Effect of initial texture on dynamic recrystallization of AZ31 Mg alloy during hot rolling

Wedge-shaped AZ31 plates with two kinds of initial textures were rolled at 573 K to investigate the effect of initial texture on dynamic recrystallization (DRX). The results indicated that the initiation and nucleation of DRX were closely related to the initial texture. The initiation and completion of DRX in the TD-plate were significantly retarded compared with that in the ND-plate. Twin related DRX nucleation was mainly observed in the ND-plate samples; while gain boundary related DRX nucleation was mainly observed in the TD-plate samples. The different DRX behavior between the TD- and ND-plates was attributed to the different deformation mechanism occurring before DRX initiation. For the ND-plate, dislocation glide was considered as the main deformation mechanism accompanied with {1 0 −1 1}-{1 0 −1 2} double twin, which led to the increment of a faster increasing stored energy within the grains. And {1 0 −1 1}-{1 0 −1 2} double twin was mainly found to be DRX nucleation site for the ND-plate. For the TD-plate, {1 0 −1 2} extension twin was the dominant deformation mechanism which resulted in a basal texture with the c-axis nearly parallel to ND. The stored energy caused by dislocation motion was relatively small in the TD-plate before a basal texture was formed, which was considered as the main reason of that DRX was retarded in the TD-plate compared with that in the ND-plate. Based on the difference in deformation mechanism and DRX mechanism caused by the different initial texture, the variation in grain size, micro-texture and misorientation angle distribution in the ND and TD plates were discussed.     

Microstructure evolution during dynamic recrystallization of hot deformed superalloy 718

Microstructure evolution during dynamic recrystallization (DRX) of superalloy 718 was studied by optical microscope and electron backscatter diffraction (EBSD) technique. Compression tests were performed at different strains at temperatures from 950 °C to 1120 °C with a strain rate of 10⁻¹ s⁻¹. Microstructure observations show that the recrystallized grain size as well as the fraction of new grains increases with the increasing temperature. A power exponent relationship is obtained between the dynamically recrystallized grain size and the peak stress. It is found that different nucleation mechanisms for DRX are operated in hot deformed superalloy 718, which is closely related to deformation temperatures. DRX nucleation and development are discussed in consideration of subgrain rotation or twinning taking place near the original grain boundaries. Particular attention is also paid to the role of continuous dynamic recrystallization (CDRX) at both higher and lower temperatures.     

Transition of dynamic recrystallization mechanism during hot deformation of Incoloy 028 alloy

Dynamic recrystallization(DRX) plays significant roles in manipulating of microstructures during hot deformation and the result mechanical properties;however,the underling mechanism leading to multi scale-microstructures remains poorly understood.Here,the DRX mechanism under wide processing conditions(i.e.950-1200℃,0.001-10 s~(-1)) in Incoloy 028 alloy was investigated,where the relationships among flow stress,Z parameter and grain size,as well as the evolution of characteristic microstructures(grain size,sub-grain boundaries,and high angle grain boundaries),are established.As the values of Z parameters decrease(corresponding to decreased flow stresses),three typical softening mechanisms successively occur,ranging from continuous DRX controlled by dislocation glide,discontinuous DRX dominated by dislocation motion(climb and cross/multiple slip) and grain boundary migration,to dynamic normal/abnormal grain growth resulting from grain boundary migration,with transition regions where two adjacent mechanisms occur simultaneously.Correspondingly,these above three softening mechanisms result in ultrafine,fine and coarse grains,respectively.The present findings demonstrate a comprehensive understanding of DRX mechanism over a wide range of processing conditions,and further provide a new guideline for preparing single crystals.     

A two-site mean field model of discontinuous dynamic recrystallization

The paper describes a new model of discontinuous dynamic recrystallization (DDRX) which can operate in constant or variable thermomechanical conditions. The model considers the elementary physical phenomena at the grain scale such as strain hardening, recovery, grain boundary migration, and nucleation. The microstructure is represented through a set of representative grains defined by their size and dislocation density. It is linked to a constitutive law giving access to the polycrystal flow stress. Interaction between representative grains and the surrounding material is idealized using a two-site approach whereby two homogeneous equivalent media with different dislocation densities are considered. Topological information is incorporated into the model by prescribing the relative weight of these two equivalent media as a function of their volume fractions. This procedure allows accounting for the well-known necklace structures. The model is applied to the prediction of DDRX in 304 L stainless steel, with parameters identified using an inverse methodology based on a genetic algorithm. Results show good agreement with experimental data at different temperatures and strain rates, predicting recrystallization kinetics, recrystallized grain size and stress-strain curve. Parameters identified with one initial grain size lead to accurate results for another initial grain size without introducing any additional parameter.     

Constitutive modeling and prediction of hot deformation flow stress under dynamic recrystallization conditions

Simple modeling approaches based on the Hollomon equation, the Johnson–Cook equation, and the Arrhenius constitutive equation with strain-dependent material’s constants were used for modeling and prediction of flow stress for the single-peak dynamic recrystallization (DRX) flow curves of a stainless steel alloy. It was shown that the representation of a master normalized stress–normalized strain flow curve by simple constitutive analysis is successful in modeling of high temperature flow curves, in which the coupled effect of temperature and strain rate in the form of the Zener–Hollomon parameter is considered through incorporation of the peak stress and the peak strain into the formula. Moreover, the Johnson–Cook equation failed to appropriately predict the hot flow stress, which was ascribed to its inability in representation of both strain hardening and work softening stages and also to its completely uncoupled nature, i.e. dealing separately with the strain, strain rate, and temperature effects. It was also shown that the change in the microstructure of the material at a given strain for different deformation conditions during high-temperature deformation is responsible for the failure of the conventional strain compensation approach that is based on the Arrhenius equation. Subsequently, a simplified approach was proposed, in which by correct implementation of the hyperbolic sine law, significantly better consistency with the experiments were obtained. Moreover, good prediction abilities were achieved by implementation of a proposed physically-based approach for strain compensation, which accounts for the dependence of Young’s modulus and the self-diffusion coefficient on temperature and sets the theoretical values in Garofalo’s type constitutive equation based on the operating deformation mechanism. It was concluded that for flow stress modeling by the strain compensation techniques, the deformation activation energy should not be considered as a function of strain.     

Effect of the size and dispersion of precipitates formed in hot rolling on recrystallization texture in ferritic stainless steels

In the present paper, the size and dispersion of precipitates in ferritic stainless steels have been varied by applying different hot rolling processes, the effect of which on the evolution of recrystallization textures was investigated. The precipitate characterization was observed and studied by transmission electron microscopy and the texture evolution processes were characterized by X-ray diffraction and electron backscattering diffraction. The results show that low temperature finish rolling can promote the formation of a large number of fine and dense TiC precipitates in hot band. These fine and dense precipitates can be inherited in the final sheet, and are beneficial to facilitating the nucleation of randomly oriented grains by promoting the formation of inhomogeneous cold rolled microstructure, strongly suppressing the growth of recrystallized grains by pinning grain boundary migration, thereby weakening the formation of γ-fiber recrystallization texture and deteriorating the formability of final sheet. By contrast, strong γ-fiber recrystallization texture is developed in the sample with sparsely distributed coarse precipitates. Therefore, the size and dispersion of precipitates formed in hot rolling have significant effects on the nucleation of randomly oriented grains and the growth of recrystallized grains during recrystallization annealing, which play important roles in controlling the γ-fiber recrystallization texture in ferritic stainless steels.     

An analytical model for the prediction of hot roll temperatures in a hot rolling process

Temperature variations in the hot roll during a hot rolling process were analysed by solving heat conduction equations for boundary conditions using an analytical method. The analysis was conducted in a steady‐state regime, taking into account the effects of process parameters such as the contact surface, roll velocity and various cooling boundary conditions. Assuming the periodicity of the process, the development of a solution in the Fourier series was employed to solve the governing equations. The temperature and its gradient distributions in the roll depth were analytically expressed according to the process parameters. The accuracy of the predicted results was examined through comparison with predictions presented in the literature (finite element solutions and measurements). Results showed that an increase in the rolling speed leads to a shorter contact time, which decreases the temperature field in the work‐roll.     

Influence of Bi addition on dynamic recrystallization and precipitation behaviors during hot extrusion of pure Mg

Low material cost and high extrudability for ensuring price competitiveness with Al alloys, as well as excellent mechanical properties, are essential for expanding the application range of Mg extrudates. Bi is a promising alloying element for developing extruded Mg alloys that satisfy such requirements. Bi is inexpensive, exhibits a high solubility limit, and forms a thermally stable Mg_3Bi_2 phase, which improves the commercial viability and enables the high-speed extrusion of Mg–Bi alloys. In this study, the effects of Bi addition on the dynamic recrystallization(DRX) and dynamic precipitation behaviors during hot extrusion of a pure Mg and the resultant microstructure and mechanical properties of the extruded materials were investigated. The addition of 6 wt% and 9 wt% Bi to a pure Mg yielded numerous fine Mg_3Bi_2 precipitates during the early stage of hot extrusion. Consequently, the area fraction of dynamic recrystallized(DRXed) grains decreased because of DRX-behavior suppression by the Zener pinning effect.However, the DRXed grain size was substantially reduced through the grain-boundary pinning effect.The size and number of undissolved Mg_3Bi_2 particles in the homogenized billets increased when the Bi content was increased, which resulted in increased DRX fractions owing to the enhanced levels of particle stimulated nucleation. Bi addition yielded considerable strength improvement of the extruded pure Mg. However, the extruded Mg–Bi binary materials were less ductile than the extruded pure Mg material. This lower ductility resulted from the cracking at twins formed in the coarse un DRXed grains of the Mg-6Bi material and the cracking at large undissolved Mg_3Bi_2 particles in the Mg-9Bi material.     

The identification of dynamic recrystallization and constitutive modeling during hot deformation of Ti55511 titanium alloy

In the present paper, an internal-variable identification approach has been proposed to investigate the dynamic recrystallization (DRX) behavior during hot deformation and corresponding constitutive model has been constructed. Isothermal compression experiments of Ti55511 titanium alloy were conducted for verification. Plastic behavior is determined by dislocation evolution in many cases while deforming. The comparison between saturated and DRX critical dislocation density was made to distinguish the occurrence of dynamic recrystallization/recovery (DRV) during hot deformation. The influence of deformation parameters on DRX behavior was illustrated by dislocation evolution map, validated by the power dissipation efficiency distribution. DRX process during hot deformation of Ti55511 alloy tends to occur under moderate temperatures and low strain rates. In addition, a physical-based Arrhenius constitutive formula has been derived for DRX criticality. The strain-rate sensitivity coefficients during hot deformation were fixed as a constant equal to 1/6 and the deformation activation energy was related to the material's self-diffusion activation.     

Multi-phase-field simulations for dynamic recrystallization

A dynamic recrystallization multi-phase-field (MPF-DRX) model that can approximately take into account grain deformation during dynamic recrystallization (DRX) has been developed, where the grain deformation was introduced by changing the size of a finite difference grid so as to keep the volume constant. The accuracy of the developed model was confirmed by simulating a single DRX grain growth. As a result of MPF-DRX simulations with grain deformation, it was clarified that, from microstructure evolutions, the appropriate deformation during DRX can be reproduced by the developed model. However, it was also concluded that the macroscopic stress–strain relationship and variations in grain size are not affected by the grain deformation introduced here. Furthermore, the DRX cycle was defined and the relationship between the DRX cycle fraction and the stress–strain curve was discussed. As a result, it was concluded that the stress–strain curve with multiple peaks is observed when the strain at the first valley of the stress–strain curve is smaller than that corresponding to the maximum first DRX cycle fraction.     

Multiscale modeling of dynamic crack propagation

A multiscale approach is employed to investigate a center-cracked specimen with the purpose to redefine fracture toughness from the atomistic perspective and to simulate different modes of crack propagation. The specimen is divided into three regions: (1) far field, modeled by classical fracture mechanics, (2) near field, modeled by a multiscale field theory and analyzed by a generalized finite element method, and (3) crack tip atomic region, modeled by molecular dynamics (MD). The exact and analytical solution of the far field is utilized to specify boundary conditions at the interface between the far field and the near field. The interaction between the near field and the crack tip region is described by full-blown interatomic forces. In this work, crystals of perovskite (Barium Titanate) and rocksalt (Magnesia) have been studied. Fracture toughness is defined as a material property associated with instability of the MD simulation. Mode I, Mode II, and mixed mode fracture have been investigated and numerical results will be presented and discussed.     

制备立方织构Ni片的研究

用轧制－再结晶方法制备立方织构镍片,研究了不同的轧制工艺对立方织构形成的影响,用极图和三维取向分布分布函数（ＯＤＦ）对织构进行了测量,分析表明对纯镍,在大变形量下,轧制过程中取向空间的β线是最终稳定线,最终轧制织构中主要织构分量为Ｃ｛１１２｝〈１１２〉、Ｓ｛１２３｝〈６３４〉、Ｂ｛１１０｝〈１１２〉,并分析了这三种织构对丙结晶立方织构的影响,再结晶织构主要为立方织构｛１００｝〈００１〉和少量Ｒ织构     

Mathematical model of deformation and microstructural evolution during hot rolling of aluminium alloy 5083

Abstract

A mathematical model to predict the through thickness temperature, strain and strain rate distributions during hot rolling and the subsequent microstructure evolution was developed using the commercial finite element package ABAQUS. Microstructure evolution predictions included the amount of recrystallisation through the thickness of the sheet based on its thermomechanical history during rolling and thermal history after rolling. The equations used to predict the microstructure evolution were based on semiempirical relationships found in the literature for a 5083 aluminium alloy. Validation of the model predictions was done using comprehensive experimental measurements which were conducted using the Corus research multimill, a pilot scale experimental rolling facility, in Ijmuiden, The Netherlands. The results indicate that the through thickness temperature and strain distribution predictions for the rolling operation are reasonable. Hence, the boundary conditions used in the finite element model adequately represent the interface heat transfer and friction conditions. Microstructure predictions using the literature based equations significantly underestimate the amount of recrystallisation occurring in the sheet. A sensitivity analysis indicates that the recrystallisation kinetics are extremely sensitive to the fitting parameters used in the microstructure equation, and that the gradient in the recrystallisation kinetics is the result of the temperature gradient experienced by the specimen during deformation.     

一种基于时序窗口的动态热点话题提取模型

针对新闻领域的专题组织进行了研究,提出了一种基于时序窗口的动态热点话题提取模型。该模型整合了热点话题的两个特点。一方面关注主题词在新闻文本中的广泛性,衡量标准为多频道播报特征项的频率综合,词频越高其广泛性越高;另一方面考虑新闻流主题词的突发性,表现为特定时间段内主题词出现频率显著异常于其它时间段。引入时序窗口进行上升和下降突发模式提取,并结合TF-DF作为主题词赋权值依据。实验结果表明,这种基于时序窗口的动态热点话题提取模型对新闻文本进行主题抽取具有很好的性能。     

铝热连轧尾部跑偏控制模型及其仿真

跑偏是铝板带在强压变形下宽度方向逐渐失去横向对称性,中心线偏离并急剧扩大的横向失稳演变过程．跑偏会引起铝带拉断、卷取时不对中、工作辊损伤等,跑偏严重时还会引发安全事故,造成停产．在板带轧制过程中,轧件尾部由于缺少张力控制,容易产生跑偏．针对轧件跑偏,采用回归分析方法,建立轧制力差和跑偏量之间的线性回归模型,得到稳定轧制的轧制力差区间．对于偏离稳定轧制力差区间的轧制过程,给出调整轧机两侧辊缝差进行纠偏的调整模型．通过对工业现场工艺条件和运行数据分析,轧件尾部跑偏以F3,F4机架跑偏较为严重,利用MATLAB软件对F3,F4机架尾部进行仿真,验证跑偏模型的准确性．     

Mathematical modelling of hot rolling steel strip

Abstract

Thermal and microstructural evolution during hot rolling of low carbon steel in a continuous six stand mill is simulated with a two-dimensional explicit finite difference model in which the cross-sectional area of the strip is divided into small elements of equal volume. The heat transfer coefficients at the surface of the strip are allowed to change as it is assumed that the strip is in air or is being descaled or deformed. Results of the microstructural modelling indicate that austenite is able to undergo dynamic recrystallisation when the conditions within the roll gap are propitious. This model also allows for the occurrence of metadynamic and static recrystallisation once the material leaves the gap and for grain growth after their completion. From this, it is concluded that the most important controlling mechanism is grain growth. The thermal portion of the model was validated with measurements made on a six stand continuous mill. It was not possible to obtain a direct validation of the microstructural algorithms, but they are considered to be correct, since it was possible to achieve a good correlation between the separation forces predicted by the model and those recorded experimentally during actual production once the kinetics of the different mechanisms were incorporated into the model.