Hot Flow Stress Research Articles

Effect of slow cooling treatment on hot deformation behavior of GH4742 superalloy

The very poor hot workability of high-alloyed GH4742 superalloy is represented in the aspects of very narrow available deformation temperature range, high deformation-resistant force and poor ductility. In this paper, a special heat treatment to change the ingot microstructure is proposed to improve the hot workability of the GH4742 superalloy. The γ′ phase with an appropriate size and morphology and tortuous grain boundary were obtained by slow cooling in the γ + γ′ two phases region after solution treatment. Modified microstructure induces that the hot deformation flow stress of GH4742 superalloy was decreased effectively and hot deformation plasticity was increased obviously.

Hot deformation behaviors and flow stress model of GCr15 bearing steel

The hot deformation behaviors of GCr15 bearing steel were investigated by isothermal compression tests, performed on a Gleeble-3800 thermal-mechanical simulator at temperatures between 950 °C and 1 150 °C and strain rates between 0.1 and 10 s−1. The peak stress and peak strain as functions of processing parameters were obtained. The dependence of peak stress on strain rate and temperature obeys a hyperbolic sine equation with a Zener-Hollomon parameter. By regression analysis, in the temperature range of 950−1 150 °C and strain rate range of 0.1−10 s−1, the mean activation energy and the stress exponent were determined to be 351 kJ/mol and 4.728, respectively. Meanwhile, models of flow stress and dynamic recrystallization (DRX) grain size were also established. The model predictions show good agreement with experimental results.

Role of Cu 2O during hot compression of 99.9% pure copper

Several authors have noticed that relatively pure coppers with different oxygen levels present a higher hot flow stress as the oxygen content increases. This work attempts to demonstrate that a large contribution to the observed increase of stress is due to precipitation of fine Cu 2O particles during hot working. Three fire-refined 99.9% pure coppers with different oxygen levels (26 ppm, 46 ppm, and 62 ppm) were hot compressed at temperatures 600–950 °C and at a strain rate range 0.001–0.3 s −1. A comparison of transmission electron micrographs prepared from coppers compressed at 750 °C and 900 °C was performed to characterize the possible precipitates. Fine Cu 2O precipitates were found only in the two coppers with higher oxygen and at the lower temperature, where stress differences were larger. Precipitation–hardening theories were adapted for higher temperature and used to iteratively determine if the precipitate sizes and calculated volume fractions could be indeed causing an increase of stress.

Extrusion Limits of Magnesium Alloys

Magnesium alloys are generally found to be slower to extrude than aluminum alloys; however, limited quantitative comparisons of the actual operating windows have been published. In this work, the extrusion limits are determined for a series of commercial magnesium alloys (M1, ZM21, AZ31, AZ61, and ZK60). These are compared with the limits established for aluminum alloy AA6063. The maximum extrusion speed of alloy M1 is shown to be similar to AA6063. Alloys ZM21, AZ31, ZK60, and AZ61 exhibit maximum extrusion speeds 44, 18, 4, and 3 pct, respectively, of the maximum measured for AA6063. For AZ31, the maximum extrusion speed is increased by 22 pct after homogenization and by 64 pct for repeat extrusions. The variation in the extrusion limits with changing alloy content is rationalized in terms of differences in the hot working flow stress and solidus temperature.

Effects of Precipitation during Dynamic Recrystallization of Copper with Different Oxygen Levels

Previous research works assert that the observed increase in hot flow stress of commercially pure copper is attributed to the interactions between solute atoms and dislocations, specifically by interstitial oxygen. This work shows TEM images of the formation of Cu2O precipitates after warm working temperatures that in part help explain the increase of stress during hot compression of 99.9% pure copper. Three commercially pure large-grained coppers with 26, 46 and 62ppm of oxygen were tested at different temperatures (600°C-950°C) and strain rates (0.3s-1- 0.001s-1). At temperatures below 850°C, the stress differences between coppers, tested at same the strain rate, became increasingly higher. A correlation between stress increase and oxygen content was found. Precipitation of nanometric Cu2O did not show any difference in dynamically recrystallized grain size; however hardness tests showed that the final properties were modified. This work discusses the effect precipitation of Cu2O has on the hot flow curve and the final microstructure of hot formed 99.9% pure copper with different oxygen levels.

A Model for Multi Peak Dynamic Recrystallization in Copper

Modelling hot flow stress during grain refinement operations of fcc metals has largely included the use of an Avrami type equation to describe the decrease in stress due to Dynamic Recrystallization (DRX). However when refining large-grained copper, the processing temperatures and strain rates often produce a multi peak behaviour, which is not predictable by an Avrami equation alone. If an initial grain size, D0, is greater than the stable dynamically recrystallized grain size, Drex, which is a function of the Zener-Hollomon Parameter, Z, then the material will tend to refine. However if the current the Zener-Hollomon value, given by current temperature and strain rate conditions, is lower than a critical value, Zc, which depends on D0, then a multi peak stress behaviour is expected while refining. The latter Relative-Grain-Size model (i.e. the D0-Zc and Drex-Z relationships plotted on the same log-log graph) is a practical model that allows determination of whether a material will grain coarsen or refine and whether the dynamic recrystallization behaviour will be monotonic or with multi peaks. The present authors devised a dynamic recrystallization algorithm to measure the stress due to the diminishing initial grain volume and to measure the correction stress due to recrystallizing grains. Analysis on the hot (600°C-950°C) compression data of a 99.9% pure copper inductively lead to the use of an Avrami type equation to describe the stress contribution produced by the deformation of the remaining initial grain volume and a damped cosine equation to describe the stress contribution of the synchronized volume of new grains. This work discusses the experimental evidence and analytical findings that inductively support the mathematical description of the stress-strain curve given by a Damped Cosine Avrami Model for discontinuous DRX.

Hot deformation behavior and flow stress model of F40MnV steel

Single hit compression tests were performed at 1 223–1 473 K and strain rate of 0.1–10 s−1 to study hot deformation behavior and flow stress model of F40MnV steel. The dependence of the peak stress, initial stress, saturation stress, steady state stress and peak stain on Zener-Hollomon parameter were obtained. The mathematical models of dynamic recrystallization fraction and grain size were also obtained. Based on the tested data, the flow stress model of F40MnV steel was established in dynamic recovery region and dynamic recrystallization region, respectively. The results show that the activation energy for dynamic recrystallization is 278.6 kJ/mol by regression analysis. The flow stress model of F40MnV steel is proved to approximate the tested data and suitable for numerical simulation of hot forging.

Prediction of Hot Flow Stress of CrMoV Steel Using Artificial Neural Network (ANN)

Hot flow stress of CrMoV steel was extracted using hot compression test. Test was carried out in strain range of 0.1–0.9, with strain rate of 0.1–5 s−1 and the temperature of 900–1200°C. Flow stress was predicted by using conventional regression method, i.e., Zenner–Holloman parameter with hyperbolic function. The results showed low accuracy of prediction with a high relative error. Several two hidden layers Artificial Neural Networks (ANN) architectures with back propagation algorithm and momentum learning process were applied using Matlab software. They used strain, strain rate and temperature as input and flow stress as output. It was found that an optimum architecture of 3–9–10–1 shows proper prediction with respect to the conventional regression method, i.e., the relative error reached −0.13% in place of 11.52%. This ANN method is also capable of generating high precision output for unseen deformation conditions if proper initial weights and biases are used.

Influence of initial microstructure on the hot working flow stress of Mg–3Al–1Zn

The hot working flow stress is examined for wrought and as-cast magnesium alloy AZ31, deformed in both compression and torsion. It is found that the hot deformation behaviour is sensitive to the deformation conditions, initial microstructure and the deformation mode. The peak stresses in compression are higher for as-cast material at low strain rates and high temperatures, whereas they are higher for fine-grained wrought material at high strain rates and low temperatures. Also, the stress–strain curves in torsion exhibit considerably higher strains to the peak flow stress than in compression, particularly at lower temperatures. Most noticeably in the compression of the wrought material, the shape of the stress–strain curves change considerably as the temperature is reduced and the strain rate is increased. These key features of the deformation behaviour are explained in terms of initial grain size, texture, twinning and dynamic recrystallization.

Measurement of flow stress in hot axisymmetric compression tests

This paper describes good practice for the measurement of hot flow stress in metallic materials. It is applicable to hot (isothermal) uniaxial axisymmetric compression tests at medium to high rates of strain (10−4–102 s−1) at deformation temperatures below the solidus.Guidance is provided on appropriate testpiece geometries and methods of verifying the temperature distribution along the length of the testpiece. Flow diagrams are given showing all the steps that are necessary, including the correction factors that need to be applied for deformational heating and for the determination of true stress. Details are given of the calibration procedures that should be followed to provide traceability to the National Measurement System.Technical input to the document has been provided by a steering group containing academic researchers and representatives of industrial users and producers of a wide range of engineering materials. An experimental and modelling programme on an aluminium alloy, AA5052; on 316 stainless steel and a nickel alloy, Nimonic 901, over a range of temperatures and strain rates has been conducted during the preparation of the document to underpin the procedures in the guide.

Modeling the Hot Flow Stress of Commercial Purity Coppers with Different Oxygen Levels

Modeling the Hot Flow Stress of Commercial Purity Coppers with Different Oxygen Levels

Flow stress of carbon steel 08F in temperature range of warm-forging

In this paper, the influence of flow stress models on FEM simulation results is discussed; and the flow stress of carbon steel 08F at different T, ε ̄ ̇ and ε ̄ in the temperature and of warm-forging is measured by a uniaxial compression experiment with a Thercmastor-Z Hot-simulator. The experimental results show that flow stress is very complex within the temperature range of warm-forging; and that the traditional interpolation method adopted in FEM simulation on the basis of cold or hot flow stress models is not scientific. To improve FEM simulation precision, a flow stress model should be calibrated by experiment. Finally, the calibrated flow stress model of carbon steel 08F is applied in the FEM simulation of the warm-forging process of the generator poles of an automobile: comparison between the results of FEM simulation and the warm-forging experiment indicates that the flow stress model calibrated by experiment is scientific and effective.

Influence of deformation conditions and texture on the high temperature flow stress of magnesium AZ31

The evolution of hot working flow stress with strain is examined in torsion, uniaxial compression and channel die compression. The flow stress was found to be strongly dependent on texture and deformation mode. At low strains this dependency accounted for a difference in flow stress of up to a factor of two. At higher strains the influence of texture and deformation mode was less marked. The stresses corresponding to an equivalent strain of 0.5 were modelled using a power law expression with an activation energy of 147 kJ/mol and a strain rate exponent of 0.15. The influence of texture and deformation mode on flow stress is rationalised in terms of the influence of prismatic slip, twinning and dynamic recrystallisation on deformation stress and structure.

A new model describing the hot stress–strain curves of HSLA steel at high deformation

The torsion test is often applied for the determination of the hot flow stress of steels. An originally derived system of equations has been used to describe the basic parameters of the torsion test and has enabled precision to be obtained in the simulation of flat rolling. Because of the relatively low strain rates obtained with a torsion plastometer, it is necessary to extrapolate the laboratory results into the range of real working conditions. In this case, only a limited number of demanding and expensive interrupted tests have to be done. The more important role is then played by computer prediction based on equations that incorporate the results of simple, but mainly continuous tests. Examples of such mathematical models are presented, including the influence of dynamic or static softening in hot rolling.

Determination of Softening Kinetics in a Material by Measuring the Evolution of Hot Flow Stress

Abstract A new technique for estimating the time and temperature-dependent softening response of a deformed material is proposed. The hot flow stress is measured at various intervals in a quasistatic softening experiment to determinse the kinetics of softening. A major advantage of the proposed technique is that a single specimen can be used to estimate the evolution of softening without having to cool and reheat the specimen for each measurement. High resolution strain measurement apparatus is used in the experiments, as minimizing strain is a critical requirement in property performing the measurement. Test control requires optimal performance from the servohydraulic system which must operate near its threshold for reliable information to be obtained. Experimental results indicate that age hardening response as well as recovery and recrystallization kinetics can be measured using this method.