Material flow in butt friction stir welds in AA2024-T3

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Acta Materialia 54 (2006) 1199–1209

Material flow in butt friction stir welds in AA2024-T3

H.N.B. Schmidt a,*, T.L. Dickerson b, J.H. Hattel a

a Process Modelling Group, Department of Manufacturing Engineering and Management, Technical University of Denmark,

Producktionstorvet building 425, 2800 Lyngby, Denmarkb Cambridge University, Engineering Department, UK

Received 7 May 2005; received in revised form 28 October 2005; accepted 28 October 2005Available online 22 December 2005

Abstract

The properties of a workpiece joined by friction stir welding (FSW) are directly related to the material flow around the tool. In thepresent work, the material flow is investigated by traditional metallography as well as X-ray and computer tomography (CT). By intro-ducing a thin copper strip in the workpiece and welding through it, thus, acting as a marker material, detailed information about the flowfield is gathered. The two- and three-dimensional CT images are used in parallel with micrographs for visualization of the flow field. Twoprocedures for estimating the average velocities for material flowing through the shear layer are presented. The procedures depend on theconfiguration of marker material relative to the welding direction, i.e. longitudinal and transverse. As such, the present work constitutesthe first attempt in the literature to estimate flow velocities in FSW based on thorough experimental investigations.� 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Friction stir welding; Flow visualization; Marker technique; Shear layer

1. Introduction

The objective of the present work is to illustrate thematerial flow in the highly deformed zone in butt frictionstir welding of AA2024-T3 panels. A marker techniqueusing a thin copper foil is used to trace the material flow.The friction stir welding (FSW) tool is extracted from theworkpiece by the stop-action, i.e. the tool is unscrewed,leaving an exit hole in which the material flow is frozen.Investigations of the material flow pattern using a specificmarker material (MM) have been presented previously inthe literature. Traditional metallographical investigationof the position of the MM has provided detailed informa-tion regarding the patterns, but the combination of timeconsuming experiments and a limitation to a planar (2D)examination of the weld has lead to the introduction ofX-ray tomography to reveal the spatial position of theMM in the weld specimen. By embedding tracer particleswith a different density than the parent material, the con-

1359-6454/$30.00 � 2005 Acta Materialia Inc. Published by Elsevier Ltd. All

doi:10.1016/j.actamat.2005.10.052

* Corresponding author. Tel.: +45 4525 4712; fax: +45 4593 4570.E-mail address: hs@ipl.dtu.dk (H.N.B. Schmidt).

trast/highlighting in the X-ray pictures indicates the mar-ker material. Colligan [1] used steel shots with B

0.38 mm as marker material, but since the diameter ofthe shots was comparable with the thread spacing, thereis some concern that the flow was not properly represented.The steel shots would also be prevented from entering thehighly deformed shear layer at the tool/matrix interface,as this is in the same size range as the shots. Several inves-tigations with consumable marker material have been car-ried out. By consumable MM is meant material, which hassimilar thermomechanical properties to the workpiece, e.g.in the case of aluminium, thin copper foils, bars/slices ofmaterial with other composition than the base metal. Thus,the MM deforms and flows as if it is part of the workpiecematrix itself; however, the presence of a foreign materialcould result in an inexpedient effect on the properties thatcontrol the flow, e.g. the contact condition and flow prop-erties of the base material. In that case, the flow patternrevealed by the MM is not representative of the �original�flow. Reynolds et al. [2] used embedded marker materialat the faying surface at different locations through thethickness. Guerra et al. [3] used a copper foil inserted at

rights reserved.

Rotation

Plunge

Traverse

Weld scar

Retreating

TrailingLeadingo

o

o

oAdvancing

Fig. 1. Orientations relative to the tool rotation and the welding direction.

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the faying surface/joint line of the butt-welded panels inaddition to overlapped welding of two dissimilar materials,from which the vertical flow pattern was examined. Lately,London et al. [4] have used a composite alloy, inserted atthe joint-line, containing Al–SiC and Al–W particles,which are registered by X-ray tomography.

The different experiments show some general character-istics regarding the flow in FSW. The material is forcedaround the retreating side of the probe, resulting in anon-symmetrical flow field. The shoulder affects the mate-rial flow in the upper region of the workpiece. In the midregion, the flow patterns are similar at different levels. Atthe root region, the probe tip and root/backing plate influ-ence the patterns considerably. Under the shoulder/matrixinterface a shear layer is present. In the mid region, thedeformation has been localized to two zones near theprobe/matrix interface, i.e. the transition and rotationzones, which will be discussed further in this paper. Behindthe probe (in the wake of the weld), the deposition processresults in a banded structure, which previously has beendenoted a nugget or a thermomechanically-affected zone(TMAZ). Additionally, Guerra et al. [3] reported a vortexflow, where the material located between the threadsrotates due to the vertical flow/screwing effect.

The objective of the present work is to investigate the2D and 3D flow patterns in the highly deformed zone atthe tool/matrix interface in friction stir welding (FSW)using marker material. This technique has been describedby Dickerson et al. [5,6]. The contact condition is thoughtto influence both the heat generation and flow field aroundthe tool. By including MM at appropriate locations, it ispossible to estimate the average velocity of the MM duringthe deformation process. Previous work by Schmidt et al.[7–11] suggests that the sticking condition is most likelypresent for the specific welding experiment considered inthe present work.

1.1. Experiment

In the present work, all welds described are FSW buttwelds of AA2024-T3 panels (60 · 150 · 3 mm each) with a0.1 mm copper strip as MM inserted in different configu-rations. This enables characterization of the flow patternaround the probe (as the stirring takes place) and in thefully welded region after the probe has passed. For somespecimens, the stop-action is used to freeze the flow pat-tern, leaving an exit hole as an undisturbed foot printof the tool for further investigation. The stop-actionenables investigation of the flow pattern around the tool,whereas a traditional �welding past� procedure gives thefinal position of the MM. Three weld configurations aredescribed in the following, where the copper strip isdenoted marker material. All welds were made with a toolwith a threaded probe and conical shoulder. During theexperimental welding the reaction forces, torque and tem-peratures were monitored to evaluate the influence fromthe presence of MM �inside� the weld on the key welding

parameters that characterize the welding process. Only lit-tle or no difference was observed in these parameterswhen the tool entered the region containing MM fromthe region without MM. This supports the contentionthat the flow patterns illustrated by the MM resemblethe �real� flow patterns. For further information on themarker technique, see [5,6].

The welding speed is 2 mm s�1 and the rotational speedis 400 revolutions per minute (rpm) corresponding to41.8 rad s�1. The tool diameter is 18 mm with a concavityof 10� and the tool probe is a M6 thread forming tool,i.e. a diameter of 6 mm with a thread spacing of 0.8 mm.

1.2. Weld setup – configuration of marker material

In the weld setup denoted E1 and E2, the MM isinserted in the longitudinal direction, i.e. in the joint-linebetween the two weld panels. In this case, the MM indi-cates a stream line entering in front of the probe whichthen flows around the retreating side of the probe. How-ever, as the tool traverses along the joint-line, the intensedeformation eventually leads to a break-up of the contin-uous copper strip into small segments. The breaking up ofthe MM strip occurs after the material passes the retreat-ing side of the probe. From this point, the traces of thestream line vanish, which makes the characterization pro-cess difficult. Seen in a coordinate system moving togetherwith the tool, the MM is flowing towards the probe withthe welding speed, which corresponds to having a rotatingnon-traversing tool. Since the MM strip flows towards theleading edge of the probe, this position is described by theangle of h = 0�, see Fig. 1. In parallel, welds with the mar-ker material inserted transverse to the welding directionwere made. In the weld setup denoted E5, the tool is tra-versed through the transverse MM strip, leaving a fullywelded region of matrix where segments of MM are dis-tributed. In the weld denoted E6, the tool is extractedas the tool center aligns with the transverse cross-section

H.N.B. Schmidt et al. / Acta Materialia 54 (2006) 1199–1209 1201

containing the MM. This resembles taking a snapshot ofthe flow pattern just as the incoming transverse MM linealigns with the tool rotation axis. In this case, each sec-tion of the MM is exposed to a different amount of defor-mation depending on the location in the transverse plane.Fig. 2 and Table 1 summarize the configuration of theMM and the tool extraction positions in each specimen.

Position of tool extraction-the exit hole

Position of marker material

Region examined by X-ray, i.e. specimen

E1and E2 E5 E6

Fig. 2. Schematic view of the specimens with indication of MMorientation and extraction position.

Table 1Specimens with MM orientation and extracting position

Specimen MM position Tool exit position

E1 Longitudinal End of panelE2 Longitudinal End of panelE5 Transverse Past the MME6 Transverse When tool center

aligns with MM

Fig. 3. 3D CT model of specimen E1 with different transparency. Gray is aretreating side corresponding to a (1�1�1) view. (a) No transparency. (b) Halfseen from the front (100 view). (e) Note the continuous shape as seen from thelegend, the reader is referred to the web version of this article.)

The nomenclature of the specimens is adopted from thepaper by Dickerson et al. [6].

2. Results and discussion

2.1. Flow fields – 3D

The 3D flow in FSW is complex as it depends on tooldesign, e.g. threads, flutes and shoulder characteristics,welding parameters, such as rotation and welding speedand direction of rotation (clockwise or counterclockwise),tilt angle, workpiece properties and the contact conditionat the tool/matrix interface. Also the thermomechanicalstate and microstructural properties in the welding zoneplay an important role in the efficiency of the joint. Allthese factors influence the material flow around the probe,i.e. how the original joint-line disrupts and the material istransported from the leading to the trailing side. In the fol-lowing section a general description of the 3D flow fieldsfor the three specimens with different configurations ofthe MM is presented.

2.1.1. Longitudinal MM – specimen E1

Fig. 3 shows a 3D computer tomography (CT) volumemodel of the exit hole in specimen E1 having the MMlocated in the longitudinal direction. In Fig. 3a, the exte-rior/skin of the workpiece is grey and the MM is light.By changing the transparency of the matrix, it is possibleto look �inside� the workpiece to reveal the �computed�position of the MM. As shown in Fig. 3, the MM flows

luminium and yellow is copper. The view point is located at the leadingtransparency. (c) Full transparency. (d) Notice the S- or C-shaped trace asside (010 view). (For interpretation of the references to colour in this figure

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towards the tool as a continuous strip at the leading side,therefore representing a flow line of the surroundingmatrix. As it interacts with the probe and shoulder, itdeflects around the retreating side of the probe. At thesame time, the material in front of the probe is extrudedupwards at the upper region of the plate, which is an effectof forcing the material into the cavity under the conicalshoulder. In the lower region, some of the material flowsdownwards under the probe tip. These movements in thevertical directions occur simultaneously to the tangentialflow around the retreating side of the probe. In Fig. 3c,all the aluminium is removed from the CT model, leavingthe MM only. From this, it is seen how the continuous stripof MM gets disrupted as it flows around the probe, thus, itbreaks up into small convoluted segments. This is due tothe extensive deformation occurring along the paththrough the shear layer in combination with insufficientelongation properties of the MM, i.e. copper. As theMM flows towards the front of the probe/tool seen fromthe moving coordinate system, it is deflected around theleading retreating side in the direction of rotation. As theMM travels into the shear layer, it gradually comes closerto the tool as a result of the material accumulation due tothe asymmetry of the shear layer. The resulting accelera-tion gives rise to extensive shearing and thinning of theMM making the segments smaller than the resolution ofthe digital CT/X-ray pictures (�0.05 mm). At the trailingside of the probe the deposition process takes place. It isobserved that MM interacts with the tool further awayfrom it at upper levels than at mid and lower levels. Thiseffect is attributed to the influence of the tool shoulder.At the mid thickness this influence vanishes and the flowis to a large extent controlled by the probe only. At thelower levels near the root the flow is influenced by boththe roundness of the tool tip and the root gap, whichenforce extrusion underneath the probe, as well as a con-straint against material flow because of the backing plate.

At the trailing side the traverse motion of the toolleaves a �cavity� into which material in the shear layer atthe retreating side immediately displaces. This is referredto as the deposition process. When the material passesthe retreating side of the probe it is deposited out ofthe shear layer into the wake of the weld. Here, the widelydistributed MM is the evidence of complex deposition;however, there is a tendency that MM originally locatedas a vertical longitudinal plane in front of the tool even-tually forms the characteristic (when welding withoutflutes) geometrically ‘‘S’’ or ‘‘C’’ shaped trace, as seen inFig. 3d, where the top of the trace is at the advancingedge of the shoulder and the bottom at the centerline.As seen from the 3D image, in Fig. 3e the trace is in gen-eral continuous along the wake of the weld. The deceler-ation of the material occurs by shearing between thematerial already being slowed down and the materialrotating in the shear layer at the trailing side of the probe.This is reflected in the way the material entering at theleading side is unwound at the trailing side. If the flow

would have been 2D and stationary only, i.e. no influencefrom the shoulder, threads or tip/root/backing plate, thematerial would be deposited at the same transverse posi-tion as it entered. This is not the case and the offsettingof the MM towards the advancing side at the top andtowards the retreating side at the mid and bottom areattributed to the material flow near the shoulder. Underthe shoulder interface, the MM segments are swept allthe way to the advancing side, where they are deposited;so does the surrounding matrix. However, close to theshoulder interface a shear layer is observed where no orvery little MM segments are observed. Similarly, in thewake of the weld MM is found submerged relatively tothe weld scar. This indicates that there is a shear layerat the shoulder interface with a low density of MMsegments.

In the wake of the weld, a steady state flow is observedwhich corresponds to a stationary deposition process whenlooking at the general pattern of the MM. However, adetailed view shows a periodic distance of 0.3 mm betweenthe MM segments in the wake of the weld, which comesfrom a cyclic deposition. The reason for this is still notcompletely understood; however, the influence of thethreads might at first view result in this characteristic. Sincethis phenomenon is also observed in steel welds with acylindrical tool probe, it might more likely be due to achange in the contact condition, i.e. sliding, sticking or par-tial sliding/sticking [7]. This comes from a dynamic balancebetween the hardening and the heat softening of the matrix,which in turn could lead to a collapse of the shear layer, inwhich case the rotation layer is deposited.

Close to the tool probe/matrix interface several MMsegments are observed at the leading retreating side. Theseare located in a matrix inside the shear layer and since theyare closer to the contact interface than the continuousMM, the MM segment has gone through more than onerotation. This layer will be referred to as the rotation layer,see Section 2.3. However, it is not possible to estimate theflow rate or how many rotations the MM segments havecompleted with this type of setup.

2.1.2. Traverse MM – specimen E6

Fig. 4 shows the specimen with the MM located in thetransverse direction and where the tool is extracted as thecenter of the tool aligns with the plane of MM. This MMsetup is distinguished from longitudinal by showing thedeflection of an �incoming� plane, whereas in E1 the MMresembles a stream line entering right in front of the tool.The transverse position enables characterization of theextension of the shear layer, i.e. the boundary betweenthe highly deformed region near the tool and less deformedregions. The deformation in the less deformed regions atthe advancing and retreating sides are characterized by adeflection/deformation of the originally planar MM intoa twisted but still continuous MM strip. This zone isdenoted the deflection zone. Close to the probe inside the

Fig. 4. 3D CT model of specimen E6 with different transparency. Gray is aluminium and yellow is copper. (a) (1�1�1) view. No transparency.(b) (1�1�1) view. Half transparency. (c) (1�1�1) view. Full transparency. (d) Front view (100). Full transparency. (e) Advancing side view (010)(welding towards right). Full transparency. (f) Top view (00�1) (welding towards right). Full transparency. (For interpretation of the references to colourin this figure legend, the reader is referred to the web version of this article.)

H.N.B. Schmidt et al. / Acta Materialia 54 (2006) 1199–1209 1203

shear layer, the MM breaks up into segments; however, theMM strip is continuous some distance along the deforma-tion path at the retreating side. Thus, the transverse MMcan give information regarding the deformation fields fur-ther behind the tool as compared to the longitudinal con-figuration used in specimens E1 and E2.

2.2. Flow fields – 2D

2.2.1. CT slides

Fig. 5 shows three CT-slices, where Fig. 5a shows theCT slice at the mid-section (at z = 1.5 mm) in specimenE1, where the MM is positioned in the longitudinal direc-tion, i.e. in the joint-line. Figs. 5b and c show the CT slices

Fig. 5. 2D CT slices at mid-plane. (a) Specimen E1 with MM positioned in thethe continuous MM line corresponds to a stream line. (b) Specimen E5 with MNotice the traces of MM segment deposited as far as two probe diameters atransverse direction. Tool stopped when MM aligns with the rotation axis.

at the mid-section in specimen E5 and E6, where the MM ispositioned in the transverse direction and welded past andto the intersection point, respectively.

Fig. 5a shows the MM flow path for E1 around theretreating side of the tool probe. This shows how theMM line enters the �front� of the shear layer at the leadingside of the probe and how it is deposited at the �back� of theshear layer at the trailing side of the probe. Fig. 5b showsthe transverse MM line in E5 after the tool has welded pastit. Notice that for this specimen, the flow field should beinterpreted as having the tool moving towards the trans-verse (stationary) line of MM, and therefore the interpreta-tion of being in a �moving� coordinate system having thematerial flowing towards the probe cannot be used for thisconfiguration. Several probe diameters in front of the MM

longitudinal direction, i.e. in the joint-line. On the leading retreating sideM positioned in the transverse direction. Tool traversed through the MM.fter the intersection point. (c) Specimen E6 with MM positioned in the

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line there are traces of MM segments. These MM segmentsindicate that during the path through the shear layer theyhave entered first the transition zone, then the rotationzone and then back to the transition zone from which theyfinally are deposited, i.e. there is an exchange of MM overthe border between the rotation and transition zone.Fig. 5c shows the deformation of the transverse MM lineas the tool intersects the original position. The positionof the MM at the trailing side (behind the probe) will be

Fig. 6. Micrographs at different xy-planes through the thickness of specimen E(b) xy-Plane at z = 2.0 mm. (c) Close look at the MM in the shear layer at th

Fig. 7. Micrographs at the mid-plane of specimen E5 and E6. (a) Specimen E5 wMM. Notice the traces of MM segment deposited as far as two probe diametertransverse direction. Tool stopped when MM aligns with the rotation axis.

used for estimating the average velocity of the MM as itflows through the shear layer.

2.2.2. Metallography

Fig. 6 shows micrographs of different xy-planes in spec-imen E2. Fig. 6a shows the microstructure at the mid-plane, whereas Fig. 6b shows the microstructure slightlybelow the mid-plane at z = 2.0 mm. Fig. 7 shows micro-graphs of the mid-planes in specimen E5 and E6.

2 with longitudinal MM. (a) xy-Plane at z = 1.5 mm, i.e. at the midplane.e retreating side.

ith MM positioned in the transverse direction. Tool traversed through thes after the intersection point. (b) Specimen E6 with MM positioned in the

Fig. 8. Schematic views. (a) Definition of the rotation layer and transition layer as well as definition of stream line parameter w. (b) Schematic view ofdeformed transverseMMused formeasuring deposition distances for specific stream line positions (procedure 1). (c) Schematic view illustrating procedure 2.

H.N.B. Schmidt et al. / Acta Materialia 54 (2006) 1199–1209 1205

2.3. Definition of characteristic flow zones

Based on the experimental investigation presented inthis work, as well as numerical flow models (computationalfluid dynamics, CFD) [11] and thermomechanical work[8,9], the presence of different characteristic flow zones orlayers are proposed. The different zones are closely relatedto the corresponding flow, see Fig. 8a, and they are definedin the present work as:

� Rotation zone – the zone in the shear layer closest to theprobe consisting of the rotation flow.

� The rotation flow consists of material which has passedthe advancing side of the shear layer, i.e. the rotationalflow equals the advancing flow or counter flow.

� Transition zone – the zone in the shear layer between therotation zone and the matrix/shear layer border consist-ing of the transition flow.

� The transition flow is defined inside the shear layer andit consists of the material which enters the shear layer atthe leading side, i.e. the transition volume flow at theretreating side equals the welding flow.

� Deflection zone – layer characterized by a low deforma-tion surrounding the transition layer.

� The material at the leading side of the deflection zoneenters the transition zone. The material at the retreatingand advancing sides of the deflection zone does not enterthe transition zone. This corresponds to the TMAZ sur-rounding the nugget in an as weld cross-section.

� The welding flow is the �band� in front of the shear layerhaving the welding velocity.

Strictly speaking, the shear layer consists of the deflec-tion zone, transition zone and rotation zone; however, insome of the cases only the high deformation region con-taining pronounced shear thinning is denoted the shearlayer. Thus, including the transition and rotation zones,only, see Fig. 8a. It should be noted that a somewhat dif-ferent definition of the rotation zone has previously beenproposed by Guerra [3].

2.4. Estimation of average speed through shear layer

This analysis is made in a moving co-ordinate systemthat has its origin in the tool center and moves with thewelding speed. Based on this, stream lines of marker mate-rial segments are used to describe the material flow.

2.4.1. Definition of the stream line parameter wWhen following a marker material entering the shear

layer it is exposed to an acceleration and deceleration, thatis, at the leading and trailing side, respectively. Using thestream line model by Schmidt and Hattel [10], it is possibleto estimate the path of an incoming stream line inside theshear layer analytically.

The identification of a stream line can be character-ized by its position relative to the effective width (inthe transverse direction) of the deformation field. A

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parameter w is defined to describe this relative positionas

w ¼ d�

d¼ rprobe þ wretr � sin hðrprobe þ whÞ

d; ð1Þ

where d* is the distance from the stream line to the streamline aligning with the matrix/shear layer border at theretreating side and d is the effective width of the deforma-tion field, see Fig. 8a. The stream lines entering so theyalign with the shear layer/matrix border at the advancingand retreating side gives wadv = 1 and wretr = 0, respec-tively. The stream line entering at the centerline is denotedwcl, where wcl = (rprobe + wretr)/d and (1 � wcl) = (rprobe +wadv)/d.

When following a stream line as it enters at the leadingside of the shear layer, it is first positioned in the vicinity ofthe shear layer. A marker point in the stream line (whichresembles the MM used in the present work) then flowstowards the inner region of the shear layer, as new materialin front of and further towards the retreating side of theprobe enters the shear layer. Inside the shear layer, thestream line under investigation has material at each sidewhich has less values of w at the outer side and larger val-ues at the inner side (referring to Eq. (1)).

2.4.2. Definition of the relative average velocity

A relative measure for the average velocity in the shearlayer with respect to the periphery velocity of the toolprobe can be defined as

#w ¼ vwxtoolrprobe

¼ �xw

xtool

; ð2Þ

where vw is the average velocity of the stream line for a gi-ven w, �xw is the average angular velocity of a materialpoint following the stream line and xtool is the tool rotationspeed. The velocity of the stream line entering at the center-line is vcl and has the average relative velocity of #cl.

2.4.3. Procedure 1: Measuring of deposition distance

The distance which the tool travels during the time ittakes from a material segment enters till it exits the shearlayer can be related to the welding parameters, the shearlayer thicknesses and the average velocity of the specificstream line. By measuring the deposition distance, the lon-gitudinal thickness of the shear layer, the tool traveldistance, stream line path distance, an expression of theaverage velocity is proposed. For this, a definition of themeasured quantities are presented.

2.4.4. Deposition distance

The deposition distance Lw is the longitudinal distancebetween the deposition position at the trailing side andthe original position. In the case of a transverse MM asin E5 and E6, this depends on the ‘‘incoming position’’,i.e. which w or hin each material point has. In other words,the incoming plane of MM is deflected. Fig. 8b shows aschematic view of the deposition distances Lw=0, Lw�0.25,

Lw�0.50, Lw�0.75 and Lw=1.00 for the stream lines w = 0,w � 0.25, w � 0.50, w � 0.75 and w = 1.00, respectively.Note that L should be given with the proper sign accordingto the L-axis.

2.4.5. Longitudinal secant length of deformation fieldThe length of the secant to the periphery of the deforma-

tion field in the longitudinal direction, P, is schematicallyshown in Fig. 8b. This secant length can be measured fromthe exit hole in Fig. 7b and to some extent in Fig. 5c (as CTpicture does not reveal microstructural changes character-istic for identifying the border of the shear layer). At thecenterline, Pcl = Pw�0.5 = rprobe lead + rprobe trail + wlead +wtrail, see Fig. 9. Note that rprobe lead and rprobe trail dependon the actual cross-section, i.e. they vary and the sum is lessthan the outer projection, B6.

2.4.6. Tool travel distance

The tool travel distance Tw is the distance the tool trav-els with the welding speed uweld along in the longitudinaldirection during the time tw a material point travels whenfollowing a stream line through the transition layer. Thus,the tool travel distance is given by

T w ¼ uweldtw. ð3ÞAs seen in Fig. 8b, the tool travel distance Tw is related tothe deposition distance Lw and longitudinal thickness ofthe deformation field Pw as Sw = Pw � Lw. For theadvancing stream line, Pw=1 = 0, i.e. Tw=1 = �Lw=1,where L < 0.

2.4.7. Stream line path distance

The stream line path distance is the length of the path amaterial point travels when following a stream line throughthe transition zone. This distance depends on the angularposition where the stream line enters and exits the shearlayer. Additionally, the path depends on the shear layercharacteristics (velocity profile and shear layer thickness,etc.). This can be observed in the schematic view of the cen-ter stream line in Fig. 8b where the dotted line to the leftrepresents the path in the moving co-ordinate system andthe dotted line to the right represents the path in the sta-tionary co-ordinate system. The former can be assumedas a part of a circle; however, it is the path in the stationaryco-ordinate system which is of interest as this takes intoaccount the influence of the welding speed. The rightdotted line shows a parabolic type of path for the centerstream line, i.e w � 0.5, however it should be mentionedthat it is a schematic view. Since the present model consti-tutes a first attempt to systematically quantify the materialflow around the probe in FSW, it is assumed for the calcu-lation of the average velocity that the stream line at thecenter line gives a circular stream line path. Strictly speak-ing, this is only exactly correct when the rotation speed isinfinitely large as compared to the welding velocity. Hence,as a first estimation, the stream line path distance is givenby

Fig. 9. Measurements of the distance the tool advances during transport of the transverse MM from the leading to the trailing side.

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Sw ¼ ðhout � hinÞr; ð4Þwhere the effective average radius of the transition layer isr � rprobe þ wretr. As seen in Figs. 5b and c, the MM firstentering the shear layer is that positioned right in frontof the tool, i.e. at hin � 0. At the trailing side, the MM withthe largest deposition distance is that exiting at hout � p.This is consistent with the analytical stream line [10] andthe 2D CFD models [11] by Schmidt and Hattel.

2.4.8. Average velocity

The average velocity of a material point flowing along astream line through the transition layer is defined as thestream line path distance divided by the time tw, i.e.

vw ¼ Sw

tw. ð5Þ

By combining Eqs. (3) and (5), the average velocitythrough the shear layer for a given stream line is given by

vw ¼ uweld � Sw

T w; ð6Þ

where Tw = Pw � Lw, see Fig. 8b.For simplicity, let us follow the stream line at the cen-

terline, i.e w � 0.5. Using the analytical stream line modelpresented in [10], the stream line enters at an anglehin = 0� and exits or is deposited at an angle hout = 180�.For thin shear layers, i.e. wh �rprobe and for high rota-tional speeds as compared to the welding speed, the circu-lar stream line path can be assumed. This gives a streamline path distance through the shear layer of Scl ¼ Sw�0:50

� pr, where r is the average radius of the path. Assumingthat r � rprobe gives an average velocity of the centerstream line of

vw�0:5 ¼prprobeuweld

T cl

. ð7Þ

By substituting v ¼ #xrprobe (Eq. (2)) in the left hand sideof Eq. (7), the relative average velocity of the center streamline is given by

#w�0:5 ¼puweldT w�0:5x

. ð8Þ

Using that x = RPM · 2p/60, Eq. (7) can be expressed forrotation speeds given in rpm instead of in rad s�1. Thus,the average velocity of the center stream line is given by

#w�0:5 ¼30uweld

T w�0:5rpm; ð9Þ

where Tw�0.5 = Pw�0.5 � Lw�0.5. This enables a calculationof the average velocity (both absolute and relative) throughthe shear layer based on a measured deposition distance.

2.4.9. Results – procedure 1 – E6

From the transverse MM setup it is possible to estimatethe flow rate of the MM by measuring the deposition dis-tance relative to the original position.

As seen in Fig. 9 the effective probe diameter or lon-gitudinal width of the deformation field, Pcl, for speci-men E6 is measured to 6.0 mm. The depositiondistance Lcl is measured in Fig. 9 to 4.63 mm, which cor-responds to a tool travel distance Tcl of 1.37 mm takingtcl = 0.685 s with uweld = 2 mm, Eq. (3). Assuming thecircular stream line path, Eq. (4) gives that Scl ¼rp � 9:4 mm, when the radius of the circular stream linepath is estimated to r � 3:0 mm. Using Eq. (6), the aver-age velocity through the leading retreating transitionlayer is estimated to 13.8 mm s�1. This corresponds to

1208 H.N.B. Schmidt et al. / Acta Materialia 54 (2006) 1199–1209

a relative average velocity of # � 0.11 (using Eq. (2)),which means that the tool rotates �4.56 rotations duringthe time the MM enters at the leading side and depositsat the trailing side. The fact that it is only �4.56 insteadof 1/0.11 � 9 rotations is because the flow path is onlyhalf a rotation. The threads on the probe are thoughtto introduce some cyclic flow pattern. Having 4 threadsalong the probe length, a MM segment on the centerlinewill be exposed to approx. 18 cyclic responses as it flowsthrough the shear layer. This could be one explanationfor the disruptions and dispersion of the MM shown inthe micrographs in this work. The vertical motion ofthe tool threads and the eccentric motion of the cross-section leads to a transient shape of the shear layerwhich is not captured by the simple 2D ‘‘models’’ pre-sented here. This could force the MM segments to enterthe rotation zone, hence making multiple rotationsbefore being deposited.

The 3D effects from the threads and the shoulder influ-ence the flow such that the MM is not deposited at thecenter section of the trailing side as predicted by thestream line model [10]. The part of the transverse MMline in specimen E6 that first interacts with the shear layeris that located at h = 0�. Therefore, the MM segment thatfirst enters the shear layer (and is deposited first) is thatfollowing the center stream line. The MM segments onthe advancing side of the transverse MM line has alreadybeen lead through the transition zone closer to the probecenter. Similarly, the MM segments on the retreating sideis lead into the transition zone after the that at the center-line. The path for each MM segment at different trans-verse locations has been estimated by the stream linemodel [10], i.e. by following the corresponding streamline.

Notice, that a deformation field of the transverse MMline similar to that observed in specimen E6 is also presentin specimen E5. Strictly speaking, the measurement of thedeposition distance should by made in E5 to ensure thatthe MM segments are deposited out of the shear layer,hence they are at rest (in the stationary coordinate system)and the deposition is finished. However, it is seen by com-parison of the micrographs in Figs. 7a and b or the CTslides in Figs. 5b and c that the deposition has completedeven though the MM line just aligns with the tool center.This is not the case for other regions such as near the shoul-der and root.

2.5. Procedure 2: MM area comparison by counting pixels

The material flow in the horizontal mid plane can bedescribed by an average velocity in the transition layer asexplained earlier. It is assumed that the flow in the mid-sec-tion is primarily planar and quasi-stationary. Thus, it ispossible to estimate the average velocity of the MM inthe shear layer based on an area comparison procedure.Fig. 6c shows a close up view of the MM distribution inthe shear layer around the retreating side of the probe in

specimen E2. It is seen how the copper strip is disruptedinto segments. Since the MM strip is no longer continuous,it is not possible to use the thickness relationship of theMM segment relative to the incoming strip to estimatethe average flow rate in the transition zone. However, itis possible to use a gross flow estimation over a region ofthe transition layer. Of special interest is the region posi-tioned at the leading retreating side. Despite the break upof the strip into segments, it is possible to estimate the areaof MM in that region. This is done by counting the numberof pixels �containing� MM in the bitmap picture shown inFig. 6c. The difficulty is then to distinguish between seg-ments positioned in the transition and rotation zone, asthe border is difficult to define. Based on this procedure,the average velocity of the MM as it flows through theleading retreating side of the shear layer is estimated inthe following. The MM from the center line has an averagevelocity through the transition layer of

v ¼ Sh1�h2

th1�h2

; ð10Þ

where Sh1�h2 is the segment length under consideration. Thenumber of MM pixels, N h1�h2 , entering the shear layer dur-ing the time th1�h2 it takes the MM to flow though the tran-sition layer from h1 to h2, is related to the pixels in the MMstrip in front of the tool probe, i.e.

N h1�h2 ¼ wtwlqpixel ¼ wtuweldth1�h2qpixel; ð11Þ

where uweld is the welding speed, wt is the transverse widthand wl ¼ uweldth1�h2 is the longitudinal width of the un-de-formed MM strip, respectively, and Sh1�h2 is the segmentlength of the investigated region. qpixel is the pixel density(pixels/area).

By combining Eqs. (10) and (11), the average velocity ofthe center stream line in the region from h1 to h2 is esti-mated as

vclh1�h2 ¼Sh1�h2uweldwtqpixel

N h1�h2

. ð12Þ

2.5.1. Results – procedure 2 – E1 and E2

Considering Figs. 6a and b it is realized that the MM isentering the transition layer at h1 = 10� due to the presenceof the deflection layer. The segment length Sh1�h2 fromh1 = 10� to h1 = 90� is measured to 4.2 mm, which is usedfor the further analysis. Keeping in mind that it is assumedthat average velocity is symmetrical along the transverse toolcenter line, i.e. vclh ¼ 10�–90� ¼ vclh ¼ 10�–170�. This allowsa comparisonwith the average velocity found in procedure 1.

The first estimate is based on the segment lengthSh=10�-90� � 4.2 mm of the transition layer in specimen E2at position z = 1.5 mm, see Fig. 6a. In the un-deformedMM strip, 12 pixels is counted over the 0.1 mm transversewidth. This gives a pixel density of q = (12/0.1)2 =14,400 pixels/mm2. 316 pixels are counted in the transitionlayer from h1 = 10� to h2 = 100�. Using Eq. (12) gives anaverage velocity of �38 mm s�1.

H.N.B. Schmidt et al. / Acta Materialia 54 (2006) 1199–1209 1209

The second estimate using procedure 2 is made in spec-imen E2 in a plane just below, i.e. at position z = 2.0 mm(see Fig. 6c). Here, the segment length is Sh=10�-90� �4.2 mm. Two thousand three hundred and thirty seven pix-els are counted in the transition zone from h1 = 10� toh2 = 100� and the pixel density is q = (31/0.1)2 =96,100 pixels/mm2. This gives an average velocity of�34 mm s�1. These experimentally estimated velocities ofthe MM can be compared with the numerically estimatedvalue from the CFD models in [11]. Here, the maximumtangential velocity of the center stream line is found to28 mm s�1, as it passes through the retreating side.

In the present work, both the longitudinal and transverselocatedMMenable an estimation of the average velocity of amaterial segment positioned at the centerline. Two differentprocedures for estimating the average velocity are used, i.e.procedure (1) by measuring the deposition distance, i.e. thedistance between the deposited MM and the un-deformedMMand procedure (2) by relating the area ofMM segmentsinside the leading retreating side of the shear layer to the areaof theMMstrip in front of the probe entering the shear layer.The following estimates for the average velocity of the MMare summarized: �14 mm s�1 at the mid-plane in E6 usingprocedure 1, �38 mm s�1 at position z = 1.5 mm and�34 mm s�1 at position z = 2.0 mm in specimen E2 usingprocedure 2. This corresponds to relative velocities rangingfrom # � 0:1 to 0:3 of the tool periphery velocity.

Even though the sticking condition most likely is at thecontact interface the velocity experienced by the material inthe transition zone is only a fraction (approx. 0.1–0.3) ofthe rotation speed. It should be mentioned that this isbased on the average velocity, but as indicated in thestream line model [10] and the CFD model [11] based onthe same experiment as presented here, the actual velocityof the MM as it passes the retreating side is higher thanthe average value.

3. Conclusion

� Three configurations of marker material are used to

� The combination of X-ray imaging, CT tomographyand metallurgical sectioning enabled the flow patternsaround the tool to be investigated.

� Two procedures for estimating the average velocity of amarker material through the shear layer are presented.

� The present work shows the first experimental resultsthat support the presence of the sticking condition atthe tool/matrix interface.

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