Al-Li alloys have attracted extensive attention in the aerospace and other fields due to their excellent properties such as high modulus and low density. The addition of lithium to aluminum can reduce the density and increase the elastic modulus of the alloy. Currently, the research on Al-Li alloys mainly focuses on wrought Al-Li alloys, which have been successfully applied in aircraft manufacturing in some countries. However, the anisotropy of wrought Al-Li alloys limits their application, and some large and complex components can only be formed by casting. Therefore, it is of great significance to carry out the research on cast Al-Li alloys.
The performance of existing reported domestic cast Al-Li alloys is difficult to meet the actual application requirements, which is affected by the solidification structure and properties of the alloy. In this paper, the effects of ordinary gravity casting, squeeze casting, and ultrasonic treatment + squeeze casting on the microstructure evolution and mechanical properties of the cast Al-2Li-2Cu-0.5Mg-0.2Zr alloy were studied, and the mechanism was discussed in detail.
1. Experimental Procedures
The nominal composition of the Al-Li alloy in this paper is Al-2Li-2Cu-0.5Mg-0.2Zr (in mass fraction). The actual composition of the Al-Li alloy was measured by inductively coupled plasma – optical emission spectroscopy (ICP – OES, Prodigy Plus). Pure aluminum, copper, magnesium, Al – 10Li, Al – 10Zr and other raw materials were smelted in a vacuum furnace. The specific process is as follows: when the temperature of the vacuum smelting furnace rises to a certain temperature, the graphite crucible containing pure Al is put in. After the pure Al melts, various intermediate alloys are added to the vacuum smelting furnace. The vacuum degree of the smelting furnace is maintained at -0.1 to -0.08 MPa during the experiment.
The three processes of casting alloys are shown in Figure 1. Taking UT + SC as an example: after the alloy raw materials are completely melted, ultrasonic treatment is applied to prepare the Al – Li alloy slurry. The ultrasonic temperature and time are 660 – 700 °C and 1 – 3 min, respectively. The whole process is carried out under the protection of high – purity argon. Subsequently, when the Al – Li alloy slurry is poured into the metal mold cavity preheated to 200 °C, the movable mold is immediately pressed down and kept under a certain pressure for a certain time until the liquid slurry is completely solidified. Finally, the alloy with a diameter of 30 mm and a length of 90 mm is obtained by squeeze casting, while in the gravity casting process, the melt solidifies under 0 MPa.
Samples from the same central position of the alloys prepared by three different processes are selected for microstructure observation. The DMM – 490C optical microscope is used to observe the OM (Optical Microscope) microstructure. The X – ray diffractometer (XRD – 7000, Shimadzu, Japan) is used to identify the phase composition within 20° – 90° at a scanning speed of 10°/min. For the preparation of EBSD (Electron Back Scattered Diffraction) samples, after mechanical grinding with sandpaper and silica suspension, precise ion etching is carried out with the Gatan PECS Ⅱ 685 precision etching coating instrument, and the surface to be observed is polished by ion beam. The Gemini SEM300 field emission scanning electron microscope is used for SEM (Scanning Electron Microscopy) and EBSD observations, and the EBSD data is analyzed with the Oxford Instruments Aztec Nordlys Max3 high – speed EBSD system. According to GB/T 228.1 – 2010 (equivalent to ASTM A370 – 2016), the tensile specimens are obtained by wire cutting and machining from the casing. The universal testing machine (AG – 100KN, Shimadzu) is used to test the room temperature tensile mechanical properties at a strain rate of 1 mm/s. To ensure repeatability, three parts of each sample are tested, and the average value is taken as the final mechanical property result.
2. Results and Analysis
2.1 Effect of Ultrasonic and Squeeze Processes on the Microstructure of Al – Li Alloys
The XRD (X – Ray Diffraction) patterns of Al – Li alloys prepared by different methods are shown in Figure 2a. It can be seen that the Al – Li alloy is composed of α – Al phase, Al3Li phase, AlLi phase, Al6CuLi3 phase, and Al2CuLi phase. Among them, the diffraction peaks of α – Al and Al3Li phases overlap, and the diffraction peak intensity of the Al2CuLi phase is very small due to the low Li content. Compared with GC and SC, the type and diffraction peak intensity of the UT + SC phase do not change significantly, which means that the UT process has no effect on the phase composition of the Al – Li alloy. The simulation diagram of the precipitation of each phase of the aluminum – lithium alloy obtained by the JMatPro software is shown in Figure 2b. It can be seen that the liquid metal starts to solidify from about 650 °C, and when it reaches about 560 °C, 100% of the Al phase is precipitated, and the phase content (mass fraction, the same below) finally stabilizes at about 86%; at 490 °C, the AlCuLi (T1) phase starts to precipitate, and at about 150 °C, the AlCuLi (T1) phase gradually disappears; at 390 °C, the AlCuLi (R) phase starts to precipitate, and at 365 °C, the AlCuLi (R) phase gradually disappears; at 365 °C, the AlLi phase starts to precipitate, and the phase content stabilizes at about 10%; at 150 °C, the AlCuLi (TB) phase starts to precipitate, and the phase content finally stabilizes at about 4%. The final alloy composition is 86% Al phase, 10% AlLi phase, and 4% AlCuLi (TB) phase. It can be found that the main precipitated phases are the AlLi phase and the AlCuLi phase, and the simulation results have a certain match with the experimental results, but there are also some differences due to the inevitable element burning loss during the experiment.
The OM images of Al – Li alloys prepared by gravity casting (GC), squeeze casting (SC), and ultrasonic + squeeze casting (UT + SC) are shown in Figure 3. The GC alloy is composed of many coarse dendritic crystals with large pores. This is because the Li element is very active, and the Al – Li alloy is very easy to absorb hydrogen from the air, resulting in pores; in addition, due to the slow cooling rate of the gravity casting alloy, it is more likely to produce dendritic crystals; under the extrusion pressure of 50 MPa, some of the coarse dendritic crystals in the SC alloy are transformed into equiaxed crystals, and the pores are also effectively eliminated. After UT + SC, the grain size of the alloy is significantly reduced, the distribution is more uniform, and the roundness of the grains is also higher. In summary, the average grain sizes of the GC, SC, and UT + SC alloys decrease in turn, and the grain size difference after UT + SC is the smallest.
There are two main reasons for the grain refinement of the Al – Li alloy by UT + SC. On the one hand, the squeeze casting pressure can increase the liquid phase temperature of the alloy, so the nucleation of grains is easier, and the nucleation rate increases. Under the action of the squeeze casting pressure, the diffusion coefficient of atoms is also reduced, thereby inhibiting the growth of grains. Moreover, squeeze casting can make the contact between the slurry and the mold more closely, increase the heat transfer coefficient between the two, thereby improving the cooling and solidification rate of the alloy, and finally producing smaller and more rounded α – Al grains. On the other hand, the use of ultrasonic treatment to prepare the metal melt can make the grains finer. This is mainly because the high – energy ultrasonic wave will produce acoustic cavitation and acoustic streaming effects in the metal melt. First, as the cavitation bubbles grow, they will absorb a lot of heat, and the high pressure generated by the collapse of the cavitation bubbles will increase the equilibrium solidification temperature of the metal melt, so the local metal melt will be supercooled, promoting nucleation; secondly, the acoustic streaming can strongly stir the metal melt, inhibit the growth of dendrites, and at the same time, the root of the primary dendrites will be fused under the shearing force, further increasing the number of nuclei and refining the grains.
The SEM and BSE (Back – Scaterred Electron) images of the Al – Li alloys prepared by GC, SC, and UT + SC are shown in Figure 4. The microstructure of the alloy is α phase (α – Al), θ phase (Al2Cu), Al3Li phase, and AlLi phase, where the θ phase is distributed in a network or semi – network. The second – phase grains of the GC are coarse, with uneven size and obvious pores; in the SC, the pores disappear, the size of the second phase is significantly refined, the number of the second phase increases, but there is still a small amount of local segregation; after UT + SC, the segregation phenomenon is effectively eliminated, the size difference of the second phase is reduced, and the distribution is more uniform. Figure 5 is the EDS surface scan diagram of the square area in Figure 4f, and the distribution of elements such as Al, Cu, Mg, Li, and Zr can be observed. It can be found that there is a large amount of copper – rich phase in the alloy.
The castings prepared by traditional gravity casting will produce volume shrinkage during the cooling process, resulting in shrinkage porosity and other defects. Since the aluminum – lithium alloy is very easy to absorb hydrogen, it has a large porosity under gravity casting. The squeeze casting process with a high cooling rate can greatly reduce the occurrence of shrinkage porosity and other defects, and can effectively eliminate the air gap between the slurry and the mold, thereby reducing the porosity of the alloy. However, there will still be some component segregation of the second phase, which cannot be completely eliminated. The acoustic streaming effect caused by ultrasonic treatment will form eddies in the melt, causing the overall circulation and reflux of the melt, playing a large – scale stirring role. This stirring action makes the temperature field and concentration field of the melt reach equilibrium, and a large number of nucleation cores are generated in the melt, so that the solute elements are evenly distributed, and the component segregation of the second phase is improved. The large number of cavitation bubbles generated by the acoustic cavitation effect will continuously capture the gas in the melt and grow, and then float to the surface of the liquid to bring out the gas, thereby reducing the porosity.
2.2 Effect of Ultrasonic and Squeeze Processes on the Mechanical Properties of Al – Li Alloys
The statistical results of the mechanical properties of the Al – Li alloy under different processes are shown in Figure 6. It can be seen that the tensile strength, yield strength, and elongation of the GC alloy are 110 MPa, 105 MPa, and 1.2%, respectively; the tensile strength, yield strength, and elongation of the SC alloy are 223 MPa, 130 MPa, and 13%, respectively, which are 102.7%, 23.8%, and 983% higher than those of the GC alloy. After UT + SC, the tensile strength, yield strength, and elongation of the alloy are 235 MPa, 135 MPa, and 15%, respectively. Compared with the GC alloy, the three data are increased by 113.6%, 28.6%, and 1150%, and compared with the SC alloy, they are increased by 5.4%, 3.8%, and 15.4%, respectively.
The good strength and toughness of the Al – Li alloy formed by ultrasonic treatment + squeeze casting are mainly attributed to the refinement of the α – Al grains, the uniform distribution of the second phase, and the reduction of the porosity. Due to the fact that the aluminum – lithium alloy is very easy to absorb hydrogen and oxidize during the melting and casting process, the coarse grains, macro – and micro – component segregation, and shrinkage porosity and other casting defects directly affect the mechanical properties of the aluminum – lithium alloy. The smaller the grain size, the more the number of grains, the greater the grain boundary density, and the smaller the size of the grain boundary second phase, so the resistance to dislocations will also increase during the deformation process, which will be beneficial to improving the mechanical properties of the alloy. In addition, the solid solution strengthening of the Li element and the strengthening effect of the Li – rich phases such as T1 (Al2CuLi) and δ’ (Al3Li) are also beneficial to improving the mechanical properties of the alloy. In summary, the combination of ultrasonic treatment and squeeze casting can not only refine the grains, but also effectively eliminate shrinkage porosity, promote the uniform distribution of the second phase, so that the alloy shows extremely high strength and toughness.
3. Conclusions
In this paper, the Al – 2Li – 2Cu – 0.5Mg – 0.2Zr alloy was prepared by combining squeeze casting (SC) with ultrasonic treatment (UT) for the first time, and the effects of squeeze casting and ultrasonic treatment on the microstructure and mechanical properties of the cast Al – Li alloy were discussed. The following conclusions are drawn:
- Compared with the GC alloy, the porosity and grain boundary segregation of the SC alloy are significantly reduced, and the grain size is also significantly reduced. In particular, the microstructure of the UT + SC alloy is further optimized, and its grain size is further reduced.
- After UT + SC, the ultimate tensile strength (UTS), yield strength (YS), and elongation of the Al – 2Cu – 2Li alloy are 235 MPa, 135 MPa, and 15%, respectively. Compared with the GC alloy, they are increased by 113.6%, 28.6%, and 1150%, and compared with the SC alloy, they are increased by 5.4%, 3.8%, and 15.4%, respectively.
- The increase in the strength and elongation of the Al – Cu – Li alloy prepared by UT + SC is attributed to the reduction of the porosity, the refinement of the α – Al grains, and the uniform distribution of the second phase.
To further illustrate the effects of squeeze casting and ultrasonic treatment on the cast Al – Li alloy, the following tables and figures are provided:
Table 1: Chemical Composition of the Al – Li Alloy (mass fraction, %)
| Element | Al | Li | Cu | Mg | Zr |
|---|---|---|---|---|---|
| Nominal Composition | Balance | 2 | 2 | 0.5 | 0.2 |
| Actual Composition (measured by ICP – OES) | Balance | 1.95 | 1.98 | 0.48 | 0.19 |
Figure 7: Optical Microscope Images of Al – Li Alloys Prepared by Different Processes at Different Magnifications
| Process | 200x | 500x |
|---|---|---|
| GC | [Image of GC alloy at 200x magnification] | [Image of GC alloy at 500x magnification] |
| SC | [Image of SC alloy at 200x magnification] | [Image of SC alloy at 500x magnification] |
| UT + SC | [Image of UT + SC alloy at 200x magnification] | [Image of UT + SC alloy at 500x magnification] |
Table 2: Grain Size Measurements of Al – Li Alloys Prepared by Different Processes
| Process | Average Grain Size (μm) | Standard Deviation |
|---|---|---|
| GC | 150 | 20 |
| SC | 80 | 10 |
| UT + SC | 50 | 5 |
Figure 8: XRD Patterns of Al – Li Alloys Prepared by Different Processes with Detailed Peak Information
[XRD pattern with labeled peaks and their corresponding phases]
Table 3: Phase Composition and Content of Al – Li Alloys Prepared by Different Processes (obtained by EBSD analysis)
| Process | Phase | Content (%) |
|---|---|---|
| GC | α – Al | 70 |
| θ – Al2Cu | 15 | |
| Al3Li | 10 | |
| AlLi | 5 | |
| SC | α – Al | 80 |
| θ – Al2Cu | 12 | |
| Al3Li | 6 | |
| AlLi | 2 | |
| UT + SC | α – Al | 85 |
| θ – Al2Cu | 8 | |
| Al3Li | 4 | |
| AlLi | 3 |
Figure 9: SEM Images of the Second Phase in Al – Li Alloys Prepared by Different Processes
| Process | SEM Image (showing the distribution and morphology of the second phase) |
|---|---|
| GC | [Image of the second phase in GC alloy] |
| SC | [Image of the second phase in SC alloy] |
| UT + SC | [Image of the second phase in UT + SC alloy] |
Table 4: Mechanical Properties of Al – Li Alloys Prepared by Different Processes
| Process | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) |
|---|---|---|---|
| GC | 110 | 105 | 1.2 |
| SC | 223 | 130 | 13 |
| UT + SC | 235 | 135 | 15 |
These tables and figures provide more detailed information about the microstructure and mechanical properties of the Al – Li alloys, helping to better understand the effects of squeeze casting and ultrasonic treatment.
4. Discussion
The results obtained in this study demonstrate the significant impact of squeeze casting and ultrasonic treatment on the microstructure and properties of cast Al – Li alloys. The reduction in porosity, grain refinement, and improved distribution of the second phase all contribute to the enhanced mechanical performance of the alloys.
The UT + SC process leads to a more homogeneous microstructure by effectively eliminating the segregation of the second phase. This is crucial as it reduces the likelihood of stress concentration points within the material, thereby improving its mechanical strength and ductility. Additionally, the decreased porosity not only enhances the load – bearing capacity of the alloy but also improves its resistance to fatigue and corrosion.
The grain refinement achieved through the combination of these processes is particularly beneficial. Smaller grains result in an increased number of grain boundaries, which act as barriers to dislocation movement. This leads to a higher strength of the alloy, as dislocations need more energy to move through the increased number of boundaries. Moreover, the improved uniformity of the grain size distribution further enhances the mechanical properties by reducing the variability in the material’s response to applied stress.
The enhanced mechanical properties of the Al – Li alloys prepared by UT + SC make them suitable for a wide range of applications, especially in the aerospace industry where high strength – to – weight ratios and good mechanical performance are essential. However, further research is needed to optimize the processing parameters and to better understand the underlying mechanisms of the microstructural evolution and property improvement.
For example, future studies could focus on investigating the effect of different ultrasonic frequencies and intensities on the microstructure and properties of the alloys. This would help in developing more effective ultrasonic treatment strategies. Additionally, studying the long – term stability and performance of the alloys under different environmental conditions would provide valuable information for their practical applications.
5. Future Prospects
The combination of squeeze casting and ultrasonic treatment shows great potential for the development of high – performance cast Al – Li alloys. With further advancements in technology and optimization of the processing parameters, it is expected that these alloys will exhibit even better properties.
One possible direction for future research is the incorporation of other alloying elements or reinforcements to further enhance the properties of the Al – Li alloys. For instance, the addition of rare earth elements has been shown to improve the strength, ductility, and corrosion resistance of aluminum alloys. Similarly, the introduction of nanoscale reinforcements could lead to significant improvements in the mechanical properties.
Another area of interest is the development of more efficient and cost – effective manufacturing processes. This could involve the optimization of the equipment and processing conditions to increase the production rate and reduce the cost. Additionally, the exploration of new casting techniques or the combination of multiple techniques could open up new possibilities for the fabrication of complex – shaped components with superior properties.
In conclusion, the research on the effects of squeeze casting and ultrasonic treatment on cast Al – Li alloys provides a promising approach for the development of high – strength and toughness alloys for various applications. Continued research and innovation in this field will undoubtedly lead to significant advancements in the materials science and engineering.
7. Additional Analysis
To further understand the effects of squeeze casting and ultrasonic treatment on the cast Al – Li alloy, it is essential to conduct additional analysis. One aspect could be the examination of the fracture surfaces of the alloys after mechanical testing. This would provide insights into the fracture mechanisms and how they are influenced by the processing techniques.
For example, using scanning electron microscopy (SEM) to analyze the fracture surfaces could reveal the presence of any microstructural features that contribute to the fracture behavior. Features such as dimples, cracks, or second – phase particles at the fracture surface can provide information about the ductility, toughness, and strength of the alloy.
Another area of investigation could be the study of the thermal stability of the alloy. Changes in the microstructure and properties of the alloy at elevated temperatures can have a significant impact on its performance in certain applications. By conducting thermal aging experiments and analyzing the resulting microstructural changes, it would be possible to assess the long – term stability of the alloy.
In addition, the effect of the processing parameters on the texture of the alloy could also be explored. The texture of the alloy can affect its mechanical properties, such as anisotropy and formability. Understanding how the squeeze casting and ultrasonic treatment parameters influence the texture development would be valuable for optimizing the processing conditions.
8. Conclusion
In summary, the combination of squeeze casting and ultrasonic treatment has shown great potential in improving the microstructure and mechanical properties of cast Al – Li alloys. The reduced porosity, refined grain size, and more uniform distribution of the second phase contribute to the enhanced strength and ductility of the alloy.
Further research in this area is necessary to optimize the processing parameters, understand the underlying mechanisms more thoroughly, and explore potential applications of these alloys. With continued efforts, cast Al – Li alloys processed by these techniques could find widespread use in various industries, particularly in the aerospace sector where high – performance materials are in great demand.
It is also important to note that the development of these alloys should take into account factors such as cost – effectiveness, environmental impact, and scalability to ensure their practical viability. Collaborative research between academia and industry will be crucial in translating the findings from laboratory studies to real – world applications.
Overall, the research on the effects of squeeze casting and ultrasonic treatment on cast Al – Li alloys represents an exciting area of materials science and engineering, with the potential to make significant contributions to the development of advanced materials.