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
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