Stamping formability of pure titanium sheets.pdf
【机械类毕业论文中英文对照文献翻译】冲压纯净的钛板料的可锻模性
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机械类毕业论文中英文对照文献翻译
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【机械类毕业论文中英文对照文献翻译】冲压纯净的钛板料的可锻模性,机械类毕业论文中英文对照文献翻译,机械类,毕业论文,中英文,对照,文献,翻译,冲压,纯净,板料,锻模
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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 Tisheetsatvarioustemperatures.Forminglimittests,V-bendtests,andcupdrawingtestswerealsoperformedtoexaminethestampingformabilityof 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. IntroductionDue to its lightweight and high specific strength, com-mercially pure titanium (CP Ti) has been a potential mate-rial for structural components, and attracts much attentionfrom the electronics industry recently. The principal manu-facturing process of CP Ti has been press forming becauseof its competitive productivity and superior performance.Among the fabrication processes of press forming, stamp-ing of CP Ti sheets is especially important for the productionof thin-walled structural components used in the electron-ics products, such as the cover cases of notebook, mobilephone, etc. The CP Ti sheet usually exhibits limited ductilityat room temperature because of its hexagonal close-packed(HCP) structure. Although the formability can be improvedat elevated temperatures, a manufacturing process at roomtemperature is always desired for the cost-effective reason.However,mostresearchofCPTiisfocusedonmicrostructure14,andtheliteratureregardingformabilityofstampingCPTi sheets is not profound.Corresponding author. Tel.: +886 2 33662701; fax: +886 2 3631 755.E-mail address: .tw (F.-K. Chen).In the present study, the formability of stamping CP Tisheetswasinvestigatedusingtheexperimentalapproach.ThemechanicalpropertiesofCPTisheetsatvarioustemperaturesrangingfromroomtemperatureto300Cwereobtainedfromexperimentalresults.Inaddition,theimportantformingchar-acteristicsofCPTisheets,suchasforminglimit,springback,and limiting drawing ratio, were also examined by experi-ments.2. Mechanical properties tests at varioustemperaturesThe stressstrain relations are the fundamental infor-mation for the study of formability of a sheet metal. Asmentionedabove,theformabilityofCPTisheetsislimitedatroom temperature and can be improved at elevated formingtemperatures. In order to examine the variety of mechanicalproperties of CP Ti sheets at different temperatures, tensiletests were performed at various temperatures ranging fromroom temperature to 300C and under different strain ratesof 0.1, 0.01, 0.001, and 0.0001/s, respectively. The tensiletest specimens made of JIS Grade 1 CP Ti sheets of 0.5mm0924-0136/$ see front matter 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2005.05.004182F.-K. Chen, K.-H. Chiu / Journal of Materials Processing Technology 170 (2005) 181186Fig. 1. True stressstrain relations at room temperature obtained from spec-imens in the three directions.thickness were prepared according to the ASTM standards.The specimens were cut along planes coinciding with therolling direction (0), and at angles of 45and 90to therolling direction. The specimens were wire cut to avoid burrsalong the edge.The tensile tests were conducted using an MTS 810 testmachine.Fortestsatelevatedtemperatures,aheatingfurnacewas mounted on the MTS810 test machine. The specimenswere heated to 100, 200, and 300C before the tensile testswere performed. During tests, the temperature of specimenwaskeptconstantuntilthespecimenwasstretchedtofailure.Inthepresentstudy,theengineeringstressstrainrelationswere first obtained from the experimental data and then wereconverted into the true stressstrain relations according to =0(1+e) and =ln(1+e), where and were true stressand true strain, 0and e were engineering stress, and engi-neering strain, respectively. The true stressstrain relationsfor CP Ti sheets at room temperature obtained from speci-menscutinthethreedifferentorientationsareshowninFig.1.The anisotropic behavior is observed in Fig. 1. It is seen inFig. 1 that the 0specimen has a higher yield strength and alarger elongation than the specimens in the other two direc-tions, the difference in elongation being more significant. Itis also observed that the 0specimen displays a significantwork-hardening property among the specimens in the threedirections. These results are consistent with those obtainedby Ishiyama et al. 5. They found that the slip deformationoccursinboththe0and90directionsinthebeginningstageof the test. During further deformation stage, the twinningdeformation increases faster in the 0direction and produceshigher resistance against the slip of dislocations, resultingin larger values in yield strength, work hardening, and elon-gation. The average yield stress and elongation of the CPTi sheet at room temperature are about 352MPa and 28%,respectively.Thoughthevaluesofyieldstressandelongationof the CP Ti sheet at room temperature are not favorable ina deep drawing process compared to those of carbon steels,they are feasible for stamping of relatively shallow productsfrom the formability point of view.Fig. 2 shows the original and deformed specimens in thethree directions. It is noticed in Fig. 2 that the 0specimenFig. 2. Original and deformed specimens in the three directions.undergoesuniformdeformationbeforefracture,whilethe90specimen displays an obvious necking, and the deformationmodeof45specimenliesbetweenthoseofothertwomodes.In order to examine the effect of strain-rate on the defor-mation of CP Ti sheets, the tensile tests were also performedat room temperature under different ram speeds, resulting indifferent strain-rates of 0.1, 0.01, 0.001, and 0.0001, respec-tively. The true stressstrain relations at various strain-ratesfor the 0specimen are shown in Fig. 3. A significant dropin the stressstrain curves from strain-rate 0.1 to 0.001 isnoticed in Fig. 3, and the stressstrain curves become closeto each other afterwards. The same trends are also observedin the tensile tests for the 45and 90specimens. It indicatesthat a stable stressstrain relations for CP Ti sheets can beobtained under the strain-rates smaller than 0.001.The true stressstrain relations of CP Ti sheets at vari-ous temperatures ranging from room temperature to 300Cfor the specimen of 0direction are shown in Fig. 4. Therelations shown in Fig. 4 are obtained from the tests per-formed at strain-rate of 0.001. It is seen in Fig. 4 that theCP Ti sheet exhibits better formability at elevated temper-atures. The stressstrain curves get lower proportionally tothe increase of testing temperature. It is to be noted in Fig. 4that the elongation of the specimen does not increase fromroom temperature to 100C as expected, on the contrary, theelongation gets smaller when the specimen is heated up to100C. However, the elongation becomes larger at testingFig. 3. True stressstrain relations at various strain-rates (1/s) for 0speci-men at room temperature.F.-K. Chen, K.-H. Chiu / Journal of Materials Processing Technology 170 (2005) 181186183Fig. 4. True stressstrain relations at various temperatures for 0specimen.Fig.5. Truestressstrainrelationsatvarioustemperaturesfor45specimen.temperatures higher than 100C. The greater elongation atroomtemperatureisquiteunusual.Butthisphenomenononlyhappens to the 0specimen. For the 45and 90specimens,the elongation continuously increases as the testing temper-ature gets elevated, as shown in Figs. 5 and 6, respectively.The greater elongation at room temperature occurred in the0specimen might be due to the fast increase of the twin-ning deformation in the 0direction at room temperature,producing higher resistance against the slip of dislocations,and resulting in a larger elongation.Another index of anisotropy is the plastic strain ratio, i.e.r-value, which is defined as the ratio of plastic strain in thetransverse direction to that in the thickness direction in auniaxial tensile test. In the present study, the r-value wasobtained from the tensile tests for specimens of 0, 45, andFig.6. Truestressstrainrelationsatvarioustemperaturesfor90specimen.90directions at room temperature. The r-values measuredfrom specimens stretched to 20% are 4.2, 2.2, and 2.1 for the0, 45, and 90specimens, respectively. Since a higher r-value indicates better drawability, it shows that CP Ti sheetsexhibit better deep drawing quality in the rolling directionthan the other two directions. Also the anisotropy of CP Tisheets was confirmed again from the significant difference ofr-values.3. Stamping formability of CP Ti sheetsIn addition to the basic mechanical properties, the stamp-ing formability of CP Ti sheets was also examined. In thepresent study, the forming limit tests at room temperature,and the V-bend tests and circular cup drawing tests at varioustemperatureswereperformed.Thetestresultswerediscussedrelating to the forming properties of CP Ti sheets in a stamp-ing process.3.1. Forming limit testsSince Keeler and Backofen 6 introduced the concept offorming limit diagram (FLD) in 1963, it has been a widelyaccepted criterion for the fracture prediction in the sheet-metal forming. To determine an FLD, stretching tests wereperformed for sheet-metal specimens of different widthsusing a semi-spherical punch. The specimens were firstelectrochemically etched with circular grids that would bedeformed into ellipses after being stretched. The engineer-ing strains measured along the major- and minor-axes of theellipse are termed the major- and minor-strain, respectively.They are also the principal strains on the plane where thestrains are measured.In the present study, rectangular specimens having thesame length of 100mm, but with different widths rangingfrom 10 to 100mm in an increment of 10mm, were tested.Similar to tensile tests, the CP Ti sheet was cut at three ori-entations to the rolling direction, i.e., 0, 45, and 90, foreach size of specimen. During the tests, specimens clampedat periphery were stretched to failure over a 78mm semi-spherical punch. The engineering major- and minor-strainsmeasured in the location closest to the fracture for eachspecimen were recorded. The major- and minor-strains wereplotted against one another with the major strain as the ordi-nate, and the curve fitted into the strain-points was defined asthe forming limit curve. The diagram showing this forminglimit curve is called the forming limit diagram. The FLD isa very useful criterion for the prediction of the occurrence offracture in a stamping process.According to the previous analysis, the CP Ti sheet couldbe formed at room temperature. In order to further confirmits feasibility, the forming limit tests were performed at roomtemperature. Fig. 7 shows the forming limit curve obtainedfrom the test results. It is seen in Fig. 7 that the major strainat the lowest point of the curve, which is also the plane strain184F.-K. Chen, K.-H. Chiu / Journal of Materials Processing Technology 170 (2005) 181186Fig. 7. Forming limit curve at room temperature.deformation mode, is 0.34. Compared with cold-rolled steelsor stainless steels, this value is a little lower. However, forstampingofshallowproducts,theforminglimitcurveshownin Fig. 7 indicates a greater possibility of forming of CP Tisheets at room temperature. This makes it possible to man-ufacture electronics components at room temperature usingCP Ti sheets.3.2. V-bend testsSince CP Ti has a lower value of elastic modulus than thatof steel, springback could be much significant in a bendingprocess.Inthepresentstudy,theV-bendtestswereperformedtoexaminethespringbackpropertyofCPTisheetsatvariousforming temperatures. The tooling used in the V-bend testsis shown in Fig. 8. It can be seen in Fig. 8 that the lower diehas an opening angle of 90. In order to study the effect ofpunch radius on springback, the tooling sets with punch radiifrom 0.5 to 5.0mm, in an increment of 0.5mm, were pre-pared. The CP Ti sheet with a thickness of 0.5mm, a lengthof 60mm, and a width of 15mm was used as specimens.For tests at elevated temperatures, both tooling and speci-mens were enclosed in a heating furnace. No lubricant wasused in the V-bend test since the frictional condition has aninsignificant effect on the springback occurred in the V-bendtest. The bending tests were conducted at room temperature,100, 200, and 300C, respectively. After bending tests, theFig. 8. Tooling used in the V-bend tests.Fig. 9. Relations between springback and punch radius at room temperaturefor specimens of three directions.angles of bent specimens were measured by a CMM, and thespringback angles were calculated.Figs. 9 and 10 show the relationships between springbackand punch radius at room temperature and 300C, respec-tively. It is seen in both figures that the springback decreasesforsmallerpunchradiiregardlessoftemperaturechange.Thesmaller punch radius causes larger plastic deformation at thebend, and hence reduces the effect of springback. It is alsonoted in both Figs. 9 and 10 that negative values of spring-back occur for smaller punch radii. This is because that thesheet on the straight sides of V-shape is deformed into anarc at the beginning of bending process, and the load appliedto flatten the arc at the end of bending process results in acomplex stress distribution that causes a negative value ofspringback 7. Comparing both figures, it is observed thatFig. 10. Relations between springback and punch radius at 300C for spec-imens of three directions.Fig. 11. Punch and die used in circular cup drawing tests.F.-K. Chen, K.-H. Chiu / Journal of Materials Processing Technology 170 (2005) 181186185Fig. 12. Drawn cups at various forming temperatures.springback decreases as the forming temperature increasesregardless of the dimension of punch radius. It indicates thatCP Ti sheets not only have better formability but also expe-rience less springback at higher forming temperatures. It isknownthatspringbackisaffectedbyboththeelasticmodulusand the yield stress of the material. Since the elastic modulusdoes not vary too much with the change of temperature, andthe yield stress of CP Ti sheets decreases with the increaseof temperature, the decrease of springback at higher form-ing temperatures is due to the lower yield stress of CP Ti atelevated temperatures.3.3. Circular cup drawing testsThe limiting drawing ratio (LDR), which is defined as theratio of the largest diameter of circular blank (Do) to thepunch diameter (Dp) in a successful circular cup drawingprocess, is a popular index used to describe the formabilityof sheet metals. A larger value of LDR implies a larger draw-ing depth, that is, a better formability. In the present study,the punch and die shown in Fig. 11 were used for the circularcup drawing tests. Tests were performed at room tempera-ture, 100, and 200C, respectively. The heating apparatusused in the tensile tests was adopted for the tests at elevatedtemperatures. In order to obtain a successful drawing pro-cess, the blank size and blank-holder force were adaptivelyadjustedtoeliminatethedefectssuchasfractureandwrinkle.If the fracture appeared in a drawing test, the blank-holderforce would be adjusted to a smaller value until the fracturewas eliminated without the occurrence of wrinkles. Whenthe adjustment of blank-holder force failed to eliminate thefracture, an attempt of reducing the blank size would be triedsimultaneously to avoid the fracture. A reverse methodologycould be adopted to suppress the occurrence of wrinkles ina drawing test. However, in an LDR test, the blank size isalso acting as a parameter to determine the value of LDRin addition to the use of the above adjustment. Since thepunch diameter is 35mm, the blank diameter is increasedin an increment of 3.5mm from 70mm to the largest possi-ble diameter for the convenience of calculating the values ofTable 1Test results of circular cup drawingTemperatureBlank diameter(mm)LDRBlank-holderforce (kN)Drawingdepth (mm)Room temperature772.22.7520100C842.43.529200C40LDR. MoS2was used as lubricant in all circular cup drawingtests conducted in the present study, and the drawing speedis 0.2mm/s.Fig.12showsthedrawncupsatvarioustemperatures.Itisclearly seen in Fig. 12 that the drawing depth increases as theincrease of forming temperature. It is also to be noted in thisfigure that the earing shapes of the drawn cup formed at vari-ous temperature are quite different. The earing phenomenonbecomessignificantathigherformingtemperatures.Theval-ues of LDR, drawing depth, and related process parametersare listed in Table 1 for the tests conducted at various tem-peratures. It is noticed in Table 1 that all values increase asthe forming temperature increases. However, the increase ofLDR and drawing depth is not so significant in the rangefromroomtemperatureto100C,butgetslargerfrom100to200C. It is also noted in Table 1 that a larger blank-holderforce is required for the larger blank size to be successfullydrawn at a higher temperature. The value of LDR of CP Tisheetsis2.2atroomtemperature,whichiscomparabletothatof carbon steels, indicating that stamping of CP Ti sheets atroom temperature is feasible.4. Concluding remarksThe formability of stamping CP Ti sheets at various form-ingtemperatureswasinvestigatedinthepresentstudybycon-ducting various experiments. The mechanical properties ofthe CP Ti sheet at various temperatures were first examined,and the stressstrain relations obtained from the experimentsindicate that the CP Ti sheet has a higher yield stress and186F.-K. Chen, K.-H. Chiu / Journal of Materials Processing Technology 170 (2005) 181186a smaller elongation at room temperature, but proportionallydecreasesinyieldstressandincreasesinelongationwhenthesheet is heated to an elevated temperature up to 300C. It isto be noted that the stressstrain relations obtained from thetensiletestsatroomtemperatureindicatethattheCPTisheetcould be formed into shallow components at room tempera-ture, although the yield stress is a little higher. The forminglimitdiagramoftheCPTisheetobtainedatroomtemperatureis not so high as those of cold-rolled steels, but the minimummajor strain of 0.34 also provides an optimum possibility forthe CP Ti sheet to be formed at roo
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