New general double ellipsoid heat source model

Abstract

Based on the tridimensional Gauss distribution of power density, a general double ellipsoid welding heat source model has been developed. This model not only consists of all the characteristics of a double ellipsoid model, but also can deal with the situation where, under an external disturbance, the arc's backbone is not perpendicular to the work surface. This general double ellipsoid model is validated by measured results of the temperature field during twin wire welding. Using the non-linear finite element software Marc, the temperature field during twin wire welding was calculated using both the double ellipsoid heat source and the general double ellipsoid heat source. The thermal cycling curves and the weld pool cross-section obtained by the general double ellipsoid heat source tally well with the experimental results.     

Effects of latent heat of fusion on thermal processes in laser welding of aluminium alloys

Abstract

Based on the Green's function method, a mathematical model allowing for the latent heat of fusion and solidification is developed to describe the steady state, two-dimensional heat flow during welding of thin plates. It is demonstrated that the latent heat has a pronounced effect on shape and size of the weld pool and mushy zone. The thermal efficiency of base metal fusion by a line heat source η _t can exceed 0·4839 considerably if the latent heat is taken into account. It is shown that the known simplified approaches for considering the latent heat can introduce large errors into the estimation of η _t. The calculated and experimental weld pool shapes are compared.     

Study on heat source model in twin-arc GMAW with a common weld pool

The heat input from arcs to weld pool in twin-arc gas metal arc welding (GMAW) with a common weld pool is investigated by high-speed photography. The characteristics of arc shapes and droplet transfer are studied and then the models for heat flux distribution on top surface of weld pool and enthalpy distribution of metal droplets transferred into weld pool are established. By using the model, 3-D geometries of weld pools in twin-arc GMAW with a common weld pool are predicted. Corresponding welding experiments on mild steel plates are carried out and the results indicate that the predicted shape of weld bead on cross section shows good agreement with measured one.     

Applicable conditions of instantaneous source used for welding heat conduction

Abstract

This paper deals with the sufficient conditions under which instantaneous heat source can be considered as welding heat source instead of moving source. Temperature rises calculated by instantaneous source and moving one were compared. The applicable conditions of the instantaneous heat source with errors of 1% and 5% were found. This enables us to select either moving or instantaneous source with information of welding speed, thermal diffusivity and the distance from the weld centre line to the interested location. Approximate equations to determine the maximum temperature rise using a moving source were proposed and the accuracy of their calculation was proved to be ～99%.     

Coupled arc and droplet model of GMAW

Abstract

A model of gas metal arc welding was developed that solves the magnetohydrodynamic equations for the flow and temperature fields of the molten electrode and plasma simultaneously, to form a fully coupled model. A commercial finite-element code was extended to include the effects of radiation, Lorentz forces, Joule heating and thermo-electric effects. The model predicts the shape of the free surfaces of the molten metal as the droplets form, detach, and merge with the weld pool. It also predicts the flow, temperature, and electric field. Material properties and the welding parameters are the input variables in the model. The geometry of the numerical model was constructed to fit an experimental apparatus using an aluminium electrode and an argon shielding gas. Droplet frequency measurements were used to verify the model's predictions. For a typical arc, the temperature of the plasma can range up to 20 000 K, where there is more uncertainty in the thermophysical properties of the plasma, and the properties in this range are highly non-linear. For this range, the material properties of the model were adjusted to obtain a better fit between the numerical and the experimental results. The model and experimental results were comparable.     

Simulation of shape and temperature field evolution of solder joints during reflow solidification process

Abstract

A large proportion of research on soldered joints focuses on the effects of stress on the service life of a solder joint. However, the shape of a joint also has a great effect on its service life. During the reflow process, the solder joint experiences heating, melting, cooling and then solidification. Conventionally, the shape of a solder joint is often predicted using the energy method after the reflow process. This, however, does not consider the evolution of the solder alloy during reflow. In this study, a computer aided analytical system is developed based on computational heat transfer and fluid dynamics techniques to simulate the shape evolution of solder joints during reflow. Experimental observations of the shape evolution of the joint during reflow were also conducted. The simulation results obtained for a solder ball 760 μm in diameter on a rectangular pad were found to be consistent with the experimental observations.     

Validation of three-dimensional finite element model for electron beam welding of Inconel 718

Abstract

A three-dimensional finite element model for the prediction of the distortion and residual stresses induced during electron beam welding is described. The model is validated by butt welding experiments on two Inconel 718 plates. A particular effort is made to determine a good model for the heat input. A combined conical and double ellipsoid heat source is used to model the deep penetration characteristic of the electron beam and this source is calibrated using the results from a separate thermodynamic simulation, using the ELSIM finite difference code. Parallel computation is used to reduce the overall simulation time in the coupled thermomechanical simulation of welding. The agreement between calculations and experiments is good with respect to the residual stresses. Measured and computed deformations agree qualitatively although they differ in magnitude.     

Simulation of role of precipitate in creep void occurrence in heat affected zone of high Cr ferritic heat resistant steels

Abstract

A three-dimensional (3D) precipitate and matrix model was used to investigate the role of precipitates as nucleation sites of creep void in the heat affected zone (HAZ) of a welded joint of high Cr ferritic heat resistant steels at high temperature and low stress. Several factors affecting creep void initiation, such as Young's modulus of precipitate and precipitate size, were studied. The material properties of the matrix were investigated for the regions of coarse grained HAZ, fine grained HAZ and base metal in the welded joint. The results show that creep void occurs easily in the fine grained HAZ region because of the deterioration in material properties and coarsened precipitates in this zone     

Quantitative simulations of microstructure evolution in single crystal superalloys during solution heat treatment

Abstract

This work presents a modelling approach for the simulation of the microstructure evolution during solution heat treatment in technical, multi component superalloys. This model is based on a theoretical approach and calclations. All calculations start with the simulation of solidification, as the as-cast microstructure is the starting condition to simulate the subsequent heat treatment. The model takes into account typical length scales, diffusion, morphological and constitutional aspects of the alloys. Simulations are performed for an alloy with several alloying elements. It is proposed that with the aid of the simulations optimised solution heat treatments can be derived.     

脉冲GMAW熔滴过渡动态过程的解析模型

在脉冲GMAW过程中，电流的突然转变会引起熔滴在焊丝末端的明显振荡，合理的利用向下振荡的动量可控制熔滴过渡。该文基于熔滴受力分析，建立了脉冲GMAW熔滴过渡的解析模型，并对熔滴的振荡和脱离过程进行分析，定量计算焊接工艺参数对熔滴过渡过程动态行为的影响，可用于指导脉冲GMAW主动控制模式下一脉一滴过渡工艺参数的优化设计。     

Analytical thermal model of conduction mode double sided arc welding

Abstract

An analytical thermal model of conduction mode double sided arc welding (DSAW) has been derived and used to predict the weld pool dimensions and shapes and temperatures within 2˙5 and 1˙15 mm thick AA5182 Al alloy sheets as functions of the primary DSAW parameters. Separate Gaussian distributed arc heat sources from a plasma arc welding and gas tungsten arc welding torch were assumed to act on the top and bottom surfaces of the sheets. There was excellent correlation between observed and predicted DSAW weld pool dimensions and shapes provided that suitable values for arc efficiencies and distribution coefficients for the two separate arcs were used in the model. The model is capable of predicting weld pool dimensions and shapes of both full and partial penetration conduction mode DSAW welds made in Al alloy sheet, the welding speed at which there is a transition from full to partial penetration welding and the speed above which no melting occurs.     

基于新型控制法的逆变GMAW工艺特点

随着熔化极气体保护焊的应用与推广,那些以飞溅小和成形好为特点的通用型焊机常常不能适应新的环境和要求,如西气东输中的管道打底焊、集装箱的高速焊、汽车制造中的短焊缝焊接、角焊缝的尺寸控制和狭窄空间的熔深控制等,还有超薄板焊接、双脉冲焊、双丝高速焊、TIME高效焊和磁控MAG焊等,这些具有特殊功能的焊接方法,我们称为个性化焊接.这些个性化焊接方法均具有自己的工艺特点,并能完成一些特殊的焊接要求.可见这些个性化焊接方法必将成为GMAW推广应用的新的增长点.     

Development of circumferential TIG welding process model: a simulation model for welding of pipe and plate

Abstract

It is useful to develop a numerical model for various welding positions in order to understand welding phenomena. Welding simulation models for a particular application could help in offline robot programming and online robot control. The objective of this study was to develop a tungsten inert gas (TIG) welding simulation model for various workpieces of different shapes. The present model may be easily applied to several workpiece configurations. This article deals with a model for circumferential welding of a pipe to a plate of mild steel, and aluminium alloys. The calculated results clarify that a homogeneous weld is hardly obtained at a fixed pipe welding position due to gravity, and that weld bead shape varies sensitively with wall thickness in pipe to plate welding. It is concluded that the model proposed in this article is useful to simply simulate TIG welding and provides a powerful means to estimate the optimum process parameters in welding practice.     

基于复合视觉和形态学模糊检测的GMAW焊过程质量控制

视觉图像检测是实现GMAW焊接质量控制的重要步骤。针对焊接图像稳定性差、噪声大的特点,提出了一种基于复合视觉和形态学模糊检测的算法。该算法采用双CCD采集焊接视觉图像并使用数学形态学理论处理图像,通过将该图像检测算法应用于焊接动态过程中的熔池图像处理,结合闭环模糊PID控制理论,一套完整的GMAW焊过程质量控制系统得以实现。MAG焊工艺试验表明,该控制系统可以有效地克服焊接过程中的外界干扰,从而保证焊接质量的稳定。     

Effect of section thickness and modification on thermal analysis parameters of A357 alloy

Abstract

Thermal analysis technique relies on the cooling curve obtained when the sample is cooled in a sampling cup. This may not represent the cooling behaviour of the real casting. The microstructure developed during solidification depends not only on the nucleation and modification potential of the melt but also on the thermal gradient imposed during solidification by the mould. The factors affecting the thermal gradient are the mould material and casting section thickness. In the present investigation the effect of modification melt treatment, cooling rate and casting section thickness on the thermal analysis parameters of A357 alloy was studied. It is found that the dimensionless heat flux parameter is high for small section thickness castings. The metal/mould interfacial heat flux is high in a copper mould. Thermal analysis parameters of A357 alloy are found to be affected significantly by the combined action of modification, chilling and section thickness.     

Effect of two step heating process on joint microstructure and properties during transient liquid phase bonding

Abstract

45MnMoB steel was joined by transient liquid phase (TLP) bonding using three two step heating processes. The joining area was first heated up to 1250°C and kept for 5 s, and then cooled down to 1210, 1220 and 1230°C named second holding temperature respectively and kept for 120 s. The interface morphology of the bond made using two step heating process was also compared with that of conventional TLP bond at a constant temperature. The results show that two step heating process can produce a cellular interface at the initial stage, which is different from the planar interfaces associated with conventional TLP bonding. No bond interface can be found in the final joint by two step heating process and the microstructure of joint is similar to that of base metal. With the increase in second holding temperature, oxide scale particles and porosities in the bond region are decreased noticeably both in size and amount, and the bond strength is increased. The bond fails at the base metal and no failure occurs when the joint is tensioned and bent to 180° at 1250 and 1230°C respectively. Therefore, two step heating process can produce a homogenous joint, free of bond interface and identical in composition and properties to that of the base metal.     

基于复合热源模型的Al-Mg-Zn铝合金脉冲MIG焊接模拟

针对脉冲MIG焊接试验过程建立了复合热源模型,并采用有限元软件ABAQUS对板厚15 mm的Al-Mg-Zn铝合金进行了多层多道脉冲MIG焊接的传热过程数值模拟. 根据复合热源和单一双椭球热源进行计算对比. 结果表明,该组合热源可以较好的模拟脉冲MIG焊接不同道次的熔池形貌,并且降低热源校准难度. 同时,基于建立的复合热源模型,文中引入非线性弹性边界条件表征实际夹具工装,对脉冲MIG焊接过程的应力和变形分布进行仿真模拟,其中焊后变形分布规律和试验测量具有较好的一致性.     

基于MSPC方法的GMAW在线监测

熔化极气体保护焊(GMAW)广泛地应用于自动焊中，其焊接过程在干扰存在的情况下易产生焊接质量问题，因此，自动焊中焊接质量的在线监测已成为制造业中亟待解决的问题之一。为实现焊接产品质量的“零缺陷”及生产过程质量监测的目标，采用电弧传感获取的电信号，从单变量统计过程控制的应用结果出发，提出了把多变量统计过程控制(MSPC)应用于GMAW过程的在线质量监测。并在自我开发的一套在线监测系统上，通过试验验证了MSPC方法的可行性。     

基于GMAW动态模型的短路过渡过程稳定性分析

基于已建立的CO₂焊短路过渡模型,模拟了短路过渡的动态过程;预测了不同送丝速度下短路过渡频率、接触熔滴和残余熔滴等效半径、熔滴的速度和位移以及液桥的变化;分析了上述参数与稳定性之间的关系,并成功预测了最佳电弧点,为焊接过程的参数优化提供依据.结果表明,当熔滴过渡频率最大时,并未达到最佳焊接条件;在最优焊接参数下,残余熔滴等效半径达到最小,且熔滴振荡速度和位移变化范围较小;接触熔滴等效半径和液桥曲率半径越小,越有利于提高短路过渡的稳定性.     

Effect of droplet heat content distribution on humping formation in high speed GMAW

The momentum of strong backward flowing melt jet and the thermal action from transferred droplets are two dominating factors affecting the formation of humping bead in high speed gas metal arc welding (GMAW). Appropriate describing the influence of the distribution mode of droplet heat content in the weld pool is essential to understand the physical mechanism of humping bead formation. Based on the experimental results, four kinds of droplet heat content distribution modes are proposed and employed to calcu...