铝合金LF6变形工艺与微观组织关系的研究

研究了防锈铝合金LF6热变形工艺对组织的影响.实验表明：随变形程度的增大,组织呈带状分布现象变得明显：在热加工过程中,试样组织发生了动态回复或动态再结晶;随温度的升高,出现晶粒长大的现象.     

铝合金LF6变形工艺与微观组织关系的研究

研究了防锈铝合金LF6热变形工艺对组织的影响。实验表明:随变形程度的增大,组织呈带状分布现象变得明显;在热加工过程中,试样组织发生了动态回复或动态再结晶;随温度的升高,出现晶粒长大的现象。     

Computational Investigation of Hardness Evolution During Friction-Stir Welding of AA5083 and AA2139 Aluminum Alloys

A fully coupled thermo-mechanical finite-element analysis of the friction-stir welding (FSW) process developed in our previous work is combined with the basic physical metallurgy of two wrought aluminum alloys to predict/assess their FSW behaviors. The two alloys selected are AA5083 (a solid-solution strengthened and strain-hardened/stabilized Al-Mg-Mn alloy) and AA2139 (a precipitation hardened quaternary Al-Cu-Mg-Ag alloy). Both of these alloys are currently being used in military-vehicle hull structural and armor systems. In the case of non-age-hardenable AA5083, the dominant microstructure-evolution processes taking place during FSW are extensive plastic deformation and dynamic re-crystallization of highly deformed material subjected to elevated temperatures approaching the melting temperature. In the case of AA2139, in addition to plastic deformation and dynamic recrystallization, precipitates coarsening, over-aging, dissolution, and re-precipitation had to be also considered. Limited data available in the open literature pertaining to the kinetics of the aforementioned microstructure-evolution processes are used to predict variation in the material hardness throughout the various FSW zones of the two alloys. The computed results are found to be in reasonably good agreement with their experimental counterparts.     

Recrystallization behavior of tungsten processed by equal channel angular extrusion at low homologues temperature: Microstructure,hardness, and texture

This work examines the recrystallization behavior of bulk pure tungsten subjected to severe plastic deformation by Equal Channel Angular Extrusion at low temperature. Grain size, morphology, and orientation are examined as a function of subsequent heat treatment temperature. Four temperature ranges are identified wherein the material undergoes recovery, boundary migration, recrystallization, and grain growth. A comparison between this material and warm-worked material is also made, illustrating the effects of stored energy on recovery and recrystallization. Experimentally determined Hall-Petch values agree with previous work. Finally, the plastic deformation behavior of worked and recovered materials are compared with bend test results showing that heat treatment can be used to lower strength while maintaining ductility in heavily deformed tungsten.     

Increase in grain size during recrystallization of heat resistant nickel alloys

Conclusions We have shown that the critical increase in grain size during recrystallization occurs only when recrystallization proceeds during plastic deformation or during the cooling of the alloy after plastic deformation or under certain conditions (heating rate) during repeated heating. If recrystallization does not occur during plastic deformation (cold deformation) and the rate of repeated heating is such that polygonization processes have time to occur then there is no critical increase in the grain size.Central Scientific Institute of Ferrous Metallurgy Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 22–24, January, 1966     

A Model to Predict Recrystallization Kinetics in Hot Strip Rolling Using Combined Artificial Neural Network and Finite Elements

A thermo-mechanical model has been developed to establish a coupled heat conduction and plastic flow analysis in hot-rolling process. This model is capable of predicting temperature, strain, and strain rate distributions during hot rolling as well as the subsequent static recrystallization fraction and grain size changes after hot deformation. Finite element and neural network models are coupled to assess recrystallization kinetics after hot rolling. A new algorithm has been suggested to create differential data sets to train the neural network. The model is then used to predict histories of various deformation variables and recrystallization kinetics in hot rolling of AA5083. Comparison between the theoretical and the experimental data shows the validity of the model.     

Computational Analysis of Material Flow During Friction Stir Welding of AA5059 Aluminum Alloys

Workpiece material flow and stirring/mixing during the friction stir welding (FSW) process are investigated computationally. Within the numerical model of the FSW process, the FSW tool is treated as a Lagrangian component while the workpiece material is treated as an Eulerian component. The employed coupled Eulerian/Lagrangian computational analysis of the welding process was of a two-way thermo-mechanical character (i.e., frictional-sliding/plastic-work dissipation is taken to act as a heat source in the thermal-energy balance equation) while temperature is allowed to affect mechanical aspects of the model through temperature-dependent material properties. The workpiece material (AA5059, solid-solution strengthened and strain-hardened aluminum alloy) is represented using a modified version of the classical Johnson-Cook model (within which the strain-hardening term is augmented to take into account for the effect of dynamic recrystallization) while the FSW tool material (AISI H13 tool steel) is modeled as an isotropic linear-elastic material. Within the analysis, the effects of some of the FSW key process parameters are investigated (e.g., weld pitch, tool tilt-angle, and the tool pin-size). The results pertaining to the material flow during FSW are compared with their experimental counterparts. It is found that, for the most part, experimentally observed material-flow characteristics are reproduced within the current FSW-process model.     

A Fully Coupled Thermomechanical Model of Friction Stir Welding(FSW) and Numerical Studies on Process Parameters of Lightweight Aluminum Alloy Joints

This paper presents a new thermomechanical model of friction stir welding which is capable of simulating the three major steps of friction stir welding(FSW) process, i.e., plunge, dwell, and travel stages. A rate-dependent Johnson–Cook constitutive model is chosen to capture elasto-plastic work deformations during FSW. Two different weld schedules(i.e., plunge rate, rotational speed, and weld speed) are validated by comparing simulated temperature profiles with experimental results. Based on this model, the influences of various welding parameters on temperatures and energy generation during the welding process are investigated. Numerical results show that maximum temperature in FSW process increases with the decrease in plunge rate, and the frictional energy increases almost linearly with respect to time for different rotational speeds. Furthermore, low rotational speeds cause inadequate temperature distribution due to low frictional and plastic dissipation energy which eventually results in weld defects. When both the weld speed and rotational speed are increased, the contribution of plastic dissipation energy increases significantly and improved weld quality can be expected.     

电塑性摩擦焊接过程的动态再结晶数值模拟

摩擦焊过程中界面的塑性变形是摩擦焊的核心，中以宇航工程常用的构件材料LY12合金为研究对象，建立了电场条件下棒状试件摩擦焊的热力耦合塑性成形有限元分析模型，获得了焊接过程中焊接界面处材料的温度场、应变场、应力场、电场强度等物理参量场，并应用Yada模型建立了LY12合金摩擦焊接过程显微组织的演化模型，计算了摩擦焊接过程动态再结晶区的分布及再结晶区晶粒的尺寸，并分析研究了上述场变量对电场条件下连续驱动摩擦焊成形工艺及成形件质量的影响。     

BT25钛合金β相区动态再结晶行为及数值模拟

利用Gleeble-3500型热模拟试验机对BT25钛合金进行单道次等温压缩实验,分别采用Najafizadeh-Jonas加工硬化率模型和Cingara-McQueen流变应力模型研究了合金在变形温度1040~1100℃,应变速率0.001~1 s^-1和最大压下率为60%的条件下动态再结晶的临界条件,分析真应力-真应变曲线,计算加工硬化率并建立了临界应变模型;同时通过线性回归法计算材料参数,构建JMAK动态再结晶动力学方程,并采用该模型模拟了BT25钛合金在热变形过程中动态再结晶行为。结果显示:流动应力表现出对应变速率和变形温度非常敏感;高温和低应变速率有利于DRX发生;有限元模型对DRX体积分数的预测误差在10%以内。该模型具有良好的预测能力,为工业生产中塑性变形和微观结构的预测提供了有效的工具。     

固溶预处理对AZ80镁合金流变行为和组织的影响

对AZ80镁合金在等温热压缩前采用固溶预处理工艺,通过等温热压缩试验、金相显微镜、扫描电镜、透射电镜、X射线衍射分析、电子背散射衍射等多种试验与分析手段,分析了固溶预处理对AZ80镁合金在不同等温压缩条件下的流变行为和再结晶行为的影响。结果表明:经过410 ℃×2 h固溶预处理后的材料,组织中粗大的Mg₁₇Al₁₂相溶解,在各变形条件下流变应力整体上均低于同条件下的初始锻态材料,峰值流变应力降低10~30 MPa,表明固溶预处理使材料在热压缩过程中得到了一定软化,提高了加工时的塑性变形能力。410 ℃×2 h固溶预处理后,在300 ℃及以下的变形中温区,材料的再结晶机制由不连续动态再结晶转变为连续动态再结晶,使再结晶体积分数明显提高,晶粒细化更充分,塑性变形能力提高。     

SA508-3钢动态再结晶过程的相场模拟

采用多相场(Multi-phase-field,MPF)模型模拟动态再结晶晶粒的生长过程,并用Kocks-Mecking(KM)方程模拟其力学行为。用热力模拟机对SA508-3钢进行了不同温度和应变速率下的热压缩试验,从热压缩流动应力-应变曲线中提取SA508-3钢动态再结晶特征参数并用于计算动态再结晶模型参数。利用所得参数对SA508-3钢的动态再结晶过程进行了多相场模拟,预测了热塑性变形过程中的组织演变和真应力-真应变曲线,与试验结果吻合较好。试验和数值结果均表明,流动应力随应变速率的增大及变形温度的降低而增大。本文的方法可用于研究其它材料的动态再结晶行为,为优化热锻工艺提供指导。     

Optimization of process parameters for the manufacturing of rocket casings: A study using processing maps

Maraging steels possess ultrahigh strength combined with ductility and toughness and could be easily fabricated and heat-treated. Bulk metalworking of maraging steels is an important step in the component manufacture. To optimize the hot-working parameters (temperature and strain rate) for the ring rolling process of maraging steel used for the manufacture of rocket casings, a systematic study was conducted to characterize the hot working behavior by developing processing maps for γ-iron and an indigenous 250 grade maraging steel. The hot deformation behavior of binary alloys of iron with Ni, Co, and Mo, which are major constituents of maraging steel, is also studied. Results from the investigation suggest that all the materials tested exhibit a domain of dynamic recrystallization (DRX). From the instability maps, it was revealed that strain rates above 10 s⁻¹ are not suitable for hot working of these materials. An important result from the stress-strain behavior is that while Co strengthens γ-iron, Ni and Mo cause flow softening. Temperatures around 1125 °C and strain rate range between 0.001 and 0.1 s⁻¹ are suitable for the hot working of maraging steel in the DRX domain. Also, higher strain rates may be used in the meta-dynamic recrystallization domain above 1075 °C for high strain rate applications such as ring rolling. The microstructural mechanisms identified from the processing maps along with grain size analyses and hot ductility measurements could be used to design hot-working schedules for maraging steel.     

AZ31B镁合金塑性变形动态再结晶的实验研究

通过不同应变速率和不同温度下的轴对称压缩试验,研究了AZ31B镁合金塑性变形与动态再结晶的相互依赖关系。研究证实,温度T在200℃～400℃区间、变形程度ε约0.2左右时,开始出现动态再结晶(DRX)现象。随变形程度的增加,DRX晶粒不断增多,材料呈现明显的软化趋势,流动应力下降。当DRX过程完成以后,继续变形,材料又出现硬化行为。为镁合金塑性变形组织演变的定量研究打下了基础。     

温度和应变率对低银无铅焊料力学行为的影响

在不同工况下,采用电子万能材料试验机和分离式霍普金森压杆装置(SHPB)对低银无铅焊料Sn0.3Ag0.7Cu分别进行准静态和动态实验,分析了应变率和温度对Sn0.3Ag0.7Cu动态力学性能的影响。结果表明:低银焊料Sn0.3Ag0.7Cu的应力-应变曲线具有温度软化效应与应变率硬化效应。在不同的温度范围内,应变率硬化效应与温度软化效应对低银焊料Sn0.3Ag0.7Cu的塑性变形的影响是不同的。基于Johnson-Cook模型对实验数据进行拟合、修正得到低温和中高温下Sn0.3Ag0.7Cu的动态本构关系,并且与实验数据进行比较,两者在材料的塑性平台区表现出高度的一致性。     

Recrystallization and the possibility of the realization of the α-γ diffusionless transformation during the ultrarapid laser heating of steels

Analysis is given of phase and structural transformations occurring upon ultrarapid laser heating in steels with different initial structures, namely, after annealing, after preliminary quenching, quenching and tempering, and after quenching with subsequent deformation and tempering. It is shown that a significant suppression of diffusion processes occurs during laser heating; this circumstance substantially affects the nature of the phase and structural transformations proceeding during laser processing. Special attention is given to studying the process of recrystallization and to the phenomenon of structural heredity during laser heating. The process of recrystallization during laser heating is considered as consisting of two stages, namely, an ordered lattice rearrangement (α-γ transformation) and the recrystallization of austenite that suffered phase-transformation-induced hardening (“phase naklep”). The effect of tempering and plastic deformation on the recrystallization of a preliminarily quenched steel consists in the intensification of the second stage, i.e., of the recrystallization of the transformation-hardened austenite. It is shown that the α-γ transformation during the laser heating of steels with the initial structure of lath martensite occurs by the “mechanism of recovery,” i.e., via the formation and growth of austenite nuclei. In steels with the initial structure of pearlite, the nucleation of austenite during laser heating can occur by a shear martensite-like diffusionless mechanism with the observance of characteristic orientation relationships between the initial ferrite and the newly formed austenite.     

两种冶炼工艺下A286高温合金的热变形行为

为了探究真空感应+真空自耗(VIM+VAR)和电炉+精炼+真空自耗(EAF+LF+VAR)两种工艺冶炼A286高温合金的热变形行为,利用Gleeble-3800热模拟试验机在温度950~1150 ℃和应变速率0.01~10 s^-1范围内进行热压缩试验。基于摩擦和绝热加热修正后的真应力-真应变曲线和应变硬化率曲线建立了A286合金的Arrhenius本构方程,确定了VIM+VAR合金和EAF+LF+VAR合金的热激活能分别为358.15和372.54 kJ·mol^-1。利用临界应变和动态再结晶体积分数50%应变引入动态再结晶速度参数k_v,建立新的动态再结晶模型。采用Prasad 准则绘制两种钢在应变0.2、0.5和0.9下的热加工图,并结合组织分析,确定VIM+VAR合金的最佳热加工工艺条件为1050~1100 ℃,0.01~1 s^-1和1100~1150 ℃,0.1~10 s^-1;EAF+LF+VAR合金的最佳热加工工艺条件为1050~1100 ℃,0.01~1 s^-1和1100~1150 ℃,0.1~3 s^-1,得出VIM+VAR合金的热加工区间较宽,其热加工性能优于EAF+LF+VAR合金。     

Recrystallization and Grain Growth in Iodide Zirconium

A systematic investigation has been made of the plastic deformation, recrystallization, and grain growth characteristics of swaged crystal bar zirconium. It was observed that zirconium can be cold worked and annealed for fabrication processes without difficulty, but that severe reductions produce a well defined fiber texture which is not removed by recrystallization. Deformation and growth processes affecting the microstructure are discussed in detail. The activation energy for recrystallization was found to vary with the degree of deformation, while a unique activation energy for grain growth was not obtained. Abnormal grain coarsening was observed after annealing severely worked material at 750°C.     

Changes in the grain structure of metallic materials upon plastic treatment

Experimental data on structural changes in the process of plastic deformation of polycrystalline copper and stainless steel have been reviewed. It has been shown that the mechanisms of the formation of a new grain structure, i.e., dynamic recrystallization, depend on the conditions of treatment or flow stresses at the steady-state stage, which are determined unambiguously by the temperature and strain rate. Upon hot deformation (low flow stresses), it is local migration (bulging) of grain boundaries and subsequent growth of these bulgings that seem to be the main mechanism of formation of new grains. Since the nucleation of new grains in this case is of a heterogeneous character, the microstructure evolution can be classified as a discontinuous dynamic recrystallization. Upon worm or cold deformation (high flow stresses), we deal with a dynamically recrystallized structure only after very high degrees of deformation. The new grain structure is formed due to the growth of angular misorientation between subgrains with an increase in the total degree of deformation, which can be considered as a continuous dynamic recrystallization. Changes in the mechanisms of dynamic recrystallization with changes in the conditions of treatment lead to a bimodal dependence of the size of dynamically recrystallized grains on the flow stresses.