ORIGINAL ARTICLENumerical modeling of friction stir welding
process:a literature reviewDiogo Mariano Neto & Pedro
NetoReceived: 14 February 2011 / Accepted: 9 April 2012 / Published
online: 6 May 2012#Springer-Verlag London Limited 2012Abstract
Thissurveypresentsaliteraturereviewonfric-tionstirwelding(FSW)modelingwithaspecial
focusontheheat generationduetothecontact
conditionsbetweentheFSWtool andtheworkpiece. Thephysical
processisdescribed and the main process parameters that are
relevanttoits modelingare highlighted. The contact
conditions(sliding/sticking) are presented as well as an analytical
mod-el that allows estimating the associated heat generation.
Themodeling of the FSW process requires the knowledge of
theheatlossmechanisms,whicharediscussedmainlyconsid-eringthemorecommonlyadoptedformulations.
Differentapproachesthathavebeenusedtoinvestigatethematerialflow are
presented and their advantages/drawbacks are dis-cussed.
AreliableFSWprocessmodelingdependsonthefinetuningofsomeprocessandmaterialparameters.Usu-ally,
these parameters are achieved with base on experimen-taldata.
Thenumerical
modelingoftheFSWprocesscanhelptoachievesuchparameterswithlesseffort
andwitheconomic advantages.KeywordsFrictions stir welding.FSW
.Modeling .Numerical simulation .Heat generation .Heat transfer
.Metal flow .Review1 Introduction1.1 Friction stir welding
processFrictionstir welding(FSW) isanovel solidstatejoiningprocess
patented in 1991 by The Welding Institute, Cambridge,UK[1]. One of
the mainadvantages of FSWover theconventional
fusionjoiningtechniquesisthat nomeltingoccurs. Thus, the FSWprocess
is performed at muchlower temperatures than the conventional
welding. Atthe same time, FSWallows to avoid many of the
environmen-tal andsafetyissuesassociatedwithconventional
weldingmethods [2]. In FSW, the parts to weld are joined by
forcinga rotating tool to penetrate into the joint and moving
across theentire joint. Resuming, the solid-state joiningprocess
ispromoted by the movement of a unconsumable tool(FSWtool)
throughtheweldingjoint.FSW consists mainly in three phases, in
which each
onecanbedescribedasatimeperiodwheretheweldingtoolandtheworkpiecearemovedrelativetoeachother.Inthefirstphase,therotatingtoolisverticallydisplacedintothejoint
line(plungeperiod). Thisperiodisfollowedbythedwell period in which
the tool is held steady relative to theworkpiece but still
rotating. Owing to the velocity differencebetween the rotating tool
and the stationary workpiece, themechanical interaction produces
heat by means of frictionalwork and material plastic deformation.
This heat is
dissipat-edintotheneighboringmaterial,promotinganincreaseoftemperature
and consequent material softening.After thesetwo initial phases,
the welding operation can be initiated bymovingeither thetool or
theworkpiecerelativetoeachother alongthejoint line.
Figure1illustratesaschematicrepresentation of the FSW setup
[3].TheFSWtool consistsofarotatingprobe(alsocalledpin)
connectedtoashoulder piece, as showninFig.
2.Duringtheweldingoperation,thetoolismovedalongthebuttingsurfacesof
thetworigidlyclampedplates(work-piece),whicharenormallyplacedonabackingplate.Theverticaldisplacementofthetooliscontrolledtoguaranteethattheshoulderkeepscontactwiththetopsurfaceoftheworkpiece.
Theheat generatedbythefrictioneffect andplasticdeformationsoftens
thematerial beingwelded. AD. M. Neto (*):P. NetoDepartment of
Mechanical Engineering (CEMUC)-POLO II,University of
Coimbra,3030-788 Coimbra, Portugale-mail: [email protected] J
Adv Manuf Technol (2013) 65:115126DOI
10.1007/s00170-012-4154-8severeplasticdeformationandflowof
plasticizedmetaloccurs when the tool is translated along the
weldingdirection. In this way, the material is transported fromthe
front of the tool to the trailing edge (where it is forgedinto a
joint) [4].The half-plate in which the direction of the tool
rotationis the same as the welding direction is called the
advancingside, whiletheotherisdesignatedasretreatingside.
Thisdifferencecanleadtoasymmetryinheattransfer,materialflow, and in
the mechanical properties of the weld.1.1.1 Process parametersThe
welding traverse speed (Vtrans), the tool rotational
speed(),thedownward force(F),thetiltangleofthetool,andthe tool
design are the main variables usually used to controltheFSWprocess
[4]. Therotationof thetool results instirring of material around
the tool probe while the transla-tion of the tool moves the stirred
material from the front tothe back of the probe. Axial pressure on
the tool also affectsthequalityoftheweld. It meansthat
veryhighpressureslead to overheating and thinning of the joint,
whereas verylow pressures lead to insufficient heating and voids.
The tiltangleofthetool, measuredwithrespect totheworkpiecesurface,
isalsoanimportant parameter, especiallytohelpproducing welds with
smooth tool shoulders [5].As mentionedbefore, tool design
influences heat gener-ation, plastic flow, the power required to
perform FSW, andthe uniformityof the weldedjoint. Generally,
twotoolsurfaces are neededtoperformthe
heatingandjoiningprocessesinFSW.
Theshouldersurfaceistheareawherethe majority of the heat by
friction is generated. This is validfor relatively thin plates;
otherwise, the probe surface is theareawherethemajorityoftheheat
isgenerated. Figure3presents a schematic example of an FSW tool
with conicalshoulderandthreadedprobe. Inthiscase,
theconicaltoolshoulder helps to establish a pressure under the
shoulder, butalso operates as an escape volume for the material
displacedby the probe due to the plunge action. As the probe tip
mustnot penetrate the workpiece or damage the backing plate, inall
tool designs the probe height is limited by the workpiecethickness
[3].1.1.2 Weld
microstructureFSWinvolvescomplexinteractionsbetweensimultaneousthermomechanical
processes. Theseinteractionsaffect theheatingandcoolingrates,
plasticdeformationandflow,dynamicrecrystallizationphenomena,
andthemechanicalintegrityof thejoint [4]. Thethermomechanical
processinvolvedunderthetool resultsindifferent
microstructuralregions (see Fig. 4). Some microstructural regions
are com-montoallformsofwelding,whileothersareexclusiveofFSW
[5].& The stir zone (alsocalled nugget) is a region of
deeplydeformed material that corresponds approximately to
thelocation of the probe during welding. The grains withinthe
nugget are often an order of magnitude smaller thanthe grains in
the base material.& The thermomechanicallyaffected zone(TMAZ)
occurson either side of the stir zone. The strain and
temperaturelevels attained are lower and the effect of welding on
thematerial microstructure is negligible.& The heat-affected
zone (HAZ) is common to all weldingprocesses. This region is
subjected to a thermal cycle butit is not deformed during
welding.1.2 Numerical
modelingSeveralaspectsoftheFSWprocessarestillpoorlyunder-stoodandrequirefurtherstudy.
Manyexperimentalinves-tigations have already been conducted to
adjust input FSWparameters(toolspeed,feedrate,and
tooldepth),contraryFig. 1 Friction stir welding setup [3]Fig. 2
Schematic illustration of the FSW process [4] Fig. 3 FSW tool with
a conical shoulder and threaded probe [3]116 Int J Adv Manuf
Technol (2013) 65:115126to numerical investigations, which have
been scarcely used forthese purposes. Computational tools could be
helpful to betterunderstand and visualize the influence of input
parameters onFSW process. Visualization and analysis of the
material flow,temperaturefield, stresses,
andstrainsinvolvedduringtheFSW process can be easily obtained using
simulation resultsthan using experimental ones. Therefore, in order
to attain thebest weld properties, simulations can help to adjust
and opti-mize the process parameters and tool design [5].One of the
main research topics in FSW is the evaluationof the temperature
field[6]. Althoughthe
temperaturesinvolvedintheprocessarelowerthanthemeltingpointsof
theweldmaterials, theyarehighenoughtopromotephasetransformations.
Thus, it isveryimportant toknowthetimetemperaturehistoryof
thewelds. Usually, FSWtemperature is measured using thermocouples
[7, 8]. How-ever, the process of measuring temperature variations
in thenugget zone using the technique mentioned above is a
verydifficult task. Numerical methods can be very efficient
andconvenient for this studyandinfact, alongthelast fewyears,
theyhavebeenusedinthefieldofFSW[9]. Riahiand Nazari present
numerical results indicating that the highgradient
intemperature(for analuminumalloy) isintheregion under the shoulder
[10].In the process modeling, it is essential to keep the goalsof
the model in view and at the same time it is also importanttoadopt
anappropriatelevel ofcomplexity.
Inthissense,bothanalyticalandnumericalmethodshavearoletoplay[11].
Usually, two types of process modeling techniques areadopted:
fluiddynamics(simulationof material flowandtemperature
distribution) and solid mechanics (simulation
oftemperaturedistribution,stress,andstrain).Bothsolidandfluidmodelingtechniques
involvenonlinear phenomenabelonging to the three classic types:
geometric, material, orcontact nonlinearity.Thesimulationof
material flowduringFSWhasbeenmodeled using computational fluid
dynamics (CFD) formu-lations. In this scenario, the material is
analyzed as a viscousfluid flowing across an Eulerian mesh and
interacting with arotatingtool [12]. Other authors
havealsousedaCFDapproachtodevelopaglobal thermal model
inwhichtheheat flowmodel includes parameters related with theshear
material andfrictionphenomenon[13]. Oneof themajor disadvantages of
CFDmodels has todowiththedefinition of the material properties
(residual stressescannot bepredicted)
[7].Solidmechanicsmodelsrequiretheuseof
Lagrangianformulationduetothehighdeformationlevels. However,the
high gradient values of the state variables near to the probeand
the thermomechanical coupling imply a large number ofdegrees of
freedomin FSWmodeling, which is costly in termsof CPUtime[14].
Recent researchdemonstratedthat thecomputational
timecanbereducedbyrecurringtohigh-performance computing techniques
[15]. Nevertheless, in or-der tofacethelongcomputational
timesassociatedtothesimulationof theFSWprocess,
theadaptivearbitraryLa-grangian Eulerian (ALE) formulation has been
implementedbysomeauthors[16, 17]. VanderStelt et al.
useanALEformulationtosimulatethematerial flowaroundthepinduring
FSWprocess[16].Thesemodelsoftheprocesscanpredict the role played by
the tool plunge depth on the forma-tion of flashes, voids, or
tunnel defects and the influence ofthreads on the material flow,
temperature field, and weldingforces [14]. Lagrangian, Eulerian,
and ALE approaches havebeenusedtonumericallysimulatetheFSWprocess,
usingsoftware such as FORGE3 and THERCAST [18], ABAQUS[10], DiekA
[16], WELDSIM [19], and SAMCEF [20].2 Heat generationThe heat
generated during the welding process is equivalenttothepower input
introducedintotheweldbythetoolminussomelossesdue tomicrostructural
effects[21].Theperipheralspeedoftheshoulderandprobeismuchhigherthan
the translational speed (the tool rotates at high
speeds).FSWprimarilyuses viscousdissipationintheworkpiecematerial,
driven by high shear stresses at the
tool/workpieceinterface.Therefore,theheatgenerationmodelingrequiressome
representation of the behavior of the contact interface,together
with the viscous dissipation behavior of the mate-rial. However,
the boundary conditions in FSWare complextodefine. Material at
theinterfacemayeithersticktothetool (it has the same local velocity
as the tool) or it may slip(thevelocitymaybelower)[11].
Ananalytical model
forheatgenerationinFSWbasedondifferentassumptionsinterms of contact
condition between the rotating tool surfaceand the weld piece was
developed by Schmidt et al. [3]. Thismodel will be discussed in the
following sections.2.1 Contact conditionWhen modeling the FSW
process, the contact condition is acritical part of the numerical
model [22]. Usually, the Cou-lombfrictionlawis
appliedtodescribetheshear forcesFig.4
Differentmicrostructuralregionsinatransversecross-sectionof FSW
[5]Int J Adv Manuf Technol (2013) 65:115126 117between the tool
surface and the workpiece. In general, thelaw estimates the contact
shear stress as:tfriction p 1where is the friction coefficient and
p is the contact pressure.Analyzing the contact condition of two
infinitesimal surfacesegments in contact, Coulombs law predicts the
mutual mo-tion between the two segments (whether they stick or
slide).The normal interpretation of Coulombs law is based on
rigidcontact pairs, without taking into account the internal
stress.However, this is not sufficiently representative for the
FSWprocess. Thus, three different contact states were developed
atthetool/workpieceinterface, andtheycanbecategorizedaccording to
the definition presented by Schmidt et al. [3].2.1.1 Sliding
conditionIf the contact shear stress is smaller than the internal
matrix(material to be welded) yield shear stress, the matrix
segmentvolumeshearsslightlytoastationaryelasticdeformation(sliding
condition).2.1.2 Sticking conditionWhen the friction shear stress
exceeds the yield shear stress oftheunderlyingmatrix,
thematrixsurfacewill sticktothemoving tool surface segment. In this
case, the matrix segmentwill accelerate along the tool surface
(receiving the tool ve-locity), until the equilibrium state is
established between thecontact shear stress and the internal matrix
shear stress. At thispoint,
thestationaryfull-stickingconditionisfulfilled. Inconventional
Coulombs friction law terms, the static frictioncoefficient relates
the reactive stresses between the surfaces.2.1.3 Partial
sliding/sticking conditionThelast
possiblestatebetweenthestickingandslidingconditionisamixedstateofboth.Inthiscase,
thematrixsegmentacceleratestoavelocitylessthanthetoolsurfacevelocity.
Theequilibriumis
establishedwhenthecontactshearstressequalstheinternal
yieldshearstressduetoaquasi-stationaryplasticdeformationrate(partial
sliding/stickingcondition).Insummary,theslidingconditionpro-motes
heat generation by means of friction and the
stickingconditionpromotes heat generationbymeans of
plasticdeformation. Inpractice, we have these twoconditionstogether
(partial sliding/sticking condition).2.1.4 Contact state
variableItisconvenienttodefineacontactstatevariable, whichrelates
the velocity of the contact workpiece surface with thevelocity of
the tool surface. This parameter is a dimension-less slip rate
defined by Schmidt et al. [3] as:d vworkpiecevtool 1 gvtool2g
vtoolvworkpiece3whereVtoolisthevelocityof thetool
calculatedfromr(being the angular velocity and r the radius),
Vworkpiece
isthelocalvelocityofthematrixpointatthetool/workpiececontact
interface, andg isthesliprate. Furthermore, theassumptionthat
theweldingtransversespeeddoesnot
in-fluencethesliprateand/orthedeformationrate,resultsinthat all
workpiece velocities can be considered tangential tothe rotation
axis. It is then possible to define as:d wworkpiecewtool4where
workpiece is the angular rotation speed of the
contactmatrixlayerandtoolistheangularrotationspeedofthetool. Ulysse
uses this relationship to prescribe a slip
bound-aryconditioninhisCFDmodelsof thematerial flowinFSW[23].
Therelationshipbetweenthedifferent contactconditions is summarized
in Table 1.2.2 Analytical estimation of heat generationDuring the
FSW process, heat is generated close to the contactsurfaces, which
can have complex geometries according to thetool geometry. However,
for theanalytical model, it isas-sumed as a simplified tool design
with a conical or horizontalshouldersurface,
averticalcylindricalprobesurface,andahorizontal probe tip surface.
The conical shoulder surface ischaracterized by the cone angle ,
which in the case of a flatshoulder assumes the value zero. The
simplified tool design ispresentedinFig. 5, whereRshoulderis
theradius of theTable 1 Definition of contactcondition,
velocity/shearrelationship, and state variable(" strain rate)
[24]Contact condition MatrixvelocityToolvelocityContact shear
stress StatevariableSticking vmatrix0vtoolvtool0r tcontact tyield""
01Sticking/sliding vmatrix