Brazing processes

A predominant distinction is made between brazing and soldering (please also refer to the foundations of brazing). The following section takes a closer look at the brazing processes - subdivided by the respective energy source.

Brazing with an electric current can be further subdivided - according to image 1 - by the three process groups of induction brazing, resistance brazing and furnace brazing, which are each explained individually.

Soldering is used in particular in electronics manufacturing, and is a separate area of specialisation which is not subject to further investigation in the following.

 

Verfahren des Hartlötens
Image 1: The brazing processes [1]

Brazing using liquids for heating

With brazing that uses liquids, the component to be brazed is submerged in a vat of molten filler metal, various salts or molten flux, thus heating it to the brazing temperature. This method is, however, very rarely used in practice.

Flame brazing

Flame brazing is one of the most commonly used brazing methods. It is used both manually and for fully automated brazing equipment for a broad variety of materials and applications. The burners used for flame brazing are operated using different combustible gases (e.g. hydrogen, acetylene, propane). There are also different types of oxygen feed (e.g. pure oxygen, air pressure, induction air) and a diverse range of burner designs, in order to achieve the optimum heating depending on the component to be brazed.

Applications for flame brazing include:

  • copper pipes used in installation technology
  • cemented carbide tools
  • steel, copper and aluminium tubes
  • compressors for refrigerators and air-conditioning units
  • valves for refrigeration and air-conditioning technology

Brazing with an electric arc/brazing using radiation

Bild: Kfz-Karosserie
Image 4: Car body

Arc brazing - as with laser beam brazing - is used above all in automotive manufacturing. These processes are particularly suitable for joining surface finished thin steel sheets, and for structural components. As brazing alloys mainly special copper and aluminium alloys are used. With the use of these processes in automotive manufacturing, it was possible to implement the following applications [2]:

  • Joining of the ABC pillars to the roof
  • Laser beam or plasma brazing on C pillar
  • Laser beam brazing on the trunk cover
  • MIG brazing of the fuel tank tube
  • MIG brazing on the structure above the axles and seat rails
  • Laser beam brazing on the roof
  • MIG brazing of the longitudinal traverses

Electron-beam brazing is conducted in a fine or high vacuum and is used, for example, for components with extremely narrow brazing joints, or which necessitate local energy use with a high-power density, and for which high heating speeds within a short space of time must be achieved [3].

Induction brazing

Image 5 Induction brazing
Image 5 Induction brazing of a cemented carbide

Induction brazing involves heating the components to be brazed to the brazing temperature using induction heating. This is based on the following physical principle: if a metal object, which is to be heated, is exposed to an electromagnetic alternating field, an electric current is induced therein. The flow of current subsequently heats the metal. This simple procedure and the associated physics can be explained using Joule’s law. It states: if a current I flows into a metal object with the electrical resistance R, the electrical charge P in this object is converted according to

P = R • I2

into heat output. If current I flows through the metal object during a defined time period t, therefore in this time the electrical energy W is converted according to

W = P • t = R • I2 • t

into heat Q = W.

This then generates the so-called Joule’s heating. Inductive heating is classed as one of the direct heating methods. The heat is generated within the work piece itself and does not need to be transferred from outside by way of heat conduction, convection or heat radiation [4].

In ferromagnetic materials, additional re-magnetisation losses occur, which also contribute to the heating process. In non-magnetic materials (e.g. copper, aluminium, brass, stainless steel) eddy-current losses have an exclusive effect.

Bild: Induktionslötanlage mit Drahtvorschub und Pyrometer
Image 6: Induction heating device with wire feed and pyrometer

Induction brazing can be conducted in contact with air using fluxes as well as in protective gas atmospheres (e.g. hydrogen, nitrogen, argon) or in vacuum.

Induction brazing is used e. g. for following applications:

  • brazing of cemented carbides (e.g. drills, circular saw blades, paper knives, milling cutters) in the tooling industry
  • brazing of metallic eyeglass frames, in particular made from titanium in the eyeglasses manufacturing industry 
  • brazing of contact materials for all types of switch in the electrical industry
  • brazing of thin-walled tubes within the automotive industry
  • brazing of compressor connectors for refrigeration and air-conditioning technology
  • brazing of pipe connections for refrigeration and air-conditioning technology

Resistance brazing

Bild: Prinzip der Widerstandserwärmung
Image 7: Principle of resistance heating, left indirect heating, right direct heating [5]

With resistance brazing, the current flows directly into the components to be brazed. The heat which is generated by the resistance of the conductors through which the current is passed (Joule effect) is thereby used for brazing [5].

The brazing joints can subsequently be heated indirectly or directly (image 7).

With indirect heating (image left) the necessary amount of heating required for brazing can be generated by the inner electrical resistance of a work piece (inner resistance heating). The heat which is generated by the resistance is then transported by heat conduction to the brazing joint. This form of heating is preferably used with materials which have a high degree of electrical resistance. Typical application cases include brazing of steel and cemented carbides in the tooling industry [5].

With direct resistance heating (image right) the work pieces to be joined are lined up together in a row. The contact resistance between the electrode and the work piece is kept to the lowest possible level as a result of the contact pressure and the conductivity of the electrodes and their design. The heating is achieved above all via the contact resistance between the parts to be brazed and via the material resistances. Typical application cases are brazing of copper or brass and contact materials in the electrical industry.

Furnace brazing

Bild: Metall-Metalloxid-Gleichgewichte in Wasserstoff als Funktion von Taupunkt und Temperatur
Image 8: Metal-metal oxide balances in hydrogen as a function of dew point and temperature [5]

In most instances furnace brazing involves a flux-free brazing process in a controlled furnace atmosphere. Suitable furnace atmospheres are standard atmosphere (seldom with the support of flux agent), reducing gas, inert gas and vacuum [5].

Furnace brazing is characterised by:

  • the brazing of components with several joints in one brazing procedure
  • the even heating of components
  • minimal distortion
  • minimal thermal stress
  • the combination of brazing treatment with subsequent heat treatment in one process

Through the use of reducing gases the existing metal oxides are removed through a chemical reaction with the reducing components of the protective gas, primarily hydrogen and carbon monoxide. The selection of a metal oxide is primarily dependent on the stability of a metal oxide and the oxygen affinity of the metal. With the assistance of metal-metal oxide balance curves (image 6) which display the balances as a function of dew point and temperature, the conditions for a reduction of the oxide in a protective gas atmosphere can be read.

The dew point is the temperature at which a wet gas is saturated with water vapour. The lower the dew point, the lower the water vapour content. According to image 8, for the reduction of Cr2O3 with a dew point of -40°C a working temperature of at least 1000°C must been reached.

Bild: Unterschiedliche Bauteile, gelötet im Schutzgasdurchlaufofen
Image 9: Different components, brazed in protective gas conveyor belt furnaces

In the use of inert gases (e.g. argon, helium and, for certain materials, even nitrogen) there is no reaction between the gases and the base materials. In inert gas atmospheres, the components to be brazed must therefore be freed before the brazing process, either mechanically or chemically, from surface layer oxides. The new formation of oxides during the heating process in the brazing furnace is then drastically slowed down to the extent that no visible oxide layers are able to form. The removal of oxide layers in inert gas atmospheres is achieved by the rupturing of the residual thin oxide films at higher temperatures, due to the significant differences in the thermal expansion of metal and metal oxide. A liquid filler metal is then able to wet the metallic base material at these tears, and infiltrate the oxide layer [6, 7].

Chamber furnaces or conveyor belt furnaces are used for furnace brazing with protective gases. The correct furnace type depends on the size and number of components to be brazed.

Furnace brazing in protective gas atmospheres is suitable for components made from different materials. In particular, mass-produced parts within the automotive industry (e.g. aluminium radiators, tubes etc.) as well as cemented carbide-steel tools are brazed in protective gas conveyor belt furnaces.

Bild: Kaltwand-Vakuumofen mit Molybdän-Heizleitern und -Strahlungsblechen
Image 10: Cold wall vacuum furnace with molybdenum heating conductors and sheet metal heat deflectors

Furnace brazing in a vacuum is done in a fine vacuum (1 – 10-3 mbar) or a high vacuum (10-3 – 10-7 mbar) in chamber furnaces. It is particularly suitable for materials with very stable oxide layers which do not dissolve in protective gas atmospheres (e.g. titanium and aluminium oxide). The removal of oxide layers is also achieved by the rupturing of the residual thin oxide films at higher temperatures, due to the significant differences in the thermal expansion of metal and metal oxide. The filler metal subsequently wets the tears and infiltrates the oxide films [6].

With furnace brazing in a vacuum, both conventional metallic materials and special materials such as ceramics are successfully joined. Due to the fact that, with this process, no flux is utilised, perfect brazed joints are created as a rule. These are characterised by a high degree of filling, no foreign particles or gas pores, no oxidation and bare brazed joints.

The filler metals which can be used for furnace brazing in a vacuum must not contain elements which possess a low vapour pressure, due to the fact that they would outgas in the furnace during the brazing process. With the filler metals being used here it primarily concerns pure copper and copper alloys, zinc-free silver brazing solders, gold- and palladium-based solders as well as nickel and cobalt-based solders which are used depending on the type of application.

Literature

[1] ISO 857-2 Welding and allied processes – Vocabulary –
Part 2: Soldering and brazing processes and related terms
[2] Brazing solutions for car body production

Publication of voestalpine Böhler Welding
[3] Löten von Kupfer und Kupferlegierungen

Deutsches Kupferinstitut
[4] Auftraglöten verschleißfester Hartstoff-/Hartlegierungs-Verbundsysteme 

Hartmut Schmoor

Verlag Mainz, Aachen, 1996
[5] Hartlöten – Eine Einführung

Erarbeitet vom Arbeitskreis „Schulungsunterlagen“ und der Arbeitsgruppe V6.1 „Hartlöten“ im Ausschuss für Technik des DVS

Herausgegeben von der Fachgesellschaft „Löten“ im DVS

ISBN 978-3-87155-839-9, DVS Media GmbH, Düsseldorf
[6] Hart- und Hochtemperaturlöten

Die Schweißtechnische Praxis, Band 20

Paul Zaremba

DVS Verlag GmbH Düsseldorf, 1988
[7] Industrial Brazing Practice, Second Edition

Phil Roberts

CRC Press 2013