FIELD

The present invention relates to sheet metal and more particularly to sheet metal made from aluminum alloys that is used for joining operations in the fabrication of structures, such as heat exchangers and heat exchanger components, by controlled atmosphere brazing (CAB) without flux.

BACKGROUND

Brazing of metals is well known, wherein metal surfaces are brought into close proximity and heated in the presence of a filler metal, melting the filler metal, which flows into the interface between adjoining parts by capillary action and hardens upon cooling to join the metal parts. For brazing of aluminum parts to be successful, the surfaces to be joined and the filler metal need to be protected from oxidation during the various stages in the brazing process. Vacuum brazing is a furnace joining technology that protects the parts from oxidation by effectively removing most of the oxygen from the brazing chamber by a combination of evacuating the chamber and by use of oxygen getters such as Mg. The dominant method of furnace brazing today is Controlled Atmosphere Brazing (CAB) in which the parts are heated in a muffle furnace under a protective inert gas atmosphere (such as Nitrogen) with relatively low O2content. In CAB brazing, a flux is used to remove the oxide from the parts to be joined and to then protect the clean metal surfaces from oxidation by the residual O2in the furnace atmosphere. One such flux extensively used today is Nocolok™ flux. CAB brazing relies heavily upon flux to deal with the oxide on the parts and to control oxidation in the furnace but there are various reasons why it is desirable to avoid the use of flux in CAB brazing. These include concerns around environmental health and safety (EHS) and problems caused by the residual flux that remains on the part after brazing. This has driven efforts to develop materials that can be joined in a flux-free CAB brazing process.

International patent application publication No. WO 2013/180630 (also U.S. patent application Ser. No. 14/404,093) describes a brazing sheet for fluxfree CAB brazing that comprises an aluminum alloy core material which is covered by an aluminum alloy interlayer comprising ≤1.0% Si, 0.1-2.5% Mg, the interlayer being covered by an Al—Si braze alloy comprising 5-14% Si, 0.01-1.0% Bi, <0.8% Fe, <6% Zn, <0.1% Sn, <0.1% In, <0.3% Cu, ≤0.15% Mn and <0.05% Sr. The core material and the interlayer material have a higher melting temperature than the braze alloy, e.g., the melting point of the interlayer and the core may be >615 C. The thickness of the interlayer may be 2.5% to 25% of the thickness of the sheet.

European Patent EP 1430988B1 describes a method of brazing on the inside hollow portion of a structure formed from aluminum alloy materials without applying flux. Brazing is done in an inert atmosphere and the sheet used to form the hollow structure has an aluminum alloy core clad with an Al—Si based brazing alloy on one or both sides with a thickness “a” microns. The core contains X % Mg (≥0.1% Mg) and the braze alloy contains Y % Mg with the respective composition of the two satisfying the relationship: (X+Y)≤a/60+0.5 and X>Y. A 4-layer brazing sheet structure that incorporates a diffusion prevention layer between the core layer and the filler layer on the outer side of the formed hollow is also disclosed. Notwithstanding, alternative brazing methods and materials remain of interest.

SUMMARY

The disclosed subject matter relates to a sheet material for fluxfree CAB brazing, including: a core layer of 3XXX, 1XXX or 6XXX series aluminum alloy; an interliner layer having 0.25 to 1.7 wt % Mg, 0.3-1.1 wt % Si, 0.09-0.5 wt % Bi, and aluminum; a brazing liner layer of 4XXX aluminum alloy, wherein the interliner layer is disposed between the core layer and the brazing liner layer, the core layer, the interliner layer and the brazing liner layer conjoined together forming the sheet material.

In another embodiment, the interliner layer further includes up to 1.8 wt % Mn, up to 0.9 wt % Cu, up to 0.7 wt % Fe, up to 0.18 wt % Ti, up to 3.0 wt % Zn, up to 0.25 wt % Cr, and up to 0.15 wt % Zr.

In another embodiment, the brazing liner layer has a composition of Mg≤0.05 wt %.

In another embodiment, the brazing liner layer has a composition of Bi<0.010 wt %.

In another embodiment, the core layer has 0.08-1.1 wt % Si, 0.15 to 0.7 wt % Fe, 0.05-1.0 wt % Cu, 0.4-1.8 wt % Mn, up to 0.5 wt % Mg, up to 2 wt % Zn, up to 0.25 wt % Cr, and up to 0.15 wt % Zr.

In another embodiment, the braze liner layer has 5-12 wt % Si, up to 0.7 wt % Fe, up to 3 wt % Zn, aluminum and incidental impurities.

In another embodiment, the core has from 60% to 90% of a total thickness of the sheet material, the interliner layer from 3% to 20% of the total thickness and the braze liner layer from 3% to 20% of the total thickness.

In another embodiment, the interliner layer is a first interliner layer and further including a second interliner layer having a composition like that of the first interliner layer, the second interliner layer being disposed on the core opposite to the first interliner layer.

In another embodiment, the brazing liner layer is a first brazing liner layer and further including a second brazing liner layer disposed over the second interliner layer distal to the core.

In another embodiment, an additional aluminum alloy layer is disposed on the core opposite the interliner layer, wherein the additional alloy layer is a 3XXX alloy, a 3XXX+Zn alloy, a 1XXX alloy, a 7XXX alloy or a 4XXX alloy.

In another embodiment, a structure includes: a first part made from aluminum; a second part made from the sheet material described above, the first part and the second part joined together by a brazed joint at least partially formed from the brazing liner layer of the second part.

In another embodiment, the structure is a portion of a heat exchanger such as an oil cooler or radiator.

In another embodiment, the heat exchanger has an internal hollow devoid of flux residue.

In another embodiment, the Mg present in the interliner layer composition is from 0.25 to 0.95 wt %.

In another embodiment, the Mg present in the interliner layer composition is from 1.0 to 1.7 wt %.

In another embodiment, the Si present in the interliner layer composition is from 0.3 to 0.65 wt %.

In another embodiment, the Si present in the interliner layer composition is from 0.7 to 1.1 wt %.

In another embodiment, the Bi present in the interliner layer composition is from 0.05 to 0.20 wt %.

In another embodiment, the Bi present in the interliner layer composition is from 0.25 to 0.50 wt %.

In another embodiment, a method for brazing, includes: providing a first aluminum alloy material; providing a second aluminum alloy material having a core layer of 3XXX, 1XXX or 6XXX series aluminum alloy, an interliner layer having 0.25 to 1.7 wt % Mg, 0.3-1.1 wt % Si, 0.09-0.5 wt % Bi, aluminum, and a brazing liner layer of a 4XXX aluminum alloy with less than 0.05% Mg and less than 0.010% Bi, wherein the interliner layer is disposed between the core layer and the brazing liner layer, the core layer, the interliner layer and the brazing liner layer being conjoined to form the sheet material; bringing the first material into contact with the second material, with the brazing liner layer of the second sheet facing the first sheet; brazing the first material to the second material in an inert atmosphere brazing furnace without using flux.

In another embodiment, the step of brazing is conducted in a controlled atmosphere where an oxygen level is reduced to a level in the range of 1 ppm to about 35 ppm.

In another embodiment, the first aluminum alloy material has the same composition as the second aluminum alloy material.

In another embodiment, a sheet material has: a core layer of 3XXX series aluminum alloy; an interliner layer of 0.25 to 1.7 wt % Mg, 0.3-1.1 wt % Si, 0.09-0.5 wt % Bi, up to 1.8 wt % Mn, up to 0.9 wt % Cu, up to 0.7 wt % Fe, up to 0.18 wt % Ti, up to 3.0 wt % Zn, up to 0.25 wt % Cr, and up to 0.15 wt % Zr, remainder aluminum and impurities; a brazing liner layer of 4XXX aluminum alloy, wherein the interliner layer is disposed between the core layer and the brazing liner layer, the core layer, the interliner layer and the brazing liner layer conjoined together forming the sheet material.

