Welding characteristics of stainless steel and aluminum plate specimens of 6.0 mm thickness by a 15 kHz ultrasonic butt welding system were studied.
Aluminum and stainless steel specimens, and stainless steel specimens of 6.0 mm thickness were welded end to end using a 15 kHz ultrasonic butt welding equipment with a vibration source using eight bolt-clamped Langevin type PZT transducers of 70 mm diameter and a 50 kW static induction thyristor power amplifier. The aluminum, aluminum alloy and stainless steel plate specimens were joined with almost equal to aluminum strength. The stainless steel plate specimens electrolytically polished were joined with welding strength almost
equal to the material strength under rather large vibration amplitude of 25 lm (peak-to-zero value), static pressure 70 MPa and welding time of 1.0–3.0 s.
Hardness of aluminum was found to decrease near the welded surface. Hardness of stainless steel decreases at the areas approximately 2 mm away from the welded surface but increases by approximately 20% near the welded surface. The phenomenon of increasing hardness due to ultrasonic vibration was observed although only decreasing hardness was noted in previous studies. The hardness increasing effect may be applied to the partial hardening of materials due to vibration; in contrast, the hardness decreasing effect may be applied to the relaxation of residual stress at the welded metal surface.
The combinations of aluminum–stainless steel and stainless–stainless steel specimens were reported as that the materials are possible to join each other by ultrasonic welding, but there were no report about welding conditions of large specimens with welded strength near to the material strength excepting small specimens as yet. Using the ultrasonic butt welding equipment, as an example, the required vibration velocities revealed in this paper are real values without slippage between the welding tip and the upper specimen in the case of a conventional ultrasonic lapped welding equipment. The obtained data are fundamental welding conditions for joining ring or spot shape parts of the products such as small and thin motors of hard disk drive and the others. Of course, adequate welding equipment must be developed for special applications including complex vibration systems.