GTAW on Carbon Steel and Marine Aluminum

Why GTAW Is the Preferred Process for Precision Marine Work

Gas Tungsten Arc Welding (GTAW), commonly called TIG, produces the cleanest, most controlled arc in the shop. Because no flux is used and the welder adds filler by hand, heat input is precise and spatter is virtually absent. Shipyards rely on GTAW for pipe penetrations, thin hull plating, aluminum superstructures, and anywhere the finished weld must meet tight visual and bend requirements. This lab develops the two most common GTAW setups you will use throughout your career: DCEN on carbon steel and AC on aluminum.

Polarity and What It Does

Direct Current Electrode Negative (DCEN) places the electrode (tungsten) on the negative terminal and the workpiece on the positive terminal. Electrons flow from tungsten to work, concentrating the majority of the heat in the base metal. This gives deep, narrow penetration and keeps the tungsten cool, which is exactly what you want on carbon steel.

Alternating Current (AC) reverses polarity many times per second. The electrode-positive half-cycle blasts the aluminum oxide layer from the surface — this is the cleaning action visible as a bright etching zone beside the puddle. The electrode-negative half-cycle puts heat into the work and allows the tungsten to cool slightly. Without AC, the tenacious aluminum oxide (melting point roughly 3700 °F, far above aluminum's roughly 1220 °F) would prevent fusion. AC is therefore non-negotiable for aluminum GTAW.

Tungsten Selection and Preparation

Carbon steel / DCEN: Use 2% ceriated tungsten (gray or gray-banded). Before welding, grind the tungsten to a sharp point on a dedicated diamond wheel, grinding with the long axis of the rod (not across it) to produce longitudinal lines that help stabilize the arc. A sharp point focuses the arc precisely and is stable under DCEN.

Aluminum / AC: Use pure tungsten (green band) or zirconiated tungsten (white band). Do not pre-grind a point. Strike an arc briefly on a copper or scrap piece and allow the heat to form a smooth, hemispherical ball at the tip — this is normal and desirable. The ball distributes heat evenly during the cleaning half-cycle. If the ball is irregular or has satellites, the tungsten is contaminated; break off the end and re-ball.

Never use the same grinding wheel for tungsten as you use for carbon steel; cross-contamination ruins both the tungsten and the weld.

Shielding Gas

100% argon is the correct shielding gas for both processes covered today. Argon is heavier than air, pools in the weld zone effectively, and provides the stable arc characteristics needed for GTAW. Flow rates for this lab are typically in the range of 15–20 cubic feet per hour (CFH); your instructor will set the final flow rate for the specific torch and cup size in use. Check the regulator, hose connections, and gas lens or collet body for leaks before striking an arc.

Welding Carbon Steel Butt Joints (3/16-inch plate, flat position)

Tack the coupons flat and allow them to cool enough to handle. Hold the torch at roughly 75–80° from the plate surface (approximately 10–15° from vertical), pointed in the direction of travel. Maintain a consistent arc length close to the diameter of the tungsten being used — too long an arc widens the bead and reduces penetration; too short risks tungsten contact and contamination. Feed ER70S-2 filler rod at a low angle (about 15°) from the opposite side, dipping the rod into the leading edge of the puddle in a smooth, rhythmic motion. ER70S-2 contains deoxidizers (silicon, manganese, zirconium, and titanium) that handle mill scale and light surface contamination common on structural steel.

Target a bead that shows full penetration (slight reinforcement on the root side), uniform ripple spacing, and no undercut. AWS D1.1 visual acceptance requires the weld face to be free of cracks, the reinforcement to not exceed the code limit, and undercut to not exceed the allowable depth.

Welding Aluminum Butt Joints (1/8-inch 5052, flat position)

Aluminum preparation is critical. Wire-brush the joint area with a stainless steel brush reserved exclusively for aluminum, then wipe with acetone and a clean cloth immediately before welding. Handle cleaned aluminum only with clean cotton gloves to prevent skin oils from recontaminating the surface.

Set the machine to AC, select the appropriate balance control if available (more cleaning action for heavily oxidized stock; more penetration for clean stock), and use ER4043 filler rod. Watch for the bright etching zone — a clean, evenly-wide zone on both sides of the puddle indicates good cleaning action. Puddle width on 1/8-inch material grows quickly; keep travel speed up and filler addition consistent to avoid burn-through. At the end of each bead, use the crater-fill control or taper down amperage manually to fill the crater — unfilled craters crack during cooling and are a reject under AWS D1.2.

Self-inspect each coupon against the D1.2 visual acceptance card before calling the instructor over.

Safety in This Lab

• Eye protection: GTAW produces intense UV and visible radiation. Use the shade recommended for the amperage range in use — typically shade 10–12 for the amperages in this lab. Side shields are required.
• Ventilation: Aluminum fumes require local exhaust ventilation. Never weld in a confined or stagnant-air space.
• Acetone: Flammable solvent. Use away from arc sources, cap the container when not in use, and dispose of rags per shop policy.
• Hot metal: GTAW coupons look identical to cold coupons. Mark hot work with soapstone and allow cooling before handling without gloves.
• Compressed gas cylinders: Chain cylinders upright at all times. Never use cylinders without a regulator.
• Lockout/Tagout: If you need to open the machine or clear a fault, follow LOTO procedures before touching internal components.