Influence of ultrasonic vibration on micro-extrusion

Influence of ultrasonic vibration on micro-extrusionCristina Bunget,Gracious Ngaile ⇑Mechanical and Aerospace Engineering Department,North Carolina State University,Raleigh,NC 27695,USAa r t i c l e i n f o Article history:Received 5November 2007Received in revised form 1January 2011Accepted 2January 2011Available online 5January 2011Keywords:Ultrasonic forming Micro-forming Micro-extrusion Tribologya b s t r a c tMicro-forming is a miniaturization technology with great potential for high productivity.Some technical challenges,however,need to be addressed before micro-forming becomes a commercially viable manu-facturing process.These challenges include severe tribological conditions,difficulty in achieving desired tolerances,and short tool-life due to inability of available die materials to withstand the forces exerted on miniature dies and punches.Some of these problems can be mitigated using ultrasonic technology.The principal objectives of this work were to investigate the possibility of applying ultrasonic vibra-tions in the micro-forming process,to design a set of tooling for ultrasonic micro-extrusion and to observe experimentally how ultrasonic oscillations influences the forming load and the surface finish.The test results showed a significant drop on the forming load when ultrasonic vibrations were imposed,and also a significant improvement in the surface of the micro-formed parts.Based on the preliminary test results,the study demonstrated high potential for using ultrasonic oscillations as a way to overcome the difficulties brought by the miniaturization.Ó2011Elsevier B.V.All rights reserved.1.IntroductionThe production of micro-parts has become more important in fields such as electronics,computers,communication,biotechnol-ogy,medicine,optics and environmental monitoring.Typical example for such parts are pins for IC-carriers,fasteners,micro-screws,lead frames,sockets,and any kind of connecting element (Fig.1).Traditionally these precision mechanical parts are fabri-cated by machining.The net shape or near net shape processes,such as metal forming processes,are best suited for miniaturiza-tion because of the higher productivity and accuracy.Despite the advantages of micro-forming,full utilization of this process has not been realized.Besides the problems that appear in conventional forming processes,like tool design and wear,there are also specific problems related with the miniaturization itself,such as material behavior,higher friction and fabrication of tools.When the process is scaled down,the microstructure and the sur-face topography remain unchanged resulting in different material flow as compared to conventional process.A series of tests in sheet micro-forming has shown that flow stress of material decreases with miniaturization and also tribological conditions are more se-vere at the micro-scale level.The influence of size on flow stress has thoroughly been investigated and well understood [2,3].The tribological effects are,however,not well understood.The dou-ble-cup extrusion test was used to study lubrication conditions for different specimen sizes ranging from 4mm down to 0.5mm diameter.The results showed an increase in apparent friction fac-tor by a factor of 20when the specimen size decreased to 0.5mm diameter [4].The authors formulated a mechanical–rheological model which assumes that the friction stress is affected by minia-turization.This approach was based on the generalized friction law given in Eq.(1).s f ¼f Áa RC Ák ;ð1Þwhere s f is the friction factor,k is the shear yield strength of the material,f is the friction factor,and a RC is the real contact area ratio.They showed that both f and a RC are not constant,but they are size dependent and become larger as the process is scaled down.Another approach of investigating the effect of miniaturization on friction was proposed by Bunget [5].Finite element analysis was used to simulate backward-cup extrusion tests for the same specimen size and different dimensions of the punch,going from 6mm (macro-forming)down to 0.5mm in the cup wall thickness (micro-forming).The ratio of the frictional energy to the total energy at different percent of the stroke was calculated.A compar-ative analysis between deformation in macro-forming and micro-forming can be drawn by considering the deformation energy associated with metal forming as expressed by the functional [6]:Y¼I 1þI 2ÀI 3¼ZVr d _ edV þk ZV_eV d _e V dV ÀZSFF i d u i ds ð2Þwhere k is a penalty constant,_eV is the volumetric strain rate, r is effective stress, e is effective strain,_ eis effective strain rate,and F i represents traction.The 1st term represents the energy due to deformation,the 2nd term volumetric strain energy,and the 3rd0041-624X/$-see front matter Ó2011Elsevier B.V.All rights reserved.doi:10.1016/j.ultras.2011.01.001Corresponding author.Tel.:+19195155222.E-mail address:gracious_ngaile@ (G.Ngaile).term energy due to friction at the tool/workpiece interface.In the presence of good lubrication (i.e.,shear friction factor m =0.1–0.15),which could result to non-defective parts,the ratio between frictional energy and deformation energy (R macro =I 3/I 1)associated with classic macro-forming is usually of the order of 0.1or less.With micro-forming,however,the ratio of friction energy to defor-mation energy increases significantly (Fig.2).This dramatic in-crease implies that much better lubrication is imperative in micro-forming because the interface friction can become dominant in the process.Furthermore,the typical lubricants used in cold macro-forming,such as zinc phosphate +metal soap,become inef-fective for micro-forming because the lubricant film thickness (e.g.,20l m)may exceed the tolerance level of the micro-parts.The investigations have also shown an increase in scattering with miniaturization [3],and therefore difficulty in achieving de-sired tolerances.Other problems that appear in micro-forming are the fabrication of the tools,which need to be very small with very close tolerances and a good surface quality.The fact that mi-cro-forming tools are also subjected to forces that may exceed the capacity of available die materials presents a great challenge as these tools are prone to short tool life.New manufacturing meth-ods have been developed,such as micro-electrical discharging machining:micro-wire EDM,micro die sinking,electrical dis-charge drilling,micro-electrical discharge milling and grinding,but the problem of the high forces exerted on the tools still needs to be resolved [7].There is a great potential to overcome some of the drawbacks of the miniaturization by applying the ultrasonic vibration during mi-cro-forming process.The application of ultrasonic vibration in me-tal forming at macro-scale has been discussed for many years and some benefits were observed,such as a reduction in the flow stress and a considerable reduction of the external friction between tool and workpiece.The flow stress was observed experimentally to be considerably lowered by the ultrasonic oscillations superimposed on the compressive forming process [8].This phenomenon was attributed to dynamical effects of stress,velocity and acceleration due to vibration,and also thermal effects due to heat generation.Earlier studies of the effects of the longitudinal oscillations im-posed on the wire drawing revealed two possible effects of the ultrasonic oscillations superimposed on the forming process:vol-ume effects and surface effects [8–10].The volume effects are re-lated with a decrease in the flow stress.The surface effects are related with the changing of the frictional conditions at the die/specimen interface.The application of longitudinal,radial and tor-sional oscillations in rolling processes resulted in changes in the apparent friction coefficient,as a result of the pumping of the lu-bricant phenomenon,softening or melting the asperities,separa-tion of the surfaces,allowing a re-distribution of the lubricant [11].Investigations on superimposing ultrasonic oscillations on the tube drawing showed a significant reduction in the flow stress [12].Other investigations were carried out to study the effect of the shape and length of the concentrator,or the tool attachment,an important part in the ultrasonic system.The concentrator shapes that were found to be more effective were conical,stepped and exponential [13].Most studies on the applications of ultra-sonic vibrations on metal forming seem to reach the same conclu-sions regarding the benefit of ultrasonic oscillations:reduction of the forming forces,reduction of the flow stress,reduction of the friction between die and workpiece and production of better sur-face qualities and higher precision.Despite these benefits wide application of this technology is still limited.This is due to high cost of designing a ultrasonic system and the cost associated with energy that is required to induce the vibrations [14].The objectives of this study are to investigate the possibility of applying the ultrasonic vibrations in the micro-forming process and to investigate their influence on material flow,forming load and tribological characteristics.It is anticipated that ultrasonic technology can overcome some of the difficulties brought by miniaturization.2.Ultrasonic micro-extrusion process mechanismThe following sections examine the physics of the extrusion process when longitudinal ultrasonic oscillations are superim-posed on the dies.Three different extrusion processes will be dis-cussed:forward-rod extrusion,double-cup extrusion,and forward-rod-backward-cup extrusion.A schematic diagram of the ultrasonic forward-rod extrusion pro-cess is shown in Fig.3.The punch pushes down the billet,forcing it to go through the die cavity,in order to form a rod with a reduced diameter.The workpiece goes through three zones:the guiding zone (Zone I ),the deformation zone (Zone II ),and the exit zone (Zone III ).All three zones are characterized by sliding friction,but the deformation zone also experiences plastic deformation.In the ultrasonic extrusion process,the ultrasonic oscillation can be im-posed on the die or on the punch.It is usually more practical to oscillate the dies,since the purpose is to have a large relative mo-tion between the die and workpiece.In this case,the ultrasonic oscillations are imposed on the die,and it is assumed that the vibratory motion is not transmitted to the punch.The material will enter with velocity v 0,given by the punch,but will exit with adif-Fig.1.Extruded micro-parts [1].15%20%25%30%35%40%n e r g y / T o t a l E n e r g y[%]Wall thickness 0.5Wall thickness 2.551(2011)606–616607ferent velocity,v e .The die will vibrate with an oscillatory velocity v D ,and an amplitude a (Fig.3).This velocity is larger than the velocity of the material to be deformed.Because of the relative velocity between the die and material,micro gaps between die and material are created and closed at every cycle of oscillation.This changes the contact pressure characteristics at the interface.It can be observed that under ultrasonic oscillations the direction of the friction force reverses for every period of oscillation,thus changing the friction characteristic at the tool-workpiece interface.Therefore,with proper design of an ultrasonic micro-forming sys-tem,tribological performance can be enhanced.The schematic of ultrasonic double cup backward extrusion pro-cess is shown in Fig.4.The billet is placed in the die,between the two punches.The lower punch is stationary and the upper punch is moving down,forcing the workpiece material to flow and to form the two cups.There are three distinctive zones:zones I and III where the lower and upper cups are formed respectively,characterized by sliding friction and Zone II where plastic deforma-tion occurs.The application of ultrasonic oscillations can be accomplished in a similar manner as forward-rod extrusion dis-cussed above.In the classical double-cup extrusion process,the material flows easier in the upper cup in the presence of friction.But the periodic change in direction of the friction force causedIn examining various extrusion processes,emphasis was on identifying changes pertaining to the relative velocities between the die and the workpiece that are associated with the introduction of ultrasonic oscillations.Altering relative motion in time and space from motions that could be observed in classical metal form-ing without ultrasonic necessarily affects friction characteristics,interface pressure characteristics,and other process variables.These changes/transformations can be exploited to enhance the process by carefully designing an ultrasonic forming system.Other key aspects pertaining to the application of ultrasonic technologies include;(a)the ultrasonic oscillations are brought in the deforma-tion zone through a concentrator connected to a transducer as shall be discussed in the next section,(b)the ultrasonic oscillations could be imposed on the die or the punch,and in rare cases on the workpiece because the workpiece undergoes changes in dimensions and shape during the deformation process.In general,it is preferred to impose the oscillations on the dies,(c)Because of high relative velocity between the die and deforming billet a signif-icant heat can be generated at the interface.The increase in the temperature at the die-specimen interface will influence the behavior of the lubricant,and (d)for classical metal forming,the application of ultrasonic oscillations is practical only when the sav-ings in the energy required for providing the forming load is larger Fig.3.Ultrasonic forward-rod extrusion process schematic.Fig.4.Ultrasonic double cup backward extrusion process schematic.a unified design which offers the possibility of changing the form-ing tools to accommodate all variants of micro-extrusion processes shown in Figs.3–5,(b)be able to micro-extrude billets made from aluminum,brass and copper materials with billet dimension rang-ing from 0.5to 2mm diameter and 1to 5mm length,(c)conceive a design that will attain maximum relative velocity at the die-work-piece interface and that the transmission of the oscillations to the rest of the structure should be minimal,(d)the setup should be de-signed for 2kW ultrasonic generator at frequencies between 20kHz and 21kHz,(e)the setup should be designed for an ultra-sonic transducer which operates at a natural frequency of 20kHz and oscillation amplitude of 10l m,and (f)the set-up should pro-duce useful mode shapes,i.e.longitudinal and radial-torsional at a natural frequency of 20kHz.In order to fulfill the above design requirements a preliminary design shown in Fig.6was conceived.The main parts of the ultra-sonic micro-extrusion system are,the ultrasonic generator which is connected to the ultrasonic transducer,the concentrator which takes the oscillations from the transducer and transmits them to the lower forming unit,the upper and lower forming units,and the hydraulic actuation cylinder which provides the forming load tothe upper part of the forming unit.The actuation cylinder is con-nected to the pump and the control system.Fig.7a shows a concentrator assembly which is comprised of several parts;die,punch,outside cone,inside cone,lock nut and the concentrator body.A simplified model of the concentrator is shown in Fig.7b.The simplified model shows that a longitudinal wave transmitted by the piezoelectric transducer at the base of the concentrator attains maximum amplitude at the deformation zone.The figure also shows that a radial wave transmitted into the disc attains zero amplitude at the clamping zone.As shown in Fig.7b,the geometric parameters to be established for the con-centrator are L i ,L m ,L o ,/d ,/i ,/m ,/o ,and t .Some of the concentrator dimensions are imposed by the practical aspects of the design such as dimensions of the dies and punches,provision for clamping the concentrators,etc.Table 1shows material properties and other parameters that were used for the design.It can be seen from Table 1that a wave length of 260mm will be exhibited at the desired fre-quency of 20,000Hz.To attain zero displacement amplitude at the clamping zone (Fig.7b)the diameter of the disc should be equal to the wave length.All other geometric parameters were iterated with the aid of the finite element method as discussed in the next section.3.1.Modal analysisIn the course of optimizing the ultrasonic micro-extrusion sys-tem,finite element analysis was carried out.In order to ensure that the natural frequency is close to the excitation frequency given by the transducer,several finite element iterations were carried out to determine the remaining dimensions of the system.Fig.8shows the boundary conditions used in the modal analysis.At location A,all nodes on the bottom surface of the base plate were con-strained in all degree of freedom.The model was also constrained at location B.This is the location where the ultrasonic transducer will be connected.In order to mimic the type of the boundary con-ditions resulted from the application of the ultrasonic oscillations all nodes on the connection surface were constrained in the axial direction.Several useful mode shapes were found in the range between 19,000Hz and 21,000Hz:Fig.9a shows a radial mode shape at 20,176Hz,and Fig.9b shows a longitudinal mode shape at 20,354Hz.Both mode shapes may result in enhancement of the tribological conditions at the die-workpiece interface.Some of the benefits of longitudinal mode shape are due to the fact that this mode shape will result in friction force reversal and formation of gaps,particularly,for forward extrusion and forward-rod-back-ward-cup extrusion processes.The benefit emanating from radial mode shape includes enhancing uniform distribution of lubricant at theinterface.Fig.5.Ultrasonic forward rod backward extrusion process schematic.Actuation Ultrasonic Ultrasonic Hydraulic pump forming unit forming unit transducerand ControlConcentratorto the superimposed oscillations,and to check if the stresses inthe structure are within permissible limits.Similarly to the previ-ous analyses,the bottom surface of the base plate was constrained Ultrasonic oscillations Dieimposed by piezoelectric Lower punch Outside cone Inside cone Lock nutConcentrator body DiscL oL iL m Ød =λftØo ØmØiTable 1Material and ultrasonic oscillation parameters used in concentrator design.Material Steel typeFig.8.Boundary conditions applied in the modal analysis [ANSYS Workbench].610 C.Bunget,G.Ngaile /Ultrasonics 51(2011)606–616100Hz step size.The frequency responses of the structure taken atpoint Rs(top surface of die holder)for three input displacements are plotted in Fig.10.The maximum displacement is reached at the resonance frequency of20,400Hz,and all the mode shapes cor-responding to the three input displacement were longitudinal. Fig.10shows that for input displacement of1l m,5l m and 10l m,the die exhibited longitudinal displacement amplitudes of 47l m,237l m and474l m respectively.3.3.Conceived tooling setupFig.11shows the developed ultrasonic micro-extrusion set-up. Before carrying out the experiment the actual displacement of the die due to ultrasonic vibration had to be established.Strain gauges were mounted on the top part of the die holder(outer cone,see Fig.7a)and on the top head of the piezoelectric transducer.The strain gauges provided local elongation and contraction of the die holder as the wave propagated.These values were later correlated with harmonic response analysis.Fig.12shows displacement amplitude at the location where the strain gauges were mounted for the outer cone and the transducer head.The gauge on the trans-ducer shows a displacement of1.3l m and the strain gauge on the die holder shows an average displacement of10l m.Harmonic analysis was then carried out for transducer input displacement of1.3l m.The frequency response curve for the top surface of the die holder and the displacement map are shown in Fig.13.A maximum displacement of61.6l m is observed.From the values given on the displacement map scale,local elongation of the die holder can be determined.For example,by subtracting the top two displacement values on the scale(61.626–54.094=7.5l m) we can obtain local elongation of the die holder.This implies that, the top5mm length of the die holder exhibited a maximum elon-gation of about7.5l m.The strain gauges mounted at this location gave an average of10l m.Since these two values are close,we could assume that the displacement values obtained from har-monic analysis are reasonable.I.e.,the die can therefore be as-sumed to oscillate at maximum amplitude of61.6l m.During the experiment the tooling exhibited a resonance frequency of 20,490Hz.As shown from Fig.13the resonance frequency fromharmonic analysis was 20,400Hz.Only a slight difference of 90Hz is observed between experiment and simulation.3.4.Experimental set-up and test proceduresThe developed test set-up discussed above was used to study the influence of lubricant on the ultrasonic vibrations for forward extrusion (FE),double-cup extrusion (DCE)and forward backward extrusion (FBCE)processes.As shown in Fig.11,the main compo-nents of the test set-up are;punch and die assemblies,lever for actuating the forming load,2kW ultrasonic generator and 20kHzHarmonic response analysis for a 1.3l m transducer displacement input (frequency response is taken at the top surface of the die holder).Fig.14.Dies and punches for (a)FE and (b)DCE.Table 2Test matrix and lubricant types.LubricantNumber of experiments FEDCE FBCE Lub 1[Polymeric lubricant]6138Lub 2[Dailube DR-38]344Lub 3[Lubsol W-72SK]111116Polymeric lubricant (C –84%,O –15%,Na –0.2%,Si –0.05%,P –0.3%,S –0.45%).Dailube DR-38.Lubsol W-72SK (C –84.74%,O –12.75%,Na –1.25%,Si –0.08%,P –0.22%,S –0.98%).612 C.Bunget,G.Ngaile /Ultrasonics 51(2011)606–616tools are a combination of those used in the other two tests.Thematerials used for the experiments were aluminum(AA1100) and brass.The samples were cut from a long wire of2mm diame-ter in small pieces of5mm length for forward-rod extrusion,4mm for forward-rod-backward-cup extrusion tests,and3mm length for double-cup extrusion test.Table2presents the test matrix, specifying the number of experiments carried out for each case and the three lubricants used in the experiments.Before the test,the diameter and length of each billet were measured.After that,the billet is dipped in the lubricant.In order tofind thefilm thickness,the billet is weighed before and after lubricant application.Table3presents the lubricantfilm thickness for some of the billets.Just before the forming load is applied, ultrasonic vibration is activated from the generator.As discussed in Section3.3the ultrasonic amplitude applied to the die for all the tests was61l m.The time for application of the ultrasonic oscillations is10s.Fig.15presents the micro-parts obtained from the experiments.After the experiments,micrographs were taken using scanning electron microscopy(HITACHI S3200NSEM)which was used to study the influence of the ultrasonic oscillations on the surface quality.Table3Film thickness for different lubricants and specimens.Lubricant Billet size(mm)Weight lubricant(g)Surface billet(mm2)Film thickness(g/mm2) Lub1£1.91Â5.110.00636.370.1650Â10À3Without Ultrasonic With UltrasonicWithout UltrasonicWith UltrasonicWithout Ultr asonicWith Ultr asonicC.Bunget,G.Ngaile/Ultrasonics51(2011)606–616613614 C.Bunget,G.Ngaile/Ultrasonics51(2011)606–616cants in all tests was enhanced by ultrasonic vibrations.The lubri-cant found more effective was lubricant1for DCE test,and lubri-cant3for FE and FBCE tests.4.2.Influence of ultrasonic oscillations on surfacefinishUsing scanning electron microscope,a comparison between the surfacefinish of the specimens obtained after the tests with and without ultrasonic oscillations was carried out.This aspect of the surface is a key factor in understanding the lubrication mechanism. Micrographs were taken for all types of tests and all lubricants.The surface quality was observed to be improved in all ultrasonic tests.Fig.20brings together the micrographs for forward-rod extru-sion tests and forward–backward-cup extrusion with and without ultrasonic oscillation[6].Sample locations where micrographs were taken are also shown.Fig.20shows a significant improve-ment in the surface quality when ultrasonic vibration was used.The differences between the surfacefinish obtained after classic double-cup micro-extrusion test and the ultrasonic test are pre-sented in Fig.21[6].Micrographs were taken in different zones. The surfaces obtained in non-ultrasonic conditions presents scars which indicate sticking conditions and localized smearing with subsequent shearing of the junctions.The surfaces obtained when ultrasonic oscillations were used show a significant improvement in the lubrication regimes.There are some isolated scars in zone 1.Zone3does not show much scars,rather this zone exhibit roughening and formation of tiny lubricant pockets,indicating a much more favorable lubrication condition.The differences in the surface quality are attributed to various factors,some of which can be understood by studying the possible lubrication mechanisms in non-ultrasonic and ultrasonic test. Ultrasonic oscillation results in high instantaneous relative veloci-ties at the tool/workpiece interface.This leads to reduction of adhesive bond formation and hence better lubrication conditions.At the same time,because of the change in the direction of the rel-ative movement,there is a change in the direction of the friction force that helps the material toflow easier than in the classic test.Also because of the relative velocity,the pockets formed by the surface roughness trap the lubricant and keep it in the deformation zone.The lubricant is better distributed at the interface and keeps up with the expansion of the surface and the generation of the new surface during the deformation process.Due to these changes,the lubrication conditions are improved leading to reduction in the friction coefficient at the tool-workpiece interface.5.ConclusionsIn this paper the influence of ultrasonic vibration on micro-extrusion processes is investigated.An ultrasonic micro-forming tooling was developed and micro-extrusion experiments were con-ducted.The forming load and the surfacefinish after deformation were some of the aspects studied.Based on results of the tests, the following conclusions were reached:Ultrasonic oscillations influence significantly the friction and lubrication mechanism at the die/workpiece interface.Higher instantaneous sliding velocities help to shift a boundary lubri-cation regime to a mixed-film regime and sticking conditions to a boundary regime.Better lubrication conditions and better surfacefinish with ultrasonic oscillations suggest that ultrasonic micro-forming isa viable process for difficult-to-lubricate materials.The magnitude of the ultrasonic oscillations effects depends on the type of the extrusion process,but is also influenced by the lubricant composition.The reduction in friction will result in a reduction in load,thus in less energyconsumption.In conclusion,the study has demonstrated that there is high po-tential for using ultrasonic vibration as a way to overcome some the difficulties brought by the miniaturization. AcknowledgmentsThe authors would like to acknowledge the National Science Foundation,Project#CMMI0900148,through which this work was partially funded.Any opinions,findings,and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Sci-ence Foundation.Sincere gratitude should also go to Mr.Florian Scherer,who participated in the early stage of tooling design. 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