What is Lack of sidewall fusion (LOSWF) in Welding

Welding is a fabrication process that fuses two materials by melting them and allowing them to call – resulting in fusion. The fusion process is essential and central to strong welds. Lack of sidewall fusion in welding is when the two mating metals have not fused properly, creating a defect in the weld.

Lack of sidewall fusion is a well known and well-studied welding defect due to its occurrence in welded joints. Many failures in welded joints occur as a result of an unidentified lack of fusion.

Characteristics of Lack Of Sidewall Fusion

Following metallurgical studies of many welds is understood that there are two types of lack of sidewall fusion that can be found.

1. Pure lack of fusion

2. Lack of fusion consisting of non-metallic inclusions

Pure lack of sidewall fusion is a type of structural defect. Pure lack of sidewall fusion occurs due to insufficient melting of the parent metal. The inadequate melting of the parent material results in an internal joint between the solid and liquid phases.

In extreme cases, a substantial void in the joint is found, a large structural concern.

Unfortunately, in most cases, the lack of sidewall fusion is not detectable by non-destructive testing.

During welding, internal stresses are present due to uneven cooling rates and density differences between solid and liquid states. Due to the internal stresses, accompanied by contractions from cooling, a void can appear between the weld material and the parent material. A lack of fusion of this type is detectable of non-destructive testing.

Like pure lack of fusions, lack of fusion consisting of non-metallic inclusions is also a result of the parent material undergoing inadequate melting. The stunted amount of melting results in a uniform distribution of oxide inclusions across the surface, where the lack of fusion has occurred. If the non-metallic inclusions melt they have a bubble/circular appearance.  

Detecting Lack of Sidewall Fusion in Welding

As mentioned above, it is difficult, and even sometimes not possible, to detect the lack of sidewall fusion in welds. However, the best method to detect welding defects is ultrasonic testing. Ultrasonic testing to the standard BS EN ISO 1760 ‘Non-destructive testing of welds – Ultrasonic testing’ states that one of the probes angles used during the assessment will allow the weld fusion faces to be scrutinised at, or as close as possible, to normal incidence.

Ultrasonic testing welds in this manner maximises the probability of detecting a lack of sidewall fusion (LOSWF) defect.

What Causes Lack Of Sidewall Fusion in Welding?

Fusion occurs when mating materials are melted, forming a molten weld pool. Following its creation, the weld pool cools and solidifies. The process fuses the two materials. Therefore, a lack of sidewall fusion results in a weaker joint, which will lead to detrimental failure.

Joint Preparation

With all welding processes, preparing the material and joint is essential to obtaining the desired outcome. The most common joint preparation flaw is one that is too narrow.

If the clearance between the two materials is too small, the arc is attracted to one side of the joint more than the other. As a result, the side of the joint that the arc is not attracted to undergoes less melting, giving a lack of fusion. Depending on the welding process, the same issue applies to lapping joints, where there is a lack of fusion with the bead deposit below.

A similar process that results in the arc being attracted to one side more than the other is an arc length, too long.

Additionally, a clearance between the materials, which is too small, limits access to the joint. Commonly, limited access to a joint can result in a lack of sidewall fusion.

Magnetic Arc Blow

Magnetic arc blow, also known as ‘arc wander’ or ‘arc blow’, is a welding phenomenon that deflects the direction of the arc due to the distortion of the magnetic field produced by the arc. The source of the distortion can stem from a variety of sources:

• Residual magnetism within the material. Residual magnetism can be left through handling and potential previous NDT assessments. A method of measuring residual magnetism in metals is by using a Gauss meter, commonly used after MPI’ing components.

• Earths magnetism

• The position of the current return cable clamp.

Welding Parameters

Controlling and selecting the correct welding parameters for and during welding is crucial for obtaining a solid joint. The speed at which welding occurs will increase/decrease the probability of imperfections, depending on if the correct speed is selected. Welding too quickly will result in poor heat penetration and a lack of fusion. 

However, welding too slow will create a flooding effect of the weld pool, which goes ahead of the arc. As a result, poor or non-uniform penetration occurs.

The other influential welding parameter is the amount of current used. If a low current is used, the arc will not have sufficient joint penetration into previous weld beads of the sidewalls of the joint resulting in a lack of fusion. A current too high has a similar effect as welding too slowly – weld pool flooding.

Insufficient current is the most probable cause for a lack of fusion in welds.

How to Prevent Lack of Sidewall Fusion in Welding

The causes of a lack of sidewall fusion are generally straightforwardly overcome.

Welding Parameters

Selecting the correct welding parameters is paramount for achieving a good solid weld and it being defect-free. Using an appropriately high current level matched with a suitable welding speed will encourage good heat penetration and, hence provide good fusion into the sidewall and any previous weld deposits.

Joint Preparation

Joint preparation, like welding parameters, help provide a solid base for providing a good weld. Joint preparation can be as simple as ensuring that the surfaces are clean – removing any impurities like grease and dirt. It is surprising how often simple joint preparation is overlooked before welding.

Another aspect of joint preparation is ensuring you have the correct joint type. For example, a butt weld is performed on a variety of joints such as a V joint, double U joint etc. To determine which joint is best is dependent on the application of the component and the orientation of the materials. If the incorrect joint is selected it will lead to poor welds and restricted access into the joint, which will result in a lack of sidewall fusion.

How is Lack of Side Wall Fusion Repaired

As with many weld defects, the best way to repair a lack of sidewall fusion is by grinding back the weld to remove the defect. Following its removal, reweld the area back up to its original location. To understand whether the lack of sidewall fusion has not been repeated, inspect the joint to BS EN ISO 1760 ‘Non-destructive testing of welds – Ultrasonic testing’, which increase the probability of detecting a lack of sidewall fusion.

Summary

Lack of sidewall fusion in welding is a well known and well-studied weld defect. It can occur for a variety of reasons, which are easily prevented. Preparation before welding is one of the common factors that cause a lack of sidewall fusion via lack of joint preparation and incorrect weld parameters.

The weld defect is difficult to detect depending on the severity, however, following the appropriate standards increase the probability of detecting the defect and preventing catastrophic failure.

• Related Posts:
• What is a Butt Weld
• What is Porosity in Welds
• What are Weld Craters and Crater Cracks

The Graduate Engineer Social Media