Welding Processes PartII

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

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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.


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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.


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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


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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.


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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


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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

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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


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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.


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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.


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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

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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.


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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.


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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.


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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.


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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.


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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.


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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.


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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.


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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

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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


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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


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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.


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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


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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


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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.


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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


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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


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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


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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


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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)


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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.


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End of the chapter….

Thank you


Welding Technology

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Welding Processes PartII