residual stress simulia.pdf

8/13/2019 residual stress simulia.pdf

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2004 ABAQUS Users Conference 347

Residual Stresses in Friction Stir Welding

Z.M. Hu, P. Blackwell & J.W. Brooks

Future Systems Technology

QinetiQCody Technology Park

Ively Road, Farnborough

Hampshire, GU14 0LX, UK

Abstract: Friction stir welding is an innovative joining process being developed for use in

aerospace manufacturing. Like most welding processes there exist problems with post-weld

residual stresses and subsequent geometrical distortion. The residual stress-state of a weldedcomponent can be modified using suitably applied mechanical tensioning and thus distortion can

be reduced or eliminated. In the present study, the finite element software, ABAQUS, has been

used to model the friction stir welding process of aluminium alloy plates and to investigate theeffects of process conditions on the residual stress. The finite element modelling has been carried

out in two steps: thermal analysis and stress analysis. In the thermal analysis, the heat input from

the friction stir welding operation was modelled using a surface heat flux. The calculated

temperature field/history was then used for stress analysis. The main focus of the work was to

examine the residual stresses generated from various welding conditions. The results suggested

that the application of appropriate mechanical tensioning during welding would reduce theresidual stress or even change the residual stresses from tensile state to compressive state. The

calculated residual stresses were also compared with experimental measurements and goodagreements were found between them.

Keywords: Friction Stir Welding, Residual Stress, Finite Element, Heat Transfer.

1. Introduction

Friction stir welding (FSW) is an innovative joining process developed in 1991 by The Welding

Institute (UK) primarily for aluminium alloys (Thomas et al, 1991). In this welding process, a

rotating welding tool is driven into the material at the interface of two adjoining plates and thentranslated along the interface. Friction stir welding offers ease of handling, precise external process

control and high levels of repeatability, thus creating very homogeneous welds.

As a solid-state welding process, FSW reduces the problems of solidification cracking, porosity,

distortion, etc. associated with fusion welding. However, the rapid heating and cooling in localised

regions of the work during welding still result in thermal expansion and contraction, which cause

residual stresses in the weldment and distortion of the welded assembly. Experiments have shown

that the use of cooling systems or mechanical tensioning during the welding process can reduce theresidual stresses. In the current study, a finite element model has been developed to help improve

the understanding of this effect.


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348 2004 ABAQUS Users Conference

Despite significant advances in the application of FSW as a new welding technique for aluminium

alloys, a fundamental knowledge of the process is still being developed. Considerableexperimental work has been reported in the published literature, but modelling work of the stir

welding process appears to be relatively scarce.

Heat input from friction stir welding can be described using a distributed heat source, based on themodel proposed by Goldak et al (1983). This heat distribution model ignores convective heat flow

and restricts heat transfer to conduction, thus simplifying the problem. Hence this model and its

refinement have been widely used for its simplicity. An analytical heat flow model for friction stirwelding was developed by Gould et al (1996). It was based on the well-known Rosenthal equation

which describes the quasi-steady state temperature field in a semi-infinite plate due to a moving

heat source. This relates the temperature field in the weld to process variables such as the tool and

welding speeds. Previously, an energy balance approach has been used to approximately predictthe shape of the weld and temperatures within the weld (Stewart et al, 1998). The Finite Difference

method has also been used for thermal modelling of FSW (Song et al, 2003), the three-dimensionalheat transfer model was described in a moving coordinate system to reduce the difficulty of

modelling the moving tool.

Peel et al (2003) investigated the microstructure, mechanical properties and residual stress by

examining four aluminium AA5083 friction stir welds produced under varying conditions. It was

found that the weld properties were dominated by the thermal input rather than the mechanicaldeformation by the tool. Ulysse et al (2002) used three-dimensional visco-plastic FE to model the

stir welding process with a focus on butt joints for aluminium thick plates. Parametric studies have

been conducted to determine the effect of tool speeds on plate temperatures and to validate themodel predictions with available measurements.

A three-dimensional finite element analysis was used to study the thermal history and thermo-mechanical processes in the butt-welding of aluminium alloy 6061-T6 (Chen et al, 2003). Themodel incorporated the mechanical reaction of the tool and thermo-mechanical process of the

welded material. The heat source incorporated in the model involved the friction between the

material, the tool and the tool shoulder. The thermal history and the evolution of longitudinal,lateral, and through-thickness stress in the friction stirred weld were also simulated. The X-ray

diffraction (XRD) technique was used to measure the residual stress of the welded plate and to

validate the model.

Investigation of the potential of mechanical tensioning to reduce the magnitude of residual stresses

in Friction Stir Welds (FSW) and to eliminate buckling distortion has been carried out at BAE

SYSTEMS (Williams et al, 2004). Welds were produced from the aluminium alloy AA2024, with

different levels of tensile stress applied either during or after welding. The resulting welds havebeen characterised in terms of their out-of-plane distortion, residual stresses and microstructure.

Mechanical tensioning has been shown to eliminate buckling distortion in FSWs 3.2mm thick

when the plates are stretched to ~35% of the materials yield stress during welding. Themagnitude of the tensioning stress required varied according to the FSW parameters that were


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used, with welds produced at higher line energies requiring higher mechanical tensioning stressesto overcome the weld-induced distortion. By mechanically tensioning plates during FSW, defect-

free welds have been produced at much faster tool traverse speeds.

In the present study, the finite element software ABAQUS has been used to model the friction stirwelding of two aluminium alloy plates. The welding was simulated with and without mechanical

tension. The finite element modelling has been carried out in two steps: thermal analysis and stress

analysis. In the thermal analysis, the heat input from the friction stir welding operation wasmodelled using a surface heat flux. The calculated temperature field/history was then used for

stress analysis. The main focus of the work was to examine the residual stresses generated from

various welding conditions.

2. FSW process and FE model

2.1 Material Properties

Thermal properties of the aluminium alloy are shown in Figure 1, which includes thermal

conductivity, specific heat and thermal expansion coefficient.

The stress and strain distribution and distortion of a weld is strongly dependent on the plastic yieldstrength of the material. As the hot zone is moving and generating rapid temperature changes, the

plastic properties of the plate also change. The plastic properties were classified in three different

zones: parent, heat affected zone (HAZ) and weld zones, for a more accurate prediction of the

residual stress. The plastic flow properties in the three zones of a weld were defined by testing the

relevant material from those regions. In the stress analysis, the ABAQUS user subroutine;USDFLD, has been employed to deal with the material property changes during the modelling

process. It was assumed that (temperature corrected) parent metal properties could be used forthose regions of the plate that did not experience temperatures above 250C. HAZ properties were

used for those regions that went above 250C but remained below 502C. Beyond this the material

starts to melt and weld nugget properties would have been used however in FSW the maximum

temperature remains below the solidus (i.e. 502C).


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Figure 1 Thermal properties of the aluminium alloy

Thermal conductivity of 2024-T3

100

125

150

175

200

0 100 200 300 400 500 600 700

Temperature (oC)

Conductivity(W/m)

Thermal expansion of 2024-T3

2.00E-05

2.20E-05

2.40E-05

2.60E-05

2.80E-05

3.00E-05

0 100 200 300 400 500 600 700

Temperature (oC)

Expansioncoefficient

Specific heat of 2024-T3

800

900

1000

1100

1200

1300

1400

0 100 200 300 400 500 600

Temperature (oC)

Specificheat(J/kg)


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Figure 2 FE mesh used for thermal and stress analysis of FSW

2.2 FE mesh model

The welding process is analysed by modelling the thermal effects from the friction stir tool and

calculating the stresses induced from the thermal fields.

The simulation involved the welding of two aluminium alloy plates of 375mm 125mm 6mm.To investigate the effects of mechanical tension on the residual stresses, one simulation was

carried out with no mechanical tension applied, and a second run with an applied tensionequivalent to 70% of the yield stress at room temperature. Because of symmetry, only one side of

the weld had to be modelled. Figure 2 depicts the FE mesh for the plate. The holes at either end

were used for the application of the mechanical tension.

The finite element modelling has been carried out in two steps: a thermal analysis and a

subsequent stress analysis. The thermal analysis was used to calculate the temperature field and

history caused by the heat source from the friction stir welding tool. Stress analysis was to predict

the stress and distortion according the temperature field from the thermal analysis. The same meshwas used for thermal and stress analyses. Seven section points were used across the thickness of

the shell elements.

2.3 Thermal analysis

A pure heat transfer analysis was carried out considering the heat generated from the FSW tool

and heat loses due to convection and radiation on the free surfaces. Experiments using the sameprocessing conditions were conducted at BAE Systems: the temperatures inside the welded plates

were recorded using thermocouples at several positions and surface temperatures near the welding

region were measured using infrared imaging.

The heat input is modelled using a surface heat model where a Gaussian type distribution is

assumed for the heat source defined in the following equation and shown in Figure 3.

Weld centreline


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Figure 3 Surface heat model flux model

+

=

2

2

2

2

expb

y

a

xFFAKF

where

a and b are the principal radii of an ellipse, a = b = 0.5d

AK is the power density and its value is determined by matching the measured and calculatedtemperatures at thermocouple positions

F is a factor defining the proportion of the heat source contained within the ellipse

Standard radiation and convection exist on the top and bottom surfaces of the plates. ABAQUS

user subroutines to implement surface heat flux (DFLUX) and convection coefficient (FILM) werewritten for the analysis. The temperature field from thermal analysis was written in an ABAQUS

result file to be used for the stress analysis.

2.4 Stress Analysis

Stress analyses were conducted for the above FSW using the temperature field from the thermal

analysis. The device to apply mechanical tension during FSW is shown Figure 4, where the twosides of the plates are clamped during welding and mechanical tension can be applied on the two

ends. The mesh structure is the same as for the thermal analysis, but uses different types of

elements and boundary conditions. As it was difficult to directly apply pressure on the edges of theshell elements, a displacement corresponding to the specified amount of mechanical tension was

applied at the end of the plate.


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(b) FE calculated temperature contours on the top surface (oC)

(a) Infra-red image of the top surface of the FSW

21.2C

351.0C

200

LI01

21.2

351

0

50

100

150

200

250

300

350

0 50 100 150 200 250 300 350 400

Time (s)

Temperature(oC)

T1 T3T2 T4

T5

T6

FE

Test

Figure 5 Temperature profiles during FSW

Figure 6 Measured and calculated temperatures at thermocouple positions


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Figure 7 Residual stress of FSW plate with no mechanical tension (MPa)

Residual stress in mid-layer (FSW 2024 plate)

-50

-25

0

25

50

75

100

125

150

-125 -100 -75 -50 -25 0 25 50 75 100 125

Distance from weld centreline (mm)

Stress(MPa)

y

no mechanical tension

x

longitudinal stress x transverse stress y

(a) Contour plots of stresses in the mid layer

+162

+150

+100+50

0

-50

-100

-150

-215

+80

+50

0

-50

-100

-150

-200

-248

(b) Stress values on the central transverse section


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Figure 8 Residual stress of FSW plate under mechanical tension (MPa)

+100

+50

0

-50

-100

-150

-200

-250

-300

-426

+122

+100+50

0

-50

-100

-150

-200

-250

-300

-412

longitudinal stress x transverse stress y

(a) Contour plots of stresses in the mid layer

(b) Stress values on the central transverse section

Residual stress in mid-layer (FSW 2024 plate)

-60

-50

-40

-30

-20

-10

0

10

20

-125 -100 -75 -50 -25 0 25 50 75 100 125

Distance from weld centreline (mm)

Stress(MPa)

y

mechanical tension of 70% yield stress

x


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It has been shown, therefore, that by judicious use of mechanical tensioning it is possible to notonly reduce, but to partially reverse the residual stress distribution in FSW plate. This has

important implications for not only FSW but also other welding techniques. The results shown

have been extensively validated using neutron diffraction measurements to determine the actualresidual stress levels. An excellent agreement was obtained. More recent work has demonstrated

that a similar technique works equally well with arc welds.

The overall concept has important implications for the reduction of weld distortion. Usually any

distortion is removed following welding and this can be a costly and labour-intensive process.

Mechanical tensioning during the welding process could eliminate the need for this and thus

generate a considerable time and cost saving. If the residual stress levels can be controlled thenthis also has implications for the stress-corrosion properties in those alloys prone to this particular

problem. High tensile residual stresses would tend to generate poor stress-corrosion resistance. If

these stresses can be significantly reduced or transformed into compressive stresses, the stress-

corrosion and fatigue resistance will be improved.

4. Conclusions

From the analysis of the friction stir welding of aluminium alloy plates, the following conclusions

can be made;

Friction stir welding can be modelled using finite element packages like ABAQUS

Heat source from FSW can be modelled using the surface heat flux model

Judicious use of mechanical tensioning during FSW will improve the state of the residual

stresses and reduce distortion.

5. Acknowledgement

The authors gratefully acknowledge funding from the Department of Trade and Industry (DTI)

through CARAD. They would also like to thank the project partners: BAE SYSTEMS, AIRBUSUK and BOC Gases Limited for their collaboration and technical help during the project.

6. References

1. Chen C.M. and Kovacevic R., Finite Element Modeling of Friction Stir WeldingThermaland Thermomechanical Analysis, International Journal of Machine Tools & Manufacture,

Vol.43, pp. 13191326, 2003

2. Goldak J., Chakravarti A. and Bibby M., A New Finite Element Model for Welding HeatSources, Metallurgical Transactions, pp. 299-305, 1983

3. Gould J.E., Feng Z. and Ditzel P., Preliminary Modelling of the Friction Stir-WeldingProcess, Conference on Joining of High Performance Materials, Columbus, Ohio, p. 297,

1996.


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4. Peel M., Steuwer A., Preuss M.and Withers P.J., Microstructure, Mechanical Properties andResidual Stresses as a Function of Welding Speed in Aluminium AA5083 Friction Stir Welds,Acta Materialia 51, pp. 47914801, 2003

5. Song M. and Kovacevic R., Thermal Modelling of Friction Stir Welding in A Movingcoordinate System and Its Validation, International Journal of Machine Tools & Manufacture,

Vol. 43, pp. 605615, 2003

6. Stewart M.B., Adams G.P., Nunes A.C. and Romine P., A Combined Experimental andAnalytical Modeling Approach to Understanding Friction Stir-Welding, Developments in

Theoretical and Applied Mechanics, SECTAM XIX, p472, 1998.

7. Thomas, W.M., Nicholas E.D., Needham J.C., Murch M.G., Templesmith P. and Dawes C.J.,Friction Stir Butt Welding International Patent Application No. PCT/GB92/02203 and GB

Patent Application No. 9125978.8, December 1991.

8. Williams, S.W., Price, D.A, Wescott, A., Harrison, C.J.C., Staron, P. and Koak, M.,Distortion Control in Welding by Stress Engineering, Welding and Brazing for Aerospace

Structures - Modern Applications and Materials for New and In-Service Parts, 12/13 May

Berlin 2004

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