The casting properties of aluminum alloys are generally understood as a combination of those properties that are most prominent during the processes of mold filling, crystallization, and cooling. These properties include fluidity, shrinkage, airtightness, casting stress, and gas absorption. These characteristics of aluminum alloys depend on the alloy composition, but are also related to casting factors, alloy heating temperature, mold complexity, gating system, and gate shape.

1. Fluidity

Fluidity refers to the ability of the liquid alloy to fill the mold. The degree of fluidity determines whether the alloy can be cast into complex castings. Eutectic alloys have the best fluidity among aluminum alloys.

Many factors affect fluidity, mainly composition, temperature, and the presence of solid particles of metal oxides, metal compounds, and other contaminants in the alloy liquid. However, the fundamental external factors are the pouring temperature and pouring pressure (commonly known as pouring head).

In actual production, with the alloy already determined, in addition to strengthening the melting process (refining and slag removal), it is also necessary to improve the mold processability (permeability of sand molds, venting of metal molds, and temperature), and increase the pouring temperature without affecting the quality of the castings to ensure the fluidity of the alloy.

2. Shrinkage

Shrinkage is one of the main characteristics of cast aluminum alloys. Generally speaking, from liquid pouring to solidification, and until cooling to room temperature, there are three stages: liquid shrinkage, solidification shrinkage, and solid shrinkage. The shrinkage of the alloy has a decisive influence on the quality of the casting; it affects the size of shrinkage cavities, the generation of stress, the formation of cracks, and changes in Size. Castings usually undergo volumetric shrinkage and linear shrinkage; in actual production, linear shrinkage is generally used to measure the shrinkage of the alloy.

3. Hot Cracking

The generation of hot cracks in aluminum castings is mainly due to the casting shrinkage stress exceeding the bonding force between the metal grains, mostly occurring along the grain boundaries. Observation of the fracture surface shows that the metal at the crack is often oxidized and loses its metallic luster. The cracks extend along the grain boundaries, with a sawtooth shape, wider on the surface and narrower inside, some penetrating the entire end face of the casting.

Different aluminum alloy castings have different tendencies to crack. This is because the greater the difference between the temperature at which a complete crystalline framework begins to form during the solidification of the cast aluminum alloy and the solidification temperature, the greater the shrinkage rate of the alloy, and the greater the tendency to produce hot cracks. Even with the same alloy, the tendency to produce hot cracks varies due to factors such as mold resistance, casting structure, and pouring process. In production, yielding molds are often used, or the pouring system of the cast aluminum alloy is improved to prevent cracks from forming in aluminum castings. The hot crack ring method is usually used to detect hot cracks in aluminum castings.

4. Airtightness

The airtightness of cast aluminum alloys refers to the degree to which cavity-type aluminum castings do not leak under the action of high-pressure gas or liquid. Airtightness actually characterizes the degree of compactness and purity of the internal structure of the casting.

5. Casting Stress

Casting stress includes thermal stress, phase transformation stress, and shrinkage stress. The causes of various stresses are different.

6. Gas Absorption

Aluminum alloys easily absorb gases, which is a major characteristic of cast aluminum alloys. This is caused by the absorption of hydrogen gas produced by the reaction of liquid aluminum and aluminum alloy components with moisture contained in the furnace charge, organic matter combustion products, and molds.