Apart from the welding processes explained earlier there is also a multitude of special welding processes. One of them is stud welding. Figure 13.1 depicts different stud shapes. Depending on the application, the studs are equipped with ei-ther internal or ex-ternal screw threads; also studs with pointed tips or with corrugated shanks are used.
In arc stud welding, a distinction is basically made between three process varia-tions. Figure 13.2. depicts the three variations – the differences lie in the kind of arc ignition and in the cycle of motions during the welding process.
Figure 13.1
Figure 13.2
The switching arrangement of an arc stud welding unit is shown in Figure 13.3.
Besides a power source which produces high currents for a short-time, a control as well as a lifting device are necessary.
In drawn-arc stud welding the stud is first mounted onto the plate, Figure 13.4. The arc is ignited by lifting the stud and melts the entire stud diameter in a short time.
When stud and base plate are fused, the stud is dipped into the molten weld pool while the ceramic ferrule is forming the weld. After the solidification of the liquid weld pool the ceramic ferrule is knocked off.
Figure 13.3
Figure 13.4
Figure 13.5 illustrates tip ignition stud welding. The tip melts away immediately after touching the plate and allows the arc to be ignited. The lifting of the stud is dis-pensed with. When the stud base is
molten, the stud is positioned onto the partly molten workpiece.
Studs with diameters of up to 22 mm can be used. Welding currents of more than 1000 A are necessary.
The arc stud welding process allows to join different materials, see Fig-ure 13.6. Problematic are the different melting points and the heat dissipation of the individual materials. Aluminium studs, for example, may not be welded onto steel.
The relatively high welding currents in the arc stud welding process cause the somewhat troublesome
side-effects of the arc blow. Figure 13.7 depicts different arrangements of current contact points and cable runs and illustrates the developing arc deflection (B,C,E).
A, D and F show possible counter-measures.
Figure 13.5
Figure 13.6
In high-frequency welding of pipes the energy input into the workpiece may be carried out via sliding contacts, as shown in Figure 13.8, or via rollers, as shown in Figure 13.9. Only the high-frequency technique allows a safe current transfer in spite of the scale or ox-ide layers.
Through the skin effect the current flows only condi-tionally at the sur-face. Therefore no thorough fusion of thick-wall pipes may be achieved.
Figure 13.7
Figure 13.8 Figure 13.9
Only welding of small wall thicknesses is profitable – as the weld speed must be greatly reduced with increasing wall thicknesses, Figure 13.10.
In induction welding – a process which is used frequently nowadays – the energy input is received contact-less, Figure 13.11. Varying magnetic fields produce eddy currents inside the workpiece, which again cause resis-tance heating in the slotted tube. A distinction is made between coil in-ductors (left) and line inin-ductors (right).
Also in case of induction welding flows the current flows only close to the surface ar-eas of the pipe. Only the current part which reaches the joining zone and causes to
fill the gap may be utilised. Fig-ure 13.12 illus-trates two current paths. On the left side: the useful current path, on the right side: the useless current path which does not contribute to the fusion of the edges.
Figure 13.10
Figure 13.11
Figure 13.13 shows the effective depth during the inductive heating for different materials, in de-pendence on the frequency. As soon as the Curie tem-perature point is reached, the effec-tive depth for ferritic steels increases.
Figure 13.12
Figure 13.13 Figure 13.14
The application of the induction weld-ing method allows high welding speeds of more than 100m/min, Figure 13.14.
Aluminothermic fusion welding or cast welding is mainly used for joining railway tracks on site. A crucible is filled with a mixture consisting of aluminium powder and iron oxide. An exothermal reaction is initiated by an igniter – the aluminium oxidises and the iron oxide is reduced to iron, Figure 13.15. The molten iron flows into a ceramic mould which matches the contour of the track. After the melt has cooled, the mould is knocked off. Fig-ure 13.16 shows the process as-sembly.
Figure 13.15
Figure 13.16
Explosion weld-ing or explosion cladding is fre-quently used for joining dissimilar materials, as, for example, unal-loyed steel/alunal-loyed steel, cop-per/aluminium or steel/aluminium.
The materials which are to be joined are pressed together by a shock wave.
Wavy transitions develop in the joining area, Fig-ures 13.17 and 13.18.
The determined cladding speed must be strictly adhered to during the welding proc-ess. If the welding speed is too low, lack of fusion is the result. If the welding speed is exceeded, the development of the waves in the joining zone is erratic. Figure 13.19 shows the critical cladding speeds for different material combinations.
Figure 13.17
Figure 13.18
Figure 13.20 shows a diagrammatic rep-resentation of a diffusion welding unit.
Diffusion welding, like ultrasonic weld-ing, is welding in the solid state. The surfaces which are to be joined are cleaned, polished and then joined in a vacuum with pressure and temperature.
After a certain time (minutes, right up to several days) joining is achieved by diffu-sion processes.
The advantage of this costly welding method lies in the possibility of joining dissimilar materials without taking the risk of structural transformation due to the
Figure 13.19 Figure 13.20
Figure 13.21