Welding Fundamentals - MIG & TIG

Welding joins metal pieces by melting them together with heat and filler material. MIG (Metal Inert Gas, formally GMAW - Gas Metal Arc Welding) and TIG (Tungsten Inert Gas, formally GTAW - Gas Tungsten Arc Welding) are two of the most widely used welding processes in fabrication, manufacturing, construction, and repair work. This guide gives you the detailed, hands-on knowledge you need to set up equipment, prepare joints, run quality beads, troubleshoot common problems, and work safely from your very first day in a welding shop.

Safety First - Before You Strike an Arc

Welding produces intense ultraviolet and infrared radiation, molten metal spatter, toxic fumes, and high electrical current. Every welder must take safety seriously every single time.

Required Personal Protective Equipment (PPE)

• Welding helmet - Auto-darkening with shade 10 to 13 for arc welding. Shade 10 for lower amperage TIG work (under 100A), shade 12 to 13 for higher amperage MIG and heavy TIG. The helmet must have ANSI Z87.1 certification. Test the auto-darkening sensor before every use by holding the helmet up to a bright light.
• Safety glasses - Wear ANSI Z87.1-rated safety glasses underneath your helmet at all times. When you flip the hood up to inspect a weld, your eyes are still protected from flying debris.
• Welding gloves - Heavy leather gauntlet gloves for MIG welding. TIG welding requires thinner, more dexterous leather gloves (often goatskin or deerskin) so you can feel the filler rod. Never use synthetic gloves - they melt to your skin.
• Clothing - Wear flame-resistant (FR) cotton or leather. Long sleeves, buttoned at the wrist and collar, with no cuffs or open pockets that can catch sparks. Pants should cover the tops of your boots with no cuffs. Never wear polyester, nylon, or any synthetic fabric near a welding arc. They ignite and melt instantly.
• Boots - Leather work boots, at minimum 6 inches tall. Steel-toed if required by the shop. No laces hanging loose. Spatter drops straight down and will go inside low-cut shoes.
• Ear protection - Earplugs in noisy shops and anytime you grind. Spatter can also enter the ear canal.

Ventilation and Fume Control

• Weld in a well-ventilated area. If indoors, use a fume extractor positioned 6 to 12 inches from the arc. Flexible arm extractors are most common in shop settings.
• Welding galvanized steel, stainless steel, or coated metals produces especially hazardous fumes. Zinc fumes from galvanized metal cause metal fume fever - flu-like symptoms that appear 4 to 12 hours after exposure. Use a powered air-purifying respirator (PAPR) or a half-face respirator with P100 filters when welding these materials.
• Never weld in a confined space without proper ventilation and atmospheric monitoring. Follow OSHA 1910.146 confined space entry procedures.
• Keep your face out of the plume. Position yourself so that natural airflow or the fume extractor pulls the plume away from your breathing zone.

Fire Prevention

• Remove all flammable materials within 35 feet of the welding area, or use fire-resistant blankets and shields to protect them.
• Have a charged fire extinguisher (ABC type, minimum 10 lbs) within 10 feet of your work area.
• Maintain a fire watch for at least 30 minutes after welding is complete. Sparks and hot slag can smolder in cracks, insulation, and debris.
• Check behind walls, below floors, and above ceilings when welding near combustible structures.
• Never weld on containers that have held flammable materials without proper purging and gas testing.

Electrical Safety

• Never weld in wet conditions or while standing on wet surfaces. Water conducts welding current, which can be fatal.
• Inspect cables, connectors, and the welding torch daily for damaged insulation, frayed wires, or loose connections. Replace damaged components immediately.
• Never wrap welding cables around your body.
• The work clamp must make a solid connection to the workpiece or the welding table. A poor ground connection causes erratic arc behavior and creates shock hazards.

Understanding MIG Welding (GMAW)

MIG welding feeds a consumable wire electrode continuously through a welding gun while shielding gas protects the molten weld pool from atmospheric contamination. It is the most common process in production shops because it is relatively easy to learn, produces clean welds quickly, and works on a wide range of materials and thicknesses.

MIG Equipment and Components

• Power source - MIG welders are constant-voltage (CV) machines. Most shop machines are 200 to 300 amps. Smaller 110V machines run 90 to 140 amps and handle material up to about 3/16 inch.
• Wire feeder - Built into the machine or a separate unit. Houses the wire spool and drive rolls that push wire through the cable and out the contact tip. Drive roll tension should be set so the wire feeds smoothly without slipping or kinking. Over-tightening deforms the wire.
• Welding gun - Contains the contact tip, gas nozzle, and liner. The trigger activates wire feed, gas flow, and welding current simultaneously.
• Contact tip - A copper tube that transfers current to the wire. The tip bore must match the wire diameter exactly. Tips wear out. Replace when the hole elongates, which causes erratic arc and inconsistent feeding.
• Gas nozzle - Directs shielding gas around the arc. Clean spatter from the inside of the nozzle regularly. A clogged nozzle causes poor gas coverage and porosity. Use anti-spatter spray or dip on the nozzle before welding.
• Liner - A tube inside the cable that guides the wire from the drive rolls to the contact tip. Steel liners are used for steel wire. Teflon liners are required for aluminum wire. Replace liners when you notice inconsistent feeding despite correct drive roll tension.
• Shielding gas - Delivered from a cylinder through a regulator and flowmeter. Set the flow rate to 25 to 35 CFH (cubic feet per hour) for most indoor work. Increase to 40 to 50 CFH if working in a drafty area or outdoors, but too much gas creates turbulence that sucks in air. Use a gas lens if wind is a problem.

MIG Wire Selection

The wire electrode is your filler material. Selection depends on the base metal:

• ER70S-6 - The most common wire for mild (carbon) steel. The "70" means 70,000 PSI tensile strength. The "S-6" indicates a solid wire with silicon and manganese deoxidizers for cleaner welds. Use this as your default for structural steel, pipe, plate, and general fabrication.
• ER70S-3 - A cleaner wire with fewer deoxidizers. Use on clean, well-prepared base metal.
• ER308L - For welding 304 stainless steel. The "L" stands for low carbon, which reduces sensitization (chromium carbide formation) in the heat-affected zone.
• ER309L - For welding dissimilar metals such as stainless to mild steel.
• ER4043 - For welding most aluminum alloys. Good general-purpose aluminum wire.
• ER5356 - For higher-strength aluminum weldments and better color match after anodizing.

Common wire diameters:

• 0.023" - Thin sheet metal, 24 to 20 gauge
• 0.030" - Light fabrication, 20 to 16 gauge. The most versatile size.
• 0.035" - General shop work, 16 to 3/8 inch. The most common production size.
• 0.045" - Heavy fabrication, 1/4 inch and thicker

Shielding Gas Selection

• 75% Argon / 25% CO2 (C25) - The standard gas mix for mild steel MIG welding. Good penetration, smooth arc, minimal spatter. This is what you will use 90% of the time on carbon steel.
• 90% Argon / 10% CO2 - Produces less spatter and a smoother arc. Good for thin materials and appearance-critical welds.
• 100% CO2 - Deepest penetration and lowest cost but produces more spatter and a harsher arc. Used in structural and heavy fabrication where penetration matters more than appearance.
• 100% Argon - Required for aluminum MIG welding. Also used for stainless steel in some tri-mix formulations.
• Tri-mix (90% He / 7.5% Ar / 2.5% CO2) - Sometimes used for stainless steel for better wetting and penetration.

MIG Machine Settings

MIG welding has two primary controls:

• Voltage - Controls the arc length. Higher voltage = longer arc = wider bead. Lower voltage = shorter arc = narrower, more focused bead. Start with the manufacturer's chart on the machine for your wire size and material thickness, then fine-tune by sound and bead appearance.
• Wire feed speed (WFS) - Controls amperage. Faster wire feed = higher amperage = more heat and penetration. Slower wire feed = less heat.

A good starting point for 0.035" wire on 1/4" mild steel: 21 to 22 volts, 280 to 320 IPM (inches per minute) wire feed speed.

The sound test: A properly tuned MIG weld sounds like steady bacon frying or a continuous buzzing hum. If it sounds like popcorn popping, your voltage is too high or wire speed too low. If the wire stubs into the work repeatedly, increase voltage or decrease wire speed.

Running a MIG Bead - Step by Step

1. Position yourself - Get comfortable. Brace your arms. You cannot run a consistent bead if you are off balance or reaching. Use your free hand to support the gun hand.
2. Angle - Hold the gun at a 15 to 20 degree angle from vertical in the direction of travel (drag or push angle). Your nozzle-to-work distance (stickout) should be 3/8 to 1/2 inch.
3. Push vs. pull - Pushing (forehand) the gun away from the completed weld gives a flatter, wider bead with shallower penetration and better gas coverage. Pulling (backhand, dragging) toward the completed weld gives deeper penetration and a more convex bead. Most structural work uses the pull technique. Sheet metal work often uses push.
4. Start the arc - Position the wire at your starting point, squeeze the trigger, and establish the arc. Hold for a second to build the puddle before moving.
5. Travel speed - Move at a steady, consistent speed. Watch the puddle, not the arc. The back edge of the puddle should stay about 3/8 inch behind the arc. If the puddle gets ahead of you, slow down. If it falls too far behind, speed up.
6. Weave patterns - For wider beads or filling gaps, use a slight side-to-side weave. Pause briefly at each edge to allow the puddle to wet into the base metal. Common patterns: straight stringer (no weave), slight crescent, zigzag, or circular. Keep weaves tight and controlled. Excessive weaving creates undercut and lack of fusion.
7. Ending the bead - Do not just release the trigger. Slow down slightly at the end to fill the crater, or use the burnback function if your machine has one. An unfilled crater is a crack initiation point.

Common MIG Defects and Fixes

Defect	Appearance	Common Causes	Fix
Porosity	Small holes in or on the bead surface	Contaminated base metal, insufficient gas coverage, drafts, wet shielding gas	Clean metal thoroughly, check gas flow and nozzle, block drafts
Undercut	Groove melted into the base metal along the toe of the weld	Travel speed too fast, voltage too high, incorrect angle	Slow down, reduce voltage, adjust angle to direct heat toward the joint
Lack of fusion	Weld metal sitting on top of the base metal without melting into it	Heat too low, travel speed too fast, wrong angle	Increase WFS, slow travel speed, direct the arc into the joint root
Excessive spatter	Metal droplets scattered around the weld	Voltage too low, WFS too high, contaminated metal, poor gas coverage	Increase voltage, clean metal, check gas, use anti-spatter
Burn-through	Holes melted through the base metal	Too much heat for the material thickness	Reduce WFS and voltage, increase travel speed, use thinner wire
Cold lap	Weld bead rolls over without fusing at the edges	Travel too fast, insufficient heat, wrong angle	Slow down, increase heat, direct arc into the base metal

Understanding TIG Welding (GTAW)

TIG welding uses a non-consumable tungsten electrode to create the arc and a separate hand-fed filler rod to add material. The process produces the cleanest, most precise welds of any arc welding method. TIG is used for aerospace, food and beverage piping, artistic metalwork, thin-wall tubing, and any application where weld appearance and quality are critical.

TIG requires more skill and coordination than MIG. Both hands work independently. The dominant hand holds the torch and controls arc position. The other hand feeds the filler rod. A foot pedal or finger control modulates the amperage in real time.

TIG Equipment and Components

• Power source - TIG welders are constant-current (CC) machines. Most shop machines are 200 to 350 amps. AC/DC machines can weld both steel and aluminum.
• TIG torch - Air-cooled torches handle up to about 200 amps. Water-cooled torches are smaller, lighter, and handle higher amperage. The torch holds the tungsten electrode in a collet. A ceramic or gas lens cup directs shielding gas.
• Gas lens - A mesh screen inside the cup that produces a smooth, laminar gas flow. Gas lenses dramatically improve gas coverage and allow you to extend the tungsten farther from the cup for better visibility in tight joints. Use a gas lens whenever possible.
• Tungsten electrodes - The electrode creates the arc but does not melt into the weld.
  • 2% Lanthanated (blue band) - Best all-purpose tungsten. Works on AC and DC. Good arc starts, stable arc, long life.
  • 2% Ceriated (gray band) - Excellent for DC welding on steel and stainless at low to moderate amperages.
  • 2% Thoriated (red band) - Historically popular but mildly radioactive. Being phased out in favor of lanthanated and ceriated.
  • Pure tungsten (green band) - Used for AC aluminum welding on older machines. Balls easily.
• Tungsten diameters - Match to the amperage range: 1/16" (10-80A), 3/32" (60-160A), 1/8" (100-250A).
• Shielding gas - Pure argon, 15 to 25 CFH. Argon/helium mixes (25-75% helium) provide more heat for thicker aluminum and copper. Never use CO2 or mixed gases with TIG.
• Filler rod - Chosen to match the base metal. Common rods:
  • ER70S-2 or ER70S-6 - Mild steel
  • ER308L - 304 stainless steel
  • ER316L - 316 stainless steel
  • ER4043 - Aluminum (general purpose, easier to feed)
  • ER5356 - Aluminum (higher strength, better for 5000-series alloys)
• Rod diameters: 1/16", 3/32", 1/8", 3/16". Match the rod diameter to the material thickness and joint size.

Preparing Tungsten

For DC welding on steel and stainless:

• Grind the tungsten to a point on a diamond grinding wheel dedicated to tungsten only. Grind lengthwise (along the axis), not radially, to create grinding marks that run from tip to back. This helps the arc track straight.
• The taper length should be about 2 to 2.5 times the electrode diameter. A 3/32" tungsten gets a taper about 3/16" to 1/4" long.
• A sharper point concentrates the arc for thinner material and precise work. A blunter point spreads the arc for wider beads and higher amperage.

For AC welding on aluminum:

• Start with a pointed tungsten. When you strike the AC arc, the positive half-cycle will melt the tip into a small ball. This is normal and desirable.
• On modern inverter machines, you can use a pointed tungsten with the AC balance adjusted to 65-75% electrode negative (EN). This keeps the point sharper while still providing the cleaning action aluminum needs.

Running a TIG Bead - Step by Step

1. Set up - Select your tungsten, cup, and gas flow. Set the machine to the correct polarity (DCEN for steel/stainless, AC for aluminum). Set the starting amperage based on material thickness. A good rule of thumb: 1 amp per 0.001 inch of thickness on steel.
2. Position - Hold the torch like a pencil at a 15 to 20 degree angle from vertical, tilted away from the direction of travel. The tungsten tip should be about 1/8 inch from the workpiece. Rest the cup on the workpiece or use the "walking the cup" technique.
3. Start the arc - Use a high-frequency start or lift-start (touch the tungsten to the workpiece, then lift off). Never scratch-start TIG - it contaminates the tungsten.
4. Build the puddle - Hold the arc in place until a small, shiny molten puddle forms. On steel, the puddle should be about 1.5 to 2 times the material thickness in diameter before you start adding filler.
5. Add filler - Hold the filler rod at about a 15 to 20 degree angle from the workpiece surface, approaching the puddle from the leading edge. Dip the rod into the front edge of the puddle, then pull it back out. The filler should melt from the heat of the puddle, not from the direct arc. "Dip, pull back, advance the torch, dip, pull back, advance" - this is the fundamental TIG rhythm.
6. Keep the filler rod in the gas shield - If you pull the rod too far back, the hot tip oxidizes and introduces contamination into your next dip. Keep the tip within an inch or two of the puddle.
7. Control heat with the foot pedal - Roll off the pedal as you approach the edge of thin material to prevent burn-through. Roll on more amperage when you hit a tack weld or a thicker section.
8. End the weld - Slowly reduce amperage with the foot pedal while continuing to add filler. This fills the crater and prevents crater cracking. Post-flow gas continues for 5 to 10 seconds after you release the pedal to protect the cooling weld and tungsten.

TIG Troubleshooting

• Tungsten contamination (gray or balled tip, erratic arc) - You dipped the tungsten into the puddle. Stop. Break off the contaminated section or regrind. Contaminated tungsten deposits tungsten inclusions in the weld.
• Sugaring (black, crusty oxidation on the back side of the weld) - No purge gas on the back side. On pipe and tube, purge with argon. On open joints, use a backing strip or copper backing bar.
• Gray or discolored weld - Insufficient gas coverage. Increase gas flow, check for leaks, use a gas lens, or reduce drafts. A good stainless TIG weld should be straw to light gold colored. Gray or blue means too little gas. Black means severe contamination.
• Porosity - Contaminated base metal, moisture on the filler rod, or gas coverage issues. Clean everything with acetone, store filler rods in a dry location, and verify gas setup.
• Lack of penetration - Increase amperage, slow travel speed, or reduce filler rod diameter. The puddle should show clear fusion into both sides of the joint.
• Uneven bead width - Inconsistent travel speed or torch angle. Practice maintaining a steady rhythm. Use a metronome app set to your dip-and-advance timing.

Joint Preparation for Both Processes

The quality of your weld starts before you strike the arc. Poor preparation is the number one cause of weld defects.

Cleaning

• Remove all rust, mill scale, paint, grease, oil, and coatings from at least 1 inch on each side of the joint. Use a grinder with a flap disc, a wire wheel, or chemical cleaner (acetone for oil and grease).
• For stainless steel, use a stainless wire brush and stainless-dedicated grinding discs. Carbon steel contamination on stainless causes rust staining and corrosion.
• For aluminum, remove the oxide layer immediately before welding with a stainless wire brush or chemical etch. Aluminum oxide melts at 3,700 degrees F while aluminum melts at 1,200 degrees F - the oxide acts as an insulating layer that prevents fusion.

Fit-Up and Gap

• Parts should fit together tightly with consistent contact or consistent gap along the entire joint length. Variable fit-up produces variable weld quality.
• For butt joints, a tight fit (zero gap) works for thin material. For thicker material, a root gap of 1/16 to 3/32 inch allows full penetration.
• For lap joints, clamp the pieces together tightly with no gap between the surfaces.
• For T-joints and fillets, the vertical piece should sit flush against the horizontal piece with no gap at the root.

Tack Welding

• Place small tack welds to hold the joint in alignment. Tacks should be small (1/4 to 3/8 inch long on thin material, up to 1 inch on heavy plate).
• Space tacks every 4 to 6 inches on thin material, 6 to 12 inches on thicker material.
• If a tack cracks or does not look right, grind it out and redo it. Bad tacks become defects in the finished weld.
• On critical work, grind the starting and stopping ends of each tack to a taper so the weld bead transitions smoothly.

Beveling

• Material over 3/16 inch thick requires a bevel for full penetration butt joints. Common bevel angles are 30 to 37.5 degrees on each side (total included angle of 60 to 75 degrees).
• Leave a root face (land) of 1/16 to 1/8 inch at the bottom of the bevel. This flat area prevents burn-through during the root pass.
• Use a grinder, beveling machine, or oxy-fuel torch (with grinding to clean the cut surface) to prepare bevels.

Common Joint Types and When to Use Them

• Butt joint - Two pieces placed edge to edge. Used for plates, pipe, and structural members. Requires beveling on thicker material.
• Lap joint - Two pieces overlapping, welded along the edge of the top piece. Used for sheet metal, brackets, and non-critical joints. Easy to fit up.
• T-joint (fillet weld) - One piece perpendicular to another, forming a T. The most common structural joint. The weld size (leg) is specified on the drawing.
• Corner joint - Two pieces meeting at an angle, usually 90 degrees. Used for boxes, enclosures, and frames.
• Edge joint - Two pieces placed side by side with edges aligned and welded along the top. Used for thin sheet metal and non-structural applications.

Welding Positions

AWS defines four basic positions. Practice all of them:

• 1F/1G (Flat) - Welding on the top surface with gravity pulling the puddle into the joint. Easiest position.
• 2F/2G (Horizontal) - Welding on a vertical surface with the bead running horizontally. Gravity pulls the puddle downward, so you must adjust angle and speed to keep the bead from sagging.
• 3F/3G (Vertical) - Welding up or down on a vertical surface. Vertical up gives better penetration and is preferred for structural work. Vertical down is faster and used on thin material.
• 4F/4G (Overhead) - Welding on the underside of a surface. Gravity pulls the puddle away from the joint. Use lower heat, faster travel speed, and shorter arc length. Wear full leather PPE - spatter drops directly onto you.

"F" designates fillet welds, "G" designates groove welds.

Essential Tools for Every Welder

• Welding helmet (auto-darkening)
• Safety glasses
• Gloves (heavy leather for MIG, thin leather for TIG)
• Chipping hammer and wire brush
• Angle grinder with flap discs, grinding discs, and cut-off wheels
• C-clamps, locking pliers (Vise-Grips), and welding magnets
• Tape measure, soapstone or silver streak marker, combination square
• Fillet weld gauge set
• Wire brush (stainless for stainless steel work, carbon steel for carbon steel)
• Pliers for pulling hot metal and adjusting clamps
• Tip cleaner files (for MIG nozzle maintenance)

Tips from Experienced Welders

• "Get comfortable before you start." Bracing your arms and finding a stable body position is the single most important thing for bead consistency. If you cannot reach the full length of the joint from one position, stop, reposition, and restart.
• "Watch the puddle, not the arc." The puddle tells you everything - penetration, travel speed, heat input, and bead profile. The arc is just the heat source.
• "Practice on scrap until the sound is right." A good MIG weld sounds like steady bacon frying. A good TIG weld is silent except for a quiet hiss of gas. Learn to weld by ear.
• "Clean it like you mean it." Five extra minutes of grinding and cleaning saves an hour of rework. Every experienced welder has learned this the hard way.
• "When in doubt, run a test coupon." Before welding on an actual project, run a practice bead on the same material and thickness to verify your settings.

Inspecting Your Welds

Every welder should visually inspect every weld before moving on:

• Uniform width and height - The bead should be consistent from start to finish. Variations indicate inconsistent speed, angle, or heat.
• Smooth transition at the toes - The edges of the weld should blend smoothly into the base metal with no undercut (grooves melted into the base metal) or overlap (weld metal sitting on top without fusion).
• No visible porosity - Look for pinholes on the surface. If you see porosity on the surface, there is likely more inside.
• No cracks - Cracks are the most serious defect. A cracked weld has zero strength at the crack. Grind it out and reweld.
• Proper size - Use a fillet gauge to verify that the weld leg size meets the drawing specification. Under-sized welds are structurally deficient.
• Full craters - The end of every weld should be filled. Craters are stress concentrators and crack initiation points.

Key Takeaways

• Safety is non-negotiable. Helmet, gloves, flame-resistant clothing, ventilation, and fire prevention are required every time.
• MIG is faster and more forgiving - choose it for production work and thicker materials. TIG is slower and more precise - choose it for thin material, critical joints, and appearance-critical work.
• Clean metal and proper fit-up are the foundation of every quality weld.
• Learn to read the puddle. It tells you everything you need to know about what is happening in the joint.
• Practice one position, one process, and one material at a time until you are consistently producing good beads before moving on.