Abstract
In tin bronze castings, gross shrinkage defects are hard to occur, so far as melting is carried out carefully, hence it has often been misunderstood that there is little trouble in producing sound tin bronze castings. This seems to be due to the fact that the shrinkage cavities of bronze castings are often so microscopically fine and dispersed that the porosities may not be detectable. The purpose of this research is to clear up the mode of shrinkage defects and to investigate the effect of risers and end portion on the soundness of tin bronze castings. The specimens of tin bronze castings were made under various casting conditions, and X-ray inspection of porosities were carried out on comparatively thin slices which were cut off from each specimen. Porosities revealed in castings subjected to radiographic inspection and effects of riser and cooling action of the end portion of castings on the soundness of tin bronze castings were also investigated. Some physical properties of castings were also measured in relation to the defects. Furthermore, a similar experiment was carried out on brass and aluminum bronze castings for a comparison with tin bronze castings.
Results obtained are as follows. In tin bronze, feeding of metal during solidification is essentially difficult because of its wide freezing temperature range, and the occurrence of fine shrinkage porosities is inevitable. Shrinkage defects are varied by the mode of crystalline growth, and the growth of equiaxials is accompanied with dispersed microporosities, while columnar crystals cause centerline shrinkage cavities. In either case, riser has little effect upon the soundness of castings except the initial stage of solidification so far as shrinkage cavities are concerned. In columnar crystal structure castings, shrinkage cavities are concentrated to the centerline portion, and the sound end zone produced is longer than the case of equiaxial one. The sound end zone is about 4D mm long in plate castings and 10√D mm in bar castings, so far as crystals grow in columns. And the length of zone is affected by the pouring procedures. Horizontal pouring is more effective than vertical. If pouring is made vertically, top gating is more desirable than bottom gating, and pouring should be done slowly. And also, gases dissolved in tin bronze have detrimental effects upon the soundness of castings. As to casting defects, brass and aluminium bronze show different behavior from tin bronze, and feeding of metal can be made so effectively that riser effect is obvious and the occurrence of shrinkage defects in these alloys resembles to that of steel castings.
