Vol.:(0123456789) 1 3 Applied Physics A (2020) 126:833 https://doi.org/10.1007/s00339-020-04013-3 A computational study of porosity formation mechanism, fow characteristics and solidifcation microstructure in the L‑DED process Arvind Chouhan 1  · Akash Aggarwal 1  · Arvind Kumar 1 Received: 6 August 2020 / Accepted: 21 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract Laser-assisted directed energy deposition is an additive manufacturing process used to manufacture metallic parts. The gas porosity is one of the prominent deposition defects in the processed parts. This infuences the mechanical properties which can cause the part failure. In this work, the mechanism of gas porosity formation at low energy density is addressed using computational modeling. An investigation is carried out to capture the powder particles interaction with the melt pool and resulting porosity formation, molten pool hydrodynamics, and solidifcation microstructure in the L-DED process. The numerical results reveal that the stagnant zone in the melt pool leads to entrapment of bubbles which eventually forms poros- ity. This bubble entrapment phenomenon is studied by varying the powder mass fow rate, and it is found that increasing the mass fow rate results in rapid bubble formation which increases the chances of gas porosity formation. The temperature gradient and cooling rates are used for solidifcation analysis and prediction of as-solidifed grain morphology. Using the empirical relation, the efect of local thermodynamic solidifcation conditions on the size of the dendritic microstructure is analyzed. The predicted melt pool geometry and porosity morphology agree with the experimental results. Keywords Computation · Defects · Lasers · Melting · Microstructure · Modeling · Molten · Porosity · Solidifcation 1 Introduction Laser-assisted directed energy deposition (L-DED) pro- cess is one of the promising methods for metal additive manufacturing (AM), which is extensively used for fab- ricating net-shaped and near-net-shaped parts. Using this technique, extremely complex parts of high-performance materials can be manufactured directly from the CAD data. In the L-DED process, the metal powder is fed coaxi- ally or through a set of radially symmetric nozzles in the molten pool generated by laser energy. This deposition is done in a layered approach to fabricate a 3D part. The process has a huge potential in the aerospace and avia- tion industry for fabricating functional parts of titanium and nickel-based alloys [1, 2], porous surface structures in biomedical implants [3], functionally graded materials [4], in maintenance, repair, and overhaul technology [5, 6], and in the production of steel components for petrochemical industries [7, 8]. However, porosity formation is one of the major bottlenecks inhibiting the usage of the L-DED fabricated parts. In the powder bed fusion-based processes (e.g., selective laser melting), the gas porosity mainly forms due to the collapse of keyholes with the applica- tion of high energy density lasers [9], but in the L-DED process, there are several means for gases to get entrapped. During processing, a shielding gas such as argon is used as a powder carrier gas as well as to prevent the oxidation of the molten metal. Ng et al. [5] carried out a comprehensive experimental analysis of porosity formation in the L-DED process and found that the porosity originates from the inert gas trapped inside the melt pool. They found that the powder feed rate is an important parameter causing porosity formation in the deposited parts. The powder stream can trap the shielding gas resulting in the entrap- ment of gas bubbles inside the melt pool. Several other experimental and numerical studies [10] have been car- ried out by researchers, but the underlying mechanism of porosity formation is still poorly known. Though post- build characterization of the L-DED fabricated sample can provide information regarding the mechanical properties, microstructure, and porosity morphology (shape, size, * Arvind Kumar arvindkr@iitk.ac.in 1 Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India