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Effects of eutectic modification and T4 heat treatment on mechanical properties and corrosion resistance of an Al–9 wt%Si casting alloy
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Materials Chemistry and Physics 106 (2007) 343–349
Effects of eutectic modification and T4 heat treatment on mechanical properties and corrosion resistance of an Al–9 wt%Si casting alloy Wislei R. Os´orio, Leonardo R. Garcia, Pedro R. Goulart, Amauri Garcia ∗ Department of Materials Engineering, State University of Campinas, UNICAMP, P.O. Box 6122, 13083-970 Campinas, SP, Brazil Received 12 December 2006; received in revised form 31 May 2007; accepted 13 June 2007
Abstract It has been reported that the mechanical properties and the corrosion resistance of metallic alloys depend strongly on the final microstructural arrangement. In order to achieve a desired level of final properties, the correlation of microstructural parameters with corrosion behavior and mechanical properties can be very useful for planning the manufacturing process. The aim of the present work was to investigate the influence of microstructural parameters of an Al–9 wt%Si alloy in as-cast, modified (eutectic) and heat-treated conditions on the resultant mechanical properties and corrosion resistance. Experimental results include ultimate tensile strength, yield strength, hardness, electrochemical impedance parameters and corrosion rates. Corrosion behavior was evaluated by both the electrochemical impedance spectroscopy technique and by Tafel plots carried out in a 0.5 M NaCl test solution at 25 ◦ C. The impedance parameters were obtained from equivalent circuits provided by the ZView software. It was found that the ultimate tensile strength increases both with the addition of a eutectic modifier and with the T4 heat treatment. Yield strength seems to be independent of such processes. On the other hand, it was also found that the corrosion resistance has significantly decreased with the eutectic modification. The T4 heat treatment has provided a recovery on the corrosion resistance. © 2007 Elsevier B.V. All rights reserved. Keywords: Eutectic modification; T4 heat treatment; Mechanical properties; Corrosion resistance; Al–Si casting alloys
1. Introduction Although the metallurgical and micromechanical aspects of the factors controlling microstructure, unsoundness, strength and ductility of as-cast alloys are complex, it is well known that solidification processing variables are of high order of importance. The effect of microstructure on metallic alloys properties has been highlighted in various studies, particularly the influences of dendrite arm spacing on mechanical properties and, recently, on the corrosion resistance [1–8]. The dendrite fineness can be even of more importance in the prediction of mechanical properties than grain size. Aluminum alloys with silicon as a major alloying element constitute a class of alloys that provides the most significant part of all shaped castings manufactured, especially in the aerospace and automotive industries [9]. This is mainly due to the outstanding effect of silicon in the improvement of casting
∗
Corresponding author. Tel.: +55 19 35213320; fax: +55 19 32893722. E-mail address: [email protected] (A. Garcia).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.06.011
characteristics, combined with other physical properties such as mechanical properties and corrosion resistance. Aluminum–silicon alloys can have significant improvements in mechanical properties by inducing structural modification in the normally occurring eutectic. In general, the greatest benefits are achieved in alloys containing from 5 wt%Si up to the eutectic concentration. Mechanical properties of Al–Si casting alloys depend not only on their chemical composition but are also significantly dependent on microstructural features such as the morphologies of the Al–rich ␣-phase and of the eutectic Si particles. Eutectic silicon in untreated Al–Si foundry alloys has often a very coarse and plate-like morphology, leading to poor mechanical properties, particularly ductility [10]. Significant improvement in mechanical properties can be achieved when the aluminum–silicon alloys undergoes a change in morphological characteristics of silicon both in eutectic and primary form by modification with certain additions made to the molten alloy, e.g., antimony, strontium, sodium and titanium [11–13] or when a specific and adequate heat treatment is applied [14]. Particularly, the addition of sodium causes the disappearance of primary silicon with the formation of solid solution dendrites
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(␣-Al) and fine fiber eutectic silicon instead of plate-like structures, resulting in a highly branched filamentary form with a better distribution of Si particles. On the other hand, the T4 heat treatment which consists on a sequence of solution heat treatment and water-quenching, generally provides a microstructural modification resulting on improvement of mechanical properties. The resultant microstructure is characterized by spheroidised silicon particles which lead to less stress concentration when compared to flake or plate silicon morphologies. Besides, spheroidised Si particles and refined precipitates can provide a retard in cracks nucleation resulting in much improved casting ductility [15,16]. There is a lack of consistent studies and experimental reports on the effect of the modification in eutectic silicon morphology and temper on surface corrosion behavior of hypoeutectic Al–Si casting alloys. The present study aims to contribute to the understanding of the role of the eutectic modification and T4 heat treatment on mechanical properties and corrosion behavior. Modified and unmodified Al–9 wt%Si samples were cast under similar operational conditions in order to parameterize the cooling rates along solidification. Samples were also subjected to the T4 heat treatment in order to investigate the modifications on the microstructural arrangement and its corresponding effects on mechanical and corrosion properties. After metallographic procedures and microscopy analysis, the corrosion resistance was evaluated by both the electrochemical impedance spectroscopy technique and the Tafel extrapolation method carried out in a 0.5 M NaCl test solution at 25 ◦ C. The impedance parameters
were obtained from equivalent circuits provided by the ZView software. Experimental results include secondary dendrite arm spacings (λ2 ), corrosion potential (Ecorr ), corrosion rate (icorr ), polarization resistance (R1 ), capacitances values (ZCPE ), ultimate tensile strength (UTS), yield strength (YS) and Vickers hardness (HV). 2. Experimental procedure Al–9 wt%Si casting alloy samples were prepared from commercially pure metals: Al (99.72 wt%) and Si (99.58 wt%). The experimental assembly used to cast such alloy samples is shown in Fig. 1(a). The casting chamber was properly insulated by refractory walls in order to ensure a dominant horizontal and unidirectional heat flow during solidification. A low carbon steel chill was used at a normal environment temperature of about 25 ◦ C (initial mold temperature), with the heat-extracting surface being polished. The Al–9 wt%Si alloy was melted in an electric resistance-type furnace, flux-degassed with argon for 5 min and then poured into the casting chamber with a pouring temperature of about 10% above the liquidus temperature. In the case of the modified samples, simultaneously to the degassing procedure, the modifier amount was sprinkled against the liquid metal surface. The melt was then stirred in order to incorporate this modifier into the liquid which was allowed to remain in contact with the melt for approximately 8 min. The used chemical modifier was a commercial salt based on sodium fluoride (Modimil® ). According to the literature [11], 1% of the melt weight of Modimil® flux is enough to achieve the eutectic modification. Modified and unmodified as-cast samples were T4 heat-treated (solution heat treatment at 540 ◦ C for 6 h, water-quenched to 27 ◦ C and held at that temperature for 10 min) [17,18]. Fig. 1(b) and (c) exhibit the longitudinal and the transversal specimens that were taken from the ingots for optical metallographic examination and tensile, corrosion and hardness testings, respectively. The tensile testing was performed
Fig. 1. (a) Casting assembly, (b) location of specimens for metalographic procedures and (c) specimens for tensile, hardness and corrosion testings.
W.R. Os´orio et al. / Materials Chemistry and Physics 106 (2007) 343–349 according to the specifications of ASTM Standard E 8M. In order to provide mean values of ultimate and yield tensile strengths, three specimens were tested for each condition (as-cast, modified and heat-treated). The mean values of Vickers hardness were also obtained from triplicate specimens. All samples were carefully solidified under similar cooling conditions. The corresponding microstructures were revealed by grinding, polishing and etching selected regions of the samples. The chemical etchant used was a solution of 0.5%HF in distilled water. Image processing systems Neophot 32 (Carl Zeiss, Esslingen, Germany) and Leica Quantimet 500MC (Leica Imaging Systems Ltd., Cambridge, England) were used to acquire the microstructures. Secondary dendrite arm spacings (λ2 ) were measured by averaging the distance between adjacent side branches. Electrochemical impedance spectroscopy (EIS) and polarization tests were carried out in triplicate for modified, unmodified and heat-treated samples. The EIS tests were carried out in a 0.5 M (3.0%) NaCl solution at 25 ◦ C and a neutral pH range (6.5). A potentiostat coupled to a frequency analyzer system, a glass corrosion cell kit with a platinum counter electrode and a saturated calomel reference electrode (SCE) were used to perform the EIS tests. The working electrodes consisted of Al9 wt%Si alloy samples (unmodified, modified and heat-treated), which were positioned at the glass corrosion cell kit, leaving a circular 1 cm2 metal surface in contact with the electrolyte. The potential amplitude was set to 10 mV in open-circuit potential and the frequency range was set from 100 mHz to 100 kHz. The samples were further ground to a 600 grit SiC finish, followed by distilled water washing and air-drying before measurements. EIS measurements began after an initial delay of 10 min for the sample to reach a steady-state condition. Polarization tests were also carried out in a 0.5 M (3.0%) NaCl solution at 25 ◦ C using a potentiostat. The polarization curves were determined by stepping the potential at a scan rate of 0.2 mV s−1 from −250 mV (SCE) to +250 mV (SCE) related to open-circuit potential. Using an automatic data acquisition system, the potentiodynamic polarization curves were plotted and both corrosion rate and potential were estimated by the Tafel extrapolation method. Impedance parameters such as polarization resistance (R1 ) and capacitance values (ZCPE ) were obtained by using the equivalent circuit technique.
3. Results and discussion Fig. 2 shows the microstructures obtained by optical microscopy for unmodified, modified and T4 heat-treated Al–9 wt%Si samples. The unmodified sample (as-cast) has a microstructure characterized by an Al-rich dendritic matrix (␣Al phase) and a eutectic mixture in the interdendritic region formed by silicon particles, which are coarse and distributed in a plate-like morphology, set in an Al-rich phase as shown in Fig. 2. By analysing the sample in which sodium-modifier was added to the melt a considerable eutectic modification can be observed. Its microstructural features result in fine and fibrous eutectic silicon, as shown in Fig. 2. In contrast, the observed mean dendrite arm spacings (λ2 ) and the Al-rich dendritic matrix appear to be unaffected, as expected. Unmodified-T4 and modified-T4 samples are also shown in Fig. 2. The unmodified-T4 sample presents a slight modification in its microstructure which appears
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as short plate-like Si particles with a mean width similar to that of the unmodified sample (of about 5 m). On the other hand, the modified-T4 sample exhibits a significant globular shaped effect on silicon particles (mean diameter of about 3.0 m). 3.1. Different sample conditions and corresponding mechanical properties The different processes to which the Al9 wt%Si alloy samples have been subjected, i.e., chill casting (unmodified), modification (sodium fluoride addition) and T4 heat treatement (in both unmodified and modified samples) have provided variations in the resulting microstructures. It is also expected that such microstructural changes will induce differences on the final mechanical properties. Table 1 presents the experimental results of ultimate tensile strength (UTS), yield strength (YS: 0.2% proof stress), tensile elongation (δ) and Vickers hardness (HV) for unmodified and modified Al–9 wt%Si alloy samples in both treated and non-treated conditions. The results of UTS, YS and HV for unmodified and heat-treated conditions are consistent with those found in the literature concerning Al–Si commercial casting alloys [7,8,15,19]. Although the modified and unmodified as-cast samples present very similar secondary dendrite arm spacings, the addition of a eutectic modifier has increased UTS, YS and HV. Such tendency of enhance on UTS presented by the modified alloy sample seems to be associated with an increase of obstacles to slip due to the more extensive distribution of ␣-Al phase/fibrous silicon particles boundaries. It can also be observed in Table 1 that the elongation decreases with the eutectic modification. It is important to remark that the secondary dendritic spacing has an important role on mechanical properties, particularly in the UTS, as recently reported [2,3,7]. By comparing the corresponding results of unmodified (as-cast) and modified Al–9 wt%Si samples, it can be seen that the latter exhibits a significant improvement in UTS and HV. On the other hand, experimental results of UTS and YS for modified, unmodified heat-treated and modified heat-treated samples are very similar, i.e., the eutectic modification of Al–Si casting alloys can provide a better cost versus benefit relationship compared to that of heat-treated alloys, as far as tensile properties are concerned. 3.2. Different sample conditions and corresponding corrosion behavior In order to investigate the corrosion resistance of unmodified, modified and heat-treated Al–9 wt%Si alloy, a number
Table 1 Ultimate and yield tensile strengths, tensile elongation, Vickers hardness and secondary dendrite arm spacing of unmodified, modified and heat-treated samples Sample
UTS (MPa)
YS (MPa)
δ (%)
HV
λ2 (m)
Unmodified Modified Unmodified-T4 Modified-T4
138 (126–144) 212 (203–225) 209 (200–222) 232 (225–246)
64 (61–67) 87 (82–91) 94 (91–96) 96 (94–101)
8 (7–9) 4 (3–6) 7 (6–8) 6 (5–7)
70 (66–75) 85 (83–86) 92 (88–96) 94 (93–96)
32 (28–36) 34 (29–38) 35 (27–39) 34 (29–37)
Numbers in parentheses represent maximum and minimum values.
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Fig. 2. Typical microstructures of unmodified, modified and T4 heat-treated Al–9 wt%Si samples and the corresponding secondary dendrite arm spacings.
of samples were subjected to corrosion tests. The corrosion behavior of Al–Si foundry alloys have been examined and compared previously in the literature only by polarization methods (potentiostatic or potentiodynamic) [17]. Although polarization methods can provide important information concerning the corrosion behavior, in the present study in addition to the impedance parameters obtained from experimental EIS diagrams, an equivalent circuit analysis has also been encompassed in order to permit a more complete spectrum of results to be attained.
Fig. 3 shows a comparison of experimental results of corrosion tests carried out in a 0.5 M NaCl solution concerning EIS diagrams and potentiodynamic polarization curves for unmodified, modified and heat-treated conditions. Impedance parameters obtained from the equivalent circuit analysis corresponding to each EIS diagram, corrosion potential and current density are shown in Table 2. Fig. 3(b) shows potentiodynamic polarization curves for modified, unmodified and heat-treated Al–9 wt%Si alloy
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Table 2 Corrosion potential, current density and impedance parameters for unmodified, modified and heat-treated Al–9 wt%Si alloy samples in a 0.5 M NaCl test solution Parameters
Unmodified (as-cast)
Modified
Unmodified and heat-treated
Modified and heat-treated
Ecorr (mV) (corrosion potential) Icorr (A cm−2 ) (corrosion rate)
−722 0.98
−755 3.43
−798 1.35
−825 1.75
Rel ( cm−2 ) ZCPE(1) (106 1 s−n cm2 ) ZCPE(2) (106 1 s−n cm2 ) n1 n2 R1 ( cm−2 ) R2 ( cm−2 ) χ2
20.13 40.56 (±2.8) 835.17 (±5.3) 0.80 0.75 5280 2 × 1015 56 × 10−4
19.39 50.71 (±3.1) 547.58 (±45.4) 0.82 0.85 2380 2 × 1015 23 × 10−4
19.27 26.92 (±2.8) 180.63 (±19.4) 0.89 0.97 4052 3366 11 × 10−3
18.98 34.93 (±3.6) 191.43 (±23.1) 0.84 0.68 4986 3179 49 × 10−4
samples. It can be observed a current density (Icorr ) of about three times higher for the modified sample (around 3.43 A cm−2 ) compared to that of the unmodified (as-cast) sample (0.98 A cm−2 ). The modified sample has also presented a corrosion potential (Ecorr ) slightly more active (or less-noble) than the unmodified one.
Fig. 3. Experimental results for unmodified, modified and heat-treated Al–9 wt%Si alloy samples in: (a) EIS tests (Bode and Bode-phase diagrams) and (b) potentiodynamic polarization curves.
A comparison between the heat-treated samples permits to observe that both the unmodified and the modified samples exhibit similar Icorr values which are intermediate to those Icorr values of the untreated samples (unmodified and modified). Although the T4 heat treatment has provided a displacement on the corrosion potential toward a less-noble potential, it is possible to affirm that such heat treatment provides a better corrosion resistance than that provided by modification, as shown in Fig. 3(b) and Table 2. An analogue discussion can also be taken when analyzing impedance and equivalent circuit quantitative results, as shown in Table 2. The fitting quality for experimental results and those calculated and interpreted by the ZView software is represented by chi-squared (χ2 ) values [20]. The electronic equivalent circuit consists of a capacitance component (ZCPE1 ) in parallel to series resistors R1 and R2 and other capacitance component (ZCPE2 ) in parallel to R2 . The impedance parameter of the equivalent circuit Rel corresponds to the electrolyte resistance which in Bode plot is expressed in a high frequency limit (F > 1 kHz). It is proposed that R1 and R2 correspond to the polarization resistances of the samples (modified and unmodified conditions) and oxide film, respectively. These two parameters represent the sum of existing resistances in low frequency limit (F < 1 Hz). The parameters n1 and n2 are correlated to the phase angle, varying between −1 and 1. ZCPE(1) and ZCPE(2) correspond to the capacitances (1 Hz < F < 1 kHz) of the experimentally examined samples and their oxide layer formation, respectively. In the literature ZCPE generally denotes the impedance of a phase element as ZCPE = [C(jw)n ]−1 [21,22]. The used equivalent circuit is shown in Fig. 4. Analyses of the impedance parameters, shown in Table 2, confirm the observations concerning the qualitative aspects of
Fig. 4. Proposed equivalent circuit for modeling impedance data for all Al–9 wt%Si alloy samples.
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Fig. 5. Typical SEM dendritic microstructure of Al–Si hypoeutectic alloys: (a) general view of the dendritic arrangement and (b) detail of secondary arm spacings and interdendritic region with localized deformation points between Al–rich phase and Si particles (white arrows).
experimental EIS results. It can be seen that, the unmodified Al–9 wt%Si as-cast sample has a capacitance ZCPE(1) of about 40 × 106 1 s−n cm2 (or 40 F cm−2 ) and a polarization resistance R1 of about 5280 cm−2 . On the other hand, the modified Al–9 wt%Si sample presents a higher capacitance and a lower polarization resistance than the unmodified sample, as shown in Table 2. Comparing unmodified heat-treated and modified heattreated samples a similarity between capacitances (ZCPE(1) ) and polarization resistances (R1 ) can be observed. The parameters ZCPE(1) vary from 27 to 35 F cm−2 and R1 range from 4 to 5 k cm−2 . This indicates that the corrosion behavior is very similar for both samples and being located in an intermediate position when compared with those of unmodified and modified samples. It can also be observed that the T4 heat treatment provides smaller capacitances than the results presented by the untreated samples. The parameters R2 indicate that less homogeneous oxide films have grown on heat-treated samples compared to those of the untreated samples. The afore-mentioned experimental and calculated parameters provide sufficient information which permits to conclude that the use of the sodium-based modifier has provoked a deleterious effect on the corrosion behavior of an Al–9 wt%Si casting alloy. Such reduction on corrosion resistance presented by the modified alloy sample seems to be associated with an increase of boundaries (between the Al–rich (␣) phase and Si particles during growth from the melt) when compared with the unmodified alloy. It seems that the modified eutectic regions are more susceptible to the corrosion action, due to localized deformation regions at such boundaries, as recently reported [8]. Fig. 5 shows a typical SEM dendritic microstructure of Al–Si hypoeutectic alloys showing the localized deformation between Al and Si particles. On the other hand, it seems that the level of localized deformation between Al and Si particles has decreased due to the spheroidizing effect on silicon particles provided by the T4 heat treatment. This can explain the recovery on corrosion resistance for the modified heat-treated sample, as shown by the results of Table 2.
Although the addition of eutectic modifiers to Al–Si alloys is widely applied by major foundry industries as an alternative way to produce components with better mechanical properties or to increase the fluidity of such alloys, the corrosion resistance can be significantly affected as previously discussed. It seems that when a compromise between good corrosion resistance and good mechanical properties is essential, an adequate surface treatment must be provided in order to improve the surface corrosion resistance of modified Al–Si alloys. In order to achieve a desired level of final properties, the correlation of microstructural parameters with corrosion behavior and mechanical properties can be very useful for planning the manufacturing process. 4. Conclusions The following main conclusions can be drawn from the present experimental investigation: (a) The experimental values of ultimate tensile strength (UTS), yield strength (YS), Vickers hardness (HV) for an Al–9 wt%Si alloy have shown that the addition of a sodium fluoride eutectic modifier increases UTS, YS and HV. It seems that such tendency of increase on mechanical properties is associated only to the eutectic modification, since the measured dendritic spacings were very similar in any condition experimentally examined. (b) Experimental results of UTS for unmodified, modified and modified heat-treated samples are very similar, i.e., the level of mechanical properties attained by the eutectic modification of an Al–9 wt%Si casting alloy can provide a better cost versus benefit relationship compared to that of heat-treated alloys. (c) The increase on UTS values after the T4 heat treatment can be attributed to the globular shaped effect on silicon particles provided by such treatment that leads to less stress concentration when compared to flake or plate silicon morphologies.
W.R. Os´orio et al. / Materials Chemistry and Physics 106 (2007) 343–349
(d) The experimental corrosion tests results have shown that unmodified samples of as-cast and heat-treated conditions tend to yield higher corrosion resistance than modified samples, and that this is associated with the morphological changes of the interdendritic eutectic mixture. On the other hand, it seems that the T4 heat treatment provides a recovery on the corrosion resistance due to the spheroidizing effect on silicon particles. (e) In order to achieve a desired level of final properties, the correlation of microstructural parameters with corrosion behavior and mechanical properties can be very useful for planning the manufacturing process. Acknowledgements The authors acknowledge financial support provided by FAPESP (The Scientific Research Foundation of the State of S˜ao Paulo, Brazil), FAEPEX-UNICAMP and CNPq (The Brazilian Research Council). References [1] N.J. Petch, J. Iron Steel Inst. 174 (1953) 25–31. [2] P. Donelan, Mater. Sci. Technol. 16 (2000) 261–269. [3] W.R. Os´orio, C.A. Santos, J.M.V. Quaresma, A. Garcia, J. Mater. Proc. Technol. 143 (2003) 703–709.
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[4] W.R. Os´orio, C.M. Freire, A. Garcia, J. Alloys Compd. 397 (2005) 179–191. [5] W.R. Os´orio, C.A. Siqueira, C.M. Freire, A. Garcia, Rev. Metal. Madrid Extr. (2005) 176–180. [6] W.R. Os´orio, J.E. Spinelli, N. Cheung, A. Garcia, Mater. Sci. Eng. A 420 (2006) 179–186. [7] W.R. Os´orio, P.R. Goulart, G.A. Santos, C. Moura Neto, A. Garcia, Metall. Mater. Trans. 37A (2006) 2525–2538. [8] P.R. Goulart, J.E. Spinelli, W.R. Os´orio, A. Garcia, Mater. Sci. Eng. A 421 (2006) 245–253. [9] S.G. Shabestari, H. Moemeni, J. Mater. Proc. Technol. 153/154 (2004) 193–198. [10] L. Heusler, W.G. Schneider, J. Light Metals 2 (2002) 17–26. [11] L. Lu, K. Nogita, A.K. Dahle, Mater. Sci. Eng. A 399 (2005) 244–253. [12] M.M. Haque, A.F. Ismail, J. Mater. Proc. Technol. 162/163 (2005) 312–316. [13] J.A. Lozano, B.S. Pe˜na, Scripta Mater. 54 (2006) 943–947. [14] B.L. Ou, C.H. Shen, Scand. J. Metall. 34 (2005) 318–325. [15] X.P. Niu, B.H. Hu, I. Pinwill, H. Li, J. Mater. Proc. Technol. 105 (2000) 119–127. [16] M. Harun, L.A. Talib, A.R. Daud, Wear 194 (1996) 54–59. [17] C. Park, S. Kim, Y. Kwon, Y. Lee, J. Lee, Mater. Sci. Eng. A 391 (2005) 86–94. [18] W.D. Fei, Z.J. Li, Y.B. Li, Mater. Chem. Phys. 97 (2006) 402–409. [19] S.A. Kori, B.S. Murty, M. Chakraborty, Mater. Sci. Eng. A 280 (2000) 58–61. [20] S.L. Assis, S. Wolynec, I. Costa, Electrochim. Acta 51 (2006) 1815–1819. [21] M. Kliskic, J. Radosevic, S. Gudic, M. Smith, Electrochim. Acta 43 (1998) 3241–3255. [22] S. Gudic, J. Radosevic, M. Kliskic, Electrochim. Acta 47 (2002) 3009–3016.
Materials Chemistry and Physics 106 (2007) 343–349
Effects of eutectic modification and T4 heat treatment on mechanical properties and corrosion resistance of an Al–9 wt%Si casting alloy Wislei R. Os´orio, Leonardo R. Garcia, Pedro R. Goulart, Amauri Garcia ∗ Department of Materials Engineering, State University of Campinas, UNICAMP, P.O. Box 6122, 13083-970 Campinas, SP, Brazil Received 12 December 2006; received in revised form 31 May 2007; accepted 13 June 2007
Abstract It has been reported that the mechanical properties and the corrosion resistance of metallic alloys depend strongly on the final microstructural arrangement. In order to achieve a desired level of final properties, the correlation of microstructural parameters with corrosion behavior and mechanical properties can be very useful for planning the manufacturing process. The aim of the present work was to investigate the influence of microstructural parameters of an Al–9 wt%Si alloy in as-cast, modified (eutectic) and heat-treated conditions on the resultant mechanical properties and corrosion resistance. Experimental results include ultimate tensile strength, yield strength, hardness, electrochemical impedance parameters and corrosion rates. Corrosion behavior was evaluated by both the electrochemical impedance spectroscopy technique and by Tafel plots carried out in a 0.5 M NaCl test solution at 25 ◦ C. The impedance parameters were obtained from equivalent circuits provided by the ZView software. It was found that the ultimate tensile strength increases both with the addition of a eutectic modifier and with the T4 heat treatment. Yield strength seems to be independent of such processes. On the other hand, it was also found that the corrosion resistance has significantly decreased with the eutectic modification. The T4 heat treatment has provided a recovery on the corrosion resistance. © 2007 Elsevier B.V. All rights reserved. Keywords: Eutectic modification; T4 heat treatment; Mechanical properties; Corrosion resistance; Al–Si casting alloys
1. Introduction Although the metallurgical and micromechanical aspects of the factors controlling microstructure, unsoundness, strength and ductility of as-cast alloys are complex, it is well known that solidification processing variables are of high order of importance. The effect of microstructure on metallic alloys properties has been highlighted in various studies, particularly the influences of dendrite arm spacing on mechanical properties and, recently, on the corrosion resistance [1–8]. The dendrite fineness can be even of more importance in the prediction of mechanical properties than grain size. Aluminum alloys with silicon as a major alloying element constitute a class of alloys that provides the most significant part of all shaped castings manufactured, especially in the aerospace and automotive industries [9]. This is mainly due to the outstanding effect of silicon in the improvement of casting
∗
Corresponding author. Tel.: +55 19 35213320; fax: +55 19 32893722. E-mail address: [email protected] (A. Garcia).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.06.011
characteristics, combined with other physical properties such as mechanical properties and corrosion resistance. Aluminum–silicon alloys can have significant improvements in mechanical properties by inducing structural modification in the normally occurring eutectic. In general, the greatest benefits are achieved in alloys containing from 5 wt%Si up to the eutectic concentration. Mechanical properties of Al–Si casting alloys depend not only on their chemical composition but are also significantly dependent on microstructural features such as the morphologies of the Al–rich ␣-phase and of the eutectic Si particles. Eutectic silicon in untreated Al–Si foundry alloys has often a very coarse and plate-like morphology, leading to poor mechanical properties, particularly ductility [10]. Significant improvement in mechanical properties can be achieved when the aluminum–silicon alloys undergoes a change in morphological characteristics of silicon both in eutectic and primary form by modification with certain additions made to the molten alloy, e.g., antimony, strontium, sodium and titanium [11–13] or when a specific and adequate heat treatment is applied [14]. Particularly, the addition of sodium causes the disappearance of primary silicon with the formation of solid solution dendrites
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W.R. Os´orio et al. / Materials Chemistry and Physics 106 (2007) 343–349
(␣-Al) and fine fiber eutectic silicon instead of plate-like structures, resulting in a highly branched filamentary form with a better distribution of Si particles. On the other hand, the T4 heat treatment which consists on a sequence of solution heat treatment and water-quenching, generally provides a microstructural modification resulting on improvement of mechanical properties. The resultant microstructure is characterized by spheroidised silicon particles which lead to less stress concentration when compared to flake or plate silicon morphologies. Besides, spheroidised Si particles and refined precipitates can provide a retard in cracks nucleation resulting in much improved casting ductility [15,16]. There is a lack of consistent studies and experimental reports on the effect of the modification in eutectic silicon morphology and temper on surface corrosion behavior of hypoeutectic Al–Si casting alloys. The present study aims to contribute to the understanding of the role of the eutectic modification and T4 heat treatment on mechanical properties and corrosion behavior. Modified and unmodified Al–9 wt%Si samples were cast under similar operational conditions in order to parameterize the cooling rates along solidification. Samples were also subjected to the T4 heat treatment in order to investigate the modifications on the microstructural arrangement and its corresponding effects on mechanical and corrosion properties. After metallographic procedures and microscopy analysis, the corrosion resistance was evaluated by both the electrochemical impedance spectroscopy technique and the Tafel extrapolation method carried out in a 0.5 M NaCl test solution at 25 ◦ C. The impedance parameters
were obtained from equivalent circuits provided by the ZView software. Experimental results include secondary dendrite arm spacings (λ2 ), corrosion potential (Ecorr ), corrosion rate (icorr ), polarization resistance (R1 ), capacitances values (ZCPE ), ultimate tensile strength (UTS), yield strength (YS) and Vickers hardness (HV). 2. Experimental procedure Al–9 wt%Si casting alloy samples were prepared from commercially pure metals: Al (99.72 wt%) and Si (99.58 wt%). The experimental assembly used to cast such alloy samples is shown in Fig. 1(a). The casting chamber was properly insulated by refractory walls in order to ensure a dominant horizontal and unidirectional heat flow during solidification. A low carbon steel chill was used at a normal environment temperature of about 25 ◦ C (initial mold temperature), with the heat-extracting surface being polished. The Al–9 wt%Si alloy was melted in an electric resistance-type furnace, flux-degassed with argon for 5 min and then poured into the casting chamber with a pouring temperature of about 10% above the liquidus temperature. In the case of the modified samples, simultaneously to the degassing procedure, the modifier amount was sprinkled against the liquid metal surface. The melt was then stirred in order to incorporate this modifier into the liquid which was allowed to remain in contact with the melt for approximately 8 min. The used chemical modifier was a commercial salt based on sodium fluoride (Modimil® ). According to the literature [11], 1% of the melt weight of Modimil® flux is enough to achieve the eutectic modification. Modified and unmodified as-cast samples were T4 heat-treated (solution heat treatment at 540 ◦ C for 6 h, water-quenched to 27 ◦ C and held at that temperature for 10 min) [17,18]. Fig. 1(b) and (c) exhibit the longitudinal and the transversal specimens that were taken from the ingots for optical metallographic examination and tensile, corrosion and hardness testings, respectively. The tensile testing was performed
Fig. 1. (a) Casting assembly, (b) location of specimens for metalographic procedures and (c) specimens for tensile, hardness and corrosion testings.
W.R. Os´orio et al. / Materials Chemistry and Physics 106 (2007) 343–349 according to the specifications of ASTM Standard E 8M. In order to provide mean values of ultimate and yield tensile strengths, three specimens were tested for each condition (as-cast, modified and heat-treated). The mean values of Vickers hardness were also obtained from triplicate specimens. All samples were carefully solidified under similar cooling conditions. The corresponding microstructures were revealed by grinding, polishing and etching selected regions of the samples. The chemical etchant used was a solution of 0.5%HF in distilled water. Image processing systems Neophot 32 (Carl Zeiss, Esslingen, Germany) and Leica Quantimet 500MC (Leica Imaging Systems Ltd., Cambridge, England) were used to acquire the microstructures. Secondary dendrite arm spacings (λ2 ) were measured by averaging the distance between adjacent side branches. Electrochemical impedance spectroscopy (EIS) and polarization tests were carried out in triplicate for modified, unmodified and heat-treated samples. The EIS tests were carried out in a 0.5 M (3.0%) NaCl solution at 25 ◦ C and a neutral pH range (6.5). A potentiostat coupled to a frequency analyzer system, a glass corrosion cell kit with a platinum counter electrode and a saturated calomel reference electrode (SCE) were used to perform the EIS tests. The working electrodes consisted of Al9 wt%Si alloy samples (unmodified, modified and heat-treated), which were positioned at the glass corrosion cell kit, leaving a circular 1 cm2 metal surface in contact with the electrolyte. The potential amplitude was set to 10 mV in open-circuit potential and the frequency range was set from 100 mHz to 100 kHz. The samples were further ground to a 600 grit SiC finish, followed by distilled water washing and air-drying before measurements. EIS measurements began after an initial delay of 10 min for the sample to reach a steady-state condition. Polarization tests were also carried out in a 0.5 M (3.0%) NaCl solution at 25 ◦ C using a potentiostat. The polarization curves were determined by stepping the potential at a scan rate of 0.2 mV s−1 from −250 mV (SCE) to +250 mV (SCE) related to open-circuit potential. Using an automatic data acquisition system, the potentiodynamic polarization curves were plotted and both corrosion rate and potential were estimated by the Tafel extrapolation method. Impedance parameters such as polarization resistance (R1 ) and capacitance values (ZCPE ) were obtained by using the equivalent circuit technique.
3. Results and discussion Fig. 2 shows the microstructures obtained by optical microscopy for unmodified, modified and T4 heat-treated Al–9 wt%Si samples. The unmodified sample (as-cast) has a microstructure characterized by an Al-rich dendritic matrix (␣Al phase) and a eutectic mixture in the interdendritic region formed by silicon particles, which are coarse and distributed in a plate-like morphology, set in an Al-rich phase as shown in Fig. 2. By analysing the sample in which sodium-modifier was added to the melt a considerable eutectic modification can be observed. Its microstructural features result in fine and fibrous eutectic silicon, as shown in Fig. 2. In contrast, the observed mean dendrite arm spacings (λ2 ) and the Al-rich dendritic matrix appear to be unaffected, as expected. Unmodified-T4 and modified-T4 samples are also shown in Fig. 2. The unmodified-T4 sample presents a slight modification in its microstructure which appears
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as short plate-like Si particles with a mean width similar to that of the unmodified sample (of about 5 m). On the other hand, the modified-T4 sample exhibits a significant globular shaped effect on silicon particles (mean diameter of about 3.0 m). 3.1. Different sample conditions and corresponding mechanical properties The different processes to which the Al9 wt%Si alloy samples have been subjected, i.e., chill casting (unmodified), modification (sodium fluoride addition) and T4 heat treatement (in both unmodified and modified samples) have provided variations in the resulting microstructures. It is also expected that such microstructural changes will induce differences on the final mechanical properties. Table 1 presents the experimental results of ultimate tensile strength (UTS), yield strength (YS: 0.2% proof stress), tensile elongation (δ) and Vickers hardness (HV) for unmodified and modified Al–9 wt%Si alloy samples in both treated and non-treated conditions. The results of UTS, YS and HV for unmodified and heat-treated conditions are consistent with those found in the literature concerning Al–Si commercial casting alloys [7,8,15,19]. Although the modified and unmodified as-cast samples present very similar secondary dendrite arm spacings, the addition of a eutectic modifier has increased UTS, YS and HV. Such tendency of enhance on UTS presented by the modified alloy sample seems to be associated with an increase of obstacles to slip due to the more extensive distribution of ␣-Al phase/fibrous silicon particles boundaries. It can also be observed in Table 1 that the elongation decreases with the eutectic modification. It is important to remark that the secondary dendritic spacing has an important role on mechanical properties, particularly in the UTS, as recently reported [2,3,7]. By comparing the corresponding results of unmodified (as-cast) and modified Al–9 wt%Si samples, it can be seen that the latter exhibits a significant improvement in UTS and HV. On the other hand, experimental results of UTS and YS for modified, unmodified heat-treated and modified heat-treated samples are very similar, i.e., the eutectic modification of Al–Si casting alloys can provide a better cost versus benefit relationship compared to that of heat-treated alloys, as far as tensile properties are concerned. 3.2. Different sample conditions and corresponding corrosion behavior In order to investigate the corrosion resistance of unmodified, modified and heat-treated Al–9 wt%Si alloy, a number
Table 1 Ultimate and yield tensile strengths, tensile elongation, Vickers hardness and secondary dendrite arm spacing of unmodified, modified and heat-treated samples Sample
UTS (MPa)
YS (MPa)
δ (%)
HV
λ2 (m)
Unmodified Modified Unmodified-T4 Modified-T4
138 (126–144) 212 (203–225) 209 (200–222) 232 (225–246)
64 (61–67) 87 (82–91) 94 (91–96) 96 (94–101)
8 (7–9) 4 (3–6) 7 (6–8) 6 (5–7)
70 (66–75) 85 (83–86) 92 (88–96) 94 (93–96)
32 (28–36) 34 (29–38) 35 (27–39) 34 (29–37)
Numbers in parentheses represent maximum and minimum values.
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Fig. 2. Typical microstructures of unmodified, modified and T4 heat-treated Al–9 wt%Si samples and the corresponding secondary dendrite arm spacings.
of samples were subjected to corrosion tests. The corrosion behavior of Al–Si foundry alloys have been examined and compared previously in the literature only by polarization methods (potentiostatic or potentiodynamic) [17]. Although polarization methods can provide important information concerning the corrosion behavior, in the present study in addition to the impedance parameters obtained from experimental EIS diagrams, an equivalent circuit analysis has also been encompassed in order to permit a more complete spectrum of results to be attained.
Fig. 3 shows a comparison of experimental results of corrosion tests carried out in a 0.5 M NaCl solution concerning EIS diagrams and potentiodynamic polarization curves for unmodified, modified and heat-treated conditions. Impedance parameters obtained from the equivalent circuit analysis corresponding to each EIS diagram, corrosion potential and current density are shown in Table 2. Fig. 3(b) shows potentiodynamic polarization curves for modified, unmodified and heat-treated Al–9 wt%Si alloy
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Table 2 Corrosion potential, current density and impedance parameters for unmodified, modified and heat-treated Al–9 wt%Si alloy samples in a 0.5 M NaCl test solution Parameters
Unmodified (as-cast)
Modified
Unmodified and heat-treated
Modified and heat-treated
Ecorr (mV) (corrosion potential) Icorr (A cm−2 ) (corrosion rate)
−722 0.98
−755 3.43
−798 1.35
−825 1.75
Rel ( cm−2 ) ZCPE(1) (106 1 s−n cm2 ) ZCPE(2) (106 1 s−n cm2 ) n1 n2 R1 ( cm−2 ) R2 ( cm−2 ) χ2
20.13 40.56 (±2.8) 835.17 (±5.3) 0.80 0.75 5280 2 × 1015 56 × 10−4
19.39 50.71 (±3.1) 547.58 (±45.4) 0.82 0.85 2380 2 × 1015 23 × 10−4
19.27 26.92 (±2.8) 180.63 (±19.4) 0.89 0.97 4052 3366 11 × 10−3
18.98 34.93 (±3.6) 191.43 (±23.1) 0.84 0.68 4986 3179 49 × 10−4
samples. It can be observed a current density (Icorr ) of about three times higher for the modified sample (around 3.43 A cm−2 ) compared to that of the unmodified (as-cast) sample (0.98 A cm−2 ). The modified sample has also presented a corrosion potential (Ecorr ) slightly more active (or less-noble) than the unmodified one.
Fig. 3. Experimental results for unmodified, modified and heat-treated Al–9 wt%Si alloy samples in: (a) EIS tests (Bode and Bode-phase diagrams) and (b) potentiodynamic polarization curves.
A comparison between the heat-treated samples permits to observe that both the unmodified and the modified samples exhibit similar Icorr values which are intermediate to those Icorr values of the untreated samples (unmodified and modified). Although the T4 heat treatment has provided a displacement on the corrosion potential toward a less-noble potential, it is possible to affirm that such heat treatment provides a better corrosion resistance than that provided by modification, as shown in Fig. 3(b) and Table 2. An analogue discussion can also be taken when analyzing impedance and equivalent circuit quantitative results, as shown in Table 2. The fitting quality for experimental results and those calculated and interpreted by the ZView software is represented by chi-squared (χ2 ) values [20]. The electronic equivalent circuit consists of a capacitance component (ZCPE1 ) in parallel to series resistors R1 and R2 and other capacitance component (ZCPE2 ) in parallel to R2 . The impedance parameter of the equivalent circuit Rel corresponds to the electrolyte resistance which in Bode plot is expressed in a high frequency limit (F > 1 kHz). It is proposed that R1 and R2 correspond to the polarization resistances of the samples (modified and unmodified conditions) and oxide film, respectively. These two parameters represent the sum of existing resistances in low frequency limit (F < 1 Hz). The parameters n1 and n2 are correlated to the phase angle, varying between −1 and 1. ZCPE(1) and ZCPE(2) correspond to the capacitances (1 Hz < F < 1 kHz) of the experimentally examined samples and their oxide layer formation, respectively. In the literature ZCPE generally denotes the impedance of a phase element as ZCPE = [C(jw)n ]−1 [21,22]. The used equivalent circuit is shown in Fig. 4. Analyses of the impedance parameters, shown in Table 2, confirm the observations concerning the qualitative aspects of
Fig. 4. Proposed equivalent circuit for modeling impedance data for all Al–9 wt%Si alloy samples.
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Fig. 5. Typical SEM dendritic microstructure of Al–Si hypoeutectic alloys: (a) general view of the dendritic arrangement and (b) detail of secondary arm spacings and interdendritic region with localized deformation points between Al–rich phase and Si particles (white arrows).
experimental EIS results. It can be seen that, the unmodified Al–9 wt%Si as-cast sample has a capacitance ZCPE(1) of about 40 × 106 1 s−n cm2 (or 40 F cm−2 ) and a polarization resistance R1 of about 5280 cm−2 . On the other hand, the modified Al–9 wt%Si sample presents a higher capacitance and a lower polarization resistance than the unmodified sample, as shown in Table 2. Comparing unmodified heat-treated and modified heattreated samples a similarity between capacitances (ZCPE(1) ) and polarization resistances (R1 ) can be observed. The parameters ZCPE(1) vary from 27 to 35 F cm−2 and R1 range from 4 to 5 k cm−2 . This indicates that the corrosion behavior is very similar for both samples and being located in an intermediate position when compared with those of unmodified and modified samples. It can also be observed that the T4 heat treatment provides smaller capacitances than the results presented by the untreated samples. The parameters R2 indicate that less homogeneous oxide films have grown on heat-treated samples compared to those of the untreated samples. The afore-mentioned experimental and calculated parameters provide sufficient information which permits to conclude that the use of the sodium-based modifier has provoked a deleterious effect on the corrosion behavior of an Al–9 wt%Si casting alloy. Such reduction on corrosion resistance presented by the modified alloy sample seems to be associated with an increase of boundaries (between the Al–rich (␣) phase and Si particles during growth from the melt) when compared with the unmodified alloy. It seems that the modified eutectic regions are more susceptible to the corrosion action, due to localized deformation regions at such boundaries, as recently reported [8]. Fig. 5 shows a typical SEM dendritic microstructure of Al–Si hypoeutectic alloys showing the localized deformation between Al and Si particles. On the other hand, it seems that the level of localized deformation between Al and Si particles has decreased due to the spheroidizing effect on silicon particles provided by the T4 heat treatment. This can explain the recovery on corrosion resistance for the modified heat-treated sample, as shown by the results of Table 2.
Although the addition of eutectic modifiers to Al–Si alloys is widely applied by major foundry industries as an alternative way to produce components with better mechanical properties or to increase the fluidity of such alloys, the corrosion resistance can be significantly affected as previously discussed. It seems that when a compromise between good corrosion resistance and good mechanical properties is essential, an adequate surface treatment must be provided in order to improve the surface corrosion resistance of modified Al–Si alloys. In order to achieve a desired level of final properties, the correlation of microstructural parameters with corrosion behavior and mechanical properties can be very useful for planning the manufacturing process. 4. Conclusions The following main conclusions can be drawn from the present experimental investigation: (a) The experimental values of ultimate tensile strength (UTS), yield strength (YS), Vickers hardness (HV) for an Al–9 wt%Si alloy have shown that the addition of a sodium fluoride eutectic modifier increases UTS, YS and HV. It seems that such tendency of increase on mechanical properties is associated only to the eutectic modification, since the measured dendritic spacings were very similar in any condition experimentally examined. (b) Experimental results of UTS for unmodified, modified and modified heat-treated samples are very similar, i.e., the level of mechanical properties attained by the eutectic modification of an Al–9 wt%Si casting alloy can provide a better cost versus benefit relationship compared to that of heat-treated alloys. (c) The increase on UTS values after the T4 heat treatment can be attributed to the globular shaped effect on silicon particles provided by such treatment that leads to less stress concentration when compared to flake or plate silicon morphologies.
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