Effect of Substrate Bed Temperature on Solute Segregation and Mechanical Properties in Ti-6Al-4V Produced by Laser Powder Bed Fusion

Titanium alloys are particularly sensitive to temperature during additive manufacturing processes, due to their dual phase microstructure and sensitivity to oxygen uptake. In this paper, laser powder bed fusion (LPBF) was used in conjunction with a heated substrate bed at 100 degrees C, 570 degrees C and 770 degrees C to produce specimens of Ti-6Al-4V, to investigate the change in mechanical properties and segregation of alloying elements. An initial increase in ductility was observed when increasing the temperature from 100 degrees C to 570 degrees C, followed by a significant loss in ductility when samples were produced at 770 degrees C. A suite of multi-scale characterisation techniques revealed that the as-printed microstructure was drastically different across the range of temperatures. At 100 degrees C, alpha + alpha ' phases were identified. Deformation twinning was extensively observed in the a phase, with Al and V segregating at the twin interfaces. At 570 degrees C (the most ductile sample), alpha ', alpha and nano-particles of beta were observed, with networks of entangled dislocations showing V segregation. At 770 degrees C, no martensitic alpha ' was identified. The microstructure was an alpha + beta microstructure and an increased volume fraction of tangled dislocations with localised V segregation. Thermodynamic modelling based on the Gibbs-free energy of formation showed that the increased V concentration at dislocations was insufficient to locally nucleate beta phase. However, b-phase nucleation at grain boundaries (not dislocations) caused pinning of grain boundaries, impeding slip and leading to a reduction in ductility. It is likely that the increased O-content within specimens printed at increased temperatures also played a key role in high-temperature embrittlement. Building operations are therefore best performed below sub-transus temperatures, to encourage the growth of strengthening phases via solute segregation, and the build atmosphere must be tightly controlled to reduce oxygen uptake within the samples.

Automated summary

This study investigates the effect of temperature on additive manufacturing process of Ti-6Al-4V alloy using laser powder bed fusion. Results show that there is an initial increase in ductility when the temperature is raised to 570 degrees C, followed by a significant decrease at 770 degrees C. Microstructural analysis reveals significant differences across the temperature range, with different phases and segregation of alloying elements observed. It is recommended to perform building operations at temperatures below the sub-transus range and control the build atmosphere to reduce oxygen uptake.