Practical Guidelines for Hot Stamping Simulations with LS-DYNA David Lorenz DYNAmore GmbH

Transcription

1 Practical Guidelines for Hot Staming Simulations with LS-DYNA David Lorenz DYNAmore GmbH 1

2 Outline 1. Imortant rocess stes in hot staming 2. Transfer and gravity simulation in hot staming 3. How to model roer material behavior 4. Thermal couling effects 5. Notes on thermal contact 6. Solution methods for cooling simulations 2

3 Imortant Process Stes in Hot Staming gravity loading closing & forming cooling & quenching Transfer of the hot blank to the die gravity loading of the hot blank on the die 3

4 Transfer Ste in Hot Staming Simulation During transfer from furnace to the ress blank temerature dros due to radiation and convection We can run this ste thermal-mechanical couled to account for the shrinkage of the blank due to thermal strains *CONTROL_SOLUTION $ SOLN 2 Couled solution *CONTROL_IMPLICIT_GENERAL $ IMFLAG DT imlicit solver choose reasonable time ste *CONTROL_IMPLICIT_INERTIA_RELIEF $ IRFLAG THRESH Inertia relief Threshold frequency *INTERFACE_SPRINGBACK_LSDYNA 4

5 Transfer Ste in Hot Staming Simulation *CONTROL_IMPLICIT_INERTIA_RELIEF static solution without alying SPCs advantageous in unconstrained sringback calculation eliminates all rigis body modes from the stiffness matrix All eigenfrequencies below the threshold frequency are treated as rigid body modes and are eliminated DYNA runs an eigenvalue analysis rior to the static solution Why not using SPCs alied to single nodes of the blank? all SPCs are written into the dynain file these SPCs become redundant in following gravity and forming simulation if you do not notice that SPCs are in the dynain file you may run into convergence trouble in the gravity ste 5

6 Gravity Ste in Hot Staming Simulation gravity deformation aears immediately blank tyically remains 1 3 s in ^thid osition till uer die moved down run in a few couled stes to account for temerature loss in contact 6

7 Gravity Ste in Hot Staming Simulation gravity deformation aears immediately blank remains tyically 1 3 s in till uer die moved down run in a few couled stes to account for temerature loss in contact *CONTROL_SOLUTION $ SOLN 2 *CONTROL_IMPLICIT_GENERAL $ IMFLAG DT *CONTROL_IMPLICIT_FORMING $ TYPE 1 enhanced static solution *CONTROL_IMPLICIT_AUTO $ IAUTO DTMIN DTMAX *CONTROL_THERMAL_TIMESTEP $ TS TIP ITS DTMIN DTMAX DTEMP automatic time steing both mechanics & thermal 7

8 Modelling Material Behavior Why is the gravity simulation not in agreement with real rocess? the elastic modulus of hot steel is still higher than cold aluminum but the yield oint at high temeratures is at very low stress level Do we accurately cature this effect in our material inut? coarse resolution yield oint determination not very accurate these exeriments aimed to meassure the yield curve u to high lastic strains Source: LFT University of Erlangen 8

9 Modelling Material Behavior Solving this shortcoming in material inut lower the yield oint for the relevant temeratures bring your simulation into better agreement with your observations and exeriences in real rocess or make a simle exeriment validate your material inut in agreement to exeriment 9

10 10 Modelling Material Behavior How to get the Cower Symonds Parameters from given yield curves? + = C ε σ σ & C C = = ε ε σ σ σ & & logarithmizing gives an easy to solve linear equation C ln 1 ln 1 ln 0 0 = ε σ σ σ & calculate C and from sloe m and intercet b m 1 = b e C =

11 Modelling Material Behavior How to get the Cower Symonds Parameters from given yield curves? calculate C and at different lastic strains (0.1, 0.2, 0.3, ) we need equally saced yield curves at different strain rates curve fit of each yield curve (Swift, Gosh, Hocket-Sherby etc.) necessary We end u with C and as functions of ε l ,0 0,1 0,2 0,3 0,4 0,5 3,40 3,38 3,36 3,34 3,32 3,30 0,0 0,1 0,2 0,3 0,4 0,5 Choose one value for C and one for Rate effects are imortant in the onset of local necking Choose C and for the higher lastic strains ( >0.2 ) 11

12 Modelling Material Behavior What if we havenot enough data (Numisheet Benchmark)? yield cuves rovided by Numisheet BM03 0,01 0,1 1, set C to a constant number C = 10 Find to match curves for 1.0s -1 5,0% 4,0% 3,0% 2,0% 1,0% 0,0% -1,0% -2,0% -3,0% -4,0% -5,0% 650 C 800 C 0,05 0,1 0,15 0,2 0, C -3,2% 0,8% 1,7% 1,3% 0,9% 800 C -2,0% -1,0% 0,1% 0,4% 1,4% 10,0 8,0 6,0 4,0 2,0 σ = σ base line for table definition in MAT_ & ε C 10s -1 0, exonential interolation e T temerature 12

13 Thermal Couling Effects Do we need to account for lastic work to heat conversion? w l = ρc T = η ε eq σ eq dε eq can cause trouble if strain localization starts localization results in high local strain rates Cower-Symonds scales u stress and thus lastic work high local temerature rates thermal solver reduces time ste blank temerature can climb above initial value we won t loose accuracy if we neglect this effect simulation is more robust without work to heat conversion 13

14 Thermal Couling Effects Is it necessary to include friction heat? friction coefficient is very high ( ) seems reasonable to include it F N d but very high local contact forces due to mass and seed scaling simle coulomb law redicts high friction energy can cause local temerature eaks in contact surface temerature fringes do not look reasonable in real life friction force is limited by blank yield stress more reliable without friction energy conversion 14

15 Thermal Couling Effects Alication of couling effects in cold staming of high strength steel work to heat in blank friction to heat in die 15

16 Notes on Thermal Contact Use of thermal contact to enhance our modelling skills Die Surface Geometry accurately modeled with Shell Elements Die Volume Geometry modeled with Volume Elements Alignment of meshes? Shell and Volume Mesh couled with contact definition indeendent meshing of surface and volume Penetrations between Volume Elements and Blank Shells are ignored in the mechanical contacts heat transfer from blank to die surface shell by thermal contact heat dissiation into the dies by thermal contact between shell and volume mesh 16

17 Notes on Thermal Contact Use of thermal contact to enhance our modelling skills *CONTACT_TIED_SURFACE_TO_SURFACE_OFFSET_THERMAL_ID $ CID CONTACT INTERFACE TITLE 6Punch 2-21 $ SSID MSID SSTYP MSTYP SBOXID MBOXID SPR MPR $ FS FD DC V VDC PENCHK BT DT $ SFS SFM SST MST SFST SFMT FSF VSF $ K HRAD HCONT LMIN LMAX CHLM BC_FLAG 1_WAY &HTOOL Set HTOOL to a very high number to get a thermal equivalent to tied contact HTOOL ~ W/m 2 K 17

18 Notes on Thermal Contact How to model ga heat transfer? h h ga k = + frad + T L ga ( )( 2 2 T + T T ) ga heat transfer very sensitive to small gas Kelvin scale necessary d closed contact do not use radiation term with C scale 18

19 Notes on Thermal Contact How to model ga heat transfer? h = ga k L ga h d closed contact k = 0.10 W/mK higher sohisticated formulation may give better agreement 19

20 Notes on Thermal Contact How accurate is ga heat transfer? Euroean standard EN h = ga k L ga h d closed contact nominal thickness of Numisheet BM mm USIBOR as delifered R 0.2 = MPa uncertainty in nominal thickness has strong imact higher sohisticated formulations overstate second order effect of ga heat transfer 20

21 Cooling Simulation Solution Methods 21

22 Cooling Simulation Solution Methods? F F? 1 2 F 1 h d closed contact? elastic or rigid dies? thermal only couled rigid couled elastic 1.0 s 22

23 Cooling Simulation Solution Methods? F F? 1 2 F 1 h d closed contact? Elastic or rigid dies? thermal only couled rigid couled elastic 3.7 s 23

24 Questions? Dynamore GmbH Industriestraße Stuttgart htt:// David Lorenz david.lorenz@dynamore.de 24