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

这项为期两年的研究的目的是建立一种更准确地测量快速凝固组织与冷却速度之间关系的方法，并根据这种关系来考虑快速凝固组织的形成机理。首先，尝试了落管技术，即直径50-100m的合金颗粒在下落时通过激光熔化，然后通过强制对流换热进行快速冷却。然而，在室温下，由于颗粒向气体气氛中的热传递太大，用5kW的激光熔化颗粒是困难的。因此，采用了电加热器对流加热。此外，还开发了另一种技术，其中静止的粒子被激光熔化，冷却曲线由光传感器测量。该激光技术被应用于玻璃覆盖的颗粒的熔化和固化，以防止表面非均匀成核。然而，由于颗粒与玻璃之间存在间隙，很难快速冷却。通过铝合金和非晶态合金颗粒的冷却技术，获得了冷却速度(10^4-10^5K/S)与凝固组织关系的一些数据。在此基础上对快速凝固组织的形成机理进行了分析，结果表明，过冷度不仅与晶体形核有关，而且与晶体生长速度有关，对凝固组织也有很大影响。

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.