Cast aluminum alloy is an aluminum alloy that can be directly obtained by the metal casting forming process. The alloying element content of such alloys in aluminum alloy castings is generally more than that of the corresponding deformed aluminum alloys.
Classification of cast aluminum alloys Advantages and disadvantages of cast aluminum alloys
1. Classification of cast aluminum alloys
According to the difference in main alloying elements, there are four types of cast aluminum alloys.
(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 most used 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 alloys, containing 4.5% to 5.3% copper, have the best strengthening effect. Appropriate addition of manganese and titanium can significantly improve 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 lowest density (2.55g/cm3) and the highest strength (about 355MPa), containing 12% magnesium, and the strengthening effect is the best. 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 deformed aluminum alloys, and have the same strengthening mechanism as deformed aluminum alloys (except for strain strengthening). Their main difference is that the content of alloying element silicon in cast aluminum alloys exceeds that of most deformed aluminum alloys Silicon content in aluminum alloys. In addition to containing strengthening elements, cast aluminum alloys must also contain a sufficient amount of eutectic elements (usually silicon), so that the alloy has considerable fluidity and is easy to fill the shrinkage joints of the castings during casting. At present, there are only the following 6 basic alloys;
① AI-Cu alloy, ② AI-Cu-Si alloy, ③ AI-Si alloy, ④ AI-Mg alloy, ⑤ AI-Zn-Mg alloy, ⑥ AI-Sn alloy.
2. The advantages and disadvantages of cast aluminum alloy
Advantages of cast aluminum alloy:
(1). Good product quality: high dimensional accuracy of castings, generally equivalent to 6~7 grades, or even up to 4 grades; good surface finish, generally equivalent to 5~8 grades; high strength and hardness, and the strength is generally 25 higher than that of sand casting. ~30%, but the elongation is reduced by about 70%; dimensionally stable and good interchangeability; it can die-cast aluminum thin-walled and complex castings. For example, the small wall thickness of the current zinc alloy die-casting aluminum parts can reach 0.3mm; the aluminum alloy castings can reach 0.5mm; the small casting hole diameter is 0.7mm; the small pitch is 0.75mm.
(2). High production efficiency: high machine productivity, for example, a domestic JIII3 horizontal cold air die-casting aluminum machine can die-cast aluminum 600-700 times on average in eight hours, and a small hot-chamber die-casting aluminum machine can die-cast aluminum 3000-7000 times on average every eight hours; The die-casting aluminum mold has a long service life. One pair of die-casting aluminum molds and die-casting aluminum bell alloys have a lifespan of hundreds of thousands of times or even millions of times; it is easy to realize mechanization and automation.
(3). Excellent economic effect: due to the precise size of die-casting aluminum parts, the surface is smooth and clean. Generally, it is not used directly without mechanical processing, or the processing volume is very small, so it not only improves the metal utilization rate but also reduces a large number of processing equipment and man-hours; castings are cheap; combined die-casting aluminum can be used with other metal or non-metallic materials. . Saves both assembly man-hours and metal.
Disadvantages of cast aluminum alloy:
A. Oxidation of slag inclusions
Defect features: Oxidation slag inclusions are mostly distributed on the upper surface of the casting, at the corners of the casting mold that are not ventilated. Fractures are mostly gray-white or yellow, which can be found by X-ray fluoroscopy or machining and can also be found in alkali cleaning, pickling, or anodizing.
cause:
1. The charge is not clean, and the amount of returned charge is too much
2. Poor design of gating system
3. The slag in the alloy liquid is not cleaned up
4. Improper pouring operation, bringing in slag inclusions
5. The standing time after refining and metamorphism is not enough
Prevention method:
1. The charge should be sandblasted, and the amount of returned charge should be appropriately reduced
2. Improve the design of the gating system and improve its slag holding capacity
3. Use appropriate flux to remove slag
4. When pouring, it should be stable, and pay attention to slag blocking
5. After refining, the alloy liquid should be allowed to stand for a certain period of time before pouring.
B. Stomatal bubbles
Defect characteristics: The pores in the wall of the three castings are generally round or oval, with a smooth surface, usually shiny oxide skin, and sometimes oily yellow. Surface pores and air bubbles can be found by sandblasting, and internal air pores and air bubbles can be found by X-ray fluoroscopy or mechanical processing.
cause:
1. The pouring alloy is not stable, and the gas is involved
2. Forming (core) sand mixed with organic impurities (such as coal dust, grass-roots horse manure, etc.)
3. Poor aeration of molds and cores
4. There are shrinkage holes on the surface of the cold iron
5. Poorly designed gating system
Prevention method:
1. Correctly control the pouring speed and avoid getting involved in the gas.
2. The molding (core) sand shall not be mixed with organic impurities to reduce the gas generation of the molding material
3. Improve the venting capacity of (core) sand
4. Correct selection and handling of cold iron
5. Improved gating system design
C. shrinkage
Defect characteristics: shrinkage porosity of aluminum castings generally occurs in the thick part of the root of the fly riser near the inner runner, the thickness transition of the wall, and the thin wall with a large plane. In the as-cast state, the fracture is gray, and the light yellow is gray-white, light-yellow, or gray-black after heat treatment. It is cloudy on the X-ray film. Serious filamentous shrinkage can be found by X-ray, fluorescence low-magnification fracture, and other inspection methods.
cause:
1. Poor riser feeding
2. Charge gas content is too high
3. Overheating near the runner
4. Too much moisture in the sand mold, and the sand core is not dried
5. Alloy grains are coarse
6. Improper placement of the casting in the mold
7. The pouring temperature is too high and the pouring speed is too fast
Prevention method:
1. Refill the molten metal from the riser to improve the design of the riser
2. The charge should be clean and free of corrosion
3. The riser is set at the shrinkage of the casting, and the cold iron or the cold iron is used in combination with the riser.
4. Control sand moisture, and core drying
5. Take measures to refine grains
6. Improve the position of the casting in the mold Reduce the pouring temperature and pouring speed
D. crack
Defect Features :
1. Casting cracks. It develops along the grain boundary, often accompanied by segregation. It is a kind of crack formed at a higher temperature. It is easy to appear in alloys with larger volume shrinkage and castings with more complex shapes.
2. Heat treatment cracks: Transgranular cracks are often caused by overheating or overheating during heat treatment. Alloys with large stress and thermal expansion coefficient are often cooled too much. or other metallurgical defects
cause:
1. The structural design of the casting is unreasonable, there are sharp corners, and the thickness of the wall changes too much
2. Sand mold (core) has a poor concession
3. Local overheating of mold
4. Pouring temperature too high
5. Premature removal of castings from molds
6. Heat treatment is overheated or overburned, and the cooling rate is too extreme
Prevention method :
1. Improve the structural design of castings, avoid sharp corners, strive for uniform wall thickness, and smooth transition
2. Take measures to increase the concession of sand (core)
3. Ensure simultaneous solidification or sequential solidification of all parts of the casting, and improve the design of the gating system
4. Appropriately reduce the pouring temperature
5. Control mold cooling time
6. The thermal correction method is used when the casting is deformed
7. Correctly control the heat treatment temperature and reduce the quenching cooling rate
3. Porosity analysis of die casting defects
Among the defects of die castings, many pores appear.
Stomatal features. There are smooth surfaces and the shape is round or oval. The manifestations can be on the surface of the casting, in subcutaneous pinholes, or inside the casting.
(1) Gas source
1) Precipitated gas from alloy liquid—a is related to raw materials and b is related to the smelting process
2) The gas involved in the die-casting process ¬—a is related to the die-casting process parameters b is related to the mold structure
3) The release agent decomposes to generate gas ¬—a is related to the characteristics of the coating itself; b is related to the spraying process
(2) Analysis of raw materials and gas generated in the smelting process
The gas in the molten aluminum is mainly hydrogen, accounting for about 85% of the total gas.
The higher the melting temperature, the higher the solubility of hydrogen in molten aluminum, but the solubility in solid aluminum is very low, so during the solidification process, hydrogen is precipitated to form pores.
Source of hydrogen:
1) Water vapor in the atmosphere, the metal liquid absorbs hydrogen from humid air.
2) The raw material itself contains hydrogen, the surface of the alloy ingot is wet, and the return material is dirty and oily.
3) Tools and flux are wet.
(3) Analysis of the gas generated during the die-casting process Because the pressure chamber, gating system, and cavity are all connected to the atmosphere, and the molten metal is filled with high pressure and high speed, if the orderly and stable flow state cannot be achieved, the molten metal will generate eddy currents, which will The gas gets involved.
The following issues need to be considered in the formulation of the die casting process:
1) Whether the molten metal flows cleanly and smoothly in the gating system without separation and eddy currents.
2) Are there sharp corners or dead zones?
3) Is there any change in the cross-sectional area of the gating system?
4) Are the positions of the exhaust groove and overflow groove correct? Is it big enough? Will it be blocked? Can the gas be discharged effectively and smoothly?
The application of computer simulation of the filling process is to analyze the above phenomena and to make judgments to select reasonable process parameters.
(4) Analysis of coating gas generated coating performance: if the gas generation amount is large, it will have a direct impact on the porosity of the casting.
Spraying process: Excessive use, resulting in a large amount of gas volatilization, too much lubricant on the punch, or scorched, all are the source of the gas.
(5) Solutions to die-casting blowholes
First, analyze what causes the stomata, and then take corresponding measures.
1) Dry and clean alloy material.
2) Control the melting temperature, avoid overheating, and carry out degassing treatment.
3) Reasonable selection of die casting process parameters, especially the injection speed. Adjust the high-speed switching start point.
4) Sequential filling is conducive to the discharge of cavity gas, and the sprue and runner have sufficient length (>50mm) to facilitate the smooth flow of the alloy liquid and the opportunity to discharge the gas. The thickness of the gate and the direction of the gate can be changed, and the overflow groove and the exhaust groove can be set at the position where the air hole is formed. The sum of the cross-sectional areas of the overflow products cannot be less than 60% of the sum of the cross-sectional areas of the inner gates, otherwise, the slag discharge effect will be poor.
5) Select the coating with good performance and control the spray amount.
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