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Mechanical properties and damping capacity after grain refinement in A356 alloy
Materials Letters 59 (2005) 2174 – 2177 www.elsevier.com/locate/matlet
Mechanical properties and damping capacity after grain refinement in A356 alloy Yijie ZhangT, Naiheng Ma, Yongkang Le, Songchun Li, Haowei Wang The State Key Laboratory of Metal Matrix Composites of Shanghai Jiaotong University, Shanghai 200030, P.R. China Received 24 October 2004; accepted 19 February 2005 Available online 11 March 2005
Abstract JR-6 nano-grain refiner was employed to investigate mechanical properties and damping capacity of A356 alloy after grain refinement. Experimental findings indicate that a-Al dendritic arm spacing reduced from 44 Am to 23 Am in size after grain refinement. With T6 heat treatment, gliding fracture only was observed by SEM on fracture region after refinement, which filled with dimple fully. Tensile testing results show that A356 alloy after refinement has better mechanical properties, which increased by 30 MPa, 24 MPa, 4.1% in tensile strength, yield strength and elongation, respectively. Damping measurement shows damping capacity of A356 alloy after grain refinement is higher than that of without refinement. Moreover, damping capacity increases with increasing the temperature and decreases with increasing frequency. With testing condition of room temperature and frequency of 0.5 Hz damping capacity of A356 alloy after grain refinement is 13 10 3, increased by 5 10 3 compared to A356 without refinement in same case. Also damping mechanisms are discussed basing on experimental results. D 2005 Elsevier B.V. All rights reserved. Keywords: Mechanical properties; Damping capacity; Grain refinement; Nano-grain refiner
1. Introduction Al–Si alloys, which comprise 85% to 90% of the total aluminum-cast parts produced, exhibit excellent castability, mechanical and physical properties [1]. The microstructure and alloy constituents are necessitated to achieve optimum mechanical properties. Some of the critical microstructural features are grain size, dendritic arm spacing and silicon morphology in the eutectic phase [2]. In solidification process of aluminium alloy casting three different grain characteristics are formed possibly, namely, equiaxed, columnar and twinned columnar(TCG) [3]. It has been reported that the presence of silicon promotes the formation of TCG. So grain refinement is very important for Al–Si alloy in order to obtain fine equiaxed grain. Equiaxed grain structure ensures uniform mechanical properties, reduced ingot cracking, improved feeding to eliminate shrinkage T Corresponding author. Tel.: +86 21 62932569. E-mail address: [email protected] (Y. Zhang). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.058
porosity, distribution of second phases and microporosity on a fine scale as well as improved machinability of castings [4]. But little literature concerns on damping capacity of Al– Si alloys after grain refinement so far. With so many advantages mentioned above, Al–Si alloys are widely used in mobile, aerospace industry and transport systems and so on. Common concepts are that Al–Si alloys possess relative low damping capacity. The damping capacity of a material refers to its ability to convert mechanical vibration energy into thermal energy or other energies. It was reported that damping capacity and mechanical properties are the contradictory factors in a materials. Materials possessed high damping capacity will exhibit poor mechanical properties and vice verse [5]. It means that techniques were adopted at present to improve damping capacity at the expense of mechanical properties. In some case both high damping capacity and good mechanical properties are needed simultaneously, thus advanced approaches are urged to improve damping capacity of Al– Si alloys without sacrificing mechanical properties.
Y. Zhang et al. / Materials Letters 59 (2005) 2174–2177
2175
Table 1 Chemical composition of A356 alloy Material
Composition (wt.%) Si
Mg
Ti
Fe
Others
Al
A356
7.0
0.35
0.11
0.08
b0.01
Bal.
The purpose of present investigation is to study the variation of mechanical properties and damping capacity after grain refinement in A356 alloy by utilizing JR-6 nano-grain refiner. And hope to explore an effective way to improve both damping capacity and mechanical properties. Fig. 2. Microstructure of A356 alloy as-cast state after grain refinement.
2. Experimental A356 alloy was used for the present purpose, the compositions were given in Table 1, which was supplied by China Baotou aluminum Co. Ltd. For studying the effect of grain refinement, JR-6 nano-grain refiner was employed, which was supplied by Shanghai Zhumei Co. Ltd. (China). The microstructure of A356 alloy samples before and after refining treatment was observed by Olympus LECO optical microscope. Fracture region observation carries out on scanning electron microscopy (SEM). A356 alloy was melted in graphite crucibles by resistance furnace, and degassed processing was done by 1 wt.% hexachloroethane at 730 8C. Then the melt was cast into standard metal mould preheated to 200 8C for tensile sample. Subsequently 0.3 wt.% JR-6 nano-grain refiner was added to the melts. The melt was maintained at 725 F 5 8C and slow stirred needed to keep grain refiner disperse uniformity in the melt for about 30 s with a silicon carbide rod after addition was completed melting. The melt was held for 30 min and then was poured into the same mould preheated to 200 8C also. Microanalysis sample was cut out of the tensile sample. After being ground mechanically on 200 grit and 400 grit water-proof adhesive paper and polished with 1 Am diamond grinding paste, the sample were etched in the
Fig. 1. Microstructure of A356 alloy as-cast state without grain refinement.
0.5% HF solution for about 2 min to display the grain feature by using an Olympus LECO optical microscope. The damping capacity measurements were performed on DMTA using three-point bending testing mode. Rectangular bar samples for the damping capacity measurements with dimensions of 50 5 1 mm were obtained by spark machining. The measure of damping capacity utilized is loss tangent (tan(/)) in this study, at least three samples for each testing mode were tested to verify repeatability. The damping capacity, in terms of loss tangent (tan(/)), is accordingly calculated from tan/ ¼ E W=E V
ð1Þ
where EW is loss modulus and EV is storage modulus or dynamic modulus.
3. Result and discussion Figs. 1 and 2 show the microstructure of A356 alloy ascast state before and after grain refinement, respectively. It can be seen that after refinement a-Al dendritic arm spacing
Fig. 3. Fracture morphology of sample without grain refinement.
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Y. Zhang et al. / Materials Letters 59 (2005) 2174–2177
0.06 f=0.5Hz heating
0.05
ε =0.005%
without grain refinement tan(φ)
0.04
30minutes after grain refinement
0.03 0.02 0.01 0.00
50
100
150
200
Temp(°C)
Fig. 5. Damping capacity of A356 alloy with T6 heat treatment before and after grain refinement in testing frequency of 0.5 Hz.
decreases significantly in size. Image analysis by Olympus LECO reveals that the amount of a-Al dendritic is 520 per mm2 and the size is 44 Am. However, the amount of a-Al dendritic is 1815 per mm2, the size is 23 Am after grain refinement. Figs. 3 and 4 indicate SEM observation of the fracture surface morphology of A356 alloy without and with grain refinement, respectively. The fracture characteristic of A356 alloy without refinement is mixture of cleavage fracture and gliding fracture, whereas gliding fracture only after grain refinement. This different fracture mechanism is attributed to good performance of grain refinement. Mechanical properties of A356 alloy before and after refinement are shown in Table 2. All samples are subjected to T6 heat treatment. It can be seen that after grain refinement the value of tensile strength, yield strength and elongation are increased by 30 MPa, 24 MPa and 4.1%, respectively. The damping capacity as a function of temperature during heating for A356 alloy without and with grain refinement as-T6 state is shown in Fig. 5. The strain amplitude keeps on 5 10 5 consistently and testing frequency is 0.5 Hz for temperature sweep tests. It can be seen from Fig. 5 that the damping capacity of these two materials is increased with increasing the temperature over the studied temperature range. It should be noted that the damping capacity of A356 alloy with grain refinement is higher than that of without grain refinement. The data in Table 2 Mechanical properties of A356 alloy without and with grain refinement State of sample
Without refinement With refinement
Tensile strength (MPa)
Yield strength (MPa)
Elongation (%)
350 380
295 319
3.9 8
Heat treatment state T6 T6
Fig. 5 indicates that damping capacity value of A356 alloy without refinement is 8 10 3 and 13 10 3 after grain refinement at room temperature. This shows that the increasing of damping capacity is linked with the change of grain size. Fig. 6 shows measurements of tan(/) vs. temperature for A356 alloy with T6 heat treatment after grain refinement in different testing frequency ( f = 0.5, 2 and 10 Hz) during ascending temperature (T = 3 8C min 1). It can be seen that the damping capacity of A356 alloy after grain refinement is decreased with increasing frequency. Keˆ [6] reported that the damping capacity of purity aluminum is the function of (G.S.) f exp(H/RT), where G.S. is grain size, f is frequency, T is temperature and H is activation energy. According to the relations mentioned above damping capacity is proportional to temperature and has inverse relation to frequency and grain size. The experimental results of present investigation are well in agreement with the achievement of Keˆ. Some literatures reported that the damping in metal and its alloys can be attributed to thermoelastic damping and defect damping [7]. Thermoelastic damping increases with increasing frequency when testing frequency below Zener 0.05 after grain refinement heating ε =0.005%
0.04 f=0.5Hz
tan(φ)
Fig. 4. Fracture morphology of sample with grain refinement.
f=2Hz
0.03
f=10Hz 0.02
0.01
0.00
50
100
150
200
Temp(°C) Fig. 6. Damping capacity of A356 alloy with T6 heat treatment after grain refinement in different testing frequency.
Y. Zhang et al. / Materials Letters 59 (2005) 2174–2177
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relaxation frequency of 160 Hz. In present study, damping capacity is decreased with increasing frequency over the frequency studied. So thermoelastic damping has no contribution to improving damping capacity of A356 alloy. Among defect damping, included point defects (e.g. vacancies), line defects (e.g. dislocation), surface defects (e.g. grain boundaries) and volume defects (e.g. inclusions), dislocation damping and grain boundary damping are considered to play a dominated role for Al–Si alloys. Dislocation damping is noteworthy because it plays a critical role, not only in the damping response of crystalline materials, but also in the overall mechanical behavior of the materials [8]. Keˆ [9] reported that a polycrystalline Al showed a higher damping than a single crystal Al, the difference of grain boundary damping between the polycrystalline Al and the single crystal Al manifest when the testing temperature exceeded 200 8C. In the present study, the a-Al dendritic arm spacing and grain size after refinement are smaller than that of A356 alloy without grain refinement. The fine grained microstructure may play a partial role on the dissipation of elastic strain energy. This is evident from Fig. 5 that A356 alloy after refinement exhibited higher damping capacity than that of without refinement at elevated temperatures because of the effects of the grain boundary damping.
studied simultaneously. The major conclusions and suggestions drawn from the results are as follows. After refinement a-Al dendritic decreases from 44 Am to 23 Am in size and mechanical properties increase by 30 MPa, 24 MPa, 4.1% in tensile strength, yield strength and elongation, respectively. Damping capacity of A356 alloy, as a function of temperature, increases with increasing the temperature over studied temperatures range. Moreover, it was noted that damping capacity tends to decrease with increasing frequency. Damping capacity of A356 alloy after grain refinement has been elevated differently corresponding to different testing frequency, its value at room temperature in 0.5 Hz up to 13 10 3 , increased by 5 10 3 compared to A356 alloy without refinement. The damping mechanisms associated with A356 were ascribed to dislocation damping and grain boundary damping. In addition, fracture mechanism is sliding fracture only compared to mixture of cleavage fracture and sliding fracture without refinement.
4. Conclusion
[5] [6] [7] [8] [9]
In the present investigation, mechanical properties and damping capacity after grain refinement in A356 alloy are
References [1] [2] [3] [4]
J.U. Ejiofor, R.G. Reddy, JOM 49 (1997) 31. P.S. Mohanty, J.E. Gruzleski, Acta Mater. 44 (1996) 3749. P.S. Mohanty, J.E. Gruzleski, Acta Metall. Mater. 43 (1995) 2001. S.A. Kori, B.S. Murty, M. Charkraborty, Mater. Sci. Eng., A 280 (2000) 58. R. Schaller, J. Alloys Compd. 355 (2003) 131. Keˆ Tingsui, Science Publishing House, Beijing, 2000. J. Zhang, R.J. Perez, E.J. Lavernia, Acta Metall. Mater. 42 (1994) 395. J. Zhang, R.J. Perez, E.J. Lavernia, J. Mater. Sci. 28 (1993) 835. T.S. Keˆ, Phys. Rev. 71 (1947) 533.
Mechanical properties and damping capacity after grain refinement in A356 alloy Yijie ZhangT, Naiheng Ma, Yongkang Le, Songchun Li, Haowei Wang The State Key Laboratory of Metal Matrix Composites of Shanghai Jiaotong University, Shanghai 200030, P.R. China Received 24 October 2004; accepted 19 February 2005 Available online 11 March 2005
Abstract JR-6 nano-grain refiner was employed to investigate mechanical properties and damping capacity of A356 alloy after grain refinement. Experimental findings indicate that a-Al dendritic arm spacing reduced from 44 Am to 23 Am in size after grain refinement. With T6 heat treatment, gliding fracture only was observed by SEM on fracture region after refinement, which filled with dimple fully. Tensile testing results show that A356 alloy after refinement has better mechanical properties, which increased by 30 MPa, 24 MPa, 4.1% in tensile strength, yield strength and elongation, respectively. Damping measurement shows damping capacity of A356 alloy after grain refinement is higher than that of without refinement. Moreover, damping capacity increases with increasing the temperature and decreases with increasing frequency. With testing condition of room temperature and frequency of 0.5 Hz damping capacity of A356 alloy after grain refinement is 13 10 3, increased by 5 10 3 compared to A356 without refinement in same case. Also damping mechanisms are discussed basing on experimental results. D 2005 Elsevier B.V. All rights reserved. Keywords: Mechanical properties; Damping capacity; Grain refinement; Nano-grain refiner
1. Introduction Al–Si alloys, which comprise 85% to 90% of the total aluminum-cast parts produced, exhibit excellent castability, mechanical and physical properties [1]. The microstructure and alloy constituents are necessitated to achieve optimum mechanical properties. Some of the critical microstructural features are grain size, dendritic arm spacing and silicon morphology in the eutectic phase [2]. In solidification process of aluminium alloy casting three different grain characteristics are formed possibly, namely, equiaxed, columnar and twinned columnar(TCG) [3]. It has been reported that the presence of silicon promotes the formation of TCG. So grain refinement is very important for Al–Si alloy in order to obtain fine equiaxed grain. Equiaxed grain structure ensures uniform mechanical properties, reduced ingot cracking, improved feeding to eliminate shrinkage T Corresponding author. Tel.: +86 21 62932569. E-mail address: [email protected] (Y. Zhang). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.058
porosity, distribution of second phases and microporosity on a fine scale as well as improved machinability of castings [4]. But little literature concerns on damping capacity of Al– Si alloys after grain refinement so far. With so many advantages mentioned above, Al–Si alloys are widely used in mobile, aerospace industry and transport systems and so on. Common concepts are that Al–Si alloys possess relative low damping capacity. The damping capacity of a material refers to its ability to convert mechanical vibration energy into thermal energy or other energies. It was reported that damping capacity and mechanical properties are the contradictory factors in a materials. Materials possessed high damping capacity will exhibit poor mechanical properties and vice verse [5]. It means that techniques were adopted at present to improve damping capacity at the expense of mechanical properties. In some case both high damping capacity and good mechanical properties are needed simultaneously, thus advanced approaches are urged to improve damping capacity of Al– Si alloys without sacrificing mechanical properties.
Y. Zhang et al. / Materials Letters 59 (2005) 2174–2177
2175
Table 1 Chemical composition of A356 alloy Material
Composition (wt.%) Si
Mg
Ti
Fe
Others
Al
A356
7.0
0.35
0.11
0.08
b0.01
Bal.
The purpose of present investigation is to study the variation of mechanical properties and damping capacity after grain refinement in A356 alloy by utilizing JR-6 nano-grain refiner. And hope to explore an effective way to improve both damping capacity and mechanical properties. Fig. 2. Microstructure of A356 alloy as-cast state after grain refinement.
2. Experimental A356 alloy was used for the present purpose, the compositions were given in Table 1, which was supplied by China Baotou aluminum Co. Ltd. For studying the effect of grain refinement, JR-6 nano-grain refiner was employed, which was supplied by Shanghai Zhumei Co. Ltd. (China). The microstructure of A356 alloy samples before and after refining treatment was observed by Olympus LECO optical microscope. Fracture region observation carries out on scanning electron microscopy (SEM). A356 alloy was melted in graphite crucibles by resistance furnace, and degassed processing was done by 1 wt.% hexachloroethane at 730 8C. Then the melt was cast into standard metal mould preheated to 200 8C for tensile sample. Subsequently 0.3 wt.% JR-6 nano-grain refiner was added to the melts. The melt was maintained at 725 F 5 8C and slow stirred needed to keep grain refiner disperse uniformity in the melt for about 30 s with a silicon carbide rod after addition was completed melting. The melt was held for 30 min and then was poured into the same mould preheated to 200 8C also. Microanalysis sample was cut out of the tensile sample. After being ground mechanically on 200 grit and 400 grit water-proof adhesive paper and polished with 1 Am diamond grinding paste, the sample were etched in the
Fig. 1. Microstructure of A356 alloy as-cast state without grain refinement.
0.5% HF solution for about 2 min to display the grain feature by using an Olympus LECO optical microscope. The damping capacity measurements were performed on DMTA using three-point bending testing mode. Rectangular bar samples for the damping capacity measurements with dimensions of 50 5 1 mm were obtained by spark machining. The measure of damping capacity utilized is loss tangent (tan(/)) in this study, at least three samples for each testing mode were tested to verify repeatability. The damping capacity, in terms of loss tangent (tan(/)), is accordingly calculated from tan/ ¼ E W=E V
ð1Þ
where EW is loss modulus and EV is storage modulus or dynamic modulus.
3. Result and discussion Figs. 1 and 2 show the microstructure of A356 alloy ascast state before and after grain refinement, respectively. It can be seen that after refinement a-Al dendritic arm spacing
Fig. 3. Fracture morphology of sample without grain refinement.
2176
Y. Zhang et al. / Materials Letters 59 (2005) 2174–2177
0.06 f=0.5Hz heating
0.05
ε =0.005%
without grain refinement tan(φ)
0.04
30minutes after grain refinement
0.03 0.02 0.01 0.00
50
100
150
200
Temp(°C)
Fig. 5. Damping capacity of A356 alloy with T6 heat treatment before and after grain refinement in testing frequency of 0.5 Hz.
decreases significantly in size. Image analysis by Olympus LECO reveals that the amount of a-Al dendritic is 520 per mm2 and the size is 44 Am. However, the amount of a-Al dendritic is 1815 per mm2, the size is 23 Am after grain refinement. Figs. 3 and 4 indicate SEM observation of the fracture surface morphology of A356 alloy without and with grain refinement, respectively. The fracture characteristic of A356 alloy without refinement is mixture of cleavage fracture and gliding fracture, whereas gliding fracture only after grain refinement. This different fracture mechanism is attributed to good performance of grain refinement. Mechanical properties of A356 alloy before and after refinement are shown in Table 2. All samples are subjected to T6 heat treatment. It can be seen that after grain refinement the value of tensile strength, yield strength and elongation are increased by 30 MPa, 24 MPa and 4.1%, respectively. The damping capacity as a function of temperature during heating for A356 alloy without and with grain refinement as-T6 state is shown in Fig. 5. The strain amplitude keeps on 5 10 5 consistently and testing frequency is 0.5 Hz for temperature sweep tests. It can be seen from Fig. 5 that the damping capacity of these two materials is increased with increasing the temperature over the studied temperature range. It should be noted that the damping capacity of A356 alloy with grain refinement is higher than that of without grain refinement. The data in Table 2 Mechanical properties of A356 alloy without and with grain refinement State of sample
Without refinement With refinement
Tensile strength (MPa)
Yield strength (MPa)
Elongation (%)
350 380
295 319
3.9 8
Heat treatment state T6 T6
Fig. 5 indicates that damping capacity value of A356 alloy without refinement is 8 10 3 and 13 10 3 after grain refinement at room temperature. This shows that the increasing of damping capacity is linked with the change of grain size. Fig. 6 shows measurements of tan(/) vs. temperature for A356 alloy with T6 heat treatment after grain refinement in different testing frequency ( f = 0.5, 2 and 10 Hz) during ascending temperature (T = 3 8C min 1). It can be seen that the damping capacity of A356 alloy after grain refinement is decreased with increasing frequency. Keˆ [6] reported that the damping capacity of purity aluminum is the function of (G.S.) f exp(H/RT), where G.S. is grain size, f is frequency, T is temperature and H is activation energy. According to the relations mentioned above damping capacity is proportional to temperature and has inverse relation to frequency and grain size. The experimental results of present investigation are well in agreement with the achievement of Keˆ. Some literatures reported that the damping in metal and its alloys can be attributed to thermoelastic damping and defect damping [7]. Thermoelastic damping increases with increasing frequency when testing frequency below Zener 0.05 after grain refinement heating ε =0.005%
0.04 f=0.5Hz
tan(φ)
Fig. 4. Fracture morphology of sample with grain refinement.
f=2Hz
0.03
f=10Hz 0.02
0.01
0.00
50
100
150
200
Temp(°C) Fig. 6. Damping capacity of A356 alloy with T6 heat treatment after grain refinement in different testing frequency.
Y. Zhang et al. / Materials Letters 59 (2005) 2174–2177
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relaxation frequency of 160 Hz. In present study, damping capacity is decreased with increasing frequency over the frequency studied. So thermoelastic damping has no contribution to improving damping capacity of A356 alloy. Among defect damping, included point defects (e.g. vacancies), line defects (e.g. dislocation), surface defects (e.g. grain boundaries) and volume defects (e.g. inclusions), dislocation damping and grain boundary damping are considered to play a dominated role for Al–Si alloys. Dislocation damping is noteworthy because it plays a critical role, not only in the damping response of crystalline materials, but also in the overall mechanical behavior of the materials [8]. Keˆ [9] reported that a polycrystalline Al showed a higher damping than a single crystal Al, the difference of grain boundary damping between the polycrystalline Al and the single crystal Al manifest when the testing temperature exceeded 200 8C. In the present study, the a-Al dendritic arm spacing and grain size after refinement are smaller than that of A356 alloy without grain refinement. The fine grained microstructure may play a partial role on the dissipation of elastic strain energy. This is evident from Fig. 5 that A356 alloy after refinement exhibited higher damping capacity than that of without refinement at elevated temperatures because of the effects of the grain boundary damping.
studied simultaneously. The major conclusions and suggestions drawn from the results are as follows. After refinement a-Al dendritic decreases from 44 Am to 23 Am in size and mechanical properties increase by 30 MPa, 24 MPa, 4.1% in tensile strength, yield strength and elongation, respectively. Damping capacity of A356 alloy, as a function of temperature, increases with increasing the temperature over studied temperatures range. Moreover, it was noted that damping capacity tends to decrease with increasing frequency. Damping capacity of A356 alloy after grain refinement has been elevated differently corresponding to different testing frequency, its value at room temperature in 0.5 Hz up to 13 10 3 , increased by 5 10 3 compared to A356 alloy without refinement. The damping mechanisms associated with A356 were ascribed to dislocation damping and grain boundary damping. In addition, fracture mechanism is sliding fracture only compared to mixture of cleavage fracture and sliding fracture without refinement.
4. Conclusion
[5] [6] [7] [8] [9]
In the present investigation, mechanical properties and damping capacity after grain refinement in A356 alloy are
References [1] [2] [3] [4]
J.U. Ejiofor, R.G. Reddy, JOM 49 (1997) 31. P.S. Mohanty, J.E. Gruzleski, Acta Mater. 44 (1996) 3749. P.S. Mohanty, J.E. Gruzleski, Acta Metall. Mater. 43 (1995) 2001. S.A. Kori, B.S. Murty, M. Charkraborty, Mater. Sci. Eng., A 280 (2000) 58. R. Schaller, J. Alloys Compd. 355 (2003) 131. Keˆ Tingsui, Science Publishing House, Beijing, 2000. J. Zhang, R.J. Perez, E.J. Lavernia, Acta Metall. Mater. 42 (1994) 395. J. Zhang, R.J. Perez, E.J. Lavernia, J. Mater. Sci. 28 (1993) 835. T.S. Keˆ, Phys. Rev. 71 (1947) 533.