The microstructure and morphology of materials play an important role in determining their mechanical, thermal and electrical properties. It is the main idea to improve the high temperature properties of cast aluminum alloy by proper alloying and heat treatment.
Alloying is the main means to improve the high temperature properties of cast aluminum alloy. The high temperature strength, creep resistance and thermophysical properties of aluminum alloy can be significantly improved by adding appropriate amount of transition group elements and rare earth elements to refine grain size, improve thermophysical properties, reduce stacking fault energy, increase solid solubility of matrix, and form the second phase with high melting point, high hardness and good thermal stability. Therefore, the alloying elements added should meet the following principles as far as possible:
(1) The diffusion coefficient in the matrix is small (the diffusion coefficient and solid solubility of some alloying elements in the aluminum matrix);
(2) The crystal structure is similar to that of the matrix and the mismatch is small;
(3) The strengthening phase with good thermal stability can be formed;
(4) The casting and cutting properties of the alloy are not affected;
(5) It has low coefficient of thermal expansion. Among them, the second phase with high melting point, high hardness and good thermal stability has the most obvious strengthening effect.
These fine second phases are dispersed at the grain boundary of the matrix, which effectively hinder the dislocation slip and the movement of the grain boundary, and inhibit the coarsening of the matrix grain at high temperature, thus significantly improving the high temperature mechanical properties of the aluminum alloy.
Mg can effectively improve the corrosion resistance of the alloy while improving the mechanical properties of the alloy. The heat resistance temperature of q-al5mg8si6cu2 phase formed by Mg and Cu in aluminum alloy can reach 250 ℃, which has good room temperature and high temperature strengthening effect. Compared with Mg, Mn can form more excellent thermal stable phases. Liao Hengcheng, et al. Have carried out a long-term and in-depth study on the microstructure regulation and strengthening mechanism of Mn on Al Si alloy, and concluded that the second phases formed by Mn in aluminum alloy are s-al2cumn, t-al20cu2mn3, t-al12cu3mn2, al50si30mn20, al15mn3si2 and Al 2O α- Al15(Fe,Mn)3Si2。 At the same time, it is proved that a115mn3si2 will not melt and coarsen during the whole heat treatment process, and it is the best heat-resistant phase, which makes the tensile strength of al-12si-4cu-1.2mn alloy in T6 state reach 124.9 MPa at 300 ℃. In addition, Mn can eliminate the harmful effect of Fe. Shabestari et al. Found that Mn can make acicular Fe rich phase when studying the effect of Fe and Mn on al-12.7si alloy β- A15fesi is completely transformed into Chinese characters with higher thermal stability α- Fe (al15 (Fe, Mn) 3si2) can effectively prevent the needle like phase from splitting and further improve the heat resistance of the alloy. Liu Xiangfa et al. [32,39-40] showed that Ni was mainly distributed in aluminum alloy ε- Al3Ni、 δ- Al3CuNi、 γ- Al7cu4ni and other intermetallic compounds with good thermal stability exist. When the content of Cu is low, the main existing form of Ni is lath ε- Al3Ni； With the increase of Cu content, it gradually changes into the network and semi network structure δ- Al3CuNi； When the content of Cu continues to increase, the final closed or semi closed structure will be formed γ- Al7Cu4Ni。 Among them, reticular and semi reticular structures are the most common δ- Al3cuni has the most significant effect on improving the heat resistance of the alloy, followed by the closed or semi closed alloy γ- Al7cu4ni, the last is lath ε- Al3Ni。 Wu Yuying et al.  found that Ni can promote needle like growth β- A15fesi becomes fishbone α- Al8ni2fe, and it is proved that α- Al8ni2fe is a quasi stable phase whose formation temperature is above the eutectic point of Al Si. Shaha et al. Used Cr, Ti, V and Zr to strengthen al-7si-cu-0.5mg alloy. It was found that Ti can exist in the form of irregular disk al10.7siti3.6, acicular al6.7si1.2tizr1.8 and massive al21.4si3.4ti4.7vzr1.8. At the same time, the tensile strength of the modified alloy composed of multiple second phases reached 196 MPa at 300 ℃. Xiao Daihong et al.  studied al-5.3cu-0.8mg-0.3ag alloy with high Cu / Mg ratio and found that the presence of CE can accelerate the aging process, shorten the peak time and improve the hardening level. At the same time, it is demonstrated that CE can also reduce the diffusion rate of Cu and Mg atoms, effectively reduce the growth rate of Ω – al2cu and refine Ω – al2cu. In addition, the effect of rare earth elements on improving the properties of aluminum alloy is very significant and diverse. Firstly, rare earth elements can form many second phases with excellent thermal stability, such as al11la3, al11pr3, al3nb, Al3Zr, al3er, al3sc, etc. These second phases not only have good thermal stability, but also can be increased obviously θ The number and size of the phase are reduced and refined α- Al, primary Si phase and eutectic Si phase can effectively inhibit grain boundary migration and dislocation movement. In addition, the low thermal expansion coefficient of rare earth elements can effectively improve the thermal expansion characteristics of the alloy. Rare earth elements can also increase the undercooling of alloy composition during solidification, improve the casting fluidity and play the role of refining degassing.
2. Heat treatment
Heat treatment can further control the as cast structure and properties of cast aluminum alloy. Solution aging is the most common heat treatment method for cast aluminum alloy, and T6 heat treatment method of artificial aging is the most widely used. After solution and aging heat treatment, the casting internal stress is eliminated, the corrosion resistance and dimensional stability are enhanced, and the microstructure and composition are homogenized. At the same time, the second phase with low melting point precipitates the transition phase which is coherent / semi coherent to the matrix after dissolution, so as to effectively improve the high temperature mechanical properties of the alloy. The detailed heat treatment system varies with the composition and properties of the alloy, and most of them are based on the DSC experimental results. The optimum solution temperature of 319 aluminum alloy was systematically explored by Samuel et al in the range of 490 ~ 540 ℃. It is found that with the increase of solution temperature, the composition of the alloy becomes more uniform and the tensile strength increases continuously. However, when the solution temperature exceeds 520 ℃, although the composition tends to be more uniform, the strength decreases due to local overburning. Finally, the optimal solution temperature is 505 ℃ × 8 h+520 ℃ × 2 h two-step solid solution. Wang Guangliang et al systematically explored the heat treatment system of al-11.9si-3.5cu-1.7ni-0.8mg alloy, and finally determined that the best scheme was 495 ℃ × 2 h + 515 ℃ × 8 h two-step solid solution and 200 ℃ × After 4 h aging, the room temperature tensile strength of the alloy is increased from 212 MPa to 367 MPa, and the high temperature tensile strength at 300 ℃ is increased from 110 MPa to 160 MPa. At the same time, the thermal exposure property is also improved. Qi et al. Improved the aging system of Al Cu mg Ag alloy, and the aging temperature was 515 ℃ × 495 ℃ was introduced after 1.5 h solid solution × 2h + 515 ℃ × After 8 h intermittent aging treatment, the ductility and corrosion resistance of the alloy are greatly improved with a slight decrease in strength.
3. Si phase modification
The eutectic Si in Al Si cast aluminum alloy is needle like, while the primary Si is large and mostly polygonal or plate like. The coarse Si phase will not only cleave α- Al matrix is easy to cause stress concentration, which eventually becomes the source of crack initiation and leads to premature fracture of the alloy. Therefore, when the silicon content in Al Si alloy is higher than 6 ~ 8 wt%, it is necessary to add modifier for modification. At present, Na, Sr, Sb, Ba and re are widely used in the modification of eutectic Si, making it from coarse needle flake to fine fibrous or lamellar. However, P can reduce the grain size of primary Si, and make the primary Si evolve into a relatively regular polygon or granular structure, and disperse uniformly in the matrix.
4. Casting process optimization
In order to improve the high temperature properties of aluminum alloy, many transition group elements or rare earth elements, such as Cr, SC, Gd, are often added to the alloy. However, these elements have very high melting point, which will obviously change the solidification process of the alloy when they are added in the form of master alloy, and tend to produce a variety of phase transformation processes with large temperature range, seriously damage the fluidity of the alloy, and more easily form casting defects such as shrinkage porosity, shrinkage cavity and segregation. These casting defects can be the source of crack initiation, leading to premature fracture of the alloy [52-53]. Therefore, it is very important to select appropriate casting method and design reasonablefor improving the high temperature properties of aluminum alloy. According to the specific composition of the alloy, the casting process optimization means such as correct parting, proper selection of pouring method, reasonable design of gating system specification, and improvement of riser feeding scheme can effectively ensure the microstructure and density of the alloy. In addition, squeeze casting crystallized under pressure can also significantly reduce shrinkage porosity and shrinkage cavity defects, while electromagnetic casting solidified under electromagnetic field can effectively improve the surface quality of castings and greatly reduce element segregation.