The Effects of Mechanical Properties on Fatigue Behavior of ECAPed AA7075

Microstructure characterization

The OM images, SEM and EDS images, TEM and SAED images of ECAPed and casted AA7075 were carried out. The OM images were given in Figure 1. The microstructure of the ECAPed AA7075 exhibits oriented grain structure. Grain structure is thinner to each pass of ECAPed samples. Hence the microstructure of the ECAPed AA7075 exhibits lying parallel grain structure to the ECAP pass direction.

Figure 1:

Optical micrograph of AA7075: ECAP one passed (a), ECAP two passed (b), ECAP three passed (c), ECAP four passed (d) and casting (e).

When the OM images are examined which is given in Figure 1, grain structure of ECAP samples have been seem smaller than only casting sample microstructure. In examined microstructure of one–four-pass ECAPed samples have seen that small and oriented of grain size. In particularly, given the examination of experimental specimens four-pass ECAPed, the grain boundary separation and orientation of the grains can be seen clearly in Figure 1(d). When the casting sample’s SEM images are analyzed, casting microstructure (e) is composed of coarse grains and has dendritic grain structure. Dendritic grain structure is in certain areas of microstructure (A) in the form of secondary dendrite arms shows the structure of the grain growth. Investigated to the other studies on A356 alloys about with ECAP process; there is shown that same properties and oriented with before studies of microstructure images at the end of shaping process [30, 31].

In this study ECAPed one–four-pass and casting of AA7075 sample SEM images and EDS (electron diffraction spectrometry) analyzed are shown in Figures 2 and 3. When investigation to ECAP’ ed samples (Figure 3) have various passes number in grain boundaries more clearly can seen together with increase of passes number. ECAPed sample grain sizes with SPD effect in casting structure encountered of dendritic structure, with the number of deformation pass have band-like alignment and cannot shown dendritic structure are obtained more coaxial grain shapes. The investigation of the microstructure and mechanical properties of ECAP routes with materials formed by ECAP in studies examining the route depending on the specific grains are oriented at an angle and is illustrated by SEM analysis [32]. Formed in samples with grain orientations increase in the number of passes with a finer grain structure has been transformed into a band-shaped directional grain structure. In one form of the resulting microstructure of the material formed during the ECAP process with dislocation density increases and shears to the mechanical properties of grain obtained by breaking it is stated that additional smaller particles [32]. ECAPed AA7075 SEM images are given in Figure 3. Inspection of each passes in SEM images appears to be much smaller than the grain size to shown in casting microstructure of grain sizes. In the SEM images macroscopic orientations are understood especially in Figure 3(b) and (d). Samples’ small grain structure at first-pass numbers changes to smaller size grains with the increase of pass numbers.

Figure 2:

The SEM microstructure of the casting AA7075 (a), EDS graphic (b).

Figure 3:

The SEM microstructure of the ECAPed AA7075: one passed (a), two passed (b), three passed (c) and four passed (d).

Grain sizes for the ECAP process applied samples can be reduced up to nanolevel. The TEM and SAD images of ECAPed one-, two-, four-pass AA7075 samples are shown in Figure 4. ECAPed samples’ grain structures are seen as band shape arrangement. Deformation flow lines and grain boundaries are also seen clearly with the effect of processing temperature. The majority of the microstructure after each passes consists of somewhat elongated regions subdivided by the dislocation boundaries oriented to the ECAP pass direction. These boundaries mostly have a low-angle misorientation. At ECAPed samples together with the increasing number of passes, there has been a reduction in grain sizes and ultrafine grain structure (UGS).

Figure 4:

The TEM microstructure of the ECAPed AA7075: one passed (a1–a2), two passed (b1–b2), four passed (c1–c2).

Together with the deformation processes that are applied to the metals, deformation hardening is occurred. Even if SPD process provides grain size reduction, it also provides decreasing at re-crystallization temperature. However, decreasing re-crystallization temperature causes to occur bigger grain sizes at deformation mechanisms made of lower temperature. Increasing of the grain sizes causes a decrease in strength [33]. In ECAP process, the increases in the number of passes in the samples are increased in proportion with the deformation. Therefore, with increasing number of passes and the grain structure formed in the deformation leads to smaller grain sizes.

Casting microstructures observed in the coaxial grain structure in optical microscope (OM) images, together with the increasing number of ECAP pass, band-shaped structure has evolved into one lined up. SPD process together with the newly developed grain boundaries and then the reduction unit is formed in ECAP process. With the effect of plastic deformation in atomic plane to samples realized in shearings’ and has occurred dislocation motion. The resulting dislocation movements increased with the increase of the number of ECAP passes and the resulting increase in deformation at the grain boundaries has created an increase in the hardness of concentrating the sample. This situation has been made by Kumar et al. in their study [2]. Also in the ECAP’ ed samples, selected area diffraction (SAD) images obtained from the diffraction images were used and the crystal lattice structure of the samples were examined in the deformed state. Taken images of SAD in the first and second passes, there is a regular spot array of arrays of four passes a point seems to be more regular. APD deformation of the crystal lattice structure of the material by the action of angular orientations depending on the irregular point shifts in alignment and point directly proportional to the deformation ratio is known to convert a structure [34]. In this study, the irregular structure of the SAD images to be caused by the intensity of the deformation rate can be expressed.

Mechanical properties

An important improvement in mechanical properties of AA7075 obtained after ECAPed compared to casted bulk AA7075. Figure 5 shows the effect of casting and ECAPed over hardness properties of the AA7075. The hardness of the ECAPed samples has increased from 105 to 137 HV (nearly 31% increase) due to the increase of pass number. However, the hardness of the as-cast sample is 97 HV and this hardness value is increased to 137 HV after ECAP four-passed processing (nearly 42% increase) as shown in Figure 5. In order to improve the mechanical properties of the casting AA7075, the samples were subjected to ECAP process. The hardness data of the AA7075 alloy samples measured after each pass.

Figure 5:

Vickers hardness of ECAPed and casting AA7075 samples.

In consequence of the values of casting process applied samples’ hardness was obtained at the lowest level (97 HV) according to ECAPed samples’ hardness values. ECAPed samples’ measured hardness values were shown to increase with the increase of the pass number and the highest hardness value (137 HV) was obtained at four-pass applied sample. Effect of material hardness to pass numbering done with a variety of materials has been investigated by ECAP studies. The results obtained vary according to the ECAP process applied material. The highest hardness value reached at four passes. In studies on subject were observed to decreases in hardness values after this pass number [35–38]. Therefore, there is degradation and recovery of crystalline materials. Dislocations because of a lock as SPD have occurred and actual hardness may provide a compression between the atoms and create an increase [33]. The greatest effect of the increase of hardness values because of dislocation density. The studies on finer grain structure and dislocation density were confirmed to this study [10, 14, 39–42].

High-cycle fatigue properties

As cast, ECAP one-passed, ECAP two-passed, ECAP three-passed and ECAP four-passed AA7075 samples are tested to four-point bending fatigue test equipment (ASTM E 466). The graph of the results is given in Figures 6 and 7. Figures 6 and 7 show the stress versus number of cycles to failure curve for the casted and ECAPed AA7075. It is observed that at an alternating stress level of 100 MPa, the number of cycles to failure is 6.5 × 10⁶, whereas at alternating stress of 85 MPa, the number of cycles to failure increased to 6.9 × 10⁶; and at alternating stress of 70 MPa, the number of cycles to failure is increased to 7.5 × 10⁶ for casting AA7075. On the other hand, for ECAP one-pass AA7075 sample at stress level of 140 MPa, the number of cycles to failure is 1.8 × 10⁷, whereas at 120 MPa it undergoes 2.2 × 10⁷ cycles to failure and at 100 MPa it undergoes 2.7 × 10⁷ cycles to failure. Also for ECAP two-pass AA7075 sample at stress level of 140 MPa, the number of cycles to failure is 2.8 × 10⁷, whereas at 120 MPa it undergoes 3.3 × 10⁷ cycles to failure and at 100 MPa it undergoes 3.7 × 10⁷ cycles to failure. On the other hand, for ECAP three-pass AA7075 sample at stress level of 140 MPa, the number of cycles to failure is 3.9 × 10⁷, whereas at 120 MPa it undergoes 4.4 × 10⁷ cycles to failure and whereas at 100 MPa it undergoes 4.7 × 10⁷ cycles to failure. Finally, for ECAP four-pass AA7075 sample at stress level of 140 MPa, the number of cycles to failure is 4.5 × 10⁷, whereas at 120 MPa it undergoes 5.0 × 10⁷ cycles to failure and at 100 MPa it undergoes 5.4 × 10⁷ cycles to failure.

Figure 6:

S–N curve of casting of AA7075 samples.

Figure 7:

Comparative S–N curve of ECAP one pass, two pass, three pass, four pass of AA7075 samples.

Fatigue fracture surface morphology

The fracture surface morphologies of casted and ECAPed samples investigated under SEM and EDS are given in Figures 8 and 9. According to the ECAP samples that can be seen ductile-brittle fracture in examination of the fracture surfaces of the casting sample in Figure 8(a-1) and 8(a-2). The resulting sharp edges have from dendritic and brittle structure during the production of casting materials. The number of passes observed with the increase in the pits smaller in size fracture has occurred to ECAPed samples. For this reason, disintegration of grain sizes with SPD and a microstructure material with small grain size obtained. In their study of Das et al. [14, 43] have made the cast AA7075 alloy fracture surface by shown that ductile-brittle fracture surfaces and can be seen as a gradual breaking of fatigue fracture surface. In examination to fracture surfaces of the fatigue tested samples in the area of crack formation to structure of casting sample and cannot move strain of sample cross section with notch effect of fatigue crack by sudden breakage of a structure is observed. This structure of optical and SEM microscopic examinations as shown (Figures 1(e) and 8) and dendritic structure exhibits ductile brittle.

Figure 8:

The SEM image of fracture surface morphology of casting AA7075 after high-cycle fatigue test.

Figure 9:

SEM images of fracture surface morphology of ECAPed AA7075 after high-cycle fatigue test; one passed (a1–a2), two passed (b1–b2), three passed (c1–c2) and four passed (d1–d2).

Especially shearing bands are given attention to investigation of the fracture surface morphology of ECAPed AA7075 samples in Figure 9. This structure shows that during ECAP “C” route using which is applied to the sample and number of passes depends on the deformation bands formed in the shape stems from the finer grain structure. Especially with first pass obtained in the sample consisting of fatigue crack propagation direction, while the nominal stress value is changed to second pass the interruption lines and stable crack growth are observed. In ECAP third pass with stable crack growth is shown as surface structure with projections and recesses. Fort this reason, according to the obtained samples ECAP one and two passes with APD to smaller grain structure. In ECAP four pass is sudden fracture and crack formation can be seen more clearly. The reason for this clarity of the ECAP grain structure one, two and three passes to be smaller and smaller particles than a single show is the fact that the structural behavior. It is considered that ECAP passes between the shear band and the fatigue crack propagation in terms of crack formation and sudden fracture evolving toward the structural progress number of passes depending on the sample in microstructure units thinning and hardness increase in the value (Figure 9).