Influence of Annealing Process on Earing Rate of 8011 Cast-Rolled Aluminum Alloy for Bottle Caps

Abstract

In this paper, 8011 cast-rolled blanks are adopted to manufacture aluminum sheets for bottle caps, and the effects of homogenization annealing and intermediate annealing processes on product properties are investigated.

The results show that bottle cap aluminum sheets meeting customer requirements can be produced from cast-rolled blanks by adopting appropriate homogenization annealing processes.

Introduction

Due to the inherent characteristics of the cast-rolling process, cast-rolled blanks exhibit obvious anisotropy, which restricts their application in deep-drawing products.

In this work, a reasonable homogenization annealing process is adopted to alleviate composition segregation and anisotropy of 8011 cast-rolled blanks induced by the cast-rolling process and optimize the internal microstructure.

Combined with appropriate intermediate annealing and reasonable cold reduction of the final rolling pass in subsequent processing, the proportions of various textures in the sheets are balanced, material anisotropy is reduced, and bottle cap aluminum sheets satisfying deep-drawing requirements are successfully fabricated.

1 Experimental Scheme

1.1 Experimental Materials

The raw material is 8011 cast-rolled coil with an initial blank thickness of 6.0–6.5 mm, and the target finished thickness of the product is 0.21 mm.

The grain size of cast-rolled coils shall reach Grade 1.

Chemical Composition of 8011 Alloy for Bottle Caps (mass fraction, %)

1.2 Experimental Procedures

With fixed cold rolling passes, homogenization annealing and intermediate annealing temperatures are set as variable parameters. The complete process route is as follows:

Cast-rolled blank → cold rolling to 3.8 mm → homogenization annealing → cold rolling to 0.43 mm → intermediate annealing → cold rolling to finished thickness of 0.21 mm

Three comparative process schemes are designed:

Scheme 1: Homogenization at 480 °C for 4 h for 3.8 mm thick blanks; intermediate annealing at 360 °C for 2 h for 0.43 mm strips

Scheme 2: Homogenization at 480 °C for 4 h for 3.8 mm thick blanks; intermediate annealing at 280 °C for 2 h for 0.43 mm strips

Scheme 3: Homogenization at 530 °C for 4 h for 3.8 mm thick blanks; intermediate annealing at 320 °C for 2 h for 0.43 mm strips

1.3 Test Items

Tensile strength \(R_m\), elongation A and earing rate e of specimens after intermediate annealing and finished products are tested in accordance with national standards.

Finished sheets produced by the three schemes are delivered to customers for cup drawing tests, and the optimal industrial production process is determined based on practical stamping feedback.

2 Experimental Results

2.1 Test Results of Intermediate Annealing

Specimens cold-rolled to 0.43 mm are annealed in a box furnace (put into the furnace after the set temperature is reached, holding time: 2–4 h) to test mechanical properties and earing characteristics.

1. Within the temperature range of 230–250 °C, the tensile strength drops sharply while the elongation rises significantly, indicating full recrystallization occurs in this temperature interval. Dramatic changes in earing behavior take place during recrystallization: after annealing at 240 °C for 2 h, earing mainly occurs along the 45° direction with slight 0°/90° earing; when the holding time is extended to 4 h, earing completely shifts to the 0°/90° direction.
2. After full recrystallization, further increasing annealing temperature causes no obvious change in earing rate; when the temperature exceeds 360 °C, the elongation of sheets decreases evidently. Therefore, the suitable temperature range for intermediate annealing is 250–360 °C.
3. Comparison of homogenization temperatures: Specimens homogenized at low temperature (480 °C) show higher earing rates and faster transition of earing directions after intermediate annealing, while specimens homogenized at 530 °C exhibit overall lower earing rates.

Actual Properties after Intermediate Annealing

2.2 Properties of Finished Products

After intermediate annealing, strips are cold-rolled to the finished thickness of 0.21 mm with a cold reduction of approximately 51%.

The mechanical and earing performance data of finished products are listed as follows.

Properties of Finished Products

Customer Trial Results

Test conclusion: The finished products of Scheme 1 and Scheme 2 homogenized at 480 °C have an earing rate close to 10%, failing to meet deep-drawing requirements for bottle caps.

The finished products of Scheme 3 homogenized at 530 °C have an earing rate of about 5% without stamping cracking, and have been supplied to multiple bottle cap manufacturers in mass production.

3 Analysis and Discussion

3.1 Correlation Between Texture and Earing Rate

The earing behavior of aluminum sheets is dominated by internal crystal textures.

Cold rolling produces deformation textures, which lead to 45° earing during cup drawing.

After annealing of sheets subjected to large cold reduction, recrystallization textures mainly consist of cube texture and R texture.

• Cube texture: induces earing along the 0°/90° directions;
• R texture: induces earing along the 45° direction.

The optimization idea for deep-drawing aluminum sheets is to regulate the proportion of the two textures, offset the earing effects induced by different textures mutually, reduce the overall earing rate and improve deep-drawing formability.

3.2 Regulation Mechanism of Homogenization Temperature on Cast-Rolled Microstructure

The cast-rolling process features high supercooling degree and rapid cooling, resulting in a large number of nonequilibrium second phases and severe solid-solution segregation of Si and Fe elements in the aluminum matrix after solidification.

Homogenization annealing promotes the transformation of nonequilibrium phases, precipitation of supersaturated solid solutions and elimination of composition segregation.

1. Metallographic comparison (Figure 3 & Figure 4)
   • Homogenization at 530 °C: Intermetallic compounds are fine and uniformly dispersed, and grains are typical equiaxed grains.
   • Homogenization at 480 °C: Precipitated phases are coarse, grain sizes are non-uniform, and local coarse grains exist.
2. Competition between Precipitation and RecrystallizationAt low homogenization temperature (480 °C), precipitation occurs prior to recrystallization. Precipitated particles pin dislocations and hinder the nucleation of recrystallization, eventually forming coarse and uneven recrystallized microstructures.At high homogenization temperature (530 °C), recrystallization takes precedence, and subsequent precipitates restrain excessive grain growth, yielding fine and uniform equiaxed grain structures.

3.3 Regulation Effect of Fe/Si Precipitates on Textures

The main alloying elements of 8011 alloy are Fe and Si. Rapid cooling in cast rolling makes a large amount of Fe and Si supersaturated in the aluminum matrix.

During homogenization, Fe and Si precipitate out to form dispersed \(FeAl_3\) particles:

1. \(FeAl_3\) particles can refine recrystallized grains, render annealed microstructures dense and uniform, and weaken material anisotropy.
2. Under homogenization at 530 °C, Fe and Si precipitate sufficiently. Dispersed second-phase particles promote in-situ recrystallization during intermediate annealing. Part of the cold rolling deformation texture is retained in this process, while the formation of R texture is facilitated and the excessive development of cube texture is inhibited. The proportions of the two recrystallization textures are balanced, significantly reducing the earing rate of finished products.

4 Conclusions

1. Increasing the homogenization annealing temperature from 480 °C to 530 °C enables uniform and dispersed alloy precipitates and improves the consistency of recrystallized grain sizes. The cube texture and R texture generated after intermediate annealing mutually counteract the deformation texture formed in the final cold rolling pass, remarkably reducing the earing rate of 8011 cast-rolled aluminum finished sheets.
2. After full recrystallization is completed via intermediate annealing, its process parameters exert limited influence on earing rate. Homogenization annealing process is the core decisive factor governing the earing rate of 8011 cast-rolled aluminum sheets for bottle caps.