Lack of fusion (LOF) means the weld metal didn’t actually melt into the base metal or into a previous weld pass. The bead looks like it was laid on top of the plate without bonding. It’s one of the most serious weld defects because the joint has zero strength at the unfused interface. Every welding code rejects LOF, and every inspector looks for it.
LOF happens when there isn’t enough heat at the point where the weld meets the base metal, when the electrode angle directs heat away from the fusion zone, or when contamination (usually oxide) prevents the metals from bonding. It’s a technique problem, not an equipment problem.
Types of Lack of Fusion
Sidewall LOF
The weld bead doesn’t fuse into the bevel face (groove sidewall) on a multi-pass groove weld. The bead fills the groove but doesn’t bond to the sides. This is the most common type of LOF in multi-pass welds.
Where it happens: Between fill passes and the groove sidewall, particularly at the bottom of the bevel face where the electrode angle makes contact difficult.
Interpass LOF (Cold Lap)
A subsequent weld pass doesn’t fuse into the previous pass. One bead sits on top of another without bonding. This is often called “cold lap” because the weld rolled over the previous bead without melting into it.
Where it happens: Between adjacent beads in fill passes, especially at the toe of the previous bead where slag can accumulate.
Root LOF
The root pass doesn’t fuse into one or both root faces. The root bead fills the gap but doesn’t bond to the base metal at the root edges.
Where it happens: In the root of groove welds, particularly with backing bars where the welder can’t see whether the root is actually fusing into both plates.
Causes of Lack of Fusion
| Cause | Mechanism | Fix |
|---|---|---|
| Insufficient heat input | Amperage too low to melt the base metal at the fusion zone | Increase amperage to the upper range of the WPS |
| Wrong electrode angle | Arc directed away from the fusion zone; heat doesn't reach the sidewall | Direct electrode into the bevel face or previous bead toe |
| Travel speed too fast | Weld puddle moves past the joint before the base metal melts | Slow down enough for the puddle to wet into the base metal |
| Groove too narrow | Electrode can't reach the sidewalls in a tight groove | Widen the groove angle or use a smaller electrode |
| Slag entrapment | Slag from a previous pass blocks fusion of the next pass | Clean every pass completely before welding the next |
| Oxide contamination | Aluminum oxide (on aluminum) prevents wetting | Clean with stainless steel brush immediately before welding |
| Magnetic arc blow | Arc wanders away from the joint due to magnetic fields | Demagnetize, use AC, change ground location |
| Large weld puddle | Puddle rolls ahead of the arc, covering unfused metal | Reduce puddle size by lowering heat or increasing speed |
LOF vs. LOP: The Distinction
Both defects involve incomplete bonding, but they occur at different locations:
| Characteristic | Lack of Fusion (LOF) | Lack of Penetration (LOP) |
|---|---|---|
| Location | Anywhere: sidewall, interpass, root face | Root only |
| Definition | Weld metal present but not bonded to adjacent material | Weld metal didn't reach the root of the joint |
| Typical cause | Low heat, wrong angle, contamination | Root face too thick, root opening too small, low heat |
| Common with | Multi-pass groove welds (any process) | Open root groove welds, backing bar joints |
| Code treatment | Always rejectable | Always rejectable on CJP welds |
LOP causes: Root face (land) too thick, root opening too small, amperage too low for the root pass, electrode too large for the root gap, wrong root pass technique.
Detection Methods
Visual Inspection
Toe LOF sometimes shows as a visible line between the weld and base metal. The weld looks like it’s overlapping the plate rather than melting into it. Run your finger along the weld toe. LOF often feels like a sharp edge or a step rather than a smooth transition.
However, sidewall and interpass LOF inside a multi-pass weld are invisible from the surface.
Ultrasonic Testing (UT)
UT is the best method for detecting LOF. The planar nature of the unfused interface produces a strong reflection. A crack or LOF defect oriented perpendicular to the sound beam gives a clean, sharp echo that’s easy to identify.
UT can locate the defect precisely in the weld cross-section, telling the repair welder exactly how deep to grind.
Radiographic Testing (RT)
RT struggles with LOF unless the unfused surfaces have a gap between them. If the weld metal is in tight contact with the base metal but not bonded (no gap, just no fusion), RT may not detect it. The X-ray contrast depends on a density difference, and tight LOF doesn’t always provide enough contrast.
Magnetic Particle Testing (MT)
MT can detect LOF at or near the surface (within about 1/4 inch). The unfused boundary disrupts the magnetic field just like a crack. But deep sidewall LOF in a thick multi-pass weld is beyond MT’s depth range.
Bend Testing
On test coupons, bend tests reveal LOF immediately. The unfused interface opens up during bending, showing as a crack or separation on the bent surface. This is why certification tests include bend specimens.
LOF on Specific Materials
Aluminum
Aluminum is particularly prone to LOF because of its tenacious oxide layer. Aluminum oxide melts at about 3,700F while the base metal melts at about 1,220F. If the oxide isn’t removed, the weld metal melts and flows but can’t bond to the base metal underneath the oxide.
Prevention: Clean with a dedicated stainless steel brush immediately before welding. Use AC on TIG (the DCEP half-cycle breaks up the oxide). Run hot enough to break through the oxide on MIG (spray transfer helps).
Stainless Steel
Stainless is less prone to LOF than aluminum, but high-chromium grades form a chromium oxide layer that resists wetting. Keep surfaces clean and use adequate heat.
Carbon Steel
LOF on carbon steel is almost always a technique or heat problem, not a material problem. Carbon steel fuses readily if the heat and angle are correct.
Repair Procedure
- Locate the defect. Use UT to determine the depth, length, and position of the LOF.
- Remove the defect. Grind or gouge from the accessible side until you’ve removed all unfused material. If the defect is deep, back-gouging (grinding from the back side) may be needed.
- Verify removal. PT or MT the excavation to confirm no LOF remains at the cavity walls.
- Identify the cause. Don’t repeat the same technique that caused the LOF.
- Re-weld. Per the original WPS, with corrected technique (higher heat, better angle, clean surfaces).
- Re-inspect. UT or RT the repair to verify the defect is gone.
Prevention Strategies
Electrode Angle Technique
The most effective prevention is directing the arc into the fusion zone. On multi-pass groove welds:
- First fill bead: Angle the electrode 10-15 degrees toward the bevel face. The arc should impinge on the sidewall, not the center of the groove.
- Center fill beads: Keep the electrode nearly perpendicular to the plate, directing heat equally into both edges of the previous beads.
- Last fill bead before cap: Angle toward the remaining bevel face to ensure sidewall fusion before the cap covers it.
Heat Input Verification
If LOF is recurring, check that your amperage and travel speed produce adequate heat input. Calculate heat input as (amps x volts x 60) / (travel speed in inches per minute x 1,000) for kilojoules per inch. If heat input is below the WPS minimum, the base metal isn’t reaching fusion temperature.
Interpass Cleaning Protocol
For processes that produce slag (SMAW, FCAW), implement a strict cleaning protocol:
- Chip all visible slag with a chipping hammer
- Wire-brush the entire bead
- Visually inspect the cleaned surface under good lighting
- If any slag remains in the toes, grind it out before the next pass
- Verify no porosity or cracks exist before proceeding
For MIG and TIG, wire-brush to remove any silica islands or oxidation between passes.
Common Mistakes
Assuming fusion from the surface appearance. A smooth, well-shaped weld bead can still have LOF inside. Surface appearance tells you nothing about sidewall fusion. Only NDT or destructive testing reveals internal LOF.
Not directing the arc into the sidewall. On multi-pass groove welds, the natural tendency is to run the bead down the center of the groove. You need to deliberately angle into the bevel face to drive fusion into the sidewall.
Skipping interpass cleaning. Slag left between passes is the most common cause of interpass LOF. Chip and brush every pass on stick and flux-core welds. Wire-brush TIG and MIG passes.
Welding too cold on aluminum. Being afraid of burn-through on aluminum leads to running too cold, which leads to LOF. Aluminum needs hot, fast passes. The puddle should be fluid and bright, not sluggish.
Using too narrow a groove. If the included angle is tight (45 degrees or less), the electrode can’t reach the bottom of the sidewalls. Widen the groove to at least 60 degrees for SMAW.
For other critical weld defects, see the porosity guide, undercut guide, and weld spatter guide. For inspection methods that detect LOF, visit the ultrasonic testing guide. Return to weld defects or the welding techniques pillar.