EVALUATION OF FRICTION CONDITIONS AT COMBINED BACKWARD CAN FORWARD ROD COLD EXTRUSION

Transcription

1 EVALUATION OF FRICTION CONDITIONS AT COMBINED BACKWARD CAN FORWARD ROD COLD EXTRUSION *Prof.Dr. Karl Kuzman *Dr. Darja Rupnik Poklukar *Assist.Prof.Dr. Tomaž Pepelnjak **M.Sc. Sebastja Mašera **M.Sc. Vid Krušič *Faculty of Mechanical Engineering Ljubljana University of Ljubljana, Slovenia **Iskra Avtoelektrika d.d., Nova Gorica, Slovenia

2 CONTENTS 1. Introduction 2. Methods for friction determination 3. Combined backward can forward rod extrusion 4. Numerical process analysis 5. Conclusions and outlook

3 1. Introduction HISTORY OF DETERMINATION OF FRICTION COEFFICIENTS Male, Cockcroft: 1964/65: A method for the determination of the coefficient of friction under conditions of bulk plastic deformation. J.Inst. Metals 93 M. Burgdorf: 1967: Ring compression test, also calibration curves J. Danckert, T. Wanheim: 1988: Frictional ring test and stresses at bulk metal forming Buschhausen, A., Lee, J.Y., Weinmann, K. and Altan: 1992: Double can extrusion test K. Kuzman, E. Pfeifer, N. Bay, J. Hunding: 1996: Forward rod backward can extrusion T. Nakamura, N.Bay: 1998: Forward conical can - backward straight can extrusion X. Tan, N. Bay, W. Zhang: 2003: Frictional measurement and modelling of forward rod extrusion test T. Schrader, M. Shirgaokar, T. Altan: 2007: Critical evaluation of double can extrusion test

4 1. Introduction FRICTION MODELS The Coulomb friction model : τ = µ* p Constant shear model: τ = m * k General friction model developed by Wanheim et all: τ = f * α * k

5 Methods for friction determination DETERMINATION OF FRICTION COEFFICIENTS BURGDORF RING COMPRESSION TEST d' i = d' + d' i,0 i,90 2 d'' i = d'' + d'' i,0 i,90 2 d i = d' i + 2d'' 3 i DRAWBACK: Appropriate for lower pressures only.

6 Methods for friction determination BURGDORF RING COMPRESSION TEST FACTORS m

7 Methods for friction determination METHODS FOR FRICTION DETERMINATION Backward can forward rod extrusion (various authors different geometries) Backward can forward can extrusion (various authors different geometries) AIM: Free material flow in two directions can be a measure of friction during the bulk forming process.

8 Combined backward can forward rod extrusion BACKWARD CAN FORWARD ROD EXTRUSION (BCFRE) TEST Free divided material flow friction factor m can be obtained from geometrical changes of the speceimen. Force measurements are not necessary. Diagram of material-lubricant ratios h p /d 2 versus h c /h r is created. Calibration curves were determined for various friction factors m based on the constant friction law. Analyses were performed for combinations of two materials and two lubricants.

9 Combined backward can forward rod extrusion Determination of calibration curves is performed with FEM program DEFORM. SIMULATION OF BCFRE TEST Extrusion process Fixed values of friction factor m are used for each simulation -> calibration curves could be extracted from various extrusions.

10 Combined backward can forward rod extrusion CALIBRATION CURVES OF BCFRE TEST FOR QSt MATERIAL h c /h r h p /d QSt32-3/MoS2 QSt32-3/Na-soap

11 Combined backward can forward rod extrusion CALIBRATION CURVES OF BCFRE TEST FOR 16MnCr5 MATERIAL h c /h r h p /d MnCr5/MoS2 16MnCr5/Na-soap

12 Combined backward can forward rod extrusion TOOL FOR BCFRE TEST QSt32-3 extruded with MoS2 QSt32-3 extruded with Na-soap

13 Numerical process analysis NUMERICAL PROCESS ANALYSIS The statistical model h p h m = f, d 2 h σˆ p c r + ε The equation was fit to data using a linear regression routine with cubic B- splines on both independent variables A (1-α)%. Confidence interval for the fitted value mˆ is constructed from the standard deviation of the prediction and is given by α α mˆ t(, ν ) ˆ σ, ˆ + (, ) ˆ p m t ν σ p 2 2, where t(q,ν) represents the qth quantile of the t distribution with ν degrees of freedom.

14 Numerican process analysis PERSPECTIVE AND CONTOUR PLOTS OF PREDICTED FRICTION FACTOR m Material: QSt32-3 Calibration curves Experimental values for lubricants MoS2 and Na-soap

15 Numerican process analysis LINE FITTING OF MEASURED VALUES OF FRICTION FACTOR m Material: QSt32-3 MoS 2 Na-soap Predicted values for m : for MoS 2 lubricant: , min=0.1260, max=0.1669, σ d = for Na-soap lubricant: , min=0.1241, max=0.2432, σ d =

16 Numerican process analysis PERSPECTIVE AND CONTOUR PLOTS OF PREDICTED FRICTION FACTOR m Material: 16MnCr5 Calibration curves Experimental values for lubricants MoS 2 and Na-soap

17 Numerican process analysis LINE FITTING OF MEASURED VALUES OF FRICTION FACTOR m Material: 16MnCr5 MoS 2 Na-soap Predicted values for m : for MoS 2 lubricant: , min= , max= , σ d = for Na-soap lubricant: , min= , max= , σ d =

18 Numerican process analysis AVERAGE FRICTION FACTORS m OBTAINED FROM BCFRE TEST MoS2 Na - soap QSt MnCr5 Current results Source [17] Source [19] Current results Source [17] Source [19] Source [17]: Krušič, V. et. all.: The impact of the forming system parameters on tool service life and product accuracy in cold forging, ICIT 2007, Celje 2007, Source [19]: T. Pepelnjak, V. Krušič, K. Kuzman: Analize koeficientov trenja pri masivnem preoblikovanju (Analyses of friction coefficients at bulk metal forming). Proc. of SLOTRIB conference, Ljubljana 2006, (in Slovene).

19 Numerican process analysis PREDICTED m VERSUS h p /d 2 FOR VARIOUS COMBINATIONS OF MATERIALS AND LUBRICANTS Predicted lines: QSt MoS 2 : m = * h p /d QSt soap : m = * h p /d MnCr5 + MoS 2 : m = * h p /d MnCr5 + soap : m = * h p /d 2

20 Conclusions and outlook CONCLUSIONS AND OUTLOOK Backward can forward rod extrusion test was selected for the determination of friction factors m in bulk metal forming. DEFORM simulation was implemented as a tool for determination of calibration curves for two materials and two lubricants. Based on the obtained calibration curves the statistical model for the friction factor m was created. Measurements of BCFRE test with combinations of QSt32-3 and 16MnCr5 material lubricated with MoS 2 and Na-soap have shown better and more stable lubrication with MoS 2 in particular at higher pressures. Experimental evaluation of numericaly obtained calibration curves showed good agreaments, diferences were only at high deformations therefore in the future the increase of contact surface extension and its influence on factor m should be cosidered as well. In the future the increase of surface extension and heat generation and their influence on factor m should also be considered.