Experimental And Theoretical Study On Liquid Metal Film Characteristic Of Armature/Rail Contact Interface In An Electromagnetic Launching

Electromagnetic railgun is an extreme sliding electrical contact with large current and hypervelocity. Under the pulse current action, solid armature moves forward at a high speed by electromagnetic thrust. Current density and thermal intensity of the armature/rail contact interface (A/R contact interface) are approaching the material limitation, and material melts would be happened on the armature contact interface. In this situation, the A/R contact status changes from solid-solid contact to solid-liquid-solid contact, which would be significant for the stability of A/R contact interface. Due to the dynamic characteristics of liquid layers, A/R contact interface could be lost contact because of liquid layer instability, and then transition is occurred. Transition is the main bottleneck of restricting the electromagnetic railgun application, and it is related to the metal liquid layer stability closely, so study on molten metal liquid layer’s generation characteristics, dynamic characteristics and stability, have a very important significance on the search for transition mechanisms and restraining transition occurrence.This paper begins with the mechanisms study of metal liquid layer, which is caused by A/R contact interface heat. Current melt wear effect and friction melt wear effect are studied by electromagnetic launching experiments, i.e., current melt wear is happened at low speed stage, and friction melt wear is at high speed stage. After lunching experiments, armatures are recovered, and then armature contact interface profile is measured by contourgraph and the contact surface morphology before and after launching is compared. The patterns of current melt wear and the laws of friction melt wear are summarized. The processes of metal liquid layer generation and development are displayed, and then, the generating characteristic and mechanism of metal liquid layer are analyzed at the same time.A magneto-elastohydrodynamic model is established for the whole metal liquid layer, and an equivalent beam model of armature tail is used to simulate the dynamic characteristics of liquid layer. Combining with electromagnetic field module, elastic fluid mechanics module, solid mechanics module and heat transfer module coupling simulation, the film thickness and pressure distribution of metal liquid layer achieve a balance. In this geometric model, Energy equation is presented to calculate temperature distribution in the metal liquid layer, and then, heat distribution and melting rate are calculated by the temperature distribution.According to the theoretical analysis of magneto-elastohydrodynamic model, it can be achieved by the COMSOL multhiphysic with multi-physics coupling iterative. Through a lot of calculation and comparison with different A/R relative speeds, the dynamic characteristic of metal liquid layer is analyzed. With the A/R relative velocity increased, the film thickness and pressure of metal liquid layer increased, the peak film thickness is194μm at the armature tail ending, and the maximum pressure is3.3X108Pa, located at the armature tail leading. The quantity of liquid layer deposition is calculated by the effective deposition height, and is compared with metal liquid layer generation quantity, finally, a criterion is proposed for the metal liquefied layer stable operation at the A/R interface.Analyze the model inadequacies, some model parameters are revised reasonably, such as the viscosity of liquid layer increased from0.0045Pa-s to0.017Pa-s, the thermal conductivity of liquid layer increased from90.97W/m/K to363.88W/m/K, the equivalent thermal conductivity of rail reduced from400W/m/K to40W/m/K, and the accumulated body temperature of armature increased from300K to673K. In the elastohydrodynamic model with revised parameters, the pressure, pressure gradient, film thickness and the effective deposition height of liquid layer are all increased, and the calculated metal liquid layer quantity very close with experiment result.Compared metal liquid layer deposition quantity with production, if taking the experiment result as metal liquid layer generation quantity, when the armature speed reaches2050m/s, liquid layer deposition quantity is more than liquid layer generation quantity, and then the electrical contact of A/R interface will not stable. If taking the calculated result of magneto-elastohydrodynamic model with revised parameters as metal liquid layer generation quantity, when the armature speed reaches2000m/s, liquid layer deposition quantity is more than liquid layer generation quantity, and then the electrical contact of A/R interface will not stable, transition may occur.