Workshop Practice

Lecture 7-10
By Mansoor Dawson
Machine Shop
Welding

• A welded join is a
permanent joint,
obtained by the
fusion of the edges of
the two parts to be
joined together, with
or without the
application of
pressure and a filler
material
Welding Classification

1. Fusion welding - coalescence is accomplished by melting

the two parts to be joined, in some cases adding filler
metal to the joint
Examples: arc welding, resistance spot welding,
oxyfuel gas welding

2. Solid state welding - heat and/or pressure are used to

achieve coalescence, but no melting of base metals
occurs and no filler metal is added

Examples: forge welding, diffusion welding, friction

welding
Welding Classification
Welding Classification
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 F (5500 C), hot enough to melt any metal

Most AW processes add filler metal to increase volume

and strength of weld joint
Arc Welding (AW)
What is an Electric Arc?

An electric arc is a discharge of electric current across

a gap in a circuit

It is sustained by an ionized column of gas

(plasma) through which the current flows

To initiate the arc in AW, electrode is brought into

contact with work and then quickly separated from it
by a short distance
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
Two Basic Types of AW Electrodes

Consumable – consumed during welding process

Source of filler metal in arc welding

Nonconsumable – not consumed during welding

process

Filler metal must be added separately if it is added

Consumable Electrodes

Forms of consumable electrodes

Welding rods (a.k.a. sticks) are 9 to 18 inches and 3/8

inch or less in diameter and must be changed
frequently
Weld wire can be continuously fed from spools with
long lengths of wire, avoiding frequent interruptions

In both rod and wire forms, electrode is consumed

by the arc and added to weld joint as filler metal
Nonconsumable Electrodes

Made of tungsten which resists melting

Gradually depleted during welding (vaporization is

principal mechanism)

Any filler metal must be supplied by aseparate wire

fed into weld pool
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
Flux
Flux

A substance that prevents formation of oxides and

other contaminants in welding, or dissolves them
and facilitates removal

Provides protective atmosphere for welding

Stabilizes arc

Reduces spattering
Power Source in Arc Welding

Direct current (DC) vs. Alternatingcurrent (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 Electrode AW Processes

Shielded Metal ArcWelding

Gas Metal Arc Welding (GMAW or MIG)

Flux-Cored ArcWelding

Electrogas Welding

SubmergedArc 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"

Power supply, connecting cables, and electrode

holder available for a few thousand dollars
Welding Electrode or Stick
Composition of filler metal usually close to base metal

Coating: powdered cellulose mixed with oxides and

carbonates, and held together by a silicate binder

Welding stick is clamped in electrode holder connected to

power source

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
Weld Appearances
Weld Appearances
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 as pool

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 manual grinding and cleaning of slag
GMAW Torch
GMAW Advantages over SMAW
Better arc time because of continuous wire electrode

Sticks must be periodically changed in SMAW

Better use of electrode filler metal than SMAW

End of stick cannot be used in SMAW

Higher deposition rates

Eliminates problem of slag removal

Can be readily automated

Weld Appearances
Nonconsumable Electrode Processes

Gas Tungsten Arc Welding (TIG Welding)

Plasma Arc Welding

Carbon Arc Welding

Stud Welding
 Gas Tungsten Arc Welding
(GTAW/ TIG)
Uses a nonconsumable tungsten electrode and an inert gas
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: aluminum and stainless steel mostly

Advantages and Disadvantages

Advantages:

High quality welds for suitable applications

No spatter because no filler metal througharc

Little or no post-weld cleaning because noflux

Disadvantages:

Generally slower and more costlythan consumable electrode

AW processes
Welding Positions
Welding Defects

Cracks

Cavities

Solid inclusions

Imperfect shape or unacceptable contour

Incomplete fusion

Miscellaneous defects
Welding Defects
Welding Defects
Welding Joints

• Butt
• Corner
• Lap
• Tee
• Edge
Butt Joint
Tee Joint
Lap Joint
Corner Joint
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)
Resistance Welding

Resistance welding,
showing 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
Resistance Spot Welding (RSW)

Resistance welding process in which fusion of faying

surfaces of a lap joint is achieved at one location by
opposing electrodes

Used to join sheet metal parts

Widely used in mass production of automobiles, metal

furniture, appliances, and other sheet metal products

Typical car body has ~ 10,000 spot welds

Spot Welding Cycle

(a) Spot welding cycle

(b) Plot of force and
current
Cycle: (1) parts
inserted between
electrodes, (2)
electrodes close, (3)
current on, (4) current
off, (5) electrodes
opened
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
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 (ethyne) 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
Acetylene (C2H2)

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
Oxyacetylene Torch

Maximum temperature reached at tip of inner cone, while

outer envelope spreads out and shields work surface from
atmosphere

Shown below is neutral flame of oxyacetylene torch

indicating temperatures achieved
Safety Issue in OAW
Together, acetylene and oxygen are highly flammable

C2H2 is colorless and odorless

Storage cylinders are packed with porousfiller 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
Natural Gas