Laser drilling of hollow sphere structures

A special composite and cellular material – the metallic hollow sphere structure (MHSS) – represents an advanced material. The core element of the structure is a hollow sphere with a wall out of porous steel. This shell allows a high geometric reproducibility. It is an interesting base material for lightweight design. However, processing technologies for further manufacturing of metallic hollow sphere structure are necessary. Laser beam drilling is a highly efficient technique with heat input concentrated locally at the structure. Laser drilling is applied to metallic hollow sphere structures. The influence of different geometries and joining technologies of the spheres on the drilling behaviour is investigated. Percussion drilling is used to form holes with a CO₂‐laser into the metallic hollow sphere structure. The investigated mean laser power is 400 W. Two different focal lenses with focal lengths of 5″ and 7.5″ allow investigations with different focal diameters and Rayleigh lengths. Diameter and roundness of the laser drilled holes are measured in five layers of different drill depths z. The maximum drilled depth is about 40 mm with a 5″ lense and about 50 mm with the f = 7.5″ lens. The relative roundness is determined and decreases with increasing drill depth. The diameter d(z) as a function of the drill depth follows the shape of an isophote, a line of constant intensity inside the laser beam. The brazed hollow sphere structures have considerably less drill depth than the sintered.     

Production of hollow spheres (HS) and hollow sphere structures (HSS)

In our paper we focus on component materials, production and prospective applications of metallic hollow spheres. We give an insight into technology, production, and the hollow spheres' diversity. It is our aim to provide an overview of the technology and enable the reader to evaluate whether the hollow spheres technology meets their specific needs.     

Prediction of the elastic properties of syntactic perforated hollow sphere structures

This work investigated the elastic 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 for a matrix material. The effective elastic properties of syntactic (i.e. spheres completely embedded in a matrix) perforated hollow sphere structures in a primitive cubic (PC) arrangement of unit cell models are numerically evaluated for different hole diameters, matrix volume fractions and different base materials. The results are compared to typical configurations without perforation. In the scope of this paper, three-dimensional finite element analysis is used in order to investigate these unit cell models.     

Simulation and experimental validation of acoustic properties of hollow sphere structures

Customers nowadays regard the noise and vibration behavior as an essential product property. Cellular character materials, in particular hollow sphere structures, are predestined to absorb sound in a very efficient manner due to their cellular character. Depending on the constituent material, the geometric parameters like the diameter of the spheres, the thickness of the walls and the assembling schema of single spheres, the absorption coefficient can be reduced to very low levels. In contrast to other cellular materials, the frequency and bandwidth can actively be influenced by the variation of the above mentioned parameters. In order to predict the acoustic behavior of a structure, FE or CFD analyses are used as standard tools. In addition, there exist some parameter based models, e.g. the BIOT theory, which characterizes the absorption, transmission and reflection coefficients using a few macroscopic parameters. Within this contribution, the acoustic properties of hollow sphere structures are investigated by a so‐called virtual material laboratory GeoDict (by Math2Market GmbH, originally by the Fraunhofer Institute for Industrial Mathematics). The results for the absorption and reflection coefficients are compared to those gained by classical analysis methods and experiments based on Kundt's tube.     

Numerical analyses of the thermal conductivity of random hollow sphere structures

This paper presents Lattice Monte Carlo (LMC) and Finite Element (FE) analyses on the effective thermal conductivity of sintered Metallic Hollow Spheres Structures (MHSS). A novel analysis technique applying LMC permits the utilisation of large high resolution models and can be based on Computed Tomography (CT) images. As a consequence, simulations are performed using the real geometry and no simplifications, such as the use of model structures, need to be introduced. In the first part, the influence of the micro-porosity on the effective thermal conductivity of the cell wall material is determined. Using these results, the second part of the analyses directly addresses the thermal properties of sintered MHSS. The findings of the LMC analyses are compared with FE results.     

Experimental determination of the macroscopic fatigue properties of metal hollow sphere structures

Fatigue properties of a rather large range of metal hollow sphere structures were determined by mean of conventional fatigue tests. All the data were collected to build S–N curves. The constitutive material, the density, the thicknesses of the walls, the diameters and the fatigue loading mode were varied to determine the effect of each parameter.The constitutive material and the density are the two parameters that have the most important effect on the fatigue strength. The diameter of the spheres acts on the slope of the S–N curve. The fatigue strength measured in compression is twice the one measured in tension.     

On the acoustical properties of metallic hollow sphere structures (MHSS)

Well-known advantages of cellular metals are their high ability for energy absorption, good damping behavior and sound absorption at a high specific stiffness. This paper has a focus on the absorption coefficient representing the acoustical property. The absorption coefficient is analysed experimentally by an impedance tube. Results fit well to theoretical investigations described by Champoux and Allard.     

Thermal conductivity of metallic hollow sphere structures: An experimental,analytical and comparative study

The thermal conductivity of metallic hollow sphere structures is analyzed in terms of their main structural parameters (sphere diameter, shell thickness and constituent metal) by using analytical/empirical models to estimate the relative density and admitted scaling laws to determine the thermal conductivity. These results are compared with experimental values of such structures determined by the transient plane source method. In addition, a thermal conductivity map comparing these results with the typical thermal conductivity of other cellular metallic materials is provided.     

Guest editorial: Molybdenum alloying: more than hardenability

The full text can be downloaded at https://link.springer.com/content/pdf/10.1007/s40436-019-00261-6.pdf     

Finite element simulation of the thermal conductivity of perforated hollow sphere structures (PHSS): Parametric study

This paper investigates the thermal 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 effective thermal conductivity of perforated sphere structures in a primitive cubic arrangement is numerically evaluated for different hole diameters and different dimensions of the joining elements.