A weld can look sound on the outside and still contain a hidden discontinuity along a fusion boundary. That is why lack of fusion welds remain an important concern in fabrication, repair and inspection.
Unlike a surface defect that can often be identified visually, an internal lack of fusion can exist between the weld metal and parent material or between successive weld passes.
For inspectors, the challenge is not simply finding a flaw. The inspection method must interact with the discontinuity in a way that makes it detectable. This is where ultrasonic testing (UT) becomes particularly valuable.
What Is Lack of Fusion in Welding?
A sound weld depends on proper fusion between the molten weld metal and the surfaces that form the joint. When this fusion does not occur, an area remains where the materials have not formed the intended metallurgical connection.
Lack of fusion can occur at several locations within a weld, including:
- Along the sidewall of the joint
- Between individual weld passes or runs
- Around the root area
- At other fusion boundaries where the weld metal has not properly bonded to the adjacent surface
TWI describes sidewall fusion as the failure of weld metal to fuse completely with the joint sidewall, while inter-run fusion refers to inadequate fusion with a previous weld bead.
The location matters because it can influence both how the defect forms and how effectively an inspection method can detect it.
Lack of Fusion vs Lack of Penetration
Lack of fusion and lack of penetration are related welding imperfections, but they are not the same defect.
Lack of penetration occurs when the weld metal does not extend sufficiently through the joint thickness, typically at the root. Lack of fusion, by comparison, occurs when the weld metal fails to bond properly with an adjacent surface or previous weld pass.
Both conditions can have similar causes, such as inadequate heat input, but their geometry and location are different.
| Defect | Typical Location | Basic Problem |
|---|---|---|
| Lack of fusion | Sidewall or between weld passes | Adjacent surfaces do not fuse properly |
| Lack of penetration | Root of the joint | Weld metal does not extend sufficiently through the joint |
Why Is Lack of Fusion Difficult to Detect?
One of the main challenges with lack of fusion is its geometry. The condition can form as a relatively thin, planar discontinuity along a fusion boundary. It may remain completely enclosed within the weld, leaving no obvious indication on the finished surface.
Research on lack of fusion has identified different forms of the condition, including pure lack of fusion, open lack of fusion and conditions involving non-metallic inclusions.
This distinction is important because a pure lack-of-fusion condition may exist without a detectable void. In such cases, conventional non-destructive testing may have difficulty identifying the discontinuity. Other forms, particularly those involving a small gap or inclusions at the interface, may produce detectable indications.
Inspection results should therefore be interpreted carefully. A relatively large discontinuity does not automatically guarantee a strong inspection signal. Defect orientation, interface condition and the direction from which inspection energy reaches the discontinuity can all influence detectability.
Visual examination still has an important role in weld quality control. During welding, an experienced supervisor may identify conditions that are likely to produce poor fusion. After welding, however, visual examination cannot reliably reveal an internal fusion boundary that remains completely enclosed within the weld.
A smooth or acceptable-looking weld profile therefore does not prove that the internal weld is free from lack of fusion.
Which NDT Methods Can Detect Lack of Fusion?
Different non-destructive testing methods detect different types and locations of discontinuities. No single method provides the same information in every situation.
Visual Testing
Visual testing (VT) is useful for assessing weld surface condition, weld profile and visible discontinuities. It can also help identify welding conditions that may increase the likelihood of incomplete fusion.
However, visual inspection cannot directly examine a fusion interface buried beneath sound weld metal.
Penetrant and Magnetic Particle Testing
Penetrant testing can reveal discontinuities that extend to the surface. Magnetic particle testing (MPT) can identify suitable surface and near-surface indications in ferromagnetic materials.
Neither method provides a general solution for a completely internal lack-of-fusion condition because the discontinuity may remain inaccessible from the surface.
Radiographic Testing
Radiography has an established role in weld inspection and can reveal many internal imperfections. However, planar lack-of-fusion defects can present a particular challenge because their detectability depends strongly on their orientation relative to the radiation beam.
Research has found that radiography can reveal larger forms of lack of fusion associated with voids or inclusions, but it does not necessarily reveal every lack-of-fusion condition itself.
Ultrasonic Testing
Ultrasonic testing approaches the problem differently. Instead of producing an image from transmitted radiation, UT sends mechanical waves into the material and evaluates energy reflected from interfaces and discontinuities.
This difference can be particularly useful when the suspected defect has a planar shape. By directing the ultrasonic beam through the weld at a controlled angle, inspectors can examine fusion boundaries that cannot be evaluated visually.
How Does Ultrasonic Testing Detect Lack of Fusion?
Before the examination begins, the inspection surface is prepared so that the probe can maintain suitable contact and coupling with the material.
The selected probe then introduces ultrasonic energy into the weld. In many weld-inspection applications, angle-beam techniques are used because they allow the sound path to enter and travel through the weld at a controlled angle.
As the ultrasonic beam travels through the weld, some of its energy can be reflected when it encounters a boundary or discontinuity. A lack-of-fusion interface may therefore produce a response on the inspection instrument.
The inspector moves the probe through the required scanning pattern and evaluates how the indication changes with probe position, angle and other inspection variables.
A UT Indication Still Requires Interpretation
A UT indication is not, by itself, a final diagnosis of lack of fusion.
The inspector needs to consider factors such as:
- Calibration
- Inspection sensitivity
- Scanning coverage
- Weld geometry
- Probe characteristics
- Indication location
- Applicable inspection standards
- Acceptance criteria
Weak or repeating indications can still deserve careful attention when lack of fusion is suspected. Research has shown that relatively large lack-of-fusion discontinuities can sometimes produce weak ultrasonic responses because of their orientation and the condition of the contacting surfaces.
Advanced Ultrasonic Methods
Conventional angle-beam UT remains an important technique for weld inspection, but advanced ultrasonic methods can provide additional information.
Phased array ultrasonic testing (PAUT) uses multiple elements to steer ultrasonic beams through a range of angles. This can provide imaging that helps inspectors visualise and evaluate indications.
Time of flight diffraction (TOFD) uses diffracted ultrasonic signals to support the detection and sizing of suitable discontinuities.
The appropriate technique depends on the weld configuration, material, applicable code, inspection access and inspection objective.
UT or Radiography: Which Is Better for Lack of Fusion?
There is no universal answer. The most appropriate inspection method depends on the expected defect type, weld geometry, applicable code, access conditions and required information.
| Inspection Consideration | Ultrasonic Testing | Radiographic Testing |
|---|---|---|
| Internal weld examination | Yes | Yes |
| Planar lack of fusion | Often well suited | Detectability can depend strongly on orientation |
| Location information | Strong positional information | Provides a different form of information |
| Radiation controls | Not required | Required |
| Inter-run fusion | Particularly useful | Can be challenging in some cases |
For lack of fusion, the advantage of UT is not simply that it is a newer inspection technology. Its usefulness comes from the ability to direct an ultrasonic beam through the weld and examine the response from a fusion interface.
For planar discontinuities, this can provide an important inspection advantage.
Radiography remains valuable for many applications, particularly for suitable volumetric imperfections. The choice should therefore follow the defect being investigated rather than assuming that one NDT method is always superior.
What Causes Lack of Fusion in Welding?
Most lack-of-fusion conditions begin during welding. The welding parameters, joint preparation, technique and material condition can all influence whether the weld pool produces proper fusion.
Insufficient Heat Input
Insufficient heat input is a common cause. If the welding current is too low for the welding conditions or the travel speed is too high, the arc may not provide enough heat to the sidewall or previous weld pass.
The result can be inadequate melting and incomplete bonding at the intended fusion surface.
Poor Joint Preparation
Joint preparation can directly affect access to the fusion surfaces. A narrow groove, unsuitable joint geometry or restricted access can make it difficult for the arc to reach the area that needs to be fused.
Proper preparation helps ensure that the welding arc and molten pool can reach the intended surfaces effectively.
Incorrect Electrode or Gun Technique
The angle and manipulation of the electrode or welding gun influence where heat is delivered. Poor technique can cause one side of the joint to receive insufficient heat or prevent the weld pool from properly washing against the sidewall.
Travel speed and bead placement can also affect whether successive weld passes fuse correctly.
Contamination and Inadequate Cleaning
Contamination, inadequate cleaning and poor interpass preparation can interfere with the formation of a sound connection between the weld metal and the adjacent surface or previous weld pass.
Maintaining suitable surface and interpass conditions is therefore an important part of preventing incomplete fusion.
Magnetic Arc Blow
Magnetic arc blow can create another challenge when welding ferromagnetic materials. Residual magnetism, the position of the current return connection and other magnetic effects can deflect the arc and disturb the intended weld path.
Depending on the situation, corrective measures may include changing the return cable position or demagnetising the material.
What Happens When a Weld Has Lack of Fusion?
A lack-of-fusion condition interrupts the continuity that the welded joint was designed to provide. Its significance depends on factors including its size, location, orientation, service conditions and the requirements governing the weld.
An internal unfused area can act as a stress-raising discontinuity. Under demanding loading conditions, it may provide a location where cracking can initiate or propagate.
In pressure-containing applications, a defect that reaches or opens toward the surface can also create leakage concerns.
Research has identified lack of fusion as a serious weld defect and has documented cases in which undetected welding discontinuities contributed to structural failures. It has also shown that some lack-of-fusion conditions can produce weak NDT indications despite being relatively large.
What Happens After UT Finds Lack of Fusion?
A UT indication does not automatically mean that the weld must be rejected. The indication first needs to be evaluated according to the applicable procedure, code, specification and acceptance criteria.
If a lack-of-fusion condition requires repair, the general process may involve:
- Identifying and evaluating the affected area.
- Removing the defective material.
- Preparing the area for repair welding.
- Completing the repair using the appropriate approved welding procedure.
- Reinspecting the repaired area using the specified NDT method or methods.
Localized gouging or grinding followed by rewelding is commonly used for remedial work on suitable imperfections. The repaired area should then undergo the required inspection to confirm that the condition has been adequately addressed.
The exact repair process must follow the applicable welding procedure, specification and acceptance requirements.
Final Thoughts
A finished weld containing lack of fusion does not always reveal what happened beneath its surface. The discontinuity can remain hidden along a sidewall, between weld passes or elsewhere within the joint, making the selection of an appropriate NDT method important.
Ultrasonic testing is particularly useful for many of these conditions because the ultrasonic beam can be directed through the weld toward a suspected fusion interface. This can be especially valuable when investigating planar discontinuities and inter-run fusion.
At the same time, UT has limitations. Reliable results depend on appropriate equipment, inspection procedures, access, calibration, scanning coverage and qualified interpretation.
For critical welds, the objective is not simply to find a signal. It is to understand what that signal represents and determine whether the weld satisfies the applicable requirements.
Effective welding practices combined with appropriate NDT inspection provide the strongest approach to controlling the risk of hidden lack-of-fusion defects.
Key Takeaways
- Lack of fusion can remain hidden beneath an otherwise acceptable-looking weld surface.
- It can occur along sidewalls, between weld passes and in other areas where proper fusion has not been achieved.
- Ultrasonic testing can detect many internal planar discontinuities associated with incomplete fusion.
- Insufficient heat input and excessive travel speed can contribute to lack of fusion.
- Proper joint preparation helps ensure that sufficient heat reaches the intended fusion surfaces.
- Electrode or welding-gun technique can influence how effectively the weld pool fuses with the joint.
- UT allows inspectors to direct ultrasonic beams toward suspected fusion boundaries within a weld.
- Radiography remains valuable, but planar defects can present orientation-related detection challenges.
- A UT indication must be evaluated according to the applicable code, specification and inspection procedure.
- Effective welding practices combined with suitable NDT inspection can improve weld reliability and reduce the risk of undetected discontinuities.
Frequently Asked Questions
What is the meaning of lack of fusion?
Lack of fusion means that the weld metal has failed to properly fuse with the base metal, joint sidewall or an earlier weld pass. This creates an unfused area within the joint that can affect weld integrity. The condition may occur along a sidewall, between weld runs or around the root area.
What causes lack of fusion in welding?
Several welding conditions can cause lack of fusion, including insufficient heat input, excessive travel speed, incorrect electrode or torch angle, poor joint preparation, contamination and inadequate interpass cleaning. Magnetic arc blow can also contribute in suitable ferromagnetic materials.
Correct welding parameters, appropriate joint preparation and controlled welding technique help reduce the risk of incomplete fusion.
How do you fix incomplete fusion in welding?
Incomplete fusion should first be evaluated against the applicable welding code, specification and acceptance criteria. Where repair is permitted, the affected material may be removed by grinding or gouging, followed by suitable joint preparation and rewelding under an approved welding procedure.
The repaired area should then undergo the required inspection to verify that the defect has been adequately removed and that the repaired weld meets the applicable requirements.
Does low welding current cause poor fusion?
Yes. Welding current that is too low for the applicable welding conditions can provide insufficient heat to melt the sidewall or previous weld pass properly. This can contribute to lack of fusion, particularly when combined with excessive travel speed or unsuitable joint preparation.
Current settings should be selected according to the approved welding procedure and the material, electrode, joint design and required heat input.