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.