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Effect of squeeze casting parameters on the mechanical properties of AZ91–Ca Mg alloys
Materials and Design 31 (2010) S50–S53
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of squeeze casting parameters on the mechanical properties of AZ91–Ca Mg alloys C.S. Goh a,*, K.S. Soh b, P.H. Oon a, B.W. Chua a a b
Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore Nanyang Technological University, School of Materials Engineering, 50 Nanyang Avenue, Singapore 639798, Singapore
a r t i c l e
i n f o
Article history: Received 10 July 2009 Accepted 18 November 2009 Available online 22 November 2009 Keywords: Magnesium Calcium Squeeze casting Microstructure
a b s t r a c t A study on the effect of squeeze casting parameters on the mechanical properties of AZ91 Mg alloy with 2 wt.% Ca incorporated was conducted. The parameters studied include squeeze casting pressure, melt temperature and mould temperature. It was found that a squeeze casting pressure of 111 MPa and a melt and mould temperature of 800 °C and 200 °C respectively gave a good combination of tensile and hardness properties in AZ91–2Ca Mg alloy. This was due primarily to the effective mould filling, microstructural refinement and good heat transfer between the molten metal and the mould. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium alloys have been widely used in the electronics, automotive and aerospace industries due to their high specific stiffness and strength. One of the technical challenges in processing Mg is its high flammability in the molten state at atmospheric conditions. Protective gases are usually required during processing to prevent the oxidation of Mg. Up to 5 wt.% of Ca is commonly added as an alloying element into Mg to reduce its flammability, improve its oxidation and corrosion resistance and to enhance its room and high temperature mechanical properties [1–3]. You et al. has found that the enhanced oxidation resistance of Ca bearing Mg alloys is due to the dense and compact oxide layer that is formed on the surface of the alloy [4]. Studies by Ninomiya et al. have shown that Ca can improve the hardness of Mg–Al alloys [5]. High pressure die casting, which is one of the most common methods for producing Mg alloys components, cannot be used to process Mg–Ca alloys. This is due to the high viscosity of the Mg–Ca melt, which results in poor mould filling, and therefore, thin parts cannot be produced. Squeeze casting of Mg–Ca alloys is an attractive alternative technique for producing near net shape Mg components with good mechanical properties that are free from macro defects. This process involves the transfer of Mg melt into a die cavity and solidification under direct high pressure. To ensure that high strength Mg–Ca components can be achieved, a study on the effect of the squeeze casting parameters is essential.
* Corresponding author. Tel.: +65 67938582; fax: +65 67925362. E-mail address: [email protected] (C.S. Goh). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.11.039
Accordingly, this work aims to determine a good combination of applied pressure, Mg melt temperature and the mould temperature for squeeze casting Mg–Ca alloys. Mechanical properties of the liquid forged samples at different squeeze casting parameters were characterised and correlated with their microstructures. 2. Experimental methodologies Commercial AZ91 Mg alloy ingots were placed together with 2 wt.% of Ca granules in a graphite crucible (2 wt.% of Ca is used because in previous our previous study, it was found when the weight percentage of Ca exceeds 2 wt.%, the room temperature ductility of Mg starts to deteriorate rapidly due to presence of higher amount of Al2Ca). The crucible was then placed in an electric resistance furnace with a constant flow of argon gas. The Mg alloys together with the Ca granules were melted at a temperature of 750 °C. The molten mixture was continuously stirred at a speed of 300 rpm for 5 min in order to ensure a uniform distribution of the alloying element. After stirring, the melt was squeeze cast to produce samples of AZ91–2 wt.% Ca (AZ91–2Ca) with dimensions of 65 mm 65 mm 5 mm. Lubricants were sprayed onto the mould to allow easy release of samples before each squeeze casting run. The squeeze casting procedure is shown in detail in Fig. 1. The squeeze casting parameters that were varied in this study are shown in Table 1. The different parameters chosen for this study are based on numerous trial runs conducted previously. For example, a maximum mould temperature of 250 °C is used because above this temperature, softening of the mould occurs and the mould life will drop drastically. The parameters chosen may
C.S. Goh et al. / Materials and Design 31 (2010) S50–S53
Punch
Molten Mg Melt
Die
(b)
(a)
Finished Mg Part
for all the three squeeze casting runs are shown in Table 2, together with the macrohardness test results. It can be seen that when the squeeze casting pressures were varied, with mould and melt temperatures set at 200 °C and 750 °C, tensile properties and macrohardness peak at a pressure of 111 MPa. A comparison was made between the microstructures of AZ91–2Ca squeeze cast using pressures of 83 MPa, 95 MPa and 111 MPa respectively as shown in Fig. 2. As the applied pressure increases, the dendritic arm spacing decreases and smaller dendrites are observed. The smallest dendrites are found at the highest pressure of 111 MPa, and this pressure also coincides with the best tensile and macrohardness properties results. Increasing applied pressure not only helps to reduce shrinkage porosity, it may also alter the microstructure due to better heat transfer into the surrounding mould material and lead to possible grain refinement effect following extensive undercooling of the melt. This effect is a result of the change in phase diagram according to the Clausius– Clapeyron equation [6]:
DT f =mP ¼ T f ðV l V S Þ=DHf
(d)
(c)
S51
ð1Þ
Ejector
Fig. 1. Schematic diagram showing the sequence of steps during squeeze casting process: (a) pre-heated die and punch, (b) molten metal poured into the mould cavity, (c) application of direct pressure and (d) ejection of solidified liquid forged Mg part.
Table 1 Squeeze casting parameters. Run no.
Squeeze casting parameters
Variables
1 2 3
Squeeze casting pressure/MPa Melt temperature/°C Mould temperature/°C
83, 95, 111 700, 750, 800 150, 200, 250
not give the best combination of tensile properties for the Mg–2Ca alloy, as all the different combinations of parameters are not exhausted. However, it does give a useful approximation of a good combination of processing parameters within three squeeze casting trials. In the first squeeze casting run, squeeze casting pressures were varied at 83, 95 and 111 MPa, while the mould and melt temperatures were kept constant at 200 °C and 750 °C respectively. In run 2, the squeeze casting pressure that gave the highest tensile and hardness properties in the first run was fixed, and the melt temperature was varied at 700 and 800 °C. In this run, the mould temperature was set at 200 °C. In run 3, the squeeze casting pressure and melt temperature from the first and second run that gave the best mechanical properties were used, and the mould temperature was varied at 150 °C and 250 °C. Specimens for microstructural examination were polished using oil-based diamond slurry to 0.5 lm mirror finish and micrographs were taken using optical microscopy. Etching using acetic and picric acid was conducted on the specimens to reveal the dendritic structures. Flat specimens with gauge dimension of 3 mm 6 mm and gauge length of 25 mm were machined from the squeeze cast samples to prepare for tensile tests at room temperature. Tensile specimens were loaded on the Instron 4505 tensile tester and tested at a strain rate of 1.33 103 s1 to determine their tensile properties. Macrohardness tests were conducted on an Indectec 8150SK hardness testing machine using a superficial Rockwell load of 15 kgf with a 1/16 in. diameter steel ball. An average of ten values was taken for macrohardness testing. 3. Results and discussion The tensile test results, which includes the yield strength (YS), ultimate tensile strength (UTS) and % strain of AZ91–2Ca samples
where Tf is the equilibrium freezing temperature, Vl and VS are the specific volumes of the liquid and solid respectively, and DHf is the latent heat of fusion. The effect of pressure on the freezing point may be roughly estimated as [7]:
P ¼ P0 expðDHf =RT f Þ
ð2Þ
where P0, DHf and R are constant, and according to Eq. (2), Tf increases with increasing pressure during solidification. With a higher external pressure on the melt, the phase diagram changes according to Eqs. (1) and (2), and the undercooling in the melt is higher than one with a lower applied pressure. This larger amount of undercooling stimulates more existing nuclei in the melt to start spontaneous heterogeneous solidification, resulting in a much refined microstructure than one that is solidified at lower pressure. The higher cooling rate at increased pressure, coupled with large undercooling, resulted in improvement in both the tensile and hardness properties of AZ91–2Ca with the largest applied pressure. Accordingly, for squeeze casting run 2, the pressure is set at 111 MPa with a mould temperature of 200 °C. When the melt temperatures were varied, it was observed that the maximum mechanical properties occur at 800 °C. The dendritic structures of the AZ91–2Ca alloys squeeze cast at 700 °C, 750 °C and 800 °C are shown in Fig. 3. It could be seen that the samples squeeze cast at 800 °C generally show more refined microstructure than those processed at lower temperatures. At 700 °C melt temperature (Fig. 3a), the dendritic structures are acicular and under higher magnification, large plate-like structures can be discerned which are detrimental to the mechanical properties of AZ91–2Ca. The dendritic structures at 750 °C and 800 °C are smaller and more homogeneous than those found at 700 °C. This could be the result of superheating where the molten Mg alloy is heated to a temperature well above the liquidus line. During the ladling process, when the Mg is transferred from the crucible to the mould, cooling of the melt occurs, and nucleant particles present in the melt can facilitate microstructural refinement [8]. Fox and Lardner [9] proposed that potential nucleant particles were likely to be a compound of Al and impurity elements. These include Al–Fe, Fe–Mn and/or Al–Fe–Mn, and in this case possible Al2Ca intermetallic compounds. These nucleant particles provide nucleation sites for grain growth and lead to more refined and homogeneous microstructure. As the superheating temperature increases, the microstructural refinement is also more significant and the mechanical properties such as yield strength and hardness are also improved accordingly. In the third run, with the pressure and melt temperature fixed at 111 MPa and 800 °C respectively, higher tensile properties were observed at a mould temperature of 200 °C. When the mould tem-
S52
C.S. Goh et al. / Materials and Design 31 (2010) S50–S53
Table 2 Tensile and macrohardness test results of squeeze cast AZ91–2Ca samples. YS/MPa
Strain/%
Macrohardness/HR15T
1.3 ± 0.4 2.1 ± 0.5 2.2 ± 0.1
72.7 ± 0.8 73.0 ± 1.1 73.8 ± 0.4
Run 2: Variable melt temp, squeeze casting pressure: 111 MPa, mould temp: 200 °C Melt temp/°C 700 102 ± 9 164 ± 13 750 114 ± 4 194 ± 8 800 123 ± 5 205 ± 5
1.4 ± 0.5 2.2 ± 0.1 2.7 ± 0.5
72.1 ± 1.3 73.8 ± 0.4 74.2 ± 0.6
Run 3: Variable mould temp, squeeze casting pressure: 111 MPa, melt temp: 800 °C Mould temp/°C 150 112 ± 10 180 ± 14 200 123 ± 5 205 ± 5 250 111 ± 11 194 ± 14
1.6 ± 0.5 2.7 ± 0.5 2.5 ± 0.7
74.5 ± 0.7 74.2 ± 0.6 73.8 ± 0.9
Run 1: Variable squeeze casting Squeeze casting pressure/MPa 83 95 111
UTS/MPa
pressure mould temp: 200 °C melt temp: 750 °C 115 ± 4 111 ± 7 114 ± 4
177 ± 9 178 ± 12 194 ± 8
Magnified views of dendritic structures
Dendrites with large dendrite arm spacing
20 µm
50 µm
20 µm
Plate-like dendritic structures
(a)
(a)
(a)
20 µm 50 µm
20 µm
(b)
(b)
(b) Increasing applied pressure produces smaller dendrites with shorter dendrite arm spacing
20 µm 50 µm
20 µm
(c) Fig. 2. Microstructures of AZ91–2Ca squeeze cast using pressures of: (a) 83 MPa, (b) 95 MPa and (c) 111 MPa.
perature was set at 150 °C, droplets of the water-based lubricant can still be discerned on the mould surface, which indicates that vaporisation of the water in the lubricant is incomplete. The lower mould temperature prevents effective mould filling through inhomogeneous coverage of lubricant and premature solidification of molten Mg melt due to the lower mould surface temperature. These factors could have contributed to the lower mechanical properties of AZ91–2Ca liquid forged at a mould temperature of
(c)
(c) Fig. 3. Dendritic structures of AZ91–2Ca alloys squeeze cast at melt temperature of: (a) 700 °C, (b) 750 °C and (c) 800 °C with pressure of 111 MPa and mould temperature of 200 °C.
150 °C. At a mould temperature of 250 °C, the cooling rate was comparatively lower and the amount of undercooling was insufficient due to the gentler temperature gradient. Therefore, refinement of the dendritic arms was not significant as could be seen in Fig. 4, where the dendrites are comparatively bigger than those samples liquid forged at 200 °C (Fig. 2c). The best mechanical properties within the parameters varied were observed when the
C.S. Goh et al. / Materials and Design 31 (2010) S50–S53
Larger dendrites as compared to samples squeeze cast using 200°C mould temperature
S53
2. A good combination of tensile and macrohardness properties was obtained at 111 MPa squeeze casting pressure, and melt and mould temperature of 800 °C and 200 °C respectively. 3. Microstructural refinement due to undercooling dictated by the mould temperature and superheating due to the relatively high melt temperature results in better mechanical properties for the present AZ91–2Ca alloy.
20 µm
References Fig. 4. AZ91–2Ca Mg samples squeeze cast using 250 °C mould temperature.
mould temperature was set at 200 °C, with a melt temperature of 800 °C and a pressure of 111 MPa. When these squeeze casting parameters were chosen, the heat transfer and mould filling resulted in microstructural features that gave a good combination of tensile and hardness properties. 4. Conclusions The following conclusions can be drawn from this study: 1. A study on the squeeze casting parameters has been successfully carried out on AZ91–2Ca Mg alloy.
[1] Wu GH, Fan Y, Gao HT, Zhai CQ, Zhu YP. The effect of Ca and rare earth elements on the microstructure, mechanical properties and corrosion behaviour of AZ91D. Mater Sci Eng A 2005;408:255–63. [2] Amberger D, Eisenlohr P, Göken M. Microstructural evolution during creep of Ca-containing AZ91. Mater Sci Eng A 2009. doi:10.1016/j.msea.2008.04.115. [3] Wang QD, Chen WZ, Zeng XQ, Lu YZ, Ding WJ, Zhu YP, et al. Effects of Ca addition on the microstructure and mechanical properties of AZ91 Mg alloy. J Mater Sci 2001;36:3035–40. [4] You BS, Park WW, Chung IS. The effect of Ca additions on the oxidation behaviour in Mg alloys. Scripta Mater 2000;42:1089–94. [5] Ninomiya R, Ojiro T, Kubota K. Improved heat resistance of Mg–Al alloys by the Ca addition. Acta Metall Mater 1995;43:669–74. [6] Ghomashchi MR, Vikhrov A. Squeeze casting: an overview. J Mater Process Technol 2000;101:1–9. [7] Cao P, Qian M, StJohn DH. Mechanisms for grain refinement of Mg alloys by superheating. Scripta Mater 2007;56:633–6. [8] Fox FA, Lardner E. J Inst Met 1945;71:1.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of squeeze casting parameters on the mechanical properties of AZ91–Ca Mg alloys C.S. Goh a,*, K.S. Soh b, P.H. Oon a, B.W. Chua a a b
Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore Nanyang Technological University, School of Materials Engineering, 50 Nanyang Avenue, Singapore 639798, Singapore
a r t i c l e
i n f o
Article history: Received 10 July 2009 Accepted 18 November 2009 Available online 22 November 2009 Keywords: Magnesium Calcium Squeeze casting Microstructure
a b s t r a c t A study on the effect of squeeze casting parameters on the mechanical properties of AZ91 Mg alloy with 2 wt.% Ca incorporated was conducted. The parameters studied include squeeze casting pressure, melt temperature and mould temperature. It was found that a squeeze casting pressure of 111 MPa and a melt and mould temperature of 800 °C and 200 °C respectively gave a good combination of tensile and hardness properties in AZ91–2Ca Mg alloy. This was due primarily to the effective mould filling, microstructural refinement and good heat transfer between the molten metal and the mould. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium alloys have been widely used in the electronics, automotive and aerospace industries due to their high specific stiffness and strength. One of the technical challenges in processing Mg is its high flammability in the molten state at atmospheric conditions. Protective gases are usually required during processing to prevent the oxidation of Mg. Up to 5 wt.% of Ca is commonly added as an alloying element into Mg to reduce its flammability, improve its oxidation and corrosion resistance and to enhance its room and high temperature mechanical properties [1–3]. You et al. has found that the enhanced oxidation resistance of Ca bearing Mg alloys is due to the dense and compact oxide layer that is formed on the surface of the alloy [4]. Studies by Ninomiya et al. have shown that Ca can improve the hardness of Mg–Al alloys [5]. High pressure die casting, which is one of the most common methods for producing Mg alloys components, cannot be used to process Mg–Ca alloys. This is due to the high viscosity of the Mg–Ca melt, which results in poor mould filling, and therefore, thin parts cannot be produced. Squeeze casting of Mg–Ca alloys is an attractive alternative technique for producing near net shape Mg components with good mechanical properties that are free from macro defects. This process involves the transfer of Mg melt into a die cavity and solidification under direct high pressure. To ensure that high strength Mg–Ca components can be achieved, a study on the effect of the squeeze casting parameters is essential.
* Corresponding author. Tel.: +65 67938582; fax: +65 67925362. E-mail address: [email protected] (C.S. Goh). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.11.039
Accordingly, this work aims to determine a good combination of applied pressure, Mg melt temperature and the mould temperature for squeeze casting Mg–Ca alloys. Mechanical properties of the liquid forged samples at different squeeze casting parameters were characterised and correlated with their microstructures. 2. Experimental methodologies Commercial AZ91 Mg alloy ingots were placed together with 2 wt.% of Ca granules in a graphite crucible (2 wt.% of Ca is used because in previous our previous study, it was found when the weight percentage of Ca exceeds 2 wt.%, the room temperature ductility of Mg starts to deteriorate rapidly due to presence of higher amount of Al2Ca). The crucible was then placed in an electric resistance furnace with a constant flow of argon gas. The Mg alloys together with the Ca granules were melted at a temperature of 750 °C. The molten mixture was continuously stirred at a speed of 300 rpm for 5 min in order to ensure a uniform distribution of the alloying element. After stirring, the melt was squeeze cast to produce samples of AZ91–2 wt.% Ca (AZ91–2Ca) with dimensions of 65 mm 65 mm 5 mm. Lubricants were sprayed onto the mould to allow easy release of samples before each squeeze casting run. The squeeze casting procedure is shown in detail in Fig. 1. The squeeze casting parameters that were varied in this study are shown in Table 1. The different parameters chosen for this study are based on numerous trial runs conducted previously. For example, a maximum mould temperature of 250 °C is used because above this temperature, softening of the mould occurs and the mould life will drop drastically. The parameters chosen may
C.S. Goh et al. / Materials and Design 31 (2010) S50–S53
Punch
Molten Mg Melt
Die
(b)
(a)
Finished Mg Part
for all the three squeeze casting runs are shown in Table 2, together with the macrohardness test results. It can be seen that when the squeeze casting pressures were varied, with mould and melt temperatures set at 200 °C and 750 °C, tensile properties and macrohardness peak at a pressure of 111 MPa. A comparison was made between the microstructures of AZ91–2Ca squeeze cast using pressures of 83 MPa, 95 MPa and 111 MPa respectively as shown in Fig. 2. As the applied pressure increases, the dendritic arm spacing decreases and smaller dendrites are observed. The smallest dendrites are found at the highest pressure of 111 MPa, and this pressure also coincides with the best tensile and macrohardness properties results. Increasing applied pressure not only helps to reduce shrinkage porosity, it may also alter the microstructure due to better heat transfer into the surrounding mould material and lead to possible grain refinement effect following extensive undercooling of the melt. This effect is a result of the change in phase diagram according to the Clausius– Clapeyron equation [6]:
DT f =mP ¼ T f ðV l V S Þ=DHf
(d)
(c)
S51
ð1Þ
Ejector
Fig. 1. Schematic diagram showing the sequence of steps during squeeze casting process: (a) pre-heated die and punch, (b) molten metal poured into the mould cavity, (c) application of direct pressure and (d) ejection of solidified liquid forged Mg part.
Table 1 Squeeze casting parameters. Run no.
Squeeze casting parameters
Variables
1 2 3
Squeeze casting pressure/MPa Melt temperature/°C Mould temperature/°C
83, 95, 111 700, 750, 800 150, 200, 250
not give the best combination of tensile properties for the Mg–2Ca alloy, as all the different combinations of parameters are not exhausted. However, it does give a useful approximation of a good combination of processing parameters within three squeeze casting trials. In the first squeeze casting run, squeeze casting pressures were varied at 83, 95 and 111 MPa, while the mould and melt temperatures were kept constant at 200 °C and 750 °C respectively. In run 2, the squeeze casting pressure that gave the highest tensile and hardness properties in the first run was fixed, and the melt temperature was varied at 700 and 800 °C. In this run, the mould temperature was set at 200 °C. In run 3, the squeeze casting pressure and melt temperature from the first and second run that gave the best mechanical properties were used, and the mould temperature was varied at 150 °C and 250 °C. Specimens for microstructural examination were polished using oil-based diamond slurry to 0.5 lm mirror finish and micrographs were taken using optical microscopy. Etching using acetic and picric acid was conducted on the specimens to reveal the dendritic structures. Flat specimens with gauge dimension of 3 mm 6 mm and gauge length of 25 mm were machined from the squeeze cast samples to prepare for tensile tests at room temperature. Tensile specimens were loaded on the Instron 4505 tensile tester and tested at a strain rate of 1.33 103 s1 to determine their tensile properties. Macrohardness tests were conducted on an Indectec 8150SK hardness testing machine using a superficial Rockwell load of 15 kgf with a 1/16 in. diameter steel ball. An average of ten values was taken for macrohardness testing. 3. Results and discussion The tensile test results, which includes the yield strength (YS), ultimate tensile strength (UTS) and % strain of AZ91–2Ca samples
where Tf is the equilibrium freezing temperature, Vl and VS are the specific volumes of the liquid and solid respectively, and DHf is the latent heat of fusion. The effect of pressure on the freezing point may be roughly estimated as [7]:
P ¼ P0 expðDHf =RT f Þ
ð2Þ
where P0, DHf and R are constant, and according to Eq. (2), Tf increases with increasing pressure during solidification. With a higher external pressure on the melt, the phase diagram changes according to Eqs. (1) and (2), and the undercooling in the melt is higher than one with a lower applied pressure. This larger amount of undercooling stimulates more existing nuclei in the melt to start spontaneous heterogeneous solidification, resulting in a much refined microstructure than one that is solidified at lower pressure. The higher cooling rate at increased pressure, coupled with large undercooling, resulted in improvement in both the tensile and hardness properties of AZ91–2Ca with the largest applied pressure. Accordingly, for squeeze casting run 2, the pressure is set at 111 MPa with a mould temperature of 200 °C. When the melt temperatures were varied, it was observed that the maximum mechanical properties occur at 800 °C. The dendritic structures of the AZ91–2Ca alloys squeeze cast at 700 °C, 750 °C and 800 °C are shown in Fig. 3. It could be seen that the samples squeeze cast at 800 °C generally show more refined microstructure than those processed at lower temperatures. At 700 °C melt temperature (Fig. 3a), the dendritic structures are acicular and under higher magnification, large plate-like structures can be discerned which are detrimental to the mechanical properties of AZ91–2Ca. The dendritic structures at 750 °C and 800 °C are smaller and more homogeneous than those found at 700 °C. This could be the result of superheating where the molten Mg alloy is heated to a temperature well above the liquidus line. During the ladling process, when the Mg is transferred from the crucible to the mould, cooling of the melt occurs, and nucleant particles present in the melt can facilitate microstructural refinement [8]. Fox and Lardner [9] proposed that potential nucleant particles were likely to be a compound of Al and impurity elements. These include Al–Fe, Fe–Mn and/or Al–Fe–Mn, and in this case possible Al2Ca intermetallic compounds. These nucleant particles provide nucleation sites for grain growth and lead to more refined and homogeneous microstructure. As the superheating temperature increases, the microstructural refinement is also more significant and the mechanical properties such as yield strength and hardness are also improved accordingly. In the third run, with the pressure and melt temperature fixed at 111 MPa and 800 °C respectively, higher tensile properties were observed at a mould temperature of 200 °C. When the mould tem-
S52
C.S. Goh et al. / Materials and Design 31 (2010) S50–S53
Table 2 Tensile and macrohardness test results of squeeze cast AZ91–2Ca samples. YS/MPa
Strain/%
Macrohardness/HR15T
1.3 ± 0.4 2.1 ± 0.5 2.2 ± 0.1
72.7 ± 0.8 73.0 ± 1.1 73.8 ± 0.4
Run 2: Variable melt temp, squeeze casting pressure: 111 MPa, mould temp: 200 °C Melt temp/°C 700 102 ± 9 164 ± 13 750 114 ± 4 194 ± 8 800 123 ± 5 205 ± 5
1.4 ± 0.5 2.2 ± 0.1 2.7 ± 0.5
72.1 ± 1.3 73.8 ± 0.4 74.2 ± 0.6
Run 3: Variable mould temp, squeeze casting pressure: 111 MPa, melt temp: 800 °C Mould temp/°C 150 112 ± 10 180 ± 14 200 123 ± 5 205 ± 5 250 111 ± 11 194 ± 14
1.6 ± 0.5 2.7 ± 0.5 2.5 ± 0.7
74.5 ± 0.7 74.2 ± 0.6 73.8 ± 0.9
Run 1: Variable squeeze casting Squeeze casting pressure/MPa 83 95 111
UTS/MPa
pressure mould temp: 200 °C melt temp: 750 °C 115 ± 4 111 ± 7 114 ± 4
177 ± 9 178 ± 12 194 ± 8
Magnified views of dendritic structures
Dendrites with large dendrite arm spacing
20 µm
50 µm
20 µm
Plate-like dendritic structures
(a)
(a)
(a)
20 µm 50 µm
20 µm
(b)
(b)
(b) Increasing applied pressure produces smaller dendrites with shorter dendrite arm spacing
20 µm 50 µm
20 µm
(c) Fig. 2. Microstructures of AZ91–2Ca squeeze cast using pressures of: (a) 83 MPa, (b) 95 MPa and (c) 111 MPa.
perature was set at 150 °C, droplets of the water-based lubricant can still be discerned on the mould surface, which indicates that vaporisation of the water in the lubricant is incomplete. The lower mould temperature prevents effective mould filling through inhomogeneous coverage of lubricant and premature solidification of molten Mg melt due to the lower mould surface temperature. These factors could have contributed to the lower mechanical properties of AZ91–2Ca liquid forged at a mould temperature of
(c)
(c) Fig. 3. Dendritic structures of AZ91–2Ca alloys squeeze cast at melt temperature of: (a) 700 °C, (b) 750 °C and (c) 800 °C with pressure of 111 MPa and mould temperature of 200 °C.
150 °C. At a mould temperature of 250 °C, the cooling rate was comparatively lower and the amount of undercooling was insufficient due to the gentler temperature gradient. Therefore, refinement of the dendritic arms was not significant as could be seen in Fig. 4, where the dendrites are comparatively bigger than those samples liquid forged at 200 °C (Fig. 2c). The best mechanical properties within the parameters varied were observed when the
C.S. Goh et al. / Materials and Design 31 (2010) S50–S53
Larger dendrites as compared to samples squeeze cast using 200°C mould temperature
S53
2. A good combination of tensile and macrohardness properties was obtained at 111 MPa squeeze casting pressure, and melt and mould temperature of 800 °C and 200 °C respectively. 3. Microstructural refinement due to undercooling dictated by the mould temperature and superheating due to the relatively high melt temperature results in better mechanical properties for the present AZ91–2Ca alloy.
20 µm
References Fig. 4. AZ91–2Ca Mg samples squeeze cast using 250 °C mould temperature.
mould temperature was set at 200 °C, with a melt temperature of 800 °C and a pressure of 111 MPa. When these squeeze casting parameters were chosen, the heat transfer and mould filling resulted in microstructural features that gave a good combination of tensile and hardness properties. 4. Conclusions The following conclusions can be drawn from this study: 1. A study on the squeeze casting parameters has been successfully carried out on AZ91–2Ca Mg alloy.
[1] Wu GH, Fan Y, Gao HT, Zhai CQ, Zhu YP. The effect of Ca and rare earth elements on the microstructure, mechanical properties and corrosion behaviour of AZ91D. Mater Sci Eng A 2005;408:255–63. [2] Amberger D, Eisenlohr P, Göken M. Microstructural evolution during creep of Ca-containing AZ91. Mater Sci Eng A 2009. doi:10.1016/j.msea.2008.04.115. [3] Wang QD, Chen WZ, Zeng XQ, Lu YZ, Ding WJ, Zhu YP, et al. Effects of Ca addition on the microstructure and mechanical properties of AZ91 Mg alloy. J Mater Sci 2001;36:3035–40. [4] You BS, Park WW, Chung IS. The effect of Ca additions on the oxidation behaviour in Mg alloys. Scripta Mater 2000;42:1089–94. [5] Ninomiya R, Ojiro T, Kubota K. Improved heat resistance of Mg–Al alloys by the Ca addition. Acta Metall Mater 1995;43:669–74. [6] Ghomashchi MR, Vikhrov A. Squeeze casting: an overview. J Mater Process Technol 2000;101:1–9. [7] Cao P, Qian M, StJohn DH. Mechanisms for grain refinement of Mg alloys by superheating. Scripta Mater 2007;56:633–6. [8] Fox FA, Lardner E. J Inst Met 1945;71:1.