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Fabrication of Al-Zn solid solution via friction stir processing
Materials Characterization 136 (2018) 221–228
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Fabrication of Al-Zn solid solution via friction stir processing ⁎
Devinder Yadav , Ranjit Bauri, Niraj Chawake
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Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Friction stir processing Metals and alloys Mechanical alloying Microstructure Electron microscopy Hardness
The present study demonstrates that friction stir processing (FSP) can be used as a mechanical alloying tool to process solid solution. Zn was incorporated in commercially pure aluminium by multi-pass FSP leading to its dissolution and formation of supersaturated solid solution which was confirmed by X-ray diffraction and scanning electron microscopy. The hardness across the stir zone matched with the distribution of Zn and was significantly improved compared to the base metal. The supersaturated solid solution that formed in the first pass, partially decomposed during the second pass, leading to the formation of fine Al-Zn precipitates. Thermal stability of the microstructure against abnormal grain growth was also improved after the second pass due to the pinning effect of the grain boundary precipitates. FSP also caused grain refinement of the alloy yielding fine equiaxed grains in the range of 4–6 μm with a high fraction (> 67%) of high angle grain boundaries. Such microstructure is conducive for isotropic properties and the fact that the microstructure is stable against thermal cycles make the process suitable for selective alloying for location specific property enhancement.
1. Introduction Since its advent a decade ago, friction stir processing (FSP) has emerged as a useful tool for material processing and microstructure modification [1]. This solid state technique has been primarily used for grain refinement of aluminium and magnesium alloys [2–3]. It has also found its applications in microstructure homogenization [4–6] and composite fabrication [2,7–9]. FSP uses a cylindrical rotating tool with a shoulder and pin to produce localized heating and plastic deformation to process the material. Apart from the material transport and mixing action, the process also involves severe plastic deformation and is thermo-mechanical in nature. The potential of all these aspects can be fully realized if the process is utilized for making solid solutions that may provide opportunities for new material development. The thermomechanical aspect of FSP is utilized in fabricating Al based in-situ composites where transitional metals (Cu, Zr, Fe, Ni) react with Al to form intermetallics during the process [10–13]. The methodology adopted to process such composites involves mixing elemental powders, cold compacting them into billets and pre-sintering before carrying out FSP on the billet. There is a fleeting reference of alloy developments using FSP in the literature. Hu et al. processed Al-Zn alloy by FSP. The authors started with elemental powders of Al and Zn and cold compacted them into billets and carried out FSP using two different tools [14]. Ball milling have been utilized over the years as a mechanical alloying tool to process solid solutions. There is hardly any
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report of other mechanical means of producing alloys in solid state. Among the alloying metals used in Al, Zn has the highest solid solubility (82.8 wt% at 381 °C) and does not form any intermetallic phases due to its weak interaction with Al. Also, the melting point of Zn (420 °C) is lower than Al and is in the range of temperature reported during FSP of Al alloys [2]. Al-Zn alloys have found wide acceptance in many applications [15]. Zn is known to wet the Al grain boundaries and enhance the grain boundary sliding and fine grained Al-Zn alloys exhibit high ductility both at ambient and high temperature [16–17]. In the present study, it is demonstrated that FSP can be used as a mechanical alloying tool to process solid solutions in a single step. The developed method can also be used for surface modification by selective alloying. The main objective here is to utilize the thermo-mechanical nature of FSP to process Al-Zn solid solution in solid state and understand it through microstructural and X-ray diffraction characterization and hardness property evaluation. The thermal stability of the processed alloy microstructure was also evaluated. 2. Materials and Methods A groove (1 mm wide, 2 mm deep and 50 mm long) was made on a commercially pure (99.2%) aluminium plate and was filled with Zn powder (− 100 Mesh, 99.9%, Alfa Aesar) with average particle size of 120 μm. The particle morphology is shown in Fig. 1. FSP was carried out along the groove using a tool made of M2 tool steel with shoulder
Corresponding author at: Materials Science and Engineering Program, Department of Mechanical Engineering, University of Colorado Boulder, Boulder, CO 80309, USA. E-mail address: [email protected] (D. Yadav). Present address: Erich Schmid Institute of Materials Science, Leoben, Austria.
https://doi.org/10.1016/j.matchar.2017.12.022 Received 11 October 2017; Received in revised form 21 November 2017; Accepted 14 December 2017 Available online 14 December 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.
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Fig. 1. SEM image showing the particle morphology of the Zn powder.
Fig. 2. XRD pattern of the FSPed Al-Zn alloy. Inset shows the shift in the high angle peak (420) of Al.
diameter, pin diameter and pin length of 15 mm, 5 mm and 3.5 mm, respectively. A tool rotation speed of 1500 rpm and traverse speed of 24 mm/min was used. The process parameters were optimized after several trials. Defects were observed in the cross section when the ratio of tool rotation speed to traverse speed was decreased either by decreasing the rotation speed (to 1200) or by increasing the traverse speed (to 60 mm/min). A higher ratio would generate more heat and can cause grain growth. Hence, a minimum ratio which was just enough to get a defect free stir zone was used to obtain fine grains. A second pass was carried out on the same track (with 100% overlap) with the same parameters, however, in the reverse direction, i.e. the advancing side in the first pass was the retreating side in the second pass. FSP was also carried out on pure Al with the same parameters for comparison and this sample is designated as FSPed Al. X-ray diffraction (XRD) was carried out for phase analysis in a PanAnalytical diffractometer using Cu-Kα radiation. The microstructure was characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The metallographically polished samples were observed in FEI Quanta FEG SEM equipped with BSE and EDS detector on the surface and in the cross section of the stir zone. For EBSD, the samples were electropolished in a mixture of perchloric acid and methanol at −10 °C and 10 V. EBSD was carried out in an FEI Quanta FEG SEM equipped with TSL-OIM software using a step size of 250 nm. For TEM, 3 mm diameter discs were sliced from the stir zone (parallel to the surface) and metallographically polished to a thickness of around 80 μm. The discs were then subjected to twin jet polishing and observed in a Philips CM12 microscope operating at 120 kV. Vickers hardness was measured across the surface of the stir zone on a Wilson Wolpert hardness tester using a load of 100 g and a dwell time of 9 s. Thermal stability of the solid solution microstructure was assessed by exposing the samples to high temperature (> 400 °C) in a tubular furnace for different periods of time in argon atmosphere. The samples were quenched to freeze the microstructure after each exposure and were subjected to EBSD analysis.
any detectable peak of Zn. The peaks of Al-Zn were shifted to higher angles in comparison to FSPed Al, indicating decrease in the lattice parameter. The inset in the figure shows the shift in one of the high angle (420) peaks. Using the Nelson-Riley extrapolation method [18] the lattice parameter of Al-Zn after first and second pass was found to be 0.40484 nm and 0.40480 nm, respectively compared to 0.40516 nm of pure Al. The lattice constant of FSPed Al (without Zn) was 0.40505 nm. Atomic radius of Zn (1.332 Å) is smaller compared to that of Al (1.432 Å) and hence, when Zn atoms replace some of the Al atoms in the lattice it leads to decrease in the lattice parameter of the resultant Al(Zn) solid solution [19]. Therefore, the observed decrease in the lattice parameter after FSP indicates the formation of solid solution. The processed material was further analyzed by SEM and TEM. 3.2. Microstructural Characterization 3.2.1. SEM Analysis Fig. 3(a) and (b) show the collage of SEM (BSE) images on the surface and the cross section of the stir zone (SZ), respectively after the first pass. The compositional contrast demarcates the stir zone and the base metal (BM). The interface between SZ and BM on the advancing side is sharp and shows more Zn content than that on the retreating side. However, regions rich and poor in Zn, as evident from the compositional contrast difference, can be observed. Though the cross section shows a good distribution of Zn, some regions on the surface (in the middle) were completely free of Zn as seen in Fig. 3(a). The uniform contrast regions had a Zn content of 9–11 wt% (3.9–4.9 at.%) as found from the EDS analysis. The SEM (BSE) image in Fig. 3(c) shows a complex material flow pattern during FSP. No Zn particle of initial size and morphology were observed in the microstructure. However, when looked at high magnification, some small particles in the size range of 2–3 μm were observed as shown in Fig. 3(d). EDS analysis on the particles confirmed them to be of Zn. These are the fragmented or undissolved pieces of original Zn particles. These left over Zn particles contribute to the Zn peak observed in XRD after first pass. The second pass was carried out over the first one (with complete overlap) to improve the Zn distribution across the stir zone. A more homogeneous distribution of Zn can be seen after the second pass on the surface and in the cross section (Fig. 4(a) and (b), respectively). The smaller Zn particles left over after the first pass also went into the solution and the Zn-free regions were completely eliminated. The Zn content across the stir zone was found to be 5–7 wt% (2–3 at.%) from the EDS analysis.
3. Results and Discussion 3.1. XRD Analysis Fig. 2 shows the XRD pattern of the samples subjected to first pass (Al-Zn FP) and second pass (Al-Zn SP) of FSP. Though the first pass sample shows minor peaks of Zn, the second pass sample did not exhibit 222
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Fig. 3. Collage of SEM (BSE) images after first pass of FSP showing (a) surface, (b) cross section of the stir zone, (c) SEM (BSE) image showing complex material flow pattern on the surface and (d) Zn particles at high magnification.
materials flow and mixing during FSP, the interface between the Zn particles and the plasticized Al is not fixed and hence the diffusion is not a steady state type [25]. The severe plastic deformation aspect of FSP results in mechanical activation. The mechanical activation can effectively break the atomic bonds in one hand and on the other hand, can provide translational mobility to the atoms through transfer of kinetic energy by dislocation movements [26]. The former will help forming new bonds with the Zn atoms and later can activate the atoms for faster diffusion by lowering the activation energy [25], either way facilitating formation of the solid solution. Hence a combined effect of thermal and mechanical mixing leads to accelerated dissolution of the Zn particles in Al.
However, when looked at high magnification, some fine precipitates were observed mostly along the grain boundaries as shown in Fig. 4(c). The EDS line scan analysis across them (Line 1 and 2), confirms them to be Zn rich as shown in Fig. 4(d). The origin of the precipitates is discussed in detail in the next section. FSP is a thermo-mechanical process during which the material flows around the pin at elevated temperature with strain rate in the range of 10–100 s− 1 [2,20] and strain of up to 40 [21]. It is reported that the material flows in a non-symmetric way and experience different combination of strain, strain rate and temperature on the advancing, center and retreating side of the stir zone during FSP [2,22–23]. These gradients lead to the non-uniform distribution of Zn into Al during the first pass. It should be remembered that the advancing and retreating sides switch during the second pass and hence, the material flow pattern also reverses. Thus, a homogeneous solid solution formed after the second pass. Using Arbegast and Hartley's equation [24] temperature in the range of 472–566 °C is expected with the process parameters used in the present study. During FSP, as the material flow around the pin, the combination of high strain and strain rate breaks the oxide film around the Zn particles and the atomically clean surface comes in contact with plasticized aluminium facilitating atomic diffusion. Moreover, the temperature in the stir zone is higher than the melting point of Zn, paving the way for enhanced diffusion of Zn atoms into Al. Due to the
3.2.2. EBSD Analysis The microstructure developed after FSP was characterized in detail by EBSD. The EBSD map at the interface on the advancing side after first pass is shown in Fig. 5(a). A clear demarcation is observed between the base metal and the stir zone based on the grain size difference. In the stir zone, the grains were fine and equiaxed with an average grain size of 5 μm compared to the average grain size of 70 μm of the base Al plate (Fig. 5(b)). It can be seen that FSP not only incorporated Zn into Al to form solid solution but also caused significant grain refinement of the processed material. Though the distribution of Zn improved after second pass, there was 223
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Fig. 4. Collage of SEM (BSE) image across the surface of stir zone after second pass on (a) surface and (b) in the cross section, (c) SEM (BSE) image showing Zn rich precipitates along grain boundaries and (d) EDS line scan across line 1 and 2 marked in (b).
aluminium during FSP. However, as described later, Zn-rich precipitates form during the second pass. These precipitates may retard the motion of dislocations and may delay their arrival to the subgrain boundaries. This is perhaps reflected in the lower fraction of HABs in the center of the stir zone in the second pass sample. The strain towards the advancing and retreating sides may be high enough for the dislocation to overcome the obstacles (precipitates). In the present case, dislocations in the process of arranging themselves to form subgrain boundary were observed (Fig. 6(a)). Fig. 6(b) shows subgrain boundaries composed of array of dislocations. Dynamic recovery (DRV) readily occurs in Al due to its high stacking fault energy. Dislocations rearrange themselves into subgrain boundaries (2–5° misorientation) by DRV. Further, addition of dislocations in the subgrain boundaries will increase the misorientation across them and gradually turn them into low angle grain boundary (5–15° misorientation). Further dislocation absorption and/or subgrain rotation will increase the misorientation across the low angle grain boundaries thereby turning them into high angle grain boundary (> 15° misorientation). These observations point towards possible occurrence of continuous dynamic recrystallization (CDRX) driven by DRV. However it should be noted that the present observations are based on the final microstructure. The second pass sample also exhibited similar dislocation features within the grains. However, fine precipitates in the size range of 20–400 nm were observed across the grains. The precipitates at the grain boundaries were bigger than the grain interior ones. Fig. 7(a) and (b) shows the precipitates along the high and low angle grain
no significant change in the grain size as seen from the EBSD map in Fig. 5(c). It has been reported before that the final grain size after FSP depend on the process parameters and not on the initial grain size, hence, subsequent passes carried out with same process parameters may not affect the grain size significantly [27]. The grain size distribution was narrow and the fraction of high angle grain boundaries (HABs) was > 67%. The microstructural features of the processed materials are summarized in Table 1. 3.2.3. TEM Analysis TEM observations revealed fine grained microstructure with grains having varying dislocation content and dislocation substructures inside them. The fine grains during FSP are formed by dynamic recrystallization (DRX) process. Different mechanisms, which include discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX), geometric dynamic recrystallization (GDRX) and dynamic recovery (DRV) are reported to be operating during FSP of Al alloys [2,23,28–31]. Stacking fault energy is one of the most important parameters which influence the recovery mechanism during high temperature deformation, since it dictates the ease of dislocation climb and cross-slip process. Generally alloying decreases the stacking fault energy of Al. However, Zn being a high stacking fault energy (220 mJ/m2) metal itself, does not change the stacking fault energy of Al (188 mJ/m2) significantly on alloying [32–33] and hence it is expected that addition Zn will not affect the overall mechanism of grain refinement of 224
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Fig. 5. EBSD (IPF + grain boundary) map across (a) interface between stir zone and base metal on the advancing side after first pass, at the center on surface after (b) first pass and (c) second pass.
and composition (Al and Zn) of the precipitates also indicate that they are not the original Zn particles. As the equilibrium solubility of Zn in Al at room temperature is < 3 wt% [34], a part of the solid solution may be supersaturated on cooling after first pass of FSP. The Zn-rich areas observed after the first pass, for example, can contain supersaturated solid solution. There is a driving force for the Zn atoms to diffuse out into Al and form stable equilibrium phases during the thermal cycle it experience during the second pass. Dislocations act as easy nucleation site for the precipitates [35] and also provide an easy path for the diffusion of Zn atoms by pipe diffusion. Pipe diffusion of Zn in Al is reported to be three orders of magnitude higher than bulk diffusion at 400°C [36]. Also, the activation energy for grain boundary diffusion of Zn in Al through low-angle subgrain boundaries, formed by dislocation rearrangement, is much lower (60 kJ/mol) compared to that through high angle grain boundary (118 kJ/mol) [37]. As discussed earlier, the fine grains during FSP are formed by dislocation rearrangement and absorption process and hence, it can be said that different boundaries formed after first pass (subgrain, low angle and high angle) will be enriched with Zn atoms. During second pass as the microstructure evolves by similar process, dislocation interaction and annihilation in the boundaries would lead to the nucleation of the Zn-
Table 1 Summary of microstructure after first and second pass of FSP, data taken from EBSD analysis. Advancing side
Center
Retreating side
Al-Zn first pass of FSP Avg. grain size (μm) Fraction of HABs in %
5 67
5 75
4.5 67
Al-Zn second pass of FSP Avg. grain size (μm) Fraction of HAGBs in %
6 73
5 69
4 72
boundaries, respectively. The dark field image and the diffraction pattern corresponding to Fig. 7(b) are shown in Fig. 7(c). These precipitates were found to be rich in Al and Zn by TEM-EDS analysis (Fig. 7(d)). Fig. 7(e) shows very fine disc shaped precipitates in the size range of 20–40 nm inside the grain. The fraction of these precipitates was too small to be detected by XRD. It is to be noted that the precipitates observed after first pass were pure Zn particles and those observed after second pass were rich in both Zn and Al as shown by TEM-EDS analysis. The morphology (disc shape)
Fig. 6. TEM image after first pass showing (a) dislocations rearranged to form subgrain boundary inside the grain and (b) subgrain boundaries formed by array of dislocations.
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Fig. 7. TEM image after second pass showing Zn rich precipitates along (a) high angle grain boundary, (b) low angle grain boundary, (c) dark field image corresponding to (b), (d) TEMEDS spectrum of the precipitates and (e) fine disc shaped precipitates inside the grain. Inset shows one fine precipitate.
rich precipitates or in other words partial decomposition of the solid solution. Zn atoms migrate towards the growing precipitates by grain boundary diffusion and pipe diffusion. The fine precipitates observed inside the grains are formed by bulk diffusion or lattice diffusion and hence are much smaller. Whether the precipitates are the equilibrium β (Zn) precipitates is yet to be ascertained. It may also be noted that at this point there is some uncertainty in assessing the origin of these particles and further investigations are needed. Decomposition of supersaturated solid solution during severe plastic deformation has been reported in the literature [38–40]. Straumal et al. [38] and Mazilkin et al. [39] reported complete decomposition and formation of equilibrium phases during high pressure torsion of Al-Zn and Al-Mg supersaturated solid solutions. Similarly, Tavoosi et al., reported decomposition of supersaturated Al-Zn solid solution during long milling hours of Al and Zn powders [40]. These studies also reported a decrease in hardness of the alloy due to the decomposition process. Fig. 8. Hardness profile across the surface of the stir zone. Inset shows the indents across the base metal and stir zone after the second pass.
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Fig. 9. EBSD (IPF + grain boundary) map of first pass sample (a) before heat treatment, (b) after heat treatment at 400 °C for 10 min and (c) after heat treatment at 400 °C for 40 min.
developed. The microstructure was stable without any change in the grain size till 350 °C even after 60 min of exposure for both the first and second pass sample. There was marked difference in the microstructure as the temperature was further increased to 400 °C. The first pass sample underwent abnormal grain growth (AGG) after a thermal exposure of 10 min at 400 °C. The EBSD map of the same area, before and after heat treatment is shown in Fig. 9(a) and (b), respectively. Island grains can be seen inside the abnormally growing grains. Further exposure of 40 min at 400 °C led to consumption of the island grains into the abnormally growing grains as shown in Fig. 9(c). Grains as large as 0.5–1 mm were observed after the heat treatment. FSPed Al also underwent AGG at 400 °C after 10 min of thermal exposure indicating incorporation of Zn does not have an effect on the AGG temperature of FSPed microstructure. The second pass sample was stable at 400 °C and suffered from AGG at 420 °C after 10 min of exposure. Rise in the AGG temperature (by 20 °C) in the second pass sample can be attributed to the pinning effect of the fine Zn rich precipitates which were observed along the grain boundaries after the second pass. Above 420 °C, the grain boundaries possibly have sufficient energy to overcome these precipitates or the precipitates themselves may dissolve at that temperature. For the method to have potential industrial applications, it is worth mentioning two important points here. First, the temperature for AGG of the processed Al-Zn solid solution is much higher than the solutionising temperature (200–250 °C) of the alloy with 8–10 wt% of Zn [34]. Therefore, the processed alloy can be heat treated (solutionised and aged) without compromising the fine grain size to get desired
3.3. Hardness Fig. 8 shows the hardness profile across the surface of the stir zone after first and second pass. A large variation in the hardness can be seen after the first pass which is due to the inhomogeneous distribution of Zn across the stir zone. Regions rich in Zn showed a higher hardness (82 Hv) value compared to the Zn free regions (43 Hv) with the average hardness being 57 Hv. The hardness curve appears more uniform after the second pass and the average value across the stir zone was 50 Hv. The inset in Fig. 8 shows the indents across the stir zone after second pass. The improvement in hardness can be attributed to a combined effect of grain refinement and solid solution hardening. Since there was no change in the grain size after second pass the uniformity in hardness values can be attributed to the improved distribution of the solute atoms (Zn). The drop in the hardness from the peak value obtained after the first pass is also an indication of uniform distribution of Zn and partial decomposition of the solid solution. Nevertheless, the hardness after the second pass was still higher than both pure Al (29 Hv) and FSPed Al (39 Hv). An improvement in the hardness of the alloy by 11 Hv comes from the addition of Zn.
3.4. Thermal Stability of the Microstructure The FSPed microstructure has been reported to undergo abnormal grain growth when exposed to high temperatures [9,41–42]. Hence, the FSPed Al-Zn solid solution samples were subjected to series of heat treatments to evaluate the thermal stability of the microstructure 227
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combination of mechanical properties. Secondly, the volume or depth of the material into which Zn can be incorporated depends on the tool size (shoulder diameter and pin length). A predetermined amount of powder can be incorporated into Al by controlling the groove size and tool dimensions to obtain a desired composition. We propose that Zn can be used as a filler material during FSW of Al alloys to get improved mechanical properties in the weld nugget. It is worth mentioning here that thermally stable intermetallic particles are also reported to dissolve in Mg alloy matrix during FSP [43]. As a concluding remark it can be said that the process can be used for selective alloying on a particular region or surface of an engineering component to tune the properties like hardness and strength preferentially. Alternatively, the alloy layer can be formed selectively for making functionally graded material. The method thus provides a wider scope of material development and surface engineering. However, more work is required to fully understand the dissolution, decomposition and the precipitation processes.
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4. Conclusions It is shown in the present study that FSP can be used as a tool for mechanical alloying to process solid solution. The following conclusions can be drawn from the present study. 1. Zn particles were incorporated in Al by FSP forming a solid solution. A second pass of FSP homogenized the solid solution. 2. The microstructure was characterized by equiaxed fine grains in the size range of 4–6 μm with > 67% boundaries being high angle. The microstructure appears to evolve through DRV assisted CDRX process. 3. Zn-rich precipitates formed during the second pass due to partial decomposition of the solid solution. 4. Hardness profile across the stir zone matches the distribution of the solute. Hardness of the solid solution formed after the second pass was higher than pure Al and FSPed Al (without Zn). 5. The microstructure of the processed solid solution was thermally stable. Abnormal grain growth occurred at a temperature (400 °C and 420 °C for the first and second pass sample, respectively) which is higher than the solutionising and ageing temperature of Al-Zn alloys. Acknowledgements The authors would like to thank the faculty at the Metal Joining Laboratory, IIT Madras for providing access to the NRB sponsored FSP facility. References [1] R.S. Mishra, R.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukherjee, High strain rate superplasticity in a friction stir processed 7075 Al alloy, Scr. Mater. 42 (2002) 163–168. [2] R.S. Mishra, Z.Y. Ma, Friction stir welding and processing, Mater. Sci. Eng. R 50 (2005) 1–78. [3] Z.Y. Ma, Friction stir processing technology: a review, Metall. Mater. Trans. A 39 (2008) 642–658. [4] Z.Y. Ma, S.R. Sharma, R.S. Mishra, Effect of friction stir processing on the microstructure of cast A356 aluminium, Mater. Sci. Eng. A 433 (2006) 269–278. [5] P.B. Berbon, W.H. Bingel, R.S. Mishra, C.C. Bampton, M.W. Mahoney, Friction stir processing: a tool to homogenize nanocomposite aluminium alloys, Scr. Mater. 44 (2001) 61–66. [6] Z.Y. Ma, S.R. Sharma, R.S. Mishra, Microstructural modification of as-cast Al–Si–Mg alloy by friction stir processing, Metall. Mater. Trans. A 37 (2006) 3323–3336. [7] R.S. Mishra, Z.Y. Ma, I. Charit, Friction stir processing: a novel technique for fabrication of surface composite, Mater. Sci. Eng. A 341 (2003) 307–310. [8] D. Yadav, R. Bauri, Nickel particle embedded aluminium matrix composite with high ductility, Mater. Lett. 64 (2010) 664–667. [9] D. Yadav, R. Bauri, A. Kauffmann, J. Freudenberger, Al–Ti particulate composite: processing and studies on particle twinning, microstructure and thermal stability,
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Fabrication of Al-Zn solid solution via friction stir processing ⁎
Devinder Yadav , Ranjit Bauri, Niraj Chawake
T
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Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Friction stir processing Metals and alloys Mechanical alloying Microstructure Electron microscopy Hardness
The present study demonstrates that friction stir processing (FSP) can be used as a mechanical alloying tool to process solid solution. Zn was incorporated in commercially pure aluminium by multi-pass FSP leading to its dissolution and formation of supersaturated solid solution which was confirmed by X-ray diffraction and scanning electron microscopy. The hardness across the stir zone matched with the distribution of Zn and was significantly improved compared to the base metal. The supersaturated solid solution that formed in the first pass, partially decomposed during the second pass, leading to the formation of fine Al-Zn precipitates. Thermal stability of the microstructure against abnormal grain growth was also improved after the second pass due to the pinning effect of the grain boundary precipitates. FSP also caused grain refinement of the alloy yielding fine equiaxed grains in the range of 4–6 μm with a high fraction (> 67%) of high angle grain boundaries. Such microstructure is conducive for isotropic properties and the fact that the microstructure is stable against thermal cycles make the process suitable for selective alloying for location specific property enhancement.
1. Introduction Since its advent a decade ago, friction stir processing (FSP) has emerged as a useful tool for material processing and microstructure modification [1]. This solid state technique has been primarily used for grain refinement of aluminium and magnesium alloys [2–3]. It has also found its applications in microstructure homogenization [4–6] and composite fabrication [2,7–9]. FSP uses a cylindrical rotating tool with a shoulder and pin to produce localized heating and plastic deformation to process the material. Apart from the material transport and mixing action, the process also involves severe plastic deformation and is thermo-mechanical in nature. The potential of all these aspects can be fully realized if the process is utilized for making solid solutions that may provide opportunities for new material development. The thermomechanical aspect of FSP is utilized in fabricating Al based in-situ composites where transitional metals (Cu, Zr, Fe, Ni) react with Al to form intermetallics during the process [10–13]. The methodology adopted to process such composites involves mixing elemental powders, cold compacting them into billets and pre-sintering before carrying out FSP on the billet. There is a fleeting reference of alloy developments using FSP in the literature. Hu et al. processed Al-Zn alloy by FSP. The authors started with elemental powders of Al and Zn and cold compacted them into billets and carried out FSP using two different tools [14]. Ball milling have been utilized over the years as a mechanical alloying tool to process solid solutions. There is hardly any
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1
report of other mechanical means of producing alloys in solid state. Among the alloying metals used in Al, Zn has the highest solid solubility (82.8 wt% at 381 °C) and does not form any intermetallic phases due to its weak interaction with Al. Also, the melting point of Zn (420 °C) is lower than Al and is in the range of temperature reported during FSP of Al alloys [2]. Al-Zn alloys have found wide acceptance in many applications [15]. Zn is known to wet the Al grain boundaries and enhance the grain boundary sliding and fine grained Al-Zn alloys exhibit high ductility both at ambient and high temperature [16–17]. In the present study, it is demonstrated that FSP can be used as a mechanical alloying tool to process solid solutions in a single step. The developed method can also be used for surface modification by selective alloying. The main objective here is to utilize the thermo-mechanical nature of FSP to process Al-Zn solid solution in solid state and understand it through microstructural and X-ray diffraction characterization and hardness property evaluation. The thermal stability of the processed alloy microstructure was also evaluated. 2. Materials and Methods A groove (1 mm wide, 2 mm deep and 50 mm long) was made on a commercially pure (99.2%) aluminium plate and was filled with Zn powder (− 100 Mesh, 99.9%, Alfa Aesar) with average particle size of 120 μm. The particle morphology is shown in Fig. 1. FSP was carried out along the groove using a tool made of M2 tool steel with shoulder
Corresponding author at: Materials Science and Engineering Program, Department of Mechanical Engineering, University of Colorado Boulder, Boulder, CO 80309, USA. E-mail address: [email protected] (D. Yadav). Present address: Erich Schmid Institute of Materials Science, Leoben, Austria.
https://doi.org/10.1016/j.matchar.2017.12.022 Received 11 October 2017; Received in revised form 21 November 2017; Accepted 14 December 2017 Available online 14 December 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.
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Fig. 1. SEM image showing the particle morphology of the Zn powder.
Fig. 2. XRD pattern of the FSPed Al-Zn alloy. Inset shows the shift in the high angle peak (420) of Al.
diameter, pin diameter and pin length of 15 mm, 5 mm and 3.5 mm, respectively. A tool rotation speed of 1500 rpm and traverse speed of 24 mm/min was used. The process parameters were optimized after several trials. Defects were observed in the cross section when the ratio of tool rotation speed to traverse speed was decreased either by decreasing the rotation speed (to 1200) or by increasing the traverse speed (to 60 mm/min). A higher ratio would generate more heat and can cause grain growth. Hence, a minimum ratio which was just enough to get a defect free stir zone was used to obtain fine grains. A second pass was carried out on the same track (with 100% overlap) with the same parameters, however, in the reverse direction, i.e. the advancing side in the first pass was the retreating side in the second pass. FSP was also carried out on pure Al with the same parameters for comparison and this sample is designated as FSPed Al. X-ray diffraction (XRD) was carried out for phase analysis in a PanAnalytical diffractometer using Cu-Kα radiation. The microstructure was characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The metallographically polished samples were observed in FEI Quanta FEG SEM equipped with BSE and EDS detector on the surface and in the cross section of the stir zone. For EBSD, the samples were electropolished in a mixture of perchloric acid and methanol at −10 °C and 10 V. EBSD was carried out in an FEI Quanta FEG SEM equipped with TSL-OIM software using a step size of 250 nm. For TEM, 3 mm diameter discs were sliced from the stir zone (parallel to the surface) and metallographically polished to a thickness of around 80 μm. The discs were then subjected to twin jet polishing and observed in a Philips CM12 microscope operating at 120 kV. Vickers hardness was measured across the surface of the stir zone on a Wilson Wolpert hardness tester using a load of 100 g and a dwell time of 9 s. Thermal stability of the solid solution microstructure was assessed by exposing the samples to high temperature (> 400 °C) in a tubular furnace for different periods of time in argon atmosphere. The samples were quenched to freeze the microstructure after each exposure and were subjected to EBSD analysis.
any detectable peak of Zn. The peaks of Al-Zn were shifted to higher angles in comparison to FSPed Al, indicating decrease in the lattice parameter. The inset in the figure shows the shift in one of the high angle (420) peaks. Using the Nelson-Riley extrapolation method [18] the lattice parameter of Al-Zn after first and second pass was found to be 0.40484 nm and 0.40480 nm, respectively compared to 0.40516 nm of pure Al. The lattice constant of FSPed Al (without Zn) was 0.40505 nm. Atomic radius of Zn (1.332 Å) is smaller compared to that of Al (1.432 Å) and hence, when Zn atoms replace some of the Al atoms in the lattice it leads to decrease in the lattice parameter of the resultant Al(Zn) solid solution [19]. Therefore, the observed decrease in the lattice parameter after FSP indicates the formation of solid solution. The processed material was further analyzed by SEM and TEM. 3.2. Microstructural Characterization 3.2.1. SEM Analysis Fig. 3(a) and (b) show the collage of SEM (BSE) images on the surface and the cross section of the stir zone (SZ), respectively after the first pass. The compositional contrast demarcates the stir zone and the base metal (BM). The interface between SZ and BM on the advancing side is sharp and shows more Zn content than that on the retreating side. However, regions rich and poor in Zn, as evident from the compositional contrast difference, can be observed. Though the cross section shows a good distribution of Zn, some regions on the surface (in the middle) were completely free of Zn as seen in Fig. 3(a). The uniform contrast regions had a Zn content of 9–11 wt% (3.9–4.9 at.%) as found from the EDS analysis. The SEM (BSE) image in Fig. 3(c) shows a complex material flow pattern during FSP. No Zn particle of initial size and morphology were observed in the microstructure. However, when looked at high magnification, some small particles in the size range of 2–3 μm were observed as shown in Fig. 3(d). EDS analysis on the particles confirmed them to be of Zn. These are the fragmented or undissolved pieces of original Zn particles. These left over Zn particles contribute to the Zn peak observed in XRD after first pass. The second pass was carried out over the first one (with complete overlap) to improve the Zn distribution across the stir zone. A more homogeneous distribution of Zn can be seen after the second pass on the surface and in the cross section (Fig. 4(a) and (b), respectively). The smaller Zn particles left over after the first pass also went into the solution and the Zn-free regions were completely eliminated. The Zn content across the stir zone was found to be 5–7 wt% (2–3 at.%) from the EDS analysis.
3. Results and Discussion 3.1. XRD Analysis Fig. 2 shows the XRD pattern of the samples subjected to first pass (Al-Zn FP) and second pass (Al-Zn SP) of FSP. Though the first pass sample shows minor peaks of Zn, the second pass sample did not exhibit 222
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Fig. 3. Collage of SEM (BSE) images after first pass of FSP showing (a) surface, (b) cross section of the stir zone, (c) SEM (BSE) image showing complex material flow pattern on the surface and (d) Zn particles at high magnification.
materials flow and mixing during FSP, the interface between the Zn particles and the plasticized Al is not fixed and hence the diffusion is not a steady state type [25]. The severe plastic deformation aspect of FSP results in mechanical activation. The mechanical activation can effectively break the atomic bonds in one hand and on the other hand, can provide translational mobility to the atoms through transfer of kinetic energy by dislocation movements [26]. The former will help forming new bonds with the Zn atoms and later can activate the atoms for faster diffusion by lowering the activation energy [25], either way facilitating formation of the solid solution. Hence a combined effect of thermal and mechanical mixing leads to accelerated dissolution of the Zn particles in Al.
However, when looked at high magnification, some fine precipitates were observed mostly along the grain boundaries as shown in Fig. 4(c). The EDS line scan analysis across them (Line 1 and 2), confirms them to be Zn rich as shown in Fig. 4(d). The origin of the precipitates is discussed in detail in the next section. FSP is a thermo-mechanical process during which the material flows around the pin at elevated temperature with strain rate in the range of 10–100 s− 1 [2,20] and strain of up to 40 [21]. It is reported that the material flows in a non-symmetric way and experience different combination of strain, strain rate and temperature on the advancing, center and retreating side of the stir zone during FSP [2,22–23]. These gradients lead to the non-uniform distribution of Zn into Al during the first pass. It should be remembered that the advancing and retreating sides switch during the second pass and hence, the material flow pattern also reverses. Thus, a homogeneous solid solution formed after the second pass. Using Arbegast and Hartley's equation [24] temperature in the range of 472–566 °C is expected with the process parameters used in the present study. During FSP, as the material flow around the pin, the combination of high strain and strain rate breaks the oxide film around the Zn particles and the atomically clean surface comes in contact with plasticized aluminium facilitating atomic diffusion. Moreover, the temperature in the stir zone is higher than the melting point of Zn, paving the way for enhanced diffusion of Zn atoms into Al. Due to the
3.2.2. EBSD Analysis The microstructure developed after FSP was characterized in detail by EBSD. The EBSD map at the interface on the advancing side after first pass is shown in Fig. 5(a). A clear demarcation is observed between the base metal and the stir zone based on the grain size difference. In the stir zone, the grains were fine and equiaxed with an average grain size of 5 μm compared to the average grain size of 70 μm of the base Al plate (Fig. 5(b)). It can be seen that FSP not only incorporated Zn into Al to form solid solution but also caused significant grain refinement of the processed material. Though the distribution of Zn improved after second pass, there was 223
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Fig. 4. Collage of SEM (BSE) image across the surface of stir zone after second pass on (a) surface and (b) in the cross section, (c) SEM (BSE) image showing Zn rich precipitates along grain boundaries and (d) EDS line scan across line 1 and 2 marked in (b).
aluminium during FSP. However, as described later, Zn-rich precipitates form during the second pass. These precipitates may retard the motion of dislocations and may delay their arrival to the subgrain boundaries. This is perhaps reflected in the lower fraction of HABs in the center of the stir zone in the second pass sample. The strain towards the advancing and retreating sides may be high enough for the dislocation to overcome the obstacles (precipitates). In the present case, dislocations in the process of arranging themselves to form subgrain boundary were observed (Fig. 6(a)). Fig. 6(b) shows subgrain boundaries composed of array of dislocations. Dynamic recovery (DRV) readily occurs in Al due to its high stacking fault energy. Dislocations rearrange themselves into subgrain boundaries (2–5° misorientation) by DRV. Further, addition of dislocations in the subgrain boundaries will increase the misorientation across them and gradually turn them into low angle grain boundary (5–15° misorientation). Further dislocation absorption and/or subgrain rotation will increase the misorientation across the low angle grain boundaries thereby turning them into high angle grain boundary (> 15° misorientation). These observations point towards possible occurrence of continuous dynamic recrystallization (CDRX) driven by DRV. However it should be noted that the present observations are based on the final microstructure. The second pass sample also exhibited similar dislocation features within the grains. However, fine precipitates in the size range of 20–400 nm were observed across the grains. The precipitates at the grain boundaries were bigger than the grain interior ones. Fig. 7(a) and (b) shows the precipitates along the high and low angle grain
no significant change in the grain size as seen from the EBSD map in Fig. 5(c). It has been reported before that the final grain size after FSP depend on the process parameters and not on the initial grain size, hence, subsequent passes carried out with same process parameters may not affect the grain size significantly [27]. The grain size distribution was narrow and the fraction of high angle grain boundaries (HABs) was > 67%. The microstructural features of the processed materials are summarized in Table 1. 3.2.3. TEM Analysis TEM observations revealed fine grained microstructure with grains having varying dislocation content and dislocation substructures inside them. The fine grains during FSP are formed by dynamic recrystallization (DRX) process. Different mechanisms, which include discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX), geometric dynamic recrystallization (GDRX) and dynamic recovery (DRV) are reported to be operating during FSP of Al alloys [2,23,28–31]. Stacking fault energy is one of the most important parameters which influence the recovery mechanism during high temperature deformation, since it dictates the ease of dislocation climb and cross-slip process. Generally alloying decreases the stacking fault energy of Al. However, Zn being a high stacking fault energy (220 mJ/m2) metal itself, does not change the stacking fault energy of Al (188 mJ/m2) significantly on alloying [32–33] and hence it is expected that addition Zn will not affect the overall mechanism of grain refinement of 224
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Fig. 5. EBSD (IPF + grain boundary) map across (a) interface between stir zone and base metal on the advancing side after first pass, at the center on surface after (b) first pass and (c) second pass.
and composition (Al and Zn) of the precipitates also indicate that they are not the original Zn particles. As the equilibrium solubility of Zn in Al at room temperature is < 3 wt% [34], a part of the solid solution may be supersaturated on cooling after first pass of FSP. The Zn-rich areas observed after the first pass, for example, can contain supersaturated solid solution. There is a driving force for the Zn atoms to diffuse out into Al and form stable equilibrium phases during the thermal cycle it experience during the second pass. Dislocations act as easy nucleation site for the precipitates [35] and also provide an easy path for the diffusion of Zn atoms by pipe diffusion. Pipe diffusion of Zn in Al is reported to be three orders of magnitude higher than bulk diffusion at 400°C [36]. Also, the activation energy for grain boundary diffusion of Zn in Al through low-angle subgrain boundaries, formed by dislocation rearrangement, is much lower (60 kJ/mol) compared to that through high angle grain boundary (118 kJ/mol) [37]. As discussed earlier, the fine grains during FSP are formed by dislocation rearrangement and absorption process and hence, it can be said that different boundaries formed after first pass (subgrain, low angle and high angle) will be enriched with Zn atoms. During second pass as the microstructure evolves by similar process, dislocation interaction and annihilation in the boundaries would lead to the nucleation of the Zn-
Table 1 Summary of microstructure after first and second pass of FSP, data taken from EBSD analysis. Advancing side
Center
Retreating side
Al-Zn first pass of FSP Avg. grain size (μm) Fraction of HABs in %
5 67
5 75
4.5 67
Al-Zn second pass of FSP Avg. grain size (μm) Fraction of HAGBs in %
6 73
5 69
4 72
boundaries, respectively. The dark field image and the diffraction pattern corresponding to Fig. 7(b) are shown in Fig. 7(c). These precipitates were found to be rich in Al and Zn by TEM-EDS analysis (Fig. 7(d)). Fig. 7(e) shows very fine disc shaped precipitates in the size range of 20–40 nm inside the grain. The fraction of these precipitates was too small to be detected by XRD. It is to be noted that the precipitates observed after first pass were pure Zn particles and those observed after second pass were rich in both Zn and Al as shown by TEM-EDS analysis. The morphology (disc shape)
Fig. 6. TEM image after first pass showing (a) dislocations rearranged to form subgrain boundary inside the grain and (b) subgrain boundaries formed by array of dislocations.
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Fig. 7. TEM image after second pass showing Zn rich precipitates along (a) high angle grain boundary, (b) low angle grain boundary, (c) dark field image corresponding to (b), (d) TEMEDS spectrum of the precipitates and (e) fine disc shaped precipitates inside the grain. Inset shows one fine precipitate.
rich precipitates or in other words partial decomposition of the solid solution. Zn atoms migrate towards the growing precipitates by grain boundary diffusion and pipe diffusion. The fine precipitates observed inside the grains are formed by bulk diffusion or lattice diffusion and hence are much smaller. Whether the precipitates are the equilibrium β (Zn) precipitates is yet to be ascertained. It may also be noted that at this point there is some uncertainty in assessing the origin of these particles and further investigations are needed. Decomposition of supersaturated solid solution during severe plastic deformation has been reported in the literature [38–40]. Straumal et al. [38] and Mazilkin et al. [39] reported complete decomposition and formation of equilibrium phases during high pressure torsion of Al-Zn and Al-Mg supersaturated solid solutions. Similarly, Tavoosi et al., reported decomposition of supersaturated Al-Zn solid solution during long milling hours of Al and Zn powders [40]. These studies also reported a decrease in hardness of the alloy due to the decomposition process. Fig. 8. Hardness profile across the surface of the stir zone. Inset shows the indents across the base metal and stir zone after the second pass.
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Fig. 9. EBSD (IPF + grain boundary) map of first pass sample (a) before heat treatment, (b) after heat treatment at 400 °C for 10 min and (c) after heat treatment at 400 °C for 40 min.
developed. The microstructure was stable without any change in the grain size till 350 °C even after 60 min of exposure for both the first and second pass sample. There was marked difference in the microstructure as the temperature was further increased to 400 °C. The first pass sample underwent abnormal grain growth (AGG) after a thermal exposure of 10 min at 400 °C. The EBSD map of the same area, before and after heat treatment is shown in Fig. 9(a) and (b), respectively. Island grains can be seen inside the abnormally growing grains. Further exposure of 40 min at 400 °C led to consumption of the island grains into the abnormally growing grains as shown in Fig. 9(c). Grains as large as 0.5–1 mm were observed after the heat treatment. FSPed Al also underwent AGG at 400 °C after 10 min of thermal exposure indicating incorporation of Zn does not have an effect on the AGG temperature of FSPed microstructure. The second pass sample was stable at 400 °C and suffered from AGG at 420 °C after 10 min of exposure. Rise in the AGG temperature (by 20 °C) in the second pass sample can be attributed to the pinning effect of the fine Zn rich precipitates which were observed along the grain boundaries after the second pass. Above 420 °C, the grain boundaries possibly have sufficient energy to overcome these precipitates or the precipitates themselves may dissolve at that temperature. For the method to have potential industrial applications, it is worth mentioning two important points here. First, the temperature for AGG of the processed Al-Zn solid solution is much higher than the solutionising temperature (200–250 °C) of the alloy with 8–10 wt% of Zn [34]. Therefore, the processed alloy can be heat treated (solutionised and aged) without compromising the fine grain size to get desired
3.3. Hardness Fig. 8 shows the hardness profile across the surface of the stir zone after first and second pass. A large variation in the hardness can be seen after the first pass which is due to the inhomogeneous distribution of Zn across the stir zone. Regions rich in Zn showed a higher hardness (82 Hv) value compared to the Zn free regions (43 Hv) with the average hardness being 57 Hv. The hardness curve appears more uniform after the second pass and the average value across the stir zone was 50 Hv. The inset in Fig. 8 shows the indents across the stir zone after second pass. The improvement in hardness can be attributed to a combined effect of grain refinement and solid solution hardening. Since there was no change in the grain size after second pass the uniformity in hardness values can be attributed to the improved distribution of the solute atoms (Zn). The drop in the hardness from the peak value obtained after the first pass is also an indication of uniform distribution of Zn and partial decomposition of the solid solution. Nevertheless, the hardness after the second pass was still higher than both pure Al (29 Hv) and FSPed Al (39 Hv). An improvement in the hardness of the alloy by 11 Hv comes from the addition of Zn.
3.4. Thermal Stability of the Microstructure The FSPed microstructure has been reported to undergo abnormal grain growth when exposed to high temperatures [9,41–42]. Hence, the FSPed Al-Zn solid solution samples were subjected to series of heat treatments to evaluate the thermal stability of the microstructure 227
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combination of mechanical properties. Secondly, the volume or depth of the material into which Zn can be incorporated depends on the tool size (shoulder diameter and pin length). A predetermined amount of powder can be incorporated into Al by controlling the groove size and tool dimensions to obtain a desired composition. We propose that Zn can be used as a filler material during FSW of Al alloys to get improved mechanical properties in the weld nugget. It is worth mentioning here that thermally stable intermetallic particles are also reported to dissolve in Mg alloy matrix during FSP [43]. As a concluding remark it can be said that the process can be used for selective alloying on a particular region or surface of an engineering component to tune the properties like hardness and strength preferentially. Alternatively, the alloy layer can be formed selectively for making functionally graded material. The method thus provides a wider scope of material development and surface engineering. However, more work is required to fully understand the dissolution, decomposition and the precipitation processes.
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4. Conclusions It is shown in the present study that FSP can be used as a tool for mechanical alloying to process solid solution. The following conclusions can be drawn from the present study. 1. Zn particles were incorporated in Al by FSP forming a solid solution. A second pass of FSP homogenized the solid solution. 2. The microstructure was characterized by equiaxed fine grains in the size range of 4–6 μm with > 67% boundaries being high angle. The microstructure appears to evolve through DRV assisted CDRX process. 3. Zn-rich precipitates formed during the second pass due to partial decomposition of the solid solution. 4. Hardness profile across the stir zone matches the distribution of the solute. Hardness of the solid solution formed after the second pass was higher than pure Al and FSPed Al (without Zn). 5. The microstructure of the processed solid solution was thermally stable. Abnormal grain growth occurred at a temperature (400 °C and 420 °C for the first and second pass sample, respectively) which is higher than the solutionising and ageing temperature of Al-Zn alloys. Acknowledgements The authors would like to thank the faculty at the Metal Joining Laboratory, IIT Madras for providing access to the NRB sponsored FSP facility. References [1] R.S. Mishra, R.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukherjee, High strain rate superplasticity in a friction stir processed 7075 Al alloy, Scr. Mater. 42 (2002) 163–168. [2] R.S. Mishra, Z.Y. Ma, Friction stir welding and processing, Mater. Sci. Eng. R 50 (2005) 1–78. [3] Z.Y. Ma, Friction stir processing technology: a review, Metall. Mater. Trans. A 39 (2008) 642–658. [4] Z.Y. Ma, S.R. Sharma, R.S. Mishra, Effect of friction stir processing on the microstructure of cast A356 aluminium, Mater. Sci. Eng. A 433 (2006) 269–278. [5] P.B. Berbon, W.H. Bingel, R.S. Mishra, C.C. Bampton, M.W. Mahoney, Friction stir processing: a tool to homogenize nanocomposite aluminium alloys, Scr. Mater. 44 (2001) 61–66. [6] Z.Y. Ma, S.R. Sharma, R.S. Mishra, Microstructural modification of as-cast Al–Si–Mg alloy by friction stir processing, Metall. Mater. Trans. A 37 (2006) 3323–3336. [7] R.S. Mishra, Z.Y. Ma, I. Charit, Friction stir processing: a novel technique for fabrication of surface composite, Mater. Sci. Eng. A 341 (2003) 307–310. [8] D. Yadav, R. Bauri, Nickel particle embedded aluminium matrix composite with high ductility, Mater. Lett. 64 (2010) 664–667. [9] D. Yadav, R. Bauri, A. Kauffmann, J. Freudenberger, Al–Ti particulate composite: processing and studies on particle twinning, microstructure and thermal stability,
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