Characterization Of Flow Stress Research Articles

Inverse flow stress characterization in hot rolling

Inverse flow stress characterization in hot rolling

42CrMo4 steel flow behavior characterization for high temperature closed dies hot forging in automotive components applications

The application of new forming processes as the high temperature hot forging in closed dies in an industrial environment still requires further investigation due to the lack of flow stress data at these temperatures. To determine the flow behavior of the 42CrMo4 steel at high temperatures hot compression tests have been carried out in a Gleeble® 3800 thermomechanical tester for a temperature range that covers the material behavior from the hot forging until the Nil Ductility Temperature (1250 °C-1375 °C) and for three different orders of magnitudes for the strain rates (0.1 s−1, 1 s−1 and 10 s−1). Then, the Hansel-Spittel model, widely used in automotive commercial software as FORGE®, has been employed to obtain the adequate constants of the constitutive equation for high temperatures.Finally, the newly obtained flow behavior model has been validated by comparison between experimental and simulated compression tests and by the process simulation of a commercial automotive component comparing the results of the simulation with the already made experimental tests in a laboratory cellule of the new technology.Hence, this paper shows the procedure for the determination and the obtention of a new constitutive model for the 42CrMo4 steel flow stress characterization at a temperature range between 1250 °C–1375 °C. This will contribute in the knowledge of material flow stress behavior models at high temperatures and will allow the prediction or simulation of high temperature hot forging in closed dies processes, enhancing the possibility of the application of these technologies from an industrial point of view.

Flow stress characterization of magnesium alloys at elevated temperatures: A review

The flow behaviors of magnesium alloys are too complicated to be simply formulated in a mathematical form. Most researches have based metallurgically or phenomenologically on specific functions with many constants, which could be applied only to the limited magnesium alloys under specific conditions. In this study, a review on the studies of flow stress characterization of magnesium alloys is conducted and the possibility of using the traditional piecewise C-m model and its extension to characterize the magnesium alloys is emphasized. The formulations of major flow models are given with three typical applications to magnesium alloy AZ80 and its characteristics are demonstrated through comparison of the fitted flow behaviors with their associated experiments and various flow stress models including Arrehenius model, four Ludwik family models (Johnson Cook, Modified Johnson-Cook, Hensel-Spittel, Sutton-Luo), two Voce family models (Ebrahimi et al., Razali et al.) and C-m models.

Flow stress characterization of carbon steel S25C in the temperature range of cold forming with an emphasis on dynamic strain aging

In this study, a new material model for carbon steel S25C, which exhibits typical dynamic strain aging (DSA), is presented. We detail the development of a closed-form function model. The total flow stress is divided into state variable-effective and DSA-effective flow stresses. The two flow stresses are first formulated as functions of temperature, with unknown material constants, at various strains and strain rates. They are then calculated using an optimization technique to minimize error. The material constants calculated at the sample points are fitted as functions of state variables and thus become material parameters; these are eventually used to describe the flow stress of S25C (with DSA) as a closed-form function. The proposed material model with fitted material parameters and constants was compared to the experimental flow stresses; the average and maximum error rates were 2.9 % and 10.3 %, respectively. Finally, we optimized the material constants and parameters to minimize the maximum errors of the sample state variables. We posed the problem as a min-max problem, and obtained average and maximum error rates of 1.4 % and 6.1 %, respectively. The importance of the closed-form function is emphasized in terms of its application to non-isothermal analyses, especially automatic multi-stage cold forging.

Characterization of flow stress at ultra-high strain rates by proper extrapolation with Taylor impact tests

This paper is to provide a novel systematic procedure to obtain the dynamic flow stress of a material at ultra-high strain rates ranging from 104 s−1 to 106 s−1 where hardening behaviors are difficult to acquire from conventional experiments. Uniaxial material tests with AISI 4340 steel are performed at a wide range of strain rates from 10−3 s−1 to 103 s−1 by using the INSTRON 5583, a high-speed material testing machine (HSMTM), and a tension split Hopkinson pressure bar (SHPB) testing machine. From the uniaxial tests above, stress–strain curves are obtained at the strain rates ranging from 10−3 s−1 to 103 s−1. However, stress–strain curves cannot be obtained at the strain rates higher than 104 s−1 due to the lack in experimental techniques. In order to characterize hardening behaviors at strain rates ranging from 104 s−1 to 106 s−1, Taylor impact tests are performed when the speed of a projectile is 200 m/s, 253 m/s, and 305 m/s, which entail ultra-high strain rates, high temperature, and large plastic deformation. Flow stresses at the ultra-high strain rates are characterized through an inverse optimization process by comparing the numerical simulation results with the experimental results of the sequentially deformed shapes of a projectile during the Taylor impact test. The thermal softening effect at different strain rates is also considered due to the elevated temperature caused by large plastic deformation. The flow stresses calibrated by the comparison are implemented to numerical simulation resulting in a good coincidence with the Taylor impact tests at different impact velocities. It is noted from the comparison that the yield stress and the comprehensive hardening curves proposed well describe the deformation behavior up to the strain rate of 106 s−1 beyond the strain rate range for conventional material testing.

Approving Restoration Mechanism in 7075 Aluminum Alloy through Constitutive Flow Behavior Modeling

The present study deals with the flow stress characterization of 7075 aluminum alloy using process maps and constitutive flow behavior modeling. The latter was performed employing the work hardening‐DRV and DRX based models. The hot compression tests data in a wide range of temperature and strain rate were used to develop the activation energy map, strain rate sensitivity map, and the processing maps. Furthermore, in comparison with previously hired models, the DRX based model showed a good compatibility with the experimental results and therefore it was approved as a novel technique to justify the main restoration mechanism in 7075 aluminum alloy.

Energy based phenomenological model for optimizing the sheared edge in the trimming of steels

Attributes related to the dimensional quality of hot rolled steels are very important in commercial sectors that make direct use of this product, because delay or equipment damage can be avoided when forming in downstream operations. In this research, the steel sheet edge trimming process and its relationship with the defect known as broken edge is experimental and numerically studied. The type of material, horizontal clearance between knives and the energy spent during the cutting process are analyzed in detail. A metal-mechanical study is carried out for obtaining a microstructural hardness and flow stress characterization. Consequently, the edge trimming process is FEM simulated and its results in relation to knife penetration and shear stress lead to determining the energy spent during the cutting process. A mathematical model is determined under the consideration that minimum energy gives the optimum cutting conditions. The model proposes a reliable value for the horizontal clearance (Hc), between knives, taking as the principal factors: energy consumed during the edge trimming process, sheet thickness (Th), carbon content (C) and/or its ultimate tensile strength, expressed as: Hc=α+βTh−γC. A comparison of the recommended numerical results with the best practical conditions is carried out and a high coincidence is successfully found. This model is expected to be easily adopted as a tool where operators can adjust and control the parameters of process, and then, as a result, produce a sheet without edge trimming defects as well as a reduction in efficiency costs.

Strain Rate Dependent Flow Stress Characterization Using Piezo-actuated Micropress

The effect of strain rate on the mechanical response during microforming deformation is investigated for commercially pure titanium and aluminum. Piezoelectric actuated micropress is used to generate controlled constant strain rate between 0.1 and 10 sec-1 in a compression experiment using three different sizes of specimens. Investigations suggest that the increase of free and constrained surface grains results in hardening-softening competition with miniaturization. In addition, loading rate in micro compression significantly influence the work hardening behavior for both materials.

Characterization of Flow Stress for Commercially Pure Titanium Subjected to Electrically Assisted Deformation

Uniaxial tension tests were conducted on thin commercially pure (CP) titanium sheets subjected to electrically assisted deformation using a new experimental setup to decouple thermal–mechanical and possible electroplastic behavior. The observed absence of stress reductions for specimens air-cooled to near room temperature motivated the need to reevaluate the role of temperature on modeling the plastic behavior of metals subjected to electrically assisted deformation, an item that is often overlooked when invoking electroplasticity theory. As a result, two empirical constitutive models, a modified-Hollomon and the Johnson–Cook models of plastic flow stress, were used to predict the magnitude of stress reductions caused by the application of constant dc current and the associated Joule heating temperature increase during electrically assisted tension experiments. Results show that the thermal–mechanical coupled models can effectively predict the mechanical behavior of commercially pure titanium in electrically assisted tension and compression experiments.

Modeling the hot deformation behavior of Al alloy 3003

The hot deformation behavior of Al alloy 3003 under a wide range of temperatures and strain rates is modeled by a modified Voce type model. The isothermal hot compression tests, in a wide range of temperatures ranging from 300°C to 500°C and strain rates ranging from 0.01s−1 to 1s−1 were conducted. The results of true stress–strain curves reveal that the flow stress decreases with the increase of the temperature or decrease of the strain rate. The curves present peak stresses at a particular strain, and are followed by the flow softening. A modified Voce type model was employed to describe the flow stress characterization in a wide range of strain, including three parameters, namely the peak flow stress, the rising coefficient, and the dropping coefficient. The Arrhenius-type equation was investigated to characterize the effects of strain rate and temperature on the peak flow stress. The effects of strain rate and temperature on the rising coefficient and the dropping coefficient were analyzed empirically by multiple linear functions. The constitutive equations were verified by comparing the experimental results and predicted results, and good agreement was achieved.

Rapid Flow Stress Characterization of Steel

This rapid flow stress characterization concept involves rapid heating to an initial test temperature, T1, followed by loading and short-time stress relaxation measurement, followed by heating to a higher temperature, T2, followed by loading and short-time stress relaxation measurement, followed by heating to T3, and so on. This test sequence can generate stress-strain rate-temperature data over a wide spectrum, with a single specimen. The principal advantage of this method of flow stress characterization is its short-time format. The cost of specimen preparation is modest, as well. Beyond this, the test methodology provides very accurate temperature control. It also tests a given metallurgical structure, with minimal complications from structural evolution due to plastic deformation. This study has demonstrated the feasibility of this method on plain carbon and austenitic stainless steels, using a Gleeble testing machine. This includes demonstrated consistency between single-test-per-specimen data and data derived from sequential testing on a single specimen, as well as consistency with conventionally developed flow stress data.