THERMO-MECHANICAL FATIGUE ANALYSIS ON FORGING TOOLS

Transcription

1 THERMO-MECHANICAL FATIGUE ANALYSIS ON FORGING TOOLS STÉPHANE ANDRIETTI DIRECTOR OF SOFTWARE PRODUCTION DEPARTMENT TRANSVALOR SA - FRANCE

2 OUTLINE INTRODUCTION DIE WEAR MODELING CASE STUDY#1 : CONSTANT VELOCITY JOINT DIE STRESS ANALYSIS CASE STUDY#2 : HEAVY FORGING CRANKSHAFT CONCLUSION

3 INTRODUCTION Die lifetime is a major concern in the forging industry Wear Gross cracking or plastic deformation Mechanical or thermal fatigue Gross cracking Courtesy of: Premature cracks due to mechanical fatigue Thermal fatigue cracking

4 INTRODUCTION Cost of tooling remains high Up to 15% of the final price is due to die works Challenge is to : extend die life by increasing the number of parts per tool set reduce production costs (maintenance & resink tool changing) Environmental matters : fewer tool manufacturing goes green How process simulation can help? Understand die failure mechanism Provide efficient modeling for die wear & die stress analysis Consider thermo-mechanical fatigue phenomena At the very end, designers are expecting a comprehensive response

5 OUTLINE INTRODUCTION DIE WEAR MODELING CASE STUDY#1 : CONSTANT VELOCITY JOINT DIE STRESS ANALYSIS CASE STUDY#2 : HEAVY FORGING CRANKSHAFT CONCLUSION

6 DIE WEAR MODELING Standard basic model Based on normal stress & interface velocity Does not depend on temperature Applicable with simple rigid die analysis Provide a qualitative response for abrasion wear The Archard s model Dependent on : normal stress & interface velocity hardness (temperature dependent) tooling properties Applicable with fully coupled analysis Provide a quantitative response

7 DIE WEAR MODELING Abrasion wear on die Based upon standard basic model Qualitative approach : red areas show where the abrasion is the largest Abrasion wear on lower die by the end of forging

8 OUTLINE INTRODUCTION DIE WEAR MODELING CASE STUDY#1 : CONSTANT VELOCITY JOINT DIE WEAR -> ARCHARD MODEL -> STEADY-STATE TEMPERATURE DIE STRESS ANALYSIS CASE STUDY#2 : HEAVY FORGING CRANKSHAFT CONCLUSION

9 CASE STUDY#1 : CONSTANT VELOCITY JOINT Automotive CV Joint Material : 38MnSV4 steel grade Warm forging : ~ 800 C Final height : ~ 110 mm Gross weight : ~ 1,5kg Objectives Abrasion wear prediction on punch Initial preform Basic wear vs Archard s model Impact of initial die temperature Benefits of a temperature steady-state distribution Final shape

10 ABRASION WEAR : STANDARD BASIC MODEL 10mm prior to end of forging 6mm prior Conditions : Rigid die computation Die constant temperature 200 C 4mm prior Wear modeling : standard basic End of forging

11 ABRASION WEAR : ARCHARD MODEL Abrasion wear on punch Conditions : Deformable die computation Initial die temperature 200 C 10mm prior to end of forging 6mm prior 4mm prior End of forging Wear modeling : Archard s model Kw, Kf : constant Hv : surface hardness Temperature evolution on punch σn : normal stress v : tangential velocity

12 WEAR PREDICTION : STD BASIC VS. ARCHARD MODEL Configuration #1 No temperature dependency Vs. Standard basic model Configuration #2 Temperature dependent Punch initial temperature = 200 C Archard abrasion wear model is temperature dependent. Location of maximum wear are slightly the same, but the intensity is significantly different. Archard model

13 MODELING OF STEADY-STATE TEMPERATURE Objective : predict the stabilized temperature distribution on the punch after several forgings Step #1 Recording of the thermal loading during the 1 st cycle Step #2 Apply on the punch the recorded thermal loading Step #3 Iterate until convergence is reached on maximum temperature Resting (5 sec) Forging Lubrication (2 sec)

14 TEMPERATURE DISTRIBUTION AFTER STEADY-STATE Temperature distribution after one single part Stabilized temperature distribution after ~ 80 parts Punch nose Punch nose Noticeable difference on temperature distribution once steady-state is reached

15 ABRASION WEAR : IMPACT OF STEADY-STATE TEMPERATURE Vs. Configuration#1 : Archard model C as initial temperature Configuration#2 : Archard model - steady-state temperature Drastic impact of the initial temperature on the abrasion wear prediction with Archard s model

16 OUTLINE INTRODUCTION DIE WEAR MODELING CASE STUDY#1 : CONSTANT VELOCITY JOINT DIE STRESS ANALYSIS CASE STUDY#2 : HEAVY FORGING CRANKSHAFT CONCLUSION

17 DIE STRESS ANALYSIS Different types of analysis are available Rigid die analysis Decoupled analysis Fully coupled analysis (with or without steady-state approach) Type Description Strength Outputs Understanding RIGID DIE No die deflection Constant temperature on die Quick & Easy Contact pressure Largest compressive zones Abrasion wear Abrasion areas Contact time Temperature gradient DECOUPLED ANALYSIS Deformable dies (stress & deflection) Additional analysis on dies only Based on mechanical loading recorded during rigid die Elastic or elasto-plastic behaviour for the die Compatible with shrink-fit dies Displacement Hydrostatic pressure Effective stress Deflection Inner tension & compression Plastic deformation 1 st & 3 rd principal stress & vector Tensile & compression areas, crack propagation direction FULLY COUPLED ANALYSIS Billet & dies are modeled as deformable bodies Interaction between billet & dies Temperature & stress calculation Same ones as decoupled analysis + Fully coupled thermo-mechanical resolution at each time step Displacement Stress components Temperature & damage criteria Abrasion wear with advanced modeling Deflection Comprehensive die stress analysis Thermal & mechanical cracking Quantitative wear prediction Number of cycles before cracking Die lifetime

18 DIE STRESS ANALYSIS Level of understanding : Rigid vs Decoupled vs Fully coupled approach Accuracy CPU rigid analysis decoupled analysis fully coupled analysis Comprehension

19 OUTLINE INTRODUCTION DIE WEAR MODELING CASE STUDY#1 : CONSTANT VELOCITY JOINT DIE STRESS ANALYSIS CASE STUDY#2 : HEAVY FORGING CRANKSHAFT FULLY COUPLED ANALYSIS -> THERMO-MECHANICAL FATIGUE CONCLUSION

20 CASE STUDY#2 : HEAVY FORGING CRANKSHAFT Crankshaft 4 cylinders Preform : 1160mm length - 150mm outer diameter Gross weight : ~ 160 kg Alloy steel : 34 CrMo4-4 (DIN SAE 4130) Preform Forging conditions Blocker stage Billet temperature : ~ 1150 C Crank press : 8000 tons Water& graphite lubrication Final flash thickness : 8mm Halfway End of forging

21 STANDARD DIE STRESS ANALYSIS Results obtained on lower die Maximum deflection (mm) Effective stress distribution (Mpa) Maximum die deflection => elastic distorsion Effective stress => areas subject to plastic deformation

22 STANDARD DIE STRESS ANALYSIS Results obtained on lower die Zoom on critical areas 1 st principal stress (Mpa) 1 st principal stress => areas in tension Principal stress vector => direction of tension Principal stress vectors

23 DIE LIFETIME PREDICTION Types of die failure Wear Plastic deformation Thermal & mechanical fatigue

24 DIE LIFETIME PREDICTION MODELING A thermo-mechanical fatigue model for hot forging Developed by RWTH-Aachen B.-A. Behrens, A. Bouguecha, T. Hadifi, G. Hirt, M. Franzke, M. Lopez Santaella FEM-Simulaton der Werkzeugversagens bei Warmmassivumformprozessen infolge thermisch-mechanischer Materialermüdung Schmiedejournal September 2011, Industrieverband Massivumformung e.v., Hagen, Germany, Page (ISSN ) e total = e mechanical + e thermal e mechanical = σ 1 max / Young modulus (at die temperature) e thermal = linear thermal expansion x temperature Number of cycles until crack initiation (empirical approach) N = C 2 ε total 2C 1 with C 1 & C 2 constants depending on the tooling

25 DIE LIFETIME PREDICTION - RESULTS Prediction of the number of forging cycles prior to crack initiation Deformation during blocker stage Cycles until crack initiation (minimum ~ 1000 parts - see blue areas)

26 DIE LIFETIME PREDICTION - RESULTS Prediction of the number of forging cycles prior to crack initiation The scale indicates the number of parts before crack initiation In the blue areas, the risk of crack initiation due to thermo-mechanical fatigue is maximal

27 SIMULATION VS. EXPERIMENT Other example with courtesy of RWTH-IBF Aachen

28 OUTLINE INTRODUCTION DIE WEAR MODELING CASE STUDY#1 : CONSTANT VELOCITY JOINT DIE STRESS ANALYSIS CASE STUDY#2 : HEAVY FORGING CRANKSHAFT CONCLUSION

29 CONCLUSION About abrasion wear prediction - Models must be temperature dependent and it is critical to calculate the steady-state temperature on the die. - The Archard s model associated with a steady-state temperature distribution on the die provides an excellent response. About die stress analysis - A fully coupled approach guarantees accuracy & a large variety of output results. - An innovative model based on thermo-mechanical fatigue is now implemented into FORGE software and it predicts the number of cycles until crack initiation. Areas of improvement - Consider surface treatment (nitriding, ). - Introduce self-adaptive meshing / remeshing to trap localized phenomena.

30 THANK YOU FOR YOUR ATTENTION ADDRESS: 694 avenue du Dr. Maurice Donat Parc de Haute Technologie Mougins cedex - France CONTACT: +33 (0) (0) marketing@transvalor.com