Detailed explanation of cast aluminum alloys, there are four types of cast aluminum alloys according to the main alloying element differences.
(1) Aluminum-silicon alloys, also called "silicon-aluminum-ming" or "silicon-aluminum-ming". It has good casting performance and wear resistance, and has a small thermal expansion coefficient. Among the cast aluminum alloys, there are the most varieties and the largest amount of alloys, and the silicon content is 4% to 13%. Sometimes silicon-aluminum alloys with 0.2% to 0.6% magnesium are added, which are widely used in structural parts, such as shells, cylinders, boxes and frames. Sometimes adding an appropriate amount of copper and magnesium can improve the mechanical properties and heat resistance of the alloy. Such alloys are widely used to make components such as pistons.
(2) Aluminum-copper alloy, the alloy containing 4.5% to 5.3% copper has the best strengthening effect. Appropriate addition of manganese and titanium can significantly improve the room temperature, high temperature strength and casting performance. It is mainly used to make sand castings with large dynamic and static loads and uncomplicated shapes.
(3) Aluminum-magnesium alloy, a cast aluminum alloy with the smallest density (2.55g/cm3) and the highest strength (about 355MPa), containing 12% magnesium, with the best strengthening effect. The alloy has good corrosion resistance in the atmosphere and seawater, and has good comprehensive mechanical properties and machinability at room temperature. It can be used for radar bases, aircraft engine cases, propellers, landing gear and other parts, as well as decorative materials.
(4) Aluminum-zinc alloys are often added with silicon and magnesium elements in order to improve their properties, which are often called "zinc-silicon-aluminum-ming". In the casting condition, the alloy has a quenching effect, that is, "self-quenching". It can be used without heat treatment. After heat treatment, the casting has higher strength. After stabilization treatment, the size is stable, and it is often used to make models, templates and equipment brackets.
Cast aluminum alloys have the same alloy system as wrought aluminum alloys, and have the same strengthening mechanism as wrought aluminum alloys (except for strain strengthening). Their main difference is that the maximum content of alloying element silicon in cast aluminum alloys exceeds that of most deformed aluminum alloys. Silicon content in aluminum alloys. In addition to strengthening elements, cast aluminum alloys must also contain a sufficient amount of eutectic elements (usually silicon) to make the alloy have considerable fluidity and easily fill the shrinkage joints of the casting during casting. At present, there are only the following 6 basic alloys;
1. The refining effect of rare earth in aluminum alloy
Adding an appropriate amount of rare earth elements to the aluminum alloy can promote the refining effect. Rare earth elements can improve inclusion morphology and purify grain boundaries. The effect of Al RE master alloy on the fluidity of A356 alloy was studied by vacuum suction casting method. The experimental results show that adding an appropriate amount of rare earth elements into the alloy melt can reduce the temperature difference between solid and liquidus and reduce the tendency of paste solidification of the alloy. , and reduce the surface tension of the alloy melt, in addition to the refining effect of degassing and removing impurities, which will improve the fluidity of the melt and reduce the viscosity, which is beneficial to the exclusion of inclusions and gases.
A new type of aluminum alloy flux containing rare earth compounds has been researched and developed. Through a series of physical and chemical reactions, the flux can not only make the hydrogen content of the A356 alloy melt at 720 °C from more than 0.30ml/100g (Al) When it drops below 0.10 ml/100g (Al), the degassing effect is remarkable, and the room temperature tensile strength of A356 alloy is increased by 7.27% and the elongation is increased by 85.58%. However, excess rare earth elements also aggravate the aggregation of RE-rich phases and become inclusions, thereby reducing the fluidity of the alloy melt.
2. Refinement effect of rare earth on aluminum alloy
Purposefully suppressing the growth of columnar and bicolumnar crystals and promoting the formation of fine equiaxed crystals is called grain refinement. Due to the grain refinement, the properties of the alloy are improved, and at the same time, defects such as shrinkage porosity, thermal cracking, and pinholes are reduced. The Z basic method of refining treatment is to suppress nucleation and to add extraneous nucleation particles of grain refiners to the melt. At present, the method of adding refiners has become the most effective and practical method. There are three types of grain refiners commonly used in cast aluminum alloys: binary Al-Ti alloys, binary Al-B alloys and ternary Al-Ti-B alloys. The master alloy (grain refiner) is added into the aluminum alloy melt to dissolve, releasing the intermetallic compound phase and becoming the foreign nucleation core.
Adding rare earths to aluminum alloys can not only refine the grains, but also significantly refine the dendrite structure (reduce the secondary dendrite spacing). The best effect corresponds to different rare earth contents. However, its refining effect is weaker than that of Ti, B and other elements. The critical value of rare earth addition is closely related to the melting and casting conditions of the alloy. Only under certain production process conditions, a certain amount of rare earth will have the best refining effect.
Using general refiners, with the extension of the standing time of the aluminum liquid, the refining effect gradually declines; using the Al-5Ti-1B-10RE master alloy, rare earth elements can prevent the aggregation and precipitation of the refining elements, which has a negative effect on the properties of Ti and B. The refining effect has a certain promotion effect, which can effectively inhibit the decline of the grain size of the aluminum-silicon alloy during the long-term standing process, and is suitable for mass production of automotive aluminum alloy castings.
3. Metamorphic effect of rare earth on Al-Si alloy
The Si phase in the cast Al-Si alloy will grow into a block or flake brittle phase under natural growth conditions, which severely splits the matrix and reduces the strength and plasticity of the alloy, so it needs to be changed into a favorable form. The modification treatment changes the eutectic Si from coarse flakes to fine fibrous or lamellar flakes, thereby improving the properties of the alloy. So far, it has been found that K, Na in alkali metals, Ca, Sr in alkaline earth metals, rare earth elements Eu, La, Ce and miscellaneous rare earths, nitrogen group elements Sb, Bi, oxygen group elements S, Te, etc. all have metamorphic effects. In the Al-Si alloy, adding aluminum rare earth intermediate alloy or rare earth chloride and fluoride can make the eutectic Si phase change from lamellar to spherical. Different rare earths have different metamorphic abilities. Generally speaking, as the atomic radius changes from large to small, the metamorphic ability changes from strong to weak.
Rare earth modifier has good long-term effect and remelting stability, small inhalation tendency, no pollution, simple adding process and no corrosion effect. The research results show that the alloy with La content of 0.056% is remelted 10 times, and the metallographic examination is carried out for each sample. It is found that Z still has a metamorphic effect, and the final concentration of La is still 0.035%, which is still in the best metamorphosis. within the range. 0.3 % mixed rare earth modified alloy was remelted for 5 times, and it was found that Z still had a good modification effect in the end.
The metamorphic process directly affects the metamorphic effect of rare earths. For Al-Si alloys, the key to obtaining a stable metamorphic structure is to reduce the burning loss of rare earths, prevent the segregation of rare earths, and make rare earths diffuse into the molten aluminum quickly and evenly. Rare earth metamorphism has an incubation period, that is, it must be kept at high temperature for a certain period of time before rare earth can play the greatest metamorphic effect.
