BRAZING

Brazing is a process of permanently bonding metals by adding brazing alloy which melts at above 450° C or below that temperature (soldering) with the sue of the capillary effect. This effect is described as the force due to which the brazing gap is filled by the liquid filler metal. The main advantage of brazing is the possibility of bonding materials of different melting temperatures and thicknesses, and maintaining the geometric and physical properties due to the relatively low process temperature.

The brazing process is a capillary phenomenon, that being the penetration of the brazing gap by molten filler metal. In order to fill the gap properly, the properties of the base metal as well as the brazing alloy should be considered. The correct size of the brazing gap must be maintained for a brazing temperature.

The many advantages of of the brazing translating into such common use in many branches of industry include:

• wide scope of process temperatures,
• large selection of filler metals allowing for bonds of desired properties,
• large selection
• possibility of bonding most metals and alloys,
• highly aesthetic bonds
• the possibility of obtaining bonds with strength similar in many cases to the strength of bonded materials
• the possibility of making structures consisting of many bonds within the scope of one operation
• easy and simple automation
• possibility of manufacturing complex structures
• possibility of bonding metals with glass, porcelain and ceramics
• cost effectiveness process
• possibility of bonding materials with low wetability

Brazing process stages:

• shaping and cleaning of the surfaces of bonded elements,
• heating them up to a temperature close to filler metal melting temperature,
• application of flux, melting of filler metal and introduction between the bonded surfaces,
• mutual diffusion of the bonded metals and liquid filler metal,
• cooling and setting of filler metal.

In practice the particular stages usually overlap and sometimes they even take place concurrently (e.g.: heating up the parts and melting of the filler metal).

The strength of the brazed bond, to a large extent. depends on, the adherence of the filler metal to the surface which in turn is dependent on the wetability property, that is the ability to cover the brazed surfaces by a thin, uniform, uninterrupted layer of liquid filler metal. A condition of wetability is, for the adhesion forces between the particles of the liquid filler metal and the particles of the brazed metals were larger than the cohesion forces between the particles of the liquid filler metal.