Discussion and prediction on decreasing flow stress scale effect

Based on crystal plasticity theory and surface layer model, relation of flow stress to billet dimension and grain size was built, and rationality of derived relation was verified with tensile tests of different size billets. With derived expressions, relation of decreasing flow stress scale effect to billet dimension, grain size as well as billet shape was discussed and predicted. The results show that flow stress is proportional to billet size; with decrease of grain size, flow stress is less influenced by billet dimension. When both cross section area and grain size are same, flow stress decrease of rectangular section billet or sheet is larger than that of circular section billet.     

AZ80合金初始晶粒对其流变应力影响的研究

在Gleeble-3800热模拟机上分别对铸态和预变形态AZ80合金进行了等温压缩,研究了初始晶粒对其流变应力的影响。结果表明,相同条件下,初始晶粒细小的预变形态AZ80合金,其峰值流变应力比铸态合金可降低20%左右,同时变形后的组织也较细密;随着变形量(真实应变)的增大,晶粒尺寸对流变应力的影响减弱。在此基础上,提出了铸态镁合金预变形省力成形方法,为降低镁合金成形力提供了新的思路。     

晶体塑性模型分析微圆柱墩粗变形尺寸效应机理(英文)

为了分析单个晶粒变形行为对微成形的影响,将自由表面的晶粒看作单晶体构建晶体塑性模型。基于率相关晶体塑性理论,考虑试样尺寸、初始晶体取向及其分布,分析微圆柱体墩粗变形中尺寸效应机理。结果表明,流动应力随着试样尺寸的减小而明显减小,晶体取向的分布对试样流动应力具有显著影响,并随着塑性变形的进行而减小。由于单晶体的各向异性,在试样表层发生了明显的非均匀变形,这将导致表面粗糙度的增加,小尺寸试样则更加明显。过渡晶粒的存在使得晶粒各向异性对表面形貌的影响减小。模拟结果得到了实验验证,这表明所建立的模型适合于以尺寸依赖性、流动应力分散性和非均匀变形为特点的微成形工艺分析。     

Theoretical study of hardness variation with pulse parameters in composite coatings electrodeposited by pulsed currents

Theoretical aspects of the electrodeposition of composite coatings prepared by pulsed currents and also the variation of coatings hardness with the pulse parameters have been presented in this paper. The effect of the grain size, polarisation, pulse-plating parameters and incorporation of ceramic particles on the hardness or flow stress are also reported. The theoretical relationship obtained showed that with increasing polarisation and particle incorporation, the hardness of the coatings was increased. In addition, by decreasing the grain size, frequency and duty cycle the hardness was increased.     

变形条件对SiCp/2014Al复合材料力学行为和晶粒度的影响

研究了变形温度,应变速率和变形程度等热加工参数对ＳｉＣｐ／２０１４Ａｌ复合材料力学行为和晶粒度的影响。结果表明,该材料的流动应力对变形温度和应变速率比较敏感。变形每升高４０℃,流动应力降低５￣１０ＭＰａ,应变速率每提高一个数量级,流动应力增加１．３￣１．８们。提高应变整编有利于获得细晶组织。以足够的变形程度锻造以及适当提高锻后冷却速度均有利于获得细晶组织。根据实验结果确定了该材料的流动应力与热加工     

A multi-region model for numerical simulation of micro bulk forming

Due to the small billet size in micro forming, each grain, especially that on the surface layer, has direct influence on the deformation behavior of the billet. The multi-region model was proposed for simulating the micro bulk forming process in this paper. The object in the model is divided into three different regions: the inner polycrystal region, the grain interior of the surface region and the grain-boundary layer in the surface region. Each surface grain has different orientation. Depending on the Hall-Petch formula, which is applicable to describe polycrystal materials, and introducing scale parameters, the constitutive equation of each region is deduced based on the unidirectional compression tests data of the copper specimen. Using the multi-region model, the coining process with micro feature is simulated to investigate the size effect in micro forming. Moreover, an experiment of coining process with micro-feature is performed to verify the correctness of the multi-region simulation model. The experimental results show a good agreement with those in numerical simulation.     

微小尺度镦挤复合成形研究

为了研究晶粒尺寸对微成形工艺的影响,在常温下对具有不同晶粒尺寸微型铜圆柱体坯料进行了微型镦挤复合成形,获得了微小尺度下材料流动特点及相关工艺参数。所得结果表明,所成形的微型杆-法兰件挤出杆端随着材料晶粒尺寸的增大而逐渐呈现出凹凸不平的现象,即出现非均匀流动;随着晶粒尺寸的增大,凸模单位压力逐渐减小;晶粒尺寸的变化对于各个阶段成形力变化趋势没有明显的影响。     

微小尺寸纯铜镦粗试验及其成形特性

主要研究了晶粒尺寸、变形速度对微镦粗工艺的影响。不同的晶粒尺寸可采用重结晶的方式获得。本试验是在常温下以不同变形速度对微型铜圆柱体进行镦粗试验,结果表明：材料流动呈现出各向异性的特点,并随压下量增大而更明显;屈服应力降低并出现较大波动;材料出现先硬化后软化现象;在相同变形量下,流动应力呈现出明显波动;变形速度对材料流动、初始屈服应力、流动应力影响不明显。     

微细薄板尺度依赖的材料本构模型的构建(英文)

通过表面层理论和金属晶体塑性变形原理解释微细薄板材料在塑性变形中产生尺寸效应的内在机理。引入尺度参数,对经典的Hall-Petch公式进行修正,建立基于表面层模型理论的尺度依赖材料模型。利用所建立的材料模型分析微细薄板厚度及其晶粒尺寸对材料成形流动性能的影响。在晶粒尺寸一定的情况下,随着微细薄板厚度的减小,材料流动应力逐渐降低;晶粒尺寸越大的微细薄板,其流动变形的尺寸效应现象越明显。利用不同厚度的不锈钢和纯铜箔材的微细薄板拉伸真应力-应变曲线对所建立的材料模型进行验证,计算结果与实验结果比较吻合,验证了所建立的材料模型的合理性。     

齿轮钢大圆坯的铸态组织及轧制遗传性

基于φ500 mm大圆坯连铸连轧生产22CrMoH汽车桥齿钢的工艺,通过对铸坯低倍组织及高倍金相组织、轧后带状组织系统分析,开展了齿轮钢大圆坯铸态组织及轧制遗传性研究。研究发现,22CrMoH齿轮钢连铸大圆坯近表面细晶区、柱状晶区、中心等轴晶区的枝晶尺寸存在明显差异;自表面至芯部二次枝晶间距逐渐增加,退火状态下枝晶主干位置以铁素体为主,枝晶间以珠光体为主;轧后带状组织形态主要受铸态组织二次枝晶间距和轧制变形的影响;轧后退火态铁素体带宽自表面至芯部呈现增加趋势。     

Analytical and computational description of effect of grain size on yield stress of metals

Four principal factors contribute to grain-boundary strengthening: (a) the grain boundaries act as barriers to plastic flow; (b) the grain boundaries act as dislocation sources; (c) elastic anisotropy causes additional stresses in grain-boundary surroundings; (d) multislip is activated in the grain-boundary regions, whereas grain interiors are initially dominated by single slip, if properly oriented. As a result, the regions adjoining grain boundaries harden at a rate much higher than grain interiors. A phenomenological constitutive equation predicting the effect of grain size on the yield stress of metals is discussed and extended to the nanocrystalline regime. At large grain sizes, it has the Hall–Petch form, and in the nanocrystalline domain the slope gradually decreases until it asymptotically approaches the flow stress of the grain boundaries. The material is envisaged as a composite, comprised of the grain interior, with flow stress σ_fG, and grain boundary work-hardened layer, with flow stress σ_fGB. The predictions of this model are compared with experimental measurements over the mono, micro, and nanocrystalline domains. Computational predictions are made of plastic flow as a function of grain size incorporating differences of dislocation accumulation rate in grain-boundary regions and grain interiors. The material is modeled as a monocrystalline core surrounded by a mantle (grain-boundary region) with a high work hardening rate response. This is the first computational plasticity calculation that accounts for grain size effects in a physically-based manner. A discussion of statistically stored and geometrically necessary dislocations in the framework of strain-gradient plasticity is introduced to describe these effects. Grain-boundary sliding in the nanocrystalline regime is predicted from calculations using the Raj–Ashby model and incorporated into the computations; it is shown to predispose the material to shear localization.     

Constitutive model based on dislocation density and ductile fracture of Monel 400 thin sheet under tension

In micro-scaled plastic deformation, material strength and ductile fracture behaviors of thin sheet in tension are quite different from those in macro-scale. In this study, uniaxial tensile tests of Monel 400 thin sheets with different microstructures were carried out to investigate the plastic deformation size effect in micro-scale. The experimental results indicate that the flow stress and fracture strain departure from the traditional empirical formula when there are only fewer grains across the thickness. And the number of dimples on the fracture surface is getting smaller with the decreasing ratio of specimen thickness to grain size. Then, a constitutive model based on dislocation density considering the free surface effect in micro-scale is proposed to reveal the mechanism of the flow stress size effect. In addition, a model is proposed considering the surface roughening inducing the thickness nonuniform and the decrease of micro-voids resulting from the reduction of grain boundary density with the decreasing ratio of specimen thickness to grain size. The interactive effects of the surface roughening and the decrease of micro-voids result in the earlier fracture in micro tension of the specimen with fewer grains across the thickness.     

轻卡转向节锻造成形有限元分析

提出了卡车转向节卧式锻造一火成形新工艺。基于刚粘塑性有限元理论,对转向节的一火锻造成形的各个工序建立了三维热力耦合有限元模型。利用大型有限元分析软件对各工序成形过程进行数值模拟,优化得出制坯的合理形状及尺寸,预、终锻过程中金属在型腔的流动过程及充填情况和等效应变及载荷—行程曲线分布,物理实验与模拟结果基本吻合,该工艺已用于实际锻件生产。     

喷射沉积7090/SiCp复合材料SiC颗粒分布均匀性研究

制备了喷射共沉积及粉末冶金SiCp/7090Al复合材料,以不同的挤压比进行热挤压成形,研究了两种复合材料中SiC增强颗粒的分布特点。实验结果表明,沉积坯经过热挤压后,沿挤压方向SiC颗粒存在明显的带状分布特征,在横截面上具有同心圆环组织。而粉末冶金坯料经过热挤压后则避免了这一现象的产生,合理的热挤压工艺可以显著改善粉末冶金复合材料中的SiC增强颗粒的形貌,细化SiC增强颗粒的粒度。增强颗粒细化后得到的细小等轴颗粒更易随着基体的塑性流动而呈弥散均匀分布,改善或消除微区域内增强体颗粒的偏聚,减弱了偏聚的程度。室温力学性能测试结果表明,随着挤压比的增加,挤压态粉末冶金复合材料的抗拉强度由挤压比为11时的362.84MPa,增大到挤压比为25时的425.27MPa。     

行波磁场作用下空心管坯的两相凝固数值模拟

利用等轴球晶两相凝固模型计算了行波磁场作用下空心管坯的凝固过程.分析了施加不同强度的行波磁场时,Sn-3.5%Pb管坯凝固过程的温度梯度、冷却速率、溶质浓度分布以及晶粒大小的变化情况.结果表明:金属液在行波磁场的作用下产生纵截面上的大环流,随着搅拌速度的加大,金属液凝固过程的冷却速率逐渐降低,熔体内的温度梯度减小,使管坯内部的晶粒得到细化,改善了铸坯凝固组织,但是强制流动导致管坯内产生宏观偏析,且搅拌速度越大,宏观偏析越明显,因此,在生产过程中应该严格控制搅拌速度的大小.     

H2/Ar流量比对纳米金刚石形貌和结构的影响

目的研究不同H_2/Ar流量比对纳米金刚石形貌和结构的影响。方法采用MPECVD法制备了质量较好的纳米金刚石薄膜,并通过SEM、XRD对金刚石薄膜的形貌、结构以及晶粒尺寸进行了测试,还使用Raman对金刚石D峰、纳米金刚石特征峰(TPA)的变化趋势进行了分析。结果当Q(H_2):Q(Ar)=50:49时,制备的金刚石晶粒为亚微米范畴,其平均晶粒尺寸为250 nm,表面平整度较差,出现堆积层错现象,但金刚石特征峰(D峰)最强,生长速率达到最大,约为125 nm/h;Q(H_2):Q(Ar)=10:89时,表面平整度高,二次形核现象明显,平均晶粒尺寸为20 nm;进一步减小H_2/Ar流量比为0时,可发现晶粒由纳米变为超纳米,二次形核更为明显,表面平整更高,其平均晶粒尺寸为3 nm,另外Raman测试发现金刚石特征峰强度随H_2/Ar流量比的减小而减小,而纳米金刚石特征峰随H_2/Ar流量比的减小而增大。结论随着H_2/Ar流量比的增加,金刚石表面平整度逐渐变差,表面粗糙度也在逐渐增大,同时金刚石的晶粒尺寸和生长速率在Q(H_2):Q(Ar)=50:49时达到最大。     

变形程度对7A04合金半固态触变组织的影响

获得具有触变特性的均匀的、近球形的显微组织是半固态触变成形工艺的基础和关键。以挤压态7A04合金SIMA法生成触变组织为例．研究变形程度对其半固态触变组织的影响。结果表明：在加热前的变形程度对显微组织的影响很大，随着变形程度的增加，晶粒尺寸减小，晶粒形状越趋于球形；当变形程度超过0．3时，晶粒尺寸及其球形化速度逐渐减慢并趋于稳定值。此外，经过冷变形后，晶粒内及晶粒间的偏析减小，晶界易被低熔点液相浸润．更易获得均匀的、近球形的触变组织。     

Microstructure optimization in design of forging processes

A new approach based on sensitivity analysis for optimizing the microstructure development during the forging processes is proposed in this work. The analytical sensitivities of the recrystallization volume fraction and dynamically recrystalized grain size with respect to the design variables are derived. The mean grain size in each finite element is introduced so that the complex recrystallization mechanics, such as no recrystallization, partial recrystallization and complete recrystallization are all considered. The objective is to minimize a function describing the variance of mean grain size and the average value of mean grain size in the whole final product. Two constraints are imposed on die underfill and excessive material waste. Two different kinds of design variables are considered, including state parameter (initial shape of billet) and process parameter (die velocity). The optimization scheme is demonstrated with the design of a turbine disk made of Waspaloy in non-isothermal forging process. The optimal initial shape of billet and the die velocity are obtained.     

喷射沉积过程中半固态流变组织演变

分析了喷射沉积过程中半固态流变组织演变过程,提出喷射沉积合金细小的等轴晶的凝固组织是飞行阶段快速冷却和沉积阶段沉积体表面糊状凝固层中半固态流变组织演变的结果,沉积体凝固过程是一种微米级流变铸造过程:不同大小、不同状态、不同温度、不同的运动速度的雾滴以高速高频的雾滴撞击沉积体表面,产生足够的剪切应力和剪切速率,使沉积体半固态糊状凝固层具有流变特性,飞行阶段形成快速凝固组织在沉积阶段沉积体表面糊状凝固层中发生变温粗化,最终形成细小的等轴晶。     

Flow stress anisotropy and Bauschinger effect in ultrafine grained copper

The effect of strain path and magnitude on the flow stress anisotropy and Bauschinger effect (BE) in ultrafine grained (UFG) copper was investigated. The material billets were deformed via multipass equal channel angular extrusion (ECAE) following several deformation routes. The monotonic stress–strain responses under tension and compression and forward compression/reverse tension response along three perpendicular directions were determined in each billet. It was observed that, in certain cases, the strong tension/compression asymmetry was in favor of tension as opposed to what has so far been reported for UFG materials, and an increase in the number of ECAE passes caused a decrease in yield strength along certain sample directions. Finally, the BE was found to be more pronounced for the lower number of passes. It was shown that crystallographic texture and grain size differences cannot be the only factors responsible for these unexpected observations. Grain morphology and grain boundary character are argued to be additional parameters that have to be taken into account. How these factors affect tension/compression asymmetry, flow anisotropy and BE in UFG copper and how they can help elucidating the observations are discussed.