Search

This study demonstrates the application of a new experimental technique for laboratory-scale simulation of the open-die forging process, known as cogging, an intermediate hot-working process necessary to design an optimised microstructure in the advanced engineering titanium alloy Ti-6Al-4V. Small test-bars of Ti-6Al-4V alloy were subjected to multi-directional cogging operations at elevated temperatures (950–1050 °C). The as-received material, prior to forging, underwent heat treatments to coarsen the initial grain structure, to better simulate the industrial-scale intermediate microstructure (i.e., β recrystallised) and to help prove the capability of the set-up to achieve microstructure modification via globularisation (below β-transus), and recrystallisation (dynamic and static) and recovery mechanisms (above β-transus) within the cogged material. The influences of hot working parameters on deformation localisation, width of α platelets, and globularisation within the resulting microstructure variation have been investigated using light microscopy (LM), Vickers hardness (HV) testing, and electron backscatter diffraction (EBSD). The cogged Ti-6Al-4V alloy specimens underwent various microstructural evolution stages after hot forging, thus indicating successful application of the designed miniaturised open-die forging apparatus for high temperature experimentation and characterisation studies. This will be suitable for low-cost small-scale trials to determine the key process parameters affecting the onset of microstructure evolution during open-die forging (e.g., ingot-to-billet conversion) of the Ti-6Al-4V alloy, prior to large-scale trials which are rather more expensive.

Development of cracks along the center axis of large high strength steel bars commonly occurs during the forging and leads to excessive part rejections. The present investigation aims to develop a better understanding of the evolution of stress-strain states during the forging operation and in particular the effect of deformation path illustrated by die geometry, on the evolution of damage during the cogging of an AISI H13 steel. Hot compression and tensile tests were performed using Gleeble-3800 thermo-mechanical simulator to develop the optimum material model which was then implemented in the finite element (FE) code Forge NxT 3.2® using a developed user subroutine. Normalized Cockcroft and Latham damage criterion and maximum shear stress (Tresca's) theory of failure were used to predict the damage and failure in the center axis of the shaft through FE analysis with three different die shapes: concave, flat, and convex. A comparative study between the three die geometries was conducted to quantify the effects of each of them on the sensitivity to central burst damage. FE model was validated using industrial data. The lowest and highest damage values were found to occur in the case of cogging with concave and flat die, respectively. The coefficient of variation (CoV) is employed as a measure of heterogeneity and it was found that the concave die provides more uniform deformation and most favorable results for the cogging compared to the flat and convex dies. The novel approach, application of concave die successfully implemented at the industrial scale cogging.

This study presents a novel experimental set-up for laboratory-scale simulation of cogging and open-die forging processes during ingot-to-billet conversion of advanced engineering alloys. The experimental set-up is designed to be cost-effective, employing a remotely operated manipulator assembly constructed from readily available “off-the-shelf” components used in conjunction with a conventional uni-axial load-frame - equipment that is available in most materials testing laboratories. Small test-bars of C101 copper alloy were subjected to multi-stroke cogging operations with intermittent rotation at ambient and elevated temperatures (20–600 °C). Prior to forging, the as-received material underwent heat treatments to coarsen the starting grain structure, and to help demonstrate the capability of the apparatus to achieve grain refinement via recrystallisation (dynamic and static) and recovery processes within the deformed material. The resulting microstructural evolution and mechanical property changes of the forged material have been investigated using light microscopy (LM), Vickers hardness (HV) testing, and electron backscatter diffraction (EBSD). The deformed C101 alloy exhibited measurable grain refinement after forging at elevated temperatures, thus demonstrating the effectiveness of the designed miniaturised open-die forging set-up.

The paper presents the results of modeling and testing of a heavy weight part made of Cr-Mo, which was V-modified ultrahigh strength steel grade AISI 4140, processed through a novel open-die forging program and two alternative routes of twostage heat treatment cycles designed to meet requirements of high-duty components for energy sector. By using unconventional forging conditions based on the assumption of large feed and reduction ratio and modifying the chemical composition, better control of the austenite grain was achieved to minimize abnormal grain growth and/or strain uniformity problems. Using the Finite Element Modeling, the multi-stage sequence of upsetting and the cogging strain distribution were optimized to minimize the strain variation along the length to a range 2.2÷2.7, and correlated with the microstructure generated at each main stage on the large cross-sections of the shaft. Mechanical and thermal processing cycles designed using the finite element method were fully verified physical modeling using a 16 ton forging block, including two alternative quenching strategies: oil vs. water spray and air. The material was studied in the as-forged, normalized and heat-treated states to observe the behavior of the hot-formed material and the effects of cooling conditions on the microstructure during the final heat treatment. It was found that the use of large feed ratios on cogging and varied cooling allowed to suppress the adverse effects of the inevitable abnormal grain growth, resulting in 1–2 ASTM in forged condition and reaching 6 ASTM and 8/9 ASTM after quenching in oil and water spray, respectively, which allowed a corresponding notched impact strength of 44÷48 and 85÷122 J/cm2 in the critical region of the forged shaft after tempering.

The use of continuously cast round blooms as a substitute for cast ingots has attracted wide interest because it can improve yields and reduce operating costs through continuous operation. However, central voids and segregations, which are the most common internal defects in continuously cast round blooms, must be eliminated through the cogging process to ensure adequate mechanical properties. In general, because the diameter of a round bloom is not large owing to the limited facility capacity, the permissible amount of forging reduction may be insufficient to completely close the internal defects. Accordingly, it is necessary to apply a more efficient cogging method to eliminate internal defects, particularly central voids, in a round bloom. Therefore, an innovative square cogging process using a square-shaped billet was implemented in this study to concentrate deformations within the central region of a Ø 1000 mm round bloom. A comparison of shop-floor operations between round and square cogging processes for the manufacture of main shafts shows that the square cogging operation noticeably increases the void closure efficiency with a reduced number of forging blows. Furthermore, the square-cogged products show improved mechanical properties as a result of the enhanced forging effect. Finite element simulations of the square cogging process also confirm the experimental results and validate the feasibility of the square cogging process. [ABSTRACT FROM AUTHOR]

In this paper, a hybrid approach is proposed for Multiphase Halbach Array Permanent Magnet Generator (MHAPMG) in the direct driven wind turbine. The proposed approach is the consolidation of Archimedes optimization algorithm (AOA) and Radial Basis Function Neural Network (RBFNN) named AOA-RBFNN approach. The proposed approach achieves higher efficiency at low speed and it is utilized to improve the lesser Cogging torque with torque ripple and fault tolerance. The proposed generator is compared with Multiphase Surface Mount conventional Permanent Magnet Generator (MSMPMG) for its improved outputs. The proposed generator is used to chose the turbine design and to estimate the output of 1000 W with a speed of 440 RPM and the wind speed of 9 m/s. The proposed approach optimizes the electromagnetic performance of Cogging and back electromotive force. The performance of the proposed Halbach array is analyzed with the regular sequence with multiphase topology based on the induced electromotive force, Cogging Torque, Air gap Flux density, and Harmonic Components. The proposed generator is investigated by finite element analysis (FEA). By then, the performance of the proposed system is activated on MATLAB/Simulink platform. The efficiency of WT using proposed technique, HS, TS, and PSO becomes 98%, 80%, 77%, and 82%, respectively. [ABSTRACT FROM AUTHOR]

Based on the crack phenomena observed in an actual cogging experiment of the new as-cast GH4151 alloy, the mechanism of crack generation and propagation during cogging was studied by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results reveal that there were massive and spherical precipitates at the original grain boundary, large-scale holes at the edge of the precipitates and other areas, tiny pores near the grain boundary, and secondary microcracks along the grain boundary. These phenomena all promoted the generation and propagation of cracks. The fracture surface of the crack was determined as intergranular fracture, and local cleavage fracture was found to occur at the grain intersection. Moreover, the strain energy concentration at the grain boundary was evident, and the deformation of billets was found to primarily depend on grain boundary slip and grain rotation. [ABSTRACT FROM AUTHOR]

The paper presents a study of the permanent magnet brushless DC machine, from two perspectives - from authors’ own experience in designing and manufacturing such motors, as well as from actual published research. Various constructive topologies and how they influence BLDC operation, windings used with emphasis on slot, concentrated windings, are also presented. The following part describes current techniques used for enhancing BLDC limited maximum speed, such as phase advance and dwell control, somewhat similar to flux weakening in AC permanent magnet brushless motors. The paper concludes with presentation of several methods used for sensing BLDC rotor position. Overall, the authors’ intention publishing this paper w as t o provide an insight regarding current BLDC development, as well as to assist in making documented choices when using BLDC in specific applications. [ABSTRACT FROM AUTHOR]

This article defines process reserves for improving the efficiency of producing milling balls by the example of AO EVRAZ ZSMK and also defines the regulatory parameters of their production on ball-rolling mills. [ABSTRACT FROM AUTHOR]

The basic features of A-100 steel, such as relationship between cogging process and mechanical properties, relationship among the forging process, grain size and mechanical properties, secondary hardening, and fatigue properties were discussed. The high-temperature homogenization and high deformation at first step technique were developed on the foundation of multiple upsetting and stretching, high forging ratio technique used for 300M steel, and became the technique foundation of cogging process in A-100 steel. The fracture toughness of A-100 steel was tended to be influenced by hot working process. The grain size grew heavily, and mixed grain structure was appeared after heating at 1140℃ and above with deformation amount below 20%, the fracture toughness was also decreased. The secondary hardening performance of A-100 steel was changed after the deformation at low temperature. The tensile strength peak temperature was changed to 468℃,the tensile strength was decreased slowly when over aging. A-100 steel was cyclic hardened, and its fatigue crack growth properties were better than 300M steel. The high cycle fatigue property was heavily deteriorated when tested in 3.5%NaCl solution.

In this article, a new manner of cogging a forging (type: shaft), consisting in the application of a two-stage process composed of preliminary shaping in convex anvils, and also principal forging in flat or shaped anvils, is presented. A new manner of forging brought about the formation of favorable conditions for achieving the maximum values of the effective strain in the central part of a forging, accompanied by a simultaneous absence of tensile stresses, which was exerting a favorable influence upon reforging the axial zone of an ingot. What was determined, was the effective geometric shapes of convex anvils; the efficiency of different technological parameters in the case of the intensity of reforging the axial zone of an ingot was analyzed as well. The investigations were complemented by means of predicting the formation of ductile fractures in the course of forging with the application of three different ductile fracture criteria. The comparison of theoretical and experimental outcomes of investigations indicates a good level of being commensurate.

The manufacturing of sound forgings from large steel ingots requires that internal cavity defects generated during the steel ingot solidification process be compressed by open die forging. The forging ratio that is generally recommended to remove internal defects in large forged products is 3S (threefold); however, the practice lacks a theoretical basis. In this study, a forging experiment and a finite element analysis were performed to investigate the correlation between the forging ratio for large steel ingots (3S) and the cavity closure behavior. First, a hot compression experiment was performed by varying the temperature and strain rate, and the flow stress data observed in the experiment was applied to the finite element analysis. In the experiment for the cogging process, the forging ratio was applied to an actual non-compressive defect material. The finite element analysis was performed using the same forging path as the forging experiment. In the cogging experiment, cavity closure was found by ultrasonic inspection at the forging ratio of 2.9S. The finite element analysis showed that the size of the cavity was significantly decreased at the forging ratio of 2.9S. A finite element analysis was also performed to investigate effective strain and hydrostatic stress at the forging ratio of 2.9S. Finally, this article provides the theoretical basis for the limitation of the internal defect size in initial materials, the threshold effective strain, and the limiting forging ratio of forged products to ensure the internal soundness of large forged products. [ABSTRACT FROM AUTHOR]

Calculation time of some material forming processes is tremendously expensive which makes reducing computational time one of the most urgent challenges in this domain. Among strategies that have been developed to speed-up calculations, one of the most flexible solutions is to utilize enhanced linear solvers such as Multi-Grid algorithm. It consists in using several levels of meshes of the same domain in order to more efficiently solve the systems of equations derived from the discretized problem. The speed-up results from the efficiency of coarse meshes in computing the low frequencies of the residual while fine meshes are more efficient in reducing the high frequencies of the residual. The method is integrated in the commercial software Forge® and applied to the industrial cogging process. The obtained results show that the speed-up depends on the number of nodes; for an industrial scale mesh of 50,000 nodes, the multigrid technique allows dividing the computational time by a factor of two. [ABSTRACT FROM AUTHOR]

Abstract AD730™ is a newly developed Ni-based superalloy for turbine disk applications produced by cast and wrought processes. Conventional ingot-to-billet conversion is an expensive and very complex operation. Because of the difficulties to achieve a uniform strain for recrystallization, large unrecrystallized grains are retained along this process. Heterogeneities of grain size have a negative impact on subsequent forging processes as well as ultrasonic inspectability of the destination part. The main aim of this work is to analyse the impact of both forging direction (billet, cogging) and strain (ε = 0.3–2) on the microstructural evolution of AD730 at subsolvus temperatures from a semi-finished product (billet) under conditions representative of both cogging and hot forging operations. Special attention to the presence of large unrecrystallized grains was paid. Double truncated cones (DTCs) were hot forged at subsolvus temperatures followed by air cooling. SEM and EBSD analysis were conducted in the as-received (billet) and the as-forged conditions. AD730 alloy presents a complex microstructure characterized by the presence of large unrecrystallized grains, aligned in the billet direction, with remarkable differences in γ′ precipitates distribution as compared to recrystallized structures. The fine distribution of primary γ′ precipitates (pinning effect) plays a key role on grain size control but also on the recrystallization behaviour. Continuous dynamic recrystallization (CDRX) mechanism was found to be operating in the large unrecrystallized grains, promoting the formation intragranular DRX grains and the gradual recrystallization of these grains. Strain presents a strong effect on the microstructural evolution of AD730, increasing the recrystallization fraction and refining the structure. By contrast, no significant effect associated to the forging direction was found. Conditions representative of cogging operations (ε ≤ 0.6) at subsolvus temperatures were translated into large fractions of unrecrystallized structures (strain accumulation). Highlights • Large unrecrystallized grains present a fine distribution of intragranular γ′ precipitates; • Intragranular precipitates play a key role on strain distribution and subsequent recrystallization behaviour for AD730; • Continuous dynamic recrystallization (CDRX) promotes the gradual consumption of large unrecrystallized grains. • Undissolved secondary precipitates play a key role on the microstructural evolution of AD730 • No significant effect associated to the forging direction on the recrystallization behaviour of AD730 was found. [ABSTRACT FROM AUTHOR]

To create a rational technology of cogging process and to determinate the optimal values of the angles of tilt and single reduction the stress-strain state (SSS) of the blank during cogging in the flat dies was analyzed. By using the finite element method and program MSC.SuperForge quantitative data are obtained and the basic patterns of distribution of SSS, the temperature during the simulation of tilting in flat dies with different angles of tilting and the amount of reduction were established. Sustainable experimental-industrial technology of forging of two-phase titanium alloys was developed and tested.

The hot deformation behavior of Waspaloy alloy has been investigated by two-pass hot cogging process. The paper presents theoretical end experimental analysis of deformations and microstructural evolutions. The results of a thermo-mechanical simulation for the spatial hot cogging process on the shaped anvils with the application of the three–dimensional finite element method, are presented. The numerical calculation gave an assessment of the effective strain, mean stress and temperature distributions in the work-piece. Models for predicting the evolution of microstructure were developed for dynamic recrystallization and grain growth phenomena. The influence of shape of the anvils on the grain size after dynamic recrystallization was analyzed. The numerical analysis was performed using a commercial program "DEFORM 3D" with thermo-mechanical and microstructural evolution coupled. The results are compared with the experimental data, a good agreement between the predicted and experimental results was obtained.

Asymmetries in permanent magnet synchronous machines (PMSM) can result from manufacturing tolerances or optimization techniques that deliberately introduce them on rotor, stator, or both. This paper analyzes the impact of such unbalance on the air gap pressure, that is radial and tangential to the direction of motion, by means of Maxwell Stress Tensor; also, their consequences on torque pulsations and unbalanced magnetic pulling. Neglecting the manufacturing variations in the analysis of high power density PMSMs underestimates the resultant torque ripple. Additionally, new vibration modes occur due to unbalanced radial pressure. On the other hand, a geometry optimized to minimize torque ripple through controlled asymmetry is investigated, the approach compensates harmonics in torque effectively, but such compensation does not have the same effect on radial forces. Finite element analysis and experimentation are used to support the analysis and it is found that an asymmetric PMSM with low torque ripple will not necessarily produce lower vibrations and noise. [ABSTRACT FROM PUBLISHER]

SUMMARY This paper presents a method for reducing cogging thrust in permanent-magnet-type linear synchronous motors. A phase shift method and supplement tooth method are proposed. The methods do not affect the original thrust performance of the linear motor, nor do they greatly modify its basic shape. The methods are confirmed through experiments and FEM analysis. This paper also discusses the effect of construction errors on cogging thrust. [ABSTRACT FROM AUTHOR]

This work addresses the systematic modeling of a linear feed drive based on a linear synchronous motor, a helpful step in control design for precise machine tools using linear motors. The model considers the electrical dynamics, ripple, cogging effects, and friction. For ripple and cogging, periodic and aperiodic behaviors are analyzed, and simple models are proposed to reflect the observed behavior. Friction is represented by the generalized Maxwell-slip model Al-Bender (IEEE Trans Autom Control 50:1883-1887, ), and the particular manner in which pre-rolling parameters and Stribeck curve were determined for the current system is shown here for completeness. Finally, the model shows a good performance both in simulation and feedforward control. [ABSTRACT FROM AUTHOR]

A Multi-Mesh Multi-Physics (MMMP) method is developed to reduce the very long computational time required for simulating incremental forming processes such as cogging or ring rolling. It consists in using several finite element meshes on the same domain to solve the different physics of the problem. A reference mesh is used to accurately store the results and history variables, while the different computational meshes are optimized to solve each physic of the problem. The MMMP algorithm consists in two main key-steps: the generation of the different unstructured meshes and the data transfer between the meshes. The accuracy of the method is supported by using meshes that are embedded by nodes. The method is applied to the simulation of the cogging metal forming process for which it shows as accurate and more than ten times faster than the standard method with a single mesh. [ABSTRACT FROM AUTHOR]

Abstract: In order to study the hot deformation behavior and fracture characteristics of hard-deformed superalloy GH4720Li during cogging, physical simulation tests were conducted using MTS. The experimental results showed that the thermal plasticity of GH4720Li ingots was sensitive with deforming temperatures and strains. A fracture prediction model with different temperatures was promoted based on the physical simulation results. Using this model, numerical simulation for cogging process of GH4720Li was also carried out by Deform software to avoid fracture. Based on the results of this study, the technological parameters of cogging could be optimized. [Copyright &y& Elsevier]

This paper presents the optimal design of a surface mounted permanent-magnet (PM) Brushless direct-current (BLDC) motor meant for spacecraft applications. The spacecraft applications requires the choice of a motor with high torque density, minimum cogging torque, better positional stability and high torque to inertia ratio. Performance of two types of machine configurations designed viz Slotted PMBLDC and Slotless PMBLDC with Halbach array and radial magnetization are compared with the help of analytical and finite-element (FE) methods. It is found that unlike a Slotted PMBLDC motor, the Slotless type with Halbach array develops zero cogging torque without reduction in the developed torque, suiting spacecraft applications. Moreover, the machine being coreless provides high torque to inertia ratio and zero magnetic stiction confirming to the requirements of spacecraft applications. [ABSTRACT FROM AUTHOR]

Abstract: The internal state of an ingot structure manufactured by the casting process has many loose structures. Generally, loose ingot structures make it difficult to obtain sound material properties. Unsound factors such as voids, porosity and shrinkage in the ingot can be improved by the forging process. The closing of the internal voids depends on the forging process. In this study, numerical analysis was carried out to find the upsetting and cogging processes increasing the internal forging effect of an ultra-large ingot (520tons). Various parameters such as the ratio of height to diameter of ingot, shifting method, die staggering, and pressing depth were investigated. In addition, verification tests were performed visually using a 1/25 scale model test with pure lead. Results from the numerical analysis and the scaled-down model experiment were compared. The optimal forging process was applied to manufacture a mock-up product. An efficient forging process was suggested to improve the void closing and to achieve the uniform forging effect in the upsetting and cogging process of an ultra-large ingot. [Copyright &y& Elsevier]

In order to reduce the very long computational time required for the simulation of incremental processes such as cogging or ring rolling, a Bimesh method is proposed. It consists in using different finite element meshes for the resolution of the different physical problems: a main fine mesh to store the results and to carry out the thermal computations, and an intermediate coarser mesh for the mechanical calculations. It makes it possible to take advantage of the localised deformations that characterize incremental processes to coarsen the mechanical mesh and consequently reduce the computational time. After presenting the main components of these Bimesh method, the building of the embedded meshes and the data transfer between the meshes, its extension for parallel calculations are discussed, before analysing the obtained speed-ups for several cogging applications, both in sequential and in parallel. [ABSTRACT FROM AUTHOR]

• The isothermal compression of four typical microstructures for Ti-6Al-4V is studied. • A modified H-S CE considering the alpha/beta structure is proposed and calibrated. • Compared with the H-S and S-C-A CEs, the W and Δ% of the modified CE are smaller and more concentrated. • The modified H-S CE is embedded in the finite element code, and good agreement with the experimental results. The isothermal compression of four typical microstructures in the cogging process for Ti-6Al-4V was conducted at strain rates ranging from 0.01 to 1 s−1 with varying temperatures. The flow behavior of different microstructures was compared. The Hansel-Spittel (H-S) and strain compensated Arrhenius (S-C-A) constitutive equations (CEs) were calibrated. To improve prediction ability, a modified H-S CE considering the evolution of microstructure was proposed and embedded in the finite element code. The original, modified H-S, and S-C-A CEs were evaluated based on the average absolute error (W) and relative error ratio (Δ%). Compared with the original H-S and S-C-A CEs, the W (−2.5%~2.5%) of the modified CE was found to be smaller and the Δ% was more concentrated. Furthermore, hot forging experiment and finite element simulation (FES) at different speeds were performed. The average relative error (AARE) of load-stroke for the H-S and modified H-S CEs is 13.51% and 3.67%, respectively. The comparisons between the FES and hot forging experiment indicate that the modified CE is more accurate for the simulation of the cogging process. [ABSTRACT FROM AUTHOR]

Abstract: Microstructural changes during the cogging of an Alloy 718 ignot processed by (vacuum induction melting)/(vacuum arc re-melting) were predicted using the combined approach of three-dimensional finite element simulation and modeling for the recrystallization and grain growth. In order to establish recrystallization and grain growth models for Alloy 718 ingot, hot compression and isothermal heat treatment tests were carried out at different temperatures and strain rates. The constitutive equations for metadynamic and static-recrystallization behavior were generated using the experimental data collected from various references. From the experimental results, it was found that the dissolution temperature of δ-phase (Ni3Nb) played an important role in the microstructural refinement of Alloy 718 ingot. The predicted grain size and its distribution were compared with the microstructures of the cogged billet, and the reliability of the prediction was discussed. [Copyright &y& Elsevier]

For the calculation of torque in synchronous motors a local method is analysed, based on the Maxwell stress theory and the filtered contributions due to the harmonics of the magnetic vector potential in the motor air-gap. By considering the space fundamental field only, the method can efficiently estimate the average synchronous torque for a variety or motor topologies, including concentrated winding designs. This approach employs an analytical filter for the Maxwell stress tensor and `frozen permeability' technique. The proposed method is validated by comparison with FE results for several synchronous motor types: interior permanent magnet motors, wound field motor, synchronous reluctance motor. [ABSTRACT FROM AUTHOR]

Three-phase permanent magnet brushless dc motors are widely used. As a function of the rotor position, the torque produced by these machines has a pulsating component in addition to the de component. This pulsating torque has a fundamental frequency corresponding to six pulses per electrical revolution of the motor. The shape of the torque waveform and, thus, the frequency content of the waveform can be influenced by several factors in the motor design, and construction. This paper addresses the various factors that influence the torque waveshape. It is shown that in addition to the basic induced electromotive force (EMF) waveshape, the magnetic saturation in the stator core, and the accuracy in the skewing are also key factors in determining the torque waveshape. Computer simulation using finite element technique has been conducted to study the torque waveform. Simulation results successfully duplicated the torque waveforms measured in experiments under different excitation currents. [ABSTRACT FROM AUTHOR]

The phenomenon of strain-induced porosity (SIP) in extra-low interstitial (ELI) grade Ti-6Al-4V with a transformed β starting microstructure is investigated to understand its origin during α-β cogging. For this purpose, the constitutive behavior of the material is coupled with finite-element method (FEM) simulations of the cogging process. Two distinct types of SIP relevant to cogging speeds and temperatures, viz., shear cracking and void nucleation, are identified. While the former occurs at the prior β grain boundaries below 825 °C, the latter occurs at the prior colony boundaries when the deformation temperature is close to the β transus. The FEM simulations have shown that deformation conditions existing in the midregion of the billet are favorable for void nucleation. The mechanism of void growth in the presence of tensile residual stress and temperature during resoaking is modeled using the Cocks-Ashby coupled growth model. Repeated cogging and resoaking steps cause multiplication of void population in large numbers. To avoid both types of defects in any region of the billet, a practical solution has been developed by introducing a differential temperature profile from the surface to the center, and the validity of the proposed scheme is verified with FEM heat-transfer simulations. [ABSTRACT FROM AUTHOR]

In this article, a new manner of cogging a forging (type: shaft), consisting in the application of a two-stage process composed of preliminary shaping in convex anvils, and also principal forging in flat or shaped anvils, is presented. A new manner of forging brought about the formation of favorable conditions for achieving the maximum values of the effective strain in the central part of a forging, accompanied by a simultaneous absence of tensile stresses, which was exerting a favorable influence upon reforging the axial zone of an ingot. What was determined, was the effective geometric shapes of convex anvils; the efficiency of different technological parameters in the case of the intensity of reforging the axial zone of an ingot was analyzed as well. The investigations were complemented by means of predicting the formation of ductile fractures in the course of forging with the application of three different ductile fracture criteria. The comparison of theoretical and experimental outcomes of investigations indicates a good level of being commensurate. [ABSTRACT FROM AUTHOR]