Cast aluminum alloy is an aluminum alloy that can directly obtain parts by metal casting forming process. The content of alloy elements in aluminum alloy castings is generally higher than that of corresponding deformed aluminum alloys.
Classification of casting aluminum alloys
1、 Classification of cast aluminum alloys
There are four types of cast aluminum alloys according to the difference of main alloy elements.
(1) Aluminum silicon alloy is also called "silicon aluminum alloy" or "silicon aluminum alloy". The alloy with good casting property and wear resistance, small thermal expansion coefficient, the largest variety and amount of aluminum alloy in casting, and silicon content of 4%~13%. Sometimes, silicon aluminum alloy with 0.2% ～ 0.6% magnesium added is widely used in structural parts, such as shells, cylinder blocks, boxes and frames. Sometimes, the mechanical properties and heat resistance of the alloy can be improved by adding an appropriate amount of copper and magnesium. This kind of alloy is widely used to make piston and other parts.
(2) Aluminum copper alloy and alloy containing 4.5%~5.3% copper have the best strengthening effect. Proper addition of manganese and titanium can significantly improve the room temperature, high temperature strength and casting properties. It is mainly used to make sand castings with large dynamic and static loads and uncomplicated shapes.
(3) Aluminum magnesium alloy, the casting aluminum alloy with the lowest density (2.55g/cm3) and the highest strength (about 355MPa), containing 12% magnesium, has the best strengthening effect. The alloy has good corrosion resistance in the atmosphere and sea water, and has good comprehensive mechanical properties and machinability at room temperature. It can be used as radar base, aircraft engine case, propeller, landing gear and other parts, and also as decorative materials.
(4) Al Zn alloy is often added with silicon and magnesium to improve its performance, which is often called "Zn Si Al Ming". Under casting conditions, the alloy has quenching effect, namely "self quenching". It can be used without heat treatment. After modification and heat treatment, the castings have high strength. After stabilization, the size is stable, and it is often used to make models, templates and equipment supports.
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 in most wrought aluminum alloys. In addition to the strengthening elements, the cast aluminum alloy must also contain enough eutectic elements (usually silicon), so that the alloy has considerable fluidity and is easy to fill the contraction joint of the casting during casting. At present, there are only six 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、 Advantages and Disadvantages of Cast Aluminum Alloys
Advantages of cast aluminum alloy:
1. Good product quality: high dimensional accuracy of castings, generally equivalent to level 6~7, or even level 4; Good surface finish, generally equivalent to Grade 5~8; The strength and hardness are relatively high. The strength is generally 25~30% higher than that of sand casting, but the elongation is about 70% lower; Stable size and good interchangeability; Die casting aluminum thin-wall complex castings. For example, the minimum wall thickness of current zinc alloy die-casting aluminum parts can reach 0.3mm; Aluminum alloy casting can reach 0.5mm; The minimum casting hole diameter is 0.7mm; The minimum pitch is 0.75 mm.
2. High production efficiency: high machine productivity. For example, the domestic J Ⅲ 3 horizontal cold air die casting aluminum machine can die cast aluminum 600~700 times per eight hours on average, and the small hot chamber die casting aluminum machine can die cast aluminum 3000~7000 times per eight hours on average; The service life of die-casting aluminum mold is long. One pair of die-casting aluminum mold and die-casting aluminum bell alloy can reach hundreds of thousands of times, 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 for machining, or the processing amount is very small, so it not only improves the utilization rate of metal, but also reduces a lot of processing equipment and man hours; The casting is cheap; Combination die cast aluminum and other metal or non-metallic materials can be used. It saves both assembly time and metal.
Disadvantages of cast aluminum alloy:
A Oxidation slag inclusion
Defect characteristics: oxidation slag inclusions are mostly distributed on the upper surface of the casting, at the corners where the mold is not ventilated. The fracture is mostly grayish white or yellow, which can be found through x-ray fluoroscopy or during machining, or during alkali cleaning, acid cleaning or anodizing
Cause:
1. The furnace charge is not clean, and the amount of recycled material is too much
2. Poor design of gating system
3. The slag in the alloy liquid is not removed completely
4. Slag inclusion caused by improper pouring operation
5. Insufficient standing time after refining and metamorphic treatment
Prevention method:
1. The furnace charge shall be blown with sand, and the usage of the returned material shall be reduced appropriately
2. Improve the design of gating system to improve its slag retaining capacity
3. Use proper flux to remove slag
4. The pouring shall be stable and attention shall be paid to slag blocking
5. The alloy liquid shall be kept for a certain time after refining and before pouring
B Air hole bubble
Defect characteristics: the pores in the wall of three castings are generally round or oval, with smooth surface, generally shiny oxide skin, sometimes oily yellow. The surface pores and bubbles can be found by sandblasting, and the internal pores and bubbles can be found to be black on the X-ray film by X-ray fluoroscopy or machining.
Cause:
1. The casting alloy is unstable, and gas is involved
2. Organic impurities (such as coal chips, grass root horse dung, etc.) are mixed in the molding (core) sand
3. Poor ventilation of mold and sand core
4. Shrinkage hole on the cold iron surface
5. Poor design of gating system
Prevention method:
1. Correctly master the pouring speed to avoid gas entrapment.
2. Organic impurities shall not be mixed in the molding (core) sand to reduce the gas generation of molding materials
3. Improve the exhaust capacity of (core) sand
4. Correct selection and treatment of cold iron
5. Improve the gating system design
C Shrinkage porosity
Defect characteristics: shrinkage porosity of aluminum castings generally results in the thick part of the riser root near the ingate, the thick thin transition part of the wall and the thin wall part with a large plane. In the as cast state, the fracture is gray, and the light yellow is grayish white and light yellow or grayish black after heat treatment. The serious filamentous shrinkage porosity on the x-ray film can be found by X-ray, fluorescence low magnification fracture and other inspection methods.
Cause:
1. Poor feeding effect of riser
2. Too much gas content in furnace charge
3. Overheating near the ingate
4. The sand mold has too much moisture and the sand core is not dried
5. Coarse alloy grains
6. Improper position of casting in the mold
7. The pouring temperature is too high and the pouring speed is too fast
Prevention method:
1. Pour liquid metal from riser and improve riser design
2. The furnace charge shall be clean and free from corrosion
3. A riser shall be set at the shrinkage porosity of the casting, and cold iron shall be placed or used together with the riser
4. Control molding sand moisture and sand core drying
5. Take measures to refine grains
6. Improve the position of the casting in the mold to reduce the pouring temperature and pouring speed
D Crack
Defect characteristics:
1. Casting cracks. It develops along the grain boundary, often accompanied by segregation. It is a kind of crack formed at high temperature, which is easy to appear in alloys with large volume shrinkage and castings with complex shapes
2. Heat treatment crack: It is usually transgranular crack caused by over burning or overheating of heat treatment. The alloy with high stress and thermal expansion coefficient is often cooled excessively. Or other metallurgical defects
Cause:
1. The casting structure design is unreasonable, with sharp corners, and the wall thickness changes too widely
2. Poor yield of sand mold (core)
3. Local overheating of the mold
4. The pouring temperature is too high
5. The casting is taken out too early from the self casting mold
6. The heat treatment is overheated or overburned, and the cooling speed is too high
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 yield of sand mold (core)
3. Ensure that all parts of the casting solidify at the same time or in sequence, and improve the design of the gating system
4. Properly reduce the pouring temperature
5. Control the mold cooling time
6. Thermal correction method shall be adopted in case of casting deformation
7. Properly control the heat treatment temperature and reduce the quenching cooling speed
3、 Porosity Analysis of Die Casting Defects
Air hole is the most common defect in die castings.
Pore characteristics. It has a smooth surface and is round or oval in shape. The expression can be on the surface of the casting, or subcutaneous pinhole, or inside the casting.
(1) Gas source
1) Gases evolved from alloy liquid - a related to raw materials b related to 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 die structure
3) The release agent decomposes to produce gas, a, which is related to the characteristics of the coating itself, b, which is related to the spraying process
(2) Analysis of gas generated from raw materials and smelting process
The gas in aluminum liquid is mainly hydrogen, accounting for about 85% of the total gas.
The higher the smelting temperature is, the higher the solubility of hydrogen in molten aluminum is, but the solubility in solid aluminum is very low. Therefore, during solidification, hydrogen precipitates to form pores.
Source of hydrogen:
1) Water vapor in the atmosphere, liquid metal absorbs hydrogen from moist air.
2) The hydrogen content of raw materials, the surface of alloy ingots is wet, and the returned materials are dirty and greasy.
3) Tools and fluxes are wet.
(3) Analysis of gas generated in die casting process Since the pressure chamber, gating system and mold cavity are all connected with the atmosphere, and the liquid metal is filled at high pressure and speed, if the orderly and stable flow state cannot be achieved, the liquid metal will generate eddy current, which will roll in the gas.
The following issues need to be considered when formulating the die casting process:
1) Whether the liquid metal can flow cleanly and stably in the gating system without separation and eddy current.
2) Are there sharp corners or dead zones?
3) Does the gating system have a change in cross-sectional area?
4) Are the exhaust grooves and overflow grooves positioned correctly? Is it big enough? Will it be blocked? Can the gas be effectively and smoothly discharged?
The purpose of computer simulation of filling process is to analyze the above phenomena and select reasonable process parameters for judgment.
(4) Analysis of coating gas generation: if the gas generation is large, the porosity of the casting will be directly affected.
Spraying process: excessive use will result in large gas volatilization, too much punch lubricant, or scorching, which are all sources of gas.
(5) Solution to Air Hole in Die Castings
First analyze what causes the blowholes, and then take corresponding measures.
1) Dry and clean alloy.
2) Control smelting temperature, avoid overheating and conduct degassing treatment.
3) Reasonably select die casting process parameters, especially injection speed. Adjust the starting point of high-speed switching.
4) Sequential filling is conducive to the cavity gas discharge. The sprue and runner have sufficient length (>50mm) to facilitate the smooth flow of the alloy liquid and the opportunity for gas discharge. The thickness and direction of the gate can be changed, and the overflow groove and exhaust groove can be set at the position where the air hole is formed. The total sectional area of overflow products shall not be less than 60% of the total sectional area of the ingate, otherwise the slag removal effect is poor.
5) Select the paint with good performance and control the spraying amount.
