冲压纯净的钛板料的可锻模性@中英文翻译@外文翻译

Journal of Materials Processing Technology 170 (2005) 181186Stamping formability of pure titanium sheetsFuh-Kuo Chen, Kuan-Hua ChiuDepartment of Mechanical Engineering, National Taiwan University, Taipei 10764, Taiwan, ROCReceived 20 October 2003; received in revised form 12 April 2005; accepted 4 May 2005AbstractBecause 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 varioustemperatures was studied by the experimental approach. Tensile tests were first conducted to investigate the mechanical behavior of CP Tisheets at various temperatures. Forming limit tests, V-bend tests, and cup drawing tests were also performed to examine the stamping formabilityof CP Ti sheets at various temperatures. The experimental results indicate that CP Ti sheets could be formed into shallow components at roomtemperature, although the formability is limited in cold forming. In addition, the results obtained from the V-bend tests reveal that springbackcan be reduced at elevated forming temperatures. The experimental results obtained in the present study can be of help to the die design ofstamping CP Ti sheets. 2005 Elsevier B.V. All rights reserved.Keywords: Pure titanium sheet; Formability; Forming limit; V-bend; Springback1. Introduction In the present study, the formability of stamping CP TimerciallyrialfromfofAmongingoficsphone,at(HCP)attemperatureHo14T0924-0136/$doi:10.1016/j.jmatprotec.2005.05.004Due to its lightweight and high specific strength, com-pure titanium (CP Ti) has been a potential mate-for structural components, and attracts much attentionthe electronics industry recently. The principal manu-acturing process of CP Ti has been press forming becauseits competitive productivity and superior performance.the fabrication processes of press forming, stamp-of CP Ti sheets is especially important for the productionthin-walled structural components used in the electron-products, such as the cover cases of notebook, mobileetc. The CP Ti sheet usually exhibits limited ductilityroom temperature because of its hexagonal close-packedstructure. Although the formability can be improvedelevated temperatures, a manufacturing process at roomis always desired for the cost-effective reason.wever, most research of CP Ti is focused on microstructure, and the literature regarding formability of stamping CPi sheets is not profound.Corresponding author. Tel.: +886 2 33662701; fax: +886 2 3631 755.E-mail address: .tw (F.-K. Chen).pertiestestsroomoftest see front matter 2005 Elsevier B.V. All rights reserved.was investigated using the experimental approach. Theproperties of CP Ti sheets at various temperaturesfrom room temperature to 300C were obtained fromxperimental 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 variousesThe stressstrain relations are the fundamental infor-for the study of formability of a sheet metal. Asabove, the formability of CP Ti sheets is limited attemperature and can be improved at elevated formingIn order to examine the variety of mechanicalof CP Ti sheets at different temperatures, tensilewere performed at various temperatures ranging fromtemperature to 300C and under different strain rates0.1, 0.01, 0.001, and 0.0001/s, respectively. The tensilespecimens made of JIS Grade 1 CP Ti sheets of 0.5 mm182 ProcessingFig.imensthicknessTherollingrollingalongmachine.wwerewerewwereconandneeringformensTheFig.lartions,iswdirections.byoccursofdeformationhigheringation.TrespectiofathefromthreeF.-K. Chen, K.-H. Chiu / Journal of Materials1. 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 thedirection (0), and at angles of 45and 90to thedirection. The specimens were wire cut to avoid burrsthe edge.The tensile tests were conducted using an MTS 810 testFor tests at elevated temperatures, a heating furnaceas mounted on the MTS810 test machine. The specimensheated to 100, 200, and 300C before the tensile testsperformed. During tests, the temperature of specimenas kept constant until the specimen was stretched to failure.In the present study, the engineering stressstrain relationsfirst obtained from the experimental data and then wereverted into the true stressstrain relations according to= 0(1 + e) and = ln(1 + e), where and were true stresstrue strain, 0and e were engineering stress, and engi-strain, respectively. The true stressstrain relationsCP 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 in1 that the 0specimen has a higher yield strength and ager elongation than the specimens in the other two direc-the difference in elongation being more significant. Italso observed that the 0specimen displays a significantork-hardening property among the specimens in the threeThese results are consistent with those obtainedIshiyama et al. 5. They found that the slip deformationin both the 0and 90directions in the beginning stagethe test. During further deformation stage, the twinningincreases faster in the 0direction and producesresistance against the slip of dislocations, resultinglarger values in yield strength, work hardening, and elon-The average yield stress and elongation of the CPi sheet at room temperature are about 352 MPa and 28%,vely. Though the values of yield stress and elongationthe CP Ti sheet at room temperature are not favorable indeep drawing process compared to those of carbon steels,y are feasible for stamping of relatively shallow productsthe formability point of view.Fig. 2 shows the original and deformed specimens in thedirections. It is noticed in Fig. 2 that the 0specimenunderspecimenmodemationatdiftiforinnoticedtointhatobtainedousforrelationsformedCPatures.thethatroomelongation100Fig.menTechnology 170 (2005) 181186Fig. 2. Original and deformed specimens in the three directions.goes uniform deformation before fracture, while the 90displays an obvious necking, and the deformationof 45specimen 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 performedroom temperature under different ram speeds, resulting inferent strain-rates of 0.1, 0.01, 0.001, and 0.0001, respec-vely. The true stressstrain relations at various strain-ratesthe 0specimen are shown in Fig. 3. A significant dropthe stressstrain curves from strain-rate 0.1 to 0.001 isin Fig. 3, and the stressstrain curves become closeeach other afterwards. The same trends are also observedthe tensile tests for the 45and 90specimens. It indicatesa stable stressstrain relations for CP Ti sheets can beunder the strain-rates smaller than 0.001.The true stressstrain relations of CP Ti sheets at vari-temperatures ranging from room temperature to 300Cthe specimen of 0direction are shown in Fig. 4. Theshown in Fig. 4 are obtained from the tests per-at strain-rate of 0.001. It is seen in Fig. 4 that theTi sheet exhibits better formability at elevated temper-The stressstrain curves get lower proportionally toincrease of testing temperature. It is to be noted in Fig. 4the elongation of the specimen does not increase fromtemperature to 100C as expected, on the contrary, thegets smaller when the specimen is heated up toC. However, the elongation becomes larger at testing3. True stressstrain relations at various strain-rates (1/s) for 0speci-at room temperature.ProcessingFig.Fig.temperaturesroomhappenstheatureThe0ningproducingandrtransvuniaxialobtainedFig.90fromF.-K. Chen, K.-H. Chiu / Journal of Materials4. True stressstrain relations at various temperatures for 0specimen.5. True stressstrain relations at various temperatures for 45specimen.higher than 100C. The greater elongation attemperature is quite unusual. But this phenomenon onlyto the 0specimen. For the 45and 90specimens,elongation continuously increases as the testing temper-gets elevated, as shown in Figs. 5 and 6, respectively.greater elongation at room temperature occurred in thespecimen might be due to the fast increase of the twin-deformation in the 0direction 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 theerse direction to that in the thickness direction in atensile test. In the present study, the r-value wasfrom the tensile tests for specimens of 0,45, and6. True stressstrain relations at various temperatures for 90specimen.0vethansheetsr3.ingpresentandtemperaturesrelatinging3.1.formingacceptedmetalperformedusingelectrochemicallydeformedingellipseThestrainssamefromSimilarentationseachatsphericalmeasuredspecimenplottednate,thelimitafracturebeitstemperature.fromatTechnology 170 (2005) 181186 183directions at room temperature. The r-values measuredspecimens stretched to 20% are 4.2, 2.2, and 2.1 for the,45, and 90specimens, respectively. Since a higher r-alue indicates better drawability, it shows that CP Ti sheetsxhibit better deep drawing quality in the rolling directionthe other two directions. Also the anisotropy of CP Tiwas confirmed again from the significant difference of-values.Stamping formability of CP Ti sheetsIn addition to the basic mechanical properties, the stamp-formability of CP Ti sheets was also examined. In thestudy, the forming limit tests at room temperature,the V-bend tests and circular cup drawing tests at variouswere performed. The test results were discussedto the forming properties of CP Ti sheets in a stamp-process.Forming limit testsSince Keeler and Backofen 6 introduced the concept oflimit diagram (FLD) in 1963, it has been a widelycriterion for the fracture prediction in the sheet-forming. To determine an FLD, stretching tests werefor sheet-metal specimens of different widthsa semi-spherical punch. The specimens were firstetched with circular grids that would beinto ellipses after being stretched. The engineer-strains measured along the major- and minor-axes of theare termed the major- and minor-strain, respectively.y are also the principal strains on the plane where theare measured.In the present study, rectangular specimens having thelength of 100mm, but with different widths ranging10 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, forsize of specimen. During the tests, specimens clampedperiphery were stretched to failure over a 78 mm semi-punch. The engineering major- and minor-strainsin the location closest to the fracture for eachwere recorded. The major- and minor-strains wereagainst one another with the major strain as the ordi-and the curve fitted into the strain-points was defined asforming limit curve. The diagram showing this formingcurve is called the forming limit diagram. The FLD isvery useful criterion for the prediction of the occurrence ofin a stamping process.According to the previous analysis, the CP Ti sheet couldformed at room temperature. In order to further confirmfeasibility, the forming limit tests were performed at roomFig. 7 shows the forming limit curve obtainedthe test results. It is seen in Fig. 7 that the major strainthe lowest point of the curve, which is also the plane strain184 Processing Technology 170 (2005) 181186deformationorstampinginsheetsufCP3.2.ofprocess.toformingishaspunchfrompared.ofFmensusedinsignificanttest.100,Fig.foranglesspringbackandtiF.-K. Chen, K.-H. Chiu / Journal of MaterialsFig. 7. Forming limit curve at room temperature.mode, is 0.34. Compared with cold-rolled steelsstainless steels, this value is a little lower. However, forof shallow products, the forming limit curve shownFig. 7 indicates a greater possibility of forming of CP Tiat room temperature. This makes it possible to man-acture electronics components at room temperature usingTi sheets.V-bend testsSince CP Ti has a lower value of elastic modulus than thatsteel, springback could be much significant in a bendingIn the present study, the V-bend tests were performedexamine the springback property of CP Ti sheets at varioustemperatures. The tooling used in the V-bend testsshown in Fig. 8. It can be seen in Fig. 8 that the lower diean opening angle of 90. In order to study the effect ofradius on springback, the tooling sets with punch radii0.5 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 length60 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 wasin the V-bend test since the frictional condition has aneffect on the springback occurred in the V-bendThe bending tests were conducted at room temperature,200, and 300C, respectively. After bending tests, theFig. 8. Tooling used in the V-bend tests.forsmallerbend,notedbacksheetarctocomplespringbackFig.imens9. Relations between springback and punch radius at room temperaturespecimens of three directions.of bent specimens were measured by a CMM, and theangles were calculated.Figs. 9 and 10 show the relationships between springbackpunch radius at room temperature and 300C, respec-vely. It is seen in both figures that the springback decreasessmaller punch radii regardless of temperature change. Thepunch radius causes larger plastic deformation at theand hence reduces the effect of springback. It is alsoin both Figs. 9 and 10 that negative values of spring-occur for smaller punch radii. This is because that theon the straight sides of V-shape is deformed into anat the beginning of bending process, and the load appliedflatten the arc at the end of bending process results in ax stress distribution that causes a negative value of7. Comparing both figures, it is observed that10. Relations between springback and punch radius at 300C for spec-of three directions.Fig. 11. Punch and die used in circular cup drawing tests.ProcessingspringbackreCPrienceknoanddoestheofingele3.3.ratiopunchprocess,ofingthecupture,usedtemperatures.cess,adjustedIfforcewthefracture,simultaneouslycouldaalsoinpunchinbleF.-K. Chen, K.-H. Chiu / Journal of MaterialsFig. 12. Drawn cups at variousdecreases as the forming temperature increasesgardless of the dimension of punch radius. It indicates thatTi sheets not only have better formability but also expe-less springback at higher forming temperatures. It iswn that springback is affected by both the elastic modulusthe yield stress of the material. Since the elastic modulusnot vary too much with the change of temperature, andyield stress of CP Ti sheets decreases with the increasetemperature, the decrease of springback at higher form-temperatures is due to the lower yield stress of CP Ti atvated temperatures.Circular cup drawing testsThe limiting drawing ratio (LDR), which is defined as theof the largest diameter of circular blank (Do) to thediameter (Dp) in a successful circular cup drawingis a popular index used to describe the formabilitysheet 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 circulardrawing tests. Tests were performed at room tempera-100, and 200C, respectively. The heating apparatusin the tensile tests was adopted for the tests at elevatedIn order to obtain a successful drawing pro-the blank size and blank-holder force were adaptivelyto eliminate the defects such as fracture and wrinkle.the fracture appeared in a drawing test, the blank-holderwould be adjusted to a smaller value until the fractureas eliminated without the occurrence of wrinkles. Whenadjustment of blank-holder force failed to eliminate thean attempt of reducing the blank size would be triedto avoid the fracture. A reverse methodologybe adopted to suppress the occurrence of wrinkles indrawing test. However, in an LDR test, the blank size isacting as a parameter to determine the value of LDRaddition to the use of the above adjustment. Since thediameter is 35 mm, the blank diameter is increasedan increment of 3.5 mm from 70 mm to the largest possi-diameter for the convenience of calculating the values ofTTTRoom100200LDR.testsisclearlyincreasefigureousbecomesuesareperatures.theLDRfrom200forcedrasheetsofroom4.ingductingtheandindicateTechnology 170 (2005) 181186 185forming temperatures.able 1est results of circular cup drawingemperature Blank diameter(mm)LDR Blank-holderforce (kN)Drawingdepth (mm)temperature 77 2.2 2.75 20C 84 2.4 3.5 29C 101.5 2.9 4.0 40MoS2was used as lubricant in all circular cup drawingconducted in the present study, and the drawing speed0.2 mm/s.Fig. 12 shows the drawn cups at various temperatures. It isseen in Fig. 12 that the drawing depth increases as theof forming temperature. It is also to be noted in thisthat the earing shapes of the drawn cup formed at vari-temperature are quite different. The earing phenomenonsignificant at higher forming temperatures. The val-of LDR, drawing depth, and related process parameterslisted in Table 1 for the tests conducted at various tem-It is noticed in Table 1 that all values increase asforming temperature increases. However, the increase ofand drawing depth is not so significant in the rangeroom temperature to 100C, but gets larger from 100 toC. It is also noted in Table 1 that a larger