Carolin Körner

Cellular Ti-6Al-4V structures with interconnected macro porosity for bone implants fabricated by selective electron beam melting.

Selective electron beam melting (SEBM) was successfully used to fabricate novel cellular Ti-6Al-4V structures for orthopaedic applications. Micro computer tomography (microCT) analysis demonstrated the capability to fabricate three-dimensional structures with an interconnected porosity and pore sizes suitable for tissue ingrowth and vascularization. Mechanical properties, such as compressive strength and elastic modulus, of the tested structures were similar to those of human bone. Thus, stress-shielding effects after implantation might be avoided due to a reduced stiffness mismatch between implant and bone. A chemical surface modification using HCl and NaOH induced apatite formation during in vitro bioactivity tests in simulated body fluid under dynamic conditions. The modified bioactive surface is expected to enhance the fixation of the implant in the surrounding bone as well as to improve its long-term stability.

Processing of Metal Foams – Challenges and Opportunities

Considerable progress has been made recently in the production of metallic foams. Scale up has progressed so far that widespread commercial use may become a reality in the near future. In this review the merits of the various fabrication techniques are highlighted, while particular emphasis is placed on the demands of the various applications and the suitability of each process to meet these demands.

Physical and material aspects in using visible laser pulses of nanosecond duration for ablation

Interaction phenomena of 50 ns copper vapour laser pulses (λ = 511/578 nm) with matter are investigated. The basic ablation process is classified into four fundamental classes. On basis of this classification processing results are connected with specific material properties like the brittleness, the viscosity of the melt or the optical properties. Knowing these properties a prognosis of the expected fundamental process is possible. In order to generate a geometrically defined structure via ablation in a given material-specific process, strategies have to be developed. Typical examples for process strategies are given.

Compression-compression fatigue of selective electron beam melted cellular titanium (Ti-6Al-4V).

Regular 3D periodic porous Ti-6Al-4V structures intended to reduce the effects of stress shielding in load-bearing bone replacement implants (e.g., hip stems) were fabricated over a range of relative densities (0.17-0.40) and pore sizes (approximately 500-1500 μm) using selective electron beam melting (EBM). Compression-compression fatigue testing (15 Hz, R = 0.1) resulted in normalized fatigue strengths at 10(6) cycles ranging from 0.15 to 0.25, which is lower than the expected value of 0.4 for solid material of the same acicular α microstructure. The three possible reasons for this reduced fatigue lifetime are stress concentrations from closed porosity observed within struts, stress concentrations from observed strut surface features (sintered particles and texture lines), and microstructure (either acicular α or martensite) with less than optimal high-cycle fatigue resistance.

In vivo performance of selective electron beam-melted Ti-6Al-4V structures.

Highly porous titanium structures are widely used for maxillofacial and orthopedic surgery because of their excellent mechanical properties similar to those of human bone and their facilitation of bone ingrowth. In contrast to common methods, the generation of porous titaniumproducts by selective electron beam melting (SEBM), an additive manufacturing technology, overcomes difficulties concerning the extreme chemical affinity of liquid titanium to atmospheric gases which consequently leads to strongly reduced ductility of the metal. The purpose of this study was to assess the suitability of a smooth compact and a porous Ti-6Al-4V structure directly produced by the SEBM process as scaffolds for bone formation. SEBM-processed titanium implants were placed into defects in the frontal skull of 15 domestic pigs. To evaluate the direct contact between bone and implant surfaces and to assess the ingrowth of osseous tissue into the porous structure, microradiographs and histomorphometric analyses were performed 14, 30, and 60 days after surgery. Bone ingrowth increased significantly during the period of this study. After 14 days the most outer regions of the implants were already filled with newly formed bone tissue (around 14%). After 30 days the bone volume inside the implants reached almost 30% and after 60 days abundant bone formation inside the implants attained 46%. During the study only scarce bone-implant contact was found around all implants, which did not exceed 9% around compact specimens and 6% around porous specimens after 60 days. This work demonstrates that highly porous titanium implants with excellent interconnectivity manufactured using the SEBM method are suitable scaffolds for bone ingrowth. This technique is a good candidate for orthopedic and maxillofacial applications.

Additive manufacturing of metallic components by selective electron beam melting — a review

Selective electron beam melting (SEBM) belongs to the additive manufacturing technologies which are believed to revolutionise future industrial production. Starting from computer-aided designed data, components are built layer by layer within a powder bed by selectively melting the powder with a high power electron beam. In contrast to selective laser melting (SLM), which can be used for metals, polymers and ceramics, the application field of the electron beam is restricted to metallic components since electric conductivity is required. On the other hand, the electron beam works under vacuum conditions, can be moved at extremely high velocities and a high beam power is available. These features make SEBM especially interesting for the processing of high-performance alloys. The present review describes SEBM with special focus on the relationship between process characteristics, material consolidation and the resulting materials and component properties.

The investigation of morphometric parameters of aluminium foams using micro-computed tomography

The development of different manufacturing techniques for foams, most especially metallic foams have opened a new market of opportunities for multifunctional cellular metallic materials. However, it is yet unclear how to quantitatively characterise the microstructure and internal architecture of foams precisely. In this paper, the potential of using micro-computed tomography in characterising the morphometric parameters of foams are explored. 3D morphometric parameters of different types of closed cell aluminium alloy foam have been characterised, and the effects of the measurement resolution, and the integration time on the measured morphometric parameters have been studied. The model based and model independent morphometric parameters are comparable with measurements using other characterisation techniques. We conclude that the new generations of micro-computed tomography equipment have versatile capability for a non-invasive and -destructive characterisation of foams.

Fundamental consolidation mechanisms during selective beam melting of powders

During powder based additive manufacturing processes, a component is realized layer upon layer by the selective melting of powder layers with a laser or an electron beam. The density of the consolidated material, the minimal spatial resolution as well as the surface roughness of the resulting components are complex functions of the material and process parameters. So far, the interplay between these parameters is only partially understood.In this paper, the successive assembling in layers is investigated with a recently described 2D-lattice Boltzmann model, which considers individual powder particles. This numerical approach makes several physical phenomena accessible, which cannot be described in a standard continuum picture, e.g. the interplay between capillary effects, wetting conditions and the local stochastic powder configuration. In addition, the model takes into account the influence of the surface topology of the previous consolidated layer on the subsequent powder layer.The influence of the beam power, beam velocity and layer thickness on the formation and quality of simple walls is investigated. The simulation results are compared with experimental findings during selective electron beam melting. The comparison shows that our model, although 2D, is able to predict the main characteristics of the experimental observations. In addition, the numerical simulation elucidates the fundamental mechanisms responsible for the phenomena that are observed during selective beam melting.

Design of Auxetic Structures via Mathematical Optimization

The design of auxetic structures is traditionally based on engineering experience, see, for instance, refs. [1,2]. The goal of this article is to demonstrate how an existing auxetic structure can be improved by structural optimization techniques and manufactured by a selective electron beam melting (SEBM) system. Auxetic materials possess negative Poisson’s ratios ν which is in contrast to almost all engineering materials (e. g. metals ν ≈ 0.3). The negative Poisson’s ratio translates into the unusual geometric effect that these materials expand when stretched and contract when compressed. This behavior is of interest for example for fastener systems and fi lters. [ 2 , 3 ] It also results in interesting mechanical behavior like increased penetration resistance for large negative Poisson’s ratios [ 4 ] and in the case of isotropy for higher shear strength relative to the Young’s modulus [ 2 ] of the structure. The auxetic behavior can always be traced back to complex mesoor microscopic geometric mechanisms, e. g. unfolding of inverted elements or rotation of star shaped rigid units relative to each other. [ 5 – 7 ] These geometries can be realized on vastly different scales from specifi cally designed polymers to reactor cores. [ 2 , 8 ]

Parallel Lattice Boltzmann Methods for CFD Applications

The lattice Boltzmann method (LBM) has evolved to a promising alternative to the well-established methods based on finite elements/volumes for computational fluid dynamics simulations. Ease of implementation, extensibility, and computational efficiency are the major reasons for LBM’s growing field of application and increasing popularity. In this paper we give a brief introduction to the involved theory and equations for LBM, present various techniques to increase the single-CPU performance, outline the parallelization of a standard LBM implementation, and show performance results. In order to demonstrate the straightforward extensibility of LBM, we then focus on an application in material science involving fluid flows with free surfaces. We discuss the required extensions to handle this complex scenario, and the impact on the parallelization technique.