s9-analysis of material form ability in incremental forming

8/3/2019 S9-Analysis of Material Form Ability in Incremental Forming

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Analysis of Material Formability in Incremental FormingL. Filicel, L. Fratin?, F . Micari' (2)

Dipartimento di M eccanica, University of Calabria, ItalyDipartimento di Tecnologia e Produzione Meccanica, University of Palermo, Italy

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AbstractIncremental forming is an innovative sheet metal forming technology in which a blank is plasticallydeformed through the progressive action of a small-size punch, whose movement is governed by a CNCmachine. In this way the tool locally deforms the m aterial through an almost p ure stretching deformationmechanics.The paper is focused on m aterial formability in incremental forming. S everal tests were d eveloped, aim edto the achievem ent of different straining cond itions in the m aterial and consequently to the determinationof Forming Limit Diagrams for progressive forming operations. The features and the application of suchFLD are discussed in the pa per.Keywords:Forming, Sheet, Incremental

1 INTRODUCTIONTraditional metal stamping processes involve relevantcosts linked to the equipments to be util ized and to therequired set-up t imes. The dies and the punches aremanufactured to be close to the shape to be producedand are used to impose the desired deformation to theblank. Therefore the industrial application of traditionalmetal stamping technology can be economically justifiedonly for large scale production (mass production) ofautomotive and electric appliance components, wherethe costs linke d to die m anufacture and set-up aredistributed over a very large number of stamped parts.Furthermore traditional processes permit to achieve asatisfying level of hard automation, b ut they strongly la ckflexibility.On the other hand in the last few decades metal formingindustries have to face new relevant and impellentneeds: among them the production of small batches ofsheet metal components, the increasing demand ofprocess flexibil ity, the necessity to reduce the time tomarket of the products are probably the mo st important.The mentioned needs are inconsistent with the abovedescribed features of traditional metal stampingprocesses. As a consequence n ew forming proce sseswhich do not require expensive conventional equipmentsand t ime consuming set-up operations have beenrecently proposed, usually classified as incremental (orprogressive) forming p rocesses.The basic idea of incremental forming operations is toobtain the desired shape of the part through themovement of a small-size punch along an user-specifiedpath. Thus a CNC lathe or mill ing machine can beutil ized to plastically deform the blank, imposing to thetool an assigned trajectory to be controlled by computer.By this way no conventional punches and dies arerequired and the final shape of the part only depends onthe trajectory assigned to the tool, because it isdetermined by the "sum" of the local deformationsinduc ed by the pun ch along its path.

Such basic considerations explain that incrementalforming permits a relevant reduction of the costs linkedto die manufacture and set-up, as well as it allows toachieve a very high level of process flexibil ity. Of coursethe forming cycle time is remarkably higher than thetraditional processes one, thus permitting an economicuse of this technology only for very small batches ofproduction. Furthermore incremental forming showsinteresting a nd promising perspe ctives for a possible useas a rapid-prototyping technique: in this case theadvantages offered by this technology in order to reducethe time to market of the products are clearly discernible.The former attempt to develop an incremental-typeforming process on a CNC machine is probably due toMatsubara [I2]. Most recently Kitazawa and Nakajima[3] proposed a cylindrical incremental drawing proce ss toproduce a cylindrical shell without using dedicated dies.Tanaka et al. [4] proposed to util ize a pair of rigid orelastic standard tools to press a small area of the sheetmetal blank. Finally lseki [5] suggested to util ize highspeed water jet controlled by computer instead of amechanical tool.Very recently some researchers have focused theirattention on process mechanics in incremental formingand, more in detail, on the issue of material formability.lseki [6] proposed an incremental bulge test using a ballroller and derived a set of forming limit curves. Shim andPar k [7] developed a new forming tool containing a freelyrotating ball and characterized the formability of anannealed Aluminium alloy taking into account a verysimple incremental forming operation, in which the toolpath was a square loop.In this p aper some results of a wide research activity onincremental forming are presented: in particular theissues of process mechanics and material formability aretaken into account following a systematic approach.Several incremental forming tests were carried out on1050-0 Aluminium Alloy blanks util izing a properlydesigned fixture mounted on a 3-axis controlled CNCmilling machine. The tests were aimed at the


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achievement of different straining conditions on thematerial and, for each of them, at the de tection of criticalstrains at fracture. In this way material formability wasfully characterized through the determination of thematerial Forming Limit Diagram for progressive formingoperations. Such FLD shows remarkable differenceswith respect to the traditional one due to the differentprocess m echanics: these issues, as well as an effectiveComputer Aided Engineering of a typical incrementalforming operation, are presented and discussed in detailin the next paragrap hs.2 DEVELOPMENT OF EFFECTIVE TESTS TOAll the tests were carried out on a 3-axis controlled CNCmilling machine; a properly designed fixture was utilizedto clamp the blank as shown in Figure 1.

EVALUATE MATERIAL FORMABILITY

Figure 1: The experimental equipmentAccording to the fixture design, square blanks I m m thick(24Ox240mm) were subjected to plastic deformation. Theblank material was AA 10 50- 0, whose flow rule wasdetermined through a preliminary set of tensile tests:

[MPaI0 14o = l 1I&Throughout the tests a cylindrical rotating punch with a12mm diameter hemispherical head was used as tool. Afilm of oil was applied to the blank to reduce friction. Thetool path was specified on the CNC milling machinethrough a proper routine. It is worth pointing out thatsuch path includes both the movement in the horizontalplane (i.e. the x-y table of the milling machine) an d thepenetration of the punch in the vertical direction (i.e. themovement along the z-axis). Therefore the punchdetermines an almost pure stretching deformationmechanics on the fully clamped sheet. Some previousinvestigations [8] showed in fact that, apart from somebending effects close to the clamping area, the blank ismainly stretched by the local action of the punch.The m ain objective of the expe riments was to investigatematerial formability in incremental forming taking intoaccount a wide range of straining conditions typical ofthese processes. Therefore different tests were designedin order to cover a set of straining conditions rangingfrom pure uni-axial stretching to fully bi-axial stretchingand consequently to derive an effective FLD forprogressive forming operations.Pure uni-axial stretching was obtained assuming, as toolpath, a square loo p consisting of four straight lines (TestA). During each loop the punch applies the verticaldisplacement to the blank (i.e. the fixed depth), while,between two subsequent loops, an horizontal (i.e. on thex-y plane) displacement toward the center of the blank isimpo sed to the punch. In this way a pyramidal shell isobtained at the end of the test (Figure 2). It is worthpointing out that almost pure uni-axial stretchingconditions occur along the straight edges of the shell, asConfirmed by the analysis of a grid of small circlesimpressed on the blank surface. Furthermore theheaviness of the straining conditions depends on the

ratio of the punch depth for each loop vs. the horizontaldisplacement imposed to the punch between twosubsequent loops. At increasing the mentioned ratio theblank undergoes heavier deformations, stretching andthinning increase up to ductile fracture occurrence.

Figure 2: Uni-axial stretching conditions (Test A).Bi-axial stretching was obtained selecting, as tooltrajectory, a simple cross consisting of two p erpendicularstraight lines (Test B shown in Figure 3). During itsmovement along each line, the punch imposes the fixeddepth to the blank. In this way almost pure bi-axialstretching conditions are achieved at the centre of thecross as confirmed by the impressed grid of smallcircles. The test is carried out u p to fracture at the centreof the cross, permitting to detect the c ritical strains for bi-axial stretching conditions (Figure 4).

Figure 3: Tool path in the bi-axial stretching test (Test B).

Figure 4: Fracture occurrence d uring the bi-axialstretching test (bottom view of the stamped part).Finally a test was developed to investigate materialformability for straining conditions in the range betweenuni-axial and bi-axial stretching. The basic idea derivedfrom the observation that, in the previous Test A,different straining conditions occurred in the cornersdepending on the corner radius. In particular if the corne rradius was simply equal to the one of the hemisphericalhead of the punch, near bi-axial stretching conditionswere achieved in the material; at increasing the cornerradius the hoop strain decreased, thus giving rise todifferent values of the ratio b etween the applied strains.Thus a new test (Test C) was developed, in which theassigned tool path is a three-dimensional spiral (see


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Figure 5): at each loop the punch moves both in the z-direction (applying the fixed depth to the blank), as wellas it has an inward radial displacement. In this way aconical shell is obtained at the e nd of the test.

Figure 5: Three-dimensional spiral tool pathAnalysing the small circles impres sed on the surface ofthe blank, it is worth pointing out that the ratio of themajor strain (i.e. the one along the axial direction of thecone) vs. the minor strain (i.e. the hoop strain) dependson the loop diameter. Such ratio increases at increasingthe mentioned diameter. Thus a set of tests at thevarying of the diam eter D of the first (and largest) loop ofthe spiral was carried out. Utilizing, for instance, adiameter of the first loop equal to 200mm the strainingconditions were not so far from uni-axial stretching with aminor strain close to zero. On the other hand, when thediameter of the first loop was rather small, bi-axialstretching conditions were approached. The lowestinvestigated D value was equal to 24mm ; in this case thestraining conditions were near to bi-axial stretching.At increasing the depth vs. radial displacement for loopratio stretching conditions and thinning became heavierup to ductile fracture (Figure 6); therefore such testspermitted to detect the critical strains and to investigatematerial formability over a wide range of strainingconditions.

Figure 6: Typical fracture occ urrence in a C-type Test.G a j o r [%I

G i n o r[%I-30 -10 10 30 50 70 90

- - Conventional FLC- ncremental FLCFigure 7: AA 10 50 -0 Forming Limit Diagram forincremental forming processes.

The results of the tests are summarized in Figure 7,where the AA 1050-0 Forming Limit Diagram forincremental forming processes is reported. In order tocarry out an useful comparison the conve ntional FLD forthe same material obtained by means of thehemispherical D ome Test is reported as well [9][10][1 I ] .It is worth pointing out that, due to the strainingconditions, the incremental forming curve covers thepositive emlnoride only: the straining conditions, in fact,rang ed from uni-axial to bi-axial stretching.3 DISCUSSION ON THE RESULTS AND

APPLICATION OF THE FORMING LIMIT DIAGRAMFigure 7 clearly shows that the forming limit curve inincremental forming is quite different from thecorresponding one in conventional forming. Much higherstrains may be achieved in incremental forming than intraditional processes. Such circumstance ca n be justifiedtaking into account the peculiarity of the processmechanics. Plastic deformation induced by the small-size punch is strongly localized and confined to the closevicinity of the contact area; then it incrementallyprogresses as the tool moves along the assigned path.As a consequence higher strains can be attained in thematerial before that fracture o ccurs.Furthermore the forming limit curve for incrementalforming processes typically has the shape of a straightline with a negative slope in the first quadrant of theForming Limit Diagram. In particular the statisticalanalysis of the experimental data permitted to derive thefollowing equation for the interpolating straight line:emlor = -1 .040 eminor+ 10 8.3 ( 2 )bein g emlor and emlnor he major a nd the minorengineering strain respectively. For the investigatedmaterial the value of the slope was very close to -1, i.e.the angle of inclination with the horizontal direction wasvery c lose to 4 4 .The above Forming Limit Diagram represents a veryeffective tool to the Computer Aided Engineering of anycomplex incremental forming process. The approach tobe followed is not very unlike the o ne usually utilized forconventional stamping processes: the strain datasupplied by the numerical simulation have to becompared with the material FLD in order to verify theprocess effectiveness.It is worth pointing out that the specific deformationmechanics of the incremental forming process requires afully three-dimensional numerical analysis since nosymmetry conditions can be assumed in the progressiveinteraction between the tool path and the clamped blank.In order to reduce CPU time and to improve contactconditions the use of an explicit dynamic model appearsto be the most effective choice [12]; furthermore the useof a geometric remeshing algorithm all along thenumerical simulation is very reliable to further reduce thecomputing weight. In this way, in fact, the deformingelements are subdivided in smaller ones as the movingrigid punch approaches them and determines adeformation level larger tha n a critical threshold.According to the above considerations the PC-Dynaformcode [ I3 1 was utilized to carry out the Compu ter AidedEngineering of an incremental forming process aimed tothe production of a typical cross-shaped component. ACAD view of such part is reported in Figure 8.As far as the numerical simulation of the process isconcerned, the punch feed rate was artificially increasedto 40 m/s as usual when explicit models are utilized;therefore a continuous check of the ratio of the kineticvs. the deformation energy, to be l imited below l o % ,was developed during the simulations in order to avoid


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the insurgence of inertia effects which would haveaffected the effectiveness of the num erical results.

-IFigure 8: CAD sketch of the con sidered part (all thedimensions are in mm).Figures 9 and 10 report some results of the numericalsimulations. In particular Figure 9 shows the formed partand the thickness distribution at the end of the process.Figure 10 reports the calculated strains on the materialFLD: all the points remain in the safe region, below theforming limit curve, indicating the effectiveness of theinvestigated operation.Figure 9: Calculated thickness distribution.

emajor [%I

-30 -10 10 30 50 70 90erninor[%I

Figure 10 : Calculated strains on material FLD

Figure 11: Experimental part.

Such prediction has been experimentally verified,manufacturing the real component with the equipmentdescribed in the previous paragraph; as shown in Figure11, the o btained part is free from defects, thusconfirming the effectiveness of the prop osed approach.4 CONCLUSIONSAccording to the above reported results it is possible todraw the following conclusions:0 incremental forming is characterized by a localstretching deformation mechanics which determinesa forming limit curve quite different from thetraditional on e;0 such FLC has a linear shape with a negative slopein the positive emlnoride of the FL D;0 the obtained FLC can be utilized as a CAE tool todesign industrial incremental forming processes.5 ACKNOWLEDGMENTSThis work has been performed with funding from MlUR(Italian Ministry for Instruction, University an d Research).6 REFERENCES

Matsubara, S., 1994, Incremental backwardforming of a sheet metal with a hemispherical headSawada, T., Matsubara, S., Sakamoto, M.,Fukuhara, G., 1999, Deformation analysis forstretch forming of sheet metal with CNC machinetool, Advanced Technology of Plasticity, Vol.Kitazawa, K., Nakajima, A,, 1999, Cylindricalincremental drawing of sheet metals by CNCincremental forming process, AdvancedTechnology of Plasticity, Vol. 11:1495-1500.Tanaka, S., Nakamura, T., Hayakawa, K., 1999,Incremental sheet metal forming using elastic tools,2000, Advanced Technology of Plasticity,Iseki, H., 1999, A simple deformation analysis forincremental bulging of sheet metal using highspeed water jet, Advanced Technology of Plasticity,lseki H., 2000, As experimental and theoreticalstudy on a forming limit curve in incrementalforming of sheet metal using spherical roller,Proceedings of Metal Formin g 2000:557-562.Shim, M.-S., Park J.-J., 2001, The formability ofaluminum sheet in incremental forming, J. ofMaterial Processing Technology, Vol. 113:654-658.Filice, L ., Fratini, L ., Micari, F. , 2001, New trends insheet metal stamping processes, Proc. of PRIME2001 Conference: 143-148.Xu, S.G., Weinmann, K.J., 1998, On predicting theforming limit diagram for automotive aluminiumsheet, Annals of the CIRP, Vol. 47/1:177-181.

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s9-analysis of material form ability in incremental forming

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