Aluminium Brazing with Non-corrosive Fluxes State of the Art and Trends in NOCOLOK® Flux Technology

Dr. Hans – Walter Swidersky Solvay Fluor und Derivate GmbH Hans-Boeckler-Allee 20 – 30173 Hanover, Germany Presented at the 6th International Conference on Brazing, High Temperature Brazing and Diffusion Bonding (LÖT 2001), Aachen, Germany (May 2001) – Revised Text Abstract This paper summarises the general development and the current status of aluminium brazing with non-corrosive fluxes. Based on the most common manufacturing practices, present-day brazing operations are described. Numerous improvements to aluminium brazing technology have been made in recent years. The most significant devel- opments and trends are addressed, particularly process reengineering (e.g., cleaning, flux application), cost savings (e.g., water, flux, energy), and aluminium alloy improvements (e.g., high strength, good formability, and long-life).

Table of contents Success or failure in CAB production relies on several • Introduction factors. The starting point is good product fit-up. Parts 1. Cleaning and Flux Application to be metallurgically joined must have intimate contact 2. Flux Application Methods at some point along the joint. An adequate (but not 3. Wet Flux Application excessive) quantity of filler metal must be available to 4. Dry/ Electrostatic Flux Application fill the joints. Capillary forces pull the filler into the 5. Post Braze Flux Residue joints. The gap tolerance is 0.1 to 0.15 mm for non-clad 6. Filler Metal Alloys components. When clad products are used, intimate 7. Brazing Alloys and Brazing Sheet contact is recommended; the clad layer(s) will act as a 8. Furnace Conditions gaping tool. • Summary Another essential for reliable brazing results is a uni- form flux coating on all surfaces involved in the joint Introduction formation. The main focus for achieving this task is on the cleaning and fluxing procedure. Since the early 1980’s, Controlled Atmosphere Brazing (CAB) with non-corrosive fluxes has evolved as the Equally important are the furnace conditions, i.e. tem- leading technology for manufacturing aluminium heat perature profile, temperature uniformity, and atmos- exchangers in the automotive industry. Today, more pheric conditions. than 400 CAB furnaces are in operation throughout the world using the NOCOLOK® process. 1. Cleaning and Flux Application This technology offers the benefits of a flux for success- Cleaning and flux application procedures depend on ful oxide removal, working at atmosphere pressure product designs and personal choice. Heat exchangers while avoiding the disadvantages of post braze treat- are classified in two groups: ments and corrosion susceptibility. The non-hygros- a) no internal brazing required copic and non-corrosive potassium fluoroaluminate flux b) internal brazing required does not react with aluminium in the molten or solid state and the post braze flux residues have a very low No internal brazing is necessary with the following water solubility 1. types: The process in most brazing operations includes the • Radiators (welded tubes) following steps: • Parallel Flow Condensers [PFC] (extruded tubes) • Component Forming and Assembly • Serpentine Evaporators • Cleaning and Flux Application • Heater Cores (welded tubes) • Brazing • The process sequence in brazing operations is de- Intercoolers (extruded tubes) pendent on: Internal joint formation (and therefore internal flux appli- cation) is needed on these types: • Heat Exchanger Design • Cleaning Method • Radiators (folded tubes) • Flux Application Method • Condensers (folded tubes) • Heater Cores (folded tubes) • Other Cleaning Methods: • Oil Coolers In some small volume manufacturing, solvent cleaning • Plate Evaporators is used. However, health safety and environmental issues are critical with many of these chemicals. Va- • Intercoolers (with turbulators) pour degreasing has been phased out almost com- For the majority of brazed heat exchangers, assembly pletely world-wide. is followed by cleaning and subsequently by flux appli- Some consideration has been given to totally eliminat- cation. However, when internal surfaces need to be ing the cleaning step under certain conditions. How- fluxed, the production sequence may follow a different ever, the question of how much residue (contamination) order, determined by when and how the flux is applied. is acceptable must be resolved before such decision • Cleaning Methods: can be made. The purpose of cleaning is to remove fabricating oils and lubricants as well as other contaminants from the 2. Flux Application Methods surfaces. The cleaning procedure must allow for ade- There is a wide variety of different fluxing methods in- quate flux retention and render the surfaces suitable for cluding: brazing. Cleaning results have great influence on the - Low Pressure Spray brazing results, post braze product appearance and - Flooding (Cascade) corrosion performance. - Dipping or Submerging The following cleaning methods are commonly utilised: - High Pressure (atomised) Spray • Aqueous Cleaning: - Brushing (Paste Application) Aqueous cleaning of high volume assemblies is usually - Dry or Electrostatic by spray. The parts travel on a belt through a multi- chamber cabinet, and are spray-cleaned by an aqueous 3. Wet Flux Application solution, spray-rinsed with warm and cold water The most common flux application method in CAB is by (2 - 3 x). The excess water is then blown off with air. spraying an aqueous suspension. Constantly agitated In most cases, the cleaning solution contains a mild flux slurries with concentrations of approximately 10 – alkali ingredient and detergents (acid-based cleaning 35% solids are pumped from tanks to fluxing booths. solutions seem less common). The cleaning solution All aluminium surfaces involved in the brazing process not only removes lubricants from the surfaces, but also are coated with the slurry, resulting in a uniform flux has a mild "etching" action (i.e. producing a microscopic layer. Excess flux slurry is removed with a high-volume roughness). This etching action renders the surface of air blow; the excess is then collected, recycled and aluminium wettable by lowering the surface tension. reused in the fluxing booth. The cleaning efficiency in aqueous cleaning is a func- Capillary effects throughout the unit can result in non- tion of: uniform flux distribution. Flux slurry tends to be held in - Concentration (Cleaning Solution) fin/ tube joints, fin louvers etc. This excess is difficult to - Time and Temperature blow off with only one curtain or knife air system. Slurry - Contact Pressure (usually 1–2 kg/cm2) also collects on the bottom surface of units in the shad- owed area that doesn’t see direct air impingement. The An alternate cleaning method often used is: excess in those areas is wasted flux, as a uniform • Thermal Degreasing: 5 g/m2 load is sufficient to ensure good brazing. If wet- With the use of special "evaporative/ vanishing oils/ tability is poor on the headers, flux load tends to be low. lubricants", a heat exchanger can be degreased simply The relatively plain header surfaces generally hold less by heating. These lubricants volatilise when heated to flux slurry. Most fluxers are designed with two flux approximately 200 – 250°C. The heat exchanger sur- slurry concentrations to ensure sufficient coverage in face temperature should not exceed 300°C, to avoid the header area by applying a 10 to 15% higher slurry the formation of high temperature oxide (see also Prod- concentration. uct Dehydration). Excess flux slurry can be blown off by installing a sec- An important consideration is that this degreasing ond tandem overhead blow-off system or additional air method leaves the heat exchanger "non-wettable", i.e. knifes for cutting off droplets from the bottom and side a uniform flux distribution in wet fluxing can be difficult. surfaces. Reducing excess slurry on the core in the When a product is thermally degreased, the flux slurry fluxer (slurry is recyclable) is more economical than requires the addition of a surfactant (wetting agent) to removing dried flux from a drier (due to hot air currents lower the surface tension of the water, thus ensuring in the drier), which is not recyclable. more uniform flux distribution. • Product Dehydration: When wet fluxing is utilised, the heat exchangers are dried prior to brazing in a separate dry-off oven. Prod- uct entering the brazing furnace must be completely dry • High humidity causes physical adsorption of from water introduced via aqueous cleaning or flux moisture to the fine powder dust in the booth. slurry coating. This can easily be confirmed initially by This can result in agglomerations. weighing the product as it exits the dry-off oven, run- • Recovering, recycling and reusing flux requires ning a second and third time through the oven, and special attention. weighing after each run. If the product weight has sta- Some operations electrostatically apply flux before they bilised, drying is acceptable. If further weight loss oc- thermally degrease the units. The thin layer of residual curs, more time in the drier or higher temperature will lubricants seems to improve powder adhesion 3. be necessary. However, the surface temperature of the parts in the drier should not exceed 250°C. High tem- Special flux qualities with adjusted particle distribution perature oxide is formed at about 300°C. are commercially available, and contribute to improved electrostatic application. Oxide thickness increases with temperature, time at temperature and particularly in the presence of mois- 5. Post Braze Flux Residue ture. Oxide formation will affect clad fluidity2. There is only a small drop in brazeability when the oxide thick- It is generally accepted that the presence of flux resi- ness increases from 40 to 220 Angstroms at 5 g/m2 flux dues on a heat exchanger enhances its corrosion resis- load. However, there is an appreciable drop in clad tance 4 5 6. However, it has always been difficult to fluidity as the oxide thickness increases from 40 to 100 quantify the level of corrosion resistance enhancement. Angstroms at 2 g/m2 flux. A reduction in brazeability In corrosion testing of flat panels or coupons coated can even be noted as the oxide thickness is increased with flux residue, there is no doubt that there is a bene- from 40 to 60 Angstroms. ficial effect. With heat exchangers on the other hand, Proper flux loads should be maintained since low flux the general trend shows a longer corrosion life, but load appears to be quite sensitive to even small factors such as uniformity of flux residue coverage and changes in oxide thickness. variations in flux load sometimes confuse the corrosion test data. 4. Dry/ Electrostatic Flux Application There is no indication of interactions between flux resi- due and coolants, refrigerants7, turbine oils, and polyal- Over the past five to ten years, some users of CAB cylene glycol lubricants. technology have successfully implemented dry flux application methods. Based on the principles of pow- 6. Filler Metal Alloys der paint technology, an application technique alterna- tive was introduced in the brazing industry. Commercial filler metals are aluminium silicon alloys containing from 6.8% to 13% silicon. The benefits of electrostatic application are directly related to the problems of wet application: • Alloy Si 8 Start Fully Braze No need to mix slurries AA Melting Molten Range • No need to monitor slurry concentration 4343 6.8-8.2% 577°C 613°C 593-610°C • No need for a surface wettability concept (i.e., 4045 9-11% 577°C 591°C 588-604°C surface treatment or wetting agent) 4047 11-13% 577°C 582°C 582-600°C • No separate drying step required to remove Table 1: Filler Metal Alloy Characteristics moisture • No waste water effluent The solidus (or the point at which melting begins) is Particularly when dry fluxing is used in combination with 577°C for all filler metal alloys. However, melting oc- thermal degreasing (i.e. evaporative oils and lubri- curs in a range. The temperature above which the filler cants), the objective is to completely eliminate or sig- metal is completely molten is called the liquidus. In nificantly reduce water consumption in the process. between the solidus and the liquidus, the filler is partly molten, existing as both solid and liquid. Before the In dry flux application, the following difficulties have filler metal becomes fully liquid and starts to flow, more been described by users when operating with conven- than 60% of the clad material must melt. This requires tional flux qualities: a minimum (threshold) temperature for each filler metal • Powder fluidisation and material transport is dif- alloy in the brazing process. (Note: AA 4047 cannot be ficult. Vibration or stirring is necessary to im- used for furnace brazing. It is too liquid at brazing tem- prove on these characteristics. perature and will flow down. It may even be drawn out • Inconsistency of flux flow and non-uniformity of of the joints by gravity forces. Applications for 4047 are applied flux. induction and flame brazing.) • Adhesion of deposited flux is inferior when com- Recent developments for filler metal alloys are focused pared with wet application. on the control of fluidity and flow pattern9 10. By adding specific trace quantities of alloying elements (e.g., Li, Na), brazing characteristics improve. These effects appear to be related to reduced surface tension. Under Long life alloys utilise the feature of silicon diffusion certain conditions, brazing can be accomplished with from the cladding to form an intermediate layer between reduced flux loads or with no flux at all11. It has not yet the cladding and the core alloy. This generates a been determined if the alloying of additional elements to dense band of precipitate that acts sacrificially to the the filler metal is feasible on a high volume scale. core alloy, restricting corrosion to within this layer, and overcoming inter-granular attack. • Core Alloy Dissolution/ Erosion: Long life alloys are now widely used in many heat ex- During the brazing cycle, silicon from the clad alloy changer components. diffuses into the core and causes dissolution of base metal. This condition results in a reduction in thickness • Magnesium Containing Alloys: of the core. The extent of erosion is increased by: For added strength and machineability, certain alloys - higher silicon levels in the clad, contain Mg. Most notably are the 6000 series alloys - longer than recommended braze cycles, (up to 1% Mg). These are used for fittings and ma- - excessive peak brazing temperatures, chined components as well as some long life alloys (up - excessive thickness of the clad alloy, and to 0.3% Mg). - a design which allows pooling of the braze metal. During the braze cycle, Mg diffuses to the surface and In some instances, filler metal erosion can be so severe reacts with the surface oxide to form MgO and that the entire thickness of the base metal (tube or fin) MgO:Al2O3 (spinel). These oxides have reduced solu- is consumed, resulting in total failures. bility in the molten flux. Furthermore, Mg and/or MgO The most common factors leading to excessive erosion can react with the flux forming compounds such as are not design-related; rather, they are linked to the MgF2, KMgF3 and K2MgF4. All of these serve to poison process. Brazing beyond the recommended maximum the flux, significantly reducing its effectiveness (i.e. peak temperature and/or dwelling at brazing tempera- resulting in smaller joints or no joint formation at all). ture are the leading causes of erosion. The limit to the amount of Mg tolerated in furnace flux brazing is 0.5%; while around 1% Mg is tolerable for 7. Brazing Alloys and Brazing Sheet flame brazing. It should be noted that the brazing toler- The core alloys in common use for heat exchanger ance to Mg is the total sum of the Mg concentrations in manufacturing include, but are not limited to: AA 3003, both components to be joined: AA 1100, AA 1145, AA 1070, AA 3005, AA 3105, [Mg]component 1 + [Mg]component 2 = [Mg]total AA 6951, AA 1050, AA 1435, AA 3102, AA 6063. Improved brazing results have been reported with mag- Aluminium producers are now offering so-called "Long nesium-containing aluminium alloys (up to 0.6% Mg) Life Alloys". These are modified AA 3XXX series al- when the flux formulation contains some cesium (i.e. loys, offering superior strength and corrosion resis- approximately 2%)12. Cesium reacts as a buffer for tance. Most of these alloys have not been designated magnesium by forming CsMgF3 and Cs4Mg3F10, which by the Aluminum Association and are known only by reduces the flux inhibition. These compounds melt at their manufacturers’ designations. Some examples of lower temperatures and interfere less with aluminium long life alloys are: X800, X900, HE45, K315, K319, brazing13. MD267 etc. • Flux Pre-coated Brazing Sheet/ Components: A general trend in the industry is to use thinner gauges for the design of heat exchanger components. Thinner The concept of a brazing sheet which is supplied with a gauges not only save on material (i.e. costs) but also on flux coating is very plausible. Such material would sig- weight and space. Based on this development, there is nificantly change the way heat exchangers are currently strong demand for higher strength alloys with good manufactured in that the flux application step would be formability. eliminated. Several patent applications have recently been filed on • Long Life Alloys: this premise 14 15 16. Its greatest challenge is the best Historically, Al-Mn based alloys such as AA 3003 and way to make the flux adhere to the metal surface. AA 3005 have been used for the manufacturing of heat Throughout the forming process of the components, exchanger components. However, these alloys are uniform coverage and strong adhesion are equally im- susceptible to inter-granular corrosion that is acceler- portant. The latter is the origin of yet another concept. ated by the diffusion of silicon along grain boundaries Flux coating technologies have been developed for from the cladding alloy during brazing. (pre-)formed components, which are primarily adapted The automotive industry has stipulated that their heat for heat exchangers with internal brazing required, i.e. 17 exchangers last a minimum of 10 years. This encour- plate evaporators . Flux application with a binder sys- aged the development of more corrosion resistant al- tem allows coating of specific surface areas with a pre- loys. cise flux amount. It also reduces flux fall-off during assembly. Binders used for pre-fluxing must evaporate during the side atmosphere, i.e. high moisture and oxygen from process without interfering with the brazing perform- back-streaming, into the furnace. Higher dew point and ance or leaving any contamination on the surfaces. oxygen in the furnace require higher flux load on the Exhaust treatment may need to be adjusted to the parts to handle the degraded atmosphere. Curtains presence of binder fumes. must be kept in the recommended condition; they • should not be "sculptured" to accommodate product Clad-less Brazing: height. There are several suggestions for brazing technologies • Muffle Mesh Belt: where the filler metal is generated during the brazing cycle from a coating layer on a plain aluminium sheet or The furnace mesh belt has a considerable surface area. on extrusion material. It exits the muffle and returns to the entrance outside One method involves a mixture of flux with silicon pow- the furnace atmosphere. During this time, it substan- der, the NOCOLOK® Sil Flux process18. At brazing tially cools and picks up moisture, which is driven off temperature, the silicon powder diffuses into the alu- inside the muffle, thus contributing to degrading the minium substrate and generates Al-Si filler metal for furnace atmosphere. A higher dew point not only cre- joint formation. Sil flux can be applied with a binder to ates a potential necessity for higher flux load; it can specific component surfaces, e.g. extruded tubes. In also be a factor in increased muffle corrosion at the this case, the filler metal would be supplied from the entrance of the furnace. tube and a clad fin sheet is not necessary. • Muffle Integrity: In Composite Deposition technology (CD process)19, the filler metal for joint formation is derived from a com- Holes or leaks in the muffle allow the potential of out- posite powder, a compound consisting of potassium side atmosphere being drawn by the Venturi effect into fluoroaluminate flux and Al-Si alloy. The CD powder is the muffle. If air is drawn in from outside, moisture and "deposited" selectively and accurately on heat ex- oxygen levels will increase. changer components prior to assembly and brazing. • Flux Build-up: The common challenge for all clad-less approaches is Molten flux has a very low surface tension and can the development of a reliable coating procedure. easily drip down from the aluminium surfaces of heat exchangers and fixtures, especially when the flux load 8. Furnace Conditions is high. These flux droplets solidify under the conveyor Specifications for acceptable CAB furnaces (with the belt in the furnace zone where the temperature is below use of 5 g/m2 uniform flux load) are: the flux freezing point (usually between the brazing and the cooling zone). A second source for flux powder • Furnace Atmosphere: residue is re-condensed KAlF4 vapour. The quantity of - Oxygen level below 100 ppm KAlF4 vapour generated during the brazing cycle de- - Dew point below –40°C pends on time at temperature. • Brazing Temperature and Time: Under normal production conditions, there will always - Ideally a uniform 600°C ± 5°C (heat exchanger be some flux build-up on the conveyor belt and, even- surface temperature) tually, some build-up on the furnace bottom. If this build-up exceeds a certain height, the conveyor belt can - Ideally 3 minutes ± 0.5 min from 580°C heat-up be deflected. Therefore, the flux build-up on the muffle to 605°C on cool-down bottom must be removed during regular maintenance Currently, many production sites work with flux loads of cleaning operations. 3 g/m2. As flux coating goes down, furnace atmos- pheric conditions become more critical. Most CAB Summary furnace manufacturers produce furnaces which rou- tinely operate in the 20 – 50 ppm oxygen and dew point Controlled atmosphere brazing using non-corrosive flux levels of –45 to –50°C (i.e., 92 – 67 ppm H2O). A low is the dominant process for making aluminium heat dew point will keep the formation of hydrogen fluoride exchangers for the automotive industry. according to equation (1) to a minimum20: There are several factors determining the success of

3 KAlF4 + 3 H2O → K3AlF6 + Al2O3 + 6 HF (1) aluminium brazing: • Product Fit-up and Assembly A suitable treatment for the furnace exhaust (i.e. dry • scrubber) is required. Component Cleanliness • Flux Application • Furnace Curtains: • Furnace Atmosphere Furnace curtains are installed in the muffle or at the • Brazing Temperature Uniformity ends to maintain a positive nitrogen pressure in the • Brazing Time at Temperature furnace. This is required to prevent the ingress of out- Recent developments in brazing technology focus on improvements in aluminium alloy composition (i.e. Brazing Seminar, Detroit (October 2000) Confer- higher strength, good formability and higher corrosion ence Proceedings resistance). 12 Garcia, J. et al., Brazeability of Aluminum Alloys Research and development activities throughout the Containing Magnesium by CAB Process Using industry have resulted in concepts for: Cesium Flux, International Invitational Aluminum • new flux qualities more suitable in dry/ electro- Brazing Seminar, Detroit (October 2000) Confer- static flux application and for furnace brazing of ence Proceedings Mg-containing alloys (up to 0.6%) 13 Seseke, U.: New Developments in Non-corrosive • clad-less brazing technologies Fluxes for Innovative Brazing. First International • pre-fluxed brazing sheet/ components Congress Aluminium Brazing, Düsseldorf (May, • reduction of consumables (e.g. flux, water) 2000) Conference Proceedings 14 Kilmer, R. and Eye, J.: A Method of Depositing Future developments for flux brazing technology will Flux or Flux and Metal onto a Metal Brazing Sub- include manufacturing more heat exchanger types and strate. International Patent Application WO extended product ranges, some of which are currently 00/52228 still produced with other processes and materials. 15 Sucke, N.: Partial or Complete Coating of Alumi- num and Aluminum Alloy Structural Parts With a 1 Field, D. J. and Steward, N. I.: Mechanistic As- Braze Flux, and Bonding Agent Prior to Brazing. pects of the NOCOLOK® Flux Brazing Process, German Patent Application DE 19859735 A1 SAE Paper # 870186, Warrendale, PA 16 Wittebrood, A.: Composite Sheet Material for Braz- 2 Gray, A., Flemming A.J.E. and Evans J.M.: Opti- ing. International Patent Application WO 00/64626 mising the Properties of Long-life Brazing Sheet A1 Alloys for Vacuum and NOCOLOK Brazed Com- 17 Kojima, M. et al.: Flux Composition for Brazing of ponents, VTMS 4 Conference Proceedings, Lon- Aluminum Material and Method for Brazing of don (May 1999) Aluminum Material. European Patent Application 3 Luo, Y. and Hutchins, H.: Method for Braze Flux EP 0936024 A1 Application. European Patent Application EP 18 Timsit, R. S. and Janeway, B. J.: A Novel Brazing 0931619 A1 Technique for Aluminum. Welding Journal, Weld- 4 Nakazawa, T., Tanabe, K., Ushikubo, K., and Hi- ing Research Supplement, (June 1994): 119 raga, M., Performance Evaluation of Serpentine 19 Coombs, J. S.: Production of Powder. International Evaporator for Automotive Air-conditioning Sys- Patent Application WO 94/17941 and European tem, SAE Paper # 840384, Warrendale, PA Patent Specification EP 0682578 B1 5 Claydon, D. and Sugihara, A., Brazing Aluminum 20 Lauzon, D., Belt. H.-J. and Bentrup, U.: HF Gen- Automotive Heat Exchanger Assemblies Using a eration in NOCOLOK® Flux Brazing Furnaces. Non-Corrosive Flux, SAE Paper # 830021, War- 1998 International Invitational Aluminum Brazing rendale PA Seminar, Conference Proceedings 6 Cooke, W. and Bowman, A., Tunnel Furnace Braz- ing of Aluminum Using a Non-Corrosive Flux, Welding Journal, (October 1980) Reprint 7 Meurer, C., Lauzon, D. C. and König, H.: Stability of R-134a in the Presence of NOCOLOK® Flux Residues. SAE Technical Paper # 980052, War- rendale, PA 8 Registration Record of International Alloy Designa- tions and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys, The Aluminum Association, Inc. (July 1998) 9 Okamoto, I. and Takemoto, T.: Brazability of Alu- minum using Al-Si Filler Alloys with Different Com- positions and Microstructures. Transactions of JWRI. 10, 2 (1981): 35 10 Sekulić, D. P.: Behavior of Aluminum Alloy Micro Layer During Brazing. Recent Res. Devel. Heat, Mass & Momentum Transfer, 2 (1999): 121 11 Childree, D.: Fluxless Brazing in a Controlled At- mosphere Furnace With a New Filler Alloy Con- taining Na. International Invitational Aluminum