Welding Notes

Welding Overview

Welding is a joining process where two or more parts are coalesced at their contacting surfaces (faying surfaces) by applying heat and/or pressure.
Filler material can be added to facilitate the process.
The assembled parts joined by welding are called a weldment.
Faying surface: the surface in contact at a joint (interface).
Weldment: a part created from an assembly of smaller components joined by welding.

Welding Importance

Welding provides a permanent joint, making the welded parts a single entity.
The weld joint can be stronger than the parent materials.
It is an economical way to join components in terms of material usage and fabrication costs.
Welding can be used to join similar or dissimilar metals.
It is not restricted to a factory environment and can be done "in the field."

Welding Limitations

Manual welding processes are expensive due to labor costs.
Welding can be dangerous due to the use of high energy.
It does not allow for convenient disassembly.
Welded joints can have quality defects that are difficult to detect, reducing joint strength.

The Weld Joint

The weld joint is the junction of edges or surfaces of parts joined by welding.
Two main issues concerning weld joints are:
- Types of joints
- Types of welds

Joint Types

Main joint types include:
- Butt joint
- Lap joint
- T joint
- Corner joint
- Edge joint

Weld Types

Common weld types include:
- Fillet Weld
- Groove (Butt) Weld
- Plug Weld
- Slot Weld
- Spot Weld
- Seam Weld
If the weld is between two workpieces, it is a groove weld.
If the weld is beside the two workpieces, it is a fillet weld.
A groove refers to a cut or indentation on a piece of metal designed to create a specific joint type, facilitating the welding process.

Fusion Welding

Fusion is the most common means of achieving coalescence in welding.
To accomplish fusion, a high-density heat energy source must be supplied to the faying surfaces.
Resulting temperatures cause localized melting of the base (parent) metals and filler metal, if used.
For metallurgical reasons, it is desirable to melt the metal with minimum energy but high heat densities.

Power Density

Energy sources with high-power densities, like laser beams or plasma arcs, can generate very high temperatures in a small area, allowing rapid melting and fusion of the metal.
This can result in faster welding speeds, increasing productivity and reducing costs.
Formula for power density: $PD = \frac{P}{A}$
- $PD$ = power density [W/mm²]
- $P$ = power entering surface [W]
- $A$ = surface area over which energy is entering [mm²]

Fusion Welding Spectrum

There is a practical range of power density values for welding.
If power density is too low, heat is conducted into the work, and melting may not occur.
If power density is too high, the metal vaporizes in the affected region.
$PD \sim 10^3$ melts metal in < 25 seconds.
$PD \sim 10^6$ vaporizes metal in µ seconds.

Principal Zones in a Fusion Welded Joint

Fusion zone: A mixture of filler metal and base metal melted together homogeneously, similar to casting; it exhibits columnar grain growth.
Weld interface (aka fusion line): A narrow boundary that solidifies immediately after melting.
Heat-Affected Zone (HAZ): The area below melting temperature but with substantial microstructural change, even with the same chemical composition as the base metal; it undergoes heat treating, which can lead to degradation in mechanical properties.
Unaffected Base Metal Zone (UBMZ): Contains high residual stress.

Heat Affected Zone

Metal experiences temperatures below the melting point but high enough to cause microstructural changes in the solid metal.
Chemical composition is the same as the base metal, but this region has been heat-treated, altering its properties and structure.
The effect on mechanical properties in the HAZ is usually negative.
Welding failures often occur in this zone.

Welding Residual Stresses

Stress exists in a weldment after all external loads are removed.
It is primarily caused by nonuniform heat flow during welding.
Can lead to defects such as distortion and warping.
Avoiding or minimizing residual stress and distortion in weldments often adds significant cost to production.
Heat treatments can improve dimensional stability and reduce susceptibility to cracking (e.g., hydrogen, fatigue cracking).
Controlling distortion may require expensive tooling, fixturing, and post-weld machining.

Minimising Distortion

Use welding fixtures to physically restrain parts.
Apply heat sinks to rapidly remove heat.
Use tack welding at multiple points along the joint to create a rigid structure prior to seam welding.
Select welding conditions (speed, amount of filler metal used, etc.) to reduce warpage.
Preheat base parts to reduce $\Delta T$
Apply stress relief heat treatment of welded assembly.
Properly design the weldment.

Classification of Fusion Welding Processes by Energy Source

Electrical
- Arc
  - SMAW (Shielded Metal Arc Welding)
  - SAW (Submerged Arc Welding)
  - GMAW (Gas Metal Arc Welding)
  - FCAW (Flux-Cored Arc Welding)
  - EGW (Electrogas Welding)
  - ESW (Electroslag Welding)
  - GTAW (Gas Tungsten Arc Welding)
- Resistance
  - RSW (Resistance Spot Welding)
Chemical
- OAW (Oxyacetylene Welding)
- OHW (Oxyhydrogen Welding)
Radiation
- EBW (Electron Beam Welding)
- LBW (Laser Beam Welding)
Gas
- AAW (Atomic Arc Welding)

Fusion Welding Modes

Autogenous Welding: No filler material is added.
Homogeneous Welding: Filler material has the same composition as the base material.
Heterogeneous Welding: Filler material has a different composition than the base material.

Classification of Fusion Welding Processes by Filler Material

A filler metal may be added to facilitate the joining process and provide bulk and strength to the welded joint.
Autogenous weld: no filler is added.
Homogeneous weld: filler equals (=) parent material.
Heterogeneous weld: filler different (≠) parent material.

Arc Welding (AW)

Fusion welding process in which an electric arc is created between an electrode (metal rod or wire) and the workpiece.
The heat generated by the arc melts the base metal and the electrode, forming a molten metal pool that cools to form a solid joint.
A filler metal can help increase the volume and strength of the weld joint.
Temperatures of 6000°C+ can be produced, sufficient to melt any metal.

Electrodes

Consumable electrodes provide the source of the filler metal in arc welding; available as rods (sticks) and wire.
Non-consumable electrodes are made of tungsten (or carbon, rarely), which resists melting by the arc.
Gradual erosion (burn-off) can occur during the welding process (vaporization is the principal mechanism), analogous to the gradual wearing of a cutting tool in a machining operation.
Electrode diameters can vary between 0.5 and 6.4 mm, and their lengths can range from 75 to 610 mm.

Protection of Welds

Hot metals are reactive to their environments (e.g., air).
They react with O2.
N2 and H2 are very soluble in molten metals.
Hydrogen leads to hydrogen cracking.
Nitrides are relatively benign.
Most become supersaturated solids at high temperature.
Welding processes must include means of protection.
Fluxes:
- Consumable, e.g., SMAW (Shielded Metal-Arc-Welding) - carbon dioxide
- Separate flux feed, e.g., submerged arc welding (SAW)
Gas shielding:
- GTAW (gas tungsten arc welding) - Argon gas
- GMAW (gas metal arc welding) - Argon or carbon dioxide

Fluxes

A substance formulated to serve several additional functions:
- Fluxing agents: Promote wetting.
- Impervious layer on top of formed weld:
  - Thermal blanket
  - Seals joint from atmosphere (slag)
- Arc stabilizers: Provide stability and directionality of arc.
- Gas formers: Decompose to form inert gas (CO2).
Upon cooling, the slag solidifies and must be removed later by chipping or brushing.

Flux Delivery Techniques

Pouring granular flux onto the welding operation.
Using a flux-coated stick electrode that melts during welding to cover the operation.
Using tubular electrodes in which flux is contained in the core and released as the electrode is consumed.