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Effect of cooling rate on the thermal and electrical conductivities of an A356 sand cast alloy
Journal of Alloys and Compounds 808 (2019) 151756
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of cooling rate on the thermal and electrical conductivities of an A356 sand cast alloy J.H. Jeon, D.H. Bae* Department of Materials Science and Engineering, Yonsei University, Seoul, 03722, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 May 2019 Received in revised form 3 August 2019 Accepted 7 August 2019 Available online 7 August 2019
This study evaluates the effect of cooling rate on the microstructures and conduction properties of an A356 sand cast alloy. To evaluate effect of cooling rate, computer recordable sand mold with four stepthickness (5 mme 20 mm) was used and microstructures of T6 heat treated specimens with different cooling rates were observed. It was found that with increasing cooling rate, eutectic reaction temperature shifted to lower temperature and the supercooling also increased. Based on the coarsening model via solute diffusion, when the cooling rate increase, secondary dendrite arm spacing decreases, providing the enhanced thermal and electrical conductivities. © 2019 Elsevier B.V. All rights reserved.
Keywords: Metals and alloys Electrical transport Heat conduction Microstructure
1. Introduction Recently, the automotive industry is focusing on the research and development of lightweight parts to increases fuel efficiency and satisfy environmental regulations. Specifically, AleSi alloys including A356 are increasingly used because they exhibit high specific strength, corrosion resistance, and thermal conductivity [1e3]. In the Al industry, sand casting is one of the oldest casting methods to manufacture metal casting products or parts, and research continue to focus on high value casting. An extremely important factor for AleSi sand cast products is the cooling rate. In the case of small-sized Al casting with a high cooling rate, microstructure control during solidification is easy in the cases involving alloying elements that are added to improve of characteristics. However, in the case of large-sized Al castings, microstructure control is difficult due to complicated shape and low cooling rate. Therefore, in order to cast excellent large-size Al products, it is necessary to control the microstructure while consider the casting condition, alloy composition, and heat capacity of the alloy via the cooling rate [4]. The factor that most directly influencing the microstructure corresponds to the secondary dendrite arm spacing (SDAS) depend on the cooling rate during solidification. Several extant studies
* Corresponding author. E-mail address: [email protected] (D.H. Bae). https://doi.org/10.1016/j.jallcom.2019.151756 0925-8388/© 2019 Elsevier B.V. All rights reserved.
established theories for the relationship, and the theories suggest that SDAS (l) is proportional to the solidification. This is expressed as follows [5]:
l ¼ KT 1=3 f
(1)
where K and Tf denote the material constant and solidification time, respectively. The theoretical approach to the SDAS presents it as a function of cooling rate and material composition. A certain degree of supercooling is necessary before nucleation commences, the solid particles grains are nucleated in the supercooled liquid phase, and the latent heat of coagulation is transferred to the liquid phase. When the solidified first branch becomes longer, its surface becomes unstable, and thus the second branch and the third branch are formed. This is termed as the dendrite, and the interval of the second branch is termed as the SDAS [6]. In the case of the AleSi hypoeutectic alloy (in which the content of Si is lower than that in the eutectic composition), the Al liquid is generated at the initial stage of solidification and grows to the a-Al phase, and the residual liquid with the eutectic composition solidifies to (Al þ Si) eutectic phase with a needle-like Si phase. In the case of important elongation in mechanical properties, several variables include grain size, distribution and size of precipitates, porosity, and impurity content, although it is known to be especially affected by SDAS [7,8]. On the other hand, Al alloys exhibit excellent thermal and electrical conductivities, and they constitute key materials to
2
J.H. Jeon, D.H. Bae / Journal of Alloys and Compounds 808 (2019) 151756
improve the efficiency of engine parts and increase service life by effectively dissipating generated heat energy [9]. Both electrical conductivity and thermal conductivity occur due to the movement of free electrons at a relatively low temperature, and thus a linear proportional relationship is generally established via WiedemanneFranz law [10,11]. Movement of free electrons is affected by factors including alloy composition, secondary phases, and process technology. Alloy composition is the most fundamental factor, and there are two main mechanisms by which elements reduce electrical conductivity and thermal conductivity. The first mechanism involves the alloy elements that are saturated in the Al matrix to cause lattice distortion. This type of lattice distortion interferes with free electron transport and causes a decrease in conductivities. The other mechanism is that the alloy elements in the molten Al readily react with Al to produce an intermetallic compound and this reduces the number of free electrons, and thereby deteriorates conductivities [12]. Hence, it is extremely important to increase thermal conductivity based on the microstructure of the heat sinks or automotive parts. As mentioned above, previous studies are focused on the effect of thickness-dependent cooling rate on mechanical properties. However, there is a paucity of studies that examine relationship between cooling rate and thermal conductivity. In the present study, change in thermal conductivity due to the variation of the cooling rate based on the mold thickness was evaluated and compared with electrical conductivity. Additionally, a model related to dendrite growth was presented to clarify the relationship between cooling rate and thermal conductivity. 2. Materials and methods The AleSi alloy used in the study corresponded to commercial A356 alloy (Hanjin-metal Co.). Table 1 shows the chemical composition of commercial A356 alloy analysed by X-ray fluorescence (XRF, ZSX100e, Rigaku). First, approximately 30 kg of the commercial A356 ingots were charged into an electrical resistance furnace and melted at 760 C. Subsequently, the melt was degassed with inert Ar gas for 10 m to eliminate gases and inclusions in molten A356 alloy. After a stabilizing time of 30 m, the molten A356 alloy was poured into a four-step sand mold with a thickness ranging from 5 mm to 20 mm as shown in Fig. 1. The mold include a computational recordable K-type thermocouple with a frequency of 100 Hz. Given sufficient solidification time, the sand cast mold was carefully broken, and the specimen was obtained The as-cast specimens were annealed with solid solution heat treatment at 545 C for 6 h, since temperature variations of ± 5 C are allowed during heat treatment and the solidus temperature of an A356 alloy is 555 C [13]. In addition, that the risk of melting of the Al2Cu phase does not reach here due to its low copper content. The specimens then were followed by artificial aging at 160 C for 8 h. In order to examine the microstructure of the specimens (which were cut 10 mm above the side of the casting), each specimen was grounded via SiC paper up to 2000 grit and polished with diamond paste. Fine polished samples were then etched for 10 s to clearly reveal SDAS with a solution of Keller's reagent (2.5 ml HNO3, 1.5 ml HCl, and 1.0 ml HF in 95 ml H2O). Microstructural analysis was performed via an optical microscope (OM, LV150, Nikon). Secondary dendrite arm spacing (SDAS) of alloy with different cooling rates was measured
Table 1 Chemical composition of commercial A356 alloy.
wt.%
Si
Mg
Fe
Cu
Ti
Mn
Cr
Al
6.98
0.391
0.118
0.029
0.109
0.005
0.005
Bal.
Fig. 1. Schematic diagram of the four-step sand mold.
via IP-Win 32 software, and SDAS measurement was performed on ten different sections of each specimen minimize the experimental errors. In order to evaluate the mechanical property, specimens (with a gauge length of 25 mm, gauge width of 6 mm, and gauge thickness of 2 mm) were machined and quasi-static uniaxial tensile tests (ASTM-E8) were conducted via an Instron-type machine (Universal Test Machine, R&B Inc.) at a strain rate of 1:0 103 =s. In order to evaluate the conduction properties, thermal conductivity of specimens (12.7 mm diameter and, 2 mm thickness) was measured via the laser flash analysis method (LFA447, NETZSCH). All specimens were coated with a graphite lubricant and measured 10 times each to ensure precise measurements. Additionally, the electrical conductivity of specimens (with a diameter of 30 mm, and thickness of 2 mm) was measured via an eddy current electrical conductivity meter (AEC-670, AJR NDT Co.). Each specimen was measured ten times to ensure reliability.
3. Result and discussion Fig. 2(a) shows cooling curves, as measured by K-type thermocouples, for commercial A356 alloy with different sand mold thickness. The solidification process of A356 alloy is as follows: Liquid / a-Al dendritic structure / (Al þ Si) eutectic phase / other intermetallic phase. The cooling curves rapidly change when solidification begins or when the temperature reaches the eutectic composition. When the thickness of the mold decreased, primary-a reaction temperatures and eutectic reaction temperatures of the A356 alloy shifted to a lower temperature. Fig. 2(b) describes the characteristics of the cooling curve [14]. Furthermore, the typical solidification parameters of the cooling curve obtained at different cooling rates are shown in Table 2. The liquidus temperature (TLiq) and eutectic temperature (TEut) of the A356 alloy used in the study were evaluated to be about 886 K and 850 K, respectively [13]. When the cooling rate increased, supercooling of the solidification temperature and supercooling of the eutectic temperature increased. The solidification time (Tf) is defined as the time until when the liquid phase becomes a solid phase through the a-Al dendritic structure and (Al þ Si) eutectic phase. The cooling rate (V) was divided by the time required to reach the end of solidification (TEnd) from liquidus temperature (TLiq). In the study, mold thicknesses corresponding to 20 mm, 15 mm, 10 mm and 5 mm were calculated at cooling rates of 0.3 K/s, 0.5 K/s, 0.8 K/s, and 1.9 K/s, respectively. The increase in the solidification temperature after supercooling was due to the recalescence phenomenon in which the latent heat owing to the phase
J.H. Jeon, D.H. Bae / Journal of Alloys and Compounds 808 (2019) 151756
3
Fig. 2. (a) Cooling curves based on mold thickness and, (b) typical cooling curve characteristics.
Table 2 Supercooling and cooling rate obtained from the cooling curves for different mold thicknesses. Thickness
DTLiq (K)
DTEut (K)
Solidification time (Tf, s)
Cooling rate (K/s)
5 mm 10 mm 15 mm 20 mm
41.0 39.9 37.4 36.2
6.9 5.1 4.1 3.7
18.6 44.6 80.0 116.4
1.9 K/s 0.8 K/s 0.5 K/s 0.3 K/s
transformation was released, and also the recalescence temperatures increased with the increases in the cooling rates [6]. This is well explained by the classical theories of Gibbs’ phase rule and heterogeneous nucleation [15e17]. The optical microstructure of T6 heat treated A356 alloy with different cooling rates is shown in Fig. 3. Microstructure of alloys are typically composed of primary Al, plate like (Al þ Si) eutectic phase, and a few other intermetallic compounds. Generally, the Si platelets are sphericalized due to the high temperature solid solution during T6 heat treatment, however, spherodization of large eutectic Si platelets by slow cooling rate are unfavourable compared to fast cooling rate. Additionally, it was confirmed that the dendrites were uniformly developed. The results of the SDAS measurement indicated that increase in the cooling rate led to finer
Fig. 3. Microstructures of T6 heat treated A356 alloy with cooling rates of (a) 0.3 K/s, (b) 0.5 K/s, (c) 0.8 K/s, and (d) 1.9 K/s.
SDAS. The relationship between SDAS and cooling rate based on thermodynamic data is widely known, and the mathematical equation for the hypo-eutectic composition proposed by Kattamis and Flemings's model is expressed as follows [6,18e20]:
0 B
l ¼ 5:5B @
DT GDL ln
Ceut C0
11=3
C C mð1 kÞðCeut C0 ÞA
V 1=3
(2)
where DT denotes the difference between the liquid temperature and eutectic temperature, G ¼ s=DSfus denotes the GibbseThomson coefficient, s denotes the interfacial energy between solid and liquid phases, DSfus denotes the entropy of fusion, DL is the diffusivity of solute atoms in liquid, Ceut denotes the eutectic composition, C0 denotes the solute composition, m denotes the slope of liquidus temperature, and k denotes the equilibrium distribution coefficient. The model is based on the binary alloy as specified by the formula for SDAS via the solute concentration difference, and thus the correlation between SDAS and cooling rate is well deduced. The SDAS results of the A356 alloy with different cooling rates tested in the study are shown in Fig. 4(a). Additionally, Eq. (2) is extrapolated in conjunction with and compared with experimental results in the present study. In this case, G ¼ 1:96 107 K,m, D ¼ 6:45 109 m2 =s, and k ¼ 0:13 were used as physical properties [21]. Results were in agreement with a well-known model, namely the KattamiseFlemings logarithmic function, despite the relatively slow cooling rate due to the sand mold. The results of tensile flow curve are shown in Fig. 4(b) to depict the effect of cooling rate and microstructural changes due to T6 heat treatment on mechanical properties. By increasing the cooling rate, the tensile strength and elongation to failure increased, but the yield strength was almost constant under all conditions. Bardes et al. [22] reported that the rate of solidification exhibited a greater effect on the deformation of DAS than the alloy composition. They also reported that yield strength did not affect DAS, although elongation and tensile strength of AleSi alloys were sensitive to the variation of DAS. As indicated by Melo et al. [23], the mechanical properties of Al alloy products depend on the microporosity formation during solidification. Microporosities are inevitably formed by the amount of dissolved hydrogen gases and the difference in thermal expansion coefficient between Al and Si when forming the primary and secondary dendrite structures, and it corresponds to the variation in pore radius as function of the cooling rate. Hence, microporosity formed over a critical size causes premature cracks in stress
4
J.H. Jeon, D.H. Bae / Journal of Alloys and Compounds 808 (2019) 151756
Fig. 4. (a) Measured values of SDAS based on the cooling rate. The dotted line is the theoretical prediction using KattamiseFlemings model, (b) Flow curves of uniaxial tensile test relative to different cooling rates.
generation and premature failure. DAS also affects mechanical properties by changing the eutectic (Al þ Si) phase. As reported in Pedersen et al. [24], brittle fracture of AleSi alloys occurs along the brittle Si existing in the grain boundaries of the alloy. When the cooling rate increases, the aspect ratio decreases and the uniformly distributed Si phase can delay brittle fracture. Thermal conductivity and electrical conductivity based on the cooling rate are shown in Fig. 5(a) and (b), respectively. In a metal, conduction mechanisms are mostly carried by movement of free electrons within the metal. Since free electrons also carry heat energy, according to the Wiedemann-Fran law thermal conductivity tracks electrical conductivity as follows [12]:
k = s ¼ LT ¼ kr
(3)
where k, s, L, T, and r are thermal conductivity, electrical
conductivity, Lorenz number, absolute temperature, and electrical resistivity, respectively. At a given temperature, the thermal conductivity and electrical conductivity of metals exhibit a proportional relationship, and thus this was in good agreement with the experiment at 298 K as shown Fig. 5(c). Evidently, both conductivities increased when the cooling rate increased. We interpret that this is potentially related to the grain coarsening model [25,26] wherein, the redistribution of solutes is related to the growth of dendrites. When the liquid was in contact with the SDAS during solidification, the increases in the thickness of SDAS in liquid solute concentration exceeded that of thin SDAS due to the difference in the radius of curvature. Thus, the solute in the liquid diffused towards the thinner SDAS due to the solute concentration gradient. Thus, the thin SDAS was dissolved and thinned to inhibit the increase in solute concentration, while the thick SDAS was solidified to stop the decrease in solute concentration. In the process, solutes
Fig. 5. Measurement values of (a) thermal conductivity, and (b) electrical conductivity with different cooling rates. (c) Relationship of thermal conductivity and electric conductivity relative to different cooling rates, and the dotted line is the theoretical prediction using WiedemanneFranz model. (d) Schematic drawing of the formation of the solute layer.
J.H. Jeon, D.H. Bae / Journal of Alloys and Compounds 808 (2019) 151756
pile up at solideliquid interface and form a solute boundary layer [27,28]. Eq. (2) specifies the relationship between the cooling rate and SDAS obtained by modifying the grain coarsening model, and the schematic diagram is shown in Fig. 5(d). Thus, electron scattering is caused by a large area of the solute layer when the cooling rate is slow although, it is barely affected by the solute layer when the cooling rate is high as shown in Fig. 5(d). 4. Conclusion In summary, microstructure, mechanical properties, electrical conductivity and thermal conductivity were evaluated at different cooling rates. When the cooling rate increased, shrinkage defects and pore defects decreased, and the role of the solute layer in preventing electron scattering decreased, thereby increasing thermal and electrical conductivities. The role of the solute layer in electron scattering was explained based on the proposed grain coarsening model. The model provided good fundamental insights and a new perspective on the effect of cooling rate on electron scattering. Acknowledgement This work was supported by National Research Foundation of Korea (NRF) grant funded by the Korea government (MISP) (No. NRF-2017R1A2B2007062). References [1] S.K. Singh, K. Chattopadhyay, G. Phanikumar, P. Dutta, Experimental and numerical studies on friction welding of thixocast A356 aluminum alloy, Acta Mater. 73 (2014) 177e185. [2] S.R. Sharma, Z.Y. Ma, R.S. Mishra, Effect of friction stir processing on fatigue behavior of A356 alloy, Scr. Mater. 51 (2004) 237e241. [3] H. Ye, An overview of the development of Al-Si-Alloy based material for engine applications, J. Mater. Eng. Perform. 12 (2003) 288e297. [4] J.G. Kaufman, E.L. Rooy, Aluminum Alloy Castings: Properties, Processes, and Applications, Asm International, 2004. [5] K.P. Young, D.H. Kerkwood, The dendrite arm spacings of aluminum-copper alloys solidified under steady-state conditions, Metall. Trans. A 6 (1975) 197e205.
5
[6] M.C. Flemings, Solidification Processing, Materials Science and Technology, 2006. [7] X. Jian, H. Xu, T. Meek, Q. Han, Effect of power ultrasound on solidification of aluminum A356 alloy, Mater. Lett. 59 (2005) 190e193. [8] J. Wannasin, R. Martinez, M. Flemings, Grain refinement of an aluminum alloy by introducing gas bubbles during solidification, Scr. Mater. 55 (2006) 115e118. [9] J.R. Davis, Aluminum and Aluminum Alloys, ASM international, 1993. [10] S. Bakhtiyarov, R. Overfelt, S. Teodorescu, Electrical and thermal conductivity of A319 and A356 aluminum alloys, J. Mater. Sci. 36 (2001) 4643e4648. [11] J. Koh, A. Fortini, Prediction of thermal conductivity and electrical resistivity of porous metallic materials, Int. J. Heat Mass Transf. 16 (1973) 2013e2022. [12] E.S. Toberer, L.L. Baranowski, C. Dames, Advances in thermal conductivity, Annu. Rev. Mater. Res. 42 (2012) 179e209. [13] A.S.M. International, C. Handbook, ASM Handbook. Volume 2, vol. 2, 1990. [14] D.M. Stefanescu, Thermal analysisdtheory and applications in metalcasting, Int. J. Metalcast. 9 (2015) 7e22. [15] B. Murty, S. Kori, M. Chakraborty, Grain refinement of aluminium and its alloys by heterogeneous nucleation and alloying, Int. Mater. Rev. 47 (2002) 3e29. [16] D. Turnbull, Kinetics of heterogeneous nucleation, J. Chem. Phys. 18 (1950) 198e203. [17] D.A. Porter, K.E. Easterling, M. Sherif, Phase Transformations in Metals and Alloys, (Revised Reprint), CRC press, 2009. [18] R. Grugel, Secondary and tertiary dendrite arm spacing relationships in directionally solidified Al-Si alloys, J. Mater. Sci. 28 (1993) 677e683. [19] M. MC FLEMINGS, Microsegregation in iron-base alloys, in: J Iron Steel Inst (London), 1970. [20] T.Z. Kattamis, Influence of coarsening on dendrite arm spacing of aluminumcopper alloys, Trans. Metall. Soc. AIME 239 (1967) 1504e1511. [21] H. Zhang, K. Nakajima, H. Lei, J. He, Restrictions of Physical Properties on Solidification Microstructures of Al-Based Binary Alloys by Cellular Automaton, 2010. [22] B. Bardes, M. Flemings, Dendrite arm spacing and solidification time in a cast aluminum-copper alloy, Mod. Cast. 50 (1966) 100e106. [23] M.d.L.N.M. Melo, E. Rizzo, R. Santos, Predicting dendrite arm spacing and their effect on microporosity formation in directionally solidified Al-Cu alloy, J. Mater. Sci. 40 (2005) 1599e1609. [24] L. Pedersen, L. Arnberg, The effect of solution heat treatment and quenching rates on mechanical properties and microstructures in AlSiMg foundry alloys, Metall. Mater. Trans. A 32 (2001) 525e532. [25] H.D. Brody, Solute redistribution in dendritic solidification, in: Massachusetts Institute of Technology, 1965. [26] S. Ganesan, D. Poirier, Solute redistribution in dendritic solidification with diffusion in the solid, J. Cryst. Growth 97 (1989) 851e859. [27] M.J. Aziz, Model for solute redistribution during rapid solidification, J. Appl. Phys. 53 (1982) 1158e1168. [28] M.J. Aziz, T. Kaplan, Continuous growth model for interface motion during alloy solidification, Acta Metall. 36 (1988) 2335e2347.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of cooling rate on the thermal and electrical conductivities of an A356 sand cast alloy J.H. Jeon, D.H. Bae* Department of Materials Science and Engineering, Yonsei University, Seoul, 03722, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 May 2019 Received in revised form 3 August 2019 Accepted 7 August 2019 Available online 7 August 2019
This study evaluates the effect of cooling rate on the microstructures and conduction properties of an A356 sand cast alloy. To evaluate effect of cooling rate, computer recordable sand mold with four stepthickness (5 mme 20 mm) was used and microstructures of T6 heat treated specimens with different cooling rates were observed. It was found that with increasing cooling rate, eutectic reaction temperature shifted to lower temperature and the supercooling also increased. Based on the coarsening model via solute diffusion, when the cooling rate increase, secondary dendrite arm spacing decreases, providing the enhanced thermal and electrical conductivities. © 2019 Elsevier B.V. All rights reserved.
Keywords: Metals and alloys Electrical transport Heat conduction Microstructure
1. Introduction Recently, the automotive industry is focusing on the research and development of lightweight parts to increases fuel efficiency and satisfy environmental regulations. Specifically, AleSi alloys including A356 are increasingly used because they exhibit high specific strength, corrosion resistance, and thermal conductivity [1e3]. In the Al industry, sand casting is one of the oldest casting methods to manufacture metal casting products or parts, and research continue to focus on high value casting. An extremely important factor for AleSi sand cast products is the cooling rate. In the case of small-sized Al casting with a high cooling rate, microstructure control during solidification is easy in the cases involving alloying elements that are added to improve of characteristics. However, in the case of large-sized Al castings, microstructure control is difficult due to complicated shape and low cooling rate. Therefore, in order to cast excellent large-size Al products, it is necessary to control the microstructure while consider the casting condition, alloy composition, and heat capacity of the alloy via the cooling rate [4]. The factor that most directly influencing the microstructure corresponds to the secondary dendrite arm spacing (SDAS) depend on the cooling rate during solidification. Several extant studies
* Corresponding author. E-mail address: [email protected] (D.H. Bae). https://doi.org/10.1016/j.jallcom.2019.151756 0925-8388/© 2019 Elsevier B.V. All rights reserved.
established theories for the relationship, and the theories suggest that SDAS (l) is proportional to the solidification. This is expressed as follows [5]:
l ¼ KT 1=3 f
(1)
where K and Tf denote the material constant and solidification time, respectively. The theoretical approach to the SDAS presents it as a function of cooling rate and material composition. A certain degree of supercooling is necessary before nucleation commences, the solid particles grains are nucleated in the supercooled liquid phase, and the latent heat of coagulation is transferred to the liquid phase. When the solidified first branch becomes longer, its surface becomes unstable, and thus the second branch and the third branch are formed. This is termed as the dendrite, and the interval of the second branch is termed as the SDAS [6]. In the case of the AleSi hypoeutectic alloy (in which the content of Si is lower than that in the eutectic composition), the Al liquid is generated at the initial stage of solidification and grows to the a-Al phase, and the residual liquid with the eutectic composition solidifies to (Al þ Si) eutectic phase with a needle-like Si phase. In the case of important elongation in mechanical properties, several variables include grain size, distribution and size of precipitates, porosity, and impurity content, although it is known to be especially affected by SDAS [7,8]. On the other hand, Al alloys exhibit excellent thermal and electrical conductivities, and they constitute key materials to
2
J.H. Jeon, D.H. Bae / Journal of Alloys and Compounds 808 (2019) 151756
improve the efficiency of engine parts and increase service life by effectively dissipating generated heat energy [9]. Both electrical conductivity and thermal conductivity occur due to the movement of free electrons at a relatively low temperature, and thus a linear proportional relationship is generally established via WiedemanneFranz law [10,11]. Movement of free electrons is affected by factors including alloy composition, secondary phases, and process technology. Alloy composition is the most fundamental factor, and there are two main mechanisms by which elements reduce electrical conductivity and thermal conductivity. The first mechanism involves the alloy elements that are saturated in the Al matrix to cause lattice distortion. This type of lattice distortion interferes with free electron transport and causes a decrease in conductivities. The other mechanism is that the alloy elements in the molten Al readily react with Al to produce an intermetallic compound and this reduces the number of free electrons, and thereby deteriorates conductivities [12]. Hence, it is extremely important to increase thermal conductivity based on the microstructure of the heat sinks or automotive parts. As mentioned above, previous studies are focused on the effect of thickness-dependent cooling rate on mechanical properties. However, there is a paucity of studies that examine relationship between cooling rate and thermal conductivity. In the present study, change in thermal conductivity due to the variation of the cooling rate based on the mold thickness was evaluated and compared with electrical conductivity. Additionally, a model related to dendrite growth was presented to clarify the relationship between cooling rate and thermal conductivity. 2. Materials and methods The AleSi alloy used in the study corresponded to commercial A356 alloy (Hanjin-metal Co.). Table 1 shows the chemical composition of commercial A356 alloy analysed by X-ray fluorescence (XRF, ZSX100e, Rigaku). First, approximately 30 kg of the commercial A356 ingots were charged into an electrical resistance furnace and melted at 760 C. Subsequently, the melt was degassed with inert Ar gas for 10 m to eliminate gases and inclusions in molten A356 alloy. After a stabilizing time of 30 m, the molten A356 alloy was poured into a four-step sand mold with a thickness ranging from 5 mm to 20 mm as shown in Fig. 1. The mold include a computational recordable K-type thermocouple with a frequency of 100 Hz. Given sufficient solidification time, the sand cast mold was carefully broken, and the specimen was obtained The as-cast specimens were annealed with solid solution heat treatment at 545 C for 6 h, since temperature variations of ± 5 C are allowed during heat treatment and the solidus temperature of an A356 alloy is 555 C [13]. In addition, that the risk of melting of the Al2Cu phase does not reach here due to its low copper content. The specimens then were followed by artificial aging at 160 C for 8 h. In order to examine the microstructure of the specimens (which were cut 10 mm above the side of the casting), each specimen was grounded via SiC paper up to 2000 grit and polished with diamond paste. Fine polished samples were then etched for 10 s to clearly reveal SDAS with a solution of Keller's reagent (2.5 ml HNO3, 1.5 ml HCl, and 1.0 ml HF in 95 ml H2O). Microstructural analysis was performed via an optical microscope (OM, LV150, Nikon). Secondary dendrite arm spacing (SDAS) of alloy with different cooling rates was measured
Table 1 Chemical composition of commercial A356 alloy.
wt.%
Si
Mg
Fe
Cu
Ti
Mn
Cr
Al
6.98
0.391
0.118
0.029
0.109
0.005
0.005
Bal.
Fig. 1. Schematic diagram of the four-step sand mold.
via IP-Win 32 software, and SDAS measurement was performed on ten different sections of each specimen minimize the experimental errors. In order to evaluate the mechanical property, specimens (with a gauge length of 25 mm, gauge width of 6 mm, and gauge thickness of 2 mm) were machined and quasi-static uniaxial tensile tests (ASTM-E8) were conducted via an Instron-type machine (Universal Test Machine, R&B Inc.) at a strain rate of 1:0 103 =s. In order to evaluate the conduction properties, thermal conductivity of specimens (12.7 mm diameter and, 2 mm thickness) was measured via the laser flash analysis method (LFA447, NETZSCH). All specimens were coated with a graphite lubricant and measured 10 times each to ensure precise measurements. Additionally, the electrical conductivity of specimens (with a diameter of 30 mm, and thickness of 2 mm) was measured via an eddy current electrical conductivity meter (AEC-670, AJR NDT Co.). Each specimen was measured ten times to ensure reliability.
3. Result and discussion Fig. 2(a) shows cooling curves, as measured by K-type thermocouples, for commercial A356 alloy with different sand mold thickness. The solidification process of A356 alloy is as follows: Liquid / a-Al dendritic structure / (Al þ Si) eutectic phase / other intermetallic phase. The cooling curves rapidly change when solidification begins or when the temperature reaches the eutectic composition. When the thickness of the mold decreased, primary-a reaction temperatures and eutectic reaction temperatures of the A356 alloy shifted to a lower temperature. Fig. 2(b) describes the characteristics of the cooling curve [14]. Furthermore, the typical solidification parameters of the cooling curve obtained at different cooling rates are shown in Table 2. The liquidus temperature (TLiq) and eutectic temperature (TEut) of the A356 alloy used in the study were evaluated to be about 886 K and 850 K, respectively [13]. When the cooling rate increased, supercooling of the solidification temperature and supercooling of the eutectic temperature increased. The solidification time (Tf) is defined as the time until when the liquid phase becomes a solid phase through the a-Al dendritic structure and (Al þ Si) eutectic phase. The cooling rate (V) was divided by the time required to reach the end of solidification (TEnd) from liquidus temperature (TLiq). In the study, mold thicknesses corresponding to 20 mm, 15 mm, 10 mm and 5 mm were calculated at cooling rates of 0.3 K/s, 0.5 K/s, 0.8 K/s, and 1.9 K/s, respectively. The increase in the solidification temperature after supercooling was due to the recalescence phenomenon in which the latent heat owing to the phase
J.H. Jeon, D.H. Bae / Journal of Alloys and Compounds 808 (2019) 151756
3
Fig. 2. (a) Cooling curves based on mold thickness and, (b) typical cooling curve characteristics.
Table 2 Supercooling and cooling rate obtained from the cooling curves for different mold thicknesses. Thickness
DTLiq (K)
DTEut (K)
Solidification time (Tf, s)
Cooling rate (K/s)
5 mm 10 mm 15 mm 20 mm
41.0 39.9 37.4 36.2
6.9 5.1 4.1 3.7
18.6 44.6 80.0 116.4
1.9 K/s 0.8 K/s 0.5 K/s 0.3 K/s
transformation was released, and also the recalescence temperatures increased with the increases in the cooling rates [6]. This is well explained by the classical theories of Gibbs’ phase rule and heterogeneous nucleation [15e17]. The optical microstructure of T6 heat treated A356 alloy with different cooling rates is shown in Fig. 3. Microstructure of alloys are typically composed of primary Al, plate like (Al þ Si) eutectic phase, and a few other intermetallic compounds. Generally, the Si platelets are sphericalized due to the high temperature solid solution during T6 heat treatment, however, spherodization of large eutectic Si platelets by slow cooling rate are unfavourable compared to fast cooling rate. Additionally, it was confirmed that the dendrites were uniformly developed. The results of the SDAS measurement indicated that increase in the cooling rate led to finer
Fig. 3. Microstructures of T6 heat treated A356 alloy with cooling rates of (a) 0.3 K/s, (b) 0.5 K/s, (c) 0.8 K/s, and (d) 1.9 K/s.
SDAS. The relationship between SDAS and cooling rate based on thermodynamic data is widely known, and the mathematical equation for the hypo-eutectic composition proposed by Kattamis and Flemings's model is expressed as follows [6,18e20]:
0 B
l ¼ 5:5B @
DT GDL ln
Ceut C0
11=3
C C mð1 kÞðCeut C0 ÞA
V 1=3
(2)
where DT denotes the difference between the liquid temperature and eutectic temperature, G ¼ s=DSfus denotes the GibbseThomson coefficient, s denotes the interfacial energy between solid and liquid phases, DSfus denotes the entropy of fusion, DL is the diffusivity of solute atoms in liquid, Ceut denotes the eutectic composition, C0 denotes the solute composition, m denotes the slope of liquidus temperature, and k denotes the equilibrium distribution coefficient. The model is based on the binary alloy as specified by the formula for SDAS via the solute concentration difference, and thus the correlation between SDAS and cooling rate is well deduced. The SDAS results of the A356 alloy with different cooling rates tested in the study are shown in Fig. 4(a). Additionally, Eq. (2) is extrapolated in conjunction with and compared with experimental results in the present study. In this case, G ¼ 1:96 107 K,m, D ¼ 6:45 109 m2 =s, and k ¼ 0:13 were used as physical properties [21]. Results were in agreement with a well-known model, namely the KattamiseFlemings logarithmic function, despite the relatively slow cooling rate due to the sand mold. The results of tensile flow curve are shown in Fig. 4(b) to depict the effect of cooling rate and microstructural changes due to T6 heat treatment on mechanical properties. By increasing the cooling rate, the tensile strength and elongation to failure increased, but the yield strength was almost constant under all conditions. Bardes et al. [22] reported that the rate of solidification exhibited a greater effect on the deformation of DAS than the alloy composition. They also reported that yield strength did not affect DAS, although elongation and tensile strength of AleSi alloys were sensitive to the variation of DAS. As indicated by Melo et al. [23], the mechanical properties of Al alloy products depend on the microporosity formation during solidification. Microporosities are inevitably formed by the amount of dissolved hydrogen gases and the difference in thermal expansion coefficient between Al and Si when forming the primary and secondary dendrite structures, and it corresponds to the variation in pore radius as function of the cooling rate. Hence, microporosity formed over a critical size causes premature cracks in stress
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Fig. 4. (a) Measured values of SDAS based on the cooling rate. The dotted line is the theoretical prediction using KattamiseFlemings model, (b) Flow curves of uniaxial tensile test relative to different cooling rates.
generation and premature failure. DAS also affects mechanical properties by changing the eutectic (Al þ Si) phase. As reported in Pedersen et al. [24], brittle fracture of AleSi alloys occurs along the brittle Si existing in the grain boundaries of the alloy. When the cooling rate increases, the aspect ratio decreases and the uniformly distributed Si phase can delay brittle fracture. Thermal conductivity and electrical conductivity based on the cooling rate are shown in Fig. 5(a) and (b), respectively. In a metal, conduction mechanisms are mostly carried by movement of free electrons within the metal. Since free electrons also carry heat energy, according to the Wiedemann-Fran law thermal conductivity tracks electrical conductivity as follows [12]:
k = s ¼ LT ¼ kr
(3)
where k, s, L, T, and r are thermal conductivity, electrical
conductivity, Lorenz number, absolute temperature, and electrical resistivity, respectively. At a given temperature, the thermal conductivity and electrical conductivity of metals exhibit a proportional relationship, and thus this was in good agreement with the experiment at 298 K as shown Fig. 5(c). Evidently, both conductivities increased when the cooling rate increased. We interpret that this is potentially related to the grain coarsening model [25,26] wherein, the redistribution of solutes is related to the growth of dendrites. When the liquid was in contact with the SDAS during solidification, the increases in the thickness of SDAS in liquid solute concentration exceeded that of thin SDAS due to the difference in the radius of curvature. Thus, the solute in the liquid diffused towards the thinner SDAS due to the solute concentration gradient. Thus, the thin SDAS was dissolved and thinned to inhibit the increase in solute concentration, while the thick SDAS was solidified to stop the decrease in solute concentration. In the process, solutes
Fig. 5. Measurement values of (a) thermal conductivity, and (b) electrical conductivity with different cooling rates. (c) Relationship of thermal conductivity and electric conductivity relative to different cooling rates, and the dotted line is the theoretical prediction using WiedemanneFranz model. (d) Schematic drawing of the formation of the solute layer.
J.H. Jeon, D.H. Bae / Journal of Alloys and Compounds 808 (2019) 151756
pile up at solideliquid interface and form a solute boundary layer [27,28]. Eq. (2) specifies the relationship between the cooling rate and SDAS obtained by modifying the grain coarsening model, and the schematic diagram is shown in Fig. 5(d). Thus, electron scattering is caused by a large area of the solute layer when the cooling rate is slow although, it is barely affected by the solute layer when the cooling rate is high as shown in Fig. 5(d). 4. Conclusion In summary, microstructure, mechanical properties, electrical conductivity and thermal conductivity were evaluated at different cooling rates. When the cooling rate increased, shrinkage defects and pore defects decreased, and the role of the solute layer in preventing electron scattering decreased, thereby increasing thermal and electrical conductivities. The role of the solute layer in electron scattering was explained based on the proposed grain coarsening model. The model provided good fundamental insights and a new perspective on the effect of cooling rate on electron scattering. Acknowledgement This work was supported by National Research Foundation of Korea (NRF) grant funded by the Korea government (MISP) (No. NRF-2017R1A2B2007062). References [1] S.K. Singh, K. Chattopadhyay, G. Phanikumar, P. Dutta, Experimental and numerical studies on friction welding of thixocast A356 aluminum alloy, Acta Mater. 73 (2014) 177e185. [2] S.R. Sharma, Z.Y. Ma, R.S. Mishra, Effect of friction stir processing on fatigue behavior of A356 alloy, Scr. Mater. 51 (2004) 237e241. [3] H. Ye, An overview of the development of Al-Si-Alloy based material for engine applications, J. Mater. Eng. Perform. 12 (2003) 288e297. [4] J.G. Kaufman, E.L. Rooy, Aluminum Alloy Castings: Properties, Processes, and Applications, Asm International, 2004. [5] K.P. Young, D.H. Kerkwood, The dendrite arm spacings of aluminum-copper alloys solidified under steady-state conditions, Metall. Trans. A 6 (1975) 197e205.
5
[6] M.C. Flemings, Solidification Processing, Materials Science and Technology, 2006. [7] X. Jian, H. Xu, T. Meek, Q. Han, Effect of power ultrasound on solidification of aluminum A356 alloy, Mater. Lett. 59 (2005) 190e193. [8] J. Wannasin, R. Martinez, M. Flemings, Grain refinement of an aluminum alloy by introducing gas bubbles during solidification, Scr. Mater. 55 (2006) 115e118. [9] J.R. Davis, Aluminum and Aluminum Alloys, ASM international, 1993. [10] S. Bakhtiyarov, R. Overfelt, S. Teodorescu, Electrical and thermal conductivity of A319 and A356 aluminum alloys, J. Mater. Sci. 36 (2001) 4643e4648. [11] J. Koh, A. Fortini, Prediction of thermal conductivity and electrical resistivity of porous metallic materials, Int. J. Heat Mass Transf. 16 (1973) 2013e2022. [12] E.S. Toberer, L.L. Baranowski, C. Dames, Advances in thermal conductivity, Annu. Rev. Mater. Res. 42 (2012) 179e209. [13] A.S.M. International, C. Handbook, ASM Handbook. Volume 2, vol. 2, 1990. [14] D.M. Stefanescu, Thermal analysisdtheory and applications in metalcasting, Int. J. Metalcast. 9 (2015) 7e22. [15] B. Murty, S. Kori, M. Chakraborty, Grain refinement of aluminium and its alloys by heterogeneous nucleation and alloying, Int. Mater. Rev. 47 (2002) 3e29. [16] D. Turnbull, Kinetics of heterogeneous nucleation, J. Chem. Phys. 18 (1950) 198e203. [17] D.A. Porter, K.E. Easterling, M. Sherif, Phase Transformations in Metals and Alloys, (Revised Reprint), CRC press, 2009. [18] R. Grugel, Secondary and tertiary dendrite arm spacing relationships in directionally solidified Al-Si alloys, J. Mater. Sci. 28 (1993) 677e683. [19] M. MC FLEMINGS, Microsegregation in iron-base alloys, in: J Iron Steel Inst (London), 1970. [20] T.Z. Kattamis, Influence of coarsening on dendrite arm spacing of aluminumcopper alloys, Trans. Metall. Soc. AIME 239 (1967) 1504e1511. [21] H. Zhang, K. Nakajima, H. Lei, J. He, Restrictions of Physical Properties on Solidification Microstructures of Al-Based Binary Alloys by Cellular Automaton, 2010. [22] B. Bardes, M. Flemings, Dendrite arm spacing and solidification time in a cast aluminum-copper alloy, Mod. Cast. 50 (1966) 100e106. [23] M.d.L.N.M. Melo, E. Rizzo, R. Santos, Predicting dendrite arm spacing and their effect on microporosity formation in directionally solidified Al-Cu alloy, J. Mater. Sci. 40 (2005) 1599e1609. [24] L. Pedersen, L. Arnberg, The effect of solution heat treatment and quenching rates on mechanical properties and microstructures in AlSiMg foundry alloys, Metall. Mater. Trans. A 32 (2001) 525e532. [25] H.D. Brody, Solute redistribution in dendritic solidification, in: Massachusetts Institute of Technology, 1965. [26] S. Ganesan, D. Poirier, Solute redistribution in dendritic solidification with diffusion in the solid, J. Cryst. Growth 97 (1989) 851e859. [27] M.J. Aziz, Model for solute redistribution during rapid solidification, J. Appl. Phys. 53 (1982) 1158e1168. [28] M.J. Aziz, T. Kaplan, Continuous growth model for interface motion during alloy solidification, Acta Metall. 36 (1988) 2335e2347.