Transcript Introduction to Materials Joining

Arc Welding Processes
Arc Welding Processes
Lesson Objectives
When you finish this lesson you will
understand:
• The similarities and difference between
some of the various arc welding processes
• Flux and gas shielding methods
• Advantages and disadvantages of the arc
welding processes
• Need to select between the processes
Learning Activities
1. Read Handbook Pp
1-16,
2. Look up Keywords
3. View Slides;
4. Read Notes,
5. Listen to
6. lecture
7. Do on-line
workbook
8. Do homework
Keywords
Welding Flux, Inert Shielding Gas, Shielded Metal Arc Welding
(SMAW), Gas Metal Arc Welding (GMAW), Metal Transfer Mode,
Flux Cored Arc Welding FCAW), Submerged Arc Welding (SAW),
Linnert, Welding Metallurgy,
AWS, 1994
Arc Welding Processes
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Welding processes that employ an electric arc are
the most prevalent in industry
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Shielded Metal Arc Welding
Gas Metal Arc Welding
Flux Cored Arc Welding
Submerged Arc Welding
Gas Tungsten Arc Welding
Electric Arc
These processes are associated with molten metal
Linnert, Welding Metallurgy,
AWS, 1994
Protection of the Molten Weld Pool
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Molten metal reacts with the atmosphere
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Oxides and nitrides are formed
Discontinuities such as porosity
Poor weld metal properties
All arc welding processes employ some means of
shielding the molten weld pool from the air
Welding Flux
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Three forms
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Granular
Electrode wire coating
Electrode core
Fluxes melt to form a protective slag over the weld pool
Other purposes
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Contain scavenger elements to purify weld metal
Contain metal powder added to increase deposition rate
Add alloy elements to weld metal
Decompose to form a shielding gas
Shielding Gas
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Shielding gas forms a protective atmosphere over the
molten weld pool to prevent contamination
Inert shielding gases, argon or helium, keep out oxygen,
nitrogen, and other gases
Active gases, such as oxygen and carbon dioxide, are
sometimes added to improve variables such as arc
stability and spatter reduction
Argon
Helium
Oxygen
Carbon Dioxide
Turn to the person sitting next to you and discuss (1 min.):
• What would happen if there was no flux on the wire to
decompose into gas or no inert shielding gas was provided?
• What would the weld metal look like?
Shielded Metal Arc Welding (SMAW)
SMAW Electrode Classification Example
E7018
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E indicates electrode
70 indicates 70,000 psi tensile strength
1 indicates use for welding in all positions
8 indicates low hydrogen
E7018-A1-H8R
•ANSI/AWS - 5.1 : Specification for Covered Carbon Steel
•ANSI/AWS - 5.5 : Specification for Low Alloy Steel
•ANSI/AWS - 5.4 : Specification for Corrosion Resistant Steel
AWS Website:
http://www.aws.org
Coating Materials -Partial List
Arc Stabilizers
Titania TiO2
Gas-Forming Materials
Wood Pulp
Limestone CaCO3
Slag-Forming Materials
Alumina Al2O3
TiO2
SiO2
Fe3O4
Slipping Agents to Aid Extrusion
Clay
Talc
Glycerin
Binding Agents
Sodium Silicate
Asbestos
Starch
Sugar
Alloying and Deoxidizing Elements
Si, Al, Ti, Mn, Ni, Cr
Linnert, Welding Metallurgy
AWS, 1994
Linnert, Welding Metallurgy
AWS, 1994
Shielded Metal Arc Welding
SMAW Advantages
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Easily implemented
Inexpensive
Flexible
Not as sensitive to part
fit-up variances
Advantages
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Equipment relatively easy to use, inexpensive, portable
Filler metal and means for protecting the weld puddle are
provided by the covered electrode
Less sensitive to drafts, dirty parts, poor fit-up
Can be used on carbon steels, low alloy steels, stainless
steels, cast irons, copper, nickel, aluminum
Shielded Metal Arc Welding
Quality Issues
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Discontinuities associated
with manual welding
process that utilize flux
for pool shielding
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Slag inclusions
Lack of fusion
Other possible effects on
quality are porosity, and
hydrogen cracking
Shileded Metal Arc Welding
Limitations
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Low Deposition Rates
Low Productivity
Operator Dependent
Other Limitations
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Heat of welding too high for lead, tin, zinc, and their
alloys
Inadequate weld pool shielding for reactive metals such
as titanium, zirconium, tantalum, columbium
Turn to the person sitting next to you and discuss (1 min.):
• Wood (cellulose) and limestone are added to the coating on
SMAW Electrodes for gas shielding. What gases might be
formed?
• How do these gases shield?
Gas Metal Arc Welding
Gas Metal Arc Welding
Gas Metal Arc Welding
GMAW Modes of Metal Transfer
Spray
Globular
Short Circuiting
Pulsed Spray
Gas Metal Arc Welding
GMAW Filler Metal Designations
ER - 70S - 6
Electrode
Rod (can be used
with GMAW)
Composition
6 = high silicon
Solid Electrode
Minimum ultimate tensile
strength of the weld metal
AWS Specifications for GMAW Wire
AWS A5.18 - Carbon Steel Electrodes
AWS A5.28 - Low Alloy Steel Electrodes
Gas Metal Arc Welding
Shielding Gas
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Shielding gas can affect
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Ar
Ar-He
He
CO2
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Weld bead shape
Arc heat, stability, and
starting
Surface tension
Drop size
Puddle flow
Spatter
Gas Metal Arc Welding
GMAW Advantages
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Deposition rates higher
than SMAW
Productivity higher than
SMAW with no slag
removal and continuous
welding
Easily automated
Gas Metal Arc Welding
Quality
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Spatter
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Droplets of electrode
material that land outside
the weld fusion area and
may or may not fuse to the
base material
Porosity
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Small volumes of
entrapped gas in solidifying
weld metal
Gas Metal Arc Welding
Limitations
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Equipment is more
expensive and complex
than SMAW
Process variants/metal
transfer mechanisms
make the process more
complex and the process
window more difficult to
control
Restricted access
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GMAW gun is larger than
SMAW holder
Turn to the person sitting next to you and discuss (1 min.):
• When comparing processes that have spray and globular
metal transfer, which type of transfer mode do you thnk
results in more spatter? Why?
Flux-Cored Arc Welding
Flux Cored Arc Welding (FCAW)
Linnert, Welding Metallurgy,
AWS, 1994
Flux-Cored Arc Welding
FCAW Electrode Classification
E70 T - 1
Electrode
Minimum UTS
70,000 psi
Position
Type Gas, Usability
and Performance
Flux Cored /Tubular
Electrode
American Welding Society Specification
AWS A5.20 and AWS A5.29.
Linnert, Welding Metallurgy
AWS, 1994
Flux-Cored Arc Welding
Advantages
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High deposition rates
Deeper penetration than
SMAW
High-quality
Less pre-cleaning than
GMAW
Slag covering helps with
larger out-of-position
welds
Self-shielded FCAW is
draft tolerant.
Flux-Cored Arc Welding
Limitations
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Slag must be removed
More smoke and fumes
than GMAW and SAW
Spatter
FCAW wire is more
expensive
Equipment is more
expensive and complex
than for SMAW
Turn to the person sitting next to you and discuss (1 min.):
• What do you suppose would happen if the powder inside
the core did not get compacted good?
Submerged Arc Welding
Submerged Arc Welding
Submerged Arc Welding
SAW Flux / Filler Metal Compositions
F7A2-EM12K
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F indicates flux
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70-95 ksi UTS, 58 ksi minimum yield strength, 22% elongation
A - as welded; P - postweld heat treated
2 - minimum impact properties of 20 ft-lbs @ 20°F
E indicates electrode (EC - composite electrode)
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M - medium manganese per AWS Specifications
12 - 0.12% nominal carbon content in electrode
K - produced from a heat of aluminum killed steel
Submerged Arc Welding
Advantages
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High deposition rates
No arc flash or glare
Minimal smoke and fumes
Flux and wire added
separately - extra dimension of
control
Easily automated
Joints can be prepared with
narrow grooves
Can be used to weld carbon
steels, low alloy steels,
stainless steels, chromiummolybdenum steels, nickel
base alloys
Submerged Arc Welding
Limitations
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Flux obstructs view of
joint during welding
Flux is subject to
contamination porosity
Normally not suitable for
thin material
Restricted to the flat
position for grooves - flat
and horizontal for fillets
Slag removal required
Flux handling equipment
Do Homework Assignment 2, “Arc Welding
Processes” from the Assignment Page of the WE300
Website. Turn in next Class Period.