• Ultrasonic process of welding is shown in Fig. 2.18. The core of magnetostrictive ultrasonic vibrations generator (15-60 kHz) is connected to the work through a horn having a suitable shaped welding tip to which pressure is applied. The combination of ultrasonic vibrations with moderate pressure causes the formation of a spot weld or seam weld (with modified apparatus). The deformation caused is less than 5 percent.
Fig. 2.18(a) Ultrasonic welding
Motion of welding tip
• Friction between the interface surfaces, along the axis of the welding tip, causes the
removal of surface contaminants and oxide film exposing the clean metallic surface
in contact with each other which weld together due to applied pressure. Weld produced is as strong as parent metal.
• Some local heating may occur and some grains may cross the interface but not melting or bulk heating occurs.
The process is briefly discussed in the following paragraphs:
1.It is solid state joining process for similar or dissimilar metals in the form of thin strips or foils to produce, generally lap joints.
2. H. F. (15000 - 75000 Hz) vibratory energy gets into the weld area in a plane parallel to the weldment surface producing oscillating shear stresses at the weld interface, breaking and expelling surface oxides and contaminants.
3. This interfacial movement results into metal-to-metal contact permitting coalescence and the formation of a sound welded joint.
Fig. 2.18(b) Ultrasonic welding (detailed sketch)
4. Before welding the machine is set for clamping force, time and power and overlapping plates are put on the anvil sonotrode is then lowered and clamping force is built to the desired amount (a few Newton to several hundred Newton) and ultrasonic power of sufficient intensity is then introduced. Power varies from a few watts for foils to several thousand watts for heavy and hard materials and is applied through the sonotrode for a pre-set time. Power is then automatically, cutoff and weldment released, time taken is less than 1 sec.
5. Continuous seams can also be produced using disc type rotary sonotrode and disc type or plain anvil.
6. Machine parameters are adjusted for each material and thickness combination.
7. Materials from very thin foils and plates upto 3 mm thickness can be welded.
8. Advantages and applications include.
(a) The process is excellent for joining thin sheets to thicker sheets.
(b) Local plastic deformation and mechanical mixing result into sound welds.
(c) Ring-type continuous welds can be used for hermetic sealing.
(d) Many applications in electrical/electronic industries, sealing and packaging, air craft, missiles, and in fabrication of nuclear reactor components.
(e) Typical applications of the process include: welding of ferrous metals, aluminium, copper, nickel, titanium, zirconium and their alloys, and a variety of dissimilar metal combinations. It is applicable to foils and thin sheets only.
(f) Other applications include: almost all commonly used armatures, slotted commuters, starter motor armatures, joining of braded brush wires, to brush plates, and a wide variety of wire terminals.
(g) With newly developed solid-state frequency converters, more than 90% of the line power is delivered electrically as high frequency power to the transducer.
(h) In the case of ceramic transducers as much as 65 - 70% of the input electrical line power may be delivered to the weldmetal as acoustical power.
Energy required to weld
Energy required to weld a given meterial increases with material hardness and thickness. This relationship for spot welding is given by
Ea = 63 H372 t15 where Ea = acoustical energy in joules
H = Vicker’s microhardness number t = material thickness adjascent to active in inches.
This equation is valid for Aluminium, Steel, Nickel and Copper for thicknesses upto
0. 81 mm.