Rehme, O: Cellular Design for Laser Freeform Fabrication

the implant and surrounding tissue. Lattice structures obtained from the SLM process can meet these requirements. This thesis contributes to the understanding of the mechanical properties of the new material class of SLM lattice structures. Their future incorporation in engineering designs requires a profound knowledge of failure mechanisms and operational limits. Therefore, a comprehensive summary is given on the state-of-the-art of cellular materials followed by a dedicated analysis on Laser Freeform Fabrication and an in-depth validation of the Selective Laser Melting capabilities. Readers with advanced knowledge on cellular materials or Laser Freeform Fabrication may skip sections 2 or 3, respectively. Next, all process constraints and boundary conditions for the manufacture of SLM lattice structures are elaborated. Then a bilateral approach was chosen to derive scaling laws and optimize the SLM lattice structures for given tasks. Firstly, a theoretical analysis comprises the examination of structural hypotheses for isotropic cellular materials before a generalized theory is developed for anisotropic SLM lattice structures. Different cubic, polyhedral and rhombic cell types are evaluated towards their producibility. Some of these cell types are preselected and are subject to numerical analysis where their mechanical properties are derived on the basis of the space framework theory. Secondly, an extensive experimental evaluation of test specimens is given. This includes examinations of the properties of SLM solids, the producibility of SLM lattice structures in terms of dimensions and testing of their mechanical properties such as strength and elasticity in compression, tension and shear load. The test procedures are divided in three stages. The first stage comprises the examination of the specific strength in dependence of the cell type to narrow down few optimum cell types for different applications. In the second and third stage these cell types are investigated towards their elasticity and strength in dependence of the cell size. Finally, this thesis concludes with scaling laws provided in accordance with the theoretical and experimental results. Opposed to simple power laws used for cellular materials these newly developed scaling laws consider leaps in properties at higher, so-called critical relative densities which can be obtained from SLM due to its high degree of design freedom. At the critical relative density SLM lattice structures cease being frameworks and become rather solids with pores. For future applications these scaling laws can be applied by design engineers to match particular requirements that can only be fulfilled by Laser Freeform Fabrication and its degrees of freedom in design. For the sake of completeness some sample applications in the field of medical implants are given in this thesis, which involve these scaling laws.