When I read about titanium cookware, it says the exterior is titanium. The interior is usually DuPont Autograph 2. What is the advantage of the titanium? I thought that hardening the cooking surface so that it won't be damaged was the main goal. Also the pan itself is usually aluminum (anodized). Isn't aluminum unsafe to cook with? I want to buy new cookware and don't want to regret my purchase later.