Big Bertha Fusion Irons incorporate superior weight-shifting Fusion Technology and three distinct materials. The unique head shape provides a clean, refined look and unprecedented performance, yet provides the playability and confidence at address once reserved for oversize irons. It's another patented breakthrough, deeply rooted in our relentless endeavor to create the most scientifically advanced equipment for your game.The face is a lightweight, formed 6-4 titanium face insert. This grade of titanium is one of the strongest titanium alloys; the "6" refers to the percentage of aluminum (6%) and the "4" refers to the percentage of vanadium (4%) in the alloy. Using this material in the face of Callaway Big Bertha Fusion irons creates a clubface that is 35% lighter than a comparable steel face.