Comparison of microstructures and mechanical properties between forging and rolling processes for commercially pure titanium

In order to reveal the differences caused by forging and rolling process for titanium ingots, hot compression behavior, mechanical properties and the microstructures of forged billets and rolled ones were investigated in detail using Gleeble-1500 thermal mechanical simulator, universal testing machine and optical microscope （OM）. The compression deformation experimental data of commercially pure titanium （CP-Ti） were mapped to be a T vs lg diagram in which data fall into three distinct regions, i.e., three-stage work hardening, two-stage work hardening and flow softening, which can be separated by border lines at 17.5 and 15.4 for lg Z, where Z represents the Zener-Hollomon parameter. The deformation twin is found to have higher Z-value corresponding to the work hardening region. The differences in microstructures and mechanical properties for two kinds of billets indicate that forged billet consists of deformation twins and some twin intersections, and many twins cross the grain boundaries. However, nearly no twins can be seen in the microstructure of billet formed by rolling under optical microscope （OM）, but there are equiaxed and platelike grains. Tensile tests and Vickers hardness test indicate that yield strength, tensile strength and microhardness of the samples after forging are higher than those after rolling.

Modeling of Microstructure Evolution and Mechanical Properties of Steel Plates Produced by Thermo-Mechanical Control Process and Its On-line Application

An integrated metallurgical model was developed to predict microstructure evolution and mechanical properties of low-carbon steel plates produced by TMCP. The metallurgical phenomena occurring during TMCP and mechanical properties were predicted for different process parameters. In the later passes full recrystallization becomes difficult to occur and higher residual strain remains in austenite after rolling. For the reasonable temperature and cooling schedule, yield strength of 30 mm plain carbon steel plate can reach 310 MPa. The first on-line application of prediction and control of microstructure and properties (PCMP) in the medium plate production was achieved. The predictions of the system are in good agreement with measurements.

Modeling of microstructure evolution of AZ80 magnesium alloy during hot working process using a unified internal state variable method

In this paper, a unified internal state variable(ISV) model for predicting microstructure evolution during hot working process of AZ80 magnesium alloy was developed. A novel aspect of the proposed model is that the interactive effects of material hardening, recovery and dynamic recrystallization(DRX) on the characteristic deformation behavior were considered by incorporating the evolution laws of viscoplastic flow, dislocation activities, DRX nucleation and boundary migration in a coupled manner. The model parameters were calibrated based on the experimental data analysis and genetic algorithm(GA) based objective optimization. The predicted flow stress, DRX fraction and average grain size match well with experimental results. The proposed model was embedded in the finite element(FE) software DEFORM-3 D via user defined subroutine to simulate the hot compression and equal channel angular extrusion(ECAE) processes. The heterogeneous microstructure distributions at different deformation zones and the dislocation density evolution with competitive deformation mechanisms were captured.This study can provide a theoretical solution for the hot working problems of magnesium alloy.

MICROSTRUCTURAL MODEL OF GATORIZED WASPALOY IN THE ISOTHERMAL FORGING PROCESS

A microstructural model, which was used to predict the evolution of microstructure of Gatorized Waspaloy in the isothermal forging process, was developed in terms of dynamic recrystallization and grain growth. The following three steps of experiment were conducted during developing the model. (1) Specimens were compressed in the MTS testing machine. (2) Dynamic recrystallization and grain growth were discussed. (3) Dynamic recrystallization model and grain growth model were set up. The simulated results are in good agreement with the experimental data.

Approximate-model Based Estimation Method for Dynamic Response of Forging Processes

Many high-quality forging productions require the large-sized hydraulic press machine（HPM） to have a desirable dynamic response. Since the forging process is complex under the low velocity, its response is difficult to estimate. And this often causes the desirable low-velocity forging condition difficult to obtain. So far little work has been found to estimate the dynamic response of the forging process under low velocity. In this paper, an approximate-model based estimation method is proposed to estimate the dynamic response of the forging process under low velocity. First, an approximate model is developed to represent the forging process of this complex HPM around the low-velocity working point. Under guaranteeing the modeling performance, the model may greatly ease the complexity of the subsequent estimation of the dynamic response because it has a good linear structure. On this basis, the dynamic response is estimated and the conditions for stability, vibration, and creep are derived according to the solution of the velocity. All these analytical results are further verified by both simulations and experiment. In the simulation verification for modeling, the original movement model and the derived approximate model always have the same dynamic responses with very small approximate error. The simulations and experiment finally demonstrate and test the effectiveness of the derived conditions for stability, vibration, and creep, and these conditions will benefit both the prediction of the dynamic response of the forging process and the design of the controller for the high-quality forging. The proposed method is an effective solution to achieve the desirable low-velocity forging condition.

Prediction of tri-modal microstructure under complex thermomechanical processing history in isothermal local loading forming of titanium alloy

To control the tri-modal microstructure and performance,a prediction model of tri-modal microstructure in the isothermal local loading forming of titanium alloy was developed.The staged isothermal local loading experiment on TA15alloy indicates that there exist four important microstructure evolution phenomena in the development of tri-modal microstructure,i.e.,the generation of lamellarα,content variation of equiaxedα,spatial orientation change of lamellarαand globularization of lamellarα.Considering the laws of these microstructure phenomena,the microstructure model was established to correlate the parameters of tri-modal microstructure and processing conditions.Then,the developed microstructure model was integrated with finite element(FE)model to predict the tri-modal microstructure in the isothermal local loading forming.Its reliability and accuracy were verified by the microstructure observation at different locations of sample.Good agreements between the predicted and experimental results suggest that the developed microstructure model and its combination with FE model are effective in the prediction of tri-modal microstructure in the isothermal local loading forming of TA15alloy.

Effects of non-isothermal annealing on microstructure and mechanical properties of severely deformed aluminum samples:Modeling and experiment

In order to investigate the evolution of microstructure and flow stress during non-isothermal annealing,aluminum samples were subjected to strain magnitudes of 1, 2 and 3 by performing 2, 4 and 6 passes of multi-directional forging. Then, the samples were non-isothermally annealed up to 150, 200, 250, 300 and 350 ℃. The evolution of dislocation density and flow stress was studied via modeling of deformation and annealing stages. It was found that 2, 4 and 6 passes multi-directionally forged samples show thermal stability up to temperatures of 250, 250 and 300 ℃, respectively. Modeling results and experimental data were compared and a reasonable agreement was observed. It was noticed that 2 and 4 passes multi-directionally forged samples annealed non-isothermally up to 350 ℃ have a lower experimental flow stress in comparison with the flow stress achieved from the model.The underlying reason is that the proposed non-isothermal annealing model is based only on the intragranular dislocation density evolution, which only takes into account recovery and recrystallization phenomena. However, at 350℃ grain growth takes place in addition to recovery and recrystallization,which is the source of discrepancy between the modeling and experimental flow stress.

Effects of Quenching Process on Microstructure and Mechanical Properties of Low Carbon Nb-Ti Microalloyed Steel

The low carbon Nb-Ti mieroalloyed tested steel was prepared by the process of vacuum induction furnace smelting, forging and hot rolling. The new steel aims to meet the demand of high strength, high toughness and high plasticity for building facilities. The effects of quenching process on microstructure and mechanical properties of tested steel were investigated. The results showed that prior austenite grain size, phase type and precipitation behavior of （ Nb, Ti） （ C, N） play important roles in mechanical properties of the steel. Through modified appropriately, the model of austenite grain growth during heating and holding is d^5.7778 = 5. 6478^5.7778 ＋ 7.04 × 10^22t^1.6136 exp（- 427. 15 ×10^3 /（RT））. The grain growth activation energy is Qg = 427. 15 kJ. During quenching, the microscopic structures are mainly martensite and lath bainite which contains lots of lath substructure and dislocations. The content of phases, fine and coarsening （ Nb, Ti ） （ C, N ） precipitated changes during different quenching temperatures and holding time. Finally compared with the hardness value, the best quenching process can be obtained that heating temperature and holding time are 900 ℃ and 50 mins, respectively.

Cellular automaton modeling of semisolid microstructure formation

Computer modeling of semi-solid structure formation is of significance in both understanding the mechanisms of globular structure formation and determining the effect of solidification conditions on final microstructure.A modified cellular automaton(mCA) model has been developed,which is coupled with macroscopic models for heat transfer calculation and microscopic models for nucleation and grain growth.The mCA model is applied to A356 Al alloy-one of the most widely used semi-solid alloys,to predict grain morphology and grain size during semi-solid solidification,and determines the effects of pouring temperature on the final microstructure.The modeling results show that the lower the initial temperature,the finer grain size will be obtained.In addition,the model can be used to predict the solutal micro-segregation.

MICROSTRUCTURAL EVOLUTION AND CONTROL IN SUPERALLOY FORGINGS

Optimum properties of high temperature superalloys are derived from interactions of the composition,thermomechanical deformation, and heat treatment.Nickel-base alloys contain a variety of alloying elements in a large number of combinations to produce various desired effects. The general characteristics of any specific nickelbase superalloy is determined by composition.The control of the microstructure in wrought processed superalloys can further influence the resultant component properties.The relationship between microstructure and properties is key in advanced design and manufacture of turbine engine components, and requires extensive alloy and process characterization to achieve desired results.Various forging methods have been developed and employed throughout the years to process superalloy components.Hammer forging, Press forging,and advanced isothermal press forging equipment and processes have been designed and manufactured to process speciality metals which often have very narrow processing windows.Combination processes have also been introduced into turbine engine applications,which result in a hybrid of resultant structures and properties.One such speciality,combination process is the Ladish Iso Con process which utilizes isothermal forging and conventional forging to achieve unique properties not matched by any other single process.Ladish utilizes forging presses for conventional forging, hot-die forging and for isothermal forging, in addition to hammers and ring rolling mills for wrought processing.This wide range of equipment allows flexibility in process design and optimization.Variations in temperature, strain and strain rate have been shown to significantly influence resultant grain sizes and microstructures.Tools are being developed to allow forging processes and specific forging parameters to be evaluated and optimized with regard to final microstructure before shop triale are required. Sophisticated finite element method (FEM) models are allowing the design of forging processes on computers rather than by iterative trial and error methods.

Microstructure Evolution of Different Forging Processes for12%Cr Steel During Hot Deformation

Five forging experiments were designed and conducted to investigate the effect of process parameters on microstructure evolution during hot deformation for X12CrMoWVNbN10-1-1 steel.The experimental results indicated that average grain size became finer with the increasing number of upsetting and stretching.Especially,the size of stretching three times with upsetting twice had the most remarkable effect on refinement,and the size was only 27.36%of the original one.Moreover,the stress model was integrated into the software and finite element models were established.Simulation results demonstrated that the strain at center point of workpiece was far larger than critical strain value in each process,so that dynamic recrystallization(DRX) occurred in each workpiece,which implied DRX could occur for several times with the increasing number of upsetting and stretching,and uniform finer microstructure would be obtained.However,the results also showed that higher temperature was an unfavorable factor for grain refinement,so the times of heating should be limited for workpiece,and as many forging processes as possible should be finished in once heating.

Selection of Heat Treatment Process and Wear Mechanism of High Wear Resistant Cast Hot-Forging Die Steel

Dry sliding wear tests of a Cr-Mo-V cast hot-forging die steel was carried out within a load range of 50--300 N at 400℃ by a pin-on-disc high temperature wear machine. The effect of heat treatment process on wear resistance was systematically studied in order to select heat treatment processes of the steel with high wear resistance. The morphology, structure and composition were analyzed using scanning electron microscopy （SEM）, X-ray diffraction （XRD） and energy dispersive spectroscopy （EDS） ; wear mechanism was also discussed. Tribo-oxide layer was found to form on worn surfaces to reduce wear under low loads, but appear inside the matrix to increase wear under high loads. The tribo-oxides were mainly consisted of Fe3O4 and Fe2O3, FeO only appeared under a high load. Oxidative mild wear, transition of mild-severe wear in oxidative wear and extrusive wear took turns to operate with increasing the load. The wear resistance strongly depended on the selection of heat treatment processes or microstructures. It was found that bainite presented a better wear resistance than martensite plus bainite duplex structure, martensite structure was of the poorest wear resistance. The wear resistance increased with increasing austenizing temperature in the range of 920 to 1 120 ℃, then decreased at up to 1 220 ℃. As for tempering temperature and microstructure, the wear resistance increased in following order： 700℃ （tempered sorbite）, 200 ℃ （tempered martensite）, 440 to 650 ℃ （tempered troostite）. An appropriate combination of hardness, toughness, microstructural thermal stability was re- quired for a good wear resistance in high-temperature wear. The optimized heat treatment process was suggested for the cast hot-forging steel to be austenized at 1020 to 1 120 ℃, quenched in oil, then tempered at 440 to 650℃ for 2 h.

重卡传动轴用高强7075铝合金热流变行为及径向锻造微观组织演变

为研究重型卡车轻量化传动轴用高强7075铝合金在径向锻造时的热变形行为,将大直径7075铝合金挤压棒料预热退火后,取样进行了不同变形条件下的等温压缩试验。分别考虑温度与应变速率对应变强化及软化的影响,构建了适用于中大型7075铝合金传动轴热加工的改进Hansel-Spittel本构模型。根据动态材料模型理论获得了7075铝合金的热加工图,并据此分析了7075铝合金棒料在典型径向锻造工艺中的组织变化。研究结果表明:改进Hansel-Spittel模型能更准确地模拟预热退火态7075铝合金的峰值和稳态流变应力,预测值与试验值的平均相对误差仅为2.71%;7075铝合金在温度为350~450℃、应变速率为0.01~10 s^(-1)范围内具有较高的热加工稳定性,高温及低应变速率的变形条件有利于材料的微观组织演化;预热退火态7075铝合金棒料在经过截面收缩率为55.6%的径向锻造工艺和T6热处理后,原始板条晶粒转变为细化大转角晶粒组织,抗拉强度提高至651 MPa,延伸率提高至12.5%,综合性能得到显著改善。

深轮辐齿坯多工位闭式模锻材料流动规律研究

探索闭式热模锻成形时材料的流动规律是控制充填不满等成形缺陷、获得高精度模锻件的理论基础。针对深轮辐齿坯件热模锻成形时易出现填充不满的问题,以某SCr420H深轮辐齿坯为研究对象,基于齿坯件结构分析,设计了“下料-加热-镦粗-预锻Ⅰ-预锻Ⅱ-终锻”的多工位闭式热模锻工艺方案;基于Deform软件,构建了深轮辐齿坯件多工位闭式热模锻全流程有限元模拟模型;采用试验验证了有限元模型的可靠性,并对多工位闭式热模锻成形过程中材料的流动规律进行了研究。结果表明:镦粗比为3.7时,可获得外形良好的圆饼形预制坯,镦粗时受上下表面摩擦效应的影响,材料径向流动不均匀,在坯件腰部出现鼓形;预锻Ⅰ工位成形出用于定位的底面内凹结构,同时减小了后续工位材料填充难度;预锻Ⅱ和终锻成形时的材料流动规律相似,成形前期材料集中流向轮缘部分、成形中期主要流向轮毂部分、末期流向圆角处;终锻成形时未出现材料填充不满的缺陷,在成形后期,少量金属材料从终锻上下模的导向间隙中流出,形成纵向飞边。通过生产试验,成形出填充良好的深轮辐齿坯件,模拟数值获得温度与实际生产数值最大误差不超过3.15%,纵向飞边高度小于0.5 mm,零件各尺寸偏差均小于0.2 mm,验证了所设计多工位闭式模锻工艺的合理性。

基于机器学习的TC18钛合金起落架锻造成形工艺智能优化

针对TC18钛合金加工窗口窄、零件成形质量与性能的预测以及工艺参数的优化设计困难的问题,通过热压缩试验分析了TC18钛合金的热变形行为与微观组织演变规律。基于Bayesian算法优化构建了TC18钛合金热变形流动应力和微观组织演变的深度神经网络(DNN)模型。通过对UG和Deform的二次开发,完成了不同工艺参数下起落架锻造的自动建模与仿真,并建立了预锻件的尺寸-工艺-质量数据库。结合DNN和遗传算法(GA)、粒子群算法(PSO)、快速非支配排序遗传算法(NSGA2)确定了最优的预锻件工艺参数。结果表明,采用NSGA2优化后锻件最大成形力R_(Tmax)可降低40.6%,目标截面的平均初生α相含量为0.207,接近最优含量20.0%。

锻造变形均匀性的支持向量机模型及应用

针对超大尺寸高强度钛合金棒材锻造成形过程中存在变形不均匀的难题,以物理热压缩模拟试验和数值模拟为基础,采用机器学习方法,建立了钛合金棒材锻造变形均匀性的支持向量机模型。结合锻造工艺参数的归一化处理,获得了锻造变形均匀性优化模型;提出了实际锻造温度和应变分布均匀性评价函数和锻造工艺参数的多目标优化模型;采用优化算法,获得了基于实际锻造温度和应变分布均匀性的锻造工艺参数。将上述模型应用于钛合金棒材的锻造过程,以锻造温度、锻造速度和压下量为优化变量,实际锻造温度与应变分布均匀性为优化目标,优化了直径Φ400mm的1300 MPa钛合金棒材7道次锻造工艺参数组合。

镍基高温合金热变形过程组织预测研究综述

本文综述了镍基合金的高温塑性变形行为及建模、微观组织演变规律及建模的研究进展,阐述了利用有限元技术预测热变形显微组织的研究现状,得到如下结论:镍基合金的高温塑性变形行为表征模型主要有唯象本构模型和神经网络模型;动态再结晶模型能够准确描述镍基合金热变形过程的微观组织演变,结合有限元模拟技术有助于实现热变形过程的组织预测及调控。

PHSICAL MODELLING AND PROCESSING OF Ti-6Al-4V ELI

Titanium has been widely used as the implant materials of joint prostheses. This paper simulates the microstructures of the hot forged titanium knee joint component by physical modeling. Ti-6Al-4V ELI were deformed by GLEEBLE 2000 hot deforma- tion simulator in compression mode to determine the hot workability, in the range of 900 to 1100℃, with the strain rates between 0.05 to 5 s^(-1). Test results derived from specimens processed by on-cooling test and on-heating test were compared, thereby estimating the effect due to thermal history. The forging process of Ti-6Al-4V ELI tibial base plate of knee joint prostheses was design based on the workability data obtained.

三重热处理对径向锻造Ti-6Al-4V钛合金组织和性能的影响

研究了几种三重热处理制度对径向锻造Ti-6Al-4V钛合金棒材组织和性能的影响。结果表明,在接近相变点进行三重热处理时,组织为网篮状组织;第一重热处理形成的网篮组织,经过两相区的二重固溶和三重时效后组织状态变化不大;随着固溶温度的增加,合金的强度逐渐上升,且距相变点越近,增加的幅度越大;伸长率和断面收缩率有下降趋势,在接近相变点时,下降幅度更大。在两相区进行三重热处理时,Ti-6Al-4V钛合金组织是典型的双态组织,包括近等轴α相和β转变组织,β转变组织内部为针状的次生α相和残余β相;当固溶温度在920~950℃时,强度和塑性呈现出良好的匹配关系,在热处理制度为950℃/1 h,AC+950℃/1 h,OQ+560℃/4 h,AC时,材料获得最佳高强度和高塑性匹配关系。

基于数值模拟的变速箱轴套精锻工艺评估及试验研究

目的针对汽车变速箱轴套在精锻工艺设计阶段成形方案选择多、成形结果不确定的现象,解决试验成本高,开发周期长的问题。方法采用有限元模拟和工艺评价数学模型进行协同研究。首先,针对各工艺方案建立有限元分析模型,分别进行成形模拟,根据成形后零件的填充情况和截面流线状况来评估工艺的合理性;其次,利用工艺评价数学模型将各方案模拟获得的最大应力、最大应变、最大损伤值、最大载荷等重要参数进行整合,以零件质量和模具寿命作为综合评估对象,根据评价值确定最优方案,同时对各方案下的载荷情况进行分析;最后,按照最优方案完成制模和试模生产。结果获得的变速箱轴套成形饱满,质量好,表面无明显缺陷,与模拟结果一致,验证了开发过程的准确性,且尺寸符合要求,内部探伤无裂纹。结论提出的开发工艺可靠,与切削工艺相比,生产效率大幅提升,同时材料利用率提高到92.7%。