Effect of solution time on microstructure and mechanical properties of 2219 aluminum alloy

The effects of solution time on the microstructure and mechanical properties of 2219 aluminum alloy were studied using mechanical properties tests, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and XRD analysis. The results show that when the solution temperature is 535 °C and the solution time is less than 1.5 h, with the increase of solution time, the Cu-containing crystalline phase in the matrix dissolves back into the matrix, and does not segregate, which increases the supersaturation of the matrix and is beneficial to the improvement of the mechanical properties of the alloy. When the solid solution time is more than 2h, the Cu-containing crystalline phase is segregated. It grows up, resulting in a decrease in the mechanical properties of the alloy, especially the elongation. Therefore, when the solution temperature is 535 °C, the suitable solution time of the alloy is 1.5h. After (175 °C × 18h) aging treatment, the tensile strength R_m, yield strength R_p0.2, and elongation A of the alloy are 393.5MPa, 305.9Mpa, and 8.7 %, respectively.

2219 aluminum alloy is a high-strength aluminum alloy of the Al-Cu-Mn series. It has the characteristics of low density, good low temperature performance, excellent heat resistance, low crack tendency, and a wide working temperature range. It is widely used in aerospace and rail transportation. With the development of the aerospace industry, higher requirements are put forward for aluminum alloy forgings, that is, higher strength while maintaining a higher level of elongation. For the industrial production of 2219 aluminum alloy large forgings, the forged matrix inevitably has a coarse undissolved phase due to the ingot composition, uniformity control, and other reasons. These coarse undissolved phases easily cause stress concentration during plastic deformation, which is not conducive to improving the toughness of forgings. To further improve the comprehensive properties, such as strength, toughness, and corrosion resistance of 2219 aluminum alloy forgings, solid solution treatment is often used to improve them. As a widely used heat treatment process, solid solution treatment aims to increase the supersaturation of solute atoms in the matrix, prepare for subsequent aging precipitation, and improve the strength and toughness of the alloy. In the solution treatment of forgings, the solute atoms in the matrix tend to be evenly distributed in the matrix under the diffusion effect. The second phase will aggregate and grow if the holding time is too long. Therefore, under the premise of determining the solid solution temperature of forgings, it is also necessary to determine the appropriate solid solution time.

For 2219 aluminum alloy, the current research mainly focuses on friction stir welding, casting process, and thermomechanical treatment process, and the research on solution treatment time is relatively few. In this paper, the effects of solution time on the microstructure and mechanical properties of 2219 aluminum alloy were systematically studied using mechanical properties test, microstructure observation, and XRD test, which provided a good process basis for the subsequent aging treatment of forgings. By comparing and analyzing the changes in mechanical properties and microstructure before and after different solution time treatments, the microscopic mechanism of the effect of solution time on the mechanical properties of the alloy was discussed, which provided an experimental basis for improving the performance of large-sized forgings.

1. Test materials and methods

The nominal composition of 2219 aluminum alloy forgings used in the test is shown in Table 1. The main alloying element is Cu, adding Mn, Zr, V, Ti, and a small amount of Fe, Si, Mg, Zn, and other impurity elements introduced in the metallurgical process. The impurity elements Fe and Si content is strictly controlled by high-purity aluminum ingot. The forging is generally formed by melting and casting ingot billet, homogenization annealing, multi-directional forging, punching and horse frame reaming, ring rolling forming, and other processes to form a ring forging of a certain size. After machining into different sizes, different time solution treatment + 175 °C × 18h aging treatment is carried out, and then the performance test is carried out.
Tab.1 Chemical composition of 2219 aluminum alloy (mass fraction, %)

The DSC test was carried out on the STA449C synchronous thermal analyzer. The sample mass was not more than 20 mg, the protective atmosphere was hydrogen, and the heating rate was 10K/min. The mechanical properties test was conducted on the Shimadzu universal tensile testing machine according to GB/T228.1-2021 “Metallic Materials – Tensile test – Part 1: Room temperature test method”. The tensile speed was 2mm/min, and the average value of three samples for each state test was taken. Figure 1 is a tensile specimen prepared according to GB/T16865-2013 “Tensile test specimens and methods for deformed aluminum, magnesium, and their alloy processed products”. X-ray diffraction was analyzed on a Japanese Rigaku D/max2000 18KW rotating target X-ray diffractometer with a light tube power of 40kV and 300mA. Copper target, sampling step width 0.020. The diffraction angle range was selected as 2θ = 20 ° -90 °, and the MDI-Jade6.5 software was used for phase qualitative analysis to study the change of solid solubility and phase composition of matrix residual crystalline phase. The fracture scanning analysis was carried out on the Sirion200 field emission scanning electron microscope, and the acceleration voltage was 20 kV.

Fig.1 Size of tensile specimen

Fig.2 DSC curve of forged 2219 aluminum alloy

2. Test results and analysis

2.1 Phase transition point test of forgings

To determine the solid solution temperature of 2219 aluminum alloy forgings and ensure the maximum solid solution strengthening effect of forgings without overburning, it is necessary to test the phase transition point of the eutectic phase. DSC method was used to test the phase transition point of 2219 aluminum alloy forgings. The results are shown in Fig. 2. It can be seen from the forged DSC curve that a major exothermic peak can be observed during the heating process of the alloy, and the temperature is about 545 °C. According to the binary phase diagram of Al-Cu alloy, the eutectic point temperature in Al₂Cu phase Al-Cu alloy is about 548 °C, so it can be considered that the dissolution of the low melting point Al₂Cu phase causes the exothermic peak. To maximize the supersaturation of the solid solution without overburning, the solid solution temperature of the test alloy should not exceed 540 °C. In this experiment, the solid solution temperature was 535 °C.

2.2 Mechanical Properties test

Fig.3 shows the tensile strength R_m, yield strength R_p0.2, and elongation A of 2219 aluminum alloy forgings after T6 (175 °C × 18h) peak aging treatment after solid solution at 535 °C for different times. It can be seen that with the increase of solution time, the tensile strength of the alloy increases gradually. When the solution time is extended from 0.5 h to 1.5 h, the corresponding tensile strength and yield strength increase from 377.5 MPa and 292.3 MPa to 393.6 MPa and 305.9 MPa, respectively, which are increased by about 5 %, while the elongation after fracture decreases from 9.3 % to 8.7 %. After that, the tensile strength and yield strength remain unchanged with the increase of solution time, but the elongation after fracture decreases with the increase of holding time. When the holding time reaches 2.5 h, the elongation after fracture decreases to 7.9 %.

Fig.4 SEM morphology of tensile fracture of 2219 aluminum alloy after solid solution at different times and aging at 175 °C × 18h
(a) 0.5h; (b) 1h; (c) 1.5h; (d) 2h; (e) 2.5h

Fig.3 Effect of solution time on mechanical properties of 2219 aluminum alloy
For 2219 aluminum alloy forgings, simply prolonging the solution time does not guarantee the improvement of mechanical properties. Still, it will reduce the elongation of forgings after fracture. Therefore, to obtain good comprehensive performance, ensure that the forgings have good tensile strength and the elongation after fracture is not too low, the solid solution time should be short, and the solid solution treatment between 1.5-2h should be selected.

2.3 SEM analysis of tensile fracture

Fig.4 shows the SEM morphology of the tensile fracture of the sample after solution treatment at different times and then aging at 175 °C × 18h. Figure 4 shows that with the increase of solution time, the number and size of dimples in the fracture of the tensile specimen forging decrease, and the fracture platform in the matrix increases continuously. When the solid solution time reaches 2h, the number of dimples distributed in the matrix is greatly reduced, the proportion of fracture platform is further increased, and many microcracks appear, which is not conducive to improving alloy plasticity. This can explain why the solid solution time of forgings is prolonged, and the elongation after fracture is reduced.

2.4 Effect of solution time on solid solubility of 2219 aluminum alloy

Figure 5 is the XRD analysis of 2219 aluminum alloy. It can be seen from Fig.5 that there are many undissolved Al2Cu phases in the forged alloy. With the extension of the solid solution time, the diffraction peak of the Al2Cu phase gradually weakens, indicating that the undissolved Al2Cu phase in the forging is re-dissolved into the matrix, and the supersaturation of the matrix increases. When the solid solution time is extended from 1.5 h to 2.5 h, the diffraction peak of the Al2Cu phase does not change much, indicating that the undissolved residual phase in the alloy has been fully dissolved after the solid solution at 535 °C for 1.5 h. From this point of view, for 2219 aluminum alloy forgings, to ensure that the matrix is fully dissolved and the undissolved phase does not segregate, the solid solution time of the forging should not be too long. The appropriate solid solution time should be selected between 1.5-2h.

Fig.5 Effect of solution time on the second phase particles of 2219 aluminum alloy matrix
Fig.6 is the DSC analysis after solution treatment at 535 °C for 1.5 h. It can be seen that compared with the forged curve, the phase transition temperature of the alloy remains unchanged after solution treatment, which may be because 2219 aluminum alloy is a hypereutectic aluminum alloy. After solution treatment, the matrix still has an undissolved eutectic phase, but its exothermic peak area is significantly reduced. In addition, it can be found that there are three exothermic peaks in the DSC curve after solution treatment: the first exothermic peak A appears at 150-200 °C. According to the literature, the exothermic peak corresponds to the G.P.region as the dissolution peak; the second exothermic peak B appears at 275-325 °C, corresponding to the dissolution of the transition phase θ′; the third exothermic peak C appears at 400-450 °C, corresponding to the dissolution of the equilibrium phase θ. These exothermic peaks are almost not observed on the DSC curve of the forged aluminum alloy. This phenomenon indicates that after the solution treatment, most of the residual crystalline phases in the matrix can be dissolved back into the matrix, and the area of the exothermic peak is significantly reduced. That is to say, after solution treatment at 535 °C for 1.5 h, most of the residual crystalline phases in the matrix dissolve back into the matrix. To ensure that the forging has good tensile strength and the elongation after fracture is not too low, the solution time should not be too long.

Fig. 6 DSC curve of 2219 aluminum alloy after solution treatment at 535 °C for 1.5 h.

3. Analysis and discussion

The 2219 aluminum alloy studied in this paper is a heat-treatable strengthening aluminum alloy. The supersaturated solid solution formed after solution treatment precipitates strengthening phases during subsequent aging hinders dislocation movement, and improves the strength of the alloy. By increasing the supersaturation of solute elements in the α (Al) matrix, the driving force of the subsequent aging precipitated strengthening phase is increased, the number of precipitated phases is increased, and the solid solution strengthening effect is improved. In the case of ensuring no over-burning, it is beneficial to the improvement of alloy strength.
For 2219 aluminum alloy, when the solution temperature is constant and the solution time is different, the movement of solute atoms in the alloy structure conforms to Fick’s first diffusion law, as shown in formula (1):

In the formula:

• J is the diffusion flux;
• d₀ is the diffusion constant;
• q is the diffusion activation energy;
• x is the distance along the diffusion direction;
• ∂c/∂x is the volume concentration gradient.

2219 aluminum alloy is a hypereutectic alloy, and the excess phase cannot be completely dissolved into the matrix. At the same time, because the Cu-containing phase is a strengthening phase, the coarse undissolved phase can easily cause stress concentration and split the matrix in the subsequent pressure processing process. In the solution treatment process, with the prolongation of solution time, the Cu content in the matrix gradually increases and reaches saturation and is evenly distributed in the matrix, the concentration gradient decreases, thereby reducing the driving force of diffusion, and the diffusion rate decreases. At this time, continue to extend the solution time; with the continuous movement of solute atoms, the Cu-containing phase will aggregate and grow, and the Cu-containing phase will be coarsened, thereby reducing the strengthening effect and not conducive to the toughness of the alloy. From the mechanical properties test (see Fig.3), it can be seen that when the solid solution time reaches 1.5h, the strength of the alloy hardly changes with the extension of the solid solution time, but the elongation after fracture decreases. Combined with the analysis of the XRD pattern (see Fig.5), the alloy’s supersaturation does not change with the extension of the solid solution time. In this process, with the extension of the solid solution time, the Cu-containing eutectic phase in the matrix segregates and grows, which leads to the decrease of the strengthening effect and the decrease of the toughness.
When the solute atoms form a solid solution in the matrix, it will hinder the dislocation movement in the matrix, thereby increasing the yield strength of the alloy. Reference pointed out that after the formation of a solid solution in the test alloy if the average concentration of solute atoms in the matrix is c, the yield strength increment ΔR_p0.2 is:

ΔR_p0.2 = kc^2/3 (2)

In the formula:
For 2219 aluminum alloy, k is a constant, and its value is related to the size of solute atoms, atomic mismatch, and elastic modulus.
Therefore, the higher the concentration of solute atoms in the solid solution, the better the solid solution strengthening effect of the alloy, which also explains that the performance of the alloy is improved with the extension of time in the initial stage of solid solution treatment. The uniformity of the solid solution composition will also affect the subsequent aging process. If its composition is not uniform, the precipitated phase is easy to precipitate and coarsen in the position of large supersaturation of solute elements, and it is difficult to precipitate in the area where the solute elements are depleted, resulting in uneven microstructure, which is not conducive to the improvement of mechanical properties of the alloy. When the solid solution temperature of the test alloy is constant, with the extension of the solid solution time, it is beneficial to the uniform distribution of solute elements in the matrix, increases the number of precipitated strengthening phases during the aging process, and makes them more uniformly dispersed in the matrix, which is beneficial to enhance the strengthening effect. However, when the solid solution time exceeds a certain extent, it is easy to cause the segregation and growth of the copper-containing phase during the heat preservation process, resulting in a decrease in the mechanical properties of the alloy, especially the elongation. The experimental results show that when the solid solution temperature is 535 °C, the optimal solid solution time of 2219 aluminum alloy is 1.5-2h. In addition, the solution treatment time also needs to be coordinated with the temperature so that the solute atoms are fully dissolved in the matrix, and the grain size is not too large to obtain excellent comprehensive performance.

4. Conclusion

• 1) When the solution temperature of 2219 aluminum alloy is 535 °C, the best solution time is 1.5h. After solution treatment and aging at 175 °C × 18h, the tensile strength is 393.6MPa, the yield strength is 305.9MPa, and the elongation is 8.7 %.
• 2) After solution treatment at 535 °C for 1.5 h, the Cu-containing crystalline phase in the matrix of 2219 aluminum alloy dissolves back to the matrix without obvious agglomeration, which provides a driving force for subsequent aging and improves mechanical properties. Continuing to extend the solution time, the Cu-containing crystalline phase segregates and grows, decreasing the alloy’s mechanical properties, especially the elongation after fracture.

Author: Wang Huimin