The effects of superimposed ultrasonic vibration on the plastic deformation of 99.99% pure polycrystalline Cu are studied both during (temporary) and after (residual) the application of ultrasound (US) using deformability measurements acquired from an automated wire bonding machine and microhardness testing. It is found that if ultrasonic irradiation is applied during the deformation of the 100 μm diameter Cu free air balls (FABs) the Cu becomes softer with increasing US power compared to Cu FABs that are deformed without US. When comparing this temporary acoustic softening of Cu to that of Au, it is found that the amount of softening is similar between the two materials. After the US is turned off, a residual acoustic softening remains. This residual softening effect increases with increasing the US power. With residual acoustic softening, a maximum increase of 13% in deformability is measured for Cu during wire bonding compared to a maximum increase of 8% for Au during wire bonding. Stronger residual acoustic softening effects are obtained in Cu than in Au with a maximum decrease in microhardness of 19% and 9%, respectively. Dynamic annealing and dislocation theory are used to explain both the temporary and residual effects of US on the deformation of Cu and Au.
In this paper, a computational fluid mechanics model is developed for full penetration laser welding of titanium alloy Ti6Al4V. This has been used to analyze possible porosity formation mechanisms, based on predictions of keyhole behavior and fluid flow characteristics in the weld pool. Numerical results show that when laser welding 3 mm thickness titanium alloy sheets with given laser beam focusing optics, keyhole depth oscillates before a full penetration keyhole is formed, but thereafter keyhole collapses are not predicted numerically. For lower power, lower speed welding, the fluid flow behind the keyhole is turbulent and unstable, and vortices are formed. Molten metal is predicted to flow away from the center plane of the weld pool, and leave a gap or void within the weld pool behind the keyhole. For higher power, higher speed welding, fluid flow is less turbulent, and such vortices are not formed. Corresponding experimental results show that porosity was absent in the melt runs made at higher power and higher welding speed. In contrast, large pores were present in melt runs made at lower power and lower welding speed. Based on the combination of experimental results and numerical predictions, it is proposed that porosity formation when keyhole laser welding may result from turbulent fluid flow behind the keyhole, with the larger the value of associated Reynolds number, the higher the possibility of porosity formation. For such fluid flow controlled porosities, measures to decrease Reynolds number of the fluid flow close to the keyhole could prove effective in reducing or avoiding porosity.
The effects of superimposed ultrasound vibration on plastic deformation of gold are studied both during and after the vibration using an ultrasonic ball bonding machine. It is found that when ultrasonic irradiation is applied along with mechanical force, the metal is softer than when deformed without the vibration. After ultrasound is turned off, the deformed metal remains softer if previously deformed with ultrasound. Possible mechanisms for the acoustic residual softening are discussed as compared to residual hardening. The acoustic residual effect is attributed to the net balance between ultrasound’s dynamic annealing and its potential opposing effect on activating and multiplying dislocations.
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