7.6 Transition Current, Mode Comparison & Pulsed Waveform Design

4. Critical Transition Current ($I_{\text{crit}}$) Mechanics

The critical transition current ($I_{\text{crit}}$) is the minimum direct current required to transform globular transfer into stable axial spray transfer in an argon-rich shielding gas.

   Droplet Detachment Frequency (Hz)
        ^
    500 |                                                  / Spray Transfer
        |                                                 /  (100 - 500+ drops/s)
    400 |                                                /
        |                                               /
    300 |                                              /
        |                                             /
    200 |                                            /   <-- Abrupt jump in detachment frequency
        |                                           /        and collapse in droplet diameter
    100 |                                          /
        |  Globular Transfer                      /
      0 +---+---+---+---+---+---+---+---+---+----+-----------------> Current (A)
           50  100 150 200 250 300 350 400 450
                           ^
                        I_crit (Transition Current Threshold)

Metallurgical & Geometric Factors Dictating $I_{\text{crit}}$

1. Electrode Wire Diameter ($d_w$): Because transition is governed by current density at the liquid neck, $I_{\text{crit}}$ scales nearly linearly with wire diameter. Larger wires require substantially higher current to achieve the threshold magnetic pinch force.
2. Electrode Material Properties:
   • Lower surface tension ($\gamma$) lowers $I_{\text{crit}}$.
   • Higher electrical resistivity ($\rho_e$) increases Joule preheating, lowering $I_{\text{crit}}$.
   • Aluminum has a lower $I_{\text{crit}}$ than steel due to its low melting point and lower surface tension.
3. Electrode Extension ($L_{\text{ext}}$): Increasing stickout increases $I^2 R$ resistance preheating. While this elevates the wire temperature upon entering the arc and increases deposition rate, it only slightly lowers $I_{\text{crit}}$ (by $5%\text{ to }10%$).
4. Shielding Gas Composition: Spray transfer requires a minimum of $80%\text{ Argon}$ (e.g., $85%\text{ Ar} / 15%\text{ CO}_2$, $90%\text{ Ar} / 10%\text{ CO}_2$, or $98%\text{ Ar} / 2%\text{ O}2$). Adding $2%\text{ to }5%\text{ Oxygen}$ reduces the liquid steel surface tension ($\gamma$), stabilizing the anode root and lowering $I{\text{crit}}$ by $15\text{ to }25\text{ A}$.

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5. Comprehensive Comparison: GMAW Metal Transfer Modes

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6. Comprehensive Worked Numerical Example: Force Balance & GMAW-P Synergic Waveform Design

Problem Statement

A welding engineer is qualifying a pulsed GMAW (GMAW-P) procedure for vertical-up ($3\text{G}$) structural fabrication on ASTM A572 Grade 50 steel plate using $1.20\text{ mm}$ AWS A5.18 ER70S-6 wire under $90%\text{ Ar} / 10%\text{ CO}_2$ shielding.

Part A: Static Force Balance Analysis For a $1.20\text{ mm}$ wire ($r_w = 0.60\text{ mm} = 6.00 \times 10^{-4}\text{ m}$) with liquid steel surface tension $\gamma = 1.20\text{ N/m}$:

1. Calculate the maximum retaining surface tension force ($F_\gamma$) assuming shape factor $f_s = 1.0$.
2. Calculate the gravity detaching force ($F_g$) acting on an oversized globular droplet of diameter $d_d = 2.40\text{ mm}$ (liquid density $\rho_m = 7000\text{ kg/m}^3$). Compare $F_g$ to $F_\gamma$ to demonstrate why globular transfer cannot detach fine droplets.
3. Using Amson's simplified electromagnetic pinch force formulation: $F_{\text{em}} \approx \frac{\mu_0 I^2}{4\pi} \ln\left(\frac{r_d}{r_n}\right)$ calculate $F_{\text{em}}$ during a peak pulse of $I_p = 420\text{ A}$ when the molten neck is throttled to $r_n = 0.20\text{ mm}$ with droplet radius $r_d = 0.60\text{ mm}$. Show that $F_{\text{em}} > F_\gamma$.

Part B: Synergic Waveform & ODAP Design To maintain puddle control in the $3\text{G}$ position, the target average welding current is set to $I_{\text{avg}} = 160.0\text{ A}$ (well below the steady spray transition threshold of $220\text{ A}$). The pulse parameters are specified as:

• Peak current: $I_p = 420.0\text{ A}$
• Peak duration: $t_p = 2.40\text{ ms} = 0.00240\text{ s}$
• Background current: $I_b = 55.0\text{ A}$

Calculate:

1. The required background duration ($t_b$), total waveform period ($T$), pulse frequency ($f$), and duty cycle ($D$).
2. If the synergic controller enforces ideal One-Drop-Per-Pulse (ODAP) transfer where each current pulse detaches exactly one spherical droplet of diameter $d_{\text{drop}} = 1.05\text{ mm}$, compute the resulting wire feed speed ($WFS$ in $\text{m/min}$ and $\text{ipm}$) required to maintain continuity.

Step-by-Step Solution

Part A: Force Balance Calculations

Step 1: Compute retaining surface tension force ($F_\gamma$) $F_\gamma = 2\pi \cdot r_w \cdot \gamma \cdot f_s = 2\pi (6.00 \times 10^{-4}\text{ m})(1.20\text{ N/m})(1.0) = 4.524 \times 10^{-3}\text{ N} = 4.524\text{ mN}$

Step 2: Compute gravity force on globular droplet ($d_d = 2.40\text{ mm}$) Droplet radius $r_d = 1.20\text{ mm} = 1.20 \times 10^{-3}\text{ m}$. Droplet volume: $V_d = \frac{4}{3} \pi r_d^3 = \frac{4}{3} \pi (1.20 \times 10^{-3}\text{ m})^3 = 7.238 \times 10^{-9}\text{ m}^3$ Droplet mass: $m_d = V_d \cdot \rho_m = (7.238 \times 10^{-9}\text{ m}^3)(7000\text{ kg/m}^3) = 5.067 \times 10^{-5}\text{ kg}$ Gravity force: $F_g = m_d \cdot g = (5.067 \times 10^{-5}\text{ kg})(9.81\text{ m/s}^2) = 4.970 \times 10^{-4}\text{ N} = 0.497\text{ mN}$ Ratio of gravity to surface tension: $\frac{F_g}{F_\gamma} = \frac{0.497\text{ mN}}{4.524\text{ mN}} = 0.1098 \quad (10.98\%$

Physical Deduction: Even when the droplet grows to twice the wire diameter ($2.4\text{ mm}$), gravity supplies only $11%$ of the force needed to detach it! In the absence of electromagnetic pinch forces, the droplet must grow to over $4.0\text{ mm}$ before gravity can overcome surface tension, explaining why globular transfer produces gigantic, unstable drops.

Step 3: Compute electromagnetic pinch force ($F_{\text{em}}$) during peak pulse $F_{\text{em}} = \frac{\mu_0 I_p^2}{4\pi} \ln\left(\frac{r_d}{r_n}\right) = \frac{(4\pi \times 10^{-7})(420)^2}{4\pi} \ln\left(\frac{0.60\text{ mm}}{0.20\text{ mm}}\right)$ $F_{\text{em}} = 10^{-7} \times 176,400 \times \ln(3.0) = 0.01764 \times 1.0986 = 0.01938\text{ N} = 19.38\text{ mN}$ Comparing forces: $F_{\text{em}} (19.38\text{ mN}) > F_\gamma (4.524\text{ mN})$ The electromagnetic pinch force exceeds the retaining surface tension force by a factor of $4.3$, rapidly severing the neck in under a millisecond and projecting the droplet axially across the gap.

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Part B: Synergic Waveform & ODAP Design

Step 1: Compute waveform timing parameters The average current across one cycle is: $I_{\text{avg}} = \frac{I_p \cdot t_p + I_b \cdot t_b}{t_p + t_b} = \frac{I_p \cdot t_p + I_b \cdot t_b}{T}$ Rearranging to solve for background time ($t_b$): $I_{\text{avg}} (t_p + t_b) = I_p \cdot t_p + I_b \cdot t_b$ $(I_{\text{avg}} - I_b) t_b = (I_p - I_{\text{avg}}) t_p$ $t_b = t_p \left( \frac{I_p - I_{\text{avg}}}{I_{\text{avg}} - I_b} \right)$ Substitute the numerical parameters: $t_b = 0.00240\text{ s} \times \left( \frac{420.0 - 160.0}{160.0 - 55.0} \right) = 0.00240 \times \left( \frac{260.0}{105.0} \right) = 0.00240 \times 2.4762 = 0.005943\text{ s} = 5.943\text{ ms}$ Total period ($T$): $T = t_p + t_b = 2.400\text{ ms} + 5.943\text{ ms} = 8.343\text{ ms} = 0.008343\text{ s}$ Pulse frequency ($f$): $f = \frac{1}{T} = \frac{1}{0.008343\text{ s}} = 119.86\text{ Hz} \approx 120\text{ Hz}$ Duty cycle ($D$): $D = \frac{t_p}{T} = \frac{2.400\text{ ms}}{8.343\text{ ms}} = 0.2877 \quad (28.77\%$

Step 2: Calculate wire feed speed for One-Drop-Per-Pulse (ODAP) Each pulse detaches one droplet with diameter $d_{\text{drop}} = 1.05\text{ mm} = 1.05 \times 10^{-3}\text{ m}$. Volume of one droplet: $V_{\text{drop}} = \frac{\pi d_{\text{drop}}^3}{6} = \frac{\pi (1.05\text{ mm})^3}{6} = 0.6061\text{ mm}^3$ At frequency $f = 119.86\text{ pulses/s}$, the volumetric deposition rate is: $\dot{V} = f \cdot V_{\text{drop}} = 119.86\text{ s}^{-1} \times 0.6061\text{ mm}^3 = 72.65\text{ mm}^3/\text{s}$ Compute the cross-sectional area of the $1.20\text{ mm}$ wire: $A_w = \frac{\pi d_w^2}{4} = \frac{\pi (1.20\text{ mm})^2}{4} = 1.1310\text{ mm}^2$ To satisfy conservation of mass, wire must enter the arc at velocity: $WFS = \frac{\dot{V}}{A_w} = \frac{72.65\text{ mm}^3/\text{s}}{1.1310\text{ mm}^2} = 64.23\text{ mm/s}$ Convert to industrial feed speed units: $WFS = \frac{64.23\text{ mm/s} \times 60\text{ s/min}}{1000\text{ mm/m}} = 3.854\text{ m/min}$ $WFS = \frac{3.854\text{ m/min} \times 39.37\text{ in/m}}{1} = 151.7\text{ ipm}$

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7. Real-World Engineering Scenarios & Exam Pitfalls

Industrial Case Study: Chronic Cold Lap on Heavy Crane Booms

A heavy machinery manufacturer experienced recurring ultrasonic inspection (UT) rejections on $25\text{ mm}$ ($1.0\text{ in}$) quenched-and-tempered steel crane booms. The joint was a single-V butt weld welded in the vertical-up ($3\text{G}$) position. To avoid puddle sagging, welders used conventional short-circuit GMAW (GMAW-S) with $1.2\text{ mm}$ wire at $140\text{ A}$ and $18\text{ V}$.

Failure Analysis: Microstructural cross-sections revealed widespread lack of sidewall fusion (cold lap). Because short-circuit GMAW operates with low instantaneous arc energy, heat was rapidly extracted into the thick $25\text{ mm}$ steel plates. The advancing molten droplet rolled over cold plate edges without melting them.

Engineering Solution: The welding engineer converted the production line to synergic Pulsed GMAW (GMAW-P) using $90/10\text{ Ar/CO}2$ shielding at $I{\text{avg}} = 165\text{ A}$, $I_p = 420\text{ A}$, and $f = 125\text{ Hz}$. The high peak current pulses delivered intense electromagnetic arc force that dug into the sidewalls and established robust dilution, while the low background current permitted immediate puddle freezing without sagging in the $3\text{G}$ position. Non-destructive UT rejections dropped from $12.4%$ to zero.

Common CWEng Exam Traps

Exam Trap 1: Attempting Spray Transfer in 100% CO2 An exam question may present a table of welding parameters and ask which combination produces axial spray transfer. If any option uses $100%\text{ CO}_2$ shielding, it is mathematically and physically impossible, regardless of whether current is $300\text{ A}, 400\text{ A}$, or $500\text{ A}$. Axial spray strictly requires an argon-rich mixture (minimum $80%\text{ Ar}$).

Exam Trap 2: Believing GMAW-P Average Current Must Exceed Transition Current Candidates often assume that because spray transfer requires $I > I_{\text{crit}}$ ($220\text{ A}$ for $1.2\text{ mm}$ steel), the average current in pulsed GMAW must also exceed $220\text{ A}$. This is completely false. The fundamental engineering benefit of GMAW-P is achieving spray transfer at average currents as low as $80\text{ to }160\text{ A}$, because only the peak current ($I_p \approx 400-500\text{ A}$) needs to exceed $I_{\text{crit}}$.

Exam Trap 3: Incorrectly Computing Pulsed Heat Input When calculating heat input for pulsed GMAW, candidates frequently multiply average voltage by average current: $P = V_{\text{avg}} \times I_{\text{avg}}$. This leads to severe errors (underestimating power by $10%\text{ to }25%$) because the mean of the product of instantaneous voltage and current does not equal the product of their means for pulsed waveforms. ASME Section IX and AWS guidelines mandate using true instantaneous power ($P = \frac{1}{T}\int v(t)i(t)dt$) or an external calibrated energy meter.