Defect feature: Oxidized slag is mostly distributed on the upper surface of the casting, at the corner where the mold is not ventilated. Fractures are mostly grey or yellow, found by x-ray or during mechanical processing and can also be found in caustic washing, pickling or anodizing
cause:
1. The charge is not clean, excessive use of returned charge
Poor injection system design
3. The slag in the alloy liquid is not removed
4. Improper pouring, slag inclusion
5. Insufficient standing time after refining and deterioration
Prevent method:
1. Burden should be blown, and the amount of recycled material should be properly reduced
2. Improve the design of the pouring system and improve its slag blocking capacity
3. Use appropriate flux to remove residue
4. Pouring should be smooth and should pay attention to slag blocking
5. Pre-cast alloy liquid should be left standing for a certain time after refining
Two air bubbles
Defect feature: The three-casting wall pores are generally round or oval, with a smooth surface, usually shiny oxide, and sometimes oily yellow. Surface pores and bubbles can be found by sandblasting. The internal pores can be found by X-ray or mechanical processing. The pores are black on the X-ray film.
cause:
1. Pouring alloy is not smooth and gets involved in gas
2. Type (core) sand mixed with organic impurities (such as coal dust, grass root horse dung, etc.)
3. Poor mold and core ventilation
4. Cold iron surface has shrinkage holes
5. Bad pouring system design
Prevent method:
1. Correctly control the pouring speed and avoid getting caught in gas.
2. No organic impurities can be mixed in the type (core) sand to reduce the amount of gas generated from the molding material
3. Improve the exhaust capacity of (core) sand
4. Correct selection and handling of cold iron
5. Improved pouring system design
Three shrink loose
Defect feature: The aluminum alloy casting shrinkage generally occurs in the vicinity of the inner runner, where the roots of the riser are thick, where the wall is thick and thin, and where there is a large plane. In the as-cast state, the fracture is gray, and the light yellow is grayish yellow or grayish black after heat treatment. The severe filamentous shrinkage on the x-ray negative film can be found by inspection methods such as X-ray and fluorescence low magnification fractures <br>
cause:
1. Poor feeding effect
2. Too much charge gas
3. Overheating near the sprue
4. Excessive moisture in sand, sand core not dried
5. Alloy grain coarse
6. Improper placement of the casting in the mold
7. Pouring temperature is too high, casting speed is too fast
Prevent method:
1. Refill metal liquid from riser and improve riser design
2. The charge should be clean and free from corrosion
3. Set up the riser at the shrinkage part of the casting, place the cold iron or cold iron together with the riser
4. Control sand moisture, and sand core drying
5. Take measures to refine products
6. Improve the position of the casting in the mold Reduce the pouring temperature and casting speed
Four cracks
Defect features:
1. Cast cracks. Along the grain boundary development, often accompanied by segregation, is a crack that forms at a higher temperature and tends to appear in alloys with larger volume shrinkage and more complex shapes
2. Heat treatment cracks: Due to overheating caused by heat treatment or overheating, cracks often appear through the crystal. Often the alloys that produce stress and have a large thermal expansion coefficient are overdone. Or when there are other metallurgical defects
cause:
1. Casting structure design is irrational, there are sharp corners, the thickness of the wall changes too much
2. Bad sand type (core) conceded
3. Overheated mold
4. Pouring temperature is too high
5. Remove casting prematurely from cast
6. Heat treatment or overheating, excessive cooling rate
Prevent method:
1. Improve casting structure design, avoid sharp corners, wall thickness strives for uniform, smooth transition
2. Take measures to increase the concession of sand type (core)
3. To ensure that all parts of the casting are solidified at the same time or sequentially, improving the design of the pouring system
4. Properly reduce the pouring temperature
5. Control mold cooling time
6. Thermal correction method for casting deformation
7. Correctly control the heat treatment temperature and reduce the quench cooling rate
Stomatal analysis
Among the defects in die castings, there are more pores.
Stomatal characteristics. There is a smooth surface, the shape is round or oval. The form of expression can be on the casting surface, or subcutaneous pinholes, and may also be inside the casting.
(1) Gas source
1) Precipitation gas of alloy liquid—a related to raw materials b related to smelting process
2) The gas involved in the die casting process is related to the parameters of the die casting process b. It is related to the structure of the die.
3) The release agent decomposes to produce gas. -a is related to the properties of the coating. b. Related to the spraying process.
(2) Gas analysis of raw materials and smelting process
The gas in aluminum liquid is mainly hydrogen, which accounts for about 85% of the total gas.
The higher the melting temperature, the higher the solubility of hydrogen in the aluminum liquid, but the solubility in the solid aluminum is very low, so during the solidification process, hydrogen precipitates to form pores.
Hydrogen source:
1) Water vapor in the atmosphere. Molten metal absorbs hydrogen from humid air.
2) The amount of hydrogen contained in the raw material itself, the surface of the alloy ingot is wet, the material returned to the furnace is dirty, and the oil is dirty.
3) The tool and flux are wet.
(3) Analysis of gas generated during die casting
Since the pressure chamber, the pouring system, and the cavity are all in contact with the atmosphere, the metal liquid is filled with high pressure and high speed. If the orderly and stable flow state cannot be achieved, the metal liquid will generate eddy currents and the gas will be entrained.
The following issues need to be considered when formulating the die casting process:
1) Whether the molten metal can flow cleanly and smoothly in the gating system will not produce separation and turbulence.
2) Is there a sharp corner or dead area?
3) Does the gating system have a change in cross-sectional area?
4) Is the exhaust slot and overflow slot in the correct position? Is it big enough? Will it be blocked? Can gas be discharged efficiently and smoothly?
The use of a computer to simulate the filling process is to analyze the above phenomena and make a judgment to select a reasonable process parameter.
(4) Gas Analysis of Coatings
Coating properties: If the gas volume is large, it has a direct effect on the porosity of the casting.
Spraying process: excessive use, resulting in a large amount of gas volatilization, too much punch lubricant, or scorched, are all sources of gas.
(5) Solution to Blowholes in Die Castings
First analyze what causes the stomata, and then take the appropriate measures.
1) Dry, clean alloy material.
2) Control the smelting temperature to avoid overheating and degassing.
3) Reasonably select the die casting process parameters, especially the injection speed. Adjust the starting point for high-speed switching.
4) Sequential filling is beneficial to the discharge of gas from the cavity, and the sprue and runners are of sufficient length (>50mm) to facilitate the smooth flow of the alloy fluid and the opportunity for gas to escape. The thickness of the gate, the direction of the gate, and the overflow slot and exhaust slot can be set at the position where the air hole is formed. The total cross-sectional area of ​​the overflow product must not be less than 60% of the total cross-sectional area of ​​the in-gate. Otherwise, the slag discharge effect is poor.
5) Select a good performance paint and control the spray amount.
The solution to the flaw
As each kind of defect originates from a number of different influencing factors, it is necessary to solve the problem in actual production. In the face of many reasons, in the end is the non-advanced first adjustment? Or first reload? Or modify the mold first? According to the degree of difficulty, it is simple and complicated to deal with, its order:
1) Clean the parting surface, clean the cavity, clean the ejector rod; improve the coating and improve the spraying process; increase the clamping force and increase the amount of pouring metal. These can be implemented by simple operations.
2) Adjust process parameters, injection force, injection speed, mold filling time, mold opening time, pouring temperature, mold temperature, etc.
3) Refueling, select high-quality aluminum alloy ingots, change the ratio of new material and return material, and improve the smelting process.
4) Modify the mold, modify the gating system, increase the inner gate, add overflow tanks, exhaust slots, etc.
For example, the causes of flashing in die castings are:
1) Die casting machine problem: The clamping force is not adjusted correctly.
2) Process problems: The injection speed is too high and the pressure shock peak is too high.
3) Mold problems: deformation, debris on the parting surface, wear of the inserts and sliders, and insufficient strength of the template. Resolve the order of the flashing: clean the parting surface → increase the clamping force → adjust the process parameters → repair the worn parts of the mold → increase the mold stiffness. From easy to difficult, every step to improve, first test its effect, not to proceed to the second step.
Die Casting Common Defects Influencing Factors
Common Influencing Factors
Undercast Bubble Deformation Shrinkage Pore Crack Cold Blanket Residue Viscosity Scratch Factor Category Origin
比压√ √ B die casting machine
Shot speed √ √ B
Pressure build time √ √ B
Pressure chamber fullness √ √ B
1-2 speed transfer point √ √ √ B
Solidification time √ √ B
Mold temperature √ √ √ √ C mold
Exhaust from the mold √ √ √ A
Pouring system is incorrect √ √ A
Poor mold surface treatment √ √ A
Casting slope is not enough √ √ √ √ A
Casting hardness is not enough √ √ A
Pouring temperature √ √ C On-site operation
Pouring amount of metal √ C
Metal impurities √ C
Coatings √ √ √ √ √ √ √ C
Note: Type A factor: depends on mold design and manufacturing.
Class B factors: Mostly depend on die casting machine performance and die casting parameters selection