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Materials

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Materials and Design 29 (2008) 791–797

& Design

On the role of axial load and the effect of interface position on thetensile strength of a friction stir welded aluminium alloy

K. Kumar, Satish V. Kailas *

Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, India

Available online 27 January 2007

Abstract

This investigation highlights the influence of axial load, and the effect of position of the interface with respect to the tool axis on tensilestrength of the friction stir welded joint. The axial load is continuously varied by linearly increasing the interference between the toolshoulder and the surface of the base material. The interface position with respect to the tool axis is continuously changed by keepingthe feed direction at an angle to the interface. The base material used in this study is Al–Zn–Mg alloy, 7020-T6, of 4.4 mm thickness.It is found that there is an optimal axial load, above which the weld is defect-free, with joint efficiency of 84%. There is a tolerancefor interface position; i.e., the tool can be allowed to deviate away from the interface without deteriorating joint efficiency of the weld.The tool can be allowed to deviate from the interface in either side, but the tolerance is higher when the interface is located in the advanc-ing side of the tool.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Friction stir welding; Precipitation hardenable aluminium alloy; FSW parameters

1. Introduction

Friction stir welding (FSW) is a relatively new solid statewelding process which is used for butt and lap joints. FSWwas invented by The Welding Institute, Cambridge, UK in1991 [1] and has emerged as a candidate process for weldingof aluminum alloys. This process has made possible to welda number of aluminum alloys that were previously not rec-ommended (2000 series & copper containing 7000 series alu-minium alloys) for welding. Because the material subjectedto FSW does not melt and re-solidify, the resultant weldmetal is free of porosity with lower distortion. An addedadvantage is that it is an environmentally friendly process.

FSW is a solid state, localized thermo-mechanical, join-ing process. The process and the terminology are schemat-ically explained in Fig. 1. In FSW a non-consumablerotating shouldered-pin-tool is plunged into the interfacebetween two plates being welded, until the shoulder

0261-3069/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.matdes.2007.01.012

* Corresponding author. Tel.: +91 80 2293 2301; fax: +91 80 2360 0648.E-mail address: satvk@mecheng.iisc.ernet.in (S.V. Kailas).

touches the surface of the base material, and is then tra-versed along the weld line. In FSW, frictional heat is gen-erated by rubbing of tool shoulder and base materialsurface, and by deformation. During traversing, softenedmaterial from the leading edge moves to the trailing edgedue to the tool rotation and the traverse movement ofthe tool, and this transferred material is consolidated inthe trailing edge of the tool by the application of an axialforce. FSW parameters are tool geometry, axial force, rota-tional speed, traverse speed and, tool tilt angle [1]. Generalconvention is, where the direction of the velocity vector ofthe tool and traverse direction are same that side is calledthe advancing side of the weld, and when the direction ofthe velocity vector opposite to the traverse direction, it iscalled the retreating side.

In general, microstructure of friction stir weld is classi-fied into three major regions, namely weld nugget (WN),thermo mechanically affected zone (TMAZ) and heataffected zone (HAZ). The weld nugget is characterized byseverely deformed and dynamically recrystallized region [2].The TMAZ is a region that undergoes plastic deformation

Fig. 1. Schematic diagram shows friction-stir welding process and terminology.

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but does not have a recrystallized microstructure. TheHAZ region is affected only by heat, generated during thewelding process, and not by plastic deformation.

Many studies have been conducted on FSW of precip-itation hardenable and non-heat treatable aluminiumalloys with respect to microstructural characterization[1–4], and the effect of welding parameters on mechanicalproperties [5–7]. Emphasis has been given to the effectwelding parameters on hardness, fatigue strength, residualstress and microstructural evolution [1,3,4,8,9]. However,there is not much information available on correlationof welding parameters with evolution of defect-free weld.In order to produce a defect-free weld the optimizationof welding parameters is extremely important. In thisinvestigation the role of axial load on weld formation isanalyzed and the optimal axial load for producing adefect-free weld is found out. In general, during frictionstir welding the tool is plunged at the interface of thematerials to be welded in such a way that the tool axisand interface are aligned. It is difficult to maintain thisalignment at all times during high speed production weld-ing, especially while welding of non-linear profiles. It isalways possible to encounter tool deviation from the inter-face. But, the effect of tool deviation from the interface onthe quality of the weld has not been studied. In this inves-tigation the effect of tool deviation from the interface onthe joint efficiency is experimentally analyzed for a set ofwelding parameters and material.

The base material used in this study is 7020-T6, which isartificially aged to peak hardness. Aluminium alloy 7020 isa precipitation hardenable Al–Zn–Mg alloy, which is usedin aerospace and automobile industries for structural appli-cations. This alloy gains its strength form the complex pre-cipitation of GP zones and g 0 (semi-coherent MgZn2)phase. This is achieved by solution treatment and con-trolled aging. These phases are stable in the temperaturerange of 20–120 �C and 120–250 �C, respectively [10–12].Above the stable temperature range the equilibrium gphase (non-coherent MgZn2) is precipitated from the

matrix or is transformed from the existing g 0 phase. For-mation of g phase reduces the material strength.

2. Experimental methods

The base material used in this study is aluminium alloy 7020-T6. Thenominal chemical composition is Zn – 4.63 wt%, Mg – 1.3 wt% and rest alu-minium. The base material tensile strength is 401 MPa with ductility of14.5%. FSW trials are carried out on a vertical milling machine with squarebutt joint configuration. A pair of work pieces of dimension 300 mm ·75 mm · 4.4 mm are abutted and clamped rigidly on the backing platefor welding. The tool geometry used for this study is 20 mm diameter flatshoulder with chamfered edge, and frustum shaped pin with 6 mm topdiameter and 4 mm bottom diameter. The frustum shaped pin profile is cho-sen according to the self-optimized tool geometry suggested by Prado andMurr [13]. Welding speed, tool rotation speed and backward tool tilt angleare kept constant at 80 mm/min, 1400 rpm and 2� respectively.

The axial load is varied by linearly increasing interference between thetool and the base material. This is done by keeping the backing plate atan angle to the tool axis in the welding direction, as shown in Fig. 2a. Axialloads for the corresponding tool interference are measured using a load cellmounted bellow the backing plate, in another experiment. For continuouslyvarying the interface position, the tool is initially plunged about 3 mm awayfrom the interface with the interface located on the retreating side of thetool. The tool is then traversed at an angle to the interface such that the weldends with interface located 3 mm away on the advancing side, as shown inFig. 2b. This experiment is carried out at the axial load of 7.8 kN. Thermo-couples are inserted in the line perpendicular to the weld in both advancingand retreating side, and the temperature data is acquired.

The tensile and metallographic samples are alternatively machinedfrom the weld coupon as shown in Fig. 3. The hardness profile was mea-sured across the weld line with 1 kg load and 10 s dwell time in Vicker’smicro harness tester. The weld cross section is prepared by standard metal-lographic practice for metallographic studies, and diluted Keller’s reagentis used for etching. The tensile testing is conducted using Hounsefieldtenso-meter with 3.2 mm cross head speed after 30 days of natural agingat room temperature.

3. Results and discussion

3.1. Microstructure details

The base material microstructure is shown in Fig. 4a. Itshows that the grains are elongated in the rolling direction

Fig. 2. Schematic diagrams show the schemes of experiments. (a) The axial–load varying experiment, where the tool interaction with the base material iscontinuously increased. (b) Experiment for varying interface position, where the tool plunged away from the interface and traversed in the weld line at anangle to the interface.

Fig. 3. Line diagram shows the scheme for sampling (WD – Welding direction). (a) Alternative sampling of metallographic and tensile samples. (b) Tensiletesting sample dimensions.

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with size varying from 300 to 500lm. The weld nuggetmicrostructures are shown in Fig. 4b–d, for the weld pro-duced at axial loads of 4.0, 4.6 and 8.1 kN, respectively.The figures show that the initial elongated grains are trans-formed into fine equiaxed grains in the weld nugget. It canalso be seen that the grain size in the weld nugget increasesas the axial load increases. The grain size in the weld nuggetincreased from �3 to �10 lm when the load increasedfrom 4.0 to 8.1 kN.

The transformation of initial elongated grain structure isinto fine equiaxed grains in the weld nugget is reported tobe dynamic recrystallization [1–3]. The increase in axialload increases the heat input to the weld as the frictionalforce increases. The increased heat input raises the nuggetzone temperature and this could lead to static grain growthof dynamically recrystallized grains. Similar kind of grain

growth has been observed by Sato et al. for FSW of AA6063 as function of tool rotation speed [14].

4. Role of axial load in the weld formation

Fig. 5a–h shows the evolution of a defect free frictionstir weld as a function of the axial load. It can be seen thatthe unfilled region is present only in the top portion of theadvancing side of the weld, and the unfilled region gradu-ally decreases as the axial load increases and is completelyeliminated at an axial load of 8.1 kN. As the axial loadincreases it can be seen that the amount of materialdrawn-in from the retreating side increases, and the weldlosses its symmetry about weld centerline. The tensile testresults are shown in Fig. 6, it shows that the tensile samplestaken from the weld made at axial load from 4.0 to 6.7 kN

Fig. 4. Micrographs show the microstructure modification and the effect of axial load on grain size in FSW. (a) Base material, (b) weld nugget produced atthe axial load 4.0 kN, (c) weld nugget produced at the axial load 4.6 kN and (d) Weld nugget produced at the axial load 8.1 kN.

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fractures in the weld nugget region, while samples takenfrom the weld made at axial load above 7.4 kN fracturesaway from the weld nugget. It can also be seen that the ten-sile strength is lower, and more or less constant, when theaxial loads are less than 6.0 kN. When the axial loadincreased from 6.0 to 7.4 kN the tensile strength increasessuddenly, and is more or less constant above 7.4 kN.

FSW is a solid state, thermo-mechanical joining process.Solid state welding processes are a group of welding pro-cesses which produces coalescence at temperatures essen-tially below the melting point of the base material. Insolid state welding process bonding occurs when a pair ofcontamination free nascent surfaces is brought togetherin the vicinity of inter-atomic-forces [15]. The major con-cern is the forces required to bring the surfaces in the vicin-ity of inter-atomic-forces over a large area. Adequatecontact can be attained by the application of compressivehydrostatic stress in the weld nugget region, and the hydro-static pressure should be essentially higher than the flowstress of the materials of the mating surfaces. Since the flowstress reduces as the temperature increases, force requiredto make the adequate contact between the surfacesdecreases. Hence, the formation of defect free solid stateweld requires optimum temperature and hydrostaticpressure.

As the axial load increases, both the hydrostatic pres-sure beneath the shoulder and the temperature in the weldnugget increases. From the graph shown in Fig. 6 the ten-sile strength increases drastically from 188.9 to 337.7 MPaas the load is increased from 6.0 to 7.4 kN. It is clearlyunderstood that this is the range of axial load which is

required generate adequate hydrostatic pressure and tem-perature in the weld region. It is interesting to note thatin Fig. 5a–f the material flow in the top portion of the weldnugget reaches the advancing side base material and makesthe bonding, when the axial load is increased from 6 to7.4 kN. Hence, when the axial load is lesser than 6.8 kNthe there is a defect in the weld because of insufficient coa-lescence of transferred material. Consequently, the weldedsamples fracture in the weld nugget due to this defect in theweld nugget.

The welds produces at axial loads above 7.4 kN frac-tured away from the weld, but the efficiency is only 84%due to softening of the HAZ. The softening of the HAZis directly related to modification of precipitates. The mod-ification of precipitates refers to dissolution or growth. Ithas been reported that for 7020 alloy when the peak tem-perature exceeds 390 �C precipitate dissolution occurs[16]. The strengthening precipitates are unaffected untilthe peak temperature reaches 200 �C. When the tempera-ture exceeds 200 �C precipitate growth and dissolutionwas observed.

Peak temperature distribution across the weld is shownin Fig. 7, and it shows the maximum temperature in theweld nugget is 400 �C. Hardness profile across the weld isshown in Fig. 8 for a weld produced at axial load of8.1 kN. The base material hardness is 164VHN1kg,andthe average weld metal hardness is around 140VHN1kg.It can be seen from the hardness profile that the minimumhardness is around 120VHN1kg, which occurs 20 mm awayfrom the weld. The hardness minima are observed in bothsides of the weld in the HAZ. The minimum temperature

Fig. 5. The macrographs show the evolution of a defect-free FSW as function of axial load. The welds (a–h) are produced at axial load of 4.0, 4.6, 5.3, 6.0,6.7, 7.4, 8.1 and 8.8 kN, respectively.

Fig. 6. The graph shows the effect of axial load on tensile strength. Clearevidence can be seen in the graph that as axial load increases from 6 to7.4 kN, tensile strength increases sharply as the defect are eliminated.

Fig. 7. The graph shows the peat temperature distribution across the weld(Negative direction in the x axis indicates the retreating side).

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Fig. 8. Graph shows the hardness profile across the weld (Negativedirection in the x axis indicates the retreating side).

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required modify the strengthening precipitates in Al–Zn–Mg alloys is 200 �C, and which exists around 20 mm awayfrom the weld. In this particular temperature the strength-ening precipitates grows in size. Because of post weld ther-mal cycle the re-precipitation is possible in the materialwhere the precipitates are dissolved in the matrix and thehardness can be recovered. But hardness cannot be recov-ered in the region where the precipitates grow. Re-appear-ing of precipitates in the weld nugget and TMAZ is causefor hardness recovery. In the HAZ precipitates are not dis-solved in the matrix, and consequently there is no reap-pearance of precipitates due to post weld thermal cycle.This causes the lowest hardness and consequently fractureduring tensile testing in the HAZ.

4.1. The effect of interface position

The presence of un-bonded interface when the tool isaway from the interface is shown in Fig. 9a–d. Fig. 9ashows the micrograph when the weld interface is3.1 mm from the tool axis and in the retreating side

Fig. 9. The micrographs show the top portion of un-bonded interface in the3.1 mm in the retreating side, (b) 3.1 mm in the advancing side, (c) 2.5 mm in

and Fig. 9b shows the micrograph when the weld inter-face is 3.1 mm from the tool axis and in the advancingside. It can be seen from Fig. 9a and b that the interfacein the retreating side is not connected to the weld nugget,whereas the top potion of the interface in the advancingside is connected to the weld nugget. This indicates thatthe top portion of the interface in the advancing side isstirred by the pin and is eliminated. It can see seen fromFig. 9c and d that when the interface is in the advancingside the surfaces are getting closer. While the interface isin the retreating side the surfaces in the interface movesapart from each other.

The tensile test results are shown in Fig. 10. When theinterface offset is more than 1 mm in the retreating sidefrom the tool axis the welded samples fracture at the inter-face. Whereas the interface offset can be allowed 1.6 mm tobe in the advancing side, above which the welded samplesfracture at the weld interface. The safe range of interfacedeviation from the tool is 1 mm in the retreating side,and 1.6 mm in the advancing side. It is evident that the saferange of interface deviation from the tool axis is differentwhen the tool is in advancing or retreating side. The totaldeviation that can be allowed in either side of the interfaceis 2.6 mm, whereas the bottom diameter of the frustumshaped pin is 4 mm. This indicates that the safe range ofdeviation is lower than the pin diameter.

The graph shown in Fig. 10 indicates that the strength ofthe weld is uniform on the safe range of tool axis deviationfrom the interface. When the tool deviates slightly awayfrom the safe range the tensile strength reduces sharply.Since the interface is not eliminated, fracture occurs atthe location of weld interface. When the interface is locatedin the safe range the interface is eliminated, and fractureoccurs in the HAZ.

cross section of the weld. Interface is offset from the tool axis about: (a)the retreating side, and (d) 2.6 mm in the advancing side.

Fig. 10. The graph shows the effect of interface position on tensilestrength. (The negative values indicate the interface located in retreatingside of the weld).

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5. Conclusions

In this study the role of axial load on weld formationand the effect of interface position on joint efficiency of aFSW of aluminium alloy 7020-T6 are studied using simpleexperimental techniques. The results derived from theseexperiments lead to the following conclusions. For thepresent set of experimental parameters.

1. The initial elongated grain structure of the base materialis transformed in to fine recrystallized equiaxed grains.And the recrystallized grain size in the weld nuggetincreases as the axial load increases.

2. The defect free aluminum alloy 7020-T6 weld is made bycontinuously increasing the axial load. The optimal axialload is found to be 8.1 kN, above which the weld is freeof defects.

3. The maximum strength of weld produced by varyingaxial load technique is 340 MPa (84% of the base mate-rial strength) with ductility of 7.3% at 8.8 kN

4. The safe range of tool deviation from the interface isfound out for a frustum shaped tool. The safe tool devi-ation from the interface is 1 mm in the advancing sideand 1.6 mm in the retreating side for a frustum shapedtool. When the tool deviates from the safe rage the ten-sile strength decreases sharply.

5. The difference in the tolerance limit in the advancingside and retreating side arises from the asymmetric nat-ure of material flow in FSW.

Acknowledgements

Authors would like to thank STC/IISc and DRDO forsponsoring the project, Mr. G.M. Gururusamy and Mr.

P. Srinivasa Rao of LPSC/ISRO for providing material,Prof. S. Shesan, Prof. R. Narasimhan and Mr. Reddy ofME/IISc for supporting with experimental facilities, andMr. K.B. Teggi, Mr. K. Manoharan, Mr. V. Gangannaand Mr. V. Saravana of CPDM/IISc for their support inworkshop practices.

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