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Acta Metall Sin  2020, Vol. 56 Issue (2): 182-192    DOI: 10.11900/0412.1961.2019.00205
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Effect of Heat Treatment Parameters on Microstructure and Hot Workability of As-Cast Fine Grain Ingot of GH4720Li Alloy
WANG Tao,WAN Zhipeng(),LI Zhao,LI Peihuan,LI Xinxu,WEI Kang,ZHANG Yong
Science and Technology on Advanced High Temperature Structural Materials Laboratory, AECC Beijing Institute of Aeronautical Materials, Beijing 100095, China
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Abstract  

GH4720Li was Ni-Cr-Co base precipitation strengthened superalloy and widely used for high performance applications such as disks and blades of either aircraft engines or land-based gas turbines attributing to its excellent properties including resistance to creep and fatigue, together with corrosion, fracture and microstructural stability for the intended applications. Compared with the double-melting process (vacuum induction melting (VIM)+electroslag remelting (ESR) or VIM+vacuum arc remelting (VAR), a triple-melting process (VIM+ESR+VAR) can eliminate the segregation coefficient of the alloying elements and reduce the content of impurity elements, while the ingot fabricated by the triple-melting process also exhibited lots of shortcomings, such as the coarse grains, dendritic structure, microstructure defects and high forging temperature. The as-cast fine grain ingot prepared by grain refining casting process can eliminate the microscopic shrinkage, reduce the differences among three crystalline regions and improve the hot workability as a result. However, it was hardly to avoid the microstructure defects by simply improving the casting process attributing to its large number of alloying elements. Therefore, the homogenization treatment was always performed on the superalloy ingot. In this work, the optimized homogenization parameter was identified according to the investigation on the microstructure evolution under various homogenization treatment conditions and hot workability of as-cast fine grain ingot after homogenization treatment. The relationships of one-stage as well as two-stage homogenization treatment parameters and segregation coefficient as well as volume fraction of eutectic phase were investigated indepth. The hot workability of the homogenized samples under various conditions was also analyzed with the help of hot compression tests. Experimental results revealed that the increased homogenization treatment temperature and extended holding time were able to decrease the volume fraction of eutectic phase and segregation coefficient of the alloying element significantly. Hot compression tests by the Gleeble 3800 dynamic thermal-mechanical testing machine indicated that the samples suffered two-stage homogenization treatment followed by the slowly cooling rate (1140 ℃, 12 h+1170 ℃, 10 h, 0.2 ℃/min furnace cooling to 1010 ℃, and then air cooling) exhibited better hot workability (the maximum reduction rate of 50% deformed at 1120 ℃, 1 s-1). Discontinuous dynamic recrystallization was identified as the mainly nucleation mechanism of the alloy, and the recrystallized grains preferred to nucleate at the boundaries of the original grains according to the microstructure observation of hot compressed samples. In additions, the M(C, N) type precipitates were able to promote the occurrence of dynamic recrystallization behavior. Homogenization treatment experiments and microstructure observation suggested that the optimized treatment parameters of the as-cast fine grain ingot was 1140 ℃, 12 h+1170 ℃, 10 h, 0.2 ℃/min furnace cooling to 1010 ℃, and then by air cooling.

Key words:  as-cast fine grain ingot of GH4720Li alloy      homogenization heat treatment      microstructure evolution      hot workability     
Received:  21 June 2019     
ZTFLH:  TG146.1  
Corresponding Authors:  Zhipeng WAN     E-mail:  waynedapeng@163.com

Cite this article: 

WANG Tao,WAN Zhipeng,LI Zhao,LI Peihuan,LI Xinxu,WEI Kang,ZHANG Yong. Effect of Heat Treatment Parameters on Microstructure and Hot Workability of As-Cast Fine Grain Ingot of GH4720Li Alloy. Acta Metall Sin, 2020, 56(2): 182-192.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2019.00205     OR     https://www.ams.org.cn/EN/Y2020/V56/I2/182

No.One stage heat treatment
OSHT 11140 ℃, 1 h, WC
OSHT 21140 ℃, 8 h, WC
OSHT 31140 ℃, 16 h, WC
OSHT 41160 ℃, 1 h, WC
OSHT 51160 ℃, 14 h, WC
OSHT 61160 ℃, 24 h, AC
OSHT 71160 ℃, 14 h, 0.2 ℃·min-1 FC to 1010 ℃, and then AC
Table 1  One-stage homogenization heat treatment (OSHT) schedules for the GH4720Li alloy
No.Two stage heat treatment
TSHT 11130 ℃, 8 h+1160 ℃, 4 h, WC
TSHT 21130 ℃, 16 h+1160 ℃, 8 h, WC
TSHT 31160 ℃, 8 h+1180 ℃, 2 h, WC
TSHT 41140 ℃, 12 h+1170 ℃, 10 h, AC
TSHT 51140 ℃, 12 h+1170 ℃, 10 h, 0.2 ℃·min-1 FC to 1010 ℃, and then AC
Table 2  Two-stage homogenization heat treatment (TSHT) schedules for the GH4720Li alloy
Fig.1  Macrostructures of as-cast fine grain ingot of GH4720Li alloy(a) overall profile of ingot (b) horizontal direction (c) vertical direction
Fig.2  OM (a) and SEM (b) images of as-cast fine grain ingot of GH4720Li alloy at R/2 region (Insets show the morphology of γ' precipitates in the interdendritic and dendrite regions, R—radius of the as-cast fine grain ingot)

Element

Mass fraction / %

Segregation coefficient (K)

InterdendriticDendrite core
Al2.472.181.133
Ti12.134.672.597
Cr8.4016.84-2.005
Co11.5914.95-1.290
Ni63.1356.431.119
Mo1.833.06-1.672
W0.461.86-4.043
Table 3  Element segregation analyses of initial as-cast fine grain ingot of GH4720Li alloy at R/2 between dendrite core and interdendritic regions
Fig.3  Microstructures of as-cast fine grain ingot of GH4720Li alloy under heat treatment temperatures of 1130 ℃ (a), 1140 ℃ (b), 1150 ℃ (c), 1160 ℃ (d), 1170 ℃ (e), 1180 ℃ (f), 1200 ℃ (g) and 1220 ℃ (h) for 1 h (Insets show the morphologies of incipient melting phase)
Fig.4  Volume fraction of eutectic phase of GH4720Li alloy under different heat treatment temperatures for 1 h
Fig.5  Microstructures of as-cast fine grain ingot of GH4720Li alloy under one-stage homogenization heat treatment parameters (Insets show the morphologies of eutectic γ+γ' phase and γ' precipitate)(a) OSHT 1 (b) OSHT 2 (c) OSHT 3 (d) OSHT 4 (e) OSHT 5 (f) OSHT 6 (g) OSHT 7
Fig.6  Volume fraction of eutectic phase during various one stage homogenization treatment parameters of as-cast fine grain ingot of GH4720Li alloy
Fig.7  Elemental segregation coefficient of as-cast fine grain ingot of GH4720Li alloy under different homogenization heat treatment parameters
Fig.8  Microstructures of as-cast fine grain ingot of GH4720Li alloy under different two-stage homogenization heat treatment parameters (Insets show the morphologies of eutectic γ+γ' phase and γ' precipitate)(a) TSHT 1 (b) TSHT 2 (c) TSHT 3 (d) TSHT 4 (e) TSHT 5
Fig.9  The volume fraction of eutectic phase of as-cast fine grain ingot of GH4720Li alloy under different two-stage homogenization treatment parameters

No.

1120 ℃1140 ℃1160 ℃
0.01 s-10.1 s-11 s-10.01 s-10.1 s-11 s-10.01 s-10.1 s-11 s-1
ACFGI303020303020303020
OSHT 6303030504040504040
TSHT 4404040505040505040
TSHT 5404050505060505060
Table 4  Critical reduction for crack initiation of homogenized as-cast fine grain ingot of GH4720Li alloy samples during hot compression tests (%)
Fig.10  True stress-strain curves of as-cast fine grain ingot of GH4720Li alloy under homogenization treatment parameter of TSHT 5 at 1140 ℃ (a) and 0.01 s-1 (b)
Fig.11  Microstructures of as-cast fine grain ingot of GH4720Li alloy under homogenization treatment parameter of TSHT 5 (Inset shows the morphology of locally magnified microstructure, T—temperature, ε˙—deformation rate, ε—strain, DRX—dynamic recrystallization, CDRX—continuous dynamic recrystallization, DDRX—discontinuous dynamic recrystallization)(a) T=1160 ℃, ε˙=1 s-1, ε=0.3 (b) T=1160 ℃, ε˙=1 s-1, ε=0.8(c) T=1160 ℃, ε˙=0.01 s-1, ε=0.8 (d) T=1140 ℃, ε˙=0.01 s-1, ε=0.8
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