General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from orbit.dtu.dk on: Jul 24, 2020 Analysis of the risk of galling in sheet metal stamping dies with drawbeads Moghadam, Marcel; Nielsen, Chris Valentin; Bay, Niels Published in: Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture Link to article, DOI: 10.1177/0954405420911307 Publication date: 2020 Document Version Peer reviewed version Link back to DTU Orbit Citation (APA): Moghadam, M., Nielsen, C. V., & Bay, N. (Accepted/In press). Analysis of the risk of galling in sheet metal stamping dies with drawbeads. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture. https://doi.org/10.1177/0954405420911307
Transcript
General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
Users may download and print one copy of any publication from the public portal for the purpose of private study or research.
You may not further distribute the material or use it for any profit-making activity or commercial gain
You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Downloaded from orbit.dtu.dk on: Jul 24, 2020
Analysis of the risk of galling in sheet metal stamping dies with drawbeads
Moghadam, Marcel; Nielsen, Chris Valentin; Bay, Niels
Published in:Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture
Link to article, DOI:10.1177/0954405420911307
Publication date:2020
Document VersionPeer reviewed version
Link back to DTU Orbit
Citation (APA):Moghadam, M., Nielsen, C. V., & Bay, N. (Accepted/In press). Analysis of the risk of galling in sheet metalstamping dies with drawbeads. Proceedings of the Institution of Mechanical Engineers, Part B: Journal ofEngineering Manufacture. https://doi.org/10.1177/0954405420911307
Analysis of the risk of galling in sheetmetal stamping dies with drawbeads
Marcel Moghadam , Chris Valentin Nielsen and Niels Bay
AbstractSheet metal stamping of complex geometries normally involves the use of drawbeads to control the metal flow in theforming die. Drawbeads are, however, often the most tribologically severe part of the stamping dies. Selection of a suit-able tribosystem for this type of forming operation depends on parameters such as local contact pressures, sliding speed,tool/workpiece interface temperature, tool and workpiece materials, and surface topographies. Furthermore, it dependson the required tool life and acceptable maintenance costs. This study demonstrates a methodology for offline evaluationof tribosystem applicability for a specific production platform for stamping of a three-dimensional component using aforming die with drawbeads. Based on an industrial case study, this work combines experimental and numerical analysesof the risk of galling in the different regions of an industrial forming die.
KeywordsDie wear, drawbeads, stamping, die tryout, limits of lubrication
Date received: 3 May 2019; accepted: 15 February 2020
Introduction
Galling is a mechanism of surface damage commonlyencountered in sheet metal forming processes, causedby relative motion between tribologically stressed sur-faces. The occurrence of galling is a major issue instamping industry, since it increases the productioncosts due to die maintenance, die replacements, andscrap production. Understanding the wear behaviorand evaluating the efficiency of different tribosystemsare therefore valuable in order to facilitate a robustsheet metal stamping production. The nature of thewear mechanism is, however, highly dynamic anddependent on a range of different processes and mate-rial parameters, such as tool geometry, tool/workpiecematerial and surface conditions, process loads, speeds,and temperatures. This makes the selection of a suitabletribosystem for a specific production platform difficult,in terms of evaluation of the necessary tool life andmaintenance costs.1 Several studies have highlightedthe wear issues stemming from the adaption of newhigh-strength steel grades in the automotive industry,2
further emphasizing the importance of selecting suitabletribosystems for sheet metal forming operations. Anumber of studies have been conducted for investiga-tion of tool wear in sheet metal stamping with numeri-cal simulation. Eriksen3 presented a study wherethe optimization of a die geometry, in terms of
minimization of the occurring tool wear, was achievedby finite element method (FEM) simulation of the wearindex (which is identical to the wear work introducedlater by equation (4) in this article). This was based ona bending under tension test, which has also beenapplied for determining friction coefficients byAndreasen et al.4 Several recent studies have similarlyshown that an adaptation of the Archard wear equa-tion gives an accurate description of the developmentof the tool wear. Pereira et al.5 studied the influence ofthe contact sliding distance in a typical sheet metalstamping process. The analysis of contact conditionsshowed that a specific region in the tool interface issubjected to long sliding distances combined with highcontact pressures, resulting in critical conditions interms of the overall wear development. Wang et al.6
investigated the influence of several process variableslike binder pressure, friction coefficient, and tool coat-ings on the tool wear distribution of a stamping die bythe numerical simulations. Based on the numerical
Department of Mechanical Engineering, Technical University of Denmark,
Kgs. Lyngby, Denmark
Corresponding author:
Marcel Moghadam, Department of Mechanical Engineering, Technical
simulations, selection of appropriate process para-meters was tailored for optimization of the tool life.Wang and Masood7 investigated the influence of vari-ous die profiles on the wear properties of forming tools.Ersoy-Nurnberg et al.8 presented a method of wearsimulation with a modified Archard model, where avariable wear coefficient is implemented. Numericalsimulation with the modified Archard model gave anaccurate description of the wear development at the dif-ferent stages of the tool life. Other wear models havealso been proposed, for instance, Josue da Silva andAlvares9 investigated tool wear for single-point incre-mental forming.
This study aims at outlining an overall methodology,based on an industrial case study, for characterizationof the tribological severity of a production platformand selection of suitable tribosystems. Critical die fea-tures are recognized from the numerical simulations,and the corresponding simulative tests are carried outfor testing the performance of different tribosystems.
Wear modeling
A large number of models have been developed for thedescription of friction and wear phenomena based oneither empirical studies or basic principles of contactmechanics.10 The Archard wear model11 is the mostcommonly applied wear model in commercial FE soft-ware for modeling of wear in manufacturing processes.The wear model states that the volume loss w (mm3)due to wear is governed by the product of the normalforce FN (N) and the accumulated sliding length L(mm) and is inversely proportional to the surface hard-ness H (HRC). The wear coefficient K is determined bythe material properties of the contact pair. The wearmodel is then given as
w=KFNL
Hð1Þ
For adaption of the wear formula into FE calcula-tions, the wear volume change per unit area _w at a
certain time step is expressed by the normal pressure P(MPa) and the relative sliding velocity vt
_w=KPvtH
ð2Þ
The total wear volume is obtained by integration ofthe wear volume rate over time
w=K
H
ð
t
Pvtdt ð3Þ
The integral in equation (3) introduces the wear work Z(MPa�mm), which allows for a qualitative evaluation ofthe wear severity independent of the material propertiesof the contact pairs, as the wear work is directly propor-tional to frictional energy dissipated in the contactingsurfaces.12 The wear work is thereby
Z=
ð
t
Pvtdt ð4Þ
Holm13 and Rabinowicz14 described wear modelswith structures similar to Archard’s wear model fordescribing atomic wear in electrical contacts and abra-sive wear, respectively.
Experimental and numerical procedure
This study is an industrial case based on the productionof an automotive component for exhaust gas recircula-tion (EGR). The component is made in a one-stepstamping operation of a 1.5-mm-thick blank of EN1.4301 stainless steel. The forming process is carried outwith a manually operated hydraulic, single-action presswith a draw cushion, where the forming die is lowereddown over a stationary draw punch. The productionrate of the component is approximately 150 parts perhour. The forming die has a stepped drawbead geome-try around the perimeter of the die opening to controlthe flow of the sheet metal during the forming opera-tion. The forming tool is shown in Figure 1. The final
(a) (b)
Figure 1. Overview of the forming tools by (a) identification of the punch, forming die, and the blank holder, where the steppeddrawbead is visible, and (b) schematic cross section of the drawbead geometry around the perimeter of the die opening.
2
geometry of the drawbead was obtained through a trialand error procedure in the die tryout stage.
Scanning of blank holder and forming die
A three-dimensional (3D) scan of the forming die wasmade using an ATOS Triple Scan optical scanner. Theoptical scanner measures the geometry of the die by tri-angulation of a fringe pattern that is captured using atwo-camera system. For measurement of the formingdie, a series of 3mm reference points were placed onthe die surface outside the areas of interest. Since theforming die has a highly polished surface, a thin layerof titanium oxide was sprayed onto the die in order tominimize reflection and enhance the definition of thecaptured surface. Calibration of the optical system wasdone according to the recommended procedure fromthe manufacturer. This involved measurement of a cali-bration panel from varying distances, positions, anddifferent orientations of the camera system. The mea-surements were carried out with a MV560 (560 3 4203 420mm3) lens setup at a measuring distance ofapproximately 830mm in a temperature-controlledlaboratory with air temperature of 20.5 �C6 0.5 �C.Two separate series of scans were conducted for theblank holder and the die geometry, each consisting of10–12 partial scans that were merged into one finalmodel. Post processing of the obtained die geometrywas done with the MeshLab software. Post processingof the scanned die geometry consisted of the removalof redundant scanned features and down sampling ofthe generated point cloud with a Poisson-disk algo-rithm in order to reduce the point cloud to approxi-mately 40–50,000 data points. This was done in orderto facilitate further processing of the scanned model,while still maintaining an accurate discretization of thedie geometry.
FEM model
The scanned model was imported into LS-DYNA forFE analysis of the stamping process. For the FE analy-sis, the forming tools were modeled as elastic steel toolswith a variable mesh size of 0.5–1mm around thedrawbead region, in order to adequately simulate thebending action introduced by the drawbeads. The geo-metry of the blank was modeled with a quadrilateralmesh with an initial mesh size of 2.5mm with adaptiveremeshing, with three levels of mesh refinement duringsimulation. The simulation was set up with a standardshell element formulation (Belytschko–Tsay formula-tion). Characterization of the strain hardening behaviorof the sheet material was done with a plane strain com-pression test, where a Voce hardening curve of sf=135 + (17852 135)(12 e22.5e) (MPa) was determinedfor the material, where sf is the flow stress of the mate-rial and e is the equivalent strain. The Coulomb frictionmodel was adapted for the FE setup with a frictioncoefficient of m=0.1.
Results and discussion
Analysis of drawbead geometry
While several simplified guidelines exist for the designof drawbeads,15 the actual drawbead design implemen-ted in the stamping dies is highly dependent on thecomponent geometry, the sheet thickness, and the expe-rience of the die manufacturer. In the die tryout stage,the die manufacturer modifies the drawbead geometryby manual grinding in order to adjust the flow of thesheet material, while trying to avoid a drawbead geo-metry that introduces excessive thinning of the sheetmaterial and forming defects such as cracks, wrinkles,and orange peeling. This trial and error approachresults in a wide range of different drawbead
Figure 2. Overview of the rounding radii of the drawbead geometry of (a) the blank holder and (b) the forming die.
Moghadam et al. 3
geometries used in industrial forming dies, which areoften prone to galling, since die features like the dieshoulder and drawbeads are often subjected to high-localized normal pressures.16 A general overview of thedrawbead geometry of the forming die in this study isshown in Figure 2.
Four zones, grouped by the size of the rounding radiiof the drawbead, are identified on the blank holder andthe forming die in Figure 2. Zone 1 and zone 2 containrelatively small rounding radii, which result in heavyrestraining of the material flow. Zone 3 and zone 4 havelarger rounding radii, resulting in less restrictions of thematerial flow. The influence of the drawbead geometryon drawing behavior of the stamped component is illu-strated in Figure 3 by comparing the outer contour ofthe flange to the initial blank contour. Figure 4, further-more, shows the thickness distribution of the formed
component with the initial drawbead geometry and thegeometry obtained during the die tryouts.
Figure 3(a) shows the restraining action of the initialdrawbeads geometry, where the rounding radii of thedrawbead are equal to the thickness of the sheet mate-rial along the entire perimeter of the die cavity. Thelarge restriction in the material flow, due to the smallrounding radii of the drawbead, results in a large flangeremaining after the forming operation and substantialthinning of the formed component (see Figure 4). Thelarge restriction of the material flow greatly exceeds the20%–30% formability guideline for the thicknessreduction, commonly used for automotive componentsin stainless steel. The excessive thinning will possiblyresult in fracturing on the top part of the component,where the largest reduction in the thickness of the sheetmetal takes place. Forming the same component with-out drawbeads, also shown in Figure 3(a), will conver-sely result in excessive draw-in of the flange materialand will not induce sufficient work hardening of thesheet material. Forming the component without draw-beads will, furthermore, increase the risk of wrinklingdue to insufficient stretching of the sheet material.Figure 4(b) shows that the drawbead geometryobtained in the die tryout introduces a variablerestraining force along the perimeter of the sheet metalblank, seen in the variable sliding length of the sheetmaterial. The variable draw-in of the sheet materialensures a uniform thickness of the formed componentand that the component has a remaining flange thatenables proper trimming of the formed component forthe subsequent production steps. Figure 4, further-more, shows that the drawing behavior of the sheetmetal can be approximated well with numerical simula-tions with the implementation of the scanned formingdie. Minor deviations between the numerical resultsand the formed component are found, most noticeably,
Figure 3. Initial blank and outer contour of component after stamping with (a) the initial drawbead geometry and stamping withoutdrawbeads by simulations and (b) the final drawbead geometry by simulation and production.
(a) (b)
Figure 4. Thickness distribution of the formed componentwith (a) the initial drawbead geometry and (b) the finaldrawbead geometry.
4
at the upper end of zone 1 where the flow of the sheetmaterial is completely restricted and in zone 4 wherethe flow of the sheet material is underestimated in thesimulation. The variable forming conditions introducedby the drawbeads impose different tribological loadson the forming tools. For the evaluation of the severityof the forming process, a simulation of the distributionof the normal pressure and the wear work was made(see Figures 5 and 6).
From the simulated pressure distribution in Figure 5,it is confirmed that the zones with smaller roundingradii of the drawbeads are exposed to higher normalpressures. From the wear work in Figure 6, it is similarlyseen that the combination of high local normal pressuresand moderately long sliding lengths (as observed inFigure 4) results in severe wear conditions in zone 2,with a wear work of more than 8000MPa�mm. Anothercritical region, in terms of wear, is the die shoulder inzone 3, which is subjected to moderately high normalpressures combined with a very long sliding length atthe center of the component. The planar contactbetween the die and the blank holder, in zone 3, is
similarly exposed to increased wear work due to the lon-ger sliding length of the sheet material.
Offline screening of suitable lubricants
Based on the analysis of the drawbead geometry andthe resulting tribological loads the drawbead designintroduces, a tool has been designed to experimentallyreplicate the drawbead conditions in zone 2, whichexhibits the most severe tribological loads as seen in thesimulated wear work. A schematic illustration of thetool design and the experimental testing parameters isshown in Figure 7. The drawbead test was conductedon the universal sheet tribotester (UST) developed atthe Technical University of Denmark.17 The USTenables simulative testing with process times that repli-cate the actual production speed of the productionplatform in order to maintain a similar temperatureprofile in the forming tools. The offline screening ofalternative lubricants was made with an untreatedSleipner die material and with a Tenifer� quench polishquench (QPQ) surface treatment of the die material,
(a) (b)
Figure 6. Simulated wear work in (a) the blank holder and (b) the forming die.
(a) (b)
Figure 5. Simulated normal pressure in (a) the blank holder and (b) the forming die.
Moghadam et al. 5
respectively, to emulate the two versions of the formingdie used for the production of the EGR component.The manufacturer specifies that a forming lubricant forthe specified production platform should support aminimum of 200 strokes without deterioration of thesurface quality of the formed component due to lubri-cant film breakdown.
The presented test setup was made with sheet mate-rial of 1mm thickness. Scaling down the drawbeaddimensions with the same factor as the sheet thicknessallows to emulate comparable drawing characteristicsin terms of surface strains, drawbead restraining force,and contact pressures in the tool/workpiece inter-face.15,18 The testing parameters were, furthermore,designed to emulate the actual production parametersof a manually operated press in terms of process times
and the sliding speed of the forming tools. The mea-sured drawing force during testing and the developedtool wear after 300 strokes are shown in Figures 8–10for the tested tribosystems.
From Figure 9, it is seen that the test series with theRhenus LA 722086, Rhenus SU 200 A, and theIRMCO 980 080 lubricants experience a rampantdevelopment of the drawing force within the first 50strokes. This corresponds well with the visible scoringof the tested sheet metal surface. Compared withthe surface structure of the tool after 300 strokes(Figure 9), it is seen that a substantial amount ofpickup of workpiece material has been accumulated onboth the rounding of the drawbeads and the plane con-tact that emulates the contact in the blank holderregion of the forming die. The BWN 205 lubricant,
Figure 7. Overview of the tool design and experimental testing parameters.
(a) (b)
Figure 8. Measured drawing force during testing with Sleipner tool material (a) without any surface treatment and (b) with aTenifer� QPQ surface treatment.
6
which is a highly chlorinated, heavy-duty deep drawinglubricant, has a stable drawing force during testing.The surface structure of the tools shows only minorsigns of initial run-in wear without any friction junc-tions formed on the tool. None of the selected, environ-mentally friendly lubricants could, therefore, replacethe BWN 205 when conducting the tests with uncoatedtools. The selected lubricants were, however, found toexhibit a significantly improved tribological perfor-mance in the test series conducted with the TeniferQPQ–treated tools. The test series conducted with theTenifer QPQ surface treatment were found to exhibitstable drawing forces and smooth surface structure of
the sheet material after testing, for all the tested lubri-cants. Evaluation of the tool surface indicates onlyminor signs of abrasive wear without any sign ofpickup of sheet material on the tools. This is seen inFigure 10, where only minor scratches are found on thetool surface due to run-in wear of the iron nitride sur-face layer formed with the Tenifer QPQ surface treat-ment. The presented results indicate that certain,tribologically severe forming operations require adeeper refinement of the tribosystem in order to imple-ment environmentally friendly, non-chlorinated lubri-cants. For the EGR component, the combined functionof the Tenifer QPQ surface treatment and an
Figure 10. Tool surface, with a Tenifer� QPQ surface treatment, after 300 strokes with the tested lubricants: (a) Rhenus LA72208, (b) Rhenus SU 200 A, and (c) IRMCO 980 080.
Figure 9. Tool surface, without surface treatment of the tools, after 300 strokes with the tested lubricants: (a) BWN 205,(b) Rhenus LA 72208, (c) Rhenus SU 200 A, and (d) IRMCO 980 080.
Moghadam et al. 7
alternative, environmentally friendly forming oil wasfound to exhibit sufficient tribological integrity to replacethe heavy-duty, chlorine-based BWN 205 forming oil.
Conclusion
This study showcases a methodology on how criticaldie features in a forming die can be evaluated for thecharacterization of the wear behavior. An industrialtool for sheet metal forming has been 3D scanned andimported into an FE simulation software for analysis ofdrawing in, normal pressures, and die wear. The simu-lated drawing of the flange shows the influence of astepped drawbead by comparing (1) the absence of thedrawbead, (2) a drawbead with a constant radius equalto the sheet thickness, and (3) an optimized drawbeadgeometry. The simulation also identifies critical zonesalong the die shoulder and the stepped drawbead interms of wear severity. These zones were emulated bysimulative testing for offline evaluation of different tri-bosystems. For the specific production platform, it wasfound that only the hazardous, chlorinated lubricantcould facilitate a stable production with an uncoatedtool steel. The tribological function of the Tenifer QPQsurface treatment combined with different forming oilscould form the basis for an alternative, environmentallyfriendly tribosystem for replacement of the hazardous,chlorinated lubricant currently used in production. Fornew tool designs, the presented method can be used tooptimize the die geometry and reduce the time and costsrelated to the die tryout process. The optimization willencompass an evaluation of the tool life and the draw-ing behavior of the component.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interestwith respect to the research, authorship, and/or publi-cation of this article.
Funding
The author(s) received no financial support for theresearch, authorship, and/or publication of this article.
