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The effect of mischmetal and heat treatment on the microstructure and tensile properties of A357 Al–Si casting alloy
Materials Science & Engineering A 556 (2012) 573–581
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The effect of mischmetal and heat treatment on the microstructure and tensile properties of A357 Al–Si casting alloy G.S. Mousavi, M. Emamy n, J. Rassizadehghani Center of Excellence for High Performance Materials, School of Metallurgy and Materials, College of Engineering, University of Tehran, Tehran, Iran
a r t i c l e i n f o
abstract
Article history: Received 30 November 2011 Received in revised form 27 June 2012 Accepted 6 July 2012 Available online 20 July 2012
The effects of La-based mischmetal (MM) and heat treatment on the microstructure and tensile properties of two different sections of A357 Al–Si casting alloy have been investigated in this study. Different concentrations (0–1 wt%) of the MM were added to the molten alloy directly. The tensile testing was employed to measure the quality index (Q.I. ¼ UTS þ 150 log (elongation)) for evaluating the modification efficiency of the alloy with different MM contents. T6 heat treatment was applied before tensile testing. The results showed that the optimum levels of added mischmetal are 0.1 wt% and 0.3 wt% for thin and thick section castings respectively. A new AlSiLa intermetallic was detected through the microstructural studies at higher MM levels. Further results demonstrated that T6 heat treatment improves tensile properties of the modified alloy significantly. The differences observed in the fracture behavior were attributed to the microstructural changes as well as morphological aspects of Si particles. & 2012 Elsevier B.V. All rights reserved.
Keywords: Aluminum alloy Casting Fracture Electron microscopy
1. Introduction Aluminum–silicon (Al–Si) cast alloys are increasingly becoming one of the most popular commercial aluminum alloys being used in the aerospace and automotive industries mainly because of their high strength to weight ratio, excellent castabililty, high corrosion and wear resistance, low coefficient of thermal expansion. These alloys possess high tensile, impact and fatigue properties after using an appropriate heat treatment process [1–3]. Mechanical properties of near-eutectic and eutectic Al–Si cast alloys depend not only on the chemical composition, but also on the microstructural features such as the morphology and size of eutectic Si and intermetallics which may be presented in the microstructure. Under normal cooling conditions, Si particles grow as coarse acicular needles. The needles act as crack initiators and weaken the mechanical properties significantly [1,2,4]. Modification of the eutectic Si from an acicular to a fine fibrous structure can be achieved in three different ways: (1) chemical modification (addition of certain elements), (2) quench modification (rapid cooling rate) and (3) thermal modification (heat treatment). Several elements are known to be responsible for chemical modification; for example Sr, Na and Sb are the most commonly used elements in industry today [5–8]. Addition of rare-earth metals has also been reported to cause modification in both hypo and hypereutectic Al-Si alloys [9–12]. Mischmetal
n
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(MM) is a combination of rare earth metals (Ce, La, Pr and Nd), and has been reported to act as a modifier in Al–Si alloys. In 2008, El Sebaie et al. studied the combined modification effect of Sr and mischmetal on A356.2 casting alloy at both high and low cooling rates. They showed the positive effect of mischmetal on the eutectic Si particles [1]. It was found that La or Ce can act as a modifying agent in Al–Si alloys to improve the morphology of the eutectic Si particles and thus the tensile strength [3,13]. An appropriate solution treatment process, if applied to Al–Si alloys, can lead to significant improvements in the morphology of eutectic Si particle. This alloy system is used in T6 condition, which consists of solutionizing at high temperatures (540– 550 1C), then quenching in water and finally artificial aging at 155–210 1C [14,15]. The optimum levels of strength and ductility after the thermal treatments is attributed to the changes in Si particle characteristics resulting from the solution treatment and to the formation of non-equilibrium precipitates of b0 (Mg2Si) as a result of aging heat treatment [16,17]. This study was carried out in order to investigate (a) the effect of La-based mischmetal as a modifier, (b) the combined effect of mischmetal and T6 heat treatment on the size and morphology of eutectic Si particles and tensile properties, and (c) comparison between these characteristics and properties of thin (|6 mm) and thick (|30 mm) cylindrical sections of A357 Al–Si cast alloy. For evaluating the modification efficiency of the alloy with different mischmetal contents, the quality index value (Q.I.¼UTSþ150 log (El.%)) was employed as well [3,18–22]. In other words, the quality index Q.I. balances the properties ultimate tensile strength and elongation to fracture.
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2. Experimental procedures Industrially pure metals (Al, Mg and Si) were used as starting materials to prepare Al–7% Si ingots. All materials were heated in an electrical resistance furnace using a 10 kg SiC crucible. Table 1 shows the chemical composition of A357 alloy studied in this work. The parent ingots were cut into small pieces, with the approximate dimensions of 40 mm 30 mm 20 mm, appropriate for a 2 kg SiC crucible. Then the alloy was remelted in another electrical resistance furnace. When the temperature reached 770 1C, mischmetal, with the composition shown in Table 2, was added to the molten alloy in 9 concentrations (0.01, 0.03, 0.05, 0.07, 0.1, 0.3, 0.5, 0.7 and 1 wt%) for the thin section, and 6 concentrations (0.05, 0.1, 0.3, 0.5, 0.7 and 1 wt%) for the thick section castings. Ten minutes after adding mischmetal, the molten alloy was hand stirred with a graphite rod for about 1 min to ensure
Table 1 Chemical composition of A357 casting alloy (wt%). Alloy
Si
Mg
Fe
Cu
Mn
Zn
A357
6.8
0.45
0.12
0.01
0.01
0.05
Table 2 Chemical composition of mischmetal. Element
wt%
La Ce Nd Pr
70.8 10.2 6.3 12.7
complete mixing. Degassing was conducted by submerging dry C2Cl6 containing broken tablets (0.3 wt% of the molten alloy). After stirring and cleaning off the dross, alloys with different MM levels were poured into thin and thick section cast iron molds (Fig. 1a and b). The thin section mold was prepared according to B108-03a ASTM standard. The main advantage of these types of mold is the application of an appropriate uphill filling system and feeding design, providing a low turbulence manner of fluid flow which consequently results in reduced oxide films, gas entrapment and porosity in cast specimens. For each section, the specimens were divided into two groups. The samples in one group were subjected to T6 treatment and were heated to 540 1C for 8 h for solid solution, then quenched in water to room temperature, and finally artificially aged at 170 1C for 7 h. For microstructural observations, the cut sections were polished and then etched by HF (5%). Samples were also deep etched in NaOH (20%) for 45 min to dissolve the aluminum matrix and reveal the eutectic Si structure for observation by SEM. Quantitative data on the microstructures were determined using an optical microscope equipped with an image analysis system (Clemex Vision Pro. Ver. 3.5.025). For minimizing the errors, the measurement was done on 50 different areas of each microstructure. The microstructural characteristics of the specimens were examined by scanning electron microscopy performed in a Vega&Tescan SEM equipped with the energy dispersive X-ray analysis (EDX) accessory. X-ray diffraction (XRD) studies for phase analysis were performed using PHILIPS-binary diffractometer applying Cu Ka radiation. Tensile test bars were machined, according to ASTM B557M02a sub-size specimens (Fig. 1c). Tensile tests were carried out on a computer controlled MTS tension machine, equipped with a strain gauge extensometer, at a constant cross-head speed of 1 mm/min. The fracture surfaces of tensile test specimens were also examined by SEM.
Fig. 1. Schematic of (a) thin, (b) thick section cast iron mold and (c) tensile test dimensions.
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3. Results and discussion 3.1. Microstructural characterization The basic microstructures of the alloy, shown in Fig. 2, is typical of hypoeutectic Al–Si alloy in both thin and thick sections, with primary a-Al dendrites and eutectic Si particles distributed around the Al dendrites to form a cell pattern, periodically repeated across the metallographic surface.
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The as-cast and heat treated microstructures of A357 alloy without and with La-base mischmetal in both thin and thick sections are shown in Figs. 3 and 4 respectively. Tables 3 and 4 also list Si particle characteristics in various A357 alloy samples solidified under high and low cooling rates, for as-cast and T6-heat treated conditions. From Fig. 3 and Table 3, it can be seen that in thin section castings, the addition of mischmetal (up to 0.1 wt%) reduces the particle length, area and the aspect ratio of Si particles. This can be described according to an impurity
Fig. 2. Optical micrographs of the A357 alloy casting in (a) thin and (b) thick section mold.
Fig. 3. The microstructures of thin section A357 alloys with different concentrations of mischmetal. (a and d) unmodified; (b and e) 0.1 wt% MM; (c and f) 1 wt% MM alloys in the (a–c) as-cast condition and (d–f) after heat treatment.
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Fig. 4. The microstructures of thick section A357 alloys with different concentrations of mischmetal. (a and d) unmodified; (b and e) 0.3 wt% MM; (c and f) 1 wt% MM alloys in the (a–c) as-cast condition and (d–f) after heat treatment.
Table 3 Si particle characteristics of various A357 alloy samples obtained at high cooling rates.
Table 4 Si particle characteristics of various A357 alloy samples obtained at low cooling rates.
Sample
MMa (wt%)
Particle length (mm)
Particle area (mm2)
Aspect ratio (%)
Sample
MMa (wt%)
Particle length (mm)
Particle area (mm2)
Aspect ratio (%)
Non-heat treated
0.00 0.05 0.10 0.50 1.00 0.00 0.05 0.10 0.50 1.00
12.0 7 4.3 11.9 7 6.2 8.5 7 4.4 10.9 7 3.1 11.2 7 4.1 8.9 7 2.0 8.5 7 1.5 6.3 7 2.4 8.07 1.6 8.2 7 1.2
21.6 78.2 16.2 74.1 13.9 74.0 14.9 76.2 15.2 74.3 39.2 710.3 38.4 78.5 23.8 79.4 33.6 73.5 37.2 712.0
5.5 7 0.7 5.2 7 0.5 3.3 7 0.7 4.8 7 1.1 5.1 7 0.9 3.4 7 1.3 2.1 7 0.6 1.6 7 0.8 1.8 7 1.2 1.8 7 1.0
Non-heat treated
0.00 0.05 0.10 0.30 0.50 1.00 0.00 0.05 0.10 0.30 0.50 1.00
31.2 7 9.4 29.0 7 8.6 28.9 7 4.2 25.8 7 4.5 27.4 7 8.2 29.3 7 6.6 21.5 7 6.2 17.7 7 6.5 15.9 7 5.2 12.9 7 4.5 14.8 7 7.2 20.0 7 9.4
80.7 733.3 74.5 711.2 70.4 724.8 64.2 725.8 65.3 736.6 80.1 733.1 161.1 749.3 119.0 710.8 96.4 734.8 70.4 76.2 131.2 729.0 156.3 750.7
6.9 7 1.5 6.1 7 1.3 5.4 7 1.8 4.2 7 0.8 6.2 7 1.3 7.2 7 1.1 5.1 7 1.6 4.9 7 1.5 4.6 7 1.1 2.7 7 0.6 3.4 7 0.6 4.6 7 0.9
Heat treated
a
Heat treated
MM ¼mischmetal. a
induced twinning theory proposed by Lu and Hellawell [23]. They suggested that modified Si fibers contain more twins than the unmodified ones and have a rough microfaceted surface. The Si fibers are crystallographically very imperfect, and each surface imperfection is a potential site for branching to occur under solidification conditions. As a result, fibers in the chemically modified eutectic are able to bend, curve and split to create a fine microstructure. Lu mentioned that a growth twin is created
MM¼ mischmetal.
at the interface when the atomic radius of the element has the correct size relative to the radius of Si (relement:rSi ¼ 1.65). Tsai et al. [3] showed that La element falls within the atomic radius range proposed to cause chemical modification. However, adding higher amounts of mischmetal is expected to introduce large intermetallic compounds. Fig. 5b shows the characteristics of the
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Fig. 5. (a) SEM photograph of Al–Si–La compound intermetallic in A357 alloy with 5 wt% mischmetal casting in thick section mold and (b) EDS spectra showing the distribution of Al, Si and La in the intermetallic.
Tables 3 and 4 also present the effect of T6 heat treatment on the size, aspect ratio and morphology of eutectic Si of samples which were solidified under high and low cooling rate conditions. From Tables 3 and 4, it can be seen that T6 heat treatment has a profound effect on the spheroidization of eutectic Si, especially in the La-based MM added alloys. It is observed that at low cooling rates (thick section castings), the characteristics of Si particles for both modified and unmodified specimens are affected more significantly after heat treatment compared to the thin section cast specimens [24]. It has been also reported that the presence of MM (in its optimum level) reduces the coarsening of the Si particles during solution heat treatment [1]. 3.2. Tensile properties Fig. 6. XRD pattern of a deep-etched A357 alloy specimen with 1 wt% mischmetal.
intermetallic phase in a specimen containing 5 wt% mischmetal. Concentration profile of the intermetallic formed in the microstructure of the alloy after the addition of 5 wt% mischmetal shows that it contains La, Si and Al atoms, which can be represented as a ternary AlSiLa according to the atomic percentages shown in Fig. 5b. The X-ray diffraction pattern of A357 alloy containing 1 wt% mischmetal is seen in Fig. 6. This is in agreement with the theoretical and experimental investigations carried out by Tsai et al. [3]. They showed that AlSiLa intermetallics can be formed when high concentrations of La are added into A356 Al alloy. On the other hand, by considering Fig. 4 and Table 4, it can be concluded that in thick section casting, similar behavior is seen by adding La-based mischmetal. However, its optimum level for improving the eutectic Si particles was found to be different (0.3 wt% MM). According to the results obtained from microstructural parameters in Tables 3 and 4, in thick section, the maximum reductions in particle length, area and the aspect ratio of the as-cast samples by the addition of mischmetal are 17.3%, 20.4% and 39.1%, respectively. While, these values for the thin section castings are 29.1%, 35.6% and 40.0%. This indicates that MM addition is more effective in modifying the microstructure of thinner sections than that of thick section castings [1].
Fig. 7 shows the stress–strain curves of thin and thick section samples, containing different concentrations of mischmetal (MM), before and after T6 heat treatment. It is seen that in both as-cast and heat treated alloys, samples which solidify under high cooling rate condition have higher tensile strength and elongation values. Besides, in each section, adding mischmetal at its optimum level improves tensile properties. Moreover, applying T6 treatment to the specimens of both sections leads to an enhancement of tensile properties. Figs. 8 and 9 show UTS and El.% values of the as-cast and heat treated A357 alloys with increasing mischmetal content in both thin and thick sections. Fig. 8a indicates that the addition of 0.1 wt% mischmetal to the thin section samples causes a significant improvement in UTS results (about 12.3% in non-heat treated and 13.6% in T6 heat treated specimens). But the use of higher mischmetal concentrations (more than 0.1 wt%) lead to a considerable reduction of UTS values. The formation of a new intermetallic phase (AlSiLa) with acicular morphology, as shown in Figs. 3– 5, may cause high levels of stress concentration and consequently the reduction of tensile properties. Fig. 8b shows that in thick section specimens, the level of added mischmetal which introduces the optimum UTS values, is 0.3 wt%, which shows about 40% improvement for all specimens before and after applying T6 heat treatment. According to Fig. 9a, addition of 0.1 wt% mischmetal to the alloy in thin section specimens increases the El.% values by about
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Fig. 7. Stress–strain diagrams of (a, b) thin and (c, d) thick section samples with different contents of mischmetal in the (a, c) as-cast and (b, d) T6-treated condition.
Fig. 8. UTS values of as-cast and heat treated A357 alloy samples, obtained at (a) high cooling rate and (b) low cooling rate condition, as a function of MM concentration.
10% in non-heat treated and 15% in T6 heat treated alloys. But, the formation of intermetallics in the existence of higher contents of mischmetal can cause deleterious effect on it. On the other hand, in thick-section specimens (Fig. 9b), addition of 0.3 wt% mischmetal improves the elongation values of the alloys for about 51% and 40%, before and after applying T6 heat treatment. The higher effect of mischmetal in improving tensile properties of
thick section specimens is due to the longer period of time provided during which solidification occurs [24]. By comparing Fig. 9a and b, it can be seen that although applying T6-heat treatment to the thick section samples leads to the reduction in El.% values, it has an adverse effect on the thin section samples. This phenomenon can be described according to what has been suggested by Martin and Dohetry [25].
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Fig. 9. Elongation percentage to fracture of as-cast and heat treated A357 alloy samples, obtained at (a) high cooling rate and (b) low cooling rate condition, as a function of MM concentration.
Fig. 10. Quality Index values of as-cast and heat treated A357 alloy samples, obtained at (a) high cooling rate and (b) low cooling rate condition, as a function of MM concentration.
They reported that when eutectic structure is subjected to a thermal treatment at elevated temperatures (such as what occurs during solution treatment), shape perturbations in the second phase constituent (Si particles) increase until, ultimately, the particles are broken into a series of nearly spherical particles. Such perturbations or interfacial instabilities cannot readily occur in plate-like, i.e. unmodified eutectic structures, exactly the same as those which form at low cooling rate conditions (Table 4), making spheroidization extremely difficult in such cases. On the other hand, it is easy for Si particles which solidified under high cooling rate conditions and are shorter in length to alter into small spheroidized particles after the solution stage. To evaluate the tensile test data, a quality index value, Q.I., which is defined as Q.I.¼UTSþ a log (El.%) was calculated, where a is 150 for Al–Si–Mg alloys [3,18–22]. This concept arose from the consideration of the relationship between ultimate tensile strength and elongation of Al–Si–Mg alloys. Since the quality index combines both strength and ductility, it is much more descriptive of the true tensile properties of a casting than either the tensile strength or the elongation alone. According to the results of quality index values, shown in Fig. 10, it can be suggested that in the as-cast thin section samples, addition of 0.1 wt% mischmetal leads to an increase of about 10% in Q.I. Besides, in thick sections, the Q.I. of the sample modified by
adding 0.3 wt% mischmetal, is about 38% higher than that of unmodified alloy. After T6 treatment, Q.I. increases up to 12.4% for thin sections and 40% for thick sections. Therefore it can be concluded that T6-heat treatment plays a crucial role in the modification effect of mischmetal. 3.3. Fragtography Fig. 11 shows the fracture surfaces of the as-cast and heat treated alloys solidified in the thin section mold. It can be seen that in both as-cast and heat treated conditions, addition of 0.1 wt% mischmetal decreases the size and increases the number of dimples slightly. But, addition of 1 wt% mischmetal leads to the formation of some cleavage planes and therefore a brittle fracture. The presence of intermetallics in these samples also provides a good reason for such a brittle fracture. The EDS spectra in Fig. 12b, showing the presence of alloying elements in the intermetallic, imply the existence of an AlSiLa intermetallic in the fracture surface of the alloy. The fracture surfaces indicate a much more ductile failure mode for the heat treated alloys, compared with that of the as-cast alloys. Fig. 13 presents SEM fractographs of the as-cast and T6 heat treated specimens solidified at the lower cooling rate condition. It can be seen that in both as-cast and heat treated conditions,
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Fig. 11. Fractographs of thin section tensile samples. (a and d) unmodified, (b and e) 0.1 wt% MM and (c and f) 1 wt% modified alloys in the (a–c) as-cast and (d–f) T6-treated condition.
Fig. 12. (a) SEM micrograph of Al–Si–La intermetallic compound in A357 alloy and (b) EDS spectra showing the presence of Al, Si and La in the intermetallic.
with the addition of 0.3 wt% MM, the fracture surface indicates the presence of more dimples as compared with the unmodified alloy, indicating a ductile fracture phenomena leading to higher tensile elongation. On the other hand, some irregular cleavage planes are apparent on the entire fracture surface of the 1 wt% mischmetal modified alloy as shown in Fig. 13c and f. It indicates that the fracture characteristics exhibit quasi-cleavage fracture which results in lower elongation values of the specimens. This is because of the acicular La-based intermetallics, which act as crack initiators and weaken the mechanical properties significantly. By comparing the as-cast and heat treated thick section specimens, it can be seen that applying T6 treatment makes the dimples larger to some extent and thus, the elongation decreases which is in agreement with the values of elongation percentage obtained from tensile tests. As reported previously by Caceres et al. [26] and Wang and Caceres [27], in the microstructures of the alloys solidified at low cooling rates, the cell boundaries are well defined by a high density of Si particles, while in the thin section samples, the small and widely spaced Si particles barely define the interdendritic
boundaries. The structure of the cell boundaries determines the degree of interaction with the plastic deformation. In thick section samples, it is seen that although the slip trace is the same for all cells in a given grain, the slip bands are interrupted by the cell boundaries. In contrast, in alloys solidified at high cooling rates, the rather open cell boundaries do not interrupt the slip bands, which are now much longer than the dendritic cells. The Si particles interact with the slip bands individually rather than collectively as in the case of distinct cell boundaries. The structure of the cell boundaries suggests that the fracture path is determined by the relative continuity of the dendritic cells boundaries. This can cause a more ductile fracture in thin section alloys as compared to the thick section castings (Figs. 11a and 13a).
4. Conclusions A study was carried out to determine the effects of La-based mischmetal and T6 heat treatment on the eutectic Si particle characteristics and tensile properties of A357 Al–Si alloy in thin
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Fig. 13. Fractographs of thick section tensile samples, (a and d) unmodified, (b and e) 0.3 wt% MM and (c and f) 1 wt% modified alloys in the (a–c) as-cast and (d–f) T6-treated condition.
and thick casting sections. Based on the results obtained and their analysis, the following conclusions were drawn: 1) The addition of mischmetal to the A357 alloy modified the eutectic Si crystals in both thin and thick section castings. The effects of modification on the microstructure were more evident in thin sections. A new AlSiLa intermetallic phase was found in the microstructures of the modified alloy with higher MM contents (Z0.3 wt% for all sections). 2) The addition of mischmetal improved the tensile strength and elongation values of the A357 alloy. To have optimal microstructural characteristics and tensile properties, the mischmetal content was found to be 0.1 wt% for thin and 0.3 wt% for thick sections. The reduction in the tensile properties of the modified alloy at higher level of MM was attributed to the effect of the intermetallic compound segregated at the eutectic areas. It was also found that the effect of MM addition on the improvement of tensile strength is more considerable in thick sections. 3) T6 heat treatment improved the UTS and Q.I. values of the modified and non-modified specimens significantly which was attributed to the effect of mischmetal on preventing Si particles from coarsening during heat treatment. 4) More dimples were found on the fracture faces of the MM modified (with optimum levels) and T6 heat treated thin section specimens, when compared with the non-modified cast alloy. New intermetallic compound was found to be responsible for the reduced tensile properties of the alloy with higher MM levels.
Acknowledgment The authors would like to thank the University of Tehran for financial support of this research.
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The effect of mischmetal and heat treatment on the microstructure and tensile properties of A357 Al–Si casting alloy G.S. Mousavi, M. Emamy n, J. Rassizadehghani Center of Excellence for High Performance Materials, School of Metallurgy and Materials, College of Engineering, University of Tehran, Tehran, Iran
a r t i c l e i n f o
abstract
Article history: Received 30 November 2011 Received in revised form 27 June 2012 Accepted 6 July 2012 Available online 20 July 2012
The effects of La-based mischmetal (MM) and heat treatment on the microstructure and tensile properties of two different sections of A357 Al–Si casting alloy have been investigated in this study. Different concentrations (0–1 wt%) of the MM were added to the molten alloy directly. The tensile testing was employed to measure the quality index (Q.I. ¼ UTS þ 150 log (elongation)) for evaluating the modification efficiency of the alloy with different MM contents. T6 heat treatment was applied before tensile testing. The results showed that the optimum levels of added mischmetal are 0.1 wt% and 0.3 wt% for thin and thick section castings respectively. A new AlSiLa intermetallic was detected through the microstructural studies at higher MM levels. Further results demonstrated that T6 heat treatment improves tensile properties of the modified alloy significantly. The differences observed in the fracture behavior were attributed to the microstructural changes as well as morphological aspects of Si particles. & 2012 Elsevier B.V. All rights reserved.
Keywords: Aluminum alloy Casting Fracture Electron microscopy
1. Introduction Aluminum–silicon (Al–Si) cast alloys are increasingly becoming one of the most popular commercial aluminum alloys being used in the aerospace and automotive industries mainly because of their high strength to weight ratio, excellent castabililty, high corrosion and wear resistance, low coefficient of thermal expansion. These alloys possess high tensile, impact and fatigue properties after using an appropriate heat treatment process [1–3]. Mechanical properties of near-eutectic and eutectic Al–Si cast alloys depend not only on the chemical composition, but also on the microstructural features such as the morphology and size of eutectic Si and intermetallics which may be presented in the microstructure. Under normal cooling conditions, Si particles grow as coarse acicular needles. The needles act as crack initiators and weaken the mechanical properties significantly [1,2,4]. Modification of the eutectic Si from an acicular to a fine fibrous structure can be achieved in three different ways: (1) chemical modification (addition of certain elements), (2) quench modification (rapid cooling rate) and (3) thermal modification (heat treatment). Several elements are known to be responsible for chemical modification; for example Sr, Na and Sb are the most commonly used elements in industry today [5–8]. Addition of rare-earth metals has also been reported to cause modification in both hypo and hypereutectic Al-Si alloys [9–12]. Mischmetal
n
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0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.07.029
(MM) is a combination of rare earth metals (Ce, La, Pr and Nd), and has been reported to act as a modifier in Al–Si alloys. In 2008, El Sebaie et al. studied the combined modification effect of Sr and mischmetal on A356.2 casting alloy at both high and low cooling rates. They showed the positive effect of mischmetal on the eutectic Si particles [1]. It was found that La or Ce can act as a modifying agent in Al–Si alloys to improve the morphology of the eutectic Si particles and thus the tensile strength [3,13]. An appropriate solution treatment process, if applied to Al–Si alloys, can lead to significant improvements in the morphology of eutectic Si particle. This alloy system is used in T6 condition, which consists of solutionizing at high temperatures (540– 550 1C), then quenching in water and finally artificial aging at 155–210 1C [14,15]. The optimum levels of strength and ductility after the thermal treatments is attributed to the changes in Si particle characteristics resulting from the solution treatment and to the formation of non-equilibrium precipitates of b0 (Mg2Si) as a result of aging heat treatment [16,17]. This study was carried out in order to investigate (a) the effect of La-based mischmetal as a modifier, (b) the combined effect of mischmetal and T6 heat treatment on the size and morphology of eutectic Si particles and tensile properties, and (c) comparison between these characteristics and properties of thin (|6 mm) and thick (|30 mm) cylindrical sections of A357 Al–Si cast alloy. For evaluating the modification efficiency of the alloy with different mischmetal contents, the quality index value (Q.I.¼UTSþ150 log (El.%)) was employed as well [3,18–22]. In other words, the quality index Q.I. balances the properties ultimate tensile strength and elongation to fracture.
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2. Experimental procedures Industrially pure metals (Al, Mg and Si) were used as starting materials to prepare Al–7% Si ingots. All materials were heated in an electrical resistance furnace using a 10 kg SiC crucible. Table 1 shows the chemical composition of A357 alloy studied in this work. The parent ingots were cut into small pieces, with the approximate dimensions of 40 mm 30 mm 20 mm, appropriate for a 2 kg SiC crucible. Then the alloy was remelted in another electrical resistance furnace. When the temperature reached 770 1C, mischmetal, with the composition shown in Table 2, was added to the molten alloy in 9 concentrations (0.01, 0.03, 0.05, 0.07, 0.1, 0.3, 0.5, 0.7 and 1 wt%) for the thin section, and 6 concentrations (0.05, 0.1, 0.3, 0.5, 0.7 and 1 wt%) for the thick section castings. Ten minutes after adding mischmetal, the molten alloy was hand stirred with a graphite rod for about 1 min to ensure
Table 1 Chemical composition of A357 casting alloy (wt%). Alloy
Si
Mg
Fe
Cu
Mn
Zn
A357
6.8
0.45
0.12
0.01
0.01
0.05
Table 2 Chemical composition of mischmetal. Element
wt%
La Ce Nd Pr
70.8 10.2 6.3 12.7
complete mixing. Degassing was conducted by submerging dry C2Cl6 containing broken tablets (0.3 wt% of the molten alloy). After stirring and cleaning off the dross, alloys with different MM levels were poured into thin and thick section cast iron molds (Fig. 1a and b). The thin section mold was prepared according to B108-03a ASTM standard. The main advantage of these types of mold is the application of an appropriate uphill filling system and feeding design, providing a low turbulence manner of fluid flow which consequently results in reduced oxide films, gas entrapment and porosity in cast specimens. For each section, the specimens were divided into two groups. The samples in one group were subjected to T6 treatment and were heated to 540 1C for 8 h for solid solution, then quenched in water to room temperature, and finally artificially aged at 170 1C for 7 h. For microstructural observations, the cut sections were polished and then etched by HF (5%). Samples were also deep etched in NaOH (20%) for 45 min to dissolve the aluminum matrix and reveal the eutectic Si structure for observation by SEM. Quantitative data on the microstructures were determined using an optical microscope equipped with an image analysis system (Clemex Vision Pro. Ver. 3.5.025). For minimizing the errors, the measurement was done on 50 different areas of each microstructure. The microstructural characteristics of the specimens were examined by scanning electron microscopy performed in a Vega&Tescan SEM equipped with the energy dispersive X-ray analysis (EDX) accessory. X-ray diffraction (XRD) studies for phase analysis were performed using PHILIPS-binary diffractometer applying Cu Ka radiation. Tensile test bars were machined, according to ASTM B557M02a sub-size specimens (Fig. 1c). Tensile tests were carried out on a computer controlled MTS tension machine, equipped with a strain gauge extensometer, at a constant cross-head speed of 1 mm/min. The fracture surfaces of tensile test specimens were also examined by SEM.
Fig. 1. Schematic of (a) thin, (b) thick section cast iron mold and (c) tensile test dimensions.
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3. Results and discussion 3.1. Microstructural characterization The basic microstructures of the alloy, shown in Fig. 2, is typical of hypoeutectic Al–Si alloy in both thin and thick sections, with primary a-Al dendrites and eutectic Si particles distributed around the Al dendrites to form a cell pattern, periodically repeated across the metallographic surface.
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The as-cast and heat treated microstructures of A357 alloy without and with La-base mischmetal in both thin and thick sections are shown in Figs. 3 and 4 respectively. Tables 3 and 4 also list Si particle characteristics in various A357 alloy samples solidified under high and low cooling rates, for as-cast and T6-heat treated conditions. From Fig. 3 and Table 3, it can be seen that in thin section castings, the addition of mischmetal (up to 0.1 wt%) reduces the particle length, area and the aspect ratio of Si particles. This can be described according to an impurity
Fig. 2. Optical micrographs of the A357 alloy casting in (a) thin and (b) thick section mold.
Fig. 3. The microstructures of thin section A357 alloys with different concentrations of mischmetal. (a and d) unmodified; (b and e) 0.1 wt% MM; (c and f) 1 wt% MM alloys in the (a–c) as-cast condition and (d–f) after heat treatment.
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Fig. 4. The microstructures of thick section A357 alloys with different concentrations of mischmetal. (a and d) unmodified; (b and e) 0.3 wt% MM; (c and f) 1 wt% MM alloys in the (a–c) as-cast condition and (d–f) after heat treatment.
Table 3 Si particle characteristics of various A357 alloy samples obtained at high cooling rates.
Table 4 Si particle characteristics of various A357 alloy samples obtained at low cooling rates.
Sample
MMa (wt%)
Particle length (mm)
Particle area (mm2)
Aspect ratio (%)
Sample
MMa (wt%)
Particle length (mm)
Particle area (mm2)
Aspect ratio (%)
Non-heat treated
0.00 0.05 0.10 0.50 1.00 0.00 0.05 0.10 0.50 1.00
12.0 7 4.3 11.9 7 6.2 8.5 7 4.4 10.9 7 3.1 11.2 7 4.1 8.9 7 2.0 8.5 7 1.5 6.3 7 2.4 8.07 1.6 8.2 7 1.2
21.6 78.2 16.2 74.1 13.9 74.0 14.9 76.2 15.2 74.3 39.2 710.3 38.4 78.5 23.8 79.4 33.6 73.5 37.2 712.0
5.5 7 0.7 5.2 7 0.5 3.3 7 0.7 4.8 7 1.1 5.1 7 0.9 3.4 7 1.3 2.1 7 0.6 1.6 7 0.8 1.8 7 1.2 1.8 7 1.0
Non-heat treated
0.00 0.05 0.10 0.30 0.50 1.00 0.00 0.05 0.10 0.30 0.50 1.00
31.2 7 9.4 29.0 7 8.6 28.9 7 4.2 25.8 7 4.5 27.4 7 8.2 29.3 7 6.6 21.5 7 6.2 17.7 7 6.5 15.9 7 5.2 12.9 7 4.5 14.8 7 7.2 20.0 7 9.4
80.7 733.3 74.5 711.2 70.4 724.8 64.2 725.8 65.3 736.6 80.1 733.1 161.1 749.3 119.0 710.8 96.4 734.8 70.4 76.2 131.2 729.0 156.3 750.7
6.9 7 1.5 6.1 7 1.3 5.4 7 1.8 4.2 7 0.8 6.2 7 1.3 7.2 7 1.1 5.1 7 1.6 4.9 7 1.5 4.6 7 1.1 2.7 7 0.6 3.4 7 0.6 4.6 7 0.9
Heat treated
a
Heat treated
MM ¼mischmetal. a
induced twinning theory proposed by Lu and Hellawell [23]. They suggested that modified Si fibers contain more twins than the unmodified ones and have a rough microfaceted surface. The Si fibers are crystallographically very imperfect, and each surface imperfection is a potential site for branching to occur under solidification conditions. As a result, fibers in the chemically modified eutectic are able to bend, curve and split to create a fine microstructure. Lu mentioned that a growth twin is created
MM¼ mischmetal.
at the interface when the atomic radius of the element has the correct size relative to the radius of Si (relement:rSi ¼ 1.65). Tsai et al. [3] showed that La element falls within the atomic radius range proposed to cause chemical modification. However, adding higher amounts of mischmetal is expected to introduce large intermetallic compounds. Fig. 5b shows the characteristics of the
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Fig. 5. (a) SEM photograph of Al–Si–La compound intermetallic in A357 alloy with 5 wt% mischmetal casting in thick section mold and (b) EDS spectra showing the distribution of Al, Si and La in the intermetallic.
Tables 3 and 4 also present the effect of T6 heat treatment on the size, aspect ratio and morphology of eutectic Si of samples which were solidified under high and low cooling rate conditions. From Tables 3 and 4, it can be seen that T6 heat treatment has a profound effect on the spheroidization of eutectic Si, especially in the La-based MM added alloys. It is observed that at low cooling rates (thick section castings), the characteristics of Si particles for both modified and unmodified specimens are affected more significantly after heat treatment compared to the thin section cast specimens [24]. It has been also reported that the presence of MM (in its optimum level) reduces the coarsening of the Si particles during solution heat treatment [1]. 3.2. Tensile properties Fig. 6. XRD pattern of a deep-etched A357 alloy specimen with 1 wt% mischmetal.
intermetallic phase in a specimen containing 5 wt% mischmetal. Concentration profile of the intermetallic formed in the microstructure of the alloy after the addition of 5 wt% mischmetal shows that it contains La, Si and Al atoms, which can be represented as a ternary AlSiLa according to the atomic percentages shown in Fig. 5b. The X-ray diffraction pattern of A357 alloy containing 1 wt% mischmetal is seen in Fig. 6. This is in agreement with the theoretical and experimental investigations carried out by Tsai et al. [3]. They showed that AlSiLa intermetallics can be formed when high concentrations of La are added into A356 Al alloy. On the other hand, by considering Fig. 4 and Table 4, it can be concluded that in thick section casting, similar behavior is seen by adding La-based mischmetal. However, its optimum level for improving the eutectic Si particles was found to be different (0.3 wt% MM). According to the results obtained from microstructural parameters in Tables 3 and 4, in thick section, the maximum reductions in particle length, area and the aspect ratio of the as-cast samples by the addition of mischmetal are 17.3%, 20.4% and 39.1%, respectively. While, these values for the thin section castings are 29.1%, 35.6% and 40.0%. This indicates that MM addition is more effective in modifying the microstructure of thinner sections than that of thick section castings [1].
Fig. 7 shows the stress–strain curves of thin and thick section samples, containing different concentrations of mischmetal (MM), before and after T6 heat treatment. It is seen that in both as-cast and heat treated alloys, samples which solidify under high cooling rate condition have higher tensile strength and elongation values. Besides, in each section, adding mischmetal at its optimum level improves tensile properties. Moreover, applying T6 treatment to the specimens of both sections leads to an enhancement of tensile properties. Figs. 8 and 9 show UTS and El.% values of the as-cast and heat treated A357 alloys with increasing mischmetal content in both thin and thick sections. Fig. 8a indicates that the addition of 0.1 wt% mischmetal to the thin section samples causes a significant improvement in UTS results (about 12.3% in non-heat treated and 13.6% in T6 heat treated specimens). But the use of higher mischmetal concentrations (more than 0.1 wt%) lead to a considerable reduction of UTS values. The formation of a new intermetallic phase (AlSiLa) with acicular morphology, as shown in Figs. 3– 5, may cause high levels of stress concentration and consequently the reduction of tensile properties. Fig. 8b shows that in thick section specimens, the level of added mischmetal which introduces the optimum UTS values, is 0.3 wt%, which shows about 40% improvement for all specimens before and after applying T6 heat treatment. According to Fig. 9a, addition of 0.1 wt% mischmetal to the alloy in thin section specimens increases the El.% values by about
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Fig. 7. Stress–strain diagrams of (a, b) thin and (c, d) thick section samples with different contents of mischmetal in the (a, c) as-cast and (b, d) T6-treated condition.
Fig. 8. UTS values of as-cast and heat treated A357 alloy samples, obtained at (a) high cooling rate and (b) low cooling rate condition, as a function of MM concentration.
10% in non-heat treated and 15% in T6 heat treated alloys. But, the formation of intermetallics in the existence of higher contents of mischmetal can cause deleterious effect on it. On the other hand, in thick-section specimens (Fig. 9b), addition of 0.3 wt% mischmetal improves the elongation values of the alloys for about 51% and 40%, before and after applying T6 heat treatment. The higher effect of mischmetal in improving tensile properties of
thick section specimens is due to the longer period of time provided during which solidification occurs [24]. By comparing Fig. 9a and b, it can be seen that although applying T6-heat treatment to the thick section samples leads to the reduction in El.% values, it has an adverse effect on the thin section samples. This phenomenon can be described according to what has been suggested by Martin and Dohetry [25].
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Fig. 9. Elongation percentage to fracture of as-cast and heat treated A357 alloy samples, obtained at (a) high cooling rate and (b) low cooling rate condition, as a function of MM concentration.
Fig. 10. Quality Index values of as-cast and heat treated A357 alloy samples, obtained at (a) high cooling rate and (b) low cooling rate condition, as a function of MM concentration.
They reported that when eutectic structure is subjected to a thermal treatment at elevated temperatures (such as what occurs during solution treatment), shape perturbations in the second phase constituent (Si particles) increase until, ultimately, the particles are broken into a series of nearly spherical particles. Such perturbations or interfacial instabilities cannot readily occur in plate-like, i.e. unmodified eutectic structures, exactly the same as those which form at low cooling rate conditions (Table 4), making spheroidization extremely difficult in such cases. On the other hand, it is easy for Si particles which solidified under high cooling rate conditions and are shorter in length to alter into small spheroidized particles after the solution stage. To evaluate the tensile test data, a quality index value, Q.I., which is defined as Q.I.¼UTSþ a log (El.%) was calculated, where a is 150 for Al–Si–Mg alloys [3,18–22]. This concept arose from the consideration of the relationship between ultimate tensile strength and elongation of Al–Si–Mg alloys. Since the quality index combines both strength and ductility, it is much more descriptive of the true tensile properties of a casting than either the tensile strength or the elongation alone. According to the results of quality index values, shown in Fig. 10, it can be suggested that in the as-cast thin section samples, addition of 0.1 wt% mischmetal leads to an increase of about 10% in Q.I. Besides, in thick sections, the Q.I. of the sample modified by
adding 0.3 wt% mischmetal, is about 38% higher than that of unmodified alloy. After T6 treatment, Q.I. increases up to 12.4% for thin sections and 40% for thick sections. Therefore it can be concluded that T6-heat treatment plays a crucial role in the modification effect of mischmetal. 3.3. Fragtography Fig. 11 shows the fracture surfaces of the as-cast and heat treated alloys solidified in the thin section mold. It can be seen that in both as-cast and heat treated conditions, addition of 0.1 wt% mischmetal decreases the size and increases the number of dimples slightly. But, addition of 1 wt% mischmetal leads to the formation of some cleavage planes and therefore a brittle fracture. The presence of intermetallics in these samples also provides a good reason for such a brittle fracture. The EDS spectra in Fig. 12b, showing the presence of alloying elements in the intermetallic, imply the existence of an AlSiLa intermetallic in the fracture surface of the alloy. The fracture surfaces indicate a much more ductile failure mode for the heat treated alloys, compared with that of the as-cast alloys. Fig. 13 presents SEM fractographs of the as-cast and T6 heat treated specimens solidified at the lower cooling rate condition. It can be seen that in both as-cast and heat treated conditions,
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Fig. 11. Fractographs of thin section tensile samples. (a and d) unmodified, (b and e) 0.1 wt% MM and (c and f) 1 wt% modified alloys in the (a–c) as-cast and (d–f) T6-treated condition.
Fig. 12. (a) SEM micrograph of Al–Si–La intermetallic compound in A357 alloy and (b) EDS spectra showing the presence of Al, Si and La in the intermetallic.
with the addition of 0.3 wt% MM, the fracture surface indicates the presence of more dimples as compared with the unmodified alloy, indicating a ductile fracture phenomena leading to higher tensile elongation. On the other hand, some irregular cleavage planes are apparent on the entire fracture surface of the 1 wt% mischmetal modified alloy as shown in Fig. 13c and f. It indicates that the fracture characteristics exhibit quasi-cleavage fracture which results in lower elongation values of the specimens. This is because of the acicular La-based intermetallics, which act as crack initiators and weaken the mechanical properties significantly. By comparing the as-cast and heat treated thick section specimens, it can be seen that applying T6 treatment makes the dimples larger to some extent and thus, the elongation decreases which is in agreement with the values of elongation percentage obtained from tensile tests. As reported previously by Caceres et al. [26] and Wang and Caceres [27], in the microstructures of the alloys solidified at low cooling rates, the cell boundaries are well defined by a high density of Si particles, while in the thin section samples, the small and widely spaced Si particles barely define the interdendritic
boundaries. The structure of the cell boundaries determines the degree of interaction with the plastic deformation. In thick section samples, it is seen that although the slip trace is the same for all cells in a given grain, the slip bands are interrupted by the cell boundaries. In contrast, in alloys solidified at high cooling rates, the rather open cell boundaries do not interrupt the slip bands, which are now much longer than the dendritic cells. The Si particles interact with the slip bands individually rather than collectively as in the case of distinct cell boundaries. The structure of the cell boundaries suggests that the fracture path is determined by the relative continuity of the dendritic cells boundaries. This can cause a more ductile fracture in thin section alloys as compared to the thick section castings (Figs. 11a and 13a).
4. Conclusions A study was carried out to determine the effects of La-based mischmetal and T6 heat treatment on the eutectic Si particle characteristics and tensile properties of A357 Al–Si alloy in thin
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Fig. 13. Fractographs of thick section tensile samples, (a and d) unmodified, (b and e) 0.3 wt% MM and (c and f) 1 wt% modified alloys in the (a–c) as-cast and (d–f) T6-treated condition.
and thick casting sections. Based on the results obtained and their analysis, the following conclusions were drawn: 1) The addition of mischmetal to the A357 alloy modified the eutectic Si crystals in both thin and thick section castings. The effects of modification on the microstructure were more evident in thin sections. A new AlSiLa intermetallic phase was found in the microstructures of the modified alloy with higher MM contents (Z0.3 wt% for all sections). 2) The addition of mischmetal improved the tensile strength and elongation values of the A357 alloy. To have optimal microstructural characteristics and tensile properties, the mischmetal content was found to be 0.1 wt% for thin and 0.3 wt% for thick sections. The reduction in the tensile properties of the modified alloy at higher level of MM was attributed to the effect of the intermetallic compound segregated at the eutectic areas. It was also found that the effect of MM addition on the improvement of tensile strength is more considerable in thick sections. 3) T6 heat treatment improved the UTS and Q.I. values of the modified and non-modified specimens significantly which was attributed to the effect of mischmetal on preventing Si particles from coarsening during heat treatment. 4) More dimples were found on the fracture faces of the MM modified (with optimum levels) and T6 heat treated thin section specimens, when compared with the non-modified cast alloy. New intermetallic compound was found to be responsible for the reduced tensile properties of the alloy with higher MM levels.
Acknowledgment The authors would like to thank the University of Tehran for financial support of this research.
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