Fusion welding can be applied to processes in which metals are heated to the temperature at which they melt and are then joined without hammering or the application of pressure. The joint can be formed without the use of a filler metal, but usually a filler metal in the form of a wire or rod is employed to fill the joint.

The filler metal has the same composition as the parent metal, but may contain alloying metals to improve its fluidity in the molten condition or to produce a fine grained weld structure. The wire or rod of filler metal may be sheathed in a special coating. Such coatings perform one or more of various functions : serve as a flux, remove oxides or other disturbing substances that may be present, improve the wettability of the material surface, protect the weld against external influences, prevent excessively rapid cooling, and stabilize the arc.

The composition of the coating depends more particularly on the material to be welded and on the welding method. Mixtures of oxides of iron, manganese and titanium, alkaline earth carbonates, fluorite, and organic compounds are used for coatings. Sources of heat employed in fusion welding are gas, electricity, chemical reactions, etc. Gas welding (Fig.1) uses a flame produced by the burning in oxygen of acetylene (oxyacetylene welding) or sometimes another fuel gas (e.g., propane, butane, hydrogen) to heat and liquefy the metal at the joint to be welded. This is a very widely employed method of welding iron, steel, cast iron, and copper. The flame is applied to the edges of the joint and to a wire of the appropriate filler metal, which is melted and runs into the joint.

A fairly recent development is the electroslag process (Fig.2) in which the metal at the joint is melted in an electrically conducting (ionized) molten-slag bath whose temperature is above the melting temperature of the metal. The welds are executed as vertical welds; with this method it is, for instance, possible to form butt welds in very thick plates quickly and economically.

The current is supplied to the slag bath through bare metallic electrodes, which melt away and provide the filler metal. The molten filler metal sinks in the slag, fills the gap of the joint and slowly solidifies in it, from the bottom upwards. The gap is bridged by water-cooled copper shoes which, together with the faces of the joint, form a mold for the molten metal. The shoes move upwards along the joint during welding.

The most important and most widely used fusion-welding technique is arc welding, which employs an electric arc to melt the parent metal and the filler metal. The latter may be provided in the form of an electrode which melts away or it may be melted thermally i.e., without carrying the welding current.

The general technique can be subdivided into three categories : open-arc welding, covered arc welding, and gas-shielded-arc welding. Open-arc welding by Benardo’s method (Fig.3a) employs direct current, the arc being formed between the parent metal and a carbon electrode.

In Zerener’s method (Fig 3b) the arc is formed between two carbon electrodes; the heat of the arc is concentrated on the workpiece by the action of a magnetic coil. The method now most widely used was originated by Slavjanov (Fig.3c): the arc is formed between a metallic electrode, which gradually melts away to supply the filler metal, and the workpiece.

The process known as firecracker welding (Fig.4) is an example of a covered-arc method. A heavily coated electrode is laid horizontally on the joint to be welded and is covered with an insulating layer of paper and a covering bar of copper or some other metal. The workpiece is connected to one pole and the electrode is connected to the other pole of a current source. An arc is struck between the end of the electrode and the joint, and burns along the length of the electrode

Another form of covered-arc welding is submerged-arc welding (Fig.5). The flux is supplied separately in the form of powder which blankets the arc. The powder melts and protects the molten filler metal from atmospheric contamination. Any powder not melted is recovered by suction and reused. When cool, the fused powder forms a slag, which peels off the weld.

Shielded-arc welding is based on the principle of protecting the molten filler metal by an envelope of chemically inert gas, which may be helium (heliarc process), argon (argonarc process) or carbon dioxide. In atomic-hydrogen welding (Fig.6a) the heat liberated by monatomic hydrogen when recombining into molecules is used to fuse the metal.

An alternating-current arc is maintained across two tungsten electrodes. A stream of hydrogen gas is passed through the arc, in which the hydrogen molecules are split up into atoms. Outside the actual arc these atoms recombine into molecules. This produces great heat, which melts the parts to be welded and unites them, with or without the addition of a filler metal. The inert-gas tungsten-arc process (Fig.6b) and the inert-gas metal-arc process (Fig.6c) are two shielded-arc welding processes that are used both for manual techniques and for automatic welding by mechanized equipment.

Thermit welding (Fig.7) has already been referred to in connection with pressure welding. It is also used as a fusion-welding process, more particularly for iron and steel castings and forgings. The source of heat is not electricity or gas but a chemical reaction that produces intense heat (3000oC): the combustion of a mixture of aluminum powder and iron oxide by which the aluminum is converted into aluminum oxide and the iron oxide is reduced to molten iron (or steel). The parts to be joined are surrounded by a sand-lined mold. The powder mixture is packed in a conical crucible and ignited. The molten iron flows in and around the joint, where it fuses with the preheated parent metal.