troubleshooting MIGed aluminum

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experienced aluminum TIG welder having difficulty with attempting first time aluminum MIG welding (pushed wire; no spoolgun).   setup is Millermatic 250x with U-drive rolls, M25 gun w/Teflon liner.  Running .030 ER5356 wire thru .035 contact tip with flush .625 nozzle.   straight argon set at 20cfm.    Using a push technique with nozzle angle 15 to 30 degrees.   I am getting an ugly weld.   It has black soot and pops and the wire blows back and lodges in the contact tip.     I'm thinking it might be lack of  shielding gas.  Should the contact tip be recessed in the nozzle?   any suggestions.  thanks.
Reply:I think you have the right answer.  I bet your argon is getting lost.  I have only done a limited amount of aluminum mig, but I have seen the same thing you are talking about.  Black welds with lots of snap, crackle and pops.  Recessed tip might keep the gas close to the wire and direct it better at the workpiece, but you also probably want to turn the gas up a bit more than when you tig and get closer to the workpiece.  I had problems with burnback into the tip and had to crank up the wire speed also.  I havent totally worked out these problems with my feeder, but it's better now than earlier.  I hate to say it, but your average spool guns really work better than your average feeders, but they are just harder for me to get used to.Smithboy...if it ain't broke, you ain't tryin'.
Reply:Also, couldn't hurt to go with a .030 tip.
Reply:Tip should be recessed a bit but it sounds more like wire feed and voltage issues. More than likely wire feed. Sounds like you need more to prevent the burnback. What kind of Al and what thickness are you doing? Black soot or smut is normal unless you are using the more modern technology of "pulse on pulse" mig technology.I mig 1/8" - 1/4" 5052 and 3003 aluminum almost a couple hours each day and I can tell you proper wire feed is paramount. I use 5356/ .035 by the way. - Good luck!
Reply:I'd match the contact tip to the wire size.  Too much clearance will cause the wire to arc against the sides erractically and stop the feed momentarily which causes the burnback and tip meltdown.
Reply:Tips for aluminum wire are generally about .005" oversized, or thereabouts, so you're actually probably pretty good with a .035 tip and .030 wire.Does the wire feed consistently through the gun without an arc?  It sounds like you may have feeding issues.  Unfortunately, aluminum wire just doesn't feed well in a push system.  You could try a shorter lead on your gun, or try to keep the the lead as straight as possible and don't use too much tension on the wire.-Heath
Reply:I agree with matching the tip to the wire diameter.  As Dendog said, too much clearance will allow the wire to wobble around the inside of the tip, and I have seen the wire arc against the inside of the tip and stop feeding.  Generally this occurs when you first strike the arc.
Reply:I mig aluminum all day everyday. I think if you use an oversized tip you subject the inside of the tip to not only arcing but spatter which seems to jam up the works. I'd also say turn up the gas to thirty pounds and make sure you have no leaks draining gas from where you want it. Make sure the tip is recessed in the nozzle a bit so the the gas is properly focused. Black welds can also be a result of too cold welds, too much wire speed, welding too dirty aluminum or the rare tank of dirty gas. Good luck.