Microstructural characterisation of friction stir processed aluminium

Microstructural characterisation of friction stirprocessed aluminium

D. Yadav and R. Bauri*

Friction stir processing was carried out on commercially pure aluminium, and a detailed

microstructural characterisation was performed by electron backscattered diffraction and

transmission electron microscopy. Friction stir processing resulted in significant grain refinement

with narrow grain size distribution. The microstructure showed fine and equiaxed grains, with

some ultrafine grains being also observed. Electron backscattered diffraction studies showed

majority of the boundaries to be high angle, confirming the occurrence of dynamic

recrystallisation (DRX). Transmission electron microscopy observations revealed dislocation

arrangement into subgrain boundaries, grains having different dislocation densities and in

different stages/degrees of recovery. Electron backscattered diffraction analysis also revealed a

progressive transformation of sub-grain boundaries into high angle grain boundaries. A

multimechanism of dynamic recovery, continuous DRX and discontinuous DRX seems to be

operating during the process. The microstructure is not affected by changing the rotation speed

from 640 to 800 rev min21, except that the grain size was marginally larger for higher rotational

speed.

Keywords: Friction stir processing, Microstructure, Grain refinement, Electron backscattered diffraction, Transmission electron microscopy, Dynamicrecrystallisation

IntroductionAluminium and its alloys are used widely in transporta-tion, aerospace and construction applications. A majorrequirement of such applications is high strength alongwith low density. The strength of Al can be increased byalloying additions, heat treatment and thermomechani-cal processing. However, there is a limit to whichstrength can be increased by these conventional meth-ods. Grain refinement is a very important and widelyaccepted method to increase the strength of metals andalloys in accordance with the Hall–Petch relationship.This has led to the development of ultrafine grained oreven nanocrystalline metals and alloys.1,2 Several severeplastic deformation (SPD) methods have been developedfor manufacturing ultrafine grained/nanocrystallinealloys. However, these methods have their own dis-advantages, which are difficult to overcome. Friction stirprocessing (FSP), which basically uses the principle offriction stir welding, is a relatively new solid statethermomechanical process developed by Mishra et al.3

The technique can be used for a variety of microstruc-tural modifications besides just joining two plates.4

During FSP, a specially designed non-consumablecylindrical tool, rotating at high speed, is traversed intothe material along a particular length at a desired

traverse speed. The side in which the tangential velocityof the tool surface is parallel to the traverse direction isdefined as the advancing side (A), and the antiparallelside is defined as the retreating side (R), as shown inFig. 1. In principle, FSP involves SPD of the material atelevated temperatures, typically .0?5Tm, where thematerial softens and can be deformed plastically. Thecombination of deformation and temperature signifi-cantly modifies, refines and homogenises the initialmicrostructure in several ways and gives equiaxed anddynamically recrystallised fine grains.5,6

Friction stir processing has been shown to be aneffective technique to improve the strength, ductility andhardness of various aluminium and magnesium alloys7,8

and has also been used to impart superplasticity,9,10

eliminate casting defects11 and fabricate composites.12,13

It is generally believed that the grain refinementduring FSP takes place by dynamic recrystallisation(DRX) process. However, the exact mechanism of grainrefinement is not well understood. Some understandinghas been derived based on the microstructure developedduring friction stir welding,14,15 and some recent studieson FSP suggested different forms of DRX occurringduring the process.16–19 However, as it appears, there isno consensus on a definite operative mechanism of grainrefinement during the FSP of aluminium.

Since the microstructure plays a major role in definingthe properties, it is imperative to understand themicrostructure developed in such a thermomechanicalprocess and present a clear picture of the substructure anddislocation features present inside the grains. The aim of

Department of Metallurgical and Materials Engineering, Indian Institute ofTechnology Madras, Chennai 600 036, India

*Corresponding author, email [email protected]

� 2011 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 10 September 2010; accepted 7 January 2011DOI 10.1179/1743284711Y.0000000006 Materials Science and Technology 2011 VOL 27 NO 7 1163

the present investigation is to develop fine grained Al byFSP and to characterise the microstructure developedafter single pass FSP. The effect of rotation speed on themicrostructure has also been studied.

ExperimentalCommercially pure aluminium plate of thickness 12 mmwas taken for the study. Friction stir processing wascarried out using a cylindrical tool made of M2 steel witha concave shoulder of diameter 12 mm, pin diameter of3 mm and pin length of 2?1 mm. A schematic of the toolis shown in Fig. 1. Two different rotation speeds of thetool, i.e. 640 and 800 rev min21, were used. In both cases,the traverse speed of the tool was kept as 150 mm min21.A downward force of 5 kN was applied to the tool.

The microstructure was characterised by electronbackscattered diffraction (EBSD) and transmission elec-tron microscopy (TEM). Electron backscattered diffrac-tion studies were performed on the surface and also alongthe depth of the stir zone. On the surface, samples werecut from three different locations of the stir zone, namely,centre and 4 mm from the centre on either side(advancing and retreating), and in order to study thecross-section (along the depth), samples were taken atdepths of 1 and 2 mm from the surface. These sampleswere electropolished using a mixture of 20% perchloricacid and methanol at 212uC and 11 V and analysed in anFEI Quanta field emission gun scanning electron micro-scope equipped with TSL-OIM software using a step sizeof 250 nm. For TEM studies, thin samples were cutparallel to the surface at the midplane and polished to athickness of 90 mm followed by twin jet electropolishingusing a mixture of 20% perchloric acid and methanol at220uC and 15 V. Observations were made using a PhilipsCM-20 microscope operating at 200 kV.

Results and discussion

Electron backscattered diffraction analysisFigure 2a shows the EBSD map (IPFzgrain boundary)of the base material. The average grain size was found tobe 75 mm, and the misorientation angle distribution inFig. 2b shows that majority of the boundaries are highangle. A single pass FSP at 640 rev min21 resulted insignificant grain refinement. The EBSD (IPFzgrainboundary) images in Fig. 3 show fine and equiaxed grains

1 Schematic of FSP tool

2 a electron backscattered diffraction (IPFzgrain boundary) map of base metal and b corresponding misorientation

angle distribution

Yadav and Bauri Characterisation of friction stir processed aluminium

1164 Materials Science and Technology 2011 VOL 27 NO 7

at the advancing side, centre and retreating side on thesurface of the stir zone respectively, indicating theoccurrence of DRX during the process. A similarmicrostructure was observed in the cross-section of thestir zone at depths of 1 and 2 mm, as shown in Fig. 4.Figure 5a shows the grain size distribution at the centre ofthe stir zone. The grain size distribution was narrow, andit is similar at all the other locations studied. The grainstructure is summarised in Table 1. A small variation ingrain size was found from the advancing side to theretreating side and also along the depth, as shown inTable 1. This could be attributed to the temperaturegradient across the stir zone. This thermal gradient playsa role in dynamic recovery (DRV) during the process.20,21

The material experiences peak temperature on the surfaceof the stir zone, towards the advancing side, and itdecreases as the depth from the surface increases. Themisorientation angle distribution in Fig. 5b shows a high

fraction of high angle boundaries in the centre. The lowangle peak (2u) observed in Fig. 5b corresponds to thesubgrain boundaries formed by dislocation arrangement.Similar characteristics were observed at all the locationsof the stir zone. However, some variation in fraction ofthe high angle boundaries was found at differentlocations of the stir zone (Table 1). This can also beattributed to the temperature gradient. As the surface ofthe stir zone is in direct contact with the shoulder ofthe tool, it experiences relatively higher temperature.Moreover, it also experiences larger deformation due tothe same reason. Therefore, the surface undergoes a highrate of DRV by which the low angle boundaries (formedby dislocation rearrangement and absorption) are trans-formed into high angle boundaries.

Increasing the rotation speed of the tool from 640 to800 rev min21 resulted in a similar microstructure withnarrow grain size distribution and with high fraction of

a advancing side; b centre; c retreating side3 Electron backscattered diffraction (IPFzgrain boundary) map on surface of stir zone

4 Electron backscattered diffraction map of cross-section at depths of a 1 mm and b 2 mm


Materials Science and Technology 2011 VOL 27 NO 7 1165

high angle boundaries, as shown in Fig. 6. However, thegrain size increased marginally due to the increase inrotation speed of the tool (Table 1). This can beattributed to the higher heat input. The amount of heatinput into the material depends on the ratio of toolrotation to traverse speed, with higher ratios leading tohigher heat input and consequently more grain growth.It is worth mentioning here that the rotation speeds of640 and 800 rev min21 were chosen after optimisationthrough several trial and error experiments. A rotationspeed below 640 rev min21 was not found suitable. Forexample, when the material was processed at560 rev min21, it gave rise to defects in the stir zone.There exists a minimum ratio of tool rotation to traversespeed for a given material. Below this, the heat producedby friction and stirring may not be sufficient to softenand plasticise the material, and it may not flow aroundthe rotating tool, making it difficult to produce acontinuous defect free stir zone.

An important point to be noted here is that a singlepass FSP resulted in significant grain refinement with a

narrow grain size distribution throughout the volume ofthe stir zone, which is 12 mm in width and 2?1 mm indepth (the length depends on the traverse distance,

5 a grain size distribution and b misorientation angle dis-

tribution at centre of stir zone

Table 1 Summary of microstructure developed after FSP

Rotation speed/rev min21 Traverse speed/mm min21 Average grain size/mm Percentage of high angle boundaries (.15u)

640 150 Advancing side, 2.79 Advancing side, 90.0Centre, 2.58 Centre, 83.6Retreating side, 2.57 Retreating side, 87.7Cross-section atdepth of 1 mm, 2.46 depth of 1 mm, 82.24depth of 2 mm, 2.37 depth of 2 mm, 73.9

800 150 Advancing side, 3.60 Advancing side, 86.3Centre, 3.34 Centre, 79.2Retreating side, 3.44 Retreating side, 81.3Cross-section atdepth of 1 mm, 3.22 depth of 1 mm, 75.7depth of 2 mm, 3.13 depth of 2 mm, 71.9

6 a electron backscattered diffraction (IPFzgrain bound-

ary) map, b grain size distribution and c misorientation

angle distribution of 800 rev min21 sample



which is 100 mm in this case). Most of the SPD andconventional thermomechanical processes involve sev-eral steps or several passes, and although they result ingrain refinement, the grain size distribution is wide,22,23

which may give rise to anisotropy in the properties.Moreover, the fraction of high angle boundariesproduced by FSP (.75%) is significantly higher thanthat produced by other SPD process (typically 60–65%).24,25 This may make the microstructure developedafter FSP stable against further thermal cycles as themobility of high angle boundaries is independent of themisorientation angle.

Transmission electron microscopy analysisFigure 7a shows the TEM image of the refined micro-structure in the stir zone of the sample processed at640 rev min21. The average grain size calculated fromTEM observations matches well with that obtained fromEBSD analysis. Some ultrafine grains were also observedin the microstructure, as shown in Fig. 7b. These ultrafinegrains were free of dislocations and bounded by high anglegrain boundaries, as seen from diffraction contrast(Fig. 7b). Individual grains were observed by tilting thesample, and the images were captured for maximumdislocation density orientation. Dislocations were seen tobe prone to arranging themselves into subgrain bound-aries, as shown in Fig. 8a. Figure 8b clearly shows that thesubgrain boundary is composed of an array of dislocations(marked by arrows). Owing to the high stacking faultenergy of aluminium, DRV readily occurs, and disloca-tions arrange themselves into a low angle sub-boundaryconfiguration. Figure 8b also shows that dislocations arebeing absorbed in the sub-boundary. Absorption of everydislocation into the subgrain boundary increases itsmisorientation and progressively turns them into lowangle grain boundaries. The diffraction contrast observedacross the subgrain boundaries, as seen from Fig. 8c,

confirms that misorientation has increased. This is alsosupported by the EBSD results. A detailed EBSD analysisrevealed three different types of boundaries based on themisorientation, as shown by different colours in the grainboundary map (superimposed with image quality map) inFig. 9a. These are low angle boundaries from 2 to 5u (red),low angle grain boundaries from 5 to 15u (green) and highangle (.15u) grain boundaries (blue). The first type of lowangle boundaries (2–5u) shown in red in Fig. 9a isessentially subgrain boundaries formed by dislocationrearrangement. Figure 9b shows subgrain boundaries withmixed orientation (mixed colours, red and green), indicat-ing that they are in the process of transforming into lowangle grain boundaries. Figure 9b also shows boundarieswith mixed green and blue orientations (low and highangles respectively). This indicates that the misorientationprogressively increases from sub-boundary to high angle.These mixed coloured boundaries were observed at all theother locations of the stir zone. A similar microstructurewas observed for the 800 rev min21 sample, with similardislocation features being observed inside the grains, asshown in Fig. 10. In the present work, free dislocations

7 Image (TEM) showing a refined microstructure and

b ultrafine grain

8 Image (TEM) showing a dislocation arrangement into

subgrain boundary, b dislocations being absorbed into

subgrain boundary (arrows indicate array of disloca-

tions) and c diffraction contrast across subgrain

boundaries



were also observed inside the grains (Fig. 11a), which isgenerally not expected in a high stacking fault energymaterial like aluminium after such a thermomechanicalprocess.26 Hence, to an extent, the presence of freedislocations also shows that the microstructure developedafter FSP may be stable against further thermal cyclesoccurring during FSP or post-FSP heat treatment orthermal exposure. Different dislocation densities anddifferent degrees of recovery were also observed in theneighbouring grains (Fig. 11b). This indicates the presenceof a possible strain gradient during the process. The grainorientation spread function estimated from EBSD analysisshows that the adjacent grains exhibit different amounts oforientation spread (Fig. 12). The bigger grains showed anin-grain misorientation gradient of up to 5u, whereas thefiner ones have up to 1u. This again points towards theexistence of a strain gradient across the grains.

Various forms of DRX have been reported byvarious authors as the mechanism of grain refinementduring FSP of Al. These include discontinuous DRX(DDRX), continuous DRX (CDRX) and geometricDRX.15–19 Discontinuous DRX takes place by theclassical mechanism of nucleation of strain free grainsand the sweeping motions of boundaries (growth).27 Itssustenance requires a high density of free dislocations(build-up of strain). Continuous DRX, on the otherhand, takes place by the gradual increase in misor-ientation between subgrain boundaries.28 As described

earlier, in the present study, the subgrains form bydislocation rearrangement, and absorption of disloca-tion into the subgrain boundaries progressively in-creases the misorientation to turn them into grainboundaries. Therefore, a CDRX driven by DRV seemsmore likely to occur in this case. However, the ultrafinegrains observed in the microstructure were free of anysuch dislocation structure and bounded by high anglegrain boundaries (Fig. 7b), and hence, they seem toevolve through a different recovery path. It appearsthat these ultrafine grains are formed by the classicalnucleation and growth mechanism as in DDRX. Thevarying dislocation density observed by TEM indicatesthe presence of a strain gradient during the process.Owing to this strain gradient, in some regions of thevolume of the stir zone, DRV reduces the build-up ofstrain energy by preventing dislocation accumulationand allows the dislocations to arrange into low angleboundaries, whereas in the highly strained regions, dy-namic nucleation can occur due to dislocation accu-mulation. The nuclei for the DDRX process are part ofthe heavily deformed matrix. A small recovered regionin the deformed grains can be considered as a potentialnucleus if the relative difference in stored energybetween the potential nucleus and its surrounding is

9 a grain boundary map showing boundaries with differ-

ent colours based on their misorientation angle and

b boundaries with mixed colours (orientations) indicat-

ing progressive transformation in orientation (white

arrow indicates subgrain to low angle grain boundary, and

black arrow indicates low to high angle transformation).

(For interpretation of the colours in this figure, the reader

is referred to the online version of this article)10 Images (TEM) of 800 rev min21 sample showing

a refined microstructure, b dislocations arranging into

subgrain boundary and c diffraction contrast observed

across subgrain boundary



high. The ultrafine grains might form in such highlystrained regions, which can sustain a high dislocationdensity required for DDRX.29

ConclusionsFriction stir processing of commercially pure aluminiumresulted in significant grain refinement. The grainstructure is characterised by a narrow grain size distribu-tion and a high fraction of high angle grain boundaries.The microstructure shows dislocation arrangement intosubgrain boundaries, grains having different dislocationdensities and in different stages/degrees of recovery. Amultimechanism of DRV, CDRX and DDRX seems tobe operating during the process. The ultrafine grains areformed by nucleation process in the highly strainedregions due to dislocation accumulation, while the finegrains are formed by a CDRX process driven by DRV.

Increasing the rotation speed from 640 to 800 rev min21

does not change the microstructure evolution path. As aconcluding remark, it should be noted here that theobservations are based on the final microstructuredeveloped after FSP, and several mechanisms or stepsmay be involved at different stages of the microstruc-ture development. In addition, the mechanism of grainrefinement may vary from one alloy to another,depending on its purity, alloying elements and natureof strengthening precipitates.

Acknowledgements

The authors would like to thank Professor K. PrasadRao, IIT Madras, for providing the NRB supportedFSP facility. Thanks are also due to Professor I.Samajdar, IIT Bombay, for providing access to theDST supported national facility for texture and OIM forEBSD studies.

References1. R. Z. Valiev, A. V. Korznikov and R. R. Mulyukov: Mater. Sci.

Eng. A, 1993, A168, 141–148.

2. R. Valiev, Y. Estrin, Z. Horita, T. Langdon, M. Zechetbauer and

Y. Zhu: JOM, 2006, 58, 33–39.

3. R. S. Mishra, M. W. Mahoney, S. X. McFadden, N. A. Mara and

A. K. Mukherjee: Scr. Mater., 2000, 42, 163–168.

4. R. S. Mishra and Z. Y. Ma: Mater. Sci. Eng. R, 2005, R50, 1–78.

5. J. Q. Su, T. W. Nelson and C. J. Sterling: Scr. Mater., 2005, 52,

135–140.

6. C. I. Chang, X. H. Du and J. C. Huang: Scr. Mater., 2007, 57, 209–212.

7. Y. J. Kwon, I. Shigematsu and N. Saito: Scr. Mater., 2003, 49,

785–789.

8. K. Nakata, Y. G. Kim, H. Fujii, T. Tsumura and T. Komazaki:

Mater. Sci. Eng. A, 2006, A437, 274–280.

9. Z. Y. Ma and R. S. Mishra: Scr. Mater., 2005, 53, 75–80.

10. L. B. Johannes and R. S. Mishra: Mater. Sci. Eng. A, 2007, A464,

255–260.

11. Z. Y. Ma, A. L. Pilchak, M. C. Juhas and J. C. Williams: Scr.

Mater., 2008, 58, 361–366.

12. R. S. Mishra, Z. Y. Ma and I. Charit: Mater. Sci. Eng. A, 2003,

A341, 307–310.

13. D. Yadav and R. Bauri: Mater. Lett., 2010, 64, 664–667.

14. R. W. Fonda, J. F. Bingert and K. J. Colligan: Scr. Mater., 2004,

51, 243–248.

15. K. V. Jata and S. L. Semiatin: Scr. Mater., 2000, 43, 743–749.

16. J. Q. Su, T. W. Nelson, R. S. Mishra and M. W. Mahoney: Acta

Mater., 2003, 51, 713–729.

17. T. R. McNelley, S. Swaminathan and J. Q. Su: Scr. Mater., 2008,

58, 349–354.

18. J. Q. Su, T. W. Nelson and C. J. Sterling: Mater. Sci. Eng. A, 2005,

A405, 277–286.

19. J. Q. Su, T. W. Nelson and C. J. Sterling: Philos. Mag., 2006, 86, 1–24.

20. Y. Li, L. E. Murr and J. C. Mclure: Mater. Sci. Eng. A, 1999, A271,

213–223.

21. M. W. Mahoney, C. G. Rhodes, J. G. Flintoff and R. A. Spurling:

Metall. Mater. Trans. A, 1998, 29A, 1955–1964.

22. A. P. Zhilyaev, B. K. Kim, J. A. Szpunar, M. D. Baro and T. G.

Langdon: Mater. Sci. Eng. A, 2005, A391, 377–389.

23. A. Gholinia, P. B. Prangnell and M. V. Markushev: Acta Mater.,

2000, 48, 1115–1130.

24. E. A. El-Danaf: Mater. Sci. Eng. A, 2008, A487, 189–200.

25. R. Kapoor, N. Kumar, R. S. Mishra, C. S. Huskamp and K. K.

Sankaran: Mater. Sci. Eng. A, 2010, A527, 5246–5254.

26. G. Zlateva and Z. Martinova: ‘Microstructure of metals and alloys:

an atlas of transmission electron microscopy images’, 1st edn, 33;

2008, New York, CRC Press.

27. L. S. Toth and J. J. Jonas: Scr. Metall. Mater., 1992, 27, 359–363.

28. S. Gourdet and F. Montheillet: Acta Mater., 2003, 51, 2685–

2699.

29. F. J. Humphreys and M. Hatherly: ‘Recrystallization and related

annealing phenomena’, 2nd edn, 371; 1996, Oxford, Elsevier.

11 Image (TEM) showing a free dislocations inside grain

and b different dislocation densities and degrees of

recovery in neighbouring grains

12 Grain orientation spread map showing misorientation

spread across individual grains. (For interpretation of

the colours in this figure, the reader is referred to the

online version of this article)



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Microstructural characterisation of friction stir processed aluminium