Oxide Reduction

Other gases such as chlorine, ﬂuorine, and bo-ron triﬂouride are more effective than hydrogen and carbon monoxide at removing surface ox-ides of particular metals, as is clearly indicated on the relevant Ellingham diagram s [Wicks and Block 1963]. Such gases operate partly by con-verting the oxide to a halide that is volatile at the joining temperature and vaporizes during the heating cycle. These halide atmospheres also chemically attack the underlying metal and physically undermine the oxide, as occurs in the

ﬂuxing of aluminum. The use of halide ﬂuxes in aluminum brazing is discussed in section 3.2.2.

Unfortunately, these and other chemically cor-rosive gases tend also to attack furnace linings, furniture, and seals. They also require complex gas handling and exhaust scrubbing equipment in order to comply with health and safety leg-islation. Consequently, they tend not to be widely used.

3.2 3.2 Che Chemic mical al Flu Fluxes xes for for Bra Brazin zingg

Successful brazing is largely dependent on the ability of the filler to wet and spread on com-ponent surfaces. A major barrier to wetting is presented by stable nonmetallic films and coat-ings on the surfaces, in particular, oxides and carbonaceous residues. Oxide films often endow beneficial attributes to metals, such as corrosion resistance, but their presence on the faying sur-faces, and on the surfaces of the brazes, present more than a nonmetallic barrier to wetting and spreading. Oxides are generally poor thermal conductors, compared with metals, and impede heat transfer, thereby exacerbating temperature gradients present and delaying fusion of the braze with the parent metal. Fluxes are chemical agents that are used to remove these layers and thereby promote wetting by the molten filler. In order to be effective in exposing a bare metal surface to the filler, a flux must be capable of fulfilling the following functions:

● Remove oxides and othe r films that exist on surfaces to be joined by either chemical or physical means. Chemical mechanisms in-volve reaction of the flux with surface oxides to effect reduction or dissolution. Physical removal of a layer of oxide by a flux is con-tingent on its ability to weaken adhesion of the oxide film. Normally, the oxide is under-mined as a result of penetration by the flux through naturally occurring pores, fissures, and other flaws in the layer, followed by electrolytic action at the interface between the oxide and the parent material.

● Protect the cleane d joint from reoxidation during the joining cycle

● Be displaced by the molten ﬁller as the latter spreads over the faying surfaces.

While molten, ﬂuxes form a thermal blanket around the joint that helps to spread the heat evenly during the heating cycle. The ﬂux also tends to reduce the surface tension between the

118 / Principles of Brazing 118 / Principles of Brazing

braze and the joint surfaces, thereby enhancing wetting.

Ideally, the flux should leave no residues or produce residues that are removed easily by, for example, being soluble in water. Fluoborate flux residues are soluble in water, but borosilicate fluxes require a solution containing 5 to 10%

sulfuric acid to facilitate their removal. For stub-born deposits, sodium dichromate may be added or a phosphate solution used. Boric acid, a com-mon constituent of many fluxes, is soluble only in hot water so cleaning solutions are often warmed. Fluxes should also be compatible with the filler and substrate materials. For example, so-called “black fluxes,” because they contain elemental boron, are not suitable for use with aluminum, magnesium, and titanium compo-nents and filler metals owing to the ready for-mation of borides of these metals.

Chemical fluxes always function while in a gaseous or liquid form, although they are fre-quently solid at room temperature. If the flux is liquid at the joining temperature, it has to wet the joint surfaces in order to be effective. A flux that is liquid can beneficially help suppress the volatilization of high vapor-pressure constitu-ents of filler metals and thereby improve joint quality. This is particularly true in dip brazing.

Fluxes can be introduced to the joint in a num-ber of ways, some of which are discussed sub-sequently. Brazing fluxes are usually applied in the form of a powder or paste immediately prior to the heating cycle. The joint is then heated to the required bonding temperature, by which point solid fluxes have become molten, ideally just before the filler metal melts. When

design-ing a joint, allowance must be made for the ﬂux to escape. Flux entrapment at sharp corners of internal features is a common problem.

A flux can also be placed within or adjacent to the joint together with the filler metal as a preform and the assembly heated to the bonding temperature. As a properly chosen flux will melt at a temperature below the melting point of the filler, the molten flux is able to spread over the joint surfaces and clean them before the filler

metal melts and displaces the ﬂux.

Another method involves introducing the flux together with the filler into a joint already held at the bonding temperature, in the form of a flux-coated brazing rod. Although this technique is widely practiced because it is fast and conve-nient, it is not recommended because the heated component surfaces are unprotected until the filler is applied. More aggressive fluxes are then

required, which in turn tends to accentuate cor-rosion and clea ning problems. The reason for the flux being placed on the outside of brazing rods but on the inside of solder wire owes to the rela-tive ratios of flux-to-filler metal required for these two processes. Because brazing is con-ducted at higher temperatures, reaction rates are faster and so a larger quantity of flux is required to protect the parent materials and fillers against oxidation during the joining process. However, at least one manufacturer sells preforms of braz-ing alloy in which the brazbraz-ing flux is internal.

The reduced quantity of ﬂux means that they function best in furnace brazing with a con-trolled atmosphere. Here, the principal attraction is simpliﬁcation of preparation prior to brazing.

Fluxes can be applied to the faying surfaces, together with the ﬁller metal, prior to the heating cycle, in the form of tapes, pastes, and creams, which are normally proprietary formulations.

They comprise mixtures of the ﬁller metal, which is present as a powder of a prescribed grain size range together with a ﬂux and a water-based or organic binder that is selected to pro-duce the desired viscosity and to dry or burn off without leaving contaminating residues. Poly-isobutylene is often used as a binder ingredient because the degree of “stickiness” and viscosity can be adjusted by altering the length of the polymer chain, and when thermally degraded, it does not leave a carbonaceous residue. Other common carrier liquids are petroleum-based and polyethylene-glycol-based. The higher viscosity and longer life after application of these pastes is useful in some manufacturing environments.

Pastes and creams are particularly used in au-tomated reflow brazing operations because they can be screen printed or dispensed using syrin-ges. Because of the large surface area of the powdered filler metal in contact with the flux, corrosion is inevitable. Therefore, these prod-ucts have a finite shelf life. Selec ted braze pastes also are available in the form of tape, which comprises a clean-burning binder material in the form of sheet that incorporates powdered braze and flux. Braze tapes tend to be fragile and must be handled with care. Most brazing fluxes are creamy white in color and are virtually indistin-guishable to the naked eye. As mentioned be-fore, the exceptions are those fluxes that contain elemental boron and, consequently, have a gray-black hue.

Certain ﬂuxes are totally soluble in water and so are applied to the faying surfaces as a liquid.

These may also be fed into the fuel gas stream

Chapter 3: The Joining Environment / 119 Chapter 3: The Joining Environment / 119

during torch brazing so that fresh ﬂux is being applied continuously to the joint area through the heating cycle.

Several different ﬂuxing mechanisms cover the majority of brazing operations that are en-countered. Even these are sufﬁciently complex not to be understood in detail at the pres ent time.

However, ﬂuxing mechanisms can be classiﬁed according to whether they remove the nonme-tallic surface coating by physical or chemical means.

A ﬂux can chemically remove a surface coat-ing by:

● Dissolving the coating

● Reacting with the coating to form a product that is unstable at the bonding temperature

● Reducing the coating to metal in an ex-change reaction, such as occurs when reduc-ing gases are effective in removreduc-ing oxides A surface coating can also be physically re-moved. This usually occurs through:

● Erosion of the underlying metal. In this mechanism the ﬂux does not react with the surface coating itself but is able to percolate through it and react with the underlying metal, thereby causing detachment of the coating.

● Wetting of the coating in a manner that causes it to spall off. This mode of fluxing applies to joining processes where compo-nents are subject to the thermal shock that cracks the coating due to the relatively brittle nature of oxide layers. Immersion in molten salts and fluidized bed furnaces are examples of this type of process. The process usually relies on the filler metal wetting the parent material through the fissures and spreading underneath the nonmetallic skin to complete its removal.

Many fluxes function by a combination of mech-anisms and for this reason, fluxing action is best illustrated with reference to specific examples.

The requirements on brazing fluxes, in partic-ular, the service temperature, greatly restrict the choice of materials that can be used. Conse-quently, whereas braze Standards (e.g., EN 1044, 1999) list 93 alloy compositions, the matching flux Standard (EN 1045, 1999) con-tains only seven entries. The rated working tem-perature range of a flux generally assumes a brazing time of up to 30 seconds in air. There-fore, a flux with a working temperature close to

the liquidus temperature of the braze can be used in a controlled atmosphere for longer times. If large components are to be brazed in air, a ﬂux with a higher working temperature will be nec-essary to allow for the slower rate of heating.

Sometimes, particularly for brazing at high tem-perature, a dual ﬂux system may be used so that some protection is provided to the workpiece at moderate temperatures and a second, high-working-temperature ﬂux then takes over at the brazing temperature. Molybdenum may be brazed in air using this technique.

3.2.1 Brazing Flux Chemistry

Although brazing ﬂux chemistry is a fairly so-phisticated science, a few common ingredients account for the vast proportion of the market.

The limitation arises on account of the high tem-peratures involved in brazing, which totally ex-cludes all aqueous and organic materials from consideration. Instead, glass complexes must be used. These possess low volatility and generally also have a low permeability to air and so can provide the clean ed surfaces of the joint and ﬁller metal with the necessary prote ction against reoxidation.

At the lower end of the brazing temperature range, (i.e., 600–800 C, or 1110–1470 F), the glass carrier is based on borates [-B _xO_y], with the ratio of oxygen to boron optimized to pro-vide a balance between viscosity and perme-ability to oxygen. In general, the higher is the oxygen to boron ratio, the higher is the viscosity, and the permeability is reduced correspondingly.

The requirement for the ﬂux to be displaced by the molten braze places a limit on the viscosity that can be tolerated. Above 800 C (1470 F), borates alone are too permeable and need to be replaced partly by silicates [SixOy] so that the resulting glass is a borosilicate that has a higher viscosity and is therefore able to protect work-pieces to higher temperatures. However, boro-silicate residues are largely insoluble in water, necessitating more rigorous cleaning proce-dures, in contrast with those of the simple bo-rates that can be dissolved in water, making them more convenient to use.

In addition to providing pro tection against ox-idation, borates have the ability to dissolve a limited fraction of oxides from the surfaces of steel and copper component s. This accounts for the effectiveness of borax (sodium tetraborate) as a ﬂux at temperatures above about 750 C

120 / Principles of Brazing 120 / Principles of Brazing

Fig. 3.8

B:K:F atom ratios of common brazing ﬂuxes

(1380 F). Adding boric acid or boric oxide to borax lowers the melting point of the ﬂux be-cause it reduces the oxygen to boron ratio. The resulting increased chemical activity means that these ﬂuxes facilitate improved wetting by sil-ver-base and silver-free brazes on carbides and alloys that form refractory oxides though having constituents that include chromium, nickel, and cobalt.

When heated by a torch, sodium salts, includ-ing borax, produce a bright yellow glare, which is unpleasant to operators. The glare is reduced substantially by partly or totally substituting the borax with the corresponding potassium salt, potassium tetraborate, albeit with a small cost penalty.

Borates on their own are insufficiently active to clean surfaces of many metals. Therefore, fluxes intended for general use contain a pro-portion of halides. Glare considerations have fa-vored potassium halides, in particular, potassium fluorides, over the equivalent sodium salts. The improved fluxing action results largely from the greater oxide dissolution ability of the fluorides.

Indeed, the dominant mechanism responsible for surface cleaning by the ﬂuoborate type ﬂuxes is believed to be direct dissolution of the oxides,

and there is little evidence for any of the other recognized types of cleaning action referred to previously. At the same time, the fluorides re-duce the melting point of the flux because they disrupt the cross linkages in the borate network structure [Eustathopoulos, Nicholas and Drevet 1999, p 355–56]. The fluoborate fluxes espe-cially suit the low-melting-point silver-base qua-ternary alloy brazes, which melt at temperatures down to about 590 C (1095 F), because they can be used at comparable temperatures. The fluoride addition does, however, reduce the up-per working temup-perature limit of these fluxes be-cause it increases the permeability to oxygen and reduces their thermal stability, owing to the for-mation of hydrofluoric acid on heating, which is volatile. Commercial fluoborate fluxes fall into the elemental composition range B:F ^1:0.75 to 1.5; B:K ^1:0.55 to 1.1; F:K ^1:0.55 to 0.8, by atomic ratio. This range is represen ted in Fig. 3.8.

Wetting agents are not strictly necessary for brazing ﬂuxes because, at the elevated joining temperatures used, organic residues will have decomposed, leaving carbonaceous deposits that will be either eliminated through oxidation or cleaned off the surface by the ﬂux. Nevertheless,

Chapter 3: The Joining Environment / 121 Chapter 3: The Joining Environment / 121

wetting and rheological agents are added to ﬂux pastes to produce a smooth consistency, which aids application to the workpiece. Because many brazing ﬂuxes have water-based carriers, it is good practice to thoroughly degrease all com-ponents prior to brazing and thereafter handle all items only with gloves or tools that have been similarly cleaned.

Interaction between brazing fluxes, metal ox-ides, and brazing alloys is often more complex than suggested by the preceding discussion. For example, it is well known that in torch brazing it is easier to make joints that are free of defects to oxidized copper components with silver-cop-per-zinc brazes than with zinc-free brazes (e.g., silver-copper-tin), under cover of a fluoborate flux. It transpires that copper oxides dissolve in fluoborates slowly so that during the course of a normal brazing cycle, there is insufficient time to remove a thick film of oxide. The flux can dissolve the outer layer of cupric oxide (CuO), but not the bulk of the film, which is predomi-nantly of cuprous oxide (Cu2O). The flux is, however, effective at removing the oxide skin from the surface of both types of brazing alloy mentioned previously. Removal of this skin per-mits the brazing alloy to wet the cuprous oxide, which is then removed by a combination of dis-solution and reduction by the braze. For this rea-son, silver-copper brazes that contain an element with a high affinity for oxygen (i.e., zinc and cadmium) wet and spread better on oxidized sur-faces of copper and its alloys than brazes with-out these ingredients. Thus, when brazing cop-per components in air, the fluxing action amounts to prevention of the formation of scale on the components, but the removal of oxides from the surfaces of the brazing alloy only. In furnace brazing, with a controlled atmosphere and well-prepared components, the extended cy-cle time and relative thinness of the oxide skin on the copper components means that the flux is able to dissolve all of the cuprous oxide, and the differentiation between zinc-containing and zinc-free brazes is less marked.

Commercial fluxes are proprietary formula-tions that contain specific ingredients tailored to application requirements and incorporate vari-ous subtleties. By way of example, many braz-ing operators require a flux that will coat a heated rod of the braze when dipped into a tub of the flux powder. This flux-coated rod is then applied to the workpiece, and the brazing opera-tion is carried out using a torch in air. To satisfy this mode of application, which speeds up the

joining procedure considerably, fluxes that con-tain close to 70% of a hydrated potassium fluo-borate compound have been formulated. This compound releases sufficient moisture, when heated, to form a sticky paste that will adhere to a metal rod. This example illustrates the finer points affecting user preference and helps to ex-plain why it is best to consult suppliers when considering which flux to use for a particular application. It is also logical that fluxes intended for one combinati on of parent material and braz-ing alloy may not be as effective for a different combination, especially if the second brazing al-loy melts at a different temperature.

Common brazing ﬂuxes are only suitable for use at temperatures up to about 1200 C (2200

F). The higher temperature fluxes comprise a mixture of sodium fluoborate and tetraborate with a significant proportion of silica, boric acid, and elemental boron. Their main use is brazing of copper-, iron-, and nickel-base alloys. Above this temperature, gas atmospheres with sufficient reductive potential are available and the brazes themselves tend to contain active elements (see Chapter 4, section 4.1.2.2 and Chapter 7, section 7.2). The higher the brazing temperature, the more difficult it is to achieve satisfactory joints

In document Principles of Brazing. Jacobson, Humpson (Page 129-134)