Research on microstructure and properties control of 2A14 aluminum alloy forged cylinder during rolling forming

In order to solve the problems of uneven mechanical properties, significant anisotropy, and unsatisfactory single point performance during the production process, a study on the free forging billet and rolling process of 2A14 aluminum alloy forged cylinder was carried out, and an integral forged cylinder with a height of 1300mm was successfully rolled. In response to manufacturing difficulties such as difficulty in refining the microstructure and obvious fiber flow direction after deformation of 2A14 aluminum alloy, a high temperature and large deformation multi-directional forging method was used to prepare billets combined with medium temperature rolling to obtain ring billets with small anisotropy differences. After heat treatment, the rolled microstructure underwent recrystallization, and a uniform and fine subgrain structure was formed inside the grains. The tensile strength in all three directions is around 450MPa, the yield strength is greater than 380MPa, and the axial elongation rate is increased to about 8%. The difference in performance in all three directions is relatively small. The overall performance of the components has been significantly improved, which can meet the demand for high-quality structural components in aerospace products.

1. Introduction

Aluminum alloy forged cylinder is a typical structure in aerospace products. Its function is to install and load various instruments and power devices, and to connect other parts, making aerospace products a whole. During launch, flight, and transportation, the shell section will be subjected to various loads, and the stress situation is complex. It is an important component of aerospace products. There are mainly several types of shell segment structures, including hard shell, semi hard shell, and integral wall panel. The comparison of various structural forms of shell segments is shown in Table 1. It can be seen that the overall strength and stiffness of shell segments produced by overall machining are good, with a small number of parts and a smooth appearance, which is beneficial for reducing air resistance. However, limited by the diameter size of the blank, they can only be applied in small shell segments.
Table.1 Comparison of Shell Segments in Various Structural Forms

In recent years, China’s equipment manufacturing and process technology level has developed rapidly, and gradually has the manufacturing technology of large tonnage, high-precision, full-automatic radial axial CNC ring mill. The forging ring manufacturing technology has made a major breakthrough in recent years. In 2014, Factory 211 developed China’s first Super heavy-lift launch vehicle super diameter aluminum alloy forging ring jointly with several units; Mirror milling system is an advanced skin manufacturing process developed in recent years, which can achieve milling of complex curved sheet metal parts. It has real-time thickness detection and compensation functions during the processing process. The multi head mirror milling system has multiple sets of milling modules, which can simultaneously process different parts of forged cylinders. The development of the above equipment technology has led to the increasingly widespread application of forged cylinders formed by high height integral rolling in the aerospace field.
This article summarizes the manufacturing technical difficulties of forging cylinders and develops forming process parameters and control strategies to address quality issues such as uneven mechanical properties, significant anisotropy, and unsatisfactory single point performance that often occur during the production process. Through high-temperature and large deformation multi-directional forging, uniform deformation of the billet is ensured to reduce the tendency towards anisotropy; By accumulating more deformation energy through medium temperature rolling, the deformed structure undergoes recrystallization during solid solution, forming a uniform and fine subgrain structure. The height of the test forgings reached 1300mm, and the mechanical properties were significantly improved, with a significant improvement in uniformity and consistency.

2. Manufacturing Technology Challenges for Forged Cylinders

The whole manufacturing process of forged cylinder includes ring blank preparation, rolling forming, heat treatment and other processes, which need to be heated, cooled and plastic deformed for several times. The history of hot working is complex. There are significant genetic and mutation characteristics in the material organization structure between the upper and lower processes, resulting in the final organizational performance being subject to the coupling effect of multiple process parameters, making it difficult to control the stability of the workpiece. There are the following problems in the production process:

• (1) The mechanical properties of the parts vary greatly, and the mechanical properties of different parts are uneven. The maximum difference in strength indicators is between 40-50MPa;
• (2) The components have obvious anisotropy, with axial and radial strength indicators 30-50MPa lower than the tangential direction, and elongation only 40% -60% of the tangential direction;
• (3) The radial and axial mechanical properties often exhibit single point failure, resulting in unstable product performance. It can be seen that there are still shortcomings in the manufacturing technology of forged cylinders. In order to improve the performance of shell segments and meet the demand for high-quality structural components in aerospace products, the following technical challenges need to be solved.

2.1 Difficulty in effectively refining grains

Fine grain strengthening is a strengthening mechanism for deformed metal materials. The smaller the grain size, the larger the grain boundary area, and the more tortuous it is, which is less conducive to crack propagation. This can enable the material to simultaneously improve plasticity and toughness while improving strength. In Figure 1, (a) – (c) show the as-cast microstructure of 2A14 aluminum alloy, the microstructure of ring billets prepared by free forging, and the microstructure after rolling and heat treatment. The initial state is typical as cast microstructure, with a grain size is still around 200 μm; After free forging, local grains are refined, and there are still coarser grains, resulting in uneven deformation; After rolling and forming, the grains have undergone a certain degree of deformation, but have not been further refined, and the grain size is still around 100 μm.

Figure.1 Microstructure of 2A14 Aluminum Alloy in Different States

2.2 Anisotropy of parts

As an important structural component in aerospace products, forged cylinders withstand significant axial loads, and structural components have high requirements for axial performance. Figure 2 (a) shows the tangential structure of a rolled piece, where the grains are elongated, forming obvious fiber flow lines. The anisotropy of the structure leads to the anisotropy of mechanical properties. The anisotropic properties of the piece are shown in Figure 2 (b), and the tangential performance indicators are significantly better than the other two directions. The tensile strength reaches 440MPa, while the axial and tangential tensile strength are 410MPa and 385MPa, respectively. The elongation differences in the three directions are more significant, with 12%, 6%, and 4.5% in the tangential, axial, and radial directions, respectively, The tangential elongation is 2-3 times that of the other two directions.

Figure.2 Structure and Properties of a 2A14 Aluminum Alloy Rolling Part

3. Control strategy and forming test

In order to improve performance uniformity and improve the qualification rate of the product, it is necessary to consider coordinating deformation in different directions and controlling grain size. The use of high-temperature and large deformation multi-directional forging technology can ensure that the as-cast structure is fully broken, and large deformation is achieved in all three directions, reducing anisotropy; The use of medium temperature rolling forming process can improve the deformation energy storage of materials, increase dislocation density, promote recrystallization of materials during solution treatment, and form fine subgrain structures. The manufacturing process of the forging cylinder includes heating of aluminum alloy ingots, multi-directional forging with high temperature and large deformation, punching, mandrel elongation, expanding, medium temperature rolling forming, heat treatment, and physical and chemical testing.

3.1 High temperature and large deformation multi-directional forging process

Aluminum alloy ingots at room temperature are composed of matrix Al and eutectic phase Al₂Cu. The coarse Al₂Cu phase is distributed in a network at the grain boundaries (Figure 3), and these coarse network phases are brittle. If they cannot be effectively broken and reduced, it will have adverse effects on the performance of the workpiece. At high temperatures, Cu element can be fully dissolved in the Al matrix, and the number of grain boundary phases will be greatly reduced, which will be conducive to the crushing and refinement of Al₂Cu.

Figure.3 Scanning Electron Microscope and Energy Spectrum of 2A14 Aluminum Alloy Ingot

The multi-directional forging process is shown in Figure 4 (a). After repeated upsetting and drawing in the X, Y, and Z directions of the ingot, the ingot undergoes a deformation in all three directions, restoring its initial direction. This process can ensure uniform deformation in all directions of the ingot, and the grains are broken without serious anisotropy, resulting in synchronous improvement of the three directional performance of the workpiece. Figure 4 (b) shows the production process of multi-directional forging with high temperature and large deformation. The temperature set for this experiment is 490 ℃.

Figure.4 Schematic diagram of multi-directional forging principle and production process

3. Medium temperature rolling deformation process

After high-temperature deformation and multi-directional forging, the grain size of the material is generally between 50 and 150 μm. The organizational foundation has been laid for the subsequent production process, but the deformation amount during the rolling process is relatively small, and the possibility of further refining the grains through deformation is relatively small. During the solid solution treatment process, static recrystallization may occur in the microstructure, resulting in the formation of finer subgrains within the grains. The occurrence of recrystallization depends on whether the deformation energy storage can achieve the nucleation driving force to induce nucleation. Metal materials deform at lower temperatures, resulting in a sharp increase in dislocation density, which accumulates a large amount of deformation energy storage and promotes recrystallization of the material during solution treatment, forming fine subgrain structures. Figure 5 shows the medium temperature rolling forming process, with 5 (a) -5 (c) showing the ring blank before rolling, the rolling forming process, and the forging cylinder after rolling, with a shell section height of up to 1300mm.

Figure.5 Rolling Forming Process

4. Organizational performance analysis

4.1 Organizational Analysis

Figures 6 (a) -6 (c) show the tangential, axial, and radial microstructure of the ring blank after multi-directional forging. It can be seen that after forging, the grains deform and break, showing polygonization and size reduction. The original cast structure (Figure 3) no longer exists, and the differences in the three directions are small, without significant fiber flow line structure. This indicates that multi-directional forging with high temperature and large deformation causes significant deformation in all three directions of the ingot, resulting in uniform deformation.

Figure.6 Three dimensional microstructure state of material after multi-directional forging

After the rolling forming is completed, it is subjected to heat treatment. Figure 7 shows the metallographic structure and scanning electron microscope photos after heat treatment. Figure 7 (a) shows the formation of a large number of uniform and fine cellular subgrains within the grains, which is significantly different from the conventional temperature rolling microstructure in Figure 1 (c). This indicates that the deformed microstructure after medium temperature rolling undergoes significant recrystallization during solution treatment, resulting in a more ideal microstructure. Figure 7 (b) shows that after heat treatment, there are uniformly dispersed small second phases within the grains, which are Al₂Cu formed during the aging process.

Figure.7 Metallographic and scanning images after heat treatment

4.2 Performance Analysis

To comprehensively test the mechanical properties of the workpiece, samples are cut from multiple positions uniformly distributed along the circumference, and the tangential, axial, and radial mechanical properties of each measurement point are tested separately. The maximum values of the various indicators in different parts are shown in Table 2. Now, a comparative analysis of the mechanical properties is conducted from the following perspectives.

• (1) Comparison of performance indicators in different parts: In terms of tensile strength, the axial maximum difference is the largest, reaching 15MPa; In terms of yield strength, the tangential maximum difference is the largest, reaching 20MPa; In terms of elongation, the tangential maximum difference is the largest, reaching 3%. Overall, there is little difference in the same mechanical performance index between different parts, indicating that the deformation of the workpiece is uniform and the degree of deformation in each part is consistent.

Table.2 Mechanical Properties of Shell Segments

• (2) Comparison of performance differences in three directions: The average tensile strength in the tangential, axial, and radial directions is 460MPa, 457MPa, and 447MPa, respectively. The performance difference in the tangential and axial directions is relatively small; The average yield strength in three directions is 388MPa, 377MPa, and 383MPa, respectively, with the maximum being tangential and the minimum being axial, and the overall difference is small; The average three-way elongation is 14%, 11.3%, and 7%, respectively, with some differences, but there is a significant increase in axial and tangential elongation. The significant reduction in the difference in three-dimensional performance indicates that high-temperature and large deformation multi-directional forging ensures coordination of three-dimensional deformation, and there is no obvious anisotropic structure.
• (3) Compared with the standard value, there is little difference in the three dimensional strength indicators, with tensile strength around 450MPa and yield strength around 380MPa, which is better than the tangential tensile strength of 430MPa and yield strength of 350MPa in the standard; The three-way elongation index meets the standard, and the axial elongation index reaches 11.3%, which is far superior to the standard of 4%. The overall mechanical performance indicators have been significantly improved, which is inseparable from the recrystallization of the workpiece during heat treatment after medium temperature rolling, resulting in uniform and fine microstructure.

5. Conclusion

Under the new process of high-temperature large deformation multi-directional forging and medium temperature rolling forming, 2A14 aluminum alloy forged cylinders with a height of up to 1300mm have been successfully produced. The three-dimensional grains of the ring billet are uniform, with unclear orientation. After heat treatment, recrystallization occurs, and uniform and fine subgrain structures are formed inside the grains; The overall tensile strength of the product is around 450MPa, the yield strength is between 380-390MPa, and the elongation in the tangential, axial, and radial directions is 14%, 11.3%, and 7%, respectively. This has achieved the improvement of the microstructure and performance of 2A14 aluminum alloy forging cylinders.
Author: Gao Jianxin