Tig Welding aluminum boat propellers and castings....
I am not talking about gross surface porosity that can easily be corrected by proper cleaning of the metal and adjusting a few parameters like shielding gas.
I am talking about fine porosity that shows up when you blend off the weld hoping for a smooth clean surface ready for plating, painting, or polishing. A good example of this is when tig welding aluminum boat propellers. When someone runs their aluminum boat prop into the lake bottom , usually it can be repaired by replacing the worn or chipped metal with weld metal. The problem is the any aluminum that has been in service like a boat prop or engine crankcase, soaks up some crap. Its like a sponge. Aluminum boat props always have some corrosion going on in the microscopic crevices of the metal and it seems to want to escape during the welding process. Sometimes, you dont notice it while you are welding and you think its all good until you blend off the weld. Then you see all these little pecker tracks that will look like fido's ass when its painted. So how do you get rid of it?
There are a few tips that help buy on aluminum castings like boat props and crankcases, you will probably never have a weld that is completely free of porosity.
Clean, clean , and clean , use a coarse carbide burr to dig out the rough fractured or worn surface. After that scotch-brite type abrasives work for removing paint and corrosion without embedding sanding or grinding grit. Follow with a good acetone wipe.
1. A good low heat bake cycle on a old gas grill turned on low heat sometimes helps. It gives you an excuse to drink a beer too. it takes exactly one beer to get warm enough. Dont wander off and dont drink 2 beers.
2. use a higher silicon filler rod for welding.4145 has about 12 % silicon and melts at a slightly lower temperature than 4043. When you ask the guy behind the counter at the welding supply store for 4145 aluminum tig rod, he is going to look at you the same way a teenager does when you talk about 8 track tapes but dont worry, he will figure it out and order it for you.
3. before you actually start laying down weld metal, take the tig torch and using low amperage, clean the area with the arc. Dont melt it just run the arc over the weld area and let the cathodic etching magic chase some of the demons out.
The key is PRACTICE PRACTICE PRACTICE before you even think about doing your own, expensive boat prop. Start learning on NEW 1/8" aluminum strips. Then, your best bet is to go to the junkyard and get some scrap aluminum parts like timing chain covers, or even aluminum heads. and PRACTICE running beads on some of the thinner edges. This will give you an idea of what happens on any aluminum casting when you light up on it with a tig torch.