Silver Solders Silver-flo Range continued

Flux selection

The selection of a flux for use with the Silver-flo alloys should be based on two main criteria:
1. The melting range of the brazing alloy: both solidus and liquidus should be well within the quoted working range of the flux.
2. The parent metals to be joined: some alloys such as aluminium bronze require special fluxes.

Thus, the standard recommendation when working on copper, brass or mild steel would be to use Easy-flo Flux Powder or Silver-flo Flux Paste with alloys having a liquidus below 775ºC.
Similarly, with alloys having a liquidus between 775 and 900ºC, Tenacity No. 5 Powder, the residues of which are insoluble in water, would generally be recommended.

Where the brazing cycle is particularly long or short, it may be beneficial or necessary to use a non-standard flux, e.g. with very short heating cycles Silver-flo 20 can be used satisfactorily with Easy-flo Flux Powder with the advantage that the flux residues are water-soluble. Alternatively, when brazing a heavy assembly which takes a long time to reach brazing temperature, it might be necessary to use Tenacity No.5 Flux with Silver-flo 40.

When joining materials such as stainless steel, tungsten carbide and aluminium bronze, special fluxes must be considered. Please see our data sheet on the various fluxes or consult with our sales engineer.

Joint design

Brazed joints should be designed to be loaded in shear in order to obtain the best results in terms of joint strength and leak tightness. The joint gap recommended for lap and sleeve joints will largely depend on the alloy to be used. Brazing alloy fillets at the edge of the joint should be kept to a minimum as they do not add significantly to joint strength and can prove costly in terms of filler metal consumption.

The recommended joint gap for use with Silver-flo 55 and 45 – free-flowing alloys with short melting ranges – is 0.05 – 0.015 mm on both ferrous and non-ferrous materials. With the remainder of the Silver-flo range, joint gaps of 0.05 – 0.2 mm are recommended when working on ferrous materials. When brazing copper and its alloys there is a greater degree of inter-alloying during the brazing operation and, while gaps of 0.05 – 0.2 mm are recommended for alloys with a liquidus below 800˚C, gaps of 0.075 – 0.25 mm are recommended for the highest melting point alloys.

The use of butt joints is not generally recommended when brazing, but it is recognized that they can prove more than adequate when the stresses on a joint are low, and their use can greatly simplify fabrication of components. Where a butt joint is stressed in such a way that the members tend to bend and subject the joint to severe tearing stresses (peel stresses), substantial fillets are often effective in preventing undue stress concentration. Alloys with long melting ranges, which are suitable for building up large fillets, include Silver-flo 43, 302 and 25.

Fluxes are not designed or intended for the primary removal of oxides, coatings, oil, grease, dirt or other foreign materials from the parts to be brazed. All parts must be subjected to appropriate cleaning operations prior to brazing.

In making a braze, the flux is often needed to combine with, remove or dissolve those unwanted residual compounds or products of the brazing operation which would otherwise impair filler metal flow.

Oxides are the principal source of surface contamination. The dissolution and removal of oxides during brazing are the most common functions of a flux. So as not to impair metal flow, the flux must also be fluid and displaced readily by the molten brazing filler metal.

Fluxes are normally selected on the basis of two main criteria:

• The melting range of the brazing alloy – both solidus and liquidus should fall well within the quoted working range of the flux.
• The parent metals to be jointed – some alloys, such as aluminium bronze, require special fluxes. Copper, brass and mild steel are effectively cleaned by all PROBRAZE METALS fluxes.

The wrong flux or a poor application technique can have a dramatic effect on joint quality.

There are also special brazing fluxes for brazing tungsten carbide, stainless steel brazing,
induction brazing, brazing aluminium bronze, manufacturing flux coated rods, reducing red
staining on brass and brazing refractory metals.

Probraze’s Tenacity fluxes have become invaluable in medium and high temperature brazing
operations, on large components and in demanding brazing conditions above 850ºC.

Easy-flo Flux powder
A general purpose flux with good fluxing activity and long life at temperature. The powder has
good hot-rodding characteristics.

Easy-flo Flux paste
A paste preparation of controlled viscosity, ensuring economic and efficient performance.
Easy-flo Flux dipping grade
A highly active and fluid flux which exhibits a minimum of bubbling, making it ideal for induction
brazing. This paste is formulated to give a thin, stable consistency suitable for dipping.

Easy-flo Flux stainless steel grade
More active than Easy-flo flux. Recommended for use on stainless steel with the lower melting
brazing alloys.

Easy-flo Flux aluminium bronze grade
Similar to Easy-flo flux but modified for use on alloys containing up to 10% aluminium.

Tenacity Flux no. 6
An opaque flux due to the addition of elemental boron. Recommended for tungsten carbide,
refractory metals and stainless steel. This flux is however unsuitable for use on stainless steel
where crevice corrosion is likely to be a hazard in service.

Tenacity Flux no. 5
Recommended for stainless steel and heavy assemblies, where flux exhaustion is likely to occur
due to prolonged heating.

Tenacity Flux no. 5A
A variation of Tenacity no. 5 for use on materials rich in chromium carbide and sintered powder
metal compacts containing chromium carbide, titanium carbide and tantalum carbide.

Tenacity Flux no. 125
A general-purpose high temperature flux for use with the PROBRAZE METALS bronze alloys.

Tenacity Flux no. 10 and 12
Developed for high-temperature applications such as the brazing of rock drills.

Tenacity Flux no. 300
A high temperature flux for longer heating cycles with Probraze bronze alloys.

Tenacity Flux no. 600
A high temperature low bulking flux well suited to induction brazing with Probraze bronze alloys.

Workshop Practice

This particular form (powder / paste) selected depends upon individual work requirements, the brazing process, and the brazing procedure.

Frequently, powdered flux is mixed with water or alcohol to make a paste. The powder may also be used in the dry form and sprinkled onto the joint, although adherence in this form isn’t good when the joint is cold.

Dry powdered flux may be applied to the heated end of a filler metal rod simply by dipping the rod into the flux container and applying it to heated surfaces prior to brazing. Paste is the most commonly used form for applying brazing flux. Paste is typically applied by brushing onto the base metals. It can be applied with good adherence to a joint and filler metal before brazing.

Flux residue generally is removed to avoid corrosion from remaining active chemicals. The residue obtained from a flux, particularly when considerable oxide removal has occurred, is a form of glass. Less formation of glass makes for easier flux residue removal.

Removing flux from properly cleaned, brazed parts can usually be accomplished by washing in hot water accompanied by light brushing. Preferably this rinse should be done immediately after the brazing operation. Following the rinse, thorough drying is recommended.

For residues that are difficult to remove or surfaces that are subject to staining, special practices are required such as using proprietary compounds available for this purpose.

How to apply the flux – as a premixed paste or as a powder?
It is recommended that flux should be applied as a paste to the parts to be joined. Powders can be made into pastes by stirring in water until the mixture has the consistency of thick cream. A few drops of liquid detergent added to the mixture will often improve the wetting of flux onto clean parent metals. The flux should be applied to both joint surfaces before assembly.

Application of flux after assembly places great demands on the fluidity of the molten flux and its ability to penetrate capillary joints.

Brushing and dipping components with flux is recommended. Brushing is an effective method of applying a thin film of paste to the joint itself and to surrounding component surfaces. Brazing flux may be applied by dipping one or more components of an assembly into a container of flux. This is most effective with a paste of a thin consistency, e.g. Probraze’s Easy-flo dipping grade flux paste.

Automatic Fluxing can be effective for mass production. Automatic application of flux is possible and can be used for mass production. Consult a Probraze representative for more details.

Hot Rodding is effective and quick but does not always result in good flux wetting and alloy penetration.

This is a technique where a warm brazing rod is dipped into flux powder and the flux adhering to the rod is transferred to the joint area. This is an effective fluxing method but it is difficult to achieve good penetration of capillary joints. A refinement on this technique is the use of flux coated brazing rods.

Flux Coated Rods are ideal for site work and quick brazing technique. Brazing flux can be applied by using a Probraze flux coated rod.

Availability
Powders and pastes from 250g to 10kg containers.

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