this IMHO
6010 root pass....no backing
fills with E70 GMAW 100% CO2
how we did it the old days
what a lie we filled with 7018
but times change
but what your asking calls for 100% CO2 for 9mm pipe with backing E70 GMAW
sincerely,
Kent

There are several ways to go.

If your pipe with backing is in the flat position (meaning rotated while welding), it is advisable to use GMAW in the spray transfer mode...  This could possibly be done with your 80/20  Ar/Co2 mixture, but would be more practical with say 90/10 or 85/15.   Spray transfer is a higher current alternative that would help ensure good fusion on your CJP Joint.

If you were working out of position, (pipe not rotated) than GMAW short circuiting transfer is probably what you are considering... This material thickness "9mm" is at the top of the range for dependable performance with short circuiting transfer and would require a much higher skill level from the welders than spray transfer on rotated pipe.   The gas used could be either the Co2 100%  or the 80/20...   The Straight C02 would have deeper penetration at a given wire feed speed, but in order to keep the joint spatter free, your operators would need very tight parameter controls and supervision.   The 80/20 would short circuit as well and have less spatter, but would sacrifice to some extent the penetration when compared to straight C02.

In order to get the best advice, you need to more clearly articulate the exact operations you wish to perform.   Unless you do, we are just guessing when we give you answers.

Short circuiting transfer on CJP structural steel components with backing is known to be problematic, to the extent that the AWS D1.1 code for structural steel requires both welders and procedures to be specially qualified by testing per Clause 4 to use short circuiting transfer GMAW on code compliant work of any kind.

had time to think
going to say 8" sch 40 pipe mild steel
here is how I would do it
open root
3/32" land
1/16" root opening
15 degree bevel on each pipe
for 30 degree bevel total
1/8" 6010 root pass DCEP @ 110 amps ( amps depend on the machine )
1/8" 7018 hot pass @ 135 amps DCEP ( same as above)
1/8" 7018 for fill and cover @ 125 amps A/C ( if you have an A/C machine ) if not 120 amps DCEP
use weave or wash style for fill and cover
I know better people here will tell I am wrong and stupid
just my thoughts
sincerely,
Kent
edit: this is from a old timer...the youngsters can tell how it is done now....my ideas and work thoughts go back to 1970... and then they had came from the 1930's...and yes I did know James F. Lincoln and did test his first welder
2nd edit: just OAW
swap the root and land 1/16 for land and 3/32 for root opening
9 psi for oxy, 5 psi for fuel
#2 tip
1/8" filler E70
that is OLD school