Laser weld pool management through diffractive holographic optics

Abstract

Conduction laser welding involves initiating a melt pool by exposure to high power laser induced light and controlled thermal conduction. Existing welding techniques generally provide enough energy to join the component but have no real control over the melt pool. This process can invariably lead to overheating in adjacent areas or even the melt pool itself, often causing unavoidable effects, such as ‘burn through’. The present work presents a procedure in which a desired melt pool shape is conceived, and a bespoke beam irradiance distribution is designed to match. The beam is shaped not by conventional lenses but by a diffractive holographic optical element (DHOE). The DHOE utilises holography to wholly create highly complex three-dimensional energy distributions through constructive and destructive interference. This technique allows novel beam irradiance distributions to be applied to conduction mode laser welding, with the melt pool transverse profile being shaped to a specific design. Holographic conduction laser welding has been shown to be successful and represents a significant step forward in the industry, as demonstrated in this case in both mild and stainless steels. The fusion zone is shown to be particularly influenced by the shape of the illuminating laser beam profile, and many of the welds demonstrate a highly novel weld profile because of this. The use of a bespoke beam irradiance distribution allows control of the heat flow to the workpiece, and this allows greater control over material migration due to surface tension effects. Many of the welds demonstrate unique surface solidification patterns directly linked to the beam profile used. The DHOE also presents a number of additional advantages, such as an increased usable depth of field, allowing for less stringent set-up tolerances. Comprehensive metallography has been performed on samples of these welds through the use of optical microscopy, electron microscopy, electron backscatter diffraction and energy dispersive (X-ray) spectroscopy. These techniques offer in depth analysis of crystal size, shape, orientation and phase. By incorporating DHOEs into a laser welding process, not only does the melt pool shape become controllable, but also the crystal growth is highly influenced. Many of the undesirable attributes of a conventional laser weld are reduced by using a beam distribution created by a DHOE, bringing the microstructure of the weld pool closer to that of the parent material.     

Keyhole depth instability in case of CW CO<Subscript>2</Subscript> laser beam welding of mild steel

The study of keyhole (KH) instability in deep penetration laser beam welding (LBW) is essential to understand welding process and appearance of weld seam defects. The main cause of keyhole collapse is the instability in KH dynamics during the LBW process. This is mainly due to the surface tension forces associated with the KH collapse and the stabilizing action of vapour pressure. A deep penetration high power CW CO₂ laser was used to generate KH in mild steel (MS) in two different welding conditions i.e. ambient atmospheric welding (AAW) and under water welding (UWW). KH, formed in case of under water welding, was deeper and narrower than keyhole formed in ambient and atmospheric condition. The number and dimensions of irregular humps increased in case of ambient and under water condition due to larger and rapid keyhole collapse also studied. The thermocapillary convection is considered to explain KH instability, which in turn gives rise to irregular humps.     

等离子弧焊接熔池和小孔形成过程数值分析

目的 研究等离子弧焊接穿孔过程中熔池内部的金属流动情况和小孔动态变化过程。方法 通过“传热-熔池流动-小孔”之间的相互耦合关系,建立了等离子弧焊接穿孔过程的数值分析模型,通过VOF方法追踪了小孔界面,采用FLOW-3D软件模拟了等离子弧焊接熔池和小孔的形成过程,定量计算了等离子弧焊接温度场、熔池流场及小孔形状;分析了等离子弧焊接熔池和小孔行为;并通过等离子弧焊接实验数据验证了模拟结果。结果 当焊接时间为0~1.0 s时,小孔深度曲线与熔深曲线几乎相同,小孔底部紧贴熔池底部;在2.8 s以后,小孔深度曲线与熔深曲线有一定距离,小孔深度曲线在一定范围内波动,等离子弧焊接电弧挖掘作用到达极限,电弧压力与其他力达到平衡状态。模拟的焊缝熔深为8.04 mm、熔宽为13.20 mm,实验测得的焊缝熔深为8.00 mm、熔宽为13.42 mm。结论 构建的随小孔动态变化的曲面热源模型和电弧压力模型可以描述等离子弧焊接过程中的电弧热-力分布;模拟出了等离子弧焊接熔池和小孔动态演变过程;模拟得到的等离子弧焊接焊缝形貌与实验测得的焊缝形貌基本吻合。     

Contact mechanical studies on continuous wave CO2 laser beam weld of mild steel with ambient and under water medium

Laser induced keyhole (KH) and weld pool formation during laser beam welding (LBW) can alter mechanical properties of weld seams. In this regard, deep penetration high power continuous wave (CW) CO₂ laser induced KH was generated in mild steel (MS) in both ambient and under water medium. Under water, KH was deeper and narrower as compared with KH formed in ambient condition. Contact mechanical studies of both the types of weld zones were carried out by measuring hardness and frictional properties. A significant rise in hardness was observed in fusion zone (FZ) regardless of the medium used. The KH and weld pool mediated LBW fusion zone showed lower coefficient of friction. Scratch size was also found to decrease in FZ due to increased hardness. The mechanical properties of FZ such as hardness, coefficient of friction and scratch size were correlated with microstructure composed of fine grained structure in the weld zones.     

Model for CO2 laser welding of stainless steel,titanium, and nickel: parametric study

Abstract

The present work has been carried out experimentally and theoretically with the aim of studying the effect of the operative pressure on the weld beads of different materials (AISI 304, Ti–6Al–4V, and Ni), welded using a CO₂ laser beam. At the same time, the reliability of a properly developed analytical model of the laser beam welding process has been confirmed. Such a model, taking into account the dynamics of the process itself, describes the laser induced thermal fields in terms of two heat sources, the first one representing the keyhole effect and the second one, the role played by the plume. A comparison between the experimental data and the beads predicted using the model gives satisfactory results, with average errors less than 5% for Ti–6Al–4V and ~10% for AISI 304 and Ni. The model allows the quantitative evaluation of the power distribution between the keyhole and the plume and a deeper understanding of the entire process.

MST/1591     

Process control of laser conduction welding by thermal imaging measurement with a color camera

Conduction welding offers an alternative to keyhole welding. Compared with keyhole welding, it is an intrinsically stable process because vaporization phenomena are minimal. However, as with keyhole welding, an on-line process-monitoring system is advantageous for quality assurance to maintain the required penetration depth, which in conduction welding is more sensitive to changes in heat sinking. The maximum penetration is obtained when the surface temperature is just below the boiling point, and so we normally wish to maintain the temperature at this level. We describe a two-color optical system that we have developed for real-time temperature profile measurement of the conduction weld pool. The key feature of the system is the use of a complementary metal-oxide semiconductor standard color camera leading to a simplified low-cost optical setup. We present and discuss the real-time temperature measurement and control performance of the system when a defocused beam from a high power Nd:YAG laser is used on 5 mm thick stainless steel workpieces.     

单束与双束串行激光填丝焊特性对比

以铝合金为研究对象,研究了双束串行激光填丝焊不同能量比对焊缝成形及焊缝气孔率的影响,并与单束激光填丝焊工艺进行对比。进一步借助高速摄像机对双束串行激光填丝焊能量比R为20/80以及单束激光填丝焊的熔池、匙孔以及等离子体的变化进行对比分析,获得了双束串行激光填丝焊能量比R为20/80时焊缝气孔率降低的原因。结果表明,对于双束串行激光填丝焊,当能量比R为20/80时焊缝气孔率较低。与单束激光填丝焊相比,双束激光填丝焊能量比R为20/80的焊缝气孔率降低了38%,而且焊接过程中焊丝熔化后沿熔池边缘流入,可大幅降低对匙孔的冲击作用,匙孔始终处于张开状态,焊接过程中等离子体的形态波动相对较小,表明焊接过程的稳定性较好。     

Simulation on welding thermal effect of AZ61 magnesium alloy based on three-dimensional modeling of vacuum electron beam welding heat source

The deep-penetration thermal effect of keyhole and surface thermal effect of high-temperature metal vapor by the direct-acting mechanism during vacuum electron beam welding were analyzed. According to the thermal effect, a composite source model working for magnesium alloy welding was developed. This model was composed of Gaussian surface source and conical heat source. By the welding experiments on AZ61 magnesium alloy, it can be obtained those two key factors, which were welding heat input and focus coil current (I_f), affected the keyhole thermal effect and weld shape significantly. In order to simulate the shape of keyhole and weld, the varying of focus conditions in model were realized by the power coefficient of composite welding heat source. It can be seen that there was a good consistency between the calculation results and experimental results.     

Numerical simulation of fluid flow and temperature field in keyhole double‐sided arc welding process on stainless steel

Double‐sided arc welding process powered by a single supply is a type of novel high‐production process. In comparison with the conventional single‐sided arc welding, this process has remarkable advantages in enhancing penetration, minimizing distortion and improving welding production. In this paper, a three‐dimensional steady numerical model is developed for the heat transfer and fluid flow in plasma arc (PA)–gas tungsten arc (GTA) double‐sided keyhole welding process. The model considers the surface tension gradient, electromagnetic force and buoyancy force. A CCD camera is used to observe the size and shape of the keyhole and weld pool. The acquired images are analysed through image processing to obtain the surface diameters of the keyhole on the two sides. A double‐V‐shaped keyhole geometry is then proposed and its characteristic parameters are derived from the images and cross‐section of weld bead. In the numerical model, the keyhole cavum within the weld pool is treated as a whole quality, whose temperature is fixed at the boiling point of the workpiece material. The heat exchange between the keyhole and weld pool is treated as an interior boundary of the workpiece. Based on the numerical model, the distributions of the fluid flow and temperature field are calculated. A comparison of cross‐section of the weld bead with the experimental result shows that the numerical model's accuracy is reasonable. Copyright © 2005 John Wiley & Sons, Ltd.     

T型接头旋转激光+GMAW复合焊熔池动态行为数值分析模型

目的 研究T型接头旋转光纤激光+GMAW复合焊熔池的温度场和流态特征,揭示气孔缺陷的产生及抑制机理。方法 依据光学、电磁学、传热学及流体动力学机理,建立T型接头旋转光纤激光+GMAW复合焊熔池数值分析模型。使用Fluent软件对旋转频率分别为50 Hz和100 Hz的T型接头旋转激光+GMAW复合焊进行温度场以及流态特征的模拟,对比不同频率下T型接头横、纵截面,从工艺和焊缝成形角度出发,针对不同频率对熔池、小孔成形以及气孔抑制的影响进行讨论。结果 当旋转频率为50 Hz时,纵截面内小孔最大深度为5.4 mm,横截面熔池内小孔开口直径相对较大,旋转一周后,小孔远离气泡,气泡无法逸出,形成气孔;当旋转频率为100 Hz时,纵截面内小孔深度显著降低,熔池体积明显减小,横截面内小孔最大开口直径和深度均降低,熔池尺寸也有所减小,在时间为0.097 s时,小孔上方区域出现的顺时针涡流不仅能抑制气孔,还能改善熔池的下垂以及立板焊趾处的咬边。结论 随着旋转频率的增大,小孔的最大开口直径和深度均降低,还对熔池具有搅拌作用,使熔池体积变小。     

Numerical simulation of variable polarity vertical-up plasma arc welding process

Three-dimensional transient governing equations were developed based on conservation laws of energy, momentum and mass. These equations described physical phenomena of convection in weld pool and heat transfer in workpiece during variable polarity vertical-up plasma arc welding process. Boundary conditions for the developed governing equations were given. Welding energy input for variable polarity vertical-up plasma arc welding process was quantitatively expressed. Free surface deformation of the keyhole molten pool was coupled into calculation. Effect of wire filling on the geometry of molten pool and weld reinforcement was considered in the simulation. Correlations of temperature and thermophysical properties for aluminum alloy 2219 were quantitatively established. A control volume based finite difference method was used to solve the discrete governing equations. Moreover, dynamic evolutions of geometrical profile, dimension and fluid flow for the molten pool and keyhole were simulated through the developed computational routines, which achieved transient solution of fluid flow field coupling with thermophysical properties, temperature field and weld pool free surface deformation. Besides, the effect of the workpiece thickness on the moments of keyhole formation and stable keyhole establishment was analyzed, and thermal cycles for the main welding stage were calculated. In addition, experiments via variable polarity vertical-up plasma arc welding technique were conducted, and the established models were experimentally verified through weld cross-section profiles.     

Nd:YAG激光与半导体激光复合焊接对铝合金焊缝组织和性能的影响

目的 提高铝合金的焊缝抗拉强度,解决铝合金焊接过程中的裂纹缺陷。方法 采用脉冲Nd:YAG激光与半导体激光复合焊接铝合金,先用Nd:YAG激光形成焊接熔池,然后用半导体激光对熔池进行加热保温,获得无裂纹的焊缝,并对焊缝进行抗拉强度测试。结果 与单独的Nd:YAG激光焊接相比,Nd:YAG激光与半导体激光复合焊接的铝合金焊缝抗拉强度提高了50%,达到193 MPa,为母材抗拉强度的90%。结论 2束激光的结合延长了熔池的冷却凝固时间,从而有效避免了热裂纹,减少了焊接缺陷,提高了焊接质量。     

Direct measurement of reflectance from aluminum alloys during CO(2) laser welding

A simple method for measuring the reflected amount of a CO(2) laser beam from the surface of a welded aluminum alloy is described. This device can be used for cases in which an integrating sphere is impractical to use. A pyroelectric detector measures the amount of reflected laser radiation at prespecified locations that define a semisphere on top of the weld spot. The spatial distribution of a reflected laser beam is obtained. It is found that ~80% of the total incident power is reflected by the alloy until a welding keyhole is developed. However, once the keyhole is formed, the reflected amount is as low as 4-8%.     

Employment of fiber laser technology to weld austenitic stainless steel 304 l with aluminum alloy 5083 using pre-placed activating flux

The overlapping welding was carried out in keyhole mode between austenitic stainless steel 304 l and aluminum alloy 5083 using a low power fiber laser in continuous irradiation. The significant content of magnesium as the alloying element with low boiling point and high vapor pressure inside the AA 5083 matrix can induce the spatter formation and depression on surface of the weld beads upon laser beam absorption and temperature growth which can deteriorate the mechanical properties and appearance of the joints. To reduce these defects, a variety of single and multi-components activating fluxes including oxide-based TiO₂ and halide-based CaF₂ flux powders were pre-placed on the surface of welding material prior to laser welding. The EDX and XRD analyses in addition to microhardness and shear tests were carried out to characterize the joints. The obtained results showed that, the oxide and halide activating fluxes can significantly improve the joints' strength up to 1.48 and 1.85 times in average respectively compared with autogenous joint. It was deduced that the simultaneous effect of significant decrease in joints' surface depression leading to welds' geometry improvement in addition to less formation of interfacial Fe–Al intermetallics, were the major causes for considerable strength improvements.     

焊接位置对激光-电弧复合焊接底部驼峰的影响

目的 通过改变焊接位置来抑制底部驼峰的形成进而降低生产成本。方法 使用高速摄像机观察平向焊接和横向焊接2种不同焊接位置下熔池的匙孔和底部驼峰的形成过程,并对底部凸起受到的金属蒸气反冲压力、熔融金属重力以及表面张力进行受力分析。结果 在S355J2W耐候钢的激光-电弧复合焊接平焊过程中,受金属蒸气反冲作用力的影响,熔融金属迅速从匙孔底部排出,流入匙孔底部凸起区域,底部凸起的增长速度极快,仅需7.5 ms即可形成大尺寸的驼峰。而在横向焊接中,匙孔前壁凸起数量减少,导致金属蒸气反冲压力减小,匙孔排出的熔融金属量减少,底部凸起的增速减缓,在13.6 ms时才达到最大高度,使驼峰在底部熔池凝固之前难以形成。结论 采用横向焊接的方式可以有效避免驼峰的产生,降低生产成本。同时,该方式在焊缝背部成形效果方面也表现良好,为优化焊接工艺提供了重要参考。     

双束激光焊接的研究现状

双束激光焊接技术是高能束焊接领域的研究热点之一,其主要目的是解决激光焊接时存在的问题。文章首先根据单束激光焊接冷却速度过快、装配要求精度过高等缺点,提出了采用双束激光焊接技术进行改善,并对其机理进行了概括分析;其次介绍了多种双束激光焊接同种/异种材料的研究现状,包括不同的工艺参数(束间距、光强比、热源布置方式等)、匙孔内的等离子体行为、焊缝性能以及焊接缺陷等方面。     

Influence of welding parameters on lap joint between Al and Ti alloys using low power fiber laser

Fiber laser beam welding has always been a user‐friendly and flexible method to join dissimilar materials despite differences in thermal coefficient. Many industrial applications such as automotive has replaced the conventional joining methods towards this because of the flexibility and reduction in time consumption. In the present study, dissimilar titanium alloy; Ti6Al4 V and aluminum alloy; AA2024‐0 were laser welded through a lap joint technique using a low power Yb‐fiber laser without any additional filler. The influence of welding speed on weld morphology was investigated using optical microscopy (OM) and scanning electron microscope (SEM). The cross‐section of the joints revealed that the fusion zone (FZ) and heat affected zones (HAZ) are wider when welding speed decreases with lower laser power. This result shows that the low power fiber laser has sufficient energy to melt the base materials, forming a liquid bridge to facilitate the smooth flow of molten metal between the top and bottom layer. Therefore, at lower welding speeds with constant low laser power, it was shown that there are possibilities of laser welding between two non‐ferrous metals.     

功率密度对DP590钢激光焊缝熔深及组织的影响

目的研究相同热输入(功率/速度)下激光功率密度(功率/光斑面积)变化对焊缝熔深及组织的影响。方法在保持热输入不变的条件下,对不同功率下1.8 mm厚的DP590钢板进行光纤激光焊接试验,在光学显微镜下检测不同条件下的熔深,在扫描电镜下观察不同条件下的焊缝组织。然后,采用FLOW-3D软件对不同条件下焊接熔池/小孔行为与激光能量吸收进行了计算研究。结果随着功率密度的增加,焊缝熔深总体不断递增,但在速度为0.055 m/s和0.065 m/s时突变。结论通过模拟发现,匙孔吸收的能量影响了焊缝熔深变化。同时,焊缝冷却速率随着焊缝吸收能量的增加而降低,使得焊缝区马氏体组织大小出现差异。     

TIG焊接熔池表面变形对流场与热场的影响

利用流体力学理论和变分法原理,根据熔池本身重力、电弧压力和表面张力之间的动态平衡,推导出了 TIG 焊接熔池表面变形的计算公式。建立了熔池表面存在变形的流场与热场的数学模型。采用 SIMPLER 方法对不锈钢试件焊接熔池内的流场与热场进行了数值分析。焊接工艺试验表明,该模型计算的熔池成形与实验值吻合良好。     

变极性等离子弧穿孔熔池受力及焊缝成形稳定性

通过YB005-01型压力变送器测定相同参数条件下正极性等离子电弧力大于反极性等离子电弧力,并建立了铝合金VPPA焊接穿孔熔池受力模型,分析了在不对称正、反极性等离子电弧力的作用下,穿孔溶池稳定性及其焊缝成形机理.同时进一步分析铝合金VPPA力学特性,掌握了焊接电流和离子气流量等重要焊接参数对其影响.经穿孔焊工艺实验,...