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Analysis of macro segregation in twin-roll cast aluminium strips via solidification curves
Journal of Alloys and Compounds 486 (2009) 168–172
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Analysis of macro segregation in twin-roll cast aluminium strips via solidification curves Yucel Birol ∗ Materials Institute, Marmara Research Center, TUBITAK, Gebze, 41470 Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 22 May 2009 Received in revised form 24 June 2009 Accepted 26 June 2009 Available online 4 July 2009 Keywords: Metals Casting Differential Scanning Calorimetry
a b s t r a c t Differential Scanning Calorimetry is employed to analyze the solidification behaviour of twin-roll cast 1050, 3003, 5754 and 6016 alloys and to identify its impact on the occurrence of macro segregation. The results of the metallographic analysis of the macro segregation patterns in the four alloys investigated are in full agreement with the predictions from the analysis of their solidification curves. Of the four alloys investigated, 3003 and 5754 are judged to be more vulnerable to macro segregation. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In spite of improved homogeneity achieved by high solidification rates, twin-roll cast (TRC) aluminium strips reveal macro as well as micro segregation [1–3]. The latter is inherited from dendritic solidification and scales with dendrite arm spacing which is in the order of several microns. Micro segregation is not a serious threat to the quality of the sheet products processed from TRC aluminium strips and is thus tolerated unless customer applications involve demanding forming operations [2]. Besides, high temperature annealing treatments employed at the start of down stream processing help to homogenize dendritic segregation. Macro segregation in TRC aluminium strips, on the other hand, is most pronounced near the centre plane of the strip and cannot be taken care of with a homogenization cycle [4]. This type of segregation could range from a mild and gradual coarsening of the Al–Fe based intermetallic particles to solute-rich channels running more or less parallel to the casting direction [1–3]. The latter is referred to as the centreline segregation and is analogous to the channel segregation often encountered in ingots and slabs [2]. The severity of centreline segregation is claimed to be linked with the solidification behaviour which in turn is dictated by the alloy composition [1]. Segregation problems can largely be eliminated through controlling the solidification process [3]. Understanding the solidification curves of aluminium alloys, i.e. solid fractions versus temperatures during solidification, under conditions which approx-
∗ Tel.: +90 262 6773084; fax: +90 262 6412309. E-mail address: [email protected]. 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.167
imate twin-roll casting process, are crucial for the control of solidification structures [5–8]. Fraction solid is a key parameter in any casting process to optimize the metallurgical soundness of the casting. Differential Scanning Calorimetry (DSC) method is employed in the present work to analyze the solidification behaviour of several popular wrought aluminium alloys and to identify its impact on their macro segregation tendency. 2. Experimental Four wrought aluminium alloys, 1050, 3003, 5754, 6016, popular in various applications ranging from architectural to automotive structural sheet, were used in the present study (Table 1). Strips of these alloys were cast industrially with the TRC technology at a gauge of 5.5 mm. Full-width cast strip samples used for profile measurements were first heavily etched in Tucker’s solution to reveal the macroscopic features of macro segregation patterns. Later, several pieces were systematically sectioned from the original sample for metallographic analysis. These pieces were prepared with conventional metallographic techniques to identify the morphology and the intensity of segregation zones further. The solidification curves were obtained with DSC experiments performed on 4 mm discs weighting approximately 50 mg in a dynamic argon atmosphere (1 l h−1 ) and with an empty reference pan. The strip sample was first heated to 700 ◦ C at 10 ◦ C min−1 where it was held for 15 min to achieve temperature equilibration. The sample cell was then cooled over the range of 700–500 ◦ C at two different rates: 2.5 and 40 K min−1 . The heat transfer model proposed by Gray [9] was adapted to obtain the solidification curves from respective DSC thermograms [10,11].
3. Results and discussion Latent heat is released in the sample cell upon solidification. Energy supply is adjusted to maintain the sample and the reference cells at the same temperature and the energy supplied is precisely recorded by the DSC unit. Assuming uniform temperature distribution in both cells and resistance to heat transfer only between the
Y. Birol / Journal of Alloys and Compounds 486 (2009) 168–172 Table 1 Chemical compositions of wrought aluminium alloys used in the present work (wt%). Alloy
Si
Fe
Mn
Mg
Cu
Ti
Al
1050 3003 5754 6016
0.103 0.249 0.117 1.002
0.269 0.617 0.263 0.189
0.005 1.028 0.036 0.075
0.002 0.017 2.675 0.429
0.001 0.076 0.005 0.0003
0.030 0.035 0.021 0.028
99.56 97.93 96.84 98.25
sample holder and the sample container [9], the energy conservation in the sample cell can be expressed as, Cs
dT s
dt
=
dq dh s s dt
+
(1a)
dt
where Cs and Ts are the heat capacity and the temperature of the sample cell, respectively. t is the time, qs is the energy supply to the sample cell and hs is the latent heat generated by the sample upon solidification. Likewise, we can write for the reference cell which is empty during the experiment, Cr
dT r
dt
=
dqr dt
(1b)
where Cr and Tr are the heat capacity and the temperature of the reference cell, respectively. qr is the energy supply to the reference cell. The heat generated by the sample can now be isolated as,
dh s
dt
= Cs
dT s
dt
− Cr
dT dq dq r r s dt
+
dt
−
dt
(2)
The rate of energy supply to the sample and reference cells can be described by Newton’s Law as, (Tp − Ts ) dqs = Rs dt
(3a)
(Tp − Tr ) dqr = Rr dt
(3b)
169
respectively. Tp is the temperature of the pan and is the same in both the sample and the reference cells while Rs and Rr are the thermal resistance values through which the heat flows to and from the sample and the reference, respectively. From Eqs. (3a) and (3b), we can write for Ts and Tr , Ts = −Rs Tr = −Rr
dq s
dt
dq r
dt
+ Tp
(4a)
+ Tp
(4b)
The difference between the rates of energy supply to the sample cell (dqs /dt) and the reference cell (dqr /dt) (dq/dt), is measured during the DSC experiment. Since there is no phase change in the reference cell, Eq. (2) can be rewritten as, dh =− dt
dq dt
+ (Cs − Cr )
dTp − Rs Cs dt
d2 qs dt 2
(5)
Eq. (5) can be simplified further, since (Cs − Cr )dTp /dt, the deviation from the baseline, is eliminated via calibration, dh =− dt
dq dt
− Rs Cs
d2 qs dt 2
(6)
and can be written in the finite-difference form and solved numerically: h q =− − Rs Cs t t
(qs /t(t + t)) − (qs /t(t)) t
(7)
t at a specific Tp is known thanks to a constant cooling rate, dTp /dt. The latent heat evolution, h, as a function of the reaction time, t, can be obtained from Eq. (7) once Rs Cs is estimated by an iteration method considering that full solidification implies a solid fraction value of 1. The fraction of solid during solidification can
Fig. 1. The change in solid fraction with temperature across the solidification interval at cooling rates of 2.5 and 40 K min−1 for (a) 1050, (b) 3003, (c) 5754, and (d) 6016 alloys.
170
Y. Birol / Journal of Alloys and Compounds 486 (2009) 168–172
Table 2 Rs Cs values for wrought aluminium alloys at two different cooling rates. Alloy
Cooling rate, ◦ C min−1
1050
40 2.5
3.1 2.5
3003
40 2.5
39.6 1.05
5754
40 2.5
0.8 0.14
6016
40 2.5
Rs Cs
1.2 11.9
then be obtained, assuming the amount of latent heat evolution to be directly proportional to the solid fraction. The Rs Cs values estimated for the wrought aluminium alloys at two different cooling rates are listed in Table 2. The solidification curves, i.e. the change in solid fraction with temperature across the solidification interval, are illustrated in Fig. 1 at two cooling rates different by over an order of magnitude. The lower of the two cooling rates employed in the present work is typical of conventional continuous casting processes [12]. The secondary dendrite arm spacing near the centre of the strips used in the present study was found by line intercept method to be in the neighbourhood of 10 microns in all alloys cast at nearly the same gauge, typical of TRC strips cast at this gauge [13,14]. A cooling rate of approximately 100 K s−1 is thus estimated from the empirical equation proposed to relate the cooling rate during solidification to the scale of the dendritic structure [15]. While this is much slower with respect to the cooling of the surface layers which solidify in contact with the water-cooled rolls, it is indeed a fair approximation of the cooling of the strip centre and is not too much different from the higher scan rate employed in the present work. The data obtained from the solidification curves regarding the solidification process are listed in Table 3. The displacement of the liquidus points with higher cooling rate is only several degrees and the gap becomes increasingly larger with progressing solidification, finally producing a substantial displacement in the solidus points (Fig. 1). The solidification interval is more than doubled with over an order of magnitude increase in cooling rate. 1050 alloy has the smallest freezing range of the four alloys tested at both cooling rates. 5754 has the largest solidification interval at 2.5 K min−1 while 3003 ranks first in this regard at 40 K min−1 . It has been well established that long freezing range alloys are difficult to cast with the TRC process and are particularly prone to macro segregation [16]. High cooling rates involved in TRC expand the solidification interval and could promote macro segregation. However, the segregation behaviour cannot be accounted for by the size of the solidification interval alone. Fig. 2 is a dimensionless binary plot which shows the ratio of elapsed to full solidification interval at each solid fraction value across the solidification interTable 3 Solidification features of the wrought aluminium alloys. ◦
Cooling rate, C min−1
Liqidus point, ◦ C
Solidus point, ◦ C
Solidif. interval, ◦ C
1050
2.5 40
654.1 650.3
637 597.4
17.1 52.9
3003
2.5 40
652.1 650.2
622.6 552
29.5 98.3
5754
2.5 40
643.1 639.7
590.6 545.9
52.5 93.3
6016
2.5 40
647 643.7
603.8 574
43.2 69.7
Alloy
Fig. 2. Dimensionless binary plot which shows the portion of the solidification interval elapsed at each solid fraction value across the solidification interval at the cooling rate relevant for the TRC process.
val at the cooling rate relevant for the TRC process. The survival range of the liquid phase nearer to the end of the solidification reaction is the key parameter that impacts the segregation behaviour. A wide temperature range remaining until complete solidification at a high solid fraction is a threat to a segregation-free, sound cast strip. It is clear from Fig. 2 that alloys 3003 and 5754 still have half of the solidification interval left until full solidification at a liquid fraction only as much as 20%. In other words, the solidification of the remaining 20% liquid phase will take much longer in these two alloys with respect to 1050 and 6016, thus promoting segregation. It is thus inferred from Fig. 2 that the segregation tendency of 3003 and 5754 is markedly higher than that of 1050 and 6016. The features of macro segregation on transverse sections of strip-cast alloys investigated in the present work are shown in Fig. 3. There is hardly any evidence of macro segregation in the 1050 alloy which reveals typical features of TRC strips with a gradual structural coarsening from the surface to the centre (Fig. 3a and b). 3003 alloy strip, on the other hand, has apparently suffered extensive segregation (Fig. 3c and d). A striking feature of the transverse section of the cast strip is an array of segregates which could be readily identified upon over-etching of the profile samples. These segregates are spaced more or less equally from one edge of the strip to the other and have roughly marked the centreline of the strip along its width. It is inferred from an analysis of a series of transverse sections, each approximately 10 cm apart, that these segregates have formed continuous channels which run more or less parallel to one another along the length of the strip. Each segregate is at least an order of magnitude larger in size than the dendritic grains in its immediate vicinity. Some of them are as wide as a fraction of a millimetre with a spacing of only a hundred microns. These segregation channels reveal predominantly eutectic features implying a shift in the composition of the liquid fraction all the way to the eutectic range. These features suggest that the liquid flow responsible for centreline segregation in TRC 3003 cast strip is far greater in volume than the interdendritic flow and is a forced one unlike that of channel segregation in ingots where liquid flow is gravitational. The hot rolling/extrusion component of the TRC process apparently acts to squeeze the mushy zone in between the already solidified skins which keep growing thicker as they move into the roll gap. The liq-
Y. Birol / Journal of Alloys and Compounds 486 (2009) 168–172
171
Fig. 3. Macro and microstructural features on the transverse sections of TRC strips of (a and b) 1050, (c and d) 3003, (e and f) 5754 and (g and h) 6016 alloys. Casting direction is normal to the plane.
uid, rich in alloying elements, is thus forced to move away, opposite to the casting direction, as suggested in [1]. It is trapped, however, when the two solid skins finally start welding together at some point in the centre, leading to well defined solute-rich channels. The macro segregation features in the 5754 and 6016 alloys are very similar to those observed in the 3003 alloy. The solute-rich channels are placed asymmetrically with respect to the centre plane in the former, possibly due to the variations in the vertical positioning of the tip table during the casting operation. The width of the
segregation channels is relatively larger in 5754 as inferred from the metallographic analysis of transverse sections while the number of channels per unit length is less (Fig. 3e and f). However, it is fair to conclude from the size, number and distribution of segregation channels that the intensity of macro segregation in 3003 and 5754 is nearly the same while that of 6016 is markedly less. It is worth noting that the intensity of macro segregation in TRC strips is in good correlation with the chemistry of the alloys. Those with a higher content of alloying elements clearly suffer more extensive segregation as expected.
172
Y. Birol / Journal of Alloys and Compounds 486 (2009) 168–172
The results of the metallographic analysis of the macro segregation patterns in the four alloys investigated in the present work are in reasonable agreement with the predictions of the solidification curve analysis. The two alloys, 3003 and 5754, judged to be most vulnerable to macro segregation in view of their solidification curves, are the two strips with the most intense macro segregation. As little can be done regarding the chemistry, special measures ought to be taken during the casting process. Casting parameters such as set-back, tip orifice, roll cooling practice, casting gauge and the casting speed need to be fine tuned to minimize, if not to completely eliminate, the occurrence of macro segregation. Slower than desired casting speeds might have to be tolerated to reduce macro segregation when casting alloys such as 5754 and 3003. 4. Summary Differential Scanning Calorimetry is employed to analyze the solidification behaviour of twin-roll cast 1050, 3003, 5754 and 6016 alloys and to identify its impact on the occurrence of macro segregation. The results of the metallographic analysis of the macro segregation patterns in the four alloys investigated are in full agreement with the predictions from the analysis of solidification curves. Of the four alloys investigated, 3003 and 5754 are judged to be more vulnerable to macro segregation.
Acknowledgements It is a pleasure to thank F. Alageyik for his help in the experimental part of the work. ASSAN Aluminium Co. is thanked for supplying TRC strips. References [1] I. Jin, L.R. Morris, J.D. Hunt, in: J.E. Andersen (Ed.), Light Metals 1982, AIME, New York, 1981, pp. 873–888. [2] Y. Birol, Aluminium 74 (1998) 318. [3] S. Ertan, M. Dundar, Y. Birol, K. Sarioglu, E. Ozden, A.S. Akkurt, G. Yildizbayrak, S. Hamer, C. Romanowski, in: R.D. Peterson (Ed.), Light Metals 2000, TMS, 2000, pp. 667–672. [4] R. Kamat, JOM 48 (1996) 34. [5] I. Minkoff, Solidification and Cast Structure, John Wiley & Sons, Chichester, U.K, 1986. [6] S.-W. Chen, Y.-Y. Chuang, Y.A. Chang, M.G. Chu, Metall. Trans. A 22A (1991) 2837. [7] R.J. Claxton, J. Metals 27 (1975) 14. [8] S.A. Metz, M.C. Flemings, AFS Trans. 77 (1969) 329. [9] A.P. Gray, in: R.S. Porter, J.F. Johnson (Eds.), Analytical Calorimetry, Plenum Press, New York, 1968, pp. 209–218. [10] S.W. Chen, C. Lin, C. Chen, Met. Mater. Trans. 29A (1998) 1965. [11] J. Dumas, J. Phys. D: Appl. Phys. 11 (1978) 1. [12] J.R. Davis (Ed.), Aluminum and Aluminum Alloys, ASM International, Materials Park, Ohio, 1996. [13] Y. Birol, Z. Metallkunde 89 (1998) 501. [14] Y. Birol, F. Sertc¸elik, Z. Metallkunde 90 (1999) 329. [15] H. Matyja, B.C. Giessen, N.J. Grant, J. Inst. Metals 96 (1968) 30. [16] Y. Birol, S. Ucuncuoglu, O.M. Acarseki, H.G. Zeybekouglu, Proc. 11th International Metallurgy and Materials Congress, I˙ stanbul, 2002, pp. 1628–1634.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Analysis of macro segregation in twin-roll cast aluminium strips via solidification curves Yucel Birol ∗ Materials Institute, Marmara Research Center, TUBITAK, Gebze, 41470 Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 22 May 2009 Received in revised form 24 June 2009 Accepted 26 June 2009 Available online 4 July 2009 Keywords: Metals Casting Differential Scanning Calorimetry
a b s t r a c t Differential Scanning Calorimetry is employed to analyze the solidification behaviour of twin-roll cast 1050, 3003, 5754 and 6016 alloys and to identify its impact on the occurrence of macro segregation. The results of the metallographic analysis of the macro segregation patterns in the four alloys investigated are in full agreement with the predictions from the analysis of their solidification curves. Of the four alloys investigated, 3003 and 5754 are judged to be more vulnerable to macro segregation. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In spite of improved homogeneity achieved by high solidification rates, twin-roll cast (TRC) aluminium strips reveal macro as well as micro segregation [1–3]. The latter is inherited from dendritic solidification and scales with dendrite arm spacing which is in the order of several microns. Micro segregation is not a serious threat to the quality of the sheet products processed from TRC aluminium strips and is thus tolerated unless customer applications involve demanding forming operations [2]. Besides, high temperature annealing treatments employed at the start of down stream processing help to homogenize dendritic segregation. Macro segregation in TRC aluminium strips, on the other hand, is most pronounced near the centre plane of the strip and cannot be taken care of with a homogenization cycle [4]. This type of segregation could range from a mild and gradual coarsening of the Al–Fe based intermetallic particles to solute-rich channels running more or less parallel to the casting direction [1–3]. The latter is referred to as the centreline segregation and is analogous to the channel segregation often encountered in ingots and slabs [2]. The severity of centreline segregation is claimed to be linked with the solidification behaviour which in turn is dictated by the alloy composition [1]. Segregation problems can largely be eliminated through controlling the solidification process [3]. Understanding the solidification curves of aluminium alloys, i.e. solid fractions versus temperatures during solidification, under conditions which approx-
∗ Tel.: +90 262 6773084; fax: +90 262 6412309. E-mail address: [email protected]. 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.167
imate twin-roll casting process, are crucial for the control of solidification structures [5–8]. Fraction solid is a key parameter in any casting process to optimize the metallurgical soundness of the casting. Differential Scanning Calorimetry (DSC) method is employed in the present work to analyze the solidification behaviour of several popular wrought aluminium alloys and to identify its impact on their macro segregation tendency. 2. Experimental Four wrought aluminium alloys, 1050, 3003, 5754, 6016, popular in various applications ranging from architectural to automotive structural sheet, were used in the present study (Table 1). Strips of these alloys were cast industrially with the TRC technology at a gauge of 5.5 mm. Full-width cast strip samples used for profile measurements were first heavily etched in Tucker’s solution to reveal the macroscopic features of macro segregation patterns. Later, several pieces were systematically sectioned from the original sample for metallographic analysis. These pieces were prepared with conventional metallographic techniques to identify the morphology and the intensity of segregation zones further. The solidification curves were obtained with DSC experiments performed on 4 mm discs weighting approximately 50 mg in a dynamic argon atmosphere (1 l h−1 ) and with an empty reference pan. The strip sample was first heated to 700 ◦ C at 10 ◦ C min−1 where it was held for 15 min to achieve temperature equilibration. The sample cell was then cooled over the range of 700–500 ◦ C at two different rates: 2.5 and 40 K min−1 . The heat transfer model proposed by Gray [9] was adapted to obtain the solidification curves from respective DSC thermograms [10,11].
3. Results and discussion Latent heat is released in the sample cell upon solidification. Energy supply is adjusted to maintain the sample and the reference cells at the same temperature and the energy supplied is precisely recorded by the DSC unit. Assuming uniform temperature distribution in both cells and resistance to heat transfer only between the
Y. Birol / Journal of Alloys and Compounds 486 (2009) 168–172 Table 1 Chemical compositions of wrought aluminium alloys used in the present work (wt%). Alloy
Si
Fe
Mn
Mg
Cu
Ti
Al
1050 3003 5754 6016
0.103 0.249 0.117 1.002
0.269 0.617 0.263 0.189
0.005 1.028 0.036 0.075
0.002 0.017 2.675 0.429
0.001 0.076 0.005 0.0003
0.030 0.035 0.021 0.028
99.56 97.93 96.84 98.25
sample holder and the sample container [9], the energy conservation in the sample cell can be expressed as, Cs
dT s
dt
=
dq dh s s dt
+
(1a)
dt
where Cs and Ts are the heat capacity and the temperature of the sample cell, respectively. t is the time, qs is the energy supply to the sample cell and hs is the latent heat generated by the sample upon solidification. Likewise, we can write for the reference cell which is empty during the experiment, Cr
dT r
dt
=
dqr dt
(1b)
where Cr and Tr are the heat capacity and the temperature of the reference cell, respectively. qr is the energy supply to the reference cell. The heat generated by the sample can now be isolated as,
dh s
dt
= Cs
dT s
dt
− Cr
dT dq dq r r s dt
+
dt
−
dt
(2)
The rate of energy supply to the sample and reference cells can be described by Newton’s Law as, (Tp − Ts ) dqs = Rs dt
(3a)
(Tp − Tr ) dqr = Rr dt
(3b)
169
respectively. Tp is the temperature of the pan and is the same in both the sample and the reference cells while Rs and Rr are the thermal resistance values through which the heat flows to and from the sample and the reference, respectively. From Eqs. (3a) and (3b), we can write for Ts and Tr , Ts = −Rs Tr = −Rr
dq s
dt
dq r
dt
+ Tp
(4a)
+ Tp
(4b)
The difference between the rates of energy supply to the sample cell (dqs /dt) and the reference cell (dqr /dt) (dq/dt), is measured during the DSC experiment. Since there is no phase change in the reference cell, Eq. (2) can be rewritten as, dh =− dt
dq dt
+ (Cs − Cr )
dTp − Rs Cs dt
d2 qs dt 2
(5)
Eq. (5) can be simplified further, since (Cs − Cr )dTp /dt, the deviation from the baseline, is eliminated via calibration, dh =− dt
dq dt
− Rs Cs
d2 qs dt 2
(6)
and can be written in the finite-difference form and solved numerically: h q =− − Rs Cs t t
(qs /t(t + t)) − (qs /t(t)) t
(7)
t at a specific Tp is known thanks to a constant cooling rate, dTp /dt. The latent heat evolution, h, as a function of the reaction time, t, can be obtained from Eq. (7) once Rs Cs is estimated by an iteration method considering that full solidification implies a solid fraction value of 1. The fraction of solid during solidification can
Fig. 1. The change in solid fraction with temperature across the solidification interval at cooling rates of 2.5 and 40 K min−1 for (a) 1050, (b) 3003, (c) 5754, and (d) 6016 alloys.
170
Y. Birol / Journal of Alloys and Compounds 486 (2009) 168–172
Table 2 Rs Cs values for wrought aluminium alloys at two different cooling rates. Alloy
Cooling rate, ◦ C min−1
1050
40 2.5
3.1 2.5
3003
40 2.5
39.6 1.05
5754
40 2.5
0.8 0.14
6016
40 2.5
Rs Cs
1.2 11.9
then be obtained, assuming the amount of latent heat evolution to be directly proportional to the solid fraction. The Rs Cs values estimated for the wrought aluminium alloys at two different cooling rates are listed in Table 2. The solidification curves, i.e. the change in solid fraction with temperature across the solidification interval, are illustrated in Fig. 1 at two cooling rates different by over an order of magnitude. The lower of the two cooling rates employed in the present work is typical of conventional continuous casting processes [12]. The secondary dendrite arm spacing near the centre of the strips used in the present study was found by line intercept method to be in the neighbourhood of 10 microns in all alloys cast at nearly the same gauge, typical of TRC strips cast at this gauge [13,14]. A cooling rate of approximately 100 K s−1 is thus estimated from the empirical equation proposed to relate the cooling rate during solidification to the scale of the dendritic structure [15]. While this is much slower with respect to the cooling of the surface layers which solidify in contact with the water-cooled rolls, it is indeed a fair approximation of the cooling of the strip centre and is not too much different from the higher scan rate employed in the present work. The data obtained from the solidification curves regarding the solidification process are listed in Table 3. The displacement of the liquidus points with higher cooling rate is only several degrees and the gap becomes increasingly larger with progressing solidification, finally producing a substantial displacement in the solidus points (Fig. 1). The solidification interval is more than doubled with over an order of magnitude increase in cooling rate. 1050 alloy has the smallest freezing range of the four alloys tested at both cooling rates. 5754 has the largest solidification interval at 2.5 K min−1 while 3003 ranks first in this regard at 40 K min−1 . It has been well established that long freezing range alloys are difficult to cast with the TRC process and are particularly prone to macro segregation [16]. High cooling rates involved in TRC expand the solidification interval and could promote macro segregation. However, the segregation behaviour cannot be accounted for by the size of the solidification interval alone. Fig. 2 is a dimensionless binary plot which shows the ratio of elapsed to full solidification interval at each solid fraction value across the solidification interTable 3 Solidification features of the wrought aluminium alloys. ◦
Cooling rate, C min−1
Liqidus point, ◦ C
Solidus point, ◦ C
Solidif. interval, ◦ C
1050
2.5 40
654.1 650.3
637 597.4
17.1 52.9
3003
2.5 40
652.1 650.2
622.6 552
29.5 98.3
5754
2.5 40
643.1 639.7
590.6 545.9
52.5 93.3
6016
2.5 40
647 643.7
603.8 574
43.2 69.7
Alloy
Fig. 2. Dimensionless binary plot which shows the portion of the solidification interval elapsed at each solid fraction value across the solidification interval at the cooling rate relevant for the TRC process.
val at the cooling rate relevant for the TRC process. The survival range of the liquid phase nearer to the end of the solidification reaction is the key parameter that impacts the segregation behaviour. A wide temperature range remaining until complete solidification at a high solid fraction is a threat to a segregation-free, sound cast strip. It is clear from Fig. 2 that alloys 3003 and 5754 still have half of the solidification interval left until full solidification at a liquid fraction only as much as 20%. In other words, the solidification of the remaining 20% liquid phase will take much longer in these two alloys with respect to 1050 and 6016, thus promoting segregation. It is thus inferred from Fig. 2 that the segregation tendency of 3003 and 5754 is markedly higher than that of 1050 and 6016. The features of macro segregation on transverse sections of strip-cast alloys investigated in the present work are shown in Fig. 3. There is hardly any evidence of macro segregation in the 1050 alloy which reveals typical features of TRC strips with a gradual structural coarsening from the surface to the centre (Fig. 3a and b). 3003 alloy strip, on the other hand, has apparently suffered extensive segregation (Fig. 3c and d). A striking feature of the transverse section of the cast strip is an array of segregates which could be readily identified upon over-etching of the profile samples. These segregates are spaced more or less equally from one edge of the strip to the other and have roughly marked the centreline of the strip along its width. It is inferred from an analysis of a series of transverse sections, each approximately 10 cm apart, that these segregates have formed continuous channels which run more or less parallel to one another along the length of the strip. Each segregate is at least an order of magnitude larger in size than the dendritic grains in its immediate vicinity. Some of them are as wide as a fraction of a millimetre with a spacing of only a hundred microns. These segregation channels reveal predominantly eutectic features implying a shift in the composition of the liquid fraction all the way to the eutectic range. These features suggest that the liquid flow responsible for centreline segregation in TRC 3003 cast strip is far greater in volume than the interdendritic flow and is a forced one unlike that of channel segregation in ingots where liquid flow is gravitational. The hot rolling/extrusion component of the TRC process apparently acts to squeeze the mushy zone in between the already solidified skins which keep growing thicker as they move into the roll gap. The liq-
Y. Birol / Journal of Alloys and Compounds 486 (2009) 168–172
171
Fig. 3. Macro and microstructural features on the transverse sections of TRC strips of (a and b) 1050, (c and d) 3003, (e and f) 5754 and (g and h) 6016 alloys. Casting direction is normal to the plane.
uid, rich in alloying elements, is thus forced to move away, opposite to the casting direction, as suggested in [1]. It is trapped, however, when the two solid skins finally start welding together at some point in the centre, leading to well defined solute-rich channels. The macro segregation features in the 5754 and 6016 alloys are very similar to those observed in the 3003 alloy. The solute-rich channels are placed asymmetrically with respect to the centre plane in the former, possibly due to the variations in the vertical positioning of the tip table during the casting operation. The width of the
segregation channels is relatively larger in 5754 as inferred from the metallographic analysis of transverse sections while the number of channels per unit length is less (Fig. 3e and f). However, it is fair to conclude from the size, number and distribution of segregation channels that the intensity of macro segregation in 3003 and 5754 is nearly the same while that of 6016 is markedly less. It is worth noting that the intensity of macro segregation in TRC strips is in good correlation with the chemistry of the alloys. Those with a higher content of alloying elements clearly suffer more extensive segregation as expected.
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The results of the metallographic analysis of the macro segregation patterns in the four alloys investigated in the present work are in reasonable agreement with the predictions of the solidification curve analysis. The two alloys, 3003 and 5754, judged to be most vulnerable to macro segregation in view of their solidification curves, are the two strips with the most intense macro segregation. As little can be done regarding the chemistry, special measures ought to be taken during the casting process. Casting parameters such as set-back, tip orifice, roll cooling practice, casting gauge and the casting speed need to be fine tuned to minimize, if not to completely eliminate, the occurrence of macro segregation. Slower than desired casting speeds might have to be tolerated to reduce macro segregation when casting alloys such as 5754 and 3003. 4. Summary Differential Scanning Calorimetry is employed to analyze the solidification behaviour of twin-roll cast 1050, 3003, 5754 and 6016 alloys and to identify its impact on the occurrence of macro segregation. The results of the metallographic analysis of the macro segregation patterns in the four alloys investigated are in full agreement with the predictions from the analysis of solidification curves. Of the four alloys investigated, 3003 and 5754 are judged to be more vulnerable to macro segregation.
Acknowledgements It is a pleasure to thank F. Alageyik for his help in the experimental part of the work. ASSAN Aluminium Co. is thanked for supplying TRC strips. References [1] I. Jin, L.R. Morris, J.D. Hunt, in: J.E. Andersen (Ed.), Light Metals 1982, AIME, New York, 1981, pp. 873–888. [2] Y. Birol, Aluminium 74 (1998) 318. [3] S. Ertan, M. Dundar, Y. Birol, K. Sarioglu, E. Ozden, A.S. Akkurt, G. Yildizbayrak, S. Hamer, C. Romanowski, in: R.D. Peterson (Ed.), Light Metals 2000, TMS, 2000, pp. 667–672. [4] R. Kamat, JOM 48 (1996) 34. [5] I. Minkoff, Solidification and Cast Structure, John Wiley & Sons, Chichester, U.K, 1986. [6] S.-W. Chen, Y.-Y. Chuang, Y.A. Chang, M.G. Chu, Metall. Trans. A 22A (1991) 2837. [7] R.J. Claxton, J. Metals 27 (1975) 14. [8] S.A. Metz, M.C. Flemings, AFS Trans. 77 (1969) 329. [9] A.P. Gray, in: R.S. Porter, J.F. Johnson (Eds.), Analytical Calorimetry, Plenum Press, New York, 1968, pp. 209–218. [10] S.W. Chen, C. Lin, C. Chen, Met. Mater. Trans. 29A (1998) 1965. [11] J. Dumas, J. Phys. D: Appl. Phys. 11 (1978) 1. [12] J.R. Davis (Ed.), Aluminum and Aluminum Alloys, ASM International, Materials Park, Ohio, 1996. [13] Y. Birol, Z. Metallkunde 89 (1998) 501. [14] Y. Birol, F. Sertc¸elik, Z. Metallkunde 90 (1999) 329. [15] H. Matyja, B.C. Giessen, N.J. Grant, J. Inst. Metals 96 (1968) 30. [16] Y. Birol, S. Ucuncuoglu, O.M. Acarseki, H.G. Zeybekouglu, Proc. 11th International Metallurgy and Materials Congress, I˙ stanbul, 2002, pp. 1628–1634.