1 Effect of intermediate annealing on the grain structure of 5052 aluminum alloy The grain structure of the longitudinal section of the cold-rolled plate to 1.8mm thickness and the recrystallization annealed plate at 350°C×3h after cold rolling is shown in Figure 1. The grain size distribution of the plate after recrystallization annealing is shown in Fig. 2 . 20 μm, the average grain diameter is 15 μm; the recrystallized grain diameter of the plate after intermediate annealing is mainly distributed in 14-22 μm, and the average grain diameter is 17 μm. The grain size of the plate after intermediate annealing is larger, indicating that the plate completed recrystallization earlier under the same annealing conditions.
2 Effect of intermediate annealing on the texture of 5052 aluminum alloy Figure 3 shows the distribution of the macroscopic texture of the cold-rolled to 1.8mm thick plate. From the cross-sectional diagrams of orientation distribution functions φ2=45° and φ2=90°, it can be seen that both cold-rolled sheets contain obvious rolling texture components, and the rolling texture strength of cold-rolled sheets without intermediate annealing is higher Some. In addition, there is a small amount of cubic texture components in the cold-rolled sheet after intermediate annealing, which is because the cubic-oriented grains formed after intermediate annealing are not completely rotated to the rolling direction during the cold rolling process, thus leaving some cubic oriented grains. Research by Engler et al. pointed out that the cubic texture formed during recrystallization mainly originates from the cubic band in the deformed structure, and the subgrains in the cubic band are generally cubic or rotated cubic. The recovery speed of the cubic subgrain during recrystallization The subgrain speed is faster than other orientations, so it provides good conditions for the nucleation of cubic texture. In addition, since the cubic-oriented grains have an orientation relationship of 40°<111> with the main cold-rolled texture S texture, they have a faster growth rate than grains with other orientations in the subsequent growth process, So you end up with a lot of cubic texture. The plate after intermediate annealing has some residual cubic oriented grains or subgrains after cold rolling, and these residual cubic oriented grains serve as the core of the recrystallization process to promote the completion of the recrystallization process.
Figure 4 shows the macro texture distribution of the cold-rolled sheet after 350℃×3h recrystallization annealing. From the cross-sectional diagrams of the orientation distribution functions φ2=45° and φ2=90°, it can be seen that after recrystallization annealing, the rolling texture inside the two sheets is transformed into a cubic texture, and the cubic texture of the intermediate annealed sheet is The strength is slightly higher. From Figure 1(a) and Figure 1(b), it can be seen that there are second phase particles with a size of about 2 μm inside the grains and at the grain boundaries. The energy spectrum analysis of the second phase is carried out, and the results are shown in Table 2 As shown, the second phase of the alloy contains more Al, Fe, Mg and other elements. During the plastic deformation process, the dislocations interact with the coarse second-phase particles, and a strong lattice rotation is generated near the second-phase particles, resulting in a strong strain gradient in this region. The orientation of the deformed region is random, and the recrystallization is completed. The orientation of the recrystallized grains formed later is also random. The non-intermediate annealed plate contains more second-phase particles, and more randomly oriented grains are formed after recrystallization, and the corresponding cubic texture strength is lower
3 Effect of intermediate annealing on the recrystallization activation energy of 5052 aluminum alloy The Vickers hardness of the 5052 aluminum alloy cold-rolled sheet without intermediate annealing and intermediate annealing treatment is 106.2HV and 96.5HV respectively, indicating that the internal dislocation density of the sheet without intermediate annealing is relatively high , The deformation storage energy is more than that of the plate after intermediate annealing. Figure 5 shows the relationship between the hardness and annealing time of the 5052 aluminum alloy cold-rolled sheet without intermediate annealing and intermediate annealing at different temperatures during recrystallization annealing. Comparing the hardness versus time curves of intermediate annealed and non-intermediate annealed sheets at the same temperature annealing, it can be found that the recrystallization incubation period of the intermediate annealed sheet is shorter, and the time required to complete recrystallization is also shorter.
The recrystallization process has an incubation period. The recrystallization speed is slow at the beginning, then gradually becomes faster, and the speed is the fastest when the recrystallization process is completed by 50%, and then the speed gradually slows down. The nucleation rate N during constant temperature recrystallization decays exponentially with time, which is usually described by the Aframi equation (1).
XR=1-exp(-Ktn)
(1)
Where K and n are constants, and XR is the volume fraction of recrystallization. Since there is no obvious boundary between the recovery stage and the recrystallization stage of the hardness curve, the recrystallization volume fraction XR is expressed by the softening fraction of the hardness value, namely:
XR=HV0-HVtHV0-HVa
(2)
In the formula: HV0 is the hardness of the plate before recrystallization, and HVa is the hardness of the plate after complete recrystallization.
Taking the logarithm on both sides of equation (1), we can get
lg(-ln(1-XR))=lgK+nlgt(3)
The formula shows that there is a linear relationship between lg(-ln(1-XR)) and lgt, and the obtained hardness test data are brought into equations (2) and (3), and the data in the recrystallization stage are linearly fitted. The K and n values obtained by linear fitting are listed in Table 3.
Analysis of the recrystallization curves of the same sample in Figure 5 when it is annealed at different temperatures shows that the higher the annealing temperature, the shorter the recovery process and the shorter the time required to complete recrystallization. The recrystallization process can be considered as a thermally activated process, and the effect of temperature T on the recrystallization rate v during isothermal annealing can be expressed by the Arrhenius formula, namely
v=Ae-Q/(RT)(4)
The recrystallization rate v is inversely proportional to the time t required to produce a certain volume fraction XR, so
1t=A'e-Q/(RT)(5)
In the formula: A' is a constant; Q is the recrystallization activation energy; R is the gas constant; T is the thermodynamic temperature. Taking the logarithm on both sides of the above formula, we can get
ln1t=lnA'－QR·1T(6)
Apply common logarithm (2.3lgx=lnx) to transform equation (6) to get
lgt=Q2.3R1T－lgA'(7)
Taking XR=0.5 and bringing it into equation (3) can obtain the lgt when the recrystallization is half completed, that is, lgt0.5. During recrystallization annealing at different temperatures, lgt0.5 and 1/T conform to a linear relationship, and the two are linearly fitted. The slope of the fitted line is Q/2.3R, and the recrystallization activation energy Q of different samples can be calculated. value. After calculation, it can be seen that the recrystallization activation energy of the plate after intermediate annealing treatment and the plate without intermediate annealing treatment are 164.3kJ/mol and 200.5kJ/mol, respectively, and the intermediate annealing treatment reduces the recrystallization activation energy of HTCR5052 alloy.