EXPERIMENTAL STUDY OF FRICTION STIR … STUDY OF FRICTION STIR WELDING OF 6061-T6 ... Special welding fixture fixed on conventional milling machine has been ... chain as shown in the

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Int. J. Mech. Eng. & Rob. Res. 2012 Qasim M Doos and Bashar Abdul Wahab, 2012

EXPERIMENTAL STUDY OF FRICTION STIRWELDING OF 6061-T6 ALUMINUM PIPE

Qasim M Doos1* and Bashar Abdul Wahab1

*Corresponding Author: Qasim M Doos,[email protected]

Friction Stir Welding (FSW) is a relatively new joining process that has exhibited many advantagesover traditional arc welding processes, including greatly reducing distortion and eliminatingsolidification. The present work aims to determine the feasibility to weld two pieces of aluminumpipe by friction stir welding process and study the effect on the mechanical properties of weldingjoints. Special welding fixture fixed on conventional milling machine has been conducted toattempt this welding and group of welding parameters. Three tool rotational speeds (500, 630,800 rpm) with four welding speeds (0.5, 1, 2, 3 mm/sec) for each rotational speed had beenused to study the effect of each parameters (tool rotation, weld speed) on mechanical andmicrostructure properties of welded joints. Mechanical properties of welded joints wereinvestigated using different mechanical tests including non destructive test (visual inspection,X-ray) and destructive test (tensile test, microhardeness and microstructure). Based on the stirwelding experiments conducted in this study the results show that aluminum pipe (AA 6061-T6)can be welded by (FSW) process with a maximum welding efficiency (61.7%) in terms of ultimatetensile strength, using 630 (RPM) rotational speed, 1 (mm/sec) traveling speed.

Keywords: Friction Stir Welding, 6061-T6 Al Pipe, Feasibility, Mechanical properties

INTRODUCTIONFSW is a rapidly maturing solid state joiningprocess that offers significant benefits overconventional joining processes. Invented byThe Welding Institute (TWI) in 1991, FSW usesa combination of frictional heating andcompressive loading to join metal plates thatare butted against each other and tightly

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Int. J. Mech. Eng. & Rob. Res. 2012

1 Faculty of Engineering/Mechanical Department, University of Baghdad, Baghdad, Iraq.

clamped to the anvil of the machine. Extensiveresearch has been accomplished ondeveloping the friction stir welding process foraluminum alloys, but there are relatively fewpapers about aluminum pipes welding.Gerçekcioglu et al. (2005). Investigated thefriction behavior on the worn surface of AA6063-T6 tubes welded via friction stir welding

Research Paper

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method. In this study, the effective of rotationalspeed of stirrer on the outer surface of frictionstir welding has been investigated.

Dubourg et al. (2009), the study shows thepreliminary results on Friction Stir Welding(FSW) of 2024-T3 aluminum alloy tubes andthe impact of the welding process on weldquality. Welding was performed on tubes withsimilar thickness. The mechanical propertiesof the welds were assessed by hardness andtensile measurements on as-welded and heattreated tubes.

This paper will focus on the mechanicalproperties of the welded joints of AA 6061-T6tubes after friction stir welding.

EXPERMINTAL WORKDue to of the non availability of specialistmachine in Iraq for FSW process. Aconventional vertical milling machine was usedto attempt the welding process as shown inthe Figure 1. The machine must has the abilityto apply significant pressure on z axisdirection, wide range of spindle speed,enough space for its working table to holdingthe welding assembly and rigidly during thewelding operation.

The milling machine used has rotationalspeed on the head that is suitable to fixingwelding tool on it, but it has linear speed ontable that is not match with the objective of thestudy (need circular rotational feed) so thatsome apparatus were adapted on its workingtable. Figure 2 shows schematically FrictionStir Welding (FSW) apparatus andequipments.

Figure 1: Milling Machine

Figure 2: Illustration of Friction StirWelding Equipment

A rotating clamping fixture where designedto clamps and hold the two segments of pipetogether for butt joint welding.

The clamping fixture consist the following:

• Gear box with a rotating clamp hold the pipefrom one side and can be fixed tightly to theworking table of the milling machine asshown in Figure 3, the reduction ratio forthe gearbox is 40 to 1 rpm.

• Fixing mandrel hold the other side of pipeand tightly forced to the two pieces,preventing them from any horizontalmovement during welding process.Themandrel can freely rotate 360 degree withrotational speed of pipe and also can befixed on the working table of the millingmachine as shown in Figure 4.

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• To provide the internal pipe support toprevent wall collapse due to the highpressures exerted by the FSW process, aninternal, anvil was designed. anvil fixedinside the two pieces of pipes beforewelding operation by use hydraulic press.Figures 5 and 6 show the anvil, fixinghydraulic press

• An external anvil used to prevent bendingof the two pieces of pipe due to the forceapplied from the tool and tool shoulderduring welding operation. Figures 7 and 8show the external anvil position duringwelding operation.

Figure 3: Rotating Clamp Gearbox

Figure 4: Fixing MandrelFigure 6: Hydraulic Press

Figure 7: External Anvil

Figure 5: Internal Anvil

Piece ofPipe

InternalAnvil

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• To have the rotational speed for theclamping gearbox, driving gearboxconnected to it by means of two gears andchain as shown in the Figure 9.

a mixture of frictional heating and forgingpressure, most of heat generation related tothe tool shoulder while the probe Whendescended to the part, the rotating pin contactsthe surface producing frictional heating whichsoftens a small column of metal.

Shoulder diameter 20 mm and it fit to theholder of milling machine.

Pin has conical shape start from diameter5 mm to 3 mm and length of pin is 4.2 mm.

Set of parameters were used as shown inthe Table 1.

Figure 8: External Anvil Position

Figure 9: Driving Gearbox

Specification of driving gearbox. Reductionratio 1500 to 80 rpm.

Motor 380V, 50HZ, 0.75hp.

• To control speed of driving gearbox aninverter AC driver fixed on the driving gear.

High speed steel material was used forwelding tool. Figure 10 show the tool geometrywhich is consist from two main part shoulderand probe. The cylindrical shoulder produces

Figure 10: Welding Tool

Stir Pin

Shoulder

500 0.5

1

2

3

630 0.5

1

2

3

800 0.5

1

2

3

Table 1: Variable Parameter Used

Rotational Speed (rpm) Weld Speed (Feed)(mm/sec)

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RESULT AND DISCUSSIONSVisual Inspection Results

Visual inspection was performed on all weldedsamples were obtained from different weldingparameters in order to verify the presence ofpossible macroscopic external defects, suchas surface irregularities, excessive flash, andlack of penetration or surface-open tunnels.

Visual Inspection Prior WeldingAfter the preparation the two segments ofpipes and fixed it on the welding fixture Somefeatures that might be affect the quality ofwelding should be inspected as following.

• The edges for the two segments thatwelding occur on it should be good finish,faced and cleaned surface from oil and dirtto get defect free welded joints.

• Internal anvil securely fixed inside the twosegments of pipe without any gap betweenthe welding edges, this gap if it exists effectsweld quality and then the welded joint maybe has a defect in the welded line.

• Tightly clamped the two segments on thewelding fixture preventing the assemblefrom any movement during weldingoperation.

• Positioning accurately the external anvildawn the interface of the two edges on thesame axis of welding tool (z axis),otherwisebending occurred due to force applied fromtool shoulder and probe on the pipe thatproduce gab dawn the interface of twosegments.

Visual Inspection During WeldingProcess

The Second stage on visual inspection wasperformed during welding process to check:

• Initial starting point is the most importantthat pin must be come to the interface bothpipes. Besides, the bond desired betweenthe both pipes cannot be taken place well.And also, some gaps and cracks can beoccurred as shown in Figure 11.

Figure 11: The Welding Surface Nearthe Initial Point

• Excessive lateral flash was also observedin most of the welds resulting from theoutflow of the plasticized material fromunderneath of the shoulder due to highplunging tool depth Figure 12.

Figure 12: Lateral Flash in the Joint

Lateral Flash

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• External surface behavior (rough or smooth)and that depends on welding parametersfrom rotational speed of tool and weldingspeed.

Visual Inspection After WeldingProcess

Final Welding joint inspected in this stage fromweld face and root. The visual defects fromthis inspection are:

• In the weld area Figure 13, the thicknessreduction from 5 to 4.8 mm was alwaysobserved on advancing side, even for theoptimized welding parameters. This defectwas named under fill and this formation canbe explained considering contactconditions between the welding tool and thepipe.

• Inspection the final weld faces for anydefects found due to tool shoulder.

• Tunnel type defect and crack whereobserved due to inadequate flow of materialcaused by insufficient heat input when highrotational tool speed (800 rpm) or hightraveling speed (2 mm/sec) where used fordifferent welding parameter.

Radiography Inspection Results

According to radiography inspection results(which conducted using 100 KVA and 5 mAsource, Exp. time 36 sec), only two joints hadbeen accepted from the inspector (sample 5,8 which is represent the experiment, FSW6and 7) others have internal defect in weld line.The type of defect is evaluated as incompletefusion (tunnel or worm hole), incomplete cap.

It is believed that these defects areattributable to the combination of the followingparameters: insufficient or excessive rotationalspeed or welding speed combined with toolow downward force. In such case the weldedparts cannot be correctly stirred and mixedtogether and a tunnel (also called “worm hole”)is created, running along the entire weld(Adamowski and Szkodo, 2007).

Radiography inspection results did not (asexpected) reveal any type of porosity or blowholes or any type of hot cracking because nomelting and freezing occurs in welding processdue to the solid state nature of (FSW) process.

Figures 14 show samples of radiographicinspection film, which were conducted usingthe above inspection parameters.

Figure 13: Cross Sectionsof a Circumferential Weld

Under Fill

RS AS

Figure 14: X-Ray Radiographic Image forSelected Sample

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Tensile Test Results

Tensile tests were conducted for each weldingjoint that produced from different weldingparameters. The test results compared withthe tensile properties of base metal and thewelding efficiency (based on ultimate tensilestrength) for friction stir welding have beencalculated for each experiment.

Table 2 show the tensile tests results andwelding efficiency for each weldingparameters.

with different welding speed (0.5, 1, 2, 3 mm/sec) and from above results we see increasingin tensile strength with increase the weldingspeed from 0.5 to 1 mm/sec but whenincreasing the welding speed to 2 and 3mm/s the tensile properties of the jointsseriously deteriorate and reach to minimumvalue 38.25 mpa for welding speed 3 mm/swith 800 rpm, joint efficiency 13.18.

Its seen that decreasing in tensile strengthdue to inadequate flow of material caused byinsufficient heat input.

The faster welding speed leaves less heatto the work piece and thus is generally called“cold weld” (Weis, 2008).

It can be explained that the weld material isunable to accommodate the extensivedeformation during welding. This may result inlong, tunnel defects running along the weldwhich may be surface or subsurface. Lowtemperatures may also limit the forging actionof the tool and so reduce the continuity of thebond between the materials from each side ofthe weld (Kumbhar and Bhanumurthy, 2008).

Figure 16 show the relation between therotational speed for the stir tool with tensilestrength of the welded joint. From the result

Base Metal – – 290.18 –

FSW 1 500 0.5 102.56 35.30

FSW 2 500 1 163.49 56.34

FSW 3 500 2 FAIL –

FSW 4 500 3 FAIL –

FSW 5 630 0.5 142.98 0.49

FSW 6 630 1 179.00 61,7.00

FSW 7 630 2 163.4 56,34.00

FSW 8 630 3 57.60 19,86.00

FSW 9 800 0.5 136.39 47.00

FSW 10 800 1 114.21 39.38

FSW 11 800 2 45.68 15.75

FSW 12 800 3 38.25 13.18

Table 2: Tensile Test Results

FS

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Ro

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(rp

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Wel

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gS

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(m

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Ten

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(N/

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fTe

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(%)

In FSW, the properties of the weld joints arehighly dependent on the operationalparameters and the materials to be welded.Two of the most important welding parametersare the rotational speed and the welding speed(travel speed).

Figure 15 show the tensile test results forthree rotational speed (500, 630, 800 rpm)

Figure 15: Relation Between theRotational Speed with Tensile Strength of

Welded Joints

0.5 mm/sec

Rotation Speed (rpm)

Ten

sile

Str

en

gth

N/m

m2

1 mm/sec

2 mm/sec

3 mm/sec

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above the tensile strength increase when therotational speed increase from 500 to 630 rpmand reach maximum value 179 mpa with jointefficiency 61.7%, but increase the weldingspeed from 630 to 800 rpm decrease thetensile strength of the welding joint and thatattribute to increase the heat input to thewelding area due to higher tool rotation ratesgenerate higher temperature and softeningtakes place in the nugget zone because of thedissolution or growth of strengtheningprecipitates during the welding thermal cycle,thus resulting in the degradation of themechanical properties of the joints (Liu et al.,2003b).

Microstructure Results

Friction Stir Welding (FSW) processgenerates large local deformations and heatcycles, which consequently modify themicrostructure and mechanical properties ofseveral zones in the joint.

The examination of many friction stir weldsin aluminum alloys has revealed that there arefour major micro structural zones, asindicated in Figure 17. The microstructure ofthe weld is complex and highly dependent onthe position within the welded zone. The fine

grains are found in the nugget zone, incontrast to the TMAZ and HAZ zones wherecoarsened grains are observed(Soundararajan et al., 2006).

The thermo-mechanically affected zone(TMAZ) is an area that has been plasticallydeformed by the friction stir welding tool, andthe heat from the process will also have exertedsome influence on the material (Soundararajanet al., 2006).

The Heat Affected Zone (HAZ) lies furtherfrom the weld center. The material hasexperienced a thermal cycle, andmodifications in mechanical properties andmicrostructure are noticed. However, no plasticdeformation occurs in this zone.

The parent material has not been deformedand may have experienced a thermal cyclefrom the weld but has not been affected by theheat in terms of microstructure or mechanicalproperties (Soundararajan et al., 2006).

Figures 18-28 shows the microstructure ofnugget zone (WN) for all parameters used inthis study, it shows the degree of refining grainsize for every type of welded joints.

Figure 16: Relation Between WeldingSpeed with Tensile Strength of Welded

Joints

500 rpm

Welding Speed (mm/s)

Ten

sile

Str

en

gth

N/m

m2

630 rpm

800 rpm

Figure 17: Friction Stir WeldingMacrostructure Zones

With of Tool Shoulder

Note: A – Unaffected Material, B – Heat-Affected Zone (HAZ),C – Thermo-Mechanically Affected Zone (TMAZ), D –Weld Nugget (WN).

Source: Indira et al. (2011)

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Figure 21: WN FSW5 400X





Figure 18: Base Metal 400X

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Figure 24: WN FSW8 400X Figure 27: WN FSW11 400X


Micro Hardness Test Results

To examine local variat ions in themechanical properties that occur in andaround the friction stir weld zone, thehardness of the welding zone was tested.The Vickers hardness profiles across theWeld Nugget (WN), Thermo-MechanicallyAffected Zone (TMAZ), Heat Affected Zone(HAZ) and partial base material wasmeasured under the load 0.5 kg for 15 sec.along the centerlines of The spacemen andVickers indentations with a spacing of 1 mmwere used as shown in the Figure 29.

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Figure 29: Positions of Micro HardnessTest Indentations (1 mm) Spacing

Weld Center

It can be seen from (Figures 30 to 39) thata hardness degradation region, i.e., softenedregion, has occurred in each joint, thus thetensile properties of the joint are lower thanthose of the base material. It can believed thatFSW creates a softened region around theweld center in a number of precipitation-hardened aluminum alloys [6061-t6]. It wassuggested that such a softening is caused bycoarsening and dissolution of strengtheningprecipitates during the thermal cycle of theFSW (Mishra and Ma 2005).

Another observation of the micro hardnessresults is the hardness on one side of weldcenter differs from the other side(unsymmetrical weld). This difference can beexplained as follows: In the leading side

Figure 30: Microhardness Profilein Welded Joint/FSW1

Distance from Weld Center (mm)

Vic

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s(H

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00

)



Vic

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s(H

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00

)Figure 32: Microhardness Profile

in Welded Joint/FSW5


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V5

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)Figure 38: Microhardness Profile

in Welded Joint/FSW11


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(advancing side) for the rotating tool where therotational velocity vector and the forwardmotion vector are in the same direction anddue to this there is higher heating on one sideof the weld center and hence lower hardens.

Microhardenss profiles below show, thereare two low-hardness zones on both sides ofthe weld center and they correspond to the twoHAZs in the joint (Liu et al., 2003a).

Considerable softening was observed inthe weld zone. However, the hardness in theweld nuggt and the thermal-mechanicallyaffected zones is higher than that in the heat-affected zone. This difference is attributableto the dynamic recrystallization that took placewithin the weld zone as a result of the frictionheat and mechanical work . In the heat-affectedzone, the hardness increases with increasingdistance from the weld center and approachesthat of the base metal at 12-15 mm from theweld center (Wang et al., 2000).

Another indication from microhardenssresults that increase with increasing weldingspeed for the same rotational speed for allwelding parameter and it can be attribute tothe low heat input when increasing weldingspeed during welding operation and thatresulted in less material softening and it’s thesame observation seen in the paper (Huijieet al., 2003).

CONCLUSIONAccording to results of the present study of(FSW) process on selected Al-alloy, severalconclusions can be written regarding alloyweldability, mechanical and microstructural.

• Aluminum alloy (6061-T6) are weldableusing different (FSW) parameters givingdifferent welding efficiencies.

• The maximum weld strength obtained in thisstudy was (179 MPa) of (61.7%) weldefficiency is recorded and this result hadbeen obtained with welding parameters(rotational speed 630 rpm,1 mm/secwelding speed and 0.2 mm plunging depth).

• FSW defects as indicated on nondestructive tests are related to the weldingparameters, defect free weld of FSWobtained by using optimum weldingparameter.

• Fine grain size microstructure obtained onweld nugget of all FSW joints.

• Microhardness drop was observed in theweld region of FSW joints and an increasein values of microhardeness whenincreasing welding speed.

REFERENCES1. Adamowski J and Szkodo M (2007),

“Friction Stir Welds (FSW) of AluminumAlloy AW6082-T6”, Journal ofAchievements in Materials andManufacturing Engineering, Vol. 20.

2. Huijie Liu, Fujii H, Maeda M and Nogi K(2003), “Tensile Properties and FractureLocations of Friction-Stir Welded Jointsof 6061-T6 Aluminum Alloy”, Journal ofMaterials Science Letters, Vol. 22.

3. Indira Rani M, Marpu R N and Kumar A CS (2011), “A Study of Process Parametersof Friction Stir Welded AA 6061 AluminumAlloy in O and T6 Conditions”, Journal ofEngineering and Applied Sciences.

4. Kumbhar N T and Bhanumurthy K (2008),“Friction Stir Welding of Al 6061 Alloy”,Asian Journal Experimental Sciences,Vol. 22, No. 2.

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5. Liu H J, Fujii H, Maeda M and Nogi K(2003a), “Comparative Study on FractureLocations of Friction Stir Welded Jointsfor Different Types of Aluminum Alloys”,Proc. 4th Int. Friction Stir Welding Symp.,CD-ROM: SESSION 05B, TWI Ltd., ParkCity, Utah, USA.

6. Liu H J, Fujii H, Maeda M and Nogi K(2003b), “Tensile Properties and FractureLocations of Friction-Stir-Welded Joints of2017-T351 Aluminum Alloy”, Journal ofMaterials Processing Technology,Vol. 142.

7. Maria Posada, Jennifer P Nguyen, DavidR Forrest and Johnnie J DeLoach (2003),“Friction Stire Welding Advances JoiningTechnalogy”, AMPTIAC Quarterly, Vol. 7,No. 3.

8. Mishra R S and Ma Z Y (2005), “FrictionStir Welding and Processing”, MaterialsScience and Engineering, R 50, pp. 1-78.

9. Rowe C E D and Wayne Thomas(2007), “Advance in Tooling Materialsfor Friction Stir Welding”, TWI andCedar Metals Ltd.

10. Soundararajan V, Valant M and KovacevicR (2006), “An Overvew of R&D Work inFriction Stir Welding at SMU”, Associationof Metallurgical Engineers of Serbia,Vol. 12, No. 4.

11. Wang X-L, Feng Z, David S A, SpoonerS and Hubbard C R (2000), “NeutronDiffraction Study of Residual Stresses inFriction Stir Welds”, Oak Ridge NationalLaboratory, Oak Ridge and EdisonWelding Institute.

12. Weis Olea C A (2008), “Influence ofEnergy Input in Friction Stir Welding onStructure Evolution and MechanicalBehaviour of Precipitation-Hardening inAluminium Alloys (AA2024-T351,AA6013-T6 and Al-Mg-Sc)”, Institute ofMaterials Research.

EXPERIMENTAL STUDY OF FRICTION STIR … STUDY OF FRICTION STIR WELDING OF 6061-T6 ... Special welding fixture fixed on conventional milling machine has been ... chain as shown in the

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