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What Is a Fillet Weld?

Since fillet welds are used so frequently, there are several factors that you should consider before fabricating such a weld. This article will review several aspects of fillet welds. In addition to beginners, even experienced welders will hopefully gain some insights through this in-depth treatise on fillet weld joints. 

So what is a fillet weld? Fillet welds are some of the most widely used joints in the welding field. Fillet welds account for an overwhelming majority of joints fabricated through arc welding. Corner, T, and lap joints are some of the most common fillet weld joints. 

There are also other permanent joining techniques besides welding that use filled welds. Non-fusion joining methods like soldering, braze welding, and brazing use fillets welds for connections. These techniques will not be discussed in this article.

What are common filled weld shapes? How are they described?

Some of the simplest and most commonly used fillet weld joints are the T joint, corner joint, and lap joint. 

Welding specifications and standards employ specific terms to discuss fillet welds. These terms include throat thickness, leg length, and deep penetration throat thickness. The deep penetration throat thickness is not so easy to measure. This is because it includes part of the heat-affected zone whose extent can be difficult to define with a high degree of precision. 

Fillet welds exist in several different shapes. They are as follows:

  • Miter: The cross-section of the miter is in the form of a right-angled triangle. 
  • Concave: The shape of this fillet weld cross-section is similar to what was described above, with one crucial difference. Instead of being a completely flat plane, the exposed surface of this fillet weld has a concave shape, as the name implies. Just like a concave lens, the exposed plane of the weld bends inwards. The throat thickness of the concave fillet weld is less than the corresponding value of miter weld. 
  • Convex: In this case, the exposed surface of the weld bends outward. The exposed weld plane, therefore, bends in the opposite direction to the concave weld. As a result, the throat thickness is more in the case of the convex fillet weld compared to both miter and concave shapes. 

What are the potential T joint fillet weld problems?

One of the most important factors of successful fillet welding is to obtain the right weld profile dimensions. The throat thickness and leg length need to be within a specific range for a reliable fillet weld. 

It is the designer’s responsibility to compute the optimum dimensions for the filled weld profile. Designers may include a safety factor as a result of which, the weld shown on the fabrication diagram may be more substantial compared to design considerations. 

The weld size is represented using the right weld symbol.

In the UK, it is customary to represent the leg length by ‘z.’ The number suffixed to ‘z’ indicates the leg length size in terms of millimeters. 

In Europe, the convention is to represent throat thickness with the letter ‘a’ suffixed by a number, which is the leg length in millimeters.

After the drawing is sent to the shop floor, the inspector or welder may include an additional safety factor to reinforce the weld joint. Expanding the leg length size will increase the strength of the weld according to certain people. 

The outcome is greater leg length than what was designed. For instance, the leg length may be increased to 8 mm for an original leg length of 6 mm. You might be surprised to note that the increased volume as a result of the additional 2mm accounts for more than 80 percent of the total volume. This represents significantly increased costs and electrode consumption. 

You should bear in mind that the designer has already included a safety factor in their original weld design. The additional safety factor will, therefore, produce a weld that is excessively larger compared to the required weld. 

Several benefits can be achieved if the final weld leg length is according to the original design. These advantages include lower operational costs, lower filler wire consumption, reduced product weight, and higher productivity. 

Another benefit materializes when leg length is fabricated according to design. Since the original leg length is smaller, it entails faster travel speed. Faster travel speed leads to increased weld penetration for greater actual throat thickness. 

Oversized welds do not have greater strength than the design; in fact, they are often weaker due to low weld penetration. They also entail higher operating costs, wasteful use of consumables, and other fabrication issues, such as increased distortion. 

What are potential lap joint fillet weld problems?

As shown previously, oversized welds are commonly fabricated due to popular misconceptions. Lap joints are no exception to this phenomenon. The designer may state a leg length that has the same size as the material thickness. 

If strength considerations are taken into account, then the fillet weld size need not have the same size as plate thickness. It doesn’t even need to be anywhere close to it. Additionally, the weld may also suffer from further problems. For instance, during the welding process, the upper corner of the lap joint may melt as a result of welding. This will reduce leg length. Consequently, an undersized weld may be created due to lower design throat value. 

The welding operator must, therefore, take care that the top corner of the upper plate is not melted down during the welding process. The ideal condition is for the weld to be at least 0.5 mm lower than the top corner of the upper plate. To accomplish this, the designer has to specify a leg length size that is smaller in comparison to the part thickness. 

The designer may specify a deep penetration fillet weld to compensate for reduced throat thickness. Suitable weld tests have to be carried out to confirm increased penetration. The welding fabrication process will also need additional controls to make sure that this increased penetration is being consistently being achieved. 

Other problems can transpire during fillet weld fabrication. There is the risk of weld toe overlap as a result of the bigger weld pool size. The weld face may be highly concave due to which the weld toe will have sharp notches. 

Due to the poor weld profile that results from these defects, the fatigue life of the fillet weld joint may be reduced owing to the greater toe angle. The higher toe angel will concentrate on stresses, which will weaken the weld more quickly. When single weld passes produce such shapes, then there may also be the potential for reduced root penetration. 

Poor fit-up may also lead to lower throat thickness. For instance, if the sections are not perfectly parallel, then the actual throat may be smaller compared to the design throat. 

Fillet weld joints are one of the most widely fabricated weld joints in the industry. But they are also one of the most difficult to create with a high level of consistency. Compared to butt joints of the same dimensions, fillet welds may need greater heat input to be created. Welding operators with less experience and lower skills may use a poor technique that creates fusion defects and lowered penetration. Such errors, unfortunately, cannot be identified through visual inspection or non-destructive testing techniques. 

Fillet weld joints cannot be subjected to non-destructive testing techniques at all times. This can be problematic owing to inspection difficulties. Ultrasonic testing of fillet welds may be time-consuming. The results of these tests may not be easy to interpret. 

What are the different non-destructive testing techniques for fillet welds? Are there any drawbacks to these techniques?

The issue with penetrant testing, magnetic testing, and visual testing is that these testing techniques scrutinize the surface and are therefore superficial. They cannot ascertain the inner structure of the weld or discover defects hiding beneath the surface. Also, a lot of effort is expended towards measuring the weld size instead of identifying other facets of quality. 

Consequently, welding fillet joints is harder, and so is inspecting them volumetrically. Quite frequently, the fabricated fillet welds are more significant than the original design size. Fillet welds thus produced may have poor profile shape, which can adversely affect their reliability and durability. 

Welding operators must, therefore, take scrupulous care to follow the design weld specifications exactly, as shown in the drawing sheet. Also, welding operators should have the skills and training to make the right fillet weld profile to avoid joint defects that can arise due to the wrong shape.  

Related questions

What is a fillet weld symbol?

The fillet weld symbol is denoted as a triangle. The leg dimension of the fillet weld is put to the left of the fillet symbol. The majority of, however not all fillet welds are of equivalent legs.

How is a fillet weld measured?

The very first step in determining a fillet weld is to identify the density of the items of metal being fused by the fillet weld. To perform this you are going to position the metal gauge against every piece of metal in order to figure out density. The throat of the fillet weld describes the front of the weld that looks out from the weld joint.

What is effective length of fillet weld?

The effective length of a fillet weld might be taken as the total size of the full-size fillet much less one leg length, s, for every end which does not continue around an edge. A fillet weld with an efficient size much less than 4s or much less than 40 mm must not be utilized to carry load.


About Pierre Young

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Hey, I'm Pierre Young a qualified AWS Certified Welder. I got into welding in 2009 as a side hustle. Ever since then, I've been doing all kinds of welds - both for business and pleasure. While immersing myself in this wonderful hobby, I've learned from hands-on experience what welding gear works and what doesn't. Welding Headquarters is the site where I share everything I've learned.

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