Thermo-Mechanical Fatigue of Cast 319 Aluminum Alloys

Transcription

1 Thermo-Mechanical Fatigue of Cast 319 Aluminum Alloys Huseyin Sehitoglu, 1 Carlos C. Engler-Pinto Jr. 2, Hans J. Maier 3,Tracy J. Foglesong 4 1 University of Illinois at Urbana- Champaign, USA, 2 Ford Motor Company, Dearborn, 3 Universität- GH Paderborn, Germany, 4 Exxon, Houston Research Supported by Ford Motor Company

2 Use of Aluminum Alloys in engine blocks and cylinder heads Thermo-Mechanical Fatigue Results Summary Modeling Studies (Precipitation hardened aluminum alloys) Outline

3 Percentage of Vehicles with Aluminum Engine Blocks and Heads (*) Heads Passenger cars 78% 85% 95% Light trucks 20% 40% 60% Blocks Passenger cars 13% 30% 50% Light trucks 5% 10% 20% (*) Delphi VIII Study, 1996

4 Advantages of cast aluminum Lightweight V-8 Engine Block: 150 lbs Cast Iron vs. 68 lbs Aluminum Cast into complex shapes Increased thermal conductivity

5 Practical Application Cylinder Heads Spark Plug Inlet Valve Exhaust Valve

6 Al319-T7B Nominal Composition in weight percentage Al Si Cu Mg Mn Fe Zn Ti Cr Bal * 0.25 max 0.25 max 0.05 max (*) WAP319: max 0.4% Fe - EAP319: max 0.8% Fe Thermal treatment solutionizing at 495 C for 8 hours followed by precipitating at 260 C for 4 hours)

7 Thermo-Mechanical Fatigue Cycles T ( C) Simultaneously changing strain and temperature (T) In-Phase: max-strain at max-t Out-of-Phase: max-strain at min-t ε mech (%) IP OP ε mech (%) IP OP time (min) T( C)

8 Thermo-Mechanical Fatigue Fatigue of materials subjected to simultaneously changing temperature and strain. Temp. ε tot = ε th + ε mech TMF OP T max TMF IP Terminology in-phase out-of-phase ε min ε max Mech. Strain T min

9 TMF Testing Micro-Computer T vs. t T F ε ε vs. t pyrometer load cell Induction Heating extensometer Hydraulic Testing Machine

10 Experimental Procedures IsothermalLCF 20 C, 150 C, 250 C and 300 C s -1, s -1 and s -1 Thermo-Mechanical Fatigue C s -1

11 TMF Loops Stress (MPa) Out-of-Phase ε m =0.6% 300 C Cycle Mechanical Strain (%) 100 C N f = In-Phase ε m = 0.54% 100 C 300 C N f = Mechanical Strain (%)

12 TMF OP C 1.0% C 100 Stress (MPa) C C STRESS1-2fea STRESS300fea Stress1-2 Stress300-6x Strain

13 Fatigue Life Criterion 120 Maximum Stress (MPa) % Load Drop LCF C - 0.3% hz WAP319-T7B 0 Number of Cycles N f 5000

14 Precipitate Coarsening Yield Strength (MPa) Initial Exposure at 300 C time (h) cycles / ~25 hours (300 C, ε m =0.2%).

15 TMF Loops IP Inelastic Strain (%) OP IP 100 C -200 OP Stress (MPa) IP Stress (MPa) C OP Inelastic Strain (%)

16 TMF Peak Stresses OP IP σ max -σ min 100 C Stress (MPa) C Inelastic Strain Range (%)

17 Stress Range (MPa) TMF Stress Range Evolution EAP319-T7B IP N f /2 Initial Behavior OP N f /2 TMF OP TMF IP Number of Cycles

18 Cyclic Stress-Strain Strain Curves Stress Range (MPa) EAP319 TMF OP TMF IP 300 C 0.5 hz 300 C 5x10-5 s -1 WAP319 TMF OP TMF IP WAP319 EAP Inelastic Strain Range

19 TMF Life Mechanical Strain Range Room Temperature 150 C 40 hz 250 C 40 hz 250 C 0.5 hz 250 C 5x10-5 s C 5x10-5 s C 0.5 hz TMF OP TMF IP EAP319-T7B Cycles to Failure

20 TMF Life Stress Range (MPa) TMF IP 5x10-5 s C 2x10-3 s -1 Room Temperature TMF OP 5x10-5 s C 2x10-1 s C 2x10-3 s EAP319-T7B 300 C 5x10-5 s Cycles to Failure

21 TMF Life 2 OP + LCF Inelastic Strain Range IP EAP C 0.5 hz 250 C 5x10-5 s C 0.5 hz 300 C 5x10-5 s -1 TMF OP TMF IP WAP319 TMF OP TMF IP Cycles to Failure

22 EAP319-T7B TMF-OP C ε m =0.6% N f =2460 c.

23 EAP319-T7B TMF-IP C ε m =0.54% N f =390 c.

24 5 4 3 a 0 = 70 µm TMF-IP TMF-OP WAP EAP Prediction µm Mehanical Strain Range N f

25 Summary 1. TMF stress-strain behavior is identical for both IP and OP loading conditions. TMF-IP lives are shorter than TMF-OP (based on the mechanical or inelastic strain range) lives. 2. Creep damage dominates for TMF-IP loading and in the high strain range regime. 3. The secondary alloy (EAP319) is softer than the primary alloy (WAP319), but TMF lives are very similar.

26 Aluminum-Copper Alloys Precipitate-dislocation interactions Anisotropy on plastic flow behavior (Hosford & Zeisloft 72, Bate et al. 81, Barlat & Liu 98, Choi&Barlat 99) Bauschinger effect (Abel & Ham 66, Moan & Embury 79, Wilson 65) Coherent particles - GP zones and θ'' (Price and Kelly 64) Higher yield stress than Al shearing of particles Comparable work hardening rates and deformationtoal Semi-coherent - θ' ( P & K 64, Russell & Ashby 70) Highyieldstressandhighworkhardeningrates Incoherent particles - θ ( P & K 64, R & A 70) Low initial yield stress Highest rates of work hardening

27 Precipitate Development GP zones * Peak aged, θ' Very over aged, coarse θ *Sato & Takahashi, 1983 Over aged, θ'&fineθ

28 Limitations of current models No implicit consideration of aging treatment. Models were developed for one specific aging treatment Peak aged, θ' No inclusion of length scale Volume fraction, precipitate size, mean free path etc. with aging treatment Empirical hardening models with a microstructural basis.

29 Proposed Hardening Law Single crystal formulation - one precipitate type θ' Ýτ = K oα 2 µ 2 b 2( τ τ o )d +θ τ s τ o τ s τ o k Ý γ k d 3 Polycrystal formulation More than one type of precipitate Incorporate grain size length scale Ýτ = µ 2 b 2 τ τ o ( ) 2 α 1 K 1 + α 2 2 K 2 + α 2 3 d 1 d 2 d 3 K 3 θ d 2 + θ τ s τ o τ s τ o3 k Ýγ k d 1 grain

30 Constitutive Equations Relate stress and strain rate at single crystal and polycrystal level. Ýε i in = Ý γ o S s =1 m s s i m j s τ c s m k σ k s τ c Can be written in pseudo-linear form. Assume overall polycrystal response described by law similartothatofsinglecrystal. n 1 i Ýε in c(sec) = M ij ( σ ) σ j ÝE i in = M ij (sec) ( Σ ) Σ j σ j wherei =1, 5

31 Hardening with Precipitates Start with dislocation evolution equation Ýρ = k K o db + k 1 ρ k 2 ρ Ý γ k Geometric storage term due to boundaries / obstacles Dynamic recovery of dislocations Statistical storage of dislocations Combine with the Bailey-Hirsch relationship for flow stress. τ = τ o +αµb ρ

32 Stress, (MPa) Experiment and Simulation for Al - 4% Cu Single Crystals [123] Orientation - Different Aging Conditions 190C, 24 hrs No Aging 190C, 3 hrs Solid line - Experiment Dashed line - Simulation Inelastic Strain, (%)

33 Summary The hardening law including the effects of precipitates on the deformation behavior of binary Al - Cu alloys is physically based and accounts for precipitate size, orientation, and mean free path. The model incorporates hardening law and predicts single crystal behavior of pure Al and Al- Cu alloys, it also predicts polycrystalline experiments from knowledge of single crystal behavior.