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