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Instruments / Technical Area	Publication Details	Published	Actions
	Pore evolution mechanisms during directed energy deposition additive manufacturing Kai Zhang , Yunhui Chen , Sebastian Marussi , Xianqiang Fan , Maureen Fitzpatrick , Shishira Bhagavath , Marta Majkut , Bratislav Lukic , Kudakwashe Jakata , Alexander Rack , Martyn A. Jones , Junji Shinjo , Chinnapat Panwisawas , Chu Lun Alex Leung , Peter D. Lee Nature Communications 15 DOI: 10.1038/s41467-024-45913-9 Open Access Show Abstract ▼ Abstract: Porosity in directed energy deposition (DED) deteriorates mechanical performances of components, limiting safety-critical applications. However, how pores arise and evolve in DED remains unclear. Here, we reveal pore evolution mechanisms during DED using in situ X-ray imaging and multi-physics modelling. We quantify five mechanisms contributing to pore formation, migration, pushing, growth, removal and entrapment: (i) bubbles from gas atomised powder enter the melt pool, and then migrate circularly or laterally; (ii) small bubbles can escape from the pool surface, or coalesce into larger bubbles, or be entrapped by solidification fronts; (iii) larger coalesced bubbles can remain in the pool for long periods, pushed by the solid/liquid interface; (iv) Marangoni surface shear flow overcomes buoyancy, keeping larger bubbles from popping out; and (v) once large bubbles reach critical sizes they escape from the pool surface or are trapped in DED tracks. These mechanisms can guide the development of pore minimisation strategies.	Feb 2024
I12-JEEP: Joint Engineering, Environmental and Processing	Unravelling dynamic recrystallisation in a microalloyed steel during rapid high temperature deformation using synchrotron X-rays Kai Zhang , Tim Wigger , Rosa Pineda , Simon A. Hunt , Ben Thomas , Thomas Kwok , David Dye , Gorka Plata , Jokin Lozares , Inaki Hurtado , Stefan Michalik , Michael Preuss , Peter D. Lee , Mohammed A. Azeem Acta Materialia 10 DOI: 10.1016/j.actamat.2024.120265 Diamond Proposal Number(s): [23749] Show Abstract ▼ Abstract: Microstructure evolution during high-strain rate and high-temperature thermo-mechanical processing of a 44MnSiV6 microalloyed steel is investigated using in situ synchrotron high-energy powder X-ray diffraction. The conditions selected replicate a newly developed near solidus high-strain rate process designed for reducing raw material use during the hot processing of steels. High temperatures (exceeding 1300 °C) and high strain rate  = 9 s-1 processing regimes are explored. The lattice strains and dislocation activity estimated from diffraction observations reveal that the microstructure evolution is primarily driven by dynamic recrystallisation. A steady-state stress regime is observed during deformation, which develops due to intermittent and competing work hardening and recovery processes. The texture evolution during the heating, tension, shear deformation and cooling stages is systematically investigated. The direct observation of phase evolution at high-temperature and high-strain rate deformation enables a comprehensive understanding of new manufacturing processes and provides deep insights for the development of constitutive models for face-centred cubic alloys.	Aug 2024

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