【作者】 方俊飞；

【导师】 吴林； 陈彦宾；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2007， 博士

【摘要】 匙孔和熔池相互作用是激光深熔焊接中的核心问题。本文通过分析Nd:YAG激光焊接过程中匙孔、熔池及等离子体的温度、电子密度特征,建立了同轴视觉传感系统,对焊接过程中熔池和匙孔区域的光辐射进行了实时检测。利用匙孔和熔池区域的光辐射特征,结合焊缝的特征对薄板激光深熔焊接的熔透模式进行了深入的研究,系统地分析了不同熔透模式下匙孔、熔池和焊缝的特征随工艺参数变化的规律,从理论上揭示了熔透模式转变的物理机制和对焊缝成形的影响规律。本文将焊接过程中匙孔和熔池的光辐射特征与焊缝的横截面特征联系起来,对全熔透激光焊接的熔透模式进行了划分。根据匙孔是否穿透工件的背面将激光深熔焊接分为匙孔穿透型和匙孔未穿透型两种熔透模式。匙孔穿透型熔透时的焊缝典型的横截面形状为“X”型,而匙孔未穿透模式时的典型焊缝横截面形状为“V”型。通过分析匙孔内部的受力平衡,对两种熔透模式相互转化时匙孔底部的力学平衡的变化进行了探讨;结合焊接工艺参数对匙孔存在和扩展的影响规律,确定了工艺参数对熔透模式转变的影响,通过熔池的受热状态和流体驱动力两个方面对两种熔透模式下焊缝的形成机制进行了阐述。对于试验获得的同轴视觉传感图像,利用中值平滑滤波、同态滤波及线性灰度变换等图像预处理方案对图像进行了除噪和增强处理,使用Canny算子实现了匙孔和熔池的边缘检测,并通过Hough变换最终获得了匙孔和熔池的形状特征。对匙孔、熔池和焊缝特征随工艺参数变化的研究表明:相对于激光功率,焊接速度是影响匙孔和熔池角度的决定因素;熔合线下部与激光轴线的夹角、焊缝背面宽度与正面宽度的比Rw可以作为判断熔透模式的特征量。在熔透模式为匙孔未穿透型时,熔池下部与激光轴线的夹角为正,一旦实现了匙孔穿透,该角度即变为负角度。工件厚度为1.4mm时,Rw大于0.55即可获得稳定的匙孔穿透型焊接模式。采用从同轴视觉传感图像所获得的熔池边缘特征来进行焊缝正面熔宽的检测,检测的结果与试验的结果具有较高的吻合度;利用匙孔背面开口的特征进行焊缝背面熔宽的检测,在匙孔背面开口较小的时候,两者近似呈现线性关系,但随着背面半径的增加,焊缝背面的宽度渐渐趋向稳定。在工艺试验的基础上,通过考虑匙孔机制在激光深熔焊接中的作用,将由同轴视觉传感系统获得的匙孔特征引入到数值计算中,并在此基础上建立了移动热源作用下的激光深熔焊接熔池传热传质的三维数学模型。研究了不同熔透模式下熔池内部的温度场和速度场的分布,并对熔池流动驱动力的作用进行了量化的分析。模拟的结果表明,不同熔透模式下,由于匙孔形状的差别使得熔池内部的流体运动产生了较大的差别,从而是两种焊缝形状形成的内在原因;在熔池内部的驱动力中,浮力所引起的流体运动速度较慢,对熔池流动的影响很弱;蒸气摩擦力主要影响的是匙孔周围熔池的流动,促使熔池金属由匙孔壁向熔池的边缘流动;表面张力是熔池中流体流动的最主要的驱动力,在其作用下熔池表面的液态金属由熔池中心流向边缘,并在熔池后方边缘处形成涡流。匙孔穿透工件时,表面张力作用下的熔池形成束腰,是形成“X”型焊缝的主要原因。更多还原

【Abstract】 The interaction between the keyhole and weld pool is the core problem of the laser keyhole welding. By analysis of the keyhole, weld pool and plasma temperature character and electron density character, a coaxial visual sensing system was established to detect the optical emission of the keyhole and weld pool in deep penetration laser welding process. Using the optical character of the keyhole and the weld pool, incorporate with the weld cross section character, the penetration mode of the thin sheet laser welding was studied. And the variation of the keyhole, weld pool and weld character with the processing parameter was analyzed, the physical mechanics of the penetration mode transition and its effect on the weld shape was represented theoretically.In this paper, the optical emission character of the keyhole and the weld pool was related to the weld cross section character, and the penetration mode of the laser full penetration welding was classified. Based on the situation of the keyhole whether full penetrated the workpiece, two penetration modes of the laser full penetration welding was represented, as open keyhole penetration mode and close keyhole penetration mode. The typical weld cross section of the open keyhole penetration mode is“X”type, the typical weld cross section of the close keyhole penetration mode is“V”type. With the transition of the penetration mode, the variety of the force equilibration at keyhole bottom was discussed. Combined with the relation between the process parameter to the keyhole existing and expansion, the effect of the process parameter on penetration mode transition was illuminated, and the weld formation mechanics for different penetration mode was explained by the heating situation and fluid driving force of the weld pool.For the coaxial visual sensing image in experiments, the Media filter, homomorphic filter and Linear gray stretch were used to eliminate noise and enhance processing. The Canny operator was applied to detect the edge of the keyhole and the weld pool. Finally, the Hough transformation was utilized to obtain shape character of the keyhole and the weld pool surface. The result of the keyhole, weld pool and the weld cross section shape characters according to the process parameter shown that the welding speed is the governing factor of the keyhole and weld pool shape angle, the included angle between undersides fusion-line and laser beam and the weld width ratio can be the characters to identify the penetration mode. In close keyhole penetration mode, the included angle between undersides fusion-line is positive, as the penetration mode transit to open keyhole penetration mode, the corresponding included angle is negative. As the workpiece thickness is 1.4mm, the weld width ratio greater than 0.55 indicates that the stable open keyhole penetration mode was obtained.The detection of the weld obverse width was achieved by the weld pool edge characters, which is represented in the coaxial visual sensing image. The detection result was agreed with the experimental result fairly. The character of the keyhole rear opening was applied to detection of the weld rear width. As the smaller keyhole rear opening size, the relation between the keyhole rear opening size and the weld rear width is approximate lineally. As the increasing of the keyhole rear opening size, the weld rear width tends to stable.Based on the technological experiment, the effect of the keyhole mechanics on welding process was considered, and the keyhole characters of the coaxial visual sensing image were introduced to the numerical simulation process, a three-dimensional mathematic model of the heat and mass transfer with the moving heat source in laser keyhole welding was developed. The temperature field and fluid flow vector field for different penetration mode was calculated, and the effect of the fluid flow driving forces was analyzed quantificational. The result of the numerical simulation demonstrated that the difference of the keyhole shape lead to the difference of the weld pool fluid flow pattern, which is the internal reason for the weld shapes formation in different penetration mode. For the driving force of the fluid flow, as the flow speed caused by the buoyancy is feeble, the effect of the buoyancy on the weld pool flow can be neglected. The fluid flow caused by the vapor friction is limited around the keyhole, which drive the fluid from the keyhole wall flow to the outer edge of the weld pool. The surface tension is the main driving force in the weld pool. Under the govern of the surface tension, the liquid metal of the weld pool surface flowed from the weld pool center to the edge, and an eddy flow was formed at the back of the weld pool. In open keyhole penetration mode, the surface tension is the key factor for the formation of the“X”type weld cross section.更多还原