Key Engineering Materials Vol. 742

Abstract: Aluminium-Matrix-Nanoparticle-Composites were produced by ball milling of micro scale Aluminium powder with various nanoscales ceramic powders like Silicon Carbide, Alumina and Boron Nitride with subsequent consolidation by hot extruding. The composites were investigated by amplitude dependent damping tests, tensile tests at elevated temperatures, hardness measurements, imaging methods and electric conductivity tests. All tested samples were machined out of hot extruded rods. The Amplitude dependent damping of bending samples was determined by measuring the strain dependent logarithmic decrement of free decaying vibrations of bending beams at room temperature. These tests were done after successive step by step isochronal heat treatments. Some samples show substantial improvement of the mechanical properties due to dispersion hardening or grain refinement. It can be concluded that the results are mainly influenced by dislocation effects like Orowan-effect, work-hardening, grain-size-hardening, recrystallization, and creation of dislocations at ceramic particles due to thermal mismatch. Moreover some results can be attributed to fatigue during mechanical cycling namely crack nucleation, crack growth and fraction. The electric conductivity was measured indirectly by permeability tests with a digital hysteresis recording devise. The results show the low influence of nano-particle dispersion hardening to conductivity in comparison of work-hardening.

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: Short fibre reinforced aluminium was produced using the Temconex^® process which is a continuous extrusion using a mixture of metal powder and ceramic short fibre as feedstock. The Temconex^® process was derived and further developed from the Conform^TM process which uses metal rod rather than powder as feedstock. In the present paper the effect of the prechamber length on the mechanical properties was examined. As material Al99.7 powder with different volume fractions of milled carbon fibres was used. Distribution, orientation and geometry of the embedded fibres were examined using light microscopy. The mechanical properties were determined via tensile testing and resonance frequency analysis. An important increase of the Young’s modulus is observed because of the introduction of fibres. It can be rationalized based on Clyne’s Shear Lag model. Results show that an extension of the prechamber enhances the Young’s modulus and the elongation of fracture due to reduced fibre fracture and better fibre alignment.

Abstract: Magnesium powder in micron scale and various volume fractions of SiC particles with an average diameter of 50 nm were co-milled by a high energy planetary ball mill for up to 25 h to produce Mg-SiC nanocomposite powders. The milled Mg-SiC nanocomposite powders were characterized by scanning electron microscopy (SEM) and laser particle size analysis (PSA) to study morphological evolutions. Furthermore, XRD, TEM, EDAX and SEM analyses were performed to investigate the microstructure of the magnesium matrix and distribution of SiC-reinforcement. It was shown that with addition of and increase in SiC nanoparticle content, finer particles with narrower size distribution are obtained after mechanical milling. The morphology of these particles also became more equiaxed at shorter milling times. The microstructural observation revealed that the milling process ensured uniform distribution of SiC nanoparticles in the magnesium matrix even with a high volume fraction, up to 10 vol%.

Abstract: The outstanding performance of many aluminum matrix composites (AMCs) regarding specific stiffness makes AMCs attractive materials for lightweight construction. Low density boride compounds promise both an increase in stiffness and decrease in composite density. Therefore for this study AlB₂, B and B₄C were chosen for composite manufacturing. The composites were fabricated with the stir casting process. To avoid gas entrapment during mixing and ensure nonporous composites, partial vacuum was adapted during particle feeding and stirring. Poor wettability of used particle material in contact with liquid aluminum hindered particle incorporation, but alloying elements such as titanium were shown to affect wettability and particle incorporation for B₄C. Zn had no influence on wettability or reactivity and did not improve particle incorporation. In contrast to Zn, Ti improved adhesion and wettability, but particle incorporation was improved exclusively for B₄C. Besides alloying Ti, the use of high-shear force mixers improved particle incorporation enabling uniform particle distribution. AMCs with up to 12 vol.% of B₄C particles were produced via stir casting without alloying Ti.

Abstract: During the last years, several studies proved the high potential of metallic glasses to be used as reinforcements in lightweight alloys. Thereby, focus was mostly on particle reinforced composites or three-dimensional and omnidirectional glass arrays within the composite. Using a specific layered structure of the entire ribbons as reinforcement to design direction-dependent tailored properties is a novel approach. The composites in this study were produced by gas pressure infiltration of a layered stack of metallic glass ribbons. Ribbons of the metallic glass Ni₆₀Nb₂₀Ta₂₀ were used as reinforcements and aluminum alloy AlSi12 as matrix. Mechanical tests like four point bending and tensile tests as well as elastic analysis using ultrasound phase spectroscopy (UPS) were performed to classify composite’s properties. Further, micro computed tomography (µCT) analysis and metallographic investigations were carried out on the four point bending samples after testing to reveal occurring damage mechanisms.

Abstract: Ni-based superalloys, in both single and polycrystalline varieties, are extensively used in high pressure turbine blades. But contrary to single crystal variants, the polycrystalline forms present easier manufacturing and offer higher potential for improvement in metal matrix composites (MMCs). To benefit from this opportunity, an Inconel X-750 superalloy reinforced with TiC particles is proposed, having a polycrystalline microstructure and the possibility for weight reduction in turbine elements application. The metallic powder with an addition of 15 vol.% of 3.7 μm_d TiC particles was prepared through low energy mixing, uniaxial pressing and sintering, followed by a triple heat treatment. The microstructure was analyzed with SEM and XRD techniques. Compressive creep tests were performed at 800 °C with 200 MPa, on both original and reinforced alloys. The study shows how the inclusion of a highly compatible particle reinforcement does not only improves the creep resistance, but also reduces the material weight, thus having potential to promote further reduction in the creep rate on turbine blades submitted to centripetal forces.

Abstract: Combining aluminum and carbon fiber reinforced plastic (CFRP) has been a key focus in realizing lightweight concepts. Manufacturing technologies for high load-bearing and ultra-lightweight CFRP structures have reached a high level of innovation. The same goes for near-net-shape high pressure die casting (HPDC) aluminum components, which can be mass-produced in a highly efficient manner. Yet for hybrid composites of these materials, the solutions to date have relied on conventionally mechanical or adhesive joining techniques. The direct joining of these two materials is problematic, due to their electrochemical intolerance and the resulting corrosive degradation. The joining technology therefore is at the center of this challenge. The DFG-sponsored joint research project of Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, Fiber Institute FIBRE, and Bremen Institute for Mechanical Engineering BIME, all three institutes located at the campus of the university in Bremen, aims at combining aluminum and thermoplastic CFRP into an intrinsic hybrid composite. This is to be achieved in a single-step primary shaping process, avoiding conventional joining techniques like adhesive bonding or riveting. To this end, CFRP structures are to be recast with aluminum, creating an electrochemically decoupling layer between the two materials. This decoupling layer can therefore be considered as a key factor for realizing hybrid composites. It also needs to have a high process reliability and be long-term and mechanically stable. Polyetheretherketone (PEEK) thermoplast was identified as a suitable material for that purpose, given its stability at high temperatures and electrochemical insulation effect. First test results show the possibility of incorporating CFRP accordingly by HPDC, resulting in a continuous intact decoupling layer of PEEK. The trend indicated that different thermal treatments as well as different aluminum thicknesses of the hybrid casted sample influence the joint strength. On average, in tensile shear tests a joint strength approximately in the range of current single lap adhesive bonds could be achieved.

Abstract: Sliding electrical contacts are traditionally produced by conventional compacting technologies. Employing the powder injection moulding process (PIM) as a new manufacturing method can offer several advantages such as the fabrication of complex net-shaped parts, cost-effectiveness and high volume productions. The PIM process route consists of the following steps: powder processing, compounding, injection molding, debinding and sintering. A two-stage process consisting of solvent debinding and thermal debinding is often used to remove the moulding binder. In the present paper, the suitability of the powder metallurgical processes: mechanical alloying and powder mixing for the preparation of bronze-graphite powder mixtures for the compounding and injection moulding of sliding contacts is discussed. The use of a suitable binder is of central importance for the preparation of injection-moldable feedstocks. For this purpose, two commercial ready-to-use binder systems were utilized and evaluated. The essential challenge of the process route is to optimize all parameters of the subprocesses to achieve a damage-free debinding and sintering of the injection-moulded parts. First results on the influence of the graphite content, the binder fraction, the debinding and sintering parameters are presented and discussed.

Abstract: At the Institute of Structures and Design of German Aerospace Center (DLR) in Stuttgart, C/C-SiC sandwich structures based on continuous fiber reinforced folded cores and skin panels have been developed via Liquid Silicon Infiltration (LSI) and in situ joining method. The resulting lightweight structures offer a high potential in various application areas such as optical benches of satellites or charging racks for high temperature furnaces.A major impediment for the new development and practical application of ceramic sandwich structures is the lack of know-how of characterization and simulation of their mechanical properties. In this study, several types of C/C-SiC sandwich structures with folded cores with different fiber orientations (0°/90° and ±45°) in the core structure were manufactured and mechanically tested in four point bending. The mechanical properties of the different sandwich structures were correlated to analytical calculation and numerical (finite element) simulation. The comparison showed good correlation. The proposed evaluation methods are suitable to determine and simulate the mechanical properties of C/C-SiC sandwich structures and are a versatile tool for further product development.