Augmentation of Fatigue and Tensile Strength of AA-6061 Processed through Equal Channel Angular Pressing

ECAP (Equal Channel Angular Pressing) is a technique used to enhance the strength of material by grain refinement. In this research, an aerospace grade aluminum alloy-6061 is investigated. The specimens were pressed through ECAP die channels, intersecting each other at an angle of 90C where a shear plane of 45C was developed, that results grains refinement. Fatigue strengths and CGR (Crack Growth Rate) for the stress ratio R 0.7 and 0.1 are found and compared with the as-received material.It was observed that the CGR is slower at stress ratio R=0.1, as compared to stress ration R=0.7. An electric furnace was embedded with ECAP die to regulate the material flow through this die. The temperature of the die was maintained at 450C during ECAP pressing and the specimens were also preheated at this temperature using another furnace. The ECAP die consistsof two channels intersecting at 90 provided with safe inner and outer corner radius to avoid scaling.The microstructural observations revealed that the deformation was perfectly plastic. The ECAPed and as-received materials were also characterized by tensile tests, micro-hardness tests, and 3-point bend fatigue tests.


INTRODUCTION
metals is the use of SPD (Severe Plastic Deformation). There are many metal strengthening techniques used by the researchers since the material development history. Some of them are conventional (forging, rolling, extrusion etc.), it is obvious that these conventional techniques do not play an important role for grain refinement and hence strength increased up to a certain limit. To overcome this problem,some advanced techniques known as SPD techniquessuch as HPT (High Pressure Torsion) [1], DCAP (Dissimilar Channel Angular Pressing) [2], ARB (Accumulative Roll Bonding) [3], ECAP [4] etc. were introduced. ECAP is typically used to strengthen light aluminum alloys through grain refinement [5][6][7]. ECAP method was introduced by Segal in the early 1980s. The material deformation of the material in ECAP isinfluenced by the non-linearity of the metals, temperature, moldgeometry, ram speed, friction between specimen and die, and plungersshape [8][9][10]. The FCGR (Fatigue Crack Growth Rate) was also investigated for as-received and ECAPed specimens. FCGR is extension in the crack length per unit cycle, (da/dN) and is determined by slope of the FCG curve in Stage-II Region. Collini et. al. showed that specimens prepared from ECAP material had higher fatigue resistance compared to the base material [11][12][13][14].

EXPERIMENTAL SET UP
There are two channels in ECAP die, explicitlyoutlet and inlet channel as shown in Fig. 1. At the intersection of these passages two channels are formed. The inside angle is die angle () and the outside angle is corner angle (). The ECAP die and plunger were manufactured using tool steel AISI H13.
A hole was machined near the top edge of the plunger that provides connection with press ram using a bolt. The ECAP die was designed by considering the parameter: (i) shape of specimen, (ii) maximum pressing load, (iii) working temperature of specimen, (iv) size of specimen, (v) the die temperature.
At intersecting channels of die, corner radius of 8.45 mm, inner fillet radius of 3 mm was provided by preserving corner angle () of 22 degree. These angles avoid scaling of the billet. To heat the specimen along with die, a temperature controlled furnace shown in Fig. 1, which was fabricated on the bed of hydraulic press for a smooth flow of material through ECAP die. The die and specimen was heated up to 450°C for a period of two hours. A HTIW (High Temperature Insulation Wool) was used for safety measurements to avoid heat dissipation. The die temperature was controlled by a control unit fitted with 4x950W heating elements. The overall capacity of hydraulic press was 100 tons.
The specimens for ECAP were prepared from as received material AA-6061, a plate with 22 mm thickness and specimens were prepared from this plate with dimension 20x20x120 mm 3 .
The mechanical properties of the tested material are shown in Table 1.
Before pressing these samples in ECAP die, the composition of the material was determined by XRF (X-Ray Fluorescence) and is given in A clip-on-gauge was installed on notch to determine CMOD (Crack Mouth Opening Displacement) during fatigue testing.
Using the values of CMOD, stress intensity was found. The obtained results were evaluated and crack ratio (a/w) for every crack length was determined. The plots of CMOD and FCGR were plotted.The 3-point bend fatigue set-up is shown in Fig. 3, in which the specimen is supported at two roller supports and load is applied with the third roller at the midpoint of specimen. The clip on gauge is fixed to measure crack growth rate Rockwell and microhardness tests were performed to measure hardness before and after ECAP. A mirror-like surface was achieved by polishing according to ASTME384 standard and micro-hardness tests were carried out for the load of 100 gram across the length of specimens.
The initial advancement of the current investigation is shown by step wise understanding of the ECAP process, shown in shows that circular rings is converted into an elliptical form. Fig. 4(b) is the specimen made fromTeflon wit circular slots filled with shoots and its right side Fig. 4(b) shows that these circular rings is converted into an ellipse when processed through ECAP die. Fig. 4(c) shows the specimen made from the research material and its deformation is on its right side Fig. 4(c). The deformation of the grains was assessed by numerical simulation [12][13][14].
A circle was introduced in meshing and the arrangement of the circle after ECAP. This was confirmed using a billet of Nylon material. Then the experiments for AA 6061 carried out and specimens from the processed material were obtained for material characterization.

RESULTSAND DISCUSSION
The results of tensile test of as-received and ECAPed specimens are presented in Tables 3-4.
It is very clear that yield strength is increased by14.0%, Rockwell hardness by 24% whereas tensile strength is increased by 34.4% of ECAPed specimens. Equation (1) is helpful to determine stress intensity factor K.The crack started early for smaller values ofK for asreceived whereas having higher magnitude for the material processed by ECAP. This is because the ECAP process has enhanced fracture toughness.
The fatigue crack length "a" was determined by installing a digital microscope on fatigue testing apparatus MTS.
The stress intensity factor of 3-point bend test of 6061-T6 Aluminum alloy was determined by Equation (3) The Paris curves for both ECAP materials and as-received were drawn [18][19]   TheK values are more than 28 MPam, and CGR shows a sudden acceleration in as-received sample whilein ECAP specimens, an acceleration in CGR was seenup to 28 MPam of K but for higher values, it is slowdown. The results suggest that the higher values of K give less distributed data points as compared to the smaller K values. This variance is because of the interaction between cyclic plastic zones and their consequence on fatigue damage. The CGR is abrupt in as-received material than the ECAPed samples.
The association of crack length and crack ratio is presented in Fig. 7. The ASTM E399 standard cannot be pragmatic to estimate crack length by CMOD if large crack closure exists as this only allows up to a limited crack length [16]. At the stress ratio of R 0.1, the difference between the samples of as-received and ECAPed materials indicates that CMOD is more in later samples. This difference is due to the finetuning of the grains and therefore strengthening of the material. For the stress ratio R 0.7 crack ratio (a/w) of asreceived material is smaller than the ECAPed material. The slope of the trend line of as-received is more than the ECAPed material which is although very close that of ECAPed material.
In Fig. 8, The behavior of CGR against K max for as-received material and ECAPed samples is given. The curvesshow that the rift started at the estimated identical K max for both. However, CGR of the ECAPed material is comparatively slower than that of as-received material, which manifests improvement in the toughness by ECAP.

Augmentation of Fatigue and Tensile Strength of AA-6061Processedthrough Equal Channel Angular Pressing
The CGR is slower in ECAPed samples than the as-received material and is associated to the augmentation of material strength in terms of its hardness, toughness, yield strength, ultimate strength and fatigue strength by ECAP processing of the investigated material.
The appearance of the grain structure of the samples is given in Fig. 9.

CONCLUSIONS
In this work, the ECAP of the aluminum alloy 6061 was carried out. The mechanical properties such as yield strength, ultimate tensile strength, micro hardness, CGR and fatigue strength were determined and compared with asreceived material. Following are the main outcomes: (i) Firstly, the method of ECAP is conceptualized through simulation and processing of relatively softer material, in which, internal plasticity of the material can be visualized in a simple way.
(iii) The improvement in the fatigue strength was also found ECAP process.
(iv) Stress intensity of the investigated material after ECAP is measured with respect to CGR. It is found that stress intensity is quite low in the ECAPed material. Also the effect of stress ratio is found negligible.
(v) Paris curve for the ECAPed and as-received material are determined for the stress ratios of 0.1 and 0.7. It is found that the CGR in the ECAPed material is low in ECAPed material for both stress ratios.
(vi) SEM observations are clearly proving the grain size refinement by ECAP of Aluminum alloy 6061 in current study.