introduction to material joining

8/2/2019 Introduction to Material Joining

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Arc Welding Processes


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Lesson Objectives

When you finish this lesson you will

understand:

The similarities and difference betweensome of the various arc welding processes

Flux and gas shielding methods

Advantages and disadvantages of the arcwelding 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),
http://et1.net.ohio-state.edu:7070/ramgen/UTS/we300/we3001-1.rmhttp://we019.eng.ohio-state.edu/we300/w300t/we300.htmhttp://we019.eng.ohio-state.edu/wenc/wenc.cfm


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Linnert, Welding Metallurgy,

AWS, 1994


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Welding processes that employ an electric arc are

the most prevalent in industry Shielded Metal Arc Welding

Gas Metal Arc Welding

Flux Cored Arc Welding

Submerged Arc Welding

Gas Tungsten Arc Welding

These processes are associated with molten metal

Electric Arc
http://encarta.msn.com/find/Concise.asp?z=1&pg=2&ti=761575532


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AWS, 1994


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Protection of the Molten Weld Pool

Molten metal reacts with the atmosphere

Oxides and nitrides are formed Discontinuities such as porosity

Poor weld metal properties

All arc welding processes employ some means ofshielding the molten weld pool from the air


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Welding Flux

Three forms

Granular Electrode wire coating

Electrode core

Fluxes melt to form a protective slagover the weld pool Other purposes

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


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Shielding Gas

Shielding gas forms a protective atmosphere over the

molten weld pool to prevent contamination Inertshielding gases, argon or helium, keep out oxygen,

nitrogen, and other gases

Activegases, such as oxygen and carbon dioxide, aresometimes added to improve variables such as arcstability and spatter reduction

Argon Helium Oxygen Carbon Dioxide
http://encarta.msn.com/find/Concise.asp?z=1&pg=2&ti=761574216http://encarta.msn.com/find/Concise.asp?z=1&pg=2&ti=761577400http://encarta.msn.com/find/Concise.asp?z=1&pg=2&ti=761562418http://encarta.msn.com/find/Concise.asp?ti=065B8000


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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?


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Shielded Metal Arc Welding (SMAW)


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SMAW Electrode Classification Example

E7018

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


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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


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Coating Materials -Partial List

Arc Stabilizers

Titania TiO2

Gas-Forming Materials

Wood Pulp

Limestone CaCO3

Slag-Forming Materials

Alumina Al2O3

TiO2SiO2Fe3O4

Slipping Agents to Aid Extrusion

Clay

Talc

Glycerin

Binding Agents

Sodium SilicateAsbestos

Starch

Sugar

Alloying and Deoxidizing Elements

Si, Al, Ti, Mn, Ni, Cr


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Linnert, Welding Metallurgy

AWS, 1994


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AWS, 1994


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SMAW Advantages

Easily implemented

Inexpensive Flexible

Not as sensitive to part

fit-up variances

Shielded Metal Arc Welding


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Advantages

Equipment relatively easy to use, inexpensive, portable

Filler metal and means for protecting the weld puddle areprovided by the covered electrode

Less sensitive to drafts, dirty parts, poor fit-up

Can be used on carbon steels, low alloy steels, stainlesssteels, cast irons, copper, nickel, aluminum


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Quality Issues

Discontinuities associated

with manual weldingprocess that utilize fluxfor pool shielding

Slag inclusions Lack of fusion

Other possible effects onquality are porosity, andhydrogen cracking

Shielded Metal Arc Welding


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Limitations

Low Deposition Rates

Low Productivity Operator Dependent

Shileded Metal Arc Welding


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Other Limitations

Heat of welding too high for lead, tin, zinc, and theiralloys

Inadequate weld pool shielding for reactive metals such

as titanium, zirconium, tantalum, columbium


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Wood (cellulose) and limestone are added to the coating onSMAW Electrodes for gas shielding. What gases might be

formed?

How do these gases shield?


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G M l A W ldi


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GMAW Modes of Metal TransferSpray Globular

Short Circuiting Pulsed Spray


G M t l A W ldi
http://www-iwse.eng.ohio-state.edu/we300/kurz.avihttp://www-iwse.eng.ohio-state.edu/we300/puls.avihttp://www-iwse.eng.ohio-state.edu/we300/sprueh.avi


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GMAW Filler Metal Designations

ER - 70S - 6Electrode

Rod (can be usedwith GMAW) Minimum ultimate tensile

strength of the weld metal

Solid Electrode

Composition6 = high silicon



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AWS Specifications for GMAW Wire

AWS A5.18 - Carbon Steel Electrodes

AWS A5.28 - Low Alloy Steel Electrodes



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Shielding Gas

Shielding gas can affect

Weld bead shape Arc heat, stability, and

starting

Surface tension

Drop size

Puddle flow

Spatter

Ar Ar-He He CO2




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GMAW Advantages

Deposition rates higher

than SMAW Productivity higher than

SMAW with no slag

removal and continuouswelding

Easily automated




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Quality

Spatter

Droplets of electrodematerial that land outsidethe weld fusion area andmay or may not fuse to the

base material Porosity

Small volumes of

entrapped gas in solidifyingweld metal




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Limitations Equipment is more

expensive and complex

than SMAW

Process variants/metaltransfer mechanisms

make the process morecomplex and the processwindow more difficult to

control Restricted access

GMAW gun is larger than

SMAW holder



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When comparing processes that have spray and globularmetal transfer, which type of transfer mode do you thnk

results in more spatter? Why?

Flux-Cored Arc Welding


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Flux Cored Arc Welding (FCAW)

Flux Cored Arc Welding


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AWS, 1994



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FCAW Electrode Classification

E70 T - 1Electrode

Minimum UTS70,000 psi

Position

Flux Cored /Tubular

Electrode

Type Gas, Usabilityand Performance

g

American Welding Society Specification

AWS A5.20 and AWS A5.29.


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AWS, 1994



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Advantages

High deposition rates

Deeper penetration thanSMAW

High-quality

Less pre-cleaning thanGMAW

Slag covering helps with

larger out-of-positionwelds

Self-shielded FCAW is

draft tolerant.



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Limitations

Slag must be removed

More smoke and fumesthan GMAW and SAW

Spatter

FCAW wire is moreexpensive

Equipment is more

expensive and complexthan for SMAW


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What do you suppose would happen if the powder insidethe core did not get compacted good?


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SAW Flux / Filler Metal Compositions

F7A2-EM12K F indicates flux

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 @ 20F

E indicates electrode (EC - composite electrode)

M - medium manganese per AWS Specifications

12 - 0.12% nominal carbon content in electrode

K - produced from a heat of aluminum killed steel



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Advantages High deposition rates

No arc flash or glare

Minimal smoke and fumes Flux and wire added

separately - extra dimension ofcontrol

Easily automated Joints can be prepared with

narrow grooves

Can be used to weld carbonsteels, low alloy steels,stainless steels, chromium-molybdenum steels, nickelbase alloys



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Limitations Flux obstructs view of

joint during welding

Flux is subject to

contamination porosity

Normally not suitable forthin material

Restricted to the flatposition for grooves - flat

and horizontal for fillets

Slag removal required

Flux handling equipment


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Do Homework Assignment 2, Arc WeldingProcesses from the Assignment Page of the WE300

Website. Turn in next Class Period.

introduction to material joining

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