TIG vs MIG vs Spray-Arc Welding: When Each Process Wins for Defense Weldments

Quick Answer: TIG (Gas Tungsten Arc Welding / GTAW) wins on thin-gauge stainless, aluminum, sanitary finishes, and code-critical root passes. It produces the cleanest, lowest-hydrogen weld with the highest visual quality but at the lowest deposition rate. MIG (Gas Metal Arc Welding / GMAW) in short-circuit or globular transfer wins on production-volume mild steel at moderate thickness. It's the everyday workhorse for defense steel weldments.

Spray-Arc transfer GMAW wins on heavy structural welding requiring high deposition rates, typically mild steel plate thicker than 1/4". For defense weldments, process selection is a metallurgical and economic trade-off: TIG for quality, MIG for production volume, spray-arc for heavy structure. The right defense Welding Procedure Specification specifies the process per weld symbol.

A design engineer releasing a defense weldment drawing has two choices: specify the welding process on the drawing, or leave the process selection to the fabricator. The first choice tightens the metallurgical outcome and pre-resolves the AWS Welding Procedure Specification (WPS) qualification path. The second choice opens the cost-quality optimization window for the fabricator, but introduces variability the design engineer has not bounded.

The right answer depends on which characteristic of the weld actually drives the part's function. This post is a working selection guide for the three arc-welding processes that cover roughly 95% of defense fabricated weldments: Gas Tungsten Arc Welding (GTAW, commonly TIG), Gas Metal Arc Welding (GMAW, commonly MIG), and spray-arc transfer GMAW.

What each process actually is

Gas Tungsten Arc Welding (GTAW / TIG) uses a non-consumable tungsten electrode shielded by an inert gas (typically argon, sometimes argon-helium for thicker aluminum). Filler metal is added separately by the welder. The process produces a precise, controlled arc with minimal spatter and an exposed weld pool the welder manipulates directly. TIG is the lowest-deposition-rate process of the three but produces the highest visual quality and lowest hydrogen content.

Gas Metal Arc Welding (GMAW / MIG), short-circuit and globular modes, uses a consumable wire electrode fed continuously through a torch, with shielding gas (typically argon-CO₂ blends for steel, argon for aluminum). Short-circuit transfer (low voltage, low amperage) is the standard mode for thin-gauge work and out-of-position welding; the wire touches the puddle and shorts hundreds of times per second.

Gas Metal Arc Welding, spray-arc transfer mode, is the same process as short-circuit GMAW but operates at significantly higher voltage and amperage, where the wire vaporizes as fine droplets that "spray" across the arc to the work without ever shorting. Spray-arc operates only above a process-specific transition current (roughly 220 A for 0.045-inch wire on steel with 90/10 argon/CO₂ shielding) and produces a smooth, fast-deposition weld on thick plate in flat and horizontal positions.

When TIG wins

Thin-gauge stainless steel for food-contact, sanitary, or cryogenic applications. TIG produces a weld bead with minimal heat input and no spatter, which preserves the passive chromium-oxide layer that gives stainless its corrosion resistance. Post-weld pickling, passivation, and polishing operations are faster and more consistent after TIG than after MIG. For AWS D1.6 stainless welding at 16 gauge or thinner, TIG is typically the default specification.

Aluminum thinner than 3/16 inch. Aluminum is highly conductive thermally, and TIG's controllable heat input prevents burnthrough on thin sections. AC TIG (with high-frequency arc start) breaks up the surface aluminum oxide layer in real time, producing clean weld pools without pre-cleaning steps.

Cosmetic-critical applications. A weld visible on the finished assembly, such as fire apparatus pump panels, food-grade equipment surfaces, or exposed cab trim on tactical vehicles, generally requires the cosmetic quality only TIG delivers. The downstream finishing cost savings frequently exceed the TIG productivity penalty.

Root passes on critical-thickness joints. A multi-pass weld on thick plate often uses a TIG root pass for full penetration and metallurgical quality, followed by MIG or spray-arc fill and cap passes for productivity.

Trade-off: TIG deposition rates are typically 1 to 4 lb/hr of filler metal, roughly one-third to one-quarter the deposition rate of MIG, and one-tenth the rate of spray-arc. On any weld where deposition rate drives cost, TIG is the most expensive process.

When MIG (short-circuit and globular transfer) wins

Thin-to-medium-gauge structural carbon steel. Short-circuit MIG on 14-gauge through 1/4-inch carbon steel is the highest-volume welding process in the typical defense fabrication shop. AWS D1.1/D1.1M:2025 prequalified WPS coverage is broad. Welders qualified in 1G/2G or 3G/4G positions can run production efficiently.

Out-of-position welding (vertical, overhead). Short-circuit transfer's low heat input and rapid solidification allow welders to manage the puddle in vertical-up, vertical-down, and overhead positions where higher-energy spray-arc would run off the work.

High-mix, low-volume work. MIG setup time is fast: load the spool, dial in voltage and wire-feed speed, strike the arc. For job-shop work with frequent material and joint changes, MIG's setup flexibility usually beats the higher productivity of spray-arc per weldment.

Trade-off: Short-circuit MIG has the highest spatter rate of the three processes and the most variable bead profile. Visible welds may require post-weld grinding and finish work. Lack-of-fusion defects in heavy plate are more common with short-circuit transfer than with spray-arc or TIG.

When spray-arc wins

Thick plate (1/4 inch and above) in the flat (1G) and horizontal (2G) positions. Spray-arc transfer's high deposition rate (typically 8 to 18 lb/hr depending on wire diameter, shielding gas, and amperage) makes it the productivity leader on thick structural weldments. Tactical vehicle subframes, fire apparatus chassis members, and heavy industrial equipment frames running prequalified D1.1 joints frequently route to spray-arc for the fill passes after a TIG or short-circuit MIG root.

Multi-pass production welds where fusion is critical. Spray-arc transfer's high arc energy produces deep penetration and strong sidewall fusion, addressing the failure mode most likely to be missed on a visual inspection and caught only at non-destructive testing. For weldments where AWS D1.1 Section 4 prequalified joint geometry is used, spray-arc gives the most consistent weld metal quality at production speed.

Robotic welding cells. Spray-arc is the default mode for most robotic welding integrations because its stable arc and predictable bead geometry simplify path teaching and reduce in-process adjustment.

Trade-off: Spray-arc requires the work to be in flat or horizontal position. Vertical-up and overhead spray-arc is generally not practical at production current levels. Heat input is significantly higher than short-circuit MIG, producing a wider heat-affected zone and more thermal distortion on the workpiece.

How a procurement officer specifies process selection at drawing release

Three options for handling process selection at drawing release:

1. Specify the process per weld symbol on the drawing. Tightest control. Pre-resolves WPS qualification path. Reduces fabricator process flexibility, which can extend lead time when the fabricator's first-choice machine is loaded.
2. Specify the process category by joint type in a general note. "Cosmetic welds: TIG. Structural welds: GMAW. Heavy plate fill passes: spray-arc transfer." Balances control with fabricator flexibility.
3. Defer process selection to the fabricator, with mandatory WPS submission for review before production. Maximum fabricator flexibility. Highest variability unless the customer's review process is rigorous. Appropriate when the fabricator has demonstrated AWS-certified WPS library coverage on similar prior work.

Option 2 is generally the right default for new Tier-2 supplier relationships. Option 1 is appropriate for safety-critical or cosmetically-critical components. Option 3 is appropriate for established suppliers with demonstrated process discipline.

How NTM matches process to weld

New Tech Metals maintains AWS Certified Welders qualified across the D1-series codes (D1.1 steel, D1.2 aluminum, D1.3 sheet steel, D1.6 stainless, D9.1 non-structural sheet metal) with documented WPS coverage spanning TIG, MIG short-circuit, and spray-arc transfer modes. Process selection per weldment is driven by the engineering drawing requirements, the joint geometry, the position, and the production volume, not by a single shop-default process.

NTM's compliance footprint, spanning ISO 9001:2015, AWS Certified Welders, ITAR, DFARS Material Compliant, NIST and CMMC, and DDTC registered, supports the documentation chain that defense welding programs require.

Action

Before releasing a defense weldment drawing to RFQ, decide which of the three process-selection options is appropriate for the part. A general-note approach that categorizes welds by function (cosmetic, structural, heavy-plate fill) typically captures the design intent without over-constraining the fabricator's process choice, and reduces quote variance across competing suppliers.

To request a multi-process welding quote, contact New Tech Metals.