Unit 6 Comprehensive Quiz: MIG/TIG Welding

Unit: 06 - MIG/TIG Welding Duration: 30-45 minutes Passing Score: 70% Format: Multiple choice covering all modules Questions: 12

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Instructions

This comprehensive quiz covers all modules in the MIG/TIG Welding unit. You should complete all module assessments before attempting this unit quiz. The quiz tests both factual recall and application of concepts across modules.

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What is the fundamental difference between MIG (GMAW) and TIG (GTAW) welding processes?

MIG uses a non-consumable electrode; TIG uses a consumable wire

MIG feeds a consumable wire electrode that melts into the joint; TIG uses a non-consumable tungsten electrode with a separate filler rod added by hand

MIG is for thin metals only; TIG is for thick metals only

There is no significant difference — they use the same process with different names

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Explanation: MIG (Metal Inert Gas / GMAW) continuously feeds a consumable wire electrode through the torch — the wire melts and becomes the filler material. TIG (Tungsten Inert Gas / GTAW) uses a tungsten electrode that does not melt; instead, the welder manually feeds a separate filler rod into the puddle with their other hand. MIG is faster and easier to learn; TIG produces higher-quality, more precise welds but requires significantly more skill and coordination.

What is the purpose of shielding gas in both MIG and TIG welding?

Shielding gas cools the weld to prevent warping

Shielding gas adds strength to the weld bead

Shielding gas displaces atmospheric oxygen and nitrogen from the weld zone, preventing porosity, oxidation, and embrittlement of the molten metal

Shielding gas is only needed for aluminum — steel can be welded without it

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Explanation: Molten metal is highly reactive with atmospheric gases. Oxygen causes porosity (gas pockets trapped in the weld) and oxide inclusions that weaken the joint. Nitrogen causes embrittlement. Shielding gas (typically 75/25 Ar/CO₂ for MIG on steel, pure argon for TIG) creates an inert atmosphere around the weld puddle. Inadequate shielding — from wind, incorrect flow rate, or a damaged gas nozzle — is one of the most common causes of weld defects.

A student's MIG weld shows a series of small holes (porosity) throughout the bead. What are the THREE most likely causes?

The wire feed speed is too fast, the voltage is too high, and the metal is too thick

Contaminated base metal (oil, paint, rust), insufficient shielding gas flow or wind blowing gas away, and moisture in the welding wire or gas line

The contact tip is too large, the liner is too long, and the ground clamp is too small

Porosity is normal in MIG welding and does not indicate a problem

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Explanation: Porosity forms when gas is trapped in solidifying weld metal. The top causes are: (1) Surface contamination — oil, paint, rust, or mill scale decompose under the arc, releasing gas into the molten puddle. Always clean the weld zone with a grinder or solvent. (2) Shielding gas disruption — wind, low flow rate (<15 CFH), or a blocked nozzle allows atmosphere to reach the puddle. (3) Moisture — water on the workpiece, in the wire spool, or in the gas line creates hydrogen porosity. Each cause requires a different fix, so diagnosis is critical.

What is "wire feed speed" in MIG welding, and how does it relate to amperage?

Wire feed speed controls the travel speed of the torch along the joint

Wire feed speed and amperage are independent settings with no relationship

Wire feed speed controls how fast the consumable wire is fed into the arc — increasing wire feed speed directly increases amperage because more wire requires more current to melt

Wire feed speed only matters when welding aluminum

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Explanation: On most MIG welders, wire feed speed (measured in inches per minute, IPM) is the primary control for heat input. The power source automatically adjusts voltage/amperage to maintain arc stability as wire speed changes. Faster wire = more filler deposited = more current required = hotter arc. For 0.030" wire on mild steel: ~150 IPM ≈ 90A (thin material), ~300 IPM ≈ 150A (medium), ~450 IPM ≈ 200A (thick). Setting wire speed correctly for material thickness is the single most important MIG setup step.

When TIG welding aluminum, why must you use AC (alternating current) polarity instead of DC?

AC produces a narrower, more focused arc for precision work

DC would melt the tungsten electrode on aluminum

AC provides a "cleaning action" during the electrode-positive half-cycle that breaks up the aluminum oxide layer (melting point 3,700°F) — without this, the oxide prevents fusion with the base aluminum (melting point 1,220°F)

AC is required for all TIG welding regardless of material

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Explanation: Aluminum forms a tenacious oxide layer (Al₂O₃) that melts at 3,700°F — far above aluminum's 1,220°F melting point. On DC electrode-negative (standard TIG polarity for steel), this oxide layer stays intact and blocks fusion. AC alternates between electrode-negative (penetration) and electrode-positive (cleaning). During the EP half-cycle, electrons flow from the workpiece to the electrode, disrupting and removing the oxide layer through cathodic etching. This is why TIG machines with AC capability and high-frequency start are essential for aluminum.

What is the correct angle and technique for running a basic MIG bead on a flat butt joint?

Hold the torch at 90° perpendicular and push away from your body as fast as possible

Hold the torch at a 10-15° travel angle (slightly tilted in the direction of travel), maintain 3/8"-1/2" contact-tip-to-work distance, and use a steady push or slight weave technique at consistent speed

Hold the torch at 45° and pull toward your body with a circular motion

The torch angle doesn't matter — only wire feed speed affects bead quality

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Explanation: Proper MIG technique on flat butt joints: (1) Travel angle of 10-15° in the push direction gives good gas coverage and visibility. (2) Contact-tip-to-work distance (CTWD) of 3/8"-1/2" maintains stable arc length — too close causes the wire to stub, too far creates a long, unstable arc. (3) Consistent travel speed produces uniform bead width and penetration. A "push" technique (torch leading the puddle) gives slightly less penetration but better gas coverage; "drag" (torch trailing) gives deeper penetration but more spatter. Beginners should master push technique first.

What minimum shade number is required for the auto-darkening welding helmet lens during MIG and TIG welding?

Shade 3-5 is sufficient for all welding

Shade 8 for both MIG and TIG

Shade 10 minimum for MIG welding (shade 10-13 depending on amperage); shade 10-12 for TIG — higher amperage requires darker shade to protect against UV/IR radiation

Shade 14 is required for all welding processes

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Explanation: Welding arcs produce intense UV and IR radiation that can cause "arc flash" (photokeratitis — essentially a sunburn of the cornea) in seconds of unprotected exposure. OSHA/ANSI standards require minimum shade 10 for MIG/TIG at typical amperages. Higher current = brighter arc = darker shade needed: 100A → shade 10, 200A → shade 11-12, 300A+ → shade 13. Auto-darkening helmets switch from shade 3-4 (for visibility before striking) to the set welding shade in ~0.0004 seconds. Always verify the auto-darkening function before each use.

What is "tungsten contamination" in TIG welding, and how does it occur?

Tungsten contamination is rust forming on the tungsten electrode

Tungsten contamination occurs when the tungsten electrode touches the molten weld puddle, depositing tungsten particles into the weld — this creates hard inclusions that weaken the joint and is visible as a sudden unstable arc

Tungsten contamination is caused by using the wrong shielding gas

Tungsten contamination only occurs with AC polarity

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Explanation: If the tungsten electrode contacts (dips into) the molten puddle, the tip contaminates — picking up weld metal and depositing tungsten into the joint. The signs are immediate: the arc becomes erratic and unfocused, and the tungsten tip shows a balled, contaminated appearance instead of a clean point. The fix: stop welding, remove the contaminated tungsten, regrind to a proper taper (20-30° included angle for DC, rounded ball for AC aluminum), and restart. The contaminated section of weld should be ground out and re-welded. This is the most common TIG beginner mistake.

What is a "T-joint" and what weld type is used to join it?

A T-joint requires a groove weld with full penetration

A T-joint is welded with a plug weld through the vertical member

A T-joint is formed when one piece meets another at a perpendicular angle, creating a "T" shape — it is joined with fillet welds on one or both sides of the vertical member

T-joints cannot be welded — they must be bolted or riveted

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Explanation: T-joints are one of the five basic weld joint configurations (butt, T, lap, corner, edge). The fillet weld deposited in the 90° corner between the two members is triangular in cross-section. Key considerations: (1) both sides should be welded for full strength when possible, (2) the weld leg size must match the design requirement (typically equal to the thinner member's thickness), (3) gravity affects puddle behavior — horizontal T-joints require different technique than vertical. T-joints and fillet welds account for approximately 80% of all structural welding.

Why must welding never be performed on or near containers that previously held flammable materials?

The heat will weaken the container walls

The welding spatter will damage the container surface

Residual flammable vapors inside the container can ignite or explode when exposed to the welding arc's heat, even if the container appears empty — an explosion can be fatal

It is safe to weld on empty containers as long as the lid is removed

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Explanation: "Empty" containers that held gasoline, solvents, paint, or other flammables still contain residual vapors mixed with air — often in the explosive range. A welding arc instantly ignites these vapors, causing a violent explosion that can be lethal. Even containers that have been "cleaned" may have absorbed flammables into porous surfaces (rust, scale). The only safe procedure is professional cleaning, purging with inert gas, and gas testing before any hot work. This is a non-negotiable safety rule — container explosions kill welders every year.

What is the difference between "short-circuit transfer" and "spray transfer" in MIG welding?

Short-circuit is for aluminum; spray transfer is for steel

They produce identical welds at different speeds

Short-circuit transfer uses lower voltage/wire speed — the wire touches the puddle and shorts, depositing small amounts of metal in rapid cycles (good for thin material and all positions). Spray transfer uses higher parameters — a continuous stream of tiny droplets crosses the arc without shorting (higher deposition rate, but limited to flat/horizontal positions)

Short-circuit transfer is an outdated technique no longer used

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Explanation: Transfer mode dramatically affects weld characteristics. Short-circuit (SCT): wire feed 100-200 IPM, 16-20V — produces a distinctive crackling sound. The wire physically touches the puddle 90-200 times per second. Low heat input makes it ideal for thin material (22 ga-3/16") and out-of-position welding. Spray transfer: wire feed 300+ IPM, 24-30V — produces a smooth hissing sound. A continuous spray of fine droplets crosses the arc, delivering high heat and fast fill rates. Limited to flat/horizontal due to the large, fluid puddle. Most classroom MIG welding uses short-circuit transfer.

What pre-weld preparation is required for a proper weld joint, and what happens if you skip it?

No preparation is needed — modern welding processes can burn through any surface contamination

Only the filler metal needs to be clean; the base metal can have paint and rust

The weld zone must be cleaned of all oil, paint, rust, mill scale, and moisture within 1" of the joint — contamination causes porosity, lack of fusion, cracking, and weakened joints that may fail under load

Pre-weld preparation is only required for TIG welding, not MIG

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Explanation: Weld quality starts before the arc is struck. Contaminants react with the molten metal: (1) oil/grease burn off as hydrocarbons, causing porosity and hydrogen embrittlement, (2) paint produces toxic fumes (especially zinc-based primers) and porosity, (3) rust/mill scale traps moisture and prevents proper fusion, (4) moisture introduces hydrogen, causing cold cracking hours or days after welding. Proper prep: grind or wire brush to bright metal, degrease with acetone, and ensure the joint fit-up is within tolerance. A 5-minute prep investment prevents a failed weld and costly rework.

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Last Updated: 2026-03-19