Influence of the Final Annealing Temperature on Al-Fe-Si Alloy Foil Microstructure and Properties

Abstract

This study systematically investigates the effects of the final annealing temperature on the microstructural evolution and mechanical properties of an Al-Fe-Si alloy aluminum foil. Scanning electron microscopy (SEM) characterization and tensile tests are employed for analysis. As the annealing temperature is elevated from 240 °C to 360 °C, the average grain size increases monotonically from 5.2 μm to 9.6 μm. Continuous recrystallization is identified as the predominant grain growth mechanism. Tensile deformation exhibits the homogeneous plastic behavior without localized necking. The tensile strength decreases significantly in the range of 240–300 °C and subsequently undergoes a recovery stage at 300–360 °C. Significant elongation anisotropy is observed. The maximum elongation reaches 30–34% in the 45° direction, relative to the rolling direction (RD), which is approximately 1.5 times that along the RD (0°). Comparative analysis of the anisotropy indices demonstrates that the aluminum foil annealed at 240 °C achieves the minimal tensile strength anisotropy (13.0 MPa) and elongation anisotropy (−4.2%). This indicates an optimal comprehensive mechanical performance. These findings provide a theoretical rationale for the industrial optimization of the annealing processes for Al-Fe-Si alloy foils. They are particularly valuable for balancing microstructural regulation and mechanical property enhancement in lithium-ion battery soft-packaging applications.

1. Introduction

In recent years, lithium-ion batteries have been widely adopted in consumer electronics (e.g., mobile phones, laptops, Bluetooth headsets) and high-end manufacturing sectors (e.g., electric vehicles, aerospace) [1,2]. Their popularity stems from multiple advantages: high energy density (up to 100 Wh/kg), low self-discharge rate, high operating voltage (above 3.5 V), long cycle life (exceeding 1000 cycles), and environmental friendliness [3]. Lithium batteries are primarily categorized into three types, based on their packaging: cylindrical lithium batteries, prismatic aluminum-cased lithium-ion batteries, and pouch lithium-ion batteries. Soft-pack lithium batteries, encapsulated with aluminum-plastic film, offer great flexibility in shape and size. They also exhibit excellent safety performance, especially at higher capacities. By adjusting their form and dimensions flexibly, soft-pack lithium-ion batteries meet the market demands for thinner and smaller products, thereby achieving a higher energy density. However, this flexibility imposes higher requirements on the performance of lithium-ion battery packaging materials.

Al-Fe-Si alloy aluminum foil plays a key role in the aluminum–plastic film for lithium battery soft packaging, due to its excellent barrier properties, strength, and processability [4]. Research indicates that the microstructure and mechanical properties of aluminum foil are significantly affected by the processing parameters. Currently, the research on Al-Fe-Si alloys mainly focuses on alloy composition regulation, processing technology research, and intermediate annealing processes. For example, an increase in the Fe content leads to a decrease in the grain size of cold-rolled sheets, and as the annealing temperature rises, recrystallized grains gradually grow, eventually forming irregular equiaxed grains [5]. However, critical gaps remain for both scientific research and industrial application. Most previous works ignored the final annealing process, a core industrial step, for high-deformation Al-Fe-Si foil with a reduction rate of 98% and above, and none quantified the link between continuous recrystallization, the dominant mechanism in high-deformation alloys, and mechanical anisotropy, nor identified the optimal final annealing temperature to balance anisotropy and overall performance [6,7].

After the Al-Fe-Si alloy aluminum foil undergoes high cold rolling deformation (98% reduction rate), a large amount of deformation energy is stored internally. Its recrystallization behavior during annealing differs significantly from that of conventional aluminum alloys. The existing literature points out [8] that factors such as the second-phase particle distribution, the initial grain size, and the deformation amount may inhibit the traditional nucleation process. This promotes the dominance of the continuous recrystallization mechanism. However, the laws governing how continuous recrystallization affects the anisotropic mechanical properties of the aluminum foil remain unclear. Therefore, this study focuses on the Al-Fe-Si alloy aluminum foil. It systematically analyzes the grain structure evolution characteristics under different final annealing temperatures. Combined with tensile property tests, the influence mechanism of the annealing temperature on the mechanical property anisotropy is elucidated. The aim is to provide theoretical guidance for industrial production.

2. Materials and Methods

The experimental material is an Al-Fe-Si alloy sheet provided by a domestic enterprise, and its chemical composition is shown in

Table 1. Chemical composition of the experimental plate material (wt.%).

Element	Si	Fe	Cu	Mn	Mg	Cr	Ti
Composition	0.86	1.12	≤0.05	≤0.05	≤0.05	≤0.05	≤0.02

. The initial thickness of the sheet is 7 mm. After 65% cold rolling, the sheet is subjected to homogenization annealing at 540 °C for 10 h. It is finally cold rolled to 40 μm aluminum foil for the laboratory annealing process research.

The final annealing performed after rolling the aluminum foil to its finished thickness is termed final annealing. In actual production, a low-temperature degreasing annealing process is typically employed to remove rolling oils from the foil surface. It also enhances the mechanical properties of the foil simultaneously. According to Reference [9], cold-rolled aluminum foil exhibits high strength and low ductility after severe plastic deformation. To further process the foil into final products, it must possess high elongation and cup test values. Therefore, cold-rolled aluminum foil typically undergoes low-temperature final annealing at 150–400 °C. This study selected a heating rate of 40 °C/h to raise temperatures to 240 °C, 270 °C, 300 °C, 330 °C, and 360 °C, respectively. After holding at each temperature for 2 h, the foil was air-cooled to room temperature.

The microstructure testing of the aluminum foil utilized an Apreo C field emission scanning electron microscope (SEM) from the FEI Company (Hillsboro, OR, USA). The samples were electropolished with an electrolyte ratio of HClO₄:C₂H₅OH = 1:9. The polishing parameters were as follows: voltage 20 V, current 0.4~0.6 A, and time 30~40 s. Images were acquired under an accelerating voltage of 20 kV and a working distance of 10 mm, and TSL-OIM Analysis 7.3 software was used to analyze the grain size and orientation distribution.

The mechanical property testing of the aluminum foil was conducted using an INSTRON 5967 universal testing machine (Norwood, MA, USA) equipped with a 1 kN load sensor. Tensile samples were prepared with a double-blade cutter. The samples were strip-shaped, with a length of 200 mm and a width of 15 mm. The tensile rate was set to 25 mm/min. Tensile tests of the aluminum foil were performed in accordance with GB/T 16865-2023 standard [10]. At least three parallel samples were tested in each of the following directions: along the rolling direction (0°), at a 45° angle to the rolling direction (45°), and perpendicular to the rolling direction (90°). The average value of the three room-temperature tensile experiments was taken as the final result.

3. Results

3.1. Influence of the Final Annealing Temperature on the Second-Phase Particles

Figure 1 shows the microstructure of the aluminum foil after 98% cold rolling and annealing at 240~360 °C. As observed from Figure 1a–e, the second-phase compounds in the foil matrix are dispersed under different annealing temperatures, mainly granular with a small amount of short rod-like or blocky forms. Based on size, they fall into three categories. The first is spherical dispersed phases (≈1 μm), precipitated during homogenization annealing with a small size and uniform distribution. The second is short rod-like second phases (1~2 μm). The third is blocky second phases (2~3 μm). The latter two evolve from the aggregation and growth of fine granular compounds during homogenization.

Figure 2 shows the second-phase size distribution of the aluminum foil annealed at 240 °C, 270 °C, 300 °C, 330 °C and 360 °C, respectively. Image-Pro 6.0 software was used for the quantitative statistics of the number of second phases within 0.01 mm². It can be found that the second-phase sizes are mainly concentrated in the ranges of 0~1 μm and 1~2 μm. At an annealing temperature of 240 °C, the number of second phases larger than 1 μm is the smallest. This indicates the minimum number of PSN nucleation sites at this temperature. At 300 °C, the number density of second phases is significantly higher than that at other temperatures. When the temperature gradually increases to 360 °C, the number of second phases in the range of 0~1 μm is the smallest, while that larger than 1 μm is the largest. According to the PSN nucleation mechanism, the nucleation effect is the strongest at this temperature.

3.2. Influence of Final Annealing Temperature on Microstructure

According to references [11,12], during the aluminum foil rolling process, when the cold rolling reduction exceeds 88%, the microstructure exhibits a fine equiaxed grain structure. Meanwhile, the proportion of high-angle grain boundaries increases rapidly. Figure 3 presents grain data collected via EBSD for the aluminum foil annealed at 240 °C, 270 °C, 300 °C, 330 °C, and 360 °C. The data was processed using the TSL-OIM analysis software, where different colors represent distinct grain orientations. As shown in Figure 3, the evolution of the microstructure is a gradual process. As indicated by the black arrows in Figure 3, at 240 °C, the matrix contains a small number of recrystallized grains. These grains exhibit irregular equiaxed shapes and retain some orientation clustering characteristics, distributed along the rolling direction. When the temperature increases from 270 °C to 300 °C, more recrystallized grains appear in the matrix (Figure 3b,c). At 330 °C, new grains completely replace the deformed structure, with significant grain growth evident (Figure 3d). As the annealing temperature further increases to 360 °C, recrystallized grains continue to grow and become uniformly distributed (Figure 3e). Studies on the microstructural evolution of the aluminum foil at different annealing temperatures indicate that recrystallization begins after annealing at 240 °C for 2 h, with relatively small grain sizes. As the annealing temperature increases, grains begin to coalesce, leading to gradual grain growth. When the annealing temperature exceeds 300 °C, the coalescence intensifies, resulting in significantly enlarged grains.

Figure 4 quantitatively characterizes the evolution of the average grain size in the aluminum foil under different final annealing temperatures. As shown in Figure 4, the grain size changes can be divided into two stages: (1) 240 °C ≤ T ≤ 300 °C—small recovery grains and large recrystallized grains coexist in the sample, with the recrystallized grain size increasing slowly with the rising annealing temperature and (2) T > 300 °C—recrystallization completes at 330 °C, with an average grain size of ~9.2 μm. Grains continue to grow as the annealing temperature increases, reaching ~9.6 μm at 360 °C. These experimental results demonstrate that the annealing temperature significantly influences the grain size of the aluminum foil after recrystallization. Considering the impact of the microstructure on foil properties, controlling the microstructure—and consequently, the properties—of the aluminum foil in industrial production can be achieved by regulating the final annealing temperature.

The grain size substantially affects the elongation and tensile strength of the aluminum foil. Effectively improving the foil quality can be achieved by controlling the grain size. Figure 5 shows the recrystallized grain size distribution of the aluminum foil within the final annealing temperature range of 240–360 °C. The horizontal axis represents the grain size (units: μm), while the vertical axis indicates the area fraction occupied by grains. At 240 °C, grain sizes are primarily concentrated in the 2–4 μm and 4–6 μm ranges. Grains smaller than 6 μm account for 72% of the area fraction, with the maximum size not exceeding 16 μm. As the annealing temperature increases to 270 °C and 300 °C, smaller grains grow continuously. The area fractions of grains below 6 μm decrease to 50% and 44%, respectively, while some grains exceed 20 μm in size. At higher annealing temperatures (330 °C and 360 °C), grains below 2 μm disappear entirely. Grain growth accelerates significantly, with grains exceeding 20 μm accounting for 0.03% and 0.02% of the total area fraction, respectively. Throughout the annealing process, some grains reach sizes exceeding 27 μm. This phenomenon may be attributed to the pinning effect of dispersed phases in the matrix on grain boundaries or sub-grain boundaries. This hinders recrystallization nucleation and leads to the formation of coarse grains, as illustrated in Figure 5d,e.

The study on the microstructural evolution of the aluminum foil under different annealing temperatures indicates that after annealing at 240 °C for 2 h, the aluminum foil has already initiated recrystallization, with a relatively small grain size. As the annealing temperature increases, the grains exhibit coalescence through boundary migration, and grow slowly. When the annealing temperature exceeds 300 °C, the swallowing phenomenon intensifies, leading to significant grain growth.

For aluminum alloy materials subjected to cold working plastic deformation, when the annealing temperature is in the stage dominated by recrystallization, the recrystallization nucleation rate during annealing is positively correlated with the annealing temperature. That is, as the annealing temperature gradually increases, the nucleation rate increases accordingly. This phenomenon usually leads to the refinement of the grain structure after recrystallization [13]. However, in this study, the grain structure after complete recrystallization exhibits finer characteristics at relatively lower annealing temperatures. The literature reports [12] that factors such as the initial grain size of the aluminum alloy matrix, the size of second-phase particles, and the amount of plastic deformation have a significant impact on the recrystallization process. When the initial grain size in the aluminum alloy matrix is small, while the second-phase particle size is large and the cold rolling deformation amount is high, no obvious “nucleation” stage occurs during subsequent recrystallization annealing. At this time, the evolution of the microstructure is more similar to conventional grain growth. This special recrystallization mode is called continuous recrystallization. As shown in Figure 3, when the heating temperature reaches 240 °C, the grains of the aluminum foil grow to ~5.2 μm. This indicates that small sub-grains already exist inside the aluminum foil before annealing. In addition, the aluminum foil matrix is rich in numerous coarse iron-containing second-phase particles. The total reduction rate of the aluminum foil during cold rolling is as high as 98%, which further affects the microstructure of the aluminum foil. Based on these specific processing conditions and the observed changes in grain size, it can be inferred that the continuous recrystallization mechanism may dominate the recrystallization annealing process of the aluminum foil. It is worth noting that similar phenomena have also been reported in some other 8xxx series alloys [14].

In the process of continuous recrystallization, no obvious “nucleation” stage occurs during heating; instead, direct grain growth takes place. Its growth rate primarily depends on the following formula [15]:

$$\mathrm{G}={\displaystyle \frac{{\mathrm{D}}_{\mathrm{B}}}{\mathrm{KT}}}·{\displaystyle \frac{{\mathrm{E}}_{\mathrm{S}}}{\mathsf{\lambda }}}$$

(1)

where $\mathrm{G}$ represents the grain growth rate, ${\mathrm{D}}_{\mathrm{B}}$ denotes the grain boundary self-diffusion coefficient, $\mathsf{\lambda }$ is the boundary width, $\mathrm{K}$ is the Boltzmann constant, and ${\mathrm{E}}_{\mathrm{S}}$ stands for the molar deformation storage energy. According to the theoretical derivation of Formula (1), as the deformation storage energy increases, the grain growth rate $\mathrm{G}$ also shows an upward trend, leading to an increase in grain size after recrystallization.

After the completion of the recrystallization process, the grain growth rate mainly depends on the grain boundary migration rate. This relationship is expressed as follows [16]:

$${\mathrm{V}}_{\mathrm{g}\mathrm{b}}={\mathrm{M}}_{\mathrm{g}\mathrm{b}}\times {\mathrm{P}}_{\mathrm{g}\mathrm{b}}$$

(2)

where $V_{gb}$ represents the grain boundary migration speed, $M_{gb}$ denotes the grain boundary mobility, and $P_{gb}$ stands for the driving force acting on the grain boundary. According to the literature reports [17], the grain boundary mobility is positively correlated with the temperature. Thus, as the annealing temperature continues to increase, the grain size exhibits a continuous growth trend. After completing recrystallization at 330 °C, the grain size of the aluminum foil continues to increase, as illustrated in Figure 3d,e.

3.3. Effect of Final Annealing Temperature on Texture

Figure 6 shows the volume fraction distribution of typical recrystallization texture (Cube texture) and rolling texture (S, Cu, Brass) at different annealing temperatures. It can be seen from Figure 6 that with the increase in the annealing temperature, the volume fraction of the recrystallization texture is about 1.5% with insignificant variation. This indicates that the final annealing temperature has a slight effect on the recrystallization texture. When 240 °C ≤ T≤ 270 °C, the total volume fraction of the rolling texture (S, Cu, Brass) increases from 56.3% to 73.9%. When 270 °C< T≤ 330 °C, it decreases to 55.05%. When T > 330 °C, the total volume fraction increases gradually to 72.8%.

3.4. Study on Plastic Deformation Behavior of Aluminum Foil

Figure 7 presents the true stress–strain curves of the aluminum foil tensile-tested along the 0°, 45°, and 90° directions under the final annealing temperatures of 240 °C, 270 °C, 300 °C, 330 °C, and 360 °C, respectively. As shown in Figure 7, under different annealing temperature conditions, the tensile curves exhibit obvious elastic deformation and uniform plastic deformation characteristics. The yield stress in the 0° direction is generally higher than that in the 45° and 90° directions. The curves are dominated by uniform deformation, followed directly by concentrated instability and fracture. No obvious necking occurs before fracture, indicating the absence of the localized necking stage that is common in metal materials. After annealing at 240 °C, the fracture strain in the 90° direction (0.30) is higher than that in the 0° direction (0.26), with a difference of 0.04. The fracture strain in the 45° direction is slightly higher than that in the 90° direction (difference: 0.01). Under different loading directions, the differences in the true stress–strain curves reflect obvious plastic deformation anisotropy. At other annealing temperatures (270~360 °C), the same trend is observed: the fracture strain is smallest in the 0° direction, intermediate in the 90° direction, and largest in the 45° direction. The magnitude of the fracture strain difference among the three directions varies with the annealing temperature.

3.5. Influence of Final Annealing Temperature on Properties of Aluminum Foil

3.5.1. Influence of Final Annealing Temperature on Mechanical Properties of Aluminum Foil

Figure 8 presents the variation curves of tensile strength and elongation of the aluminum foil with the annealing temperature, with the average values of tensile strength and elongation calculated, respectively. As shown in Figure 8a, after 98% cold rolling deformation and annealing at different temperatures, the tensile strength of the aluminum foil in the 0°, 45°, and 90° directions follows basically consistent variation trends. The tensile strength is highest in the 0° direction, intermediate in the 90° direction, and lowest in the 45° direction. With an increasing annealing temperature, the tensile strength of the aluminum foil first decreases and then increases. In the range of 240~300 °C, the tensile strength drops sharply. This is because during low-temperature annealing below 300 °C, the aluminum foil matrix mainly undergoes continuous recrystallization. Dislocations and point defects introduced by plastic deformation undergo rearrangement or annihilation, leading to gradual property recovery toward the pre-deformation level. When the annealing temperature further increases to 360 °C, the tensile strength shows an upward trend, with the average tensile strength reaching a maximum of 122 MPa.

Figure 8b presents the curve of aluminum foil elongation, varying with annealing temperature. As shown in the figure, the elongation is lowest in the 0° direction (≈17~23%) and decreases gradually with the increasing annealing temperature. The elongation in the 90° direction is intermediate (≈25~32%), showing a trend of first increasing and then decreasing with the rising annealing temperature. The elongation is highest in the 45° direction (≈30~34%), which also follows a first-increase-then-decrease trend and is almost 1.5 times that in the rolling direction (0°). A comprehensive analysis indicates that during annealing at 240~330 °C, the average elongation remains relatively stable at (28 ± 1)%. However, the minimum elongation value of 25.9% is more decisive for characterizing the material.

According to the classic Hall–Petch relationship, metallic materials show a negative correlation between tensile strength and the square root of grain size, with grain growth typically reducing the tensile strength [18]. In this study, the tensile strength of the Al-Fe-Si alloy foil drops sharply at 240~300 °C, which conforms to this relationship. However, a slight recovery in tensile strength is observed at 300~360 °C, despite continuous grain growth. This is a unique mechanical response that is different from that of conventional aluminum alloys during annealing [19]. This anomalous change is not driven by the grain size alone, but by the synergistic effect of multiple microstructural factors [20]. Recrystallization completion realizes microstructural homogenization, rolling texture evolution optimizes grain orientation distribution, and second-phase particles pin grain boundaries to inhibit excessive grain coarsening. These factors collectively offset the strength softening effect caused by grain growth, resulting in a slight recovery of the tensile strength in this temperature range.

3.5.2. Influence of Final Annealing Temperature on Anisotropy of Aluminum Foil

To represent the degree of anisotropy of tensile properties in different orientations of the material, the anisotropy index is used for characterization, as shown in the following formulas [21]:

$$∆\mathrm{UTS}={\displaystyle \frac{({\mathrm{UTS}}^0+{\mathrm{UTS}}^{90})-2{\mathrm{UTS}}^{45}}{2}}$$

(3)

$$∆\mathrm{EL}={\displaystyle \frac{({\mathrm{EL}}^0+{\mathrm{EL}}^{90})-2{\mathrm{EL}}^{45}}{2}}$$

(4)

In the formulas, ΔUTS denotes the tensile strength anisotropy index, and ΔEL represents the elongation anisotropy index. UTS0°, UTS45°, and UTS90° are the tensile strengths of the sample in the 0°, 45°, and 90° directions, respectively. EL0∘, EL45°, and EL90° are the fracture elongations of the sample in the corresponding directions. The tensile strength and elongation anisotropy indices of the aluminum foil under different annealing temperatures, calculated via Formulas (1)–(3) and Formulas (1)–(4), are presented in

Table 2. Anisotropy indices of strength and elongation in aluminum foil under different annealing temperatures.

Temperature	Tensile Strength Anisotropy Index/MPa	Anisotropy Index of Elongation/%
240 °C	$13.0\pm$ 0.2	$-4.2\pm$ 0.2
270 °C	$13.7\pm$ 0.3	$-8.4\pm$ 0.4
300 °C	$13.2\pm$ 0.2	$-7.8\pm$ 0.3
330 °C	$14.5\pm$ 0.4	$-11.2\pm$ 0.6
360 °C	$15.2\pm$ 0.4	$-9.3\pm$ 0.5

.

At annealing temperatures of 240 °C, 270 °C, 300 °C, 330 °C, and 360 °C, the tensile strength anisotropy indices are 13.0 MPa, 13.7 MPa, 13.2 MPa, 14.5 MPa, and 15.2 MPa, respectively.

The elongation anisotropy indices are −4.2%, −8.4%, −7.8%, −11.2%, and −9.3%, respectively.

The tensile strength anisotropy degree gradually increases with the rising annealing temperature, with the smallest index at 240 °C. In contrast, the elongation anisotropy index first increases and then decreases with the annealing temperature, reaching the maximum at 330 °C (three times that at 240 °C). Based on the anisotropy degree and average elongation of the aluminum foil, the material exhibits an optimal comprehensive performance when annealed at 240 °C.

4. Discussion

4.1. Microstructural Evolution and Mechanical Property Regulation Mechanism

The Al-Fe-Si alloy foil with 98% cold rolling reduction stores substantial deformation energy, leading to dominant continuous recrystallization during annealing (no obvious nucleation stage, direct grain growth from sub-grains). This is driven by coarse iron-containing second-phase particles, a small initial grain size, and high deformation—which are consistent with the observations in 8xxx series alloys. Grain growth exhibits two stages: slow growth (240~300 °C, 5.2~6.8 μm, coexisting recovery/recrystallized grains) and rapid growth (>300 °C, 6.8~9.6 μm, completed recrystallization). Second-phase particle pinning causes inhomogeneous grain distribution (max size > 27 μm). Mechanical properties are microstructure-dependent: tensile strength decreases (240~300 °C, dislocation annihilation), then slightly recovers (>300 °C, uniform recrystallized structure).

Elongation shows directional differences: 0° direction (17~23%) decreases continuously, while 45° (30~34%) and 90° (25~32%) directions first increase then decrease. This anisotropic behavior is directly related to the rolling texture and crystallographic orientations in the foil. TSL-OIM analysis confirms a strong {110}<112> rolling texture in the cold-rolled state, which persists after annealing. For aluminum alloys, slip system activation ({111}<101>) depends on the loading direction, relative to grain orientations. The 45° direction has ~42% of grains with favorable orientations for multi-slip system activation, enabling higher elongation. The 0° direction has ~38% of grains with {110}<112> orientations that restrict slip, leading to lower elongation. The 90° direction shows an intermediate ~30% favorable grains, resulting in moderate elongation. This aligns with the anisotropy indices in Table 2.

The anisotropy indices increase with the temperature (tensile strength: 13.0~15.2 MPa; elongation peaks at −11.2% at 330 °C). An elevated annealing temperature weakens the rolling texture but retains orientation inhomogeneity, maintaining significant anisotropy. Optimal comprehensive performance is achieved at 240 °C (minimal anisotropy, average elongation 28.21%), as moderate annealing balances recrystallization and texture retention, minimizing the slip system activation discrepancies across directions. This makes 240 °C the optimal annealing temperature for battery soft-packaging, where a uniform mechanical response is critical for processing and service reliability.

4.2. Engineering Significance and Application Prospects

This study provides important industrial guidance for the optimization of the final annealing process of Al-Fe-Si alloy aluminum foil: controlling the final annealing temperature at approximately 240 °C can effectively improve the processability of the aluminum foil for aluminum-plastic films, reduce the risk of rupture of lithium-ion battery packaging during processing and cycling, and thus improve the safety performance of pouch lithium-ion batteries. The quantitative research results on the continuous recrystallization mechanism and mechanical anisotropy of the high-deformation Al-Fe-Si alloy foil also provide a theoretical basis for the microstructural and property regulation of other high-deformation aluminum alloys.

Future industrial optimization research can focus on the adjustment of the heating rate and holding time during the final annealing process, as well as the regulation of the distribution of second-phase particles in the Al-Fe-Si alloy matrix. Notably, in actual industrial production, the fluctuations of raw material composition, the uniformity of cold rolling deformation, and the annealing atmosphere all need to be subjected to a targeted process verification to ensure the stability of the microstructure and mechanical properties of the Al-Fe-Si alloy aluminum foil products.

5. Conclusions

(1)

As the annealing temperature rises from 240 °C to 360 °C, the grain size of the Al-Fe-Si alloy aluminum foil increases from ~5.2 μm to ~9.6 μm. When the annealing temperature exceeds 300 °C, the grain swallowing phenomenon intensifies, leading to obvious grain growth.

(2)

The tensile deformation of the aluminum foil is dominated by uniform plastic deformation, with almost no localized necking before fracture. The fracture strain is smallest in the 0° direction, intermediate in the 90° direction, and largest in the 45° direction (30~34%), showing significant plastic deformation anisotropy.

(3)

With an increasing annealing temperature, the tensile strength of the aluminum foil first decreases and then increases, reaching the minimum at ~300 °C. Regarding elongation:

• Elongation along the rolling direction (0°) is the lowest (≈17~23%), decreasing gradually with a rising annealing temperature;
• Elongation perpendicular to the rolling direction (90°) is intermediate (≈25~32%), showing a trend of first increasing and then decreasing with temperature;
• Elongation in the 45° direction is the highest (≈30~34%), also following a first-increase-then-decrease trend and nearly 1.5 times that in the rolling direction.

(4)

With an increase in annealing temperature, the tensile strength anisotropy degree of the aluminum foil gradually increases. The anisotropy index is smallest at 240 °C, with ΔUTS = 13.0 MPa and ΔEL = −4.2%. Thus, the aluminum foil exhibits an optimal comprehensive performance when annealed at 240 °C.