Some rivets might be made of aluminum, but most car stuff is steel. And again you'd end up with more sand. powdering aluminum isn't easy, if you use a grinder, you can cause the aluminum to ignite.. it's a combustible dust.. I've heard that old etch a sketchs have it in them. There's a YouTube video about thermite where a guy takes aluminum foil and folds it up and crushes it in a hydraulic press before shredding it somehow.