The foundations of brazing

The following article focuses on the foundations of brazing, and is intended as an initial introduction to this topic. This includes a definition of brazing, the significant terms critical to an understanding of brazing and the prerequisites and overviews of brazing methods, filler metals and fluxes.

Definition of brazing

Bild: Schmelztemperaturen von Grundwerkstoffen und Lot
Image 1: Melting temperatures of base materials and brazing alloy, schematic

Brazing is an insoluble, firmly bonded joining technology like welding and gluing [1] and is internationally defined as follows in the International Standard ISO 857-2 (key terminology is explained below):

“Joining processes in which a molten filler material is used that has a lower liquidus temperature than the solidus temperature of the parent material(s), which wets the surfaces of the heated parent material(s) and which, during or after heating, is drawn into (or, if pre-placed, is retained in) the narrow gap between the components being joined” [2].

Brazing is typically used for metals. However, non-metals such as, for example, ceramics, can also be brazed. The following explanations deal exclusively with metal brazing.

According to the definition, brazing involves the preheating of the component parts to be joined together to a temperature at which only the filler metal melts, but not the base material(s). This is the significant difference to the welding process.

Liquidus und solidus temperature

Bild: Solidus- und Liquidustemperatur sowie Schmelzbereich des Lotes
Image 2: Solidus and liquidus temperature as well as melting range of filler metal Ag 145

The terms liquidus and solidus temperature originate from physical metallurgy. While pure metals melt at an exact temperature (e.g. silver at 961°C), materials consisting of different metals possess so-called melting ranges in which they are neither completely solidified or liquid. This applies in particular to filler metals, due to the fact that these are often compounds of two or more metals.

Materials which consist of two or more metals are also referred to as alloys.

The term liquidus temperature refers to the upper temperature of the melting range of a material. The material is completely liquid above this temperature.

The term solidus temperature refers to the lower temperature of the melting range of the material. The material is completely solidified below this temperature.

Image 2 explains these temperature terms using the silver filler metal Ag 145 (short code according to International Standard ISO 17672, brazing - filler metals [3]).

 

Wetting

The term “wetting” with reference to brazing relates to the “spreading and adhesion of a continuous layer of molten filler metal to the surface of the components being joined”. [2]

Molten filler metal wets a surface only under the following conditions [4]:

  • The brazing surfaces and the filler metal must be “bare metal”.
  • The brazing surfaces and the filler metal must at least have reached the brazing temperature.
  • At least one alloy element of the filler metal must be able to form an alloy with the base material.

“Bare-metal” denotes a surface which is free from impurities, oils, grease and oxide layers. This condition must also be insured during heating, which necessitates particular measures for the removal and prevention of new oxide layers forming. This includes the utilisation of flux agents and the application of protective gas - or vacuum atmospheres.

The brazing temperature corresponds to the “temperature at the joint where the filler metal wets the surface or where a liquid phase is formed by boundary diffusion and there is sufficient material flow” [2].

As a rule, filler metals consist of a mixture of several metals (e.g. silver, copper and zinc) which is why they are described as brazing alloys. At least one of the components found in the filler metal must also be miscible with the base material (e.g. iron), to form an alloy. This applies, for example, for iron with copper and zinc, but not, however, for iron with silver.

The wetting angle α indicates the degree of wettability of a base material by a liquid filler metal. This describes the angle established between a liquid drop of filler metal and the even base material surface.

Bild: Benetzungswinkel
Image 3: Wetting angle

For α < 90° wetting has occurred (optimal wetting upwards of α < 30°).
For α = 0° comprehensive wetting has occurred
For α = 90° the drop of filler metal has wetted but not spread.
For α > 90° there is no wetting.

Capillary attraction

Image 4: Capillary filling pressure in dependency on the gap width
Image 4: Capillary filling pressure in dependency on the gap width

As mentioned in the definition for brazing, a gap is located between the components to be joined into which the liquid filler metal flows. This so-called capillary attraction is responsible for the flow of a filler metal into the gap. This refers to the specific behaviour of liquids determined by surface tensions, which is visible in the narrow hollow spaces of fixed objects such as, for example, gaps, pores, tubes (capillaries). If the liquid can wet the solid body (or the filler metal the base material), the liquid rises upwards in a narrow tube from the solid body against gravity (= the filler metal flows into the gap).

The capillary attraction is defined as follows: “force, caused by surface tension, which draws the molten filler metal into the gap between the components being joined, even against the force of gravity.” [2]

The capillary filling pressure indicates the level of force. This is dependent on the gap width and the geometry of the brazing gap. In principle, it applies that the capillary filling pressure increases, the narrower the gap becomes.

Classification of brazing processes

Bild: Einteilung der Verfahren durch Weich- und Hartlöten
Image 5: Graphical classification of soldering and brazing processes [2]

The classification of brazing processes can be made according to different criteria, and different approaches are also taken in the literature. In the now redundant German Industry Standard DIN 8505 Part 2 (replaced by DIN ISO 857-2), classification takes place according to:

  • the liquidus temperature of the filler metals
  • the type of brazing geometry
  • the type of oxide removal
  • the type of filler metal application
  • the type of production
  • the energy sources

The definition of the corresponding terms and methods is according to the categories stated above.

The now valid International Standard ISO 857-2 picks up on the classification stipulated in German Industry Standard DIN 8505 Part 2 and defines the terms and processes directly without assigning titles to the classification. A method description based on energy sources is provided in the informative attachment to the standards.

A significant detail for the classification of brazing processes is the differentiation between
Soldering – a “joining process using filler metal with a liquidus temperature of 450 °C or less” [2] and
Brazing – a “joining process using filler metal with a liquidus temperature above 450 °C” [2]

Furthermore, the term high temperature brazing refers to a flux-free brazing process under the exclusion of air (vacuum, protective gas) which uses filler metals whose liquidus temperature is above 900°C. This term is, however, no longer listed under the International Standard ISO 857-2.

Filler metals

In accordance with the classification in terms of soldering and brazing as explained above, a differentiation is also made between soldering filler metals and brazing filler metals. Soldering filler metals are standardised in the International Standard ISO 9453 [5]; for brazing filler metals the International Standard ISO 17672 applies.

Bild: Schmelztemperaturbereiche der Hartlotgruppen nach DIN EN ISO 17672
Image 6: Melting temperature ranges of brazing filler metal groups pursuant to ISO 17672

Soldering filler metals are subdivided into lead-based and lead-free filler metals [5]. These contain different subgroups which are aligned with one another according to their composition. Their melting ranges are between 145°C and 370°C (lead-based soldering filler metals) and 118°C and 380°C (lead-free soldering filler metals) respectively. The primary field of application for soldering is electronics manufacturing and, in terms of its scope and significance, represents a separate area of specialisation which shall not be the subject of further detail here.

Brazing filler metals are categorised according to [3] into the following groups:

Group AI Aluminium and magnesium brazing filler metals
Group Ag Silver brazing filler metals
Group CuP Copper-phosphorus brazing filler metals
Group Cu Copper brazing filler metals
Group Ni Nickel (and cobalt) brazing filler metals
Group Pd Palladium bearing brazing filler metals
Group Au Gold bearing brazing filler metals

 

Fluxes

As mentioned in the chapter on “Wetting”, both the brazing surfaces and the filler metal surface must be “bare metal”, i.e. free from impurities, oils, grease and oxide layers, for the brazing to be successful. The removal of layers of oxide in particular, as well as the prevention of new layers forming during the heating process, requires special measures. In addition to the use of protective gas or vacuum atmospheres fluxes can also be used.

ISO 857-2 defines flux  as a “non-metallic material which, when molten, promotes wetting by removing existing oxide or other detrimental films from the surfaces to be joined and prevents their re-formation during the joining operation”.

Standards for fluxes required in brazing are listed in European Standard EN 1045. In principle, a differentiation is made between fluxes used in the brazing of heavy metals (class FH) and fluxes used in the brazing of light metals. The fluxes are supplied in powder, paste or liquid form, as well as a filler metal-flux mixture (paste-like or powder-like) [according to 6].

Type Flux base Working temperature range Application Behaviour of flux residues
FH10 Boron compounds, fluorides 550°C – approx. 800°C Multi-purpose flux Corrosive
FH11 Boron compounds, fluorides, chlorides 550°C – approx. 800°C Cu-Al materials Corrosive
FH12 Boron compounds, fluorides, boron 550°C – approx. 850°C Stainless and high alloyed steels, cemented carbides Corrosive
FH20 Boron compounds 700°C – approx. 1000°C Multi-purpose flux Corrosive
FH21 Boron compounds 750°C – approx. 1000°C Multi-purpose flux Non-corrosive
FH30 Boron and silicon compounds, phosphates > 1000°C Cu or Ni brazing filler metals Non-corrosive
FH40 Chlorides, fluorides (boron free) 600°C – approx. 1000°C Boron-free applications Corrosive
FL10 Chlorides, fluorides
Lithium compounds
> 550°C Light metals Corrosive
FL20 Fluorides > 550°C Light metals Non-corrosive
Table 1: Flux groups used for brazing metal materials pursuant to EN 1045

In order to both reduce and prevent the new formation of oxide layers, minimum amounts of flux are required on the surfaces to be brazed. Consequently, it applies that when brazing with fluxes the brazing gap must not become too small, due to the fact that otherwise an insufficient amount of flux is available for successful brazing. For this reason, a minimum brazing gap of 0.05 mm is used for brazing with fluxes in practice (please also refer to image 4).

Literature

[1] 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
[2] ISO 857-2 Welding and allied processes – Vocabulary –
Part 2: Soldering and brazing processes and related terms
[3] DIN EN ISO 17672 Brazing – Filler metals
[4] Principles of Brazing and Soldering
Daniel Schnee
Publication of Umicore AG & Co. KG
[5] DIN EN ISO 9453, Soft solder alloys - Chemical compositions and forms
[6]

EN 1045 Brazing - Fluxes for brazing -
Classification and technical delivery conditions