Numerical modelling and metallurgical characterization of Cr-Mo steels processed by directed energy deposition




Cooke, Shaun

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Additive manufacturing (AM) provides unique opportunities to push the boundaries of material properties and free form fabrication. However with this novel manufacturing technique a number of defects not commonly found in conventional processes such as machining or casting can arise. Both experimental and numerical studies can help better understand the printed material on a more fundamental level in order to optimize the process and mitigate these defects. Electron microscopy can provide essential information about the as-built microstructure and characteristic defects while numerical modelling can help determine a correlation between process parameters and the resulting properties. First, an initial investigation of directed energy deposition (DED) processed 4140 steel was conducted using various microscopy methods to better understand the defects and microstructure of the printed alloy. A martensite dominate microstructure within a bainitic matrix with increasing degrees of tempering further down the build was revealed. Additional sample preparation was conducted with a focused ion beam and analyzed with the transmission electron microscope to investigate features such as grain boundaries, mechanical twins and interplanar spacing. This interplanar spacing was measured for a number of different diffraction images and compared with the theoretical values. The deviation between the measured and theoretical values can be attributed to defects such as residual stress which causes lattice strain and consequently a smaller or larger spacing between atomic planes. Lastly, diffraction images were characterized and compared with the literature to determine the Miller indices and the specific zone axis orientations. A thermo-mechanical-metallurgical finite element model for 42CrMo4 steel was then developed in ABAQUS to identify the correlation between processing parameters and resulting properties by predicting the temperature history, and resulting residual stresses and metallurgical phase fractions for the DED process. A pre-processing framework was implemented in order to allow the modelling of complex geometries and laser trajectories while experiments were conducted to validate the fidelity of the model. Four separate cases were fabricated with varying processing parameters and geometries. In addition to in-situ temperature measurements, post-build residual stress and substrate distortion data was also collected. Furthermore, metallurgical analysis was performed for each case and compared with the simulated phase fractions. The accuracy of the distortion profile increased with increasing dwell time while the accuracy in predicting the metallurgical phase fractions and residual stresses demonstrated the opposite trend.



Additive Manufacturing, Directed Energy Deposition, Numerical Modelling, Metallurgy, Steel