Tube Twist Pressing (TTP) as a New Severe Plastic Deformation Method

Tube twist pressing (TTP) as a new severe plastic deformation method for processing tubular parts was presented. The commercially pure aluminum tubes successfully were processed by TTP method. Microstructural examination by XRD analysis of the processed tubes revealed the formation of fine grains in the average size of 1.1 μm after four TTP passes. Also, the obtained results of mechanical tests showed a notable increase in microhardness, yield and ultimate strengths. The capabilities of TTP method were verified via comparison of the obtained results with the results of other SPD processes. To further investigate the TTP method, FE modeling was carried out using the Abaqus/Explicit to study the macroscopic deformation and microstructural evolution (the evolution of dislocation density and grain size) during TTP via continuous dynamic recrystallization. In the FE model, the strain hardening behavior of the material was related to microstructure quantities based on the micromechanical constitutive model. The FEM simulated grain refinement behavior was consistent with the experimentally obtained results.     

Control of UFG Microstructure in Welded Carbon Steel Tubes by Cold Drawing and Annealing

The present study is made to develop ultra fine grained microstructure in welded steel tubes, through multiple cold drawing passes followed by an annealing treatment. The average ferrite grain size is reduced from 16 to 1.9 μm. SAE 1019M steel grade used for a typical automotive driveline component is studied. Strains between 0.3 and 1.4 followed by annealing at 400, 450 and 500 °C are considered to optimize the combination of cold drawing strain and temperature required to produce ultra fine grained microstructure in steel tubes. At a strain value of 1.4 and annealing temperature, 500 °C, polygonal ferrite grains and fine carbide particles are obtained. This microstructure is found to be suitable owing to its combination of high strength and good ductility in steel tubes. Tensile strength as high as 1,061 MPa and 9 % elongation is obtained due to microstructural refinement. The strength is increased by 350 MPa compared to the strength of conventional cold-drawn welded tubes.     

晶粒细化对奥氏体不锈钢高温力学性能的影响

摘要：对粗晶201LN奥氏体不锈钢采用60%冷变形结合700℃退火120s工艺制备超细晶奥氏体不锈钢,研究晶粒细化对奥氏体不锈钢高温力学性能的影响。利用光学显微镜、扫描电子显微镜、透射电子显微镜、电子背散射衍射技术对粗晶和超细晶奥氏体钢进行了组织表征,并使用万能试验机测试20和650℃环境下力学性能。结果显示粗晶奥氏体不锈钢经过冷变形结合退火工艺处理,平均晶粒尺寸由18μm细化为0.9μm,屈服强度由383MPa提高到704MPa,而伸长率由63.8%下降到46.3%,表明晶粒细化能有效提高奥氏体不锈钢屈服强度的同时较小损害塑性,TEM证实其形变机制均为形变诱导马氏体和孪生协同作用。当温度由20℃提高到650℃时,粗晶奥氏体不锈钢屈服强度和伸长率分别下降到180MPa和28.1%,超细晶奥氏体不锈钢屈服强度和伸长率分别为384MPa和24.2%。这表明在650℃高温环境下细晶强化作用仍然有效,粗晶和超细晶奥氏体不锈钢也有较好的塑性,其形变机制分别变为位错滑移和位错滑移+层错+孪生。     

Ultrafine Grained Steels with Improved Ductility Fabricated by Repetitive Thermo‐Mechanical Processing

Ultrafine grained (UFG) steels with bimodal grained microstructure consisting of ultrafine grains in the size of 400 nm and micrometer‐sized grains were obtained by repetitive thermo‐mechanical processing. The microstructure evolution and mechanical properties were studied. It was revealed that the formation of the bimodal grained microstructure is attributed to the combination of continuous and discontinuous recrystallization. Compared with the uniform UFG steel with average grain size of about 400 nm, the ductility was remarkably improved by introducing micrometer‐sized grains into the UFG microstructure, providing better strength‐ductility balance.     

Modelling of plastic flow in coarse grained nickel-base superalloy compacts under isothermal forging conditions

A constitutive relationship for predicting the flow stress and the evolution of microstructure during isothermal forging of coarse grained P/M Ni-base superalloy compacts, forged below their γ′ solvus temperatures, is derived. These coarse grained compacts gradually transform to microduplex γ−γ′ microstructures during forging and this leads to softening during plastic flow. To model the transformation and the resulting softening, the material is considered as a composite material consisting of hard (untransformed) and soft (transformed to the fine microduplex γ−γ′ grains) regions. A rate equation for this material is written in terms of that for the hard and the soft regions assuming that both carry the same stress but each region is subjected to different strain rates. This is consistent with the occurrence of flow localization in the transformed microduplex regions as observed experimentally. A single grain microstructural model and modified forms of established transformation kinetics relationships for grain boundary nucleated reactions are proposed for modelling the deformation dependent transformation. The constitutive relationship is consistent with necklace structure formation typically characteristic of these materials, and it is suggested that it can be used to predict the development of grain size gradients and shear instabilities in forgings.     

Ultrafine Grain Formation in a Ti-6Al-4V Alloy by Thermomechanical Processing of a Martensitic Microstructure

In the current study, ultrafine equiaxed grains with a size of 150 to 800 nm were successfully produced in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure. This was achieved through a novel mechanism of grain refinement consisting of several concurrent processes. This involves the development of substructure in the lath interiors at an early stage of deformation, which progressed into small high-angle segments with increasing strain. Consequently, the microstructure was gradually transformed to an equiaxed ultrafine grained structure, mostly surrounded by high-angle grain boundaries, through continuous dynamic recrystallization. Simultaneously, the supersaturated martensite was decomposed during deformation, leading to the progressive formation of beta phase, mainly nucleated on the intervariant lath boundaries.     

低合金高强钢针状铁素体组织特征和形成机理

针状铁素体是一种具有大角度晶界、高位错密度的板条状中温转变组织,该组织能有效细化晶粒并具有良好的强韧性匹配.因此,通常在低合金高强度钢焊缝和粗晶区中,利用细小的夹杂物来诱导针状铁素体形成,形成有效晶粒尺寸细小的针状铁素体联锁组织或者针状铁素体和贝氏体的复合组织,使其具有良好的韧性.然而,相关研究对针状铁素体组织的形成机理和控制原理的解释并不十分清楚,对于针状铁素体的定义和理解也存在差异.总结了针状铁素体的本质、相变、形核、形态、晶体学取向关系、长大行为、细化机理和力学性能等方面的特征,归纳了奥氏体晶粒尺寸、转变温度、冷却速度、夹杂物类型和尺寸等对针状铁素体形成的影响,提出了针状铁素体组织形态和转变机理方面几个仍需深入研究的问题和方向.     

剧烈塑性变形对块状镁合金微观组织和力学性能的影响

等通道转角挤压（Equal channel angular pressing, 简称ECAP）可以使镁合金产生较大的塑性变形.通过有限元方法模拟了等通道转角挤压工艺及其相关工艺参数,研究了工件的应变和载荷分布情况,并建立了累积变形结果、微观组织细化和力学性能的数学模型.通过分析得到了晶粒细化和力学性能的关系,对累积变形的特点分析,预测了晶粒细化后的尺寸和力学性能.      

Grain Refinement of TRIP‐Assisted Multiphase Steels through Strain‐Induced Phase Transformation

To develop high performance steels for automotive applications, enhanced strengthening mechanisms are required. This study aims at assessing the critical parameters leading to the refinement of the strain‐induced ferrite matrix of thermomechanically processed multiphase steels. Hot rolling simulations allowed the definition of the temperature, strain and cooling rate conditions bringing about the formation of strain‐induced ferrite with a reduced grain size. The relationship between the deformation and the concurrent or subsequent phase transformations is highlighted thanks to a thorough characterisation of the generated microstructures. It is also shown that the prior austenite grain size influences the distribution of the second phases within the finely grained ferrite matrix.     

Microstructure,Texture, and Tensile Properties of Ultrafine/Nano-Grained Magnesium Alloy Processed by Accumulative Back Extrusion

An AZ31 wrought magnesium alloy was processed by employing multipass accumulative back extrusion process. The obtained microstructure, texture, and room temperature tensile properties were characterized and discussed. Ultrafine-grained microstructure including nano-grains were developed, where the obtained mean grain size was decreased from 8 to 0.5 μm by applying consecutive passes. The frequency of both low angle and high angle boundaries increased after processing. Strength of the experimental alloy was decreased after processing, which was attributed to the obtained texture involving the major component lying inclined to the deformation axis. Both the uniform and post-uniform elongations of the processed materials were increased after processing, where a total elongation of 68 pct was obtained after six-pass deformation. The contribution of different twinning and slip mechanisms was described by calculating corresponding Schmid factors. The operation of prismatic slip was considered as the major deformation contributor. The significant increase in post-uniform deformation of the processed material was discussed relying on the occurrence of grain boundary sliding associated with the operation of prismatic slip.

     

高能球磨-放电等离子烧结法制备双晶分布钛

以平均粒径约150μm的球形钛粉为原料,采用高能球磨结合放电等离子烧结技术制备由双尺度晶粒组成的高致密纯钛块体材料,研究高能球磨过程中钛粉的形貌、尺寸及显微组织的变化,分析球磨钛粉放电等离子烧结时的致密化行为和显微组织的演变规律,测试烧结钛块体材料的室温压缩性能。结果表明:钛粉在球磨初期发生剧烈的塑性变形并相互焊合,形成层片状团聚粉末。球磨10 h时,钛粉的部分晶粒细化至40~100 nm。放电等离子烧结过程中,随烧结温度升高和烧结时间延长,烧结钛的密度逐渐增大。在烧结温度为800℃、保温时间为4 min、烧结压力为50 MPa的条件下,烧结钛的密度达到4.489 g/cm3,接近全致密,其显微组织由双尺度的等轴晶组成,细晶区晶粒尺寸为1~2μm,粗晶区晶粒尺寸为5~20μm,二者呈层状交替分布;该试样在室温压缩条件下的综合力学性能与铸锻Ti-6Al-4V合金相当。     

Dynamic Recrystallization of Hot Deformed 3Cr2NiMnMo Steel: Modeling and Numerical Simulation

Hot compression tests of 3Cr2NiMnMo steel were performed at temperatures in the range of 850 to 1100 °C and with strain rates of 10^?2s^?1 to 1s^?1. Both the constitutive equations and the hot deformation activation energy were derived from the correlativity of flow stress, strain rate and temperature. The mathematical models of the dynamic recrystallization of 3Cr2NiMnMo steel, which include the dynamic recrystallization kinetics model and the crystallization grain size model, are based on Avrami's law and the results of thermosimulation experiments. By integrating derived dynamic recrystallization models with the thermal-mechanical coupled finite element method, the microstructure evolution in hot compressive deformation was simulated. The distribution of dynamic recrystallization grains and grain sizes were determined through a comparison of the simulation results with the experimental results. The distribution of strain and dynamic recrystallization grain is also discussed. The similarity between the experimental results and the simulated results indicates that the derived dynamic recrystallization models can be applied effectively to predict and analyze the microstructure evolution in hot deformed 3Cr2NiMnMo steel.     

Microstructure Refinement and Property Improvement of Metastable Austenitic Manganese Steel Induced by Electropulsing

Grain refinement efficiency of electropulsing treatment(EPT)for metastable austenitic manganese steel was investigated.The mean grain size of original austenite is 300μm.However,after EPT,the microstructure exhibits a bimodal grain size distribution,and nearly 70vol.%grains are less than 60μm.The refined austenite results in ultrafine martensitic microstructure.The tensile strengths of refined austenitic and martensitic microstructures were improved from 495to 670,and 794to 900MPa respectively.The fine grained materials possess better fracture toughness.The work-hardening capacity and wear resistance of the refined austenitic microstructure are improved.The reasonable mechanism of grain refinement is the combination of accelerating new phase nucleation and restraining the growth of neonatal austenitic grain during reverse transformation and rapid recrystallization induced by electropulsing.     

Coupling Thermomechanical and Microstructural FE Analysis in Plate Rolling Process

Using three-dimensional rigid-viscoplastic finite element method （FEM）, a coupling multivariable numerical simulation model for steel plate rolling has been established based on the physical metallurgy microstructural evolution rule and experiential equations. The effects of reduction, deformation temperature, and rolling speed on the deformation parameters and microstructure in plate rolling were investigated using the model. After a typical rolling process of steel plate 16Mn is simulated, the strain, temperature, and microstructure distributions are presented, as well as the ferrite grain transformation during the period of cooling. By comparing the calculated ferrite grain sizes with measured ones, the model is validated.     

TEM studies on recovery and recrystallisation in Equal Channel Angular Extrusion processed Al-3%Mg alloy

Equal Channel Angular Extrusion (ECAE) is a promising severe plastic deformation (SPD) process which can produce polycrystalline materials with ultra-fine grains (UFG) of sub micrometer range or nanometer range. Large plastic shear deformation induced by the high applied pressure in ECAE material processing is the prime reason behind the grain refinement. The focus of the present work is to study the evolution of dislocation microstructure during dynamic recovery (due to intense strain deformation) and static recovery (due to static annealing after deformation) in commercial Al-3%Mg alloy processed by ECAE. It is observed that local concentrations of shear strain can take place and high angle boundary (HAGB) segments are formed initially at random locations. When thermal energy is provided, during static annealing, the boundary segments get further defined and extended. This leads to the formation of very fine size grains with high mis-orientations which subsequently develop into an ultra-fine grain distribution in the material. Also, it appears dynamic recrystallisation (DRX) occurring during the deformation itself is a general phenomenon leading to refinement of grains. Transmission Electron Microscopy (TEM) is the characterizing tool used in the present study. The influence of precipitates/second phase particles on the deformation characteristics and on the increased degree of grain fragmentation is also detailed.     

Development of ultra-fine grained dual phase microalloyed steels through severe cold rolling and intercritical annealing

A Nb-microalloyed structural steel with ferrite-pearlite microstructure was subjected to cold rolling and intercritical annealing to produce ultra-fine grained dual phase microstructure. Optical and transmission electron microscopy techniques were employed to characterise the microstructure. Initial results showed that the intercritical annealing (at 790°C for 90s) of samples rolled to a true strain of 2.4 resulted in a significant grain refinement from the average initial grain size of 20 μm to 1–2 microns. The microstructure primarily consisted of UFG ferrite matrix with homogeneously distributed islands of plate martensite with volume fraction of 27%.     

高速线材生产过程组织性能预测模型仿真

 采用计算机对高速线材生产过程进行模拟,开发出具有较高准确度同时具有对不同轧制工艺有较好通用性的高线生产仿真系统。利用有限元方法计算了线材在待轧、轧制、水冷及风冷过程的温度场;通过对再结晶动力学模型的解析,得到了静态再结晶、动态再结晶的分数以及奥氏体晶粒在轧制过程中的变化情况;通过组织演变模型和温度模型的耦合计算,模拟出斯太尔摩风冷线上线材的组织变化过程;建立了利用初始化学成分和组织组成预测高线产品力学性能的BP神经网络模型,通过生产过程数据的训练,实现了对线材力学性能的预测。仿真计算的结果对线材控轧、控冷工艺的改进有一定的指导意义。     

Texture and Microstructural Evolution in Pearlitic Steel During Triaxial Compression

This article presents the deformation behavior of high-strength pearlitic steel deformed by triaxial compression to achieve ultra-fine ferrite grain size with fragmented cementite. The consequent evolution of microstructure and texture has been studied using scanning electron microscopy, electron back-scatter diffraction, and X-ray diffraction. The synergistic effect of diffusion and deformation leads to the uniform dissolution of cementite at higher temperature. At lower temperature, significant grain refinement of ferrite phase occurs by deformation and exhibits a characteristic deformation texture. In contrast, the high-temperature deformed sample shows a weaker texture with cube component for the ferrite phase, indicating the occurrence of recrystallization. The different mechanisms responsible for the refinement of ferrite as well as the fragmentation of cementite and their interaction with each other have been analyzed. Viscoplastic self-consistent simulation was employed to understand deformation texture in the ferrite phase during triaxial compression.     

Effect of Microstructure and Mechanical Properties of Al–Mg Alloy Processed by ECAP at Room Temperature and Cryo Temperature

This research primarily focuses on improving the strength of Al 5083 alloy by both the ECAP and Cryo ECAP methodology. Equal Channel Angular Pressing (ECAP) is one of the best technologies that enable the direct transformation of conventional macro grained metals into sub-micron, ultra-fine and nano grained materials. Fine grain size increases the strength and the fracture toughness of the material and provides the potential for super plastic deformation at moderate temperatures and at high strain rates. The microstructure evolution in Al 5083, subjected to Room Temperature ECAP and Cryo ECAP were analysed. ECAP was carried out using an optimized die with Channel angle ‘?’ = 90°and corner angle ‘Ψ’ = 20° through processing route A and C up to four passes. The results were thoroughly studied using TEM, SEM, and optical microscopic images. Initially the annealed sample had the grain size of 80 µm with the equi-axed grains. In Room Temperature, the hardness values and the mechanical strength were found to be increased from 88 to 410 HV and 306 to 453 MPa after four passes in route A and in route C the strength increased from 390 to 416 MPa after four ECAP passes. Moreover, in Cryo Condition, the sample was processed up to four ECAP passes at route A and route C. The hardness of 153 HV was obtained after four passes in route C and 164 HV obtained after four passes on route A. Additionally, fracture behaviour using SEM, grain size using TEM and crystallite size by X-ray diffraction studies were analyzed. It was observed that the Cryo ECAP showed marginal improvements in mechanical properties relative to the RT ECAP in case of Al 5083.     

不同锻造变形量对管状溅射靶材晶粒组织的影响

选用常规钼粉作为原料,采用锻造加工方式加工出一定规格的管状溅射靶材,在不同退火温度下观察晶粒组织,得出变形量为80%、1 100℃工艺下加工出的溅射靶材晶粒大小均匀,组织性能较好,符合溅射靶材对晶粒度的要求。