Bi系高温超导带材中的塑性微成形技术

随着塑性成形技术向着"极端"方向发展,塑性微成形方法的研究成为学术界和工业界中的前沿领域之一,Bi系高温超导带材作为目前唯一的一种已实现工业化生产的超导材料,它的加工过程就是典型的多芯多场塑性微成形过程;文章从实验和计算方面介绍该塑性微成形的特征,涉及超导粉体材料的本构行为、拉拔和轧制的加工过程,并对Bi系高温超导带材中的塑性微成形的研究现状进行评述.     

多芯铋系超导材料轧制过程中的变形分布研究

塑性变形是铋系超导带材加工的必要手段,多芯超导体在成形过程中的变形分布对成形质量具有重要影响。文章对多芯超导带材在轧制过程中不同加工道次变形横截面的形貌进行处理,给出了超导带材多芯的变形形貌及其变化规律,涉及各超导芯在轧制过程中的轧制方向和横向的应变分布;并结合塑性加工的变形理论对分布规律进行分析。取得的研究结果可以为开展铋系高温超导带材的数值模拟提供实验基础,并为改进超导带材生产工艺、提高带材性能提供重要参考。     

铋系超导线材拉拔过程中的粉体变形行为

单芯和多芯拉拔工艺是铋系高温超导带材(Bi-2223/Ag)制备过程中必不可少的重要环节。为研究超导粉体在拉拔过程中的变形行为,对单芯超导线材的拉拔工艺进行理论分析,建立了工艺参数与粉体密度之间的关系,并通过试验验证其合理性。同时,采用大截面减缩率单芯拉拔工艺来实现铋系超导线材的节能高效成形。对1.86mm61芯Bi-2223/Ag线材进行显微硬度测试,并结合数值模拟进行三道次拉拔加工,分析认为:在加工过程中,各超导芯之间及芯内部均存在密度分布不均匀性,且各层粉体密度随拔制过程不断波动,将不利于后续轧制工艺。为此,提出渐进跑道形三道次拉拔工艺,并应用于铋系超导带材的后期拉拔加工,可提高最终带材的临界电流密度10.1%。     

铋系高温超导带材的先进成形技术

铋系(Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223/Ag)高温超导带材的传统制备工艺难以实现其性能提升。为了改善带材综合性能,从超导带材成形过程中的缺陷及问题出发,介绍了高强度金属外包套材料、多芯线材跑道模拉拔和热等静压三种先进技术。提出了可应用于实际工程的方案,以提高铋系高温超导带材的力学性能。     

超薄板料微弯曲成形过程中尺度效应的数值研究

近年来,大量的实验研究表明,金属材料在微观尺度下的力学行为不同于经典连续介质力学的预测.由于微米或亚微米量级金属构件在MEMS系统中的广泛应用,这对微构件加工成形过程中工艺参数选取的理论依据提出了迫切的要求.应变梯度塑性理论能够较好地反映材料的尺度效应,文章采用该理论,对超薄板料弯曲成形过程中的尺度效应现象进行了数值研究.计算结果表明,对于微米量级的薄板,其力学行为受板厚的影响很大.当板厚接近于材料特征长度时,其挠度曲线、应力应变分布与经典塑性理论预测有很大的区别;当板厚远大于材料特征长度时,梯度理论预测结果与经典塑性理论预测一致.因此,在超薄板料的塑性成形过程中,工艺参数的选取需要考虑其尺度效应才能达到设计要求.     

金属微成形中尺度效应的探讨

微成形研究领域中一些概念和称谓的定义存在理论根据不足或不够清晰的问题,因此重新给出了微成形中微尺度效应的定义.在相似理论的基础上,提出了相似差和相似精度的概念,作为微尺度效应的量化评价指标.提出了微尺度效应的新的分类方法,即将微尺度效应分为材料本征微尺度效应和工艺条件微尺度效应,利用已有的实验数据,举例说明了以上量化方法的应用.最后,分析了对微尺度效应进行分类和量化的工程意义.     

微成形技术在塑性加工中的应用研究

快速增长的MEMS市场为微系统工程提供了广阔的发展和应用空间,对微金属零件的需求也日益增多,结合微成形技术和传统塑性加工方法,可以提高微零件精度和生产率,实现批量生产,但是也对微成形技术和塑性加工提出了新要求.尺度效应的影响使得微塑性成形成为一个全新的领域,传统的塑性加工理论无法继续使用.本文在介绍微成形概念的基础上、对其在塑性加工中的应用、进展以及研究方法进行了分析总结,并阐述了相关的关键技术.     

尺寸效应对黄铜镦粗微成形影响规律的研究

微成形是微细加工技术群体中的一项技术。由于尺寸效应的影响，微成形比传统的塑性成形更为复杂。零件微型化导致的尺寸效应，使传统的塑性加工工艺不能直接应用于塑性微成形。在微成形过程中材料不能再被看作是均质的，且材料的流动变形规律由于尺寸效应的影响而表现得非常敏感。以H62黄铜（直径0．5～4．0mm）进行室温镦粗实验的结果表明：由于尺寸效应的影响，随着试样尺寸等比例减小，材料的流动应力呈明显的减小趋势，采用表面层模型对实验结果给予了详细深入的理论探讨。     

微细塑性成形工艺实现及最新研究进展

阐述了微细塑性成形工艺的基本概念,从材料性能、摩擦、工艺、模具加工及成形装置等方面分析了微小尺度零件成形加工的技术特点,指出其中的关键问题是微细尺度下材料性能的变化规律,研究了微细尺度摩擦规律以及微细模具和微成形装置的设计制造。指出了微细成形的研究要充分利用现有的技术,不断深入研究,最终可实现微细成形工艺的实际应用。     

温度对黄铜微成形镦粗的影响规律研究

微成形是微细加工技术群体中的一项技术.由于尺寸效应的影响,微成形比传统的塑性成形更为复杂.研究表明：零件微型化导致了尺寸效应,使传统的塑性加工工艺不能直接应用于微成形,材料不能再看作是均质的,而提高温度进行加工有可能抵消掉非均质材料特性.本文通过H62黄铜（直径从4mm到0.5mm）的加热镦粗实验,其结果与冷成形时进行比较,表明温度影响成形结果,加热使得材料流动趋向于均匀,流动应力降低,因此微成形能通过提高加工温度改进成形结果.     

Effects of Post-weld Heat Treatment on Microstructure,Mechanical Properties and the Role of Weld Reinforcement in 2219 Aluminum Alloy TIG-Welded Joints

In as-welded state, each region of 2219 aluminum alloy TIG-welded joint shows diff erent microstructure and microhardness due to the diff erent welding heat cycles and the resulting evolution of second phases. After the post-weld heat treatment, both the amount and the size of the eutectic structure or θ phases decreased. Correspondingly, both the Cu content in α-Al matrix and the microhardness increased to a similar level in each region of the joint, and the tensile strength of the entire joint was greatly improved. Post-weld heat treatment played the role of solid solution strengthening and aging strengthening. After the post-weld heat treatment, the weld performance became similar to other regions, but weld reinforcements lost their reinforcing eff ect on the weld and their existence was more of an adverse eff ect. The joint without weld reinforcements after the post-weld heat treatment had the optimal tensile properties, and the specimens randomly crack in the weld zone.     

The First Phase of the SRIF Industrialized Base in ＂West Area＂ Has Been Put into Operation

After nearly two years＇ tense construction, the first phase of industrialized base of Shenyang Research Institute of Foundry （SRIF）, located at the Tiexi Casting and Forging Industrial Park in the west of Tiexi District, has now been completed and formally put into operation.     

Thermodynamics Study on the Decomposition of Chromite with KOH

Institute of Process Engineering, Chinese Academy of Sciences, China, has proposed a method for oxidative leaching of chromite with potassium hydroxide. Understanding the mechanism of chromite decomposition, especially in the potassium hydroxide fusion, is important for the optimization of the operating parameters of the oxidative leaching process. A traditional thermodynamic method is proposed and the thermal decomposition and the reaction decomposition during the oxidative leaching of chromite with KOH and oxygen is discussed, which suggests that chromite is mainly destroyed by reactions with KOH and oxygen. Meanwhile, equilibrium of the main reactions of the above process was calculated at different temperatures and oxygen partial pressures. The stable zones of productions, namely, K2CrO4 and Fe2O3, increase with the decrease of temperature, which indicates that higher temperature is not beneficial to thermodynamic reactions. In addition, a comparison of the general alkali methods is carried out, and it is concluded that the KOH leaching process is thermodynamically superior to the conventional chromate production process.     

Influence of Heat Treatment on Damping Behaviour of the Magnesium Wrought Alloy AZ61

The effect of isochronal heat treatments for 1h on variation of damping, hardness and microstructural change of the magnesium wrought alloy AZ61 was investigated. Damping and hardness behaviour could be attributed to the evolution of precipitation process. The influence of precipitation on damping behaviour was explained in the framework of the dislocation string model of Granato and Lücke.     

Effect of Rare Earth Doping on Impedance,Modulus and Conductivity Properties of Multiferroic Composites:0.5(BiLa_xFe_(1-x)O_3)–0.5(PbTiO_3)

The Lanthanum-doped bismuth ferrite–lead titanate compositions of 0.5(Bi LaxFe1-xO3)–0.5(Pb Ti O3)(x = 0.05,0.10,0.15,0.20)(BLxF1-x-PT) were prepared by mixed oxide method.Structural characterization was performed by X-ray diffraction and shows a tetragonal structure at room temperature.The lattice parameter c/a ratio decreases with increasing of La(x = 0.05–0.20) concentration of the composites.The effect of charge carrier/ion hopping mechanism,conductivity,relaxation process and impedance parameters was studied using an impedance analyzer in a wide frequency range(102–106Hz) at different temperatures.The nature of Nyquist plot confirms the presence of bulk effects only,and non-Debye type of relaxation processes occurs in the composites.The electrical modulus exhibits an important role of the hopping mechanism in the electrical transport process of the materials.The ac conductivity and dc conductivity of the materials were studied,and the activation energy found to be 0.81,0.77,0.76 and 0.74 e V for all compositions of x = 0.05–0.20 at different temperatures(200–300 °C).     

Quantitative Analysis of the Crystallographic Orientation Relationship Between the Martensite and Austenite in Quenching–Partitioning–Tempering Steels

The orientation relationships(ORs)between the martensite and the retained austenite in low-and medium-carbon steels after quenching–partitioning–tempering process were studied in this work.The ORs in the studied steels are identified by selected-area electron diffraction(SAED)as either K–S or N–W ORs.Meanwhile,the ORs were also studied based on numerical fitting of electron backscatter diffraction data method suggested by Miyamoto.The simulated K–S and N–W ORs in the low-index directions generally do not well coincide with the experimental pole figure,which may be attributed to both the orientation spread from the ideal variant orientations and high symmetry of the low-index directions.However,the simulated results coincide well with experimental pole figures in the high-index directions{123}_(bcc).A modified method with simplicity based on Miyamoto’s work was proposed.The results indicate that the ORs determined by modified method are similar to those determined by Miyamoto’method,that is,the OR is near K–S OR for the low-carbon Q–P–T steel,and with the increase of carbon content,the OR is closer to N–W OR in medium-carbon Q–P–T steel.     

Growth Kinetics of Single Inclusion Particle in Molten Melts

On the basis of the single-particle framework, a new theory on inclusion growth in metallurgical melts is developed to study the kinetics of inclusion growth on account of reaction and collision. The studies show that the early growth of inclusion depends on reaction growth and Brawnian motion collision, and where the former is decisive, the late growth depends on turbulence collision and Stokes＇ collision, and where the former is dominant; collision growth is very quick during the smelting process, lessened in the refining process, but nearly negligible in the continuous casting process.     

Theoretical Analysis for the Motion and Temperature of Melt Droplets in Spray Degassing Process

The motion of melt droplets in spray degassing process was analyzed theoretically. The height of the treatment tank in spray degassing process could be determined by the results of theoretical calculation of motion of melt droplets. To know whether the melt droplets would solidify during spraying process, the balance temperature of melt droplets was also theoretically analyzed. Then proof experiments for theoretical results about temperature of melt droplets were carried. In comparison, the experimental results were nearly similar to the calculation results.     

Using Finite Element and Contour Method to Evaluate Residual Stress in Thick Ti-6Al-4V Alloy Welded by Electron Beam Welding

This work was to reveal the residual stress profile in electron beam welded Ti-6Al-4V alloy plates(50 mm thick) by using finite element and contour measurement methods.A three-dimensional finite element model of 50-mmthick titanium component was proposed,in which a column–cone combined heat source model was used to simulate the temperature field and a thermo-elastic–plastic model to analyze residual stress in a weld joint based on ABAQUS software.Considering the uncertainty of welding simulation,the computation was calibrated by experimental data of contour measurement method.Both test and simulated results show that residual stresses on the surface and inside the weld zone are significantly different and present a narrow and large gradient feature in the weld joint.The peak tensile stress exceeds the yield strength of base materials inside weld,which are distinctly different from residual stress of the thin Ti-6Al-4V alloy plates presented in references before.     

Microstructures and Tensile Properties of Ultrafine-Grained Ni–(1–3.5) wt% SiC_(NP) Composites Prepared by a Powder Metallurgy Route

Silicon carbide nanoparticle-reinforced nickel-based composites(Ni–Si CNP),with a Si CNPcontent ranged from1 to 3.5 wt%,were prepared using mechanical alloying and spark plasma sintering.In addition,unreinforced pure nickel samples were also prepared for comparative purposes.To characterize the microstructural properties of both the unreinforced pure nickel and the Ni–Si CNPcomposites transmission electron microscopy(TEM) was used,while their mechanical behavior was investigated using the Vickers pyramid method for hardness measurements and a universal tensile testing machine for tensile tests.TEM results showed an array of dislocation lines decorated in the sintered pure nickel sample,whereas,for the Ni–Si CNPcomposites,the presence of nano-dispersed Si CNPand twinning crystals was observed.These homogeneously distributed Si CNPwere found located either within the matrix,between twins or on grain boundaries.For the Ni–Si CNPcomposites,coerced coarsening of the Si CNPassembly occurred with increasing Si CNPcontent.Furthermore,the grain sizes of the Ni–Si CNPcomposites were much finer than that of the unreinforced pure nickel,which was considered to be due to the composite ball milling process.In all cases,the Ni–Si CNPcomposites showed higher strengths and hardness values than the unreinforced pure nickel,likely due to a combination of dispersion strengthening(Orowan effects) and particle strengthening(Hall–Petch effects).For the Ni–Si CNPcomposites,the strength increased initially and then decreased as a function of Si CNPcontent,whereas their elongation percentages decreased linearly.Compared to all materials tested,the Ni–Si CNPcomposite containing 1.5% Si C was found more superior considering both their strength and plastic properties.