电子束焊接工件的传导电流特性

讨论了电子束焊接过程中电子失去动能的能量转换和电子定向流动的电流传导这两种物理过程，提出并定义了工件传导电流概念，设计了专用的多通道电流信号采集系统，从复杂的电子束焊接动态过程中剥离并定量检测出包涵焊接信息的工件传导电流的动态行为。全面系统地研究了工件传导电流的时域、频域、排序特性、传导比等特性；发现并证明了传导比可定量地描述焊接动态过程的临界穿透状态；利用研究获得的传导比均值与输入束流的函数关系建立了可描述电子束焊接过程的动态特性判据。     

电子束变焦——临界温度极值焦点测量法

针对电子束焦点难以直接测量的问题,提出一种利用电子束流与金属粉末相互作用产生的熔池温度极值效应测量电子束流焦点的方法.文中分析讨论了电子束流焦点位置的影响因素,通过试验研究了电子束加热过程中粉末熔池温度与聚焦电流的函数规律,发现温度—聚焦电流关系函数的极大值即为聚焦电子束能量密度分布状态的临界转变点.基于扫描电子束粉末烧结过程的这种临界温度特性,提出了一种测量电子束加工过程动态焦点的方法,即变焦-临界温度极值检测的焦点测量法.结果表明,这种方法可以实现电子束焦点位置的快速、高效地检测与定位控制.     

电子束流焦点和测量方法进展及分类

回顾了焦点定义和测量方法,首次明确将电子束流焦点状态与焊接过程建立直接联系,揭示了具有焊接特征焦点与测量方法的内在关系,并根据束流与金属是否产生作用和作用的程度,将焦点分类成静态焦点、准动态焦点和动态焦点。指出静态焦点是轨迹形状焦点,AB法实质就是斜面-熔宽极值焦点测量法,认识了焦点测量方法本身定义的焦点与静态焦点的本质区别,使得在电子束焊接工程中焦点概念应用更清晰,焦点参数使用更方便,并且为进一步开发新的焦点测量方法提供了理论指导。     

斜面-熔池极值法测量电子束流准动态焦点

分析了利用电子束流与金属相互作用效应产生的熔池极值效应测量电子束流焦点的方法,总结了测量电子束流焦点的要素,讨论了利用电子束流焊接效应测量得到的焦点与电子束流的轨迹形状焦点的区别,提出了电子束动态焦点的概念,并且将束流焦点分类为静态焦点、准动态焦点和动态焦点。基于以上理论分析,进一步采用类AB法测量了电子束焊机的准动态焦点,讨论并分析了上坡焊和下坡焊方法对所测焦点的影响。     

厚件电子束动态焦点焊接数值模拟

焊深波动是影响厚件电子束动态焦点焊(也称电子束变焦焊)质量的一个重要因素。根据电子束动态焦点焊热源特点,建立了由圆锥体热源和峰值功率递增式旋转高斯曲面体热源组成的复合热源模型,并通过ANSYS软件自带的APDL语言编程实现了束流焦点在纵向上周期性下移的热源加载。进行了变焦频率为52 Hz的50 mm厚合金钢板电子束动态焦点焊数值模拟和试验。结果表明:厚件电子束焊接中焊深存在波动,动态焦点焊接模拟的焊缝形貌、焊深波动与变焦焊试验结果相吻合。     

电子束聚焦特性对束流品质的影响研究

电子束聚焦特性是影响电子束焊接束流品质的主要因素之一,对焊缝质量影响较大。本文采用束流品质测量系统,在不锈钢材料上研究了不同聚焦特性下电子束能量密度分布及焦斑特性等束流品质特征,并采用金相显微镜对焊缝成形特性进行了表征。研究结果表明:在一定的电子束电流下,当束流在试件表面为表面聚焦状态时,电子束焦斑直径最小,平均束流密度最高,所得到的焊缝深度最大;随着束流在表面状态向上聚焦或下聚焦状态流散转变,电子束在工件表面焦斑直径增大,平均束流密度降低,焊深减小。     

聚焦电流对电子束填丝焊接焊缝几何特征的影响

通常情况下,电子束焊接工艺研究时将聚焦电流作为焦点位置的主要表征参数.电子束填丝焊接时,通过改变聚焦电流,可以控制束流的焦点位置.不同的焦点位置决定了束流到达工件表面时的能量分布,直接影响到焊丝及母材的熔凝状况,进而对电子束填丝焊接焊缝截面几何形状具有重要影响.通过改变聚焦电流、其余参数不变,能够得到聚焦电流与焊缝截面形状、焊缝横截面面积、熔深、表面熔宽、半熔深处熔宽、余高等焊缝截面几何参量之间的关系,对优化电子束填丝焊接工艺参数具有积极参考意义.     

电子束流的动态焦点和深穿极值效应

通过分析具有焊接特征的焦点测量方法，利用简单熔池模型说明了具有焊接特征的不同焦点测量方法之间的内在关系，建立了电子束流的动态焦点概念，讨论了动态焦点的影响因素，进一步分析了动态焦点的性质，其中最重要的性质是深穿极值效应。     

电子束非穿透焊中束流品质对焊缝质量的影响

针对电子束非穿透焊接,通过工艺试验研究束流品质与焊缝内部质量之间的关系,发现影响非穿透焊中产生钉尖缺陷的主要因素是束流品质的下部宽度和峰值能量,并进行了理论分析。采用Design-Expert软件响应面分析法,建立束流品质与焊缝熔深之间的数学模型。     

束参数及真空度对电子束形状的影响

本文通过电子束形状的测定试验,研究了低真空下束流、偏压和聚焦电流变化对束焦点直径及其位置的影响,由于束流增大时焦点位置上移,指出了厚板电子束焊接中传统的“小束流对中对焦,大束流焊接”工艺方法的不合理性。本文还着重研究了在13.332Pa、133.32Pa 的低真空范围内,工作室真空度变化对于束形状的影响。并提出了确定束焦点直径及其位置的经验公式。     

Characterization of electrical properties of resistance welding machines

Due to the individual electrical and mechanical characteristics of resistance welding machines, choice of the right machine and welding parameters for an optimized production is often difficult. This is especially the case in projection welding of complex joints. In this paper, a new approach of characterizing the electrical properties of AC resistance welding machines is presented, involving testing and mathematical modelling of the weld current, the firing angle and the conduction angle of silicon controlled rectifiers with the aid of a series of proof resistances. The model predicts the weld current and the conduction angle (or heat setting) at each set current, when the workpiece resistance is given.     

A numerical model for deep penetration welding processes

The general features of a numerical model, and of its extensions, for calculating the temperature and fluid velocity field in a three-dimensional workpiece undergoing deep penetration laser beam welding are described. In the current model, the deposition of power from the beam is represented by time-dependent boundary conditions on the equations of energy and momentum transfer. These boundary conditions are specified at each timestep on a surface whose configuration can change with time and upon which energy is deposited according to a specified power distribution. This model also includes the effects of the buoy-ancy force on the melt pool and of the surface tension gradient on the surface of the fluid. The coupled equations of energy, momentum transfer, and continuity combined with the time-dependent boundary conditions representing the keyhole and the moving boundaries of the workpiece are solved by using a specific implementation of the SIMPLE algorithm. The important features of the numerical methods used in the model are discussed. Isotherms and convection patterns calculated using the current model are presented, and their significance for predicting weldment properties is discussed. A significant result of the simulations is that they demonstrate the overwhelming influence of the keyhole vapor/liquid inter-face on fluid convection and conduction in deep penetration welding.     

Evaluation of molten pool geometry with induced plasma plume absorption in laser-material interaction zone

The presence of plasma affects laser material processing technology because, according to process parameters, a large portion of the energy emitted by the laser source is absorbed by the plasma plume without hitting the workpiece. The only way to avoid a significant reduction in process efficiency, due to plasma absorption, is thus to decrease the plasma formation by controlling the working parameters.An original analytical system for the prediction of the actual energy transmitted to the workpiece was developed by modelling the plasma plume physical state related to the process parameters. In this way, by determining the laser beam energy lost in the plasma plume and the conduction energy transmitted to the workpiece, an evaluation of the laser material interaction could be carried out.The developed model allows to evaluate the geometry of the molten pool by means of the computation of the interface between the solid and the remelted material. The effect of the plasma plume presence, by comparison with a modelisation without plasma implemented in similar way by the authors, was to reduce the molten pool and in particular the penetration depth and it permits to have close simulation results to experimental data.For the model validation several experiments were performed on an austenitic stainless steel with a CW CO₂ laser source. The experimental activity was developed by varying process speed and power level up to 1200 W when in the range of conduction welding.     

Simulation of deep penetration welding of stainless steel using geometric constraints based on experimental information

Results of a numerical simulation of deep penetration welding of 304 stainless steel are presented. This numerical model calculates the temperature and fluid velocity fields in a three-dimensional workpiece undergoing deep-penetration electron beam welding. The deposition of power from the beam and energy outflow at the model-system boundaries is effected by means of time-dependent boundary conditions on the equations of energy and momentum transfer. The vapor-liquid interface defining the keyhole is represented by a surface whose temperature is that of vaporization for the steel. On this surface, are specified boundary conditions for the momentum transfer equations such that the component of the velocity normal to the keyhole vapor-liquid interface is zero. In addition, this study introduces two new numerical procedures. These procedures are based on the inclusion of experimental information concerning beam spot size and weld pool geometry into the model system via constraints and the deduction of effective keyhole shape via an inverse mapping scheme.     

Optimisation of welding using two Nd–YAG laser beams combined at workpiece surface

Abstract

Two Nd–YAG laser beams were combined at a certain point on the workpiece surface to increase weld penetration depth. One of the beams was a pulsed laser beam, and the other was a continuous wave laser beam or a modulated laser beam. Using this combination of laser beams, a wide range of welding conditions, such as average power, peak power, and power density, could be selected. A high peak power pulsed laser beam would play a significant role in forming a keyhole, but a severe spatter loss problem could be encountered under high peak power laser conditions, thus the conditions necessary to prevent spatter loss were investigated. The greatest penetration depth is obtained under the critical conditions for spatter loss. Critical conditions for spatter loss are controlled by the peak power of a pulsed laser beam, thus deeper weld penetration is obtained using a pulsed laser beam with higher average power, that is, of longer pulse width and/or a higher repetition rate within the limit of the oscillator output. Moreover, spatter loss is reduced under conditions providing large molten zones in the weld, thus a higher peak power pulsed laser beam can be employed under such conditions. Large molten zones are obtained using a modulated laser beam of a high average power and/or low welding speeds.