Joining process

Joining is the connecting of two or more pieces by means of joining elements or formless fabrics (DIN 8580). A possible classification of typical joining processes is shown in Fig. 10.3. 

---

Glass has a very low thermal expansion coefficient the materials joined with glass have to be similar in expansion or must be duetile, while staying vacuum tight. Even with best-matched materials skilled craftsmanship is asked for the joining process. 

In any brazing/soldering process, a molten alloy comes in contact with a surface of solid, which may be an alloy, a ceramic, or a composite material (see Ceramics Composite materials). For a molten alloy to advance over the soHd surface a special relationship has to exist between surface energies of the hquid—gas, soHd—gas, and Hquid—soHd interfaces. The same relationships should, in principle, hold in joining processes where a molten alloy has to fill the gaps existing between surfaces of the parts to be joined. In general, the molten alloy should have a lower surface tension than that of the base material. 

Fluxes also contain agents that faciHtate appHcation, such as petroleum jeUy or alcohol. These bum or evaporate at elevated temperatures before the soldering temperature is attained, leaving behind a uniform coating of flux. Fluxes become molten before the joining process is reached. The molten flux flows or spreads to form a continuous coating over the surfaces to which they have been appHed. 

A difference in potential may result from differences in structure or stress brought about during or subsequent to the joining process. 

Furthermore, many joining processes lead to a crevice, with the consequent possibility of crevice corrosion. 

Heat-affected Zone that part of the parent metal which is metallurgically affected by the heat of the joining process, but not melted. 

A snap joint is economical in two respects it allows the structural member to be molded simultaneously with the molded product, and it allows rationalizing the assembly, compared with such other joining processes as screws. Table 3-4 provides a comparison of its advantages and disadvantages. Some examples of the various types and their design considerations are shown in Figs. 3-23 to 3-25. 

Figure 12.1. Overall coefficients (join process side duty to service side and read V from centre scale)...

Soldering. Soldering is one of the metal joining processes which is obtained by melting a lower melting point alloy which will wet or alloy with the joint surface and then freeze in place. 

Thermojunctions may be formed by welding, soldering or pressing the materials together. Such junctions give identical emfs (by law (iii)), but may well produce different currents as the contact resistance will differ depending on the joining process utilised. Whilst many materials exhibit thermoelectric effects, only a small number are employed in practice. The characteristics of the more common thermocouple materials are listed in Table 6.4. 

Sometimes conventional welding or a mechanical joining process is just not possible. Substrate materials may be incompatible for metallurgical welding due to their thermal expansion coefficients, chemistry, or heat resistance. The end product may not be able to accept the bulk or shape required by mechanical fasteners. 

This chapter certainly does not consider all possible substrates. However, the guidance that is offered should be sufficient for the user to select candidate joining processes and epoxy adhesive materials, no matter what substrate or combination of substrates is involved. 

Because acetals absorb a small amount of water, the dimensions of molded parts are affected by their water content, which varies with the relative humidity of the environment. The absorbed water should be considered when bonding because it could migrate to the interface during exposure to elevated temperatures from the joining process or from the service environment. This released moisture could create a weak boundary layer. Dimensional changes must also be considered when acetal parts are bonded to other substrates and then exposed to changing temperature and humidity conditions. 

The high degree of workmanship displayed in a piece as complex as the Achemenide pendant shows that the goldsmith of 2500 years ago recognized and used different temperatures for various joining processes. 

A simpler (one-step) and more economical joining process is direct brazing in a furnace under a vacuum or inert gas atmosphere through the use of active filler metals [Mizuhara Cl al., 1989]. An active element such as the commonly used Ti in the filler metal forms a true alloy with the base metal. The difference in the thermal expansion coefficients between the ceramic membrane and the metal housing can lead to high stress at the... 

Initiation. Protein synthesis in bacteria begins by the association of one 308 subunit (not the 708 ribosome), an mRNA, a charged tRNA , three protein initiation factors, and guanosine 5 -triphosphate (GTP). These molecules make up the 308 preinitiation complex. Association occurs at an initiator AUG codon, whose selection was described above. A 508 subunit joins to the 308 subunit to form a 708 initiation complex (Figure 25-11). This joining process requires hydrolysis of the GTP contained in the 308 preinitiation complex. There are two tRNA... 

The measurement seems to be helpful to estimate the joining process for preparation ofFGM. 

---