SOLDERING AND BRAZING

Meaning of Soldering and Brazing

Soldering is a method of joining two metals surfaces by melting an alloy between them. It is a process in which two or more metal items are joined together by melting and flowing a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal.

Brazing is a metal joining process whereby a filler metal is heated above melting point and distributed between two or more close fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere, usually a flux.

It can also be defined as a process of joining similar metals using a filler metal that typically includes a base of copper combined with silver, nickel, zinc or phosphorus.

Brazing is sometimes referred to as hard soldering. It is used when a strong joint is required.

 

Soldering and Brazing Tools and Materials

Soldering Tools

The essential soldering tools are the soldering iron and solder. There are other soldering accessories such as: soldering iron tips, soldering iron stands, clamps, magnifying glass, solder wick, solder sockets and fume extractors.

1. Soldering Iron: A soldering iron is a hand tool used in soldering. It supplies heat to melt the solder so that it can flow into the joint between two work pieces. It is composed of a heated metal tip and an insulated handle. Heating is often achieved electrically, by passing an electric current through a resistive heating element.
2. Solder: There are two types of solder. These are – rosin core and acid core. The rosin is the kind of flux, which cleans as you solder, it comes in three types – 50/50, 60/40 and 63/37. These numbers represent the amount of tin and lead percentage present in the solder. The acid core solder are tricky – the flux is an aggressive type designed to solder steel as well as other metals. This method should be avoided for beginners.

 

Brazing Tools

The essential brazing tools are: brazing rod, acetylene torch or oxygen, brazing torch and brazing flux.

Brazing materials include: silver, brass and copper.

 

Filler Materials

A variety of alloys are used as filler metals for brazing depending on the intended use or application method.

Some of the common types of filler metals used are: aluminium, silicon, copper, copper-silver, copper-zinc (brass), gold-silver, nickel alloy and silver.

 

Operations, Types and Uses of Soldering and Brazing

How to join Metals by Soldering

Soldering is similar to welding, but it is quite different. In soldering, we make use of solder (alloy of metals) and soldering iron. For a good soldered joint, clean the components that need to be joined, then connect the two joints mechanically. Finally, the two joints connect electrically by melting some solder on top using soldering iron. When soldering, simply melt a low temperature alloy on top of the terminals you are joining together to make a reliable electrical bridge between them. The terminal themselves do not actually melt and are not usually changed in any way, the solder just sits on top.

 

Soldering Operations

Soldering operations can be simply classified into four general forms.

1) Sweat Soldering: A sweated joint can be utilised when relatively large surfaces of the components are to be joined or when a work is too small and cannot be joined by normal soldering operations.

The process involves lining the faces to be joined and the work is brought together under light pressure. Heat is then applied until the solder runs and melts.

 

2) Tinning the Bits: This is done by:

• Heating the soldering iron or bit to the correct temperature in a gas furnace.
• Cleaning the soldering bit with a file or wire brush.
• Applying some flux by dipping the bit inside the flux.
• Coating the bit with a film of solder by rubbing a stick of solder over it.

 

3) Cleaning the surface: The cleaning of the area or surfaces to be joined or after joining is very important. Different ways of cleaning the surfaces to be joined include:

• Filling: This is done before heat is applied to the metal to be joined.
• Using Emery Paper or Cloth: This is also done before heat is applied to the metal to be joined.
• Using Fluxes: This depends on the metal to be used and the operations involved.

 

4) Washing: The finished work can be washed in hot water or hot soda in order to remove flux that can cause corrosion.

 

How to join Metals by Brazing

The brazing process produces a much stronger joint than that of soldering. The process makes use of solder referred to as ‘spelter as a filler rod. Spelter is an alloy of copper and zinc, which are harder than solder (tin and lead). It has a higher melting point than solder (i.e. 900°C).

For brazing, the whole workpiece is heated to 900°C and the molten spelter flows into the joint. The spelter has a lower melting temperature than the materials to be joined.

The only flux used during brazing is called borax and it performs the following function.

1. It dissolves the oxides that form on the surface of the work to be joined.
2. When the required temperature is attained (red heat), the borax verifies and forms a film over the surface of the metal, preventing it from atmospheric actions. The equipment used in brazing is blow pipe or blow torch.

The process involved in brazing joints with blow torch and brazing rod is as follows:

1. The two pieces of steel to be brazed must be cleaned so as to remove grease and dirt. Wire wool or emery cloth are the most suitable abrasives. A borax flux (powder) is mixed with water to produce a paste which is brushed along the joint. Flux prevents oxidation on the metal surfaces as this would make the brazing process unsuccessful.
2. A compressor is turned on and this pressurises the gas and air. As the gas air control is slowly turned on, gas is fed through the nozzle and this is ignited by the pilot light on the nozzle or by a match.
3. Once lighted, the gas-air control is turned to allow more air or gas through the nozzle which gives a longer and more fierce flame. The length of the flame can be adjusted with the control until the desired type is achieved. As a rough guide, a blue section of flame will appear near the nozzle, the end of this is the hottest part of the flame.
4. The two pieces of steel should have already been placed on the rotating table. Fire bricks are used to raise both pieces slightly off the surface of the table so that heat can flow round it. Fire bricks should also be placed at the back and sides of the metal so that heat does not escape. The steel is given a gentle overall heating first which raises the temperature slowly. This allows the steel to expand slowly and for the water in the flux to evaporate without moving the steel out of position. The flame is moved around the joint fairly quickly throughout this first stage of heating.
5. The flame is then moved forward with the blue tip of the flame nearly touching the steel. The focus of heat should now be on the joint as the flame is slowly moved backward and forward along it. The joint will entirely become so hot that it becomes red in colour.
6. A brazing rod (copper-zinc alloy) is then pushed gently against the joint and if the temperature is right, the end of the rod will melt and begin to run along the joint. The rod is fed into the joint until a brazed joint is complete. The steel is allowed to cool slowly because if cooled quickly, such as dipping in water, the joint can crack or become distorted.