friction-stir-welding

name	friction-stir-welding
description	Friction stir welding (FSW) — tool design (shoulder/pin), process parameters (RPM/feed/tilt), material flow zones, heat generation, microstructure (TMAZ/HAZ/nugget), defects, AWS D17.3, aluminum/titanium/steel FSW, FSW-to-MIG comparison.
metadata	{"priority":7,"promptSignals":{"phrases":["friction stir welding","FSW","FSW tool design","friction stir weld defects","AWS D17.3","FSW aluminum"],"minScore":3}}

Friction Stir Welding (FSW) — Complete Skill

FSW Process Fundamentals

Process: rotating non-consumable tool (shoulder + pin) plunged into joint; tool traverses along joint; frictional + adiabatic heat softens material; plasticized material flows around pin → solid-state bond

Advantages over fusion welding:

• No melting → no hot cracking, porosity, or solidification defects
• Low distortion (lower peak temperature than arc welding)
• Fine grain microstructure in weld zone (recrystallization from severe plastic deformation)
• No filler metal, shielding gas, or flux required
• Suitable for alloys difficult to fusion weld (6xxx, 7xxx, 2xxx aluminum)

Limitations:

• Requires rigid fixturing (high tool forces: 5–30 kN downward)
• Exit hole from tool retract (remedied by retractable pin tools or runoff tab)
• Not suitable for complex 3D geometries (traditionally linear welds)
• Limited to softer materials (Al, Mg, Cu, Ti with special tools; steel is challenging)

Tool Design

Shoulder

Function: generates most of the frictional heat; contains plasticized material; forge workpiece surface Shoulder diameter D_s: D_s / t ≈ 3–5 (t = plate thickness); typical D_s = 15–30 mm for 3–10 mm Al Shoulder features:

• Flat: simple; effective for thin sheet
• Concave (scrolled): improves material containment; reduces flash; most common
• Convex: for filling T-joints or lap welds
• Scroll channels: improves material flow inward → better containment

Shoulder material: H13 tool steel for Al; WC-Co or Si₃N₄ for titanium/steel (harder)

Pin (Probe)

Function: plunges to root of joint; stirs through-thickness; provides threaded or featured path for material transport

Pin geometry types:

• Cylindrical smooth: simplest; lowest heat input; basic research
• Cylindrical threaded: most common; enhances material flow axially
• Tapered threaded: conical; better penetration, reduced forces
• MX-Triflute (The Welding Institute patented): three flutes + right/left hand threads → improved flow, reduced defects
• Trilobite / threaded with flats: non-circular cross-section → pulsating action → improved mixing

Pin length: L_pin = 0.9–0.95 × t (sheet thickness) [leave small unfused root: ≤ 0.5 mm] Pin diameter d_p: d_p = 0.3–0.4 × D_s [shoulder-to-pin diameter ratio: ~2.5–3.5]

Process Parameters

Tool rotation speed N [RPM]: 300–1500 RPM for Al (higher for thin sheet; lower for thick) Weld speed v [mm/min]: 50–800 mm/min for Al; slower for thick or harder materials Tilt angle: 1–3° (tool tilted from perpendicular toward trailing edge) → promotes forging action

Pitch (mm/rev): v / N [mm/revolution; lower pitch = more heat/energy per unit length]

Heat generation model: Q = 2π²/3 × μ_friction × P_axial × N × (D_s³ - D_p³) / (60 × 1000) [W; simplified shoulder heat] μ_friction ≈ 0.3–0.5 (sliding friction); P_axial = axial tool pressure [MPa]

Shoulder heat fraction: ~80–90% of total heat from shoulder; 10–20% from pin

Heat index (process heat input): HI = N² / v [RPM²/(mm/min); higher HI = hotter weld; can cause defects if too high] Or: Energy/length = Tool torque × 2π × N / (60 × v) [J/mm]

Weld Zone Microstructure

Zone Definitions

Nugget (weld core/stir zone): Direct contact with pin; severe plastic deformation + heat → dynamic recrystallization Result: very fine equiaxed grains (1–5 μm in Al); uniform microstructure Properties: high strength (precipitation may be dissolved); hard in some alloys

TMAZ (Thermomechanically Affected Zone): Adjacent to nugget; influenced by both heat and plastic strain Deformed grains (not recrystallized in Al); elongated grain structure visible Lower strength than nugget; transition zone

HAZ (Heat Affected Zone): Thermal influence only; no plastic strain For precipitation-hardened alloys (7075, 2024): over-aging in HAZ → strength reduction of 10–30% HAZ width: 5–20 mm depending on parameters; minimum with high weld speed

Parent material: unaffected beyond HAZ

Microstructure-Property Relationships

Hardness profile (7075-T6 typical): Parent: HV 180; nugget: HV 100–120 (dissolution); HAZ minimum: HV 90–110 (over-aged) UTS: nugget ≈ 80% of parent; joint efficiency = σ_weld_UTS / σ_parent_UTS × 100% Typical joint efficiency: 70–90% for 2xxx/7xxx Al; 90–98% for 5xxx/6xxx Al

Defects

Wormhole/void: insufficient material flow; caused by too-low heat input (high weld speed or low RPM) Detection: UT (phased array), X-ray; fatal defect

Kissing bond (cold lap at root): weak oxide layer not disrupted at root; requires sufficient plunge depth; detected by UT

Flash: excess material expelled at surface; too-high heat (excessive shoulder pressure or too-slow travel)

Nugget collapse: high heat + insufficient forging force → material flows out; hollow nugget

Surface galling/roughness: tool wear or incorrect shoulder geometry; affects surface finish

Lack of penetration (LOP): pin too short; root unfused → notch effect; critical fatigue location Prevention: verify L_pin = 0.9 × t; use PAUT (phased array UT) for inspection

Inspection and Quality

NDE methods: Phased array UT (PAUT): most effective; detects internal voids, kissing bonds, LOP X-ray: detects voids; cannot detect kissing bonds (planar defect) Dye penetrant: surface cracks only

AWS D17.3 (Aerospace FSW): Inspection: 100% UT on all welds; accept/reject criteria per standard Weld class A, B, C: different defect allowances for different criticality levels Cross-section metallography required for process qualification

Tensile testing per ASTM E8: Longitudinal tensile: weld parallel to gauge (checks parent + weld zone) Transverse tensile: weld perpendicular to gauge; measures joint efficiency

FSW of Non-Aluminum Materials

Magnesium (Mg): Similar to aluminum; lower forces; tool: H13 or WC; N = 500–2000 RPM Joint efficiency: 85–95% typical

Copper: Higher flow stress; H13 tool wears quickly → WC-Co or PCBN tools N = 200–500 RPM; v = 50–150 mm/min

Titanium: Very high flow stress + high temperature → tool wear severe PCBN or Si₃N₄ tools; inert gas shielding required; N = 100–400 RPM; v = 30–100 mm/min Joint efficiency: 90–100%; microstructure: fine α+β in nugget

Steel (low-carbon): PCBN or WC-Re tools; short tool life N = 100–300 RPM; v = 50–200 mm/min; preheating may help Limited industrial use due to tool cost; research active

Standards

Standard	Scope
AWS D17.3/D17.3M	Friction stir welding for aerospace
AWS D1.2	Structural welding code — aluminum (includes FSW)
ISO 25239	Friction stir welding — general requirements
ASTM E2700	Contact ultrasonic testing for friction stir welds
MIL-STD-1839	Defense material joining (includes FSW criteria)

Output

Provide: material(s) and thickness [mm], joint configuration (butt/lap/T), tool design (shoulder D_s [mm], pin d_p [mm], L_pin [mm], geometry type), process parameters (N [RPM], v [mm/min], tilt angle [°]), heat input [J/mm], weld zone characterization (nugget grain size [μm], HAZ width [mm]), microhardness profile (nugget/HAZ/parent HV), joint efficiency [%] (UTS_weld/UTS_parent), defects detected (type, size), NDE method used, AWS D17.3 weld class (if aerospace), and applicable standard (AWS D17.3, ISO 25239).