Dear Roshan Sebastian

Heat extracts from both mechanisms!

if you ask heat goes from melt pool mainly via bottom side cold solids or from top gas, I think heat extracts mainly from bottom solid part (Conduction).

Sir, let assume around 30 microns of the melt pool solidified and remaining 30 microns of liquid left to solidify.

What is the effect of the extracted heat from liquid melt region on the already solidified 30 microns width region with respect to the microstructure?

Dear Roshan Sebastian

heat extracts not only by previous solidified power but also via initial solid substrate.

OK, lets come back to micro-structure evolution. In fact, after melting, we can have two different kind of grain; new grains nucleated at melt pool boundary, and previous grain from already solidified region extended till new melt pool boundary. During the solidification, both new grains and previous grains can growth toward new melt pool (which is solidifying) .

But if you ask about the effect of heat comes from new melt pool into already solidified region; you should know grain structure can be form in a range of temperature, for example for powder material A with melt temperature 1000oC, the grain evolution can be form in rang of 1000-995oC, with under-cooling temperature (5 oC).

then if already solidified region be colder than 995oC and heat comes from melt pool , it just can lead two effects:

1- change the phase distribution inside already solidified region

2- causes the grain growth ( courses grains)

But please not that in can be different for different materials.

Please feel free to ask anything you like.

I explain with details here: