  Inverse design of high-resolution and fine-detailed 3D lightweight mechanical
structures is notoriously expensive due to the need for vast computational
resources and the use of very fine-scaled complex meshes. Furthermore, in
designing for additive manufacturing, infill is often neglected as a component
of the optimized structure. In this paper, both concerns are addressed using a
de-homogenization topology optimization procedure on complex engineering
structures discretized by 3D unstructured hexahedrals.
  Using a rectangular-hole microstructure (reminiscent to the stiffness optimal
orthogonal rank-3 multi-scale) as a base material for the multi-scale
optimization, a coarse-scale optimized geometry can be obtained using
homogenization-based topology optimization. Due to the microstructure
periodicity, this coarse-scale geometry can be up-sampled to a fine physical
geometry with optimized infill, with minor loss in structural performance and
at a fraction of the cost of a fine-scale solution. The upsampling on 3D
unstructured grids is achieved through stream surface tracing which aligns with
the optimized local orientation. The periodicity of the physical geometry can
be tuned, such that the material serves as a structural component and also as
an efficient infill for additive manufacturing designs.
  The method is demonstrated through three examples. It achieves comparable
structural performance to state-of-the-art methods but stands out for its
significant computational time reduction, much faster than the base-line
method. By allowing multiple active layers, the mapped solution becomes more
mechanically stable, leading to an increased critical buckling load factor
without additional computational expense. The proposed approach achieves
promising results, benchmarking against large-scale SIMP models demonstrates
computational efficiency improvements of up to 250 times.

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