Joining Mechanism

TM 750-245-4

8-2. Joining Mechanism

Figure 8-2 illustrates the two basic welds.

a. The joining mechanism of welding may be

b. Fusion occurs when the temperature is great

classified as being either a fusion or a forging action.

enough to cause melting of the weld materials at the

The fundamental difference between the two

point of interface. The molten materials are confined

mechanisms is the temperature at which the joining

within the weld materials and, upon cooling, solidity to a

weld occurs. The heat generated is a function of the

cast structure, termed a "nugget," which binds the

thermal and electrical characteristics of the materials to

materials together. Lighter metals and materials such

be welded. These characteristics govern which joining

as iron, steel, and nickel exhibit this type of weld.

mechanism takes place.

c. The forge weld is basically a solid state one.

The temperature reached is not high enough to cause

melting but high enough to cause the materials to reach

a plastic state. The pressure exerted by the electrodes

forces the materials into intimate contact, and the

proximity of the atoms of the two materials at the

interface causes a solid state bond. There is no

evidence of a nugget in the forge process., Copper and

other metals possessing low resistivity and high thermal

conductivity are joined by this type of weld, since it is

difficult to localize the heat of the interface.

8-3. Preparation of Metal for Welding

a. The properties of a welded joint depend partly

on the correct preparation of the edges being welded.

All scale, rust, oxides, and other impurities must be

removed from the joint edges or surfaces to prevent

their inclusion in the weld metal. Edges should be

prepared to permit fusion without excessive melting.

Care should be taken to keep to a minimum the heat

loss due to radiation into the base metal from the weld.

Properly prepared joints will give a minimum of

expansion on heating and contraction on cooling.

b. The preparation of the metal for welding is

governed by form, thickness, kind of metal, the load

which the weld will be required to support, and the

available means for preparing the edges to be joined.

Figure 8-2. Basic types of welds.

Section II. TYPES OF WELDS AND PROCESSES

electrodes. Welds are made with or without application

8-4. General

of pressure and with or without filler metals.

This section is included so that you may become

familiar with all types of welds and their processes. he

a. Metal Electrodes.

more knowledge you possess, the better qualified you

(1) Bare metal arc welding. The arc is drawn

will be to inspect.

between a bare or lightly coated electrode and the

workpiece. Filler metal is obtained from the electrode

8-5. Arc Welding

and neither shielding nor pressure are used.

In this process, the weld is produced by the extreme

(2) Stud welding. The arc is drawn between a

heat of an electric arc drawn between an electrode and

metal stud and the workpiece. The molten surfaces to

the workpiece, or in some cases, between two

8-2