Mechanical properties of additively manufactured metallic parts are strongly dependent on defects and microstructural characteristics. Since the fabrication process occurs very rapidly in a very small scale, multi-physics and multi-scale modelling can be a strong methodology to understand the relevant phenomena toward optimisation. In this talk, topics will be presented which are related to additive manufacturing (AM) processing both in laser powder bed fusion (L-PBF) and directed energy deposition (DED). They include elemental mixing in in-situ alloying, microstructure evolution during solidification, pore dynamics and its active elimination, microstructure control by nano-particle addition, etc [1,2]. Melting to solidification involves heat transfer, mass transfer, dynamic fluid flow/particle motion and crystallisation, which is a combination of complicated phenomena and the high-fidelity modelling approach is key to identifying the mechanisms along with experimental observations. Relevance to mechanical response will be also discussed, comparing several cases under different process conditions. Finally, future directions will be discussed in terms of modelling which may to lead to enhanced AM quality and reliability. REFERENCES [1] J. Shinjo, A. Kutsukake, A. Arote, Y.T. Tang, D.G. McCartney, R.C. Reed, C. Panwisawas: “Physics-based thermal-chemical-fluid-microstructure modelling of in-situ alloying using additive manufacturing: Composition-microstructure control,” Additive Manufacturing 64 (2023) 103428. https://doi.org/10.1016/j.addma.2023.103428 [2] K. Zhang, Y. Chen, S. Marussi, X. Fan, M. Fitzpatrick, S. Bhagavath, M. Majkut, B. Lukic, K. Jakata, A. Rack, M.A. Jones, J. Shinjo, C. Panwisawas, C.L.A. Leung, P.D. Lee: “Pore evolution mechanisms during directed energy deposition additive manufacturing,” Nature Communications 15 (2024) 1715. https://doi.org/10.1038/s41467-024-45913-9