Annealing Effect on the Microstructure and Mechanical Properties of AA 5182 Aluminum Alloy

aUniversidade Estadual do Amazonas, Faculdade de Engenharia, Departamento de Engenharia de Materiais, Avenida Darcy Vargas, 1200, Parque Dez de Novembro, CEP 69065-020, Manaus, AM, Brasil. bUniversidade Estadual do Amazonas, Faculdade de Engenharia, Departamento de Engenharia Mecânica, Avenida Darcy Vargas, 1200, Parque Dez de Novembro, CEP 69065-020, Manaus, AM, Brasil. cUniversidade Estadual de Campinas, Faculdade de Engenharia Química, Departamento de Engenharia de Materiais e Bioprocessos, Rua Albert Einstein, 500, Cidade Universitária Zeferino Vaz, Barão Geraldo, CEP 13083-852, Campinas, SP, Brasil.


Introduction
Aluminum alloys are widely used in the areas of packaging, electronic technology, automotive body structure, engines and wheels, and wind and solar power generation, among others, due to advantages such as high specific strength, high processability, corrosion resistance 1 , high conductivity, and ease of recycling 2 . The mechanical properties of aluminum alloys vary in accordance with their applications, and heat treatments are used to modify the microstructures, which has an effect on their properties 3 .
Aluminum alloys are distinguished by their chemical composition, each of which has a specific application. The 2xxx series has Cu as its main element and is used in aviation and aerospace. The 3xxx series comprises Mn as its main element and isutilized in sheets for civil construction and fins. In 6xxx series, the main elements are Mg and Si and applications are in tools and the automotive industry. The alloy of the 5xxx series possessesMg as its main element and, among its variations, the alloy AA5182 is highlighted here due to its use in can end stock (CES). The strength of the non-heat treatable AA 5xxx series aluminum alloy is due to a combination of solid solution hardening with Mg and dispersion hardening by Mn and other alloying elements in the formation of dispersoids 4,5 . Another type of increase in mechanical strength of this aluminum series is due to plastic deformation which affectsthe grain size 6 , displacement density 7 and texture developed during the deformation process 8 .
Beverage cans are produced in aluminum and are composed of two parts, the body (cup), which is usually made from AA 3004,and the lid, which is madefrom AA 5182. The manufacturing process of the lids is carried out by cold forming thin sheets (0.208 mm thickness). To maintain the quality of the product,either annealing or accelerated aging is carried out,which simulates six months of exposure of the final product to normalenvironmental conditions. Then, the mechanical buckle and manual lid opening tests are performed. Figure 1 shows the finished product and the "residual metal" region in which the annealing heat treatment affects. The aluminum sheets used in this study are used in the manufacture of packaging caps.
In the search for better performance of products obtained from AA 5182 aluminum, several studies have sought to understand the microstructural and mechanical changes of this alloy when subjected to the annealing process. *e-mail: jmacedo@uea.edu.br Zhang et al. 9 submitted cold-rolled AA 5182 alloy to 454 °C and 540 °C and the effect on the initial recrystallization grain and mechanical properties were investigated. The authors found that the fine-grained alloy recrystallized faster and more homogeneously. The results showed that strength and flow limits were reduced for those with fine and coarse grains by increasing the annealing temperature.
Another study showed the effect of annealing and hot rolling on the chemical composition and morphology of AA5182 alloy dispersoids used in automobiles. For the heat treatment, temperatures of 530 °C for 3h were used, followed by cooling in the oven for 1h and 480 °C for 2h. After the homogenization annealing, dispersoids containing finely dispersed Fe and Mn were obtained 10 .
In another study, a sheet of AA5182 aluminum alloy was annealed at 454 °C for 3 h and another, a sheet of direct-chill (DC), was cold rolled and annealed at 260 °C for 2 h, 288 °C for 10 min and 343 °C for 10 s. The kinetics and texture of recrystallization were analyzed. The authors noted that the texture depends heavily on the annealing temperature. At low annealing temperatures, the recrystallization texture was characterized by an R orientation and a weak cube orientation. At high annealing temperatures the texture of recrystallization was entirely by the cube component 11 .
This study shows the changes that occurred in the microstructure and mechanical properties of AA5182 aluminum alloy sheets after annealing with temperatures of 50 °C, 100 °C, 150 °C, 200 °C, 250 °C and 300 °C for 10 minutes. Microstructural characterization was performed by optical and electronic microscopy with energy dispersive spectroscopy (SEM-EDS). Details of the behavior of the sheet fractures that were obtained by the tensile test were also shown by electron microscopy (SEM-EDS).

Materials:
The aluminum alloy used was AA 5182 (Al-Mg), which was cold rolled and had a of thickness 0.208 mm ± 0.005 mm. This was supplied in the form of 32 x 29.5 cm plates (Figure 2b).

Chemical composition:
For the analysis of the composition, optical emission spectroscopy was performed (Spectro, model MaxxLMF15). Table 1 shows the chemical composition of AA 5182 alloy. Three readings were performed on the plate ( Figure 2c) and the mean and standard deviation were calculated. The main elements of this alloy are Si, Fe, Cu, Mn, Mg Zn and Ti, and magnesium is the principal element of this alloy. The percentage of magnesium in the samples are not in accordance with the literature for this type of alloy 12 .
Heat treatment: The heat treatment used was that of annealing, which was performed at temperatures of 50, 100, 150, 200, 250, 300 °C, with a fixed time of 10 minutes for each temperature (Figure 2a). For this step, we used a Quimis oven (model Q318M).The annealing temperatures of AA5182 aluminum were based on the temperatures that the industry already uses, i.e., between 90 and 150 °C. Scanning electron microscopy (SEM): To obtain the images of the fractures, the tested samples were manually cut at 3 cm parallel to the faces of the fractures. Then, the samples were inserted vertically into a sample holder and metallized for 4 minutes using a Jeol metallizer (Model: DII-29010SCTR Smart coater) to improve the quality of the images. For precipitate images, the samples used for optical microscopy were metallized before SEM analysis. To obtain the images by SEM, a Jeol microscope (Model: JSM -IT500HR) was used.     Figure 4a, it is possible to observe that up to 200 ºC there was a gentle reduction in the resistance limit and an increase in elongation. Between the temperatures of 200 and 250 ºC, there was a greater reduction in the resistance limit and an increase in elongation. In Figure 4b, it can be observed that between the temperatures of 200 and 250 ºC, there was a reduction in the yield stress and elastic modulus. As the annealing temperature increased, a gradual reduction of the yield stress and resistance limit occurred due to the recovery effect. Between the temperatures of 200 and 250 °C, there was a reduction in the yield stress, resistance, elastic modulus, and a drastic increase in elongation. This behavior was due to the occurrence of the beginning and end of recrystallization which reduces the density of dislocation in the alloy 13 . After the temperature of 250 °C, behavior of the yield stress, resistance and elastic modulus was constant 9,11,14 . In Figure 4c, it is observed that, between the temperatures of 200 and 250 ºC, there was a reduction in the load and an increase in the rupture time, which indicates the occurrence of the beginning and end of recrystallization. In Figure 6e, it is observed that there was an increase in the number of Al dispersoids (Fe, Mn) when the temperature increased to 250 °C 18 . Figure 6f shows that there was a reduction in the number of dispersoids with the increase in temperature to 300 °C. This reduction indicates that there was a dilution of the precipitates and an increase in the concentration of manganese in the matrix 10 . The increase in the quantity and size of dispersoids inhibits the recrystallization of the alloy 19 . This reflected in the mechanical properties of the alloy when heated to 300 °C, since it achieved greater elongation. As the dispersoids presented a non-coherent interface and high energy in the matrix, they caused greater mobility and promoted their dissolution 20 .  in the aspect of fractures 21 .The reduction of the number of dispersoids for the sample annealed at 300 °C caused a fracture aspect with 90°. Figures 8a, b, c, d, e and f show the images obtained using SEM of the fractures of the annealed samples at temperatures of 50, 100, 150, 200, 250 and 300 °C, respectively. While Figures 9a-b show the SEM-EDS fractures images just for 50 and 100 °C. In all the images, large and small dimples are observed, which are characteristic of ductile fracture. It is possible to observe that the larger dimples decrease as the annealing temperature increases. The larger dimples are due to the higher triaxial state of stress. Constituent particles are seen on the inside of the dimples at all temperatures, but in some they are not seen and may have fallen or be on the opposite surface. In each particle, it is assumed that they are sites of nucleation of dimples due to particle fracture and/or dissociation between the particles and the matrix 22,23 . The fracture mechanisms found are similar for all recrystallization temperatures and consist of nucleation, growth and coalescence of voids around the constituent particles of Al(Fe, Mg), Al(Mg) as seen in Figure 9. Nucleation occurred by creating a crack from a debondingin an interface of the second phase particles. The incipient cracks associated with the particles grow and coalesced by enlargement and linkage of the voids 24 .A large number of constituent particles, which appear to be small dispersoids, can be observed in the fractures. The size distribution of the constituent particles, as well as their distribution, are key elements that determine the tensile ductility of aluminum alloys 22 . It was observed that the number of dispersoids decreases as the annealing temperature increases, and this may have affected the elongation of the alloy.

Conclusions
The effect of annealing on the mechanical and microstructural properties of AA 5182 aluminum alloy was investigated and the following conclusions were obtained: • Using optical spectroscopy, it was observed in the studied samples that the chemical composition of magnesium (the main characteristic element of AA 5182 alloy) presented below 14%, which is the standard for this type of alloy. • Between the temperatures of 200 and 250 ºC, there was an inversion of the values of the related mechanical properties, limit of resistance and elongation, elastic modulus and yield stress, load and rupture time. This behavior was due to the beginning and end of recrystallization, which caused a relative reduction in mechanical resistance. • After precipitation, there is a formation of extremely small and evenly distributed dispersoid particles.    can apparently be seen, at the nucleation sites of the dimples due to particle fracture or discohesion, growth and coalescence of voids surrounding the particles and the matrix. • The sizes of the constituent particles, as well as their distribution, are key elements for determining the tensile ductility of aluminum alloys. • As the annealing temperature increases the number of dispersoids will decrease, affecting the elongation of the alloy.