Numerical simulation of molten pool dynamics in high power disk laser welding

A single-phase problem is solved rather than a multiphase problem for numerical simplicity: and the solution is based on the assumption that the region of gas or plasma can be treated as a void because solid or liquid steel has a greater density level than gas or plasma. The volume-of-fluid method, which can calculate the free surface shape of the keyhole, is used in conjunction with a ray-tracing algorithm to estimate the multiple reflections. Fresnel's reflection model is simplified by the Hagen-Rubens relation for handling a laser beam interaction with materials. Factors considered in the simulations include buoyancy force, Marangoni force and recoil pressure; furthermore, pore generation is simulated by means of an adiabatic bubble model, which can also lead to the phenomenon of a keyhole collapse. Models of the shear stress on the keyhole surface and of the heat transfer to the molten pool via a plasma plume are introduced in simulations of the weld pool dynamics. Analysis of the temperature profile characteristics of the weld bead and molten pool flow in the molten pool is based on the results of the numerical simulations. The simulation results are used to estimate the weld fusion zone shape; and the results of the simulated fusion zone formation are compared with the results of the experimental fusion zone formation and found to be in good agreement. The effects of laser beam profile (Gaussian vs. measured), vapor shear stress, vapor heat source and sulfur content on the molten pool behavior and fusion zone shape are analyzed.     

Numerical simulation of full penetration laser welding of thick steel plate with high power high brightness laser

Full penetration laser welding was carried out on a 10 mm steel plate using a 16 kW maximum power continuous wave thin disk laser. Upper surface and lower surface of molten pool were observed synchronously with two high speed CCD cameras during the welding process. The lower surface was much longer and more unstable than the upper one. A three dimensional laser deep penetration welding model in which volume of fluid (VOF) method was combined with a ray-tracing algorithm was used to simulate the dynamic coupling between keyhole and molten pool in laser full penetration welding. The calculated weld cross-section morphology and molten pool length on both upper side and lower side agree well with experimental results. Evolution of molten pool in lower side during full penetration laser welding was analyzed, periodical features of energy coupling, molten pool behavior and keyhole dynamics in laser full penetration welding were identified and discussed.     

Modeling of temperature field and pool formation during linear laser welding of DP1000 steel

A rotary-Gauss body heat source was employed in the study to model the laser welding of DP1000 steel. The condition of heat dissipation during the welding has a significant effect on the temperature field as well as the shape and size of the laser weld. A series of welding experiments were performed, and good agreement was observed between the calculated weld dimensions and the experimental results. The microhardness values across the welded joint were measured to determine the range of the soft zone in the heat affected zone (HAZ) and simultaneously the temperature range experienced in this region. The results indicated that the soft zone significantly affects the mechanical performance of the welded joint. The width of the soft zone and its distance from the weld center increase with increasing laser power, while the width of the soft zone and its distance from the weld center decrease with increasing welding speed.     

1420铝锂合金激光焊接气孔形成机理

1420铝锂合金与传统的2000、7000系列铝合金相比，密度低12％，弹性模量高8％左右，具有优异的抗腐蚀性和高温性能。激光焊接作为一种快速高效的焊接方法，在焊接1420铝锂合金时存在一个问题，即具有严重的气孔倾向。针对1420铝锂合金激光焊接中气孔产生的原因进行了比较系统的分析。分析认为，表层物质是焊接过程中氢的主要来源，一定要采用适当的方法进行彻底的清除，而Mg、Li等合金元素不仅增加了熔池吸氢倾向，还增加了匙孔末端的不稳定性，应该采用合适的熔透模式，改善熔池的流动，将这种不稳定性降到最低。     

1420铝锂合金激光焊接气孔抑制技术

分别采用气体和固体两种类型、三台激光器对1420铝锂合金消除气孔的焊接工艺进行了试验研究.结果表明,1420铝合金表面氧化膜对产生气孔有很大影响,化学清理可以获得气孔较少的焊缝.保护气体种类、气体流量、焊接速度对1420铝锂合金气孔都有影响.适宜的双激光束焊接工艺可以获得成形美观且无气孔的优质1420铝锂合金焊缝,是1420铝锂合金焊接较为理想的焊接工艺.     

Inherent instability investigation for low speed laser welding of aluminum using a single-mode fiber laser

The causes of instability are investigated for low speed welding of aluminum from 10 mm/s down to 1 mm/s using a 300 W single-mode fiber laser. Results show that the welding is stable until the speed drops below a certain threshold (∼1 mm/s) at which there was a significant change in the process mechanism, causing shallow, inefficient welds with many defects. A power distribution model and several tests are used to examine different types of power losses at low speeds. It is then hypothesized that, at low speeds, the CW laser beam mainly irradiates at the molten pool, which absorbs a large portion of the beam energy near the surface. The majority of this absorbed energy subsequently is either lost via evaporation or transferred into the bulk material via convection and conduction without being used for melting the solid at the welding front. A laser pulsing scheme was used to test the above hypothesis. It was found that, through proper control of the duty cycle and frequency to prevent overheating of the molten pool, a high aspect ratio weld shape can be restored at low speeds, thus, confirming the hypothesis. In addition, the 1 mm/s low speed threshold is found to be related to the initial molten pool propagation speed, which is found to be approximately 1.4 mm/s. Although this paper does not propose a solution to restore process stability, the understanding of the instability origin will be helpful in the search of such a solution to overcome the process instability for slow speed welding of aluminum.     

高功率激光去除薄金属构件中缺陷的研究

针对薄金属构件中的缺陷,为了探讨激光焊接去除缺陷之可行性与焊道的纯化效应机理,利用激光熔覆技术或不当焊接工艺来制作含缺陷的试样.薄金属构件中缺陷的去除系采用一次或多次keyhole激光焊接方式.利用X射线探伤技术对所得试片进行检测,利用扫描电镜进行组织观察和成分分析,并对缺陷去除前后的试样进行V型缺口冲击试验.结果表明,keyhole激光焊接可以通过蒸发或分解夹杂物而有效地去除夹杂物颗粒,并显著提高材料的冲击韧性.     

Prediction of molten area in laser transmission welding of thermoplastic polymers

The prediction of the molten area is of great significance for laser transmission welding (LTW) of polymers. The purpose of the present study is to develop a mathematical model for LTW of polymers. The cross-sectional molten area of weld can be calculated through this model. The mathematical model was developed and deduced on the basis of the physical heat calculation formula. Subsequently, the model was validated by laser irradiating polyamide-66 (PA66) plaque containing 30 wt-% carbon fibre (CF), as well as LTW of PA66 containing 30 wt-% glass fibre with PA66 containing 30 wt-% CF. A viewpoint that the melt area of the upper layer to the bottom layer ratio (S_U/S_L) depends on the allocation proportion of the line energy between the interface during LTW of thermoplastic polymers was proposed. The model can well predict the molten area and help optimise process parameters during LTW of polymers.     

A 3D transient model of keyhole and melt pool dynamics in laser beam welding applied to the joining of zinc coated sheets

In order to get a deeper understanding of laser beam welding, a process model was developed at the Chair of Manufacturing Technology. It is based on the continuity equation, the equation of heat conduction and the Navier–Stokes equation. The model includes effects of Fresnel absorption, vapor pressure, surface tension, melting and evaporation enthalpy and energy loss due to evaporating material. This paper presents the results of a three-dimensional, transient finite volume simulation of a laser beam deep penetration welding process based on this model. The simulations show periodic keyhole oscillations and the complex fluid dynamics of the melt pool. A comparison of the evaporation rates calculated from the simulations and the experimentally observed process emissions shows good correlation. Furthermore, the simulations show pore formation at higher feed rates, the influence of a gap on the welding process and give an explanation for the welding behavior of zinc coated steel sheets.     

铝合金激光深熔焊气孔形成机理与控制技术

观察了铝合金激光深熔焊气孔的分布特征和形貌特征,深入分析了气孔的形成机理,研究了双光点能量分布的激光对铝合金激光深熔焊气孔的控制效果.结果表明,铝合金激光深熔焊焊缝中存在分布特征和形貌特征不同的两类气孔,即冶金类气孔和工艺类气孔.冶金类气孔的形成与氢在熔池中的析出、聚集与合并有关,而工艺类气孔产生的根本原因是焊接过程中匙孔的瞬间失稳.采用双光点能量分布的激光焊接铝合金可以扩大匙孔张口和根部直径,改善匙孔壁的波动状态,增强匙孔的稳定性,从而减少工艺类气孔的产生,但对冶金类气孔没有明显影响.     

铝及铝合金MIG焊接预防气孔的措施

针对铝及铝合金MIG焊接过程中容易产生气孑L的情况,分析了产生气孔的原因。这些气孑L主要是氢气孑L。分析了氢的来源,包括母材和焊丝表面的油污、水分及碳氢化合物,焊丝原材料中的氢,施焊环境中的氢。在焊接工艺方面采用亚射流过渡形式,采用合适的焊接速度、焊丝伸出长度、焊枪角度、气体流量、预热温度和层问温度,采用摩擦系数小的特氟龙送丝软管,可以有效的预防和减少铝及铝合金MIG焊接过程中产生的气孔。     

Study on the application for electromagnetic controlled molten pool welding process in overhead and flat position welding

Abstract

In fusion welding, gravity makes a molten metal flow downward and it sometimes causes an irregular shaped weld bead and weld defects such as an undercut. To solve this problem, the authors propose a new electromagnetic controlled molten pool welding process method which controls the molten metal flow by using upward electromagnetic forces, and the applicability of this method to industry is examined. In flat position welding with excessive heat input, the molten metal tends to sag down and an undercut defect is likely to occur. It is found that the upward electromagnetic force given by adjusting the conditions of magnetic field can lift the molten metal up, resulting in the remarkably improved shape of a penetration bead. It is further found that, even in overhead position welding, a well shaped penetration bead without undercuts is obtained by adjusting the welding touch angle as well as magnetic field conditions.     

光纤激光焊接熔池和小孔的高速摄像与分析

采用主动光源和光学窄带滤光片等辅助器件,利用高速摄像技术对光纤激光焊接过程中的熔池和小孔进行了拍摄,获得了较为清晰的熔池和小孔图像,以及不同激光功率下光纤激光焊接熔池和小孔的实际尺寸,可以为光纤激光焊接熔池和小孔的模拟提供可靠的参考依据。对高速摄像图片和焊缝熔深波动以及焊缝形貌进行了分析。结果表明:小孔前沿附近是光纤激光焊接过程中飞溅产生的主要区域;利用高速摄像可以监测焊缝的熔深变化;熔池温度最低的区域为熔池后部的中轴线两侧,而非熔池边界处。     

Keyhole formation and its characteristics in laser welding mode transition

Keyhole is the most important characteristic for laser deep penetration welding, and its formation indicates the beginning of laser deep penetration welding mode. The keyhole developing process was analyzed and the keyhole formation time was calculated according to welding speed and the length of weld bead formed in the keyhole formation process. The results showed that the keyhole forms in 40-70 ms at different rate of change of laser power. In laser deep penetration welding process, the variation of light intensity radiated by laser induced plasma can identify the keyhole formation, but it can not be used to estimate the keyhole formation time because of delay effect.     

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.     

HJP-01型激光拼焊设备及其关键技术

介绍了轿车底板用板材的HJP-01型激光拼焊设备及其关键技术。该设备集机械、电子、激光和液压技术于一体,采用计算机控制,具有较高的自动化程度。     

Scaling law for penetration depth in laser welding

A simple scaling law for penetration depth in laser welding is proposed considering heat flow characteristics and multiple reflections. First, a process parameter is identified that is proportional to the surface temperature during laser processing, and the parameter is modified by accounting for the effect of multiple reflections. As a result, the normalized penetration depth is expressed as a function of a single parameter that is a combination of laser intensity, interaction time and an indicator of the strength of multiple reflections. The obtained scaling law is applicable not only to conduction mode welding but also to keyhole mode welding, and provides insight into why and how penetration depth changes in a particular way. Systematic and extensive welding experiments were conducted using a 2 kW multi-mode fiber laser and two types of steels. The experimental results were in good agreement with the proposed scaling law.     

Effect of rotating arc process on molten pool control in horizontal welding

This paper describes the effect of rotating arc process on molten pool control in narrow gap horizontal welding. From energy strategy and force strategy, technical advantages of rotating arc process in molten pool control are explored. On the one hand, because of the decrease of temperature and holding time in weld centre, rotating arc process limits the tendency of molten pool sagging with the arc rotating frequency no more than 10 Hz, which is indicated from the simulation results of heat distribution characteristics. On the other hand, rotating arc process induces the flow of molten metal in the weld width direction by the periodical effect of arc force and droplets impingement, which is observed by the high speed photography system. This oscillation propels the molten metal at lower side against the effect of gravity to flow to the upper side, which is beneficial for the molten pool formation of horizontal welding.