Towards an integrated experimental and computational framework for large-scale metal additive manufacturing

Using the Metal Big Area Additive Manufacturing (MBAAM) system, a thin steel wall was manufactured from a low carbon steel wire. The wall was then characterized comprehensively by high-throughput high-energy X-ray diffraction (HEXRD), electron backscatter diffraction (EBSD), and in-situ HEXRD tensile tests. With the predicted temperature histories from the finite element-based additive manufacturing process simulations, the correlations between processing parameters, microstructure, and properties were established. The correlation between the final microstructure with the predicted temperature history is well explained with the material's continuous cooling transformation (CCT) diagram calculated based on the composition of the low carbon steel wire. The final microstructure is dependent on the cooling rate during austenite to ferrite/bainite transformation during initial cooling and the subsequent reheating cycles. Fast cooling rate resulted in small ferrite grain size and fine bainite structure at the location closest to the base plate. Slower cooling rate at the side wall location and repeated reheating cycles to the ferrite-pearlite regions resulted in all allotriomorphic (equiaxed) ferrite with medium grain size with small amount of pearlite. With no reheating cycles, the top location has the slowest cooling rate and a large grained allotriomorphic ferrite and bainitic structures. The measured mechanical strength is then related to the microstructural feature size (grain or lath size) observed in those locations. A good correlation is found between the mechanical properties, microstructure features and the temperature history at various locations of the printed wall.