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Effect of Pressure Difference on the Quality of Titanium Casting

Department of Biomaterials Science, Baylor College of Dentistry-Texas A&M University System, 3302 Gaston Ave., Dallas, Texas 75246

J.H. Watkins

Department of Biomaterials Science, Baylor College of Dentistry-Texas A&M University System, 3302 Gaston Ave., Dallas, Texas 75246

H. Nakajima

Department of Biomaterials Science, Baylor College of Dentistry-Texas A&M University System, 3302 Gaston Ave., Dallas, Texas 75246

M. Atsuta

Department of Fixed Prosthodontics, Nagasaki University School of Dentistry, Nagasaki, Japan

T. Okabe

Department of Biomaterials Science, Baylor College of Dentistry-Texas A&M University System, 3302 Gaston Ave., Dallas, Texas 75246

In casting titanium using a two-compartment casting machine, Herø et al. (1993) reported that the pressure difference between the melting chamber and the mold chamber affected the soundness of the castings. This study tested the hypothesis that differences in pressure produce castings with various amounts of porosity and different mechanical properties values. Plastic dumbbell-shaped patterns were invested with an alumina-based, phosphate-bonded investment material. Both chambers of the casting machine were evacuated to 6 x 10-2 torr; the argon pressure difference was then adjusted to either 50, 150, 300, or 450 torr. The porosity of the cast specimens was determined by x-ray radiography and quantitative image analysis. Tensile strength and elongation were measured by means of a universal testing machine at a strain rate of 1.7 x 10^-4/s. The fractured surfaces were examined by SEM. Changes in Vickers hardness with depth from the cast surface were measured on polished cross-sections of the specimens. Raising the argon pressure difference to 300 and 450 torr caused a significant increase in internal porosity and a resultant decrease in the engineering tensile strength and elongation. The highest tensile strength (- 540 MPa), elongation (- 10%), bulk hardness (HV_50g 209), and lowest porosity level (- 0.8%) occurred in the specimens cast at 150 torr. Turbulence of the metal during casting was thought to be responsible for the increase in porosity levels with the increase in argon pressure difference. By choosing an argon pressure difference (around 150 torr) suitable for this geometry, we could produce castings which have adequate mechanical properties and low porosity levels.

Key Words: titanium • casting pressure difference • mechanical properties • porosity.

Journal of Dental Research, Vol. 76, No. 3, 773-779 (1997)
DOI: 10.1177/00220345970760031001


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