In my experience I have found that it it is very good practice to purge tubing I.D.'s on these materials when high quality is desired. In fact, many specifications and some construction codes require a back purge for 2.25% & higher Cr. base materials. I have not ran procedure qualifications with and without back purge for a tensile, ductility, charpy impact comparision. As far a CFM, this is trial and error. It depends on diameter, run lengths, etc. Your CFM rates should not have to be too high for 1" and 2" diameters (say, 10 - 25). What you are looking for is an I.D. inert atmosphere without creating turbliance. Be careful not to pressurize the pipe or tube I.D. As this will give you concave roots and a bunch of other problems. Normal procedure is to fill from the bottom (low point) and vent at the top (high point).

I rekon it will take 10 to 15 seconds to displace air inside a 2" x 6' tube at a flow rate of 1cfm (vol=Pi x Rsquared x length)(dissolving of (heavier) Ar into the air excluded; minimized by filling from bottom? Do we fill from top with He?)

I also have no experience upon which to refer. In that context is it necessary to use what seems to be a high flow rate (in comparison to static number above) to insure convection or breezes do not pollute the inert shield? What are the dynamics in question? Can I use the static numbers as a base to determine a ratio (considering VOLUME, not diameter) so we can compute a reasonable start point? (60 volumes per minute?)

Thanks in advance

Can't help with the aeromat, but in regard to the purging on aluminum, it depends on the weld joint. Fillet weld, no. Butt weld yes and no.
If you have an open root you will need to purge, there is a way to perform a butt weld without backing called an extended land bevel. While using any method the base material must be CLEAN to get a high quality weld. The best way to confirm proper purging is to use an oxygen analyser. A good one is approximately 3000USD.

Best regards, Brad