In-situ following of damage in titanium alloys obtained by laser powder bed fusion: effect of work hardening capacity

The advent of additive manufacturing has opened the door to the production of previously inaccessible geometries. However, laser powder bed fusion tends to generate defects (porosities, hot cracking) as well as high internal stresses, resulting in a significant loss of mechanical properties of printed parts compared to their forged counterparts.
In this work, we consider β-metastable titanium alloys exhibiting both TRIP and TWIP effects simultaneously as more suitable alloys for additive manufacturing processing. Indeed these alloys exhibit a very high strain hardening rate as well as high resistance to damage nucleation, resulting in very high ductility.

The binary Ti-12 wt% Mo grade produced by mixing the pure powders was chosen as the case study. The mechanical properties of the printed parts were first verified by tensile tests. Uniform elongation and work hardening similar to those in the wrought state (casting, rolling, recrystallisation) were achieved after a simple flash heat treatment, while the yield strength of the printed samples was increased by 20%. In situ X-ray tomography follow-up of certain tensile tests also revealed that the presence of defects (porosities, unmelted molybdenum particles) does not have a strong impact on the fracture mechanism of this alloy. In addition, a comparison with printed TA6V shows that although the void nucleation rate is greater in printed Ti12Mo due to the breakage of unmelted molybdenum particles, the acceleration of growth only occurs later. This delay in coalescence can be explained by the strain-hardening of the material ligaments present between the cavities, as revealed by the nano-indentation tests carried out along these ligaments.

Speaker: Marion Coffigniez defended her PhD on additive manufacturing of 3D architectured metallic biomaterials by robocasting in 2021 at MATEIS, INSA-Lyon. Now she is working on additive manufacturing of beta metastable titanium alloys presenting both TRIP and TWIP effects.