Extraction of Weld Pool Width Feature Based on Visual Sensing during High-Power Fiber Laser Welding

In a high-power fiber laser welding process, the thermal radiation of a weld pool contains plenty of information for welding quality, in which the pool width can reflect the welding stability. Thus, extracting the welding pool width of high-power fiber laser welding based on infrared thermal imaging is an important method for monitoring the weld seam quality. In this paper, we studied the 304 stainless steel welded by a 10kW high-power fiber laser continuously. A near-infrared high-speed sensing camera was used to capture the weld pool images. Image algorithms such as median filtering, gray scale stretching, cutting, dynamic threshold mathematical morphology were applied to extract the weld pool image edge, analyze and detect the weld pool width. Welding experimental results showed that the proposed methods could extract the weld pool width, which could reflect the stability status of high-power fiber laser welding process accurately.

Detection of Weld Pool Width during Strong Arc Disturbance Environment

Detecting the weld pool width is prerequisite to control the welding quality. However, in an arc welding process, it is difficult to obtain a clear image of weld pool due to strong arc disturbance. A method is proposed to lessen the arc disturbance and obtain the width of a weld pool. A camera was used to capture the weld pool images during the arc welding process. The frequency domain filter combined with spatial processing algorithms were used to depress the noises in the pool image. During arc welding, the weld pool was in a liquid state and showed specular reflection due to the arc light effects. Because of the specular reflection, edges of the weld pool exhibited different brightness from that of the internal weld pool where a camera hangs. According to this phenomenon, it was easy to recognize the outlines of the internal and external weld pool. After the image noise of the weld pool was eliminated by an image filtering algorithm, then the line scanning on the image was carried out. It was crystal-clear that the external edge and internal edge demonstrate different grey levels. Therefore the method could be utilized to identify and extract the internal weld pool width and external weld pool width.

Stability of keyhole in plasma arc welding

Stability of the keyhole plays a fundamental role in producing quality welds in keyhole plasma arc welding. Some studies have assumed that keyhole diameter is a stability measurement. However, recent experimental observation by the authors revealed that width is not correlated with stability. To measure stability, this study uses the keyhole width as the lower limit of the weld pool width required to prevent keyhole collapse. Because the maximal width of the weld pool before burn-through is approximately fixed for particular applications, it can be used as the upper limit of the weld pool width for preventing burn-through. Hence, the margins of the keyhole process from collapse and burn-through can be measured based on how close the pool width is to its upper and lower limits. The stability distance and stability factor are then defined based on the margins to quantity the state of the stability of the keyhole process.

Vision-Based Weld Pool Width Control

Methods for controlling weld penetration for arc welding processes from top-side measurements have long been sought. One indirect variable that has been reported to correlate with penetration is weld pool geometry. A system which uses weld pool geometry sensing for controlling weld penetration is described in this paper. The system uses a miniature camera mounted in a modified coaxial viewing torch to view the weld pool. A robust machine vision algorithm has been developed for this system to measure weld pool width. The algorithm was designed to locate the edges of the weld pool despite the presence of other edges caused by the heat affected zone, scratches, marks, and weld pool impurities. The algorithm uses a matched edge filter and a majority voting scheme to measure the width of the pool. A control system was developed to regulate weld pool width in the presence of disturbances caused by such items as incorrect parameter settings, small variations in material composition, and material thickness changes. Experiments were conducted to test the control system by simulating some of these disturbances. The experiments demonstrated that for certain classes of materials, this technique works quite well. However, for other materials such as stainless steel 304, surface impurities in the weld pool visually obscure the weld pool and its edges to such a degree that the system fails to lock onto the edges of the pool.