The numerical simulation of the movements and thermal states of atomized metal droplets in the spray forming process

Numerical simulation was performed in an atomizing chamber to investigate the movements and thermal states of the atomized metal droplets in the spray forming process. The velocity, temperature and solid fractions of metal droplets with different diameters and under different atomizing pressures were investigated. The results indicate that a higher atomizing pressure results in the increased flying velocity of the metal droplets and a decrease in the cone-shaped angle formed by their flight paths. Synchronously, the cooling of the metal droplets is accelerated and the time of the complete solidification process is shortened. Under the same atomization pressure, large metal droplets have a lower flying speed and a lower rate of temperature decrease in the atomizing chamber than small metal droplets. In addition, metal droplets flying along the edge of the atomizing region cool faster than those flying in the core region.

Numerical Simulation of Molten Metal Droplet Behavior in Gas Metal Arc Welding by Three-Dimensional Incompressible Smoothed Particle Hydrodynamics Method

The numerical model was developed using a SPH (Smoothed Particle Hydrodynamics) method and the projected transfer phenomena during a GMA (Gas Metal Arc) welding were simulated by the model to clarify mechanisms of the phenomena. As a result, the droplet transfer mode obtained from this calculation was regarded as a projected transfer mode in which the liquid column grew about 1 mm and a droplet grew up until its diameter became large the same as a wire diameter,?after that it was detached from the tip of the column. In addition, 10 droplets were formed for 0.1 s through these growth and detachment processes at the tip of a wire. To compare with the numerical results, actual GMA welding was carried out and molten metal droplet transfers were taken by high speed camera. The diameter of a wire, the length of a liquid column, the velocity of a droplet right before it reached a weld pool obtained by simulation showed good agreement with experiment.

A Simple Method to Measure the Contact Angle of Metal Droplets on Graphite

The determination of solid–liquid interfacial tension plays an important role in science and technology.Here,we propose a simple method to directly measure the contact angle between metal droplets and a graphite substrate for the determination of metal–graphite interfacial tension.The proposed method involves the synthesis of micro-and nanosized metal droplets on graphite by arc melting.Owing to its small volume,the rapid cooling of the prepared metal droplets on the graphite substrate leads to the freezing of equilibrium contact confguration after solidifcation.We observe that the measured contact angle between micro-and nanosized Au(or Ag)particles and the graphite substrate is almost size independent,even though the size of the particles synthesized herein is 1–3 orders of magnitude smaller than that studied in previous works.In addition,the interfacial tensions of Au and Ag on the step edges(edge plane)of graphite are found to be larger than that on the(0001)plane(basal plane).The proposed method provides a simple approach to determine the solid–liquid interfacial tension and may be efective in the study of interface related science and technology.

Hard-magnetic liquid metal droplets with excellent magnetic field dependent mobility and elasticity

Magnetic liquid metal droplets(MLMDs)have been proven to be very important in many fields such as flexible electronics and soft robotics.Usually,soft magnetic particles such as nickel(Ni)and iron(Fe)are mixed or suspended into the liquid metal to obtain soft MLMDs(S-LMDs),which can be easily manipulated under the magnetic field due to the favorable deformability and flexibility.In addition,hard magnetic particles such as neodymium iron boron(Nd Fe B)with a high residual magnetization can also be dispersed into the liquid metal and the hard MLMDs(H-LMDs)become more compact due to the interaction between internal particles induced by remanence.This work reports a kind of H-LMDs with high surface tension,high flexibility and mechanical robustness,whose electrical conductivity and strength are better than the S-LMDs.Under the magnetic field,the H-LMDs have a faster response time(0.58 s)and a larger actuating velocity(4.45 cm/s)than the S-LMDs.Moreover,the H-LMDs show excellent magnetic controllability,good elasticity and favorable mobility,as demonstrated by magnetically actuated locomotion,bounce tests and rolling angle measurements.Finally,the droplets can be further applied in wheeldriven motors and micro-valve switches,which demonstrates their high application potential in robotic manipulation and microfluidic devices.

Liquid metal spring： oscillating coalescence and ejection of contacting liquid metal droplets

与相当高的表面紧张，房间温度液体金属可以与常规液体不能拥有的意外行为继承。这里，我们经由高速度的照相机揭示了液体金属微滴的结合和喷射现象。当轻轻地联系时，它试验性地被发现那(而非碰撞) 有相同尺寸的二金属微滴一起在 NaOH 答案，震荡的结合将发生它就像在接口以后的春天一样的跑破裂并且形成毛状的波浪。为有显然不同的直径的二金属微滴，结合导致相当不平常的喷射现象。大微滴将咽下小的部分然后逐出另一小得多的微滴。如此的现象在金属微滴上为电的双层的存在提供一条直接证据。影响行为的动力学液体通过从记录电影，和液体金属微滴之间的基本差别处理图象被确定，水微滴的被澄清。与事件有关的理论机制 preliminarily 被解释。发现的礼品使液体金属微滴的基本理解清醒，它也建议使用如此的基本效果描绘粘性，表面紧张，金属液体的电的双层和微滴形成的潜在的价值。

基于工程应用的CW-GMAW熔滴过渡形态表征

综述了涉及工程应用的冷丝熔化极气体保护焊(Cold wire gas metal arc welding,CW-GMAW)熔滴过渡形态特征。结果表明,在大电流、强规范、富氩混合气体保护下,CW-GMAW工艺的熔滴过渡形态呈喷射过渡;当电流较小、电弧电压较低时,可能为滴状过渡,甚至在弧压很低时,呈现短路过渡形态。该工艺电弧发生偏向冷丝的位移,弧长变短甚至发生短路,与冷丝送进速率比增高及冷丝在电弧中产生大量金属蒸气时弧柱电阻下降有关。在具有富氩混合保护气体的相同工艺参数下,CWGMAW转变电流比GMAW降低了4%~7%。焊接工艺参数对CW-GMAW和GMAW工艺熔滴过渡形态的影响规律大致相近,但前者因涉及冷丝送进速率比和电极焊丝送进速度,以及它们的匹配等,使焊接电流的影响更为复杂。

喷印金属双液滴撞击固体形成气泡的模拟研究

喷墨打印镓金属液滴撞击固体表面时会产生空气夹带,造成气泡沉积出现喷墨打印导线不互联现象,深入理解气泡形成的影响因素是解决此问题的关键。为消除气泡,提高打印导线互联性,本研究使用Ansys Fluent软件,采用VOF耦合水平集方法,建立了二维镓金属双液滴撞击固体铜基板过程的仿真模型,研究了水平壁面温度、双液滴间距、倾斜壁面角度、倾斜壁面温度等参数对金属双液滴撞击固体壁面形成气泡过程的影响规律。研究结果表明,对于水平基板,在不同基板温度下气泡的大小呈现双向变化,金属双液滴间距的增加更有利于液滴铺展。当金属双液滴间距S=D_(0)-2.38D_(0)时,有气泡形成;当S=2.38D_(0)-2.5 D_(0)时,液滴铺展出现断裂,且气泡随温度的增加没有明显变化;当金属双液滴间距S=2.6 D_(0)时,随着温度的增加不再产生气泡,此时喷墨打印液滴表现出最佳精度,提高了打印导线的互联性。对于倾斜基板,使用参数韦伯数(We⊥)和参数(K⊥)表征金属双液滴形成的气泡,当撞击倾斜壁面韦伯数We⊥>15、K⊥>0.035时,液滴会在固体表面上形成气泡;当We⊥>16、K⊥>0.04时,形成的气泡直径随壁面温度的增加而减小;当15<We⊥<16、0.035<K⊥<0.04时,液滴出现铺展断裂,并且气泡直径随着壁面温度的增加而减小;当5<We⊥<15、0.015<K⊥<0.035时,液滴铺展断裂且气泡直径变大;当We⊥<5、K⊥<0.01时,液滴在基板形成的气泡消失,此时为喷墨打印液滴的最佳精度,可提高打印导线的互联性。该研究为优化喷墨打印金属液滴的工艺提供了重要理论依据,有望在实际应用中消除气泡,进一步提高喷墨打印导线的互联性。

金属有机框架多面体微颗粒自组装超结构的设计策略

周期性超结构通常具有优异的光学性能,在光子晶体学、催化、传感等领域具有重要意义。在此,采用了声悬浮法和点滴法来探索多维有序金属有机框架(MOF)超结构的组装策略,并建立了组装参数与超结构维度和有序性之间的关系。结果表明:声悬浮法对MOF颗粒分散液浓度和颗粒的单分散性包容性极强,适用于三维超结构的自组装;点滴法对MOF颗粒分散液浓度和颗粒的单分散性有较高的要求,能组装出大面积二维MOF单层膜。该研究为利用各向异性微粒组装多维超结构提供了新的策略。

车用薄镀锌板CMT搭接工艺特性

针对车用薄镀锌板材料,为了进一步降低冷金属过渡(cold metal transfer,CMT)时产生的飞溅,研究分析了薄镀锌板CMT搭接焊接工艺特性.采用Baumer HX 13工业级高速摄像机拍摄了CMT焊接熔滴过渡图像,利用NI PXI数据采集系统采集了CMT焊接电流电压信号.主要研究了短路前期阶段与短路后期阶段电流电压对CMT熔滴过渡的影响规律,同时分析了焊枪倾角对薄镀锌板CMT搭接焊接熔滴过渡行为的影响.结果表明,当CMT短路阶段电流小于基值阶段电流时,电信号波形未出现扰动现象,焊接过程稳定;当降低短路后期电流约5 A时,能够有效降低CMT搭接产生的飞溅;当焊枪与垂直平面呈30°夹角时,能够获得平稳的熔滴过渡,焊接飞溅最小,焊缝熔深最大,焊缝成形最佳.故短路阶段电流小于基值阶段电流且降低短路后期电流约5 A时,采用焊枪倾角30°的焊接工艺能够实现较小的焊接飞溅,为生产实践提供理论依据.

正负极性CMT铝焊丝熔滴体积增长量分析

为了定量研究铝焊丝在正负极性冷金属过渡时熔滴体积的变化情况,搭建了VP CMT熔滴过渡图像及电信号采集平台.利用数字图像处理技术,对拍摄得到的熔滴过渡图像进行处理,得到了仅包含焊丝、熔滴轮廓的二值图像.结合焊丝直径与熔滴径向尺寸之间的关系特征,将熔滴与焊丝之间“固-液”分界面的判定分为焊丝直径大于、等于、小于熔滴径向尺寸3种情况进行讨论,提出了判别熔滴与焊丝之间“固-液”分界面的具体方法.首先基于此方法得到了不同工艺参数下,铝焊丝在燃弧阶段不同时刻的瞬时熔滴体积.依据燃弧阶段末期与燃弧阶段初期的熔滴体积,计算得到了不同极性下的熔滴体积增长量.发现在相同的工艺参数下,焊丝极性接负(EN)阶段的熔滴体积增长量明显大于焊丝极性接正(EP)阶段.之后给出了燃弧阶段输入功与熔滴体积增长量之比(W/ΔV)参量,并定量对比了在不同的工艺参数下,EP和EN阶段的W/ΔV变化.结果表明,熔化相同体积的铝焊丝,EP阶段所需要的燃弧阶段输入功约为EN阶段的2.2倍.最后结合焊丝作为阴(阳)极时的产热以及电弧在EN阶段沿焊丝产生“上爬”现象对上述结果进行了解释.该研究为后续精确控制溶滴尺寸,进而控制焊缝成形提供了借鉴.

Study of porosity tendency in the Cr-Ni austenitic weld based on inclusion’s characteristic in the droplet

The effect of nonmetallic inclusions in the droplet of the stainless steel covered electrode on the porosity was researched.The result shows that the nonmetallic inclusions in the droplet are spherical,their composition is different from the one of slag and the inclusions have the character of “inner formation”.When the ratio of rutile to ilmenite in the coated material is increased, the droplet becomes coarse, the content of nonmetallic inclusion in the droplet decreases,and the porosity sensitivity in the weld metal also decreases.When the ratio of fledspar to ilmenite in the coated material is increased, the droplet becomes fine,the content of nonmetallic inclusion in the droplet increases, and the porosity sensitivity in the weld metal increases. When the ratio of Fe_2O_3 to ilmenite in the coated material is increased, the droplet becomes fine, the content of nonmetallic inclusion decreases, while the porosity sensitivity does not reduce.

Mesoscopic numerical computation model of air-diffusion electrode of metal/air batteries

This work creates a droplet battery model based on the electrolyte performance in the porous electrode, studies the current density on the mesoscopic scale, and explains how the mesoscopic structure of the porous electrode influences the current density on the air-diffusion electrode. Near the three-phase line, there is a strong band containing nearly 80% current. For porous electrodes, the total current is proportional to the length of the strong band. Thus, it can be inferred that on the macroscopic scale, the longer the total length of the strong band on unit area is, the larger the current density is.

焊丝中稀土金属对纯CO_(2)保护气体GMAW熔滴过渡形态的影响

综述了焊丝中稀土金属对纯CO_(2)保护气体GMAW熔滴过渡形态的影响。结果表明,含稀土金属的焊丝以正极性纯CO_(2)保护气体焊时,由于电弧中阴极斑点面积扩大(位于熔滴上方的电弧呈圆锥形),被细化的熔滴形成轴向喷射过渡形态,飞溅很小,工艺性优良。稀土金属降低了阴极电子逸出功,同时高的电磁夹持力使熔滴以更小的尺寸轴向过渡,还有作用在熔滴上的主导力方向有利熔滴过渡,最终纯CO_(2)保护气下完全满足了熔滴喷射过渡形成条件。含有稀土金属的焊丝在电流波形控制新型电源中的应用,使J-STAR焊接方法在一个较宽电流范围内实现了超低飞溅,拓宽了该工艺的应用范围。

STUDY ON METAL TRANSFER MODES OF SELF-SHIELDED FLUX-CORED WELDING WIRE

Metal transfer behavior of six kinds of self shielded flux cored wire(SSFCW) is studied using the apparatus of SSFCW high speed photography self made. Six kinds of metal transfer modes of SSFCW were obtained through observation for high speed photograph film and analysis. It is believed that the research is of magnificent for improving operative performance and mechanical properties of SSFCW and dynamics characteristic of welding power.

焊丝成分对高氮钢CMT+P焊工艺性的影响

为提升高氮钢焊接质量和优化焊接工艺,研究焊丝氮、锰含量带来的焊接工艺稳定性。采用冷金属过渡加脉冲(Cold Metal Transfer plus Pulse,CMT+P)焊技术对5种高氮钢焊丝进行焊接试验,研究焊丝成分对电信号、熔滴过渡、飞溅率的影响。研究结果表明:氮含量的增加会引起电信号波动变大且分布离散,而锰含量的变化对电信号的影响较小,焊丝中氮含量对高氮钢CMT+P焊接稳定性影响大于锰含量的影响;随着氮含量的增加,熔滴过渡模式由一脉一滴转变为多脉一滴,熔滴形状不规律且尺寸变大,焊丝工艺性变差;当焊丝中氮、锰含量较小,分别为0.42%、7.19%时,焊接工艺稳定性较好;氮逸出、锰蒸发导致高氮钢熔滴剧烈爆炸产生大量飞溅,焊接飞溅率随着氮、锰含量的增加而不断增大。

直接激光沉积的金属液滴凝固特性数值模拟研究

液滴凝固过程是直接激光沉积制造的重要部分,其在制造面上的最终形态将直接影响制造层的成型质量。为了明晰直接激光沉积制造过程中金属液滴的凝固现象及成型机理,建立了金属液滴在冷壁面上凝固的两相流动及传热数值模型,并在模型正确性验证的基础上对强弱辐射作用两种情况下液滴的凝固过程开展研究。结果表明:弱表面辐射作用下,液滴在制造壁面上呈振荡式铺展及收缩,液滴前缘最大铺展速度可达25 mm/s,其凝固呈由壁面区域向中心区域、边缘区域向中心区域凝固的态势;强表面辐射作用下,液滴凝固呈由四周外表面向内部凝固的态势,液滴无明显铺展及收缩运动,液滴凝固速度相较于弱辐射情况极大提升,但随着凝固向内部的深入,凝固率呈线性增大趋势。

水成膜泡沫剂对液滴冷却高温金属表面的影响

向细水雾中添加水成膜泡沫灭火剂成分能明显提升针对B类火灾的灭火效果,但添加剂能否提升水雾对火灾区域内固体表面的冷却效果尚不清楚。分别从金属表面温度、We数和是否添加泡沫灭火剂3个方面设置了试验条件,开展了单液滴撞击高温金属表面模拟试验并对冷却效果进行了定量计算。结果表明,液滴的最大铺展因子与撞击动量正相关,含添加剂液滴表面张力降低且流变特性呈现剪切变稀特征。当Leidenfrost现象发生时,碰撞试验表明泡沫溶液液滴相较于纯水液滴的最大铺展系数更小且回弹速率更大,因此泡沫灭火剂的添加不利于提升吸热效率。研究结果可为泡沫细水雾技术的灭火实践提供理论指导。

均匀金属液滴喷射微制造技术的研究现状

金属液滴喷射沉积增材制造技术是利用均匀金属液滴逐点沉积成形制件的快速成型技术。根据喷射原理可分为连续喷射和按需喷射两类。对这两种技术的喷射原理、相关特点及其在微制造领域的应用研究及发展概况进行了介绍,阐述了以压电换能器和气体压力作为驱动方式的两种均匀液滴喷射成形设备的工作原理与相关特点,提出了两种技术在喷射成形低熔点金属、高熔点金属中的关键技术。在此基础上,探讨了两种工艺的发展方向及在直接金属快速成型、电子封装、喷射沉积金属涂层以及喷射印刷电路等方面的应用,并提出了该技术的研究方向。

基于金属3D打印技术成形嵌套零件工艺研究

针对传统采用多个零件拼接而成具有悬垂嵌套结构的零件,提出了金属均匀喷射熔滴沉积"双喷头"3D打印成形工艺,开发了金属微喷熔滴沉积成形试验平台,通过对新型支撑材料的配比和成形工艺研究,实现了金属以及支撑材料成形的联动控制,成形出较高精度悬垂嵌套的金属制件,与传统成形工艺相比,具有成形周期短,制造成本低等优点,实现了此类零件的即用即打,此类零件成形提供了实现的途径与条件.