The Effect of Anvil Geometry and Welding Energy on Microstructures in Ultrasonic Spot Welds of AA6111-T4
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INTRODUCTION
ULTRASONIC metal spot welding (USW) uses cyclic vibration to form a lap joint between two metal sheets under static clamping pressure. The direction of vibration is in the plane of weld interface as compared to that typically used for ultrasonic joining of thermoplastic polymers. The vibration frequency for the majority of metal welders is typically 20 kHz and above, although some commercial welders are available to operate at lower frequencies. The electrical energy required for welding two AA6111-T4 coupons of 0.9mm thickness by single-transducer wedge-reed welders is approximately 800 J. This is one to two orders of magnitude lower than the 50 to 100 kJ required for resistance spot welding. Additionally, in contrast to fusion welding technologies, USW does not produce a heat-affected zone that can degrade the strength of the metals being joined.[1] The mechanism by which ultrasonic welding produces strong welds is not well understood, despite past research efforts in microelectronics[2,3,4] and multilayer prototype components.[5] Many published hypotheses are based on contact mechanics[6,7,8] and incorporate the associated frictional heating. Although contact mechanics models sufficiently approximate thermomechanical interactions at the weld interface, they ignore the nonlinear stress waves excited by cyclic vibrations and R. JAHN, Technical Specialist and R. COOPER, and D. WILKOSZ, Research Engineers are with Ford Research and Advanced Engineering, Ford Motor Company, Dearborn, MI 48121, USA. Contact e-mail: [email protected] Manuscript submitted May 19, 2006. Article published online April 6, 2007. 570—VOLUME 38A, MARCH 2007
dynamic instability[9,10] in solid materials. Consequently, it is difficult to explain or predict microstructural changes that USW produces in welds between relatively thick sheets. Recently, some experimental evidence of material changes induced by high intensity ultrasonic vibration at 20 kHz in steel[11] has been reported using scanning electron microscopy (SEM). Nanofeatures of unspecified nature were observed to nucleate at grain boundaries, but not inside the grains. The dominant microstructural mechanism for strong welds is likely to depend on the domain of welding parameters, which vary widely among applications and alloy thicknesses. The USW is strongly affected by the design of the gripping surface of the sonotrode tip in single-transducer wedge-reed welders. It is likely that the contact geometry of the anvil cap, such as the contact area and knurl pattern, could also influence the performance of USW. Additionally, the weld microstructures vary with the input welding energy. These effects of anvil contact geometry and welding energy have not been systematically investigated and will be described in this article.
II.
EXPERIMENTAL PROCEDURE
Sheets of AA6111-T4 alloy were obtained from Alcan in as-rolled condition. These sheets were sheared into weld coupons of 101.6 mm · 25.4 mm · 0.9 mm. The coupon length dimension was parallel to the rolling direction. The average surface ro
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