Shell Materials and Casting Methods for Casting Titanium
The casting of Titanium alloys can be done with various primary, intermediate, and backup shell materials. There are also different methods of casting these alloys. Recommendations are given to minimize Alpha Case formation. Two case studies of Yttria and Zirconia prime layers of the shell are presented. Yttria generates far less Alpha Case. The economics of using Yttria is discussed.
The main difficulty with casting Titanium alloys is their reactivity with common elements in air like oxygen and nitrogen. Melting and casting of Titanium alloys needs to be done in furnaces with very high vacuum or inert gas to avoid dissolving oxygen and nitrogen in the molten Titanium. Oxygen and Nitrogen are quite soluble in molten Titanium metal. Upon cooling, these elements form oxides and nitrides that have a negative effect on the mechanical properties of the metal. In addition to atmospheric elements, molten Titanium is reactive with all ceramic oxides. This reactivity with ceramics is a major challenge for the investment casting foundry assuming the foundry has a suitable casting furnace. Molten Titanium alloys will reduce or react with ceramic oxides freeing oxygen which dissolves into the Titanium and upon cooling forms a contaminated surface layer commonly referred to as Alpha Case.