Unified simulation of hardening and softening effects for metals up to failure

Toward accurately simulating both hardening and softening effects for metals up to failure,a new finite strain elastoplastic J2-flow model is proposed with the yield strength therein as a function of the plastic work in the explicit form.With no need to identify any adjustable parameters,the uniaxial stress-strain response predicted from this new model is shown to automatically and accurately match any given data from monotonic uniaxial extension tests of bars.As such,the objectives in three respects are achieved for the first time,i.e.,(i)both the hardening and softening effects up to failure can be simulated in the sense of matching test data with no errors,(ii)the usual tedious implicit procedures toward identifying numerous unknown parameters need not be involved and can be totally bypassed,and(iii)the model applicability can be ensured in a broad sense for various metallic materials with markedly different transition effects from hardening to softening.With the new model,the complete response features of stretched bars and twisted tubes up to failure are studied,including the failure effects of bars under monotonic extension and tubes under monotonic torsion and,furthermore,the fatigue failure effects of bars under cyclic loading.The results show accurate agreement with the uniaxial data,and the results for both the shear stress and the normal stress at the finite torsion display realistic hardening-to-softening transition effects for the first time.

FEM modeling of softened base metal in narrow-gap joint by CMT+P MIX welding procedure

A required finite element method（FEM） model applicable for narrow gap CMT and CMT＋P MIX welding was established based on the interactions between arc,base metal and filler metal.A novel method of simplifying wire feeding pulses and heat input pulses was supposed under the conduction of equivalent input.The method together with composed double-ellipse heat sources was included in the model.The model was employed in the investigation of thermal cycling and the identification of the softened zone of AA7A52 base plates.Low-frequency behavior emerged in the form of low-cooling rate sects,which were not expected under experimental conditions.The softened zone including the quenched zone and averaging zone of the base plate was much wider internal the base plate than that close to the surfaces.The reliability of the predictions in thermal cycling was supported by infrared imaging test results of the thermal cycle process.

Metallurgical characteristics of armour steel welded joints used for combat vehicle construction

Austenitic stainless steel(ASS) and High nickel steel(HNS) welding consumables are being used for welding Q&T steels, as they have higher solubility for hydrogen in austenitic phase, to avoid hydrogen induced cracking(HIC) but they are very expensive. In recent years, the developments of low hydrogen ferritic steel(LHF) consumables that contain no hygroscopic compounds are utilized for welding Q&T steels. Heat affected zone(HAZ) softening is another critical issue during welding of armour grade Q&T steels and it depends on the welding process employed and the weld thermal cycle. In this investigation an attempt has been made to study the influence of welding consumables and welding processes on metallurgical characteristics of armour grade Q&T steel joints by various metallurgical characterization procedures. Shielded metal arc welding(SMAW) and flux cored arc welding(FCAW) processes were used for making welds using ASS, LHF and HNS welding consumables. The joints fabricated by using LHF consumables offered lower degree of HAZ softening and there is no evidence of HIC in the joints fabricated using LHF consumables.

Analysis of damage localization for ductile metal in process of shear band propagation

Distribution of localized damage in shear band can’ t be predicted theoretically based on classical elastoplastic theory. The average damage variable in shear band was considered to be a non-local variable. Based on non-local theory, an analytical expression for the localized damage in strain-softening region of shear band in the process of shear band propagation was presented using boundary condition and symmetry of local damage variable, etc. The results show that dynamic shear softening modulus, dynamic shear strength and shear elastic modulus influence the distribution of the localized damage in shear band. Internal length of ductile metal only governs the thickness of shear band. In the strain-softening region of shear band, the local damage variable along shear band’s tangential and normal directions is non-linear and highly non-uniform. The non-uniformities in the normal and tangential directions of shear band stem from the interactions and interplaying among microstructures and the non-uniform distribution of shear stress, respectively. At the tail of the strain-softening region, the maximum value of local damage variable reaches 1. This means that material at this position fractures completely. At the tip of shear band and upper as well as lower boundaries, no damage occurs. Local damage variable increases as dynamic shear softening modulus decreases or shear elastic modulus increases, leading to difficulty in identification or detection of damage for less ductile metal material at higher strain rates.

Grain boundary stability governs hardening and softening in extremely-fine nanograined metals

Subject Code:E01 Conventional polycrystalline materials become harder with decreasing grain size,following the classical Hall—Petch relationship,i.e.,strength increases reversely proportional to the square root of the grain size.Strengthening occurs due to dislocation pileups at grain boundaries that prevent the dislocations

Age softening phenomenon and microscopic mechanism of Li−B alloy

The age softening phenomenon of the Li−B alloy was investigated by a series of tensile tests under various aging conditions.The results show that the tensile strength of the rolled Li−B alloy decreases with increasing aging time.Higher aging temperature accelerates the softening process,and leads to an early end of aging.A model was proposed to analyze the mechanism of the age softening.The vacancy annihilation is the control factor of age softening.The strength−temperature relationship was estimated by the newly proposed model and a standard heat treatment,annealing at 150℃ for 1 h,was designed to eliminate the age softening of rolled Li−B alloy.

非晶合金中剪切温升的研究进展

区别于传统的晶态金属材料,非晶合金(BMGs)不具备长程有序结构,其塑性变形载体为剪切带。剪切带一旦形成,便很快发展成为裂纹,引发材料的灾难性断裂。剪切不稳定性的研究有助于非晶合金塑性变形机理的理解,并可为非晶合金塑性变形能力的提高提供设计思路。近年来,基于非晶合金的结构特点,科研工作者努力探究非晶合金的剪切不稳定性,主要提出了结构软化诱导的剪切不稳定性和热软化引发的剪切不稳定性两种机制。本文重点总结了非晶合金中剪切温升的研究进展,介绍了测试应变速率、外部约束、试验机刚度和测试温度对剪切温升的影响,指明非晶合金中剪切引入的热远低于玻璃转变温度,暗示热软化对剪切不稳定性的影响是微弱的。本文最后对非晶合金中剪切不稳定性机制的研究方向进行了展望。

6061-T6铝合金DP-MIG焊轧复合成形接头组织与力学性能

为解决6061-T6铝合金焊后热影响区软化导致力学性能下降的问题,基于焊接接头余高大变形强化过时效软化区的思想,采用双轧辊系统对6061-T6薄板铝合金直流双脉冲熔化极气体保护焊(DC double pulse metal inert gas welding,DP-MIG焊)接头进行同步双面轧制,并分析了焊轧复合成形接头组织与力学性能.结果表明,焊缝晶粒受到竖直方向的轧制力发生大塑性变形,内部气孔消失,焊缝区域变宽,邻近焊缝的热影响区受到来自焊缝的挤压力发生协同宏观变形,远离焊缝的过时效软化区组织受到来自焊缝处的压应力,从沿垂直焊缝方向产生部分协同形变强化;接头的硬度明显提升,过时效软化区的硬度由60~70 HV提升至80~90 HV,过时效软化区的拉伸性能也有所提升,当焊接速度为800 mm/min时,抗拉强度最佳可达到母材的83.6%,相较于焊接接头提升19.4%,进而实现了对过时效软化区的强化效果.

结晶工艺在废水处理领域的应用与发展

结晶水处理工艺具有占地面积小、不产生污泥、可回收资源等优势,近年在国内外备受关注,随着理论和工艺技术的不断完善,其应用范围逐渐扩大。到目前为止,国外已将结晶水处理工艺成功应用于众多实际工程,但国内对其的研究仍局限于实验室阶段。基于上述原因,本文较全面地综述了结晶工艺在废水处理领域的发展现状,并重点对结晶工艺在磷酸盐、重金属的去除与回收以及工业防垢领域的理论研究和工程应用进展进行讨论,同时探讨了目前结晶工艺存在的几个问题以及下一步的发展方向。

PCBN刀具的发展性能及应用

从材料性能角度介绍PCBN刀具的特点及应用领域 。

Cu-Al异种金属超声焊接过程模拟

利用有限元方法建立了Cu-Al超声焊三维热—力耦合模型.热源包括摩擦热和塑性变形热,其热流密度与焊接不同阶段材料的振幅相关.焊接过程超声变软影响温度场分布、应力、应变场和齿的嵌入,模拟合理考虑了铜、铝超声变软的数学模型.模拟结果表明,铜和铝的块体温度均低于熔点,最高温度出现在焊头与铜板的接触面中心;Cu-Al连接界面最高温度出现在焊接区域中心处.同时也模拟了齿的嵌入,焊接过程中焊头齿完全嵌入铜表面,但底座齿并未完全嵌入铝中.与热电偶测温和焊接截面形貌对比验证表明,模拟结果与实际基本吻合,较好地模拟了超声变软、热和力三者之间的作用.

金属薄板包辛格效应的试验研究

针对薄板包辛格效应力学参数难于测量的问题,在试验中设计并加工了一套夹具来避免材料反向加载时的弯曲变形。通过薄板拉伸压缩循环加载试验,获取了材料在不同预应变下的包辛格效应曲线,计算了包辛格效应的力学参数,分析了试验中出现的反向流变曲线圆化、反向屈服应力减小、永久软化等现象。

精密硬态切削过程金属软化效应与表面塑性侧流的研究

由于普通CNC车床上实施精密硬态车削仍存在被加工表面机械性能不稳定的因素 ,从而导致该工艺的实施仍面临挑战且不易实现良好的加工表面质量。本文通过实验研究了精密硬态车削淬硬GCr15轴承钢 ,讨论了PCBN刀具后刀面磨损对已加工表面材料塑性侧流的作用。实验结果表明 ,由于刀具后刀面磨损量增大导致高的切削温度从而使金属软化是决定材料塑性侧流形式的决定性因素 ,当后刀面磨损量超过 0 2mm时 ,发生严重塑性流动 ,已加工表面与原始表面微观组织无明显变化 。

PCBN刀具硬态切削过程中金属软化效应的研究

通过分析PCBN刀具切削淬硬GCr15轴承钢时,工件硬度和切削速度对切削力和切削温度的影响,给出了产生金属软化效应的条件。对比金属软化效应产生前后已加工表面粗糙度。硬化层深度和表层基体组织,得出了金属软化效应对已加工表面质量不存在负面影响的结论。

弹丸膛内运动过程中弹带表层热软化机理分析

为研究弹带表层金属在急剧升温条件下的热软化机理与组织演变过程,基于再结晶动力学理论分析表层金属发生再结晶及晶粒长大的可能性,研究了动态再结晶对弹带表层热软化的影响,采用金相显微镜和扫描电镜(SEM)观察并分析了回收砂弹的弹带细观与微观组织.实验中发现了存在于弹带表面薄层的再结晶组织,且部分晶粒未显著长大,这表明再结晶是弹带表层热软化的微观机理.

流态化诱导结晶沉积法处理无机废水的研究进展

介绍了流态化诱导结晶沉积法的原理,综述了该方法在无机废水处理中的应用研究,着重介绍其在重金属离子、磷酸盐、氟离子以及水的软化处理方面的研究进展。在重金属废水处理上的实际研究工作表明,该技术具有反应快、占地面积小、无污泥产生等优点,具有广泛应用前景。

诱导沉淀结晶技术在水处理中的应用

介绍了诱导沉淀结晶技术的原理,综述了该技术在水质软化及去除废水中的重金属离子、磷酸盐、氟离子方面的应用情况。实践表明,该技术结晶速度快、反应器容积小、固液分离快,具有较好的应用前景。

PCBN刀具干湿切削淬硬钢对比试验研究

通过PCBN刀具干、湿切削淬硬轴承钢GCr15对比试验,对两种条件下的切削力随刀具磨损的变化、刀具寿命及被加工工件表面粗糙度进行了对比。试验表明:干切削状态下金属软化效应占主导地位,湿切削可获得更好的表面粗糙度,切削液在刀具磨损后期开始发挥作用。

富含Bi_2O_3微晶封接玻璃的制备及性能分析

制备富含Bi_2O_3的微晶封接玻璃,分别用XRD、膨胀仪、光学显微镜等仪器对样品进行表征。结果表明,富含Bi_2O_3系统可形成玻璃,经造粒成型、热处理后可微晶化。微晶玻璃的转变温度T_g和软化温度T_f分别为438℃和475℃,在420℃时电阻率达6.4×10~7Ω·m,在30～300℃膨胀系数α达8.82×10^(-6)/℃,可取代金属封接用的传统含铅封接玻璃,并且满足环保对无铅的要求。

不锈钢管道焊缝金属240℃循环变形行为的试验研究

基于增量步和成组法低周疲劳试验，研究了 1Cr18Ni9Ti不锈钢管道焊缝金属在 240℃下的循环变形行为。结果表明，材料为循环软化材料，具有轻微 Non masing行为特征。相同总应变范围的循环应力应变 (CSS)行为存在很大分散性，意味着任何外载，即使恒幅载荷也将导入一个随机应变载荷史，现有确定性方法可能导致偏于危险的设计分析，有必要引入概率方法表征材料的循环变形行为。基于验证的应力幅统计分布--正态分布，建议了概率 Ramberg Osgood CSS曲线的描述方法。文中试验结果分析验证了方法的有效性。