Yes, on steel and alu both. I can't count the number of owner operaters and fleet owners that have asked over the last 15 years or so if I could patch up the old junk they own. At some point (and it is usually at the exact same time that the equipment appears on the market for sale, if it wasn't nickel and diming the owner to death, he wouldn't be selling it) dump beds (and tractors as well) are only usefull as scrap. I can't make a call on your dump bed from afar, but judging from your description of the numbers of repairs, it is probably well past it's usefull income producing life.
Then you can get away with a lot of things a for profit trucking co. can't. Like frequent down time. It's like the difference between a business that needs to do some welding every once and awhile, and a company that runs their welding machines all day long and starts losing money the minute a machine goes down. The former can get by nicely with semi reliable equipment that would kill the later.
They haul rock/dirt/asphalt/sand just about everywhere so finding a welding company that is experienced in alu dump bed repairs should not be too hard. An awful lot of portable welding outfits have a spoolgun tucked away for this sort of thing. Finding one that wants to run a class to teach you how, might be more difficult : )
JTMcC.