Fusion is the melting and joining together of two or perhaps more pieces of metal to make a continuous and solid joint. This is one of the most fundamental welding concepts and fusion must occur to create a strong and durable connection between disparate metal parts. When there is a lack of fusion in welding, the molten metal will not mix and bond with the base metal which may create weak welds and the potential for structural failure.
So, during fusion welding, a welder must control the heat input, use proper welding techniques, and maintain the welding parameters to achieve the required level of fusion for each welding application.
Fusion begins to occur when the welding process generates sufficient heat to melt the edges of the metal pieces that are being joined together. The exact temperature when fusion occurs varies depending on the welding method and the type of metal.
Certain welding processes, such as arc, gas, and laser welding will generate heat differently. When the metal edges melt the molten metal from both pieces mix to create a metallurgically bonded joint.
In the joint area, a bead or pool of molten metal is formed and when the heat source is removed this area will cool and solidify. When the molten metal is fully solidified and cooled a complete joint has been created. The newly solid metal is known as a weld bead or nugget and ideally, this will be a strong and continuous connection.
Now that we understand the importance of fusion it’s important to know how to identify a lack of fusion in welding processes, here are some common indicators:
Cracks and/or Voids:
A lack of fusion can create cracks, voids, and other openings in between the base metal and the weld. In some cases, this will be immediately obvious, but these openings can be smaller and harder to detect. On closer inspection, you may notice thin gaps along the entire weld line.
This is when the weld metal doesn’t penetrate fully into the root of the weld or joint. You may notice that the weld metal has not reached the entire thickness of the weld joint.
This is a depression or groove that runs along the weld bead and it may be a sign that there was a lack of fusion at the weld edges.
An Uneven Profile:
The weld profile may have visible deviations or irregularities from the weld bead shape which may make it look uneven.
The Toe of the Weld:
If a lack of fusion has occurred at the toe of the weld where the base and weld metal should have fused together the pieces may be separated.
If a bend test is performed any cracks or fractures may be revealed.
Radiographic and Ultrasonic Inspections:
In a critical application these two types of inspections can detect internal flaws that may have occurred due to a lack of fusion.
Close Visual Inspection:
Even a cursory look at the weld bead could reveal irregularities and discontinuities caused by incomplete fusion in the weld joint.
A lack of fusion can lead to unreliable and weak welds that could potentially fail under pressure or stress.
Causes of Lack of Fusion in Welding
Now that you can identify a lack of fusion, weld defect causes and how to avoid them becomes a priority for welders that want to make solid and durable weld joints. The most common cause is insufficient heat input, there simply isn’t enough heat energy to melt and fuse the metals together.
This may happen if the voltage of the welding current is too low or if the welder uses a fast travel speed. If the joint surfaces are poorly prepared with insufficient bevels it’s much harder to create a weld pool that can fuse with the base metal.
When the base metal is dirty or contaminated with rust, dirt, oil, and other contaminants this can prevent contact between the base metal and the molten weld pool.
Proper welding technique is important
If the contact is inconsistent or weaving it’s harder to achieve fusion. The welding torch or electrode must be manipulated correctly and if the wrong size or type of electrode is used inadequate fusion may occur.
If thicker materials and certain alloys are not given sufficient preheat or a proper interpass temperature cannot be maintained this may result in a lack of fusion. The welder must have the skill and experience to achieve proper fusion, newer welders may struggle to adequately control the welding process.
If the welding position is awkward the molten metal may not fuse properly with the base metal and this is especially true in overhead or vertical welding.
In gas tungsten arc and gas metal arc welding the proper shielding gas must be used and the gas flow rates must be maintained to protect the weld pool. If there is insufficient gas shielding, a lack of fusion may occur.
Prevention and Remediation–To ensure that proper fusion occurs When Welding
The welding environment should be clean and clear. The base metal needs to be free from contaminants, such as rust, oil, paint, dirt, and anything else that could prevent fusion.
Welding codes, standards, and guidelines that are relevant to the specific application should be carefully followed.
The joint must be prepared properly, the edges should be clean and uncontaminated, and grooving, beveling, or chamfering may be needed to facilitate fusion. Joint fit-up should be properly aligned and tight because misalignment and gaps will inhibit proper fusion.
Set aside some time to develop and practice your welding skills and take guidance from instructors or more experienced welders.
It’s important to select the correct welding process for the joint design and the materials you want to bond together. Various welding processes have different characteristics that make them the best option for specific applications.
Choosing the correct electrode or filler material and consumables is also essential to achieve proper fusion. The electrode or welding torch angle must be correct. Refer to welding procedure specifications for further guidance if you’re unsure.
Heat input is critical to melt the base metal and filler material and the correct voltage, current, and travel speed settings should be followed.
Travel speed needs to match the joint thickness and the welding process. If the movement is too fast, insufficient fusion will occur, and moving too slowly will lead to excessive heat input which can cause other problems.
The arc must be appropriate and consistent to maintain the stability and size of the weld pool. Proper penetration is important; the weld needs to penetrate evenly and fully into the joint. Good welding technique with oscillation or weaving patterns that match the chosen welding process is essential for even heat distribution that will promote fusion.
Proper Checks and Adjustments While Welding
During the welding process, the weld bead should be visually inspected during and after completion. This is the best way to identify the aforementioned lack of fusion and to find any other welding defects.
To achieve proper fusion it may be necessary to fine-tune the settings and make adjustments that are appropriate for the materials and joint configurations for that specific welding process.
In a critical application, you may want to consider using radiographic or ultrasonic non-destructive testing methods. These will verify that proper fusion has occurred and that the weld quality is good.
Lack of Fusion in Welding: In Conclusion
A lack of fusion in welding can lead to weak welds that are more likely to fail under stress and pressure. The molten metal must mix and bond with the base metal to create a viable bond. Controlling factors such as the technique, parameters, heat input, and proper preparation are important to promote the desired level of fusion.