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Research on electromagnetic centrifugal casting process and its influence on alloy solidification structure

1 introduction in recent years, the application of electromagnetic effect in industry, agriculture, medical and health care has become more and more widely and has aroused great interest. In particular, the application of electromagnetic effect in metallurgical production has made great progress after decades of exploration, and has been paid more and more attention by scientific workers. In 1982, the International Union of theoretical and Applied Mechanics (IUTAM) held the first International Conference on the application of electromagnetic fluid dynamics (MHD) in metallurgical production in Cambridge, England, pointing out that MHD can innovate the production process of metal materials [1]. After more than ten years of efforts, the International Conference on electromagnetic processing of materials (EPM) was held in Nagoya, Japan, in October 1994, which further established the status of the application of electromagnetic effect in metallurgical production, and also marked that it has become an independent research field. The influence of electromagnetic effect on metal solidification process is the most important aspect. For foundry workers, improving the quality of castings is the most important task to directly carry out a/d conversion without amplification, and the key to improving the quality of castings is to improve the solidification process of castings. Different from traditional methods, electromagnetic effect opens up a broad prospect for effectively controlling the solidification process of castings. The basis of studying the application of electromagnetic action in metallurgical production is electromagnetic hydrodynamics. Previous studies mainly focused on the flow caused by electromagnetic action in liquid metal, and then the flow changed the solidification process of metal. The influence of electromagnetic action on the physical properties of liquid metals will also affect the solidification process of metals, and because different substances have different electromagnetic properties, the influence of electromagnetic action on them will be different under the same process conditions. Therefore, it is necessary to explain several effects of electromagnetic action in liquid metals and the basic theory of electromagnetic hydrodynamics. 1.1 effect of electromagnetic action in liquid metal 1.1.1 magnetic properties of liquid metal metals or alloys often show different magnetic properties in liquid state from that in solid state, such as iron and carbon steel show ferromagnetism below Curie point, while paramagnetism above Curie point and liquid state. In most cases, liquid metals show diamagnetism or weak paramagnetism, but some alloy paper strength testing machines containing transition metal solutes, also known as paper strength testers, show greater paramagnetism in liquid state. At present, ferromagnetism and antiferromagnetism have not been found [2]. In general, the magnetization effect of magnetic field on liquid metal is very weak. 1.1.2 electromagnetic induction because the liquid metal is an electrical conductor, when there is current passing through the cylindrical liquid metal, the magnetic field intensity of the current at x away from the surface of the liquid column is [3]:h = I/2 π A2 (A-X) 1.1 when the liquid metal is under the action of alternating or moving magnetic field, or when the liquid metal moves and cuts the magnetic line of force in a constant magnetic field, the current will be induced in the liquid metal, and the induced current density is: J = σ V × B 1.2 high frequency induced current has skin effect, that is, the penetration depth of alternating magnetic field is related to its change frequency. The higher the frequency, the smaller the penetration force; The lower the frequency, the greater the penetration. 1.1.3 electromagnetic force (Lorentz force) the interaction between current and magnetic field in liquid metal generates electromagnetic force in liquid metal. The electromagnetic force is the volume force, and the electromagnetic force per unit volume is: F = J × B 1.3 the current in the liquid metal includes the current generated by the applied electric field and the induced current, and sometimes the current generated by the thermoelectric effect, that is, the thermoelectric effect current generated when there is a temperature gradient in the liquid metal, and the current flows from the cold end to the hot end. 1.1.4 when a current is applied to a liquid metal (conductor) with Hall effect and a magnetic field orthogonal to it is applied, an electric field perpendicular to both the current and the magnetic field will be generated. 1.1.5 thermal effect because the liquid metal has a certain resistance, Joule heat is generated when the current passes through the liquid metal. The heat released per unit time is: p=i2 R Ω 1.4 the thermal effect increases the temperature of liquid metal, decreases the viscosity and improves the fluidity. 1.1.6 changes in physical properties of liquid metal under electromagnetic action, due to the effects mentioned above, the physical properties of liquid metal will inevitably change. It has been found that the surface tension of liquid metal decreases under pulsed magnetic field [4], and the permeability and conductivity are affected by the frequency of magnetic field change [5][6]. At present, the influence of electromagnetic action on other physical properties of liquid metal is not clear. The in-depth study of this aspect will lay a good foundation for the application of electromagnetic fluid mechanics in metallurgical production. 1.2 introduction to application of electromagnetic fluid dynamics the science of studying the relationship between electromagnetic action and fluid flow is called electromagnetic fluid dynamics, also known as magnetohydrodynamics (MHD). It includes classical electrodynamics, magnetodynamics and hydrodynamics. Electromagnetic fluid mechanics mainly studies the phenomena and laws of magnetic fluid in macro and cosmic space, while the research in industry, agriculture, health care and other practical aspects is relatively a local (finite) space problem, so it is called applied electromagnetic fluid mechanics. The development of application of electromagnetic fluid mechanics can be traced back to 1823, when Farady envisaged using the induced electromotive force between electrodes to measure the motion law of fluid in the magnetic field. In 1923, CK applied for the patent of "smooth suspension melting after rough polishing". In 1932, aunbeck realized that rotating magnetic field can make fluid rotate. However, it was not until the 1960s that electromagnetic fluid dynamics was systematically developed to realize the development of recycling. After that, the principle of MHD was widely used in the metallurgical process of ferrous and non-ferrous metals, and progress was made in many aspects [1]. 1.2.1 basic equation of electromagnetic fluid mechanics Maxwell equation EH Δ× E=- μ 0 1.5 et Δ× H=J 1.6 Δ· H = 01.7 Ohm's law J = σ (E+ μ 0 V × H) 1.8 the electromagnetic force on liquid metal is: F = σ (E+V × B) × B 1.9 under steady state, the motion equation of Newtonian fluid is: ρ (V· Δ) V=- Δ p+ ηΔ 2 V + F 1.10 due to the slow flow rate of the fluid in the metallurgical device, the influence of the flow on the electromagnetic field is small (magnetic Reynolds number = μ 0 σ VL (1, L-length), can be ignored [7]. 1.2.2 the main applications of electromagnetic fluid mechanics in metallurgical production are electromagnetic stirring (EMS), electromagnetic pump, electroslag remelting (ESR), electroslag welding (ESW); electromagnetic casting (EMC) controls the shape of liquid metal; and

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