high form factor leads to the growth of columnar grains upward directed from the side wall to the weld centre & lesser form factor results in the growth of columnar grains downward. why?
•Weld pool shape can be expressed by a "form factor".
•
Ø Form factor is the ratio of the weld pool width to its maximum depth.
Ø Width is the root opening plus the total penetration into the base metal.
Ø Depth is the distance from the top of the molten weld pool to the lowest level of the liquid-solid interface.
So, if the depth of the weld pool increase the which leads to in shifting of center of mass in downward direction and also as it in molten state so more depression of molten metal taken place at the middle of weld pool compared to other surrounding portion of molten pool. Due to this the columnar grains follow the downward direction.
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