How it is confirmed that these are bubbles? These are may be pores.

It is highly likely, that the 'some bubbles' are pores which appeared due to high content of gases was in the base metal before welding

Also this problem may be due to poor edge preparation (presence of oxide layers, dirt, etc.)

Thanks Mikhail, Yes my first thought was oil residues on the surface.

This was supposed to be "high purity" copper (normally there should have been much oxyde) so it either a misuse of sample from our part or something wrong from where we bought the copper.

thanks again.

@mukul we just saw some bubble under a skin of copper as you and mikhail said there is surely porosity.