機(jī)械外文文獻(xiàn)翻譯-重疊定向?qū)︽V合金板料攪拌摩擦焊縫疲勞行為的影響【中文8980字】【PDF+中文WORD】,中文8980字,PDF+中文WORD,機(jī)械,外文,文獻(xiàn),翻譯,重疊,定向,鎂合金,板料,攪拌,摩擦,焊縫,疲勞,行為,影響,中文,8980,PDF,WORD Effect of overlap orientation on fatigue behavior in friction stir linearwelds of magnesium alloy sheetsJ.F.C.Moraesa,R.I.Rodrigueza,J.B.Jordona,X.SubaDepartment of Mechanical Engineering,The University of Alabama,Tuscaloosa,AL 35401,USAbFord Motor Company,Dearborn,MI 48124,USAa r t i c l ei n f oArticle history:Received 22 September 2016Received in revised form 23 January 2017Accepted 19 February 2017Available online 21 February 2017Keywords:FatigueFriction stir weldingMagnesiumFractographya b s t r a c tIn this work,we investigate the effect of sheet stacking orientation on fatigue behavior in friction stir lin-ear welding of AZ31 Mg alloy.It is well known that during friction stir welding,the advancing andretreating flow of the material generated by the tool creates asymmetrical weld features resulting is ani-sotropic mechanical behavior.As such,friction stir welding of overlap joints was carried out on 2 mmthick sheets,where the orientation of the pull direction of the coupon was varied with respect to the toolrotation direction.Subsequently experimental fatigue tests were performed to evaluate this effect of thesheet stacking orientation on cyclic behavior.The fatigue results showed that the overlap joints loaded onthe retreating side exhibited superior performance compared to the advancing side.Post-mortem anal-ysis coupled with finite element results suggest that the geometrical shape of the faying surface producedby the advancing and retreating material flow largely determines the number of cycles to failure in thesefriction stir linear welded overlap joints.?2017 Elsevier Ltd.All rights reserved.1.IntroductionRecently,there has been renewed interest in reducing theweight of vehicles in the automotive industry in order to meetthe stringent fuel and green house gas emissions standards.Oneway to achieve this goal is lightweighting the designs throughthe use of materials with enhanced strength-to-weight ratios 1.For example,materials such as Al and Mg alloys,which are knownto have high strength-to-weight ratios,are seeing increased use inthe body-in-white production.In fact,the employment of light-weight materials,like Mg alloys,can drastically reduce vehicleweight while maintaining satisfactory structural performance.However,traditional welding techniques,such as resistance spotwelding(RSW),which is widely used to join traditional metals likesteels,are difficult to implement for joining magnesium alloys1,2.This difficulty is mainly due to the high electrical and thermalconductivity 3 characteristics of Mg alloys,which results in highelectrical currents and thus can produce poor weld quality.Whilemany alternative joining techniques including fasteners for Mgalloys exist,friction stir welding(FSW)is ideally suited for joiningMg alloys 4.FSW is a solid-state process which limits tempera-tures below the melting point of the material and thus mostlyeliminates or significantly reduces problems such as solidification,formation of second phases,porosity,embrittlement and cracking5.Moreover,the relatively low temperature at which the processoccurs enables the FSW joint to achieve lower distortion and resid-ual stresses 5 compared to fusion welding.In automotive manufacturing,the overlap is a commonly usedjoint configuration.In friction stir linear welding(FSLW),whichis a variant of FSW,two sheets can be assembled in an overlap con-figuration,as shown in Fig.1.In this welding configuration,a rotat-ing tool is plunged into the two materials at a predetermineddepth,having the tool shoulder in contact with the top sheet asthe tool transverses along the welding segment and the lap linearweld is completed when the tool is retracted 6.Due to the mate-rial flow and the tools direction of travel,two distinctive weld fea-tures are produced in overlap welding:the advancing side;and theretreading side.The advancing side(AS)is the side of the toolwhere the point velocities are faster(rotation plus translation),whereas the retreating side(RS)is the side of the tool where thepin velocities are slower(rotation minus translation).In FSLW,the faying surface(surfaces in contact in the joint)ofthe AS usually remains exterior to the weld nugget and pointsupwards along the weld nugget boundary 7.On the other hand,the RS curves up and terminates in the nugget.Since the AS sideof the weld takes on a hook-like form,it is typically referred toas the hooking defect;whereas,the RS is known as cold lap defecthttp:/dx.doi.org/10.1016/j.ijfatigue.2017.02.0180142-1123/?2017 Elsevier Ltd.All rights reserved.Corresponding author.E-mail address:bjordoneng.ua.edu(J.B.Jordon).International Journal of Fatigue 100(2017)111Contents lists available at ScienceDirectInternational Journal of Fatiguejournal homepage: is known that these macro features depend on a combinationof tool geometry and welding parameters.Furthermore,it has alsobeen established that these features affect the weld strength anddecrease the joint load capacity and/or influence crack nucleationand propagation 8.As the formation of these macro featuresdepends on the heat generated from the friction between the tooland the pieces to be joined,and material flow during the FSW pro-cess,characteristics of FSLW highly depend on tool geometry610.For example,Yang et al.6 studied different tool geome-tries and process conditions and their influence on shear strengthof AZ31 Mg alloy friction stir lap welds.One of the key results oftheir research was that a higher tensile load is reached when load-ing the top sheet of the lap joint configuration on the RS versusloading on the AS.A similar study by Yuan et al.8 evaluatedthe effect of different tool designs and weld variables on shearstrength in FSLW lap joints of AZ31 Mg alloy.In their study,theyfound that the RS of the joint achieved higher loads compared tothe AS with the same process parameters.However,they did notexplore the effect of RS and AS configuration on fatigue behaviors.While there exists a few published studies focused on the effectof welding parameters on the static strength of the FSLW in Mgalloys,to the best of the authors knowledge,the effect of sheetstacking orientation on fatigue performance has not been eluci-dated.As such,the objective of this paper is to quantify the effectsof stacking orientation on fatigue characteristics of AZ31 Mg alloyjoinedbyFSLWthroughbothexperimentalandnumericalapproaches.2.Materials and experimentsFor this study,commercial grade 2 mm thick AZ31 Mg alloysheets were employed,which contained a nominal chemical com-position of Mg-3.0 wt%Al-1.0 wt%Zn,with a base material yieldstrength of 250 MPa,and an ultimate strength of 342 MPa 11.For welding purposes,the sheets were cut to a width of 75 mmand length of approximately 1500 mm.The plates were assembledin an overlap configuration and welded at a tool rotational speed of2000 rpm and a travel speed of 4.6 mm s?1.A FSW tool having aconcave scroll shoulder with 13 mm diameter,a 3.5 mm long triflatthreaded pin,a 4.7 mm pin tip diameter,and 6.0 mm pin root wasused to weld the lap-shear samples.As shown in Fig.1a and b,forthe same traveling direction and rotation of the tool,the side of theweld to be loaded is defined according to the orientation of thesheets relative to the pull direction.Two sets of FSLW were createdin an overlap configuration.The only difference between the twosets of coupons is the overlap orientation(i.e.the orientation ofthe top and bottom sheets in order to have the advancing orretreating side on the free edge of the top sheet).A schematicdrawing of the coupons is shown in Fig.2a.After welding was com-pleted,the FSLW overlap plates were cut into 30 mm wide by120 mm long coupons for mechanical testing purposes.Fig.2bshows the configuration for the coupons oriented to the RS and AS.A MTS servo-hydraulic load frame was used to perform lapshear tensile test(quasi-static)for each sheet stacking configura-tion in order to obtain a representative average ultimate load car-ried by the joint.Three coupons were tested per configuration anda grip-to-gripdistance of 60 mm,at an actuator speedof1 mm min?1was used during tensile testing.For fatigue testing,the coupons were tested with the same grip-to-grip distance inan MTS servo-hydraulic load frame with a 2.2 kN load cell,underload control condition with a sinusoidal waveform at load ratioR=0.1 and a frequency of 20 Hz.Shims were used in both thequasi-static and fatigue tests in order to avoid additional bendingmoments and loads on the test samples.Inadditiontomechanicaltesting,analysisoftheweldmicrostructure and postmortem analysis were conducted in thisstudy.Mechanically untested and tested coupons were sectionedparallel to the loading direction,cold mounted in epoxy,ground,and polished.The final polishing was done on a neoprene pad withalumina 0.05 m in glycol slurry.In order to characterize themicrostructure,the mounted coupons were etched using a solutioncomposed of 4.2 g picric acid,10 ml acetic acid,10 ml H2O and70 ml ethanol 12,13.An optical digital microscope KeyenceVHX-1000 was used to evaluate size and shape of the weld fea-tures,the effective sheet thickness,and the transverse crackFig.1.Coupon layout of friction stir lap welding;(a)retreating side,(b)advancing side.2J.F.C.Moraes et al./International Journal of Fatigue 100(2017)111propagation under different loading conditions.Microtexture char-acterization of the FSLW coupons was performed using a JEOL7000 scanning electron microscope(SEM)equipped with a detec-tor for electron backscatter diffraction(EBSD).All samples wereelectro-polished at 3 V for 20 s using H3PO4diluted in ethanol(3:5 ratio).EBSD analysis was conducted using 20 kV beam voltagein 0.9lm steps.Microtexture data was acquired using the AZTECsoftware from Oxford Instruments and post-processing was doneusing the HKL Channel 5 package.Microhardness measurements were conducted on the cross sec-tion of the top and bottom sheets with increments of approxi-mately 0.5 mm,using a Wilson hardness testing machine.A loadof 100 g with a dwell time of 5 s was applied in order to obtainthe Vickers hardness(HV)across the weld nugget.For crack nucle-ation and propagation analysis,fractured surfaces of the fatiguetested coupons were examined in the Jeol 7000 SEM.3.Results and discussion3.1.Geometrical featuresA representative cross-section of a FSLW coupon is presented inFig.3.As noted earlier,the faying surface on the advancing side(AS)usually exhibits the shape of a hook and curves upwards alongthe nugget periphery.On the opposite side of the weld,the fayingsurface on the retreating side(RS)extends through the weld nug-get toward the AS,where this feature is generally referred as cold-lap feature 8.These distinct features are a result of trapped oxidefilms that are on the surface of the sheets prior to welding.Thesetrapped oxide film features depend on relative velocities betweenthe tool and work material.The material in front of the rotatingtool is pushed upward due to the tool tilt angle.The amount ofmaterial being driven upward on the leading side flows aroundFig.2.(a)Configuration of the friction stir linear welded(FSLW)lap-shear coupon.(b)Schematic of loading configurations:retreating side(RS)and advancing side(AS).Dimensions are in millimeters.Fig.3.(a)Cross-section view of a representative friction stir linear weld(FSLW)in overlap configuration where AS is the advancing side and RS is the retreating side.Magnified views of the(b)hooking feature of the AS,(c)cold-lap features in the stir zone,(d)and in the RS.J.F.C.Moraes et al./International Journal of Fatigue 100(2017)1113the pin in the rotation direction 1416,resulting in a hook featurethat points upwards(Fig.3b).As this material flow decelerates onthe trailing side,it accumulates resulting in flow away from thetool pin 15,thus leading to the downward pointing lap-feature,as shown in Fig.3c.Fig.3d shows the peak height of cold-lapfeature.3.2.Microstructure and hardnessFig.4 shows the results of the microstructure characterizationby EBSD measurements.In particular,inverse pole figures(IPF)illustrate the grain orientation,as well as grain size distributionof the FSLW coupon.Fig.4a shows the locations of the EBSD mea-surements.For both the base material(BM)and the stir zone(SZ),astrong texture can be observed in Fig.4b.This strong texture is dueto the rolling process of the Mg sheet material in the BM and thelarge shear deformation caused by the tool in the SZ.Lastly,Fig.4cshows the grain size distribution comparison between the BZ andthe SZ,where the average grain size of SZ was only slightly finerthan the BZ.Fig.5 shows a representative hardness profile of the FSLW cou-pons.The horizontal axis represents the distance from the center ofthe nugget of the weld in mm.The vertical axis represents themeasured Vickers(HV)hardness value.As shown in Fig.5,the cen-ter of weld nugget exhibited higher hardness compared to outeredges of the SZ.As shown in Fig.5,hardness values change signif-icantly across the weld.However,the hardness profile of the FSLWexhibited symmetry from the center of the weld outward,showingsimilar hardness values for the AS and RS.Moreover,there was nota significant difference of hardness measurement in the areasFig.4.(a)Locations of EBSD analysis on the cross-section of friction stir linear welded coupon.(b)Inverse pole figures of BM(base material)and SZ(stir zone).(c)Grain sizedistribution plot.Fig.5.Microhardness profile measurements on a representative friction stir linearweld coupon.Hardness of base material:59.82 2.69 HV.4J.F.C.Moraes et al./International Journal of Fatigue 100(2017)111where the fatigue cracks initiated in the AS and RS of the weld.Thiswill be discussed later in this paper.3.3.Lap-shear tensile behaviorLap-shear tensile tests were conducted to evaluate the jointstrength of the AS and RS orientation,where three coupons weretested in both the AS and RS orientations.It is noted that,in a sim-ilar study on AZ31 Mg alloy joints,Yuan et al.8 reported that theRS orientation achieved higher lap-shear strength when comparedto the AS produced under the same welding parameters.While inthis study,the welding parameters were slightly different from thework by Yuan et al.8,the trend in mechanical behavior of thejoint performance in this study was similar.In fact,in this study,the average ultimate load of the RS was approximately 50%greaterthan the AS orientation.Representative load-displacement curvesunder quasi-static lap-shear testing of the RS and AS coupons areshown in Fig.6.Regarding the fracture behavior of the coupons under tensileloading,representative cross-sections of fractured coupons areshown in Fig.7.Fig.7ac show optical cross-sectional views of afractured AS coupon,while Fig.7df show optical cross-sectionalviews of a fractured RS coupon.The darker areas of the opticalimages in Fig.7b and e show the twinning distribution due tothe large scale deformation under monotonic loading conditions.Fig.7c and f show high magnification of the twinning density indetail.Similar results were reported by Yang et al.6,where thistype of failure is due to localization of deformation,indicated bymechanical twins near the fractured surface.3.4.Fatigue behaviorFig.8 shows the experimental results of the fatigue tests of theFSLW lap-shear coupons in the RS and AS orientation.In this figure,the vertical axis represents the load range applied to the joint andthe horizontal axis is the corresponding number of cycles to failure,where we define failure as the complete separation of the joint.Thearrows on the plot indicate run-outs.It can be observed that the RSorientation exhibited superior fatigue lifetimes compared to the ASorientation at the same cyclic load.In addition,Fig.8 indicates anearly linear offset of the fatigue behavior of the RS as comparedto the AS orientation.This result suggests a strong correlation tothe ultimate strength of the joint and fatigue behavior,which willbe discussed later in the paper.Regarding the fracture behavior of the coupons under fatigueloading,Figs.9 and 10 show the cross-sections of representativefailed coupons.For clarification purposes in this study,we classifythe fatigue loading into low cycle fatigue(10,000)regimes.The cross-sections of failed coupons in the low cycle fatigueregime are depicted in Fig.9.The low cycle fatigue failure of theAS is shown in Fig.9a where failure occurred at 503 cycles at amaximum load of 2069 N.The low cycle fatigue failure of the RSoccurred at 2620 cycles at a maximum load of 2069 N as shownin Fig.9d.In both cases,cracks grew in mode I propagation directlythrough the stir zone and into the surface of the top sheet as shownin detail in Fig.9b and e.Also,less twinning was observed com-pared to monotonic loading due to lower severity of plasticity ascan be seen in detail in Fig.9c and f.The high cycle fatigue failures are shown in Fig.10,where fail-ure of the AS presented in Fig.10a occurred at 353,589 cycles atFig.6.Representative load versus displacement curves of FSLW lap-shear tests ofthe retreading side(RS)and advancing side(AS)orientated coupons.50 m a)Tensile AS b)d)Tensile RS c)e)f)Fig.7.Failed lap-shear coupons loaded on:(a)advancing side,(d)retreating side.Twinning distribution near the facture:(b)advancing side,(e)retreating side.Highermagnification image showing mechanical twining in detail:(c)advancing side,(f)retreating side.J.F.C.Moraes et al./International Journal of Fatigue 100(2017)1115maximum load of 371 N.Fig.10d shows the failure of the RS at437,661 cycles at a maximum load of 570 N.It is important to notethat at lower applied amplitudes,the crack propagates through theboundary between the SZ and TMAZ in a mixed-mode(I+II)behavior in both conditions,and evidence of twinning was notoptically observed as shown in Fig.10b and e.Fig.10c and f showin detail multiple crack propagation directions.Additionally,crackbranching was observed that was not observed in the low cyclefatigue samples.This may indicate relatively low residual stressesdue to the welding process not having a significant influence on thebehavior of the fatigue lives,which is shown in Fig.8.Figs.11 and 12 show SEM images of fatigue fracture surfaces ofthe AS and RS orientated coupons.Fig.11a shows the AS,wherefailure occurred at 43,368 cycles at a maximum load of 580 N.The ratchet marks indicate that the cracks initiated at multiplelocations across the hook tip,and then grew toward the top surfacein the direction indicated by the white arrows.A further evaluationFig.8.Experimental results of load range versus the number of cycles to failure ofthe FSLW lap-shear coupons tested at a load ratio R=0.1.Fig.9.Representative cross-sectional views of fractured low cycle fatigue coupons loaded on:(a)advancing side(503 cycles),(d)retreating side(2620 cycles).Magnifiedview of the crack growth path through the stir zone:(b)advancing side:(e)retreating side.High magnified view of the crack path near rupture:(c)advancing side,(f)retreating side.Load ratio was R=0.1.Fig.10.Representative cross-sectional views of fractured high cycle fatigue coupons loaded on:(a)advancing side(failure at 353,589 cycles),(d)retreating side(failure at437,661 cycles).Magnified view of crack path that grew between the stir zone and the thermo-mechanically affected zone:(b)advancing side:(e)retreating side.Highmagnified view of secondary cracks:(c)advancing side,(f)retreating side.Load ratio was R=0.1.6J.F.C.Moraes et al./International Journal of Fatigue 100(2017)111was made by analyzing crack initiation in the areas close to thehook tip(Fig.11d)and steady crack growth regime some distancefrom the hook tip(Fig.11b).Also note that the striation spacing issmall in areas near to the hook tip as shown in Fig.11e,and largerin areas further from the hook tip as observed in Fig.11c.Theseresults agree with the proposed crack initiation location and crackpropagation direction presented previously.In Fig.12a,the RS coupon