Abstract
Directed Energy Deposition (DED) Additive Manufacturing process for metallic parts are becoming increasingly popular and widely accepted due to their potential of fabricating parts of large dimensions. The complex thermal cycles obtained due to the process physics results in accumulation of residual stress and distortion. However, to accurately model metal deposition heat transfer for large parts, numerical model leads to impractical computation time. In this work, a 3D transient finite element model with Quiet/Active element activation is developed for modeling metal deposition heat transfer analysis of DED process. To accurately model moving heat source, Goldak’s double ellipsoid model is implemented with small enough simulation time increment such that laser moves a distance of its radius over the course of each increment. Considering thin build-wall of Stainless Steel 316L fabricated with different process parameters, numerical results obtained with COMSOL 5.6 Multi-Physics software are successfully validated with experiment temperature data recorded at the substrate during the fabrication of 20 layers. To reduce the computation time, elongated ellipsoid heat input model that averages the heat source over its entire path is implemented. It has been found that by taking such large time increments, numerical model gives inaccurate results. Therefore, the track is divided into several sub-tracks, each of which is applied in one simulation increment. In this work, an investigation is done to find out the correct simulation time increment or sub-track size that leads to reduction in computation time (5–10 times) but still yields sufficiently accurate results (below 10% of relative error on temperature). Also, a Correction factor is introduced that further reduces computation error of elongated heat source. Finally, a new correlation is also established in finding out the correct time increment size and correction factor value to reduce the computation time yielding accurate results.
Highlights
Directed Energy Deposition (DED) is an additive manufacturing process in which focused thermal energy is used to fuse materials by melting as they are being deposited
Finite Element Method (FEM) techniques to model LDED process is inspired by prior research done on multi-pass weld modeling (Brickstad and Josefson, 1998; Lindgren et al, 1999; Deng and Murakawa, 2006; Bézi and Szávai, 2014; Bonnaud and Gunnars, 2015), because welding process, that has been studied in depth is quite similar to LDED process (Lindgren, 2001a; Lindgren, 2001b; Lindgren, 2001c)
The proposed model significantly reduces the problem of temporal discretization originated due to moving heat source and leads to lesser computation time (Irwin and Michaleris, 2016)
Summary
Directed Energy Deposition (DED) is an additive manufacturing process in which focused thermal energy is used to fuse materials by melting as they are being deposited. An Elongated Ellipsoid Heat Source Model is developed and demonstrated to reduce the computation time for Selective Laser Melting (SLM) process (Irwin and Michaleris, 2016). This work serves to validate a strategy by introducing efficient material activation by utilising Elongated Ellipsoid Heat Source model in order to reduce the computation time of FEM for LDED process. To reduce the computation time, fixtures clamp is not included, for these surfaces, a higher heat loss is modelled through conductive to convective equivalent heat loss expression similar to Newton’s law TABLE 6 | Cases examined for Thermal Model Validation (Double Ellipsoid). The thermal conductance at the contact surfaces is h 150 (W.m−2.K−1) to model the conductive heat transfer from substrate to the metallic fixture clamps as suggested in (Lu et al, 2019)
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