When the CO2 ceramic liner is single-sidedly welded and bottomed, a shrinkage cavity will appear in the center of the back of the arc. The main reason for the shrinkage cavity is that the thermal conductivity of the ceramic liner is smaller than that of the base material, and the molten metal in the upper part of the molten pool solidifies first due to good heat dissipation conditions, while the molten metal in the lower part of the molten pool has poor heat dissipation conditions and finally solidifies. During solidification, the decrease in temperature causes volume shrinkage, and at this time other parts of the metal have solidified, and there is no metal to supplement this shrinkage, thus forming shrinkage cavities. In order to eliminate this shrinkage cavity, the method of current attenuation is first adopted. Now general CO2 welding machines have adjustment knobs for filling arc crater attenuation specifications. Before welding, adjust this knob to an appropriate attenuation value. When the arc is to be closed, press the button on the gun twice. At this time, the welding current and welding voltage will automatically attenuate from the original parameters to a smaller value. After the last molten metal fills the pit, the arc is directed onto the solidified weld. Release the button of the welding torch at this time, the arc is finally extinguished, and the shrinkage cavity will not be generated.

In the installation and construction of steel structures, the corner joints between columns and beams are full-penetration welds, which belong to class II welds and require 20% UT flaw detection . This type of welded joint has high quality requirements and is basically high-altitude welding on site, so welding is difficult. In order to facilitate the welding construction, ensure the welding quality and improve the welding efficiency, the method of adding a welding backing to the back of the weld is adopted. Put the liner on the back of the groove of the specified shape and size of the steel plate and the workpiece, and weld from the front, which can form both sides at one time, the back weld is full, the weld traces are neat, and the shape is beautiful. With this method, the arc is stable during welding, the welding quality is guaranteed, the work efficiency is improved, the working conditions of the welders are greatly improved, and the construction period is shortened.