Selection of filler metals and flux

The selection of a suitable filler metal and, potentially a flux for one of the following brazing tasks depends on several criteria, which can also have an interdependent influence on one another. As a rule, the filler metal is always selected first and then the flux, if required. A requirements profile for the brazing joints to be produced can be useful when selecting the filler metal, in which the technical and economic requirements are determined. The criteria for the selection of filler metals and fluxes are listed and explained in more detail in the following article. The list of criteria is by no means exhaustive; in individual cases a consultation with an industry professional is recommended. The terms used in this article are explained in the article on “The foundations of brazing”.

Criteria for selecting filler metals

Kriterien zur Auswahl von Loten
Thomas Knauer - Fotolia

The following criteria should be taken into consideration when selecting filler metals [1]:

  • type and treatment condition of the base materials
  • dimensions and production tolerances of the work pieces
  • the available operating equipment to be used for carrying out brazing work
  • load bearing capacity of the brazing joint
  • corrosive media at the brazed joints
  • further processing of brazed components
  • rules of technology and occupational health & safety
  • Economic efficiency of the brazing process

Type and treatment condition of the base materials

Art und Behandlungszustand der Grundwerkstoffe
Thaut Images - Fotolia

Due to the fact that with brazing the base material is not melted, the filler metal must have a lower liquidus temperature than the base material. As a rule, the liquidus temperature of the filler metal should be at least 50-100°C below the solidus temperature of the base material. If two different base materials are being used, this applies to the base material with the lowest melting point. It can, however, occur that a filler metal must be selected which melts at just a few degrees lower than the base material. This applies above all to the brazing of aluminium work pieces.

The filler metal must be able to wet the base material being used, i.e. at least one of the metals found in the filler metal must be miscible with the base material.

The base material can adopt special characteristics through heat treatments (e.g. hardening, homogenisation, strain hardening) before the brazing process. The filler metal must be chosen with regard to the brazing temperature in such a way that, during the heating process the characteristics are not impaired, to the greatest extent possible.

Dimensions and production tolerances of the work pieces

For larger - and therefore heavier - components the use of brazing filler metals with a low melting point is recommended, due to the fact that the heating of the components at high temperatures can be too complicated.

Generally speaking, a narrow brazing gap is recommended when brazing (0.05 - 0.2mm) so that the filler metal flows into this as a result of capillary action. If the manufacturer tolerances of the work pieces require a wider brazing gap, the use of viscous filler metals is recommended. This viscous condition can be created by processing the filler metal below the liquidus temperature. From heat transfer point of view this is all-the-more easy, the wider the melting interval of the filler metal. For this reason, a filler metal with a wide melting interval is used for a wide gap, and vice versa: the melting interval of the filler metal should therefore become narrower, the narrower the gap becomes [1].

The available operating equipment to be used for brazing

Verfügbare Betriebseinrichtungen für die Lötarbeiten

If operating equipment used for brazing (e.g. flame brazing systems, furnaces etc.) is available at the company, the filler metal must be processable with this equipment. Due to the fact that not all filler metals may be used for every heating process, this point is to be taken into consideration. This means that, for example, filler metals containing zinc may not be used in furnace brazing, since during these processes the zinc can be outgassed from the filler metal. This applies in particular to brazing in vacuum furnaces.

Load bearing capacity of the brazed joint

Brazed joints are subject to different stresses when in use. When selecting a filler metal, the mechanical stresses, operating temperature and potentially the operating pressure are to be taken into consideration.

Mechanical stresses can be either static or dynamic. In order to ensure that the brazed joint is suitable for use as a result of the filler metal used, these stresses should be known and defined in the requirements profile for the brazing joint. Generally speaking, the following strengths for appropriately designed and executed brazed joints with a gap filling level of 80% can be expected [1, 2, 3]:

Tensile strength: 200 MPa (N/mm2)
Shear strength: 100 MPa (N/mm2)

Detailed information on strengths are contained in part in the delivery programs and technical data sheets of filler metal suppliers. Separate inspections are to be carried out as required.

It must also be considered as to whether the brazing join is suitable for use in the prevailing operating temperatures. The information provided by the brazing manufacturer is also to be taken into consideration here. The value stated in the delivery programs or technical data sheets should not be exceeded in case of continuous load-bearing. Short-term breaches in temperature may be permissible if, upon increased temperatures no significant stress is placed on the brazed joint [1, 3]. Under no circumstances may the solidus temperature of the filler metal be exceeded.

Existing working pressure leads to mechanical stress of the brazed joint. This must be converted into a mechanical stress for the design in question; the filler metal may be selected thereafter. The operating temperature is also to be noted in this case.

If a vacuum density is required for the brazed joint, the vacuum consistency of the brazing filler metals is to be taken into consideration. This is inspected using vapour pressure curves. Correctly executed brazed joints are typically vacuum tight at room temperature. If vacuum integrity is also required at increased temperatures, zinc and cadmium-free filler metals are to be used [1, 3].

 

Corrosive media at brazed joints

Lötverbindungen

When selecting the filler metal, it is necessary to assess which media (air, gases, liquids) will come into contact with the brazed joint. This includes the media in use which comes into contact with the brazed component. Under certain circumstances, other media which come into contact with the brazed joint during manufacturing, transport and warehousing must also be taken into consideration.

When in contact with air practically all filler metals may be used.

When joining gas pipes, the rules of technology apply (e.g. DVGW worksheet GW 2, ISO 9539).

If brazed joints - which were manufactured using copper-phosphorous filler metals - come into contact with sulphur-based media, they can be destroyed through selective corrosion [4]. These media include sulphur-based industrial atmospheres, sulphur-based liquids, sulphur-based motor oils etc. The use of silver brazing solders is recommended in these cases.

If the brazed joint is to come into contact with liquids, the suitability of the filler metal is also to be assessed. Distilled water, organic solvents, alcohols and ammonia-free cooling agents do not require any special filler metal. The use of brazed joints in acidic solutions or in alkalines can require special filler metals depending on the pH value. In principle, the combination with the base material being used is always to be assessed when in contact with liquids. This applies in particular to the use of corrosion-resistant steels in contact with moisture, as well as the use of the brazed joint in contact with sea and brackish water.

Further processing of brazed components

Depending on the requirements placed on the finished component, this can be subject to further processing procedures such as coating, heat treatment etc. following the brazing process.

If the components are to be given a galvanized coating after brazing, filler metals should be used with which the removal of flux residues can be executed without any issue. This applies, for example, to brazing filler metals with a low melting temperature.

For the following enamelling processes or heat treatments, the temperature of the respective process is to be noted and a filler metal is to be selected, which guarantees sufficient rigidity at that temperature.

Metallic carbide tools in particular are sometimes also coated with a wear-resistant titanium nitride layer after the brazing process. This coating process is carried out in a vacuum at temperatures of approximately 500°C. Filler metals are to be selected for this process which do not contain components which experience outgassing at this temperature in the vacuum. As a rule, special zinc-free silver brazing filler metals are used for this particular application.

Rules of technology and Occupational Health & Safety

Depending on the component and application special laws, standards, rules of technology etc. are to be observed, as well as the specifications of the customer. These are to be checked with the respective specifications of the filler metal.

As with all technical processes the laws governing Occupational Health & Safety are to be observed. The safety data sheets for the filler metals contain information on the safe handling of the filler metals, and are therefore to be reviewed before use.

Economic efficiency of the brazing process

Wirtschaftlichkeit und Fertigung des Lötprozesses

The economic efficiency of a brazing process depends on several factors which are to be taken into consideration as part of the entire manufacturing process. This includes the following points (without any claim to completeness):

  • the costs for necessary brazing gap tolerances
  • filler metal costs
  • flux costs
  • costs for the potential use of auxiliary materials (e.g. cleaning agents)
  • energy costs
  • investments and/or depreciations of the necessary brazing systems
  • manufacturing costs (preparation of components, assembly, heating, post-processing etc.)

 The cost-based assessment of the entire brazing process is recommended, along with a comparison with alternative processes.

Criteria for selecting flux

Auswahl von Flussmitteln
sumire8-Fotolia

If the component is to be brazed in contact with air, the use of a flux is required - with some exceptions. The first criterion for selecting the flux is the brazing temperature of the selected filler metal. This temperature must be within the effective temperature range of the flux.

The next criterion is the base material to be brazed. There exist also different fluxes for different base materials within the same effective temperature range. Initial information is provided by the European Standard EN 1045 [5]. Further information can also be found in the delivery programs of filler metal and flux providers.

Further criteria such as, for example, the form of the flux (e.g. paste, powder, liquid, dosing capability etc.) are also to be defined by the user, depending on the brazing task.

Literature

[1] Selection Rules of Brazing Alloys and Fluxes 

Publication of Umicore AG & Co. KG
[2] Hart- und Hochtemperaturlöten
Die Schweißtechnische Praxis, Band 20
Paul Zaremba
DVS Verlag GmbH Düsseldorf, 1988
[3] Technik die verbindet Nr. 36
Veröffentlichung der Degussa AG
[4] Technik die verbindet Nr. 28
Veröffentlichung der Degussa AG
[5] EN 1045 Brazing - Fluxes for brazing -
Classification and technical delivery conditions