The ideal shielding gas is Helium since it has good thermal conductivity, does not ionize, and is inert. You will get the deepest penetration welds with Helium. Since it is expensive, it is often only used with CW CO2 laser welding where the likelihood of ionization is very high. If you are using a YAG laser, Argon is a fine choice. You may also try Nitrogen for Be-Cu welds since it will be even cheaper than Argon.
The shielding gas flow (direction and flow rate) should be controlled to make sure that any products of ionization (gas, metal vapors, etc.) are quickly washed away. Gas flow should be optimized; if it is too high and coming out of a small nozzle, it can acutually drag oxygen into the weld. A diffuser would be a good idea.
Filler materials are not typically used in laser welding. If you do need to use a filler for metallurgical reasons, you can place it in the joint in the form of a washer.
Girish
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