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Investigationofmicrostructure,surfaceandsubsurfacecharacteristicsintitaniumalloyfrictionstirweldsofvariedthicknesses

ARTICLEinSCIENCEANDTECHNOLOGYOFWELDING&JOINING·JULY2009

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Investigation of microstructure, surface andsubsurface characteristics in titanium alloyfriction stir welds of varied thicknesses

P. D. Edwards1,2 and M. Ramulu*1

Friction stir welding of titanium alloy (Ti–6Al–4V) was demonstrated on 3, 6, 9 and 12 mm

thickness square groove butt joints. Complete microstructural and microhardness evaluations

were conducted in addition to surface and subsurface examinations for each case. The 3 mm

welds exhibited an extremely fine grained microstructure with evidence of processing

temperatures below the beta transus temperature of the alloy. The 6, 9 and 12 mm samples

possessed larger grains formed by a slower cooling rate from above the beta transus

temperatures. The thick section weld exhibited a nearly uniform microhardness, while the thinner

welds showed a slight, 6%, increase in hardness compared with the parent material.

Keywords: Friction stir welding, Titanium, Microstructure, Microhardness

IntroductionTi–6Al–4V is the most used of all the titanium alloys. Itaccounts for more than 50% of worldwide titaniumusage.1 Furthermore, of all the titanium alloys, Ti–6Al–4V is considered to be the most highly weldable viastandard fusion welding processes such as arc, laser orelectron beam. These joining processes produce largegrained, martensitic, cast type microstructures thatresult in adverse effects on the mechanical propertiesof the joints.1 Solid state joining processes such asfriction stir welding (FSW) are able to retain themicrostructural integrity of the parent material in thewelded joint, thereby producing mechanical propertiesthat are more comparable with those of the parentmaterial. Friction stir welding has been successfullyapplied to the joining of metals such as aluminium,magnesium, copper and even steel.2,3 Only recently hasFSW been applied to high strength, high temperaturematerials such as steel and titanium mainly due to thedifficulties associated identifying tooling materials thatcan withstand the temperatures and loads involved withwelding such materials.

Mishra et al.2,3 and Nandan et al.4 have recentlyprovided excellent reviews of the FSW process, weldcharacterisation, modelling and its applications for avariety of materials including aluminium and titanium.However, because the application of FSW to titaniumalloys is relatively new and information available isscarce, there still exists an opportunity for processdevelopment and joint characterisation. The purpose ofthis paper is to review the developments in titanium

FSW to date and present the results of a current studyintended to examine the process conditions required toproduce high quality square groove butt joint config-uration welds in a wide range of material thicknessesalong with the surface and subsurface characteristics ofthose joints.

Brief review of titanium friction stirweldsThere have been several papers published on the FSW oftitanium.5–17 Titanium alloys friction stir welded includeCP-Ti, Ti 15-3-3-3, Ti 17, Ti 6-2-4-2, Ti 17 and beta 21S.Welded thicknesses reported range from 1?2 to 12 mm.However, process conditions and tooling informationavailable were rather limited. Since the most widely usedtitanium alloy is Ti–6Al–4V, it was of particular interest.From the limited literature available, the thicknesseswelded and processing conditions used in Ti–6Al–4V aresummarised in Table 1. In most cases, welds wereperformed in position control using a simply shapedtungsten tool and some employed the use of thermalmanagement.5,6 Other tool materials such as TiC,7

PCBN8 and Mo based9 alloy were used, but these werefar less successful than the tungsten based tools. Tooldimensions used ranged from 15–19 mm shoulders and3–8 mm diameter pins.9,10 In some cases, the pin tipswere tapered from 5 mm at the shoulder to 3 mm at thetip.9

Characterisation of Ti–6Al–4V FSWs typicallyincluded macro- and microstructural evaluations alongwith microhardness and tensile testing. Macrodefectsobserved in welds made under non-optimal conditionsinclude undercut, root voids and lack of penetration.9

Microstructural evaluations typically identified a refinedweld nugget microstructure compared with the parentmaterial with evidence of exceeding the beta transus

1University of Washington, Seattle, WA, USA2The Boeing Company, Seattle, WA, USA

*Corresponding author, email ramulum@u.washington.edu

� 2009 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 17 January 2009; accepted 10 February 2009DOI 10.1179/136217109X425838 Science and Technology of Welding and Joining 2009 VOL 14 NO 5 476

temperature during processing.5,6,9,10,13 Microstructuralevaluations of the heat affected zones (HAZ) andthermomechanically affected zones (TMAZ) indicatedthat the peak temperatures in these portions of the weldare below the beta transus temperature10 and the TMAZis relatively small compared with aluminium FSWs.13

Microhardness evaluations showed that the hardness ofthe weld nugget is greater than the surrounding parentmaterial.9,10 Reported hardness measurements of theHAZ varied. Some reported softening in the HAZ,9

while others found hardening10 in the HAZ. It isgenerally expected that the weld nugget will be harderthan the parent material due to the refined grain size,while the HAZ will be softer due to the enlarged grainscaused by exposure to elevated temperatures.9 Tensiletesting typically resulted in lower elongations andstrengths with failure occurring in the HAZ.9,10 Othertensile property studies14,15 observed lower to equivalentelongations and strengths of the weld specimen com-pared with the parent material with failure occurring ineither the base metal or weld depending on the quality ofthe joints. Tensile testing of the weld nugget aloneshowed increased strength and elongation comparedwith the parent material.9 Again, this is assumed to bedue to the microstructural differences among the HAZ,weld nugget and base material.9

Experimental procedure

MaterialThe material used this investigation is the Ti–6Al–4Vtitanium alloy possessing a average grain size that istypically on the order of 8–10 mm. The thicknesses usedare 3, 6, 9 and 12 mm. The chemical composition forthis material is 6Al–0?8C–0?15H–0?4Fe–0?05N–0?2O–9Ti–4V and typical representative microstructures at low(Fig. 1a) and high (Fig. 1b) magnifications are shown in

Fig. 1. All materials were machined along the abuttingedges and chemically etched before welding. Testmaterials were welded then evaluated visually andmicrostructurally to determine the joint quality. Testparts were welded using the facilities at the EdisonWelding Institute (EWI). This development effort was acollaborative effort among EWI, the Boeing Companyand the University of Washington.

Friction stir weldingThe tooling design configurations, tooling materials,process conditions and thermal management techniquesutilised in the work by Sanders11,12 were the baselineused in this study. A tapered, tungsten lanthanide, pintool with a relatively small shoulder diameter was usedthrough out this study. Thermal management of the pintool was also used to control the high temperaturesproduced during the Ti FSW process.

In general, spindle speed, travel speed, shoulderengagement, penetration ligament tool tilt, coolant flowthrough the weld tool and the coolant flow through thebacking anvil are the process parameters that dictate thequality of the joint. Tool tilt, penetration ligament andshoulder engagement are defined in Fig. 2. The processparameters and basic tooling configuration dimensionsused during weld development effort are summarised inTable 2.

Microstructural, surface and subsurfacecharacterisationMicrostructural evaluations were carried out in order toexamine the grain structure of the weld nugget, the HAZand the base metal after the FSW process. Toaccomplish this, the samples were first cut, via abrasivewater jet, from a titanium plate that had been frictionstir welded. The sample included a cross-section of theweld nugget oriented perpendicularly to the joining line.The samples were then polished on 5, 1 and finally

a b

a optical micrograph of standard Ti–6Al–4V microstructure; b scanning electron micrograph of standard Ti–6Al–4Vmicrostructure

1 Standard Ti–6Al–4V micrographs

Table 1 Summary of process conditions used for FSW of Ti–6Al–4V

Thickness, mm Spindle speed, rev min21 Feedrate, mm min21 Ref. no.

1?5 325 100 112 200 100 5, 62 300, 400, 500 60 96 275 100 10

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0?3 mm alumina oxide polishing wheels. The sample wasthen etched to expose the microstructure. A JEOL JSM-840A scanning electron microscope (SEM) was used inconjunction with a JEOL DSG Plus digital scan imagingsystem to digitally capture the SEM images. First, aseries of low magnification (610) optical macrographswere taken to encompass the whole sample. This wasfollowed by a series of higher magnification (650 and6500) optical micrographs. These were taken atapproximately the mid-thickness of each weld. Scanningelectron images were captured at 62000 magnificationunder a 10 kV accelerating voltage.

Surface topography was investigated using opticalmicroscopy and surface profilometry. Surface profiles ofthe weldments and average roughness parameters wereobtained using MarSurf XR 20 surface analyser. ThisXR 20 is equipped with a stylus based, 2?54 mm radiusconical diamond tip with 5 nm resolution. The unit hashigh sensitivity capability to capture exceptionally smallchanges in surface roughness as it traverses across aprofile. The diamond tip is necessary to avoid damagewhen measuring hard metal alloys with rough surfaces,such as the sharp peaks of notches present on thetitanium FSW coupons. Surfanlyser is that it automa-tically stores all of the data to disk and then it computesthe standard roughness parameters. Longitudinal passesalong specimen’s centreline were obtained with a cutofflength of 0?8 mm and a traverse length of 5?6 mm. Inorder to quantify the surface topology, the surfaceroughness parameters Ra, the arithmetic average rough-ness height Ry, the maximum peak to valley height andRz, the 10 point height as calculated by measuring thedistance between the five highest peaks and the fivelowest valleys in the sampling length were calculatedfrom the recorded surface profile.

Microhardness tests were utilised to assess the surfaceand subsurface characteristics of both the base materialand the weld nugget. These samples were prepared in thesame manner as discussed for the microstructuralevaluations. The samples were tested with a 100 g loadapplied for 15 s using an automated Vickers hardnesstester. The size of the resulting indent was then mea-sured to determine the microhardness of the materialat that point. For each weld cross-section, a two-dimensional microhardness grid was laid out over thespecimen. The microhardness indent spacing wasy500 mm in the through thickness and transverse direc-tions. Depending on the weld thickness, this resulted in1000–6000 indents per sample. The first microhardnesspoint for a given traverse was started at the left edge ofthe sample, in the base material, and then moved to theHAZ, the weld nugget, back into the HAZ and finallyinto the base material on the opposite side of the weldnugget again.

ResultsProcess parameters for welding 3, 6, 9 and 12 mm Ti–6Al–4V were identified and evaluated. The optimalprocessing conditions for each thickness based on thisdevelopment effort are given in Table 3. Macrographsfor each of these optimal welding conditions in eachthickness are shown in Fig. 3.

In addition to macrographic examinations, each ofthe welds made under the best set of process conditionsidentified was prepared for microstructural examinationvia optical microscopy and SEM as described pre-viously. Low magnification (650) optical photographsof the weld nugget–base metal transition areas, whichinclude HAZ and TMAZ were taken for each weldthickness along with higher magnification (6500)optical photographs of the microstructure in the centreof the welds. Representative optical micrographs forthe 6 and 9 mm weld cases are shown in Fig. 4.Representative SEM images from the centre of the 3and 12 mm welds are given in Fig. 5. Scanning electronmicroscope images (62000) which show the grain sizedistribution through the thickness of the 3 and 12 mmwelds are shown in Fig. 6. Finally, a 62000 SEM imageof the HAZ in a 9 mm weld sample is shown in Fig. 7.

The surface texture of the friction stir welded sampleswas characterised with optical macrographs and withsurface roughness profiles. As the FSW tool shoulder

Table 2 Summary of Ti–6Al–4V FSW parameters tested

Joint thickness, mm Spindle speed, rev min21 Feedrate, mm min21 Shoulder diameter, mm Pin tip diameter, mm

3 300 50–130 20 86 250–320 45–100 25 109 250–285 65–100 25 1012 140–190 40–75 30 10

Table 3 Optimal FSW processing parameters for giventool design and thickness

Joint thickness,mm

Spindle speed,rev min21

Feedrate,mm min21

3 300 756 280 1009 270 6512 170 65

2 Schematic defining tool tilt, shoulder engagement and

penetration ligament

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rotates and translates along the workpiece surface, itleaves behind tool markings similar to feed marks inmachining. These surface characteristics were charac-terised as they induce surface stress concentrationfactors that may have sever impacts on the mechanicalproperties of the joints, particularly high cycle fatiguelife. Figure 8 shows typical cross-section of these toolmarkings left behind the FSW tool at the crown of theweld joint along with typical surface roughness profiles

recorded for the 3 and 9 mm weld samples. For the3 mm sample, values of Ra and Ry were approximately22 and 137 mm respectively. For the 12 mm sample, theRa and Ry values were approximately 25 and 107 mmrespectively.

Extensive microhardness testing was also conductedon the welds produced during this study. After themicrostructural examination was complete, the sampleswere repolished and two dimensional microhardness

3 Macrographs of a 3 mm, b 6 mm, c 9 mm and d 12 mm FSW cross-sections and e photograph of typical weld surface

4 Optical micrographs of advancing side base metal to weld nugget transition zones in a 6 mm and c 9 mm welds, and

higher magnification optical micrographs of the same regions in b 6 mm and d 9 mm welds

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grids were laid out on the mounts. Figure 9 shows theresults of this microhardness study as contour plots foreach of the weld thicknesses. Figure 10 shows one-dimensional plots for mid-thickness microhardnesstraverses extending from the base metal on one side ofthe weld nugget, all the way through the heat affectedzone, weld nugget and into the base metal on theopposite side for the 3 and 12 mm weld cases. In Fig. 10,the approximate locations of the microhardness pointstaken in the base material (BM), heat affected zone(HAZ) and weld nugget (WN) are indicated on the plots.

DiscussionThere were several aspects of the welds produced duringthis study which were uncovered via metallurgicalcharacterisation. First, it was found that these weldspossess an extremely small HAZ, on the order of 200–400 mm, as shown in Figs. 3 and 4. Furthermore, it wasdifficult to distinguish between the HAZ and anyTMAZ, even at higher magnification (Fig. 4b and d).Essentially, the TMAZ in titanium FSW is negligiblysmall. This is in strong contrast to aluminium frictionstir welds which possess distinct and rather larger HAZand TMAZ. This is likely attributed to the high strengthand low thermal conductivity of titanium compared withaluminium. In the Ti FSW process, the thermalconductivity of the material is so low, the heat generatedby the process is not transferred away from the weldzone fast enough to create the large HAZ that would beseen in fusion welded joints. Because heat is nottransferred far from the weld zone, the cooler andstronger material beside the weld zone resists mechanicaldeformation that would create a TMAZ, like inaluminium FSW.

Another characteristic of the titanium FSW nugget isthe grain size distributions (Fig. 6). It was observed thatthe grain size is quite uniform across the width of theFSW nugget at any given depth (Fig. 4). However,optical and scanning electron microscopy revealed aclear grain size distribution through the thickness and ageneral grain size difference between the welds of

a 12 mm weld; b 3 mm weld5 Images (SEM) taken from centre of weld nugget

6 Scanning electron images showing grain size distribution through thickness of a 12 mm weld and b 3 mm weld

7 Scanning electron image of HAZ in 9 mm weld sample

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different thicknesses. Optical and SEM images haveshown that the grain size in the welds increases withthickness. Very small grains were observed in the thinwelds and quite large grains were found in the thickwelds. Additionally, there is a clear grain size distribu-tion through the thickness of any given weld section(Fig. 6). The grains in the welds are relatively large nearthe crown, or top, or the weld and become small andvery fine near the root, or bottom, or the weld.

All of these grain size distribution characteristics canbe attributed to the thermomechanical nature of theprocess. The grains are large at the top of the weldbecause that is where the shoulder of the tool isgenerating the most heat. In the root of the weld, thesmall tip of the tool generates much less heat, and due tothe low thermal conductivity of the material, the heatgenerated at the top by the shoulder is not conducted

into the root. Thus, the material at the root of the jointis subject to high degrees of thermomechanical work,with minimal heating, while the material at the top ofthe weld is subjected to much greater temperatures,allowing grain growth post-stirring. The grains in theregion of the weld corresponding to the material directlyunder the pin tip are typically extremely fine, even innanoscale. The grain size variation in thickness is likelydue to heating and cooling rates. The thinner welds coolmuch faster than the thick section welds, restrictinggrain growth post-stirring. Additionally, the tools usedin the thinner section welds have a much smaller surfacearea, producing less heat than the thick section weldtools. This also contributes to the smaller grain size.

These microstructural characterisations can also beused to make a rough estimate of the temperatures in theweld zone during processing. The SEM image of the

8 a photograph of 3 mm weld surface, b surface roughness profile in 3 mm sample, c photograph of 12 mm weld sur-

face and d surface roughness profile in 12 mm sample

9 Microhardness contour plots for 3, 6, 9 and 12 mm thick FSWs

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12 mm sample (Fig. 5) shows a transformed betamicrostructure with grain boundary alpha. This suggeststhat the weld temperature in this section exceed the betatransus temperature of 1065uC (1950 F). The SEMimage of the 3 mm sample (Fig. 5) exhibits a muchsmaller transformed beta microstructure, with whatcould be some untransformed, or primary, alpha. Thissuggests that the temperature in the thin section weld isnear the beta transus temperature, but not completelythrough it, leaving some untransformed alpha in theresulting microstructure. The SEM images taken for the6 and 9 mm samples are similar to that of the 12 mmsample and all suggest that the beta transus temperaturewas exceeded in the weld zone during processing. For allwelds, there was noticeable untransformed alpha in theHAZ (Fig. 7), indicating that the temperatures in theHAZ were near, but did not completely exceed the betatransus temperature, regardless of the weld thickness.

The surface texture left behind the FSW tool is quiterough (Fig. 6). There are peaks and valleys correspond-ing to the material flow around the pin tool and underthe shoulder. Most alarming is the sharp shape of thevalleys of these flow lines. These will greatly deteriorate

the fatigue properties of these welds because these sharpradii features will cause high stress concentrations andsites for early crack initiation. Based on the macro-graphic analysis, a minimum of 500 mm (200 mm for thefeature plus a safety factor of 2?5) is suggested to bemachined off of any Ti FSW surface to prevent fatigueperformance degradations due to the tool markings onthe surface of the welds.

For the microhardness evaluations, the thin weldsshow a higher hardness in the weld nugget than theparent material, while the thicker section welds have anearly uniform hardness in the weld nugget comparedwith the parent material (Fig. 7). This is also likely dueto the grain size variations in the welds resulting fromthe peak temperature and cooling rate differencesbetween the thick section welds and thin welds. Thethick welds cool at a much slower rate, allowing thegrains to grow to a size more similar to the parentmaterial resulting in more uniform hardness distribu-tions. Conversely, the grains in the thin welds aregenerally smaller than the surrounding parent materialdue to the faster cooling rates and lower heat inputs,resulting in higher microhardness. It is expected that aproper heat treatment cycle will successfully removethese microhardness differences in the as welded sec-tions. It was also noted that the thinner welds (Fig. 10a)show a hardness decrease in the HAZ which is inagreement with previous microhardness evaluations.

In general, the spindle speed (rev min21) and travelspeed (mm min21) have the most dominate effects onthe quality of the joint. The spindle speed contributesmost to heating and the travel speed has the largesteffect on how well the root of the joint is stirred forpenetration. The most common defects encountered area lack of penetration (LoP) defect and small cavities inthe root of the weld. LoP defects are caused byinsufficient heating and deformation at the root of thejoint which leaves behind an unstirred prior jointinterface (Fig. 11). Root cavities (Fig. 11) are causedby improper combinations of spindle speed and travelspeed. A relatively high spindle speed with a relativelylow travel speed will lead to overstirring of the root anddefect formation. Also, a low spindle speed with a hightravel speed will result in insufficient heating andworking of the root, also leaving behind void defects.

There is a delicate balance among all of the FSWprocessing parameters that must be achieved to producea defect free joint in Ti–6Al–4V. Further work needs to

10 Single microhardness traverse plots across 3 and

12 mm weld samples at mid-thicknesses of 1?5 and

6 mm respectively

a lack of penetration defect; b root void defect11 Typical FSW defects

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be conducted to establish relationships between all of therelevant processing parameters and the resulting qualitycharacteristics of the joint. Implementing modelling andexperimental measurement techniques to characterisethe thermal gradients and deformation zone formationduring the process will also be valuable for establishingthis relationship, which is essential to identifying theoptimal processing parameter limits for each thicknessin order to ensure that the parameters being used aresufficiently robust for a production process.

ConclusionsFriction stir welding of Ti–6Al–4V alloy has beendeveloped and demonstrated in butt weld thicknessesranging from 3 to 12 mm. Extensive metallurgical andmicrohardness characterisations have been performedon welds made under the optimal processing conditions,which are defined as those that produce defect freewelds. Based on this experimental study, the followingkey conclusions were made:

1. Metallurgical characterisations observed grain sizedistributions through the thickness of the welds andbetween weld thicknesses. However, the grain size wasquite uniform across the width of each weld at any givendepth. In the 6, 9 and 12 mm welds, the microstructureappears to have crossed the beta transus temperaturedue to the presence of alpha prime in the weld nuggetmicrostructure. The 3 mm weld is likely to have not fullycrossed the beta transus temperature because of remnantprimary alpha in the microstructure.

2. Microhardness results showed a hardness increasein the thin section welds compared with the surroundingbase material and a more uniform hardness in the thicksection welds. These microhardness characteristics arelikely due to the grain size variations noted previously.The thin section welds have grains that are smaller thanthe parent material while the thick section welds havemuch larger grains.

Acknowledgements

The authors of this paper would like to thank theBoeing Company for the support and Dr D. Sanders

for his encouragement throughout this research project.The authors’ sincere thanks are also extended toJ. Bernath, Edison Welding Institute, for providingthe welded specimens and assistance with processdevelopment.

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