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1、Journal of Materials Processing Technology 211 (2011) 644649 Contents lists available at ScienceDirect Journal of Materials Processing Technology journal homepage: Cold stamping formability of AZ31B magnesium alloy sheet undergoing repeated unidirectional bending process Lei Zhanga,b, Guangsheng Hua

2、nga,b, Hua Zhanga,b, Bo Songa,b aNational Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400030, China bCollege of Material Science and Engineering, Chongqing University, Chongqing 400030, China a r t i c l ei n f o Article history: Received 11 April 2010 Received

3、in revised form 7 November 2010 Accepted 28 November 2010 Available online 7 December 2010 Keywords: Magnesium alloy sheet RUB Texture Stamping formability Cell phone housing a b s t r a c t The repeated unidirectional bending (RUB) process was carried out on an AZ31B magnesium alloy in order to inv

4、estigate 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 confi rmed that cell phone housings can be stamped successfully

5、 in crank press using the RUB processed AZ31B magnesium alloy sheet. The improvement of the stamping formabilityatroomtemperaturecanbeattributedtothetexturemodifi cations,whichledtoaloweryield strength, a larger fracture elongation, a smaller Lankford value (r-value) and a larger strain hardening ex

6、ponent (n-value). 2010 Elsevier B.V. All rights reserved. 1. Introduction Nowadays,theproductsofmagnesiumalloys,mainlyformedby castinganddie-casting,areusedintheaerospace,automobile,civil- ian household appliances. Compared with casting and die-casting, plastic forming technology seems to be more at

7、tractive because of its competitive productivity and performance. Among the fabri- cation 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, magne- sium alloy sheets have lo

8、w 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 draw- ingofcommercialmagnesiumalloysheets,theydemonstratedthat the limiting drawing ratio of rolled AZ31 magnesium alloy sheets an

9、nealed at 773K can reach 1.7. Mori et al. (2009) have shown that a two-stage cold stamping process are also helpful for forming magnesiumalloycups.Watanabeetal.(2004)suggestedtheductil- ityofmagnesiumalloysheetscanbeimprovedbyreducing(0002) basal texture at room temperature. The limiting drawing rat

10、io for the cold deep drawing of commercial magnesium alloy sheets can beimprovedfrom1.2to1.4byreducing(0002)basalplanetexture Corresponding author at: National Engineering Research Center for Magne- sium Alloys, College of Material Science and Engineering, Chongqing University, 174 Shazheng Street,

11、Chongqing 400030, China. Tel.: +86 23 65112239; fax: +86 23 65102821. E-mail address: (G. Huang). (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 signifi cantly the tensile elongation (Kim et

12、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 tex- ture obtained by cross-roll rolling (Chino et al., 2006) and different speed rolling (DSR) process (X.S. Huang et al., 2009), exhibit higher stamping formabili

13、ty 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 form- ing refer to

14、 a “special bending sheet,” which was produced by Dow Magnesium. The special bending sheet with a modifi 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 form

15、ing of magnesium alloy sheets by weakening basal texture of sheets. The Erichsen values of the RUB processed sheet signifi 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 formabil

16、ity 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 technolog

17、y of magnesium alloy. 0924-0136/$ see front matter 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2010.11.019 L. Zhang et al. / Journal of Materials Processing Technology 211 (2011) 644649645 Fig. 1. Schematic diagram of the RUB process. In this paper, an investigation of the drawa

18、bility of RUB pro- cessed 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 p

19、reparation of experimental material Commercial AZ31B magnesium alloy sheets with a thickness of 0.8mm, cut into 1000mm100mm (lengthwidth) pieces, were used in the experiments. Fig. 1 shows the schematic diagram of the RUB process. The radius of the cylindrical support was 1mm and thebendinganglewas9

20、0.Themagnesiumalloysheetwasbenton acylindricalsupportunderaconstantforceTwithaconstantspeed 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 b

21、ending orientation was also changed in the next pass. The RUB processed sheets were annealed at 533K for 60min, and then were subjected to tensile tests, deep drawing, and cold stamping of cell phone housing investigation. Previous studies (Song et al., 2010; Huang et al., 2010) have investigated mi

22、crostructure and texture evolution of AZ31 mag- nesium alloy sheets underwent RUB. For the as-received sheet, the grains were fi ne. After the magnesium alloy sheet underwent RUB was annealed at 260C, the grains near the surface of sheet grew obviously, while those in the central region had little g

23、rowth. The average grain size of two state sheets was almost the same. Fig. 2 shows 0002 pole fi gures of two state sheets. The as- received 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 co

24、ntrast, the RUB processed sheets Fig. 3. Schematic diagram of mold. Table 1 Parameters of punch and die used in the experiment. Punch diameter, dp (mm) Punch shoulder radius, rp(mm) Die clearance, z (mm) Die shoulder radius, rd(mm) 5051.289.1 exhibit a large inclination of c-axis around the normal d

25、irection (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 57mm, a width of 12.5mm and a thickness of 0.8mm. The specimens were cut along planes coinciding with at the angles of 0(RD) and 45 and 90(TD)

26、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

27、 3102s1to exam- inethemechanicpropertiessuchastheyieldstrength,theultimate tensile strength and the fracture elongation. The strain hardening exponent values (n-value) were obtained by power law regression (? =?n) of the tensile test data within a uniform strain of =15%. The Lankford values (r-value

28、), r = w/t, where the variables w and tdenote the strains in the tensile specimens transverse and thickness directions, respectively, were measured on the speci- mens at a uniform plastic deformation of =15%. 2.3. Limiting drawing ratio (LDR) tests To evaluate the deep drawability of the RUB process

29、ed AZ31 magnesium alloy sheet, limiting drawing ratio (LDR) tests were carried out on a 600kN hydraulic press to examine the stamp- ing 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 pro

30、cessed into circular Fig. 2. 0002 Pole fi gures of as-received sheet and RUB sheet. (a) as-RUB sample, max density=8.66; (b) RUB sample, max density=7.31. 646L. Zhang et al. / Journal of Materials Processing Technology 211 (2011) 644649 specimens with various diameter dimensions using wire-cutting.

31、Before deep drawing, all circular specimens should be polished by theabrasivepaperinordertoavoidcrackinthem.Specialposition- ing ring was adopted to fi x the specimens. A rigid blank holder was used on the molds, which can offer suffi cient blank holder force to press the blank tightly by adjusting

32、the spring. Consequently, the blank holder and die were uniformly lubricated with oil. The punch was not lubricated. 2.4. Cold stamping of cell phone housing The as-received sheets and the RUB processed AZ31 magne- sium alloy sheets with a thickness of 0.6mm were used in these experiments; three set

33、s 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 structureofdeepdrawingdiewasmorecomplex.Themainparam- eters of deep drawing die were as follows: punch radius rp=1mm; die radius rd=2

34、mm; die clearance in the straight wall C=0.6mm; dieclearanceinthecornerC=0.66mm.Thethreesetsofdiesdriven by the crank press completed the blanking, the deep drawing and the piercing process in turn. 3. Results 3.1. Mechanical properties Fig. 4 shows that the true stressstrain curves of the as-receiv

35、ed specimens and the RUB processed specimens in the tensile direc- tions of RD, 45and TD. Compared with the as-received specimens, the RUB processed specimens exhibit larger in-plane anisotropy, and the signifi cant differences can be observed from the true stressstrain curves at the beginning stage

36、 of the tensile defor- mation. The work-hardening effects are stronger for the tensile specimens in the tensile directions of RD, 45and 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 spe

37、cimens are nearly the same as that of the as-received specimensregardlessofthetensiledirections.Whileyieldstrength of the RUB processed specimens is signifi cantly lower than that of the as-received specimens especially in the RD. These results indi- cate that the RUB process has a strong effect on

38、the yield strength but not the tensile strength. Additionally, the fracture elongations of the RUB processed specimens are improved in the tensile direc- tions of RD, 45and TD in comparison with those of the as-received specimens, especially in the RD with the largest increase from 19.2% to 26.7%. T

39、hese are mainly due to the RUB processed spec- Fig. 4. The true stressstrain curves of the as-received specimens and the RUB pro- cessed specimens in the tensile directions of RD, 45and TD (RD, rolling direction; TD, transverse directions). imens with stronger work-hardening effects which contribute

40、 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. Ther-valueandthen-valueoftheas-receivedspecimensandthe RUB processed specimens are sho

41、wn 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 decreasesfrom2.15to0.92andincreasesfrom0.20to0.29,respec- tively. The difference between r-values as well as that between n-values of the as

42、 received specimens and the RUB processed specimens decreases with increasing the tensile angle. The aver- age r-value ( r = (rRD+ 2r45+ rTD)/4) falls from 2.45 to 1.36, and the average n-value ( n = (nRD+ 2n45+ nTD)/4) rises from 0.175 to 0.225 in comparison with those of the as-received specimens.

43、 The decrease in r indicates that it is easier to reduce or increase the thicknessofsheetduringtheplasticdeformation.Furthermore,the improvementinthefractureelongationwasmainlyduetothehigh n which resulted in a low sensitivity to strain localization in the form of necking. 3.2. LDR Drawingratio(DR)i

44、scommonlyexpressedbyRD=d0/dp,where d0and dpare the blank diameter and punch diameter, respectively. The LDR is the one when the specimen is on the verge of fracture. Fig. 5. (a) Tensile strength and yield strength, (b) fracture elongation of the as-received specimens and the RUB processed specimens

45、in the tensile directions of RD, 45and TD. L. Zhang et al. / Journal of Materials Processing Technology 211 (2011) 644649647 Fig. 6. r-Value and n-value of the as-received specimens and the RUB processed specimens in the tensile directions of RD, 45and TD. Fig. 7 shows cold deep drawn cups of the as

46、-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 7.2mm. However, the drawn cup of the RUB processed specimens showed a good appearance at a drawing depth of 11.8mm. Compared with the as-received

47、specimens, the RUB pro- cessed specimens show better stamping formability. These are mainly due to the RUB processed specimens with a tiled tex- ture, which contribute to the increase in the drawing depth. If the drawing depth went up to 14.8mm, the fracture occurred at the edge of the fl ange for t

48、he RUB processed specimens dur- ing deep drawing. Yang et al. (2008) investigated die as shown in Fig. 8(a), the force was not applied onto the edge using the fl at blank holder. To apply the force onto the edge even in pass- ing though the die corner, the blank holder was exchanged for that having

49、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 bendingunbending as the material passes over the die radius. Those previous observations po

50、int 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 twinning- induced tensioncompression strength asymmetry in two sides of the bending blank (Agnew et al., 2006). The b

51、lank holder with a ring-shaped projection is employed instead of the fl at bank holder after the edge of the fl ange breaking out of the fl 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 holderwitharing

52、-shapedprojectioninanintermediatestageofthe 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 6mm, thus the subsequent cold stamping proces

53、s of cell phone housing is carried out using one-step and fl at blank holder. 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.

54、 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 fl ange of the cup resulted in the increase in thickness Fig. 7. Cold deep drawn cups with different drawing dept

55、h of as-received specimen and the specimen undergoing RUB process for DR=1.5. 648L. Zhang et al. / Journal of Materials Processing Technology 211 (2011) 644649 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

56、 blank holder force. Fig. 9. Cold deep drawn cup using the blank holder with a ring-shaped projection. Fig. 10. Distributions of wall thickness strain of drawn cups for =1.5. during deep drawing. For the RUB processed sheets with a tilted basal texture, the thickness strain can be generated by basal

57、 slip. Fig.11. Theresultsofcoldstampingofcellphonehousings:(a)as-receivedsample; (b) the RUB processed specimen. 3.3. Cold stamping of cell phone housings PreliminaryexperimentalresultsdemonstratethattheRUBpro- cess has an important infl uence on the stamping formability of AZ31 magnesium alloy shee

58、ts. Fig. 11 shows the results of cold stamping of cell phone housings. The as-received specimen was drawnunsuccessfully,asshowninFig.11(a).Itcanbefoundthatthe critical section at the punch shoulder was broken before the fl ange of the specimen was fully dragged into the die cavity. While the RUB pro

59、cessed specimen was drawn successfully, the critical sec- tion at the punch shoulder and the fl ange was excellent, as shown in Fig. 11(b). The experimental results show that the RUB pro- cessimprovedtheshallowdrawingformabilityofmagnesiumalloy sheets. Besides, certainly, cell phone housings can be obtained suc- cessfully in crank press using the RUB processed AZ31 specimens by the cold stamping process. 4. Discussion G.S.Huangetal.(2009)revealedthatmechanicalpropertiesand stretch formability of magnesium alloy sheets with a tilted basal texture obtained by the RUB process were imp