Discontinuity
Incomplete fusion
Weld metal that lay against the joint face or a previous bead without melting into it, leaving a tight unbonded plane that carries no load even though the joint looks completely filled.
What it looks like
- A flat, tight line between weld metal and groove face, or between two adjacent passes
- Nothing visible at the surface in most cases, since the cap can be perfectly formed above it
- On film, a straight dark line following the groove face, usually with sharp parallel edges
- On a sectioned macro, weld metal simply resting against unmelted parent metal with no fusion line
Causes
- Insufficient heat input, so the arc melted filler metal without raising the joint face to melting temperature
- Poor electrode angle or manipulation that let the puddle run ahead of the arc onto cold metal
- Travel speed too fast for the current, giving the arc no time to wet the joint face
- Slag or oxide left on a previous pass acting as a barrier between the new deposit and the old
- Narrow groove angles or deep joints that keep the arc from reaching the sidewall directly
Prevention
- Set current high enough to melt the joint face, and direct the arc at the leading edge of the puddle rather than the middle
- Clean every pass thoroughly before depositing the next, especially in deep narrow grooves
- Open the included angle or reduce bead size so the arc has direct line of sight to the sidewall
- Slow the travel and keep the arc at the fusion face long enough to wash it in properly
How it is found
- UT
- The unbonded plane is a flat reflector, and an angle beam striking it near perpendicular returns a strong, well-defined echo.
- RT
- Detectable only when the beam is close to parallel with the unbonded plane, so film alone can miss sidewall fusion problems entirely.
- VT
- Limited to cases where the lack of fusion breaks out at the weld toe or root and can be seen on a cleaned surface.
How inspectors think about it
- Incomplete fusion is planar, so it is judged on the scale used for cracks rather than the more forgiving scale used for rounded gas pockets.
- The dangerous part is that it costs load-carrying area without any warning at the surface, so a perfectly formed cap gives no assurance the joint is sound.
- Because ultrasonic testing is far better suited to flat vertical reflectors than radiography, the choice of examination method can decide whether the flaw is found at all.
General reasoning only. The accept/reject limits for your work come from the code or specification governing the job, which this site does not reproduce.
Related discontinuities
Incomplete joint penetrationA groove weld that did not reach through the joint as the design required, leaving unfused root faces and an effective throat smaller than the drawing calls for.Slag inclusionNon-metallic slag trapped inside the weld instead of floating to the surface, most often left in a corner or undercut of a previous pass and then buried by the next one.OverlapWeld metal that has rolled over the toe and sits on unmelted base metal, forming a lip with a sharp crevice underneath that carries no load and hides an unfused interface.Excessive reinforcementA weld face built up far higher than the joint needs, producing an abrupt change of section at the toes that concentrates stress despite the extra metal looking like extra strength.
Can you name it on sight?
The gauge trainer's identification mode shows you a weld and four names. Reading about porosity and picking it out of a line-up are not the same skill.