Effect Of DC Magnetic Field On Thermoelectric Power And Resistance Of Al-X Binary Alloy Melts

A lot of theories and technology practices show that electromagnetic field can significantly change the solidification behavior of metallic materials and can be applied to regulate and control the solidification structure. Therefore, the application of electromagnetic field in the process of solidification and casting has been developed rapidly and becomes an important means of metallic material preparation. However, the current researches mostly focus on its technological influences such as solidification refinement and macro-segregation depression or developing related new casting processes, few research work on the principles and mechanisms of electromagnetic field influence. In fact, the effect of electromagnetic field on metallic materials solidification is usually associated with its effects on the metallic melt structures, and finally with the effects on the behavior of nucleation and grains growth. So investigation of the effects of electromagnetic field on the melt microstructure and its corresponding solidification microstrucutre would contribute to understanding the mechanism of solidification behaviors under the electromagnetic field. As sensitive physical parameters to metallic melt structure, thermoelectric power and electric resistance are important parameters to characterize structural variation of metallic melt. Therefore, investigating in the influence of DC magnetic field on the rmoelectric power and electric resistance of the Al-X binary alloy melts through measuring their variations under DC magnetic field, and investigating in the influence of DC magnetic field on the structure of the metallic melt through the theories of metallic melt structure as well, which would provide basic data for better understanding the physical mechanism of alloy melt solidification behavior impacted by DC magnetic field and for further optimizing its effects in this paper.The article focuses on the effects of the metallic melt temperature, DC electromagnetic field strength and its applying time, and Fe content on the rmoelectric power and electric resistance of the Al-X binary alloy melts, and also the effect of DC electromagnetic field strength on the rmoelectric power and the electric resistance of the Al-Cu, Al-Ni and Al-Si binary alloy melts, by a self-designed device which can be used to real time detect both of the thermoelectric power and the electric resistance synchronously effected by DC electromagnetic field. The main conclusions are as follows:(1) The method and the corresponding apparatus are proposed and designed respectively, which can be used to real-time detect the thermoelectric power of molten metal and the resistance variation under several electromagnetic fields accurately and synchronously.(2) The results show that the metallic melt structure caused by electromagnetic field which is characterized by thermoelectric power detection has a higher sensitivity and can reflect more information about changes of the metal melt structure compared with the resistance.Melt variation that is caused by electromagnetic field can be explained by the theory of localized states-extended state transition and melt clusters.(3) The results show that the DC magnetic field can not change thermoelectric power and resistance of the solid metal, which should be related that the DC magnetic field can not change microstructure and electronic states of solid metal in the experimental conditions.DC magnetic field can change the metal melt thermoelectric power and resistance. Thethermoelectric potential difference transient variation△U(t) and lasting variation Au(t) are important parameterswhich respectively characterize extent of molten metal structure change and metal melt structure persistent change caused by electromagnetic field during loading electromagnetic field in the constant temperature conditions.After closing the magnetic field,variation of the thermoelectric potential difference Udecreasesand there are three stages: dramatic recovery, slow recovery and steady stage.The time△t1it takes to reach the stable stage is an important parameter that reflects the stability of melt structural changes caused by electromagnetic field.The thermoelectric potential difference transient maximum variation△U and residual volume△u are important parameters that reflect the greatest degree of melt structural changes and residual changes caused by electromagnetic field.The sample resistance maximum variation AR also reflects the degree of metal melt changes caused by electromagnetic field to some extent during loading electromagnetic field.(4) The effect of DC magnetic field on thermoelectric power and resistance of Al-Fe is related with Fe content. When the composition of molten metal is aluminum, hypoeutectic or near-eutectic composition, in the liquid region, thermoelectric potential difference U is reduced at low temperature with the DC magnetic field applying. On the contrary, U increases at high temperatures. When the composition of molten metal is hypereutectic, U is reduced at low temperature with the DC magnetic field applying. The maximum transient variation of thermoelectric potential difference△U is reduced in the solid-liquid phase region under any temperature, while it increases in liquid region. The higher magnetic field intensity is, the greater△U is. The effect of Magnetic field on△U increases with the prolongation of time. However, there is a limiting value;△R reaches the maximum value when the composition of molten metal is near-eutectic composition.(5) DC magnetic field has different influence on the thermoelectric power of binary alloy melt with different components, such as Al-xFe, Al-2.44%Cu, Al-7.37%Ni and Al-15.5%Si and so on.The change of electric potential difference is more obvious when DC magnetic field acts on the Al-xFe and Al-7.37%Ni alloy melts compared with Al-2.44%Cu and Al-15.5%Si alloy. The resistance of different Al-X melts all increases to some extent.