Electron beam brazing reflow technique

Barrie Dunn, Luca Soli, Elio Picchione

Research output: Contribution to specialist publicationArticle

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Abstract

A technique to join molybdenum housing pieces has been developed by means of a novel process: vacuum brazing, performed using Electron Beams (EBs) as the heat source. This development has been made in the frame of a LISA Pathfinder spacecraft equipment application. Here, the Electrode Housing (EH) is a 3D capacitive sensor machined from molybdenum plates of complex geometry which contain recesses for the attachment of sapphires crystals optics. This equipment has extremely demanding requirements such as tolerance alignments in microns, magnetic cleanliness, molecular contamination, ultra-high vacuum and thermo-elastic stability. Two processes, each based on EB brazing, have been studied in an attempt to join the various molybdenum (Mo) sections of the EH in order to achieve both mechanically strong, and fully hermetic joints. Direct EB welding of Mo-to-Mo was not used because preliminary tests indicated this would cause excessive heating and damage to the internally mounted sapphire crystals, and the test joints themselves suffered from hot cracking. Instead, the test programme progressed to EB brazing (using low-temperature melting Au-based alloys) with very low thermal impact was tried. Finally, a medium temperature EB brazing process (using pure titanium foils as filler metal) was used, as this promised higher strength joints. Both EB brazing processes were performed in a modern electron beam welding facility, where the electrons were used solely for heating, but not melting the Mo piece-parts. The low temperature reflow process has been implemented and successfully proven on EH qualification models. This technique may be extended to other applications in aerospace or ground equipment, such as Microelectronics Hybrids, where reliable joints are needed, but cleanliness, contamination, low thermal stresses and precision are equally critical
Original languageEnglish
Pages317-327
Number of pages11
Volume2016
No.3
Specialist publicationRivista Italiano della Saldatura
PublisherItalian Institute of Welding
Publication statusPublished - 1 May 2016

Keywords

  • Electron beam
  • Spacecraft materials
  • Aerospace
  • Brazing

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