Introduction
Cast Al-Si alloy has been widely used. Hypoeutectic Al-Si alloy is represented by A356 aluminum alloy, which has the advantages of excellent casting performance, good fluidity, low shrinkage, low hot cracking tendency, light weight and high recovery. It is mainly used in the manufacture of automobile and motorcycle wheels. Hypereutectic Al-Si alloy is an ideal material for manufacturing cylinders and pistons because of its wear resistance, heat resistance, corrosion resistance, low coefficient of thermal expansion and good volume stability. If the alloy is directly used in casting, the performance will be very poor because of the uneven distribution of internal grains, which can not meet the standard of normal use. After element control and heat treatment, the microstructure can be changed and more excellent properties can be obtained. In this paper, the effect of alloy elements on the microstructure and properties of Al-Si alloy, the strengthening mechanism and heat treatment process of Al-Si alloy are introduced in detail.
1. Microstructure and properties of as-cast Al-Si alloy.
The as-cast Al-Si alloy is mainly composed of α-Al dendrite and coarse eutectic silicon. For hypereutectic Al-Si alloy, there is primary silicon, in which the α dendrite is oval dendritic. For bulk polygonal primary silicon, the larger the particle size is and the more irregular the shape is, the lower the strength is, and it is easy to crack preferentially in the tensile process. Huang Caimin and others found that there is a local temperature gradient during cooling and solidification of high temperature liquid aluminum, and the different cooling rates lead to composition segregation between the dendrites of as-cast A356 alloy. At the same time, the matrix also has some defects such as porosity, voids, inclusions, shrinkage and oxide film. The eutectic silicon of unmodified A356 alloy is coarse needle-like. Mg2Si is precipitated strengthening phase, but the amount of Mg2Si phase in as-cast state is small and small, so it is not easy to be found. A large number of smooth quasi-cleavage surfaces appear in the tensile fracture morphology of as-cast A356 alloy, while the local area is mixed with dimples of different sizes, most of which are small and shallow, and the number of dimples is relatively small. the reason why the alloy shows the characteristics of this cleavage plane is that cracks occur at the joint between eutectic silicon and matrix, which are distributed in the eutectic region after continuous expansion. Yifan Wang et al. found that covalent bonds are formed between Al and Si atoms at the Al-7Si-0.6Mg interface, which plays a key role in the bonding strength of the interface. According to Griffith fracture theory, cracks first form and propagate in the Al precipitates, and the interface can be used as a protective layer to prevent crack propagation. Through the fracture surface of as-cast A356 aluminum alloy, Lou Huashan et al found that when the crack growth is blocked by eutectic silicon, the crack will cut off the eutectic silicon particles and form a long crack as the small cracks grow and connect together. then the crack propagates and propagates along the weakest part of the grain boundary (lamellar structure), and finally shows brittle fracture. At the same time, S.Samat et al found that the decrease of plasticity is related to the microstructure characteristics of harmful acicular β-AlFeSi intermetallics and the existence of micro-pores during solidification. For hypereutectic Al-Si alloy, coarse primary silicon as hard point can improve the wear resistance of the alloy, but because of its hardness and brittleness and serious splitting of the matrix, the mechanical properties of the alloy are reduced and the workability becomes worse.
2. Effect of alloying elements on microstructure and properties of cast Al-Si alloy.
The addition of alloying elements is an important way to improve the microstructure and properties of Al-Si alloy. Mg, Cu, Mn, Sr and RE are often added to Al-Si alloy.
Mg element can be dissolved into α-Al to cause lattice distortion and play the role of solid solution strengthening. At the same time, Mg and Si form Mg2Si phase, which is a strengthening phase, which improves the hardness of the alloy. The amount of Cu in Al-Si alloy reaches 2.5%. The number of Al _ 2Cu phase increases, which is distributed at the interface between α-Al and eutectic silicon, which plays a strengthening role, but the coarse morphology and non-uniformity of strengthening phase make the elongation of the alloy decrease. Mn can reduce the amount and size of primary silicon in Al-Si alloy, and eutectic silicon becomes shorter needle-like structure. Mn-containing hypereutectic Al-Si alloy will precipitate dispersed phase particles containing Mn in the homogenization process, which has high density and high thermal stability, refines recrystallized grains, and becomes the nucleation core of aging strengthening phase, which has a great influence on the mechanical properties and workability of the alloy. Sr can change the morphology of eutectic Si phase from acicular to fibrous, and after adding Mn and Sr elements, AlFeSi phase in Al-Si alloy is uniformly distributed in α-Al dendrite, and the effect of Mn on improving the morphology of acicular Fe phase is greater than that of Sr. A certain amount of Ba has a good modification effect on ZL109 eutectic silicon, and has good resistance to deterioration and remelting properties, and the modified alloy can obtain higher strength; but when the Ba content is more than 0.125%, a small amount of acicular phase appears in the structure, and the properties are reduced accordingly. With the increase of Fe content, the size of iron-rich phase in A356 aluminum alloy increases, the morphology changes from bone to needle, and the tensile strength of A356 aluminum alloy decreases. Large flake iron-rich intermetallic compound particles in high iron aluminum alloy castings promote fatigue crack initiation, which is one of the sources of cracks. However, the increase of Fe content will increase the high temperature and short-term tensile strength of the alloy. After the modification of Sb with A356, the density of the alloy is increased, and the modification effect is long-term, and Zr can effectively refine the grains and inhibit recrystallization. With the addition of Zn element to a certain amount, eutectic clusters are formed in the microstructure of the modified Al-Si alloy. With the increase of Zn content, the hardness of the alloy increases and the elongation decreases. When phosphorus salt is added to hypereutectic Al-Si alloy, the heterogeneous core of A1P is formed, the size of primary silicon is reduced, and the shape changes from plate to polygonal or agglomerate. The hypereutectic Al-Si alloy modified by phosphate salt-strontium salt has good mechanical properties, wear resistance and casting properties.
Nd can refine the α-Al matrix in Al-Si alloy, but when the content of Nd is too high, the rare earth-rich phase appears in the alloy and its mechanical properties decrease significantly. Gd can refine the grains of A357 alloy and reduce the secondary dendrite spacing, and can also effectively refine the eutectic silicon in the alloy. The mechanical properties of the alloy have been significantly improved. Ce can refine the grain size of Al-Si alloy, and the grain size is the smallest when the content of Ce is 1%. Compared with the original Al-Si alloy, the average grain size decreases from 80 μ m to 25 μ m, and the eutectic silicon size decreases significantly and the degree of dispersion increases. The fracture surface of Al-Si alloy without Ce is lamellar structure and brittle fracture occurs, while the fracture surface of Al-Si alloy without Ce has dimple structure and mixed fracture. La can change eutectic silicon from flake and needle to dot and short rod, and can refine primary silicon to achieve the effect of modification. P-RE composite modification refines eutectic silicon and primary silicon particles of Al-20Si alloy, which obviously improves the strength, plasticity and impact toughness of the alloy. In the cast Al-Si alloy, the strength and plasticity of the alloy are significantly improved with the increase of the amount of RE, but too much addition will make the alloy form a large number of AlxV3CuRE compounds, which will lead to the deterioration of the mechanical properties of the cast Al-Si alloy.
3. Effect of heat treatment on microstructure and properties of cast Al-Si alloy.
The heat treatment of cast Al-Si alloy mainly includes solid solution treatment and aging treatment. Heat treatment will not change the external shape and overall chemical composition of the material, but adjust the internal structure of the material, so as to improve the performance of the material. The effect of heat treatment on α-Al lies in the solid solution of alloy elements and the precipitation of strengthening phase in α-Al matrix.
3.1 solid solution treatment
The purpose of solid solution treatment of Al-Si alloy is to fully dissolve Mg and Si into α-Al matrix, which provides preparation conditions for the precipitation of more strengthening phases in subsequent aging, and to make the morphology of eutectic silicon phase change from fibrous to granular, so as to improve the mechanical properties of the alloy.
3.1.1 effect of solid solution treatment on Microstructure of cast Al-Si Alloy
When the solution temperature is constant, with the extension of solution time, the more primary Mg2Si particles in the casting alloy dissolve, the more sufficient the diffusion of elements such as Si and Mg, and the easier it is for α-Al matrix to reach saturation state. In the solid solution process of Al-Si alloy, some parts of eutectic silicon, such as surface depression, take the lead in necking and begin to decompose and fracture under the action of heat. With the further extension of solution treatment time, the higher the spheroidization degree of eutectic Si particles is, the smaller the size and number of silicon is, and the more the area fraction of eutectic Si particles is. At the same time, the spheroidization of eutectic silicon was also found in the modified A356 aluminum alloy, and the spheroidization was more obvious than that of the unmodified alloy. However, when the solution time is too long, the grains grow, and the alloy softens to a certain extent, which leads to the growth and aggregation of eutectic Si. Therefore, the choice of solution time should first ensure the full dissolution of the strengthening phase, obtain the maximum supersaturation, and avoid the growth of eutectic silicon as far as possible. For hypereutectic Al-Si alloy, when the solution temperature and holding time are suitable, the distribution uniformity of primary silicon in the alloy is good and its shape is close to circular.
3.1.2 effect of solid solution treatment on Properties of cast Al-Si Alloy
For the hardness of the alloy after solution treatment, the influence of solute atoms is very great. When the solution time is too long, the supersaturated solid solution atoms in the matrix are dissolved and precipitated under the action of thermal activation after heating. The solid solution strengthening effect of supersaturated solid solution atoms on the matrix is weakened, and the hardness of the matrix is slightly reduced. The solid solution time has a significant effect on the strength and elongation, which is closely related to the change of the morphology of silicon phase during the solution process. When Cao Guojian and others studied the effect of solution treatment on 4004 aluminum alloy, it was found that when the solution time was short, the eutectic Si was not completely broken, so the elongation of the alloy was low, and the alloy elements were not completely dissolved, which reduced the precipitation of strengthening phase and weakened the strength of the alloy after aging. However, when the solution time is too long, the granulated eutectic Si will coarsen with the prolongation of holding time, showing massive and angular characteristics, which will enhance the splitting effect on the matrix and reduce the strength and elongation of the alloy. The refinement and uniform distribution of eutectic Si particles are beneficial to improve the elongation of Al-Si alloy. Therefore, the selection of appropriate holding time is a very important link in solid solution treatment. In addition to the solution time, the choice of solution temperature also has a very important influence on the effect of solution treatment. With the increase of solution temperature, the strengthening phase dissolves, the concentration of strengthening elements in the matrix increases, the size of eutectic silicon decreases, and the mechanical properties of the alloy gradually improve, but when the solution temperature is too high, eutectic Si appears coarse, segregation and other phenomena, and this phenomenon becomes more obvious with the increase of solution temperature. As a result, the mechanical properties of the alloy decrease, so the appropriate temperature is also very important for the solid solution treatment, and the choice of the solid solution temperature is generally lower than the solidus temperature of 10-15 ℃. When Yao Heng and others studied the Al-Si alloy containing scandium, they found that the silicon phase was hard and brittle, and when it was finely distributed, the deformation of α phase was greatly limited and the hardness increased, but with the increase of solution temperature, the eutectic silicon of the alloy became coarse. the binding force on matrix deformation decreases, and the deformation of α phase is easier, so the hardness of the alloy decreases gradually. When Chang Fang-E and others studied the effect of heat treatment on Al-Si alloy without solidification shrinkage, it was found that the hardness increased at first and then decreased with the increase of solution temperature. when the solution temperature exceeded a certain value, the microstructure was locally overheated and the hardness of the alloy decreased.
3.2 Aging treatment
The purpose of aging of Al-Si alloy is: (1) uniform precipitation of Mg2Si; (2) homogenization of microstructure and elimination of internal stress; (3) improvement of machinability.
3.2.1 effect of Aging treatment on Microstructure of cast Al-Si Alloy
The main strengthening elements in A356 aluminum alloy are Si and Mg. Because Mg2Si is incoherent with the matrix, the strengthening phase can not be precipitated directly, so the precipitation of the strengthening phase is in a process. The precipitation sequence is as follows: supersaturated solid solution ("GP Ⅰ (Mg/Si clusters)" GP Ⅱ (Mg5Si6 transition phase), forming solute atomic segregation region, and then the distortion of crystal lattice increases continuously, forming coherent strain region. Improve the strength and hardness of the alloy) "β" phase (Mg9Si5 metastable phase, with the increase of the degree of distortion in this region, the greater its aging strengthening effect) "β" phase (Mg2Si stable phase, has its own independent lattice, distortion disappears, strength decreases), in which β 'phase can be regarded as the formation of Mg and Si atoms enriched on the basis of β' phase. The strength of the material can be improved by increasing the aspect ratio of the precipitated phase, because when the precipitation with large aspect ratio is sheared, the extra energy is consumed on the generation of the new interface. During the aging of Al-Si alloy, the formation of GP zone and metastable phase is partially overlapped and crossed. The morphology and amount of precipitated strengthening phase Mg2Si also changed significantly after aging treatment, which was mainly affected by aging time and aging temperature. Through the transmission electron microscope, it can be seen that when the aging time is short, most of the precipitates show a slender and short rod shape, and the number of precipitates is less; with the increase of aging time, the number of second phase strengthening phase increases and distributes dispersively; further increase the aging time, the equilibrium phase gathers and coarsens each other. With the increase of aging temperature, the precipitated phase began to change from long rod to spherical, and some of the spherical precipitates precipitated directly from the matrix. With the increase of aging temperature, the long rod precipitates basically disappeared, and the spherical precipitates tended to grow further.
3.2.2 effect of Aging treatment on Properties of cast Al-Si Alloy
Proper aging treatment temperature and time can significantly improve the microstructure uniformity and the morphology of precipitated phase, thus increasing the strength of the alloy, but too high temperature or too long aging time will reduce the strength of the alloy.
Among the factors affecting the mechanical properties of A356 aluminum alloy, aging time has the greatest influence on tensile strength, yield strength and elongation, and the magnitude of these properties increases at first and then decreases with the increase of aging time. When the aging time is too long, the grain coarsening and shape change directly reduce the hardness of the material. Secondly, the aging time is too long to form a continuous coarse brittle Mg2Si phase, which also reduces the mechanical properties of the alloy. The precipitated phase Mg2Si is a hard and brittle intermetallic compound, which can effectively pin dislocations, stabilize substructure, prevent grain boundary slip, make a good combination of strength, plasticity, toughness and hardness, and increase the recrystallization temperature of the matrix, thus inhibiting recrystallization. In addition, it also improves the strength of the matrix. The stable precipitation hardening phase produced by the aging cast Al-Si alloy will not dissolve into the matrix again, which prevents the long-range movement of the dislocation, so the thermal fatigue resistance of the alloy is improved. The fatigue properties of the alloy are mainly affected by the morphology and size of Si particles, both of which are controlled by adjusting heat treatment. The alloy with heat treatment has excellent fatigue properties because of a large number of fine Si spheroidizing. Fine silicon particles exist in the cell structure, and they can limit fatigue crack propagation and delay fatigue fracture by changing the propagation direction. The tensile fracture dimples of low pressure casting A356 aluminum alloy after two-stage aging are smaller, the small dimples are divided by smaller dimples, and there are no large dimples on the edge of the dimples, and its uniformity is better than that of the tensile fracture after T6 heat treatment. Therefore, the elongation of the alloy after two-stage aging is better than that of T6 process. After T6 treatment, the fracture surface of A356 alloy is mixed with cleavage surface and some dimples, which is easy to form brittle cracks, which is caused by the weak bond between agglomerated particles and matrix and fracture at the interface. For hypereutectic Al-Si alloy, the aging temperature affects the boundary dissolution and diffusion of alloy elements. with the increase of aging temperature, the boundary dissolution and diffusion of alloy elements accelerate, which is beneficial to improve the mechanical properties of the alloy. Proper aging treatment will improve the wear resistance of the alloy. Sun Yu et al studied the effect of heat treatment on strontium modified near eutectic Al-Si casting alloy and found that aging treatment will reduce the plasticity of the material. Liu et al found that aging treatment can improve the impact toughness of Al-20%Si alloy, which is related to the change of the shape of primary silicon and eutectic silicon and the strengthening of matrix.
4. Conclusion
With the development of science and technology and the improvement of the performance requirements of Al-Si alloy, there are some problems in the regulation of alloy structure and solution aging heat treatment at the present stage. for example, the cost of rare earth elements is too high, the utilization rate of rare earth elements is low, the heat treatment cost is high, the processing efficiency is low, the processing time is long, and the strength and plasticity are difficult to be improved organically. In the future development trend of solid solution aging heat treatment, short-time, high efficiency, low cost, strength and plasticity are improved organically, and controlling the amount of rare earth elements, the morphology and size of Si phase during solid solution, and the speed, size and quantity of Mg2Si strengthening phase precipitation during aging will become the development direction of alloy element control and solid solution aging heat treatment process in the future.
