Short Answer:

Incomplete fusion is a type of welding defect that occurs when the weld metal fails to properly fuse with the base metal or between layers of weld metal. This defect creates a weak bond, resulting in poor strength and sometimes cracks in the welded joint. It is mainly caused by low heat input, improper electrode angle, incorrect welding speed, or poor cleaning of the metal surface before welding.

This defect is serious because it reduces the load-bearing capacity of the welded structure and may lead to joint failure under stress or vibration. Proper heat control, correct electrode handling, and surface cleaning can prevent incomplete fusion.

Detailed Explanation :

Incomplete Fusion

Incomplete fusion is a welding defect that occurs when the molten weld metal does not completely fuse with the base metal or with the preceding layer of weld metal. It results in a lack of proper bonding between the materials being joined. This defect weakens the structural integrity of the joint, as the unfused areas act as discontinuities where cracks or fractures can develop when the welded component is subjected to load or vibration.

This defect is especially common in manual metal arc welding, gas metal arc welding, and submerged arc welding if the process parameters are not properly controlled. The causes of incomplete fusion are usually related to low heat input, improper welding technique, or poor preparation of the work surface.

Causes of Incomplete Fusion

1. Insufficient Heat Input:
   The most common cause of incomplete fusion is inadequate heat input. When the welding current or voltage is too low, the base metal and filler metal do not reach the required melting temperature, resulting in poor fusion.
2. Improper Electrode Angle:
   If the electrode is not positioned correctly, the arc may not strike directly at the joint interface. This causes the molten metal to be deposited without fully melting the edges of the base material.
3. Fast Welding Speed:
   Welding too fast does not allow sufficient time for heat to penetrate into the base metal. As a result, the weld bead forms on the surface without properly fusing to the material below.
4. Oxides, Dirt, or Rust on Surface:
   When the joint surfaces are not clean before welding, the presence of contaminants like rust, oil, or oxides acts as a barrier to fusion between molten metal and base material.
5. Improper Joint Preparation:
   If the bevel angle or root gap in the joint is too small, the welding torch or electrode may not reach the bottom or sides properly, leading to unfused zones.
6. Incorrect Arc Length:
   Maintaining too long or too short an arc length affects the concentration of heat at the joint, which may cause incomplete melting and poor bonding.
7. Improper Position or Technique:
   Wrong welding position or poor manipulation of the electrode can prevent the molten metal from flowing and fusing evenly throughout the joint.

Types of Incomplete Fusion

Incomplete fusion can occur in different areas of the weld joint, depending on the cause and welding technique used:

1. Side Wall Fusion:
   This occurs when the weld metal fails to fuse with one or both sidewalls of the joint. It often happens due to an improper electrode angle or insufficient heat.
2. Root Fusion:
   Root fusion defects occur when the bottom of the weld (root) is not fully fused. This is common in joints with poor root gap or when the penetration is too shallow.
3. Inter-run Fusion:
   In multi-pass welding, sometimes the newly deposited layer does not completely fuse with the previous layer, resulting in inter-run incomplete fusion.

Each of these defects can significantly reduce the strength of the weld and must be avoided through proper control of welding parameters and techniques.

Effects of Incomplete Fusion

Incomplete fusion is one of the most dangerous defects because it can lead to hidden weaknesses inside the weld. The main effects include:

• Reduced Strength: The joint cannot carry the designed load, as some areas remain unfused.
• Crack Formation: Unfused zones act as starting points for cracks during stress or vibration.
• Fatigue Failure: Under cyclic loads, incomplete fusion accelerates fatigue damage.
• Leakage in Pressure Systems: In tanks or pipelines, incomplete fusion can cause leaks.
• Poor Structural Integrity: The overall performance and life of the welded structure are reduced.

Prevention of Incomplete Fusion

1. Proper Heat Control:
   Use the recommended welding current and voltage as per the metal thickness and process type to ensure proper melting of both base and filler metal.
2. Clean the Joint Surface:
   Remove rust, paint, grease, and oxides before welding to allow proper bonding.
3. Correct Joint Design:
   Maintain appropriate bevel angle and root gap to ensure proper access and penetration of the weld.
4. Control Welding Speed:
   Avoid excessively fast welding that reduces heat penetration. A steady speed allows uniform melting and fusion.
5. Maintain Proper Electrode Angle:
   Position the electrode correctly to ensure the arc directly reaches the joint sides and root.
6. Use Suitable Technique for Multi-pass Welding:
   When doing multi-layer welds, clean each layer properly and ensure full fusion with the previous pass.
7. Proper Training and Supervision:
   Skilled operators can adjust settings and detect improper fusion during welding itself, reducing chances of defect formation.

Detection and Repair

Incomplete fusion can be detected through non-destructive testing (NDT) methods like ultrasonic testing (UT), radiographic testing (RT), and magnetic particle testing (MT). These methods help identify hidden defects within the weld metal.

To repair incomplete fusion, the defective section is usually ground or gouged out and re-welded with proper parameters to ensure full fusion. The re-welded joint is then inspected again to confirm defect-free bonding.

Conclusion

Incomplete fusion is a serious welding defect that results from insufficient heat, improper electrode angle, or poor joint preparation. It causes weak bonding between the weld and base metal, leading to structural failure if not corrected. Proper control of welding parameters, cleaning, and skilled workmanship are essential to prevent this defect. Regular inspection and adherence to correct welding practices ensure strong, reliable, and safe welded joints.