Eco-friendly brazing

Flux-free brazing thanks to AMAG TopClad® PURE FF

The world of heat exchangers is evolving rapidly, with new and increasingly complex geometries emerging all the time. At the same time, there is a growing focus on cost-efficient solutions that are also environmentally friendly. This makes the development of a flux-free brazing material for controlled atmosphere brazing an incredibly exciting and important challenge.

Oxidschicht_EN — **Figure 1:** Aluminium surface [5]

There’s no doubt about it: aluminium is the material of choice for automotive heat exchangers. Why? Quite simply, because it offers so many advantages in terms of strength, weight, thermal conductivity, formability and corrosion resistance. And when it comes to producing automotive heat exchangers such as oil coolers, condensers, evaporators and battery cooling plates, aluminium brazing is the preferred method. This makes it possible to create material-to-material bonds in geometrically complex components with multiple joints in a single operation. [1]

An essential part of successful brazing is removing or breaking up the natural oxide layer that forms on all aluminium surfaces. Industrially established techniques use different approaches to achieve this, including controlled atmosphere brazing (CAB) and vacuum brazing (VB). [1], [2]

CAB is performed in an inert gas atmosphere and uses a non-corrosive, non-hygroscopic flux containing potassium fluoride and sodium fluoride, which partially dissolves the oxide layer in the molten state to enable material flow. In CAB, it is essential to accurately control the amount of flux being used. Excessive post-braze flux residues can negatively impact the material’s visual appearance, impede the functionality of fin packs by accelerating the degradation of certain coolant solutions and diminish thermal exchange performance.Vacuum brazing does not require the use of flux. It does, however, require sufficient magnesium in the cladding layer (1.0-2.0%) and a high vacuum in the brazing chamber (approx. 10-⁵ mbar). The magnesium vaporizes as the temperature increases and disrupts the oxide layer during brazing, which enables the filler alloy to flow.

The aluminium brazing industry has long been searching for a convenient method by which products can be easily brazed under an inert atmosphere without the need of flux, thereby combining the advantages of CAB and VB. The development of a flux-free brazing system - i.e. the material, the type of heat exchanger and the brazing furnace - has also been driven by the need for better cleanliness inside a brazed heat exchanger, the use of high-strength brazing sheets, and health and environmental concerns regarding the use of fine particulate fluoride salts. [2]AMAG is thrilled to announce that we have developed a revolutionary new material processing route, AMAG TopClad® PURE, and an innovative flux-free filler (FF) alloy. We are proud to present the new AMAG TopClad® Pure FF, which represents a revolutionary combination of these two cutting-edge approaches!

Oberflächen_EN — **Figure 2:** Schematic illustration of the additional processing steps in the production of AMAG TopClad® PURE

Processing route for AMAG TopClad® PURE FF

The thickness of the oxide layer has a significant impact on the brazing results. The thicker the layer, the more challenging it is to break up. Achieving a satisfactory braze joint means keeping the oxide layer as thin as possible. [2]In addition to the natural oxide layer, hot rolled coils are also covered by an ultra-thin coating of impurities (Figure 1). This coating consists mainly of metallic aluminium, oxidic impurities and small quantities of carbon-containing residues. The thickness of the roll coating is built up by constantly releasing and re-accumulating particles, which are then transferred onto the slab surface of the cladding.The morphology of the oxides is dependent on the chemistry of the clad filler material and also of the hot rolling emulsion. [3], [4]

The final brazing quality is influenced by the cleanliness of the surface. High levels of impurities on the surface reduce the flowability and wettability of the liquid filler, which negatively impacts the quality of the brazing joint. This will inevitably result in a higher number of rejects due to leakage or inadequate results when the material is subjected to burst pressure and thermal shock tests. Furthermore, surface accumulation of oxidic impurities will result in field failure due to selective corrosion.AMAG TopClad® PURE is subjected to an additional surface treatment (etching), which removes the roll coating from the hot rolling process as far as possible. The etching step is performed on AMAG’s brand new continuous surface treatment line (see also AluReport 1/2024), which can handle both hot-rolled and cold-rolled materials. During the etching process, the natural aluminium oxide layer and the roller coating from the hot rolling step are removed. As the etching period increases, the etching material also affects the aluminium base material. It is decisive that only the aluminium matrix is removed and the silicon particles in the clad filler material remain unaffected. This process step significantly reduces the level of unwanted surface impurities and the thickness of the oxide layer while also increasing the Si-particle quantity on the surface. Following the etching step, at least one further cold-rolling step is performed to achieve final thickness by rolling the exposed Si particles back into the Al matrix. Chemical degreasing prior to the final annealing is also advantageous, as it minimizes carbon residues on the finished brazing sheet and thereby achieves maximum surface purity. This, in turn, ensures effective wetting and good filler flow during brazing. This surface treatment has no effect on the material’s corrosion properties or storage life.

Alloying concept of AMAG TopClad® PURE FF

The brazing process begins with melting the filler alloy below the surface and natural fracturing of the oxide layer.This melting occurs at temperatures above 577°C. Chemical transformations within the filler and differences in the thermal expansion coefficients of the molten metal and the oxide layer make it possible to break up the oxide layer. The basis for the new filler material is the aluminium-silicon alloy AlSi10, which is modified with 0.10-0.4% magnesium and 0.1-0.4% bismuth. Adding magnesium disrupts the oxide layer during brazing and further reduces the already low oxygen level in the brazing furnace. In a low-oxygen environment, magnesium transforms the Al₂O₃ in the oxide layer into Al and MgAl₂O₄.

TopClad® PURE FF	Si	Fe	Cu	Mn	Mg	Zn	Ti	Bi	je	total

Min.	9,0	-	-	-	0,10	-	-	0,1	-	-
Max.	11,0	0,6	0,2	0,2	0,4	0,2	0,2	0,4	0,05	0,15

others ≤ 0,03 each, total ≤ 0,10 Table 1: Chemical composition of the AMAG TopClad® Pure FF filler alloy

These new complexes occupy different spatial volumes within the oxide layer, causing the oxidic skin to fracture and allowing the flow- and wetting of molten filler metal. The formation of low-melting Mg₂Si within the aluminium-silicon filler alloy also significantly increases filler metal activity and the formation of brazed joints. It is almost impossible to obtain a brazing joint with a magnesium content below 0.10% because the Al₂O₃ oxide layer is not sufficiently broken up. Conversely, when the magnesium content exceeds 0.4%, the thickness of the Mg oxide film increases significantly at higher temperatures and inhibits brazing. The addition of bismuth in the range of 0.1-0.4% significantly reduces the surface tension of the molten filler metal, thereby improving its flow and wettability.

AMAG TopClad® Pure FF is available as a one-side clad, double-side clad or multilayer compound, independent of the core alloy. This means that any 3xxx or 6xxx alloy from the AMAG product portfolio (see AluReport 02/2024) can be clad with the new patented filler alloy (see Table 1) as a coil or sheet in thicknesses ranging from 0.4 mm to 3.5 mm. The new option to use heat-treatable 6xxx aluminium as the core alloy for flux-free CAB facilitates an additional post-brazing heat treatment step to achieve higher material strength. As an additional benefit, the material can be produced in Al4® ever quality in accordance with ISO 14066/14067 upon request. The new cladding alloy in combination with the special processing route is already patented and available for series production. Table 1 shows the chemical composition range of AMAG TopClad® Pure FF.

umweltschonender-loetprozess-abb-3 — **Figure 3:** Brazed plate-and-bar heat exchanger following burst pressure testing

umweltschonender-loetprozess-abb-4 — **Figure 4:** Brazing of the turbulators and separating plate within the heat exchanger (oil side)

Brazing tests with AMAG TopClad® PURE FF in industrial furnaces

Brazing tests with AMAG TopClad® PURE FF have been conducted in an industrial CAB tunnel furnace with plate-and-bar heat exchangers. After brazing, a burst pressure test was performed. This determines the pressure a component can withstand before it bursts and fails. Real operating pressures are far below the burst pressures measured in testing. The result of the test was positive, as the brazing joints withstood the requisite burst pressure. As Figure 3 shows, the maximum pressure was determined due to material failure in the turbulators rather than failure of the brazing joints. After cutting open a brazed heat exchanger, the satisfying brazing result between the separating plate (AMAG TopClad® PURE FF) and turbulators (oil side of the heat exchanger) is clearly visible (Figure 4). The braze fillets are well-pronounced with no delamination of the turbulators from the separating plate. The formation of the braze fillet is evident on the surface of the air fins; however, further testing is planned to achieve a broader brazing fillet on the outer surface. Upon closer examination, the braze fillets inside the heat exchanger appear well pronounced (Figure 5) and comparable with the areas where flux was used.Close cooperation between AMAG and customers will be essential to achieve optimal brazing results with the new AMAG TopClad® PURE FF. The material, the geometry of the heat exchanger and the brazing process itself require careful coordination.

umweltschonender-loetprozess-fig-5_EN — **Figure 5:** Brazing joint between a cooling fin and the separating plate. A) Brazing joint at the outer edge, exposed to furnace atmosphere B) Brazing joint 1.5 mm from the outer edge

Customer benefits

This material makes it possible to braze in CAB conditions without the need of flux, with following beneficial effects:

reduces costs (no flux procurement costs, no post-brazing flux deposits and cleaning, lower production costs than VB),
increases heat exchanger performance (no clogging) and avoids the environmental impacts of harmful flux (no flux dusts in production lines,
no flux in sewage after cleaning, and longer service lives for heat exchangers due to elimination of clogging).

AMAG TopClad® Pure FF increases the range of alloys that can be used in heat exchanger applications.

References:

[1] Altenpohl, Dietrich. Aluminium von innen: das Profil eines modernen Metalles. Aluminium-Verlag, 1994.[2] Hawksworth; D. K.; Fluxless brazing of aluminium; Advances in brazing; pp. 566 - 584 Woodhead Publishing Limited, 2013.[3] V. R. Howes, M. P. Amor: Roll coatings formed during the hot rolling of aluminium with rolls of different material“, Wear 79 (1982) 375 - 384[4] J. A. Schey: „Lubrication in the rolling of light metals“, Light Metal Age, April 198[5] A. Gray, Butler C., Nadin A. C. (2014): The Influence of Thermomechanical Processing (TMP) on the Brazeability of Clad Heat Exchanger Materials. In: DVS Berichte Aluminium Brazing 8th international Congress.

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