Leung, Chu Lun Alex; (2018) X-ray imaging of powder consolidation during laser additive manufacturing. Doctoral thesis (Ph.D), University of Manchester.

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Abstract

The laser-matter interaction and powder consolidation in laser additive manufacturing (LAM) occur on very short time scales (10-6 - 10-3 s); and they have proven difficult to characterise. A better understanding of the underlying mechanisms during LAM is crucial for prediction and optimisation of part properties. This thesis highlights the development and applications of a LAM process replicator (LAMPR), combined with in operando high-speed synchrotron X-ray imaging and image analysis to study these mechanisms. Using this setup, the sequential powder consolidation phenomena were revealed in LAM of stainless steel (SS316L), a Fe-Ni alloy (Invar36) and bioactive glass (13-93). The consolidation mechanisms of alloy powders are driven by molten pool wetting and vapour-driven powder entrainment. The principal consolidation mechanism of 13-93 bioactive glass is driven by viscous flow. The evolution of porosity and spatter were revealed during LAM of virgin and oxidised Invar 36 powders under different build conditions. The oxide films altered the Marangoni convection from centrifugal to centripetal, restricted the melt flow (and gas transport) in the melt track and promoted pore growth. Several new pore mechanisms were uncovered, including pore migration, dissolution, dispersion, and bursting. The laser-induced gas/vapour jet promoted the formation of melt tracks and denuded zones while ejecting spatter at velocities up to 1 m/s along the argon gas flow and laser scanning directions. In addition, a new spatter mechanism has been discovered; spatter can be formed by laser-driven gas expansion. Laser re-melting of large pre-existing pores can result in two extreme outcomes: (1) pore healing by Marangoni flow and (2) formation of droplet spatter and open pore. These results have clarified the physics behind previous hypotheses and proposed new mechanisms in LAM, which are critical for the development of LAM and simulation models of the process.

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