Metal joining. Joining materials by thermal processes

Thermal processes in common use for the joining of metals and their alloys include:

1. Soft soldering.

2. Hard soldering or brazing.

3. Braze welding.

4. Full fusion welding.

5. Resistance welding.

One common factor in all the above processes is that each of them is dependent upon a suitable source of heat energy. The application or choice of a particular thermal joining process may vary according to the temperatures used to produce an efficient joint and, of course, according to the materials being joined.

Soft soldering

Soft soldering is a low temperature thermal process in which the actual metals being joined are not melted. The process involves the use of a suitable low-melting-temperature alloy of tin and lead which is “bonded”, by the application of heat, to an unmelted parent material.

Soft solders, therefore, must have a lower melting point than the metals they join.

It is an essential feature of a soft soldered joint that each of the joined surfaces is “tinned” by a film of solder and that these two films of solder are made to “fuse” with the solder filling the space between them.

Basically the action of a soft solder when applied to a prepared metal joint which has been heated to the required temperature is to:

a) flow between the parent metal surfaces which remain unmelted;

b) completely fill the space between the surfaces;

c) adhere thoroughly to the surfaces;

d) solidify.

In a correctly soldered joint, examination under a microscope has shown that in the action of tinning metals, such as brass, copper and steel, a definite chemical reaction takes place. The metal surface and the tin in the solder react together to form an “intermetallic compound” which, to some extent, becomes part of the parent metal.

In general there are two stages in making a soft-soldered joint:

1. Tinning the metal surface.

2. Filling the space between the tinned surfaces with solder.

Preparing the joint

The “tinning action” of solder cannot take place unless the two surfaces to be joined are chemically clean. Not only must the mating surfaces be free from dirt and grease, but also of any oxide film.

Cleaning for soldering

Prior to any soft soldering, brazing or welding operations it is necessary to expose bare metal at the mating or joining surfaces. Dirt, grease, oil or oxides are themselves unsolderable and would act as barriers between the molten solder and the metal surfaces to be joined.

Oxygen combines with substances much more rapidly at elevated tempera­tures. Therefore, it is very important that metal surfaces must be prevented from oxidizing whilst they are being heated to the soldering temperature. A suitable soldering flux is used for this purpose.

Soldering fluxes

The requirements of a good soldering flux are:

1. It must remain liquid at soldering temperature.

2. In its liquid state it must act as a cover over the joint and exclude the air.

3. It must dissolve any oxide film present on the surfaces being joined.

4. It should be readily displaced from the metal surfaces by molten solder.

Fluxes used for soft soldering operations may be classified as active or inactive (passive).

The active fluxes will quickly dissolve the oxide film on a metal and, at the same time act as a barrier to prevent further oxidation.

All active fluxes have a corrosive action. Although any active flux residue which is present along the edges of the joint after soldering may at first appear to be dry and harmless, it must he removed as soon as the soldering has been completed. These apparently dry flux residues which remain after soldering absorb water readily from the atmosphere. Within a few hours any active flux residue, if not removed, will become wet spots at which corrosion will take place rapidly. Corrosive flux residues may be removed by washing in hot water containing a few drops of hydrochloric acid, rinsing in a hot dilute soda solution, and finally in hot water to remove the salts formed. If only hot water is available, the soldered part should be thoroughly rinsed.

The inactive fluxes protect a previously cleaned metal surface and have little effect on oxides.

Because of the difficulty of avoiding corrosion the use of active fluxes is banned in electrical and other work that cannot be washed effectively.