Inhibition of Cusp magnetic field on stray-crystal formation in platform region during directionally solidified single-crystal superalloy

doi:10.1016/j.jmst.2021.06.086

Abstract

Abstract:

The stray crystal in the platform region is one of the common main defects in single-crystal superalloy blades. The simple and effective method to eliminate this defect is urgent to be explored. This work found that the Cusp magnetic field can effectively inhibit the stray-crystal formation in the platform. The tendency of stray-crystal formation decreases as the magnetic-field strength increases at a certain withdrawal rate and temperature-gradient. The suppressing effect decreases as the withdrawal rate or the temperature-gradient increases. Finally, the inhibiting mechanism on the stray-crystal formation from the Cusp magnetic field is proposed based on the experiments and the numerical simulation. The magnetic-field application strengthens the flow velocity and changes the flow structure near the liquid-solid interface, and further reduces the radial temperature difference. Accordingly, the secondary dendrites in the heat-conduction undercooled zone expands towards the corner in a faster speed, which reduces the stray-crystal formation in the platform corner. This study provides an effective and simple method for decreasing the stray-crystal formation during the preparation of single-crystal with platform region.

Key words: Single-crystal superalloy, Cusp magnetic field, Stray crystals, Platform region

Weili Ren, Tao Zhou, Xiaotan Yuan, Ming Jian, Congjiang Zhang, Biao Ding, Jianchao Peng, Tianxiang Zheng, Yunbo Zhong. Inhibition of Cusp magnetic field on stray-crystal formation in platform region during directionally solidified single-crystal superalloy[J]. J. Mater. Sci. Technol., 2022, 106: 183-194.

Figures/Tables 19

Fig. 1. The single-crystal sample with the platform (The red and yellow dotted lines indicate the observation positions of the cross and the longitudinal section, respectively).

Fig. 2. The schematic diagram of experimental device.

Fig. 3. The cross section structures and their corresponding EBSD images of the single-crystal superalloy at 5 mm above the platform under different Cusp magnetic-field strength (The withdrawal rate is 100 μm/s and the temperature-gradient is 65 K/cm). The parts circled by the red dotted line in the metallographic figures are stray crystals. The yellow dotted line indicates the cutting position of longitudinal section. The red arrows represent the dendrite arrangement direction: (a) without magnetic field; (b) 50 A; (c) 75 A; (d) 100 A.

Fig. 4. The longitudinal section structure and corresponding EBSD images of the single-crystal superalloy near the platform region under different magnetic-field strength (The withdrawal rate is 100 μm/s and the temperature-gradient is 65 K/cm). The parts circled by the red dotted line in the metallographic figures are stray crystals. The red arrows represent the dendrite arrangement direction: (a) without magnetic field; (b) 50 A; (c) 75 A; (d) 100 A.

Fig. 5. The stray-crystal ratio on cross section and longitudinal section under different Cusp magnetic field strength (The red arrow indicates the reduced ratio of stray crystals compared with 0 A magnetic field).

Fig. 6. The EBSD images of the cross section at 5 mm above the platform region when the withdrawal rate increases at the same temperature-gradient of 65 K/cm: (a) without magnetic field; (b) 100 A Cusp magnetic field; (a1, b1) 50 μm/s; (a2, b2) 75 μm/s; (a3, b3) 100 μm/s; (a4, b4) 150 μm/s.

Fig. 7. The stray-crystal ratio in the cross section at 5 mm above the platform with 0 and 100 A magnetic fields when the withdrawal rate increases (The red arrow indicates the reduced ratio of stray crystals).

Fig. 8. The EBSD images of the cross section at 5 mm above the platform when the temperature-gradient increases at the same withdrawal rate of 100 μm/s: (a) without magnetic field; (b) Cusp magnetic field with 100 A current; (a1, b1) 50 K/cm; (a2, b2) 65 K/cm; (a3, b3) 80 K/cm.

Fig. 9. The stray-crystal ratio in the cross section at 5 mm above the platform with 0 and 100 A magnetic fields when the temperature-gradient increases (The red arrow indicates the reduced ratio of stray crystals).

Fig. 10. The isotherm diagram of liquid-solid interface when directional solidification reaches to the platform.

Fig. 11. The flow field in the sample with the platform under 0 and 100 A Cusp magnetic field when directional solidification goes to 250 and 590 s (The white arrows represent the flow direction. The red dotted lines indicate the observation positions of the cross section): (a) without magnetic field; (b) Cusp magnetic field with 100 A current; (a1, b1) 250 s; (a2, b2) 590 s.

Table 1. The mean radial velocity and the mean overall velocity (μm/s) at the cross section of the lower part in the flow cell near the liquid-solid interface.

Time (s)	Magnetic field strength (A)	Mean radial velocity (MRV)	Mean overall velocity (MOV)	The ratio of MRV to MOV (%)
250	0	0.95	1.52	63
	100	2.60	2.90	90
590	0	3.40	4.70	72
	100	15.8	17.2	92

Fig. 12. The mean flow velocity in the zone far away from and near the liquid-solid interface without the magnetic field and with the 100 A Cusp magnetic field when directional solidification reaches to 250 s and 590 s: (a) 250 s; (b) 590 s.

Fig. 13. (a, b) The sample position and magnetic field distribution (100 A current) when solidified to the platform (The blue arrow is magnetic induction line. The white frame line indicates the real sample position).

Fig. 14. The flow field only with magneto damping under 0 and 100 A Cusp magnetic field when the directional solidification goes to 590 s (The white arrows represent the flow direction. The red dotted lines indicate the observation positions of the cross section): (a) without magnetic field; (b) Cusp magnetic field with 100 A current.

Fig. 15. (a) The thermal current distribution; (b) The thermoelectric magnetic force; (c) The thermoelectric magnetic convection; (d) The schematic diagram of thermal current $\vec{J}$, magnetic field $\vec{B}$, and their angleθ in the different positions at the liquid-solid interface.

Fig. 16. (a) The longitudinal temperature field under without magnetic field; (b) The longitudinal temperature field under the Cusp magnetic field (1-3 represent the thermal-radiation undercooled zone, the hot-spot zone, and the heat-conduction undercooled zone, respectively). (c) The temperature difference between the center and the edge of the sample at different times when solidified to the platform.

Fig. 17. The temperature differences between the center and the edge of the sample at the platform under different conditions: (a) The current intensity applied in the magnetic coils increases as the withdrawal rate (100 μm/s) and the temperature-gradient (65 K/cm) keep unchanged; (b) The withdrawal rates change at a certain temperature-gradient (65 K/cm); (c) The temperature-gradient increases at a certain withdrawal rate (100 μm/s).

Fig. 18. The temperature-field evolution with times near the platform: (a) without magnetic field; (b) 50 A Cusp magnetic field; (c) 100 A Cusp magnetic field.

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