Observation of Rapidly Solidified Structure Obtained by Drop-tube and Laser Beam Melting Techniques

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The purpose of this two-year research is to develop a method whichcan measure more accurate relationship between rapidly solidified structure and cooling rate, and to conside the fornation mechanism of rapidly solidified structure based on the relation.First,a drop tube technique has been tried,where an alloy particle of 50-100 m in diameter is melted by laser irradiation during falling and subsequently cooled rapidly by forced convection heat transfer. However,it was difficult to melt the particle by 5KW YAG laser because the heat transfer from the particle to gas atmosphere at the room temperature is too large. Therefore, convection heating by electric heater was used. Further,another technique was developed where a particle at rest is melted by the laser and the cooling curve is measured by a photosensor. This laser technique was applied for melting and solidification of a particle covered by glass to prevent surface heterogeneous nucleation. However,it was difficult to cool it rapidly, because a gap has developed between the particle and the glass.Some data on the relation between cooling rate(10^4-10^5K/S) and solidified structure have been obtaied by the techniques for Al-alloys and amorphous-alloy particles. The consideration of the formation mechanism of repidly solidified structure based on the data revealed that not only the crystal nucleation but also the rate of the crystal growth has large effect on undercooling ,hence on the solidified structure.