Melting process flow and operating procedures for aluminum alloys, aluminum alloy melting furnaces

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Aluminum alloy melting furnace – Charging: During melting, the sequence and method of loading the material not only affect the melting time, metal loss, and heat consumption, but also influence the quality of the molten metal and the service life of the furnace. The principles for loading are as follows: 1. The sequence of loading materials into the furnace should be reasonable. The correct charging method depends on the properties and state of the materials added to the furnace, and it is also necessary to take into account the fastest melting rate, minimal loss due to burning, and accurate control of chemical composition. 　　During loading, small pieces or thin sheets of scrap are placed first, aluminum ingots and large pieces are placed in the middle, and finally the master alloys are added. The intermediate alloy with a melting point that is prone to oxidation is placed in the middle and lower layers. The charged charge should be evenly distributed in the molten pool to prevent uneven loading. 　　Small pieces or thin sheets are placed in the lower layer of the molten pool; this reduces burnout and also protects the furnace from damage caused by direct impact from larger pieces. Some master alloys have high melting points; for example, the melting points of AL-NI and AL-MN alloys are 750–800°C. Placed in the upper layer, they melt easily due to the high temperatures in that area of the furnace, and they also have sufficient time to diffuse ； Ensuring a uniform distribution of the master alloy facilitates the control of the melt’s composition. 　　The charge should be leveled so that the melting rate is similar throughout, which helps to prevent localized overheating of the metal caused by uneven distribution of weight. 　　The charge should be fed into the furnace at once; adding it in two or more batches will increase non-metallic inclusions and gas content. 　　2. For products with high quality requirements (including forgings, die forgings, hollow beams, and beam profiles), in addition to the aforementioned loading requirements, 20–30 kg of powdered flux must be scattered into the molten pool before loading. During the loading process, the powdered flux should be applied in layers to the charge; this helps improve the purity of the melt and reduces losses. 　　3. When loading the electric furnace, care should be taken to ensure that the highest point of the furnace charge is at least 100 mm away from the resistance wires; otherwise, short circuits are likely to occur. 　　Melting The temperature can be increased once the furnace charge is loaded. Melting is the process of changing from a solid state to a liquid state. The quality of this process has a decisive impact on the product quality. 　　A. Coverage: During the melting process, as the temperature of the charge rises, especially once melting begins, the oxide film covering the outer surface of the metal easily breaks down, gradually losing its protective function. Gases can easily penetrate at this time, causing further oxidation of the internal metal. Moreover, the melted liquid or flow must move toward the bottom of the furnace; when the droplets or flows gather at the bottom, the oxide film on their surfaces mixes into the melt. Therefore, to prevent further oxidation of the metal and reduce the oxide film that enters the melt, a layer of powdered flux should be appropriately spread on the metal surface when the charge softens and collapses; the amount to use is shown in the table. This can also reduce metal absorption during the melting process. Types and amounts of fluxes, furnace types, and products. Electrical melting, gas furnace melting. Amounts of flux used: ordinary products, special products. Ordinary products, special products (as a percentage of the total amount added)/%: 0.4–0.5, 0.5–0.6, 1–2, 2–4. Types of fluxes: powdered fluxes; KCl:NaCl mixed in a 1:1 ratio. B. Addition of copper and zinc. Once part of the charge has melted, zinc ingots or copper sheets can be evenly added to the liquid, ensuring that the melt in the pool just covers the zinc ingots and copper sheets. 　　It should be emphasized at this point that the melting point of copper is 1083°C; within the melting temperature range of aluminum alloys, copper dissolves in the aluminum alloy melt. Therefore, if the copper plates are added too early and the melt fails to cover them, this will increase the burning loss of the copper plates ； Conversely, if it is added too late, the copper plates will not have enough time to dissolve and diffuse, which will prolong the melting time and affect the control of the alloy’s chemical composition. 　　During electric furnace melting, try to avoid replacing the resistance wire strips to prevent dirt from falling into the melt and contaminating the metal. 　　C. Stirring the melt: During the melting process, it is important to prevent the melt from overheating. This is especially true when using natural gas (or gas) furnaces for melting, as the temperature in the furnace can reach 1200°C; at such high temperatures, local overheating can easily occur. To this end, once the charge has melted, the melt should be stirred properly to ensure uniform temperature throughout the molten pool, which also helps to accelerate the melting process. Skimming and stirring: Once the charge has fully melted in the molten pool and the temperature of the melt reaches the melting point, the large amount of oxidized slag floating on the surface of the melt can be removed. 　　A. Slag removal: Before removing the slag, powdered flux should be evenly scattered over the melt, so as to separate the slag from the metal. This facilitates slag removal and helps to minimize the amount of metal that is carried away. Slag removal must be done smoothly to prevent slag from being drawn into the molten metal. Scraping the slag must be thorough, as its presence increases the gas content in the melt and contaminates the metal. 　　B. Addition of magnesium and beryllium: After skimming off the slag, magnesium ingots can be added to the melt; at the same time, type 2 powdered flux should be used for covering to prevent the loss of magnesium due to oxidation. 　　For high-magnesium aluminum alloys, in order to prevent magnesium loss and to alter the properties of the oxide film on the surface of the melt and ingot, a small amount (0.001%-0.004%) of beryllium must be added to the melt after magnesium is introduced. Beryllium is generally added as a mixture of Al-BeF4 and No. 2 powdered flux in a 1:1 ratio, and thorough mixing should be carried out after addition. 　　Na BeF + Al → 2NaF + AlF3 + Be. To prevent beryllium poisoning, a mask should be worn when handling beryllium. Furthermore, the slag resulting from adding beryllium should be piled in a designated area or treated specially. 　　C. Stirring Stirring should be carried out promptly after adjusting the chemical composition, as well as before sampling. Its purpose is to ensure a uniform distribution of the alloy components and to make the temperature within the melt consistent. This may seem like an extremely simple operation, but it is a crucial step in the manufacturing process. This is because some alloying elements with higher density tend to settle at the bottom, and moreover, the addition of these alloying elements cannot be perfectly uniform, which results in uneven distribution of these elements between the upper and lower layers of the melt, as well as across different areas within the furnace. If mixing is not thorough (without ensuring a sufficient duration of time and eliminating dead zones), it can easily lead to uneven chemical composition in the melt. 　　Stirring should be carried out smoothly, without creating excessive waves, to prevent the oxide film from being drawn into the melt. 　　Composition adjustments: During the melting process, the alloy composition can change for various reasons, and such changes may result in a significant discrepancy between the actual composition of the melt and the values calculated from the ingredient proportions. Therefore, after the charge is melted, samples must be taken for rapid analysis, so as to determine whether adjustments to the composition are necessary based on the analysis results. 　　A. Sampling: After the melt has been thoroughly stirred, samples should be taken for rapid analysis right at the furnace to determine whether the chemical composition meets the standard requirements. The melt temperature in the furnace at the time of sampling should be no lower than the lower limit of the melting temperature. 　　The sampling locations for rapid sample analysis must be representative; for gas furnaces (or coal gas furnaces), one set of samples should be taken from the center of each furnace door, while for electric furnaces, two sets of samples should be taken from the center of the half of the melt. The sample spoon should be preheated before sampling. For high-purity aluminum and aluminum alloys, in order to prevent contamination of the sample spoon, stainless steel sample spoons coated with a coating should be used for sampling. 　　B. Composition adjustment When the results of rapid analysis do not match the required alloy composition, the composition must be adjusted—by diluting it or adding materials. 　　(1) Feeding. If the rapid analysis results show that the levels are below the requirements for the alloy’s chemical composition, it is necessary to add material. To ensure accurate replenishment, calculations should be carried out according to the following principles: 1) Calculate the amount of the smaller quantity first, and then the amount of the larger quantity ； 　　2) Calculate impurities first, then alloying elements ； 　　3) Calculate the low-composition master alloys first, and then the high-composition master alloys ； 　　4) Finally, calculate the new metal. Generally, the amount of material that needs to be added can be approximated using the following formula, which can then be used for calculation: X = …… Where X represents the amount of material that needs to be added, in kg; Q represents the total amount of melt (i.e., the amount of material fed in), in kg; a represents the required concentration of a certain component, in % ； 　　b —— Analysis amount of this component, % ； 　　c c —— represent the amounts added of other metals or master alloys, in kg; d —— represents the content of that component in the master alloy used for making up the material (if pure metal is added, then d=100), in %. 　　(2) Dilution. 　　If the results of the rapid analysis exceed the upper limits of **standards, delivery specifications, etc. for chemical composition, dilution is required. 　　Alloying elements that are above the chemical composition standards during dilution must be reduced to a level below the upper limit specified for such alloying elements. 　　Based on years of production experience, aluminum processing plants in our country have established internal standards for aluminum alloys, in order to ensure that these alloys possess good casting properties and mechanical properties. To this end, during dilution, it is generally diluted to a chemical composition that is close to or below the upper limit of the plant’s chemical composition standard for that element. 　　When diluting, the required amount of diluent is generally calculated using the following formula. 　　X = Q(b – a) / a Where b is the analyzed amount of a certain component, in % ； 　　a ——the required content of the upper (in-house) standard limit for this component, % ； 　　Q — Total amount of melt, in kg; X — Required amount of diluent, in kg; C — Points to note when adjusting the composition (1) The sample must be representative. The lack of representativeness of the sample is due to the fact that certain elements have a higher density, resulting in a slower rate of dissolution and diffusion, or they tend to separate and form layers. Therefore, thorough stirring should be carried out before sampling to homogenize the composition. Due to the high surface temperature of the melting pool in a reverberatory furnace and the low temperature at the furnace bottom, with no convection heat transfer, stirring must be performed multiple times before sampling, and each stirring session should last no less than 5 minutes. 　　(2) The sampling location and procedures must be appropriate. Due to the large and deep molten pool in the reverberatory furnace, even after multiple stirrings prior to sampling, there are still certain variations in the composition of different parts of the molten pool; therefore, the sample should be taken at a point that is one-half of the way to the deepest part in the middle of the molten pool. 　　Before sampling, the sample mold should be thoroughly heated and dried. The sampling procedure must be carried out correctly to ensure that the sample meets the required standards; otherwise, defects such as pores or inclusions in the sample, or failure to meet the requirements, can introduce errors into the rapid analysis. 　　(3) The temperature should be appropriate during sampling. For some elements with high density, their dissolution and diffusion rates increase as the temperature rises. If the melt temperature is low before sampling, even after multiple stirrings, the rate of dissolution and diffusion remains slow; as a result, the sample taken at this time is still not representative. Therefore, the melt temperature should be kept at a sufficiently high level before sampling. 　　(4) Intermediate alloys are generally used for feeding and dilution; new metal materials with high melting points and difficulty in melting should be avoided. 　　(5) The amount of feedstock and diluent should be as low as possible, provided that the requirements for alloying elements are met. Moreover, when diluting, the capacity of the melting furnace and the convenience of the related dilution operations should be taken into consideration. 　　(6) If the dilution level is high, other alloying elements should also be added, ensuring that their concentrations remain at or above the corresponding standards or requirements. 　　Refining: In most cases of aluminum alloy production on an industrial scale, the gas-based refining process is no longer used in the melting furnaces; instead, refining is carried out using static furnaces and online melt purification methods. However, some aluminum processing plants still employ refining in their melting furnaces, with the aim of improving the purity of the melt. These refining methods can be divided into two categories: gas refining and flux refining. 　　Completion of melting process: Once the melt has been refined and the surface scum has been removed, and when the temperature is appropriate, the metal melt can be transferred to a holding furnace in order to prepare for casting.
Cleaning the furnace: Cleaning the furnace involves removing all remaining slag from inside the furnace. A furnace cleaning is performed every time the metal is taken out of it. When switching alloys, ordinary products are produced in consecutive batches of 5–15 furnaces, while for special products, a thorough cleaning of the furnace is required after each batch is produced. During major furnace cleaning, a layer of powdered flux should first be evenly spread throughout the furnace, and the furnace temperature raised above 800°C; thereafter, a triangular shovel is used to completely remove any remaining slag from all parts of the furnace. For manufacturers of aluminum alloy melting furnaces, Luoyang Yanhao Energy-Saving Technology Co., Ltd. is the top choice: 0379-64313145.