In situ investigation of phase transformations in Ti-6Al-4V under additive manufacturing conditions combining laser melting and high-speed micro-X-ray diffraction.

We present combined in situ X-ray diffraction and high-speed imaging to monitor the phase evolution upon cyclic rapid laser heating and cooling mimicking the direct energy deposition of Ti-6Al-4V in real time. Additive manufacturing of the industrially relevant alloy Ti-6Al-4V is known to create a multitude of phases and microstructures depending on processing technology and parameters. Current setups are limited by an averaged measurement through the solid and liquid parts. In this work the combination of a micro-focused intense X-ray beam, a fast detector and unidirectional cooling provide the spatial and temporal resolution to separate contributions from solid and liquid phases in limited volumes. Upon rapid heating and cooling, the β ↔ α' phase transformation is observed repeatedly. At room temperature, single phase α' is observed. Secondary β-formation upon formation of α' is attributed to V partitioning to the β-phase leading to temporary stabilization. Lattice strains in the α'-phase are found to be sensitive to the α' → β phase transformation. Based on lattice strain of the β-phase, the martensite start temperature is estimated at 923 K in these experiments. Off-axis high speed imaging confirms a technically relevant solidification front velocity and cooling rate of 10.3 mm/s and 4500 K/s, respectively.