GB2175235A - Superplastic forming of hollow articles - Google Patents

Abstract

A fluid container or pressure vessel is made by superplastic expansion of a preform made either by machining a hollow space within a solid piece of metal or alternatively by extruding a tubular preform and blanking off at least one of its ends. The preform 4 illustrated is made by machining a bore in a rod, and attaching a plug and spout 3 by welding. The preform 4 is located in a two-part mould 5,5' and superplastically expanded at high temperature by pressurised argon gas supplied through spout 3. Shielding gas is supplied to the interior of a muffle box 6. In an alternative embodiment, a pressure vessel having two compartments, Figure 4 (not shown) is superplastically formed. <IMAGE>

Description

SPECIFICATION Superplastic forming This invention relates to the manufacture of hollow articles such as fluid containers and pressure vessels, by the superplastic expansion of a preform made of superplasticallydeformable metal.

It is known in the art to form hollow articles by expanding preforms fabricated from superplastic material in sheet form. In such methods, one, two or more sheets of superplastic material are fusion welded together aroundtheir peripheriesto form an envelope which is then superplastically expanded againsta mould tool to form a structure. The appli cantshavemadevessels in this way and itisfound that the local microstructures resulting from thefusion welding do not exhibit su perplastic properties to any great degree.In addition, the presence of a seam is thought to degrade the structural integrity of the container and moreoverthefinished productwill have an external flange remaining adjacenttheseam which will need to be machined away if the container isto have a smooth outer surface.

According to one aspect ofthe invention, there is provided a method of making a hollow article, for example a fluid container or pressurevessel,which method includes making a pre-form by machining a hollow space within a solid piece of superplastically deformable metal and then superplastically expanding the preform to the required shape of said article by applying pressure within said hollow space while maintaining the preform at a temperature permitting superplastic deformation of said metal.

According to a second aspect, there is provided a method of producing a container forfluid, said method including the steps of; (i) selecting a solid blank of a superplastic metal material, (ii) forming an elongate passage in said blankthereby to create a preform element, (iii) introducing said preform into a mouldtool, (iv) expanding said preform superplastically under selected conditions oftemperature and pressure to conform to said mould tool.

Preferably, said preform comprises a length of rod having a bore machined therein.

Conveniently, said method further includes the steps of attaching a hollow plug to the mouth of said blind passage by welding, and the step of introducing pressurised fluid into said passage via said plug, thereby to expand said preform.

Preferably said mould tool isofsimilaraxial extent to said preform.

The invention also extends to a pressure vessel whenever manufactured in accordance with the above method.

All of the prior art examples of structures known to the applicants areformedfrom a preform which is fabricated from sheet or plate material. The Applicants themselves previously thought it impossible, or at least highly undesireable, to use any material other than sheet because it was thought necessarythatthe material to undergo superplastic forming should have generally isotropic properties (which can be developed in sheet) rather than the unidirectional properties of bar which arise as a consequence of its formed ing process. For convenience, the phrase "superplastic material" is used to described material capable of undergoing superplastic deformation.

By way of example only, an embodiment of pressure vessel manufactured in accordance with this i nvention will now be described in detail, reference being made to the accompanying drawings, in which: Figure lisa schematic section view of a preform element, Figure2 is a schematic section view of the preform element located in the mouldtool priortoforming, Figure3 is a schematic section view of the pressure vessel formed by expansion ofthe preform element, and Figure 4 is a sectioned view of another preform element located in its mould tool prior to forming.

Referring to the drawings, the preform comprises a main body portion 1 which is formed by cutting a length of rod and drilling or machining it to form a blind bore. The rod is hot-rolled, centreless ground pickled and produced by normal metallurgical methods from the alloy Ti 6A1 4V.

An end plug 2 is turned from the same rod material as the main body portion 1 and a spout 3 is formed of the same material. The main body portion 1, end plug 2andspout3arethoroughlydegreasedandthen electron beam welded together to form in combination a preform 4.

The preform is cleaned and degreased and then placed in a mould tool comprising mould tool por tions5,51.Themould is formed of mild steel and prior to insertion ofthe preform the interior of the mould is degreased, spray coated with Yttria release agent in a solvent base, and then baked in an oven to drive off the solvent.

The assembled mould tool with the preform inside is placed in a stainless steel muffle box 6 which is subsequently sealed by welding, but having inlets for shielding gas and for forming gas. The muffle box and its contents are then placed in a cold press. Low pressure gas is allowed to flood the muffle box via inlet 7 and the cold press is then heated electricallyto a temperature of 930 C+ FC and allowed to stabilize.

Oncethetemperature of the preform has reached the superplastic forming temperature (typically between 900at and 950'C) argon gas at pressure is admitted to the preform via spout 3. The pressure is controlled to increase gradually over a period from atmospheric to the forming pressure (typically 200400 psi) whereupon this pressure is maintained for a period to su it the superplastic forming strain rate, and then released (the whole process taking approxi matelythree hours).

The mould is then allowed to cool to a temperature sufficiently low to allow handling and the muffle box and its contents are removed from the press and disassembled to allow removal of the formed component.

It is emphasised that in the above example the ranges of values of temperature and pressure are relevantonlyforthe example under consideration.

Appropriate parametersfor other preforms can be calculated from knowledge of the flow stress of the superplastic material concerned and assessment of the internal pressure required to generate an approximately equivalent hoop stress.

In the above example Ti 6A1 4V isthe material employed; however, it is believed thatthe above described method has applications with othersuperplastic material.

In design of an appropriate preform, it is the ratio of wall thickness to internal diameterthatisthecritical factor in determining the forming pressure cycle. The preform may be shaped internally and externally by conventional machining methods, to form, for exam- ple, external lugs or internal ribs.

In the above example, the mould tool comprises two portions of mild steel. It can however be formed of stainless steel and indeed this is preferred for it obviates the need for a muffle box.

The rod from which the body portion 1, plug 2 and spout3 ofthe preform 4are made can be produced by hot rolling as mentioned earlier or alternatively could be made by a forging process. As a further possibility, the body portion 1 could be produced by extrusion as an open-ended hollow cylinder which is then fitted with plug 2 and spout3 at one end while a solid blanking plug (not shown) is welded in place at its other end.

For the best results, the length of rod and/orthe details of the process (ie the particular parameters of the hot rolling, forging or extrusion process) is or are selected so that the metallurgical grain dimensions of the wall of the preform are not too elongated in any one direction, ie axially or circumferentially.

Particularly where the article to be produced isto have one or more fairly sharp corners, such as the peripheral corners where the cylindrical part of the completed pressure vessel shown in Figure 3 meets the domed end faces of the vessel, it may be preferred to so control the application ofthe pressure within the preform thatthe major part ofthe expansion ofthe preform occurs by a superplastic deformation pro cesswhilethe remainer,iethefinal expansion ofthe preform into the sharp corners of the mould, occurs by a creep deformation process.For example, while maintaining the preform at its superplastic deformation throughoutthe process, the pressure applied within the preform can be controlled to maintain a strain ratewithinthe preform material in or around the range 10-3 to 10-4, this giving the superplastic deformation stage ofthe process, and then the pressure is maintained constant, orvery gradually increasing, at a value producing a strain rate of around 1 or6 for quite a large proportion (around one third say) of the overall deformation processtimeto produce the final creep deformation stage.

Figure 4 illustrates the manufacture of another hollow article, namelyatwo-compartmentpressureves- selforuseinthemanufactureofcolddryair.The preform comprises two cylindrical pieces of rod 41 and 42 each of which has been drilled from one endto form a blind holetherein.Aradial hole is also drilled in each pieceandthetwoarejoinedbyalengthof titanium alloy tube 43 fixed by welding into therespective radial holes. The open end ofthe hole in the piece 41 is closed by welding a solid titanium alloy plug 44therein while the open end ofthe hole in piece 42 is fitted with a plug 2 and spout 3 of Figure 1 .Both pieces 41 and 42 are then superplastically expanded by the application of pressure to spout 46so thatthey take up the shape of the mould 47. After expansion, plug and spout 45 and 46 are replaced by a solid plug or possibly the spout 46 is simply sealed offand exit and entry holes (not shown) forthe air to be processed are drilled in the originally closed ends of the pieces 41 and 42.

Claims (3)

1. According to one aspect of the invention, there isprovideda methodofmaking a hollowarticle,for example a fluid container or pressure vessel, which method includes making a preform by machining a hollow space within a solid piece of superplastically deformable metal and then superplastically expanding the preform to the required shape of said article by applying pressure within said hollow space while maintaining the preform at a temperature permitting superplasticdeformation of said metal.

2. A method of making a hollow article, for example a fluid container or pressure vessel, which method includes making a tubular preform by an extrusion process, closing off at least one end of the interior ofthe preform, and then superplastically expanding the preform to the required shape of said article by applying pressure within said interiorwhile maintaining the preform at a temperature permitting superplasticdeformation of said metal.

3. A method of producing a containerforfluid, said method including the steps of: (i) selecting a solid blank of a superplastic metal material, (ii) forming an elongate blind passage in said blank therebyto create a preform element, (iii) introducing said preform into a mould tool, (iv) expanding said preform superplasticallyunder selected conditions oftemperature and pressure to conform to said mould tool.