MICROSTRUCTURE EVOLUTION OF PARTIALLY MELTED ZONE OF TIG WELDING JOINT OF Ni-BASED INCONEL-718 SUPERALLOY
YE Xin1,2, HUA Xueming1,2(), WANG Min1,2, LOU Songnian1,2
1 Welding Engineering Institute, College of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240 2 Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai Jiao Tong University, Shanghai 200240
Cite this article:
YE Xin, HUA Xueming, WANG Min, LOU Songnian. MICROSTRUCTURE EVOLUTION OF PARTIALLY MELTED ZONE OF TIG WELDING JOINT OF Ni-BASED INCONEL-718 SUPERALLOY. Acta Metall Sin, 2014, 50(8): 1003-1010.
Element segregation, such as Nb in Ni-based Inconel-718 superalloy, causes the precipitation of low melting point phase during solidification. The actual base metal can melt in a lower temperature and the structural continuity is damaged during welding thermo cycle curve. The liquid film easily generates between austenite grains and leads to stress concentration before solidifying into the low melting point phase. Microstructure evolution of Inconel-718 welding joint increases the hot crack sensitivity and changes mechanical property of the joint. The partially melted zone (PMZ) is close to the molten metal in the fusion zone, which is the most liquation crack sensitive region of welding joint heat affected zone (HAZ). Different microstructures exists among wrought, as-cast and homogenization Inconel-718 superalloy inducing weldability differences of these material. Especially, the solidus-liquidus curve differences of low melting point phase in PMZ notably affect the high temperature mechanical property of welding joint. The wrought, as-cast and homogenization Inconel-718 superalloy sheets were respectively welded by tungsten inert gas arc welding (TIG) with different heat inputs. The microstructure of PMZ was observed by OM and SEM. Alloy element content of intradendritic austenite, interdendritic segregated region and Laves phase was investigated by EDS. The theoretical solidus-liquidus temperature of these phases was calculated by Thermo-Calc software. The melting and solidification temperature of austenite and Laves in PMZ of different base metal was investigated to analyze the temperature range for the formulation of liquid film. The results show that the microstructure heredity phenomenon obviously exists in the PMZ of Inconel-718 welding joint. The equiaxed grains remain in the PMZ of wrought joint, and the dendritic structure is still kept in the PMZ of as-cast and homogenization joint. The slender Laves and particle MC phase precipitate along the boundary of the austenite in PMZ of welding joint. But the segregated region originally existed in base metal disperses. The calculating results show that the maximum solidus-liquidus temperature range is in as-cast base metal, secondary in homogenization, minimum in wrought. The width of PMZ is increased with the increasing heat input and PMZ of as-cast is larger than PMZ of wrought and homogenization Inconel-718 superalloy.
Fig.2 OM (a) and SEM (b) images of base metal for wrought Inconel-718 superalloy
Fig.3 OM (a) and SEM (b) images of weld joint for wrought Inconel-718 superalloy
Fig.4 OM (a) and SEM (b) images of base metal for as-cast Inconel-718 superalloy
Fig.5 OM (a) and SEM (b) images of weld joint for as-cast Inconel-718 superalloy
Fig.6 OM (a) and SEM (b) images of base metal for homogenization Inconel-718 superalloy
Fig.7 OM (a) and SEM (b) images of weld joint for homogenization Inconel-718 superalloy
Heat input / (kJ?cm-1)
PMZ width /mm
Wrought
As-cast
Homogenization
2.4
0.218±0.022
0.335±0.022
0.247±0.028
3.6
0.292±0.019
0.426±0.028
0.339±0.016
4.8
0.401±0.013
0.541±0.015
0.423±0.031
Table 1 Effect of heat inputs on PMZ width of welding joint for Inconel-718 superalloy with different treatments
Fig.8 Austenite (a) and carbide (b) equilibrium mass fraction diagrams for Inconel-718 superalloy
Fig.9 Ni-Nb (a) and (b) Ni-C equilibrium binary phase diagrams for Inconel-718 superalloy
Type
Zone
Region
Mass fraction /%
Solidus
Liquidus
Fe
Nb
K
K
As-cast
Base metal
Laves
16.19
26.02
1391
1621
Segregated γ
17.30
8.17
1429
1586
Intradendriticγ
24.87
2.69
1553
1668
Homogenization
Laves
12.12
27.62
1376
1635
Segregated γ
20.59
10.19
1423
1556
Intradendriticγ
25.21
4.87
1462
1631
As-cast
PMZ
Laves
21.54
26.30
1419
1625
Intradendriticγ
29.25
1.91
1593
1693
Homogenization
Laves
21.50
24.86
1430
1600
Intradendriticγ
28.13
2.32
1572
1683
Table 2 Element contents and theoretical solidus and liquidus temperatures on as-cast and homogenization in base metal and PMZ zone for Inconel-718 superalloy
Base metal
Liquidus of austenite
Solidus of low melting phase
Temperature range
K
K
K
Wrought
1630
1451
179?
As-cast
1668
1391
277
Homogenization
1631
1376
255
Table 3 The theoretical solidus and liquidus temperature and solid-liquid coexistence temperature range of base metal for Inconel-718 superalloy
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