am-fatigue-metal

name	am-fatigue-metal
description	Metal AM fatigue — LPBF/DED fatigue properties, porosity effects, surface finish, residual stress, HIP treatment, S-N comparison to wrought, ASTM E466, crack initiation mechanisms.
metadata	{"priority":7,"promptSignals":{"phrases":["AM fatigue","additive manufacturing fatigue","LPBF fatigue","metal AM fatigue properties","SLM fatigue","additive fatigue","HIP fatigue"],"minScore":3}}

Metal AM Fatigue — Complete Skill

Fatigue in Metal AM Parts

Key challenge: AM parts exhibit wide scatter in fatigue life due to:

• Porosity (lack of fusion, keyhole pores, gas pores)
• Rough surface finish (upskin Ra 5–15 μm; downskin Ra 15–40 μm)
• Residual tensile stress (surface tensile stress accelerates crack initiation)
• Columnar microstructure (anisotropic, direction-dependent)
• Oxidation inclusions and lack-of-fusion defects

Typical Fatigue Performance vs. Wrought

Ti-6Al-4V (LPBF, as-built):

• σ_endurance (10^7 cycles, R = -1): 200–350 MPa
• Wrought Ti-6Al-4V: 500–600 MPa
• As-built: 40–60% of wrought endurance limit
• Large scatter: scatter band 2–5× in life at given stress

After HIP + heat treat:

• σ_endurance: 400–500 MPa → 80–90% of wrought
• Remaining deficit: surface finish and residual defects

Inconel 625 (LPBF, as-built):

• σ_endurance: 250–350 MPa
• Wrought: 350–450 MPa
• As-built: ~75% of wrought

AlSi10Mg (LPBF, as-built):

• σ_endurance: 50–80 MPa
• Wrought A357: 100–140 MPa
• After T6 treatment: 80–120 MPa

Porosity Effects on Fatigue

Pore types:

• Lack-of-fusion (LOF) pores: large (50–500 μm); irregular; worst for fatigue; occur at low energy density
• Keyhole pores: spherical; 30–100 μm; high energy density; less harmful than LOF
• Gas pores: spherical; < 30 μm; from entrapped gas; minor effect

Stress intensity from pore: K_pore ≈ 0.5 × σ × √(π × √(area_pore)) [Murakami's √area parameter] √area_pore [μm] = √(projected area of pore perpendicular to principal stress)

Fatigue limit reduction (Murakami): σ_endurance = C × (HV + 120) / (√area_pore)^(1/6) C = 1.43 for surface defect; 1.56 for internal defect HV = Vickers hardness; √area in μm

Acceptable pore size (Ti-6Al-4V, HV = 350): σ_endurance_target = 400 MPa → √area_max ≈ 40 μm (sub-surface) or 25 μm (surface)

Residual Stress Effects

As-built LPBF: tensile surface residual stress (100–700 MPa depending on material and geometry) Tensile residual stress ↑ mean stress → reduced fatigue life (Goodman/Gerber)

Effective stress ratio: R_eff = (R_applied × σ_max + σ_residual) / (σ_max + σ_residual) Tensile residual stress → R_eff increases → fatigue life decreases

Stress relief (SR): Reduces residual stress by 50–80% at 650°C for Ti-6Al-4V, 4 hr Fatigue improvement after SR: typically 20–40% improvement in life

Surface Finish Effects

As-built surface with Ra = 15 μm (downskin): Stress concentration factor K_t ≈ 2–4 at deep valleys Fatigue limit ≈ σ_endurance_smooth / K_t,surface (conservative)

Machined surface (Ra < 0.8 μm): Near-smooth surface; fatigue approaches bulk material minus residual stress

Shot peening as-built surface:

• Removes sharp notches; introduces compressive residual stress
• Fatigue improvement: 30–80% increase in life
• Used for aerospace AM components as standard post-process

Electropolishing / chemical etching:

• Reduces Ra from 15 → 3 μm; removes sharp surface features
• Fatigue improvement: 20–50%; gentler than machining for complex geometry

HIP (Hot Isostatic Pressing)

Closes internal pores: 120–200 MPa, 900–1200°C (material-dependent), 2–4 hr Eliminates LOF and keyhole pores (> 99% closure of pores < 200 μm)

Ti-6Al-4V HIP cycle: 920°C / 1000 bar / 2 hr → closes all pores; alters microstructure (annealed structure) Follow HIP with solution treat + age to recover strength

Limitations: HIP does NOT close surface-connected pores (open channels remain) HIP does NOT eliminate surface roughness

After HIP + heat treat + machined surface: Ti-6Al-4V fatigue life → 90–100% of wrought (best achievable for AM)

Fatigue Test Methods (ASTM E466)

Test setup: R = -1 (fully reversed) or R = 0.1 (tensile-tensile) Frequency: 10–100 Hz; specimen: hourglass smooth or notched; Kt = 2.0–3.0

Specimen orientation: Test in both XY (horizontal) and Z (vertical) direction to capture anisotropy Z-direction typically 10–30% lower fatigue limit than XY

S-N data needed: ≥ 10 specimens per condition; runout at 10^7 cycles; statistics: mean ± 2σ (B-basis allowable)

Comparative S-N (schematic at R = -1, 10^7 cycles):

Condition	Ti-6Al-4V σ_e [MPa]
Wrought	500–600
LPBF as-built	200–350
LPBF + SR	280–400
LPBF + HIP	380–480
LPBF + HIP + machined	450–520

Design Allowables for AM Parts

A-basis (99% exceedance, 95% confidence): Use when single failure = catastrophic (aerospace critical) Requires ≥ 30 specimens per condition

B-basis (90% exceedance, 95% confidence): Minimum for most structural applications σ_B = σ_mean - K_B × σ_std [K_B = tolerance factor from MIL-HDBK-5 table]

Recommendation: use B-basis from testing; apply additional safety factor of 1.25–1.5 for as-built surfaces

Output

Provide: fatigue endurance limit σ_e [MPa] at 10^7 cycles (by direction: XY and Z), comparison to wrought [%], dominant crack initiation mechanism (pore/surface/microstructure), porosity level [%] by CT scan, Murakami √area of critical defect [μm], residual stress state (tensile/compressive), recommended post-processing (SR/HIP/machining/shot peening), fatigue life improvement from each post-process [%], design allowable basis (A-basis/B-basis), and applicable test standard (ASTM E466/ASTM E468).