Experimental and numerical investigation on under-water friction stirwelding of armour grade AA2519-T87 aluminium alloyS. SREE SABARI a, S. MALARVIZHI a, V. BALASUBRAMANIAN a,*,

G. MADUSUDHAN REDDY b

a Centre for Materials Joining and Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar, 608 002 TamilNadu, India

b Metal Joining Section, Defense Metallurgical Research Laboratory (DMRL), Hyderabad, India

Received 7 January 2016; revised 2 February 2016; accepted 15 February 2016

Available online 28 March 2016

Abstract

Friction stir welding (FSW) is a promising welding process that can join age hardenable aluminium alloys with high joint efficiency. However,the thermal cycles experienced by the material to be joined during FSW resulted in the deterioration of mechanical properties due to the coarseningand dissolution of strengthening precipitates in the thermo-mechanical affected zone (TMAZ) and heat affected zone (HAZ). Under water frictionstir welding (UWFSW) is a variant of FSW process which can maintain low heat input as well as constant heat input along the weld line. The heatconduction and dissipation during UWFSW controls the width of TMAZ and HAZ and also improves the joint properties. In this investigation, anattempt has been made to evaluate the mechanical properties and microstructural characteristics of AA2519-T87 aluminium alloy joints made byFSW and UWFSW processes. Finite element analysis has been used to estimate the temperature distribution and width of TMAZ region in boththe joints and the results have been compared with experimental results and subsequently correlated with mechanical properties.© 2016 China Ordnance Society. Production and hosting by Elsevier B.V. All rights reserved.

Keywords: Aluminium alloy; Friction stir welding; Underwater friction stir welding; Mechanical properties; Microstructural characteristics; Finite elementanalysis

1. Introduction

In recent days armour grade steels in the military vehicle isreplaced by lighter materials for better mobility [1,2]. Alu-minium alloy AA2519-T87 is a potential candidate, to replacesteel in a few defence applications due to its high specificstrength. In addition, it has excellent tensile properties, fracturetoughness properties and ballistic immunity. Researchers con-firmed that friction stir welding (FSW) can produce soundjoints in heat-treatable aluminium alloys [3–7]. However, thethermal cycles exerted during FSW resulted in the reduction ofmechanical properties of the joints due to coarsening and dis-solution of strengthening precipitates in the thermo mechani-cally affected zone (TMAZ) and heat affected zone (HAZ) ofthe joints [8,9].

In most of the age hardenable aluminium alloys, TMAZ andHAZ are identified as the weak regions [10–13]. The strength ofthe parent metal and weld joints of aluminium alloys depend onthe presence of the precipitates. The thermal conditions pre-vailed during heating and cooling cycle of welding affect theprecipitation behaviour. In FSW, the heat generated in the stirzone and heat conducted to TMAZ and HAZ causes theprecipitates to be either solutionized or coarsened. The loss ofprecipitates during solutionizing and coarsening of precipitatesresulted in poor joint properties [14]. In addition, the highthermal conductivity of aluminium creates variation in thethermal gradient and heat build ups along the longitudinaldirection, especially in joining of thick plates. It results innon-uniformity along the weld region. In order to overcome theheat related problems created in TMAZ and HAZ and toimprove the strength of FSW joints, the heat should be dissi-pated readily using a cooling method. The water coolingmethod is more efficient than the other cooling method in termsof uniform cooling and high heat transfer coefficient.

Peer review under responsibility of China Ordnance Society.* Corresponding author. Tel.: +91 04144 239734.E-mail address: visvabalu@yahoo.com (V. BALASUBRAMANIAN).

http://dx.doi.org/10.1016/j.dt.2016.02.0032214-9147/© 2016 China Ordnance Society. Production and hosting by Elsevier B.V. All rights reserved.

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Liu et al. carried out an investigation on under water frictionstir welding (UWFSW) ofAA2219-T6 aluminium alloy and theinvestigation concluded that the UWFSW improved thestrength of the joint by restricting the coarsening and dissolu-tion of strengthening precipitates [15]. Fu et al. also observedthe improvement in the tensile strength of the UWFSW jointsof AA7075 T87 aluminium alloy by reducing the width ofHAZ [16]. Fratini et al. simulated the temperature histories ofUWFSW process and finds that the water cooling resulted inreduction of thermal flow adjacent to the tool [17]. Huijie et al.developed mathematical modelling and optimization proce-dures for underwater friction stir welding of a heat-treatablealuminium alloy. The optimization result indicated that amaximum tensile strength of 360 MPa can be obtained throughUWFSW, which is 6% higher than the maximum tensilestrength obtained in FSW [18]. From the literature it is under-stood that the mechanical properties of the FSW joints could beimproved by the water cooling.

Studying the thermal histories of the weld joints is useful toexplore the fundamental aspects of UWFSW. In order to mapthe temperature distribution and to predict the thermal historiesof welding processes, finite element (FE) method was used bymany researchers [19,20]. However, very few researchers havecarried out experimental investigation and finite element analy-sis of UWFSW of aluminium alloys. Moreover, the previousinvestigations were focused on the microstructural character-ization, mechanical properties evaluation and FE analysis of theconventional FSW process [21,22]. Very few publications areavailable on the comparison of experimental and numericalinvestigations of FSW and UWFSW of aluminium alloys.Hence, this investigation was carried out to evaluate themechanical properties of FSW and UWFSW joints of highstrength, armour grade AA2519-T87 aluminium alloy. Finiteelement analysis was also used to estimate temperature distri-bution during FSW and UWFSW of AA2519-T87 aluminiumalloy. Further, width of the TMAZ region was estimated forboth the joints (FSW and UWFSW) by FE analysis and com-pared with the experimental results.

2. Experimental work

Fig. 1 shows the experimental details employed for the fab-rication of FSW and UWFSW joints. Fig. 1(a) shows the sche-matic representation of the UWFSW process. The workpiecewas rigidly clamped on the backing plate in the tank. The wateris discharged into the tank through the inlet to the level abovethe tool shoulder. During welding, the inlet and outlet valves areadjusted to control the water flow in such a way to maintain thetemperature of water below 60 °C near to the welding location.From the previous investigation, the author found that the taperthreaded tool was well suited for joining this material and so itis used for this investigation [23]. The dimension of the tool isshown in Fig. 1(b). The photographs of the FSW and UWFSWsetups are shown in Fig. 1(c) and 1d respectively. AA2519-T87aluminium alloy was used as the parent metal. Joint configura-tion of 150 × 150 × 19 mm was used in this investigation(Fig. 1(e)). The chemical composition of the parent metal wasconformed using vacuum spectrometer and presented in

Table 1. The process parameters and the welding conditionsused for fabricating FSW and UWFSW joints are shown in theTable 2. Tensile test was carried out using Instron made servohydraulic controlled universal testing machine. ASTM E8M-04(Standard test methods for tension testing of metallic materials)guidelines were followed for the extraction (Fig. 1(e)) andtesting of the tensile samples. The transverse tensile propertiessuch as yield strength, ultimate tensile strength and elongationwere evaluated from the tensile test. Vickers microhardness

Fig. 1. Experimental details (a) Schematic diagram of UWFSW process (b)Tool dimensions (c) Photographs of FSW setup (d) Photographs of UWFSWsetup (e) Joint configuration and specimen extraction diagram.

Table 1Chemical composition (wt %) of AA 2519 T87 aluminium alloy.

Al Cu Mg Mn Fe V Si Ti

93.32 5.71 0.47 0.27 0.1 0.05 0.04 0.02

Table 2Process parameters and tool dimensions used in this investigation.

Process parameters Values

FSW UWFSW

Tool rotational speed/rpm 150 800Welding speed/(mm·min−1) 7.5 10Pin length/mm 18 18Tool shoulder diameter/mm 30 30Pin diameter/mm 12-6 12-6Axial force/kN 12 14Pin profile Taper threaded cylindricalTool material Hardened super high speed steel

325S. Sree Sabari et al. /Defence Technology 12 (2016) 324–333

tester was used to measure the microhardness at the variousregion of the weld joint with a load of 0.5 N and dwell time of15 s. The sample preparations and the testing procedures forthe microindentation were followed as per the guidelinesfrom ASTM E384-99 standard (Standard Test Method forMicroindentation Hardness of Materials).

Microstructural examination was carried out using anoptical microscope. The specimens for metallographic exami-nation were initially polished by rough emery and subsequentlypolished using different grades of emery papers to get mirrorpolish. The specimens were etched with modified Keller’sreagent as per the ASTM E407 standard (Standard Practice forMicroetching Metals andAlloys). Transmission electron micro-scope was used to characterize the microstructure of TMAZ andHAZ. The samples of 3 mm diameter is extracted and polishedto 10 μm thick using ion milling process. Line intercept methodwas employed to measure the average grain size of differentregions of the weld joint.

3. Finite element analysis

This paper presents three-dimensional thermal analysisusing the finite element (FE) code COMSOL. Dissimilarmeshed model of varying size and element types were used inthe FE analysis (Fig. 2). The meshed model composed of quad-rilateral, triangular and square elements. The model is meshedinto 6821 finite elements. In order to reduce the computationaltime and achieve accurate results, finer mesh was carried outnear the tool and coarser mesh was carried out for the regionaway from the tool. The input parameters used for FE analysisis tabulated in Table 3.

3.1. Thermal boundary condition

The heat generated during welding is lost due to three modesof heat transfer which greatly reduce the availability of heatrequired to weld the work piece. In welding, the heat is trans-ferred by conduction, convection and radiation modes. Thegoverning differential equation for three dimensional heat con-duction equations for a solid in Cartesian coordinate system isgiven by [24]

δδ

δδ

δδ α

δδ

2

2

2

2

2

2

1T

x

T

y

T

z

q

k

T

t+ + + =

⎛⎝⎜⎜⎜

⎞⎠⎟⎟⎟

g (1)

where αρ

=k

Cp

thermal diffusivity of the material, qg is the

heat generation per unit volume in W/m3, ρ is density of thematerial in kg/m3, Cp is specific heat in J/kgK and k are thermalconductivity W/mK.

Heat transfer from the work piece due to convection wasapplied in the free surface. The corresponding heat flux expres-sion for free surface is [25]

q h T T T Tup down up down amb, ,= − −( )+ −( )04 4εσ (2)

where hup and hdown are the convection coefficient of top andbottom surface respectively, T0 is the reference temperature,Tamb is the ambient temperature, σ is the Stefan–Boltzmannconstant and ε is the surface emissivity. The natural convectionbetween aluminium and air was experimentally found by Choaet al. as 15 W/(m2 °C) for FSW [26]. Similarly Zhang et al.found the heat transfer coefficient of UWFSW at top andbottom as 2000 W/(m2 °C) and 1000 W/(m2 °C) respectively[27]. Temperature based material properties were consideredfor the finite element analysis. In this investigation, the thermalmodel was considered for the analysis and the material flowbehaviour was not accounted. The degree of heterogeneitybetween the advancing side (AS) and retreating side (RS)mainly depends on the material flow and the microstructurerather on the temperature distribution and hence symmetricmodel was considered for the analysis. The mid-plane ofthe butt joint was assumed to be a plane of symmetry in theanalysis. Zero displacement conditions were used forconstraining the butt joint which resembling the complete fixedfixture.

3.2. Heat source modelling

The heat generated is concentrated locally and propagatesrapidly into subsequent regions of the plates by heat conductionaccording to Eq. (1) as well as convection and radiation throughthe boundary. Constant heat fluxes were applied as the heatsource in the tool shoulder-workpiece interface and tool pin-workpiece interface.

Heat generation in shoulder-work piece interface is the func-tion of major elements such as axial load, area subjected tofriction, coefficient of friction and angular velocity. The heatflux can be mathematically expressed by Song and Kovacevic[28]

q F Rnshoulder = × × × × ×2 π μ ω (3)

Fig. 2. 3-Dimensional meshed model.

Table 3Input parameters used in FE analysis.

Parameter Notations FSW UWFSW

Ambient temperature/°C T0 30 30Melting temperature/°C Tmelt 660 660Heat transfer coefficient/(W·m−2 °C) [18] hupside 15 2000Heat transfer coefficient/(W·m−2 °C) [19] hdownside 200 1000Welding speed/(mm·min−1) uweld 7.5 10Friction coefficient μ 0.3 0.3Rotation speed/rpm ω 150 800Normal force/kN Fn 12 14Pin radius/mm rpin 12-6 12-6Shoulder radius/mm rshoulder 30 30

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where Fn is the normal force (kN), As is the shoulder’s surfacearea (mm2), μ is the friction coefficient, R is the distance fromthe center axis of the tool (mm) and ω is the tool rotation speed(rpm). The mathematical expression of heat generated at theinterface of pin and work piece is adapted from Colegrove [29]

q r Y Tp p=× +( )

×( )× ( )μ

μω

3 1 2(4)

where μ is the friction coefficient, rp is the pin radius (mm), ωis to the angular velocity (rad/s), and Y(T) is the average shearstress of the material (N/mm2). The input parameter used for theFEM simulation of FSW and UWFSW are presented in Table 3.

4. Results

4.1. Tensile properties

Fig. 3 shows the superimposed stress strain curves of FSWand UWFSW joints. From the stress strain curves, the trans-verse tensile properties like yield strength, tensile strength,percentage of elongation and joint efficiency of FSW andUWFSW joints were calculated and presented in Table 4. Thetensile strength was measured as 452 MPa for parent material,271 MPa for UWFSW joint and 248 MPa for FSW joint. Theyield strength was measured as 427 MPa for parent material,223 MPa for UWFSW joint and 198 MPa for FSW joint.UWFSW joint exhibited higher tensile and yield strength thanFSW joint. However, both the joints showed lower strengthvalues than the parent material. The joint efficiency was calcu-lated by finding the ratio of tensile strength of welded joint andtensile strength of the unwelded parent metal. The UWFSWjoint yielded a higher joint efficiency of 60% and the FSW joint

exhibited lower joint efficiency of 55%. From the percentage ofelongation value, the degree of ductility of the weld joint can beunderstood. The elongation was measured as 11.2% for parentmaterial and 4.56% for UWFSW joint.

4.2. Macro and microstructure

Fig. 4 shows the cross-sectional macrographs of FSW andUWFSW joints. Defect free trapezoidal shape stir zone isobserved in FSW joint (Fig. 4(a)). Fig. 4(b) shows themacrograph of UWFSW joint consisting defect free stir zonecharacterized by large sized onion ring formation extendedfrom the mid thickness region (MTR) to the pin influencedregion (PIR). FSW joint reveals a wider TMAZ in both AS andRS compared to UWFSW joint. The width of RS-TMAZ ismeasured as 4.1 mm and 1.7 mm for FSW and UWFSW joints,respectively. Fig. 5 shows the optical microstructure of parentmetal. It comprises of elongated grains of 49 μm (average graindiameter) oriented towards the rolling direction. The stir zone isclassified as upper shoulder influenced region (SIR), middlemid-thickness region (MTR) and lower pin influenced region(PIR) and their respective optical micrographs are shown inFig. 6.All the three regions of stir zone reveal fine recrystallizedequi-axed grains in both the joints. The average grain size ofvarious regions are measured and presented in the Table 5. The

Fig. 3. Stress strain curves.

Table 4Transverse tensile properties of parent metal and welded joints.

0.2% Yieldstrength/MPa

Tensilestrength/MPa

Elongation in50 mm gaugelength /%

Jointefficiency/%

Parent material 427 452 11.2 -FSW 198 248 9.02 55UWFSW 223 271 4.56 60

Fig. 4. (a) Macrostructure of FSW joint (b) Macrostructure of UWFSW joint.

Fig. 5. Optical micrograph of parent metal.

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grain size variation in all the three regions of stir zone is not sosignificant in FSW joint (Fig. 6(a), (c) and (e)). But in UWFSWjoint, the grain size is larger in the SIR, relatively smaller inMTR and very fine in PIR. Comparing the average grain size ofstir zone, UWFSW joint consists of marginally larger grainsthan FSW joint.

Fig. 7 shows the micrographs of TMAZ and HAZ of advanc-ing side (AS) and retreating side (RS). In TMAZ, the grains arepulled upward towards the stir zone in both FSW and UWFSWjoints (Fig. 7(a)–(d)). But the grains are more lineated towardsstir zone in UWFSW than FSW joints. The extent of strainingof TMAZ is higher in UWFSW than FSW joint. Figs. 7(e) and

(f) show the HAZ micrographs in which FSW joint exhibitslightly larger grains than the UWFSW joint. The averagegrain size of HAZ is measured as 55 μm and 49 μm for FSWand UWFSW joints respectively. The grain size of HAZ ofUWFSW is equal to the grain size of the parent metal.

The precipitation behaviour of TMAZ and HAZ are aloneanalysed with the transmission electron microscopy (TEM).Fig. 8 shows the TEM images of various regions of FSW andUWFSW joints. Fig. 8(a) shows the TEM image of parentmetal, comparing needle like precipitates normally oriented toeach other. The precipitates are densely distributed and finer in

Fig. 6. Optical micrographs of stir zone at various locations (a) and (b) Shoul-der influenced region (SIR) (c) and (d) Mid-thickness region (MTR) (e) and (f)Pin influence region (PIR).

Table 5Average grain size (μm) of various regions of welded joints.

Stirzone

AS-TMAZ RS-TMAZ AS-HAZ RS-HAZ Basematerial

FSW 3.95 33 56 55 55 49UWFSW 4.5 38 54 49 49 49

Fig. 7. Optical micrographs of TMAZ and HAZ regions (a) and (b) Advancingside TMAZ (c) and (d) Retreating side TMAZ (e) and (f) Advancing side HAZ(g) and (h) Retreating side HAZ.

328 S. Sree Sabari et al. /Defence Technology 12 (2016) 324–333

size (of 50 nm in diameter). These Al-Cu alloys are strength-ened by the presence of θ‘-CuAl2 precipitates [30–32].TMAZof FSW joint exhibits precipitate free zone (PFZ) and very fewcoarsened precipitates of 500 nm in size (Fig. 8(b)). Similarly,TMAZ of UWFSW joint exhibits precipitate free zone (PFZ)and dislocation cell structures (Fig. 8(c)). The presence of densedislocation refers the extent of work hardening in the TMAZ. InFSW joint, HAZ has undergone coarsening of precipitates andreduction in the number of precipitates (Fig. 8(d)). Compared tothe parent metal, the precipitates in HAZ are transformed fromθ’ precipitates to large sized equilibrium θ precipitates of200 nm in size. In UWFSW joint, the size and distribution ofprecipitates are almost similar to the parent metal but very fewprecipitates are coarsened. The XRD analysis was made toidentify the evolution of the precipitates during FSW andUWFSW joints (Fig. 9). The Al and CuAl2 phases are observedin the parent metal, FSW and UWFSW joints. The new phaseAl4Cu9 is observed in the FSW and UWFSW joints with sig-nificant intensities.

4.3. Microhardness

Fig. 10 shows the microhardness plot for FSW and UWFSWjoints. The stir zone shows fluctuating hardness values rangingfrom 108 HV to 131 HV in UWFSW joint. Similarly the hard-

ness values are varying from 108 HV to 120 HV in the stir zoneof FSW joint. A drop in hardness value is observed in theTMAZ. In both joints the RS-TMAZ exhibit lower hardnessand therefore this region is termed as lower hardness distribu-tion region (LHDR). The LHDR of FSW and UWFSW jointsexhibited lower hardness of 90 HV and 94 HV respectively. Themicrohardness plots shows increasing trend from TMAZ toHAZ. The increment in the hardness value is gradual from the

Fig. 8. Transmission electron micrographs of various regions of weld joints (a)Parent metal (b) RS-TMAZ of FSW joint (c) RS-TMAZ of UWFSW joint (d)RS-HAZ of FSW joint (e) RS-HAZ of UWFSW joint.

Fig. 9. XRD results.

Fig. 10. Microhardness plots of the joints (a) FSW joint (b) UWFSW joint.

329S. Sree Sabari et al. /Defence Technology 12 (2016) 324–333

TMAZ to HAZ in FSW joint. But, an abrupt increment isobserved in UWFSW joint. The HAZ exhibit hardness values of132 HV and 144 HV in FSW and UWFSW joints respectively.The parent metal region recorded maximum hardness of158 HV. The higher microhardness values are recorded in thestir zone and HAZ of UWFSW joint than the FSW joint.

4.4. Fracture surface analysis

The fractograph of UWFSW joint shows fine populateddimples with uniform size oriented towards the loading direc-tion (Fig. 11). In contrast, FSW joint has both small and largesize dimples. The secondary cracks are observed inside thedimples. From the fractographs, it could be inferred that boththe joints failed under ductile mode. The TMAZ near weldperiphery undergone high grain deformation, lower hardnessand fracture localization.

4.5. Finite element (FE) analysis

Fig. 12 illustrates the top surface temperature contours forFSW and UWFSW joints. In FSW joint, the temperature dis-tributed to the leading and trailing edge of the tool along thelongitudinal direction seemed to be asymmetrical. Symmetrictemperature distribution is observed along the longitudinaldirection for the UWFSW joint. Fig. 13 illustrates the thermalcontours in the cross-section of the weld. In FSW joint, thetemperature experienced by SIR, MTR and PIR are almostsame around 525 °C (Fig. 13(a)). But in UWFSW joint, there

exists a variation in SIR, MTR and PIR. Maximum temperatureof 545 °C is witnessed in the SIR and gradually reducedtowards PIR of 512 °C (Fig. 13(b)). This is clearly evident fromthe colour contours which show intense blue colour in the SIRand dull blue colour in the PIR. Similar to the longitudinaltemperature contour map, the isothermals of UWFSW jointalso remarkably move towards the tool axis in contrast to theFSW joint. The entire stir zone of FSW experienced sameamount of heat. In UWFSW joint, the maximum temperature isat the SIR and the temperature is decreasing towards the PIR.TMAZ, the region next to stir zone exhibits band of temperaturevalues fluctuating around 400 °C as shown in light blue colourcontour. The material experienced heat in this region is softenedand deformed by the stirring action of tool. The FSW joint havewider range of temperature values than the UWFSW. Fig. 13(c)shows the temperature plot along the transverse direction. Themaximum temperature is not recorded at the weld center but itis observed 25 mm away from the center line, the periphery ofshoulder-work piece interface region.

5. Discussion

5.1. Thermal analysis

During tool traverse, the leading edge of the tool gets con-tacted with the cold zone which was generally referred aspreheat zone. The high yield strength of the cold zone offershigh resistance against the material flow. Due to tool rotation,intense frictional heat was generated in the tool-work piececontact area (Fig. 13). The cold zone or preheat zone conductsthe heat from the interface. In UWFSW process, the heat con-ducted to preheat zone was readily dissipated by the convectiveheat transfer of water. The temperature attained in the preheatzone was too low for UWFSW joint than FSW joint. Therefore,

Fig. 11. Fractographs of the joints (a) FSW joint (b) UWFSW joint.

Fig. 12. Simulated temperature contour at top surface of the joints (a) FSWjoint (b) UWFSW joint.

330 S. Sree Sabari et al. /Defence Technology 12 (2016) 324–333

high yield stress was created which in turn results high heat fluxin the shoulder-work piece contact area. This was one of thereasons for the higher heat flux created in the shoulder-workpiece contact area of UWFSW.

The material in the trailing edge of the tool experiencedgradual drop in the temperature. Because of thermal softening,the material in the leading edge got dropped in yield strengthand got easily transported to the rear end. The plasticized mate-rial offers poor friction between the tool and work piece andtherefore relatively low temperature is observed in the rear end.As the tool traverses, cooling cycle takes place in the weldedregion. Hence, low thermal gradient was observed at the rearside of the tool in FSW joint. The isothermal was larger at therear and smaller at front of the tool. This was because of theheat build-up behind the tool during tool traversing. But inUWFSW process, the heat was readily dissipated by the waterand hence high thermal gradient was observed at the rear end.Moreover, similar isothermals were observed at both the frontand rear end of the tool during UWFSW (Fig. 12).

The temperature of isothermals can be correlated with thedegree of thermal softening of the material. High temperatureisothermal results in high degree of thermal softening and vice-versa. The isothermal was confined around the tool in UWFSW

joint but for FSW joint, widespread isothermal was noted.Consequently thermal softening was also confined near the toolin UWFSW joint and the extent of thermal softening is higherfor FSW joint. The softened band of material around the toolgets deformed to form stir zone and TMAZ. Because of thenarrowed softening, the extent of region undergoing plasticdeformation was limited in UWFSW joint. So a narrow TMAZwas formed in the UWFSW. All these facts are even moreclearly evident from the cross sectional temperature mapping(Fig. 13).

From the literature it was identified that the TMAZ of alu-minium alloys experiences the temperature range of around400 °C [33]. The cross sectional thermal contour map in theFig. 13 shows the peak temperature attained in the variousregion. FSW joint shows a wider TMAZ region and the tem-perature range was around 400 °C. At this temperature, thematerial gets softened and plasticized to form wider TMAZ.But in UWFSW joint, only a narrowed region experiences thesame temperature. This was the main reason for achieving con-stricted TMAZ in the water cooled-FSW joint. The adjacentregion was identified as HAZ region, which has peak tempera-ture of around 300 °C. The temperature value was enough toalter the grain size and precipitates in the material. In FSWjoint, a wider region experienced this temperature. But, thesame range of temperature values cannot be observed in thecontour map of UWFSW joint. Hence from the FEM results, itcould inferred that the temperature range affecting the micro-structure in HAZ was controlled significantly due to the highconvection heat transfer nature of water.

5.2. Microstructure

Microstructural investigation shows the grain size variationsalong different region. Comparing, the grain size in the SIR ofUWFSW joint is higher than the FSW joint. In UWFSW, thehigher tool rotation speed of 800 rpm results high friction forcebetween the tool shoulder and work piece interface. Hence, dueto the high intense frictional heat, the grains are large sized inSIR. Nearly 80% of total heat is generated at the tool shoulderand work piece interface. But in FSW joint, due to high thermalconductivity the heat generated is evenly distributed over thestir zone. The low tool travel speed of FSW process also allowstime to conduct the temperature over stir zone. Hence, the grainsize over SIR, MTR and PIR are similar in size. The rate of heattransfer to the backing plate is minimal because it previouslyexperienced heat due to conduction. So heat is build up in thePIR of FSW joint. In contrast, the backing plate experiencedminimal amount of heat in UWFSW. The high rate of heattransfer to cold backing plate does not allow heat to build up inPIR. So regardless of high temperature in SIR, PIR experienceslow temperature. Therefore, fine grains are formed in the PIR aslike FSW joint. Despite of the difference in temperature distri-bution and grain size, the stir zone microstructure exhibit fineequi-axed grains in the entire region due to the dynamic recrys-tallization aided by the interaction effect of plastic deformationand heat generation during FSW and UWFSW process.According to the Hall Petch relationship, the finer grain size inthe stir zone exhibits higher hardness than the other. It is

Fig. 13. Temperature profiles (a) Cross sectional temperature contour of FSWjoint (b) Cross sectional temperature contour of UWFSW joint (c) Thermalplots.

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interesting to observe that the peak temperature is noted in theweld periphery and not in the weld center for both the joints.This is because at the weld center the material experiences lessamount of tool rubbing than the region near to the peripheryregion of shoulder.

5.3. Precipitation behaviour

The precipitation sequence of Al–Cu alloy is as follows:supersaturated solid solution (SSSS) → Guinier–Preston zone(GPZ) → θ”→ θ’ → θ (stable) [34]. In both FSW and UWFSWprocess, the temperature attained in the stir zone was higherthan the solutionization temperature. Therefore the precipitatesget dissolved to form SSSS. From the precipitation sequence, itis understood that after solutionization, GPZ was formed in thealuminium matrix. During the cooling cycle of FSW andUWFSW process, GPZ is formed in the stir zone following thesolutionization. The formation of GPZ cannot be observed inTEM microstructures because it was few Angstrom in size.Hence, it was difficult to differentiate GPZ from PFZ in theTEM image.

From the thermal analysis, it can confirm that the heat avail-able in the TMAZ was not sufficient to reprecipitate under bothcooling conditions. By the sequence of precipitation, it wasagain confirmed that there was no possible formation of GPZand only solutionization occurred in the TMAZ irrespective ofthe cooling medium. The dissolution of precipitations results insolid solution strengthening of TMAZ. It was also observed thatthe thermal cycles prevailed in FSW enables the diffusion of Cuatom to form Cu rich Al4Cu9 phase in the weld joint. Moreoverthe presence of dislocations in the TMAZ results in strainhardening. From the OM, it was noticed that the TMAZ ofUWFSW underwent severe plastic deformation than the FSWjoint; therefore, more dense dislocations are observed (Fig. 8).

The HAZ region experiences the peak temperature of 200 °Cto 400 °C, which results in over aging. The coarsened of θ’precipitates to θ precipitates confirms the over aging of HAZ.Here, the heat conducted is insufficient to solutionize orreprecipitate. In the HAZ of UWFSW joint, the transformationfrom θ’ to θ precipitate was greatly reduced and the precipitatedistribution was more or less similar to the parent metal. Thisindicates that the over aging was greatly limited due to watercooling. The over aging of HAZ and the decrement in thehardness was also reported by Zhang et al. [35].

The presence of θ’ precipitate was preferable for the bettermechanical properties. During loading, the dislocations move-ment is hindered by the θ’ precipitate and thus the hardening ofmaterial takes place. The stable θ precipitate do not createsufficient strain misfit around the precipitate and also allows thedislocations to bypass through during loading. Similarly, PFZdeteriorates the mechanical properties because no precipitatesare available to hinder the dislocations. The PFZ are strength-ened by solid solution strengthening because of dissolution ofsolute particle in the matrix. However, the strengthening effectis lower than the precipitation strengthening. In external watercooling FSW process, the coarsening of precipitates and nar-rowing of LHDR were limited and thereby results in superiorproperties than the air cooled FSW joints.

5.4. Mechanical properties

The region of fracture can be correlated with the LHDR ofmicrohardness plot. In both the joints, the location of failurewas found to be exactly in the TMAZ-stir zone interface. Fromthis, it can infer that the extent of softening in LHDR of thejoint was a vital factor deciding the tensile properties. From theTEM analysis, it can understand that the softening of TMAZ isattributed to the presence of coarsened precipitates and precipi-tate free zone (Fig. 8). However, the hardness of the LHDR ofUWFSW is higher because of the high extent of precipitatehardening and strain hardening than the FSW joint. A similarstrengthening effect was previously reported by Liu et al. [36].The lower hardness of HAZ of FSW joint was attributed to thepresence of high volume fraction of coarse θ’ precipitates. Theover aging of HAZ was greatly reduced in the UWFSW whichresults in higher hardness than FSW joint. This was attributed tothe high cooling rate and the attainment of lower temperature inthe HAZ. During hardness indentation, the hardening precipi-tates were not available in the TMAZ to limit the plastic defor-mation. During tensile loading, the strain gets localized at thesofter TMAZ among the various regions of weld joint. Becauseof the strain localization, both FSW and UWFSW joints exhibitlower elongation than the parent metal (Table 2). Moreover,absence of necking formation was seen due to the severe strainlocalization. The SEM characterization of fracture surfacesrevealed dimples in both the UWFSW and FSW joints whichindicates that both the joints undergone ductile mode of failure.However UWFSW joint exhibits fine dimples than FSW joint.During tensile loading, the second phase precipitates act asthe nucleation sites for the voids formation. The voids werecoalesced to form the final fracture. Relatively less coarse andhigh volume fraction of precipitates in the UWFSW jointsprovides more nucleation sites than FSW joints and thereby finepopulated dimples were found.

6. Conclusions

The experimental and numerical investigation on underwaterfriction stir welding (UWFSW) and friction stir welding(FSW) of armour grade, high strengthAA2519-T87 aluminiumalloy was conducted and following important conclusions arederived:

1) The peak temperature experienced by the UWFSW jointis 547 °C which is higher compared to the peak tempera-ture experienced by FSW joint. However, UWFSW jointresulted in higher cooling rate and higher temperaturegradient than FSW joint due to severe and even heatabsorption capacity of the water cooling system.

2) The coarsening and dissolution of precipitates resulted inTMAZ as the weaker zone. The thermal gradient alongthe transverse and longitudinal axis of the joint is con-trolled by the water cooling and thereby the weakerTMAZ and HAZ are appreciably narrowed. Ultimatelyreduction of width of weaker zone and reduction of overaging of HAZ are the reasons for the marginal increase intensile properties.

332 S. Sree Sabari et al. /Defence Technology 12 (2016) 324–333

3) Among the various regions, RS-TMAZ exhibited lowerhardness of 90 HV and 94 HV in FSW and UWFSWjoints respectively. The difference in the microhardnesscannot be attributed with the grain size rather it is attrib-uted to the presence of heterogeneous precipitates acrossthe weld joints.

4) Under water FSW joint exhibited higher tensile strengthof 271 MPa and higher joint efficiency of 60% than con-ventional FSW joint. The controlling of thermal historiesand its subsequent effect on precipitation behaviour arefound to be the main reasons for the enhancement in thestrength of UWFSW joints.

Acknowledgement

The authors gratefully acknowledge the financial supportof the Directorate of Extramural Research & IntellectualProperty Rights (ER&IPR), Defense Research DevelopmentOrganization (DRDO), New Delhi through a R&D project no.DRDO-ERIPER/ERIP/ER/0903821/M/01/1404 to carry outthis investigation. The authors also wish to record their sincerethanks to M/S Aleris Aluminium, Germany for supplying thematerial to carry out this investigation

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