Grain structure and cracks behaviors of tungsten with different geometrical shapes and support structure prepared by laser powder bed fusion

In order to explore the application of tungsten (W) fabricated by laser powder bed fusion (LPBF), W with different geometrical shapes and different support structures were prepared, and the corresponding microstructural characteristics were investigated. Coarse columnar grains and fine grains exist simultaneously in the cylinder or bulk W specimens fabricated by bi-directional scanning. The single-directional scanning used in wall-shape specimen fabrication facilitates the epitaxial growth of W grains, forming regular elongated columnar grains. Cracks were found in all the specimens, the formation of cracks in W during LPBF is determined by a combination of the localized stress distribution and the intrinsic strength/ductility of the W phase. Cracks are concentrated at the grain boundaries where the coarse columnar grain and fine grains are adjacent. With the specimen size increased from cylinder (slender cylinder, squat cylinder), wall (thin wall) to the bulk specimen (with 0–2 mm support), more cracks were found due to higher residual stress. Cracks could relieve the residual stress dramatically, which leads to the decrease of the dislocation density, therefore, with the increase in specimen size, both low angle grain boundaries (LAGBs) and the geometrically necessary dislocation (GND) density decrease. Adding support structure in bulk W could decrease the cooling rate and increase the cooling time. Low cooling rate can reduce the stress level as well as the generation of cracks. Long cooling time could provide sufficient time for dislocation nucleation and movement to form LAGBs structure. Therefore, there are fewer cracks and higher GNDs density/LAGBs portion inside the W phase with a higher support structure.