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Die Casting Aluminum Alloy Defects And Analysis Solutions
Jul 17, 2016

Nitrous oxide slag
Defect characteristics: oxidizing slag mostly distributed in the upper surface of the casting, the parts of the corner of the mold without ventilation. The fracture showed mostly off-white or yellow, x-ray or found when machining can also be found in the alkaline cleaning, pickling, or anodized
Reason:
1. Burden dirty, the back charge to use excessive
2. Poor gating system design
3. Alloy liquid slag is not clean
4. Improper operation of pouring into the slag
5 standing time is not enough. Refined modification


Prevention method:
1. Charge after blowing sand, scrap appropriate to reduce the use of
2. To improve gating system design, to improve its ability to slag
Appropriate flux to slag
4. It should be smooth and pouring slag should be noted
5. Refined before pouring the liquid alloy should be allowed to stand for a certain period of time


Second stomatal bubbles
Defect feature: the pores within the wall of the three castings generally round or oval-shaped, and has a smooth surface, usually the oxide skin is shiny, and sometimes an oil yellow. Surface porosity, bubbles can be found by the blast, the internal porosity bubbles through X ray or machining to find stomata bubbles on X-ray film was black
Reason:
1. Casting alloys are not smooth, involved in gas
2. Type (core) sand mixed with organic impurities (such as coal dust, grass-roots horse manure, etc.)
3. Mold and sand core poor ventilation
4. Cold iron surface cratering
5. Poor gating system design
Prevention method:
1. Correctly grasp the casting speed, and avoid getting involved in the gas.
2. Shall not be mixed with organic impurities to reduce gas evolution of the shape of the material type (core) sand
3. The improvement (core) sand exhaust ability
4. Correct selection and processing of cold iron
5. Improved gating system design


Three Shrinkage
The defect characteristics: aluminum casting shrinkage generally produce including runner near fly riser roots Heavy parts at the wall thickness adapter and has a large flat-screen thin-walled office. Fracture cast gray, light yellow after heat treatment for pale yellow or gray-black in the x-ray film was cloudy filamentous shrinkage grave was found by X-ray fluorescence low fracture inspection methods <br>
Reason:
1. Poor riser Feeding role
2. Burden containing gas too
3. Within runner near overheating
4. Sand too much water, the sand core is not drying
5. Coarse grain alloy
6. Improper casting mold
7. Pouring temperature is too high, too fast pouring
Prevention method:
1. Pouring liquid metal from the riser, improved riser design
2. Charge should be clean and free of corrosion
3. The casting shrinkage porosity at the set riser, placed cold iron or cold iron riser associated with
4. Control molding sand, water, and sand core drying
5. Measures to refine grain products
6. Improvement of the position of the casting mold to reduce the pouring temperature and casting speed


Four cracks
Defect characteristics:
1. Casting crack. Along grain boundaries, often accompanied by segregation, is a crack is formed at higher temperatures in the volume shrinkage larger alloys and more complex shape castings prone
2. Heat treatment cracks: heat treated to burn or overheating caused, often transgranular cracks. Often produces stress and thermal expansion coefficient larger alloy cooling drastic. Or the presence of other metallurgical defects arising
Reason:
1. Unreasonable casting structure design, sharp corners, wall thickness change is too great
2. Poor sand (core) concession
3. Cast local overheating
4. Pouring temperature is too high
5. Remove from mold castings premature
6. Hot or burning heat treatment, cooling rate excesses
Prevention method:
1. To improve the structural design of castings, avoid sharp corners, wall thickness strive uniform, smooth transition
2. Measures to increase the sand (core) Climb
3. To ensure casting solidification or solidification, improved gating system design
4. Appropriate to reduce the pouring temperature
5. Control mold cooling out time
6. Casting correction method using thermal deformation
7. Correctly control the heat treatment temperature, reducing the speed of the quench and cooling
Stomatal analysis


Die casting defects, most stomata.
Stomatal characteristics. Have a smooth surface, the shape is circular or elliptical. Manifestations can be provided in the casting surface, or subcutaneous pinhole, may be provided in the internal casting.
(1) the source of gas
1) alloy liquid precipitation related to gas-a raw material b and smelting processes
2) involved in the gas - a casting process parameters b and mold structure die casting process related
3) release agent decomposition of gas - a & Coatings characteristics about b spraying process
(2) raw materials and smelting process gas analysis
Hydrogen gas in liquid aluminum, accounting for about 85% of the total gas.
The higher the melting temperature, the hydrogen solubility in the liquid aluminum in the higher, but the solubility in solid aluminum is very low, so in the solidification process, hydrogen precipitated to form pores.


The source of hydrogen:
1) atmospheric water vapor, liquid metal hydrogen absorption from the moist air.
2) hydrogen content in the raw materials themselves, wet the surface of the alloy ingots, scrap dirty oil.
3) tools, moisture flux.
(3) die-casting process gas analysis
Gating system due to the pressure chamber, the cavity are vented to the atmosphere, the liquid metal in a high-pressure, high-speed filling, if you can not achieve an orderly, steady flow state, the liquid metal eddy will gas volume inside.
The die casting process design needs to consider the following questions:
1) liquid metal pouring system ability to clean, smooth flow, not separation and eddy.
2) there is no the cusp area or dead zones exist?
3) whether there are changes in the cross-sectional area of the gating system?
4) exhaust ducts overflow tank position is correct? Is big enough? Whether it will be blocked? The gas can effectively smooth discharge?
The application of computer simulation of the filling process, that is, in order to analyze the above phenomenon, for the judgment to choose reasonable parameters.
(4) coating a gas analysis
Coating performance: as big casting porosity, gas evolution.
Spraying process: excessive usage, resulting in a large amount of volatile gases, too much punch lubricants, or charred, are the source of the gas.
(5) to solve the die casting porosity approach


The first analysis of what causes the stomata, come back to take the appropriate measures.
1) dry, clean alloy material.
2) control the melting temperature to avoid overheating, in addition to gas processing.
3) reasonable choice of casting process parameters, especially the speed of injection. Adjust the high-speed switching starting point.
4) filled in the order conducive cavity gas discharge, the sprue and runner is of sufficient length (> 50mm), in order to facilitate the smooth flow and the gas of the liquid alloy, the opportunity to discharge. Can change the thickness of the gate, the gate directions overflow tank is provided, the location of the formation of pores, the exhaust groove. Overflow product cross-sectional area not less than the sum of inner gate cross-sectional area of the sum of 60%, otherwise poor slagging.
5) Select a good performance coatings and control the amount sprayed.


Ideas to solve the defects
Causes a defect from a number of different factors, the actual production to solve the problem in the end face of many reasons the Appraisement first tune the machine? Or the first refueling? Or modify the mold first? Proposed according to the degree of difficulty, Simple complex to deal with its order:
1) to clean parting surfaces, clean the cavity, clean up mandrel; improve paint to improve spraying process; increase the clamping force, increase the amount of casting metal. These measures can be implemented by simple operation.
2) adjusting the process parameters, injection pressure, injection speed, filling time, mold, pouring temperature, mold temperature.
3) refueling, select high quality aluminum alloy ingots, changing the proportion of new material and scrap, improved smelting process.
4) Modify the mold, to modify the gating system to increase within the gates, additional overflow tank, exhaust ducts.


For example, die castings The flash of reasons:
1) die-casting machine: clamping force adjustment does not.
2) process issues: the injection speed is too high, the formation pressure shock peak too high.
3) mold problems: deformation, the parting surface debris, inserts, sliders wear is not flush with the template strong enough. Fly side measures order: clean parting surface → → → adjusting the process parameters repair mold wear parts → improve mold stiffness increase clamping force. From easy to difficult, each step improved test its effects first, not second step.

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