Book Description

Common failure modes to Big Area Additive Manufacturing (BAAM) are the phenomenon of slumping and excessive distortion. Slumping or sagging usually occurs when the printed structure retains excessive heat. This phenomenon is commonly seen when the build has insufficient cooling between layers and, therefore, inadequate mechanical strength due to the high-temperature material properties to support the layers above. Distortion is the planar deviation from the desired geometry. Significant residual stresses typically distort BAAM builds. Stresses often occur due to the thermal cycling and large temperature gradients found in additively manufactured parts. This study developed a transient thermal and structural simulation model to predict the slumping phenomenon and distortion, specifically applicable to overhanging features. A pyramidal model was crafted in Ansys Workbench software to simulate a large layer overhang to investigate the necessary slumping conditions. The pyramid was designed to have 53 layers and utilized symmetry to reduce the pyramid to one-quarter of the overall size and was modeled using standard ABS material. The simulation model matches the dimensions in the experimental pyramid, which had bead dimensions of 12.5 mm wide with a thickness of 5 mm. The overall structure size was 1.06 m by 0.77 m by 0.43 m. Each layer in the model independently allows for element birth/death commands and individual layer mesh parameters. The built-in element birth/death commands enable the layers to activate and progress the same way as the experimental build. As each new layer is activated, a temperature input of 200°C is applied then turned off just as the next layer is activated. The feedstock material selected for this study is Acrylonitrile Butadiene Styrene (ABS), which was selected based on the physical properties and the availability of the temperature-dependent material properties. The availability of these temperature-dependent material properties is essential to consider during simulation since the thermo-physical properties will significantly impact the accuracy of both the thermal and structural models. The transient thermal finite element analysis (FEA) simulation allows for customization of dwell times, allowing designers to determine when the threat of slumping is no longer predicted. The transient thermal FEA analysis reported in this work showed consistent results with the numerical method for layer temperature. This agreement shows the assumptions made in both evaluations are valid and can be used in other studies to compare against experimental builds for other materials and geometries. The experimentally measured part was also in excellent agreement with the temperature profile results from the thermal model simulation. The experiment and simulation compared results agreed within 5 %. Additionally, the results collected in this work show an exponential temperature distribution of the build as it cools. Exponential cooling was expected as it is often seen with similar materials and offers increased stability in the printed part since the material will solidify quickly and reach low-temperature material properties soon after. It was also observed that as the print height increased, more of the immediate layers underneath had increased temperatures. The increase in temperature is due to the reduced layer print times as the height increases, reducing the overall cooling time. A mechanical structural analysis determined the deflection of each layer as the build progressed. The in-plane deflection for both the simulation and experimental build was in good agreement, and both showed more significant deflection in the areas of high heat retention. The increase in the height of the model also increased the number of layers above the glass transition temperature. This observation was consistent with the experimental and thermal simulations results and is likely the cause for slumping. The structural simulation was used to determine the local distortion created during a BAAM build. The simulation was built off the thermal simulation coupling the two simulations. The model was set up to recreate the environment and printing process of the experimentally produced pyramid. The model used ABS temperature-dependent material properties reported in literature, gravity, and the time-history thermal profile as the inputs. The thermal-history results from the thermal simulation were coupled with the structural simulation to effectively relate the temperature profile to distortion estimated by the structural simulation. The simulation results and experimental build measured distortions were in good agreement. Additionally, the stress profile of the model indicates that slumping is caused by geometry, heat retention, and material properties.