In the welding process, the drug skin plays a complex metallurgical reaction and physical and chemical changes. It basically overcomes the problem of the light welding rod appearing in the welding. So it is also one of the main factors to determine the quality of the weld metal. 1) mechanical protection (1) gas protection During welding, a large amount of gas is enveloped in the welding area and molten pool after melting, and the molten metal is basically separated from the air. Most of these gases are reductive gases (CO, H, etc.), which can form a good protective layer around the arc and molten pool to prevent the invasion of oxygen and nitrogen in the air, and play a role in protecting molten metal. Slag protection In the process of welding, the skin of the traditional Chinese medicine is melted at high temperature to form molten slag, which covers droplets and molten pool metal. This will not only cut off oxygen and nitrogen in the air, protect the weld metal, but also slow down the cooling rate of the weld, promote the discharge of gas in the weld metal, reduce the possibility of generating the pores, and improve the forming of the weld. Crystallization, play the role of slag protection. 2) metallurgical effect Through the metallurgical reaction of molten slag and molten metal, the harmful impurities (such as oxygen, hydrogen, sulfur, phosphorus) and the addition of beneficial alloy elements have been added to make the weld meet the required mechanical properties. Although the drug skin has the effect of mechanical protection, the liquid metal is inevitable to be intruded and oxidized by a small amount of air. In addition, some substances in the skin are decomposed and released by high temperature arc. The alloy elements in the liquid metal are burned and the quality of the weld is reduced. Therefore, some reductant should be added in the skin to restore the oxide so as to ensure the quality of the weld. In addition, some dehydrogenation and sulphur removal materials are added to improve the crack resistance of the weld metal according to the different properties of the electrode. Because of the high temperature of the arc, some alloy elements contained in the weld metal are burned (oxidized or nitrided), which will reduce the mechanical properties of the weld. By adding iron alloy or pure alloy element to the skin of the electrode, it is transferred to the weld metal with the melting of the skin, in order to make up the burning of the alloy elements and improve the mechanical properties of the weld metal. 3) improve the welding process performance, make the arc stable combustion, less spatter, good weld formation, easy slag removal and high deposition efficiency. In the process of welding, it is necessary to ensure the normal and stable combustion of the arc. In addition to the electron emission from the cathode, it is necessary to ionize the gas in the arc space, the more easily the gas is ionized, the better the gas conductivity is, the more stable the arc combustion is. For this reason, the low ionization potential material is added to the skin to improve the stability of the arc combustion. The melting point of the electrode is slightly lower than the melting point of the welding core (about 100^-250 C), but the core is in the center of the arc, the temperature is high, so the flux is first melted, and the skin of the powder melts a little later. In this way, a small length of casing is formed at the end of the electrode. The bushing makes the arc heat more concentrated and can stabilize the arc combustion, which is conducive to the transition of droplets to the molten pool and improves the deposition efficiency. In short, the role of the skin is to ensure that the weld metal obtains the required chemical composition and mechanical properties and enables the electrode to have good welding performance.