Investigation of the Mechanical Punch Loads during Fine Blanking of High-Strength Steels with Cemented Carbide

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Procedia Manufacturing (2019) 000–000 Procedia Manufacturing 3400 (2019) 90–100 Procedia Manufacturing (2019) 000–000 Procedia Manufacturing 00 00 (2017) 000–000

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Procedia Manufacturing 00 (2019) 000–000 47th SME North American Manufacturing Research Conference, NAMRC 47,www.elsevier.com/locate/procedia Pennsylvania, USA www.elsevier.com/locate/procedia 47th SME North American Manufacturing Research Conference, NAMRC 47, Pennsylvania, USA 47th SME North American Manufacturing Research Conference, NAMRC 47, Pennsylvania, USA 47th SME North American Manufacturing Research Conference, NAMRC 47, Pennsylvania, Investigation of the Manufacturing Mechanical Punch Loads during Fine Blanking of 47thSME SME North North American Research Conference, NAMRC 47,State Pennsylvania, USA 47th American Manufacturing Research Conference, Penn BehrendUSA Erie, Investigation of the Mechanical Punch Loads during Fine Blanking of 47th SME North American Manufacturing Research Conference, NAMRC 47, Pennsylvania, USA Investigation of the Mechanical Punch Loads during Fine Blanking of Pennsylvania, High-Strength Steels with 2019 Cemented Carbide

Investigation of Punch Loads Fine InvestigationHigh-Strength of the the Mechanical Mechanical Punch Loads during during Fine Blanking Blanking of of Steels with Cemented Carbide Steels with Cemented Carbide a a a,∗ a InvestigationHigh-Strength of the Mechanical Punch Loads during Fine Blanking of High-Strength Steels Cemented Carbide O. Baera , A. Feuerhack , H. Voigts , T. Bergs High-Strength Steels with with Cemented Carbide a a,∗ a Manufacturing Engineering Society International Conference 2017, MESIC 2017, 28-30 June O. Baer , A. Feuerhack , H. Voigts , T. Bergs aaEngineering aa Aachen a,∗ aa Laboratory for MachineHigh-Strength Tools andO. Production (WZL) of RWTH University, Campus-Boulevard 30, 52074 Aachen, Germany a,∗ Steels with Cemented Carbide Baer Bergs a ,, A. a ,, H. a,∗,, T. a O. Baer2017, A. Feuerhack Feuerhack H. Voigts Voigts T. Bergs Vigo (Pontevedra), Spain a

a Laboratory a Laboratory a Laboratory a Laboratory

O. Baer , A. Feuerhack , H. Voigts , T. Bergs O. Baer , A. Feuerhack , H. Voigts , T. Bergs

for Machine Tools and Production Engineering (WZL) of RWTH Aachen University, Campus-Boulevard 30, 52074 Aachen, Germany for 30, a Aachen a,∗ Campus-Boulevard a for Machine Machine Tools Tools and and Production ProductionaEngineering Engineering (WZL) (WZL) of of RWTH RWTH Aachen University, University, Campus-Boulevard 30, 52074 52074 Aachen, Aachen, Germany Germany for Machine Tools and Production Engineering (WZL) of RWTH Aachen University, Campus-Boulevard 30, 52074 Aachen, Germany

Costing models for capacity optimization in Industry 4.0: Trade-off between used capacity and operational efficiency

a Laboratory for Machine Tools and Production Engineering (WZL) of RWTH Aachen University, Campus-Boulevard 30, 52074 Aachen, Germany Abstract Abstract Abstract The increasing ecological demands require emissions reduction of automotive vehicles. This challenges the automotive industry for the increased Abstract Abstract The increasing ecological demands require emissions reductionsteels. of automotive vehicles. This challenges the brings automotive for the processes increased application of light-weight components made of high-strength The processing of high-strength steels manyindustry manufacturing The increasing ecological demands require emissions reduction ofmaterials automotive vehicles. This challenges challenges the automotive industry for the the processes increased application of light-weight components made ofconventional high-strength steels. The processing of high-strength steels many manufacturing The ecological demands emissions reduction of automotive This the automotive industry for increased to itsincreasing mechanical limitations. In finerequire blanking, tool lackvehicles. on compressive strength inbrings order to separate high-strength sheet Abstract The increasing ecological demands require emissions reduction of automotive vehicles. This challenges the automotive industry for the increased application of light-weight components made of high-strength steels. The processing of high-strength steels brings many manufacturing processes to its mechanical limitations. In fine used blanking, conventional tool materials lack on with compressive strength in order to separate high-strength sheet a a,* b b application of light-weight components made of high-strength steels. The processing of high-strength steels brings many manufacturing processes metal. Nowadays, high-speed steels for tool applications must be substituted materials having a higher compressive strength, such as application of light-weight components made ofconventional high-strength steels. The processing of high-strength steels brings many manufacturing to its mechanical limitations. In fine blanking, tool lack on compressive strength in order to separate high-strength sheet The increasing ecological demands require emissions reduction ofmaterials automotive vehicles. This challenges industry for the processes increased metal. Nowadays, high-speed steels used for tool applications must be substituted with materials havingthe a automotive higher compressive strength, such as to its mechanical limitations. In fine blanking, conventional tool materials lack on compressive strength in order to separate high-strength sheet cemented carbides. to its mechanical limitations. In fine blanking, conventional tool materials lack on compressive strength in order to separate high-strength sheet metal. Nowadays, high-speed steels used for tool applications must be substituted materials having aa brings higher compressive strength, such as application ofwith light-weight components made of high-strength steels. The processingwith oflower high-strength steels many manufacturing processes cemented carbides. metal. Nowadays, high-speed steels used for must be substituted with materials having higher compressive strength, such as However, the increased compressive strength the carbides show a significantly ductility. During a repeated stripping off process, the atool applications metal. Nowadays, high-speed In steels used for tool applications must be substituted with materials having in a higher strength, such as University of Minho, 4800-058 Guimarães, Portugal cemented carbides. to itsductility mechanical limitations. fineleads blanking, conventional tool materials lack onthis compressive strength order tocompressive separate high-strength sheet However, with the increased compressive strength the carbides show aand significantly lower ductility. During a repeated stripping offThe process, the cemented carbides. low of cemented carbides to a premature punch fracture with reduced economic feasibility of the process. stripping b cemented carbides. Unochapecó, 89809-000 Chapecó, SC, Brazil However, with the increased compressive strength the carbides show a significantly lower ductility. During a repeated stripping off process, the metal. Nowadays, high-speed steels used for tool applications must be substituted with materials having a higher compressive strength, such as low ductility of cemented carbides leads to a premature punch fracture and with this reduced economic feasibility of the process. The stripping However, with the the increased compressive strength the carbides carbides show significantly lower ductility. During aa repeated repeated stripping off process, process, the offHowever, force required for increased the final punch ejectionstrength is generated by the show clamping force applied by ductility. elastic deformation of the scrap material. Currently, with compressive the aaand significantly lower During stripping off the low ductility of cemented carbides leads to aa premature punch fracture with this reduced economic feasibility of the process. The stripping cemented carbides. off force required for the final punch ejection isapproximate generated byprediction the clamping force applied by elastic deformation of the scrap material. Currently, low ductility of cemented carbides leads to premature punch fracture and with this reduced economic feasibility of the process. The stripping phenomenological models exist allowing only of the clamping force. The precise prediction is not possible, since the lowHowever, ductility of cemented carbides leads to astrength premature punch fracture and with applied this reduced economic feasibility of the process.offThe stripping off force required for the final punch ejection generated by the clamping force by elastic deformation of the scrap Currently, with the increased compressive carbides show aof significantly lower ductility. During a repeated process, the phenomenological models exist allowing onlyis approximate prediction the clamping force. Theas precise isstripping notmaterial. possible, sincenest the off force required for the punch ejection is generated by the clamping force applied by deformation of scrap material. Currently, clamping force depends plenty parameters, such asthe sheet material strength and thickness, well punch prediction geometry, ambient temperature, off force required for theonfinal final punch ejection is generated by the clamping force applied byaselastic elastic deformation of the the scrap material. Currently, phenomenological models exist allowing only approximate prediction of the clamping force. The precise prediction is not possible, since the low ductility of cemented carbides leads to a premature punch fracture and with this reduced economic feasibility of the process. The stripping clamping force depends on plenty parameters, such as sheet material strength and thickness, as well asprecise punch geometry, ambient temperature, nest phenomenological models exist allowing only approximate prediction of the clamping force. The prediction is not possible, since the positioning, etc. A comprehensive investigation of the influence of these parameters on the punch load is strictly required. phenomenological models exist allowing onlyis approximate prediction of the clamping force. Theas precise prediction is notmaterial. possible, sincenest the clamping force depends on plenty parameters, such as sheet material strength and thickness, as well punch geometry, temperature, off force required the final punch ejection generated by the clamping force applied elastic deformation ofhigh-strength the ambient scrap Currently, Abstract positioning, etc. Afor comprehensive investigation of the influence of these parameters onmaterial thebypunch load is strictly required. clamping force depends on plenty parameters, such as sheet material strength and thickness, as well as punch geometry, ambient temperature, nest In order to enhance the applicability of cemented carbides as an alternative punch in fine blanking of steels, a tool for clamping force depends on plenty parameters, such as sheet material strength and thickness, as well as punch geometry, ambient temperature, nest positioning, etc. A comprehensive investigation of the influence of these parameters on the punch load is strictly required. phenomenological models exist allowing only approximate prediction of the clamping force. The precise prediction is not possible, since the In order to enhance the applicability of cemented carbides as an alternative punch material in fine blanking of high-strength steels, a tool for positioning, etc.quantification, A comprehensive comprehensive investigation of the the influence of theseduring parameters on the the punch punch load load is strictly strictly required. process forces such investigation as clamping and stripping offof forces one cutting-ejection cycle, must berequired. provided. To achieve this goal, positioning, etc. A of influence these parameters on is In order to enhance the applicability of cemented carbides as an alternative punch material in fine blanking of high-strength steels, aathis tool for clamping force depends onof plenty parameters, such as sheet material strength and thickness, as well asto punch geometry, ambient temperature, nest process forces quantification, such asthe clamping andof stripping off forces during one cutting-ejection cycle, must be provided. To achieve goal, In order to enhance the applicability of cemented carbides as an alternative punch material in fine blanking of high-strength steels, tool for a comprehensive numerical study on influence process parameters such as sheet metal strength, sheet thickness and cemented carbide grade Under the concept "Industry 4.0", production processes will be pushed be increasingly interconnected, In order toetc. enhance the applicability of cemented carbides asof anthese alternative punch material in fine blanking ofrequired. high-strength steels, athis tool for process forces quantification, suchon asthe clamping and stripping off forces during one cutting-ejection cycle, must be provided. To achieve achieve goal, positioning, Anumerical comprehensive investigation of the influence parameters onmetal the punch load is strictly a comprehensive study influence of process parameters such as sheet strength, sheet thickness and cemented carbide grade process forces quantification, such as clamping and stripping off forces during one cutting-ejection cycle, must be provided. To this goal, on the cutting, clamping and stripping off forces is performed. The developed three-dimensional numerical model of a fine blanking process with information based on a real time basis and, necessarily, much more efficient. In this context, capacity optimization process forces quantification, such as clamping and stripping off forces during one cutting-ejection cycle, must be provided. To achieve this goal, aaon comprehensive numerical study on the influence of process parameters such as sheet metal strength, sheet thickness and cemented carbide grade In order to enhance the applicability of cemented carbides as an alternative punch material in fine blanking of high-strength steels, a tool for the cutting, clamping andstudy stripping offof forces isof performed. The developed three-dimensional numerical model of aand fine blanking processdiscs. with comprehensive numerical on the influence process parameters such as sheet metal strength, sheet thickness cemented grade an elastic punch is validated by means the experimentally measured punch force during an industrial fine blanking process of carbide spacer aon comprehensive numerical study on influence of process parameters such asone sheet metal strength, sheetmust thickness cemented carbide grade goes beyond the traditional aim of capacity maximization, contributing also forthe organization’s profitability and value. the cutting, clamping and stripping off is performed. The developed numerical model of aaand fine blanking process with process forces quantification, such asthe clamping and stripping off forces during cutting-ejection cycle, be provided. To achieve this goal, an elastic punch is validated by means ofaforces the experimentally measured punchthree-dimensional force during an industrial fine blanking process of spacer discs. on the cutting, clamping and stripping off forces is performed. The developed three-dimensional numerical model of fine blanking process with Based on the obtained numerical results, basic correlation between the clamping force during cutting step and the stripping off force during on the cutting, clamping andstudy stripping offof forces isof performed. The developed three-dimensional numerical model of aand fine blanking processdiscs. with elastic punch is validated by means the experimentally measured punch force during an industrial fine blanking process of spacer aan comprehensive numerical onand the influence process parameters sheet metal strength, sheet thickness cemented grade Indeed, lean management improvement approaches suggest capacity optimization instead of Based on the obtained numerical results, acontinuous basic correlation the such clamping force during the cutting step and theand stripping offcarbide force during an elastic punch is by means of the experimentally measured punch force during an industrial fine blanking process of spacer discs. the ejection step are analyzed. identified arebetween used to create anasanalytical model of the clamping force a regression model of an the elastic punch is validated validated byThe means ofaforces thetendencies experimentally measured punch force during an the industrial fine blanking process of spacer discs. Based on the obtained numerical results, basic correlation between the clamping force during cutting step and the stripping off force during on cutting, clamping and stripping off is performed. The developed three-dimensional numerical model of a fine blanking process with the stripping ejection step are analyzed. The identified tendencies arebetween used to create anofanalytical model of the clamping force and a regression model of Based on the obtained numerical results, a basic correlation the clamping force during the cutting step and the stripping off force during maximization. The study of capacity optimization and costing models is an important research topic that deserves off forces. The developed models allow for improved prediction the expected process forces during fine blanking of high-strength Based on the obtained numerical results, a basic correlation between the clamping force during the cutting step and the stripping off force during the ejection step are analyzed. identified are used to create an model the clamping force aa regression model of an elastic is for validated byThe means of appropriate thetendencies experimentally measured punch force an of industrial fine blanking process of high-strength spacer discs. the stripping offthis forces. The developed allow forgrade improved prediction the expected process during fineand blanking of ejection step are analyzed. The identified tendencies are of used to create anofanalytical analytical model of theforces clamping force and regression model of steels and punch with the selection of models the cemented carbide asThis a during punch material. contributions from both the practical and theoretical perspectives. paper presents and discusses a mathematical the ejection step are analyzed. The identified tendencies are used to create an analytical model of the clamping force and a regression model of the stripping off forces. The developed for improved prediction of the expected process forces during Based on the obtained results, basicallow correlation between the clamping during the cutting step andfine the blanking stripping of offhigh-strength force during steels and with this for numerical the selection of models thea appropriate grade of cemented carbide asforce a punch material. the stripping off forces. The developed models allow for improved prediction of the expected process forces during fine blanking of high-strength the stripping off forces. The developed models allow for improved prediction of the expected process forces during fine blanking of high-strength model for capacity management based on different costing models (ABC and TDABC). A generic model has been steels and with this for the selection of the appropriate grade of cemented carbide as a punch material. the ejection stepthis arefor analyzed. The identified tendencies are of used to createcarbide an analytical model of the clamping force and a regression model of steels and with the selection of the grade cemented as aa punch material. © 2019 The Authors. Published by Elsevier B.V. steels and with this the selection of models the appropriate appropriate ofand cemented carbide asexpected punch material. the stripping off forces. The developed allow forgrade improved prediction ofstrategies the process forces during fine blanking of high-strength developed and itfor was used to idle capacity to design towards the maximization of organization’s © 2019 The Authors. Published by analyze Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) steels and with this forPublished the selection ofmaximization the appropriate grade of cemented carbide as a punch material. © 2019 The Authors. by Elsevier B.V. © 2019 The Authors. Published by Elsevier B.V. value. The trade-off capacity vs operational efficiency is highlighted and it is shown that capacity This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) © 2019 The Authors. Published by B.V. Peer-review under responsibility of Elsevier Scientific Committee of NAMRI/SME. © 2019 The Authors. Published by Elsevier B.V.BY-NC-ND This isisan an open access article under the Scientific CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) This anopen open access article under theBY-NC-ND CC (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the Committee oflicense NAMRI/SME. This is access article under CC license (http://creativecommons.org/licenses/by-nc-nd/3.0/) optimization might hide operational inefficiency. This is an open access article under the Scientific CC carbide; license Peer-review under responsibility ofcemented the Committee of (http://creativecommons.org/licenses/by-nc-nd/3.0/) NAMRI/SME. © 2019 The Authors. Published by Elsevier B.V. Committee Keywords: Fine blanking; tool load; FEMCommittee Peer-review under responsibility ofScientific theBY-NC-ND Scientific of NAMRI/SME. Peer-review under responsibility of the of NAMRI/SME. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility ofcemented the Committee of (http://creativecommons.org/licenses/by-nc-nd/3.0/) NAMRI/SME. Keywords: Fine blanking; tool load; FEM license This is an open access article under the Scientific CC carbide; BY-NC-ND Keywords: Fine blanking; tool load; load; cemented carbide; FEMcommittee Peer-review under responsibility of Scientific thecarbide; scientific of the Manufacturing Engineering Society International Conference Peer-review under responsibility the of NAMRI/SME. Keywords: blanking; tool FEM Keywords: Fine Fine blanking; tool load;ofcemented cemented carbide;Committee FEM

A. Santana , P. Afonso , A. Zanin , R. Wernke

2017.

ring processes to its limitations. In fine blanking conventional Keywords: Fine blanking; tool load; cemented carbide; FEM ring processeslack to its Instrength fine blanking conventional 1. Introduction and motivation tool materials onlimitations. compressive in order to separate ring processes to its limitations. In fine blanking conventional Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency ring processes to its limitations. In fine blanking conventional 1. Introduction and motivation tool materials lack on compressive strength in order to separate the sheet metal.toConventionally used high speed steels must be ring processes its limitations. In fine blanking conventional 1. Introduction and motivation tool materials lack on compressive strength in order to separate 1. Introduction and motivation tool materials lack on compressive strength in order to separate the sheet metal. Conventionally used high speed steels must be substituted with materials having higher compressive strength, 1.1. Initial situation 1. Introduction and motivation tool materials lack on compressive strength in order to separate ring processes to its limitations. In fine blanking conventional the sheet metal. metal. Conventionally used highcompressive speed steels steelsstrength, must be be the sheet used high speed must substituted with Conventionally materials having higher 1.1. Initial situation such as cemented carbides. Nowadays cemented carbides the sheet metal. Conventionally used high speed steels mustare be 1. Introduction and motivation tool materials lack on compressive strength in order to strength, separate substituted with materials having higher compressive 1.1. Initial situation substituted with materials having higher compressive strength, such as cemented carbides. Nowadays cemented carbides are 1.1. Initial situation 1. Introduction successfully applied as tool materials for cutting, forming [1] ecological demands require emissions reduction. substituted with materials having higher compressive strength, 1.1.Increasing Initial situation the sheet metal. Conventionally used high speed steels mustare be such as cemented carbides. Nowadays cemented carbides such as carbides. Nowadays cemented carbides are successfully applied as materials for cutting, forming [1] Increasing ecological demands require reduction. and machining tools [2],tool where their hardness, high compresThis challenges the automotive industry foremissions the increased applisuch as cemented cemented carbides. Nowadays cemented carbides are substituted with materials having higher compressive strength, 1.1.Increasing Initial situation successfully applied as tool materials for cutting, forming [1] ecological demands require emissions reduction. successfully applied as tool materials for cutting, forming [1] and machining tools [2], where their hardness, high compresIncreasing ecological demands require emissions reduction. This challenges the for the increased appli- for companies sive strength and wear resistance allows ancutting, improvement with cation ofcost light-weight components ofemissions high-strength steels. The of idleautomotive capacity isindustry amade fundamental information and their management of extreme importance successfully applied as tool materials for forming [1] Increasing ecological demands require reduction. suchmachining as cemented carbides. Nowadays cemented carbides are and tools where their hardness, high compresThis challenges the automotive industry for the increased appliand machining tools [2], where their hardness, high compressive strength and wear[2], resistance allows an can improvement with This challenges the automotive industry for the increased application of light-weight components made of high-strength steels. respect to conventional tool steels. Cemented carbides as a tool The processing of high-strength steels brings many manufactuand machining tools [2], where their hardness, high compresThis challenges the automotive industry for the increased appliin modern production systems. In general, it is defined as unused capacity or production potential and be measured successfully applied as resistance tool materials forancutting, formingwith [1] Increasing ecological demands made require emissions reduction. sive strength and wear improvement cation of light-weight components of high-strength steels. sive strength and wear resistance allows an improvement respect toinconventional tool steels.allows Cemented carbides as thickawith tool cation of components made of high-strength steels. The processing of high-strength steels brings many manufactumaterial fine blanking of conventional steels with sheet sive strength and wear resistance allows anthe improvement with cation of light-weight light-weight components made ofavailable high-strength steels. in several ways: tons of production, hours of manufacturing, etc. The management of idle capacity and machining tools [2], where their hardness, high compresThis challenges thehigh-strength automotive industry for themany increased applirespect to conventional tool steels. Cemented carbides as a tool The processing of steels brings manufacturespect tool steels. Cemented carbides as aa tool material fine of conventional steels with sheet The processing of high-strength steels brings many manufactunesses sto upconventional toand 6blanking mm areresistance industrially applied respect toin conventional tool steels.allows Cemented carbides as thicktool The processing of high-strength steels brings many ∗* Corresponding sive strength wear an [3]. improvement with cation of Afonso. light-weight components made of high-strength steels. Paulo Tel.: +351 253 510 761; fax: +351 253 manufactu604 741 author. Tel.: +49-241-80-24979 ; fax: +49-241-80-22293. material of conventional steels with sheet thickmaterial in fine blanking of conventional steels with sheet thicknesses s in upfine to 6blanking mm are industrially applied [3]. ∗ Corresponding author. Tel.: +49-241-80-24979 ; fax: +49-241-80-22293. To evaluate requirements on fine blanking punches made material in fine blanking of conventional steels with sheet thickE-mail address: [email protected] (H. Voigts). respect to conventional tool steels. Cemented carbides as a tool E-mail address: [email protected] The processing of high-strength steels brings many manufactunesses s up to 6 mm are industrially applied [3]. ∗ Corresponding author. Tel.: +49-241-80-24979 ; fax: +49-241-80-22293. nesses ss up to 6 mm industrially applied [3]. To evaluate requirements on fine blanking punches made ∗ E-mail address:author. of cemented a numerical finite-element (FE) analysis [email protected] (H. Voigts). nesses upfine tocarbide 6blanking mm are are industrially applied [3]. Tel.: +49-241-80-24979 ;; fax: +49-241-80-22293. material in of conventional steels with sheet thick∗ Corresponding To evaluate requirements on fine blanking punches made Corresponding author. Tel.: +49-241-80-24979 fax: +49-241-80-22293. E-mail address: address: [email protected] [email protected] (H. (H. Voigts). Voigts). To requirements blanking punches made of cemented carbide a numerical finite-element (FE) analysis must resulton the analysis, punch loads E-mail Tobeevaluate evaluate onoffine fine blanking punches made 2351-9789 ©©2019 Authors. Published by Elsevier B.V. B.V. nesses s performed. up tocarbide 6 requirements mmAs areaindustrially applied [3].the 2351-9789 2017The The Authors. Published by(H. Elsevier address: [email protected] Voigts). of cemented finite-element analysis ∗ E-mail Corresponding author. Tel.: +49-241-80-24979 ; fax:B.V. +49-241-80-22293. of cemented carbide numerical finite-element (FE) analysis must be performed. Asaaa anumerical result of the analysis, the(FE) punch loads 2351-9789 © 2019 The Authors. Published by Elsevier This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) of cemented carbide numerical finite-element (FE) analysis Peer-review under responsibility of the scientific committee the Manufacturing Engineering Society Conference 2017. 2351-9789 © 2019 The Authors. Published Elsevier of B.V. Tobe evaluate requirements onoffine blankingthe punches made must performed. As aaInternational result the analysis, punch loads E-mail address: [email protected] (H.by Voigts). 2351-9789 © 2019 The Authors. Published by Elsevier B.V. must be performed. As result of the analysis, the punch loads This is an open access article under CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the Scientific Committee of NAMRI/SME. 2351-9789 © 2019 The Authors. Published by Elsevier B.V. must be performed. Asa anumerical result of the analysis, the(FE) punch loads Thisisisanan access article under CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) of cemented carbide finite-element analysis 2351-9789 ©open 2019 The Authors. Published bythe Elsevier B.V. This open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the Scientific Committee of (http://creativecommons.org/licenses/by-nc-nd/3.0/) NAMRI/SME. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the Scientific Committee of NAMRI/SME. This is an open access article under the Scientific CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) must be performed. As a result of the analysis, the punch loads Peer-review under responsibility of the Committee of NAMRI/SME. 2351-9789 ©under 2019responsibility The Authors.of Published by Elsevier B.V. of NAMRI/SME. Peer-review the Scientific Committee 10.1016/j.promfg.2019.06.125 Peer-review under responsibility of the Scientific Committee of NAMRI/SME. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the Scientific Committee of NAMRI/SME.



O. Baer et al. / Procedia Manufacturing 34 (2019) 90–100 O. Baer et al. / Procedia Manufacturing 00 (2019) 000–000

must be derived for the expected range of process parameters. The knowledge of the punch load will allow the selection of the appropriate carbide grade as well as an estimation of the tool life. FE analysis is commonly used to optimize fine blanking processes in terms of required part surface quality and reasonable tool life. Many works exist on the simulation of fine blanking processes. The majority of simulation models imply two-dimensional geometrical models due to axial symmetry of the fine blanking tools and consider the tools as rigid bodies. Hambli [4] performed a fine blanking process simulation for a sheet metal separation analysis with an exponential continuum damage approach. The results showed a good applicability of the proposed damage model for crack initiation and propagation simulation. Often a FE-simulation is used to predict the effect of process parameters such as die clearance [5] on the shear planes in fine blanked parts. Chen et al. [6] have successfully used a mixed pressure-displacement FE-method to predict stress state in the fine blanking parts. The correct representation of the compressive hydrostatic stress for adequate damage prediction is emphasized in the work. Thipprakmas et al. [7] investigated a secondary sheared zone in fine blanked parts. By means of axisymmetric two-dimensional simulations the relationship between the process conditions such as die radii, blank holder force and fine blanked surface characteristics were obtained. Fine blanking of aluminum sheet is investigated to enable the manufacturing of high-precision parts for light-weight applications [8]. Tanaka et al. [9] performed a two-dimensional simulation of a fine blanking process of high-strength steels. The dependencies of the die-roll on the blank holder force as well as die clearance are derived based on the obtained numerical data and experiments. Li et al. [10] have numerically considered a fine blanking process without the use of a vee-ring. The required compressive state in the metal sheet was generated by means of the applied blank holder force. The numerical analysis allowed to derive a recommendation for the process parameters and corresponding cutting surface characteristics. Stanke et al. [11] used data from two-dimensional axisymmetric FE-simulations of a fine blanking process to train an artificial neuronal network. The developed parametrized numerical model allowed for the automatic variation of process parameters such as punch edge, die edge radius or die edge geometry, blank holder and counter punch force. The trained artificial neuronal network was used to predict the die-roll height depending on the varied input parameters. In order to predict the tool loads in a fine blanking process a consideration of elastic tool stresses is required. A numerical simulation of tensile stresses in fineblanking punches on a two-dimensional model was performed by Krobath et al. [12]. A correlation to the fatigue strength was performed and the region of the punch with increased wear was determined. Another two-dimensional simulation considering elastic tool behavior is presented in a work of Klocke et al. [13]. The authors investigated the influence of the die-clearance and friction on the dieroll as well as tool stresses to predict the tool life. Due to complexity of the process leading to high stress and strain gradients in the cutting zone, only a few numerical works exist, considering the three-dimensional geometry of the fine

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blanking tool and sheet. A dual mesh approach employing Arbitrary Lagrangian Eulerian FE-formulation is used to perform a fine mesh in the cutting region as well as to adequately represent tool edge radius [14]. A blanking process in three-dimensional formulation is considered in the work of Bohdal et al. [15] to qualify the blank’s sheared edge. The three-dimensional formulation allows to capture material and process non-linearities inherent to a real blanking process, which is neglected in a two-dimensional approach. Yang et al. [16] performed a threedimensional FE-analysis of the fine blanking of helical gears. The three-dimensional formulation was strictly required by the geometry of gear tooth. 1.2. Objective The applicability of cemented carbides as tool materials for fine blanking of high-strength steels requires the determination of the process forces acting on the tool. The analysis must cover the wide range of process parameters such as sheet metal thickness, material’s tensile strength and cemented carbide grade in order to describe main interdependencies. The comprehensive analysis will allow for establishing of a physically-motivated model for the prediction of the process forces acting on fine blanking punches. The main objective of the current work is to establish such a model and as a result to allow the knowledgebased selection of cemented carbides regarding their tensile and fatigue strength. 1.3. Methodology As a first step, the analysis of the mechanical properties of selected cemented carbides with varying binder content and grain size as potential tool material based on their microstructure is performed. The high-strength steels selection as sheet materials based on their mechanical properties is done in Section 2. The analyzed cemented carbides cover the wide range of industrially available materials and allow direct correlation between the cemented carbide microstructure, their mechanical properties and finally their endurance as fine blanking punch. As a second step, a numerical model of the fine blanking process of the industrially produced spacer discs by means of the FE-method is constructed in Section 3. The numerical model validation is performed with measured data of the punch force in an industrial process in Section 3.4. The third step is the model verification on a range of sheet materials and sheet metal thicknesses performed in Section 3.5. Based on the numerically generated data the analysis of the clamping and stripping off forces is performed in a fourth step. As a result, an analytical model for the prediction of the clamping force based on shafthub theory, see Section 4.2, as well as a regression model for the prediction of the stripping off forces, see Section 4.3, are derived. 2. Analysis of material properties In the current section a selection of the tool and sheet materials for the fine blanking process simulation is justified. The 2

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main micro mechanical features of the carbides affecting mechanical properties such as binder content fc and average grain size L, are considered to characterize the carbide mechanical properties. Additionally, the selection of high-strength steel materials regarding to their tensile strength is performed in order to enable the numerical data generation in a wide spectrum of tensile strength.

The resulting value of tensile strength depending on the specimen microstructure varied in the range from 1,500 to 2,900 MPa, showing significant scatter. A resistance to bending increases with increased Co-content and the maximum can be found at the middle grain size due to relatively low resistance to the crack initiation and propagation [1]. In order to cover a wide range of the cemented carbides and with this a wide range of the mechanical properties of interest, four cemented carbide grades were selected and analysed: CF-F35Z [21], CTF40E [22], CTM12D [23] and CTS12L [24] according to the specification of Ceratizit company. Their representative microstructure is exemplary given in Figure 2.

2.1. Analysis of cemented carbides properties The advantages of cemented carbides are their high hardness in comparison to the tool steels, high compression strength, wear resistance as well as high strength under high-temperature conditions. Such excellent mechanical properties of cemented carbides are determined by their composite microstructure, see Figure 1. It consists of hard particles such as tungsten carbides (WC), having hexagonal structure and hard particles of TiC or TaC, having cubic structure. Hard carbide particles are surrounded by a binder phase consisting of Co, Ni and Fe as schematically presented in Figure 1.

CF-F35Z

CTF40E

20 µm

20 µm CTS12L

CTM12D

hard particles (WC – hexagonal) hard particles (TiC, TaC – cubic)

20 µm

binder phase (Co, Ni, Fe)

20 µm

Fig. 2. Representative microstructure of the selected cemented carbides

The selection of cemented carbide grades was done based on the averaged grain size L and Co-content fc as it is presented in Figure 3.

Fig. 1. Schematic representation of cemented carbide microstructure [17]

Averaged grain size L [µm]

The microstructure consisting of strong, rigid carbides, embedded in a ductile matrix allows to ensure enhanced effective mechanical properties of the macro material. Presence of Ni in the binder phase increases corrosion resistance of the cemented carbide [18]. However, with the increased compressive strength, the cemented carbides show a significantly lower ductility, resulting in low tensile and bending strength. Due to the lack of tensile strength, an application of the cemented carbide as tool material is limited to axially symmetric tools without excessive tensile stress loadings. Taking into account high costs of cemented carbides as well as their processing to a tool, only the manufacturing of a high number of parts is economically reasonable. Typical applications for cemented carbides as tool materials are extrusion dies and blanking punches. Typically, fracture of cemented carbides takes place due to excessive static loads and respective loss of tensile strength. Another widely spread cause of carbides’ fracture is fatigue due to cycling loads [19] and subsequent loss of fatigue strength. Due to exceptional hardness and brittleness of cemented carbides, the direct determination of mechanical properties such as yield strength, ultimate tensile strength and fatigue strength is complicated [20] that is why these values are not provided by manufacturers in the documentation sheet [21]. Kl¨unsner et al. [20] investigated tensile properties of cemented carbides depending on the WC grain size L and binder (Co) content fc .

3.5

CF-F35Z CTF40E CTM12D CTS12L

3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0

5.0

10.0 15.0 Co-content fc [%]

20.0

25.0

Fig. 3. Representation of the four selected cemented carbides depending of their Co-content fc and averaged grain size L

Figure 3 demonstrates, that the selected cemented carbides cover submicron grain size (L < 0.8 µm) as well as coarse grain size (L = 2.5 − 6.0 µm). The minimal content of the binder phase fc is of about 6 and 20%. This leads to respective variation of the compression strength σm from 3,900 (CF-F35Z and CTF40E) to 7,200 MPa (CTS12L) and variation of the elastic modulus E from 480 (CF-F35Z) to 630 MPa (CTS12L). The selected carbide grades are used for numerical analysis with FE-method, see Section 3. 3



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Tensile strength Rm [MPa]

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2.2. Selection of high-strength sheet materials In order to investigate the influence of the sheet material strength on the punch loadings during fine blanking, four highstrength steel materials with tensile strength Rm varied in range from 750 to 1,000 MPa were used for numerical modeling. The selected materials are C45E, C86D, 34CrNiMo6 and 42CrMo4 steels, given in order of tensile strength decrease. The elasticplastic properties of the selected steels were modeled by means of definition of elastic modulus E, Poisson’s ratio ν and flow curves, see Section 3.2. Based on the available flow curves in a temperature range of cold forming, the ultimate tensile strength was determined for each material according to the phenomenological relationship [25]: Rm = Knn ,

42CrMo4 34CrNiMo6 C86D C45E

ν [−] 0.3 0.3 0.3 0.3

800 700 600 500

0

100

200

300

400

on the cutting force prediction with a phenomenological relationship [28].

(1)

Rm [MPa] 753 805 922 1,002

900

Fig. 4. Temperature dependency of the tensile strength Rm of the simulated sheet materials

3.1. Geometrical model and mesh The geometry of the industrially produced spacer disc is presented in Figure 5(a). The main spacer disc dimensions are the outer diameter D = 102.3 mm and width w = 10 mm. The experimentally measured punch force during fine blanking of the spacer disc from C45E steel with a thickness s of 3.2 mm was available from an industrial partner. The C45E cold-rolled steel (EN 10132-3) has an experimentally determined tensile strength Rm ranged from 966 to 982 MPa, with this referred to the high strength steel grade [29].

Table 1. Selected mechanical properties of the modeled steels at room temperature [26, 27]

E [GPa] 210 220 210 220

C45E C86D 34CrNiMo6 42CrMo4

1,000

Temperature T [◦C]

where K [MPa] is the strength coefficient and n [−] is the strain hardening exponent of the flow curve for the corresponding steel at given temperature. The ultimate tensile strength Rm estimated from the flow curves for the investigated steels at room temperature together with elastic modulus E, Poisson’s ratio ν and density ρ are given in Table 1.

Material

1,100

ρ [g/cm3 ] 7.85 7.83 7.81 7.85

punch guide slug ejector

Using Equation (1), tensile strength evolution with the temperature was determined in the range relevant for fine blanking processes. The tensile strength data was generated at temperature intervals of 25◦ C to obtain a smooth trend from the flow curves at given temperatures. The flow curves for given temperatures were generated using Equations (2), (3) and (4) and fitted with the power law parameters K and n. These parameters together with Equation (1) were used for tensile strength calculation. The evaluated trends of tensile strength with temperature, see Figure 4, showed that the tensile strength of the materials C45E and 34CrNiMo6 decrease more strongly with temperature than those of the sheet materials C86D and 42CrMo4. This dependency of the tensile strength on the temperature was included in the modeling of clamping and stripping off forces in Sections 4.2 and 4.3.

main punch blank holder sheet

D

inner punch

die

w s a

ejector

b

Fig. 5. Industrial data of the fine blanking process: (a) spacer disc geometry of the thickness s; (b) tool active elements

Disregarding drillings and faces, an assembly of the active tool elements together with a sheet metal has an axisymmetric shape, see Figure 5(b). The fine blanking process of the spacer disk takes place in axial direction. The investigated clamping forces acting on the punch are generated by the stresses acting in the radial direction of the punch. In order to capture these radial stresses a three-dimensional model formulation is required. However, the full three-dimensional modeling of the fine blanking process, accounting elastic behavior of the punch is challenging in terms of computational power. In order to reduce the computational costs and at the same time obtain a reasonable precision of the simulated stress state, 1/16th section of the geometry is modeled. The geometrical model of the active tool elements was established by means of CAD data import in Forge software, see Figure 5(b). The CAD data was adopted in terms of adding the

3. Finite-element analysis of the fine blanking process To capture stress-strain distribution in the punch during the fine blanking process a FE-analysis was performed. An industrial process of fine blanking of spacer discs was considered. The experimentally measured punch force during fine blanking of the spacer disc from C45E steel with a thickness of 3.2 mm was used to validate the model. Numerical model verification for other sheet thicknesses and sheet materials was done based 4

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edge fillet radius on the main punch as rmp = 0.05 mm, internal punch and die radii as rip = rdie = 0.3 mm. The entire tool elements except of the main punch were modeled as rigid bodies, since their deformation is negligibly small in comparison to the deformation of the metal sheet. In order to reduce the computational time, the entire fillet and drilling were removed, having a negligible effect on the resulting stress distribution in the model. The main punch was modeled as an elastic body since it is required to capture contact stresses and the resulting contact forces between the punch and metal sheet at the end of the cutting process. To mesh the rigid bodies slug ejector, blank holder, die, ejector and inner punch, surface elements are used. For the main punch and metal sheet volumetric elements are used, to capture the deformation of the entire volume. The selected elements have triangular shape, allowing meshing of the complex geometries. This is especially important, since forming simulation assumes large deformations leading to element distortions. This requires a repeated remeshing during the simulation. The triangular element shape ensures accurate automatic remeshing of complex geometrical shapes. The standard remeshing algorithm implemented in Forge software with the remeshing on deformation set to 1 is used. This mode was selected as giving accurate simulation results as well as smooth mesh with a reasonable computational time.

properties were set in terms of Young’s modulus and Poisson’s ratio, see Table 1. The elastic properties were given for room temperature (RT) conditions and their temperature dependency was neglected. In order to simulate material flow of metal sheet during fine blanking, the flow curves of the selected steels were integrated in the model. The flow data for C45E steel was taken from experimental work of Hoppe [26]. The flow curves for C45E are described in terms of a modified Swift model [30], taking into account temperature and strain rate dependency of the flow stress. The modified model has the form [26]: ˙ Ψ(T ), σF = (K1 (B1 + ε)n1 + η · ε)

(2)

where K1 , B1 , n1 and η are material parameters describing the influence of the strain ε and strain rate ε˙ on the flow stress σF . The temperature influence Ψ(T ) on the flow stress is described by the function below [26]:   T − T0 , (3) Ψ(T ) = exp −β1 Tm where β1 is the material parameter, T 0 is an ambient or reference temperature and T m is the melting temperature of modeled steel. The fitted material model parameters for C45E steel are listed in Table 2. Table 2. Material model parameters of C45E steel [26]

K1 [MPa] 1,390

3.2. Material model To complete the numerical model of the fine blanking process, the material model of the punch made of cemented carbide and of the metal sheet are defined. The elastic behavior is prescribed to the punch, based on the data described in Section 2. The metal sheet is modeled to deform elastic-plastic in the temperature and strain rate region relevant for fine blanking. The other tool elements except of the main punch are modeled as rigid. The standard material model for the elastic punch made of tungsten carbide available in Forge software is used. The material model is given through the definition of the density ρ and elastic modulus E. The CTS12L material with the elastic modulus E of 630 GPa, Poisson’s ratio ν of 0.24 and density ρ of 14.78 g/cm3 is modeled. Simulations with selected sheet materials and thicknesses were performed with the other three cemented carbides, described in Section 2.1, the results showed negligible variations of the cutting and stripping off forces. However, the reduction of the elastic modulus of the main punch must lead to intensification of the spring back effect due to increased elastic strains and with this influence the clamping forces. The absence of the effect in the current work is explained by the used model formulation. To enable this effect in the analysis, improved contact conditions in the numerical model must be used in future work. High strength steels C45E, C86D, 34CrNiMo6 and 42CrMo4 were investigated in the current research. The material behavior of the four materials was modeled as elasticplastic. Consideration of elasticity was required in order to capture the spring back effect of the scrap after cutting. Elastic

B1 [−] 0

n1 [−] 0.21

η [MPa · s] 0.02

β1 [−] 1.43

T0 [◦ C] 20

Tm [◦ C] 1,428

By means of the proposed parameters the tabular data is created for the expected ranges of temperature and strain rate 20 – 300 ◦ C and 0.1 – 1,000 s−1 respectively. The tabular data for the flow stress is implemented in Forge software. The flow curves for 34CrNiMo6, 42CrMo4 and C86D steels were taken from the Forge software material database [27], where the coefficients of the Hensel–Spittel model were given [31]. The Hensel–Spittel equation for the case of cold forming has the form [31]: σF = A1 · expm1 T · εm2 · expm4 /ε · ε˙ m2

(4)

Material model parameters m1 , m2 , m3 , m4 for 34CrNiMo6, 42CrMo4 and C86D steels are listed in the Table 3. Table 3. Material model parameters of 34CrNiMo6, 42CrMo4 and C86D steels [27]

Material 34CrNiMo6 42CrMo4 C86D

A1 [MPa] 1,009.7 929.8 1,257.2

m1 [10−3 ] -1.1 -0.64 -0.71

m2 [10−1 ] 1.28 0.89 1.57

m3 [10−2 ] 1.02 1.14 1.0

m4 [10−4 ] -0.2 -80 3.2

The Latham–Cockcroft criterion [32] is used to represent material separation during cutting. The so-called trigger value is used to determine the criterion for element deletion and is set equal to the sheet thickness for each simulation. With this setting the material separation will take place at the end of cutting process. The cutting surface consisting of smooth shearing 5



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1,600

Force F [kN]

zone and fracture zone is not represented. Such settings allows for significant reduction of the calculation time and does not affect the final process forces and thus is acceptable for the current investigation. 3.3. Process kinematics and contact conditions The entire fine blanking process is modeled in the two steps: cutting and stripping off. The steps ejection of the part and scrap for the next operation are eliminated since at these stages no forces are applied to the main punch. During both, cutting and stripping off steps, the die, inner punch and slug ejector, see Figure 6(a) and (b) are modeled as fixed in space. vc

y = 287.5x − 0.0016 800

400

0 2.8

3.2

3.6

4

4.4

4.8

5.2

Sheet thickness s [mm] Fig. 7. Process forces applied for numerical analysis

The Coulomb’s friction law is used to determine the friction condition between the sheet and tool elements. The friction coefficient µ = 0.1 is used as a recommended value for fine blanking processes [5]. No contact condition is defined between the main punch and tool elements. Finally, a bilateral sticking condition is determined between the punch guide and main punch to transfer velocity boundary conditions.

Fbh

Z

Fe

1,200

y = 105.1x − 5.67

vc

Fbh

blank holder force Fbh ejector force Fe measured values

3.4. Model validation X

a

The numerical FE-model of the fine blanking process of the spacer disc was validated by means of existing process data of the punch force Fp . The process data was provided for the sheet material C45E and the sheet thickness s = 3.2 mm by an industrial partner. The maximum punch force was measured as Fp = 1,553 kN. The FE-analysis with the generated model was carried out with different element sizes and remeshing strategies in order to determine the optimum numerical model parameters in relation to the resulting punch force. A comparison of the simulated force profile during cutting of the model with the maximum punch force (filled circle) measured during the industrial process is presented in Figure 8. The standard remeshing procedure together with the initial element size equal to 0.2 mm led to a good agreement between the measured and simulated maximum punch force and showed a relative deviation of 3.8%. In this way determined numerical model parameters were used for further simulations, see Section 3.5.

b

Fig. 6. Process kinematics during the (a) cutting and (b) stripping off step

The blank holder force Fbh is applied on the blank holder, acting in −Z direction. Additionally during the cutting step, see Figure 6(a), the ejector presses on the sheet with the constant force Fe and moves together with the main punch in −Z direction. The values of blank holder Fbh and ejector forces Fe for sheet thickness s = 3.2 mm were measured in an industrial process. Since no process data from the industrial partner were available for other selected sheet thicknesses and materials, the blank holder and ejector process forces were calculated by linear interpolation. The verification was carried out by comparison of the numerically determined maximum values of the punch force with the values calculated according to an existing phenomenological approach. The resulting summary on applied process forces depending on sheet metal thickness is presented in Figure 7, the experimentally measured values are highlighted in the box. Since 1/16th section of the geometry is modeled, the 1/16th of the force values are applied in the model. According to the Forge software specification, no press kinematics can be directly applied to the deformable body. To solve this problem, a punch guide was included in the model additional two-dimensional rigid body to represent main punch kinematics. Connected with the top of the main punch by bilateral sticking contact condition the punch guide provides the cutting velocity vc to the main punch. According to the available process data the cutting velocity vc is set to 25 mm/s for all sheet thicknesses. During the stripping off step, see Figure 6(b), no ejector force is applied on the sheet and the movement of the deformable punch is prescribed in +Z direction.

Punch force Fp [kN]

1,600

discrepancy range

-3.8% 1,200 800 400

Simulation, s = 3.2 mm Exp. data, s = 3.2 mm

0 0

1

2

3

4

Punch stroke h [mm] Fig. 8. Comparison of the numerically calculated progression of punch force Fp with the measured maximum value during fine blanking of the spacer disc with a thickness of 3.2 mm made of C45E sheet metal

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and outer σr,outer sides of the punch. The radial stresses on the main punch were examined at the end of the cutting process, see Figure 10, to take into account the spring-back effect of the sheet scrap.

3.5. Numerical model verification for other sheet thicknesses and materials

Max. cutting force Fcmax [kN]

Radial stress σr , [MPa]

In order to investigate the dependency of clamping and stripping off forces on sheet thickness and tensile strength, the developed simulation model was extended to the other sheet thicknesses of 3, 4 and 5 mm as well as to the materials C86D, 34CrNiMo6 and 42CrMo4. Since no process data from the industrial partner were available for the selected sheet thicknesses and materials, blank holder and ejector forces were calculated by linear interpolation. The cutting force was calculated using existing analytical formulas [28]. The assumption was made that the ejection and blank holder forces are not dependent on the sheet material strength. The fine blanking FE-simulation model was executed with the validated numerical parameters from Section 3.4. The numerically calculated values of the maximum cutting force Fcmax , see Figure 9, were compared with the forces calculated according to the phenomenological Equation (7).

1,000 750

800

850

900

950

low σr,outer

Fig. 10. Distribution of radial stresses on the inner and outer sides of the main punch at the contact zone with the sheet scrap at the end of the cutting process

The simulated radial stress distribution on the inner and outer surfaces of the main punch in Figure 10 shows an uneven distribution due to numerical issues. Additionally, the contact area height does not coincide with the sheet thickness, but is reduced to the upper region of the scrap-punch contact. To calculate the clamping force, the actual surfaces of the radial stress peaks distribution corresponding to the contact surfaces were considered on the inner Ainner and outer surfaces Aouter . These contact surfaces Ainner and Aouter were determined manually for each simulation by calculating the current height of contact surface between scrap and the main punch. The element radial stresses in the areas Ainner and Aouter were determined manually and an averaged value was calculated. Thereby the averaged values of σr,inner and σr,outer were determined and the resulting clamping forces on the inner and outer surfaces were calculated as follows:

sim., s=3.0 sim., s=3.2 sim., s=4.0 sim., s=5.0 mod., s=3.0 mod., s=3.2 mod., s=4.0 mod., s=5.0

1,500

main punch

σr,inner

2,500

2,000

high

1,000

Tensile strength Rm [MPa] Fig. 9. Numerically calculated and phenomenologically predicted dependency of the maximum cutting force Fcmax on material tensile strength during fine blanking of the spacer discs of various thicknesses s given in mm

Fk,inner = σr,inner · Ainner ; Fk,outer = σr,outer · Aouter 4. Analysis of process forces

The total clamping force was then calculated as the sum of the internal and external forces:

In the following section the loading acting on the fine blanking punch during the cutting and stripping off step are analyzed. Special attention is payed to the dependency of the clamping force on the sheet material and sheet thickness. An analytical model based on the shaft-hub connection theory is proposed to predict the clamping force during fine blanking of the highstrength sheet materials. The relationship between the sheet thickness s, the tensile strength Rm as well as the punch geometry was analyzed and an analytical model for the clamping force was derived.

Fk,total = σr,total · Atotal

(5)

4.2. Derivation of the analytical model of clamping force The clamping force was derived analytically based of the shaft-hub connection theory. The joining pressure p [N/mm2 ] of a cylindrical shaft-hub-press connection is calculated according to the following equation, which can be found elsewhere [33]: Fax · S r , (6) p= π · Dj · lj · µax where Fax – axial force [N], S r – slip resistance [−], Dj – diameter of the joining [mm], lj – joining length [mm] and µax – friction coefficient in axial direction [−]. In the case of the fine blanking of the investigated spacer disc, the total joining pressure p consists of the pressure on the inner pinner and outer pouter surfaces of the main punch, see Figure 10. This results in

4.1. Clamping force analysis and modeling The numerical analysis of the fine blanking process was performed for sheet thicknesses of 3, 4 and 5 mm as well as for sheet materials C45E, C86D, 34CrNiMo6 and 42CrMo4. To determine the total clamping force Fktotal acting on the main punch, the radial stresses were determined on the inner σr,inner 7



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the following expression:

regression analysis was performed. The relationship between the contact area height hk and the sheet thickness s can be represented as follows:

p = pinner + pouter The joining pressure on the inner pinner and outer pouter surfaces of the main punch is produced by the clamping forces on the corresponding surfaces and thus applies: Fk,outer Fk,inner ; pouter = pinner = Ainner Aouter After the calculation of the contact surfaces Ainner and Aouter and the joining pressure according to Equation (6) the resulting relationship is Fax · S r Fk,inner = Fk,outer = µax Combined with the Equation (5), the total force is calculated as 2 · Fax · S r Fktotal = . µax The axial force Fax is derived from the cutting force Fc as:

hk = 0.67 · s − 0.44

4.2.2. Dependency on the tensile strength The simulation of the fine blanking process of spacer discs showed a significant temperature increase in the shearing zone up to the maximum value T c at the end of cutting. The temperature increase leads to a significant decrease of the tensile strength Rm , see Figure 4. Therefore, the use of the tensile strength Rm at room temperature in Equation (7) is not appropriate. In order to make an accurate prediction of the clamping force Fk , the value of the tensile strength at the actual temperature at the end of the cutting process Rcm had to be defined. The numerically determined peak temperature values in ◦ C showed a linear dependency on the sheet thickness s, which was defined by means of a linear regression as follows:

Fax = Fc · f3 , where f3 = 0.1 is an empirical factor [34]. The maximum cutting force Fcmax is calculated according to the relationship [28]: Fcmax = lc · s · Rm · f1 ,

(7)

where lc is the total length of the cutting line and f1 = 0.6 is an another empirical factor [34].

T c = 35.08 · s + 143.57

800 700

4

600 500

3

400

2

300 1

200

100

0 2

3

4

5

6

7

8

Sheet thickness s [mm]

9

Tensile strength Rm [MPa]

Contact area height hk [mm]

900

5

(9)

Considering the temperature in the cutting zone at the end of the cutting process T c and the temperature dependency of the tensile strength Rm , see Figure 4, the Equation for calculating the material tensile strength at the cutting zone at the end of the cutting process Rcm has the form:

4.2.1. Dependency on the contact height The simulation results have shown that at the end of the cutting process, the contact does not exist over the entire sheet thickness s. Therefore, the value of sheet thickness s in Equation (7) was replaced by the current contact area height hk . The current values for each material and each sheet thickness were determined numerically and the results are shown in Figure 11 (hollow circle markings). 6

(8)

The obtained relationship, see Equation (8), is valid for the geometry of the investigated spacer disc. The general conclusion can be made, that independently on the part geometry, a linear interdependency between the sheet thickness and the resulting height of the contact area is expected. Such linear interdependency is rather low affected by the strength of the metal sheet material.

Rcm = b1 · T c2 + b2 · T c + b3

(10)

The parameters b1 , b2 and b3 in the description of the temperature dependency of the tensile strength were determined for each material from experimental data. The determined values of the parameters b1 , b2 and b3 for the investigated steels are listed in Table 4.

sim. hk , C45E sim. hk , C86D sim. hk , 34CrNiMo6 sim. hk , 42CrMo4 arithm., mean hk sim. Rm , C45E sim. Rm , C86D sim. Rm , 34CrNiMo6 sim. Rm , 42CrMo4 mod. Rm , C45E mod. Rm , C86D mod. Rm , 34CrNiMo6 mod. Rm , 42CrMo4 mod. hk , all mater.

Table 4. The values of parameters b1 , b2 and b3 determined from the experimental data for the description of the temperature dependency of the tensile strength Rm

Material C45E C86D 34CrNiMo6 42CrMo4

Fig. 11. Numerically calculated and modeled dependencies of the contact surface height hk on tensile strength Rm and sheet thickness s

b1 3 · 10−4 −7 · 10−4 5 · 10−4 −1 · 10−4

b2 9.8 · 10−1 1.6 · 10−1 9.2 · 10−1 3.9 · 10−1

b3 1.021 · 103 9.255 · 102 8.236 · 102 7.608 · 102

Updated Equation (7) for the cutting force results in the relation for the total clamping force: 2 · f1 · f3 · S r · lc · hk (s) · Rcm (T c , s) Fktotal = . µax With the coefficient of friction in the axial direction µax , assumed to be 0.2, and the slip resistance factor S r , set to 0.26, the total clamping force Fktotal for the sheet thicknesses s = 3 to 9 mm was modeled, as shown in Figure 12.

The numerically determined values of the contact surface height hk showed a clear dependency on the sheet thickness s. However, the dependency on the material tensile strength Rm was low and thus was neglected in the modeling. In order to create a phenomenological formula for the description of the relationship between the current contact area height hk and the sheet thickness s, an average value was calculated and a linear 8

O. Baer et al. / Procedia Manufacturing 34 (2019) 90–100

98

9

400

sim., s=3 sim., s=4 sim., s=5 mod., s=3 mod., s=4 mod., s=5 mod., s=6 mod., s=7 mod., s=8 mod., s=9

300 200 100 0 740

Max. stripping off force Famax [kN]

Total clamp. force Fktotal [kN]

O. Baer et al. / Procedia Manufacturing 00 (2019) 000–000

830

920

Tensile strength Rm [MPa]

14 10

1,010

Fktotal

Fig. 12. Evolution of the total clamping force as a function of the tensile strength Rm , modeled for the sheet thicknesses s = 3 to 9 mm and compared with the simulated values

8 6 4

2 0 750

850

950

1.050

Tensile strength Rm [MPa]

Fig. 13. Numerically calculated and modeled course of the maximum stripping off force Famax , determined for different sheet thicknesses s given in mm as a function of the tensile strength Rm

The increased coefficient of friction compared to the coefficient of friction set in the simulation was selected to consider the increase in roughness of the scrap material, contacting the punch. The model captures well the variations of the clamping force as a function of tensile strength as well as the increase of the clamping force with increasing sheet thickness. For a sheet thickness s = 3 mm, the simulation results show large fluctuations in the values of the clamping force and the correspondent prediction with the developed model is rather poor. The simulated and modeled values for the sheet thicknesses s = 4 and 5 mm show good agreement. Due to the low range of values of the clamping force produced by the sheet thickness s = 3 mm, no critical value of the stripping off force will be generated and thus no punch fracture is expected. The model shows a good agreement with the simulation results for larger sheet thicknesses, which represent the relevant range for the determination of the stripping off force and thus the critical loads for punches from cemented carbides.

Similar to the results for the clamping force, the simulation results for a sheet thickness s = 3 mm, show a large variation in the values of the stripping off force. Due to the low value of the stripping off force at a sheet thickness s = 3 mm, no critical load on the cemented carbide punch is applied. The model shows a good agreement with the simulation results of the larger sheet thicknesses, which represents the relevant range for determining the critical load of a carbide punch. For the fine blanking of the sheet material C86D with a thickness of s = 9 mm, the created model predicts a clamping force of 350 kN, see Figure 12. According to the model of the stripping off force produced, such a clamping force corresponds to a stripping off force of 14 kN. According to the phenomenological prediction for the stripping off force in fine blanking [28] as 10 % of the maximum cutting force Fcmax , the stripping off force for these sheet parameters must reach values up to 220 kN. The significant underestimation of the stripping off force with the model created can be explained by the inaccuracy of the numerical data. Due to a lack of experimental data for the stripping off force, the simulation was not validated with respect to this process parameter. The data of the experimentally measured stripping off force during fine blanking of high-strength sheets is required to validate the numerical simulation regarding the stripping off force and specify the parameters a1 and a2 accordingly. The form of the function in Equation (11) will remain unchanged. The improvement of the contact formulation of the numerical model is recommended to enable consideration of the tool components effect of the elastic punch. Such an improved contact formulation will allow to reproduce the die clearance effect on the cutting and with this clamping and stripping off forces. However, the consideration of the contact between the tool components and punch will lead to increase of the required calculation efforts.

4.3. Derivation of the regression model of stripping off force Based on the numerical results obtained with the FE-model described in Section 3 and the model for the clamping force from Section 4.1, a model of stripping off force Fa was derived using a regression analysis. The maximum stripping off force Famax and the tensile strength Rm were numerically determined for sheet thickness s equal to 3, 4 and 5 mm at the beginning of the stripping off step and are shown in Figure 13 as filled markings. In order to model the dependency of the maximum stripping off force on the sheet thickness s and tensile strength Rm , the following relationship was determined: Famax = a1 · lc · s · Rcm (T c , s) + a2 ,

sim., s=3 sim., s=4 sim., s=5 mod., s=3 mod., s=4 mod., s=5 mod., s=6 mod., s=7 mod., s=8 mod., s=9

12

(11)

where a1 and a2 are regression coefficients. The temperature dependency of the material tensile strength at the cutting zone at the end of the cutting process Rcm , defined in Equation (10) for the clamping force model, was used. Together with the regression coefficients a1 = 4.83 · 10−6 and a2 = 5 the stripping off force for the different sheet thicknesses and tensile strengths is modeled, see Figure 13. The model correctly captures the variations of the stripping off force as a function of the tensile strength at the cut zone and the increase of the stripping off force with increasing sheet thickness.

5. Conclusions and outlook The result of this work is the knowledge about the relationship between the fine blanking process parameters and clamping forces acting on the punch as well as the resulting stripping off forces. Using a three-dimensional FE-simulation model of 9



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fine blanking with an elastic punch, the numerical data for the stress state in the punch were generated as a function of the sheet thickness s and the tensile strength Rm of the material. This enabled determination of the relationship between clamping and stripping off forces, sheet thickness and tensile strength. The analytical model of the clamping force Fk was derived using the shaft-hub-connection theory. The model was validated on the basis of the numerically generated data on the clamping forces acting on elastic punch. The developed model is based on the physical values of the sheet thickness s and the tensile strength of the sheet material at the cutting zone at the end of the cutting process Rcm . A consideration of the tensile strength of the sheet material at the cutting zone at the end of the cutting process Rcm in the model leads to a significant increase of the predictability of the clamping force in comparison to the consideration of the tensile strength at room temperature Rm . The obtained relationship for the contact area height hk is valid for the geometry of the investigated spacer disc. The determined linear dependency in Equation (8) between the sheet thickness s and the resulting height of the contact area hk is expected to be found for other geometries of the fine blanked parts. The slope of the linear curve is rather low affected by the strength of the sheet metal material. The dependency of the peak temperature at the end of cutting process T c on the sheet thickness s in Equation (9) has also a linear character. The precise values of the parameters can be measured experimentally or determined numerically for the given case of the part geometry. If the peak temperature at the end of cutting process for the given fine blanking process parameters is known, the corresponding tensile strength value can be evaluated form the available flow curve data for a given material and the resulting dependency can be determined. With the known dependencies for the contact area height hk and material tensile strength at the cutting zone at the end of the cutting process Rcm the Equation (11) for the definition of the clamping force Fktotal can be used in its present form. A regression model of the stripping off force Fa was generated as a function of the sheet thickness s and the tensile strength of the sheet material at the cutting zone at the end of the cutting process Rcm . The form of the function was chosen to fit the numerically determined values of the stripping off force in the best way. Due to missing validation data on the stripping off force, the created model underestimates the generated stripping off force. The experimental tests, including the determination of clamping and stripping off forces must be conducted in the future. On the basis of the test results, the model parameters determined for clamping and stripping off forces can be specified more precisely or extended for other part geometry cases. The entire simulations were performed considering the elastic modulus of CTM12L cemented carbide for a punch material. The simulations with the elastic modulus variations, representing other selected cemented carbides showed no significant difference in the variation of the cutting, stripping off and clamping forces. Thus at the current stage, the developed models of the clamping and stripping off forces do not consider influence of the elastic modulus of the cemented carbide. To improve the numerical model, the consideration of more com-

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plex contact conditions is required. This will allow for consideration of the effect of punch elastic strains and die clearance effect. Additionally, the influence of bending loading on the punch strength during fine blanking must be considered in future since all four selected cemented carbides have various levels of bending strength. Under consideration of pure tensile loads, the developed model in its current state predicts that all four selected carbides are suitable for fine blanking of the high strength steels. Acknowledgments This work was supported by the German Research Foundation and Karl Scharrenbroich GmbH & Co. KG. References [1] F. Klocke. Fertigungsverfahren 4. Umformen. SpringerVieweg, Berlin, 2017. In German. [2] V. Bonache, E. Ray´on, M.D. Salvador, and D. Busquets. Nanoindentation study of WC–12Co hardmetals obtained from nanocrystalline powders: Evaluation of hardness and modulus on individual phases. Materials Science and Engineering: A, 527(12):2935 – 2941, 2010. ISSN 0921-5093. [3] L¨osungen f¨ur den Werkzeugbau. Ceratizit Deutschland GmbH, 2016. [4] R. Hambli. Finite element simulation of fine blanking processes using a pressure-dependent damage model. Journal of Materials Processing Technology, 116(2):252 – 264, 2001. [5] T.S Kwak, Y.J Kim, and W.B Bae. Finite element analysis on the effect of die clearance on shear planes in fine blanking. Journal of Materials Processing Technology, 130–131:462 – 468, 2002. [6] Z.H. Chen, C.Y. Tang, T.C. Lee, and L.C. Chan. Numerical simulation of fine-blanking process using a mixed finite element method. International Journal of Mechanical Sciences, 44(7):1309–1333, 2002. [7] S. Thipprakmas, M. Jin, K. Tomokazu, Y. Katsuhiro, and M. Murakawa. Prediction of fineblanked surface characteristics using the finite element method (fem). Journal of Materials Processing Technology, 198(1):391 – 398, 2008. [8] W. Wieckowski, P. Lacki, and J. Adamus. Modelling of fine blanking process of the aluminium sheets. Key Engineering Materials, 473:290–297, 2011. [9] T. Tanaka, S. Hagihara, Y. Tadano, S. Yoshimura, T. Inada, T. Mori, and K. Fuchiwaki. Analysis of shear droop on cut surface of high-tensilestrength steel in fine-blanking process. Materials Transactions, 52(3):447– 451, 2011. [10] Y. T. Li, Y. H. Luo, W. Z. Dong, and L. Chen. Numerical simulation of fine blanking on plane blankholder. In 2010 International Conference on Digital Manufacturing Automation, volume 2, pages 22–25, 2010. [11] J Stanke, D Trauth, A Feuerhack, and F Klocke. Setup of a parameterized fe model for the die roll prediction in fine blanking using artificial neural networks. Journal of Physics: Conference Series, 896(1):012096, 2017. [12] M. Krobath, T. Kl¨unsner, W. Ecker, M. Deller, N. Leitner, and S. Marsoner. Tensile stresses in fine blanking tools and their relevance to tool fracture behavior. International Journal of Machine Tools and Manufacture, 126: 44 – 50, 2018. ISSN 0890-6955. [13] F. Klocke, K. Sweeney, and H.-W. Raedt. Improved tool design for fine blanking through the application of numerical modeling techniques. Journal of Materials Processing Technology, 115(1):70 – 75, 2001. [14] N. Manopulo, L. Tong, C. Karadogan, and P. Hora. A dual-mesh strategy for the 3d simulation of fineblanking processes. International Journal of Material Forming, 2(1):589, 2009. [15] Ł. Bohdal, L. Kukielka, K. Kukielka, A. Kułakowska, L. Malag, and R. Patyk. Three dimensional finite element simulation of sheet metal blanking process. Applied Mechanics and Materials, 474:430 – 435, 2014.

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