Testhouse Cold Welding

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ESA Testhouse – Cold welding in Launch

Project Funding:
Project Coordination:
AAC, Austria
10 months

During launch space­crafts are subjec­ted to heavy vibra­tions. This may harm any hold-down point, as fret­ting in the contact may occur which may lead to cold weld­ing: metals can be welded when rubbing them together in vacuum (as no oxide layers inhibit close metal-to-metal contact). Recently, cold weld­ing tests under fret­ting were done for customer applic­a­tions on a hold down mech­an­ism. They showed unex­pec­ted high adhe­sion forces between Ti-alloy and steel in envir­on­ment with even poor vacuum pres­sures, i.e. at 1 and 10mbar. Even under ambi­ent pres­sure some hints on cold weld­ing were iden­ti­fied, which is not usual. The exist­ing cold weld data base does not cover inform­a­tion on such high pres­sures, as liter­at­ure was show­ing for stain­less steels that above 10-3 mbar no adhes­ive beha­viour should be found. This means that oxid­a­tion is faster than form­ing welds between the metal­lic bodies (to be more precisely: at frequen­cies in range of some hundred Hertz. Of course welds can be formed in air during “ultra­sonic bond­ing”, but herein frequen­cies about 10.000 Hz are used.). Similar effects were seen by AAC on cast iron in the recent past. In the two recently closed stud­ies a few common mater­ial combin­a­tions were screened for their cold weld­ing beha­viour in launch envir­on­ment, i.e. under test dura­tions being close to launches. This covered low test dura­tions of 5 minutes in each of three envir­on­ments: ambi­ent (humid) air, low vacuum (10mbar) and high vacuum. It was shown that test­ing in one single envir­on­ment does not reflect the beha­viour when test­ing is done in a launch sequence. Especially, when using solid-lubric­ant coat­ings, life-time under high vacuum is strongly reduced when pre-damage in air and low vacuum occurs. Image from a SEM (Scanning Electron Microscope): Left: uncoated metal surface – strong destruc­tion by cold weld­ing in high vacuum Right: surface with proper coat­ing: in good conditions