The casting properties of aluminum alloys are generally understood to be a combination of those properties that are most prominent during mold filling, crystallization, and cooling. Fluidity, shrinkage, air tightness, casting stress, suction.
1. Activity
Mobility is the ability of the alloy liquid to fill the mold. The size of the activity determines whether the alloy can forge large castings. Eutectic alloy has the best activity in aluminum alloy.
Many factors affect the activity, the important is the identity, temperature and the presence of metal oxides, metal compounds and other pollutants in the alloy liquid solid particles, but the external basic factors for pouring temperature and pouring pressure (commonly known as pouring head) rugged.
In the actual production, in the confirmed environment of the alloy, in addition to strengthening the smelting process (essence and slag removal), it is necessary to improve the casting process (sand mold permeability, metal mold exhaust and temperature), and improve the casting temperature without affecting the casting quality, to ensure the mobility of the alloy.
2. Contractility
Shrinkage is one of the main characteristics of cast aluminum alloy. Generally speaking, alloy from liquid pouring to solidification, until cold to room temperature, there are three stages, namely liquid shrinkage, solidification shrinkage and solid shrinkage. The shrinkage property of the alloy has a decisive influence on the casting quality. It affects the size of the shrinkage cavity, the generation of stress, the formation of crack and the change of dimension. Usually, casting shrinkage is divided into bulk shrinkage and linear shrinkage, and linear shrinkage is generally used to measure the shrinkage of alloy in actual production. Aluminum alloy shrinkage, usually expressed as a percentage, known as the shrinkage rate.
(1) The shrinkage of aluminum alloy casting body includes liquid shrinkage and solidification shrinkage.
Casting alloy liquid from pouring to solidification, in the final solidification place there will be macro or micro shrinkage, the macroscopic shrinkage caused by shrinkage can be seen by naked eyes, and divided into concentrated shrinkage and dispersed shrinkage. Concentrated shrinkage cavity is large and concentrated and distributed at the top of the casting or at the hot spot with large section thickness. The dispersed shrinkage cavity and porosity is dispersed and fine, most of which are distributed in the casting axis and hot spot. Microshrinkage cavity is hard to see by naked eye. Most microshrinkage cavity is distributed under grain boundary or between dendrites of dendrites.
Shrinkage cavity and porosity is one of the main defects of casting, which is caused by liquid shrinkage is greater than solid shrinkage. Found in production, casting aluminium alloy solidification range is smaller, the formation of concentrated shrinkage, solidification range is wide, the more easy dispersion shrinkage cavity formation, therefore, in the design must conform to the principle of directional solidification casting aluminum alloy, the casting liquid is to the body during the solidification shrinkage should be alloy liquid supplement, is loose focus on the outside of the castings and shrinkage cavity at the mouth. For aluminum alloy castings which are easy to be dispersed and loose, the number of risers is more than that of concentrated shrinkage cavity, and the chill iron is set in the place which is easy to be loose, so as to increase the local cooling speed and make it solidify at the same time or quickly.
(2) The shrinkage line of aluminum alloy casting will directly affect the casting quality. The greater the linear shrinkage is, the greater the tendency of aluminum casting to crack and stress is. After cooling, the size and shape of castings change greatly.
For different casting aluminum alloy has different shrinkage rate, even if the same alloy casting, the shrinkage rate is different, in the same casting, its length, width, height shrinkage rate is different. It depends on the specific situation.
3. Inhalability
Aluminum alloy is easy to absorb gas and is an important characteristic of forging aluminum alloy. Liquid aluminum and aluminum alloy components and charge, organic combustion products and casting, and other water produced by the reaction of hydrogen produced by the absorption of aluminum liquid.
The higher the temperature of molten aluminum alloy, the more hydrogen is absorbed. At 700℃, the hydrogen solubility in every 100g of aluminum is 0.5 ~ 0.9; when the temperature rises to 850℃, the hydrogen solubility increases by 2 ~ 3 times. When containing alkali metal impurities, the solubility of hydrogen in aluminum solution increases obviously.
4. Air tightness
Air tightness of casting aluminum alloy refers to the degree of no leakage of cavity type aluminum casting under the action of high pressure gas or liquid.
The air tightness of the cast aluminum alloy is related to the properties of the alloy. The smaller the solidification range, the smaller the tendency of looseness and the smaller the precipitation porosity, the higher the air tightness of the alloy. The air tightness of the same cast aluminum alloy is also related to the casting process, such as lowering the casting temperature of the cast aluminum alloy, placing the chiller to speed up the cooling speed, and solidifying and crystallizing under pressure. The leakage void can also be plugged by leaching to improve the air tightness of the casting.
5. Thermal cracking
The main reason for thermal cracks in aluminum castings is that the shrinkage stress of the castings overtakes the cooperation between the metal grains, and most of the cracks occur along the grain boundary. The crack extends along the grain boundary and is serrated in shape. It is broad in appearance and narrow in the interior. Some cracks ChuangTou the whole end face of the casting.
6. Inhalability
Aluminum alloy can absorb gas easily and is the main characteristic of cast aluminum alloy. Liquid aluminum and aluminum alloy components and charge, organic matter combustion products and casting and other moisture content of hydrogen produced by the reaction caused by the absorption of aluminum liquid.
The higher the temperature is, the more hydrogen is absorbed. At 700℃, the hydrogen solubility in every 100g of aluminum is 0.5 ~ 0.9; when the temperature rises to 850℃, the hydrogen solubility increases by 2 ~ 3 times. The solubility of hydrogen in aluminum solution increases significantly when alkali metal impurity is present.
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