The keyhole status is a determining factor of weld quality in laser-metal active gas arc (MAG) hybrid welding process. For a better evaluation of the hybrid welding process, three different penetration welding experiments: partial penetration, normal penetration (or full penetration), and excessive penetration were conducted in this work. The instantaneous visual phenomena including metallic vapor, spatters and keyhole of bottom surface were used to evaluate the keyhole status by a double high-speed camera system. The Fourier transform was applied on the bottom weld pool image for removing the image noise around the keyhole, and then the bottom weld pool image was reconstructed through the inverse Fourier transform. Lastly, the keyhole bottom was extracted from the de-noised bottom weld pool image. By analyzing the visual features of the laser-MAG hybrid welding process, mechanism of the closed and opened keyhole bottom were revealed. The results show that the stable opened or closed status of keyhole bottom is directly affected by the MAG droplet transition in the normal penetration welding process, and the unstable opened or closed status of keyhole bottom would appear in excessive penetration welding and partial penetration welding. The analysis method proposed in this paper could be used to monitor the keyhole stability in laser-MAG hybrid welding process.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Owing to the advantages in the highly focused intensity of laser with deep penetration and joint filling capability of MAG, the Laser-MAG hybrid welding technology has been widely used in the industrial manufacturing [1–3]. Laser-MAG hybrid welding technology consists of laser welding and arc welding. The laser has two welding modes: conduction welding and deep penetration welding. The laser is in the deep penetration mode in laser-MAG hybrid welding. During deep penetration welding, a “keyhole” is generated in the molten pool, and the hydrodynamics of molten pool is a very complex process . Under the stable welding condition, the keyhole would stay in equilibrium among the different driving forces such as ablation pressure, metallic vapor pressure, surface tension, gravity and possible induced electromagnetic forces. Those same driving forces also produce the flow of the molten pool. Once the equilibrium is broken, the keyhole would collapse . During MAG welding the metal droplet would transfer to molten pool and flow with the molten pool. In laser-MAG welding, the laser heat resource is the main factor of penetration and the MAG determines the weld width. However, the welding defects such as porosity and undercut are tend to be generated in laser welding of thick plate. The porosity is formed by a bubble which mainly resulted from the keyhole collapse , and the unstable keyhole would form undercut . Recently, high power disk laser-GMA hybrid welding has been developed for thick metal plates welding . Because the two different kinds of welding processes, the laser-MAG hybrid welding process is more complex than a single heat resource welding . It is usually considered that the irregular flow of molten metal around the keyhole is related to the instability of keyhole in laser welding [9–11]. However, the effect of interaction between laser welding and MAG welding on keyhole status has not been clearly revealed . Therefore, it is of great significance to investigate the keyhole status in laser-MAG hybrid welding process.
The highly dynamic phenomena in welding process has increased the difficulty of keyhole status evaluation, many efforts have been made to experimentally observe the keyhole shape in laser welding. The keyhole characteristic has been observed between austenitic stainless steel and GG17 glass in deep penetration laser welding , and the interaction between the laser beam and keyhole wall was investigated by observing the keyhole wall . However, there was little research work focused on analyzing the keyhole status in laser-MAG hybrid welding process. The keyhole was observed by an X-ray transmission system in laser-MAG hybrid welding , but the X-ray image could not describe the effects of interaction between laser and MAG on keyhole status. Many researches were focused on the mechanism of laser-MAG hybrid welding, the interaction of laser beam and arc [15, 16], and the effect of welding parameters on laser-arc hybrid welding . Up to now, it is still difficult to effectively evaluate the keyhole status in laser-MAG hybrid welding process.
As an important analysis method for process monitoring, fault detection and diagnosis have been widely studied in welding process by using signal processing approaches . Multiple physical phenomena such as metallic vapor, spatters and the flow of weld pool are directly related to the weld status [19, 20], which could be used to diagnose the stability of welding process. Hence, several sensing techniques have been proposed for weld process monitoring, such as optics sensing , x-ray transmission , spectrum , and acoustic sensor . One of the most effective technique for welding process monitoring is visual sensing. A double high-speed camera sensing system has been used in laser welding in our previous work . This system could be used to capture the physical phenomena to the computer to evaluate the keyhole status in laser-MAG hybrid welding process.
This work aims to evaluate the keyhole status by observing the visual physical phenomena in laser-MAG hybrid welding process of three different penetration conditions. By applying a Fourier de-noising operator on bottom image, the keyhole bottom was extracted from the bottom of the weld pool. The mechanism of laser-MAG hybrid welding process was also analyzed from visual images. Lastly, four kinds of visual features: 1) average pixel of keyhole bottom, 2) area of keyhole bottom, 3) centroid coordinate x and 4) centroid coordinate y of keyhole bottom in different penetration conditions were used to evaluate the keyhole status.
2. Experimental procedures
2.1 Experimental setup
The experimental setup is shown in Fig. 1. The experiments were performed with a laser-MAG hybrid welding process monitoring system. The system consists of a hybrid welding system and a double high-speed camera system. A double high-speed camera system was used to capture the visual phenomena in welding process. One camera was placed above the weldment and perpendicular to the welding direction to capture the visual phenomena of the top weld surface and the other camera was setup under the weldment to capture the visual phenomena of bottom weld surface using a mirror. The camera frame rate was 2000 fps and image resolution was 512 pixel × 384 pixel. In the hybrid welding system, the laser was focused 2 mm below the surface and the process gas was 20%CO2 + 80%Ar with a flow rate of 40 L/min. The separation of laser beam and MAG arc was 2 mm with MAG torch leading. The weldment dimensions were 150 mm × 60 mm × 6 mm on 316 stainless steel that was fastened to the welding test bed. The gap of two weldments in butt joint welding was 0.05 mm and the diameter of weld wire was 1.2 mm.
The fundamental mechanism of laser-MAG hybrid welding process is presented in Fig. 2. The first step of welding process has divided into two part. As shown in Fig. 2(a), the laser beam transfers the heat to melt the weldment. And then a keyhole is generated under the laser beam as illustrated in Fig. 2(b). On the other hand, the wire is melted under the arc and a molten droplet is formed in the tip of wire. The second step of welding process is shown in Figs. 2(c) and 2(d), the molten droplet transfers to the molten pool with different status which directly affect the penetration status. Lastly, the welding process turns to the first step again.
2.2 Visual feature extraction
For monitoring the keyhole bottom status, this work extracts the visual feature in laser-MAG hybrid welding of different penetration conditions. The scheme of keyhole bottom extraction method is shown in Fig. 3. The region of interest (ROI) was selected from the original bottom image. The flow of bottom weld pool and the metallic vapor ejected from keyhole bottom resulted in image noise in ROI, which decreased the accuracy of keyhole extraction. As shown in Fig. 4(a), the image noises are of the high-frequency characteristic. The image noises are removed by applying a fast Fourier transformation (FFT), and then the bottom weld pool image is reconstructed through the inverse Fourier transform. The processing effect of the bottom image de-noising method is presented in Fig. 4(b). Lastly, the visual features of keyhole bottom are extracted by using morphological method which has been used for plasma feature extraction in laser welding process . There are four kinds of visual features, extracted from the image: AvePixel, CeX, CeY, and Area. AvePixel represents the average pixel of the whole keyhole bottom; the CeX and CeY is the centroid coordinate (x,y), respectively; Area is the area of keyhole bottom.
3. Results and discussion
3.1 Visual phenomena in welding process
Laser-MAG hybrid welding is a highly dynamic process, and the multiple physical phenomena such as spatter, metallic vapor and the droplet transition in welding process are of great kinetic energy. Those physical phenomena directly relate to the weld quality. A normal penetration welding process shown in Fig. 5, the visual phenomena from top and bottom weldment surface are captured by a double high-speed camera system. At t ms, both top and bottom surface are welding in a stable status. Droplet transition occurs at t + 0.5 ms, which is accompanied by a blast of molten droplet and results in an explosive reaction, but the keyhole is still in a stable status. At t + 1 ms, a white point is appearing in the bottom image, and the explanation could be that the metallic vapor is ejecting from the bottom surface of keyhole. At this moment, the newly added molten metal generated from droplet transition is covering the top surface of keyhole, and the equilibrium of keyhole is breaking. Therefore, the instantaneous welding status at t + 1 ms is in Fig. 2(c). At t + 3ms, the metallic vapor largely eject from the bottom surface of keyhole, and the explanation could be that the equilibrium of keyhole is broken and lead to the phenomenon of keyhole collapse. Therefore, the instantaneous welding status at t + 3 ms is in Fig. 2(d). At t + 5.5 ms, the keyhole is reached an equilibrium status again. A new droplet transition starts at t + 9.5 ms, the metallic vapor isn’t ejecting from the keyhole bottom until t + 13 ms. The instantaneous welding status is in Fig. 2(b) during this period of time (from t + 5.5 ms to t + 13 ms). Which indicates that the keyhole bottom is keeping a closed status. When the metallic vapor eject from the bottom surface of keyhole, the status of keyhole bottom can be seen as opened. In full-penetrated laser-MAG hybrid welding experiment, an opened keyhole bottom is likely to occur with the droplet transition. Though the droplet transition sometimes couldn’t break the equilibrium of keyhole (i.e. the period of droplet transition at t + 9.5 ms in Fig. 5), the droplet transition is the main factor to break the equilibrium of keyhole, which resulting an unstable welding status in laser-MAG hybrid welding process.
3.2 Different penetration welding process
Laser-MAG hybrid welding process can be presented as four different conditions, which is shown in Fig. 2. Two kinds of full-penetrated hybrid welding are conducted in this section, the corresponding experimental condition are number 2 and number 3 in Table 1. As a comparison group, the partial penetration welding also presented, the corresponding experimental condition is number 1 in Table 1. For analyzing the visual feature more intuitive, the original signals of features were normalized. The unit of features is a.u. (arbitrary unit). As analyzed in section 3.1, the droplet transition directly affect the equilibrium of keyhole, which resulting in an opened and closed status of keyhole bottom. Features AvePixel is the average pixel of keyhole bottom, the value of AvePixel approximate to 1 means that the keyhole bottom is very bright and the instantaneous bottom surface of keyhole is opened. When the keyhole bottom is very dark and the instantaneous keyhole bottom is closed, the value of AvePixel approximate to zero. Features CeX and CeY means the centroid coordinate of keyhole bottom, which reflects the fluctuation of position of the keyhole bottom. And Area indicates the area of keyhole bottom.
A partial penetration welding process is shown in Fig. 6, 2kW laser power was not high enough to obtain a fully-penetrated weldment in 6 mm thickness of stainless steel at experimental number 1 in Table 1. Metallic vapor could not eject from the keyhole bottom as shown in Figs. 2(a) and 6. Keyhole bottom was hold in an approximate consistent status, both centroid coordinates and area were constant. Feature AvePixel also approximately kept an unchanged value.
As analyzed in section 3.1, the opening keyhole bottom is likely to occur with the droplet transition. When the opening and closing status of keyhole bottom changes in a stable frequency, the keyhole bottom in laser-MAG hybrid welding process is stable. The visual features in laser-MAG hybrid welding process of normal penetration condition are presented in Fig. 7. However, there are continuous lower values exist in green box, in which the keyhole bottom keep in a continuous closed status. By analyzing the global data of AvePixel in Fig. 7, keyhole bottom approximately periodically changes between the opened and closed status except for a continuous closed status in green box. In our opinion, the opened and closed status of keyhole bottom are mainly resulted from the droplet transition. Once the continuous opened or closed keyhole bottom occurs in some period of time, which means that the keyhole bottom in corresponding time is unstable. Therefore, under the normal penetration condition in laser-MAG hybrid welding, the keyhole bottom approximately keep in a stable status except a period of time in green box.
Another full-penetrated hybrid welding experiment is presented in Fig. 8, and the weldment is of imperfect weld surface. Due to the increase of laser power, the absorbed energy of keyhole is increased, and resulting in a more impressionable equilibrium of keyhole. By comparing the features in Fig. 8 and Fig. 7, the periodic performance in Fig. 7 is stronger than Fig. 8. The feature curve AvePixel have several boxes in Fig. 8, in which the green box represents the continuous closed status of keyhole bottom and the red box is the continuous opened status of keyhole bottom. The opened and closed keyhole bottom occurs continuously, it indicates that the welding process in Fig. 8 is more unstable than Fig. 7. In our opinion, the unstable keyhole status in Fig. 8 is mainly resulted from the severe movement of molten metal, which has absorbed much laser energy. Furthermore, by observing centroid coordinates of red box and green box in Fig. 8, the CeX of red box is lower than the CeX of green box. Laser is known as high density and high directivity, which ensures the correctness of the opened keyhole’s position. Therefore, the drifting of centroid coordinate is existed in closed keyhole bottom in Fig. 8.
In order to analyze the keyhole bottom status of laser-MAG hybrid welding process, the physical phenomena in different penetration welding processes were observed by a double high-speed camera system. The mechanism of the equilibrium of keyhole also be analyzed from visual sensors, and four kinds of visual features were defined to evaluate the status of keyhole bottom. During the normal penetration laser-MAG hybrid welding process, the droplet transition is the main factor to affect the keyhole status, and an opened keyhole bottom sometimes occurs with the droplet transition. With the increase of laser power, the keyhole bottom tends to become unstable, and the continuous opened or closed keyhole bottom would appear with more frequency in the welding process. Furthermore, the centroid coordinates of keyhole bottom would drift along the welding direction and the area of keyhole bottom is larger when the keyhole bottom is closed. By decreasing the laser power, a partial penetration weldment is obtained. Comparing the visual feature in partial and full penetrations, four features such as the centroid coordinates, area, and the average pixels are approximately kept constant in the partial penetration condition. The results show that the proposed analysis method could be used to monitor the keyhole stability in laser-MAG hybrid welding process.
National Natural Science Foundation of China (NSFC) (51675104); Science and Technology Planning Project of Guangzhou, China (201510010089); Science and Technology Planning Public Project of Guangdong Province, China (2016A010102015).
Many thanks are given to Katayama Laboratory of Osaka University, for their assistance of laser-MAG hybrid welding experiments.
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