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Cold stamping formability of AZ31B magnesium alloy sheet undergoing repeated unidirectional bending process
Lei Zhanga,b, Guangsheng Huanga,b, * , Hua Zhanga,b , Bo Song a,b
a National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400030, China
b College of Material Science and Engineering, Chongqing University, Chongqing 400030, China
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a r t i c l e i n f o
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Article history:
Received 11 April 2010
Received in revised form 7 November 2010
Accepted 28 November 2010
Available online 7 December 2010
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a b s t r a c t
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The repeated unidirectional bending (RUB) process was carried out on an AZ31B magnesium alloy in order to investigate its effects on the cold stamping formability. The limiting drawing ratio (LDR) of the RUB processed magnesium alloy sheet with an inclination of basal pole in the rolling direction can reach 1.5 at room temperature. It was also con?rmed that cell phone housings can be stamped successfully in crank press using the RUB processed AZ31B magnesium alloy sheet. The improvement of the stamping formability at room temperature can be attributed to the texture modi?cations, which led to a lower yield strength, a larger fracture elongation, a smaller Lankford value (r-value) and a larger strain hardening exponent (n-value).
? 2010 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy sheet;RUB;Texture;Stamping formability；
Cell phone housing
1. Introduction
Nowadays, the products of magnesium alloys, mainly formed by casting and die-casting, are used in the aerospace, automobile, civilian household appliances. Compared with casting and die-casting, plastic forming technology seems to be more attractive because of its competitive productivity and performance. Among the fabrication
processes of plastic forming, stamping of magnesium alloy sheets is especially important for the production of thin-walled structural components (Chen and Huang, 2003). However, magnesium alloy sheets have low ductility at room temperature due to its
strong (0002) basal texture, as shown in the literature (Doege and Droder, 2001). Mori and Tsuji (2007) investigated cold deep drawing of commercial magnesium alloy sheets, they demonstrated that the limiting drawing ratio of rolled AZ31 magnesium alloy sheets
annealed at 773 K can reach 1.7. Mori et al. (2009) have shown that a two-stage cold stamping process are also helpful for forming magnesium alloy cups. Watanabe et al. (2004) suggested the ductility of magnesium alloy sheets can be improved by reducing (0002) basal texture at room temperature. The limiting drawing ratio for the cold deep drawing of commercial magnesium alloy sheets can be improved from 1.2 to 1.4 by reducing (0002) basal plane texture (Iwanaga et al., 2004). It is well-known that equal channel angular pressing (ECAP) is an effective method to obtain a tilted basal texture, which improved signi?cantly the tensile elongation (Kim et al., 2003). But it is hard for ECAP to fabricate thin sheet. Recently, it is reported that a rolled magnesium alloy sheet, with a tiled texture obtained by cross-roll rolling (Chino et al., 2006) and different speed rolling (DSR) process (X.S. Huang et al., 2009), exhibit higher stamping formability compared with a rolled magnesium alloy sheet by normal-roll rolling. It is therefore important to improve the formability at room temperature for a wide use of magnesium alloy sheets by changing or weakening the basal texture.
Older versions of the ASM Metals Handbook (1969) on forming refer to a “special bending sheet,” which was produced by Dow Magnesium. The special bending sheet with a modi?ed crystallographic texture, had better forming characteristics than conventional AZ31 sheet.
Previous study (G.S. Huang et al., 2009) revealed that the RUB process also improved the stretch forming of magnesium alloy sheets by weakening basal texture of sheets. The Erichsen values of the RUB processed sheet signi?cantly increased from 3.53 to 5.90 in comparison with the cold-rolled magnesium alloy sheet. However, up to now, few researchers made efforts to study the cold stamping formability of the magnesium alloy sheets. Cold stamping products, such as housings of laptop computers and cell phones, have not been reported in other investigations. Hence, it is important to investigate the cold deformation behaviors so as to establish fundamental knowledge of the cold forming technology of magnesium alloy.
In this paper, an investigation of the drawability of RUB processed AZ31 magnesium alloy sheet was performed at room temperature using uniaxial tensile tests, deep drawing and cold stamping of a cell phone housing. The performance of RUB sheet was compared with that of the as-received sheet.
2. Experimental material and procedure
2.1. The preparation of experimental material
Commercial AZ31B magnesium alloy sheets with a thickness of 0.8 mm, cut into 1000 mm × 100 mm (length × width) pieces, were used in the experiments. Fig. 1 shows the schematic diagram of the RUB process. The radius of the cylindrical support was 1 mm andthe bending angle was 90 ? . The magnesium alloy sheet was bent on a cylindrical support under a constant force T with a constant speed v. There was six-pass bending, which indicated that there were six bending operations in all at two orientations in the experiment. This meant that after each bending pass, the sheet was turned over and the bending orientation was also changed in the next pass. The RUB processed sheets were annealed at 533 K for 60 min, and then were subjected to tensile tests, deep drawing, and cold stamping of cell phone housing investigation.
Fig. 1. Schematic diagram of the RUB process.
Fig. 2. {0002} Pole ?gures of as-received sheet and RUB sheet. (a) as-RUB sample, max density = 8.66; (b) RUB sample, max density = 7.31.
Previous studies (Song et al., 2010; Huang et al., 2010) have investigated microstructure and texture evolution of AZ31 magnesium alloy sheets underwent RUB. For the as-received sheet, the grains were ?ne. After the magnesium alloy sheet underwent
RUB was annealed at 260 ? C, the grains near the surface of sheet grew obviously, while those in the central region had little growth. The average grain size of two state sheets was almost the same. Fig. 2 shows {0002} pole ?gures of two state sheets. The asreceived
sheets exhibit a strong basal texture, where the majority of grains are oriented with their
{0002} basal planes parallel to the rolling plane of the sheet. In contrast, the RUB processed sheets exhibit a large inclination of c-axis around the normal direction
(ND) towards the RD, which weakens basal texture of the sheet.
2.2. Uniaxial tensile tests
The specimens for tensile tests had a parallel length of 57 mm, a width of 12.5 mm and a thickness of 0.8 mm. The specimens were cut along planes coinciding with at the angles of 0 ? (RD) and 45 ? and 90 ? (TD) to the rolling direction. Prior to testing, all specimens were polished by the abrasive paper to remove major scratches to avoid fracture occurring at an undesired location of the specimen. The uniaxial tensile tests were carried out on a CMT6305-300 KN testing machine with an initial strain rate of 3 × 10 -2 s -1 to examine the mechanic properties such as the yield strength, the ultimate tensile strength and the fracture elongation. The strain hardening exponent values (n-value) were obtained by power law regression (_ = _e n ) of the tensile test data within a uniform strain of e =15%. The Lankford values (r-value), r= e w /e t , where the variables e w and e t denote the strains in the tensile specimens’ transverse and thickness directions, respectively, were measured on the specimens at a uniform plastic deformation of e =15%.
2.3. Limiting drawing ratio (LDR) tests
To evaluate the deep drawability of the RUB processed AZ31 magnesium alloy sheet, limiting drawing ratio (LDR) tests were carried out on a 600 kN hydraulic press to examine the stamping formability at room temperature. The schematic diagram and geometry dimension of mold are shown in Fig. 3 and Table 1, respectively. Magnesium alloy sheets were processed into circular specimens with various diameter dimensions using wire-cutting. Before deep drawing, all circular specimens should be polished by the abrasive paper in order to avoid crack in them. Special positioning ring was adopted to ?x the specimens. A rigid blank holder was used on the molds, which can offer suf?cient blank holder force to press the blank tightly by adjusting the spring. Consequently, the
blank holder and die were uniformly lubricated with oil. The punch was not lubricated.
Fig. 3. Schematic diagram of mold.
Table 1 Parameters of punch and die used in the experiment.
Punch diameter, d p
(mm)
Punch shoulder
radius, r p (mm)
Die clearance,z (mm)
Die shoulder
radius, r d (mm)
50
5
128
9.1
2.4. Cold stamping of cell phone housing
The as-received sheets and the RUB processed AZ31 magnesium alloy sheets with a thickness of 0.6 mm were used in these experiments; three sets of stamping dies for cell phone housing manufacture were used, the blanking die, deep drawing die and piercing die. Compared with the blanking and piercing die, the structure of deep drawing die was more complex. The main parameters of deep drawing die were as follows: punch radius r p = 1 mm; die radius r d = 2 mm; die clearance in the straight wall C = 0.6 mm; die clearance in the corner C = 0.66 mm. The three sets of dies driven by the crank press completed the blanking, the deep drawing and the piercing process in turn.
Fig. 4. The true stress–strain curves of the as-received specimens and the RUB processed
specimens in the tensile directions of RD, 45 ? and TD (RD, rolling direction;
TD, transverse directions).
3. Results
3.1. Mechanical properties
Fig. 4 shows that the true stress–strain curves of the as-received specimens and the RUB processed specimens in the tensile directions of RD, 45 ? and TD. Compared with the as-received specimens, the RUB processed specimens exhibit larger in-plane anisotropy,
and the signi?cant differences can be observed from the true stress–strain curves at the beginning stage of the tensile deformation. The work-hardening effects are stronger for the tensile specimens in the tensile directions of RD, 45 ? and TD after the yield deformation. The yield strength, tensile strength and the fracture elongation are shown in Fig. 5. The tensile strengths of the RUB processed specimens are nearly the same as that of the as-received specimens regardless of the tensile directions. While yield strength of the RUB processed specimens is signi?cantly lower than that of the as-received specimens especially in the RD. These results indicate that the RUB process has a strong effect on the yield strength but not the tensile strength. Additionally, the fracture elongations of the RUB processed specimens are improved in the tensile directions of RD, 45 ? and TD in comparison with those of the as-received specimens, especially in the RD with the largest increase from 19.2% to 26.7%. These are mainly due to the RUB processed spec-imens with stronger work-hardening effects which contribute to the increase in the fracture elongation. Above all, the inclination of the c-axis toward the RD lowers the yield strength but elevates work-hardening effects which contribute to improve the uniform elongation.
The r-value and the n-value of the as-received specimens and the RUB processed specimens are shown in Fig. 6. Compared with the as received specimens, the RUB processed specimens show a much smaller r-value and a larger n-value especially in the RD, which decreases from 2.15 to 0.92 and increases from 0.20 to 0.29, respectively.
The difference between r-values as well as that between n-values of the as received specimens and the RUB processed specimens decreases with increasing the tensile angle. The average r-value (ˉ r = (r RD + 2r 45 ? + r TD )/4) falls from 2.45 to 1.36, and the average n-value ( ˉ n = (n RD + 2n 45 ? + n TD )/4) rises from 0.175 to 0.225 in comparison with those of the as-received specimens. The decrease in ˉ r indicates that it is easier to reduce or increase the thickness of sheet during the plastic deformation. Furthermore, the improvement in the fracture elongation was mainly due to the high
ˉ n which resulted in a low sensitivity to strain localization in the
form of necking.
Fig. 5. (a) Tensile strength and yield strength, (b) fracture elongation of the as-received specimens and the RUB processed specimens in the tensile directions of RD, 45 ? and
Fig. 6. r-Value and n-value of the as-received specimens and the RUB processed
specimens in the tensile directions of RD, 45 ? and TD.
3.2. LDR
Drawing ratio (DR) is commonly expressed by RD = d 0 /d p , where d 0 and d p are the blank diameter and punch diameter, respectively. The LDR is the one when the specimen is on the verge of fracture.
Fig. 7 shows cold deep drawn cups of the as-received specimens and the RUB processed specimens for DR = 1.5. The as-received specimens fractured at the punch shoulder, and the drawing depth was only 7.2 mm. However, the drawn cup of the RUB processed specimens showed a good appearance at a drawing depth of 11.8 mm. Compared with the as-received specimens, the RUB processed specimens show better stamping formability. These are mainly due to the RUB processed specimens with a tiled texture, which contribute to the increase in the drawing depth. If the drawing depth went up to 14.8 mm, the fracture occurred at the edge of the ?ange for the RUB processed specimens during deep drawing. Yang et al. (2008) investigated die as shown in Fig. 8(a), the force was not applied onto the edge using the ?at blank holder. To apply the force onto the edge even in passing though the die corner, the blank holder was exchanged for
that having a ring-shaped projection in an intermediate stage of the deep drawing as shown in Fig. 8(b) (Mori and Tsuji, 2007). Additionally, for magnesium alloy sheets, the fracture happened in the top of the cup during bending–unbending as the material passes over the die radius. Those previous observations point out that compared with aluminum-alloy sheets (including AA2024, 6061,7075), magnesium alloys exhibit poor bending ductility due to their strong in-plane anisotropy and mechanical twinninginduced
tension–compression strength asymmetry in two sides of the bending blank (Agnew et al., 2006). The blank holder with a ring-shaped projection is employed instead of the ?at bank holder after the edge of the ?ange breaking out of the ?at bank holder, which is helpful to improve unbending ductility of the sheet in the die corner. Fig. 9 shows cold deep drawn cup using the blank holder with a ring-shaped projection in an intermediate stage of the
deep drawing as shown in Fig. 8(b). The LDR of the RUB processed specimens is 1.5 under present experimental conditions. However, compared to a circular cup deep drawing, the depth of cell phone housing is only 6 mm, thus the subsequent cold stamping process
of cell phone housing is carried out using one-step and ?at blank holder.
Fig. 7. Cold deep drawn cups with different drawing depth of as-received specimen and the specimen undergoing RUB process for DR = 1.5.
Fig. 8. The edge of the blank passes though the corner of the die at different pressure situations: (a) No blank holder force; (b) action of blank holder force.
Fig. 9. Cold deep drawn cup using the blank holder with a ring-shaped projection.
Fig. 10 shows the thickness strain at the angles of 0 ? (RD), 45 ? and 90 ? (TD) to the rolling direction of cold deep drawn cup for the RUB processed specimens. The valleys of the curves represent the sections of the cup corners. Despite of the different r-values in the
three directions, the values at the cup corners are approximately the same. It is well known that the stresses in the hoop directions around the ?ange of the cup resulted in the increase in thickness during deep drawing. For the RUB processed sheets with a tilted basal texture, the thickness strain can be generated by basal
slip.
Fig. 10. Distributions of wall thickness strain of drawn cups for ? = 1.5.
3.3. Cold stamping of cell phone housings
Preliminary experimental results demonstrate that the RUB process has an important in?uence on the stamping formability of AZ31 magnesium alloy sheets. Fig. 11 shows the results of cold stamping of cell phone housings. The as-received specimen was drawn unsuccessfully, as shown in Fig. 11(a). It can be found that the critical section at the punch shoulder was broken before the ?ange of the specimen was fully dragged into the die cavity. While the RUB processed specimen was drawn successfully, the critical section at the punch shoulder and the ?ange was excellent, as shown in Fig. 11(b). The experimental results show that the RUB process improved the shallow drawing formability of magnesium alloy sheets. Besides, certainly, cell phone housings can be obtained successfully in crank press using the RUB processed AZ31 specimens by the cold stamping process.
Fig. 11. The results of cold stamping of cell phone housings: (a) as-received sample;
(b) the RUB processed specimen.
4. Discussion
G.S. Huang et al. (2009) revealed that mechanical properties and stretch formability of magnesium alloy sheets with a tilted basal texture obtained by the RUB process were improved at room temperature. Agnew and Duygulu (2005) and Koike et al. (2003) have
noted that for magnesium alloy sheets with a very strong basal texture, the width strain e w can be generated by prismatic slip, while the thickness strain is generated by pyramidal slip and twinning. Therefore, this led to the high r-value and the poor deformation capability of sheet thinning for the as-received sheets in the work of X.S. Huang et al. (2009). In contrast, the thickness strain of magnesium alloy sheets, with a tilted basal texture obtained by the RUB process, can be generated by basal slip, which resulted in a lower r-value. It is generally expected that high r values favor sheet
formability and will lead to higher limiting drawing ratios (Lee, 1984). However, the RUB processed sheet exhibits a lower r value and better drawability at room temperature. The results indicate that the relationship between the r values and sheet formability of
magnesium alloys should be interpreted in a different way than is usually done for cubic metals. The lower r value means the tendency of increase in the thickness strain, which favors the formability of the drawn cup corners. Previous studies (Cheng et al., 2007; Yi et al., 2010) have reached the same conclusion, but the relation between drawability and a lower r value is unclear and further research is needed. It is reported that the sheets with a favored texture for the basal slip exhibited a superior formability in both stretch forming (G.S. Huang et al., 2009) and deep drawing (Cheng et al., 2007). Therefore, formability of magnesium alloy sheets can be improved by the RUB process weakening basal texture of the sheet. Compared with the as-received sheets, the Erichsen values of the RUB processed sheets increased to 5.90 from 3.53, which increased by 67% at most. The LDR of the RUB processed sheets can reach 1.5 from 1.2 which was proved in other study of Chino et al. (2006) at room temperature. The larger Erichsen values for the RUB processed sheets were attributed to the larger n-value and the smaller r-value, which enhanced the capability of sheet thinni