or localized
intersections of parts
 Numerical
1) A resistance welding performed on two pieces of 2.5 mm thick sheet steel uses
12000A for a 0.2 s duration. The electrodes are 6 mm in diameter. Resistance is
approx. is 0.0001 Ohm, and the resulting weld nugget is 6 mm in diameter, and 3
mm in thickness. The specific melting energy for the metal is 12 J/mm3. How
much of the heat generated was used to form the weld nugget, and how much
was dissipated into workpiece, surrounding air and electrodes?

Heat generated in the operation = I2Rt = 2880 J

Volume of weld nugget = 84.8 mm3

Heat required to melt the weld nugget = 84.8 x 12 = 1018 J

Heat Lost to electrodes, workpiece and surroundings = 2880 – 1018 = 1862 J