The
aluminum alloy casting process is a widely used technique in metal casting, and its performance directly impacts the quality and application of the castings. The process performance of
aluminum alloy casting typically includes factors like fluidity, shrinkage, airtightness, casting stress, and gas absorption. These characteristics are influenced not only by the alloy composition but also by various factors during the casting process, such as mold design, heating temperature, mold complexity, and gating system.
Fluidity is one of the most important properties in aluminum alloy casting. It refers to the ability of the molten alloy to fill the mold. Good fluidity ensures that aluminum alloys can be cast into complex shapes. Generally, eutectic alloys have better fluidity because they have a narrow solidification temperature range. Several factors affect fluidity, including the alloy composition, temperature, mold design, and the presence of metal oxides and solid-phase particles in the molten alloy. External factors such as pouring temperature and pouring pressure (head pressure) are also critical. If the pouring temperature is too low, fluidity will be poor, affecting the casting quality.
In actual production, besides optimizing the melting process (such as refining and slag removal), it is essential to improve the mold's processability. This can be achieved by enhancing the permeability of sand molds, improving the venting properties of metal molds, and appropriately increasing pouring temperatures to ensure good fluidity, thus improving casting quality.
Shrinkage is another crucial property to consider in the aluminum alloy casting process. As aluminum alloy transitions from liquid to solid, it undergoes three stages of shrinkage: liquid shrinkage, solidification shrinkage, and solid shrinkage. The shrinkage of the alloy significantly affects the quality of the casting. Excessive shrinkage may lead to defects like shrinkage cavities, cracks, and dimensional changes. Shrinkage is typically divided into volumetric shrinkage and linear shrinkage, with linear shrinkage often used in production to measure the alloy's shrinkage.
Different aluminum alloys have varying shrinkage rates, which can affect casting quality. To prevent shrinkage cavities and porosity, the casting design should follow the principle of sequential solidification. Specifically, the volumetric shrinkage during the liquid-to-solid transition should be compensated by additional alloy liquid to prevent shrinkage defects from concentrating within the casting. For alloys prone to porosity, adding more risers and using chill bars can help accelerate cooling, ensuring more uniform solidification.
Hot cracks in aluminum alloy castings are mainly caused by shrinkage stress exceeding the bonding strength between the metal grains. When a casting shrinks during cooling, excessive stress may cause cracks along the grain boundaries. Hot cracks typically appear along the grain boundaries, displaying a serrated pattern, with wider surface cracks and narrower internal cracks. The tendency for hot cracking is closely related to the solidification temperature and shrinkage rate of the alloy. The larger the shrinkage rate of the alloy, the greater the tendency for hot cracking.
To prevent hot cracking in aluminum alloy castings, improvements can be made in mold design or gating systems. Additionally, using flexible molds or selecting appropriate aluminum alloy compositions can effectively reduce the occurrence of hot cracks.
Airtightness refers to the ability of a casting to withstand high-pressure gas or liquid without leakage. It is a key indicator of the density and purity of the internal structure of a casting. The airtightness of aluminum alloys is significantly influenced by alloy composition and casting process. Alloys with smaller solidification ranges tend to produce fewer porosities, resulting in better airtightness.
To improve the airtightness of aluminum alloy castings, casting processes can be optimized by lowering pouring temperatures, using chill bars to accelerate cooling, and solidifying under pressure. Additionally, methods such as impregnation can be used to seal the pores, effectively improving airtightness.
During the aluminum alloy casting process, three main types of casting stresses occur: thermal stress, phase change stress, and shrinkage stress. Thermal stress is caused by uneven geometrical shapes and inconsistent cooling; phase change stress arises from volume changes during solidification, while shrinkage stress results from the hindrance caused by molds or cores during solidification.
Casting stresses not only affect the mechanical properties of the casting but can also lead to cracks. To reduce the impact of residual stresses, annealing treatments are commonly used to eliminate residual stress in the aluminum casting, ensuring its dimensional accuracy and mechanical performance.
Aluminum alloys have a strong tendency to absorb gases, especially hydrogen, during the melting process. This occurs because the molten aluminum reacts with the furnace material, fuel, and moisture in the mold at high temperatures, generating hydrogen gas, which is absorbed by the aluminum melt. The amount of gas absorption is closely related to the melting temperature; the higher the temperature, the greater the absorption.
To reduce gas absorption in the molten aluminum, refining processes can be applied to lower hydrogen content, or covering agents can be used during melting to protect the aluminum melt from gas absorption. Excessive gas absorption can lead to porosity in the casting, which not only affects airtightness but also reduces its mechanical properties.
The performance of the aluminum alloy casting process is a comprehensive indicator that involves fluidity, shrinkage, airtightness, casting stress, and gas absorption. To ensure high-quality aluminum alloy castings, it is necessary to consider factors such as alloy composition, casting process, and mold design. By optimizing the casting process and alloy composition, improving the fluidity, shrinkage, and airtightness, and effectively controlling casting stresses and gas absorption, the overall performance of aluminum alloy castings can be significantly enhanced, meeting the diverse demands of various industries.
