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Deep drawing of ZK60 magnesium sheets fabricated using ingot and twin-roll casting methods
Materials and Design 110 (2016) 214–224
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Deep drawing of ZK60 magnesium sheets fabricated using ingot and twin-roll casting methods Jae-Hyung Cho a,b,⁎, Sang Su Jeong c, Suk-Bong Kang a a b c
Korea Institute of Materials Science, 797 Changwondaero, Seongsan-gu, Changwon, Gyeongnam 642-831, South Korea University of Science and Technology, 217 Gajeong, Yuseong, Daejeon 305-350, South Korea KHVatec, 293 Gondan-dong, Gumi-si, Gyeongbuk, South Korea
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Evolution of microstructure and texture of ZK60 (Mg-5.5Zn-0.5Zr) was examined during deep drawing. • Two different ZK60 alloys of ingot casted (IC) and twin-roll casted (TRC) were compared. • Major deformation modes between the IC and TRC sheets were different during drawing process. • Tensile twinning and shear bands were frequently observed in IC cups. • Peak broadening along the TD and a peak split along the RD were more observed in TRC cups.
a r t i c l e
i n f o
Article history: Received 18 May 2016 Received in revised form 13 July 2016 Accepted 28 July 2016 Available online 1 August 2016 Keywords: ZK60 magnesium alloys Deep drawing Microstructure and texture Twinning EBSD TEM
a b s t r a c t Deep drawing was carried out on two different ZK60 (Mg-Zn-Zr) magnesium sheets fabricated via ingot and twin-roll casting methods. The effects of the microstructure and texture on the deep drawability and the associated microstructural evolution of ZK60 alloys were examined. Twin-roll casted sheets with refined particles were stronger and showed greater elongation than ingot-casted sheets. Moreover, they were fully deep-drawn at lower temperatures than ingot-casted sheets, and the deep drawability of the ingot-casted sheets was better at a high deformation rate compared to that of the twin-roll casted sheets. During the deep drawing process, the ingot-casted sheets with large grains experienced more tensile twinning than the twin-roll casted sheets, and shear bands were observed. The twin-roll casted sheets revealed peak broadening of the basal texture along the TD and peak splitting along the RD. Thickness variations of the drawn cups showed that the twin-roll casted sheets resisted wall thinning more effectively than the ingot-casted sheets. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction
⁎ Corresponding author at: Korea Institute of Materials Science, 797 Changwondaero, Seongsan-gu, Changwon, Gyeongnam 642-831, South Korea. E-mail address: [email protected] (J.-H. Cho).
http://dx.doi.org/10.1016/j.matdes.2016.07.131 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
Industrial applications of wrought magnesium alloys have recently drawn increased amounts of attention due to the low density and high specific strength levels of these alloys. Magnesium alloys, however, inherently have poor formability given their limited slip activation at room temperature. The critical resolved shear stresses (CRSSs) of
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magnesium slip systems strongly depend on the temperature [1–3]. The CRSS of the basal 〈a〉 slip is significantly lower than that of other nonbasal slip systems at room temperature. At elevated temperatures, non-basal slip systems of magnesium alloys can be more easily activated. This results in better formability of magnesium alloys; thus, the forming of magnesium alloys is usually carried out at elevated temperatures. In addition to the slip, twinning frequently enhances the formability of magnesium alloys [4]. The deep drawing process has been used for various industrial applications, such as automobile parts, electrical appliances, and cellular phones. To carry out the deep drawing of magnesium alloys successfully, careful attention should be paid to processing parameters (e.g., temperatures of the dies, the blank, and the blank holder; the punch speed; the blank holding force; and the lubrication amounts and methods used) and certain metallurgical aspects (texture and microstructure). The forming temperature is one of the most significant parameters affecting the deep drawability of magnesium alloys. During the rolling of magnesium alloys, strong basal textures usually develop, resulting in poor workability. The crystallographic texture is associated with the planar anisotropy of sheet metals, and it is important to consider it for successful deep drawing [5–9]. To modify the typical texturing, certain elements such as Mm (misch metal), can be added to magnesium alloys. Soluble Zn and RE additions (Y) weaken the rolling texture of magnesium alloys [10]. The addition of Ce or Li to magnesium alloys results in weak basal textures [11–13]. Thermo-mechanical processing can alter the texture and microstructure of magnesium alloys. Asymmetric or differential speed rolling process caused weak basal textures [14]. Twin-roll casting (TRC) followed by an asymmetric rolling process can more effectively modify the microstructure and texture of sheets as compared to conventional ingot casting (IC) with a subsequent symmetric rolling process [15,16]. AZ91 sheets manufactured by roll strip casting and a subsequent hot rolling process have been successful for warm deep drawing [17]. Enhanced mechanical properties of TRC sheets have also been reported in many magnesium alloys as compared to those of conventional IC sheets [18,19]. In this study, the deep drawing of two different ZK60 sheets fabricated by TRC and IC processes was carried out at elevated temperatures. Under various working temperatures and punch speeds, the deep drawability of ZK60 sheets was examined. Using HR-SEM/EBSD (highresolution scanning electron microscopy/electron backscatter diffraction) and TEM (transmission electron microscopy), the evolution of the microstructure and texture and the distribution of the second phases were also investigated to gain a better understanding of the mechanical responses during the deep drawing of ZK60 magnesium alloys. 2. Experimental 2.1. Materials and heat treatments Two different ZK60 magnesium sheets were separately fabricated by TRC and IC processes. The overall chemical compositions of the ZK60 alloys used here are given in Table 1. The rolling speed of the strip caster was approximately 3 rpm (revolution per minute). The manufactured TRC strips were approximately 4.0 mm thick, 180 mm wide and more than 10 m long. For the IC samples, molten alloys at 993 K (720 °C) were poured into a steel mold 180 mm × 160 mm × 25 mm in size. The IC slabs were warm-rolled down to a thickness of 4 mm. The homogenization heat treatment for both samples (TRC and IC) consisted of two steps: holding at a temperature of 623 K (350 °C) for 10 h and Table 1 Overall chemical composition of ZK60 (Mg-Zn-Zr) magnesium alloys [wt.%].
ZK60
Al
Zn
Mn
Si
Fe
Cu
Ni
Ca
Zr
Mg
0.01
5.47
0.009
0.022
0.003
0.003
0.007
0.005
0.58
Bal.
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then holding at 723 K (450 °C) for 5 h to avoid cracking due to the presence of Zn and Zr intermetallic compounds during the rolling process. Detailed information about the TRC system and the working process is available in the literature [20]. To fabricate the blank sheets used in the deep drawing process, seven or eight passes of warm rolling from 4 mm down to 0.6 mm were carried out on the ZK60 TRC and IC sheets. The roller used, with a diameter of 200 mm, was preheated to 523 K (250 °C) and operated at a speed of 3.5 rpm. The ZK60 sheets were reheated at 573 K (300 ° C) for 30 min; the intermediate annealing time and temperature between the rolling passes were 5 min and 573 K, respectively. Final annealing of the ZK60 sheets with a thickness of about 0.6 mm was carried out at 648 K (375 °C) for 1000 s. 2.2. Deep drawing The deep drawing processes were carried out using a universal sheet testing machine (USTM). The USTM consisted of a die, a punch, and a blank holder. The punch diameter was 37 mm and the inner diameter of the die was 39 mm. The clearance between the punch and the die was 1 mm. The diameter of the blank was 74 mm, and a drawing ratio (DR) of 2.0 was applied. The blank holder and die were heated to specific temperatures using embedded electrical heating elements. The blank was laid between the die and the blank holder and was held for about 5 min to stabilize the temperature distribution of the blank sheets. The blank holding force (BHF) during the deep drawing process was varied from 5 to 20 kN depending on the working conditions. At the same temperature, the same BHF was applied, regardless of the punch speed. In order to ensure successful deep drawing, the BHF was increased with a decrease in the temperature. A low BHF of 5 kN was applied at an elevated temperature of 623 K, and a high BHF of 20 kN was applied at a low temperature of 423 K. Lubricant was used to reduce the friction during the deep drawing processes. The deep drawing conditions of the ZK60 sheets are summarized in Table 2. 2.3. Microstructural measurements and mechanical testing Analyses of the microtexture and microstructure were carried out using an automated high-resolution EBSD (HKL Channel5) attached to an HR-SEM device (JEOL7001F). Triclinic sample symmetry was applied to create pole figures. The step size during the EBSD mapping was 0.5 μm for the IC samples and 0.25–0.28 μm for the TRC samples. The EBSD samples were mechanically polished and then electropolished using a solution of butyl cellosolve (50 ml), ethanol (10 ml), and perchloric acid (5 ml) at a voltage of 10 V and a temperature of 258 K (− 15 °C) to 253 K (− 20 °C). Intermetallic compounds or particles were examined using a JEM-2100F TEM operating at 200 kV. The TEM samples were prepared by mechanical polishing down to approximately 100 μm followed by electropolishing using methanol (60 ml), glycerin (30 ml), and nitric acid (10 ml). Electropolishing was carried out at temperatures and voltages of 263 K (−10 °C) to 258 K (−15 °C) and 20 V to 25 V, respectively. The TEM and SEM sample preparation steps were finalized by ion milling when necessary to prevent oxidation and other contamination of surfaces. Energy dispersive spectroscopy (EDS) was also carried out on the particles. Table 2 Deep drawing conditions of the ZK60 magnesium alloys. Punch diameter [mm] Inner diameter of the die [mm] Specimen thickness [mm] Drawing speed [mm/min] Temperature [K] Blank holding force [kN] Blank diameter [mm]
37 39 0.6 30, 40, 50 398–623 5–20
74
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Uniaxial tension tests (ASTM E8 standard) of the ZK60 sheets were carried out along the rolling direction (RD) using a standard universal testing machine (Instron 4206). The gauge length and width of the samples were 12.5 mm and 3 mm. The strain rates of the ZK60 alloys were 0.0025/s at room temperature. Three samples were tested and their mechanical properties were averaged. 3. Results and discussion 3.1. Microstructure and mechanical properties of the initial ZK60 sheets Various microstructural features, including the particles, crystallographic orientations, and grain structure, can affect the deep drawability of ZK60 sheets. In order to examine distribution of the particles of ZK60, a TEM analysis was carried out on the TRC and IC sheets, as shown in Fig. 1. The IC sheet contains bulk particles, and the TRC sheet possesses refined and homogeneously distributed particles. Refined particles found in TRC sheets usually improve the mechanical properties related to the strength and elongation of the sheets. The EDS results of the particles are shown in Fig. 1(c) and (d). The bulk particles in the IC sheet consist of Zn and Zr, which formed during the solidification of the melt. The fine particles found in the TRC sheet also mainly contain Zn and Zr. Fig. 2 presents inverse pole figure (IPF) maps and recalculated pole figures (PFs) as measured from the initial ZK60 sheets. Grain identification (GID) angles greater than 15° are specified with thick lines,
representing high angle grain boundaries. GID angles between 5° and below 15° associated with low angle grain boundaries or subgrain boundaries are represented with thin lines. GID angles below 5° were ignored as grain boundaries. The microstructure of the initial state of the IC sheet is given in Fig. 2(a). The average grain size is approximately 7.1 μm. The recalculated pole figures (PFs) of ð0001Þ; ð1120Þ; andð1010Þ are shown in Fig. 2(c). A strong basal peak which mainly formed during the warmrolling step was noted along the normal direction (ND) of the sheet (or at the center of the (0001) PF). The maximum peak is tilted from the ND of the sheet toward the bottom left at about 10°. Tilting of the basal peaks is frequently observed in magnesium sheets [21]. Certain aspects of inhomogeneous deformation, such as a shear band, are among the reasons why the basal peaks deviate from the ND. In addition, the angular distribution of the basal peak in the (0001) PF is slightly broader toward the transverse direction (TD) than toward the rolling direction (RD). The shape and intensity of the basal peaks in magnesium alloys are associated with many factors, such as the chemical compositions, dominant deformation modes, and the thermo-mechanical processing characteristics. Basal 〈a〉 and prismatic 〈a〉 slips in addition to tensile twinning contribute to the basal intensity with a broad TD distribution [4]. Both of the prismatic planes of ð1120Þ, and ð1010Þ lying on the sheet plane possess only slight distinctively preferred orientations along the perimeter of the PFs. The maximum peak intensity of the initial IC sheet is approximately 10.0 MUD (multiples of a uniform distribution). In the pole figures, the half-width value controls how much the pole is spread out over the
Fig. 1. Various particles and their EDS results of ZK60 sheets. TEM micrographs of the particles: (a) IC and (b) TRC. EDS results of particles consisting of Zn and Zr: (c) IC and (d) TRC.
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Fig. 2. Microstructure and texture of the initial ZK60 sheets: (a) inverse pole figure (IPF) maps and (c) recalculated pole figures (PFs) for the IC sheet, (b) IPF maps and (d) recalculated PFs for the TRC sheet.
surface of the projected sphere, and clustering speeds up the calculations. Detailed information about creating pole figures from EBSD data can be found in the HKL Channel 5 manual [22]. The microstructure of the initial ZK60 sheets fabricated by TRC also possesses both equiaxed and elongated grains, as shown in Fig. 2(b). Subgrain boundaries are mainly found in the elongated grains. The average grain size of the TRC ZK60 sheet is approximately 4.3 μm. The recalculated PFs of the TRC sheet are shown in Fig. 2(d). The (0001) PF shows that the basal intensity of the TRC sheet spreads intensely from the ND to the TD. The angular distribution of the basal peak is much broader toward the TD than toward the RD, compared to that of the IC sheet. It was also noted that there is a peak split along the RD in the center of the (0001) PF. The prismatic planes of ð1120Þ, and ð1010Þ show preferred orientations along the perimeter of PFs. The maximum basal intensity of the TRC sheet is about 7.0 MUD. Overall, the TRC sheet possesses smaller grains and a lower basal intensity level than the IC sheet. The angular distribution of the basal planes (0001) and the perimetric distribution of the prismatic planes ð1120Þ and ð1010Þ of the TRC and IC sheets also differed from each other.
The mechanical properties of two different ZK60 sheets (TRC and IC) were examined in uniaxial tension tests. Fig. 3 shows the tensile flow curves along the RD of the ZK60 sheets. The tensile strength (TS), yield strength (YS) and elongation (El) of the TRC sheets are approximately 330 MPa, 277 MPa and 18%, respectively. The corresponding values for the IC sheets are 320 MPa, 235 MPa, and 14 %, indicating the superiority of TRC sheets in terms of the mechanical properties as compared to the IC sheets. The microstructural features of small grains and refined particles obtained during the TRC process with a high solidification rate are usually coincident with improved mechanical properties. 3.2. Deep drawing at different temperatures and punch speeds Under various working temperatures ranging from 398 K (125 °C) to 623 K (350 °C) and punch speeds ranging from 30 to 50 mm/min, the deep drawing of the ZK60 magnesium sheets was carried out. The overall working window, or the deep drawability, is represented in Fig. 4. Fig. 4(a) shows symbols representing the failure (×), or success (○) of the deep drawn cups. Drawn cups with some wrinkles in the top region
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Fig. 3. Tensile stress-strain curves of the ZK60 sheets.
are specified with a triangle, △. The wrinkles reflect the instability of the metal flow, which is caused by compressive strain along the circumferential or hoop direction. At working temperatures between 448 K (175 °C) and 623 K and punch speeds between 30 mm/min and 40 mm/min, deep drawing was successful for both the TRC and the IC sheets. For the TRC sheets, minor wrinkles were found at temperatures of 448 K and 473 K (200 °C) at a punch speed of 50 mm/min. To avoid these types of wrinkles, greater BHF should be applied, after which successful drawing becomes much more likely. At 423 K (150 °C), the increased BHF resulted in successful deep drawing at all punch speeds. For the IC sheets, wrinkles near the top of the cup appeared at a working temperature of 423 K regardless of the punch speed. The different wrinkling behaviors between IC and TRC sheets observed at 423 K reflect the different levels of plastic instability. Twin-roll casted sheets were fully deep-drawn at lower temperatures than ingot-casted sheets, and the drawability of the ingot-casted sheets was better at a high deformation rate than that of the twin-roll casted sheets. The deep drawability of ZK60 magnesium alloys is closely associated with the working parameters of the temperature and the deformation rate. When considering plastic deformation and various deformation modes of slip and twinning, the temperature-dependency of the associated CRSS values should also be considered. It is known that basal 〈a〉 and {1012} tensile twinning are fairly immune to the temperature. Prismatic〈a〉, pyramidal〈c + a〉, and {1011} compressive twins, however, are more temperature-dependent. At elevated temperatures, the CRSS values of the non-basal slip systems and compressive twins decrease, and these systems can easily be activated. In addition, the deformation rate plays an important role in activating twinning. With an increase in the deformation rate, the flow stress increases. Twinning activation usually increases with an increase in the flow stress [23]. The microstructural features of the grain size and orientation also strongly affect the deep drawability characteristics. Microstructural evolution during deep drawing and its effect on the deep drawability will be discussed in greater detail later.
Fig. 4. Deep drawability of ZK60 magnesium alloys: (a) index of the drawn cups. Deep drawability for (b) IC and (c) TRC sheets.
3.3. Mechanical responses during deep drawing The punch load and the wall thickness of the drawn cups were investigated to gain a better understanding of the mechanical responses during the deep drawing of ZK60, as shown in Fig. 5. The working conditions of the temperature and the punch speed were 448 K (175 °C) and 30 mm/min, respectively. The distribution of the punch load reflects several combined effects by the work hardening of blank sheets, by the frictional force between the blank and the blank holder or the blank and the die, and sometimes
by ironing on the cup wall during the deformation process. The punch load increases sharply with an increase in the punch stroke at the beginning of the drawing process (with a punch stroke ranging from 0 to 7 mm), as shown in Fig. 5(a). There was some difference in the punch load between the TRC and IC sheets. With the assumption of similar amounts of friction between the ZK60 sheets (both IC and TRC) and the die, this difference mainly stems from differences in the work hardening. During deep drawing or deformation, both slip and twins are activated, and work hardening occurs with strain.
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Fig. 5. Variations of the punch loads and thickness during the deep drawing of ZK60: (a) punch load, and (b) thickness.
The thickness variation of the drawn cups is presented in Fig. 5(b). The inset in Fig. 5(b) illustrates the locations along the drawn cup walls, where the thickness was measured. The cup walls near the bent region undergo negative strain or a thickness reduction during the deep drawing process. Regions 3 and 4 are usually thinnest in the cups. Therefore, most failures or necking are observed in the negative strain regions for both the TRC and IC sheets. The thickness of the cup walls gradually increases along the top of the cup due to the increased compressive strain along the hoop direction in the flange. The thickness of region 7 showed the greatest value among all measured positions. The thickness strain levels of the TRC sheets along positions of 4, 5, and 6 were greater than those of the IC sheets. Particularly, the IC sheet shows negative strain at positions 4 and 5, followed by a sharp increase at positions 6 and 7. The TRC drawn cup, however, shows positive strain at the lower part (positions 4 and 5). This implies that there was some difference in thickness gain caused by the compressive strain and thickness reduction stemming from the tensile strain between the TRC and the IC sheets. We will discuss deformation modes of IC and TRC sheets in the following section. 3.4. Texture and microstructure of deep-drawn ZK60 The texture and microstructure of the deep-drawn cups were investigated for a better understanding of the deep drawability of ZK60 under the various working conditions discussed in the previous section. Most blank regions except for the cup bottom experience circumferential compression, with radial tension occurring in the flange, followed by bending and tensile deformation along the wall to form drawn cups.
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The linking parts or nearly bent areas connecting the cup bottom to the cup wall simply undergo bending and tensile deformation. The tensile deformation mainly decreased the thickness, increasing the possibility of a failure of the linking part. The microstructural evolution along the linking part was examined in detail using EBSD. Fig. 6 presents the inverse pole figure (IPF) and band contrast (BC) maps with tensile twinning boundaries obtained from the drawn cups of the IC sheets. During the deep drawing process, the linking regions revealed a deformed grain structure. Under a head speed of 30 mm/min at 423 K (Fig. 6(a)), many twins were observed inside large grains. Small grains are located between large grains. The working temperature of 423 K is somewhat low for dynamic recrystallization (DRX), or even for lowtemperature DRX [24]. Therefore, those small grains delineated with yellow lines in Fig. 6(b) appear to be associated with inhomogeneous shear bands during drawing. The microstructure under a working condition of 50 mm/min at 423 K in Fig. 6(C) reveals few twins. At a high temperature of 623 K, twinning is also dominant for both head speeds of 30 and 50 mm/min. The microstructure of the drawn cup at 50 mm/min is shown in Fig. 6(e). DRX during the deep drawing of IC sheets is not notably clear either, even at 623 K. Image quality or band contrast maps are helpful when attempting to analyze microstructural features. The degree of band contrast reflects the perfection of the crystal lattice. It depends on many factors, such as the local dislocation, orientation, and hardware and software setup used to capture Kikuchi patterns [25]. With the same type of mapping, it can also be used as a qualitative measure of the relative strain. Some grains with low band contrast imply the existence of high levels of stored energy. The red lines represent tensile twin boundaries, and large grains mostly contain the twin boundaries. The green and blue lines correspond to double and compressive twins, respectively. During the deep drawing of the IC sheets, the numbers of double and compressive twins were negligible. Numerous tensile twins were observed under most working conditions. With a working condition of 50 mm/ min at 423 K (Fig. 6(d)), profuse twinning was expected considering the low temperature and high deformation rate. The EBSD results, however, indicated that there was little twinning under this condition, as discussed above. Pole figures (PFs) computed from IPF maps in Fig. 7 are presented in Fig. 2(c). The initial state mainly showed a strong basal texture, as depicted in Fig.2(c). During deep drawing, the basal intensity levels increased in the center of the {0001} PFs in all working conditions. A second peak which formed by tensile twinning appeared at the border of the {0001} PFs, except under the condition of 50 mm/min and 423 K, as shown in Fig. 7(b). This implies that the major deformation modes during deep drawing are basal 〈a〉 slip and f1012g tensile twinning. Shear bands also contribute to deformation to some degree. To accommodate negative strain along the thickness direction during the deep drawing of the rolled magnesium sheets, non-basal slip systems or twins are required [3]. Particularly, 〈c + a〉 slip and f1011g compressive twinning can rotate the lattice orientations of the grains; thus, basal 〈a〉 and f1012g tensile twinning can easily be activated. During the cup drawing process here, the working condition 50 mm/min at 423 K led to little twinning activation. Considering that the overall thickness strains of the drawn cups ranged from approximately 10% to 15%, other deformation mechanisms, such as shear bands or non-basal slips, should be activated. In Fig. 6(d), deformed grains or shear bands are homogeneously spread in the samples, which is advantageous for more uniform straining before a fracture. In the IC sheets, tensile twins and shear bands appeared to play important roles in the accommodation of the thickness strain. Fig. 8 summarizes the maximum peak intensity of the (0001) PFs as obtained from the various IC drawn cups along with the fraction of twinning boundaries corresponding to the IPFs. Basal 〈a〉 and f1012g tensile twinning are soft deformation modes, with low CRSS values. Strong activity of basal 〈a〉 increases the basal intensity. The increase
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Fig. 6. Inverse pole figure (IPF) and band contrast (BC) maps with tensile twinning boundaries measured from the drawn cups (IC sheets). Different head speeds and working temperatures were applied during deep drawing: (a) and (b) IPF and BC for 30 mm/min at 423 K, (c) and (d) IPF and BC for 50 mm/min at 423 K, and (e) and (f) IPF and BC for 50 mm/min at 623 K.
in the basal peak intensity from the initial value during the drawing process mainly resulted from basal 〈a〉 slip. The strongest peak intensity was obtained under a working condition of 50 mm/min at 423 K, with few tensile twins. The peak intensity is slightly lower at a temperature of 623 K as compared to 423 K at an identical deformation rate. At elevated temperatures, the CRSS of the non-basal slips usually decreases, possibly resulting in the activation of non-basal slip systems. In addition, more tensile twins are observed at 623 K; this can also contribute to decreasing the peak intensity. At a temperature of 423 K, the fraction of the tensile twin boundaries observed from each drawn cup decreased with an increase in the drawing speed. At a drawing speed of 50 mm/min, the fraction of the tensile twin boundaries increased with an increase in the temperature. Twinning usually increases under working conditions causing high twinning
stress, i.e., a low temperature or a high deformation rate [23]. The current results, however, are not in good agreement with this. It is interesting to note that at the same working temperature, a low deformation rate of 30 mm/min led to possess more tensile boundaries compared to the rate of 50 mm/min. Our previous works on compression experiments using ingot-casted ZK60 alloys revealed some implications with regard to the deformation mechanism of ZK60 [26]. At a deformation rate of 5 mm/min, stress relaxation due to tensile twinning was observed from 298 K to 698 K. At a higher deformation rate of 100 mm/min, stress-strain flow curves were ordinarily obtained only above 448 K. Below this temperature, little plastic deformation occurred. These findings imply that there is some correlation between the deformation rate and the temperature to activate twinning, although it must be considered that the deep drawing process included
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Fig. 7. Pole figures (PFs) computed from the EBSD maps shown in Fig. 6 (IC sheets). Different head speeds and working temperatures were applied during the deep drawing process: (a) 30 mm/min at 423 K (150 °C), (b) 50 mm/min at 423 K (150 °C), and (c) 50 mm/min at 623 K (350 °C).
the complex deformation modes of tension, compression, and bending. In fact, the nucleation of deformation twins in HCP polycrystals is complicated [27–29], though a more in-depth discussion of this is beyond the scope of this study.
Fig. 8. Maximum peak intensity of the pole figures and fraction of twinning boundaries obtained from the relevant EBSD measurements (IC drawn cups). TTW represents f1012g tensile twinning with a misorientation angle and an axis of 86 h1210i 7:5 . CTW is f1011g compressive twinning with 56 h1210i 7:5 . DTW denotes f1011g−f1012g double twinning with 38 h1210i 7:5 .
The microstructural evolution during the deep drawing of the TRC sheets is presented in Fig. 9. Compared with the IC cups (Fig. 6), the TRC cups possess more refined grain structures. In addition, grains aligned with the basal planes (red color) decrease, and various grain orientations are observed. The most noticeable difference in the microstructure between the IC and TRC drawn cups is twinning. Twinned grains in the TRC cups are rarely observed. As noted above, the grain size is one of the most important factors affecting the activation of twinning, and it appears that the small grain size in the TRC sheets resulted in little twinning. Overall, the grain size of the drawn cups at the high temperature of 623 K is smaller than that of the drawn cups at the low temperature of 423 K. The smallest grains are observed with the drawn cup at the working condition of 50 mm/min at 623 K. Band contrast maps with the twinning boundaries of TRC drawn cups are also presented. Tensile twinning (red lines) is not very active. A small amount of double twinning (green lines) was also observed. Most small and equiaxed grains drawn at a temperature of 623 K (Fig. 9(f)) reveal bright band contrast, which implies that they are dynamically recrystallized and possess low levels of stored energy. The pole figures (PFs) computed from the IPF maps of the TRC drawn cups are presented in Fig. 10. The initial TRC sheet possessed a broad basal peak mainly spread along the TD and minor double peaks along the RD, as shown in Fig. 2(d). During deep drawing, the overall peak intensity in the TRC sheets is smaller than that in the IC sheets. Double peaks along the RD are also clearly observed in the {0001} PFs of the drawn cups. In addition to a peak split, the overall shape of the basal
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Fig. 9. Inverse pole figure (IPF) maps and band contrast (BC) maps with tensile twinning boundaries measured from the drawn cups (TRC sheets). Different head speeds and working temperatures were applied during the deep drawing process: (a) and (b) IPF and BC for 30 mm/min at 423 K (150 °C), (c) and (d) IPF and BC for 50 mm/min at 423 K (150 °C), and (e) and (f) IPF and BC for 50 mm/min at 623 K (350 °C).
peaks are much broader than that of the initial TRC sheet. DRX with orientation spread from the basal textures appears to contribute to peak broadening. Under the working condition of 50 mm/min at 423 K (Fig. 10(b)), an off-basal texture without a basal peak split is observed. The lack of a peak split results in strong basal peak intensity as compared to the levels observed in other experimental cases. Exceptional textural evolution during deep drawing under the working condition of 50 mm/min at 423 K was also observed in the IC sheets (Fig. 7(b)). As discussed earlier, the activation of tensile twinning under these working conditions is unlikely. There are several previous reports of double peaks along the RD in magnesium alloys. Double twinning, f1011g−f1012g, with f1011g compressive twinning followed by f1012g tensile twinning, can explain the double peaks [13]. Based on a study of the slip system activity, the splitting of the basal poles along the RD can be caused by the high activation of pyramidal 〈c + a〉 slip [4]. At high temperatures, 〈c + a〉 can easily be activated due to the lower CRSS value, contributing to the formation of double peaks. Fig. 11 summarizes the maximum peak intensity of the PFs obtained from the various TRC drawn cups and the fraction of the corresponding
twin boundaries. Tensile twins were highly active in the IC drawn cups (Fig. 8) but were relatively weak in the TRC drawn cups. The effect of the grain size on twinning is more dominant during tensile twinning than it is during compressive twinning [30]. This appears to be one of the reasons for the minor amount of tensile twinning in the TRC sheets. Instead, double twinning is observed more in the TRC cups than in the IC cups, although the fraction in this case is not great, compared to that associated with tensile twinning. Considering the characteristics of compressive twins, the volume fraction of double twinning is meaningful even with the minor values. Note that a large fraction of double twining (30 mm/min at 623 K) or tensile twinning (50 mm/min at 448 K and 623 K) results in a lower basal intensity level compared to the other working conditions. At a drawing speed of 50 mm/min, elevated temperatures of 448 K and 623 K produced more tensile twins compared to the temperature of 423 K. As noted with the IC sheets, the correlation between the temperature and the deformation rate appears to be important for the activation of the tensile twinning of TRC sheets (even with small grains). A low fraction of double twinning boundaries is observed with the working conditions of 50 mm/min at 423 K, 40 mm/min at 423 K and 50 mm/min at 448 K.
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Fig. 10. Pole figures (PFs) computed from the EBSD maps shown in Fig. 6 (TRC sheets). Different head speeds (millimeter per minute, mmpm) and working temperatures were applied during the deep drawing process: (a) 30 mm/min at 423 K (150 °C), (b) 50 mm/min at 423 K (150 °C), and (c) 50 mm/min at 623 K (350 °C).
Fig. 12 shows the distribution of the grain size. In the IPFs (Figs. 6 and 9), large and small grains are mixed. We arbitrarily divided all grains into two groups, large and small grains, using the number of mapping pixels. Approximately, 300 pixels was effective to separate all grains into two groups in this experiment. In the IC sheets, the volume of large grains was greater than that of small grains. In the TRC sheets, the volume of small grains was greater than that of large grains. This
reflected that the average grain size of all grains in the IC sheets was greater than that in the TRC sheets. Particularly, the average grain size of the large grains in the IC sheets was clearly greater than that in the TRC sheets, which implied easier twinning in the IC sheets. 4. Conclusions The evolution of the texture and microstructure during the deep drawing of two different ZK60 (Mg-Zn-Zr) sheets fabricated by means of ingot casting (IC) and twin-roll casting (TRC) was investigated
Fig. 11. Maximum peak intensity of the pole figures and fraction of twinning boundaries obtained from the relevant EBSD measurements (TRC drawn cups). TTW represents f1012g tensile twinning with a misorientation angle and an axis of 8 6 h1210i 7:5 . CTW is f1011g compressive twinning with 56 h1210i 7:5 . DTW denotes f1011g−f1012g double twinning with 38 h1210i 7:5 .
Fig. 12. Grain size and volume fraction of the small and large grains.
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using TEM/EDS and EBSD. There were some differences in the microstructure and texture noted between the IC and TRC sheets and the drawn cups. Variations of the punch load and of the thickness of the cup walls were also examined. • Particles of second phases measured from TRC sheets were finer than those measured from IC samples. Most particles mainly consisted of Zn and Zr. The TS, YS and El of the TRC sheets are approximately 330 MPa, 277 MPa and 18%, respectively. The corresponding values for the IC sheets are 320 MPa, 235 MPa, and 14%, indicating the superiority of TRC sheets. • The grains of the IC samples were larger than those of the TRC samples. The volume fraction of the basal texture of the IC samples was also greater than that of the TRC samples. The TRC sheets initially showed basal peak broadening along the TD and basal peak split along the RD. • At working temperatures of 448 K to 623 K and deformation rates of 30 mm/min to 50 mm/min, the deep drawing of ZK60 sheets was successfully carried out on both the IC and the TRC samples. At 423 K, the TRC sheets were more favorable for deep drawing than the IC samples, and the deep drawability of the TRC sheets was better than that of the IC sheets at the low temperature. • Some differences in the major deformation modes between the IC and TRC sheets during the deep drawing process were found. Tensile twinning and shear bands were frequently observed in the cups made of IC sheets with large grains. Peak broadening of the basal texture along the TD and a peak split along the RD were mainly observed in the cups made of TRC sheets. Dynamic recrystallization was also evident in the TRC sheets. • The greatest deformation rate and the lowest temperature in the current study resulted in little tensile twinning during the cup drawing process for both the IC and the TRC sheets. This finding implies that a high deformation rate or a low temperature does not always result in easy twinning. Acknowledgement This research was supported by the Fundamental Research Program of the Korea Institute of Materials Science (KIMS, PNK4970). References [1] S. Ion, F. Humphreys, S.H. White, Dynamic recrystallisation and the development of microstructure during the high temperature deformation of mgnesium, Acta Metall. 30 (1982) 1909–1919. [2] A. Jain, S. Agnew, Modeling the temperature dependent e_ect of twinning on the behavior of magnesium alloys AZ31B sheet, Mater. Sci. Eng. A 462 (2007) 29–36. [3] A. Chapuis, J.H. Driver, Temperature dependency of slip and twinning in plane strain compression magnesium single crystals, Acta Mater. 59 (2011) 1986–1994. [4] S.R. Agnew, M.H. Yoo, C.N. Tome, Application of texture simulation to understanding mechanical behavior of mg and solid solution alloys containing Li and Y, Acta Mater. 49 (2001) 4277–4289.
[5] S.H. Choi, J.H. Cho, K.H. Oh, K. Chung, F. Barlat, Texture evolution of FCC sheet metals during deep drawing process, Int. J. Mech. Sci. 42 (2000) 1571–1592. [6] T. Walde, H. Riedel, Simulation of earing during deep drawing of magnesium alloy AZ31, Acta Mater. 55 (2007) 867–874. [7] S. Yi, J. Bohlen, F. Heinemann, D. Letzig, Mechanical anisotropy and deep drawing behavior of AZ31 and ZE10 magnesium alloy sheets, Acta Mater. 58 (2010) 592–605. [8] H. Zhang, G. Huang, J. Fan, B.X.H.J. Roven, Deep drawability and drawing behaviour of AZ31 alloy sheets with di_erent initial texture, J. Alloys Compd. 615 (2014) 302–310. [9] W. Tang, S. Huang, D. Li, Y. Peng, Mechanical anisotropy and deep drawing behaviors of AZ31 magnesium alloy sheets produced by unidirectional and cross rolling, J. Mater. Process. Technol. 215 (2015) 320–326. [10] J. Bohlen, M.R. Nurnberg, J.W. Senn, D. Letzig, S. Agnew, The texture and anisotropy of magnesium-zine-rare earth alloy sheets, Acta Mater. 55 (2007) 2101–2112. [11] Y. Chino, M. Kado, M. Mabuchi, Compressive deformation behavior at room temperature - 773 K in Mg–0.2 mass% (0.035 at.%)Ce alloy, Acta Mater. 56 (2008) 387–394. [12] L. Mackenzie, M. Pekguleryuz, The recrystallization and texture of magnesium-zinecerium alloys, Scr. Mater. 59 (2008) 665–668. [13] L. Mackenzie, M. Pekguleryuz, The influence of alloying additions and processing parameters on the rolling microstructures and textures of magnesium alloys, Mater. Sci. Eng. A 480 (2008) 189–197. [14] J.H. Cho, S.S. Jeong, H. Kim, S. Kang, Texture and microstructure evolution during the symmetric and asymmetric rolling of AZ31B magnesium alloys, Mater. Sci. Eng. A 566 (2013) 40–46. [15] X. Gong, S.B. Kang, S. Li, J.H. Cho, Enhanced plasticity of twin-roll cast ZK60 magnesium alloy through di_erential speed rolling, Mater. Des. 30 (2009) 3345–3350. [16] X. Gong, L. Hao, S.B. Kang, J.H. Cho, S. Li, Microstructure and mechanical properties of twin-roll cast Mg-4.5Al-1.0Zn sheets processed by di_erential speed rolling, Mater. Des. 31 (2010) 1581–1587. [17] H. Watari, T. Haga, N. Koga, K. Davey, Feasibility study of twin roll casting process for magnesium alloys, J. Mater. Process. Technol. 192-193 (2007) 300–305. [18] S. Park, G. Bae, D. Kang, B. You, N.J. Kim, Superplastic deformation behavior of twinroll cast Mg-6Zn-1Mn-1Al alloy, Scr. Mater. 61 (2009) 223–226. [19] Y. Wang, S.-B. Kang, J.-H. Cho, Microstructure and mechanical properties of Mg-AlMn-Ca sheet produced by twin roll casting and sequent warm rolling, J. Alloys Compd. 509 (2011) 704–711. [20] Y.S. Lee, H.W. Kim, J.H. Cho, Process parameters and roll separation force in horizontal twin rollcasting of aluminum alloys, J. Mater. Process. Technol. 218 (2015) 48–56. [21] J.H. Cho, H.M. Chen, S.H. Choi, H.W. Kim, S.B. Kang, Aging e_ect on texture evolution during warm rolling of ZK60 alloys fabricated by twin-roll casting, Metall. Mater. Trans. A 41 (2010) 2575–2583. [22] HKL Channel 5 Manual, Oxford Instruments HKL, Majsmarken1, 9500 Hobro, Denmark, 2007. [23] M.A. Meyers, O. Vohringer, V.A. Lubarda, The onset of twinning in metals: a constitutive description, Acta Mater. 49 (2001) 4025–4039. [24] Z. Liu, S. Bai, S. Kang, Low-temperature dynamic recrystallization occuring at a high deformation temperature during hot compression of twin-roll-cast Mg-5.51Zn0.49Zr alloy, Scr. Mater. 60 (2009) 403–406. [25] S. Claves, A. Deal, Orientaion dependence of ebsd pattern quality, Microsc. Microanal. 11 (suppl.2) (2005) 514–515. [26] J.H. Cho, S.B. Kang, Deformation and recrystallization behaviors in magnesium alloys, in: P. Wilson (Ed.), Recent Developments in the Study of Recrystallization, InTECH press 2013, pp. 139–159. [27] I. Beyerlein, C. Tome, A probabilistic twin nucleation model for hcp polycrystalline metals, Proc. R. Soc. A 466 (2010) 2517–2544. [28] I. Beyerlein, L. Capolungo, P. Marshall, R. McCabe, C. Tome, Statistical analyses of deformation twinning in magnesium, Philos. Mag. 90 (2010) 2161–2190. [29] S.R. Niezgoda, A.K. Kanjarla, I.J. Beyerlein, C.N. Tome, Stochastic modeling of twin nucleation in polycrystals - an application in hexagonal close-packed metals, Int. J. Plast. 56 (2014) 119–138. [30] Y. Wang, H. Choo, Influence of texture on Hall-Petch relationships in an Mg alloy, Acta Mater. 81 (2014) 83–97.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Deep drawing of ZK60 magnesium sheets fabricated using ingot and twin-roll casting methods Jae-Hyung Cho a,b,⁎, Sang Su Jeong c, Suk-Bong Kang a a b c
Korea Institute of Materials Science, 797 Changwondaero, Seongsan-gu, Changwon, Gyeongnam 642-831, South Korea University of Science and Technology, 217 Gajeong, Yuseong, Daejeon 305-350, South Korea KHVatec, 293 Gondan-dong, Gumi-si, Gyeongbuk, South Korea
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Evolution of microstructure and texture of ZK60 (Mg-5.5Zn-0.5Zr) was examined during deep drawing. • Two different ZK60 alloys of ingot casted (IC) and twin-roll casted (TRC) were compared. • Major deformation modes between the IC and TRC sheets were different during drawing process. • Tensile twinning and shear bands were frequently observed in IC cups. • Peak broadening along the TD and a peak split along the RD were more observed in TRC cups.
a r t i c l e
i n f o
Article history: Received 18 May 2016 Received in revised form 13 July 2016 Accepted 28 July 2016 Available online 1 August 2016 Keywords: ZK60 magnesium alloys Deep drawing Microstructure and texture Twinning EBSD TEM
a b s t r a c t Deep drawing was carried out on two different ZK60 (Mg-Zn-Zr) magnesium sheets fabricated via ingot and twin-roll casting methods. The effects of the microstructure and texture on the deep drawability and the associated microstructural evolution of ZK60 alloys were examined. Twin-roll casted sheets with refined particles were stronger and showed greater elongation than ingot-casted sheets. Moreover, they were fully deep-drawn at lower temperatures than ingot-casted sheets, and the deep drawability of the ingot-casted sheets was better at a high deformation rate compared to that of the twin-roll casted sheets. During the deep drawing process, the ingot-casted sheets with large grains experienced more tensile twinning than the twin-roll casted sheets, and shear bands were observed. The twin-roll casted sheets revealed peak broadening of the basal texture along the TD and peak splitting along the RD. Thickness variations of the drawn cups showed that the twin-roll casted sheets resisted wall thinning more effectively than the ingot-casted sheets. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction
⁎ Corresponding author at: Korea Institute of Materials Science, 797 Changwondaero, Seongsan-gu, Changwon, Gyeongnam 642-831, South Korea. E-mail address: [email protected] (J.-H. Cho).
http://dx.doi.org/10.1016/j.matdes.2016.07.131 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
Industrial applications of wrought magnesium alloys have recently drawn increased amounts of attention due to the low density and high specific strength levels of these alloys. Magnesium alloys, however, inherently have poor formability given their limited slip activation at room temperature. The critical resolved shear stresses (CRSSs) of
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magnesium slip systems strongly depend on the temperature [1–3]. The CRSS of the basal 〈a〉 slip is significantly lower than that of other nonbasal slip systems at room temperature. At elevated temperatures, non-basal slip systems of magnesium alloys can be more easily activated. This results in better formability of magnesium alloys; thus, the forming of magnesium alloys is usually carried out at elevated temperatures. In addition to the slip, twinning frequently enhances the formability of magnesium alloys [4]. The deep drawing process has been used for various industrial applications, such as automobile parts, electrical appliances, and cellular phones. To carry out the deep drawing of magnesium alloys successfully, careful attention should be paid to processing parameters (e.g., temperatures of the dies, the blank, and the blank holder; the punch speed; the blank holding force; and the lubrication amounts and methods used) and certain metallurgical aspects (texture and microstructure). The forming temperature is one of the most significant parameters affecting the deep drawability of magnesium alloys. During the rolling of magnesium alloys, strong basal textures usually develop, resulting in poor workability. The crystallographic texture is associated with the planar anisotropy of sheet metals, and it is important to consider it for successful deep drawing [5–9]. To modify the typical texturing, certain elements such as Mm (misch metal), can be added to magnesium alloys. Soluble Zn and RE additions (Y) weaken the rolling texture of magnesium alloys [10]. The addition of Ce or Li to magnesium alloys results in weak basal textures [11–13]. Thermo-mechanical processing can alter the texture and microstructure of magnesium alloys. Asymmetric or differential speed rolling process caused weak basal textures [14]. Twin-roll casting (TRC) followed by an asymmetric rolling process can more effectively modify the microstructure and texture of sheets as compared to conventional ingot casting (IC) with a subsequent symmetric rolling process [15,16]. AZ91 sheets manufactured by roll strip casting and a subsequent hot rolling process have been successful for warm deep drawing [17]. Enhanced mechanical properties of TRC sheets have also been reported in many magnesium alloys as compared to those of conventional IC sheets [18,19]. In this study, the deep drawing of two different ZK60 sheets fabricated by TRC and IC processes was carried out at elevated temperatures. Under various working temperatures and punch speeds, the deep drawability of ZK60 sheets was examined. Using HR-SEM/EBSD (highresolution scanning electron microscopy/electron backscatter diffraction) and TEM (transmission electron microscopy), the evolution of the microstructure and texture and the distribution of the second phases were also investigated to gain a better understanding of the mechanical responses during the deep drawing of ZK60 magnesium alloys. 2. Experimental 2.1. Materials and heat treatments Two different ZK60 magnesium sheets were separately fabricated by TRC and IC processes. The overall chemical compositions of the ZK60 alloys used here are given in Table 1. The rolling speed of the strip caster was approximately 3 rpm (revolution per minute). The manufactured TRC strips were approximately 4.0 mm thick, 180 mm wide and more than 10 m long. For the IC samples, molten alloys at 993 K (720 °C) were poured into a steel mold 180 mm × 160 mm × 25 mm in size. The IC slabs were warm-rolled down to a thickness of 4 mm. The homogenization heat treatment for both samples (TRC and IC) consisted of two steps: holding at a temperature of 623 K (350 °C) for 10 h and Table 1 Overall chemical composition of ZK60 (Mg-Zn-Zr) magnesium alloys [wt.%].
ZK60
Al
Zn
Mn
Si
Fe
Cu
Ni
Ca
Zr
Mg
0.01
5.47
0.009
0.022
0.003
0.003
0.007
0.005
0.58
Bal.
215
then holding at 723 K (450 °C) for 5 h to avoid cracking due to the presence of Zn and Zr intermetallic compounds during the rolling process. Detailed information about the TRC system and the working process is available in the literature [20]. To fabricate the blank sheets used in the deep drawing process, seven or eight passes of warm rolling from 4 mm down to 0.6 mm were carried out on the ZK60 TRC and IC sheets. The roller used, with a diameter of 200 mm, was preheated to 523 K (250 °C) and operated at a speed of 3.5 rpm. The ZK60 sheets were reheated at 573 K (300 ° C) for 30 min; the intermediate annealing time and temperature between the rolling passes were 5 min and 573 K, respectively. Final annealing of the ZK60 sheets with a thickness of about 0.6 mm was carried out at 648 K (375 °C) for 1000 s. 2.2. Deep drawing The deep drawing processes were carried out using a universal sheet testing machine (USTM). The USTM consisted of a die, a punch, and a blank holder. The punch diameter was 37 mm and the inner diameter of the die was 39 mm. The clearance between the punch and the die was 1 mm. The diameter of the blank was 74 mm, and a drawing ratio (DR) of 2.0 was applied. The blank holder and die were heated to specific temperatures using embedded electrical heating elements. The blank was laid between the die and the blank holder and was held for about 5 min to stabilize the temperature distribution of the blank sheets. The blank holding force (BHF) during the deep drawing process was varied from 5 to 20 kN depending on the working conditions. At the same temperature, the same BHF was applied, regardless of the punch speed. In order to ensure successful deep drawing, the BHF was increased with a decrease in the temperature. A low BHF of 5 kN was applied at an elevated temperature of 623 K, and a high BHF of 20 kN was applied at a low temperature of 423 K. Lubricant was used to reduce the friction during the deep drawing processes. The deep drawing conditions of the ZK60 sheets are summarized in Table 2. 2.3. Microstructural measurements and mechanical testing Analyses of the microtexture and microstructure were carried out using an automated high-resolution EBSD (HKL Channel5) attached to an HR-SEM device (JEOL7001F). Triclinic sample symmetry was applied to create pole figures. The step size during the EBSD mapping was 0.5 μm for the IC samples and 0.25–0.28 μm for the TRC samples. The EBSD samples were mechanically polished and then electropolished using a solution of butyl cellosolve (50 ml), ethanol (10 ml), and perchloric acid (5 ml) at a voltage of 10 V and a temperature of 258 K (− 15 °C) to 253 K (− 20 °C). Intermetallic compounds or particles were examined using a JEM-2100F TEM operating at 200 kV. The TEM samples were prepared by mechanical polishing down to approximately 100 μm followed by electropolishing using methanol (60 ml), glycerin (30 ml), and nitric acid (10 ml). Electropolishing was carried out at temperatures and voltages of 263 K (−10 °C) to 258 K (−15 °C) and 20 V to 25 V, respectively. The TEM and SEM sample preparation steps were finalized by ion milling when necessary to prevent oxidation and other contamination of surfaces. Energy dispersive spectroscopy (EDS) was also carried out on the particles. Table 2 Deep drawing conditions of the ZK60 magnesium alloys. Punch diameter [mm] Inner diameter of the die [mm] Specimen thickness [mm] Drawing speed [mm/min] Temperature [K] Blank holding force [kN] Blank diameter [mm]
37 39 0.6 30, 40, 50 398–623 5–20
74
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Uniaxial tension tests (ASTM E8 standard) of the ZK60 sheets were carried out along the rolling direction (RD) using a standard universal testing machine (Instron 4206). The gauge length and width of the samples were 12.5 mm and 3 mm. The strain rates of the ZK60 alloys were 0.0025/s at room temperature. Three samples were tested and their mechanical properties were averaged. 3. Results and discussion 3.1. Microstructure and mechanical properties of the initial ZK60 sheets Various microstructural features, including the particles, crystallographic orientations, and grain structure, can affect the deep drawability of ZK60 sheets. In order to examine distribution of the particles of ZK60, a TEM analysis was carried out on the TRC and IC sheets, as shown in Fig. 1. The IC sheet contains bulk particles, and the TRC sheet possesses refined and homogeneously distributed particles. Refined particles found in TRC sheets usually improve the mechanical properties related to the strength and elongation of the sheets. The EDS results of the particles are shown in Fig. 1(c) and (d). The bulk particles in the IC sheet consist of Zn and Zr, which formed during the solidification of the melt. The fine particles found in the TRC sheet also mainly contain Zn and Zr. Fig. 2 presents inverse pole figure (IPF) maps and recalculated pole figures (PFs) as measured from the initial ZK60 sheets. Grain identification (GID) angles greater than 15° are specified with thick lines,
representing high angle grain boundaries. GID angles between 5° and below 15° associated with low angle grain boundaries or subgrain boundaries are represented with thin lines. GID angles below 5° were ignored as grain boundaries. The microstructure of the initial state of the IC sheet is given in Fig. 2(a). The average grain size is approximately 7.1 μm. The recalculated pole figures (PFs) of ð0001Þ; ð1120Þ; andð1010Þ are shown in Fig. 2(c). A strong basal peak which mainly formed during the warmrolling step was noted along the normal direction (ND) of the sheet (or at the center of the (0001) PF). The maximum peak is tilted from the ND of the sheet toward the bottom left at about 10°. Tilting of the basal peaks is frequently observed in magnesium sheets [21]. Certain aspects of inhomogeneous deformation, such as a shear band, are among the reasons why the basal peaks deviate from the ND. In addition, the angular distribution of the basal peak in the (0001) PF is slightly broader toward the transverse direction (TD) than toward the rolling direction (RD). The shape and intensity of the basal peaks in magnesium alloys are associated with many factors, such as the chemical compositions, dominant deformation modes, and the thermo-mechanical processing characteristics. Basal 〈a〉 and prismatic 〈a〉 slips in addition to tensile twinning contribute to the basal intensity with a broad TD distribution [4]. Both of the prismatic planes of ð1120Þ, and ð1010Þ lying on the sheet plane possess only slight distinctively preferred orientations along the perimeter of the PFs. The maximum peak intensity of the initial IC sheet is approximately 10.0 MUD (multiples of a uniform distribution). In the pole figures, the half-width value controls how much the pole is spread out over the
Fig. 1. Various particles and their EDS results of ZK60 sheets. TEM micrographs of the particles: (a) IC and (b) TRC. EDS results of particles consisting of Zn and Zr: (c) IC and (d) TRC.
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Fig. 2. Microstructure and texture of the initial ZK60 sheets: (a) inverse pole figure (IPF) maps and (c) recalculated pole figures (PFs) for the IC sheet, (b) IPF maps and (d) recalculated PFs for the TRC sheet.
surface of the projected sphere, and clustering speeds up the calculations. Detailed information about creating pole figures from EBSD data can be found in the HKL Channel 5 manual [22]. The microstructure of the initial ZK60 sheets fabricated by TRC also possesses both equiaxed and elongated grains, as shown in Fig. 2(b). Subgrain boundaries are mainly found in the elongated grains. The average grain size of the TRC ZK60 sheet is approximately 4.3 μm. The recalculated PFs of the TRC sheet are shown in Fig. 2(d). The (0001) PF shows that the basal intensity of the TRC sheet spreads intensely from the ND to the TD. The angular distribution of the basal peak is much broader toward the TD than toward the RD, compared to that of the IC sheet. It was also noted that there is a peak split along the RD in the center of the (0001) PF. The prismatic planes of ð1120Þ, and ð1010Þ show preferred orientations along the perimeter of PFs. The maximum basal intensity of the TRC sheet is about 7.0 MUD. Overall, the TRC sheet possesses smaller grains and a lower basal intensity level than the IC sheet. The angular distribution of the basal planes (0001) and the perimetric distribution of the prismatic planes ð1120Þ and ð1010Þ of the TRC and IC sheets also differed from each other.
The mechanical properties of two different ZK60 sheets (TRC and IC) were examined in uniaxial tension tests. Fig. 3 shows the tensile flow curves along the RD of the ZK60 sheets. The tensile strength (TS), yield strength (YS) and elongation (El) of the TRC sheets are approximately 330 MPa, 277 MPa and 18%, respectively. The corresponding values for the IC sheets are 320 MPa, 235 MPa, and 14 %, indicating the superiority of TRC sheets in terms of the mechanical properties as compared to the IC sheets. The microstructural features of small grains and refined particles obtained during the TRC process with a high solidification rate are usually coincident with improved mechanical properties. 3.2. Deep drawing at different temperatures and punch speeds Under various working temperatures ranging from 398 K (125 °C) to 623 K (350 °C) and punch speeds ranging from 30 to 50 mm/min, the deep drawing of the ZK60 magnesium sheets was carried out. The overall working window, or the deep drawability, is represented in Fig. 4. Fig. 4(a) shows symbols representing the failure (×), or success (○) of the deep drawn cups. Drawn cups with some wrinkles in the top region
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Fig. 3. Tensile stress-strain curves of the ZK60 sheets.
are specified with a triangle, △. The wrinkles reflect the instability of the metal flow, which is caused by compressive strain along the circumferential or hoop direction. At working temperatures between 448 K (175 °C) and 623 K and punch speeds between 30 mm/min and 40 mm/min, deep drawing was successful for both the TRC and the IC sheets. For the TRC sheets, minor wrinkles were found at temperatures of 448 K and 473 K (200 °C) at a punch speed of 50 mm/min. To avoid these types of wrinkles, greater BHF should be applied, after which successful drawing becomes much more likely. At 423 K (150 °C), the increased BHF resulted in successful deep drawing at all punch speeds. For the IC sheets, wrinkles near the top of the cup appeared at a working temperature of 423 K regardless of the punch speed. The different wrinkling behaviors between IC and TRC sheets observed at 423 K reflect the different levels of plastic instability. Twin-roll casted sheets were fully deep-drawn at lower temperatures than ingot-casted sheets, and the drawability of the ingot-casted sheets was better at a high deformation rate than that of the twin-roll casted sheets. The deep drawability of ZK60 magnesium alloys is closely associated with the working parameters of the temperature and the deformation rate. When considering plastic deformation and various deformation modes of slip and twinning, the temperature-dependency of the associated CRSS values should also be considered. It is known that basal 〈a〉 and {1012} tensile twinning are fairly immune to the temperature. Prismatic〈a〉, pyramidal〈c + a〉, and {1011} compressive twins, however, are more temperature-dependent. At elevated temperatures, the CRSS values of the non-basal slip systems and compressive twins decrease, and these systems can easily be activated. In addition, the deformation rate plays an important role in activating twinning. With an increase in the deformation rate, the flow stress increases. Twinning activation usually increases with an increase in the flow stress [23]. The microstructural features of the grain size and orientation also strongly affect the deep drawability characteristics. Microstructural evolution during deep drawing and its effect on the deep drawability will be discussed in greater detail later.
Fig. 4. Deep drawability of ZK60 magnesium alloys: (a) index of the drawn cups. Deep drawability for (b) IC and (c) TRC sheets.
3.3. Mechanical responses during deep drawing The punch load and the wall thickness of the drawn cups were investigated to gain a better understanding of the mechanical responses during the deep drawing of ZK60, as shown in Fig. 5. The working conditions of the temperature and the punch speed were 448 K (175 °C) and 30 mm/min, respectively. The distribution of the punch load reflects several combined effects by the work hardening of blank sheets, by the frictional force between the blank and the blank holder or the blank and the die, and sometimes
by ironing on the cup wall during the deformation process. The punch load increases sharply with an increase in the punch stroke at the beginning of the drawing process (with a punch stroke ranging from 0 to 7 mm), as shown in Fig. 5(a). There was some difference in the punch load between the TRC and IC sheets. With the assumption of similar amounts of friction between the ZK60 sheets (both IC and TRC) and the die, this difference mainly stems from differences in the work hardening. During deep drawing or deformation, both slip and twins are activated, and work hardening occurs with strain.
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Fig. 5. Variations of the punch loads and thickness during the deep drawing of ZK60: (a) punch load, and (b) thickness.
The thickness variation of the drawn cups is presented in Fig. 5(b). The inset in Fig. 5(b) illustrates the locations along the drawn cup walls, where the thickness was measured. The cup walls near the bent region undergo negative strain or a thickness reduction during the deep drawing process. Regions 3 and 4 are usually thinnest in the cups. Therefore, most failures or necking are observed in the negative strain regions for both the TRC and IC sheets. The thickness of the cup walls gradually increases along the top of the cup due to the increased compressive strain along the hoop direction in the flange. The thickness of region 7 showed the greatest value among all measured positions. The thickness strain levels of the TRC sheets along positions of 4, 5, and 6 were greater than those of the IC sheets. Particularly, the IC sheet shows negative strain at positions 4 and 5, followed by a sharp increase at positions 6 and 7. The TRC drawn cup, however, shows positive strain at the lower part (positions 4 and 5). This implies that there was some difference in thickness gain caused by the compressive strain and thickness reduction stemming from the tensile strain between the TRC and the IC sheets. We will discuss deformation modes of IC and TRC sheets in the following section. 3.4. Texture and microstructure of deep-drawn ZK60 The texture and microstructure of the deep-drawn cups were investigated for a better understanding of the deep drawability of ZK60 under the various working conditions discussed in the previous section. Most blank regions except for the cup bottom experience circumferential compression, with radial tension occurring in the flange, followed by bending and tensile deformation along the wall to form drawn cups.
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The linking parts or nearly bent areas connecting the cup bottom to the cup wall simply undergo bending and tensile deformation. The tensile deformation mainly decreased the thickness, increasing the possibility of a failure of the linking part. The microstructural evolution along the linking part was examined in detail using EBSD. Fig. 6 presents the inverse pole figure (IPF) and band contrast (BC) maps with tensile twinning boundaries obtained from the drawn cups of the IC sheets. During the deep drawing process, the linking regions revealed a deformed grain structure. Under a head speed of 30 mm/min at 423 K (Fig. 6(a)), many twins were observed inside large grains. Small grains are located between large grains. The working temperature of 423 K is somewhat low for dynamic recrystallization (DRX), or even for lowtemperature DRX [24]. Therefore, those small grains delineated with yellow lines in Fig. 6(b) appear to be associated with inhomogeneous shear bands during drawing. The microstructure under a working condition of 50 mm/min at 423 K in Fig. 6(C) reveals few twins. At a high temperature of 623 K, twinning is also dominant for both head speeds of 30 and 50 mm/min. The microstructure of the drawn cup at 50 mm/min is shown in Fig. 6(e). DRX during the deep drawing of IC sheets is not notably clear either, even at 623 K. Image quality or band contrast maps are helpful when attempting to analyze microstructural features. The degree of band contrast reflects the perfection of the crystal lattice. It depends on many factors, such as the local dislocation, orientation, and hardware and software setup used to capture Kikuchi patterns [25]. With the same type of mapping, it can also be used as a qualitative measure of the relative strain. Some grains with low band contrast imply the existence of high levels of stored energy. The red lines represent tensile twin boundaries, and large grains mostly contain the twin boundaries. The green and blue lines correspond to double and compressive twins, respectively. During the deep drawing of the IC sheets, the numbers of double and compressive twins were negligible. Numerous tensile twins were observed under most working conditions. With a working condition of 50 mm/ min at 423 K (Fig. 6(d)), profuse twinning was expected considering the low temperature and high deformation rate. The EBSD results, however, indicated that there was little twinning under this condition, as discussed above. Pole figures (PFs) computed from IPF maps in Fig. 7 are presented in Fig. 2(c). The initial state mainly showed a strong basal texture, as depicted in Fig.2(c). During deep drawing, the basal intensity levels increased in the center of the {0001} PFs in all working conditions. A second peak which formed by tensile twinning appeared at the border of the {0001} PFs, except under the condition of 50 mm/min and 423 K, as shown in Fig. 7(b). This implies that the major deformation modes during deep drawing are basal 〈a〉 slip and f1012g tensile twinning. Shear bands also contribute to deformation to some degree. To accommodate negative strain along the thickness direction during the deep drawing of the rolled magnesium sheets, non-basal slip systems or twins are required [3]. Particularly, 〈c + a〉 slip and f1011g compressive twinning can rotate the lattice orientations of the grains; thus, basal 〈a〉 and f1012g tensile twinning can easily be activated. During the cup drawing process here, the working condition 50 mm/min at 423 K led to little twinning activation. Considering that the overall thickness strains of the drawn cups ranged from approximately 10% to 15%, other deformation mechanisms, such as shear bands or non-basal slips, should be activated. In Fig. 6(d), deformed grains or shear bands are homogeneously spread in the samples, which is advantageous for more uniform straining before a fracture. In the IC sheets, tensile twins and shear bands appeared to play important roles in the accommodation of the thickness strain. Fig. 8 summarizes the maximum peak intensity of the (0001) PFs as obtained from the various IC drawn cups along with the fraction of twinning boundaries corresponding to the IPFs. Basal 〈a〉 and f1012g tensile twinning are soft deformation modes, with low CRSS values. Strong activity of basal 〈a〉 increases the basal intensity. The increase
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Fig. 6. Inverse pole figure (IPF) and band contrast (BC) maps with tensile twinning boundaries measured from the drawn cups (IC sheets). Different head speeds and working temperatures were applied during deep drawing: (a) and (b) IPF and BC for 30 mm/min at 423 K, (c) and (d) IPF and BC for 50 mm/min at 423 K, and (e) and (f) IPF and BC for 50 mm/min at 623 K.
in the basal peak intensity from the initial value during the drawing process mainly resulted from basal 〈a〉 slip. The strongest peak intensity was obtained under a working condition of 50 mm/min at 423 K, with few tensile twins. The peak intensity is slightly lower at a temperature of 623 K as compared to 423 K at an identical deformation rate. At elevated temperatures, the CRSS of the non-basal slips usually decreases, possibly resulting in the activation of non-basal slip systems. In addition, more tensile twins are observed at 623 K; this can also contribute to decreasing the peak intensity. At a temperature of 423 K, the fraction of the tensile twin boundaries observed from each drawn cup decreased with an increase in the drawing speed. At a drawing speed of 50 mm/min, the fraction of the tensile twin boundaries increased with an increase in the temperature. Twinning usually increases under working conditions causing high twinning
stress, i.e., a low temperature or a high deformation rate [23]. The current results, however, are not in good agreement with this. It is interesting to note that at the same working temperature, a low deformation rate of 30 mm/min led to possess more tensile boundaries compared to the rate of 50 mm/min. Our previous works on compression experiments using ingot-casted ZK60 alloys revealed some implications with regard to the deformation mechanism of ZK60 [26]. At a deformation rate of 5 mm/min, stress relaxation due to tensile twinning was observed from 298 K to 698 K. At a higher deformation rate of 100 mm/min, stress-strain flow curves were ordinarily obtained only above 448 K. Below this temperature, little plastic deformation occurred. These findings imply that there is some correlation between the deformation rate and the temperature to activate twinning, although it must be considered that the deep drawing process included
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Fig. 7. Pole figures (PFs) computed from the EBSD maps shown in Fig. 6 (IC sheets). Different head speeds and working temperatures were applied during the deep drawing process: (a) 30 mm/min at 423 K (150 °C), (b) 50 mm/min at 423 K (150 °C), and (c) 50 mm/min at 623 K (350 °C).
the complex deformation modes of tension, compression, and bending. In fact, the nucleation of deformation twins in HCP polycrystals is complicated [27–29], though a more in-depth discussion of this is beyond the scope of this study.
Fig. 8. Maximum peak intensity of the pole figures and fraction of twinning boundaries obtained from the relevant EBSD measurements (IC drawn cups). TTW represents f1012g tensile twinning with a misorientation angle and an axis of 86 h1210i 7:5 . CTW is f1011g compressive twinning with 56 h1210i 7:5 . DTW denotes f1011g−f1012g double twinning with 38 h1210i 7:5 .
The microstructural evolution during the deep drawing of the TRC sheets is presented in Fig. 9. Compared with the IC cups (Fig. 6), the TRC cups possess more refined grain structures. In addition, grains aligned with the basal planes (red color) decrease, and various grain orientations are observed. The most noticeable difference in the microstructure between the IC and TRC drawn cups is twinning. Twinned grains in the TRC cups are rarely observed. As noted above, the grain size is one of the most important factors affecting the activation of twinning, and it appears that the small grain size in the TRC sheets resulted in little twinning. Overall, the grain size of the drawn cups at the high temperature of 623 K is smaller than that of the drawn cups at the low temperature of 423 K. The smallest grains are observed with the drawn cup at the working condition of 50 mm/min at 623 K. Band contrast maps with the twinning boundaries of TRC drawn cups are also presented. Tensile twinning (red lines) is not very active. A small amount of double twinning (green lines) was also observed. Most small and equiaxed grains drawn at a temperature of 623 K (Fig. 9(f)) reveal bright band contrast, which implies that they are dynamically recrystallized and possess low levels of stored energy. The pole figures (PFs) computed from the IPF maps of the TRC drawn cups are presented in Fig. 10. The initial TRC sheet possessed a broad basal peak mainly spread along the TD and minor double peaks along the RD, as shown in Fig. 2(d). During deep drawing, the overall peak intensity in the TRC sheets is smaller than that in the IC sheets. Double peaks along the RD are also clearly observed in the {0001} PFs of the drawn cups. In addition to a peak split, the overall shape of the basal
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Fig. 9. Inverse pole figure (IPF) maps and band contrast (BC) maps with tensile twinning boundaries measured from the drawn cups (TRC sheets). Different head speeds and working temperatures were applied during the deep drawing process: (a) and (b) IPF and BC for 30 mm/min at 423 K (150 °C), (c) and (d) IPF and BC for 50 mm/min at 423 K (150 °C), and (e) and (f) IPF and BC for 50 mm/min at 623 K (350 °C).
peaks are much broader than that of the initial TRC sheet. DRX with orientation spread from the basal textures appears to contribute to peak broadening. Under the working condition of 50 mm/min at 423 K (Fig. 10(b)), an off-basal texture without a basal peak split is observed. The lack of a peak split results in strong basal peak intensity as compared to the levels observed in other experimental cases. Exceptional textural evolution during deep drawing under the working condition of 50 mm/min at 423 K was also observed in the IC sheets (Fig. 7(b)). As discussed earlier, the activation of tensile twinning under these working conditions is unlikely. There are several previous reports of double peaks along the RD in magnesium alloys. Double twinning, f1011g−f1012g, with f1011g compressive twinning followed by f1012g tensile twinning, can explain the double peaks [13]. Based on a study of the slip system activity, the splitting of the basal poles along the RD can be caused by the high activation of pyramidal 〈c + a〉 slip [4]. At high temperatures, 〈c + a〉 can easily be activated due to the lower CRSS value, contributing to the formation of double peaks. Fig. 11 summarizes the maximum peak intensity of the PFs obtained from the various TRC drawn cups and the fraction of the corresponding
twin boundaries. Tensile twins were highly active in the IC drawn cups (Fig. 8) but were relatively weak in the TRC drawn cups. The effect of the grain size on twinning is more dominant during tensile twinning than it is during compressive twinning [30]. This appears to be one of the reasons for the minor amount of tensile twinning in the TRC sheets. Instead, double twinning is observed more in the TRC cups than in the IC cups, although the fraction in this case is not great, compared to that associated with tensile twinning. Considering the characteristics of compressive twins, the volume fraction of double twinning is meaningful even with the minor values. Note that a large fraction of double twining (30 mm/min at 623 K) or tensile twinning (50 mm/min at 448 K and 623 K) results in a lower basal intensity level compared to the other working conditions. At a drawing speed of 50 mm/min, elevated temperatures of 448 K and 623 K produced more tensile twins compared to the temperature of 423 K. As noted with the IC sheets, the correlation between the temperature and the deformation rate appears to be important for the activation of the tensile twinning of TRC sheets (even with small grains). A low fraction of double twinning boundaries is observed with the working conditions of 50 mm/min at 423 K, 40 mm/min at 423 K and 50 mm/min at 448 K.
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Fig. 10. Pole figures (PFs) computed from the EBSD maps shown in Fig. 6 (TRC sheets). Different head speeds (millimeter per minute, mmpm) and working temperatures were applied during the deep drawing process: (a) 30 mm/min at 423 K (150 °C), (b) 50 mm/min at 423 K (150 °C), and (c) 50 mm/min at 623 K (350 °C).
Fig. 12 shows the distribution of the grain size. In the IPFs (Figs. 6 and 9), large and small grains are mixed. We arbitrarily divided all grains into two groups, large and small grains, using the number of mapping pixels. Approximately, 300 pixels was effective to separate all grains into two groups in this experiment. In the IC sheets, the volume of large grains was greater than that of small grains. In the TRC sheets, the volume of small grains was greater than that of large grains. This
reflected that the average grain size of all grains in the IC sheets was greater than that in the TRC sheets. Particularly, the average grain size of the large grains in the IC sheets was clearly greater than that in the TRC sheets, which implied easier twinning in the IC sheets. 4. Conclusions The evolution of the texture and microstructure during the deep drawing of two different ZK60 (Mg-Zn-Zr) sheets fabricated by means of ingot casting (IC) and twin-roll casting (TRC) was investigated
Fig. 11. Maximum peak intensity of the pole figures and fraction of twinning boundaries obtained from the relevant EBSD measurements (TRC drawn cups). TTW represents f1012g tensile twinning with a misorientation angle and an axis of 8 6 h1210i 7:5 . CTW is f1011g compressive twinning with 56 h1210i 7:5 . DTW denotes f1011g−f1012g double twinning with 38 h1210i 7:5 .
Fig. 12. Grain size and volume fraction of the small and large grains.
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using TEM/EDS and EBSD. There were some differences in the microstructure and texture noted between the IC and TRC sheets and the drawn cups. Variations of the punch load and of the thickness of the cup walls were also examined. • Particles of second phases measured from TRC sheets were finer than those measured from IC samples. Most particles mainly consisted of Zn and Zr. The TS, YS and El of the TRC sheets are approximately 330 MPa, 277 MPa and 18%, respectively. The corresponding values for the IC sheets are 320 MPa, 235 MPa, and 14%, indicating the superiority of TRC sheets. • The grains of the IC samples were larger than those of the TRC samples. The volume fraction of the basal texture of the IC samples was also greater than that of the TRC samples. The TRC sheets initially showed basal peak broadening along the TD and basal peak split along the RD. • At working temperatures of 448 K to 623 K and deformation rates of 30 mm/min to 50 mm/min, the deep drawing of ZK60 sheets was successfully carried out on both the IC and the TRC samples. At 423 K, the TRC sheets were more favorable for deep drawing than the IC samples, and the deep drawability of the TRC sheets was better than that of the IC sheets at the low temperature. • Some differences in the major deformation modes between the IC and TRC sheets during the deep drawing process were found. Tensile twinning and shear bands were frequently observed in the cups made of IC sheets with large grains. Peak broadening of the basal texture along the TD and a peak split along the RD were mainly observed in the cups made of TRC sheets. Dynamic recrystallization was also evident in the TRC sheets. • The greatest deformation rate and the lowest temperature in the current study resulted in little tensile twinning during the cup drawing process for both the IC and the TRC sheets. This finding implies that a high deformation rate or a low temperature does not always result in easy twinning. Acknowledgement This research was supported by the Fundamental Research Program of the Korea Institute of Materials Science (KIMS, PNK4970). References [1] S. Ion, F. Humphreys, S.H. White, Dynamic recrystallisation and the development of microstructure during the high temperature deformation of mgnesium, Acta Metall. 30 (1982) 1909–1919. [2] A. Jain, S. Agnew, Modeling the temperature dependent e_ect of twinning on the behavior of magnesium alloys AZ31B sheet, Mater. Sci. Eng. A 462 (2007) 29–36. [3] A. Chapuis, J.H. Driver, Temperature dependency of slip and twinning in plane strain compression magnesium single crystals, Acta Mater. 59 (2011) 1986–1994. [4] S.R. Agnew, M.H. Yoo, C.N. Tome, Application of texture simulation to understanding mechanical behavior of mg and solid solution alloys containing Li and Y, Acta Mater. 49 (2001) 4277–4289.
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