Cast heat-resistant aluminum alloy is a kind of aluminum alloy which is prepared by casting, has good oxidation resistance and excellent mechanical properties at high temperature, has good creep resistance under certain temperature and load (dynamic or static) for a long time, and has good thermal conductivity and low density. Cast heat-resistant aluminum alloys can be roughly divided into Al Si cast aluminum alloys, Al Cu cast aluminum alloys and Al Mg cast aluminum alloys. However, Al Mg cast aluminum alloys are rarely studied and used because of their dependence on expensive SC strengthening.
1. Al Si cast heat resistant aluminum alloy
Al Si cast aluminum alloy is the most widely used cast aluminum alloy at present, and its production amount accounts for more than 90% of the total amount of cast aluminum alloy. Due to the existence of Si element, Al Si alloys have excellent casting properties such as good air tightness, fluidity, small casting shrinkage and low hot cracking tendency. However, the strength and heat resistance of unalloyed Al Si alloy are not high, and the service temperature is generally not higher than 180 ℃. In actual production, the mass fraction of Si is generally between 4-22 wt.%, and the as cast structure is mainly composed of silicon α- It is composed of Al, primary Si, eutectic Si and heat-resistant phase. Cu, Ni, Mg, Mn and rare earth elements are usually added to achieve the purpose of high-temperature service.
Mg element in α- The solid solubility of Al at room temperature is about 0.34 wt.%, and the limiting solid solubility is about 14.9 wt.%. Two strengthening phases, Mg2Si and mg5si6, can be formed. Among them, Mg2Si is the main strengthening phase of Al Si Mg alloy, which presents network shape in as cast state and fine granular transition phase after heat treatment β Juehe β They are distributed in different areas α- In Al matrix, the room temperature strength of the alloy is effectively improved. However, the thermal stability temperature of m-mg2si is only 180 ℃, which will coarsen rapidly and seriously reduce the heat resistance of the alloy. Therefore, the working temperature of Al Si mg cast heat-resistant aluminum alloy is generally lower than 185 ℃. Cu in α- The solid solubility of Al at room temperature is about 0.2 wt%, and the limiting solid solubility is about 5.65 wt%, which can form the Al with thermal stability up to 225 ℃ θ- CuAl2 is a heat-resistant phase. At the same time, proper heat treatment can make θ- CuAl2 transforms into a dispersed transition phase θ Juehe θ ꞌꞌ, effectively improve the room temperature and high temperature strength of the alloy [16-18]. At present, the most widely used Al Si Cu mg cast heat-resistant aluminum alloy is formed by the co addition of Mg and Cu. This alloy is used in the cylinder block and piston of most automobile engines, so it is also known as “piston alloy”. This kind of alloy has many advantages θ- In addition to CuAl2 and m-mg2si phases, there are complex strengthening phases such as q-al5mg8si6cu2 and w-al4mg5si4cu. The heat resistance temperature of the alloy can reach 250 ℃. The heat resistance of Al Si Cu mg cast aluminum alloy can be further improved by adding Ni element and forming a variety of Ni rich phases.
The composition and properties of Al Si commercial aluminum alloys widely used at home and abroad are summarized. Among them, Al Si mg cast aluminum alloy and Al Si Cu cast aluminum alloy represented by 319, A356 and A380 are widely used in engine cylinder block and cylinder head with stable service environment; Al Si Cu mg cast aluminum alloys, such as m124, m142, ZL117 and A390, are widely used in engine pistons with relatively poor service conditions.
2. Al Cu cast heat resistant aluminum alloy
Compared with Al Si cast aluminum alloy, Cu is the main element in Al Cu cast aluminum alloy, generally in the range of 3 ~ 11 wt.%. The large amount of Cu element makes Al Cu cast aluminum alloy show high thermal stability and good mechanical properties at room temperature and high temperature after heat treatment. However, due to the lack of Si element, the casting fluidity of this kind of alloy is very poor, and it is easy to produce hot cracking and casting defects, with poor corrosion resistance and high coefficient of linear expansion [25]. At present, Al Cu cast aluminum alloys are mostly used in castings with simple shape, thick wall, small volume, and service temperature between 250 ℃ and 350 ℃, such as engine cylinder head and cylinder head.
When the content of Cu is 4.0 ~ 5.0 wt.% θ- CuAl2 and α- The results show that the eutectic structure of Al is a continuous network, while the eutectic structure of Al is a closed network when the content of Cu is more than 10wt.%. After heat treatment, fine and dispersed particles precipitate in the alloy matrix θ Juehe θ These two phases have a coherent / semi coherent relationship with the matrix, which can effectively hinder the movement of dislocations and pin grain boundaries, thus improving the high temperature strength and creep resistance of the alloy. Mn is a strengthening element often added in Al Cu cast heat-resistant aluminum alloy. It mainly exists in the form of Mn rich phase al6mn, t-a120cu2mn3 and t-al12cu3mn2, and t-a120cu2mn3 is the most common. AG is also an effective strengthening element, but it is seldom used because of its high price. The addition of Ag can change the aging precipitation process of Al Cu alloy, so that GP zone is not formed or only a small amount of GP zone is formed at the initial stage of aging, and then the GP zone is formed at the beginning of aging α- The {111} of Al α A new Ω – al2cu phase precipitates on the slip surface, resulting in the formation of Ω – al2cu and a small amount of α – al2cu θ The composite strengthening phase of ꞌ. Compared to θ- The results show that al2cu, Ω – al2cu phases have a more strict co orientation relationship with the matrix, which has a stronger effect on the movement of dislocation, and can significantly improve the mechanical properties of the alloy at room temperature and high temperature. Many studies [29-30] show that SC is the most important element to improve the heat resistance of Al Cu cast aluminum alloy at present, and al3sc phase can be precipitated during heat treatment. It can not only stabilize the substructure of the alloy, but also significantly refine the grains, thus greatly improving the mechanical properties of the alloy at room temperature and high temperature. However, due to the high price of SC, it is rarely used to strengthen aluminum alloy in actual production.
The composition and properties of commercial Al Cu alloys which are widely used at home and abroad are summarized. The Al Cu cast aluminum alloys represented by ZL205A, zl207 and 206 are mostly used in engine parts with relatively simple structure. Among them, the domestic ZL205A Alloy has good room temperature and high temperature properties: the room temperature strength is 510 MPa, and the high temperature strength at 300 ℃ is 175 MPa. This shows that Al Cu cast aluminum alloy has great potential in the development of new heat resistant and high strength aluminum alloy in the future.
3. Al Mg cast heat resistant aluminum alloy
Compared with Al Si alloy and Al Cu alloy, Al Mg cast heat-resistant aluminum alloy has better corrosion resistance and machinability, so it is also called corrosion-resistant aluminum alloy. However, the strength of Al Mg cast aluminum alloy is relatively low due to the improvement of room temperature strength due to the absence of eutectic Si and the strengthening of relative high temperature properties due to Cu rich. The current scientific research and practical application show that SC can effectively improve the high temperature properties of Al Mg cast aluminum alloy. In view of the high cost of SC element, Al Mg SC cast aluminum alloy is only used in the aerospace field. Some studies show that SC element changes the distribution of solute atoms near the liquid phase in the front of dendrite during solidification, thus refining the as cast structure of Al Mg cast aluminum alloy. In addition, SC can be formed in Al Mg cast aluminum alloy and the matrix α- The results show that Al is coherent al3sc phase, and the al3sc phase with nanometer size can pin dislocations and grain boundaries strongly. At the same time, it has excellent thermal stability, which greatly improves the high temperature strength of Al Mg cast aluminum alloy. At present, the main Al Mg cast aluminum alloy is ZL301. The content of Mg in the alloy is as high as 9.5 ~ 11.0 wt.%.
