Abstract
Effect of aging treatment on mechanical properties of an age-hardenable aluminum alloy after equal channel angular pressing at room temperature has been investigated using hardness, stress–strain behavior and surface fractography. Aluminum alloy 7075 was pressed after solution treatment. Yield stress, ultimate stress and hardness of pressed samples have increased significantly compared with those of coarse grain, but the elongation to failure has decreased. Also the pressed specimens were subjected to aging treatment at room temperature and temperatures of 80 °C, 100 °C, 120 °C and 140 °C to obtain the optimized strength and ductility. The results indicated that post–equal channel angular pressing aging at 80 °C has resulted in the maximum strength, and natural aging has resulted in good ductility and acceptable strength. It confirmed the fact that there is a potential in obtaining high strength and good ductility in age-hardenable alloys employing severe plastic deformation and subsequent aging.
Introduction
High strength aluminum alloys, have a high strength to weight ratio, being widely used in aerospace and structural engineering. There, is nevertheless, much significance to improve the mechanical properties (strength and ductility) of these materials using engineering processes. Equal channel angular pressing (ECAP), as a severe plastic deformation (SPD) process, produces significant plastic strain into materials without reducing the cross-sectional area. Sample, in the form of a rod or bar, is placed so that it can be pressed through the die using a plunger. Nature of the imposed deformation is simple shear which occurs as the sample passes through the die as shown schematically in Figure 1, the theoretical shear zone is shown between two elements within the sample numbered 1 and 2, and these elements are transposed by shear as depicted in the lower part of the diagram. 1 Despite the introduction of a very intense strain as the sample passes through the shear plane, it ultimately emerges from the die without experiencing any change in the cross-sectional dimensions. ECAP process also introduces non-equilibrium condition in the microstructure of the alloys such as high dislocation density and large number of low angle grain boundaries. 2
The principle of ECAP showing the shearing zone within the die.
Although there are some limitations for ECAP as a manufacturing process (like other manufacturing processes) such as the workpiece length, there are some capabilities in significantly increasing the strength and fatigue endurance during this process. Also products manufactured by this technology meet the basic pre-requisites for their subsequent use at super-plastic forming. So the ECAP process can be regarded as an unconventional forming method which would be useful namely in automotive and aerospace industry. Yanagida et al. 3 practically applied ECAP as a pre-forming process in manufacturing of a micro-bolt with ultrafine-grained (UFG) carbon steel. They compared formability and also tensile strength of the bolts with that of bolts formed by a conventional process, involving cold rolling followed by swaging. Djavanroodi et al. 4 proposed and experimented a new method for production of UFG wire-shaped workpiece based on the conventional ECAP die. The results indicated that the number of passes can be, respectively, chosen if hardness or electrical resistivity being individually considered as favorable criterion. Tran 5 used ECAP process to improve hardness and electrical conductivity of copper to fabricate and improve working performance of copper as electrode for resistance spot brazing application on nickel substrate.
Most investigations, including ECAP process on aluminum alloys, have concentrated on work hardening of Al–Mg alloys6,7 and little attempts have been made for strengthening Al–Zn–Mg alloys. 8 On the one hand, cold working after solution treatment (ST) has been found to be inconsequent in strengthening of Al–Zn–Mg alloys, 9 and performing SPD processes on solutionized alloys at room temperature is generally difficult, 10 but on the other hand for age-hardenable alloys, it is worth to investigate whether the precipitation strengthening effect can be added to the hardening effect of SPD processes. Increasing the process temperature11–13 or the channel angle within the ECAP die 14 may eliminate or reduce these problems, but it leads to a decrease in the strengthening effect. For Al–Zn–Mg alloys, there is an additional difficulty in successful processing, where these alloys are so sensitive to natural aging (NA) which may occur rapidly prior to SPD processing. 15 So, there are few investigations on the improvement in mechanical properties of solutionized aluminum alloy 7075 processed by ECAP at room temperature. 16
Performing SPD processes leads to increase the strength of metals and alloys, but the tensile ductility decreases after the first pass. 17 Decreasing the ductility can be attributed to the lack of ability to accumulate dislocations.18,19 Recently, some investigations have focused on the improvement of both strength and ductility of age-hardenable SPD processed aluminum alloys. Kim et al. 20 investigated the effect of post-ECAP aging on aluminum alloy 2024 and obtained high strength and a moderate level of ductility due to effects of fine-particle distribution in the aged matrix. Zheng et al. 15 studied the strengthening of solutionized aluminum alloy 7050 and identified that post-ECAP low-temperature aging has significantly improved the strength. Kim et al., 12 for the modified aluminum alloy 7075, reported that yield stress and ultimate strength have significantly increased when the post-ECAP aging treatment was applied at best aging conditions. Zhao et al. 21 have proposed a post-SPD aging treatment for simultaneously increasing ductility and strength of the cryorolled aluminum alloy 7075. Panigrahi and Jayaganthan 22 investigated the influence of different aging temperatures and reported that the cryorolled sample after aging treatment shows a significant increase in yield stress and ultimate strength.
The objective of this study is to investigate the effects of ECAP and subsequent aging treatment on mechanical properties of aluminum alloy 7075. The alloy was processed by ECAP at room temperature immediately after ST and then aged at room temperature and at different elevated temperatures. The interactions between strain hardening, age hardening and recovery at elevated temperatures and their effects on both strength and ductility are studied to investigate the effects of aging conditions to achieve high strength and good ductility.
Experimental methods
Chemical composition of aluminum alloy 7075 used in this study is given in Table 1. Initial rods of 19.3 mm in diameter and 70 mm in length were first subjected to ST at 490 °C for 4 h and then water quenched, resulting in supersaturated solid solution. Then, samples were pressed through an ECAP die. The ECAP die used in this work consisted of two channels with equal cross-sections that intersect at an angle of Φ = 90° and outer curvature angle of Ψ = 20°. For this die design, the effective strain accrued on a single pass through the die is approximately 1. 23 The ECAP process was carried out through one pass at room temperature and a pressing speed of 1 mm/s. Molybdenum disulfide (MoS2) was used as lubricant. These ST and pressed samples were then subjected to NA and artificial aging at four different temperatures of 80 °C, 100 °C, 120 °C and 140 °C with the aging durations sufficient for saturation of hardness to study the influence of age hardening on mechanical behavior of equal channel angular pressed aluminum alloy 7075. Also, ECAP has performed on an annealed sample which was heated at 350 °C for 2 h and then furnace cooled to investigate the effect of pre-ECAP heat treatment on mechanical properties.
Chemical compositions of aluminum alloy 7075.
In order to study the effect of different aging treatment conditions on strength and ductility of the equal channel angular pressed samples, Rockwell B hardness and tensile tests were carried out. Hardness was measured on the plane parallel to the longitudinal axis. The average value of the hardness is based on six separate measurements at different randomly selected positions. All of the tensile tests were performed at room temperature using a Dartec-type testing machine operating at a constant rate of crosshead displacement at strain rate of 4 × 10−3 L/s. The dimensions of tensile specimens according to sub-size dimensions of ASTM E8 are shown in Figure 2. The fracture surfaces of the tensile test segmented specimens were characterized by a Philips-XL30 type scanning electron microscope (SEM).
Dimensions of tensile specimens (mm).
Results and discussion
Mechanical properties
One of the aims of this study is to investigate the effect of pre-ECAP heat treatment on mechanical properties of pressed specimens. Figure 3 shows the effect of NA on hardness of solutionized alloy. The specimen was heated at 490 °C for 4 h and then water quenched. The results indicated that the alloy is very sensitive to NA and the hardness has increased rapidly which could be attributed to the early formation of Guinier–Preston (GP) zones after quenching. 16 As an example, the hardness of ST specimen has increased about 60% in 24 h. The problem arises when the alloy is prepared for ECAP since the mechanical properties of solutionized alloy 7075 and also pressed specimen are very dependent on the delay time before ECAP. One way to overcome this problem is to impede the process to achieve stable properties, but increase in forming forces is inevitable, where the precipitates are strong enough to suppress the dislocation motion. 24 For this alloy, according to Figure 3, the properties of solutionized alloy are so sensitive to the delay time up to about 24 h and after this time the properties will be stable for about 3 days. This way, however, the application of SPD is often complicated, and because of the combined effect of precipitates and shear bands, samples often break during ECAP at room temperature. Therefore, in several cases, supersaturated samples of Al–Zn–Mg alloys were processed by ECAP at elevated temperatures. 25 On the other hand, at high temperatures, the precipitation is often uncontrolled and a recovery of the UFG microstructures may occur. 26 Another way is to keep the solutionized samples at low temperatures before performing the ECAP process or perform the ECAP immediately after ST. In our experiments, the samples were kept in a freezer for 2 h and then placed in the die to perform the ECAP process.
Hardness of solutionized aluminum alloy 7075 versus natural aging time.
ST and also annealed specimens were then subjected to ECAP. Figure 4 shows the effect of pre-ECAP heat treatment on hardness of the specimens. As shown in this figure, the hardness of annealed and ST specimens has increased up to four and two times, respectively, after the ECAP process. For annealed sample, the increase in hardness can be attributed to the strain hardening effects including the increase in the dislocation density and grain refinement 27 and for the ST sample, the precipitation hardening and the dynamic strain aging can also be considered. Furthermore, the hardness of solutionized specimen after ECAP is greater than the annealed one. Also the hardness of specimen after T6 treatment (88 RB) is slightly higher than the as-ECAP specimen (85 RB). But after about 2 months of NA, the hardness of pressed sample has increased up to 92 RB. The increase in hardness is consistent with earlier reports for the aluminum alloy 7075 where after ECAP and 1 month of NA the reported hardness is about 200 Hv (93 RB). 21
Effect of ECAP and heat treatment on hardness of aluminum alloy 7075.
The results demonstrate that processing a solutionized specimen by ECAP, even by a single pass, significantly improves the hardness and the maximum hardness can be obtained using ECAP and subsequent aging. This finding may have important practical significance, because of its advantage in industrial applications to press through a minimum number of passes when using a continuous manufacturing. The application of a small number of passes leads not only to a significant savings in time, but also it reduces the possibility of micro-crack formation which often occurs during processing by SPD. 26 For instance, Young et al. 28 continuously applied the ECAP to a commercial multi-stage forming process at a production rate of 30 r/min at room temperature to produce high strength aluminum bolts without modifying conventional material.
It is worthwhile to note that the possibility of performing the second ECAP pass on the solutionized specimen at room temperature is ambiguous, because of the combined effect of GP zones (created by dynamic aging during ECAP and NA after ECAP) and SPD which resulted in limited ductility. The high density of dislocations produced already in the first ECAP pass can strongly accelerate the formation of GP zones. Therefore, there may be a detrimental effect of GP zones in subsequent passes, where the strain localization may cause the formation of cracks during ECAP. 24 Figure 5 shows the macroscopic, even catastrophic, shear bands created during the second pass 1 week after the first pass. However, it may be possible to perform the second pass immediately after the first 16 or at elevated temperatures. 25
Pre-ECAP solutionized specimen after the second pass.
In order to improve the mechanical properties of equal channel angular pressed specimens, samples were subjected to post-ECAP aging treatment. Figure 6 shows the effect of NA on hardness of pre-ECAP solutionized alloy. The hardness of pressed sample is sensitive to NA and has increased about 3% after 40 h and about 6% increase after 2 months.
Hardness of equal channel angular pressed aluminum alloy 7075 versus natural aging time.
To study the effects of artificial aging, samples were placed in furnace at temperatures of 80 °C, 100 °C, 120 °C and 140 °C. The hardness data both for ST and equal channel angular pressed samples during artificial aging are shown in Figure 7. It is indicated that, after ECAP process, the required time to gain the maximum hardness has been decreased. Also, reducing the aging temperature has resulted in increasing the maximum hardness, but the aging time to reach the maximum hardness is increased. For instance, when samples are subjected to aging treatment at 80 °C (Figure 7(d)), the hardness of solutionized sample has increased from 41.3 to 89.8 RB in 220 h (117% increase) and for the pressed samples, the hardness has increased with aging time of 140 h from 86.3 to 94.6 RB (9% increase).
Hardness of equal channel angular pressed aluminum alloy 7075 versus artificial aging time at (a) 140 °C, (b) 120 °C, (c) 100 °C and (d) 80 °C.
The maximum hardness has been obtained by post-ECAP aging at 80 °C and 100 °C, where the aging time for maximum hardness was less at 100 °C. Obtaining maximum hardness at lower temperatures of artificial aging can be attributed to hardening effect of fine precipitations produced by low-temperature aging and decreasing the effect of recovery and particle coarsening at low temperatures.21,29 The effect of recovery and particle coarsening are significant at higher temperatures and also increase with aging time. So the maximum hardness at 120 °C and 140 °C is smaller than the maximum hardness at lower temperatures and the recovery and particle coarsening cause the hardness to decrease after saturation time, when these effects dominate the precipitation effect. Similar observations are reported for cryorolled aluminum alloy 7075 and the maximum hardness of 210 Hv (95 RB) was obtained using 100 °C for aging treatment. 22 Furthermore, the results clearly show that aging phenomenon is accelerated for the equal channel angular pressed material. It can be attributed to the increased dislocation density which is one of the microstructural effects of SPD. The increased dislocation density facilitates the nucleation of strengthening precipitates. So, the incubation time for precipitate nucleation and the aging time to obtain maximum hardness are reduced for the pressed alloy as compared to the ST alloy. 20
Four different post-ECAP aging conditions have performed on the equal channel angular pressed samples to investigate the effect of post-ECAP aging on the strength and ductility; NA for 2 months, artificial aging at 80 °C, 100 °C and 140 °C all for 100 h. Three tensile samples have been machined out for each aged specimen (Figure 8). The tensile stress–strain curves and data of these samples are shown in Figures 9 and 10 and Table 2.
Position of tensile samples in the equal channel angular pressed specimen.
Effect of aging on stress–strain behavior of equal channel angular pressed aluminum alloy 7075 after aging at (a) room temperature, (b) 80 °C, (c) 100 °C and (d) 140 °C.
Comparison between stress–strain behaviors of central (B) samples.
Effect of artificial aging on mechanical properties of equal channel angular pressed aluminum alloy 7075.
UTS: ultimate tensile strength; ST: solution treatment; ECAP: equal channel angular pressing.
As compared to the ST samples, the average yield strength (YS) and the ultimate tensile strength (UTS) of the post-ECAP natural aged samples have increased significantly, in agreement with the hardness result in Figure 4. The high strength of post-ECAP aged samples compared to ST samples can be attributed to the influences of precipitation hardening, grain refinement and dislocation strengthening. 29 For a ST specimen, if the second ECAP pass be performed immediately after the first pass and at room temperature, the maximum strength could be obtained. As an instance, after two passes by route Bc at room temperature and 1 month NA of aluminum alloy 7075, the engineering YS of the specimen has improved from 320 to 650 MPa and the ductility decreased from 20.5% to 8.4%. 16 Comparing the results with the strength of pre-ECAP annealed specimen, 27 it could be indicated that the strength of the pre-ECAP solutionized specimen after only one pass of ECAP is significantly greater than the strength of pre-ECAP annealed specimen after four passes, where the UTS and the ductility have been reported to be 425 MPa and 7.9%, respectively. Although the ECAP forces are much less for annealed specimens, increase in number of passes will result in significant decrease in ductility. Furthermore, decreasing the number of passes will reduce the negative effects of SPD such as creation of micro-cracks. This result can be useful in application of ECAP combined with ST as a part of the manufacturing process, where only one pass of ECAP after ST can improve the strength of material significantly and also to increase the production rate.
Since the precipitates density after artificial aging is higher than that of NA, its strengthening effect is more and the yield stress and ultimate strength have increased up to 510 and 546 MPa after artificial aging at 100 °C. This fact could also be deduced from Figures 6 and 7, where the maximum hardness of the artificial aged samples is higher than natural aged sample.
The effects of post-SPD artificial aging on mechanical properties (hardness, strength and ductility) can be attributed to the interaction between two major parameters: 22 (1) presence of precipitates in the artificial aged samples (precipitation effect), in which dislocations can be accumulated in the surrounding of the nanosized precipitates during tensile test. Increase in density of the precipitates can increase the possibility for accumulation of higher amounts of dislocations causing more tensile straining, contributing to the enhanced ductility and can also enhance the strength, where the particles can also act as obstacles for dislocation movement. (2) Annihilation of dislocations during artificial aging (recovery effect), which provides additional possibility for dislocation accumulation during tensile test, which increases the ductility and can decrease the strength because of decreasing the dislocation density. Also if the aging temperature is high enough, the concurrent particle coarsening can decrease the strength. So, when the heavily deformed samples are subjected to aging treatment, the effect of precipitation hardening and recovery occurs simultaneously and the effect of aging temperature on mechanical properties can be explained by the interaction between precipitation hardening and recovery. Decreasing the aging temperature to 80 °C, the YS and UTS have reached to 520 and 576 MPa, respectively, because of the greater effect of precipitation hardening, where the aging temperature is not sufficient for the recovery effect. However, increasing the aging temperature up to 140 °C has resulted in significant decrease in both YS and UTS as a result of particle coarsening and more recovery.
Along with the improvement of strength, the ductility has decreased after ECAP from 18.4% to 11.82%. The decrease in ductility continues after aging treatment at 80 °C down to 9.24% but, aging at 100 °C has resulted in a slight improvement of ductility up to 10.12%, where the decrease in the YS and UTS is not considerable. So, there may be conditions in which the ductility could be improved without significant decrease in strength. This result may be attributed to three major aspects of the aging treatment. 29 First, high dislocation density after the ECAP provides appropriate sites for the precipitation and distribution of nanoscale particles. Second, the reduction in internal stress and development of more homogeneous microstructure are due to the recovery occurring during aging. Third, the low temperature of aging treatment is not sufficient for major particle coarsening. However, aging at 140 °C sacrificed the strength enhancement of SPD to some extent and has improved the ductility up to 11.23% which is clearly the effect of recovery dominance. It indicates that when the post-ECAP aging temperature is not sufficient for the recovery effect, the ductility could be improved without significant decrease in strength. A proper post-ECAP heat treatment allows the pressed alloy to have a very high strength with a moderate level of ductility. Similar observations were reported in literature. Improvement from 550 to 616 MPa in YS and from 4.5% to 10.5% in ductility for cryorolled aluminum alloy 7075, 21 improvement from 545 to 607 MPa in YS and from 5.6% to 9.5% in ductility for cryorolled aluminum alloy 7075, 22 improvement from 690 to 725 MPa in YS and from 14.5% to 17% in ductility for high temperature equal channel angular pressed aluminum alloy 7075 containing Sc 12 have been reported after artificial aging.
Surface fractography
The SEM fractographs of tensile samples before and after ECAP are shown in Figure 11. It reveals that natural aged sample has experienced ductile fracture, consisting of many ductile dimples and voids indicating the ductile failure. The dimples are the result of void nucleation and subsequent coalescence. In addition, it can be seen that the average dimple size of the initial specimen is coarse. Decrease in dimple size and dominance of cleavage facets are observed in the equal channel angular pressed sample. The combined brittle and ductile fracture can be attributed to the grain refinement and work hardening, which are in accordance with the similar reports for the SPD samples.24,27
Tensile fracture surfaces of aluminum alloy 7075 after (a) natural aging, (b) ECAP and natural aging.
Conclusion
Processing a solutionized aluminum alloy 7075 specimen by ECAP, even by a single pass, significantly improves the hardness, YS and UTS. This finding may have important practical significance because of its advantage in industrial applications with considerable saving in time.
Hardness of solutionized and equal channel angular pressed specimen increased from 86.3 to 92 RB after 2 months NA and increased up to 94.6 after artificial aging. The maximum hardness was obtained by post-ECAP aging at 80 °C, where the effects of recovery and particle coarsening are negligible.
The YS of solutionized specimen was increased from 240 to 451 MPa after ECAP and 2 months of NA. The maximum YS of 520 MPa was obtained by post-ECAP aging at 80 °C.
The ductility of solutionized specimen decreased significantly after ECAP and 2 months NA. The ductility decreased again after artificial aging at 80 °C down to 9.24% but increased up to 10.12% after aging at 100 °C. It confirmed the fact that, there may be conditions in which the ductility could be improved without significant decrease in strength.
The ductility increased with aging at 140 °C but with significant decrease in the strength, where the aging temperature was high enough for particle coarsening.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship and/or publication of this article.