[DonRotolo]

As others have noted, it is possible to bend 1.5" square tubing. But it is not easy unless you have the right machinery, and that is expensive. You also need the right kind of aluminum, and 6061-T6 is not it. Do your homework to find a better material.

Your best bet is cut and re-join using gusset plates at the corners.

That being said, you can make a fixture that will improve your chances of success. The first part is two flat plates to prevent the top and bottom walls of the bend from splaying outward or folding. The second part is filling the tube tightly with dry sand to help maintain even pressure and, again, help prevent folding. The last part is a curved wooden "buck" to bend against to maintain a reasonable radius.

So, this fixture might look like two 12" square pieces of plywood, screwed together through a 1.5" thick piece of wood cut to a curve of about 6" radius, and held tightly in a vise. The sand is held in place with wooden plugs or a few layers of duct tape. Use a large piece of heavy pipe to make the bend, and you need to figure out a way to hold the tube as you pull on it.

Even then, practice first, because that's not trivial to do.