Metal treatment – Stock – Aluminum base
Reexamination Certificate
2000-01-22
2001-10-16
Wyszomierski, George (Department: 1742)
Metal treatment
Stock
Aluminum base
C148S440000, C420S541000
Reexamination Certificate
active
06302973
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an aluminium-magnesium-zinc-silicon alloy, preferably in the form of semi-fabricated products like extrusions, plates, coil or sheets, which is particularly suitable to be used in welded or brazed structures and all other applications where the beneficial combination of properties is helpful. For example, the extrusions of this invention can be used in the welded construction of marine transportation vessels such as catamarans or monohull type, fast ferries, but also in bridges and offshore structures. An example of coil and sheet products would be the use as the core alloy for brazing sheet to be brazed in for example heat exchangers. Sheets may have a thickness in the range of less than 6 mm, e.g. 0.5 mm. The plates of the alloy of this invention can be used for example in the construction of ships, offshore structures, silos, bridges and armoured vehicles. Plates may have a thickness in the range of a few mm, e.g. 5 mm, up to 400 mm. Further the alloy in accordance with the invention can be used as weld filler wire.
DESCRIPTION OF THE RELATED ART
Extrusions and rolled products of Al-Mg-Si-type alloys are being used extensively in the construction of marine vehicles and other applications resulting in the construction of large welded structures. In case of marine vessels, AA6082 alloy is being used in the construction of primarily hulls and to a lesser extent in superstructures. The advantage with the AA6082 alloy is that it represents the most widely used, commercially available strongest marine grade alloy. However, this alloy suffers from the following disadvantages: in the case of extrusions low extrudability (as compared to AA6063), lower strength after welding (for example: as compared to 7XXX) and quench sensitivity. In extrusions the reduced extrudability of this alloy consequently limits the shape and the minimum wall thickness of the profiles that can be produced using this alloy. Any increase in Mg or Si or Mn levels of AA6082 causes drastic reduction in the extrudability. Although commercially available 7XXX series alloys can be stronger than AA6082 in the welded condition, they do not qualify as marine grade alloys due to severe corrosion in the heat affected zone. One of the known methods of controlling the stress corrosion in 7XXX series alloys is to reduce the Zn and Mg levels. However, this approach leads to substantial strength loss in both welded and unwelded conditions. Another known method is to add Cu to 7XXX series alloys to a level of more than 0.4 wt.%, which in turn deteriorates resistance to pitting. There exists up to the present no 6XXX-series or 7XXX-series alloy, that as extrusions is better extrudable than AA6082, and having higher strength after welding plus a corrosion resistance similar to that of AA6082 as extrusions and rolled products.
For brazing applications to produce aluminium heat exchangers usually clad products with a combination of 3XXX-series core alloys and a cladding of alloy 4XXX-series are used extensively. After mechanical assembling of stamped parts these heat exchangers are brazed in vacuum or in flux controlled atmosphere combinations at temperatures where the 4XXX-series alloys due to their eutectic composition is melting. The 3XXX-series alloy remains solid after brazing but is softened to very low strength due to the exposure to temperatures around 600° C. during brazing. These low properties of the core alloy limits the minimum thicknesses and weight of the heat exchangers. Therefore, alloys which give higher strength after brazing would be beneficial for downgauging. Such alloys have to be in a melting range which is at least slightly above the brazing temperature. The core alloy should have a good corrosion resistance after brazing especially in salt atmosphere. Such corrosion behaviour is influenced by Si diffusion of the clad 4XXX-series alloy during brazing. Alloys which avoid extensive Si-diffusion e.g. by keeping non-recrystallised or pan-cake structures or by other microstructural means, are beneficial.
Some other disclosures of Al—Mg—Zn—Si alloys found in the prior art literature will be mentioned below.
EP-B-0173632 describes an alloy for extruded or rolled products having a microstructure which is essentially non-recrystallised, and has a preference for high Cu levels. The chemical composition is, in wt. %:
Si 0.9-1.2
Mg 0.7-1.1
Mn 0.25-0.75
Cu 0.30-1.1, preferably 0.8-1.0
Zr 0.07-0.20
Zn 0.10-0.70, preferably 0.30-0.60
Fe <0.30
balance Al.
WO-A-96/12829 describes a method for the production of AlSiMgCu alloy products in the T6-temper having enhanced intercrystalline corrosion resistance. The chemical composition of the product is, in wt. %:
Si 0.7-1.3
Mg 0.6-1.1
Mn 0.3-0.8
Cu 0.5-1.1
Zr <0.20
Fe <0.30
Zn <1.0
Ag <1.0
Cr <0.25
balance Al, and with the proviso that Mg/Si <1.
The proviso Mg/Si<1 is required to obtain the desired corrosion resistance levels, and the high Cu content is required to obtain the desired strength levels and to obtain thermal stability of the alloy. The examples, both comparative and within the invention, all have a maximum Zn content of 0.15% and a Cu content of 0.8% or more.
SUMMARY OF THE INVENTION
One object of the present invention is to provide an Al—Mg—Zn—Si alloy rolled product or extrusion with substantially improved strength in the welded condition as compared to the standard AA6082 alloy and having a corrosion resistance at least equivalent to that of the standard AA6082 alloy. It is also an object to provide alloy rolled products or extrusions which can offer ductility, weldability, and formability at least equivalent to those of AA6082.
A further object of the present invention is to provide an Al—Mg—Zn—Si alloy rolled product or extrusion with substantially improved strength after brazing as compared to the standard 3XXX-series alloys, e.g. AA3003. It is also an object to provide alloy rolled products or extrusions which have a melting range which is above brazing temperature, having a good corrosion resistance, and can offer ductility, weldability (in order to produce tubes prior to brazing), and formability at least equivalent to those of standard 3XXX-series alloys.
According to the invention there is provided an Al—Mg—Zn—Si alloy in the form of a plate or a sheet or an extrusion, having the following composition in weight percent:
Mg 0.5-1.5
Zn 0.1-3.8
Si 0.05-1.5
Mn 0.2-0.8
Zr 0.05-0.25
Cr 0.3 max.
Cu <0.3
Fe 0.5 max.
Ag 0.4 max.
Ti 0.2 max.
balance Al and inevitable impurities.
The present invention we can provide alloy plate or extrusion having higher strength than AA6082, and particularly the welded joints of the present alloy can have higher strength than the standard AA6082 welds, while maintaining a good corrosion resistance.
The present invention can further provide alloy sheet or extrusion having higher post-braze strength than the standard 3XXX-series alloys.
The invention also provides in a welded structure having at least one welded plate or extrusion of the alloy set out above. Preferably the proof strength of the weld is at least 135 MPa.
The invention also provides in a brazed structure having at least one sheet or extrusion of the alloy set out above. The alloy is capable of obtaining in the post-brazing state proof strength of at least 70 MPa, and in a more preferred embodiment of at least 90 MPa.
It is believed that the improved properties available with the invention, particularly higher strength levels in both welded condition and post-braze condition, result from a combination of a fine distribution of GP-zones, MgZn
2
-type and Mg
2
Si-type precipitates, as well as Mg and Zn solutes and Al
3
Zr dispersoids. The deleterious effect of Zn addition on the stress corrosion cracking, particularly in extrusions, is compensated by keeping the level of Zn to a value below 3.8 wt. % and using combination of appropriate levels of Si and Cu in solid solution.
The alloy plates, coils and sheets of the invention can be manufactured by preheating, hot rolling, cold rolling wi
Haszler Alfred Johann Peter
Sampath Desikan
Combs-Morillo Janelle
Corus Aluminium Walzprodukte GmbH
Stevens Davis Miller & Mosher LLP
Wyszomierski George
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