MIG vs TIG welding: differences, applications and advantages

MIG and TIG are the two arc welding processes you will meet most often in a workshop. Both melt the edges of two parts with an electric arc and protect the molten metal with a shielding gas. The difference is in how the filler metal gets into the joint, and that one difference changes the speed, the finish, the cost and the skill each process needs.

How arc welding works

An arc is a continuous electric spark between an electrode and the workpiece. It runs at several thousand degrees, so it melts a small pool of metal where it touches. As the torch moves along the joint, the pool moves with it and the metal behind it freezes into a weld bead that fuses the two parts together.

Molten steel or aluminium reacts with the oxygen and nitrogen in air, which leaves the weld porous and brittle. Both MIG and TIG prevent this by blowing a shielding gas over the pool through the torch.

MIG welding (GMAW)

MIG stands for metal inert gas. Its formal name is gas metal arc welding (GMAW). A spool of solid wire is pushed by feed rollers through the torch, and the wire itself is the electrode. The arc melts the tip of the wire, and the molten wire becomes the filler metal in the joint. When the shielding gas contains CO₂, as it usually does for steel, the process is strictly called MAG (metal active gas), but most people still say MIG.

The process, step by step

1. Clamp the earth lead to the workpiece and set the voltage and the wire feed speed for the material and thickness.
2. Hold the torch at about 10 to 15 degrees from vertical, with a short length of wire sticking out of the contact tip.
3. Press the trigger. Gas starts to flow, the wire feeds forward and the arc strikes as soon as the wire touches the work.
4. Move steadily along the joint. The machine keeps the arc length constant, so the welder mainly controls travel speed and torch angle.
5. Release the trigger at the end of the joint, hold the torch in place for a moment so the gas protects the cooling crater, then clean off any spatter.

Advantages

• Fast. The wire feeds continuously, so MIG deposits metal several times faster than TIG.
• Easy to learn. One hand holds the torch and the machine does much of the work.
• Good on thicker sections. Steel from about 1 mm up to heavy plate, in several passes where needed.
• Easy to automate. Most robotic welding cells in car and equipment factories run MIG.

Limitations

• Less precise. It is harder to control heat on very thin sheet, where the wire can burn through.
• Spatter. Small droplets of metal land around the weld and often need grinding off.
• Rougher finish. The bead is acceptable on structures but rarely good enough for visible, cosmetic joints.
• Sensitive to wind. A draught blows the gas away, so outdoors it needs screens or a flux-cored wire.

Where MIG is used

Structural steel frames, trailers and chassis, agricultural and construction equipment, sheet-metal enclosures, car body repair and almost any production welding where speed matters more than appearance.

TIG welding (GTAW)

TIG stands for tungsten inert gas, formally gas tungsten arc welding (GTAW). The arc comes from a tungsten electrode, which has such a high melting point (about 3,400 °C) that it does not melt. If the joint needs extra metal, the welder dips a separate filler rod into the pool with the other hand. The heat is usually set with a foot pedal, so all four limbs are busy.

The process, step by step

1. Grind the tungsten to a point (for DC welding of steel) and set the current. Steel and stainless steel are welded on DC; aluminium is welded on AC, which breaks up the oxide skin on its surface.
2. Clean the joint thoroughly. TIG has no flux, so oil, paint and oxide end up in the weld.
3. Start the arc without touching the work (high-frequency start) and build a small, shiny pool.
4. Dip the filler rod into the front edge of the pool, pull it back, and move the torch forward a little. Repeat in a steady rhythm.
5. Ease off the pedal at the end to fill the crater, and keep the torch over the weld until the post-flow gas stops.

Advantages

• Precise and clean. No spatter, no slag and very neat beads, often left as they are on visible parts.
• Works on almost any metal. Stainless steel, aluminium, titanium, copper alloys and very thin sheet, down to about 0.5 mm.
• Full control of heat. The pedal lets the welder adjust the heat while welding, which avoids burning through thin material.
• High quality. Welds are dense and strong, so TIG is used where a joint must not fail or leak.

Limitations

• Slow. Deposition is much lower than MIG, so it costs more per metre of weld.
• Needs skill. Coordinating torch, rod and pedal takes long practice.
• Needs very clean material. Contamination shows up straight away as porosity or a dirty weld.
• Also sensitive to wind, like MIG.

Where TIG is used

Stainless steel food and pharmaceutical equipment, aerospace parts, bicycle and motorsport frames, stainless exhaust systems, aluminium fabrications, thin sheet, repairs on tools and dies, and root passes on critical joints that are then filled faster with another process.

MIG vs TIG comparison

Which one should you choose?

Start from the part, not the process. Ask three questions.

• What is the material and how thick is it? Thin stainless steel or aluminium points to TIG. Mild steel from 2 mm upwards points to MIG.
• Will the weld be seen? A visible joint on a product, a handrail or a food machine is usually TIG. A weld hidden inside a frame is usually MIG.
• How many parts are there? For a production run, MIG’s speed and suitability for robots usually win. For one-off parts or repairs, TIG’s control can be worth the extra time.

Designing parts for welding

The choice of process starts on the drawing, so a few design decisions make welding easier and cheaper whichever process the shop uses.

• Leave access for the torch. A MIG torch is bulky. Check in the CAD assembly that the torch can reach every weld at a sensible angle.
• Show welds with standard symbols. Use ISO 2553 or AWS A2.4 weld symbols on the drawing (type, size, length and side), rather than a note that says “weld all round”.
• Prepare the edges. Thicker plates need a bevel (a V or J preparation) so the weld penetrates the full thickness. Model or note the preparation on the part drawing.
• Plan for distortion. Welds shrink as they cool and pull the part out of shape. Balanced welds on both sides, tack welds and fixtures keep it straight. Leave machining allowance on faces that must be accurate after welding.
• Avoid welding dissimilar metals. Steel cannot be welded directly to aluminium with MIG or TIG; join them mechanically or with a special transition insert.