Investigation on Yield Behavior of 7075-T6 Aluminum Alloy at Elevated Temperatures

The material investigated in this work is a commercial peak aged (T6 tempered) aluminum alloy sheet, AA7075-T6, with a thickness of 2.0 mm. Uniaxial tensile specimens were modified following the ASTM Standard E8/E8M [21] and were cut by wire electro discharge machining (WEDM) at 0° (rolling direction, RD), 45° (diagonal direction, DD) and 90° (transverse direction, TD) to the rolling direction. In order to reduce the heat transfer from heated specimens to the hydraulic grips of the testing machine, the length of the gripper ends in specimen was extended, as shown in Figure 1. Uniaxial tensile tests were performed on the universal testing machine MTS E45.105 with the aid of induction heating at four temperatures, namely 25, 130, 180 and 230 °C, at a nominal strain rate of 0.001/s. The variations of tensile rates are detailed in Table 1. Digital image correlation (DIC) techniques were used to measure strains during the tests. To prepare the speckle pattern for DIC measurements, the following standard procedure was applied. The specimen surface was cleaned with alcohol, and a layer of white paint was sprayed onto the cleaned surface and then black paint dots with sizes of ~0.3 mm were applied onto the dried white paint layer. The DIC system was set on one side of specimen, and the induction heating coil and infrared thermal imager were set on the other side, as illustrated in Figure 2. A graphite paint layer was applied to the measured area of specimen and the infrared thermal imager provided real-time monitoring and closed-loop control of the temperatures of heated specimens. The temperature of the hot spot in the measured area was used as the feedback temperature in the induction heating system. Figure 3 presents the temperature contours of a modified specimen at 230 °C. It is clear that the temperature decreases from the central area of the gage section to the gripper ends, and a uniform temperature distribution section with an approximate length of 8 mm was noted as depicted by the dashed square in Figure 3, where the temperature difference was controlled within 5 °C below the testing temperature. The specimens were heated to the testing temperatures followed by a holding time of 2 min to ensure a homogeneous temperature distribution on specimens, and then the specimens were pulled until localized necking or fracture happened. It should be noted that the strain data from DIC calculation within the dashed square in Figure 3 were used in the post-processing with the Vic-3D software to acquire stress vs. strain curves.

To obtain the yield loci of AA7075-T6 at elevated temperatures, biaxial tensile tests of cruciform specimens at 25, 130, 180 and 230 °C were carried out on the testing machine MTS BIA5105 equipped with an induction heating system. The dimensions of cruciform specimen are illustrated in Figure 4, where the central gage section was designed following Hanabusa et al. [22] and ISO 16842 [23]. Specimens were cut by laser, and the same operation was implemented for strain measurements using DIC techniques. The horizontal and vertical directions were defined as the x and y-axes, respectively. The RD of specimens was along x-axis, and the force ratios (\(F_{x} :F_{y}\)) were set to 1:2, 1:1, and 2:1. The DIC system was set up in front of the tested specimen, and the induction heating coil was placed 10 mm from the specimen on the other side, as shown in Figure 5. In order to ensure the feasibility of applying induction heating to biaxial tensile testing, the temperature contours of a cruciform specimen were measured by an infrared camera at 230 °C in Figure 6(a). A graphite paint layer was applied to the measured area of specimen. It can be found that the temperature decreased from the central area of the specimen to the surrounding arms gradually, while the temperature contours in the central square gage area (45 mm by 45 mm) was relatively uniform. The temperature profiles extracted along the x-axis and y-axis are presented in Figure 6(b), where the origin of the coordinate system located in the cruciform center. Within the range of 45 mm along the x-axis and the y-axis, the maximum temperature was 234 °C and the lowest temperature was 222 °C. Hence, the temperature fluctuation is smaller than ± 8 °C. During the tensile testing, a thermocouple was taped to the center of the specimen using a high temperature adhesive, and the temperature at this center was monitored and used in the feedback control of the induction heating system. The heating rate was set to ~ 150 °C/min.

The biaxial true strain components were extracted and averaged over the gage area (45 mm by 45 mm) of cruciforms in the DIC software. True stress components and plastic strain components of the deformed gage regions were calculated based upon the Hooke’s law and the law of volume constancy in plastic deformation. The detailed processing of stress vs. strain data in the biaxial tensile testing refers to Min et al. [24].

Figure 7 shows the true stress (\(\sigma\)) vs. equivalent plastic strain (\(\overline{\varepsilon }_{\text{p}}\)) curves obtained from uniaxial tensile tests along different directions at the temperatures of 25 °C, 130 °C, 180 °C and 230 °C. The equivalent plastic strain (\(\overline{\varepsilon }_{\text{p}}\)) is defined by Eq. (1) to evaluate plastic deformation and impose plastic work equivalency between various stress states,

where \(\sigma_{0}\) is the yield stress under uniaxial tension along the RD, and \(\theta\) is the angle between the length direction of specimens and the RD. The initial yield stress decreases with an increase of temperature. Work hardening was observed at temperatures from 25 to 180 °C, while \(\sigma\) vs. \(\overline{\varepsilon }_{\text{p}}\) curves are almost flat at 230 °C and softening is dominated at this temperature. The softening effect is due to the increasing dislocation mobility caused by dynamic recovery at warm temperatures [25].

As shown in Figure 8(a), fracture occurred on the arm of cruciform specimens at 25 °C. The plastic deformation in the gage area of cruciform specimens was limited, and only the initial yield locus (with \(\overline{\varepsilon }_{\text{p}}\) of 0.002) can be obtained at 25 °C. To enlarge the plastic deformation in the gage region of cruciform, scholars modified the cruciform by reducing thickness of the gage or by strengthening the arms. Liu et al. [26] developed an optimal shape with a flat circular thickness reduction in the gauge region, which permits to reach an experimental biaxial stress vs. strain curve for AA5086 up to 0.30 of equivalent plastic strain. An arm strengthening method via laser deposition using materials compatible with the test material was developed by Hou et al. [27]. This method was verified using various materials to significantly enlarge the obtainable maximum plastic strain in the gage of cruciforms of aluminum alloy AA5754-O [28], dual-phase steels (e.g., DP590, DP780 and DP980) [29] and quenching and partitioning steel QP980 [30, 31].

Fracture occurred at the end of slits close to the central gage area at elevated temperatures, as shown in Figure 8(b), which led to relatively larger plastic deformation. The maximum equivalent plastic strains achieved in the gage area of specimen at the four investigated temperatures and three loading ratios are listed in Table 2. According to the data processing procedure proposed by Min et al. [24], experimental yield loci of AA7075-T6 associated with \(\overline{\varepsilon }_{\text{p}}\) = 0.002 are presented in Figure 9. Note that the data associated with stress ratios (\(\sigma_{22} :\sigma_{11}\)) of 0:1 and 1:0 are from the uniaxial tensile tests of specimens along RD and TD, respectively. It can be found from Figure 9 that the experimental yield locus at initial yielding decreases with increasing temperature.

To compare the shape of yield loci at various temperatures as well as their evolving behavior as a function of \(\overline{\varepsilon }_{\text{p}}\), the yield loci are normalized by the yield stress (\(\sigma_{0}\)) under uniaxial tension along the RD. Figure 10 presents the normalized yield loci associated with \(\overline{\varepsilon }_{\text{p}} = 0.002\) and \(\overline{\varepsilon }_{\text{p}} = 0.01\) at elevated temperatures. It can be found that the shapes of yield loci are almost temperature-independent at both \(\overline{\varepsilon }_{\text{p}} = 0.002\) and \(\overline{\varepsilon }_{\text{p}} = 0.01\).

The normalized yield loci associated with different \(\overline{\varepsilon }_{\text{p}}\) at 130, 180 and 230 °C are presented in Figure 11. At the same temperature, it is clear that the yield locus expands with an increase of \(\overline{\varepsilon }_{\text{p}}\), especially under the equi-biaxial tension. This is attributed to the fact that the work hardening rate under biaxial tension is higher than that under uniaxial tension, e.g., as shown in Figure 12 when the testing temperature was 180 °C. It is also observed that the evolving characteristics of yield loci is more pronounced at 180 and 230 °C.