Iron has always been considered a major harmful impurity in cast aluminum alloys. Various countries and professional standards have clearly restricted it, and the standards of various enterprises have stricter controls on it. This is mainly because as the iron content increases, needle and flaky brittle iron phases with high hardness will be formed in the metallographic structure. Its existence splits the matrix of the aluminum alloy and reduces the mechanical properties of the alloy, especially the toughness. It also makes the machining of parts more difficult, with severe wear of knives and cutting tools, poor dimensional stability, etc. However, the iron content in low-quality aluminum alloy ingots is inherently high. With the reuse of alloy charge, iron crucibles, tools, and presets are required to be produced. The use of parts, etc. makes alloy iron increase inevitable. It has attracted the majority of foundry workers to study for many years. The role of iron in Al-Si alloy and its weakening and elimination countermeasures will be discussed below.
1 The role of iron in cast Al-Si alloys
1.1 The existence form of iron in cast Al-Si alloys
Table 1 shows aluminum-silicon alloys Among the existing forms of iron, α-AlFeSi and β-AlFeSi are the two common forms. However, ρ-AlMgFeSi and δ-AlFeSi are not very common. Among them, the crystal structure characteristics of AlFeSi and Al(Fe, Cr)Si are currently not very detailed. As for what kind of phase is formed, in addition to the iron content in the alloy, it is also closely related to the cooling rate of the casting, the amount and type of alloying elements, etc. Compared with Al-Si alloys, the Chinese character-shaped α-AlFeSi can increase the strength and hardness, but does not reduce the toughness much, while the needle-shaped β-AlFeSi phase severely splits the matrix and significantly reduces the toughness of the alloy, especially the impact toughness. According to reports, When Fe>1%, the entire alloy itself can become brittle.
Table 1 Iron phase morphology in Al-Si alloys
Category Crystal structure Melting temperature/°C Shape α-AlFeSi hexagonal crystal 860 Kanji-shaped β-AlFeSi single crystal 870 Needle, flake ρ -AlMgFeSi cubic crystal δ-AlFeSi tetragonal crystal 1.2 Effect of iron on the mechanical properties of aluminum-silicon alloy
1.2.1 Effect on room temperature mechanical properties
On Al-Si For binary alloys, when Fe>0.5%, the flaky β phase can improve the strength of the alloy and slightly reduce its elongation; when Fe>0.8%, the elongation begins to decrease significantly. When Fe increases from 0.4% to 1.2%, the increase in strength is minimal, but it significantly reduces the elongation from 4% to 1%. For Na-modified Al-Si crystalline alloy Every 0.1% increase in Fe can reduce the elongation by more than 1%.
1.2.2 Effect on high-temperature properties
Although iron reduces the room-temperature mechanical properties of Al-Si piston alloy, it improves its high-temperature mechanical properties. This is mainly Since the strength of the matrix itself decreases a lot with the increase of temperature at high temperatures, the iron phase existing in the form of network, Chinese characters and fine needles is basically unchanged at around 316°C and is a stable compound phase. The presence of it increases the tensile strength of the sample at high temperatures. For Al-Si-Cu-Mg alloy, when Fe>0.95%, σ300℃ is 92MPa.
1.2.3 Effect on wear resistance and corrosion resistance
Iron improves the wear resistance of Al-Si alloys. This is due to the hard acicular iron phase. The matrix is ??strengthened to resist deformation and at the same time plays a supporting role to improve wear resistance. At the same time, the iron phase causes the oxide film on the surface of the alloy to lose continuity, making electrochemical corrosion prone to occur. Iron reduces the corrosion resistance of the alloy.
1.2.4 Effect on casting properties
As the iron content increases, when the alloy crystallizes, the β phase interferes with the flow between dendrites, so the porosity increases. At the same time, the hot cracking tendency of the alloy is increased. However, for die-cast aluminum alloys, a certain amount of Fe can prevent mucosa. However, there are also reports that a certain amount of Fe increases the fluidity of the alloy.
1.2.5 Effect on machining performance
The iron phase deteriorates the machining performance, increases the wear of cutting tools, and worsens the dimensional stability.
2 Methods to eliminate and inhibit the harmful effects of iron
2.1 Mechanical methods
Commonly used mechanical iron removal methods include filtration, precipitation, and centrifugal casting Methods, etc., they all use alloying elements such as Mn, Cr, Ni, Zr, etc. to be added to the melt to form large compounds with iron. Since its density is different from that of aluminum alloy, precipitation will occur. The method of using precipitation is called precipitation method. , which can reduce iron by 0.5%. The method of filtering bulk compounds through filter cloth, filter mesh, and plate is called the filtration method. It can reduce Fe by 0.7%. The melt with alloying elements added will, under the action of centrifugal force, due to the density d The difference causes the iron phase to move to the edge, while the internal iron content can be reduced from 2.07% to 0.27%, with a reduction efficiency of 87%. Different rotation speeds and different Fe/Mn ratios also have an impact on iron removal efficiency. Mechanical methods used in production are generally used in combination, such as filtration and precipitation, precipitation first and then filtration, and the combination of filtration and centrifugal casting will achieve better results.
2.2 Melt treatment method
2.2.1 Adding alloy elements to neutralize the effect of Fe (modification treatment)
Add alloy to the melt Elements are used to change the morphology of the iron phase, weaken the effect of iron, increase the strength of the alloy, and improve the elongation. The commonly added elements are: Mn, Cr, Co, Be, Mo, Ni, S, Mg, Re, etc., which are analyzed one by one below:
a. Mn: It is the most commonly used element. Adding Mn can significantly reduce the number and size of the iron phase, and even make the iron phase disappear completely. Since the addition of Mn expands the α-iron phase area, the iron phase transforms into the α-iron phase. , the amount of Mn added to neutralize the iron phase cannot yet be determined. It is said that adding 0.5% Mn to Al-Si13 alloy can transform the needle and flake iron phase into alpha iron in the alloy containing 1.5% Fe. Some people recommend adding Mn according to Mn%=2 (%Fe-0.5). In short, by adding Mn, the number and size of the β-Fe phase can be gradually reduced until it no longer appears.
b. Cr: Adding Cr to ZAlSi7Mg alloy can transform the coarse flaky β phase into a Chinese character α iron phase. Adding 0.2% to 0.6% Cr can prevent the brittleness of Al-Si13 alloy containing Fe>1%. Breaking, adding 0.2% to 0.3% Cr to the Al-5Si-1.5Cu-0.5Mg alloy increases the elongation of the alloy containing 0.4% iron from 1.7% to 3.8 %, adding 0.4Cr can increase the elongation of the alloy containing 0.75% Fe from 0.8% to 2.6%.
c. Co: The role of Co is similar to Mn, but it needs to be added slightly to make the iron-rich phase spherical. Some people suggest that the ratio of Fe/Co should be 1:2. At the same time, the segregation of Co added to itself is small, so its effect is excellent. Yu Mn.
d. Be: It can also be used as a neutralizing agent. When the amount of Be added is >0.4%, a tight AlFeBe phase can be formed. At the same time, because Be is a good antioxidant, it can improve the performance of Al alloys. Sand castings can increase the tensile strength of AlSi0.6Mg alloy by 5% to 10% without reducing its elongation. It is also reported that adding 0.05% to 0.5% Be to Al-6Si alloy will The shape of the Fe impurity phase changes from long needles to less harmful spherical or nearly spherical shapes, thereby improving the plasticity of the alloy.
e. Mo: can be used to neutralize the harmful effects of Fe. Its effect is better than Mn. It is an effective modifier for Fe in Al-Si alloy. Add 0.2% Mo and 0.1% to an alloy containing 1.2% Fe. % S can increase the elongation of the alloy from 1% to 2.8%, and the tensile strength from 160MPa to 180MPa.
f. Mg: It can also neutralize the harmful effects of impurity iron. When the content reaches a certain level, an AlFeSiMg compound phase will be formed, thereby reducing the formation of β iron phase.
g． Ni and S: are also neutralizers for the harmful effects of iron. S can also be used as a modifier for aluminum alloys. It is reported that adding sulfur can make most of the iron phase into short rods and Chinese characters, and a small amount into spherical and lump shapes. shape. However, the effect is not ideal when added alone. It must be combined with other elements such as Mn, Cr, rare earths, etc. to achieve obvious effects.
h． Rare earth RE: Rare earth is a good Fe phase modifier. It is reported that adding 0.04% to 0.06% Sr to 413 alloy can effectively reduce the number and size of β iron phase. For 6063 alloy, when adding After adding 0.05% Sr, the existing iron phase compounds took on the shape of Chinese characters and were refined. Japanese patents have also reported that adding 0.005% to 0.10% Sr and the same amount of Zn can reduce the number and size of beta iron, and it has been confirmed in many Al-Si series and profile alloys. This is mainly due to RE It is a kind of modifier and alloy purifier. Its addition can effectively remove the harmful effects of iron.
In short, for deterioration neutralizer, it can reduce and eliminate the formation of β-iron phase, but it cannot remove the harmful effects of Fe by itself. It only has a slowing effect, and the dosage of deterioration used increases with the amount of Fe. It also increases and reduces the toughness of the alloy to a certain extent. Moreover, the formation of various complex compounds will bring other related side effects. Therefore, we advocate the use of modifiers, and the use of complex comprehensive modifiers, adding as little as possible .
2.2.2 Melt superheating and rapid cooling treatment
a. Melt overheating
It is reported that overheating treatment can reduce the nucleation core of the iron-rich phase. This is because the nucleation core of the β-iron-rich phase at high temperatures is γ (Al), and γ (Al) It exists at low temperatures. When the temperature reaches a certain level (≥85°C), the γ (Al) phase transforms into α (Al), which is not conducive to the nucleation of the β iron phase, thereby inhibiting the emergence of the β iron phase. At the same time, it was found that as the superheat of the melt increases, the iron-rich intergranular compounds in the casting become finer. When the pouring temperature is greater than 800°C, the flaky β iron phase in the alloy transforms into the α iron phase, and this process is irreversible. , that is, once the melt is overheated to a temperature sufficient to produce the α phase, subsequent treatment and standing have no effect on the morphology of the iron phase, and when the amount of iron is higher, it becomes more and more difficult to change it by superheating. In actual operation, because the melt absorbs air after overheating and is severely oxidized, it is rarely used.
b. Rapid cooling treatment
Rapid cooling treatment can weaken the harmful effects of iron. This is recognized by everyone. This is why the quality of sand casting stipulated in the national professional standards is smaller than that of metal molds. During rapid cooling, there are many nucleation cores in the alloy liquid, and the interface advancement speed is fast. Under the same conditions, the harmful iron phase formed should be short and thin, and even the needle-like phase cannot be seen. At the same time, the Mn required in the alloy to neutralize the Fe phase The amount also changes with the cooling rate during the solidification process, and the cooling rate also has a great influence on the Fe phase morphology. When the cooling rate is <0.1°C/s, it helps to form the β-iron phase; when the cooling rate is >10°C/s, it inhibits the formation of β-iron phase.
3 Discussion
(1) Should the Fe content in the alloy meet the national standard?
Under the condition of alloying treatment method and increasing cooling rate, we can reduce or even eliminate the harmful effects of the needle-like iron phase, so that its organizational properties can meet the requirements of the national standard. At this time, the iron content in the alloy has exceeded the standard. It even seriously exceeds the standard. So should the ingredients be the main focus at this time, or should the performance be the main focus? We advocate that the harmful effects of Fe have been eliminated, and its content or iron content equivalent (that is, the iron content at this time is equivalent to the usual national standard) should be used as a reference only, mainly based on organizational properties, and the ingredients should not have veto power. Compared with foreign foundry developed countries, the Fe content stipulated in my country's national standards is obviously stricter than abroad, so we hope that corresponding standards will emerge in my country's professional industry standards.
(2) How to reduce the harmful effects of iron in production?
In actual production, overheating is rarely used because it will cause severe burning loss of elements and serious air inhalation. However, centrifugal casting requires centrifuge and other equipment, which is still suitable for professional alloy manufacturers. And ordinary manufacturers cannot invest in equipment just for it. The most practical and feasible method is alloying and modification treatment and increasing the cooling rate. In modification treatment, the use of small addition amounts with compound effects should be promoted. One element has multiple functions or several element compound agents. At the same time, mechanical and Composite processing of deterioration methods.