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Conclusion
Introduction Aluminum alloys have been the most widely used structural materials in aerospace industry on account of their high stiffness/weight ratio and strength/weight ratio for several decades [1,2]. Two types of alloys, 2000 series (Al–Cu–Mg) and 7000 series (Al–Zn–Mg–Cu) alloys are age-hardened aluminum alloys (artificial aging) with high strength. Many experiments have been conducted to investigate the compressive or tensile stress–strain behavior of aluminum alloy under the static and dynamic loading [3–6]. However, in some applications, the aluminum alloy structures are easily fractured due to impact loading, such as the impact of debris during take-off or landing of a plane. Therefore, it is of major importance to understand and predict the dynamic fracture behaviors of high-strength aluminum alloys for actual engineering application. A comprehensive characterization of the fracture behaviors of aluminum alloy under quasi-static and dynamic loading has prompted numerous investigations into its fracture behaviors in recent years. Pedersen et al. [7] studied the fracture mechanisms of AA7075-T651 aluminum alloy under various loading conditions, and discussed the influence of stress triaxiality on its fracture behavior through the introduction of a notch in the tensile specimen. Mostafavi et al. [8] made a series of uniaxial, biaxial and triaxial tests for AA2024-T361, and investigated the effect of stress state on the fracture of AA2024-T361. Chen et al. [9] explored the dynamic fracture behavior of extruded aluminum alloy by using an instrumented Charpy test machine and V-notch specimens. It was found that the dissipated candesartan cilexetil manufacturer is practically invariant to specimen orientation and notch direction for the recrystallized alloy. For the extruded alloys, the dissipated energy is lower when the longitudinal direction of specimen is at a 90° angle to the extrusion direction. Dumont et al. [10] studied the relationship among microstructure, strength and toughness of 7000 series aluminum alloy. They examined the influences of quenching rate and aging treatment on the precipitate microstructure and the associated compromise between yield strength and fracture toughness. Han et al. [11] studied the effects of the pre-stretching and aging on the strength and fracture toughness of 7050 aluminum alloy. The results show that the peak-aged 7050 aluminum alloy possesses a higher strength, but its fracture toughness is poor. Recently, Børvik et al. [12] investigated the quasi-brittle fracture of AA7075-T651 aluminum alloy plate in plate impact test. In fact, the fracture behaviors of aluminum alloys are different from those of materials processed by rolling, extruding and heat treatment due to their complex and inhomogeneous microstructures [13]. How the microstructure affects the fracture toughness and crack propagation is of special interest [14]. Even though the aforementioned authors studied the fracture behavior of aluminum alloy plate, they did not paid enough attention to the fracture behavior of extruded aluminum alloy rod, especially under impact loading. The aim of this paper is to study the dynamic fractures of age-hardened 2024-T4 and 7075-T6 aluminum alloys. A series of dynamic three-point bending tests of the notched specimen were carried out by using Instron Ceast 9350-HV drop tower. Specifically, the effects of loading rate (expressed as the time rate of change of the stress intensity factor [15]) on the initiation fracture toughness and propagation fracture toughness were investigated. The metallurgical investigations of fracture surfaces by using optical microscopy (Keyence VHX-E1000) and desk-top scanning electron microscopy (Phenom-World BV Phenom G2 Pro) are presented below.
Materials Two different age-hardened aluminum alloys, 2024-T4 temper and 7075-T6 temper, are investigated in this research. The alloys are provided as extruded rods with 25 mm in diameter produced by Aluminium Company of America (ALCOA). Their chemical compositions are listed in Table 1. The polished and etched microstructures of 2024-T4 and 7075-T6 aluminum alloys are shown in Fig. 1(a) and (b), which are the tri-planar optical micrographs of the extruded rods along the three orthogonal directions, respectively. For 2024-T4, many coherent CuMgAl2 precipitate phases and Al–Cu–Mn dispersions in the grain interiors are presented, as shown in Fig. 1(a). The coarse recrystallized grains with elongated irregular shape along the extrusion direction are observed in the longitudinal section, while the nearly equiaxed and evenly distributed grains are observed in the transverse section. For 7075-T6, seen from Fig. 1(b), many hardening phase η′ (coherent MgZn2) in the grain interiors and precipitates η (noncoherent MgZn2) at the grain boundaries are elongated along the extrusion direction, too. The precipitate-free zones adjacent to the grain boundaries are formed, as shown in Fig. 1(a) and (b). These zones are softer than the matrix so that the trend towards the formation of strain localization appears during deformation. The formation of grain boundary precipitates depends on the cooling condition at the solid solution temperature [10,14].