Fracture toughness of titanium alloys fabricated by high-power laser-directed energy deposition: Fractal analysis and prediction model

Laser additive-manufactured (AM) metallic components typically have superior uniaxial tensile strength to their conventional processing counterparts. However, the strength and toughness trade-off for most AM-fabricated metallic parts remains unsolved. Generally, the heat treatment processes can enhance the elongation and toughness of as-deposited AM samples. In this work, the fracture toughness of high-power (7600 W) laser directed energy deposition Ti–6Al–4V (Ti64) + heat treatment (short as Ti64 DED-HT) samples, were studied using fracture property tests. Combining electron backscatter diffraction (EBSD), confocal laser scanning microscope, and fractal geometry theory, we investigated their fracture mechanism and proposed a new prediction model between plane-strain fracture toughness (K_Ic) and conventional tensile properties. The results show that the plane-strain fracture toughness value in four states (two scanning speeds and two directions) is 81.3±0.7 MPa m^1/2, higher than that of the wrought counterparts (∼65 MPa m^1/2). This high plane-strain fracture toughness results from the combination of relatively fine columnar β grains and coarse α laths of the deposited parts after a specific heat-treated process. Combined with a confocal laser scanning microscope and fractal geometry analysis theory, we found that the rough surface profile leads to high fractal dimension values. In addition, we proposed a modified analytical prediction model, which can effectively predict the plane-strain fracture toughness value of AM Ti64 titanium alloys. These findings provide a guideline for obtaining a high strength–toughness and reliably predicting its K_Ic value in AM titanium alloys.