Failure to complete amalgamation with parent metal or between the weld runs, is referred to as lack of fusion in welding. The weld metal fails to blend with parent metal or sometimes two weld beads are not completely fused to make as a whole. The problem occurs when the work piece not receives adequate heat for fusion. Normally there are three locations where lack of fusion may occur.
- Along the joint wall
- Between the weld runs
- At the root
Following are the possible causes of lack of fusion and the practicable ways to control them.
1. High Heat Input
As it is stated earlier that less heat supply results in poor fusion. We know heat input is the relation of welding voltage, current and travel speed. It has direct relation with current and voltage and that of inverse with travel speed. Low voltage/current settings don’t supply enough heat. On the other hand too faster travel induces the same problem. Adjust the maximum allowable current and blend weld metal with moderate pace. Here great skills are expected from a welder.
2. Scale/oxide removal
Weld joints can be subjected to atmospheric contamination. Any unwanted particle or presence of oxide film may impede the process of fusion and may harm weld properties. Therefore it is recommended to remove any foreign overlay on the joint. Joint cleaning prior to welding makes it easier to eliminate lack of fusion.
3. Correct Angles
Electrode and work angles play an important role in achieving best quality results. Make sure the right electrode angle with moderate pace because an incorrect electrode handling makes liquid weld to interrupt arc activity.
4. Bead Orientation
Wrong positioning of weld beads along the joint wall creates slot and fissure between them. The development of slot may lead to slag inclusions and consequently lack of fusion takes place. A welder must be trained enough, to put the beads in right place and ensure the complete blend of weld along the side wall.
5. Position welding
Root pass with vertical down position creates disruption in the process and may result in partial fusion. In order to prevent this, vertical up position is preferred.
6. Edge Preparation
Well prepared edge is an essential part of perfect welding. If the joint pieces are not aligned in the same plane, variation occurs in root attributes. Furthermore if the root face is too large, weld metal fails to fuse completely at root. Fix this problem by preparing small root face edge. On the other hand, small root opening also results in unfused output by influencing the aspects of fusion and it is recommended to confirm the correct root opening before commencing the welding procedure.
7. Correct voltage and Inductance
During short circuiting mode in MIG/MAG process, inductance may exceed up to the certain level which can cause severe fusion problems. It is only the matter of fact that the magnetic field produced due to immoderate inductance, resists the welding current to rise. In this case metal at the wire tip is not heated adequately and fails to fuse with parent metal. Appropriate adjustment of voltage and inductance settings are required to prevent spattering and lack of fusion, as too low inductance also causes severe spatter. Use of modern equipment with advance power source (that provides regulated voltage and inductance values) is appreciable.