Incomplete root fusion or penetration

Identification

Incomplete root fusion is when the weld fails to fuse one side of the joint in the root. Incomplete root penetration occurs when both sides of the joint are unfused. Typical imperfections can arise in the following situations:

• an excessively thick root face in a butt weld (Fig. 1a)
• too small a root gap (Fig. 1b)
• misplaced welds (Fig. 1c)
• failure to remove sufficient metal in cutting back to sound metal in a double sided weld (Fig. 1d)
• incomplete root fusion when using too low an arc energy (heat) input (Fig. 1e)
• too small a bevel angle,
• too large an electrode in MMA welding (Fig 2) 

Fig. 1 Causes of incomplete root fusion

 a)b)
c)d)
e)
a) Excessively thick root face 
b) Too small a root gap 

c) Misplaced welds
d) Power input too low
e) Arc (heat) input too low

Fig. 2 Effect of electrode size on root fusion

a)b)

a) Large diameter electrode
b) Small diameter electrode

Causes

These types of imperfection are more likely in consumable electrode processes (MIG, MMA and submerged arc welding) where the weld metal is 'automatically' deposited as the arc consumes the electrode wire or rod. The welder has limited control of weld pool penetration independent of depositing weld metal. Thus, the non consumable electrode TIG process in which the welder controls the amount of filler material independent of penetration is less prone to this type of defect.

In MMA welding, the risk of incomplete root fusion can be reduced by using the correct welding parameters and electrode size to give adequate arc energy input and deep penetration. Electrode size is also important in that it should be small enough to give adequate access to the root, especially when using a small bevel angle (Fig 2). It is common practice to use a 3.25mm diameter electrode for the root so the welder can manipulate the electrode for penetration and control of the weld pool. However, for the fill passes where penetration requirements are less critical, a 4 or 5mm diameter electrode is used to achieve higher deposition rates.

In MIG welding, the correct welding parameters for the material thickness, and a short arc length, should give adequate weld bead penetration. Too low a current level for the size of root face will give inadequate weld penetration. Too high a level, causing the welder to move too quickly, will result in the weld pool bridging the root without achieving adequate penetration.

It is also essential that the correct root face size and bevel angles are used and that the joint gap is set accurately. To prevent the gap from closing, adequate tacking will be required.

Best practice in prevention

The following techniques can be used to prevent lack of root fusion:

• In TIG welding, do not use too large a root face and ensure the welding current is sufficient for the weld pool to penetrate fully the root
• In MMA welding, use the correct current level and not too large an electrode size for the root
• In MIG welding, use a sufficiently high welding current level but adjust the arc voltage to keep a short arc length
• When using a joint configuration with a joint gap, make sure it is of adequate size and does not close up during welding
• Do not use too high a current level causing the weld pool to bridge the gap without fully penetrating the root.

The limits for lack of penetration are specified in BS EN ISO 5817 for the three quality levels.
Lack of root penetration is not permitted for Quality Level B (stringent) and Level C (intermediate). For Quality Level (moderate) short lack of penetration imperfections are permitted.
Incomplete root penetration is not permitted in the manufacture of pressure vessels but is allowable in the manufacture of pipework depending on material and wall thickness.

Remedial actions

If the root cannot be directly inspected, for example using a penetrant or magnetic particle inspection technique, detection is by radiography or ultrasonic inspection. Remedial action will normally require removal by gouging or grinding to sound metal, followed by re-welding in conformity with the original procedure.

BS EN ISO 5817:2003 Welding - fusion-welded joints in steel, nickel, titanium and their alloys (beam welding excluded) - Quality levels for imperfections.
BS EN 30042: 1994 Arc welded joints in aluminium and its weldable alloys - Guidance on quality levels for imperfections.
Read also Lack of Side Wall Fusion & Lack of Inter-run Fusion