Bonding Zone Microstructure and Mechanical Properties of Forging-Additive Hybrid Manufactured Ti-6Al-4V Alloy
MA Jiankai, LI Junjie(), WANG Zhijun, WANG Yujian, WANG Jincheng()
State Key Laboratory of Solidification Technology, Northwestern Polytechnical University, Xi'an 710072, China
Cite this article:
MA Jiankai, LI Junjie, WANG Zhijun, WANG Yujian, WANG Jincheng. Bonding Zone Microstructure and Mechanical Properties of Forging-Additive Hybrid Manufactured Ti-6Al-4V Alloy. Acta Metall Sin, 2021, 57(10): 1246-1257.
The forging-additive hybrid manufacturing technology combines the advantages of traditional manufacturing in terms of efficiency and cost with the refined, flexible, and rapid prototyping characteristics of additive manufacturing. It provides an effective solution for efficient forming of large components. The bonding zone between the wrought-substrate and laser-deposition zones is the fundamental key in the properties of the entire component. In this study, laser solid forming (a powder-feeding laser additive manufacturing technology) was used to deposit bulk samples on a wrought Ti-6Al-4V substrate that contained a bi-modal microstructure. The microstructure in the bonding zone between the substrate and laser-deposition zones under different inputs of linear energy density were studied. The results show that the microstructure in the bonding zone varied from the bottom to the top due to the different influence extents of the heat source. Because of the lower peak temperature, the bi-modal microstructure at the bottom of the bonding zone still retained the initial morphology but contained a certain degree of coarsening. A mixed structure that contained equiaxed α, lamellar α, and a large number of secondary α in the middle of the bonding zone occurred with the increase in temperature and prolonging of the holding time. Meanwhile, the peak temperature of the upper part exceeded the β-phase transition temperature, which exhibited a Widmanstätten structure consisting of lamellar α and the so-called ghost area that was formed due to insufficient element diffusion. In the tensile tests, the fracture position of all bonding samples fabricated with various linear energy densities were very far from the bonding zone, indicating a better strength of the bonding zone than that of the wrought substrate and laser-deposition part. In addition, when the linear energy density was 100 J/mm, the yield and tensile strengths of the composite fabricated sample were larger than that with linear energy densities of 133 and 200 J/mm because the feature size of the α phase in the bonding and additive zones was smaller. Both the yield and tensile strengths of the hybrid fabricated specimen decreased with the increase in the linear energy density, whereas the elongation increased.
Fund: National Key Research and Development Program of China(2018YFB1106003);National Natural Science Foundation of China(51874245)
About author: LI Junjie, associate professor, Tel: (029)88492374, E-mail: lijunjie@nwpu.edu.cn WANG Jincheng, professor, Tel: (029)88492374, E-mail: jchwang@nwpu.edu.cn
Table 1 Chemical compositions of the wrought Ti-6Al-4V substrate and powder
Fig.1 Schematics of forging-additive hybrid manufactured processing (a), hybrid manufactured samples (substrate size: 100 mm × 45 mm × 55 mm, deposition size: 60 mm × 20 mm × 60 mm) (b), sampling position of tensile specimen in bonding zone (c), and the size of tensile specimen (d)
Sample
P
W
v
mm·s-1
d
mm
R0
%
Vf
g·min-1
ΔZ
mm
El
J·mm-1
1
1500
15
5
50
12-13
0.7
100
2
2000
15
5
50
12-13
0.7
133
3
2000
10
5
50
12-13
0.7
200
Table 2 Parameters of the forging-additive hybrid manufactured processing
Fig.2 Microstructures of the bonding zone in forging-additive hybrid manufactured Ti-6Al-4V alloy under linear energy densities of 100 J/mm (a), 133 J/mm (b), and 200 J/mm (c), and high magnified image of WSZ in Fig.2a (d) (WSZ—wrought substrate zone, LDZ—laser deposition zone, BZ—bonding zone)
Fig.3 Microstructures (a-c) and feature sizes (d-f) of α phase of the laser deposition zone in forging-additive hybrid manufactured Ti-6Al-4V alloy under linear energy densities of 100 J/mm (a, d), 133 J/mm (b, e), and 200 J/mm (c, f) (Insets in Figs.3a-c show the enlarged views. α: dark area, β: bright bar, αGB—grain boundary α phase)
Fig.4 Microstructures of the bonding zones in forging-additive hybrid manufactured Ti-6Al-4V alloy corresponding to the square areas in Figs.2a (a), 2b (b), and 2c (c), and locally magnified images of bottom-BZ (a1, b1, c1), mid-BZ (a2, b2, c2), and up-BZ (a3, b3, c3) (The dashed lines in Figs.4a, b, and c show the interfaces between the up-BZ and LDZ. The dotted lines in Figs.4a3, b3, and c3 show the morphologies of equiaxed β grains formed during heating)
Fig.5 The sizes of different microstructures of the bonding zone under different linear energy densities in forging-additive hybrid manufactured Ti-6Al-4V alloy
Fig.6 Microstructures of the middle of bonding zone in forging-additive hybrid manufactured Ti-6Al-4V alloy under linear energy densities of 100 J/mm (a), 133 J/mm (b), and 200 J/mm (c) (αs—secondary α)
Fig.7 Microstructures of the up of bonding zone in forging-additive hybrid manufactured Ti-6Al-4V alloy under linear energy densities of 100 J/mm (a), 133 J/mm (b), 200 J/mm (c), and element content changes in ghost structure along line in Fig.7b (d)
Fig.8 EBSD of the deposition zone (a), the reconstruction of β grain in Fig.8a (b), and scatter pole figures of α phase (c) and β phase (d) of A grain in forging-additive hybrid manufactured Ti-6Al-4V alloy (A grain represents β grain in Fig.8a, arrows in Figs.8c and d show the pole positions)
Fig.9 TEM images of α and β phases (a), corresponding electron diffraction patterns of β phase (b) and α phase (c) in the deposition zone
Fig.10 Stress-strain curves of the bonding zone samples in forging-additive hybrid manufactured Ti-6Al-4V alloy under different linear energy densities
Sample
UTS / MPa
YS / MPa
EL / %
1
954.64 ± 0.4
861.74 ± 3.2
12.48 ± 2.25
2
918.00 ± 8.1
815.60 ± 5.5
12.80 ± 0.24
3
915.86 ± 10.5
828.23 ± 7.8
15.43 ± 0.45
Table 3 Tensile properties of the forging-additive hybrid manufactured Ti-6Al-4V alloy under different linear energy densities
Fig.11 Low (a, c, e) and locally high (b, d, f) magnified images showing fracture positions of the bonding zone samples in forging-additive hybrid manufactured Ti-6Al-4V alloy under line energy densities of 100 J/mm (a, b), 133 J/mm (c, d), and 200 J/mm (e, f)
Fig.12 Low (a1-c1) and locally high (a2-c2) magnified fracture morphologies of bonding zone in forging-additive hybrid manufactured Ti-6Al-4V alloy under linear energy densities of 100 J/mm (a1, a2), 133 J/mm (b1, b2), and 200 J/mm (c1, c2)
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