Repair welding process of friction stir welding groove defect

Available online at www. sciencedirect.comTransactions ofP“ScienceDirectNonferrous MetalsSociety of ChinaELSEVIER PressTrans. Nonferrous Met. Soc. China 19(2009) 563-567www .nmsc.cnRepair welding process of friction stir welding groove defectLIU Hui-jie(刘会杰), ZHANG Hri-je(张会杰)State Key Laboratory of Advanced Welding Production Technology,Harbin Institute of Technology, Harbin150001, ChinaReceived 17 June 2008; accepted 27 October 2008Abstract: The groove defect formed in the friction stir welding dramatically deteriorates weld appearances and mechanicalproperties of the joints owing 10 its larger size and penetration. Therefore, the friction stir repair welding was utilized to remove sucha groove defect, and the focus was placed on the mechanical properties and microstructural charaterstics of the repair joints so as toobtain an optimum repair welding process. The experimental results indicate that the groove defect can be removed by friction stirrepair welding, and the offset repair welding process is superior to the symmetrical repair welding process. In the symmetrical repairwelding process, a large number of fine cavity defects and an obvious aggregation of hard-britle phase Al2Cu occur, accordingly themechanical properties of the repair joint are weakened, and the fracture feature of repair joint is partially britle and partially plastic.A good-quality repair joint can be obtained by the offset repair welding process, and the repair joint is fractured near the interfacebetween the weld nugget zone and thermal-mechanically affected zone.Key words: friction stir welding; groove defect; repair welding process; mechanical propertiesdeteriorates weld appearances and mechanical properties1 Introductionof the joints owing to its larger size than the others[11],accordingly the groove defect is chosen as a study objectAs a new and promising solid state welding processfor repair welding in the present work, and FSW is stilldeveloped in recent years[1-2]， friction stir weldingused as the repair welding process. The focus is placed(FSW) is characterized by the high welding quality, lowon the mechanical properties and microstructuralproduction cost and low welding distortion, and thus cancharacteristics of the repair joints in order to obtain anbe utilized to weld some materials that are difficult tooptimum repair welding process and reveal the decisivefusion weld[3- -6]. However, in the fast development andfactor for controlling the mechanical properties of repairapplication of the FSW process, it has been known thatjoints.welding defects may be formed during FSW whenimproper welding parameters or technological conditions2 Experimentalare used[7- 10], and such defects as groove, cavity andkissing bond have significant influence on theThe base metal used in this study was a 2219mechanical properties of the joints[11-13]. Therefore,aluminum alloy plate of 7.5 mm in thickness, whosethe repair welding process of defect-existent jointschemical compositions and mechanical properties areattracts much attention, but the related research is lack.listed in Table 1. The plate was cut and machined intoIn the two published papers, one is on the repair weldingrectangular welding samples of 300 mm in length andtechnology of the defect produced during friction stir100 mm in width. After cleaned by acetone, the sampleswelding owing to the rupture of pin tool[14], and thewere longitudinally butt-welded and then repair-weldedother is on the friction stir processing technology of theusing an FSW machine (FSW- 3LM- -003). The weldingcrack formned in the aircraft wing structure[15]. Amongtool中国煤化工- stum contour andthe defects mentioned above, groove defect dramaticallypossesYHCNMHGFoundation item: Project suppored by the Program of Excellent Team in Harbin Instiute of Technology, China; Project(2006BAF04B09) supported by theNational Key Technology Research and Development Program of ChinaCorreponding author; LIU Hui-jie, Tel: +86- 451-86413951; E-mail: liuhj@hit.edu.cnDOI: 10.1016/51003-6326(08)60313-1564LIU Hui-jie, et al/Trans. Nonferrous Met. Soc. China 19(2009) 563-567Table 1 Chemical compositions and mechanical properties of 2219 aluminum alloyChemical composition (mass fraction)%Mechanical propertiesYieldElongation/AICuMnFeVZnZstrength/MPa strength/MPaB6.480.32 0.230.060.08 0.040.40.2043231511For the convenience, we define that the joint or(Instron-1 186), and the tensile properties of each jointweld directly formed from the base metal is initial jointwere evaluated using three tensile specimens cut fromor initial weld, while the joint or weld formed after repairthe same joint. After tensile test, the fracture features ofwelding of the initial joint or initial weld is repair joint orthe joints were analyzed by OM and SEM mentionedrepair weld. With regard to repair welding, there wereabove.two repair processes. One was symmetrical repairwelding process in which the centerine of the repair3 Results and analysesweld was consistent with that of the initial weld withgroove defect; the other was offset repair welding3.1 Tensile properties of jointsprocess in which the centerline of the repair weld wasFig.1 shows the tensile test results of the initiallocated in the groove position. In order to decrease thjoints and corresponding repair joints. It can be seen thatdifference of microstructures and mechanical propertiesthe initial joint with groove defect has considerably lowon the advancing side (AS) and retreating side (RS) ofmechanical properties, namely, the tensile strength ofthe repair weld, the direction of repair welding was152 MPa and the clongation of 1.6%. This implies thatconsidered to be opposite to that of the initial welding.the groove defect has resulted in an enormousBased on the previous experimental results, the initialdeterioration of the mechanical properties of the joints.joints with groove defect and the good-quality repairjoints can be produced by using the technologicala)parameters listed in Table 2.Table 2 Welding technological parameters for initial joints with00-groove defect and good-quality repair jointsRotationWeldingJoint sortspeed/3 Pressure/Tilt200cNangle/(C)(r-min ) (mm:min )Initial joints00with groove 1 0004.62.5defectGood-quality600.6Arepair jointsJoint“(6)After welding, all the joints were cross-sectioned12-perpendicular to the welding direction for metallographic10Fanalyses and tensile tests using an electrical-dischargecutting machine (DK- 7718B -CG). The cross-sections of8the metallographic specimens were polished with adiamond paste, etched with Keller's reagent (150 mLwater, 3 mL nitric acid, 6 mL hydrochloric acid and 6 mLhydrofluoric acid), and analyzed by optical microscope(OM, Olympus -MPG3) and scanning electronmicroscope (SEM， Hitachi- S4700) with an energyDdispersive spectrometer (EDS).The configuration and size of the transverse tensile中国煤化工specimens were prepared with reference to ChinaFig,1.DCHCNM H G'and repair jointsNational Standard (GB2625- -89). The room temperature- Symmetrical repairtensile test was carried out at a crosshead speed ofjoint; C- -Offset repair joint; D- Good-quality initial joint):1 mm/min using a computer-ontrolled testing machine(a) Tensile strength; (b) ElongationLIU Hui-jie, et alTrans. Nonferrous Met. Soc. China 19(2009) 563-567565After the groove defect is removed by different repairwelding processes, the mechanical properties of thejoints are improved to different extents.When the symmetrical repair welding process ischosen, the tensile strength and elongation of the repairjoint are 308 MPa and 4.8%, respectively. However,ASRSwhen the offset repair welding process is utilized, thetensile strength of the repair joint is up to 335 MPa,equivalent to 78% of that of the base metal, and theelongation of the repair joint is up to 8.4%, equivalent to77% of that of the base metal. Obviously, both repairwelding processes are able to improve the mechanicalproperties of the joints with groove defect, but the offsetrepair welding process is superior to the symmetrical oneand can produce good-quality repair joints that meet theengineering requirements.3.2 Microstructural characteristics of jointsFig.2 shows typical cross-section photographs of theinitial joint with groove defect and corresponding repairjoints, in which the retreating side and the advancing sideare denoted by RS and AS, respectively. When unsuitableFSW parameters are used (see Table 2), the plasticized[C)White curve bandmaterial cannot be refilled to the AS from the RS nearthe back surface of tool pin, therefore, a continuouslydistributing groove defect is formed on the AS (seeFig.2(a)), thus resulting in a degradation of mechanicalproperties. After repair welding, the groove defectdisappears in the repair joints, therefore, the mechanicalproperties of the joints increase.It can be seen from Fig.2 that the offset repair jointhas a relatively large weld zone because the new weld3 mmnugget formed in the repair welding and the old oneformed in the initial welding are partially overlappedFig.2 Cross sectional morphologies of initial joint with groove(see Fig.2(b)), while the symmetrical repair joint has adefect and its repair joints: (a) Initial joint; (b) Offset repairsmaller weld zone than the offset one (see Fig.2()).joint; (C) Symmetrical repair jointMoreover, there is a white curve band over the weldnugget zone of the symmetrical repair joint (seeoccurs in the repair joint. Therefore, the white curveFig2()), but no white curve band can be seen in theband is a weak location and weakens the mechanicaloffset repair joint (see Fig.2(b)).properties of the symmetrical repair joint.Fig.3 shows a further analytical result of the whitecurve band. The white curve band is composed of white3.3 Fracture features of jointsphases, and such white phases are almost continuouslyThe initial joint with groove defect is fractured atdistributed in the white curve band and extend to thethe defect location during the tensile test, and the repairweld surface of the symmetrical repair joint. In thejoints are fractured on the RS of the welds. However, themeanwhile, a large number of fine cavities, 1-5 um inspecific fracture locations of the repair joints aresize, exist around the white curve band (see Fig.3()).different due to different repair welding processes, asThe EDS analysis reveals that the chemical compositionshown in Fig.4. The fracture path of the offset repairof the white phases is consistent with that of thedegree to the weldprecipitate phase Al2Cu in the 2219 aluminum alloy (seesurfac中国煤化工near the interfaceFig3(). That is to say, although the original groovebetweYHC N M H Grmal-mecaicalydefect is removed by the symmetrical repair weldingaffecteu zcoIc (scc Tig.+(a)) Iuc syuumetrical repair jointprocess, a large number of fine cavity defects are formedis fractured at the weld nugget zone on the RS of theand an obvious aggregation of hard-bittle phase Al2Cuweld, and the fracture path is zigzag, composed of AB,566LIU Hui-jie, et al/Trans. Nonferrous Met. Soc. China 19(2009) 563-567RSASI White curve bandb6)b)DBA30um3 mmCATFig.4 Fracture locations of repair joints: (a) Ofiset repair joint;(b) Symmetrical repair jointIn order to further reveal the fracture features of the|ctwo kinds of repair joints, the tensile fracture surfaces ofthe repair joints are analyzed, as shown in Fig.5. It canbe seen that the offset repair joint and AB section of thesymmetrical joint have almost the same plastic fracturefeatures, ie. larger dimples and embedded precipitates atCuthe bottom of the dimples (see Figs.5(a) and (b)),0246indicating that an extensive plastic deformation hasE/keVoccurred. In contrast to AB section, BC section isFig.3 Morphologies of white curve band and its EDS result ofcharacterized by the small and shallow dimples,symmetrical repair joint: (a) OM image of weld cross-section;implying that the plastic deformation resistance increases(b) SEM image of white curve band; (c) EDS spectrum ofand the ductility decreases. With regard to CD section,white phasethere is no dimple on the fracture surface and the fractureis britle, thus confirming that joint is fractured along theBC and CD sections (see Fig.4(b)). It can be seen thatwhite curve band.AB section is consistent with the fracture path of theffet repair joint, ilustrating that AB section is the4 Conclusionsintrinsic weak location in the friction stir welded joint.Because the location of CD section is corresponding to1) The groove defect that has a significant influencethat of the white curve band shown in Fig.2(c), it can beon the mechanical properties of the joints can bededuced that the symmetrical repair joint is fracturedremoved by friction stir repair welding, but differentalong the white curve band during the tensile test, furtherrepair welding processes exhibit different repair results.confirming that the white curve band is the weak locationThe offset repair welding process is superior to theof the symmetrical repair joint, as mentioned above. BCsymm中国煤化工，and the tensilesection is a transition one between AB section and CDstrengFquivalent to that ofsection, and its deformation and fracture are inevitablethe gCNMHGto78%ofthatofafter AB and CD sections are first deformed andthe base metal.fractured.2) In the symmetrical repair welding process, aLIU Hui-jie, et alTrans. Nonferrous Met. Soc. China 19(2009) 563-567567I 150V 10 8nnx 00k SEM150V11.3mx1.0kSEM500ml 15 ow 10 Bnmxt 00k SEMFig.5 Morphologies of fracture surfaces of repair yoints: (a) Offset repair joint; (b), (C) and (d) AB, BC and CD sections ofsymmetrical repair joint in Fig. 4(b), respectivelylarge oumber of fine cavity defects are formed and analys [1]. J Mater Eng Perform. 2002. 11(1): 51-63.obvious aggregation of hard-bittle phase Al2Cu occurs[7] LIUHJ, CHEN Y C, FENG J c.fet of zigzag line on themechanical properties of friction stir welded joins of an AI-Cu alloyin the repair joint. Therefore, the mechanical properties[小Scripta Matcrialia, 2006. 55: 231-234.of the repair joint are weakened, and the repair joint[8] LEONARD AJ, LOCKYER s A. Flaws in friction stir welds [CVIexhibits zigzag fracture path during the tensile test, and4th International Symposium on Friction Stir Welding, Uiah,the fracture feature is partially brittle and partially[9] ZHANG H, LINS B. WUL. FENG JC, MA s L. Defects frmation2003:14-16plastic.procedure and mathematic model for defect free friction stir welding3) A good-quality repair joint can be obtained by theof magnesum aly [0], Materials and Design. 2006. 27: 805-809offset repair welding process. The repair joint is[10] KIM Y G, FUII H, TSUMURA T. Three defect types in friction stirfractured near the interface between the weld nuggetwelding of aluminum die casting alloy [U]. Material Science andzone and thermal-mechanically affected zone during theEngineering A.2006.415: 250 254.tensile test, and the large dimples occur on the fracture1] PAN Q. Welding defects and mechanical properties of friction stirsurface or the fracture feature is plastic.welded 2219 aluminum aloy joints (D] Harbin: Harbin Institute ofTechnology, 2007. (in Chinese)[12] DICKERSON T L, PRZYDATEK J. Fatigue of friction stir welds inReferencesaluminium alys that contain root flaws [U]. International Journal ofFatigue, 2003. 25: 1399 -1409.[] JOHNSEN M R Friction stir welding takes off at Boeing [0].[13] JAMES M N, BRADLEY G R. HATTINGH D G HUGHES D 1」,Welding Joumnal, 1999. 78(2): 35-39.WEBSTER P J. Residual stesses, defects and farigue cyeling in[2] IMMARIGEON J P, HOLT R T, KOUL A K. ZHAO L, WALLACEfriction stir butt welds in 5383-H321 and 5083-H321 aluminumW. BEDDOES J C. Lightweight materials for aircraft applications [小alloys []. Materials Science Forum, 2003, 426(4): 2915- 2920.Materials Characterization, 1995, 35(1): 41-67.[3] MISHRA R s, MA Z Y. Friction stir welding and processing [].[14] JOHNSON R, THREADGILL P L. Processing implicaios fomevaluating repaired friction stir welds [CW 3rd IntemationalMaterials Science and Engineering. 2005, R50(2): 1-78.[4LEE W B, YEON Y M. JUNG S B. The improvement of mechanicalSymposium on Fiction Stir Welding. Kobe, 2001:1320-1345properties of fition-stir-welded A356 Al alloy [] Materials[15]中国煤化工，solid phase welding tScience and Engineering A. 2003, 355(1/2): 154-159.m aircraft wing structure[JOELJ D. The friction sti; advantage []. Welding Journal,CNMHGFrction Stir Welding200. 80(530-343453[6] OUYANG J H. KOVACEVIC R. Material flow and microstructure(Edited by YaNG Bing)in the frictio stir butt welds of the same and disimilar alumnum