Welding cast alumnium to 6061.

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I have a job i can take on.  I have to weld up a hole on the top of an alum transmission case.  PTO shaft broke and windowed the the top of this transmission.  There is a hole clear through about the size of a silver dollar and many cracks coming off of it like a cracked windshield.  I would like to weld all the cracks up, then wled a plate over the hole area so it doesn't leak anymore.  Im going to tig weld it.  What is the process for welding cast alum, and what rod should i use?  Any help would be greatly appreciated.Tom Rockwell Race Carswww.trracecars.com
Reply:Cast Al, not too different to wrought sometimes. Some castings weld cleanly, poor quality ones can be a pain with regards to dirt/contamination- especially older ones that have gone very porous and soaked up oil etc. I have been in situations where castings literally 'bleed' oil when heated!Need to know the alloy used for the casting to be definitive but 4043 is a good bet. 4047 is sometimes a better choiceAfter cleaning the area I find the ends of the cracks and drill a small hole before veeing out the cracks with a burr.Preheat depends on the complexity of the casting- complex castings i preheat properly (150-200C depending on just how complex) simple ones i just warm thoroughly. If i suspect there will be lots of contamination to deal with i run an arc over the area (just barely melting it) which can help pull the crap to the surface. A lowish heat input helps with regard to 'cooking' crap out of the casting
Reply:Trans casting alloy is A356 or derivative thereof.Trans, bellhousings, etc. weld okay with 4000 or 5000 series rod. 5356 is stronger, which normally is my choice.Expect you will see significant porosity in the casting. This is normal for any automotive castings. The newer ones are really, really porous.A pic or two would help, you know.Now, for 'THE BIGGEY':-Is the trans. repair expected to be done, while bolted into the vehicle?(It really should be out of the  vehicle, to remove the stress that's on it, when bolted in. Ideally, the part should be unbolted and repaired, as such--to prevent the below scenarios from biting you in the a**.)-depending on where the repair is on the casting and just how restrained this casting is.....you can encounter serious cracking--following right behind the weld bead..... or during your preheat  (which I do little to none of)....or welding--the casting decides to bust up some more. Sometimes heat is not our friend. There's ways to control and mitigate damaging heat input; which I've commented on in other posts, including  http://weldingweb.com/vbb/showthread.php?t=21408This is a 'Best-Effort' repair, depending on the unknowns I've mentioned, operator competency and equipment used.Blackbird
Reply:Yup..5356 filler.The yuck will bubble up to the top..Grind it off and repeat untill the weld is clean..Not a big deal.....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:zap s right   weld  clean weld clean you can do it slow an steady
Reply:I would highly recommend, prior to fixing the box, to first; drain it, degrease it, clean it of combustibles, and then prep for welding....... I've seen flames shoot out of the sides of a box becasue the gear oil got a little to hot...5356 has worked well for me on gearboxes from the 50's-2005....can you fill the hole with a rose bud rather than patching it ?-Graham-Mechanical EngineerAutosport Mechanic/Fabricator
Reply:Thanks for all the helpfull info.  Should be staring the jobe early next week maybe sooner.  I'll let you know how it goes.Tom Rockwell Race Carswww.trracecars.com