5083铝合金的热变形组织演变及晶粒度模型

通过等温压缩实验、光学显微镜与透射电镜研究了变形温度300~450℃、应变速率0.01~1s-1、真应变0.36~1.2范围内变形条件对5083铝合金热变形组织演变的影响。结果表明:升高热变形温度或降低应变速率均可促进5083铝合金的动态再结晶发生,使变形后5083铝合金位错密度降低,再结晶晶粒尺寸增大;随着应变量的增加,变形后合金的位错密度降低,动态再结晶程度增大。根据唯象理论的指数模型,利用线性回归方法建立了5083铝合金动态再结晶晶粒度模型,模型计算值与实测值吻合良好,平均相对误差仅为4.6%。     

Effect of strain-induced precipitation on the low angle grain boundary in AA7050 aluminum alloy

Effects of the particles induced by strain on dynamic recrystallization and microstructure of the AA7050 aluminum alloy were investigated during hot deformation using X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD). Experimental results showed that partial recrystallized grains containing little sub-structure were produced during the solution treatment. Numerous particles were successfully obtained by the strain-induced precipitation during first-pass deformation at 573 K. The deformation promoted spheroidization and refinement of the precipitate particles. Then these particles pinned dislocations and grain boundaries inhibiting dynamic recrystallization during second-pass high-temperature deformation at 673 K and low angle grain boundary fraction was increased significantly to 83.8%. Furthermore, the tensile test indicated that microstructure with numerous low angle boundaries (LAGBs) and 5 μm sub-grains had increased the strength and ductility of the AA7050 aluminum alloy.     

Recrystallization kinetics of Al–Mg alloys AA 5056 and AA 5083 after hot deformation

Abstract

Slabs of two commercial Al–Mg alloys were hot rolled on a laboratory mill. A wide range of processing parameters was used in the experimental design and the effect of those parameters on the annealing kinetics was established. The effects of each parameter are described by simple empirical relationships. The recrystallization time required for a given volume fraction recrystallized was found to decrease with increasing total hot strain and with decreasing initial grain size. The recrystallization time also decreased with increasing temperature compensated strain rate. The annealing temperature also has a profound effect. Empirical relationships incorporating all the process variables are presented, and it is shown that the alloy AA 5056 recrystallizes more readily than does alloy AA 5083. It is concluded that this observation arises because of the larger volume fraction of precipitates present in the homogenized 5083 alloy.

MST/357     

Fractal Characteristics and Prediction of Ti-15-3 Alloy Recrystallized Microstructure

Grain shape of the hot deforming alloy is an important of material. The fractal theory was applied to analyze index to character the microstructure and performance the recrystallized microstructure of Ti-15-3 alloy after hot deformation and solution treatment. The fractal dimensions of recrystallized grains were calculated by slit island method. The influence of processing parameters on fractal dimension and grain size was studied, It has been shown that the shapes of recrystallized grain boundaries are self-similar, and the fractal dimension varies from 1 to 2. With increasing deformation degree and strain rate or decreasing deformation temperature, the fractal dimension of grain boundaries increased and the grain size decreased. So the fractal dimension could characterize the grain shape and size. A neural network model was trained to predict the fractal dimension of recrystallized microstructure and the result is in excellent agreement with the experimental data.     

奥氏体不锈钢热轧加工性能的数学模型研究

钢的热加工性能是钢的热轧工艺设计的基础.奥氏体钢在热加工中涉及到众多的物理现象,如动态回复、动态再结晶、静态回复、亚动态再结晶、静态再结晶和晶粒长大.一个优秀的描述钢的热加工性能的数学模型可以优化热轧工艺,提高生产效率,改善产品质量.综述了奥氏体不锈钢在热加工中发生的各类物理现象及其相对应的数学模型,讨论了变形温度、变形参数与流变应力、再结晶以及再结晶晶粒度之间复杂的关系,并分析了在工业多道次轧制工艺中,如何应用这些数学模型模拟和预测轧钢过程中残余应变和其内部组织的演变过程.     

Effect of impurity reduction on rollability of AZ31 magnesium alloy

The effect of impurity reduction on the hot rolling behavior of AZ31 magnesium alloy was systematically investigated in this study. In the as-cast alloys, the total content of main impurity elements such as Fe, Si, Cu, and Ni was varied from 0.0462 to 0.0163 wt% by changing the purity of used raw magnesium metals. The alloys after homogenization were subjected to hot rolling at 300 °C with a reduction of 20% per pass. It was found that the initiation of edge cracks is postponed with reducing impurity level in the alloys. And the maximum rolling reduction prior to edge cracking increases from 34 to 58% as the impurity content drops from 0.0462 to 0.0163 wt%. Microstructural observations showed that smaller grains are present in the alloy with lower impurity content in the cast and homogenization states. Moreover, decreasing impurity content leads to a reduced number of deformation twins and an enhanced volume fraction of small recrystallized grains in the as-rolled microstructure, which indicates that impurity reduction is beneficial to the recrystallization process and subsequent plastic deformation. Based on the results, the enhancement in hot rollability of the AZ31 sheet by impurity reduction should be due to finer grain size, the reduced number of deformation twins and the enhanced extent of recrystallization.     

Microstructure development in hot deformed AA6082

Age hardenable aluminium alloys are suitable for hot deformation at temperatures between 300 and 600 °C, depending on the alloy and on the process. During thermo mechanical processes, dynamic and/or static restoration mechanisms take place. In this work, hot compression tests of hot rolled AA6082 and of a hot extruded AA6082 were carried out by means of a Gleeble^® 1500 thermo-mechanical system, between 450 and 550 °C and at 0.1-10 s⁻¹ of strain rate followed by fast cooling. The materials were thermo-mechanically processed before hot deformation to transform the complex initial microstructure into a fully recrystallized microstructure. Some compression tests of non recrystallized samples were carried out for comparison. All the samples were examined using electron backscatter diffraction (EBSD), followed by a determination of grain size and subgrain size distributions as well as of the local misorientation distribution. The as received samples showed a duplex microstructure consisting of elongated grains oriented in the rolling and extrusion directions, and some small recrystallized grains. The main restoration mechanism for all the materials is shown to be dynamic recovery, and the Zener-Hollomon parameter, as well as the flow stress was correlated to the subgrain size. The hot rolled material flow behaviour was sensitive to the initial microstructure, and geometric dynamic recrystallization was found.     

Improvement of superplasticity in AA5083 by equal channel angular extrusion and rolling

Al–Mg–Mn alloy AA5083 has been processed by equal channel angular extrusion (ECAE) followed by hot and cold rolling. The grain structure, crystallographic texture, intermetallic phases and superplasticity were investigated and compared with a conventionally hot and cold rolled AA5083 sheet. The intensive shear strain in ECAE has a very strong breakdown effect on the dendritic cast structure, which effectively changes the volume fraction and size of the intermetallic particles that provide particle stimulated nucleation upon recrystallisation. Consequently, the combination of ECAE and rolling leads to a finer and more thermally stable grain structure, with fewer very coarse constituent particles, resulting in a significant improvement in superplasticity through enhanced grain boundary sliding, with retarded formation and linkage of cavities.     

Recrystallized grain size prediction in a particulate reinforced metal matrix composite

The recrystallized grain size following cold rolling and annealing of an Al alloy (AA2014) and a particulate reinforced AA2014 composite was investigated. The composite contained 20 vol% alumina particulates of average diameter 15 m. The recrystallized grain size in the composite was finer than in the alloy, for a given set of conditions; this was most notable for material strained less than 50% cold-work. This behaviour was attributable to a higher nucleation efficiency in the matrix adjacent to coarse alumina particles in the composite. A model was presented for the composite to predict the recrystallized grain size as a function of strain, with respect to the size distribution and number density of alumina particles. This model predicted the strain dependence of the recrystallized grain size and, in particular, the grain size insensitivity to strain at moderate-to-high levels of cold-work.     

低碳钢奥氏体再结晶模型的建立

为了描述低碳钢变形过程的组织演化,建立了一套完整的奥氏体动态再结晶、静态再结晶、亚动态再结晶模型.本文利用Gleeble试验机研究不同初始晶粒度、变形温度、应变和应变速率对奥氏体再结晶量和晶粒尺寸变化的影响.流变应力模型考虑了变形条件对模型系数的影响.利用测得的应力-应变曲线及晶粒度由多元非线性回归得出了奥氏体再结晶模型系数,并且由模型计算的峰值应变、稳定应变、硬化区流变应力、再结晶体积分数、晶粒尺寸和实际接近.     

Particle effects on recrystallization in magnesium-manganese alloys: Particle pinning

Dispersoid particles are widely used in wrought aluminium alloys to control grain structure during thermomechanical processing. The aim of this work was to investigate whether this approach could be utilized in wrought magnesium alloys to obtain better control of recrystallization. A binary magnesium-manganese alloy was heat treated to produce a fine dispersion of manganese precipitates. The effect of this dispersion on dynamic and static recrystallization during channel die deformation (at a slow strain rate), hot rolling, and annealing was studied and compared with that of an alloy free of fine particles. It was found that the presence of particles did not suppress dynamic recrystallization during channel die deformation. Fine particles did lead to a much reduced recrystallized fraction after hot rolling, attributed to a retardation of static recrystallization kinetics. Although the presence of pinning particles greatly slowed recrystallization kinetics on annealing, for no conditions studied was it possible to prevent recrystallization of the as-deformed structure using particles.     

A strategy for creating ultrafine-grained microstructure in magnesium alloy sheets

An Mg-3Al-1Zn alloy with fully recrystallized microstructure and a mean grain size of 1 μm has been produced by high-ratio differential speed rolling under the condition that the cold sheet is subjected to rolling with hot rolls preheated to 473 K, resulting in a total thickness reduction of 68% after two-step rolling. No surface or internal cracks were developed. The microstructure was homogenous along the thickness direction. A bimodal grain size distribution was obtained in which approximately 40% of the grains were ultrafine with submicron size coexisting with other grains with a size of several microns. The proposed processing method holds great potential for continuous production of ultrafine-grained magnesium alloy sheets.     

高Nb-TiAl合金的高温变形行为及其板材的性能

对高Nb-TiAl合金进行多步热压缩,研究其高温变形行为及其板材的性能。结果表明,热压缩变形后高Nb-TiAl合金的组织中等轴γ晶粒和α晶粒的增多、层片晶团的体积分数和尺寸降低,使其变形能力提高。根据这些结果确定了最优轧制工艺为应变速率低于0.5 s^-1、道次变形量前期应不高于25%、变形温度高于1150℃。选用上述工艺对其其进行5道次大变形量轧制,制备出表面质量良好、无缺陷的高Nb-TiAl合金板材,其尺寸为600 mm×85 mm×3 mm。这种板材具有双态组织,平均晶粒尺寸小于5 μm,其室温屈服强度、抗拉强度和塑性分别为948 MPa、1084 MPa和0.94%,800℃下抗拉强度为758 MPa。     

冷轧前热处理对6016-T4P铝合金组织和力学性能的影响

通过拉伸试验、电子背散射衍射(EBSD)和透射电子显微镜(TEM)等检测手段,研究了在冷轧前分别进行固溶处理和过时效处理工艺对6016-T4P板材组织和力学性能的影响.结果表明:冷轧前进行固溶处理的6016-T4P板材比冷轧前进行过时效处理的6016-T4P板材塑性、强度和平面各向异性指数更高,晶粒度更小,晶界分布更均匀;而冷轧前进行过时效处理会弱化6016-T4P板材的再结晶织构强度并改变再结晶织构类型,进而改变板材平面内不同方向上的屈服强度并减小平面各向异性指数.     

Finite element modelling on microstructure evolution during multi-pass hot compression for AZ31 alloys using incremental method

Based on the principle of piecewise linearization, the incremental forms of microstructure evolution models were integrated into the thermo-mechanical coupled finite element(FE) model to simulate nonlinear microstructure evolution during multi-pass hot deformation. This is an unsteady-state deformation where dynamic recrystallization(DRX), meta-dynamic recrystallization(MDRX), static recrystallization(SRX) and grain growth(GG) take place during hot deformation or deformation interval. The distributions of deformation and microstructure for cylindrical AZ31 sample during single-pass and double-pass hot compressions were quantitatively calculated and compared with the metallographic observation. It is shown that both the deformation and microstructure are non-uniformly distributed due to the presence of friction between the die and the flat end of sample. The average grain size and its standard deviation under the double-pass hot compression are slightly smaller than those under single-pass compression.The simulated average grain sizes agree well with the experiments, which validates that the developed FE model on the basis of incremental forms of microstructure evolution models is reasonable.     

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.     

镁合金 AZ40M 再结晶晶粒尺寸与硬度模型研究

目的研究变形温度及变形速率对镁合金AZ40M再结晶晶粒尺寸以及硬度的影响。方法在gleeble-1500D热模拟机上进行热物理模拟压缩实验,变形温度为250~400℃,变形速率为0.001~1 s-1,通过金相法观测AZ40M镁合金在不同变形条件下的组织形貌,采用维氏硬度计测出镁合金热变形后的硬度值。结果当升高变形温度或降低变形速率时,材料的晶粒尺寸增大且硬度减小。结论得出了再结晶晶粒尺寸的变化规律,建立了AZ40M镁合金的晶粒尺寸与硬度的关系模型。     

Recrystallized cube grains in an Al–Mg–Si alloy dependent on prior cold rolling

This paper reports on the experimental investigation of an industrial Al–Mg–Si alloy, which was subjected to different cold rolling reductions and subsequently solution annealed. Based on large-scale electron backscatter detection (EBSD) measurements, it provides an analysis of the area fraction, size and number density of cube grains in the fully recrystallized microstructure. The area fraction and number density of recrystallized cube grains increase continuously with increasing strain, but the cube grain size equals the average grain size independent of prior strain. The recrystallization advantage of cube grains decreases rapidly with increasing misorientation from the ideal cube component. The technological relevance of this misorientation dependence and its possible micro-mechanical origins are discussed.     

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.     

Inconel 718镍基高温合金热加工动态再结晶建模与计算

目的 建立Inconel 718镍基合金动态再结晶组织演变三维介观尺度计算模型.方法 采用Gleeble-1500型热力模拟试验机进行恒温恒应变速率压缩实验,获得Inconel 718高温合金热变形的真应力-应变曲线,并结合光学显微镜分析热变形后的材料的塑性流动规律与组织演化特征,获得材料参数.结果 基于流变和组织行为...