Experimental study of weld position detection based on keyhole infrared image during high power fiber laser welding

Keyhole is one of the important phenomena in high-power laser welding process. By studying the keyhole characteristic and detecting the seam offset during high-power fiber laser welding, an infrared sensitive high-speed camera arranged off-axis orientation of laser beam was applied to capture the dynamic thermal images of a molten pool. The 304 austenitic stainless steel plate butt joint welding experiment with laser power 10 kW was carried out. Through analyzing the keyhole infrared image, the weld position was calculated. Least squares method was used to determine the actual weld position. Image filtering technique was used to process the keyhole image, and the keyhole centroid coordinate were calculated. Also, least squares method was used to fit the keyhole centroid curve equation and establish a nonlinear continuous model which described the deviation between keyhole centroid and weld seam. The heat accumulation effect affected by the infrared radiation was analyzed to determine whether the laser beam focus spot deviated from the desired welding seam. Experimental results showed that the keyhole centroid has related to the seam offset, and can reflect the welding quality.     

Mechanism of laser beam welding for SiCp／6063Al composite

The laser beam welding technique was used to process SiC particles/6063A1 alloy matrix composite, the influence of laser power and welding speed on the properties of joint was studied. Decreasing the laser beam power with same welding speed can make the quantity and size of A14C3 decreased, and the interactive mechanism of the reinforcing particles and the matrix in the joint and the causes for joint strength reduction were analyzed.Increasing welding speed properly can improve the distribution of energy and restrain the interfacial reaction in the molten pool, and measures for improving were proposed.     

Prediction of wire transfer behaviors in laser hot wire welding

With preheating wire by resistance heat, laser hot wire welding improves process stability and wire deposition efficiency, which gives broad potential applications in sugracing and narrow gap welding. It is a critical issue to control the temperature of preheated wire in this process. The temperature which is so high that the wire fuses outside molten pool or so low that the wire cannot melt timely in the molten pool, results in poor wire transfer stability and bad weld formation. This paper is purposed to calculate the wire temperature for the prediction of wire transfer behavior under various welding parameters. A heat conduction model is set up. Heat sources of the wire include resistance heat and reflected laser, and the heat source of molten pool is laser. The calculated temperature of wire part outside the molten pool is verified by infrared ratio temperature measurement. The calculated temperature of wire part in the molten pool is verified by measurement of the molten pool size. Analyzing the wire temperature and welding process observed by the high speed video imaging, the temperature criteria of wire transfer behaviors are obtained. Thus, numerical simulation of the wire temperature can be used to predict wire transfer behaviors in laser hot wire welding.     

Hump formation mechanism in gas-jet-assisted keyhole laser welding of HR-2 stainless steel

Gas-jet-assisted keyhole laser welding offers the possibility of a breakthrough in the limitations of penetration depth in laser welding,which currently suffers from equipment restrictions.A gas jet of sufficient intensity to assist the keyhole should be used to obtain suppressed plasma,a deepened keyhole,and increased penetration depth.However,an excessively strong gas jet gives rise to humps.The incident angle of the keyhole-assisted gas jet is 60°,with a nozzle ahead of the laser beam.A series of experiments were carried out with different welding velocities and gas parameters by using HR-2 hydrogen-resistant stainless steel and a slab CO_2 continuous-wave laser welding machine.The weld profiles can be categorized into four types,welds of traditional laser welding,welds of enhanced laser welding,undercut welds,and humping welds with increased gas pressure.A high-speed camera was employed in the experiments to monitor the formation of humps under an excessively strong gas jet.The results of analysis show that hump formation can be divided into six stages.Its main driving force is the intense turbulence of gas jet.There are two main reasons for hump formation:premature solidification of the molten pool caused by the large temperature gradient between the front and rear parts of the molten pool,and the emergence of a thin layer liquid bridge with one-directional flow under the enhanced cooling effect of excessively strong gas.     

Critical power of keyhole formation in CW Nd：YAG laser deep penetration welding

The energy model was founded to calculate the critical power of keyhole formation by using the limit principle in CW （ continuous wave ） Nd： YAG laser deep penetration welding process. The model was validated by experiments. The results show that ＇.there are two errors between the calculated critical power of keyhole formation and that of experiments ： one is that the calculated results is less than those of experiments, which is caused by not considering the energy loss by heat conduction in the model of keyhole formation. The other is that there is 0. 9 mm error between the axis of the calculated curve of critical power with location of laser focus and that of experimental curve, which is induced by the excursion of laser focus in laser deep penetration welding. At last, the two errors were revised according to the analyses of the errors.     

Simulation on the Melt Flow in Laser Full Penetration Welding with a Model Including a Non-Rotational Symme-try Keyhole Based on Energy Balance

A three dimensional model including a non-rotational symmetry keyhole of which the geometry is calculated base on the energy balance at keyhole wall is created to simulate the heat transfer and fluid flow within the molten pool during laser full penetration welding of titanium alloy plate.In order to take into account the effects of multiple reflections within the keyhole,a ray tracing method is employed and the coordinate dependent heat flux obtained is exerted on keyhole wall during the simulation on the heat transfer and fluid flow within molten pool.Furthermore,the computed weld cross sectional profile is compared with the experimental result to verify the model and it is fond that the simulation result is consists with experimental result.     

Influence of welding parameters on temperature field characteristics during laser welding of TA15 by infrared thermography

Using an infrared thermographic technique,the temperature field during laser welding of TA15 is quantitatively measured and investigated. The influence of two welding parameters on the weld temperature distribution is analyzed and the mechanism is discussed. New conclusions are drawn that are different from conventional weld temperature distributions. For the face of the weld,changes in welding speed induce changes in the temperature distribution because of different heat inputs and the cooling effect. Similar temperature features in the welds are observed for all speeds,which exhibit a relatively low temperature area below 1 500 ℃ between the high temperature area in the position of laser incidence and the sub-high temperature area at the end of the molten pool. For the influence of laser power on the face of the weld,the temperature on the weld is higher for P = 2. 8 k W compared to P = 2. 0 k W,especially the temperature in the sub-high temperature area. However,for the temperature field of the back of the weld,the influence of welding speeds is quite different compared to the results for the face of the weld. The highest temperature does not locate in the keyhole area,but instead in the middle of the molten pool. And there are different temperature features at different speeds. When v =6 m/min,the temperature field is like a uniform color belt and the temperature along the center of the weld fluctuates between 300 and450 ℃. When v = 4 m/min,the transient temperature distribution is not uniform and is unstable at different times. However,for v =2. 4 m/min and lower speeds,the temperature field becomes stable. The influence of laser power on the back of the weld temperature field is more complex. The measured temperature does not increase with increasing laser power,which seems to conflict with the conventional thermal conduction theory. This may be related with the characteristics of the keyhole area.     

Microstructural evolution during direct laser sintering of multi-component Cu-based metal powder

A multi-component Cu-based metal powder was chosen for direct laser sintering. The powder consists of a mixture of high-purity Cu powder, pre-alloyed CuSn and CuP powder. Liquid phase sintering with complete melting of the binder （CuSn） but non-melting of the cores of structural metal （Cu） proves to be a feasible mechanism for laser sintering of this powder system. The microstructural evolution of the sintered powder with variation of laser processing parameters was presented. High sintering activities and sound densification response were obtained by optimizing the laser powers and scan speeds. Using a high laser power accompanied by a high scan speed gives rise to baUing effect. At a high laser power with a slow scan speed the sintering mechanism may change into complete melting/solidification, which decreases the obtainable sintered density. The role of additive phosphorus in the laser sintering process is addressed. Phosphorus can act as a fluxing agent and has a preferential reaction with oxygen to form phosphatic slag, protecting the Cu particles from oxidation. The phosphatic slag shows a concentration along grain boundaries due to its light mass as well as the short thermal cycle of SLS.     

Welding characteristics in different laser-TIG hybrid manners

An experiment for determining the laser-TIG hybrid welding characteristics was carried oat in three kinds of hybrid methods: CO2 laser-TIG coaxial hybrid, CO2 laser-TIG paraxial hybrid and Nd: YAG laser-TIG paraxial hybrid. The experimental results indicate that hybrid welding has two welding mechanisms in CO2 laser-TIG hybrid welding: deep penetration welding and heat conduction welding. As the effect of the laser-induced keyhole, the arc root is condensed, the current density and penetration depth increase significantly, the welding characteristic is apt to deep penetration welding.When current increases to some degree, the keyhole induced by laser disappears, which produces a shallow penetration and wide bead. The weld exhibits heat conduction welding characteristics. Furthermore, the arc images and weld bead crosssections of three kinds of hybrid manners were also compared and analyzed at different welding currents, which established the foundation for understanding the welding characteristics of laser-TIG hybrid welding comprehensively.     

Research for the method of image acquisition of the molten pool in the TIG welding of aluminum alloy

Obtaining the image of molten pool aluminum alloy‘ s tungsten inert gas (TIG) welding becomes a challenging problem in the welding field. In this paper, a bran-new optical sensor based analyzing the light spectrum was designed, and the clear image of the molten pool during the aluminum alloy‘ s welding using the common industrial CCD camera was obtained. And with the new algorithm provided by myself, the desirable characteristic parameters of the molten pool of aluminum alloy‘ s welding were obtained, and it provides a good base for advanced monitor welding quality.     

激光焊接不锈钢微间隙焊缝偏差角点检测法

针对大功率(10 kW)光纤激光焊接304不锈钢紧密对接微间隙焊缝(焊缝间隙小于0.1 mm),通过高速像机摄取熔池近红外热像并分析其特征,分析和处理熔池热像特征,提取激光束偏离焊缝位置的信息,探索激光束与焊缝偏差的信息表征.利用激光深熔焊的匙孔效应,研究焊缝在固态与液态交界处不稳定边缘特征点,提出一种角点检测法实现微间隙焊缝偏差的检测.结果表明,熔池红外热像角点密集分布中心与焊缝偏差有密切的关系,通过角点分布密度可以有效判断焊缝偏差状态.     

激光深熔焊熔池X光图像恢复与特征分析

X光对于焊件具有很强的穿透性,可以利用其观测和分析激光深熔焊过程焊件内部熔池形态以及匙孔能量传递效果,实现对焊接过程有效地实时监测.在大功率光纤激光焊接304不锈钢过程中,由于X光图像采集过程中的噪声干扰,导致熔池X光图像存在严重退化现象,清晰度不高,识别性较差.为此研究一种图像恢复和增强算法,对熔池X光退化图像进行恢复和增强处理,使其在处理后能够更好地被识别.以盲去卷积法试验估计出熔池X光图像退化的点扩散函数,并以此函数为参数对熔池X光图像进行维纳滤波恢复,再进行灰度变换以增强图像,进而对熔池动态特性进行分析.结果表明,所提出的方法能够有效改善激光焊过程熔池X光图像质量,突出熔池边缘轮廓特征.     

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。     

激光焊接不锈钢微间隙焊缝偏差小波变换检测法

针对大功率(激光功率10 kW)光纤激光焊接304不锈钢间隙小于0.1 mm的紧密对接焊缝,研究一种基于小波变换的焊缝偏差检测方法. 为了分析焊缝偏差特征细节,采用红外传感高速摄像机摄取焊接区域熔池红外热像,利用小波函数对熔池图像感兴趣区域进行多尺度分解. 定义小波变换后的高频分量极值的横坐标作为匙孔形变参数,以匙孔形变参数作为特征值,分析特征值和焊缝偏差之间的关系. 结果表明,匙孔形变参数与焊缝偏差之间存在密切关联,能有效判断和检测焊缝偏差.     

大功率光纤激光焊熔池形态及焊接稳定性分析

在大功率光纤激光焊接过程中,熔池热辐射含有丰富的焊接质量信息,熔池形态及其变化特征与焊接稳定性密切相关.研究一种基于红外热像的大功率光纤激光深熔焊熔池形态分析及焊缝质量稳定性评价的方法.以10 kW大功率光纤连续激光焊接304不锈钢为试验对象,应用近红外传感高速摄像机摄取熔池动态热像,通过动态自动阈值和数学形态学方法重...     

Porosity formation mechanism and its prevention in laser lap welding for T-joints

A high-speed camera and X-ray transmission observation system were used to observe the keyhole and molten pool dynamic behavior in laser lap welding T-joints. The oscillation frequency of the molten pool and the keyhole increases with increasing gap. The lower keyhole becomes slant with the large gap and large quantities of bubbles are formed at the bottom tip of the keyhole. The molten pool is divided into three different zones by the large gap and a small eddy is formed at the lower molten pool. The bubbles are difficult to escape from the lower molten pool and the gap when the gap is large, resulting in the formation of porosity at the gap and root of weld seam. The distribution characteristics of porosity in different gap have an excellent agreement with the keyhole and the molten pool dynamic behavior. Porosity can be suppressed by maintaining a small gap or adopting high welding speed. The paper provides fundamental insights into the mechanism of porosity formation during laser lap welding T-joints and guidance to aid in its elimination.     

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.     

基于视觉的不锈钢Nd：YAG激光焊焊接熔池和小孔的图像处理

在激光焊接过程中存在众多干扰因素,为保证焊接质量,需要对激光焊进行质量控制。实现焊接质量控制的关键是表征焊接质量信息的获取,而焊接熔池和小孔包含丰富的焊接质量信息。针对不锈钢Nd：YAG激光焊,构建了同轴视觉检测系统,获得了熔池和小孔图像。根据熔池和小孔图像的特点,设计了图像处理算法,并分别针对熔池和小孔确定了搜索起始点和搜索准则。     

大功率光纤激光焊熔透状态模糊聚类识别方法

熔透是评价激光焊接质量的重要因素,但在焊接过程中难以直接检测熔透状态.以大功率光纤激光焊接304不锈钢紧密对接焊缝为试验对象,研究一种基于熔池红外热像的熔透状态识别方法.采用红外摄像机摄取焊接区域熔池动态图像,提取熔池特征量,应用模糊聚类算法分析熔池特征量与熔透状态之间的关系,使用模糊C-均值（FCM）和Gustafson-Kessel（GK）两种模糊聚类法建立熔透状态识别模型.激光焊接试验结果表明,熔池表征与熔透状态之间存在密切关联,通过GK模糊聚类算法建立的模型对熔透状态能达到78%以上的识别率,为大功率光纤激光焊接过程熔透状态的识别和控制提供试验依据.     

激光-MIG复合焊熔透状态视觉检测方法的研究

激光-MIG复合对接焊中,熔透状态是反映焊缝成形质量的重要指标.为获得反映熔透状态的信息,在此采用视觉传感器时焊接过程中的熔池正面、小孔图像、熔池背面分别进行了检测.结果表明,在复合对接焊条件下,采用熔池正面图像检测或小孔热辐射图像检测的方法,不能判断出熔透状态.采用主动照明的拍摄方式并选用合适滤光片的熔池背面图像的视觉检测方法,能够获得反映焊缝熔透状态的图像.