Brazing Fundamentals

The principles of brazing as a process are well established and documented. At VBC Group we follow these when we are supporting our customers with activities such as: advising on new brazing jobs; problem solving; providing technical training and; undertaking consultancy work.

Here you’ll find some fundamental information relating to the brazing process.

  • What is the Difference Between Welding, Brazing and Soldering?

    • Welding – is a thermal joining process used to make joints between materials, usually metals or thermoplastics. A heat source is used to melt the base (or parent) materials together and on cooling they fuse together. Some welding techniques use pressure in conjunction with heat, or by itself, to produce the weld.

      In many welding processes a filler material of the same or a compatible chemistry is introduced between the base materials forming a weld pool which becomes the joint. Depending on the weld configuration (butt, full penetration, fillet, etc.) the resulting welded joint can be stronger than the base material.

      Another general feature of welding is that it requires a form of shield to protect the filler metals or melted metals from being contaminated or oxidized.

      Welding is distinct from lower temperature metal-joining techniques such as brazing and soldering, which do not melt the base metal.

      Brazing – is a thermal joining process in which base or parent materials, metal alloys, cermets or ceramics, can be joined by introducing a molten filler metal between their surfaces. In many instances capillary attraction is the force which causes the molten braze filler material to be drawn between the parent materials.

      By definition brazing takes place below the melting point of the parent materials but above 450°C. Because the parent materials are not melted during the brazing process the heat affected zone resulting can be less than for other thermal joining processes.

      Brazing is a versatile process that can be used to join dissimilar materials such as copper and its alloys, precious metals, ferrous metals, nickel alloys, aluminium, tungsten carbide, PCD and a range of ceramics.

      Brazing relies of the parent materials to be sufficiently free from oxides to alloy the molten brazing alloy to form a bond with the parent materials. This is achieved by using a chemical flux, a protective or chemically reducing gas or a vacuum.

      Soldering – is a thermal joining process which is similar to brazing in many ways. By definition soldering takes place below the melting point of the parent materials and below 450°C. Because the soldering alloys (soft solders or solders) melt at comparatively low temperatures soldering can be used to join materials that cannot be exposed to high temperatures or where a large heat affected zone is undesirable.

      Solders tend to be comparatively soft and not strong materials and in turn soldered joints are not as strong as welded or brazed joints. Like brazing, soldering often makes use of a flux or a protective atmosphere to achieve an oxide free surface upon which the solder can form a bond with the parent materials.

      Soldering is a widely used process in the electronics industry and in a wide range of industrial applications.

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  • Definition of Brazing and Soldering

    • Brazing and Soldering are thermal joining process in which a molten filler metal is drawn into a capillary gap between the materials being joined. By convention brazing filler metals have a melting point above 450˚C whereas solders melt below this temperature. Both brazing and soldering alloys melt below the melting point of the materials being joined. Brazing is a versatile joining process it can be used to join most metals and alloys commonly used in engineering.

  • Standards in Brazing

    • ISO 17672: 2016 - Brazing Filler Metals is the key international standard used to define the composition of brazing filler metals. It defines the families of brazing alloy types, applies limits to major elements and on the impurities known to be detrimental to the performance of the brazing filler metal. It lists melting ranges (rounded to 5°C), lists special vacuum grade impurity levels and covers forms of supply. This standard does not place any requirements on the properties of the filler metals, recommended brazing temps are not given.

      ISO 3677: 2016 is a standard or codex intended for use in providing a designation for brazing and soldering alloys that are not covered by any other standard. This standard does not provide a specification for the alloys described by it, so it does not for example specify the absolute composition, impurity limits or properties of the alloys it can be used to designate. 

      AWS A5.8 is the American Welding Society's standard for brazing filler metals. It specifies the composition of the filler metals, applies limits to major elements and impurities known to be detrimental to the performance of the brazing filler metal. Potentially this standard could have minor differences to the ISO standard. It details special vacuum grade impurity levels (in a different way to ISO 17672) and outlines forms of supply. Melting ranges are not given, there are no recommended brazing temps and no physical or mechanical properties are specified.

       

  • Terminology Connected with Brazing Products

    • Brazing Alloy / Brazing Filler Metal - A filler metal which melts above 450˚C and below the melting point of the materials to be joined.

      Soft Solder / Solder / Soldering Alloy - A filler metal which melts below 450˚C and below the metals being joined.

      Silver Solder / Hard Solder – a historic terms used by silversmiths for over 100 years and some hobbyists to describe the silver brazing alloys that that group uses.

      Brazing Flux - A chemical substance generally used when brazing in air which is applied to the parent metals to prevent oxide formation and build up during heating, promoting flow and wetting of the brazing filler metal.

      Active Brazing Alloy – A term used to describe a brazing product which contains a reactive element or substance allowing it to form a bond with non-metallic parent materials. Active elements used in this way include titanium, titanium hydride, lithium and zirconium and they can be alloyed with the brazing alloy or included in a hybrid product.

      Tri-foil / Ply-metal/ ‘Sandwich’ Alloy – A hybrid brazing foil product containing an interlayer of a higher melting point material (usually copper) with a (silver) brazing alloy on the two faces of the foil. These products are designed to ‘cushion’ a joint absorbing stresses built up during the brazing process or in subsequent operations.

  • Brazing Terminology

    • Parent Materials / Metals - The materials to be joined by brazing.

      Wetting - When molten brazing alloy flows over and adheres to the parent metals. The degree of wetting can be judged by the contact angle of the braze deposit.

      Joint Gaps - The gap between the parent metals to be joined into which the brazing alloy will flow

      Capillary Attraction - The force that draws a molten brazing alloy into the joint gap.

      Solidus - The highest temperature at which a filler metal is completely solid.

      Liquidus - The lowest temperature at which a filler metal is completely liquid.

      Melting Range - The temperature range over which the filler metal melts.

      Eutectic - An alloy with a single melting point rather than a melting range is known as a eutectic alloy. This can apply to a brazing or soldering alloy that melts at a single point on the temperature scale.

      Working Range of a Flux – The range of temperature over which a flux is generally considered to be active and able to reduce metal oxides.

  • Common Heating Methods and Techniques

    • Flame Brazing – is a very common and generally low cost way, of heating components that need to be brazed in air. Flame heating is often used for brazing with silver brazing alloys, copper phosphorus brazing alloys, copper based or aluminium brazing materials. Various fuel gasses can be used such as; air / natural gas; air / propane; air acetylene; oxy-propane; oxy-acetylene; and more recently, hydrogen-oxygen mixes produced from either the electrolysis of water or cracking of water across a proton exchange membrane. Most flame brazing involvs the use of a hand torch however the use of fixed burners, which can be computer controlled, is an effective way to build up a heat pattern suitable for brazing.

      Induction Brazing - is a well established, highly effective, repeatable and safe method of brazing. The parts to be brazed are introduced into an electro magnetic field which induces eddy current in them and causes them to heat up.

      Induction brazing in air, using a silver brazing alloy and a flux, is a widely used process. Specialised applications have been developed in which a chamber containing a vacuum, an inert gas or a reducing gas such as dry hydrogen / nitrogen mix. In these applications there is no need for a brazing flux.

      Resistance Brazing - is a less common heating method for brazing. An electrical current is passed through the components to be brazed causing them to heat up and then for the brazing alloy to melt. Resistance heating is particularly effective when brazing copper components with a silver-copper-phos brazing alloy preplaced in the joint.

      Vacuum Brazing – An industrial batch process in which components are heated in a vacuum allowing a successful brazed joint to be made due to the absence of oxygen

      Partial Pressure Furnace Brazing – A technique in which an amount of inert gas, often dry argon, is introduced into a vacuum furnace to suppress the volatilisation elements with a low vapour pressure from the molten braze alloy during brazing.

      Controlled / Reducing Atmosphere Brazing - is a continuous brazing process in which pre-assembled parts are loaded onto a belt and enter a furnace with a reducing or 'controlled' gas atmosphere. The parts travel through a series of heating zones until they reach brazing temperature. The reducing gas atmosphere breaks down any pre-existing oxides and keeps the parts free from further oxidation allowing the braze alloy to form a successful brazed joint. The parts emerge bright and free from oxidation.

  • Metallurgical Terms Associated with Brazing

    • Liquation - When a brazing filler metal with a long melting range is heated too slowly, the phase with the lowest melting point begins to flow first. The material left behind has a changed composition and a higher melting point. It will not flow readily. An unsound and unsightly joint is the usual result of liquation.

      Dezincification - This is a form of galvanic corrosion, generally associated with two phase brass alloys, in which the zinc rich beta phase is selectively leached out of the brass, leaving a sponge like matrix of the copper rich phase. This can occur when brazed joints are exposed to salt or seawater.

      Intergranular Penetration - Nickel and nickel-based alloys are prone to cracking during brazing with silver brazing filler metals. The cracking is known as intergranular penetration or stress cracking. It usually follows the grain boundaries and only occurs when components are subject to high stresses either in the material or created through the brazing process.

      Crevice or Interfacial Corrosion - An uncommon form of corrosion which can occur when silver brazed joints made on certain grades of stainless steel are exposed to water or humidity in service, joint failure may result from a specific form of interfacial or crevice corrosion along the stainless steel - brazing filler metal interface. Certain brazing alloys and forms of stainless steel are less susceptible to this form of corrosion.

  • The Properties of a Successfully Brazed Joint

      • Strong and ductile - well made brazed joints can be at least as strong as the parent metals being joined and will withstand demanding service conditions.
      • Leak tight - brazing produces leak tight joints widely used on liquid and gas pipe-work installations.
      • Electrical conductivity - brazed joints offer good electrical conductivity and are used in applications where this property is important.
      • Joint appearance - brazed joints can have smooth, neat fillets.
      • An outstanding feature of brazing is its ability to join different metals and components of dissimilar size and mass.
      • Brazing is also capable of joining ceramics and cermets such as tungsten carbide, PCD, diamond, quartz, sapphire, silicon nitride and similar non-metallic materials.
      • In practice brazing processes span a wide range of filler metals and temperatures from aluminium brazing staring at 450˚C, through silver brazing, 650˚C-900˚C, up to nickel and precious metal brazing over 1200 ˚C.