Key Engineering Materials Vol. 809

Abstract: The properties of a resin transfer molded sheet of strand-reinforced composite for automotive applications are investigated at the microscopic level. Three components of the composite can be identified in the bright field micrograph – glass fibers, epoxy matrix and binder. The latter having been added in manufacturing process. Accelerated Nanoindentation with 64.451 single indentation experiments is performed at room temperature to generate a mechanical property map of an area containing the 3 components. The distribution of properties, mean value and standard deviation, is determined for each component. Two locations in the composite are selected for a study of the local glass transition behavior by performing dynamic indentation experiments while simultaneous variation of the temperature of the indenter tip and sample within a micro-heating chamber.

Abstract: Rotational moulding (RM) of plastics is predominantly used to produce hollow, singlepiece products. Polyethylene (PE) in its various forms, is the most commonly used material for this process. Researchers have been conducting numerous experiments trying to incorporate reinforcements attempting to improve the mechanical performance of RM products and overcome the material limitations posed by design parameters. One of the most common problems with reinforcement in RM is the migration of the filler towards the inside of the mould and agglomerations. In order to find a competitive material which is desirable by industry, RM experiments were conducted with various composite reinforcements; high density polyethylene (HDPE), numerous types of glass fibres (GF), carbon fibres (CF) and carbon nanotubes (CNT). In particular, the influence of low weight fractions of reinforcement on the mechanical performance, tensile, flexural and impact properties of HDPE were investigated.

Abstract: Composite peening describes a modified process based on micro shot peening. This process allows the controlled penetration of ceramic particles into areas of metallic matrix materials close to the surface layer. Composite material produced by composite peening promises a high application potential in the fields of lightweight, wear-resistant and durable materials. The use of ceramic reinforcing particles is expected to significantly improve thermal stability compared to conventional surface hardening processes. In addition, composite peening offers the possibility of cost-effectively reinforcing components and can even be applied subsequently in highly stressed surface layers. The material combination selected for this study was technically pure aluminum as model and matrix material and alumina as abrasive respectively reinforcement material. The influence on the particle density and the particle gradient was achieved by varying the process parameters, such as the process temperature and the peening pressure. A maximum penetration depth of almost 30 μm could be observed at high homologous temperatures. In light and scanning electron microscopy it was observed that the ceramic particles might break on impact with the surface of the blasting material. This causes a drastic reduction of the particle size, which initially had a size of 10 μm. This reduction of particle size promises advantages, particularly with cyclic loads.

Abstract: Metal Matrix Composites (MMCs) are known for their remarkable properties, by combining materials from different classes. Ni-based MMCs are a promising group of heat-resistant materials, targeting aerospace applications. A discontinuously reinforced Inconel X-750/TiC 15 vol.% MMC was proposed for use in lighter, creep resistant turbine elements, with the aim to endure service temperatures up to 1073 K (800 °C). However, their microstructural stability at high temperatures for long periods of time remained to be further investigated. To address this need, specimens were produced by both conventional hot pressing and spark plasma sintering, using powders milled by low and high energy processes, followed by long isothermal aging. The treatments were conducted at 973 and 1073 K, for times between 50 and 1000 hours. The resulting samples were investigated with XRD and EDS techniques for phase analysis. In addition, measurements of hardness were made to monitor changes in mechanical behavior. It was found that, for each different manufacturing process, the amount, distribution and size of γ’ and other precipitates notably vary during the overaging process. Consequently, the amount of elements kept in solid solution also shifted with time. Furthermore, the study shows how distinct initial microstructures, resulting from diverse fabrication processes, differently impact the microstructural stability over long times of exposure to high temperatures.

Abstract: MMCs exhibit a high potential in modern structures due to many positive attributes. One of these attributes is a heightened creep resistance compared to conventional single or multi-phase alloys. The following paper focuses on the creep resistance of a spray-compacted Al-based alloy (DISPAL^® S270). This alloy, designed for high temperature applications like turbochargers, cylinder bore liners or pistons can be subjected to temperatures up to 450°C, which is considerably higher compared to unreinforced Al-alloys. DISPAL^® S270 microscopically consists of Si-particles with round shapes in an Al-rich α solid solution, which is additionally reinforced by complex precipitates, including Al₉FeNi and Al₃Ti, among others. The particle sizes and ratios differ with the heat treatment. The same applies to the precipitates in the matrix. The investigated heat treatments are F (after extrusion) and T6 (additional peak aged). The alloy is tested at 250°C using different loads at constant force with stress values between 125 and 200 MPa. The measured minimal creep rates are in the range of _min. = 10^-5-10^-8 1/s. In comparison to conventional single or multiple phase alloys MMCs reveal some advantages, since the modified threshold stresses and direct strengthening often reduce the minimum creep rate. The stress exponent of the MMCs has higher values ​​compared to aluminum alloys, as known from previous MMC studies. The specimens are analyzed with LM, SEM, EDS and XRD. It could be shown that an additional heat treatment (peak aged T6) enhances the creep properties by introducing very fine precipitates containing the Mg₅Si₆ phase.

Abstract: For the first time, metal matrix composite (MMC) layers on parts made of highly conductive copper alloys have been generated by laser melt injection (LMI). In order to ensure a sufficient absorption efficiency, different kinds of surface modification were investigated. Welding speeds up to 7.5 m/min can be obtained. When increasing the dispersing rate, the process efficiency, which is the product of absorption efficiency and thermal efficiency, increases. At high dispersing rates, some spherical fused tungsten carbide (SFTC) particles are slightly deformed or partly fused together without decreasing the hardness.

Abstract: Cladding of steel is mainly carried out by hot rolling. This process is very labor-intensive and, therefore, expensive. Cold plating has been used successfully to produce bimetals and could also be an alternative manufacturing process for cladded carbon steel composites. So far, however, only thin narrow IF-steels sheets were successfully cold plated. Different pretreatments and process windows have been used to successfully produce a cold roll-cladded composite of various steel grades on a cold rolling test facility. While joining two similar steels was relatively easy, the combination of different steel alloy compositions was more difficult. Higher necessary forces and edge cracks complicated the experiments. A slight warming of the sheets before joining had a positive effect on the production of the composite. From today's perspective, the required high rolling forces do not allow scaling up to large-scale production.

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: Understanding thermos-physical properties of MMCs includes considering interfacial processes and interactions between the constituents in MMCs. In this context, interfacial bonding is of vital interest for a deeper understanding of composites. Neutron diffraction experiments on Al/diamond composites were performed and reconciled with their thermo-physical properties and quantification of interfacial carbides formation. To create different interfacial conditions both, the contact time during processing the MMCs by liquid metal infiltration and the nominal composition of the matrix were changed, thus creating different amounts of interfacial Al₄C₃ carbides. Neutron diffraction showed the increase in contact time and the addition of Si to Al both increase the bonding strength, although going with a significant decrease of the composite`s thermal conductivity.

Abstract: For the first time tungsten carbide particles were deliberately agglomerated by two-step laser melt injection. After a regular laser melt injection, a second laser melt injection was performed with additional tungsten carbide particles in order to generate a large local agglomerate. The influence of laser power, laser spot diameter, powder feeding rate and pulse time on the agglomeration process was examined and depended on the energy input and tungsten carbide quantity introduced into the metal matrix composite surface. Chemical composition of the agglomerates corresponded with the values of tungsten carbide and the agglomerate hardness was slightly lower. It is intended to apply this agglomerated tungsten carbide for deep drawing tools under Dry Metal Forming conditions.