Aluminium alloy for use as core material in brazing sheet

Details Aluminium alloy for use as core material in brazing sheet Aluminium alloy for use as core material in brazing sheet

1999-03-24

2001-09-25

Zimmerman, John J. (Department: 1775)

Stock material or miscellaneous articles

All metal or with adjacent metals

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

C420S535000, C420S551000, C420S552000

Reexamination Certificate

active

06294272

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an aluminium alloy for use as a core material in brazing sheet, to a brazing sheet comprising the aluminium alloy as core material, to the use of the aluminium alloy as core material of a brazing sheet in a brazed assembly and to a brazed assembly containing at least one member having the aluminium alloy as core material. The aluminium alloy is of the AA3xxx type. The invention has the particular advantage that the alloy is non-heat-treatable, i.e. it does not require post-brazing ageing treatment. A principle use of brazing sheet containing such alloy is in heat exchangers, such as vehicle radiators, in which water is one heat-exchanging medium. Herein the term sheet material includes tube material.
DESCRIPTION OF THE PRIOR ART
Many proposals for an aluminium alloy to be used as a core material in brazing sheet have been made. Generally, in the prior art, the alloy is used as a core layer in a brazing sheet with a clad layer on at least one face. The clad layer provides corrosion protection.
WO 94/22633 describes such an alloy, having the composition in wt. %:
Mn
0.7-1.5
Cu
0.5-1.0, preferabiy >0.6-0.9
Fe
0.4 max.
Si
0.15 max.
Mg
≦0.8
V and/or Cr
≦0.3, preferably ≦0.2
Ti
≦0.1
balance Al and impurities.
This is used as core material with corrosion-resistant clad layers containing Si. The high Cu content is to improve post-brazing sag resistance. Ti is preferably not deliberately added, though is typically present from source material. Preferably Zr is not deliberately added. Cr and/or V are said not to improve post-brazing corrosion resistance, but contribute to post-brazing strength and sag resistance.
U.S. Pat. No. 4,761,267 describes a core alloy for brazing, which has improved secular corrosion resistance due to a sacrificial anode or a filler metal on the water-contacting side. Thus the core alloy is used with clads on both sides. The composition is in wt. %:
Mn
0.6-1.0
Cu
0.5-1.0
Fe
0.3 max.
Si
0.1 max.
Mg
optionally 0.05-0.4
Ti
0.1-0.3
Cr
optionally 0.05-0.4
Zr
optionally 0.05-0.4
balance Al and impurities.
Cu is present to allow the filler metal and sacrificial anode to demonstrate the sacrificial anode effect. It also increases strength. Cu is said to decrease the corrosion resistance of the core alloy itself. Mg, Cr and Zr are optional elements for increasing strength. In none of the examples are Mg and Cr both present, and in only one are Cr and Zr present.
Another approach to obtaining corrosion resistance in core alloys is shown in EP-A-492796, where the core alloy contains in wt. % either
Mn 0.5-1.5
at least one of: Ti (0.05-0.3) and
Zr (0.05-0.4)
or
Mg 0.05-1.0
Si 0.05-0.3, preferably 0.1-0.2
at least one of: Ti (0.05-0.3) and
Zr (0.05-0.4).
Also optional in either case are Fe (0.03-1.0) and Cu (0.05 to 0.2). In particular, Si is said to increase strength and is therefore preferably at 0.1-0.2. Cu at above 0.2 is said to be disadvantageous. Cr is not used. This core alloy is provided with clad layers.
JP-A-4-297541 discloses a core alloy for brazing tube, of composition:
Mn
0.3-1.5
Cu
0.2-0.9
Mg
0.2-0.5
Si
0.1-0.3
Fe
0.1-0.7
Ti
0.1-0.3
optionally either or
both of:
Zr
(0.05-0.2)
Gr
(0.05-0.2)
balance Al and impurities.
This core alloy is assembled into tube with clad layers. Cu is for strength and to give corrosion resistance with the clad layers. Si is used for strength improvement, but at less than 0.1% has insufficient effect. Zr and Cr are for strength improvement, and in the examples where they are employed Cu is 0.5%.
JP-A-63-186847 discloses a core alloy for brazing material used with a sacrificial anode clad layer on one face. The composition is, in wt. %:
Mn
1.0-1.5
Cu
0.3-0.6
Mg
0.1-0.5
Cr
0.05-0.35
Zr
0.05-0.35
optionally, one or both of
V
(0.05-0.35)
Ti
(0.05-0.35)
Fe
0.5 max.
Si
0.5 max.
Cr and Zr increase strength, and in the examples given total at least 0.3 wt. %. Si is an impurity, present at 0.15 wt. % in examples, and reducing corrosion resistance above 0.5 wt. %.
Corrosion resistance is of high importance especially in brazing sheet for use in automobile radiators. Long-life alloys are those which in the SWAAT test without perforations according to ASTM G-85 exceed 10-12 days (see K. Scholin et al., VTMS 1993, SAE P-263).
SUMMARY OF THE INVENTION
The object of the present invention is to provide a core alloy for brazing sheet which has improved properties over the alloys of the prior art, and in particular provides a combination of high post-brazing strength and high corrosion resistance. It is a particular feature that the core alloy of the invention does not require post-brazing ageing treatment, and that the core alloy has high corrosion resistance in the absence of the sacrificial anode coating layer which is generally employed in the prior art.
According to the invention, there is provided an aluminium alloy for use as a core material in brazing sheet, consisting of, in weight %:
Mn
0.7-1.5
Cu
0.6-1.0
Fe
not more than 0.4
Si
less than 0.1
Mg
0.05-0.8
Ti
0.02-0.3
Cr
0.1-0.25
Zr
0.1-0.2
balance Al and unavoidable
impurities
wherein 0.20≦(Cr+Zr)≦0.4, the alloy being capable of obtaining in the post-brazing state 0.2% yield strength of at least 65 MPa and having a corrosion life of more than 11 days in a SWAAT test without perforations in accordance with ASTM G-85.
This aluminium alloy has good mechanical properties and a good corrosion resistance of over 11 days in the SWAAT test without perforations according to ASTM G-85. In the best examples, this corrosion resistance is more than 16 days. This level of corrosion resistance qualifies the alloy as a long-life product. The alloy is a non-heat-treatable alloy. It is believed that the excellent properties are the result of the specific combination of the contents of particularly Cu, Si, Ti, Cr and Zr, which are all within relatively narrow ranges. Notably, the alloy has a high corrosion resistance in a brazing sheet without the presence of a clad layer acting as a sacrificial anode on the side contacting aqueous cooling fluid in use.
The aluminium alloy is of the AA3xxx type, Mn being the main alloying element in order to obtain the desired corrosion resistance. At least 0.7% is required for obtaining the desired corrosion resistance, while an Mn content of over 1.5% does not produce any significant improvements in respect of the strength because coarse Al—Mn-containing compounds are formed. A further disadvantage of coarse Al—Mn-containing compounds is that they reduce the rollability of the aluminium alloy. More preferably the Mn content is in a range of 0.8-1.2%.
Fe is present in all known aluminium alloys but in the aluminium alloys in accordance with the invention it is not a required alloying element and is not deliberately added. With a high Fe content among other things the corrosion resistance decreases. The admissible Fe content is 0.4% maximum and preferably 0.2% maximum.
The admissible Si content is 0.1% maximum. This low Si content is critical for the excellent long-life corrosion performance of the core alloy, because the low Si content promotes the formation of a sacrificial precipitation layer at the original filler-core interface during the brazing cycle. The filler in this case is a Si-containing low melting brazing alloy of conventional type. If the precipitate band known as ‘brown band’ is not present then in SWAAT testing the corrosion proceeds in an accelerated intergranular manner. When a precipitate band is present, the sacrificial nature of the band deflects the corrosion in a lateral manner, i.e. along the direction of the band parallel to the original filler-core interface, preventing through thickness penetration.
In the alloy with Si <0.1%, during brazing Si diffuses from the filler into the core. It appears that increasing the Si content reduces the solid solubility of Mn in aluminium, and hence the precipitation of Mn containing precipitates, i.e. &agr;-MnSiAl, is enhanced. This favoured precipitation results in a dense band of precipitates, typically 40 to 70 &mgr;m thick in this re

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