Papers by Keyword: Electromagnetic Processing of Materials (EPM)

Abstract: In this paper, some of our recent results in phase equilibrium, microstructure, texture and precipitation resulting from the application of an external high magnetic field during diffusional phase transformation in both medium carbon and high carbon steels have been summarized Their potential engineering applications are foreseen.

Abstract: The recrystallization behaviors of cold rolled aluminum alloys in electric field up to 400kV/mm and the phase transformation processes of proeutectoid steels under magnetic field up to 14 Tesla have been experimentally examined. It has been found that both the electric field and the magnetic field have influence on the evolution of texture and microstructure characteristics. During the recrystallization annealing under the electric field of the cold-rolled 3104 aluminum alloy sheets, the electric field postpones the recovery and recrystallization progress. First principle calculation was performed to study the electric structures of aluminum atoms and vacancies. It shows that vacancies that are helpful for recovery are electrically negative. As the sample worked as anode during electric field annealing, it was covered with positive surface charges that attract the electronegative vacancies in the vicinity of the free surface and annihilate them. In this way, the recovery and then the recrystallization are postponed. The magnetic field applied changes the precipitation sequence of transitional carbides during low temperature tempering that makes the relatively high-temperature monoclinic χ-Fe5C2 carbide precipitated without following the usual precipitation sequence, i.e. by skipping the precipitation of the usual orthorhombic η-Fe2C carbide. To reveal the working mechanism of this phenomenon, first principle calculations were performed to study the formation energies of the two iron-carbide systems and their electronic and magnetic structures and properties. Calculation results show that η-Fe2C has lower formation energy, which is proved by the formation sequence observed during the usual low temperature tempering process. However, χ-Fe5C2 has the higher magnetic moment, which enhances the stability under the magnetic field through magnetization. Therefore, under the magnetic field its precipitation tendency is increased.

Abstract: A static magnetic field and an alternating current are imposed on a metallic alloy during solidification for a crystal alignment of the primary phase. A Sn-10%Pb is selected as a sample because its primary phase is expected to have an anisotropic nature in magnetic susceptibility. In the x-ray diffraction pattern of the sample solidified without the magnetic field, the first and second highest peaks are (101) and (211) planes. On the other hand, those solidified with the magnetic field are (200) and (220) planes which are magnetically preferred planes. That is, the primary phase crystals in the sample solidified with the magnetic field are aligned to the specific direction.

Abstract: In the present study, the influencing factors such as the intensity and the direction of gradient magnetic fields, the magnetic susceptibilities of non-magnetic metals on the structures are studied theoretically and experimentally. In the theoretical analyses, the influences of high gradient magnetic fields on nucleation and structures are investigated. In the experimental research, high gradient magnetic fields are imposed on paramagnetic material Al and diamagnetic one Sn during their solidification processes. Then the macro- and microstructures of these samples are examined and the influences of magnetic susceptibilities of metals, the intensity and the direction of high magnetic fields are analyzed in details. It is found that solidified structures could be refined when the magnetization force and gravity were in the same directions, while the solidified structures could be coarsened and the coarse dendrites grew along the direction of the imposed magnetic fields when the directions of these two forces were opposite. Those phenomena could be explained from the views of reduced gravity and elevated gravity effects caused by magnetization force and the convection suppression effect caused by high magnetic fields. The results indicate that high magnetic fields can be applied to control the solidified structures of metals and then improve the quality and the properties of materials for different purposes.

Abstract: The new phase equilibrium of Fe-C diagram under magnetic field has been theoretically calculated. Results show that the magnetic field mainly shifts the γ⁄α+γ equilibrium line and the eutectoid point to the high carbon and high temperature sides. Based on this result, an experimental setup has been launched to investigate the effect of magnetic field on austenite decomposition in medium carbon and high carbon steels. The thermodynamic and kinetic effects of the high magnetic field on proeutectoid transformation at different cooling rates have been studied. It was found that for medium carbon steels, the magnetic field increases the amount of proeutectoid ferrite and accelerates the diffusional decomposition of austenite at medium and relatively fast cooling rates (10°C/min and 46°C/min). But there is no special grain growth along the field direction. The results led to a proposal of a new rapid annealing under a high magnetic field. However, when cooling is slow (2°C/min), the magnetic field shows a strong tendency to promote the proeutectoid ferrite grains to grow along the field direction through the magnetic dipolar interaction, which leads to the formation of an elongated grain structure. Moreover, the magnetic field also exhibits influence on the austenite decomposition in hypereutectoid steel by changing the amount of secondary cementite and lamellar spacing of pearlite.

Abstract: The distribution and solidified structure of alloying elements are important for the quality and the properties of alloys. In the present study, the solidification behavior of aluminum-rich alloys is studied under various high magnetic field conditions, and the influences of uniform and gradient magnetic fields with different intensity and direction on the distribution and the morphology of solute elements of Al-Cu and Al-Mg alloys are investigated. It is found that because of the differences of the electromagnetic force (Lorentz and magnetization forces) acting on Cu element and Mg element with different physical properties in the matrix, the regularities of distribution for Cu element and Mg element are opposite just in the intracrystalline and intergranular under high uniform magnetic field condition, and not only the content but the distributions of Cu and Mg elements are obviously different under high gradient magnetic field conditions as well. It can be concluded that high magnetic field has different effect on the solute distribution in alloys with different physical properties such as density, susceptibility, conductivity, etc. And the experimental results indicate that it is possible to control the terminal solubility and morphology of the solute elements in alloys by high magnetic fields.

Abstract: A new technology relating to crystal orientation and structure alignment has emerged by the development of superconducting technologies. Now, a high magnetic field covering a rather large space is available even in small-scale laboratories. Under this circumstance it has been found that the crystal orientation in materials can be controlled by imposition of the high magnetic field. This principle due to a magnetization force can be applied not only to magnetic materials but also to non-magnetic materials with asymmetric unit cells. In this paper, three novel processes for the crystal orientation of ceramics and metals are described.

Abstract: A 12-Tesla magnetic field was applied during the transformation from austenite to ferrite and then pearlite in a medium plain carbon steel at two different cooling rates. Results show that when cooling is slow, the magnetic field shows an effect of promoting proeutectoid ferrite grains to grow along the field direction that results in an elongated grain microstructure. However, when cooling is fast, the magnetic field mainly shows an effect of reducing the amount of low angle misorientations and increasing the amount of CSL boundaries. In addition, the magnetic field exhibits a slight enhancement of the <001> texture component in the direction that is perpendicular to the field direction (TFD).

Abstract: High magnetic fields were applied to the austenite to proeutectoid transformation and tempering process in a 42CrMo steel. The thermodynamic and kinetic effects of the high magnetic field on the austenite decomposition show that it can obviously increase the amount of the product ferrite and accelerate the transformation by enhancing the Gibbs free energy difference between the parent and product phases. Moreover, the magnetic field can considerably lower the amount of low angle misorientations of ferrite in pearlite colonies and obviously increase the frequency of S3-29 coincidence boundaries, especially S3 boundaries, of the ferrite. But it has no obvious effect on crystallographic orientation distribution. When the field is applied to the high temperature tempering process, it can effectively prevent the directional growth of cementite along martensite plate boundaries and twin boundaries by increasing both the cementite/ferrite interfacial energy and the magnetostrictive strain energy. Finally, particle-like cementite is obtained. The magnetic field also obviously retards the formation and growth of the ‘distortion-free’ regions of the matrix.