laser-welding

name	laser-welding
description	Laser beam welding — keyhole vs. conduction mode, power density threshold, weld geometry (aspect ratio, penetration depth), Rosenthal solution, HAZ, hot cracking, porosity prevention, fiber/CO₂/disk laser selection, filler wire, dissimilar metal welding, AWS D17.1/ISO 15614-11, automotive battery tab welding.
metadata	{"priority":7,"promptSignals":{"phrases":["laser welding","laser beam welding","LBW","keyhole welding","laser weld","fiber laser welding"],"minScore":3}}

Laser Beam Welding (LBW) — Complete Skill

Welding Modes

Conduction Mode

Power density: < 10⁶ W/cm²; no keyhole Heat flow: surface heating by absorption; conduction melts metal Weld geometry: shallow, wide bead; aspect ratio (depth/width) < 0.5 Application: thin sheet sealing (<1 mm), jewelry, electronics Absorption: depends entirely on surface reflectivity (significant loss for metals)

Keyhole Mode

Power density: > 10⁶ W/cm² (typically 10⁶–10⁸ W/cm²) Mechanism: laser vaporizes metal → recoil pressure creates cavity (keyhole) surrounded by molten pool; vapor column transmits energy deep into metal Weld geometry: narrow, deep; aspect ratio (depth/width) 2–10+ Key advantage: high aspect ratio → deep penetration with small heat input → minimal distortion

Keyhole stability: Keyhole oscillates at 100–10,000 Hz → causes porosity (keyhole collapses, trapping vapor pockets) Mitigation: beam oscillation, modulated power, weld speed optimization, vacuum (eliminates surface plasma)

Energy Balance and Penetration

Power Density Threshold

Threshold power density for keyhole: I_threshold = σ_SB × T_vap⁴ + h_vap × ρ × v_weld / t_beam × beam_area [approximate; complex] Practical: I > 10⁶ W/cm² → keyhole guaranteed for metals; I = 10⁵–10⁶ W/cm² → transitional

Laser power on target: P_target = η_coupling × P_laser [η_coupling = 0.7–0.95; losses from optics, reflection] I = P_target / A_spot = P_target / (π/4 × d_spot²) [W/cm²]

Rosenthal Solution (Analytical Temperature Field)

Moving point source — temperature distribution: T(x,y,z) - T₀ = (P / (2π k)) × (1/r) × exp(-v(x + r)/(2κ)) [steady state; moving along x at speed v] r = √(x² + y² + z²) [distance from heat source] κ = λ/(ρ c_p) = thermal diffusivity [m²/s] k = thermal conductivity [W/(m·K)] P = power [W]; v = travel speed [m/s]; T₀ = ambient temperature [K]

Peak temperature (on fusion boundary): T_fusion at r_fusion (depth of fusion): where T = T_liquidus

Simplified Rosenthal for thin plate (2D case): T(r) - T₀ = (P / (2π k t)) × K₀(v r / (2κ)) [K₀ = modified Bessel function of 2nd kind; t = plate thickness]

HAZ width estimate: r_HAZ: where T = T_AC1 (austenite start ~723°C for carbon steel) r_HAZ ≈ √(P / (π k v × (T_AC1 - T₀))) [rough estimate; HAZ boundary]

Penetration Depth

Empirical for keyhole welding: d = C × (P/v)^n / (ρ c_p T_vap)^m [process-dependent; C, n, m from experiment or simulation]

Simplified depth rule (fiber laser, steel): d [mm] ≈ 2.5 × P [kW] / (v [m/min] × d_spot [mm]^0.5) [rough empirical; varies by alloy]

Penetration ratio: For 4 kW, v = 5 m/min, d_spot = 0.4 mm → d ≈ 2.5 × 4 / (5 × 0.63) = 3.2 mm Full penetration required: P × (P/v) > threshold for given material thickness

Process Parameters and Weld Quality

Key Parameters

Power (P): primary determinant of energy input; higher P → deeper weld Travel speed (v): faster → less heat input → narrower weld; too fast → lack of fusion; too slow → excessive distortion and vaporization Line energy (heat input): E_L = P/v [J/m]; lower than MIG/TIG → key advantage Spot size (d_spot): smaller → higher intensity; important for threshold crossing Focus position: surface vs. below surface — affects penetration (negative defocus increases penetration in keyhole mode for some applications)

Process window: Minimum line energy for fusion at given thickness: from Rosenthal (T > T_liquidus required) Maximum line energy before blow-through (full evaporation of thin material): empirical; typically 5–10 × minimum Operating range: 2–4× the minimum line energy (for process robustness)

Shielding Gas

Purpose: protect molten pool from atmospheric O₂, N₂, H₂O; suppress plasma plume He (helium): best plasma suppression; highest ionization potential; expensive; recommended for CO₂ laser (prevent inverse Bremsstrahlung absorption by plasma) Ar (argon): good shielding; cheaper than He; plasma suppression less critical for fiber laser (shorter λ → less plasma interaction) N₂: used for stainless (austenitic); slight nitriding effect; acceptable Flow rate: 15–40 L/min; directed to molten pool and weld pool keyhole

Joint Fit-Up Requirements

Gap tolerance: very tight (< 10% of beam diameter; often < 0.1 mm) for autogenous (no filler) Misalignment: ±25% of spot size maximum for fusion; beam tracking required Filler wire: resolves fit-up gap (up to 0.3 mm with wire); also adjusts weld chemistry Common fillers: ER308L (stainless), ER70S-6 (mild steel), R4043 (aluminum) Wire feed rate: 1–5 m/min; must match melt pool consumption rate

Metallurgical Considerations

HAZ and Microstructure

Narrow HAZ advantages: LBW HAZ: 0.1–1 mm wide vs. MIG/TIG 2–5 mm Shorter time above A₁/A₃ → finer austenite grain; less grain growth; better toughness

Martensitic transformation (high-strength steel): Rapid cooling after keyhole close → martensite in HAZ; potentially hard and brittle Pre-heat (100–200°C): reduces cooling rate → bainite instead of martensite → improved toughness Post-weld heat treatment (PWHT): 550–650°C × 1 hr → temper martensite → restore toughness Assessment: hardness traverse (Vickers) per ISO 9015; limit per application (e.g., max 380 HV10 for piping)

Stainless steel sensitization: 304/316: if heated 450–850°C for > 0.1 s → Cr₂₃C₆ precipitates at grain boundaries → sensitized LBW fast thermal cycle: reduced sensitization vs. TIG; but not zero Use L grades (316L, 304L) or stabilized grades (321, 347) for high-temperature service

Hot Cracking (Solidification Cracking)

Mechanism: liquid film between solidifying dendrites → tensile stress from contraction → crack along grain boundary Susceptible alloys: Al 6xxx, high-Si stainless, some Ni alloys Ferrite number (δ-ferrite) in stainless: FN > 3–5 prevents hot cracking; below → austenitic solidification → susceptible Mitigation: Filler wire with higher dilution (ER308 adds Cr/Ni balance → FN adjustment) Reduce heat input → faster solidification → smaller mushy zone Modify joint geometry → reduce restraint

Porosity

H₂ porosity (aluminum): H₂ from moisture; high solubility in liquid Al; rejected on solidification → pores Prevention: pre-bake aluminum (200°C/30 min); shield with dry Ar; clean surface (acetone)

Keyhole collapse porosity: Keyhole oscillates → collapses on itself → vapor pockets trapped → 200–500 μm spherical pores Prevention: beam oscillation (10–400 Hz; 0.2–1 mm amplitude) → stabilize keyhole; vacuum welding

Acceptance criteria: ASTM E1032/E2375 (X-ray); pore size limit depends on application; typically < 20% of weld width or < 1 mm diameter; zero porosity for nuclear/pressure vessel

Dissimilar Metal Welding

Challenge: different melting points, thermal conductivities, thermal expansion coefficients → mixing defects Intermetallic formation: Al-Fe: FeAl₃, Fe₂Al₅ → brittle → cracks; must minimize mixing Cu-Fe: complete miscibility; acceptable in some conditions

Offset beam technique: Place laser spot 0.5–1 mm toward the lower-melting material (Al for Al-Fe joint) → limit Fe dissolution into Al Key: keep intermetallic layer < 10 μm for adequate bond strength

Transition interlayers: Cu foil (0.1–0.5 mm) between Al and steel → Cu melts and wets both; no direct Al-Fe mixing Ni interlayer: commonly used for steel-Al dissimilar electron beam and laser welds

Battery Tab Welding (EV Application)

Materials: Al tab to Cu busbar (or Al/Cu cell terminal); Ni-coated Cu; Al 1xxx/3xxx Challenge: very thin (0.1–0.5 mm tabs); high reflectivity; dissimilar metals; overlap joint Laser: green (532 nm) or blue (450 nm) lasers: higher absorptivity in Cu → less power needed; avoid IR (1064 nm) for Cu Parameters: 500–3,000 W; 10–1,000 ms pulse duration; 1–5 mm weld diameter Joint strength requirement: pull-out > 150 N (typical for 1.5 mm spot) for safety in EV pack

Weld inspection: X-ray CT (non-destructive); cross-section visual; contact resistance measurement < 0.1 mΩ

Standards

Standard	Scope
AWS D17.1	Fusion welding for aerospace (includes LBW)
ISO 15614-11	Qualification of welding procedures for electron beam and LBW
ISO 13919-1	Electron beam and laser welds — imperfections and quality levels
ISO 9015-1	Destructive testing — hardness testing of welds
ISO 13919-2	Laser welds in aluminum
EN ISO 4063	Numbering system — laser welding = process 52
ASTM E1032	Radiographic examination of weldments

Output

Provide: laser type (fiber/CO₂/disk) and wavelength [μm], power P [kW] and travel speed v [m/min], spot size d_spot [mm], line energy E_L [J/mm], power density I [W/cm²], welding mode (keyhole/conduction), penetration depth d [mm] and aspect ratio (d/w), HAZ width [mm] from Rosenthal estimate, shielding gas (type and flow rate [L/min]), joint fit-up tolerance [mm], filler wire (grade and feed rate [m/min] if used), hardness traverse result [HV10] vs. maximum allowable, porosity rate [%] (from radiography), hot cracking susceptibility assessment, PWHT required (yes/no; temperature × time), and applicable standard (AWS D17.1, ISO 15614-11, ISO 13919-1).