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Effect of processing parameters on the corrosion behaviour of friction stir processed AA 2219 aluminum alloy
Solid State Sciences 11 (2009) 907–917
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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Effect of processing parameters on the corrosion behaviour of friction stir processed AA 2219 aluminum alloy K. Surekha, B.S. Murty*, K. Prasad Rao Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 February 2008 Received in revised form 19 November 2008 Accepted 19 November 2008 Available online 6 December 2008
The effect of processing parameters (rotation speed and traverse speed) on the corrosion behaviour of friction stir processed high strength precipitation hardenable AA 2219-T87 alloy was investigated. The results indicate that the rotation speed has a major influence in determining the rate of corrosion, which is attributed to the breaking down and dissolution of the intermetallic particles. Corrosion resistance of friction stir processed alloy was studied by potentiodynamic polarization, electrochemical impedance spectroscopy, salt spray and immersion tests. Ó 2008 Elsevier Masson SAS. All rights reserved.
Keywords: Friction stir processing AA 2219 aluminum alloy Corrosion Dissolution of intermetallics
1. Introduction AA 2219(Al–6.3%Cu) is a precipitation hardenable alloy which is used for strategic applications because of good weldability and high strength to weight ratio. CuAl2 is the major intermetallic, which imparts strength to this alloy at the same time decreasing the corrosion resistance due to the formation of galvanic cells between the noble CuAl2 and the Al matrix (solution potential of 0.64 and 0.73 V, respectively). Heat treatable aluminum alloys have two types of second phase particles, viz., intermetallic phases formed during casting and those formed during aging. Both these second phase particles influence corrosion. Corrosion being a surface phenomenon one can adopt an approach of reducing or removing these second phase particles from the surface. In such a situation, the corrosion resistance of the alloy is expected to be improved. The composition of microconstituents, their size, quantity, location, continuity and corrosion potential relative to that of the adjacent aAl matrix, is the important aspect of microstructure that affects the corrosion behaviour of the alloy [1]. Surface modification techniques can improve the corrosion resistance of the alloy by altering the above-mentioned properties. One such attempt is envisaged in the present work by friction stir processing (FSP). FSP is an offshoot of friction stir welding (FSW) which is a solid state welding technique patented by The Welding institute of United Kingdom [2] in 1991. Mishra et al. [3,4] extended the * Corresponding author. Tel.: þ91 44 2257 4754; fax: þ91 44 2257 4752. E-mail address: [email protected] (B.S. Murty). 1293-2558/$ – see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.11.007
principle of FSW to FSP. FSP, unlike its forerunner FSW, which is used to join two plates imparts variety of benefits like improved corrosion and wear resistance, improved ductility and induces even super plasticity. Further, FSP technique has been used to produce surface composite on aluminum substrate [5], homogenization of powder metallurgy aluminum alloy [6], microstructural modification of metal matrix composites [7] and property enhancement in cast aluminum alloys [8]. Liu and Ma [9] achieved low temperature super plasticity of 350–540% at 200–350 C in Al–Zn–Mg–Cu alloy by FSP. Ma et al. [10] have reported that Al and Mg alloys with coarse second phase particles and precipitates are ideal for FSP. Elangovan and Balasubramanian [11,12] studied the effect of tool profile and rotation speed on AA 2219 and AA 6061 and found that square tool profile gave sound welds. The range of parameters and tool profile for the present work were selected based on this work [11]. Hsu et al. [13] achieved ultrafine grained Al–Al2Cu composite by FSP, which has high Young’s modulus, good compressive strength and reasonably good compressive ductility. Cavaliere and Squillace [14] studied the super plastic behaviour of FSP AA 7075 by hot tensile tests at different strain rates obtaining high values of the strain rate sensitivity at the higher strain rates. Santella et al. [15] studied the effect of FSP on the mechanical properties of cast A356 and A319 Al alloys and observed that the cast dendritic structure was replaced with fine equiaxed structure in the stir zone and the tensile strength, ductility and fatigue life of both alloys improved by FSP. Surekha et al. [16] studied the effect of multipass friction stir processing on the corrosion behaviour of AA 2219 and found that
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Table 1 Parameters used for FSP. Samples
Rotation speed (rpm)
Travel speed (mm/s)
SS SF MS MF FS
800 800 1200 1200 1600
0.37 0.76 0.37 0.76 0.37
corrosion resistance increased with increase in number of passes. Mishra and Ma [17] have described the basic principle of FSP and the various alloys which can be subjected to FSP. Even though the principles for FSW and FSP are same, the volume of material which acts as effective heat sink is higher in FSP
compared to FSW. Information with respect to the effect of process parameters in FSP on any property is very scanty. Hence, the present work is undertaken to study the effect of process parameters on the corrosion resistance of friction stir processed AA 2219 T87 alloy. FSP is an environment, energy friendly technique as opposed to its conventional counterpart laser melting, a fusion based technique, which is also used for improving corrosion resistance. 2. Experimental procedure The material used in this work is AA 2219-T87 alloy with the nominal composition (in wt.%) of Cu – 6.1, Mn – 0.25, Zr – 0.16, V – 0.09, Ti – 0.05, Fe – 0.2 and Si – 0.15, rest – Al. The AA 2219-T87
Fig. 1. Optical micrographs of (a) BM, (b) SS, (c) MS, (d) FS, (e) SF and (f) MF samples.
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(T87-solutionized at 535 C, cold worked and artificially aged at 190 C for 18 h) plates (250 150 5 mm in size) were friction stir processed, with an indigenously developed machine (3000 rpm, 15 HP and 25 kN) at a constant axial force of 12 kN with a nonconsumable threaded tool made up of high-speed tool steel. Rotation speeds and traverse speeds were varied in the present study. Three rotation speeds 800 (slow-S), 1200 (medium-M), 1600 (fast-F) rpm and two traverse speeds (0.37 (slow-S) and 0.76 (fastF) mm/s) were used. The processing depth was 2 mm in a 5 mm thick plate. Samples processed at a rotation speed of 800 rpm and traverse speed of 0.37, 0.76 mm/s are named SS and SF, respectively. Similarly, all other joints were named according to the parameters used for the process and their nomenclature is shown in Table 1. FF
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samples are not shown in the table as it was not possible to get a crack free processing with this parameter. The microstructural analysis of the friction stir processed samples was carried out by optical microscope and scanning electron microscope (SEM). The samples were etched with Keller’s reagent to reveal the grain boundaries. Specimens for transmission electron microscopy (TEM) were taken from the stir zone and studied using a Philips CM20 TEM with EDX microanalysis facility. Micro-hardness measurements on the top surface on various regions were carried out using Vicker’s micro hardness tester at 25 g load for 30 s. Grain size and particle size measurements were carried out using an image analyzer. SEM images were used to find the grain size of a-Al and the particle size. Grain size of SS and SF
Fig. 2. SEM micrographs of (a) BM, (b) SS, (c) MS, (d) FS, (e) SF and (f) MF samples.
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samples was measured by Heyn’s intercept method. Differential scanning calorimetry (DSC) measurements were carried out by differential scanning calorimeter to find the amount of CuAl2 dissolved during FSP. Software based PAR basic electrochemical system was used to conduct potentiodynamic polarization tests as per ASTM G3 and electrochemical impedance tests (EIS) as per ASTM B457 standards. A flat cell was used for all the experiments and the 5 mm wide stir zone is used as working electrode, graphite is used as auxiliary electrode and saturated calomel electrode as reference electrode. 3.5% NaCl prepared by dissolving 35 g of NaCl in double distilled water and adding 0.4 g of NaOH to maintain the pH at 10 was used as electrolyte. Tests were conducted at a scan rate of 10 mV/min. The breakage of the passive film is denoted by Epit value and hence the corrosion current increases drastically with the applied voltage after Epit. For the electrochemical impedance tests, the samples were immersed in the electrolyte for 30 min before the test. The samples were exposed (0.16 cm2) such that only the stir zone is subjected to the corrosion tests and the rest of the areas were masked. EIS measurements were carried out in the frequency ranging from 10 mHz to 100 kHz. To determine the intergranular corrosion resistance, immersion corrosion tests were performed on etched samples according to ASTM standard G110 in a solution of 57 g/l (0.98 M) NaCl and 10 ml/l H2O2 (0.09 M) for 6 h and the extent of corrosion attack was observed in SEM. Salt Spray corrosion (ASTM B117) test was carried out in 5% NaCl for 100 h to find the general corrosion resistance. The samples of size 25 12 5 mm were masked on all sides except the top surface. The weight of the samples was measured before and after the corrosion tests. The weight loss measurements were used as a measure of corrosion rate. The
samples were cleaned ultrasonically to remove the salt sticking to the surface and then examined under SEM to know the extent of corrosion. 3. Results and discussion 3.1. Microstructural characterization Fig. 1(a)–(f) shows the optical micrographs of the base metal (BM) and stir zones of SS, MS, FS, SF and MF samples, respectively. Since at the end of FSP, the processed plate will have the microstructure of the stir zone alone, in the present work only the stir zone is subjected to all studies. Significant grain refinement can be noticed in the alloy on FSP in comparison to the base metal. It is also observed that the grain size increased with increase in rotation speed and there was no significant change in grain size with the traverse speed within the range studied. Since the second phase particles were not discernible by optical microscopy, SEM studies were carried out. Fig. 2(a)–(f) shows SEM micrographs of BM, SS, MS, FS, SF and MF samples respectively. In high strength Al alloys the stir zone contains both soluble and insoluble second phase particles [18,19]. In AA 2219 alloy, CuAl2, Al7Cu2Fe and Al6Mn precipitates are soluble and Al3Zr and dispersoids formed with Ti and V are insoluble. The second phase particles in Fig. 2 are identified as CuAl2 particles by EDX microanalysis. The histograms of average grain sizes of FS, MS and MF samples are shown along with that of BM in Fig. 3. The BM shows a large average grain size of 67.4 mm while the friction stir processed samples showed fine grains. The average grain sizes of SS, MS, FS, SF and MF samples were 3.3, 6.2, 9.6, 3.4 and 6.1 mm, respectively. The formation of fine grains during FSP can be attributed to dynamic
Fig. 3. Average grain size of (a) BM, (b) MS, (c) FS and (d) MF samples.
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Fig. 4. Average CuAl2 particle size in (a) BM, (b) SS, (c) MS, (d) FS, (e) SF and (f) MF samples.
recrystallization [20,21]. Hassan et al. [22] have reported that a low heat input during FSW results in an exceptionally fine grain structure along with dissolution of the precipitates. When the friction stir processing is carried out with higher heat inputs, the grains in the stir zone are coarser due to grain growth. Hence the grains in SS, SF samples are finer compared to MS, MF and FS samples. Fig. 4 shows a significant reduction in the size of the second phase particles with increase in rotation speed and there was no change in the particle size with the traverse speed as observed earlier by Hassan et al. [22]. Average size of second phase particles in BM reduced from 20.9 to 4.6, 5.2, 7.6, 5.3 and 7.6 mm by FS, MS, SS, MF and SF, respectively. The average particle and grain
Table 2 Average grain and particle sizes in the stir zone of different friction stir processed samples along with the base metal. Alloy condition
Average grain size (mm)
Average particle size (mm)
BM SS MS FS SF MF
67.4 3.3 6.2 9.6 3.4 6.1
20.9 7.6 5.2 4.6 7.6 5.3
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Fig. 5. TEM images of (a) BM and stir zone in (b) SS, (c) MS and (d) FS samples.
sizes were the mean values obtained by considering hundred plus grains and particles. Table 2 shows the average grain and particle sizes at various parameters. The decrease in particle size is by the combined action of the mechanical stirring by the pin and the heat produced by the pin and the shoulder during FSP. The mechanical stirring breaks the coarser particles into finer ones and the heat
produced dissolves the finer particles. The presence of finer particles further increases the solubility of second phase particles in Al matrix. Similar increase in dissolution of Ti in Al by decreasing the particle size of TiAl3 and TiB2 was observed in Al–Ti–B grain refiners under severely stressed conditions [23]. At a fixed rotation speed, changing the traverse speed does not change the heat input and hence the particle and grain sizes. TEM analysis was carried out to show the dissolution of second phase particles formed during aging since they cannot be resolved by SEM studies. Fig. 5 shows the TEM images of SS, MS and FS samples along with the base metal. TEM studies also showed that the size and volume fraction of the precipitates decrease with increase in the rotation speed. The second phase particles formed during casting and those formed during aging influence corrosion and hence it has been established by SEM and TEM studies that size and volume fraction of second phases, formed during casting and those formed during aging decreased with increase in rotation speed. DSC studies were carried out to find quantitatively the amount of precipitates (second phase formed during both casting and
Table 3 DSC results showing the amount of CuAl2 dissolved during FSP.
Fig. 6. DSC traces of SS, MS, FS, SF and MF samples along with the BM.
Process parameter
% of CuAl2 dissolved (DSC result)
BM SS MS FS SF MF
– 17.4 23.1 27.5 17.6 24.3
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Table 4 Corrosion values of FSP samples after pitting, impedance and salt spray tests. Alloy condition
Epit (mV)
Icorr (mA)
Z (kU cm2)
Corrosion rate in mpy
BM SS MS FS SF MF
587 544 501 485 538 498
869.6 388.9 37.7 11.2 343.0 77.8
0.484 1.1 9.6 16.6 1.3 9.7
15.7 11.5 4.5 4.0 11.6 4.6
3.2. Hardness
Fig. 7. Average hardness value of the processed alloy at various rotation and traverse speeds.
aging) dissolved during FSP. In the DSC traces shown in Fig. 6, for different FSP samples the peak corresponds to the dissolution of second phase particles. The amount of precipitates dissolved was estimated from DSC traces by considering the area under the endothermic peak corresponding to the precipitate dissolution. For a particular FSP parameter, the difference between the area under the endothermic peak of the base metal and the processed metal divided by the area under endothermic peak of the base metal gives the fraction of precipitates present (x) at a particular FSP condition. From this, the fraction of second phase dissolved is calculated by subtracting x from one. Similar dissolution of particles on heavy deformation has been reported earlier [23, 24]. The calculations show (Table 3) that the dissolution is higher with higher rotation speeds.
Fig. 8. Potentiodynamic polarization curves of SS, MS and FS samples along with the BM.
In heat treatable alloys, the precipitates only impart strength to the alloy. Dissolution of these strengthening precipitates impairs the mechanical properties. To have an insight into the mechanical properties, hardness profiles were taken. The average hardness is shown in Fig. 7. The processed alloy, in all the conditions studied, showed a lower hardness compared to the base metal. This could be attributed to the dissolution of precipitates during FSP, which was confirmed by DSC results. FSW/FSP creates a softened region around the weld center/stir zone in a number of precipitationhardened aluminum alloys. Hulbert et al. [25] and others [26,27] observed a lowered hardness in FSW Al alloys and they attributed the softening of material in and near the FSW zone to the overaging of the Cu rich second phase inherit in the alloy. Similar overaging (particle coarsening and particle dissolution) is possible in the present alloy also during FSP. However, severe plastic deformation could break the coarsened precipitates due to overaging, which could be the reason for not observing coarse precipitates in the FSP condition. In addition, the base metal is already in the deformed condition (T87 involves cold working) and hence the temperature raise during FSP can lead to recovery of the cold worked structure of the base metal, which could also lead to a decrease in hardness. 3.3. Corrosion behaviour Fig. 8 shows the potentiodynamic polarization curves of FS, MS and SS samples along with the base metal. Table 4 shows the Epit values at various process parameters. It was found that the pitting
Fig. 9. Effect of rotation speed on Epit.
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resistance has improved at all parameters compared to the base metal (Epit – 587 mV). The samples at higher rotation speeds have a wide passive range compared to medium and slow speed samples which can be noticed from the Epit values. The Epit values of FS, MS and SS samples are 485, 501 and 544 mV, respectively. The pitting potentials of MF and SF samples are 498 and 538 mV. It can be noticed that the pitting potentials did not change with the traverse speed. From these results it can be seen that the pitting resistance increased significantly with increase in rotation speed and there was no change with the variation in traverse speed. The Icorr values decreased with increase in rotation speed thus indicating better resistance. The values are tabulated in Table 4. Similar results were observed by Jariyaboon et al. [28] by FSW of AA 2024. Fig. 9 shows the variation of Epit values with rotation speed. Fig. 10 shows the influence of the amount of precipitates (ppt) on the Epit values. Localized galvanic cells formed between the Al matrix and the second phase particles are the main reason for corrosion of AA 2219 alloy. Dissolution of the precipitates decreases the sites for galvanic coupling and hence increases the corrosion resistance. Dissolution of the precipitates was confirmed by DSC technique. EIS results for base material and the processed alloys exposed to 3.5% NaCl solution for 30 min are plotted in Fig. 11. The Z values at various parameters are shown in Table 3. Higher impedance value at low frequency indicates better corrosion resistance. It was found that FS exhibits higher electrochemical corrosion resistance (16.6 kU cm2) than the base material (484.6 U cm2). It was also found that the electrochemical resistance decreased with decrease in the rotation speed. The impedance values of MS, MF, SS and SF samples were 9.6, 9.7, 1.1 and 1.3 kU cm2, respectively. In EIS test also, it is observed that the traverse speed does not have influence on corrosion resistance. The number of galvanic sites did not change with the variation in traverse speed and hence the corrosion resistance did not change. Fig. 12 shows the influence of ppt dissolved on the Z values. Salt spray tests in 5% NaCl solution for 100 h showed pits of very small diameter on the surface and a continuous decrease in thickness over the entire surface area of the metal throughout the corrosion test. Fig. 13 shows the SEM images of the SS, MS, FS, SF and MF samples after salt spray test along with the base metal. Table 3 shows the corrosion rate at various parameters. The base metal has corroded very severely and corrosion products are seen throughout the surface. Also in SS and SF, the rate of attack is very high. With increase in rotation speed noticeable increase in corrosion resistance is observed. In FS sample only a few pits are
Fig. 10. Influence of amount of precipitate (ppt) dissolved on Epit values.
Fig. 11. EIS curves for SS, MS, FS, and FS samples along with the base metal.
seen. It can further be seen that the density and size of pits in FS are lower in comparison to MS and MF samples. The SS and SF samples showed poor resistance to corrosion as the number of galvanic sites is very high. The corrosion rate in the base metal is 15.7 mpy whereas it is 11.5, 4.5, 4.0, 11.6 and 4.6 mpy in SS, MS, FS, SF and MF samples, respectively. From these results, it can be inferred that with the increase in rotation speed the corrosion resistance decreases. The corrosion product formed is known to be Al(OH)3. EDX microanalysis of the corroded products was carried out. However, as this technique cannot reveal the presence of hydrogen, the presence of Al(OH)3 could not be proved at the moment. Attempts are on to carry our FTIR studies to prove the presence of Al(OH)3 as a part of future work. To investigate the susceptibility to intergranular attack, 6 h immersion test was carried out in a solution containing 57 g/l NaCl and 10 ml/1 H2O2 (30 vol.%). Fig. 14 shows the SEM images of the samples after immersion test. The results are consistent with the impedance, potentiodynamic and salt spray tests. The rotation speed is the primary factor in determining the rate of attack. The corrosion attack is less in FS sample compared to SS, MS, SF, MF
Fig. 12. Influence of amount of precipitate (ppt) dissolved on impedance values.
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Fig. 13. SEM images after salt spray test of BM, SS, MS, FS, SF and MF samples.
samples and the BM. The base metal has corroded intergranularly whereas only a few pits were seen in the processed alloys. Intergranular corrosion will occur only when the following three conditions are simultaneously met [29]. 1. Presence of a corrosive medium. 2. Difference in potential in the order of 100 mV between the intermetallics and the matrix. 3. Continuous network of the intermetallics at the grain boundaries such that intergranular cracks can propagate.
All the above three conditions are met in the case of base metal. Hence, the base metal corroded intergranularly. The corrosion products are identified as Cu2O in this case based on the EDX microanalysis. Copper enrichment (54–58 wt.%) in the corrosion deposit was observed, which was due to the selective dissolution of copper rich intermetallic followed by redeposition of copper on the surface in the form of oxide. In the case of FSP alloys, continuous network of CuAl2 at grain boundaries is not observed due to the breakage and dissolution of the intermetallic particles. Hence, there was no copper enrichment in the
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Fig. 14. SEM images after G110 corrosion of BM, SS, MS, FS, SF and MF samples.
corrosion products and only a few pits were observed. The propagation of intergranular corrosion starts at pits. Hence, the processed alloys have better corrosion resistance compared to the base metal. The improved corrosion resistance of the processed alloys is attributed to the refinement in the size and distribution of strengthening particles and the decrease in precipitate free zone (PFZ). In AA 2219, precipitation of CuAl2 at the grain boundaries causes a depletion of copper near the grain boundaries, making those regions anodic to the grain centre. Fig. 15 shows the TEM images of the base metal along with the processed alloy. Fig. 15(a) shows a distinct PFZ in the base metal, whereas the PFZ is not seen in the processed alloys (Fig. 15(b)–(d)). Instead sub-grain formation
is noticed in the processed alloy with the CuAl2 precipitates inside the grains rather on the boundaries. This redistribution of the precipitates could be one of the reasons for the improved corrosion resistance of the processed alloys. 4. Conclusions 1. With the present range of parameters chosen, only the rotation speed has influence on the corrosion behaviour, while the traverse speed does not show any influence. The corrosion resistance increased with the increase in rotation speed. 2. The effect of parameters on corrosion behaviour in friction stir processed alloys is similar to friction stir welded samples.
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Fig. 15. TEM images of (a) BM with PFZ (b) SS, (c) MS and (d) FS samples showing sub-grain formation.
3. Dissolution of the CuAl2 particles during FSP reduces the number of sites available for galvanic coupling and hence increases the corrosion resistance. Amount of dissolution increases with rotation speed and hence the corrosion resistance also increases with rotation speed. Acknowledgements The authors gratefully acknowledge the help and support rendered in FSP by Prof. V. Balasubramanian of Department of Manufacturing Engineering, Annamalai University, Chidambaram. References [1] E.H. Dix, Corrosion of light metals, Am. Soc. Metal. (1946) 15. [2] W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Templesmith, C.J. Dawes, G.B. Patent Application No. 9125978.8, 1991. [3] R.S. Mishra, M.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukherjee, Scr. Mater. 42 (2000) 263. [4] Z.Y. Ma, R.S. Mishra, M.W. Mahoney, Acta Mater. 50 (2002) 4419. [5] R.S. Mishra, Z.Y. Ma, I. Charit, Mater. Sci. Eng., A 341 (2002) 307. [6] P.B. Berbon, W.H. Bingel, R.S. Mishra, C.C. Bampton, M.W. Mahoney, Scr. Mater. 44 (2001) 61. [7] J.E. Spowart, Z.Y. Ma, R.S. Mishra, in: K.V. Jata, M.W. Mahoney, R.S. Mishra, S.L. Semiatin, T. Lienert (Eds.), Friction Stir Welding and Processing II, TMS, 2003, p. 243.
[8] Z.Y. Ma, S.R. Sharma, R.S. Mishra, M.W. Manohey, Mater. Sci. Forum 426–432 (2003) 2891. [9] F.C. Liu, Z.Y. Ma, Scr. Mater. 58 (2008) 677. [10] Z.Y. Ma, A.L. Pilchak, M.C. Juhas, J.C. Williams, Scripta Mater 58 (2008) 361. [11] K. Elangovan, V. Balasubramanian, Mater. Sci. Eng., A 459 (2007) 7. [12] K. Elangovan, V. Balasubramanian, Mater. Des. 29 (2008) 362. [13] C.J. Hsu, P.W. Kao, N.J. Ho, Scr. Mater. 53 (2005) 341. [14] P. Cavaliere, A. Squillace, Mater. Charact. 55 (2005) 136. [15] M.L. Santella, T. Engstrom, D. Storjohann, T.Y. Pan, Scr. Mater. 53 (2005) 201. [16] K. Surekha, B.S. Murty, K. Prasad Rao, Surf. Coat. Technol. 202 (2008) 4057. [17] R.S. Mishra, Z.Y. Ma, Mater. Sci. Eng. R50 (2005) 1. [18] F.J. Humphreys, M. Hatherly, Recrystallization and Related Annealing Phenomenon, Pergamon, 1995, p. 314. [19] M.W. Mahoney, C.G. Rhodes, J.G. Flintoff, W.H.S. Bingel, Metall. Mater. Trans. 29A (1998) 1955. [20] A.A. Hassan, P.B. Prangnell, A.F. Norman, D.A. Price, S.W. Williams, Sci. Technol. Weld. Joining 8 (2003) 257. [21] K.V. Jata, S.L. Semiatin, Scr. Mater. 43 (2000) 743. [22] Kh.A.A. Hassan, A.F. Norman, D.A. Price, P.B. Prangnell, Acta Mater. 51 (2003) 1923. [23] K. Venkateswarlu, M. Chakraborty, B.S. Murty, Mater. Sci. Eng., A 364 (2004) 75. [24] M. Muruyama, Z. Horita, K. Hono, Acta Mater. 49 (2000) 21. [25] D. Hulbert, C. Fuller, M. Mahoney, B. London, Scr. Mater. 57 (2007) 269. [26] Y.S. Sato, H. Kokawa, M. Enmoto, S. Jogan, Metall. Mater. Trans. 30A (1999) 2429. [27] G. Liu, L.E. Murr, C.S. Niou, J.C. McClure, F.R. Vega, Scr. Mater. 37 (1997) 355. [28] M. Jariyaboon, A.J. Davenport, R. Ambat, B.J. Connolly, S.W. Williams, D.A. Price, Corros. Sci. 49 (2007) 877. [29] C. Vargel, M. Jacques, M.P. Schmidt, Corrosion of Aluminium, Elsevier, 2004, p. 125.
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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Effect of processing parameters on the corrosion behaviour of friction stir processed AA 2219 aluminum alloy K. Surekha, B.S. Murty*, K. Prasad Rao Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 February 2008 Received in revised form 19 November 2008 Accepted 19 November 2008 Available online 6 December 2008
The effect of processing parameters (rotation speed and traverse speed) on the corrosion behaviour of friction stir processed high strength precipitation hardenable AA 2219-T87 alloy was investigated. The results indicate that the rotation speed has a major influence in determining the rate of corrosion, which is attributed to the breaking down and dissolution of the intermetallic particles. Corrosion resistance of friction stir processed alloy was studied by potentiodynamic polarization, electrochemical impedance spectroscopy, salt spray and immersion tests. Ó 2008 Elsevier Masson SAS. All rights reserved.
Keywords: Friction stir processing AA 2219 aluminum alloy Corrosion Dissolution of intermetallics
1. Introduction AA 2219(Al–6.3%Cu) is a precipitation hardenable alloy which is used for strategic applications because of good weldability and high strength to weight ratio. CuAl2 is the major intermetallic, which imparts strength to this alloy at the same time decreasing the corrosion resistance due to the formation of galvanic cells between the noble CuAl2 and the Al matrix (solution potential of 0.64 and 0.73 V, respectively). Heat treatable aluminum alloys have two types of second phase particles, viz., intermetallic phases formed during casting and those formed during aging. Both these second phase particles influence corrosion. Corrosion being a surface phenomenon one can adopt an approach of reducing or removing these second phase particles from the surface. In such a situation, the corrosion resistance of the alloy is expected to be improved. The composition of microconstituents, their size, quantity, location, continuity and corrosion potential relative to that of the adjacent aAl matrix, is the important aspect of microstructure that affects the corrosion behaviour of the alloy [1]. Surface modification techniques can improve the corrosion resistance of the alloy by altering the above-mentioned properties. One such attempt is envisaged in the present work by friction stir processing (FSP). FSP is an offshoot of friction stir welding (FSW) which is a solid state welding technique patented by The Welding institute of United Kingdom [2] in 1991. Mishra et al. [3,4] extended the * Corresponding author. Tel.: þ91 44 2257 4754; fax: þ91 44 2257 4752. E-mail address: [email protected] (B.S. Murty). 1293-2558/$ – see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.11.007
principle of FSW to FSP. FSP, unlike its forerunner FSW, which is used to join two plates imparts variety of benefits like improved corrosion and wear resistance, improved ductility and induces even super plasticity. Further, FSP technique has been used to produce surface composite on aluminum substrate [5], homogenization of powder metallurgy aluminum alloy [6], microstructural modification of metal matrix composites [7] and property enhancement in cast aluminum alloys [8]. Liu and Ma [9] achieved low temperature super plasticity of 350–540% at 200–350 C in Al–Zn–Mg–Cu alloy by FSP. Ma et al. [10] have reported that Al and Mg alloys with coarse second phase particles and precipitates are ideal for FSP. Elangovan and Balasubramanian [11,12] studied the effect of tool profile and rotation speed on AA 2219 and AA 6061 and found that square tool profile gave sound welds. The range of parameters and tool profile for the present work were selected based on this work [11]. Hsu et al. [13] achieved ultrafine grained Al–Al2Cu composite by FSP, which has high Young’s modulus, good compressive strength and reasonably good compressive ductility. Cavaliere and Squillace [14] studied the super plastic behaviour of FSP AA 7075 by hot tensile tests at different strain rates obtaining high values of the strain rate sensitivity at the higher strain rates. Santella et al. [15] studied the effect of FSP on the mechanical properties of cast A356 and A319 Al alloys and observed that the cast dendritic structure was replaced with fine equiaxed structure in the stir zone and the tensile strength, ductility and fatigue life of both alloys improved by FSP. Surekha et al. [16] studied the effect of multipass friction stir processing on the corrosion behaviour of AA 2219 and found that
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Table 1 Parameters used for FSP. Samples
Rotation speed (rpm)
Travel speed (mm/s)
SS SF MS MF FS
800 800 1200 1200 1600
0.37 0.76 0.37 0.76 0.37
corrosion resistance increased with increase in number of passes. Mishra and Ma [17] have described the basic principle of FSP and the various alloys which can be subjected to FSP. Even though the principles for FSW and FSP are same, the volume of material which acts as effective heat sink is higher in FSP
compared to FSW. Information with respect to the effect of process parameters in FSP on any property is very scanty. Hence, the present work is undertaken to study the effect of process parameters on the corrosion resistance of friction stir processed AA 2219 T87 alloy. FSP is an environment, energy friendly technique as opposed to its conventional counterpart laser melting, a fusion based technique, which is also used for improving corrosion resistance. 2. Experimental procedure The material used in this work is AA 2219-T87 alloy with the nominal composition (in wt.%) of Cu – 6.1, Mn – 0.25, Zr – 0.16, V – 0.09, Ti – 0.05, Fe – 0.2 and Si – 0.15, rest – Al. The AA 2219-T87
Fig. 1. Optical micrographs of (a) BM, (b) SS, (c) MS, (d) FS, (e) SF and (f) MF samples.
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(T87-solutionized at 535 C, cold worked and artificially aged at 190 C for 18 h) plates (250 150 5 mm in size) were friction stir processed, with an indigenously developed machine (3000 rpm, 15 HP and 25 kN) at a constant axial force of 12 kN with a nonconsumable threaded tool made up of high-speed tool steel. Rotation speeds and traverse speeds were varied in the present study. Three rotation speeds 800 (slow-S), 1200 (medium-M), 1600 (fast-F) rpm and two traverse speeds (0.37 (slow-S) and 0.76 (fastF) mm/s) were used. The processing depth was 2 mm in a 5 mm thick plate. Samples processed at a rotation speed of 800 rpm and traverse speed of 0.37, 0.76 mm/s are named SS and SF, respectively. Similarly, all other joints were named according to the parameters used for the process and their nomenclature is shown in Table 1. FF
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samples are not shown in the table as it was not possible to get a crack free processing with this parameter. The microstructural analysis of the friction stir processed samples was carried out by optical microscope and scanning electron microscope (SEM). The samples were etched with Keller’s reagent to reveal the grain boundaries. Specimens for transmission electron microscopy (TEM) were taken from the stir zone and studied using a Philips CM20 TEM with EDX microanalysis facility. Micro-hardness measurements on the top surface on various regions were carried out using Vicker’s micro hardness tester at 25 g load for 30 s. Grain size and particle size measurements were carried out using an image analyzer. SEM images were used to find the grain size of a-Al and the particle size. Grain size of SS and SF
Fig. 2. SEM micrographs of (a) BM, (b) SS, (c) MS, (d) FS, (e) SF and (f) MF samples.
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samples was measured by Heyn’s intercept method. Differential scanning calorimetry (DSC) measurements were carried out by differential scanning calorimeter to find the amount of CuAl2 dissolved during FSP. Software based PAR basic electrochemical system was used to conduct potentiodynamic polarization tests as per ASTM G3 and electrochemical impedance tests (EIS) as per ASTM B457 standards. A flat cell was used for all the experiments and the 5 mm wide stir zone is used as working electrode, graphite is used as auxiliary electrode and saturated calomel electrode as reference electrode. 3.5% NaCl prepared by dissolving 35 g of NaCl in double distilled water and adding 0.4 g of NaOH to maintain the pH at 10 was used as electrolyte. Tests were conducted at a scan rate of 10 mV/min. The breakage of the passive film is denoted by Epit value and hence the corrosion current increases drastically with the applied voltage after Epit. For the electrochemical impedance tests, the samples were immersed in the electrolyte for 30 min before the test. The samples were exposed (0.16 cm2) such that only the stir zone is subjected to the corrosion tests and the rest of the areas were masked. EIS measurements were carried out in the frequency ranging from 10 mHz to 100 kHz. To determine the intergranular corrosion resistance, immersion corrosion tests were performed on etched samples according to ASTM standard G110 in a solution of 57 g/l (0.98 M) NaCl and 10 ml/l H2O2 (0.09 M) for 6 h and the extent of corrosion attack was observed in SEM. Salt Spray corrosion (ASTM B117) test was carried out in 5% NaCl for 100 h to find the general corrosion resistance. The samples of size 25 12 5 mm were masked on all sides except the top surface. The weight of the samples was measured before and after the corrosion tests. The weight loss measurements were used as a measure of corrosion rate. The
samples were cleaned ultrasonically to remove the salt sticking to the surface and then examined under SEM to know the extent of corrosion. 3. Results and discussion 3.1. Microstructural characterization Fig. 1(a)–(f) shows the optical micrographs of the base metal (BM) and stir zones of SS, MS, FS, SF and MF samples, respectively. Since at the end of FSP, the processed plate will have the microstructure of the stir zone alone, in the present work only the stir zone is subjected to all studies. Significant grain refinement can be noticed in the alloy on FSP in comparison to the base metal. It is also observed that the grain size increased with increase in rotation speed and there was no significant change in grain size with the traverse speed within the range studied. Since the second phase particles were not discernible by optical microscopy, SEM studies were carried out. Fig. 2(a)–(f) shows SEM micrographs of BM, SS, MS, FS, SF and MF samples respectively. In high strength Al alloys the stir zone contains both soluble and insoluble second phase particles [18,19]. In AA 2219 alloy, CuAl2, Al7Cu2Fe and Al6Mn precipitates are soluble and Al3Zr and dispersoids formed with Ti and V are insoluble. The second phase particles in Fig. 2 are identified as CuAl2 particles by EDX microanalysis. The histograms of average grain sizes of FS, MS and MF samples are shown along with that of BM in Fig. 3. The BM shows a large average grain size of 67.4 mm while the friction stir processed samples showed fine grains. The average grain sizes of SS, MS, FS, SF and MF samples were 3.3, 6.2, 9.6, 3.4 and 6.1 mm, respectively. The formation of fine grains during FSP can be attributed to dynamic
Fig. 3. Average grain size of (a) BM, (b) MS, (c) FS and (d) MF samples.
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Fig. 4. Average CuAl2 particle size in (a) BM, (b) SS, (c) MS, (d) FS, (e) SF and (f) MF samples.
recrystallization [20,21]. Hassan et al. [22] have reported that a low heat input during FSW results in an exceptionally fine grain structure along with dissolution of the precipitates. When the friction stir processing is carried out with higher heat inputs, the grains in the stir zone are coarser due to grain growth. Hence the grains in SS, SF samples are finer compared to MS, MF and FS samples. Fig. 4 shows a significant reduction in the size of the second phase particles with increase in rotation speed and there was no change in the particle size with the traverse speed as observed earlier by Hassan et al. [22]. Average size of second phase particles in BM reduced from 20.9 to 4.6, 5.2, 7.6, 5.3 and 7.6 mm by FS, MS, SS, MF and SF, respectively. The average particle and grain
Table 2 Average grain and particle sizes in the stir zone of different friction stir processed samples along with the base metal. Alloy condition
Average grain size (mm)
Average particle size (mm)
BM SS MS FS SF MF
67.4 3.3 6.2 9.6 3.4 6.1
20.9 7.6 5.2 4.6 7.6 5.3
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Fig. 5. TEM images of (a) BM and stir zone in (b) SS, (c) MS and (d) FS samples.
sizes were the mean values obtained by considering hundred plus grains and particles. Table 2 shows the average grain and particle sizes at various parameters. The decrease in particle size is by the combined action of the mechanical stirring by the pin and the heat produced by the pin and the shoulder during FSP. The mechanical stirring breaks the coarser particles into finer ones and the heat
produced dissolves the finer particles. The presence of finer particles further increases the solubility of second phase particles in Al matrix. Similar increase in dissolution of Ti in Al by decreasing the particle size of TiAl3 and TiB2 was observed in Al–Ti–B grain refiners under severely stressed conditions [23]. At a fixed rotation speed, changing the traverse speed does not change the heat input and hence the particle and grain sizes. TEM analysis was carried out to show the dissolution of second phase particles formed during aging since they cannot be resolved by SEM studies. Fig. 5 shows the TEM images of SS, MS and FS samples along with the base metal. TEM studies also showed that the size and volume fraction of the precipitates decrease with increase in the rotation speed. The second phase particles formed during casting and those formed during aging influence corrosion and hence it has been established by SEM and TEM studies that size and volume fraction of second phases, formed during casting and those formed during aging decreased with increase in rotation speed. DSC studies were carried out to find quantitatively the amount of precipitates (second phase formed during both casting and
Table 3 DSC results showing the amount of CuAl2 dissolved during FSP.
Fig. 6. DSC traces of SS, MS, FS, SF and MF samples along with the BM.
Process parameter
% of CuAl2 dissolved (DSC result)
BM SS MS FS SF MF
– 17.4 23.1 27.5 17.6 24.3
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Table 4 Corrosion values of FSP samples after pitting, impedance and salt spray tests. Alloy condition
Epit (mV)
Icorr (mA)
Z (kU cm2)
Corrosion rate in mpy
BM SS MS FS SF MF
587 544 501 485 538 498
869.6 388.9 37.7 11.2 343.0 77.8
0.484 1.1 9.6 16.6 1.3 9.7
15.7 11.5 4.5 4.0 11.6 4.6
3.2. Hardness
Fig. 7. Average hardness value of the processed alloy at various rotation and traverse speeds.
aging) dissolved during FSP. In the DSC traces shown in Fig. 6, for different FSP samples the peak corresponds to the dissolution of second phase particles. The amount of precipitates dissolved was estimated from DSC traces by considering the area under the endothermic peak corresponding to the precipitate dissolution. For a particular FSP parameter, the difference between the area under the endothermic peak of the base metal and the processed metal divided by the area under endothermic peak of the base metal gives the fraction of precipitates present (x) at a particular FSP condition. From this, the fraction of second phase dissolved is calculated by subtracting x from one. Similar dissolution of particles on heavy deformation has been reported earlier [23, 24]. The calculations show (Table 3) that the dissolution is higher with higher rotation speeds.
Fig. 8. Potentiodynamic polarization curves of SS, MS and FS samples along with the BM.
In heat treatable alloys, the precipitates only impart strength to the alloy. Dissolution of these strengthening precipitates impairs the mechanical properties. To have an insight into the mechanical properties, hardness profiles were taken. The average hardness is shown in Fig. 7. The processed alloy, in all the conditions studied, showed a lower hardness compared to the base metal. This could be attributed to the dissolution of precipitates during FSP, which was confirmed by DSC results. FSW/FSP creates a softened region around the weld center/stir zone in a number of precipitationhardened aluminum alloys. Hulbert et al. [25] and others [26,27] observed a lowered hardness in FSW Al alloys and they attributed the softening of material in and near the FSW zone to the overaging of the Cu rich second phase inherit in the alloy. Similar overaging (particle coarsening and particle dissolution) is possible in the present alloy also during FSP. However, severe plastic deformation could break the coarsened precipitates due to overaging, which could be the reason for not observing coarse precipitates in the FSP condition. In addition, the base metal is already in the deformed condition (T87 involves cold working) and hence the temperature raise during FSP can lead to recovery of the cold worked structure of the base metal, which could also lead to a decrease in hardness. 3.3. Corrosion behaviour Fig. 8 shows the potentiodynamic polarization curves of FS, MS and SS samples along with the base metal. Table 4 shows the Epit values at various process parameters. It was found that the pitting
Fig. 9. Effect of rotation speed on Epit.
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resistance has improved at all parameters compared to the base metal (Epit – 587 mV). The samples at higher rotation speeds have a wide passive range compared to medium and slow speed samples which can be noticed from the Epit values. The Epit values of FS, MS and SS samples are 485, 501 and 544 mV, respectively. The pitting potentials of MF and SF samples are 498 and 538 mV. It can be noticed that the pitting potentials did not change with the traverse speed. From these results it can be seen that the pitting resistance increased significantly with increase in rotation speed and there was no change with the variation in traverse speed. The Icorr values decreased with increase in rotation speed thus indicating better resistance. The values are tabulated in Table 4. Similar results were observed by Jariyaboon et al. [28] by FSW of AA 2024. Fig. 9 shows the variation of Epit values with rotation speed. Fig. 10 shows the influence of the amount of precipitates (ppt) on the Epit values. Localized galvanic cells formed between the Al matrix and the second phase particles are the main reason for corrosion of AA 2219 alloy. Dissolution of the precipitates decreases the sites for galvanic coupling and hence increases the corrosion resistance. Dissolution of the precipitates was confirmed by DSC technique. EIS results for base material and the processed alloys exposed to 3.5% NaCl solution for 30 min are plotted in Fig. 11. The Z values at various parameters are shown in Table 3. Higher impedance value at low frequency indicates better corrosion resistance. It was found that FS exhibits higher electrochemical corrosion resistance (16.6 kU cm2) than the base material (484.6 U cm2). It was also found that the electrochemical resistance decreased with decrease in the rotation speed. The impedance values of MS, MF, SS and SF samples were 9.6, 9.7, 1.1 and 1.3 kU cm2, respectively. In EIS test also, it is observed that the traverse speed does not have influence on corrosion resistance. The number of galvanic sites did not change with the variation in traverse speed and hence the corrosion resistance did not change. Fig. 12 shows the influence of ppt dissolved on the Z values. Salt spray tests in 5% NaCl solution for 100 h showed pits of very small diameter on the surface and a continuous decrease in thickness over the entire surface area of the metal throughout the corrosion test. Fig. 13 shows the SEM images of the SS, MS, FS, SF and MF samples after salt spray test along with the base metal. Table 3 shows the corrosion rate at various parameters. The base metal has corroded very severely and corrosion products are seen throughout the surface. Also in SS and SF, the rate of attack is very high. With increase in rotation speed noticeable increase in corrosion resistance is observed. In FS sample only a few pits are
Fig. 10. Influence of amount of precipitate (ppt) dissolved on Epit values.
Fig. 11. EIS curves for SS, MS, FS, and FS samples along with the base metal.
seen. It can further be seen that the density and size of pits in FS are lower in comparison to MS and MF samples. The SS and SF samples showed poor resistance to corrosion as the number of galvanic sites is very high. The corrosion rate in the base metal is 15.7 mpy whereas it is 11.5, 4.5, 4.0, 11.6 and 4.6 mpy in SS, MS, FS, SF and MF samples, respectively. From these results, it can be inferred that with the increase in rotation speed the corrosion resistance decreases. The corrosion product formed is known to be Al(OH)3. EDX microanalysis of the corroded products was carried out. However, as this technique cannot reveal the presence of hydrogen, the presence of Al(OH)3 could not be proved at the moment. Attempts are on to carry our FTIR studies to prove the presence of Al(OH)3 as a part of future work. To investigate the susceptibility to intergranular attack, 6 h immersion test was carried out in a solution containing 57 g/l NaCl and 10 ml/1 H2O2 (30 vol.%). Fig. 14 shows the SEM images of the samples after immersion test. The results are consistent with the impedance, potentiodynamic and salt spray tests. The rotation speed is the primary factor in determining the rate of attack. The corrosion attack is less in FS sample compared to SS, MS, SF, MF
Fig. 12. Influence of amount of precipitate (ppt) dissolved on impedance values.
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Fig. 13. SEM images after salt spray test of BM, SS, MS, FS, SF and MF samples.
samples and the BM. The base metal has corroded intergranularly whereas only a few pits were seen in the processed alloys. Intergranular corrosion will occur only when the following three conditions are simultaneously met [29]. 1. Presence of a corrosive medium. 2. Difference in potential in the order of 100 mV between the intermetallics and the matrix. 3. Continuous network of the intermetallics at the grain boundaries such that intergranular cracks can propagate.
All the above three conditions are met in the case of base metal. Hence, the base metal corroded intergranularly. The corrosion products are identified as Cu2O in this case based on the EDX microanalysis. Copper enrichment (54–58 wt.%) in the corrosion deposit was observed, which was due to the selective dissolution of copper rich intermetallic followed by redeposition of copper on the surface in the form of oxide. In the case of FSP alloys, continuous network of CuAl2 at grain boundaries is not observed due to the breakage and dissolution of the intermetallic particles. Hence, there was no copper enrichment in the
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Fig. 14. SEM images after G110 corrosion of BM, SS, MS, FS, SF and MF samples.
corrosion products and only a few pits were observed. The propagation of intergranular corrosion starts at pits. Hence, the processed alloys have better corrosion resistance compared to the base metal. The improved corrosion resistance of the processed alloys is attributed to the refinement in the size and distribution of strengthening particles and the decrease in precipitate free zone (PFZ). In AA 2219, precipitation of CuAl2 at the grain boundaries causes a depletion of copper near the grain boundaries, making those regions anodic to the grain centre. Fig. 15 shows the TEM images of the base metal along with the processed alloy. Fig. 15(a) shows a distinct PFZ in the base metal, whereas the PFZ is not seen in the processed alloys (Fig. 15(b)–(d)). Instead sub-grain formation
is noticed in the processed alloy with the CuAl2 precipitates inside the grains rather on the boundaries. This redistribution of the precipitates could be one of the reasons for the improved corrosion resistance of the processed alloys. 4. Conclusions 1. With the present range of parameters chosen, only the rotation speed has influence on the corrosion behaviour, while the traverse speed does not show any influence. The corrosion resistance increased with the increase in rotation speed. 2. The effect of parameters on corrosion behaviour in friction stir processed alloys is similar to friction stir welded samples.
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Fig. 15. TEM images of (a) BM with PFZ (b) SS, (c) MS and (d) FS samples showing sub-grain formation.
3. Dissolution of the CuAl2 particles during FSP reduces the number of sites available for galvanic coupling and hence increases the corrosion resistance. Amount of dissolution increases with rotation speed and hence the corrosion resistance also increases with rotation speed. Acknowledgements The authors gratefully acknowledge the help and support rendered in FSP by Prof. V. Balasubramanian of Department of Manufacturing Engineering, Annamalai University, Chidambaram. References [1] E.H. Dix, Corrosion of light metals, Am. Soc. Metal. (1946) 15. [2] W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Templesmith, C.J. Dawes, G.B. Patent Application No. 9125978.8, 1991. [3] R.S. Mishra, M.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukherjee, Scr. Mater. 42 (2000) 263. [4] Z.Y. Ma, R.S. Mishra, M.W. Mahoney, Acta Mater. 50 (2002) 4419. [5] R.S. Mishra, Z.Y. Ma, I. Charit, Mater. Sci. Eng., A 341 (2002) 307. [6] P.B. Berbon, W.H. Bingel, R.S. Mishra, C.C. Bampton, M.W. Mahoney, Scr. Mater. 44 (2001) 61. [7] J.E. Spowart, Z.Y. Ma, R.S. Mishra, in: K.V. Jata, M.W. Mahoney, R.S. Mishra, S.L. Semiatin, T. Lienert (Eds.), Friction Stir Welding and Processing II, TMS, 2003, p. 243.
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