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=AP
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∼103 melts metal in < 25 seconds.
PD∼106 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 ΔT
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.
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.