Effect Of Electric Current Pulse On Solidification Structure Of Metal

The electric current pulse (ECP) remarkably influences on solidification processes of metal. In recent years, studying effect and mechanism of ECP on solidification structure is an important item in field of material preparation. The effect of ECP on solidification process and structure of pure Al, pure Cu, and Al-Si, Al-Cu alloys is experimentally investigated by using self-designed pulse current equipment, and the refinement mechanism of ECP on solidification structure is analysed.Using pulse current with lower voltage and higher frequency investigates the effect of pulse current peak value, frequency and pulse width on solidification structure of pure Al and Cu. The results show that the solidification structure of pure Al transforms from coarse columnar crystal into fine globular crystal with increase of peak value current density and frequency or decrease of pulse width. However, the refining effect of pulse current with higher voltage and lower frequency on solidification structure of pure Cu is more than that of pulse with lower voltage and higher frequency. It is indicated that refinement of solidification structure of pure Cu must use a lager ECP, which reflects there is a closely relationship between effect of ECP on solidification structure and characteristics of metal.The influence of electric current pulse on solidification structure of hypoeutectic Al-Si and Al-Cu alloy is investigated. The results show that their macrostructure is remarkably refined by using electric current pulse. Moreover, with an increase of the peak value and frequency of current, not only the size of grain is decreased , but also primary a-Al phase of microstructure is transformed from dendritic crystals into equiaxed ones. For the microstructure of Al-Si alloy, quantity of primary a-A\ phase increases, and eutectic Si phase is changed short strip and well-proportioned distribution with ECP from long strip without ECP. For the microstructure of Al-Cu alloys, the secondary-arm gradually disappears. In addition, with the increase of Cu content of hypoeutectic Al-Cu alloys, the refining effect of ECP on macrostructure is weakened, which illustrates farther that there is a closely relationship between effect of ECP on solidification structure and characteristics of metal.The refining mechanism of ECP on solidification structure is investigated by studying effect of ECP on crystal nucleation and growth. Firstly, eight ECP treatment techniques are designed according to different solidification stage of pure Al. The experimental results show that the solidification structure can not be refined by exerting ECP on high temperature liquid phase. The ECP has also no obvious influence on the solidification structure when it is applied during crystal growth. However, the very fine structure is obtained by applying ECP during the nucleation of the melt. These verify that refining effect of ECP on structure of metal is promoting the nucleation rate. For making out the reason of enhancing nucleation rate, secondly, the experiments of wire netting beforehand putting into each sand mould and preheating mould are designed. The reason of enhancing nucleation rate is certify that the nuclei created on the mould wall are broken off and fall into the melt, and conformed that the nucleation rate increases by retarding the formation of the solidification shell.Based on the above results, the effect of ECP on solidification nucleation process is studied by theories of electrodynamics and electromagnetics, and established the mathematic model of nucleus multiplication by ECP. These show that refinement of structure relate with not only parameter of ECP but also characteristic of melt and material of sand mould, which make clear why has a different refining effect for various metal with same parameter of ECP, And make out the formation reason of shape of globular crystal zone by analyzing movement of nucleus under ECP.The effect of ECP on interface front of temperature field, solute field and interface morphology and stability is investigated during directional solidification. The results show that ECP can decrease solute concentration and solute boundary layer thickness nearby front of solid-liquid interface, and increase remarkably liquid temperature gradient at interface. Application of ECP could makes solid-liquid interface change from cellular to planar interface at the puling rate of 3μm/s, from cellular-dendritic to cellular interface at the puling rate of 14μm/s. When the puling rate is 96μm/s, application of ECP suppresses the secondary dendrites, and makes them shorten even disappear.The ECP improves the solid-liquid interface stability and increases the critical growth velocity that the interface morphology transforms planar crystal to cellular crystal and cellular crystal to cellular dendrite. It also shows that the cellular spacing and the mushy zone depth decrease with increasing current density.