Thermal deformation behavior and processing maps of 7075 aluminum alloy sheet based on isothermal uniaxial tensile tests

Highlights

• Temperature rise of AA7075 generated from plastic deformation was calculated.

• Work hardening rate of the alloys was analyzed with the constructed model.

• The processing maps were used to optimize the hot stamping process.

• The main softening mechanism of the “safe” domain is DRV.

Abstract

Process parameters and their effects on thermal-mechanical properties of aluminum alloys are vitally important for the successful manufacture of aluminum alloy panel components under hot forming condition. In this study, firstly, thermal-mechanical properties of 7075 aluminum alloy sheets under hot forming conditions were comprehensively investigated using a series of isothermal uniaxial tensile tests at different temperatures and strain rates on a Gleeble 3500 thermal mechanical simulator. Based on the experimental results, the temperature rise ΔT generated from plastic deformation was further calculated considering the fraction of plastic work converted into heat, β, and the fraction of deformation heat appearing as a temperature rise, δ. ΔT increases with an increase in strain rate and a decrease in deformation temperature, with a maximum temperature rise ΔT_max reaching 33.5 K. Furthermore, taking ΔT into consideration, the relationship model between the work hardening rate, temperature and strain rate was constructed according to the Zener-Hollmon parameter. The variation of the work hardening rate with strain rate and temperature can be interpreted using this model. Finally, to determine the optimal processing parameters, the processing maps of 7075 aluminum alloy were established subsequently under the experimental conditions. The optimal processing parameters for 7075 aluminum alloy are located within the windows: (1) temperature, 573–680 K and strain rate, 0.368–8.1 s⁻¹; (2) temperature, 695–723 K and strain rate, 0.05–1 s⁻¹. According to the microstructure observations, it can be concluded that the main softening mechanism of “safe” domains is dynamic recovery (DRV) and the continuous dynamic recrystallization (DRX) can be effectively retarded due to the presence of precipitated particles. The work performed in this research, for the first time, provides quantitative evaluations of process parameters on the hot deformation of aluminum alloy sheet forming using processing maps considering corresponding microstructural evolutions, which enables to provide useful guides for process designers of sheet metal forming.

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).