Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2001-01-23
2003-05-20
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S697000, C148S701000
Reexamination Certificate
active
06565684
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a manufacturing process for hollow pressure vessels, particularly compressed gas cylinders made of aluminum alloy AlZnMgCu, in other words in the 7000 series according to the Aluminum Association's nomenclature.
DESCRIPTION OF RELATED ART
The use of aluminum alloys in the 7000 series for manufacturing hollow pressure vessels has been known for many years; these alloys have a high mechanical strength in the heat treated temper, such that the weight of the manufactured product can be reduced. Manufacturing includes casting and homogenization of billets, reverse extrusion of a cylindrical casing, tapering of the cylinder neck and heat treating by solution heat treatment, quenching and aging. The other properties required for this application are formability, particularly for the cylinder necking operation, good resistance to stress corrosion and inter-crystalline corrosion, and obtaining a ductile behavior during bursting tests under internal hydraulic pressure.
Patent FR 2510231 filed by the applicant describes the use of a 7475 type alloy for this application with the following composition (% by weight):
Zn=5.6-6.1, Mg=2.0-2.4, Cu=1.3-1.7, Cr=0.15-0.25, Fe<0.10, Fe+Si<0.25. The reverse extrusion operation may be done hot or cold.
Patent EP 0081441 filed by the applicant describes a process for manufacturing high strength and high toughness extruded products made of 7049A alloy with the following composition:
Zn=7.2-9.5, Mg 2.1-3.5, Cu=1.0-2.0, Cr=0.07-0.17, Mn=0.15-0.25, Fe<0.10, Si<0.08, Zr=0.08-0.14.
The product is extruded at a temperature of the order of 400° C.
Patent EP 0257167 filed by the applicant describes the use of a 7060 alloy with the following composition:
Zn=6.25-8.0, Mg=1.2-2.2, Cu=1.7-2.8, Cr=0.15-0.28, Fe<0.20, Fe+Si<0.40, Mn<0.20.
Patent EP 0589807 is a variant of the previous patent in which Cr is replaced by Zr (0.10-0.25%). Cylinders made of 7060 are produced industrially by hot extrusion.
Patent application WO 94/24326 by Alcan International describes a process for manufacturing a hollow pressure vessel starting from an alloy with composition Zn=5.0-7.0, Mg=1.5-3.0, Cu=1.0-2.7, Fe<0.30, Si<0.15, a recrystallization inhibitor (particularly Cr or Zr) with a content 0.05-0.4, with a microstructure such that the fraction of the S phase (CuMgAl
2
) by volume is kept below 1%, and preferably below 0.2%. According to the application, this microstructure is obtained by homogenization of the billet at about 475° C. with a low rate of temperature rise when getting close to this value. Preferably, for cost reasons, extrusion is done cold or slightly warm. Here, aging is an overaging which is applied such that the yield strength is about 20% below the peak to improve the toughness, fatigue resistance, and resistance to crack propagation and stress corrosion. An alloy with the claimed composition was subsequently recorded at the Aluminum Association with the designation 7032.
Patent application EP 0670377 made by Pechiney Recherche applies to alloys with a high mechanical strength and the following composition:
Zn=7-13.5, Mg=1.0-3.8, Cu=0.6-2.7, Mn<0.5, Cr<0.4, Zr<0.2
possibly transformed by extrusion to obtain hollow vessels. Homogenization and solution heat treating are carried out at 10° C. below and preferably 5° C. below the incipient melting temperature under conditions such that in T6 temper, the absolute value of the specific energy associated with the DTA (differential thermal analysis) signal is less than 3 J/g.
For some applications, it is desirable to use very high strength alloys to give the minimum weight of cylinders, and also to reduce manufacturing costs; for example this is the case for portable extinguishers. One means of lowering the cost is to use cold extrusion, in other words in which the metal at ambient temperature at the beginning of extrusion, or slightly warm extrusion in which the metal is heated before extrusion to a temperature of less than 300° C., which is significantly more economic than hot extrusion in which the metal is heated to between 350 and 450° C. before extrusion.
However, cold extrusion of high strength alloys such as 7060 causes very high extrusion forces that are often incompatible with extrusion presses usually used for this type of product, or in any case reducing the life of extrusion tools. Furthermore, application of the information in WO 94/24326 to the 7060 alloy concerning the billet homogenization temperature (more than 470° C.) often means that the alloy incipient melting temperature is reached during homogenization.
Thus, the purpose of the invention is to develop a procedure for manufacturing high strength hollow pressure vessels made of a 7000 alloy, such as 7060 alloy, by cold or slightly warm extrusion under acceptable industrial conditions, in order to give a high mechanical strength without prejudice to other properties required for this application.
SUMMARY OF THE INVENTION
The object of the invention is a process for manufacturing hollow pressure vessels, particularly compressed gas cylinders, comprising the following steps:
a) cast a billet using an alloy with the following composition (% by weight) Zn=6.25-8.0, Mg=1.2-2.2, Cu=1.7-2.8, Fe<0.20, Fe+Si<0.40, Mn<0.10, at least one of the elements belonging to the group consisting of Cr, Zr, V, Hf, Sc in the proportion 0.05-0.3, other elements <0.05 each and <0.15 total,
b) homogenization of this billet using a temperature profile such that the metal temperature is slightly less than its incipient melting temperature at all times,
c) softening annealing with a duration of 20 to 40 h between 200 and 400° C. with cooling at less than 50° C./h down to a temperature of below 100° C., such that the hardness <54 HB,
d) cutout a slug,
e) cold or slightly warm extrusion (extrusion start temperature <300° C.) of a casing,
f) necking the casing,
g) solution heat treting at a temperature slightly below the incipient melting temperature, with a duration such that the absolute value of the specific energy associated with the DTA signal is less than 3 J/g (and preferably <2 J/g)
h) quenching in cold water,
i) aging at between 100 and 200° C. for a duration between 5 and 25 h.
DESCRIPTION OF THE INVENTION
The chemical composition of the alloy is within the limits defined in patents EP 0257167 (chromium alloy) and EP 0589807 (zirconium alloy). Chromium and zirconium may be replaced by vanadium, hafnium or scandium. Preferably, the contents will be (individually or in combination) Zn>6.75%, Mg<1.95%, Fe<0.12%, Fe+Si<0.25%, Mn<0.10%.
The alloy is cast in billets in a manner known per se, for example by semi-continuous casting.
Homogenization is done with a temperature profile such that the alloy temperature is a few degrees C below the incipient melting temperature of the alloy, that may vary from 470 to 485° C. depending on the alloy composition, at all times. It is important that homogenization is sufficient, otherwise there is a risk of seeing cracks appearing during extrusion due to the alignment of coarse copper phases (for example AlCuZn) and causing dissolution of local melting, leading to decohesions, burning or porosity. The homogenization quality may be evaluated by differential enthalpic analysis. Insufficient homogenization will cause initial melting with a large endothermic peak, indicating metastable eutectic melting (&agr;Al+S, M, T). It can be estimated that this quality is good when, as described in patent EP 0670377, the DTA thermogram indicates an absolute value of the specific energy associated with the melting peak equal to less than 3 J/g, and preferably less than 2 J/g. It will also be possible to make this check on the solution heat treated product only, and then judge the quality of the homogenization—solution treatment pair.
It is important that
Sainfort Pierre
Sigli Christophe
Combs-Morillo Janelle
Dennison, Schultz & Dougherty
Societe Metallurgique de Gerzat
Wyszomierski George
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