6.5.08 by dr.phanikumar

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Welding Processes &Welding defects

ASNT Level III Refresher Course

Dr. G. PhanikumarDept of Metallurgical and Materials Engg.,

Indian Institute of Technology Madras



Acknowledgement

Some images are from the Internet

Sindo Kou's book on “Welding Metallurgy”

ASM Handbook vol. 6 on “Welding,Brazing and Soldering”



Contents

Welding Processes, bonds, common processes, hard surfacing, solid state

brazing, soldering - brief outline, Fusion

and pressure bonding, Distortion and

stresses, Heat for welding, Manual and

automotive welding, Special welding

processes – Electron beam welding, Plasma

arc, Ultrasonic welding, Basic design – welding symbols, electrodes, weld defects



What is welding?

• Welding is a joining technique

• Permanent union of metallic surfaces by

establishing atom to atom bonds between

the surfaces

• Distinction from fastening & adhesive

bonding

• Formation of Metallurgical Bond



More distinctions

• (Fusion) Welding : base materials melt

• Soldering : only the filler material melts

(T < 450 oC)

• Brazing : same as soldering

(T > 450o

C)



Soldering Has been around for 1000’s of years, but a lot of it is still an

art Need right balance of surface energies, solder, flux, and

base material

Need a flux that will properly wet the liquid solder and solid base material but not have such a strong affinity for the base

material that the liquid solder cannot displace it soldering metals, since they have a high surface energy, will

bond with almost anything, but need a right balance ofinterfacial energies.

Soldering v. brazing: temperature below 425 oC issoldering, above is brazing

The distinction is because no common metal melts betweenthe range of zinc and aluminum alloys



Brazing Higher temperatures (>425 oC)

More flexibility in choice of fluxes solder fluxes tend to use organics

brazing fluxes can use anhydrous salts, more aggressive

More reactive fluxes mean more base metals/non-metalscan be brazed

most base metals can be brazed, many ceramics

can almost always find a flux that will work for brazing

often used for joining dissimilar metals where welding is a problem

Room temperature strength of filler metal is greater (5,000-

10,000psi) Volatization of alloy elements can be a problem.

Intermetallics may form.

Thermal stresses may be severe.

Erosion of the base metal may occur.



Hard surfacing

Weld overlay

Single piece over which a desired material isclad

Flexibility of welding processes Flexibility of choice of overlay materials



Welding Processes



When does welding take place?

• When the materials are brought to intimate

contact with each other

• Enabled by absence of surface

contamination

• Enabled by fusion

• Enabled also by pressure (and heat)



A word about the power density

• Metals have good thermal

diffusivity

• Rate of heating must bemore than rate of heatremoval by thermal

diffusion to be able tomelt

• About 1000 W/cm2 isnecessary to melt metals

• Heating sources are

available from 100 W/cm2 to 107 W/cm2 (gas flameto laser / electron beam)

Heat

time x area



Classes of Welding Processes

• Fusion Welding: where base materials melt

• Diffusion Welding

• Pressure Welding

• Resistance Spot Welding



Some abbreviations

• SMAW: Shielded Metal Arc Welding

• MMAW: Manual Metal Arc Welding

• FCAW: Flux Cored Arc Welding

• GTAW: Gas Tungsten Arc Welding• GMAW: Gas Metal Arc Welding

• SAW: Submerged Arc Welding

• FRW: Friction Welding

• FSW: Friction Stir Welding

• LBW: Laser Beam Welding

• EBW: Electron Beam Welding



Arc Welding Processes



Fusion Welding

• Two materials join bymelting at the mating

surfaces• Adequate heat

intensity

• Moving heat source

for a continuous joint• Protection from

environment



Fusion Welding Process

Example of SMAW



Shielded Metal Arc Welding



Arc Welding Electrodes Arc welding electrodes are identified using the A.W.S, (American Welding Society)

numbering system

Ex: a welding rod identified as an 1/8" E6011 electrode. The electrode is 1/8" in diameter

The "E" stands for arc welding electrode.

Next will be either a 4 or 5 digit number stamped on the electrode. The first twonumbers of a 4 digit number and the first 3 digits of a 5 digit number indicate theminimum tensile strength (in thousands of pounds per square inch) of the weld that therod will produce, stress relieved.

For example, E60xx would have a tensile strength of 60,000 psi E110XX would be110,000 psi

The next to last digit indicates the position the electrode can be used in.

1. EXX1X is for use in all positions

2. EXX2X is for use in flat and horizontal positions

3. EXX3X is for flat welding

The last two digits together, indicate the type of coating on the electrode and thewelding current the electrode can be used with. Such as DC straight, (DC -) DC reverse(DC+) or A.C.



GTAW



Polarities

DCEN, DCEP, ACWork function



Automation in welding Precision and productivity

Improves weld repeatability.

Welds in more than one axis and whereaccessibility is difficult

Automating the torch motions decreases the error potential which means decreased scrap and rework.

A fully equipped and optimised robot cell can work

for 24 hours a day, 365 days a year without breaksmakes it more efficient than a manual cell.



Submerged Arc Welding



Plasma Arc welding process

Discharging hightemperature ionized

plasma through an

orifice.

A non consumableelectrode is used.

The plasma, at

temperatures as high

as 10,000 °C at itscore

§ deeper penetration with reduced heat-affected zones



Beam assisted welding processes



Electron Beam Welding



Features of EBW

Entire chamber to be at high vacuum

Easily vaporising metals?

X-Ray shielding necessary

Needs alignment of joint with beam traversal

High capital cost

Flexibility of power density

Welds very thick or thin sheets in one go!



EBW vs GTAW

L B W ldi



Laser Beam Welding



Comparison of welding processes



Spot Welding Processes



Resistance Spot Welding

Sequence of operation in Resistance



Sequence of operation in ResistanceSpot Welding



Ultrasonic Welding

(a) Wedge Reed System(b) Lateral Drive System



Features of Ultrasonic Welding

Suitable for thin sheets

Essentially spot welds

Series of spot welds to givea continuous weld



Friction Welding Processes



Friction Stir welding

B fit f FSW



Benefits of FSW

Low distortion, even in long welds

Excellent mechanical properties as proven by fatigue, tensile and bendtests.

No fumes

No porosity

No spatter

Low shrinkage

Can operate in all positions

Energy efficient

Non consumable tool

No filler wire

No gas shielding

Some tolerance to imperfect welds preparation

S h ti di f i t t l



Schematic diagram of microstructuralzones in FSW of Al

Joint Configurations possible with



Joint Configurations possible withFSW



Heat obtained frommechanically-induced slidingmotion

This process can beaccurately controlledwhen speed, pressure, andtime are closely regulated.

FRICTION

WELDINGA solid state welding process



FRW Applications

Process is used to fabricate

axle cases, drills, pipes,steering columns, hydrauliccylinders and piston rods,general tractor components,half-shafts, fasteners, engine

valves, and many others.

FRW can weld almost allmetallic materials. The

basic requirement is some plasticity at higher

temperature and thermalstability



Diffusion Joining



Diffusion Bonding Diffusion Bonding

Squeeze and add heat,

Heat helps deform surface asperities to get better surface contact,

Heat also helps diffuse away the surfacecontaminants in some cases

Cold welding with heat, but usually don’t

have significant shear example: hollow titanium air foils, diffusion

bonded at interfaces for F-22 fighter planes

St i diff i B di



Stages in diffusion Bonding process

a) Initial contactlimited to fewasperities at roomtemperature

b) Deformationof surfaceasperities by

plastic flow andcreep



C) Grain boundary diffusionof atoms to the voids and grain

boundary migration

d) Volume diffusionof atoms to the voids



Characteristics of Diffusion Bonding

Bonding temperature is typically 0.6 to 0.8 of melting point

Tm (K)

Want a high enough temperature to get deformation at bonding pressure

Bonding pressures 500-5000psi

Some materials (like aluminum and magnesium) are notdiffusion bondable (unless interpose say a copper layer eventhen very difficult)

Sometimes use thermal expansion mismatch to create the

pressure, special steel and titanium, or molybdenum

This is commonly used for high value parts, aerospaceindustry and not generally used in automotive industry dueto high cost associated with the furnaces.



Weld Defects



Casting during welding !

Melt pool re-solidifiesafter the heat sourcemoves away

Mini casting

Casting defects need

to be extended tosolidified welds



Filler material

Almost necessary above 3 mm thick welds

Weld bead shape and geometry depends on

processing parameters

Variable thicknesses possible

Compatibility with the base material oneither side of the weld



Properties relevant to Solidification

• Most metallic alloys shrink when theysolidify

• They also shrink when they cool down• If adequate liquid metal is not available to

compensate the shrinkage, porosity will

result• Shrinkage leads to residual stresses

Development of Weld Microstructure




Microporosity

Connected porosityLiquation crackingetc.,



Welding Defects

Cracks In weld metal or HAZ

Porosity

Gas bubbles entrapped in weld metal

Incomplete fusion (interrun or at preparation edge)

Inclusions

Slag or other matter entrapped in weld

Defective profile

Under-weld, over-weld, lack of penetration, overlap,undercut



Causes of weld defects

Poor design of weldment

Lack of access to make weld

Mistakes by welder Lack of skill

Poor placement of weld metal

Inadequate cleaning

Poor welding procedure

D f t i W ld



Defects in Welds

Defects in weldments



Defects in weldments

Incomplete fusion Incomplete joint penetration

Groove welds and defects



Cracks in Welds



Solidification cracking

• Contraction strains

cause rupture of the

weld at the pointwhere the last

material solidifies.

– Solidification range

– Weld pool size &

shape

Distortion in Weldments



Distortion in Weldments

Distortion is mainly caused by highly localizedheating and cooling of the metal being joinedtogether.

Remedies for angular distortion



Remedies for angular distortion

Place weld around neutralaxis

Prefer single pass deep penetration to reduce volume ofweld metal



Effect of Residual stresses

Distortion of work piece.

Reduction in dimensional stability

Results in weld cracking

Results in brittle fracture

Affects fatigue strength adversely

Lowers creep strength

Out of plane distortion causes reduction in buckling strength



Welding Positions

Four standard positions of welding

based on thedirection of gravity



Basic Welding Joints



Typical Weld Joint Variations

Weld Symbols



Weld Symbols

A standard welding symbol consists of

a reference line

becomes the foundation of the welding symbol

. used to apply weld symbols, dimensions, and other data to theweld

an arrow

arrow simply connects the reference line to the joint or area to bewelded

direction of the arrow has no bearing on the significance of thereference line.

a tail.

used only when necessary to include a specification, process, orother reference information

Elements of a Weld Symbol



Elements of a Weld Symbol



Symbols applied toreference line

Specifyingweld

locations



Concluding Remarks

Attention to process details

Consider material characteristics

Localized nature of the process

Geometrical complexity

Critical nature of the function of a weld in astructure

6.5.08 by dr.phanikumar

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