Refining and refining of recycled aluminum

When the metal melting component is adjusted, the next step is the aluminum refining process. The purpose of aluminum alloy refining is to obtain high cleanliness and low gas content alloy liquid after degasification and impurity removal measures are taken. Refining has the following methods:

Chloride (ZnCl2, MnCl2, AlCl3, C2Cl6, TiCl4, etc.), venting (passing N2, Cl2, or a mixture of N2 and Cl2), vacuum treatment, addition of a non-toxic refining agent, and ultrasonic treatment.

According to its principle, the refining process has two functions: for dissolved hydrogen, it mainly relies on diffusion to remove hydrogen from the aluminum liquid, and oxide inclusions are removed mainly by the addition of flux or bubbles to the surface of the medium.

(a) out of gas

In general, the floatation method is used for degassing. The principle is that a certain hydrogen-free gas is introduced into the aluminum liquid to generate bubbles, and these bubbles are used to take dissolved hydrogen out of the aluminum liquid during the floatation process and escape into the atmosphere. In order to obtain a better refining effect, the iron pipe for introducing gas should be pushed into the depth of the molten pool as far as possible. The lower end of the iron pipe is 100mm to 150mm away from the bottom of the crucible, so that the stroke of the air bubble rises longer, and it does not sink into the aluminum. Inclusions at the bottom of the liquid stir.

When the gas is introduced, the iron pipe should be slowly moved laterally in the aluminum liquid so that air bubbles pass through the molten pool. Use lower ground pressure and speed as much as possible, because the smaller bubbles thus formed enlarge the surface area of ​​the bubbles, and because the bubbles are small and the float speed is slow, more inclusions and gases can be removed.

At the same time, in order to ensure a good refining effect, the selection of the refining temperature should be appropriate. If the temperature is too high, the generated bubbles are large and float quickly, and the refining effect is deteriorated. When the temperature is too low, the viscosity of the aluminum liquid is relatively large, which is not conducive to the full discharge of the gas in the aluminum liquid, and also reduces the refining effect.

Ultrasonic treatment of liquid aluminum can also effectively degas. Its principle is that by introducing elastic waves into the aluminum liquid, a "hole" phenomenon is caused in the aluminum liquid, thus destroying the continuity of the aluminum liquid structure and producing countless microscopic holes, which are dissolved in the aluminum liquid. Hydrogen rapidly escapes into these cavities and becomes a bubble core, and as it grows up, it escapes as a bubble to escape the aluminum liquid, thereby achieving a refining effect.

(b) Removal of impurities

For non-metallic inclusions, the use of gas refining methods can be effectively removed, and for the more demanding materials can also be used in the casting process using a filter method or the melt through the molten flux layer mechanical filtration, etc. to remove.

For metallic impurities, the general treatment method is to turn harmful factors into favorable factors. That is, it becomes a beneficial second phase by the alloying method so as to facilitate the performance of the material. If it must be removed, in most cases it is the high-temperature and low-pressure selective distillation using different boiling points of different elements to achieve the purpose of removing metal impurities.

Aluminum alloys smelted from aluminum-containing scrap often contain excessive metal elements that should be removed as much as possible. Selective oxidation can be used to remove various metal impurities having a higher oxygen affinity than aluminum and oxygen from the melt. For example, elements such as magnesium, zinc, calcium, and zirconium accelerate the oxidation of these impurity elements by stirring the melt. These metal oxides are insoluble in molten aluminum and enter the slag, so that they can be melted from the aluminum by slagging. Removed from the body.

Metal impurities in the alloy can also be removed by utilizing the difference in solubility. For example, an aluminum alloy contaminated with impurities is co-melted with a metal that can dissolve aluminum without dissolving impurities, and then the aluminum alloy liquid is separated by filtration, and the added metal is removed by vacuum distillation. Aluminum, iron, silicon and other impurities are usually removed by the addition of magnesium, zinc, mercury, and then these added metals are removed by vacuum distillation.

For example, if an aluminum alloy contaminated with impurities is eutecticized with 30% magnesium, the alloy is allowed to stand for a period of time at near-eutectic temperature, and the precipitated crystal phase containing iron and silicon is removed by filtration, and is then vacuum-removed at 850°C. In the case of magnesium, impurities with high vapor pressure, such as zinc and lead, are also removed together with magnesium, and the pure aluminum alloy after magnesium removal can be ingot-cast.

In order to further improve the liquid quality of aluminum alloys, or when certain brands of aluminum alloys require strict control of hydrogen content and inclusions, a combined refining method may be used, that is, two refining methods may be used simultaneously. For example, chlorine salt-filter combined refining and argon-flux combined refining can achieve better results than single refining.

(III) Organization control and degeneration

1. Metamorphic treatment of hypoeutectic and eutectic aluminum-silicon alloys

The silicon phase in the aluminum-silicon alloy eutectic grows into flakes in a self-growth condition, and even a thick, polygonal plate-like silicon phase appears. These forms of silicon phase will severely split the Al matrix at the tip and corners of the Si phase. At locations where stress concentration occurs, alloys tend to crack along the grain boundaries, or plate-like Si itself cracks to form cracks, making the alloy brittle, and the mechanical properties, particularly the elongation, are significantly reduced, and the cutting function is not good. In order to change the existing state of silicon and improve the mechanical properties of the alloy, modification techniques have been used for a long time.

2, metamorphic elements

The metamorphic effect elements of eutectic silicon include: Na, Sr, S, La, Ce, Sb, and Te. At present, the research mainly focuses on several kinds of metamorphic agents such as sodium, strontium, and rare earth.

(1) Sodium metamorphism (Na)

Sodium is the earliest and most effective metamorphic element of eutectic silicon. It can be added in three ways: metal sodium, sodium salt and sodium carbonate.

The initial modifier used for sodium metal is sodium metal. The sodium has the best metamorphic effect. It can effectively refine the eutectic structure and add a small amount (about 0.005% to 0.01%) to the eutectic silicon phase from the needle. The metamorphoses into a completely uniform fibrous form. However, there are some disadvantages in the use of Na metal modification. First, the metamorphic temperature is 740° C., which is close to the boiling point of sodium (892° C.). Therefore, the aluminum liquid easily boils and splashes occur, which promotes the oxidation and absorption of the aluminum liquid and is unsafe to operate. Secondly, the proportion of sodium is small (0.97), and it is enriched in the surface layer of aluminum liquid when it is degraded, causing the upper layer of aluminum liquid to degenerate excessively and the bottom to be deficient in quality. The effect of deterioration is extremely unstable. At the same time, sodium reacts with water vapor to generate hydrogen, which increases the gas content of aluminum liquid. Sodium is chemically very active, easily reacts with oxygen in the air, and is generally stored in kerosene. Kerosene must be removed before use. This is also a difficult task, but it will not be removed. Bring gas and inclusions.

Sodium salt. The commonly used modificator in production is a mixture containing a halogen salt such as NaF, which reacts with sodium to produce sodium and metamorphose. However, these sodium salts can easily bring moisture into the air, which increases the oxidation tendency of the alloy. At the same time, these sodium salts have a corrosive effect on the environment and cause damage to the health of the body.

Sodium carbonate. The metamorphic agent mainly composed of sodium carbonate is a non-environmental damage agent that should be developed by overcoming the environmental problems caused by the deterioration of the above sodium salt. In other words, sodium carbonate and aluminum and magnesium react at a high temperature to produce sodium and undergo metamorphism. The reaction process and reaction products are non-toxic. Similarly, such pollution-free modifiers also have the problem of water absorption and increase the tendency of the aluminum alloy to inhale and oxidize.

The use of sodium metamorphism also has a shortcoming that cannot be ignored, that is, it has a short duration of metamorphic effect and is a non-permanent metamorphic agent. The effective period of sodium salt modifier is only 30min to 60min. After this time, the effect of metamorphism disappears on its own. The higher the temperature, the faster the failure. Therefore, the molten aluminum that must be changed must be used within a short period of time. When remelted, it must be degraded again. Moreover, it is difficult to accurately control the process of sodium metamorphism, so the sodium metamorphism is gradually being replaced by some long-term metamorphic methods.

(2) Deterioration (Sr)

Sr (Sr) is a long-term metamorphic agent with metamorphic effect equivalent to that of sodium, and it does not have the disadvantage of sodium deterioration. It is a promising metamorphic agent. In the United Kingdom, the Netherlands, and other countries, the use of the Sr metamorphic method began in the early 1980s. At present, many studies have been conducted at home and abroad for metamorphic metamorphism. The use of strontium (Sr) instead of sodium or sodium has also been increasing.

Metamorphism has the following advantages: 1 good metamorphic effect, long effective period; 2 metamorphic process, no smoke, no poison, no environmental pollution, no corrosion of equipment, tools, does not harm health, easy operation; 3 easy to obtain satisfactory mechanical properties; 4 The recycled materials have certain remelting and metamorphic effects; 5 castings have high yields and the overall economic benefits are significant. However, practice has shown that metamorphic alloys tend to produce shrinkage, increase the pinhole degree of the castings, reduce the compactness of the alloy, and exhibit a mechanical deterioration.

(3) Deterioration (Sb)

Sb can change eutectic silicon from acicular to lamellar. In order to obtain a lamellar sheet, the optimum addition range is usually 0.15% to 0.2%. Its deterioration is not as good as sodium and cesium. A prominent advantage of twisting metamorphism is a long time of deterioration (more than 8 hours). The melting point of niobium is 630.5°C and the density is 6.68g/cm3. Therefore, it is easier to control the niobium content, and it is not easy to cause deterioration and deterioration, and does not increase the tendency of inhalation and oxidation inclusion of aluminum liquid. However, the effect of its deterioration is greatly affected by the cooling rate, and it has a good metamorphic effect on the metal type and the casting with rapid cooling, but the effect on the slow cooling of the thick-walled sand type casting is not obvious, and the use is subject to certain restrictions.

(4) Deterioration (Te)

Te (Te) is a successful metamorphic agent for Chinese research. The effect of metamorphic metamorphism is similar to that of metamorphic metamorphism, which is to promote the refinement of silicon in the form of flake-like branches, and it cannot be turned into fibrous, and the metamorphic effect is more repulsive. The metamorphic effect has a long-lasting effect and does not change after 8 hours or remelting. Similarly, its metamorphic effect is also greatly affected by the cooling rate.

(5) Deterioration (Ba)

Tantalum has a good metamorphic effect on eutectic silicon. Compared with sodium, barium, and strontium, the metamorphic effect of earthworms is relatively long-lasting. A wide range of adding amounts, adding 0.017% to 0.2% of earthworms, can obtain a good metamorphic structure. After the addition of niobium, the tensile strength of the alloy is significantly increased, continuous remelting is performed, the metamorphic effect can still be maintained, and the metamorphic effect is satisfactory. The insufficiency of the use of thorium metamorphism is that it has a great sensitivity to the wall thickness of the casting, and has poor effect on the deterioration of the thick-walled casting. In order to obtain a good metamorphic effect, it must be cooled quickly. At the same time, niobium is sensitive to chloride and is generally not refined with chlorine or chlorine salts.

(6) Rare earth metamorphism

The countries where rare earths were used in aluminum and aluminum alloys were earlier in Germany, and Germany had successfully used rare earth-containing aluminum alloys during the First World War. Rare-earth elements can achieve similar metamorphic effects as sodium and strontium, which can change eutectic silicon from flakes into short rods and spheres, improving the properties of the alloy. In addition, metamorphism of rare earths has relatively long-term and remelting stability, and its metamorphic effect can be maintained for 5 to 7 hours. After the La metamorphic lifetime was examined, a 0.056% metamorphic alloy containing La was repeatedly melted and solidified 10 times. There are still deteriorating effects.

Rare earth due to its chemical nature of the active nature, easily react with O2, N2, H2, etc., and thus play a role in dehydrogenation, deoxidation, descaling and so on, which can purify aluminum liquid.

In short, rare earths have the dual effects of refining and modification in Al-Si alloys, and their metamorphic effects are quite long-lasting and remelting stable. The addition of rare earth elements improves the fluidity of the alloy, improves the casting properties of the alloy, and optimizes the intrinsic quality of the alloy. Another major advantage is that the addition of rare earth does not produce smoke, does not cause pollution to the environment, and complies with the needs of the development of the times.

(d) Selection of modifiers

At present, the most widely used aluminum alloy casting production is the sodium salt modifier, which consists of sodium and potassium halide salts. The use of such modifiers is reliable and the effect is stable. In the composition of the modifier, NaF can be metamorphic. After the contact with the aluminum liquid, the following reaction occurs: 3NaF+Al→AlF3+3Na, and the sodium generated by the reaction enters into the aluminum liquid, that is, it acts as a metamorphism. Since NaF has a high melting point (992° C.), NaCl and KCl are added to the modifying agent in order to reduce the metamorphic temperature and reduce the suction and oxidation of the aluminum liquid at high temperature. Add a certain amount of ternary ternary modifier composed of NaCl and KCl. The melting point is below 800°C. Under the general metamorphic temperature, it is in the molten state, which is conducive to the deterioration and improves the speed and effect of metamorphism.

In addition, the metamorphic agent in the molten state easily forms a continuous coating layer on the liquid surface, which improves the covering effect of the modifying agent. For this reason, NaCl and KCl are also called fluxes.

Some modifiers include a certain amount of cryolite (Na3AlF6). This modifier has a deteriorating, refining, and covering effect and is generally referred to as a "universal modifier". This modifier is often used when pouring important castings or when the metallurgical quality of the molten aluminum is high.

In production, the deterioration process is generally performed after refining and before casting. The metamorphic temperature should be slightly higher than the casting temperature, and the melting point of the modifying agent is preferably between the metamorphic temperature and the casting temperature so that the metamorphic agent is in a liquid state when it is degraded, and the metamorphic agent can be cast after being deteriorated, so that the metamorphic agent will not deteriorate for a long time. Failure. In addition, after the modification process is completed, the degraded slag has become a very thick solid, which facilitates skimming, so that the residual flux is not poured into the casting mold to form flux slag.

When selecting a modificator, the melting point and metamorphic temperature of the modificator are generally determined according to the required pouring temperature, and then the appropriate modificator component can be selected according to the selected mordant melting point.

(5) Influence of deterioration process factors

The main factors of deterioration process are: metamorphic temperature, metamorphic time, type and amount of modifier.

1. Metamorphic temperature. Higher temperature, favorable for metamorphic reactions, high recovery of sodium, rapid deterioration, and good results. However, the metamorphic temperature should not be too high. If the temperature is too high, the oxidation and absorption of molten aluminum will be drastically increased, and the iron impurities in the molten aluminum will increase, which will reduce the service life of the crucible. In general, the metamorphic temperature should be chosen to be slightly higher than the pouring temperature. This avoids the high metamorphic temperature, can reduce the time to adjust the temperature after deterioration, and is conducive to improving the metamorphic effect and the metallurgical quality of the aluminum liquid.

2. Deteriorating time. The higher the metamorphic temperature and the better the contact of the molten aluminum with the modificator, the shorter the required deterioration time. The deterioration time should be determined on the basis of the experiment according to the specific circumstances. If the metamorphic time is too short, the metamorphic reaction will be incomplete; if the metamorphic time is too long, the burning of the modificator will increase and the inhalation and oxidation of the alloy will increase.

The metamorphic time is composed of two parts: the variator covering time is generally 10 to 15 minutes, and the pressing time is generally 2 to 3 minutes.

3. Kind and amount of modifiers. The type and amount of modificators should be selected according to the type of alloy, the casting process, and the specific requirements for the control of the structure. Selecting non-toxic, non-polluting and long-lasting metamorphic effect modifiers is the current development direction of aluminum alloy melting process.

In the production practice, it should be considered that the mutator reaction may be incomplete, so the amount of modificator should not be too small, otherwise the effect of deterioration is not good. However, the amount of modificators should not be too much, too much will cause deterioration. Therefore, the amount of modifier is generally specified as 1% to 3% of the weight of the charge. In production, usually adding 2% can guarantee good deterioration. For metal castings, the amount of modifier can be reduced. When a general-purpose modificator is used, besides taking into consideration the effect of deterioration, the requirements for the coverage and refining ability of the modificator should also be considered. Usually, the amount of the modificator is 2% to 3% of the weight of the aluminum liquid.

(VI) Pre-inspection of deteriorating treatment

Cast the sample, knock it open after cooling, and judge the effect of deterioration according to the shape of the fracture. If the deterioration is insufficient, the crystal grains are coarse, the fracture surface is dark gray, and bright silicon crystal grains are visible; if the deterioration is normal, the crystal grains are fine, the fracture is white velvet, there is no silicon crystal grain; if the deterioration is excessive The grain is coarse, the fracture is blue-gray, and there is a bright crystal point of silicon.

(7) Modification of hypereutectic Al-Si alloys

Hypereutectic Al-Si alloy due to the large amount of silicon, so that the alloy's thermal expansion coefficient decreases, increased wear resistance, suitable for internal combustion engine piston and other wear parts. Plate-like primary crystal and needle-like eutectic silicon exist in the hypereutectic Al-Si alloy. The primary crystal silicon as a hard point can improve the wear resistance of the alloy, but because it is hard and brittle, it is detrimental to the mechanical properties of the alloy and deteriorates the machining performance of the alloy. Therefore, the total amount of silicon in the hypereutectic Al-Si alloy Both crystalline silicon and primary silicon must be modified.

For a long time, the refinement of primary silicon has been studied in depth. Ultrasonic vibration crystallization method, quenching method, superheat melting, low temperature casting can achieve certain results. However, the effect is the most stable, and the most useful value in the industry is to add a modifying agent.

At present, the modificator actually used for production is phosphorus monolith. Red phosphorus is used earliest. When the amount added is 0.5% of the alloy weight, primary silicon can be refined. However, due to the low ignition point of phosphorus (240°C), the transportation is not safe. When it is degraded, phosphorus will burn fiercely, generate a lot of smoke, pollute the air, and also make the aluminum liquid absorb more gas. Therefore, phosphorus is mixed with other compounds. The industrially more common method is to add in the form of a Cu-P master alloy. The content of phosphorus in the master alloy is generally 8% to 10%, and the addition amount is between 0.5% and 0.8%.

With respect to the mechanism of phosphor modification of Al-Si alloys, it is generally believed that phosphorus forms a large number of AlP particles with high melting point in the alloy solution. The crystal structure of AlP and Si is similar, the lattice constants are similar, and AlP is a sphalerite-type structure. The lattice constant a is 5.451, the melting point is 1060°C, and the lattice constant of a silicon crystal is a=5.428. The distance between AlP and silicon is also very similar. Silicon is 2.44 and AlP is 2.56. AlP can be used as non-spontaneous primary silicon. The core, thereby refining primary silicon.