8.4 Gas Tungsten (GTAW) & Plasma Arc Welding (PAW): Electrodes & Keyholing

8.3 Gas Tungsten (GTAW) & Plasma Arc Welding (PAW): Electrodes & Keyholing

Quick Answer: Gas Tungsten Arc Welding (GTAW) and Plasma Arc Welding (PAW) utilize non-consumable tungsten electrodes to generate highly stable, slag-free electric arcs. In GTAW, arc morphology is governed by electrode tip geometry (vertex angle and flat diameter) and thermionic emission: sharp grind angles ($<30^\circ$) disperse the arc plasma, yielding shallow, wide beads, whereas blunt angles ($60^\circ–90^\circ$) concentrate current density and electromagnetic Lorentz pinch forces, producing deep narrow penetration. Rare-earth oxide additions (ceria, lanthana, thoria) lower the electron work function ($\phi$), enhancing arc re-ignition and tip life. PAW advances arc physics by constricting the plasma column through a water-cooled copper orifice nozzle; this constriction elevates plasma velocity and temperature ($>25,000\text{ K}$), enabling "keyhole mode" single-pass full-penetration welds on plates up to $12\text{ mm}$ thick without joint beveling.

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GTAW Process Physics & Thermionic Electron Emission

In GTAW, the electric arc is sustained between a non-consumable tungsten electrode and the workpiece inside an inert gas blanket. The electrode functions as a thermionic cathode under Direct Current Electrode Negative (DCEN, straight polarity):

                         (-) DCEN Power Supply
                                  |
                                  V
                        +-------------------+
                        | Tungsten Electrode|
                        +---------+---------+
                                 / \
                                /   \  Vertex Angle (θ)
                               /  |  \
                              +---+---+
                              |d_flat |
                                ( | ) Arc Flame
                               ( ( ) ) High Current Density Core
       =======================[ M-Pool ]======================= (+) Anode Workpiece

The Richardson-Dushman Thermionic Emission Equation

The current density of electrons emitted from the heated tungsten cathode tip ($J_e$) is governed by the Richardson-Dushman relationship:

$J_e = A_R T^2 \exp\left(-\frac{\phi}{k_B T}\right)$

where:

• $A_R$ is Richardson's constant (theoretically $1.20 \times 10^6\text{ A}/(\text{m}^2\cdot\text{K}^2)$),
• $T$ is absolute surface temperature of the cathode (K),
• $\phi$ is the electronic work function of the electrode surface ($\text{eV}$),
• $k_B$ is the Boltzmann constant ($8.617 \times 10^{-5}\text{ eV/K}$).

Because electron emission varies exponentially with $-\phi/k_B T$, lowering the work function ($\phi$) dramatically increases electron current density at a given temperature, or allows the electrode to operate at a significantly cooler temperature for the same current. This minimizes tungsten erosion, eliminates spitting, and stabilizes the cathode spot.

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Tungsten Electrode Classifications & Metallurgy (AWS A5.12 / ISO 6848)

Tungsten has the highest melting point of all metals ($3422^\circ\text{C}$ / $6192^\circ\text{F}$). To optimize thermionic performance, manufacturers dope pure tungsten with $1%–2%$ of stable rare-earth metal oxides.

AWS A5.12 Tungsten Electrode Specifications

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Electrode Tip Geometry and Arc Pressure Mechanics

The preparation of the tungsten tip dictates current density distribution, magnetic arc constriction, and weld puddle geometry.

   SHARP TIP (θ = 15° - 30°)                    BLUNT TIP (θ = 60° - 90° with Flat)

             | |
            /   \  Small Arc Flame                   | |
           /     \ (Dispersed Jet)                  /     \    Intense Lorentz Pinch
          /   .   \                                /   _   \   Constricted Plasma Core
         +----+----+                              +---+-----+  (Deep Digging Force)
             ( )                                      | |
           (     ) Broad, Shallow Arc                (   )
       ===[ Puddle ]===                          ====[ | ]==== Narrow, Deep Penetration

Vertex Angle ($\theta$) and Truncation Flat ($d_{\text{flat}}$)

1. Sharp Vertex Angle ($\theta < 30^\circ$):

   • Spreads the cathode attachment area over a longer conical surface.
   • Produces a divergent, wide arc cone with lower current density.
   • Results in a shallow, wide weld bead profile.
   • Prone to tip overheating, erosion, and tungsten inclusion at high current.

2. Blunt Vertex Angle ($\theta = 60^\circ–90^\circ$ with a Truncated Flat):

   • Forces the cathode emission spot to attach to the tiny flat end ($d_{\text{flat}} \approx 0.25–0.8\text{ mm}$).
   • Greatly intensifies the self-induced magnetic pinch force: $P_{\text{arc}} \approx \frac{\mu_0 I^2}{4 \pi r_{\text{spot}}^2}$
   • Generates a stiff, high-velocity plasma jet directed downward into the pool.
   • Produces deep, narrow joint penetration with a high depth-to-width aspect ratio, reducing angular joint distortion.

3. Grinding Direction:

   • Grinding marks must run longitudinally (parallel to the electrode axis). Transverse grinding creates circumferential ridges that force the arc to jump erratically between peaks, causing arc wander, high-frequency instability, and premature tungsten disintegration.

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Shielding Gases & Arc Initiation Systems

Shielding Gas Physics in GTAW

• Pure Argon (Ar): Benchmark gas. Low thermal conductivity and low ionization potential provide effortless arc starting, smooth cathode cleaning on AC, and stable arc operation. Concentrates heat in the center line.
• Helium and Ar-He Mixtures ($25–75%\text{ He}$): Helium has twice the thermal conductivity of Argon and a higher ionization potential ($24.6\text{ eV}$). At equivalent arc lengths, pure Helium increases arc voltage by $60–100%$, tripling total heat input. Essential for thick aluminum, high-conductivity copper, and high-speed tube mills.
• Argon-Hydrogen Mixtures ($95%\text{ Ar} / 5%\text{ H}_2$): Hydrogen dissociates in the arc and releases intense heat upon recombining at the plate. Adding $2–5%\text{ H}_2$ increases travel speeds by $50%$ and produces exceptionally clean, oxide-free beads on 300-series austenitic stainless steels and nickel alloys. Strict Warning: Hydrogen additions cause catastrophic cold cracking in carbon, low-alloy, and martensitic steels, and porosity in aluminum and copper.

Arc Initiation Systems

  1. HIGH-FREQUENCY (HF) SPARK              2. LIFT-ARC (MICRO-CURRENT)
     No Physical Contact                       Physical Touch -> Controlled Lift

       Tungsten Tip                              Tungsten Tip
            |                                         |
       - - -|- - - (Ionized HF Spark)                 V Touches Plate (Low Current: ~10A)
       ============ Base Metal                   =====+===== (Retracts -> Full Current Arc)

• High-Frequency (HF) Start: A high-voltage ($3000–5000\text{ V}$), high-frequency ($>1\text{ MHz}$) spark breaks down the dielectric resistance of the shielding gas gap, ionizing the gas and establishing the welding arc without physical contact. Avoids tungsten contamination, but HF radiation causes severe electromagnetic interference (EMI) with nearby CNC controllers and computers.
• Lift-Arc Initiation: Microprocessor-controlled touch start. The electrode touches the work with a tiny sensing current ($<10\text{ A}$). Once contact is detected, the welder lifts the torch; upon lift-off, the machine instantly ignites the main arc. Eliminates tungsten contamination and EMI.
• Scratch Start: Striking the electrode like a match. Strictly prohibited on nuclear, aerospace, and high-purity piping because it deposits brittle tungsten inclusions in the joint root.

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Plasma Arc Welding (PAW): Orifice Constriction & Modes

Plasma Arc Welding is an advanced extension of GTAW where the arc column is constricted by a water-cooled copper orifice nozzle.

                       GTAW (TIG)                       PLASMA ARC (PAW)
                     Unconstricted Arc                   Constricted Arc

                     +---------------+                 +---------------+  Electrode
                     |   Tungsten    |                 |   Tungsten    |  Inside Nozzle
                     +-------+-------+                 +-------+-------+
                             |                                 |
                             V                                 V Orifice Gas (Ar)
                                                       [=====|===|=====] Water-Cooled
                            / \                              |   |       Copper Orifice
                           /   \ Constriction ->             | P | Collimated Plasma Jet
       ===================[ Puddle ]====================    [=====] Shielding Gas Nozzle
         Divergent Arc Cone (Low Energy Density)         ====[Keyhole]==== High Energy Density

Orifice Physics and Arc Constriction

In standard GTAW, the arc cone is free to diverge, resulting in temperatures of $10,000–16,000\text{ K}$ and an energy density of approximately $10^4\text{ W/cm}^2$. In PAW, forcing the plasma through a nozzle orifice ($d_{\text{orifice}} = 1.0–3.5\text{ mm}$) restricts radial expansion. To sustain current conduction through this tiny channel:

1. The plasma gas undergoes severe thermal ionization.
2. Column temperature surges to $20,000–30,000\text{ K}$.
3. Plasma gas velocity accelerates to near-sonic speeds.
4. Energy density exceeds $10^5–10^6\text{ W/cm}^2$.
5. The arc emerges as a collimated cylinder, making arc voltage and penetration virtually immune to variations in torch standoff distance.

Transferred vs. Non-Transferred Arc

• Transferred Arc: The tungsten electrode is the cathode ($-$), and the workpiece is the anode ($+$). The plasma jet extends across the gap to melt the joint. Used for all structural welding, deep keyholing, and cutting.
• Non-Transferred Arc: The tungsten electrode is the cathode ($-$), and the constricting orifice nozzle itself is the anode ($+$). The arc terminates inside the torch; only the hot exiting plasma flame reaches the work. Used for low-heat brazing, thermal spraying, and non-conductive materials.

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PAW Keyhole Mode Mechanics and Force Balance

When plasma gas flow and current exceed threshold values, PAW transitions from traditional "melt-in" mode into keyhole mode.

                                PAW KEYHOLE MECHANICS

                                  Plasma Jet (High Velocity)
                                           |  |
                                           V  V
           Base Metal                 +------------+                 Base Metal
           -------------------------> |  KEYHOLE   | <-------------------------
           |      Molten Pool         |  (Vapor    |         Molten Pool      |
           |   Flows Around Hole      |   Cavity)  |      Flows Around Hole   |
           |                          +------------+                          |
           --------------------------------------------------------------------
                                           |  |
                                           V  V Exit Orifice Flame

The Hydrodynamic Force Balance

In keyhole welding, the stagnation pressure of the plasma jet ($P_{\text{arc}} + P_{\text{plasma}}$) physically pierces through the entire plate thickness, creating an open vapor cavity. Molten metal is held in equilibrium along the vertical cavity walls by surface tension ($\gamma$). As the torch moves forward, surface tension forces the liquid metal to flow symmetrically around the keyhole cavity and coalesce at the trailing edge to form a defect-free, full-penetration bead.

The dynamic equilibrium condition to maintain a stable keyhole without blowout is:

$P_{\text{arc}} + P_{\text{plasma}} + P_{\text{vapor}} = \frac{2\gamma}{r_{\text{keyhole}}} + \rho g h$

where:

• $r_{\text{keyhole}}$ is the radius of the keyhole vapor channel,
• $\gamma$ is the surface tension of the molten alloy ($\text{N/m}$),
• $\rho$ is the liquid metal density ($\text{kg/m}^3$),
• $g$ is gravitational acceleration ($9.81\text{ m/s}^2$),
• $h$ is plate thickness (m).

If plasma pressure exceeds surface tension support ($P_{\text{plasma}} > \frac{2\gamma}{r} + \rho g h$), the keyhole blows out catastrophically, expelling liquid metal and creating an elongated through-hole (blowout defect). If plasma pressure is insufficient, the keyhole collapses, resulting in incomplete penetration.

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Comprehensive Worked Numerical Example: Keyhole Force & Energy Density Modeling

Problem Statement

A titanium aerospace tank shell ($6.0\text{ mm}$ thick Grade 2 Titanium) is welded in a single pass using keyhole PAW. The welding parameters are:

• Current $I = 185\text{ A}$ (DCEN)
• Voltage $V = 31.0\text{ V}$
• Travel Speed $v = 280\text{ mm/min}$
• Orifice nozzle diameter $d_{\text{orifice}} = 2.4\text{ mm}$
• Thermal efficiency $\eta_{\text{paw}} = 0.70$
• Titanium liquid surface tension $\gamma = 1.65\text{ N/m}$
• Liquid density $\rho = 4110\text{ kg/m}^3$
• Keyhole radius $r_k = 1.4\text{ mm}$

Calculate: (1) the average electrical power density ($q_{\text{density}}$) entering the constricting orifice, (2) the net heat input per unit length ($H$), and (3) the minimum critical internal arc pressure ($P_{\text{crit}}$) required to balance the ferrostatic head and surface tension of the titanium puddle.

Step-by-Step Engineering Solution

Step 1: Calculate Electrical Power Density ($q_{\text{density}}$) in the Orifice Orifice cross-sectional area:

$A_{\text{orifice}} = \frac{\pi d^2}{4} = \frac{\pi (2.4\text{ mm})^2}{4} = 4.524\text{ mm}^2 = 4.524 \times 10^{-2}\text{ cm}^2$

Total arc electrical power:

$P_{\text{elec}} = V \cdot I = 31.0\text{ V} \cdot 185\text{ A} = 5735\text{ W}$

Power density entering the constricted nozzle:

$q_{\text{density}} = \frac{P_{\text{elec}}}{A_{\text{orifice}}} = \frac{5735\text{ W}}{4.524 \times 10^{-2}\text{ cm}^2} = 126,768\text{ W/cm}^2 \approx 1.27 \times 10^5\text{ W/cm}^2$

Observation: This energy density exceeds that of unconstricted GTAW ($~10^4\text{ W/cm}^2$) by an order of magnitude.

Step 2: Calculate Net Heat Input ($H$)

$H = \frac{\eta_{\text{paw}} \cdot V \cdot I \cdot 60}{1000 \cdot v} = \frac{0.70 \cdot 31.0 \cdot 185 \cdot 60}{1000 \cdot 280} = \frac{240,870}{280,000} = 0.860\text{ kJ/mm}$

Step 3: Calculate Critical Keyhole Equilibrium Pressure ($P_{\text{crit}}$) The surface tension capillary pressure opposing keyhole collapse is:

$P_{\text{cap}} = \frac{2\gamma}{r_k} = \frac{2 \cdot 1.65\text{ N/m}}{1.4 \times 10^{-3}\text{ m}} = \frac{3.30}{0.0014} = 2357.1\text{ Pa}$

The hydrostatic head of the liquid puddle is:

$P_{\text{hydro}} = \rho \cdot g \cdot h = (4110\text{ kg/m}^3) \cdot (9.81\text{ m/s}^2) \cdot (0.006\text{ m}) = 241.9\text{ Pa}$

Total required balancing pressure:

$P_{\text{crit}} = P_{\text{cap}} + P_{\text{hydro}} = 2357.1 + 241.9 = 2599.0\text{ Pa} \approx 2.60\text{ kPa}$

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Industrial Scenarios & Certified Welding Engineer Exam Pitfalls

Real-World Engineering Failure Scenario

An automated manufacturing plant fabricated thin-wall ($1.5\text{ mm}$) 316L stainless steel heat-exchanger tubing using high-speed mechanized GTAW. The line experienced a sudden spike in weld porosity and lack of penetration. Investigation revealed that the shop had switched from ground 2% Ceriated electrodes to Pure Tungsten (EWP, green) to reduce procurement costs, and operators were sharpening electrodes on a standard pedestal grinding wheel with circumferential rotations. The pure tungsten electrodes eroded rapidly under DCEN, developing large balled ends that caused the arc to wander across the seam. Furthermore, circumferential grind marks induced continuous high-frequency arc jumping. The engineer mandated a switch to 1.5% Lanthanated electrodes (EWLa-1.5), installed a dedicated longitudinal diamond grinding fixture with a $50^\circ$ vertex angle and $0.5\text{ mm}$ truncation flat, and added $2%\text{ H}_2$ to the argon shielding gas. The rejection rate dropped to zero.

Common Exam Traps

Exam Trap 1: Using Ar-H2 Shielding Gas on Carbon or Ferritic Steels While Argon-Hydrogen ($95%\text{ Ar} / 5%\text{ H}_2$) delivers exceptional benefits for austenitic stainless steels (304, 316), applying this gas to carbon steel, HSLA steel, or duplex stainless steel introduces catastrophic amounts of diffusible hydrogen directly into a susceptible martensitic/ferritic lattice, causing immediate Hydrogen-Induced Delayed Cracking (HIC).

Exam Trap 2: Balled vs. Pointed Tungsten for Polarity A frequent exam misconception is assuming balled tips are used for deep penetration. A rounded/balled tip is strictly formed on Pure or Zirconiated tungsten when welding on AC (Alternating Current) to distribute heat during the DCEP cleaning half-cycle. For all DCEN welding, the electrode tip must be ground to a truncated cone to focus arc current density.

Exam Trap 3: Confusing Orifice Gas and Shielding Gas in PAW Plasma Arc Welding uses two completely separate gas streams: (1) Orifice Gas, which flows through the constricting nozzle around the tungsten to form the high-velocity plasma column (almost exclusively pure Argon at low flow rates, $1–5\text{ L/min}$), and (2) Shielding Gas, which flows from an outer cup to protect the weld puddle from the atmosphere (typically Argon or Ar-He at higher flow rates, $10–25\text{ L/min}$). Swapping or mixing these flow rates will either instantly melt the constricting copper nozzle or blow the weld puddle out of the joint.