In the actual production of aluminum die casting, the defects of porosity and oxidation slag are one of the most widespread defects. Explore the source of gas and the method of controlling the size, location and quantity of stomata; The morphology of oxidized slag is an eternal topic in die casting process. At present, there are many enterprises to improve the casting porosity through the mold pouring system design, but there is still a large chunk of gas generated in the aluminum alloy smelting.
一. Gas and impurities in the alloy
- Understanding of holes and air
● Single large air hole: mainly from the barrel, flow passage, cavity, release agent volatilization, generally has nothing to do with the quality of liquid aluminum
● Pinhole: mainly derived from hydrogen in liquid aluminum, when the temperature of liquid alloy drops sharply, the solubility of hydrogen drops sharply, thus forming pinhole from liquid aluminum
● Shrinkage hole: the formation of shrinkage hole has nothing to do with the gas, the main reason is that the alloy temperature is not consistent leading to the different shrinkage speed and the formation of vacuum holes
- Understanding of oxidation slag inclusion
Analysis of formation:
● Primary oxide inclusion: aluminum liquid before die casting
● Primary oxide inclusions: existing in liquid aluminum before die casting (related to melting)
● Secondary oxide inclusions: generated in the pouring process, mostly distributed in the corner of the casting wall and the final solidified part (unrelated to melting)
- Source analysis of gas and oxidized slag inclusion in liquid aluminum
Hydrogen and hydrogenated inclusions in liquid aluminum are mainly derived from the reaction between liquid aluminum and water vapor in furnace gas
Of all the furnace gas components, only hydrogen can be dissolved in large quantities in liquid aluminum. According to the determination, the presence of gas in aluminum alloy, hydrogen accounted for more than 80%, so “gas content” can be regarded as “hydrogen content” synonym
Source of impurities
Aluminum alloy is usually melted in the atmosphere. When the liquid aluminum and the N2, O2, H2O, CO2, CO, H2 and CmHn in the furnace gas come into contact, the process of combination, separation, dissolution and diffusion will occur. The final products of many reactions are mostly AI2O3. The chemical stability of AI2O3 is very high, the melting point is as high as 2015℃±15℃, no longer decomposed in liquid aluminum, is the main oxide inclusion in aluminum castings.
- Solubility of hydrogen
Since most of the gas in the liquid alloy is hydrogen, it is necessary to have a better understanding of the solubility of hydrogen
At the melting point of aluminum, the solubility of hydrogen decreases sharply when it is changed from liquid to solid, reaching ≈0.68mL/(100g) in liquid aluminum and ≈0.03mL/(100g) in solid aluminum. The difference between the two is ≈0.64mL/(100g), which is equivalent to 1.73% of liquid aluminum volume. In short, the solubility of hydrogen increases with the increase of the temperature of the alloy liquid, and changes sharply between the solid and liquid phase
At the melting point of aluminum, the solubility of hydrogen decreases sharply when it is changed from liquid to solid, reaching ≈0.68mL/(100g) in liquid aluminum and ≈0.03mL/(100g) in solid aluminum. The difference between the two is ≈0.64mL/(100g), which is equivalent to 1.73% of liquid aluminum volume. In short, the solubility of hydrogen increases with the increase of the temperature of the alloy liquid, and changes sharply between the solid and liquid phases
The effect of alumina on the solubility of hydrogen:
At the melting temperature, the surface of liquid aluminum will react with furnace gas to form AI2O3, AI2O3 and alloy liquid contact surface is dense, and the contact surface of furnace gas is rough and contains water gas and impurities. The higher the content of alumina in liquid aluminum, the higher the solubility of hydrogen (both the higher the hydrogen content).
Effects of alloying elements on hydrogen solubility:
● The higher the magnesium element in the alloy, the higher the solubility of hydrogen
● The higher the silicon and copper content, the lower the solubility of hydrogen
Effect of time on solubility:
Smelting aluminum alloy in the atmosphere, aluminum liquid is constantly oxidized, the longer the smelting time, the more oxidation inclusions generated, the more serious the suction. Therefore, in production, the principle of “rapid smelting” should be followed to avoid the long-term stay of molten aluminum in the furnace
二.In addition to gas
- Analysis of hydrogen precipitation process
Formation of bubbles → floating → breakthrough of oxide film → overflow of melt
Bubble travel analysis:
Aluminum oxide is used as the surface of nuclear attachment, and the bubbles are formed by passing into the external inert gas. Hydrogen diffuses into the bubbles
Analysis of buoyancy:
The aluminum liquid caused by the bottom of the molten pool to the liquid surface directional movement, travel convection, can improve the diffusion coefficient of hydrogen, increase the diffusion speed of hydrogen, promote the rapid generation of bubbles, grow up at the same time, quickly float to the liquid surface and be removed, improve the degassing effect.
The hydrogen bubble escaping through the surface oxide film is the final stage of dehydrogenation. The surface oxide film of liquid aluminum has dense structure and high strength. Therefore, the rate of bubble escape depends on the oxide film structure present at the phase interface. The solvent that can break and dissolve the oxide film on the surface can eliminate the barrier of bubble escape and improve the dehydrogenation rate of liquid aluminum.
Summary of effective ways to improve degassing effect:
● Increase the number of bubbles as far as possible, reduce the bubble diameter as far as possible, increase the effective contact area between liquid aluminum and bubble parts.
● Reduce oxide, according to the principle of alumina and hydrogen solubility.
● Increase convection
- Degassing measures – rotating rotor air injection method
Principle of rotating rotor air injection method:
Inert gas through the dry, through the rotating nozzle into the aluminum alloy solution to form bubbles, hydrogen and oxidation slag adsorption on its surface, with the buoyancy of the bubbles excluded.
Analysis of characteristics:
● Break the bubble, make the bubble diameter smaller.
● The bubble rotates and rises under the combined action of buoyancy and centrifugal force
● The horizontal radial rotation of the rotor, basically will not have too much damage to the oxide film.
● Adjustable bubble size, floating bubble up to Φ2 – Φ3 can directly break through the surface tension of the alloy liquid and oxide film escape.
- Auxiliary measures
- Function: dissolve and destroy the surface oxide film.
2, the solvent requirements: not with liquid aluminum chemical reaction, do not dissolve each other
The melting point is lower than the refining temperature, and the fluidity is easy to form a continuous covering layer on the surface of the liquid aluminum to protect the liquid aluminum. The best melting point is higher than the firing and injection temperature, which is easy to remove slag.
It can absorb and dissolve
3, commonly used solvents; NaCL, NaF, KCI, Na3AIF6, Na2SiF6, CaF2, etc.
- Adjustment of alloy properties
Sodium modification is an effective measure to improve the microstructure of eutectic AI-Si alloy, which can effectively improve the strength and plasticity of casting.
● Dosage: In order to cover liquid aluminum and considering that NaF can not be completely decomposed, the dosage is generally 1% — 3%. For low silicon content, high diameter ratio of crucible or metal casting is desirable lower limit, otherwise take the upper limit, but need to prevent deterioration at this time.
● Operation method: In degassing stage, Al-Si alloy can directly cover the surface with solvent.
Observation port: when metamorphism is insufficient, the port grain is coarse, the bright silicon grain is clear and countable, and the whole fracture is dark gray. Normal metamorphic fracture, very fine grain without silicon bright spot
Pattern identification: The surface is bright before metamorphism, and the surface is composed of Na2O film after metamorphism, and the pattern formed after the rupture of NaO film is distributed, and the color is dark. The thicker the pattern is, the more developed the branches are, and the more complete the metamorphism is.
Four, smelting operation rules
Furnace preheating → charging → melting → check before furnace → adjusting composition → refining and slag removal → temperature regulation → pouring into the holding furnace