Papers by Author: Markus Merkel

Abstract: Additive manufacturing processes offer the ability to manufacture highly complex geometries, but are limited in terms of the achievable surface quality. These limitations are based on physical restrictions, especially the need of support and the powder-bed environment, and economic decisions. In this study, the development of the morphology of surfaces with varying inclination angles was investigated on the example of 316L stainless steel. Surfaces with low inclination angles to the manufacturing plane suffered extensively from the process related staircase effect, whereas perpendicular side faces revealed high dependencies on the interaction with the powder-bed.

Abstract: The innovative core of this paper is the development, simulation and experimental validation of a modular tool set for the production of different window contour types made of wood. Caused by the modular structure and the high rotational speed in wood machining, an essential objective in the development of a modular tool set is to achieve close customer defined tolerances. This can only be achieved with a good balancing grade. To determine the unbalance, a parametric 3D-CAD-model of the validation model is imported into a Multi Body Simulation (MBS) software and analyzed numerically. The results of these simulations are then validated by experimental analysis on a balancing machine.

Abstract: Heat conductivity is a well-known energy transfer method and it is here applied to the study of metal foams and laser processing. Metallic hollow sphere structures (MHSS), a relatively new group of advanced composite materials, combine the advantages of cellular metals without major scattering of their material parameters. They are characterised by high geometry reproduction leading to relatively constant mechanical and physical properties.The laser processing technology provides not only a laser cutting but also a laser soldering procedure. Within this work a laser cutting process is applied to MHSS. Laser beam cutting is a highly efficient technique to cut materials, because the relatively small amount of heat affects only a small heating zone.Numerical simulation is used in order to define proper process parameters for a large variety of MHSS. The finite element method based simulation covers material parameters as well as process parameters like the cutting velocity. Heat conduction and convection are taken into account and the phase change from solid to liquid state as well. Within the simulations the concept of representative volume element (RVE) is applied. The temperature distribution is the fundamental result.

Abstract: This paper investigates the mechanical properties of a new type of hollow sphere structure. For this new type, the sphere shell is perforated by several holes in order to open up the inner sphere volume and surface. The mechanical behaviour of perforated sphere structures under large deformations and strains in a primitive cubic arrangement is numerically evaluated by using the finite element method for different hole diameters and different joining techniques.

Abstract: Hollow sphere structures are a new group of advanced lightweight materials for multifunctional applications. Within the scope of this paper, the uniaxial deformation behaviour in the regime of large deformations is investigated. Appropriate computational models are developed to account for the deformation mechanisms occurring under high deformations. Macroscopic stress-strain curves are derived and the influence of different material parameters is investigated.

Abstract: This paper investigates the uniaxial mechanical properties of a new type of hollow sphere structures. For this new type, the sphere shell is perforated by several holes in order to open the inner sphere volume and surface. The mechanical properties, i.e. elastic properties and initial yield stress, of perforated hollow sphere structures in a primitive cubic arrangement are numerically evaluated for different hole diameters and different sphere wall thicknesses.

Abstract: This paper investigates the thermal properties of a new type of hollow sphere struc- tures. For this new type, the sphere shell is perforated by several conical holes in order to open the inner sphere volume. The effective thermal conductivity of perforated sphere structures in a primitive cubic arrangement is numerically evaluated for different material combinations and compared to sphere structures without perforation.

Abstract: This paper investigates the structural properties of sintered hollow sphere struc- tures. First of all, the packing density is analysed using three different methods: namely, liquid infiltration, Archimedes' principle, and image processing of micrographs. In addition, the pore fraction in the metallic sphere shells is characterised based on micrographs and the density of the structure and the base material is determined. In the final part, the geometrical characteristics of the sphere structure and the material composition of the base material are analysed.

Abstract: The paper investigates the mechanical properties of sintered hollow sphere struc- tures. First of all, the Young's modulus, plastic modulus, initial flow stress, initial flow strain, plateau stress and densifcation strain are derived from compressive tests. In addition, the material behaviour during the compressive testing is described. Furthermore, the influence of different specimen shapes on the results is discussed.