Simulation and Experimental Study on Temperature and Flow Field in Friction StirWelding of TC4 Titanium Alloy Process

Yiming Qi, Junping Li, Yifu Shen+ and Wentao Hou

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics,29 Jiangjun Road, Nanjing 211106, PR China

In this study, numerical simulation method was used to investigate the temperature field and the flow field of FSW for TC4 titanium alloy.According to the contact condition between the tool surface and the weld piece, a scientific heat source calculation method was built and itsreliability has been verified by experiments. The temperature distribution characteristics of welding zone were investigated. The simulationresults shows that the welding materials around the friction head flows like funnel-shape in a whole, similar to the shape of the friction head. Theflow condition of welding materials is asymmetric with respect to the weld line, and the flow velocity increases with the greater distance from thecenter of the weld. As the depth of the weld increases, the flow capacity of the material gradually becomes weaker.[doi:10.2320/matertrans.MT-M2020017]

(Received January 9, 2020; Accepted August 4, 2020; Published November 25, 2020)

Keywords: numerical simulation, friction stir welding, temperature field, TC4

1. Introduction

Friction Stir Welding (FSW) is a solid phase weldingtechnique invented by the British Welding Institute (TWI)in 1991.1) There was no melting during the welding processcompared to the conventional welding method. Therefore,defects such as pores and cracks could be avoided to someextent.2) FSW is mostly used for the connection of metalssuch as copper, titanium, steel and their alloys and dissimilarmetals. During the welding process, the stirring pin wasinserted into the plate with a certain rotating speed, and thefriction-heat generation between the friction head and theplate made the material in the weld area plasticized. As thetool moves, the plastic material in the weld was squeezedand connected together.

Titanium alloy was widely used in aviation, aerospace andmedical field. The research on FSW of titanium alloys weremainly focuses on the optimization of welding parameters,analysis of weld microstructure and mechanical properties.In recent years, people began to pay more attention tothe numerical simulation of FSW of titanium alloy. Forexample, McClure et al.3) proposed the Rosenthal model thatconsidered the work-piece as an infinite object, and treatedthe problem as a common heat conduction problem. And itused a moving point heat source and a line heat source todescribe the heat input of the friction stir welding. Gouldet al.4) also proposed a similar analytical model and verifiedit with experimental results. The model only considered thatthe heat generated by the shoulder, and controlled thetemperature by adjusting the friction coefficient and thepressure. They also found significant dynamic recrystalliza-tion and grain growth in the weld nugget zone, indicated thatthe material in the zone didn’t melt. Frigaard et al.5) used thefinite element method to study the FSW process of TC4 alloy.In the simulation process, only the frictional heat generationof the shoulder was considered, and the heat generation of thestirring pin was neglected. The study results indicated thatthere was a relatively large temperature gradient in the joints.

Schmidt et al.6,7) proposed an analytical model for the heatgeneration of FSW, which described the contact conditionsas viscous, sliding or partially sliding partially viscous. Thecorrectness of the model was confirmed by the FSW test on2024 aluminum alloy. It was found that the heat productionwas not directly proportional to the force. Therefore, theybelieved that the viscous contact must be taken intoconsideration, which was important for further research onthe FSW. Zhang et al.8,9) have researched that the heat in thewelding process mainly came from the friction between thetool and the workpiece. The faster the welding speed and theweaker the stirring effect of the welding tool. This researchalso pointed out that the temperature field was basicallysymmetrical about the center of the weld, and the rotationof the shoulder could accelerate the flowing of the surfacematerial and the deformation of the material.

In this study, we explored the temperature field andmaterial flow characteristics of TC4 titanium alloy sheetduring friction stir welding by means of experiments andnumerical simulations, and proposed a formula for generatingheat of stirring pin, and gave a detailed calculation method.Using the results of numerical simulation, the mechanism ofmaterial flow was studied. The defect area is proposed, andthe formation mechanism of the defect area is explained bystudying the flow characteristics of the material.

2. Numerical Model Developed

Compared to traditional experimental methods, numericalsimulation has great advantages. The flow of material andthe change in temperature during the FSW process can beobserved very intuitively. In this research, it established theheat source model of FSW of TC4 plate, and set reasonablewelding parameters to study the heat generation, temperaturedistribution and material flow mechanism. The software ofMarc and Fluent were selected as finite element simulationsoftware in this study.

The heat generation in the FSW process is a complexprocess that a coupling process of friction heat generation andphase change latent heat. It is very difficult to analyze the+Corresponding author, E-mail: yfshen_nuaa@hotmail.com

Materials Transactions, Vol. 61, No. 12 (2020) pp. 2378 to 2385©2020 The Japan Institute of Metals and Materials

influence of various influencing factors in together. So thisstudy established a heat generation model that ignored thelatent heat of phase change during the welding process.Thus the heat in the welding process is considered that theheat was only generated by friction between the tool andworkpiece.10­12)

Table 1 and Table 2 are the thermal conductivity andspecific heat capacity of TC4 titanium alloy respectively.The software will automatically use these data during thesimulation. The measurement environment of these data isconsistent with the experimental environment.

The geometric model used to study the FSW of TC4 plateis showing in Fig. 1. The workpiece are TC4 plates with2mm thickness. The tool rotating speed is 800 r/min andwelding speed is 30mm/min. The axial pressure of frictionhead is 12MPa. The selection of all these parameters wasbased on the preliminary experimental studies of researchgroup.13) It defined that the plate surface where the workpiecewas in contact with the tool as a heat source. So as showingin Fig. 2, the heat source contained three parts: tool shouldersurface (SS), pin side surface (PSS) and pin tip surface (PTS).In the actual welding process, the tool shoulder has a certainpenetration amount, so a part of the shoulder side in contactwith the workpiece, but the heat contribution in this contactarea was small. So in this study, the heat generated in theshoulder side area was ignored. The tool sizes were presentedin Table 3.

According to the heat source formula proposed byHamilton et al.,14) the heat generation could be calculatedby the following:

Qtotal ¼ ¤Qsticking þ ð1� ¤ÞQsliding ð1ÞHere Qtotal represents the total heat generated during thewelding process, Qsticking represents the heat generated bythe plastic deformation of the workpiece during the welding

process, and Qsliding represents the heat generated by thefriction between the stirring pin and the workpiece duringthe welding process. Here ¤ is a contact state function. When¤ = 0, all heat is generated by the friction between theworkpiece and the tool. When ¤ = 1, all heat is generatedby plastic deformation of the material.13) The value of ¤ isan empirical parameter, which is determined by comparisonbetween a large number of numerical simulation experimentsand actual experiments.

As showing in Fig. 2, according to study of the FSW of2024 aluminum alloy by Schmidt,6,7) it believed that theviscous contact must be considered, so the basic condition ofthe shoulder and the workpiece was the partial sliding part.According to the study by Zhang et al.,14) the heat generatedat the shoulder surface (QSS) can be expressed as:

QSS ¼ ½½¤SSð¸b � ®PÞ þ ®P�Z R1

R2

2³r2dr ð2Þ

The surface heat flux density of the shoulder area is:

qSS ¼ QSS

³R12 � ³R2

2

Table 1 Thermal conductivity of TC4 titanium alloy.

Table 2 Specific heat capacity of TC4 titanium alloy.

Fig. 1 FSW model illustration.

Fig. 2 Schematic diagram of the friction tool.

Table 3 Dimensions of the tool.

Simulation and Experimental Study on Temperature and Flow Field in Friction Stir Welding of TC4 Titanium Alloy Process 2379

¼ 2½½¤SS¸b þ ½ð1��SSÞ®P� � ðR13 � R2

3Þ3ðR1

2 � R22Þ ð3Þ

Where ¤SS is the contact state variable, which was assumedto be 0.35 in this study; ® is the coefficient of friction. Thecoefficient of friction is measured in a high-temperaturefriction and wear test machine. The coefficient of frictionwill change with the change of temperature, but the rangeof change within the temperature range of the experiment isvery small, which is approximately 0.4; P is the plungepressure (Pa); ½ is the tool angular velocity (rad/s).

The heat generated at the pin side surface (QPSS) isexpressed as

QPSS ¼ 2³¤PSS½¸b

Z H

0

ðR3 þ h tan ¡Þ2dh

þ 2³®P1½ � ð1� ¤PSSÞcos ¡

Z H

0

ðR3 þ h tan ¡Þ2dh

¼ 2½³ðR23 � R3

3Þ3 sin ¡

½¤PSS¸b cos ¡þ ð1� ¤PSSÞ®P1� ð4Þ

P1 is the pressure on the side of the stirring pin (Pa).The surface heat flux density of the pin side surface (qPSS)

is:

qPSS ¼ QPSSZ H

0

2³ðR3 þ h tan ¡Þcos ¡

dh

¼ 2½ðR22 þ R3

2 þ R2R3Þ3ðR2 þ R3Þ

� ½¤PSSð¸b cos ¡� ®P1Þ þ ®P1� ð5ÞHere ¤PSS is the contact state variable, which is assumed tobe 0.5 in this study; H is the tool pin height; h is the heightvariable, ¡ is the conic angle; P1 and P are approximatelyequal.

The heat generated at the pin bottom surface (PBS) isexpressed as:

QPBS ¼ ¤PBS½

Z R3

0

ð¸b � ®PÞ � 2³r2drþ ®P½

Z R3

0

2³r2dr

¼ 2³½R33

3½¤PBS¸b þ ®Pð1� ¤PBSÞ� ð6Þ

The heat flux density at the pin bottom surface (PBS) isexpressed as:

qPBS ¼2½R3

3½¤PBSð¸b � ®PÞ þ ®P� ð7Þ

Here ¤PBS is the contact state variable, which is assumed to be0.35 in this study.

It also should be considered that the heat exchange wasbetween the workpiece and the external environment. Theway of the heat exchanged between the surface of workpieceand the external environment was mainly convective heattransfer mode. The convection heat transfer formula is:

k@T

@z

����top

¼ htðT� T0Þ ð8Þ

Here ht is the heat dissipation coefficient of the surface of thework-piece, which was assumed to be 30Wm¹2K¹1 in this

study. T0 is the ambient temperature, which is assumed to be300K in this study, k is the thermal conductivity of weldmaterials.15)

The boundary conditions at the bottom of the workpieceare formulated as:

k@T

@z

����bottom

¼ hbðT� T0Þ ð9Þ

Here hb is the heat dissipation coefficient at the bottom ofthe workpiece, which is assumed to be 50Wm¹2K¹1 in thisstudy. The values of ht and hb are estimated based on thechange speed of the workpiece temperature and the area ofthe workpiece.

The thermal energy conservation equation is given as:16)

μCp

@ðuiTÞ@xi

¼ �μCp®weld

@T

@xiþ @

@xik@T

@xi

� �þ Qv ð10Þ

Equation (10) is a quasi-steady heat conduction equation;here Cp is the specific heat capacity, which is obtained byfitting the material property parameters; ®weld is the weldingspeed; Qv is the energy source term, which is defined as:

Qv ¼QPSS þ QPBS

Vpin

ð11Þ

Here Vpin is the volume of the pin.The fluidity of the material is also an important parameter

in this study. The governing continuity equations for thematerial flow can be expressed as:17)

@ui@xi

¼ 0 ð12Þ

μ@ui@uj@xi

¼ � @P

@xiþ @

@xi©@uj@xi

þ ©@ui@xj

� �� μ®weld

@uj@xi

ð13Þ

where xi is the distance along the i direction, i = 1, 2, 3representing x-direction, y-direction, and z-direction respec-tively; ®i, ®j are the velocity components along the i and jdirections respectively; ®weld is the welding speed; μ is thematerial density; © is the non-Newtonian viscosity and Pis the plunge pressure.

The non-Newtonian viscosity © can be expressed as:

© ¼ ·ðT; �¾Þ3�¾

ð14Þ

where · is the flow stress; T is the temperature, effectivestrain rate. �¾ can be regarded as the mesh strain rate innumerical simulation.

According to a study by Sheppard T et al.,18,19) the flowstress can be expressed as:

© ¼ 1

3�¾¡ln

�¾ exp

�Q

RT

�

A

26664

37775

�1=n8>>>><>>>>:

þ 1þ�¾ exp

�Q

RT

�

A

0BBB@

1CCCA

2=n26664

37775

1=29>>>>=>>>>;

ð15Þ

Y. Qi, J. Li, Y. Shen and W. Hou2380

Where A, ¡, and n are temperature independent constant; Q isthe temperature-independent activation energy. Equation (15)was called by the user define function (UDF). The materialparameters are presented in Table 4.20,21)

3. Results and Discussion

3.1 Temperature fieldAt different stages of welding, the overall distribution of

the temperature field is the same, and the peak temperaturewill change. The main purpose of this study is to qualitativelyanalyze the distribution characteristics of the temperaturefield. Therefore, the temperature field at the stable stage ofwelding is selected for research. Figure 3 is the temperaturefield distribution diagram under three process parameters.Except for the difference in peak temperature, the temper-ature distribution under different process parameters isroughly the same. Since the thermal conductivity of TC4 isrelatively poor, the temperature gradient around the frictionhead is difference. Behind the pin, the workpiece materialis affected by heat conduction and heat input. So the hightemperature area is relatively large in this area, and thetemperature is relatively lower due to the influence of thethermal conductivity. After the tool passed, the temperaturedecreased slowly and the temperature gradient is relativelydecreased too. The material in front of the tool was onlyaffected by the friction, the temperature rises rapidly in ashort time, and the temperature gradient is also relativelylarger. The two sides of the tool are transitional zones,showing a distinct asymmetric temperature profile betweenthe AS and RS. This is because the material flow directionof AS is opposite to the direction of tool rotation. Theseresults were also in well agree with the literature.22,23)

In order to explore the temperature distribution of thewelding zone in the welding process and verify theeffectiveness of the model, a series of characteristic pointswere selected to test the temperature change in the actualFSW trials and simulation model. As shown in Fig. 4, aseries of points are taken along line a and line b on the lowersurface of the workpiece, line a is 1mm from the center ofthe pin, line b is 6mm from the center of the pin. In the FSWexperiment, the temperature for these points was measuredby thermocouple thermometers. The temperature change testresults are shown in Fig. 5. From this picture we can see thatthe actual temperature on the retreating side is slightly higherthan the temperature of the simulation experiment. The causeof this difference may be that the material on the retreatingside is heated by extrusion, which is ignored in the simulationexperiment. The experimental results are well agreements

with the numerical results, which indicate that this modelcould be used to predict the temperature profiles in realFSW process. As can be seen from the Fig. 6, the surfacetemperature of the workpiece is characterized by rising firstlyand then falling from the forward side to the backward side.Clearly, the peak temperature of line a is not at the center ofthe weld, but at a distance of about 4mm from the weld

Table 4 Material constants and property values of the TC4 titanium alloy.

(a)

(b)

(c)

Fig. 3 Distribution of the simulated temperature: (a) 800 r/min, 30mm/min, (b) 600 r/min, 30mm/min, (c) 400 r/min, 30mm/min.

Simulation and Experimental Study on Temperature and Flow Field in Friction Stir Welding of TC4 Titanium Alloy Process 2381

line. This is because of the farther away from the center ofshoulder, the greater the line speed of the shoulder, so themore heat generated by the friction. Thus the heat generatedat the edge of the shoulder is larger than that of the center ofweld. But the heat lost is also larger at the edge of tool, so thehighest temperature point isn’t at the edge line of weld butbetween the center weld line and the edge line. The heatinput at the midpoint of line b is relatively more, the peaktemperature of line b is higher than the peak temperature ofline a, and the position of the peak temperature is closer tothe soldering surface.

In order to fully understand the temperature change in theworkpiece during the welding process, some of the featurepoints are taken for analysis. As shown in Fig. 7(a), somepoints on the upper surface with a distance of 4mm fromthe center of the weld were selected to investigate thetemperature characteristic. The interval between each pointwas 5mm. It can be seen from the Fig. 7(b) that thetemperature curves at different points are roughly with thesame trend, and the rate of temperature rise is greater thanthat of temperature decrease. This is due to the thermalconductivity of the TC4 is relatively poor, so the decreaseof temperature is relatively slow and the temperature curve isrelatively smooth.

3.2 Flow fieldFigure 8 is a simulation model for the flow field of FSW of

TC4. In this model, the flowable area (FCR) represents the

welding area. The width of the FCR is set to be 1mm widerthan the shoulder and the depth of the FCR is 0.5mm deeperthan the weld. The groove region (GR) of workpiece meansnot welded. So there is almost no material flow in this area,thus the GR has the function of transferring heat andmaintaining the plasticized material.

In this experiment, it assumed that heat generation andmaterial flow were stable in the welding. And the elasticdeformation of the material was neglected, and the plasticizedmaterial was regarded as an incompressible viscoplasticmaterial. It was also considered that only plastic deformationand fluid flow occurred during the welding process.

The materials in the FCR and GR regions are the sameduring the welding process and are considered viscoplasticfluids in this study, the difference being the difference inviscosity. For FCR, viscosity is primarily dependent ontemperature and strain rate. The GR material did not flow andwas set to a very large value in the experiment.

During the FSW process, the material flows plasticallyunder the influence of the tool, and the flow velocity of thematerial is affected by the contact conditions of the tool andthe workpiece. Figure 9 is the simulation result of the FSWofTC4 plate with a given tool. As can be seen from Fig. 9(a),the entire material stream is funnel-shaped, similar in shapeto the surface in contact with the agitating head and theworkpiece. As can be seen from Fig. 9(b), the materials nearthe tool surface have a higher flow velocity, and the velocitynear the outer circle of tool is greater than that of inside.

Fig. 4 Schematic diagram of the lower surface feature points of theworkpiece.

(a) (b)

Fig. 5 Comparison between calculated and experimental temperature of the measured points: (a) ai points, (b) bi points.

Fig. 6 Temperature of characteristic point of workpiece bottom surface.

Y. Qi, J. Li, Y. Shen and W. Hou2382

The velocity value generally shows the variation tendency.As the depth increases and the distance from the axis ofthe stirring head decreases, the flow velocity of the materialgradually decreases. This is because the linear velocity ofthe tool surface increases as the diameter increases, and theflow of materials is driven by the tool, so the velocity of thenodes of materials also exhibits the same characteristics. So itcould be concluded that the friction tool is the main source ofmaterial momentum in the FSW process.

Figure 10 is the two-dimensional material flow behaviorof the weld zone under three process parameters. In theFig. 10(a), it indicates that the material in the front of the toolflows to the RS side of the tool as the tool rotating, and thatfinally deposits behind the tool. This process consists of threeparts: (1) the material is softened by the heat conductionbefore it comes into contact with the rotating tool; (2) thesoftened material sticks to the surface of the tool and flowswith the rotation of the tool before final deposition; (3) withthe tool continuous move forward, the material that behindthe tool is split, and form a moon-like cavity. When theflowing material passes through the region A, it may beaccumulated and overflow in the region A, which is the mainreason for the formation of the welding flash during thefriction stir welding process.8) As shown in Fig. 10(c), thefluidity of the region B is poor, and defects such as groovesand voids were generated usually in this region. This regionis called a defect-prone area (DPR). Thus the cause of defects

Fig. 8 Model of the flow characteristics of FSW.

(a)

(b)

Fig. 9 Material flow velocity in the weld zone (a) overall distribution of velocity vectors (b) material flow velocity of weld cross section.

(a)

(b)

Fig. 7 Schematic diagram of simulation result of temperature: (a) characteristic point distribution, (b) profile of temperature over time ofcharacteristic point.

Simulation and Experimental Study on Temperature and Flow Field in Friction Stir Welding of TC4 Titanium Alloy Process 2383

such as grooves and voids was insufficient fluidity of theplastic materials. As can be seen from Fig. 10(a) andFig. 10(b), as the rotational speed increases, the fluidity ofthe material in the B region becomes better. Figure 11 is thejoint morphology under three process parameters. It can beseen from Fig. 11(c) that there is a tunnel defect on the ASaside and some flash on the RS aside, which is well agreewith the simulate result that the groove defect usuallygenerates in DPR region. At a rotation speed of 800 rpm, theresults of numerical simulation show that the flow field linesin area A are very dense. It can be seen from Fig. 11(a) thata large number of flashes appeared; At a rotation speed of

600 rpm, a welded joint with good surface formation wasobtained. So it means that this simulation model for FSW ofTC4 alloy is valid.

4. Conclusions

A detailed calculation model for the heat generation ofthe FSW of TC4 was developed. Combined with numericalsimulation and welding experiment results, the temperaturefield distribution and flow characteristics during the frictionstir welding process were analyzed, and the followingconclusions could be drawn:(1) In this study, the model of TC4 flat titanium alloy

friction stir welding was established. Through themeasurement and comparison of the temperature duringthe welding test, it was found that the temperature valueand temperature curve of the simulation experimentwere in good agreement with the welding experiment,which indicated the numerical model is reasonable. Thismodel could be used for the prediction of temperatureduring FSW.

(2) Both numerical simulation and actual measurementresults show that there is an asymmetric temperaturedistribution between AS and RS, but this asymmetry isrelatively small. The peak temperature on the forwardside is slightly higher than the peak temperature on thereverse side.

(3) The peak temperature of the weld zone is not at thecenter of the weld, nor at the edge of the tool, butbetween the center weld line and the edge line, whichwas due to the joint action of friction heat and heatlost.

(a)

(b)

(c)

Fig. 10 Material flow behavior: (a) 800 r/min, 30mm/min, (b) 600 r/min,30mm/min, (c) 400 r/min, 30mm/min.

(a)

Flash

Flash

RS

AS

Welding velocity

Welding velocity

RS

ASTunnel defect

(b)

Flash

(c)

RS

AS

Welding velocity

Fig. 11 Surface morphology of welding sample: (a) 800 r/min, 30mm/min, (b) 600 r/min, 30mm/min, (c) 400 r/min, 30mm/min.

Y. Qi, J. Li, Y. Shen and W. Hou2384

(4) The soften materials were accumulated and overflow inthe retreat side during FSW process, which was themain reason for the formation of the welding flashduring the FSW process. The fluidity of the defect-prone area is poor, and defects such as grooves andvoids were generated usually in this region actually,because of insufficient fluidity of the plastic materials.

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Simulation and Experimental Study on Temperature and Flow Field in Friction Stir Welding of TC4 Titanium Alloy Process 2385