TIG, MIG, MAG or stick welding: which welding process should you choose?

Arc welding joins two parts by locally melting them, with or without filler metal, using an electric arc. The four common processes differ in the electrode (non-consumable or consumable), in how the weld pool is protected (a gas or the rod coating) and in how the filler metal is supplied. The ISO 4063 standard gives each a number, which you sometimes find on drawings and welding procedures.

The difference between MIG and MAG lies solely in the gas: inert for MIG, active (a mix of argon and carbon dioxide, or pure CO2) for MAG. In the workshop, people often say “MIG-MAG” or “semi-automatic” for both. For a welding and plate fabrication workshop, the choice of process is a compromise between quality, productivity and joint accessibility.

TIG welding (Tungsten Inert Gas) strikes the arc between the workpiece and a tungsten electrode, which does not melt. An inert gas, generally argon, protects the pool. The filler metal, a rod held in the other hand, is optional: on thin sheet, you can weld without filler (autogenous welding).

The welder controls heat and filler separately, which gives clean, spatter-free beads suited to thin sheet, tubes and visible welds. It is the reference process for stainless steel, aluminium and titanium. On aluminium, alternating current is used: it breaks up the alumina layer, which melts at around 2,050 °C while the metal melts at around 660 °C. On stainless steel and titanium, the back of the joint is protected by a purge gas, otherwise it oxidises.

The downside is slowness and demands: TIG needs a highly skilled welder and costs more per metre of weld than wire processes. It is justified when quality, appearance or the fineness of the joint require it: piping and tanks for the food industry, tubes and vessels, thin or visible parts.

In MIG-MAG, a solid wire, fed continuously by the torch, acts as both electrode and filler metal, under gas shielding. The welder has only one hand to manage: the process is fast, easy to automate and robotise, and deposits are regular.

• MAG on steel. The argon-CO2 mix (or pure CO2) gives good penetration at low cost: it is the most common process for frames, structures and structural steel plate fabrication. Stainless steel can also be MAG welded, with a mildly active argon-based gas.
• MIG on aluminium. Argon (sometimes with added helium) and a wire suited to the alloy give long, regular beads; aluminium wire, being soft, is trickier to feed.
• Flux-cored wire. A tubular wire filled with flux increases deposition and suits heavy steel thicknesses.

Depending on the energy, the metal transfers by short-circuit (thin sheet, low heat input), in pulsed mode or in spray mode (heavy thicknesses, flat position). The limits: gas shielding is vulnerable to wind, which requires a shelter outdoors; spatter needs cleaning; and, in short-circuit mode on heavy thicknesses, insufficient setting can leave a lack of fusion, a defect invisible at the surface. The bead appearance is less neat than with TIG.

Stick welding

A metal rod covered with a coating serves as electrode and filler metal. As it melts, the coating produces a shielding gas and a slag that covers the bead. The equipment is simple, light and needs no gas cylinder: the process withstands wind and suits site work, repairs and heavy steel thicknesses. It is less productive, leaves slag to remove, requires some electrodes to be kept dry and remains poorly suited to sheet under about 2 mm. Bead quality depends heavily on the welder.

Production and heavy-thickness processes

• Submerged arc (121): wire and powdered flux, for long straight welds in the flat position on heavy thicknesses (tanks, beams).
• Resistance (spot) welding: joining thin sheets by overlap, very fast in production (enclosures, sheet metal work).
• Laser and plasma: automated welding with very concentrated heat input, hence little distortion, for stable production runs.

These processes require specific resources and are mainly justified for production runs: ask the workshop whether it masters them.

The choice depends first on the material, then on the thickness and the quality required. The table gives common practice; the workshop chooses the process it masters and has qualified.

Thickness then steers the process: below about 1.5 mm, TIG, short-circuit MAG or spot welding limit the risk of burning through; beyond a few millimetres, butt welds require bevel preparation and several passes. For a part to be galvanised or painted, weld before treatment and provide vent holes: see anodising, powder coating or galvanising.

The process does not guarantee the quality of a weld: the welder, the procedure and the level of requirement count just as much. Four notions are worth knowing before writing your request.

• Welder qualification. ISO 9606-1 defines the qualification of welders on steels; other parts of the series cover aluminium, copper, nickel and titanium.
• Welding procedure. The WPS (welding procedure specification) sets the parameters; its qualification (WPQR) falls, for example, under ISO 15614-1 for steels.
• Imperfection quality level. ISO 5817 defines three levels for steels: B (the most severe), C and D; ISO 10042 plays this role for aluminium. The more severe the level, the more costly the inspection and rework.
• Inspections. Visual examination is systematic. Dye penetrant and magnetic particle testing reveal surface defects; ultrasonics and radiography reveal those hidden inside the bead.

At company level, ISO 3834 sets quality requirements specific to welding (parts 2, 3 and 4: comprehensive, standard or elementary requirements). For steel or aluminium structures, EN 1090 governs execution and CE marking, with four execution classes (EXC1 to EXC4). The rail industry has its own standard, EN 15085. Require only what the use justifies: see the guide on quality certifications.

• the material, grade and thickness of each part, with the material certificate if necessary;
• the general drawing with weld symbols (joint type, continuous or intermittent bead, bead size), and the 3D model if possible;
• the quality level of the welds (ISO 5817: B, C or D) or, failing that, the use of the part and its constraints (pressure, load, fatigue, hygiene);
• the qualifications required: welders, procedure, reference standard;
• the inspections expected (visual, dye penetrant, radiography, etc.) and the associated report;
• the finish: ground or brushed beads, pickling and passivation for stainless steel, surface treatment after welding;
• the tolerances and the risk of distortion, which determines clamping and welding sequence;
• the quantity, rate and lead time.

The clearer these points, the more accurately the workshop quotes. On Meaflow, posting a request is free and commission-free: you compare quotes from several workshops and talk by messaging before choosing.