constant,so the fusion rate of prefused flux mainly depends on the superheat. Because the temperature of liquid metal in ingot is usually about 1500C and the temperature of melting point of flux is only 1150-1250C,once the heat transfer makes the liquid temperature exceed its melting point,fusion process starts.Even if the superheat is not too high,the fusion rate will also be great. 3.2 Effect of Particle Size and Heat Transfer According to the classical theory,in the casc of using mold fluxes of same chemical compositions,the fusion rate of them becomes higher with the reducing in particle size.This effect is obvious until the particle size of flux is less than 150 mesh.This conclusion is presented based on the observation of the "point" melting behaviour.However,the measured results of "volume"melting behaviour of the prefused powder flux is opposite to it.It is presented that the reason of this phcnomenon is connected with the influence of particle size on heat transfer When temperature reaches the designed melting temperature of flux,(m), the thickness of fusion layer in the vertical direction is described respectively as h or hp in the case that two kinds of flux system which have different particle size.Because is grcater than h,according to the theory of heat transfer, flux mixed up with large particle powder is favourable to increase the fusion rate as a result of causing a large fusible interval.But the fusion rate of simply mixed powder flux is depentent on the dissolution rate of solid powder in liquid,especially for the flux whose particle size is larger,the dissolution will become the limiting step of fusing process.So it is impossible to obtain a conclu- sion that the larger the particle,the faster the fusion of flux.This is connected with the "point"melting behaviour.As long as the particle size of flux reaches a certain extent and has no influence on the dissolution rate,it is possible for flux which is comprised of large particle powder to have a strong effect on heat transfer and lead to a great change in fusion bchaviour. 4 Conclusions (1)The melting temperature of prefused flux is lower than that of simply mixed flux,and the tendency to form glass and the homogenization are also better in the case even the two kinds of fluxes have same chemical compositions and same particle size. (2)The difference of melting temperalure between them was reduced with the decrease in particle size.The sensitive extent of the melting temperature to the particle size of simply mixed flux is greater than that of prefused flux. (3)The expcrimental results of "volume"melting behaviour show that the 524， · 几 “ 了 一 “ ， ， 一 ， 。 ， ， “ 、 ， · 、 ， “ ” · 〕 · ， 二 ， 、 、 、 几。 ， ， 又 “ 一 ， ， · 一 ， · 、 “ ” · ， 才 月卜 、 ， 、 。 ‘ · 、 。 · ， “ ，， 尹