Lack of Penetration & Root Fusion

Lack of penetration and root fusion are two similar – and very common – imperfections that can occur during the welding process.

Lack of fusion comes in several forms:

Lack of root fusion
Lack of sidewall fusion
Lack of inter-run fusion
Partial penetration

Lack of fusion in the root area is generally considered unacceptable, however there are some applications where partial root penetration may be allowed due to the fabrication design.

Problems caused by lack of root fusion are best resolved by careful selection and control of welding parameters; good joint design; and attention to welding technique.

Lack of penetration happens when both sides of the root fail to fuse to the weld; for example, when the groove of the metal is not filled completely.

Neither situation is a great outcome.

Partial penetration may be acceptable in some butt-welding applications, but full penetration is often required for improved welding performance – particularly when it comes to fatigue loading and corrosion resistance.

The stresses caused by these issues can eventually make the material crack. Fortunately, it’s possible to avoid root fusing and penetration problems.

What causes penetration and root fusing problems?

Penetration defects often result from following incorrect procedures, but are also caused by poor preparation of the joint, or even operating conditions (such as using the wrong materials or techniques) that aren’t up to scratch.

Here are some of the factors that may cause imperfect root fusion and penetration:

Root gap narrowing or closing up as welding progresses
Excessively thick root face in a butt weld – or a root gap that is too small
Misplaced welds, or excessive welding speed
Failure to remove sufficient metal in a double-sided weld

Energy (heat) input is too low
Using a bevel angle that is too small
Use of a diameter electrode that is too large

How can root fusion and penetration problems be avoided?

There are specific techniques for dealing with penetration imperfections. These are:

Correct joint design – using V or compound joint profiles.
Correct preparation of the joint and base metal surfaces. Ensure root is clean and without oxide, rust or coatings before welding.
Using tack welds or restraint to maintain an even root gap. Also using procedures that involve back grinding and filling from the underside of the joint.

Choosing temporary – e.g. ceramic – or permanent backing.
Selecting a more appropriate process such as plasma keyhole or MMA welding with cellulosic electrodes.

When it comes to preventing root fusion – a problem commonly associated with high-deposition rate, low-heat input processes such as MIG welding – various alternative approaches can help to avoid problems and achieve a better weld:

Increase the average current setting on the welding power source.
Avoiding molten pool from flooding the weld. Use a suitable diameter electrode, not too big or small, and the correct welding current.

Controlling the operating technique, using the correct electrode diameter and angle; and using a suitable travel speed to allow time for root fusion.
Selecting different consumables; such as helium additions to the shielding gas for aluminium or stainless steel, or increased CO2 levels in argon-based mixtures for carbon-manganese steels).

How to remedy root fusion or penetration problems

In this instance, prevention is far better than cure.

Often the root cannot be directly inspected – for instance using a penetrant or magnetic particle inspection technique – so detection must be done by radiography or ultrasonic inspection.

Remedial action usually involves removing the work by gouging or grinding to sound metal, followed by a re-welding using the correct procedure.