Grain boundary topology engineering enables crack-free additive manufacturing of tungsten

Additive manufacturing (AM) of tungsten is severely limited by intergranular cracking, rooted in its intrinsic brittleness and coarse solidification microstructure. Here we demonstrate that grain-boundary topology engineering, enabled by multi-cycle local rescanning in laser powder bed fusion (LPBF), produces crack-free, high-performance bulk tungsten without extreme preheating or alloying. Controlled thermomechanical cycling introduces well-recovered low-angle dislocation boundaries that progressively reconstruct straight solidification grain boundaries into a tortuous network rich in large-dihedral-angle triple junctions. Experiments and finite-element modeling reveal that this reconstruction is driven by cyclic high-temperature plasticity beneath the melt pool. Large-scale molecular dynamics simulations show that these large-angle triple junctions act as potent crack arrestors, promoting crack-tip blunting and dislocation-mediated plasticity. The resulting tungsten exhibits full density, complete crack suppression, and mechanical properties comparable to wrought material. Our results establish grain-boundary topology engineering as a general design principle for introducing toughness into brittle crystalline materials through deliberate control of boundary network geometry.