A Novel Brazing Technique for Aluminum

WELDING RESEARCH SUPPLEMENT TO THE WELDING JOURNAL, JUNE 1994 Sponsored by the American Welding Society and Ihe Welding Research Council A Novel Brazing Technique for Aluminum A simplified and cost-effective method using an alloy powder mixture instead of a clad surface has been developed for brazing aluminum, copper and brass BY R. S. TIMSIT AND B. J. JANEWAY ABSTRACT. This paper describes a novel Introduction eutectic composition such as AA4045, brazing technique for aluminum, in AA4047 or AA4343 (Ref. 4). These alloys which at least one of the contacting alu The joining of metal parts by brazing contain 9 to 1 3 wt-% of Si and are char minum surfaces is coated with a powder- often involves the use of a filler metal acterized by a melting temperature (in a mixture consisting of silicon and a potas characterized by a liquidus temperature narrow range near 577°C) (Ref. 5) con sium fluoroaluminate flux. Brazing is above 450°C (842°F) but appreciably siderably lower than that of the core alloy carried out by heating the joint to ap below the solidus temperatures of the (~660°C). Joining is carried out at ap C proximately 600 C in nitrogen gas at core materials. On melting, the filler proximately 600°C (1112°F) in the pres near-atmospheric pressure over a time metal spreads between the closely fitted ence of a noncorrosive flux such as a flu interval of a few minutes. During heating, surfaces, forms a fillet around the joint oroaluminate salt (Refs. 1, 6) to remove the flux melts at 562°C and dissolves the and yields a metallurgical bond on cool native surface oxide films from the con surface oxide layers on the aluminum, ing. Over the past several years, nitrogen tacting aluminum surfaces. Oxide re thus allowing the silicon particles to furnace brazing (Refs. 1, 2) has been used moval enhances wetting by the molten come into intimate contact with the bare in preference to other techniques for the Al-Si eutectic alloy at the brazing tem metal. At temperatures exceeding 577°C, large-scale joining of aluminum parts perature and eases liquid-metal penetra the silicon dissolves rapidly into the alu such as automobile heat exchangers and tion of the joint. minum and generates in-situ a layer of air-conditioning condensers (Refs. 2, 3). The manufacture of clad aluminum Al-Si liquid alloy of eutectic composi In these applications, at least one of the brazing sheet is a multistep process that tion. The filler metal forms a fillet around aluminum components is clad with filler involves casting of the filler metal ingot, the joint by capillary action. A metallur metal consisting of an Al-Si alloy of near- subsequent cladding to the aluminum gical bond is formed on cooling. The ad core material by hot rolling and final cold dition of Zn powder to the Si/flux powder rolling of the composite material to the mixture allows diffusion of Zn during desired thickness. The preparation of alu brazing and modifies the resistance to minum brazing sheet is thus relatively corrosion of the brazed assembly. KEY WORDS complicated and the manufacturing cost The novel brazing technique may be is correspondingly high. The present exploited using elements other than sili Brazing work was motivated by the need to sim con for generating filler metal. The only Aluminum plify aluminum brazing sheet manufac requirement on these elements, such as Eutectic Reaction ture. The development of a brazing tech Cu, Ge, Zn, etc., is that they form a rela Intermediary Powder nique that obviates the use of clad tively low-temperature eutectic alloy Si/Flux Powder Mix material, without greatly affecting the in with aluminum. The technique has been Oxide Removal dustrial brazing practice evolved over used successfully for brazing alu- Temperature Relation the past several years for aluminum, minum/Cu, Cu/Cu and Cu/brass in addi Filler Metal Generation would obviously be attractive. The pre tion to aluminum/aluminum joints. Si, Cu, Ge vs. Mg sent paper describes such a technique. Low-Temp. Eutectic The process is carried out in nitrogen gas R. S. TIMSIT is with AMF of Canada Ltd., and has been used successfully for braz Markham, Ont., Canada,and B. J. JANEWAY ing aluminum/Cu, Cu/Cu and Cu/brass in are with Alcan International Ltd., Kingston R & D Center, Kingston, Ont., Canada. addition to aluminum/aluminum joints. WELDING RESEARCH SUPPLEMENT I 119-s to yield a good metallurgical bond. Be S, particle flux cause fillets are formed through capillary flow of the filler metal, brazing requires only minimal contact force at the joint in terface. Procedures and Results 562°C < T < 577°C Preparation of Surfaces and Joints (c) The successful use of the novel braz liquid ing technique requires a uniform coating alloy of Si-flux powder on the metal surfaces prior to brazing. In the present work, the uniform coating could be deposited from T>577°C a water-based slurry after cleaning the (d) surfaces chemically prior to dipping. Chemical cleaning was carried out as fol -*- residual flux lows: the surfaces were first immersed for "*" oxide 5 s in a caustic bath consisting of 5% by weight NaOH in water at 65°C (149°F), rinsed in cold water, desmutted for ~2 s in an acid bath consisting of 50 wt-% HNOj, rinsed in cold water and finally dried for a few minutes in a forced-air fur nace. This procedure removes oil and Fig. I — Successive steps in the novel brazing process. A — Deposition of a Si/flux powder mix grease residues and yields an aluminum on the aluminum surface. The Si particle dimensions range from ~l to 100 pm; the flux particle surface that is uniformly wettable by dimensions do not exceed I pm; B — melting of the flux at 562°C and dissolution of surface oxide water. The aluminum sheet was then films; C — at 562°C < T < 577°C, solid-state interdiffusion of Si and aluminum; D — af T>577°C, rapid dissolution of Si to form localized pools of filler metal of near-eutectic composition, fol dipped for a few seconds into a water- lowed by coalescence of liquid metal pools; E — end of filler metal generation and solidification. based slurry composed of 50 wt-% of the Si/flux powder and 50% distilled water at room temperature. Excess slurry was al pm. Brazing is carried out by heating the Brazing Process lowed to drip off following withdrawal, joint at approximately 600°C in nitrogen thus leaving the uniform powder coating The present brazing technique uses gas at near-atmospheric pressure for a on the sheet surface. The Si powder con the "eutectic bonding" approach de few minutes. During temperature ramp- sisted of commercial-grade material of scribed in earlier publications (Refs. 7, 8), up, the flux melts at ~562°C (1043°F) and 99.1% purity, with Fe as the major con but avoids the need to coat the base dissolves the surface oxide layers on alu taminant. metal surface with an intimately adher minum (Ref. 11), as illustrated in Fig. 1 B. ing layer of the eutectic-forming metal by Sufficient flux must always be present to The aqueous slurries were found to be electroplating or vacuum deposition. In remove these oxides. Oxide dissolution remarkably stable when used with the present technique, at least one of the must occur more rapidly than reoxida- cleaned aluminum surfaces. No effects of aluminum surfaces is coated with a thin tion of the aluminum surface, and it al slurry aging on the brazing process were layer of a powder mix consisting of an el lows the Si particles to come into inti detected over a time interval of two ement capable of forming a low-temper mate contact with the bare metal. At this years. Powder deposition was always ature eutectic with aluminum (e.g., Si, juncture, the large elemental concentra performed after agitating the slurry to Cu, Ge, Zn) and a flux capable of dis tion gradients at the aluminum/Si inter generate a reasonably uniform suspen solving surface oxide films (Ref. 9), as il face cause the aluminum and Si to inter sion of the heavy Si/flux powder in water. lustrated in Fig. IA. A commonly avail diffuse — Fig. 1C. At temperatures The Si/flux weight ratio in the slurry was able noncorrosive flux (Ref. 6) was used exceeding 577°C (1070°F), it is found varied between 1:2 and 1:3. Depending in the present work. This flux consists of that the Si particles diffuse rapidly into on the concentration of solids in suspen a mixture of KAIF4 and K2AIF5 H20 pow the aluminum surface and generate in sion, surface coverage by the brazing mix ders in a molar ratio of the respective situ a layer of Al-Si liquid alloy of near ranging from 10-J to 8 x 10~2 kg m-2 could salts of approximately 1 3:1 (Ref. 10), and eutectic composition — Fig. 1 D. The easily be obtained on aluminum. After with a particle dimension of the order of filler metal penetrates the joint of interest dipping, the coating was dried in a cir 1 pm. For reasons to be addressed later, by capillary action and forms a fillet, thus culating air oven at 1 70°C (338°F) for 1 we will focus most of our attention on producing a metallurgical bond on cool min. The deposit thus obtained was very brazing using Si. ing. Any unused filler metal remains on uniform. The mating surface in the joint The surface coverage by the Si pow the aluminum surface to form a layer of was similarly cleaned but was not neces der may range from a few to several tens Al-Si alloy of near-eutectic composition sarily coated with powder.

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