# Quality Monitoring for Laser Welding Based on High-Speed Photography and Support Vector Machine
## 2 Experimental Method
### Features of the Welding Sample  
Figure 4 shows the top view of the welded seam. The welded seam was divided into three regions according to the image serial number, region A [521, 980], region B [981, 1500], and region C [1501, 2000], corresponding to the relevant region in Figure 4. Welding quality is mainly related to several factors,  
Figure 3: The extracted features: (**a**) the area of metal vapor plume; (**b**) the height of metal vapor plume; (**c**) the average intensity of metal vapor plume; (**d**) the vertical coordinate of plume centroid; (**e**) the horizontal coordinate of plume centroid; (**f**) the number of spatters. including the fluctuation in the welding process, the uniformity of the bead width of the weld, the number of internal pores, as well as the number and the size of spatters. From the surface appearance of the welded seam, the surface weld width of region A and region C were relatively stable and of high welding quality, while the weld width of region B became narrow suddenly and was of poor weld quality. As such, region A and region C were treated as a class, denoted with R, and region B was treated as another class, denoted with E.  
As shown in Figure 4, three different cross sections of the sample marked with 1, 2, and 3 were selected for further observation of the internal quality. The bead width (the width of the welded seam in the horizontal direction on the sample surface) and the weld penetration of the three cross sections denoted 1, 2 and 3 were shown in Figure 5a-c. As shown in Figure 5 and Table 2, the depth-width ratio of the second cross sections was the largest, and the quality of its underfilled surface was the worst. Observing the cross sections through a metallurgical microscope (XJP-6/6A), several pores were found in the cross section, as shown in Figure 6. The captions of each image were named as _i-j_, which meant the _j_-th pores in the _i_-th cross sections. Two pores were found in the first cross section, as shown in Figure 6(1-1),(1-2), five pores were found in the second cross section, as shown in Figure 6(2-1)-(2-5), and two pores were found in the third cross section, as shown in Figure 6(3-1),(3-2). Most of the pores were rounded. The number of pores found in the second cross section were more than the other two sections, and the diameter of the pores were bigger than those of the other two sections. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Region** & **1** & **2** & **3** \\ \hline The depth (mm) & 8.05 & 8.3 & 8.4 \\ The width (mm) & 1.66 & 1.43 & 1.66 \\ The depth-to-width ratio & 4.85 & 5.80 & 5.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The depth and the depth-to-width ratio of the three traditional cross sections. Figure 4: Top view of the welded seam. Three regions A, B, C were divided, and three different cross sections marked with 1, 2, and 3 were selected. Figure 5: The traditional cross sections of the welded seam: (a) corresponding to region A; (b) corresponding to region B; (c) corresponding to region C.  
The first cross section belonged to region A, whose surface width was a bit wider. As can be seen from Figure 3, from the 771 st to the 790 th image, it was found that the area and the height of the metal vapor plume both underwent a sharp upward mutation. There existed two obvious raised knots on the surface of the welded seam in region A, which are some tumor-like particles caused by spatters, but only two pores were found in the corresponding cross section, which indicated that the sudden increase of the area and height of the metal vapor plume corresponded to the formation of the knots on the surface, but it had little relation with the change of the pores. We observed the trail of the spatters from the real-time video, and thought that the formation of knots was due to two reasons. First, when the spatters were big enough, the movement trail of spatters was mainly in the vertical direction under the action of its gravity, leading to the sudden increase of the area and height of metal vapor plume.