COLD WELDING OF COPPER UNDER ULTRAHIGH VACUUM
Semi-annual technical rept., 1 Oct 1966-31 Mar 1967
FRANKLIN INST RESEARCH LABS PHILADELPHIA PA
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The cohesion of copper under ultrahigh vacuum in the range 10 to the minus 11th power to 10 to the minus 9th power torr was investigated using the technique of cold welding specimens previously fractured in the vacuum. It was found that the cohesion coefficient increased with the compressive load during cold welding but was relatively independent of the purity and structure of the copper and in turn the hardness and strength, time of contact over the range 5 sec to 900 sec and exposure to the vacuum environment over the range of 10 to the minus 9th power to 0.0001 torr-sec. No voids were detected at magnifications up to 1000X along the weld interface of fine-grained 6 - 10 microns specimens which had been cold welded at loads approaching the virgin fracture load. Voids were, however, observed for coarse-grained 100 - 450 microns specimens at such loads and for all specimens at loads appreciably below the initial fracture load. From ultrasonic transmission studies it was deduced that the cohesive strength was proportional to the area of contact developed during cold welding. The effect of compressive load on the cohesive strength is explained by the increase in contact area resulting from elastic and plastic strains in the vicinity of the interface. This interpretation suggests that the cohesion coefficient should be proportional to the reciprocal of the elastic modulus, which was shown to be the case when comparing Cu, Ag and Fe. The fact that the cohesion coefficient was independent of exposure to the vacuum environment up to 0.0001 torr-sec is shown to be in accord with adsorption theories.
- Properties of Metals and Alloys
- Fabrication Metallurgy