Band 65, Heft 2

Magnesium alloy materials are often affected by cyclic stress, resulting in fatigue fracture due to changes in mechanical properties. In this study, external cyclic loads were introduced to simulate the loading of AZ91D (Mg-9Al-0.5Zn) parts. Tensile tests under static and external circulation loads were performed on the same specimens, and the effect of loading conditions on the mechanical properties of the specimens (tensile strength, yield strength and elongation) were studied. The internal microstructure of the test specimens was analyzed, and the influence of cyclic loading on the microstructure change of the specimens was discussed. Compared with static load, under the condition of cyclic loading, the tensile strength and yield strength of the specimens are increased by more than 15%, and the elongation rate is increased by 3 times. In addition, the microstructure images showed that many twins and the second phase β-Mg17Al12 were generated inside the AZ91D microstructure under cyclic loading. The former results in specimens crack initiation and it is the root cause of fatigue fracture. And the latter accounts for the strengthening of the mechanical properties of specimens.

Selective laser melting is a production method that results in a large amount of residual stress due to high cooling rates and high thermal gradients. Although there are many studies examining the effects of process parameters on residual stress or mechanical properties in the literature, there are a few studies investigating the effects of changing laser power and scanning velocity (exposure time) at constant energy density on residual stress or mechanical properties and these studies have different results. This is a comprehensive study in this field that includes detailed comparisons with the results of similar studies in the literature. In this study, firstly specimens were produced at different process parameters and it was tried to find the process parameters that will obtain the highest relative density among the trials. Then at the constant energy density (85.0 J mm −3 ), which the maximum density has been obtained the effects of changing laser power and scanning velocity on residual stress, mechanical properties, microstructure and relative density were investigated. It was observed that at constant energy density, increasing or decreasing laser power and scanning velocity did not increase or decrease residual stress, tensile strength, % elongation and relative density monotonously.

The aim of this study is to characterize E-glass/epoxy unidirectional fiber reinforced composites using the digital image correlation method with virtual extensometer, which is a less laborious method than strain gauges, compare the results and investigate whether virtual extensometers can be used instead of strain gauges. Measurements in tensile and Iosipescu shear tests were made with both strain gauge and virtual extensometer. Unlike full-field strain measurements in literature, the strains were measured using virtual extensometers. Tensile test and in-plane shear test results gave very consistent results. The differences between the strain gauge and the virtual extensometer for the tensile and in-plane shear tests were less than 3% in the linear region. However, the out-of-plane shear test showed a larger difference of 8.6%. This study showed that the 2D digital image correlation method with virtual extensometers is highly sufficient to find the elasticity moduli and shear moduli in tensile and shear tests in the linear region. In addition, after the damage has started, more measurement data can be obtained with virtual extensometers than with strain gauges.

The accuracy of high-temperature constitutive models is essential for the establishment and optimization of thermomechanical processing, and is one of the most relevant factors determining the reliability of finite element simulation results. In this study, attention is paid to propose a new coupled Johnson–Cook constitutive model for predicting the flow behaviors of titanium alloy TA31 composed of bimodal microstructure, and to implement it in the finite element software Abaqus. The proposed model not only takes into account the strain softening phenomenon of titanium alloy but also the coupled interactions of strain, strain rate and temperature. The correlation coefficient R  and average absolute relative error AARE of the modified constitutive equation were 0.9885 and 3.6%, respectively. The mathematical formulation of the new constitutive model allows for accurate predictions of the flow stress of TA31 titanium alloy, while retaining a low identification cost of the unknown coefficients. A subroutine VUHARD of this model was developed, and its reliability was verified in the case of the modeling of one element test. It is helpful to understand the deformation behavior of titanium alloy TA31 and provide valuable references for optimizing the thermomechanical processing.

In this paper, epoxy acrylate/nickel-plated carbon nanotubes composites with different structural orientations (X, Z) were prepared by digital light procession 3D, and the orientation of nickel-plated carbon nanotubes in the magnetic field and its effect on the mechanical properties of the composites were systematically studied. The scanning electron microscope shows the orientation of the fiber in the magnetic field. The experimental results show that the mechanical properties and friction coefficient of the material show obvious anisotropy, but the anisotropy of wear rate is not obvious. The tensile strength of the X sample is about 18% higher than that of the Z sample when the content of nickel-plated carbon nanotubes was 0.15 wt%. The impact strength of the X sample is about 15% higher than that of the Z sample when the content of Ni-MWNTs was 0.1 wt%. The friction coefficient on the plane (X, Y) is about 20% lower than that on the plane (Y, Z) when the content of carbon nanotubes is 0.05 wt%.

Optimization of engineering discipline problems are quite a challenging task as they carry design parameters and various constraints. Metaheuristic algorithms can able to handle those complex problems and realize the global optimum solution for engineering problems. In this article, a novel generalized normal distribution algorithm that is integrated with elite oppositional-based learning (HGNDO-EOBL) is studied and employed to optimize the design of the eight benchmark engineering functions. Moreover, the statistical results obtained from the HGNDO-EOBL are collated with the data obtained from the well-established algorithms such as whale optimizer, salp swarm optimizer, LFD optimizer, manta ray foraging optimization algorithm, hunger games search algorithm, reptile search algorithm, and INFO algorithm. For each of the cases, a comparison of the statistical results suggests that HGNDO-EOBL is superior in terms of realizing the prominent values of the fitness function compared to established algorithms. Accordingly, the HGNDO-EOBL can be adopted for a wide range of engineering optimization problems.

A new AlCrFeSrTiBSi master alloy was manufactured by in situ synthesis in Al melt, and compared with AlBSr master alloy. Microstructures of A356 alloy modified with AlCrFeSrTiBSi master alloy were investigated, and the structural details of the new cast A356 alloy were evaluated by X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), microhardness and differential thermal analysis (DTA). AlBSr master alloy typically formed α-Al and granular SrB 6 phases in A356, and the samples inoculated with AlCrFeSrTiBSi master alloy consisted of additionally AlCrFeSi phase with irregular blocks. The addition of 0.1 wt%AlCrFeSrTiBSi alloy to the A356 alloy significantly refined and modified the grain structure together. The structure of eutectic Si converted from acicular form to fibrous. The size of α-Al dendrites declined from ∼1000 μm to ∼100 μm. The strength values of the A356 alloy were developed by ∼70% with the addition of 0.1 wt%AlCrFeSrTiBSi master alloy.

Ni-based G-NiCr28W alloy, which is widely preferred in high-temperature applications, has an austenite matrix as well as carbides dispersed in the structure. The morphology of the carbides in the structure varies depending on the chemical composition and heat treatment. Carbide phases commonly seen in these alloys are MC, M 7 C 3 , and M 23 C 6 . These carbides can transform into each other and the mechanical properties of the alloy can be affected by the carbide transformation. In this study, the effects of the carbides on the morphology, distribution, and strength of the alloy were investigated following the solution treatment applied at different temperatures and durations. Samples were characterized by light metal microscopy, SEM, EDS, XRD analysis, hardness, and potentiodynamic polarization tests. After the process, due to the excess of more stable carbides and their homogeneous distribution within the structure, the highest hardness and corrosion values were revealed in the sample that was processed at 1000 °C for 100 h.

Twisted variable channel angular pressing (TV-CAP) is a novel method. While it combines the advantages of equal channel angular pressing (ECAP), twist extrusion and direct extrusions, also it eliminates the disadvantages of these methods. Finite element analysis was also carried out in order to examine the design parameters, material flow and examine the effective strain values. Hardness and tensile tests were performed to examine the effect of TV-CAP on the mechanical properties of AA6082. In addition, optic microscope, SEM and TEM images were taken respectively and XRD, EDS and EBSD analyses were accomplished in order to investigate the microstructural analysis. As a result of this study, it has been observed that the material has hardened approximately 3 times compared to the annealed material and became 1.5 times stronger in terms of ultimate tensile strength. It was also concluded that, this new method is more efficient than twist extrusion and multi-pass equal channel angular pressing processes.

Dry friction and wear behavior at high temperatures was investigated against an Al 2 O 3 ball in order to characterize the wear rate, coefficient of friction (COF), hardness and thermal friction behavior at different temperatures in the wear-treated zones of the CuCrZr copper alloy. Experiments were fulfilled using a pin-on-disk wear device with the aid of a temperature modulated wear device at room temperature, 100, 200 and 300 °C. The loads applied during wear are given as 3N, 5N and 7N, respectively. In order to examine the morphological structure of the worn surfaces, appropriate samples were prepared and SEM (scanning electron microscopy) device was used. Similarly, EDS (energy dispersive x-ray spectrometry) was applied to worn surfaces to examine the abrasive-material interaction. Against the hardness of the material at 25 °C, the hardness value in measurements at 300 °C showed a small increase. The COF value increased at 100 °C and then decreased, while the wear rate increased significantly as the temperature increased. The temperature increase starting from RT up to 300 °C weakened the internal structure of the CuCrZr alloy and caused an increase in the wear rate. XRD test was used to show the phases that may occur in CuCrZr alloy due to temperature and heat treatment.

AISI 1040 and AISI 304 steel plates of 10 mm were joined without pretreatment by double-sided TIG arc welding (DSAW). Joints were manufactured by using welding currents of 420, 440, and 460 A. The microstructural variations in the interface of the weld samples were defined by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and microhardness analysis. V-notch impact and tensile tests were done to detect the weld strength of the weld samples. In DSAW welding of different steels, a full penetration joint was achieved without opening the weld edge. The current intensity had a major effect on the symmetrical and hourglass shape in welds. Welding at 460 (A) showed acceptable joint quality. Tensile and impact energy quantities of welded joints had significant ratios. Fractures in the weld metal of the samples were ductile mode.

In this study, mechanical and tribological properties of the borided dual-phase α  +  β type Ti6Al4V titanium alloy were examined. For this purpose, Ti6Al4V alloy samples were borided for 6 h at a temperature of 1100 °C by the powder-pack boriding process. As a result of boriding, a boride layer consisting of TiB 2 with a thickness of max ∼25 µm and TiB phases with a thickness of max ∼10 µm was obtained on the Ti6Al4V sample surfaces. As a result of the boride layer’s nanoindentation tests carried out using the Berkovich indenter, it was found to have an elastic modulus of 534.255 GPa and a hardness of 36.537 GPa. Wear tests were carried out using the pin-on-disc method under a load of 10 N and with a sliding distance of 1000 m. Whereas the dominant type of wear in non-borided samples was abrasive wear, oxidative mild wear was generally observed in borided samples. In borided samples, as a result of becoming of surface smoother by hard asperities breaking and increasing the actual contact area, the friction coefficients increased. It was determined that with boriding, the wear performance of Ti6Al4V alloy improved ∼46.8 times against the Al 2 O 3 counterpart and ∼4.57 times against WC-6Co counterpart.

Delamination is the most important problem encountered in drilling composite materials. This problem can be minimized by optimizing the cutting parameters. In this study delamination behavior of glass/carbon/aramid fiber reinforced hybrid composite was investigated during the drilling operation. Experiments were planned according to Taguchi’s L27 orthogonal array. Cutting velocity (20 m min −1 , 40 m min −1 , and 60 m min −1 ), feed rate (0.04, 0.08, and 0.12 mm rev −1 ), and cooling type (wet, air, and dry) were selected as cutting parameters. Thrust force was measured during the drilling operation, and the delamination factor for entrance was calculated after the tests. Optimization of the parameters was evaluated using the signal/noise ratio approach. The effectiveness of parameters on results was determined by analysis of variance (ANOVA). In addition, regression equations were obtained with the response surface method (RSM). According to the optimization results, the thrust force can be reduced with low cutting velocity and low feed rate under dry conditions. It was concluded that low cutting velocity and the feed rate should be applied under dry conditions to reduce delamination.

The selective laser sintering process is widely used to fabricate polyamide matrix composites. Although various reinforcements are used to produce polyamide matrix composites, metallic-based reinforcements are used to improve the mechanical properties of the structure. Also, selective laser sintering process parameters significantly affect the microstructural and mechanical properties of the composite products. In this study, polyamide 12 (PA 12) matrix composites were produced with Cu particulates by selective laser sintering methods using various energy density values. Moreover, dry sliding wear of the energy density dependent PA 12/Cu composites was investigated. Worn samples were characterized by scanning electron microscopy and 2D profile analysis. The examinations revealed that the lower energy density values improve the microstructural properties and wear behavior of the PA 12/Cu composites. It was obtained that the lowest energy value provides the highest wear resistance for the PA 12/Cu composites.