3.4 Incomplete Fusion and Incomplete Joint Penetration
Key Takeaways
- Incomplete fusion = failure to fuse between weld metal and base metal (or between passes) at any location
- Incomplete joint penetration = weld metal does not extend through full thickness of a CJP groove weld
- Both are planar, crack-like discontinuities — treated seriously by all codes
- Main causes: insufficient heat input, improper electrode angle, fast travel speed, poor joint geometry
- IJP is only a defect in CJP groove welds — PJP welds are designed with intentional partial penetration
- Prevention: proper amperage, correct electrode angle, adequate root opening, appropriate joint design
3.4 Incomplete Fusion and Incomplete Joint Penetration
Incomplete fusion and incomplete joint penetration are among the most serious discontinuities after cracks because they create planar, crack-like interfaces that act as severe stress concentrators.
Incomplete Fusion (IF)
Incomplete fusion (also called lack of fusion or LOF) occurs when the weld metal fails to fuse to the base metal or to a preceding weld bead. There is a physical separation — an unfused interface — within the joint.
Types of Incomplete Fusion:
| Type | Location | Description |
|---|---|---|
| Sidewall lack of fusion | Between weld metal and groove face | Weld metal did not melt into the sidewall |
| Inter-run lack of fusion | Between weld passes | New pass did not fuse with previous pass |
| Root lack of fusion | At the root of the joint | First pass did not fuse with the root face |
Causes:
| Cause | Mechanism |
|---|---|
| Insufficient heat input | Not enough energy to melt the base metal at the fusion interface |
| Improper electrode angle | Arc directed away from the joint surface |
| Travel speed too fast | Insufficient time for base metal to melt |
| Excessive weave width | Arc dwells at center of weave rather than sidewalls |
| Magnetic arc blow | Arc deflected away from the intended fusion area |
| Oxide/contamination on surface | Oxide layer prevents fusion even though metal appears molten |
| Cold lapping | Molten metal flows onto unmelted base metal (especially in vertical-down) |
Incomplete Joint Penetration (IJP)
Incomplete joint penetration occurs when the weld metal does not extend through the full thickness of a complete joint penetration (CJP) groove weld. The root of the joint is not fully fused.
Causes:
| Cause | Mechanism |
|---|---|
| Root opening too small | Insufficient gap for the arc to reach the root |
| Root face too large | Thick root face blocks penetration |
| Insufficient amperage | Not enough heat to achieve full penetration |
| Electrode too large | Large electrode cannot access the root area |
| Travel speed too fast | Insufficient heat input at the root |
| Improper joint design | Groove angle too narrow, restricting arc access |
| Misalignment (high-low) | Offset root faces prevent proper fusion |
CJP vs. PJP Groove Welds
| Joint Type | Full Penetration Required? | IJP = Defect? |
|---|---|---|
| CJP (Complete Joint Penetration) | Yes — weld must extend through full thickness | Yes — any IJP is rejectable |
| PJP (Partial Joint Penetration) | No — designed to fuse only through a specified depth | No — IJP below the specified depth is expected |
For the Exam: Know the difference between incomplete fusion (failure to fuse at any interface) and incomplete joint penetration (failure to achieve full root penetration in a CJP weld). Both are planar, crack-like discontinuities and are treated seriously by all codes.
What is the PRIMARY difference between incomplete fusion and incomplete joint penetration?
A welder is producing welds with sidewall lack of fusion. Which corrective action is MOST likely to resolve the problem?