On healing metal damages using high-energy pulsed electromagnetic field

The concentration of the field takes place on the structural defects of the material, if it is affected by the electromagnetic field. In particular, it initiates electrical, thermal and mechanical processes in the vicinity of micro-defects (cracks, pores, inclusions, etc.). The transformation and interaction of defects are investigated in the article, e.g. the flat intergranular micro-cracks with linear dimensions of the order of 10 microns. These processes occur in the material when the metal samples are treated with a high-energy pulsed electromagnetic field which induces a short pulse of a high density electrical current in the material. The study uses the numerical coupled model related to the impact of the high-energy electromagnetic field on the pre-damaged thermal elastoplastic material with defects. This model considers the metal's melting and evaporation, as well as the dependence of its physical and mechanical properties on temperature. The system of equations is solved numerically using the finite elements method on adaptive lattices using the alternative method of Euler-Lagrange. The simulation shows that the treatment by the short pulse of current results in the welding of the crack and healing of the micro-defects. The healing occurs due to a simultaneous reduction of length, ejection of the molten metal into the cracks and closing of micro-crack edges. It leads to the fact that the edges of the crack start to contact the jet stream of the molten material, and, finally, the stream becomes completely jammed by the crack's edges. Meanwhile the volume of the micro-cracks starts to decrease in time. In this paper, the material healing and damage parameters are introduced for the macroscopic description of the healing process. The parameter of healing is determined as a relation of the micro-crack's change of volume to the initial micro-crack's volume at a time when the material is affected by the electromagnetic field. The damage (porosity) is understood as a ratio of the micro-crack's volume at a time to the volume of the representative element. The healing of micro-cracks increases the material's healing parameter and reduces its damage. The paper studies the changes in the material's healing and damage parameters depending on time under the action of the current pulse. The issues of selecting the preferred regions of integration in modeling the considered processes are researched. It is investigated how the distance between the micro-cracks and their mutual arrangement influence the healing and damage over time. The simulation of the considered processes in the entire investigated range of distances between the defects (or, for any initial damage equivalently) have shown that the dependences of the healing and damage on the time will not be different, no matter if we calculate these dependences in the regions of integration consisting of one or several representative elements. The arrangement of micro-cracks relative to each other and the distance between them do not affect the dependences of the healing and damage on the time under the current pulse. These changes are affected by the value of the initial damage only. The dependences of healing and damage on time will be practically the same for all different mutual arrangements of micro-cracks provided that the initial damages are equal for these different mutual arrangements of defects. Based on the simulation results, the approximate piecewise-linear dependences of healing and damage on time and the initial damage are obtained. It is clear that until a certain moment all the micro-cracks in the material (regardless of the initial damage) are not healed or damaged when they are affected by the current. After this moment, the process of micro-cracks' healing starts. Meanwhile, under the action of the current, the material's damage decreases over time at a constant rate (independent of the initial damage), while the healing increases over time at a rate inversely