In Situ Recrystallization and Phase Transformations in Laser Processing of Metals and Alloys : Microstructure Design and Monitoring
Laser Powder Bed Fusion (LPBF) is a method of Additive Manufacturing extensively used for metal 3D printing. It utilises a laser to selectively melt metallic powder particles based on a Computer Aided Design (CAD) model. This technique enables the construction of parts with intricate geometries and fine precision, as well as the formation of microstructures that differ from conventional processing. These unique microstructures can lead to enhanced mechanical properties, such as increased dislocation density, better strength-ductility trade-off, and improved formability. However, controlling and attaining the desired microstructure is challenging due to the rapid heating and cooling rates inherent to the process.
In this thesis, a way of tuning and monitoring LPBF microstructures through different types of diffusive transformations was investigated in crystalline and amorphous alloys, specifically 316L austenitic stainless steel and Zr-based bulk metallic glasses (BMGs). In crystalline alloys such as 316L, increased stored energy from dislocations and stresses tends to favour recrystallisation upon further heat treatments. This diffusive transformation may be desired to randomise crystallographic texture and reduce anisotropy of mechanical properties. However, the sluggish recrystallisation observed in LPBF 316L introduces the need for a deep understanding of its kinetics.
Accordingly, an online monitoring system able to identify microstructural events was developed through in situ synchrotron X-Ray Diffraction (XRD). Simultaneously, acoustic sensors acquired corresponding signals so that the acoustic footprint of these critical events could be identified and used as an effective monitoring method. The kinetics of fast recrystallisation during operando laser heat treatments on a highly deformed sample was followed by examining XRD peak narrowing over time and correlated to different stages of microstructure transformation.
To promote recrystallisation in AM 316L, the chemical composition was modified by adding aluminium (Al), known to influence dislocation pinning by heavy elemental segregation at cellular walls. The impact of this Al addition was twofold: first, the duplex nature of this newly designed alloy allowed exceptional mechanical properties, effectively competing with high-strength steels; second, it enabled the transition from columnar textured grains to a refined and more equiaxed microstructure. It was concluded that the new 316L steel/Al system was suitable for fabricating composite-like microstructures.
BMGs are metallic alloys with an atomic structure lacking long-range order, allowing them to solidify in an amorphous state. LPBF is increasingly used for manufacturing these alloys due to its ability to achieve rapid cooling rates, which can prevent crystallisation and result in exceptional properties associated with the glassy state. However, the precise conditions and mechanisms linked to crystal formation have not been fully elucidated yet and exhibit some randomness. Similar to recrystallisation in 316L, monitoring methods are needed to identify BMG crystallisation during printing and assist processing map construction. Hence, acoustic signals recorded near the processing zone were analysed to identify statistical crystal formation in Zr-BMGs. Compared to 316L, signals were significantly more discontinuous. These discontinuities were interpreted as rapid changes in stress state, with some promoted by crystallization.
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