L P i Eff t F i ti Sti W ldiLaser Peening Effects on Friction Stir Welding

Omar HatamlehNASA Johnson Space Center

Contents

Outline &Background1

FSW & Peening2

Residual Stresses3

Mechanical Properties4

Fatigue Crack Growth35

2

Backgroundg Friction Stir Welding (FSW) is a welding technique that uses frictional

heating combined with forging pressure to produce high strength bondsheating combined with forging pressure to produce high strength bonds.

Attractive for aerospace applications– Can result in considerable cost and weight savings by reducingCan result in considerable cost and weight savings, by reducing

riveted/fastened joints, and part count – Can weld metals that are difficult to weld with conventional methods

Space shuttle external tank

Although residual stresses in FSW are generally lower when compared to conventional fusion welds, recent work has shown that significant tensile residual stresses can be present in the weld after fabrication

Residual tensile stresses in the weld can lead to:– Faster crack initiation

Faster crack propagation– Faster crack propagation – Could also result in stress corrosion cracking (SCC)

Therefore, laser shock peening was investigated as a means of moderating th t il id l t d d d i ldi

3

the tensile residual stresses produced during welding

Backgroundg

Nugget or the stirred zone– The grain structure usually fine and equiaxed

Recrystalization from the high temperatures Extensive plastic deformation

Thermo-mechanical affected zone (TMAZ) – Lesser degree of deformation and lower temperatures – Recrystallization does not take place– The grain structure in elongated, with some considerable distortions

Heat affected zone (HAZ)– Unaffected by mechanical effects, and is only affected by the friction heat

U f FSW i di d i l i i ld d j i b i d i i i l l d b i Use of FSW is expanding and is resulting in welded joints being used in critical load bearing structures

4

Friction Stir Weldingg

The alloy selected for this investigation was a 1.25 cm thick 2195-T8 aluminum lithium alloy. Possess many superior properties and is well suited for many aerospace applications due to its low density,

high strength, and corrosion resistance. For the welding process, a rotational speed of 300 RPM in the counter-clockwise direction and a translation

speed of 15 cm/min were used. The dimensions of the FSW panels were 91 cm x 30 cm x 1.25 cm. To verify the integrity of the weld, several bending tests using strip specimens were performed. The FSW specimens were inspected visually afterward with no crack indications revealed.

5

Microstructure

6

Laser Peeningg

7

Shot Peeningg

8

Peening Methodsg

FSW

Laser Peening• 1 mm thick laminar tamping layer• Samples covered using a 0 22 mm

Shot Peening• 0.59 mm glass beads• Almen intensity of 0.008-0.012

• Samples covered using a 0.22 mm thick aluminum tape

• Applied using a square laser spot• Laser power density of 5 GW/cm2

• 18 ns in duration

• Both faces of samples were peened

• 18 ns in duration• Spots were overlapped 3%• Applied at a frequency of 2.7 Hz • Using a 1 micrometer wavelength

B th f f l d

9

• Both faces of samples were peened

Residual Stresses

S f R id l StXRDXRD

Surface Residual StressesDetermined by the x-ray diffraction technique

ContourContour Through Thickness Residual StressesDetermined by the contour method

11

Contour Method

111. Sectioning the Sample - Sample is fixed to a rigid backing plate - Sample is cut along the measurement plane with an 11 p g p

EDM wire

2. Measuring Deformation Af i i d f d f h i d d

22- After sectioning a deformed surface shape is produced

-Resulting from the relaxed residual stresses-The displacement is measured on both sectioned surfaces using a coordinate measuring machine (CMM)

Through-thicknessResidual Stresses

surfaces using a coordinate measuring machine (CMM)

3. Estimating the Residual Stresses - The displacements from both cutting surfaces is averaged

33- The noise in the measurements is filtered - The original residual stresses are calculated from the measured contour using a finite element model (FEM)

12

Residual Stresses in FSW Specimen

13

Residual Stresses in FSW Specimen

Surface residual stresses

100

200

irect

ion)

UnpeenedShot PeeningLaser Peening

-100

0-5 -4 -3 -2 -1 0 1 2 3 4 5

Long

itudi

nal d

-300

-200

Stre

ss, M

Pa (L

-500

-400

Res

idua

l

Advancing SideRetreating Side

Distance From Weld Centerline (cm)

Residual stresses for the various peened FSW specimens

14

Through Thickness Residual Stress

Two-dimensional map of the measured residual stress for the unpeened FSW specimen

Two-dimensional map of the measured residual stress for the shot peened FSW specimen

Two-dimensional map of the measured residual stress for the laser peened FSW specimen

15

Through Thickness Residual Stress

16

Mechanical Propertiesp

Investigate the effects of peening

Mi h d

Tensile Properties Surface Effects

Microhardness

18

Peening Conditionsg

No Peening Laser Peening(1 layer)Shot Peening Laser Peening

(3 layers)Laser Peening

(6 layers)

19

Tensile Propertiesp

Conventional transverse tensile testing only provides the Conventional transverse tensile testing only provides the overall strain experienced by the sample

It is necessary to determine local strains and equivalent tensile properties across the weld p p– Evaluated at different regions of the weld using an ARAMIS system

20

ARAMIS Systemy

Step 1:•A random or regular pattern with good contrast is applied to the surface of the test object and is deformed along with the

Step 1Step 1 Step 2Step 2 Step 3Step 3object and is deformed along with the object.

•As the specimen is deformed under load, the deformation is recorded by the cameras and evaluated using digital image processing.

Step 2:•The initial image processing defines a set of unique correlation areas known as macro-image facets, typically 5-20 pixels across.

Step 3:•These facets are then tracked in each successive image with sub-pixel accuracy.

Intrinsic Tensile

Properties

21

•Strains are calculated at different regions across the weld region.

Tensile Properties for 2195p

400

As welded condition

300

350

200

250

ss (M

Pa)

100

150Stre

s

HAZ (Advancing Side)TMAZ (Advancing Side)Weld NuggetTMAZ (Retreating Side)

A ( i Si )

0

50

0 0 2 0 4 0 6 0 8 0 10 0 12 0

HAZ (Retreating Side)

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Strain (%)

Tensile properties at different regions of the weld for a FSW 2195 AA

22

Tensile Propertiesp

The weld nugget exhibited the lowest tensile properties when The weld nugget exhibited the lowest tensile properties when compared to other locations across the weld– Strengthening precipitates in 2195 AA were no longer present in the weld g g p p g p

nugget Temperature during joining was above the solution temperature of the hardening

precipitatesprecipitates – This region of the weld will therefore be relatively ineffective in inhibiting

dislocation motion The localized strain in the softened area of the weld will result in lower

mechanical properties

23

Tensile Properties at Weld Nugget

450

350

400

250

300

(MPa

) No Peening

Shot Peening

L P i (1 L )

150

200

Stre

ss Laser Peening (1 Layer)

Laser Peening (3 Layers)

Laser Peening (6 Layers)

50

100

00 2 4 6 8 10 12

Strain (%)

Tensile properties at the weld nugget under different peening conditions

24

Tensile properties at the weld nugget under different peening conditions

Tensile Properties at TMAZp

450

350

400

450

250

300

MPa

) No Peening

Shot Peening

150

200

Stre

ss (M

g

Laser Peening (1 Layer)

Laser Peening (3 Layers)

Laser Peening (6 Layers)

50

100

00 1 2 3 4 5 6 7 8

Strain (%)

25

Tensile properties at the TMAZ under different peening conditions

Global Yield and Ultimate Stresses

350410

250

300

350

MPa

)

UnpeenedShot PeenedLaser Peened (1 Layer)Laser Peened (3 Layers)Laser Peened (6 Layers)

380

390

400

410

MPa

)

UnpeenedShot PeenedLaser Peened (1 Layer)Laser Peened (3 Layers)Laser Peened (6 Layers)

150

200

Stre

ss (M

350

360

370

380

Stre

ss (M

100Yield Stress

340Ultimate Stress

The yield stress (0.2% offset) for different peening conditionsThe ultimate tensile strength for different peening conditions

20 cm

26

Strain distribution across the weld

27

EBSD Grain Size Difference

Grain size histogram for laser peened specimen Grain size histogram for unpeened specimen

28

Tensile Propertiesp

Laser peened demonstrated: Laser peened demonstrated:– 60% increase in the yield strength in the weld nugget in the FSW joint– 11% increase to the ultimate tensile strength in the weld nugget in the FSW joint

In contrast shot peening e hibited onl modest impro ement to the tensile– In contrast, shot peening exhibited only modest improvement to the tensile properties (3%)

Th i i h i l ti f th l i The increase in mechanical properties from the laser peening was mainly attributed to:

– High levels of compressive residual stresses introduced during the high energy g p g g gypeening that can reach significantly deeper than shot peening

– Increase in dislocation density from the peening

Laser peening using six layers resulted in a 35% reduction to ductility

29

Tensile Properties (360 F)p ( )

325

375

Laser Peening (6 layers)Laser Peening (3 layers)Shot Peening

275

a)

Shot PeeningNo Peening

175

225

Stre

ss (M

Pa

75

125

25

75

0 2 4 6 8 10 12 14 16

St i (%)

30

Strain (%)

Tensile Properties (-150 F)p ( )

425

475

Laser Peening (6 layers)Laser Peening (3 layers)Laser Peening (1 layer)

325

375

a)

Laser Peening (1 layer)Shot PeeningNo Peening

225

275

Stre

ss (M

Pa

125

175

25

75

0 2 4 6 8 10 12 14 16 18

St i (%)

31

Strain (%)

Microhardness (Top)( p)

230

Top Side-NP Top Side-SP Top Side-L1 Top Side-L3 Top Side-L6

200

210

220

300g

)

FSW Tool Shoulder

170

180

190

200

ng (K

noop

3

150

160

170

dnes

s R

eadi

n

120

130

140

Mic

roha

rd

Ad i Sid

100

110

-36 -32 -28 -24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24 28 32 36

Distance from Weld Centerline (mm)

Advancing SideRetreating Side

32

Distance from Weld Centerline (mm)Microhardness profile across the top side of the weld for different peening methods

Microhardness

Significant hardness was achieved by processing the FSW 2195 AA g y p gsamples with laser peening

– Hardness increase around 28% in the top surface– Hardness increase around 21% in the bottom side of the weld nugget region

Hardness levels due to laser peening increased proportionally with the number of peening layers in the 2195 aluminum alloy

The polishing that takes place prior to microhardness measurement can wipe out all the hardness effects produced by the shot peening.

– This is because the hardening effects from shot peening only affect a shallow depth in the material.s s because t e a de g e ects o s ot pee g o y a ect a s a ow dept t e ate a .

Hardness profiles across the weld were wider on the top side of the weld compared to the bottom surface.

– That was attributed to bottom plate contact with the backing plate which acts as a heat sink, therefore reducing the metallurgical transformations that take place at high temperatures.

33

Surface Roughnessg

34

Shot Peening Laser peening

Surface Roughnessg

ConditionCondition RaRa RpkRpk RvkRvk NomenclatureNomenclature

2.884 µm5.761 µm5.029 µmShot Peened Ra: Roughness average

0.93 µm1.429 µm1.087 µmUnpeened

1.328 µm1.815 µm1.336 µmLaser Peened (6 layers)

Ra: Roughness averageRpk: Maximum peak heightRvp: Maximum valley depth

35

Surface Roughnessg

36

Surface Roughnessg

37

Surface Roughnessg

38

Surface Wear/Friction

DifferentSpeeds

Different Loads

DifferentPeening

DifferentMaterials

39

Friction

•Testing done with identical contact force (10 N) and speed (0 5 cm/sec)•Testing done with identical contact force (10 N) and speed (0.5 cm/sec)• Shot peening appears to provide higher steady state friction coefficients. •Long term friction coefficients for laser peened and bare surfaces were comparable

0.80.9

comparable.

0.50.60.70.8

Shot Peening

0.20.30.4

0 5

00.10.2

0 20 40 60 80 100

Base MaterialLaser Peening

40

Lap

Wear

No peening Laser peening Shot peening

Influence of Depth Wear Resistance on Al 2195 as a function of peening

30.0

35.0

40.0

45.0

m)

2195 Bottom 2N

2195 Bottom 10N

5.0

10.0

15.0

20.0

25.0

Dep

th (µ

m

41

0.0No Peening Laser Peening Shot Peening

Fatigue Crack Growth Ratesg

Room TemperatureFCGR

Elevated Temperatures (360 F)FCGR p ( )FCGR

Cryogenic Temperatures (-150F)FCGR

Shot Peening Laser PeeningNo Peening

43

Shot Peening Laser PeeningNo Peening

Fatigue Samples g p

iThrough Thickness Cracks

44

a vs. N for R=0.1 (RT)( )

40

35

40

No Peening

Laser Peening

25

30

mm

)

15

20

Cra

ck L

engt

h (m

5

10

0

5

0 20000 40000 60000 80000 100000 120000 140000 160000 180000 200000

Number of Cycles (N)

45

y ( )

a vs. N for R=0.7 (RT)( )

30

25

No PeeningLaser PeeningBase Material

20

h (m

m)

10

15

Cra

ck L

engt

h

5

10

00 100000 200000 300000 400000 500000 600000 700000 800000 900000

Number of Cycles (N)

46

Number of Cycles (N)

A vs. N for R=0.1 (180c)( )

1.8

1.4

1.6

No Peening (180c)

Shot Peening (180c)

Laser Peening (180c)

1

1.2

(in)

0.8

1

Cra

ck L

engt

h (

0.4

0.6

0

0.2

0 10000 20000 30000 40000 50000 60000

Number of Cycles (N)

47

Number of Cycles (N)

A vs. N for R=0.1 (-100c)( )

1.2

1

No Peening (-100c)Laser Peening (-100c)

0.8

(in)

0.6

Cra

ck L

engt

h (

0.2

0.4

00 100000 200000 300000 400000 500000 600000 700000

Number of Cycles (N)

48

Number of Cycles (N)

Fractured Surfaces

49

Conclusions

– The laser peening process can result in considerableThe laser peening process can result in considerable improvement to crack initiation, propagation, and mechanical properties in FSW Longer hardware service life

– Improve processed hardware safetyImprove processed hardware safety By producing higher failure tolerant hardware, & reducing risk

– Lower hardware maintenance cost Longer hardware service life, and lower hardware down time

Application of this proposed technology will result in substantial benefits and savings throughout the life of the treated components

50

treated components