This article focuses on the influence of Sc and Zr on the microstructure and mechanical properties of squeeze-cast ZL205A alloy. Through experimental research and analysis, it was found that with the increase in Sc content, the grain structure of the alloy was refined, and the tensile strength first increased and then decreased. The combined addition of Sc and Zr had a more significant effect on the refinement of the alloy structure and the improvement of mechanical properties. The squeeze-cast alloy was used to manufacture connecting rod components, which exhibited good surface quality and excellent mechanical properties. The research results provide a reference for optimizing the microstructure and improving the comprehensive performance of high-strength and tough cast Al-Cu alloys.
1. Introduction
In recent years, with the increasing emphasis on resource conservation and environmental protection worldwide, lightweight technology has received extensive attention, promoting the research and development of lightweight alloy materials. Among them, cast aluminum alloys represented by ZL205A have become one of the research hotspots due to their good casting performance, low density, low cost, and high yield rate. ZL205A aluminum alloy is a typical high-strength cast aluminum alloy of the Al-Cu system, suitable for various casting methods such as gravity casting, squeeze casting, and centrifugal casting. However, the large crystallization interval of ZL205A aluminum alloy, reaching 105 °C, often leads to microstructure and performance defects in the formed parts, making it difficult to meet the actual application requirements. With the increasing demand for the medium and high-temperature properties of alloy materials in industrial production, it is necessary to further optimize the comprehensive performance of ZL205A aluminum alloy.
Microalloying is an effective and convenient method to improve the performance of aluminum alloys. The addition of rare earth elements such as Y, Sc, and Zr to aluminum alloys can shorten the crystallization interval, optimize the casting structure, improve the hot cracking resistance, and enhance the comprehensive mechanical properties. Previous studies have shown that the addition of Sc and Zr to aluminum alloys can refine the grain size, improve the strength and plasticity, and enhance the heat resistance and corrosion resistance of the alloys. However, the effects of Sc and Zr on the microstructure and properties of ZL205A aluminum alloy under squeeze casting conditions still need to be further studied.
2. Experimental Materials and Methods
2.1 Experimental Materials and Preparation
The base alloy used in this study was ZL205A alloy, and its main chemical composition is shown in Table 1. By adding a certain amount of Al-4Zr and Al-2Sc master alloys to the ZL205A base alloy, the effects of Zr and Sc elements on the microstructure and mechanical properties of the base aluminum alloy were studied. The specific process parameters for modifying the composition of the ZL205A alloy are shown in Table 2.
| Element | Cu | Mn | Ti | Cd | V | Zr | Al |
|---|---|---|---|---|---|---|---|
| Mass fraction/% | 4.6 – 5.3 | 0.5 – 1.0 | 0.8 – 1.0 | 0.15 – 0.35 | 0.05 – 0.30 | 0.05 – 0.2 | Remainder |
| Sample No. | Mass fraction/% | Preparation Process | Sample No. | Mass fraction/% | Preparation Process | ||
|---|---|---|---|---|---|---|---|
| Sc | Zr | Sc | Zr | ||||
| 1 | – | – | Gravity Casting | 6 | 0.40 | 0.10 | Squeeze Casting |
| 2 | – | – | Squeeze Casting | 7 | 0.40 | 0.15 | Squeeze Casting |
| 3 | 0.20 | – | Squeeze Casting | 8 | 0.40 | 0.20 | Squeeze Casting |
| 4 | 0.20 | 0.15 | Squeeze Casting | 9 | 0.60 | – | Squeeze Casting |
| 5 | 0.40 | – | Squeeze Casting | 10 | 0.60 | 0.15 | Squeeze Casting |
The appropriate amount of ZL205A base alloy was placed in a graphite crucible coated with ZnO and melted in an SG2 well-type crucible resistance furnace. When the temperature of the aluminum alloy melt reached 700 °C, a covering agent (NaCl + KCl) was added to the surface to prevent oxidation. After the melt was heated to 740 °C and the ZL205A base alloy was completely melted, the Al-4Zr and Al-2Sc master alloys were added, and the alloy melt was stirred thoroughly to ensure the uniform distribution of Zr and Sc elements. C₂Cl₆ refining agent was added to the alloy melt for degassing and refining. Then, the alloy melt was poured into the cavity of the squeeze casting mold, solidified under pressure, and ejected by the ejector rod. The schematic diagram of the aluminum alloy squeeze casting mold assembly is shown in Figure 1. The squeeze casting was carried out on a THP16-200T hydraulic press with a loading speed of 10 mm/s, a forming pressure of 100 MPa, and a mold preheating temperature of 250 °C.
2.2 Sample Characterization and Analysis
Samples were cut from the squeeze-cast aluminum alloy billet by wire cutting for microstructure characterization and mechanical property testing. The metallographic samples were polished and etched with Keller reagent (2.5% HNO₃ + 1.5% HCl + 1% HF + 95% H₂O) for 10 s, and the microstructure of the samples was observed using an OLYMPUS-GX71 metallurgical microscope. The tensile mechanical properties of the squeeze-cast aluminum alloy billet at room temperature (20 °C) and 150 °C were tested using a Shimadzu-AGXplus universal testing machine with a tensile rate of 1 mm/min. The schematic diagram of the tensile sample size is shown in Figure 2. The fracture morphology of the tensile samples was observed using a SUPRA55 field emission scanning electron microscope. The Vickers hardness of the alloy billet was tested using a 310HVS-5 digital microhardness tester with a test load of 2.94 N and a loading time of 10 s. The Vickers hardness value of each sample was the average of 10 test values.
3. Experimental Results and Analysis
3.1 Effect of Sc Content on the Microstructure and Properties of ZL205A Alloy
Figure 3 shows the metallographic structures of the modified ZL205A alloys with different Sc contents. It can be seen from Figures 3a and 3b that the metallographic structure of the gravity-cast ZL205A alloy without Sc mainly consisted of white α-Al dendrites and black eutectic phases. Compared with the metallographic structure of the squeeze-cast ZL205A alloy, the grains were coarser and the grain boundaries were not clear. With the increase in the Sc content in the squeeze-cast ZL205A alloy from 0 to 0.4%, the dendrites of the base alloy were significantly refined, the secondary dendrites gradually decreased, and the black eutectic phase structure was significantly reduced and became finer. Sc can react with Al to form the second phase Al₃Sc with an L1₂ structure. During the solidification of the aluminum alloy, the Al₃Sc phase can act as a heterogeneous nucleation site, increasing the nucleation rate of the aluminum alloy. At the same time, the Al₃Sc phase exists in the aluminum alloy matrix in a coherent form with the matrix phase, which can pin dislocations and refine grains, thereby hindering the coarsening and growth of the aluminum alloy matrix grains. When the Sc content in the squeeze-cast ZL205A alloy continued to increase to 0.6%, the matrix grains coarsened, and the grain boundaries exhibited a discontinuous phenomenon similar to that of the gravity-cast alloy. Moreover, compared with the alloy without Sc addition, the black eutectic structure significantly increased. Therefore, an appropriate amount of Sc has a significant modification effect on the ZL205A alloy, which can refine the grain size of the base alloy, pin dislocations and refine subgrains, thereby improving the mechanical properties of the aluminum alloy. However, excessive Sc can weaken or even eliminate this modification effect.
| Figure | Microstructure |
|---|---|
| 3a | Gravity-cast ZL205A alloy without Sc |
| 3b | Squeeze-cast ZL205A alloy without Sc |
| 3c | Squeeze-cast ZL205A alloy with 0.2% Sc |
| 3d | Squeeze-cast ZL205A alloy with 0.4% Sc |
| 3e | Squeeze-cast ZL205A alloy with 0.6% Sc |
Figure 4 shows the room-temperature mechanical properties of the modified ZL205A alloys with different Sc contents. It can be seen that the comprehensive mechanical properties of the gravity-cast ZL205A alloy were poor, while the comprehensive properties of the aluminum alloy could be significantly improved after squeeze casting and pressure solidification. This is because squeeze casting can basically eliminate the microscopic defects such as shrinkage porosity and shrinkage cavity in the gravity-cast billet structure and increase the undercooling degree of the alloy, promoting the nucleation of the aluminum alloy during the solidification stage and thereby refining the as-cast grain structure. According to the Hall-Petch formula, the grain size has a significant relationship with the strength of the alloy. Within a certain range, the finer the grains, the higher the strength of the alloy. In addition, the fine grain structure shortens the length of dislocation slip and dislocation pile-up groups during the deformation process of the alloy, reduces the stress concentration at the grain boundaries, and also improves the plasticity of the alloy.
| Figure | Mechanical Property |
|---|---|
| 4a | Tensile strength |
| 4b | Elongation |
| 4c | Hardness |
After adding an appropriate amount of Sc to the ZL205A aluminum alloy, although the elongation of the aluminum alloy decreased slightly, its tensile strength was significantly increased. As shown in Figure 3d, the grain structure of the aluminum alloy matrix with 0.4% Sc was finer, so the modification strengthening effect of Sc was fully exerted, and the tensile strength of the alloy reached 389.5 MPa. However, excessive Sc led to further coarsening of the aluminum alloy matrix grain structure, and its modification strengthening effect was weakened to some extent. As shown in Figure 4a, when the Sc content in the aluminum alloy was 0.6%, its tensile strength decreased by 7.3%. With the increase in Sc content, the hardness value of the aluminum alloy first increased and then decreased. When the Sc content reached 0.4%, the hardness (HV) of the aluminum alloy reached the maximum value of 122.9. This is because when the Sc content in the aluminum alloy is less than 0.4%, the main strengthening precipitates in the aluminum alloy are uniformly and dispersedly distributed in the matrix, increasing the hardness of the alloy. When the Sc content further increased to 0.6%, the hardness (HV) of the alloy decreased to 112.7 (a decrease of 8.3% compared with the aluminum alloy with 0.4% Sc), which is due to the aggregation of the second phase leading to a decrease in the properties of the alloy.
3.2 Effect of Zr and Sc Contents on the Microstructure and Properties of ZL205A Alloy
Figure 5 shows the metallographic structures of the squeeze-cast ZL205A alloys with different Zr contents when the Sc content was 0.40%. It can be seen that with the increase in Zr content from 0 to 0.15%, the microstructure of the aluminum alloy was significantly refined, and some grains gradually changed from dendrites to near-spherical grains. However, when the Zr content further increased to 0.20%, the matrix grains of the aluminum alloy did not continue to be refined, and their grain sizes were basically the same as those when the Zr content was 0.15%. Zr and Sc have the same lattice type, and the combined addition of the two elements can enhance the modification strengthening effect of Sc. This is because during the solidification of the aluminum alloy, Zr will replace part of the Sc positions in the Al₃Sc phase to form a new Al₃(Sc, Zr) phase. The Al₃(Sc, Zr) phase has a strong modification effect, and its precipitation density is significantly increased and the distribution is more dispersed compared with Al₃Sc, so it has a stronger ability to inhibit grain coarsening.
| Figure | Microstructure |
|---|---|
| 5a | Squeeze-cast ZL205A alloy with 0.40% Sc and 0% Zr |
| 5b | Squeeze-cast ZL205A alloy with 0.40% Sc and 0.10% Zr |
| 5c | Squeeze-cast ZL205A alloy with 0.40% Sc and 0.15% Zr |
| 5d | Squeeze-cast ZL205A alloy with 0.40% Sc and 0.20% Zr |
Figure 6 shows the room-temperature tensile mechanical properties of the squeeze-cast modified ZL205A alloys with different Zr and Sc contents. It can be seen that when Zr and Sc were added together, the tensile strength of the ZL205A aluminum alloy was significantly improved. That is, adding an appropriate amount of Zr on the basis of Sc addition could further improve the comprehensive properties of the aluminum alloy. When 0.15% Zr was added to the aluminum alloy with Sc addition amounts of 0.20%, 0.40%, and 0.60%, the tensile strength of the aluminum alloy increased by 2.2% – 4.2%, and the elongation remained stable. Considering the tensile strength and elongation of the alloy, the optimal element content was Zr of 0.15% and Sc of 0.40%. After composition modification, the tensile strength of the aluminum alloy reached 398.3 MPa, which was 14.4% higher than that of the unmodified ZL205A aluminum alloy.
| Figure | Mechanical Property |
|---|---|
| 6a | Tensile strength |
| 6b | Elongation |
Figure 7 shows the tensile mechanical properties of the squeeze-cast modified ZL205A alloys with different Zr and Sc contents at 150 °C. It can be seen that with the increase in the Sc content in the aluminum alloy from 0 to 0.40%, the comprehensive mechanical properties of the aluminum alloy continuously improved. When the Sc content was 0.40%, the tensile strength at 150 °C increased by 20.4% and the elongation increased by 59.6% compared with the base alloy. Similar to the room-temperature mechanical properties, when the Sc content continued to increase to 0.60%, the comprehensive mechanical properties of the aluminum alloy slightly decreased. This is because Zr and Sc form primary Al₃(Sc, Zr) particles, which have a strong grain refinement effect and also have a dispersion strengthening effect, can pin dislocations and inhibit the recrystallization of the aluminum alloy. When the deformation temperature increases, the hindering effect of the Al₃(Sc, Zr) particles on dislocation movement weakens, and plastic deformation is more likely to occur. However, when the Sc content is too high, the number of Al₃(Sc, Zr) phases in the aluminum alloy is relatively large, and they gradually change from a dispersed particulate state to a continuous network structure, resulting in a decrease in the mechanical properties of the aluminum alloy.
| Figure | Mechanical Property |
|---|---|
| 7a | Tensile strength |
| 7b | Elongation |
