1. Physical properties
The physical properties of alloys refer to their response to various physical phenomena, such as temperature changes, electromagnetic effects and so on.
2. Chemical properties
The chemical properties of alloys refer to the ability to chemically react with other elements in various media, mainly corrosion resistance. For example, heating equipment, steam turbines, jet engines, etc., must choose good corrosion resistance alloy to manufacture.
3. Mechanical properties
Mechanical parts will be subjected to various external forces during use, such as static load, impact load or alternating load, so that mechanical parts are stretched, compressed Aluminum castings and distorted.
The mechanical properties of the alloy refer to the characteristics of its resistance to external forces, such as strength, hardness and elasticity, which is a sign to measure the performance of the alloy.
4. Process performance
The process properties of alloys are the properties that make them easy to process and form. Mainly includes casting performance, casting performance is a comprehensive concept, mainly including fluidity, shrinkage and the formation of thermal cracking, stress, segregation, suction and other tendencies.
1. Liquidity
Alloy fluidity refers to the ability of the alloy to fill the cavity. Castable alloys are easy to die cast parts with complex shapes and thin walls, and can obtain a clear, full profile.
As far as the die casting conditions are concerned, the fluidity mainly depends on the temperature of the casting soup, the speed of injection and the pressure. Under the same conditions, firstly, increasing the temperature of the casting soup can increase the alloy fluidity, reduce the viscosity and enhance the filling ability, but the suction is serious at the same time, and the oxidation is added. Second, the use of higher injection speed, can improve the alloy filling capacity, but the temperature is too high is easy to vorticity and enrolling, thus affecting the quality of the casting. Third, the injection pressure can be increased to strengthen the filling capacity of the alloy.
In summary, the methods to improve the fluidity of the alloy are as follows:
(1) Properly adjust the alloy composition, strictly control the melting process, purify the alloy liquid, reduce the non-metallic inclusions and gases in the alloy liquid, add trace elements, and refine the grain.
(2) Increase the mold overflow exhaust system to improve slag removal and exhaust capacity.
(3) Reasonably set the pouring system to improve the pouring temperature and injection speed without affecting the performance of the casting.
(4) Improve the casting structure to make it have better casting performance.
2. Contractility
During the process of casting from liquid to solidification and continued cooling, the volume and size changes will occur, that is, shrinkage, which can usually be divided into three stages, namely liquid shrinkage, solidification shrinkage, and solid shrinkage. Liquid shrinkage and solidification shrinkage have great influence on casting shrinkage, solid shrinkage and casting cracking, size change; Solidification shrinkage and solid shrinkage affect the hot cracking of castings. Solidification shrinkage and liquid shrinkage determine the shrinkage of the cast, and solid shrinkage is closely related to the size of the casting, which is called linear shrinkage.
The size of the shrinkage of die casting mainly depends on the type of alloy, chemical composition, pouring temperature, injection speed, pressure holding and mold opening time, injection ratio pressure size, casting conditions and casting structure.
Shrinkage hole and shrinkage porosity are one of the common defects in castings. The basic condition of production is that the liquid shrinkage and solidification shrinkage of alloy are much larger than the solid shrinkage.
3. Hot cracking
The hot crack of casting refers to the crack formed in the alloy at high temperature. Formed at the end of the solidification of the casting, the shape of the hot crack is tortuous and irregular, the crack surface is strongly oxidized, and there are external cracks and internal cracks in the casting.
Generally speaking, the larger the difference between the temperature at which the complete crystalline framework begins to form and the temperature at which the solidification ends, the larger the thermal cracking tendency of the alloy. For example, aluminum, copper alloy, aluminum, magnesium alloy castings, generally than aluminum, silicon alloy castings have a greater tendency to crack.
However, for the same alloy, whether the casting produces hot cracking, it mainly depends on the casting resistance, casting structure, die casting process and other factors, so in the casting solidification process, all the measures to reduce the shrinkage force, internal stress, and increase the high temperature of the alloy will help prevent the casting of hot cracking. For example, improving the casting structure, improving the pouring system and other effective ways to avoid casting hot crack defects.
4. Casting stress
Casting stress can be divided into thermal stress, phase change stress and shrinkage stress.
The thermal stress is caused by the different order of shrinkage due to the uneven wall thickness of the casting.
The phase change stress is the result of the phase change of the alloy during the cooling process after solidification, which is accompanied by the volume change and causes the casting size to change as well.
Shrinkage stress is caused by the resistance of the cast and shape when the casting shrinks.
5. Segregation
The phenomenon of uneven chemical composition in castings is called segregation. Segregation is a casting defect, due to the chemical composition of the casting is inconsistent, is bound to make its mechanical and physical properties are inconsistent, which will affect the working effect and service life of the casting, segregation can be divided into three types of in-grain segregation, regional segregation and specific gravity segregation in die casting production, mainly due to the alloy after melting in the furnace for a long time or each furnace alloy liquid is not all used up and often add material, The alloy liquid at the bottom of the furnace is left for a long time and produces specific gravity segregation, which causes the chemical composition of the castings produced successively to be inconsistent, and also causes the chemical composition of the cast soup and the castings to be inconsistent.
Measures to prevent alloy segregation:
(1) Try to use up the alloy liquid in the furnace and then add new material;
(2) The alloy liquid should be fully stirred before being sent into the insulation furnace;
(3) Add some trace alloying element for modification, refine the grain and change the initial crystal state.
(4) The remaining material at the bottom of the furnace, the general grain is coarse, the segregation is serious, and the casting can not be produced.
(5) Stir the cast soup of the machine side melting furnace (6-7 times from the bottom of the furnace), and the interval is calculated once every one to two hours.
(6) For the cast soup of the machine side melting furnace, inject nitrogen from the bottom of the furnace to stir, the interval is one to two hours.
6. Inhale
All kinds of cast colored gold have the characteristics of suction.
When the alloy is solid, the solubility of the gas is very small, and the solubility increases with the increase of temperature. When the gas solubility reaches the maximum, the gas solubility decreases sharply. When the alloy liquid reaches the boiling point, the gas solubility is almost zero. Under normal circumstances, the melting process of cast non-ferrous alloys is in the stage where the solubility of gas increases rapidly with the increase of alloy temperature, and all have a tendency to inhale. In this way, if the alloy absorbs more gas during the melting process, and even reaches saturation, after filling the cavity, the gas will be precipitated with the decrease of temperature. When it is too late to run out of the cavity, pores are formed, which is more obvious for aluminum alloy pinhole defects.
The gas absorbed by the casting soup in the die casting mainly comes from the water in the charge and various auxiliary tools, the furnace gas crucible, melting tools, etc. For the highly aspirating non-ferrous alloys, such as aluminum alloy and magnesium alloy, in the process of melting and casting, all the opportunities to connect with the gas and water will lead to inhalation (mainly hydrogen).
Measures to reduce breathing:
1) Strictly control the quality of auxiliary materials and auxiliary tools (avoid contact with water);
2) Minimize the heat preservation of alloy liquid at high temperature to avoid overheating of alloy liquid;
3) For the very aspirating alloy, it is dissolved under the protection of the covering agent and degassed after melting (central melting furnace).
7. Air tightness
The air tightness of the alloy refers to the ability of the casting to withstand the action of high pressure gas or liquid without leakage, which usually reflects the inner density of the casting.
The general rule is that the narrower the solidification temperature of the alloy, the smaller the tendency of the casting to produce loose, the less gas precipitated during the solidification process, the fewer precipitated pores, the higher the air tightness of the alloy.
Methods to improve the air tightness of castings:
1) Reasonable selection of alloy grades.
2) Reduce the alloy casting temperature as much as possible to avoid overheating of the alloy.
3) Properly design the casting process and take high pressure crystallization measures (local extrusion) during rapid cooling.
4) For aluminum alloy castings with leakage defects, the infiltration process can also be used to save, but it is limited to holes of about 0.4 mm and is not a part with high pressure test requirements.