Brazing sheet and method of making same

Details Brazing sheet and method of making same Brazing sheet and method of making same

2000-03-31

2002-03-05

Koehler, Robert R. (Department: 1775)

Stock material or miscellaneous articles

All metal or with adjacent metals

Composite; i.e., plural, adjacent, spatially distinct metal...

C148S523000, C148S535000, C148S552000, C148S691000, C148S692000, C148S438000, C420S537000, C420S538000, C420S550000, C420S551000, C420S552000, C420S553000

Reexamination Certificate

active

06352789

ABSTRACT:

FIELD OF THE INVENTION
The invention is directed to brazing sheet having a core sheet of an aluminium alloy core material and a roll-bond or clad brazing layer of an aluminium alloy having silicon as the main alloying element, typically in the range of 5 to 14 weight %, on at least one side of the core sheet. The invention further relates to a method of manufacturing such a brazing sheet, and also to an assembly thus manufactured.
DESCRIPTION OF THE RELATED ART
In the prior art, aluminium alloys are the alloys of choice for heat exchanger applications. These alloys are selected for their desirable combination of strength, low weight, good thermal and electrical conductivity, brazeability, corrosion resistance, and formability.
Brazing sheet of this type set out above is well known in the art and is used extensively for automotive radiators, evaporators, condensers, charge air coolers, among other products. Two brazing methods, known as Controlled Atmosphere Brazing (CAB) employing a NOCOLOK (trade mark) flux and Vacuum Brazing, are conventional and need not be described here. The flux is a non-corrosive flux made up of a mixture of potassium and fluoro-aluminates. Such brazing takes place at a temperature of about 600° C. as determined by the aluminium silicon alloy of the brazing layer.
Commercial available brazing sheet which has very good brazeability by means of flux brazing, e.g. NOCOLOK brazing, comprises for example a core alloy cladded on one side with a brazing alloy, the core alloy having a composition, in weight %:
Mn
0.65-1.0
Cu
0.5-0.7
Si
max. 0.3
Fe
max. 0.5
Zn
max. 0.10
Ti
0.08-0.10
balance aluminium and impurities. Such brazing sheet is capable of obtaining a post-braze 0.2% yield strength typically in the range of up to 40 MPa and up to 48 MPa when the core is respectively subjected to a homogenisation treatment and not subjected to a homogenisation treatment between the processing steps of casting and hot deformation by means of hot-rolling. Moreover, the brazing sheet has a long-life corrosion performance as tested in a SWAAT-test in accordance with ASTM G-85. Long-life alloys are those which in the SWAAT test without perforations according to ASTM G-85 exceed 10 to 12 days.
There is a demand for brazing sheet which meet the requirements of excellent brazeability during flux brazing, while having improved post-brazed strength and simultaneously having a good corrosion resistance.
Alloys which have a high post-brazed strength and have a good corrosion resistance are known in the art. From EP-A-0718072, brazing sheet is known having a core sheet of an aluminium alloy core material and on at least one side thereof a brazing layer of an aluminium alloy containing silicon as a main alloying element, wherein the aluminium alloy of the core sheet has the composition (in weight %):
Mn
0.7-1.5
Cu
0.2-2.0
Mg
0.1-0.6
Si
>0.15, most preferably >0.40
Ti
optional, up to 0.15
Cr
optional, up to 0.35
Zr
and/or V optional, up to 0.25 in total,
balance aluminium and unavoidable impurities, and with the proviso that (Cu+Mg)>0.7, and most preferably (Cu+Mg)>1.2.
The alloying elements Cu and Mg are added to provide in combination a sufficient mechanical strength and corrosion resistance to the brazing sheet obtained. Although this brazing sheet may be processed by means of flux brazing, some difficulties are encountered due to the relatively high Mg content in the alloy which might influence the brazing flux applied during the brazing cycle. Further disadvantages of having a too high Mg-level in the core alloy, are that flow and/or wettabillity is decreased when applying the NOCOLOK brazing flux during the brazing cycle. However, lowering the Mg level in this known aluminium core material would drastically lower the strength levels obtainable after brazing.
Some other disclosures of literature will be mentioned below. JP-A-07003370 describes an aluminium core clad with an Al-Si filler metal to form a brazing sheet. The core alloy comprises, in weight %:
Fe
0.4 to 1.5
Mn
0.7-1.7
with the proviso that (Fe+Mn)<2.4
Si
1.3 max.
Cu
1.5 max.
with the proviso that (Si+Cu)>1.5 and with the further proviso that [Fe]+[Mn]>1.7[2.5 ([Si]+[Cu])] -4.2 balance aluminium and inevitable impurities.
GB-A-2321869 discloses an aluminium alloy brazing sheet comprising an Al-Si series filler alloy clad on one or both surfaces of an aluminium alloy core material comprising 0.3 to 1.5 wt. % of Cu and 0.03 to 0.3 wt. % of Sn, the balance of the alloy being substantially Aluminium. Corrosion resistance is improved by the combined addition of Cu and Sn in the given ranges.
From EP-A-0712681 a brazing sheet is known for a heat-exchanger tube with a three layer structure in a total thickness of not more than 0.25 mm, in which the core alloy comprises (in weight %):
Si
0.2-2.5
Fe
0.05-2.0
Cu
0.7-2.5
Mn
0.05-2.0
Mg
optionally, not more than 0.5
Cr
optionally, not more than 0.3
Zr
optionally, not more than 0.3
Ti
optionally, not more than 0.3
balance aluminium and inevitable impurities, and is cladded on one side with a brazing material, and mandatorily cladded on the other side having a sacrificial anode clad layer with a thickness in the range of 46 to 70 micron, and said sacrificial anode clad layer comprises (in weight %)
Zn
3.0-6.0
Mg
0.05-2.5 
balance aluminium and inevitable impurities,
During manufacture, all three layers are subjected after casting to a homogenisation treatment within a temperature range from 450 to 600° C. The hot rolling of the three layer structure is carried out at a temperature not lower than 450° C. Further Fe is deliberately added to the alloy in order to distribute the coarse intermetallic compounds into the alloy.
SUMMARY OF THE INVENTION
An object of the invention is to provide a brazing sheet having a core sheet made of an aluminium alloy core material and on one or both sides an aluminium brazing layer, providing improved brazeability in a flux brazing process, and having at least a 10% improvement of the post-braze 0.2% yield strength in both the situation where the core is being subjected to a homogenisation treatment and not being subjected to a homogenisation treatment, while maintaining a good corrosion resistance.
This object is achieved according to the invention, by providing a brazing sheet with a two-layer structure or a three-layer structure, having a core sheet made of an aluminium alloy core material and on one side or both sides thereof a brazing layer of an aluminium alloy containing silicon as a main alloying element, wherein the aluminium alloy of the core sheet has the composition (in weight %):
Mn
0.5 to 1.5
Cu
0.5 to 2.0
Si
0.3 to 1.5
Mg
<0.05
Fe
<0.4
Ti
<0.15
Cr
<0.35
Zr and/or V
<0.35 in total
Zn
<0.25
balance aluminium and unavoidable impurities.
This brazing sheet has good mechanical properties in the post-brazing state and is capable of providing an increase in post-braze 0.2% yield strength of at least 10% in both the situation where at least the core material is either or not subjected to a homogenisation treatment between the casting and the hot deformation processing step as compared to the known prior art brazing sheet set out above having a core alloy consisting of (in weight %):
Mn
0.65-1.0
Cu
0.5-0.7
Si
max. 0.3
Fe
max. 0.5
Zn
max. 0.10
Ti
0.08-0.10
balance aluminium and impurities,
and having a post-braze 0.2% yield strength of typically in the range of up to 40 MPa and up to 48 MPa when the core is respectively subjected to a homogenisation treatment and not being subjected to a homogenisation treatment. In the case where the material is subjected to the homogenisation treatment the brazing sheet is capable of achieving in an 0-temper a post-braze 0.2% yield strength of at least 50 MPa, and in the best examples of at least 55 MPa. In the case where the material is not being subjected to the homogenisation treatment, the brazing sheet is capable of achieving, in an H24-temper, a post-braze 0.2% yield strength of at le

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