Introduction of Casting Aluminum Alloy Technology

Aluminum alloy casting process, aluminum alloy shrinkage size, usually expressed as a percentage, known as shrinkage.

Body contraction

Body contraction includes fluid contraction and coagulation contraction.

Casting alloy liquid from casting to solidification, in the final solidification of the place there will be a macro or micro shrinkage, this shrinkage caused by macroscopic shrinkage visible to the naked eye, and divided into concentrated shrinkage and shrinkage. Concentrated shrinkage of the aperture large and concentrated, and distributed in the casting at the top or section of the thick section of the hot section. Dispersed crater topography is scattered and small, mostly distributed in the casting axis and hot section. Microscopic shrinkage hole is difficult to see the naked eye, most of the microscopic shrinkage holes in the grain boundary or dendrite between the dendrites.

Shrinkage and loose is one of the main defects of the casting, the reason is the liquid shrinkage greater than the solid contraction. Found in the production, casting aluminum alloy the smaller the solidification range, the more likely to form a concentrated shrinkage, the wider the solidification range, the more likely to form dispersive crater, so the design must be consistent with the principle of casting aluminum alloy casting, casting Liquid to solidification during the body shrinkage should be added to the alloy liquid is shrinkage and loose concentrated in the casting external riser. For easy to produce loose loose aluminum alloy castings, set the number of riser than the concentration of shrinkage to be more, and easy to loose place to set cold iron, increase the local cooling rate, so that the same time or rapid solidification.

Line contraction

Line shrinkage will directly affect the quality of the casting. The greater the line shrinkage, the greater the tendency of the aluminum castings to produce cracks and stresses; the greater the size and shape of the castings after cooling.

For different casting aluminum alloys have different casting shrinkage, even if the same alloy, different casting, shrinkage is also different in the same casting, the length, width, and high shrinkage is also different. Should be based on specific circumstances.

Aluminum casting thermal cracking, mainly due to the shrinkage stress of castings over the grain of the metal binding force, mostly along the grain boundary fracture observed from the crack at the metal is often oxidized, loss of metallic luster. Cracks along the grain boundary extension, the shape of a sawtooth-shaped, wide surface, the internal narrow, and some are penetrating the entire casting end.

Different aluminum alloy castings tend to crack is different, because the aluminum alloy solidification process begins to form a complete crystalline frame temperature and solidification temperature difference between the greater the greater the alloy shrinkage, the greater the tendency to produce hot cracking , Even if the same alloy due to casting resistance, casting structure, casting process and other factors have different hot cracking tendencies. Production often use concessionary cast, or improve casting aluminum alloy casting system and other measures to make aluminum casting to avoid cracks. Hot cracking is usually used to detect hot cracking of aluminum castings.

Air tightness

Cast aluminum alloy air tightness refers to the cavity type aluminum casting in the high-pressure gas or liquid under the action of non-leakage degree, air tightness actually characterizes the casting internal organization density and purity degree.

The airtightness of the cast aluminum alloy is related to the properties of the alloy. The smaller the solidification range of the alloy is, the smaller the tendency of the alloy is, and the smaller the precipitate porosity is, the higher the airtightness of the alloy is. The same kind of aluminum alloy air tightness is good or bad, but also with the casting process, such as reducing the casting aluminum alloy casting temperature, placing cold iron to speed up the cooling rate and solidification under pressure crystallization, can make the aluminum castings air tightness improve. Infiltration can also be used to plug the leakage gap to improve the air tightness of the casting.

Casting stress

Casting stress, including thermal stress, phase transformation stress and shrinkage stress three. The causes of various stresses are not the same.

Thermal Stress 

Thermal stress is due to different geometric shapes at the intersection of the thickness of the cross-section uneven cooling caused by inconsistency. A compressive stress is created at the thin wall, resulting in a residual stress in the casting.