Lack of fusion occurs when the weld metal fails to bond completely with the parent material or previous weld passes; lack of penetration occurs when the weld fails to extend fully through the joint thickness. Both are serious defects because they significantly reduce joint strength and may not be visible from the surface. They are commonly caused by insufficient welding current, excessive travel speed, poor torch angle or incorrect joint preparation.
This guide explains both defects, their causes, and how to prevent them with the correct technique, parameters and joint design.
What is lack of fusion?
Lack of fusion occurs when the weld metal fails to bond completely with the parent material or previous weld passes. It is considered a serious welding defect because it significantly reduces joint strength and may not always be visible from the surface.
Correct welding technique and parameter selection are essential to avoid lack of fusion.
What causes lack of fusion?
Common causes include insufficient welding current, excessive travel speed, poor torch angle, incorrect joint preparation or contamination. The weld may appear acceptable externally while containing serious internal defects.
Following the correct welding procedure and ensuring adequate heat input helps achieve complete fusion.
How do I prevent lack of fusion?
Use sufficient welding current, maintain the correct travel speed and ensure the weld pool fully wets into both sides of the joint. Clean the material thoroughly and prepare joints correctly before welding.
Regular inspection and adherence to approved welding procedures help minimise the risk.
What causes lack of penetration?
Lack of penetration occurs when the weld fails to extend fully through the joint thickness. It is commonly caused by insufficient current, narrow joint preparation, excessive travel speed or incorrect root gap.
Selecting suitable welding parameters and joint design helps ensure full penetration.
How do I improve weld penetration?
Improving penetration may involve increasing welding current, reducing travel speed, adjusting the torch angle or modifying the joint preparation. Choosing the correct wire diameter, filler metal and welding process also influences penetration.
Following the recommended welding procedure ensures the weld achieves the required strength and performance.
Getting the parameters and consumables right
Both defects share the same fixes: enough heat input, correct travel speed, a sound torch angle and proper joint preparation — backed by the right wire diameter and process for the material. For deeper penetration on thicker sections, larger wire diameters and processes such as flux-cored welding can help.
For wire and process selection advice, see our MIG and TIG filler wire comparisons and process and equipment comparisons, or contact the Weldability SIF technical team.