The layer-wise fashion of DED imposes a cyclic thermal history, with reported cooling rates between 10 2–10 5 K s −1, affecting phase evolution, microstructure formation and ultimately the achievable properties 3.Ī material with a high technical relevance in the context of AM is the (α + β) dual phase titanium alloy Ti-6Al-4V 1, 4. Compared to PBF, higher build rates are achieved and larger components up to several meters in size can be produced. The complete processing head and the work piece are then moved along the tool path to create the geometrically complex part. In a typical DED process, material is fed as powder or wire into the melt pool heated by a laser, arc or electron beam. While PBF is restricted to a flat building plane due to its gravity-compacted powder bed, DED allows for processing comparable to CNC machining with freedom of movement in multiple axes. The beam-based AM technologies for metals can be distinguished into powder-bed fusion (PBF) and directed energy deposition (DED) technologies where material is fed into the melt pool 1, 2. 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.Īdditive manufacturing (AM) of metals is increasingly applied to produce complex parts in aerospace, defence, medical and automotive industries.
Based on lattice strain of the β-phase, the martensite start temperature is estimated at 923 K in these experiments. Lattice strains in the α′-phase are found to be sensitive to the α′ → β phase transformation. Secondary β-formation upon formation of α′ is attributed to V partitioning to the β-phase leading to temporary stabilization. At room temperature, single phase α′ is observed. Upon rapid heating and cooling, the β ↔ α′ phase transformation is observed repeatedly. 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. Current setups are limited by an averaged measurement through the solid and liquid parts. 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.
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.