am-residual-stress

name	am-residual-stress
description	AM residual stress — LPBF/DED stress mechanisms, temperature gradient mechanism (TGM), distortion prediction, stress relief heat treatment, XRD/neutron diffraction measurement, support optimization.
metadata	{"priority":7,"promptSignals":{"phrases":["AM residual stress","additive manufacturing residual stress","LPBF residual stress","thermal distortion AM","AM distortion","residual stress metal AM"],"minScore":3}}

AM Residual Stress — Complete Skill

Origin of Residual Stress in LPBF

Temperature Gradient Mechanism (TGM)

Phase 1 — Heating: Laser heats small zone rapidly; surrounding cool material constrains thermal expansion → compressive stress in heated zone (may yield in compression)

Phase 2 — Cooling: Heated zone contracts; constraints induce tensile stress in cooling region Net result: tensile residual stress at surface; compressive in bulk interior

Magnitude: σ_surface (tensile) ≈ 100–700 MPa (material and scan strategy dependent) SS316L: 300–700 MPa; Ti-6Al-4V: 200–600 MPa; Inconel 718: 400–800 MPa

Cool-Down Shrinkage Mechanism (CSM)

Each solidified layer shrinks on cooling; constrained by underlying layers → tensile stress in upper layers; compressive in lower layers

Dominant in DED (lower cooling rate, deeper melt pool than LPBF)

Distortion Patterns

Cantilever/thin plate: Bending due to thermal gradient; lift/curl after removal from plate Typical deformation: d = C × σ × L² / (E × t) [mm; L = length; t = thickness; C ≈ 1.5 for fixed-free]

LPBF SS316L bridge specimen: Standard distortion test: 70 mm × 10 mm × 5 mm bridge with 40 mm span Typical uplift after release: 0.5–3.0 mm depending on scan strategy

Distortion factors: Scan length: longer vectors → more temperature gradient → more distortion Island scanning (5×5 mm chessboard): reduces distortion vs. long stripes 67° rotation per layer: reduces in-plane stress anisotropy

Scan Strategy Effects

Long stripe (bidirectional): Highest residual stress; largest anisotropy; simplest strategy

Chessboard (island): Shorter scan vectors; more uniform stress; reduces max principal stress by 20–40% Optimal island size: 3–7 mm (tradeoff between short vectors and stripe re-melting)

Zigzag with 67° rotation: Stress more isotropic; reduces preferred texture

Preheating build plate: Higher T_plate (100–500°C depending on material): reduces ΔT during process → reduces stress Ti-6Al-4V: plate at 200°C → σ_residual reduces 30–50% Inconel: plate at 100–200°C; above martensite start T for crack-prone alloys

Measurement Methods

X-Ray Diffraction (XRD)

sin²ψ method: d(ψ) = d₀ + [(d₀ × (1+ν)/E) × σ × sin²ψ] d(ψ) = lattice spacing at tilt ψ; d₀ = unstressed spacing Slope of d vs. sin²ψ → σ

Depth penetration: CuKα: 5–20 μm penetration (surface only) CrKα: 3–10 μm; MoKα: 20–50 μm

Application: map σ_x, σ_y vs. position on surface; surface stress state

Neutron Diffraction

Full 3D residual stress mapping: Penetration: 50–100 mm in steel; 200 mm in Al Gauge volume: 1–4 mm³; provides interior stress profile Facility: ISIS, ILL, NIST, SNS (beamtime limited; expensive)

Use for: validation of FEM models; thick section investigation

Hole Drilling (ASTM E837)

Semi-destructive: drill 1–2 mm hole; measure strain relief around hole; calculate residual stress Depth profile: 0–1 mm from surface

Modified layer removal: alternative; remove electropolished layers → stress relaxation → calculate

Contour Method

Destructive: cut part by wire EDM; measure surface deformation after cut → calculate σ_z Provides 2D stress map on cut cross-section; unique for internal stress profiling in AM

Stress Relief Heat Treatment

Ti-6Al-4V: 650°C / 2–4 hr / vacuum or argon → reduces σ by 60–80%; retains duplex (α+β) microstructure 800°C / 2 hr → near full stress relief; begins to soften (avoid for max strength)

Inconel 718: 870°C / 1 hr → stress relieve without precipitation; then solution + double age for max strength

SS316L: 1050–1100°C / 30 min → full annealing; softens material; used for maximum ductility

AlSi10Mg: 300°C / 2 hr → partial stress relief; 325°C → over-ages (strength loss); keep ≤ 300°C if strength needed

Finite Element Simulation (Residual Stress Prediction)

Inherent strain method (fast, approximate): Assign plastic strain eigenvalues to each layer → solve structural problem → get distortion Calibrate inherent strains from simple coupon experiments

Thermomechanical FEM (high fidelity): Track temperature field layer by layer → convert to thermal stress → accumulate Software: Simufact Additive, Netfabb, Amphyon, ANSYS Additive

Validation: compare simulated distortion to bridge specimen measurement; typical accuracy ±30%

Support Structure Optimization for Stress Reduction

Support purpose: Anchor part to build plate (prevents lift-off); dissipate heat; control temperature gradient

Support density effect: High density support: better thermal conduction → lower ΔT → lower stress → less distortion But: more material → more post-processing; design tradeoff

Support optimization tools: Topology optimization of support layout (Materialise Magics, Autodesk Netfabb) Minimize stress while maintaining build integrity

Tree support: single touch points → low material but poor stress management Solid support: high thermal conductivity to build plate → best for stress management

Output

Provide: dominant residual stress mechanism (TGM/CSM), estimated surface residual stress magnitude [MPa] (tensile/compressive), scan strategy recommendation (chessboard island size, rotation angle), build plate preheat temperature [°C], predicted distortion [mm] for given geometry (from inherent strain or FEM), stress relief heat treatment cycle (T [°C], time [hr], atmosphere), measurement method for verification (XRD/neutron/hole-drill), post-treatment residual stress level [MPa], and support strategy for minimum distortion.