Influence of Grinding Depth on the Surface Integrity and Fatigue Property of γ-TiAl Alloy
NI Mingjie1,2, LIU Renci1,2(), ZHOU Haohao3, YANG Chao4, GE Shuyu3, LIU Dong1,2, SHI Fengling3, CUI Yuyou1,2, YANG Rui1,2
1 Shi -changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China 3 AECC Shenyang Liming Aero-Engine Co. Ltd., Shenyang 110043, China 4 AECC Commercial Aero-Engine Co. Ltd., Shanghai 200241, China
Cite this article:
NI Mingjie, LIU Renci, ZHOU Haohao, YANG Chao, GE Shuyu, LIU Dong, SHI Fengling, CUI Yuyou, YANG Rui. Influence of Grinding Depth on the Surface Integrity and Fatigue Property of γ-TiAl Alloy. Acta Metall Sin, 2024, 60(2): 261-272.
γ-TiAl is a family of promising structural materials with low density, high stiffness, and good oxidation and creep resistances at elevated temperatures. They can replace heavier nickel-based alloys at 600-800oC; thus, they are used in the construction of low-pressure turbine blades for aeroengines, i.e., General Electric next-generation and leading edge aviation propulsion. Grinding is an important processing step in blade production to ensure the accuracy of assembling. However, the limited ductility and fracture toughness at room temperature and low thermal conductivity of γ-TiAl alloys narrow the parameter windows of the grinding process. Cracks often form on the surface when the processing parameters are not well controlled. Additionally, grinding greatly influences the surface integrity (i.e., roughness, microstructure, and hardness), which influences the mechanical properties, especially those of brittle γ-TiAl alloys that are sensitive to notch. Grinding depth is a major parameter in blade production because it influences quality and efficiency. Investigating the effect of grinding depth on the surface integrity and fatigue properties of γ-TiAl samples is necessary to optimize the grinding process and identify the major factors of surface integrity that guarantee optimal mechanical properties. In this work, cast γ-TiAl alloy (Ti-45Al-2Nb-2Mn-1B, atomic fraction, %) samples were ground with different depths. The surface integrity (surface roughness, microstructure, and microhardness) and fatigue properties of the samples were compared. Cracks were detected in samples ground to 0.5 and 1 mm depths, while no cracks were detected in samples ground to 0.2 mm or less depths, this is related to the tensile stress induced by temperature increase caused by deformation heat. With the increased grinding depth, the number and depth of grooves increased and for the surface roughness parameters, arithmetic mean deviation and 10-point mean roughness (Rz) increased, while skewness decreased. The γ + α2 lamellae bended in the surface layer and layer thickness increased with the increased grinding depth. The microhardness initially decreased and then increased from the surface to the interior. The rotating bending fatigue life at 650oC under a load of 440 MPa decreased with the increased grinding depth: it was > 106 cyc at 0.05 mm grinding depth but dropped to ~104 cyc at 0.2 mm grinding depth. Fracture surface analysis showed that the cracks mainly nucleated at the surface grooves caused by grinding, which resulted in stress concentration and reduced the fatigue life of samples ground to 0.2 mm depth. The fatigue life decreased with increasing Rz, but remained above 106 cyc when Rz was less than 4 μm. A nonlinear relationship between fatigue life and Rz was shown.
Fund: National Natural Science Foundation of China(51701209);CAS Project for Young Scientists in Basic Research(YSBR-025);Major Special Science and Technology Project of Yunnan Province(202302AB080009)
Corresponding Authors:
LIU Renci, professor, Tel: (024)83970951, E-mail: rcliu@imr.ac.cn
Fig.1 Schematics of grinding setup of γ-TiAl plate (a) and fatigue test sample (b) (vs—linear speed of grinding wheel, vw—workpiece linear speed, ap—grinding depth; T represent the transversal section of sample, and L represent the longitudinal section of sample)
Fig.2 Fluorescent penetration inspect (FPI) results of γ-TiAl plates ground with 0.05 mm (a, b), 0.1 mm (c, d), 0.2 mm (e, f), 0.5 mm (g), and 1 mm (h) depths
Fig.3 Surface morphologies of γ-TiAl plates ground with 0.2 mm (a, b) and 0.5 mm (c, d) depths under low (a, c) and high (b, d) magnifications (Inset in Fig.3c shows the morphology of the whole grinding surface of plate, the rectangle shows the region observed in Fig.3c)
Grinding depth / mm
Ra / μm
Rz / μm
Rsk
Rku
0.05
0.66
4.30
0.281
2.97
0.1
0.84
4.43
0.176
2.98
0.2
0.93
4.87
-0.137
2.37
0.5
1.01
4.98
-0.079
1.84
1
1.21
5.01
0.011
2.33
Table 1 Surface roughness parameters of plates ground with different depths
Fig.4 Surface topographies of γ-TiAl plate ground with 0.05 mm (a), 0.1 mm (b), 0.2 mm (c), 0.5 mm (d), and 1 mm (e) depths and surface profile perpendicular to grinding direction (f) (Inset in Fig.4f shows the magnification of 400-500 μm profiles of 0.05 and 0.1 mm depths)
Fig.5 Surface topographies of fatigue test γ-TiAl samples ground with 0.05 mm (a), 0.1 mm (b), and 0.2 mm (c, d) depths
Grinding depth / mm
No.
Ra / μm
Rz / μm
Rsk
Rku
Nf / cyc
0.05
1
0.525
2.819
-0.134
2.52
1978066
2
0.669
4.222
-0.574
3.73
188264
3
0.590
3.364
-0.472
2.83
2963463
4
0.491
2.885
0.390
2.85
2950000
5
0.469
2.956
-0.882
4.20
7530000
0.1
1
0.563
3.233
-0.586
3.19
1860544
2
0.747
4.232
-0.452
2.92
2900787
3
0.631
3.582
-0.083
2.79
9709
4
0.773
4.286
0.232
2.64
16000
5
0.452
2.717
-0.452
3.29
1960000
0.2
1
1.351
8.325
-0.485
3.62
35200
2
2.831
13.573
-0.236
2.55
12200
3
1.695
9.565
-1.010
4.29
15500
4
1.389
7.466
-0.406
2.64
13284
5
1.649
9.941
-0.921
3.77
5149
Table 2 Surface roughness parameters and fatigue life at 650oC loaded 440 MPa of γ-TiAl samples ground with different grinding depths
Fig.6 OM images near the surface of γ-TiAl plates ground with 0.05 mm (a, f), 0.1 mm (b), 0.2 mm (c), 0.5 mm (d), and 1 mm (e) depths in the L sections (a-e) and T section (f)
Fig.7 Band contrast map (a), phase map (b), inverse pole figure (IPF) map (c), and kernel average misorientation (KAM) map (d) near the surface of plate ground with 1 mm depth in the L section
Fig.8 Variations of microhardness with distance to the surface of plates at different grinding depths
Fig.9 Low (a, c, e, g) and high (b, d, f, h) magnified SE-SEM images showing fracture surfaces of fatigue γ-TiAl samples ground with 0.05 mm (a, b), 0.1 mm (c-f), and 0.2 mm (g, h) depths (a, b) Nf = 7.53 × 106 cyc (c, d) Nf = 1.86 × 106 cyc (e, f) Nf = 9709 cyc (g, h) Nf = 1.22 × 104 cyc
Fig.10 Schematic of stress and temperature in the grinding surface layer of γ-TiAl plate (Fp—grinding force, α—angle between radius and ch-ord or angle between chord and grinding direction, Fa—axial component force of Fp, Ft—tangential component force of Fp, Fn—normal component force of Fp, R—grinding wheel radius, Ta—average temperature in the surface of the workpiece contacted with wheel, fn—tensile stress along depth caused by thermal contraction, fr—tensile stress parallel to grinding direction caused by thermal contraction)
Fig.11 Relationship between Rz and Nf tested at 650oC under a load of 440 MPa
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