The smelting and pouring of alloys are key processes in casting production. Strictly controlling the entire smelting and pouring process plays an important role in preventing casting defects such as pinholes, inclusions, misruns, cracks, porosity, and shrinkage.
Since molten aluminum has a strong tendency to absorb hydrogen, strong oxidizing properties, and easily dissolves iron, simple yet careful precautions must be taken during melting and casting processes to produce high-quality castings.
1. Cooling of Aluminum Alloy Furnace Materials and Quality Control
In order to produce high-quality molten aluminum, 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. Production practice has shown that if raw materials (including metallic and auxiliary materials) are not strictly controlled, batches of castings may be scrapped.
(1) Raw materials must have qualified chemical composition and structure, with the specific requirements as follows:
In addition to analyzing the main components and impurity content of the alloy ingots entering the factory, inspections are also conducted for low-alloy structures and fracture surfaces. Practice has proven that using aluminum melt containing severe shrinkage cavities, pinholes, and bubbles makes it difficult to obtain dense castings and can even result in the scrapping of an entire furnace or batch of castings.
When studying the effect of aluminum-silicon alloy ingots on aluminum alloy porosity, it was found that no porosity appeared when using test blocks made of molten pure sand-cast molds. However, after adding low-quality and substandard aluminum-silicon alloy ingots, the test blocks showed severe porosity and coarse grains. The reason for this is due to the material's inheritance effect. For aluminum-silicon alloys, the inheritance effect increases with the content, becoming significant when the silicon content reaches 7%. Continuing to increase the silicon content to the eutectic composition slightly reduces the inheritance effect. To resolve casting defects caused by the inheritance effect of furnace stock, it is necessary to select high metallurgical quality aluminum ingots, intermediate alloys, and other furnace materials. The specific standards are as follows:
1) The fracture surface should not have pinholes or gas holes
Pinholes should be within grade three, and locally (no more than 25% of the inspected area) should not exceed grade three. If it exceeds grade three, remelting must be performed to reduce the pinhole level. The remelting and refining method is the same as for general aluminum alloy melting. The casting temperature should not exceed 660°C. For aluminum ingots or alloy ingots with large original grains, a lower mold temperature should be used first to make them solidify quickly and refine the grains.
(2) Charge Material Handling
Before use, the charge materials should be sandblasted to remove surface rust, grease, and other contaminants. Aluminum alloy ingots and metallic scrap with a relatively clean surface and short storage time may not require sandblasting, but any iron filters or embedded components mixed in the charge must be removed. All charge materials should be preheated before being placed in the furnace to remove surface moisture and reduce the melting time by more than 3 hours.
(3) Management and Storage of Furnace Materials
Proper storage and management of furnace materials are important for ensuring the quality of the alloys. Furnace materials should be stored in warehouses with little temperature variation and dry conditions.
2. Preparation of Crucibles and Smelting Tools
(1) For casting aluminum alloy, common iron crucibles are used, and crucibles made of cast steel or welded steel plates can also be used.
Both new crucibles and old crucibles that have not been used for a long time should be sandblasted before use and heated to 700–800°C, maintaining this temperature for 2–4 hours to burn off moisture and combustible substances adhering to the inner wall of the crucible. When cooled to below 300°C, carefully clean the inner wall of the crucible, and apply the coating when the temperature is no lower than 200°C.
The crucible should be preheated to a dark red color (500–600°C) before use and kept at this temperature for more than 2 hours. For a new crucible, it is best to first melt a batch of recycled material of the same grade before smelting the actual material.
(2) Preparation of smelting tools
Bell cover, smashing lid, stirring spoon, pouring bag
Before use, molds and other equipment should be preheated, coated with a protective layer at a temperature of 150–200°C, and thoroughly dried. The drying temperature should be 200–400°C, with a holding time of more than 2 hours. After use, any oxides and fluorides adhering to the surface should be completely removed (sandblasting is recommended).
3. Control of Melting Temperature
If the melting temperature is too low, it is not conducive to the dissolution of alloying elements and the removal of gases and inclusions, increasing the tendency for segregation, cold shuts, and misruns. It can also result in insufficient heat in the riser, preventing proper feeding of the casting. According to some references, the melting temperature of all aluminum alloys should reach at least 705°C and stirring should be performed. On the other hand, an excessively high melting temperature not only wastes energy but, more importantly, leads to increased hydrogen absorption, coarser grains, more severe aluminum oxidation, and greater loss of some alloying elements. Consequently, the mechanical properties of the alloy deteriorate, casting and machining performance worsen, the effectiveness of heat treatment is reduced, and the gas-tightness of the casting decreases.
Production practice has proven that rapidly heating the alloy melt to a higher temperature and performing proper stirring helps dissolve all alloying elements (especially refractory metal elements). After removing the floating slag, lowering the temperature to the pouring point minimizes segregation, reduces dissolved hydrogen, and helps obtain a uniform, dense alloy with high mechanical properties. Since the temperature of the aluminum melt is difficult to judge with the naked eye, no matter what type of melting furnace is used, temperature should be controlled with a measuring instrument. The measuring instruments should be regularly calibrated and maintained. Thermocouple sleeves should be periodically cleaned with a metal brush and coated with protective paint to ensure accurate temperature measurements and long service life.
4. Control of Melting Time
To reduce the oxidation of the aluminum melt, gas absorption, and dissolution of iron, the residence time of the aluminum melt in the furnace should be minimized, and melting should be carried out rapidly. From the start of melting to the completion of pouring, the duration should not exceed 4 hours for sand casting, 6 hours for metal mold casting, and 8 hours for die casting.
To accelerate the melting process, medium-sized scrap with a lower melting point and aluminum-silicon intermediate alloys should be added first, so that a melt pool quickly forms at the bottom of the crucible. Then, larger pieces of scrap and pure aluminum ingots are added, allowing them to gradually immerse into the expanding melt pool and melt quickly. After the main portion of the charge has melted, a small amount of higher-melting intermediate alloys is added, and the temperature is raised and stirred to accelerate melting. Finally, the temperature is lowered and easily oxidized alloying elements are pressed in to minimize losses.
5. Transfer and Pouring of the Melt
Although the density of solid alumina is approximately equal to that of molten aluminum, once it enters the aluminum melt, it takes a sufficiently long time to settle to the bottom of the crucible. In contrast, the alumina film formed on the surface of aluminum after oxidation has a dense side in contact with the aluminum melt, while the side exposed to air is loose and contains numerous pores with diameters of 60–100 Å. This film has a large surface area and strong adsorption properties, which makes it easily adsorb water vapor and even tend to float. Therefore, since the density difference between this oxide film and the aluminum melt is small, mixing it into the melt results in very slow sinking and floating, making it difficult to remove from the melt, and leading to the formation of gas pores and inclusions in the castings. Consequently, when transferring aluminum melts, it is essential to minimize stirring of the molten metal and to reduce the melt's exposure to air as much as possible.
When using a tilting crucible to pour molten metal, in order to avoid mixing the molten metal with air, the ladle should be placed as close to the furnace spout as possible and tilted so that the molten metal flows along the side wall of the ladle, preventing it from directly hitting the bottom of the ladle, which could cause agitation or splashing.
Using the correct and reasonable pouring method is one of the important conditions for obtaining high-quality castings. Production practice shows that paying attention to the following points is very effective in preventing and reducing casting defects.
(1) Before pouring, carefully check the furnace melt temperature, the pouring ladle capacity, and the dryness of the coating layer on its surface, as well as whether other tools are adequately prepared. The metal pouring cup should be placed on the sand mold 3–5 minutes before pouring. At this time, the temperature of the pouring ladle should not exceed 150°C. If placed too early or if the temperature is too high, a large amount of gas can accumulate in the pouring channel, posing a risk of explosion during pouring.
(2) Pouring should not be conducted in locations with drafts, where the molten metal may strongly oxidize or burn, which could cause oxidation inclusions and other defects in the casting.
(3) When obtaining molten metal from the crucible, first gently remove the oxide film or flux layer on the surface of the melt with the bottom of the ladle, then slowly immerse the ladle into the molten metal, scoop the melt with the wide mouth of the ladle, and lift the ladle steadily.
(4) When carrying the ladle, avoid tilting it with your palm; walk steadily. The ladle should not be lifted too high, and the metal level inside must remain stable and undisturbed.
(5) Just before pouring, remove any slag from the ladle to prevent bringing slag, oxide films, and other impurities into the mold during pouring.
(6) During pouring, ensure that the flow of molten metal remains stable; it should not be interrupted or directed to the bottom of the sprue directly. The sprue should be filled from top to bottom, and the liquid surface must remain steady. Control the pouring speed appropriately. Typically, start pouring slightly slower to allow the melt to fill evenly, then increase the speed slightly and maintain a relatively consistent pouring rate.
(7) During the pouring process, keep the distance between the ladle spout and the sprue as close as possible, not exceeding 50 mm, to avoid excessive oxidation of the molten metal.
(8) For a sprue with a plug, the plug should not be removed too early. After the sprue is filled with molten metal, gradually remove the plug at an angle to prevent vortex formation in the mold channels.
(9) Molten metal less than 60 mm above the bottom of the crucible should not be used for casting.
