Optimisation of friction stir processing parameters to produce sound ...

Optimisation of friction stir processing parameters to produce sound and fine grain layers in pure magnesium D. Ahmadkhaniha, M. Heydarzadeh Sohi* and A. Zarei-Hanzaki The effects of friction stir processing (FSP) parameters such as rotational, traverse speeds and tool penetration depth on the formation of fine and defect free magnesium layers were investigated. The achieved microstructures were optically studied, and the microhardness profile of the optimised workpiece was measured. The results show that rotational and traverse speeds as well as their ratio play key roles in achieving a sound friction stir processed workpiece of pure Mg. In addition, at constant rotational and traverse speeds, when the penetration depth increases, the title angle must also increases in order to have a defect free workpiece. At optimum conditions, one pass FSP significantly refined the grain size from 3 mm in the as received magnesium to 14?6 mm in friction stir process layer. The microhardness of the fabricated layer reached to about 1?6 times that of the base metal. Keywords: Pure magnesium, Friction stir processing, Microhardness, Microstructure

Introduction Magnesium and its alloys have received considerable attention as biodegradable implants in recent years. Unfortunately, their biomedical applications are still limited by some shortcomings including their poor corrosion resistance.1,2 In addition, high degradation rate of Mg based materials deteriorates their mechanical integrity during the healing process. Many techniques have been developed to improve corrosion resistance of magnesium alloys, such as alloy modification,3 producing composites4–8 and surface treatment.9–11 Among this variety of methods, the one which can enhance both the corrosion and mechanical properties of magnesium would be more interesting. Previous studies have shown that the grain refinement can improve mechanical properties12,13 and increase corrosion resistance.14–16 In recent years, a lot of researchers have focused on friction stir processing (FSP), which can significantly refine structure,17–22 improve superplasticity23–25 and is a route to form intermetallics22,26–28 and fabricate composites.22,29–31 FSP requires a careful design of process parameters in order to achieve a defect free workpiece that is consistently reproducible. Defects in friction stir processed workpieces have already been classified as flow or geometric related. The geometric related defects usually occur due to insufficient pin penetration depth. Flash formation, surface galling, lack of fill, wormholes and nugget collapse or lack of

School of Metallurgy and Materials, College of Engineering, University of Tehran, Tehran, PO Box 1155-4563, Iran *Corresponding author, email [email protected]

consolidation are categorised in the flow related defects.32 Most of the researches on FSP have been focused on specific types of aluminium and magnesium alloys such as AZ31 and AZ91. Azizieh et al.33 studied the effect of rotational speed and probe profile on microstructure and hardness of AZ31/Al2O3 nanocomposites. Asadi et al.34 evaluated the effects of FSP parameters on the formation of a defect free AZ91/SiC composite. Balasubramanian35 investigated the relationship between base metal properties and friction stir welding process parameters. Elangovan and Balasubramanian36 studied the influence of tool pin profile and tool shoulder diameter on the formation of FSP zone in AA 6061 aluminium alloy. During FSP, the friction between the tool and the workpiece and the plastic deformation occurring around the tool result in heat generation. It has been shown that the heat generated during friction stir welding was influenced mainly by rotational to traverse speed ratio (w/v). Arbegast and Hartley17 established a relationship for aluminium alloys between the maximum welding temperature and the processing parameters. Commin et al.37 also showed that by increasing the shoulder diameter or the tool rotational speed or decreasing traverse speed, the heat generated during the processing of AZ31 was increased. There are limited works on FSP of magnesium for biomedical application. However, there have been no reports on the effect of the processing parameters on the quality of the treated zone in pure Mg. Since FSP possesses the ability to severely refine the grains, it would be a powerful candidate to enhance the corrosion resistance of pure Mg. However, as magnesium holds hexagonal close packed structure, the successful achievement of fine grains in pure Mg by

ß 2014 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 4 November 2013; accepted 2 December 2013 DOI 10.1179/1362171813Y.0000000186

Science and Technology of Welding and Joining

2014

VOL

19

NO

3

235

Ahmadkhaniha et al.

Producing sound and fine grain layers on pure magnesium by FSP

FSP might be very difficult due to its limited slip systems. Therefore, this work has been conducted to study the effects of FSP parameters, such as rotational w and traverse v speeds, tool penetration depth (PD) and tilt angle on preparation of a defect free friction stir processed pure magnesium workpiece.

Experimental The workpieces with dimension of 106567 mm were prepared from a pure Mg ingot. The predetermined FSP were applied on the surface of the workpieces using a conventional miller machine. The tapered cylindrical FSP tool with shoulder diameter of 15 mm was machined from H13 tool steel (Fig. 1). In this study, the FSP tool design was held constant, but the other parameters including tool’s rotational and traverse speeds were subjected to change (Table 1). As was 1 Image of FSP tool

a w51000 rev min21, v563 mm min21, PD50?2 mm; b w51000 rev min21, v531?5 mm min21, w51000 rev min21, v512 mm min21, PD50?2 mm; d w51250 rev min21, v512 mm min21, PD50?4 mm 2 Surface appearance of friction stir processed specimens at different conditions with tilt angle of 2?5u

PD50?2 mm;

c

Table 1 Friction stir processing parameters Code number

Rotational speed w/rev min21

Traverse speed v/mm min21

w/v

Tilt angle/u

PD/mm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1000 1000 1000 1250 1250 1250 1250 1250 1600 1600 1600 1600 1600 1600 1600 1600

63 31?5 12 63 31?5 12 12 12 12 31?5 31?5 31?5 31?5 31?5 31?5 63

15?87 31?74 83?33 19?84 39?68 104?16 104?16 104?16 133?33 50?79 50?79 50?79 50?79 50?79 50?79 25?39

2?5 2?5 2?5 2?5 2?5 2?5 2?5 2?5 2?5 2?5 2?5 2?5 3 1?5 1?5 2?5

0?2 0?2 0?2 0?2 0?2 0?2 0?3 0?4 0?2 0?2 0?3 0?4 0?2 0?2 0?1 0?2


2014

VOL

19

NO

3

236

Ahmadkhaniha et al.


a w51000 rev min21, v512 mm min21, PD50?2 mm, tilt angle52?5u; b w51250 rev min21, v563 mm min2-1, PD50?2 mm, tilt angle52?5u; c w51250 rev min21, v531?5 mm min21, PD50?2 mm, tilt angle52?5u; d w51250 rev min21, v512 mm min21, PD50?2 mm, tilt angle52?5u; e w51250 rev min21, v512 mm min21, PD50?3 mm, tilt angle52?5u; f w51600 rev min21, v563 mm min21, PD50?2 mm, tilt angle52?5u; g w51600 rev min21, v531?5 mm min21, PD50?2 mm, tilt angle53u 3 Cross-sectional macroviews of friction stir processed specimens at different conditions

mentioned in section on ‘Introduction’, heat input is proportional to the rotational and traverse speeds. Accordingly, different combinations of w and v were chosen to investigate the effect of heat input on the friction stir processed workpiece. As shown in Table 1, a number is given to each condition and will be referred to in this manuscript. To study the microstructure and the microhardness, the cross-section of friction stir processed workpieces were prepared by standard metallographic procedure and etched. Then, the microstructure of the workpiece was characterised by optical microscopy (model GipponGDCE-30). A Vickers microhardness tester (HV-5, Laizhou Huayin Testing Instrument Co. Ltd) was used to measure the hardness of the workpieces with an applied load of 200 g for 10 s.

Results and discussion Qualification of friction stir processed workpieces The friction stir processed workpiece’s surfaces created at a tilt angle equal to 2?5u, PD50?2 mm and various rotational and traverse speeds are shown in Fig. 2. The

cross-sectional macro- and microstructures of the friction stir processed workpieces, fabricated at different processing conditions, are also shown in Figs. 3 and 4. Figure 2a shows the related surface of the workpiece, which was treated with 1000 rev min21 and 63 mm min21 speeds. The observed galling and grooves on the surface of workpiece are attributed to the poor material flow due to the low heat input.18 By decreasing the traverse speed to 31?5 mm min21, the amount of heat input is increased. Referring to Fig. 2b, the processing zone is cracked in this processing condition (Fig. 2b). Since Mg has a hexagonal closed pack structure, its deformation capability is very sensitive to temperature. In this condition, the amount of heat input was not sufficient to make the material soft enough; hence, a brittle fracture occurred. By lowering the traverse speed to 12 mm min21, the surface appearance improved (Fig. 2c), but tunnelling defect was noticed in its cross-section (Figs. 3a and 4a), which is due to insufficient heat input. Therefore, to overcome this problem, the rotational speed was increased to 1250 rev min21 in order to increase the heat input. As seen in Figs. 3b–d and 4b–d, tunnelling defect or voids remained in the cross-section of the workpieces even by decreasing the traverse speed to 12 mm min21. These defects are formed where the


2014

VOL

19

NO

3

237

Ahmadkhaniha et al.


a w51000 rev min21, v512 mm min21, PD50?2 mm, tilt angle52?5u; b w51250 rev min21, v563 mm min2-1, PD50?2 mm, tilt angle52?5u; c w51250 rev min21, v531?5 mm min21, PD50?2 mm, tilt angle52?5u; d w51250 rev min21, v512 mm min21, PD50?2 mm, tilt angle52?5u; e w51250 rev min21, v512 mm min21, PD50?3 mm, tilt angle52?5u; f w51600 rev min21, v563 mm min21, PD50?2 mm, tilt angle52?5u; g w51600 rev min21, v531?5 mm min21, PD50?2 mm, tilt angle53u 4 Cross-sectional micro views of friction stir processed specimens at different conditions

amount of heat and pressure is not adequate to fill the space behind and below the FSP tool. Since the amount of w/v ratio in this condition (w51250 rev min21, v512 mm min21) is high enough (Table 1), it was decided to increase PD to forge the material behind the FSP tool and fill the voids. By comparing Fig. 3d and e and Fig. 4d and e, it is noticed that increasing the PD from 0?2 to 0?3 mm reduced the size of the void. Further increase in the PD to 0?4 mm was ended to sticking of magnesium to the FSP tool (Fig. 2d). On the whole, it was found that by decreasing the traverse speed from 25 to 12 mm min21 and increasing the PD, the size of voids was slightly reduced, but at the same time, the material’s surface would be damaged. It was therefore decided to follow the FSP at higher rotational speeds. Figure 5a discloses that w5 1600 rev min21 and v512 mm min21 have resulted in material sticking to the FSP tool, apparently due to the excessive heat generated at this condition. Increasing traverse speed to 31?5 mm min21, thereby reducing the heat input, has improved the surface appearance and resulted in a defect free and sound FSP tracks (Fig. 5b). Although the surface appearance by increasing the traverse speed to 63 mm min21 remained satisfied, tunnelling defect was formed again in the cross-section (Figs. 3f and 4f). According to these observations and

Table 1, it is concluded that defining a lower and upper limit of heat input for FSP of pure Mg is not easy. These results demonstrate that rotational and traverse speeds can change friction mode between the tool and the workpiece, and they hold significant influence in fabricating a sound friction stir processed layer. In addition, it appears that FSP’s window process for pure Mg is quite narrower than that of AZ alloys, since Asadi et al.,34 Commin et al.37 and Ramesh Babu et al.38 achieved sound FSP workpieces with extended ranges of rotational and traverse speeds in AZ alloys. The optimised rotational and traverse speeds in this study appear to be 1600 rev min21 and 31?5 mm min21 respectively. Hence, the effect of tilt angle and PD was studied in this condition. Figure 5c shows the effect of increasing PD (from 0?2 to 0?4 mm) on surface quality of the friction stir processed workpiece. By increasing PD, actual contact area between the FSP tool and the workpiece increases, and it approaches the apparent one. In this case, sliding friction disappears and sticking friction dominates. By comparing Fig. 5b to Fig. 5e, it is concluded that where PD is low, defect free workpiece can be achieved by decreasing the tilt angle and vice versa. Figure 4g is a cross-sectional view of the workpiece, which has been treated by w51600 rev min21, v531?5 mm min21, PD50?2 mm and tilt angle53u,


2014

VOL

19

NO

3

238

Ahmadkhaniha et al.


a w51600 rev min21, v512 mm min21, PD50?2 mm, tilt angle52?5u; b w51600 rev min21, v531?5 mm min2-1, PD50?2 mm, tilt angle52?5u; c w51600 rev min21, v531?5 mm min21, tilt angle52?5u, PD gradually increased from 0?2 to 0?4 mm according to the given graph; d w51600 rev min21, v531?5 mm min21, PD50?2 mm, tilt angle51?5u; e w51600 rev min21, v531?5 mm min2-1, PD50?1 mm, tilt angle51?5u 5 Surface appearances of specimens treated at different conditions

a overview image; b NZ; c NZ and TMAZ; d TMAZ; e heat affected zone and base metal; f higher magnification of TMAZ for better observation of twinning 6 Optical micrographs of cross-section of workpiece processed at optimised conditions of w51600 rev min21, v531?5 mm min21, tilt angle52?5u and PD50?2 mm


2014

VOL

19

NO

3

239

Ahmadkhaniha et al.


in hexagonal close packed metals, the relative activity of these mechanisms and the fraction of recrystallised grains. The heat and strain induced in TMAZ are lower than those in NZ, and hence, the grain size in TMAZ is coarser than that in NZ (Table 2 shows the mean grain size values of different zones). Since the twinning stress increases with decreasing grain size, the finer grain size can be the most important cause for the suppression of twinning in NZ; hence, twinning is not favoured over slip in NZ due to the combination of grain refinement and high processing temperature.39,40

Microhardness

7 Microhardness proﬁles of friction stir processed workpiece at different depths from surface, at condition of w51600 rev min21, v531?5 mm min21, PD50?2 mm and tilt angle52?5u

showing a few voids in the treated zone, although its surface appearance was as good as the one shown in Fig. 5b. Since the contact area between the shoulder and the workpiece in a constant PD decreases with increasing the tilt angle, pressure is not sufficient to forge materials; therefore, voids may appear in the workpiece cross-section. Figure 5d and e demonstrate that if tilt angle decreased to 1?5u, defect free workpieces can be achieved by decreasing the PD to 0?1 mm. Consequently, at higher tilt angle, a higher PD is required to supply the process heat and forge the materials behind the tool.

The hardness distributions across the friction stir processed zone, at 2?5, 4 and 6 mm below the surface, are shown in Fig. 7. The higher microhardness of NZ in comparison to that of the base metal can be attributed to grain refinement in NZ. As seen in Fig. 7, the highest microhardness was achieved at 4 mm below the surface. The hardness variation through the thickness can be explained on the basis of temperature reached at different depths. Since shoulder holds more impression in producing heat than pin,17 it is responsible for the formation of larger grains close to the top surface. This result is supported by the mean grain size values in Table 2, which was also observed by Sato et al.,41 Darras et al.,42 Yu et al.43 and Chang et al.44 The grain boundaries can hinder the dislocation slip, thereby resulting in higher hardness achieved at the centre of the workpiece. At 6 mm distance from the top, there was not sufficient strain and microhardness changed in a narrower width. In this work, the microhardness of one pass friction stir processed Mg reached a maximum of 1?6 times that of the as cast one, while in a previous study by Chang, similar amount of increase in hardness was achieved with four passes of equal channel angular pressing.45

Microstructure Figure 6a shows an optical overview of a cross-section of the workpiece processed at the optimised conditions of w51600 rev min21, v531?5 mm min21, tilt angle5 2?5u and PD50?2 mm. Figure 6b–e shows the microstructures of corresponding different zones, i.e. nugget zone (NZ), thermomechanically affected zone (TMAZ) and heat affected zone. Figure 6b demonstrates that FSP has resulted in the formation of a fine grained NZ with average grain size of 14?6 mm that is 205 times smaller than that of the untreated magnesium. This is related to the occurrence of dynamic recrystallisation as a result of high input strain at high temperature. The presence of distinct grain boundaries in Fig. 6b and c also confirms the occurrence of dynamic recrystallisation during FSP. In addition, some twins are observed within the grains in TMAZ (Fig. 6d and f). Figure 6b–e shows the variation of twinning frequency from NZ to base metal. As is well established, the deformation modes are highly dependent on the temperature, strain, strain rate and crystal structure. Increasing the temperature would alter the deformation mechanism from twinning to slip Table 2 Mean grain size value at different zones Base material

Top of NZ

Middle of NZ

Bottom of NZ

TMAZ

3 mm

14?6 mm

13?6 mm

15?4 mm

22?8 mm

Conclusions This study shows that FSP’s window process for pure Mg is quite narrower than that of AZ alloys. Achieving a defect free layer of friction stir processed pure Mg is very sensitive to the processing temperature and friction mode. Rotational and traverse speeds play key roles in achieving a sound friction stir processed pure Mg layer. In addition, at constant rotational and traverse speeds, when the PD increases, the title angle must also increase in order to have a defect free workpiece. One pass FSP significantly refined the grain size from 3 mm in the as cast magnesium to 14?6 mm in the fabricated friction stir processed layer at optimum conditions and increased the microhardness of the fabricated layer to about 1?6 times that of the base material.

References 1. F. Witte: ‘The history of biodegradable magnesium implants: a review’, Acta Biomater., 2010, 6, 1680–1692. 2. M. P. Staiger, A. M. Pietak, J. Huadmai and G. Dias: ‘Magnesium and its alloys as orthopedic biomaterials: a review’, Biomaterials, 2006, 27, 1728–1734. 3. E. Zhang, L. Yang, J. Xua and H. Chen: ‘Microstructure, mechanical properties and bio-corrosion properties of Mg–Si (Ca, Zn) alloy for biomedical application’, Acta Biomater., 2010, 6, 1756–1762. 4. F. Witte, F. Feyerabend, P. Maier, J. Fischer, M. Stormer, C. Blawert, W. Dietzel and N. Hort: ‘Biodegradable magnesium– hydroxyapatite metal matrix composites’, Biomaterials, 2007, 28, 2163–2174.


2014

VOL

19

NO

3

240

Ahmadkhaniha et al.

5. X. Gu, W. Zhou, Y. Zheng, L. Dong, Y. Xi and D. Chai: ‘Microstructure, mechanical property, bio-corrosion and cytotoxicity evaluations of Mg/HA composites’, Mater. Sci. Eng. C, 2010, C30, 827–832. 6. A. Feng and Y. Han: ‘Mechanical and in vitro degradation behavior of ultrafine calcium polyphosphate reinforced magnesium-alloy composites’, Mater. Des., 2011, 32, 2813–2820. 7. K. Abdelrazek Khalil and A. A. Almajid: ‘Effect of high-frequency induction heat sintering conditions on the microstructure and mechanical properties of nanostructured magnesium/hydroxyapatite nanocomposites’, Mater. Des., 2012, 36, 58–68. 8. A. Feng and Y. Han: ‘The microstructure, mechanical and corrosion properties of calcium polyphosphate reinforced ZK60A magnesium alloy composites’, J. Alloys Compd, 2010, 504, 585–593. 9. H. Hornberger, S. Virtanen and A. R. Boccaccini: ‘Biomedical coatings on magnesium alloys – a review’, Acta Biomater., 2012, 8, 2442–2455. 10. L. Li, J. Gao and Y. Wang: ‘Evaluation of cyto-toxicity and corrosion behavior of alkali-heat-treated magnesium in simulated body fluid’, Surf. Coat. Technol., 2004, 185, 92–98. 11. Y. Wang, M. Wei and J. Gao: ‘Improve corrosion resistance of magnesium in simulated body fluid by dicalcium phosphate dihydrate coating’, Mater. Sci. Eng. C, 2009, C29, 1311–1316. 12. Y. J. Kwon, I. Shigematsu and N. Saito: ‘Mechanical property improvements in aluminum alloy through grain refinement using friction stir process’, Mater. Trans., 2004, 45, 2304–2311. 13. W. M. Gan, M. Y. Zheng, H. Chang, X. J. Wang, X. G. Qiao, K. Wu, B. Schwebke and H. G. Brokmeier: ‘Microstructure and tensile property of the ECAPed pure magnesium’, J. Alloys Compd, 2009, 470, 256–262. 14. H. Wang, Y. Estrin, H. Fu, G. Song and Z. Zu´berova´: ‘The effect of pre-Processing and grain structure on the bio-corrosion and fatigue resistance of magnesium alloy AZ31’, Adv. Eng. Mater., 2007, 9, 967–972. 15. Y. Chino, T. Hoshika and M. Mabuchi: ‘Mechanical and corrosion properties of AZ31 magnesium alloy repeatedly recycled by hot extrusion’, Mater. Trans., 2006, 47, 1040–1046. 16. G. Ben Hamua, D. Eliezer and L. Wagner: ‘The relation between severe plastic deformation microstructure and corrosion behavior of AZ31 magnesium alloy’, J. Alloys Compd, 2009, 468, 222–229. 17. W. J. Arbegast and P. J. Hartley: ‘Friction stir weld technology development at Lockheed. Martin Michoud Space Systems- An Overview’, in: Proceedings of the Fifth International Conference on Trends in Welding Research, Pine Mountain, GA, USA, June 1–5, 1998, 541. 18. R. S. Mishra and M. W. Mahoney: ‘Friction stir welding and processing’; 2007, Materials Park, OH, ASM International. 19. C. I. Chang, X. H. Dua and J. C. Huang: ‘Achieving ultrafine grain size in Mg–Al–Zn alloy by friction stir processing’, Scr. Mater., 2007, 57, 209–212. 20. Y. Zhang, Y. S. Sato, H. Kokawa, S. Hwan, C. Park and S. Hirano: ‘Stir zone microstructure of commercial purity titanium friction stir welded using pcBN tool’, Mater. Sci. Eng. A, 2008, A488, 25–30. 21. T. R. McNelley, S. Swaminathan and J. Q. Su: ‘Recrystallization mechanisms during friction stir welding/processing of aluminum alloys’, Scr. Mater., 2008, 58, 349–354. 22. Z. Y. Ma: ‘Friction stir processing technology: a review’, Metall. Mater. Trans. A, 2008, 39A, 642–658. 23. Z. Y. Ma, F. C. Liu and R. S. Mishra: ‘Superplastic deformation mechanism of an ultrafine-grained aluminum alloy produced by friction stir processing’, Acta Mater., 2010, 58, 4693–4704. 24. Z. Y. Ma, R. S. Mishra and M. W. Mahoney: ‘Superplastic deformation behaviour of friction stir processed 7075Al alloy’, Acta Mater., 2002, 50, 4419–4430. 25. I. Charit and R. S. Mishra: ‘High strain rate superplasticity in a commercial 2024 Al alloy via friction stir processing’, Mater. Eng. A, 2003, A359, 290–296.


26. C. J. Hsu, C. Y. Chang, P. W. Kao, N. J. Ho and C. P. Chang: ‘Al– Al3Ti nanocomposites produced in situ by friction stir processing’, Acta Mater., 2006, 54, 5241–5249. 27. L. Ke, C. Huang, L. Xing and K. Huang: ‘Al–Ni intermetallic composites produced in situ by friction stir processing’, J. Alloys Compd, 2010, 503, 494–499. 28. C. J. Hsu, P. W. Kao and N. J. Ho: ‘Ultrafine-grained Al–Al2Cu composite produced in situ by friction stir processing’, Scr. Mater., 2005, 53, 341–345. 29. D. K. Lima, T. Shibayanagi and A. P. Gerlich: ‘Synthesis of multiwalled CNT reinforced aluminium alloy composite via friction stir processing’, Mater. Sci. Eng. A, 2009, A507, 194–199. 30. M. Dixit, J. W. Newkirk and R. S. Mishra: ‘Properties of friction stir-processed Al 1100–NiTi composite’, Scr. Mater., 2007, 56, 541– 544. 31. M. Barmouz, P. Asadi, M. K. BesharatiGivia and M. Taherishargh: ‘Investigation of mechanical properties of Cu/SiC composite fabricated by FSP: effect of SiC particles’ size and volume fraction’, Mater. Sci. Eng. A, 2011, A528, 1740–1749. 32. R. M. Leal, C. Leita˜o, A. Loureiro, D. M. Rodrigues and P. Vilaca: ‘Material flow in heterogeneous friction stir welding of thin aluminium sheets: effect of shoulder geometry’, Mater. Sci. Eng. A, 2008, A498, 384–391. 33. M. Azizieh, A. H. Kokabi and P. Abachi: ‘Effect of rotational speed and probe profile on microstructure and hardness of AZ31/ Al2O3 nanocomposites fabricated by friction stir processing’, Mater. Des., 2011, 32, 2034–2041. 34. P. Asadi, G. Faraji and M. Besharati: ‘Producing of AZ91/SiC composite by friction stir processing (FSP)’, Int. J. Adv. Manuf. Technol., 2010, 51, 247–260. 35. V. Balasubramanian: ‘Relationship between base metal properties and friction stir welding process parameters’, Mater. Sci. Eng. A, 2008, A480, 397–403. 36. K. Elangovan and V. Balasubramanian: ‘Influences of tool pin profile and tool shoulder diameter on the formation of friction stir processing zone in AA6061 aluminum alloy’, Mater. Des., 2008, 29, 362–373. 37. L. Commin, M. Dumont, J. E. Masse and L. Barrallier: ‘Friction stir welding of AZ31 magnesium alloy rolled sheets – influence of processing parameters’, Acta Mater., 2009, 57, 326–334. 38. S. Ramesh Babu, V. S. Senthil Kumar, G. Madhusudhan Reddy and L. Karunamoorthy: ‘Microstructural changes and mechanical properties of friction stir processed extruded AZ31 alloy’, Proc. Eng., 2012, 38, 2956–2966. 39. J. Jiang and A. Ma: ‘Bulk ultrafine-grained magnesium alloys by SPD processing: technique, microstructures and properties’, (ed. F. Czerwinski); 2011, Jiangsu, Hohai University, 187–218. 40. S. Biswas, S. Singh Dhinwal and S. Suwas: ‘Room-temperature equal channel angular extrusion of pure magnesium’, Acta Mater., 2010, 58, 3247–3261. 41. Y. S. Sato, M. Urata, H. Kokawa and K. Ikeda: ‘Hall–Petch relationship in friction stir welds of equal channel angular-pressed aluminum alloys’, Mater. Sci. Eng. A, 2003, A54, 298–305. 42. B. M. Darras, M. K. Khraisheh, F. K. Abu-Farha and M. A. Omar: ‘Friction stir processing of commercial AZ31 magnesium alloy’, J. Mater. Process. Technol., 2007, 191, 77–81. 43. S. R. Yu, X. J. Chen, Z. Q. Huang and Y. H. Liu: ‘Microstructure and mechanical properties of friction stir welding of AZ31B magnesium alloy added with cerium’, J. Rare Earths, 2010, 28, 316– 320. 44. C. I. Chang, C. J. Lee and J. C. Huang: ‘Relationship between grain size and working strain rate and temperature during friction stir processing in AZ31 Mg alloy’, Scr. Mater., 2004, 51, 509–514. 45. S. Y. Chang, S. W. Lee, K. M. Kang, S. Kamado and Y. Kojima: ‘Improvement of mechanical characteristics in severely plasticdeformed Mg alloys’, Mater. Trans., 2004, 45, 488–492.


2014

VOL

19

NO

3

241