Grain boundary discontinuity and performance improvement mechanism of wire arc additive manufactured Ti–6Al–4V

Highlights

• The effect of the grain boundary discontinuity on tensile properties in wire arc additive manufactured titanium alloys was investigated.

• The improvement mechanism of tensile properties was elucidated.

• A quantitative model has been successfully applied to investigate the anisotropic ductility.

Abstract

Most recent studies for improving tensile properties of wire arc additive manufactured Ti–6Al–4V focused on layer-by-layer plastic deformation but with complicated operation. Previous study demonstrated that a sub-transus solution increased the strength and decrease the anisotropic ductility by generating a new micro-lamellae recrystallization to discontinue α grain boundaries (α_GB). The current study investigated the effects of near β transus solution and aging treatment on the microstructure and tensile properties. Both α lamellae recrystallization and α' martensite decomposition were observed, which induced a fine lamellar (α + β) structure and a discontinuous α_GB. These changes increased the yield strength (YS) up to 904 MPa while maintaining a high elongation above 15.4%. According to the data, the Hall-petch relationship between the YS and α lamellae width was established, where the quadratic relationship between the elongation and α lamellae width was found. The anisotropic ductility disappeared when the discontinuous ratio of α_GB was higher than 0.6 because the discontinuous α_GB changed the failure mode of horizontal sample from opening to sliding modes similarly with that of vertical sample. The results would provide a pathway to improve the tensile properties of additive manufactured Ti–6Al–4V without complicated layer-by-layer plastic deformation.

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 (α_GB) [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.