Thermal deformation behavior and processing maps of 7075 aluminum alloy sheet based on isothermal uniaxial tensile tests
Introduction
With an increasing awareness of global warming, environmental pollution and the scarcity of fossil fuel resources, many researchers and manufacturers have been looking for sustainable solutions to improve the efficiency of fuel consumption [1,2]. The usage of lightweight materials in transportations, such as aluminum and magnesium alloys, is a feasible approach. Aluminum alloys are attractive materials that have been widely used for structural components in automotive and aerospace industries. This is mainly due to their advantageous combination of properties, such as high strength-to-density ratio, high fracture toughness and resistance to stress corrosion cracking (SCC) [3,4]. Unfortunately, it is extremely difficult to form aluminium alloy panel components with complex geometries at room temperature, resulting from their poor formability and high springback [5,6]. To address this limitation, increasing forming temperature contributes to increasing alloy ductility enabling the manufacture of complex-shaped high strength aluminium alloy components becomes possible [7,8]. Among these elevated temperature forming techniques, a hot stamping technology, solution Heat treatment, hot Forming and Quenching, (HFQ®), proposed by Lin et al. [9], is believed to be a leading technology. The process of hot stamping involves a hot blank that is hot formed and quenched with cold dies. Such a process integrates heat treatment and hot forming at one operation, enabling higher straining and guaranteeing post-formed properties simultaneously.
During hot stamping process, thermal-mechanical properties and processing parameters of aluminum alloy determine the post-formed property and quality of the final components. Thermal-mechanical properties mainly refer to flow stress level, strain and strain rate hardening and ductility, which determines the deformation uniformity and initiation of tearing. Therefore, a thorough understanding of them for specific alloys during hot deformation is vitally important for the design of hot stamping processes. Hot deformation behaviors of aluminum alloys at elevated temperatures have been extensively investigated. Li et al. [7] studied the uniaxial tensile deformation behavior of three different aluminum alloy sheets at warm forming temperatures and found that the enhanced ductility at elevated temperature was contributed primarily from the post-uniform elongation. Degner et al. [10] analyzed the mechanical behavior during isothermal tensile tests of AA6111 and AA7075, and elaborated that both yield stress and strain hardening exponent were sensitive to temperature. Taheri-Mandarjani [11] studied the mechanical behavior of an extruded 7075 aluminum alloy using the tensile and compression testing method in a wide range of temperatures and strain rates, and the results indicated that the ductility was monotonically increased by increasing deformation temperature owing to the decrease of the volume fractions of the second phase particles at higher temperature. Xiao et al. [12] investigated the hot formability of AA7075 experimentally and numerically. The results have shown that the formability of AA7075 increased with decreasing deformation temperature from 773 K to 673 K and with increasing strain rate from 0.1 s−1 to 10 s−1. However, to the date, effects of process parameters on the thermal-mechanical properties of aluminium alloys at hot stamping conditions are still lack of quantitative analysis, and an industrial-friendly processing designs is urgently required.
Processing maps, which were proposed by Prasad and co-workers [[13], [14], [15], [16]], have been considered an important tool to obtain the optimum processing parameters and understand hot deformation behavior based on the dynamic materials model (DMM), and have been extensively investigated for metallic materials, such as aluminium alloys, magnesium alloys [17], Ti17 alloy [18], metal matrix composite [19], steel [20]. The “safe” domains suiting for hot forming and “unsafe” domains unsuiting for hot forming can be predicted by using the processing maps. Rajamuthamilselvan et al. [21] established processing maps of AA7075 using hot compression tests. Corresponding deformation behaviors and microstructural evolutions under different hot working conditions were identified. The results indicated that dynamic recrystallization (DRX) occurred in the temperature range of 613–663 K and strain rate range of 0.013–0.12 s−1. The optimum hot working condition for 7075 aluminum alloy was 623 K and 0.1 s−1. Lin et al. [22] also performed similar work for 7075 aluminum alloy using hot compression tests. The resulting optimum processing parameters were 623–723 K and 0.001–0.05 s−1. Kai et al. [23] investigated the hot compression workability of 6 × 82 aluminum alloy and established the processing maps. It was found that the optimum processing window of the alloy was 738–808 K and 0.09–1.2 s−1 in which exhibited a typical DRX microstructure with fine grains. Wu et al. [24] developed the processing maps of a new Al-Zn-Mg-Er-Zr alloy by applying isothermal compression tests. It was found that the processing maps exhibited two “safe” regions with high power dissipation efficiency, and the main deformation mechanism in the “safe” domains was dynamic recovery (DRV).
From the literatures described above, most studies explored the direct relationships between the deformation behavior of aluminum alloys and the process parameters including forming temperature, strain rate and strain. The effects of deformation conditions on flow behavior and corresponding mechanism determined by quantitative characterization of the deformation heat and work hardening rate of the materials were rarely discussed. As for processing maps, the current research mainly focused on bulk metal forming based on compression tests, and few studies considered the sheet metal forming processes at elevated temperatures based on tensile tests, which has the similar deformation behavior as the process of hot stamping. Hence, it is necessary to thoroughly understand effects of processing parameters on the thermal-mechanical behaviors of aluminum alloys under hot stamping conditions, and construct a processing map specific for hot stamping aluminum alloy sheets based on isothermal uniaxial tensile tests to determine the optimal deformation conditions and provide manufacturing guides for industrial volume production.
To compensate the research gap discussed above, a series of uniaxial tensile tests were carried out to obtain the flow behavior of 7075-T6 aluminum alloy at different temperature and strain rates. Effects of deformation conditions on the heat generated from plastic deformation and the work hardening rate of alloy were investigated. Then, for the first time, the processing maps of 7075 aluminum alloy sheet were further established to optimize the hot processing parameters based on the dynamic materials model (DMM).
Section snippets
Materials and experiments
The material tested in this study was 7075 aluminum alloy sheet in T6 condition with a thickness of 2 mm. 7075 aluminum alloy is a commercial Al-Zn-Mg-Cu alloy supplied by aluminum corporation of China limited. The chemical composition (wt%) of the alloy is given in Table 1. The optical microstructure of the as-received material in the longitudinal section is shown in Fig. 1. Bandlike grains elongated along the rolling direction can be seen. The tensile specimens were machined in the rolling
Flow stress behavior of 7075 aluminum alloy
Fig. 5 shows the true stress-true strain curves of 7075-T6 aluminum alloys sheet obtained from the elevated temperature tensile tests at different deformation temperatures of 573–723 K and strain rates of 0.01–10 s−1. It is evident that the flow stress of alloy is strongly dependent on the strain, strain rate and deformation temperature. The hot tensile deformation is an interactive process of work hardening, dynamic softening, precipitation and voids or cracks development [27,28]. As observed
Conclusions
In this study, the thermal mechanical properties and processing maps of 7075-T6 aluminum alloy at elevated temperatures were investigated. The following conclusions can be drawn:
- (1)
The rise of temperature ΔT, generated from plastic deformation, increases markedly with an increase in strain. ΔT is greater at lower deformation temperatures and higher strain rates, and the maximum temperature rise is 33.5 K, obtained at 573 K/10 s−1. In addition, ΔT has an almost linear relationship with strain at a
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant no. 51675392); China Automobile Industry Innovation and Development Joint Fund (Grant no. U1564202); the 111 Project (Grant no. B17034); and the Innovative Research Team Development Program of Ministry of Education of China(Grant no. IRT17R83). The authors would like to acknowledge Miss Rong Jiang and Miss Tingting Luo (Center for Materials Research and Analysis, Wuhan University of Technology) for
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