|
Influence of the welding energy
The amount of ultrasonic energy to be transferred to the parts is nearly proportional to the welding time, the amplitude and the pressure force. By increasing parameters, the introduced welding energy will increase immediately. To weld workpieces together a minimum amount of energy is necessary. Once the welding process is started, it must be insured that a successful coupling between the sonotrodes and the upper welding part exists and that the removal of possible contamination and disturbing layers on the surface of the workpieces is guaranteed. Avoid excessive welding energy If an insufficient amount of energy is used, the bond between the parts will be weak. Welding force and amplitude have to be properly set for the weld time to be effective and to achieve a good weld.
Figure 35: Welding an enameled wire with copper wire by ultrasonics Influence of mechanical stress
Figure 36: Enameled wire welded to a contact plate (without coating) using ultrasonics With an oscillation frequency of 20 kHz, the number of load cycles on the workpieces to be welded can be as much as 10,000 within 0.5 seconds. The parts to be welded are also subjected to large plastic deformations. The longer the welding time, the higher the possibility severe damage will occur due to the cyclic load exerted by the alternation of traction and pressure. The magnitude of mechanical stress depends both on amplitude and mechanical pressure. The larger the amplitude, the stronger the displacement of the layers near the surface of the workpieces in the bonding area.
Figure 37: Magnet wire welded to a stranded wire by ultrasonics
Influence of thermal stress The thermal stress on the parts to be welded depends on a variety of influences such as welding time, oscillation length of the sonotrode (amplitude) and mechanical pressure. Due to the small frictional contact of the parts to the welded in the joining area, lower welding forces enable a higher relative movement between these parts. During this process, the majority of the supplied energy is transformed into frictional heat. The contact temperature starts to increase. Even longer welding times or greater amplitudes lead to an increase in temperature due to the higher release of energy into the parts to be welded. On the other hand, the welding time and consequently the increase in temperature are decreased considerably when the amplitude increases.
|
The necessary minimum energy has to be insured by selection of proper values for pressure and amplitude. This is the only way to obtain optimum welds. An excess of welding energy leads to a severe deformation of the parts to be welded and thus to a reduction of the cross-section or even damage of the work-pieces. This weakens the bond and reduces the tensile strength.
A study of mechanical stress in the parts to be welded has to consider both the time of exposure and magnitude of the stress. The welding time dictates the number of oscillations acting on the parts to the welded.
If the boundary of elasticity is exceeded, it will either lead to material viscosity or to significant material damage. Unlike the dynamic oscillation load, the static load exerted by the mechanical pressure can be largely ignored.
