Continuous observation of twinning and dynamic recrystallization in ZM21 magnesium alloy tubes during locally heated dieless drawing

Compared to cold drawing,dieless drawing has shown great potential for manufacturing biodegradable Mg alloy microtubes due to the large reduction in area acquired in a single pass.However,owing to the local heating and local deformation,the deformation mechanism during dieless drawing is not clear,and thus causing difficulties in controlling the microstructure of dieless drawn tubes.For the purpose of acquiring a desired microstructure.in this study the deformation mechanism of ZM21 Mg alloy tube was clarified by conducting continuous observation of the microstructural evolution during dieless drawing.The results show that both SRX and DRX occurred during dieless drawing.SRX occurred before the plastic deformation to soften dieless drawn tubes.With increase of feeding speed,the deformation mechanism changed accordingly:(1) At the low-speed of 0.02 mm/s,the deformation mechanism was dominated by twin-slip sliding,during which {10-12} tension twins were generated inside grains to accommodate the plastic deformation by changing the crystal orientation.(2) At the intermediate-speed of 2 mm/s,a twin-DRX process related to {10-12} tension twin was observed,which was characterized by the generation of abundant {10-12} tension twins and the evolution of misorientation angle of {10-12} tension twins.Moreover,the transformation from twin-DRX to CDRX can be observed at the late stage of plastic deformation,which was attributed to the inhomogeneous conditions of dieless drawing.(3) At the high-speed of 5 mm/s,a CDRX process was observed,during which grain boundary sliding and grain tilting were observed,in addition to the gradual rotation of subgrains.These results show that during dieless drawing,DRX is not only a temperature-dependent phenomenon,but also influenced by the variation of feeding speed.

Microstructure and performance of biodegradable magnesium alloy tubes fabricated by local-heating-assisted dieless drawing

Magnesium(Mg)alloy stents are expected to be the next generation of stents because of good biocompatibility and biodegradability.Compared with cold drawing,dieless drawing with local heating is an effective method for manufacturing the Mg alloy microtubes since a large reduction in area can be achieved in a single pass.However,the microstructure and properties of dieless drawn tubes have not been clarified,leading to the problems in practical application of dieless drawn tubes.In this study,the microstructure and performance of dieless drawn tubes are clarified.The results show that temperature and speed in the dieless drawing process are two factors in determining the grain size of dieless drawn tubes since decreasing the temperature or increasing the speed promotes the generation of fine-grained microstructure.Twins are also generated during the dieless drawing process,which 1)disintegrates grains leading to refinement and 2)causes Hall-Petch law effect on dieless drawn tubes.Tensile tests show that grain size is the main factor in determining the mechanical properties of dieless drawn tubes,namely,0.2%proof stress 135-180MPa,ultimate tensile strength(UTS)200-250MPa,and elongation 8-12%.In 0.9 wt%NaCl solution,localized corrosion is the key factor in initiating the corrosion of dieless drawn tubes,but refined grains and fewer twins can alleviate local corrosion.These results imply that dieless drawn tubes are promising in the clinical application of Mg alloy stents for cardiovascular disease.

Velocity Field and Variation of Wall Thickness During Tube Dieless Upsetting

Theoretical analyses show that the variation of wall thickness is in direct proportion to outer-diameter, inter-diameter, and (1+R_s)^(1/2) (Rs is sectional increase ratio of tube), the theoretical calculated results of wall thickness during tube dieless upsetting are very approximate to the experimental one. As the width of deformation field increases, both the variation of wall thickness and the derivative of wall thickness variation to width of deformation field (to/tf) reduce.

Processing limit maps for the stable deformation of dieless drawing

Tin bronze wires were produced by dieless drawing. The effects of heating power, the distance between cooler and heater as well as feeding speed on the diameter, the temperature field, and the deformation region profile of the wires were investigated. The results indi-cated that each processing parameter exhibited both lower and upper limits of stable deformation based on the criterion of stable deformation with the diameter fluctuation of ±0.05 mm. Both the temperature and its gradient of the deformation region increased with increasing heating power under stable deformation, but decreased with an increase in feeding speed. As the distance between cooler and heater increased, the temperature of the deformation region increased and the slope of the deformation region profile decreased. The processing limit map of sta-ble deformation exhibited a closed curve and the unstable deformation consisted of wire breakage and diameter fluctuations.

A fractal-based model for the microstructure evolution of silicon bronze wires fabricated by dieless drawing

The back-propagation neural （BPN） network was proposed to model the relationship between the parameters of the dieless draw- ing process and the microstrecmres of the QSi3-1 silicon bronze alloy. Combined with image processing techniques, grain sizes and grain-boundary morphologies were respectively determined by the quantitative metallographic method and the flactal theory. The outcomes obtained show that the deformed microstructures exhibit typical fractal features, and the boundaries can be characterized quantitatively by ffactal dimensions. With the temperature of 600-800℃ and the drawing speed of 0.67-1.00 mm-s-1, either a lower temperature or a higher speed will cause a smaller grain size together with an elevated fractal dimension. The developed model can be capable for forecasting the microstructure evolution with a minimum error. The average relative errors between the predicted results and the experimental values of grain size and fractal dimension are 3.9% and 0.9%, respectively.

Deformation behavior of Cu-12wt%Al alloy wires with continuous columnar crystals in dieless drawing process

The microstructure and mechanical properties of Cu-12wt%Al alloy wires which are composed of continuous columnar crystals after dieless drawing forming at drawing speed of 1.0–1.4 mm/s and deformation temperature of 600–900°C were analyzed, and deformation behavior of the alloy during dieless drawing forming was experimentally investigated. The results showed that in the above-mentioned conditions, recrystallization phenomenon was not found during dieless drawing forming. When a drawing speed of 1.0 mm/s was used, the grain boundaries were out of straight gradually with increasing deformation temperature from 600°C to 900°C, and tensile strength of the dieless drawn Cu-12wt%Al alloy wires increased while elongations decreased with increasing deformation temperature. At drawing speed of 1.1–1.2 mm/s and deformation temperature of 600°C, the effect of dieless drawing forming process on the microstructure of the alloy was inconspicuous, and when drawing speed was up to 1.3–1.4 mm/s, the grain boundaries of continuous columnar crystals became zigzag while there was little effect of drawing speed of 1.1–1.4 mm/s on the elongation and tensile strength of the alloy wires.

Numerical Simulation of Dieless Drawing Process with Non-steady Processing Parameters

Since processing parameters have always been assumed to be stable in the current finite element numerical simulation of dieless drawing process, the simulation results for the product dimension tend to stabilize gradually. In fact, the dimension fluctuation exists in the forming process all the while. A mathematical model of Gauss distribution for processing parameters was employed and a finite element numerical model of dieless drawing process with non-steady processing parameters was established. Dieless drawing processing of Ni-Ti alloy wire was conducted for verifying the proposed model. The results indicated that the non-steady processing parameters model had higher simulation accuracy of the wire diameter than that given by the steady parameters model. Furthermore, the model could also be used to analyze the fluctuation characteristics in the whole dieless drawing process.

Microstructure and properties of continuous casting Ag-28Cu-8Sn alloy fabricated by dieless drawing

Ag-28Cu-8Sn(wt%)alloy is a widely used brittle silver-based brazing filler metal.The wire of brazing filler metal was prepared by continuous casting process and dieless drawing technology.The phase structure was measured by X-ray diffraction(XRD),and the microstructure of wetting interface,cast states,processing states and fracture morphologies were characterized by the optical microscopy(OM)and scanning electron microscopy(SEM),respectively.The electrical conductivity,hardness,tensile strength and elongation rate were tested as well.Furthermore,the solid-liquid phase temperature was measured by a differential scanning calorimeter(DSC),and the wettability of brazing filler metal was tested by spreading method.The outcomes obtained show that the as-cast microstructure is a typical three-zone structure,including region of surface fine grain,zone of columnar grain and region of center equiaxed crystal.Ag-28Cu-8Sn alloy is mainly composed of Ag-rich a-phase,Cu-rich pphase and intermediate compounds.Grain refinement appears in the cross section,as for grains of the longitudinal section,the shape is changed from ribbon to fiber to form a silk texture.The strength and hardness improve with the increase in the true strain,while the conductivity and elongation are reduced.Furthermore,the solid-phase temperature is 605.9℃,and the liquid-phase temperature is 725.1℃.The spreading area of Ag-28Cu-8Sn brazing filler metal is 174 mm^2,and the metallurgical bonding occurs between Ag-28Cu-8Sn brazing filler metal and Cu matrix.In addition,compared with cold drawing process,there are not any microcracks at the fracture morphology for the alloy fabricated by dieless drawing.The dieless drawing process overcomes some processing defects of traditional cold drawing,and the processing performance of Ag-28Cu-8Sn alloy is improved.

微细金属管制造技术现状与研究进展

微细金属管具有微环境下的微通道封闭、定向导向、稳流导流等功能,被广泛应用于医疗、电工电子、传热、光学等领域。受加工工具小型化、尺寸效应对材料性能和摩擦行为的改变等限制,微细金属管的制备极具挑战性。总结了微细金属管常用制备技术的制备原理、特点、制造能力以及优缺点,包括热挤压成形、冷拔成形、无模拉拔成形以及电铸成形,并对微细金属管制造技术的未来发展趋势与研究重点进行了展望。

无模无铆连接接头的塑性变形行为及工艺参数研究

为克服传统无铆连接工艺模具结构复杂、对中要求高等不足,提出了一种新型无模无铆连接工艺,并针对2 mm厚AA5052-H32铝合金板材开展了无模无铆接头塑性变形行为及工艺参数的研究。通过单轴拉伸实验和参数拟合获取了铝合金材料模型,并以此为材料数据建立了无模无铆接头成形过程有限元模型。此外,还分析了成形过程中板料的等效塑性应变、等效应力和材料流动情况,研究了冲头工艺参数对接头塑性变形行为的影响规律。实验与仿真结果表明,所建立的有限元模型具有较好的精度,[JP3]可以有效预测接头的成形过程,接头的颈部厚度、互锁值和凸起高度的预测误差均在10%以内,分别为9.4%、1.8%和0.1%;冲头直径、冲头圆角半径、冲头高度和冲头拔模斜度4种冲头工艺参数对接头的塑性变形行为和最终的接头形状有重要影响。

激光熔覆成形技术的研究进展

激光熔覆成形技术是近年来发展起来的一种新型的先进制造技术。该技术将激光熔覆表面强化技术和快速原型制造技术相结合,具有成形零件复杂、结构优化、组织性能优良、加工材料范围广泛、柔性化程度高、能实现梯度功能和无模近终成形等独特优点,可广泛应用于复杂零件的直接制造和修复,具有广阔的应用前景。介绍激光熔覆成形技术的原理、特点和应用,从几种典型激光熔覆成形系统、成形零件组织性能研究、温度场数值模拟、成形过程检测与控制和成形过程残余应力和裂纹产生的原因及对策等方面对激光熔覆成形技术的研究进展进行综述,指出该技术存在的问题,并对其发展方向进行预测和展望。激光熔覆成形技术在硬件系统、工艺研究和熔池检测与控制等方面都取得了可喜的进步,但成形精度和裂纹仍然是该技术领域非常棘手的问题,有待进一步突破。

金属板材无模多点成形专用/CAM与CAT软件的开发

给出了金属板材三维曲面自动成形压力机专用无模多点成形ＣＡＤ／ＣＡＭ与ＣＡＴ软件的开发过程。该软件主要由三维曲面造型、无模多点成形工艺ＣＡＤ、ＣＡＭ以及ＣＡＴ等４个模块构成。以鞍形曲面制品为例，显示了这４个模块的实际运行结果。

无模胀球的原理与研究进展

介绍了无模胀球技术的基本原理和主要工序， 以及无模胀球技术的最新研究进展和在工程中的应用情况。对椭球壳体液压胀形时的应力分布进行了理论分析和图形描述， 并进行了一系列的椭球壳整体无模液胀成形试验研究， 同时通过有限元数值模拟分析了壳体在胀形过程中塑性变形的发生部位及扩展过程。

汽车大型覆盖件的数字化成形技术

本文介绍金属板料数字化成形的方法 ,这是一种用CAD模型直接驱动 ,实现设计与制造一体化的柔性快速成形方法。其采用快速原型制造技术的“分层制造”思想 ,将复杂的三维形状分解成一系列二维层上的数据。并根据工件的几何形状信息 ,用三轴数控设备控制一个成形工具头作三维曲线运动 ,逐层对板料进行塑性加工 。

增量成形研究进展

重点综述了新型增量成形技术中的4类典型成形工艺,即局部不均匀加载下板面内弯曲增量成形、单点增量成形、多点柔模成形、高速冷滚打成形就其成形原理、工艺、设备及研究现状等方面的研究成果,并提出了这类新型增量成形的研究方向。

管材无模拉伸壁厚变化规律实验研究

对管材无模拉伸时壁厚变化规律进行了实验研究.研究结果表明,管材无模拉伸时,管材壁厚之比(t_f/t_o)、内径之比(D_(if)/D_i)、外径之比(D_(of)/D_O)以及(1-R_)^(1/2)(R_—断面减缩率)彼此成正比.且当相对厚度(t_o/D_o)足够小时,其比例系数 K_1,K_2,K_3均接近于1.另外,冷热源间距对管材壁厚变化也有影响.

镍钛形状记忆合金线材无模拉拔成形过程的电磁场和温度场模拟

无模拉拔成形过程的电磁场和温度场数值模拟对于正确制定加工工艺, 精确控制线材质量十分重要.利用ANSYS有限元软件, 对镍钛形状记忆合金线材无模拉拔成形过程的电磁场和温度场进行了数值模拟, 获得了镍钛形状记忆合金线材无模拉拔成形时的电磁场和温度场信息, 并进行了讨论.提出了较合理的镍钛形状记忆合金线材无模拉拔成形工艺参数范围: 电流频率59～61 kHz、电流密度(2.95～3.05)×107 A·m-2、冷却水与感应加热线圈之间的距离19～22 mm、冷却水流量22～26 ml·s-1.

金属板材无模渐进成形板厚变化仿真与实验研究

板厚变化是板材渐进成形工艺涉及的主要问题,与成形角度、层间距等工艺参数密切相关。针对成形角度分别为45°和60°的方锥件,结合数值模拟和实验结果证明余弦定理可用于预测成形零件的板材厚度,但预测值较实际值偏大。运用无模渐进成形有限元模型,分析了工具头尺寸和层间距对板材厚度变化的影响,结果表明大的工具头尺寸和小的层间距有助于提高板厚的均匀化程度,不同的工具头尺寸虽然导致板厚均匀区域大小有所差别,但最终的板厚尺寸趋于一致,随着层间距的增大,余弦定理的预测误差增大。

金属板料单点无模渐进成形数值模拟及分析

有限元数值模拟对单点无模渐进成形技术的研究起着重要的理论指导作用。本文介绍了数值模拟的基本步骤,重点介绍了圆/方锥台形件模拟轨迹的改进,轨迹环采用正逆相间方式可避免成形件发生扭曲变形。此外,基于Matlab软件编程,可快速生成不同工艺参数的模拟轨迹。最后通过多次数值模拟得出,工具头半径越小,工件成形角度越大,板料发生的等效应变就越大。结论与实验结果完全吻合。

金属板材无模成形加工轨迹生成的研究

介绍了一种采用快速原型制造原理的金属板材无模成形技术 根据快速成形技术中分层制造的思想 ,从板材无模成形的技术特点出发 ,采用基于STL文件的金属板材无模成形的加工轨迹生成方法 ,通过构造STL文件的等距模型并对其进行切片 。