Modified Inherent Strain Method for Predicting Residual Deformation in Metal Additive Manufacturing

Additive manufacturing (AM) of metal components has seen rapid development in the past decade, since arbitrarily complex geometries can be manufactured by this technology. Due to intensive heat input in the laser-assisted AM process, large thermal strain is induced and hence results in significant residual stress and deformation in the metal components. To achieve efficient simulation for metal printing process, the inherent strain method (ISM) is ideal for this purpose, but has not been well developed for metal AM parts yet. In this dissertation, a modified inherent strain method (MISM) is proposed to estimate the inherent strains from detailed process simulation. The obtained inherent strains are employed in a layer-by-layer static equilibrium analysis to simulate residual distortion of the AM part efficiently. To validate the proposed method, single-walled builds deposited by directed energy deposition (DED) process are studied first. The MISM is demonstrated to be accurate by comparing with full-scale detailed process simulation and experimental results. Meanwhile, the MISM is adapted to powder bed fusion (PBF) process to enable efficient yet accurate prediction for residual stress and deformation of large components. The proposed method allows for calculation of inherent strains accurately based on a small-scale simulation of a small representative volume. The extracted mean inherent strains are applied to a part-scale model layer-by-layer to simulate accumulation of residual deformation. Accuracy of the proposed method for large components is validated by comparison with experimental results, while excellent computational efficiency is also shown. As further applications, the MISM is extended to deal with efficient simulation for residual deformation of thin-walled lattice support structures with different volume densities. To achieve this goal, asymptotic homogenization is employed to obtain the homogenized inherent strains and elastic modulus given the specific laser scanning strategy and process parameters for fabricating those thin-walled lattice support structures. Accuracy of the homogenization-based layer-by-layer simulation have been validated by experiments. Moreover, the enhanced layer lumping method (ELLM) is developed to further accelerate the layer-by-layer simulation to the largest extent for metal builds produced by PBF. By using tuned material property models, good accuracy can be ensured while directly lumping the equivalent layers in the layer-wise simulation

Competition-Induced Sign Reversal of Casimir-Lifshitz Torque: An Investigation on Topological Node-Line Semimetal

The dispersion of quasiparticles in topological node-line semimetals is significantly different in different directions. In a certain direction, the quasiparticles behave like relativistic particles with constant velocity. In other directions, they act as two-dimensional electron gas. The competition between relativistic and nonrelativistic dispersions can induce a sign reversal of Casimir-Lifshitz torque. Three different approaches can be applied to generate this sign reversal, i.e., tuning the anisotropic parameter or chemical potential in node-line semimetal, changing the distance between this material and substrate birefringence. Detailed calculations are illustrated for the system with topological node-line semimetal Ca$_3$P$_2$ and liquid crystal material 4-cyano-4-n-pentylcyclohexane-phenyl.Comment: 7 pages, 5 figure

Anisotropic Magneto-conductance of InAs Nanowire: Angle Dependent Suppression of 1D Weak Localization

The magneto-conductance of an InAs nanowire is investigated with respect to the relative orientation between external magnetic field and the nanowire axis. It is found that both the perpendicular and the parallel magnetic fields induce a positive magneto-conductance. Yet the parallel magnetic field induced longitudinal magneto-conductance has a smaller magnitude. This anisotropic magneto-transport phenomenon is studied as a function of temperature, magnetic field strength and at an arbitrary angle between the magnetic field and the nanowire. We show that the observed effect is in quantitative agreement with the suppression of one-dimensional (1D) weak localization