Grain boundary discontinuity and performance improvement mechanism of wire arc additive manufactured Ti–6Al–4V
Introduction
Wire arc additive manufacture (WAAM) is one of the new techniques that fabricate full density near-net shape metal components with wires as the depositing materials [1], [2], [3]. It provided a new method for high-efficiency and high-quality fabrication of large-scale titanium alloy components due to its low cost and short production cycle [4], [5]. Usually, the macrostructure of WAAMed Ti–6Al–4V exhibited columnar prior-β grains and layered structure, while the microstructure showed basket weave and colony structures [6], [7], [8]. These features led to the lower ultimate tensile strength (UTS) and yield strength (YS) compared to the forged samples, as well as the obvious anisotropy in the elongation (EL) [9], [10], [11], [12], [13], [14]. Baufeld found that the EL of the horizonal sample (H-sample) was 8% lower than that of the vertical sample (V-sample) [13]. Carroll claimed that the anisotropic ductility was attributed to the long and thin prior-β grains [14]. These undesirable mechanical properties limit the aviation application of WAAMed Ti–6Al–4V components.
In order to improve the strength and decrease the anisotropic ductility of WAAMed Ti–6Al–4V, several methods with plastic deformation have been employed [15], [16], [17], [18]. It was found that the columnar prior-β grains transformed to the equiaxed grains when rolled layer-by-layer in the deposition of WAAMed Ti–6Al–4V [15], [16], which increased the tensile properties due to the formation of fine lamellar (α + β) structure and the breakage of grain boundary α (αGB) [16]. It was reported that the layer-by-layer ultrasonic impact induced the similar microstructure transformation [18]. However, all these methods both decreased the deposition rate and increased the manufacturing cost because of the complicated and impractical operations.
It was notable that some new phenomena in the microstructure and mechanical properties were found in the heat treatment of WAAMed titanium alloys. Baufeld claimed that the strength was increased and the EL was decreased via the heat treatment over β transus temperature [19], [20]. On the other hand, in selective laser melted (SLMed) Ti–6Al–4V, the ultrafine lamellar (α + β) structures were created via α' martensite decomposition, inducing higher YS and EL than those of forged sample [21], [22]. It was difficult to control the martensite decomposition and obtain the ultrafine lamellar (α + β) structure in WAAMed Ti–6Al–4V because of the slower cooling rate (less than 102 K s−1) [21], [23]. These researches raise a question: Could such high strength and low anisotropic ductility of WAAMed titanium alloys be produced only through heat treatment rather than complicated layer-by-layer plastic deformation?
The present work investigated the effect of heat treatment on the microstructure and tensile properties of WAAMed Ti–6Al–4V. Some new phenomena about grain boundary discontinuity and microstructure evolution induced by both recrystallization and martensite decomposition were observed. The relationship between the microstructure characteristics and its improvement on tensile properties were established.
Section snippets
Experimental procedures
As shown in Fig. 1, the WAAM system used consisted of a Fronius Magic Wave 4000 plasma welder, a Fronius PTW3500 plasma torch with a gas shielding chamber, and a Fanuc M-710iC robot. A 1.2 mm-diameter Ti–6Al–4V wire (wt%: 6.1 Al, 4.0V, 0.15 Fe, 0.01C, 0.01N, 0.002H,0.16O, balance Ti) was used for deposition. The substrate was a hot-rolled Ti–6Al–4V plate with the size of 400 × 400 × 20 mm3 (Length × Width × Thickness). The deposition was shielded with pure argon. In order to prevent the
Macrostructure observation
As shown in Fig. 2a, the as-deposited sample was composed of multiply coarse columnar prior-β grains epitaxially growing through deposited layers. The width of two adjacent grains was about 1.5–2.2 mm. The growth behavior of columnar prior-β grains was determined by the crystal structure and the maximum direction of the thermal temperature gradient. The body-centered cubic β-Ti grew along the< 001 > direction under the maximum thermal temperature gradient in WAAM deposition. The 550-AC sample
Conclusions
In the present study, the effect of the grain boundary discontinuity on tensile properties in WAAMed Ti–6Al–4V via heat treatment was studied. The following conclusions can be drawn:
- 1.
Both the as-deposited sample and the 550-AC sample showed the continuous αGB grew up through the colony structure and basket weave structure. The as-deposited sample and heat-treated sample showed coarse columnar prior-β grains.
- 2.
The recrystallization of α lamellae and the α' martensite decomposition destroyed the
CRediT authorship contribution statement
Yong Xie: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing - original draft, Writing - review & editing. Mengcheng Gong: Formal analysis, Investigation, Methodology, Writing - original draft, Writing; Ruize Zhang: Formal analysis, Methodology; Ming Gao: Conceptualization, Resources, Supervision, Writing; Xiaoyan Zeng: Project administration, Resources; Fude Wang: Conceptualization, Project administration, Resources, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research is financially supported by the National Natural Science Foundation of China (grant nos. 51775206, 51475183 and 51429501). Thanks for the State key Laboratory of Materials Processing and Die & Mould Technology, and the Analysis and Testing Center of Huazhong University of Science and Technology for the tests. The authors have declared that no conflict of interest exists.
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