Interfacial Microstructure and Mechanical … Microstructure and Mechanical Properties of Dissimilar Friction Stir Welds 975 significantly fine compared with the initial grain size

Interfacial Microstructure and Mechanical Properties of Dissimilar Friction

Stir Welds between 6061-T6 Aluminum and Ti-6%Al-4%V Alloys*

Ki-Sang Bang1;2, Kwang-Jin Lee1, Han-Sur Bang2 and Hee-Sun Bang2

1Automotive Components Center, Korea Institute of Industrial Technology,Korea 1110-9 Oryong-dong, Buk-gu, Gwangju 500-480, Korea2The Institute of Joining and Manufacture, Chosun University,Korea 375 Seosuk-dong, Dong-gu, Gwangju 501-759, Korea

Friction stir welding (FSW) was performed about AA6061-T6 and Ti-6Al-4V alloy sheets. A unique shaped tool with circular truncatedcone of probe was used. Mechanical properties and interfacial microstructure were evaluated using tensile test, hardness test, optical microscopy(OM), scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM), respectively. Root area of probe in stirzone (SZ) reveals a mixture of finely recrystallized grains of Al and Ti particles pushed away from the base metal by strong stirring of probe. Thejoint interface of tip area of probe was relatively flat because stirring between aluminum and titanium alloy was not occurred due to the gap of theprobe and titanium alloy front. It is considered that the insufficient stirring due to inclined side of the probe was contributed to the decrease ofweld strength. After tensile test, fracture surface was analyzed by SEM. In the probe root area, dimples of Al were observed. In the probe tip area,the initial surface of titanium alloy plate was observed. However, in the middle area, similar amount of Ti and Al was detected. As result, it wasconfirmed that the fracture sequence was very complex and the fracture position was different according to the probe position.[doi:10.2320/matertrans.L-MZ201114]

(Received October 1, 2010; Accepted November 29, 2010; Published April 20, 2011)

Keywords: friction stir welding (FSW), AA6061-T6, Ti-6Al-4V, interfacial microstructure, mechanical properties

1. Introduction

High performance and concurrent weight and cost reduc-tion become more important in almost industries.1) Thereforethe demand for dissimilar metal joining has been increased inmany industrial fields including transportation systems andelectronics. As an implication there is a need for a weldingmethod allowing for joining dissimilar materials such asTi and Al that high strength, low weight and low cost arecharacterized.2) In the case of aerospace industry, the joiningof aluminum alloy and titanium alloy could have a majorapplication in the body structure where high strength andlight weight are desirable. Especially Titanium and titaniumalloys are widely used because of their high corrosionresistance and high specific strength.3) Serveral access toweld aluminum and titanium have been made in the past.Conventional fusion welding methods such as arc and laserwelding have been tried. Unfortunately, however, soundjoints with acceptable strength have not been obtainedbecause of the formation of intermetallic compounds in weldregion and/or weld interface. And all these representativeexamples exhibited problems like the necessity for shieldinggas, sophisticated equipment and geometrical limitations ofthe welding interface.

The welding institute (TWI) was developted friction stirwelding process in 1991.4–13) FSW is a soild-state joiningtechnique in which a rotating tool is traversed along the weldpath, plastically deforming the surrounding material to formthe weld and generating significant heat around the toolduring FSW.14) Since its development in 1991, the frictionstir weld process has increased acceptance as a joining

method of choice for industry such as automotive, shipbuild-ing, aerospace using aluminum alloy.

In the present study, dissimilar materials joints betweenAA6061-T6 and Ti-6Al-4V alloy sheets were fabricated byusing friction stir welding method. Microstructure andmechanical properties of weld zone was examined. Espe-cially, the interfacial microstructure and element distributionwas precisely investigated by STEM.

2. Experimental Procedure

Five millimeters-thick sheets of Ti-6Al-4V alloy andAA6061-T6 were successfully friction stir welded using 2-dimensional precision FSW machine. The chemical compo-sition and mechanical properties of the base materials werelisted in Table 1 and Table 2, respectively. The butt joiningwas carried out using a FSW tool consisting of tapered from6 mm at the probe root area to 4 mm at the probe tip area witha screw. The shoulder diameter and probe length is 15 mmand 4.5 mm, respectively. The tool was made of standard

Table 1 Chemical composition of AA6061-T6 and Ti-6Al-4V base metals

(at%).

Si Mg Cu Fe C V Others Al Ti

6061-T6 0.56 0.98 0.31 0.29 — — 0.09 Bal. —

Ti-6Al-4V — — — 0.40 0.10 3.95 0.04 6.62 Bal.

Table 2 Mechanical properties of AA6061-T6 and Ti-6Al-4V base metals.

YS (Mpa) UTS (Mpa) El. (%)

6061-T6 310 342 17

Ti-6Al-4V 880 950 14*The Paper Contains Partial Overlap with the ICAA12 Proceedings by

USB under the Permission of the Editorial Committee.

Materials Transactions, Vol. 52, No. 5 (2011) pp. 974 to 978Special Issue on Aluminium Alloys 2010#2011 The Japan Institute of Light Metals

steel (SKD61). Butt joints were made along the RD of thesheet with tool traveling speed of 2.5 mm/s and tool rotationspeed of 1000RPM. During the FSW, 3� tilt angle and aplunge depth of 0.2 mm were applied to the tool. Figure 1shows a schematic illustration of friction stir welding processbetween AA6061-T6 and Ti-6Al-4V alloy. During FSW,advancing side is aluminum alloy and retreating side istitanium alloy. Contrary to conventional friction stir buttwelding, the tool probe center was nearly shifted by the proberadius towards the aluminum plate. Therefore, except for afew tenth miilimerter, the stirring action of the probe occuredon the aluminum plate of the joint. This was conducted toprevent probe wear and over heating of the aluminum alloy.This welding process was similar to the FSW of steel-aluminum joints. Figure 2 shows a schematic illustration ofjoint interface. To evaluate interfacial microstructure andmechanical properties in accordance with the probe position,probe positions were varied along thickness direction of basemetals. Probe root area was equivalent to the weld surfaceand the probe was completely inserted in titanium alloy ofthis region. However, probe tip area was equivalent to to theweld bottom and the probe was not contact to taitanium alloy.Interfacial microstructure was inspected by optical microsco-py (NIKON, EPIHOT200), scanning electron microscopy(JEOL, JSM-700F) and scanning transmission electron mi-croscopy (FEI, TECNAI); sections taken perpendicular to thewelding direction were polished and etched (keller’s reagent)by using conventional methods. TEM specimens on a cross-section perpendicular to welding direction were fabricated byfocused ion beam (FEI, QUANTA3D). The bright fieldimages of the weld interface were observed. The high angleannular dark field (HAADF) images were observed andconsequently elements mapping was carried for the samefield. Tensile test was carried out by using an instron-typetester under a cross head speed of 1:7� 10�5 m/s at roomtemperature. Tensile test specimens (gage length: 45 mm,width: 7.5 mm) were machined perpendicular to the weldingdirection from joint. After tensile test, fracture surfaces wereinspected by a scanning electron microscopy equipped withX-ray spectroscopy analysis system (EDX). Vickers hardness

profiles were measured each of 0.05 mm from titanium sheetto aluminum sheet using indenter and a load of 490 mN.And hardness test were conducted on a cross-section 1/4tperpendicular to the welding direction.

3. Result and Discussion

3.1 Mechanical propertiesHardness profile for the welding cross-section evaluated

by Vickers hardness tester is shown in Fig. 3. The hardnesslevel of Ti-6Al-4V alloy side amounts to circa 350 HV. Sharpdecrease of hardness level generated at the stir zone.Hardness level of stir zone is slightly smaller than that ofaluminum alloy base metal, except when the indenter hitedtatinum particles. HAZ has a lowest hardness level. It isconsidered that annealing effect of HAZ was caused byfriction heating. The stress-strain curves of the tensile testspecimens are shown in Fig. 4. The ultimate tensile strengthreached 134 Mpa representing 35% by that of aluminumalloy base metal. The all joints expressed lower strengthand elongation than base metal. This is because probe tiparea could not be affected by probe stirring action. The alljoints are fractured in the weld region during transversetensile test.

3.2 Interfacial microstructureFigure 5 shows that the cross-section image of friction stir

welded AA6061-T6 and Ti-6Al-4V alloy (a) and the opticalmicroscopy (OM) images of the cross-section (b-g). The stirzone occurs maninly on the aluminum side of joint. Becausetool was shifted towards AA6061-T6, SZ was formed mostlyon the AA6061-T6 of the weld zone. Ti-6Al-4V alloyfragments were observed in SZ (Fig. 5(a)). SZ was composedof finely recrystallized aluminum alloy grains and titaniumalloy fragments pushed away from the titanium base metaldue to the stirring effect of the probe. Therefore SZ has acomposite structure of aluminum alloy refoinced by titaniumparticles. Middle area of the weld interface evaluated by OMis shown in Fig. 5(c). Titanium alloy fragments were alsoobserved in SZ. However, they were not found in the probetip area of the weld interface (Fig. 5(d)). This is why theprobe did not stir the titanium alloy in direction. Crystalgrains of the aluminum alloy in SZ (Fig. 5(e)) became

Fig. 1 Schematic illustrating for friction stir welding of AA6061-T6 and

Ti-6Al-4V.

Fig. 2 Schematic illustrating for joint interface.

Fig. 3 Hardness profile of the cross-section taken perpendicular to welding

direction.

Interfacial Microstructure and Mechanical Properties of Dissimilar Friction Stir Welds 975

significantly fine compared with the initial grain size ofaluminum alloy base metal (Fig. 5(f)). Figure 5(g) showsTitanium alloy base metal around the weld interface. It wasnot affected by the stirring action of the probe during FSW.

Figure 6 shows SEM images of the weld interface. Lotsof coarse fragment of titanium alloy were observed in theprobe root area, Fig. 6(a). It thought that the strong stirringwas occurred in this region. In the middle area, smallfragment of the titanium alloy and the particles of thealuminum alloy were observed, Fig. 6(b). No evidence of thestirring action by the probe was found in the probe tip areaof the weld interface, Fig. 6(c). This reason was expressedabove, already. And irregular lamellar structures wereformed in interfacial zone due to plastic flow of base metalby probe stirring action. After tensile test, fracture surfacewas observed by SEM. Fracture sections exhibit differentaspect along the plate thickness direction. Figure 7(a) is forthe probe root area, dimples were observed. It thought thatductile fracure occurred on the probe root area. As the resultof EDAX analysis of Fig. 7(a), these area display analuminum content of about 100%. From this result, it isconsidered that the fracture of this region occurred in SZof aluminum alloy. In the probe tip area, the initial surfaceof titanium alloy sheet was observed. It was confirmed thatthe fracture sequences are very complicate and the fractureposition depends on the probe position.

TEM specimens fabricated by FIB are shown in Fig. 8.Interfacial zone could be observed in these specimens.Figure 9 shows bright filed images of probe root area andprobe tip area. In the weld interface of probe root area, wavymorphogical interface was observed at aluminum alloy side.It is considered that the wavy morphology was formed by

Fig. 5 Macro image of weld zone (a), probe root area of interface (b), middle area of interface (c), probe tip area of interface (d), SZ of

AA6061-T6 (e), BM of AA6061-T6 (f), BM of Ti-6Al-4V (g).

Fig. 4 Strain-Stress curve of friction stir welded AA6061-T6 and Ti-6Al-

4V joint.

976 K.-S. Bang, K.-J. Lee, H.-S. Bang and H.-S. Bang

spiral fabrication in the surface of probe. However, the wavymorphology was not observed at the weld interface in theprobe tip area. This is because of effect from the tool withcircular truncated shape. Titanium alloy of probe tip areacould not contact with probe. High angle annular dark field(HAADF) images and elements mapping of probe root areain FSW welds are shown in Fig. 10. Various contrasts weredected at the interfacial zone along thickness direction inHAADF images. It thought that compostion of interfacialzone did not be homogenous and specified compositions wereconcentrated irregulary at interfacial zone. In bright area with

500 nm width in HAADF image, Ti and V element wasconcentrated at the weld interface and Al element wasconcertrated at the dark area. As the result, it was consideredthat metal flow caused by stirring action of high rotation toolwas resulted in lamellar structure. Intermetallic compoundwhich was observed in the fusion weld zone was not found inthe present weld interface. It was considered that friction stirweld process of the present study did not occur in melting andsolidification. At the Irregular lammellar and/or feather-likedistrubuton of Al, Ti and V component was also detected.However, it did not found special trend to Mg distribution.According to the elements mapping result, Si was concen-trated at the weld interface having wavy morphology. TheHAADF image and elements mapping of probe tip areaare also shown in Fig. 11. In the probe tip area, wavymorphological interface was not observed. However, weakconcentration of Si component was detected at the weldinterface along the sheet thickness direction.

4. Conclusion

In the present study, butt joining of AA-6061T6 and Ti-6Al-4V plates was performed using a 2-dimentioal precisionfriction stir welding machine. The morphology of theinterfacial zone formed between SZ of aluminum alloy andthe edge of titanium alloy was examined. A morphologicalinvestigation and element mapping analysis were preciselyevaluated for the interfacial zone. The mutual relationshipbetween the probe position and interfacial microstructure wasinvestigated and the following results were made clear.

In the probe root area, the SZ reveals finly recrystallizatedgrains of aluminum alloy and fragments of titanium alloyformed by the contact and consequent stirring between thefront of titanium alloy plate and the inclined flank of theprobe. Dimples of Al were observed at the fracture surface bytensile test in this case. This suggests that the weld interfacebetween the edge of the SZ and the front of titanium alloywas sound. The formation of wavy morphological interfacewas also confirmed. It is considered that this was due to spiralwork on the flank of probe. According to the TEM-HAADFverification and element mapping analysis, it was confirmedthat the weld interface reveals very complicate sequenceincluding lamella structure with Al and Ti-V. Feather-likedistrubuton of Ti-V component was also detected. In theprobe tip area, however, the initial front (and/or surface) oftitanium alloy plate was observed. It thought that actualjoining reaction was not occurred in the present area and thisresulted in the strength decrease.

Fig. 6 SEM images of probe root area (a), middle area (b), tip area (c) in the weld interface.

Fig. 7 SEM images for fracture surface (a) probe root area and probe tip

area (b).

Fig. 8 TEM specimen fabricated by FIB of probe root area (a) and probe

tip area (b).

Fig. 9 Bright field image of probe root area (a) and probe tip area (b).

Interfacial Microstructure and Mechanical Properties of Dissimilar Friction Stir Welds 977

In conclusion, it was clearly revealed that mechanicalproperties and interfacial microstructure of AA6061-T6/Ti-6Al-4V but joints manufactured by FSW are affected by theprobe position. Further, it was also confirmed that the probemust be shifted toward titanium alloy plates to get soundjoints.

Acknowledgement

The authors would like to express their thanks to WinxenCo., Ltd. for offering FSW equipments. The present studywas financially supported by the Project (10-JE-2-0001) forConstructing Infrastructure of Advanced Materials & Parts ofthe Ministry of Strategy and Finance, Korea.

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Fig. 10 High angle annular dark field image (a) and elements mapping result of probe root area; Ti (b), V (c), Mg (d), Si (e), Al (f).

Fig. 11 High angle annular dark field image (a) and elements mapping result of probe tip area; Ti (b), V (c), Mg (d), Si (e), Al (f).

978 K.-S. Bang, K.-J. Lee, H.-S. Bang and H.-S. Bang