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Effects of squeeze casting parameters on density, macrostructure and hardness of LM13 alloy
Materials Science and Engineering A 428 (2006) 135–140
Effects of squeeze casting parameters on density, macrostructure and hardness of LM13 alloy A. Maleki, B. Niroumand ∗ , A. Shafyei Isfahan University of Technology, Isfahan, Iran Received 28 November 2005; accepted 25 April 2006
Abstract In this research the effects of applied pressure, melt and die temperatures on the density, macrostructure and hardness of squeeze cast LM13 alloy were investigated. The results showed that the density of the samples decreased with application of a 20 MPa external pressure but it increased steadily for higher applied pressures up to about 106 MPa after which it became almost constant. Increasing the applied pressure resulted in smaller grain size and improved hardness. A decrease in the melt or die temperature rendered similar effects on the macrostructure and hardness of the samples. The results were explained based on the changes in nucleation rate, cooling rate, and operation of feeding mechanisms brought about by the application of pressure during solidification. © 2006 Elsevier B.V. All rights reserved. Keywords: Squeeze casting; Solidification; Aluminum alloy; Macrostructure; Hardness
1. Introduction Castings are produced via different processes each of which enjoys certain advantages and disadvantages. A main disadvantage of conventional casting processes such as high-pressure die casting is formation of such defects as gas and shrinkage porosities which decrease mechanical properties, integrity, and reliability of the products [1–4]. Squeeze casting is one of the modern casting processes, which has been invented to address these shortcomings and has a high potential to produce sound castings. In this process, which can be regarded as a combination of casting and forging processes [1,5], a high pressure is applied on the melt during solidification. This external pressure is held on the melt until the end of solidification. North American Die Casting Association (NADCA) describes squeeze casting as a casting process that uses low die filling velocity to create minimum turbulence and pressurizes the melt to produce high quality castings without a solution heat treatment [3]. The fact that castings can be used in as cast condition and after a limited finishing treatment has secured a place for squeeze casting among other near net shaping methods [1,6–8].
∗
Corresponding author. Tel.: +98 311 391 2750; fax: +98 311 391 2752. E-mail address: [email protected] (B. Niroumand).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.04.099
Application of pressure on a molten metal during solidification may cause the following effects: 1. Change in the melting point. Melting point (liquidus temperature) of most metals and alloys increases under pressure [2]. The increase satisfies Clausius–Clapeyron equation [1,2,9]. This characteristic can be utilized to create sudden large undercooling in the melt upon application of pressure if the melt temperature and timing of pressure application are accurately controlled. 2. Change of solidification rate. In most casting processes an air gap is formed shortly after pouring between the die and the solidified outer shell of the casting [6]. This is due to simultaneous contraction of the shell and expansion of the die. Air gap formation changes heat transfer mechanism from conduction to convection and radiation and causes a significant decrease in heat transfer rate and consequently decreases the cooling rate [5,10]. In squeeze casting, air gap formation is eliminated as a result of the applied pressure on the casting and therefore heat transfer and cooling rate increases considerably. Higher cooling rate, especially if coupled with a prompt large undercooling as mentioned above, can causes significant improvements in the structure and mechanical properties of the castings. 3. Structural changes. Applied pressure causes structural changes through affecting cooling rate of the melt and its
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undercooling. These structural changes include decrease in dendrite arm spacing (DAS), more homogeneous distribution of structural features and refinement and modification of intermetallic phases. These factors cause improvement in the mechanical properties of squeeze cast components [11]. 4. Reduction of gas and shrinkage porosities. It has been shown that gas solubility in the melt increases under an applied pressure [12]. This makes gas bubble nucleation more difficult [13]. It is also well established that application of an external pressure during solidification of a casting activates the different feeding mechanisms and hinders the shrinkage porosity formation [14]. Consequently, if a high enough pressure is used, formation of both gas and shrinkage porosities may be completely eliminated. As a result of the above changes, squeeze cast components can have superior mechanical, micro and macrostructural characteristics compared with the conventional ones. The key to this task is control of the processing variables. The most important squeeze casting process parameters that affect the quality of castings are intensity of applied pressure, melt temperature and die preheating temperature. Effects of these three parameters on the density, macrostructure and hardness of LM13 alloy were investigated in this work. Authors have not found any published data on the squeeze casting of this alloy.
Fig. 1. Schematic drawing of the squeeze casting die. Table 2 Experimental conditions used for squeeze casting of the samples
Pressure, P (MPa) Melt temperature, Tm (◦ C) Die temperature, Td (◦ C)
Fixed parameters
Variable parameter
Tm = 730, Td = 200 P = 171, Td = 200 P = 171, Tm = 730
P = 0, 20, 53, 106, 171, 211 Tm = 630, 680, 730, 780 Td = 150, 200, 250, 300
2. Experimental LM13 is a heat treatable cast Al–Si alloy with good bearing properties, good fluidity and low coefficient of thermal expansion. It is a common alloy used in production of automotive pistons and other automobile parts. Table 1 shows the chemical composition of this alloy. A pre-measured amount of the alloy was melted in an electrical furnace. The crucible was then removed from the furnace and the melt temperature was controlled with a thermocouple before pouring into a cylindrical die made of H13 steel. The die had an inner diameter of 50 mm and height of 100 mm as schematically shown in Fig. 1. It was preheated with an electric heater, which enabled a uniform heating, and accurate control of the die temperature. Pressure was applied by means of a 100 tonnes
Table 1 Chemical composition of LM13 alloy (BS 1490LM13)
hydraulic press. The die was coated with a graphite suspension before each experiment. After some preliminary tests, the experimental conditions shown in Table 2 were chosen to study the effects of processing parameters on the density, macrostructure and hardness of the alloy. All samples were heat treated after casting. In the absence of any published data on the heat treatment of squeeze cast LM13 alloy, the heat treatment of the samples was carried out according to that of a die cast LM13 alloy. The treatment was a solution annealing for 4 h at 525 ◦ C, quenching in 65 ◦ C hot water and aging at 200 ◦ C for 5 h. Density measurements were made using weight loss method. Hardness measurements and macrostructural investigations were carried out on a transverse section cut from the middle of the samples. 3. Results and discussion
Element
wt.%
Cu Mg Si Fe Mn Ni Zn Pb Sn Ti Al
0.7–1.5 0.8–1.5 10–13 <1 <0.5 <1.5 <0.5 <0.1 <0.1 <0.2 Reminder
3.1. Effect of pressure on density Effect of applied pressure on the density of samples squeeze cast with melt temperature of 730 ◦ C and die temperature of 200 ◦ C is illustrated in Fig. 2. Although the density is expected to increase continuously with increasing the applied pressure, Fig. 2 shows a drop in the density at 20 MPa applied pressure. The position and size of porosities in the samples are shown in Fig. 3. Fig. 3a reveals that the main part of shrinkage for the sample solidified under atmospheric pressure has been accommodated by formation of a large shrinkage pipe on the top surface
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it is postulated that pressures more than about 100 MPa are able to fully eliminate gas and shrinkage porosities. 3.2. Effect of pressure on macrostructure
Fig. 2. Effect of pressure on the density of squeeze cast LM13 alloy (Tm = 730 ◦ C and Td = 200 ◦ C).
which is not accounted as a part of the internal porosity of the sample. It is evident from Fig. 3b and c that with the application of external pressure, such concentrated shrinkage cavities are pushed down toward the bulk of the samples. Consequently, if the applied pressure is not large enough to eliminate such cavities, smaller density values may be attained. With further increase in the applied pressure, gas and shrinkage porosities decrease toward zero (Fig. 3d and e) and density approaches its theoretical value. Based on the results shown in Figs. 2 and 3,
Fig. 4 shows the macrostructure of samples solidified under different applied pressures with a melt and die temperature of 730 and 200 ◦ C, respectively. Macrostructure of the sample solidified under atmospheric pressure (Fig. 4a) consists of a band of thick columnar grains surrounding some large equiaxed grains in the center. Application of external pressure results in finer columnar as well as equiaxed grains and limits the extent of columnar growth from the die surface. This is caused by a better contact between the metal and the die surface during solidification which renders improved heat transfer across the metal/die interface. The difference in the macrostructures is particularly pronounced for external pressures of up to 100 MPa (Fig. 4b–d). At higher pressures (more than 100 MPa) increasing the pressure exhibits no significant effect on the grains structure (Fig. 4e and f). This appears to be due to the complete contact of metal/die surface at a pressure of about 100 MPa. Consequently further increase in the applied pressure cannot cause any noticeable improvement in the heat transfer and the macrostructure.
Fig. 3. Effect of external pressure on the position and size of shrinkage porosities in squeeze cast LM13 alloy: (a) 0 MPa, (b) 20 MPa, (c) 53 MPa, (d) 106 MPa, and (e) 171 MPa (Tm = 730 ◦ C and Td = 200 ◦ C).
Fig. 4. Effect of pressure on the macrostructure of squeeze cast LM13 alloy: (a) 0 MPa, (b) 20 MPa, (c) 53 MPa, (d) 106 MPa, (e) 171 MPa, and (f) 211 MPa (Tm = 730 ◦ C and Td = 200 ◦ C).
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Fig. 5. Effect of external pressure on the hardness of squeeze cast LM13 alloy (Tm = 730 ◦ C and Td = 200 ◦ C).
3.3. Effect of pressure on hardness Fig. 5 depicts the effect of applied pressure on the hardness of as cast samples. Hardness of the samples steadily increases from 97 HBN for the sample solidified under atmospheric pressure to about 110 HBN at an external pressure of 171 MPa and becomes constant at higher applied pressures. The slop of the curve is steeper at lower applied pressures and becomes less steep at applied pressures of more than about 100 MPa. This is due to the sudden increase of cooling rate caused by the improved contact between the metal and the die surface. The structural investigation revealed that this would not only result in refinement of the macrostructure of samples but in the refinement of their microstructure and modification of eutectic silicon particles as well [15]. All these structural changes bring about the increase in hardness as shown in Fig. 5. It is postulated again that 100 MPa is the external pressure at which a complete contact between the metal and die surface is realized. 3.4. Effect of melt temperature on macrostructure Fig. 6 illustrates the macrostructures of samples squeeze cast under an applied pressure of 171 MPa, die preheating temperature of 200 ◦ C and different melt temperatures. The general trend of increase in grain size with increase in melt temperature is clearly noticed from the figure. This trend is usually attributed
to lower cooling rate of samples during solidification and fading capability of heterogeneous nucleation sites as the pouring temperature is increased. It is evident form the figure that as the melt temperature is lowered from 780 to 730 ◦ C and then to 680 ◦ C, the macrostructures gradually become finer and grains become smaller. However further decrease of melt temperature to 630 ◦ C results in a striking change. The structure consists of very fine and uniform equiaxed grains and little sign of columnar grains is observed. It was mentioned in Section 1 that a sudden large undercooling could be created in the melt upon application of pressure, if the melt temperature and timing of pressure application were accurately controlled. Theoretically, largest melt undercooling would be achieved if the pressure were applied when the melt temperature in the die was lower than its liquidus temperature and just above the temperature required for explosion of nucleation (for example about 0.98 of melting point of the alloy for the case of heterogeneous nucleation) [16]. It is suggested that for the sample poured with melt temperature of 630 ◦ C, the melt temperature inside the die might have had the chance to fall to the temperature range mentioned above before the application of pressure. This has rendered a sudden undercooling in the melt and a very fine grain size in the solidified sample. However one must bear in mind the adverse effect of low melt temperature on its fluidity. Although it has been shown that fluidity is a less crucial issue for squeeze casting compared to the conventional casting processes [17], but still the shape and complexity of the castings has to be considered in choosing a suitable melt temperature. 3.5. Effect of melt temperature on hardness Fig. 7 illustrates the effect of melt temperature on the hardness of samples squeeze cast using an external pressure of 171 MPa and die temperature of 200 ◦ C. As it is seen the hardness of the sample poured at 630 ◦ C is somewhat higher while those of other samples are almost the same. This is consistent with the observations made on the macrostructures, that is, the sample with the smallest grain size has the highest hardness. The figure shows that hardness is not affected significantly at melt temperatures above 680 ◦ C.
Fig. 6. Effect of melt temperature on the macrostructure of squeeze cast LM13 alloy: (a) 630 ◦ C, (b) 680 ◦ C, (c) 730 ◦ C, and (d) 780 ◦ C (P = 171 MPa and Td = 200 ◦ C).
A. Maleki et al. / Materials Science and Engineering A 428 (2006) 135–140
Fig. 7. Effect of melt temperature on the hardness of squeeze cast LM13 alloy (P = 171 MPa and Td = 200 ◦ C).
3.6. Effect of die temperature on macrostructure Fig. 8 shows the macrostructures of samples squeeze cast under an applied pressure of 171 MPa, melt temperature of 730 ◦ C and different die preheating temperatures of 150, 200, 250 and 300 ◦ C. Increasing the die temperature has resulted in larger grains due to slower heat transfer and smaller cooling rate during solidification. Effect of die temperature is particularly noticeable on the extent of the chill zones. Samples poured with die temperature of 150 or 200 ◦ C have limited chill zones. In the former, for instance, this zone covers about 6–8% of the sample diameter. The chill zones have widened in the samples poured with die temperature of 250 or 300 ◦ C. In the latter, for instance, this zone covers about 12–14% of the sample diameter. Considering the refinement of the macrostructures in one hand and decreasing the extent of solidification before pressure application as well as increasing the die life on the other hand, the most suitable die preheating temperature seems to be about 200 ◦ C. 3.7. Effect of die temperature on hardness Fig. 9 illustrates the effect of die temperature on the hardness of the samples squeeze cast under an applied pressure of 171 MPa and melt temperature of 730 ◦ C. Hardness decreases with an increase in the die temperature due to smaller cooling
139
Fig. 9. Effect of die temperature on the hardness of squeeze cast LM13 alloy (P = 171 MPa and Tm = 730 ◦ C).
rate during solidification of samples. A comparison between Figs. 7 and 9 suggests that die temperature would have a greater influence on the hardness of the squeeze cast components than melt temperature. 4. Conclusion The following results were obtained in this work on squeeze casting of LM13 alloy: 1. Density of the samples decreased with application of 20 MPa external pressure. However it increased steadily for higher applied pressures up to about 100 MPa above which it approached its theoretical value and became practically constant. It is postulated that pressures more than about 100 MPa are able to fully eliminate gas and shrinkage porosities. 2. Increasing the applied pressure resulted in smaller grain size and improved hardness. 3. It appears that 100 MPa is also the external pressure at which a complete contact between the metal and die surface is realized. 4. A decrease in the melt or die temperature rendered similar effects as that of increasing the external pressure on the macrostructure and hardness of the samples. This is due to the increased cooling rate during solidification of the squeeze cast samples. 5. Squeeze casting could produce considerable improvements in the macrostructure and hardness of the samples.
Fig. 8. Effect of die temperature on the macrostructure of squeeze cast LM13 alloy: (a) 150 ◦ C, (b) 200 ◦ C, (c) 250 ◦ C, and (d) 300 ◦ C (P = 171 MPa and Tm = 730 ◦ C).
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Acknowledgment The authors would like to acknowledge the financial support from the office of deputy research of Isfahan University of Technology (IUT)/Iran (Grant No. 1MSA801). References [1] M.R. Ghomashchi, A. Vikhrov, J. Mater. Process. Technol. 101 (2000) 1–9. [2] J.R. Morton, J. Barlow, The Foundryman (1994) 23–28. [3] M.A. Savas, S. Altintas, Mater. Sci. Eng. A 137 (1993) 227–231. [4] F. Weinberg, Proceedings of an International Conference Organized by the Applied Metallurgy and Metals Technology on Solidification Technology in the Foundry and Cast house, 1980, pp. 131–136. [5] J.R. Franklin, A.A. Das, British Foundryman 77 (3) (1984) 150– 158. [6] T.M. Yue, G.A. Chadwick, Met. Mater. 5 (1) (1989) 6–12. [7] J.L. Dorcic, S.K. Verma, ASM Handbook, vol. 15, 1999, pp. 323–326.
[8] P. Krishna, A study on interfacial heat transfer and process parameters in squeeze casting and low pressure permanent mold casting, Ph.D. Thesis, University of Michigan, 2001. [9] T.M. Yue, G.A. Chadwick, J. Mater. Process. Technol. 58 (1996) 302–307. [10] G. Williams, K.M. Fisher, Proceedings of an International Conference Organized by the Applied Metallurgy and Metals Technology on Solidification Technology in the Foundry and Cast House, 1980, pp. 137– 142. [11] S. Chatterjee, A.A. Das, The British Foundryman 65 (1972) 420– 425. [12] V.M. Plyatskii, Extrusion Casting, Primary Sources Inc., New York, 1965. [13] J. Benedyk, W. Shaw, Light Met. Age 27 (5) (1969) 6–8. [14] J. Campbell, Castings, Elsevier Butterworth-Heinemann, Oxford, 1993. [15] A. Maleki, M.Sc. Thesis, Isfahan University of Technology, 2004. [16] G.J. Davies, Solidification and Casting, Applied Science Publisher, London, 1973. [17] ASM Handbook, vol. 15, 9th ed., 1991.
Effects of squeeze casting parameters on density, macrostructure and hardness of LM13 alloy A. Maleki, B. Niroumand ∗ , A. Shafyei Isfahan University of Technology, Isfahan, Iran Received 28 November 2005; accepted 25 April 2006
Abstract In this research the effects of applied pressure, melt and die temperatures on the density, macrostructure and hardness of squeeze cast LM13 alloy were investigated. The results showed that the density of the samples decreased with application of a 20 MPa external pressure but it increased steadily for higher applied pressures up to about 106 MPa after which it became almost constant. Increasing the applied pressure resulted in smaller grain size and improved hardness. A decrease in the melt or die temperature rendered similar effects on the macrostructure and hardness of the samples. The results were explained based on the changes in nucleation rate, cooling rate, and operation of feeding mechanisms brought about by the application of pressure during solidification. © 2006 Elsevier B.V. All rights reserved. Keywords: Squeeze casting; Solidification; Aluminum alloy; Macrostructure; Hardness
1. Introduction Castings are produced via different processes each of which enjoys certain advantages and disadvantages. A main disadvantage of conventional casting processes such as high-pressure die casting is formation of such defects as gas and shrinkage porosities which decrease mechanical properties, integrity, and reliability of the products [1–4]. Squeeze casting is one of the modern casting processes, which has been invented to address these shortcomings and has a high potential to produce sound castings. In this process, which can be regarded as a combination of casting and forging processes [1,5], a high pressure is applied on the melt during solidification. This external pressure is held on the melt until the end of solidification. North American Die Casting Association (NADCA) describes squeeze casting as a casting process that uses low die filling velocity to create minimum turbulence and pressurizes the melt to produce high quality castings without a solution heat treatment [3]. The fact that castings can be used in as cast condition and after a limited finishing treatment has secured a place for squeeze casting among other near net shaping methods [1,6–8].
∗
Corresponding author. Tel.: +98 311 391 2750; fax: +98 311 391 2752. E-mail address: [email protected] (B. Niroumand).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.04.099
Application of pressure on a molten metal during solidification may cause the following effects: 1. Change in the melting point. Melting point (liquidus temperature) of most metals and alloys increases under pressure [2]. The increase satisfies Clausius–Clapeyron equation [1,2,9]. This characteristic can be utilized to create sudden large undercooling in the melt upon application of pressure if the melt temperature and timing of pressure application are accurately controlled. 2. Change of solidification rate. In most casting processes an air gap is formed shortly after pouring between the die and the solidified outer shell of the casting [6]. This is due to simultaneous contraction of the shell and expansion of the die. Air gap formation changes heat transfer mechanism from conduction to convection and radiation and causes a significant decrease in heat transfer rate and consequently decreases the cooling rate [5,10]. In squeeze casting, air gap formation is eliminated as a result of the applied pressure on the casting and therefore heat transfer and cooling rate increases considerably. Higher cooling rate, especially if coupled with a prompt large undercooling as mentioned above, can causes significant improvements in the structure and mechanical properties of the castings. 3. Structural changes. Applied pressure causes structural changes through affecting cooling rate of the melt and its
136
A. Maleki et al. / Materials Science and Engineering A 428 (2006) 135–140
undercooling. These structural changes include decrease in dendrite arm spacing (DAS), more homogeneous distribution of structural features and refinement and modification of intermetallic phases. These factors cause improvement in the mechanical properties of squeeze cast components [11]. 4. Reduction of gas and shrinkage porosities. It has been shown that gas solubility in the melt increases under an applied pressure [12]. This makes gas bubble nucleation more difficult [13]. It is also well established that application of an external pressure during solidification of a casting activates the different feeding mechanisms and hinders the shrinkage porosity formation [14]. Consequently, if a high enough pressure is used, formation of both gas and shrinkage porosities may be completely eliminated. As a result of the above changes, squeeze cast components can have superior mechanical, micro and macrostructural characteristics compared with the conventional ones. The key to this task is control of the processing variables. The most important squeeze casting process parameters that affect the quality of castings are intensity of applied pressure, melt temperature and die preheating temperature. Effects of these three parameters on the density, macrostructure and hardness of LM13 alloy were investigated in this work. Authors have not found any published data on the squeeze casting of this alloy.
Fig. 1. Schematic drawing of the squeeze casting die. Table 2 Experimental conditions used for squeeze casting of the samples
Pressure, P (MPa) Melt temperature, Tm (◦ C) Die temperature, Td (◦ C)
Fixed parameters
Variable parameter
Tm = 730, Td = 200 P = 171, Td = 200 P = 171, Tm = 730
P = 0, 20, 53, 106, 171, 211 Tm = 630, 680, 730, 780 Td = 150, 200, 250, 300
2. Experimental LM13 is a heat treatable cast Al–Si alloy with good bearing properties, good fluidity and low coefficient of thermal expansion. It is a common alloy used in production of automotive pistons and other automobile parts. Table 1 shows the chemical composition of this alloy. A pre-measured amount of the alloy was melted in an electrical furnace. The crucible was then removed from the furnace and the melt temperature was controlled with a thermocouple before pouring into a cylindrical die made of H13 steel. The die had an inner diameter of 50 mm and height of 100 mm as schematically shown in Fig. 1. It was preheated with an electric heater, which enabled a uniform heating, and accurate control of the die temperature. Pressure was applied by means of a 100 tonnes
Table 1 Chemical composition of LM13 alloy (BS 1490LM13)
hydraulic press. The die was coated with a graphite suspension before each experiment. After some preliminary tests, the experimental conditions shown in Table 2 were chosen to study the effects of processing parameters on the density, macrostructure and hardness of the alloy. All samples were heat treated after casting. In the absence of any published data on the heat treatment of squeeze cast LM13 alloy, the heat treatment of the samples was carried out according to that of a die cast LM13 alloy. The treatment was a solution annealing for 4 h at 525 ◦ C, quenching in 65 ◦ C hot water and aging at 200 ◦ C for 5 h. Density measurements were made using weight loss method. Hardness measurements and macrostructural investigations were carried out on a transverse section cut from the middle of the samples. 3. Results and discussion
Element
wt.%
Cu Mg Si Fe Mn Ni Zn Pb Sn Ti Al
0.7–1.5 0.8–1.5 10–13 <1 <0.5 <1.5 <0.5 <0.1 <0.1 <0.2 Reminder
3.1. Effect of pressure on density Effect of applied pressure on the density of samples squeeze cast with melt temperature of 730 ◦ C and die temperature of 200 ◦ C is illustrated in Fig. 2. Although the density is expected to increase continuously with increasing the applied pressure, Fig. 2 shows a drop in the density at 20 MPa applied pressure. The position and size of porosities in the samples are shown in Fig. 3. Fig. 3a reveals that the main part of shrinkage for the sample solidified under atmospheric pressure has been accommodated by formation of a large shrinkage pipe on the top surface
A. Maleki et al. / Materials Science and Engineering A 428 (2006) 135–140
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it is postulated that pressures more than about 100 MPa are able to fully eliminate gas and shrinkage porosities. 3.2. Effect of pressure on macrostructure
Fig. 2. Effect of pressure on the density of squeeze cast LM13 alloy (Tm = 730 ◦ C and Td = 200 ◦ C).
which is not accounted as a part of the internal porosity of the sample. It is evident from Fig. 3b and c that with the application of external pressure, such concentrated shrinkage cavities are pushed down toward the bulk of the samples. Consequently, if the applied pressure is not large enough to eliminate such cavities, smaller density values may be attained. With further increase in the applied pressure, gas and shrinkage porosities decrease toward zero (Fig. 3d and e) and density approaches its theoretical value. Based on the results shown in Figs. 2 and 3,
Fig. 4 shows the macrostructure of samples solidified under different applied pressures with a melt and die temperature of 730 and 200 ◦ C, respectively. Macrostructure of the sample solidified under atmospheric pressure (Fig. 4a) consists of a band of thick columnar grains surrounding some large equiaxed grains in the center. Application of external pressure results in finer columnar as well as equiaxed grains and limits the extent of columnar growth from the die surface. This is caused by a better contact between the metal and the die surface during solidification which renders improved heat transfer across the metal/die interface. The difference in the macrostructures is particularly pronounced for external pressures of up to 100 MPa (Fig. 4b–d). At higher pressures (more than 100 MPa) increasing the pressure exhibits no significant effect on the grains structure (Fig. 4e and f). This appears to be due to the complete contact of metal/die surface at a pressure of about 100 MPa. Consequently further increase in the applied pressure cannot cause any noticeable improvement in the heat transfer and the macrostructure.
Fig. 3. Effect of external pressure on the position and size of shrinkage porosities in squeeze cast LM13 alloy: (a) 0 MPa, (b) 20 MPa, (c) 53 MPa, (d) 106 MPa, and (e) 171 MPa (Tm = 730 ◦ C and Td = 200 ◦ C).
Fig. 4. Effect of pressure on the macrostructure of squeeze cast LM13 alloy: (a) 0 MPa, (b) 20 MPa, (c) 53 MPa, (d) 106 MPa, (e) 171 MPa, and (f) 211 MPa (Tm = 730 ◦ C and Td = 200 ◦ C).
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Fig. 5. Effect of external pressure on the hardness of squeeze cast LM13 alloy (Tm = 730 ◦ C and Td = 200 ◦ C).
3.3. Effect of pressure on hardness Fig. 5 depicts the effect of applied pressure on the hardness of as cast samples. Hardness of the samples steadily increases from 97 HBN for the sample solidified under atmospheric pressure to about 110 HBN at an external pressure of 171 MPa and becomes constant at higher applied pressures. The slop of the curve is steeper at lower applied pressures and becomes less steep at applied pressures of more than about 100 MPa. This is due to the sudden increase of cooling rate caused by the improved contact between the metal and the die surface. The structural investigation revealed that this would not only result in refinement of the macrostructure of samples but in the refinement of their microstructure and modification of eutectic silicon particles as well [15]. All these structural changes bring about the increase in hardness as shown in Fig. 5. It is postulated again that 100 MPa is the external pressure at which a complete contact between the metal and die surface is realized. 3.4. Effect of melt temperature on macrostructure Fig. 6 illustrates the macrostructures of samples squeeze cast under an applied pressure of 171 MPa, die preheating temperature of 200 ◦ C and different melt temperatures. The general trend of increase in grain size with increase in melt temperature is clearly noticed from the figure. This trend is usually attributed
to lower cooling rate of samples during solidification and fading capability of heterogeneous nucleation sites as the pouring temperature is increased. It is evident form the figure that as the melt temperature is lowered from 780 to 730 ◦ C and then to 680 ◦ C, the macrostructures gradually become finer and grains become smaller. However further decrease of melt temperature to 630 ◦ C results in a striking change. The structure consists of very fine and uniform equiaxed grains and little sign of columnar grains is observed. It was mentioned in Section 1 that a sudden large undercooling could be created in the melt upon application of pressure, if the melt temperature and timing of pressure application were accurately controlled. Theoretically, largest melt undercooling would be achieved if the pressure were applied when the melt temperature in the die was lower than its liquidus temperature and just above the temperature required for explosion of nucleation (for example about 0.98 of melting point of the alloy for the case of heterogeneous nucleation) [16]. It is suggested that for the sample poured with melt temperature of 630 ◦ C, the melt temperature inside the die might have had the chance to fall to the temperature range mentioned above before the application of pressure. This has rendered a sudden undercooling in the melt and a very fine grain size in the solidified sample. However one must bear in mind the adverse effect of low melt temperature on its fluidity. Although it has been shown that fluidity is a less crucial issue for squeeze casting compared to the conventional casting processes [17], but still the shape and complexity of the castings has to be considered in choosing a suitable melt temperature. 3.5. Effect of melt temperature on hardness Fig. 7 illustrates the effect of melt temperature on the hardness of samples squeeze cast using an external pressure of 171 MPa and die temperature of 200 ◦ C. As it is seen the hardness of the sample poured at 630 ◦ C is somewhat higher while those of other samples are almost the same. This is consistent with the observations made on the macrostructures, that is, the sample with the smallest grain size has the highest hardness. The figure shows that hardness is not affected significantly at melt temperatures above 680 ◦ C.
Fig. 6. Effect of melt temperature on the macrostructure of squeeze cast LM13 alloy: (a) 630 ◦ C, (b) 680 ◦ C, (c) 730 ◦ C, and (d) 780 ◦ C (P = 171 MPa and Td = 200 ◦ C).
A. Maleki et al. / Materials Science and Engineering A 428 (2006) 135–140
Fig. 7. Effect of melt temperature on the hardness of squeeze cast LM13 alloy (P = 171 MPa and Td = 200 ◦ C).
3.6. Effect of die temperature on macrostructure Fig. 8 shows the macrostructures of samples squeeze cast under an applied pressure of 171 MPa, melt temperature of 730 ◦ C and different die preheating temperatures of 150, 200, 250 and 300 ◦ C. Increasing the die temperature has resulted in larger grains due to slower heat transfer and smaller cooling rate during solidification. Effect of die temperature is particularly noticeable on the extent of the chill zones. Samples poured with die temperature of 150 or 200 ◦ C have limited chill zones. In the former, for instance, this zone covers about 6–8% of the sample diameter. The chill zones have widened in the samples poured with die temperature of 250 or 300 ◦ C. In the latter, for instance, this zone covers about 12–14% of the sample diameter. Considering the refinement of the macrostructures in one hand and decreasing the extent of solidification before pressure application as well as increasing the die life on the other hand, the most suitable die preheating temperature seems to be about 200 ◦ C. 3.7. Effect of die temperature on hardness Fig. 9 illustrates the effect of die temperature on the hardness of the samples squeeze cast under an applied pressure of 171 MPa and melt temperature of 730 ◦ C. Hardness decreases with an increase in the die temperature due to smaller cooling
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Fig. 9. Effect of die temperature on the hardness of squeeze cast LM13 alloy (P = 171 MPa and Tm = 730 ◦ C).
rate during solidification of samples. A comparison between Figs. 7 and 9 suggests that die temperature would have a greater influence on the hardness of the squeeze cast components than melt temperature. 4. Conclusion The following results were obtained in this work on squeeze casting of LM13 alloy: 1. Density of the samples decreased with application of 20 MPa external pressure. However it increased steadily for higher applied pressures up to about 100 MPa above which it approached its theoretical value and became practically constant. It is postulated that pressures more than about 100 MPa are able to fully eliminate gas and shrinkage porosities. 2. Increasing the applied pressure resulted in smaller grain size and improved hardness. 3. It appears that 100 MPa is also the external pressure at which a complete contact between the metal and die surface is realized. 4. A decrease in the melt or die temperature rendered similar effects as that of increasing the external pressure on the macrostructure and hardness of the samples. This is due to the increased cooling rate during solidification of the squeeze cast samples. 5. Squeeze casting could produce considerable improvements in the macrostructure and hardness of the samples.
Fig. 8. Effect of die temperature on the macrostructure of squeeze cast LM13 alloy: (a) 150 ◦ C, (b) 200 ◦ C, (c) 250 ◦ C, and (d) 300 ◦ C (P = 171 MPa and Tm = 730 ◦ C).
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Acknowledgment The authors would like to acknowledge the financial support from the office of deputy research of Isfahan University of Technology (IUT)/Iran (Grant No. 1MSA801). References [1] M.R. Ghomashchi, A. Vikhrov, J. Mater. Process. Technol. 101 (2000) 1–9. [2] J.R. Morton, J. Barlow, The Foundryman (1994) 23–28. [3] M.A. Savas, S. Altintas, Mater. Sci. Eng. A 137 (1993) 227–231. [4] F. Weinberg, Proceedings of an International Conference Organized by the Applied Metallurgy and Metals Technology on Solidification Technology in the Foundry and Cast house, 1980, pp. 131–136. [5] J.R. Franklin, A.A. Das, British Foundryman 77 (3) (1984) 150– 158. [6] T.M. Yue, G.A. Chadwick, Met. Mater. 5 (1) (1989) 6–12. [7] J.L. Dorcic, S.K. Verma, ASM Handbook, vol. 15, 1999, pp. 323–326.
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