Wire-arc additive manufacturing (WAAM) combines an electric arc as a heat source and a wire as feedstock to build a layer-by-layer component. In WAAM, process parameters are important characteristic, which can affect the mechanical properties and microstructure of the deposited material. This paper demonstrates improvements in specimens made of Ti-6Al-4V manufactured by WAAM treated by various methods (heat input, deposition speed and cryogenic vaporised Ar shielding/cooling).
Heat input: A high heat input (106 J/m, Specimen H) produced a columnar grain exhibiting a large anisotropic tensile strength. However, a low heat input (5×105 J/m, Specimen L) transformed the columnar grains to equiaxed grains owing to the accelerated solidification rates. The thermal history of the WAAM deposits was simulated using the finite-element method. The faster cooling rates in the solidification range (1600–1660 °C) in Specimen L resulted in a larger fraction of the equiaxed grains and significant reduction of tensile strength anisotropy. Meanwhile, due to more thermal cycles and rapid cooling rates during the secondary-α transformation below the β-transus temperature (700–1006 °C), Specimen H produced larger amounts of nitrogen, α’ martensite, and fine-secondary α with a tangled dislocation, thereby exhibiting a larger tensile strength and hardness than Specimen L.
Deposition speed: Equiaxed prior-β grains developed in the faster deposition speed specimen (F) and mixture prior-β grains (equiaxed and columnar) took shape in the middle and slow deposition speed specimen (M and S), which is consistent with the anisotropic property as observed in the fast deposition speed specimen with lower anisotropic tensile strength than others. Meanwhile, simulation results demonstrated that a faster deposition speed led to a lower temperature gradient (G) and a higher solidification rate (R) resulted in a larger number of equiaxed grains compared to the slower deposition speed specimen.
Cryogenic vaporised Ar shielding/cooling: The cooling time of cryogenic vaporised Ar shielding/cooling of Ti-6Al-4V deposits was ~10 times faster than that of normal Ar gas shielding/cooling. The columnar β grains in rapid cooling condition were observed to be in the longitudinal direction of the deposits same as that in natural cooling condition. Under the rapid cooling, a narrow heat-affected zone band (~70 μm) and a wide vanadium-segregation band was observed, along with a reduced aspect ratio of the α plate, which led to improved isotropic-tensile properties along the deposition and building directions. Rapid cooling condition also retained a significant amount of O and N, which enhanced its hardness and tensile strength. Thus, cryogenic vaporised Ar shielding/cooling can produce Ti-6Al-4V wire-arc additive manufacturing deposits with better tensile properties compared to normal Ar gas shielding.