It is known from experience that wire-splices of the same type sometimes have relatively large mechanical and metallurgical tolerances.

These different initial conditions may result in different weldings. In order to make the outcome independent of an unknown parameter, the inserted amount of copper wires serves as the basis for control of the welding process. The volume of copper wires represents a value of 100% with their nominal cross-section. After inserting the wires into the welding tool, the anvil presses them together under welding pressure conditions. The cross-section measured just before the ultrasonics is triggered (anvil width x distance between sonotrode and anvil) is a combination of wires, cavities and impurities and usually represents 120% of the nominal cross-section, i.e. the cross-section measured before the welding takes place is 20% above the nominal cross-section. The splices here have a compression rate of 0% (figure 41).

Figure 41: Compression rates with ultrasonic wire-splice welding

A non-contact measuring sensor registers the condition and compares it with the pre-set value in order to ascertain the correct combination of wires. In addition, the definition of the compression rate (%) serves as a process variable and a basis for determining the deformation under the influence of ultrasonics.

During the sonic-exposed welding time the anvil, which is subjected to the welding pressure, reduces the height of the wire-filled welding area by displacing cavities and contamination.

Figure 42: Ultrasonic welding of multi-layer flat copper splices

Figure 43: Contact spring, welded to brass plate using ultrasonics

Figure 44: Nickel pins welded to a copper contact bar using ultrasonics

This also reduces the copper volume of the splices in the welding area, which comes close to the nominal cross-section of the copper wires -- 100%. A compaction of the individual wires has already been achieved. Further bonding and deformation finally lead to a rate of compression which is close to or above the rated cross-section.

Measuring the deformation

The deformation work is measured during the welding process. It serves to retrace the welding time, to determine the energy flow and the end of the welding process as soon as optimum and predetermined conditions are achieved.

The following machine data are predefined

• preset welding parameters which do not deviate during welding

  -- welding pressure
  -- welding amplitude
  -- tool width

• parameters for quality control

  -- height of splice nugget before welding
  -- height of splice nugget after welding (compression)
  -- welding energy during welding time

For these parameters upper and lower limits are defined. If the welding has not taken place within these preset limits, the bond is considered faulty and a visual and/or audible alarm signal is triggered.

Alarm for faulty bonds

The setting of the machine and control parameters takes place initially on the basis of experience. Mainly tensile tests are made to confirm these settings, or to make adjustments, if necessary.

Apart from tensile tests, tests on the mechanical fatigue strength (bending strain and oscillations stress reversal), the thermal stability, the electrical conductivity as well as corrosion test and micrographs may be conducted.

Metallographical tests show a plastic deformation and a reduction of the grain size. Diffusion takes place in the area of the welding.

If the compression of the welded parts is too strong, it leads to an excessive reduction of the cross-section and thus to a reduction of mechanical strength (see figure 39).