高速电弧喷涂雾化熔滴传热过程数值分析Ⅱ.工艺参数对熔滴传热过程的影响

在高速电弧喷涂雾化熔滴传热过程数学模型的基础上，用Fe-Al合金进行了数值计算，分析了工艺参数对熔滴传热过程的影响。结果表明，熔滴尺寸越小，在一定喷涂距离上的对流换热系数则越大、熔滴温度越高、固相分数越小、冷却速度越大；雾化气流压力和喷涂电流越大，在一定的喷涂距离上熔滴温度也就越高，熔滴中的固相分数越低，且其凝固过程也越长；熔滴的冷却速度对熔滴尺寸和喷涂距离的变化十分敏感，而对雾化气流压力和喷涂电流的变化不太敏感；Fe-Al合金熔滴的液态冷却速度达10^5～10^7K／s数量级，预示涂层将具有快速凝固组织特征。     

高速电弧喷涂雾化熔滴的热传输行为

提出了高速电弧喷涂(HVAS)雾化过程熔滴的热传输理论模型,并用一种FeAl合金进行数值分析。结果表明,雾化过程中熔滴的液态冷却速度在105~107K穝-1数量级,预示涂层将具有快速凝固组织特征;熔滴尺寸、雾化气流初始速度、熔滴过热度及喷涂距离对雾化熔滴的热传输行为均有很大的影响;在一定范围内增大雾化气流压力,增大熔滴过热度,缩短喷涂距离,可以有效地改善高速电弧喷涂层的性能。     

钛铝双丝超音速电弧喷涂层氮氧含量的分析与仿真

为研究喷涂工艺参数对钛铝双丝超音速电弧喷涂层氮氧含量的影响规律,利用SAS-Ⅱ型超音速电弧喷涂设备和直径为2mm的钛(TA2)、铝(L2)丝材为喷涂材料,在0.6～0.8MPa的压缩空气下,测定了丝材熔化速率与喷涂电压和喷涂电流之间的关系,根据空气动力学原理确定了初始熔滴的尺寸及形成时间,建立了涂层氮、氧含量与喷涂工艺参数之间的仿真分析模型,并利用氧氮分析仪对涂层氮氧含量进行测试.结果表明,当电流一定时,随喷涂电压的升高,涂层的N、O含量逐渐下降;当电压一定时,随电流的增大,涂层的N、O含量逐渐增大;仿真结果与测试比较接近.     

高速电弧喷涂熔滴速度的数值模拟及试验

熔滴速度是电弧喷涂涂层性能的主要影响因素之一。本基于空气动力学和二相流流体力学理论建立了高速电弧喷涂雾化气流化和熔滴速度的数学模型，并进行了数值模拟；同时用试验方法测试了气流速度及Al,3Cr13熵滴在不同喷涂距离处的平均速度；数值计算结果与试验数据基本吻合。结果表明，雾化气流的速度和距喷嘴一定距离内将保持初始速度（约650m/s)，然后随喷涂距离的增大而衰减，这与超音速气流通过Laval喷管后所产生的膨胀波和压缩波相互作用有关；熔滴在雾化飞行过程中经历了先加速后减速的过程，小熔滴能在较短的距离内被加速到最大速度；达到最大速度之后，小熔滴由于惯性力较小而迅速减速，而大熔滴则因较大的惯性力而减速不明显；熔滴速度的变化是由熔滴的Reynolds数决定的。Al和3Cr13熵滴的最大速度在0．3m喷涂距离之内均超过音速。     

Q235钢基表面超音速电弧喷涂WC-Co涂层冲蚀磨损性能研究

采用超音速电弧喷涂工艺(HVAS)在A3钢基体上制备了WC-Co涂层,对所制备的涂层在不同冲蚀角度下的冲蚀磨损性能进行了研究。结果表明:冲蚀磨损时,喷涂电流为100 A和200 A的涂层质量损失都随冲蚀角的增大而增加,冲蚀角为30°时磨损量最小,冲蚀角为90°时磨损量最大,说明冲蚀过程中冲蚀粒子的锤击作用对材料的质量损失影响更大。喷涂电流为100 A时的冲蚀磨损量要高于喷涂电流为200 A的冲蚀磨损量,喷涂电流增大,弧区温度升高,容易形成细小熔滴,在压缩空气作用下粒子飞行速度增大,动能提高,促使涂层表层硬度提高,从而提高了涂层的耐磨性。     

高速电弧喷涂电流波动对涂层性能的影响

电弧喷涂时的电压和电流是对喷涂层性能有重要影响的两个电参数。由于喷涂过程中电压的波动范围不大 ,而电流的波动范围较大 ,因此喷涂电流的波动对电弧喷涂过程的稳定性及涂层质量的影响更为突出。在保持其它参数 (如雾化气体压力、喷涂距离、喷枪角度等 )不变的前提下 ,研究了高速电弧喷涂时的电流波动率对涂层的孔隙率、显微硬度、耐磨性等方面的影响 ,当电流波动幅度减小时 ,涂层的综合性能将会得到提高。文中从理论上对电流波动影响涂层性能的机理进行了分析 ,认为电流波动造成电弧喷涂雾化粒子尺寸、温度、空间分布等不均是造成涂层综合性能不高的主要原因。     

双丝间接电弧氩弧焊的熔滴过渡

采用氙灯背光高速摄像系统及示波器对双丝间接电弧氩气保护焊的熔滴过渡及其对应的电压、电流波形进行了研究.结果表明,双丝间接电弧氩弧焊焊接电流与电弧电压的不同匹配选择,熔滴具有短路过渡、大滴过渡、混合过渡、射滴过渡、射流过渡等不同过渡形式.随着焊接电流的增大熔滴尺寸减小,熔滴细化,随电弧电压的增大,熔滴尺寸减小.熔滴过渡形式与电压、电流的波形之间有很好的对应关系.     

高速电弧喷涂雾化熔滴传热过程数值分析Ⅰ.数学模型及传热参数变化规律

采用流体力学理论、凝固理论和牛顿冷却模式,提出了高速电弧喷涂雾化熔滴传热过程的数学模型,并用一种Fe-Al合金进行数值计算,用Spray watch-2i热喷涂监控系统测试不同喷涂距离处熔滴平均温度的变化,以验证数学模型的正确性,并分析了雾化熔滴传热参数的变化规律.结果表明,计算结果与实测数据基本吻合.雾化过程中熔滴的对流换热系数、温度、固相分数及冷却速度等传热参数呈规律性变化.直径为34 μm的Fe-Al合金雾化熔滴的初始液态冷却速度达2.5×106K/s,预示涂层将具有快速凝固组织特征.     

FeCrBSiNb粉芯丝材电弧喷涂的弧区动态行为

电弧喷涂粉芯丝材是由金属外皮包覆金属或非金属的复合粉末组成,喷涂过程中在弧区发生冶金反应并雾化成熔滴,大量熔滴沉积在基体表面最终形成涂层。粉芯丝材的电弧喷涂是一个高度动态的传热传质过程,采用高速摄像技术研究了喷涂过程中两根粉芯丝材交汇处的电弧、丝材熔化与熔滴形成等行为。另外,通过高速摄像试验分析了喷涂电流和电压对电弧形态及丝材熔化的影响规律。结果表明:喷涂时间歇出现燃弧、熄弧、再燃弧的循环变化;不同于实心丝材,粉芯丝材在阴阳极上的电弧都发散,这有利于丝材外皮和粉芯间的冶金反应;阴极丝材主要表现为抽吸作用下以细小熔滴或片状挤出物等形式形成熔滴,阳极丝材主要以片状液带的形式脱离并雾化成小熔滴。     

高速电弧喷涂雾化熔滴传热过程数值分析Ⅰ.数学模型及传热参数变化规律

采用流体力学理论、凝固理论和牛顿冷却模式,提出了高速电弧喷涂雾化熔滴传热过程的数学模型,并用一种Fe-Al合金进行数值计算,用Spraywatch-2i热喷涂监控系统测试不同喷涂距离处熔滴平均温度的变化,以验证数学模型的正确性,并分析了雾化熔滴传热参数的变化规律。结果表明,计算结果与实测数据基本吻合。雾化过程中熔滴的对流换热系数、温度、固相分数及冷却速度等传热参数呈规律性变化。直径为34μm的Fe-Al合金雾化熔滴的初始液态冷却速度达2.5×10⁶ K/s,预示涂层将具有快速凝固组织特征。     

基于高速CCD摄像的短路过渡焊接熔滴检测与分析

建立了基于高速CCD摄像的熔滴图像检测和焊接电流、电弧电压同步采集系统,在给出短路过渡模式下的熔滴尺寸定义并简述基于MATLAB平台的熔滴尺寸与电弧信号分析系统的基础上,对平特性电源短路过渡CO2焊接熔滴尺寸变化特征及其与工艺性能间的关系进行了试验研究.结果表明,熔滴尺寸呈分散性较大的正态分布(1～2倍焊丝直径),过大或过小的熔滴尺寸均不利于短路过渡焊接过程的稳定性.根据熔滴的形成和过渡过程,初步分析了影响熔滴尺寸的主要因素及控制熔滴尺寸的途径,即短路过程结束后焊丝端部的残余液态金属量和燃弧能量的随机性导致了熔滴尺寸的不确定性,对其进行有效控制将提高熔滴尺寸和短路过渡过程的一致性,进而改善短路过渡CO2焊接的工艺性能和焊接质量.     

In-Flight particle measurements of twin wire electric arc sprayed aluminum

A real-time, nonintrusive measurement technique was successfully applied to a Tafa Model 9000 (TAFA Incorporated, Concord, NH) twin wire electric arc thermal spray system to simultaneously measure particle size, velocity, and temperature within the spray plume. Aluminum wire was sprayed with the current varied from 100 to 300 amp, and the gun pressure (air flowrate) varied from 40 to 75 psia. For all cases, the average sizes of the molten aluminum particles along the spray centerline range from 33 to 53 μm. The particles accelerate to peak velocities between 130 and 190 m/s, then decelerate slightly as they travel downstream. The average centerline particle temperature ranges from 2004 to 2056 °C, and the temperature profile remains fairly flat throughout transport to the substrate. A stagnation pressure probe was used to quantify the gas flow regime in the unladen jet. The wires were found to have a pronounced effect on the flow, resulting in a complex three-dimensional flowfield with mixed regions of subsonic and supersonic flow.     

Study on arc characteristics and behavior of metal transfer in pulse current flux-cored arc welding of mild steel

The variation in arc characteristics and behavior of metal transfer with the change in pulse parameters has been studied by high speed video camera during pulse current flux cored arc weld deposition. A comparative study of similar nature has also been carried out during flux-cored arc weld deposition in globular and spray transfer modes. The effect of pulse parameters has been studied by considering their mean current and arc voltage. The arc characteristics studied by its root diameter, projected diameter and length, and the behavior of metal transfer noted by the metal transfer model and the droplet diameter have been found to vary significantly with the pulse parameters. The observation may help in understanding the arc characteristics with respect to the variation in pulse parameters which may be beneficial in using pulse current FCAW to produce desired weld quality.     

短路过渡CO2焊接熔滴尺寸控制

分析了短路过渡CO2焊接的熔滴形成过程及焊接电流波形对熔滴形状及短路过渡历程的影响规律,对采集获得的焊接过程中的焊接电流信号进行数学处理,采用最小二乘法建立了波形控制短路过渡焊接的焊丝熔化模型.在此基础上,通过控制燃弧初期脉冲电流的宽度调整燃弧能量的大小,对熔滴尺寸控制进行了试验研究.结果表明,随着燃弧脉冲电流宽度的增加,熔滴尺寸单调增加,这说明改变燃弧初期脉冲电流的宽度可以有效地控制燃弧能量和熔滴尺寸的大小.     

无镀铜焊丝熔滴过渡形态与工艺质量的关系

综述了镀铜和无镀铜焊丝熔滴过渡形态与工艺质量的关系。两种焊丝GMAW焊接时,熔滴有大滴过渡、喷射过渡和短路过渡3种形态。在富氩混合气时都存在滴状向喷射过渡的转变电流。无镀铜焊丝在不同保护气体时的电弧改善、熔滴细化、转变电流均低于镀铜焊丝。焊接电流和电弧电压的正确匹配是获得满意过渡形态的重要条件。焊丝的工艺质量除了受焊丝和涂层成分及母材焊接性控制之外,主要受焊接工艺条件控制。通过工艺参数匹配的变化建立了熔滴过渡形态与焊丝工艺质量间的关系,其内在联系主要是熔滴尺寸和转变电流的变化。     

Random nature of droplet size and its origins in short circuit CO2 arc welding

Abstract

The nature of the droplet size variation and its effect on the process behaviour were investigated for short circuit CO₂ arc welding using a power supply with constant voltage output. To obtain the droplet images and record the welding current and voltage transient waveforms in phase, an integrated experimental system was established by combining a high speed charge coupled device camera based on laser shadowing and a related optical system with a dual channel analog/digital converter. The droplet size in short circuit transfer mode is defined as the maximum diameter of the molten droplet just before short circuiting. The experimental results reveal that the distribution of droplet size typically has a random nature with a range of between 1 and 2 times the electrode diameter. Excessively large or small droplet size is markedly unfavourable to the process stability. The droplet shape just before short circuiting and the short circuiting experience are also critical factors affecting the process behaviour. The main factors influencing the droplet size and shape are analysed in detail and approaches to control their evolution are then proposed.     

Arc characteristics and behaviour of metal transfer in pulsed current GMA welding of stainless steel

The variation in arc characteristics, stability in shielding of arc environment and behaviour of metal transfer with a change in pulse parameters have been studied by high speed video-photography during pulsed current gas metal arc (P-GMA) weld deposition using austenitic stainless steel filler wire. A comparative study of similar nature has also been carried out during gas metal arc (GMA) weld deposition in globular and spray transfer modes. The effect of pulse parameters has been studied by considering their hypothetically proposed summarized influence defined by a dimensionless factor ? = [(I_b/I_p) ft_b], mean current and arc voltage and correlation between welding parameters and arc characteristics have been established. The arc characteristics studied by its root diameter, projected diameter, length and stiffness measured in terms of arc pressure and the behaviour of metal transfer noted by the droplet diameter and velocity of droplet at the time of detachment have been found to vary significantly with the variation in ?. At a given ? the experimentally measured values of the behaviour of metal transfer are found well in agreement to their corresponding theoretical values estimated through mathematical expressions reported earlier. The increase of ? and the ratio of (I_b/I_p) have been found to adversely affect the stability of shielding jacket and arc profile especially at high arc voltage.     

Characterization of Cold Spray Titanium Supersonic Jet

Titanium is widely used in aerospace, highly corrosive environments, and implants due to unique properties such as high strength to weight ratio and excellent corrosion resistance. Cold gas dynamic spray (cold spray) technology, in contrast to current fabrication technologies, has provided the potential for titanium to be utilized in broader industrial applications and at lower cost. Particle velocity is the most important parameter in the cold spray process that leads to successful deposition of titanium at supersonic speeds. In this study, particle image velocimetry (PIV) is utilized to characterize supersonic flow field for a commercially pure (CP) titanium powder. The results represent experimentally determined velocity for titanium particles under supersonic conditions with respect to propellant gas, spray temperature, and stagnation pressure. The high velocity flow region outside of the cold spray nozzle was significantly extended using helium. An increase in stagnation temperature results in a high velocity region close to the axis of the cold spray nozzle. In contrast, an increase in pressure expands the high velocity regions in the cold spray plume. The PIV that is a whole-flow-field process is a practical characterization technique for optimization of parameters and validation of the future models for the cold spray process.     

Hybrid laser-arc welding of advanced high-strength steel

This study investigated the synergetic effect between laser beam and electrical arc during hybrid welding by using a spectral diagnostic technique. The synergetic effect increased the energy density in the keyhole and deepened the weld penetration, resulting in a lower plasma electron temperature. The metal transfer mode was a globular one at a small offset distance while a spray mode was achieved with an increase in the offset distance. The decrease in the arc voltage and arc current due to the synergetic effect caused this transition from spray to globular modes. Globular transfer mode destabilized the molten pool and keyhole with the large droplet impingement, leading to the formation of porosity in the corresponding weld bead. The presence of porosity was on-line detected by identifying serious fluctuations in the Fe I electron temperature signals based on the fact that the instability of the molten pool and keyhole is strongly related to the signals coming from the plasma.     

Effects of shielding gas composition on arc characteristics and droplet transfer in tandem narrow gap GMA welding

The effects of shielding gas composition in tandem narrow gap gas metal arc welding were studied. The shielding gas included argon, carbon dioxide and helium. The arc characteristics and droplet transfer process were analysed. The results show that in the same welding parameters, the trail wire welding current is higher than the lead wire welding current. With the increase of carbon dioxide content, the welding currents of two wires decrease, and the trail wire droplet transfer mode transforms from spray transfer to projected transfer. With the increase of helium content, the welding currents increase and the lead wire droplet transfer mode transforms from projected transfer to spray transfer. The weld width is the largest when the shielding gas mixture is 80%Ar10%CO₂10%He.