Microstructure and texture characterization of 7075 Al ...

Materials Characterization 106 (2015) 390–403

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Microstructure and texture characterization of 7075 Al alloy during the SIMA process B. Binesh, M. Aghaie-Khafri ⁎ Faculty of Materials Science and Engineering, K.N. Toosi University of Technology, 1999143344, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 2 March 2015 Received in revised form 15 June 2015 Accepted 17 June 2015 Available online 18 June 2015 Keywords: Isothermal treatment SIMA Semi-solid Texture 7075 Al alloy

a b s t r a c t The effect of SIMA process parameters on the crystallographic texture and microstructure of 7075 aluminum alloy was investigated. Specimens of 7075 were subjected to 10–55% uniaxial compression strain at ambient temperature and then semi-solid treatment was carried out at a range of 600 to 620 °C for 5–35 min. The fiber textures of b 110N║CD and b111N║CD were dominant textures in compressed samples. The textures were transformed to a randomized texture following the formation of equiaxed and globular grains. Intense segregation of Cu and Si and appreciable depletion of Mg were observed at grain boundaries during soaking time. The grain boundary composition approaches Al–Cu and Al–Si binary eutectics, which facilitates grain boundary melting. Effects of temperature and strain on both the coarsening kinetics of solid particles and the pinning of grain boundary liquid films are also discussed. © 2015 Elsevier Inc. All rights reserved.

1. Introduction In recent years, semi-solid state forming has been widely studied and accepted as an effective process which is capable of forming metals into a near net shape. It combines advantages of conventional casting and forging processes [16,24,36,54]. Semi-solid metal processing can be divided into two main groups: thixoforming and rheoforming [31,35,36]. Thixoforming requires an intermediate solidification step during processing and the metal is solid at the beginning of the forming process. However, the raw material is brought completely to a liquid state before semi-solid forming in the rheoforming process [34,35]. The key factor in the semi-solid forming processes is the presence of a thixotropic microstructure including equiaxed and spherical grains before the final forming operation [36,51]. The ideal microstructure should have an appropriate volume fraction of globular solid grains that are distributed uniformly within the liquid matrix [23,34]. It was found that fine and spherical solid grains lead to better thixotropic properties [52]. Nowadays, various processes have been developed to achieve a thixotropic microstructure in metallic alloys [28]. These processes are generally divided into two main categories [28,35]. The first category includes processes in which the main microstructure modification occurs during solidification and there is no need for reheating, e.g., mechanical and magneto-hydrodynamic (MHD) stirring. The second category includes processes in which the structural modification is carried out through remelting, e.g., remelting of fine powder particles in the ⁎ Corresponding author. E-mail address: [email protected] (M. Aghaie-Khafri).

http://dx.doi.org/10.1016/j.matchar.2015.06.013 1044-5803/© 2015 Elsevier Inc. All rights reserved.

powder metallurgy process, the thermomechanical strain induced melt activation (SIMA) process and the thixomolding process [35]. The SIMA process has the advantages of simplicity and low equipment costs. There is no need to treat the molten metal or for the capability to process both high and low melting point engineering alloys [36, 55]. The SIMA process was first introduced by Young et al. [58] in the 1980s. This process involves cold or warm working and then heating and remelting in the semi-solid range [30,57]. If the stored strain energy is sufficient, recrystallization occurs in the microstructure of the alloy and leads to the formation of fine equiaxed grains. Thus, the deformation–recrystallization mechanism is the main dominant mechanism responsible for the formation of globular microstructure during the SIMA process [57]. The effects of coupling equal-channel angular pressing (ECAP) and heating at semi-solid temperature on the microstructure of an aluminum alloy were investigated by Aghaie-Khafri [1]. The conventional forming process of the wrought aluminum alloys is generally accompanied by costly machining operations and considerable wastage of materials. Thus, the semi-solid forming of these alloys, including the high-strength 7075 alloy which is widely used in the aerospace and automotive industries, has received great attention in recent years [9,14,15,37,53]. A few studies [11,42] have been conducted on the semi-solid feedstock production of 7075 alloy. However, these were mainly restricted to the effect of processing parameters on the size and sphericity of solid grains. Although there are extensive researches [18,21,27,41,48] on the texture evolution of aluminum alloys during thermomechanical processes, no significant attempt has been made to investigate the texture of semi-solid samples during isothermal treatment.

B. Binesh, M. Aghaie-Khafri / Materials Characterization 106 (2015) 390–403

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In the present work, the effects of SIMA process parameters on the microstructure and texture of 7075 Al alloy were studied during isothermal heating. The coarsening kinetics of compressed samples during isothermal treatment at various temperatures and the effect of pre-deformation on the coarsening rate of solid particles were also investigated. 2. Experimental procedure 2.1. Materials and thermal analysis A commercial extruded round bar of 7075-T6 aluminum alloy was used as the starting material, the chemical composition of which is shown in Table 1. Metler Star SW 10 deferential scanning calorimeter (DSC) apparatus was used to determine the solidus and liquidus temperatures and the solid volume fraction versus temperature curve of 7075 alloy. Samples of the material (30 mg) were put into the alumina pan and then heated to 750 °C at 10 °C/min under nitrogen atmosphere. The solidus and liquidus temperatures based on the DSC curve were 486 and 645 °C, respectively. The solid fraction versus temperature curve was obtained by integration of the DSC curve, as shown in Fig. 1. Although the melting range of 7075 alloy is relatively wide (~160 °C), the thixoforming window appears to be small owing to the melting that occurs above 600 °C. The results were used to determine the isothermal heat treatment temperatures in the SIMA process. 2.2. SIMA process In the present research, the SIMA process was carried out to obtain semi-solid slurry of 7075 Al alloy. Cylindrical samples with a diameter of 30 mm and a height of 35 mm were machined from the starting material, stress relieved at 460 °C for 1 h, and then air cooled. Compression of cylindrical samples with compression ratios of 10, 20, 40 and 55% was carried out at room temperature using a high strain rate (~15 s−1) hammer apparatus. The compressed specimens were machined to samples with a diameter of 20 mm and a height of 15 mm to ensure uniform strain in the core section. Isothermal treatments of samples were carried out at three different temperatures of 600, 610 and 620 °C with an accuracy of ±1 °C for 5–35 min in a resistance furnace. Heating cycles were interrupted at predetermined interval times and then samples were quenched to study the microstructure. In the present study, the various SIMA cycles are referred to by compression ratio % / temperature °C / time min.

Fig. 1. DSC and solid volume fraction versus temperature curves for 7075 Al alloy.

size was determined by division of the line length (L) by the number of intercepted grains (N). The shape factor of the solid grains was measured by means of Clemex-Professional Edition image analyzer software and using Eq. (1) [46]: XN F¼

N¼1

4πA=P2

N

ð1Þ

where A and P are area and perimeter of the solid grains, respectively, and N is number of grains. For each sample, measurements were taken from the whole sectioned area including 300–400 solid particles per sample. Texture evolution of α-Al particles during isothermal heating in the semi-solid range was investigated by carrying out an X-ray diffraction (XRD) experiment using a Philips X'pert model apparatus with Cu (kα) target and a wave length of 0.15406 nm. Axially sectioned samples were used for texture analysis. During the test, as shown in Fig. 2, CD (compressing direction) was in the direction of the sample axis, TD (transverse direction) was parallel to the radial direction and ND (normal direction) was perpendicular to the testing plane. Defocusing and background errors were omitted using powder samples. In order to obtain more accurate results, the crystalline texture component was determined using orientation distribution function (ODF). Finally, {111}, {110} and {100} pole figures were determined by computer software. 3. Results and discussion 3.1. Effects of isothermal treatment on the microstructure of strained samples

2.3. Microstructure and texture characterization The microstructure of the quenched samples was examined using the standard metallography method. The surface perpendicular to the direction of the compression was ground with standard SiC abrasive papers and polished with 0.25 μm diamond paste. Samples were etched using modified Keller etchant solution (3 ml HF–2 ml HCL–20 ml HNO3–175 ml H2O). Microstructural investigations were carried out using a Neophot 32 optical microscope and Vega©Tescan and a Mira3Tescan scanning electron microscope equipped with an EDS detector. Linear intercept method was used to determine the average grain size. A series of straight lines with a specified length was considered on the optical micrographs for each sample and the average grain

Table 1 Chemical composition of wrought 7075 Al alloy in wt.%. Al

Mn

Fe

Cr

Cu

Mg

Zn

Si

Bal.

0.28

0.28

0.13

1.58

2.41

5.31

0.14

Fig. 3 shows SEM secondary electron images of the as-received 7075 aluminum alloy. It can be observed that the microstructure of the alloy consists of directed α-Al grains and coarse precipitate particles in the direction of the initial extrusion process. The EDS analysis results showed that these precipitates are mainly Al2CuMg, Al6(Cu,Fe) and Al7Cu2Fe intermetallic particles. It is worth noting that Zn, Cu and Mg are the main alloying elements in 7075 alloy which play an important role in the formation of precipitates. In 7xxx aluminum alloys, when the Zn:Mg ratios are between 1:2 and 1:3, MgZn2 with hexagonal crystalline structure precipitates at aging temperatures below 200 °C. MgZn2 is the main strengthening factor in 7xxx aluminum alloys [5]. The precipitates have dimensions of a few tens of nanometers which can only be revealed by TEM technique. Microstructures of 55% strained samples following isothermal treatment are shown in Fig. 4. The initial microstructure (annealed and then compressed) consisted of plastically deformed grains (Fig. 4(a)). It can be observed that grains gradually transform to a globular microstructure of spherical and equiaxed grains after heating and partial remelting

392


Fig. 2. Schematics showing sample orientation for compression and sample extraction and orientation for texture measurements.

at 600 °C. Concerning the micrographs of Fig. 4(a) and (b), heating the strained sample at 600 °C for 5 min leads to no appreciable variation in the microstructure. Prolonging heating up to 10 min results in fine equiaxed grains which were somewhat spherical in shape. Moreover, grain boundaries show discontinuity. With longer heating time (Fig. 4(d)–(f)), the average size and sphericity of α-Al solid grains increased. Therefore, it can be concluded that forming of polygonal equiaxed grains commences with heating at 600 °C for 10 min, and prolonged heating results in coarse and spherical grains, as shown in Fig. 5. In addition, some intragranular islands which are claimed to be liquid pools were observed just before quenching within the solid grains. The presence of fine liquid droplets is a common feature of semi-solid microstructures and has been reported for several alloys [34]. Fig. 6(a) and (b) show SEM images of the sample at 40%/600 °C/ 25 min, and Figs. 6(c) and (d) show the microstructure of areas 1 and 2 of Fig. 6(a) and (b) at higher magnifications, respectively. The EDS results of the marked areas on the SEM image are summarized in Table 2. An intense concentration of Cu at the entrapped liquid droplets within the solid grains (point A) and at the grain boundaries (point B) is observed. In addition, Si intensely segregated at other regions of the grain boundaries (point C). Although the former has also been reported in the literature [12,30], the latter is an unreported phenomenon in the semi-solid microstructure of 7075 Al alloy. Solid grains are depleted from Cu as a consequence of the segregation of Cu at grain boundaries.

This causes the increase of the solidus temperature at these regions and the decrease of the temperature at grain boundaries. Finally, the chemical composition approaches Al–Cu eutectic composition. The segregation of Si at grain boundaries leads to grain boundary chemistry closer to the Al–Si eutectic composition. The low melting point eutectic phases were fused during soaking and penetrated between the grains. Thus, a continuous intergranular network formed after quenching. It is clear that the eutectic phase becomes the predominant feature at grain boundaries following semi-solid soaking (Fig. 4(f)). Therefore, it can be concluded that the formation of low melting phases at grain boundaries is mainly influenced by high content of Cu and Si at grain boundaries. Moreover, considering Al–Cu and Al–Si phase diagrams [4], the melting point of Al–Cu eutectic compound is lower than Al–Si eutectic (548 °C compared with 577 °C). Consequently, Al–Cu eutectics are melted at grain boundaries during heating at the semi-solid temperature. The EDS analysis of the precipitates at grain boundaries (point D) in Fig. 6(d) shows a chemistry close to Al6(Cu,Fe) intermetallic phase. These particles cannot be dissolved at 600 °C [61] and appear mainly as square or irregular shaped particles at grain boundaries in the micrographs. It can be concluded from the above discussion that the distribution of alloying elements in the microstructure is significantly changed during the isothermal treatment of 7075 samples. Segregation and distribution of alloying elements have a great influence on microstructural

Fig. 3. The SEM secondary electron images of the as-received 7075 aluminum alloy.


393

Fig. 4. SEM image of samples compressed 55% and isothermally heat treated at 600 °C for (a) 0, (b) 5, (c) 10, (d) 15, (e) 25 and (f) 35 min.

Fig. 5. The effects of holding time on the average size and shape factor of solid grains for 55% strained samples at 600 °C.

evolution during semi-solid treatment. Moreover, this affects the mechanical properties of the subsequent thixoformed parts [3,13,38]. Fig. 7 shows the EDS mapping of the main alloying elements of a 40%/ 600 °C/25 min sample. The EDS map analysis of Cu and Si elements (Fig. 7(c) and (d)) shows considerable segregation of Cu and Si at grain boundaries. In addition, an appreciable depletion of Mg was observed at grain boundaries (Fig. 7(e)). However, there was no appreciable change in Zn content at various points of the microstructure (Fig. 7(f)). This is in contrast to the results of research conducted by Bolouri et al. [12] that reported the segregation of Zn at grain boundaries of a semi-solid 7075 alloy. Moreover, as can be observed in Fig. 7(g), grain boundary precipitates are rich in Fe. Considering the EDS analysis shown in Table 2, these precipitates are Al6(Cu,Fe) intermetallic particles which can pin grain boundary liquid films. This issue will be extensively discussed in Section 3.4. Finally, an overview of the alloying elements' distribution in the semi-solid microstructure of 7075 Al alloy can be observed in Fig. 7(h). The solid fraction of 0.5 to 0.7 has been reported as an optimum solid volume fraction for thixoforming processes by Atkinson et al. [6]. In this study, using the obtained solid fraction versus temperature curve for

394


Fig. 6. SEM images of 40%/600 °C/25 min semi-solid sample.

7075 alloy (Fig. 1), the isothermal heating temperatures were selected based on Atkinson et al.'s [6] research. According to Fig. 1, the solid volume fractions at 600, 610 and 620 °C are 0.8, 0.7 and 0.55, respectively. As can be observed from the solid fraction profile (Fig. 1), the gradient

Table 2 SEM-EDS analysis results of 7075 semi-solid sample of 40% /600 °C /25 min. Element

Al Mg Si Cr Cu Zn Fe Mn

Liquid droplet (point A)

Grain boundary (point B)

Grain boundary (point C)

Grain boundary precipitates (point D)

wt.%

at.%

wt.%

at.%

wt.%

at.%

wt.%

at.%

79.41 0.73 0.17 0.02 16.26 3.34 0.04 0.03

89.43 0.92 0.18 0.01 7.85 1.57 0.02 0.02

42.34 1.71 0.24 0.00 52.32 3.27 0.12 0.00

62.18 2.79 0.34 0.00 32.63 1.98 0.08 0.00

74.01 2.31 16.42 0.00 1.31 5.87 0.09 0.00

77.61 2.69 16.54 0.00 0.58 2.54 0.04 0.00

71.92 0.46 0.32 0.36 4.00 2.04 17.59 3.32

84.03 0.59 0.36 0.22 1.98 0.98 9.93 1.91

change of the solid fraction increases with the increase of temperature from 600 to 620 °C. In thixoforming processes, selecting the appropriate heating temperature and achieving the optimum solid (or liquid) fraction is important. Due to the lower heating rate, control of the thixoforming parameters will be easier if the solid fraction is less sensitive to temperature variations. However, it should be noted that a too low heating rate increases the viscosity, which in turn results in formation of some imperfections during semi-solid forming. Fig. 8 shows the quenched microstructures of specimens with different compression ratios heated at 600, 610 and 620 °C for 25 min. It is clear that increasing the isothermal holding temperature brings about greater size and sphericity of solid grains. Following coarsening of the solid grains during isothermal heating, the liquid fraction also increases and grain boundaries become thicker. The eutectic phase formation at the grain boundaries during semi-solid treatment is attributed not only to the increase of the liquid fraction but also to the coarsening of α-Al solid grains. Both the grain boundary areas and the amount of eutectic phase per unit boundary decrease with coarsening of the solid grains. This facilitates full wetting of all grain boundaries. It is worth noting that the number of entrapped liquid droplets within the grains


395

Fig. 7. (a) SEM image, (b) Al, (c) Cu, (d) Mg, (e) Zn, (f) Si, (g) Fe and (h) all alloying elements X-ray image analysis of 40%/600 °C/25 min semi-solid sample.

decreases and the droplets become larger by increasing heating temperature and time (Figs. 4 and 6). Considering the intragranular contrast in Fig. 4(a)–(f), the former observation can be attributed to the gradual dissolution of fine precipitates during the heating process. Moreover, following the prolonged heating process, some of the fine liquid droplets join the larger liquid droplets. Subsequently, these new liquid droplets become more spherical to decrease the solid/liquid interfacial energy. Considering the results obtained in the present study, the microstructural evolution of the 7075 samples during the SIMA process can be divided into two main steps. The first step includes the recovery, recrystallization and partial remelting which mainly occurs at low heating temperatures and in short times (Fig. 4(c)). During recovery and recrystallization, vacancies are combined and dislocations are rearranged by climbing or cross-slipping to decrease the free energy of the strained structure, and subgrain boundaries are created. In this step, grains of high dislocation density are replaced by new subgrains with less

dislocation density. Moreover, because of the high holding temperature, which is higher than the eutectic line, partial remelting also occurs [11]. The second step in the SIMA process is spheroidization and coarsening of solid particles. Plenty of liquid phases are produced and discrete solid grains are created by increasing the isothermal heating time (Fig. 4(c)–(f)). These particles grow and coarsen during further heating. The growth and coarsening of solid particles in the SIMA process are controlled by two mechanisms of coalescence and Ostwald ripening. Considering the coarsening of the microstructure, the diffusion of the solid material from regions of high curvature to low curvature points provides the required driving force for spheroidization of solid particles [57]. As long as the grains are not spherical, the coarsening process leads to change of the particles' shape while numbers per unit volume remain constant. Following this stage, smaller particles which have a lower melting point are melted in favor of the larger particles, and the numbers per unit volume are decreased. The microstructure of samples

396


Fig. 8. Optical micrographs of samples 10%/(a) 600 °C and (b) 610 °C, (c) 620 °C/25 min, 20%/(d) 600 °C and (e) 610 °C, (f) 620 °C/25 min, 40%/(g) 600 °C and (h) 610 °C, (i) 620 °C/25 min and 55%/(j) 600 °C and (k) 610 °C, (l) 620 °C/25 min.

that have been subjected to partial remelting in the SIMA process will be changed simultaneously and independently by the two mentioned mechanisms. The sphericity of solid particles and the amount of liquid volume fraction both have a considerable effect [51]. Ostwald ripening and grain coalescence operate simultaneously and independently as soon as liquid is formed. It has been found that the coalescence frequency is proportional to the number of adjacent solid grains [32,50]. Since the number of neighboring grains decreases with the increase in the volume fraction of liquid, growth by coalescence of solid grains is dominant at lower volume fractions in the liquid phase shortly after the liquid is formed. Therefore, it is expected that Ostwald

ripening is dominant over longer times and at high volume fractions of the liquid phase. However, the coalescence mechanism is dominant over short times after the liquid is formed, i.e., at low volume fractions of the liquid phase [32,50,51]. In the present study, small liquid volume fractions are observed in the early stages of the isothermal heating operation. Thus, solid grains are easily in contact with each other and coalescence is the dominant mechanism in the coarsening of the microstructure. This can be verified by considering the microscopic images that are shown in Figs. 4(d)–(f). It can be observed in Fig. 4(d) that due to the relatively low liquid fraction in samples heated at 600 °C for 15 min, coalescence is the dominant


mechanism. At low heating temperatures and shorter holding times, the liquid fraction is small and some grain boundaries are removed due to the coalescence of solid grains. Therefore, it can be concluded that growing and coarsening of solid grains mainly occur through the coalescence mechanism. At longer holding times, the liquid fraction increases and Ostwald ripening is active, as shown in Figs. 4(e) and (f). When the majority of solid grains gain uniform surface curvature through these two mechanisms, the growth of spherical grains will commence. Therefore, spheroidization and further growth of solid grains by the two competing mechanisms mentioned are expected in the SIMA process [51].

397

are in good agreement with the calculated theoretical values of psi = 90° and phi = 0°. As can be observed in Fig. 10(a)–(c), the maximum intensity of (110) pole figure is higher than those of the (100) and (111) pole figures. Thus, it is clear that the b110N║CD texture (α-fiber) has been created in samples that were strained by 55%. In contrast, after 20% deformation of samples, the maximum intensity of the (111) pole figure was higher than those of the (100) and (110) pole figures (Fig. 10(d)–(f)). This reveals that the b 111N║CD texture is the dominant texture in 20% compressed samples. These results are obviously consistent with those observed from the XRD patterns of 20% and 55% compressed samples in Fig. 9.

3.2. Texture evaluation of SIMA processed samples 3.2.1. Texture of deformed samples The crystallographic texture of an alloy depends on its chemical composition, microstructure, crystallography and processing conditions. Various textures appear in metals and alloys depending on deformation modes and deformation and recrystallization mechanisms [29, 43]. During cold rolling of aluminum alloys a typical fcc rolling texture develops [27], which is composed of two fibers: (i) the α-fiber (b100 N parallel to the normal direction, ND, which connects the {011}b 112 N “B” orientation with the {011}b011N “G” (Goss) orientation) occurring at low strains; and (ii) the β-fiber which aligns the main three texture components C (copper), S and B (brass). Uniaxial deformations such as tension, compression or extrusion lead to textures that are of fiber type [49]. During uniaxial compression and equiaxed forging, the b 110N direction is the stable orientation and the commonly preferred fiber texture component [26]. Shaha et al. [47] also showed that the compression deformation of as-cast, solution treated and T6 aged Al–Si–Cu–Mg alloys causes the development of {001}b110N and {111}b110N texture components. X-ray diffraction patterns of 20 and 55% compressed samples are shown in Fig. 9. It can be observed in Fig. 9(a) that the relative intensity of the 111 reflection of the Al matrix is much stronger than the other peaks for the 20% compressed sample. This indicates the presence of a strong texture in the matrix. However, considering the XRD pattern of the 55% compressed sample (Fig. 9(b)), the relative intensity of the 220 reflection of the Al matrix is much stronger than the other peaks, which obviously corresponds to a textured microstructure. A set of (111), (100) and (110) pole figures of 20 and 55% compressed samples are shown in Fig. 10. Considering the pole figures of compressed samples and the symmetry of the uniaxial compression process, it can be concluded that the texture of the strained samples is a fiber texture which can be represented as b hklN║CD. The obtained results showed that the psi and phi of the (110) pole figure for 55% compressed sample are 87° and 0°, respectively, which

3.2.2. Texture evolution following SIMA process Fig. 11 shows (110) pole figures of 55% strained samples after isothermal heating in the semi-solid range. Following isothermal heating of strained samples at 600 °C for 5 min, a weak deformation texture developed (Fig. 11(a)). This can be attributed to the recovery and initiation of the recrystallization process. Following heating time of up to 15 min and formation of equiaxed grains (Fig. 4(d)), the deformation texture was completely eliminated and replaced by a random texture as shown in Fig. 11(b). As expected, further increase of the heating temperature to 620 °C causes weaker random texture (Fig. 11(c)) compared to the sample that was heat treated at 600 °C (Fig. 11(b)). A weak primary basal deformation texture and strong random texture in semi-solid samples can be attributed to the particle stimulated nucleation (PSN) mechanism during the recrystallization of the alloy. Studies have indicated that alloys with coarse precipitates, typically above 1–2 μm, generate randomized texture due to PSN [17,20]. Similar conclusions were reported by Engler and Hirsch [22] and Benum and Nes [8] for Al–Mg–Si alloy sheet during cold rolling. They showed that particles with a size larger than 1 μm could provide stimulation at the deformation zones which interact with slip dislocations and weaken the texture intensity. This observation coincides with the present findings in this research. The X-ray diffraction patterns of 20% and 55% compressed samples in Fig. 9 show that secondary phase particles of MgZn2, Al2CuMg, Al7Cu2Fe and Al6(Cu,Fe) are produced in different samples. These are common precipitates in Al–Zn–Mg–Cu alloys which are also observed in the microstructure of wrought 7075 sample (Fig. 3), and were similar to those reported by other researchers [39,56,59]. Dimensions of these precipitates are commonly greater than 1 μm, and therefore during deformation a special zone develops in the vicinity of such non-deformable particles, which is characterized by a high local strain and high misorientation gradient. This makes them preferred nucleation sites for recrystallization during the subsequent heat treatment [45].

Fig. 9. XRD patterns of (a) 20% and (b) 55% compressed specimens.

398


Fig. 10. (a) {111}, (b) {100} and (c) {110} pole figures of 55% compressed sample and (d) {111}, (e) {100} and (f) {110} pole figures of 20% compressed sample.

3.3. The effect of pre-deformation on the semi-solid microstructure The microstructures of 55% compressed samples which were cut perpendicular to the applied pressure direction, as shown in Fig. 4(a), consisted of deformed α-Al grains. Microstructures of samples strained at various compression ratios and heat treated for 25 min at different temperatures are also shown in Fig. 8. Microstructures of 10% compressed samples after isothermal treatment consisted of coarse and

non-equiaxed solid grains, and were elongated in shape. Considering Fig. 8(d)–(l), the average grain size decreased and the sphericity of solid grains significantly increased with further increase of the compression ratio up to 55% at different temperatures. Fig. 12 shows variations of the average grain size and shape factor of samples with different compression ratios, heat treated at various holding temperatures and times. Concerning 10 to 20% compression strain, the average grain size decreased and the shape factor increased

Fig. 11. {110} pole figures of (a) 55%/600 °C/5 min, (b) 55%/600 °C/15 min and (c) 55%/620 °C/15 min samples.


399

Fig. 12. Variations in the average grain size and shape factor versus compression ratio for compressed samples heated at (a, b) 600 °C, (c, d) 610 °C and (e, f) 620 °C for various times.

significantly. With further extension of the compression ratio up to 55%, the grain size shows no significant change (Fig. 12(a), (c) and (e)). However, a slight rise in the grain size was observed in the case of 40% compressed samples. Moreover, the shape factor slightly reduced with a rise in the compression ratio to 40%, and then significantly increased with further extension of the compression ratio up to 55% (Fig. 12(b), (d) and (f)). Therefore, it can be concluded that samples compressed 10% and heat treated at different temperatures and times consist of coarse grains which show very low sphericity. Thus, such a microstructure cannot be considered an appropriate one for thixotropic

applications. The average grain sizes of 20% and 55% compressed samples show no significant differences. However, the shape factors of 55% compressed samples are significantly greater than those of 20% compressed ones. For instance, the shape factors of 20% compressed samples heated at 610 °C for 15, 25 and 35 min were 0.75, 0.79 and 0.81, respectively, and those for 40% compressed samples reduced to 0.74, 0.75 and 0.78, respectively. The shape factors for samples with 55% compression ratio increased to 0.78, 0.82 and 0.84, respectively. Therefore, the compression ratio of 55% can be considered an optimum cold work value for the SIMA process.

400


Results of research conducted by Vaneetveld et al. [53] showed that the semi-solid feedstock with lower liquid fraction has better formability in the semi-solid range and also significantly prevents the formation of some defects such as porosity and shrinkage during the forming process. The results obtained in the present research showed that the liquid fraction is relatively high at 620 °C (see Fig. 1). Thus, heating samples at such a temperature leads to undesirable coarsening of the solid grains. Consequently, the semi-solid microstructure obtained by isothermal heating at 620 °C is inappropriate for thixotropic applications. However, low liquid fraction was observed following heating at temperatures of 600 and 610 °C. The average grain size smaller than 80 μm and the shape factor greater than 0.75 were obtained for 55% compressed samples heated at 600 and 610 °C for 15 and 25 min. Therefore, the isothermal heating temperature range of 600 to 610 °C and holding time of 15– 25 min can be considered the optimum semi-solid treatment temperature range. Tensile or compressive plastic deformation promotes internal strain energy due to the increase of dislocation density and the formation of lattice defects such as vacancies. This provides the required driving force for recovery and recrystallization during the isothermal treatment [2,57]. With extension of the compression ratio, much strain energy is stored in the structure and the thermodynamic instability increases. The increase of the lattice strain energy according to Eq. (2) results in decrease of the critical size of steady recrystallization nuclei and a simultaneous rise in the number of nucleation site [60]. R ¼ 2γ

ΔE

ð2Þ

where R⁎ is the critical size of the recrystallized grains, γ is the surface interfacial energy and ΔE is the stored internal energy. The enhancement of the internal strain energy results in faster stress assisted atomic diffusion and this in turn leads to extensive grain sphericity. Besides this, more compression strain causes the material structure to be more fragmented and therefore increases the grain and subgrain boundaries. This in turn leads to the acceleration of the nucleation rate [60]. The unexpected variations observed in the grain size and shape factor for 40% compressed samples (Fig. 12) can be explained in two ways. I) The recrystallization mainly comprises the nucleation and growth of the recrystallized nuclei. These processes are responsible for the size variation of the recrystallized grains [36]. We may expect that during holding time in the sample with 40% compression ratio, very fine equiaxed grains are formed and then grain growth and agglomeration lead to grain coarsening. When the compression ratio increases to 40%, the accumulating rate of the strain energy descends by degrees in the cold work which possibly weakens the effect of recrystallization in the evolution of globular grains in the semi-solid microstructure [36]. II) The second possibility is that vacancies, lattice defects and dislocations generated by increasing compression ratio may be neutralized. For example, two dislocations with opposite Burgers vectors will neutralize each other [11,57]. Consequently, some of the stored energy is released and causes the grain size and the shape factor to be decreased. It is worth mentioning that similar explanations have been given by other researchers [11,36,57] for such variations in the grain size and sphericity of semi-solid microstructures. 3.4. Coarsening kinetics of strained 7075 samples Ostwald ripening is the dominant mechanism for the coarsening of solid particles. The coarsening kinetics can be expressed by the Lifshitz–Slyozov–Wagner (LSW) relationship [7,25,40]: Dn −Dn0 ¼ kt

ð3Þ

where D and D0 are the final and initial grain sizes, respectively, t is the isothermal holding time, k is the coarsening rate constant and n is the

Fig. 13. Variations of the cube of average grain size versus isothermal holding time for 55% compressed samples at heating temperatures of 600–620 °C where R2 is the regression coefficient.

power exponent. It has been found that n is 3 for volume diffusion controlled systems in the semi-solid state [57]. In the present research, the coarsening rate constant (k) was calculated by fitting a power relationship to the experimental results. Fig. 13 shows the cube of the average grain size variations versus isothermal holding time for 55% compressed samples heated at 600, 610 and 620 °C, where D0 is the average grain size when the holding time is 15 min. The regression coefficient of the fitted equations is close to 1. Thus, it can be concluded that the coarsening kinetics of solid particles during isothermal heating of deformed 7075 samples at the semi-solid temperature range have a good correlation with the LSW equation. Generally, it is expected that by extending the heating temperature the coarsening rate of solid particles will accelerate due to the development of a fast diffusion path as a result of an increase of the liquid fraction. Annavarapu and Doherty [3] showed that the coarsening rate accelerates with an increase of the solid fraction (fs) for fs higher than 0.6. They suggested the liquid film migration model for the coarsening process of the solid particles in the semi-solid state. However, the research conducted by Manson-Whitton et al. [40] showed the opposite results for higher solid fractions (fs ≳ 0.7) in the case of spry formed Al–4%Cu. Therefore, they proposed the modified liquid film migration model by considering the solid–solid contact effect during coarsening and by defining a transition solid fraction. They found that i) the liquid film migration model is valid for solid fractions lower than the transition value (f s ~ 0.7); ii) the coarsening rate constant increases with the increase of the solid fraction; and iii) for solid fractions higher than those of the transition value, k reduces with the increase of the solid fraction which conflicts with the liquid film migration model. The microstructural investigations carried out by Manson-Whitton et al. [40] showed that for solid fractions greater than 0.7, a large number of solid grains come into contact with each other instead of being separated by liquid film and solid–solid grain boundaries are formed. On the other hand, [33] and Boettinger et al. [10] observed that the coarsening process of Mo–Ni–Fe and Al–Sn alloys with high solid fractions mainly occurs by diffusion through the liquid phase instead of the connection of solid particles. Consequently, connection between solid particles leads to the decrease of the coarsening rate constant as a result of the decrease in the solid/liquid interfacial area. In the present study, the calculated coarsening rate constants for 10– 55% compressed 7075 alloys (Fig. 14) show that the k values are highly dependent on the isothermal holding temperature and compression ratio. An increase of the holding temperature (decrease of the solid fraction) for 55% compressed samples results in the increase of the coarsening rate constant. However, for samples with low compression ratio


Fig. 14. Coarsening rate constants of compressed samples versus isothermal holding temperature.

401

(10–40%), k reduced with increase of the holding temperature up to 610 °C (solid fraction of 0.7) and significantly increased with further increase of the holding temperature up to 620 °C (solid fraction of 0.55). The obtained results revealed that the coarsening process of solid particles in 10–40% compressed samples for solid fractions greater than 0.7 is similar to the model proposed by Manson-Whitton et al. [40]. However, an unexpected increase in k values is observed for solid fractions lower than 0.7 which is similar to the result observed by Bolouri et al. [12] for 30% compressed 7075 alloy. The transition solid fraction can be considered 0.8 based on the experimental finding of the present research. Considering Fig. 3, it seems that up to the temperature of 610 °C (decreasing the solid fraction to 0.7), the effect of the coalescence mechanism which is more effective at 600 °C is dismissed as a result of fewer solid–solid contacts. Therefore, the coarsening rate constant appreciably decreases. A different observation in samples with 55% compression ratio can be attributed to the high liquid fractions formed in the samples. Zhang et al. [60] showed that for AZ91D alloy extending the compression ratio brings about a higher liquid fraction. The grain boundary areas and therefore the melting routes were improved by compression

Fig. 15. (a) and (b) SEM backscattered electron images and (c) the EDS analysis of A (grain boundary precipitates) in (b) of 40%/600 °C/25 min sample.

402


straining due to the decrease of the grains' size. This finally results in more liquid fraction during the heating process. It can be concluded that 55% strained samples have greater liquid fraction compared to 10–40% samples. Thus, in 55% strained samples superior atomic diffusion and excess liquid fraction due to the increase of temperature to 610 °C compensate for the lack of an effective coalescence mechanism. This leads to an increase of the coarsening rate constant. High values of the coarsening rate constant are observed up to a temperature of 620 °C (solid fraction of 0.55). This can be attributed to the further increase of both atomic diffusion and liquid fraction and the effective Ostwald ripening mechanism. Another important cause of the severe increase of the coarsening rate is the decreasing retardation effect of the precipitate particles with the increase of the holding temperature. The presence of the square solid particles along the grain boundaries and the triple solid grain junctions was the key feature of the microstructures that were heated at low temperatures, as shown in Fig. 15(a). The EDS analysis results (Fig. 15(c)) showed that these precipitates have composition close to Al6(Cu,Fe) intermetallic phase. Al6(Cu,Fe) particles in the semi-solid microstructure of 7075 alloy precipitate at the grain boundaries at relatively low heating temperatures during the SIMA process (Fig. 15(a)). These particles bring about convolution of grain boundaries, as marked by arrows in Fig. 15(b), suggesting the pinning and retardation of the grain boundary liquid film migration during the coarsening of solid grains. The effect of the presence of Al9FeNi and AlFeMn intermetallic precipitation at the grain boundaries on the coarsening rate of AA2618 and AA2024 aluminum alloys has also been reported by Manson-Whitton et al. [40] and De Freitas et al. [19]. The results obtained in the present research indicate that Al6(Cu,Fe) precipitates mainly dissolve or become smaller than the thickness of the liquid film with the increase of the heating temperature up to 620 °C. Thus, the movement of grain boundaries can easily occur and lead to greater values of the coarsening rate constant. 4. Conclusions The effects of cold work and isothermal heating parameters on the microstructure and texture of 7075 Al alloy were investigated during the SIMA process. The following results can be drawn from the analysis, – Al–Si eutectic in addition to Al–Cu eutectic was observed at grain boundaries of the semi-solid microstructure. – Fiber textures of b 110N║CD and b 111N║CD were preferred textures in compressed samples which were transformed to a random texture following the isothermal heating. – Following straining to 10 to 20%, the average grain size becomes smaller and the shape factor rises significantly. The variation of the grain size and shape factor declined with further increase of the compression ratio up to 55%. The 40% compressed samples show an appreciable rise of the grain size and decrease of the shape factor. – Microstructural investigation results showed that the compression ratio of 55% and the isothermal heating temperature range of 600 to 610 °C for 15–25 min can be considered an optimum condition for the SIMA process. – The transition solid fraction according to the modified liquid film migration model was determined as 0.8. The presence of Al6(Cu,Fe) dispersoids at grain boundaries' liquid films pins the migrating liquid films and results in retardation of the coarsening rate. Dispersoids smaller than the thickness of the liquid film increase with isothermal heating temperature and become ineffective in the retardation phenomenon.

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