Laser Vacuum Welding of Aluminium

[mrpom]

I am currently welding joints on Aluminium EN AW 2219 without porosity, by welding cylindrical parts through a cylindrical vaccum chamber.

The vaccum chamber is made of PMMA (Plexiglas - Altuglas), thickness 2± 0.1mm, the intern presure is 5.10-3 mbar.

I am using fibre delivered CW 4kW ND:YAG.
I use a twin spot, beam diameter is 300µm and the two beams are separate of 300µm.

The distance between part's surface to weld and the intern wall of vacuum chamber is about 15 mm. The beam diameter on PMMA wall is about 6 mm

Effects on porosity are good (compared with welding under argon shielding) but I note some fumes deposited on each side of the weld joint surface.
In addition I note a light degradation of the polymer on intern side of the PMMA vaccum chamber

Is this fumes comes from the PMMA or of the aluminium?
Which is the transmittance to 1064nm for polished PMMA?
Does the beam degrade the PMMA on vaporize it?
In this case where is going the PMMA fumesn in weld pool or on it surface?

Any suggestions gratefully received!

Excuse me for my english, I'm not very good.