Volume 58, Issue 1

This study demonstrates how the synchrotron X-ray absorption spectroscopy can be applied for the quantification of precipitation in high strength low alloyed steel. The fraction of vanadium in different forms of compounds, as well as in the solid solution, was measured using a linear combination fit of the spectral weights. The technique confirms substantial enhancement of precipitation by lowering the cooling rate. A significant fraction of vanadium atoms of over 0.6 remained in the solid solution when the cooling rate was as high as 60 K × s −1 . In contrast, the precipitation of vanadium was almost completed by decreasing the cooling rate to 0.1 K × s −1 . The results corresponded to the proof by scanning transmission electron microscopy.

In this study, the wear behavior of uncoated, TiN and AlTiN coated cold work tool steel 1.2379 which is widely used in the mold industry was investigated experimentally. Heat treatment was applied to the specimens, then TiN and AlTiN PVD coating process was performed. The wear tests were carried out at 0.5 m × s −1 sliding speed, 5, 10 and 15 N loads and 120 m sliding distance by using reciprocating abrasion device. The microhardness was measured and metallographic tests of the samples were investigated by SEM and EDS analysis. In order to examine the effect of process parameters on wear results, a statistical method such as analysis of variance (ANOVA) was employed. A mathematical model was created by using regression analysis based on both linear model and quadratic model for predicted wear value. Process parameters were optimized using response surface methodology with desirability function analysis. It is observed that the uncoated specimens worn approximately two times more than AlTiN coated and one time more than TiN coated. In the SEM images and EDS analysis, it is seen that the coating is spread uniformly on the materials.

In this paper, the friction stir welding process was numerically investigated for 6 mm thick aluminum alloy AA2024-T3. The finite element software COMSOL Multiphysics was used to calculate the transient thermal field during welding and the mechanical reaction depending on different mechanical clamping conditions and hardening models subsequently. A thermal pseudo-mechanical (TPM) heat source was implemented. Softening effects of the material due to precipitation hardening dissolution caused by the frictional heat were accounted for. The transient temperature evolution measured by thermocouple elements at various locations was compared to the numerical results. A good agreement was found for the thermal field. A sensitivity study of the mechanical models showed the strong influence of the clamping conditions and the softening model.

The method of Bayesian analysis of confidence in the testing evidence collected during ultrasonic automated testing of high quality aluminum ingots for minute defects of sub-millimeter size is discussed. These test results are assessed in the view of true or false evidence. The analysis demonstrates the significant influence of false evidence on the confidence in the test outcome where the evidence is skewed in the direction of false positive. At the same time the influence of the false negative evidence is shown to be more muted. To minimize the risk of false rejection of ingots with “ghost” indications not representing the physical defects, the off-line ingot retesting by an A-scan with defect waveform verification is implemented. Retesting of fabricated blanks made of the ingots previously passed the ingot testing is used to minimize the risk of passing products with defects earlier undetected.

Active thermography with lock-in excitation is a nondestructive testing method that is also feasible for testing of carbon fiber reinforced polymers (CFRP). For validating the method extensive investigations were done during a research project in order to advance a standardization process. The most important parameters of optical and ultrasonic excitation thermography were investigated. For example, the appropriate selection of spectral sensitivity of the used infrared camera systems is important for recording undisturbed thermal signals. Regarding excitation, influences of excitation power and ultrasonic frequency were studied. Furthermore, material parameters such as CFRP layup are known for strongly affecting the results of measurement.

Although the number of experimental studies investigating the effect of friction stir welding (FSW) parameters on joining properties have increased recently, there are not sufficient numbers of studies on the effect of the stirring tool material in friction stir welding. This study investigated the effect of stirring tool materials on microstructure, mechanical properties and residual stress of joints. Samples of 7075-T651 Al alloys were joined by FSW using uncoated and TiN-coated X210Cr12 alloy steel stirring tools. The welding processes were performed at rotational speeds of 900, 1250 and 1600 rpm and at a welding speed of 60 mm × min −1 . Mechanical and metallographic tests were applied to the welded joints and residual stress analysis was performed using the hole drilling method. The best mechanical properties were determined in the welded samples joined by the uncoated tool at a rotational speed of 900 rpm. It was also determined that the TiN-coated tool negatively affected the mechanical and metallurgical properties of the weldements. Moreover, the highest longitudinal residual stresses were specified in the joining performed by the uncoated tool.

Extrusion-shear (ES), a novel severe plastic deformation (SPD) method which includes initial forward extrusion and a subsequent one step shearing process, has been developed to fabricate fine grained AZ61 Mg alloys in a Gleeble 1500D thermomechanical simulator for the first time. The effects of the ES process on the grain refinement of an as cast magnesium alloy were investigated at different extrusion speeds and the microstructures of the as processed billet and rods were investigated at right, central and left positions. It was found that the extrusion force caused by the ES process at 5 mm × s −1 is about 5 times of that caused at an extrusion speed of 1 mm × s −1 . Besides, the increase of extrusion speed could improve the deformation stress. The ES extrusion causes serve plastic deformation and improves the dynamic recrystallization during the process. The results confirmed that the ES process can be used to refine the grains effectively and to produce serve plastic deformation. The microstructural investigation showed that grain sizes are finer in the bottom parts than in the other parts of the rod.

Nickel-based alloys are a crucial class of materials because of their excellent corrosion resistance. In the present study, single layer and two layers alloy 625 weld overlays were deposited by GTAW process on A516 grade 70 carbon steel. The dilution in terms of Fe, Ni, Mo and Nb content was calculated in 30 points of weld overlay. Microstructure observations showed that alloy 625 had austenitic structure with two types of Laves and NbC secondary phases. The uniform and pitting corrosion resistance of alloy 625 weld overlay as casted and as forged were evaluated in accordance with ASTM G48-2011 standard at different temperatures to determine the weight loss and critical pitting temperature. For achieving a better comparison, samples from alloy 625 as casted and as forged were tested under the same conditions. The results point out that single layer alloy 625 weld overlay is not suitable for chloride containing environments, two layers alloy 625 weld overlay and alloy 625 as casted have acceptable corrosion resistance and almost the same critical pitting temperature. Alloy 625 as forged has the best corrosion resistance and the highest critical pitting temperature among all test specimens. Also, the corrosion behavior was evaluated in accordance with ASTM G28 standard. The corrosion rate of single layer weld overlay was unacceptable. The average corrosion rate of two layers weld overlay and in casted condition were 35.82 and 33.01 mpy, respectively.

Due to their excellent thermomechanical properties, as well as their good media resistance, phenolic molding compounds are outstanding materials for high temperature applications, such as components for the engine compartment. An additional post-curing process, after injection molding, shifts the glass transition temperature towards high temperatures up to 300 °C. The objective of this work is to analyze the influence of the post-curing process on the chemical structure of the material. This structure is responsible for the mechanical and thermal properties of the material. Therefore tensile test bars were molded with a highly filled phenolic molding compound and different post-curing cycles under different atmospheres were done. The tensile properties and the glass transition temperatures (TMA) were measured. To get detailed information of the chemical structure ATR-FTIR and NMR analyses were done. The mechanical properties decreased up to 25 % with higher post-curing temperature (under air atmosphere) although the glass transition temperatures (TMA) and the grades of curing (NMR) increased. Furthermore, in the ATR-FTIR measurements, oxidation products were detected on the surface of the sample which were post-cured under air atmosphere. In comparison, no oxidation products could be detected on the samples post-cured under nitrogen atmosphere and the mechanical properties just decreased about 4 %. Oxidation processes could be provided by post-curing under nitrogen atmosphere which led to higher mechanical properties, in connection with a high grade of cure and a higher glass transition temperature in comparison with post-curing under air atmosphere.

In this study, Co-based Cr-Mo powder alloy was produced at different pressing temperatures by using hot pressing technique and abrasive wear behaviors of the produced specimens were examined. Produced specimens were exposed to abrasive wear experiment using block on disc wear test device by applying a load of 50 N with 100-mesh SiC abrasive paper. Each specimen was investigated at 25, 50, 75 and 100 m. At the end of the experiment, abrasive wear results of the specimens were determined by calculating their mass losses. Microstructural properties of the specimens which were produced at different pressing temperatures were investigated by optical and SEM examinations and their wear resistances were examined by abrasive wear experiments. Consequently, it was observed that in Co-based Cr–Mo powder alloy produced by hot pressing technique; as sintering temperature increased, size of neck formations between the powder grains increased, porosity decreased and abrasive wear resistance increased.

In this study, micro-size drillability of Hardox 500 was investigated by the electro-discharge machining (EDM) method. For this purpose, holes were drilled by using different machining parameters (discharge current, electrode rotational speed, dielectric spray pressure) and the effect of these parameters on machining performance and hole profile was specified. At the end of the tests, it was detected that discharge current and electrode rotational speed increased material removal rate (MRR), electrode wear rate (EWR) and hole dimensional error (DE) whereas dielectric spray pressure increased the MRR values and decreased the EWR and DE values.

In literature, a lot of research works have been presented on crashworthiness in order to develop crash performance of vehicles and thin-wall structures. In this research, a new hybrid optimization algorithm based on gravitational search algorithm and Nelder-Mead algorithm is introduced to improve crash performance of vehicles during frontal impact. The results show that the hybrid approach is very effective to develop crash performance of the vehicle components and thin-wall structures.

The fuel and cost efficiency in automotive industry is related to the designing of light-weight vehicle components. Main aim of this paper is to show effectiveness of two new optimization algorithms in solving structural design optimization problems. The gravitational search algorithm (GSA) and charged system search algorithm (CSS) are applied to the optimum design of a vehicle component. The results show that both GSA and CSS have a better search capability than the genetic algorithms to find optimum solutions.

Nine different contents of sodium hydroxide solutions, silt, and ordinary Portland cement were selected as basic materials to make the cemented soil blocks. The electrical resistivity under different current frequencies and unconfined compressive strengths were measured after curing for 3, 7, 14, 28, 60 and 90 days, respectively. The results show that the alternating current electrical resistivity decreases with the increase in current frequency for all content values. The higher the current frequency is, the smoother the electrical resistivity. The current frequency range of 5 × 10 4 to 10 6 Hz is recommended to reduce the error resulting from the slight variation of current frequencies. The strength and electrical resistivity increase logarithmically with the increase in curing time. With the increase in content, the strength first increases linearly and then decreases linearly. The strength reaches the peak value at the content of 7.5 g × kg −1 . The electrical resistivity decreases exponentially. A useful prediction formula for the strength is proposed, which was established according to the linear relationship between the strength and the electrical resistivity. Based on the influencing degree of sodium hydroxide on the strength of cemented soil, an evaluation standard for the electrical resistivity of cement-solidified sodium hydroxide-contaminated soil is also proposed.