745 Welding, Cutting, and Brazing Safety

Coalescence

Coalescence is the process in which two metal surfaces merge to form a single, continuous body of material when heated and brought into close contact. This occurs at a microscopic level, where atomic bonding takes place due to heat and pressure.

The two fundamental requirements for coalescence are:

1. Heat, to increase atomic mobility
2. Intimacy of contact to ensure proper surface interaction

Coalescence is a key principle in welding, brazing, and soldering, as it facilitates the formation of strong, defect-free joints.

Heat

Welding processes differ depending on the source of heat, the manner in which heat is applied or generated, and the intensity of the heat. The fuel used as a heat source may include: acetylene or hydrogen in air or in oxygen, an electric arc, or a chemical reaction.

The intensity of heat applied or generated at the joint varies according to the metals being joined and to the welding process being used. All welding processes, except brazing, use temperatures high enough to melt the base metals.

Intimacy of Contact

In the second basic requirement for coalescence, intimacy of contact, the welding processes are divided into two groups: pressure processes and non-pressure processes.

• Pressure Processes: In pressure processes, no space exists between the surfaces being joined. Welders apply pressure while the contact surfaces are at a temperature high enough to allow plastic flow of the metal.
• Non-pressure Processes: In non-pressure processes, space exists between the joined surfaces and molten metal fills the space. The molten metal may be produced by:
• A filler metal (welding rod or electrode)
• Melting the surfaces to be joined
• Combining a filler metal and melted base metal

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