Then make it 2mm thick. Not only will you have a hard time welding to 1.5mm without blowing through it, but a vertical hit to it will make a decent dent if not go through. I'd also advise stainless steel if possible. You can pick some stainless tubing up pretty cheap on ebay (it's used for those stupid boy racer exhausts).

I wouldn't use aluminium. You can't weld steel teeth to it.