First, the safety measures for magnesium alloy smelting 1. Personal protection Personal protection is the basic conditions for magnesium alloy work. Under normal circumstances, the following protective equipment is the basic protection of magnesium alloy die casting personnel: overalls, helmets, protective masks, Hot asbestos gloves, fire-retardant garments are hot and heat-resistant at 700°C. The operator must wear protective equipment according to the above requirements before carrying out work. Persons not wearing protective equipment should not be close to the work area and can not operate. 2. Safe Operation of the Furnace The safe operation of the magnesium alloy furnace mainly involves such points. 1 Untrained personnel must not enter the equipment area. 2 There is danger of explosion if there is an open flame or smoking at the point where there is dust of magnesium, magnesium or magnesium. 3 The parts of the equipment are live. If the electrical equipment is not properly maintained and the grounding wire is not tightened, it may cause personal injury or death. 4 Modifying the equipment at will will reduce the safety of the equipment and may result in injury or death. 5 Dangerous and erroneous operation of the equipment may result in injury or death. 6 Adding tidal temperature or dirty magnesium ingots and magnesium dust, magnesium powder, and magnesium slag to the melting furnace has a risk of explosion. Therefore, the magnesium alloy added to the crucible should be dry, oil-free, dirt-free, and not less than 1500C-preheated. The most important thing is to keep the site dry and clean. The mold should be preheated to more than 150°C before each start-up. Do not spray too much paint during spraying to avoid water accumulation in the cavity and cause danger. In addition, the cooling of the punch and die should not be water-cooled. The cooling of the punch can be air-cooled, and the heating and cooling of the mold generally use high-temperature oil. Magnesium alloy die-casting punch speed is also higher than that of aluminum alloy die-casting. In order to avoid flying material injury, sometimes the fly baffle is installed on the parting surface of the die. The melted magnesium overflowing from the 7th bowl can cause people to burn or die, so wear protective clothing when melting or adding magnesium alloys. 3. Fire-fighting safety of magnesium alloys The fire extinguishing agents used for magnesium alloys include dry sand, covering agents and Class D fire extinguishers. These fire extinguishing equipment should be placed in a conspicuous place and convenient for on-site emergency use. Dry sand and covering agent should be stored in containers. Prevent moisture and always check it. Fire extinguishing agents that can be used for magnesium alloys include dry sand, covering agents, Class D fire extinguishers, and it is important to remember that water cannot be used and common fire extinguishers can catch fire on magnesium alloys. These fire extinguishing equipment should be placed in a conspicuous place and convenient for emergency use on site. Dry sand and covering agent should be stored in the container to prevent moisture and should be checked frequently. For a small amount of magnesium burning, you can use dry sand to extinguish, or you can quickly scoop up and transfer it to the empty place or put it into the slag box. If the dispersion of magnesium cannot be scooped up, the covering agent can be evenly sprinkled on the magnesium liquid, and the extinguishing effect on the magnesium alloy is also better. Due to the high price of Class D fire extinguishers, they should not be used as much as possible under normal circumstances. The key to the safe production of magnesium alloy die casting is to keep the production site clean and dry, so that on-site operation personnel are well trained in safe operation, and timely handling magnesium chips and dust in production according to the correct safety operation regulations. In pressure casting, magnesium alloys are high-pressure and high-speed at the time of injection. Therefore, the requirement for die-casting equipment is high, and the inferior die-casting equipment has potential dangers. In the die-casting operation, molten magnesium is injected into the cavity at a speed of 70-100 m / s. Because the molten magnesium is flammable and explosive, it will react violently with oxygen, explode with water, encounter rust, moisture-contained concrete, silicon-containing refractories, etc., and will react violently, and it is difficult to extinguish in the event of a fire. A large fire can not be saved, so its performance, reliability, and safety requirements for die-casting complete sets of equipment are extremely high, and poor quality equipment can easily cause disaster accidents. In the course of exploring applications, there have been many major safety incidents caused by equipment problems at home and abroad. In order to ensure safe production, long-term production use and reliability under harsh conditions of use, the quality requirements for magnesium alloy die-casting equipment are extremely high. Unlike ordinary mechanical equipment, inferior equipment that does not have strong comprehensive technical strength is produced. It can easily cause major disasters. For example, if the low quality site collapses above 650°C, the Ibb layer will be produced at 7000°C for a long period of time under high temperature conditions. The outer layer is susceptible to long-range oxidation, and the inner layer will be quickly eroded due to the corrosion of magnesium liquid and the corrosion of SF6 protective gas. The outflow of molten magnesium will cause a major disaster after a fire and explosion. The protection of the cover with traditional methods of soil will exacerbate the corrosion process. The crucible of high-quality equipment is made of specially developed composite materials. The inner layer is resistant to corrosion, the outer layer is heat-resistant, and it is resistant to high-temperature oxidation, which can avoid serious perforation accidents. The protective gas control of high-quality die-casting equipment is accurate and stable, and the gas composition and flow rate are stable enough. It has a special automatic protection device in the event of unexpected power outages, unexpected accidents, etc., which is extremely safe. Inadequate control of gas composition and flow rate in poor quality equipment can easily result in fire in the furnace due to low concentration and flow rate, rapid corrosion of the furnace due to high concentration and high flow rate, and fire explosion due to magnesium liquid leakage, and lack of reliable automatic protection measures under special circumstances. . å©åŸš Poor materials and unstable protective gas components all affect the composition and performance of the alloy, resulting in a drop in the intrinsic quality of the die casting, such as a rapid increase in the hot and cold cracking tendency, a decrease in the overall mechanical properties, and a failure to meet corrosion resistance requirements. Unstable performance of the injection system can also result in loose internal structure of the casting, unstable mechanical properties of the die casting product, and various other die casting defects. II. Cleaning of the equipment All the crucibles used for the melting of magnesium alloys contain metal residues when they are removed from use. The same applies to the tools used in metal liquids. The residue can be removed by diluting it with 1:10 hydrochloric acid in water. The tube is usually removed by pumping acid. Purging with hydrochloric acid will produce nitrogen, and it must have smooth ventilation. Third, the typical magnesium alloy smelting process 1. AZ91 alloy smelting process can use flux method and fluxless melting steam smelting method. 1 flux method. Preheat the crucible to dark red ((400-50000, evenly sprinkle a layer of powder RJ-2 domain RJ-1) on the inner wall and the bottom of the crucible.) The charge is preheated to above 150°C, and the charge and magnesium are successively added. Ingots, aluminum ingots, and sprinkle a layer of RJ-2 flux on the charge, when the charge is taken, the amount of flux accounts for 1%-.2% of the weight of the charge, heating and smelting, when the melt temperature reaches 700-720°C, add the master alloy In the case of charging and smelting, once it is found that the molten metal is exposed and combusted, RJ-2 flux should be immediately refilled, and after the charge is completely melted, stir it vigorously for 5-8 minutes to make the composition uniform, and then cast the spectrum sample. Pre-furnace analysis: If the composition is unqualified, adjust the addition until it is acceptable. The temperature of the melt was raised to 7300C, the slag was removed, and it was soaked with a layer of RJ-2 flux for insulation treatment. The magnesite mineral that will account for 0.4% of the total weight of the charge will be broken into 2-3 packs of pre-crushed oysters with a diameter of about 10 mm. The foils will be wrapped in aluminum foil and placed in a bell jar in batches. 3, and the level of smooth movement, so that the boiling liquid until the complete decomposition of the modifier 6-12miO. If C2C16 is used for modification, the addition amount is 0.5%-0.8% of the total weight of the charge, and the treatment temperature is 740-7600C. After the modification treatment, the surface slag is removed and sprinkled with a new RJ-2 flux. The temperature was adjusted to 710-.730°C for refining. Stir the melt for 10-30 minutes so that the melt tumbling from bottom to top without splashing, and continue to spray the refining agent on the peak of the melt. The amount of refining agent depends on the amount of oxide inclusions in the melt, which is typically 1.5% to 2.0% of the weight of the charge. After the refining is completed, the slag on the surface of the alloy solution, the crucible wall, the nozzle and the baffle plate is removed, and then the RJ-2 flux is sprinkled. The temperature of the melt was raised to 755-7700C, and the sample was held in heat for 20-60 minutes. The fracture specimen was poured and the fracture was inspected to be dense and silver white. Otherwise, it needs to be degraded and refined. After passing the molten metal to the pouring temperature is usually 720,780 persuasive, cast out. The purpose of heating and standing after refining is to reduce the density of the melt and the 4-t degree to accelerate the precipitation of the slag, and also allow the slag to have a sufficient time to settle out of the magnesium melt and not be mixed into the casting. . Overheating is also beneficial to grain refinement. If necessary, it can be superheated to 800-8400C and then rapidly cooled to the pouring temperature to improve the grain refinement effect. The smelted molten metal should be poured in lh after standing. Otherwise, it is necessary to re-cast the sample and inspect the fracture. After inspection, the casting can be continued. If it is unqualified, it must be degraded and refined. If the fracture inspection is repeated twice unqualified, the melt can only be cast and cannot be cast. The entire smelting process includes 3%-.5% of the total weight of the charge. (2) Flux-free smelting (ballast method). The preparation of raw materials and smelting tools for fluxless method is basically the same as that for flux smelting. The difference is: 1 using SF6. CO2 and other protective gases, C2C16 metamorphic refining, nitrogen purging, and its The technical requirements are shown in Table 12-5. 2 The melt tools are cleaned and preheated to 200-30000 spray paint. 3 The total weight of the secondary and tertiary return materials is not more than 40% of the total weight of the charge during the batching process. The returned material shall not be greater than 20% of the operating process. First, the melting crucible shall be preheated to dark red, 500-6000C, filled with the preheated charge, and the charging sequence shall be: alloy ingot, magnesium ingot, aluminum ingot, return material, intermediate alloy Zinc and other crucibles cannot be loaded at once, but part of the ingots or small batches can be returned to the charge and the alloys are melted and then added in batches. A) Cover with protective cover, pass protective gas, and warm up the melted crucible to feed SF6 gas once. - 6min. When the temperature of the melt rises to 700-720° C., the mixture is stirred for 2-5 min to make the composition uniform, after which the slag is removed and the spectral sample is poured. When the composition is not qualified, make adjustments until it passes. Raise the temperature to 730-750°C and incubate. Use 0.1% C2Ch self-dumping metamorphic refining agent. The formulation is shown in Table 12-6. The deslagging is performed after refining, and the flow rate at 730-750°C is 1- 2L / min nitrogen supplement refined blowing scouring 2-4min blowing head should be inserted under the rhyme of the melt, the amount of nitrogen through the liquid with a gentle boiling is appropriate. After the blowing of the atmosphere is finished, the liquid slag is removed and heated to 760-7800C. The heat is allowed to stand for 10-20 min. The fracture specimen is poured. If it is not qualified, it can be re-refined and replaced by 1%), but it is generally not allowed. More than 3 times. The melt is transferred to the pouring temperature for pouring, and should be poured within 2 hours after standing. Otherwise, the sample should be re-examined fracture, unqualified refining refining process. Before pouring, pass the protective gas from the sprue to the large mold for 2 - 3 minutes, and the inside and the inside for 0.5 - 1 min, and cover the riser with the asbestos plate. During pouring, protective gas is continuously transported into the ladle or at the liquid stream for protection, and the mixture of sulfur and boric acid is allowed to be spilt in a ratio of 1:1 to prevent the melt from burning during pouring. 2. ZK4. Magnesium alloy smelting process 炉, charge and other preparations are the same as AZ91 alloy, Zr is added in the form of Mg-Zr master alloy, and the charge is carefully cleaned, and never mixed with Mg-Al alloy. Melting tools are also dedicated and must not be mixed with Mg-Al alloy melting tools. Furnace composition dumping points: 10%-20% of the new material, 80%-90% of the returned material, and the primary charge of the Qizhong should be 60% of the material. Heat the crucible to dark red and sprinkle the appropriate flux at the bottom. Add magnesium ingots that have been preheated above 150°C, and return the charge, and heat up and melt. Zinc is added when the melt temperature reaches 720-7400C. Continue to heat up to 780-8100C, slowly add Mg-Zr master alloy in batches. After all was melted, it was stirred for 25 minutes to accelerate the dissolution of the crucible and make the ingredients uniform. The fracture specimen was cast at a melt temperature not lower than 760°C. If the fracture is unqualified, Mg-Zr master alloy with mass fraction of 1%-2% may be added as appropriate, and the fracture may be rechecked to allow for the second supplement. If the failure is still unacceptable, the furnace alloy may only be cast. Fractured molten metal can be adjusted to 750-7600C, the impeller of the mixer can sink to 2/3 of the melt, stir for 4 -6min, and continuously smelting flux is poured on the flow peak. The amount of flux is 1.5% of the total weight of the charge. -2.5%. Then remove the slag from the spout, baffle, crucible wall, and molten surface and spill into the new flux. Raise the temperature of the melt to 780 -820 cC, allow it to stand still, and n, and if necessary, check the fracture again until the total time for standing is 30 -50 minutes. RJ-4 flux is used for covering and refining. Throughout the smelting process, the flux consumption is between 2% and 3% of the total weight of the charge, including refining flux). The standing time after refining is not allowed to exceed 2 h, and the temperature is maintained between 780-820° C. to avoid precipitation of zirconium. Zr can also be added to the magnesium alloy in the state of chlorine acid salt or fluoride acid salt. In this case, it should be noted that: 1 The amount of salt added is generally 8-10 times of the calculated composition; When Zr is added in the state of the acid salt, it is difficult to perform overheating to the 9000C operation. 3 When the wrong state is added in the chlorocobaltate state, although the addition is difficult, the obtained casting has insufficient corrosion resistance. The key to the faulty magnesium alloy is whether Zr can be added to the magnesium alloy and whether the grain refinement effect is qualified. For this reason, first, we must carefully clean the charge and adopt a purer charge to reduce the influence of various impurities such as Fe "Si" Al. Otherwise, not only will a certain amount of Zr be lost, but also the quality of the alloy will be seriously affected. Second, the wrong temperature is not lower than 7800C, otherwise Zr is easily precipitated in the bottom of the crucible, causing an insufficient amount of the alloy. When the temperature is higher than 820°C, the surface of the melt is intensified, and nitrogen is absorbed from the atmosphere. At the same time, as the amount of dissolved iron increases, compounds formed by Zr and Fe and H increase, which also increases the loss of Zr and weakens Zr. Refinement effect. In order to avoid the precipitation of Zr, the residence time of the alloy liquid should be shortened as much as possible, especially the residence time below 760°C. Fourth, magnesium alloy casting defects easily occur in the production of magnesium alloys and aluminum alloys, although the same as the light metal, but the casting properties of the two are quite different. Due to the poor casting performance of magnesium alloys, a variety of defects easily occur in the production of magnesium alloy castings, and it is difficult to eliminate, resulting in high production costs. Common defects are sulfur sulfide, slag inclusions, cracks, and pores. Starting from raw and auxiliary materials, the quality of the molds, equipment, and processes must be controlled by specialists and inspectors. 1. Loosing the defect is easy to occur at the gate and hot section. Castings with uneven wall thicknesses tend to form multiple hot spots. If these dispersed hot spots are not fed with molten metal during the solidification process, loose defects will be formed on the castings. This defect is generally found inside the casting and is invisible to the naked eye and can be detected by X-ray inspection. Since the magnesium alloy has a wide crystallization temperature interval, it is generally crystallized in a paste-like manner, so that there is a clear dendritic structure and it is difficult to obtain feeding between dendrites. However, due to a long time in the overheated state at the inner gate and the hot section, the temperature is high during the solidification process to cause the defect. To prevent such defects, we must do the following aspects: (1) Design the pouring system rationally from the point of view of the process. After repeated practice, determine the ratio of the cross-sectional area of ​​the sprue, runner, and ingate to an optimal ratio of 1:3. : 3, the size of the riser should be calculated according to the empirical formula, strengthen the feeding effect of the riser to form the order of the solidification of the casting; 2 make full use of the chilling effect of cold iron to help feed at the hot section, generally cold iron is With the use of the mouth; 3 in the loose parts of the use of dark feeder feeding, if the effect is not good should be appropriate to increase the size of the riser or the use of insulation riser, but not easy to clear the rise of the place made easy to cut mouth. Practice has proved that these measures are effective. 2. Black inclusions are visible on the surface of the slag inclusion castings or on the sharp corners of the casting after the pickling treatment. Dark spots are surrounded by a nearly circular light area bordered by a darker ring. This slag inclusion is generally caused by the flux, which will reduce the mechanical properties of the casting, and the MgC12 in the flux will also reduce the corrosion resistance of the casting. Magnesium and magnesium alloys are easily oxidized, especially ZM series alloys are more easily oxidized. In order to prevent oxidative combustion of magnesium alloys during smelting, certain fluxes must be added during the smelting process so that they are not in contact with air and the castings are cast. Due to poor performance of the flux or improper operation of the flux, the flux on the pouring tool can easily enter the molten metal; when the molten metal is sucked improperly, the flux is brought into the molten metal; due to the careless pouring, the flux floating on the molten ladle enters. Into the cavity. It is also possible that the casting temperature is high, and the magnesium alloy solution reacts with the gas released from the core in the cavity to cause secondary oxidation slag resulting from combustion. Therefore, to eliminate this kind of defect, when using a thirteen-tube teapot with a throttling tube, it is necessary to remove the self-adhering flux. A small amount of melt at the bottom of the ladle should not be poured into the cavity. Before the liquid metal can be used at the bottom of the ladle to open the liquid surface flux and oxide layer. During the smelting process, the temperature must not be too high, causing the flux to volatilize. The pouring temperature should be appropriate, and the amount of sulfur and boraxic acid used as a protective agent in the core should be appropriately increased. If, in the process of pouring, there is a flame out of the liquid surface, a layer of sulphur powder shall be sprinkled on it. The melting of these substances and chemical reactions take place for a certain period of time, so it is necessary to wait 2-3 minutes before drawing the molten metal. . The entire operation process must be meticulous and the operation must be stable to prevent violent stirring of the molten metal. During the refining process, the flux should be sprinkled on the top of the molten magnesium to the highest point, so that the whole liquid surface is evenly covered with a layer of flux. Finally, no white liquid is turned up from the bottom of the molten pool until the liquid level is bright, due to the flux. It reacts with the oxides in the alloy and produces an adsorption effect. The resulting density is greater than the larger accumulation of alloy and sinks into the bottom of the crucible, achieving the effect of purification. The refining time is controlled at 5. 8 min. After refining, it should be sedated for 10-15 min so that inclusions have enough time to sink into the bottom of the crucible. After the refining is complete, the liquid surface of the alloy must be carefully sprayed with a mixture of sulfur and boric acid. Otherwise, oxides are easily generated. It is important to remove the residue on the concrete wall regularly. Excessive residue accumulation may produce a strong chemical reaction and affect the smelting quality. 3. Crack cracks generally occur at the transition between the thickness of the casting. The reason is that during the solidification process, the thin wall is first solidified at a thicker wall thickness, and internal stress is generated due to shrinkage resistance. Especially when there is mud core, the expansion and contraction are more easily hindered; the transition fillet of the core is too small, and the metal solidifies When there is a large stress concentration. Therefore, to prevent the occurrence of such defects, it is possible to increase the process margin or set ribs at the thin wall, increase the fillet appropriately, and set the cold iron at the thick wall to balance the solidification sequence. 4. The source of the pore gas is various: in the smelting process, the metal liquid is in contact with the atmosphere to inhale; the oil and moisture on the alloy raw material generate gas in the smelting process; the alloy liquid inhales air during the pouring process, and The mold reaction produces gas. The density of magnesium metal is about 1.74x1031.74x10 k coffee, which is 1/3 of aluminum metal. Therefore, the liquid metal filling pressure is quite low. This has certain advantages, but more important is the shortcomings. In order to let the gas in the cavity (including during the pouring process, the core and sand produced in the casting process be discharged into the atmosphere freely, the sand must have good air permeability. These gases will be discharged through the lowest pressure route. If the sand type has poor air permeability, then The generated gas enters the liquid or semi-liquid state and invades the metal to generate pore defects.In order to improve the air permeability, the sand must have a certain particle size, and should be placed in the upper and the lower type of the mold, the cast hole should be poured in time. The storage time should not be too long to prevent moisture absorption.The non-working surface of the mud core must be provided with a sufficient number of vent holes. For larger mud core cavities, coke can be filled to increase the exhaust effect. The coke is preferably dried. 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