Plasma Arc Weldlng (PAW)

• uses non-consumable tungsten electrode to produce an arc to a workpiece

• differs from TIG (GTAW) welding only in that low-volume flow of inert gas (argon
or helium) through the inner orifice in the nozzle of the welding torch constricts the
arc
• resulting in several advantages over the GTAW process including:
- greater energy concentration (i.e., higher energy density)
- higher heat content (or source intensity)
- improved arc stability
- deeper penetration capability
- higher welding speeds
- cleaner welds
• plasma generation can be
i) operating in the transferred arc mode
- occurs when the workpiece is connected electrically to the welding
torch such that it is of opposite polarity to the permanent electrode, the plasma is
drawn to the workpiece electrically.
- is usually employed for welding or cutting
ii) operating in the nontransferred mode
- occurs when the workpiece is not connected electrically to the torch,
and the plasma is simply forced to the workpiece by the force of the inert gas.
- is usually employed for thermal spraying
• Two distinctly different welding modes are possible with the plasma arc welding process:
i) melt-in or conduction mode
- refers to a weld pool similar to that which typically forms in the gas-tungsten arc welding
(GTAW) process, where a bowl-shaped portion of the workpiece material that is under the arc is
melted
- heating of the workpiece occurs by conduction of heat from the plasma’s contact with
the workpiece surface inward.
- is good for joining thin sections (0.025-1.5 mm) and making fine welds at low currents,
and for joining thicker sections (up to 3 mm) at high currents
ii) keyhole mode.
- refers when the arc fully penetrates the workpiece material, forming a nominally concentric
hole, or keyhole, through the thickness. The molten weld metal flows around the arc and
resolidifies behind the keyhole as the torch traverses the workpiece.
- the high energy density of a very high-current plasma vaporizes a cavity through
theworkpiece and creates a weld by moving the keyhole, analogous to a hot wire through
wax.
- produces high penetration since the vapor cavity tends to “trap” energy by internal
reflection.
- Molten metal surrounding the vapor cavity is drawn by surface tension or capillary
forces to fill the cavity at the trailing edge of the weld.
• disadvantage of PAW
- Power sources, gas controllers, and torches are all more complicated and expensive than for
GTAW, and the torches tend to be large, making handling difficult during manual operation
 Schematic comparison of the (a) the gas-
plasma arc welding (PAW) processes
tungsten arc welding (GTAW) and (b) plasma
arc welding (PAW) processes.
Electroslag Welding
• is a welding process that melts and joins metals by heating them with a pool
of molten slag held between the metals and continuously feeding a filler
wire electrode into it
• Is an arc welding process that produces coalescence of metals by heating
them with an arc between a continuous filler metal electrode and the work.
Molding shoes are used to confine the molten weld metal for vertical
position welding. The electrodes may be either flux cored or solid.
Shielding may or may not be obtained from an externally supplied
• is not a true arc welding process
- exists only during the initiation period of the process to heat up and
melt the flux
- the arc is then extinguished and is replaced by the resistance heating
using current passing through the slag to keep it molten.
- The weld metal is produced from a filler wire and the weld pool is
covered with molten slag and moves upward as welding progresses.
• uses a pair of water-cooled dams of copper shoes, one in the front the other
behind the workpiece to keep the weld pool and the molten slag from
running off.
• gives a higher deposition rates typically 7-13 kg per hour per electrode.
Advantages and Disadvantages
• Can have extremely high deposition rates, but only one single pass is required no
matter how thick the workpiece is.
• Doesn't produce angular distortion unlike because the weld is symmetrical with
respect to its axis.
• Produces very high heat which may deteriorate the quality of weld:
- coarse grains in the fusion and the heat-affected zone that can reduce
toughness.
• Is restricted to vertical position welding because of the very large pools of the
molten metal and slag.
Fluxed-Core Arc-Welding (FCAW)
• Is an arc welding process that uses the heat generated by an electric
arc between a tubular, flux-cored continuous filler metal electrode and
the workpiece.
• can be of two types
- gas-shielded flux-cored
- self-shielded flux-cored
core ingredients or flux contained within the electrode
* are slag formers, deoxidizers, arc stabilizers and alloying
elements
* protect the weld metal from the atmosphere by self-
shielding through the decomposition of core ingredients where the
molten weld pool are protected from the atmosphere by a slag
covering without external shielding.
• advantages
- high deposition rates
- less operator skill required than for gas-metal arc welding (GMAW)
- simpler and more adaptable than submerged arc welding (SAW)
- deeper penetration than shielded metal arc welding (SMAW)
- more tolerant of rust and mill scale than GMAW
Disadvantages
- more smoke and fume are produced in FCAW than in the GMAW and
SAW
processes
- fume extraction is generally required
- equipment is more complex and much less portable than SMAW
equipment
 flux-cored electrode manufacturing process

gas-shielded flux-cored arc welding

4. Resistance Welding
• is a process in which heat is produced by resistance to the flow of high electrical
currents (5 to 10kA) across the interface between two contacting surfaces.
• requires careful cleaning of the surfaces prior to welding.
• require specialized machinery, but do not require filler metals, shielding gases,
or flux.
• induces melting at the interface and a weld fusion zone is created.
- At lower current levels: welding occurs entirely by a solid-state
deformation
• is most commonly used to join two overlapping sheets or plates that may have
different thicknesses and can produce individual spot welds or continuous,
overlapping seam welds.
• is produced by when a pair of electrodes clamp the workpieces under pressure
to provide good electrical contact and produce molten metal in the joint.
- the production of molten metal at the joint is ascribed to the resistance to
the flow of current or heats at the faying surfaces
- the heat generated through the electrical resistance of the two bodies to be
joined is given by
H = I2Rt
where H is the heat generated in joules;
I is the current in amperes;
t is the time of current flow, in seconds.
• The actual temperature rise at the joint depends on the interrelationship of
the three component variables.
• The strength of the bond depends on the
- Pressure applied before the current is activated to ensure a good bond
- the cleanliness of the mating surfaces from oxide layers and
contaminants
- other welding processes.
• Produces rapid rate of heating and rapid rate of cooling.
• operated without shielding because of the combined effects of short time at
temperature, limited access for air, and favorable effects of forging pressure
to breakdown any oxide.
• can be classified into
- resistance spot welding (RSW)
- resistance seam welding (RSEW)
- projection welding (PW)
- Flash welding (FW)
- Seam welding
- Percussion welding
- Upset welding (UW) differs from flash welding in that no arc is
generated by the process.
• consists of
- workpiece to be welded,
- two opposing electrodes
- mechanism to apply pressure to squeeze the workpieces
- AC power supply to maintain the current
- A circuit breaker with times to stop the flowing current after a
preset time.
• .
4.1 Resistance Spot Welding
• Is a process in which faying surfaces are joined in one or more spots by the heat
generated by a short-time pulse of low-voltage, high amperage current that passes
through workpieces that are held together under force by electrodes.
- fusion of the faying surfaces of a lap joint is achieved by opposing electrodes.
• Is used for up to 3 mm thickness of workpiece.
• Couldn't produce air tight or waterproof joints.
• Is the most widely used joining technique for the assembly of sheet metal products
such as automotive body, domestic appliances, furniture, building products,
enclosures and, to a limited extent, aircraft components.
• Has a high operating speeds and suitability for automation or robotization in high-
production assembly lines at high production rates and low unit labor costs using
semiskilled operators
Equipment
• Spot welding machines are composed of three principal elements:
- electrical circuit, which consists of a welding transformer, tap switch, and a
secondary circuit
- control circuit, which initiates and times the duration of current flow and
regulates the welding current
- mechanical system, which consists of the frame, fixtures, and other devices that
hold and clamp the workpiece and apply the welding force
4.2 Resistance Seam Welding
• uses rotating wheels as welding electrodes for making a continuous series of
spot welds along the lap joint.
• the lap joint to be made is allowed to pass between rotating electrodes.
These electrodes press the workpiece and fuse it to make a continuous lap joint.
• is used in production of gasoline tanks, automobile
• produces air tight and leak proof joint.
• produces coalescence at one or more relatively small contact points on the parts.
• is precise, quick and cheaper and preferably used for mass production

Resistant seam welding

4.3 Flash Welding (FW)
• is a resistance welding process in which a butt joint weld is produced by a
flashing action and by the application of pressure.
• is a melting and a forging process.
- Heating of the faying surface is carried out by a combined resistance
and arcing process .Two parts to be welded are each made an electrode of
opposite polarity by hooking them up into an electric circuit.
- as the gap between the two faying surface is reduced,
arcing (flashing) will take place at the high points (asperities) on the faying
surface.
follows heating  softening  melting  explosion.
- Molten metal is expelled, the hot metal is plastically upset, a weld is
produced, and a “flash” of frozen expelled metal is formed.
• is used to
- join metallic parts that have similar cross sections, in terms of size
and shape.
- join all grades of steel, aluminum, brass, and copper parts, in
addition to selected dissimilar materials.
- weld materials with cross sections ranging from 65 to 13,000 mm2
4.4 Percussion Welding
- Is similar to flash welding but duration of welding cycle is very
short.
* Two surfaces to be joined are brought into contact, electric
current is passed through to heat them till melting point
* pressure is applied percussively (by percussion of one part
against the other to form the weld) during or immediately following
the electric discharge
* heating occurs due to resistance of the joint and arc occurs
between them up to some extent.
- Fast heating is accomplished by rapid discharge between the
surfaces to be joined.
- Produces very localized heating.
- Is used in electronic circuit making where ultra accuracy in heat
application is required.
- produces butt joints.