2015 Volume 33 Issue 4 Pages 365-375
This study clarifies the collective growth processes and size distributions of fume primary particles generated around an arc plasma in welding by numerical analysis using a model which can treat growth through homogeneous nucleation, heterogeneous condensation and coagulation among particles with any size distribution ranging from sub-nanometers to a few hundreds nanometers. In a representative case, almost all iron vapor is converted into iron particles in a very short time of 218 μs after nucleation starts. The particles grow mainly by nucleation and condensation in an early period, whereas coagulation becomes dominant after vapor consumption. The vapor consumption rate of condensation exhibits approximately 400 times as high as that of nucleation at their maxima. Homogeneous nucleation rate is high when both the temperature and the cooling rate are also high. In a region closer to the base metal, a fewer particles are generated because the cooling rate is lower and the vapor concentration is higher, which cause a lower homogeneous nucleation rate. As a result, heterogeneous condensation becomes more effective. Furthermore, the particles take longer time to reach their melting points. In consequence, the particles generated in the vicinity of the base metal have larger sizes.