The oxidation or combustion of the metal must be effectively prevented during the melting of the magnesium alloy and can be achieved by a fluxing or fluxless process on the surface of the metal melt. A small amount of metal is usually added to calcium to increase the oxidation resistance of the magnesium melt. Flux smelting and fluxless smelting are two basic types of basic processes for the smelting and casting of magnesium alloys. Before 1970, the melting smelting process was mainly used for the smelting of magnesium alloys. The flux removes impurities from the magnesium and forms a protective film on the surface of the magnesium alloy melt, insulating the air. However, the effect of the flux membrane to block air is not very satisfactory. The loss of magnesium caused by oxidative combustion in the smelting process is still relatively large. In addition, the flux smelting process still has some problems. On the one hand, it is easy to generate flux inclusions, resulting in a decrease in the mechanical properties and corrosion resistance of the castings, which limits the application of magnesium alloys; on the other hand, the flux reacts with the magnesium alloy liquid to generate corrosive flue gas, destroying Smelting equipment deteriorates the working environment. In order to improve the safety of the melting process and reduce the oxidation of the magnesium alloy solution, a fusionless I1 smelting process emerged in the early 1970s, and sulfur hexafluoride (SF) and nitrogen (N) or dry air were used in the smelting furnace. Mix protective gas to avoid contact between liquid and air. The content of SF6 in the mixed gas should be carefully selected. If the SF6 content is too high, it will erode and reduce its service life; if the content is too low, it cannot effectively protect the melt. In general, whether it is flux smelting or fluxless smelting, as long as the proper operation, can produce a good quality cast magnesium alloy. 10. Smelting protection process 1 flux protection melting process. Separating the melt surface from oxygen is the most basic requirement for safely performing magnesium alloy smelting. Early attempts were made to use a gas protection system, but the effect was not satisfactory. Later, people developed a flux protection smelting process. Magnesium alloy fluxes are shown in Table 12-3. In the smelting process, it is necessary to avoid the "bridge" phenomenon of the molten charge in the crucible, gradually add the remaining charge to the crucible, keep the liquid level of the molten alloy rise steadily, and gently spread the flux on the surface of the melt. Each magnesium alloy has its own special flux and must strictly comply with the supplier's guidelines for the use of flux. During the melting process, local overheating of the charge must be prevented. When melt chlorination is used to melt magnesium alloys, effective measures must be taken to collect C12. Before pouring, the melt is carefully slag-dried and de-oxidized, especially the chloride that affects the corrosion resistance. After pouring, sulfur powder is usually sprinkled on the surface of the melt to reduce its oxidation during the solidification process. 2 no flux protection process. The use of the flux smelting process in die-casting technology will bring about some operational difficulties, especially in hot-pressing chamber die casting. At the same time, flux inclusions are the most common defects of magnesium alloy castings, which seriously affect the mechanical properties and corrosion resistance of the castings, which greatly hinder the extensive application of magnesium alloys. In the early 1970s, the successful development of the fluxless smelting process was an important breakthrough in the application of magnesium alloys, and it had a revolutionary significance for the development of the magnesium alloy industry. 1 gas protection mechanism. As mentioned above, pure inert gas such as N2.Ar. Ne can play a certain role in flame retarding and protecting magnesium and its alloy melt, but the effect is not ideal. N2 easily reacts with Mg to form Mg Yin 2 powder compound, which has a loose structure and cannot stop the reaction. A: Although inert gas such as Ne does not react with Mg, it cannot prevent the evaporation of magnesium. A large number of experimental studies have shown that CO2'SO2'S and other gases can play a good protective role for magnesium and its alloy melts, of which the best effect of SF6. The melt is very slow in dry pure CO:. The equation of chemical reaction of CO and magnesium at high temperature is 2MgO+CO2=2MgO5+C. The reaction product is amorphous carbon. It can fill the gap with the oxide film and improve the compactness of the oxide film on the surface of the melt. In addition, it can strongly Suppresses the diffusional movement of magnesium ions across the surface film, thereby suppressing the oxidation of magnesium. The chemical reaction formula of SO2 and Mg is 3Mg. The reaction product of +S02=2MgO8+MgS forms a thin layer of denser MgS/MgO composite film on the surface of the melt, which can inhibit the oxidation of magnesium. In the 1970s, before the protection effect of SF6 was not recognized, SO2 gas was widely used to suppress the oxidation and combustion of magnesium alloys. S is an artificially prepared non-toxic gas, with a relative molecular mass of 164.1 and a density that is 4 times that of air. Chemical reactions may produce toxic gases and are extremely stable at room temperature. A series of complex reactions can occur with the mixed gas containing SF6 and magnesium: 2Mg(L)+02=2MgO(S) 2Mg(L)+02+SF6=2MgF2(S)+SO2+F2 2MgO(S)+SF6= The density of 2MgF2(S)+S02+F2 MgF2 is high. It can form a continuous and dense oxide film together with MgO, which has a good protective effect on the melt. It should be noted that the use of SF6-containing protective atmosphere must not contain water vapor, otherwise the presence of water will greatly aggravate the oxidation of Mg and also produce toxic HF gas. .SF6 protection atmosphere. At present, SF6 protection atmosphere is widely used in the melting and production of magnesium alloys. The SF6 protective atmosphere is a very effective protective atmosphere, which can significantly reduce melting losses and is widely used in the ingot production industry and the die casting industry. Experimental studies have shown that a gas mixture containing a volume fraction of 0.01% S can effectively protect the melt. However, in practice, SF is used to compensate for the loss caused by reaction and leakage of SF6 with the melt. The concentration is higher. In the configuration of mixed gas, generally should be used for multi-pipe, multi-outlet distribution, as close as possible to the liquid surface and uniform distribution, and the need to regularly check the pipe for clogging and corrosion. When using the SF6 protective atmosphere to smelt the alloy, the stability of the pouring temperature, melt level, and feed rate should be increased as much as possible to avoid disrupting the SF6 gas concentration above the liquid level. Also note that the protective gas reacts with the helium, otherwise reaction product eF3.Fe20) will react violently with Mg. S There are two main protective atmospheres, one is a mixture of dry air and SF6, and the other is a mixture of dry air and Co: and SF6. In the flux smelting process, due to no flux, the amount of slag at the bottom of the crucible is greatly reduced and the melt loss is relatively low. As SF6 decomposes slowly and reacts with other elements at the melting temperature of magnesium alloys, toxic gases such as S02, HF, and SF4 are generated. At 815°C, S2F10 is also highly toxic, but S2F10 decomposes at SF6 at 300-350°C. S Where, therefore, the smelting temperature of magnesium alloys is generally not more than 8000C o S. A protective atmosphere with a volume concentration of less than 0.4% can effectively protect the magnesium alloy melt, and toxic gases generated can be ignored. SF6 has a high price and potential greenhouse effect, so it is necessary to try to control the emission of S. The concentration of SF6 in the protective atmosphere must not exceed 2% by volume, otherwise it will cause helium loss. Especially at high temperatures, when the concentration of SF6 exceeds a certain volume fraction, violent reaction or even explosion may occur in the crucible. Therefore, the SF6 concentration in the mixed gas must be strictly controlled. In addition, the lidded enamel cannot be protected with a pure SF6 atmosphere. SF6 is the main factor affecting the life cycle indicators of magnesium alloys, and it is also a key factor that restricts magnesium alloys to be a green material in the 21st century. In 2000, the International Magnesium Association (Mg) assisted people in the magnesium industry to pay attention to the development of protective gas as a sand-molding process for magnesium alloys instead of SF6 4 and magnesium alloys. In general, it can be done according to the basic processes used for other metals, but Considering that magnesium alloys can rapidly oxidize in the molten state, their density and heat capacity are low. At present, in the sand casting of magnesium alloys, in order to prevent the burning of magnesium liquid and for environmental protection and human health considerations, sodium alkyl sulfonate non-toxic sand is generally used. This type of sand has passed the test of production, and it has been fully proved that it can be used as a substitute for fluorine add-on sand for casting magnesium alloys. Moreover, the sand has the advantages of small odor, good process performance, good air permeability, low cost, and wide supply of goods, and is well received by enterprises. However, if the sodium alkyl sulfonate is not well controlled, it can easily cause more casting defects in the casting. This article describes the control of the sodium alkyl sulphonate sand molding process, preparation points and preservation of the use of control points, and some of the problems that may arise in the actual production were discussed for reference. 1. Composition and Formulation of Sodium Alkyl Sulfate Sand The density of magnesium alloy is about 2/3 of that of aluminum and 1/4 of that of iron, so the pressure of metal in the cavity is quite low. In order to allow the gases generated by the sand to flow freely into the atmosphere, the sand must have good air permeability. Otherwise, the generated gas will invade the molten metal and cause casting defects. The composition of the sodium alkanesulphonate sand is as shown in Table 12-4. The sand mixing process affects the strength of the sand to a large extent and requires careful operation. The following describes the preparation process of sodium alkanesulphonate sand. Check whether the equipment is operating normally before sand preparation. Add quartz sand, bentonite, and boric acid to dry mix for 5-7 min. Add sodium alkyl sulfonate to mix for 9-10 min. Add water and mix for 8-10 min. If you need to add glycerin, stir it with water. There are three reasons for adding glycerin: 1 reduce the water content of sand; 2 increase the working time of sand by slowing down the rate of water evaporation; 3 use as a flame retardant with boric acid to form a borate that has a protective effect on magnesium. Compounds. 2. Sodium Alkyl Sulfate Sand and Its Characteristics 1 Alkyl Sulfonate is a carbon-nitrogen compound with a long carbon chain and a light yellow liquid in water. When there are two sulfonate groups on the molecule, they have a double sulfo structure, and there are two or more sulfonate structures. Therefore, an alkyl sulfonate molecule, not only cleaves during the pyrolysis process, produces one S02, but multiple so: molecules. At the same time, it also generates a certain amount of Co: gas to form a strong protective atmosphere. In addition, in the high-temperature pouring process, the cracked products are similar to the high-molecular alkanes, and there are gaseous hydrocarbons such as ethylene, ethane, propylene, propane and the like. When the carbon chain is broken, some of the carbon and hydrogen are oxidized by oxygen in the air or oxygen in the sulfonate to produce certain Co: and steam. During the casting of high-temperature molten metal, in the sand shell near the surface of the casting, due to lack of oxygen, a certain amount of free carbon will be generated when the alkyl carbon chain is broken, making the sand shell crust black and loose and fragile. The cooling shrinkage of magnesium castings has created favorable conditions. 2 The role of boric acid. Boric acid is a fine powder of white powder, which is a kind of weak acid. It is dehydrated many times after heating, and it produces boron drunk. Boron can increase the wet strength, but if the amount is too large, the permeability will decrease. In addition, boric acid and magnesium alloy surface contact with the formation of a dense protective film of magnesium boride, magnesium oxide castings do not oxidize the combustion process, and sodium alkyl sulfonate used in conjunction with each other, play an essential role in flame protection. 3 The role of bentonite. Bentonite, as the main binder, has a high wet-end drilling force when wetted with water. When properly blended with other components, bentonite has higher toughness. If the amount of private soil is too high, there will be more effective bentonite, and the amount of water needed to achieve a uniform mixture will have to increase, resulting in an excess of suitable moisture. The increase of moisture and bentonite will increase the toughness of the sand and it will be difficult to compact the shape. It is a factor that promotes mechanical sand. 4 Alkyl Sulfonate sand has better physical properties. Alkyl sulfonic acid sodium sand has a moisture permeability of not less than 80, a wet compressive strength of 0.4-0.6 MPa, and a moisture content of 4.5%-.5.5%. 3. Storage of Sodium Alkyl Sulfonate Sands Uses Sodium Alkyl Sulfonate as Weak Alkaline, should be stored in acid and alkali resistant containers to avoid long-term contact with iron. When sodium sulphonate is supplied in the whole tannage supply, it should be transferred to a separate mash for the first two days, allowed to stand, and the unsaponifiables and sodium salts floating on it should be removed before use. Because of the high content of unsaponifiables, its flame-retardant performance is significantly reduced, and at the same time it has an impact on the sand-forming process performance. The sediment at the bottom cannot be used. When adding a suitable amount of water, the amount of water contained in the sodium alkyl sulfonate added should be taken into consideration, and finally meet the moisture performance index. Moisture performance indicators should be controlled at the upper limit in summer and dry weather, and at the lower limit in winter and rainy weather. In order to slow down the rate of water evaporation and improve the process performance of sand, it is possible to add glycerin with a mass ratio of 0.1% to 0.2% to the molding sand. When adding glycerin, glycerol should be mixed into a mixture of glycerin and water in advance, and stir evenly. Modeling sand should be dry and free from coal dust and other debris, especially coal, which can cause stomata and subcutaneous pores on the casting surface. Usually in the modeling, it is divided into surface sand and sand filling. He called back sand, face sand is the new mixed sand, and the sand is mixed with regrind sand. When using recycled sand, because the components in the sand are burned, there is no need to add bentonite, just add 0.1%-0.3% of the amount, 0.2%-0.4% of sodium alkyl sulfonate and the right amount of water, mix The time for sand is slightly less than that for new sand. After the general sand is used once, the burned material is formed in the sand, which causes sand to appear in the sand and the sand becomes coarse. Therefore, it is necessary to add a certain amount of new sand. However, the new sand should not be added too much. When the amount is too large, the wet strength and dry strength will easily decrease, resulting in deterioration of the sand's thermal properties. Because the mechanical action of sand mixing will cause the sand temperature to rise, the sand temperature will rise, affecting the drilling and knotting force of the private soil agent film, which will cause casting defects. Therefore, after the sand is mixed, it should be parked for at least lh. Sand temperature requirements not more than 400C. The prepared sand should be covered with wet sacks to prevent loss of water caused by the molding sand, resulting in scumming and sandblasting defects. 4. Frequent problems in production 1 If the sodium alkyl sulfonate is stored in the iron crucible for a long time, the carcass will corrode and a dark brown suspension will be generated. The sodium alkanesulphonate was used to prepare the sand, and the casted riser had an open flame and burnt down on Shuo Street. After the castings were boxed, the riser was severely ablated, and the quality of the casting was also affected. During the pouring process, with the constant increase of the pouring temperature in the cavity, the protection of the shielding gas is required to be more objective, but actually the concentration of the shielding gas is gradually reduced from high to low, resulting in insufficient flameproof atmosphere. This phenomenon occurs. To avoid this situation, a thin layer of sulphur powder must be sprinkled around the thick and complex cavity surfaces of castings and around the riser. In the sand mold, sulfur near the liquid metal can be volatilized to sulfur vapor at a relatively low temperature. In the pouring process, sulfur quickly seizes oxygen from the air in the cavity to create a so: protective atmosphere, so: it reacts with the metal to form a surface with additional protection, improving the anti-burning effect. What's more important is that S can interact with the surface of the magnesium liquid to form a protective film of magnesium sulfide and enhance the oxidation resistance of the casting surface. 2 Burning gray spots were found on the surface of magnesium alloy castings. Most of the gray spots are on the flow path and near the gate, as well as on the top and side of the casting. In severe cases, one black spot can be found with the naked eye, with dark gray powder inside. The reasons may be as follows: 1 When the magnesium liquid is poured, the liquid flow is disordered, or the design of the pouring system is unreasonable, and there is no flow-resisting buffer and slag slag unit. During the pouring process, the magnesium liquid must collide and spray to generate oxidative combustion and combustion particles. Mgo) into the cavity, floating on the private attached to the surface of the mold; 2 sodium alkyl sulfonate in the case of high temperature oxygen, the formation of carbonation layer. When there is a portion of recycled sand that is used multiple times, this carbide layer will cause the casting to burn. However, the carbonized layer is difficult to remove. This requires a rational design of the casting casting system, and the molten metal should enter the cavity without any impact. The sodium alkanesulphonate sand has relatively high gas permeability, but its wet strength is low because the sodium alkyl sulfonate can activate the surface of water and reduce the surface tension of water. It can penetrate into the fine particles of bentonite, so that the surface tension between the bentonite solid particles is less than the surface tension between the inside, thus destroying the cohesion between the solid particles, so that the particles break into small particles dispersed into the sand. At this time, the outer layer of the microparticles is covered with a layer of hydrophilic active molecular adsorbent film, because the active molecules have the same directionality on the surface film of the particles, and thus generate a polar charge, so that the particles have a repulsive force and the result is destroyed. Bentonite's private role. Therefore, the water evaporation speed in the sand after mixing or stacking is fast, and the surface layer of the sand is dry and scattered. In the long time of styling or modification, the moisture on the surface of the cavity evaporates quickly, which brings certain difficulties to the stylist. In the casting of sodium alkanesulphonate sand-magnesium alloys, its quality control is an important part of improving the quality of castings. Only by properly controlling the composition, moisture, etc. of the various components of the sand preparation, can the quality of the sand be guaranteed, and then improve. Castings, get better economic benefits. Acorn Nuts,Hardware Nuts,Finish Cap Nuts,Acorn Nut Fastener Taizhou Wanluo Hardware Products Co., Ltd. , https://www.wlhardware.com
[Technology] Smelting Process Characteristics of Magnesium Alloy Melting