Joining: Soldering and brazing
Soldering and brazing are methods for joining different metallic materials. One can solder or braze all metals. With few exceptions (see Prerequisites section), all aluminium alloys can be joined to other materials by soldering or brazing.
The process takes place using a fill metal (solder) whose melting point is below that of the metals being joined. The workpieces remain in solid form during the process and are wetted by molten solder. At the point of soldering, the metals have to have at least reached their working temperature. The term working temperature is used to describe the lowest surface temperature at the point of soldering at which the solder can cause wetting. If the workpiece has not reached the working temperature, the solder needs to have a considerably higher temperature in order that it can flow and cause wetting.
A very small separation between the two surfaces to be joined results in the highest joint strength. Wetting is an important criterion for a successful joint. The angle between a drop of molten solder and the base material is classified as being either ‘completely wetted’ or ‘adequately wetted’ if it is between 0–30° and as only ‘partially wetted’ or ‘not wetted at all’ if it is over 30°. Ideally the angle should be around 0°.
One differentiates between brazing and soldering. The difference between the two lies in the so-called liquidus temperature or upper melting point of the solder. With brazing, the temperature is between 450 ºC and 900 ºC. With soldering, it is below 450 ºC. In addition, the processes differ in the types of process used, properties and applications.
A fundamental difficulty that arises when brazing aluminium, as compared to other metals, is its relatively low melting point. While the brazing of pure and super-purity aluminium may be undertaken without difficulties, the addition of certain alloying constitutions result in a decrease of the melting point. This is the case of wrought alloys containing higher percentage of magnesium (> 3 %) as well as in cast alloys with a silicon content of 12 % and more. Moreover, unlike steel aluminium does not exhibit an annealing colour under the effect of heat. Therefore, the filler metal itself or the fluxes often serve as an indication for when the working temperature has been reached. Finally, as aluminium features high thermal conductivity – around twice that of iron – it requires a considerably higher input of heat.