This article describes the characteristic features and principal causes of lack of sidewall and inter-run fusion. General guidelines on best practice are given so that welders can minimise the risk of imperfections during fabrication.
Lack of fusion imperfections can occur when the weld metal fails
- to fuse completely with the sidewall of the joint (Fig. 1)
- to penetrate adequately the previous weld bead (Fig. 2).
The principal causes are too narrow a joint preparation, incorrect welding parameter settings, poor welder technique and magnetic arc blow. Insufficient cleaning of oily or scaled surfaces can also contribute to lack of fusion. These types of imperfection are more likely to happen when access to the joint is restricted.
Too narrow a joint preparation often causes the arc to be attracted to one of the side walls causing lack of side wall fusion on the other side of the joint or inadequate penetration into the previously deposited weld bead. Too great an arc length may also increase the risk of preferential melting along one side of the joint and cause shallow penetration. In addition, a narrow joint preparation may prevent adequate access into the joint or encourage flooding the joint with moulting weld metal. For example, this happens in MMA welding when using a large diameter electrode, or in MIG, MAG and FCAW welding where an allowance has not been made for the diameter of the sheilding gas nozzle. Consideration should also be given to fabrication features that may obstruct the welding torch.
It is important to use a sufficiently high current for the arc to penetrate into the joint sidewall and previously deposited weld runs. Consequently, too high a welding speed for the welding current will increase the risk of these imperfections. However, too high a current or too low a welding speed will cause weld pool flooding ahead of the arc resulting in poor or non-uniform penetration.
Poor welder technique such as incorrect angle or manipulation of the electrode/welding gun, will prevent adequate fusion of the joint sidewall. Weaving, especially dwelling at the joint sidewall, will enable the weld pool to wash into the parent metal, greatly improving sidewall fusion. It should be noted that the amount of weaving may be restricted by the welding procedure specification limiting the arc energy input, particularly when welding alloy or high notch toughness steels.
Magnetic arc blow
When welding ferromagnetic steels lack of fusion imperfections can be caused through uncontrolled deflection of the arc, usually termed arc blow. Arc deflection can be caused by distortion of the magnetic field produced by the arc current (Fig. 3), through:
- residual magnetism in the material through using magnets for handling
- earth's magnetic field, for example in pipeline welding
- position of the current return cable clamp
The effect of welding past the current return cable which is bolted to the centre of the place is shown in Fig. 4. The interaction of the magnetic field surrounding the arc and that generated by the current flow in the plate to the current return cable is sufficient to deflect the weld bead. Distortion of the arc current magnetic field can be minimised by positioning the current return cable clamp so that welding is always towards or away from the clamp and, in MMA welding, by using AC instead of DC. Often the only effective means is to demagnetise the steel before welding.
Best practice in prevention
The following fabrication techniques can be used to prevent formation of lack of sidewall and interrun fusion imperfections:
- use a sufficiently wide joint preparation
- select welding parameters (high current level, short arc length, not too high a welding speed) to promote penetration into the joint side wall and previousl deposited weld runs without causing flooding
- ensure the electrode/gun angle and manipulation technique will give adequate side wall fusion
- use weaving and dwell to improve side wall fusion providing there are no heat input restrictions
- if arc blow occurs, reposition the current return cable clamp, use AC (in MMA welding) or demagnetise the steel
The limits for incomplete fusion imperfections in arc welded joints in steel are specified in BS EN ISO 5817 for the three quality levels (see Table). These types of imperfection are not permitted for Quality Level B (stringent) and C (intermediate). For Quality level D (moderate) they are only permitted providing they are intermittent and not surface breaking.
For arc welded joints in aluminium, long imperfections are not permitted for all three quality levels. However, for quality levels C and D, short imperfections are permitted but the total length of the imperfections is limited depending on the butt weld or the fillet weld throat thickness.
Acceptance limits for specific codes and application standards
|BS EN ISO 5817:2007
|Level B and C not permitted.
Level D short imperfections permitted but not surface breaking.
|BS EN ISO 10042:2005
|Levels B, C, D.
Long imperfections not permitted.
Levels C and D.
Short imperfections permitted.
|BS PD5500:2012+A1: 2012
|BS EN 14015:2004
|'l' not greater than 15mm
(depending on wall thickness)
|API 1104 (R2010)
|'l' not greater than 25mm
(less when weld length <300mm)
Detection and remedial action
If the imperfections are surface breaking, they can be detected using a penetrant or magnetic particle inspection technique. For sub-surface imperfections, detection is by radiography or ultrasonic inspection. Ultrasonic inspection is normally more effective than radiography in detecting lack of inter-run fusion imperfections.
Remedial action will normally require their removal by localised gouging, or grinding, followed by re-welding as specified in the agreed welding procedure.
If lack of fusion is a persistent problem, and is not caused by magnetic arc blow, the welding procedures should be amended or the welders retrained.
Copies of other articles in the 'Job knowledge for welders' series can be found under Practical Joining Knowledge or by using the search engine.
This Job Knowledge article was originally published in Connect, May/June 1999. It has been updated so the web page no longer reflects exactly the printed version.