In actual production of aluminum die-casting, pores and oxide inclusions are one of the most common defects. Among them, source of gas and method of controlling size, position and number of pores, and form of oxide inclusions are eternal topics in die-casting process. At present, many suppliers have many ways to improve pores of castings through design of mold casting systems, but there is still a relatively large gas generated in smelting of aluminum alloys. This article mainly introduces source, cause, understanding of gas, degassing, and slag removal in smelting of die-casting aluminum alloys.

1. Understanding of pores and gas
Gas will produce pores, but are all pores produced by gas or are gases in pores from same source? We make a simple classification based on source and cause of formation of gas in pores:
Single large pore: mainly from volatilization of barrel, runner, and cavity release agent, which is generally unrelated to quality of aluminum liquid.
Pinhole: mainly from hydrogen in aluminum liquid. When alloy liquid temperature drops sharply, solubility of hydrogen drops sharply, thus precipitates from aluminum liquid to form pinholes.
Shrinkage cavity: Shrinkage cavity formation has nothing to do with gas. Main reason for its formation is that inconsistent alloy temperature leads to different shrinkage speeds, thus forming vacuum holes.
2. Understanding of oxide inclusions
Analysis of oxide inclusion formation:
Primary oxide inclusions: Already present in aluminum liquid before die casting (related to smelting).
Secondary oxide inclusions: Generated during pouring process, mostly distributed at corners of casting wall and last solidified part (not related to smelting)
3. Analysis of source of gas and oxide inclusions in aluminum liquid
Hydrogen and oxide inclusions in aluminum liquid mainly come from reaction between aluminum liquid and water vapor in furnace gas.
Gas source: Among all furnace gas components, only hydrogen can be dissolved in large quantities in aluminum liquid. According to measurements, hydrogen accounts for more than 85% of gas present in aluminum alloys, so "gas content" can be regarded as a synonym for "hydrogen content".
Source of impurities: Aluminum alloys are usually melted in atmosphere. When aluminum liquid contacts N2, O2, H2O, CO2, CO, H2, CmHn, etc. in furnace gas, chemical combination, chemical separation, dissolution, diffusion and other processes will occur. Most of final products of many reactions are Al2O3. Al2O3 has extremely high chemical stability and a melting point of up to 2015±15°. It no longer decomposes in aluminum liquid and is main oxide inclusion in aluminum castings.
4. Solubility of hydrogen
Most of gas in the alloy liquid is hydrogen, so it is necessary to have a deep understanding of solubility of hydrogen.
Solubility: When melting point temperature of aluminum changes from liquid to solid, solubility of hydrogen will drop sharply. Solubility of hydrogen in liquid aluminum is ∼0.68 mL/(100g)
, while that in solid aluminum is only ∼0.036mL/(100g), a difference of ∼0.64mL/(100g), equivalent to 1.73% of volume of aluminum liquid. Simply put, solubility of hydrogen increases with increase of alloy liquid temperature, and changes dramatically when solid-liquid phase is transformed.
Effect of alumina on solubility of hydrogen: At melting temperature, surface of aluminum liquid will react with furnace gas to form Al2O3. Contact surface of Al2O3 with alloy liquid is dense, contact surface with furnace gas is rough, contains moisture and impurities. The higher content of alumina in aluminum liquid, the higher solubility of hydrogen will be (that is, the higher hydrogen content).
Effect of alloying elements on hydrogen solubility: The higher magnesium content in alloy liquid, the higher solubility of hydrogen;
The higher silicon and copper content, the lower solubility of hydrogen;
Effect of time on solubility: When smelting aluminum alloy in atmosphere, aluminum liquid is constantly oxidized. The longer smelting time, the more oxide inclusions are generated, and the more serious inhalation is. Therefore, in production, principle of "fast smelting" should be followed to avoid aluminum liquid staying in furnace for a long time.

Having a certain understanding of solubility of hydrogen, we further understand process of hydrogen formation, escape from alloy liquid to formulate a scientific and effective degassing method.
1. Analysis of hydrogen precipitation process
Forming bubbles → floating → breaking through oxide film → escaping aluminum liquid
Bubble formation analysis: With aluminum oxide as core attached to surface, bubbles are formed by introducing external inert gas, and hydrogen diffuses into bubbles.
Floating analysis: Aluminum liquid moves from bottom of insulation bag to liquid surface in a directional manner, forming convection, which can increase diffusion coefficient of hydrogen, increase diffusion rate of hydrogen, promote rapid generation and growth of bubbles. At the same time, they quickly float to liquid surface and are removed, thereby improving degassing effect.
Escape analysis: Escape of hydrogen bubbles in aluminum liquid through surface oxide film is final stage of dehydrogenation process. Surface oxide film of aluminum liquid is dense and has high strength. Therefore, speed of bubble escape depends on oxide film structure existing on phase interface. Flux that can break and dissolve surface oxide film can eliminate barrier for bubble escape, increase dehydrogenation rate of aluminum liquid.
Effective ways to improve degassing effect are summarized as follows:
Increase number of bubbles as much as possible, reduce bubble diameter as much as possible, increase effective contact area between aluminum liquid and bubble;
Reduce oxides;
Increase convection;
2. Degassing measures-rotating rotor jet method
There are dozens of degassing methods at present, such as vacuum melting, electromagnetic stirring, etc. Here we mainly introduce the one that is currently used more by die-casting enterprises-rotating rotor jet method.

 

2.1. Principle of rotating rotor jet method: After inert gas is dried, it enters aluminum alloy solution through rotating nozzle, forming a large number of dispersed bubbles. Hydrogen and oxide inclusions are adsorbed on its surface and discharged by buoyancy of bubbles.
Its principle diagram is as follows:

 

Feature analysis:
1) Break bubbles to reduce bubble diameter.
2) Bubbles rotate and rise under combined action of buoyancy and centrifugal force (both floating time and distance are increased, and probability of small bubbles merging is reduced).
3) Rotor rotates horizontally and radially, which basically does not cause much damage to oxide film.
4) Bubble size can be adjusted. When floating bubbles reach Φ2~Φ3, they can directly break through surface tension of alloy liquid and oxide film.

 

Introduction to main structural components of equipment
1. Nozzle structure: Blade adopts a radial straight nozzle, bubble is well refined, dispersion is high, and it is easy to cause depression, but an intermediate baffle can be added to prevent depression. Nozzle diameter is generally 1/4 of crucible diameter, and bubbles are emitted from all parts of liquid surface.
2. Nozzle material comparison: cast iron or steel nozzles are generally not used, mainly because iron will contaminate melt.
1) Graphite nozzles are pollution-free and low-cost and can be used;
2) Life of ceramics is 4-5 times longer than that of graphite, which can be considered as appropriate;
3) Titanium alloys are expensive, have long life, are pollution-free and can be used as appropriate;
3. Spray rod: Material is the same as nozzle (both are immersed in melt);
2.2. Rotary degassing process requirements:
1) Inert gas purity: Argon is selected as high-purity argon, and nitrogen is selected as ultra-pure nitrogen; general purity requirement is 99.99%. In addition, no matter how to choose, moisture and air content must be measured, then dried (silica gel, calcium chloride) treatment must be carried out. Before passing inert gas, cover solvent to destroy oxide film to facilitate escape of hydrogen.
2) Coating:Nozzle and spray rod need to be painted to extend their life. In actual production process, rotor diameter needs to be controlled, and nozzle outlet hole needs to be checked for blockage.
3) Temperature: As temperature rises, viscosity of aluminum melt decreases, which makes it easier for bubbles to escape, but air intake will increase. Taking all factors into consideration, casting temperature is generally selected to rise by 20~30℃.
4) Nozzle insertion depth: Try to extend it to the bottom, 100mm~150mm from bottom of crucible, so as to increase jet flow rate, increase gas contact time with aluminum liquid and purification efficiency.
5) Nozzle speed: As speed increases, bubble diameter decreases, but it will aggravate tumbling and depression of liquid surface. Taking all factors into consideration, buffer measures (adding baffles and forward and reverse rotation) can be taken, which can be set to 450-550r/min.
6) Forward and reverse rotation: Effectively prevent liquid surface from sinking and causing air intake. The one-way rotation time can be controlled within 10s.
7) Blowing pressure: Standard is that aluminum liquid does not flow back into spray rod, and 0.1-0.2Mpa can be taken.
8) Gas flow: Flow rate is related to nozzle speed, insertion depth, and gas pressure. It is generally 0.25-0.4m3/h; pay attention to flow monitoring and degassing effect (observe bubble size and bubbling range).
9) Blowing time: Generally, it is about 10 minutes according to aluminum liquid capacity. In actual production, in order to prevent re-inhalation, it is generally blown until it is transferred to insulation furnace or directly die-cast.
10) Standing: Floating time of bubbles takes about 3-5 minutes, and it takes 10-15 minutes to ensure that remaining gas escapes.

3.1, Refining and slag removal solvent
1) Function: dissolve and destroy surface oxide film.
2) Requirements for solvents:
a. Do not chemically react with aluminum liquid, nor dissolve each other.
b. Melting point is lower than refining temperature, and fluidity is good. It is easy to form a continuous covering layer on the surface of aluminum liquid to protect aluminum liquid. It is best that melting point is higher than pouring temperature to facilitate slag removal.
c. It can absorb, dissolve and break Al2O3 inclusions.
d. It has abundant sources and is cheap.
3) Processability requirements of refining solvents:
a. Good covering property: that is, spreading property, which refers to ability of flux to automatically spread on the surface of aluminum liquid to form a continuous covering layer.
b. Good separation performance: performance of flux and aluminum liquid to automatically separate, separation performance is good, slag is easy to remove, flux is not easy to mix into aluminum liquid and pour into casting, and it will not cause flux to include slag.
c. Strong refining performance: refers to ability of flux to absorb, dissolve and break oxidized inclusions in aluminum liquid, that is, ability to remove slag and degassing.
4) Commonly used refining flux:
Common flux components include: NaCl, NaF, KCl, Na3AlF6, Na2SiF6, CaF2, etc. Different components are made of flux in different proportions, with different melting points, different surface properties and different process properties to meet different requirements.
3.2. Filtration treatment: There are generally two types of slag filtration through filtering devices:
One type is inactive filter agents, such as graphite blocks, magnesium chips bricks, glass fibers, etc., which rely on mechanical action to remove non-metallic inclusions in aluminum liquid.
The other type is active filter agents, such as NaF, CaF2, Na3AlF6, etc., which mainly remove oxidized inclusions by adsorption and dissolution of Al2O3 in addition to mechanical action.

Various process measures to eliminate pores, remove oxidized inclusions, thus reduce scrap rate of aluminum castings can be summarized as "prevention", "discharge" and "dissolution".
Prevention: Mixing of other impurities such as moisture, oxygen, and oil.
Exhaust: Remove gas and slag through refining.
Dissolve: Increasing pressure can dissolve hydrogen in casting, so as not to form pores, and prevention is main thing.