2195铝锂合金平面应变热压缩行为

2195铝锂合金被认为是航空航天领域的理想结构材料,但对其热变形行为的研究却相对较少。本工作通过平面应变热压缩试验,研究了2195铝锂合金的热变形行为,变形温度为400~500℃,应变速率为0.01~10 s~(-1)。研究表明,材料变形呈稳态流变特征,随变形温度增高和应变速率降低,流变应力逐渐减小,合金具有正应变速率敏感性。建立了2195铝锂合金材料本构方程,其激活能值为214.937 k J/mol。通过分析加工图,得到材料的适宜加工区为应变速率接近0.01 s~(-1),温度为475~500℃。最后通过分析动态软化过程中的应力规律,得到了材料软化机制判定方程。     

高强可焊2195铝-锂合金热压缩变形的流变应力

在Gleeble-1500热模拟实验机上,采用高温等温压缩,0.001～10 s-1,变形温度为360～520℃,对2195铝-锂合金在高温压缩变形中的流变应力行为进行了研究,分析了其高温变形的物理本质.结果表明:在应变速率为1 s-1(变形温度为520℃)和应变速率为0.1、0.01、0.001 s-1(变形温度为360～520℃)时,2195铝-锂合金热压缩变形流变应力出现了明显的峰值应力,表现为连续动态再结晶特征;在其它变形条件下存在较为明显的稳态流变特征;可采用Zener-Hollomon参数的双曲正弦函数来描述2195铝-锂合金高温变形时的流变应力行为;在获得的流变应力σ解析表达式中,A、α和n值分别为2.569×1017 s-1、0.012 48 MPa-1和5.94;热变形激活能Q为250.45 kJ/mol.     

2195铝锂合金多道次热变形流变应力的模拟研究

采用G1eeble-1500热模拟实验机，对2195铝锂合金变形温度为360～520℃，应变速率为0．001-1．0s^-1的单道次热压缩及变形温度为320℃和360℃，应变速率为0．1s^-1，道次间隔时间30-180s的双道次热压缩的流变应力及静态软化规律进行了模拟研究。通过对幂指数应力函数中系数A和β与应变关系的分析，以及采用平均软化法考虑前一道次变形的残余应变对后一道次变形的影响，建立了2195铝锂合金多道次热变形的流变应力方程。     

2195铝锂合金的热变形行为

在Gleeble-1500热模拟机上实施热压缩试验,研究2195铝锂合金在变形温度360~500 ℃,应变速率0.1~10 s^-1时的热变形行为,并通过OM和EBSD研究了热变形中微观组织的演变。基于动态材料模型理论及Zener-Holloman参数,构建了2195铝锂合金的应变量为50%时的加工图及本构方程。结果表明,流变应力随变形温度降低或者应变速率的增加而提高,高温软化机制包括动态回复与动态再结晶。利用加工图及显微组织分析确定了试验参数范围内热变形过程的最佳工艺参数为480 ℃/10 s^-1;发现失稳区形变组织和再结晶组织呈层状交替分布,且随着变形温度降低,形变组织层厚度增加;稳定区的微观组织具有明显的动态再结晶特征,形变组织基本消失。     

喷射成形2195铝锂合金的热变形行为和热加工图

在变形温度300～460℃和应变速率0.01～5 s-1下,采用Gleeble3500热模拟机对喷射成形2195铝锂合金进行了高温热压缩试验。研究了该合金热变形过程中高温流动行为,利用线性回归方法拟合了峰值应力、应变速率以及变形温度间的本构关系,得到了变形激活能。结果表明:合金的变形激活能为155.049 kJ/mol,有效热加工窗口为410～460℃和0.01～0.5 s-1。     

Al-17.5Si-4Cu-0.5Mg合金在热压缩变形中的流变应力行为

采用Gleebe-1500D热压缩模拟试验机在变形温度350℃~500℃、应变速率0.001s-1~5s-1的条件下对Al-17.5Si-4Cu-0.5Mg合金进行热压缩实验,研究该合金在热塑性变形条件下的流变应力行为,并建立热变形时的本构方程。研究结果表明,随着应变速率的增加、变形温度的降低,合金的流变应力增加,为正应变速率敏感性材料;采用Znenr-Hollomon参数双曲正弦形式对合金高温塑性变形时的流变应力行为进行描述,流变应力σ解析表达式中材料常数A,σ,n分别为1.81×1019s-1,0.024MPa-1和6.37,合金的平均热变形激活能Q为308.61kJ·mol-1。     

一种新型水平连铸Al-Mn-Si-X合金热压缩变形流变应力

采用圆柱体在Gleeble-1500热模拟机上进行热压缩实验,对一种新型水平连铸Al-Mn-Si-X合金热变形流变应力行为进行研究,变形温度为350℃~500℃,应变速率为0.01s-1~10s-1。结果表明,流变应力先随应变的增大而增大,达到峰值后则逐渐减小并趋于平稳,表现出流变软化特征;而应力峰值是随着温度的升高而减小,随应变速率的增大而增大。应用包含Zener-Hollomon参数的Arrhenius双曲正弦关系描述合金热压缩变形流变应力,其变形激活能Q=159.2kJ/mol。     

ZA27合金的热变形及加工图

采用Gleeble-1500热模拟实验机进行热压缩试验,研究ZA27合金的热变形行为,在变形温度为200~350℃、应变速率为0.01~5 s-1、工程应变为60%,基于Murty准则,建立ZA27合金的加工图。结果表明:流变应力随变形温度的升高而减小,随应变速率的提高而增大;在变形温度为200~210℃、应变速率为0.01~0.1 s-1和变形温度为250~350℃、应变速率为1~5 s-1的2个区域内易产生流变失稳现象;动态再结晶是导致流变软化及稳态流变的主要原因,ZA27合金的安全热加工区域的变形温度在250~350℃之间、应变速率在0.1~1 s-1之间。     

Cu-0.8Mg合金高温变形行为与机制

利用Gleeble-1500D热模拟试验机对Cu-0.8Mg合金进行热变形试验,变形温度为500~850℃、应变速率为0.001~10 s-1,研究不同试验条件下合金流变应力的变化规律,分析合金的流变应力、应变速率和变形温度之间的关系,对合金的热加工图进行研究。结果表明:合金在热变形过程中,其流变应力曲线表现出典型的加工硬化、动态回复和再结晶特征,随着变形温度的升高和应变速率的降低,其流变应力和峰值应力也随之降低;合金热变形过程中的激活能为177.88 k J/mol,构建了合金的本构方程;合金在热变形过程中的最优加工参数为:变形温度为700~800℃、应变速率为0.01~0.1 s-1。     

基于应变补偿和修正项的新型铝锂合金温热变形本构模型

采用UTM5000电子万能拉伸试验机,在变形温度573~648K和应变速率0.001~0.1s-1条件下对2060-T8铝锂合金进行等温恒应变速率拉伸试验,得到其在变形过程中的真应力-真应变曲线,建立了基于应变补偿和修正项的温热变形本构方程。通过扫描电子显微镜(SEM)分析拉伸断口,对2060-T8铝锂合金的温热变形行为进行研究。结果表明：2060-T8铝锂合金对变形温度和应变速率具有较高的敏感性,流变应力曲线呈现出应变硬化和流变软化的特征,随着变形温度的升高和应变速率的降低,稳态流变特征逐渐消失,其在温热变形条件下的断裂形式为韧性断裂。修正的本构模型与实验值吻合度较高,可以为2060-T8铝锂合金温热变形的有限元模拟提供前提条件。     

Laser compression of nanocrystalline tantalum

Nanocrystalline tantalum (grain size ～70 nm) prepared by severe plastic deformation (high-pressure torsion) from monocrystalline [1 0 0] stock was subjected to shock compression generated by high-energy laser (～350–850 J), creating pressure pulses with initial duration of ～3 ns and amplitudes of up to ～145 GPa. The laser beam, with a spot radius of ～1 mm, created a crater of significant depth (～135 μm). Transmission electron microscopy revealed few dislocations within the grains and an absence of twins at the highest shock pressure, in contrast with monocrystalline tantalum. Hardness measurements were conducted and show a rise as the energy deposition surface is approached, evidence of shock-induced defects. The grain size was found to increase at a distance of 100 μm from the energy deposition surface as a result of thermally induced grain growth. The experimentally measured dislocation densities are compared with predictions using analyses based on physically based constitutive models, and the similarities and differences are discussed in terms of the mechanisms of defect generation. A constitutive model for the onset of twinning, based on a critical shear stress level, is applied to the shock compression configuration. The predicted threshold pressure at which the deviatoric component of stress for slip exceeds the one for twinning is calculated and it is shown that it is increased from ～24 GPa for the monocrystalline to ～150 GPa for the nanocrystalline tantalum (above the range of the present experiments). Calculations using the Hu–Rath analysis show that grain growth induced by the post shock-induced temperature rise is consistent with the experimental results: grains grow from 70 to 800 nm within the post-shock cooling regime when subjected to a laser pulse with energy of 684 J.     

Laser compression of monocrystalline tantalum

Monocrystalline tantalum with orientations [1 0 0] and [1 1 1] was subjected to laser-driven compression at energies of 350–684 J, generating shock amplitudes varying from 10 to 110 GPa. A stagnating reservoir driven by a laser beam with a spot radius of ～800 μm created a crater of significant depth (～80 to ～200 μm) on the drive side of the Ta sample. The defects generated by the laser pulse were characterized by transmission and scanning electron microscopy, and are composed of dislocations at low pressures, and mechanical twins and a displacive phase transformation at higher pressures. The defect substructure is a function of distance from the energy deposition surface and correlates directly with the pressure. Directly under the bottom of the crater is an isentropic layer, approximately 40 μm thick, which shows few deformation markings. Lattice rotation was observed immediately beneath this layer. Further below this regime, a high density of twins and dislocations was observed. As the shock amplitude decayed to below ～40 GPa, the incidence of twinning decreased dramatically, suggesting a critical threshold pressure. The twinning planes were primarily {1 1 2}, although some {1 2 3} twins were also observed. Body-centered cubic to hexagonal close-packed pressure induced-transformation was observed at high pressures (～68 GPa).The experimentally measured dislocation densities and threshold stress for twinning are compared with predictions using analyses based on the constitutive response, and the similarities and differences are discussed in terms of the mechanisms of defect generation.     

Improved coatings by isodynamic compression

Selected platelet-powder mixtures that were fabricated from spin-cast ribbon were deposited using arc plasma spraying at various velocities (from subsonic to supersonic) to form rapidly solidified coatings with mechanical-type bonding on a low alloy steel substrate. After deposition at ambient temperatures these sprayed coatings were then subjected to transformation treatments at two elevated temperatures using hot isodynamic compression (HIC) techniques. Efforts aimed at using transformation to achieve greater densities, higher strength and metallurgically bonded coatings are described for an Ni- Mo-Fe-B alloy. HIC and hot isostatic pressing approaches were compared statistically and the significant experimental variables were characterized using Vickers microhardness or electron metallography (scanning electron metallography, electron diffraction spectroscopy and energy-dispersive X-ray microanalysis).     

Tube branching by asymmetric compression beading

This paper draws on the capability of producing out-of-plane instability waves in thin-walled tubes subjected to axial loading. In terms of deformation mechanics, the aim is to identify the operative parameters, to diagnose possible sources of failure and to understand the route for selecting the most appropriate operative conditions for obtaining sound asymmetric compression beads created by out-of-plane instability waves.In terms of industrial technology, the goal is to apply the fundamentals of asymmetric compression beading for developing an innovative tube branching process to join tubes that is capable of eliminating welding and brazing in standard or user made types of connections.The presentation includes details on tool design, independent determination of the mechanical properties of the tubes, finite element modelling and experimentation under laboratory conditions. Results and observations show that tube branching by means of asymmetric compression beading is a flexible and cost-effective technology for joining tubes in standard tee connections as well as in special purpose applications involving custom sizes and dissimilar materials.     

Warm compression tests of aluminum alloy

Compression tests of aluminum alloy were experimentally investigated at specified temperatures ranging from 30 ‡C (room temperature) to 250 ‡C under a constant strain rate of 0.2 X 10^-3/s using powdered graphite as a lubricant throughout the tests. The effective stress method is found to show a significant fall in stress values beyond the barreling point, indicating a serious shortcoming over the barrel correction factor method within the tested temperature range. The compression curves obtained using the barrel correction factor method and the Bridgman remachining technique (no barreling allowed during the test) are found to have close values, even at higher temperatures. The true-stress versus true-strain curves and the barrel sizes obtained follow empirical power laws, even at higher test temperatures.     

Cu-Ni-Si-Co-Zr合金的热压缩行为

利用Gleeble-1500数控动态力学模拟试验机,对Cu-1.56Ni-0.65Si-1.12Co-0.05Zr合金进行热压缩试验,应变速率0.002～ 10 s-1,变形温度为600～900℃,总变形量为50％.结果 表明:在热压缩过程中,Cu-1.56Ni-0.65Si-1.12Co-0.05Zr合金的流变应力随着变形温度的降低和应变速率的增加而升高,应力在达到峰值之后不再发生明显变化,高温、低应变速率的变形条件更有利于合金的动态再结晶.显微组织观察表明合金的动态再结晶机制为连续动态再结晶和不连续动态再结晶共同作用,析出相主要钉扎在位错和晶界处,能够阻碍位错的运动从而增强基体.     

Higher-order gradient effects in micropillar compression

Deformation behavior in single-slip oriented micropillars subjected to compressive loading is analyzed using a three-dimensional finite element method incorporating a higher-order gradient crystal plasticity theory in which the spatial gradients of the geometrically necessary dislocation densities are assumed to give rise to backstress that impedes or promotes dislocation motion, depending on its sign. In experimental studies, it has been frequently observed that the gauge portion of micropillar samples is split into a shear zone and two nearly unstrained dead zones under compressive loading. Here it is shown that effects of a higher-order gradient are essential for the formation of this particular deformation mode.     

生物质压缩成型工艺参数

以玉米秸秆、锯末、棉杆为原料,进行改进后热压缩成型工艺试验,通过松弛密度这一指标得到最佳工艺参数范围;并利用OLYMPUS BX51-金相显微镜观察玉米秸秆成型后颗粒的微观形貌,从微观角度研究成型工艺参数与成型块微观结构之间的关系。试验结果表明,最佳参数范围是,粒度0～2mm、含水率15%～23%、温度130℃～150℃、长径比5.28;以玉米秸秆为例,微观形貌分析得出的最佳工艺参数范围与试验得出的基本一致。