Papers by Keyword: Plastic Deformation

Abstract: Aluminum alloys are important in aerospace industry, due to their mechanical properties, low specific weight and good corrosion resistance. Such properties are achieved due to a heat treatment of solubilization, quenching and aging, in order to precipitate metastables phases, which act as dislocation obstacles, increasing the strength of the alloy. In the present study, the precipitation sequence of Al-8%Ag alloy was analyzed via Vickers hardness and Transmission Electron Microscopy. The size and morphology of the precipitated particles, involved in the stages of precipitation process was characterized. It was determined the microstructure at the peak hardness, which is mainly composed of spherical GP zones with about 6 nm average diameter, which are responsible for the alloy achieve a value of 72 HVN. It was observed that this hardness value does not compete with others well known alloys, like AA 6061 and AA 2024, which can be precipitation hardened. The main reason for the low values of HVN, is because of there is no enough difference between the matrix and the precipitated particles lattice parameters, and dont cause a significant elastic strain by coherence in the matrix lattice, that could produce a substantial hardening. To ascertain this assumption, the aged material was severely plastic deformed, achieving 94 HVN, and the grain refinement and high dislocations density were the major hardening mechanisms, since the precipitates behavior was similar as the matrix, because particles were distorted instead of acting as impediment to material flow.

Abstract: High temperature forging tools are highly damaged by wear shearing under cyclic loading which reduces the life of tools. In real conditions, depending upon the tool areas, the level of wear can change. The surfaces of tools can be treated by cobalt-based hardfacing using different welding processes. This study focuses on tribological damages of Stellite 21 deposited by MIG process. Wear tests are carried out at room and high temperature on a ring on disc tribometer under high normal load. Different testing conditions are examined. The surface plastic strain due to the friction shear stresses is demonstrated by different methods like SEM observations, micro-hardness measurements and XRD analyses. More particularly, it is shown that the initial (200) crystallographic preferred orientation due to the welding process is modified into (111) crystallographic preferred orientation due to shear stresses regardless the loading. Moreover, a relationship between the gradient of the plastic shear strain at the friction subsurface and the level of the micro-hardness measurement has been established. In regard of the results, when the shear strain exceeds a threshold, the micro-hardness measurement is stabilised. The ultimate value could be induced by the stacking fault energy that is considered as weak in Stellites.

Abstract: As the scale and complexity of products such as aircraft and cars increase, demand for new functional processes to join mechanical parts grows. The use of plastic deformation for joining parts potentially offers improved accuracy, reliability and environmental safety as well as creating opportunities to design new products through joining dissimilar materials. This paper aims to provide an overview of the state of the art in such joining processes, including cold welding, friction stir welding, joining by forming, self-pierce riveting and mechanical clinching. The paper includes description of the mechanism of joint formation, joint strength and applicability.

Abstract: An evaluation of the impact forces imparted due to wind flow onto and across walls is necessary to improve the safety of stopper components in swing-type windows. A strong wind test was performed to measure the impact force imparted by wind, and the characteristics of the wind energy and resulting impact force were clarified. An impact force evaluation method based on a simple mechanical model was proposed. Furthermore, it was confirmed that the deformation of a plastic stopper component due to an impact force could be modeled via an empirical linear formula, derived experimentally using the impact force imparted in a drop weight load test. The proposed evaluation method is considered to be useful for the safety design of swing-type windows.

Abstract: For understanding the distribution of plastic deformation induced by asymmetric rolling (ASR), multi-pass ASR and symmetric rolling (SR) experiments combined with the finite element simulation were used for high-strength aluminum alloy in the present study. The influence of reduction per-pass on the shear / effective strain distributions were studied via different ASR processes. By measuring the shear angle (θ, the angle between the reference mark before and after rolling) of rolled sheets, redundant shear strain and equivalent strain were calculated. It is shown that with equal total thickness reduction for ASR and SR, ASR can induce much more shear deformation through the thickness. By calculating the evolution of redundant shear strain and total equivalent strain for different ASR routines, it indicates that small pass reduction could be much favorable to the strain accumulation than that of the large pass reduction under a same total reduction in ASR process. Also, the influence of shear stress on the strain distribution and the through-thickness strain distribution were studied and evaluated with FEM analyses.

Abstract: Flexible Pad Laser Shock Forming (FPLSF) is a new microforming process using laser-induced shock pressure and a flexible pad. This process involves high strain-rate (~10⁵ s^-1) plastic deformation of metallic foils along with the hyperelastic deformation of the flexible elastomer pad over which the foil is positioned. This paper studies the influence of flexible pad on the shockwave propagation behavior and the plastic deformation of metal foil in FPLSF using finite element analysis. The effect of flexible pad materials such as silicone rubber, polyurethane rubber and natural rubber on the deformation of copper foils has been analysed in detail. An increase in crater depth is observed with the reduction in flexible pad hardness. However, it is found that there exists an optimum hardness of the flexible pad to achieve perfect hemispherical craters on metal foils, as bending of foils at non-deformed region is observed with softer pads whereas flattening of crater surface occurs with harder pads. The effect of flexible pad thickness on the foil deformation was analyzed at six different thickness levels: 300 μm, 600 μm, 900 μm, 1200 μm, 1500 μm, and 2000 μm. Similarly, there exists an optimum flexible pad thickness to maximize the crater depth and achieve the hemispherical shapes. Analysis of flexible pad thickness indicates that the pad thickness influences the elastic recovery of the flexible-pad and hence the plastic deformation of the metallic foils.

Abstract: Shot peening is widely utilized to improve the fatigue property of mechanical parts for transportation equipment such as cars and airplanes. Also, this technology is being applied as a film-forming technology in order to improve surface quality. The authors have recently proposed new joining methods using shot peening, shot lining. In this method, the metals are bonded with the dissimilar metal by applying plastic deformation and the pressure. The thin foil can be joined to the substrate surface by the pressure generated by the hit of the shots. In this study, the formation of an Fe-Al intermetallic compound film on high-speed tool steel by shot lining and heat treatment was investigated. In the experiment, a centrifugal-type peening machine with an electrical heater was employed. The shot medium was high-carbon cast steel. The substrate was a commercial high-speed tool steel JIS-SKH51, and the foil was commercially available pure aluminium. The shot lining process of tool steel with an aluminium foil was carried out at 573K in air using a peening machine. Heat treatment was performed at diffusion temperatures from 923 to 1573K in vacuum. The lined substrates exhibited a harder layer of aluminium-rich intermetallics in the diffusion temperature range of 923 to 1173K. When the temperature of the lined substrates was more than 1273K, the surface was covered with thicker and highly anticorrosive layers of iron-rich intermetallics. We found that the present method could be used for the formation of functional films on high-speed tool steel.

Abstract: Silicon is brittle and easily cracks even under a small load. The difficulty in shaping silicon has prevented breakthroughs in the mass production of silicon lenses for terahertz and infrared technology. We developed a novel method of deforming bulk single-crystal silicon into the required shape by one-shot pressing at a temperature just below the melting point of silicon, despite its brittleness and covalent nature, and realized the near-net shaping of the material into the plano-convex shape with the curvature radius R=7.5 mm for a infrared transmission lens. The crystallographic quality of the obtained lens could be improved by primary recrystallization. The simple method of 'pressing' will enable the mass production of not only silicon lenses but also lenses with a complex shape, such as aspherical lenses, and lens arrays by using dies with desired shape.

Abstract: The information in the basic references about the relation between elastic constants and particularly Young’s modulus (E) behavior and plastic deformation indicates that this parameter is constant or almost constant. At the beginning of the XX century several authors indicated that E of some metals decreased when cold deformation increased and detected reductions up to 15% in steels, aluminum, copper, brass... In the last years this behavior is taking into account during the finite-element analysis of sheet metal stamping or other plastic deformation processes. This work includes an extensive review of papers of our research team and of other authors related with the behavior of Young’s modulus during plastic deformation of some metallic alloys. This parameter can diminish up to 10% by plastic deformation (tension test) in iron, aluminum, and stainless steel (UNS S 30403) but remains practically unaltered in aluminum alloys deformed before or after aging. Results of Young’s modulus in nanostructured copper and copper alloys determined by ultrasonic technique are also presented. Additional results of Young’s modulus of UNS G10180 and UNS G10430 steels measured during loading and unloading steps in tension test are also included. High differences in the E values were detected between both steps.

Abstract: The three dimensional grain mapping technique for polycrystalline material, which is called X-ray diffraction contrast tomography (DCT) has proposed. In the present study, the measurement of DCT was conducted in SPring-8, which is the brightest synchrotron radiation facility in Japan, and the condition of measurement and data procedure are discussed. Developed technique was applied to aluminium alloy and stainless steel. The shape and location of grain could be determined by the developed three-dimensional mapping technique using the apparatus in a bending beam line of SPring-8. To evaluate plastic deformation, the grain orientation spreads of individual grains were measured. The grain orientation spread is caused by the mosaicity, which relates to the dislocation structure in a grain. The grain orientation spread was found to increase with increasing plastic strain. Fatigue damage also could be evaluated by the grain orientation spread in the DCT measurement.