Constitutive behavior of aluminum alloy in a wide temperature range from warm to semi-solid regions
Graphical abstract
Introduction
Semi-solid processing (SSP) is a promising near net shape technology which can form complex components with relatively high mechanical properties (compared with as-cast part). SSP is based on the thixotropic behavior of materials (e.g. metallic alloys and metal matrix composites) in the semi-solid state [1]. When the materials are sheared they flow like liquid, while when allowed to stand they thicken up again [2], [3]. However, semi-solid processing can hardly provide as high mechanical properties as forging [4], and it is mainly attributed to two reasons: (a) the alloy contains some liquid phase (liquid fraction of 40%∼60%) before semi-solid processing, so the solidification of intergranular liquid can degrade the bonding strength between solid–solid grains; (b) the improvement in mechanical properties caused by work hardening is little in semi-solid processing. Therefore, a challenge for semi-solid processing is further improving the mechanical properties to compete with the wrought parts.
Combination of plastic forming and semi-solid processing (compound forming) is a feasible method. Two routes can be applied for compound forming: (a) the alloy is pre-formed in the semi-solid state firstly, and as the temperature decreased below the solidus it is deformed plastically to the final shape; (b) the billet is rapidly heated by gradient induction heating before processing, and some region is reheated into the semi-solid state while the temperatures of other regions are still below the solidus. Therefore, thixotropic flow and plastic flow exist simultaneously during the compound forming process.
Constitutive behavior of engineering materials is of importance to predict the flow stress as a function of material structure, temperature and strain rate, and is significant for optimizing the processing parameters [5]. Therefore, constitutive behavior has received much attention in recent decades, and much work has been done to study the constitutive behaviors related to forging or semi-solid processing [6]. Serajzadeh et al. [7] studied the deformation behavior of a metal matrix composite AA2017-10% SiCp in warm and hot deformation regions (room temperature to 400 °C). At temperatures lower than 250 °C, negative strain rate sensitivity was observed indicating the occurrence of dynamic strain aging, while at higher temperatures, dynamic recovery was detected and the apparent activation energy for hot deformation was calculated to be ∼303 kJ/mol. Roy et al. [8] studied the constitutive behavior of as-cast A356 aluminum alloy using compression tests performed over a wide range of deformation temperatures (30–500 °C). The results indicate that the extended Ludwik-Hollomon and Koccks-Mecking are suitable for small strain and large strain conditions, respectively. At elevated temperatures, Zener-Hollomon model provides the best prediction of flow stress. Wang et al. [9] proposed a visco-plastic constitutive equation for thixoforming aluminum alloy in the semi-solid state, and it takes into account the effect of the liquid fraction along with other factors i.e. the stress/strain relationship, strain rate and temperature.
To the best of our knowledge, although many phenomenological and physical-based constitutive models have been proposed for forging and semi-solid processing, no constitutive model has been reported to cover the wide range of processing temperatures (from warm forming temperature to semi-solid region). Therefore, it will be of significance to develop a constitutive model suitable for compound forming which combines plastic forming and semi-solid processing.
In the present work, the constitutive behavior of aluminum alloy was characterized using compression tests performed over a wide range of temperatures (350–600 °C). Mechanical testing and microstructural observations were adopted to evaluate the materials responses during deformation at elevated temperatures. This work could enable optimization for the practical application and numerical simulation of compound forming.
Section snippets
Experimental procedure
The materials used in this work were extruded and T6 heat treated 7075 aluminum alloy rods (extruded at 350 °C with extrusion ratio of 16:1). Table 1 shows the chemical composition of the starting materials, which was examined using X-ray fluorescence spectrometer.
Differential scanning calorimetry (DSC) was used to determine the solidification interval and the liquid fraction-temperature relationship. The details of DSC method were described in the previous work [10]. Fig. 1 shows the change in
Microstructures of the starting materials
Fig. 2 shows the microstructures of extruded 7075 aluminum alloy in longitudinal and transverse sections. As shown in Fig. 2(a), the grains are elongated with some dispersed particles along the extrusion direction. Some sub-grains also exist in the microstructures in transverse and longitudinal sections, indicating the alloy has been partly recrystallized after T6 heat treatment. According to recrystallization and partial melting (RAP) route [11], if such materials are reheated into the
Conclusion
The constitutive behavior of 7075 aluminum alloy in a wide temperature range from warm temperatures to semi-solid region was studied by compression testing at temperatures of 350–600 °C and strain rates of 10−3–1 s−1. The stress/strain response exhibits typical plastic deformation character at temperatures of 350–490 °C, while at temperatures above 490 °C, it exhibits typical semi-solid deformation character that the flow stress decreases rapidly after reaching the peak value, and then presents
Acknowledgments
The authors express their appreciation for the financial support of National Natural Science Foundation of China under Grant No. 51405100, Postdoctoral Science Foundation of China under Grant No. 2014M551233, Astronautical Supporting Technology Foundation of China under Grant No. 2014-HT-HGD12, and Science and Technology Development Plan of Shandong Province under Grant No. 2015GGX102023.
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