Elucidating the Fracture Toughness of Additively Manufactured and Thermo-Mechanically Treated Ti6Al4V

Additive manufacturing of workhorse Ti6Al4V aerospace alloy components by direct metal laser sintering (DMLS) offers cost-efficiency in producing complex designs. However, the rapid and complex heating and cooling cycles during manufacturing can lead to undesirable microstructures and mechanical properties that are inferior to the wrought product. The present investigation aims to study the microstructure-fracture toughness paradigm for the heat treated DMLS Ti6Al4V sample with conventional thermos-mechanically processed microstructures. To this end, DMLS Ti6Al4V samples were subjected to isothermal heat treatment at 800°C for 1 h to obtain a basketweave α+β dual phase microstructure, while hot rolled samples with equiaxed α+β microstructure were subjected to heat treatment at 1000°C for 1 h to obtain a lamellar α+β microstructure. Fracture toughness tests were performed in three-point bend geometry on fatigue pre-cracked specimens for the three distinct microstructures. The fracture toughness of the heat treated DMLS parts is comparable to the thermo-mechanically treated lamellar α+β microstructure and superior to the equiaxed microstructure. Full-field strain measurement was performed during fracture toughness testing using digital image correlation and detailed microstructural characterisation was performed using electron backscatter diffraction and synchrotron diffraction. It was revealed that the deformation within the lamellar α+β phase delays the crack nucleation, while further crack propagation through thicker α laths and prior β grain boundaries contribute to pronounced crack tortuosity or crack path deflection resulting in a more corrugated fracture surface and enhanced fracture toughness.