lecture 39 welding

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Lecture 39: Welding

ME 450Modeling Materials Processing

Professor Brian G. ThomasDepartment of Mechanical and Industrial Engineering

University of Illinois at Urbana-Champaign

Brian G. Thomas, ME450 2

Welding

• Welding microstructural phenomena

– Steel, age hardened, or cold worked metal

• Welding processes

– Solid state joining

– Liquid – solid state (soldering & brazing)

– Fusion welding (liquid state)

• Welding heat transfer

• Welding defects

– Residual stresses




From: W.D. Callister, Materials Science and Engineering,An

Introduction (6th Ed.) , Wiley and Sons, 2003, p. 357.

Temperature changes during metal welding produces heat treatmentCause microstructure and property changes in:

Fusion zone,

base metal (BM),

heat affected zone (HAZ).

Residual stresses (from thermal gradients)

Weld quality (penetration depth, shape, stress concentration, etc.)

Atmospheric contamination: porosity, oxide particles, etc.

Welding Issues


Weld Microstructures

Columnar grains Mixed dendrites

with grains

Equiaxed

Recrystallized, no GG

Weld Metal

Fusion Zone Base MetalFusion Line HAZ




Welding Processes:

solid state joining

• Weld strength depends on:pressure, temperature, contact time, cleanliness

• Atmospheric protection: from mechanical exclusion

• Diffusion bonding

– Forge welding

– Friction welding

– Resistance welding

• Plastic deformation at interface

– Cold roll bonding – Explosion welding

– Ultra-sonic welding


Solid state welding processes:

Plastic deformation at interface

• Cold roll

bonding

Explosion

welding

Ultrasonic

welding

Eg: CladdingUS quarter:75 Cu – 25 Ni

Cu

75 Cu – 25 Ni

Shock wave

Large chemical

equipment

Vibrations remove oxides

Foil packaging,

electronics

Pressure causes bonding: similar metals only




Solid state welding processes:

Diffusion Bonding

• Forge

welding

Friction

welding

Induction

welding

Ancient process

Simple but effective

Rotate one end; push;

Stop and push

Induction heating

Called “resistance

welding” if interfaces

melt

Apply heat & pressure – dissimilar metals diffuse together


Welding Processes:

liquid – solid state

• Melt the filler metal only(no changes in base metal)

• Ancient process (<3000 BC)

• Good for repairs

• Liquid flows into joints by capillary action

• Cleanliness critical (relative to other processes)

• Large contact area better

• Soldering (< 450 oC)

• Brazing (> 450 oC)




Major Welding Processes:

fusion welding (liquid state)

• Thermochemical energy (flame welding):

– Oxy-acetylene,

– thermit,

– Hydrogen welding

• Electric current energy

– Arc welding, MIG, TIG, PAW, SAW, SMAW, GMAW, MCAW

• Electric resistance energy

– Spot welding

– flash-butt welding

– seam welding

• Radiation energy beam

– Laser,

– electron beam welding

Brian G. Thomas, ME450 11From: H.B. Cary, Modern Welding Technology, Prentice-Hall, 1979, pp. 252,253.

often used for cutting

Oxy-acetylene welding




Transform to Eulerian frame

• B.C. at infinity T = T 0

• Transformation to moving frame fixed at beam

– Define ζ = z - z b

– Transform ( x, y, z ) → ( x, y, ζ)


Envision as a stationary heat source with moving

plate ( velocity -v).

edgez v t ζ = −

Define a distance from the heat s ource, r, as:

2 2 2r x y ζ = + +

Consider a hemispherical area of heat input

2(2 )

T Q k R

r

∂ π

∂

= −

From: F.V. Lawrence, Welding Notes, 1989.

Rosenthal models of Heat transfer

during fusion welding

r

R ( )Q W

2 2 R x ζ = +

(point source)

(line source)

,vζ

y

y

x




Solution: Steady Temperature

• Assume T is steady in moving frame

• Rosenthal's solution (point source)

– Recast boundary condition

– Consider hemispherical cap

– Solution


Solution Behavior

• Radial and axial part

– Hemispherical pool for V → 0

– Stretched in axial direction




Slower Travel Speed

• Reduce V from 100α to 10α

• What is T (0, 0, 0)?


For steel, “thin” is < ~5 mm thick whereas “thick” is > ~100 mm. From: F.V. Lawrence, Welding Notes, 1989.

Cooling rates along centerline:

Rosenthal Eqs. for thick & thin plates

= power input (W)

v = welding velocity (m/s)

Thin PlateThick Plate

( )2

02T v

k T T

t Q

π ∂

= − ∂

( )2

3

02

p

T vhk C T T

t Q

π ρ ∂

= − ∂

k= thermal cond.

C p= specific heat

ρ = density

T = temperature in plate

T0 = initial temperature

h = plate thickness

Q




From: J. Dantzig,

ME231, 1998.

Temperature changes during welding

Examine 650 and 750 C isothermsto find cooling rate at 700 C:

750 650

750 650

T T T v

t t

v

ζ

ζ ζ

ζ ζ

∂ ∂ ∂ ∂= =

∂ ∂ ∂ ∂

−=

−

y

x

z

z


Summary: Rosenthal welding

heat transfer models

• Point Source (thick plate solution for T)

• Line Source (thin plate solution for T)

0 0( , ) exp2 2 2

Q v vRT R T K k h

ζ ζ π α α

− = +

Bessel function off second kind

(zero order)

0( , ) exp exp2 2 2

Q vr vT r T

k r

ζ ζ

π α α

− − = +

thermal diffusivity




Fusion Welding

microstructural changes


Welding microstructures:

Cold-worked metals




From: H.B. Cary, Modern Welding Technology, Prentice-Hall, 1979, p. 399.

Micro-hardness measurements

correlate strongly with microstructure


Welding microstructures:

Steel




M a x T e

m p .

Liquidus temperatureSolidus temperature

Pearlite

LStructure at

max temp

High

hardenability

steel

Low

hardenability

steel

Martensite

Welded Steel Microstructure

Austenite temperature

Eutectoid temperature


Weldability

• Weldabil i ty : Ability of a particular alloy to bewelded without substantial embrittlement due tomartensite formation

– Generally the opposite of hardenability

– Of particular concern for high strength steels!

– High strength steels are heavily alloyed, shifting TTTcurves

– Some alloys, under severe thermal cycling, will

recrystallize, grow grains, and even age in heat-affectedzone

• Concept of effective carbon content: – Higher carbon content harder to weld

– Carbon equivalent = %C+%Mn/6 + %Ni/15 + %Cr/5 +%Mo/4 +%V/5




Recall: cooling rate at 700 oC is most

representative for TTT behavior .

Use welding temp histories & Jominy or

TTT data to predict microstructure

T (C)

400

800

1200

1600

0

A

B

C

C

B

A

0 30 60 90 120 150 180 210 240 270 300

Time (sec)

Typical TTT curves for the base metal.

Typical temperatures recorded near weld

0

10

20

30

40

50

60

1 10 100 1000

y = 79.577 * x^(-0.64633) R= 0.99839

J o m i n y D i s t a n c e ( m m )

Cooling Rate at 700oC (oC/sec.)

Jominy Bar Cooling Rate


0

100

200

300

400

500

600

700

800

0.1 1 10 100 1000 10000 100000

SAE 6150 Steel

6150615061501 mm3 mm12 mm50 mm

T e m p ( C )

Time (sec)

50 mm from quenched end



20

30

40

50

60

70

0 10 20 30 40 50

SAE 6150 Steel, Hardenability Curve

Upper Lower

R C ,

R o c k w e l l H a r

d n e s s

Normalize @ 900 CAustenitize @ 870 C

Distance from Quenched End (mm)

Recall: distance along Jominy bar

indicates microstructure & hardness

α + P + B

α + P + B + M

Mainly M

P. Kurath, me231 S03




M a x T e m p

Liquidus temperature

Solidus temperature

Solvus temperature

L

+L +θ’ +θ ”

+θ’ +θ +θ +θ ”

Base metalOveraged

zone

Fusion zone

Partial fusion zoneSolution treated zone

At peak temperature

After cooling

Precipitation Hardened Metals

Overaging temperature

Al – Cu Phase Diagram

L

+θ


Fusion welding Processes:

thermochemical energy

• Oxy-acetylene welding(manual, cheep, portable)

• Thermit welding

• Atomic hydrogen welding

3 4 2 35000 o powder

C Fe O Al Fe Al O heat + → + +

25500

2oinarcC

H H heat → +

2 2 2 2 2 23200

oC

C H O CO H heat CO H O heat + → + + → + +

Hot gases transport energy

nozzle core outer envelope




Fusion welding processes:

electric (arc) current energy

Energy transmitted:V = voltage (V)

I = current (amps)

t = time (s)

v = velocity (m/s)

J = V I t

J/m = V I / v Energy input(per length of weld)

Many different types: according to

electrode (permanent or consumed)

shielding material

flux (slag)inert gas

plasma

Typical:

I = 200 Amps

V = 20 to 50 V-DC or 60 to 110 V-AC


Fusion welding -

arc welding with gas shielding

PAW

(Plasma Arc welding)

SAW(Submerged Arc Welding)

TIG (Tungsten Inert Gas arc-welding)




From: H.B. Cary,

Modern Welding

Technology, Prentice-

Hall, 1979, p. 360.

GTAW~TIG

GMAW ~ Gas Metal Arc (MIG)

PAW~Plasma Arc Welding

SMAW~ Shielded Metal Arc

FCAW~Flux Cored Electrode Arc

EW~Electroslag Welding

SAW~Submerged Arc Welding

SW~Stud welding

CAW~Carbon Arc welding

Shielding gases:

protect weld metal from oxidation


Fusion welding –

arc welding with flux shielding

SMAW

(Shielded Metal Arc Welding)

most common

MIG(Metal Inert Gas)

GMAW(Gas Metal Arc Welding)




Arc Welding (SMAW)

• Very cheap apparatus

• Simple to switchmaterials

• Use in many

environments

• Dirty

• Slow• Slag is a pain


MIG Welding (GMAW)

• High deposition

rate• Uninterrupted weld

• Low fumes, spatter

• No slag!

• Higher skill (??)

• More complex

equipment

• Need controlled

environment

EP





Electric resistance energy

• Spot welding

•flash-butt welding

•seam welding

R = resistance (ohm)

I = current (amps)

t = time (s)

v = velocity (m/s)

J = I2 R tEnergy transmitted:



Radiation energy

• Laser welding

• Electron Beam welding

Deeper penetration

Faster;

less energy input per length;

Narrower HAZ




From: H.B. Cary,

Modern Welding

Technology, Prentice-

Hall, 1979, p. 397.

Typical arc weld

Rest From: Proceedings,Welded

High-Strength Structures,NESCO I,

EMAS Publishers, 1997, pp.

103,104.

Deeper penetration and smaller HAZ.

Laser & Electron Beam (EB) welding:

weld microstructures


• Huge thermal cycling in welding leads

to shrinkage and residual stress:

• In nearly all cases, weld metal and

HAZ end up under residual tension

Shrinkage in a butt weld

Shrinkage in a fillet weld

Residual Stress in Welds




Residual Distortion in Welds(usually accompanies residual stress)

Taking advantage of residual distortion:

Flame straightening:

apply heat to a bent metal beaminitially: beam will bend toward the heat

after cooling: beam will bend away from the heat

Butt joints T- joints


Extensive Welding

Codes: Specify: weld

metal for different

base metals,

operating conditions,

and weld setup:

Example: Groove

Proportions for arc

welding




From: H.B. Cary, Modern Welding Technology,

Prentice-Hall, 1979, p. 438.

Suggested filler metal for different

aluminum base metals


Explaining thermal distortion

1) Review thermal stress eqs

n ccr

QA

RTexp ε = σ −

ε total = ε elastic + ε inelastic + ε thermal

thermal final initial (T T )ε α = −

Where:

elastic

σ ε =

total

L

L

∆ε =

( )inelastic plastic cr t ε ε ∆ ε = +

F σ =




Thermal Stress Example #3

• Plate heated from top

– Constrained

compression

tension

– Unconstrained

compression

tension

cold

cold

heat

hot

cold

cold

cold

initial

heat

hot

cold

final

heat

hot

cold

slight compression

slight tension



• Residual stress in an elastic plate ?

• Plate bends and unbends:

zero

stress• If all elastic

NO!

cold

cold

initial

1)

compression

tension

heat

hot

cold

2)

cold

cold

3)




compression

tension

4)

cold

cold

• Residual Stress - needs inelastic strain


time

cold

cold

initial

1)

compression

tension

heat

hot

cold

2)

= 0

3)

hot

cold


ABAQUS steady

temperature results

Stress

results

Create

mesh

Problem:

Crack in auto

exhaust manifold

Application 4: thermal fatigue

From www.abaqus.com(HKS)




From: D.F. Socie, Lectures, 2001

Weld quality matters!


Advanced Topics:

Example Applications of

Computational Models

TAM 470, ME412,

ME471, ME554 ?

Professor Brian G. ThomasDepartment of Mechanical and Industrial Engineering

University of Illinois at Urbana-Champaign

© University of Illinois Board of Trustees, All Rights Reserved

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lecture 39 welding

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