International Journal of Mechanical Engineering and Materials Sciences, 4(1) January-June 2011, pp. 19-24

Corresponding author: hspatil12@rediffmail.com

Structural Stress Analysis of Friction Stir Welding Tool Pin Profiles

H. S. PATIL1 AND S. N. SOMAN2

1Department of Mechanical Engineering, Mahatma Gandhi Institute of Technical Education &Research Centre, Navsari - 396450, India

2Departments of Metallurgical & Material Engineering Faculty of Engineering &Technology M. S. University of Baroda, Vadodra-1, India

ABSTRACT

The presented paper deals with structural stress analysisof four different tool pin profiles- cylindrical, conical,square, and triangular. All these tools are made from samematerial cold work die steel with different tool pingeometry. In operation, tools are subjected to lateralbearing loads during tool transverse motion as well asthe frictional torsion loads caused by tool spinning. Lateralloads vary with respect to tool travel speed while frictionaltorsion loads are invariant in the material plastic state atsolidus temperature. Inclusive ways of judging thestrength of these tools have been established bycomparing the stresses in these tools when all subjectedto the same loading conditions. Physical tool modelssubjected to loading conditions as happened in actual FSWprocess are analysed by means FEM tool of structuralanalysis using the ANSYS12 package. The paper presentsthe material deformation and Von-Mises stresses inducedin tool pin profiles during the friction stir welding process.

Keywords: FSW process, Tool profiles, Stress analysis,Finite element method

INTRODUCTIONFriction Stir Welding (FSW) was invented andpatented by Thomas1 et al. (1991) of The WeldingInstitute in Cambridge, UK and has been widelyrecognized for its ability to provide greatly improvedweld properties over conventional fusion welds.Figure 1 illustrates the working principle of FSWprocess. During the FSW process, the pin (probe) of ashouldered tool is slowly plunged into the jointbetween the two materials to be welded and rotatedat high speed. The resulting friction creates aplasticized shaft of material around the pin. As thepin moves forward in the joint, it stirs or crushes, theplasticized material, creating a forged bond, or weld.Although the FSW process is more reliable andmaintains higher material properties thanconventional welding methods, two major drawbackswith the initial design impacted the efficacy of theprocess, the requirement for different-length pin toolswhen welding materials of varying thickness and the

reliance on a pin tool that left a keyhole at the end ofthe weld. The latter was a reliability concernparticularly when welding cylindrical items such asdrums, pipes, and storage tanks.

Figure 1: Working Principle of FSW Process

Several papers have been written on the FSWprocess, Dawes & Thomas2-3 who described the FSWprocess, summarizing its advantages anddisadvantages. Elangovan & Balasubramanian4-5

investigated the influences of different types of pinprofile & effect of rotational & welding speed onmechanical-metallurgical properties of aluminiumalloy of 2xxx series. Cavaliere6 et al. studied aboutwelding of dissimilar alloy and characterized thewelded plates mechanically and metallurgically.Balasubramanian7 established relationship betweenthe base material properties and FSW processparameters by using five different grades ofaluminium alloys.

However, although considerable experimentalpublished works have been reported, there arerelatively few papers about modeling the FSWprocess. Diego H. Santiago8 et al developed acomputational 3-D finite element model of the FSWprocess that describe the main aspects of the processand to show and evaluate the computationalrequirements needed for the appropriate capture ofthe main phenomena involved. Z. Zhang & H. Zhang9

presented numerical model that investigate energydissipations and residual stress distributions in

20 International Journal of Mechanical Engineering and Materials Sciences

friction stir welds. Nandan10 et al. modeled the 3-Dvisco-plastic flow of metals and the temperature fieldsin friction stir welding. In this the equations ofconservation of mass, momentum, and energy weresolved in three dimensions using spatially variablethermo-physical properties and non-Newtonianviscosity. Hosein11 et al studied on computationalfluid dynamics (CFD) model for simulating thematerial flow and heat transfer in the FSW of AA6061alloy and that numerical model has been used foranalysis of viscous and inertia loads applied to theFSW tool by varying the welding parameters.

Nevertheless, there is a major concern aboutlifespan of FSW tools as unexpected tool failures inthe middle of an joining operation can be harmful tothe whole operation especially for the waste of somany prepared parts and the probe of the tool orbroken pin left inside the work-piece that is notnormally compatible with the properties ofsurrounding material. To improve these mishapscaused by tool fracture in FSW welding in reality ismore difficult than what the conventional fusionwelding techniques can change, consequently thereis a strong need to maintain safety strength of its toolswhenever FSW are put into service. The more stressanalysis and better tool strength, the less cost andbetter quality of welds can we produce for use. So farmost studies and researches of FSW focus on themagnificent material properties could be generatedby the solid state melting processes, little effort hasbeen done to explore into the mechanics and thestructural ability of the tools to withstand the big loadthat this tool’s probe unavoidably have to meet. Thusthe purpose of this study is very important for FSWutilization.

THE FINITE ELEMENT MODELPro/ENGINEER part enables to design models assolids in a progressive three-dimensional solidmodeling environment. Solid models are geometricmodels that offer mass properties such as volume,surface area, and inertia. If manipulate any model,the 3-D model remains solid. We can design manydifferent types of models in Pro/ENGINEER. Themodel of all tools was created by using PRO/ENGINEER 3D product development software. Themodels developed in Pro/E for the Cylindrical-Conical and Square-Triangular tool tip are shown infigure 2 & 3 respectively.

STRESS STATE OF PLASTIC ZONE AROUNDTOOL PINThe stress states of the material encloses the tool pinis of interest, because the heating is accomplished by

friction between the tool and the workpiece andplastic deformation of workpiece. The localizedheating softens the material around the pin andcombination of tool rotation and translation leads tomovement of material from the front of the pin to theback of the pin. As a result of this process a joint isproduced in ‘solid state’ that is a plastic state and canbe better understood by the theory of plasticity.

MAXWELL-HUBER-HENCKY-VON MISESTHEORYThe von Mises Criterion12 is often used to estimatethe yield of ductile materials. Von mises stress is usedas a criterion in a determining the onset of failure inductile materials. The failure criterion states that thevon mises stress σVM should be less than the yieldstress Y of the material. In the inequality form, thecriterion may be put as;

σVM ≤ σY (1)

In material science and engineering the von misesyield criterion can be also formulated in terms of thevon mises stress or equivalent tensile stress, a scalarstress value that can be computed from the stresstensor. In this case, a material is said to start yieldingwhen its von mises stress reaches a critical valueknown as the yield strength, σy. The von mises stressis used to predict yielding of materials under anyloading condition from results of simple uniaxialtensile tests. The von mises stress satisfies the propertythat two stress states with equal distortion energyhave equal von mises stress.

Mathematically the yield function for the vonmises condition is expressed as:

Figure 2: 3-D Models of Cylindrical- Conical Tool Pin

Figure 3: 3-D Models of Square-Triangular Tool Pin

Structural Stress Analysis of Friction Stir Welding Tool Pin Profiles 21

= − =2 2( ) 0f J J k (2)

An alternative form is:

f(J2) = J2 – k2 = 0 (3)

Where, J2 is invariants of stress state, k can be shownto be the yield stress of the material in pure shear. Asit will become evident later that at the onset ofyielding, the magnitude of the shear yield stress inpure shear is √3 times lower than the tensile yieldstress in the case of simple tension. Thus, we have

σ=

3yk (4)

Furthermore, if we define the von mises stress as

σ = 23 ,r J the von mises yield criterion can beexpressed as:

= − σ = σ − σ =

2 2( ) 3,0y

v y

f J J(5)

Substituting J2 in terms of the principal stressesinto the von mises criterion, we have

σ − σ + σ − σ + σ − σ = = σ2 2 2 2 21 2 2 3 1 3( ) ( ) ( ) 6 2 yk (6)

Or

( )σ + σ + σ − σ σ − σ σ − σ σ = = σ2 2 2 2 21 2 3 1 2 2 3 1 3 3 yk (7)

In cases of plane stress, σ3 = 0. The von misescriterion reduces to,

σ − σ σ + σ ≤ σ2 2 21 1 2 2 y

This equation represents rotated ellipse in theσ1, σ2 plane as illustrated in the figure 4.

As a matter of fact, from the point of view of strainenergy stored in yielding deformation, it is possibleto separate it into energy by bulk modulus and thatby shear modulus, Hencky12 in 1924 has disclosed thatdistortion energy U is a function of shear modulus Gand yielding stress σy, that U = 2σ2

y /12G. Thus it isreasonable to consider that only the deviator stressesin the plastic zone will affect the tool’s sheardeformation and strength, as the same slip systemsand dislocation movements of normal ductilematerials also govern deformations of most metallictools. Hydrostatic pressures are thus deemed asharmless to the structure since they won’t cause anypermanent deformation and the failure accrued withit. In addition to bounding the principal stresses toprevent ductile failure, the Von Mises criterion alsogives a reasonable estimation of fatigue failure,especially in cases of repeated tensile and tensile-shearloading.

NUMERICAL METHOD FOR STRUCTURALANALYSIS OF TOOLSAn ANSYS structural analysis tool is applied for thepurpose, with suitable geometric parameterdefinitions in physical model establishments in thepreprocessor and solution modules, we can generateschemes to repeatedly change geometries for theproblem and switch loading conditions for solutionprocesses too. The results from postprocessor moduleof the parametric programming as discussed willproduce enough sets of data about the maximum VonMises’ stress that will facilitate our purpose ofgathering the information about tool strength/stressand its relation with characteristic geometric featuresand loading types. Finite element method is thenumerical means here to solve FSW tool for its stressstates and get better strength of the tool by changingtool geometries in calculation, such as radius of theshoulder and the various cross section of pin. VonMises stresses are calculated therewith for finding outthe weakest spot of tool for the bearing load due tothe transverse pin motion.

The analysis is based on the following twoassumptions:

1. The actual pressure distribution that isdistributed pressure load with a half circlepattern along one half portion of the pincircumference, where the peak value can bejust the yielding stress. The rest of the pincircumference is supposed to be free frompressure loads. This is illustrated in figure 5.

2. Eliminate hydrostatic pressure from the realnormal loads to the pin surface, as it plays noFigure 4: Von Mices-Hencky Theory

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role in permanent deformation due toyielding of ductile materials

In this analysis we are considering tool shoulderwith all degree of freedom as constraint and load isapplied on half area of the tool probe and assumingremaining side will not be affected by any pressure.

GEOMETRICAL DATA USED IN FEMSTRUCTURAL ANALYSISThe characteristic geometric features used inANSYS12 numerical analysis for the FSW tools is asfollows, Tool shoulder diameter = 18mm, Probelength = 4.7mm, Material Property Young’s modulus=E = 200e5 N/mm² & Poisson Ratio = 0.33 are keptsame for all tools.

Cylindrical tool pin profile- Probe diameter=6mm.

Conical tool pin profile - Probe big diameter=6mm,probe small diameter=4mm.

Square tool pin profile- Probe base side length=5mmwith square shape.

Triangular tool pin profile- Probe base sidelength=5mm with triangular shape.

NUMERICAL RESULTS & DISCUSSIONFigure 6-9 represent the displacement (deformation)contour due to bearing load of all four tool pinprofiles. Figure 10-13 illustrates the Von Mises Stresscontour of due to bearing load in all pin profiles. Tojudge the strength of these tools, we have to comparethe stresses produced in these four tools when theyall subjected to the same loading conditions, the lowerthe maximum stress induced in the tools body thestronger it can be. The maximum stresses for

comparison are Von Mises’ stress as they are closelyrelated to permanent deformation of the tools. Fourmajor tool strengths are quantitatively revealed fromthe numerical simulations by using the ANSYS12structural analysis package.

Figure 5: Pressure Distribution Around the Probe

Figure 6: Displacement Contour of Cylindrical Tool

Figure 7: Displacement Contour of Conical Tool

Figure 8: Displacement Contour of Square Tool

Structural Stress Analysis of Friction Stir Welding Tool Pin Profiles 23

Figure 13: Von Mises Stress Contour of Triangular Tool

Figure 9: Displacement Contour of Triangular Tool

Figure 10: Von Mises Stress Contour of Cylindrical Tool

Figure 11: Von Mises Stress Contour of Conical Tool

Figure 12: Von Mises Stress Contour of Square Tool

The Von Mises Stress analysis of tool pin profilesis represented by table 1.

Table 1Von Mises Stress Analysis of Tool Pin Profiles

Von Mises Stress (ANSYS-Results)Tool Pin Profile Stress (N/mm2) Node No

Cylindrical 148.65 405Conical 13.766 149Square 894.63 2314Triangular 2288.6 808

CONCLUSIONIn this paper it is observed that the structural analysisof FSW tools using finite element method is to be aneffective and practical way that will ensure tools’strength to stand the tremendous loads during friction-stir welding operation. A comprehensive way ofjudging the strength of these tools have beenestablished by comparing the stresses in these four toolswhen they all subjected to the same loading conditions.Four different tool configuration models subjected toloading conditions as happened in actual FSW processare analysed by using FEM tool of structural analysis,it reveals important information about how strong thetools behave with respect to different geometricfeatures. Now by comparing the maximum stresseswhich are experienced by the tool in the respectivementioned nodes, it is found that in conical tool pinprofile induced stresses are less as compared to othertool pin profiles and thus it is to be said that conicaltool profile is stronger tool as compared to othersduring FSW operation for the same loading condition.

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G., Temple-Smith P., Dawes C. J., Friction Stir ButtWelding, International Patent Application No. PCT/GB92/02203, (1991).

24 International Journal of Mechanical Engineering and Materials Sciences

[2] Dawes C. J., Thomas W. M., Friction Stir Process foraluminum alloys, Welding Journal 75, (1996), 41-45.

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[5] Elangovan K., Balasubramanian V. Influences of ToolPin Profile and Welding Speed on the Formation ofFriction Stir Processing Zone in AA2219 AluminiumAlloy; Journal of Materials Processing Technology, 200,(2008), 163–175.

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[8] Santiago D. H., Lombera G., Santiago U., Cassanelli A.,Luis A. de Vedia, Numerical Modeling of Welded Jointsby the Friction Stir Welding Process, Materials Research 7(2004), 569-574.

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