Laser Peening Effects On Friction Stir Welding

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Laser Peening Effects on FrictionStir WeldingOmar HatamlehJohnson Space Center

ContentsFatiguePropertiesIntroductionResidual StressMechanicalProperties2

BackgroundApplicableFSWFriction StirWelding (FSW) is awelding techniquethat uses frictionalheating combinedwith forgingpressure to producehigh strength bonds.Attractive for aerospaceapplicationsCan result inconsiderable costand weight savings,by reducingriveted/fastenedjoints, and partcountCan weld metalsthat are difficult toweld withconventionalmethodsSpace shuttleexternal tankRSEffectsAlthough residualstresses in FSW aregenerally lowerwhen compared toconventional fusionwelds, recent workhas shown thatsignificant tensileresidual stresses canbe present in theweld afterfabricationResidual tensilestresses in the weld canlead to:Faster crackinitiationFaster crackpropagationCould also resultin stresscorrosioncracking (SCC)Therefore, laser shock peening was investigated as a means ofmoderating the tensile residual stresses produced during welding

BackgroundFriction Stir WeldingNugget or the stirred zoneThe grain structure usually fine and equiaxedRecrystalization from the high temperaturesExtensive plastic deformationThermo-mechanical affected zone (TMAZ)Lesser degree of deformation and lower temperaturesRecrystallization does not take placeThe grain structure in elongated, with some considerabledistortionsHeat affected zone (HAZ)Unaffected by mechanical effects, and is only affected by thefriction heatUse of FSW is expanding and is resulting in welded joints beingused in critical load bearing structures4

BackgroundWelding ProcessThe alloy selected was a 1.25 cm thick 2195-T8 aluminum lithium alloy.Possess many superior properties and is well suited for many aerospace applications due toits low density, high strength, and corrosion resistance.For the welding process, a rotational speed of 300 RPM in the counter-clockwise directionand 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 were performed.The FSW specimens were inspected visually afterward with no crack indications revealed.5

BackgroundMicrostructure6

BackgroundLaser Peening7

BackgroundShot Peening8

Peening ProcessLaserPeening 1 mm thick laminar tamping layer Samples covered using a 0.22 mm thickaluminum tape Applied using a square laser spot Laser power density of 5 GW/cm2 18 ns in duration Spots were overlapped 3% Applied at a frequency of 2.7 Hz Using a 1 micrometer wavelength Both faces of samples were peenedShotPeening 0.59 mm glass beads Almen intensity of 0.008-0.012 Both faces of samples were peenedFSWSamples

Residual Stresses

Residual StressesXRDContourSurface Residual StressesDetermined by the x-ray diffraction techniqueThrough Thickness Residual StressesDetermined by the contour method11

Contour Method1. Sectioning the Sample1- Sample is fixed to a rigid backing plate- Sample is cut along the measurement plane with anEDM wire2. Measuring DeformationThrough-thicknessResidual Stresses2- After sectioning a deformed surface shape is produced-Resulting from the relaxed residual stresses-The displacement is measured on both sectionedsurfaces using a coordinate measuring machine (CMM)3. Estimating the Residual Stresses3- The displacements from both cutting surfaces is averaged- The noise in the measurements is filtered- The original residual stresses are calculated from themeasured contour using a finite element model (FEM)12

Residual Stress Quantification13

Residual Stress Relaxation14

Through Thickness ResidualStress15

Samples Used in Testing16

Residual Stress in FSWSurface residual stressesResidual Stress, MPa (Longitudinal direction)200UnpeenedShot Peening100Laser Peening0-5-4-3-2-1012345-100-200-300-400Retreating SideAdvancing Side-500Distance From Weld Centerline (cm)Residual stresses for the various peened FSW specimens17

Effects of Laser Peening onResidual Stress in FSWTwo-dimensional map of the measured residual stress for the unpeened FSW specimenTwo-dimensional map of the measured residual stress for the shot peened FSW specimenTwo-dimensional map of the measured residual stress for the laser peened FSW specimen18

Through Thickness ResidualStress Measurements19

Mechanical Properties

Mechanical PropertiesInvestigate the effects of peeningTensile PropertiesSurface EffectsMicrohardness21

Peening ConditionsNo PeeningShot PeeningLaser Peening(1 layer)22Laser Peening(3 layers)Laser Peening(6 layers)

Testing MethodsTensile PropertiesConventional SamplesConventional transverse tensileTesting only provides theoverall strain experienced bythe sampleWelded SamplesIt is necessary to determinelocal strains andequivalent tensile propertiesacross the weldEvaluated at different regions ofthe weldusing an ARAMIS system

Digital Image CorrelationStep 1:Step 1Step 2Step 3 A random or regular pattern with goodcontrast is applied to the surface of the testobject and is deformed along with theobject. As the specimen is deformed under load,the deformation is recorded by the camerasand evaluated using digital imageprocessing.Step 2: The initial image processing defines a setof unique correlation areas known asmacro-image facets, typically 5-20 pixelsacross.IntrinsicTensilePropertiesStep 3: These facets are then tracked in eachsuccessive image with sub-pixel accuracy.24 Strains are calculated at different regionsacross the weld region.

Tensile Testing Samples25

Surface Residual Stress26

Through Thickness ResidualStress27

Through Thickness ResidualStress28

Hardness vs Residual Stress29

Tensile Properties for 2195As welded condition400350Stress (MPa)300250200HAZ (Advancing Side)150TMAZ (Advancing Side)Weld Nugget100TMAZ (Retreating Side)HAZ (Retreating Side)5000.02.04.06.08.010.012.0Strain (%)Tensile properties at different regions of the weld for a FSW 2195 AA30

Tensile PropertiesTensile PropertiesThe weld nugget exhibited the lowest tensile properties whencompared to other locations across the weldStrengthening precipitates in 2195 were no longer present in the weld nuggetTemperature during joining was above the solution temperature of the hardeningprecipitatesThis region of the weld will therefore be relatively ineffective in inhibitingdislocation motionThe localized strain in the softened area of the weld will result in lowermechanical properties31

Tensile Properties at WeldNugget450400350Stress (MPa)300No Peening250Shot PeeningLaser Peening (1 Layer)200Laser Peening (3 Layers)Laser Peening (6 Layers)150100500024681012Strain (%)Tensile properties at the weld nugget under different peening conditions32

Tensile Properties at TMAZ450400350Stress (MPa)300No Peening250Shot PeeningLaser Peening (1 Layer)200Laser Peening (3 Layers)Laser Peening (6 Layers)150100500012345678Strain (%)Tensile properties at the TMAZ under different peening conditions33

Global Yield and Ultimate Stress410Stress (MPa)390300Stress (MPa)400350UnpeenedShot PeenedLaser Peened (1 Layer)Laser Peened (3 Layers)Laser Peened (6 Layers)380370UnpeenedShot PeenedLaser Peened (1 Layer)Laser Peened (3 Layers)Laser Peened (6 Layers)250200360150350340100Ultimate StressThe ultimate tensile strength for different peening conditionsYield StressThe yield stress (0.2% offset) for different peening conditions34

Strain Distribution Across theWeld35

EBSD Grain Size DifferenceGrain size histogram for laser peened specimenGrain size histogram for unpeened specimen36

Yield Stress at Various DepthsSix layers of laser peeningCrown Side (397 MPa)Middle Side (341 MPa)Yield StressRoot Side (433 Mpa)37

Tensile PropertiesTensile Properties 60% increase in the yield strength in the weld nugget in the FSW joint 11% increase in ultimate tensile strength in the weld nugget Shot peening exhibited only modest improvement in tensile properties (3%)ImprovementThe increase in mechanical properties from the laser peening wasmainly attributed to: High levels of compressive residual stresses introduced during the highenergy peening that can reach significantly deeper than shot peening Increase in dislocation density from the peening38

Tensile Properties at 360F375Laser Peening (6 layers)Laser Peening (3 layers)325Shot PeeningNo PeeningStress (MPa)27522517512575250246810121416Strain (%)39

Tensile Properties at -150F475Laser Peening (6 layers)425Laser Peening (3 layers)Laser Peening (1 layer)375Shot PeeningNo PeeningStress (MPa)3252752251751257525024681012141618Strain (%)40

Microhardness DistributionAcross Weld41

Microhardness at the TopRegion of the WeldTop Side-NPTop Side-SPTop Side-L1Top Side-L3Top Side-L6230Microhardness Reading (Knoop 300g)220210FSW Tool Shoulder200190180170160150140130120Advancing SideRetreating 36Distance from Weld Centerline (mm)Microhardness profile across the top side of the weld for different peening methods42

MicrohardnessMicrohardness EffectsSignificant Hardnessincrease wasachieved thoughLaser Peening28% Increase on Top21% increase on BottomHardness Levels forFSW 2195 increasedproportionally withnumber of LaserPeening layersThe polishing thattakes place prior tomicrohardness canwipe mitigate allhardness effectsassociated with theShot PeeningProcess43

Surface RoughnessShot PeeningLaser peening44

Surface RoughnessConditionRaRpkRvkUnpeened1.087 µm1.429 µm0.93 µmShot Peened5.029 µm5.761 µm2.884 µmLaser Peened(6 layers)1.336 µm1.815 µm1.328 µmNomenclatureRa: Roughness averageRpk: Maximum peak heightRvp: Maximum valley depth45

Surface Roughness46

Surface Roughness47

Surface Roughness48

Fatigue Crack Growth

Fatigue TestingRoom TemperatureElevated Temperature (360F)Cryogenic Temperature (-150F)No PeeningShot PeeningLaser Peening51

Testing SamplesThrough ThicknessCracks52

Residual Stress vs. Hardness53

Through Thickness ResidualStress54

Through Thickness ResidualStress55

Fatigue Crack Growth Rates for707556

FCGR For Different Conditionsfor 707557

Fatigue Crack Growth Rates for707558

Fatigue Crack Growth Rates for219559

Fatigue Crack Growth Rates at180 Degrees Celsius1.8No Peening (180c)Shot Peening (180c)1.6Laser Peening (180c)1.4Crack Length 00Num ber of Cycles (N)60

Fatigue Crack Growth Rates at 100 Degrees Celsius1.2No Peening (-100c)Laser Peening (-100c)1Crack Length 0000700000Num ber of Cycles (N)61

Fatigue Crack Growth RatesNumber of Cycles to grow a 25mm crack fromone side of the EDM notch62

Fractured Surfaces63

ConclusionsLonger hardware service lifeThe laser peening process can result in ConsiderableImprovement to crack initiation, propagation, and mechanicalproperties in FSWImprove processed hardware safetyBy producing Higher Failure Tolerant hardware, &reducing riskLower Hardware Maintenance CostLonger hardware service life, and Lowerhardware down timeApplication of this proposed technology willresult in substantial benefits and savingsthroughout the life of the treatedcomponents

Laser Peening . 28% Increase on Top . 21% increase on Bottom . Hardness Levels for FSW 2195 increased proportionally with number of Laser Peening layers . The polishing that takes place prior to microhardness can wipe mitigate all hardness effects associated with the Shot Peening Process

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