Simulation of additive manufacturing using a mechanism based plasticity model

This thesis presents finite element (FE) simulations of additive manufacturing (AM) and physically based material modeling of alloy 625 and alloy 718. In recent years, there has been an increasing interest in AM and there has been a dramatic increase in publications in the field. AM can be beneficial compared to conventional manufacturing methods in many applications. The method offers short component lead times and large design freedom with the possibility to create complex components. Alloy 625 and alloy 718 are nickel-based superalloys used in high-temperature applications owing to their high-temperature strength. The materials are difficult to manufacture by conventional machining due to rapid tool wear and low material removal rates. Thus, the alloys are appropriate for the AM technology with its near-net shape potential.Owing to the rapid heating and solidification in the AM process, residual stresses are induced in the component. This is a well-known problem and causes distortion of the samples when removing them from the build plate. The residual stresses may also deteriorate the fatigue properties. It is important for the manufacturer to understand how the choice of process parameters and scanning strategy affect the residual stresses to minimize those and improve the quality of the components. Simulation can be used as a tool while developing the process parameters and support the experimental efforts. FEM is generally the preferred method for simulation of deformations and residual stresses in AM. The simulation technique used when modeling AM has its origin from welding simulations that was performed already since the beginning of 1970. However, it is not possible in practice to simulate an AM process in the traditional way due to a large number of elements and time increments to be calculated. This is especially true for the laser-based powder bed fusion (PBF-LB) process where the process of a full-scale part may comprise many thousands of added layers, and the passes are lengthy relative to their thicknesses and widths.The aim of this thesis work is to develop FE simulation techniques that reduce the computational effort when modeling residual stresses in AM processes to enable simu-lation of full-scale parts. This has been done with thermo-mechanical FE-models using different lumping techniques e.g., lumping of layers and lumping of hatches. Lumping of layers and hatches means that several physical layers, or several physical hatches, are merged and added in one modeled layer or hatch respectively. Lumping allows fewer time steps and a coarser mesh which reduces the computational effort. An existing mechanism based flow stress model has been developed to fit the mechanisms typical for alloy 625 and alloy 718 and implemented in the FE model. Also, synchrotron X-ray diffraction was performed to measure the residual stress for comparison with the models. The stress was extracted from the diffraction data using the full Debye ring fitting method.In this work, using the lumping techniques described above, it was possible to simu-late AM processes with up to physical 1500 layers. For different process parameter sets and scan strategies, thermal behavior, deformation and residual stresses have been mod-eled and compared with experiments. Using the lumping of layer technique resulted in modeled residual stresses showing the same trend as measured stresses from synchrotron X-ray diffraction for two different process parameter sets. Utilizing lumping of hatches, the resulting deflection in a part was modeled successfully for different scanning strate-gies. In the modeling, the larger deflection was seen for the samples printed with the scanning direction parallel to the long-side which was also shown experimentally.The results in this work shows that the presented lumping approaches are promising when it comes to modeling of the deformations and residual stresses in AM. Using lumping approaches, it is also possible to simulate different scanning strategies for processes of larger parts. The description of the mechanical behavior of the material is improved, using the mechanism based material model, compared to when the flow stress was modeled with tabulated data, since it takes mechanisms as viscoplasticity and stress relaxation into account. The mechanism based model includes microstructural information as grain size and solutes and can thus more easily be combined with a microstructure model. The combination of the mechanism based material model and the use of lumping techniques is thus an advance in the development of predictive models of the AM process