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In this work, a finite element model is developed for predicting the thermo-mechanical response of Ti-6Al-4V during electron beam deposition. A three-dimensional thermo-elasto-plastic analysis is performed to model distortion and residual stress in the workpiece and experimental in situ temperature, and distortion measurements are performed during the deposition of a single-bead-wide, 16-layer-high wall built for model validation. Post-process blind hole–drilling residual stress measurements are also performed. Both the in situ distortion and post-process residual stress measurements suggest that stress relaxation occurs during the deposition of Ti-6Al-4V. A method of accounting for such stress relaxation in thermo-elasto-plastic simulations is proposed where both stress and plastic strain are reset to 0, when the temperature exceeds a prescribed stress relaxation temperature. Inverse simulation is used to determine the values of the absorption efficiency and the emissivity of electron beam–deposited, wire-fed Ti-6Al-4V, as well as the appropriate stress relaxation temperature.

T h erm o m ech an ical M o d elin g of A dditive M an u factu rin g Large PartsA finite element modeling strategy is developed to allow for the prediction of distortion accumulation in additive manufacturing (AM) large parts (on the order of meters). A 3D thermoelastoplastic analysis is performed using a hybrid quiet inactive element activa tion strategy combined with adaptive coarsening. At the beginning for the simulation, before material deposition commences, elements corresponding to deposition material are removed from the analysis, then elements are introduced in the model layer by layer in a quiet state with material properties rendering them irrelevant. As the moving energy source is applied on the part, elements are switched to active by restoring the actual material properties when the energy source is applied on them. A layer by layer coarsen ing strategy merging elements in lower layers of the build is also implemented such that while elements are added on the top of build, elements are merged below maintaining a low number of degrees of freedom in the model for the entire simulation. The effective ness o f the modeling strategy is demonstrated and experimentally validated on a large electron beam deposited Ti-6Al-4V part consisting of 107 deposition layers. The simula tion and experiment show good agreement with a maximum error of 29%.

Please cite this article as: Erik R. Denlinger, Pan Michaleris, Effect of stress relaxation on distortion in additive manufacturing process modeling, (2016), http://dx. AbstractA method for modeling the effect of stress relaxation at high temperatures during Laser Direct Energy Deposition processes is experimentally validated for Ti-6Al-4V samples subject to different inter-layer dwell times. The predicted mechanical responses are compared to those of Inconel R 625 samples, which experience no allotropic phase transformation, deposited under identical process conditions. The thermal response of workpieces in additive manufacturing is known to be strongly dependent on dwell time. In this work the dwell times used vary from 0 to 40 s. Based on past research on ferretic steels and the additive manufacturing of titanium alloys it is assumed that the effect of transformation strain in Ti6Al-4V acts to oppose all other strain components, effectively eliminating all residual stress at temperatures above 690 • C. The model predicts that Inconel R 625 exhibits increasing distortion with decreasing dwell times but that Ti-6Al-4V displays the opposite behavior, with distortion dramatically decreasing with lowering dwell time. These predictions are accurate when compared *

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