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Influence of applied pressure on microstructure and tensile properties of squeeze cast magnesium Mg–Al–Ca alloy
Materials Science and Engineering A 528 (2011) 3589–3593
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of applied pressure on microstructure and tensile properties of squeeze cast magnesium Mg–Al–Ca alloy Mohsen Masoumi ∗ , Henry Hu Dept. of Mechanical, Automotive & Materials Engineering, University of Windsor, Windsor, Ontario, N9B 3P4, Canada
a r t i c l e
i n f o
Article history: Received 8 September 2010 Received in revised form 2 December 2010 Accepted 13 January 2011 Available online 19 January 2011 Keywords: Magnesium Squeeze casting Applied pressure Tensile properties Solid solubility
a b s t r a c t This paper reports the effect of applied pressure on the tensile properties and the microstructure of squeeze cast Mg–5 wt.%Al–1 wt.%Ca (AX51) alloy. In this study, applied pressures from 3 to 90 MPa were considered. It was observed that the fraction of second phases and porosity level reduces with an increase in applied pressure. The tensile tests results indicate that ultimate tensile strength (UTS), yield strength (YS) and elongation (Ef ) of AX51 alloy increase with increasing applied pressures. The improvement in tensile properties was attributed to the casting densification and presence of higher amount of solute in the matrix. The scanning electron microscopy (SEM) fractographs reveal that the fracture modes of the squeeze cast alloy is more ductile at higher applied pressures. The crack initiation occurred mostly in the vicinity of Mg–Al–Ca particles. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Since early 90s, applications of Mg alloys have received significant attention for vehicle weight-reduction [1,2]. To date, the application of Mg alloys in cars has been mostly limited to die-cast parts, which are used at room temperature, such as steering-wheel cores, brake-pedal brackets, instrument panel and seat frames [3–6]. The use of magnesium in automotive power train has a great potential for further vehicles weight reduction [1,6]. Therefore, the next generation of magnesium alloys for automotive applications need to perform at elevated temperatures. The AM and AZ alloys, the most common Mg die-cast alloys, exhibit poor creep resistance due to presence of Mg17 Al12 phase which is metallurgically unstable at elevated temperature and may lead to creep-induced precipitation causing grain boundary migration [7,8]. The creep resistance of AM alloys has been improved significantly by addition of Ca [6,8–12]. However, the addition of calcium (at certain composition ranges) adversely affected the die-castability of AX alloys due to extensive hot-cracking and die-sticking [13,14]. Hence, it is essential to develop alternative manufacturing processes, such as squeeze casting, for Ca-containing magnesium alloys. Squeeze casting, with its promising advantages, has been demonstrated for its capability to minimize the casting defects [15–18]. Applied pressure is one of the most important pro-
∗ Corresponding author. Tel.: +1 514 518 8554; fax: +1 514 398 4492. E-mail address: [email protected] (M. Masoumi). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.01.032
cess parameters in squeeze casting. The influence of the applied pressure includes, change of phase diagram equilibrium temperatures, grain size reduction, and minimization of porosity formation [16–20]. The effect of squeeze casting parameters on the microstructure and mechanical properties of aluminum alloys and their composites have been investigated to a large extent. Although, some researcher investigated the influence of process parameters on Mg alloys [19–21], still the studies are relatively limited, and hence more investigations are needed to optimize the process parameters for Mg alloys. This paper studies the effects of applied pressure on the microstructure, tensile properties and fracture behaviours of squeeze cast alloy AX51 (Mg–5 wt.%Al–1 wt.%Ca). The mechanisms for the improvement in tensile properties are discussed based on microstructural characterization. 2. Experimental procedure Commercial magnesium alloy AM50 (Mg–5 wt.%Al– 0.38 wt.%Mn) and a Mg–30 wt.%Ca master alloy (produced by Timminco Metals) were used to produce the Mg–5 wt.%Al–1 wt.%Ca alloy. In each batch, about 1.0 Kg of the AX51 alloy was prepared in an electric resistance furnace using a steel crucible under a gas mixture of sulfur hexafluoride (SF6 ) and carbon dioxide (CO2 ). The melt was held at 760 ◦ C for about 10 min, stirred few times, and then transferred for squeeze casting of cylindrical coupons with a diameter of 100 mm and a height of 15 mm. A 75 ton, vertical hydraulic press was used for direct squeeze casting. Five different levels of applied pressures, 3, 10, 30, 60
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M. Masoumi, H. Hu / Materials Science and Engineering A 528 (2011) 3589–3593
Fig. 1. The thickness microstructure of the squeeze cast AX51 under (a) 3 MPa (b) 10 MPa (c) 30 MPa (d) 90 MPa.
and 90 MPa, were investigated. Both the upper and lower dies were preheated to 300 ◦ C prior to casting. The squeeze-cast coupons were sectioned from the center of coupons, mounted, polished, and prepared based on the standard metallographic procedures for microstructure analysis. The grain size was measured by image analyzer (Buehler 2002) according to the ASTM E112-96. Mechanical properties were evaluated via tensile testing (ASTM B557) at ambient temperature using specimens of 25 mm × 6 mm × 6 mm (gauge length × width × thickness). The tensile specimens were machined from the center of squeeze cast coupons. The tensile tests were performed at an initial strain rate of 5 × 10−3 s−1 . The tensile properties, including 0.2% yield strength (YS), ultimate tensile strength (UTS), and elongation to failure (Ef ), were obtained based on the average of three tests. Samples for density evaluation were taken from the location in which metallographic specimens were obtained. Following the measurement of the specimen weight in air and distilled water, the actual density (Da ) of each specimen was determined using
Archimedes principle based on ASTM standard D3800: Da =
Wa Dw Wa − Ww
(1)
where Wa and Ww are the weight of the specimens in air and water, respectively, and Dw is the density of water. The porosity of each specimen was calculated by the following equation: %Porosity =
D − D t a Dt
× 100%
(2)
where Dt is the density of alloy AX51 squeeze cast under 90 MPa, since the standard density of AX51 alloys is not available in the literature. The detailed features of the microstructure were also characterized by a JSM-5800LV scanning electron microscope (SEM) with a maximum resolution of 100 nm. Fractured surfaces of tensile specimens were analyzed by the SEM to ascertain the nature of fracture mechanisms.
Fig. 2. SEM micrograph showing the second phase particles and the EDS spectrum of squeeze cast AX51 under 60 MPa.
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Fig. 3. Effect of applied pressure levels on (a) density (b) porosity content of alloy AX51.
Fig. 4. (a) Typical true stress vs. strain curves of AX51 alloys squeeze cast under 3 and 90 MPa (b) effect of pressure levels on UTS, YS and elongation of AX51 alloy.
Fig. 5. SEM fractographs of squeeze cast AX51 under (a and b) 90 MPa (c and d) 3 MPa ((a and c) low magnification and (b and d) high magnification).
Fig. 6. Optical micrograph showing crack origins in AX51 squeeze cast under (a) 3 MPa (b) 90 MPa.
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3. Results and discussion 3.1. Microstructure analysis Fig. 1a–d presents the optical microstructure of the as-cast AX51 alloy, squeeze cast under different pressure levels. Second phases with plate-like morphology and round shape are observed both in the matrix and at the grain boundaries. However, the plate-like phase in sample cast under pressure of 3 MPa is much thicker than that cast under pressure of 90 MPa. The grain sizes of the alloy cast under pressures of 3 and 90 were measured as 40.2–39.0 m, respectively. In other words, this observation indicates that an increase in applied pressure does not lead to a grain size reduction, which seems to be in contrary to the results presented by other researchers [20,22]. The minor effect of applied pressures on grain size may be attributed to casting geometry related to heat transfer taking place during squeeze casting [15]. In general, during squeeze casting, heat transfer across the interface between the casting and the die is enhanced with applied pressure, which eliminates (reduces) air gaps at casting/die interface. In the present casting, the side area covers about 23% of total casting surface area, meanwhile the top and bottom surfaces cover about 77% of the casting surface area. The top and bottom surfaces are in complete contact with die during applying pressure. Therefore, despite the elimination (reduction) of the air gap between the casting side surface and die wall by the applied pressure, but due to the dominant effect of the top and bottom surfaces in heat transfer, the increase in the applied pressure does not lead to a noticeable grain refining. This confirms that the process is mainly suited to chunky components having a small aspect ratio i.e. the width and height of the casting have the same dimensions. Moreover, the die temperature influences its capability to absorb heat; the relatively high die temperature which was employed in this study minimizes the difference between the casting and die temperature, which could reduce heat transfer rate from the casting to the die. In an effort to quantitatively determine the role of applied pressure on the formation of second phases, the area fraction of second phases, fsp , in AX51 alloy cast under different levels of applied pressure was determined by image analysis. It was observed that as the applied pressure increases, the fsp decreases from 11.2% to 8.6%. The phenomenon of a reduction in fsp with an increase in applied pressure is attributed to the increase of the solute in the solid-solution phase. This result is in good agreement with that reported for Mg–Al, where the increase in pressure, increased the solid solubility of aluminum in magnesium [23]. Also, Lipchin [24] reported that by applying pressure during solidification, the solubility of the second element in matrix increases. However, for the Mg–Al system, it was reported that the solid solubility decreased with an increase in applied pressure during squeeze casting [24]. Lipchin [24] also claimed that, as the cooling rate (CR) increases from 3 to 15 ◦ K/s, the solubility of the solute in the matrix increases. This result is not in line with results reported by other researchers [25–27]. During solidification two phenomena occur in the same time (i) solute rejection from solidifying metal to liquid metal (ii) diffusion of solute from the liquid/solid interface back into the solid. It is known that at very low CR (close to the equilibrium condition), the solute can diffuse from the liquid/solid interface back into the solid phase and so the amount of the eutectic phases is close to the equilibrium fraction (fsp ) obtained by the lever rule. On the other hand, at rapid solidification rates (>200 ◦ C/s), the solute rejection from solidifying metal to liquid metal is restricted; “solute engulfing” produces a supersaturated single-phase solid solution (␣-Mg) and fsp is thus much reduced. Between these two conditions, the Scheil non-equilibrium solidification mode prevails, where backdiffusion of the solute into the solid is limited (formation of an
“undersaturated” solid-solution) and solute rejection into the liquid is high, leading to a higher amount of second phases than that of the equilibrium condition [25–27]. The cooling rates reported by Lipchin [24] are in Scheil non-equilibrium solidification mode range and, therefore, it is expected that as the cooling rate increases the solute in the solid phase decreases. In order to confirm the increase in solubility with increasing applied pressures, the microhardness of the solid-solution matrix was measured along the longitudinal directions. The microhardness measurements show that as the applied pressure increases from 3 to 90 MPa, the hardness of matrix increases from 51 to 58 HV, which could be attributed to presence of higher amount of solute in the matrix, which increases the solid-solution strengthening effect. However, the increase in microhardness measurements might be attributed to higher dislocation densities with increasing squeeze cast pressures [28]. In line with the observations above, furthermore, the semi-quantitative EDS analysis (on the matrix close to the second phases) indicates that the increase in applied pressure from 3 to 90 MPa corresponds to an increase in weight percentage of Al in solid-solution from 2.8 to 3.3 wt.%, respectively. SEM micrographs and EDS analysis of squeeze cast AX51 under 60 MPa are shown in Fig. 2a–d. The EDS spectra confirm that the matrix is ␣-Mg (solid solution of Mg–Al), and particles are eutectic phase presumably (Al,Mg)2 Ca which precipitates around grain boundaries, and the round white particles as an intermetallic phase containing aluminum and manganese presumably Al5 Mn8 . Oxygen peaks, which appear in the spectra could have resulted from surface oxidation during and after sample preparation. The results indicate that the squeeze cast AX51 alloy contains the same types of the second phases, which were presented in the similar alloy made by a die casting process [10]. 3.2. Material densification Fig. 3a and b presents the density and porosity measurements of the AX51 alloy squeeze cast under various pressure levels. It is evident from Fig. 3a that the density of squeeze cast AX51 samples increases with an increase in applied pressure. The porosity reduction in squeeze cast AX51 could be attributed to the fact that the applied pressure enables the melt to feed the microshrinkages, which form in the last solidifying region of casting. Moreover, the applied pressure suppresses the nucleation of gas pores, and significantly decreases the size of entrapped gas [17,18]. Consequently, the alloy becomes highly densified with considerably low amount of porosity. 3.3. Tensile properties Typical true stress–strain curves of AX51 alloy squeeze-cast under pressures of 3 and 90 MPa are shown in Fig. 4a. The effect of applied pressures on the tensile properties of squeeze cast AX51 alloy is shown in Fig. 4b. It is observed that an increase in pressure levels brings a significant improvement in the ductility and certain improvements in the UTS and the yield strength. The elongation, UTS and YS of the alloy cast under 90 MPa pressure is 5.4%, 184 MPa and 90 MPa, which are 66%, 20% and 13% higher than the corresponding values of the specimens cast under 3 MPa. However, increasing the applied pressure beyond 30 MPa seems to provide minute improvement in the tensile properties of squeeze cast AX51 alloy. It is well known that the mechanical properties of an alloy consisting of a ductile phase and a hard brittle phase depend on the distribution of the brittle phase in the microstructure. If the brittle phase is present as a grain boundary envelope, the alloy tends to be brittle. However, the brittleness of the alloy is reduced somewhat if the brittle phase is in the form of discontinuous particles
M. Masoumi, H. Hu / Materials Science and Engineering A 528 (2011) 3589–3593
at grain boundaries [29]. Therefore, the increase in the UTS and elongation with increasing applied pressure could be attributed, in part, to materials densification (porosity reduction) and, in part, to the reduction of the second phase fraction which form a semicontinuous network around grain boundaries. The slight increase in the yield strength might be attributed to the higher dislocation density [28] and the presence of a higher amount of solute in the matrix in samples squeeze cast under higher applied pressure. Both the increase in the number of dislocations and the increase in solute atoms increase the lattice strains that interact with active dislocations, impeding their motion and causing an increase in the yield strength of the material [29]. 3.4. Fracture behaviour The SEM fractographs of the squeeze cast AX51 are shown in Fig. 5a–d. It can be observed that the fracture behaviour is somehow influenced by the applied pressure levels during the squeeze cast process. The fracture surface morphologies of the both samples cast under 3 and 90 MPa show dimpled rupture pattern, which is considered as a characteristic of a ductile fracture mode [30]. As the pressure increases, the fracture mode of the alloy tends to be more ductile, which is characterized by the presence of deeper dimples in the 90 MPa specimen compared with 3 MPa sample. In ductile fracture mode, the dimples are formed by the localized microvoid coalescence when fractured under a continual rising load [30]. The microvoids nucleate in material at the areas of localized high plastic deformation such as that associated with second phase particles, inclusions, and grain boundaries. As the load on the material increases, the microvoids grow, coalesce, and eventually form a continuous fracture surface [30]. The less ductile fracture mode of samples squeeze cast under 3 MPa can be attributed to the higher porosity content and the higher amount of second phases. The presence of these two lead to stress concentration, which in turn results in crack initiation. The damaged microstructures underneath the fractured surfaces, Fig. 6, to some extent, show that the presence of (Al,Mg)2 Ca phase might be the main cause of the intergranular fracture. The SEM observations of the fracture surfaces are in a good agreement with the tensile behaviour of the alloy. 4. Conclusions The effect of applied pressure on microstructure and mechanical properties of squeeze cast AX51 alloy was investigated. The alloys contain primary ␣-Mg (Al,Mg)2 Ca intermetallic and Mn–Al intermetallic phases. Due to the high aspect ratio of the casting geometry, no significant improvement in grain structure was observed as the applied pressures increased. The results of the tensile testing indicate that the mechanical properties, UTS, YS, and elongation, increase with an increase in the applied pressures during solidification. The material densification and reduction in
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second phase fraction are deemed to be responsible for the increase in tensile properties. The observation via SEM fractography and tensile results indicates that, as the applied pressures increase, the fracture mode become more ductile. Acknowledgements The authors would like to take this opportunity to thank the Natural Sciences and Engineering Research Council of Canada for supporting this work. One of the co-authors (M. Masoumi) wishes to acknowledge the Government of Ontario and University of Windsor for financial support in the form of an Ontario Graduate Scholarship and a University of Windsor Tuition Scholarship, respectively. One of author would like to thank Prof. Pekguleryuz for interesting discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
H. Hu, A. Yu, N. Li, J.E. Allison, Mater. Manuf. Process. 18 (5) (2003) 687–717. E. Doege, K. Droder, J. Mater. Process. Technol. 115 (2001) 14–19. T. Kaneko, M. Suzuki, Mater. Sci. Forum 419–422 (Part 1) (2003) 67–72. B. Engl, Light Met. Age 63 (5) (2005) 14–19. E. Baril, P. Labelle, M.O. Pekguleryuz, J. Met. (JOM-US) 55 (11) (2003) 34–39. M.O. Pekguleryuz, A.A. Kaya, Adv. Eng. Mater. 5 (12) (2003) 866–878. M.M. Avedesian, H. Baker, Magnesium and Magnesium Alloys, ASM International, Materials Park, OH, USA, 1999, pp. 314. M. Pekguleryuz, M. Celikin, Int. Mater. Rev. 55 (4) (2010) 197–217. R. Ninomiya, T. Ojiro, K. Kubota, Acta Metall. Mater. 43 (2) (1995) 669–674. K.Y. Sohn, W. Jones, J.J. Berkmortel, H. Hu, J.E. Allison, Part 2 of 2 in SAE 2000 World Congress, Detroit, Michgan, 2000. pp. 2000-01-1120), doi:10.4271/2000-01-1120. M.O. Pekgulryuz, J. Renaud, Magnesium Technology, in: H. Kaplan, J. Hryn and B. Clow (Eds.), TMS, 2000, pp. 279–284. Y. Terada, R. Sota, N. Ishimatsu, T. Sato, K. Ohori, Metall. Mater. Trans. A 35A (2004) 3029–3032. J. Berkmortel, H. Hu, J.E. Kearns, J.E. Allison, Part 1 of 2 in SAE, Detroit, Michigan, 2000, pp. 2000-01-1119, doi:10.4271/2000-01-1119. M. Pekguleryuz, A. Luo, “Creep Resistant Magnesium Alloy for die casting”. Patent WO96/25529, 1995. M. Masoumi, H. Hu, N. Li, Magnesium Technology, in: R.S. Beals (Ed.), Orlando, FL, TMS 2007, pp. 109–114. D.M. Stefanescu, J.R. Davis, J.D. Destefani, Casting, ninth ed., ASM International, Metals Park, Ohio, USA, 1998. M.R. Ghomashchi, A. Vikhrov, J. Mater. Process. Technol. 101 (1–3) (2000) 1–9. H. Hu, J. Mater. Sci. A 33 (6) (1998) 1579–1589. T.M. Yue, G.A. Chadwick, J. Mater. Process. Technol. 58 (2–3) (1996) 302–307. M.S. Yong, A.J. Clegg, J. Mater. Process. Technol. 145 (2004) 134–141. C.S. Goh, K.S. Soh, P.H. Oon, B.W. Chua, Mater. Des. 31 (2010) S50–S53. M.T. Abou El-khair, Mater. Lett. 59 (8–9) (2005) 894–900. M. Akihito, M.F.M. Masaya, K. Iwao, T. Yutaka, J. Soc. Mater. Sci. (Jpn.) 52 (7) (2003) 851–856. T.N. Lipchin, Metalloved. Term. Obrab. Met. 4 (1986) 43–46. M. Masoumi, M. Pekguleryuz, Trans. Am. Foundry Soc. 117 (2009) 617–626. J.A. Sarreal, G.J. Abbaschian, Metall. Trans. A 17A (November (11)) (1986) 2063–2073. B. Dutta, M. Rettenmayr, Mater. Sci. Technol. (UK) 18 (December (12)) (2002) 1428–1434. T.N. Lipchin, P.A. Bykov, Liteinoe Proizv 3 (1973) 31–33. G.E. Dieter, Mechanical Metallurgy, McGraw-Hill, New York, 1976. K. Mills, Fractography, vol. 12, 9th ed., ASM International, Metals Park, Ohio, 1987.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of applied pressure on microstructure and tensile properties of squeeze cast magnesium Mg–Al–Ca alloy Mohsen Masoumi ∗ , Henry Hu Dept. of Mechanical, Automotive & Materials Engineering, University of Windsor, Windsor, Ontario, N9B 3P4, Canada
a r t i c l e
i n f o
Article history: Received 8 September 2010 Received in revised form 2 December 2010 Accepted 13 January 2011 Available online 19 January 2011 Keywords: Magnesium Squeeze casting Applied pressure Tensile properties Solid solubility
a b s t r a c t This paper reports the effect of applied pressure on the tensile properties and the microstructure of squeeze cast Mg–5 wt.%Al–1 wt.%Ca (AX51) alloy. In this study, applied pressures from 3 to 90 MPa were considered. It was observed that the fraction of second phases and porosity level reduces with an increase in applied pressure. The tensile tests results indicate that ultimate tensile strength (UTS), yield strength (YS) and elongation (Ef ) of AX51 alloy increase with increasing applied pressures. The improvement in tensile properties was attributed to the casting densification and presence of higher amount of solute in the matrix. The scanning electron microscopy (SEM) fractographs reveal that the fracture modes of the squeeze cast alloy is more ductile at higher applied pressures. The crack initiation occurred mostly in the vicinity of Mg–Al–Ca particles. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Since early 90s, applications of Mg alloys have received significant attention for vehicle weight-reduction [1,2]. To date, the application of Mg alloys in cars has been mostly limited to die-cast parts, which are used at room temperature, such as steering-wheel cores, brake-pedal brackets, instrument panel and seat frames [3–6]. The use of magnesium in automotive power train has a great potential for further vehicles weight reduction [1,6]. Therefore, the next generation of magnesium alloys for automotive applications need to perform at elevated temperatures. The AM and AZ alloys, the most common Mg die-cast alloys, exhibit poor creep resistance due to presence of Mg17 Al12 phase which is metallurgically unstable at elevated temperature and may lead to creep-induced precipitation causing grain boundary migration [7,8]. The creep resistance of AM alloys has been improved significantly by addition of Ca [6,8–12]. However, the addition of calcium (at certain composition ranges) adversely affected the die-castability of AX alloys due to extensive hot-cracking and die-sticking [13,14]. Hence, it is essential to develop alternative manufacturing processes, such as squeeze casting, for Ca-containing magnesium alloys. Squeeze casting, with its promising advantages, has been demonstrated for its capability to minimize the casting defects [15–18]. Applied pressure is one of the most important pro-
∗ Corresponding author. Tel.: +1 514 518 8554; fax: +1 514 398 4492. E-mail address: [email protected] (M. Masoumi). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.01.032
cess parameters in squeeze casting. The influence of the applied pressure includes, change of phase diagram equilibrium temperatures, grain size reduction, and minimization of porosity formation [16–20]. The effect of squeeze casting parameters on the microstructure and mechanical properties of aluminum alloys and their composites have been investigated to a large extent. Although, some researcher investigated the influence of process parameters on Mg alloys [19–21], still the studies are relatively limited, and hence more investigations are needed to optimize the process parameters for Mg alloys. This paper studies the effects of applied pressure on the microstructure, tensile properties and fracture behaviours of squeeze cast alloy AX51 (Mg–5 wt.%Al–1 wt.%Ca). The mechanisms for the improvement in tensile properties are discussed based on microstructural characterization. 2. Experimental procedure Commercial magnesium alloy AM50 (Mg–5 wt.%Al– 0.38 wt.%Mn) and a Mg–30 wt.%Ca master alloy (produced by Timminco Metals) were used to produce the Mg–5 wt.%Al–1 wt.%Ca alloy. In each batch, about 1.0 Kg of the AX51 alloy was prepared in an electric resistance furnace using a steel crucible under a gas mixture of sulfur hexafluoride (SF6 ) and carbon dioxide (CO2 ). The melt was held at 760 ◦ C for about 10 min, stirred few times, and then transferred for squeeze casting of cylindrical coupons with a diameter of 100 mm and a height of 15 mm. A 75 ton, vertical hydraulic press was used for direct squeeze casting. Five different levels of applied pressures, 3, 10, 30, 60
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Fig. 1. The thickness microstructure of the squeeze cast AX51 under (a) 3 MPa (b) 10 MPa (c) 30 MPa (d) 90 MPa.
and 90 MPa, were investigated. Both the upper and lower dies were preheated to 300 ◦ C prior to casting. The squeeze-cast coupons were sectioned from the center of coupons, mounted, polished, and prepared based on the standard metallographic procedures for microstructure analysis. The grain size was measured by image analyzer (Buehler 2002) according to the ASTM E112-96. Mechanical properties were evaluated via tensile testing (ASTM B557) at ambient temperature using specimens of 25 mm × 6 mm × 6 mm (gauge length × width × thickness). The tensile specimens were machined from the center of squeeze cast coupons. The tensile tests were performed at an initial strain rate of 5 × 10−3 s−1 . The tensile properties, including 0.2% yield strength (YS), ultimate tensile strength (UTS), and elongation to failure (Ef ), were obtained based on the average of three tests. Samples for density evaluation were taken from the location in which metallographic specimens were obtained. Following the measurement of the specimen weight in air and distilled water, the actual density (Da ) of each specimen was determined using
Archimedes principle based on ASTM standard D3800: Da =
Wa Dw Wa − Ww
(1)
where Wa and Ww are the weight of the specimens in air and water, respectively, and Dw is the density of water. The porosity of each specimen was calculated by the following equation: %Porosity =
D − D t a Dt
× 100%
(2)
where Dt is the density of alloy AX51 squeeze cast under 90 MPa, since the standard density of AX51 alloys is not available in the literature. The detailed features of the microstructure were also characterized by a JSM-5800LV scanning electron microscope (SEM) with a maximum resolution of 100 nm. Fractured surfaces of tensile specimens were analyzed by the SEM to ascertain the nature of fracture mechanisms.
Fig. 2. SEM micrograph showing the second phase particles and the EDS spectrum of squeeze cast AX51 under 60 MPa.
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Fig. 3. Effect of applied pressure levels on (a) density (b) porosity content of alloy AX51.
Fig. 4. (a) Typical true stress vs. strain curves of AX51 alloys squeeze cast under 3 and 90 MPa (b) effect of pressure levels on UTS, YS and elongation of AX51 alloy.
Fig. 5. SEM fractographs of squeeze cast AX51 under (a and b) 90 MPa (c and d) 3 MPa ((a and c) low magnification and (b and d) high magnification).
Fig. 6. Optical micrograph showing crack origins in AX51 squeeze cast under (a) 3 MPa (b) 90 MPa.
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3. Results and discussion 3.1. Microstructure analysis Fig. 1a–d presents the optical microstructure of the as-cast AX51 alloy, squeeze cast under different pressure levels. Second phases with plate-like morphology and round shape are observed both in the matrix and at the grain boundaries. However, the plate-like phase in sample cast under pressure of 3 MPa is much thicker than that cast under pressure of 90 MPa. The grain sizes of the alloy cast under pressures of 3 and 90 were measured as 40.2–39.0 m, respectively. In other words, this observation indicates that an increase in applied pressure does not lead to a grain size reduction, which seems to be in contrary to the results presented by other researchers [20,22]. The minor effect of applied pressures on grain size may be attributed to casting geometry related to heat transfer taking place during squeeze casting [15]. In general, during squeeze casting, heat transfer across the interface between the casting and the die is enhanced with applied pressure, which eliminates (reduces) air gaps at casting/die interface. In the present casting, the side area covers about 23% of total casting surface area, meanwhile the top and bottom surfaces cover about 77% of the casting surface area. The top and bottom surfaces are in complete contact with die during applying pressure. Therefore, despite the elimination (reduction) of the air gap between the casting side surface and die wall by the applied pressure, but due to the dominant effect of the top and bottom surfaces in heat transfer, the increase in the applied pressure does not lead to a noticeable grain refining. This confirms that the process is mainly suited to chunky components having a small aspect ratio i.e. the width and height of the casting have the same dimensions. Moreover, the die temperature influences its capability to absorb heat; the relatively high die temperature which was employed in this study minimizes the difference between the casting and die temperature, which could reduce heat transfer rate from the casting to the die. In an effort to quantitatively determine the role of applied pressure on the formation of second phases, the area fraction of second phases, fsp , in AX51 alloy cast under different levels of applied pressure was determined by image analysis. It was observed that as the applied pressure increases, the fsp decreases from 11.2% to 8.6%. The phenomenon of a reduction in fsp with an increase in applied pressure is attributed to the increase of the solute in the solid-solution phase. This result is in good agreement with that reported for Mg–Al, where the increase in pressure, increased the solid solubility of aluminum in magnesium [23]. Also, Lipchin [24] reported that by applying pressure during solidification, the solubility of the second element in matrix increases. However, for the Mg–Al system, it was reported that the solid solubility decreased with an increase in applied pressure during squeeze casting [24]. Lipchin [24] also claimed that, as the cooling rate (CR) increases from 3 to 15 ◦ K/s, the solubility of the solute in the matrix increases. This result is not in line with results reported by other researchers [25–27]. During solidification two phenomena occur in the same time (i) solute rejection from solidifying metal to liquid metal (ii) diffusion of solute from the liquid/solid interface back into the solid. It is known that at very low CR (close to the equilibrium condition), the solute can diffuse from the liquid/solid interface back into the solid phase and so the amount of the eutectic phases is close to the equilibrium fraction (fsp ) obtained by the lever rule. On the other hand, at rapid solidification rates (>200 ◦ C/s), the solute rejection from solidifying metal to liquid metal is restricted; “solute engulfing” produces a supersaturated single-phase solid solution (␣-Mg) and fsp is thus much reduced. Between these two conditions, the Scheil non-equilibrium solidification mode prevails, where backdiffusion of the solute into the solid is limited (formation of an
“undersaturated” solid-solution) and solute rejection into the liquid is high, leading to a higher amount of second phases than that of the equilibrium condition [25–27]. The cooling rates reported by Lipchin [24] are in Scheil non-equilibrium solidification mode range and, therefore, it is expected that as the cooling rate increases the solute in the solid phase decreases. In order to confirm the increase in solubility with increasing applied pressures, the microhardness of the solid-solution matrix was measured along the longitudinal directions. The microhardness measurements show that as the applied pressure increases from 3 to 90 MPa, the hardness of matrix increases from 51 to 58 HV, which could be attributed to presence of higher amount of solute in the matrix, which increases the solid-solution strengthening effect. However, the increase in microhardness measurements might be attributed to higher dislocation densities with increasing squeeze cast pressures [28]. In line with the observations above, furthermore, the semi-quantitative EDS analysis (on the matrix close to the second phases) indicates that the increase in applied pressure from 3 to 90 MPa corresponds to an increase in weight percentage of Al in solid-solution from 2.8 to 3.3 wt.%, respectively. SEM micrographs and EDS analysis of squeeze cast AX51 under 60 MPa are shown in Fig. 2a–d. The EDS spectra confirm that the matrix is ␣-Mg (solid solution of Mg–Al), and particles are eutectic phase presumably (Al,Mg)2 Ca which precipitates around grain boundaries, and the round white particles as an intermetallic phase containing aluminum and manganese presumably Al5 Mn8 . Oxygen peaks, which appear in the spectra could have resulted from surface oxidation during and after sample preparation. The results indicate that the squeeze cast AX51 alloy contains the same types of the second phases, which were presented in the similar alloy made by a die casting process [10]. 3.2. Material densification Fig. 3a and b presents the density and porosity measurements of the AX51 alloy squeeze cast under various pressure levels. It is evident from Fig. 3a that the density of squeeze cast AX51 samples increases with an increase in applied pressure. The porosity reduction in squeeze cast AX51 could be attributed to the fact that the applied pressure enables the melt to feed the microshrinkages, which form in the last solidifying region of casting. Moreover, the applied pressure suppresses the nucleation of gas pores, and significantly decreases the size of entrapped gas [17,18]. Consequently, the alloy becomes highly densified with considerably low amount of porosity. 3.3. Tensile properties Typical true stress–strain curves of AX51 alloy squeeze-cast under pressures of 3 and 90 MPa are shown in Fig. 4a. The effect of applied pressures on the tensile properties of squeeze cast AX51 alloy is shown in Fig. 4b. It is observed that an increase in pressure levels brings a significant improvement in the ductility and certain improvements in the UTS and the yield strength. The elongation, UTS and YS of the alloy cast under 90 MPa pressure is 5.4%, 184 MPa and 90 MPa, which are 66%, 20% and 13% higher than the corresponding values of the specimens cast under 3 MPa. However, increasing the applied pressure beyond 30 MPa seems to provide minute improvement in the tensile properties of squeeze cast AX51 alloy. It is well known that the mechanical properties of an alloy consisting of a ductile phase and a hard brittle phase depend on the distribution of the brittle phase in the microstructure. If the brittle phase is present as a grain boundary envelope, the alloy tends to be brittle. However, the brittleness of the alloy is reduced somewhat if the brittle phase is in the form of discontinuous particles
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at grain boundaries [29]. Therefore, the increase in the UTS and elongation with increasing applied pressure could be attributed, in part, to materials densification (porosity reduction) and, in part, to the reduction of the second phase fraction which form a semicontinuous network around grain boundaries. The slight increase in the yield strength might be attributed to the higher dislocation density [28] and the presence of a higher amount of solute in the matrix in samples squeeze cast under higher applied pressure. Both the increase in the number of dislocations and the increase in solute atoms increase the lattice strains that interact with active dislocations, impeding their motion and causing an increase in the yield strength of the material [29]. 3.4. Fracture behaviour The SEM fractographs of the squeeze cast AX51 are shown in Fig. 5a–d. It can be observed that the fracture behaviour is somehow influenced by the applied pressure levels during the squeeze cast process. The fracture surface morphologies of the both samples cast under 3 and 90 MPa show dimpled rupture pattern, which is considered as a characteristic of a ductile fracture mode [30]. As the pressure increases, the fracture mode of the alloy tends to be more ductile, which is characterized by the presence of deeper dimples in the 90 MPa specimen compared with 3 MPa sample. In ductile fracture mode, the dimples are formed by the localized microvoid coalescence when fractured under a continual rising load [30]. The microvoids nucleate in material at the areas of localized high plastic deformation such as that associated with second phase particles, inclusions, and grain boundaries. As the load on the material increases, the microvoids grow, coalesce, and eventually form a continuous fracture surface [30]. The less ductile fracture mode of samples squeeze cast under 3 MPa can be attributed to the higher porosity content and the higher amount of second phases. The presence of these two lead to stress concentration, which in turn results in crack initiation. The damaged microstructures underneath the fractured surfaces, Fig. 6, to some extent, show that the presence of (Al,Mg)2 Ca phase might be the main cause of the intergranular fracture. The SEM observations of the fracture surfaces are in a good agreement with the tensile behaviour of the alloy. 4. Conclusions The effect of applied pressure on microstructure and mechanical properties of squeeze cast AX51 alloy was investigated. The alloys contain primary ␣-Mg (Al,Mg)2 Ca intermetallic and Mn–Al intermetallic phases. Due to the high aspect ratio of the casting geometry, no significant improvement in grain structure was observed as the applied pressures increased. The results of the tensile testing indicate that the mechanical properties, UTS, YS, and elongation, increase with an increase in the applied pressures during solidification. The material densification and reduction in
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second phase fraction are deemed to be responsible for the increase in tensile properties. The observation via SEM fractography and tensile results indicates that, as the applied pressures increase, the fracture mode become more ductile. Acknowledgements The authors would like to take this opportunity to thank the Natural Sciences and Engineering Research Council of Canada for supporting this work. One of the co-authors (M. Masoumi) wishes to acknowledge the Government of Ontario and University of Windsor for financial support in the form of an Ontario Graduate Scholarship and a University of Windsor Tuition Scholarship, respectively. One of author would like to thank Prof. Pekguleryuz for interesting discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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