Microstructure and texture characterization of 7075 Al alloy during the SIMA process

    Microstructure and Texture Characterization of 7075 Al Alloy during the SIMA Process B. Binesh, M. Aghaie-Khafri PII: DOI: Reference:

S1044-5803(15)00219-3 doi: 10.1016/j.matchar.2015.06.013 MTL 7933

To appear in:

Materials Characterization

Received date: Revised date: Accepted date:

2 March 2015 15 June 2015 17 June 2015

Please cite this article as: Binesh B, Aghaie-Khafri M, Microstructure and Texture Characterization of 7075 Al Alloy during the SIMA Process, Materials Characterization (2015), doi: 10.1016/j.matchar.2015.06.013

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ACCEPTED MANUSCRIPT Microstructure and Texture Characterization of 7075 Al Alloy during the

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SIMA Process

Faculty of Materials Science and Engineering, K.N. Toosi University of Technology, Postal

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B. Binesha, M. Aghaie-Khafria,1

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Code: 1999143344, Tehran, Iran

Abstract

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The effect of SIMA process parameters on the crystallographic texture and microstructure of

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7075 aluminum alloy was investigated. Specimens of 7075 were subjected to 10-55 % uniaxial

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compression strain at ambient temperature and then semi-solid treatment was carried out at the range of 600 to 620 °C for 5-35 min. The fiber textures of <110>║CD and <111>║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.

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Corresponding author. Tel: +98 09123088389; Fax: +98 21 886 74748. E-mail address: [email protected] (M. Aghaie-Khafri).

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ACCEPTED MANUSCRIPT 1. Introduction In recent years, semi-solid state forming has been widely studied and accepted as an effective

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process which is capable of forming metals into a near net shape. It combines advantages of

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conventional casting and forging processes (Flemings, 1991; Wang et al., 1997; Chen et al., 2002; Lin et al., 2007). Semi-solid metal processing can be divided into two main groups:

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thixoforming and rheoforming (Lin et al., 2007; Jufa et al., 2010; Koeune, 2011). Thixoforming

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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

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state before semi-solid forming in the rheoforming process (Koeune, 2011; Kleiner et al., 2004). The key factor in the semi-solid forming processes is the presence of a thixotropic microstructure

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including equiaxed and spherical grains before the final forming operation (Lin et al., 2007; Tzimas and Zavaliangos, 2000a). The ideal microstructure should have an appropriate volume

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fraction of globular solid grains that are distributed uniformly within the liquid matrix (Kleiner et al., 2004; Fan, 2002). It was found that fine and spherical solid grains lead to better thixotropic properties (Tzimas and Zavaliangos, 2000b). Nowadays, various processes have been developed to achieve thixotropic microstructure in metallic alloys (Hirt and Kopp, 2009). These processes are generally divided into two main categories (Koeune, 2011; Hirt and Kopp, 2009). 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 powder metallurgy process, the thermomechanical strain induced melt activation (SIMA) process and the thixomolding process (Koeune, 2011).

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ACCEPTED MANUSCRIPT 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

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engineering alloys (Lin et al., 2007; Wang et al., 2008). The SIMA process was first introduced

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by Young et al. (1983) in the 1980s. This process involves cold or warm working and then heating and remelting in the semi-solid range (Jiang and Li, 2005; Yan et al., 2012). If the stored

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strain energy is sufficient, recrystallization occurs in the microstructure of the alloy and leads to

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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

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SIMA process (Yan et al., 2012). The effects of coupling equal-channel angular pressing (ECAP) and heating at semi-solid temperature on the microstructure of an aluminum alloy were

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investigated by Aghaie-Khafri (Aghaie-Khafri and Azimi-Yancheshme, 2012). The conventional forming process of the wrought aluminum alloys is generally accompanied

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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 (Liu et al., 2003; Birol, 2008; Chayong et al., 2005; Chayong et al., 2004; Vaneetveld et al., 2010). A few studies (Bolouri et al., 2011; Mohammadi et al., 2011) 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 (Engler, 1999; Maurice and Driver, 1997; Daaland and Nes, 1996; Sidor et al., 2011; Hirsch and Al-Samman, 2013) 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.

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ACCEPTED MANUSCRIPT 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

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samples during isothermal treatment at various temperatures and the effect of pre-deformation on

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the coarsening rate of solid particles were also investigated.

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2. Experimental procedure

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2.1. Materials and thermal analysis

A commercial extruded round bar of 7075-T6 aluminum alloy was used as the starting

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material, the chemical composition of which is shown in Table 1. Metler Star SW 10 deferential scanning calorimeter (DSC) apparatus was used to determine

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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

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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 height of 35 mm were machined from the starting material, stress relieved at 460 °C for 1 hr, and then air cooled. Compression of cylindrical samples with compression ratios of 10, 20, 40 and 55 % was carried out at room

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ACCEPTED MANUSCRIPT 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 height of 15 mm to ensure uniform

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strain in the core section. Isothermal treatments of samples were carried out at three different

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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

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to by compression ratio %/temperature °C/time min.

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quenched to study the microstructure. In the present study, the various SIMA cycles are referred

2.3. Microstructure and texture characterization

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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

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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

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investigations were carried out using a Neophot 32 optical microscope and Vega©Tescan and Mira3Tescan scanning electron microscope equipped with 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 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) (Seo and Kang, 2005):

 F

N

4 A / P 2

N 1

(1)

N

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ACCEPTED MANUSCRIPT 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-

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400 solid particles per sample. Texture evolution of α-Al particles during isothermal heating in the semi-solid range was

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investigated by carrying out an X-ray diffraction (XRD) experiment using Philips X’pert model

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apparatus with Cu (kα) target and wave length of 0.15406 nm. Axially sectioned samples were

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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

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ND (normal direction) was perpendicular to the testing plane. Defocusing and background errors were omitted using powder sample. In order to obtain more accurate results, the crystalline

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texture component was determined using orientation distribution function (ODF). Finally, {111},

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{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 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 6

ACCEPTED MANUSCRIPT alloys (Atkinson et al., 2008). The precipitates have dimensions of a few tens of nanometers which can only be revealed by TEM technique.

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Microstructures of 55 % strained samples following isothermal treatment are shown in Fig. 4.

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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

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of spherical and equiaxed grains after heating and partial remelting at 600 °C. Concerning the

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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

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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

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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

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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 (Kleiner et al., 2004). Figs. 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 Figs. 6(a) and (b) at higher magnifications, respectively. The EDS results of the marked areas on the SEM image are summarized in Table 2. 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 (Bolouri et al., 2012; Jiang and Li, 2005), the latter is an unreported phenomenon in the semi-

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ACCEPTED MANUSCRIPT 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

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these regions and the decrease of the temperature at grain boundaries. Finally, the chemical

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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

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point eutectic phases were fused during soaking and penetrated between the grains. Thus, a

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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)).

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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-

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Cu and Al-Si phase diagrams (ASM Handbook, 1992), the melting point of Al-Cu eutectic compound is lower than Al-Si eutectic (548 °C compared with 577 °C). Consequently, Al-Cu

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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 (Zhao, 2007) 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 has a great influence on microstructural evolution during semi-solid treatment. Moreover, this affects the mechanical properties of the subsequent thixoformed parts (Annavarapu et al., 1995; Liu et al., 2004; Cavaliere et al., 2004). Fig. 7 shows the EDS mapping of the main alloying elements of a 40 %/600 °C/25 min sample.

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ACCEPTED MANUSCRIPT 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

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grain boundaries (Fig. 7(e)). However, there was no appreciable change in Zn content at various

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points of the microstructure (Fig. 7(f)). This is in contrast to the results of research conducted by Bolouri et al. (2012) that reported the segregation of Zn at grain boundaries of a semi-solid 7075

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alloy. Moreover, as can be observed in Fig. 7(g), grain boundary precipitates are rich in Fe.

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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

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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).

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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. (2002). In this study, using the obtained solid fraction

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versus temperature curve for 7075 alloy (Fig. 1), the isothermal heating temperatures were selected based on Atkinson et al.’s (2002) 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 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 semisolid forming.

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ACCEPTED MANUSCRIPT 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

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temperature brings about greater size and sphericity of solid grains. Following coarsening of the

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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

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is attributed not only to the increase of the liquid fraction but also to the coarsening of α-Al solid

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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.

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It is worth noting that the number of entrapped liquid droplets within the grains decreases and the droplets become larger by increasing heating temperature and time (Fig. 4 and 6). Considering

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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

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heating process, some of the fine liquid droplets join to 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 (Bolouri et al., 2011).

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ACCEPTED MANUSCRIPT 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

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isothermal heating time (Fig. 4(c)-(f)). These particles grow and coarsen during further heating.

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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

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microstructure, the diffusion of the solid material from regions of high curvature to low

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curvature points provides the required driving force for spheroidization of solid particles (Yan et al., 2012).

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As long as the grains are not spherical, the coarsening process leads to change of particles’ shape while numbers per unit volume remain constant. Following this stage, smaller particles

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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 that have been subjected to partial

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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 (Tzimas and Zavaliangos, 2000a). 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 (Takajo et al., 1984; Kaysser et al., 1984). Since the number of neighboring grains decreases with 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

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ACCEPTED MANUSCRIPT dominant over short times after the liquid is formed, i.e., at low volume fractions of the liquid phase (Tzimas and Zavaliangos, 2000a; Takajo et al., 1984; Kaysser et al., 1984).

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In the present study, small liquid volume fractions are observed in the early stages of the

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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

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verified by considering the microscopic images that are shown in Figs. 4(d)-(f). It can be

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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

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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

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grains mainly occurs 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

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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 (Tzimas and Zavaliangos, 2000a).

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 (Jayaganthan et al., 2010; Pedersen et al., 2008). During cold rolling of aluminum alloys a typical fcc rolling texture develops (Hirsch and Al-Samman, 2013), which is composed of two 12

ACCEPTED MANUSCRIPT fibers: (i) the α-fiber (<100> parallel to the normal direction, ND, which connects the {011} <112> “B” orientation with the {011} <011> “G” (Goss) orientation) occurring at low strains;

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and (ii) the β-fiber which aligns the main three texture components C (copper), S and B (brass).

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Uniaxial deformations such as tension, compression or extrusion lead to textures that are of fiber type (Suwas and Ray, 2014). During uniaxial compression and equiaxed forging, the <110>

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direction is the stable orientation and the commonly preferred fiber texture component (Hirsch,

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2005). Shaha et al. (2014) also showed that the compression deformation of as-cast, solution treated and T6 aged Al-Si-Cu-Mg alloys causes the development of {001}<110> and

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{111}<110> texture components.

X-ray diffraction patterns of 20 and 55 % compressed samples are shown in Fig. 9. It can be

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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

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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 ║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 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 the (100) and (111) pole figures. Thus, it is clear that <110>║CD

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ACCEPTED MANUSCRIPT 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 the

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(100) and (110) pole figures (Fig. 10(d)-(f)). This reveals that the <111>║CD texture is the

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dominant texture in 20 % compressed samples. These results are obviously consistent with those

3.2.2. Texture evolution following SIMA process

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observed from the XRD patterns of 20 % and 55 % compressed samples in Fig. 9.

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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

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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

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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

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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 (Deng et al., 2010; Cheng and Morris, 2002). Similar conclusions were reported by Engler and Hirsch (2002) and Benum and Nes (1997) 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

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ACCEPTED MANUSCRIPT 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

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also observed in the microstructure of wrought 7075 sample (Fig. 3), and were similar to those reported by other researchers (Mahathaninwong et al., 2012; Xinyu et al., 2011; Yu and Li,

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2011). Dimensions of these precipitates are commonly greater than 1 μm, and therefore during

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deformation a special zone develops in the vicinity of such non-deformable particles, which is

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characterized by a high local strain and high misorientation gradient. This makes them preferred nucleation sites for recrystallization during the subsequent heat treatment (Schafer et al., 2009).

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3.3. The effect of pre-deformation on the semi-solid microstructure

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The microstructures of 55 % compressed samples which were cut perpendicular to the applied

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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

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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 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

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ACCEPTED MANUSCRIPT 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

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samples compressed 10 % and heat treated at different temperatures and times consist of coarse

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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 %

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compressed samples show no significant differences. However, the shape factors of 55 %

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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,

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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,

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0.82 and 0.84, respectively. Therefore, the compression ratio of 55 % can be considered an optimum cold work value for the SIMA process.

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Results of research conducted by Vaneetveld et al. (2010) 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

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ACCEPTED MANUSCRIPT 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.

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Tensile or compressive plastic deformation promotes internal strain energy due to the increase

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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 (Yan et

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al., 2012; Aghaie-Khafri and Mahmudi, 2005). With extension of the compression ratio, much

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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

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recrystallization nuclei and a simultaneous rise in the number of nucleation site (Zhang et al.,

E

(2)

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R *  2γ

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2007).

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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 (Zhang et al., 2007). 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 (Lin et al., 2007). 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 17

ACCEPTED MANUSCRIPT 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-

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solid microstructure (Lin et al., 2007). II) The second possibility is that vacancies, lattice defects

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and dislocations generated by increasing compression ratio may be neutralized. For example, two dislocations with opposite Burgers vectors will neutralize each other (Yan et al., 2012; Bolouri et

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al., 2011). Consequently, some of the stored energy is released and causes the grain size and the

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shape factor to be decreased. It is worth mentioning that similar explanations have been given by other researchers (Lin et al., 2007; Yan et al., 2012; Bolouri et al., 2011) for such variations in

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the grain size and sphericity of semi-solid microstructures. 3.4. Coarsening kinetics of strained 7075 samples

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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

Dn  D0n  kt

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(Manson-Whitton et al., 2002; Hardy and Voorhees, 1988; Atkinson and Liu, 2008): (3)

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 power exponent. It has been found that n is 3 for volume diffusion controlled systems in the semi-solid state (Yan et al., 2012). 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

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Generally, it is expected that by extending the heating temperature the coarsening rate of solid

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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 (1995) showed that the coarsening rate

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accelerates with increase of the solid fraction (fs) for fs higher than 0.6. They suggested the liquid

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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. (2002) showed the opposite results

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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

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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

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constant increases with increase of the solid fraction; and iii) for solid fractions higher than the transition value, k reduces with increase of the solid fraction which conflicts with the liquid film migration model. The microstructural investigations carried out by Manson-Whitton et al. (2002) 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, Kim et al. (1983) and Boettinger et al. (1987) 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.

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ACCEPTED MANUSCRIPT 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

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temperature and compression ratio. Increase of the holding temperature (decrease of the solid

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fraction) for 55 % compressed samples results in the increase of the coarsening rate constant. However, for samples with low compression ratio (10-40 %), k reduced with increase of the

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holding temperature up to 610 °C (solid fraction of 0.7) and significantly increased with further

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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

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fractions greater than 0.7 is similar to the model proposed by Manson-Whitton et al. (2002). However, an unexpected increase in k values is observed for solid fractions lower than 0.7 which

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is similar to the result observed by Bolouri et al. (2012) for 30 % compressed 7075 alloy. The transition solid fraction can be considered 0.8 based on the experimental finding of the present

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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. (2007) 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 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

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ACCEPTED MANUSCRIPT 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.

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High values of the coarsening rate constant are observed up to the temperature of 620 °C

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(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

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severe increase of the coarsening rate is the decreasing retardation effect of the precipitate

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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

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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)

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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

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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. (2002) and Freitas et al. (2004). 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 increase of the heating temperature up to 620 °C. Thus, the movement of grain boundaries can easily occur and leads to greater values of the coarsening rate constant.

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Conclusions

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The effects of cold work and isothermal heating parameters on the microstructure and texture

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of 7075 Al alloy were investigated during the SIMA process. The following results can be drawn from the analysis.

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- Al-Si eutectic in addition to Al-Cu eutectic was observed at grain boundaries of the semi-solid

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microstructure.

- Fiber textures of <110>║CD and <111>║CD were preferred textures in compressed samples

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which were transformed to a random texture following the isothermal heating.

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- 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

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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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ACCEPTED MANUSCRIPT Zhang, Q.Q., Cao, Z.Y., Zhang, Y.F., Su, G.H., Liu, Y.B., 2007. Effect of compression ratio on the microstructure evolution of semisolid AZ91D alloy. J. Mater. Process. Technol. 184, 195-

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Zhao, J., 2007. Methods for phase diagram determination, first ed. Elsevier, pp. 194-201.

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ACCEPTED MANUSCRIPT Table Captions

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

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

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Figure Captions

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Fig. 1. DSC and solid volume fraction versus temperature curves for 7075 Al alloy. Fig. 2. Schematics showing sample orientation for compression and sample extraction and

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orientation for texture measurements.

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

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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.

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

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

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55 %/(j) 600 °C, (k) 610 °C, (l) 620 °C/25 min.

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

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

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(e) {100}, (f) {110} pole figures of 20 % compressed sample.

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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.

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Fig. 12. Variations in the average grain size and shape factor versus compression ratio for

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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.

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

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Mn

Fe

Bal.

0.28

0.28

Cr

Cu

Mg

Zn

Si

0.13

1.58

2.41

5.31

0.14

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

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Grain boundary (point C) wt. % at.% 74.01 77.61 2.31 2.69 16.42 16.54 0.00 0.00 1.31 0.58 5.87 2.54 0.09 0.04 0.00 0.00

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Grain boundary (point B) wt. % at.% 42.34 62.18 1.71 2.79 0.24 0.34 0.00 0.00 52.32 32.63 3.27 1.98 0.12 0.08 0.00 0.00

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Al Mg Si Cr Cu Zn Fe Mn

Liquid droplet (point A) wt. % at.% 79.41 89.43 0.73 0.92 0.17 0.18 0.02 0.01 16.26 7.85 3.34 1.57 0.04 0.02 0.03 0.02

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Element

Grain boundary precipitates (point D) wt. % at.% 71.92 84.03 0.46 0.59 0.32 0.36 0.36 0.22 4.00 1.98 2.04 0.98 17.59 9.93 3.32 1.91

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ACCEPTED MANUSCRIPT Highlights Texture and microstructure of SIMA processed 7075 alloy was investigated. Coarsening kinetics of solid particles was investigated.

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Pinning of grain boundary liquid films were discussed.

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Effects of cold work and isothermal heating parameters on texture were studied.

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