**2. Experimental methods**

428 Recent Trends in Processing and Degradation of Aluminium Alloys

It has been reported that in AA8011 aluminium alloy sheets processed by ARB, the main deformation texture orientation is the Dillamore {4 4 11}<11 11 8> component and the shear texture is developed on the surface of the sheets (Kim et al., 2002). This surface shear texture disappears rapidly as the surface area of the material reappears in the centre of the composite sample during the next ARB cycle. The role of second phase particles on the grain size reduction of aluminium sheets during the accumulative roll bonding process has been investigated by comparing the microstructure and texture of a single phase (AA1100) and a particle containing aluminium alloy (AA3003) during various ARB cycles (Pirgazi &

Circular shaped hollow sections like tubes and cylinders, as a category of engineering components, are also expected to achieve high strengths by nanostructure. Tube spinning is used as a common tube fabricating process (Wong et al., 2003), based on which a coldbonding process titled "spin-bonding" with the advantages of ARB of sheets has been recently proposed (Mohebbi & Akbarzadeh, 2010a, 2010b) for manufacturing of high strength thin-walled tubes and cylinders. The SPD process proposed in that work is, in fact, repeatedly spin-bonding of layered tubes (accumulative spin-bonding, ASB) to induce large plastic strain on tubes similar to the ARB of sheets, Fig. 2 (Mohebbi & Akbarzadeh, 2010a).

Fig. 2. Schematic illustration of the spin-bonding (Mohebbi & Akbarzadeh, 2010a)

In many cases, the inner and outer surfaces of hollow sections are exposed to different environments, and different characteristics are required inside and outside. In these cases, various bimetallic or clad tubes of stainless steels or high-alloy steels and super alloys clad onto carbon or low alloy steels, are utilized in boilers, heat exchangers, nuclear power plants and petroleum and chemical industries (Chen et al., 2003). So far, several methods have been used to produce composite tubes and cylinders. While centrifugal casting (Sponseller et al., 1998) and extrusion (Chen et al., 2003) can be used to fabricate the thick-walled bimetallic tubes, explosive bonding (Berski et al., 2006), ball attrition (Zhan et al., 2006), thermohydraulic fit method and hydraulic expansion method (Wang et al., 2005) are capable of manufacturing thin-walled cylinders. The most widely used cladding process is roll-

Akbarzadeh, 2009).

The materials used to study the ARB process were fully annealed sheets of Al-Mn alloy (AA3003) and commercial purity aluminum (AA1100) with initial grain sizes of 40 and 34 µm. Thickness of the sheets was respectively 0.4 and 0.5 mm, and the chemical compositions are presented in Table 1. Two sheets of 150×50 mm2 were degreased (in acetone) and wire brushed (by a stainless steel brush with wires of 0.4 mm in diameter). After the surface treatment, the two sheets were stacked on top of each other and preheated to a temperature of 250 °C for 5 minutes. The plane strain rolling was performed along the longest dimension by 50% reduction in thickness at 200 °C without any lubrication and the mean strain rate was 51 sec-1. Afterwards the roll bonded sample was cut into two sheets of approximately the initial dimension and the procedure was repeated up to a total of 8 cycles so that an accumulated equivalent strain of 6.4 was achieved.


Table 1. Chemical composition of the aluminium alloys (Pirgazi & Akbarzadeh, 2009)

The microtexture and microstructure measurements were performed on the section which is perpendicular to the transverse direction of rolling and is located at the mid-thickness of the sheets. The analysis was carried out by employing an Orientation Image Microscopy (OIM) attached on a Philips XL30 ESEM microscope equipped with a LaB6 filament. The electron back-scattering diffraction (EBSD) mappings were carried out with step sizes in the range of 0.12 μm to 80 nm and the OIM software developed by TSL® was used for data acquisition and post-processing procedures.

Since the ARBed samples were severely deformed, acquiring good Kikuchi-patterns was not evident. By means of electro polishing a very smooth surface was produced and by adjusting the operating parameters of the SEM and the software (OIM Data Collection) a

Nanostructure, Texture Evolution and Mechanical Properties of

Table 2. Dimensions of tools and work piece and the process conditions

thickness of the specimens by applying the load of 0.49 N.

without etching.

**3.1 Microstructure 3.1.1 ARB processing** 

**3. Results and discussions** 

Aluminum Alloys Processed by Severe Plastic Deformation 431

EBSD patterns were obtained at 25 kV and analyzed by TSL software regarding the quantitative analysis of grain boundaries and misorientation distributions. The step size was 100 nm for the specimens after the first and second cycles and 60 nm for the specimens after the third and fourth cycles of ASB. Mechanical properties of the specimens were determined at ambient temperature with strain rate of 2.6×10−3 sec−1. The samples were prepared from the circumferential direction of the tubes after straightening and machining according to the ASTM E8M standard. Vickers microhardness values were also measured through the

The material used in ASB process was the commercial purity aluminum (AA 1050). Thickness of the tubes was 0.8 mm. The inner diameter of the internal tube and the external one were 51 and 52.8 mm, respectively and their length was 40 mm. Work pieces were annealed at 350 °C for 2 hrs before the bonding process. Since they were processed via tube spinning, work pieces were diametrically true after annealing. The bonding surfaces were degreased in acetone and scratch brushed. The time between surface preparation and bonding process was kept to less than 300 s to minimize the formation of contaminant film and a thick oxide layer on the bond surfaces of the tubes. Afterwards, the two tubes to be joined were positioned against each other. The process is schematically illustrated in Fig. 2. The bond strength of the Al/Al bimetal layers was measured using the T-peel test. Samples were 8 mm wide and 50 mm long and were cut at the longitude of the tubes so that their width was in the circumferential direction. The breaking off force per width of the sample was measured as the bond strength (N/mm). Optical microscopy was employed to examine the bond interface of the longitudinal section of deformation zone. The surfaces to be examined were prepared by standard metallographic procedure to polished conditions

Figure 3 shows the optical micrograph observed in the RD-ND section of the sample produced by 10 ARB cycles, indicating that a good bonding with no delamination has been achieved under the present ARB conditions. To investigate the microstructural evolution of

satisfying result could be obtained. In this way, after using a grain confidence index (CI) standardization (5°, two points) and neighbor orientation correlation (CI = 0.1) cleanup procedures on the raw data, confidence indices of 0.9 and 0.79 were obtained for the samples processed by 4 and 10 cycles, respectively.

The Vickers micro hardness test was utilized to investigate the mechanical properties of initial and ARB processed samples. The values reported for HV represent the average of ten measurements taken at randomly selected points across the thickness of the sheets using loads of 50 and 100 g for 20 s. The mechanical properties of initial and ARBed samples were measured by tensile tests at room temperature executed with an Instron tensile testing machine. The test specimens were prepared with the tensile axis parallel to the rolling direction. The test conditions and the specimen size were chosen according to the ASTM-E8 standard.

Orientation distribution functions (ODFs) were calculated by harmonic series expansion with truncation at L = 16. The initial texture of the fully annealed AA3003 alloy sheet prior to the ARB process was also characterized using the EBSD analysis. In order to obtain a statistically acceptable result, this measurement was carried out with a step size of 2 μm over an area with dimension of 300 μm×300 μm located at mid thickness and as a result more than 22,000 orientations were analyzed.

The texture evolution during ARB process was predicted with the Alamel model (Van Houtte et al., 2005, 2006). In this model the texture is discretized in a set of N individual orientations which are considered in pairs of two. Each pair of orientations will accommodate the externally imposed strain but shear strains can be relaxed in equal and opposite measure for each grain belonging to a pair. In the ODF calculations the orthorhombic sample symmetry was imposed which is usually assumed for the conventional rolling process. Only the {111} <110> slip systems were taken into account for the texture calculations.

The tubes used for ASB process were prepared from commercially pure aluminum (AA1050). Thickness of the tubes was 0.8 mm. Work pieces were annealed at 350 °C for 2 hrs before the first cycle. Spin-bonding process was repeated up to four cycles. In the spin bonding process two surfaces to be bonded (inner surface of the external tube and outer surface of the internal one) were degreased in acetone and wire brushed as surface treatment. After scratch brushing, surface treated tubes were positioned against each other and fitted on the mandrel for tube spinning at room temperature with conditions of Table 2 (Mohebbi & Akbarzadeh, 2010c). At this stage, while the tube and mandrel rotate about their axes, a roller with a degree of freedom about its own axis moves along the direction of the tube axis to reduce its thickness to 50% leading to bonding of the tubes. No preheating was performed in this work. More details of the spin-bonding is available in (Mohebbi & Akbarzadeh, 2010a).

The longitudinal sections of the specimens were observed by an optical microscope after polishing and etching in the Tucker solution for 15 s. Transmission electron microscopy (TEM) micrographs and the corresponding selected area diffraction (SAD) patterns were also obtained from the specimens after 1, 2 and 4 cycles of ASB. To do so, thin foils were prepared by twin-jet polishing from the mid-thickness of the tubes normal to the tube surface. The microstructural evolution was also analyzed by electron backscattering diffraction (EBSD) pattern. This was performed on the longitudinal sections perpendicular to the peripheral direction of the tubes after various cycles of ASB. The measured area was 25 μm×30 μm located at the center of mid-thickness of the tubes.

satisfying result could be obtained. In this way, after using a grain confidence index (CI) standardization (5°, two points) and neighbor orientation correlation (CI = 0.1) cleanup procedures on the raw data, confidence indices of 0.9 and 0.79 were obtained for the

The Vickers micro hardness test was utilized to investigate the mechanical properties of initial and ARB processed samples. The values reported for HV represent the average of ten measurements taken at randomly selected points across the thickness of the sheets using loads of 50 and 100 g for 20 s. The mechanical properties of initial and ARBed samples were measured by tensile tests at room temperature executed with an Instron tensile testing machine. The test specimens were prepared with the tensile axis parallel to the rolling direction. The test conditions and the specimen size were chosen according to the ASTM-E8

Orientation distribution functions (ODFs) were calculated by harmonic series expansion with truncation at L = 16. The initial texture of the fully annealed AA3003 alloy sheet prior to the ARB process was also characterized using the EBSD analysis. In order to obtain a statistically acceptable result, this measurement was carried out with a step size of 2 μm over an area with dimension of 300 μm×300 μm located at mid thickness and as a result

The texture evolution during ARB process was predicted with the Alamel model (Van Houtte et al., 2005, 2006). In this model the texture is discretized in a set of N individual orientations which are considered in pairs of two. Each pair of orientations will accommodate the externally imposed strain but shear strains can be relaxed in equal and opposite measure for each grain belonging to a pair. In the ODF calculations the orthorhombic sample symmetry was imposed which is usually assumed for the conventional rolling process. Only the {111} <110> slip systems were taken into account for

The tubes used for ASB process were prepared from commercially pure aluminum (AA1050). Thickness of the tubes was 0.8 mm. Work pieces were annealed at 350 °C for 2 hrs before the first cycle. Spin-bonding process was repeated up to four cycles. In the spin bonding process two surfaces to be bonded (inner surface of the external tube and outer surface of the internal one) were degreased in acetone and wire brushed as surface treatment. After scratch brushing, surface treated tubes were positioned against each other and fitted on the mandrel for tube spinning at room temperature with conditions of Table 2 (Mohebbi & Akbarzadeh, 2010c). At this stage, while the tube and mandrel rotate about their axes, a roller with a degree of freedom about its own axis moves along the direction of the tube axis to reduce its thickness to 50% leading to bonding of the tubes. No preheating was performed in this work. More details of the spin-bonding is available in (Mohebbi &

The longitudinal sections of the specimens were observed by an optical microscope after polishing and etching in the Tucker solution for 15 s. Transmission electron microscopy (TEM) micrographs and the corresponding selected area diffraction (SAD) patterns were also obtained from the specimens after 1, 2 and 4 cycles of ASB. To do so, thin foils were prepared by twin-jet polishing from the mid-thickness of the tubes normal to the tube surface. The microstructural evolution was also analyzed by electron backscattering diffraction (EBSD) pattern. This was performed on the longitudinal sections perpendicular to the peripheral direction of the tubes after various cycles of ASB. The measured area was

25 μm×30 μm located at the center of mid-thickness of the tubes.

samples processed by 4 and 10 cycles, respectively.

more than 22,000 orientations were analyzed.

standard.

the texture calculations.

Akbarzadeh, 2010a).


Table 2. Dimensions of tools and work piece and the process conditions

EBSD patterns were obtained at 25 kV and analyzed by TSL software regarding the quantitative analysis of grain boundaries and misorientation distributions. The step size was 100 nm for the specimens after the first and second cycles and 60 nm for the specimens after the third and fourth cycles of ASB. Mechanical properties of the specimens were determined at ambient temperature with strain rate of 2.6×10−3 sec−1. The samples were prepared from the circumferential direction of the tubes after straightening and machining according to the ASTM E8M standard. Vickers microhardness values were also measured through the thickness of the specimens by applying the load of 0.49 N.

The material used in ASB process was the commercial purity aluminum (AA 1050). Thickness of the tubes was 0.8 mm. The inner diameter of the internal tube and the external one were 51 and 52.8 mm, respectively and their length was 40 mm. Work pieces were annealed at 350 °C for 2 hrs before the bonding process. Since they were processed via tube spinning, work pieces were diametrically true after annealing. The bonding surfaces were degreased in acetone and scratch brushed. The time between surface preparation and bonding process was kept to less than 300 s to minimize the formation of contaminant film and a thick oxide layer on the bond surfaces of the tubes. Afterwards, the two tubes to be joined were positioned against each other. The process is schematically illustrated in Fig. 2.

The bond strength of the Al/Al bimetal layers was measured using the T-peel test. Samples were 8 mm wide and 50 mm long and were cut at the longitude of the tubes so that their width was in the circumferential direction. The breaking off force per width of the sample was measured as the bond strength (N/mm). Optical microscopy was employed to examine the bond interface of the longitudinal section of deformation zone. The surfaces to be examined were prepared by standard metallographic procedure to polished conditions without etching.
