Surface reaction between titanium castings and investments

Area of Science:

• Materials Science
• Biomaterials Engineering
• Casting Technology

Background:

• The mechanical properties of titanium castings are significantly influenced by the surface-reacted layer.
• Choosing appropriate investment materials is critical for optimizing titanium casting outcomes.

Purpose of the Study:

• To analyze the interfacial zone of titanium castings produced with various commercial investments.
• To investigate the relationship between the surface-reacted layer and the hardness of titanium castings.

Main Methods:

• Casting of CP titanium disks using eight different commercial investment systems (alumina-, spinel-, zirconia-, and magnesia-based).
• Vickers hardness testing at 50-micron intervals from the surface to 0.5 mm depth.
• Metallurgical observation and electron probe microanalysis (EPMA) for elemental mapping of the reacted layer.

Main Results:

• Vickers hardness at 50 microns varied significantly (312–624) among investments (p < 0.01).
• The reacted layer thickness (10–60 microns) was rich in O, Si, Al, Zr, Mg, and P.
• No correlation between reacted layer thickness and hardness (p > 0.05), but alumina- and zirconia-based investments resulted in harder castings at 500 microns compared to spinel- and magnesia-based ones.

Conclusions:

• Alumina- and zirconia-based investments produce superior hardness in titanium castings compared to spinel- and magnesia-based investments.
• Molten titanium reduces silica, alumina, and zirconia, leading to diffusion of Si, Al, and Zr into the casting, altering mechanical properties.