Perform Gas Metal Arc Welding (GMAW) and Flux-Cored Arc Welding (FCAW)

Performing Gas Metal Arc Welding (GMAW) and Flux-Cored Arc Welding (FCAW)

Introduction to the GMAW and FCAW Processes

GMAW (Gas Metal Arc Welding), also called MIG/MAG welding, is an arc welding process that uses a continuous consumable electrode wire and an external shielding gas. FCAW (Flux-Cored Arc Welding) is a similar process, but the wire is tubular and contains an internal flux that can generate its own shielding gas or require additional external gas.

These two processes are classified in the category of arc welding processes with a fusible electrode under gas shielding. According to the Canadian Electrical Code, Part I, and CSA W47.1 and CSA W59 standards, these processes are designated by process numbers 131 (GMAW) and 136 (FCAW) according to ISO 4063, although Canadian classification frequently uses AWS (American Welding Society) designations.

The Red Seal exam candidate must master the physical principles, welding parameters, equipment, shielding gases, typical defects, and regulatory requirements associated with these processes.

Fundamental Principles of GMAW

The Welding Circuit

GMAW operates on the principle of an electric arc established between the electrode wire and the workpiece. The electrical circuit includes:

Polarity is a critical parameter. In GMAW, reverse polarity (DCEP — Direct Current Electrode Positive) is the most common for carbon steel. In this mode, the wire is connected to the positive terminal (+) and the workpiece to the negative terminal (−). This polarity provides deep penetration and stable metal transfer. Straight polarity (DCEN — Direct Current Electrode Negative) is used in specific cases, particularly for welding thin sheets with self-shielded flux-cored wire.

Metal Transfer

The molten metal transfer mode is determined by voltage, current, wire diameter, and shielding gas. Four main modes are recognized:

Short-circuit transfer occurs when the wire contacts the molten pool, creating a short circuit that melts the wire and detaches a droplet. This mode is ideal for thin sheets and positional welding because the heat input is low.

Spray transfer requires a gas containing at least 80% argon. The droplets are very fine and the transfer is axial and stable. This mode produces an intense arc and deep penetration, but it is limited to flat and horizontal welding positions.

Pulsed transfer combines the advantages of spray with precise control of heat input. The current alternates between a background level (low) and a peak level (high), allowing axial transfer of droplets with each pulse.

Welding Parameters

The main GMAW parameters are:

The relationship between wire feed speed and current is approximately linear for a given wire diameter. For example, for a 1.2 mm steel wire, a feed speed of 5 m/min corresponds to approximately 150 A, while 10 m/min corresponds to approximately 280 A.

Arc voltage determines arc length. Too high a voltage produces a long arc, spatter, and a wide, flat bead. Too low a voltage produces a short arc, a narrow and crowned bead, with a risk of wire stubbing.

The FCAW Process

FCAW with Gas (FCAW-G)

FCAW-G (Gas-Shielded Flux-Cored Arc Welding) uses a tubular wire containing flux and requires an external shielding gas. The most common gas is pure CO₂ or an Ar/CO₂ mixture (75/25 or 80/20). This process offers:

FCAW-G is designated by process number 136 according to ISO 4063. The polarity used is generally DCEP.

Self-Shielded FCAW (FCAW-S)

FCAW-S (Self-Shielded Flux-Cored Arc Welding) uses a tubular wire whose flux generates internal shielding gases that protect the molten pool. No external gas is required. This process is designated by number 114 according to ISO 4063.

The advantages of FCAW-S include:

The disadvantages include:

FCAW Wire Classification

FCAW wires are classified according to AWS A5.20 for carbon steel and AWS A5.29 for low-alloy steel. The designation follows this format:

E71T-1C or E71T-1M

For example, an E71T-1C wire is a tubular wire with 490 MPa strength, all positions, with rutile flux, requiring pure CO₂ as shielding gas.

Equipment and Accessories

Power Source

Power sources for GMAW and FCAW are constant voltage (CV) sources. Unlike SMAW, which uses a constant current source, the CV source maintains a relatively stable voltage while current varies according to wire feed speed. This characteristic allows self-adjustment of arc length: if the wire approaches too close to the workpiece, current increases, which accelerates melting and restores arc length.

The source must provide an open-circuit voltage of 30 to 60 V and a rated current of 200 to 600 A depending on the application.

Wire Feed System

The wire feeder must be adapted to the wire diameter and liner type. Critical components are:

The contact tip must have a diameter slightly larger than the wire (0.1 to 0.2 mm larger). A worn or incorrectly sized contact tip causes an unstable arc and welding defects.

Shielding Gases

The choice of shielding gas depends on the base material and transfer mode:

The gas flow rate should be set between 15 and 25 L/min. Too low a flow rate causes insufficient shielding and porosity. Too high a flow rate creates turbulence that draws ambient air into the molten pool.

Operating Techniques

Joint Preparation

Joint preparation for GMAW and FCAW follows the requirements of CSA W59 (Welded Steel Construction — Arc Welding). Bevel angles, root gaps, and root faces must conform to the specified weld details.

For plate thickness greater than 6 mm, a V-bevel is generally required with a 60° angle and a root gap of 2 to 3 mm. The root face must be 1 to 2 mm to prevent burn-through.

Gun Position

The gun angle influences bead shape and penetration:

The work angle (lateral angle) should be 90° for flat fillet welds, and adjusted according to position for inside corner welds (45° for a symmetrical fillet weld).

Travel Technique

The travel technique (oscillation) depends on the desired bead width:

Arc length must be kept constant. In GMAW, arc length is controlled by voltage: higher voltage produces a longer arc. The welder must maintain the distance between the contact tip and the workpiece (stick-out) constant, typically 10 to 20 mm.

Calculations and Parameters

Calculating Wire Feed Speed

Wire feed speed is directly related to welding current. The approximate relationship is:

I = k × WFS

Where:

For a 0.9 mm wire, the constant is approximately 25; for a 1.6 mm wire, it is approximately 35.

Calculating Heat Input

Heat input is a critical parameter for controlling the heat-affected zone (HAZ) and preventing cracks:

H = (V × I × 60) / (S × 1000)

Where:

Example: V = 25 V, I = 200 A, S = 300 mm/min

H = (25 × 200 × 60) / (300 × 1000) = 300,000 / 300,000 = 1.0 kJ/mm

Calculating Deposition Rate

The deposition rate for GMAW can be estimated:

D = (WFS × ρ × A) / 1000

Where:

For a 1.2 mm wire (cross-section = π × 0.6² = 1.13 mm²) with a feed speed of 8 m/min:

D = (8 × 7.85 × 1.13) / 1000 = 0.071 kg/min = 4.26 kg/h

Welding Defects and Remedies

Porosity

Porosity is caused by insufficient gas shielding or contamination:

Excessive Spatter

Spatter is often caused by:

Lack of Fusion

Lack of fusion is a serious defect that can be caused by:

Cracks

Hot cracks in the deposited metal can result from:

Regulatory Requirements and Standards

CSA W47.1

CSA W47.1 (Certification of Companies for Fusion Welding of Steel) requires that companies performing GMAW or FCAW on steel structures be certified. This certification involves:

CSA W59

CSA W59 (Welded Steel Construction — Arc Welding) specifies requirements for:

According to CSA W59, GMAW and FCAW welds must be made with wires conforming to AWS A5.18 (solid wire) or AWS A5.20 (flux-cored wire). The welder must be qualified according to the standard's requirements for each process and position.

Canadian Electrical Code, Part I

The Canadian Electrical Code, Part I (C22.1-21) applies to electrical installations at welding stations. Rule 8-200 specifies requirements for welding circuits, including:

The welder must ensure that the ground cable is properly connected and that the return circuit is sized for the welding current.

CSA B149.1

CSA B149.1 (Natural Gas and Propane Installation Code) applies to shielding gas installations when they use combustible gases or gas distribution systems. Although shielding gases (argon, CO₂) are not combustible, the standard's requirements apply to storage and distribution installations for these pressurized gases.

Occupational Health and Safety

Electrical Hazards

GMAW and FCAW welding present specific electrical hazards:

The welder must wear dry insulating gloves, use insulated pliers, and ensure that equipment is properly grounded.

Fumes and Gases

GMAW and FCAW welding fumes contain metal oxides, carbon monoxide, ozone, and other hazardous substances. Control measures include:

Radiation

The GMAW and FCAW arc produces intense ultraviolet (UV) radiation that can cause skin burns and eye damage (arc eye). Protection includes:

Quality Control

Visual Inspection

Visual inspection is the first step of quality control. According to CSA W59, acceptance criteria include:

Non-Destructive Testing

Non-destructive testing (NDT) commonly used for GMAW and FCAW welds includes:

Welder Qualification

Welder qualification according to CSA W47.1 includes:

Qualification is valid for a specified period and must be renewed if the welder has not practiced the process for an extended period.

Pitfalls to Avoid

Summary

GMAW and FCAW are continuous wire processes that offer high productivity and great versatility. The essential points to remember for the Red Seal exam:

The candidate must be able to identify the correct parameters for a given application, calculate heat input and deposition rate, and recognize welding defects with their causes and remedies. Mastery of Canadian regulatory requirements is essential to pass the exam and to practice the trade in full compliance.