Papers by Author: Bernd Kieback

Abstract: In this study different types of multi-walled carbon nanotubes (MWCNT) were produced by the fixed bed and aerosol chemical vapor deposition (CVD) method. Nanocomposite materials were prepared by incorporation of different MWCNTs in copper matrix using powder metallurgy methods. By using hot pressing in combination with hot extrusion, the orientation of the carbon reinforcement was tuned from 3D to 1D alignment. After a selective etching process the carbon reinforcement is partially free-standing at the composite surface, but still embedded in the metal matrix. The engineered surface acts almost like a black body. The spectral evaluation of the surface functionalization will be shown for wavelengths from 200 nm to 20 µm. These results are compared to bulk copper. The free-standing MWCNT also behave like fins/pins in heat exchanger structures or surface enhancement in pool boiling. The experimental setup will be explained and the measurement described for pure copper. The theoretical heat transfer coefficient of the engineered surface is calculated depending on diameter and length of the free-standing MWCNTs. The results are compared to bulk copper.

Abstract: The integration of fibers, especially tailor fiber placement (TFP), in metal matrices offers one way to generate composite materials with increased specific strength compared to the unreinforced metal matrix. The TFP can be adapted according to the final load paths through the component and can be covered partially or fully with the metal. Following this approach load transfer elements can be built, transferring much load and having low mass. First fields of application are identified in building and automotive industry. This work includes the powder metallurgical manufacturing process using Spark Plasma Sintering (SPS) technique, the characterization of the microstructure and the tensile test of different specimens (sintered copper, TFP (as received) and TFP (Cu covered) reinforced copper). Experimental result on 19.5 vol.% TFP (Cu covered) reinforced copper shows an increase of specific strength around a factor of 2.2 compared to pure copper.

Abstract: To meet the need of high-performance thermal management materials in the field of electronic applications, heat sink materials reinforced with synthetic diamonds have been prepared via powder metallurgy. A matrix of a silver alloy with a silicon content of 0.45 wt.% was chosen out of the prediction of the thickness of a final carbide layer of about 180 nm. The volume content of the diamonds and the diamond size were kept constant. The mixed powders were consolidated by Spark Plasma Sintering (SPS) using different sintering temperatures between 800 and 870 °C with a holding time of 30 min. The maximum thermal conductivity of 680 W/(mK) measured at room temperature and 620 W/(mK) at 275 °C was obtained at 810 °C sintering temperature. The degradation of the most promising sample after one thermal cycle up to 275 °C was determined below 1 percent of the value after sintering.

Abstract: The present study addresses the need for grain refinement in free sintered titanium alloys produced by 3D screen printing. Thermohydrogen processing (THP) was used for temporary alloying Ti-6Al-4V with hydrogen to refine its microstructure. The impact on microstructure was investigated by a parameter study with varying temperatures, exposure times and hydrogen partial pressures. Heat treated specimens were examined by optical microscopy, XRD and thermal analysis. The influence of the refined microstructure on the mechanical properties was evaluated by tensile and microhardness testing. Ultrafine grained microstructures with ultimate tensile strengths of up to more than 1000 MPa could be produced.

Abstract: Powder bed additive manufacturing of titanium components offers several advantages. The high freedom of design enables the fabrication of structurally optimized, lightweight parts. Complex geometries may serve additional functions. The use of additive manufacturing has the potential to revolutionize logistics by dramatically reducing lead time and enabling a high degree of customization. Manufacturing near net shape parts reduces the loss of expensive material.For the application in safety relevant parts certainty about static and fatigue strength is critical. A challenge arises from complex influences of built parameters, heat treatments and surface quality. Ti-6Al-4V specimen built by electron beam melting (EBM) were subjected to heat treatments adapted to various employment scenarios. The results of tensile and fatigue testing as well as crack propagation and fractography will be compared to titanium manufactured conventionally and by selective laser melting (SLM). The mechanical behavior will be correlated to the microstructural evolution caused by the heat treatments

Abstract: The Development of Fiber Reinforced Plastics (FRP) offers a great opportunity for applications in automobile industry, aeronautics and consumer goods to achieve light weight structures. However, the connection technology between FRP and mainly metallic based structures is the key to use the full potential of the FRP. Out of this motivation recent developments address this aspect.Using the powder metallurgical approach to generate a metal/ FRP connection module by spark plasma sintering a great variety is possible by integration of different metal and/ or fiber components. In this work aluminum and stainless steel was chosen for the upper and lower metallic side. The fibers integrated into the metal were glass, basalt and carbon fiber in one layer, two layer and mixed layer configuration. To connect the sintered module to greater CF weaves an infiltration process with a room temperature curing resin was used in a modified vacuum infusion (MVI) setup. In not optimized configuration the shear test after infiltration indicated an initial value for module shear strength above 20 MPa which can be enhanced in future developments by optimized armor between the upper and lower metal side and the number of integrated fiber layers of the connection module. A model is predicted to calculate the module shear strength in sintered state by multiplication of the armor area with the shear strength of the armor material. First experiments additionally show the possibility to weld the connection module directly to metallic structures.

Abstract: Sintering especially of loose particle packings is accompanied by dimensional changes of the specimen. The growth of inter particle contacts under the influence of the Laplace pressure is well understood and described by the two particle model. On the other hand the understanding of other fundamental sintering phenomena i.e. cooperative material transport processes is rather vague. To overcome this limitation for near net-shape production of components by powder metallurgy an improved model of particle rearrangement processes is required. First efforts to obtain necessary experimental data were performed at 1D and 2D models. But recent improvements of high resolution synchrotron computer tomography (SCT) setups allow the acquisition of in-situ data of particle rearrangements. In-situ studies of particle rotations during sintering were conducted at the ESRF in Grenoble. The rotations during free sintering of monocrystalline particles were investigated during continuous heating up to 1050 °C or with frequent interruptions of the heating by 1 hr dwell times every 100 °C. In contrast to 1D specimens measured by Wieters the 3D specimens showed negligible rotations. This must be attributed to the constraints in 3D samples. To obtain a more detailed insight in the rotations the particles of one sample were marked. It is possible to show that the particles perform intrinsic rotations. Therefore a new rotation model is developed. The intrinsic rotations are confirmed by complementary EBSD analyses as well.

Abstract: The ideal thermal management material working as heat sink and heat spreader should have a high thermal conductivity combined with a reduced and tailorable thermal expansion. To meet these market demands copper composites reinforced with diamond particles were fabricated by a powder metallurgical method (powder mixing with subsequent pressure assisted consolidation). In order to design the interfacial behaviour between copper and the reinforcement different alloying elements, chromium or boron, were added to the copper matrix. The produced composites exhibit a thermal conductivity up to 700 W/mK combined with a coefficient of thermal expansion (CTE) of 7-8 x 10-6/K. The copper composites with good interfacial bonding show only small decrease in thermal conductivity and a relatively stable CTE after the thermal cycling test.

Abstract: The two-particle model describes the approach of particle centres and the growth of the interparticle contacts during sintering of metal powders. Unfortunately the comprehensive description of processes inside of three dimensional specimens must consider the contribution of particle rearrangements. The recent developments of combined micro focus computed tomography (CT) and 3D photogrammetric image analyzing give the opportunity to obtain the experimental data required to overcome the shortcomings of sintering theories based on the two-particle model. The analysis of spherical poly and single crystalline copper powder was performed by CT. In addition a single crystal specimen was analyzed by high resolution synchrotron radiation tomography - a more sophisticated analysis method with very limited availability. The analysis of the 3D tomographic image by photogrammetric image analyzing yielded the positions and radii of all particles and their contact partners as well. A statistical analysis of the retrieved data was performed. The formation and breaking of necks during sintering could be observed. An in-depth analysis of the particle rotation with respect to the coordination number and local density will be presented.

Abstract: The decrease of the distance between particle centers due to the growth of the sinter necks can be explained by the well known two-particle model. Unfortunately this model fails to provide a comprehensive description of the processes for 3D specimens. Furthermore, there is a significant discrepancy between the calculated and the measured shrinkage because particle rearrangements are not considered. Only the recently developed analysis of the particle movements inside of 3D specimens using micro focus computed tomography (μCT), combined with photogrammetric image analysis, can deliver the necessary experimental data to improve existing sintering theories. In this work, μCT analysis was applied to spherical copper powders. Based on photogrammetric image analysis, it is possible to determine the positions of all particle centers for tracking the particles over the entire sintering process and to follow the formation and breaking of the particle bonds. In this paper, we present an in-depth analysis of the obtained data. In the future, high resolution synchrotron radiation tomography will be utilized to obtain in-situ data and images of higher resolution.