Module 1: Welding Science Fundamentals Assessment Quiz

Module: U6M1 - Welding Science Fundamentals Duration: 25 minutes Passing Score: 70% Format: Multiple choice and scenario-based

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What is the fundamental difference between welding and brazing?

Welding melts the base metal to create a fusion joint; brazing uses a filler metal that melts below the base metal's melting point

Welding uses electricity while brazing uses gas

Welding is for thick metal while brazing is for thin metal

There is no difference — they are the same process

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Explanation: In welding, the base metals are melted and fused together (with or without filler metal). In brazing, only the filler metal melts (above 840°F but below the base metal's melting point), and it flows into the joint by capillary action. Soldering is similar to brazing but uses filler metals that melt below 840°F.

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

To protect the molten weld pool from atmospheric contamination (oxygen, nitrogen, moisture)

To cool the welding torch and prevent overheating

To provide fuel for the welding arc

To clean the surface of the base metal before welding

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Explanation: The molten weld pool is extremely reactive with atmospheric gases. Oxygen causes porosity and oxide inclusions. Nitrogen causes brittleness. Moisture causes hydrogen embrittlement. Shielding gas creates an inert or semi-inert atmosphere around the weld pool to prevent these defects.

What causes porosity in a weld?

Gas trapped in the solidifying weld metal, often from contamination, moisture, or inadequate shielding

Using too much filler metal

Welding too slowly

Using too large a filler rod diameter

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Explanation: Porosity occurs when gas bubbles become trapped as the weld pool solidifies. Sources include: moisture on the base metal or filler, oil/grease/paint contamination, inadequate shielding gas coverage (wind, wrong flow rate), and damp electrodes. It appears as small holes (pores) in the weld metal and weakens the joint.

What is the "heat-affected zone" (HAZ) in welding?

The region of base metal adjacent to the weld where the microstructure has been altered by heat but not melted

The molten weld pool during welding

The area heated by the torch before welding begins

The zone where shielding gas is effective

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Explanation: The HAZ extends from the fusion line (edge of the weld pool) into the base metal. In this zone, the metal reached temperatures high enough to change its grain structure but did not melt. The HAZ may be harder, softer, or more brittle than the original base metal, depending on the alloy and cooling rate.

Which property of metal most affects how it behaves during welding?

Thermal conductivity — determines how fast heat spreads from the weld zone

Magnetic permeability

Electrical resistance

Surface hardness

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Explanation: Thermal conductivity determines how quickly heat dissipates from the weld zone into the surrounding material. Aluminum (high conductivity: 167 W/m·K) requires more heat input because heat spreads quickly. Stainless steel (low conductivity: 16 W/m·K) retains heat, requiring less input but more care to avoid distortion and burn-through.

What does "weld penetration" mean?

The depth to which the weld fuses into the base metal from the surface

How deep the filler metal sits above the base metal surface

The distance the arc reaches from the torch

The width of the weld bead

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Explanation: Penetration is the depth of fusion into the base metal. Full penetration means the weld fuses completely through the joint thickness — critical for structural joints. Partial penetration may be acceptable for non-structural applications. Inadequate penetration creates a weak joint that may fail under load.

What type of joint is formed when two pieces of metal are placed edge-to-edge and welded?

Butt joint

Lap joint

T-joint

Corner joint

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Explanation: A butt joint places two pieces edge-to-edge in the same plane. Other common joint types: lap joint (overlapping), T-joint (perpendicular), corner joint (right angle at edges), and edge joint (parallel edges). Each joint type has specific preparation and welding technique requirements.

Why must metal be cleaned before welding?

Contaminants (oil, rust, paint, moisture) cause weld defects including porosity, cracking, and inclusions

Dirty metal conducts electricity poorly and the arc will not start

Clean metal melts at a lower temperature, saving energy

Cleaning is only necessary for appearance, not weld quality

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Explanation: Surface contaminants decompose in the heat of the arc and introduce gases and foreign material into the weld pool. Oil and grease cause porosity and hydrogen cracking. Rust (iron oxide) creates oxide inclusions. Paint can produce toxic fumes and porosity. Mill scale prevents proper fusion. Always clean to bright, shiny metal within 1" of the weld zone.

What is "distortion" in welding and what causes it?

The warping or bending of the workpiece caused by uneven heating and cooling during welding

The visual distortion seen through a welding helmet lens

The bending of the welding electrode during use

The change in color of the metal after welding

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Explanation: When metal is heated locally (as in welding), it expands. As it cools, it contracts. Because the heating is uneven, the expansion and contraction are uneven, causing the workpiece to warp. Distortion increases with: more heat input, thinner material, longer welds, and lack of clamping. Techniques to minimize distortion include tack welding, clamping, backstep welding, and balanced weld sequences.

A student welds two pieces of ¼" mild steel in a T-joint. The weld looks good on the surface but the fillet is only fused to the top plate, not the vertical plate. What is this defect called?

Lack of fusion (also called incomplete fusion or cold lap)

Undercut

Overlap

Porosity

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Explanation: Lack of fusion occurs when the weld metal does not properly fuse to the base metal or to a previous weld pass. It creates a weak point where the joint can fail. Common causes: insufficient heat input, improper torch angle (not directing heat to both pieces), too fast travel speed, or contaminated surfaces. It is often not visible on the surface — destructive testing or radiographic inspection may be needed to detect it.

What is the difference between AC and DC welding polarity, and when is each used?

DC provides steady arc direction (DCEP or DCEN); AC alternates polarity and is used for aluminum TIG welding to break oxide layer

AC is for thin metals, DC is for thick metals

AC uses less electricity than DC

There is no practical difference — they produce the same results

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Explanation: In DC welding, current flows in one direction: DCEP (electrode positive) provides more penetration for MIG, while DCEN (electrode negative) concentrates heat at the workpiece for TIG on steel. AC alternates polarity 60-120 times per second — the electrode-positive half-cycle provides a cleaning action that breaks up the aluminum oxide layer, making AC essential for aluminum TIG welding.

What is the primary purpose of filler metal in welding?

To add material to the joint, filling the gap and building up the weld to the required size and strength

To reduce the melting point of the base metal

To provide shielding gas when heated

To remove contaminants from the weld pool

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Explanation: Filler metal (wire in MIG, rod in TIG) adds material to the weld joint. It fills gaps between parts, builds up the weld bead to the required size (throat and leg dimensions for fillets), and must be metallurgically compatible with the base metal. The filler metal composition is selected to match or exceed the strength of the base metal. Some welds (autogenous) use no filler — the base metal edges are melted together directly.

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Last Updated: 2026-03-19