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Effect of the melt superheat on equiaxed solidification of Al-20 wt% Cu alloy investigated by in situ synchrotron radiography
Accepted Manuscript Effect of the Melt Superheat on Equiaxed Solidification of Al-20 wt.% Cu Alloy Investigated by In Situ Synchrotron Radiography Shifeng Luo, Guangyu Yang, Lei Xiao, Wanxia Huang, Qingxi Yuan, Wanqi Jie PII: DOI: Reference:
S0022-0248(17)30447-5 http://dx.doi.org/10.1016/j.jcrysgro.2017.07.001 CRYS 24232
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
20 January 2017 2 July 2017 3 July 2017
Please cite this article as: S. Luo, G. Yang, L. Xiao, W. Huang, Q. Yuan, W. Jie, Effect of the Melt Superheat on Equiaxed Solidification of Al-20 wt.% Cu Alloy Investigated by In Situ Synchrotron Radiography, Journal of Crystal Growth (2017), doi: http://dx.doi.org/10.1016/j.jcrysgro.2017.07.001
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Effect of the Melt Superheat on Equiaxed Solidification of Al-20 wt. % Cu Alloy Investigated by In Situ Synchrotron Radiography
Shifeng Luoa, Guangyu Yanga,*, Lei Xiaoa, Wanxia Huangb, Qingxi Yuanb, Wanqi Jiea a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, No. 127 Youyi Western Road, Xi’an 710072, PR China
b
*
Institute of High Energy Physics, The Chinese Academy of Sciences, Beijing 100039, PR China
Corresponding author:
Tel.: +86-13679228998, Fax: +86-029-88495414. Email address: [email protected]
Abstract Effect of the melt superheat on equiaxed solidification of Al-20 wt. % Cu alloy was investigated by in-situ synchrotron radiography at Beijing Synchrotron Radiation Facility. For comparison, the corresponding DSC analysis was also conducted. It was found that the grain size decreased with increasing the melt superheat. The relationship between the final mean grain size and the melt superheat can be expressed as: d 4919.3 T
0.33
. During solidification, the
mean grain size increased sharply in the first 70 s, then reached the final grain size gradually. Furthermore, with increasing the melt superheat, the mean nucleation rate increased, which can be attributed to the fact that increasing the melt superheat led to an increase in nucleation undercooling, and the growth rate and the duration of free growth stage decreased. As the melt superheat increased from 100 oC to 160 oC, the mean nucleation rate increased by 78.2% while the mean growth rate only decreased by 19.3%, which indicated that the high mean nucleation rate and the consequent low mean growth rate may be the real reasons for grain refinement. The increased nucleation density caused earlier growth deceleration due to solutal impingement effects.
Keywords: A1. Melt superheat; A1. Mean grain size; A1. Mean growth rate; A1. Mean nucleation rate; A1. Synchrotron X-ray radiography; A1. Nucleation undercooling
1. Introduction The performance of the casting alloys is greatly depended on the solidification microstructure [1, 2], which can be controlled by optimizing the process variables, such as melt treatment (including the melt superheat and the corresponding holding time) or casting parameters (including the mold preheating temperature and the pouring temperature of the melt). The effect of the melt superheat treatment on the microstructure evolution of casting alloys has been extensively studied [3-6]. Li et al. [7] studied the effect of the melt superheat on the microstructure of Al-16 Si (wt.%, used throughout the paper unless noted) alloy and concluded that a higher superheat temperature reduced the heredity of the structure. Jie et al. [8] investigated the microstructure of Al-7 Si-0.55 Mg alloy with melt superheat treatment, and found that the Si phase was modified through the melt superheat treatment, which reduces the heterogeneous nucleation of Si phase. Wang et al. [9] studied the effect of melt thermal treatment on commercial hypoeutectic Al-Si alloy (A356) and found that melt thermal treatment can significantly refine the solidification structure of the A356 alloy including the primary Alpha phase and intermetallic compounds. The maximum refining effect (e.g. the primary dendrite length decreased from 450 μm to 210 μm) was obtained in an optimal cooling rate range (5 – 6 K / s). Qin et al. [10] investigated the microstructure of Mg2Si/Al-Si-Cu composite with the different melt superheat and found that the superheat treatment increases undercooling and thereby the size of Mg2Si phase decreases from 150 μm to 40 μm as the melt superheating temperature increases from 720 oC to 1020 oC. The grain size decreases significantly from 6420 μm to 89 μm with increasing the melt superheating from 1380 o
C to 1680 oC in IN718C superalloy [11]. However, the accurate grain refinement mechanism still
remains unclear which can be attributed to the fact that solidification process is hard to investigate by in-situ method. Furthermore, with the aim to clarify the grain refinement mechanism, nucleation and growth of grains have been investigated widely [12-17]. The free growth theory, which can make quantitatively correct predictions for grain size, has been proposed by Greer et al. [12]. According to this theory, a grain grows from a refiner particle at an undercooling inversely proportional to the particle diameter. Zhang et al. [14] have been developed the edge-to-edge matching model for describing the interfacial crystallographic characteristics between two phases and predicted that Al3Ti is a powerful nucleating substrate foe Al alloy than TiC, TiB 2 and AlB2. The
interdependence theory linked grain formation and nucleant selection has been proposed by StJohn et al. [17], which assumes that grain formation is the result of the interdependence between nucleation and growth acting in concert within an environment dictated by the alloy chemistry. Although these theories can clarify the nucleation and growth of grains and predict the grain size quantitatively, the nucleation rate and growth rate of grains cannot quantitatively analyze due to the static analysis (e.g. microstructure observation after solidification). Nowadays, synchrotron radiography method can be utilized to observe the dynamic process of solidification. Mathiesen and Arnberg [18] studied the columnar dendritic growth of Al-30 Cu alloy, and analyzed the morphology and velocity of the solid-liquid interface, as well as the constitutional undercooling at the tip of the growth crystals. In situ observation on crystal fragmentation and columnar-to-equiaxed transitions in real alloy has also been reported from X-radiographic studies [19-21]. It is worth noticing that the effect of the electric current on the solidification behavior has been investigated by in-situ synchrotron radiography technique [22-24]. Furthermore, the home-lab sources were also used widely to investigate the metallic solidification with a novel solidification furnace (XRMON-GF) designed by Nguyen-Thi et al. [25]. Murphy et al. [26] studied the eutectic transformation of Al-20 Cu alloy by using laboratory-based radiography and post-solidification metallographic characterisation, and also investigated the equiaxed dendritic solidification with varying grain refiner concentrations and sample orientation at different cooling rates [27]. In order to obtain more uniform temperature in the sample, a new furnace with rotational symmetry (XRMON-SOL) has been designed by Murphy et al. [28] and isothermal equiaxed solidification of Al-Cu alloy in microgravity has been conducted on board the MASER 13 sounding rocket [28, 29]. Although synchrotron radiography and laboratory-based radiography have been used widely to study the solidification of metallic alloy [18-31], the effect of the melt superheat treatment on grain refinement has not been reported. In this paper, the solidification process of Al-20 Cu alloy with the different melt superheat was investigated by in-situ synchrotron radiography at the first-generation Beijing Synchrotron Radiation Facility (BSRF). The mean growth rate and mean nucleation rate were quantitatively analyzed. The mechanism of grain refinement was also discussed.
2. Experimental method
A home-made solidification system was designed with a window in the middle for X-ray passing through the sample, as shown in Fig. 1. The system included double furnaces, which is composed of a stainless steel shell, a heating coil, a sample holder and a fiberfrax heat insulator. The temperature of each furnace was monitored by two embedded K-type (chromel-alumel) thermocouples with the accuracy of ±0.1 oC, one on either side of the sample and 1 cm vertically apart. Thus, the system can be used for directional solidification and equiaxed solidification. The sample can move freely along the vertical axis direction of the furnace. Al-20 Cu alloy was prepared by pure Al (99.99) and Al-50 Cu master alloy in a resistant furnace, and the composition of the sample was analyzed through inductively coupled plasma atomic emission spectrum (ICP-AES) apparatus and listed in Table 1. The starting grain size of the prepared sample was about 500 μm due to high cooling rates. And it was worth noting that grain refiner was not used in the alloy. According to the Al-Cu phase diagram [32], the equilibrium liquidus temperature of the experimental alloy is 602.5 oC. The ingot was cut into rectangular slices with a size of 1.5 × 3.5 cm2 and polished down to the thickness of 200 µm. The samples were sandwiched between two 280-µm-thick Al2O3 plates, and the edge was sealed with a mixture of silica sol and Al2O3 powders. The experiments were carried out on the 4W1A beam line at Beijing Synchrotron Radiation Facility
(BSRF)
with
an
updated
synchrotron
radiation
imaging
technique,
i.e.,
diffraction-enhanced imaging (DEI). A fast-readout charge coupled device camera (CCD) with nominal spatial and temporal resolution of 7.4 µm and 300 ms, respectively, was used to collect the images. The exposure time per frame was 500 ms. During the experiments, the beam energy of 20 keV was chosen based on the alloy composition and the sample thickness. The sample was fixed upright in the furnace and heated to the desired melt superheat (100 oC, 120 oC, 140 oC, and 160 oC) by adjusting the temperature of hot and cold furnaces separately. After holding the sample in the furnace for 30 min to homogenize the melt, the solidification experiment was trigged by power down method which consists in applying simultaneously the same cooling rate on both heater elements of the furnaces, which can be achieved by two embedded K-type (chromel-alumel) thermocouples. The cooling rate was determined to be 5.0 K / min. For comparison, differential scanning calorimetry (DSC) analysis (Universal V4.1DTA) was also performed to determine the onset temperature of solidification of the experimental alloys. 20 mg
sample was heated at the speed of 20 K / min to the desired melt superheat (100 oC, 120 oC, 140 o
C, and 160 oC), then holding at that temperature for 30 min, and cooled down at the speed of 5 K
/ min under the protection of high pure argon gas. There was a weak contrast between Al-enriched dendrites and surrounding Cu-enriched liquid in the recorded images, as shown in Fig. 2(a), and some defects originating from the Al 2O3 plates were exhibited, as shown in Fig. 2(b). Therefore, the image processing was applied to improve the quality of the radiographs [25], which consisted in dividing the images taken at time t (Fig. 2(a)) by a reference image recorded at initial time t0 (Fig. 2(b)), when the sample was totally homogeneous liquid. With the image processing, the image quality was improved with an enhanced contrast and a defect free image, as shown in Fig. 2(c), where the α-Al dendrite appears in white and Cu-enriched liquid appears in dark grey.
3. Results and discussion 3.1 In situ observation of solidification process Fig. 3 shows in situ synchrotron X-ray radiographs of Al-20 Cu samples with the different melt superheat of 100 oC, 120 oC, 140 oC, and 160 oC during equiaxed solidification at the cooling rate of 5.0 K / min, respectively. It is found that the mean grain size of α-Al matrix decreases with increasing the melt superheat. It is also found that several grains were got rotated in the melt while they moved upwards away from its original location. Besides, some grains tend to grow in an irregular equiaxed morphology, as denoted by white arrows in Fig. 3. The primary dendrite arm of α-Al grew rapidly in one particular direction while being restricted in the other directions. This phenomenon was referred to as “self-poisoning” by Bogno et al. [33]. The ejected solute was carried downwards by gravity-driven fluid flow, leading to the suppression growth of dendrite arms situated aside and below during solidification. Furthermore, isothermal equiaxed solidification under microgravity conditions and on ground with the sample perpendicular to the gravity vector, which can limit dramatically the potential effects of grain buoyancy within the sample, was carried out by Murphy et al. [27-29], and the gravity-induced thermosolutal convection and buoyant grain motion have largely been avoidable. Unlike the results obtained in this work, no grain rotation was observed in the early stages of solidification in microgravity. The variation of the final mean grain size with the melt superheat is shown in Fig. 4. The
mathematical between the final mean grain size and the melt superheat by a linear regression analysis can be expressed as:
d 4919.3 T 0.33
(1)
Where, d is the final mean grain size, ΔT is the melt superheat. Clearly, the melt superheat leads to a significant grain refinement. Furthermore, the mean grain size as a function of the cooling time is shown in Fig. 5. The moment when the grain appears is marked as zero. Each grain within the field of view is measured. The mean grain size is obtained by averaging all these grains within the field of view. Clearly, the mean grain size increases sharply in the first 70 s under the experimental conditions, then reaches the final mean grain size gradually, which is in good agreement with the trends obtained by Zhu et al. [23]. According to the reference [31], the grains interaction in the late stage of equiaxed solidification is mainly due to the solutal interaction. Thus, regardless of the curve shape of the mean grain size evolution, the experimental values can be analyzed using the Kolmogorov - Johnson - Mehl Avrami (KJMA) model [34-38], which is typical of transformations in which the growth rate is diffusion controlled. The KJMA equation can be expressed as:
Lt L0 1 exp t tc
n
(2)
Where, L0 is the asymptotic value of the mean grain size, tc is the approximate time at which the growth phase starts, which represents the so-called induction period (i.e. time at which the nucleus is formed) and is not possible to determine experimentally, α is the reaction constant, n is the Avrami exponent which gives information about the nucleation and growth behavior. The values of the parameters of KJMA function for four experiments, which were obtained by the liner regression analysis, are given in Table 2. It is worth nothing that the values of Avrami exponent only exhibit a slight variation, almost close to 0.8, indicating that the nucleation and growth behavior of the four experiments are similar. Furthermore, the values of tc are less than 0, this can be due to the fact that nucleus is too small to visible for the naked eye at the first stage. Instead, in the present work, the moment of nucleation can be obtained indirectly, which is essential to determine the duration of grain growth accurately. For example, the value of tc for the melt superheat of 100 oC is close to -7.8 s, indicating that the reference time should be put forward 7.8 s.
Using this modified reference time, the duration of different growth stage can be obtained. The detail description is discussed as follow.
3.2 Mean nucleation rate and growth rate It is well accepted that the grain size is mainly controlled by the nucleation rate and the growth rate. The higher nucleation rate and the lower growth rate results in the smaller grain size. In order to further elucidate the mechanism of grain refinement by the melt superheat treatment, it is necessary to analyze the mean nucleation rate and the growth rate quantitatively. 3.2.1 Mean nucleation rate Fig. 6 shows the number of grains of Al-20 Cu alloy as a function of the cooling time, and t = 0 represents the moment when the nucleus appear. It should be noted that the reference time (t = 0) used here represents the moment when the nucleus appear, which is modified through the KJMA model and the linear regression analysis as described above. Each grain within the field of view is measured. Clearly, the number of grains increases sharply at first 40 s, then reaches the final values gradually. This is consistent with the trend for the mean grain size, as shown in Fig. 5, indicating that nucleation mostly occurs in the early stage of solidification. The mean nucleation rate can, therefore, be obtained by linear regression analysis in the early stage of solidification. In order to perform this analysis, the following assumptions should be taken: firstly, the thickness of the sample is not changed throughout the experiment; secondly, only one dendrite grew in the thickness direction of the sample since the grains did not overlap in the image, as shown in Fig. 3. The mean nucleation rates of Al-20 Cu alloy obtained for the different melt superheat are given in Table 3. Clearly, the mean nucleation rate increases with the melt superheat.
3.2.2 Mean growth rate For each of the four experiments ten primary dendrite arms in the same field of view were randomly selected. The mean growth rate is calculated by averaging the growth rates of the ten dendrite arms at corresponding times. The mean growth rate as a function of the cooling time are plotted in Fig. 7, t = 0 represents the moment when the nucleus appear. Clearly, the curves are divided into two stages: free growth stage Ⅰ and interaction stage Ⅱ. This is also consistent with the experimental result reported by Zhu et al. [23] and Bogon et al. [33]. However, this
observation is in contrast to the results obtained by Murphy et al. [27] with the horizontal sample orientation, where there is no initial acceleration regime before the onset of solutal impingement. This may be attributed to the fact that, using the horizontal sample orientation, solutal mixing in the thin sample dimension is expected to be complete and the solutal field surrounding individual grains is assumed to remain relatively uniform and unperturbed throughout solidification. The former stage (free growth stage Ⅰ) in this work corresponds to the period when the relative distance between grains is large enough to isolate grains. The latter stage (interaction stage Ⅱ) corresponds to the period when the solute diffusion field ahead of the advancing dendrite tip overlaps with the neighboring grains, reducing the growth rate and prohibiting to the growth. With the increase of the melt superheat, the maximum growth rate decreases gradually. The maximum growth rate of Al-20 Cu alloy with the melt superheat of 160 oC is 20.5 µm / s, which is lower than that with the melt superheat of 100 oC by 25.4 µm / s. Furthermore, the duration of stage Ⅰ also decreases with the increase of the melt superheat. The duration of stage Ⅰ decreases from 45.0 s to 37.9 s as the melt superheat increases from 100 oC to 160 oC. More interesting, it is almost at the same time that the growth rate reaches the maximum values and the number of grains also reaches the quasi-stable values, as shown in Fig. 6 and Fig. 7. This strongly indicates that the growth rate may decrease after the number of grains reaches the quasi-stable values.
3.3 Nucleation undercooling determined by DSC For comparison, the nucleation undercooling of Al-20 Cu alloy was also investigated by DSC. It should be noted that the heating rate and cooling rate uesd in DSC analysis are the same as those used in the radiographic experiments. The cooling DSC curves of Al-20 Cu alloy with the different melt superheat are shown in Fig. 8. The four curves show a similar trend with two different peaks. The first peak is at about 590 ～ 595 oC, corresponding to the solidification of primary α-Al, and the second peak is at about 536 ～ 541 oC, relating to the eutectic reaction. For more details, the magnified images of onset temperature of solidification were also inset. It should be noted that the onset temperature was calculated through the 1st derivative of DSC curve in this work, and the point, which deviated from linearity of the slope of the DSC curve, was determined to be the onset temperature of solidification, as shown in Fig. 8. The onset temperature decreases with increasing the melt superheat. The onset temperature decreases by about 3.44 oC with the
increase of melt superheat from 100 oC to 160 oC. The nucleation undercooling can be expressed by the difference between the equilibrium liquidus temperature and the onset temperature of solidification. The nucleation undercooling as a function of the melt superheat is shown in Fig. 9. Clearly, the nucleation undercooling increased with the increase of the melt superheat, which is in good agreement with Yin et al. [3].
3.4 Grain refinement mechanism The final grain size is determined by the nucleation rate and the growth rate. At the early stage of solidification, the nucleation will be a predominant factor. The nucleation undercooling of Al-20 Cu alloy increases with increasing the melt superheat, as shown in Fig. 8 and Fig. 9. The nucleation rate was significant sensitive to the nucleation undercooling. Therefore, the mean nucleation rate of Al-20 Cu alloy increases from 1.749 × 108 m-3s-1 to 3.117 × 108 m-3s-1 as the melt superheat increases from 100 oC to 160 oC, as listed in Table 3. At the later stage of solidification, the growth rate will be a dominant factor. The number of grains under the melt superheat of 160 oC is more than that under the melt superheat of 100 oC. The relative distance between the nuclei should, therefore, be shorter under the condition of the melt superheat of 160 o
C. Moreover, the diffusion coefficient increases exponentially with the temperature, and the
change of the diffusion coefficient is lagged after the change of temperature due to the relaxation effect, which meant that when the melt cooled down to a low temperature, the time needed for solute to achieve the new equilibrium state was larger than the time for temperature [39]. Therefore, the diffusion coefficient still remained higher even when the melt cooled to lower temperatures. The relative short distance between nuclei and high diffusion coefficient with the high melt superheat are more likely to make the solutal diffusion field overlaps with the neighboring grains, and thereby reduces the solutal undercooling between the dendrite tips, as shown in Fig. 7. Therefore, the mean growth rate will decrease with the increase of the melt superheat. Furthermore, it should be noted that when the growth rate superheat increased from 100 o
C to 160 oC, the mean nucleation rate increased by 78.2% while the mean growth rate only
decreased by 19.3%, which indicated that the high mean nucleation rate and the consequent low mean growth rate may be the real reasons for grain refinement. 4. Conclusions
The effect of the melt superheat on equiaxed solidification of Al-20 Cu alloy was investigated by in-situ synchrotron radiography at BSRF, and the corresponding DSC analysis was also conducted. The main conclusions can be drawn: (1) The primary dendrite of α-Al grew rapidly in one particular direction while being restricted in the other directions. (2) The mean grain size increased sharply in the first 70 s, then reached the final grain size gradually. The relationship between the final grain size and the melt superheat can be expressed as:
d 4919.3 T 0.33 . (3) With increasing the melt superheat, the mean nucleation rate increased and the corresponding maximum growth rate and the duration of free growth stage decreased. (4) An increase in melt superheat led to an increase in nucleation undercooling and a decrease in mean growth rate. The real reason for grain refinement may be the high mean nucleation rate and the consequent low mean growth rate.
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant nos. 51227001 and 51420105005) and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 138-QP-2015).
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[39] S.L. Sobolev, Local-nonequilrium model for rapid solidification of undercooled melts, Phys. Lett. A. 199 (1995) 383-386.
Figure captions Fig. 1 Schematic of synchrotron radiation imaging experiment Fig. 2 Equiaxed dendrite of Al-20Cu alloy (with the cooling rate of 5.0 K / min); (a) raw image during the solidification; (b) raw image at a reference time; (c) obtained image by dividing the two previous images Fig. 3 In situ synchrotron X-ray radiographs of Al-20Cu alloy with the different melt superheat during equiaxed solidification at the cooling rate of 5.0 K / min: (a) 100 oC ;(b) 120 oC ;(c) 140 o C ;(d) 160 oC, respectively Fig. 4 The mean grain size of Al-20 Cu alloy as a function of the melt superheat Fig. 5 The mean grain size of Al-20 Cu alloy as a function of the cooling time Fig. 6 The number of grains of Al-20 Cu alloy as a function of the cooling time Fig. 7 The mean growth rate of Al-20 Cu alloy as a function of the cooling time Fig. 8 DSC curves of Al-20 Cu alloy as a function of different melt superheat (a) 100 oC; (b) 120 o
C; (c) 140 oC; (d) 160 oC
Fig. 9 The nucleation undercooling of Al-20 Cu alloy as a function of the melt superheat
Table captions Table 1 Chemical composition of Al-20Cu alloy Table 2 Values of the parameters of KJMA function for four experiments Table 3 Mean nucleation rate of Al-20 Cu alloy with the different melt superheat
Figure 1 Click here to download high resolution image
Figure 2 Click here to download high resolution image
Figure 3 Click here to download high resolution image
Figure 4 Click here to download high resolution image
Figure 5 Click here to download high resolution image
Figure 6 Click here to download high resolution image
Figure 7 Click here to download high resolution image
Figure 8 Click here to download high resolution image
Figure 9 Click here to download high resolution image
Table 1 Elements
Cu
Fe
Si
Mg
Ti
Sr
Al
Composition
20.01
<0.01
<0.01
<0.01
<0.01
<0.01
Bal.
Table 2 Melt superheat (°C)
L0 (µm)
α
Correlation tc (s)
n coefficient (R)
100
1085.8
0.041
-7.8
0.85
0.9977
120
1033.7
0.062
-6.4
0.80
0.9938
140
963.7
0.054
-9.7
0.81
0.9982
160
936.3
0.061
-9.3
0.78
0.9975
Table 3 Melt superheat
Slope
Volume of liquid
Mean nucleation rate
(°C)
(s-1)
(×10-9 m3)
(×108 m-3s-1)
100
1.4237
8.1393
1.749
120
1.6996
8.1393
2.088
140
2.1887
8.1393
2.689
160
2.5373
8.1393
3.117
Highlights 1. Synchrotron radiography is a powerful technique for study of alloy solidification. 2. The mean nucleation rate and the maximum growth rate were quantitatively analyzed. 3. The melt superheat had a significant impact on the nucleation undercooling.
S0022-0248(17)30447-5 http://dx.doi.org/10.1016/j.jcrysgro.2017.07.001 CRYS 24232
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
20 January 2017 2 July 2017 3 July 2017
Please cite this article as: S. Luo, G. Yang, L. Xiao, W. Huang, Q. Yuan, W. Jie, Effect of the Melt Superheat on Equiaxed Solidification of Al-20 wt.% Cu Alloy Investigated by In Situ Synchrotron Radiography, Journal of Crystal Growth (2017), doi: http://dx.doi.org/10.1016/j.jcrysgro.2017.07.001
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Effect of the Melt Superheat on Equiaxed Solidification of Al-20 wt. % Cu Alloy Investigated by In Situ Synchrotron Radiography
Shifeng Luoa, Guangyu Yanga,*, Lei Xiaoa, Wanxia Huangb, Qingxi Yuanb, Wanqi Jiea a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, No. 127 Youyi Western Road, Xi’an 710072, PR China
b
*
Institute of High Energy Physics, The Chinese Academy of Sciences, Beijing 100039, PR China
Corresponding author:
Tel.: +86-13679228998, Fax: +86-029-88495414. Email address: [email protected]
Abstract Effect of the melt superheat on equiaxed solidification of Al-20 wt. % Cu alloy was investigated by in-situ synchrotron radiography at Beijing Synchrotron Radiation Facility. For comparison, the corresponding DSC analysis was also conducted. It was found that the grain size decreased with increasing the melt superheat. The relationship between the final mean grain size and the melt superheat can be expressed as: d 4919.3 T
0.33
. During solidification, the
mean grain size increased sharply in the first 70 s, then reached the final grain size gradually. Furthermore, with increasing the melt superheat, the mean nucleation rate increased, which can be attributed to the fact that increasing the melt superheat led to an increase in nucleation undercooling, and the growth rate and the duration of free growth stage decreased. As the melt superheat increased from 100 oC to 160 oC, the mean nucleation rate increased by 78.2% while the mean growth rate only decreased by 19.3%, which indicated that the high mean nucleation rate and the consequent low mean growth rate may be the real reasons for grain refinement. The increased nucleation density caused earlier growth deceleration due to solutal impingement effects.
Keywords: A1. Melt superheat; A1. Mean grain size; A1. Mean growth rate; A1. Mean nucleation rate; A1. Synchrotron X-ray radiography; A1. Nucleation undercooling
1. Introduction The performance of the casting alloys is greatly depended on the solidification microstructure [1, 2], which can be controlled by optimizing the process variables, such as melt treatment (including the melt superheat and the corresponding holding time) or casting parameters (including the mold preheating temperature and the pouring temperature of the melt). The effect of the melt superheat treatment on the microstructure evolution of casting alloys has been extensively studied [3-6]. Li et al. [7] studied the effect of the melt superheat on the microstructure of Al-16 Si (wt.%, used throughout the paper unless noted) alloy and concluded that a higher superheat temperature reduced the heredity of the structure. Jie et al. [8] investigated the microstructure of Al-7 Si-0.55 Mg alloy with melt superheat treatment, and found that the Si phase was modified through the melt superheat treatment, which reduces the heterogeneous nucleation of Si phase. Wang et al. [9] studied the effect of melt thermal treatment on commercial hypoeutectic Al-Si alloy (A356) and found that melt thermal treatment can significantly refine the solidification structure of the A356 alloy including the primary Alpha phase and intermetallic compounds. The maximum refining effect (e.g. the primary dendrite length decreased from 450 μm to 210 μm) was obtained in an optimal cooling rate range (5 – 6 K / s). Qin et al. [10] investigated the microstructure of Mg2Si/Al-Si-Cu composite with the different melt superheat and found that the superheat treatment increases undercooling and thereby the size of Mg2Si phase decreases from 150 μm to 40 μm as the melt superheating temperature increases from 720 oC to 1020 oC. The grain size decreases significantly from 6420 μm to 89 μm with increasing the melt superheating from 1380 o
C to 1680 oC in IN718C superalloy [11]. However, the accurate grain refinement mechanism still
remains unclear which can be attributed to the fact that solidification process is hard to investigate by in-situ method. Furthermore, with the aim to clarify the grain refinement mechanism, nucleation and growth of grains have been investigated widely [12-17]. The free growth theory, which can make quantitatively correct predictions for grain size, has been proposed by Greer et al. [12]. According to this theory, a grain grows from a refiner particle at an undercooling inversely proportional to the particle diameter. Zhang et al. [14] have been developed the edge-to-edge matching model for describing the interfacial crystallographic characteristics between two phases and predicted that Al3Ti is a powerful nucleating substrate foe Al alloy than TiC, TiB 2 and AlB2. The
interdependence theory linked grain formation and nucleant selection has been proposed by StJohn et al. [17], which assumes that grain formation is the result of the interdependence between nucleation and growth acting in concert within an environment dictated by the alloy chemistry. Although these theories can clarify the nucleation and growth of grains and predict the grain size quantitatively, the nucleation rate and growth rate of grains cannot quantitatively analyze due to the static analysis (e.g. microstructure observation after solidification). Nowadays, synchrotron radiography method can be utilized to observe the dynamic process of solidification. Mathiesen and Arnberg [18] studied the columnar dendritic growth of Al-30 Cu alloy, and analyzed the morphology and velocity of the solid-liquid interface, as well as the constitutional undercooling at the tip of the growth crystals. In situ observation on crystal fragmentation and columnar-to-equiaxed transitions in real alloy has also been reported from X-radiographic studies [19-21]. It is worth noticing that the effect of the electric current on the solidification behavior has been investigated by in-situ synchrotron radiography technique [22-24]. Furthermore, the home-lab sources were also used widely to investigate the metallic solidification with a novel solidification furnace (XRMON-GF) designed by Nguyen-Thi et al. [25]. Murphy et al. [26] studied the eutectic transformation of Al-20 Cu alloy by using laboratory-based radiography and post-solidification metallographic characterisation, and also investigated the equiaxed dendritic solidification with varying grain refiner concentrations and sample orientation at different cooling rates [27]. In order to obtain more uniform temperature in the sample, a new furnace with rotational symmetry (XRMON-SOL) has been designed by Murphy et al. [28] and isothermal equiaxed solidification of Al-Cu alloy in microgravity has been conducted on board the MASER 13 sounding rocket [28, 29]. Although synchrotron radiography and laboratory-based radiography have been used widely to study the solidification of metallic alloy [18-31], the effect of the melt superheat treatment on grain refinement has not been reported. In this paper, the solidification process of Al-20 Cu alloy with the different melt superheat was investigated by in-situ synchrotron radiography at the first-generation Beijing Synchrotron Radiation Facility (BSRF). The mean growth rate and mean nucleation rate were quantitatively analyzed. The mechanism of grain refinement was also discussed.
2. Experimental method
A home-made solidification system was designed with a window in the middle for X-ray passing through the sample, as shown in Fig. 1. The system included double furnaces, which is composed of a stainless steel shell, a heating coil, a sample holder and a fiberfrax heat insulator. The temperature of each furnace was monitored by two embedded K-type (chromel-alumel) thermocouples with the accuracy of ±0.1 oC, one on either side of the sample and 1 cm vertically apart. Thus, the system can be used for directional solidification and equiaxed solidification. The sample can move freely along the vertical axis direction of the furnace. Al-20 Cu alloy was prepared by pure Al (99.99) and Al-50 Cu master alloy in a resistant furnace, and the composition of the sample was analyzed through inductively coupled plasma atomic emission spectrum (ICP-AES) apparatus and listed in Table 1. The starting grain size of the prepared sample was about 500 μm due to high cooling rates. And it was worth noting that grain refiner was not used in the alloy. According to the Al-Cu phase diagram [32], the equilibrium liquidus temperature of the experimental alloy is 602.5 oC. The ingot was cut into rectangular slices with a size of 1.5 × 3.5 cm2 and polished down to the thickness of 200 µm. The samples were sandwiched between two 280-µm-thick Al2O3 plates, and the edge was sealed with a mixture of silica sol and Al2O3 powders. The experiments were carried out on the 4W1A beam line at Beijing Synchrotron Radiation Facility
(BSRF)
with
an
updated
synchrotron
radiation
imaging
technique,
i.e.,
diffraction-enhanced imaging (DEI). A fast-readout charge coupled device camera (CCD) with nominal spatial and temporal resolution of 7.4 µm and 300 ms, respectively, was used to collect the images. The exposure time per frame was 500 ms. During the experiments, the beam energy of 20 keV was chosen based on the alloy composition and the sample thickness. The sample was fixed upright in the furnace and heated to the desired melt superheat (100 oC, 120 oC, 140 oC, and 160 oC) by adjusting the temperature of hot and cold furnaces separately. After holding the sample in the furnace for 30 min to homogenize the melt, the solidification experiment was trigged by power down method which consists in applying simultaneously the same cooling rate on both heater elements of the furnaces, which can be achieved by two embedded K-type (chromel-alumel) thermocouples. The cooling rate was determined to be 5.0 K / min. For comparison, differential scanning calorimetry (DSC) analysis (Universal V4.1DTA) was also performed to determine the onset temperature of solidification of the experimental alloys. 20 mg
sample was heated at the speed of 20 K / min to the desired melt superheat (100 oC, 120 oC, 140 o
C, and 160 oC), then holding at that temperature for 30 min, and cooled down at the speed of 5 K
/ min under the protection of high pure argon gas. There was a weak contrast between Al-enriched dendrites and surrounding Cu-enriched liquid in the recorded images, as shown in Fig. 2(a), and some defects originating from the Al 2O3 plates were exhibited, as shown in Fig. 2(b). Therefore, the image processing was applied to improve the quality of the radiographs [25], which consisted in dividing the images taken at time t (Fig. 2(a)) by a reference image recorded at initial time t0 (Fig. 2(b)), when the sample was totally homogeneous liquid. With the image processing, the image quality was improved with an enhanced contrast and a defect free image, as shown in Fig. 2(c), where the α-Al dendrite appears in white and Cu-enriched liquid appears in dark grey.
3. Results and discussion 3.1 In situ observation of solidification process Fig. 3 shows in situ synchrotron X-ray radiographs of Al-20 Cu samples with the different melt superheat of 100 oC, 120 oC, 140 oC, and 160 oC during equiaxed solidification at the cooling rate of 5.0 K / min, respectively. It is found that the mean grain size of α-Al matrix decreases with increasing the melt superheat. It is also found that several grains were got rotated in the melt while they moved upwards away from its original location. Besides, some grains tend to grow in an irregular equiaxed morphology, as denoted by white arrows in Fig. 3. The primary dendrite arm of α-Al grew rapidly in one particular direction while being restricted in the other directions. This phenomenon was referred to as “self-poisoning” by Bogno et al. [33]. The ejected solute was carried downwards by gravity-driven fluid flow, leading to the suppression growth of dendrite arms situated aside and below during solidification. Furthermore, isothermal equiaxed solidification under microgravity conditions and on ground with the sample perpendicular to the gravity vector, which can limit dramatically the potential effects of grain buoyancy within the sample, was carried out by Murphy et al. [27-29], and the gravity-induced thermosolutal convection and buoyant grain motion have largely been avoidable. Unlike the results obtained in this work, no grain rotation was observed in the early stages of solidification in microgravity. The variation of the final mean grain size with the melt superheat is shown in Fig. 4. The
mathematical between the final mean grain size and the melt superheat by a linear regression analysis can be expressed as:
d 4919.3 T 0.33
(1)
Where, d is the final mean grain size, ΔT is the melt superheat. Clearly, the melt superheat leads to a significant grain refinement. Furthermore, the mean grain size as a function of the cooling time is shown in Fig. 5. The moment when the grain appears is marked as zero. Each grain within the field of view is measured. The mean grain size is obtained by averaging all these grains within the field of view. Clearly, the mean grain size increases sharply in the first 70 s under the experimental conditions, then reaches the final mean grain size gradually, which is in good agreement with the trends obtained by Zhu et al. [23]. According to the reference [31], the grains interaction in the late stage of equiaxed solidification is mainly due to the solutal interaction. Thus, regardless of the curve shape of the mean grain size evolution, the experimental values can be analyzed using the Kolmogorov - Johnson - Mehl Avrami (KJMA) model [34-38], which is typical of transformations in which the growth rate is diffusion controlled. The KJMA equation can be expressed as:
Lt L0 1 exp t tc
n
(2)
Where, L0 is the asymptotic value of the mean grain size, tc is the approximate time at which the growth phase starts, which represents the so-called induction period (i.e. time at which the nucleus is formed) and is not possible to determine experimentally, α is the reaction constant, n is the Avrami exponent which gives information about the nucleation and growth behavior. The values of the parameters of KJMA function for four experiments, which were obtained by the liner regression analysis, are given in Table 2. It is worth nothing that the values of Avrami exponent only exhibit a slight variation, almost close to 0.8, indicating that the nucleation and growth behavior of the four experiments are similar. Furthermore, the values of tc are less than 0, this can be due to the fact that nucleus is too small to visible for the naked eye at the first stage. Instead, in the present work, the moment of nucleation can be obtained indirectly, which is essential to determine the duration of grain growth accurately. For example, the value of tc for the melt superheat of 100 oC is close to -7.8 s, indicating that the reference time should be put forward 7.8 s.
Using this modified reference time, the duration of different growth stage can be obtained. The detail description is discussed as follow.
3.2 Mean nucleation rate and growth rate It is well accepted that the grain size is mainly controlled by the nucleation rate and the growth rate. The higher nucleation rate and the lower growth rate results in the smaller grain size. In order to further elucidate the mechanism of grain refinement by the melt superheat treatment, it is necessary to analyze the mean nucleation rate and the growth rate quantitatively. 3.2.1 Mean nucleation rate Fig. 6 shows the number of grains of Al-20 Cu alloy as a function of the cooling time, and t = 0 represents the moment when the nucleus appear. It should be noted that the reference time (t = 0) used here represents the moment when the nucleus appear, which is modified through the KJMA model and the linear regression analysis as described above. Each grain within the field of view is measured. Clearly, the number of grains increases sharply at first 40 s, then reaches the final values gradually. This is consistent with the trend for the mean grain size, as shown in Fig. 5, indicating that nucleation mostly occurs in the early stage of solidification. The mean nucleation rate can, therefore, be obtained by linear regression analysis in the early stage of solidification. In order to perform this analysis, the following assumptions should be taken: firstly, the thickness of the sample is not changed throughout the experiment; secondly, only one dendrite grew in the thickness direction of the sample since the grains did not overlap in the image, as shown in Fig. 3. The mean nucleation rates of Al-20 Cu alloy obtained for the different melt superheat are given in Table 3. Clearly, the mean nucleation rate increases with the melt superheat.
3.2.2 Mean growth rate For each of the four experiments ten primary dendrite arms in the same field of view were randomly selected. The mean growth rate is calculated by averaging the growth rates of the ten dendrite arms at corresponding times. The mean growth rate as a function of the cooling time are plotted in Fig. 7, t = 0 represents the moment when the nucleus appear. Clearly, the curves are divided into two stages: free growth stage Ⅰ and interaction stage Ⅱ. This is also consistent with the experimental result reported by Zhu et al. [23] and Bogon et al. [33]. However, this
observation is in contrast to the results obtained by Murphy et al. [27] with the horizontal sample orientation, where there is no initial acceleration regime before the onset of solutal impingement. This may be attributed to the fact that, using the horizontal sample orientation, solutal mixing in the thin sample dimension is expected to be complete and the solutal field surrounding individual grains is assumed to remain relatively uniform and unperturbed throughout solidification. The former stage (free growth stage Ⅰ) in this work corresponds to the period when the relative distance between grains is large enough to isolate grains. The latter stage (interaction stage Ⅱ) corresponds to the period when the solute diffusion field ahead of the advancing dendrite tip overlaps with the neighboring grains, reducing the growth rate and prohibiting to the growth. With the increase of the melt superheat, the maximum growth rate decreases gradually. The maximum growth rate of Al-20 Cu alloy with the melt superheat of 160 oC is 20.5 µm / s, which is lower than that with the melt superheat of 100 oC by 25.4 µm / s. Furthermore, the duration of stage Ⅰ also decreases with the increase of the melt superheat. The duration of stage Ⅰ decreases from 45.0 s to 37.9 s as the melt superheat increases from 100 oC to 160 oC. More interesting, it is almost at the same time that the growth rate reaches the maximum values and the number of grains also reaches the quasi-stable values, as shown in Fig. 6 and Fig. 7. This strongly indicates that the growth rate may decrease after the number of grains reaches the quasi-stable values.
3.3 Nucleation undercooling determined by DSC For comparison, the nucleation undercooling of Al-20 Cu alloy was also investigated by DSC. It should be noted that the heating rate and cooling rate uesd in DSC analysis are the same as those used in the radiographic experiments. The cooling DSC curves of Al-20 Cu alloy with the different melt superheat are shown in Fig. 8. The four curves show a similar trend with two different peaks. The first peak is at about 590 ～ 595 oC, corresponding to the solidification of primary α-Al, and the second peak is at about 536 ～ 541 oC, relating to the eutectic reaction. For more details, the magnified images of onset temperature of solidification were also inset. It should be noted that the onset temperature was calculated through the 1st derivative of DSC curve in this work, and the point, which deviated from linearity of the slope of the DSC curve, was determined to be the onset temperature of solidification, as shown in Fig. 8. The onset temperature decreases with increasing the melt superheat. The onset temperature decreases by about 3.44 oC with the
increase of melt superheat from 100 oC to 160 oC. The nucleation undercooling can be expressed by the difference between the equilibrium liquidus temperature and the onset temperature of solidification. The nucleation undercooling as a function of the melt superheat is shown in Fig. 9. Clearly, the nucleation undercooling increased with the increase of the melt superheat, which is in good agreement with Yin et al. [3].
3.4 Grain refinement mechanism The final grain size is determined by the nucleation rate and the growth rate. At the early stage of solidification, the nucleation will be a predominant factor. The nucleation undercooling of Al-20 Cu alloy increases with increasing the melt superheat, as shown in Fig. 8 and Fig. 9. The nucleation rate was significant sensitive to the nucleation undercooling. Therefore, the mean nucleation rate of Al-20 Cu alloy increases from 1.749 × 108 m-3s-1 to 3.117 × 108 m-3s-1 as the melt superheat increases from 100 oC to 160 oC, as listed in Table 3. At the later stage of solidification, the growth rate will be a dominant factor. The number of grains under the melt superheat of 160 oC is more than that under the melt superheat of 100 oC. The relative distance between the nuclei should, therefore, be shorter under the condition of the melt superheat of 160 o
C. Moreover, the diffusion coefficient increases exponentially with the temperature, and the
change of the diffusion coefficient is lagged after the change of temperature due to the relaxation effect, which meant that when the melt cooled down to a low temperature, the time needed for solute to achieve the new equilibrium state was larger than the time for temperature [39]. Therefore, the diffusion coefficient still remained higher even when the melt cooled to lower temperatures. The relative short distance between nuclei and high diffusion coefficient with the high melt superheat are more likely to make the solutal diffusion field overlaps with the neighboring grains, and thereby reduces the solutal undercooling between the dendrite tips, as shown in Fig. 7. Therefore, the mean growth rate will decrease with the increase of the melt superheat. Furthermore, it should be noted that when the growth rate superheat increased from 100 o
C to 160 oC, the mean nucleation rate increased by 78.2% while the mean growth rate only
decreased by 19.3%, which indicated that the high mean nucleation rate and the consequent low mean growth rate may be the real reasons for grain refinement. 4. Conclusions
The effect of the melt superheat on equiaxed solidification of Al-20 Cu alloy was investigated by in-situ synchrotron radiography at BSRF, and the corresponding DSC analysis was also conducted. The main conclusions can be drawn: (1) The primary dendrite of α-Al grew rapidly in one particular direction while being restricted in the other directions. (2) The mean grain size increased sharply in the first 70 s, then reached the final grain size gradually. The relationship between the final grain size and the melt superheat can be expressed as:
d 4919.3 T 0.33 . (3) With increasing the melt superheat, the mean nucleation rate increased and the corresponding maximum growth rate and the duration of free growth stage decreased. (4) An increase in melt superheat led to an increase in nucleation undercooling and a decrease in mean growth rate. The real reason for grain refinement may be the high mean nucleation rate and the consequent low mean growth rate.
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant nos. 51227001 and 51420105005) and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 138-QP-2015).
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Figure captions Fig. 1 Schematic of synchrotron radiation imaging experiment Fig. 2 Equiaxed dendrite of Al-20Cu alloy (with the cooling rate of 5.0 K / min); (a) raw image during the solidification; (b) raw image at a reference time; (c) obtained image by dividing the two previous images Fig. 3 In situ synchrotron X-ray radiographs of Al-20Cu alloy with the different melt superheat during equiaxed solidification at the cooling rate of 5.0 K / min: (a) 100 oC ;(b) 120 oC ;(c) 140 o C ;(d) 160 oC, respectively Fig. 4 The mean grain size of Al-20 Cu alloy as a function of the melt superheat Fig. 5 The mean grain size of Al-20 Cu alloy as a function of the cooling time Fig. 6 The number of grains of Al-20 Cu alloy as a function of the cooling time Fig. 7 The mean growth rate of Al-20 Cu alloy as a function of the cooling time Fig. 8 DSC curves of Al-20 Cu alloy as a function of different melt superheat (a) 100 oC; (b) 120 o
C; (c) 140 oC; (d) 160 oC
Fig. 9 The nucleation undercooling of Al-20 Cu alloy as a function of the melt superheat
Table captions Table 1 Chemical composition of Al-20Cu alloy Table 2 Values of the parameters of KJMA function for four experiments Table 3 Mean nucleation rate of Al-20 Cu alloy with the different melt superheat
Figure 1 Click here to download high resolution image
Figure 2 Click here to download high resolution image
Figure 3 Click here to download high resolution image
Figure 4 Click here to download high resolution image
Figure 5 Click here to download high resolution image
Figure 6 Click here to download high resolution image
Figure 7 Click here to download high resolution image
Figure 8 Click here to download high resolution image
Figure 9 Click here to download high resolution image
Table 1 Elements
Cu
Fe
Si
Mg
Ti
Sr
Al
Composition
20.01
<0.01
<0.01
<0.01
<0.01
<0.01
Bal.
Table 2 Melt superheat (°C)
L0 (µm)
α
Correlation tc (s)
n coefficient (R)
100
1085.8
0.041
-7.8
0.85
0.9977
120
1033.7
0.062
-6.4
0.80
0.9938
140
963.7
0.054
-9.7
0.81
0.9982
160
936.3
0.061
-9.3
0.78
0.9975
Table 3 Melt superheat
Slope
Volume of liquid
Mean nucleation rate
(°C)
(s-1)
(×10-9 m3)
(×108 m-3s-1)
100
1.4237
8.1393
1.749
120
1.6996
8.1393
2.088
140
2.1887
8.1393
2.689
160
2.5373
8.1393
3.117
Highlights 1. Synchrotron radiography is a powerful technique for study of alloy solidification. 2. The mean nucleation rate and the maximum growth rate were quantitatively analyzed. 3. The melt superheat had a significant impact on the nucleation undercooling.