[CHINA ALUMINUM NETWORK] Smelting and pouring of alloys is a major part of casting production. Strictly controlling the entire process of smelting and casting plays an important role in preventing casting defects such as pinholes, inclusions, undercasting, cracks, porosity, and shrinkage.
Due to the large tendency of aluminum melt to absorb hydrogen, strong oxidizing ability, and easy dissolution of iron, it is necessary to adopt simple and prudent precautions in the smelting and casting process to obtain high-quality castings.
First, aluminum alloy charge sneer and quality control In order to smelt a high-quality aluminum melt, first of all should use qualified raw materials. The raw materials must be subject to scientific management and appropriate treatment, otherwise it will seriously affect the quality of the alloy. The production practice proves that the poor control of raw materials (including metal materials and auxiliary materials) will cause the castings to be scrapped in batches.
(1) Raw materials must have qualified chemical composition and organization. Specific requirements are as follows:
In addition to the analysis of the main components and impurity content of the alloy ingots, they are still inspecting low accompaniment and fractures. Practice has proved that the use of a liquid containing a serious shrinkage, pinholes, and air bubbles, it is difficult to obtain a dense casting, and even cause the entire furnace, the entire batch of castings scrapped.
Someone was studying the effect of aluminum-silicon alloy ingots on the pinholes of aluminum alloys. It was found that pinholes did not appear when using melted pure cast sand-type test pieces. When adding low-tissue and unqualified aluminum-silicon alloy ingots, the pinholes of the test pieces were Serious and large grains. The reason is due to the heredity of the material. Al-Si alloys and hereditary properties increase with the increase of the content, and when the amount of silicon reaches 7%, the heredity is significant. Continue to increase the silicon content to the eutectic composition, and the heritability decreases slightly. In order to solve the casting defects caused by the inheritance of the charge, aluminum ingots, intermediate alloys and other charge materials with high metallurgical quality must be used. The specific criteria are as follows:
(1) There should be no pinholes on the fractures, and the pinholes of the stomata should be within three grades. Partial (not exceeding 25% of the examined area) should not exceed three grades. Those who exceed three grades must adopt remelting methods to reduce needles. Porosity. The remelting refining method is the same as general aluminum alloy smelting. The casting temperature should not exceed 660°C. For those aluminum ingots, alloy ingots, etc. with large primary crystal grains, the lower ingot temperature should be used to rapidly solidify them. grain.
(II) Before the charge processing charge is used, it should be blown with sand to remove surface rust, grease and other dirt. The aluminum alloy ingots and metal return materials with relatively clean surfaces may not be blown, but the iron filters and inserts mixed in the charge should be eliminated. All charge materials should be pre-heated before entering the furnace. Heat to remove the attached surface moisture and shorten the smelting time to more than 3 hours.
(3) The management of the charge and the reasonable preservation and management of the charge are of great importance to ensure the quality of the alloy. The charge should be stored in a dry, dry warehouse.
Second, the preparation of helium and smelting tools (a) å©åŸš commonly used casting aluminum alloy shovel, can also be used to cast steel and steel plate welding å©åŸš.
The new sorghum and long-term unused sorghum should be blown before use, and heated to 700--800 degrees for 2 to 4 hours to burn off the moisture and combustible substances attached to the inside wall of the sampan and cool to 300. When the temperature is lower, carefully clean the inside of the crucible and spray the paint at a temperature not lower than 200 degrees.
å©åŸš Before use, warm up to dark red (500--600 degrees) and keep warm for more than 2 hours. Before the new crucible is smelted, it is better to first melt the return material of the same brand.
(II) Preparation of Melting Tools Bell jars, pressure scoops, stirring spoons, pouring molds, etc. shall be preheated before use, and protective coatings shall be applied at a temperature of 150°C to 200°C and thoroughly dried. The drying temperature is 200--400 degrees, and the holding time is more than 2 hours. After use, the oxides and fluorides adhered to the surface should be completely removed (better to blow sand).
Third, the smelting temperature control Smelting temperature is too low, is not conducive to the dissolution of alloying elements and the discharge of gases, inclusions, increase the formation of segregation, cold septum, the tendency to die, but also due to lack of heat riser, the castings are not available Reasonable feeding, there is information that all aluminum alloy melting temperature to less than 705 degrees and should be stirred. Excessive smelting temperature is not only a waste of energy, but is more serious because the higher the temperature, the more hydrogen is absorbed, the coarser the crystal grains are, the more severe the oxidation of aluminum is, and the more serious the burning of some alloying elements, resulting in the mechanical properties of the alloy. With the decline, the casting performance and mechanical processing performance deteriorated, the effect of the deterioration treatment was weakened, and the hermeticity of the casting was reduced.
The production practice has proved that the molten metal is quickly heated to a relatively high temperature, and reasonable stirring is performed to promote the dissolution of all alloying elements (especially refractory metal elements), and after removing the dross, the molten metal is dropped to the casting temperature. Thus, the degree of segregation is relatively high. Small, melted hydrogen is also small, is conducive to obtain uniform dense, high mechanical properties of the alloy. Because the aluminum melt temperature is difficult to judge with the naked eye, so no matter what type of melting furnace, you should use the temperature meter temperature control. Thermometers should be regularly checked and repaired. The thermowell should be cleaned periodically with a metal brush and coated with a protective coating to ensure the accuracy of temperature measurement results and the service life of the director.
Fourth, the smelting time control In order to reduce the aluminum melt oxidation, suction and iron dissolution, should minimize the aluminum melt residence time in the furnace, rapid smelting. From the beginning of melting to the completion of casting, sand casting does not exceed 4 hours, metal casting does not exceed 6 hours, and die casting does not exceed 8 hours.
In order to speed up the smelting process, firstly, a medium block, low melting point return material, and an aluminum-silicon intermediate alloy should be added so as to form a molten pool at the bottom of the slag, and then add a large amount of return material and pure aluminum ingots. They can slowly immerse into the expanding pool and melt quickly. After the main part of the charge is melted, an intermediate alloy with a higher melting point and a smaller amount is added, and the temperature and stirring are increased to accelerate melting. Cool down later, press in easily oxidized alloy elements to reduce losses.
Fifth, the transfer of the melt and pouring Although the density of solid aluminum oxide is similar to the density of aluminum melt, after entering the aluminum melt, after a long enough time will sink to the bottom of the accompany. However, the aluminum oxide film formed after the aluminum melt is oxidized has only one side that is in contact with the aluminum melt and is dense. The side that is in contact with the air is loose and has a large number of small holes with a diameter of 60-100A, and its surface area is large. Strong, easy to absorb in the water vapor, there is a tendency to go up. Therefore, the difference between the specific gravity of this oxide film and the aluminum melt is small, and it is mixed into the melt, the float sinking speed is very slow, it is difficult to remove from the melt, and the formation of pores in the casting is too much inclusion. Therefore, the key to transferring aluminum melt is to minimize the stirring of molten metal and minimize the contact between the melt and air.
When using a tilt-type rotary transfer melt, in order to avoid mixing of the melt and air, the ladle should be placed on the furnace nozzle as far as possible, and be placed obliquely so that the melt flows down the side wall of the ladle without directly impacting the bottom of the package. Stirring, splashing, etc.
Proper and reasonable pouring method is one of the important conditions for obtaining high-quality castings. Production practice, pay attention to the following matters, to prevent and reduce casting defects is very effective.
(1) Before pouring, the temperature of the melt, the capacity of the ladle, and the degree of dryness of the surface coating layer should be carefully checked. Whether or not the preparation of other tools is desirable or not. The metal sprue cup is placed on the sand mold within 3 to 5 minutes before pouring. At this time, the temperature of the pouring package is not higher than 150 degrees, the placement is too early or the temperature is too high, and a large amount of gas is trapped in the sprue. There is a danger of explosion when pouring.
(2) It must not be poured in situations where there is a "curator style", and the melt is strongly oxidized and burned, causing the casting to produce oxide inclusions and other defects.
(3) When obtaining the melt from the crucible, the oxide scale or flux layer on the surface of the melt shall be gently poked with the bottom of the crucible, and the ladle shall be slowly immersed in the melt, and the molten metal shall be withdrawn from the wide mouth of the ladle. , and then gently raise the ladle.
(D) Do not flatten the end of the package, the pace should be stable, the ladle should not be raised too high, the ladle metal liquid surface must remain stable, not converging.
(5) When the casting is about to be completed, the slag of the net ladle should be removed so as not to bring slag, scale, etc. into the casting during pouring.
(f) In the pouring process, the melt flow will remain stable, cannot be interrupted, and the bottom hole of the mouth cannot be directly flushed. The gate should be filled with mother's end, the liquid surface must not be turned, and the pouring speed should be controlled properly. Usually, the start of pouring is slightly slower, so that the melt fills smoothly, then the speed is slightly faster, and the pouring speed is basically kept unchanged.
(g) In the pouring process, the distance between the ladle nozzle and the gate shall be as close as possible to a limit of not more than 50 mm to avoid excessive oxidation of the melt. ,
(8) If the gates with clogging are blocked, the plugging cannot be set too early. After the melt is filled in the gates, it is slowly pulled out obliquely to avoid the turbulence generated when the melt is injected into the runners.
(9) Castings below 60 mm from the bottom of the crucible shall not be cast.
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