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Flame Brazing with NOCOLOK® Flux Introduction

Flame brazing of aluminum is not new. to the fact that the years of acquired In fact the very first brazed aluminum knowledge of flame brazing aluminum assemblies were produced using a has come from corrosive chloride- chloride based flux and a flame as the based flux brazing. heat source. Unfortunately, the same techniques can What has changed over the years is not be directly applied to NOCOLOK ® the sophistication of the types of fluxes flux flame brazing. It is therefore the available and to a certain extent the alloy intention of this article to re-familiarize selection. However, even if one returns the brazing engineer with the funda- to the absolute basics of a flame, filler mentals of flame brazing aluminum metal and flux, there remains a great and use those fundamentals to realize deal to be learned about the funda- all the advantages of NOCOLOK ® flux mentals of flame brazing of aluminum. brazing. This becomes especially evident when the brazing engineer applies his tech- niques and equipment to NOCOLOK ® flux flame brazing and years of learned practice seem to fail. This is largely due

What is Flame Brazing?

According to the American Welding Society, brazing is the joining of metals using a molten filler metal, which on cooling forms a joint. The filler metal melting temperature is above 450 °C, but below the melting point of the metals. Flame brazing then implies the use of a flame as the heat source to accomplish what is described above. Flame brazing lends itself well to joining components with simple configurations such as tube-to-tube, tube-to-fitting and joints having large thermal mass differences. Since much faster heating rates are possible than in furnace braz- ing, flame brazing is versatile and as will be explained in more detail later, can Flame Brazing braze some Mg containing alloys.

2 Solvay Special Chem Flame Brazing with NOCOLOK® Flux What is NOCOLOK® Flux?

NOCOLOK ® flux is a white powder Joint Clearances ® consisting of a mixture of potassium The recommended gap tolerances for fluroaluminate salts of the general flame brazing range from 0.1 mm to formula K1-3 Al‑F4-6. The flux has a 0.15 mm. Larger gap clearances can defined melting point range of 565 °C be tolerated, but capillary action is to 572 °C, below the melting point of reduced, gravity activity is increased the Al-Si brazing alloy. The flux is non- and more filler metal may be required. corrosive and non-hygroscopic and Friction fits should also be avoided as is only very slightly soluble in water this will restrict filler metal flow and result (0.2 % to 0.4 %). The shelf and pot life of in discontinuities in the brazed joint area. the flux is therefore indefinite. The flux does not react with Al at room tempera- ture or at brazing temperature and only becomes reactive when molten.

Role of the Flux Once molten the flux works by dissolv- ing the oxide film on the Al surfaces to be joined and prevents further oxida- tion. The flux wets the Al surfaces and allows the filler metal to flow freely into the joints by capillary action. Upon cool- ing, the flux solidifies and remains on the surfaces as a thin, tightly adherent film, which need not be removed.

Flame Brazing

Flame Brazing with NOCOLOK® Flux Solvay Special Chem 3 Recommended equipment for flame brazing

Since the principles of flame braz- Hardware ing can be explained using the most Torch basic equipment, only the equipment It is critical that the joint area is brought necessary for manual flame brazing is up to temperature uniformly. For this described. From the basic principles, reason a dual headed torch capable of all other equipment is only a matter of heating the joint from 2 sides is recom- the degree of automation the end user mended. wishes to achieve. Torch Tip A multi-orifice tip generates a broader flame at the exit of the tip. This feature enhances component temperature uniformity during heat up. Pin-point flames should be avoided as burn- through can easily occur.

Dual headed torch Torch Tip

Filler metal

4 Solvay Special Chem Flame Brazing with NOCOLOK® Flux Consumables Brazing Paste Gas Commercially available brazing pastes Most commercial gas mixtures are consist of the flux, powdered filler acceptable for flame brazing Al: metal and a binder/carrier to keep eve- rything in uniform suspension. This ■■ oxygen – propane paste is all inclusive, there is no need ■■ oxygen – methane to supply flux or filler metal to the joint ■■ oxygen – natural gas separately. Brazing pastes can also be applied with automatic dispensers, ■■ oxygen – acetylene (oxyacetylene) with syringes or by brush application. Oxyacetylene combination produces Flux Paste the hottest flame and may be used, but This is very similar to brazing pastes with extreme care to avoid overheating except that there is no powdered and burn-through. filler metal present, meaning that flux Filler Metal pastes requires filler metal in one form The filler metal alloy most commonly or another to be added to the joint used for flame brazing Al is AA4047 separately. The advantage of using a which contains 11 to 13 % Si. The Al-Si flux paste is that the end user does not phase diagram shows the eutectic have to prepare his own paste. at 577 °C with 12.6 % Si. AA4047 filler In-House Paste Preparation alloy therefore has the lowest melting The least expensive and most com- temperature with the highest fluidity, mon is the in-house preparation of flux ideal properties for flame brazing Al. pastes. The flux is mixed with either The filler metal is available in a variety of water or alcohol and/or water at 40 % shapes and forms including wire, rings, to 60 % solids. Using some alcohol in foil and powder. When used as a pow- the paste formulation allows for quicker der, it is usually mixed with flux and a drying. Using pastes prepared in-house carrier to form a paste (more on pastes of course requires that the filler metal below). The filler metal wire is also avail- be supplied to the joint separately. able commercially either cored or coat- These pastes are not easily dispensa- ed with flux, precluding the application ble automatically and are most often of flux. applied with a brush. For brazing a tube-to-tube joint, the table below summarizes the complex- ity level in applying the flux and filler metal in their various forms:

Flux Cored or Coated Wire Brazing Pastes Flux Pastes In-House Paste Preparation 1. Apply or dispense paste 1. Preplace ring at the joint 1. Preplace ring at the joint 1. Prepare paste at the joint 2. Apply or dispense paste 2. Braze 2. Dry 2. Preplace ring at the joint at the joint

3. Braze 3. Dry 3. Apply paste at the joint

4. Braze 4. Dry

5. Braze

Flame Brazing with NOCOLOK® Flux Solvay Special Chem 5 Procedure

This section describes the neces- 3. Apply the Flux 5. Heating sary steps and control procedures to The flux is then applied with a small Once the flux has dried, more intense ensure a properly brazed joint. brush around the circumference of heat to begin the actual braze sequence the joint at a loading of about 25 to can be applied. The braze flame should 1. Clean the Components 30 g/m2. not be allowed to impinge on any one The joint area must be cleaned free area very long to avoid overheating and of cutting and machining lubricants. 4. Dry the Flux burnthrough. The component with the Aqueous cleaning, solvent dipping or The flux should be allowed to dry higher thermal mass should be heated wiping are acceptable procedures. before the application of intense heat more. The flame should not be allowed to begin brazing. This can be done by 2. Assemble the Components to rest on the flux or preform ring to allowing the joint to air dry or alterna- The components are assembled with avoid premature melting before the tively by gently heating the surrounding the filler alloy ring in place. There must joint area is uniformly heated to braze joint area with the flame, which will heat be intimate contact between the two temperature. The flame should be the metal and dry the flux. Intense heat components to be joined and the alloy kept moving at all times, moving back should be avoided before the flux has ring. and forth between the components dried, otherwise splattering and flux of different mass in such a way as to fall-off will occur.

1 2

Clean the Components Assemble the Components

5 5

Heating Melting point

6 Solvay Special Chem Flame Brazing with NOCOLOK® Flux bring the entire joint to temperature directly on the joint and filler metal ing minimal flux residue, the surfaces uniformly. ring. Very shortly after flux melting, the can be painted with relatively good filler metal ring begins to loose shape paint adhesion over the flux residue. There are three temperature indica- (third temperature indicator) and If absolutely desired, the flux residue tions in NOCOLOK ® flux flame brazing. begins to melt at 577 °C. The molten can be removed, but only by mechani- The first is the appearance of a yellow filler metal is quickly drawn into the cal means such as wire brushing and flame at the Al surface. This indicates joint by capillary action. As soon as the grit blasting. Removing the flux resi- that the surface is starting to overheat/ full preform ring is molten, the flame due is recommended only when joint burn, since the aluminum skin always should be quickly removed and the cleanliness is absolutely imperative. runs hotter than the component center. brazed joint allowed to cool. The flame must visit the area less fre- quently to avoid burning. The second 6. Post Braze Treatment indicator is the first sign of flux melt- After cooling, no further treatment is ing, that is the fluxed area turns from required. The flux, although visible white to clear. This indicates that is non-hygroscopic and in standard the joint temperature is about 565 °C. applications non-corrosive. With the At this point the flames can be played brazing conditions optimized, mean-

3 4

Apply the Flux Dry the Flux

6

Brazed joint

Flame Brazing with NOCOLOK® Flux Solvay Special Chem 7 Alternatives

Fluxing Prior to Assembly this window. By placing the preform The procedure described above is the ring at the joint prior to heating ensures most common and reliable method to that the flux will not dry out before the ensure good quality brazed joints, but filler begins to melt. is not the only one. For instance with a The table below lists the chemical tube-to-tube joint, with the filler metal composition and melting point ring in place, the flux can be applied to range of common alloys suitable for the lower portion of the straight tube NOCOLOK ® flux flame brazing. prior to insertion in the expanded tube. As shown in the Table, alloys that are In this case, a light flux loading around considered difficult or impossible to the circumference of the joint after braze by furnace methods can be flame assembly is still required to cover the brazed. In furnace brazing, Mg diffuses filler metal ring. to the surface and reacts with the sur- face oxide to form MgO and spinels of Feeding the Joint with MgO:Al2O3, which have reduced solu- Filler Metal bility in NOCOLOK ® flux. Furthermore, Another alternative to using a preform Mg and/or MgO can react with the flux, ring is to feed the filler metal into the rendering it less effective. The rule of joint with filler alloy wire or rod during thumb for furnace brazing is to limit the heating, after the flux is molten. This total amount to less than 0.5 wt %‑Mg. method is frequently adopted by braz- In flame brazing, higher Mg levels can ers who have had experience with be tolerated since the faster heating choride flux flame brazing or who have rates do not allow the diffusing Mg welding backgrounds. To the novice enough time to appreciably decrease brazer, this method is very difficult the beneficial effects of the flux. Up to because the window between when 1 wt %‑Mg can be brazed with relative the flux becomes molten and when the ease, while greater than 1 wt %‑Mg flux dries out is very small, when braz- may be possible under some cir- ing in air. The window of opportunity cumstances (increased flux loadings, is in the order of a few seconds and extremely fast heating rates). skilled brazers are able to work within

Alloys suitable for flame brazing

Alloy Composition (% By Weight) Approx. Melting Range

AA- Si Fe Cu Mn Mg Zn Ti Solidus (C°) Liquidus (C°) 3003 0.6 0.7 0.05 – 0.20 1.0 – 1.5 – 0.10 – 643 654 1145 0.55 Si+Fe 0.05 0.05 0.05 0.05 0.03 640 655 1070 0.20 0.25 0.04 0.03 0.03 0.04 0.03 640 655 3005 0.60 0.70 0.30 1.0 –1.5 0.20 – 0.6 0.25 0.10 640 655 3105 0.60 0.70 0.30 0.30 – 0.80 0.20 – 0.80 0.40 0.10 635 655 6951 0.20 – 0.50 0.80 0.15 – 0.40 0.10 0.40 – 0.8 0.20 – 616 654 3102 0.40 0.7 0.10 0.05 – 0.40 – 0.30 0.10 645 655 6063 0.20 – 0.6 0.35 0.10 0.10 0.45 – 0.9 0.10 0.i0 616 652 6061 0.40 – 0.8 0.7 0.15 – 0.40 0.15 0.8 – 1.2 0.25 0.15 616 652 Aluminum association alloy composition

8 Solvay Special Chem Flame Brazing with NOCOLOK® Flux Precaution This effect is worse in the outer skin Cross-Section of As-Extruded and Overheated Machineable Alloys Mg containing alloys have lower melting since this area sees a higher tem- points than those alloys containing very perature (Ts surface temperature) low or no Mg (see Table above). The than deeper within the component (Tj most common high strength machi- joint temperature) as illustrated below. neable alloys for example are AA6061 Metallurgical degradation of this type and AA6063 which typically contain can cause problems with the longev- 1 wt‑% and 0.5 wt %, respectively. ity and suitability of machined surfaces Note that both of these alloys have a such as threads in fittings. Heated to 630‑°C solidus of 616 °C. This means that these alloys, when overheated are prone to incipient grain boundary melting. The effect of overheating manifests itself as degradation of the micro- structure and a roughening of the skin, commonly known as ‘orange peel’ effect. These effects are illustrated in the photomicrographs right. As extruded

Melting point of parent material

Tube Ts Temperature

Temperature of surface Process window

Filler Metal Ring Tj

Ts Melting point of brazing alloy Ts

Tj Fitting Tj Melting point of flux

Temperature of joint

Time

Schematic of joint* * Concept courtesy of Burner Flame Technology, Ltd.

Flame Brazing with NOCOLOK® Flux Solvay Special Chem 9 Automation

The procedure described above is for Carousels Optical Pyrometry manual flame brazing using a hand Flame brazing carousels are at the upper Measurements held torch and visual indicators (flux level of automation. The carousels move Automatic flame brazing carousels are melting) for monitoring process param- assembled components to be brazed often equipped with infra-red pyrom- eters. Flame brazing is easily automat- continuously from flame station to flame eters to measure the temperature of ed and the level of automation can vary station arranged in a circular pattern the braze joint area. The information is between simple shuttle systems to fully (hence the name carousel). Usu­ally, fed to a computer and the computer automatic brazing carousels including each flame station consists of twosta- ‘tells’ the torches to flip away from the optical pyrometry temperature meas- tionary and opposing torches between joint when the proper temperature is urements. Regardless of the level of which the joint to be brazed is indexed. reached. The pyrometer however can- automation, the principles of manual As the component moves from station not determine an absolute accurate flame brazing still apply. to station, the joint temperature gets temperature. As aluminum heats up, progressively hotter. The number of sta- the oxide layer changes and therefore Shuttle Systems tions depends on the joint configuration the emmissivity of the surface chang- The shuttle system consists of a frame- and weight. The joint is thus uniformly es and it becomes dif­ficult to modify work on which one or two components heated to brazing temperature. The last the pyrometer to track these changes requiring brazing are mounted where station where brazing takes place may dynamically. It is possible to get repro- the motor driven framework laterally be equipped with an optical pyrometer ducible accurate relative temperatures shuttles the braze joints between one to monitor braze joint temperature. After of the heat affected zone, but not of or two braze stations each equipped brazing, the joint may be cooled by air or the joint itself. In other words, optical with opposing flames (the equivalent water spray. Other options include auto- pyrometry measurements can be used of a double headed torch). The shuttle matic application of filler metal and/or to monitor the consistency of the braz- is usually configured such that one or dispensing of flux (filler metal ring, flux/ ing process from part to part (for the two new components requiring braz- brazing paste). same given part), but the information ing can be installed on the return of the by itself for a single part would not be shuttle to its original position and the that useful. brazed units unloaded.

Shuttle system Carousel * Photos from Everwand & Fell

10 Solvay Special Chem Flame Brazing with NOCOLOK® Flux Bimetallic Joining

Flame brazing aluminum to another quickly consumed to form the eutec- metal such as bronze, copper, steel tic metal. Management of time and and stainless steel is possible, but temperature is critical to minimize the requires special care and attention. inter-diffusion and metal consumption. Dealing with all Al-metal combina- There is an advantage however. Since tions in detail is beyond the scope of filler metal is created in-situ, there is no this brochure, but a few comments on need to supply filler metal to the joint. Al-Cu joining is noteworthy, since this The only requirement is that the design combination is common in the refrig- of the joint allows metal consumption eration industry (copper tube to alu- without sacrificing joint integrity. minum roll-bond panel, for example). There is a eutectic between Cu and Al at 548 °C. When the flux melts and the surface oxides are removed, inter- diffusion of Al and Cu is rapid and una- voidable. This means that at braze tem- perature, the Al and Cu materials are

From left to right: Al-Cu, Al-stainless, Al-brass and Cu-Al

Conclusions

From the preceeding discussions, ■■ Preplacing the filler metal in the it is evident that NOCOLOK ® flux joint is preferred, but with some flame brazing is not a complicated skill may be fed into the joint after process and that if the fundamentals the flux melts. of manual brazing are followed, any ■■ Temperature uniformity is critical level of automation can be achieved. and excessively high surfaces The most important aspects of the temperatures must be avoided. discussion are as follows:

■■ NOCOLOK ® flux melts just before the filler metal temperature. ■■ Once molten, the flux remains active only for a short period before drying out.

Flame Brazing with NOCOLOK® Flux Solvay Special Chem 11 www.ahlersheinel.de observing all legal, administrative and regulatory procedures relating to the product, personal hygiene, and protection of human welfare and the environment. the and welfare human of protection and hygiene, personal from product, exempt the to not is user relating the case, any In procedures required. be not may regulatory and measures other that administrative or recom not legal, are all and indicated, are observing measures infringement, safety patent of all that free is use assume such not any that should user The warranty or patent. implied. or any express infringe to representation kind, any of without mendations made are responsibility or products our of use warranty possible guarantee, concerning without presented suggestions are or but reliable Statements and accurate be to believed are herein given data and information, statements, All Disclaimer: www.nocolok.com www.solvay.com Fax: Tel.: Germany Hannover, 30002 220 Postfach GmbH Fluor Solvay Europe +49 511 857-2146 511 +49 857-0 511 +49 Fax: Tel.: USA 77098, TX Houston, Avenue Richmond 3333 LLC Fluorides, Solvay America North +1 713 525-7805 +1 713 525-6000 +1 713 Fax: Tel.: Korea South 120-140, Seoul Seodaemun-gu Bukahyun-ro 150, Center R&I EWAH-Solvay LTD CO., Korea Solvay Asia/Pacific + 82 2 2125 5380 22125 + 82 5300 22125 + 82

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