| Abstract Scope |
Dissimilar metals joining opens many opportunities in a variety of fields. For example, light-weighting of vehicles by joining aluminum to steel or heat regulation in power plants by joining refractory metals to steel. These dissimilar joints expand the available materials used for a given application, opening avenues previously not considered. However, these joints also possess a fundamental problem of deleterious phase formation at the material interface. Materials may be immiscible in each other or upon alloying the phase formed may result in increased brittleness at the joint. This is often what prevents the use of dissimilar materials even when their properties together provide advantages over a single material. However, if the composition of the joint is precisely controlled, it is possible to avoid problematic phases.
This work proposes a method to avoid deleterious brittle phase formation at the interface by employing path planning techniques to circumvent those regions in multidimensional phase space. To achieve this end, CALPHAD-based thermodynamic tools have been used to generate isothermal phase diagrams that are in turn utilized to plan gradient paths that completely avoid these phases. While similar techniques have been utilized previously it has mainly focused on 3 dimensions or less, as it was dependent on human ability to visualize the path. This work employed machine learning to create functionally graded materials. To validate the functionally graded material’s planned paths additive manufacturing was employed. Additive manufacturing enables joints between materials to be compositionally controlled. In this study, a multi-material laser-based direct energy deposition (DED)-based 3D printer was employed allowing for layer-by-layer control of composition. Using a Laser Engineered Net Shaping (LENS) DED system with four powder hopper capabilities, the designed functionally grated materials were created and compositionally and microstructurally evaluated to determine the effectiveness of the design method in eliminating detrimental phases. The validation experiment demonstrated the technique on a path starting at 316L stainless steel and moving to commercially pure chromium, successfully avoiding CrNi2 and sigma phase formation. This planned path was shown to decrease sigma phase formation by over 90% from the simple linear path. The following work translated the 3-dimensional path evaluated in the validation experiment to four-dimensional space. This new path navigated from Fe9Cr to commercially pure W. This system had numerous brittle phases and a significantly more complex compositional path. Again, it was shown that it is possible to print a functionally graded material in 4-dimensional space using the CALPHAD-based path planning tool. Overall, it was shown not only is it possible to path plan around deleterious phases, but it is possible to create parts that follow the planned path effectively avoiding brittle phases. |