Comprehensive! Research Progress Of High Performance Squeeze Casting Al-Si Alloys

Keywords: squeeze casting; Al-Si alloys; alloying elements; inoculation modification; microstructure and properties
Squeeze casting technology can prepare complex, high strength and toughness net shape or near net shape thin-walled aluminum alloy castings, which has broad application prospects in the fields of automobile casting and aerospace. Al-Si alloys have good casting properties, high specific strength, good wear resistance, heat treatment, and low cost. They are the earliest researched, most developed, and most widely used aluminum alloys in cast aluminum alloys. Among Al-Si alloys, the hypoeutectic Al-Si alloy with Si content (mass fraction) of 2% to 12.6 % can ensure high plastic toughness while having high strength, while the hypereutectic Al-Si alloy with Si content higher than 12.6 % has high strength. The crystalline Al-Si alloy has the characteristics of good compression resistance, strong wear resistance, and low thermal expansion coefficient. It is an ideal material for the manufacture of automobile pistons, cylinder blocks, swash plates, and other parts. Aiming at Al-Si alloys and their composite materials, this topic summarizes the aspects of alloy composition design and inoculation modifier, which can provide a reference for the development of high-performance squeeze-casting Al-Si alloys.



1. Effects of alloying elements on the microstructure and properties of squeeze-cast Al-Si alloys



1.1 Si element

At present, the extrusion casting Al-Si alloys mainly include: hypoeutectic Al-(4~8)Si series(I), Al-(9~13)Si series(II), and hypereutectic Al-(14~ 22) Si series (III). Figure 1 summarizes the tensile properties of Al-Si alloys.(a) Tensile strength (b) Elongation

Fig.1 Statistical chart of tensile strength and elongation of Al-Si alloys

● Squeeze casting, ▲Squeeze casting + heat treatment, ★Semi-solid-squeeze casting + heat treatment



1.2 Mg and Cu elements

Mg and Cu elements have obvious effects on the microstructure and mechanical properties of Al-Si alloys. Xu Song et al. found that with the increase of Mg content from 1 % to 2 %, the morphology of the eutectic silicon phase in the extrusion-cast Al-10Si-2.5Cu-xMg alloy changed from pin-stripe to fine-grained. When the Mg content is 1.5%, the tensile strength of the alloy reaches the maximum value (about 290 MPa). Similar results were obtained in the study of JIANG B et al.: with the increase of Mg content (0.2%~2.6%), the morphology of eutectic silicon in squeeze cast Al-10Si-2.5Cu-0.8Fe-xMg alloy changed from flake-like. It is fibrous, and the Fe-containing phase changes from rod shape to fine flake shape, and then changes to rod shape; when the magnesium content is 1.38%, the tensile strength of the alloy reaches the highest value of 289 MPa.

LIU HJ et al. found that when the Cu content was 2.25% and the Cu/Mg mass ratio was 4, the strengthening phase in the heat-treated squeeze-cast Al-10.7Si-2.25Cu-0.56Mg alloy changed from θ-Al2Cu to Mg2Si, θ-Al2Cu and Q-Al5Cu2Mg8Si6, the tensile strength of the alloy increases to 426 MPa. LI R et al. found that after adding Cu and Mg elements, the tensile strength of the squeeze-cast Al-17Si-2Cu-0.4Mg-1.5Fe alloy was 225 MPa, which was 72 MPa higher than that of the squeeze-cast Al-17Si alloy. MAENG D et al. After further adding Cu and Mg, the tensile strength of squeeze cast Al-17.48Si-4.25Cu-0.6Mg can reach 315 MPa. Therefore, adding appropriate amounts of Cu and Mg can significantly improve the mechanical properties of the alloy.

1.3 Ce, Sc, and Ni elements

AO X H et al. explored the effect of the Ce element on the eutectic Si and mechanical properties of the squeeze-cast A380 alloy. When the specific pressure is 100 MPa and the Ce content is in the range of 0.1%~0.5%, the modification effect of eutectic Si is obvious: the average length of eutectic Si decreases from 24.82 μm to 10.87 μm, and the increase of the number of Al8Cu4Ce phases makes the alloy tensile strength. Strength increased from 210 MPa to 254 MPa. KITTISAK et al. found that when the Fe content is lower than 0.2% and the Sc content is 0.2%, a small amount of small and round Sc-rich intermetallic compounds can be formed in the Al-7Si-0.3Mg-Fe alloy, and the eutectic Si becomes Fine, so that the alloy hardness, yield strength and tensile strength are improved while maintaining good elongation. When the Sc content is 0.4 %, the number of Sc-rich intermetallic compounds increases and the elongation of the alloy decreases. Li et al. found that as the Ni content increased from 0 to 2.0 %, the volume fraction of Ni-containing intermetallic compounds in squeeze cast A380 alloy increased, and the yield strength and tensile strength of the alloy increased by 39% and 13%, respectively.

1.4 Melt treatment and inoculation deterioration

1.4.1 Melt Processing

Common melt purification methods include the flux method, flotation method, and filtration method. In addition, common melt treatment methods include ultrasonic treatment, vacuum magnetic suspension treatment, melt superheat treatment, electromagnetic stirring treatment and electric pulse treatment. Xia Feng et al. found that ultrasonic treatment can increase the precipitation temperature of the primary crystal phase and eutectic phase in the Al-Si alloy melt, resulting in the α-Al phase in the alloy being fine and round, and the primary Si phase changing from coarse and irregular morphology to Smaller and more evenly distributed. Zhang Liang et al. used vacuum magnetic levitation to smelt A356 alloy and found that compared with conventional smelting, the eutectic silicon phase changed from slender needles to short flakes, and the density of the alloy increased by 1.14%, and the structure was denser. Dang Bo et al. found that when the superheat temperature is lower than 960 ℃, as the superheat temperature increases, the primary Si phase in the hypereutectic Al-25Si alloy tends to grow in facet; when the superheat temperature is higher than 960 ℃ , with the increase of superheat temperature, the tendency of primary Si phase to grow in facet weakens. Miao Long used electromagnetic stirring to prepare Al-20Si alloy and found that as the stirring voltage increased, the segregation of primary silicon at the edge of the sample became more obvious. Zhang Guangan et al. applied electric pulse inoculation treatment to Al-7Si-Mg alloy and found that the columnar α-Al phase became a nearly spherical shape, and the grain refinement effect was very obvious.

Fig.2 Microstructure of Al-20Si alloy under different ultrasonic power

1.4.2 Al-Ti-B refiner

Inoculant modifier is widely used in gravity casting, high-pressure casting aluminum alloy, and other fields. Among them, Al-Ti-B grain refiner is the most widely used in aluminum alloys. Al-5Ti-B alloy is effective for deformed aluminum alloys, but it cannot play a good refining effect on mainstream cast aluminum-silicon alloys (Si≥5%). This phenomenon is called the "silicon poisoning effect". The new Al-Ti-Nb-B grain refiner developed based on the traditional Al-Ti-B refiner forms B2 (Ti, Nb) particles in the melt through the synergistic effect of Ti/Nb, which can promote The alloy melt nucleates and the grain refinement effect is obvious. In order to avoid the silicon poisoning effect of Al-5Ti-B, DONG X et al. and ZHANG YH et al. developed Al-3Ti-3B and Al-3V-B and Al-3Ti-1B refiners respectively, all of which made the "silicon poisoning effect" Significant improvement.

1.4.3 Al-P Modifier

Al-P type master alloy modifier has the characteristics of good and stable modification effect, no pollution, strong adsorption, and low cost. In addition, the use of new Al-P-Ti-TiC-Y and Al-P-Ti-TiC-Nd2O3 refiners can refine eutectic silicon to 20 μm.

1.4.4 Modifiers containing Sr

Sr-containing modifiers have the characteristics of good metamorphic effect, long effective period, recyclability, and little pollution. The research results of Wang Yong et al. show that when the Sr content is 0.06%, the morphology of the eutectic silicon in the Al-7Si alloy changes from a long needle shape to a short needle shape. JEYAKUMAR J et al. found that Sr does not affect the biplane reentrant growth mechanism of eutectic silicon, but forms a metamorphic Al2Si2Sr phase at the boundary between eutectic silicon and aluminum grains during the eutectic reaction. Further research later found that the Al2Si2Sr clusters formed in the melt would increase the degree of undercooling required for the nucleation of eutectic Si, and would also affect the growth of eutectic Si twins to modify eutectic Si. The microstructures of the unmodified and Sr modified alloys are significantly different, as shown in Figure 5.
Fig.3 Schematic diagram of the mechanism of silicon poisoning induced by Al-5Ti-B refiner

(a) Average grain size of α-Al in Al-10Si alloy ingot (b) Al-0.5Ti-2.0Nb-0.3B (c) Al-1.2Ti-2.9Nb-0.5B (d) Al-3B (e) Al-2.08Nb-0.23B

Fig.4 Grain refining effect of different kinds of grain refiners on Al-10Si alloy
(a1, a2, a3) unmodified (b1, b2, b3) Sr modified

Fig.5 Microstructures of unmodified and Sr-modified Al-12.7Si alloys

(a1) and (b1) eutectic phases (gray for α-Al and dark gray for Si); (a2) and (b2) correspond to the EBSD phase species diagrams of (a1) and (b1), respectively (α-Al is blue, Si in red), (a3) and (b3) EBSD grain orientation diagrams with color-scaled inverse pole diagrams shown in the lower left corner of (a3)

1.4.5 Compound metamorphism

The addition of Sr as a modifier in Al-Si alloys changes the morphology of eutectic Si from coarse plates or needles to fine fibers. However, the addition of Sr can lead to the formation of a long columnar dendritic α-Al phase (α-Al phase with relatively large longitudinal size), so it is also necessary to add other elements to change the long columnar dendritic α-Al phase. LIAO H C and other studies found that in the Al-13.0Si alloy, when the added Al-5Ti-1B is higher than 0.82%, it will have an adverse effect on the modification of Sr. Subsequently, the Al-5Ti-1B alloy was replaced with an Al-1B master alloy. It was found that in addition to the size of eutectic silicon, the size of α-Al dendrites in the alloy was significantly reduced, and the alloy after composite treatment had good mechanical properties. HAGHAYEGHI R et al. added Sr and Sb elements to the hypereutectic A390 alloy (Al-17.4Si-5Cu) to refine the primary silicon crystal. It was found that Sb refines the primary Si crystals by twinning and increasing the stacking fault density, while Sr refines the primary Si through the formation of kinks, which together refine the eutectic Si. CHO Y et al. added P and Sr elements to hypoeutectic Al-Si alloys and found that the Al2Si2Sr phase in the alloy would surround the AlP phase in the alloy, resulting in the poisoning or deactivation of the two phases with refined eutectic Si. , resulting in coarse eutectic silicon in Al-Si alloys with these two elements added at the same time.

Table 1 shows the mechanical properties of some Al-Si alloys. It can be seen that the types and contents of alloying elements have an important influence on the microstructure and properties of squeeze-cast Al-Si alloys. Research on the types and contents of main and micro-alloying elements is important for the preparation of high-performance squeeze-cast Al-Si alloys. significance.

2 Squeeze casting of Al-Si alloy composites



Metal matrix composites have become an important structural material in the aerospace and automotive industries. Compared with traditional materials, metal matrix composites have a high specific strength, good wear resistance, light weight of parts, and stable high-temperature strength. Among the numerous processing processes, squeeze casting is an effective method for manufacturing high-performance metal matrix composites.

2.1 Al-Si-based inorganic non-metallic composites

Inorganic non-metallic materials commonly used for compounding with aluminum alloys are: boron carbide (B4C), aluminum oxide (Al2O3), diamond, and silicon carbide (SiC). Among these materials, silicon carbide (SiC) reinforced aluminum matrix composites have better mechanical properties and lower manufacturing costs. GURUSAMY P et al. studied the effect of squeeze casting pressure on the microstructure and mechanical properties of Al/SiCp metal matrix composites. It was found that at a squeeze casting pressure of 100 MPa, the composites containing SiC particles had lower porosity, increased tensile strength by 300%, and increased hardness by 51%. GURUSAMY P et al. and ARULRAJ M et al. found that when the pouring temperature and die temperature were 850 °C and 350 °C, respectively, the particle distribution in the Al/SiCp composite was the most uniform, and the tensile strength reached the maximum value (245 MPa). However, the strain concentration near the SiC particles reduces the plasticity of the material.

MANU K S et al. prepared micron SiO2 reinforced aluminum matrix composites by squeeze casting process. Since the SiO2 particles are uniformly distributed in the matrix, the micron SiO2 reinforced aluminum matrix composites have better mechanical and corrosion resistance properties. CHOU S N et al. studied the mechanical properties of squeeze-cast Al2O3/A356 aluminum alloy composites. It was found that when the A356 alloy content was increased from 5% to 40%, the hardness (HV) of the composites decreased from 109 to 227, and the fracture strength decreased from 492.5 MPa to 457 MPa. KRISHNAN PK et al. used scrap aluminum alloy as matrix to replace Al-7Si-Mg alloy, and used waste alumina catalyst from refinery waste to replace Al2O3 as reinforcement material, which greatly reduced the composite material's strength under the premise of ensuring that the tensile strength did not drop significantly. Cost of production. JIANG L et al. also studied the interface properties of high thermal conductivity diamond/aluminum composites by squeeze casting. The results show that the lath-shaped Al4C3 grows on the diamond/aluminum alloy interface according to a trapezoidal step mechanism, and the lath-shaped Al4C3 tip interface is a rough interface, which migrates in a continuous growth mode.

2.2 Aluminum-aluminum bimetallic composites

Aluminum-aluminum bimetallic composites have unique advantages in meeting the high-performance and efficient production needs of fields such as automotive and aerospace. Among them, 6101 aluminum alloy has high strength, excellent thermal conductivity and electrical conductivity; while A356 aluminum alloy has good casting characteristics, high tensile strength and fatigue performance. The A356-6101 aluminum alloy bimetal material prepared by composite extrusion casting technology can combine the advantages of both.

Wang G et al. studied the effect of pressure on squeeze casting A356-6101 bimetallic materials influence on the microstructure and mechanical properties. It was found that the tensile strength of the A356 aluminum alloy matrix increased with increasing pressure, whereas the tensile strength of the bimetal was independent of the applied pressure. LIU G et al. found that in the squeeze cast A356/6101 bimetal after heat treatment, the matrix and transition zone of A356 aluminum alloy have obvious hardening phenomenon, and the spheroidization of silicon particles in the alloy is obvious. In addition, the interdiffusion of solutes between the two matrices is accelerated, and the interfacial shear strength increases from 76.7 MPa to 102.7 MPa.



3 Conclusion



Squeeze casting aluminum alloy can form complex thin-walled castings, and the parts have dense structure, high surface quality and good mechanical properties, and have broad application prospects in the field of automobile parts preparation. The development of Al-Si alloy squeeze casting technology can be carried out from the following aspects:

(1) The Al-Si alloy composition needs to be further optimized. Appropriate addition of Cu and Mg elements can greatly improve the strength of Al-Si alloy; the addition of rare earth metals such as Ce, Sc and Ni can significantly improve the alloy structure. Therefore, by optimizing the content of main and micro-alloying elements in the alloy, it is expected to develop a high-strength plastic squeeze-cast Al-Si alloy.

(2) It is necessary to further develop Al-Si alloy inoculation modifiers. The Al-Ti-B grain refiner has a significant effect on the α-Al grain size in the Al-Si alloy. It is particularly important to develop a variety of refiners with remarkable effects, which are cheap and easy to manufacture. Modifiers containing Sr, Sb and P have a significant effect on the size and morphology of primary Si and eutectic Si in Al-Si alloys. Therefore, it is of great significance to study more efficient refining modifiers to improve the properties of Al-Si alloys. . In addition, the composite use of various modifiers is also important to improve the performance of Al-Si alloys.

(3) Metal matrix composites are structural materials with very broad development prospects. Metal matrix composites have high specific strength, good wear resistance, light weight of parts, and stable high temperature strength. Squeeze casting technology has broad application prospects in the development of high-performance metal matrix composites.

Citation format: Xue Liwen, Zhou Wenqiang, Pu Yinan, et al. Research progress on composition optimization, inoculation modification and composite materials of Al-Si alloys in squeeze casting [J]. Special Casting and Nonferrous Alloys, 2022, 42 (1 ): 22-31.