Effect of Process Parameters on Tensile Strength of Friction Stir Welded Cast LM6 Aluminium Alloy Joints

J. Mater. Sci. Technol., Vol.25 No.5, 2009 655

Effect of Process Parameters on Tensile Strength of Friction

Stir Welded Cast LM6 Aluminium Alloy Joints

M. Jayaraman1), R. Sivasubramanian2) and V. Balasubramanian3)†1) Department of Mechatronics Engineering, Kongu Engineering College, Erode, Tamil Nadu, India2) Department of Mechanical Engineering, Coimbatore Institute of Technology, Coimbatore, Tamil Nadu, India

3) Centre for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, AnnamalaiUniversity, Annamalainagar, Tamil Nadu, India

[Manuscript received September 22, 2008, in revised form February 20, 2009]

This paper reports the effect of friction stir welding (FSW) process parameters on tensile strength of castLM6 aluminium alloy. Joints were made by using different combinations of tool rotation speed, weldingspeed and axial force each at four levels. The quality of weld zone was investigated using macrostructureand microstructure analysis. Tensile strength of the joints were evaluated and correlated with the weld zonehardness and microstructure. The joint fabricated using a rotational speed of 900 r/min, a welding speedof 75 mm/min and an axial force of 3 kN showed superior tensile strength compared with other joints. Thetensile strength and microhardness of the welded joints for the optimum conditions were 166 MPa and 64.8 Hvrespectively.

KEY WORDS: LM6 aluminium alloy; Friction stir welding; Tool rotation speed; Welding speed;

Axial force; Tensile strength

1. Introduction

Al-Si alloys are one of the most commonly usedcast alloys because they offer many advantages suchas excellent castability, high strength to-weight ratio,wear and corrosion resistance, pressure tightness andgood weldability[1]. Applications of these alloys haveincluded automotive cylinder heads, engine manifolds,aircraft components and housings. Conventional fu-sion welding of cast aluminium alloys is generally dif-ficult due to some common defects such as porosity,oxide inclusion, hot cracking which in turn reduce theweld strength[2]. In recent years, friction stir welding(FSW) was found to be very effective for the weldingof various wrought aluminum alloys. FSW was alsoemployed on cast aluminium alloys in order to im-prove the strength of the weldments. FSW was usedfor joining cast Al alloy A413 and found that there isno weld zone degradation[3]. Dissimilar plate weldingwas made with Al 6061-T651 rolled plate and A356-T6 cast plate by FSW. The elongation of the jointwas found lower than that of the uni- alloy joint[4].Friction stir processing (FSP) technique was devel-oped based on FSW for improving the mechanicalproperties of cast Al alloys. The mechanical prop-erties of cast A356 Al alloy was improved by FSP dueto the elimination of porosity and refinement of themicrostructure[5]. More over the superplasticity andfatigue properties are also improved considerably[6].

The mechanical properties of cast A319 Al alloywas improved by FSP because of reduction in the sizeof second phase particles, uniform distribution of Siparticles and reduction of percentage porosity volumedue to the intense stirring[7]. The tensile strength andhardness of ADC12 aluminum die casting alloy wereimproved by FSP which is attributed to the elimi-nation of cold flake, uniform dispersion of Si parti-cles and the grain refinement of aluminium matrix[8].There is significant effect of tool rotation rate and

† Corresponding author. Prof., Ph.D.; Tel.: +91 4144239734;Fax: +91 4144 238080/238275; E-mail address: [email protected] (V. Balasubramanian).

traverse speed on microstructure and macrostructureof friction stir processed cast A356 Al alloy[9]. Thestrength and ductility of the multi pass FSP nuggetzone in A356 are similar to those achieved in thesingle-pass FSP sample[10].

It is already known that the joint between castAl alloys has a potential for expanding the usage ofeconomic casting in aircraft and automotive applica-tions. However, the investigations on FSW of castaluminium alloys are limited. Most of the reported lit-erature on FSW of cast aluminium alloy have focusedon microstructure, hardness and tensile properties ofFSP region alone. To widen applications of FSW tocast aluminium alloys, it is necessary to study jointproperties of the cast alloys. Hence, an attempt wasmade to understand the effect of FSW process para-meters on tensile strength of cast LM6 (also knownas A413) aluminium alloy.

2. Experimental

Castings of commercial LM6 aluminium alloy weremade by sand casting method and they were machinedto rectangular plates of 175 mm×75 mm×6 mm insize. The chemical compositions of LM6 alloy are pre-sented in Table 1. Square butt joint configuration asshown in Fig. 1 was prepared to fabricate FSW joints.

Table 1 Chemical compositions of LM6 (A413) alu-minium cast alloy (wt pct)

Si Fe Cu Mn Mg Zn Al11.6 0.32 0.03 0.04 0.05 0.01 Bal.

The initial joint configuration was obtained by secur-ing the plates in position using mechanical clamps.Non-consumable tool made of high carbon steel wasused to fabricate the joints. An indigenously designedand developed friction stir welding machine (15 hp,3000 r/min; 25 kN) was used to fabricate the joints.Joints were fabricated using different combinations oftool rotation speed, traverse speed and axial force.

656 J. Mater. Sci. Technol., Vol.25 No.5, 2009

Table 2 FSW process parameters and tool dimensions

Process parameters ValuesTool rotation speed/(r/min) 800,900,1100,1300Welding speed/(mm/min) 22,40,75,100Axial force/kN 2,3,4,5Pin length/mm 5.7Tool shoulder diameter, D/mm 18Pin diameter, d/mm 6D/d ratio of tool 3.0Tool pin geometry ThreadTool material high carbon steel

Fig. 1 Dimensions of square butt joint

Fig. 2 Dimensions of tensile specimen

The process parameters and their levels and tool di-mensions are presented in Table 2. Tensile specimens(as shown in Fig. 2) were machined in the traversedirection from the welded joints. Tensile test was car-ried out in 100 kN, servo controlled Universal Test-ing Machine (UNITEK-94100 FIE-Bluestar, India).Macro and microstructural analyses were carried outusing an optical microscope (MIL-7100 MEJI, Japan)incorporated with an image analyzing software (MetalVision MVLx1.0). The specimens for metallographicexamination were sectioned to the required sizes fromthe joint comprising weld zone (WZ), thermo me-chanically affected zone (TMAZ), heat affected zone(HAZ) and base metal (BM) regions. Usual metallo-graphic procedures were followed to polish the spec-imen and Keller′s reagent was used as an etchant toreveal the microstructure.

3. Results and Discussion

3.1 Macrostructure

During FSW, the material flows around thetool pin due to the heat generated by the fric-tion and stirring action. FSW joints are prone todefects like pin hole, tunnel, cavity, kissing bond,cracks etc due to insufficient and excess heat in-put in the stir zone[11]. All the joints fabricatedin this investigation were analysed at low magni-fication (10X) by optical microscopy to reveal thequality of weld zone. The macrographs of the weldnugget for four levels of rotational speed, weld-ing speed and axial force are presented in Tables3–5. Of the four tool rotation speeds used to

Fig. 3 Optical micrograph of LM6 aluminium alloy

fabricate the joints, rotational speed of 900 r/min pro-duced defect free weld. Similarly, the joint fabricatedusing a welding speed of 75 mm/min contains no de-fect. An axial force of 3 kN yielded defect free joint. Itis also observed that the macrograph consists of WZ,TMAZ and BM regions and absence of HAZ region.

3.2 Tensile strength

Transverse tensile strength of FSW joints are pre-sented in Figs. 4–6. Three specimens were tested ateach condition and the result in average of three speci-mens is presented. From the figures, it can be inferredthat the tool rotation speed, welding speed and axialforce have significant influence on tensile strength ofthe FSW joints of LM6 aluminium alloy. Of the fourjoints, the joint fabricated with a tool rotation speedof 900 r/min showed higher tensile strength comparedwith the other joints (Fig. 4). Similarly, the joint fab-ricated with welding speed of 75 mm/min (Fig. 5) andthe joint fabricated with an axial force of 3 kN (Fig. 6)exhibited superior tensile strength compared to theircounter parts.

3.3 Microstructure and microhardness

During the tensile test, most of the specimensfailed in the weld nugget (Tables 3-5) and hencethe microstructure analysis and microhardness mea-surements were made in the weld nugget to iden-tify the reasons. Microhardness values in the weldnugget of FSW joints are presented in Figs. 7–9.The microstructure of base metal (LM6 aluminiumalloy) consists of acicular eutectic Si embedded in α-aluminium matrix (Fig. 3). Figures 10–12 show opti-cal micrographs of weld nugget of all the joints. Thereis significant breakup of Si particles, subsequentlycreating a uniform distribution of finer Si particlesin the α-aluminium matrix. It is due to the stir-ring action at plastic condition of the metal duringFSW. This phenomenon was also reported by otherinvestigators[12,13].

4. Discussion

From the experimental results of macrostruc-ture, microstructure, hardness and joint strength,it is found that the joint fabricated at 900 r/min,75 mm/min and 3 kN exhibited superior tensilestrength compared with the other joints. The rea-sons for the better performance of these joints areexplained below.





Fig. 4 Effect of rotation speed on tensile strength (weld-ing speed =75 mm/min; axial force =3 kN)

Fig. 5 Effect of welding speed on tensile strength (rota-tion speed =900 r/min; axial force =3 kN)

Fig. 6 Effect of axial force on tensile strength (rotationspeed =900 r/min; welding speed =75 mm/min)

4.1 Effect of tool rotation speed

In FSW, tool rotation speed results in stirring andmixing of material around the rotating pin which in

Fig. 7 Effect of rotation speed on microhardness (weld-ing speed =75 mm/min; axial force =3 kN)

Fig. 8 Effect of welding speed on microhardness (rota-tion speed =900 r/min; axial force =3 kN)

Fig. 9 Effect of axial force on microhardness (rotationspeed =900 r/min; welding speed =75 mm/min)

turn increase the temperature of the metal. It ap-pears to be the most significant process variable sinceit tends to influence the transitional velocity. It isknown that the maximum temperature was observedto be a strong function of rotation speed. When therotational speed increases, the heat input within thestirred zone also increases due to the higher frictionheat which in turn result in more intense stirring andmixing of materials[14]. For the given welding speed


Fig. 10 Effect of tool rotation speed on microstructure of weld nugget (welding speed =75 mm/min; axial force=3 kN)

Fig. 11 Effect of welding speed on microstructure of weld nugget (rotation speed =900 r/min; axial force =3 kN)

and axial force, the increase in rotation speed be-yond 900 r/min produces tunnel defect in the retreat-ing side due to abnormal stirring. The turbulenceof softened metal is not consolidated in the retreat-ing side (Table 3). Similar phenomenon was reportedby Ma et al. for the rotation speed of greater than700 r/min[15]. Tensile strength of the joint made with900 r/min was higher because of the sound joint withoptimum heat input in the stir zone, whereas thestrength was reduced with the increase in the rota-tion speed due to the formation of defects. The higher

tensile strength is also attributed to the uniform dis-tribution of fine eutectic Si particles in the aluminiummatrix of the stir zone (Fig. 10(b)) whereas the re-duction of the strength is due to the coarse eutecticSi particles and nonhomogeneous distribution in thematrix[16]. This is due to the turbulence of softenedmetal at higher rotation speeds in which the brokenSi particles are clustered to be coarse and segregated(Fig. 10(c) & (d)). The fracture occurred at the basemetal region during tensile test for the joint madewith rotation speed of 900 r/min due to the enhance-


Fig. 12 Effect of axial force on microstructure of weld nugget (rotation speed = 900 r/min; welding speed=75 mm/min)

ment of the strength and hardness of the stir zone.But the location of the failure for other joints is in-variably at the TMAZ of retreating side due to thepresence of defects.

4.2 Effect of welding speed

The translation of tool moves the stirred mater-ial from the front to the back of the pin. The rateof heating in a thermal cycle during FSW is a strongfunction of the welding speed[17]. During FSW ther-mal cycle, most of the Mg2Si precipitates, the pri-mary strengthening phase in A356, are dissolved intothe aluminium matrix. Fast cooling of FSW thermalcycle retains these solute atoms. Lim et al.[18] investi-gated the tensile behaviour of AA6061 aluminum alloyand showed that the main cause of the change in thetensile behaviour of friction stir welded AA 6061-T651alloy with varying welding condition was the amountof plastic flow per unit time rather than the heat gen-erated during the solid welding. Severe clustering ofcoarse Mg2Si precipitates was observed in the tensilefractured area for each welding condition and the clus-tering was more significant for the specimens joinedat low welding speed.

Of the four joints fabricated using different weld-ing speeds, the joints fabricated using a welding speedof 75 mm/min exhibited superior tensile strength dueto the formation of fine eutectic Si particles uniformlydistributed in the aluminium matrix and also absenceof defects in the stir zone (Fig. 11(c)). The jointsfabricated at welding speed of 22 mm/min consistof coarser Si particles and banded structure in thestir zone (Fig. 11(a)) which in turn, reduces the jointstrength. Similarly, the joints fabricated at weldingspeed of 40 mm/min consist of channel defect at theretreating side of the stir zone and consist of coarser

Si particles (Fig. 11(b)). The optimum heat inputand the distribution of fine Si particles are the rea-sons for the enhancement of joint strength at weldingspeed 75 mm/min under constant rotation speed of900 r/min.

4.3 Effect of axial force

Material flow in the weld zone is influenced by theextrusion process, where the applied axial force andthe motion of the tool pin propel the material after ithas undergone the plastic deformation. The shoulderforce is directly responsible for the plunge depth of thetool pin into the work piece and load characteristicsassociated with linear friction stir weld[19]. As the ax-ial load increases, both hydrostatic pressure beneaththe shoulder and the temperature in the stir zone willincrease.

It is well known that the lower axial load resultedin defects in the weld because of insufficient coales-cence of transferred material[20]. Ouyang et al.[21] re-ported that at steady state, the shoulder force variesdepending upon the rotational speed. Increase in ro-tation speed resulted in drop in initial axial force withincreasing time. The difference in the measured forcesis due to the decrease of the material flow stress atelevated weld temperature. Krishnan[22] studied themechanism of onion ring formation in the friction stirwelds of aluminium alloys and found that the degreeof material mixing and inter-diffusion, the thickness ofdeformed aluminium lamellae, and material flow pat-terns highly depend upon the geometry of the tool,welding temperature, and material flow stress in turndepends on the axial force. Hence, the axial forcemust be optimized to get FSP zone with good con-solidation of metal and without thinning of the basematerial[23].


Of the four axial force levels used to fabricatethe joints, the joint fabricated with an axial forceof 2 kN resulted tunnel defect at the middle of theweld zone due to insufficient material flow and themicrostructure consists of coarse eutectic Si particles(Fig. 12(a)), even though uniform distribution in thealuminium matrix. But the joint fabricated with 4 kNaxial force resulted in nonuniform distribution of Siparticles and thinning of weld nugget due to the excessaxial force (Fig. 12(c)). Due to the high heat input themetal gets softened and extruded as flash. Similarlythe joint fabricated with 5 kN exhibited the same pat-tern (Fig. 12(d)). But the joint fabricated with 3 kNaxial force consists of fine, eutectic Si particles withuniform distribution throughout the aluminium ma-trix (Fig. 12(b)) due to the sufficient flow of softenedmaterial. This may be the reason for higher tensilestrength of the joints fabricated with 3 kN axial forcecompared to their counterparts.

4.4 General discussion

The heat input and material flow behaviour de-cides the quality (defective or defect free) of FSWjoints. The heat input and material flow behaviourare predominantly influenced by the FSW process pa-rameters such as tool rotation speed, welding speedand axial force. The heat input increases with in-crease in rotation speed and axial force and decreaseswith the increase in welding speed. At lower rotationspeed, the heat input is not sufficient and also im-proper stirring causes a tunnel defect at the middleof the retreating side. Higher rotation speeds couldraise the strain rate and turbulence (abnormal stir-ring) in the material flow caused a tunnel defect atthe weld nugget. As the rotation speed increases, thestrained region widens, and the location of the maxi-mum strain finally moves to the retreating side fromthe advancing side of the joint. This implies that thefracture location of the joint is also affected by therotation speed.

Low welding speeds resulted in higher heat inputand excess turbulence of the plasticized metal whichcaused a tunnel defect at the top of weld nugget.Higher welding speeds are associated with low heat in-puts, which result in faster cooling rates of the weldedjoint. This can significantly reduce the extent of met-allurgical transformations taking place during weld-ing and hence the local strength of individual regionsvaries across the weld zone.

Material flow in the weld zone is influenced by theextrusion process, where the applied axial force andthe motion of the tool pin propel the material after ithas undergone the plastic deformation. The shoulderforce is directly responsible for the plunge depth ofthe tool pin into the work piece and load character-istics associated with linear friction stir weld. Recentinvestigation on various FSW alloys has shown thatthe tensile fracture site is dependent on the alloy typeand also on the welding parameters[24]. For this al-loy the retreating side of the weld zone was found tobe the major fracture site where strain localizationoccurred.

If the joints are defective, then the failure will takeplace along the defects. If the joints are defect free,then the failure will take place along the lowest hard-

Fig. 13 Microhardness profile across the weld nugget (ro-tation speed 900 r/min, welding speed 75 mm/minand axial force 3 kN)

ness region. In this case, base metal is having lowerhardness compared to the weld nugget region due topresence of porosity and coarse silicon needle like par-ticles. The hardness profile across the weld nuggetshown in Fig. 13 clearly depicts that the unaffectedbase metal is having lower hardness compared to otherregions.

During microstructure analysis, it was observedthat the weld nugget region (stir zone) and a very nar-row TMAZ region ran across the weld cross section.After TMAZ region, only the unaffected base metalregion alone was observed. Similar observations weremade by the previous investigators also[25,16]. Thematerial flow behaviour will be different for cast alloysand wrought alloys due to large difference in ductility.Usually cast alloys will have lower ductility comparedto wrought alloys due to the presence of porosity, in-clusions and higher amount of silicon. This will re-duce the ductility of cast alloys and subsequently af-fect the material flow behaviour under the action ofrotating FSW tool. Moreover, the heat distribution(heat transfer) from the weld nugget region to adja-cent regions will also be influenced by the above saidfactors. So, low ductility and low heat conduction ofcast alloys will lead to the formation of very narrowTMAZ region alone adjacent to the weld nugget re-gion and this may be one of the reasons for absenceof HAZ region in FSW of cast alloys.

To get defect free joints, the heat input mustbe optimized by optimizing FSW process parameterssuch as the rotation speed, welding speed and ax-ial force. In this investigation, the maximum tensilestrength of 166 MPa was attained for the joint fab-ricated using the welding parameters of 900 r/min,75 mm/min and 3 kN and this may be due to theoptimum heat generation under these welding condi-tions, which led to the fabrication of defect free joint.

5. Conclusions

(1) Of the twelve joints fabricated, the joint fabri-cated using the process parameters of 900 r/min (toolrotation speed), 75 mm/min (welding speed) and 3 kN(axial force) yielded higher tensile strength comparedto other joints.

(2) Defect free weld nugget, higher hardness ofweld nugget and very fine, uniformly distributed eu-tectic Si particles in the weld nugget are found to bethe important factors responsible for the higher ten-sile strength of the above joints.


AcknowledgementsThe authors are grateful to the Department of Man-

ufacturing Engineering, Annamalai University, Anna-malainagar, Tamil Nadu, India for extending the facilitiesof Metal Joining Laboratory and Materials Testing Labo-ratory to carry out this investigation.

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Effect of Process Parameters on Tensile Strength of Friction Stir Welded Cast LM6 Aluminium Alloy Joints

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