Cellular Design for Laser Freeform Fabrication

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• Publisher: Cuvillier Verlag
• Year: 2010
• eBook ISBN:
  9783736932739
• Print ISBN:
  9783869552736
• Edition: 1

Cellular materials are spread all across the world. They can be found in nature, e.g. in boneand wood, as well as in engineering applications such as honeycomb sheets and aluminumfoams to name but a few. Cellular materials have some unique properties which allow newand innovative applications beyond the scope of solid engineering materials. Especially theirlow density and therefore their outstanding stiffness-to-weight-ratio is of greatest importancein most applications. Functions of cellular materials could be lightweight structures of highstiffness, damping and absorption of mechanical energy, vibration control, acousticabsorption, heat exchange, filtering and numerous other tasks. Generally, a combination ofthese tasks in one part exhibits an optimized and therefore innovative overall performance.One recent development in production technologies is the field of Laser Freeform Fabrication(LFF) processes where parts are manufactured by application of thin layers of powder orsometimes liquid material. A laser beam melts and solidifies the material along contour linesand hatch areas according to slices of a corresponding 3D-CAD model. Among theseprocesses the Selective Laser Melting (SLM) technology was advanced based upon the workin this thesis to allow the manufacture of periodic, open-cell lattice structures fromengineering materials such as stainless steel, titanium, etc. In contrast to other cellularmaterials these lattice structures can be of well-defined, nearly arbitrary shape. Due to thelayerwise fabrication the SLM process is also capable of creating lattice cores surrounded bysolid shells allowing new degrees of geometric freedom in engineering design that was neverexperienced before in conventional machining. This allows the development of interestingnew applications such as medical implants where the main issues are the improvement ofosseointegration and realization of physioelastic material properties for an optimized bondbetween the implant and surrounding tissue. Lattice structures obtained from the SLM processcan meet these requirements.This thesis contributes to the understanding of the mechanical properties of the new materialclass of SLM lattice structures. Their future incorporation in engineering designs requires aprofound knowledge of failure mechanisms and operational limits. Therefore, acomprehensive summary is given on the state-of-the-art of cellular materials followed by adedicated analysis on Laser Freeform Fabrication and an in-depth validation of the SelectiveLaser Melting capabilities. Readers with advanced knowledge on cellular materials or LaserFreeform Fabrication may skip sections 2 or 3, respectively. Next, all process constraints andboundary conditions for the manufacture of SLM lattice structures are elaborated. Then abilateral approach was chosen to derive scaling laws and optimize the SLM lattice structuresfor given tasks. Firstly, a theoretical analysis comprises the examination of structuralhypotheses for isotropic cellular materials before a generalized theory is developed foranisotropic SLM lattice structures. Different cubic, polyhedral and rhombic cell types areevaluated towards their producibility. Some of these cell types are preselected and are subjectto numerical analysis where their mechanical properties are derived on the basis of the spaceframework theory. Secondly, an extensive experimental evaluation of test specimens is given.This includes examinations of the properties of SLM solids, the producibility of SLM latticestructures in terms of dimensions and testing of their mechanical properties such as strengthand elasticity in compression, tension and shear load. The test procedures are divided in threestages. The first stage comprises the examination of the specific strength in dependence of thecell type to narrow down few optimum cell types for different applications. In the second andthird stage these cell types are investigated towards their elasticity and strength in dependenceof the cell size. Finally, this thesis concludes with scaling laws provided in accordance withthe theoretical and experimental results. Opposed to simple power laws used for cellularmaterials these newly developed scaling laws consider leaps in properties at higher, so-calledcritical relative densities which can be obtained from SLM due to its high degree of designfreedom. At the critical relative density SLM lattice structures cease being frameworks andbecome rather solids with pores.For future applications these scaling laws can be applied by design engineers to matchparticular requirements that can only be fulfilled by Laser Freeform Fabrication and itsdegrees of freedom in design. For the sake of completeness some sample applications in thefield of medical implants are given in this thesis, which involve these scaling laws.