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Brazing - a guide to best practice Section 5. Brazing of aluminium alloys Introduction Brazeable aluminium alloys Brazing filler metals Brazing fluxes Joint clearances Brazing processes Introduction Brazing of aluminium and its alloys requires careful temperature control as the temperature gap between braze filler melting and base metal melting is narrow. Base alloys typically melt within the range 560-660°C, while most of the available braze filler alloys melt in the range 520-610°C and are brazeable in the range 580-620°C. A variety of brazing processes may be used for joining aluminium both with and without flux. Although some of the aluminium fluxes (particularly chlorides) are liable to cause corrosion, provided that they are properly cleaned off, brazed aluminium assemblies can show excellent corrosion resistance. Brazeable aluminium alloys The Aluminum Association designates aluminium alloys according to their main alloy addition: 1xxx series alloys are 99%min aluminium. Other aluminium alloy series are: 2xxx (copper), 3xxx (manganese), 4xxx (silicon), 5xxx (magnesium), 6xxx (magnesium + silicon), 7xxx (zinc), 8xxx (other elements). Table 1 lists some of the aluminium alloys, their compositions and melting ranges, and their relative brazeability. This list does not detail all the aluminium alloys; however, the following general rules give some guidance on those not listed. In general, the maximum percentages of alloying elements which may be present for the alloy to be brazeable are: Cu - 0.5%, Mg - 2.0%, Mn - 3.0%, Si - 2.0%, Zn - 6.0%. The non-heat treatable alloys most frequently brazed are found mainly in the 1xxx, 3xxx and low magnesium content (<2.5%Mg) 5xxx series of alloys. The heat-treatable alloys most frequently brazed are 6061, 6063, 6101, 6151, 6951, 7004 and 7005. Some castable aluminium alloys are brazeable; however brazing is difficult when surface finish is poor or the metal is porous. Aluminium alloys to be avoided when brazing are high Mg-content 5xxx series (such as 5154, 5083, 5086 and 5456) since the base metal is only poorly wet and yet excessive penetration and diffusion is experienced due to the low melting point. Die cast alloys are also not readily wetted by molten filler since they tend to blister on heating due to gas and lubricant entrapments. Other alloys not brazeable include 2011, 2014, 2017, 2024, 2319 and 7075 since the currently available range of filler metals does not have a low enough melting point to make brazing of these alloys achievable. Table 1 The nominal composition, melting range and brazeability of common parent alloys Aluminium Association Alloy No. Cu Si Mn Mg Zn Cr Approx. Melting Range °C Note Recommended Braze Filler 1350 99.50% minimum Aluminium 646-657 A 4043/4145 1100 0.12 99% minimum Aluminium 643/657 A 4343/4047 3003 - - 1.2 - - - 643-654 A 4343/4047 3004 - - 1.2 1.0 - - 629-652 B 4047 3005 0.3 0.6 1.2 0.4 0.25 0.1 638-657 A 4043 5005 - - - 0.8 - - 630-652 B 4047 5050 - - - 1.2 - - 627-652 B 4047 5052 - - - 2.5 - 0.25 593-649 C 4043 5154 - - - 3.5 - 0.25 593-643 D 4043 5456 - - 0.1 5.2 - 0.1 593-652 D 4043 6061 0.25 0.6 - 1.0 - 0.25 593-641 A 4047 6063 - 0.4 - 0.7 - - 616-654 A 4047 6101 0.5 - - 0.6 - - 621-654 A 4047 6151 - 1.0 - 0.6 - 0.25 588-649 A 4043/4145 6951 0.25 0.35 - 0.6 - - 616-654 A 4047 7004 - - 0.45 1.5 4.2 - 604-646 B 4047 7005 - - 0.45 1.4 4.5 0.13 607-649 B 4047 7072 - - - - 1.0 - 646-657 A 4043/4047 Cast 356.0 - 7.0 - 0.30 - - 557-613 D 4043/4145 Cast 357.0 - 7.0 - 0.50 - - 557-613 D 4043/4145 Cast A357.0 - 7.0 - 0.50 - - 557-613 D 4043/4145 Cast 359.0 - 9.0 - 0.60 - - 563-602 D 4043/4145 Cast 443.0 - 0.50 - - - - 574-632 C 4043/4145 Cast 710.0 0.5 - - 0.7 6.5 - 596-646 B 4047 Cast 711.0 0.5 - - 0.35 6.5 - 604-643 A 4047 Cast 712.0 0.25 - - 0.50 5.75 - 613-649 A 4043/4047 A Alloys readily brazed by all techniques B Alloys that can be brazed by all techniques with a little extra care C Alloys that require special care and effort D Alloys difficult to braze Brazing filler metals All of the commercially available brazing filler metals suitable for brazing aluminium are based around the Al-13wt%Si eutectic alloy, which melts at 577°C. One of the main reasons that aluminium-containing brazes are used is to minimise the risk of galvanic corrosion, to which aluminium is particularly susceptible. Filler alloys are listed in Table 2 and are described in more detail in Section 3 of this Best Practice Guide. Filler metals are supplied in the form of powder, paste, wire or thin-gauge shim stock and are either face-fed or pre-placed in the joint area. Another means of applying filler metal is to use a brazing sheet which consists of a core of aluminium clad with a lower melting filler metal. This cladding may be applied to one or both sides of the core sheet. The cladding is roll bonded to the core by hot rolling and eliminates the need for subsequent placement of the filler metal at the joint interface. For example, brazing filler metals 4343 (BAlSi-2), 4045 (BAlSi-5), 4044 and 4004 (BAlSi-7) are available pre-clad onto various thicknesses of 3003 and 6951 grades of aluminium alloy. Additions of Ti and Cu to braze filler alloys and alloy cladding can improve overall properties without significantly altering the brazeability. Ti increases the corrosion resistance of the brazing alloy on clad materials, while Cu (and Mn) can provide electrochemical corrosion protection and increase strength in critical areas. Elements such as Zn, Sn and In may be added to other areas of an assembly designed to act as a sacrificial anode. Improvements in corrosion resistance are being actively investigated and many centre around the use of Zn, or Zn-alloy coatings and their application routes. Recently, braze/solder compositions in the Sn-Ag-Ti system have also been developed for low temperature (250-480°C) fluxless brazing of aluminium and aluminium alloys. Alloys principally containing Cu and Ni additions (plus Zn) have also been developed for fluxless inert gas brazing down to 520°C. Mg and Bi additions are used especially in fluxless brazing procedures. The additions diffuse into the aluminium oxide layer and cause it to crack through differential thermal expansion. Liquid braze filler is able to penetrate the cracks, lift away the oxide layer and then wet and spread on the underlying aluminium. Magnesium is also able to diffuse into the surface layers of the base metal, which, along with Si diffusing from the filler alloy, further disrupts the oxide film through the formation of low melting Al-Si-Mg phases. Silicon diffusion must be controlled, however, as excessive diffusion can weaken brazed joints and promote corrosion. Table 2 Specified compositions (% by mass) and melt/braze temperatures of the aluminium filler metals BS (AWS) Classification Si Cu Mg Bi Fe Zn Mn Cr Ti Al Melting Range °C Brazing Range °C 4343 (BA1Si-2) 6.8-8.2 0.25 - - 0.8 0.2 0.1 - - rem 575-615 593-620 4145A (BA1Si-3) 9.3-10.7 3.35-4.7 0.15 - 0.6 0.2 0.15 0.15 - rem 520-585 570-604 4047A (BA1Si-4) 11.0-13.0 0.3 0.10 - 0.6 0.2 0.15 - 0.15 rem 575-585 582-604 4045 (BA1Si-5) 9.0-11.0 0.3 0.05 - 0.8 0.1 0.05 - 0.2 rem 575-590 588-604 (BA1Si-6) 6.8-8.2 0.25 2.0-3.0 - 0.8 0.2 0.1 - - rem 560-607 599-621 4004 (BA1Si-7) 9.0-10.5 0.25 1.0-2.0 - 0.8 0.2 0.1 - - rem 555-590 588-605 (BA1Si-8) 11.0-13.0 0.25 1.0-2.0 - 0.8 0.2 0.1 - - rem 560-580 582-604 (BA1Si-9) 11.0-13.0 0.25 0.10-0.5 - 0.8 0.2 0.1 - - rem 560-580 582-604 (BA1Si-10) 10.0-12.0 0.25 2.0-3.0 - 0.8 0.2 0.1 - - rem 560-580 582-604 4043A 4.5-6.0 0.3 0.2 - 0.6 0.1 0.15 - - rem 575-630 590-640 4104 (BA1Si-11) 9.0-11.0 0.25 1.0-2.0 0.02-0.20 0.8 0.2 0.1 - - rem 555-595 582-604 4044 7.8-9.2 0.25 - - 0.8 0.2 0.1 - - rem 577-602 593-613 Brazing fluxes In aluminium brazing, flux must be used for every process except fluxless inert gas or fluxless vacuum brazing. The flux should have a melting point ~20-50°C below that of the filler metal and remain stable at least to temperatures ~20-50°C above the maximum brazing temperature. It should exhibit minimal reaction or gas evolution at the aluminium surface and should either be easily removed after brazing or be non-corrosive. For aluminium alloys, two main types of flux are used: chloride based and fluoride based. These are formulated into several different compositions appropriate to different brazing processes. As with all fluxes they act as deoxidizers and prevent re-oxidation by coating the surfaces to be joined. By lowering surface tension, they promote wetting of the base aluminium alloy by the molten braze filler. The majority of aluminium brazing fluxes are composed of mixtures of alkali, alkaline earth chlorides and fluorides. Fluoride is the most reactive agent, but its presence increases the flux melting point and therefore other additions, such as Sb, Cd, Cr, Co, Cu, Pb, Mn, Sn and Zn chlorides are made to establish the desired melting temperatures and activity. Chloride fluxes leave a hygroscopic corrosive surface on the workpiece which must be thoroughly cleaned off after brazing. Chloride and fluoride based fluxes FB1-A and FB1-B are used for torch and furnace brazing of aluminium alloys in the temperature range 560-615°C. FB1-C is used in the range 540-615°C for dip brazing. Fluoride-based fluxes are primarily composed of K-Al-F and the first and best known type is Nocolok (a registered trademark of Alcan). The most salient feature of this flux is the absence of chloride. Provided they do not experience an ionising environment, the flux residues are not corrosive and do not need to be cleaned off after brazing. Fluoride fluxes require inert gas (N2) protection during brazing. Joint clearances When brazing aluminium alloys, the set-up of individual components with respect to alignment and particularly joint gap is very important. Too large a space between parts means that the braze will not be able to fill the entire gap and too small a space means that the surface tension effects which cause capillary flow of the molten filler metal will not operate and the braze will be unable to properly fill the joint gap. As a guide, when torch brazing fluxed lap joints above 6mm in length, a joint clearance in the range 0.2-0.25mm is recommended. For shorter lap lengths, less than 6mm, clearance should be in the range 0.05-0.2mm, while for vacuum furnace brazing or when using clad sheet, joint clearance should be less than 0.05mm. Further details on calculation of joint clearance and on appropriate joint design are provided in Section 6 of this Best Practice Guide. Brazing processes Aluminium alloys may be brazed via most standard processes (see Section 4 of this Best Practice Guide). Dip (flux) brazing, furnace brazing (air, inert gas and vacuum) and torch brazing are, perhaps, the most common processes utilised. In all cases, careful temperature control is required as brazing temperatures are usually close to parent alloy melting temperatures. Four of the most common production brazing processes are compared in Table 3. The significance of the development of non-corrosive fluxes (e.g. Nocolok) can be seen. Table 3 Comparative features of common production brazing processes Process Feature Dip brazing Chloride-fluxed furnace brazing Nocolok brazing Vacuum brazing Heat transfer 1 1 1 1 Corrosion resistance 3 1 1 3 Brazeability 1 1 1 3 Fit-up/clearance 1 1 1 2 Energy 2 2 1 2 Effluent treatment 4 4 1 1 Equipment cost 2 1 2 4 Maintenance cost 3 2 1 4 Material cost 4 2 1 2 Production cost 3 3 1 2 Where 1 = good; 4 = poor The Nocolok process, developed in the 1980s by Alcan, is by far the most important recent development in aluminium alloy brazing technology. The process is fast, clean and inexpensive and, now that the patent has expired, is frequently the chosen method. Until recently, the most favoured approach was dip brazing, or, if equipment was available, vacuum brazing. However, the cost implications of these two techniques, with regard to cleaning and capital equipment cost, have more or less eliminated their use for new production lines. The Nocolok process, however, is not universally applicable, and other techniques are still widely used. Further details are given below. Dip (flux) brazing Dip brazing has been used widely and successfully for many years, particularly for complex assemblies, and is one of the best methods of heating and fluxing aluminium joints. The process enables rapid, uniform heating and can accommodate low dimensional tolerances. However, significant post-braze cleaning is required to remove flux residues and close attention must be paid to assembly design to avoid air traps. Due to the low specific heat of flux, and to avoid retained moisture causing spattering of flux, the assemblies are pre-heated to ~540°C prior to full immersion in the ceramic-lined molten flux bath. Typically, bath temperature should be controlled within ±3°C and drop no more than 6°C when the aluminium parts are immersed. Immersion time may vary from 30 seconds to 30 minutes depending on the size and weight of the assembly being brazed. Suitably corrosion resistant materials for jigging include 304 stainless steel, Nimonic 75 and Inconel X-750. The flux is a molten mixture of chlorides of Na, K and Li with additions of fluorides of Na, Al and Mg and is periodically adjusted by further additions of molten chlorides and fluorides. It is very important that the flux residues are removed after brazing to inhibit corrosive attack of the parent material. Torch brazing Torch brazing of aluminium involves locally applied heat typically generated by a slightly reducing oxy-acetylene, oxy-hydrogen or oxy-natural gas flame. The latter is usually the preferred gas mixture for aluminium torch brazing as it is cheaper and generally more controllable. As with other aluminium brazing processes, close temperature control is important. While this is relatively straightforward in automated torch brazing, in manual brazing (particularly used for repairs) there is no colour change in the aluminium to indicate temperature. As an aid to temperature indication, some torch brazing fluxes are formulated to change colour (as well as liquefy) when the appropriate temperature is reached. Filler metal may be preplaced or face fed during brazing. Post-braze cleaning to remove chloride flux residues is required. Furnace brazing - fluxless, inert atmosphere Furnace brazing is attractive from the perspectives of low cost of equipment, the ease with which existing furnaces can be adapted to braze aluminium and the simple, minimal fixturing required. Inert gas furnaces are much cheaper and simpler than vacuum furnaces, more easily achieve uniform heating and avoid zinc or magnesium volatilisation. The process generally provides higher production rates and lower costs than can be achieved with vacuum or torch brazing, although pre-cleaning of parts is required. The process of fluxless brazing of aluminium under inert atmosphere requires that the aluminium either has a magnesium-containing cladding or is pre-coated with nickel. Magnesium works by disrupting the surface oxide layer on the aluminium while nickel protects the surface from oxidation and also reacts with the aluminium surface during heating. Both mechanisms are effective in promoting wetting by molten braze filler. The use of appropriate chemical pre-cleaning treatments and new formulation braze filler metals has opened up brazing to a wider range of aluminium alloys, including high strength precipitation hardening alloys and castables. Brazing temperatures as low as 510°C have been achieved using nitrogen (and vacuum) atmospheres. Furnace brazing - fluxed (Nocolok), N2 atmosphere Significant work has been undertaken to develop non-corrosive fluxes which may be used in N2 inert atmosphere furnaces. The result was the Nocolok flux brazing process. The process eliminates post-brazing cleaning operations and avoids the need for nickel coating. Additionally, as-brazed surfaces are suitable for further corrosion protection via chromate conversion and flux residues are non-contaminating. The process, however, is not generally applicable to aluminium alloys containing more than 0.5%Mg or to the high strength alloys. Typical base alloys used include 3003 (+Zn), 3005 (+Zn) and 7072. Filler alloys used include 4045 (+Zn) and 4343 (+Zn). Nocolok flux is a fine white powder of <200 mesh particle size and contains a eutectic mixture of K3AlF6 and KAlF4. It melts just below the melting point of the braze alloy and reacts with the aluminium oxide layer without attacking either solid or molten aluminium. Brazing is then carried out under nitrogen atmosphere at a temperature of ~600°C. With such an atmosphere, only a light flux coverage is required. After assembly and fixturing, all parts are pre-heated to ~150°C to drive moisture and alcohol out of the flux and then the entire assembly is placed in the hot nitrogen furnace (batch or continuous controlled to ±5°C). The parts are held for 3-5 minutes after they have reached brazing temperature and then removed. In general, the assembly remains in the furnace for no longer than 15 minutes. Furnace brazing - fluxless, vacuum Aluminium alloys can be successfully brazed under vacuum in both hot-wall and cold-wall types of vacuum furnace. The process requires the use of an oxygen getter, such as magnesium, placed on the material and in the furnace to react with residual oxygen. Batch, or semi-continuous furnaces may be used, provided that a uniform temperature distribution of around ±5°C can be achieved. However, it is important that the atmosphere is strictly conditioned. For example, if the vacuum is 1x10-1 torr, hardly any fillets will be formed, whilst at 5x10-3 torr, fillets are formed all over the aluminium surface. Although capital costs of vacuum brazing equipment are relatively high and not all braze and parent alloys are suitable for vacuum heat treatment, the process can be automated and, if properly controlled, has all the cost and corrosion resistance advantages associated with fluxless brazing. Vacuum brazing had been widely accepted by many volume producers of assemblies such as heat exchangers because of its economics of scale and cleanliness. However, the conventional vacuum brazing sheet alloys (e.g. 4045 + Mg or 4104 clad onto 3003 or 3005) used in such applications become sensitised to intergranular corrosion after vacuum brazing, requiring a sacrificial anode within the brazing sheet. The long heating times associated with the vacuum brazing process allow silicon to diffuse from the filler into the base alloy which aids wetting and spreading of the filler. However, if excessive, Si diffusion can lead to poor fillet formation, low joint strength and impaired corrosion resistance. Suggested Further Reading Humpston G and Jacobson D M: 'Principles of Soldering and Brazing'. ASM International, Ohio, USA 1993 ISBN: 0-87170-462-5 'Soldering and Brazing of Aluminium' publ. Aluminum Assoc., Washington, USA 1991 'Welding, Brazing and Soldering' ASM Handbook Volume 6, publ. ASM International, USA 1993 ISBN: 0-87170-382-3 'Aluminium Fluxless Vacuum Brazing' publ. Aluminum Assoc., Washington, USA 1983 'Brazing of Aluminium' publ. Aluminium-Zentrale, Dusseldorf, Germany 1987 'Aluminium Brazing Handbook' Aluminum Assoc., Washington, USA 1990 'Source Book on Brazing and Brazing Technology' ed M Schwartz, publ. ASM International, Ohio, USA 1980 ISBN: 0-87170-099-9 'Brazing for the Engineering Technologist' M Schwartz, publ. Chapman & Hall, London, UK 1995 ISBN: 0-412-60480-9 About the IIW / Mumbai Branch / Other Branches / Coming Events / Technical Lectures Mumbai Weldnet / Trends in welding / Related Websites / IIW Forum / Feedback / Home