Hi there,
When ever we weld old crank cases and sumps etc we pre-heat many times until the oil and grease stops coming out of the cracks etc,
(you know, the dirty brown marks that seep out of the casting when you heat it up)
When all this stops happening, we clean in brake cleaner thoroughly and re-heat.
Welding is usually done with pure ally rods, or 5% silicone but no Mag content.
Set the square wave on -2 on our Fronius, (it will be different on other types) and use as little amps as we can get away with.
Good luck.
Julian