摩擦系数对等通道转角挤压工艺影响的有限元分析及试验研究

采用ANSYS软件建立了等通道转角挤压有限元模型,对试样与模具接触面间不同摩擦系数条件下的等通道转角挤压过程进行了模拟,获得了摩擦应力、试样与模具内部应力的变化及分布规律。以AZ31镁合金为试验对象,进行了等通道转角挤压试验,将模拟分析结果与试验结果进行了对比。结果表明,等通道转角挤压过程中,试样与模具间的摩擦系数对摩擦应力、试样变形及弹性应力均有重要影响。随着试样与模具接触面摩擦系数的增加,试样与模具表面会产生较大摩擦应力,导致试样与模具内应力分布不均匀。摩擦应力、材料内部应力大小及分布区域随着摩擦系数的增加而增加。     

AZ31镁合金等通道转角挤压的有限元模拟及试验

采用Ansys软件建立了等通道转角挤压的有限元模型,对不同模具角度条件下等通道转角挤压过程进行了模拟,获得了摩擦应力、试样与模具内部应力与应变随模具角度的变化。以AZ31镁合金为试验对象,进行了等通道转角挤压试验,对模拟分析结果与试验结果进行了对比。结果表明,等通道转角挤压过程中试样与模具表面会产生较大摩擦应力,导致试样与模具内应力与应变分布不均匀,摩擦应力、材料内部应力及应变随着模具角度增加而减小。挤压后AZ31镁合金微观组织被细化,理论分析结果与试验结果一致。     

纯钛连续等径通道转角挤压的有限元模拟

等通道转角挤压(Equal Channel Angular Pressing,ECAP)是一种制备超细晶材料的加工方法.利用ABAQUS有限元分析软件及网格再划分对纯钛的连续等通道转角挤压变形的Bc和C方式进行了三维的计算机有限元模拟,得到了应力应变分布规律和挤压力一位移曲线.结果表明,Bc方式在1、3、4道次挤压后纯钛试样的塑性应变梯度比C方式更小,而最大的压力是2道次.     

等通道转角挤压制备超细晶材料的最新研究进展

等通道转角挤压(Equal channel angular pressing,ECAP)方法是制备性能优异超细晶材料最常见的大塑性变形方法之一。模角、挤压路径、挤压道次、挤压温度和挤压速度等因素都会影响等通道转角挤压制备超细晶材料的性能;等通道转角挤压的模具也在不断地优化,如背压-等通道转角挤压(Back pressure ECAP,BP-ECAP)模具、可加热的模具以及在等通道转角挤压基础上形成的板材连续剪切技术等,这些新的模具可以改变ECAP变形过程中的组织均匀性。本文综述了等通道转角挤压制备超细晶材料的最新研究进展,并指出了几个需要深入研究的问题及方向。     

LD10合金ECAP工艺参数的研究

等通道转角挤压(ECAP)是一种获得超细晶粒的重要制备方法,它是通过材料在等通道模具中的大剪切变形而实现晶粒细化的.研究了LDl0合金的等通道转角挤压及热处理工艺对显微组织的影响,优化了工艺参数.试验结果及分析表明,对试样在300℃下按路径B(每次挤压时试样旋转90°,旋转方向不变)挤压4次并经350℃退火1h可得到细小、均匀的等轴品.为LD10合金的晶粒细化及超塑性近净成形技术的开发提供试验依据.     

ECAE处理钛合金数值分析研究

通过等通道转角挤压法来细化晶粒是目前比较常用的方法,此法对室温下易成形材料应用较广,但对于难加工材料钛合金还处于研究阶段。传统实验法对钛合金等通道转角挤压时,工作量大、成本高。本文利用数值模拟技术分析了TC4钛合金在350℃、BC路径下1,2,4,8道次及在不同的初始挤压温度、摩擦系数及挤压速度下,等通道转角挤压过程中的载荷、温度场变化、等效应变场及微观组织的变化情况,结果显示,在挤压4道次、摩擦系数0.12及6mm·s-1的挤压速度下的晶粒细化效果较好。     

304奥氏体不锈钢在新型挤压模具下的仿真分析

通过运用设计的3种新型等通道转角挤压模具,结合现有关于等通道转角挤压的研究,对304奥氏体不锈钢材料进行三维有限元模拟,研究了挤压过程中的载荷曲线变化、等效应力以及等效应变分布。结果表明,挤压方式的改变不会影响凸模载荷的大小;同时,对比了3种不同挤压方式下模具的等效应力、等效应变后发现,旋转90°工艺的等通道转角挤压模具的等效应力值最小,使其在转角处发生拉毛的概率最小,对模具损伤也最小,并且其等效应变值最大,对试样的细化效果最好。最后,通过采取点跟踪的方式绘制应变点循迹图,更直观地论述了旋转90°工艺的等通道转角挤压模具更具有实用价值,也为今后开发实用型的等通道转角挤压模具的设计提供了理论支撑。     

等通道转角挤压对Mg-Li合金力学性能的影响概述

等通道转角挤压（ECAP）是一种高效率的大塑性变形（SPD）技术,用于生产具有优异性能的超细晶粒（UFG）材料。本文总结了经ECAP加工的各种Mg-Li合金的力学性能、加工参数的影响及其相关机理,为未来提高镁锂合金力学性能提供研究方向与支持。     

等通道转角挤压过程有限元模拟

等通道转角挤压是一种新的制备超细晶粒材料的技术。对于工业纯铝材料的等通道转角挤压过程 ,采用有限元技术进行模拟 ,分析了挤压过程中材料的应力和应变并对不同摩擦条件下的挤压变形情况进行了分析 ,将有限元分析结果与实际网格变形进行了比较 ,两者结果基本吻合     

Ti-6Al-4V钛合金等通道转角挤压有限元模拟

对Ti-6Al-4V钛合金的等通道转角挤压过程进行三维有限元模拟,分析不同的凸模下压速度对等通道转角挤压过程的影响,并结合实际的挤压过程考虑挤压后残留在模具出口通道内的残余试样对挤压下一根试样的影响.结果表明:挤压速度的提高对应力、载荷和温升的影响很大,对应变速率很敏感的钛合金应在有效细化晶粒的前提下降低挤压速度,试验中挤压速度取0.3mm·s-1;残余试样的存在使变形更均匀,但增加了挤压下一根试样时的初始阶段的载荷.     

Equal channel angular deformation process and its neuro-simulation for fine-grained magnesium alloy

Fine-grained structure of as-cast magnesium AM60 alloy was obtained by means of equal channel angular deformation(ECAD) technique. Through analyzing the relationship between the load and the displacement under dif-ferent working conditions, it is demonstrated that employment of back-pressure, multi-passages of deformation, and speed of deformation are the main factors representing ECAD working condition. As for ECAD process, a network composed of nonlinear neuro-element based on error back-propagation learning algorithm is launched to set up a pro-cessing mapping module for dynamic forecasting of load summit under different working conditions. The experimen-tal results show that back-pressure, multi-passages and deforming speed have strong correlation with ECAD pro-cessing characteristics. On the metallographs of AM60 alloy after multi-passes ECAD, a morphology that inter-me-tallic compound Mg17TAl12 precipites on magnesium matrix without discrepancy, which evolves from coarse casting ingot microstructure, is observed. And the grains are refined significantly under accumulated severe shear strain.The study demonstrates feasibility of ECAD by using as-cast magnesium alloy directly, and launches an intelligent neuro-simulation module for quantitative analysis of its process.     

Applications of ultrasonic reflection mode conversion transducers in NDE

Linearly polarized ultrasonic shear waves constitute a privileged tool for investigation in the field of mechanical behavior of materials and metallic structures as well as for the determination of acoustical properties of porous materials. These waves are often generated by a piezoelectric plate vibrating in a direction perpendicular to its thickness. However, this way of production presents some difficulties to obtain a perfectly linear polarization. To palliate to this inconvenience, shear waves can be generated by the mode-conversion of a longitudinal wave.This paper deals with the principle and some applications of two types of mode conversion transducers which permit transmitting and receiving shear waves obtained by reflection mode-conversion. These transducers are made up with straight ultrasonic probes coupled, by contact, to acoustic delay lines (or relays) of different geometry. The first type of transducer uses an acoustic relay or delay line of big length. When it is buried in a recipient containing dry sand, it permits the detection of external mechanical disturbances as vibrations and transient shocks. The second type of transducer, for which the delay line has a reduced length, is used for the detection of material anisotropy and for the study of the polarization direction of conventional normal incidence shear wave transducers.     

A nanoindentation study of serrated flow in bulk metallic glasses

Plastic deformation of two Pd- and two Zr-based bulk metallic glasses (BMGs) is investigated through the use of nanoindentation, which probes mechanical properties at the length scale of shear bands, the carriers of plasticity in such alloys. These materials exhibit serrated flow during nanoindentation, manifested as a stepped load-displacement curve punctuated by discrete bursts of plasticity. These discrete “pop-in” events correspond to the activation of individual shear bands, and the character of serrations is strongly dependent on the indentation loading rate; slower indentation rates promote more conspicuous serrations, and rapid indentations suppress serrated flow. Analysis of the experimental data reveals a critical applied strain rate, above which serrated flow is completely suppressed. Furthermore, careful separation of the plastic and elastic contributions to deformation reveals that, at sufficiently low indentation rates, plastic deformation occurs entirely in discrete events of isolated shear banding, while at the highest rates, deformation is continuous, without any evidence of discrete events at any size scale. All of the present results are consistent with a kinetic limitation for shear bands, where at high rates, a single shear band cannot accommodate the imposed strain rapidly enough, and consequently multiple shear bands must operate simultaneously.     

预置间隙对铝/钢CMT接头强度与失效模式的影响

采用CMT熔钎焊方法,对汽车用镀锌低碳钢板与6061铝合金焊接接头的强度与破坏模式进行了研究.采用不同规格的塞尺控制铝/钢搭接接头的预置间隙,以改变铝/钢异种材料熔钎焊接头的熔合面积.结果表明,通过预置间隙的方法可以有效增大熔合线长度,从而提高焊缝接头的拉剪强度.在间隙为0.5 mm时失效模式发生变化,主要原因是焊趾处铝/钢界面层中的锌元素增大了界面层的脆性.     

高密度聚乙烯管道电熔焊接头基本力学行为

焊接技术是工程塑料管道的主要连接方法，直接影响高密度工程塑料管道的安全应用。研究焊接接头在不同温度下的基本力学行为，具有重要的意义。有鉴与此，采用文中设计的试样形式，对不同温度下高密度聚乙烯燃气管道电熔焊接头的基本力学行为进行了研究。结果表明，常温下，焊缝结合面所能承受的力高于管材和套筒材料；为了得到该接头的剪切强度，必须采用在接头电熔套筒侧开人工槽的方式，来缩短有效结合面的长度；随着温度的降低，要得到接头在不同温度下的剪切强度所对应的焊缝结合面的有效长度缩短，剪切强度呈上升的趋势；随着温度的降低，高密度聚乙烯材料对尖锐缺口的敏感性急剧上升。     

Flow Stress Evaluation in Hot Rolling of Steel

In this paper, an inverse analysis technique is used to obtain the flow curve of materials in a hot rolling finishing mill. This technique is based on minimization of the differences between the experimental and computed values. The flow curves and the friction coefficients at roll/work-piece interface are derived from two different models. Model I is based on simple slab method of analysis. Model II is based on a modified slab method in which the effect of shear stress in calculating the rolling force and torque is taken into account. It is shown that the developed inverse analysis technique is reliable and can simultaneously determine a more accurate flow stress for the material as well as a better estimation for the interface friction factors.     

大型熔体输运式反重力铸造设备的研制

针对采用反重力铸造法生产超大型铝合金铸件过程中存在的设备容量不足、铸型难于起吊以及压力不易控制等技术难题,研制了一种大型熔体输运式反重力铸造设备。该设备以避免吊运超重铸型为设计出发点,采用压力罐上侧设置熔体输运口的结构形式,通过熔体转运式或熔体输送式两种不同工作方式,有效解决了熔体处理及输送与铸型准备工作的并行问题。此外,在控制软件中对PID算法进行了模糊化处理,有效提高了设备的压力控制精度。实践证明,研制的大型熔体输运式反重力铸造设备可满足超大型复杂铝合金铸件（轮廓尺寸可达4m,浇注重量超过1t）反重力铸造的生产要求。     

Recent developments in controlling the architecture for property optimization in Al-based materials

In Al materials the evolution of new material architectures involves new processing techniques to refine and control critical length scales in the structure. New casting technology is changing the capability of clad products; high shear processes are being developed to produce ultra-fine grained alloys and new composite materials providing property enhancements not available in conventional alloys.     

双剪切试样用于短纤维金属基复合材料的蠕变响应研究

利用纵向各向异性有限元模型对双剪切试样的蠕变响应进行了有限元分析，其目的是为了研究双剪切试样用于短纤维金属基复合材料（MMC）蠕变响应研究的可行性。双剪切试样的蠕变响应与试样的取向和材料的各向异性有关。详细的应力分析表明，双剪切试样在受剪切区域能提供稳定的应力状态，该应力接近于施加应力的平均值，并与试样的取向及材料的各向异性不相关，即双剪切试样可以为MMC提供一简单的复杂应力，提出了用双剪切实验结果来推得MMC蠕变性能和各向异性参数的方法，以便可以用双剪切实验来确定那些很难用拉伸和单向压缩实验确定的各向异性材料的参数。     

Catastrophic Forgetting;Catastrophic Interference;Stability;Plasticity;Rehearsal.

This paper reviews the problem of catastrophic forgetting (the loss or disruption of previously learned information when new information is learned) in neural networks, and explores rehearsal mechanisms (the retraining of some of the previously learned information as the new information is added) as a potential solution. We replicate some of the experiments described by Ratcliff (1990), including those relating to a simple 'recency' based rehearsal regime. We then develop further rehearsal regimes which are more effective than recency rehearsal. In particular, 'sweep rehearsal' is very successful at minimizing catastrophic forgetting. One possible limitation of rehearsal in general, however, is that previously learned information may not be available for retraining. We describe a solution to this problem, 'pseudorehearsal', a method which provides the advantages of rehearsal without actually requiring any access to the previously learned information (the original training population) itself. We then suggest an interpretation of these rehearsal mechanisms in the context of a function approximation based account of neural network learning. Both rehearsal and pseudorehearsal may have practical applications, allowing new information to be integrated into an existing network with minimum disruption of old information.