【作者】 王广伟；

【导师】 吴毅雄；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2008， 博士

【摘要】 短路过渡是熔化极气体保护焊中的一种重要的熔滴过渡方式,其以优质、高效、低成本、节能、生产率高的特点,广泛应用于中、薄板焊接。对于短路过渡这一燃弧-短路交替的复杂非线性时变系统,其瞬时过程决定了焊接过程中产生飞溅的可能性以及焊缝的最终成形,因此国内外广大焊接工作者对熔化极气体保护焊短路过渡(GMAW-S)的熔滴、熔池等行为过程进行了大量的研究与探索工作。但长期以来由于受试验技术、研究手段等多方面的限制,此类研究工作主要集中在基于电信号的过程建模及工艺优化、基于电弧声信号的过渡状态判断、基于图像信号的熔深熔宽控制、还有部分基于熔滴图像信号的状态分析等,研究平台相对分散,建模数据较为单一,直接性和可验证性较差。因此整合研究平台,对GMAW-S焊接过程中瞬时状态所蕴含的机理进行研究,能够进一步加深对GMAW-S焊接过程的理解,为提出新的GMAW-S焊接过程控制思路提供科学依据。论文研制了一套由背光系统、滤光系统和成像系统所组成微距高速视觉传感系统。在背光系统设计中,根据所采用的AlGaInP半导体激光空间模式,基于高斯光束传输和光学波动成像理论,通过模拟计算,确定了空间滤波器和准直扩束光学系统的最优光学参数,获得了理想的平顶光束。在滤光系统设计中,根据偏振光学原理,设计偏振光学系统取代传统的小孔光阑以达到连续滤光的目的,并与保护镜片和干涉滤光片通过螺纹式连接装配组成了同轴调节系统。在成像系统设计中,根据所采用的高速摄像机参数,采用C/F转接座配置中微距可变焦镜头,获得了成像比率较高的GMAW-S、GMAW-P的熔滴细节图像。该系统的出现突破了以往焊接过程图像高速采集所面临的设备搭建耗时、参数设置复杂、焊接规范适用性差、成像质量不高的技术瓶颈,为广泛开展基于图像监测的各类焊接过程研究提供了可能。论文构建了由焊接控制系统、保护气混配供给系统、焊接电信号实时采集系统和高速视觉传感系统四部分组成的GMAW-S焊接试验平台,并进通过基于保护气配比的GMAW-S焊接试验,研究了不同组分保护气对GMAW-S熔滴过渡状态的影响。针对GMAW-S瞬时过程图像存在大量焊接烟尘、衍射条纹等复杂背景噪声,提出了一种结合频率域运算的线性图像处理算法。通过图像频率域转换,经过衰减低通滤波、灰度映射、高斯低通滤波和Laplacian锐化,最终获得不改变图像性质、主体细节突出、背景噪声大量减少的GMAW-S瞬时过程图像。并在此基础上,根据Mallat塔式算法构造3次B样条小波,通过多尺度小波边缘检测算法实现了GMAW-S瞬时过程图像中熔滴、液桥、熔池等目标物的边缘提取。论文研究了熔滴成形瞬时过程,并建立了该过程的力学模型,通过该模型进行计算分析后认为:表面张力提供的支持力为主要的熔滴成形驱动力,焊丝在熔滴成形过程中的熔化速度与小滴体积扩展速度相符,因此小滴在扩展过程中仍能保持滴状形状。论文以磁流体动力学理论和计算流体动力学方程为基础,综合考虑熔滴所受的电磁力、表面张力、等离子流力和重力的作用,建立了二维GMAW-S熔滴受激变形瞬时过程的动态数学模型。通过对该过程的模拟计算,研究分析了熔滴在长大和电流下降后的形态演变、内部液态金属流动机制以及速度场分布规律。论文对瞬时短路过渡过程进行深入研究,建立了瞬时短路过程力平衡临界方程,通过图像分析技术结合电信号定量计算,证明该临界方程能够真实的反映瞬时短路进程,并进一步探讨了瞬时短路过程的定义,认为以往对瞬时短路的定义“电极在非常短的时间和熔池发生接触,但没有发生金属过渡”是不完全的,建议应该摒弃时间概念,定义瞬时短路过程为熔化电极和熔池接触但没有建立液桥的短路现象,而把液桥建立但没有完全铺展开的短路过渡过程定义为瞬时短路过渡过程。更多还原

【Abstract】 Short-circuit transfer is an important metal transfer mode in gas metal arc welding. Featured with high-quality, great efficiency, low-cost, energy-saving, and high-productivity, it is widely used in sheets welding. And as it belongs to the complex nonlinear time-varying systems, the possibility of the generation of the welding spatter as well as the forming of the final weld are determined by the GMAW-S instantaneous processes. Therefore, researchers over the world have carried out a lot of studies and exploratory work on the behavior of droplet and weld pool in GMAW-S. Such studies, however, have been for a long time conditioned by limited experimental techniques, researching tools and other restrictions, thus have been mainly focused on modeling and process optimization based on signal process, judgment of transferring state based on arc acoustic signal, width and penetration control based on image signal and partly state analysis based on droplet images. The research platforms are relatively dispersive; the models are simplex; the results are not obvious and hard to verify. Therefore, a thorough study of the mechanism of GMAW-S instantaneous process based on integrated research platforms will further deepen the understanding of GMAW-S process and provide scientific basis for new GMAW-S welding process control ideas.In this paper, a set of macro high speed vision sensor system which is composed of backlight system, filtering system and imaging system, was developed.In the design of backlight system, based on the spatial mode of the semiconductor laser AlGaInP adopted in the research, the optimal optical parameters of spatial filter and collimated expander were determined by the results of the calculations based on Gaussian beam transmission and the theory of fluctuations in optical imaging and the top-hat beam was eventually obtained.In the design of filtering system, according to the principles of polarized light, a polarized optical system was designed to replace the traditional pinhole for the purpose of achieving continuous filtering. Together with the protection lens and interference filter, a coaxial-conditioning system was assembled through threaded connectors.In the design of imaging system, based on the parameters of the high speed camera adopted, detailed droplet images of GMAW-S and GMAW-P of higher imaging ratio were obtained by using C/F adapter and macro zoom.The establishment of this system eliminates many technological bottlenecks in high speed image capturing of welding process, i.e. long time consumed in equipment structuring, complexity in parameter setting, poor applicability of welding parameters and low quality of image. It thus provides possibility for carrying out wider range of welding process studies based on image monitoring.In this paper, a GMAW-S welding experimental platform which is composed of welding control system, shielding gas mixing system, real-time electrical signal acquisition system and high speed vision sensor system were developed. Based on this platform, the influence of different components of shielding gas to the state of GMAW-S droplet transfer was studied based on the GMAW-S welding experiment with the same ratio shielding gas as in simulation.For the complex background noise in GMAW-S instantaneous process images such as mass welding fume, diffraction fringes, etc., a linear image processing algorithm which combines frequency-domain calculation was proposed. Through image spatial-frequency domain transformation, attenuation of low-pass filtering, gray-scale mapping and Gaussian low-pass filtering, as well as Laplacian sharpening, the GMAW instantaneous process images with distinct details and much lower background noise were obtained while the nature of the images remain unchanged. On this basis, the cube B-spline wavelet was constructed according to Mallat tower algorithm and the edge extraction of droplet, liquid bridge and weld pool in GMAW-S instantaneous process images was achieved by the means of multi-scale wavelet edge detection algorithm.In the paper, the droplet forming instantaneous process was studied and the mechanical model of this process was established. Based on the calculation of this model, the results demonstrated that the surface tension was the major driving force in droplet forming process, and the drop can maintain droplet shape in the expansion process since the wire melting speed is the same with the drop expansion speed.Based on the magneto hydrodynamics (MHD) theory and computational fluid dynamics equations, a two-dimensional dynamic mathematical model of instantaneous GMAW-S droplet stimulated deforming process was established, with consideration of electromagnetic force, surface tension, plasma flow and gravity. The evolution of droplet profile in growing in shape and falling of current, the flow mechanism of internal metal liquid and the velocity distribution were analyzed, based on numerical simulation.The instantaneous short circuits (ISC) process was thoroughly studied in this paper and an ISC force equilibrium critical equation was established. Using image analysis technology combined with quantitative calculation of electrical signals, it is proved that the critical equation can reflect the real ISC process. Further, the definition of ISC was discussed and the past definition“the electrode touches the weld pool for a very short period of time, but no metal transport takes places”was considered to be inaccurate. The time concept in definition was recommended to be abandoned, and ISC should be defined as a short-circuit phenomenon where melting electrode makes contact with the weld pool without liquid bridge formed. Moreover, a short-circuit transfer process in which liquid bridge formed but not completely extended was defined as instantaneous short circuit transfer (ISCT) process.更多还原