Activation Energy For Hot Working Research Articles

Hot Deformation Behaviour of Additively Manufactured 18Ni-300 Maraging Steel

In this article, hot compression tests on the additively produced 18Ni-300 maraging steel 18Ni-300 were carried out on the Gleeble thermomechanical simulator in a wide temperature range (900-1200 °C) and at strain rates of 0.001 10 s-1. The samples were microstructurally analysed by light microscopy and scanning electron microscopy with electron backscatter diffraction (EBSD). This showed that dynamic recrystallization (DRX) was predominant in the samples tested at high strain rates and high deformation temperatures. In contrast, dynamic recovery (DRV) dominated at lower deformation temperatures and strain rates. Subsequently, the material constants were evaluated in a constitutive relationship using the experimental flow stress data. The results confirmed that the specimens are well hot workable and, compared with the literature data, have similar activation energy for hot working as the conventionally fabricated specimens. The findings presented in this research article can be used to develop novel hybrid postprocessing technologies that enable single-stage net shape forging/forming of AM maraging steel parts at reduced forming forces and with improved density and mechanical properties.

Quasi-static hot deformation behavior of a novel third-generation Ni-based powder metallurgy superalloy

Quasi-static hot deformation behavior of a novel third-generation Ni-based powder metallurgy superalloy A3 was investigated by hot compression tests and microscopy. The activation energy for hot working of the experimental alloy was about 867 kJ/mol. An instability domain existed in the processing map when the strain was 0.2, implying a plastic instability at the temperature range of 1080–1100°C and the strain rate range of 0.016–0.1 s−1. The power dissipation efficiency in processing maps indicated that optimum parameters for large deformation could be controlled at the temperature range of 1000–1030°C and the strain rate range of 0.001–0.01 s−1. It provided a reliable suggestion for hot processing during the manufacturing of such superalloy.

On the hot deformation behavior of Ti-6Al-4V made by additive manufacturing

Innovative process chains that use additive manufacturing (AM) to produce pre-forms for hot forging have been proposed recently, but almost no scientific knowledge on the hot deformation behavior of AM material exists. The combination of AM and metal forming may allow for producing pre-forms by AM which can be forged to the final shape in a single forming operation, thus saving tooling costs compared to conventional multi-stage forging processes. In this work, Ti-6Al-4 V samples made from powder using Directed Energy Deposition (DED) and Selective Laser Melting (SLM) are investigated under typical hot working conditions. An increasing cooling rate in AM promotes the formation of martensite. When heated to forming temperature, the martensite decomposes and yields intermediate microstructures with improved hot working properties compared to conventional wrought material. Differences in the flow stress and activation energy for hot working are analyzed for conventional and as-built Ti-6Al-4 V. AM material exhibits both lower flow stresses and a faster globularization rate, which is explained by non-equilibrium phase fractions and the spatial arrangement of the β-phase. A model is proposed to predict flow stresses and microstructure evolution in Ti-6Al-4 V specimens with a conventional and AM microstructure, which takes the spatial arrangement into account. The findings can be used to design new process chains that allow for single-stage net-shape forging of Ti-6Al-4 V parts at reduced forming forces and with improved mechanical properties.

Some aspects of physical metallurgy of microalloyed steels

Some aspects of deformation, precipitation, and recrystallization behavior in medium carbon V-microalloyed and low carbon Nb/Ti-microalloyed steels are presented in the paper. Changes in microstructure are explained together with methods of quantification. The temperature of No-recrystallization (Tnr) is defined as a milestone to show the onset of retardation of recrystallization while the apparent activation energy for hot working shows the extent of this retardation. In the case of high cooling rates, this method is not sufficiently sensitive and Trl (recrystallization limit temperature) and Trs (recrystallization stop temperature) must be evaluated from softening data. Paper presented the possibility to estimate Tnr temperature on six stands finishing train at Hot Strip Mill in HBIS Iron and Steel Serbia, Smederevo as well as the activation energy for static recrystallization, QSRX, derived from Tnr temperatures.

Comparison of the Hot Working Behavior of Wrought, Selective Laser Melted and Electron Beam Melted Ti-6Al-4V

The production of high-temperature components is of great importance for the transport and energy sector. Forging of high-temperature alloys often requires expensive dies, multiple forming steps and leads to forged parts with large tolerances that require machining to create the final shape. Additive manufacturing (AM) offers the possibility to print the desired shapes directly as net-shape components. AM could provide the advantage of being more energy-efficient compared to forging if the energy contained in the machining scrap exceeds the energy needed for powder production and laser processing. However, the microstructure and performance of 3D-printed parts will not reach the level of forged material unless further processes such as hot-isostatic pressing are applied. Combining AM and metal forming could pave the way for new process chains with little material waste, reduced tooling costs and increased part performance. This study investigates the hot working properties and microstructure evolution of Ti–6Al–4V pre-forms made by selective laser melting and electron beam melting. The results show that both materials are hot workable in the as-built state. Due to its martensitic microstructure, the SLM material shows a lower activation energy for hot working than EBM and wrought material and a faster globularization during forming, which is beneficial for hot forming since it reduces the forming forces and tool loads.

Hot workability and microstructure evolution of pre-forms for forgings produced by additive manufacturing

Additive Manufacturing (AM) processes allow for near-net shape production of components without dies or substantial machining. Especially for energy-intensive materials, AM processes offer a reduction in lead-time, and in some cases also of yield loss and cost. However, the properties and microstructure of the parts fabricated by AM will not reach the level of forged material without using expensive downstream processes such as hot-isostatic pressing. Using the specific advantages of both processes, AM could be combined with forging operations to novel process chains, offering the possibility to reduce the number of forging steps and to create near-net shape forgings with desired local properties. The present study focusses on the investigation of the microstructure evolution during heat treatment and hot working of Ti–6Al–4V specimens which were produced by means of selective laser melting (SLM). The results show that the as-built SLM samples are well hot workable and have a lower activation energy for hot working, lower peak stresses as well as faster the globularization kinetics than conventional wrought material with a lamellar microstructure. These properties can be also considered favourable for hot working and may be exploited to reduce the tool load during forging.

Evaluation of hot deformation characteristics in modified AA5052 using processing map and activation energy map under deformation heating

The hot workability and deformation characteristics of modified AA5052 were evaluated by hot torsion testing in the temperature range 573–773 K and at strain rates of 0.1–10 s−1. Flow curves obtained from the hot torsion test were corrected considering deformation heating during hot deformation. The processing map and activation energy map were developed from this compensated flow data to propose the optimum processing conditions and to reveal the influence of hot processing parameters on microstructure evolution. The validities of the corrected processing map and activation energy map were also analyzed. The processing map showed that this alloy had a wide flow instability region at all working temperatures; in contrast, maximum power dissipation efficiency was shown at the highest and lowest strain rates in the large temperature region (above 723 K). The activation energy for hot working varied from 28 to 497 kJ mol−1 and showed a tendency for relatively small activation energy where higher power dissipation efficiency was observed. Microstructural observations indicated that dynamic recovery considerably contributed to a higher power dissipation efficiency and lower level of activation energy. On the other hand, the main reason for flow instability was attributed to the evolution of deformation bands or liquid phases. Thus, the hot deformation characteristics of the alloy proposed from the processing map and activation energy map were consistent with the deformed microstructure.

The characteristic of deformability and quantitative description of the microstructure of hot-deformed Ni-Fe superalloy

Abstract The paper presents the results of research concerning the influence of hot plastic working parameters on the deformability and microstructure of a Ni-Fe superalloy. The research was performed on a torsion plastometer in the range of temperatures of 900-1150°C, at a strain rates 0.1 and 1.0 s-1. Plastic properties of the alloy were characterized by the worked out flow curves and the temperature relationships of flow stress and strain limit. The structural inspections were performed on microsections taken from plastometric samples after so-called “freezing”. The stereological parameters as the recrystallized grain size, inhomogenity and grain shape have been determined. Functional relations between the Zener-Hollomon parameter and the maximum yield stress and the average grain area have been developed and the activation energy for hot working has been estimated.

Constitutive analysis of high-temperature workability of a high-nitrogen bearing steel

In this paper, the hot working behaviour of X30 CrMoN 15 1 high N steel was investigated by torsion testing. A new constitutive model based on the Garofalo equation was used to describe the flow stress dependence on temperature, strain and strain rate. Adiabatic heating was accounted for in the modelling of the flow curves. An excellent description of the experimental data was obtained by the model, which suggests that in isothermal condition the flow curves are characterised by a peak, followed by a moderate softening to a steady state. The high values of the activation energy for hot working were rationalised by considering the strengthening effect of undissolved particles hindering dislocation motion.This paper is part of a Themed Issue on Recent developments in bearing steels.

Microstructural and constitutive analysis in process modeling of hot working: The case of a Mg-Zn-Mn alloy

This study presents an analysis of the microstructural evolution and flow stress variation with strain of a Mg-2%Zn-1%Mn alloy during hot deformation, carried out through a combination of torsion experiments, electron backscatter diffraction (EBSD) studies and constitutive analysis. Straining was observed to produce dynamic recrystallization (DRX). It emerged that such DRX was sluggish at 250°C, while almost complete grain refinement occurred at 350°C. A further increase in temperature led to concurrent grain growth and dynamic recrystallization, resulting in coarse grain size. A modified strain-dependent form of the Garofalo equation was used in the constitutive analysis, obtaining a more than satisfactory description of the experimental results. The extrapolation of the model in the low strain rate/high strain regions led to an equally satisfactory description of the experimental data. The analysis of the variation of the activation energy for hot working, which was equivalent to the activation energy for self-diffusion, suggested that deformation was recovery-controlled in most of the range of the experimental conditions investigated.

The Deformability of Fe-Ni Superalloy during High Temperature Deformation

The influence of initial soaking and parameters of plastic deformation on the deformability of A-286 superalloy have been presented. The hot-torsion tests were executed at constant strain rates of 0.1 and 1.0 s-1, at testing temperatures in the range 900-1150°C and were conducted until total fracture of the samples. Plastic properties of the alloy were characterized by worked out flow curves and the temperature relationships of flow stresses and strain limits. Activation energy for hot working Q was assessed for the alloy after two variants of previous heating, i.e. 1100°C/2 h and 1150°C/2 h.

A New Constitutive Model for the Plastic Flow of Metals at Elevated Temperatures

A new constitutive model based on the combination of the Garofalo and Hensel-Spittel equations has been used to describe the plastic flow behavior of an AA6005 aluminum alloy tested in torsion. The analysis of the experimental data by the constitutive model resulted in an excellent description of the flow curves. The model equation was then rewritten to explicitly include the Arrhenius term describing the temperature dependence of plastic deformation. The calculation indicated that the activation energy for hot working slowly decreased with increasing strain, leading to thermally activated flow softening. The combined use of the new equation and torsion testing led to the development of a constitutive model which can be safely adopted in a computer code to simulate forging or extrusion.

Modelling of the flow behaviour of wrought aluminium alloys at elevated temperatures by a new constitutive equation

The hot working behaviour of an Al–0.7%Mg–0.4%Si (wt.%) aluminium alloy has been investigated by torsion testing in the 450–575°C temperature range. The behaviour of the alloy, in terms of equivalent stress vs equivalent strain, was modelled by a modified form of the Hensel and Spittel constitutive equation. In this new equation, the applied stress σ was replaced by the sinh(ασ) term, where α was a constant. The revised form of the Hensel–Spittel equation gave an excellent description of the flow curves and of the decrease in the strain rate sensitivity of the flow stress observed, at a given strain, with increasing strain rate. The model equation was then rewritten to explicitly include the Arrhenius term describing the temperature dependence of strain rate. The calculation indicated that the activation energy for hot working decreased from 218 to 198kJmol−1 in the considered range of strain.

Rate-controlling mechanisms of hot deformation in a medium carbon vanadium microalloy steel

Isothermal compression tests were carried out on a medium carbon vanadium microalloy steel (roughly Fe-0.33C-1.5Mn-0.1V, wt%) by using a Gleeble-1500 simulator. Based on constitutive analysis including an Arrhenius term, activation energy for hot working was calculated and used to evaluate the rate-controlling mechanism of hot deformation. At low strain rates (0.1–1s−1), the activation energy for hot working (287.4kJ/mol) is very close to the austenite lattice self-diffusion activation energy, indicating that the rate-controlling mechanism is dislocation climb. While at high strain rates (10–30s−1), the activation energy becomes very high (500.6kJ/mol), and activation volume is better used under such conditions. Then, activation volume analysis based on both Schöck model and Kocks–Argon–Ashby model demonstrates that the rate-controlling mechanism at high strain rates is cross slip. That is, the rate-controlling mechanisms of hot deformation for the medium carbon vanadium microalloy steel at high and low strain rates are intrinsically different. Inspired by the findings above, processing map analysis based on dynamic materials model was further preceded and different peak domains of power dissipation efficiency in high and low strain rate regimes were found.

High-temperature deformation properties of austenitic Fe-Mn alloys

The influence of the Mn content on the hot deformation properties of austenitic binary Fe-Mn alloys containing 1 to 20 mass pct Mn has been investigated for the first time. The influence of the Mn content on the constitutive equations was determined for the temperature range of 950 °C to 1250 °C and the strain rate range of 0.1 to 2 s−1. The activation energy for hot working increased with increasing Mn content, and dynamic recrystallization was observed for all the Fe-Mn binary alloys. The Mn was found to delay dynamic recrystallization. An increase in the Mn content resulted in an increase of both the peak stress σ p and the corresponding peak strain e p , thus revealing the pronounced influence of Mn on the hot deformation process.

Hot Deformation Analysis of a 2 wt.-% Si Electrical Steel by Means of Torsion Tests

Steels with high amounts of silicon are used in electrical applications due to their low mangectoestriction, high electrical resistivity and reduced energy losses, but they exhibit poor formability. A fundamental study of the workability of such materials using torsion testing may help to understand and to optimise its production. Single deformation torsion tests were carried out on a steel containing 2 wt.-% Si in a temperature range of 800 to 1100°C and strain rates in the range of 0.01 to 2 s-1. A value of 299 kJ/mol was found for the apparent activation energy for hot working after applying the hyperbolic-sine equation to the mean flow stress (MFS) values computed from the test. Multiple deformation torsion tests under continuous cooling conditions were carried out in the same temperature range at strain rates from 0.2 to 1 s-1, the strain per pass and interpass time (determining the cooling rate) were varied. Different critical temperatures, which are of importance for processing this alloy, can be calculated from the dependence of MFS with the inverse absolute temperature; such a method was used to determine the temperature at which recrystallisation stops (Tnr). It was found that this temperature depends on strain rate, pass strain and interpass time. Results of the microstructure analysis of quenched samples are in good agreement with the values of Tnr.

Hot working and multipass deformation of a 41Cr4 steel

The hot working response of a 41Cr4 steel has been investigated by means of torsion tests at temperatures of 850, 950 and 1050 °C, in order to simulate the rolling schedule for this material. The equivalent strain rates used in this part of the study ranged between 5 and 0.05 s −1. The peak stresses obtained from the flow curves were related to strain rate ( ε) and temperature ( T) by means of the well known equation ε ̇ exp Q/RT=A( sinh ασ) n where Q was the activation energy for hot working, n was the stress exponent, and A and α were material parameters. A least square analysis of the data led to n=4.3, α=0.0096 MPa −1 and Q=295 kJ mol −1. A subsequent analysis was aimed at simulating rolling practice; the strain and strain rate for each single pass were determined on the basis of the real rolling schedule, and a series of multi-step torsion tests with different finishing temperatures (1050, 800 and 750 °C) were carried out. The microstructures of the samples after deformation were then analysed by means of light microscopy, and the differences in term of morphology, distribution and amount of the various constituents are discussed.

Hot Working of Zirconium Alloys: Some Recent Developments

The published results on the hot deformation behaviour of zirconium and its alloys have been reviewed critically to ascertain the type of information available in the areas of hot workability and evolution of microstructure during hot deformation. It is observed that in a majority of these investigations, the emphasis was on the fundamental aspects of hot deformation such as identification of the rate controlling mechanisms, activation energy for hot working and identification of softening mechanisms. However, no systematic effort has been directed towards the optimization of hot workability and control of microstructure of these technologically important materials. In recent years efforts have been made to characterize the hot workability and development of microstructure during hot deformation by constructing power dissipation maps or processing maps and flow instability maps in the format of Dynamic Materials Model. The power dissipation maps and instability maps have been found very useful in optimizing the hot workability and controlling the microstructure during hot deformation of several zirconium alloys.

Effect of Interpass Time and Cooling Rate on Apparent Activation Energy for Hot Working and Critical Recrystallization Temperature of Nb-microalloyed Steel.

Two-stage linear ln[sinh(ασ)] vs. 1/T relations indicative of double QHW behavior, are obtained from anisothermal multipass flow curves generated by testing Nb/Ti microalloyed steel in torsion (in the range 1250-800oC), except for the case of the high cooling rate-short interpass time tests, which gave rise to single-stage plots, indicative of single QHW behavior. Above the Tnr, the QHWU (apparent activation energy for hot working corresponding to the upper temperature range) is little affected by test variables (interpass time and cooling rate) and type of deformation (isothermal continuous and anisothermal multipass deformation give virtually equivalent QHWU, values above the Tnr). However, below the Tnr, the QHWL, corresponding to the lower temperature range, becomes sensitive to test variables, and may show considerable deviation from QHWL obtained in continuous tests. Therefore, the temperature dependence of the flow stress, below the Tnr, relevant to a multipass hot working operation, can be well described only by the QHWL obtained from the multipass flow curves, because it is interpass time dependent. The interpass time dependence of both the QHWL and the Tnr can be divided into three regions. Within the short interpass time region (1.8 to 10 sec), the two parameters show opposite trends, while beyond 10 sec they correlate well. Within the 1.8 to 10 sec region the Tnr is assumed to be controlled by Nb in solid solution, while the QHWL is controlled by both solute and precipitation effect. Beyond 10 sec precipitation is the only controlling mechanism. In addition to the Tnr derived from Mean Flow Stress (MFS) and ln[sinh(ασ)] vs. 1/T plots, the recrystallization limit, Trl, which correlates well with both Tnr's, and the recrystallization stop, Trs, temperatures are derived from % Fractional Softening (FS) vs. 1/T plots.

Analysis of high temperature flow stress of titanium alloys IMI 550 and Ti-10V-2Fe-3AI during isothermal forging

High temperature flow stress data obtained from laboratory scale isothermal forging tests on two titanium alloys, IMI550 (Ti-4Al-4Mo-2Sn-0.5Si) and Ti-10V-2Fe-3Al, have been correlated using a generalised hot working equation. The (α + β) titanium alloy IMI 550 was studied at temperatures in the range 900–950°C, high in the (α + β) phase field, and over a range of strain rates from 4.2 × 10−4 S−1 to 4.2 × 10−2 S−l. The metastable β titanium alloy Ti-10V-2Fe-3Al was studied at temperatures in the range 740–780°C, high in the (α + β) phasefield, and also at 820–950°C in the β field, over the same range of strain rates. A good correlation between the experimental data and the hot working equation was observed for both alloys. Values of the activation energy for hot working have been considered, in conjunction with microstructural evidence, in relation to the dynamic restoration processes taking place. The activation energy for hot working of Ti-10V-2Fe-3Al in the β field has been found to be 185 kJ mol−1, which supports the view that dynamic recovery is the only restoration mechanism in the single phase field. In the(α + β) field, ‘effective’ activation energy values have been determined for both Ti-10V-2Fe-3Al and IMI 550, and these values were significantly higher than those for β forging, especially in the case of IMI 550. However, for both alloys, the dominant restoration mechanism at temperatures high in the (α + β) field is considered to be dynamic recovery of the β phase.