I have an aluminum square tube (diameter 1/2") that I would like to attach to a flat aluminum surface which has a circular hole of smaller diameter in it for water to flow through at low pressure. Would welding a flange to the square tube (with screw holes for going into the surface) be the best option, or are there better ways for making this kind of attachment? This is not a high pressure system so I don't think there will be any complications there.
If it needs to be removable, then yes, attaching a flange will be the best choice. If so, you'll need to put some sort of seal between the flange and the existing aluminum surface. The faying surfaces of aluminum will not, on their own, form a good seal. There are many types of seals, but an elastomeric gasket or O-ring will probably be your best bet. If you use an O-ring, it is best to put a groove in one or both of the mating parts so it is properly supported.
As for making the watertight aluminum-to-aluminum connection itself, brazing will probably be your best bet. Soldering is possible, but won't be as strong or reliable. Welding is possible, although a 1/2" tube is likely to have a thin wall, so you'd need a skilled TIG welder to make the joint both strong and watertight. Brazing is the best compromise because it will be reasonably strong, and doesn't take too much skill to make watertight. There will also be less of a chance of melting through the wall or collapsing the tube.
One thing that can make this kind of joint easier is running the tube through the flange, so it sticks out maybe 1/4" from the other side of the flange. This reduces the risk of deforming the tube as you heat it, and it's easy to grind or machine the extra tube off when you're done welding/brazing. If you can keep the gap reasonably small between the tube and the thickness of the flange, you'll also get some help from capillary action to make your weld stronger and more watertight.
Do keep in mind that some alloys of aluminum are significantly effected by heat, and heat as low as 400 degrees Fahrenheit can soften the aluminum permanently. If strength or hardness is important to you, you'll want to pay attention to what alloys you're using.
Yes, attaching the two parts with a flange is the first thing that comes to mind.
A few other options:
- Weld, solder, or epoxy the tube directly to the flat surface.
- Solder the square tube to a fitting whose outer diameter is the same size or slightly smaller than the hole.
- Reshape the end of the square tube to be round, and attach using one of the previously-mentioned methods.
- Seal the end of the square tube drill a hole the same size as the other hole, and join the two with appropriately-sized fittings (e.g., plumbing fittings).
Edit: You mentioned in a comment that the connection needs to be rigid, but otherwise you could clamp a hose around the square tube and attach the other end of the hose to a fitting installed through the hole in the flat aluminum surface.