Band 61, Heft 7

The effect of rhenium and cobalt additions on the microstructure and gamma prime phase stability of modified nickel-based superalloy, grade MGA1400 by means of a vacuum arc melting method with different contents of rhenium (Re) and cobalt (Co) was investigated. After being subjected to heat treatment, the size of the gamma prime particles was slightly reduced with increasing rhenium content. Then, the alloys were exposed at a temperature of 1173 K for 1800 ks. The results showed that increasing the rhenium addition leads to a slower coarsening rate of the gamma prime particles. Furthermore, the shape of the gamma prime particles changed to a more cubic shape. However, an excessive rhenium content (>1.21 wt.-%) promotes the formation of a topologically close-packed phase after a 360 ks exposure at 1173 K. It can be concluded that an increase in rhenium content can result in a lower coarsening rate of the gamma prime particles. The cobalt addition is beneficial for impeding or slowing the precipitation of topologically close-packed phases at an elevated temperature.

Glass-reinforced epoxy composite pipes were exposed to fire for predetermined time durations under dry and wet internal conditions of the pipe. Subsequently, a monotonic burst test was performed, in which the fire-damaged specimen pipes were pressurized internally. Then, a micrograph analysis was performed using scanning electron microscopy. Continuous reduction in the residual burst strength of the composite pipe was noted for both the dry and wet internal conditions. The wet specimens exhibited prolonged functionality compared with the dry specimens, as the water present inside the pipe lowered the temperature of the inner pipe walls and reduced the rate of matrix decomposition. The composite pipe failed owing to the weepage caused by the matrix cracking from the increasing intumescent char formation on the internal pipe surface.

This study focuses on the use of lightweight design and optimization methodology as a computer aided approach to enhancing the conceptual design of an impact attenuator for a Formula SAE race car. The most important question answered here is how to improve conceptual design outlines to assist a designer with respect to an impact attenuator design. In this study, different tools for lightweight design and optimization are compared to find the most suitable procedures and optimization techniques for an impact attenuator design. The geometrical features used as design variables and constraints are defined according to Formula SAE rules. The optimization problem is solved using a genetic algorithm and sequential quadratic programming methods by means of meta-modeling techniques. It is found that lightweight design and optimization can be used to enhance the conceptual design outlines of an impact attenuator through EPP foam, kriging meta-modeling and genetic algorithm optimization techniques.

The intent of this work is to synthesize Al-Fe-Cu-Si-Zn alloy (AA8079)at different milling hours, i. e. 1, 10, 20, 30 and 40 h, by mechanical aloying (MA). X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis was carried out for the milled powders. From the SEM images, a significant change in the size and shape of the particle was observed while the milling time was extended from 1 to 40 h. The effect of milling time on properties such as density, hardness and compressive strength was analyzed. After mechanical alloying, the powders were compacted at three different compaction pressures (500 MPa, 550 MPa & 600 MPa) and sintered in a furnace at different temperatures (500 °C, 550 °C & 600 °C) under a controlled atmosphere for 1 h, 2 h and 3 h. An extreme Vickers hardness of 815 MPa and compressive strength of 327 MPa were achieved for 40 h milled powders compacted at 600 MPa, sintered at a temperature of 600°C for a period of 3 h. A optical microscope (OM) and SEM analysis were conducted for the sintered AA8079 samples to study the effect of milling time and PM parameters on the microstructure.

The material properties of the heat affected zone (HAZ) in welds can significantly differ from the material properties of the base material (BM) due to recrystallization effects during heating and cooling. In this work, the microstructure of the heat affected zones of the low- and high strength steels S355J2, S690QL and S960QL was physically simulated by a Gleeble simulator. The temperature profiles for heating and cooling were taken from in-situ temperature measurements of a MAG welding process. The static and dynamic yield strengths of the simulated HAZ were determined by tensile tests under variable strain rates. The cyclic material properties according to Coffin, Manson and Morrow were determined for the HAZ and for the BM of each steel by strain- and stress controlled fatigue tests. Furthermore, the cyclic stress-strain curve of each HAZ was determined according to Ramberg and Osgood. Due to the lack of data for comparison, two approximation methods were used for the determination of the cyclic material properties and were compared to the investigated ones.

The ever increasing demand for more resource-efficient and safer vehicles in today's automotive industry makes lightweight construction techniques necessary. However, overcoming contradicting requirements arising from lightweight design and safety remains a challenging task. The extent to which lightweight measures can be applied in order to save fuel, heavily depends on the fact that rising safety requirements have to be met by increasing strength of parts. This contradicting demand for parts with high strength and low weight leads to the development of new production technologies. One example, regarding car body components, is the tailor welded blank (TWB) technology. In tailor welded blanks, materials and thicknesses are locally adapted to meet the needed strength and strain properties while keeping the weight as low as possible. While tailor welded blanks consisting of similar materials with different thicknesses are already used in vehicles, the use of TWBs with dissimilar materials, e.g. steel and aluminum, is still in development due to the problems in joining dissimilar materials. Especially when manufacturing parts made of TWBs through joining and subsequent deep drawing, the joint needs to have very good strength properties in order not to fail during forming. One way to overcome these joining difficulties is friction stir welding. In this paper, a methodology is presented to produce multi-material tailor welded blanks with varying thicknesses through friction stir welding (FSW) and deep drawing in a subsequent step. A newly developed FSW joint configuration is used to weld steel sheets in 1 mm thickness to 2 mm thick aluminum sheets. A welding parameter study is conducted to investigate the influence of the process parameters on the joint quality. Tensile and Nakajima tests show that the joint strength, obtained with optimal process parameters, exceeds the strength of the steel base material. Thus, failure occurs in the steel, whereas the joint remains intact. The friction stir welded blanks were furthermore deep drawn. Two different tool approaches were tested to compensate the different sheet thicknesses during the forming process. Using the more suitable approach, blanks were deep drawn with three different punch geometries to show the potential of friction stir welding for the manufacturing of multi-material tailor welded blanks.

This study presents results concerning the vibrational characteristics of a mild steel and carbon/epoxy composite hollow shaft system under different boundary conditions. Finite element analysis has been deployed for critical investigation. The effect of cutout profile and its geometric location over natural frequency and mode shape is studied in detail. From the results, it is clear that the introduction and increase in the number of cutouts leads to a drastic reduction in the natural frequency of the system for both the cases. Moreover, the geometric location of cutouts has a predominant effect in deciding natural frequencies and corresponding mode shapes. The results presented in this study will yield higher levels of interest for engineers dealing with topology optimization problems, where material removal is inevitable.

The abrasive wear and corrosion behavior of as-cast, heat-treated and tempered high chromium white cast iron was studied, and the results were evaluated by means of microstructure, hardness, surface roughness and corrosion resistance. The samples were subjected to wear on the ball-on-flat tribometer test machine under different loads. Corrosion tests were carried out using the potentiodynamic scanning (PDS) technique in both alkaline (3.5 % NaCl) and acidic (0.5 M H 2 SO 4 ) media. According to the results obtained, the wear rate and hardness of the as-cast alloy were relatively low. After heat treatment higher wear resistance and hardness were achieved thanks to the thinned and elongated primary carbides, excessive amount of secondary carbides and the martensitic matrix. After the tempering process, the primary carbides were rounded and shortened, and all the carbides were homogeneously dispersed. This resulted in lower material hardness. Although the hardness of the tempered sample was lower than that of the heat-treated sample, the wear resistance was higher in virtue of the microstructure. The corrosion results showed that the differences between the corrosion resistance of as-cast, heat-treated and tempered samples are only due to microstructural transformation. The differences are related to the Cr/C ratio, primary carbide content and its form in the matrix, and residual stress. In addition, the study showed that secondary carbides play an influential role at the initialization of corrosion but that the primary carbide content in the alloy are dominant in the progression and orientation of the corrosion in the structure.

The effect of mixing alumina (Al 2 O 3 ) nanoparticles in pure aluminum has been investigated, with a characterization of the mechanical properties and erosion–corrosion (E–C) behavior. Specimens of pure aluminum and Al with nanoparticles added as reinforcement have been produced by casting. Uniform dispersion of nanoparticles in the molten aluminum matrix has been achieved by mechanical stirring. Mechanical properties such as yield strength, ultimate strength, modulus of elasticity, and hardness of the new composites have been measured. E–C tests have been conducted to investigate the effect of the nanoparticle additives on the weight loss and surface properties of the new composites. Scanning electron microscopy, electron backscatter diffraction, and energy dispersive spectrometry have been employed to analyze, characterize, and compare pure aluminum and the composites. Al–Al 2 O 3 nanocomposites have been obtained successfully and with a reasonable distribution by adding 1 wt.-% of alumina nanoparticles (100 nm or 20 nm in size) to molten aluminum. The alumina nanoparticles have increased the yield strength and hardness of the new composites when compared to pure aluminum. Enhancement in E–C resistance is also observed under various conditions.

In this study, the static strain aging behavior of a bake hardening steel (BH 220), suitable for cold forming and widely used in the automotive sector was investigated. Tensile specimens prepared from BH 220 steel were first subjected to a 40 minute solution treatment at 920 °C and 970 °C and then cooled in water to generate various grain sizes. Both as-received and heat-treated samples were pre-strained for 3 % tension and then aged at 180 °C for 10, 20, 30, 60, 180 and 300 min. Tensile tests were carried out to obtain the yield strength (YS), ultimate tensile strength (UTS), elongation (%) and bake hardening value (δY) of aged as-received and heat treated samples. Results indicated that BH 220 steel exhibited static strain aging behavior under as-received and heat-treated conditions. Heat treated samples showed higher susceptibility to static strain aging as compared to the as-received samples. Increased grain size in all the as-received and heat-treated samples decreased the YS and UTS but increased the bake hardening value δY.

The cost of composite materials limits the use of these materials. For this reason, many studies have been done with composites to reduce costs. In this study, the filler effect on vinylester matrix composites was investigated in order to contribute to cost reduction. Calcium carbonate (CaCO 3 ) and aluminum hydroxide Al(OH) 3 were used as filler, and E-glass was used as reinforcement material. In addition, the filler ratio, which was added to the structure, was 5 wt.-%,10 wt.-% and 15 wt.-%. The production method was vacuum infusion, and the manufacturing of the composite plates completed. A tensile test, bending test, hardness test and ash test were performed on the samples. Also, microstructural images were examined by scanning electron microscope. Finally, it was found that 15 wt.-% CaCO 3 filler increased hardness considerablely. Tensile properties and flexural strength properties did not change dramatically with the addition of filler. Moreover, the addition of filler decreases the cost of the composite. Thus, when we compare the cost effectiveness of fillers, CaCO 3 filler on a vinylester matrix composite is advantageous for decreasing cost.

In this study, the AC impedance, the complex dielectric function, and the complex electric modulus properties of mercury based copper oxide layered cuprates, which have superconducting properties below the critical transition temperature, have been investigated at room temperature. The HgBa 2 Ca 2 Cu 3 O 8+x samples were synthesized by a conventional, solid state reaction preparation method with an identical stoichiometric ratio. The varied oxygen content of the sample was obtained by oxygen annealing. The AC impedance and the related dielectric and electric modulus properties of non-oxygen doped (i. e. as grown) and optimum oxygen doped HgBa 2 Ca 2 Cu 3 O 8+x samples, was measured by an impedance analyzer at room temperature within the frequency interval of 10 Hz–10 MHz. AC impedance spectroscopy analyses of the samples suggest Koop's bilayer model for the structure of Hg-based perovskites. Additionally, the optimally oxygen doped process increased grain and grain boundary resistance along with decreasing grain size. Moreover, AC conductivity results have also confirmed the increased resistive behavior of the sample due to oxygen doping. Furthermore, complex dielectric function analyses have determined an oxygen doping induced negative dielectric constant for the mercury cuprates. In this context, optimally oxygen doped mercury cuprate has a promising potential for metamaterial coating applications.

High chromium white cast iron (HCrWCI) is generally used in applications requiring superior wear resistance. The alloys were inoculated with 1–3 wt.-% TiBAl to rearrange the crystallography of the carbides. The alloys were subject toheat treatment at 1000 °C for 1 and 2 h. The microstructuresof the alloys was examined by SEM. Chemical composition was determined by EDS. For wear tests, alloys were tested using 10, 20 and 30 N loads under pin-on-disc wear conditions. Worn surfaces of the alloys were analyzed by SEM. The worst wear resistance was obtained in the alloy inoculated with 3 wt.-%. TiBAl.

After hip replacement surgery there will be a clear distinction between the stiffness of the metallic implant and the more supple femur bone which will cause stress shielding and bone resorption. In order to treat this phenomenon, the stiffness of the femoral stem implant must be reduced. The effect of using porous and non-porous implants of steel, titanium and FGM will be studied in this research. The assembly of the femur bone and the implant was modeled by means of the 3D finite element method. It will be noticeable from this analysis that for all stem materials using porous implant materials lead to an increase in stress in cortical and cancellous bone as compared to non-porous implant materials which lead to reduced stress shielding and bone resorption. Gradually increasing the porosity of a steel stem from the distal part of the stem to the proximal part increased maximum von Mises stress at the proximal medial cortical bone by 29 % when compared to the non-porous steel stem. By contrast, von Mises stress at the proximal medial cortical bone increased by about 21 % for porous titanium and porous FGM as compared to non-porous titanium and FGM stem implants.