"blow in" defect in CO2 laser beam welding / burning candle in the jar

[loak]

I'm trying to find the causes of the phenomenon called "blow in"

for those who don't have an idea of this phenomenon in welding application it's quite like the burning candle in a jar experiment :
put a burning candle on water then cover it with a glass the candle will stop burning and the water level in the glass will increase over 20% of the glass volume

it's the same with laser welding : when you try to weld a close chamber at the end of the weld when you stop the laser beam, you see the melt bath being soaked up inside the chamber this is "blow in"

I am trying to find the physics parameters behind thís sh... eeeerr this problem

Is there anybody who have a clue on this subject?????

thank you for answering

PS:
and thanks for having read my awful english

 

[GRoberts]

I am not quite understanding the description of the phenomenon you are experiencing with the laser welding, but as far as the candle, wouldn't the water be coming into the glass when the candle is extenguished because the air in the jar is cooling off and the pressure is being reduced, thus the water level in the jar rises to balance out the internal and external pressures on the jar?

 

[loak]

thank you for your answer

the pressure is of course the parameter the most important in fact the problem is how to provide the "blow in" or to regulate the pressure without drilling holes in the close chamber

 

[NickE]

Pull a vacuumn on the outside of the closed chamber?

ncik

 

[GRoberts]

What is in the chamber, inert gas or partial vacuum? If inert gas, can you provide a pressure relief valve (or vacuum relief valve) that is connected to an inert gas supply?

 

[sarinjk]

Hi "loak"

can you describe your chamber. material - weld configuration, penetration, laser parameters. did you try to cool the chamber? or tried to provide cooling closer to the weld joint.

Sarin
Scientist,
Centre for Laser Processing of Materials.

 

[loak]

the chamber is in fact an automotive shaft
in one side it's close by a "tulip"
on the other side it's close by a "stud"

it's a CO2 deep penetration welding
laser power: 6kW
frequency : 50kHz
feed rate : 2 m/min
the gas I combine with is He
the material is pre-heated

the problem has also to deal with the production setting limitation
so cooling/provide cooling yes I think it would work but I also have to deal with cost efficiency problem

I think the problem is not so simple to explain
I'll really need someone who was already in front of this problem

thanks to everybody for the answer