International Journal of Heat and Mass Transfer 158 (2020) 119989 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/hmt Role of impinging powder particles on melt pool hydrodynamics, thermal behaviour and microstructure in laser-assisted DED process: A particle-scale DEM – CFD – CA approach Akash Aggarwal a , Arvind Chouhan a , Sushil Patel a , D.K. Yadav b , Arvind Kumar a,∗ , A.R. Vinod c , K.G. Prashanth d,e,f , N.P. Gurao b a Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India b Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India c Centre for Additive Manufacturing and Special Manufacturing Processes, Central Manufacturing Technology Institute, Bengaluru 560022, India d Department of Mechanical and Industrial Engineering, Tallinn University of Technology, Tallinn 19086, Estonia e Eric Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A-8700, Leoben, Austria f CBCMT, School of Mechanical Engineering, Vellore Institute of Technology, Vellore – 632014, India a r t i c l e i n f o Article history: Received 29 December 2019 Revised 20 April 2020 Accepted 22 May 2020 Keywords: Laser-assisted directed energy deposition Particles impingement Molten pool hydrodynamics Discrete element method Cellular automata Grain growth a b s t r a c t High speed imaging of molten pool free-surface hydrodynamics in laser-assisted directed energy deposi- tion process clearly revealed a highly oscillatory and dynamic melt flow due to impinging powder parti- cles. Surprisingly, most of the reported computational work exclude the injection of powder particles and rather adopt a homogeneous mass and energy addition approach, and therefore provides less accurate predictions. In this work, we develop a coupled multi-physics particle-scale approach utilizing the dis- crete element method for particle trajectory prediction, the computational fluid dynamics for free-surface thermo-fluidic modelling and the cellular automata method for grain growth evolution. In the model, the governing physical phenomena, such as laser-powder interaction, in-flight particle heating, phase change (melting, vaporization and solidification), free-surface evolution, molten pool hydrodynamics and imping- ing particles-melt interaction have been considered. Experiments for the deposition of Inconel-625 on an Inconel-625 substrate are carried out, and the model predictions are validated with the experimental measurements. For the first time, the predicted thermo-fluidic simulation results reveal highly oscilla- tory, chaotic and random melt flow attributed to the impinging powder particles. During the deposition, it is found that the role of the Marangoni convection is less significant as compared to the momentum imparted by the impinging powder particles in the melt pool. Using the simulated thermal undercool- ing data, cellular automata-based grain growth simulation predicts elongated columnar dendrites in the melt pool that grows epitaxially from the melt pool interface and stretches towards the centre. Using the Kurz-Fisher model, the effect of local thermodynamic solidification conditions on the size of dendritic microstructure is also described. The predicted melt pool geometry, temperature field and grain structure compare well with the experimental measurements. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction The laser-assisted directed energy deposition (L-DED) process is one of the promising metal additive manufacturing processes ca- pable of fabricating net-shaped and near-net-shaped components directly from computer-aided design (CAD) data [1]. The process involves layered fabrication by coaxial feeding of metal powder directly into the molten pool generated by the laser beam. The ∗ Corresponding author. E-mail address: arvindkr@iitk.ac.in (A. Kumar). physics governing the process is complex and the typical physical phenomena controlling the deposition include particle-beam inter- action, substrate-beam interaction, phase transition (melting, va- porization and solidification), free-surface evolution, particle-melt interaction and melt flow [2]. There is a wide range of process pa- rameters which strongly influences the deposition. Some of these parameters are laser beam characteristics (power, traversal speed, spot radius and beam profile), particles morphology (size, shape and distribution), inert gas flow (shielding and carrier), stand-off distance and material properties (absorptivity, fusion point, ther- mal diffusivity) [3]. These parameters need to be optimized and https://doi.org/10.1016/j.ijheatmasstransfer.2020.119989 0017-9310/© 2020 Elsevier Ltd. All rights reserved.