Papers by Keyword: Flow Stress

Abstract: The effects of minor additions of alloying elements Fe/Ni/Co (0.5wt.%), B (0.01–0.1wt.%), and Y (0.2wt.%) on the superplastic behavior, microstructural evolution and mechanical properties of Ti-4 wt.%Al-3wt.%Mo-1wt.%V alloys are investigated. By increasing the high-angle grain boundary fraction and related facilitation of the grain boundary sliding, these elements reduce the flow stress values at the initial deformation stage and improve flow stability at a steady state. The most pronounced effect is found at low deformation temperatures when acceleration of recrystallization and globularization of the microstructure is critical. As a result, minor additions of the studied elements provide good superplasticity at relatively low temperatures of 625–775 °C (m≈0.50 and elongation to failure ≈ 500–1000%) and post-forming room-temperature strength (YS≈830 MPa and UTS≈990 MPa).

Abstract: Materials characterization and the knowledge of their elastic-plastic behavior are of fundamental importance for the design of industrial manufacturing processes. Nowadays, FEM simulation is the main tool used to optimize product quality and minimize scraps, and the numerical codes have evolved over the years to obtain accurate solutions with reduced computational times. Nevertheless, in order to perform reliable simulations, it is necessary to include accurate modeling of the material flow stress. Hot torsion is a powerful method for the characterization of the material flow stress because, tests can be carried out at constant speeds and temperatures, reaching large strain values, and thus getting over the limits of compression and tensile tests. In this paper the hot torsion characterization applied to AA6082 alloy is presented: tests were performed with equivalent strain rates of 0.01, 0.1, 1, and 10 s^-1, in the temperature range from 440 to 550 °C (from 713.15 to 823.15 K). The results are presented in terms of equivalent stress vs equivalent strain. Finally, the material flow stress curve was predicted by the Hyperbolic sine model and Hensel-Spittel law, and the material parameters A, m_1-9 are provided for the temperature expressed in °C and K.

Abstract: Tensile tests on AA5005 alloy were conducted on model MTS-810 tensile test machine during temperature 633-773 K and strain rate 0.0003-0.03 s^-1. The flow stress–true strain curves were obtained. In order to analyze the flow stress behavior of aluminum AA5005 alloy, the phenomenological Fields-Backofen equation based on the fitting regression analysis was developed. The flow stress values calculated by the obtained model keep coincidence with experimental values. Eventually, the statistical analysis methods (correlation coefficient (R), average absolute relative error (AARE)) were adopted to examine the credibility of the established model. Results show that the R-value is 0.99592 and the AARE is 3.3128 %, which indicates the high fitting accuracy of the Fields-Backofen equation. Consequently, the Fields-Backofen model can describe the constitutive relationship of AA5005 alloy credibly.

Abstract: Due to the continuous emergence of new alloys and the improvement of basic research requirements for aluminum alloy, researchers need to further study the flow constitutive relationship of new aluminum alloys. This paper used Model MTS-810 tensile machine to conduct tensile tests on AA5005 aluminum alloy at temperature of 360°C and strain rate of . Hollomon model, Swift model, Voce model, Voce+Voce model were used to compare the fitting accuracy with the experimental stress-strain curves, and the fitting parameters required by each formula were obtained from the fitting results, so as to obtain the constitutive model of AA5005 aluminum alloy. The Voce+Voce model which with more fitting freedom degree, lower flow stress increasing rate and saturation rate is most suitable to describe flow stress relation of aluminum alloy AA5005.

Abstract: The hot deformation of TA15 titanium alloy was studied by the hot compression test on the Gleeble-3800 thermal simulation equipment. The true stress-strain curves of TA15 titanium alloy at the temperature of 1123-1223K and the strain rate of 0.001-1s^-1 were obtained.. The results show that the flow stress increases with decreasing temperature, and increases as the strain rate increases. And the deformation process is accompanied by work hardening and dynamic recovery and dynamic recrystallization. Based on the true stress-strain curves of TA15 titanium alloy, the Arrhenius-typed constitutive equation was established. The thermal deformation activation energy of TA15 titanium alloy with a strain of 0.2 is 746.27kJ/mol.

Abstract: In cold forging, the temperature difference between the workpiece and the tool is small and thus deformation analysis is rarely coupled with thermal analysis. However, the friction coefficient of zinc phosphate coating with metal soap has a large temperature dependence. Consideration of the effects of workpiece and tool temperature change on the friction coefficient is thus expected to improve the analytical accuracy of cold forging. Thermally coupled cold forging analysis requires thermal conductivity, specific heat, heat transfer coefficient between the workpiece and the tool, in addition to the temperature dependence of flow stress and friction coefficient. The heat transfer coefficient between workpieces coated with zinc phosphate with metal soap and tools is investigated in this paper. Cold backward extrusion was performed with a 50% reduction of area, and the temperature history in the punch was measured with a thermocouple. The forging speed was 1, 3, and 10 spm. FEM analysis was performed to simulate the experiment by considering the temperature dependence of flow stress and friction coefficient. The heat transfer coefficient was estimated at 20 kW/(m2•°C) by comparing the experimental result and calibration curves.

Abstract: The δ-TRIP steel has attracted wide attention from researchers and industry, as it possesses the mechanical properties of the third-generation advanced automotive steels, and the lower density achieved by the addition of light element aluminum. However, there have been few studies on the hot deformation behavior of δ-TRIP steel at high temperature, which is critical for the design of the hot rolling process. In the present study, hot compression tests were carried out on a 980 MPa δ-TRIP steel to achieve a better understanding of the deformation behavior at high temperatures from 800°C to 1150°C. Combined with the electron backscatter diffraction (EBSD) analysis, the results showed that δ-TRIP steel was a positive strain-rate sensitive material, and its soften was contributed by dynamic re-crystallization of ferrite and austenite during deformation at high temperature. The hot deformation activation energy (Q = 327.485 kJ/mol) of the experimental steel was calculated, and the strain index was 5.65. Besides, the related constitutive equation and the flow stress equation were also established.

Abstract: In this paper, an Al-Zn-Mg-Cu alloy with a small amount of Er and Zr added was used as the research object. The homogenization annealing was carried out, and the 7N01 aluminum alloy was used at 300 °C, 350 °C, 400 °C, 450 °C and 0.1 s^-1, 1 s^-1, 10 s^-1 deformation conditions by Gleeble-3500 thermal simulator. Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM) were used for microstructure analysis. The results show that the stress-strain curve of with Er 7N01 aluminum alloy can be divided into micro-strain stage, uniform deformation stage and steady-state flow stage during the thermal compression process. The flow stress of 7N01 aluminum alloy achieved peaks at the initial stage of strain, and then increased with the increase of strain rate and the decrease of deformation temperature. With the increase of deformation temperature and the decrease of deformation rate, the recrystallization process was significantly increased.

Abstract: The article is devoted to the testing of samples for tensile, namely the formation of the neck. The objective of the study is to establish the qualitative level of influence of the nature of the hardening curve of the material on the shape and size of the neck. By using computer simulation, testing of samples of the two test materials, selected so as to provide the possibility of direct comparisons, forming the necks of the same diameter in the minimum section and the section near the neck, is worked over. It is found that the nature of the hardening curve determines the shape and size of the neck, including the radius of its curvature. This makes it possible to recover the hardening curve for large values of the strain. At the same time, it is established that the equation of the forming neck is invariant to the nature of the hardening curve of the material.

Abstract: The paper is devoted to the practical implementation of the new torsion testing method for studying rheological properties of materials in a hot state. This method involves the testing of cylindrical samples in the grips of a test setup, the angular velocity of which changes exponentially. The testing mode allows you to restore the hardening curves of a material according to the test results. This article aims to formulate the requirements for possible ways to implement the proposed testing method, and presents two different ways to obtain the specified exponential testing mode. The experience of their use on the test setup in the Ural Federal University indicates the feasibility of the new testing method, as well as the possibility of a smooth transition to the specified testing mode.