This article comprehensively studies the influence of Sc and Zr on the microstructure and properties of squeeze cast ZL205A alloy. Through a series of experiments and analyses, it is found that appropriate addition of Sc and Zr can refine the grain structure of the alloy, improve its mechanical properties, and the alloy has good application prospects in the manufacture of connecting rod components.
1. Introduction
With the increasing emphasis on resource conservation and environmental protection in the world, lightweight technology has received extensive attention, promoting the research and development of lightweight alloy materials. Among them, casting aluminum alloys represented by ZL205A have become one of the research hotspots due to their good casting performance, low density, low cost, and high productivity. However, the large crystallization interval of ZL205A aluminum alloy easily leads to microstructure and performance defects in formed parts, which is difficult to meet the actual application requirements. Therefore, it is necessary to further optimize the comprehensive performance of ZL205A aluminum alloy.
Microalloying is an efficient and convenient method to improve the performance of aluminum alloys. Adding 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. In this study, the effects of Sc and Zr and their addition amounts on the microstructure optimization and mechanical property enhancement of aluminum alloys were investigated by melt treatment + squeeze casting, and the application of modified aluminum alloys in the direction of lightweight and complex-shaped high-performance components was explored.
2. Experimental Materials and Methods
2.1 Experimental Materials and Preparation
The base alloy is 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 the modification of the ZL205A alloy composition are shown in Table 2.
The ZL205A base alloy was melted in a graphite crucible coated with ZnO using 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. The melt was further heated to 740 °C, and after the ZL205A base alloy was melted, Al-4Zr and Al-2Sc master alloys were added, and the alloy melt was充分 stirred to ensure the uniform distribution of Zr and Sc elements. C₂Cl₆ refining agent was added to the alloy melt for degassing and refining, and then the alloy melt was poured into the cavity of the squeeze casting mold. After pressure solidification, the part was ejected by the ejector rod. 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 (volume fraction: 2.5% HNO₃ + 1.5% HCl + 1% HF + 95% H₂O) for 10 s, and the microstructure was observed using an OLYMPUS – GX71 metallographic 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 fracture morphology of the tensile specimens was observed using a SUPRA55 field emission scanning electron microscope. The Vickers hardness value of the alloy billet was tested using a 310HVS – 5 digital small load Vickers hardness tester with a test load of 2.94 N and a loading time of 10 s. The Vickers hardness result of each specimen was the average of 10 test values.
3. Experimental Results and Analysis
3.1 The Influence of Sc Content on the Microstructure and Properties of ZL205A Alloy
3.1.1 Microstructure Analysis
The metallographic structures of the modified ZL205A alloys with different Sc contents are shown in Figure 3. It can be seen that the metallographic structure of the gravity-cast ZL205A alloy without Sc mainly consists of white α-Al dendrites and black eutectic phases. Compared with the metallographic structure of the squeeze-cast ZL205A alloy, the grains are coarser and the grain boundaries are not clear. With the increase of Sc content in the squeeze-cast ZL205A alloy from 0 to 0.4%, the dendrites of the base alloy are明显 refined, the secondary dendrites gradually decrease, and the black eutectic phase组织 is明显 reduced and the size is more fine. When the Sc content in the squeeze-cast ZL205A alloy继续 increases to 0.6%, the base grains are coarsened, and the grain boundaries show a discontinuous phenomenon similar to that of the gravity-cast alloy, and the black eutectic组织 is明显 increased compared with the alloy without Sc addition.
3.1.2 Mechanical Property Analysis
The room temperature mechanical properties of the modified ZL205A alloys with different Sc contents are shown in Figure 4. It can be seen that the comprehensive mechanical properties of the ZL205A alloy prepared by gravity casting are poor, while the comprehensive performance of the aluminum alloy can be greatly improved after squeeze casting and pressure solidification. After adding an appropriate amount of Sc to the ZL205A aluminum alloy, although the elongation of the aluminum alloy decreases, its tensile strength is greatly improved. When the Sc content is 0.4%, the tensile strength of the alloy reaches 389.5 MPa. However, excessive Sc will lead to further coarsening of the aluminum alloy matrix grain组织, and its modification strengthening effect is weakened to a certain extent. With the increase of Sc content, the hardness value of the aluminum alloy first increases and then decreases. When the Sc content reaches 0.4%, the hardness (HV) of the aluminum alloy reaches the maximum value of 122.9.
3.2 The Influence of Zr and Sc Contents on the Microstructure and Properties of ZL205A Alloy
3.2.1 Microstructure Analysis
The metallographic structures of the squeeze-cast ZL205A alloys with 0.40% Sc and different Zr contents are shown in Figure 5. It can be seen that with the increase of Zr content from 0 to 0.15%, the microstructure of the aluminum alloy is明显 refined, and some grains gradually change from dendrites to near-spherical grains. However, when the Zr content further increases to 0.20%, the base grains of the aluminum alloy do not continue to be refined, and the grain size is basically the same as that when the Zr content is 0.15%.
3.2.2 Mechanical Property Analysis
The room temperature tensile mechanical properties of the squeeze-cast modified ZL205A alloys with different Zr and Sc contents are shown in Figure 6. It can be seen that when Zr and Sc are added together, the tensile strength of the ZL205A aluminum alloy is明显 improved. When the addition amounts of Zr and Sc are 0.15% and 0.40% respectively, the tensile strength of the alloy reaches 398.3 MPa, which is 14.4% higher than that of the unmodified ZL205A aluminum alloy. The mechanical properties of the modified ZL205A alloys at 150 °C are shown in Figure 7. It can be seen that with the increase of Sc content from 0 to 0.40%, the comprehensive mechanical properties of the aluminum alloy at 150 °C are continuously improved. When the Sc content is 0.40%, compared with the base alloy, the tensile strength at 150 °C is increased by 20.4%, and the elongation is increased by 59.6%.
The T5 heat treatment was carried out on the ZL205A aluminum alloy with 0.15% Zr and 0.40% Sc. The tensile strength of the modified aluminum alloy at 150 °C after T5 heat treatment reaches 345.1 MPa, and the elongation is 15.8%.
The fracture morphologies of the squeeze-cast modified ZL205A alloys with different Zr and Sc contents at room temperature and 150 °C are shown in Figure 8. It can be seen that there are some shrinkage porosity and shrinkage holes in the tensile fracture of the unmodified aluminum alloy at room temperature, and the fracture is relatively flat and the dimples are relatively shallow, indicating that the plastic deformation before the fracture of the alloy is not充分. When the Zr content is 0.15% and the Sc content is 0.40%, the dimples at room temperature and 150 °C are deeper and larger, and the plastic deformation before the fracture of the tensile specimen is more充分. At this time, the second phase particles are uniformly distributed in size and shape, and the strengthening effect of the second phase particles on the comprehensive properties of the alloy is the best.
3.3 Squeeze Casting Forming and Mechanical Properties of ZL205A Aluminum Alloy Connecting Rod
Based on the lightweight requirements of automobile engines and the actual application needs of complex-shaped high-performance components, the ZL205A aluminum alloy with 0.40% Sc and 0.15% Zr was used as the raw material to design the size of the aluminum alloy connecting rod component and the connecting rod squeeze casting mold. The squeeze casting forming process of the connecting rod was carried out, and the mechanical properties of the formed connecting rod component were tested.
The results show that the squeeze-cast formed component has a complete structure, no obvious cracks, shrinkage holes, folds, and other macro-defects on the surface, and the forming is good. The tensile strength and elongation of the connecting rod component are measured, and it is found that compared with the modified ZL205A aluminum alloy squeeze casting casting, the tensile strength of the formed connecting rod component is higher. The hardness values at different positions of the connecting rod component are also measured, and it is found that the maximum hardness value (HV) is located at the front flange of the large end of the connecting rod, which is 146.6.
4. Conclusions
In this study, the effects of Sc and Zr on the microstructure and properties of squeeze-cast ZL205A alloy were investigated. The following conclusions are drawn:
- Adding an appropriate amount of Sc to the ZL205A aluminum alloy can refine the matrix grain组织 and improve the comprehensive mechanical properties. When the Sc addition amount is 0.4%, the comprehensive mechanical properties of the alloy are the best.
- Adding Zr and Sc elements to the ZL205A aluminum alloy can further refine the matrix grain组织 and improve the comprehensive mechanical properties through the formation of Al₃(Sc, Zr) phase. When the Zr addition amount is 0.15% and the Sc addition amount is 0.40%, the comprehensive mechanical properties of the alloy are the best.
- The ZL205A aluminum alloy with 0.40% Sc and 0.15% Zr has good formability in the squeeze casting of the connecting rod component, and the mechanical properties of the formed connecting rod component are improved compared with the modified aluminum alloy casting.
The research results provide a reference for the optimization of the microstructure and the improvement of the comprehensive performance of high-strength and tough casting Al – Cu alloys, and also provide a reference for the application of modified aluminum alloys in the direction of lightweight and complex-shaped high-performance components.
4.1 The Role of Sc in Refining the Microstructure of ZL205A Alloy
The addition of Sc to ZL205A alloy leads to the formation of second phase with structure. During the solidification of the alloy, phase acts as heterogeneous nucleation sites, increasing the nucleation rate of the alloy. At the same time, phase exists in the aluminum alloy matrix in a coherent form with the matrix, which can pin dislocations and refine grains, thereby hindering the coarsening and growth of the aluminum alloy matrix grains. This is the main reason why Sc can refine the microstructure of ZL205A alloy.
4.2 The Synergistic Effect of Zr and Sc on the Microstructure and Properties of ZL205A Alloy
When Zr and Sc are added simultaneously, Zr can replace some Sc positions in phase to form phase. The precipitation density of phase is significantly increased compared with phase, and the distribution is more dispersed, so it has a stronger ability to inhibit grain coarsening. This synergistic effect of Zr and Sc can further refine the microstructure of the alloy and improve its mechanical properties.
4.3 The Influence of Heat Treatment on the Properties of ZL205A Alloy
The T5 heat treatment process used in this study includes solution treatment at 535 °C for 10 h, quenching in 60 °C water immediately, and artificial aging treatment at 175 °C for 12 h. During the aging treatment, precipitates nucleate, and Sc atoms segregate at the interface between phase and the aluminum matrix, restricting the diffusion of Cu atoms from the matrix to phase, thereby 抑制 the growth of phase and further improving the aging strengthening effect of the aluminum alloy. This is why the mechanical properties of the alloy at 150 °C are improved after T5 heat treatment.
5. Application and Prospect
The modified ZL205A alloy with Sc and Zr addition shows excellent mechanical properties and good formability, especially in the manufacture of automobile engine connecting rod components. The lightweight design of automobile engines is of great significance for improving fuel efficiency and reducing emissions. The application of this high-performance aluminum alloy in the automotive industry can not only meet the performance requirements of engine components but also contribute to the lightweight of automobiles.
In the future, with the continuous development of lightweight technology and the increasing demand for high-performance materials, the research and application of modified aluminum alloys will continue to expand. It is expected that more advanced microalloying techniques and heat treatment processes will be developed to further improve the performance of aluminum alloys and meet the more stringent requirements of various industries.
In conclusion, the study of the influence of Sc and Zr on the microstructure and properties of squeeze-cast ZL205A alloy provides valuable insights for the development and application of high-performance aluminum alloys. The results of this study have important theoretical and practical significance for promoting the lightweight and high-performance development of the automotive industry and other fields.
