Slide 002: Joint Types, Weld Profiles & Metallurgy Basics

Slide Visual

Slide Overview

This slide covers the fundamental joint configurations, weld bead anatomy, and basic metallurgy concepts that every welder must understand. These concepts apply equally to MIG and TIG welding.

Instruction Notes

The Five Basic Joint Types

Joint Type	Description	Common Application	Preparation
Butt	Two pieces edge-to-edge, same plane	Pipe, plate, structural	Gap, bevel for thick material
Lap	Two pieces overlapping	Sheet metal, frames	Minimal — just overlap
T-joint	One piece perpendicular to another	Frames, brackets, structural	Clean faying surfaces
Corner	Two pieces meeting at a corner angle	Boxes, enclosures, frames	Square edges
Edge	Two parallel pieces joined at their edges	Sheet metal, flanges	Aligned edges

Weld Bead Anatomy

A completed weld cross-section shows three distinct zones:

1. Fusion Zone (Weld Metal): Where the filler and base metal melted and solidified together. The composition is a mix of filler and base metal. Grain structure is columnar — oriented toward the heat source.

2. Heat-Affected Zone (HAZ): Base metal that was heated enough to alter its microstructure but did not melt. Width varies from 0.02" to 0.25" depending on heat input and material. Can be harder (high-carbon steel) or softer (heat-treated aluminum) than the original base metal.

3. Unaffected Base Metal: Beyond the HAZ, the metal's properties are unchanged.

Fillet Weld Measurements

The fillet weld (most common weld type — used on T-joints, lap joints, and corner joints) is measured by:

• Leg size: The distance from the root to the toe along each member (e.g., ¼" fillet has ¼" legs)
• Throat: The shortest distance from the root to the face of the weld. For a standard flat-faced fillet: throat = leg × 0.707
• Reinforcement (convexity): Material above the theoretical face — slight convexity is normal and acceptable

Rule of thumb for fillet size: The fillet leg size should equal the thickness of the thinner member being joined (up to ⅜"). Above ⅜", consult engineering specifications.

Basic Metallurgy Concepts

Grain structure: Metals are composed of microscopic crystals (grains). Smaller, uniform grains = stronger, more ductile metal. Welding heat causes grain growth in the HAZ, potentially reducing strength.

Thermal expansion: All metals expand when heated. Coefficient of thermal expansion varies: aluminum expands about twice as much as steel for the same temperature increase. This drives distortion and residual stress.

Phase transformations: Some steels (medium and high carbon) undergo phase changes during the heating and cooling of welding. Rapid cooling can create martensite — a very hard, brittle phase that is prone to cracking. This is why some steels require preheat and controlled cooling.

Carbon Content and Weldability

• Low carbon steel (<0.25% C): Excellent weldability — the standard makerspace material
• Medium carbon steel (0.25-0.50% C): Requires preheat to prevent cracking
• High carbon steel (>0.50% C): Difficult to weld — requires special procedures
• Cast iron (2-4% C): Very difficult — requires specialized techniques

Key Talking Points

1. The five joint types cover 99% of all welding configurations
2. Fillet welds on T-joints and lap joints are the most common weld type
3. The HAZ is where most weld failures originate — understanding it prevents problems
4. Low-carbon mild steel is the most forgiving material to learn on
5. Weld size (leg/throat) must match the engineering requirement — bigger is not always better

Learning Objectives (Concept Check)

• [ ] Identify and describe the 5 basic joint types
• [ ] Label the three zones of a weld cross-section
• [ ] Measure and specify fillet weld leg and throat dimensions

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Last Updated: 2026-03-19