Metal treatment – Stock – Aluminum base
Reexamination Certificate
2001-02-20
2002-01-29
Ip, Sikyin (Department: 1742)
Metal treatment
Stock
Aluminum base
C148S439000, C420S542000, C420S590000
Reexamination Certificate
active
06342113
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an aluminium-magnesium alloy in the form of plates and extrusions, which is particularly suitable to be used in the construction of large welded structures such as storage containers and vessels for marine and land transportation. For example, the plates of this invention can be used in the construction of marine transportation vessels such as catamarans of monohull type, fast ferries, high speed light craft, and jet rings for the propulsion of such vessels. The alloy plates of the present invention can also be used in numerous other applications such as structural materials for LNG tanks, silos, tanker lorries and as tooling and moulding plates. Plates may have a thickness in the range of a few mm, e.g. 5 mm, up to 200 mm. Extrusions of the alloy of this invention can be used for example as stiffeners and in superstructures of marine vessels such as fast ferries.
DESCRIPTION OF THE RELATED ART
Al—Mg alloys with Mg levels >3% are extensively used in large welded constructions such as storage containers and vessels for land and marine transportation. A standard alloy of this type is the AA5083 alloy having the nominal composition, in wt %:
Mg
4.0-4.9
Mn
0.4-1.0
Zn
≦0.25
Cr
0.05-0.25
Ti
≦0.15
Fe
≦0.4
Si
≦0.4
Cu
≦0.1
others (each)
≦0.05
(total)
≦0.15
In particular, AA5083 alloy plates in the soft and work-hardened tempers are used in the construction of marine vessels such as ships, catamarans and high speed craft. Plates of the AA5083 alloy in the soft temper are used in the construction of tanker lorries, dump trucks, etc. The main reason for the versatility of the AA5083 alloy is that it provides good combinations of high strength (both at ambient and cryogenic temperatures), light weight, corrosion resistance, bendability, formability and weldability. The strength of the AA5083 alloy can be increased without significant loss in ductility by increasing the Mg % in the alloy. However, increasing the %Mg in Al—Mg alloys is accompanied by a drastic reduction in exfoliation and stress corrosion resistances. Recently, a new alloy AA5383 has been introduced with improved properties over AA5083 in both work-hardened and soft tempers. In this case, the improvement has been achieved primarily by optimising the existing composition of AA5083 alloy.
Some other disclosures of Al—Mg alloys found in the prior art literature will be mentioned below.
GB-A-1458181 proposes an alloy of strength increased relative to JISH 5083, containing a larger amount of Zn. The composition is, in wt %:
Mg
4-7
Zn
0.5-1.5
Mn
0.1-0.6, preferably 0.2-0.4
optionally, one or more of Cr
0.05-0.5
Ti
0.05-0.25
Zr
0.05-0.25
In the examples, ignoring reference examples, the Mn contents range from 0.19 to 0.44, and Zr is not employed. This alloy is described as cold fabricatable, and also as suitable for extrusion.
U.S. Pat. No. 2,985,530 describes an alloy for fabricating and welding having a much higher Zn level than AA5083. The Zn is added to effect natural age hardening of the alloy, following welding. The composition for plate is, in wt %:
Mg
4.5-5.5, preferably 4.85-5.35
Mn
0.2-0.9, preferably 0.4-0.7
Zn
1.5-2.5, preferably 1.75-2.25
Cr
0.05-0.2, preferably 0.05-0.15
Ti
0.02-0.06, preferably 0.03-0.05
In “The Metallurgy of Light Alloys”, Institute of Metallurgy, Ser. 3 (London) 1983, by Hector S. Campbell, pages 82-100, there are described the effects of adding 1% of Zn to aluminium alloys containing 3.5-6% Mg and either 0.25 or 0.8% Mn. The Zn is said to improve tensile strength and to improve stress corrosion resistance in ageing over 10 days at 100° C. but not in ageing over 10 months at 125° C.
DE-A-2716799 proposes an aluminium alloy to be used instead of steel sheet in automobile parts, having the composition, in wt %:
Mg
3.5-5.5
Zn
0.5-2.0
Cu
0.3-1.2
optionally at least one of Mn
0.05-0.4
Cr
0.05-0.25
Zr
0.05-0.25
V
0.01-0.15
More than 0.4% Mn is said to reduce ductility.
SUMMARY OF THE INVENTION
One object of the present invention is to provide an Al—Mg alloy plate or extrusion with substantially improved strength in both soft and work-hardened tempers as compared to those of the standard AA5083 alloy. It is also an object to provide alloy plates and extrusions which can offer ductility, bendability, pitting, stress and exfoliation corrosion resistances at least equivalent to those of AA5083.
According to the invention there is provided an aluminium-magnesium alloy in the form of a plate or an extrusion, having the following composition in weight percent:
Mg
5.0-6.0
Mn
>0.6-1.2
Zn
0.4-1.5
Zr
0.05-0.25
Cr
0.3 max.
Ti
0.2 max.
Fe
0.5 max.
Si
0.5 max.
Cu
0.4 max.
Ag
0.4 max.
By the invention we can provide alloy plate or extrusion having higher strength than AA5083, and particularly the welded joints of the present alloy can have higher strength than the standard AA5083 welds. Alloys of present invention have also been found with improved long term stress and exfoliation corrosion resistances at temperatures above 80° C., which is the maximum temperature of use for the AA5083 alloy.
The invention also consists 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 140 MPa.
It is believed that the improved properties available with the invention, particularly higher strength levels in both work-hardened and soft tempers, result from increasing the levels of Mg and Zn, and adding Zr.
The present inventors consider that poor exfoliation and stress corrosion resistances in AA5083 may be attributed to the increased extent of precipitation of anodic Mg-containing intermetallics on the grain boundaries. The stress and exfoliation corrosion resistances at higher Mg levels can be maintained by precipitating preferably Zn-containing intermetallics and relatively less Mg-containing intermetallics on the grain boundaries. The precipitation of Zn-containing intermetallics on the grain boundaries effectively reduces the volume fraction of highly anodic, binary AlMg intermetallics precipitated at the grain boundaries and thereby provides significant improvement in stress and exfoliation corrosion resistances in the alloys of the present invention at the higher Mg levels employed.
The alloy plates of the invention can be manufactured by preheating, hot rolling, cold rolling with or without inter-annealing and final annealing of an Al—Mg alloy slab of the selected composition. The conditions are preferably that the temperature for preheat in the range 400-530° C. and the time for homogenisation not more than 24 h. The hot rolling preferably begins at 500° C. Preferably there is 20-60% cold rolling of the hot rolled plate with or without interannealing after 20% reduction. The final and intermediate annealing is preferably at temperatures in the range 200-530° C. with a heat-up period of 1-10 h, and soak period at the annealing temperature in the range 10 min to 10 h. The annealing may be carried out after the hot rolling step and the final plate may be stretched by a maximum of 6%.
Details of extrusion processes are given below.
The reasons for the limitations of the alloying elements and the processing conditions of the aluminium alloy according to the present invention are described below.
All composition percentages are by weight.
Mg: Mg is the primary strengthening element in the alloy. Mg levels below 5.0% do not provide the required weld strength and when the addition exceeds 6.0%, severe cracking occurs during hot rolling. The preferred level of Mg is 5.0-5.6%, more preferably 5.2-5.6%, as a compromise between ease of fabrication and strength.
Mn: Mn is an essential additive element. In combination with Mg, Mn provides the strength in both the plate and the welded joints of the alloy. Mn levels below 0.6% cannot provide sufficient strength to the welded joints of the alloy. Above 1.2% the hot rolling becomes increasingly difficult. The preferred minimum for Mn is 0.7% for strength and the preferred range for Mn is 0.7-0.9% which represen
Haszler Alfred Johann Peter
Sampath Desikan
Corus Aluminium Walzprodukte GmbH
Ip Sikyin
Steven Davis Miller & Mosher LLP
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