Mechanical behaviour of additively manufactured Ti6Al4V meta-crystals containing multi-scale hierarchical lattice structures

The mimicry of crystalline microstructure at meso-scale creates a new class of architected materials, termed meta-crystals, and offers effective ways to significantly improve the toughness and eliminate the post-yield collapse of architected materials. This study investigated the mechanical behaviour of polygrain-like meta-crystals fabricated from Ti6Al4V by laser powder bed fusion. The mechanical behaviour of Ti6Al4V meta-crystals is governed by lattice structures across length-scales: the crystalline microstructure, architected crystal-like mesostructures and the quality of lattice struts. Due to the intricate architecture, significant processing defects were seen in the printed meta-crystals, in particular notch-like defects due to lack of fusion at the free surface of struts. Such defects raised stress concentration and reduced the load-bearing area of struts, hence significantly weakening the lattice struts. In addition, the as-printed condition was brittle due to the presence of acicular alpha' martensites. The defects and the as-printed brittleness led to the premature fracture of struts and compromised the benefits of the crystal-inspired mesostructures. The study subsequently conducted multiple measures to resolve this issue: increasing the strut diameter to reduce the influence of the process defects, and annealing to relieve internal stresses and regain ductility. The combination of the increase in strut diameter and annealing successfully enabled the crystal-like architected mesostructure to effectively improve the toughness of the meta-crystals.