Lack of fusion

A common challenge, lack of fusion in welding occurs when the weld metal fails to fuse with the side wall or joint, or when two weld beads fail to completely join together

At its worst, it can produce serious, crack-like defects, which reduce the load-carrying capacity of the joint.

Find out the causes and, more importantly, how to prevent lack of fusion in welding.

What causes lack of fusion?

There are two common issues that you’re likely to come across:

Lack of root fusion – when the first weld bead does not fully fuse with the parent metal
Lack of sidewall fusion – when a weld bead deposited immediately next to the parent plate fails to fully fuse with the parent.

Lack of fusion in welding typically occurs when there isn’t adequate heat for fusion, but there are a number of additional causes that it’s worth checking.

How to deal with lack of fusion in welding

Other causes of lack of fusion include:

Poor joint or root area preparation

Joints and root areas can be subjected to atmospheric contamination and, if edges aren’t correctly prepared, can fail to align properly.
A narrow joint or root

Too narrow a joint can result in the arc being attracted to just one of the side walls, while a root can be too difficult to access, or the root gap narrow further as welding progresses.
Incorrect parameter selection

Too high a welding speed in relation to the current will increase the risk of lack of fusion, while too high a current or too low a welding speed will result in the weld pool running in front of the arc, causing poor penetration.

Too great an arc length

If your arc length is too long, it can be attracted to just one side of the joint, meaning the weld doesn’t penetrate adequately enough to fuse properly.
Magnetic arc blow

Also known as ‘arc wander’, magnetic arc blow results from distortion of the magnetic field produced by the arc current. This uncontrolled deflection of the arc often results in lack of fusion.
Wrong size electrode

The electrode should be large enough for the material thickness, but can be too large if the root is narrow. Care must be taken to select the right electrode for the weld.

What can you do to minimise lack of fusion?

It is possible to take steps to reduce the risk of lack of fusion.

Check your weld current and gas mix

The usual solution for lack of fusion is to increase your mean current, improving your operating technique or choose shielding gases that provide a more fluid weld pool.
Ensure sufficient access

Make sure your joint or root are well prepared and not too narrow to weld evenly.
Use the correct welding parameters

For the best results, select a high current, short arc length and moderate welding speed, to promote fusion and avoid flooding.

For sidewall fusion, use a weaving technique

Weaving and dwell can improve side wall fusion, but be aware, it may be restricted by the procedure specification, limiting the arc energy input.
For root fusion, use a tack weld or restraint and ensure the root is clean

Maintain an even root gap and avoid contamination with oxide, rust or coatings.
Minimise arc blow by repositioning your weld clamp

Distortion of the arc magnetic field can be minimised by positioning the current return cable clamp so welding is always towards or away from the clamp.