Solidification Mechanism of Aluminium Castings and Causes of Surface Oxidation
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- Release time: 2026-01-23
Aluminium castings offer low cost, excellent workability, and resource-saving remelting for recycling, ensuring enduring application and development. Sand casting is the preferred method for aluminium castings, primarily due to its lower cost, simpler production process, and shorter production cycle compared to other casting techniques. Aluminium castings are produced by preparing pure aluminium or aluminium alloy ingots according to standard composition ratios. These are then heated manually to transform them into liquid or molten aluminium alloy. This molten material is subsequently poured into cavities using specialised moulds or corresponding processes. Upon cooling, the desired shape of the aluminium component is formed. Common aluminium casting methods include: sand casting, die casting, low-pressure casting, mould casting, and lost foam casting. Aluminium castings exhibit excellent mechanical and physical properties, offering a wide range of strength, hardness, and toughness combinations. They may also incorporate one or more specialised characteristics, such as wear resistance, high/low temperature tolerance, and corrosion resistance. Die-casting manufacturers
Aluminum castings offer low cost, good processability, and remelting for resource and energy conservation, ensuring their enduring application and development. Sand casting is preferred for aluminum castings primarily because it features lower costs, simpler production processes, and shorter production cycles compared to other casting methods. Aluminum castings are produced by proportioning pure aluminum or aluminum alloy ingots according to standard composition ratios, artificially heating them into liquid or molten aluminum alloy, and then pouring the aluminum liquid or molten alloy into molds or through corresponding processes into cavities, cooling to form aluminum parts of required shapes. Common aluminum casting methods include sand casting, die casting, low-pressure casting, permanent mold casting, and lost foam casting. Aluminum castings exhibit excellent mechanical and physical properties, offering various combinations of strength, hardness, and toughness, as well as special properties such as wear resistance, high and low temperature resistance, and corrosion resistance.
The weight and casting size range of aluminum castings is very wide, from as light as a few grams to as heavy as 400 tons; wall thickness can be as thin as 1 millimeter or exceed 1 meter; length ranges from a few millimeters to over ten meters, meeting the requirements of different industrial sectors. Generally, the surface quality of aluminum castings can be judged using comparison samples to determine surface roughness; fine surface cracks can be inspected by dye penetrant or magnetic particle methods. Internal quality can be examined and assessed using acoustic, ultrasonic, eddy current, X-ray, and gamma-ray methods. The quality of aluminum castings significantly affects the performance of mechanical products. For example, the wear resistance and dimensional stability of machine tool aluminum castings directly impact the machine's precision retention life; the dimensional accuracy, profile accuracy, and surface roughness of impellers, housings, and hydraulic component cavities directly affect pump and hydraulic system efficiency, energy consumption, and cavitation development; the strength and thermal shock resistance of engine blocks, cylinder heads, liners, piston rings, and exhaust manifolds directly influence engine service life.
Advantages of Aluminum Castings:
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Capable of producing metal parts with complex shapes, clear contours, and thin-walled deep cavities. Because molten metal maintains high fluidity under high pressure and speed, metal parts difficult to process by other methods can be obtained.
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Aluminum castings offer high dimensional accuracy, reaching IT11-13 grades, sometimes IT9 grades, with surface roughness of Ra0.8-3.2μm and good interchangeability.
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High material utilization rate. Due to the high precision of aluminum castings, only minimal machining is required for assembly and use; some aluminum castings can be assembled directly. Material utilization is approximately 60-80%, with blank utilization reaching 90%.
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High production efficiency. Because of rapid filling, short filling time, and quick metal solidification, die casting operations cycle rapidly. Among various casting processes, die casting offers the highest productivity, suitable for mass production.
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Convenient use of inserts. Positioning mechanisms can be easily set on die casting molds, facilitating insert casting to meet local special performance requirements of aluminum castings.
Solidification Modes of Aluminum Castings:
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Layer-by-layer solidification: Pure aluminum or eutectic aluminum alloys do not have a liquid-solid coexistence zone during solidification, so the outer solid layer and inner liquid layer are clearly separated by a boundary (solidification front). As temperature decreases, the solid layer continuously thickens while the liquid layer reduces until it reaches the aluminum casting. This solidification mode is called layer-by-layer solidification.
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Mushy solidification: If the aluminum alloy has a wide crystallization temperature range and the aluminum casting has a relatively flat temperature distribution, there will be no solid layer on the casting surface during a certain period of solidification. Instead, the liquid-solid coexistence zone extends through the entire cross-section, similar to cement solidification, becoming mushy before solidifying—this is called mushy solidification.
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Intermediate solidification: Most solidifications fall between layer-by-layer and mushy solidification, called intermediate solidification. Generally, aluminum casting quality is closely related to its solidification mode. When [the alloy] has strong filling capacity, it facilitates preventing shrinkage cavities and porosity; mushy solidification is the mode to obtain dense aluminum castings.
Causes and Solutions for Aluminum Casting Surface Oxidation:
I. Cause Analysis: Through our observation and research, we found many factors causing aluminum casting surface oxidation, mainly including: equipment not being properly cleaned after use, causing many material impurities to adhere to the casting surface, leading to oxidation over time; or the solution in the casting, which generally contains metal elements, oxidizing after prolonged immersion in equipment; and improper equipment operation causing damage and subsequent oxidation.
II. Solutions: Faced with the above oxidation causes, we need to take correct measures according to actual conditions: clean castings promptly after use to ensure no other impurities or contamination on the surface; improve pouring capacity and adopt appropriate production methods; promptly clean metal solutions from equipment, rinse repeatedly with clean water after thorough cleaning; handle production materials by effectively removing impurities and appropriately reducing equipment usage time.
