Study On Liquid Metal Film Stability Of Armature/Rail Contact Interface Based On The Magneto-elastohydrodynamic Model

The armature/rail contact in electromagnetic railgun is an extreme sliding electrical contact. There exists a thin liquid film in the interface between armature and rail, which is caused by melting wear of the armature material due to high interfacial Joule heating from electrical current flow concentrated at the a-spots and friction heating arising out of high contact pressure in which thermal intensity of the armature/rail contact interface(A/R contact interface) is approaching the material limitation, and the A/R contact status changes from solid-solid contact to solid-liquid-solid contact. The film instability, caused by the dynamic characteristics of liquid layers, can lead to the occurring of transition, which is an unavoidable and urgent critical bottleneck in railgun technology and is closely related to the stability of the A/R interface. Hence studying the dynamic characteristics and stability of liquid layers is of importance to reveal the nature of the film and a good way to find the possible reason to the transition.A magneto-elastohydrodynamic model is established for the whole metal liquid layer, including coupled electromagnetic field module, elastic fluid mechanics module, solid mechanics module and heat transfer module. It can be achieved by the COMSOL multhiphysic with multi-physics coupling iterative, and the dynamic characteristic of metal liquid layer is analyzed. With the A/R relative velocity increased, the overall pressure and maximum pressure, about several hundred MPa and located at the armature tail leading, of metal liquid layer increased, and the film thickness increased firstly, and it tends to saturation and even decreasing during current down-slop, and the peak film thickness at the armature tail ending is around one or two hundred ?m.A criterion based on melting deposition ratio is proposed for the metal liquefied layer stable operation at the A/R interface from the perspective of the generation and deposition of liquefied layer and the critical condition of stability is found in this paper. It is found contraction may happen in the liquid film under 'starved' lubrication conditions, in which the supply of lubricants from armature melting wear is insufficient to maintain a full film and the hydrodynamic and stability of the contracted lubrication film is also studied. Under different contraction mode, there exists certain difference in dynamics and stability of liquefied layer, the more the film contraction, the smaller the critical speed of stability and the contraction to the tail ending have a greater critical speed compared with to the tail leading in the same contraction length.A more accurate model of stability is established in considering the effect of the film thickness dynamic changes during the launching. By calculating the model it reveals changes of the liquefied layer status throughout the launch process, and gets the critical point of stability and stable region. When full-fill film reaches the critical point of stability with the increasing of sliding speed, it begins to contract, while the reached region is still unstable, the contraction will keep on going. Finally, based on the study of the influence of outer parameters to stability, the macro direction to the control of stability is given.