plasma-welding

name	plasma-welding
description	Plasma arc welding (PAW) — transferred vs. non-transferred arc, keyhole mode vs. melt-in mode, plasma torch geometry, orifice gas (argon), shielding gas, welding current, travel speed, energy input calculation, HAZ and microstructure, material compatibility (stainless steel, titanium, nickel superalloys), comparison to TIG/GTAW, plasma cutting parameters, and AWS D1.6 / D17.1 applications.
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Plasma Arc Welding (PAW) — Complete Skill

Fundamentals

Plasma Arc Generation

Plasma definition: Ionized gas at very high temperature; electrically conductive; T_plasma = 8,000–20,000 K (much higher than TIG arc ≈ 6,000 K) Generated by constricting the arc through a water-cooled orifice (copper nozzle)

Two-arc configurations: Transferred arc: arc established between electrode (W) and workpiece; plasma column traverses full gap; most common for welding and cutting; higher current density Non-transferred arc: arc between electrode and nozzle; plasma jet exits toward workpiece; used for thermal spraying, non-conducting workpieces

Electrode: Tungsten, 2% thoriated (EWTh-2) or 2% ceriated (EWCe-2); pointed tip; DC-EN (electrode negative) for steels/titanium Electrode recessed inside nozzle by 2–6 mm → prevents contamination from spatter; key difference from TIG

Torch Components

Pilot arc: DC high-frequency spark ignition from electrode to nozzle body; initiates plasma; then transferred to workpiece Orifice (constricting) gas (plasma gas): argon (most common); 0.3–8 L/min; constricts arc; increases temperature density Shielding gas: argon, argon-hydrogen, argon-helium; 10–30 L/min; protects molten pool Water cooling: mandatory for plasma nozzle; prevents melting of copper orifice Orifice diameter: 2–5 mm; smaller = more constricted = higher energy density = keyhole capability

Plasma Modes

Melt-in (microplasma) mode: Low current (0.5–50 A); gentle melting; similar to TIG but more focused beam Precise, low distortion welds; thin sections < 3 mm; electronic components, thin sheet metal Arc divergence: less than TIG → more stable at very low currents (down to 0.1 A — plasma "microplasma")

Keyhole mode: High current (100–400 A); high energy density penetrates through workpiece Full penetration in single pass; no edge preparation needed up to 6–8 mm thick Gas pressure from plasma column creates keyhole cavity → full penetration weld Suitable for: stainless steel, titanium, nickel alloys 3–8 mm thickness

Transition (between modes): 50–100 A range depending on material and geometry

Process Parameters

Current, Voltage, Travel Speed

Welding current I [A]: Microplasma: 0.5–50 A Keyhole: 100–300 A (stainless); 80–250 A (titanium, more sensitive) Arc voltage V_arc: 15–35 V (varies with arc length and gas composition) Power P = I × V_arc [kW]

Travel speed v [mm/min]: Microplasma: 150–600 mm/min (slow; precision) Keyhole: 300–1,200 mm/min (faster than TIG for same thickness) Speed balance: too slow → excessive heat input → distortion; too fast → lack of fusion

Heat input: HI = I × V × 60 / (1000 × v) [kJ/mm; I in A; V in V; v in mm/min] PAW keyhole: HI = 0.5–1.5 kJ/mm (lower per unit depth than TIG for same penetration) PAW melt-in: HI = 0.1–0.5 kJ/mm

Energy density: PAW keyhole: 10⁵–10⁶ W/cm² (comparable to laser; much higher than TIG ≈ 10⁴ W/cm²)

Gas Parameters

Plasma gas flow rate: Argon: 0.5–3 L/min (microplasma); 2–8 L/min (keyhole) Higher flow → more constriction → higher temperature → deeper penetration Argon + 5–15% H₂ (forming gas): increases arc temperature → higher productivity; not for hydrogen-sensitive materials

Shielding gas: Argon: universal; most common; excellent for titanium (must maintain inert atmosphere) Argon-helium (25–50% He): higher thermal conductivity → better sidewall fusion for aluminum and thick steel Argon-hydrogen (5–15% H₂): extra energy; better speed; for austenitic stainless; avoid for ferritic, martensite, duplex

Trailing gas shield: For titanium: additional trailing shield (argon) covers weld bead until cooled < 300°C (prevents oxidation) Color test: silver/straw = acceptable; gold/blue/grey = marginal; purple/black = rejected contamination

Keyhole Parameters

Keyhole stability: Orifice-to-workpiece distance (CTWD): 3–6 mm (tighter than TIG ≈ 5–15 mm) Plasma gas balance: insufficient → collapse; excessive → blow-through Back purge required: inert gas on root side to prevent oxidation at keyhole root

Detecting keyhole: Current × orifice_diameter combination: keyhole requires I ≥ I_keyhole_threshold I_keyhole ≈ 150 × t^0.5 (amperes; t = thickness in mm; approximate; depends on material)

Microstructure and Properties

Stainless Steel (Austenitic 304/316)

Single-pass keyhole weld (6 mm plate): HAZ width: 2–4 mm (narrower than TIG multi-pass; less total heat input) Weld microstructure: fully austenitic + delta ferrite (Ferrite Number 3–8 FN) Sensitization risk: carbon steel grades; minimize HI or use L-grade (304L, 316L)

Properties: Weld tensile strength ≥ 95% of base material Corrosion resistance: maintained if FN > 3 (prevents hot cracking) and no sensitization Hot cracking: prevented if FN ≥ 3; use ER308L or ER316L filler (if added)

Titanium (Ti-6Al-4V)

Plasma keyhole welding: Very high reactivity to oxygen and nitrogen above 300°C → requires excellent shielding Acceptable weld: silver/gold coloration (minor TiO₂ — acceptable) or silver (ideal) HAZ structure: coarsened alpha phase; beta grain growth; potential embrittlement

Trailing shield: Argon trailing torch or glove box enclosure; ≥ 99.998% Ar purity Flowrate: ≥ 15–20 L/min trailing shield; maintain until T < 300°C under shield

Properties: Tensile strength: ≥ 896 MPa (≥ 95% of base Ti-6Al-4V); if properly shielded Notch sensitivity: clean Ti weld; avoid oxide contamination (causes significant reduction in fatigue life)

Nickel Superalloys (Inconel 625, 718)

PAW keyhole for thick Inconel: HI must be controlled to avoid HAZ liquation cracking (grain boundary liquation at 1,250–1,290°C) Travel speed: higher speed (> 600 mm/min) with lower HI preferred Post-weld heat treatment: stress relieve per AMS 2770 or AMS 5664 spec Filler (if used): ERNiCrMo-3 (Alloy 625) or ERNiCrMo-14 (low C for 718)

Carbon Steel / Low-Alloy Steel

Keyhole PAW: Single pass up to 8 mm without beveling HAZ: narrow; coarse-grained HAZ; martensite if high-C steel; preheat required for CE > 0.40 Preheat: same as TIG/SMAW — per AWS D1.1 or AWS D1.6 tables; CE formula

Comparison: PAW vs. TIG (GTAW)

Parameter	PAW (Keyhole)	TIG/GTAW
Max single-pass depth	8 mm	3 mm
Travel speed	Higher (up to 2×)	Baseline
Heat input	Lower per depth	Higher per depth
HAZ width	Narrow	Wider
Equipment cost	Higher	Lower
Torch-to-work distance	Short (3–6 mm)	Larger tolerance
Wire filler needed	No (autogenous keyhole)	Often yes
Microplasma (< 1 A)	Yes	No (arc unstable)
Arc stability at low A	Excellent	Limited
Oxide cleaning	No (DCEN)	AC for aluminum

Plasma Cutting

Parameters

Plasma cutting (different from PAW; higher current; non-transferred preferred for thick plate): Current: 100–300 A (manual); 300–1,000 A (mechanized/CNC) Gas: argon-H₂ or O₂ (for steel); N₂ or air (economical); Ar-H₂ for stainless and aluminum Cut quality: ISO 9013 quality class 1–5 (squareness, roughness)

Cutting speed: 10 mm steel at 100 A: ~1,500 mm/min; 20 mm: ~600 mm/min; 50 mm: ~150 mm/min Dross: caused by too-slow or too-fast speed; optimize per material and thickness

Standoff: Manual: 3–6 mm; CNC: controlled by voltage-height (THC — torch height control) Water injection plasma (HyDefinition, Hypertherm): very fine cut; ISO 9013 class 1–2

Standards and References

Standard	Scope
AWS A5.18	Carbon steel filler metals
AWS D1.6	Structural welding — stainless steel
AWS D17.1	Aerospace welding (PAW commonly used)
AMS 2680	Plasma arc welding of aerospace parts
ISO 9013	Thermal cutting — quality classes
ASME IX (QW)	Welding procedure qualification (including PAW)
MIL-STD-2219	Fusion welding for aerospace applications

Output

Provide: mode (microplasma/keyhole/melt-in), material and thickness [mm], current I [A], voltage V [V], travel speed [mm/min], heat input HI [kJ/mm], plasma gas (type; flow rate [L/min]), shielding gas (type; flow rate [L/min]), back-purge gas and flow [L/min], trailing shield for titanium (Yes/No; flow rate [L/min]), orifice diameter [mm], CTWD [mm], penetration achieved (full/partial; single pass), expected HAZ width [mm], FN (if stainless; verify FN ≥ 3), coloration acceptance criterion (if titanium), preheat required (CE > 0.40 → preheat [°C]), filler wire (if used: AWS classification), post-weld heat treatment (PWHT) required, visual acceptance per AWS D17.1 or ASME IX, and applicable standard (AMS 2680, AWS D17.1, ASME IX, ISO 9013 for plasma cutting).