Rotating injection is an important method for purification of aluminum alloy melt． Ｒotor is the key component of rotating injection． The quality and life of the rotor have great effect on the quality of aluminum alloy melt． Oxidation and wear are the main factors affecting the rotor life． It is essential to improve the quality of the rotor． It presents the wear mechanism of the graphite rotor and several methods to improve its life．
Graphite rotor is the key component of the rotary injection degassing device for aluminum alloy melt. The quality and life of the graphite rotor have a direct impact on the continuity of aluminum alloy melt purification and the stability of the quality of aluminum products. The high-speed rotator disperses argon or nitrogen into the aluminum alloy melt, forming tiny bubbles, increasing the specific surface area of these bubbles. At the same time, the graphite rotator also promotes the diffusion of hydrogen and inclusions in the aluminum melt, increasing their contact probability with argon or nitrogen bubbles, thus maximizing the removal of hydrogen and inclusions in the aluminum melt. Achieve the purpose of purifying molten aluminum.
Most of the material of the rotor is high purity graphite, which is easy to damage and short in service life. Therefore, this paper describes the damage mechanism of graphite rotating rod, and introduces the measures for prolonging the service life of the rotating rod.
1 damage mechanism of graphite rotor
Graphite rotor including rotating rod and impeller, after a period of use, graphite rotating rod in the aluminum alloy liquid surface necking, rotating rod diameter is becoming smaller and smaller, until the fracture scrap.
In the degassing process of aluminum melt, the graphite rotating rod is effected by the erosion of aluminum liquid and high temperature oxidation. Whether it is liquid aluminum erosion or high temperature oxidation that causes the rod to shrink its neck is a matter of debate. In addition to high temperature oxidation and liquid aluminum erosion are the main reasons for the damage of the rotating rod, improper material, structure and operation of the rotating rod will shorten the life of the graphite rotating rod.
Schematic diagram of rotor degassing
1.1 high temperature oxidation
Graphite is mainly composed of carbon, which is oxidized visibly in air over 600℃. The product of carbon oxidation reaction is CO and CO2 gas. Without a protective film on carbon, the rotating rod can not be protected. Generally speaking, the rotating rod is exposed to air without inert gas protection, and the oxidation of graphite rotors is inevitable. The purifying temperature of aluminium alloy melt is between 700 ~730℃, which is much higher than 600℃. Therefore, the possibility of oxidation of graphite rotating rod is great. As a result, the diameter of rotating rod is gradually reduced until it is broken and discarded.
Graphite rotor may have many defects, such as pores, cracks and so on. Oxidation of the rotating rod surface will expose the internal pores. Oxygen in the air immersed in the graphite rotor along the pores causes internal oxidation and accelerates the oxidation and spalling of the rotating rod. The oxidation rate of carbon is directly proportional to the porosity of the graphite rotating rod, and the higher the porosity is, the greater the oxidation rate.
1.2 scouring of molten aluminum
When purifying the aluminum alloy melt, the graphite rotor immerses in the aluminum alloy melt, and generally rotates and stirs the melt at the speed of 300 r/min to 600 r/min. The aluminum alloy melt will produce strong erosion and friction to the rotor. The structure of the rotating rod is loosened by high temperature oxidation at the contact area between the rotating rod and the liquid surface. The carbon on the surface of the rotating rod will gradually peel off under the action of aluminum alloy melt, which makes the rotating rod thinner and reduces the service life of the rotor.
Aluminum alloy melt has a certain viscosity, rotating rotor and aluminum alloy melt friction erosion, so that the graphite rod layer by layer off, but this effect is quite small. The production practice shows that the impeller is completely immersed in the aluminum molten pool when it rotates, and the impeller will be subjected to greater friction erosion. However, after several days and nights of continuous operation, the impeller is still intact and no erosion is found.
2 graphite rotor reinforcement
2.1 new materials
With the continuous pursuit of rotor life, a new type of silicon nitride rotor came into being. Silicon nitride has the advantages of high temperature resistance, oxidation resistance and high hardness, so it has a good application prospect in the rotor degassing of aluminum melt. However, the impeller shape of the rotor is complex and the preparation process requirements are high. The coaxiality of larger silicon nitride rotors is difficult to guarantee and the production cost is high. Therefore, developing low cost and high strength rotors is imminent.
In practical application, it is found that the vulnerable parts of the rotating rod occur on the surface of aluminum melt, so a composite rotor is developed, that is, the upper rotating rod is made of silicon nitride, the middle and lower parts are made of graphite, or the rotating rod made of silicon nitride and the impeller made of graphite. Give full play to their advantages, not only reduce the oxidation of the rotating rod, but also can cooperate with the use of a variety of rotor shapes, prolong the life of the rotor, reduce production costs, improve the quality of aluminum alloy melt.
Schematic diagram of a compound rotor
2.2 graphite rotary rod strengthening treatment
There are many methods to improve the oxidation resistance of graphite rotor, such as vapor deposition, coating method, impregnated salt solution method. The impregnated salt solution method is simple and low cost. After impregnation, inorganic salts are tightly bonded with graphite materials to form an anti oxidation film to improve oxidation resistance. There are many formulations and impregnating processes of impregnating agent. Borate is preferred as impregnating agent abroad, and phosphate is preferred as impregnating agent in China. Aluminum dihydrogen phosphate has high temperature resistance, corrosion resistance, erosion resistance, oxidation resistance and other properties, and has good wettability with graphite materials, and poor wettability with aluminum alloy melt, does not react. Aluminum dihydrogen phosphate was immersed in the pores of graphite material in vacuum atmosphere, and then cured to convert aluminum dihydrogen phosphate into polyaluminium phosphate. The surface and pores of the impregnated graphite rotor are filled with polyaluminium phosphate (PAP). A protective film of PAP is formed on the surface of the exposed graphite particles. The graphite does not contact with air or high temperature aluminum liquid directly. The oxidation resistance of the graphite rotor and the corrosion, erosion and wear resistance of the aluminum liquid are improved. The density of the rotor increases the strength of the graphite rotor, thus greatly improving the life of the graphite rotor.
2.3 inert gas protection
By introducing inert gas, the part of the rotor exposed to aluminum liquid is in inert atmosphere, which reduces the contact surface between the rotor and oxygen, thereby reducing the oxidation process of the rotor and prolonging its service life.
Some studies have found that the wear of graphite rotating rods is mainly attributed to the oxidation of carbon. The service life of the rotating rod can be effectively improved through the research and development of new materials, impregnation strengthening treatment, inert gas protection and other measures. Develop new ceramic materials with high temperature resistance and oxidation resistance to reduce rotator oxidation; The graphite rotator bar was impregnated with aluminum dihydrogen phosphate to form a protective film. Protect the graphite rotator by inert gas, reduce the probability of contact with oxygen and reduce the oxidation of the rotator. All of the above three methods can extend the service life of the rotor.
Author: ZHANG Xiao-guang。CHENG Shu-jian，QIU Li-bao