Abstract
Pure aluminum foil (dilute Al-Fe-Si alloy series) with Cu or Mn addition was prepared by severe cold-rolling deformation, and the effect of room temperature storage or low temperature annealing on the tensile properties and microstructure was investigated through tensile tests, optical microscope, scanning electron microscope, electron back scatter diffraction, and atom probe microscope. Results show that the ultimate tensile strength and elongation simultaneously decrease after room temperature storage. The recovery mechanism of substructure, such as subgrain coalescence, leads to the decrease in tensile properties. The decrease in plasticity is more significant for the Mn-containing alloy due to the more significantly increased subgrain size. The atom cluster strengthening can compensate for the strength loss to some extent, whereas the effect of the secondary phases is negligible.
With the fast development of electrical vehicles and portable electronic devices, the demand for aluminum foils, which are mainly made from 1XXX aluminum alloys, as the positive current collector of Li ion batteries is rapidly increased. High compaction density can increase the energy density of Li ion batteries. As a result, the thinner and stronger aluminum foil is required. The thickness of aluminum foil for positive current collector can be reduced to ≤13 μm, and the typical tensile strength of over 200 MPa can be achieved with elongation of over 4%, which greatly exceed the standard for 1060-H18 temper (tensile strength≥110.32 MPa, elongation≥1%, ASTM B209-2014).
Such high strength of aluminum foil can only be achieved by applying severe plastic deformation (SPD) through repetitive cold rolling with total strain of ≥5.5. SPD techniques, such as equal channel angular press, high pressure torsion, and accumulative roll bonding, have been widely researched. However, the microstructure and properties of aluminum foil produced by industrial-scale rolling mills are rarely investigated. Different microstructures are formed when different processing techniques are applied. During SPD by cold rolling, the deformed grains with lamellar structure appea
During the production of severely deformed aluminum foil, both the tensile strength and elongation are decreased at room temperature, and the microstructure evolution mechanism of this phenomenon is still obscure. The effect of room temperature annealing on severely deformed metals has been widely researched. Yu et a
In this research, trace Cu or Mn was added into the 1XXX alloy to improve the work hardening abilit
The aluminum foil was produced from the base metal of dilute Al-Fe-Si alloy, whose composition was similar to that of the commercial AA1050 alloy. Cu or Mn with different contents was added into the base metal. The ingots were produced by the direct chill casting method. The Al-5Ti-1B master alloy was used as the grain refiner for casting. The AA1050 alloys after Cu and Mn addition were named as 1050-Cu and 1050-Mn alloys, respectively, and their nominal chemical composition is shown in
| Alloy | Fe | Si | Cu | Mn | Ti | Al |
|---|---|---|---|---|---|---|
| 1050-Cu | 0.20 | 0.05 | 0.04 | - | 0.02 | Bal. |
| 1050-Mn | 0.20 | 0.05 | - | 0.02 | 0.02 | Bal. |
The ingots of about 8 tons after direct chill casting were homogenized at 600 °C for 10 h and cooled to the initial rolling temperature (480 °C). Then, they were hot-rolled into plates of 6 mm in thickness. The hot-rolled plates were heated at 380 °C for 3 h (intermediate annealing) to achieve the foil with 2 mm in thickness, and subsequently cold-rolled to the foil with thickness of 13 μm. All the rolling processes were performed by the industrial-scale rolling mills. In order to observe the decreased tensile properties, the aluminum foil was kept at 20 °C (room temperature) for 60 d.
The specimens were cut along the rolling direction for tensile tests. The dimension of rectangular uniaxial tensile specimens was set according to ASTM E8/E8M-16a standard: the gauge length was 50 mm and the rectangular gauge cross-section was used for observation, as shown in
Fig.1 Schematic diagram of tensile specimen
Optical microscope (OM, Zeiss AX-10 metallographic microscope) and transmission electron microscope (TEM) were used for microstructure observation. Scanning electron microscope (SEM, TESCAN MIRA3 microscope) equipped with electron backscattered diffractometer (EBSD, OXFORD Nordlys Nano EBSD detector) was also used for micro-structure analysis. EBSD data was analyzed by Channel 5 software. The specimens were mechanically and electroly-tically polished before OM/SEM observation.
The specimens for atom probe microscope (APM) obser-vation were prepared by the standard two-step electro-polishing technique, and the sharp needle-like specimens were obtaine
Fig.2 TEM morphology of needle-like APM specimen
The tensile properties of different aluminum foils of 13 μm in thickness after room temperature storage for 60 d are shown in
Fig.3 Tensile properties of different aluminum foils during room temperature storage for 60 d: (a) ultimate tensile strength and (b) elongation
It can be seen that the initial ultimate tensile strength of 1050-Cu foil is higher than that of 1050-Mn foil, while the initial elongation of 1050-Mn foil is higher than that of 1050-Cu foil. The higher strength of 1050-Cu foil can be attributed to the higher solute concentration and the increased strain hardening caused by Cu solute
It is worth noting that the aluminum foils of different alloys both suffer a slight decrease in ultimate tensile strength and a significant decrease in elongation. Moreover, the decrement of elongation of 1050-Mn foil is greater than that of 1050-Mn foil.
The effect of temperature on recovery and tensile properties of 1050-Cu aluminum foil was investigated through annealing at different temperatures (80–110 °C with temperature interval of 10 °C), as shown in
Fig.4 Tensile properties of 1050-Cu foils after annealing at different temperatures: (a) ultimate tensile strength and (b) elongation
It can be observed that both the tensile strength and elongation are decreased during the annealing. However, the decreasing rate is much higher, compared with that caused by the variation in foil composition. In addition, the decreasing rate is increased with increasing the annealing temperature. Ref.[
The grain microstructures of 1050-Cu and 1050-Mn foils after intermediate annealing are shown in Fig.
Fig.5 Grain microstructures of 1050-Cu (a) and 1050-Mn (b) foils after intermediate annealing treatment
SEM morphologies of the secondary phases in 1050-Cu and 1050-Mn foils are shown in
Fig.6 SEM morphologies of the secondary phases in 1050-Cu (a) and 1050-Mn (b) foils
APM was applied to investigate the solute atom segregation of the 1050-Cu and 1050-Mn foils. For the 1050-Cu foil after room temperature storage for 60 d, the spatial distribution and solute-solute nearest-neighbor (N-N) distances of the Cu atoms are shown in
Fig.7 Spatial distribution (a) and N-N distributions of Cu solute atoms (b) in 1050-Cu foil after room temperature storage for 60 d
It can be observed that some Cu atoms form the solute atom clusters after the natural aging, and the measured N-N distances of the Cu solute atoms are slightly smaller than those of atoms with random distributio
The distribution of grain boundary misorientation angle in 1050-Cu and 1050-Mn foils before and after room temperature storage is shown in
Fig.8 Distributions of grain boundary misorientation angle in 1050-Cu (a) and 1050-Mn (b) foils before and after room temperature storage
A large number of low angle grain boundaries (LAGBs) can be observed in both alloy foils, which are normally introduced by plastic strain
The subgrain boundary distributions of 1050-Cu and 1050-Mn foils before and after room temperature storage are shown in
Fig.9 Subgrain boundary distributions of 1050-Cu (a, c) and 1050-Mn (b, d) foils before (a, b) and after (c, d) room temperature storage
Fig.10 Subgrain diameter distributions of 1050-Cu (a) and 1050-Mn (b) foils before and after room temperature storage
Ref.[
The decrease in the tensile strength is less significant than that in elongation, because the cluster of solute atoms will result in solute cluster strengthening effec
1) Both the ultimate tensile strength and the elongation of the aluminum foils are decreased after room temperature storage for 60 d or after low temperature annealing for several hours. The loss of elongation is greater in the AA1050 alloy with Mn addition.
2) The solute atom clusters are formed, which can compensate for the strength loss through solute cluster strengthening effect to some extent. The effect of Cu/Mn addition on the secondary phases is insignificant after room temperature storage.
3) The recovery of substructures, such as subgrain coalescence, occurs during the room temperature storage, resulting in the decrease in tensile properties. The increase in subgrain size is more significant in the AA1050 alloy with Mn addition, which leads to greater elongation loss.
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