📄 chemistry

Interfacial Reactions Between Calcium Titanate Investment-Casting Shells and Different Titanium Alloys

This study investigates the interfacial reactions between calcium titanate investment-casting shells and three titanium alloys, revealing that while oxygen enrichment occurs in all cases, the Ti-45Al-2Mn-2Nb alloy exhibits a significantly thinner altered zone due to the formation of a protective aluminum oxide barrier that restricts further oxygen diffusion.

Original authors: Chongyang Li, Shibing Liu, Hongju Li, Tianyi Liu, Hongyu Liu, Weilong Wang, Weichen Qiu, Hewei Qu

Published 2026-09-08

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CC BY 4.0

Original authors: Chongyang Li, Shibing Liu, Hongju Li, Tianyi Liu, Hongyu Liu, Weilong Wang, Weichen Qiu, Hewei Qu

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Titanium is a metal prized for its strength and lightness, making it a favorite for building aircraft and medical implants. However, when this metal is melted down to be poured into a mold, it behaves like a hungry sponge. It aggressively grabs onto oxygen and other elements from the very container holding it. This contamination creates a hard, brittle skin on the surface of the final part, which can cause it to crack or fail under stress. To prevent this, engineers use a special ceramic coating on the inside of the mold, acting as a shield between the molten metal and the container walls. The challenge has always been finding a ceramic that is strong enough to withstand the heat but chemically calm enough not to react with the titanium.

Researchers at the Shenyang Research Institute of Foundry Co., Ltd. set out to test a specific type of ceramic shield made from calcium titanate. They wanted to see how well this material held up when used to cast three different kinds of titanium alloys under vacuum conditions. The team poured molten commercially pure titanium, a common alloy known as TC4, and a complex aluminum-rich alloy into molds lined with this calcium titanate coating. After the metal cooled, they carefully sliced the castings open to examine the boundary where the metal met the ceramic shell. Using powerful microscopes and chemical scanners, they measured exactly how deep the contamination went and identified what new compounds had formed on the surface.

The investigation revealed that the calcium titanate shell remained remarkably stable at high temperatures. When the shell was heated before the metal was poured, it did not break down or change drastically; it stayed mostly as calcium titanate, with only tiny amounts of other stable minerals forming from the binders used to hold the coating together. This confirmed that the material itself could survive the intense heat of the casting process without falling apart. However, the interaction between the shell and the metal was not entirely silent. The researchers found that oxygen from the shell did migrate into the metal, creating a contaminated layer just beneath the surface. Crucially, they found no evidence that heavy metal ions from the shell, such as calcium or zirconium, traveled into the titanium. The contamination was driven almost entirely by oxygen moving inward, not by the shell material dissolving into the metal.

The severity of this oxygen contamination depended heavily on the type of alloy being cast. The commercially pure titanium suffered the most, developing a contaminated layer that stretched more than half a millimeter deep. The TC4 alloy fared better, with a damaged zone roughly one-third of a millimeter thick. The aluminum-rich alloy performed the best by a wide margin, showing a contaminated layer that was barely fifteen micrometers thick. The difference came down to the chemistry of the alloys. In the pure titanium and the TC4 alloy, oxygen moved freely into the metal, hardening the surface and changing its microscopic structure. In the aluminum-rich alloy, the aluminum reacted with the oxygen first, forming a thin, protective barrier of aluminum oxide. This barrier acted like a shield, stopping the oxygen from traveling deeper into the metal and keeping the contamination zone extremely narrow.

The researchers also measured the hardness of the metal from the surface inward. In every case, the metal near the surface was significantly harder than the metal in the center, confirming that the oxygen had penetrated deep enough to alter the material's properties. For the pure titanium, the hardness dropped off sharply after about half a millimeter, matching the depth of the visible damage. For the aluminum-rich alloy, the surface reaction was so thin that the hardness change was subtle, yet the protective effect was clear. The study concluded that while the calcium titanate shell is not completely inert, it offers a promising balance of stability and performance. It prevents the shell from dissolving into the metal, and when used with alloys containing aluminum, it allows the metal to form its own protective barrier, resulting in a clean, high-quality surface ready for demanding applications.

Technical Summary: Interfacial Reactions Between Calcium Titanate Investment-Casting Shells and Different Titanium Alloys

Problem Statement
Titanium and its alloys are critical for aerospace and biomedical applications due to their high specific strength and corrosion resistance. However, molten titanium is highly reactive, readily interacting with investment-casting shell materials to form a hard, brittle, oxygen-enriched $\alpha$ -case layer. This layer degrades ductility, fatigue performance, and dimensional stability. While existing refractories like $Y_2O_3$ , $ZrO_2$ , $CaO$ , and $Al_2O_3$ offer specific advantages, none simultaneously satisfy requirements for chemical stability, thermal-shock resistance, processability, and cost. $CaZrO_3$ has shown promise but requires complex synthesis. This study investigates $CaTiO_3$ (calcium titanate) as a principal face-coat refractory, evaluating its interfacial reaction behavior with different titanium alloys under realistic vacuum casting conditions.

Methodology
The study employed a shell system where $CaTiO_3$ served as the primary face-coat refractory, bonded with zirconium acetate and stabilized with ammonium metatungstate. Three distinct alloys were cast under vacuum ( $\le 0.1$ Pa) using an 80-kg consumable-electrode skull arc furnace:

Commercially pure titanium (TA1)
Ti-6Al-4V (TC4)
Ti-45Al-2Mn-2Nb (TiAl)

The molds were fired to 1050 °C. Post-casting characterization involved:

Phase Analysis: X-ray diffraction (XRD) of the shell before and after calcination, and of the casting surfaces.
Thermal Analysis: Differential scanning calorimetry and thermogravimetry (DSC-TG) on the face-coat powder.
Microstructural Analysis: Optical microscopy and Scanning Electron Microscopy (SEM) with Energy-Dispersive Spectroscopy (EDS) to measure transition zone thicknesses and elemental distributions.
Surface Chemistry: X-ray Photoelectron Spectroscopy (XPS) to identify chemical environments and oxidation states.
Mechanical Properties: Microhardness profiling from the surface inward.

Key Results

Shell Stability: After calcination, $CaTiO_3$ remained the predominant phase, accompanied by minor $ZrO_2$ and $CaWO_4$ formed from binder decomposition. DSC-TG analysis showed minimal mass loss (0.7%) and no sharp thermal events, confirming high-temperature stability.
Interfacial Reaction Zones: Significant differences in reaction severity were observed based on alloy composition:
- TA1: Exhibited the widest metallographic transition zone (503.20 $\mu$ m) and subsurface altered zone (292.00 $\mu$ m). The surface was dominated by $\alpha$ -Ti and Ti-O environments.
- TC4: Showed intermediate reaction severity with a transition zone of 318.24 $\mu$ m and altered zone of 254.88 $\mu$ m. Surface phases included Ti-O, Ti-V, and Al-O related species.
- TiAl: Demonstrated the thinnest reaction zones (15.15 $\mu$ m metallographic; 12.48 $\mu$ m SEM-observed). The surface contained $Al_2O_3$ alongside various Ti-O phases.
Elemental Migration: EDS and XPS analyses revealed that interfacial contamination was driven primarily by oxygen enrichment and inward diffusion. No continuous inward migration of shell-derived cations (Ca or Zr) was detected in any alloy.
Mechanism: Thermodynamic calculations suggest that under vacuum, the reduction of $CaTiO_3$ by molten Ti produces volatile Ca gas, driving the reaction forward while preventing Ca accumulation in the casting. In TiAl, selective oxidation of Aluminum formed a stable $Al_2O_3$ barrier layer that restricted further oxygen diffusion, resulting in the significantly thinner altered zone.
Hardness Profiles: Microhardness decreased from the surface toward the interior for all alloys, correlating with oxygen diffusion depths. The hardness gradient extended beyond the visible microstructural transition zones, particularly in TA1 and TC4.

Significance and Claims
The paper claims that $CaTiO_3$ serves as a viable face-coat refractory for titanium investment casting, offering good high-temperature stability and chemical inertness regarding cation transfer. The study highlights that the severity of interfacial reactions is not solely determined by the refractory but is strongly modulated by the alloy chemistry. Specifically, the presence of Aluminum in TiAl alloys facilitates the formation of a protective $Al-O$ barrier, effectively suppressing oxygen ingress and minimizing the $\alpha$ -case thickness.

The authors conclude that while $CaTiO_3$ is not completely inert, its interaction is governed principally by oxygen transport rather than the transfer of shell cations. The study identifies that for TA1 and TC4, oxygen diffusion remains the primary challenge, suggesting that future improvements should focus on reducing $CaTiO_3$ surface activity or constructing more stable barrier layers. Additionally, the authors note that the compatibility between $CaTiO_3$ and zirconium acetate binders is limited, indicating a need for more stable binder systems to ensure long-term slurry stability.

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