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Skin holes of titanium casting
O Miyakawa1, K Watanabe, S Okawa
1Niigata University School of Dentistry, Japan.
Dental Materials Journal
|December 1, 1993
Summary
This study investigated refractories for titanium casting, finding quartz molds superior to cristobalite. Cristobalite molds with MgO improved casting but still produced holes due to forsterite reactions.
Area of Science:
- Materials Science
- Metallurgy
- Ceramics
Background:
- Titanium (Ti) casting is prone to surface defects like skin holes.
- Phosphate-free refractories are crucial for high-quality Ti castings.
- Understanding mold-metal reactions is key to defect prevention.
Purpose of the Study:
- To evaluate quartz (Qu) and cristobalite (Cr) based refractories for Ti casting.
- To investigate the effect of MgO addition on Cr-based molds.
- To analyze the formation mechanisms of skin holes in Ti castings.
Main Methods:
- Preparation of phosphate-free molds using Qu, Cr, and MgO mixtures.
- High-temperature firing of refractory molds.
- Investigation of casting cross-sections using optical microscopy and electron probe microanalysis (EPMA).
Main Results:
- Qu molds resulted in nearly complete castings with fewer skin holes and a layered reaction zone.
- Cr molds led to incomplete castings with deep reaction zones, Ti-Si phases, cracks, and holes.
- MgO addition to Cr molds improved castability and reaction zone structure, but introduced new holes attributed to forsterite-Ti reactions.
Conclusions:
- Quartz refractories offer better performance in Ti casting compared to cristobalite.
- MgO addition mitigates some issues with cristobalite molds but does not fully eliminate defects.
- The reactivity difference between Qu and Cr is a primary factor in skin hole formation, not mold strength.