I am no expert on alum but learnen, Could it be the air die grinder contaminating the surface from the air discharge? everytime I try to use any air tools preping or cleaning alum I get little contamination from the oil-moisture.
Just a thought
Tom
Hello devo, first and foremost the words "cast aluminum" can sometimes be the kiss of death when related to welding and welding repair. You have no way of verifying the grade or quality of these castings, thus it is often a crap shoot to make successful repairs on these sorts of items. Your technique sounds very reasonable and likely will give you reasonable results for the scenarios of repair you have described. A few items could be considered to possibly increase the quality of the finished repair: you mentioned using a dynabrade belt sander, if you are using belts with a "carbide" component in the belt description this could possibly include some undesirable additions to the weld repair area. I believe the aluminum oxide belts are the preferred ones. One of the other posters mentioned pneumatic tools and the oil mist associated with their use. If the exhaust of these tools is directed towards the part then there is a possibility of some contamination because of this, so it is certainly something to consider as well, yet cleaning with a good grade of acetone should negate any undesirable issues. You will possibly find some information describing different types of acetone, some contain oils others do not, so try to select the type that doesn't include oils.
I have run into many cast aluminum repairs where there will be porosity along the toes of the welds or directly adjacent to the welds. I don't believe you can do anything about these due to their origins being in the castings themselves. A bit more for you to consider. Best regards, Allan
I've welded all kinds of cast aluminum crap,from outboard motors to Ford diesel bellhousings,transmissions,lawnmower parts,Motorcycle engine blocks everybit of it living in the nastiest unfriendly weld repair enviornment.
What we started out with,.......Simple green and smoking hot water and a stainless wire brush,Step2 was large fluted die grinder bit,3 laquer thinner,4 preheat to boil the junk out of the crack,5 more cleaning with a clean brush,6 more laquer thinner,7 put a pass in it,8 grind that pass out,9 put another pass in it,if porosity shows up,die grinder it out(used an electric grinder),10 fill er up with as many passes as it takes.
I did the skeg on my buddies outdrive when he forgot to lift it up at the launch(cause he was drunk),It took some doing but ended up good as new,like it never happend.
Steer clear of using anything with "Sand" such as polifans,bead blasters,the grinding belt your using,The grit breaks of and get's in there(because it smears rather than cuts) and creates the "porosity" look when it floats free and the weld metal solidifies around it,Same goes for welding on something that has been "Sand Cast" you'll grind and grind and while it get's less and less basically your displacing the sand with the fillermetal.Your fillermetal "patch" will be pure fillermetal but right next to where your welding will have all the original defects from the sand cast process.
In my experiance ...............unless it's one off,or a REALLY expensive part with minimal damage your better off to just get a replacement part.I spent a half day screwing around with the outdrive skeg,I spent a half day fiddling with a sidecase for a 4 wheeler, the new part was 300 bucks and the old one was HAMMERED.I got it fixed with no leaks per the dye pen but it was 5 hours in the making.If it wasn't a buddy project on my own time I'd never have taken either of them on because I'd feel guilty charging the time spent to some guy off the street.Plus those kind of "jimmy" repairs can last weeks or years depending on various factors.