A BASIC STUDY OF COLD WELDING IN ULTRAHIGH VACUUM

Abstract : The cohesion of the FCC metals Ag, Al, Cu, and Ni, under ultrahigh vacuum was investigated by cold welding specimens previously fractured in the vacuum. The cohesion strength of the weld increased with compression load for all metals; all data fell on one curve of slight positive curvature when the cohesion load (stress) and the compression load (stress) were divided by the initial fracture load (stress) of the virgin specimen. The effect of compression load on the cohesion coefficient alpha, for all metals could be described by alpha = 0.75 + 0.15 LC/LFo, where LC is the compression load and LFo is the initial fracture load. Ultrasonic measurements and microscopic examination of Cu specimens indicated that the contact area was proportional to the ratio LC/LFo and that excellent matching of the fractured surfaces was possible with the apparatus used. The cohesion results are explained on the basis that the rupture of the weld occurs at a constant value of the 'true' fracture stress. Heat treatment reduced the strength of copper welds and alloying (70-30 Brass) lowered the cohesion coefficient compared to unalloyed Cu. Prolonged exposure of copper fracture surfaces to the gases O2, CO, CO2 and air caused an appreciable reduction in the cohesion coefficient, whereas no effect was observed for N2. This is in good agreement with predictions based on absorption theory and observations reported for LEED studies. (Author)