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[en] Die-casting AZ31 Mg alloys were successfully friction stir welded at a constant welding speed and different rotation rates. More uniform and fine grains were obtained at the rotation rate of 1400 rpm due to more suitable temperature for dynamic recrystallization in this welding condition. In addition, the intensity of (0001) texture in stir zone increased with the increasing rotation rate. The results of mechanical property test indicated joint with rotation rate of 1400 rpm had better tensile property, which was associated with fine grains, uniform transition in the interface between thermo-mechanically affected zone and stir zone as well as more favorable Schmidt factor for basal slip and twinning. The corrosion resistance of joints increased with the increasing rotation rate, which is significantly related to the (0001) texture with basal plane parallel to the corroded surfaces.
[en] The results of the investigation dealing with enhancing the mechanical and functional properties of carbon fiber-reinforced polymers are presented in the paper. The scientific issue is relevant enough to the aerospace engineering where strength-to-density ratio is one of the most important properties providing sufficient strength and stiffness along with minimal weight, while electrical and heat conductivity are required for lightning strike protection and better efficiency of thermal deicing systems, respectively. In the present research, insufficient electric conductivity of CFRP is improved by adding single-wall carbon nanotubes. Measurement of electrical resistivity of modified composites as well as CNT-filled epoxy is taken. The results of mechanical testing demonstrate unchanged tensile strength, while flexural strength increases, which is attributed to higher interfacial shear strength of hybrid CFRP/CNT composites.
[en] The microstructure, texture, and mechanical properties of the Al/Ni/Cu composite during various accumulative roll bonding (ARB) cycles were studied using optical microscopy, scanning electron microscopy, x-ray diffraction, shear punch test, and hardness test. In addition, ImageJ software and Rietveld software were used in order to study microstructure and dislocation density variations, respectively. It was found that Ni and Cu layers were fractured and distributed in the Al matrix due to differences in their mechanical properties. Fracture and distribution of Cu and Ni particles after cycle five led to the alteration of the composite structure from a layered to a particle-reinforced structure. ARB process leads to the formation of strong orientation along the β-fiber and also to pronounced copper and dillamore components in both Al and Cu phases. Furthermore, the shear yield stress and ultimate shear strength of the composite increased as the ARB process advanced; however, shear elongation presented a non-uniform variation. Investigation of the fracture surfaces revealed that the mechanical properties of the composite are affected not only by the strain hardening of the Cu layer, but also by the structural change in the composite during the initial ARB cycles. During the last stages of the process, however, changes in mechanical properties were mostly governed by reinforcement particles serving as strain concentration zones and the strain hardening of the Al matrix.
[en] Residual stresses were determined through the thickness of quenched and quenched followed by cold compression 7050 aluminum alloy T-section forgings using the contour method and neutron diffraction. 7050 T-section forgings are used for the manufacture of the connecting joints of wing and fuselage. Inevitably high residual stresses are introduced during the quenching process of 7050 T-section forgings, which could be reduced by cold compression to overcome the effect of residual stress on machining deformation. The longitudinal residual stresses obtained by the two techniques achieved good agreement in both trend and magnitude in both the quenched and quenched followed by cold compression 7050 T-section forgings. The stress-free lattice spacing of Al(311), the Bragg reflection used for the neutron measurements, was obtained from a 5-mm slice using the plane-stress condition, and its variation through the thickness is insignificant in both the quenched specimen and quenched followed by cold compression specimen. A comparison of the residual stresses in the quenched specimen and quenched followed by cold compression specimen showed significant changes in range of residual stresses from (− 304, 262 MPa) induced by quenching process to (− 125, 142 MPa) due to 3% cold compression. The percentage reduction in residual stress in the quenched specimen due to 3% cold compression was (46, 59%).
[en] An aluminized coating was prepared on Ti-6Al-4V alloy by hot-dip aluminizing and subsequently diffusion treatment. Dry sliding wear tests were performed for the aluminized and uncoated Ti-6Al-4V alloy under the loads of 10-50 N at the sliding velocities of 0.5-4 m/s. The wear resistance of the titanium alloy was improved by the aluminized coating under various conditions, especially at 4 m/s. The improved wear performance was suggested to be attributed to the Ti-Al coating and tribo-oxide layer. Tribo-layers were identified to form on worn surfaces under various conditions but their influence on the wear behavior and mechanism was decided by the amount and kind of oxides. The outmost values of the microhardness distribution at subsurfaces as a function of load could be used to identify the property and stability of tribo-layers. At 4 m/s, oxide-containing tribo-layers (more TiO, TiO2 and trace Fe2O3) presented high hardness and stability, thus possessed an obvious wear-reduced function. Conversely at 2.68 m/s, no-oxide tribo-layers did not show the protection from wear because of their lower hardness and instability. The formation of tribo-layers under various sliding speeds was noticed to be a process including wear debris production, oxidation and accumulation and densification, even sintering.
[en] In this study, a heat transfer model that takes under consideration the micro-segregation phenomena and has been extensively deployed for solidification studies concerning the plant casters is also applied to show the casting speed and superheat effects upon internal soundness. Τhe solidification path of C45R, 42CrMo4, S355 grades was simulated using Thermocalc® and DICTRA software in order to examine the phases formed in crucial process temperature windows for the casting. Coupled Heat Transfer/Simple Micro-Segregation Model software has been deployed to simulate the effect of cooling rate on solidus temperature, and solid fraction during solidification. The solidus temperature results computed by the HT/SM and DICTRA were found to be in good agreement. However, the differences in the predicted solidification fractions may derive from the fact that DICTRA uses an average cooling rate whereas the heat transfer SM model takes into account also the local cooling rates in the solidification progress.
[en] To improve the mechanical properties of aluminum alloy forgings, solution treatment and quenching is necessary. However, it becomes difficult to control the residual stress and deformation after solution treatment and quenching which always results in obtaining a part with an undesirable size, especially for a long stringer forging with an existing rib. Therefore, this paper demonstrates a quenching experiment and residual stress measurements for a ribbed aluminum alloy forging; the calculated results are close to the actual convective heat transfer coefficients. In addition, the heat transfer coefficient is introduced into the quenching simulation of a long stringer forging consisting of rib-web forging and plate forging. The influence of ribs on the residual stress and deformation of the forging is compared and analyzed. The results show that the heat transfer coefficient on the web without a rib is highest and the heat transfer coefficient on the web below the rib is lowest. Compared with the plate forging, the deformation direction of the rib-web forging is opposite, and the deformation of the rib-web forging is obviously increased.
[en] The hot ductility of a low alloyed Cr-Mo steel has been investigated to evaluate the surface cracking sensitivity within the straightening or unbending regime during the continuous casting process. Tensile samples were subjected to various thermal treatments, including melting and solidification, and were tested at deforming temperatures ranging between 600 and 1100 °C using a strain rate of 10−3 s−1. Hot ductility was evaluated based on reduction in area measurement and metallographic investigations. The investigated steel exhibits a drop in ductility at around 800 °C due to intergranular cracking. Microstructural examinations and supplementary thermokinetic computer simulations were carried out to describe the evolution of the microstructure during solidification and cooling.
[en] Partial replacement of steel by Aluminium (Al) alloys is a promising approach adopted by automotive sector to improve fuel efficiency without compromising on the strength. However, this results in the generation of dissimilar welds of Al alloys and steel. Understanding the corrosion behavior of Al alloys–steel welds as a function of welding parameters is critical in the successful application of such alloys. The present study reports the intergranular corrosion behavior of T6 heat-treated AA6061 Al alloy-galvanized mild steel lap joints, welded by metal inert gas welding–brazing technique as a function of different welding parameters, viz. weld speed (4-6 m/min) and wire feed rate (0.8-0.9 m/min), following ASTM G 67-04. Weld characterization was performed by field emission scanning electron microscopy (FESEM), x-ray diffraction and nano-hardness measurement. Results indicate heavy dissolution and metal loss at the interface. A high volume fraction of Al-Fe intermetallics was precipitated at the weld interfaces, resulting in high hardness and higher localized corrosion. The thickness of Al-Fe intermetallic layer increased with increasing wire feed rate and lower weld speed which enhanced the severity of galvanic and intergranular corrosion.