Aluminum alloy melting and pouring

Alloy melting and pouring is the main link in casting production. Strict control of the whole process of melting and casting plays an important role in preventing casting defects such as pinholes, inclusions, undercasting, cracks, porosity and shrinkage.
　　
Due to the high tendency of aluminum melt to absorb hydrogen, strong oxidation ability, and easy dissolution of iron, simple and careful precautions must be taken during the melting and casting process to obtain high-quality castings.

1. Aluminum alloy charge sneering and quality control
　　In order to melt high-quality aluminum melt, qualified raw materials should be selected first. The raw materials must be scientifically managed and properly treated, otherwise the quality of the alloy will be seriously affected, and the production practice has proved that the raw materials (including metal materials and auxiliary materials) are not strictly controlled, and the castings will be scrapped in batches.
　　(1) Raw materials must have qualified chemical composition and structure, and the specific requirements are as follows:
　　In addition to analyzing the main components and impurities, the incoming alloy ingots are also inspected for low coincidence and fractures. Practice has proved that the use of liquid aluminum containing serious shrinkage holes, pinholes, and bubbles is difficult to obtain dense castings, and even will cause the whole furnace and batch of castings to be scrapped.
　　When studying the influence of aluminum-silicon alloy ingots on aluminum alloy pinholes, some people found that there were no pinholes when casting sand molds with molten pure casting, and when low structure and unqualified aluminum-silicon alloy ingots were added, the pinholes of the test blocks were serious and the grains were large. The reason for this is the heritability of the material. The aluminum-silicon alloy and heritability increase with the increase of content, and the heredity is significant when the silicon content reaches 7%. The silicon content continues to be increased to the eutectic component, and the heritability decreases slightly. In order to solve the casting defects caused by the hereditary nature of the charge, it is necessary to select aluminum ingots, master alloys and other charges with high metallurgical quality. The specific criteria are as follows:

       (1) There should be no pinholes or pores on the fracture
　　The pinhole should be within the third grade, and the local (not more than 25% of the inspected area) should not exceed the third grade, and the method of remelting must be adopted to reduce the pinhole degree if it exceeds the third grade. The remelting refining method is the same as the general aluminum alloy smelting, and the casting temperature should not exceed 660 °C.
　　(2) Charge handling
　　The charge should be blown before use to remove rust, grease and other dirt on the surface. The aluminum alloy ingots and metal-type returns with a clean surface can be treated without sand blowing, but the iron filter and inlays mixed in the charge should be eliminated, and all the charges should be preheated before entering the furnace to remove the moisture attached to the surface and shorten the melting time by more than 3 hours.
　　(3) Management and storage of charges
　　Reasonable storage and management of the charge is of great significance to ensure the quality of the alloy. The charge should be stored in a dry warehouse with little change in temperature.

2. Preparation of crucibles and smelting tools
　　(1) Iron crucibles are commonly used for crucible casting aluminum alloy, and crucibles can also be welded with cast steel and steel plates.
　　The new crucible and the old crucible that has not been used for a long time should be blown sand before use, and heated to 700-800 degrees, kept for 2-4 hours, in order to burn off the moisture and combustible substances attached to the inner wall of the crucible, when it is cooled to below 300 degrees, carefully clean the inner wall of the crucible, and spray paint when the temperature is not less than 200 degrees.
　　The crucible should be preheated to dark red (500-600 degrees) before use, and kept warm for more than 2 hours. Before smelting outside the new crucible, it is best to melt a furnace of the same grade of return material.
　　(2) Preparation of smelting tools
　　Bells, scoops, mixing spoons, pouring ladles
　　The ingot mold should be preheated before use, and coated with protective paint at a temperature of 150 degrees --- 200 degrees, and thoroughly dried, the drying temperature is 200-400 degrees, the holding time is more than 2 hours, and the oxides and fluorides attached to the surface should be completely removed after use, (it is best to blow sand).

3. Control of melting temperature

　The smelting temperature is too low, which is not conducive to the dissolution of alloying elements and the discharge of gases and inclusions, increasing the tendency to form segregation, cold isolation and undercasting, and will also cause the castings to be reasonably replenished due to insufficient riser heat, and some data point out that the smelting temperature of all aluminum alloys should reach as little as 705 degrees and should be stirred. Too high melting temperature is not only a waste of energy, but also more serious because the higher the temperature, the more hydrogen absorption, the coarser the grains, the more serious the oxidation of aluminum, the more serious the burning loss of some alloying elements, which leads to the decline of the mechanical properties of the alloy, the deterioration of casting properties and machining properties, the weakening of the effect of deterioration treatment, and the reduction of the air tightness of the casting.
　　

The production practice proves that the alloy liquid is quickly heated to a higher temperature, and reasonable stirring is carried out to promote the dissolution of all alloying elements (especially refractory metal elements), and the scum is removed and reduced to the pouring temperature, so that the degree of segregation is minimal, and the melting hydrogen is also less, which is conducive to obtaining a uniform and dense alloy with high mechanical properties. Because the temperature of the aluminum melt is difficult to judge with the naked eye, no matter what type of melting furnace is used, the temperature should be controlled with a thermometer. The temperature measuring instrument should be checked and maintained regularly. The thermowell should be cleaned with a metal brush and coated with protective paint to ensure the accuracy of the temperature measurement results and the long service life.

4.  the control of smelting time
　　In order to reduce the oxidation, gas absorption and iron dissolution of the aluminum melt, the residence time of the aluminum melt in the furnace should be shortened as much as possible and the melting should be rapid. From the beginning of melting to the completion of pouring, sand casting does not exceed 4 hours, metal mold casting does not exceed 6 hours, and die casting does not exceed 8 hours.
　　In order to speed up the melting process, the return material and aluminum-silicon master alloy with medium lumpiness and low melting point should be added first, so as to form a molten pool at the bottom of the crucible as soon as possible, and then the return material and pure aluminum ingots with a large lumpiness should be added, so that they can be slowly immersed in the gradually expanding molten pool and melted quickly. After the main part of the charge is melted, a high melting point and a small number of master alloys are added, and the temperature is raised and stirred to accelerate the melting. Finally, it is cooled and pressed into the alloying elements that are prone to oxidation to reduce losses.

5. Transferring and pouring of melt
　　Although the density of solid alumina is similar to that of the aluminum melt, it takes a long enough time to sink to the bottom of the crucible after entering the aluminum melt. The alumina film formed after the aluminum melt is oxidized, but only the side in contact with the aluminum melt is dense, and the side in contact with the air is loose and has a large number of small pores with a diameter of 60-100A, which has a large surface area and strong adsorption, and is easy to adsorb in water vapor, but has a tendency to float. Therefore, the specific gravity difference between this oxide film and the aluminum melt is small, and when it is mixed into the melt, the floating and sinking speed is very slow, and it is difficult to exclude from the melt, and the formation of pores in the casting is too inclusional. Therefore, the key in transferring the aluminum melt is to minimize the stirring of the molten metal and minimize the contact between the melt and the air.
　　When the tilting crucible is used to transfer the melt, in order to avoid the mixing of melt and air, the ladle should be placed as close to the furnace nozzle as possible and placed at an angle, so that the melt will flow down along the side wall of the ladle, so that it will not directly impact the bottom of the ladle, stirring, splashing, etc.
　　The use of correct and reasonable pouring method is one of the important conditions for obtaining high-quality castings. In production practice, it is very effective to pay attention to the following matters to prevent and reduce casting defects.

       (1) Before pouring, the melt furnace temperature, pouring ladle capacity and the dryness of the coating layer on its surface should be carefully checked, and whether the preparation of other tools is in accordance with the requirements. The metal pouring cup is placed on the sand mold within 3-5 minutes before pouring, and the temperature of the pouring ladle is not higher than 150 degrees at this time, and the placement is too early or the temperature is too high, and a large amount of gas is held in the sprue, and there is a risk of explosion during pouring.
　　(2) It cannot be poured on the occasion of "passing the hall", and the melt is strongly oxidized and burned, so that the casting has defects such as oxidation inclusions.
　　(3) When obtaining the melt from the crucible, the oxide scale or flux layer on the surface of the melt should be gently removed with the bottom of the ladle, the ladle should be slowly immersed in the melt, the melt should be scooped with the wide mouth of the ladle, and then the ladle should be lifted smoothly.
　　(4) Do not palm flat when ending the ladle, the steps should be steady, the pouring ladle should not be raised too high, and the metal liquid level in the pouring ladle must be kept stable and not moved.
　　(5) When pouring, the slag of the pouring ladle should be cleaned to avoid bringing the slag and oxide scale into the casting mold during pouring.
　　(6) In the pouring process, the melt flow is kept stable, can not be interrupted, and cannot be directly punched into the bottom hole of the mouth. The gate should be full from the mother to the end, the liquid level should not be turned, and the pouring speed should be properly controlled. Typically, the pouring starts a little slower to make the melt fill smoother, then slightly faster and basically keeps the pouring speed constant.
　　(7) During the pouring process, the distance between the ladle nozzle and the gate should be as close as possible, not exceeding 50 mm, so as to avoid excessive oxidation of the melt. 、
　　(8) The gate with blockage, the blockage can not be dialed too early, after the melt is filled with the gate arms, and then slowly diagonally dialed, so as to avoid the melt when injecting the sprue to produce vortex.
　　(9) The melt below 60 mm from the bottom of the crucible should not be poured casting.