EP0194827B1

EP0194827B1 - Superplastic forming

Info

Publication number: EP0194827B1
Application number: EP19860301667
Authority: EP
Inventors: Robert Anthony British Aerospace P.L.C. Harding; Stanley Arthur British Aerospace P.L.C. Smith
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1985-03-09
Filing date: 1986-03-10
Publication date: 1989-05-24
Also published as: DK160000C; ES552806A0; DK160000B; NO860872L; JPS61253130A; GB8605888D0; EP0194827A3; GB2175235B; DE3663458D1; ES8702187A1; NO163598C; JPH0790295B2; DK104286D0; DK104286A; GB2175235A; GB8506157D0; NO163598B; EP0194827A2

Description

This invention relates to the manufacture of hollow vessels such as fluid containers and pressure vessels, by the superplastic expansion of a preform made of superplastically deformable metal.
For convenience, the phrase «superplastic material» is used to describe material capable of undergoing superplastic deformation.
It is known in the art to form hollow vessels by expanding preforms fabricated from superplastic material in sheet form. In such methods, two or more sheets of superplastic material are fusion welded together around their peripheries to form an envelope which is then superplastically expanded against a mould tool to form the vessel. The applicants have made vessels in this way and it is found that the local microstructures resulting from tthe fusion welding do not exihibit super- plastic properties to any great degree. In addition, the presence of a seam is thought to degrade the structural integrity of the container and moreover the finished product will have an external flange remaining adjacent the seam which will need to be machined away if the container is to have a smooth outer surface.
Most of the prior art examples of structures known to the applicants are formed from a pre- form 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 the material to undergo superplastic forming should have generally isotropic properties (which can be developed in sheet) rather than the unidirectional properties of rod or bar material which arise as a consequence of its forming process.
In FR-A-22 445 428, however, it is proposed to make a hollow tubular preform by drawing or extruding a metal disc, to subject the preform to a surface finishing process, for example polishing or 'ironing' and then, after a pre-heating step, to superplastically deform the preform to the shape of a container.
According to the invention, there is provided a method of making a hollow vessel, for example a fluid container or pressure vessel, wherein a hollow pre-form made of superplastically deformable metal is expanded to the required shape of the vessel by applying pressure within the pre-form while maintaining it at a temperature permitting superplastic deformation of the metal, characterised in that, to make the pre-form, there is taken a length of solid superplastically deformable metal bar, and then the central region extending along and around the central axis of the length of bar is removed by machining so as to leave the pre-form with al wall having a grain structure suitable for superplastic deformation.
Preferably, an axially extending hole is drilled in a length of solid hot-rolled titanium alloy bar.
Advantageously, the pre-form is expanded within a mould by applying a controlled pressure within the pre-form to achieve an initial expansion of the pre-form by superplastic deformation and then, with a strain rate of the pre-form metal less than that during the initial expansion and over a substantial period of time, further expansion of the preform by a creep deformation process.
For a better understanding of the invention, reference will now be made by way of example, to the accompanying drawings, in which:

Figure 1 is a schematic section view of a preform;
Figure 2 is a schematic section view of the pre- form located in a mould prior to forming;
Figure 3 is a schematic section view of a pressure vessel formed by expansion of the preform element; and,
Figure 4 is a sectioned view of another preform element located in its mould prior to forming.

Referring to the drawings, the preform comprises a main body portion 1 which is formed by cutting a length of bar and drilling or machining it to form a blind bore. The bar 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 2 and spout 3 are thoroughly degreased and then 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 portions 5, 5^1. The mould is formed of mild steel and prior to insertion of the 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 electrically to a temperature of 930 °C ± 5 °C and allowed to stabilise.
Once the temperature of the preform has reached the superplastic forming temperature (typically between 900 °C 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 1.4-2.8 x 10⁶ Pa (200-400 psi)) whereupon this pressure is maintained for a period to suit the super-plastic forming strain rate, and then released (the whole process taking approximately three (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 relevant only for the example under consideration. Appropriate parameters for 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 is the material employed; however, it is believed that the above described method has applications with other superplastic material.
In design of an appropriate preform, it is the ratio of wall thickness to internal diameter that is the critical factor in determining the forming pressure cycle. The preform may be shaped internally and externally by conventional machining methods, to form, for example, 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 bar from which the body portion 1, plug 2 and spout 3 of the preform 4 are made can be produced by hot rolling as mentioned earlier or alternatively could be made by a forging process.
The hot rolling or forging will have been carried out so that, after the central bore is formed in the bar, the body portion 1 is left with a cylindrical wall in which the metallurgical grain dimensions are not too elongated in any one direction, i.e. axially or circumferentially.
Particularly where the article to be produced is to 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 of the pressure within the preform that the major part of the expansion of the preform occur by a super- plastic deformation process while the remainder, i.e. the final expansion of the preform into the sharp corners of the mould, occurs by a creep deformation process. For example, while maintaining the preform at its superplastic deformation temperature throughout the process, the pressure applied within the preform can be controlled to maintain a strain rate within the preform material in or around the range 10-³ to 10-⁴ S-¹, this giving the superplastic deformation stage of the process, and then the pressure is maintained constant, or very gradually increasing, at a value producing a strain rate of around 10-⁶ S-¹ for quite a large proportion (around one third say) of the overall deformation process time to produce the final creep deformation stage.
Figure 4 illustrates the manufacture of another hollow article, namely a two-compartment pressure vessel for use in the manufacture of cold dry air. The preform comprises two cylindrical pieces of bar 41 and 42 each of which has been drilled from one end to form a blind hole therein. A radial hole is also drilled in each piece and the two are joined by a length of Titanium alloy tube 43 fixed by welding into the respective radial holes. The open end of the hole in the piece 41 is closed by welding a solid Titanium alloy plug 44 therein while the open end of the 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 46 so that they 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 off and exit and entry holes (not shown) for the air to be processed are drilled in the originally closed ends of the pieces 41 and 42.

Claims

1. A method of making a hollow vessel, for example a fluid container or pressure vessel, wherein a hollow pre-form made of superplastically deformable metal is expanded to the required shape of the vessel by applying pressure within the pre- form while maintaining it at a temperature permitting superplastic deformation of the metal, characterised in that, to make the pre-form (4), there is taken a length of solid superplastically deformable metal bar, and then the central region extending along and around the central axis of the length of bar is removed by machining so as to leave the pre-form (4) with a wall having a grain structure suitable for superplastic deformation.

2. A method according to Claim 1, wherein an axially extending hole is drilled in a length of solid hot-rolled Titanium alloy bar (1).

3. A method according to Claim 1 or Claim 2, wherein the pre-form is expanded within a mould by applying a controlled pressure within the pre- form to achieve an initial expansion of the pre- form by superplastic deformation and then, with a strain rate of the pre-form metal less than that during the initial expansion and over a substantial period of time, the pre-form is further expanded by a creep deformation process.