Effect of Ti content and hot/cold extrusion on microstructure and properties of 2219 heat-resistant aluminum alloy

Figure 2 shows the SEM images of the four alloys. It is found that there are white undissolved phases in the four images, in which the undissolved phases of H1 alloy penetrates the entire grain boundary and the intergranular broadening phenomenon is severe (figure 2(a)). When a certain amount of Ti was added to H2 alloy, the undissolved phases are discontinuous and most of them exist in the triangular region between the grains. Besides, H3 alloy has the highest intergranular undissolved ratio and some undissolved particles appear on the surface of the alloy (figure 2(c)). These undissolved particles will preferentially break and affect the mechanical properties during the plastic deformation process, which indicates that Ti has a certain solid solubility and will promote more uniform distribution of the excessive second phase before exceeding the maximum solid solubility [14]. Compared with figure 2(c), the proportion of undissolved phases in figure 2(d) reduced greatly, and there is no second phase aggregation. Only a small amount of scattered undissolved phases are distributed inside the alloy, which indicates that cold extrusion can promote the redundant second phase to dissolve in the matrix.

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The undissolved phases were analyzed by EDS and the actual composition is shown in table 2. The main component in region A is Al and Cu with a small amount of titanium. According to element ratio, the undissolved phase is mainly θ (Al₂Cu) phase. The main elements in region B are Al and Ti, which were estimated to be Al₃Ti phase and the white coarse Al₃Ti phase is hardly soluble in the matrix because of excessive titanium.

Figure 3 is the metallographic images of the four alloys. Figure 4 shows the grain size distributions and average grain size of the four alloys. It is observed that H1, H2 and H3 alloys have a certain equiaxed crystal. The eutectic structures have not been fully dissolved in the matrix and some grains appear to grow, which indicate that the three alloys have recrystallized during the solution treatment. The grain size in figures 3(b) and (c) are obviously smaller than that in figure 3(a), indicating that Ti element can effectively promote the refinement of grains and enhance mechanical properties. However, there are many second phases remaining in the intergranular phases and the grain boundary widening phenomenon is serious. Besides, the triangular complex melting zone and the over-burning phenomenon happen between the grains with the increase of Ti content, which may be due to an unipolar solid solution system. The low-temperature soluble phases have no time to integrate into the matrix fully and solidify again after being in the molten state, leading to an over-burning phenomenon [15].

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Compared C3 alloy with H3 alloy, there are some undissolved black phases on their surfaces because of excessive titanium. The grain size in figure 3(d) is smaller than that in figure 3(c), indicating that cold extrusion will further refine grains. Besides, the equiaxivity of the grains in figure 3(d) is smaller than that in figure 3(c), the degree of recrystallization is light and most of the intergranular eutectic structures have been dissolved in the matrix, which shows that cold extrusion can inhibit the recrystallization and promote eutectic structures into the matrix. Thus, properties of alloys can be further improved.

Figure 5 shows the XRD pattern and half-peak width of the four alloys. Figure 6 is the XRD analysis spectrum of pure Al. Compared with pure Al, it is found that the pattern of H1 alloy is the closest, while H2 and H3 alloys have larger differences, which indicates that the addition of Ti will increase the internal texture ratio of the crystals of 2219 aluminum alloys. Besides, H3 alloy has the largest half-peak width, which indicates that Ti has little influence on the crystal orientation and the lattice strain. Compared with H3 alloy, C3 alloy has a larger crystal grain ratio (figure 5(g)) and the half-peak width reduced slightly.

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Based on the above data, the dislocation density of the four alloys and the corresponding dislocation strengthening contribution values can be calculated. The relationship can be expressed by the following equation [16].

$$\begin{eqnarray}&&\displaystyle \frac{{(\delta 2\theta )}^{2}}{{\tan }^{2}{\theta }_{0}}=25{\left\langle e\right\rangle }^{2}+\displaystyle \frac{\lambda }{d}\left(\displaystyle \frac{\delta 2\theta }{\tan \,{\theta }_{0}\,\sin \,{\theta }_{0}}\right)\end{eqnarray} \tag{ 1 }$$

According to figure 5, ${\theta }_{0}$ and FWHM($\delta 2\theta$) can be derived. Further, according to figure 7, the coherent diffraction region size (d) and the average lattice strain (${\left\langle {{\rm{e}}}^{{\rm{2}}}\right\rangle }^{1/2}$) can be calculated by the following equation [17].

$$\begin{eqnarray}&&\rho =2\sqrt{3}{\left\langle {e}^{2}\right\rangle }^{1/2}/\left(d\times b\right)\end{eqnarray} \tag{ 2 }$$

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The dislocation enhancement (${\sigma }_{\rho }$) can be calculated by the following equation [18].

$$\begin{eqnarray}&&{\sigma }_{\rho }=M\alpha Gb{\rho }^{1/2}\end{eqnarray} \tag{ 3 }$$

In the formula, b is Baise vector and the value is 0.286 nm; M is Taylor orientation factor and the value is 3.06; α represents numerical factor and the value is 0.24; G represents shear modulus and the value is 26 GPa. The dislocation parameters associated with the calculation of the above three equations are shown in table 3.

According to table 3, it is found that the addition Ti will consume some dislocations, but the phenomenon will be inhibited with further increasing the content of Ti. Besides, some studies have shown that the larger the lattice strain, the lower the hardenability [19]. Table 3 shows that the lattice strain of H1 alloy is the largest and the hardenability is the worst. With the increase of Ti, the lattice strain decreased firstly and then increased, indicating that moderate Ti has a positive effect on the hardenability.

Figure 8 shows the aging-hardness in different time periods at 191 °C. Table 4 is the hardness value and conductivity of the four alloys. It is found that the hardness of H2 alloy and H3 alloy are 12 HV higher than that of H1 alloy before aging process (about 104 HV), while the hardness of the cold-extruded C3 alloy reached up to 112HV. After 6 h aging, the hardness of the four alloys increased significantly, increasing by 16% to 28%. The first peak appeared and the hardness is 118.47 HV, 130.10 HV, 121.50 HV and 140.42 HV, respectively. After 18 h aging, the second peak appeared and the hardness of each alloy was slightly lower than that at the first peak. After 24 h aging, the hardness of the four alloys decreased, which indicates that over-aging has occurred at this time. In general, the aging time of 6 h is the peak aging of the alloy, which is the best aging time. Compared the three hot-extruded alloys, when the content of Ti is within a certain range, it can promote the hardness of alloys. As Ti content continues to increase, the hardness will decline. Besides, cold extrusion can greatly improves the hardness of alloys.

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In conclusion, the aging system adopts 191 °C × 6 h. It can be seen that for the three hot-extruded alloys, the addition of Ti can affect the electrical conductivity of alloys, ranging from 26.3% IACS alloy to 22.49% IACS, which indicates that the addition of Ti will reduce the stress corrosion resistance of alloys. Compared with H3 alloy, the electrical conductivity of C3 alloy decreases slightly [20].

Table 5 shows the mechanical properties of the four alloys. Compared with H1 alloy, the strength of H2 alloy increased by 87.01 MPa and the elongation decreases by 3.4%. When titanium reached up to 0.89 (wt%), the tensile strength of H3 dropped to 315.59 MPa. Compared with H2 alloy, there are a large number of eutectic phases not soluble in the matrix and the proportion of main strengthening phases decreased. Thus, the mechanical properties of alloys decline (figure 2). In addition, it is found that the yield strength of C3 alloy increased by 101.08 MPa and the elongation reduces to 8.6%, indicating that the cold extrusion greatly can improve the strength of alloys.

Figure 9 shows the tensile fractures of the four alloys. It is found that the fractures of the four alloys are uneven and the fractures are intergranular fractures, which are typical brittle fractures. Because the grain boundaries still have many eutectic phases that are not soluble in the matrix and excessive phases tend to be a preferential fracture source, causing the alloy to break prematurely, Thus, it exhibits a brittle fracture (figure 2). Compared the three hot-extruded alloys, it is found that the dimples of H1 alloy were relatively small and large (figures 9(a) and (b)), indicating that H1 alloy has the best plasticity alloy, which is consistent with the results of elongation in table 5. The size and number of dimples of H3 alloy are the second. The size of H2 dimples is larger than that of H3 alloy (figures 9(c) and (d)). Compared with H3 alloy, it is found that the fracture of C3 alloy is not flat and the tear hole size becomes larger. Besides, the number of dimples is less than that of H3 (figures 9(g) and (h)), which indicates that the degree of recrystallizing is lower than that of H3 alloy. Thus, the cold extrusion can increase the strength of alloys while decrease the plasticity.

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Figure 10 is the true stress-strain diagram of the four alloys. Table 6 shows the stress peaks of the four alloys. It can be seen that the true stress of material rises rapidly with the increase of strain at the beginning. When the strain reached 0.1, the peak of stress appears and the upward trend stops. The stresses of the three hot-extruded alloys show a slow downward trend, while the trend of C3 alloy is relatively flat and remains basically unchanged. This is due to the fact that work hardening and dynamic softening occur simultaneously in the process of plastic deformation. These two phenomena have the opposite effect on the performance of alloys. When alloys begin to strain, the work hardening phenomenon is leading and the rise of true stress is obvious. When the stress reaches the peak value, the dynamic softening phenomenon is aggravated and the stress does not rise, keeping a dynamic equilibrium state [21, 22]. From table 6, it is found that the strengthening effect of Ti on heat resistance of the three hot-extruded alloys increased firstly and then decreased. H2 alloy has the highest stress peak, reaching 252.76 MPa, indicating that its heat resistance is the best. Besides, the stress peak C3 alloy is slightly higher than that of H3, which indicates that the cold extrusion will promote the heat resistance enhancement effect of alloys.

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The intergranular corrosion pictures of the four alloys are shown in figure 11. Table 7 is the evaluation table of intergranular corrosion grade. It is found that the maximum corrosion depth of the four alloys is from 100 to 300 μm and the grades are all grade 4.

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Compared the three hot-extruded alloys, the corrosion depth of H1 alloy is 236.21 μm and the corrosion morphology of the outer layer is relatively flat. There is almost no phenomenon that outer crystal grains fell off. With the increase of Ti content, the corrosion depth of H2 alloy decreased by 54.65 μm. But the intergranular corrosion solution has been corroded along the outer crystal grains, showing an intergranular network corrosion morphology (figure 11(b)). Meanwhile, the intergranular corrosion resistance of H3 alloy is the worst. The outer crystal grains have been corroded and peeled off. Besides, the pores are continuous and irregular. It shows that when the content of Ti is within a certain range, it can promote the corrosion resistance of alloys. However, it is easy to form corrosion channels as Ti content continues to increase, resulting in the decrease of corrosion resistance.

Compared with H3 alloy, the corrosion resistance of C3 alloy improved greatly and the depth reduced by 81.09 μm. Besides, the corrosion of C3 alloy is smooth and some surface grains have dropped (figure 11(d)). Most of the intercrystalline second phases have been dissolved in the matrix and crystal grains are further refined. There is no large corrosion hole and only a slight intergranular network corrosion. indicating that cold extrusion can improve the mechanical properties and corrosion resistance.