The Formation Rule Of Metallic Hollow Microspheres During Quench Rapid Solidification

Metal hollow microspheres have various properties superior to the normal materials, so it becomes the hot spot of new material development. But there are many problems, such as complex processing, higher costs and unsatisfactory metal coating. In order to solve those problems, this article will prepare Ni-Pb hollow microspheres by quenching rapid solidification. The influence of different shear rate and composition on the spherical particles ratio, hollow microspheres ratio, single-hole microspheres ratio and the morphology of hollow microspheres are studied. And the formation mechanism of hollow microspheres is revealed. It shows that:The diameter of Ni100-xPbx (x=3,5,8,10, wt%) hollow microspheres are 70μm-800μm under various shear rate(dv/dy= 1.30×105 s-1,1.79×105 s-1,2.26×105 s-1). The ratio of spherical particles is about 40%-80%. For a certain Pb content, spherical particles ratio increases first and then decreases, it is the same as the certain shear rate.For a certain Pb content, both the ratio of hollow microspheres and single-hole microspheres first increase and then decrease, with increasing shear rate. The morphology of large-diameter hollow microspheres changes from irregular-shaped pores into more regular single hole and then to irregular-shaped pores. Most of small-diameter microspheres are single circular hollow microspheres, the inner surface microstructural morpholopy of which is small equiaxed grains. Under the certain shear rate, the ratio of hollow microspheres decreases, the ratio of single-hole microspheres first increases and then decreases, with increasing the Pb content. The morphology of large-diameter hollow microspheres also changes from the irregular-shaped porous into the more regular single hole and then to irregular-shaped porous. Most of small-diameter microspheres are single round hollow microspheres, too.The process of formation hollow microspheres is discussed as follows. The gas is pushed into the metal droplets by the pressure of outside turbulent gas. Under the surface tension and pressure gas, the bubble is wrapped in the droplet. Then the bubble is moved to the center of the droplet by marangoni convection and is eventually formed hollow microspheres under quenching rapid solidification.There are a lot of influencing factors during the formation process of hollow microspheres, such as shear rate, melt superheat, gas pressure, surface tension, cooling rate, melting point and viscosity. Turbulence intensity of melt flow is influenced by shear rate, melt superheat and gas pressure. Meanwhile, aeration degree is influenced by turbulence intensity of melt flow. The time of interaction between gas and liquid is influenced by surface tension, cooling rate and melting point. And the intensity of interaction between gas and liquid is influenced by viscosity.