TIG on cast aluminum

I was wondering if anybody has some good info on TIG welding cast aluminum. Every piece of cast aluminum I try to TIG weld starts to get black soot in
the weld area immediately contaminating it and blackening my ceramic gas nozzles. I know there are different types of cast aluminum, some weldable, some are not. How do you tell? Can the black soot be overcome to produce an acceptable weld? I have been welding clean 6061 T6 aluminum for some time now and can produce very decent TIG welds on it but just cant seem to do cast. I know that cast aluminum can be TIG welded because I have seen cracked aluminum heads for car engines that have been repaired by TIG. I would be happy to hear back from anyone with experience and info regarding TIG welding cast aluminum.
Thanks. -Greg-
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Greg H wrote:

Hi Greg,
Cast aluminum can be the nastiest stuff to TIG weld. First thing to keep in mind is that the stuff is porous. Unless it was vacuum cast, or centrifugally cast, it will contain voids, and impurities. Impurities also range depending on the mold material. If it something like investment casting, there will be fewer contaminants, and if they were sand cast, expect more contaminants. Finally, if a porous casting was immersed in oil, or some other nasty chemical, expect to find that material embedded in the aluminum. Beware of exposure to noxious, or dangerous chemicals.
Many casting alloys are weldable, 4043 rod will stick to just about anything. When in doubt, it often does the job. If you want more strength, you can go for the 5xxx series filler, be aware that they will typically be more finicky to work with. Also remember that if the casting is bad in one location, 4043 filler will often be a marked improvement over the existing metal in a location. Often, there is a reason a casting fails.
Cleaning is the major watchword when dealing with castings, but there is only so much you can do. If there are oils impregnating the voids in the casting, they will drastically affect welding. A good cleaning regiment, including alcohol, and extensive brushing is a must. Preheating in an oven can sometimes help in burning off contaminants, and making welding a bit easier.
Using a high helium content shielding gas is a must. This will help burning off contaminants. Sometimes the stuff is so bad, nothing beats a hot arc to just get the stuff to melt. Argon is too hard to work with when heavy contamination is present, it is very hard to maintain a productive arc.
You will want to use copper backing plates to support thin sections. Because cast aluminum varies in behavior, you will want to avoid watching the work disappear. As mentioned earlier, because of contaminants, sometimes there is no color change when the aluminum liquefies. The copper sheeting can help when building up a section by providing support for the liquefied aluminum. It also sinks heat rapidly, often preserving the integrity of machined surfaces. While it isn't perfect, it does go a long way.
Make sure the part is well supported. If you are doing a lot of build-up, you will find that some distortion is inevitable. Having the part properly supported is sometimes the difference between a fixed part, and junk.
Finally, on a practical level, it is possible that you almost have to "replace" some of the base metal, just to get enough strength out of a part. Some castings are so bad, and sometimes just in small localized areas, that you have to weld it up, grind it away, build it up some more. Sometimes you will find that you have to "pump" large amounts of metal into a section to get it to some decent level of strength. To give you some perspective, one part I had to weld several times, and it just kept breaking at very low stress levels. Only after I systematically replaced the metal by repeated weld/grinding operations was there enough good metal to provide enough structural strength. While this isn't an exacting procedure, you will know when you run into this situation. Often, one tries to be as unobtrusive as possible, but circumstances sometimes require a firm approach.
Because of the porosity in the casting material, you will often have to add metal to make up for some of that porosity collapsing as you weld it up.
Sometimes, you run into quite good castings, they weld up very well, and the aluminum cooperates.
Setting the welding machine also helps. Sometimes, a lot of penetration is more useful, other times you will want a lot of cleaning action, and only superficial penetration. Some castings will require a mix, an initial superficial penetration to provide basic structural support, followed by more intensive passes to replace metal deeper into the structure. It depends how intricate the part, and how much latitude is available to effect repair.
Good luck,
Guy
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Some good stuff there. I would add the basic rules of welding any castings, whether they be bronze, aluminum or iron.
Chemically remove any contaminants, such as oils, paints, greases or waxes. Mechanically clean all surfaces to be welded, die grinders work well for this. V-grind out the crack. Preheat the whole piece to at least 500 degF. Lay in the welds in short stitches, using the least heat possible. Allow to cool as slow as possible, by wrapping in wool blankets, or burying the piece in vermiculite or powdered lime. Pray to your god of choice.
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Oh yeah, and see if you can find some 4047 filler rod. It has twice the silicon content of 4043, and therefore has less shrinkage, better wetting, and a lower melting point than 4043.
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