Mechanism of porosity formation in transfer films in electromagnetic launchers

Peter Y. Hsieh, Chadee Persad, Gautam Ghosh, Yip Wah Chung, Qian Wang

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

Melting occurs at the armature-rail interface during electromagnetic launch due to resistive heating. The liquid metal film at the interface is thought to decrease the friction and contact electrical resistance. The liquid metal is quenched at the rail surface, leaving a solid transfer film with surface properties that may affect the efficiency of subsequent launches. Notably, the transfer film contains pores which increase the overall rail electrical resistance. Reduction of atmospheric water vapor by liquid aluminum to produce hydrogen is thought to be the main source of porosity. To further examine the pore formation mechanism, we performed launch experiments using steel armatures and copper rails lubricated with two silver-bismuth alloys. Spherical pores similar to those observed in aluminum films are also present in silver-bismuth transfer films. Neither liquid silver nor bismuth is capable of reducing water vapor to hydrogen. This strongly suggests that the porosity is not due to hydrogen in these films. Air entrainment occurs in forced wetting when the three-phase contact line velocity exceeds a critical value. The critical wetting velocity is a function of surface tension and viscosity and is estimated to be 4-40 m/s for liquid bismuth at 600K and a comparable value for liquid aluminum. Armature velocities exceed these values nearly instantaneously during launch. These results indicate that air entrainment is likely the source of porosity in transfer films.

Original languageEnglish (US)
Title of host publication2008 14th Symposium on Electromagnetic Launch Technology, EML, Proceedings
Pages251-254
Number of pages4
DOIs
StatePublished - Dec 29 2008
Event2008 14th Symposium on Electromagnetic Launch Technology, EML - Victoria, BC, Canada
Duration: Jun 10 2008Jun 13 2008

Publication series

Name2008 14th Symposium on Electromagnetic Launch Technology, EML, Proceedings

Other

Other2008 14th Symposium on Electromagnetic Launch Technology, EML
CountryCanada
CityVictoria, BC
Period6/10/086/13/08

Keywords

  • Air entrainment
  • Liquid metal
  • Lubrication
  • Monolithic armature
  • Porosity
  • Transfer film

ASJC Scopus subject areas

  • Computer Networks and Communications
  • Aerospace Engineering
  • Electrical and Electronic Engineering

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