Cyclic Fatigue Testing of Wrought Magnesium AZ80 Alloy for Automotive Wheels

Transcription

1 Cyclic Fatigue Testing of Wrought Magnesium AZ80 Alloy for Automotive Wheels MATLS 701 Oct 21, 2009 Geoff Rivers Supervisor Dr. Marek Niewczas

2 Overview Introduction Background Previous work Research objectives Methodology Results Summary Acknowledgements 2

3 Introduction New government automotive regulations promote emission reduction and increased efficiency Auto companies focusing on weight reduction to meet goals 100kg curb weight reduction = + 1 km/l [Sakurai, 2007] 3

4 Introduction Use of new materials a favored method Use of Magnesium alloys is increasing steadily Mg alloys provide high strength to weight ratio Previous success Volkswagen, Porshe, BMW engine blocks Fig 1: BMW Magnesium engine sdrive 35i used in the BMW Z4 (BMW) 4

5 Introduction Magnesium Alloy wheel Rim Target alloy is Timminco Magnesium AZ80 Element Designated Range [ASM] Al Zn Mn Fe Ni Cu 0.05 Si 0.10 Table 1 - Composition Range of AZ80 Mg Alloys (wt.%) (Report on Joint Industry and University R&D Program on Development of Mg AZ80 for Auto Wheel Forging, Scott Shook, B.J. Diak, Z. Xia, M. Niewczas, 66 th Annual World Magnesium Conference, San Francisco, USA) Spoke Fig 2: Image of prototype forged AZ80 wheel 5

6 Background - Forging AZ80 Wheel formed through closed forging process 1 Deformation σ Heated metal blank is compressed between two machined die 2 σ Work piece is deformed under high pressure Fig 3 - Simplified Closed Die Forging Process (Introduction to Deformation Processes, DoITPoMS, Department of Materials Science and Metallurgy, University of Cambridge) 6

7 Background - Effect of Forging on Microstructure Forging involves large stresses and large deformations Final product has undergone work hardening and some dynamic recovery, dynamic recrystallization Size, orientation, concentration of precipitates and dislocations vary throughout wheel geometry As does degree of work hardening, dynamic recovery, dynamic recrystallization 7

8 Background - Cyclic Fatigue Wheels are subject to cyclic loading; cyclic fatigue Operating lifetime is for large number of cycles Sudden failure of a wheel would be Dangerous, potentially fatal Poor engineering design Effects of cyclic fatigue must be accounted for Figure 4 - Static von Mises stress levels for wheel geometry under assembled loading, simulated by Finite Element Analysis (F. Ju) 8

9 Background - Cyclic Fatigue Cyclic Fatigue Progressive and localized structural damage Occurs when a material is subjected to cyclic loading Failure occurs at less than σ ys,e Localized stress concentration Develops dislocation substructures Produces nucleation of voids and cracks Accompanied by work hardening (a) b σ -5 5 Variation in material microstructure produces variation of fatigue behavior throughout wheel σ Fig 5 - Modeling simulation image of stress concentration fields of the immediate neighborhood of (a) an edge dislocation in EAM Aluminum (b) the crack tip in a gallium nitride crystal. Scale is not shared. ((a) Reconsideration of Continuum Thermomechanical Quantities in Atomic Scale Simulations E. B. Webb III, J. A. Zimmerman, S. C. Seel, Mathematics and Mechanics of Solids, 13, (2008). (b)(looking at the nanomechanics of electronic devices under the scanning electron microscope Giuseppe Pezzotti, Andrea Leto, Marco Deluca, Alessandro Alan Porporati, Atsuo Matsutani, Maria Chiara Munisso, and Wenliang Zhu, 27 June 2008)) 9

10 Background - Cyclic Fatigue Cracks grow during tensile portion of cycle Growth halted by crack tip blunting Caused by plastic deformation at crack tip Local plastic deformation at less than σ ys,e applied due to stress concentration Concentration related to crack geometry Cyclic growth Produces striations in crack Fig 6 - Simplified Crack Tip Growth: (a) unloaded crack tip (b) Tip loaded in tension, (C) maximum force applied (d) compressive force phase of cycle (e) Crack tip at maximum compression. (callister, Materials Science and Engineering 7 th Ed.) 10

11 Background - Fatigue Testing Axial loaded Desired Amplitude Peak data Peak σ vs Cycles curve Stress controlled Strain is dependent σa Sample σa +σa Stress Applied - σa Time Cylindrical samples Warm up period Stable period Valley σ vs Cycles curve Fig 7 - Simplified diagram of axial stress controlled cyclic fatigue testing on a sample, and a simplified stress vs time plot of stress applied during testing 11

12 Background Characterizing Fatigue SN curve determination Endurance limit Endurance limit A Testing provides variety of data Peak strain vs cycles Hysteresis loop Area of hysteresis vs cycles Endurance limit B (defined by σ at 10 7 cycles) Fig 8 - The Wohler SN curve diagram, shown schematically for two materials, with endurance limits shown ( doc/etb/mod/fm1/stresslife/stresslife_help.html) 12

13 Previous Work AZ80 (a) Very little available information on Mg AZ80 Most previous work is from other parts of this Auto21 project (b) γ phase Cast microstructures: grain size and precipitates TEM analysis of precipitates 100nm TEM Fig 9 - Observations of (a) solidification processed microstructures for one AZ80 alloy by SEM using electron backscatter and (b) γ phase (Mg17Al12) precipitates by TEM (Report on Joint Industry and University R&D Program on Development of Mg AZ80 for Auto Wheel Forging, Scott Shook, B.J. Diak, Z. Xia, M. Niewczas, 66th Annual World Magnesium Conference, San Francisco, USA) 13

14 Previous Work AZ80 FEA of Forging process Determined in ABAQUS Results allow design of complete forging process Leads to production of prototype wheels for further testing Fig 10 - FEA of von Mises Stress levels for wheel forging process (Report on Joint Industry and University R&D Program on Development of Mg AZ80 for Auto Wheel Forging, Scott Shook, B.J. Diak, Z. Xia, M. Niewczas, 66th Annual World Magnesium Conference, San Francisco, USA) 14

15 Previous Work AZ80 Tensile testing of samples cut from wrought prototype wheel Provides σ ys estimates, failure strain Fig 11 - True Stress v True Strain for Mg AZ80 forged wheel samples (B.J. Diak) Testing completed using samples cut from rim, spoke Location σ ys,t 0.2% (MPa) σ ys,t 0.02% (MPa) Failure Strain (%) Rim Spoke Table 2 - Results of AZ80 forged wheel tensile testing (B.J. Diak) 15

16 Previous Work Other Alloys Nearly all AZ80 research comes from this Auto21 project Looked at other alloys for independent data AM50, AZ31, AZ91D Looked at fatigue behavior of each Provided a general idea of what to expect from AZ80 Endurance limit Fig 12 - SN Curve of AZ31 under rotating bending cyclic loading (ON THE SHARP BEND IN THE S-N CURVE OF THE AZ31 EXTRUDED MAGNESIUM ALLOY, Zhenyu NAN, Sotomi ISHIHARA*, Takahito GOSHIMA, and Reiko NAKANISHI *Department of Mechanical and Intellectual System Engineering, Toyama University, Japan) 16

17 Research Objectives Determine and compare SN curves of wrought Magnesium AZ80 samples taken from different locations in the wheel Endurance limit determination Investigate fracture surface to analyze fracture process Relate local microstructure to mechanical fatigue behavior Investigate the effect of cyclic fatigue on development of microstructure of wheel material 17

18 Methodology Sets of cylindrical samples have been cut from various locations in the wheel One set also from unforged billet of AZ80 2b 3 1 2a Each set represents location of interest (a) (b) Figure 13 - (a) Sample locations superimposed on wheel cross section (Neiwczas) and (b) Static von Mises Stress Levels for wheel geometry simulated by Finite Element Analysis (Ju and Xia) 18

19 Analytical Methods Samples fatigue tested to failure Each at unique stress amplitude Only billet set and set #1 received Plotting data SN curves, hysteresis curves, hysteresis area vs cycles, peak strain vs cycles Fracture surface, SEM (begun, limited results) Microstructure, precipitates, TEM Texture analysis studies of undeformed and deformed samples (still to be done) Fig 14 - Servohydraulic tensile testing machine used for testing 19

20 Results SN Curve Fig 15 - SN curve of Set 1 samples Endurance limit approx. 98 Mpa 20

21 Results Tuning 21

22 Results Peak Strain vs Cycles 22

23 Results Hysteresis Area vs Cycles Hysteresis Area (MJ/m 3 ) σ a = 150MPa Hysteresis Area (MJ/m 3 ) σ a = 120MPa Cycles Cycles Hysteresis Area (MJ/m 3 ) σ a = MPa Hysteresis Area (MJ/m 3 ) σ a = 98MPa Cycles Cycles 23

24 Results SEM Grip Length Fractures Site 1 2 mm 2 mm Images from 135 MPa sample Transitioning fracture surface Visible uneven profile to texture gradient Implies multiple initial cracks (or nucleation points for cracks) Wide flat surfaces at base of slope Site 2 24

25 Results SEM Site 1 Site 2 50µm 200µm 500 µm 1mm Transitioning fracture surface Flat to steep slope profile Secondary crack formed 25

26 Summary SN curve of Set #1 similar to other Mg alloy curves Well defined endurance limit present Peak strain vs cycles, Hysteresis area vs cycles Provide look at work hardening processes Need better method of collecting hysteresis data Initial surface fracture SEM of 135 MPa sample shows Multiple cracks nucleating at surface of sample Possible anisotropy in sample microstructure Possible mode II stress concentration effects 26

27 Future Work Fatigue test other location sample sets SN curve construction and comparison New samples will be arriving soon Peak strain and hysteresis area vs cycles analysis Fracture surfaces SEM Look for secondary cracking, surface texture transition Compare surface texture transition to stress applied Microstructure TEM precipitates, voids, dislocations Use endurance limit sample to look at pre-fracture fatigued sample Texture analysis studies of undeformed and deformed samples 27

28 Acknowledgements Dr. Niewczas Feng Ju Brad Diak Chris Butcher, Steve Koprich Mike Jobba 28

29 Thank You Question Period 29