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引線片沖裁件沖裁工藝及模具設計【沖孔落料級進?！?沖孔落料級進模,引線片沖裁件沖裁工藝及模具設計【沖孔落料級進模】,引線,片沖裁件沖裁,工藝,模具設計,沖孔,落料級進模 Journal of Materials Processing Technology 170 (2005) 181186 Stamping formability of pure titanium sheets Fuh-Kuo Chen , Kuan-Hua Chiu Department of Mechanical Engineering, National Taiwan University, Taipei 10764, Taiwan, ROC Received 20 October 2003; received in revised form 12 April 2005; accepted 4 May 2005 Abstract Because of hexagonal close-packed (HCP) crystal structures, commercially pure titanium (CP Ti) shows low ductility at room temperature, and requires thermal activation to increase its ductility and formability. In the present study, the formability of CP Ti sheets at various temperatures was studied by the experimental approach. Tensile tests were first conducted to investigate the mechanical behavior of CP Ti sheets at various temperatures. Forming limit tests, V-bend tests, and cup drawing tests were also performed to examine the stamping formability of CP Ti sheets at various temperatures. The experimental results indicate that CP Ti sheets could be formed into shallow components at room temperature, although the formability is limited in cold forming. In addition, the results obtained from the V-bend tests reveal that springback can be reduced at elevated forming temperatures. The experimental results obtained in the present study can be of help to the die design of stamping CP Ti sheets. 2005 Elsevier B.V. All rights reserved. Keywords: Pure titanium sheet; Formability; Forming limit; V-bend; Springback 1. Introduction In the present study, the formability of stamping CP Ti mercially rial from f of Among ing of ics phone, at (HCP) at temperature Ho 14 T 0924-0136/$ doi:10.1016/j.jmatprotec.2005.05.004 Due to its lightweight and high specific strength, com- pure titanium (CP Ti) has been a potential mate- for structural components, and attracts much attention the electronics industry recently. The principal manu- acturing process of CP Ti has been press forming because its competitive productivity and superior performance. the fabrication processes of press forming, stamp- of CP Ti sheets is especially important for the production thin-walled structural components used in the electron- products, such as the cover cases of notebook, mobile etc. The CP Ti sheet usually exhibits limited ductility room temperature because of its hexagonal close-packed structure. Although the formability can be improved elevated temperatures, a manufacturing process at room is always desired for the cost-effective reason. wever, most research of CP Ti is focused on microstructure , and the literature regarding formability of stamping CP i sheets is not profound. Corresponding author. Tel.: +886 2 33662701; fax: +886 2 3631 755. E-mail address: fkchenccms.ntu.edu.tw (F.-K. Chen). sheets mechanical ranging e acteristics and ments. 2. temperatur mation mentioned room temperatures. properties tests room of test see front matter 2005 Elsevier B.V. All rights reserved. was investigated using the experimental approach. The properties of CP Ti sheets at various temperatures from room temperature to 300 C were obtained from xperimental results. In addition, the important forming char- of CP Ti sheets, such as forming limit, springback, limiting drawing ratio, were also examined by experi- Mechanical properties tests at various es The stressstrain relations are the fundamental infor- for the study of formability of a sheet metal. As above, the formability of CP Ti sheets is limited at temperature and can be improved at elevated forming In order to examine the variety of mechanical of CP Ti sheets at different temperatures, tensile were performed at various temperatures ranging from temperature to 300 C and under different strain rates 0.1, 0.01, 0.001, and 0.0001/s, respectively. The tensile specimens made of JIS Grade 1 CP Ti sheets of 0.5 mm 182 Processing Fig. imens thickness The rolling rolling along machine. w were were w were con and neering for mens The Fig. lar tions, is w directions. by occurs of deformation higher in gation. T respecti of a the from three F.-K. Chen, K.-H. Chiu / Journal of Materials 1. True stressstrain relations at room temperature obtained from spec- in the three directions. were prepared according to the ASTM standards. specimens were cut along planes coinciding with the direction (0 ), and at angles of 45 and 90 to the direction. The specimens were wire cut to avoid burrs the edge. The tensile tests were conducted using an MTS 810 test For tests at elevated temperatures, a heating furnace as mounted on the MTS810 test machine. The specimens heated to 100, 200, and 300 C before the tensile tests performed. During tests, the temperature of specimen as kept constant until the specimen was stretched to failure. In the present study, the engineering stressstrain relations first obtained from the experimental data and then were verted into the true stressstrain relations according to = 0 (1 + e) and = ln(1 + e), where and were true stress true strain, 0 and e were engineering stress, and engi- strain, respectively. The true stressstrain relations CP Ti sheets at room temperature obtained from speci- cut in the three different orientations are shown in Fig. 1. anisotropic behavior is observed in Fig. 1. It is seen in 1 that the 0 specimen has a higher yield strength and a ger elongation than the specimens in the other two direc- the difference in elongation being more significant. It also observed that the 0 specimen displays a significant ork-hardening property among the specimens in the three These results are consistent with those obtained Ishiyama et al. 5. They found that the slip deformation in both the 0 and 90 directions in the beginning stage the test. During further deformation stage, the twinning increases faster in the 0 direction and produces resistance against the slip of dislocations, resulting larger values in yield strength, work hardening, and elon- The average yield stress and elongation of the CP i sheet at room temperature are about 352 MPa and 28%, vely. Though the values of yield stress and elongation the CP Ti sheet at room temperature are not favorable in deep drawing process compared to those of carbon steels, y are feasible for stamping of relatively shallow products the formability point of view. Fig. 2 shows the original and deformed specimens in the directions. It is noticed in Fig. 2 that the 0 specimen under specimen mode mation at dif ti for in noticed to in that obtained ous for relations formed CP atures. the that room elongation 100 Fig. men Technology 170 (2005) 181186 Fig. 2. Original and deformed specimens in the three directions. goes uniform deformation before fracture, while the 90 displays an obvious necking, and the deformation of 45 specimen lies between those of other two modes. In order to examine the effect of strain-rate on the defor- of CP Ti sheets, the tensile tests were also performed room temperature under different ram speeds, resulting in ferent strain-rates of 0.1, 0.01, 0.001, and 0.0001, respec- vely. The true stressstrain relations at various strain-rates the 0 specimen are shown in Fig. 3. A significant drop the stressstrain curves from strain-rate 0.1 to 0.001 is in Fig. 3, and the stressstrain curves become close each other afterwards. The same trends are also observed the tensile tests for the 45 and 90 specimens. It indicates a stable stressstrain relations for CP Ti sheets can be under the strain-rates smaller than 0.001. The true stressstrain relations of CP Ti sheets at vari- temperatures ranging from room temperature to 300 C the specimen of 0 direction are shown in Fig. 4. The shown in Fig. 4 are obtained from the tests per- at strain-rate of 0.001. It is seen in Fig. 4 that the Ti sheet exhibits better formability at elevated temper- The stressstrain curves get lower proportionally to increase of testing temperature. It is to be noted in Fig. 4 the elongation of the specimen does not increase from temperature to 100 C as expected, on the contrary, the gets smaller when the specimen is heated up to C. However, the elongation becomes larger at testing 3. True stressstrain relations at various strain-rates (1/s) for 0 speci- at room temperature. Processing Fig. Fig. temperatures room happens the ature The 0 ning producing and r transv uniaxial obtained Fig. 90 from F.-K. Chen, K.-H. Chiu / Journal of Materials 4. True stressstrain relations at various temperatures for 0 specimen. 5. True stressstrain relations at various temperatures for 45 specimen. higher than 100 C. The greater elongation at temperature is quite unusual. But this phenomenon only to the 0 specimen. For the 45 and 90 specimens, elongation continuously increases as the testing temper- gets elevated, as shown in Figs. 5 and 6, respectively. greater elongation at room temperature occurred in the specimen might be due to the fast increase of the twin- deformation in the 0 direction at room temperature, higher resistance against the slip of dislocations, resulting in a larger elongation. Another index of anisotropy is the plastic strain ratio, i.e. -value, which is defined as the ratio of plastic strain in the erse direction to that in the thickness direction in a tensile test. In the present study, the r-value was from the tensile tests for specimens of 0 ,45 , and 6. True stressstrain relations at various temperatures for 90 specimen. 0 v e than sheets r 3. ing present and temperatures relating ing 3.1. forming accepted metal performed using electrochemically deformed ing ellipse The strains same from Similar entations each at spherical measured specimen plotted nate, the limit a fracture be its temperature. from at Technology 170 (2005) 181186 183 directions at room temperature. The r-values measured specimens stretched to 20% are 4.2, 2.2, and 2.1 for the ,45 , and 90 specimens, respectively. Since a higher r- alue indicates better drawability, it shows that CP Ti sheets xhibit better deep drawing quality in the rolling direction the other two directions. Also the anisotropy of CP Ti was confirmed again from the significant difference of -values. Stamping formability of CP Ti sheets In addition to the basic mechanical properties, the stamp- formability of CP Ti sheets was also examined. In the study, the forming limit tests at room temperature, the V-bend tests and circular cup drawing tests at various were performed. The test results were discussed to the forming properties of CP Ti sheets in a stamp- process. Forming limit tests Since Keeler and Backofen 6 introduced the concept of limit diagram (FLD) in 1963, it has been a widely criterion for the fracture prediction in the sheet- forming. To determine an FLD, stretching tests were for sheet-metal specimens of different widths a semi-spherical punch. The specimens were first etched with circular grids that would be into ellipses after being stretched. The engineer- strains measured along the major- and minor-axes of the are termed the major- and minor-strain, respectively. y are also the principal strains on the plane where the are measured. In the present study, rectangular specimens having the length of 100mm, but with different widths ranging 10 to 100 mm in an increment of 10 mm, were tested. to tensile tests, the CP Ti sheet was cut at three ori- to the rolling direction, i.e., 0 ,45 , and 90 , for size of specimen. During the tests, specimens clamped periphery were stretched to failure over a 78 mm semi- punch. The engineering major- and minor-strains in the location closest to the fracture for each were recorded. The major- and minor-strains were against one another with the major strain as the ordi- and the curve fitted into the strain-points was defined as forming limit curve. The diagram showing this forming curve is called the forming limit diagram. The FLD is very useful criterion for the prediction of the occurrence of in a stamping process. According to the previous analysis, the CP Ti sheet could formed at room temperature. In order to further confirm feasibility, the forming limit tests were performed at room Fig. 7 shows the forming limit curve obtained the test results. It is seen in Fig. 7 that the major strain the lowest point of the curve, which is also the plane strain 184 Processing Technology 170 (2005) 181186 deformation or stamping in sheets uf CP 3.2. of process. to forming is has punch from pared. of F mens used insignificant test. 100, Fig. for angles springback and ti F.-K. Chen, K.-H. Chiu / Journal of Materials Fig. 7. Forming limit curve at room temperature. mode, is 0.34. Compared with cold-rolled steels stainless steels, this value is a little lower. However, for of shallow products, the forming limit curve shown Fig. 7 indicates a greater possibility of forming of CP Ti at room temperature. This makes it possible to man- acture electronics components at room temperature using Ti sheets. V-bend tests Since CP Ti has a lower value of elastic modulus than that steel, springback could be much significant in a bending In the present study, the V-bend tests were performed examine the springback property of CP Ti sheets at various temperatures. The tooling used in the V-bend tests shown in Fig. 8. It can be seen in Fig. 8 that the lower die an opening angle of 90 . In order to study the effect of radius on springback, the tooling sets with punch radii 0.5 to 5.0 mm, in an increment of 0.5 mm, were pre- The CP Ti sheet with a thickness of 0.5 mm, a length 60 mm, and a width of 15 mm was used as specimens. or tests at elevated temperatures, both tooling and speci- were enclosed in a heating furnace. No lubricant was in the V-bend test since the frictional condition has an effect on the springback occurred in the V-bend The bending tests were conducted at room temperature, 200, and 300 C, respectively. After bending tests, the Fig. 8. Tooling used in the V-bend tests. for smaller bend, noted back sheet arc to comple springback Fig. imens 9. Relations between springback and punch radius at room temperature specimens of three directions. of bent specimens were measured by a CMM, and the angles were calculated. Figs. 9 and 10 show the relationships between springback punch radius at room temperature and 300 C, respec- vely. It is seen in both figures that the springback decreases smaller punch radii regardless of temperature change. The punch radius causes larger plastic deformation at the and hence reduces the effect of springback. It is also in both Figs. 9 and 10 that negative values of spring- occur for smaller punch radii. This is because that the on the straight sides of V-shape is deformed into an at the beginning of bending process, and the load applied flatten the arc at the end of bending process results in a x stress distribution that causes a negative value of 7. Comparing both figures, it is observed that 10. Relations between springback and punch radius at 300 C for spec- of three directions. Fig. 11. Punch and die used in circular cup drawing tests. Processing springback re CP rience kno and does the of ing ele 3.3. ratio punch process, of ing the cup ture, used temperatures. cess, adjusted If force w the fracture, simultaneously could a also in punch in ble F.-K. Chen, K.-H. Chiu / Journal of Materials Fig. 12. Drawn cups at various decreases as the forming temperature increases gardless of the dimension of punch radius. It indicates that Ti sheets not only have better formability but also expe- less springback at higher forming temperatures. It is wn that springback is affected by both the elastic modulus the yield stress of the material. Since the elastic modulus not vary too much with the change of temperature, and yield stress of CP Ti sheets decreases with the increase temperature, the decrease of springback at higher form- temperatures is due to the lower yield stress of CP Ti at vated temperatures. Circular cup drawing tests The limiting drawing ratio (LDR), which is defined as the of the largest diameter of circular blank (Do) to the diameter (Dp) in a successful circular cup drawing is a popular index used to describe the formability sheet metals. A larger value of LDR implies a larger draw- depth, that is, a better formability. In the present study, punch and die shown in Fig. 11 were used for the circular drawing tests. Tests were performed at room tempera- 100, and 200 C, respectively. The heating apparatus in the tensile tests was adopted for the tests at elevated In order to obtain a successful drawing pro- the blank size and blank-holder force were adaptively to eliminate the defects such as fracture and wrinkle. the fracture appeared in a drawing test, the blank-holder would be adjusted to a smaller value until the fracture as eliminated without the occurrence of wrinkles. When adjustment of blank-holder force failed to eliminate the an attempt of reducing the blank size would be tried to avoid the fracture. A reverse methodology be adopted to suppress the occurrence of wrinkles in drawing test. However, in an LDR test, the blank size is acting as a parameter to determine the value of LDR addition to the use of the above adjustment. Since the diameter is 35 mm, the blank diameter is increased an increment of 3.5 mm from 70 mm to the largest possi- diameter for the convenience of calculating the values of T T T Room 100 200 LDR. tests is clearly increase figure ous becomes ues are peratures. the LDR from 200 force dra sheets of room 4. ing ducting the and indicate Technology 170 (2005) 181186 185 forming temperatures. able 1 est results of circular cup drawing emperature Blank diameter (mm) LDR Blank-holder force (kN) Drawing depth (mm) temperature 77 2.2 2.75 20 C 84 2.4 3.5 29 C 101.5 2.9 4.0 40 MoS 2 was used as lubricant in all circular cup drawing conducted in the present study, and the drawing speed 0.2 mm/s. Fig. 12 shows the drawn cups at various temperatures. It is seen in Fig. 12 that the drawing depth increases as the of forming temperature. It is also to be noted in this that the earing shapes of the drawn cup formed at vari- temperature are quite different. The earing phenomenon significant at higher forming temperatures. The val- of LDR, drawing depth, and related process parameters listed in Table 1 for the tests conducted at various tem- It is noticed in Table 1 that all values increase as forming temperature increases. However, the increase of and drawing depth is not so significant in the range room temperature to 100 C, but gets larger from 100 to C. It is also noted in Table 1 that a larger blank-holder is required for the larger blank size to be successfully wn at a higher temperature. The value of LDR of CP Ti is 2.2 at room temperature, which is comparable to that carbon steels, indicating that stamping of CP Ti sheets at temperature is feasible. Concluding remarks The formability of stamping CP Ti sheets at various form- temperatures was investigated in the present study by con- various experiments. The mechanical properties of CP Ti sheet at various temperatures were first examined, the stressstrain relations obtained from the experiments that the CP Ti sheet has a higher yield stress and 186 F.-K. Chen, K.-H. Chiu / Journal of Materials Processing Technology 170 (2005) 181186 a smaller elongation at room temperature, but proportionally decreases in yield stress and increases in elongation when the sheet to tensile could ture, limit is major the cular LDR dra sheet perature. dra icant of temperature sho strain-rate e springback Also used the present study provide the fundamentals for the stamping die design of forming CP Ti sheets. Ackno Council this which Refer 1 2 3 4 5 6 7 is heated to an elevated temperature up to 300 C. It is be noted that the stressstrain relations obtained from the tests at room temperature indicate that the CP Ti sheet be formed into shallow components at room tempera- although the yield stress is a little higher. The forming diagram of the CP Ti sheet obtained at room temperature not so high as those of cold-ro

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