1. Die casting aluminum alloy, part of the cast aluminum alloy, due to the requirements of the die casting process, so the composition of the die casting alloy must be strictly controlled, and the chemical composition of the alloy should be kept within the specified range in each link of the die casting production.
Aluminum alloy classification:
Cast aluminum alloy according to its main components, can be divided into Aluminum die castings four types, namely cast aluminum-silicon alloy, cast aluminum-copper alloy, cast aluminum-magnesium alloy and cast aluminum-zinc alloy.
2. Casting aluminum alloy performance characteristics:
1. Good casting performance;
2. Low density (2.5-2.9g /cm3), high intensity (δ0/r);
3. Good heat resistance, wear resistance, thermal conductivity and electrical conductivity;
4. Aluminum-silicon alloy has a tendency to stick, and some grades have poor cutting performance;
5. Serious corrosion of metal crucible;
6. Large volume shrinkage rate, easy to produce shrinkage holes;
3.Aluminum-silicon alloys (5.25%)
The properties of AL-Si alloys are characterized by a small crystallization temperature interval due to the silicon phase contained in the alloy. Silicon has a large latent heat of solidification (396 CAL/g, while aluminum is only 1/3-1/4). The test shows that AL-Si eutectic still maintains good plasticity near its freezing point, which is not found in other aluminum alloys, so the casting performance of AL-Si alloy is generally better than that of other aluminum alloys, its filling capacity is also better, and the tendency of hot cracking and hot loosening is relatively small. Moreover, AL-Si alloy has better plasticity than other aluminum alloys.
Because of the above reasons, AL-Si alloy not only has good casting performance, but also good machining performance, foreign die-casting aluminum alloy grades, the most used is AL-Si alloy, such as the United States A360,A380,A384, Japan ADC10 and ADC12
4.The effects and effects of each element in die casting aluminum alloy are as follows:
- Silicon:
Silicon is the main element of most die-cast aluminum alloys, which can improve the casting properties of the alloy. Silicon and aluminum can form a solid solution, at 577℃, silicon in aluminum melting degree is 1.65%, at room temperature is 0.2%, silicon content to 11.7%, silicon and aluminum form a eutectic. The high temperature molding property of the alloy is improved, the shrinkage rate is reduced, and there is no tendency of hot cracking. When the silicon content in the alloy exceeds the eutectic component, and the impurities such as copper and iron are too much, there is a hard spot of free silicon, which makes the cutting and processing difficult, and the high-silicon aluminum alloy has a serious corrosion effect on the cast iron crucible. And silicon damage anode gasification, not left in the casting surface treatment.
2) Copper:
Copper and aluminum form a solid melt, and when the temperature is 5480C, the solubility of copper in aluminum should be 5.65%, and it drops to about 0.1% at room temperature. Increased copper content. It can improve the fluidity, tensile strength and hardness of the alloy, but reduce the corrosion resistance and plasticity, and the tendency of hot cracking is large.
3) Magnesium:
Adding a small amount of magnesium (about 0.2~0.3%) to high silicon aluminum alloy can improve the strength and yield limit, and put forward the machinability of the alloy. When the mass fraction of Mg in AL alloy is greater than 4%, it is easy to produce oxygen pole oxide, which is manifested as flow marks and markings on the casting.
Aluminum alloy containing magnesium 8% has good corrosion resistance, but its casting performance is poor, the strength and plasticity are low at high temperature, and the shrinkage is large when cooling, so it is easy to produce hot cracking and the formation of loose.
4) Zinc:
Zinc in aluminum alloy can improve fluidity, increase thermal brittleness, reduce corrosion resistance, so the content of zinc should be controlled within the specified range.
5) Iron:
In all aluminum alloys contain harmful impurities, because the iron content in aluminum alloys is too high, iron to FeAl3, Fe2Al7t and AL-ST-Fe sheet or needle structure exists in the alloy, reducing mechanical properties, this organization will also reduce the fluidity of the alloy. Enthusiasm increases, but because the adhesion of aluminum alloy to the mold is very strong, especially when the iron content is below 0.6%, when more than 0.6%, the adhesion phenomenon is greatly reduced, so the iron content should be controlled in the range of 0.6% —- 1% is beneficial to die casting, but the maximum can not exceed 1.5%.
6) Manganese
Manganese in aluminum can reduce the harmful effects of iron, can be combined with the aluminum alloy formed by iron sheet or needle structure into a fine crystal structure, so the general aluminum alloy allows the presence of less than 0.5% of manganese. When the manganese content is too high, it will cause segregation.
7) Nickel
Nickel in aluminum alloy can improve the strength and hardness of the alloy, reduce the corrosion resistance. The function of nickel and iron is the same, it can reduce the corrosion of the alloy on the mold, and at the same time, it can neutralize the harmful effect of iron and improve the welding performance of the alloy. When the nickel content is 1% —- 1.5%, the casting can be polished to obtain a glossy surface. Due to the lack of nickel sources, nickel-containing aluminum alloys should be used as little as possible.
5. Smelting of aluminum alloy
Non-ferrous alloys have the characteristics of low melting point, easy to overheat, easy to inhale and oxidize. For the die casting process, the alloy lasts a long time in the molten state, and the ladling soup is frequent. And the use of return charge occupies a certain proportion, these are easy to cause the alloy quality change and produce adverse results. Therefore, it is necessary to strictly abide by the alloy melting process specifications.
Alloy melting is an important part of the die casting process, metal solid into molten alloy liquid, and from liquid to solid alloy or casting, this is a complex physical and chemical reaction process, it affects the chemical composition of the alloy to varying degrees, physical and chemical properties, as well as mechanical and casting properties, follow the correct and reasonable process procedures for alloy melting, Is a prerequisite for quality die casting.
The melting process includes a certain proportion of raw materials, which are loaded into the melting furnace to melt into a liquid, through slag removal, degassing refining, metamorphic treatment, etc., so that the alloy liquid is as pure as possible, ensuring that the composition meets the standard, the grain refinement and uniform organization, and the need to have the appropriate temperature to meet the requirements of the casting process.
High-quality aluminum alloy liquid should have the following three conditions:
1) The chemical composition meets the standard and the composition is uniform.
2) Gas, oxidizing inclusion, flux slag inclusion is less, does not lead to the formation of pores and slag inclusion in the casting.
3) Good metamorphism, fine organization, casting can obtain good crystalline structure.
1. Smelting process of cast aluminum alloy (as shown below) :
Note: Figure 1 (central melting furnace).
Figure 2 (edge melting and holding furnace)
2. Measures to improve the quality of aluminum alloy
In the process of alloy melting, in order to improve the quality of alloy liquid, the following aspects should be paid attention to:
1) Prevent iron increase
In most aluminum alloys, iron is a harmful impurity. The so-called iron increase is that during smelting, the iron content in the aluminum alloy increases, which is caused by the use of improperly treated iron crucible and tools in smelting, and the continuous reuse of die-cast aluminum alloy return charge. When aluminum alloy liquid is in contact with them, the melting of iron will occur, the melting speed of iron is very fast, and with the increase of melting temperature and the growth of holding time, the melting amount of iron continues to increase, the higher the temperature, the longer the time, the more iron will increase, seriously affecting the quality of aluminum alloy liquid.
Measures to prevent iron increase are:
① All iron crucible and tool surfaces in contact with liquid aluminum must be cleaned regularly and sprayed with appropriate special paint to avoid direct contact with liquid aluminum, especially cast iron crucible.
②Choose the lowest possible melting temperature to avoid unnecessary overheating and long-term moisturizing.
2) Prevent oxidation
During the heating process, the surface of aluminum alloy is quickly oxidized, and the oxidation is intensified with the increase of temperature. As the generated oxide film is mainly insoluble oxide AL2O3. Dense coating on the surface of the alloy liquid, this layer of oxide film can prevent the continued oxidation of aluminum alloy liquid, but when the temperature exceeds 900℃, the density of the film significantly decreases, and the oxidation reaction violently increases, the oxidation of aluminum alloy liquid will produce the following adverse consequences:
① Cause the alloy to burn.
② Deterioration of alloy casting properties. Such as reducing the filling capacity, increase the formation of holes, cracks and other tendencies.
③ Reduce mechanical properties, especially impact toughness and fatigue limits.
④ Deterioration of cutting performance.
3) Prevent inhalation
① Water gas: it comes from furnace gas, insufficiently dried charge, refining agent, covering agent, metamorphic agent, insufficiently dried charge, coating on crucible and tool, and aqueous solvent remaining on crucible, tool and charge. These water gas reacts with aluminum to form 2AL+3H2O——AL2O3+6H to produce hydrogen, and hydrogen enters aluminum liquid in atomic state.
② The oil comes from the charge and tools with oil, the reaction of oil and aluminum:
4AL+CnHn=CnAL4+Hn
Hydrogen is produced, and hydrogen enters the aluminum liquid in an atomic state.
(3) There are water-bearing corrodes on the charge, such as AL2O3,H2O,AL2O3,3H2O and AL(OH)3, etc. These corrodes are decomposed by heat, and water gas is released, which reacts with AL to produce hydrogen.
④ refining
Aluminum alloy in the melting process, the removal of non-metallic inclusions (various solid oxides) and gas processes, generally called “refining” refining is a very important process in the alloy melting process.
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