Abstract
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.
Highlights
Additive manufacturing (AM) of metals is increasingly applied to produce complex parts in aerospace, defence, medical and automotive industries
We report in situ probing of the phase evolution in cyclically rapidly laser heated and cooled Ti-6Al-4V mimicking real AM directed energy deposition (DED) conditions by combined in situ synchrotron micro-focused X-ray diffraction and high-speed imaging at the Swiss Light Source (SLS)
Ar shielding gas is provided by a laminar flow nozzle in a radius of 15 mm around the specimen
Summary
Additive manufacturing (AM) of metals is increasingly applied to produce complex parts in aerospace, defence, medical and automotive industries. The layer-wise fashion of DED imposes a cyclic thermal history, with reported cooling rates between 102–105 K s−1, affecting phase evolution, microstructure formation and the achievable properties[3]. In situ synchrotron X-ray diffraction and radiographic imaging is increasingly applied to reveal the non-equilibrium nature and dynamics of AM processing[16,17,18,19,20,21]. The most advanced setup reported so far combines in situ high-speed synchrotron X-ray radiography and diffraction to reveal the dynamics of melt pool formation in PBF of Ti-6Al-4V19. We report in situ probing of the phase evolution in cyclically rapidly laser heated and cooled Ti-6Al-4V mimicking real AM DED conditions by combined in situ synchrotron micro-focused X-ray diffraction and high-speed imaging at the Swiss Light Source (SLS). The experimental setup and data analysis procedures developed in this work will provide high-value additional data on the fundamental understanding of the AM process not available from currently deployed setups
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