Gas Metal Arc Welding (GMAW / MIG)

Gas Metal Arc Welding (GMAW), commonly known as Metal Inert Gas (MIG) welding, is a semi‑automatic or automatic arc welding process that uses a continuously fed solid wire electrode and an external shielding gas to protect the weld pool. GMAW is one of the most widely used welding processes due to its high productivity, ease of use, and adaptability to a wide range of materials and thicknesses.

Overview

In GMAW, an electric arc forms between the consumable wire electrode and the workpiece, melting both to form a weld pool. The shielding gas, typically argon, CO₂, or a mixture, protects the molten metal from atmospheric contamination. The process can be operated in different transfer modes—short‑circuiting, globular, spray, and pulsed spray—depending on current, voltage, and shielding gas. GMAW is versatile, fast, and produces clean welds with minimal post‑weld cleaning.

Apparatus and Working

Apparatus

  • Power source: Constant voltage DC power supply (CV) with polarity usually DCEP.
  • Wire feeder and gun: Feeds solid wire electrode continuously into the weld zone.
  • Electrode wire: Solid wire (e.g., ER70S‑6 for carbon steel) matched to base metal.
  • Shielding gas: Argon, CO₂, or Ar/CO₂ blends depending on material and transfer mode.
  • PPE: Welding helmet, gloves, jacket, and fume extraction equipment.
  • Accessories: Gas regulator, flowmeter, and contact tips for wire guidance.

Working Steps

  1. Joint preparation: Clean base metal; remove rust, oil, and coatings.
  2. Parameter setup: Select wire type/diameter, shielding gas, voltage, and wire feed speed.
  3. Arc initiation: Trigger the gun to start wire feed and establish the arc.
  4. Welding: Maintain correct gun angle (typically 10–15° push or drag), travel speed, and stick‑out.
  5. Shielding gas coverage: Ensure proper gas flow to protect the weld pool.
  6. Inspection: Clean and visually inspect welds; perform NDT if required.

Principle

GMAW is based on the principle of generating heat through an electric arc between a consumable wire electrode and the workpiece. The electrode melts and becomes filler metal, while the shielding gas prevents oxidation and nitrogen absorption. The transfer mode (short‑circuit, globular, spray, or pulsed) determines droplet size, penetration, and spatter levels, allowing the process to be tailored to different applications.

Advantages and Disadvantages

Advantages

  • High deposition rates and productivity compared to SMAW and GTAW.
  • Minimal slag; reduced post‑weld cleaning.
  • Easy to learn and operate; suitable for automation and robotics.
  • Versatile for a wide range of materials and thicknesses.
  • Produces clean welds with good mechanical properties.

Disadvantages

  • Requires shielding gas; less suitable for outdoor/windy conditions.
  • Equipment is more complex and less portable than SMAW.
  • Higher equipment and consumable costs compared to stick welding.
  • Limited penetration on very thick sections without multiple passes.
  • More sensitive to surface contaminants than FCAW or SMAW.

Applications

  • Automotive industry: Body panels, frames, and exhaust systems.
  • Shipbuilding: Hulls, decks, and structural assemblies.
  • Construction: Structural steel, bridges, and buildings.
  • Manufacturing: Fabrication of machinery, appliances, and pipelines.
  • General repair: Maintenance welding of equipment and components.

Process Variants (Metal Transfer Modes)

  • Short‑circuiting transfer: Low heat input; suitable for thin materials and out‑of‑position welding.
  • Globular transfer: Larger droplets; more spatter; used with CO₂ shielding.
  • Spray transfer: Fine droplets; high deposition; requires higher current and Ar‑rich shielding gas.
  • Pulsed spray transfer: Alternates between high and low current; allows spray transfer at lower average heat input.

Common Defects in GMAW

  • Porosity: Caused by inadequate shielding gas or contamination.
  • Lack of fusion: Improper parameters or poor technique.
  • Excessive spatter: Incorrect voltage or shielding gas selection.
  • Burn‑through: Excessive heat input on thin materials.
  • Cracking: Improper filler selection, high restraint, or poor joint design.

Procedure and Quality Control

  • WPS/PQR compliance: Follow qualified procedures for base metal, filler, and parameters.
  • Shielding gas control: Maintain correct flow rate and nozzle condition.
  • Wire storage: Keep wire clean and dry to prevent rust and contamination.
  • Inspection: VT for bead quality; PT/MT for surface cracks; UT/RT for critical joints.
  • Automation: Use robotic GMAW for consistent quality in mass production.