室温多道次ECAP变形对7075铝合金组织性能及沉淀相分布的影响

1 Introduction

Aluminium alloys are widely used in aerospace engineering, marine vehicles and military applications due to their high specific strength, low density and easy processing. Moreover, these alloys are the driving force behind the global development of making objects and devices more lightweight [1-5]. At present, the research and development of aluminium alloys have become a key development target for defence science and technology [6-7]. Amongst these alloys, 7075 aluminium alloy is one of the strongest alloys for commercial use and has high utilisation value as a structural material. With the development of the manufacturing industry, there is now a requirement for traditional 7075 aluminium alloys to exhibit plasticity.

Fine grain strengthening is one of the effective means by which to improve the properties of metallic materials, as represented by Hall-Petch theory [8-11]:

σy=σ0+k/d12

(1)

where d is the grain diameter; σ0 is the frictional force that acts on the dislocation; σy is the yield limit of the material and k is a constant. Using the above equation, it can be seen that the strength of a material can be improved by refining its grains. However, there are some limitations to this that when the grains are refined to close to the physical limit of the dislocation itself, the above equation is not applicable. Severe plastic deformation (SPD) is favored by researchers as a unique deformation method with controllable microstructure characteristics, and can be performed at room or low temperature to obtain a material that exhibits a fine grain structure. If the amount of deformation is increased, superfine grain materials can be obtained. Equal-channel angular pressing (ECAP) is an effective SPD method that obtains bulk submicron and nanomaterials and is considered to be the most promising deformation process in SPD processing technology, with the advantages of stable deformation, multi-pass processing, and production of large bulk fine-grained metallic materials compared to other processes [12-15]. During the ECAP deformation process, the material is passed through two equal cross-sectional pipe molds with a certain angle by the top force, during which pure shear deformation occurs to break the coarse crystals into fine grains.

To date, some progress has been reported on the ECAP deformation of 7075 aluminium alloy. ZHAO et al [16] studied the relationship between the precipitation behaviour of the precipitation phase and the mechanical properties of the 7075 aluminium alloy after ECAP deformation by solid solution treatment at 480 ℃ for 5 h and ECAP deformation at 250 ℃. GHALEHBANDI et al [17] performed the single pass ECAP deformation of 7075 aluminium alloy at room temperature after solid solution treatment, where it was found that the fracture toughness of the material was significantly improved after ECAP deformation with ageing treatment.

When performing ECAP, the deformation environment temperature has a great influence on the mechanical properties of the alloy [18-21]. The 7075 aluminium alloy exhibits excellent strength and stiffness properties, but poor ductility, making it difficult to perform multi-pass ECAP at room temperature, which is why it has received less research attention.

In this study, 7075 aluminium alloys were subjected to multiple pass ECAP deformation at room temperature to investigate the effects of the process on the microstructure transformation, enhancement of the properties and the precipitation behaviour of the precipitation phase of the original material.

2 Experimental methods

Commercial 7075 aluminium alloy was used as the material of study, the composition of which is shown in Table 1. This alloy was cut into block samples of 30 mm×18 mm×170 mm size and subjected to a 30 min solid solution water cooling treatment at 477 ℃. The surface finish of the treated samples was achieved using a vertical milling machine.

Table 1  Chemical composition of 7075 aluminum alloy( wt% )

Si Fe Cu Mn Mg Cr Zn Ti Al

0.08 0.27 1.51 0.06 2.50 0.20 5.52 0.03 Bal.

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Prior to ECAP deformation, homemade lubricating fluid (graphite powder and oil mixture) was evenly applied to the samples and the extrusion rod. The process parameters for ECAP were as follows: a die angle of Φ=135°, an outer angle ψ=20°, the extrusion speed was 2.5 mm/s, and the extrusion method was route C (the sample was rotated 180° in the die for the next deformation after each process), as shown in Figure 1. The microstructure of the 7075 aluminium alloy before and after deformation was studied by electron backscatter diffraction (EBSD, NORDLYS NANO) and high resolution transmission electron microscope (HRTEM, FEI Talos F200X) to observe the grain size, grain boundary characteristics, structural evolution and precipitation phase precipitation behaviour of the alloy. The specific sampling location is the centre of the ED surface. The specific polishing parameters are shown in Table 2. The results were analyzed using Channel 5 software after completing the EBSD testing. X-ray diffractometry (XRD, D8ADVANCEA25) was used to examine the precipitated phases before and after deformation, and the physical phase analysis was performed using the Jade software. Stress-strain tensile experiments (INSTRON 8801 universal testing machine) were conducted, and the fractures in the material were observed by scanning electron microscopy (SEM). A HX-1000TM micro-Vickers hardness tester was used to examine the hardness of the ED surface before and after deformation and to construct a cloud chart. The hardness cloud point and tensile sample size are shown in Figure 2.

Figure 1  Principle diagram of ECAP

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Table 2  Parameters required for electrolytic polishing

Process condition Parameter

Polishing solution

10 % perchloric acid

alcohol solution

Polishing temperature/℃ -30

Polishing time/min 3

Polishing voltage/V 25

Polishing current/A 0.4 - 0.5

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Figure 2  (a) The required measuring points for the ED surface hardness cloud chart and (b) the size of the tensile sample (Unit: mm)

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3 Results and discussion

Figure 3 shows the grain orientation diagram before and after four passes of ECAP at room temperature, from which it can be seen that the grain shape and size changed significantly after deformation. The original base metal was a typical rolled strip structure (Figure 3(a)), and the grain distribution was coarse strip (with an average grain size of around 26 μm). After ECAP shear deformation, the strip grain was broken into a fine equiaxed structure with an average grain size of around 4.5 μm and the distribution was uniform (Figure 3(b)). Figure 4 shows the grain boundary distribution before and after the ECAP and the distribution frequency of the grain boundary size. After four passes of ECAP deformation, a large number of stress-induced dislocations were generated due to the adjustment of the lattice structure due to stress concentration, and a large number of sub-grain absorption dislocations were gradually transformed from a low-angle grain boundary (LAGB) to high angle grain boundary (HAGB) [22]. By observing the grain morphology before and after ECAP in Figure 3 and some HAGB fragments in Figure 4(b), it can be seen that continuous dynamic recrystallisation (CDRX) occurred in the material after ECAP. Therefore, the proportion of HAGB increased significantly, from 8.1% for the original base metal to 41.8%. After ECAP deformation, bimodal distribution was observed at grain boundaries, and the peak was at the LAGB (<15°) and HAGB (50°- 62°). According to a number of studies, bimodal distribution contributes to the microstructure of the material exhibiting high strength and high toughness [23].

Figure 3  Grain orientation before and after ECAP: (a) Original base metal; (b) After ECAP deformation

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Figure 4  Grain boundary distribution before and after ECAP deformation: (a) Grain boundary of the original base metal; (b) Grain boundary diagram after ECAP; (c) Grain misorientation distribution of the original base metal; (d) Grain misorientation distribution after ECAP deformation

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Figure 5 shows the distribution of the recrystallisation before and after ECAP deformation based on EBSD, from which the degree of completion of recrystallisation before and after ECAP deformation can be obtained, where blue indicates the recrystallisation area, yellow is the sub-grain line area and red is the deformation area. The percentage of the recrystallisation area increased significantly after four-passes of ECAP deformation, which indicates that the recrystallisation effect is very significant. Under the effect of the pure shear deformation of ECAP, a large number of dislocations accumulated in the deformed grains to form dislocation cells and gradually transformed into sub-structural tissues.

Figure 5  Recrystallization distribution before and after ECAP deformation: (a) Original base metal; (b) After four passes of ECAP; (c) Recrystallization fraction plot of the original base metal; (d) Recrystallization fraction plot after four passes of ECAP

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Figure 6 shows the local orientation difference (KAM) and the relative probability curve of the alloy before and after ECAP deformation, in which a high KAM value represents high dislocation density in the region (according to the analysis of the EBSD data, the dislocation density indicated by the KAM plot is mainly geometrically necessary dislocations). By observing the data shown in Figure 6, it can be concluded that the KAM values are reduced after ECAP deformation compared to the original material, which has the highest percentage at 0.6 and 0.5 after ECAP deformation. This is mainly due to the occurrence of CDRX in the material, which makes the deformed grains gradually absorb dislocations and transform into recrystallisd grains, a phenomenon that is consistent with the above description.

Figure 6  Local orientation difference distribution before and after ECAP deformation: (a) Original base metal; (b) After four passes of ECAP; (c) Relative probability curve of the average orientation differences

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Figure 7 shows the polar diagram analysis before and after ECAP deformation, from which it can be seen that the preferred orientation changed after four-passes of ECAP deformation. Figure 7(a) shows that the original material exhibits a typical rolled texture (β-fibre texture), and after ECAP deformation, the strongest point is observed for the {111} crystal plane and derives more strong points, mainly featuring a C-type shear texture, which is due to the ideal pure shear deformation of ECAP, the c-axis is tilted in the ED and ND directions, and the texture orientation is tilted 45° along the deformation system. The decrease in polar density compared to the parent material after four passes of ECAP deformation may be explained by the fact that the factors that change the crystal degree phase during the ECAP deformation process mainly include dislocation accretion and twinning, and the dislocation cell caused by dislocation accretion has relatively random phase statistics [24].

Figure 7  Pole diagram analysis before and after ECAP deformation: (a) Original material; (b) After ECAP deformation

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Figure 8 shows the XRD pattern analysis of the original base material and the alloy after the ECAP deformation. It can be seen that the precipitated phases in precipitation species are basically the same before and after deformation, mainly featuring a mixture of η phase (MgZn2), S phase (Al2CuMg) and GP region (Al2Cu). After ECAP deformation, the Al2Cu phase peak can be observed at a 2θ angle of 41.49°, and the content of the MgZn2 and Al2CuMg phases decrease at 2θ angles of 44.87° and 65.20°, which may be due to the coarsening of some insoluble precipitation phases after the solid solution and natural ageing of the original base material, and the fragmentation and refinement of  precipitating and co-gridding with the matrix due to ECAP deformation [25]. The main peaks of the precipitated phases were shifted in different directions after ECAP deformation, indicating that both precipitation phase generation and dissolution occurred.

Figure 8  XRD analysis before and after ECAP deformation

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The 7075 aluminum alloy is a precipitation-reinforced aluminum alloy, the type and formation of the second phase of which have a great influence on its materials matrix [26-29]. Figure 9 shows the TEM analysis of the original base material and the second phase after ECAP deformation. In         Figure 9(a), it can be seen that the coarse second phases (indicated by the white arrows in the figure), which are mainly Fe-containing precipitated phases, appear in the original base material, and these second phases easily generate large strain gradients around them due to their large size, thus undergoing particle stimulated nucleation (PSN). PSN nucleation can produce rotational cubic texture and P texture around it, which is one of the reasons for the low strength of the texture due to the fragmentation of the coarse second phase after the ECAP deformation. The diffusely distributed second phase plays a key role in the recovery and recrystallisation of the 7075 aluminum alloy. From Figure 9(a), it can also be observed that a diffuse phase band appears at the grain boundary (shown by the yellow arrow in the figure), and the fine second phase in the dispersion phase band can produce pinning effect on grain boundary migration, so the migration of the grain boundary along the width of the diffuse phase band exerts a large pinning force, which inhibits the occurrence of reversion and recrystallisation and therefore the dislocation density is high [30]. Figure 9(b) shows the diffraction calibration of the precipitated phase types in the pristine base material, which mainly include long rod-like MgZn2 and fine spherical Al2CuMg. In Figure 9(c), it can be seen that the rolled pristine base material contains a large number of dislocations accumulated as dislocation walls (shown by black arrows in the figure), and the interaction between the second phase and dislocations can be observed in the base material, which shows that the main strengthening modes of the base material are second phase and dislocation reinforcements. As shown in Figure 9(d), compared with the base material, the grains are elongated in the 45° direction after four passes of ECAP deformation. Moreover, the dislocation density decreases and the dislocation morphology is greatly changed. This is because after ECAP deformation, the dislocations are gradually transformed into dislocation cells under the effect of shear and dynamic recovery, and further evolve into sub-grains up to the HAGB. It can be seen from Figure 9(d) that the second phase particles are more uniformly distributed and finer in size, which according to the Orowan dispersion strengthening theory can contribute to the mechanical properties of the matrix.

Figure 9  TEM analysis of base metal and alloys after ECAP: (a) Distribution of insoluble phase in the original base metal; (b) Type of the precipitated phase of the 7075 aluminium alloy; (c) Dislocation distribution of the original base metal; (d) Dislocation distribution after ECAP deformation

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Figure 10 shows the analysis of different precipitation phases in 7075 aluminium alloy after ECAP deformation using HRTEM to determine the various precipitation phase types. The position selected in the green box in the figure is the circular precipitation phase, and the lattice constant                d=0.214 nm can be known by inverse FFT transformation, and the precipitation phase can be known as Al2Cu phase by comparing its lattice constant and morphology. The position selected in the white box in the figure is the Al matrix, and by measuring the lattice constant, it is known that its lattice constant d=0.247 nm, which is higher than the original lattice constant d=0.234 nm, so it is presumed that the Al matrix lattice constant is distorted in the vicinity of the Al2Cu phase. Notice that the yellow box checked position is the bar precipitation phase, and the lattice constant is known by measurement. The lattice constant d=0.845 nm, and combined with the phase morphology can be known as the MgZn2 phase. The area selected by the blue box is an ellipsoidal precipitation phase, and the lattice constant is measured to be d=0.247 nm, and the phase can be identified as Al2CuMg phase in combination with the morphology.

Figure 10  HRTEM analysis of different precipitation phases:(a) Al matrix lattice stripes; (b) Disc-shaped precipitated phase; (c) Disc-shaped precipitated phase lattice stripes; (d) Rod-like precipitated phase; (e) Rod-like precipitated phase lattice stripes; (f) Elliptical precipitated phase; (g) Elliptical precipitated phase lattice stripes

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Figure 11 shows the hardness cloud chart before and after ECAP deformation. It can be seen from the diagram that the hardness is significantly improved after four passes of ECAP deformation, from the original average of HV 115 to HV 145, an increase of 26.1%. It can thus be seen that the ECAP deformation process has an obvious effect on the improvement in hardness. After four passes of ECAP deformation, the microhardness distribution is relatively uniform, but the average hardness of the upper surface is slightly higher than that of the lower surface. This is due to the large shear effect on the upper surface during ECAP deformation, and the lower surface is not uniform because there is a dead zone at the corner of the die. According to previous research, the formation of the GP zone increases significantly after ECAP deformation, which is beneficial to improving the hardness of the alloy. After ECAP deformation, the grains are effectively refined. The relationship between grain size and hardness can be described according to the  equation by CABIBBO et al [31] :

Figure 11  Analysis of hardness cloud chart before and after ECAP deformation: (a) Base metal; (b) After ECAP deformation

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H=H0+kd−1/2      

(2)

where H is the current hardness of the material; H0 and k are constants; and d is the grain diameter. This equation shows that the smaller the grain size, the greater the hardness.

Figure 12 shows the room temperature tensile curves before and after ECAP deformation, with the mechanical properties shown in Table 3. The yield strength (YS), tensile strength (UTS) and elongation (EL) of the base material are 118 MPa, 153 MPa and 16.2%, respectively, while after ECAP deformation, the YS, UTS and EL are 314 MPa, 346 MPa and 6.5%, respectively. Compared to the base material, the YS and UTS were enhanced by 166.1% and 126.1%, respectively, after four-pass of ECAP deformation, but the plasticity decreased significantly, from 16.2% to 6.5%, which shows that the ECAP deformation process is an effective means to improve the strength of the metal. The effective improvement in YS and UTS after ECAP deformation may be the result of the coupling effect of fine grain strengthening and work hardening. The plasticity of ECAP is reduced due to the increase in grain boundary distortion and defects after intense plastic deformation, which readily leads to dislocation plugging and is not conducive to dislocation opening. According to the theory of fine grain strengthening, after ECAP deformation, the grain size decreases and the proportion of relative grain boundaries increases, and thus the resistance to dislocation movement increases. It is difficult to cross the grain boundaries; therefore, the strength is effectively improved.

Figure 12  Stress-strain curves before and after ECAP deformation

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Table 3  Mechanical properties before and after ECAP deformation

Material YS/MPa UTS/MPa EL/%

BM 118 153 16.2

ECAP 314 346 6.5

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During the ECAP deformation process, the 7075 aluminium alloy matrix produces numerous strengthening mechanisms, such as second-phase strengthening, fine grain strengthening and dislocation strengthening. 7075 aluminium alloy has a large number of precipitated phases, which have been determined as MgZn2 phase, Al2CuMg phase and Al2Cu phase by XRD and HRTEM above, so, the second-phase strengthening model related to the volume fraction is used to calculate its contribution to the yield strength of 7075 aluminum alloy. The measured precipitation phase volume fraction picture is shown in Figure 9(b). The measurement software is Image-Pro plus, and the equation followed is [32]:

σs=σm[Vp(S+2)2+Vm]

(3)

where σm is the matrix yield strength; Vp is the volume fraction of the precipitated phase;Vm is the matrix volume fraction; S is the aspect ratio of the precipitated phase. The incremental effect of precipitation phase strengthening on the yield strength of the material is [32]:

Δσs=σs−σi

(4)

The volume fraction Vp of the precipitated phase was 0.155 measured by Image-Pro plus software; matrix volume fraction Vm is 0.845; the aspect ratio S of the precipitated phase is 1.35. Therefore, it can be calculated that the precipitation phase reinforcement contributes 32.85 MPa to the yield strength during ECAP. Fine grain strengthening has a significant effect on the yield strength improvement of the material. To investigate the specific value of its contribution to the yield strength of the material, the effect of grain size on the yield strength of ECAP after deformation was therefore investigated according to the Hall-Petch relationship (Eq. (1)). And the strengthening effect of fine grains on the yield strength increment of the material during ECAP deformation is [33]:

Δσs=σ1−σ0

(5)

Δσs=K0(d−1/21−d−1/20)

(6)

In the aluminium alloy, K0 is 0.06-                0.28 MPa/m1/2. Therefore, it can be concluded that the maximum contribution to the material yield strength by fine grain strengthening is 76.7 MPa. According to the above KAM diagram analysis, it can be seen that the dislocation distribution inside the material matrix is relatively uniform after ECAP deformation, so the contribution of dislocation strength (Δσd) and dislocation density (ρ) to the yield strength has been studied with Taylor formula [34]:

Δσd=Mα1Gbρ−−√

(7)

where M=3.06 is the Taylor factor; α1=0.3 is a constant; G=27 GPa is the shear modulus of Al; b=0.286 nm is the Burgers vector of dislocations in the Al alloy, and the internal dislocation density after ECAP deformation can be calculated as 6.2×1013 m-2 by the KAM plot. The calculated contribution to the yield strength of the material matrix by dislocation strengthening is 54.7 MPa. Therefore, it can be deduced that during ECAP deformation, fine grain strengthening contributes most to the yield strength, followed by dislocation strengthening, and precipitation phase strengthening the least, with a ratio of 2.33:1.67:1.

Figure 13 shows the analysis of the morphology of the fracture before and after ECAP deformation, from which it can be seen that the base material exhibits typical ductile fracture characteristics. Moreover, the surface of the fracture is relatively flat and contains a large number of dimple, the size of which is larger and deeper, indicating that its plasticity is better. After ECAP deformation, the fracture surface exhibits obvious undulations. Moreover, the number of dimple is reduced, the size is smaller, their depth is shallower and the tearing ribs are thinner. In addition, the features of the deconstruction fracture and fracture along the grain, such as river pattern (as shown by the dashed yellow rectangle in Figure 13(b)) and holes (as shown by the dashed orange rectangle Figure 13(b)), infer that it is a mixed brittle-ductile fracture mode. From the high-magnification SEM observation of the fracture after ECAP deformation (Figure 13(c)), it can be observed that it contains a large number of fine second-phase particles deep in the toughness fossa, and that these fine particles play a certain role in promoting the mechanical properties. Moreover, the fine second phase exerts a certain shear force on the matrix during ECAP, which is beneficial to refining the grains. In addition, these second phases also improve the plastic toughness of the material and organize the crack expansion.

Figure 13  Fracture morphology analysis before and after ECAP: (a) Original base metal fracture; (b) Fracture after ECAP deformation; (c) Black rectangular high magnification image

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4 Conclusions

1) The ECAP deformation process has a very obvious effect on the grain refinement of 7075 aluminum alloy, the average grain size in the original rolling state of which is reduced from

26 μm to 4.5 μm, and the percentage of HAGB is increased from 8.1% to 41.8%.

2) After four passes of ECAP deformation, the degree of crystallisation of the material is improved and the deformed grains are gradually transformed into recrystallized grains through the continuous absorption of dislocations.

3) Many coarse second phases of the base material broke and refined in ECAP deformation, and transformed into fine particles, which provide a better strengthening effect for the substrate.

4) The materials strength and hardness were significantly improved after four passes of ECAP deformation, which is mainly related to the second phase strengthening and dislocation strengthening mechanism.