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Application of ultrasonic treating to degassing of metal ingots
Materials Letters 62 (2008) 4152–4154
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Application of ultrasonic treating to degassing of metal ingots Junwen Li a,b,⁎, Tadashi Momono b, Yoshinori Tayu b, Ying Fu a a b
College of Material and Chemical Engineering, Liaoning University of Technology, Jinzhou 121001, China Department of Materials Science and Engineering, Muroran Institute of Technology, Muroran 050-8585, Japan
A R T I C L E
I N F O
Article history: Received 1 March 2008 Accepted 5 June 2008 Available online 10 June 2008 Keywords: Ultrasonic vibration Aluminum alloy Porosity formation Prevention
A B S T R A C T The relations between porosity in the ingot and the effecting factors such as the ultrasonic power and the time of ultrasonic vibration (UV) treating to melt were investigated. Moreover, the mechanism of the porosity formation and the prevention method was studied. The results indicate that the effect of degasification was better when the intensity of UV is above threshold value. On the contrary, the intensity of UV below the value resulted in the increase of the gas content in the ingot and the decrease of density. It could be confirmed that there is an appropriate time on degasification by UV treating. When treating time is over the time, the density of the ingot tended to decrease. By using UV to degas with constraint cooling in the bottom of the ingot, the value of porosity volume (PV) can be decreased below 0.1 cm3/100 g and the ηdeg is near to 97%. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental methods
It is known that the crystal structure of the ingot treated by ultrasonic is changed from coarse columnar grain into equiaxed grain and its macro and micro segregation are also improved because of the ultrasonic unique acoustic effect [1–3]. As the presence of porosity in castings is extremely detrimental to the mechanical properties and corrosion resistance of the castings, it is important to eliminate the porosity and shrinkage cavity to produce high-quality castings. However, only some of the reports on ultrasonic degassing are found so far [4–6]. Several traditional degassing methods contain the vacuum degassing, the use of nitrogen or argon as a purge gas and hexachloroethane (C2Cl6) tablets. As an environment-protecting and resources-economizing method, the ultrasonic degassing has attracted a great deal of attention. The research on ultrasonic degassing was initiated in Soviet Union [7–8]. Recently, in the Oak Ridge National Laboratory of USA, the effects of humidity, melt volume and melt temperature on ultrasonic degassing have been investigated [9–10]. Therefore, it is necessary to find the effects of other factors on ultrasonic degassing. In this paper, we investigated the relationship between the porosity in the ingot and the ultrasonic power and the time of ultrasonic vibration (UV) treating to melt and studied the mechanism of the porosity formation and prevention.
The details of the ultrasonic apparatus can be found in Ref. [11]. The material used is 99.7 wt.% commercial aluminum ingots mixed with master-alloy ingots ( commercial Al–39.22 wt.% Cu, Al–24.4 wt.% Si, Al–11.0 wt.% Mn ). These ingots were held in a graphite crucible, then melted in the electric resistance furnace. Afterward, the molten alloy was stirred twice in the furnace and degassed first by using hexachloroethane tablets before pouring into another small graphite crucible for treatment. Ultrasonic probe without preheating begun to work before treating the melt and then was inserted into the melt 10 mm below liquid surface for a certain time. Densities of the specimens were measured by using Archimedes' method. The specimens were cut in the middle vertically and polished to observe macro and micro structure. The experimental conditions are summarized in Table 1.
⁎ Corresponding author. College of Material and Chemical Engineering, Liaoning University of Technology, Jinzhou 121001, China. Tel.: +86 416 4199650; fax: +86 416 4199579. E-mail address: [email protected] (J. Li). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.06.016
3. Experimental results and discussion 3.1. Effect of ultrasonic power Fig. 1 shows the relationship between ultrasonic power and the densities of three aluminum alloys with the same solute content of 1 wt.%. The values of the ultrasonic power are 0, 30, 75, 120 and 150 W, respectively. It can be seen that the trend of changes on the three kinds of aluminum alloys is similar i.e. the densities of the ingots decrease with the increase of ultrasonic power at first. When the ultrasonic power is up to 75 W, the density is minimum. However, with the continuous increase of ultrasonic power, each of the alloy densities also increases. Thus, there is a critical value at 75 W. When the power exceeds the critical value, the densities of the three aluminum alloys are increased. This phenomenon is considered to be related to the cavitation generated by ultrasonic vibration in the melt. An ultrasonic wave propagating through melts generates alternate regions of compression and rarefaction. In the rarefaction phase, it will form cavitation phenomenon. According to the investigation of G. I. Eskin, when the intensity of ultrasonic vibration is above the
J. Li et al. / Materials Letters 62 (2008) 4152–4154
4153
Table 1 Experimental conditions Item
Conditions
Ultrasonic power/W Ultrasonic frequency/kHz Pouring temperature/°C Melt temperature at the beginning of ultrasonic vibration treatment/°C Crucible temperature/°C Preheating temperature of horn/°C Insertion depth of horn/mm Ultrasonic treatment time/s
150 27.5 730 TL + 40 500 R.T. 10 0, 60, 120, 180, 240
value 2 W/cm2 (reference value in water), the cavitation reaches the developing– developed stage. It is considered to be in the sub-cavitation stage if below the value [12]. Therefore, in this test, it is in the developing–developed stage if above the critical value and sub-cavitation stage below it. In this experiment, the ultrasonic with power 150 W and frequency 27.5 kHz was carried out in graphite crucibles with Φ = 56 mm, H = 76 mm. According to P = I × S, P = 30 W and I = 1.26 W/cm2, all the energy of the ultrasonic passed through the melt surface. So, according to the dependence relationship between the minimum intensity of cavitation and its frequency, when the frequency f = 27.5 kHz, the value of Imin is 1.25 W/cm2 [13]. Here, cavitation in the melt is only in the beginning stage without pulsation of cavitation bubbles, which is the stage of sub-cavitation. At the moment, cavitation bubbles are formed in a limited range. Thus, the gas (majority is hydrogen) which has been dissolved in molten aluminum cannot diffuse towards the cavitation bubbles. As a result, those few cavitation bubbles were blockaded in the ingot with the decline of temperature during the solidification of the melt, which results in the decrease of the densities of the ingots. When the values of P and I (intensity of UV) reach 150 W and 6.33 W/cm2, respectively, the cavitation is considered to reach developing–developed stage in the melt. The state of cavitation in the melt can also be discussed from the view of acoustic intensity. The acoustic intensity of cavitation in the melt Pk can be expressed by the following expressions. Pk ¼
2PρL CL S
Fig. 2. Relationship between the application time of UV and the ingot density for pure aluminum and Al–Si-based alloys.
3.2. Effect of the ultrasonic treatment time In order to study the effect of the ultrasonic treatment time on gas content in ingots, excessive UV was carried out in the melt, and the crucible was preheated at 500 °C. Fig. 2 shows the relationship between ultrasonic treatment time and densities of the ingots. It can be seen that the densities of pure aluminum and Al–Si-based alloys reach maximum value when ultrasonic treatment times are 180 s and 120 s respectively. The densities of the ingots decline rapidly with the increase of ultrasonic treatment time. It can be explained by the fact that the solidification interface is different for pure metal and alloy. When the ultrasonic treatment time is proper, there are lots of cavitation bubbles generated. If the relationship between the cavitation bubbles in melt and ultrasonic frequency is a resonant one, they will expand obviously. The resonant frequency of the cavitation bubbles can be confirmed by the following formula [12–13]
1=2 ð1Þ
where, P represents output power of ultrasonic, (W); ρL the density of melt, (kg/m3); CL the sound velocity of ultrasonic in aluminum alloy melt, (m/s); S the area of probe end, (m2). In this experiment, when output power of ultrasonic is P1 = 75 W or P2 = 150 W, the homologous values are Pk1 = 5.25 × 106Pa and Pk2 = 7.42 × 106Pa, respectively. They are both above the threshold value Pa = 1.0 × 106Pa. Therefore, the cavitation is considered to reach the developed stage in the melt. The cavitation bubbles in this stage are extremely active, and the pressure in the cavitation bubbles is decreased obviously, resulting in the coagulation and growth of cavitation bubbles, then removed to the melt surface. The result obtained from the present experiment in the developed stage is consistent with the previous studies of O.V. Abramov on magnesium and aluminum alloys [13].
f ¼
1 πd
sffiffiffiffiffiffiffiffiffiffiffi 3kp0 ρL
ð2Þ
where, f represents frequency of the ultrasonic transducer (kHz); d the diameter of cavitation bubble, (cm); k the ratio of constant pressure specific heat/constant volume specific heat in cavitation bubble; p0 the hydraulics pressure of the melt, (Pa); ρL the density of aluminum alloy melt, (g/cm3). In this experiment, f = 25 kHz, p0 = 1 atm = 101,325 Pa, k = 1.14 and ρL = 2.4 g/cm3 are given, so that the values of the diameters of the cavitation bubbles can be obtained, i.e. d = 0.0153 cm. Suppose cavitation bubble is sphere, its radius R0 is d/2 and it rises at the rate of v in the melt of which coefficient of viscosity [14] is η (η = 1.40 × 10− 3 Ns/m2 at 800 °C), v can be calculated according to the formula of Stokes: v¼
2gR20 ðρL −ρ0 Þ 9η
ð3Þ
where, ρL represents the density of cavitation bubble (g/cm3); g the acceleration of gravity, (m/s2). So that, v = 2.2 cm/s when the height of crucible is 70 mm (i.e. H = 70 mm), the time of rising is about 3.2 s (t = H / v). In fact, there are cavitation bubbles with which their height is lower than 70 mm and they amalgamate each other. Thus, the coagulation of bubbles is more rapid and the growth of cavitation bubbles is more active. If the value of
Table 2 Comparison result with constraint cooling in the bottom of the ingot and other melt treatment methods (Al–1.65 wt.% Si) Treatment condition
Fig. 1. Relationship between the ultrasonic power and the ingot density.
No degassing and no ultrasonic vibration Using degasifying agent only Degasifying agent treatment and UV 5 min Degasifying agent treatment and UV 5 min with constraint cooling in the bottom of the ingot Al–1.65 wt.% Si theoretical density
Ingot density
Degassing efficiency
(g/cm3)
ηdeg (%)
2.6474 2.6591 2.6839 2.6898
0 26.7 83.1 96.6
2.6913
100
4154
J. Li et al. / Materials Letters 62 (2008) 4152–4154
cavitation bubbles' radius is estimated to be bigger than the first value (R0), it may come to the conclusion that the rising time of cavitation bubbles is less than or equal to 3.2 s (i.e. t ≤ 3.2 s). Subsequently, if ultrasonic vibration treatment is overmuch, with the increase of solid phase, the rising cavitation bubbles may be blocked by equiaxed grain, resulting in porosity formation i.e. the decrease of the densities of the ingots. 3.3. The efficiency of ultrasonic degassing with constraint cooling at the bottom of the ingot There are a few ultrasonic degassing schemes in the condition of constraint cooling at the bottom of the ingot. The result is shown in Table 2. The condition in this experiment is as follows: the output power and the frequency of the ultrasonic are 150 W and 27.5 kHz, respectively, the adding number of hexachloroethane tablets is 1 wt.% and ultrasonic treatment was carried out at the temperature over liquidus of 40 °C. The efficiency of ultrasonic degassing is defined as ηdeg. Its expression is shown as follows: ηdeg ¼
ρa −ρ0 100k ρt −ρ0
ð4Þ
where, ρa represents the density of the ingot, (g/cm3); ρt the theoretical density of metal or alloy, (g/cm3); ρ0 density of the ingot without any treatment, (g/cm3). From the expression, it can be seen that the ηdeg (used C2Cl6 only) is small. On the contrary, the porosity level in ingots decreases greatly in the condition of ultrasonic degassing with constraint cooling at the bottom of the ingot. In order to study the effect with or without constraint cooling at the bottom of the ingot, the value of porosity volume (PV) in the ingot (every 100 g) can be calculated by the following expressions [15]: PV ¼
1 1 − ρa ρt
100; cm3 =100 g:
ð5Þ
The result is shown in Fig. 3, it can be seen that in the case of ultrasonic degassing, the value of PV is below 0.1 cm3/100 g with constraint cooling in the bottom of the ingot, when the time of ultrasonic treatment reaches 300 s, the value of PV reaches 0.02 cm3/ 100 g and ηdeg is near to 97%, which is equal to the vacuum degassing with the pressure of 10 mm Hg (the value of ηdeg is below 0.1 cm3/100 g). According to the result, the ultrasonic degassing is considered to be a good method to obtain high-quality castings. Fig. 4 shows the micro structures of the centers of the ingots treated in various methods. Lots of interdendritic porosities and grain boundary cavities in the specimen without any treatment can be found as shown in Fig. 4 (a). There is also porosity existing in other specimens (Fig. 4 (b) and (c)). Furthermore, those porosities often distribute in the center of the ingots. But it doesn't occur in the specimen treated by ultrasonic degassing with constraint cooling in the bottom of the ingot, on contrast, there are lots of equiaxed grains existing as shown in Fig. 4 (d).
4. Conclusions The results were attained in the experiments as follows: 1) The effect of degasification was better when the intensity of UV is above the threshold value than below the value. Below the value, the gas content in the ingot was increased and the density was decreased. 2) It could be confirmed that there is an obvious effect of UV treating on degasification when the treating time of UV reaches the appropriate
Fig. 4. Comparison of Al–1.65 wt.% Si micro structure with constraint cooling in the bottom of the ingot and other melt treatment methods. (a) No degassing and no UV. (b) Using degasifying agent only. (c) Degasifying agent treatment and UV 5 min. (d) Degasifying agent treatment and UV 5 min with constraint cooling in the bottom of the ingot.
one, while the density of the ingot tended to decrease when the treating time was over the value. 3) By using the method of ultrasonic degassing with constraint cooling in the bottom of the ingot, the value of porosity volume (PV) can be decreased below 0.1 cm3/100 g and the ηdeg is near to 97%. Acknowledgements The authors are grateful to M. Kudo and M. Yuguchi for helpful discussions. This assistance of the Supported Expenses of Research Activation from the Ministry of Education, Culture, Sports, Science and Technology (Japan) is acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Fig. 3. Relationship between the UV time and the porosity volume under constraint cooling in the bottom of the ingot.
Jian, Meek T, Han Q. Scr Mater 2006;54:893. Jian, Xu H, Meek T, Han Q. Mater Lett 2005;59:190. Eskin, Sinyavskii VS, Usova VV. Prot Met 2003;39:551. Abramov, Abramov OV, Straumal BB, Gust W. Mater Des 1997;18:323. Li, Li T, Li X, Jin J. Ultrason Sonochem 2006;13:121. Osawa, Sato A. J JFS 2000;72:733. Abramov. Ultrasound in liquid and solid metals. London: CRC Press; 1994. p. 91. Eskin. Ultrasonic treatment of light alloy melts. Amsterdam: Gordon and Breach; 1998. p. 87. Naji Meidani, Hasan M. J Mater Process Technol 2004;147:311. Xu, Jian X, Meek T, Han Q. Mater Lett 2004;58:3669. Li, Momono T. Mater Technol 2005;20(4):202. Eskin. Ultrasonic and application technology. Osaka: New Japan casting and forging association press; 1991. p. 197. Abramov. Metal solidification under ultrasonic field. Osaka: New Japan casting and forging association press; 1975. p. 128. Iida T, Guthrie R. The physical properties of liquid metals. New York: Oxford University Press Inc; 1988. p. 169. Morimoto K, Awano Y, Nakamura M. IMONO 1991;63:757.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Application of ultrasonic treating to degassing of metal ingots Junwen Li a,b,⁎, Tadashi Momono b, Yoshinori Tayu b, Ying Fu a a b
College of Material and Chemical Engineering, Liaoning University of Technology, Jinzhou 121001, China Department of Materials Science and Engineering, Muroran Institute of Technology, Muroran 050-8585, Japan
A R T I C L E
I N F O
Article history: Received 1 March 2008 Accepted 5 June 2008 Available online 10 June 2008 Keywords: Ultrasonic vibration Aluminum alloy Porosity formation Prevention
A B S T R A C T The relations between porosity in the ingot and the effecting factors such as the ultrasonic power and the time of ultrasonic vibration (UV) treating to melt were investigated. Moreover, the mechanism of the porosity formation and the prevention method was studied. The results indicate that the effect of degasification was better when the intensity of UV is above threshold value. On the contrary, the intensity of UV below the value resulted in the increase of the gas content in the ingot and the decrease of density. It could be confirmed that there is an appropriate time on degasification by UV treating. When treating time is over the time, the density of the ingot tended to decrease. By using UV to degas with constraint cooling in the bottom of the ingot, the value of porosity volume (PV) can be decreased below 0.1 cm3/100 g and the ηdeg is near to 97%. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental methods
It is known that the crystal structure of the ingot treated by ultrasonic is changed from coarse columnar grain into equiaxed grain and its macro and micro segregation are also improved because of the ultrasonic unique acoustic effect [1–3]. As the presence of porosity in castings is extremely detrimental to the mechanical properties and corrosion resistance of the castings, it is important to eliminate the porosity and shrinkage cavity to produce high-quality castings. However, only some of the reports on ultrasonic degassing are found so far [4–6]. Several traditional degassing methods contain the vacuum degassing, the use of nitrogen or argon as a purge gas and hexachloroethane (C2Cl6) tablets. As an environment-protecting and resources-economizing method, the ultrasonic degassing has attracted a great deal of attention. The research on ultrasonic degassing was initiated in Soviet Union [7–8]. Recently, in the Oak Ridge National Laboratory of USA, the effects of humidity, melt volume and melt temperature on ultrasonic degassing have been investigated [9–10]. Therefore, it is necessary to find the effects of other factors on ultrasonic degassing. In this paper, we investigated the relationship between the porosity in the ingot and the ultrasonic power and the time of ultrasonic vibration (UV) treating to melt and studied the mechanism of the porosity formation and prevention.
The details of the ultrasonic apparatus can be found in Ref. [11]. The material used is 99.7 wt.% commercial aluminum ingots mixed with master-alloy ingots ( commercial Al–39.22 wt.% Cu, Al–24.4 wt.% Si, Al–11.0 wt.% Mn ). These ingots were held in a graphite crucible, then melted in the electric resistance furnace. Afterward, the molten alloy was stirred twice in the furnace and degassed first by using hexachloroethane tablets before pouring into another small graphite crucible for treatment. Ultrasonic probe without preheating begun to work before treating the melt and then was inserted into the melt 10 mm below liquid surface for a certain time. Densities of the specimens were measured by using Archimedes' method. The specimens were cut in the middle vertically and polished to observe macro and micro structure. The experimental conditions are summarized in Table 1.
⁎ Corresponding author. College of Material and Chemical Engineering, Liaoning University of Technology, Jinzhou 121001, China. Tel.: +86 416 4199650; fax: +86 416 4199579. E-mail address: [email protected] (J. Li). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.06.016
3. Experimental results and discussion 3.1. Effect of ultrasonic power Fig. 1 shows the relationship between ultrasonic power and the densities of three aluminum alloys with the same solute content of 1 wt.%. The values of the ultrasonic power are 0, 30, 75, 120 and 150 W, respectively. It can be seen that the trend of changes on the three kinds of aluminum alloys is similar i.e. the densities of the ingots decrease with the increase of ultrasonic power at first. When the ultrasonic power is up to 75 W, the density is minimum. However, with the continuous increase of ultrasonic power, each of the alloy densities also increases. Thus, there is a critical value at 75 W. When the power exceeds the critical value, the densities of the three aluminum alloys are increased. This phenomenon is considered to be related to the cavitation generated by ultrasonic vibration in the melt. An ultrasonic wave propagating through melts generates alternate regions of compression and rarefaction. In the rarefaction phase, it will form cavitation phenomenon. According to the investigation of G. I. Eskin, when the intensity of ultrasonic vibration is above the
J. Li et al. / Materials Letters 62 (2008) 4152–4154
4153
Table 1 Experimental conditions Item
Conditions
Ultrasonic power/W Ultrasonic frequency/kHz Pouring temperature/°C Melt temperature at the beginning of ultrasonic vibration treatment/°C Crucible temperature/°C Preheating temperature of horn/°C Insertion depth of horn/mm Ultrasonic treatment time/s
150 27.5 730 TL + 40 500 R.T. 10 0, 60, 120, 180, 240
value 2 W/cm2 (reference value in water), the cavitation reaches the developing– developed stage. It is considered to be in the sub-cavitation stage if below the value [12]. Therefore, in this test, it is in the developing–developed stage if above the critical value and sub-cavitation stage below it. In this experiment, the ultrasonic with power 150 W and frequency 27.5 kHz was carried out in graphite crucibles with Φ = 56 mm, H = 76 mm. According to P = I × S, P = 30 W and I = 1.26 W/cm2, all the energy of the ultrasonic passed through the melt surface. So, according to the dependence relationship between the minimum intensity of cavitation and its frequency, when the frequency f = 27.5 kHz, the value of Imin is 1.25 W/cm2 [13]. Here, cavitation in the melt is only in the beginning stage without pulsation of cavitation bubbles, which is the stage of sub-cavitation. At the moment, cavitation bubbles are formed in a limited range. Thus, the gas (majority is hydrogen) which has been dissolved in molten aluminum cannot diffuse towards the cavitation bubbles. As a result, those few cavitation bubbles were blockaded in the ingot with the decline of temperature during the solidification of the melt, which results in the decrease of the densities of the ingots. When the values of P and I (intensity of UV) reach 150 W and 6.33 W/cm2, respectively, the cavitation is considered to reach developing–developed stage in the melt. The state of cavitation in the melt can also be discussed from the view of acoustic intensity. The acoustic intensity of cavitation in the melt Pk can be expressed by the following expressions. Pk ¼
2PρL CL S
Fig. 2. Relationship between the application time of UV and the ingot density for pure aluminum and Al–Si-based alloys.
3.2. Effect of the ultrasonic treatment time In order to study the effect of the ultrasonic treatment time on gas content in ingots, excessive UV was carried out in the melt, and the crucible was preheated at 500 °C. Fig. 2 shows the relationship between ultrasonic treatment time and densities of the ingots. It can be seen that the densities of pure aluminum and Al–Si-based alloys reach maximum value when ultrasonic treatment times are 180 s and 120 s respectively. The densities of the ingots decline rapidly with the increase of ultrasonic treatment time. It can be explained by the fact that the solidification interface is different for pure metal and alloy. When the ultrasonic treatment time is proper, there are lots of cavitation bubbles generated. If the relationship between the cavitation bubbles in melt and ultrasonic frequency is a resonant one, they will expand obviously. The resonant frequency of the cavitation bubbles can be confirmed by the following formula [12–13]
1=2 ð1Þ
where, P represents output power of ultrasonic, (W); ρL the density of melt, (kg/m3); CL the sound velocity of ultrasonic in aluminum alloy melt, (m/s); S the area of probe end, (m2). In this experiment, when output power of ultrasonic is P1 = 75 W or P2 = 150 W, the homologous values are Pk1 = 5.25 × 106Pa and Pk2 = 7.42 × 106Pa, respectively. They are both above the threshold value Pa = 1.0 × 106Pa. Therefore, the cavitation is considered to reach the developed stage in the melt. The cavitation bubbles in this stage are extremely active, and the pressure in the cavitation bubbles is decreased obviously, resulting in the coagulation and growth of cavitation bubbles, then removed to the melt surface. The result obtained from the present experiment in the developed stage is consistent with the previous studies of O.V. Abramov on magnesium and aluminum alloys [13].
f ¼
1 πd
sffiffiffiffiffiffiffiffiffiffiffi 3kp0 ρL
ð2Þ
where, f represents frequency of the ultrasonic transducer (kHz); d the diameter of cavitation bubble, (cm); k the ratio of constant pressure specific heat/constant volume specific heat in cavitation bubble; p0 the hydraulics pressure of the melt, (Pa); ρL the density of aluminum alloy melt, (g/cm3). In this experiment, f = 25 kHz, p0 = 1 atm = 101,325 Pa, k = 1.14 and ρL = 2.4 g/cm3 are given, so that the values of the diameters of the cavitation bubbles can be obtained, i.e. d = 0.0153 cm. Suppose cavitation bubble is sphere, its radius R0 is d/2 and it rises at the rate of v in the melt of which coefficient of viscosity [14] is η (η = 1.40 × 10− 3 Ns/m2 at 800 °C), v can be calculated according to the formula of Stokes: v¼
2gR20 ðρL −ρ0 Þ 9η
ð3Þ
where, ρL represents the density of cavitation bubble (g/cm3); g the acceleration of gravity, (m/s2). So that, v = 2.2 cm/s when the height of crucible is 70 mm (i.e. H = 70 mm), the time of rising is about 3.2 s (t = H / v). In fact, there are cavitation bubbles with which their height is lower than 70 mm and they amalgamate each other. Thus, the coagulation of bubbles is more rapid and the growth of cavitation bubbles is more active. If the value of
Table 2 Comparison result with constraint cooling in the bottom of the ingot and other melt treatment methods (Al–1.65 wt.% Si) Treatment condition
Fig. 1. Relationship between the ultrasonic power and the ingot density.
No degassing and no ultrasonic vibration Using degasifying agent only Degasifying agent treatment and UV 5 min Degasifying agent treatment and UV 5 min with constraint cooling in the bottom of the ingot Al–1.65 wt.% Si theoretical density
Ingot density
Degassing efficiency
(g/cm3)
ηdeg (%)
2.6474 2.6591 2.6839 2.6898
0 26.7 83.1 96.6
2.6913
100
4154
J. Li et al. / Materials Letters 62 (2008) 4152–4154
cavitation bubbles' radius is estimated to be bigger than the first value (R0), it may come to the conclusion that the rising time of cavitation bubbles is less than or equal to 3.2 s (i.e. t ≤ 3.2 s). Subsequently, if ultrasonic vibration treatment is overmuch, with the increase of solid phase, the rising cavitation bubbles may be blocked by equiaxed grain, resulting in porosity formation i.e. the decrease of the densities of the ingots. 3.3. The efficiency of ultrasonic degassing with constraint cooling at the bottom of the ingot There are a few ultrasonic degassing schemes in the condition of constraint cooling at the bottom of the ingot. The result is shown in Table 2. The condition in this experiment is as follows: the output power and the frequency of the ultrasonic are 150 W and 27.5 kHz, respectively, the adding number of hexachloroethane tablets is 1 wt.% and ultrasonic treatment was carried out at the temperature over liquidus of 40 °C. The efficiency of ultrasonic degassing is defined as ηdeg. Its expression is shown as follows: ηdeg ¼
ρa −ρ0 100k ρt −ρ0
ð4Þ
where, ρa represents the density of the ingot, (g/cm3); ρt the theoretical density of metal or alloy, (g/cm3); ρ0 density of the ingot without any treatment, (g/cm3). From the expression, it can be seen that the ηdeg (used C2Cl6 only) is small. On the contrary, the porosity level in ingots decreases greatly in the condition of ultrasonic degassing with constraint cooling at the bottom of the ingot. In order to study the effect with or without constraint cooling at the bottom of the ingot, the value of porosity volume (PV) in the ingot (every 100 g) can be calculated by the following expressions [15]: PV ¼
1 1 − ρa ρt
100; cm3 =100 g:
ð5Þ
The result is shown in Fig. 3, it can be seen that in the case of ultrasonic degassing, the value of PV is below 0.1 cm3/100 g with constraint cooling in the bottom of the ingot, when the time of ultrasonic treatment reaches 300 s, the value of PV reaches 0.02 cm3/ 100 g and ηdeg is near to 97%, which is equal to the vacuum degassing with the pressure of 10 mm Hg (the value of ηdeg is below 0.1 cm3/100 g). According to the result, the ultrasonic degassing is considered to be a good method to obtain high-quality castings. Fig. 4 shows the micro structures of the centers of the ingots treated in various methods. Lots of interdendritic porosities and grain boundary cavities in the specimen without any treatment can be found as shown in Fig. 4 (a). There is also porosity existing in other specimens (Fig. 4 (b) and (c)). Furthermore, those porosities often distribute in the center of the ingots. But it doesn't occur in the specimen treated by ultrasonic degassing with constraint cooling in the bottom of the ingot, on contrast, there are lots of equiaxed grains existing as shown in Fig. 4 (d).
4. Conclusions The results were attained in the experiments as follows: 1) The effect of degasification was better when the intensity of UV is above the threshold value than below the value. Below the value, the gas content in the ingot was increased and the density was decreased. 2) It could be confirmed that there is an obvious effect of UV treating on degasification when the treating time of UV reaches the appropriate
Fig. 4. Comparison of Al–1.65 wt.% Si micro structure with constraint cooling in the bottom of the ingot and other melt treatment methods. (a) No degassing and no UV. (b) Using degasifying agent only. (c) Degasifying agent treatment and UV 5 min. (d) Degasifying agent treatment and UV 5 min with constraint cooling in the bottom of the ingot.
one, while the density of the ingot tended to decrease when the treating time was over the value. 3) By using the method of ultrasonic degassing with constraint cooling in the bottom of the ingot, the value of porosity volume (PV) can be decreased below 0.1 cm3/100 g and the ηdeg is near to 97%. Acknowledgements The authors are grateful to M. Kudo and M. Yuguchi for helpful discussions. This assistance of the Supported Expenses of Research Activation from the Ministry of Education, Culture, Sports, Science and Technology (Japan) is acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Fig. 3. Relationship between the UV time and the porosity volume under constraint cooling in the bottom of the ingot.
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