交变磁场对激光熔覆铁基复合涂层宏观形貌的影响及其微观组织演变

摘要

通过特殊设计的电路及作用线圈制作了交变磁场发生装置,并用其研究了不同磁场强度对激光熔覆铁基涂层宏观形貌和显微组织的影响.基于电磁学及金属凝固原理,揭示了激光熔覆涂层的固化过程和磁场诱发熔覆涂层柱状树枝晶向等轴晶转变的主要机制.实验结果表明:在交变磁场作用下,熔池金属液表面产生的趋肤效应和交变电磁力使凝固后熔覆层的表面形态呈波浪式,熔高和横截面积均随磁场电流的增加而减小,但熔宽变化不大.熔池内部产生的电磁力驱动熔体流动使树枝晶熔蚀和机械折断,游离的破碎枝晶成为新的形核核心,增加了形核率,从而促使熔覆层顶部组织由树枝晶向等轴晶转变.随着磁场电流的增加,等轴晶区扩大,但涂层底部的组织变化不明显.%A alternative magnetic field generator is prepared by special circuits and active coils and the effects of different magnetic field intensities on the macro morphologies and microstructures of Fe-based composite coatings are studied by laser cladding process. Based on the electromagnetic principle and liquid metal solidification theory, it describes the solidification process of laser cladding coatings and reveals the main mechanism that magnetic field induced columnar dendrites transform into equi-axed crystals. The results show that the skin effect of liquid metal molten pool surface and alternating electromagnetic force allow the solidified cladding layer surface morphology to the wave form. The height of cladding layer and cross-section area decrease with the increase of magnetic field current, but the cladding layer width changes a little. The electromagnetic force in molten pool interior drives the dendrite ablation and mechanical break, and the free broken dendrites become a new nuclei to increase the nucleation rate, by which the top microstructure of cladding layer transforms from dendrite crys-tals to equiaxed crystals. Furthermore, the equiaxed crystal zone becomes wider with the increase ofrnmagnetic field current, but the bottom microstructure has a little change.