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On influence of Ti and Sr on microstructure, mechanical properties and quality index of cast eutectic Al–Si–Mg alloy
Materials and Design 32 (2011) 1865–1871
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
On influence of Ti and Sr on microstructure, mechanical properties and quality index of cast eutectic Al–Si–Mg alloy Sergio Haro-Rodríguez a, Rafael E. Goytia-Reyes a, Dheerendra Kumar Dwivedi b,⇑, Víctor H. Baltazar-Hernández a, Horacio Flores-Zúñiga a, María J. Pérez-López c a b c
Maestría en Procesos y Materiales, Unidad Académica de Ingeniería, Universidad Autónoma de Zacatecas, Av. López Velarde 801, Zacatecas, Zac, CP 98000, Mexico Department of Mechanical & Industrial Engineering, Indian Institute of Technology Roorkee, Roorkee 24667, India Departamento de Metal-Mecánica, Instituto Tecnológico de Saltillo, Saltillo, Coahuila, Mexico
a r t i c l e
i n f o
Article history: Received 29 August 2010 Accepted 6 December 2010 Available online 13 December 2010 Keywords: C. Casting E. Mechanical G. Metallography
a b s t r a c t In this paper, the influence of grain refinement (Al–5Ti–1B) and modification (Al–10Sr) on the microstructure, the morphology of iron-containing needle shape beta phase and mechanical properties of cast Al–12Si alloy has been reported. The grain refinement and modification transforms the needle shape morphology of the b-phase into Chinese script. Grain refinement and modification by addition of Ti and Sr respectively increased the quality index (Q) of the casting. Addition of 0.06 wt.% Sr resulted in a best combination of mechanical properties such as ultimate tensile strength and ductility. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Grain refinement and modification of cast Al–Si alloys improve the mechanical properties by refining the micro-constituents namely a-Al and eutectic silicon particles [1,2]. Grain refinement of cast aluminum alloys is usually achieved by addition of Al–Ti, Al–Ti–B, Al–B and Al–TiC in form of master alloys [3–5]. Al–Ti refiners generally contain 3–10 wt.% of Ti. Al–Ti–B refiners additionally have B in such a way that Ti/B ratio is in range of 5–50 [6,7]. The modification of Al–Si alloys describes the transformation of needle shape silicon into fine fibrous form silicon embedded in Al-matrix. The modification is generally carried out by adding modifiers like sodium, calcium and strontium. Addition of strontium in a range of 0.015–0.05 wt.% is a common standard industrial practice for acceptable modification [7,8]. The commercial-grade Al alloy usually contains iron either in the form of impurity or alloying element. The iron present in Al– Si alloy forms needle shape intermetallic compound of Al–Fe–Si called b-phase. The morphology of b-phase is determined by the alloy composition, cooling rate during solidification and the heat treatment [8]. The plate shape iron containing compounds in Al– Si alloys have been reported as monoclinic b-Al5FeSi and these are considered detrimental to the mechanical properties such as notch toughness and ductility of the alloy [9,10]. Al–Fe–Si intermetallic compound is also found in form of blocky shape particle and Chinese script called a-phase (Al8Fe2Si) [9,11,12]. The a-phase is ⇑ Corresponding author. Tel.: +91 1332 285665; fax: +91 1332 285826. E-mail address: [email protected] (D.K. Dwivedi). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.12.012
not considered to be detrimental for the mechanical properties of the alloy as alpha phase causes low stress concentration at the particle–matrix interface owing to its morphology [11,12]. It has been reported [13] that the addition of Sr in Al–Si alloy refines platelets of b-phase (Al–Fe–Si) present in eutectic matrix called secondary b-phase while the morphology of the platelets of b-phase trapped between the a-Al dendrites called primary b-phase is not appreciably affected. The Ti addition produces opposite effect on the morphology of primary and secondary platelets of b-phase (Al–Fe–Si) intermetallic compounds. Combined addition of both Ti and Sr results in moderate effect on morphology of the secondary b-phase. Addition of Sr in the presence of transition elements such as Mn promotes favorable ‘‘Chinese script’’ morphology of a-phase [14]. Literature survey revealed that the effect of grain refinement and modification on fraction and morphology of Al–Fe–Si intermetallic compounds in cast alloys especially under low cooling conditions has not been studied systematically. Therefore, in present work, attempts were made to study the influence of Ti and Sr addition in cast Al–12Si–2.5Cu–0.4Mg alloy on the morphology of b-phase Al–Fe–Si compound, mechanical properties and quality (Q) of the casting under low cooling conditions. 2. Experimental procedure Chemical composition of the commercial-grade Al–12Si alloy used in the present study is shown in Table 1. The Al–12Si alloy was melted in a silicon carbide crucible using an electric furnace and was kept at 720 °C (±5 °C). The grain refinement and
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Table 1 Chemical composition of Al–12Si base alloy. Element
Si
Fe
Cu
Mn
Mg
Zn
Ti
Ni
wt.%
12.53
0.44
2.95
0.010
0.34
0.020
0.08
2.13
Table 2 Details of Ti and Sr addition. Condition
Ti (wt.%)
Sr (wt.%)
1 2 3 4 5
0.00125 0.015 – –
– – 0.04 0.06
modification of the alloy were done separately by adding Al–5Ti– 1B and Al–10Sr master alloys. The concentration of Ti and Sr added in cast Al–12Si alloy is shown in Table 2. Degassing was done using argon supplied at a flow rate of 14.6 L/h for 20 min and porous plug method. Molten alloy was poured at 710 °C into the shell sand mould having a size of 25 mm diameter and 150 mm length. The mould was preheated to a temperature of 200 °C before pouring the molten alloy. Samples for microscopy were polished using standard metallographic procedures. Polished samples were etched using a solution of K4[Fe (CN)6] (10 g), NaOH (5 g) and water (60 ml). Microstructure study was done using optical microscope and scanning electron microscope (SEM). To study the distribution of various elements in microstructure, SEM and energy dispersive X-ray analysis were carried out. The secondary electron (SE) and backscattered electron (BE) images were recorded to distinguish different phases in cast Al–Si alloy. XRD analysis was carried out to study the effect of Ti and Sr addition on transformation of b-phase particles into a-phase. The tensile testing of as cast, refined and modified Al–Si alloy was carried out according to ASTM B 557-06 [17]. All tensile tests were conducted at constant strain rate of 1 mm min 1 using a universal testing machine of 100 KN capacity. Rockwell B hardness was measured at five different locations in entire cross-section of the sample using 600 N load. The quality index of the castings was measured using approach first proposed by Drouzy et al. [15] Q = rb + klog d, where Q is the quality index, rb is ultimate tensile strength, d is fracture elongation and k is a constant and which was subsequently modified by Samuel et al. [16] as Q = rb + 150log d for Al–Si alloys. 3. Results and discussion 3.1. Microstructural study The microstructure of the as cast Al–12Si base alloy and that of grain refined and modified alloy is shown in Fig. 1a–c. It can be observed that the microstructure of cast alloys (without refiner and modifier) consists of fine eutectic Si particles and cuboid shape primary silicon particles in matrix of a-Al, besides needle shape bphase iron rich intermetallic compounds (Fig. 1a). The formation of primary Si particles in the structure of eutectic Al–12Si alloy is primarily due to low cooling rate conditions experienced by molten metal in preheated shell sand mould during solidification. It is known that cooling conditions affect the composition for the eutectic reaction. The length of needlelike b-phase particles was found to vary from 150 to 400 lm. The effect of grain refiner and modifier on the microstructure of Al–12Si base alloy is shown in Fig. 1b and c. It can be observed that in general Ti addition transforms the long needle shaped b-phase
particles into fine, fragmented, irregular shape particles while Sr addition results in fine and largely spherical shape morphology of b-phase. The effect of amount of Ti addition (0.00125 and 0.015 wt.%) in alloy is shown in Fig. 2a and b. It was observed that size and fraction of needlelike b-phase reduced appreciably with addition of Al– Ti–B master alloy. However, closer look of the microstructure at high magnification exhibited the transformation of needle shape b-phase intermetallic compounds into Chinese script morphology which is generally found in form of Al15(MnFe)3Si2.[8]. No major distinguishable feature in microstructure of alloy with 0.015 wt.% Ti (Fig. 2b) as compared to that obtained with 0.00125 wt.% Ti could be observed (Fig. 2a). The effect of amount of Sr addition (0.04 and 0.06 wt.%) in alloys is shown in Fig. 3a and b. It was observed that in general Sr addition modified needle shape b-phase particles into more favorable fine nodular and blocky shape morphology. It can be noticed that the addition of 0.04 wt.% Sr in Al– 12Si alloy resulted in fine blocky shape b-phase particles (Fig. 3a) while 0.06 wt.% Sr addition resulted in both blocky and Chinese script morphology (Fig. 3b). Effects of Ti and Sr addition in cast Al–Si alloy on iron rich bphase needle observed in this work are in agreement with earlier published literature [18–22]. It has been reported that increased solidification rate [18], addition of strontium in presence of transition elements such as manganese [14] promotes the development of the a-phase with Chinese script morphology, at expense of the b-phase. Suárez et al. [13] also reported that b-phase is refined with Sr addition and with high Sr/Ti ratio. 3.2. SEM–EDS analysis SEM–EDAX analysis of the micro-constituents of the base alloy without grain refinement and modification was carried out to show the distribution of Si and Fe in the form of elemental mappings. The maps confirmed the presence of Al–Fe–Si intermetallic compounds and their needle shaped morphology as shown in Fig. 4a–c. Further, the SEM and EDAX analysis allowed observing the relationship between eutectic silicon particles and iron-based intermetallics. The backscattered electrons image (Fig. 5), showed the nucleation of primary silicon particles on b-phase (in black circle) besides intermixed iron-base intermetallic, b-phase, with silicon particles (in white circles). These observations are in agreement with published literature [13,19]. The nucleation of silicon particles on b-phase suggests that the b-phase is formed first at high temperature during the solidification and then acted as nucleants for Si particles [20,21], while intermixed acicular-shaped b-precipitates with silicon needles in eutectic, might be formed due to rejection of the iron into residual liquid from growing dendrites of a-Al during solidification. 3.3. XRD analysis In general, intermetallic phases are identified according to their response to a chemical attack, morphology and/or energy dispersive X-ray spectra [22]. X-rays diffraction, convergent beam electron diffraction (CBED) and electron probe microanalysis (EPMA) are used to identify these phases [9,23,24]. Kral et al. [25] have pointed out that some confusion persists in the literature regarding the accurate identification of the intermetallics in Al–Si casting alloys and using EBSD technique they also identified a-phase as cubic Al19(Fe,Mn)5Si2. Electron backscatter diffraction (EBSD) patterns of the b-phase are consistent with tetragonal Al3(Fe,Mn)Si2. It was also noted that the a-phase and b-phase might not clearly exhibit dendritic (Chinese script) or plate-shape morphology in alloys with eutectic modifiers.
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(a)
(b)
100 μm 100 μm
(c)
100 μm
Fig. 1. Optical micrographs of: (a) alloy without refiner and modifier, (b) alloy with 0.00125 wt.% Ti and (c) alloy with 0.06 wt.% Sr.
(a)
(b) Al matrix
Si
eutectic Si Chinese script phase Fig. 2. Effects of addition of Ti on microstructure of the Al–12Si base alloy: (a) 0.00125 wt.% Ti and (b) 0.015 wt.% Ti addition.
The XRD patterns of base alloys and modified/refined Al–12Si alloy are shown in Fig. 6a–c. XRD pattern of Al–12Si base alloy revealed several peaks, between 43 and 47° (2h values), with very low intensities (2–4% of intensity respect to principal peak) which might be corresponding to different intermetallics. The principal peak of b-phase (Al5FeSi) is located at 46.7°, as shown in Fig. 6a. Unfortunately this could not be confirmed by the presence of the second largest peak located at 17.05°, as XRD analysis in present work was carried out in range of 20–100° only. XRD for patterns for grain refined and modified alloys were found similar to that of as cast un-modified and unrefined alloy. The XRD pattern of grain refined (Fig. 6b) and modified (Fig. 6c) alloy showed almost
the similar XRD pattern behavior i.e. without any peak corresponding to b-phase, therefore it was found difficult to confirm any bphase to a-phase transformation by XRD analysis. This may be due to very low fraction (less than 2%) of iron rich b or a-phase in Al–12Si alloys. 3.4. Mechanical properties The effect of Ti addition (as Al–5Ti–1B master alloy) on the ultimate tensile strength (UTS) and the elongation (%) of Al–12Si alloy are shown in Fig. 7. It can be observed that addition of small amount of Ti (0.00125 wt.%) increased the UTS from 93 MPa to
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Fig. 3. Effects of addition of Sr on microstructure of the Al–12Si base alloy: (a) 0.04 wt.% Sr and (b) 0.06 wt.% Sr addition.
(a)
(b)
200 µm
(c)
Si
Fe
Fig. 4. Backscattered electrons image and elemental maps of as cast Al–12Si alloy (a) Needles and platelets morphologies (bright) of the iron-base intermetallics and irregular grey colored Si-eutectic particles and Si-cuboids (arrows). EDAX elemental maps of (b) Si and (c) Fe.
143 MPa, while the ductility increased from 2.7% to 4.5%. Increase in UTS and elongation of as cast alloy can be attributed to the refinement of Al–Fe–Si intermetallic compound and the change in morphology of the needle shape b-phase intermetallic com-
pounds into blocky shape and Chinese script form as shown in Fig. 2. Further, it can be noticed that affect of Ti addition above 0.0125 wt.% on the ultimate tensile strength is marginal while ductility is reduced. Reduction in ductility might be due to presence of
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0.06% continuously increases the tensile strength while addition of Sr above 0.04% results in marginal decrease in ductility (% elongation) as shown in Fig. 8. It can also be noted that Sr addition produced higher ultimate tensile strength and ductility as compared to that of Ti addition, this is due to two factors (1) eutectic silicon modification and (2) morphology changes of b-phase into Chinese script phase. The effect of Ti and Sr additions on hardness is shown in Fig. 9. It can be observed that grain refiner and modifier addition reduced the hardness. Addition of 0.04 wt.% Sr to base metal, reduces the hardness from 62 to 51 HRB principally due to the modification of eutectic silicon and change of b-phase morphology. Similarly addition of the 0.00125 wt.% Ti also reduces the hardness from 62 to 55 HRB, due to change in morphology of b-phase. 3.5. Quality index
200 µm Fig. 5. Backscattered electrons image showing the morphology and distribution of Si-eutectic particles and iron-base intermetallics in as cast Al–12Si alloy.
some un-dissolved grain refiners (Al–Ti–B) acting as a site for stress concentration and so easy nucleation of cracks and voids during tensile test. Similarly, addition of 0.06 wt.% Sr increased the UTS from 93 MPa to 153 MPa and elongation from 2.7% to 5.3%. It can also be seen that increase in amount of Sr addition in alloy from 0 to
(a)
The quality index ‘‘Q’’ of the alloy castings in as cast, grain refined and modified conditions was calculated using approach proposed by Samuel et al. [16]. Table 3 shows alloy condition and influence of grain refinement and modification on the quality index. It can be observed that in general addition of grain refiner and modifier improved the quality index of the alloy. Moreover, the influence of modification on the quality index was found to be more than grain refinement. Low quality index of base alloy is due to poor UTS and low elongation; caused by needle shape eutectic Si particles and needle shape hard and brittle b-phase intermetallic compounds. Addition of Ti and Sr refines a-Al, eutectic silicon and b-phase particles and therefore improves the mechanical properties (UTS and % elonga-
(b)
β
Al
Si
Al Al Si
Si
Al
(c) Phases:
Al
Al Si
Al
β-(Al5FeSi) Al
Al
Si Si Si
Fig. 6. XRD patterns of: (a) Al–12Si base alloy, (b) with addition of 0.00125 wt.% Ti and (c) with addition of 0.06 wt.% Sr.
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Fig. 7. Effects of addition of Ti on the UTS and the elongation of the Al–12Si alloy.
Fig. 8. Effect of Sr addition on the UTS and the elongation of the Al–12Si alloy.
Fig. 9. Effect of Ti and Sr addition on the HRB hardness of the Al–12Si alloy.
Table 3 Influence of grain refiner and modifier addition on quality index. Condition
Ti (wt.%)
Sr (wt.%)
Quality index (Q)
1 2 3 4 5
– 0.00125 0.015 – –
– – – 0.04 0.06
158.79 242.05 234.72 254.04 261.34
tion) of the alloy. Sr modification not only refines eutectic Si particles but also b-phase particles, which have more significant effect on mechanical properties of cast Al–Si alloys while the addition of Ti refines b-phase particles and a-Al. It is known that eutectic
Si particles size and shape has more significant effect on mechanical properties than the a-Al. Hence, it can be concluded that Ti and Sr addition improves quality index. Higher quality index of Sr modified alloy than Ti refined alloy can be explained in the same line. 4. Conclusions From the present investigation, following conclusions can be drawn: (1) Grain refinement of alloy with optimum amount of Al–Ti–B master alloys (0.00125 wt.% Ti) in the base Al–12Si alloy reduces the size of Al–Fe–Si intermetallic compounds and the transforms the morphology of the needlelike b-phase intermetallic compound into blocky and Chinese script form.
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(2) Similarly, modification of the alloy with optimum amount of Al–Sr master alloys (0.06 wt.% Sr) in the base metal Al–12Si alloy refines eutectic Si particles and changes the needlelike b-phase into fine spherical shape morphology and Chinese script shape. (3) Addition of Al–Ti–B and Al–Sr master alloy promotes the development of Chinese script morphology at an expense of the needlelike particles of b-phase which in turn improves the tensile strength and elongation of the cast Al–12Si alloy. (4) Optimum addition of grain refiner (Al–Ti–B) and modifier (Al–Sr) results in significant increase in tensile strength and ductility. (5) Grain refinement and modification of the Al–12Si alloy increased the quality index (Q). Moreover, the effect of modification on quality index was more than grain refinement. (6) In view of microstructure, mechanical properties and quality index, it is recommended to add 0.06 wt.% Sr or 0.00125 wt.% Ti in Al–12Si alloy to achieve best combination of tensile strength and ductility of the alloy. Acknowledgements The authors acknowledge the support and facilities provided by CONACyT, UAZ and UANL from México also to IITR Roorkee and Department of Science and Technology (vide sanction letter DST/ INT/Mex/RPO-07/2008 dated 08-12-2009) from India; PROMEPMéxico by support the collaborative network: ‘‘Surface phenomena, deterioration and aging of materials’’. Thanks to CIMAV S. C., México, for their support in carrying out the electron microscopy also to Raúl Ochoa for his collaboration. Special thanks to Julián Ramírez, Enrique A. López, Angel González and Ricardo Leyva, technical staff. Author gratefully thanks to Rocío Estrella, Paulo Sergio, Brisa Marian and Aldo Enrico for their support and advice. References [1] Haque MM et al. In: Sastry DH et al., editors. In: Proceeding of international conference on aluminum. INCAL 98, 11–13 Febraury, New Delhi, vol. 2, 1998. p. 299. [2] Smart RF. Metallurgical aspects of Al–Si eutectic piston alloys. Br Foundry Man 1971;64:430–8. [3] Johnsson M, Bäckerud L. Nucleants in grain refined aluminium after addition of Ti- and B-containing master alloys. Z Metallkd 1992;83:774–80.
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[4] Wangli BX. Refining effect of boron on hypoeutectic Al–Si alloys. J Mater Sci Technol 2000;16:517–20. [5] Greer AL et al. Modelling of inoculation of metallic melts: application to Grain Refinement of aluminium by Al- Ti –B. Acta Mater 2000;48:2823–35. [6] Aluminum and Aluminium alloys. ASM Specialty Handbook. ASM International; 1993. p. 221–24. [7] Ray Cook. Metallurgy of aluminum. London & Scandinavian Metallurgical Co. Limited; 1998. [8] Aluminum Casting Technology. Des Plaines, AFS, 2nd. ed. USA: Illinois; 2001. p. 28–323. [9] Mondolfo LF. Aluminum alloys, structure and properties. London, Butterworths and Co. Ltd.; 1976. [10] Haizhi Ye. An overview of the development of Al–Si-alloy based material for engine applications. J Mater Eng Perform 2003;12:288–97. [11] Eklund JE. On the effects of impurities on solidification and mechanical behavior of primary and secondary commercial purity aluminum and aluminum alloys, PhD. thesis. Helsinki: University of Technology; June 1993. [12] Taylor JA, Schaffer GB, St John DH. The role of iron in the formation of porosity in Al–Si–Cu-based casting alloys. Metall Mater Trans 1999;30A:1643–62. [13] Suárez-Peña B, Asensio-Lozano J. Influence of Sr modification and Ti grain refinement on the morphology of Fe-rich precipitates in eutectic Al–Si die cast alloys. Scripta Mater 2006;54:1543–8. [14] Couture A. Iron in aluminum casting alloys: a literature survey. AFS Int Cast Metals J 1981;6:9–17. [15] Drouzy M, Jacob S, Richard M. Interpretation of tensile results by means of quality index and probable yield strength. AFS Int Cast Metals J 1980;5:43–50. [16] Samuel AM, Gauthier J, Samuel FH. Microstructural aspects of the dissolution and melting of Al2Cu phase in Al–Si alloys during solution heat treatment. Metall Mater Trans A 1996;27:1785–98. [17] Standard methods of tension testing wrought and cast aluminum and magnesium alloy products. B557-84. Annual Book of ASTM Standards. vol. 02.2. Philadelphia, PA: ASTM; 1986. p. 506. [18] Vorren O, Evensen JE, Pedersen TB. Microstructure and mechanical properties of Al–Si (Mg) casting alloys. AFS Trans 1984;92:459–66. [19] Martínez EJ, Lacaze DJ, Cisneros y M, Valtierra S. Efecto del Estroncio en las Temperaturas de Reacción Eutécticas y Microestructuras de Solidificación de una aleación Al–Si tipo A319. Rev Fac Ing – Univ Tarapacá 2004;12:21–6. [20] Narayanan LA, Samuel FH, Gruzleski JE. Crystallization behavior of ironcontaining intermetallic compounds in 319 aluminum alloy. Metall Mater Trans A 1994;25:1761–73. [21] Wen KY, Hu W, Gottstein G. Intermetallic compounds in thixoformed aluminium alloy A356. Mater Sci Technol 2003;19(6):762–8. [22] Samuel FH, Samuel AM, Doty HW, Valtierra S. Decomposition of Feintermetallics in Sr-modified cast 6XXX type aluminum alloys for automotive skin. Metall Mater Trans A 2001;32:2061–75. [23] Munson D. A clarification of the phases occurring in aluminium–rich aluminium–iron–silicon alloys, with particular reference the ternary phase a-AlFeSi. J Inst Metals 1967;95:217–9. [24] Qian Ma, Taylor JA, Yao JY, Couper MJ, St John DH. A practical method for identifying intermetallic phase particles in aluminium alloys by electron probe microanalysis. J Light Metals 2001;1:187–93. [25] Kral MV, McIntyre HR, Smillie MJ. Identification of intermetallic phases in a eutectic Al–Si casting alloy using electron backscatter diffraction pattern analysis. Scripta Mater 2004;51:215–9.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
On influence of Ti and Sr on microstructure, mechanical properties and quality index of cast eutectic Al–Si–Mg alloy Sergio Haro-Rodríguez a, Rafael E. Goytia-Reyes a, Dheerendra Kumar Dwivedi b,⇑, Víctor H. Baltazar-Hernández a, Horacio Flores-Zúñiga a, María J. Pérez-López c a b c
Maestría en Procesos y Materiales, Unidad Académica de Ingeniería, Universidad Autónoma de Zacatecas, Av. López Velarde 801, Zacatecas, Zac, CP 98000, Mexico Department of Mechanical & Industrial Engineering, Indian Institute of Technology Roorkee, Roorkee 24667, India Departamento de Metal-Mecánica, Instituto Tecnológico de Saltillo, Saltillo, Coahuila, Mexico
a r t i c l e
i n f o
Article history: Received 29 August 2010 Accepted 6 December 2010 Available online 13 December 2010 Keywords: C. Casting E. Mechanical G. Metallography
a b s t r a c t In this paper, the influence of grain refinement (Al–5Ti–1B) and modification (Al–10Sr) on the microstructure, the morphology of iron-containing needle shape beta phase and mechanical properties of cast Al–12Si alloy has been reported. The grain refinement and modification transforms the needle shape morphology of the b-phase into Chinese script. Grain refinement and modification by addition of Ti and Sr respectively increased the quality index (Q) of the casting. Addition of 0.06 wt.% Sr resulted in a best combination of mechanical properties such as ultimate tensile strength and ductility. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Grain refinement and modification of cast Al–Si alloys improve the mechanical properties by refining the micro-constituents namely a-Al and eutectic silicon particles [1,2]. Grain refinement of cast aluminum alloys is usually achieved by addition of Al–Ti, Al–Ti–B, Al–B and Al–TiC in form of master alloys [3–5]. Al–Ti refiners generally contain 3–10 wt.% of Ti. Al–Ti–B refiners additionally have B in such a way that Ti/B ratio is in range of 5–50 [6,7]. The modification of Al–Si alloys describes the transformation of needle shape silicon into fine fibrous form silicon embedded in Al-matrix. The modification is generally carried out by adding modifiers like sodium, calcium and strontium. Addition of strontium in a range of 0.015–0.05 wt.% is a common standard industrial practice for acceptable modification [7,8]. The commercial-grade Al alloy usually contains iron either in the form of impurity or alloying element. The iron present in Al– Si alloy forms needle shape intermetallic compound of Al–Fe–Si called b-phase. The morphology of b-phase is determined by the alloy composition, cooling rate during solidification and the heat treatment [8]. The plate shape iron containing compounds in Al– Si alloys have been reported as monoclinic b-Al5FeSi and these are considered detrimental to the mechanical properties such as notch toughness and ductility of the alloy [9,10]. Al–Fe–Si intermetallic compound is also found in form of blocky shape particle and Chinese script called a-phase (Al8Fe2Si) [9,11,12]. The a-phase is ⇑ Corresponding author. Tel.: +91 1332 285665; fax: +91 1332 285826. E-mail address: [email protected] (D.K. Dwivedi). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.12.012
not considered to be detrimental for the mechanical properties of the alloy as alpha phase causes low stress concentration at the particle–matrix interface owing to its morphology [11,12]. It has been reported [13] that the addition of Sr in Al–Si alloy refines platelets of b-phase (Al–Fe–Si) present in eutectic matrix called secondary b-phase while the morphology of the platelets of b-phase trapped between the a-Al dendrites called primary b-phase is not appreciably affected. The Ti addition produces opposite effect on the morphology of primary and secondary platelets of b-phase (Al–Fe–Si) intermetallic compounds. Combined addition of both Ti and Sr results in moderate effect on morphology of the secondary b-phase. Addition of Sr in the presence of transition elements such as Mn promotes favorable ‘‘Chinese script’’ morphology of a-phase [14]. Literature survey revealed that the effect of grain refinement and modification on fraction and morphology of Al–Fe–Si intermetallic compounds in cast alloys especially under low cooling conditions has not been studied systematically. Therefore, in present work, attempts were made to study the influence of Ti and Sr addition in cast Al–12Si–2.5Cu–0.4Mg alloy on the morphology of b-phase Al–Fe–Si compound, mechanical properties and quality (Q) of the casting under low cooling conditions. 2. Experimental procedure Chemical composition of the commercial-grade Al–12Si alloy used in the present study is shown in Table 1. The Al–12Si alloy was melted in a silicon carbide crucible using an electric furnace and was kept at 720 °C (±5 °C). The grain refinement and
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Table 1 Chemical composition of Al–12Si base alloy. Element
Si
Fe
Cu
Mn
Mg
Zn
Ti
Ni
wt.%
12.53
0.44
2.95
0.010
0.34
0.020
0.08
2.13
Table 2 Details of Ti and Sr addition. Condition
Ti (wt.%)
Sr (wt.%)
1 2 3 4 5
0.00125 0.015 – –
– – 0.04 0.06
modification of the alloy were done separately by adding Al–5Ti– 1B and Al–10Sr master alloys. The concentration of Ti and Sr added in cast Al–12Si alloy is shown in Table 2. Degassing was done using argon supplied at a flow rate of 14.6 L/h for 20 min and porous plug method. Molten alloy was poured at 710 °C into the shell sand mould having a size of 25 mm diameter and 150 mm length. The mould was preheated to a temperature of 200 °C before pouring the molten alloy. Samples for microscopy were polished using standard metallographic procedures. Polished samples were etched using a solution of K4[Fe (CN)6] (10 g), NaOH (5 g) and water (60 ml). Microstructure study was done using optical microscope and scanning electron microscope (SEM). To study the distribution of various elements in microstructure, SEM and energy dispersive X-ray analysis were carried out. The secondary electron (SE) and backscattered electron (BE) images were recorded to distinguish different phases in cast Al–Si alloy. XRD analysis was carried out to study the effect of Ti and Sr addition on transformation of b-phase particles into a-phase. The tensile testing of as cast, refined and modified Al–Si alloy was carried out according to ASTM B 557-06 [17]. All tensile tests were conducted at constant strain rate of 1 mm min 1 using a universal testing machine of 100 KN capacity. Rockwell B hardness was measured at five different locations in entire cross-section of the sample using 600 N load. The quality index of the castings was measured using approach first proposed by Drouzy et al. [15] Q = rb + klog d, where Q is the quality index, rb is ultimate tensile strength, d is fracture elongation and k is a constant and which was subsequently modified by Samuel et al. [16] as Q = rb + 150log d for Al–Si alloys. 3. Results and discussion 3.1. Microstructural study The microstructure of the as cast Al–12Si base alloy and that of grain refined and modified alloy is shown in Fig. 1a–c. It can be observed that the microstructure of cast alloys (without refiner and modifier) consists of fine eutectic Si particles and cuboid shape primary silicon particles in matrix of a-Al, besides needle shape bphase iron rich intermetallic compounds (Fig. 1a). The formation of primary Si particles in the structure of eutectic Al–12Si alloy is primarily due to low cooling rate conditions experienced by molten metal in preheated shell sand mould during solidification. It is known that cooling conditions affect the composition for the eutectic reaction. The length of needlelike b-phase particles was found to vary from 150 to 400 lm. The effect of grain refiner and modifier on the microstructure of Al–12Si base alloy is shown in Fig. 1b and c. It can be observed that in general Ti addition transforms the long needle shaped b-phase
particles into fine, fragmented, irregular shape particles while Sr addition results in fine and largely spherical shape morphology of b-phase. The effect of amount of Ti addition (0.00125 and 0.015 wt.%) in alloy is shown in Fig. 2a and b. It was observed that size and fraction of needlelike b-phase reduced appreciably with addition of Al– Ti–B master alloy. However, closer look of the microstructure at high magnification exhibited the transformation of needle shape b-phase intermetallic compounds into Chinese script morphology which is generally found in form of Al15(MnFe)3Si2.[8]. No major distinguishable feature in microstructure of alloy with 0.015 wt.% Ti (Fig. 2b) as compared to that obtained with 0.00125 wt.% Ti could be observed (Fig. 2a). The effect of amount of Sr addition (0.04 and 0.06 wt.%) in alloys is shown in Fig. 3a and b. It was observed that in general Sr addition modified needle shape b-phase particles into more favorable fine nodular and blocky shape morphology. It can be noticed that the addition of 0.04 wt.% Sr in Al– 12Si alloy resulted in fine blocky shape b-phase particles (Fig. 3a) while 0.06 wt.% Sr addition resulted in both blocky and Chinese script morphology (Fig. 3b). Effects of Ti and Sr addition in cast Al–Si alloy on iron rich bphase needle observed in this work are in agreement with earlier published literature [18–22]. It has been reported that increased solidification rate [18], addition of strontium in presence of transition elements such as manganese [14] promotes the development of the a-phase with Chinese script morphology, at expense of the b-phase. Suárez et al. [13] also reported that b-phase is refined with Sr addition and with high Sr/Ti ratio. 3.2. SEM–EDS analysis SEM–EDAX analysis of the micro-constituents of the base alloy without grain refinement and modification was carried out to show the distribution of Si and Fe in the form of elemental mappings. The maps confirmed the presence of Al–Fe–Si intermetallic compounds and their needle shaped morphology as shown in Fig. 4a–c. Further, the SEM and EDAX analysis allowed observing the relationship between eutectic silicon particles and iron-based intermetallics. The backscattered electrons image (Fig. 5), showed the nucleation of primary silicon particles on b-phase (in black circle) besides intermixed iron-base intermetallic, b-phase, with silicon particles (in white circles). These observations are in agreement with published literature [13,19]. The nucleation of silicon particles on b-phase suggests that the b-phase is formed first at high temperature during the solidification and then acted as nucleants for Si particles [20,21], while intermixed acicular-shaped b-precipitates with silicon needles in eutectic, might be formed due to rejection of the iron into residual liquid from growing dendrites of a-Al during solidification. 3.3. XRD analysis In general, intermetallic phases are identified according to their response to a chemical attack, morphology and/or energy dispersive X-ray spectra [22]. X-rays diffraction, convergent beam electron diffraction (CBED) and electron probe microanalysis (EPMA) are used to identify these phases [9,23,24]. Kral et al. [25] have pointed out that some confusion persists in the literature regarding the accurate identification of the intermetallics in Al–Si casting alloys and using EBSD technique they also identified a-phase as cubic Al19(Fe,Mn)5Si2. Electron backscatter diffraction (EBSD) patterns of the b-phase are consistent with tetragonal Al3(Fe,Mn)Si2. It was also noted that the a-phase and b-phase might not clearly exhibit dendritic (Chinese script) or plate-shape morphology in alloys with eutectic modifiers.
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(a)
(b)
100 μm 100 μm
(c)
100 μm
Fig. 1. Optical micrographs of: (a) alloy without refiner and modifier, (b) alloy with 0.00125 wt.% Ti and (c) alloy with 0.06 wt.% Sr.
(a)
(b) Al matrix
Si
eutectic Si Chinese script phase Fig. 2. Effects of addition of Ti on microstructure of the Al–12Si base alloy: (a) 0.00125 wt.% Ti and (b) 0.015 wt.% Ti addition.
The XRD patterns of base alloys and modified/refined Al–12Si alloy are shown in Fig. 6a–c. XRD pattern of Al–12Si base alloy revealed several peaks, between 43 and 47° (2h values), with very low intensities (2–4% of intensity respect to principal peak) which might be corresponding to different intermetallics. The principal peak of b-phase (Al5FeSi) is located at 46.7°, as shown in Fig. 6a. Unfortunately this could not be confirmed by the presence of the second largest peak located at 17.05°, as XRD analysis in present work was carried out in range of 20–100° only. XRD for patterns for grain refined and modified alloys were found similar to that of as cast un-modified and unrefined alloy. The XRD pattern of grain refined (Fig. 6b) and modified (Fig. 6c) alloy showed almost
the similar XRD pattern behavior i.e. without any peak corresponding to b-phase, therefore it was found difficult to confirm any bphase to a-phase transformation by XRD analysis. This may be due to very low fraction (less than 2%) of iron rich b or a-phase in Al–12Si alloys. 3.4. Mechanical properties The effect of Ti addition (as Al–5Ti–1B master alloy) on the ultimate tensile strength (UTS) and the elongation (%) of Al–12Si alloy are shown in Fig. 7. It can be observed that addition of small amount of Ti (0.00125 wt.%) increased the UTS from 93 MPa to
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Fig. 3. Effects of addition of Sr on microstructure of the Al–12Si base alloy: (a) 0.04 wt.% Sr and (b) 0.06 wt.% Sr addition.
(a)
(b)
200 µm
(c)
Si
Fe
Fig. 4. Backscattered electrons image and elemental maps of as cast Al–12Si alloy (a) Needles and platelets morphologies (bright) of the iron-base intermetallics and irregular grey colored Si-eutectic particles and Si-cuboids (arrows). EDAX elemental maps of (b) Si and (c) Fe.
143 MPa, while the ductility increased from 2.7% to 4.5%. Increase in UTS and elongation of as cast alloy can be attributed to the refinement of Al–Fe–Si intermetallic compound and the change in morphology of the needle shape b-phase intermetallic com-
pounds into blocky shape and Chinese script form as shown in Fig. 2. Further, it can be noticed that affect of Ti addition above 0.0125 wt.% on the ultimate tensile strength is marginal while ductility is reduced. Reduction in ductility might be due to presence of
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0.06% continuously increases the tensile strength while addition of Sr above 0.04% results in marginal decrease in ductility (% elongation) as shown in Fig. 8. It can also be noted that Sr addition produced higher ultimate tensile strength and ductility as compared to that of Ti addition, this is due to two factors (1) eutectic silicon modification and (2) morphology changes of b-phase into Chinese script phase. The effect of Ti and Sr additions on hardness is shown in Fig. 9. It can be observed that grain refiner and modifier addition reduced the hardness. Addition of 0.04 wt.% Sr to base metal, reduces the hardness from 62 to 51 HRB principally due to the modification of eutectic silicon and change of b-phase morphology. Similarly addition of the 0.00125 wt.% Ti also reduces the hardness from 62 to 55 HRB, due to change in morphology of b-phase. 3.5. Quality index
200 µm Fig. 5. Backscattered electrons image showing the morphology and distribution of Si-eutectic particles and iron-base intermetallics in as cast Al–12Si alloy.
some un-dissolved grain refiners (Al–Ti–B) acting as a site for stress concentration and so easy nucleation of cracks and voids during tensile test. Similarly, addition of 0.06 wt.% Sr increased the UTS from 93 MPa to 153 MPa and elongation from 2.7% to 5.3%. It can also be seen that increase in amount of Sr addition in alloy from 0 to
(a)
The quality index ‘‘Q’’ of the alloy castings in as cast, grain refined and modified conditions was calculated using approach proposed by Samuel et al. [16]. Table 3 shows alloy condition and influence of grain refinement and modification on the quality index. It can be observed that in general addition of grain refiner and modifier improved the quality index of the alloy. Moreover, the influence of modification on the quality index was found to be more than grain refinement. Low quality index of base alloy is due to poor UTS and low elongation; caused by needle shape eutectic Si particles and needle shape hard and brittle b-phase intermetallic compounds. Addition of Ti and Sr refines a-Al, eutectic silicon and b-phase particles and therefore improves the mechanical properties (UTS and % elonga-
(b)
β
Al
Si
Al Al Si
Si
Al
(c) Phases:
Al
Al Si
Al
β-(Al5FeSi) Al
Al
Si Si Si
Fig. 6. XRD patterns of: (a) Al–12Si base alloy, (b) with addition of 0.00125 wt.% Ti and (c) with addition of 0.06 wt.% Sr.
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Fig. 7. Effects of addition of Ti on the UTS and the elongation of the Al–12Si alloy.
Fig. 8. Effect of Sr addition on the UTS and the elongation of the Al–12Si alloy.
Fig. 9. Effect of Ti and Sr addition on the HRB hardness of the Al–12Si alloy.
Table 3 Influence of grain refiner and modifier addition on quality index. Condition
Ti (wt.%)
Sr (wt.%)
Quality index (Q)
1 2 3 4 5
– 0.00125 0.015 – –
– – – 0.04 0.06
158.79 242.05 234.72 254.04 261.34
tion) of the alloy. Sr modification not only refines eutectic Si particles but also b-phase particles, which have more significant effect on mechanical properties of cast Al–Si alloys while the addition of Ti refines b-phase particles and a-Al. It is known that eutectic
Si particles size and shape has more significant effect on mechanical properties than the a-Al. Hence, it can be concluded that Ti and Sr addition improves quality index. Higher quality index of Sr modified alloy than Ti refined alloy can be explained in the same line. 4. Conclusions From the present investigation, following conclusions can be drawn: (1) Grain refinement of alloy with optimum amount of Al–Ti–B master alloys (0.00125 wt.% Ti) in the base Al–12Si alloy reduces the size of Al–Fe–Si intermetallic compounds and the transforms the morphology of the needlelike b-phase intermetallic compound into blocky and Chinese script form.
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(2) Similarly, modification of the alloy with optimum amount of Al–Sr master alloys (0.06 wt.% Sr) in the base metal Al–12Si alloy refines eutectic Si particles and changes the needlelike b-phase into fine spherical shape morphology and Chinese script shape. (3) Addition of Al–Ti–B and Al–Sr master alloy promotes the development of Chinese script morphology at an expense of the needlelike particles of b-phase which in turn improves the tensile strength and elongation of the cast Al–12Si alloy. (4) Optimum addition of grain refiner (Al–Ti–B) and modifier (Al–Sr) results in significant increase in tensile strength and ductility. (5) Grain refinement and modification of the Al–12Si alloy increased the quality index (Q). Moreover, the effect of modification on quality index was more than grain refinement. (6) In view of microstructure, mechanical properties and quality index, it is recommended to add 0.06 wt.% Sr or 0.00125 wt.% Ti in Al–12Si alloy to achieve best combination of tensile strength and ductility of the alloy. Acknowledgements The authors acknowledge the support and facilities provided by CONACyT, UAZ and UANL from México also to IITR Roorkee and Department of Science and Technology (vide sanction letter DST/ INT/Mex/RPO-07/2008 dated 08-12-2009) from India; PROMEPMéxico by support the collaborative network: ‘‘Surface phenomena, deterioration and aging of materials’’. Thanks to CIMAV S. C., México, for their support in carrying out the electron microscopy also to Raúl Ochoa for his collaboration. Special thanks to Julián Ramírez, Enrique A. López, Angel González and Ricardo Leyva, technical staff. Author gratefully thanks to Rocío Estrella, Paulo Sergio, Brisa Marian and Aldo Enrico for their support and advice. References [1] Haque MM et al. In: Sastry DH et al., editors. In: Proceeding of international conference on aluminum. INCAL 98, 11–13 Febraury, New Delhi, vol. 2, 1998. p. 299. [2] Smart RF. Metallurgical aspects of Al–Si eutectic piston alloys. Br Foundry Man 1971;64:430–8. [3] Johnsson M, Bäckerud L. Nucleants in grain refined aluminium after addition of Ti- and B-containing master alloys. Z Metallkd 1992;83:774–80.
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[4] Wangli BX. Refining effect of boron on hypoeutectic Al–Si alloys. J Mater Sci Technol 2000;16:517–20. [5] Greer AL et al. Modelling of inoculation of metallic melts: application to Grain Refinement of aluminium by Al- Ti –B. Acta Mater 2000;48:2823–35. [6] Aluminum and Aluminium alloys. ASM Specialty Handbook. ASM International; 1993. p. 221–24. [7] Ray Cook. Metallurgy of aluminum. London & Scandinavian Metallurgical Co. Limited; 1998. [8] Aluminum Casting Technology. Des Plaines, AFS, 2nd. ed. USA: Illinois; 2001. p. 28–323. [9] Mondolfo LF. Aluminum alloys, structure and properties. London, Butterworths and Co. Ltd.; 1976. [10] Haizhi Ye. An overview of the development of Al–Si-alloy based material for engine applications. J Mater Eng Perform 2003;12:288–97. [11] Eklund JE. On the effects of impurities on solidification and mechanical behavior of primary and secondary commercial purity aluminum and aluminum alloys, PhD. thesis. Helsinki: University of Technology; June 1993. [12] Taylor JA, Schaffer GB, St John DH. The role of iron in the formation of porosity in Al–Si–Cu-based casting alloys. Metall Mater Trans 1999;30A:1643–62. [13] Suárez-Peña B, Asensio-Lozano J. Influence of Sr modification and Ti grain refinement on the morphology of Fe-rich precipitates in eutectic Al–Si die cast alloys. Scripta Mater 2006;54:1543–8. [14] Couture A. Iron in aluminum casting alloys: a literature survey. AFS Int Cast Metals J 1981;6:9–17. [15] Drouzy M, Jacob S, Richard M. Interpretation of tensile results by means of quality index and probable yield strength. AFS Int Cast Metals J 1980;5:43–50. [16] Samuel AM, Gauthier J, Samuel FH. Microstructural aspects of the dissolution and melting of Al2Cu phase in Al–Si alloys during solution heat treatment. Metall Mater Trans A 1996;27:1785–98. [17] Standard methods of tension testing wrought and cast aluminum and magnesium alloy products. B557-84. Annual Book of ASTM Standards. vol. 02.2. Philadelphia, PA: ASTM; 1986. p. 506. [18] Vorren O, Evensen JE, Pedersen TB. Microstructure and mechanical properties of Al–Si (Mg) casting alloys. AFS Trans 1984;92:459–66. [19] Martínez EJ, Lacaze DJ, Cisneros y M, Valtierra S. Efecto del Estroncio en las Temperaturas de Reacción Eutécticas y Microestructuras de Solidificación de una aleación Al–Si tipo A319. Rev Fac Ing – Univ Tarapacá 2004;12:21–6. [20] Narayanan LA, Samuel FH, Gruzleski JE. Crystallization behavior of ironcontaining intermetallic compounds in 319 aluminum alloy. Metall Mater Trans A 1994;25:1761–73. [21] Wen KY, Hu W, Gottstein G. Intermetallic compounds in thixoformed aluminium alloy A356. Mater Sci Technol 2003;19(6):762–8. [22] Samuel FH, Samuel AM, Doty HW, Valtierra S. Decomposition of Feintermetallics in Sr-modified cast 6XXX type aluminum alloys for automotive skin. Metall Mater Trans A 2001;32:2061–75. [23] Munson D. A clarification of the phases occurring in aluminium–rich aluminium–iron–silicon alloys, with particular reference the ternary phase a-AlFeSi. J Inst Metals 1967;95:217–9. [24] Qian Ma, Taylor JA, Yao JY, Couper MJ, St John DH. A practical method for identifying intermetallic phase particles in aluminium alloys by electron probe microanalysis. J Light Metals 2001;1:187–93. [25] Kral MV, McIntyre HR, Smillie MJ. Identification of intermetallic phases in a eutectic Al–Si casting alloy using electron backscatter diffraction pattern analysis. Scripta Mater 2004;51:215–9.