Tig T joints question

admin

As some of you may have read I had taken a test involving thin (.040 - .075 ) aluminumT joints using Tig.   At home I was practicing the same configuration to better result.Here is something I noticed but have never thought about till now.I noticed that I often have somewhat dirty starts at the beginning of the T joints- say thefirst 3/8 inch or so.  That area is never as clean as an inch later.   Then I tried taking the electrode and moving it awat from the corner - out about 1/2 inchout on the flat face.  Perfectly clean start.   Back in the corner - dirty start.  I ws thinking about the geometry of the corner and what the gas might be doing at it streams into the corner.   Then I remembered how a laser beam reflects 180 degreeswhen you shoot it into two mirrors placed at 90 degrees - like the T joint geometry.Here is my thought ----   could the gas stream be retro reflecting from the faces in such a way that it is hitting the face- bouncing across to the other face and then backtoward the torch cup ?   and in so doing this could this reflecting action be leaving atunnel of air near the bottom of the V of the T joint ?     Could this explain why it seemsthat getting a clean puddle in the corner is harder than outboard on the face ?????I can see that the electrical field situation down in the corner is kind of unstable -the electrode is always closer to either face than to the exact vertex of the corner joint.( just exactly reverse of how the OUTSIDE corner joint would be .... where the arcWANTS to go the the outside vertex because thats the closest point on the work )So if there is some kind of reflection going on with the shielding gas -- how do I defeat this problem ??????I have both regular cups and  gas lens types.  Any ideas ??????????If the gas is reflecting - am I the only one who DIDNT know about it ???? I am just thinking out loud here !  Trying to figure out how to do a better job !Tim
Reply:A few questions first...what gas flow rate and cup size are you using? it certainly sounds like gas turbulence is the issue.when you say the 'corner' - is the corner between 3 pieces of metal or just 2? Is it just when the cup is close to the fillet? Might help to include a sketch to show exact how your issue arises.- Mick
Reply:try lowering the flow rate..and increase the preflow to maybe 2-3 seconds..
Reply:You could also try adding run-off tabs to your plate, so you won't be starting right at the corner.
Reply:Another thing to watch is your torch angle. Out near the edges of a fillet weld a high torch angle is more likely to entrain air into the gas stream - so less 'push' angle on the torch may help too. - Mick
Reply:Usualy I start the T joint (two 1 by 3 inch cupons tacked on ends)  at a zero or very slight (compared to when I am dipping and moving )  push angle.  Normaly I start inboard about 1/4 inch from the endand alternate between the two faces untill untill the heat builds up and I am getting puddles on each face - and then I try to do a fast dip (wipe?)  to get a small ammount of filler on one puddle - then theother face and then dip between them which (hopefully)  establishes a nice clean, wetted connection wetween them.  If that went OK then I let it cool for a few secconds. - then start the bead.  Often that first dip is a little dirtier than the rest of the bead.I have tried propping another piece of scrap up on the end to form a 3 sided pyrimid but didn't seemto help at all.   Now that I consider it, on a flat piece of work where clean starts seem easy and routine - there is NO resistance to the gas stream spreading out 360 degrees equaly and uniformly.But on a T joint - north and south are kind of blocked, forcing most of the gas to go east andwest to get out of the way of the gas behind it.  I can immagine if the gas is flowing fast -thismight make the two streams kind if curl back on its self -  sort of behaving to "suck in" airfrom the top of the north-south faces.  like  entranement.   Nest time I am practiceing I will try some flat and see how low I can turn the gas till I get dirt-then switch to T joint and turn it down more and see what the results are- if I can go way lowerthan flat fefore I get black crap.  I wonder if anyone has done a computer numerical simulation of what the gas is doing so you could see it and play with the variables ??????????????????????????????????????????????I used to do a little of that in school long ago.Tim
Reply:Damn, over-thinking this possibly??I bet its dirt on your fill rod.
Reply:Did you clean both sides of the top piece and the bottom where it hits the bottom piece?Clean is EVERYTHING when it comes to aluminum.....zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:Ryan.....  Yeah maybe !Zapster......  Yes usualy but not allways maybe - but I do know better !  Thanks !Is there a special torch angle which helps when starting the first dolips on the facesand then connecting them?  Usualy I use about zero push angle......  or do I need to lay it over quite a bit ? (like 15-20 degrees ?)Tim
Reply:ideally  you get a puddle immediately from both sides..try shorter arc length.....
Reply:Originally Posted by jethroRyan.....  Yeah maybe !Zapster......  Yes usualy but not allways maybe - but I do know better !  Thanks !Is there a special torch angle which helps when starting the first dolips on the facesand then connecting them?  Usualy I use about zero push angle......  or do I need to lay it over quite a bit ? (like 15-20 degrees ?)Tim
Reply:Originally Posted by jethroI ws thinking about the geometry of the corner and what the gas might be doing at it streams into the corner.   Then I remembered how a laser beam reflects 180 degreeswhen you shoot it into two mirrors placed at 90 degrees - like the T joint geometry.Here is my thought ----   could the gas stream be retro reflecting from the faces in such a way that it is hitting the face- bouncing across to the other face and then backtoward the torch cup ?   and in so doing this could this reflecting action be leaving atunnel of air near the bottom of the V of the T joint ?     Could this explain why it seemsthat getting a clean puddle in the corner is harder than outboard on the face ?????...So if there is some kind of reflection going on with the shielding gas -- how do I defeat this problem ??????I have both regular cups and  gas lens types.  Any ideas ??????????If the gas is reflecting - am I the only one who DIDNT know about it ????
Reply:Ten:Perhaps you are taking my anlogy of light to gas too literaly.  I only threw that out to illustrate whatgave me the thought of reflection.   I would say gas can bounce and reflect - its just that those effectsmight often be overcome by the other forces involved with fluid behavior.  As you mention- at too high a flow rate - stuff happens.   Instabilitys of flow, cavitation, turbulence,boundry layers, intranement of air ... since I can't see it I really don't know.  And you are  1000% correct about it being done correctly every day- thats why I KNOW it is something I am doing different than the expert example.   I have seen my teacher do it and get a PERFECT start.  Then he would go back to his officeand I'd do it with the same settings intact and get a a little dirt.   I am only struggleing to find the out-of-whack variable so I can fix it.  It may just be too much gas.  I will take pics and attempt to figure out how to post them (the hard part)Perhaps , in general, I need to be more consious of ALL the variables when setting the settingswhen I change to a new size of work.I do admit to overthinking ...  its a brain disease I think.Try do get pics this week.T