BASIC PRINCIPLES OF WELDING

If your were to take two ice cubes from the refrigerator, the outer surfaces under the heat of the day will begin to melt to ice water. Place the two wet cubes one on top of the other back in the refrigerator and within a short time the two cubes are welded together to form one block of ice. The addition of heat has melted portion of the two parts to be joined and they both cool down to one structure, the melted section becoming an intimate part of the bond. (Fig.1)

On the other hand, in the glueing of two cubes of wood, the wood forms no part of the bond and adhesion is achieved by the strength and intimate contract of the glue with the two abutting surfaces.

While some specialised processes may offer slight variants, the generally accepted term of welding, particularly as it applies to the oxyacetylene and arc processes, implies a fusionweld wherein the base materials being joined are melted at the abutting faces and become all or part of the weld. In some cases welds are made using the oxyacetylene, carbon arc or argon arc processes, where only the base material melts to form the weld. In other instances a filler rod or compatible material is added to the joint and this occurs in the case of the manual stick electrode arc weld. here the electrode is consumed in the process and the molten metal added to the joint to form, along with the melted surfaces of the base material, the solidified mixture which is the weld metal.

You will appreciate that this is a different process to soldering and brazing, where a metallic cement melting at a lower temperature than the parts being joined flows and solidifies in intimate contact with the components to form a bond.
 

These processes of lower heat input are more akin to the glue joint in wood mentioned above. (Fig. 2)

A necessary part of a welding process is a source of intense heat sufficient to melt the material being joined (and any filler metals added) and the oxyacetylene and metallic arc processes are the most commonly employed. The very low temperature soldering process can be achieved with a soldering iron, while the high temperature soldering or brazing process using silver or copper alloys may be carried out with air or oxy‑fuel gas combinations, or carbon arc torches. The fuel gases commonly available in Australia are acetylene, liquefied petroleum gas (LPG) and, in some areas, Natural Gas.

The blacksmith's forge weld involves both heat and pressure. Here a fusion weld is not involved, but the material are heated into the plastic range and under the influence of the applied pressure (hammering), the hot working of the metal parts together ensures an intimate bond with a re-crystallization into one homogeneous mass. (Fig. 3). Resistance spot welding is a modern electromechanical version of the blacksmith's forge weld.

SOME ELEMENTARY ASPECTS OF ELECTRICITY

As we shall be using some technical terms in everyday discussion of arc welding, we should know basically to what these terms refer. In considering elementary electricity, it is sometimes simplified if we use a water system analogy.