Band 63, Heft 7

In the field of dental technology, the length of ceramic pontics is limited to avoid mechanical failure. To reduce thermal-induced residual stress within the ceramic, using smaller subcomponents and subsequent bonding with silicate-based glass solder may be a favorable approach. Thus, the bending strength of zirconia compounds bonded with different silicate-based glass solders was investigated. For this purpose, rectangular specimens made of zirconia were bonded by glass solder. Parameters such as the scarf angle (45° and 90°), two different glass solders, as well as the soldering process (pressure and surface treatment) were varied. All specimens were subjected to quasi-static four-point bending tests according to DIN EN ISO 843-1. Additionally, the quality of the glass solder connection was evaluated using μ CT and fractography. In the present study, zirconia compounds were sucessful bonded of zirconia compounds using silicate-based glass solder was. No significant differences in terms of bending strength were observed with respect to the different bonding parameters analyzed. The highest bending strength of 130.6 ± 50.5 MPa was achieved with a 90° scarf angle combined with ethanol treatment of the specimens before soldering and an additional application of a pressure of 2 bars in a dental pressure pot before subsequent soldering. Nevertheless, the bending strengths were highly decreased when compared to monolithic zirconia specimens (993.4 ± 125.5 MPa).

In this study, a Ti-22Al-25Nb alloy with nanocrystalline structure was produced by high energy mechanical alloying (HEMA) and 1 at.-% yttrium was added as a thermal stabilizer. The as-milled samples were annealed at various temperatures up to 900 °C in a protective gas atmosphere, and the samples were allowed to cool to room temperature in the furnace. The phase transformations and microstructural changes as a function of the annealing temperatures and alloy compositions were studied using room- and high-temperature X-ray diffraction (XRD), focused ion beam microscopy (FIB), and scanning electron microscopy (SEM). The mechanical properties of the samples were interpreted based on the hardness results and their correlation with the microstructures. The results showed that the as-milled nanocrystalline structure of Ti-22Al-25Nb alloy increased from 3.4 nm to 350 nm after annealing at 800 °C due to the high driving force induced by the large grain boundary area. Consequently, the as-milled hardness of the Ti-22Al-25Nb alloy dropped from 7.63 ± 0.18 GPa to 5.37 ± 0.28 GPa. The grain size stability of the Ti-22Al-25Nb alloy after annealing at elevated temperature was ensured through the addition of yttrium. Thus, the grain size remained at the level of 125 nm, and the hardness value was maintained at around 6.98 ± 0.43 GPa after annealing at 800 °C.

During hot ring rolling and subsequent air-cooling processes, the as-cast metal alloy undergoes a complicated microstructural evolution. In this paper, the grain refinement of as-cast 42CrMo ring billet during hot ring rolling and air-cooling was conducted by FEM simulation and tests. Moreover, the grain refinement mechanism of as-cast 42CrMo was also studied by comparison of single-pass deformation and multi-pass deformation with short pass interval time, with the purpose of studying the influence of the deformation process on grain refinement supported by the results of FEM simulation. As a result, effective strain and average grain size of the ring show zonal distribution characteristics The effective strain on the inner and outer layers of the ring is large, contributing to fine and homogeneous grains. In contrast, the cumulated effective strain on the interlayer of the ring is small, resulting in inhomogeneous and mixed grains and large average grain size. Grain growth occurs during subsequent air-cooling. The microstructural distribution of the hot rolled ring was confirmed by a hot ring rolling test.

Creep cracking is one of the key forms of structural material SCC damage with respect to nuclear power. Accurately obtaining the amount of creep deformation is also an important basis for estimating the service life of structural parts. However, because the primary circuit of nuclear power occurs in a high-temperature and high-pressure service water environment, it is not possible to use a conventional extensometer to obtain accurate creep of gauge length under these conditions. Considering that DCPD is an important method for monitoring crack propagation in a high-temperature water environment, by taking the austenite 304 stainless steel commonly used for nuclear power as a research object, a calibration method based on a combination of DC potential drop (DCPD) and strain testing to obtain the creep deformation of the specimen was established. By comparing theoretical research with experimental results, it can be concluded that the calculation results of the model are close to the experimental results and consistent with the theory, thus proving the feasibility of using DCPD technology to obtain the creep deformation amount.

On-site investigation, scanning electron microscope, energy dispersive X-ray spectroscopy, and cyclic potentiodynamic polarization test were carried out for failure analysis of the 316 L pipeline in this paper. The visual inspection revealed that the inner wall was covered with severe rust, and obvious misalignment and poor appearance were found in the weld. The energy dispersive X-ray spectroscopy result identified the presence of Cl in the inner wall of the pipeline. Some carbides of Cr and Mo precipitated in the heat affected zone, which partially deteriorated the corrosion resistance. The cyclic potentiodynamic polarization curve proved that the heat affected zone is not only sensitive to pitting corrosion, but also has inferior repassivation ability. Finally, the pitting preferentially occurred in the heat affected zone and gradually developed to leakage.

While X-ray based darkfield imaging with grating interferometers is already widely used, darkfield imaging with neutrons has still a relatively small user community focused mostly on magnetic materials. Here, we demonstrate the application of neutron darkfield imaging byTalbot-Lau type grating interferometry to fiber reinforced plastics. Common carbon and glass fiber composites have been investigated including characteristic damage structures. The darkfield images show a strong signal response caused by fiber delamination, suitable fiber direction, particles, pores and cracks. The basic principles of neutron darkfield imaging applied to fiber composites are highlighted.

In the study for this contribution, production of in situ synthesized TiB2 particles in iron-based composite coatings using four different submerged arc welding powders (fluxes) containing increasing amounts of ferrotitanium and ferroboron with S1 welding wire, were targeted. For this purpose, coating deposition was carried out to improve the hardness and wear properties of the AISI 1020 steel surfaces using hybrid submerged arc welding. In hybrid submerged arc welding, the welding pool is protected by both welding powders and an argon gas atmosphere. To examine the composite coatings, visual, chemical, microstructural analyses and hardness and wear tests were carried out. With the use of increasing amounts of ferrotitanium and ferroboron in the welding powders, it was observed that the microstructure of the coatings changed in terms of TiB2 particle geometries such as rectangular and hexagonal; volume fractions of TiB2 particles in the coating microstructures increased; hardness values of coatings were enhanced from 34 HRC to 41 HRC; the wear resistance of the coatings improved, and worn surface images of the coatings caused by the counter body changed from continuous with deep scratches to discontinuous with fine scratches and crater cavities.

Factors such as the surface geometry of a joint, the direction of the applied load, and the type of adhesive used have a great influence on the strength of a joint in adhesive bonding. In adhesively bonded joints (ABJ), it is possible to improve surface geometry by forming various geometric surfaces. ABJs are not very resistant to peeling stress, thus requiring that a bonding model be analyzed according to the direction of the applied load to prevent peeling stress. In this study, a butt curved joint was prepared from aluminum plates (A2024-T3) to improve the surface geometry of the joint. The mechanical behavior of the joints in three-dimensions and subjected to bending were investigated depending on an increase in the curvature radius. The adhesive DP810 was used for bonding. The finite element analysis was performed in ANSYS and cohesive zone modeling was used for a simulation of the damage growth in the adhesive layer. The results of bilinear and exponential models were found to be more appropriate to the experimental results. When the radius of curvature increases, the damage load carried decreases in the butt curved lap joints. It was seen that decreases in the curvature radius significantly decrease normal stress.

In this study, the failure mechanism of glass particle epoxy composites was investigated under compression and sliding wear. Random fiber distribution with minimum interfiber distance was modeled by representative volume elements (RVEs). Spherical and platelet type glass particles were used for the reinforcements. A numerical simulation of the elastic properties of composites was performed for a perfectly bonded interface, and the results were compared using the Mori Tanaka mean field approach. The elastic stiffness results indicated that the platelet reinforced composites bore more load than spherical ones because of the aspect ratio effects. The separation distance based cohesive zone model was applied to modeling the failure zone at the particle matrix interfaces to establish sliding wear. The effect of the perfectly bonded interface and the cohesive zone interface on overall stiffness and elasto-plastic behavior were discussed. The cohesive zone interface was found to be effective at the interface in terms of the strength and debonding characteristics of the composites. The results were compared with the sliding wear test results of glass particle reinforced composites. The numerical and sliding wear experimental results indicated that matrix yield stress, plastic strain, particle penetration at the contact interface and particle stress are found to be effective parameters for the debonding mechanism.

During the extrusion of magnesium alloys, temperature change could have a significant effect on the outcome. When this effect is not considered, some commonly known defects might be observed, such as hot cracking. In this study, all samples consist of extruded AZ31 and AM50 magnesium alloys as a solid profile, but the methods by which they are cooled, such as air cooling and water quenching, vary. The effects of cooling methods on tensile-compression behavior and the microstructural properties of the samples were investigated. Test samples were obtained in extrusion direction and perpendicular to the extrusion direction separately for mechanical tests. The main purpose of this study was to investigate the effect of different cooling methods on the mechanical properties and microstructural behavior of AZ31 and AM50 magnesium alloys after extrusion, once different cooling methods were applied. According to the microstructural investigation results, an AM50 magnesium alloy has a finer grain structure as compared with an AZ31 alloy according to both cooling methods in the extrusion process. The average grain size values of both alloys were found to be higher for water cooling. Cooling methods have significant effects on the tensile properties of both alloys, depending on their extrusion directions.

In this study, bainitic microstructure was formed via heat treatmenton R260 rail steel, which is generally used in railways. Bainitic steel, which is considered more advantageous than current rail steel, waswelded by flash butt welding, which is often used for joining rails andthe mechanical and microstructure of the samples were thenexamined and compared. Bainitic structural steel obtained by austempering heattreatment with normal rail steel was welded by flash butt welding. Flash-butt welding parameters were kept constant during the experiment. The welding capabilities of the joints were compared and the results wereevaluated. It was determined that the bainite structure obtained as a result of austempering heat treatment changes the microstructuralproperties of the samples and affects the mechanical values ​of the joints.

Stir cast aluminum based metal matrix composites (MMCs) with silicon carbide (SiC) reinforcement particles consist of cast product dendrites and large agglomerated reinforcements. The agglomeration of SiC particles creates a difference in properties in the composite system. Friction stir processing (FSP) is used to overcome the uneven distribution of SiC particles in the aluminum matrix. The friction stir processed (FSPed) microstructure is significantly influenced by the process parameters used for processing. In FSP, the effect of the tool diameter ratio (tool shoulder diameter to pin diameter) on material flow, material mixing, material consolidation is more predominant than other parameters. Hence, a perfect combination of shoulder diameter to pin diameter is required to produce sufficient material flow. In this investigation, the tool diameter ratio is varied from 2.0 to 4.0 for processing composite material and thus FSPed zones were subjected to micro structural characterization. It was concluded that a tool diameter ratio of 3.0 yielded a defect free stir zone with higher hardness compared to other ratios.

Hydrostatic measurement, a method traditionally used to measure the density of solid bodies, is not suitable for all solid bodies. This method is undesirable for solid materials that interact with water and lose their properties. In addition, this method is not suitable for porous objects because measurements in water are erroneous and can damage material samples due to the ability of some solid materials to absorb water. In this study, a new density measurement technique has been developed and evaluated to measure the density of rigid objects by means of nonstandard geometry. The density of objects with distorted geometry was measured pneumatically using communicating vessels logic and using the ideal gas equation. An experiment set has been developed, improved and evaluated. Through this technique, the measurement accuracy of the density of the sample tested was determined with an accuracy of 0.08 %.

The research in this paper is a sequel of an earlier work by the author in which experimental and CFD results were compared for an absorber plate made of iron with and without fins for two flow rates. The research yielded a good comparative result between the experimental and computational process for an optimized flow rate and the effect of the fins. The objective of this paper is to verify the effect of the overlay composite absorber plate material on a solar air heater through experimental and computational fluid dynamics. The experimental setup consists of an absorber plate as an overlay composite of aluminum and copper for enhanced heat transfer. Experiments and CFD analysis were done in three configurations. In configuration one, only the aluminum absorber plate with fins was considered. In configuration two, the overlay composite was considered with copper on the top and aluminum at the bottom as fins, and in configuration three, the overlay composite was considered with aluminum at the top and copper at the bottom as fins. A transient 8 hours CFD analysis was carried out using these configurations. While validating the results it was found that the overlay absorber plate Cu-Al was capable of generating a high outlet temperature Max of 88 °C and capable of generating 83 °C air for 5 hours and had good thermal efficiency when compared to the other materials in the other two configuration. It was found that experimental and computational analysis were in very close agreement, and the margin of error between the experimental and computational processes was less than 8 %.