Production Engineering II

AAiT

Chapter Three

Welding & Bonding Technology

Introduction to welding

• Joining is generally used for welding, brazing,

soldering, and adhesive bonding which form a

permanent joint between the parts.

• Welding is a materials joining process in which

two or more parts are coalesced at their

contacting surfaces by a suitable application of

heat and/ or pressure.

• Many welding processes are accomplished by

heat alone, others by a combination of heat and

pressure, and still others by pressure only.

• Welding process is divided into two major groups:

1. Fusion welding and

2. Solid-state welding

1. Fusion welding: use heat to melt the base metals. In many fusion welding operations, a filler metal is added to the molten pool to facilitate the process and provide bulk and strength to the welded joint. The fusion category includes: Arc welding (AW), Resistance welding (RW), Oxyfuel gass welding (OFW), Electron beam welding, and Laser beam welding.

2. Solid-state welding: joining process in which coalescence results from application of pressure alone or a combination of heat and pressure. If heat is used, the temperature in the process is below the melting

point of the metal being welded, no filler metal is utilized.

The solid state category includes: Diffusion welding (DW), Friction welding (FRW), Ultrasonic welding (USW).

Arc welding (AW)

• A fusion welding process in which coalescence of

the metals is achieved by the heat from an

electric arc between an electrode and the work

�Electric energy from the arc produces

temperatures 10,000 oF (5500 oC), hot enough

to melt any metal

�Most AW processes add filler metal to increase

volume and strength of weld joint

Arc Welding (AW)• A pool of molten metal is formed near

electrode tip, and as electrode is moved along joint, molten weld pool solidifies in its wake

Figure.1.1 Basic configuration of an arc welding process.

Types of Arc welding electrodes

i. Consumable: consumed during welding process. Source

of filler metal in arc welding

ii. No consumable: not consumed during welding

process. Filler metal must be added separately.

Arc shielding

• At high temperatures in AW, metals are chemically reactive to

oxygen, nitrogen, and hydrogen in air �Mechanical properties of joint can be seriously 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

• Flux

Flux is substance that prevents formation of oxides and other contaminants in welding, or dissolves them and facilitates removal

• Protecting the molten weld metal from atmosphere

• Stabilizes arc

• Reduces spattering

Power source in arc welding

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

� AC machines 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

Consumable electrodes -AW processes

welding processes which uses consumable electrode:

– Shielded metal AW (SMAW)

– Gas metal AW (GMAW)

– Flux-cored AW (FCAW)

– Electro gas welding (EGW)

– Submerged AW (SAW)

Non consumable electrodes- AW processes

Welding process which uses non-consumable electrodes:

•Gas Tungsten AW (GTAW)

•Plasma AW (PAW)

•Carbon AW (CAW)

Shielded metal arc welding (SMAW)

• Uses a consumable electrode consisting of a filler metal rod coated with

chemicals that provide flux and shielding

• Application: construction, pipelines, machinery structures, fabrication job

shops and repair work.

• The equipment is portable and low cost.

• Welding stick is clamped in electrode holder connected to power source.

• 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.

Gas metal arc welding (GMAW)

• 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.

• Wire diameter used for GMAW is 0.8 to 6.5mm. Gases used for shielding includes inert gas such as argon and helium, and active gases such as carbon dioxide.

• 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 manual grinding and cleaning of slag

• Used in fabrication operations for welding of ferrous and non-ferrous metals.

• It uses continuous weld wire rather than welding stick

Figure.1.3 Gas metal arc welding (GMAW).

Flux-cored arc welding

• Adaptation of shielded metal arc welding, to overcome limitations imposed by the use of stick electrodes.

• Electrode is a continuous consumable tubing that contains flux and other ingredients in its core.

there are two versions of FCAW:

�Self-shielded FCAW- core includes compounds that produce shielding gases

�Gas-shielded FCAW- uses externally applied shielding gases

Figure.1.4 FCAW, Presence or absence of externally supplied shielding gas distinguishes the two types: (1) self-shielded, in which core provides ingredients for shielding, and (2) gas-shielded, which uses external shielding gases.

Electro gas welding (EGW)

• Uses a continuous consumable electrode, flux-cored wire or bare wire with externally supplied shielding gases, and molding shoes to contain molten metal

• When flux-cored electrode wire is used and no external gases are supplied, then special case of self-shielded FCAW

• When a bare electrode wire used with shielding gases from external source, then special case of GMAW

Figure.1.5 Electro gas welding using flux-cored electrode wire: (a) front view with molding shoe

removed for clarity, and (b) side view showing molding shoes on both sides.

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 operation, preventing sparks, spatter,

and radiation

• Used in steel fabrication for structural shape (e.g. welded I-beams);

seams for large diameter pipes, tanks, and pressure vessels; and

welded components for heavy machinery.

Gas Tungsten arc welding (GTAW)

• Uses a non-consumable tungsten electrode and an inert gas for arc shielding

• Melting point of tungsten = 3410°C (6170°F)

• Tungsten inert gas welding (TIG) or (WIG)

• Used with or without filer metalWhen filler metal used, it is added to weld pool from separate rod or wire

• Application: Aluminium and stainless steel mostly

GTAWAdvantages:

• 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 28,000°C (50,000°F), due to constriction of

arc, producing a plasma jet of small diameter and very high energy density

Figure.1.8 Plasma arc welding (PAW)

Advantages of PAW:• Good arc stability and excellent weld quality• Better penetration control than other AW processes• High travel speeds• Can be used to weld almost any metals

Disadvantages:� High equipment cost � Larger torch size than other AW processes� 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)

Figure.1.9 Resistance welding, showing the

components in spot welding, the main

process in the RW group.

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

Advantages of RW:• 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 of RW:� High initial equipment cost� Limited to lap joints for most RW processes

Oxyfuel gas welding (OFW)

• Group of fusion welding operations that burn various fuels mixed with oxygen

• OFW employs several types of gases, which is the primary distinction among the members of this group

• Oxyfuel gas is also used in flame cutting torches to cut and separate metal plates and other parts

• Most important OFW process is oxyacetylene welding

Oxyacetylene Welding (OAW)

• Fusion welding performed by a high temperature flame from combustion

of acetylene and oxygen

• Flame is directed by a welding torch

• Filler metal is sometimes added

� Composition must be similar to base metal

� Filler rod often coated with flux to clean surfaces and prevent oxidation

Figure.1.10 A typical oxyacetylene welding operation (OAW)

• Most popular fuel among OFW group because it is capable of

higher temperatures than any other

� Up to 3480°C (6300°F)

• Two stage reaction of acetylene and oxygen:– First stage reaction (inner cone of flame)

C2H2 + O2 → 2CO + H2 + heat

– Second stage reaction (outer envelope)2CO + H2 + 1.5O2 → 2CO2 + H2O + heat

• The two stages of combustion are visible in the oxyacetylene flame emitted from the torch.

• When the mixture of acetylene and oxygen is in the ratio 1:1, it results neutral flame.

• Maximum temperature reached at tip of inner cone, while outer envelope spreads out and shields work surface from atmosphere.

Figure.1.11 The neutral flame from an oxyacetylene torch indicating temperatures achieved.

Safety issue in OAW• Together, acetylene and oxygen are highly flammable

• C2H2 is colorless and odorless– It is therefore processed to have characteristic garlic odor

• C2H2 is physically unstable at pressures much above 15 lb/in2 (about 1 atm)

– Storage cylinders are packed with porous filler material saturated with acetone (CH3COCH3)

– Acetone dissolves about 25 times its own volume of acetylene

• Different screw threads are standard on C2H2 and O2

cylinders and hoses to avoid accidental connection of wrong gases

Alternative Gases for OFW: Methylacetylene-Propadiene (MAPP), Hydrogen, Propylene, Propane, and natural gas.

Soldering and Brazing• Soldering and brazing process lie some where in between fusion welding

and solid state welding.

• These processes have some advantages over welding process. They canjoin metals having poor weldability, dissimilar, with very less amount ofheating needed.

• The major disadvantage is joint made by soldering and brazing has lowstrength as compared to welded joint.

• The major difference between them is huge amount of heat isrequired in case of welding, low amount in case of brazing and verylow amount in case of soldering.

• Joints made by brazing are relatively weak and soldering joints arevery weak to bear load. Generally, soldering is used to makeelectrical contacts.

Ways to choose joining methods:

• There are no hard-and-fast rules.

• In general, the controlling factors are:

– the types of metal you are joining,

– cost involved, nature of the products you are fabricating, and

– the techniques you use to fabricate them.

Because of its flexibility and mobility, gas welding is widely used for

maintenance and repair work in the field.

On the other hand, gas shielded metal arc welding to repair a critical

piece of equipment made from aluminum or stainless steel.

Welding Terminology

1. FILLER METALS

The material that you add to fill space during the welding process

is known as the filler metal, or material.

Two types of filler metals commonly used in welding are welding

rods and welding electrodes.

welding rods

1. Refers to a form of filler metal

that does not conduct an

electric current during the

welding process.

2. The only purpose of a welding

rod is to supply filler metal to

the joint.

3. This type of filler metal is often

used for gas welding.

welding electrodes

1. conducts the current from the electrode

holder to the metal being welded.

2. Consumable electrodes :not only provide a

path for the current but they also supply

fuller metal to the joint (shielded metal- arc

welding).

3. Non consumable electrodes : are only used

as a conductor for the electrical current (gas

tungsten arc welding). The filler metal for

GTAW is a hand fed consumable welding rod.

Welding Terminology2. FLUXES

• The term flux refers to a material used to dissolve oxides and releasetrapped gases and slag (impurities) from the base metal; thus the fluxcan be thought of as a cleaning agent. In performing this function, theflux allows the filler metal and the base metal to be fused.

No single flux is satisfactory for universal use; however, there are a lot of good

general-purpose fluxes for use with common metals.

In general, a good flux has the following characteristics:

A. It is fluid and active at the melting point of the fuller metal.

B. It remains stable and does not change to a vapor rapidly within the

temperature range of the welding procedure.

C. It dissolves all oxides and removes them from the joint surfaces.

D. It adheres to the metal surfaces while they are being heated and does

not ball up or blow away.

E. It does not cause a glare that makes it difficult to see progress.

F. It is easy to remove after the joint is welded.

G. It is available in an easily applied form.

Welding JointsThe weld joint is where two or more metal parts are joined by welding.

The five basic types of weld joints are the butt, corner, tee, lap, and edge.

• A butt joint is used to join two members aligned in the same

plane. This joint is frequently used in plate, sheet metal, and pipe

work.

• Corner and tee joints are used to join two members located at

right angles to each other. Various joint designs of both types have

uses in many types of metal structures.

• A lap joint, as the name implies, is made by lapping one piece of

metal over another. This is one of the strongest types of joints

available; however, for maximum joint efficiency, you should

overlap the metals a minimum of three times the thickness of the

thinnest member you are joining. Lap joints are commonly used

with torch brazing and spot welding applications.

• An edge joint is used to join the edges of two or more members

lying in the same plane. It is more frequently used in sheet metal

work

Standard Terms (Parts of Joints)1. The root of a joint is that portion of the joint where the metals are

closest to each other. The root may be a point, a line, or an area,

when viewed in cross section.

2. Groove is an opening or space provided between the edges of the

metal parts to be welded.

• The specified requirements for a particular joint are expressed in

terms as bevel angle, groove angle, groove radius, and root opening.

• Root penetration refers to the depth that a weld extends into the

root of the joint. Root penetration is measured on the center line of

the root cross section.

• Joint penetration refers to the minimum depth that a groove weld

extends from its face into a joint, exclusive of weld reinforcement.

• Weld reinforcement is a term used to describe weld metal in excess

of the metal necessary to fill a joint.

Fig: Root penetration and joint penetration

of welds

Simple weld beadWeld reinforcement

Square groove welds

Single Bevel groove welds Double Bevel groove welds

Single V groove welds Double V groove welds

Fig: Standard groove welds

Weld Quality

• Concerned with obtaining an acceptable weld

joint that is strong and absent of defects

• Also concerned with the methods of inspecting

and testing the joint to assure its quality

Welding defects:

1. Cracks

2. Cavities

3. Solid inclusions

4. Incomplete fusion

Welding cracks• Fracture-type interruptions either in weld or in base metal

adjacent to weld

• Serious defect because it is a discontinuity in the metal that significantly reduces strength

• Caused by embrittlement or low ductility of weld and/or base metal combined with high restraint during contraction

• In general, this defect must be repaired

Figure.1.13 Various forms of welding cracks.

Cavities

• Two defect types, similar to defects found in

castings:

1. Porosity - small voids in weld metal formed by

gases entrapped during solidification

- Caused by inclusion of atmospheric gases, sulfur in weld metal,

or surface contaminants

2. Shrinkage voids - cavities formed by shrinkage

during solidification

Solid inclusions

• Solid inclusions - nonmetallic material entrapped

in weld metal

• Most common form is slag inclusions generated

during AW processes that use flux

- Instead of floating to top of weld pool, globules of slag become

encased during solidification

• Metallic oxides that form during welding of

certain metals such as aluminum, which normally

has a surface coating of Al2O3

Incomplete fusion

• Also known as lack of fusion, it is simply a weld

bead in which fusion has not occurred

throughout entire cross section of joint

Figure.1.14 several forms of incomplete fusion

Inspection and testing methods1. Visual inspection

• Most widely used welding inspection method

• Human inspector visually examines for:- dimension, warpage, Cracks, cavities, incomplete fusion, and other surface defects

• Limitations:� Only surface defects are detectable

� Welding inspector must also decide if additional tests are warranted .

2. Nondestructive evaluation tests (NDT)

� Ultrasonic testing - high frequency sound waves through specimen to detect cracks and inclusions

� Radiographic testing - x-rays or gamma radiation provide photograph of internal flaws

� Magnetic particle testing – iron filings sprinkled on surface reveal subsurface defects by distorting magnetic field in part

Welding Symbols

� Special symbols are used on a drawing to specify where welds are to be

located, the type of joint to be used, as well as the size and amount of

weld metal to be deposited in the joint.

� These symbols have been standardized by the American Welding Society

(AWS).

The reference line becomes the

foundation of the welding symbol. It is

used to apply weld symbols,

dimensions, and other data to the

weld. The arrow simply connects the

reference line to the joint or area to

be welded. The tail of the welding

symbol is used only when necessary to

include a specification, process, or

other reference information.

Welding Symbols

Example: Weld symbols

applied to reference line

Filet Weld: Arrow Side

Filet Weld: Other Side

Filet Weld: Both Side

SPECIFING WELDING LOCATION DIMENSIONS APPLIED TO

SYMBOLS

And of course Do staffs Safe all the time

specially Welding!!

Use PPE (Personal Protection Equipments)

Personal protection equipments (PPE)

� Protect your eyes with safety glasses.

� Protect your body from welding spatter and arc flash with protective

clothing:

� Woolen clothing (possibly cotton) — never synthetic!

� Welding jackets

� Flame-proof apron

� Gloves

� Properly fitted clothing that is not frayed or worn

� Long-sleeve shirts

� Straight-leg trousers that cover shoes

� Fire resistant cape or shoulder covers for overhead work

� Leathers to protect specific body parts or areas

� Keep clothes free of grease and oil.

The End