Microstructural Analysis Of The Creep Behavior Of Friction .

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2018 NDIA GROUND VEHICLE SYSTEMS ENGINEERING AND TECHNOLOGYSYMPOSIUMMATERIALS & ADVANCED MANUFACTURING TECHNICAL SESSIONAUGUST 7-9, 2018 – NOVI, MICHIGANMICROSTRUCTURAL ANALYSIS AND CREEP BEHAVIOR OF 25MMTHICK FRICTION STIR WELDED AA2139-T8Uchechi OkekeDepartment of Chemical Engineering &Materials ScienceMichigan State UniversityEast Lansing, MICarl Boehlert, PhDDepartment of Chemical Engineering &Materials ScienceMichigan State UniversityEast Lansing, MITank Automotive Research,Development, and Engineering CenterWarren, MIABSTRACTMost studies conducted on friction stir welded (FSW) Al alloys are on plates that are 2.5-7 mmthick. However, the U.S. Army utilizes materials that are 25 mm thick and greater for structureand armor. In order to properly apply FSW to Al-Cu-Mg-Ag alloys for use in next generationground vehicles, data must be generated and available for model and simulation databases. Onekey type of data is the tensile-creep behavior of FSW AA 2139-T8. Creep is the time dependent,plastic deformation of a material under a constant load, usually observed at a constanttemperature where T 0.3Tm. The objective of this study is to provide information regarding thetensile-creep behavior of the stir zone in comparison to the heat affected zone (HAZ) through thedepth of the weld. The results from this research provide information on the effect of FSWprocessing on the microstructure and creep behavior. Pre- and post-deformation samples wereanalyzed via SEM and TEM and the results are discussed.INTRODUCTION AND BACKGROUNDIn 1991, The Welding Institute (TWI, UK)developed a solid-state joining technique known asfriction stir welding (FSW) [1,2]. This method hasbeen heavily investigated on Al-Cu alloys due tothe increased incorporation of Al alloys in theautomotive and aerospace industry. FSW oflightweight alloys has shown industrial advantagesincluding the reduction of component weight byeliminating additional joining materials (e.g. rivets,fasteners, and filler weld), the capability for joiningdissimilar materials, and the capability forachieving dimensional restrictions as demonstratedin the 2008 Ford GT application and the NASASpace Shuttle Super-Light Weight Tank [3,4].Among the several alluring advantages of FSW [5],the primary reason for the invested interest is that itcircumvents fusion welding issues on agehardenable Al alloys, for instance solidificationcracking.DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

Proceedings of the 2018 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS)FSW requires no additional thermal energy sourceother than that generated by the friction due to therotational stirring of the welding tool. In addition,no welding supplemental materials are required.The rotating tool descends with a downward forceinto the joint of two metal plates which are buttedtogether. The rotating tool traverses along the jointto weld the two plates together. The thermal inputand shear deformation result in three uniquemicrostructural zones listed in order from themiddle of the weld to the periphery [6]: (i) the stirzone (SZ); (ii) the thermomechanically affectedzone (TMAZ); and (iii) the heat affected zone(HAZ). The base metal (BM) region is unaffectedby the welding process and maintains its originalmicrostructure and mechanical properties. Themechanical properties, such as hardness and tensilestrength, vary across these zones and are stronglydependent on the microstructural changes causedby the heat generation and the stirring [7–10].Research on FSW has been primarily on Al alloyswith thicknesses ranging from 2-7 mm [3,7,10–16]due to private commercial industry interest. Thislimits the transferable knowledge on the effects ofprocessing on the microstructure and mechanicalproperties for Al alloys with a thickness of 25 mmand greater. There is also limited informationavailable on the effects of the SZ on the tensilecreep behavior. Creep is the sustained deformationof a material under a constant load and temperature[17]. Due to growing interests and the applicationof lightweight alloys for automotive and aerospaceindustries in public and private sectors, elevatedtemperature loading environments should beinvestigated.This study aims to provideinformation on the tensile-creep deformationbehavior of thick plate FSW Al-Cu-Mg-Ag alloysat elevated temperatures.Tensile-creep experiments were conducted on 25mm thick AA2139-T8. Samples were extracted inthe transverse direction parallel to the weldingdirection in the SZ and the HAZ. This isolationpermitted the evaluation of a particular zone. It wasassumed that the microstructure was homogenousthrough the depth of the SZ. This study, however,demonstrates that tensile-creep behavior throughthe depth of a single SZ cannot be duplicated dueto the varying microstructure. Results werecompared to the HAZ samples. In addition, someHAZ samples were extracted and tested in thetransverse direction.Electron backscattereddiffraction (EBSD), transmission electronmicroscopy (TEM), and scanning electronmicroscopy (SEM) were utilized to acquireinformation on the grain size, precipitationproducts, and failure behavior. The methodologyfor material preparation and experimentation arepresented in the experimental details section. Thepre- and post-test microstructure and creepbehavior of the HAZ and SZ are mentioned in theresults section and analyzed in the discussionsection.EXPERIMENTAL DETAILSMaterial PreparationTwo 76.2 x 45.7 cm plates of wrought, 25 mmthick AA2139-T8 were friction stir welded togetherby EWI (Columbus, OH) per the weldingspecifications displayed in Table 1 [8]. The targetcomposition of AA2139 is shown in Table 2. Theplates were welded perpendicular to the rollingdirection (i.e. in the transverse direction, TD).Samples used for analysis were extracted from theBM, HAZ, and the SZ of this plate in the TD usingelectrical discharge machining (EDM). The sampledimensions and areas of extraction are depicted inFigure 1. The gage section width and thicknesswere 12.7 mm and 1.78 mm, respectively. UsingSiC grinding paper, the samples were ground priorto testing to both remove the EDM contaminationlayer and provide a smooth surface.The nomenclature of samples machined from theSZ was used to record their location through thedepth of the weld. Samples used for creep testingwere machined from the SZ Bottom, Mid Bottom,and Middle regions as shown in Figure 2. The SZBottom samples were machined 0 to 4.2 mm fromMicrostructural Analysis and Creep Behavior of 25mm Thick Friction Stir Welded AA2139-T8, Okeke and BoehlertDISTRIBUTION STATEMENT A. Approved for public release; distribution unlimitedPage 2 of 15

Proceedings of the 2018 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS)the weld root, the very bottom of the weld. The SZMid Bottom samples were machined 4.2 to 10.5mm from the weld root. The SZ Middle sampleswere machined 10.5 to 14.8 mm from the weld root.Table 1: Welding parameters.ParameterSpecificationShoulder Diameter41.3 mmPin Length24.7 mmPlunge Depth0.51-0.13 mmSpindle Speed150-250 RPMTravel Speed5.1 CPMTotal Length45.7 mmTable 2: Target composition in wt% of AA2139 as listed inThe Teal Sheets igure 1: Image depicting how the mechanical test sampleswere extracted from the welded plates.Figure 2: Low magnification image showing the location of5 areas through the stir zone.Microscopic AnalysisImages of the fracture surface and the transversecross section of the samples were acquired using afield emission gun TESCAN Mira3 SEM at a beamvoltage of 25kV.Samples sectioned forfractography were cut approximately 1 cm fromboth fracture surfaces. One side was left to analyzethe fracture surface and the other side was mountedto analyze the sample surface at the fracture site.Images were acquired under secondary electron(SE) and backscattered electron (BSE) conditions.EBSD Orientation image maps were acquired andprocessed with an EDAX (Mahwah, NJ) EBSDdetector and TSL OIM software. TEM analysis onthe as-welded SZ and deformed samples wasperformed on a JEOL 2100F. For the untested BM,a JEOL JEM-ARM200F TEM was used. A voltageof 200kV was used for both TEMs. The precipitatevolume percent, V p , was measured using theImageJ software.Tensile TestingTensile tests were conducted on the large,rectangular dogbone samples, see Figure 1, using aMTS servo-hydraulic, thermomechanical testingmachine with a MTS Flex Test SE controller(Eden Prairie, MN). The tensile specimens had athickness of approximately 0.75 mm. The loadingaxis was parallel to the TD, see Figure 1. The testswere performed at RT at a constant displacementrate of 0.025 mm-s-1, which corresponded to astrain rate of approximately 10-3 s-1. An aluminarod extensometer was used to measure the strain.Engineering stress (σ), 0.02% offset yield strength(YS), elastic modulus (E), and the ultimate tensilestrength (UTS) were measured for eachexperiment.Creep TestingTensile-creep experiments were performed atstresses ranging from 25 to 125 MPa and attemperatures ranging from 225 C to 275 C, seeTable 3. The tests were conducted on lever-armcreep machines.Data was recorded usingMicrostructural Analysis and Creep Behavior of 25mm Thick Friction Stir Welded AA2139-T8, Okeke and BoehlertDISTRIBUTION STATEMENT A. Approved for public release; distribution unlimitedPage 3 of 15

Proceedings of the 2018 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS)LabVIEW VI software. All tests were conductedusing two general steps: heat up and creep loading.During the heat up procedure, the temperature wasincreased to the target temperature within an hour.A load between 3-8 MPa was applied to preventcompression of the sample during heat up. Whenthe target temperature was achieved, the sampleswere heat soaked for approximately 15 minutesbefore applying the creep load.The time,temperature, load, and displacement were recordedthroughout the test.slightly rotated and show some texture. Themeasured average grain size was 32, 8, and 3 µmfor the top, middle, and bottom, respectively.Figure 3: EBSD orientation map of the HAZ in thetransverse cross section.Table 3: List of creep experiments omMid-Bottom-3Temp ( C)250-275250-275225225250250Stress ostructureThe HAZ microstructure, depicted in the EBSDorientation map in Figure 3, has elongated grainsretained from the rolling during the T8 temper.Micrographs through the center of the SZ capturethe microstructure from the top of the weld to thebottom in Figure 4. The grain size decreased fromthe top of the weld, where the average grain sizewas 30 μm, to the bottom of the weld, where theaverage grain size was 6 μm. The precipitatevolume percent, V p , had an inverse relationshipwith the grain size. The lowest V p was observed atthe top (2.4%) while the highest was observed atthe bottom (10.6%).EBSD was performed on the surface normal to theweld at the top, middle, and bottom. The middle isdefined as the region in the middle of the plate andthe top sample was extracted 8 mm above it. Thebottom sample was 8 mm below the middle sample.The EBSD images are presented in Figure 5. Thegrains at the bottom of the sample in the SZ wereFigure 4: SEM micrographs through the depth of the SZ inthe transverse cross section with the measured average grainsizes (GS) and precipitate volume percent (V p ).Creep of HAZ and SZRoom temperature (RT) tensile tests wereconducted on the BM and SZ in the transversedirection to measure the yield strength, which wasused to determine the applied creep stresses. TheUTS and the YS for the SZ were both lower thanthose of the BM, see Figure 6. The BM exhibited aYS, UTS, and elongation to failure of 300 MPa, 433MPa, and 7.2%, respectively. The SZ exhibitedvalues of 265 MPa, 364 MPa, and 7.3% for the YS,UTS, and elongation to failure, respectively. The Efor BM and SZ were 76.6 GPa and 61.1 GPa,respectively.Microstructural Analysis and Creep Behavior of 25mm Thick Friction Stir Welded AA2139-T8, Okeke and BoehlertDISTRIBUTION STATEMENT A. Approved for public release; distribution unlimitedPage 4 of 15

Proceedings of the 2018 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS)a)500b)Engineering Stress (MPa)BM400SZ3002001000012345678Strain (%)Figure 6: Room temperature stress versus strain curves forthe SZ and the BM in the transverse direction.c)Figure 5: EBSD orientation map of the normal surface of theSZ region at the a) top (average grain size 32 µm), b) middle(average grain size 8 µm), and the c) bottom (average grainsize 3 µm).The stress or temperature was increased after thecreep rate versus time plot indicated that asecondary creep stage was reached. In Figure 7 andFigure 8, the creep strain versus time plot of theHAZ was compared to that of the SZ. Figure 7shows a stress increase test for the HAZ and SZMid Bottom-2. The applied stresses ranged from50-125 MPa in increments of 25 MPa. Sampleswere tested at a constant temperature of 225 C. SZMid Bottom-2 failed after the stress increase to 75MPa and approximately 350 hrs of testing. TheHAZ sample failed after the stress increase to 125MPa and approximately 700 hrs of testing. Thesecondary creep rates for SZ Mid Bottom-2 werefaster than the HAZ by approximately one order ofmagnitude. The transverse cross section of the SZMid Bottom-2 in Figure 7b shows a refined,equiaxed microstructure with some coarsenedprecipitates at the grain boundaries. The V p , Table4, was 3.5%, which was approximately half that ofFSW SZ Mid Bottom-2 (6.3%). Analysis of the SZMid Bottom-2 fracture surface in Figure 9 revealeddimples towards the lateral ends of the gage sectionwhich is indicative of a ductile fracture. The centerMicrostructural Analysis and Creep Behavior of 25mm Thick Friction Stir Welded AA2139-T8, Okeke and BoehlertDISTRIBUTION STATEMENT A. Approved for public release; distribution unlimitedPage 5 of 15

Proceedings of the 2018 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS)Creep through the SZ DepthTwo SZ samples were tested at a temperature of250 C and at applied stresses of 25, 35, and 50MPa. Although both samples were from the SZ,they exhibited different creep behavior, as seeFigure 12a.a)1oHAZ225 C-9CR 1.86x10 s0.8Creep Strain (%)exhibited intergranular features which is indicativeof a brittle fracture. The post-test microstructure ofthe HAZ exhibited intragranular and intergranularprecipitate coarsening, as seen in Figure 7c. Theprecipitate V p was measured to be 4.3% which wasapproximately twice that of the untested BM(2.1%). The fracture surface of the HAZ exhibitedductile and brittle features, as well. In Figure 10,the right side of the fracture surface exhibitedintergranular cracking and the left side exhibiteddimples. The fracture features for both sampleswere consistent with the elongation-to-failure eventhough brittle fracture features were observed. Asimilar observation was noted in the temperatureincrease experiments.The SZ Mid Bottom-1 and HAZ were tested attemperatures of 250 C and 275 C at an appliedstress of 50 MPa, see Figure 8a. The SZ MidBottom sample failed after the temperature increaseto 275 C and approximately 200 hrs of testing. TheHAZ sample failed after the temperature increaseto 275 C and approximately 320 hrs of testing. Thetransverse cross section of the HAZ in Figure 8cshows intergranular and intragranular precipitatecoarsening. The precipitate V p was 3.8% whichwas greater than the precipitate V p of the BM. Thetransverse cross section of SZ Mid Bottom-1 inFigure 8b reveals a refined, equiaxedmicrostructure with intergranular and intragranularcoarsened precipitates. The precipitate V p was7.3%, which was greater than the precipitate V p ofthe FSW SZ Mid Bottom-1, see Table 4. Dimpleswere observed on the lateral sides of the SZ MidBottom-1 fracture surface which suggests a ductilefracture and is consistent with the elongation-tofailure; however, brittle fracture features wereobserved. The center of the sample demonstratedintergranular fracture, see Figure 11.-8CR 1.33x10 s0.6-1-10CR 8.78x100.4-9CR 1.23x10 ss-1-1-10CR 8.49x10s-1σ 125MPa0.2-100-1SZ Mid Bottom-2-1CR 5.53x10 sσ 100MPaσ 75MPaσ 50MPa0100 200 300 400 500 600 700 800Time (hrs)b)c)Figure 7: Stress increase test. Creep strain versus time plotfor SZ Mid Bottom-2 and HAZ samples at 225 C and stressesranging between 50-75-100-125 MPa. Micrographs for a) SZMid Bottom-2 (0.75% creep strain) and b) HAZ (0.95% creepstrain).Microstructural Analysis and Creep Behavior of 25mm Thick Friction Stir Welded AA2139-T8, Okeke and BoehlertDISTRIBUTION STATEMENT A. Approved for public release; distribution unlimitedPage 6 of 15

Proceedings of the 2018 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS)a)2.5a)σ 50 MPaSZ Mid Bottom-12oo275 CCreep Strain (%)250 C1.5-7CR 1.23x10 s1-9CR 7.62x10 s-1-1HAZ0.5-10CR 1.5x100b)s-1-10CR 3.07x10s-1c)050100150200250300350Time (hrs)b)Figure 9: a-b) Fractograph of sample SZ Mid Bottom-2 creeptested at 225 C and 50-75 MPa. Lateral sides of the samplehave dimples. c) Intergranular features were observed at thecenter. Creep strain to failure was 0.75%.Table 4: Precipitate V p of the creep tested samples and thecenter of the SZ through the depth of the weld.SampleStress(MPa)Temp.( C)Vp(%)SZ Mid Bottom-250-752253.5HAZ50-75-100-1252254.3SZ Bottom25-35-5025012.5SZ Mid Bottom-325-35-5025011.3HAZ50250-2753.8SZ Mid Bottom-150250-2757.2BM----2.1FSW SZ Top----2.4FSW SZ Mid Top----2.9FSW SZ MidFSW SZ MidBottomFSW SZ Bottom----3.6----6.3----10.6c)Figure 8: Temperature increase test. a) Creep strain versustime plot for SZ and HAZ samples at temperatures rangingbetween 250-275 C and at stress of 50 MPa. b) SZ MidBottom-1 micrograph (2.4% creep strain). c) HAZmicrograph (0.25% creep strain).Microstructural Analysis and Creep Behavior of 25mm Thick Friction Stir Welded AA2139-T8, Okeke and BoehlertDISTRIBUTION STATEMENT A. Approved for public release; distribution unlimitedPage 7 of 15

Proceedings of the 2018 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS)a)a)b)b)c)c)Figure 11: a-b) Fractograph of sample SZ Mid Bottom-1which underwent a temperature increase creep test at 250275 C and 50 MPa.c) High magnification of theintergranular fracture of the boxed region in b). Creep strainto failure was 2.4%.Figure 10: Fractograph of the HAZ creep tested at 225 Cand 50-125 MPa. a) Left half contains dimples. b) Righthalf consists of intergranular cracking. c) Cracking at triplejunctions (arrows) and along grain boundaries (boxed area).Creep strain to failure was 0.95%.The observed maximum creep strain for SZBottom at failure was 2.4% at 560 hrs. The creeprates were 2.7x10-9 s-1, 1.3x10-9 s-1, and 2.1 x10-8 s1for 25, 35, and 50 MPa, respectively. The creeprate did not change significantly when the stresswas increased to 35 MPa. Sample SZ Mid Bottom3 experienced significant increases in creep ratefrom 7.0x10-10 s-1 to 1.1x10-8 s-1 to 1.1x10-7 s-1 at25, 35, and 50 MPa, respectively. The observedmaximum creep strain at failure was 1.6% afterapproximately 400 hrs. SZ Bottom had a highercreep strain by almost 0.5% than SZ Mid Bottom-3at 25 MPa and the creep rate was approximatelyfour times faster. SZ Mid Bottom-3 continued toincrease in creep rate at 50 MPa which was greaterthan SZ Bottom. In general, SZ Bottom exhibiteda lower creep rate and higher creep strain valuesthan SZ Mid Bottom-3.The transverse cross section of SZ Bottom and SZMid Bottom-3, see Figure 12b-c, show a tates, especially at the grain boundaries forboth samples. However, SZ Bottom appears tohave slightly more precipitate coarsening.Table 4 and Figure 4 demonstrate that there is agradient in the microstructure in the SZ. When th

MICROSTRUCTURAL ANALYSIS AND CREEP BEHAVIOR OF 25MM THICKFRICTION STIR WELDED A A2139-T8 . Uchechi Okeke Department of Chemical Engineering & . and post-deformation samples were analyzed via SEM and TEM and the results are discussed. INTRODUCTION AND BACKGROUND ; In 1991, The Welding Institute (TWI, UK) developed a solid-state joining .

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