Metal-To-Metal Bonded Joints

Metal-to-metal joints involve the use of heat, to raise the temperature of the metals to a point where, either by the use of hammering by the application of pressure or by a chemical reaction between the metals being joined, the metals fuse together and thus create the required bond.

Welding

Welding is the fusing together by heating the point or edge of contact of two or more pieces of metal (and applying a filler rod if required), making one continuous piece.

Welded joints are normally considered to be part of an aircraft’s permanent structure and they would not be dismantled during routine maintenance.

Only a trained welder, authorised by the CAA may weld component parts of a British-registered civil aircraft and that person is required to submit to the CAA, a series of test welds for examination, every twelve months. Therefore, it is beyond the scope of these course notes to consider the various forms of electric arc, gas, resistance, seam or spot welding techniques.

Soft Soldering

Soft soldering is the permanent joining of metals, using a filler metal that melts at a temperature considerably lower than the metals being joined. The filler metal is an alloy consisting mainly of lead and tin (with possibly, antimony and bismuth), mixed in varying proportions, depending on the use for which it is intended.

To ensure a satisfactory joint, the solder must form a metallic bond ('key') with the surfaces, being joined and to allow this to happen, the joint surfaces must be free of oil, grease, dust and corrosion.

It is also necessary to use of an approved substance (a ‘flux’), which is applied to the metals, to prevent the formation of potentially corrosive oxide films while the metals are being heated (usually by conduction of the heat from a soldering ‘iron’) and joined.

Hard Soldering

Hard soldering includes Silver Soldering and Brazing. In these processes, the fillers melt at higher temperatures than soft solder and provide a much stronger joint, which is also capable of operating at higher temperatures.

Silver Solder consists of an alloy of copper and silver (with a melting point almost twice that of the soft solders), while brazing uses a copper-zinc alloy with a melting point higher than that of Silver Solder.

Usually, the source of heat used for hard soldering is a direct flame and a different flux is also necessary to prevent oxidation of the joint.

Hard soldered joints have their fillers drawn into them by capillary action, therefore the gap between components must be kept uniform and closely controlled. As with all soldered joints, the surfaces being joined must be clean and free of oil, grease, corrosion, scale etc. Mechanical methods of cleaning can include emery cloth, wire brush or filing.