Fine equiaxed β grains and superior tensile property in Ti-6Al-4V alloy deposited by coaxial electron beam wire feeding additive manufacturing

doi:10.1007/s40195-020-01073-5

Abstract

Abstract:

Coarse columnar β grains result in anisotropic mechanical properties in Ti alloys deposited by additive manufacturing. This study reports that Ti-6Al-4V alloy fabricated by coaxial electron beam wire feeding additive manufacturing presents a weak anisotropy, high strength and ductility. The superior tensile property arises from a microstructure with fine equiaxed β grains (EGβ), discontinuous grain boundary α phase and short intragranular α lamellae. A large region of fine EGβ arises from a special combination of the temperature gradient and solidification rate, and attractive α morphology is caused by solid phase transformations during interpass thermal cycling and post heat treatments.

Key words: Directed energy deposition, Wire feeding additive manufacturing, Ti-6Al-4V alloy, Equiaxed β grain, Tensile properties

Jiahua Zhang, Yi Yang, Sheng Cao, Zhiqiang Cao, Dmytro Kovalchuk, Songquan Wu, Enquan Liang, Xi Zhang, Wei Chen, Fan Wu, Aijun Huang. Fine equiaxed β grains and superior tensile property in Ti-6Al-4V alloy deposited by coaxial electron beam wire feeding additive manufacturing[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(10): 1311-1320.

Figures/Tables 11

Fig. 1 Coaxial electron beam wire feeding additive manufacturing system: a schematic diagram, b machine (xBeam 3D Metal Printer)

Fig. 2 Outside view of the as-deposited structure: a overall view, b enlarged image of the squared region in a

Table 1 Chemical composition of the deposited structure (wt%)

Ti	Al	V	Fe	O	N	H	C	Y
Bal.	6.26	4.25	0.22	0.12	0.0055	0.0014	0.0091	< 0.005

Fig. 3 a Engineering tensile stress- strain curves of 2-bead wall, b ultimate tensile strength versus total elongation to failure for Ti-6Al-4V titanium alloy fabricated by different wire additive manufacturing processes

Table 2 Room temperature tensile properties of 2-bead wall after heat treatment

Direction	UTS (MPa)	YS (MPa)	EL (%)
Z	972.5 ± 2.5	842.5 ± 2.5	19.0 ± 1.0
X	982.5 ± 7.5	858.0 ± 8.0	18.5 ± 0.5

Fig. 4 SEM images of the tensile fracture surfaces for 2-bead specimen in X direction: a entire fracture surface, showing three zones, i.e., fibrous zone, radial zone and shear lip zone; b-d corresponding enlarged images of the squared regions in a

Fig. 5 Optical macrographs in Y-Z sections of 5-bead a and 2-bead walls b after three-stage heat treatment, showing large regions of fine equiaxed prior β grain zones, c microstructure of the substrate, d1-d6 magnified β grain morphologies in the rectangles labeled as 1-6 in d row in b, e1-e6 magnified β grain morphologies in the rectangles labeled as 1-6 in e row in b

Fig. 6 EBSD orientation maps of β phase and α phase morphology in Y-Z section of 2-bead wall after three-stage heat treatment: a columnar β grain zone, b equiaxed β grain zone. a-1, b-1 the 2nd layer; a-2, b-2 the 18th layer; a-3, b-3 the 32th layer. The dashed lines indicate the original β grain boundaries. The insets are (001) pole figures of β grains

Fig. 7 EBSD orientation maps of α phase in Y-Z section of 2-bead wall after three-stage heat treatment: a columnar β grain zone, b equiaxed β grain zone. a-1, b-1 The 2nd layer; a-2, b-2 the 18th layer; a-3, b-3 the 32th layer

Fig. 8 Microstructures of the as-built 2-bead wall, showing a martensitic microstructure: a, c CGβ zone, b, d EGβ zone, a, b optical macrographs, c, d SEM macrographs

Fig. 9 a Illustration of Ti-6Al-4V solidification map, showing that columnar β grains and equiaxed β grains can be obtained via controlling temperature gradient (G) and solidification rate (R). The different color shapes in a illustrating G vs. R in different sites of 2-bead wall during deposition in b

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