It isn't unusual to have porosity in an aluminum casting. As far as getting all the oil out prior to welding, try soaking the casting in acetone for a couple of days. You can also use the acetone in an ultrasonic cleaner if you have access to one. The ultrasonic cleaner will enhance the cleaning action and reduce the time it takes for the acetone to soak in and dissolve the grease and oil.

You can set the castings aside for a day or so to let the acetone evaporate. Set the casting in the hot sun to raise the temperature without introducing the danger of igniting the fumes. I don't know where you are located, but here, any metallic object will heat up to the point where you can't handle it without gloves in a matter of a half hour or less if it is left in the bright sunlight.

I would resoak the part in acetone after preparing the weld groove (assuming one is needed). You will likely expose subsurface porosity when you make the groove by filing or machining. Make sure you tap the surface with a stainless steel brush to reveal the subsurface porosity that is concealed by smeared aluminum. Tapping the surface will cause a dimple if there is porosity present. Use a sharpened rod to "pop" the dimple and reclean with acetone. I like to follow the acetone soak with a soaking in isopropyl alcohol.

I don't know what aluminum alloy the bearing carriers are, so I can't suggest a filler metal. Remember, depending on the type of alloy the casting is made of, the strength and ductility of the aluminum can be adversely affected by the welding operation.

Best regards - Al

In addition to previous good advice be reminded that some alloys of Al, including many castings, simply cannot be welded. If filler becomes an issue ESAB and others make a flux-coated rod designed for arc but suitable for oxyacetylene which is actually a brazing rod and may have suitable properties which could avoid issues actual welding may present, perhaps doing what might otherwise be impossible.

Some luck and expedience by making what I myself call a sacrificial weld which I intend to cut out, along with the contaminants, and then reweld "for keeps"... it's the best procedure I've come across. In other words, heat the part past the point at which the grease will burn off. As long as porosity does not interconnect as do the pores in an open-cell foam air filter, the sacrificial weld should be effective.

Alternatively, or perhaps in addition, vapor cleanibng followed by ultrasonic cleaning will probably be only way to clean the micro-pores, otherwise, who is to say which direction... into or out of the part... that heat is driving the contamination? My enginework machineshop can provide these cleaning services, but if unavailable to you perhaps soak i a bath of digestive cleaner might work as an early step- the type of cleaner available through NAPA &c made by GUNK and other competitors which comes in a five gallon bucket which has a dip basket, typically sold as a carbourator cleaner. The actual digestive solvent name escapes me at the moment, but it is so dangerous and volitile that it (and you) is/are protected by a layer of light parrifin-like liquid which floats over the actual solvent in the bucket. It dissolves nearly everything except metal (especially paint, rubber, neoprene, skin, eyeballs, and other organics) this is NOT a safety-solvent or Guink engine-brite degreaser available everywhere including many supermarket grocery stores, this is for professional use and is rather expensive as well as hazardous and hard to dispose of.

BTW there are procedures whereby you can SAFELY use ultrasonic cleaning with dangerous highly volatile solvents, although it is easy to defeat the fundamental theory of operation of the ulrtasonic process. It's better to stick with detergents they typically use for a variety of reasons

Good Luck & Regards,
D