Brazing and Soldering:

Both brazing and soldering are the metal joining processes in which parent metal does not melt but only filler metal melts filling the joint with capillary action. If the filler metal is having melting temperature more than 450°C but lower than the melting temperature of components then it is termed as process of brazing or hard soldering. However, if the melting temperature of filler metal is lower than 450°C and also lower than the melting point of the material of components then it is know as soldering or soft soldering.

During brazing or soldering flux is also used which performs the following functions:

• Dissolve oxides from the surfaces to be joined.

• Reduce surface tension of molten filler metal i.e. increasing its wetting action or spreadability.

• Protect the surface from oxidation during joining operation.

The strength of brazed joint is higher than soldered joint but lower than welded joint. However, in between welding and brazing there is another process termed as ‘braze welding'.

Braze Welding:

Unlike brazing, in braze welding capillary action plays no role but the filler metal which has liquidus above 450 ° C but below the melting point of parent metal, fills the joint like welding without the melting of edges of parent metal. During the operation, the edges of the parent metal are heated by oxy-acetylene flame or some other suitable heat source to that temperature so that parent metal may not melt but melting temperature of filler metal is reached. When filler rod is brought in contact with heated edges of parent metal, the filler rod starts melting, filling the joint. If edges temperature falls down then again heat source is brought for melting filler rod. The molten filler metal and parent metal edges produce adhesion on cooling resulting into strong braze weld.

The braze welding filler material is normally brass with 60% Cu and remaining Zn with small additions of tin, manganese and silicon. The small additions of elements improve the deoxidizing and fluidity characteristics of filler metal.


The most commonly used filler metal is copper base zinc alloy consisting of normally 50-60% Cu, approximately 40% Zn, 1% Ni, 0.7 % Fe and traces of Si and Mn, which is brass and termed as 'spelter'. In some cases around 10% Ni may also be added to filler alloys. Copper base alloys may be available in the form of rod, strip and wire. Silver brazing filler metal may consists of 30-55% Ag, 15-35% Cu, 15-28% Zn, 18-24% Cd and sometimes 2-3% Ni or 5% Sn. Silver brazing alloys are available in form of wire, strip, rods and powders.

Borax and boric acid are commonly used fluxes for brazing with copper base filler metals. Many other commercial fluxes may be available in the form of paste or liquid solution leading to ease of application and adherence to the surface in any position.

Various commonly used method of brazing are followings:

• Torch Brazing

Torch brazing utilizes the heat of oxy-acetylene flame with neutral or reducing flame. Filler metal may be either preplaced in form of washers, rings, formed strips, powders or may be fed manually in form of rod.

• Dip Brazing

In dip brazing components with filler metal in proper form is preplaced at the joint and assembly is dipped in bath of molten salt which acts as heat source as well as flux for brazing. Preplaced preform melts and fills the joint. Another variant is to dip assembled parts in metallic bath and metal of bath fills the joint.

• Furnace Brazing

Self fixturing assembly with preplaced filler metal is placed inside electrically heated furnace with temperature control for heating and cooling. These furnaces may also be using protective atmosphere with inert gases like argon and helium or vacuum for brazing of reactive metal components.

• Infra-red Brazing

The heat for brazing is obtained from infra-red lamps. Heat rays can be concentrated at desired area or spot with concave reflectors. Such method of brazing requires automation and parts to be joined should be self fixturing. Filler metal is to be preplaced in the joint. The operation can be performed in air or in inert atmosphere or in vacuum.

• Induction Brazing

The heat is generated by induced current into the workpiece from a water cooled coil which surrounds the workpieces to be brazed. High frequencies employed vary from 5 to 400 kHz. Higher the frequency of current, shallow is the heating effect while lower frequencies of current lead to deeper heating and so it can be employed for thicker sections. Fluxes may or may not be used during brazing.

• Resistance Brazing

In resistance brazing the heat is generated at the interfaces to be brazed by resistive heating. The components are connected to high current and low voltage power supply through two electrodes under pressure. Only those fluxes are used which are electrically conductive and filler metal is preplaced.

Fig 3.2: Typical Self Fixturing Brazing Assembly

Fig 3.3: Preplaced Brazing Material and filling of joint during Brazing.


The soldering filler metal is called solder. The most commonly used solder is lead and tin alloy containing tin ranging from 5 to 70% and lead 95 to 30%. Higher the contents of tin, lower the melting point of alloy. Other filler metal are tin-antimony solder (95% tin and 5% antimony), tin-silver solder (tin 96% and silver 4%), lead-silver solder (97% lead, 1.5 tin and 1.5 silver), tin-zinc solder (91 to 30% tin and 9 to 70% zinc), cadmium-silver solder (95% cadmium and 5% silver). These are available in the form of bars, solid and flux cored wires, preforms, sheet, foil, ribbon and paste or cream.

Fluxes used in soldering are ammonium chloride, zinc chloride, rosin and rosin dissolved in alcohol.

Various soldering methods are soldering with soldering irons, dip soldering, torch soldering, oven soldering, resistance soldering, induction soldering, infra-red and ultrasonic soldering.

Soldering iron being used for manual soldering, consists of insulated handle and end is fitted with copper tip which may be heated electrically or in coke or oil/gas fired furnace. Solder is brought to molten state by touching it to the tip of the soldering iron so that molten solder can spread to the joint surface.

Ultrasonic soldering uses ultrasonics i.e. high frequency vibrations which break the oxides on the surface of workpieces and heat shall be generated due to rubbing between surfaces. This heat melts the solder and fills the joint by capillary action.

Flux Residue Treatment:

When brazing or soldering is completed then the flux residues are to be removed because without removal the residues may lead to corrosion of assemblies.

Brazing flux residues can be removed by rinsing with hot water followed by drying. If the residue is sticky then it can be removed by thermal shock i.e. heating and quenching. Sometimes steam jet may be applied followed by wire brushing.

Soldering flux residues of rosin flux can be left on the surface of joint, however, activated rosin flux and other flux residues require proper treatment. If rosin residues removal is required then alcohol, acetone or carbon tetrachloride can be used. Organic flux residues are soluble in hot water so double rising in warm water shall remove it. Residue removal of zinc chloride base fluxes can be achieved by washing first in 2% hydrochloric acid mixed in hot water followed by simple hot water rinsing. [/justify][/left]