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Welding Processes PartII
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WELDING PROCESSES
PART-II
ME 473
WELDING TECHNOLOGY
Department of Mechanical Engineering
Instructor: Dr. Oğuzhan Yılmaz
THE UNIVERSITY OF GAZIANTEP
2
Plasma arc welding (PAW): Introduction
PAW is an arc welding process that
uses a constricted arc between a
nonconsumable electrode and the weld
pool (transferred arc) or between the
electrode and the constricted nozzle
(nontransferred arc).
The process is used without the
application of pressure. Filler metal
may or may not be used.
Shielding is obtained from the ionized
gas issuing from the torch, which may
be supplemented by an auxiliary
source of shielding gas.
PAW is also used for metal cutting and
for metal spraying.
Dr. Oğuzhan Yılmaz
Welding Technology
3
The modes of plasma arc welding
PAW: Principles of operation
If an electric arc between a tungsten electrode and the work is constricted or reduced in cross-sectional area, its temperature increases since it carries the same amount of current. This constricted arc is called plasma.
There are two modes of operation: nontransferred arc and transferred arc.
In nontransferred arc mode the current flow is from the electrode inside the torch to the nozzle containing the orifice and back to the power supply.
In the transferred arc mode the current is transferred from the tungsten electrode inside the welding torch through the orifice to the workpiece and back to the power supply.
• The transferred arc mode is the most used for
welding except for very low-current applications.
• The plasma acts as an extremely high
temperature heat source to form a molten weld
pool in the same manner as the TIG.
Dr. Oğuzhan Yılmaz
Welding Technology
4
PAW: Advantages and major uses
Advantages of PAW when compared to TIG stem from the fact that PAW has a
higher energy concentration. Its higher temperature, constricted cross-sectional
area, and the velocity of the plasma jet create a higher heat content.
• The torch-to-work distance is less critical than for TIG more freedom to observe
and control the weld.
• The HAZ and the form of the weld are more desirable. The HAZ is smaller than
with TIG, and the weld tends to have more parallel sides, which reduces angular
distortion.
• The higher heat concentration and the plasma jet allow for higher travel speeds.
PAW has deeper penetration and produces a narrower weld.
PAW is used the manufacturing of tubing, components made of thin metal, root-pass
welds on pipe.
PAW is normally applied as a manual process. Automatic and mechanized app. (limited)
Join practically all of the commercial metals.
Filler rod is used for making welds in thicker materials.
Dr. Oğuzhan Yılmaz
Welding Technology
5
PAW: Equipment required
Power source; A CC drooping characteristic power source supplying dc welding
current is recommended, however ac/dc power source can be used.
80 V open circuit voltage and max. 500 A for PAW applications.
Control circuit; The plasma torch connects to the control console or to the power
source. The control console includes a power source for the pilot arc, a timer,
water and gas valves, separate flowmeters for the plasma gas and the shielding
gas. Usually the console is connected to the power source.
A wire feeder may be used for mechanized or automatic welding.
Circuit diagram of PAW PAW torch and wire feeder
Dr. Oğuzhan Yılmaz
Welding Technology
6
PAW: Materials required
Filler metals is used except when welding the thinnest metal. The composition
of the filler metal should match the base metal. The size of the filler metal rod
depends on the thickness of the base metal and the welding current.
Plasma and shielding gas: An inert gas, either argon, helium, or a mixture, is
used for shielding the weld area from the atmosphere. Argon is more
commonly used since it is heavier and provides better shielding at lower rates.
For flat and vertical welding;
The shielding gas flow rate 7 to 14 lt/min
For overhead positions;
Argon is usually used for plasma gas and flow rate 0.5 lt/min to 2.4 lt/min
depending on the torch size and application.
Limitations: Equipment and apparatus are delicate and complex. The torch must be
water cooled. The tip of tungsten and orifice must be maintained within very close limits.
Dr. Oğuzhan Yılmaz
Welding Technology
7
Arc welding with consumable electrode
Electrode is melted and the molten
metal is carried across the gap.
A uniform arc length is maintained
between the melting end of the
electrode and weld pool.
Shielded metal arc welding
Gas metal arc welding
Flux cored arc welding
Electro slag welding
Submerged arc welding
The electrode is continiously fed
into the arc and is melted by the
heat of the arc as a deposition.
Arc region of the consumable electrode arc
Dr. Oğuzhan Yılmaz
Welding Technology
8
Arc welding with consumable electrode
Good quality of welding and high-productivity welding depend on two major factors:
The penetration of the weld into the base metal
The melt-off rate of the electrode
The maximum heat normally occurs at cathode.
When straight polarity welding (DCEN), the melt-off rate is high, but the penetration of the base metal is low.
When DCEP welding, the max. heat still occurs at cathode, deep penetration occurs.
Polarity and heat relationship
High current electrode melt-off rapidly
Low current melt-off slowly
Melt-off: The heat required to melt the electrode
is a physical relationship between the current
and the weight of metal melted, known as melt-
off or burn-off rate which is the weight of metal
melted per unir time. Dr. Oğuzhan Yılmaz
Welding Technology
9
Melt-off rate and its factors Factors that affect the melt-off rate:
Melting point of the material (i.e. Al has higher melt-of rate)
The size of electrode wire: based on current density I / cross-section area
Electrode extension: resistance to heat
Steel-Wire size
Magnesium-Wire size
Aluminium -Wire size
Dr. Oğuzhan Yılmaz
Welding Technology
10
Metal transfer across the arc
The forces that cause metal to
transfer across the arc are similar for
all the cosumable electrode arc
welding.
The metal being transferred ranges
from small droplets, smaller than the
diameter of the electrode, to droplets
much larger in diameter than the
electrode.
The mechanism of transferring liquid
metal across the arc gap is
controlled by:
Surface tension
The plasma jet
Gravity
Electromagnetic force
Electromagnetic force on drop about to transfer
•The combination of these forces that acts on the molten droplet and determines the transfer
mode.
Causes the surface of the liquid to contract to the smallest possible area
Molten metal drops in flight are accelerated toward the workpiece
Tends the detach the liquid drop
Acts to detach a molten drop at the tip of the electrode.
Dr. Oğuzhan Yılmaz
Welding Technology
11
Modes of metal transfer
The mode of metal transfer across the arc is related to the welding process;
The metal involved
The arc atmosphere
The size, type and polarity of the electrode
The characteristics of the power source
The welding position
Welding current, current density, and heat input
The most common way to classify metal transfer is according to size and
frequency and characteristics of the metal drops being transferred.
Four major types of metal transfer:
Spray transfer
Globular transfer
Short-circuiting transfer
Pulsed-spray metal transfer
There is an intermediate form of transfer in the transition zone between two modes
where both types of transfer may occur simultaneously.
Dr. Oğuzhan Yılmaz
Welding Technology
12
Modes of metal transfer
Spray transfer: The drops of molten metal are
approximately the same size as the electrode
wire. It occurs in an inert gas atmosphere, %80
argon. Smooth transfer, large weld pool, good
penetration, not used for thin materials.
Globular transfer: The molten globule can grow
in size until its dia reaches 1.5 to 3 x D electrode.
It usually occurs when CO2 shielding gas used.
Very deep penetration, only used in flat pos.
Used for heavy steel sections.
Short-circuiting transfer: It is a low-energy
mode of transfer. The molten tip may grow up to
1.5 times the electrode dia. Weld pool is small,
not used on nonferrous metals.
Pulsed-spray metal transfer: It produces
droplets of approximately the same or smaller
size than the electrode dia. It is based on a
special pulsed waveform of the welding current.
Type of pulsing is difficult to adjust, never
become popular.
Spray transfer Globular transfer
Short-circuiting
transfer Pulsed-spray
transfer Dr. Oğuzhan Yılmaz
Welding Technology
13
Shielded metal arc welding (SMAW) SAW is an arc welding
process with an arc between a
covered electrode and the
weld pool.
The process is used with
shielding from the
decomposition of the electrode
covering, without the
application of pressure, and
with filler metal from the
electrode.
SAW is also known as stick
electrode welding.
Dr. Oğuzhan Yılmaz
Welding Technology
14
SMAW: Principles of operation
It consists of an arc between a covered electrode and the base metal.
The arc is initiated by touching the electrode momentarily to the workpiece.
The heat of the arc melts the surface of the base metal to form a molten
pool. The metal melted from the electrode is transferred across the arc into
the molten pool.
The size of the weld pool and the depth of penetration determine the mass of
molten metal under the control of welder.
If current is too high, the depth of penetration will be excessive and the
volume of molten weld metal will become uncontrollable.
A higher speed of travel reduces the size of the molten weld pool.
The weld metal deposit is covered by a slag from the electrode covering.
The arc in the immediate arc area is enveloped by an atmosphere of
protective gas produced by the disintegration of the electrode coating.
Dr. Oğuzhan Yılmaz
Welding Technology
15
SMAW: Advantages and major uses
It is the most popular arc welding process.
It has maximum flexibility and can weld many metals in all positions from near minimum to maximum thickness.
The investment for equipment is small.
It is used in manufacturing and in field work for construction and maintenance.
The method of application is manual. Semiautomatic and mechanised methods are not used. Automatic method can be used.
Welding in the horizontal, vertical and overhead positions are possible depends on the type and size of the electrode, as well as the welding current and the skill of the welder.
Dr. Oğuzhan Yılmaz
Welding Technology
16
SMAW: Equipment required
The welding machine or power
source: to provide electric power of
the proper current (CC, 25A-500A)
and voltage (15 to 35V).
Electrode holder, held by the welder.
It firmly grips the electrode and
transmits the welding current to it.
Dr. Oğuzhan Yılmaz
Welding Technology
17
SMAW: Material used
The covered electrode is the only item of material normally required.
The selection of the covered electrode is based on the electrode usability
and the composition and the properties of the deposited weld.
The coating on the electrode provides:
gas from the decomposition of certain ingredients of the coating to shield the
arc from the atmosphere.
the deoxidizers for scavenging and purifying the weld deposited
slag formers to protect the deposited weld
ionzing elements to make the arc more stable
alloying elements to provide special characteristics to the deposited weld
iron powder to improve productivity of the electrode.
Dr. Oğuzhan Yılmaz
Welding Technology
18
Gas metal arc welding (GMAW)
It is an arc welding process that uses an
arc between a continious filler metal
electrode and the weld pool.
The process is used with shielding from
an externally supplied gas and without the
application of pressure.
This is also called as metal inert gas
(MIG) or metal active gas (MAG) welding.
There are many variations depending on
the type of shielding gas, the type of metal
transfer, the type of metal welded etc.
Dr. Oğuzhan Yılmaz
Welding Technology
19
MIG: Principles of operation
MIG welding utilizes the heat of an arc
between a continiously fed consumable
electrode and the work to be welded.
The heat of the arc melts the surface of
the base metal and the end of the
electrode.
The metal melted off the electrode is
transferred across the arc to the molten
pool.
The penetration is mainly controlled by
welding current.
The width of the molten pool is mainly
controlled by the travel speed.
Shielding of the molten pool, the arc,
and the surrounding area is provided
by an envelope of gas fed through the
nozzle.
• The shielding gas may be an inert gas,
an active gas, or a mixture, surrounds
the arc area to protect it from
contamination from the atmosphere.
Dr. Oğuzhan Yılmaz
Welding Technology
20
MIG: Advantages and major uses MIG welding is one of the most popular arc welding process.
Continuous wire feed
Automatic self-regulation of the arc length
High deposition rate and minimal number of stop/start locations
Welder has good visibility of weld pool and joint line
Little or no post weld cleaning
Can be used in all positions
Wide range of application: sheet metal industry, pipe welding.
Disadvantages:
High level of equipment maintenance
No independent control of filler addition
Lower heat input can lead to high hardness values
Joint and part access is not as good as TIG (tungsten inert gas) welding
• The MIG process uses semiautomatic, mechanised, or automatic equipment.
• In semiautomatic welding, the wire feed rate and arc length are controlled automatically.
• In mechanised welding, all parameters are under automatic control.
• With automatic equipment, there is no manual intervention during welding.
Dr. Oğuzhan Yılmaz
Welding Technology
21
MIG: Equipment required
A MIG system consists of:
Power source, 50-500A and 10-50V
The electrode wire feeder and control system
The welding gun and cable assembly for semiautomatic welding or the welding torch for automatic welding
The gas and water control system for the shielding gas and cooling water
Travel mechanism and guidance for automatic welding
• MIG welding requires high current at a relatively low voltage.
• Three generic types of power source are suitable for MIG welding: AC/DC transformer rectifier or inverter, and DC generator.
• Wire feed unit is to feed the consumable wire at a constant rate.
• High performance feed units are capable of delivering wire at up to 30m/min. Dr. Oğuzhan Yılmaz
Welding Technology
22
MIG: Material used Two materials are used for the MIG welding process:
The electrode: following factors govern the selection of the electrode;
Metal to be welded: the composition and mechanical properties of the base
metal are of primary importance.
Thickness and joint design: thicker sections and complex joint designs
require filler metals that provide high weld metal ductility.
Surface conditions: the surface of the base metal
Specifications or service conditions
The shielding gas
Selecting of the shielding gas involves;
Electrode,the base metal and welding position
• The shielding gas will have a substantial effect on the stability of the arc and metal
transfer and the behaviour of the weld pool, in particular, its penetration.
• General purpose shielding gases for MIG welding are mixtures of argon, oxygen and CO2,
and special gas mixtures may contain helium. The gases which are normally used for the
various materials are:
• Steels CO2, argon +2 to 5% oxygen, argon +5 to 25% CO2
• Non-ferrous argon, argon / helium
Dr. Oğuzhan Yılmaz
Welding Technology
23
Submerged arc welding (SAW) SAW is an arc welding process that uses
an arc or arcs between a bare metal
electrode or electrodes and the weld pool.
The arc and molten metal are shielded by
a blanket of granular flux on the
workpieces.
The process is used without pressure and
with filler metal from the electrode and
sometimes from a supplemental source
(welding rod, flux, or metal granules).
It is normally automatic process.
It is also known as under powder welding
or smothered arc welding
Dr. Oğuzhan Yılmaz
Welding Technology
24
SAW: Principles of operation
SAW utilizes the heat of an arc between a continiously fed electrode and the
work.
The heat of the arc melts the surface of the base metal and the end of the
electrode.
Shielding is obtained from a blanket of granular flux, which is laid directly
over the weld area.
The flux close to the arc melts and intermixes with the molten weld metal
and helps purify and fortify it.
The flux forms a glasslike slag that is lighter in weight than the deposited
weld metal and floats on the surface as a protective cover.
The electrode is fed into the arc automatically and travel can be manual or
by machine.
The metal transfer mode is less important in SAW.
Dr. Oğuzhan Yılmaz
Welding Technology
25
SAW: Advantages and major uses
High quality weld metal
Extremely high deposition rate and speed
Smooth, uniform finished weld with no spatter
Little or no smoke
No arc flash, thus minimal need for protection
High utilization of electrode wire
Easily automated for high operator factor
Highly used in heavy steel plate fabrication work:
welding of structural shapes and the longitudinal seam of larger diameter pipe
manufacture of machine components of heavy industry
vessels and tanks for pressure and storage
Shipbuilding (fabrication of subassemblies)
Steels in medium and heavy thickness
Dr. Oğuzhan Yılmaz
Welding Technology
26
SAW: Equipment required Welding machine or power source (CC, ac or dc can be supplied)
Wire feeder and control system
Welding torch for automatic welding
Flux hopper and feeding mechanism
Travel mechanism for automatic welding
Dr. Oğuzhan Yılmaz
Welding Technology
27
SAW: Material used
Two materials are used in submerged arc welding: The welding flux:
Shields the arc and molten weld metal from atmospheric oxygen and nitrogen.
The flux contains deoxidizers and scavengers, which help remove impurities from the weld metal.
Introducing alloys into the weld metal
The flux that melts and forms the slag covering must be removed from the weld (easily done after the weld cools)
The flux is selected based on the mechanical properties required of the weld deposit.
Fluxes may be neutral or active: Neutral fluxes will not produce any significant changes in weld metal chemistry and used
for multi-pass applications.
Active fluxes contain small amounts of maganese and/or silicon used to reduce porosity and weld cracking and used for single pass applications.
The consumable electrode:
In SAW, it is necessary to select and electrode and flux combination to match the base metal composition and properties.
Dr. Oğuzhan Yılmaz
Welding Technology
28
Gas welding: Oxyfuel gas welding
Oxyfuel gas welding (OFW) is a group of welding processes that produce
coalescence of workpieces by heating them with an oxyfuel gas flame.
The processes are used with or without the application the pressure, and with or
without filler metal.
Major processes are:
Oxyacetylene welding (most popular, uses acetylene as the fuel gas)
Oxyhydrogen welding (uses hydrogen as the fuel gas, not popular)
Pressure gas welding
Dr. Oğuzhan Yılmaz
Welding Technology
29
Oxyacetylene welding (OAW) OAW process consist of high temperature flame
produced by the combustion of acetylene with
oxygen and directed by a torch.
The intense heat of the flame 3482ºC melts the
surface of the base metal to form a molten pool.
Filler metal is added to fill gaps or grooves. As
the flame moves along the joint, the melted
base metal and filler metal solidify to produce
the weld.
The temperature of the oxyacetylene flame is
not uniform throughout its length and the
combustion is also different in different parts of
the flame.
The temperature is the highest just beyond the
end of the inner core and decreases gradually
toward the end of the flame. Outer envelope
Acetylene feather İnner core
Flame temp ºC
Dr. Oğuzhan Yılmaz
Welding Technology
30
OAW: Chemical reactions
The chemical reaction for a 1:1 ratio of acetylene and
oxygen plus air is:
C2H2 + O2 = 2CO + H2 + Heat
This is the primary reaction, however, both carbon monoxide
and hydrogen are combustible and will react with oxygen
from the air:
2CO +H2 + 1.5O2 = 2CO2 + H2O + Heat
This is the secondary reaction, which produces carbon
dioxide, heat and water.
There are three basic flame types:
Neutral (or balanced): 1:1 ratio of oxygen and
acetylene. It obtains additional oxygen from the air for
complete combustion. Generally preferred.
Excess acetylene (carborazing): indicated in the flame
when the inner cone has a feathery edge extending
beyond it. It may add carbon to the weld metal.
Excess oxygen (oxidising):has a shorter envelope and
a small pointed white cone. The flame tends to oxidise
the weld metal and is used only for welding specific
metals.
Carburising flame
Oxidising flame
Neutral flame
Dr. Oğuzhan Yılmaz
Welding Technology
31
OAW: Advantages and uses The equipment is very portable, relatively inexpensive, versatile, and can be used
in all welding positions.
OAW can be used for welding, brazing, soldering, and with proper attachments,
flame cutting.
The equipment can also be used for bending, forming, straightening, hardening
and so on.
Normally used as a manual. It can be mechanised (not common), Semiautomatic
applications (rarely)
Welding is possible for most of the common metals
Normally used for welding thinner materials up to 6.4mm.
Industrial applications are in the field of maintenance and repair, and welding
small-diameter pipe.
The chemical action of the oxyacetylene flame can be adjusted by changing the
ratio of the volume of oxygen to acetylene.
Gases such as propane, hydrogen and coal gas can be used for joining lower
melting point non-ferrous metals, and for brazing and silver soldering
Dr. Oğuzhan Yılmaz
Welding Technology
32
OAW: Equipment required
The apparatus and equipment:
Welding torch and tips: the function
of mixing the fuel gas with oxygen
and provides the required flame. It
contains a handle and valves for
regulating the gases.
Oxygen and acetylene hose
Oxygen and acetylene regulators:
reduce the pressure of the gas in the
cylinder or supply system to the
pressure used in the torch. Oxygen
1 to 25 psi, acetylene 1 to 12
psi, reduced working pressures.
Oxygen cylinders, 2200 psi (15.2
MPa)
Acetylene cylinders, 250psi (1.7 MPa)
Dr. Oğuzhan Yılmaz
Welding Technology
33
OAW: Material used
Gases:
Oxygen: stored within a high
temperature cylinders and supplied
through a piping system to the
welding station.
Acetylene: or fuel gases, supplied
in a cylinder to the welding station.
The acetylene may be supplied to
the piping system by manifold
cylinders or by an acetylene
generator which produces
acetylene at the plant site by the
reaction of carbide and water.
Welding rod: selected according to
base metal.
Dr. Oğuzhan Yılmaz
Welding Technology
34
End of the chapter….
Thank you
Dr. Oğuzhan Yılmaz
Welding Technology