Modeling and analysis of the wire arc additive manufacturing process of Ti-6Al-4V alloy

Heat energy is used to melt materials during Wire Arc Additive Manufacturing (WAAM). However, the applied heat leads to non-uniform, repeated thermal variations during the cyclic formation of layers which can cause changes in the microstructure of metallic materials throughout the process. Besides, high levels of undesirable residual stresses and deformations may occur on parts fabricated with WAAM depending on the thermal loading to which they are exposed. Therefore, studies to be carried out for understanding the physical interactions occurring during WAAM processes and ensuring the process control is of great importance. Within the scope of this thesis prepared with this motivation, studies have been carried out on the development and experimental verification of thermal-microstructural-mechanical (TMM) finite element method (FEM) simulation models that enable understanding of the process-structure-property relationships in parts to be produced from Ti-6Al-4V alloy by the TIG-WAAM method, taking into account the latest approaches, observations, and findings in the literature. In this context, firstly, an in-house laboratory-scale TIG-WAAM device to be used in the development and validation of process simulation models was designed and manufactured considering mechanical, electronic, and software aspects. Using this device, samples are produced with different heat inputs, layer numbers and scanning patterns, and considering their dimensions and boundary conditions, TMM finite element simulation models are developed. The ABAQUS finite element software is used to develop the simulation models, and with the help of Python macros and Fortran user subroutines, a finite element model is created where various model components (boundary conditions, element activation definitions, moving heat source model, microstructural and mechanical properties of the material, etc.) could be defined in a flexible and parametric structure. In the thermal model, heat transfer through conduction, convection, and radiation is considered, and a modified Goldak heat source model is used to define the TIG welding heat source. For predicting the microstructural transformations occurring due to temperature variations during the WAAM process, a microstructurel model that accounts for both diffusional and diffusionless solid-state phase transformations is considered which allows for the investigation of the changes in the α and β phase fractions throughout the manufacturing process. Additionally, by considering the calculated fractions and microhardness values of the phases, the change in the microhardness of the parts is calculated using the rule of mixtures. The effect of thermal variations on the mechanical behavior of the material is considered using a fully coupled modeling technique. Additionally, the effect of microstructural changes on the material's yield behavior is included into the simulation models using the Johnson-Cook material model and the rule of mixtures that considers the calculated volume fractions and yield values of the phases. Validation studies are performed firstly on single-layer wall samples produced with two different heat inputs. In this context, experimentally measured temperature values over time at two different points using thermocouples, dimensions of the melt pool and heat-affected zones and hardness values measured from the cross-sections of the samples, and displacement measurements measured from the clamped base plate are compared with the values calculated using the finite element analyses. Subsequently, similar validation studies are performed on five-layer wall samples produced with unidirectional and zigzag scanning patterns to demonstrate that the model can also produce accurate results for the multi-layer structures. The validation studies show that, the temperature values, melting pool dimensions, hardness values, and displacement results calculated using the developed simulation models are in close agreement with the experimental data.