Chengxian Li

The armature and rail (A/R) interface in railguns experiences extreme conditions of high current and velocity, leading to severe melting erosion. Experimental and numerical studies have demonstrated that convex rails exhibit superior launch performance compared with flat ones, with recovered armatures revealing central erosion patterns distinct from those observed in flat rails. In this article, a coupled Multiphysics numerical model is developed, incorporating variations in contact pressure and area, armature acceleration, and temperature dependence of electrical conductivity. The electromagnetic–mechanical–thermal characteristics of the A/R interface at the beginning stage of the melting erosion are analyzed by investigating the distribution of interfacial heating sources. The results indicate that the melting erosion process is affected by two stages, including the initial stage and the acceleration stage. The interfacial Joule heat plays an important role in the temperature rise during the initial stage, whereas the frictional heat dominates the temperature distribution as the velocity increases. Eventually, central erosion patterns are generated on the A/R interface as the temperature reaches the melting point.

During the high-speed motion phase of the armature in an electromagnetic rail launcher, the impact load may cause collision vibration between the armature’s tail and the rail, leading to a loss of contact between the armature and rail. This, in turn, can generate a transition arc, affecting the launch performance of the armature. This article establishes a dynamic calculation model of ultrahigh speed oblique impact for the armature/rail (A/R) interface based on the Johnson–Cook nonlinear constitutive model of materials. The model considers the strong coupling relationship between strain, strain rate, and pressure of the A/R materials. Based on this calculation model, the dynamic response of the armature structure during the oblique impact collision and the change in contact status between the armature and rail are studied. The critical conditions for the loss of contact at the A/R interface are analyzed, a new transition mechanism at the current falling edge is proposed, and the critical magnetic pressure and critical current line density corresponding to the transition under different impact angles and armature motion speeds are given. The findings of this article can help reduce the occurrence of transition phenomena during the electromagnetic launch process.

In a railgun, the armature and rail (A/R) interface is characterized as an imperfect sliding contact with high contact pressure and partial contact, resulting in distinct electromagnetic properties compared to those of a perfect contact. In this article, a numerical model for the imperfect A/R interface is established using a high aspect ratio thin layer, which compromises the accuracy of the Galerkin-finite element method (FEM). Linear shell element weak formulations are developed to deal with the thin-layer domain without mesh generation. Furthermore, the accuracy of the proposed formulations is validated through comparative calculations with Galerkin-FEM. The current distributions on the imperfect convex A/R interface are numerically investigated under both stationary and moving conditions. The results suggest that the current on the interface coincides with the contact pressure and area distribution, though the patterns differ between stationary and moving conditions. On the stationary interface, current diffuses from the trailing and side edges toward the central part, where the contact pressure concentrates. However, on the sliding interface, the current fails to diffuse inward, instead concentrating on the central region of the trailing edge, with partial concentration along the side edge.

The armature and rail (A/R) interface is a dynamic imperfect electrical contact (ImPEC), involving varying contact pressure and areas. In this article, the contact pressure and current distributions on the dynamic convex A/R interface are studied with boundary conditions for dynamic ImPEC. The numerical results indicate that the contact pressure has a central distribution pattern, and the contact area variation trend is similar to the driving current. Current distributions on the dynamic A/R interface with different motion states are different. On the stationary interface, the current concentrates on the trailing edge at the beginning, diffuses to the leading edge during the flat stage, and concentrates on the leading edge as the contact area contracts with the decreasing driving current. However, the current concentrates on the trailing and side edges of the sliding interface throughout the whole stages under low-velocity conditions. Comparative calculations have been conducted to investigate current evolution patterns on dynamic contacts. Results suggest that both stationary and sliding interfaces exhibit significant current fluctuations on contact area boundaries, which are concluded as forced shifts of current caused by dynamic contacts.

The armature and rail (A/R) interface in railgun is considered a dynamic electrical contact involving contact pressure and area variations, and the extreme working conditions lead to distinct electromagnetic properties from the bulk and perfect contact. This article investigates the mechanical and electromagnetic properties of the dynamic A/R interface based on imperfect contact boundary conditions. The contact area expands as the driving current increases, which increases the current distributed area and enhances uniformity. The current redistribution patterns are identified as forced shifts of current, with currents on the previous contact area boundaries decreasing and those on the existing boundaries increasing sharply. The analysis results indicate that the electric scalar potential difference and contact conductivity dominate the current variations on previous and existing boundaries, respectively. In other words, the conduction current and the contact state are the primary factors causing forced shifts of current rather than the induction current. Furthermore, the influences of dynamic contact conditions on the electric scalar potential difference are studied, and the results indicate that contact area variation has a more significant impact on the electric scalar potential difference. As the contact area changes, the conduction current paths shift with contact area boundaries, which causes variations in electric scalar potential differences, consequently, leading to the phenomenon of forced current shifts.

During the launch of the railgun, the interface of the armature/rail (A/R) will undergo severe melting and wear. The molten armature material will further solidify and deposit on the surface of the rail to form an aluminum deposition layer, which will change the original contact state and affect the subsequent armature launch performance. The evolution of the deposition layer under flat rails and D-shaped rails was experimentally studied. The study found that the deposition layer presents a changing trend of thickness on both sides and thin in the middle. With the increases in the number of experiments, the thickness of the deposition layer gradually accumulates, however, in the end, the thickness of the deposition layer tends to stabilize. Experiments have found that the horizontal distribution of the D-shaped rail deposition layer is more uniform than that of the flat rail, and the increase rate of the thickness of the deposition layer is lower than that of the flat rail. Based on the analysis of the A/R contact resistance and current density distribution, we have given an explanation for the difference in the thickness distribution of the deposition layer. In addition, the thickness of the deposition layer tends to be stable under the two rail conditions because the bond strength of the deposition layer and the rail decreases after repeated launch, and the tensile stress increases, which leads to the peeling off of the deposition layer.

The armature and rail (A/R) interface is an imperfect contact that is made at discrete asperities at the microscale resulting from high contact pressure. The current distribution of the interface differs significantly from the bulk behavior. In this paper, based on the contact layer model (CLM) and the Cooper-Mikic-Yoranovich model (CMYM), sheet element approximation and boundary conditions are proposed to analyze the electromagnetic properties of the A/R interface. Assuming zero gradients of the magnetic vector in the thickness direction, there are two ways for the approximation, which are mathematical approximation (MA) and physical approximation (PA). Results from both methods show high agreement, consistent with results from slit boundary conditions. Current distributions on both stationary and sliding A/R interfaces are numerically investigated. On the stationary interface, current diffuses from the edges to the central part of the real contact area, whereas on the sliding interface, current concentration occurs at the trailing edge due to the velocity skin effect (VSE). Furthermore, the contour of the current distribution aligns with the erosion pattern observed in experiments, validating the accuracy of the computational method.

In railgun, the melting erosion and deposition at the interface between the armature and rail have severe influences on the bore life and the launch performance. In this article, using a payload-separated method, experiments at a small-caliber rail launcher were performed to study the melting erosion and deposition on the convex rails. The erosion pattern of the armature surface, the mass loss of the armature, and the distribution of the deposition were measured. The results show that central erosion occurs, and edge erosion is significantly mitigated on armatures for convex rails, whereas flat rail presents an obvious edge erosion pattern. Moreover, convex rails reduce material losses of armatures and homogenize depositions. It is found that the initial contact pressure on the contact interface of the convex rail is less concentrated on edges, contributing to the mitigation of edge erosion, which not only allows the molten aluminum to be deposited more evenly but also prevents it from ejecting and damaging the bore insulator.

The Joule heat generated by the pulsed large current and the friction heat of the armature/rail (A/R) contact interface in the electromagnetic launcher causes the surface of the armature's tail melting. A part of the metal liquid film is deposited on the contact interface, a part of it left the barrel together with the armature, and the others was sprayed laterally, which damages the rails and the insulation. In order to lengthen the span of the launcher barrel's life, it is necessary to minimize the side-ejecting of metal liquid film. In this article, we obtained the 2-D Reynolds equation applicable to the metal liquid film. Using the finite-element software LS-DYNA and COMSOL, we have established a 3-D electromagnetic-elastic mechanics-hydrodynamics multiphysics coupling model considering the dynamic changes of magnetic pressure, liquid film's own acceleration, fluid pressure, and film thickness. We use this model to obtain the lateral film thickness, total pressure, and velocity distribution characteristics of the metal liquid film at any time. We have further studied the influence of armature velocity, stiffness of armature's tail, liquid film's own acceleration, and linear current density on the amount of side-ejecting of liquid film.

According to the lubrication theory, when the bearing capacity of the liquid metal film (LMF) is not enough to support the external load, the film will be unstable and at the risk of break. In this article, the criterion of the instability for the hydrodynamic lubrication at the armature/rail (A/R) interface is established, and the transition mechanism at current down-slope is proposed that the break of the LMF to degrade the electrical continuity across the A/R interface will lead to the transition. A 2-D Reynolds equation considering the self-acceleration of the film is derived. By coupling the electromagnetic field, stress, and fluid field, a magneto-elastohydrodynamic (MEHD) model is established completely. Based on this, the dynamic characteristics of the bearing capacity and external load are explored. The influence of the slope of the current drop on the transition moment and transition critical velocity is analyzed. The results show that: 1) at a certain moment of current down-slope, the bearing capacity of LMF is indeed less than the external load; 2) the larger the slope (absolute value) of the current drop, the earlier the transition occurs, and the lower the critical velocity of transition; and 3) the model gives an acceptable magnitude for the transition critical velocity that matches well with the experimental data, which indicates the criterion of the instability for the hydrodynamic lubrication and the transition mechanism at current down-slope are reasonable.