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Febr uar y 2013 Mast er ' sThesi s

A St udy on t heMachani cal Char act er i st i csofFr i ct i on St i r

Wel dedJoi ntf orThi n Fer r i t i c St ai nl essSt eel

Gr aduat eSchoolofChosunUni ver si t y

Depar t mentofNavalAr chi t ect ur eand Ocean Engi neer i ng

Kyoung ^­ Hak Ki m

A St udy on t heMechani cal Char act er i st i csofFr i ct i on St i r Wel dedJoi nt sf orThi n Fer r i t i c

St ai nl essSt eel

마찰교반접합기술을 이용한 박판 페라이트계 스테인리스강 접합부의 기계적 특성에 관한 연구

Febr uar y 25,2013

Gr aduat eSchoolofChosunUni ver si t y

Depar t mentofNavalAr chi t ect ur eand

Ocean Engi neer i ng

A St udy on t heMechani cal Char act er i st i csofFr i ct i on St i r Wel dedJoi nt sf orThi n Fer r i t i c

St ai nl essSt eel

Advi sor:Pr of essorHan-SurBang

A Thesi ssubmi t t edf ort hedegr eeof Mast erofEngi neer i ng

Oct ober2012

Gr aduat eSchoolofChosunUni ver si t y

Depar t mentofNavalAr chi t ect ur eand Ocean Engi neer i ng

Kyoung ^­ Hak Ki m

Kyoung­Hak Ki m' smast er t hesi si scer t i f i ed.

Commi t eeChai rChosunUni v. Pr of .Hee-SeonBang

MemberChosun Uni v. Pr of .Han-SurBang

MemberChosun Uni v. Pr of .J e-Woong Par k

November2012

Gr aduat eSchoolofChosun Uni ver si t y

CONTENTS

Li stofTabl es · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·Ⅲ Li stofFi gur es · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·Ⅳ Abst r act· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·Ⅵ

Chapt er1.I nt r oduct i on

1.1Research Backgroundand Purpose ···1

1. 1. 1Appl i cat i on ofFSW · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·4

1. 1. 2Appl i cat i on ofst ai nl essst eeli n i ndust r i es · · ·6

1.2Research Methodology ···7

Chapt er2.Theor et i calBackgr ound

2. 1. 1Pr i nci pl eofFSW · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 8

2. 1. 2Char act er i st i cmi cr ost r uct ur eofFSW · · · · · · · · · · · · · 10

2. 1. 3Gener at i on and f l ow ofheat · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 12

2. 1. 4Advant agesand l i mi t at i onsofFSW · · · · · · · · · · · · · · · · 13

2.2Classification ofStainlessSteel···15

Chapt er 3. Exper i ment al Met hod of Fr i ct i on St i r Wel di ng Pr ocess

3. 1. 1Exper i ment alequi pment · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 18

3. 1. 2Obj ect i vemat er i al· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 20

3. 1. 3Descr i pt i on oft oolmat er i alandshape · · · · · · · · · · ·22

3.2ExperimentalProcedure ···23

3.3MechanicalEvaluation ···25

3. 3. 1.Tensi l et est · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 25

3. 3. 2.Har dnesst est · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 27

3.4MetallurgicalEvaluation ···29

3.5TemperatureMeasurement ···31

Chapt er4.Resul t sandDi scussi ons

4.2MechanicalCharacteristics ···34

4. 2. 1.Tensi l est r engt h · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 35

4. 2. 2.Har dness· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 38

4.3MetallurgicalCharacteristics···39

4.4MeasuredTemperatureHistory···45

Chapt er5.Concl usi on· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·39

Ref er ence· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·47

Li stofTabl es

Table3.1 Chemicalcompositionsandmechanicalproperties···21

Table3.2 Configuration ofweldedspecimen···21

Table3.3 Chemicalcompositionsandmechanicalpropertiesoftool ···22

Table3.4 Welding parametersofFSW ···24

Table3.5 Geometry oftensiletestspecimen···26

Table3.6 Hardnesstesting condition···28

Table4.1 Beadprofilesofweldsforvarioustravelspeedindifferent rotation speed···33

Table4.2 Tensilestrengthandfracturedspecimenaftertensiletest forvarioustravelspeedindifferentrotationspeed···36

Li stofFi gur es

Fig.1.1 ApplicationofFSW tostainlesssteel···3

Fig.1.2 ApplicationofFSW in industries···5

Fig.1.3 Applicationofstainlesssteelin industires···6

Fig.2.1 Schematicdiagram ofFSW ···8

Fig.2.2 Microstructuralregionsin FSW welds···10

Fig.2.3 Classification offerriticstainlesssteels···15

Fig.2.4 Classification ofausteniticstainlesssteels···16

Fig.2.5 Classification ofmartensiticstainlesssteels···17

Fig.3.1 SpecificationsofFSW system ···19

Fig.3.2 Experimentalset-upforFSW ···19

Fig.3.3 Dimensionsandshapeoftool···22

Fig.3.4 Tensiletestmachine···26

Fig.3.5 Vickershardnesstestschemeandmachine···28

Fig.3.6 Opticalmicroscope···29

Fig.3.7 Common thermocoupletemperatureranges···31

Fig.3.8 Set-upfortemperaturemeasurement···32

Fig.3.9 Thermocouplepositionson workpiece···32

Fig.4.1 Tensiletestspecimens ···35

Fig.4.2 Comparison oftensilestrength ofFSW weldswith travel speed ···35

Fig.4.3 Comparison ofstress-strain curve ofFSW welds with travelspeed ···36

Fig.4.7 SEM imageofFSW welds···42 Fig.4.8 SEM observation offracturesurfaceaftertensiletestwith travelspeed ···43 Fig.4.9 SEM observation offracture surface aftertensile testfor endpartofweldedspecimen···44 Fig.4.10 Temperaturehistory ofFSW welds···45 Fig.4.11 ComparisonoftemperaturehistoryofFSW weldsandGTAW

welds···46

ABSTRACT

마찰교반접합기술을 이용한 박판 페라이트계 스테인리스강 접합부의 기계적 특성에 관한 연구

Kim KyoungHak

Advisor:Prof.BangHan-sur,Ph.D.

DepartmentofNavalArchitectureand OceanEngineering,

GraduateSchoolofChosunUniversity

세계적으로 대기오염 및 지구온난화 그리고 자원고갈로 인하여 21세기를 맞이한 현대사회의 각종 산업분야에서 최대의 핵심 과제는 “친환경적 녹색성 장”일 것이다.이에 따라 각종 산업분야에서는 환경보호 및 에너지 절감에 대 한 요구를 만족하기 위하여 경량부재의 적용 및 부재재질의 두께 감소 등 녹 색 기술에 집중하고 있다.또한 세계적인 경제 발달과 더불어 삶의 질의 향 상 및 편의를 위해 많은 종류의 가전제품이 개발 및 출시되고 있으며 3대 그 린가전 중의 하나인 세탁기의 경우 그 기술 개발속도가 빠르게 이루어지고 있으며,세계적인 수요 또한 꾸준히 증가하고 있는 추세이다.

스테인리스강의 경우 표면이 미려하고 부식에 대한 저항성이 우수한 특수 강으로 내식성,내구성,내화성 등 제품의 고급화를 요구하는 자동차,가전제 품의 수요산업에서 사용이 크게 증가하고 있다.STS430J1L의 경우 기존 STS430에 Cu,Nb를 첨가하여 내식성,성형성,용접성 및 고온특성이 우수하 기 때문에 자동차,가전제품,철강,플랜트 등의 합금강 산업 분야 전반에 걸 쳐 사용되고 있으며 그 활용도가 더욱 증가하고 있다.

그러나 기존 스테인리스강 용접의 경우 일반적으로 MIG 및 TIG와 같은

감소하는 단점이 있어 스테인리스강 접합에 새로운 다른 열원의 필요성이 요 구되고 있다.

이러한 문제점 등을 개선한 접합방법으로 1981년 영국의 용접연구소(The Welding InstituteoftheUnitedKingdom,이하 TWI)에서 개발한 마찰교반 접합(Friction StirWelding,이하 FSW)은 융점이하에서 고상 상태의 접합공 정으로 회전하는 Tool을 피접합체에 삽입하여 접합부에 충분한 마찰열과 강 한 소성변형이 발생하여 매우 미세한 재결정 조직 및 우수한 기계적 특성을 얻을 수 있다고 알려져 있다.이러한 공정은 기존 용융용접공정과 비교하여 용접에 의한 변형이 적고 비소모성 접합방법일 뿐만 아니라 용접결함,흄,유 해광선의 발생이 없어 환경친화형 접합방법으로 각광을 받고 있다.

하지만,이러한 장점에도 불구하고 박판재 접합에 있어서 국내·외 산업 및 응용분야에서는 대부분 용융용접법(TIG 및 MIG)을 이용하여 구현되고 있으 며,마찰교반접합을 이용한 박판재 접합에 관한 연구 및 적용은 거의 전무한 실정이다.

따라서 본 연구에서는 내식성,성형성,용접성 및 고온특성이 우수하여 공 업용으로 사용되고 있는 STS430J1L을 새로운 열원으로 마찰교반접합기술을 이용하여 접합기구상의 특성상 박판접합에 있어서 프로브(Probe)가 없는 타 입의 단순한 환봉형태의 공구(Tool)을 사용하였고,접합부의 미세조직 관찰 (OM 및 SEM)및 인장,경도시험을 통하여 기계적 특성을 평가하였다.또한 접합성을 평가하여 산업분야의 적용가능성을 고찰하고자 하였다.

Chapt er1I nt r oduct i on

1. 1Resear ch Backgr oundandPur pose

The biggestkey technology in the various industry field ofmodern society will be "eco-friendly green growth" due to the atmospheric pollution,globalwarming and resource depletion in the world.To meet the need for energy reduction and protection of the environment,the various industry field of modern society are focussing on the green technology whichhaveapplicationtothedecreaseinthicknessofmaterial and use oflightweightmaterial.Moreover,differenttypes ofhousehold applianceswith developmentoftheglobaleconomichavebeenreleasedin order to improve the quality and convenience of life.And an electric washingmachine,oneofthewhitegoods,hasbeendevelopedrapidly.

Stainless steel has been applied as high strength-to-weight ratio materialfor automobiles,aircraftand other machines.Ferrictic stainless steelhas good corrosion resistance,is used in demand-based industries including automobile and electricalappliance to meetthe demand where considerstheweightlightening and energy saving.Ferriticstainlesssteel has many advantages.Firstly,ferritic stainless steelare more economic because they do notcontain nickelwhich is an expensive alloy^.And STS430J1L hasbecamegoodasightbetterweldabilitywithCuandNb.

However,there are several problems in fusion welding of ferritic stainlesssteel.Oneoftheproblem isaboutexcessivegrain growth.Itis

problem offerritic stainless steel,anotherproblem in fusion welding is formation of sigma-phase (-phase).Severalundesirable intermetallical phasessuch as-phasemay occurwhen stainlesssteelsareexposed to 650-850℃ foraperiodoftime.The-phaseisthemostseriousofthese secondary phases due to its impacton the mechanicalproperties^.To preventthe-phase,ferritic stainless steelmustnotbe preheated over 400℃.Another solution is after welding stainless steelmustbe cooled very quick^.Buttheseproblemscan beavoided by using lowerwelding heatinputs.

Tosolvethisproblem,FrictionStirWelding(FSW)isanew solidstate joining processwhich isinventedby TheWelding InstituteoftheUnited Kingdom(TWI)in 1981.Grain growth and weld defects in FSW can be avoidedbecausethemaximum temperatureislowerthanthemeltingpoint incomparisonwithfusionwelding^.

Variousresearch workshasalready been reported abouttheapplication ofFSW process.Onfrictionstirwelded(FSWeld)AISI430FSS inwhich about95% property ofthebasemetalwasachieved^.In asimilartrend, weldability ofFSS which was due to the presence ofvery fine duplex structure of ferrite and martensite in the weld microstructure formed consequent^.And Meran and Canyurtsearched theeffectoftoolrotation speed and traverse speed on welding of AISI 304 austenitic stainless steelsbyFSW method.Theyobtainedtheoptimalstrengthwithrotational speedof950rpm and47.5mm/min^.

In this study, it was intended to investigate the weldability and mechanicalcharacteristics ofthin ferritic stainless steelby using FSW.

Dueto thin platewelding,toolofnon-probetypeisused in thisstudy.

Itsothermeritisthatfrictionalheatoccurundertheshoulderinthisthin buttjoints and frictionalheatpasses throughoutinside.Also buttjoints hasbecomegrain refinementaffecting dynamicrecrystallization ofstrong

shearingdeformationandfrictionalheat.Successfullyobtainingtheprocess of the experimental data will influence the industrial and national competitiveness.

Fig.1.1 ApplicationofFSW tostainlesssteel

1. 1. 1Appl i cat i on ofFSW

Fricition Stir Welding has been applied for automotive,rail,marine, aerospace,land transport and other machines.In the case of marine industry,FSW is used in panelfor deck,Alextrusions and helicopter landing platform.In advanced country cases,university ofadelaide was developed FSW moving system to build a lightalloy cruise ships.Also SwedenandNorwayofnortheuropeapplyforbuildingalightalloyships. These FSW technology obtain approval to apply for build from the classification ship's class(DNV, RINA, Lloyds). FSW technology is presentlyfollowedinanationalshipbuildingindustry.

In the case ofautomotive,Aluminium engine cradles and suspension strutsforstretchedLincolnTownCarwerethefirstautomotivepartsthat werefriction stiratTowerAutomotive,whousetheprocessalsoforthe engine tunnelofthe Ford GT.In Japan FSW is applied to suspension strutsatShowaDenkoandforjoining ofaluminium sheetstogalvanized steelbrackets fortheboot(trunk)lid oftheMazda MX-5.Friction stir spotwelding issuccessfully usedforthebonnet(hood)andreardoorsof theMazdaRX-8andthebootlidoftheToyotaPrius.

Also,boeing applies FSW to the Delta II and Delta IV expendable launch vehicles, and the first of these with a friction stir welded Interstagemodulewaslaunchedin1999.Theprocessisalsousedforthe Space Shuttle externaltank,forAres Iand forthe Orion Crew Vehicle testarticleatNASA[datedinfo]aswellasFalcon1andFalcon9rockets atSpaceX.Thetoenailsforramp ofBoeing C-17GlobemasterIIIcargo aircraftby Advanced Joining Technologies and the cargo barrierbeams for the Boeing 747 Large Cargo Freighter were the firstcommercially produced aircraftparts.FAA approved wingsand fuselagepanelsofthe Eclipse500aircraftweremadeatEclipseAviation.

Fig.1.2Application ofFSW inindustries

1. 1. 2Appl i cat i on ofst ai nl essst eeli n i ndust r i es

While the original form of stainless steel, (iron with around 12%

chromium)is stillin widespread use,engineersnow have a wide choice ofdifferenttype(grades).In all,therearemorethan 100differentgrades butthese are usually sub-classified into distinctmetallurgical"families"

suchastheaustenitic,ferritic,martensiticandduplexfamilies.

Althoughaustenuticstainlesssteelhascomonuse,ferriticstainlesssteel hasmany advantages.Firstly,ferritic stainlesssteels aremoreeconomic becausetheydonotcontainnickelwhichisanexpensivealloy.

Fig.1.3 Applicationofstainlesssteelin industries

1. 2Resear ch Met hodol ogy

Themethodologyofthisstudyisasinthefollowing.

1) ExperimentaldesignonFSW

·Designandfabricationofoptimaljig

·DesignandfabricationofFSW tool

2) Tofindoptimum weldingconditionsonFSW experiment

·Tiltangle(°)

·Rotationspeed(rpm)

·Travelspeed(mm/min)

·Dwelltime(s)

·Weldlength(mm)

·Back-upplate

·Evaluationofbeadonplate

3) Mechanicaltestsandmicrostructuralanalysis

·Tensiletest

·Hardnesstest

·OpticalMicroscope

·ScanningElectronMicroscope

4) Temperaturehistorytest

·Thermocoupletest

Fig.2.1Schematicdiagram ofFSW

Chapt er2Theor et i calBackgr ound

2. 1Pr i nci pl eandChar act er i st i csofFSW

2. 1. 1Pr i nci pl eofFSW

Friction StirWelding(FSW)is a solid-state joining process (the metal is notmelted)and is used when the originalmetalcharacteristics must remain unchanged as much possible.Itmechanically intermixes the two piecesofmetalattheplaceofthejoin,then softensthem so themetal can befusedusing mechanicalpressure,much likejoining clay,dough or plasticine.A constantly rotated cylindricalshouldered toolwith a profiled nib istransversely fed ata constantrate into a buttjointbetween two clamped pieces ofbutted material.The nib is slightly shorter than the welddepthrequired,withthetoolshoulderridingatoptheworksurface.

Frictional heat is generated between the wear-resistant welding componentsandtheworkpieces.Thisheat,along withthatgeneratedby themechanicalmixing processand theadiabaticheatwithin thematerial,

causethestirredmaterialstosoftenwithoutmelting.Asthepinismoved forward,aspecialprofileon itsleading faceforcesplasticised materialto therearwhereclampingforceassistsinaforgedconsolidationoftheweld.

Thisprocessofthetooltraversing along theweld linein aplasticised tubularshaftofmetalresults in severesolid statedeformation involving dynamicrecrystallizationofthebasematerial.

Fig.2.2MicrostructuralregionsofFriction StirWelding

A : Thebasemetal(BM)isnochange.

B :

The heat-affected zone (HAZ) is common to all welding processes.Asindicated by thename,thisregion issubjectedto a thermal cycle but is not deformed during welding. The temperatures are lowerthan those in the TMAZ butmay still have a significant effect if the microstructure is thermally unstable.In fact,in age-hardened aluminium alloys this region commonlyexhibitsthepoorestmechanicalproperties.

C :

Thethermo-mechanicallyaffectedzone(TMAZ)occursoneither side ofthestirzone.In thisregion the strain and temperature are lower and the effectofwelding on the microsturcture is correspondingly smaller.Unlike the stirzone the microstructure is recognizably thatofthe parentmaterial,albeitsignificantly

2. 1. 2Char act er i st i cmi cr ost r uct ur eofFSW

Thesolid-statenatureoftheFSW process,combined with itsunusual toolandasymmetricnature,resultsinacharacteristicmicrostructure.The microstructurecanbebrokenupintothefollowingzone:

deformed and rotated. Although the term TMAZ technically refersto theentiledeformed region itisoften used to describe any region notalready coveredby thetermsstirzoneandflow arm.

D :

Thestirzone(alsonugget,dynamically recrystallised zone)isa region ofheavily deformed materialthatroughly correspondsto the location ofthe pin during welding.The grains within the stirzoneareroughly equiaxed and often an orderofmagnitude smallerthan thegrainsin theparentmaterial.A uniquefeature ofthestirzoneisthecommon occurrenceofseveralconcentric rings which has been referred to as an "onion-ring"structure. Thepreciseoriginoftheseringshasnotbeenfirmlyestablished, although variations in particle number density,grain size and texturehaveallbeensuggested.

2. 1. 3Gener at i on and f l ow ofheat

Heating generation during friction stirwelding arisesfrom twosource:

friction atthe surface ofthe tooland the deformation ofthe material around the tool. The heat generation is often assumed to occur predominantly undertheshoulder,dueto itsgreatersurfacearea,and to be equalto the powerrequired to overcome the contactforces between thetooland theworkpiece.Thecontactcondition undertheshouldercan be described by sliding friction, using a friction coefficient  and interfacialpressure ,orsticking friction,based on the interfacialshear strength at an appropriate temperature and strain rate. Mathematical approximations for the totalheatgenerated by the toolshoulder _{ }

havebeendevelopedusingbothslidingandstickingfrictionmodel:

_{}  

 _{  }^ _{  }^ 

_{} 

 _{  }^ _{  }^ 

where istheangularvelocity ofthetool,_{  } istheradiusofthe toolshoulderand _{  } thatofthepin.Severalotherequationshavebeen proposedtoaccountforfactorssuch asthepin butthegeneralapproach remains the same.A major difficulty in applying these equations is determining suitable values for the friction coefficientor the interfacial shearstress.The conditions underthe toolare both extreme and very difficulttomeasure.Todate,theseparametershavebeen used asfitting parameterswhere themodelworks back from measured thermaldata to obtainasimulatedthermalfield.Whilethisapproachisusefulforcreating processmodelsto predict,forexample,residualstressesitislessuseful forprovidinginsightsintotheprocessitself.

1) AdvantagesofFricitionStirWelding

· Goodmechanicalpropertiesintheas-weldedcondition.

· Improved safety duetotheabsenceoftoxicfumesorthespatter ofmoltenmaterial

· Noconsumables

· Easily automated on simple milling machines-lower setup costs andlesstraining.

· Can operatein allposition(horizontal,vertical,etc.),asthereisno weldpool.

·

Generallygoodweldappearanceandminimalthicknessunder/over -matching,thus reducing the need forexpensive machining after welding.

· Low environmentalimpact.

2. 1. 4Advant agesand l i mi t at i onsofFSW

Thesolid-statenatureofFSW leadstoseveraladvantagesoverfusion welding methods are problems associated with cooling from the liquid phase are avoided. Issues such as porosity, solute redistribution, solidification cracking andliquation cracking donotariseduring FSW.In general,FSW has been found to produce a low concentration ofdefects andisverytolerantofvariationsinparametersandmaterials.

· Large down forces required with heavy-duty clamping necessary toholdtheplatestogether.

· Less flexible than manual and arc processes (difficulties with thicknessvariationsandnonlinearwelds).

· Often slowertraverse rate than some fusion welding techniques, althoughthismaybeoffsetiffewerweldingpassesarerequired.

1) FerricticStainlessSteels

Theseareplainchromium (10.5to18%)gradessuchasGrade430 and 409.Theirmoderatecorrosion resistanceand poorfabrication properties are improved in the higher alloyed grades such as Grades434and444andin theproprietary grade3CR12.Although austenuticstainlesssteelhascomonuse,ferriticstainlesssteelhas manyadvantages.

Fig.2.3Classification offerriticstainlesssteels

2. 2Cl assi f i ct i on ofst ai nl essst eel

The worldwide consumption ofstainless steelis increasing.There is growing demand from the building and construction industry where stainless steelis used forits attractive appearance,corrosion resistance, low maintenance and strength. Many other industries are adopting stainlesssteelforsimilarreasonsaswellasthefactthatitdoesnotneed to be treated,coated orpainted when putinto service,despite the fact thatitismoreexpensivethanplaincarbonsteels.

2) AusteniticStainlessSteels

This group contains at least 16% and 6% nickel and range through tothehigh alloy superausteniticssuch as904L and 6%

molybdenum grades.Additionalelement can be added such as molybdenum, titanium or copper, to modify or improve their properties,making them suitable for many critical applications involving high temperature as wellas corrosion resistance.This groupofsteelsalsosuitableforcryogenicapplicationsbecausethe effectofthe nickelcontentin making the steelaustenitic avoids the problems of brittleness at low temperatures, which is a characteristicofothertypesofsteel.

Fig.2.4Classification ofausteniticstainlesssteels

3) MartensiticStainlessSteels

Martensitic stainless steels are also based on the addition of chromium asthemajoralloying elementbutwith ahighercabon and generally lower chromium content than the ferritic types : Grade 431 has a chromium content of about 16%, but the microstructureisstillmartensitedespitethishigh chromium level becausethisgradealsocontains2% nickel.

Fig.2.5Classification ofmartensiticstainlesssteels

Chapt er3Exper i ment alMet hodofFr i ct i on St i rWel di ng Pr ocess

3. 1Exper i ment alDet ai l s

3. 1. 1Exper i ment alequi pment

In ordertocarry outtheFSW experiment,WINXEN FSW gantry type system isused in thisthin platewelding experiment.And FSW machine isabletomovethetriaxis(X,Y,Z).Fig.3.1showsthespecificationsof WINXEN FSW gantrytypesystem.

In comparison with existing FSW method,No probe ofused toolis Otherfeaturein thisstudy.When shoulderoftoolrotateon an axis,the frictional heat is generated on surface of metals. Tungsten carbide (WC-Co12%) toolhaving shoulder diameter of 6mm was used in the experiment.

Theback-upplateisstainlesssteel430J1L ofthesamequalitybecause thefrictionalheatlossoccurfrom thebackplate.Also,Thestainlesssteel havelow heatconductivity andthermalexpansion.A shoulderofrotating toolisforcedtoplungeintotheplatestobeweldedandmovealong the centralcontactline.

Dwelltime is thatthe materialis preheated by a stationary,rotating tooltogetasufficientheatinput.Thisperiodhelpsbuttjointstoincrease heatgeneration.And so dwelltime is very importantvariable in this studybecausetheinitialsectionofthinbuttjointscan'tbeweldeddueto the lack offrictionalheat.Fig.3.2 shows the experimentalset-up for FSW ofthinplate.

Fig.3.1SpecificationsofFSW system

Fig.3.2Experimentalset-upforFSW

3. 1. 2Obj ect i vemat er i al

STS430J1L Stainlesssteelwasused in thisstudy.Thisstainlesssteel has a typicalcontents of16.47Cr-0.286Nb-Low (C,N) in ferritic type which improved processing,anti-corrosion,heat-resistance and welding performance ofSTS430 significantly.Itincreased more chrome contents than STS430 and improved the anti-corrosion because STS430J1L has becamegoodasightbetterweldabilitywithCuandNb.

The chemicalcompositions and mechanicalproperties ofthe materials are shown in Table 3.1. Two sheets with dimensions of 0.5(t)×160(B)×150(L) mm were joined in butt joints.To minimize the formation ofburr,specimens ofthe contactside was done by milling process.Thewelding surfacewaswiped with MethylAlcoholto remove the grease before welding process.Table.3.2 shown the dimension of usedspecimen.

Table.3.1Chemicalcompositionsandmechanicalproperties

Table.3.2Configuration ofusedspecimen

3. 1. 3Descr i pt i on oft oolmat er i alandshape

The materialoftoolis made of12%Co tungsten carbide (WF20)to preventwear-resistantoftoolduetofrictionalcontactwithstainlesssteel plates while conducting FSW process.The chemicalcompositions and mechanicalpropertiesoftheused toolin thisstudy areshown in Table. 3.3.

NoprobeofusedtoolisOtherfeaturein thisstudy.When shoulderof toolrotate on an axis,the frictionalheatis generated on interface of metals.Tungsten carbide (WC-Co12%)toolhaving shoulderdiameterof 6mm wasusedintheexperiment.Fig.3.3showsthedimensionandshape oftool.

Fig.3.3Dimension andshapeoftool

Table.3.3Chemicalcompositionsandmechanicalpropertiesoftool

3. 2Exper i ment alpr ocedur e

To carry outthe FSW experimentforthin platewelding,Firstofall, key process variables were established during the FSW.In the case of back-up plate,stainless steel430J1L ofthe same quality was used to minimizetheheatlossfrom thebackplate.Andtiltanglewerekeptfixed (2°).

Secondly,rotation speed and travelspeed arevariable.Rotation speeds were chosen at 800, 850, 900rpm and travel speeds were 84, 96, 108mm/min.Asa resultofexperiment,acrosssection ofthejointwere not welded at rotation speed below 800rpm. Welding parameters are showninTable.3.4.

Thirdly,aftertheFSW hadbeenfinished,mechanicaltestofjointswas carryouttogetthetensilestrength.cross-headspeedofTensiletestwas 0.0333mm/sec to comply with the korean standard (KS B 0801 13-B). Also,hardnesstestwascarryoutbyvickerstester.

Fourthly,afterthe FSW had been finished,visualinspection ofjoints was examined to check the defects on the bead surface and then metallurgicalexaminationweredoneafterpolishingandetching.

Table.3.4Welding parametersofFSW

3. 3Mechani calEval uat i on

3. 3. 1Tensi l et est

Fundamentally,theprocess involvesplacing the testspecimen in the testing machineandapplyingtensiontoituntillitfractures.During theapplicationof tension,the elongation ofthe gauge section is recorded againstthe applied force.Thedata is manipulated so thatitisnotspecific to thegeometry of thetestsample.Theelongation measurementisusedtocalculateengineering strain,,usingthefollowingequation:

  _

∆ _

  _

where^∆ isthechangein gaugelength,_ istheinitialgaugelength, andL isthefinallength.Theforcemeasurementisusedtocalculatethe engineeringstress,,usingthefollowingequation:

  

_

where F is the force and A is the cross-section ofgauge section.The machinedoesthesecalculationsastheforceincreases,sothatdatapoints canbegraphedintoastress-straincurve.

Tensile test was carried out with Dongil-Simaz Universal Testing Machine(EHF-EG200KN-40L)usingWINSERVO program.Fig.3.4shows theEHF-EG200KN-40L andtensiletestingsetup.

The specimens are fabricated in accordancewith thekorean standards (KS B 0801 13-B).The specimen dimensions are given in Table 3.5 Tensile testwas done with Load speed 0.0333mm/sec and stress-strain

Fig.3.4Tensiletestmachine

Table.3.5Geometry oftensiletestspecimen

3. 3. 2Har dnesst est

Generally,loads of various magnitudes are applied to a flatsurface, depending on the hardness of the materialto be measured.The HV numberisthen determinedby theratioF/A whereF istheforceapplied to the diamond in kilograms-force and A is the surface area of the resulting indentation in square millimeters.A can be determined by the formula

  sin 

^

whichcanbeapproximatedbyevaluatingthesineterm togive

 ≈ 

^

where d is the average length ofthe diagonalleftby the indenter in millimeters.Hence,

  

≈ ^



whereF isinkgfanddisinmillimeters.Thecorresponding unitsofHV are then kilograms-force per square millimeter (kgf/mm²).To calculate Vickers hardness numberusing SIunits one needs to convertthe force applied from kilogram-force to newtons by multiplying by 9.80665 (standard gravity)and convertmm to m.To do the calculation directly, thefollowingequationcanbeused:

_{ }



≈ ^



whereF isnewtonanddismillimeters.

Thehardnessofwelded specimen wasmeasuredusing AkashiHM-112

Fig.3.5Vickershardnesstestschemeandmachine

Table.3.6Hardnesstesting condition

3.4 Metallurgical Evaluation

The metallurgicalinvestigations on a cross section ofthe jointwere done after polishing and etching.The cross section ofFSWelded butt jointswascutperpendiculartotheweldingdirection.Itwaspolishedwith 9,3and1㎛ diamondpaste,andthenthespecimenwaschemicallyetched in a "Aqua ria“(20mlHNO₃+60mlHCI)fora period of10 seconds to observe the macro and microstructure. Fig. 3.7 shows the optical microscope.

Fig. 3.6 Optical microscope

SecondaryelectrondetectorsarestandardequipmentinallSEMs,butitis rarethata singlemachine would have detectors forallpossiblesignals. Thesignalsresultfrom interactionsoftheelectronbeam withatomsator nearthesurfaceofthesample.Inthemostcommonorstandarddetection mode,secondary electron imaging or SEI,the SEM can produce very high-resolution imagesofa samplesurface,revealing detailslessthan 1 nm in size.Due to the very narrow electron beam,SEM micrographs have a large depth of field yielding a characteristic three-dimensional appearance usefulfor understanding the surface structure ofa sample. This is exemplified by the micrograph ofpollen shown above.A wide rangeofmagnificationsispossible,from about10times(aboutequivalent to thatofa powerfulhand-lens)to more than 500,000 times,about250 timesthemagnificationlimitofthebestlightmicroscopes.

3. 5Temper at ur eMeasur ement

A thermocouple is a sensorformeasuring temperature.Itconsists of two dissimilarmetals,joined togetheratone end.When the junction of the two metals is heated or cooled a voltage is produced thatcan be correlatedbacktothetemperature.Thethermocouplealloysarecommonly available as wire. And a thermocouple is available in different combinationsofmetalsorcalibrations.Thefourmostcommoncalibrations are J,K,T and E. The Fig.3.8 shows the common thermocouple temperatureranges.

Thetemperaturegradientatmid thicknessweremeasured atsix point locations during the FSW process using MIDILOGGER GL220-UM-851 and thermocouples (K type) which placed diameter inserted into approximately 0.3mm diameterholes drilled on top side ofspecimens.K type was chosen in this study because K type was economicaland proper.

Fig. 3.8 Set-up for temperature measurement

Fig. 3.9 Thermocouple positions on workpiece

Chapt er4Resul t sandDi scussi ons

4. 1.BeadPr of i l es

Table 4.1 shows thebead appearance and crosssection ofFSW welds with travelspeedof84,96,108mm/minindifferentrotationspeedsof800, 850,900rpm.Itwasobservedthatdefectsoccurredonthesurfaceandcross sectionofjointsatatoolrotationspeedof800,850rpm andtravelspeedof 84,96,108mm/min.However,thewidth oftheplastically deformed region wasobservedatatoolrotationspeedof900rpm andtravelspeedof84,96, 108mm/min. Distinctively,the width of the plastically deformed region decreaseddowntheverticaldirection.

Table.4.1Beadprofilesofweldsforvarioustravelspeedin different rotation speed

4. 2.Mechani calChar act er i st i cs

4. 2. 1Tensi l est r engt h

Fig.4.1.showsthetensiletestweldedspecimensobtainedfrom different travelspeedswithtravelspeedof84~108mm/minintoolrotationspeedof 900rpm. Fig.4.2 shows the comparison of tensile strength of welds fabricatedunderabovewelding conditions.From theresults,itwasfound thatmaximum tensilestrength ofweldswas450MPa undertravelspeed of96mm/min.Thetensile strength ofthe 84mm/min joints was430MPa and the tensile strength of108mm/min joints was 388MPa,respetively.

The maximum tensile strength of96mm/min was approximately 97% in comparisonwiththatofbasemetal.Table4.3andFig 4.3show fractured specimens and stress-strain curves of FSW welds at 900rpm with differenttravelspeeds.

Moreover,tensile strength ofend partofFSW welds,which regarded as the weld defectwas investigated. Itwas observed thatend partof FSW welds ofthin plate indicated the good tensile strength which was 488MPa.Itcan beinferredthatbecauseofincreasementofheatinputby dwelltimeatendpart.Fig 4.4showsthestress-straincurveofendpart ofFSW welds.

Fig.4.1Tensiletestspecimens

Fig.4.2ComparisonoftensilestrengthofFSW weldswithtravel speed

Table. 4.2 Tensile strength and fractured specimen after tensile test for various travel speed in different rotation speed

Fig. 4.3 Comparison of stress-strain curves of FSW welds with travel speed

Fig. 4.4 Stress-strain curve for end part of FSW welded specimen

4. 2. 2Har dness

Fig4.5showsthehardnessdistributionsofweldsintransversedirection ofwelding crosssection atthedistance0.25mm away from top surface. Thehardnessatweldswashigherthan thatofbasemetalwhich isdue to the strong shearing deformation and frictionalheataffecting dynamic recrystallization.Thehardnessofinterfacebetween basemetaland joints was higher than thatofbase metal.Itis thoughtthatthe regions of these hardness distributions probaly corresponds to heat affected zone (HAZ)orthermo-mechanicallyaffectedzone(TMAZ).Theotherreasonof increasinghardnessisformationofmartensiteofwelds.Itwasfoundthat maximum hardnessoccurred in stirsone(SZ)and wasapproximately 275 Hv.

Fig.4.5HardnessdistributionsofFSW welds

4. 3

Fig.4.6 shows the microstructure (OM) of FSWelded joints at tool rotation speed of 900rpm and travel speed of 96mm/min. This microstructure is splitinto fourgroups,pointA,B,C and D.However, there is little difference between the pointA and B.The point A is retreating side and point D is advancing side. The slightly finer recrystallizedgrainsinthejoints(pointC)areattributedtothegeneration of strong shearing deformation and frictional heat affecting dynamic recrystallization.

This microstructure has a clear characteristic to optical microscope images in the way thatitis hard to tellthe heataffected zone (HAZ) andthermo-mechanicallyaffectedzone(TMAZ)respectively.

The width of the plastically deformed region decreased to vertical directionbecausefrictionalheatoccurundertheshoulderin thisthinbutt joints and frictionalheat passes throughoutinside.In comparison with back-up plate used,the stainless steel430J1L of the same quality is superior to the Alalloy or mild steelat the depth of the plastically deformedregion.

Fig.4.6OpticalmicrostructureofFSW welds

The grain sizes and elements ofbuttjoints were examined using a scanning electron microscope(SEM)equipped with and energy dispersive X-ray spectroscope (EDS). Fig. 4.7 shows the SEM observation of microstructure at tool rotation speed of 900rpm and travel speed of 96mm/min.Incomparisonwithgrainsizeofbuttjoints,thegrainsizesof base metal(point A) were approximately average 20㎛.However,the grain sizes ofbuttjoints (pointC)decreased to about5㎛,because the slightly finer recrystallized grain in the joint are attributed to the

generation of strong shearing deformation and frictionalheat affecting dynamicrecrystallization.Intheprocessofmicrostructureanalysis(SEM), niobium ofalloying elementin joints has been observed.Niobium is a chemical element with the symbol Nb and atomic number 41 and paramagnetic metalin group 5 ofthe periodic table.Itis a soft,grey, ductiletransitionmetal.Althoughalloyscontainonlyamaximum of0.1%, thatsmallpercentageofniobium improvesthestrengthofthesteel.

Themicrostructureofthe fractured specimens were analyzed afterthe tensile test. Fig. 4.8 shows the microstructure (SEM) of fractured specimens.Theductilefracturewithdimplepattenandbrittlefractureare observed in the specimens attravelspeed of84mm/min and 96mm/min, but brittle fracture is observed at 108mm/min.As a result of tensile fracture surface ofend partattravelspeed of1.6mm/min,the ductile fracturewith dimpleisobserved.Fig.4.9showstheSEM observation of fracturesurfaceaftertensiletestinendpartofbuttjointsattoolrotation speedof900rpm andtravelspeed96mm/min.

Fig.4.7SEM imageofFSW welds

(a)84mm/min,900rpm

(b)96mm/min,900rpm

Fig.4.9SEM observation offracturesurfaceaftertensiletestfor endpartofweldedspecimen

4. 5

Fig.4.10showsthetemperaturehistory ofFSW welds.Asaresultof temperaturehistory,thetemperatureofstartpartis109℃,whichislower than otherpoints.Thislow temperaturehascaused poorweldability due tolackoffrictionalheat,butmaximum temperatureofendparthasbeen measuredtobe312℃,whichimprovestensilestrength.

Fig.4.10Temperaturehistory ofFSW welds

Fig.4.11showsthecomparison oftemperaturegradientofFSW welds andGTAW welds.Asaresultoftemperaturehistory,thetemperatureof FSW welds is lower than that of GTAW welds. Therefore, this temperature characteristics can be results in reduction of weld defects suchasthermaldeformation,residualstressetc.

Fig.4.11Comparison ofTemperaturegradientofFSW weldsand GTAW welds

l Although toolofnon-probetypeisused in FSW buttwelding for thin plate,FSW welded butt joints for thin plate with satisfactoryacceptablejointstrengthissuccessfully achieved.It wasobserved thatthemaximum tensile strength was450MPa with travel speed of 96mm/min and tool rotation speed of 900rpm.

l Thehardnessatthejointswashigherthan thatofbasemetal duetostrong shearing deformation andfrictionalheataffecting dynamicrecrystallization.Tobeconsiderhardnessdistributions, the hardness ofinterface between base metaland joints was higherthanthatofbasemetal.Itisthoughtthattheregionsof thesehardnessdistributionsprobaly correspondstoheataffected zone(HAZ)orthermo-mechanicallyaffectedzone(TMAZ).

l

The width of the plastically deformed region decreased to vertical direction because frictional heat occur under the

Chapt er5Concl usi on

Friction StirWeiding hasgreatpotentialasanew welding technology.

This study was intended to investigate the weldability, mechanical characteristics(tensileand hardnesstest)and microstructureanalysis of thin ferriticstainlesssteelby using FSW.The resultsofthisstudy are listedbelow.

region.Also,the microstructure has a clear characteristic to opticalmicroscopeimagesinthewaythatitishardtotellthe heat affected zone (HAZ) and thermo-mechanically affected zone(TMAZ)respectively.

l In comparison with grain size ofFSW welded buttjoints for thin plate,the grain sizes ofbase metalwere approximately average20㎛.However,thegrain sizesofbuttjointsdecreased to about5㎛,because the slightly finerrecrystallized grain in the jointare attributed to the generation ofstrong shearing deformation and frictional heat affecting dynamic recrystallization.

l As a resultoftemperature history,the temperature ofFSW welded butt joint near joint is lower than that of GTAW weldedbuttjoint.Therefore,thistemperaturecharacteristicscan be results in reduction of weld defects such as thermal deformation,residualstressetc.

l Moreover,tensile strength ofend partofFSW Welded joint, which regarded as the weld defectwas investigated. Itwas observed that end part of FSW Welded joint of thin plate indicated the good tensile strength which was 488MPa.Itcan beinferredthatbecauseofincreasementofheatinputby dwell timeatendpart.

[1] G. Mallaiah, A Kumar, P. Ravinder Reddy, G Madhusudhan Reddy : Influence of grain refining elements on mechanical properties of AISI 430 ferritic stainless steel weldments - Taguchiapproach,JournalofMaterialsand Design,36,443-450, 2012

[2] M.O.H.Amuda,S.Mridha:Comparativeevaluation ofrefinement in AISI430FSS weldsby elementalmetalpowderaddition and cryogenic cooling,JournalofMaterials and Design,35,609-618, 2012

[3] MehmetBurak Bilgin,CemalMeran :Theeffectoftoolrotation andtraversespeedonfrictionstirweldabilityofAISI430ferritic stainless steels,Journalof Materials and Design,33,376-383, 2012

[4] Sigma phase embrittlement.In : ASM international handbook committee.ASM handbook - propertiesandselection ironssteels andhighperformancealloy,vol.1,p.1657,1993

[5] X.K. Zhu, Y.J. Chao : Numerical simulation of transient temperatureand residualstressesin friction stirwelding of304L stainless steel,JournalofMaterials Processing Technology,146, 263-272,2004

[6] Sathiya P,Aravinda S,NoorulHaq A :Effectoffriction stir

Ref er ences

[7] Lakshminarayanan Ak,Balasubramanian V :An assessmentof microstructure,hardness,tensiland impact strength of friction stirweldedferriticstainlesssteeljoints,JournalofMaterialsand Design,31,4592-4600,2010

[8] C Meran,O.E.Canyurt : Friction Stir Welding of austenitic stainless steels, Journal of Achievements in materials and ManufacturingEngineering,43-1,432-439,2010

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(한글)마찰교반접합기술을 이용한 박판 페라이트계 스테인리스강 접합부의 기계적 특성에 관한 연구

(영어)A StudyontheMechanicalCharacteristicsofFriction StirWeldedJointsforThinFerriticStainlessSteel 본인이 저작한 위의 저작물에 대하여 다음과 같은 조건아래 조선대학교가 저작물을 이용할 수 있도록 허락하고 동의합니다.

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동의여부 :동의(O ) 반대( ) 2013년 2월