Study on the macrosegregation behaviour of 2524 alloy flat ingot solidified under the influence of electromagnetic field Dan Mou, Yubo , Qingfeng Zhu, , Jianzhong Cui

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Dan Mou, Yubo Zuo, Qingfeng Zhu, Lei Li, Jianzhong Cui. Study on the macrosegregation behaviour of 2524 alloy flat ingot solidified under the influence of electromagnetic field. 8th International Con- ference on Electromagnetic Processing of Materials, Oct 2015, Cannes, France. ￿hal-01331623￿

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Dan Mou, Yubo Zuo*, Qingfeng Zhu, Lei Li, Jianzhong Cui

Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang, Liaoning, 110819, China

* Corresponding author : [email protected]

Abstract The flat ingots of 2524 alloy were prepared by direct chill (DC) casting process with the application of a low frequency electromagnetic field and the macrosegregation behaviour of this alloy was investigated. The experimental results showed that there are obvious positive segregation near to the surface and a negative segregation in the centre area of the ingot. Cu shows the highest segregation tendency among the main elements of Cu, Mg and Mn. Ti shows a different segregation trend opposite to that of Cu. The application of electromagnetic field do not have significant effect on the segregation near to the ingot surface. However, with the application of electromagnetic field, the negative centreline segregation in the centre area of the ingot was evidently reduced. This paper presents these results and corresponding discussions.

Key words : electromagnetic field, DC casting, macrosegregation, 2524 alloy

1 Introduction 2524 aluminum alloy as a typical 2xxx series Al-Cu alloy has been widely used as aerospace materials. 2524 aluminum alloy could be one of the best materials for the fuselage skin. The use of 2524-T3 aluminum has become widespread because it has the same strength properties as 2024-T3, but higher fracture toughness and better damage tolerance and creep resistance[1, 2]. The commercial 2524 alloy is normally cast, hot rolled, solution heat treated, quenched and then artificially aged. The DC casting process is usually used for preparation of 2524 alloy ingots. However the macrosegregation has been a key problem for DC casting of large sized 2524 alloy ingot due to the high segregation tendency of copper in Al-Cu alloys. Macrosegregation is a phenomenon associated with solidification. It occurs over large length scale. Homogenization heat treatment and diffusion mechanisms cannot remove macrosegregation due to its large length scale. Macrosegregation can be very harmful for the material either in service or during subsequent processing. Eskin and Katgerman [3] summarized the mechanisms of macrosegregation upon direct chill (DC) casting of aluminium alloys. The macrosegregation is caused by the relative movement of liquid and solid phases during solidification. Therefore the macrosegregation mechanisms are related to thermo-solutal convection, free-moving crystals, shrinkage and deformation induced flow, and forced convection. The application of electromagnetic field in the DC casting process can significantly refine microstructure due to the formation of forced convection[4]. The electromagnetic field could affect the macrosegregation behaviour of DC cast ingot. Electromagnetic casting (EMC)[5] developed in the early 1970s is perhaps the earliest application of electromagnetic field in DC casting of aluminium alloys. Based on EMC, a CREM process was developed by Vives[6, 7] which slightly modified the conventional DC casting mould and applied the industrial frequency alternating current to generate alternating magnetic field through a coil around the mould. Based on EMC and CREM, a low frequency electromagnetic casting (LFEC) process was developed recently by Cui and his colleagues[8, 9]. In the LFEC process, a electromagnetic field with the frequency lower than the industrial frequency was used to control the fluid flow and temperature field. The aim of this work is to prepare flat ingot of 2524 aluminium alloy with the LFEC process, present the macrosegregation behaviour of 2524 alloy and the effect of LFEC on the macrosegregation.

2 Experimental procedure The commercial aluminium alloy 2524, Al-4.3Cu-1.5Mg-0.65Mn-0.02Ti (all in wt%), was used in the present work. The pure aluminum was melted in a 45 kW resistance furnace. Pure Cu, Mg and addition agent of Mn were added to the o melted aluminum at 750 C. The melt was degassed by C2Cl6, and then was poured into a DC casting mould through a filter tank and a launder. A 90 turn induction coil was arranged outside the mould and a graphite ring was set in the mould. Flat ingots with the cross section of 350mm×160mm were then cast by the conventional DC casting and the LFEC process (25 Hz and 50A) respectively, under the following conditions: casting temperature (melt temperature in the furnace), 710-720 oC; casting speed, 75 mm/min; temperature of cooling water, 17 oC; cooling water flow rate, 110 L/min. The schematic of the LFEC process is shown in Fig.1.

Fig. 1 Schematic of the LFEC process The chemical composition at different positions was measured by using a direct reading spectrometer. The flat ingot was machined layer by layer from the surface to the centre (along the thickness direction) for the composition measurement. 3 Results and discussions 3.1 Macrosegregation of DC cast ingot Fig. 2 shows the concentration distribution of Cu, Mg, Mn and Ti from the surface to the centre (along the thickness direction) of DC cast ingot. Different elements show different macrosegregation tendencies. For Cu, Mg, there is a negative (solute-depleted) segregation in the centre adjoined by a positive (solute-rich) segregation approximately at mid-thickness with a solute depletion at subsurface followed by a strong positive segregation at the surface. The macrosegregation of Cu is more serious than that of Mg. The distribution of Mn is uniform without obvious macrosegregation. The distribution of Ti exhibits a exactly different tendency from that of Cu. It shows a centreline positive segregation and a negative segregation at the surface. 4

8

3

6

2

4

Concentration of Cu (%) Cu of Concentration Concentration of Mg (%) Mg of Concentration

2 1 0 20 40 60 80 0 20 40 60 80 Distance from the surface of the ingot (mm) Distance from the surface of the ingot (mm)

1.0 0.04

0.8 0.03

0.02

0.6

0.01

0.4

Concentration of Ti (%) Ti of Concentration Concentration of Mn (%) Mn of Concentration

0.00 0.2 0 20 40 60 80 0 20 40 60 80 Distance from the surface of the ingot (mm) Distance from the surface of the ingot (mm) Fig. 2 Macrosegregation of DC cast 2524 alloy ingot It is well known that the solidification of alloys is accompanied by a certain degree of microsegregation of alloying elements due to rejection of element (K<1) during grain growth and due to the non-equilibrium nature of solidification, resulting in the composition difference between liquid and solid phases. If, subsequently, relative movement between the liquid and the solid occurs, the segregation can appear on a macro-scale, then the macrosegregation is formed. Therefore the partition coefficient, K, plays a very important role in the formation of macrosegregation. The partition coefficients of Cu, Mg, Mn and Ti are 0.17, 0.43, 0.90 and 9.0 respectively. Partition coefficient of Mn is 0.90 which is close to 1, therefore Mn shows more uniform distribution without serious macrosegregation. Partition coefficient of Cu is 0.17, so Cu shows a higher macrosegregation tendency than Mg and Mn. Ti with K > 1 exhibits an exactly different segregating tendency, which is opposite to the trend of Cu and Mg. For element of Cu, the copper concentration at the ingot surface can reach about 9%. The surface macrosegregation is mainly caused by exudation of interdendritic melt due to solidification shrinkage and the formation of air gap between the shell and the mould induced by the global solidification contraction of the ingot. The transport of interdendritic elements enriched melt to the surface leads to positive macrosegregation at the ingot surface, and consequently results in negative segregation in subsurface through which the interdendritic melt flow has taken place. Mg shows a macrosegregation tendency similar to Cu. Ti shows a results opposite to Cu due to its partition coefficient is much larger than 1. In the case of centreline macrosegregation[3, 10], there are several mechanisms relating to thermo-solutal convection, free floating grains, shrinkage and deformation induced flow, and forced convection. These factors make contributions to the formation of negative macrosegregation in the centre of ingot.

3.2 Macrosegregation of DC cast ingot under the influence of electromagnetic field With the application of electromagnetic field, the macrosegregation tendency get changed. The results are shown in Fig. 3. As discussed in 3.1, the segregation tendency of Mg and Mn is not very serious, here the segregation tendency of only Cu and Ti was studied under the influence of electromagnetic field. The electromagnetic field did not show significant effect on the surface segregation for both Cu and Ti. The ingots prepared with DC and LFEC process have obvious segregation near to the surface. However the LFEC process shows a obvious effect on the centreline segregation. With the application of electromagnetic field, both the negative segregation of Cu and the positive segregation of Ti in the centre of the ingots are evidently reduced. 10 0.04

0.03 8 DC DC LFEC, 50A LFEC, 50A 0.02

6

0.01

4

Concentration of Ti (%) Ti of Concentration Concentration of Cu (%) Cu of Concentration 0.00

2 0 20 40 60 80 0 20 40 60 80 Distance from the surface of the ingot (mm) Distance from the surface of the ingot (mm) Fig. 3 Comparison of macrosegregation of ingots prepared by DC and LFEC processes Fig. 3 only shows the concentration distribution along a line from the surface to the centre of the ingot (thickness direction). In order to more clearly observe the effect of electromagnetic field on the macrosegregation, a contour picture was platted by measuring the composition over the entire quarter of the cross section of the ingot. The contour picture of the concentration of Cu is shown in Fig. 4.

80 80

9.000 9.000 8.297 8.297 7.595 7.595 60 6.892 60 6.892 6.190 6.190 5.487 5.487 4.785 4.785 40 4.082 40 4.082 3.380 3.380

20 20

0 0

30 60 90 120 150 30 60 90 120 150

Distance from the centre of the ingot, mm ingot, the of centre the from Distance mm ingot, the of centre the from Distance Distance from the centre of the ingot, mm Distance from the centre of the ingot, mm

Conventional DC LFEC process Fig. 4 The contour picture of the concentration of Cu It can be seen from Fig. 4 that the negative segregation in the centre area of conventional DC cast ingot is significantly reduced with the application of LFEC process, and the distribution of Cu in the ingot becomes more uniform. In the LFEC process the alternative current generates a time varying magnetic field in the melt, which, in turn, gives rise to an induced current in the melt and/or ingot. Therefore, the melt is subjected to electromagnetic body forces caused by the interaction between the induced current and the magnetic field. The Lorentz force can be expressed as Eq. (1)[6, 7]. 1 1 F  J  B  (B)B  B2 (1)  2 where B and J are the magnetic induction intensity and current density generated in the melt, μ is the permeability of the melt. The first term on the right hand of Eq. (1) is a rotational component which results in a forced convection and a flow in the melt. The forced convection results in uniform temperature field, reduced sump depth[11] and in consequence uniform and refined microstructure[12]. It is accepted that the shrinkage induced flow contributes to the formation of negative centreline segregation[10]. Grain refinement can decrease the permeability of the mushy zone and in consequence reduce the shrinkage-induced flow. With the reduction of the sump depth, the solidification front during LFEC process becomes much flatter, resulting in the reduced shrinkage induced flow. Therefore the grain refinement and reduced sump depth make contributions to the reduced centreline segregation. Floating grains also contribute to negative centreline segregation[13]. Uniform and refined microstructure resulted from the application of LFEC process, does not show obvious floating grains. With the reduction of floating grains, the negative centreline segregation could be weakened.

4 Conclusions (1) The main elements in the flat ingot of 2524 alloy show a typical macrosegregation with a positive segregation at the surface and a negative segregation in the centre of the ingot. Cu shows the highest segregation tendency among the main elements of Cu, Mg and Mn. Ti shows a different segregation trend opposite to that of Cu. (2) The electromagnetic field does not show significant effect on the surface segregation of DC cast 2524 alloy. However the LFEC process shows an obvious effect on the centreline segregation. With the application of electromagnetic field, the negative segregation of Cu and the positive segregation of Ti in the centre of the ingots are evidently reduced.

Acknowledgment This work was financially supported by the National Natural Science Foundation of China (51374067), the outstanding young scholars growth plan in the colleges and universities of Liaoning Province, China (LJQ2014032) and the National Basic Research Program of China (2012CB619506).

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