Abstract
The microstructure change due to various air-vent systems in dental 84Ni-9Cr casting alloy was, similarly to the article(Part 2), investigated. As a result, cell structures which indicated equiaxed and cell-like morphologies were found at the interior and superficial layers of casting specimen in using both blind-type and closed-type air-vent systems at the 0 degree-sprue. On the contrary, cellular dendritic and cell structures which were observed in 45 degree-sprue and blind-type air-vent system, formed at the interior and superficial layers of casting specimen, respectively, which showed morphologically inhomogeneous distribution. Examining the shape and the size distribution of microstructures, the distribution of equiaxed and cell-like cell structures at 0 degree-sprue and blind-type air-vent system showed, independent of portions of casting specimen, the homogeneous state having approximate 20μm as average value. The cell-like cell structure at closed-type air-vent system(0 degree-sprue system)had, however, the inhomogeneous distribution at the superficial layers. In addition, arm length of cellular dendritic structure at 45 degree-sprue system ranged from 100 to 500μm near the sprue portion where had larger values than the other portions of casting specimen. Considering the corrosion property, cellular dendritic structure which showed strong Ni-segregation was heavily attacked and this corroded area was very larger than cell structures. Therefore, the blind-type air-vent system at 0 degree-sprue in high-melting 84Ni-9Cr alloy was optimum, which was different from the Ni-Cu-Mn alloy having the 45 degree-sprue and blind-type air-vent system as an optimum vent system.
