Development of additive manufacturing processes for multi-material metal components

This thesis reports on all applications developed and published in the field of multi-material additive manufacturing with metal powders. The topics covered include several widely used technologies in the field of Additive Manufacturing (AM), such as Direct Energy Deposition (DED) and Laser Powder Bed Fusion (LPBF). The knowledge available in the literature on these categories of processes has served as a basis for the development of innovative studies and, in particular, for the optimisation, production and characterisation of artefacts consisting of metallic materials of different natures used simultaneously in the same process. The text of the thesis has been divided into the following application categories: 1) Experiments using the LPBF process a) Case studies where the transition between different materials is immediate (clear); b) Case studies where the transition between different materials in gradual (cFGM). 2) Case studies using the DED process. The most rigorous scientific methods available in the literature and the most advanced investigation techniques in powder metallurgy and laser processes were used in all phases of the experiments. In fact, it is possible to list the instruments used by subdividing them as shown below: In the design phase of the experiment, various tools and methods are used. Bibliographic research is the first step to gather relevant information, followed by the use of statistical methods for Design of Experiments (DOE). Additionally, numerical simulations are conducted to model and predict the behavior of the system. During the setup phase, the functionality and calibration of the equipment and instruments are checked to ensure they are ready for the experiment. Then, the necessary process parameters are set to perform the tests. In the execution phase of the experiment, in-situ monitoring methods are used to control key process quantities (as geometrical features) and parameters. These include optical and thermal techniques, both on-axis and off-axis, to ensure that the process proceeds under the established conditions. Next comes the process analysis phase, where techniques such as image analysis and control charts are used to evaluate the performance and stability of the process. The characterization of specimens involves both non-destructive testing and more detailed analyses using Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD), and X-ray Diffraction (XRD). In addition to these analyses, metallographic and mechanical characterizations are performed to determine the physical properties of the samples. Finally, in the conclusion phase of the experiment, the results are analyzed to establish a correlation between the inputs and outputs using Analysis of Variance (ANOVA). Moreover, the numerical models developed during the design phase are validated. The experiments carried out demonstrated that multi-material components produced by additive manufacturing processes are a real option, offering significant advantages in providing the optimum properties where they are required. Ad hoc fixtures were designed and built to produce test specimens on machines not designed for this purpose. In-process monitoring methods have provided important information for process optimisation, especially when analysed with appropriate statistical and image analysis tools.