3.1 Welding Technology Overview & Arc Fundamentals

3.1 Welding Technology Overview & Arc Fundamentals

Quick Answer: For IWI-S, welding is a controlled joining process defined on a WPS. Most production work is fusion welding with an electric arc. Inspectors must know whether the power source is CC or CV, which polarity is required, and that open-circuit voltage is not the same as arc voltage under load.

Welding Technology (WT) is a large part of the IWI-S syllabus. Module WT1 introduces process fundamentals so an inspector can verify that the process, equipment, and parameters used on site match the qualified procedure. This chapter covers arc process families used most often in fabrication: MMA/SMAW (ISO 4063 111), MIG/MAG/GMAW (131/135), flux-cored variants (114/136), and TIG/GTAW (141). Later chapters add SAW and other processes.

Welding as a Joining Technology

Welding joins components so that load can transfer across the joint through continuous metallic (or, less often, non-metallic) material—not only by friction, fasteners, or adhesive. In steel and aluminium fabrication the usual goal is a metallurgical bond between parts, often with added filler metal.

Inspectors should keep three joining families in mind:

1. Fusion welding — parent metal (and usually filler) is melted; the joint solidifies as a weld metal / heat-affected zone structure. Arc, laser, electron beam, and many resistance processes fall here.
2. Solid-state welding — bonding occurs below bulk melting temperature by pressure, deformation, friction, or diffusion (friction stir, forge, explosion welding). No conventional weld pool.
3. Brazing and soldering — only the filler melts; parent metal remains solid. Useful for thin assemblies, but not a substitute for a structural fusion weld when the WPS calls for one.

IWI-S inspection practice is dominated by fusion arc welding. Solid-state and brazing appear mainly as awareness topics or special applications.

Fusion vs Solid-State — Inspector Relevance

Fusion welds create a weld pool, a fusion boundary, and a heat-affected zone (HAZ). That thermal cycle drives most inspection concerns: hydrogen cracking risk, hot cracking, porosity, lack of fusion, distortion, and hardness. Solid-state joints lack a conventional pool and slag, so the imperfection catalogue differs (for example, lack of bonding, flash geometry).

When you open a WPS, first identify the process number (ISO 4063) and whether the joint is fusion-based. That choice locks in power-source type, consumable form, shielding method, and which NDT methods later apply.

The Welding Arc

An electric welding arc is a sustained plasma discharge between electrode and workpiece. Current heats the electrode tip and the work; gas in the arc column ionizes and carries current. Arc temperatures are high enough to melt steel and most engineering alloys rapidly.

For the inspector, the arc is not only a heat source—it is a process control surface:

• Arc length (gap) strongly affects voltage and heat distribution in manual processes.
• Shielding (gas, flux, or slag) must exclude air; broken shielding causes porosity and nitrogen/oxygen pickup.
• Stability depends on power-source characteristic, polarity, electrode design, and operator technique.

IAB WT1 expects you to link these physical ideas to what you check on the shop floor: leads, polarity switches, gas flow, electrode condition, and the numbers written on the WPS.

Power Sources: Constant Current (CC) vs Constant Voltage (CV)

Welding power sources are classified by their static output characteristic—how voltage and current interact as the arc length changes.

Constant current (CC) — "drooping" characteristic

• Output current stays nearly constant when arc length (and thus voltage) varies within a normal range.
• Suited to manual metal arc (MMA/SMAW) and TIG/GTAW, where the welder holds the torch or electrode and arc length is not machine-regulated by wire feed.
• The welder sets current (amperage). Voltage then depends mainly on arc length and process.

Constant voltage (CV) — flat characteristic

• Output voltage is held nearly constant; current self-adjusts with burn-off rate of a continuous wire.
• Suited to MIG/MAG (GMAW) and FCAW, where wire-feed speed largely sets current.
• The welder (or robot) sets voltage and wire-feed speed; the machine self-regulates arc length for a given stick-out.

Exam and site trap: Fitting a stick electrode holder to a CV wire feeder output, or running GMAW on a pure CC stick machine without the correct mode, is a process mismatch. Modern multi-process inverters may offer both curves—the selected mode must still match the WPS process.

Polarity: DCEN, DCEP, and AC

Polarity describes which terminal is positive and how heat and electron flow are distributed.

Polarity is an essential variable on many procedures. Wrong polarity can change penetration, bead shape, electrode consumption, and—for aluminium TIG—remove the cleaning action needed to break oxide.

Open-Circuit Voltage vs Arc Voltage

• Open-circuit voltage (OCV) is the voltage measured at the output terminals with the machine on but no arc (no welding current). It must be high enough to strike and re-strike the arc, and is limited by safety and equipment design.
• Arc voltage (welding voltage) is the voltage during welding, measured across the arc under load. It is the parameter that appears in heat-input calculations and on many WPS ranges for GMAW/FCAW/TIG.

Inspectors should not treat a meter reading taken before the arc is struck as proof that welding voltage is correct. When verifying parameters, use the method required by the procedure or quality plan—often a calibrated meter during welding, or machine digital display known to be reliable for that setup.

Duty Cycle

Duty cycle is the percentage of a defined period (commonly a 10-minute cycle at a stated ambient temperature) during which a power source can deliver a given current without exceeding thermal limits. Example: 300 A at 60% duty cycle means 6 minutes of welding and 4 minutes of rest at that current within each 10-minute window (machine rating basis—not a production schedule rule).

Why inspectors care:

• Undersized machines run hot, trip thermal protection, and tempt operators to change parameters to "keep going."
• Mechanised high-deposition welding may need 100% duty cycle capability at the working current.
• Consumable and cable ratings also matter; duty cycle is not only a transformer sticker.

Why Inspectors Verify Power-Source Type Against the WPS

A WPS is a qualified recipe. Process family, current type, polarity, and parameter windows are not optional preferences. Before production (and at ITP hold points), the IWI-S should confirm:

1. Process matches ISO 4063 number and description on the WPS (e.g. 111 vs 135 vs 141).
2. Power characteristic is appropriate (CC for MMA/TIG; CV for GMAW/FCAW) or the multi-process unit is in the correct mode.
3. Polarity / current type (DCEN, DCEP, AC) matches the WPS and consumable data sheet.
4. Parameter displays (current, voltage, wire speed, gas flow) are readable and, where required, calibrated or verified.
5. Duty cycle and equipment condition support continuous production without forced parameter drift.

If equipment cannot maintain the WPS window, welding must stop until the non-conformance is resolved—not "adjusted by feel." That discipline is central to IWI-S supervision of IWI-B staff and to witnessing procedure qualification.

Link to Later Chapters

Arc fundamentals feed heat-input control, metallurgy, and acceptance. Wrong polarity or CC/CV mismatch is often the root cause behind porosity, lack of fusion, or burn-through that later appears under ISO 6520 classification. Master the energy delivery system first; then study each process family's consumables and defects.