Multi-material structural optimization for additive manufacturing based on a phase field approach

A topology optimization problem in a phase field setting is considered to obtain rigid structures, which are resilient to external forces and constructable with additive manufacturing. Hence, large deformations of overhangs due to gravity shall be avoided during construction. The deformations depend on the stage of the construction and are modelled by linear elasticity equations on growing domains with height-dependent stress tensors and forces. Herewith, possible hardening effects can be included. Analytical results concerning the existence of minimizers and the differentiability of the reduced cost functional are presented in case of a finite number of construction layers. By proving Korn's inequality with a constant independent of the height, it is shown that the cost functional, formulated continuously in height, is well-defined. The problem is numerically solved using a projected gradient type method in function space, for which applicability is shown. Second-order information can be included by adapting the underlying inner product in every iteration. Additional adjustments enhancing the solver's performance, such as a nested procedure and subsystem solver specifcations, are stated. Numerical evidence is provided that for all discretization level and also for any number of construction layers, the iteration numbers stay roughly constant. The benefits of the nested procedure as well as of the inclusion of second order information are illustrated. Furthermore, the choice of weights for the penalization of overhangs is discussed. For various problem settings, results are presented for one or two materials and void in two as well as in three dimensions.