Dr. Dmitri Kopeliovich

Brazing is a method of joining two metal work pieces by means of a filler material at a temperature above its melting point but below the melting point of either of the materials being joined.
Flow of the molten filler material into the gap between the work pieces is driven by the capillary force. The filler material cools down and solidifies forming a strong metallurgical joint, which is usually stronger than the parent (work piece) materials. The parent materials are not fused in the process.

Brazing is similar to Soldering. The difference is in the melting point of the filler alloy: brazing filler materials melt at temperatures above 840°F (450°C); soldering filler materials (solders) melt at temperatures below this point.

The difference between brazing and welding processes is more sufficient: in the welding processes edges of the work pieces are either fused (with or without a filler metal) or pressed to each other without any filler material; brazing joins two parts without melting them but through a fused filler metal.

• Surface cleaning and brazing fluxes
• Brazing filler materials
• Brazing methods
• Advantages of brazing
• Disadvantages of brazing

Capillary effect (Fundamentals of adhesive bonding#Wetting|wettability) is achieved by both: a proper Surface preparation and use of a flux for wetting and cleaning the surfaces to be bonded.
Contaminants to be removed from the part surface are: mineral oils, miscellaneous organic soils, polishing and buffing compounds, miscellaneous solid particles, oxides, scale, smut, rust.
The work pieces are cleaned by means of mechanical methods, soaking cleaning and chemical cleaning (acid etching).

A brazing flux has a melting point below the melting point of the filler metal, it melts during the heating stage and spreads over the joint area, wetting it and protecting the surface from oxidation.
It also cleans the surface, dissolving the metal oxides.
It is important that the surface tension of the flux is: 1. Low enough for wetting the work piece surface; 2. Higher than the surface tension of the molten filler metal in order to provide displacement of the flux by the fused brazing filler. The latter eliminates the flux entrapment in the joint.
The flux is applied onto the metal surface by brushing, dipping or spraying.

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• Copper filler alloys: BCuP-2 (Cu-7P), BCuP-4 (Cu-6Ag-7P). Used for brazing Copper alloys, steels, Nickel alloys.
• Aluminum filler alloys: Al-4Cu-10Si, Al-12Si, Al-4Cu-10Si-10Zn, 4043 (Al-5.2Si), 4045 (Al-10Si). Used for brazing Aluminum alloys.
• Magnesium filler alloys: BMg-1 (Mg-9Al-2Zn), BMg-2 (Mg-12Al-5Zn). Used for brazing Magnesium alloys.
• Nickel filler alloys: BNi-1 (Ni-14Cr-4Si-3.4B-0.75C), BNi-2 (Ni-7Cr-4.5Si-3.1B-3Fe), BNi-3 (Ni-4.5Si-3.1B). Used for brazing Nickel alloys, cobalt alloys, Stainless steels.
• Silver brazing alloys: BAg-4 (40Ag-30Cu-28Zn-2Ni), BAg-5 (45Ag-30Cu-25Zn), BAg-6 (50Ag-34Cu-16Zn), BAg-7 (56Ag-22Cu-17Zn-5Sn). Used for most of metals and alloys except aluminum and magnesium alloys.

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• Torch brazing utilizes a heat of the flame from a torch. The torch mixes a fuel gas with Oxygen or air in the proper ratio and flow rate, providing combustion process at a required temperature.
  

The torch flame is directed to the work pieces with a flux applied on their surfaces. When the work pieces are heated to a required temperature, filler alloy is fed into the flame. The filler material melts and flows to the gap between the joined parts.
Torch brazing is the most popular brazing method.

Torch brazing equipment:
- Fuel gas cylinder with pressure regulator;
- Oxygen cylinder with pressure regulator;
- Welding torch;
- Blue oxygen hose;
- Red fuel gas hose;
- Trolley for transportation of the gas cylinders.

• Furnace brazing uses a furnace for heating the work pieces.
• Vacuum brazing is a type of furnace brazing, in which heating is performed in vacuum.
• Induction brazing utilizes alternating electro-magnetic field of high frequency for heating the work pieces together with the flux and the filler metal placed in the joint region.
• Resistance brazing uses a heat generated by an electric current flowing through the work pieces.
• Dip brazing is a brazing method, in which the work pieces together with the filler metal are immersed into a bath with a molten salt. The filler material melts and flows into the joint.
• Infrared brazing utilizes a heat of a high power infrared lamp.

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• Low thermal distortions and residual stresses in the joint parts;
• Microstructure is not affected by heat;
• Easily automated process;
• Dissimilar materials may be joined;
• High variety of materials may be joined;
• Thin wall parts may be joined;
• Moderate skill of the operator is required.

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• Careful removal of the flux residuals is required in order to prevent corrosion;
• No gas shielding may cause porosity of the joint;
• Large sections cannot be joined;
• Fluxes and filler materials may contain toxic components;
• Relatively expensive filler materials.

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• Classification of welding processes
• Principles of arc welding
• Carbon Arc Welding
• Shielded Metal Arc Welding (SMAW)
• Submerged Arc Welding (SAW)
• Metal Inert Gas Welding (MIG, GMAW)
• Tungsten Inert Gas Arc Welding (TIG, GTAW)
• Electroslag Welding (ESW)
• Plasma Arc Welding (PAW)
• Resistance Welding (RW)
• Gas Welding (GW)
• Electron Beam Welding (EBW)
• Solid State Welding (SSW)
• Thermit Welding (TW)
• Laser Welding (LW)
• Soldering
• Welding stainless steels

to Metals
to Metal joining technologies (welding, brazing, soldering)