等离子弧焊接熔池演变过程的格子Boltzmann模拟与验证

本文应用基于分子动理论的格子Boltzmann方法,建立了描述定点等离子弧焊接熔池动态演变过程的二维数理模型,对相变过程的传热与流动现象开展模拟;根据焊接过程能量分布特点改进等离子弧的组合式热源模型,采用total-enthalpy模型求解温度、速度分布及追踪相界面。研究结果表明,模拟的熔合线形状与实验焊缝吻合,格子Boltzmann模拟得到的计算精度及计算效率均优于基于连续流体假设的有限容积法,验证了格子Boltzmann方法用于等离子弧焊接模拟的可行性和优越性;熔池中出现两个方向相反的环流,流动对焊缝形状的作用不容忽略;熔池的流动方式影响了温度场、速度场及二者协同度,直接影响固相线上的热量传递,促进了焊缝中部凸起的形成。     

等离子弧焊接穿孔、传热与流动的耦合过程

本文建立了考虑穿孔效应的等离子弧焊接传热与流动的三维数学模型。在该模型中,开发出一种新颖的随小孔连动的热源模型,即能量密度仿照实际的焊接热物理过程随小孔增长而动态变化,有效地表征了热量沿厚度方向传输过程。同时应用体积流函数(VOF)方法追踪小孔界面,将小孔深度作为热源参数调控热源分布,实现了穿孔过程与熔池内传热及流动的动态耦合。针对实验工况,数值求解了穿孔焊接过程中动态热量传输和相应的焊接温度场,并考察了小孔界面及其周围熔融金属流动的演变过程;焊接熔池形状尺寸和焊件穿孔时间的计算值与实验数据吻合较好,验证了本文模型的正确性。     

电弧特性及其对熔池形貌影响的数值模拟

通过电弧模型与熔池模型耦合数值模拟,研究了氩弧和氦弧特性及其对SUS304不锈钢钨极惰性气体保护（TIG）焊熔池形貌的影响.通过比较氩弧和氦弧的温度轮廓线以及阳极表面电流密度和热流密度分布发现,氦弧的径向距离比氩弧收缩明显,导致更多热量传递给阳极.模拟了氩弧和氦弧下浮力、电磁力、表面张力和气体剪切力分别对熔池形貌的影响.结果表明：不论是在氩弧还是在氦弧下熔池中表面张力是影响熔池形貌的最主要驱动力.在氩弧下,影响熔池形貌的另一个重要的驱动力是气体剪切力,而氦弧下则是电磁力.由于电磁力引起的内对流运动增加了熔深,从而导致相同氧含量时氦弧下的熔深和焊缝深宽比要高于氩弧下的熔深和焊缝深宽比.随着氧含量的增加,氩弧和氦弧下的焊缝深宽比均先增加而后保持不变.焊缝深宽比的模拟结果与实验结果符合较好. 关键词： 氩弧 氦弧 电弧特性 熔池形貌     

焊接热输入对高温熔池形态的影响

焊接熔池形态对焊接过程和焊接质量有着重要影响,熔池的形状不仅决定了焊缝的形状,而且对焊缝的组织、力学性能和焊接质量均有重要影响。本文使用ANSYS有限元软件,采用"生死单元"技术,对X80管线钢复合型坡口多层焊的焊接过程进行二维模拟,研究各填充焊层受后一焊层热影响后,其高温停留时间和熔深随焊接热输入的影响规律。结果表明,焊层的峰值温度随着热输入的增加而增大,填充焊层焊接热输入从13.25 kJ/cm增加到14.57 kJ/cm,高温停留时间的百分比增量是热输入从12.04 kJ/cm增加到13.25 kJ/cm的一半;而各焊层熔深的增加值不变,均为0.45 mm。     

激光穿透焊的三维数值模拟

本文对激光穿透焊进行了三维数值模拟，除考虑焊接速度影响外，还研究了Ｍａｒａｎｇｏｎｉ对流对熔池流动与传热的影响。将三维数值模拟结果与相应的二维模拟结果进行了比较，结果表明，Ｍａｒａｎｇｏｎｉ对流对熔池的形成有很大的影响，因此不可忽略。本文还给出了估计激光穿透焊中蒸气孔尺寸与焊接热效率的方法。     

不同焊接位置对5A90铝锂合金激光焊接质量的影响(英)

针对铝锂合金激光焊接产生的成形不良和气孔缺陷,探究不同焊接位置对焊缝成形及气孔的影响,并对比分析不同焊接位置熔池受力状态、熔融金属流动和小孔动态行为,解释缺陷形成及其抑制机理。研究发现:不同焊接位置熔池受力状态和熔融金属流动决定焊缝成形。平焊时焊缝成形差、下榻严重;横焊时背面焊缝不连续,有飞溅;立向上焊时背面焊缝内凹严重;立向下焊可得到最优焊缝成形和最优的气孔缺陷。焊缝气孔受小孔动态行为影响,不同焊接位置气孔随热输入的变化规律一致,气孔先增后减,在仅熔池透状态下有气孔最大值。立上焊时小孔不稳定,焊缝气孔多且分布杂乱;立向下焊时小孔稳定性高,气孔最少,主要分布在焊缝中心线上。     

不同焊接位置对5A90铝锂合金激光焊接质量的影响（英文）

针对铝锂合金激光焊接产生的成形不良和气孔缺陷,探究不同焊接位置对焊缝成形及气孔的影响,并对比分析不同焊接位置熔池受力状态、熔融金属流动和小孔动态行为,解释缺陷形成及其抑制机理。研究发现:不同焊接位置熔池受力状态和熔融金属流动决定焊缝成形。平焊时焊缝成形差、下榻严重;横焊时背面焊缝不连续,有飞溅;立向上焊时背面焊缝内凹严重;立向下焊可得到最优焊缝成形和最优的气孔缺陷。焊缝气孔受小孔动态行为影响,不同焊接位置气孔随热输入的变化规律一致,气孔先增后减,在仅熔池透状态下有气孔最大值。立上焊时小孔不稳定,焊缝气孔多且分布杂乱;立向下焊时小孔稳定性高,气孔最少,主要分布在焊缝中心线上。     

双面电弧焊接的传热模型

将等离子焊接(PAW)电弧和钨极氩弧焊(TIG)电弧串接,相对作用于工件的正反面形成双面电弧焊接(DSAW)系统,可以引导焊接电流沿工件厚度方向流过小孔,补偿等离子电弧穿透工件时消耗的能量,以有效地提高等离子弧的穿透能力.综合考虑影响双面电弧焊接正反面熔池几何形状的力学因素,建立了熔池表面变形的控制方程,以此为基础并采用帖体曲线坐标系建立了DSAW焊接传热的数学模型,分析了DSAW,PAW焊接传热的差异,从传热的角度解释了DSAW焊接熔深增加的原因.焊接工艺实验表明,计算结果与实测结果吻合良好. 关键词： 双面电弧焊接(DSAW) 传热模型 熔池表面变形模型     

MIG焊接熔池表面形状与熔滴热焓量分布的数学模型

综合考虑熔滴与熔池相互作用的物理过程,建立了描述熔池表面变形的数学模型和熔滴热焓量在熔池内部的分布模型.应用数值模拟技术分析了焊接工艺参数对熔池表面形状、熔滴热焓量分布区域、焊缝成形的相互影响规律,并进行了焊接工艺试验.     

大功率光纤激光焊焊缝跟踪偏差红外检测方法

精确控制激光束使其始终对准并跟踪焊缝是保证激光焊接质量的前提.针对大功率(激光功率10 kW)光纤激光焊接304型不锈钢紧密对接焊缝(间隙为0-0.1 mm),研究一种基于红外热像的焊缝跟踪偏差检测新方法. 采用红外传感高速摄像机摄取焊接区域熔池红外动态热像,分析激光束对准和偏离焊缝中心时的熔池温度分布和红外辐射特性,以熔池匙孔形变参数和热堆积效应参数作为激光束与焊缝中心偏差检测特征值,通过图像识别技术研究和分析特征值与焊缝偏差之间的关系. 激光焊接试验结果表明,熔池匙孔形变参数和热堆积效应参数与焊缝偏差 关键词： 大功率光纤激光焊 焊缝跟踪偏差 红外热像 检测     

A keyhole volumetric model for weld pool analysis in Nd:YAG pulsed laser welding

This study presents a new model for analyzing the temperature distribution and weld pool shape in Nd:YAG pulsed laser welding. In the proposed approach, a surface flux heat transfer model is applied in the low laser energy intensity region of the weld, while a keyhole heat transfer model based on a volumetric heat source is applied in the high laser energy intensity region of the weld. The correlation between the intensity of the laser input energy and the geometric parameters of the volumetric heat source is derived experimentally. A series of MARC finite element simulations based on the proposed single pulse model are performed to investigate the shape and size of the weld pool given different laser energy intensities. A good agreement is observed between the simulation results and the experimental results obtained under equivalent single pulse welding conditions. Thus, the basic validity of the proposed model is confirmed.     

3D transient multiphase model for keyhole,vapor plume,and weld pool dynamics in laser welding including the ambient pressure effect

The physical process of deep penetration laser welding involves complex, self-consistent multiphase keyhole, metallic vapor plume, and weld pool dynamics. Currently, efforts are still needed to understand these multiphase dynamics. In this paper, a novel 3D transient multiphase model capable of describing a self-consistent keyhole, metallic vapor plume in the keyhole, and weld pool dynamics in deep penetration fiber laser welding is proposed. Major physical factors of the welding process, such as recoil pressure, surface tension, Marangoni shear stress, Fresnel absorptions mechanisms, heat transfer, and fluid flow in weld pool, keyhole free surface evolutions and solid–liquid–vapor three phase transformations are coupling considered. The effect of ambient pressure in laser welding is rigorously treated using an improved recoil pressure model. The predicated weld bead dimensions, transient keyhole instability, weld pool dynamics, and vapor plume dynamics are compared with experimental and literature results, and good agreements are obtained. The predicted results are investigated by not considering the effects of the ambient pressure. It is found that by not considering the effects of ambient pressure, the average keyhole wall temperature is underestimated about 500 K; besides, the average speed of metallic vapor will be significantly overestimated. The ambient pressure is an essential physical factor for a comprehensive understanding the dynamics of deep penetration laser welding.     

An investigation on capability of hybrid Nd:YAG laser-TIG welding technology for AA2198 Al-Li alloy

This paper surveys the capability of the hybrid laser-arc welding in comparison with lone laser welding for AA2198 aluminum alloy experimentally. In the present research, a continuous Nd:YAG laser with a maximum power of 2000 W and a 350 A electric arc were used as two combined welding heat sources. In addition to the lone laser welding experiments, two strategies were examined for hybrid welding; the first one was low laser power (100 W) accompanied by high arc energy, and the second one was high laser power (2000 W) with low arc energy. Welding speed and arc current varied in the experiments. The influence of heat input on weld pool geometry was surveyed. The macrosection, microhardness profile and microstructure of the welded joints were studied and compared. The results indicated that in lone laser welding, conduction mode occurred and keyhole was not formed even in low welding speeds and thus the penetration depth was so low. It was also found that the second approach (high laser power accompanied with low arc energy) is superior to the first one (low laser power accompanied with high arc energy) in hybrid laser-arc welding of Al2198, since lower heat input was needed for full penetration weld and as a result a smaller HAZ was created.     

Optimization of hybrid laser – TIG welding of 316LN steel using response surface methodology (RSM)

In the present study, the hybrid laser – TIG welding parameters for welding of 316LN austenitic stainless steel have been investigated by combining a pulsed laser beam with a TIG welding heat source at the weld pool. Laser power, pulse frequency, pulse duration, TIG current were presumed as the welding process parameters whereas weld bead width, weld cross-sectional area and depth of penetration (DOP) were considered as the process responses. Central composite design was used to complete the design matrix and welding experiments were conducted based on the design matrix. Weld bead measurements were then carried out to generate the dataset. Multiple regression models correlating the process parameters with the responses have been developed. The accuracy of the models were found to be good. Then, the desirability approach optimization technique was employed for determining the optimum process parameters to obtain the desired weld bead profile. Validation experiments were then carried out from the determined optimum process parameters. There was good agreement between the predicted and measured values.     

Effect of carbon black on temperature field and weld profile during laser transmission welding of polymers: A FEM study

The influence of the carbon black on temperature distribution and weld profile, during laser transmission welding of polymers, is investigated in the present research work. A transient numerical model, based on conduction mode heat transfer, is developed to analyze the process. The heat input to the model is considered to be the volumetric Gaussian heat source. The computation of temperature field during welding is carried out for polycarbonates having different proportion of carbon black in polymer matrix. The temperature dependent material properties of polycarbonate are taken into account for modeling. The finite element code ANSYS^® is employed to obtain the numerical results. The numerically computed results of weld pool dimensions are compared with the experimental results. The comparison shows a fair agreement between them, which gives confidence to use the developed model for intended investigation with acceptable accuracy. The results obtained have revealed that the carbon black has considerable influence on the temperature field distribution and the formation of the weld pool geometry.     

Numerical analysis of plasma arc physical characteristics under additional constraint of keyhole

The physical characteristics of a plasma arc affect the stability of the keyhole and weld pool directly during keyhole plasma arc welding(KPAW). There will be significant change for these characteristics because of the interaction between the keyhole weld pool and plasma arc after penetration. Therefore, in order to obtain the temperature field, flow field, and arc pressure of a plasma arc under the reaction of the keyhole, the physical model of a plasma arc with a pre-set keyhole was established. In addition, the tungsten and base metal were established into the calculated domain, which can reflect the effect of plasma arc to weld pool further. Based on magneto hydrodynamics and Maxwell equations, a two-dimensional steady state mathematical model was established. Considering the heat production of anode and cathode, the distribution of temperature field, flow field, welding current density, and plasma arc pressure were solved out by the finite difference method. From the calculated results, it is found that the plasma arc was compressed a second time by the keyhole. This additional constraint results in an obvious rise of the plasma arc pressure and flow velocity at the minimum diameter place of the keyhole, while the temperature field is impacted slightly. Finally, the observational and metallographic experiments are conducted, and the shapes of plasma arc and fusion line agree with the simulated results generally.     

Parametric optimisation and microstructural analysis on high power Yb-fibre laser welding of Ti–6Al–4V

In this work thin sheets of Ti–6Al–4V were full penetration welded using a 5 kW fibre laser in order to evaluate the effectiveness of high power fibre laser as a welding processing tool for welding Ti–6Al–4V with the requirements of the aircraft industry and to determine the effect of welding parameters including laser power, welding speed and beam focal position on the weld microstructure, bead profile and weld quality. It involved establishing an understanding of the influence of welding parameters on microstructural change, welding defects, and the characteristics of heat affected zone (HAZ) and weld metal (WM) of fibre laser welded joints. The optimum range of welding parameters which produced welds without cracking and porosity were identified. The influence of the welding parameters on the weld joint heterogeneity was characterised by conducting detailed microstructural analysis.