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vSix Technological Properties of Aluminum Alloy Casting

Update time:2019-08-26 10:45:06Number of clicks:0次
The casting process properties of aluminium alloys are generally understood as the synthesis of those properties which are outstanding in the process of filling, crystallizing and cooling.

West Aluminum - West Aluminum Industry Telecom:The casting process properties of aluminium alloys are generally understood as the synthesis of those properties which are outstanding in the process of filling, crystallizing and cooling. Fluidity, shrinkage, air tightness, casting stress and suction. These characteristics of aluminium alloy depend on the composition of the alloy, but also on casting factors, heating temperature of the alloy, casting complexity, gating riser system, gate shape and so on.


1. Liquidity

Fluidity refers to the ability of alloy liquid to fill the mold. Fluidity determines whether the alloy can cast complex castings. Crystalline alloys have good fluidity in aluminium alloys.

There are many factors affecting fluidity, such as composition, temperature and solid particles of metal oxides, metal compounds and other pollutants in alloy liquids, but the external fundamental factors are pouring temperature and pouring pressure (commonly known as pouring head).

In actual production, in addition to strengthening smelting process (refining and slag removal), casting process (sand permeability, metal mould exhaust and temperature) must be improved, and casting temperature should be increased without affecting the quality of castings, so as to ensure the fluidity of the alloy.


2. Contractility

Shrinkage is one of the main characteristics of cast aluminium alloy. Generally speaking, alloys can be divided into three stages from liquid pouring to solidification, to cooling to room temperature, which are liquid shrinkage, solidification shrinkage and solid shrinkage. The shrinkage of alloys has a decisive effect on the quality of castings. It affects the shrinkage hole size, stress generation, crack formation and size change of castings. Generally, casting shrinkage can be divided into volume shrinkage and linear shrinkage. In practice, linear shrinkage is generally used to measure the shrinkage of alloys.

The shrinkage of aluminium alloys is usually expressed as a percentage, which is called shrinkage rate.

(1) The volume contraction includes liquid contraction and solidification contraction.

From pouring to solidification, macroscopic or microscopic shrinkage occurs at the later solidification place. The macroscopic shrinkage caused by shrinkage is visible to the naked eye and can be divided into concentrated shrinkage and dispersive shrinkage. The hole diameter of the concentrated shrinkage hole is large and concentrated, and it is distributed at the top of the casting or the hot spot where the cross section is thick. The morphology of dispersive shrinkage holes is dispersed and fine, and most of them are distributed in the axle center and hot spot of castings. Microscopic shrinkage is hard to be seen by naked eyes, and most of them are distributed under grain boundaries or between dendrites.

Shrinkage and porosity are one of the main defects in castings. The reason is that liquid shrinkage is greater than solid shrinkage. It is found in production that the smaller the solidification range of cast aluminium alloy, the easier to form concentrated shrinkage holes, the wider the solidification range, and the easier to form dispersive shrinkage holes. Therefore, in design, it is necessary to make cast aluminium alloy conform to the principle of sequential solidification, that is, the volume shrinkage of casting from liquid to solidification should be supplemented by liquid alloy, which is shrinkage holes and porosity. Focus on the riser outside the casting. For aluminium alloy castings prone to dispersed porosity, the number of risers is more than that of centralized shrinkage holes, and chills are set at the places prone to porosity, so as to increase the local cooling rate and make them solidify simultaneously or rapidly.

(2) The shrinkage of linear shrinkage line will directly affect the quality of castings. The larger the linear shrinkage is, the greater the tendency of cracks and stresses in aluminum castings will be, and the larger the size and shape of the castings will change after cooling.

For different cast aluminium alloys, casting shrinkage is different, even for the same alloy, casting shrinkage is different. On the same casting, its length, width and high shrinkage are also different. It should be based on specific circumstances.


3. Hot crackability

The main reason for hot cracks in aluminium castings is that the shrinkage stress of the castings exceeds the bonding force between the metal grains. Most of the hot cracks occur along the grain boundary. It can be seen from the fracture surface of the cracks that the metal at the cracks is often oxidized and the luster of the metal is lost. Cracks extend along grain boundaries and are serrated in shape. The surface is wider and the interior is narrower. Some of them penetrate through the end of the casting.

The tendency of cracking in different aluminum alloy castings is also different, because the larger the difference between the temperature at which the complete crystallization frame begins to form and the solidification temperature during the solidification process, the larger the shrinkage rate of the alloy and the greater the tendency of hot cracking. Even the same alloy is also due to the resistance of the casting, the structure of the castings and the pourer. Hot cracking tendencies are also different due to process and other factors. In order to avoid cracks in aluminium castings, concessional casting or improving the casting system of aluminium alloys are often used in production. Hot crack ring method is usually used to detect hot crack in aluminium castings.


4. Air tightness

Air tightness of cast aluminium alloy refers to the degree of non-leakage of cavity aluminium castings under the action of high pressure gas or liquid. Air tightness actually represents the degree of compactness and purity of the internal structure of the castings.

The air tightness of cast aluminium alloys is related to the properties of the alloys. The smaller the solidification range of the alloys, the smaller the tendency of loosening, and the smaller the precipitation pore, the higher the air tightness of the alloys. The air tightness of the same kind of cast aluminium alloy is also related to the casting process, such as lowering the pouring temperature of the cast aluminium alloy, placing chilled iron to accelerate the cooling rate and solidifying and crystallizing under pressure, which can improve the air tightness of the aluminium castings. The leaking voids can also be plugged by infiltration method to improve the air tightness of castings.


5. Casting stress

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

(1) Thermal stress and thermal stress are caused by uneven thickness of cross section and inconsistent cooling at the intersection of different geometric shapes of castings. Compressive stress is formed at the thin wall, resulting in residual stress in the casting.

(2) Phase change stress and phase change stress are caused by phase change of some cast aluminium alloys during cooling after solidification, which results in volume change. It is mainly caused by the uneven wall thickness of aluminium castings and the phase transformation of different parts in different time.

(3) The shrinkage stress of aluminium castings is caused by the tensile stress caused by the hindrance of the mould and core during shrinkage. This kind of stress is temporary, and the opening of aluminium castings will automatically disappear. However, the inappropriate opening time often causes hot cracks, especially for aluminum alloys cast in metal molds, which are prone to hot cracks under such stress.

Residual stress in aluminium alloy castings reduces the mechanical properties of the alloy and affects the processing accuracy of the castings. Residual stress in aluminium castings can be eliminated by annealing. Because of its good thermal conductivity, there is no phase change in the cooling process. As long as the structure of the castings is reasonable, the residual stress of the aluminium castings is generally small.

The higher the melt temperature of aluminium alloy is, the more hydrogen is absorbed. At 700 C, the solubility of hydrogen in 100 g aluminium is 0.5-0.9, and at 850 C, the solubility of hydrogen increases by 2-3 times. The solubility of hydrogen in liquid aluminium increases significantly when alkali metal impurities are present.



(Chongqing Xialu Xinda Aluminum Industry Co., Ltd. Publicity Department collates contributions)

 

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