Variation in Alpha-Case Thickness of Ti-xAl Castings.

Area of Science:

• Materials Science
• Metallurgy
• Surface Engineering

Background:

• Alpha-case formation is a critical issue in titanium casting, negatively impacting surface integrity and mechanical properties.
• Interfacial reactions between molten titanium and oxide molds lead to undesirable alpha-case layers.
• Understanding and mitigating alpha-case is essential for high-performance titanium components.

Purpose of the Study:

• To investigate the effect of aluminum content on alpha-case formation in titanium.
• To evaluate the reaction kinetics between titanium melts and alumina molds with varying aluminum concentrations.
• To establish aluminum addition as a strategy for improving titanium casting quality.

Main Methods:

• Plasma arc melting and drop casting with alumina-based molds were employed.
• Cooling rate measurements and thermodynamic modeling were used to assess reaction kinetics.
• Microstructural analysis, hardness testing, and electron probe microanalysis characterized the alpha-case.

Main Results:

• Increasing aluminum content (0-52 at%) significantly suppressed alpha-case formation.
• No reaction layer formed above 30 at% aluminum concentration.
• Alpha-case composition included Ti3Al, TiO2, and Ti5Si3, with oxygen as the dominant reactive element.

Conclusions:

• Aluminum addition effectively minimizes interfacial reactions and alpha-case depth in titanium casting.
• Reduced oxygen diffusivity, shorter reaction times, and lower interfacial temperatures contribute to suppressed alpha-case.
• Aluminum is a key strategy for enhancing dimensional accuracy and surface quality in titanium investment casting.