laserace,
I don't know much about laser welding and cutting, but "big" lasers are sommething I know about. I assume you are asking about a backup plate for a weld. We always used a piece of graphite for beam dumps for Multi-kilowatt class lasers. Little reflection off the surface and the surface slowely ablates when hit with a high power beam. A material called Grafoil works especially good. It is a sheet material made for vacuum (radiant) insulation. It has good heat transfer laterally, but poor thru the sheet. I had a real thin piece mounted on the wall opposite the aperature of a highly collimated 6kW laser and it only got hot. Never burned a hole in it.
Timelord