Structural anisotropy in 3D-printed products using selective laser melting technology

Using the Ti-6Al-4V alloy as an example, the influence of thermal conditions in 3D-printing on the anisotropy of its microstructure, phase composition, residual macrostresses, and mechanical properties during selective laser melting (SLM) is demonstrated. A fiber ytterbium laser with air cooling and a nominal power of 200 W, a laser beam diameter of ~45 µm, and a wavelength of 1070 ± 2 nm was employed, with a scanning speed of 500 mm/s, a layer thickness of 25 µm, and a hatch distance of 150 µm. It is shown that the synthesized microstructure consists of periodic scale-gradient layers with a banded structure, which results from the cyclic thermal history experienced by the SLM sample of Ti-6Al-4V, rather than from segregation or oxidation. The bands exhibit a Widmanstätten α-colony morphology with an average size of structural elements around (0.2–2) µm, while the nominal microstructure between the bands shows a basket-weave morphology. The gradient width of each band is determined by thermal effects, particularly the maximum heating temperature and cooling rate from this temperature, depending on the band’s position – whether in the lower, middle, or upper part of each layer and the distance of this layer from the substrate. The cooling rate of the deposited layer decreases as the number of added layers increases and is highest at the top plane of the SLM sample due to additional heat loss from convection and radiation, as well as the absence of remelting and thermal cycling experienced by the previous layers. As a result, the microstructure of the top plane is finer, and a greater probable amount of needle-like α'-martensite, a smaller OQR size, and a higher degree of deformation of the β-phase crystal lattice provide a higher microhardness value than in the lateral plane, despite the greater magnitude of tensile macrostresses. The obtained results convincingly demonstrate the feasibility of applying thermomechanical post-processing to ensure a homogeneous structure in SLM products.