This is a problem that is almost quintessential of pulsed MIG.
What you probably don't know is that for welding steels, pulsed MIG is costly and irresponsible gimmick. (Don't tell the folks from Lincoln I told you that....)
If you want to solve your severe lack-of-fusion and lack-of-penetration problems I strongly advise you to stop using pulsed MIG. Turn the pulsed mode OFF and switch to traditional CONSTANT VOLTAGE, spray transfer. (If you don't know how to do this consult your owner's manual.)
You may also wish to switch to Ar+15%CO2 shield gas. In my opinion Ar+2%Oxygen is better suited for short-circuit welds on thin gauge materials, than for spray welding of thick plate. Ar+CO2 mixes are both less oxidizing in nature, provide more weld energy, and generally provide more consistent penetration. However Ar+O mixes can be used for spray.
(EDIT: 70S-2 is perfectly FINE for this application. The two circumstances where you should choose 70S-6 is either using using 100% CO2 gas, or welding rusty, scaly steels. 70S-6 has a lot more deoxidizes (mainly extra Si+Mn) in it, which results in a slower freezing, much more fluid weld puddle. This can lead to cold laps and undercut in some circumstances.)
The optimal (CV) spray setting for .045" is around 500 ipm and 26-28 volts, which will draw about 280 amps. That may be a bit hotter than you've been accustomed to at this point. Trust me, that is a good thing. If you wish you can increase up to 570 ipm and 27-28 volts which will draw about 300-320 amps I believe. Please feel free to fine-tune your voltage until you get a nice quiet crackling sound, with very few "pops" or "plops."
Here a little extra info you may appreciate:
Pulsed MIG is an AC process. One wonders why would a person would choose to use a process that is at it's maximum current only 10% of the time, the rest of the time is at a low "background current?" With CV spray transfer, the arc is at it's maximum possible energy and maximum possible current 100% of the time. An additional problem with pulsed is that the voltage pulse used to pinch off a metal droplet also causes the wire tip to burn back several mm. This results in an excessively long, wide, low current, fairly diffuse arc that provides poor cleaning action and lacks concentrated heat in order to promote good penetration.
For steels traditional CV spray will always provide superior results for flat and horizontal welds. For out-of position welds, traditional flux-core wires like E71T-1 will always provide superior results. For welding thin gauge materials, the simple and easy-to-use short circuit mode is all you need.
Source(s): Welder and weld inspector. I don't sell crap, I buy it and then USE it.