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We have seen in one of the previous posts that, in order to obtain a <\/span>sound brazed joint<\/strong>, it is necessary to ensure that both the base and the filler metal are clean and free from any superficial film of contaminating material. An effective, economical and easy to use protective medium is <\/span>in these cases needed.<\/strong> This medium is the <\/span>flux. <\/strong><\/p>\n Fluxes <\/span>are<\/strong> <\/span>chemical compounds, usually based on complex alkaline salts mixtures, and produced in form of <\/span>liquids, powders or pastes. <\/strong> They are deposited and distributed on the surface of the pieces to be brazed, prior of their assembly and heating. Once the brazing temperature has been attained, the molten filler metal will flow and flush away the flux from the joint.After the cooling stage, in the majority of the cases, <\/span>the flux residues<\/strong> <\/span>will have to <\/span>removed<\/strong> <\/span>either mechanically or chemically, as otherwise they could promote corrosion phenomena. The most important technical property of <\/span>fluxes<\/strong> <\/span>is their <\/span>working temperature range.<\/strong> <\/span>This property is indicated by two temperatures: the lower one is the temperature at which the flux starts to be <\/span>active<\/strong>, the upper one is the temperature at which the flux is <\/span>exhaustedand<\/strong> <\/span>will not perform any more its deoxidant and protective function. It is a good rule to choose fluxes in such a way so that the lower temperature is <\/span>at least There is also an additional class of <\/span>liquid fluxes (Stella LI Series),<\/strong> <\/span>which are used in a particular process in which the flux is directly spread through the torch\u2019s flame. As it is well known, in this case, no additional flux is needed for the brazing operation. [\/et_pb_text][\/et_pb_column][\/et_pb_row][\/et_pb_section]<\/p>\n","protected":false},"excerpt":{"rendered":" BRAZING FLUX: HOW TO CHOOSE AND HOW TO USE IT We have seen in one of the previous posts that, in order to obtain a sound brazed joint, it is necessary to ensure that both the base and the filler metal are clean and free from any superficial film of contaminating material.These films, in fact, […]<\/p>\n","protected":false},"author":2,"featured_media":3077,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_et_pb_use_builder":"on","_et_pb_old_content":"","_et_gb_content_width":"","footnotes":""},"categories":[38],"tags":[],"class_list":["post-3662","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-insights"],"acf":[],"_links":{"self":[{"href":"https:\/\/stellawelding.cthosting.it\/en\/wp-json\/wp\/v2\/posts\/3662","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/stellawelding.cthosting.it\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/stellawelding.cthosting.it\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/stellawelding.cthosting.it\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/stellawelding.cthosting.it\/en\/wp-json\/wp\/v2\/comments?post=3662"}],"version-history":[{"count":5,"href":"https:\/\/stellawelding.cthosting.it\/en\/wp-json\/wp\/v2\/posts\/3662\/revisions"}],"predecessor-version":[{"id":6057,"href":"https:\/\/stellawelding.cthosting.it\/en\/wp-json\/wp\/v2\/posts\/3662\/revisions\/6057"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/stellawelding.cthosting.it\/en\/wp-json\/wp\/v2\/media\/3077"}],"wp:attachment":[{"href":"https:\/\/stellawelding.cthosting.it\/en\/wp-json\/wp\/v2\/media?parent=3662"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/stellawelding.cthosting.it\/en\/wp-json\/wp\/v2\/categories?post=3662"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/stellawelding.cthosting.it\/en\/wp-json\/wp\/v2\/tags?post=3662"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
These films, in fact, will inhibit the wetting and the flow of the filler metal inside of the joint.
Dust, grease, oil, residues of fabrication process, etc. may be easily removed by simple mechanical means such as brushing or by washing with solvents and\/or detergents.
However, the majority of metals are covered, even at ambient temperature, by a film of <\/span>superficial oxide<\/strong>, coming from the reaction of the metal with atmospheric oxygen<\/strong>, which poses a further obstacle to the flow of the filler metal.
Besides of that, the thickness of this oxide layer grows quickly and considerably when metals are heated up in air, becoming a complete barrier to the successful outcome of the brazing operation.
It is therefore necessary that, throughout all of the process, <\/span>already present oxides are dissolved<\/strong> <\/span>and the pieces are protected from <\/span>further oxidation. <\/strong>In some instances this may be obtained excluding oxygen from the brazing environment, by surrounding the pieces with <\/span>inert (argon, nitrogen) or reducing (hydrogen mixtures) atmospheres,<\/strong> <\/span>or by performing operations under <\/span>vacuum conditions. <\/strong>In the majority of the cases, however, these processes are not practical nor convenient, and the heating of the pieces takes place in the open air, <\/span>without any protective atmosphere.<\/strong><\/p>\n
During the heating stage the fluxes become active, <\/span>by chemically removing the surface oxides, preventing further oxidation and protecting the base metal from contact with the atmospheric oxygen.<\/strong><\/p>\n
50 \u00b0C lower<\/strong> <\/span>than the solidus temperature of the brazing alloy to be used, and that the upper temperature is <\/span>at least 50 \u00b0C higher<\/strong> <\/span>than the liquidus temperature of the alloy. As an example, a good general purpose flux for silver based alloys, such as <\/span>Stella Flux AG4<\/strong>, has a working temperature range of <\/span>550 \u2013 850 \u00b0C<\/strong>, which make it suitable to be used with a wide range of alloys, approximately <\/span>from Ag25 to Ag60 <\/strong>Many different types of <\/span> fluxes<\/strong> are available, each with different chemical composition, working temperature range and properties, that may be used with different alloys, different family of products and for different applications.
A comprehensive explanation of the different features of each flux will take too long and is out of the purpose of these short notes.
The necessary details can be found in the relevant data sheets <\/span>data sheets<\/strong> <\/span>di ciascun prodotto pu\u00f2 fornire gli opportuni dettagli per la scelta del tipo pi\u00f9 opportuno per ogni specifica applicazione; in caso di dubbi si consiglia di chiedere una consulenza al nostro ufficio tecnico. Si possono comunque generalmente distinguere le seguenti grandi <\/span>broad families:<\/strong><\/p>\n\n
This flux is generally used with brass filler metals, for the brazing of iron or steel elements, or with copper-phosphorus or silver-copper-phosphorus alloys for the brazing of copper pipes and fittings.
We will address this specific flux in a subsequent post.
One last comment has to be dedicated to the <\/span>brazing of copper with the copper-phosphorus and silver-copper-phosphorus filler metals.<\/strong><\/p>\n
The reason for this apparent contradiction is simple: it is the <\/span>phosphorus<\/strong> <\/span>contained in the alloy that act as the flux, and make these products self <\/span>flux.<\/strong><\/p>\n<\/div>\n<\/div>\n<\/div>\n