Method of making hollow bodies

Details Method of making hollow bodies Method of making hollow bodies

1996-01-29

1999-08-03

Ryan, Patrick

Metal treatment

Process of modifying or maintaining internal physical...

Heating or cooling of solid metal

148690, 148550, C22C 2110

Patent

active

059320370

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to a method of making a hollow body for a pressure container, using an aluminium alloy of the 7000 series. The method is particularly suitable for the manufacture of high pressure gas cylinders. There is currently competition between manufacturers of pressurised gas cylinders in aluminium, steel and composite materials.


BACKGROUND OF THE INVENTION

Basic requirements of materials for use in pressurised gas containment systems include: providing adequate fabricability to allow manufacture of the system and the capability to provide adequate strength, ductility, toughness, corrosion resistance, and resistance to all forms of time-dependence degradation of mechanical properties in the final product.
In the past, these requirements have restricted the use of aluminium alloys in commercial gas cylinders to those with peak strengths below about 450 MPa. An ill-fated attempt to exceed this strength level was made in the early 1970s, when a 7000 series aluminium alloy gas cylinder was introduced into the marketplace and resulted in the recall of all cylinders due to severe stress corrosion cracking initiating after limited service life that eventually would have led to catastrophic failures.
U.S. Pat. No. 4,439,246 (Gerzat) describes a method of making pressurised gas cylinders from 7475 alloy. A billet of the alloy was homogenised for 12 hours at 465.degree. C.; hot (or alternatively cold) extruded; necked; solution annealed and quenched; and finally aged by the two step tempering type T73 treatment.
European Patent specification 257 167 (Gerzat) reports that the products (of the aforesaid U.S. patent) were found to be unsuitable after extensive testing, despite their very high level of fracture toughness, their good mechanical strength and excellent stress corrosion resistance in the T73 condition. The problem is solved, according to the European patent specification, by use of an alloy comprising 6.25-8.0% Zn; 1.2-2.2% Mg; 1.7-2.8% Cu; 0.15-0.28% Cr; and Fe+Si preferably <0.25%. As-cast billets of this composition are subjected to hot backward extrusion; drawing; necking; solution heat treating and quenching; and precipitation heat treating to a variety of over-aged conditions.
There is a need for pressurised gas cylinders with a higher strength to weight ratio, and in which any failure is preferably confined to the cylindrical part and does not spread to or occur at either the base or the shoulder.


SUMMARY OF THE INVENTION

The present invention provides a method of making a hollow body for a pressure container, which method comprises providing a billet of composition (in wt %)


______________________________________ Zn 5.0-7.0 Mg 1.5-3.0 Cu 1.0-2.7 Recrystallisation inhibitor 0.05-0.4 Fe up to 0.30 Si up to 0.15 other impurities up to 0.05 each and 0.15 in total, Al balance said billet having a volume fraction of S phase below 1.0% extruding the billet, forming the extrusion into the shape of the desired hollow body, and over-ageing the hollow body. Preferably the alloy has the following composition: Zn 5.0-7.0 Mg 1.5-2.5 Cu 1.8-2.2 Cr and/or Zr 0.10-0.25 Fe up to 0.15 Si up to 0.08 ______________________________________
The Zn concentration is 5-7%. If the Zn concentration is too low, the alloy lacks the strength necessary to permit overageing. If the Zn content is too high, the alloy is difficult to cast by direct chill casting techniques, and the cast product is brittle and difficult to age in order to increase toughness. Alloys with higher Zn contents require higher extrusion pressures, and thus increased extrusion press costs and maintenance.
Mg acts in combination with Zn to increase hardness.
The Cu content is 1.0-2.7%, preferably 1.8-2.2%. Cu is required to permit overageing to give stress corrosion resistance. The formation of an undesired S-phase (of composition CuMgAl.sub.2) increases with increasing Cu content, but can be dealt with by homogenisation of the cast ingot (as discussed below).
Cr and/or Zr is used as a recr

REFERENCES:
patent: 3984259 (1976-10-01), Rodger et al.
patent: 4439246 (1984-03-01), Develay et al.
Chemical abstract 103:108796 Oct. 1985 Balasubramanian.
Sanders et al. Met Trans v. 9A Aug. '78 pp. 1087-1100.

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