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The need for sheet metal forming using highly resistant materials such as titanium alloys and stainless steel has increased recently. These materials possess elevated mean flow stress values, which make them difficult to draw at room temperature. To achieve a homogeneous distribution of strain in the stretched component, reduce the load required for plastic deformation, and greatly improve material formability, hot forming is helpful. The goal of the current study is to conduct stretch-forming experiments to investigate the forming characteristics of Austenitic material Stainless Steel (ASS) 304 at Hot working temperatures. Stretch forming experiments have been conducted on the Servo electrical sheet press test machine at 650 and 800°C. The formability has been estimated by constructing a Fracture forming diagram (FFLD), limiting the height of the dome (LDH) and the distribution of the strain of stretched cups. It has been discovered that the limit of forming bounds rises with the temperature reaching 800°C, while the DSA effect causes the necking region – the area between the safe and fracture limits ‒ to decrease with additional temperature rise from 800 to 900°C. Within the experimental limitations, it has been considered that the Hot forming of ASS 304 at 650°C gives the highest strain forming limits with a uniform strain distribution in the stretched cups. From the Formability limit diagram, dome height, and strain distribution, it can be observed that ASS 304 has good limiting strain up to 800°C with lower load application.

Silver products with high relief have become popular in the silver decoration industry. However, it is difficult to obtain these products through conventional processing at ambient temperature. The aim of this work is to solve this problem by increasing the deformation temperature. Detailed studies were conducted on the evolution of microstructure characteristics in bulk high-purity silver by electron backscatter diffraction (EBSD) to achieve high-relief applications at elevated temperatures. The high temperature sample is mainly composed of recrystallized and substructured grains, exhibiting a more stable state than the ambient temperature sample. More than 70% annealing twins are observed in the hot-working sample. They are characterized by the amount of Σ3n-type triple grain boundary junctions within large grain clusters formed by multiple twinning. These particular boundaries improve the intergranular corrosion resistance and degradation, which is significantly essential for high-purity silver jewelry exposed to sweat and air. The closed multi-coining processes at different temperatures were conducted subsequently. The performance of workpieces demonstrates that increasing the deformation temperature is a viable alternative for producing durable high-relief silver products. [ABSTRACT FROM AUTHOR]

A new mean-field modeling tool is developed to correctly tackle the problem of precipitation during deformation. The model is an extension of a previously developed Langer–Schwartz–Kampmann–Wagner precipitation modeling approach for multicomponent alloys. The latter is now integrated together with Thermo-Calc software and with physically based equations describing the evolution of the dislocation density during hot-working. New equations for the nucleation barrier and the critical size have also been added to correctly simulate the early stages of precipitation. The model is applied to hot compression tests, for which experimental information was available. The model shows the importance of accounting for the overall precipitation kinetics, and not only for constant values of the precipitate volume fraction or size, to investigate the mechanical property evolution. This is due to a complex interplay between the defects and the precipitates’ evolution driven by a competition between thermodynamics and kinetics. Finally, it is highlighted how the strain rate used affects the final microstructure of the material.

Silver products with high relief have become popular in the silver decoration industry. However, it is difficult to obtain these products through conventional processing at ambient temperature. The aim of this work is to solve this problem by increasing the deformation temperature. Detailed studies were conducted on the evolution of microstructure characteristics in bulk high-purity silver by electron backscatter diffraction (EBSD) to achieve high-relief applications at elevated temperatures. The high temperature sample is mainly composed of recrystallized and substructured grains, exhibiting a more stable state than the ambient temperature sample. More than 70% annealing twins are observed in the hot-working sample. They are characterized by the amount of Σ3n-type triple grain boundary junctions within large grain clusters formed by multiple twinning. These particular boundaries improve the intergranular corrosion resistance and degradation, which is significantly essential for high-purity silver jewelry exposed to sweat and air. The closed multi-coining processes at different temperatures were conducted subsequently. The performance of workpieces demonstrates that increasing the deformation temperature is a viable alternative for producing durable high-relief silver products.

The experimental flow stress curves of structural steels obtained from axisymmetric compression tests conducted under hot-working conditions very often include the frictional effects present at the tool/specimen interface. Such effects have a significant influence on the flow stress and therefore, should be corrected prior to any quantitative analysis aimed at determining the constitutive description of these materials. Commonly, such a correction is carried out by assuming a constant friction coefficient (μ) or friction factor (m) independent of deformation conditions, which is an unrealistic approach. The present investigation analyzes experimentally the frictional effects that occur when steel is deformed under axisymmetric compression conditions in the temperature range of 850 to 1200 °C at a strain rate of 0.1 s−1 and applied effective strains of 1, employing cylindrical samples with an initial diameter to initial height ratio (d0/h0) in the range of 0.5 to 2. Finite element modeling (FEM), as well as element-free Galerkin modeling (EFGM), have been employed for the analysis and prediction of the von Mises stress distribution, barreling and amount of metal folding undergone by the compression specimens. It has been shown that the increase in flow stress due to frictional effects can be corrected on the basis of either μ or m, by assuming that these parameters vary in the course of plastic deformation and are strongly dependent on deformation temperature. A novel procedure for the systematic correction of the flow stress curves, taking into consideration the changes in friction conditions during plastic deformation, has been proposed. [ABSTRACT FROM AUTHOR]

The dynamic recrystallization (DRX) behavior of 47Zr-45Ti-5Al-3V alloy was studied by using the experiment and numerical simulation method based on DEFORM-3D software and cellular automata (CA) over a range of deformation temperatures (850 to 1050 °C) and strain rates (10−3 to 100 s−1). The results reveal that the DRX behavior of 47Zr-45Ti-5Al-3V alloy strongly depends on hot-working parameters. With rising deformation temperature (T) and decreasing strain rate (ε˙), the grain size (dDRX) and volume fraction (XDRX) of DRX dramatically boost. The kinetics models of the dDRX and XDRX of DRX grains were established. According to the developed kinetics models for DRX of 47Zr-45Ti-5Al-3V alloy, the distributions of the dDRX and XDRX for DRX grains were predicted by DEFORM-3D. DRX microstructure evolution is simulated by CA. The correlation of the kinetics model is verified by comparing the dDRX and XDRX between the experimental and finite element simulation (FEM) results. The nucleation and growth of dynamic recrystallization grains in 47Zr-45Ti-5Al-3V alloy during hot-working can be simulated accurately by CA simulation, comparing with FEM.

• A new refinement mechanism was noted in a near-β titanium alloy with ultra-large β grains. • New grain boundaries were produced in 〈0 0 1〉 grains interconnecting the grain corners/protrusions. • New grain boundaries were formed via a low-angle grain boundary merging mechanism. • The new refinement mechanism plays an unignorable role in ingot breakdown. In this work, the multi-step hot-working of a near-β titanium alloy, Ti-55511 alloy with centimeter-sized grains, was performed at 1170 °C. It was confirmed that the static/metadynamic recrystallization during interpass annealing was the primary mechanism responsible for the microstructural refinement. Meanwhile, a new breakdown mechanism of the ultra-large β grains with 〈0 0 1〉 preferential orientation was observed during deformation, which was essentially different from the traditional discontinuous or continuous dynamic recrystallization. That is, new grain boundaries and even boundary networks were produced interconnecting the corners/protrusions of the 〈0 0 1〉 -orientated grains through a low-angle grain boundary merging mechanism, which progressively severed the large β grains into smaller ones. Such a breakdown mechanism may be an important complement for microstructural refinement during primary hot-working, which allowed the breakdown of the ultra-large β grains under fairly low strain level. [ABSTRACT FROM AUTHOR]

Modeling and optimization of industrial hot-working operations by means of commercial finite element computer codes require robust constitutive descriptions, which could be easily implemented. Also, it is expected that such formulations are able to describe as accurate as possible the flow stress, work-hardening and work-softening rate of the material, as a function of microstructure and deformation conditions. The present communication describes the development of a novel constitutive description of metals which exhibit dynamic recrystallization during deformation under hot-working conditions. The formulation is founded on the extension of the model earlier developed by Anand and co-workers for the constitutive description of metals which exhibit dynamic recovery as the only dynamic restoration mechanism during hot deformation. The advanced constitutive formulation has been validated by conducting axisymmetric compression tests with samples of a R260 steel employed for the manufacture of rail tracks. The specimens were deformed in a wide range of temperatures and strain rates, both under constant and variable strain rate conditions. The latter involve either an increase or a decrease in the Zener-Hollomon parameter as plastic deformation is applied. The results obtained indicate that the proposed constitutive description is able to reproduce quite accurately the experimental flow stress data. It has also been shown that the correct computation of the recrystallization time and volume fraction recrystallized dynamically, by means of an appropriate differential or incremental formulation of the Avrami equation, constitutes an essential aspect of the model. Such an approach is particularly important for predicting correctly the changes in flow stress under variable strain rate conditions. [ABSTRACT FROM AUTHOR]

The present work has been conducted in order to develop a novel approach to predict the inhomogeneous flow of a 20MnCr5 steel during an axisymmetric hot compression test, by using the element-free Galerkin (EFG) method under a differential constitutive description. The governing equations have been solved on the basis of the EFG global weak formulation. A detailed explanation concerning the characteristics inherent to the application of the EFG method to this problem, has also been provided. Furthermore, a return mapping algorithm for the solution of associative von Mises inelastic problems, has been formulated for the differential constitutive description employed in this communication, namely, a differential return mapping algorithm (DRMA). The feasibility and suitability of the EFG method for solving the axial compression problem has been shown by comparing its results with a FEM based solution employing a simple constitutive description. The reliability of the proposed DRMA has been demonstrated by a comparison of a homogeneous deformation numerical test with the experimental and direct integration results reported in a previous communication. Finally, the EFG model has been used to predict the stress, strain and volume fraction recrystallized distributions under steady and transient nominal strain rate and temperature deformation conditions. These parametric studies have been carried out by considering the differential constitutive description, but also a conventional integrated constitutive model of a 20MnCr5 steel. The results have revealed the suitability of the EFG formulation under the proposed DRMA, for predicting the performance of an axisymmetric hot compression test. The differences between the use of differential and integrated constitutive descriptions on the performance of hot-working processes under inhomogeneous deformation conditions, have also been evidenced in this research work. [ABSTRACT FROM AUTHOR]

Cu-Ni-Si alloys and their variants constitute very important engineering materials, which have found a number of applications in different fields, as potential substitutes of the toxic Cu-Be alloys traditionally employed for the manufacture of different parts and components. However, the hot-working behavior of these materials has not been deeply investigated and only few studies have reported some limited information regarding the relationship between flow stress and deformation conditions. The present communication reports the main findings of an original investigation aimed at analyzing in detail the flow stress, work-hardening and work-softening behavior of a Cu-Ni-Si alloy deformed in the temperature range of 600 °C to 950 °C, at nominal strain rates in the range of 0.1–10 s −1 . Particularly, for the first time, a general constitutive formulation able to describe the changes in flow stress, work-hardening and work-softening rates of this material as a function microstructure and deformation conditions, in the temperature range between 800 °C and 950 °C, is proposed. The nature of the constitutive description here advanced allows the computation of the flow stress either under constant and transient loading conditions, on the basis of physically-based and well established models. Such a constitutive formulation is able to reproduce the experimental values of the experimental flow stress with an accuracy of ±7 MPa and therefore, it represents a valuable tool for modeling hot-working operations conducted on this material. It has been concluded that, in order to take full advantage of the grain refining effects associated with dynamic recrystallization, hot-working of the alloy should be carried out at temperatures above 850 °C. Deformation at temperatures below 800 °C leads to a significant increase in the mechanical properties of the alloy and even fracture, as a consequence of the possible precipitation of fine secondary phases. [ABSTRACT FROM AUTHOR]

The development of sound constitutive descriptions able to predict accurately the changes in flow stress during plastic deformation, under changing temperature and strain rate conditions is of utmost importance for the correct prediction of the mechanical response of metals and alloys, which undergo deformation under hot-working conditions. Therefore, the present work has been conducted with the aim of developing a constitutive model of such characteristics, able to describe as accurate as possible the changes in flow stress of a commercial 20MnCr5 steel, when this material is deformed at elevated temperatures and different strain rates. Particular attention has been paid to the analysis of the flow stress, work-hardening and work-softening behavior when this material is subjected to transient testing conditions, that is to say, changes in strain rate and deformation temperature in the course of plastic deformation. The constitutive description is initially developed from the analysis of the stress-strain curves obtained from axisymmetric compression tests conducted under nominal constant testing conditions, which involved the deformation of the material at temperatures in the range of 1123-1473K and strain rates of 0.01-10s-1, up to effective strains spanning from approximately 0.6 to 1. The initial work-hardening and the subsequent work-softening behavior exhibited by the material during plastic deformation have been thoroughly characterized on a rational basis. The temperature and strain rate dependence of the yield stress, critical stress for the initiation of dynamic recrystallization, apparent saturation stress and actual steady-state flow stress have been described by means of the Sellars-Tegart-Garofalo model. The time to 50% dynamic recrystallization has also been described as a function of deformation temperature and strain rate, by means of a simple parametric relationship commonly employed for this purpose. The proposed constitutive description employs a single activation energy, whose value is close to that required for the self-diffusion of Fe in austenite, of approximately 284kJmol-1. Further experiments involving changes in deformation temperature and strain rate have been carried out in order to validate the constitutive model. The results indicate that the proposed approach is able to describe satisfactorily the changes in flow stress, work-hardening and work-softening of the investigated material during transient testing conditions. [ABSTRACT FROM AUTHOR]

Highlights: [•] A constitutive description of a C–Mn steel under hot-working conditions has been developed. [•] Plane strain compression tests were conducted between 850°–1100°C, at strain rates of 0.3–24s−1. [•] The temperature and strain rate dependence of the flow stress is introduced in two different ways. [•] The formulation considers the interaction of dislocations with athermal and thermal barriers. [•] The flow stress is computed from the numerical integration of the work-hardening rate equation. [ABSTRACT FROM AUTHOR]

Coupled processes of dynamic recovery and precipitation, occurring during hot-rolling and subsequent aging, lead to the formation of a unique honeycomb network of precipitates in commercial Mg–Y–Nd–Zr or WE43 alloy. The honeycomb network is developed on the (0 0 0 1)Mgbasal planes and consisted of fine Nd-richβ1platelets lying on all threeplanes presumably decorating recovery-generated dislocation subcell boundaries. Three variants ofβ1platelets are connected by Y-richprecipitates at the nodes of the hexagonal honeycomb network. [ABSTRACT FROM PUBLISHER]

Abstract: The present study has been conducted in order to formulate a rational description of the flow stress of structural steels, when these are deformed in tension under hot-working conditions. The simplified formulation that has been developed only involves the analysis of the positive work-hardening region of the stress–strain curve and is based on the concepts behind the Mechanical Threshold Stress Model. Accordingly, it is assumed that the flow stress of the material arises from three different contributions: athermal hardening and solid solution, which do not evolve in the course of plastic deformation and work-hardening, the only evolutive component of the flow stress. The evolution of work-hardening is introduced in the formulation by means of the phenomenological work-hardening law earlier proposed by Sah and co-workers, expressed in differential form. The temperature and strain rate dependence of the contributions of solid solution hardening to the flow stress, as well as that of the saturation stress, within the work-hardening law, are described by means of the kinetic model earlier advanced by Kocks. The formulations derived are able to reproduce satisfactorily both the flow stress and work-hardening rate of the material employing a relatively small number of material parameters and therefore, these could be a valuable tool for the analysis of hot-working processes. [Copyright &y& Elsevier]

Abstract: In this work crystallization kinetics of bioactive glasses 1–98 and 13–93 are discussed. Within a certain temperature–time window these glasses can be hot worked into various products without interfering with crystallization. The crystallization was studied isothermally by heating glass plates at different temperatures for different times. Phases in the samples were studied through XRD and SEM analyses. The nucleation-like curves and crystallization characteristics were measured with DTA. The temperature of maximum nucleation was measured for glass 1–98 at 725°C and for 13–93 at 700°C. The activation energy of crystallization of both glasses was 280kJ/mol. The Johnson–Mehl–Avrami exponent and the SEM micrographs of the samples suggested surface crystallization. The primary crystalline phase was wollastonite. The growth rate of the crystallized surface layer was 1 order of magnitude higher in the plates of 1–98 than in 13–93. The results can be utilized to optimize the parameters in hot-working of the glasses. [Copyright &y& Elsevier]

Microstructures, and especially grain boundary populations, are characterized in a γ/γ′ nickel-base superalloy submitted to various thermomechanical treatments involving hot deformation and subsequent supersolvus annealing. The resulting microstructures consist of equiaxed grains, including twins. Both the grain size and the amount of twin boundaries are dependent on the processing parameters, which demonstrates that grain boundary engineering is applicable to those materials. The mechanisms involved are discussed, taking into account the hot-working conditions. [Copyright &y& Elsevier]

The high-manganese steels with the austenitic structure belong to a group of modern steels predicted to use in the automotive industry. The chemical composition of the steel containing 25% Mn, solution hardened by Si and Al was developed. Microadditions of Nb and Ti introduced into the steel creating stable nitrides and carbides should act by precipitation hardening and inhibit a grain growth of recrystallized austenite. The aim of the work was to determine the influence of various hot-working conditions on a structure of the investigated steel. The processes controlling work-hardening and removing strengthening after finishing the hot-working were identified. The preliminary upset forging by the use of eccentric press with a degree of deformation in a range of 20 to 60% and at temperatures of 850 and 1000 °C was carried out. On the basis of determined conditions the multi-stage axial compression tests ensuring the finegrained austenite structure were performed. [ABSTRACT FROM AUTHOR]

The high-manganese austenitic steels are an answer for new demands of automotive industry concerning the safety of passengers by the use of materials absorbing high values of energy during collisions. The chemical compositions of two high-manganese austenitic steels containing various Al and Si concentrations were developed. Additionally, the steels were microalloyed by Nb and Ti in order to control the grain growth under hot-working conditions. The influence of hot-working conditions on recrystallization behaviour was investigated. On the basis of initial investigations realized by hot upsetting the thermo-mechanical conditions resulting in a fine-grained structure were designed. The δ-ε curves and identification of thermally activated processes controlling work-hardening by the use of the Gleeble simulator were determined. It was found that the thermo-mechanical treatment conditions influence a phase composition of the investigated steels alter solution heat treatment. [ABSTRACT FROM AUTHOR]

Textured silicon nitride (Si3N4) has been intensively studied over the past 15 years because of its use for achieving its superthermal and mechanical properties. In this review we present the fundamental aspects of the processing and anisotropic properties of textured Si3N4, with emphasis on the anisotropic and abnormal grain growth of β-Si3N4, texture structure and texture analysis, processing methods and anisotropic properties. On the basis of the texturing mechanisms, the processing methods described in this article have been classified into two types: hot-working (HW) and templated grain growth (TGG). The HW method includes the hot-pressing, hot-forging and sinter-forging techniques, and the TGG method includes the cold-pressing, extrusion, tape-casting and strong magnetic field alignment techniques for β-Si3N4 seed crystals. Each processing technique is thoroughly discussed in terms of theoretical models and experimental data, including the texturing mechanisms and the factors affecting texture development. Also, methods of synthesizing the rodlike β-Si3N4 single crystals are presented. Various anisotropic properties of textured Si3N4 and their origins are thoroughly described and discussed, such as hardness, elastic modulus, bending strength, fracture toughness, fracture energy, creep behavior, tribological and wear behavior, erosion behavior, contact damage behavior and thermal conductivity. Models are analyzed to determine the thermal anisotropy by considering the intrinsic thermal anisotropy, degree of orientation and various microstructure factors. Textured porous Si3N4 with a unique microstructure composed of oriented elongated β-Si3N4 and anisotropic pores is also described for the first time, with emphasis on its unique mechanical and thermal-mechanical properties. Moreover, as an important related material, textured α-Sialon is also reviewed, because the presence of elongated α-Sialon grains allows the production of textured α-Sialon using the same methods as those used for textured β-Si3N4 and β-Sialon. [ABSTRACT FROM AUTHOR]

Abstract: The current study analyses the influence of boron contents (between 29 and 105ppm) on the hot ductility of boron microalloyed steels. For this purpose, hot tensile tests were carried out at different temperatures (700, 800, 900 and 1000°C) at a constant true strain rate of 0.001s−1. In general, results revealed an improvement of the hot ductility of steels at increasing boron content. At 700, 900 and 1000°C the ductility is higher than at 800°C, where boron microalloyed steels exhibit a region of ductility loss (trough region). Likewise, dynamic recrystallization only occurred at 900 and 1000°C. The fracture surfaces of the tested steels at temperatures giving the high temperature ductility regime show that the fracture mode is a result of ductile failure, whereas it is ductile–brittle failure in the trough region. Results are discussed in terms of dynamic recrystallization and boron segregation towards austenite grain boundaries, which may retard the formation of pro-eutectoid ferrite and increase grain boundary cohesion. [Copyright &y& Elsevier]

The article discusses a study which examined the mechanical properties of 24Cr-14Ni stainless steel billets during hot rolling. The stainless steels were hot-rolled under various reduction ratios. The effect of the reduction ratio on the hot mechanical properties and phase transformation of the stainless steel billets were examined.

Abstract: Multielemental Si3C x O y □ z N4–(x+y+z) nanowires and nanoscrews with a diameter of a few tens of nanometers and a length up to a few tens of micrometers have been synthesized for the first time and the possible mechanism for their formation is analyzed in this investigation. The one-dimensional nanomaterial has a β-Si3N4 structure with the long axis along the 〈210〉 direction. [Copyright &y& Elsevier]

A silicon carbide, with a preferred orientation of platelet grains, was obtained by hot-forging the hot-pressed SiC with equiaxed microstructures. Increased strength (810–1150MPa), fracture toughness (4.4–5.9MPam1/2), and hardness (24.4–26.8GPa) were achieved in the hot-forged specimens compared to the hot-pressed specimens with random orientation. [Copyright &y& Elsevier]

The dynamic recrystallization (DRX) behavior of 47Zr-45Ti-5Al-3V alloy was studied by using the experiment and numerical simulation method based on DEFORM-3D software and cellular automata (CA) over a range of deformation temperatures (850 to 1050 °C) and strain rates (10−3 to 100 s−1). The results reveal that the DRX behavior of 47Zr-45Ti-5Al-3V alloy strongly depends on hot-working parameters. With rising deformation temperature (T) and decreasing strain rate ( ε ˙ ), the grain size ( d D R X ) and volume fraction ( X D R X ) of DRX dramatically boost. The kinetics models of the d D R X and X D R X of DRX grains were established. According to the developed kinetics models for DRX of 47Zr-45Ti-5Al-3V alloy, the distributions of the d D R X and X D R X for DRX grains were predicted by DEFORM-3D. DRX microstructure evolution is simulated by CA. The correlation of the kinetics model is verified by comparing the d D R X and X D R X between the experimental and finite element simulation (FEM) results. The nucleation and growth of dynamic recrystallization grains in 47Zr-45Ti-5Al-3V alloy during hot-working can be simulated accurately by CA simulation, comparing with FEM. [ABSTRACT FROM AUTHOR]

Textured silicon nitride (Si3N4) has been intensively studied over the past 15 years because of its use for achieving its superthermal and mechanical properties. In this review we present the fundamental aspects of the processing and anisotropic properties of textured Si3N4, with emphasis on the anisotropic and abnormal grain growth of β-Si3N4, texture structure and texture analysis, processing methods and anisotropic properties. On the basis of the texturing mechanisms, the processing methods described in this article have been classified into two types: hot-working (HW) and templated grain growth (TGG). The HW method includes the hot-pressing, hot-forging and sinter-forging techniques, and the TGG method includes the cold-pressing, extrusion, tape-casting and strong magnetic field alignment techniques for β-Si3N4 seed crystals. Each processing technique is thoroughly discussed in terms of theoretical models and experimental data, including the texturing mechanisms and the factors affecting texture development. Also, methods of synthesizing the rodlike β-Si3N4 single crystals are presented. Various anisotropic properties of textured Si3 N4 and their origins are thoroughly described and discussed, such as hardness, elastic modulus, bending strength, fracture toughness, fracture energy, creep behavior, tribological and wear behavior, erosion behavior, contact damage behavior and thermal conductivity. Models are analyzed to determine the thermal anisotropy by considering the intrinsic thermal anisotropy, degree of orientation and various microstructure factors. Textured porous Si3N4 with a unique microstructure composed of oriented elongated β-Si3N4 and anisotropic pores is also described for the first time, with emphasis on its unique mechanical and thermal-mechanical properties. Moreover, as an important related material, textured α-Sialon is also reviewed, because the presence of elongated α-Sialon grains allows the production of textured α-Sialon using the same methods as those used for textured β-Si3N4 and β-Sialon.