Lack of Fusion — What It Is and Why It Matters
Lack of fusion (LOF, also called incomplete fusion) occurs when the weld metal fails to properly fuse with the base metal or with a previous weld pass. It creates an unfused interface — essentially a crack-like planar defect — that dramatically reduces joint strength and acts as a failure initiation point under load.
Lack of fusion is caused by insufficient heat input, improper torch angle directing the arc away from the fusion zone, excessive travel speed, contaminated surfaces, and cold lapping (depositing weld metal on top of the base metal without actually melting into it). It is one of the most serious weld defects because it can be invisible on the surface while hiding a large unfused area beneath.
Detection requires volumetric inspection methods — radiography (X-ray) or ultrasonic testing — since LOF often lies along the sidewall of the joint where visual inspection cannot reach. Prevention focuses on adequate heat input, proper torch angle aimed into the joint, clean preparation, and technique that ensures the arc melts the base metal ahead of the advancing weld pool.
Frequently Asked Questions
How is lack of fusion different from lack of penetration?
Lack of fusion means the weld metal did not properly bond to the base metal sidewall or to a previous pass. Lack of penetration means the weld did not reach the required depth into the joint root. Both are incomplete bonding defects, but they occur at different locations within the joint.
Can lack of fusion be repaired?
Yes, but it requires excavating (grinding or gouging) the defective area down to sound metal and rewelding. The repair must address the root cause — increased heat input, better torch angle, or improved joint preparation — to prevent recurrence.