The axle tube doesn't really hold a lot of fluid when the axle is in level ground. Like Brad mentioned, the axle tube is going to be plenty to disipate the heat. I would worry about the axle seal more than the fluid since that is much closer to where your going to weld. My best advice in this situation is to get a towel wet in cold water and place it at the flange area, just keep your weld area dry. Don't stay on the bead too long where the heat just keeps building up and you'll do just fine. For a good result, after using a grinder or a wire brush on the surface where your going to weld, wipe it with a cloth that's wet with acetone or alcohol. This will clean the surface even more thus a better burn with the weld.